New Capabilities for Molecular Surface and In-Depth Analysis with Cluster Secondary Ion Mass Spectrometry

New Capabilities for Molecular Surface and in-Depth Analysis with Cluster Secondary Ion Mass Spectrometry

A thesis submitted to the University of Manchester for the degree of Doctor of Philosophy in the Faculty of Science and Engineering

2018

Huriyyah Ahmed Alturaifi

The University of Manchester

Faculty of Science and Engineering

School of Chemistry

Contents

List of Figures 6

List of Tables 15

List of Equations 17

Abbreviations 18

Abstract 20

Declaration 21

Acknowledgements 22

1. Introduction 23 1.1. Secondary Ion Mass Spectrometry (SIMS) 23 1.2. Generation of Secondary Ions 25 1.2.1. The Sputtering Process 26 1.2.2. Ionisation 29 1.2.2.1. Nascent Ion Molecule Model 29 1.2.2.2. Deposition Ionisation Model 30 1.3. Models of Operations of SIMS 31 1.3.1. Dynamic SIMS 31 1.3.2. Static SIMS 32 1.3.3. Imaging SIMS 32 1.4. Cluster SIMS 33 1.5. Damage Cross-Section 36 1.6. Cross-linking 38 1.7. Molecular Depth Profiling 41 1.8. Molecular Dynamic Simulation 58 1.9. Aims of the Study 61 1.10. References 62 2. Instrumentation 70 2.1. ToF-SIMS Instrumentation 70 2.1.1. ToF Mass Analyser 70 2.1.2. ToF-SIMS Instrumentation Development 73 2

2.1.2.1. Sample Holder 76 2.1.2.1.1. Sample Holder insertion into the Instrumentation 77 2.1.2.1.2. Sample Handling Systems 78 2.1.2.2. Instrumentation Control 79 2.1.2.3. Electron Flood Gun 80

2.1.2.4. Polyatomic C60 Source 81 2.1.2.4.1. Mass Selection (Mass Filtration) 82 2.1.2.4.2. Spatial Resolution 83 2.1.2.5. Gas Cluster Ion Beams 84 2.1.2.5.1. Cluster Size Measurement 86 2.1.2.6. The J105 Secondary Ion Optics 86 2.1.2.7. Tandem Mass Spectrometry (MS/MS) 88 2.2. Film Deposition Techniques 88 2.2.1. Spin-Coating 88 2.2.2. Thermal Evaporation Films Deposition 90 2.3. Instrumentation for Measurements of Film Thickness 90 2.3.1. Stylus Profilometry 91 2.3.2. Atomic Force Microscopy (AFM) 91 2.3.2.1. How the Atomic Force Microscopy Work 91 2.3.2.1.1. The Atomic Force Microscopy Design 91 2.3.2.1.2. Principle of the Atomic Force Microscopy 92 2.3.2.1.3. Tapping Mode (Intermittent Mode) AFM Imaging Modes 93 2.4. References 94 3. Molecular Depth Profiles of PMMA with Time-of-Flight Secondary Ion Mass + Spectrometry (ToF-SIMS) Using Different Cluster Ion Beams (20 and 40 keV C60 + and 20 keV Arn ) at Different Temperatures 97 3.1. Introduction 97 3.2. Experimental Section 99 3.2.1. Sample Preparation 99 3.2.2. ToF-SIMS 100 3.3. Results and Discussion 101 3.3.1. PMM Sample Analysis ar Room Temperature 101 3.3.1.1. Secondary Ion Spectra of PMMA 101 3.3.1.2. Secondary Ion Yields at the Pseudo-Steady-State Region 107 3.3.1.3. Depth Profiling 108

+ 3.3.1.3.1. Using 20 and 40 keV C60 Cluster Ion Beams 112

3.3.1.3.2. Using 20 keV Arn Cluster Ion Beams (n = 250-2000) - 114 3.3.2. Influence of Sample Cooling on the Depth Profiles of PMMA 122 + 3.3.2.1. Using 20 and 40 keV C60 Cluster Ion Beams 122 + + 3.3.2.2. Using 20 keV Ar500 and Ar2000 Cluster Ion Beams 128 3.4. Summary and Conclusions 135 3.5. References 137 4. Molecular Depth Profiling of Organic Semiconductors with Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) Using Cluster Ion Beams (20 and + + 40 keV C60 and 20 keV Ar250-2000 ) at Different Temperatures 140 4.1. Introduction 140 4.2. Experimental Section 141 4.2.1. Sample preparation 141 4.2.1.1. PTAA 141 4.2.1.2. TIPS-penatcene 142 4.2.2. ToF-SIMS Analysis 142 4.3. Analysis of the PTAA Sample at Room Temperature 143 4.3.1. Results and Discussion 143 4.3.1.1. Secondary Ion Mass Spectra of PTAA 143 4.3.1.2. Depth Profiles of PTAA Sample at Room Temperature 148 + 4.3.1.2.1 Using 20 and 40 keV C60 Cluster Ion Beams 178 + 4.3.1.2.2 Using 20 keV Arn Cluster Ion Beams (n =250-2000) 150 4.4. Analysis of the TIPS-pentacene Sample at Room Temperature 155 4.4.1. Secondary Ion Spectra of TIPS-pentacene 155 4.4.2. Depth Profile of TIPS-pentacene Samples at Room Temperature 161 + 4.4.2.1 Using 20 and 40 keV C60 Cluster Ion Beams 161 + 4.4.2.2 Using 20 keV Arn Cluster Ion Beams (n = 250-2000) 163 4.4.3 Influence of Sample Cooling on Depth Profiles of PTAA samples 168 + 4.4.3.1 Using 20 keV C60 Cluster Ion Beams 168 4.4.4 Influence of Sample Cooling on Depth Profiles of TIPS-pentacene 170 + + 4.4.4.1 Using 20 keV Ar500 and Ar2000 Cluster Ion Beams 170 4.5. Summary and Conclusions 176 4.6. References 178

5. Investigation of the Interface between Bi-layered Organic Materials (Semiconductor and Insulating Materials), using Molecular Depth Profiling ToF- SIMS Cluster Ion Beams 180 5.1. Introduction 180 5.2. Experimental Section 181 5.2.1. Samples Preparation 181 5.2.2. ToF-SIMS Analysis 182 5.3. Results and Discussion 182 5.3.1. TIPS-Pentacene/PMMA/Si Bi-layer Films 182 5.3.2. PMMA/TIPS-pentacene/Si Bi-layer Films 189 5.3.3. PTAA/PMMS/Si Bi-layer Films 191 5.3.4. PMMA/PTAA/Si Bi-layer Films 193 5.4. Conclusions 196 5.5. References 197 6. Conclusions and Future Work 198 6.1. Conclusions 198 6.1.1. Sputtering Yields 198 6.1.2. Secondary Ion Yields at the Pseudo-Steady-State Region 201 6.1.3. Depth Resolution 203 6.2. Future work 205 6.3. References 208 7. Appendix 209

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List of Figures Figure 1.1: A schematic representation of the SIMS sputtering process. A primary ion beam strikes the sample surface causing a series of collision cascades. The ejected secondary particles are the majority of neutral (atoms or molecules (red)) and a small number of positively or negatively charged {cations (blue), anions (orange) and electrons (dark red)}. 28 Figure 1.2: Schematic presentation of the emission of the secondary ion by the nascent ion molecule model [10]. 30 Figure 1.3: Schematic of dynamic and static modes with SIMS. 31

Figure 1.4: Currents of normalized primary ions, for Aun and Bin clusters, as a function of + + cluster size and charge. Currents of Au1 and Bi1 are normalized to 100 % [31]. 34

Figure 1.5: Mechanism of PMMA degradation with irradiation. Adapted from [60]. 40 Figure 1.6: Different categories of molecular depth profiling of thin films (a-c) and bulk materials. (a) The ideal shape of the depth profile achieved under optimum conditions. (b and c) The depth profiles obtained under non-optimum conditions and result accumulation of damage, but a greater damage occurs in (c) as compared to (b). Three regions displayed during depth profiles are: (1) the signal intensity is decreased or increased in an initial surface transient region; (2) a steady state region (a) or pseudo steady state region (b), and (3) an interfacial region, in which molecular signal decreases and substrate signal increases. In bulk samples (d) after a certain critical fluence (4) signal intensity is lost. This critical fluence is affects by the choice of ion source and other parameters e.g. energy [50].- 41

Figure 1.7: Comparison of depth profiles, for a 180 nm thick sample. The sample is a vapor- + + deposited glutamate thin film, using SF5 and Ar primary ions, under dynamic SIMS conditions. + The required SF5 primary ion dose that could penetrate up to reach the silicon substrate was 2.4 × 1015 ions/cm2 [52]. 43 Figure 1.8: Positive secondary ion intensities of an ion fragment (m/z 69) plotted as a function of sputter depth, for ~ 160 nm of PMMA film, deposited on a Si substrate, in a + + dual beam mode: 10 keV Ar , for analysis and 5 keV SF5 , for sputtering. Depth profiles were acquired at three different temperatures, as displayed inside this Figure [57]. 45

+ Figure 1.9: Positive depth profiles for bulk PMMA films, using 5 keV and 8 keV SF5 primary ions, at -100 °C. The PMMA (m/z 59) was plotted as a function of sputter depth [61]. 46 Figure 1.10: Positive ions ToF-SIMS depth profiles of a PS thin film, working with a 20 + keV C60 primary ion beam, at 48° and 76° incidence angles. The secondary ion intensity for m/z 91, of PS, was plotted as a function of sputter depth [64]. 48

+ Figure 1.11: Depth profiles of (a) PVP/PMMA and (b) P4VP/PMMA, working with C60 , for sputtering and Ga+, for analysis [65]. 50 Figure 1.12: ToF-SIMS depth profiles of a) PMMA, b) PS and c) PC in positive secondary + ion spectra, obtained in a single beam mode, working with 5.5 keV Ar700 , for both sputtering and analysis purposes [67]. 52

Figure 1.13: Sputtering yield volume versus molecular weight for: a) PS and b) PMMA, obtained via three cluster sizes of Ar, which are 1500, 3000 and 5000 atoms/primary ion, with an incident energy of 10 keV [73]. 54 Figure 1.14: Sputtering yield volume per cluster atom, Y/n, versus energy per atom, E/n in cluster for: a) PS and b) PMMA, obtained under the bombardment of a 10 keV Ar+ cluster, using selected cluster size, in the range of 1250-7000 atoms/ion [73]. The dash line as seen in a) for PS from Seah’s equation (see below note) [74]. 54

+ + + Figure 1.15: Depth profiles of Alq3 and NPD , using 5.5 keV Ar700 . (a) For ITO/Alq3 (100 nm)/NPD (100 nm), b) For Si/NPD (40 nm)/Alq3 (80 nm)/NPD (40 nm), c) Si/Alq3 (20 nm)/NPD (120 nm) with fluence [77]. 55 Figure 1.16: Sputtering yields plotted as a function of kinetic energy: 2.5, 5 and 10 keV, + + for Ar1000 and Ar2000 [69]. 56 Figure 1.17: Sputtering yields of NPB and Irganox 1010 is plotted as a function of the + sample temperature with the transition temperatures, using Ar2000 at a 5 keV energy beam [79]. 57 Figure 1.18: Molecular dynamic simulation. Cross-sectional view of the temporal evolution of a typical collision event leading to ejection of atoms, under 15 keV, Ga and C60 projectiles, bombarding Ag {111} surface, at normal incidence. The ejected atoms are coloured as the colour of their original layers in the substrate and the projectile atoms are coloured black [82]. 60

Figure 2.5: Schematic diagram of the J105 3D Chemical Imager [11]. 76 Figure 2.6: The image of the rectangle copper sample holder, in the J105 instrumentation, used to mount the substrate onto it. 77 Figure 2.7: Imaging of the J105 sample handling system (J105 Instrumentation Manual). 78 Figure 2.8: Image of Z Lift consisting of three Tiers to keep three samples (one in each Tier) in the preparation chamber of the J105 instrumentation (J105 Instrumentation Manual). 79 Figure 2.9: The image of a Dewar containing copper coils. The Dewar is continuously filled up with liquid nitrogen (LN2) to just over the level of the coils, during a cold stage process. 80

Figure 2.10: Schematic diagram of C60 gun ion optics [5]. 82

Figure 2.11: Schematic diagram of an electron ionisation source [6]. 82 Figure 2.12: Schematic diagram of a Wien filter whose purpose is to select primary ion beam based on their m/z ratios [6]. 83

+ Figure 2.13: The lateral resolution measurement using the 40 keV C60 ion beam with the 30 µm aperture is demonstrated to be 1.3 µm using a profile line scan (20-80%), 300 mesh copper grid imaged using the SED [16]. 84 Figure 2.14: The schematic diagram of gas cluster ion beams (GCIB) system [20]. 85 Figure 2.15: An Oscilloscope was used to read the time-of-flight of the selected cluster ions. The average flight time is 44.8 µs. This is achieved by measuring (using an oscilloscope) the time delay between the detected secondary electron peak (lower trace) + and the ion beam pulser (upper trace). The shown result is that of a cluster size of Ar1000 when the energy of the beam is 20 keV. 86 Figure 2.16: Schematic diagram of the buncher connecting with the reflectron of the ToF mass analyser in the J105 instrumentation [9]. 88 Figure 2.17: The type of spin-coater device (Model WS-400BX-6NPP/LITE) was utilised to prepare a film on a substrate e.g. a silicon wafer (In SIMS lab and in OMIC lab). 89 Figure 2.18: Schematic diagram of the thermal evaporation process. 90 Figure 2.19: A schematic diagram of the AFM [24]. 92 Figure 2.20: Schematic representation of the operation of the tapping mode AFM [25]. 94 Figure 3.1: The chemical structure of the repeating unit of PMMA produced from monomer methyl methacrylate MMA, using free radical vinyl polymerization. 97 Figure 3.2: A schematic diagram of the dual beam approach in ToF-SIMS depth profiling [1]. 98 Figure 3.3: Positive secondary ion mass spectra of PMMA films, obtained after an ion dose of 5 × 1012 ions/cm2 was delivered, using primary ion beams, of: (a) 40 keV and (b) + 20 keV C60 at room temperature. 104 Figure 3.4: Positive secondary ion mass spectra of PMMA films, obtained after an ion 12 2 + dose of 5 × 10 ions/cm was delivered, using cluster ion beams, of: (a) Ar250 and (b) + Ar500 , working at 20 keV incident energy and room temperature. 105 Figure 3.5: Positive secondary ion mass spectra of PMMA films, obtained after an ion 12 2 + dose of 5 × 10 ions/cm was delivered, using two cluster ion beams, of: (a) Ar1000 and + (b) Ar2000 , working at 20 keV incident energy and room temperature. 106 Figure 3.6: Positive secondary ion yields of PMMA fragments (m/z 126, m/z 139 and m/z + + + 186) with various ion beams, including 40 keV C60 , 20 keV C60 and 20 keV Ar250 , + + + 11 2 Ar500 , Ar1000 and Ar2000 , using primary ion dose of 5 × 10 ions/cm . The ion yields were determined from an average of three layers from the pseudo-steady-state regions; all experiments were performed at room temperature (25 °C). The accumulated ion dose were: 13 12 12 2 + 13 13 2.71 × 10 , 5.42 × 10 and 7.59 × 10 ions/cm at 40 keV C60 ; 1.47 × 10 , 1.78 × 10 13 2 + 13 13 13 2 and 2.03 × 10 ions/cm at 20 keV C60 ; 4.24 × 10 , 1.85 ×10 and 2.61 × 10 ions/cm + 13 13 13 2 + at 20 keV Ar250 ; 1.44 × 10 , 1.90 × 10 and 2.28 × 10 ions/cm at 20 keV Ar500 ; 1.31

13 13 13 2 + 12 12 × 10 , 1.69 × 10 and 2.05 × 10 ions/cm at 20 keV Ar1000 ; and 5.91 × 10 , × 9.32 10 13 2 + and 1.29 × 10 ions/cm at 20 keV Ar2000 . 108 Figure 3.7: Depth profiles of PMMA fragment secondary ion yields (m/z 126, m/z 139 and + m/z 186) are plotted as a function of increasing primary ion dose, using a 20 keV C60 . These profiles were performed at room temperature (25 °C), on a 34 nm thick, PMMA film. 109 Figure 3.8: Normalized secondary ion intensity of the fragment ion of PMMA, m/z 186, + 2 using a 20 keV C60 , plotted as a function of: (a) ion dose (ions/cm ) and (b) depth (nm). The horizontal red line (a) indicates the interface ion dose after decreasing the secondary ion signal to 50% of its steady-state. The vertical blue lines (b) show the interface width from 81.28% to 12.89%, and the grey box shows the depth resolution (Δz). 111 Figure 3.9: Depth profiles of secondary ion yields of m/z 186 fragment of PMMA, plotted + as a function of increasing C60 primary ion doses, working at: a) 20 keV and b) 40 keV. These profiles were performed at room temperature (25 °C), on a 34 nm ± 0.1 nm thick, PMMA films. 113 Figure 3.10: Depth profiles of the m/z 186 fragment ions of the PMMA secondary ion + yields, are plotted as a function of primary ion doses, working with (a) Ar250 and (b) + Ar500 , cluster ion dose, at 20 keV. These profiles were performed at room temperature, on a 34 nm thick, PMMA film. 117 Figure 3.11: Depth profiles of the m/z 186 fragment ions of the PMMA secondary ion + yields, are plotted as a function of primary ion doses, working with: (c) Ar1000 and (d) + Ar2000 cluster ion dose, at 20 keV. These profiles were performed at room temperature, on a 34 nm thick, PMMA film. 118

Figure 3.12: The sputtering yields of PMMA are plotted as a function of the size of Arn cluster ion beams (n = 250-2000), at room temperature with an incident energy of 20 keV. 119 Figure 3.13: Combining PMMA sputtering yields (in the current work with 20 keV and a previous study with 10 keV [27]), and PS yields (in a previous study with energies from 10 + to 20 keV [23]), are plotted as a function of the size of Arn cluster ion sizes (n = 250 to 5000). 120 Figure 3.14: Sputtering yields of PMMA are plotted as a function of the energy per atom. + + + A constant energy was 10 keV, with Ar1500 , Ar3000 and Ar5000 cluster ions (from a + + previous study) [27]. A constant energy was 20 keV, with Ar250 to Ar2000 cluster ions (from the current work). 121 Figure 3.15: Depth resolution of PMMA is plotted as a function of Ar cluster ion beams size (n = 250-2000), at room temperature, with a 20 keV beam. 122 Figure 3.16: Depth profiles of the m/z 186 fragment ions of the PMMA secondary ion + yields are plotted as a function of increasing C60 primary ion dose at: a) 20 keV and b) 40 keV. The profiles were performed at room temperature, 25 °C, (before and after the cooling process of the films) and at cryogenic temperatures (from -100 to -170 °C), on a 34 nm ± 0.1 nm thick, PMMA films. 126 Figure 3.17: The sputtering yields of PMMA are plotted as a function of temperatures + (°C), using 20 and 40 keV C60 . Symbols (*) and (^) refer to before and after the cooling of the PMMA film, respectively. 127

Figure 3.18: Depth resolution of PMMA is plotted as a function of temperatures (°C) with + 20 and 40 keV C60 cluster ion beams. Symbols (*) and (^) refer to before and after the cooling of the PMMA film, respectively. 127 Figure 3.19: A comparison of secondary ion yields of the m/z 186 PMMA film fragment ions at room temperature (before and after the cooling of the samples of PMMA) and below room temperatures, averaged from three selected layers within the pseudo-steady- + 13 13 state region with an accumulated 20 keV C60 primary ion dose of 1.26 × 10 , 1.47 × 10 13 2 + 12 and 1.68 × 10 ions/cm and an accumulated 40 keV C60 primary ion dose of ~ 4 × 10 , 7 ×1012, and 9 × 1012 ions/cm2 (Symbols (*) and (^) indicate before and after cooling the films, respectively). 128 Figure 3.20: Depth profiles of PMMA fragment, m/z 186; secondary ion yields plotted as a + + function of primary ion dose: a) Ar500 and b) Ar2000 cluster ion beams, at 20 keV. The profiles were performed at room temperature, 25 °C, (before and after the cooling process of the films), and at cryogenic temperatures (-50 and -150 °C), on a 34 nm PMMA film. 131 Figure 3.21: The sputtering yields of PMMA are plotted as a function of temperatures + + (°C), with Ar500 and Ar2000 cluster ion beams, at an incident energy of 20 keV. Symbols (*) and (^) refer to before and after cooling of the PMMA film, respectively. 132

Figure 3.22: Depth resolution of PMMA is plotted as a function of temperatures (°C), with + + Ar500 and Ar2000 cluster ion beams, at an incident energy of 20 keV. Symbols (*) and (^) refer to before and after cooling of the PMMA films, respectively. 132

Figure 3.23: Secondary ion yields at (before and after cooling of the PMMA film) and below room temperature, for m/z 186 PMMA film fragment signals, averaged from three + selected layers within the steady-state region with an accumulated 20 keV Ar500 primary cluster ion dose of 1.37 × 1013, 1.65 × 1013 and 1.93 × 1013 (ions/cm2); and an accumulated + 13 13 13 20 keV Ar2000 primary cluster ion dose of 1.16 × 10 , 1.43 × 10 and 1.69 × 10 (ions/cm2). Symbols (*) and (^) indicate before and after cooling the films, respectively. 134 Figure 4.1: Chemical structures of (a) PTAA [9] and (b) TIPS-pentacene organic semiconductor samples. 141 Figure 4.2: Positive secondary ion mass spectra of PTAA films were obtained after an ion 12 2 + dose of 5 × 10 ions/cm with C60 primary ion beams of energies: (a) 40 keV and (b) 20 keV at room temperature. The ion dose used to accumulate the spectra was 5 × 1011 ions/cm2. 145

Figure 4.3: Positive secondary ion mass spectra of PTAA films were obtained after an ion 12 2 + + dose of 5 × 10 ions/cm of: (a) Ar250 and (b) Ar500 primary ion beams using a 20 keV incident energy at room temperature. The ion dose used to accumulate the spectra was 1.68 × 1012 ions/cm2. 146

Figure 4.5: Positive secondary ion mass spectra of PTAA films were obtained after an ion 12 2 + + dose of 5 × 10 ions/cm of (a) Ar1000 and (b) Ar2000 primary ion beams using a 20 keV incident energy at room temperature. The ion dose used to accumulate the spectra was 1.68 × 1012 ions/cm2. 147

Figure 4.6: Secondary ion yields, of depth profiles of the [M-H]+ m/z 270 molecular ions and m/z 254 and m/z 168 fragment ions (related to the repeat unit of the PTAA sample) and 10

+ + + C11 from C60 projectiles, are plotted as a function of increasing C60 primary ion dose using: a) 20 and b) 40 keV. These profiles were obtained at room temperature of a 51.5 nm ± 0.8 nm PTAA film. The insert is the spectra of the last layer shown carbon deposition. 149

+ + Figure 4.7: Depth profiles of PTAA secondary ion yields, [M-CH3-2H] m/z 254, [M-3H] + + m/z 268 and [M-H] m/z 270; and secondary ion yields of Si6 m/z 168, are plotted as a + + function of increasing cluster ion dose of: (a) Ar250 and (b) Ar2000 , using 20 keV energy. These profiles were obtained at room temperature of a 51.5 nm ± 0.8 nm film thickness. 151

+ + Figure 4.8: Depth profiles of PTAA secondary ion yields, [M-CH3-2H] m/z 254, [M-3H] + + m/z 268 and [M-H] m/z 270; and secondary ion yields of Si6 m/z 168, are plotted as a + + function of increasing cluster ion dose of: (a) Ar1000 and (b) Ar2000 , using 20 keV energy. These profiles were obtained at room temperature of a 51.5 nm ± 0.8 nm film thickness. 152 Figure 4.9: Sputtering yields of PTAA are plotted as a function of Ar cluster ion beam size (n = 250-2000) at room temperature, using 20 keV. 154 Figure 4.10: Average (three selected layers from two areas) of secondary ion yields of PTAA samples in the pseudo-steady-state region at room temperature are shown as a + 14 function of eV/atom with an accumulated: Ar250 primary ion doses of 1.10 × 10 , 1.16 × 14 14 2 + 14 14 14 10 , and 1.22 × 10 ions/cm ; Ar500 ion doses of 1.19 ×10 , 1.25 × 10 , and 1.32 × 10 2 + 13 13 13 2 + ions/cm ; Ar1000 ion doses of 9.29 ×10 , 9.94 ×10 , and 1.06 × 10 ions/cm ; and Ar2000 ion doses of 4.83 × 1013, 5.40 × 1013, and 5.96 × 1013 ions/cm2, all using an incident energy of 20 keV. 155

Figure 4.11: Positive secondary ion mass spectra of TIPS-pentacene films obtained after 11 12 2 + an ion dose of: (a) 5 × 10 and (b) 5 × 10 ions/cm , using 20 keV C60 primary ion beams at room temperature. 157 Figure 4.12: Positive secondary ion mass spectra of TIPS-pentacene films obtained after 11 12 2 + an ion dose of: (a) 5 × 10 and (b) 5 × 10 ions/cm , using 40 keV C60 primary ion beams at room temperature. 158 Figure 4.13: Positive secondary ion mass spectra of TIPS-pentacene films obtained after 12 2 + + an ion dose of 5 × 10 ions/cm , using Ar cluster ion beams, (a) Ar250 and (b) Ar500 , at a 20 keV incident energy and room temperature. The ion dose used to accumulate the spectra was 5 × 1011 ions/cm2. 159 Figure 4.14: Positive secondary ion mass spectra of TIPS-pentacene films obtained after 12 2 + + an ion dose of 5 × 10 ions/cm with Ar cluster ion beams, (a) Ar1000 and (b) Ar2000 , at a 20 keV incident energy and room temperature. The ion dose used to accumulate the spectra was 5 × 1011 ions/cm2. 160

Figure 4.15: Secondary ion of depth profiles of [M]+ m/z 638 molecular ion of TIPS- + + pentacene and C11 (m/z 132) of the C60 projectiles are plotted as a function of increasing + C60 primary ion dose, using: a) 20 keV and b) 40 keV beam energies. These profiles were obtained at room temperature of a 42 nm ± 4 nm TIPS-pentacene film. Insert is a spectrum shown carbon deposition. 162 Figure 4.16: Average (twice repeated) depth profiles of [M]+ m/z 638 of the TIPS- + pentacene secondary ion yields and that of secondary ion yields of Si6 m/z 168 are plotted + as a function of increasing cluster ion dose using 20 keV beams of: (a) Ar250 and (b) 11

+ Ar500 . These profiles were obtained at room temperature of a 42 nm ± 4 nm TIPS- pentacene film. 164

Figure 4.17: Average (twice repeated) depth profiles of [M]+ m/z 638 of the TIPS- + pentacene secondary ion yields and that of Si6 m/z 168 are plotted as a function of + + increasing cluster ion dose using 20 keV beams of: (a) Ar1000 and (b) Ar2000 . These profiles were obtained at room temperature of a 42 nm ± 4 nm TIPS-pentacene film. 165 Figure 4.18: The sputtering yields of TIPS-pentacene are plotted as a function of Ar cluster ion beam size (n = 250-2000) at room temperature using 20 keV. 166 Figure 4.19: Depth resolution of TIPS-pentacene is plotted as a function of the size of Ar cluster ion beams (n = 250-2000) at room temperature using 20 keV. 167

+ + Figure 4.22: Average (three repeats at (25 °C) using Ar500 and -150 °C using Ar2000 and + + two repeats at (25 °C) and -50 °C, using Ar2000 and low temperatures using Ar500 ) depth profiles of molecular ion of [M]+ m/z 638 secondary ion yields of TIPS-pentacene, are + + plotted as a function of increasing cluster ion dose of: a) Ar500 and b) Ar2000 , at different temperatures. 171 Figure 4.23: The sputtering yields of TIPS-pentacene are plotted as a function of + + temperature (°C) using Ar500 and Ar2000 cluster ion beams at an incident energy of 20 keV (*) before and (^) after the cooling the TIPS pentacene film. 172 Figure 4.24: Depth resolution of TIPS-pentacene is plotted as a function of temperatures + + (°C) using Ar500 and Ar2000 cluster ion beams at an incident energy of 20 keV. Symbols (*) and (^) refer to before and after the cooling TIPS-pentacene film, respectively. 173 Figure 4.25: Average of secondary ion yields of three layers of the pseudo-steady-state + 13 region using: a 20 keV Ar500 (10 eV/atom) primary cluster ion dose of 1.96 × 10 2 + 13 2 (ions/cm ) and a 20 keV Ar2000 (40 eV/atom) primary ion dose of 1.30 × 10 (ions/cm ) for molecular ion [M]+ m/z 638 TIPS-pentacene as a function of temperature (at and below room temperature). Symbols (*) and (^) indicate before and after cooling the TIPS- pentacene film, respectively. 175 Figure 5.1: The structures of OFET devices: (a) top-gate/top-contact, (b) top-gate/bottom- contact (c) bottom-gate/top-contact (d) bottom-gate/bottom-contact [4]. S and D refer to source and drain, respectively. 180 Figure 5.2: Illustration of the four bi-layer films structures deposited on Si substrates. 182

Figure 5.3: Depth profiles of the bi-layer films of TIPS-pentacene on PMMA, on silicon + + + substrates. Secondary ion yields of [M] m/z 638, [C9H14O4] m/z 186 and Si6 m/z 168 are + + plotted as a function of increasing cluster ion dose of: (a) Ar250 and (b) Ar500 , using 20 keV energy beams. 184

Figure 5.4: Depth profiles of bi-layer films of TIPS-pentacene on PMMA, on silicon + + + substrates. Secondary ion yields of [M] m/z 638, [C9H14O4] m/z 186 and Si6 m/z 168 are + + plotted as a function of increasing cluster ion dose of: (a) Ar1000 and (b) Ar2000 , using 20 keV energy beams. 185 Figure 5.5: Depth resolution of the interface: between TIPS-pentacene and PMMA (Bi- layer in red) and between TIPS-pentacene and Si (Single-layer in black) was plotted as a + function of cluster ion size of Arn (n = 250-2000), at room temperature using a 20 keV energy beam. 187

Figure 5.6: Average (three layers from two areas) of secondary ion yields of the TIPS- pentacene on the PMMA film at the pseudo-steady-state region, is plotted as a function of + + 13 eV/atom with an accumulated: Ar500 and Ar1000 primary ion doses of 2.08 ×10 , 2.24 × 13 13 2 + 12 12 12 10 and 2.40 × 10 ions/cm ; and Ar2000 6.54 × 10 , 7.94 × 10 and 9.34 × 10 ions/cm2; all at an incident energy of 20 keV. 188

Figure 5.7: Depth profiles of TIPS-pentacene on PMMA, bi-layer films, on silicon + + + substrates. Secondary ion yields of [M] m/z 638, [C9H14O4] m/z 186 and C11 m/z 132, + are plotted as a function of increasing 20 keV C60 cluster ion dose. 189 Figure 5.8: Depth profiles of PMMA spin-coated on an evaporated TIPS-pentacene, bi- + + layer films, on silicon substrates. Secondary ion yields of [M] m/z 638, [C9H14O4] m/z + + 186 and Si6 m/z 168, are plotted as a function of increasing cluster ion dose of: (a) Ar250 + and (b) Ar1000 , all using 20 keV energy beams. 190

Figure 5.9: Depth profiles of PMMA on PTAA, bi-layer films, on silicon substrates. + + + Secondary ion yields of [M-H] m/z 270, [C9H14O4] m/z 186 and C11 m/z 132, are plotted + as a function of increasing C60 cluster ion dose using 20 keV energy beams. 192

Figure 5.10: Depth profiles of PTAA on PMMA, bi-layer films, on silicon substrates. + + + Secondary ion yields of [C9H14O4] m/z 186, [M-H] m/z 270 and Si6 m/z 168, are plotted + + as a function of increasing cluster ion dose of: (a) Ar250 and (b) Ar2000 , using 20 keV beams. 193

Figure 5.11: Depth profiles of PMMA on PTAA, bi-layer films, on silicon substrates. + + + Secondary ion yields of [M-H] m/z 270, [C9H14O4] m/z 186 and Si6 m/z 168, are plotted + as a function of increasing C60 cluster ion dose using 20 keV energy beams. 194

Figure 5.12: Depth profiles of PMMA on PTAA, bi-layer films, on silicon substrates. + + + Secondary ion yields of [C9H14O4] m/z 186, [M-H] m/z 270 and Si6 m/z 168, are plotted + + as a function of increasing cluster ion dose of: (a) Ar250 and (b) Ar2000 , all using 20 keV energy beams. 195 Figure 6.1: Sputtering yield per nucleon, for PMMA (top), TIPS-pentacene (middle) and PTAA (bottom) vs. energy per nucleon. 199

Figure 7.1: Synthesis of PTAA. Reagents and conditions: Pd2(dba)3, KOtBu, toluene, 105 °C, 22 h. Turner’s group has made it [9]. 209 Figure 7.2: Bombarded areas, represented as squares, signify different time/day of experimental investigations. Coloured squares, size of which is 350 µm × 350 µm per 13

represent analysis craters, due to the bombardment of the PMMA sample film, blue, sitting on a Si substrate (size 1 cm × 1 cm), with cluster ion beams, of: (a) 20 keV and (b) 40 keV, energies. The colours, detailed in the Table 7(a) below, signify different sample temperatures, in the experimental investigations, starting, as shown, from the bottom-right of the sample, going upwards, and finishing at the top left-hand of the sample. Different Si substrates were used for each cluster ion beam. 210 Figure 7.3: Bombarded areas, represented as squares, signify different time/day of experimental investigations. Coloured squares, size of which is 350 µm × 350 µm per represent analysis craters, due to the bombardment of the PMMA sample film, blue, sitting + + on a Si substrate (size 1 cm × 1 cm), with 20 keV Ar500 and Ar2000 cluster ion beams. The colours, detailed in the Table 7(b) below, signify different sample temperatures, in the experimental investigations, starting, as shown, from the bottom-right of the sample, going upwards, and finishing at the top left-hand of the sample. 211 Figure 7.4: Bombarded areas, represented as squares, signify different time/day of experimental investigations. Coloured squares, size of which is 350 µm × 350 µm per represent analysis craters, due to the bombardment of the TIPS-pentacene films, orange, + + sitting on a Si substrate (size 1 cm × 1 cm), with 20 keV Ar500 and Ar2000 cluster ion beams. The colours, detailed in the Table 7(c) below, signify different temperatures, in the experimental investigations, starting, as shown, from the bottom-right of the sample, going upwards, and finishing at the top left-hand of the sample. 212 Figure 7.5: Image of the TIPS-pentacene film. 213 Figure 7.6: The sputtering yields (a) and Depth resolution (b) of TIPS-pentacene plotted as + + a function of temperature (°C) with Ar500 and Ar2000 cluster ion beams at an incident energy of 20 keV. Symbols (*) and (^) indicate before and after cooling the TIPS- pentacene film, respectively. 215

Figure 7.7: Depth profiles of PTAA on PMMA, bi-layer films, on silicon substrates. + + + Secondary ion yields of [C9H14O4] m/z 186, [M-H] m/z 270 and Si6 m/z 168, are plotted + + as a function of increasing cluster ion dose of: (a) Ar250 and (b) Ar2000 , all using 20 keV beams. 216 Figure 7.8: Measurements of the thicknesses of the PTAA film by DektakXT. The white lines represent scratches made on the samples. The red boxes represent the areas where DektakXT was made to inspect their depths. The measurements of the depths are presented on the right, in three sub-figures, where the red tracings represent the DektakXT data as a function of the span of each area. 218

List of Tables Table 1.1: Secondary ion yields of organic materials, for secondary ions in total mass + + spectra, with 10 keV C60 and Ga , in a positive-ion mode [30]. 35

Table 1.2: Secondary ion yields (Y), cross sections of damage (σ) and efficiencies of + + + secondary ion formation, with Ar , Xe and SF5 beams of bombarding ions, hitting PS and PC. Three characteristics of secondary ions m/z, for each polymer, were selected [48].---37 Table 1.3: Sputtering yields, interface widths and topography formation (roughness), for PMMA, at different temperatures. 45 Table 2.1: Comparison of mass analysers used for SIMS [4] 73

+ Table 2.2: Different apertures of 40 keV C60 for focusing the beam and the resulting lateral resolution [17] 84 Table 2.3: A summary of solvent conditions during spin-coating, their boiling points, vapour pressure and the duration of their spinning process 89 Table 3.1: Characteristic positive secondary ion signals of the repeat unit of undamaged + + PMMA, observed using C60 and Arn (n = 250, 500, 1000 and 2000) cluster ion beams. The chemical formula and structures are obtained from [5], [6], [7]. 102 Table 3.2: Characteristic positive secondary ions of the repeat unit of damaged PMMA, + + observed using C60 and Arn (n = 250, 500, 1000 and 2000) Cluster ion beams. The structures are obtained from [2]. 103 Table 3.3: Summary of PMMA interface doses, sputtering yields and depth resolutions, + using 10, 20 and 40 keV C60 primary ion beams, obtained at room temperature, with their standard deviations. 114 Table 3.4: Summary of results of PMMA interface doses, sputtering yields and depth + resolutions, with different sizes of the Arn cluster ion beams (n = 250-2000) at an incident energy of 20 keV, performed at room temperature, with their corresponding standard deviations. 119 Table 3.5: Summary of the results of PMMA interface dose, sputtering yields and depth + resolutions, at different temperatures, using 20 and 40 keV C60 , with their standard deviations. 125 Table 3.6: Ratio of the sputtering yields, of PMMA (before and after cooling the PMMA + films) and below room temperatures (°C), using 20 and 40 keV C60 . 125 Table 3.7: Summary of the results of PMMA interface dose, sputtering yields and depth + + resolutions, using different temperatures, with 20 keV Ar500 and Ar2000 cluster ion beams, with their respective standard deviations. 130 Table 4.1: Molecular ion and fragmentation calculated and observed m/z from ToF-SIMS analysis of PTAA samples in a positive-ion mode. 144 Table 4.2: Summary of results of PTAA interface dose, sputtering yields, and depth + resolution with different sizes of Arn cluster ion beams (n = 250-2000) at an incident energy of 20 keV at room temperature with standard deviations. 154 Table 4.3: Calculated and observed m/z from spectra of ToF-SIMS analysis of molecular and fragmentation ions of TIPS-pentacene in a positive-ion mode. 156 15

Table 4.4: Summary of results of TIPS-pentacene interface dose, sputtering yields, and + depth resolutions with different Arn cluster ion beams (n = 250-2000) at an incident energy of 20 keV investigated at room temperature with standard deviations. 163 Table 4.5: Summary of results of TIPS-pentacene interface dose, sputtering yields, and depth resolution obtained of TIPS-pentacene films at different temperatures using 20 keV + + Ar500 and Ar2000 cluster ion beams with standard deviations. Measurements of the film thickness (44 nm) were obtained by AFM. 174 Table 5.1: Summary of results of TIPS-pentacene (bi-layer film of TIPS- pentacene/PMMA/Si) interface doses, sputtering yields and depth resolutions with + different Arn cluster ion beams (n = 250-2000) using an incident energy of 20 keV. 187 Table 7.1: Different temperatures and/or days of experimental investigations for the PMMA films, together with their corresponding cluster ion beam energies (20 keV and 40 + keV C60 beams), as well as, the repetition number of the experiments. 211 Table 7.2: Different temperatures and/or days of experimental investigations for the + + PMMA film, together with their corresponding 20 keV Ar500 and Ar2000 cluster ion beams), as well as, the repetition number of experiments. 212 Table 7.3: Different temperatures and/or days of experimental investigations for the TIPS- + + pentacene film, together with their corresponding 20 keV Ar500 and Ar2000 cluster ion beams), as well as, the repetition number of experiments. 213 Table 7.4: Summary of TIPS-pentacene results for interface dose, sputtering yields, and + + depth resolution at different temperatures under 20 keV Ar500 and Ar2000 cluster ion beams, with standard deviation. These measurements of the crater depth were obtained by DektakXT. 214

List of Equations Equation 1.1 25 Equation 1.2 36 Equation 1.3 37 Equation 1.4 37 Equation 1.5 53 Equation 1.6 53 Equation 1.7 53 Equation 2.1 70 Equation 2.2 83 Equation 2.3 87 Equation 2.4 92 Equation 3.1 109 Equation 3.2 109 Equation 3.3 110 Equation 3.4 110 Equation 3.5 110 Equation 3.6 110 Equation 3.7 110 Equation 3.8 110 Equation 5.1 186 Equation 6.1 200 Equation 6.2 200

List of Abbreviations

Alq3 hydroxyquinoline)aluminium

C60/SnPc fullerene-tin-phthalocyanine CID collision induced dissociation DCB 1,2-dichlorobenzene DESI desorption electrospray ionisation DPPC dipalmitoylphosphatidylcholine ESA electrostatic analysis GCIB gas cluster ion beam ITO indium tin oxide LB Langmuir-Blodgett

LN2 liquid nitrogen MALDI matrix assisted laser desorption and ionisation MCP microchannel plate MS/MS tandem mass spectrometry NPB N,N’-bis(naphthalene-1-yl)-N,N’-bis(phenyl)benzidine NPD N,N- 4,4’-bis[N-(1-naphthyl)-N-phenylamino]biphenyl OFETs organic field-effect transistors OLED organic light emitting diode model OLEDs organic light-emitting diodes OPVCs organic photovoltaic cells OTFTs organic thin-film transistors P4VP poly(4-vinylphenol) PAMS Poly (α-methyl-styrene) PC poly(carbonate) PET poly(ethyleneterephthalate) PMA poly(methyl acrylate) PMMA poly(methylmethacrylate) PS poly(styrene) PTAA poly(dimethyl-triarylamine) PTFE (polytetrafluoroethylene 18

PVD physical vapour deposition PVP poly(vinyl pyrrolidone) QCM quartz crystal microbalance SAC sample analysis chamber SED scanning electron detector SEM scanning electron microscope SIMS secondary ion mass spectrometry TIPS-pentacene 6,13-bis(triisopropylsilylethynyl)pentacene ToF time-of-flight ToF-SIMS time-of-flight secondary ion mass spectrometry TRIM Transport of ions in matter programme UHV ultrahigh vacuum

UV/O3 ultaviolet-ozone

Abstract

Energetic polyatomic cluster beams are increasingly used in materials processing and surface analysis applications. In secondary ion mass spectrometry (SIMS) such beams have previously been utilised to investigate the chemical distribution of organic molecules (polymers, biological molecules and pharmaceuticals etc). One important application is in organic electronics, where the depth-distribution of organic components is important in the device performance. Massive gas cluster ion beams (GCIBs) have produced more successful depth-profiles for organic electronic devices that smaller projectiles including + + SF5 and C60 . However, further work is needed to investigate and optimise experimental parameters to deliver the necessary SIMS performance.

This study focused on molecular depth profiling of organic insulator (PMMA) and semiconductor (PTAA and TIPS-pentacene) materials, in single and bi-layered + + combinations, utilising cluster SIMS, using C60 and Arn , at different temperatures and energies. In general, at room temperature, the best depth resolution was obtained, using large Ar-GCIBs of low energy per atom (E/n ~10 eV), in comparison with the smaller Ar- + GCIBs or with C60 beams at the same total impact energy. On materials which sputtering + under C60 bombardment, ion and neutral yields were greatest due to the higher E/n, compared with GCIBs. Data from PMMA show that the sputter yield under C60 and Arn projectiles conform to the published ‘universal’ dependence of Y/n to E/n. Depth profiling + of the semiconductor compounds were unsuccessful, using C60 projectiles. For depth profiles using large GCIB projectiles, an increase in the secondary ion yield was observed at the interface with the silicon substrate – a phenomenon which was not observed for the smaller projectiles. In general, the most successful depth profiles (i.e. more constant molecular and fragment secondary ion yields, observed at pseudo-steady-state regions) and best depth resolutions were obtained at cryogenic temperatures - conditions under which corresponding sputtering yields and secondary ion yields were suppressed.

Declaration

No portion of the work referred to in the thesis has been submitted in support of an application for another degree or qualification of this or any other university or other institute of learning. Copyright Statement

i. The author of this thesis (including any appendices and/or schedules to this thesis) owns. The certain copyright or related rights in it (the “Copyright”) and s/he has given The University of Manchester certain rights to use such Copyright, including for administrative purposes. ii. Copies of this thesis, either in full or in extracts and whether in hard or electronic copy, may be made only in accordance with the Copyright, Designs and Patents Act 1988 (as amended) and regulations issued under it or, where appropriate, in accordance with licensing agreements which the University has from time to time. This page must form part of any such copies made. iii. The ownership of any patents, designs, trademarks and any and all other intellectual property rights except for the Copyright (the "Intellectual Property Rights") and any reproductions of copyright works, for example graphs and tables ("Reproductions"), which may be described in this thesis, may not be owned by the author and may be owned by third parties. Such Intellectual Property Rights and Reproductions by cannot and must not be made available for use without the prior written permission of the owner(s) of the relevant Intellectual Property Rights and/or Reproductions. iv. Further information on the conditions under which disclosure, publication and commercialisation of this thesis, the Copyright and any Intellectual Property and/or reproductions described in it may take place is available in the University IP Policy (see http://documents.manchester.ac.uk/DocuInfo.aspx?DocID=487), in any relevant Thesis restriction declarations deposited in the University Library, The University Library’s regulations (see http://www.manchester.ac.uk/library/aboutus/regulations) and in The University’s policy on Presentation of Theses.

Acknowledgements

I would like to acknowledge the Minister of High Education in Saudi Arabia and King Khalid University in Abha for funding my PhD studies.

I would like to thank my supervisors Dr Nicholas Lockyer and Professor Michael Turner for their guidance and support during my PhD project.

I am grateful for all the support received from present and former group members of the surface analysis research centre (SARC): Giles, Samar, Irma, Sadia, Jo, Afnan, Andres, Taylor, John and Danielle. Special thanks to Dr Alex Henderson for help with data processing.

I would like to thank Prof Turner’s group for their support. More special thanks go to Dr Adam Parry for preparing the TIPS-pentacene films by thermal evaporation and Dr Daniel Tate for providing me the PTAA samples.

Finally, I would like to thank my all family in Saudi Arabia. I would like to give my utmost thanks and express my great gratitude to my sister Suckinah, my brother Omran, my husband and my mum finally my beloved children. I could not have done this project without their encouragement, support and kindness towards me during my PhD programme.

Chapter 1: Introduction Nowadays, the use of organic materials is wide-spread in many aspects of life. This study is concerned with investigating organic polymers and materials that are used in a variety of applications in many domains, such as that of the electronics. For example, many TVs and mobile phones use organic materials in their display technology. Molecular electronics consist of various small organic molecules arranged as films, in applications ranging from photovoltaic, manufacturing, biomedicine and pharmaceuticals. Moreover, organic semiconductors can be a cheaper alternative to silicon ones, and they offer impressive performances [1]. Another field of application is that of inkjet printing, whereby depositing organic semiconductors materials is carried out, as well as being investigated for thin film transistors (TFTs) fabrication purposes [2]. Therefore, the scope of this study will be to investigate poly (methyl methacrylate) (PMMA), amorphous poly(dimethyl-triarylamine) (PTAA), and 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), for depth profiling and sputtering yields, to assess the various parameters that might affect analytical measurement, such as temperature, and/or ion beam irradiation. The investigations will be carried out in single layer and bi-layers combinations. It is very important to assess the quality of the interfaces between the various layers of these organic materials so that higher performances might be achieved e.g in organic light emitting diodes OLEDs [3].

PMMA, an organic polymer, is one of these investigated materials. It is a transparent, rigid thermoplastic material that finds applications in diverse fields of applications, including in organic electronics e.g. in OLEDs [3], organic photovoltaic cells (OPVCs) [4], organic field-effect transistors (OFETs) [5], [6], in gated insulator in organic field-effect transistors (OFETs)), etc., in medical devices e.g. intraocular lenses and in microfluidic cells.

As for PTAA promising trials were carried out, on its amorphous sate, to blend it with polystyrene, for use in OFETs [1]. TIPS-pentacene was also investigated because of its high p-type mobility and having a good environmental stability. This meant an improvement to TFT performance of inkjet printed TIPS-pentacene [2].

1.1. Secondary Ion Mass Spectrometry (SIMS) In many scientific investigations, there is a need to identify the physical and/or chemical composition of the constituents of a material. To this end, scientific investigators use a

variety of techniques. For many decades, one such technique, namely microscopy, proved useful in this respect.

However, there are deficiencies and/or limitations associated with all techniques. For example, to extract useful information from, say biological samples, scientists make use of certain detailed imaging techniques. They add, for example, fluorescence tags, together with other imaging facilitators [7] to achieve a meaningful detailed image, from which information could be extracted. However, this means that an extra layer of a 'foreign' material is introduced in the form of a 'mask'. This will necessarily stain the sample and consequently entails producing a not very faithful extracted image of the original sample [7]. Optical and electron microscopy imaging, by their very nature, reveals only physical details and lacks in-depth chemical compositional information of the sample.

To overcome such deficiencies, researchers turned their attention to mass spectrometry (MS), whereby ions of constituents are transmitted through vacuum chambers in order to deflect them using electric and magnetic fields. The amount of this deflection depends on their mass-to-charge ratios, and the strength of the applied electrical and magnetic fields. This technique solves the missing mass information issue that was a feature of optical/electron microscopy imaging, since MS provides detailed molecular weight characteristic. Mass spectrometry imaging (MSI) produces detailed molecular information about a sample’s solid surface, and the distribution of atoms and molecules within a substance.

The earliest type of mass spectrometry imaging, and the subject of this thesis, is secondary ion mass spectrometry (SIMS) [8], [9], [10]. In this technique the resolution is better than that of the more recently-introduced matrix-assisted laser desorption/ionisation (MALDI) [11], [9], [10] or desorption electrospray ionisation (DESI) [12], [9], [10]. Ionised particles of the sample are ejected (emitted) from the sample’s surface that has been bombarded with energetic ion beams (called 'primary' ion beams), such as atomic + + + - + + ions of (Ar , Ga or Cs ), small cluster ions of (O2 , Au3 or Bi3 ) or polyatomic ions of + + + + (SF5 , C60 or Ar2000 or (H2O)2000 ). The emitted particles are either secondary ions (i.e. not the original bombarding ions; positive and negative, atomic and cluster ions), electrons, photons or neutral species. Most emitted secondary particles are neutral. The secondary ions are then detected and analysed by a mass spectrometer. This process records the mass spectrum of the bombarded surface at the point of primary beam interaction with the surface and therefore produces a detailed molecular surface content or solid analysis as per requirement [13]. The SIMS process is described in detail below. 24

1.2. Generation of Secondary Ions Two processes are behind the generation of ions in SIMS. One is termed sputtering, by which atoms and/or multi-atomic clusters are desorbed from a solid target material after the target is bombarded by energetic particles. The second process is ionisation, where a portion of these sputtered particles become positively or negatively charged by losing (or gaining) some electrons. These sputtering and ionisation processes will be briefly discussed later in this Chapter. However, the basic equation that relates the various factors affecting sputtered particles is overviewed first.

The yield of secondary ions is strongly influenced by the electronic state of the material being analysed, with consequent quantitative analysis complications [13]. The equation of the basic SIMS process is the following:

+ 퐼m = 퐼p푦m휎 휃m휂 Equation 1.1

where Im is the (positive) secondary ion current from the sample species m;

Ip is the primary particle flux; ym is the sputter yield, which is the total yield that is emitted due to bombardment of a target material, in the form of neutrals and ions per incident primary ion.

α+ is the ionisation probability of positive ions;

θm is the fractional concentration of the element or species m in the surface layer

η is the transmission of the analysis system

+ Clearly, sensitivity to m is governed by α and ym which, both, depend on the primary ion beam characteristics.

+ The yields of sputtering, ym, and the ionisation probability, α , of positive ions are extremely important parameters in SIMS experiments resulting from the bombardment of a target material by the primary ions. Two items have significant influences on the yields of sputtering species:

1) Primary ion beam parameters, such as mass, charge and energy, etc. [14] 2) The physical state of the target materials (i.e. target parameters), such as crystallinity, topography, atomic number, , preferential sputtering (this where different elements of the sample, tend to produce more sputtered species than other

elements of the same sample), and sputtered-induced mixing and emission of cluster particles (this is where the bombarding ions might induce some mixing of the sample elements, thus affecting the sputtering yield) [14].

As for the transmission of the analytical system, η, it has a significant role in influencing the SIMS sensitivity. This parameter depends on the configuration and the kind of selected mass analyser as summarized in Table 2.1 in Chapter 2 in section 2.1.

More details about the processes that are behind sputtered ions and neutrals are given below.

1.2.1. The Sputtering Process There have been various theories that tried to explain sputtering; one such successful theory is put forward in 1981 by Sigmund, called Sigmund’s Linear Cascade. So far, it offered the most successful model to explain the sputtering process, conditional on meeting the criteria that Sigmund's formula required [13]. These three criteria need to be observed, so that linear sputtering is to occur:

1) That the observed erosion of a material’s surface, when being bombarded by primary ions, is only due to the bombardment and nothing else, thus ignoring other mechanisms, such as, heat generation, displacement cascading, blistering, evaporation, and ionisation [15], [16]. 2) That sputtering occurs in the limit of small incident-particle current, so that no excess concentrated energy is delivered that would cause macroscopic heating, which in turn could cause sputtering via an evaporation process. 3) That sputtering occurs in the limit of small incident-particle fluence, so that the incident particles, on the individual level, are able to cause sputtering based on that individual level.

Sigmund included a fourth criterion that the target material should be homogenous. However, it was thought that this is unnecessarily restrictive [14]. These criteria restricts the mode of operation to that of static one (i.e. primary ions are of a very low current density level [17], and not dynamic whereby fluences of >1014 ions cm-2 are used [15].

Sigmund classified the mechanism that causes sputtering as being divided between:

A. A knock-on sputtering mechanism, which is due to low energy ion bombardment.

B. An electronic sputtering, due to high energy (MeV) ions hitting the sample. This particular mechanism was disregarded in the following discussion because incident energies are beyond the keV range used in this thesis.

Furthermore, knock-on sputtering (or elastic collisions) is sub-divided into three types:

1) Single knock-on occurs when low primary ion energies in the range of 10 to 100 eV are used, and low masses such as H+ and He+, are produced from direct impact. 2) Linear cascades (a series of collisions, between atoms, one being initially at rest, in a linear fashion pushing deep into the sample, afterwards some recoil back to the surface of the sample) are initiated by primary particles. 3) Spike regime (also called slow thermal sputtering) is obtained when atoms spatial density moves as a whole. This is because, in the spike regime, atoms (original sample elements) collide with atoms (recoiled species), some of which are already in motion due to increased energy density that is coming from primary ion impact, thus making an increased spatial density that moves.

In elastic collisions resulting from keV impacts, projectiles lose energy by transfer to the surface atoms. Consequently, a series of knock-on collision cascades occurs between the solid atoms (approximated to hard spheres, for Newtonian elastic collision treatment purposes and sometimes termed as nuclear stopping collisions, since they do not lose energy to electrons could reach depths of ~ 30 Å into the sample [15]. Then, some collided atoms will recoil-back to the surface. If these atoms have energy exceeding the surface binding energy, an emission will occur, i.e. sputtering occurs, as shown in Figure 1.1. This model is called linear (knock-on) collision cascade theory, which closely matches experimental results when elemental samples are used as targets, and all required conditions are met, Under these conditions the sputter yield is well-predicted [13] and dependson primary particle mass and energy, when a medium to relatively high energy primary ion beam is being used. Low energy beams would recoil back elastically i.e. not losing energy to, say, electrons or lose energy to other mechanisms, such as displacement cascades [15], [18]. Another requirement is that the sample is of a single element (i.e. single component). This is because the theory has been derived with such restriction (by definition and theoretical treatment), as mechanisms affecting multi-component materials are very complex. One reason for this is that, by nature, having multi-components mean that if one component moves within a multi-component sample, it will change the physical and/or chemical characteristic of that sample. This affects the mechanisms and probabilities of interactions between the bombarding particles and the 'newly' created 27

sample. The transport of energy in multicomponent samples will be highly directional depending on e.g. bonding and not isotropic [13] and therefore the calculations for the recoiled particles are not straight forward ones, as in the case for single-component atomic samples. In summary, elemental samples, using energies of few keVs, as well as utilising mono-atomic primary ion beams of low current and fluence, will produce a well- developed, isotropic, cascade, that conforms well to Sigmund's conditions and Sigmund's formula [18].

Models of polyatomic sputtering are still under development, principally using molecular dynamics simulations to account for the much-increased energy density in the surface region. Effects such as vibrational and rotational excitation are thought to be important parameters in determining the result of polyatomic keV impacts on organic samples. Spikes, collective-motion, molecular vibrations, pressure pulses and shock waves are some of the phenomena thought to be important in the sputter mechanisms with polyatomic projectiles [19].

Figure 1.1: A schematic representation of the SIMS sputtering process. A primary ion beam strikes the sample surface causing a series of collision cascades. The ejected secondary particles are the majority of neutral (atoms or molecules (red)) and a small number of positively or negatively charged {cations (blue), anions (orange) and electrons (dark red)}.

1.2.2. Ionisation A minute amount of sputtered particles is ionised, while the rest (typically > 99%) is neutral. The ion yield’s high dependency on the electronic properties of the sample’s matrix is called the matrix effect.

Two helpful models, which describe how ionisation of generated molecular secondary ions is achieved, are briefly described below.

1.2.2.1. Nascent Ion Molecule Model The nascent ion molecule model was developed in 1983 by Gerhard and Plog [20]. In this model, it assumed that any ion, which would be emitted from the surface of a sample, would be neutralised before these ions could escape that surface. This process of neutralisation is carried out by the effect of swift electronic transitions occurring in the surface region. However, the neutralised sputtered molecular species dissociate, at some distance from the surface, to form the secondary ions. This non-adiabatic dissociation of nascent ion molecules (neutral molecules) assists in forming the secondary ions. For example, in inorganic compounds, when oxide surfaces that are bombarded by, say Ar+, as shown in Figure 1.2, ions are formed within the sample. These ions are pairs of positive and negative types, thus are able to combine as neutral MeO molecule that try to leave the surface [13]. The origin of most neutral molecules comes from direct creation/emission as ion pairs, MeO; their molecular character is not changed after leaving the surface, when they are away from the electronic effect of that surface. Only a few molecules dissociate into their constituents, when they have sufficient energy to do so. It is thought that this energy comes from the collision cascade energy that originally gave rise to the formation of those in-solid ions in the first place [15]. The electronic influence of the surface will be much smaller because the dissociation occurs at some distance, farther away from the surface.

Figure 1.2: Schematic presentation of the emission of the secondary ion by the nascent ion molecule model [20]. 1.2.2.2. Deposition Ionisation Model In 1983, Cooks and Busch introduced the desorption ionisation model to help in the understanding of cluster, or molecular ion emissions from organic materials, while in a state of excitation due to vibrational energy, (see below) [21]. This model assumes that the two procedures of desorption and ionisation can be investigated separately and that the relative abundance of ions that are observed in the SIMS spectra can be rationalised. In this model, it is thought that during the process of initial excitation, the energy of the molecules is divided into either translational or vibrations motions. The ions that are desorbed with a very low internal energy show reduced ion fragmentation compared to those that are desorbed with high energy (dominate in the SIMS spectra)..

The model attributes the desorption ionisation process to two forms of molecular reaction, which are:

1) Fast ion/ molecule reaction or electron ionisation that takes place in the selvedge. 2) Uni-molecular dissociations that occur in free vacuum, which cause fragment ions that are due to the energy of the bombarding ion.

1.3. Modes of Operations of SIMS SIMS has three modes of operation that are used in the analyses of different materials and to address different analytical tasks: Dynamic, static, and imaging SIMS. Each of these techniques has a different function. A brief description of these modes is shown below.

1.3.1. Dynamic SIMS Secondary ion emission was first observed when bombardment of a metal surface, using a primary ion beam, was carried out in 1910 by J. J. Thomson [22]. The first modern SIMS instrument was introduced in 1949; and then secondary ion mass spectra of metals and oxides were analysed by Herzog and Vieböck, at the University of Vienna [23]. In that SIMS instrument, primary ions are accelerated to hit a target, and it was possible to isolate the secondary ions, using electric fields. The first commercial SIMS instrument was developed, in the 1960s, by Liebl and Herzog [24]. That instrument was used in a dynamic SIMS mode, for depth profiling. That set-up was utilised high primary ion doses, and therefore they eroded the sample surface layers deep enough to reveal its molecular information as a function of depth (Figure 1.3a), thus the name depth profiling [25]. Traditional dynamic SIMS with atomic primary ions provides information that relate only to elemental composition of the materials, as a function of depth. This mode is more destructive than static SIMS, (see below), because the bombarding particles penetrate the sample to some depth.

Figure 1.3: Schematic of dynamic and static modes with SIMS.

+ - + Ion beams of oxygen (O2 , O ) and caesium (Cs ), are frequently employed as primary ion sources. Moreover, they are more successfully suited for the purpose of depth profiling, in elemental compositional investigations of inorganic materials, especially semiconductor

device materials, rather than organic ones, for in a dynamic SIMS, with atomic primary ions, the chemistry of the material is damaged [13].

The secondary ions are continuously sputtered due to the effect of the continuous primary ion beam bombarding the sample. The secondary ions are then detected and analysed by either a magnetic sector mass analyser or a quadrupole mass analyser.

1.3.2. Static SIMS Benninghoven adapted the SIMS methodology for true surface analysis, at the beginning of the 1970s. This is called the Static SIMS mode [26]. This mode uses a low primary ion dose to strike the uppermost layer of a sample, to produce low secondary ion emissions, hence not damaging the analysed area. The ion dose is at, or below, the 1 × 1013 ions/cm2 level. At this amount, the quantity of the material removed, from the sample, will be less than 1 % of the top monolayer (Figure 1.3b) [13], [27]. This means that the sample’s surface could be thought of, as being unchanged, static, hence the name. Below this static SIMS dose, the same analysis area could be bombarded again without detecting damage to the sample as each primary ion is assumed to hit a new part of the surface.

In this mode, it is also possible to analyse the elemental composition of the sample, as well as, the chemical structure of the surface, for example, its crystallinity. In the 1980s, ToF mass analysers were introduced into static SIMS, to increase their sensitivity, as compared with quadrupole mass analysers. The comparison between mass analysers is summarised in Table 2.1 in Chapter 2 in section 2.1.1. Further increases in sensitivity were acquired with the development of new primary ion beams [28].

1.3.3. Imaging SIMS The third mode of SIMS operation is that of imaging. Two imaging approaches, which are microprobe and microscope, were developed, in 1984 and 1967, by Castaing and Slodzian and Liebel, respectively [29], [30]. Each approach has been employed in a different manner to detect ions coming out of the target.

In the microprobe mode, a focused primary ion beam is rastered over the surface of the sample to obtain maximum image resolution. The resulting analyte ions are fed into a mass spectrometer. These ions are detected by a microchannel plate (MCP) detector, pixel-by- pixel. Therefore, the whole sample area is examined and its mass spectra are stored and recorded for individual pixels. Most mass spectrometry imaging is operated in this mode, because it is relatively easy to employ and is flexible enough to be used with any kind of

mass analysers. Other techniques using desorption-ionisation techniques, such as MALDI or DESI, can be performed, using the scanning mode methodology.

As for the microscope mode, a defocused primary ion beam is employed on a much larger region of the target. The position of the ion desorption is maintained after ionisation and transmission through the mass analyser. An ion optical image is generated on a position-sensitive detector. The advantage of this detector is that it is capable of providing the position of the ions impacting the detector surface, not only the intensity of the ions and determination of their m/z value. Moreover, this mode is able to achieve better spatial resolutions, for MALDI, and potentially DESI where the desorption beam cannot be highly focused. This is because at this mode, resolution is dependent on the quality of the ion optics and the detector’s pixel size, instead of the spot size that is limiting the probe beam, as is the case with microprobes. However, the disadvantage of this mode lies in it not being able to be used with all types of mass analysers, with the exception of the specifically designed ToF analysers.

1.4. Cluster SIMS Many authors have researched the benefits of cluster sources , such as diatomic and triatomic primary particles, in the 1970s [31]–[34]. Cluster sources generate nonlinear enhancement in sputtering yields and secondary ion yields. These cluster or polyatomic sources were used by the SIMS community since the mid-1980s.

In 1987, Appelhans and Delmore applied SF6 neutral beams to analyse and study the surface, of a variety of non-conducting polymer samples, including polyphosphazene, poly(tetrafluoroethylene) (PTFE), pol(methylmethacrylate) (PMMA) and poly(ethyleneterephthalate) (PET) [35]. Secondary ion yields were observed to increase, three or four orders of magnitude, with the benefit of reduced damage to the sample. This was done using a SF6 neutral beam, in contrast to other equivalent energy atomic beams. Two years later, Appelhans and Delmore analysed different organic materials, (pharmaceutical compounds). They also observed similar results in their mass analysis 0,- denoted here for neutral and ion beams (as was denoted by the author), as SF6 particles [36]. At the same time, the first reported research on systematic studies on the influence of nuclearity of clusters on the yields of secondary ions were reported by Blain et al. [37]. + They used cluster (CsI)nCs ion beams on different materials. They noticed that secondary ion yields increased proportionally with the square of velocity of projectiles, which impacted the surface. They also noticed nonlinearly with increasing projectile nuclearity.

They also noticed more pronounced effects on ion yields, when they used molecular species other than atomic ions [38], [28]. + Liquid metal ion source that produce Aun clusters was first employed by Benguerba and co-workers [39], in 1991, and were used on organic surfaces, such as, phenylalanine. As metal cluster ion sources became commercially available, they were further developed for SIMS applications, in 2003, by Davies and co-workers [40] and, in 2004, by Köllmer + + [41], using Aun and Bin cluster beams, from liquid metal ion source (LMIS), respectively. These sources commonly replaced monatomic projectiles, Ga+ and In+, and in the processes, they enhanced secondary ion yields, whilst retaining ease of use and focus ability of other LMIS. Observations were recorded for non-linear enhanced behaviour of the secondary ion yields, with Au1 to Au5 clusters [39]. Köllmer, in 2004, made a comparison normalization + + of the current of the single charged monomers Au1 and Bi1 to 100 % as shown in Figure 1.4 [41]. It can be seen from the chart, the benefits of using a Bi source, in comparison to an Au source, in that it emits larger clusters; producing higher cluster current; and producing doubly charged clusters.

Figure 1.4: Currents of normalized primary ions, for Aun and Bin clusters, as a function of + + cluster size and charge. Currents of Au1 and Bi1 are normalized to 100 % [41].

The Orsay and Texas A & M groups carried out comparison of secondary ion yields enhancement between Aun (n = 2 and 4) and a number of large organic projectiles, 34

including C60 and C24H12 [42], [43]. Aun projectiles result in a non-linear increase in secondary ion yields for organic surfaces such as phenylalanine films and Langmuir- Blodgett films (cadmium arachidate). The secondary ion yields from organic projectiles, was 4-5 times higher than that of Au projectiles. Studies by Van Stipdonk and co-workers, + + + in 1996, compared molecular projectiles of C60 and monatomic projectiles of Cs and Ga , onto phenylalanine films. The authors also observed that the emission of secondary ion + yields is larger, when using C60 polyatomic projectiles than that of atomic projectiles of Cs+ and Ga+ [30].

C60 sources were first developed and commercially available in 2003 by the Vickerman group in collaboration with Ionoptika Ltd [40], [44], [45]. Weibel and co-workers [40] analysed a number of organic materials, to compare the enhancement of the secondary ion yields, stemming from C60 polyatomic ions and Ga monatomic ions, at the same 12.5 keV + + energy. The results showed that C60 ions produce higher secondary ion yields than Ga , for these organic materials as shown in Table 1.1. They, also, compared the enhancement of the secondary ion yields of PET, in terms of molecular ions and fragmentation, using + + + + three types of primary ion beams: Au , Au3 and C60 . They found out that C60 ions produce higher secondary ion yields than Au3 and Au. In 2006, Fletcher and co-workers reported the effect of using different impact energies of polyatomic primary beams, C60, on the yields of secondary ions, from a number of organic materials. The results of these experiments showed an increase in secondary ion yields, as projectiles’ energies increased from 20 to 120 keV [46]. Table 1.1: Secondary ion yields of organic materials, for secondary ions in total mass + + spectra, with 10 keV C60 and Ga , in a positive-ion mode [40]. Organic Secondary ion yields (× 10-5) for total Relative secondary ion + + materials mass spectra yields (C60 /Ga ) + + C60 Ga PET 14200 230 62 PS2000 8000 87.9 91 PTFE 14500 2100 7 Irganox 1010 23629 142 166 cyclodextrin 6129 212 29 DPPC 35700 730 49

The observed increase in the secondary ion yields produced from different types of projectiles is explained by transport of ions in matter programme (TRIM) calculation [42] estimates that a 20 keV C60 ion beam, deposits its energy in the 30 Å distance from the top of the sample, whilst an Au4 cluster, with the same energy, deposits its energy in subsurface, to a distance of nearly 115 Å. The presumed explanation is that C60 projectiles 35

are broken into individual atoms, after collision with the surface, and therefore deposit its energy closer to the surface. This has the effect of producing lager yields than those of Au clusters [42]. This process has more recently been modelled using Molecular Dynamics Simulations (see section 1.7). Yamada and co-workers have designed gas cluster ion beams (GCIB) sources, for the purpose of sample processing (sample polishing and cleaning), for the semiconductor industry [47], [48]. They made suitable modifications so that they could be used in SIMS applications [49], [50]. The authors reported enhancements of the sputtering yields and secondary ion yields, with Ar cluster primary beams, by simulation studies (see section 1.7) [51].

In 2013, water cluster ion beams were used, by the Vickerman group in the J105 3D Chemical Imager at the University of Manchester [52]. Sheraz and co-workers reported a comparison of the enhancements in secondary ion yields, between water cluster primary ion beams and Ar cluster primary ion beams at the same cluster size and the incident energy of 1 keV and 10 keV, respectively, for a number of biomolecular samples. The water cluster beams produced an approximately 10 fold (or more) increase in the secondary ion yields of molecular ions, in comparison with Ar cluster beams. Only a very few SIMS applications with water cluster primary ion beams have currently been reported for standard bio-molecules and mouse brain tissue [53]–[57].

1.5. Damage Cross-Section As mentioned above, the bombardment of the sample’s surface by primary ions causes the surface to eject secondary ions, which in turn results in some damage to this surface. This damage could be described by a disappearance (damage) cross section (σ). This is defined as the average size of the damaged surface area for a single ion impact.

The surface’s damage depends on the sample’s molecular characteristics, such as, the structure and the composition of the surface; the primary ion beam properties; and secondary ion species [58]. The damage cross section σ can be calculated with equation 1.2.

휎 = 푁des/푁0 Equation 1.2 where Ndes is the average number of molecular surface species desorbed from the surface as the result of a single primary ion impact; N0 is the total number of molecular species M, in area units (1 cm2) in the uppermost monolayer of the sample.

It must be noted that secondary ion yield Y is defined as the number of secondary ion emitted divided by the number of primary ion (equation 1.3).

푌 = 푁/푁p Equation 1.3 where N is the number of secondary ion and Np is the number of primary ion.

Therefore, it is more beneficial, concerning the calculation of damaged areas, to find the secondary ion formation efficiency, E when both Y and σ (the damaged cross-section) are known. E is defined as the ratio of Y to σ, equation1.4:

퐸 = 푌/휎 Equation 1.4 A few authors have reported the damage cross sections and the efficiencies of polymeric samples [40], [58], [59]and Irganox [40], all working in a positive-ion mode. For example, Kötter and Benninghoven reported, in 1998, damage cross sections and efficiencies for PS and PC, under three different primary ion projectiles from low atomic mass to high mass + + + and to polyatomic as Ar , Xe and SF5 having 10 keV of incident energy. The primary ion + SF5 provided the highest efficiency, but caused the highest damage cross section. Their results are summarized in Table 1.2.

polymer PS PC Mass-to-charge 51 103 178 51 103 178 ratio (m/z) Y (10-6) Ar+ 61 35 25 116 43 46 Xe+ 155 241 218 239 146 163 + SF5 1504 3160 3000 3598 2179 2575 σ (10-14 cm2) Ar+ 0.7 2.7 2.8 1.0 1.3 1.4 Xe+ 1.1 3.3 3.6 1.2 1.9 1.9 + SF5 2.8 5.6 5.8 5.4 7.6 7.6 E (108 cm2) Ar+ 82 15 9 117 33 33 Xe+ 141 72 60 207 75 84 + SF5 544 562 515 167 288 340

Other studies by Staple and co-workers, have been carried out in 2000, for monolayer poly(methyl acrylate) (PMA) films and multilayer Langmuir-Blodgett (LB) films, with + + Xe and SF5 primary ion bombardment, with an energy range of 0.5 to 10 keV [59]. The reported damage cross sections increased with increasing bombardment energies, for both primary ions, but the polyatomic ions caused higher damage cross sections, in both PMA

investigations. The secondary ion formation efficiencies in thin films were observed to be similar for both monoatomic and polyatomic ions. However, in thick films, the secondary ion efficiencies were observed to be higher with polyatomic ions than those that were for monoatomic ones. The effect of increasing energy to both primary ions did not produce significant changes in secondary ion formation efficiencies, with the exception of the 0.5 keV beam.

Weibel and co-workers, in 2003, also reported damage cross sections and efficiencies, for different samples including: PET, PS (Mw = 2000) and Irganox 1010, in thick films (multilayer) [40]. In addition, PS was investigated, in thin films (monolayer). The primary + + ion projectiles used, were Ga and C60 , both, working in 10 keV, in a positive-ion mode. + + The observed efficiencies were higher with C60 ion projectiles than those with Ga , using thin films and thick films, in both cases. The damage cross sections were found to be of similar values, for both primary ions, concerning PS thin films. However, the damage cross + + sections for thick films were greater with C60 bombardments than Ga . Moreover, PS thick films were found to achieve greater efficiencies and smaller induced damage cross + sections than their PS thin films counterparts, when C60 was used as ion projectiles. However, Ga+ projectiles, gave similar results for both values of efficiencies and damage cross sections for both thin and thick films. In addition, researchers observed lower + damage cross section and high secondary ion formation efficiencies, from C60 ion beams, + compared to SF5 , for PS monolayer, working at 10 keV.

1.6. Cross-linking

Throughout this study and other studies, cross-linking is highlighted as a process that affects sputtering yield. Therefore, it is prudent that this process is qualitatively explained in some depth. A cross-link is a chemical bond (covalent or ionic) that links molecules, or polymer chains (synthetic or natural, as in the case with proteins), one chain to the other. In essence, in polymers, it creates branches, though these are not classified as proper polymer chemical branches. This is because branches could be broken, with relative ease, but crosslinked-created branches are much more difficult to break, if at all (cross-link bonds are much stronger than the usual branch bonding in branched polymers). Therefore, cross- linking adds more bonds to existing molecular systems, making them more ‘glued’ to each other, more rigid, thus affecting physical properties of the materials. For example, when cross-linking is introduced into rubber molecules, their flexibility starts to decrease, their hardness increases (or ductility), and their melting point increases. This is why linear or 38

branched, polymers, thermoplastics types (i.e. of elastic behaviour), become thermoset plastics (i.e. of in-elastic behaviour, more rigid) whenever large amount of cross-linking are introduced in them. In the rubber example, it is termed vulcanization, and this is used to make tires perform better. Another example, is that of resins, where cross-linking their chains will solidify them, gluing them to each other. As for the chemical properties of cross-linked molecules, they could also change. This is because cross-linking affects molecular weights of the cross-linked molecules. This type of bonding could, also, make materials that are subject to ion bombardment to have higher cohesive energy (the energy gained by arranging atoms in a crystalline state) than that by chain scission (the breaking of bonds that hold chains in polymers) [60]. Moreover, cross- linking will cause a reduction in the sputtering rate [61]. The more bonds a material have the more difficult to break it into small fragments, although bigger fragments could still be created, albeit with reduced mobility. Chapiro [62] classifying polymers being, of type I and type II, depending on their irradiation interaction. Whenever type I polymers are irradiated (i.e. bombarded with electrons, alpha particles, neutrons, beta-rays, X-rays, gamma-rays, and/or accelerated ions), cross-linking will typically occur. Type I polymers have very small amount of branching and high concentration of aromatic components. As for type II polymers, they will pre-dominantly degrade, through chain scission, then eventually cross-link at high fluences. The mechanisms involved in all degradations are complex [63], involving free radical intermediates. Furthermore, Mahoney [60] noted that the beam-induced cross-linking, “oftentimes”, is the cause of non-linear sputtering rate, with detrimental consequences on depth profiling. In this context, cross-linking is undesired and must be reduced. PS, beside others, is a polymer of type I. PMMA is a type II polymer. The chemical composition of these polymers plays a role in allowing a polymer to undergo a scission, or cross-link. It is likely that polymers of CH2-CH2 or CH2-

CHR structure will cross-link, while those with plenty of quaternary carbon ions, e.g. CH2-

CR2 structure are likely to undergo a scission. The reason for these behaviour is due to a change in the tacticity (syndotactic, isotactic and atactic) structure, due to steric repulsion effects. From a different vantage point, if the temperature of the sample is reduced, cross- linking is significantly decreased [60], [64], [65], [66], [67]–[69]. This could be due to less energy being available to facilitate cross-linking, and consequently, this reduction in cross- linking reduces sputtering! This could be due to fragments being less energized to make it to the surface to sputter away.

As the temperature rises to room temperature, then more energy should, theoretically, be available for molecules/atoms within the layers of the sample to break free and possibly make it to the surface to sputter away, should they have enough energy to overcome the surface binding energy. However, higher temperature (to room temperature, and not high enough to cause de-polymerisation, which then dominates the situation [60] ) will facilitate more cross-linking, and therefore reduced sputter rate of chain fragments. Mahoney [60] notes that besides cross-linking, polymers tend to create double bonds, upon irradiation, thus making more unsaturated bonds available. The following mechanism was proposed when degradation in PMMA occurs: first the methyl ester pendent group is lost; when the fluence is low, then chain scission occurs; and finally, when fluence is increased, cross-linking and un-saturation occur. The fluence-dependency is determined by the sample material, and some of the various parameters that were mentioned above, and below (temperature, energy, ion type, etc.). Moreover, it seems that different ions have different time scales (immediate to delayed effect) to cause cross-linking [66]. Figure 1.5 is a diagram, based on that given by Mahoney [60], showing the chronological steps from projectile impact through chain scission to cross-linking and un-saturation. Extra unsaturated bonds, together with polymer chain scission, as the case with PMMA, will make more mobile fragments that are able to form more cross-linking. As for PTAA, it has aromatic components, which make it less able to break its bonds, hence forming less cross-linking. This is manifested by the results of this study, whereby the values obtained for sputtering yields of PTAA(having three aromatic molecules in its monomer) are less than those of PMMA (having no aromatic molecule in its monomer). However, the same explanation cannot be applied to TIPS-pentacene, as it is not a polymer.

Figure 1.5: Mechanism of PMMA degradation with irradiation. Adapted from [60].

1.7. Molecular Depth Profiling The benefits of polyatomic primary ion beams, in comparison to atomic primary beams, are not limited to nonlinear enhancements of sputtering yields and secondary ion yields, as discussed in Cluster SIMS section above in section 1.4. Polyatomic beams have another feature - molecular secondary ions could continue to be emitted, as a function of depth, during analysis of organic materials. In contrast, with atomic primary ions, one notices that rapid losses of signal, due to molecular secondary ion occur. Polyatomic cluster beams are suited to molecular depth profiling for numerous organic materials, because of their decreased subsurface damage, less interlayer mixing, high molecular secondary ion yields and decreased topography formation.

The organic materials of interest include polymers, organic electronics, biology and pharmaceuticals. This literature review will only focus on two types of organic materials: polymers and organic electronics. These are most relevant to the work presented in this thesis. Before discussing SIMS applications, there will be some discussion on the shapes of the graph for depth profiling and the scenarios of depth profiling.

Normally, depth profiles with organic or polymeric materials will create one of the four scenarios as shown in Figure 1.6. The removal of material during the bombardment by a primary ion beam, will show itself as going through three regions, in the graph. In first region of the depth profile, there is an initial signal intensity decrease (or sometimes an m+ increase, as in some investigations using C60 -m here denotes ionic charge number or

+ Arn >500 sources, n denotes cluster sizes). This change in the intensity correlates with near the surface sputtering values, and is known as the initial transient region (region 1). In the second region (region 2), the signal intensity is a pseudo-steady as the the bombardment continues. Finally, the interface between a sample and a substrate comes into play, whereby a sudden decrease in intensity of the secondary ions (which refers to complete removal of the film) is observed and a commensurate increase in ions of the substrate occurs [60].

Figure 1.6: Different categories of molecular depth profiling of thin films (a-c) and bulk materials (d). (a) The ideal shape of the depth profile achieved under optimum conditions. (b and c) The depth profiles obtained under non-optimum conditions and the result of accumulation of damage, but a greater damage occurs in (c) as compared to (b). Three regions displayed during depth profiles are: (1) the signal intensity is decreased or increased in an initial surface transient region; (2) a steady-state region (a) or pseudo- steady-state region (b), and (3) an interfacial region, in which molecular signal decreases and substrate signal increases. In bulk samples (d) after a certain critical fluence (4) signal intensity is lost. This critical fluence is affected by the choice of ion source and other parameters e.g. energy [60]. The first demonstration of molecular depth profiling, using cluster beams, was reported by Corent and co-workers, in 1994 [70]. The authors used massive glycerol cluster ions to bombard protein samples, and observed that constant molecular secondary ion signals

result, with increasing ion fluence, while the signal of molecular secondary ions rapidly decayed when bombarding, with Xe+ atomic beams.

Gillen and Roberson demonstrated molecular depth profiling of a polymeric sample + (PMMA) and organic films, with SF5 cluster beams, in 1998 [71]. As an example taken from their experiments, a comparison of depth profiles of a thin glutamate film was made, + + + with two ion beams, Ar and SF5 . The SF5 polyatomic beam not only generated high ion yields, but also produced molecular and fragment ion signals, belonging to the glutamate film, which remained constant throughout the film. This was followed by ion signals of the substrate Si that increased in value, when the interface with Si was reached. However, when the same sample was bombarded with Ar+ monatomic beam, the molecular and fragment ion signals decayed with increasing sputtering time. Figure 1.7 shows a + + comparison of depth profile of glutamate film that has been bombarded with SF5 and Ar . + + SF5 and Ar depth profiles looked, roughly, identical in shape as shown in Figure 1.6a and b, respectively. One notices that the Si signal shape of the Ar+ depth profiles graph is + dissimilar to the SF5 one. Gillen and co-workers, in 2001, employed carbon cluster ions - - + C2 to C8 , for depth profiling of a glutamate film, generating similar results, with SF5 [72].

Figure 1.7: Comparison of depth profiles, for a 180 nm thick sample. The sample is a + + vapor-deposited glutamate thin film, using SF5 and Ar primary ions, under dynamic + SIMS conditions. The required SF5 primary ion dose that could penetrate up to reach the silicon substrate was 2.4 x 1015 ions/cm2 [71].

+ The authors elucidated that when a cluster SF5 ion beam strike a target’s surface, its energy will be split-up between the sulfur and fluoride atoms, and consequently these atoms will be deposited near the surface. Therefore, high sputter yields can be obtained

with less damage caused to the surface region. As the bombardment continues, the samples’ top layers will be removed until the substrate is reached, and hence the sample will be removed by subsequent impacts. Throughout this continuous process, the molecular secondary ion signal is not lost, and hence depth profiling could carry on, until the interface with the silicon substrate is reached. In contrast, when a monatomic ion beam impacts a surface, its energy is delivered deep into the subsurface region. However, as will be seen later in this study, the deciding factor is the beam energy divided by its cluster size or by the nucleon number of the bombarding ion species. As a result, both, low sputter yield and high damage to the subsurface region occur. This means that molecular + secondary ion signals decay rapidly, as fluence increases. SF5 cluster ion beams had been utilized for the purpose of depth profiles’ analysis, of a variety of materials, such as, drug eluting medical devices [65], [73], polymers and multi-layer films [74]. More recently, these materials have now been investigated with Ar-GCIB (n > 500) sputter sources as shown below in this section for polymers and multi-layered materials.

Beside energy, cluster type, chemical make-up and dosage, there are other factors that have an influence on SIMS results. Temperature proved to be another factor affecting depth-profiling process. Mahoney and co-workers demonstrated that temperature plays an + important role in polymeric depth profiling, as they showed by using SF5 cluster ions in ToF-SIMS [67]–[69]. An example taken from their research, showed such an effect on depth profiling of PMMA, at three different temperatures, -75 °C, 25 °C and 125 °C, as shown in Figure 1.8. The investigators reported that the 'best' depth profiles were those are obtained at a low temperature (-75 °C). These results, as compared with results obtained at room temperature, are in line with molecular secondary ion signals, low interfacial widths and the reduction in the topography formations inside the created craters bottoms, as the ion dosewas increased (or depth, as shown in the graph). At cold temperature the cross- linking effect, of the PMMA was more reduced than that at room temperature, (cross- linking is a bond between constituents of different molecules, see the cross-linking section, for more details and for the reason for the lower value at lower temperatures). As for a temperature of 125 °C, the depth profile was also roughly in line with molecular secondary ion signal of the -75 °C temperature, however, it caused less erosion to the PMMA film than that at room temperature. In spite of the constant secondary ion signal at low temperature, sputtering yields are not significantly different from those at room temperature. However, at high temperature (at 125 °C), the sputtering yield was higher, in comparison to that of room and low temperatures. The authors suggested that the increase in sputtering yields was affected by ion beam-induced depolymerisation. Table 1.3 shows 44

the values of sputtering yields, interface widths and topography of crater bottoms (roughness of the crater bottoms after etching by the primary beam ions) at three different temperatures, from -75 °C to 125 °C, for PMMA.

Figure 1.8: Positive secondary ion intensities of an ion fragment (m/z 69) plotted as a function of sputter depth, for ~ 160 nm of PMMA film, deposited on a Si substrate, in a + + dual beam mode: 10 keV Ar , for analysis and 5 keV SF5 , for sputtering. Depth profiles were acquired at three different temperatures, as displayed inside this Figure [67]. Table 1.3: Sputtering yields, interface widths and topography formation (roughness), for PMMA, at different temperatures [67]

Temperature Sputtering yield Interface widths Topography of crater 3 + (ºC) (nm /SF5 ) (nm) bottom (roughness) (nm) -75 30 ± 4 14.9 ± 3.3 ~ 0.27 25 22 ± 3 60.9 ± 3.4 ~ 11.37 125 49 ± 10 47.3 ± 6 ~ 1.00

In the same year that Mahoney’s work was being carried out, Möllers and co-workers + also performed temperature-related studies, for polymer materials with a C60 beam (10 keV) [75]. The authors found that the rates of erosion of PMMA were increased at high temperature. These results were in-line with Mahoney’s results, at high temperatures. The sputtering yields at three different temperatures, including a low temperature of –110 °C, a

3 + 3 + room temperature of 20 °C and high temperature of 180 °C were 20 nm /C60 , 38 nm /C60 3 + + and 60 nm /C60 , respectively. The sputtering yield decreased using C60 beams when the + temperature decreased from 180 to -110 °C in contrast to the SF5 beams study where the yields slightly improved from 25 to -75 °C. The reasons for this are not clear but the size of the projectile may have an influence on the temperature dependence of the sputter yield. A + better depth profile of PMMA at room temperature was obtained with C60 beams, in + comparison to those carried out with SF5 beams. It seems that the molecular secondary ion signals are not reduced, when working at room temperature, with an increase in erosion. However, values for sputtering yields, at room temperature, were low. This could + be attributed to a form of ion-induced cross-linking effect, when using a C60 beam, at room temperature. In a further work, they employed a 25 keV Ga+, for PMMA analysis [75]. The depth profiles of PMMA were attained at high temperatures, ranging from 180 °C to 210 °C. In fact, successful depth profiling of PMMA, under this primary ion, could not be achieved at room temperature. Poly (α-methyl-styrene) (PAMS) and polystyrene (PS) were also + investigated for the effect of temperature on depth profiles, with C60 ion beam. Again, for PAMS depth profiling could not be obtained at room temperature but as temperature was increased to 160 °C, working with a 20 keV and to 180 °C, working with a 10 keV, then depth profiling was achieved, possibly due to the ion-induced depolymerisation. In terms + of PS, the depth profiles were unsuccessful at all temperatures under a C60 beam; even + though Mahoney’s work showed success with SF5 beams [67].

In 2010, Mahoney and co-workers investigated positive-ion mode for depth profiling, with two different energies for the same primary ion beams, using a 5 keV and an 8 keV of + SF5 , for thin films, at room temperature, [76]. The characteristics of secondary ions (m/z +, 59 ) (this is the ester pendant C2H3O2 part of the PMMA monomer, fragmented away due to the ion bombardment), from a film thickness value of about 200 nm of PMMA, and working under low incident energy was observed to be constant, even with an increasing ion dose, in contrast to high energy beams.

As for depth profiles of bulk PMMA films, at low temperature (-100 °C) are shown in Figure 1.9. PMMA film (m/z 59) gave a maximum erosion depth, at the craters’ bottom, of 2400 nm and 1600 nm, for 5 keV and 8 keV, respectively. Depth profile sputtering was + also carried out, for bulk PMMA films, with three different energies of C60 projectiles, 10, 20 and 40 keV, by Fisher and co-workers [66]. Erosion depths increased with increasing

energies; 600 nm, 1300 nm and 2800 nm, using energies, ranging from 10 to 40 keV. This was carried out in a negative ToF-SIMS mode.

+ Figure 1.9: Positive depth profiles for bulk PMMA films, using 5 keV and 8 keV SF5 primary ions, at -100 °C. The PMMA (m/z 59) was plotted as a function of sputter depth [76]. In 2008, Gramer and co-workers attempted to achieve the depth profile of PC and PS samples, using C60 cluster beams [77]. However, the molecular ion signal of PC reduces from the initial region until entire film erosion. Moreover, they worked with a variety of + sputtering ion energies, from 10 to 30 keV, for C60 beams, but the characteristic of the molecular depth profile was not affected. By changing the value of another parameter, + namely the angle of the incident of the beam (in this case it was a C60 beam), it was possible to achieve constant secondary ion signals, for both polymers. At an incidence angle of 76° to the surface normal, several hundred nanometres in depth profiling was achieved. Figure 1.10 shows depth profiles of PS, with a 20 keV beam, at 48° and 76° incidence angles, plotted as a function of sputter depth. As can be seen, the ion signal decays rapidly at a 48° incidence, and then steadies out, while it quickly reached a steady state in sputter depth, at a 76° incidence [78]. Other studies, investigating PMMA, have shown that the maximum sputtering yields were reached, at a 45º cluster beam angle, using + 5 keV and 10 keV, Ar2000 beams [79].

Figure 1.10: Positive ions ToF-SIMS depth profiles of a PS thin film, working with a 20 + keV C60 primary ion beam, at 48° and 76° incidence angles. The secondary ion intensity for m/z 91, of PS, was plotted as a function of sputter depth [78].

+ It must be noted that Figure 1.10, above, is for C7H7 , an aromatic structure. This is a + PS structure without CH2 , due to the bombardment of the ions. The original six membered + aromatic group of the PS will now join the outside aromatic cycle CH, to make the C7H7 molecular ion. Therefore, this is shown in the spectra as a fragment of bombardment.

As can be seen above from all the studies of analysis of polymeric samples, different parameters such as temperature (cooling and heated), incident energy and glancing angles were varied. Cooling samples, generally, created a better depth profile and depth resolution because of a reduction of the cross-linking, during the sputtering process of the sample (see the cross-linking section), but high incident energy can adversely affect sputtering yields + and depth resolution. For example, using glancing angle, in PC and PS using C60 ion beam, can yield constant depth profiling , however, sputtering yields will decrease [78]

Some researchers investigated samples that are layered. Mouhib and co-workers investigated two kinds of bi-layered polymers. This was carried out to investigate their molecular depth profiles, with a C60 cluster, in both polarities (positive and negative modes), using ToF-SIMS [80]. These bi-layer films were: poly(vinyl pyrolidone) (PVP)/poly(methylmethacrylate) (PMMA) and poly(4-vinylpyridine) (P4VP)/poly(methyl methacrylate) (PMMA). For bi-layered PVP/PMMA, the secondary ion intensity of PVP has reached a steady state and then the secondary ion of PMMA increased, when PVP ions decreased in amount. Moreover, PVP secondary ions’ signal reached a second steady state region when the PMMA signal became constant (through erosion). The authors attributed this to mass interference. Further, judging from the results of our own study, it seems that

this second steady state (and observing that it coincides with the second steady state of the second layer, PMMA) is possibly due to some mixing of both layers that occurred during the preparation of both layers at the lower part of the sample. On the contrary, for the bi- layered P4VP/PMMA, the P4VP secondary ions’ signal decreased when PMMA signal increased. The loss of P4VP secondary ion signal may be attributed to the presence of aromatic molecules, since they are more stable and makes it more difficult to fragment away.The same trend was observed for PS and PC, when attempting to obtain their depth + + profiles, with SF5 [67] and C60 [81] clusters. Again, this is because of the presence of aromatic groups. For both bi-layered polymers, Si signals were observed to increases, while PMMA signals decreased. It was noticed thatworking with both ion polarities, the same behaviour occurred, during the erosion of these films. Figure 1.11 shows depth profiles for both bi-layered polymers, working in a positive-ion mode.

(a)

Intensity

+ 14 2

C60 primary ion fluence (× 10 ions/cm )

(b)

Intensity

+ 14 2 C primary ion fluence (× 10 ions/cm ) 60

+ Figure 1.11: Depth profiles of (a) PVP/PMMA and (b) P4VP/PMMA, working with C60 , for sputtering and Ga+, for analysis [80]. Another variant of these experiments used GCIBs. Many applications of Ar-GCIBs molecular depth profiling have been reported in the past decade, especially for complex polymers [82] and semiconductors [83], [84] that proved difficult to profile with smaller + + projectiles like C60 and SF5 . In 2009, Ninomiya and co-workers demonstrated successful depth profiles for PS, using Ar-GCIBs [82]. In investigating the same polymer, and taking 50

into consideration all the parameters that were mentioned above, (except the glancing + + angle), it turned out that it is not possible to attain depth profiles, with C60 and SF5 . However, the authors obtained depth profiles for PMMA, PS and PC, using the same Ar- + + + GCIB. PMMA, PS and PC fragment ion signals of C4H5O , C7H7 and C9H11O , respectively, were observed to be stable throughout the depth profiling process. This stability carried on up to the interface between these films and the glass substrate, which was coated with indium tin oxide (ITO), as shown in Figure 1.12.

A few studies have experimented with changing the energy per atoms of the beams as well as their cluster sizes. Polymeric materials (PMMA, PS and PC) were optimized for sputtering yields with different Ar cluster sizes and energies [79]. Along these lines, other researchers investigated depth profiling for organic multilayers, such as organic light emitting diode (OLEDs) [85]. Sputtering yields were directly proportional to increased beam energies (i.e. increasing energy per atom), however, they showed a small decrease, as cluster size increased. Sputtering yields displayed linearity with increasing energy per atom, working at a constant cluster size. Furthermore, sputtering yields reached a constant volume, with an increase of polymer molecular weight [86].

It is an interesting phenomena that increasing the cluster size and decreasing energy provides a better depth resolution. This phenomena was further investigated with different materials such as OLEDs and Irganox delta layer structure [85]. The results for these materials were in good agreement with polymeric ones; working with the same Ar cluster ion beam parameters. Threshold energy, Eth, (which is the minimum energy that will produce sputtering yield) is dependent on the energy per atom in its cluster and not related to the cluster size and ions types. The threshold energies for, PMMA, PS and PC were 1.0, 1.4 and 1.7 eV/atom, respectively.

Figure 1.12: ToF-SIMS depth profiles of a) PMMA, b) PS and c) PC in positive secondary + ion spectra, obtained in a single beam mode, working with 5.5 keV Ar700 , for both sputtering and analysis purposes [82]. Other researchers investigated the effect of changing the number of atoms in a beam. In + 2014, Cristaudo and co-workers, reported depth profiles for PMMA and PS, using Arn cluster beams, while increasing the number of atoms in these beams (n = 1500 to 5000) [86]. The targets of this study were firstly to investigate the effect of the Ar cluster size; and secondly the efficiency of the sputtering process, when analysing different molecular weights of polymers. The incident beam energy was held at constant 10 keV, for all these cluster sizes within the analysis. The energy per Ar atom, in each cluster beam, was decreased (at the same time as increasing the number of9 atoms in the Ar cluster beam) from 8 eV/atom to 1.4 eV/atom. The results of the sputtering yield volumes of PS and PMMA, measured in nm3 per Ar cluster ion, were plotted against the molecular weights

+ (Mw), as shown in Figure 1.13. The results were for three different cluster ions: Ar1500 , + + Ar3000 and Ar5000 , working with a constant beam energy of 10 keV. It can be seen from Figure 1.13 that the sputtering yields decrease, as molecular weights of PS and PMMA, increase, for a given cluster size, and nearly stabilise. The same happens, i.e. a decrease of yield, at constant polymer molecular weight with increasing cluster size. Furthermore, PMMA films generated higher sputtering yield volumes than the PS films, because it is thought that they were undergoing chain-scission processes and cross-linking respectively when bombarded with ion projectiles. The sputtering yields were stable from Mw = 60 k to 140 k atomic mass unit (amu). Figure 1.14 confirms, in clear manner, the influence of the molecular weights on sputtering yields (measured per nucleon (nm3/n)), when it is plotted versus energy per atom (E/n, eV/atom).

A note must be included here (before mentioning the graphs that relate Y and E) regarding Seah's equation, which was published in 2013 [87]. This equation is one of a variety of equations, put forward to explain yield variation with some parameters. An earlier equation was presented by Matsuo and colleagues [88], [87], as given below:

퐸 푌(n, E) = 푘푛p( − 퐸 ) Equation 1.5 푛 th

where Eth = threshold energy for sputtering, p = coefficient of size effects and k = constant, n = ion cluster size.

An earlier equation by Ichiki et al. [89] was given, again including Eth, as

1.5 푌(2000[푐푙푢푠푡푒푟 푠𝑖푧푒], 퐸) = 0.0000268 (퐸 − 퐸th) Equation 1.6

Seah put a more general equation that did not include Eth, but with some parameters (A and q) that are established by fitting and the general parameters of E, Y and n, in the yield to E/n relation graph:

푌⁄푛 = [퐸⁄(퐴푛) 푞/1 + (퐸⁄(퐴푛)푞 − 1 Equation 1.7

Another note to mention here is that of glass transition temperature (Tg). Vanina

Cristaudo [86] suggested that the trend of the change of Y against Mw can be related to Tg of their investigated PS samples and that analogous considerations for PMMA can be investigated.

Figure 1.14: Sputtering yield volume per cluster atom, Y/n, versus energy per atom, E/n in cluster for: a) PS and b) PMMA, obtained under the bombardment of a 10 keV, Ar+ cluster, using selected cluster size, in the range of 1250-7000 atoms/ion [86]. The dash line as seen in a) for PS from Seah’s equation (see below note) [87]. Ninomiya and co-workers investigated depth profiling, for multilayer structures of organic semiconductor materials, using Ar-GCIBs. It was difficult to obtain good depth profiles with using C60 clusters [90]. The multilayer consists of tris(8- hydroxyquinoline)aluminium (Alq3) and 4,4’-bis[N-(1-naphthyl)-N-phenylamino]biphenyl + + (NPD). Figure 1.15 shows depth profiles of Alq3 and NPD , using Ar cluster beams. + + Shard and co-workers, in 2007, attempted to accomplish depth profiles of Alq3 , using C60 beams, to not avail [91]. 54

+ + + Figure 1.15: Depth profiles of Alq3 and NPD , using 5.5 keV Ar700 . (a) For ITO/Alq3 (100 nm)/NPD (100 nm), b) For Si/NPD (40 nm)/Alq3 (80 nm)/NPD (40 nm), c) Si/Alq3 (20 nm)/NPD (120 nm) with fluence [90]. Other materials, such as organic electronics (photovoltaic cells), were studied for molecular depth profiling, using Ar-GCIB sources, by Mouhib and co-workers, in 2013

[84], [92]. In that study, they carried out depth profiling for pure fullerite (C60) and bi-layer composed of C60 fullerite and fullerene-tin-phthalocyanine (C60/SnPc), using Ar cluster beams [84]. Pure C60 films were characterised, through depth profiling, using various + incident energies of the Ar1000 cluster ion beam, working with 5, 10 or 20 keV for + sputtering, and with 30 keV Bi3 for analysis. The lowest incident beam energies, i.e. that of 5 and 10 eV/atom, created molecular depth profile of C60 molecular ion signal intensity, + + until the interface between C60 and Si ion signals was reached. At an incident beam energy of 20 keV (20 eV/atom), it was noticed that a rapid decay of molecular ion signal 55

+ + for C60 films, occurred. At this stage, the interface between C60 and Si ion signals was 2+ reached. In contrast, the same sample was studied with C60 cluster projectiles, but the depth profiling for this sample was unsuccessful. The sputter yield volumes of C60 films, + + using Ar2000 , were almost as twice as large as that using Ar1000 , at a given Ar cluster energy, as shown in Figure 1.16. In addition, sputtering yields increased linearly with + increasing energies, for 2.5, 5 and 10 keV of Ar1000-2000 ions. The obtained depth profiles of the bi-layers of C60/SnPc, on silicon substrates, were successfully analysed, using an + + incident energy of 30 keV, Bi3 , for analysis, and 10 keV, Ar1000 , for sputtering.

Figure 1.16: Sputtering yields plotted as a function of kinetic energy: 2.5, 5 and 10 keV, + + for Ar1000 and Ar2000 [84]. Seah and co-workers analysed NPB and Irganox 1010 to investigate the effect of sample + temperature on sputtering yields and depth resolution, using a 5 keV Ar2000 cluster ion beam [93]. The sputtering yields have shown a steady increase in value, from low temperature (at -100 °C) up to transition temperature (15 °C for NPB and 0 °C for Irganox 1010). At room temperature, the sputtering yields are more enhanced than those below

room temperature and when the temperature increases above room temperature (for Irganox up to 50 °C) they increase, accordingly. However, for NPB one could still see noticeable sputtering yields increasing, up to 100 °C, as shown in Figure 1.17.

Figure 1.17: Sputtering yields of NPB and Irganox 1010 plotted as a function of the + sample temperature with the temperature transitions indicated, using Ar2000 at a 5 keV [93]. On a practical note, Seah and co-workers [93] made an observation that their sample showed roughness reduction, one day after their experiments were carried out, due to surface relaxation. While in this study, no attention was particularly paid to this very point, the multiplicity of this study's investigations bore no indications to support the relaxation effect. Furthermore, due to lack of resources, namely time (beside the elaborate time- consuming process of setting up the various instrument parameters, as well as, laboratory restrictive rules), and the fact that Seah and co-workers [93] published their paper in 2016, there was no possibility of confirming the applicability of this point to this study. Expounding this lack of resources issue further, it had an impact on the parameters that were supposed to have been investigated, such as H2O-GCIBs and CO2-GCIBs where they + were supposed to have been used alongside the Ar-GCIBs and C60 ones; as H2O-GCIBs were pointed out by some studies [52], [53], [56] that they produce higher secondary ion yields and less fragmentation. More time should have been allocated to identify the type of polymers that are being investigated, from the point of view of atacticity and Tg, as it will be pointed out below. 57

Polymer structure is expected to influence the sputtering behaviour with different sample temperatures and under different projectile parameters. There are atactic, syndiotactic, and isotactic, types of polymers. These types refer to the way the steric configuration, of the order of the pendant groups along the chain of a polymer, is laid out. For isotactic and syndiotactic (called, stereoregular polymers), the arrangement of the pendant groups is either one-sided, or following a certain pattern, respectively. As for atactic, it is of a random arrangement of such groups. These different arrangements affect the crystallinity, density and melting point of polymers. Consequently, they affect the Tg, the glass (or Tt transition) temperature, and/or Tm (melting point), hence the possibility of affecting the SIMS secondary ion yields, sputtering yields and/or depth resolutions.

1.8. Molecular Dynamic Simulation Molecular dynamic computer simulations have been used to help understand sputtering mechanisms that occur during the bombardment of a target surface with polyatomic primary beam, as well as comparing yields with that of atomic primary ion beams. The main results of these studies have been discussed in a number of review article (see for example [94], [95]. The computational time required to run these simulations means that typically only single ion trajectories are modelled. Many mechanisms and issues that affect sputtering and sample damage, such as surface melting, phonon creation, energy channelling, crystal selvedge, crystal direction, relaxations, relaxation time are beyond the scope of most MD studies.

Nevertheless, such, studies help to understand the mechanisms of polyatomic primary ion beams sputtering processes in SIMS experiments. In one of the earliest MD studies + + involving polyatomic projectiles, Winograd and Garrison utilized C60 and Ga projectiles, to bombard a silver substrate, Ag [111], with the same incident energy, as shown in Figure 1.18 [96], [97]. From this Figure, one notices that the energy deposition process by these two projectiles is completely different. The large incident energy of Ga monatomic projectiles causes atoms to penetrate deeper into the Ag surface. Consequently, the primary energy deposition traverses below the surface.

However, if one considers that the ion beam comprises of 'beams' of particles, one after other, then the further incoming ions will not find the same surface topography that the first found, but a changed topography that has a crater. This is the basis of the depth profiling experiment, but is difficult to model with current computer technology, unless a very coarse-grained approach is taken to simplify the model.

MD studies however showed the occurrence of a large amount of interlaying mixing between the primary incident Ga atom and the sample atoms and very little material ejected from the surface. However, the atomic projectiles create a narrow and deeper crater in the target sample and the crater’s shape will be roughly cylindrical. Generally, in using high energy atomic projectiles it is not possible to attain valuable molecular SIMS information, such as depth profiles, because of the physical damage to the sample.

In contrast, the C60 polyatomic (cluster) incident projectiles deposit a significant amount of primary energy, near the sample surface, because of the distribution of its energy over a number of relatively light constituent atoms, (250 eV per C atom, working under a 15 keV incident energy beam). Therefore, individual carbon atoms do not penetrate deep into the Ag sample surface. They will suffer a series of cascade collisions. This results in ejecting a large amount of material from the sample’s surface and reducing the interlaying mixing.

The C60 projectile also creates low damage to the sample and the accumulation of damage completely disappears. The cluster projectile forms a wide and less hemispherical crater in the Ag sample. The occurrence of the cluster energy deposition near the sample surface is also observed with different samples, including graphite and diamond, as pointed out by Yamada and co-workers, using with various beam energies, from 10 to 20 keV [98], [99].

Molecular dynamic simulations are also used to explain the sputtering processes, occurring in the bombarded sample surface using different polyatomic primary beams, as

Delcorte and co-workers showed [100]. Figure 1.19 shows a comparison of C60 and Arn cluster impacts, on a C60 fullerite solid sample’s surface, working with a 2.5 keV incident energy beam. It can be seen that the damage of the sample, created by C60 and that due to the small argon cluster (n < 250) projectiles, has similar effects in terms of the shape and volume of the damaged area. The energy of the cluster projectiles is deposited near the fullerite’s surface, because of the distribution of its total energy over a number of constituent atoms (e.g. 41.67 eV/atom, 138.89 eV/atom, for C60 and Ar18 clusters, respectively). The result of initiating motion of atoms in the sample leads to the formation of a crater, in the shape of a hemisphere. Fullerite sample bombardment results are in good agreement with benzene results, when employing C60 and small cluster projectiles [101]. In contrast, increasing the Ar cluster size (n > 500) creates shallower craters, which increases the lateral size of the damaged area, while their depths in the sample decrease. This is because of the decrease in energy per atom, while the total cluster energy is unchanged.

This makes the breakage of bonds, in C60 molecules, highly improbable. The same

projectiles were used on the polystyrene sample [102] and the results were similar to the fullerite’s sample.

It is clear that MD simulations will continue to offer valuable insights into the projectile-surface interactions and as computer power advances may be able to address further important issues including ionisation and chemical reaction.

Figure 1.18: Molecular dynamic simulation. Cross-sectional view of the temporal evolution of a typical collision event leading to ejection of atoms, under 15 keV, Ga and C60 projectiles, bombarding Ag {111} surface, at normal incidence. The ejected atoms are coloured as the colour of their original layers in the substrate and the projectile atoms are coloured black [97]. 60

Figure 1.19: Molecular dynamic simulation, using C60 and Arn (n = 18, 60, 250, 500, 1000, 1700 and 2500) cluster projectiles, at an incident energy of 2.5 keV. The colour scales show the energies scales, or single substrate (top) or projectiles (bottom) [100]. 1.9. Aims of this Study The main aim of the research here is to investigate the effects of bombarding different polyatomic cluster ions beams (of different chemical compositions and different physical properties) on organic materials (the insulating PMMA and the semi-conducting PTAA, and TIPS-pentecene, as these materials are heavily utilised in many fields of applications). Results of depth profiling were to be studied by the ToF-SIMS technique, utilising the J105 instrument. The objectives include carrying out comparative assessments between the various ion beams in terms of cluster size (n) and impact energies,(E), in the keV region. Single-layered materials were to be investigated first, as reference samples; obtaining their sputtering yields, depth resolutions and secondary ion yields, using different projectiles + + + (Arn , n = 250, 500, 1000, and 2000, and C60 ), of different energies (20 keV for Arn , and + 20-40 keV for C60 ). It was hoped to also perform some measurements with clusters of molecular species H2O and CO2 but this proved not to be possible due to technical difficulties and time constraints. The effect of sample cooling on these measurement parameters was studied separately. As model systems of organic electronic devices, bi- layers (of different combinations of the investigated materials) were to be investigated, to determine the optimum experimental parameters to most accurately measure their interface positions.

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Chapter 2: Instrumentation

2.1. ToF-SIMS Instrumentation 2.1.1. ToF Mass Analyser Before discussing the operation of the ToF-SIMS instrument used in this project, it is first appropriate to introduce some background on the subject and give an overview of the science behind the topic and the general performance characteristics of the tool, as well as the operating principles behind the time-of-flight (ToF) mass analyser.

The concept behind the operation of a ToF mass analyser was first reported by Stephens in 1946 [1]. The first commercial instrument available was a linear ToF mass spectrometer and was designed by Wiley and McLaren in 1955 [2].

The theoretical principle behind the separation of the ions is based on their m/z (m = mass of the ion and z = its charge number) ratio, which in turn determines their velocity or (time-of-flight) distribution [3].

In SIMS (see Chapter 1) the bombarding primary ions that cause the sputtering of particles from a sample are sourced from an ion source that supplies the ions in ‘packages’; they come with specific characteristics (discussed below). Secondary ions are then accelerated by an electric field into a free field region in the flight tube. In theory, all ions are accelerated with the same potential (V), hence gaining the same kinetic energy (K.E). The ions are then separated by their respective mass-to-charge ratios; those with high m/z ratio take longer flight time and are slower than the lower m/z values. The equation below shows how to calculate time-of-flight [4]:

푚 푡 = 퐿( )1/2 Equation 2.1 2푧푉 where: t = flight time;

L = length of free flight tube; m/z = mass-to-charge ratio, and;

V = acceleration potential

Consequently, the acquired time-of-flight spectrum, which is measured in micro- seconds, (µs) as shown along the x-axis (time scale) of Figure 2.1, is obtained. From this Figure it is possible to calibrate the instrument. Moreover, by applying a calibration routine (see Appendix for details), a change of axis to that of a mass-to-charge ratio is obtained and this is shown in the axis of Figure 2.2. This spectrum now represents that of all the detected secondary ions.

However, the way that linear ToF mass spectrometry performs analysis has shortcomings. The simple linearity of the instrument does not make it possible compensate for different ion creation times or initial kinetic energies of ions [3]. This causes poor mass resolution. To overcome these particular issues, linear ToF mass analysers were developed to have a delayed pulsed extraction process, whereby a delay, or lag, was introduced to extract all ions at the same time, regardless of the precise time they were created. Another development was to introduce a reflectron to compensate for kinetic energy spread to obtain a better mass resolution.

Three kinds of the reflectrons are used: a single-stage, two-stage and non-linear stage. When ions have the same m/z ratio but having different kinetic energy, the reflectron compensates for the energy spread in the flight tube. This is because that in this reflectron scenario, ions are deflected by a retarding electrostatic field. The higher-energy, faster ions will travel further into the reflectron than the lower kinetic energy, slower ions and therefore they will arrive at the detector at the same time as other ions of the same m/z [3]. This method cured the shortcoming of the linear ToF analyser. More details about the two methods are now given below.

Figure 2.1: A ToF-SIMS spectrum (in µs) of secondary ions.

Figure 2.2: A ToF-SIMS spectrum (in m/z ratio) of secondary ions. The theory behind delayed extraction is that when it is introduced between the ion formation and extraction steps, it will cause an additional energy spread in the ions going into the flight tube compensating for different ion creation times of initial kinetic energy in the source. Technically in this method, ions are first permitted to extend into the field-free source region. The more energetic ions move further towards the detector than the less energetic ones. Then, a delayed extraction potential pulse is applied. This will mean that more energy is fed into those ions that have spent longer time in the source. This means

that the less energetic ions receive more kinetic energy than those, which are ahead and consequently join the more energetic ions at the detector. Therefore, the mass resolution of the ToF mass analyser with this delayed extraction technique is improved thus negating the shortcoming of the linear ToF analyser [3].

There are other mass analysers, such as, quadrupole and magnetic sector SIMS. However, the performance of the ToF mass analyser has many advantages in comparison to these two as summarised in Table 2.1.

Table 2.1: Comparison of mass analysers used for SIMS [4]

Types of Mass Mass Transmission Mass Mass Relative analysers range % Resolution detection sensitivity method Time-of flight 103-104 50-100 >103 parallel 104 quadrupole <103 1-10 102-103 sequential 1 Magnetic <104 10-50 104 sequential 10 sector

2.1.2. ToF-SIMS Instrumentation Development ToF-SIMS instrumentation is very effective in analysing various types of organic and inorganic samples [5]. At present, two ToF-SIMS instruments are available at the University of Manchester in the SIMS lab (this group is called the Surface Analysis and Research Centre (SARC)). One of these instruments is known as the Bio-ToF-SIMS (a conventional ToF-SIMS design principle). It was designed by Kore technology (Cambridge, UK), Winograd group at the university of Pennsylvania State and the Vickerman group in the mid-1990’s. A schematic drawing of it is shown in Figure 2.3. The operation of the Bio-ToF-SIMS is similar to the ION-ToF-SIMS instruments (ION- ToF GmbH, Müenster, Germany). A schematic drawing of it is shown in Figure 2.4. Both of them use a short pulsed primary ion beam. The primary ion beams strike a sample surface, from which the secondary ions are emitted and then extracted by an extractor (see Chapter 1 for more details on the sputtering process). Then the ions reach the detector after travelling about 3 m through the reflecton ToF mass analyser. During the flight, different masses are separated according to their mass-to-charge ratio and are detected individually to produce a mass spectrum.

Figure 2.3: Schematic diagram of the Bio-TOF-SIMS mass spectrometer [7]. The main drawback of the conventional ToF-SIMS is that it is impossible to acquire from the obtained data the highest spatial resolution and highest mass resolution at the same time because of the consequences of having short primary ion beam pulses (for further insight, see reference [6] regarding static limit, duty cycle, and short pulses issues). Furthermore, the primary ion beam cannot be shifted to the next pixel before all the ions from the first pixel have reached the detector, limiting the speed of data acquisition.

Figure 2.4: Schematic diagram of the ION-ToF-SIMS that display the stages of operation for the conventional ToF-SIMS design [8].

A recent innovation in the instrumentation regarding ToF-SIMS was achieved when an instrument called the J105 3D Chemical Imager (Figure 2.5) was developed by the SARC group in collaboration with Ionoptika Limited (Southampton, UK), and SAI Limited (Manchester, UK) [9]–[12]. The benefits of this instrument are as follows: its capacity to acquire rapidly data; the simultaneous achievement of highest spatial and mass resolutions; and the use of continuous primary ion beams (direct current-DC). In this chapter, the J105 3D Chemical Imager will be discussed in more detail.

This J105 instrument comprises of many parts, which will be outlined as the following:

 Two sources of cluster beams; C60 and GCIB (Arn and (H2O)n)  A preparation chamber containing a sample storage component called a Z lift (which has 3 Tiers, chambers) and an arm for transferring the sample  Sample analysis chamber (SAC) containing of the XYZ sample stage and secondary electron detector (SED)  An electrostatic analysis (ESA)  A buncher (for the purpose of bunching ions together)  A harmonic-field reflectron  An ion detector  Two cameras one at the insertion point of the sample to provide an initial sample map, for navigational purposes, and another one that is located at the SAC to help in the selection of an analysis area in the sample. In the following sections, there will be more details given about the J105; the various parts of this instrument; the ions sources (their production, sizes, etc.) used in sample analysis investigations; the separation and selection processes used and their mechanism that are used to attain the required characteristics for the investigative ions and sample resolution issues.

Figure 2.5: Schematic diagram of the J105 3D Chemical Imager [11]. 2.1.2.1. Sample Holder The sample holder comprises of a copper stub. Its dimensions are 4 cm long, 3 cm wide and 1.2 cm thick. The rectangle copper holder has screw holes for a substrate to be attached to it e.g. a silicon wafer using screws. The primary ion beam current is measured at the sample holder via a Faraday cup. As the sample is heated or cooled, a PT100 temperature sensor is used to monitor the temperature and is embedded in the side of the sample as shown in Figure 2.6.

Figure 2.6: The image of the rectangle copper sample holder, in the J105 instrumentation, used to mount the substrate onto it. 2.1.2.1.1. Sample Holder Insertion into the Instrumentation To insert a new sample, the lock position must be cleared of any previous samples. To free the lock from old samples, its lid must be opened. However, to do this, the lock position must be vented. Now, the substrate of the new sample, which is mounted on the sample’s stub, is placed onto the Z lift (Tier 1). After closing the lid, pumping is needed before moving the sample into the preparation chamber. The action of pumping is done by a rotary and turbo pump. Then the Z lift is moved down into the preparation chamber after the pumping. The downward movement occurs when the pressure inside the lock reaches nearly 1 × 10-5 mbar. Once the sample is in Tier 1 up position of the preparation chamber, a digital optical image of the whole sample stub is taken by the lock camera. The sample image is saved and imported into the sample handling software to utilize it. Moreover, with this image, it is possible to navigate around the desired sample in the SIMS analysis software to select regions of interest.

When the pressure becomes less than 2 × 10-6 mbar in the preparation chamber, the gate valve between the preparation chamber and the SAC is opened to transfer the sample stub into the XYZ stage inside SAC. This action carried out by the transfer arm (the sample transfer process from Tier 1 to the sample stage is automated). Then, the transfer arm is 77

taken back to the home position and the gate valve is closed. The SAC also has a camera that shows a high magnification image of the analysis area.

2.1.2.1.2. Sample Handling Systems The J105 instrument is designed to control the transfer of the sample stub inside the surface analysis chamber automatically. This process is done through software. Three separate components are noted here: the transfer arm, the Z lift and the XYZ stage as shown in Figure 2.7.

Figure 2.7: Imaging of the J105 sample handling system (J105 Instrumentation Manual). The Z lift stays in the preparation chamber and consists of 3 Tiers, which are Tier 1, Tier 2 and Tier 3 as shown in Figure 2.8. It is moved up into the lock position and down into the preparation chamber. This is done after venting or pumping processes in the load- lock that are necessary to protect the system from damage, due to pressure discrepancies involved.

Figure 2.8: Image of Z Lift consisting of three Tiers to keep three samples (one in each Tier) in the preparation chamber of the J105 instrumentation (J105 Instrumentation Manual). The transfer arm picks up the sample stub from Z lift Tier 1 to transfer it to the XYZ stage of the surface analysis chamber (SAC). The XYZ stage is the analysis stage. The transfer arm is also used to transfer the sample stub to the other Tiers for sample storage under high vacuum if needed. All Tiers are moved at the same time due to their mechanical connection with the Z lift, but each of them has a separate room. More details about the sample holder and the insertion mechanism follow.

2.1.2.2. Instrumentation Control The Z lift stage and XYZ stage can be cooled or heated as necessary depending on the required experimental parameters. During analysis, and during the case of working under cryogenic conditions, one notes that both stages are separately connected via copper tubes that are embedded in Dewars. Dewars are filled with liquid nitrogen (LN2). Nitrogen gas passing through the coiled tubes is then cooled close to the LN2 temperature with a pressure of about 40-60 psi as shown in Figure 2.9. In the case of running the stages at an elevated temperature, there is a heating unit for this purpose. Moreover, there is a PT100 sensor to monitor the temperature, which read a range of 95 K to 700 K. The temperature controlling unit heats up and measure the temperature of the Z lift stage and the XYZ stage, at the same time to stabilize the stages and the sample at the desired temperature.

Figure 2.9: The image of a Dewar containing copper coils. The Dewar is continuously filled up with liquid nitrogen (LN2) to just over the level of the coils, during a cold stage process. 2.1.2.3. Electron Flood Gun As mentioned earlier, samples are bombarded with an ion beam to release secondary ions (besides other particles). The bombardment causes the sample’s surface potential to change, according to the charge of the ions beam and loss of secondary electrons. This raises the need for a process of neutralisation of the sample’s surface. This can be done with an electron-flood gun. An electron gun capable of flooding the samples with electrons of energies ranging between 10 eV to 1000 eV, for charge compensation, is used to counteract the aforementioned issues. However, this method must be used with caution, since it could either degrade the sample, or being not enough to neutralise it, so an issue is very delicate fine control of the dose of electrons [13]. Another problem that could arise is the emission of stimulated electron emission for higher energy incident electrons [14]. Other methods of charge compensation can be used to resolve this problem. These include coating the sample surface with a thin film of gold (or silver) and/or the use of a grid over the surface of the sample [14], [15]. However, these methods of neutralising the sample were not used in this thesis, for it was not envisaged to be necessary, since another method of compensation (i.e. controlling the bias of the sample, through software) was thought to be less damaging to the sample and easier to control.

The next section sheds some light on the constituents of the ion beams typically used for analysis in the J105.

2.1.2.4. Polyatomic C60 Source

IOG C60-20 (20 keV C60 gun) and IOG C60-40 (40 keV C60 gun) are installed in SIMS analysers, such as, the Bio-ToF-SIMS instrument and the J105 3D Chemical Imager instrument, respectively. These ion guns are manufactured by Ionoptika Ltd (Southampton,

UK). Both guns have the same C60 source with similar designs as shown in Figure 2.10. They are suitable for operating in ultra-high vacuum chambers (UHV) environment. More in-depth details regarding the use of the C60 source is found in Weibel et al. in 2003 [5].

C60 ions are produced by the action of electron bombardment in the C60 source. The amount of C60 powder placed in a copper reservoir located at the rear of the source is about

250 mg. Production of C60 vapour starts at a temperature of 350 °C, but this temperature produces an insufficient gas density for the required beam current. The source temperature -8 is then increased ~390 °C, resulting in a C60 pressure ~ 8 × 10 mbar which is admitted into an ionization cell via a nozzle. The ionization cell is surrounded by a circular filament and a concentric cylindrical grid. Electrons are accelerated from the filament into the source centre by the positive grid voltage and negative repellor voltage. Consequently, the formation of C60 ions occurs due to the collision between low-energy electrons (~70 eV) and gas-phase C60 molecules [4], [6]. The C60 primary ion beams are then extracted from the ion source and accelerated and focused to create a beam, impacting the sample surface with an energy range of between 2 to 40 keV [4]. The lifetime of the ion source may be increased by operating at lower temperatures, or the beam current maximised by operation at up to 425 °C (with reduced lifetime). Figure 2.11 shows a schematic diagram of an electron ionisation source [4], [6].

The C60 ions and other positively charged carbon ions enter into the optical column by extraction and they are accelerated by an anode potential. The optical column has two lenses. Lens 1 focuses the ion beam in an intermediate field image nearly halfway down the column between a pair of pulsing plates. The final focus of the ion beam on the sample is by Lens 2. Any neutrals are rejected from the beam by a 1̊ bend that is located between a Wien filter and Lens 1 (Figure 2.10). The ion optical column can be used with high current and low spatial resolution mode or vice versa, according to the selected aperture size.

Figure 2.10: Schematic diagram of C60 gun ion optics [5].

Figure 2.11: Schematic diagram of an electron ionisation source [6]. 2.1.2.4.1. Mass Selection (Mass Filtration) The source ions have different m/z ratio and accordingly travel at different velocities. Consequently, a Wien filter is used for mass filtering of the ion beam according to their mass-to-charge ratio. Any unwanted small carbon cluster ions are deflected out of the C60 ion beams and the required charge state for C60 ion beams is selected; the charge of the + 2+ 3+ ions can be either a single (C60 ), a double (C60 ) or a triple (C60 ) one. This device comprises of electrostatic plate and magnetic poles’ piece as shown in Figure 2.12. Equation 2.2 relates to the variables that relate to the Wien filter. It is given as: 82

퐵 × 푞 × 푣 = 퐸 × 푞 Equation 2.2 where: B is magnetic flux; E is electric field; ʋ is velocity and q is electric charge

A similar Wien filter is also employed in the GCIB. It is used for selecting the cluster sizes of the Ar cluster ion beams as discussed below in section 2.1.2.5.1

Figure 2.12: Schematic diagram of a Wien filter whose purpose is to select primary ion beam based on their m/z ratios [6]. 2.1.2.4.2. Spatial Resolution High spatial resolution is achieved by the selection of a smaller aperture in the beam path, thus a smaller beam diameter (and final spot size). It also means a smaller sample current beam. Table 2.2 shows 5 aperture sizes, their micrometer readings and their + corresponding current beam as well as their spot sizes for a 40 keV C60 beam. The best compromise selection for IGO C60-40 is often observed when using a 300 µm aperture. Maximum sample current can be achieved by increasing Lens 1 voltage. The optimum beam focus on the sample can be obtained through the adjustment of Lens 2. The smallest + diameter of the 40 keV C60 ion beam (500 nm) is obtained at using the 10 µm aperture as shown in Table 2.2.

The spot size and corresponding lateral resolution of the ion gun can be measured using a copper grid, by utilising a secondary electron detector (SED) [16]. Figure 2.13 shows the + lateral resolution with 30 µm aperture with a 300 mesh copper grid under 40 keV C60 .

+ Table 2.2: Different apertures of 40 keV C60 for focusing the beam and the resulting lateral resolution [17].

Aperture size (µm) Micrometer Beam current (nA) Beam spot size reading (mm) (µm) 1000 5.9 1.056 40 300 9.9 0.100 15 100 13.9 0.045 8 30 17.9 0.001 0.8 10 21.9 0.0006 0.5

2.1.2.5. Gas Cluster Ion Beams Gas cluster ion beams (GCIBs) are composed of between a few hundreds to a few thousands atoms or molecules per gaseous projectile [18]. On impact the cluster shatters and the total acceleration energy of the cluster is distributed between the atoms in the cluster; each atom has low energy of several eV [19] . For example, a cluster consisting of 1000 atoms with the incident energy of 10 keV, then the energy per atom (E/n) is 10 eV. The GCIB source in the J105 is capable of operating at a beam energy of between 2.5 and 20 keV. A schematic diagram of the GCIB is shown in Figure 2.14 [20].

The clusters must be ionized first before acceleration, focusing and striking the target. Therefore, firstly, Ar gas clusters are generated by feeding Ar gas into the ion source chamber through a nozzle with high pressure of about 10 to 24 bar, where it undergoes an adiabatic expansion into high vacuum [6]. At prescribed conditions, the nozzle flow will 84

experience a supersonic shock wave. This process creates, predominantly, neutral clusters. A skimmer is used to prevent the selection of outliers from the main axis of the travelling particles from entering the ioniser. Now the second stage of action starts with the ionization of these neutral clusters inside the ionization chamber. This is achieved through electron ionization. At the hot filament (opposite of the anode) electrons are created and start accelerating towards the anode, colliding as they move through the neutral cluster hence ionising them. Finally, the produced cluster ions are accelerated by the anode potential down the gun column towards the targets.

Figure 2.14: The schematic diagram of gas cluster ion beams (GCIB) system [20]. As for the size of these clusters, the desired cluster size is attained by controlling the source pressure and through a selection process, using a Wien filter (as describes above for

C60). If a large argon cluster size is required, then the gas pressure (i.e. forcing an increased chance of cluster formation) is increased from 10 to 24 bars or vice-versa. The source can produce a minimum cluster size of Ar2 to the maximum cluster size of Ar10000. Five adjustable apertures including 1000 µm, 300 µm, 100 µm, 30 µm and 10 µm are available. This means that the GCIB gun can offers the desired beam currents and spot sizes as discussed in Section 2.2.7.2 for Polyatomic C60 sources.

The same Ar-GCIB source in the J105 can be used to generate water cluster primary ion beams by adding a water reservoir to the source [21]. The water is heated above the boiling point to create a super-heated stream that is directed through a heated nozzle. The generation of water clusters also occurs via an adiabatic expansion inside the low-pressure ion source region. Finally, the water cluster beam is ionised in the ionisation chamber by electron ionisation as with the Ar cluster. The desired water cluster size is selected using a

Wein filter and the cluster sizes in the range from (H2O)2 to (H2O)8000 can be achieved depending on the boiler temperature (water pressure). 85

2.1.2.5.1. Cluster Size Measurement The size of the gas cluster beams is calculated by pulsing the beam, measuring the time- of-flight of a pulsed cluster, and by knowing the energy of the ions as well as the length of their flight to the sample, the average mass of ions can be determined. The Wien filter voltage was used to change the cluster size values as required. Figure 2.15 shows an oscilloscope reading for the flight time distribution for the required cluster beams.

Figure 2.15: An Oscilloscope was used to read the time-of-flight of the selected cluster ions. The average flight time is 44.8 µs. This is achieved by measuring (using an oscilloscope) the time delay between the detected secondary electron peak (lower trace) + and the ion beam pulser (upper trace). The shown result is that of a cluster size of Ar1000 when the energy of the beam is 20 keV. 2.1.2.6. The J105 Secondary Ion Optics When secondary ions are generated using the J105 Chemical Imager, they have to travel through two devices before being injected into the linear buncher and reflectron. These extracted ions are first cooled though collision by a suitable gas, such as nitrogen

(N2), under the influence of some RF source to focus the ions (in the form of a quadrupole) where their kinetic energy speed is reduced from 100 eV to 1 eV. Then, the energy of the secondary ion beam is filtered by an electrostatic analyser (ESA). The resulting secondary ions are then injected into the linear buncher. In the conventional ToF-SIMS, these energy cooling and filtering features are not included; instead, the extract secondary ions are pulsed directly towards the reflectron, which is located at the end of the ToF mass analyser.

The length of the buncher is 300 mm. It comprises of 30 coupled apertures each of which carries a different voltage. The secondary ions fill this device for 85 µs. After the secondary ions have filled the buncher a 100 V electric field is applied to a blanker plate preventing future ion entering during the buncher pulse period. An accelerating field is then applied every 100 µs to pulse the secondary ions out of the buncher with different energies between the entrance and the exit plate of the buncher (7 kV and 1 kV, respectively). This process generates a time focus (tightly pulsed secondary ions packets) at the entrance of the ToF mass analyser, but a very wide (6 keV) energy spread in secondary ions. Consequently, the mass resolution is now dependent only on the time- focus of the tight packet of secondary ions, and not on the sample topography (initial kinetic energy) or sputtering events (spread in ion creation times over the long primary ion pulse). After the buncher-blacking pulse is complete, more secondary ions then enter in the buncher after the first secondary ions have left, and are transmitted through it; this process is repeated every 100 µs, as mentioned above. The transmission of the secondary ions depends on the ion mass. Light fragments fly through the buncher away from the heavy ions. This means that good transmission happens for molecular ions, while the reduced transmission occurs for fragment ions. The equation concerning the working of the buncher transmission is shown below [9]:

푙 푚 푇푟푎푛푠푚𝑖푠푠𝑖표푛 = × 100 Equation 2.3 푡 √2푣푞

Where: l is length of the buncher; V is transport voltage; t is buncher filling time; m is mass of the ions and q is charge of the electron.

A 6 keV kinetic energy spread is observed in the ions because of the acceleration in the buncher. This energy spread will cause inaccurate analyses of the ions in a conventional ToF with a field-free region, so a harmonic-field ToF reflectron is required to resolve ions at the correct m/z values independent of their high energy spread. In the harmonic ToF field flight times of the secondary ions are only dependent on the mass-to-charge ratio regardless of their high kinetic energy spread, which was induced by using the buncher. Prior to the secondary ions of the same m/z reaching the detector, they spend 50% of the time in harmonic motion within the mass analyser, which is equivalent to the time spread due to the focus from the buncher. The linear buncher is illustrated in Figure 2.16.

Figure 2.16: Schematic diagram of the buncher connecting with the reflectron of the ToF mass analyser in the J105 instrumentation [9]. 2.1.2.7. Tandem Mass Spectrometry (MS/MS) Tandem mass spectrometry (MS/MS) is performed in the J105 instrument in a ToF-ToF configuration [11]. The advantage of tandem mass spectrometry is its ability to provide additional structural information; implementing necessary selectivity so to characterize unknown compounds during sample analysis. Ions traverse the collision cell, which is located between the buncher and the ToF mass analyser (Figure 2.5). The collision cell is operated at an increased pressure and is filled with a collision gas such as helium (He), nitrogen (N2) or argon (Ar). The precursor ion collides with the inert gas or molecule in a collision-induced dissociation (CID) process with collision energies ranging from 1 keV to 7 keV. Consequently, the precursor ion fragment into their product’s ions. Both the precursor and its product ions have the same velocity to keep traveling because the CID occurs in the field-free region. Then, these ions are selected by a timed ion gate. After reaching the ToF analyser, a detection process is performed.

2.2. Film Deposition Techniques Two techniques were used to make films on desired substrates in this project including spin-coating and thermal evaporation. Each of these is separately described below.

2.2.1. Spin-Coating A spin-coating is used to prepare uniform thin films of organic materials having thickness range of a few nanometres to a few micrometres. Different thicknesses of uniform films are controlled and achieved by selected spinner speed (200-7000 rpm) and a concentration of material solutions. Two types of methods are available to obtain uniform films. These are static dispense and dynamic dispense.

In a static dispense, the solution of the desired target is flowed into the centre of a substrate prior to rotation. In the dynamic dispense method, when the spin reaches the

desired speed the solution of the material flows to cover the substrate during its rotation. The drying of the complete solution during spinning depends on the type of solvent used because of the different boiling point and vapour pressures involved (see Table 2.3). Moreover, it depends on atmospheric conditions such as temperature and humidity.

To obtain homogenous films is very important when studying secondary ion yields, sputtering yields and depth resolution (depth profile and etching).

Table 2.3: A summary of solvent conditions during spin-coating, their boiling points, vapour pressure and the duration of their spinning process

Solvents Boiling point (ºC) Vapour pressure Duration of spinning chloroform 61.15 2.8 kPa (20 ºC) ~ 30 Second toluene 111 25.9 kPa (25 ºC) ~ 30 Second anisol 154 - ≥ 2 minutes DCB 180.5 1 mmHg (20 ºC) ≥ 2 minutes

The spin coater used to obtain a film on the target material in this project is shown in Figure 2.17. A static dispense method with anisol solvent used for PTAA and DCB solvent for PMMA. This device is operated under a vacuum, so that to let the coat stick to the substrate.

It must be noted that before placing the silicon wafers on the spin coater, the substrates must be washed thoroughly to remove any contaminating materials (such as dust) that could lie on the top of the substrate surface. The cleaning process is achieved by ultrasonication with the use of two or three solvents so that uniform films could be created. Contamination on substrates, if untreated it will affect films.

Figure 2.17: The type of spin-coater device (Model WS-400BX-6NPP/LITE) was utilised to prepare a film on a substrate e.g. a silicon wafer (In SIMS lab and in OMIC lab).

2.2.2. Thermal Evaporation Films Deposition Thermal evaporation is a Vapour-Phase Deposition (VPD) technique used for film deposition purpose. Some of the powder sample material is loaded onto a crucible (a heated container that contains a coil of tungsten) and is evaporated by heating the crucible to an adequately high temperature in a vacuum chamber. The evaporated material travels directly to the substrate in free path and it condenses as a solid state onto the substrate to create a thin film. A quartz crystal microbalance is used to monitor the film thickness. The instrument used in this project is that of an Edwards Auto 306 (at The University of Manchester, School of Chemistry, and Organic Materials Innovation Centre (OMIC)) and was designed for small organic materials. Figure 2.18 shows the thermal evaporation system.

Figure 2.18: Schematic diagram of the thermal evaporation process. 2.3. Instrumentation for Measurements of Film Thickness In order to calculate sputtering yields it was essential to know the depth of the craters formed, or, when profiling completely through a thin film, the film thickness. Two devices were used to measure the thickness of films in this project. One was stylus profilometry instrument, which was used to measure the PMMA, PTAA and TIPS-pentacene film thicknesses. The other one was an Atomic Force Microscope (AFM), which was used to measure TIPS-pentacene films. These devices are available for use at the School of Chemistry (at The University of Manchester, OMIC).

2.3.1. Stylus Profilometry Stylus profilometry is a technique for measuring film thickness that depends on electrical signals being passed across stylus (diamond tipped) and a sample. This is the main difference with AFM (see below), as AFM depends on Van Der Waals forces between the tip of a cantilever and the sample. The other difference is that in stylus profilometry the diamond tip continuously stays in contact with the sample, thus presenting problem with sample integrity [22]. The instrument used in the project for profilometry is a DektakXT (Bruker, in Billerica, Massachusetts, United State).

The above shortcomings of stylus profilometry make AFM a better choice for film thickness measurement, for TIPS-pentacene films.

2.3.2. Atomic Force Microscopy (AFM) The atomic force microscopy (AFM) is among a family of scanning probe microscope (SPM) instruments. Three decades ago, Binning, Quate and Gerber [23]invented the AFM instrument. A fine tip (cantilever) is utilized in the AFM to measure the morphology and properties of a surface that is formed as an interface between the tip and the sample. The AFM is capable of studying (imaging) numerous various materials such as organic and inorganic samples, biological samples (such as cells and tissues), semiconductors, polymers, metals, glasses and ceramics. Using these materials, as samples, is dependent on selecting the appropriate environments that are relevant for each one of them.

A brief description of how the AFM work (design, principles of working and tapping mode) is shown below in the following sections.

2.3.2.1. How the Atomic Force Microscopy Work 2.3.2.1.1. The Atomic Force Microscopy Design Figure 2.19 shows a schematic diagram of AFM. The benefit of this technique is its ability to improve its atomic-scale resolution by numerous improvements in the AFM design such as sharper tips, better elastic cantilevers, having a deflection sensor which is more sensitive, force feedback and higher resolution between the tip and the sample distances [24].

Figure 2.19: A schematic diagram of the AFM [24]. 2.3.2.1.2. Principle of the Atomic Force Microscopy An AFM uses a sharp tip to scan over the surface of a sample and in the process relaying some feedback information about the interaction. This feedback mechanism provides a constant force on the tip above the sample, which just rests above the sample’s surface. This feedback utilises the piezo-electric effects and is enclosed in the form of a scanner. The tip is placed at the end of a small cantilever, which is usually fabricated from either silicon nitride (Si3N4) or silicon (Si). Attractive and repulsive forces, due to the Van Der Waals’ interactions are generated within the interaction areas between the probe and the sample. These forces cause the cantilever to deflect by an amount that is consistent with Hooke’s Law as represented by this equation 2.4:

퐹 = 푘. 푋 Equation 2.4 where: F is the force, k is spring constant and X: cantilever deflection

The cantilever will move up and down; because of the constant spring tension of the cantilever. The movement of the cantilever can be measured by means of a laser beam, 92

which is deflected from the tip. The reflected laser goes into position-sensitive photodiodes (PSPDs). A constant force between the tip and the sample is maintained by a continuous feedback loop mechanism, which compensates the damage effects caused during the scanning process. This keeps the tip at a constant height. The feedback loop informs the piezoelectric device to move the tip closer (repulsive) or further away (attractive) from the surface. Therefore the force value is constant. Feedback signals from each pixel of the raster of the scanner are translated into topography images.

The cantilever tips used in AFM systems can be operated in contact (static) mode, non- contact mode or tapping mode (the two later modes are called dynamic modes) and these are implemented according to the sample surfaces that is available. The tapping mode is going to be discussed further below, as it was used in this study.

2.3.2.1.3. Tapping Mode (Intermittent Mode) AFM Imaging Modes In this mode, the cantilever is oscillated at or near its resonant frequency by a piezoelectric crystal. The amplitude of the cantilever oscillation is significantly high (from 20 to 100 nm) because of the piezoelectric motion, in the case, the tip is not in contact with the surface. The tip starts to softly strike the surface during its oscillating movement because of closeness to the sample. The tip is capable of touching the surface and lifting off alternately at frequencies of approximately 50 to 500 kHz within the scanning process. Oscillation amplitude of the tip, which continuously scans the surface topography, depends on the sample being used [25]. The amplitude of the oscillation can either decrease or increase whenever the tip undergoes a bump and a depression on the surface respectively.

A feedback loop assists the amplitude of the cantilever oscillation to remain constant within its operation. The optical system reveals the change in the tip’s oscillating amplitude. These changes are recorded in the controller’s electronics. Error signals are then generated whenever a comparison between the measured value and a set reference value occurs. As a result of the constantly oscillating amplitude through the interaction between the tip and the sample surface, the force on the sample remains constant (Figure 2.20) [25].

Figure 2.20: Schematic representation of the operation of the tapping mode AFM [25].

2.4. References [1] W. E. Stephens, “A pulsed Mass Spectrometer with Time Dispersion,” Phys. Rev., vol. 69, p. 691, 1946. [2] W. C. Wiley and I. H. Mclaren, “Time-of-Flight Mass Spectrometer with Improved Resolution,” Rev. Sci. Instrum., vol. 26, no. 12, pp. 1150–1157, 1955. [3] E. de H. and V. Stroobant, “Mass Analysers,” in Mass Spectrometry Principle and ApplicationS, 3rd ed., Chichester: John Wiley & Sons Ltd, 2007, pp. 85–173. [4] J. C. Vickerman and I. S. Gilmore, “Molecular Surface Mass Spectrometry by SIMS,” in Surface Analysis – The Principal Techniques: the principal techniques, 2nd ed., Chichester, 2009, pp. 113–199. [5] D. Weibel, S. Wong, N. Lockyer, P. Blenkinsopp, R. Hill, and J. C. Vickerman, “A C60 Primary Ion Beam System for Time of Flight Secondary Ion Mass Spectrometry: Its Development and Secondary Ion Yield Characteristics,” Anal. Chem., vol. 75, no. 7, pp. 1754–1764, Apr. 2003. [6] R. Hill, “Analysis beams used in ToF-SIMS,” in TOF-SIMS: Materials Analysis by Mass spectrometry, 2nd ed., Chichester: IM Publications LLP and SurfaceSpectra Limited, 2013, pp. 271–290. [7] R. M. Braun et al., “Performance characteristics of a chemical imaging time-of- flight mass spectrometer,” Rapid Commun. Mass Spectrom., vol. 12, no. 18, pp. 1246–1252, 1998. [8] “http://www.fkf.mpg.de/ga/machines/sims/How_does_TOF_SIMS_work.html.” . [9] R. Hill, P. Blenkinsopp, S. Thompson, J. Vickerman, and J. S. Fletcher, “A new time-of-flight SIMS instrument for 3D imaging and analysis,” Surf. Interface Anal., vol. 43, no. 1–2, pp. 506–509, 2011. [10] J. S. Fletcher, N. P. Lockyer, and J. . Vickerman, “DEVELOPMENTS IN MOLECULAR SIMS DEPTH PROFILING AND 3D IMAGING OF BIOLOGICAL SYSTEMS USING POLYATOMIC PRIMARY IONS,” Mass Spectrom. Rev., vol. 47, no. 1, pp. 142–147, 2011. [11] J. S. Fletcher et al., “A New Dynamic in Mass Spectral Imaging of Single Biological Cells,” Anal. Chem., vol. 80, no. 23, pp. 9058–9064, 2008. [12] J. C. Vickerman, “Molecular imaging and depth profiling by mass spectrometry-- SIMS, MALDI or DESI?,” Analyst, vol. 136, no. 11, pp. 2199–2217, 2011. [13] I. S. Gilmore and M. P. Seah, “Investigating the difficulty of eliminating food gun damage in TOF-SIMS,” Appl. Surf. Sci., vol. 204, pp. 600–604, 2003. [14] J. C. Vickerman, “Prologue:ToFSIMS-An evolving mass spctrometry of materials,” in TOF-SIMS: Material Analysis by Mass Spectrometry, 2nd ed., Chichester: IM Publication LLP and SurfaceSpectra Limited, 2013, pp. 1–37. [15] A. B. and N. M. R. John C. Vickerman, “SIMS DEPTH PROFILING OF SEMICONDUCTORS,” in Secondary Ion Mass Spectrometry Principles and Applications, the United States, 1989, pp. 105–148. [16] I. B. Razo, “Molecular imaging of mouse brain tissue using Cluster Time-of-Flight 95

Secondary Ion Mass Spectrometry,” PhD thesis, The university of Manchester,UK, 2015. [17] S. Rabbani, “Advances in Time-of-Flight Secondary Ion Mass Spectrometry for the Analysis of Single Cells on Sub-Cellular Scale,” PhD thesis, The University of Manchester, UK, 2010. [18] I. Yamada, “Materials processing by gas cluster ion beams,” Mater. Sci. Eng. R Reports, vol. 34, no. 6, pp. 231–295, 2001. [19] N. Toyoda, J. Matsuo, T. Aoki, I. Yamada, and D. B. Fenner, “Secondary ion mass spectrometry with gas cluster ion beams,” Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms, vol. 190, no. 1, pp. 860–864, 2002. [20] T. Seki, J. Matsuo, G. H. Takaoka, and I. Yamada, “Generation of the large current cluster ion beam,” Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms, vol. 206, pp. 902–906, 2003. [21] S. S. (née Rabbani), A. Barber, I. B. Razo, J. S. Fletcher, N. P. Lockyer, and J. C. Vickerman, “Prospect of increasing secondary ion yields in ToF-SIMS using water cluster primary ion beams,” Surf. Interface Anal., vol. 46, no. S1, pp. 51–53, 2014. [22] “Nanoscience Instrument, http://www.nanoscience.com/technology/optical-profiler- technology/how-profilometer-works/.” . [23] G. Binnig, C. F. Quate, and C. Gerber, “Atomic force microscope,” Phys. Rev. Lett., vol. 56, no. 9, p. 930, 1986. [24] D. Ricci and P. C. Braga, “How the Atomic Force Works,” in Atomic Force Microscopy Biomedical Methods and Applications, vol. 242, 2004, pp. 3–12. [25] N. Jalili and K. Laxminarayana, “A review of atomic force microscopy imaging systems: application to molecular metrology and biological sciences,” Mechatronics, vol. 14, no. 8, pp. 907–945, 2004.

Chapter 3: Molecular Depth profiles of PMMA with Time-of-flight Secondary Ion Mass Spectrometry (ToF-SIMS) using Different Cluster + + Ion Beams (20 and 40 keV C60 and 20 keV Arn ) at Different Temperatures

3.1. Introduction Poly (methyl methacrylate) PMMA is synthesised by free radical vinyl polymerization from monomer methyl methacrylate MMA, as shown in Figure 3.1.

Figure 3.1: The chemical structure of the repeating unit of PMMA produced from monomer methyl methacrylate MMA, using free radical vinyl polymerization. It is a transparent thermoplastic material that finds applications in diverse areas such as organics electronics (e.g. a gate insulator in organic field-effect transistors (OFETs)) as well as medicine (e.g. intraocular lenses, microfluidic cells).

PMMA has two methyl groups, one is called a methyl ester pendant group and another is attached to the polymer backbone. The molecular weight of repeat unit is about 100.12 g/mol.

Depth profiles of PMMA have been previously studied with SIMS using a diversity of monoatomic, polyatomic and cluster primary ion beams. In the monoatomic investigations, Ga+, for example, was used to study PMMA films, at high temperatures (180 to 210 °C). However, so far, the secondary ion yields of only the initial layers of PMMA were investigated with monoatomic primary ion beams (e.g. Ga+ [1], Au+ [2], Ar+ and Xe+ [3]) , at room temperature.

+ + As for polyatomic ion beams, such as, SF5 and C60 they have been successfully used + for depth profiling of PMMA, at different temperatures. With SF5 , sputtering yields and + depth resolutions were obtained. As for C60 , results were obtained only when PMMA was

deposited on Au substrates and bombarded with a 20 keV beam. At room temperature, + + secondary ion yields of the initial layer of PMMA were obtained with SF5 [3] and C60 [2]. Also, Ar+ cluster beams were used to depth profile of PMMA, at room temperature, but depth resolutions were not calculated [4].

The instruments used in the literature above to analyse samples of PMMA, previously operated with pulsed primary ion beams that is working in a dual beam mode, to provide high spatial, mass resolution and high sensitivity. In Figure 3.2, a schematic diagram is + shown of an example of a dual beam depth profile experiment using C60 (in DC mode) to + sputter a large area, and a Bix beam for analysis in a smaller area. This technique (the dual beam mode) is time-consuming, and there will be a loss of data from layers during sputtering cycles.

Figure 3.2: A schematic diagram of the dual beam approach in ToF-SIMS depth profiling [5]. The use of conventional monoatomic ions in the bombardment of a sample, usually, entails some beam-induced-damage to that sample, which consequently, affects the values for sputtering yields [6]. Therefore, to avoid this phenomenon, one needs to operate SIMS in static mode, i.e. working under the critical fluence limit of 1 × 1013 ions/cm2. Operating above this limit means that SIMS is operating in dynamic mode, which results in bombardment damage. Using more constituent atoms in the projectiles will spread the energy of the impact and hence reduce the possibility of damage. This is achieved by using cluster ions, which will significantly enhance sputtering yield [6]. Therefore, in this study, cluster ions were used to achieve the aims of the research. Moreover, and in order not to have high energy that will cause the ions to penetrate the sample and cause deep damage to its layers, a suggested E/nuc threshold (obtained by simulation) was taken into 98

consideration, the value of which was put at ~ 0.1-1 eV/nuc by Delcorte et. al. [7], [8]. This facilitates the achievement of successful depth profiling. Therefore, using Ar (atomic + weight 40) and a 10 keV beam energy of a cluster of 2000 Ar2000 (the biggest routinely available) will produce an E/nuc value of 0.25 eV/nuc (i.e. well within the limit). These requirements are also confirmed by Shen et.al. [9] that the highest kinetic energy and the largest cluster, ought to produce an E/nuc value of slightly over 5 eV/atom. Although, in + our Ar2000 , 10 keV beam, the E/nuc was 5 eV, thus abiding by this requirement, it must be noted that the authors based their value by investigating trehalose, which is not what we are investigating (PMMA, PTAA, and TIPS-pentacene). Although our highest choice of + Ar250 , 20 keV beam energy meant that the E/atom was 80 eV, the trehalose investigation authors did not put their value as a limit, but rather as a "best situation" value. Further, the + 80 eV/atom (Ar250 ) is a 2 eV/nuc, which is higher than the simulation threshold [Delcorte et. al.], thus not contradicting this requirement. It must be said that [10], experimentally, put the threshold, for PMMA, as 1.0 eV/nuc.

It would have been interesting to see how the behaviour of the investigated materials vary with other (higher cluster sizes), but due to time limitations it was not possible to pursue such investigations. This reason is also valid for the values of temperatures that this study did not cover, which will be discussed below.

Therefore, in this study, ion beams of different cluster sizes, were employed in DC + + + + + mode (single beam setup); C60 , Ar250 , Ar500 , Ar1000 and Ar2000 , were used to depth profile PMMA, using the J105. The influence of the temperature on the sputtering and secondary ion yields and depth resolutions, of the different cluster sizes, has been investigated. Secondary ion yields of PMMA were selected at the pseudo-steady-state region. PMMA analysis is used as a reference material (as a single-layer) and to compare with previous published results under different conditions.

3.2. Experimental Section 3.2.1. Sample Preparation PMMA (MW ~ 120 kDa purchased from Sigma-Aldrich) was dissolved in 1,2- dichlorbanzene (DCB), to obtain a 2.5% w/v solution. Silicon wafers 10 × 10 mm2 (purchased from IDB, Technologies, Ltd, UK) were sonicated in acetone and isopropanol, for 15 minutes (in each solvent). Then, the washed silicon wafers were rinsed with isopropanol and were blown dry with a stream of nitrogen gas. To remove organic contaminants, the wafers were subjected to 20 minutes of UV/O3 treatment. Finally, 75 µL

of PMMA solution was spun onto each wafer at 3000 rpm, for 3 minutes to get uniform films. A DektakXT Stylus profiler (Bruker), with a 3 mg stylus force, was used to measure thicknesses of the films, in three areas (34.0 nm ± 0.1 nm).

3.2.2. ToF-SIMS Analysis A J105 3D Chemical Imager ToF-SIMS instrument was used to analysis of the PMMA sample. The instrumentation is described in more details, in Chapter 2.

Mahoney [11] noted that, investigating some samples for depth profiling, was greatly improved when cryogenic temperatures were chosen. This together with accumulated knowledge that controlling the temperature of a sample has an important effect on such analytical investigations, mean that the more wide-spread choice of temperatures, in these studies, the better. Since investigating samples above room temperature means approaching Tg, the glass temperature (at which the samples structure changes start to affect sputtering), and the fact that many other studies had already covered this temperature range, it was felt prudent that in this study one ought to investigate at room temperature, and below it. This temperature range is important because free radicals reactivity is decreased at these temperatures [11], which in turn affects cross-linking. It must be pointed out that some of this cross-linking is created and/or diffused immediately upon primary ion bombardment of the sample, and/or later as a secondary cross-linking-creation phase [11]). Now since sputtering yield is affected by the presence/loss of cross-linking, then qualifying and quantifying this link is important. In general, lower temperatures mean reduced mobility of the samples species that are produced during ion bombardments, which is beneficial in depth profiling activity of molecules. Mahoney, Seah, Möller, have investigated low temperature effects by choosing -75 °C, and 0 °C, -25 °C, -50 °C, -75 °C, - 100 °C, and -110 °C, respectively. This suggested that investigating in-between temperatures (i.e. between 0 °C and -110 °C, such as, -50 °C, -100 °C, -125 °C) and further cryogenic ones (-150 °C and -170 °C) will be useful to assess the possibility of different temperatures effects, on the investigated samples. Moreover, going back to room temperature investigations mean that 'deferred cross-linking' [11] could be assessed. Although some of the measurements were identical to those of Mahoney, Seah and/or Möller, different ion beam energy, different projectiles and different cluster sizes were used, so that no repetition of already investigated parameters were carried out. Moreover, different sample thicknesses (as lower than 80 nm thickness were observed to have less + signal degradation, and not at all if C60 ions are used [11] were employed, as well as, a different instrumentation (the J105). 100

+ + Therefore, C60 cluster beams (at 20 and 40 keV) and four different sizes of Ar cluster + + + + beams (Ar250 , Ar500 , Ar1000 , and Ar2000 at 20 keV) were used to analyse the sample of PMMA, initially at room temperature. Liquid nitrogen were used to cool the PMMA films (the mechanism of which has been explained earlier in Chapter 2, section 2.1.2.2) to -100 + °C, -125 °C and -170 °C, for analysis with C60 and to -50 °C and -150 °C, for analysis + + with Ar500 and Ar2000 .

All the experiments were carried out in a positive-ion mode with 350 × 350 µm2 field- of-view (the raster area) and 16 × 16 pixels. To confirm that beam current was stable during the experiments, the primary ion dose (which was 5 × 1011 ions/cm2) was measured before and after exposing the sample to cluster beams. The spectral regions were selected from 30 to 900 Da (J105 'low-mass' range) after accumulation of ion dose 5 × 1012 ions/cm2 (ion dose at the tenth analysis layer of the film).

3.3. Results and Discussion 3.3.1. PMMA Sample Analysis at Room Temperature 3.3.1.1. Secondary Ion Spectra of PMMA Positive secondary ion mass spectra of samples of PMMA were obtained, with different + + + + + cluster ion beams, namely, C60 (at 20 and 40 keV) and Ar250 Ar500 , Ar1000 and Ar2000 (at 20 keV), Figures 3.3, 3.4 and 3.5. The secondary ion signals of undamaged and damaged (typically aromatic hydrocarbon species) samples of PMMA are summarized in Table 3.1 and 3.2, respectively. These results are consistent with what have been observed by Leggett and Vickerman, using monatomic Ar+ beam (2 keV) [12] and by Wagner, + using SF5 (5 keV) [13] . Some of these signals were also observed in polystyrene samples [14], as most of these prominent signals are thought to be due to the cyclic and poly-cyclic nature of hydrocarbon ions [14].

Aggregating the obtained spectra from all the PMMA investigations, Table 3.1 lists the molecular weights of all the highest yield peak fragments (secondary ion structures) that were sputtered away, together with their chemical formulae and structures, if known. These peaks, being the highest yield, represent the biggest amount of ions that sputtered away, signifying the most probable fragmentation. Table 3.1 shows both observed and theoretical molecular weights of the major ions observed.

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Table 3.1: Characteristic positive secondary ion signals of the repeat unit of undamaged + + PMMA, observed using C60 and Arn (n = 250, 500, 1000 and 2000) cluster ion beams. The structures are obtained from [15], [16], [17].

m/z m/z Chemical Secondary ion structure calculated observed formula

+ 126.068 126.067 [C7H10O2]

+ 139.076 139.080 [C8H11O2]

+ 186.089 186.093 [C9H14O4]

+ 200.105 200.099 [C10H16O4]

+ 213.113 213.112 [C11H18O4] ----- + 233.118 233.116 [C14H17O3] ----- + 235.133 235.125 [C14H19O3] -----

To explain the aromaticity of the molecules, in Table 3.2, it is noted that irradiation of polymers tend to create double bonds. This means that the irradiated polymer will probably have more unsaturated bonds after irradiation than before it [6]. Moreover, since irradiation also can cause cyclization (due to cross-linking), then the effect of double bond formation can and eventually produce aromatic structures [6], assuming that all other requirements for aromaticity are satisfied. Although the precise mechanism for this has not been determined, it is worth mentioning that cyclisation increases in materials that have more crystallinity, [6], and since in this study we are working at temperatures less than Tg, the glass transition temperature, then the materials will have more crystallinity, thus more tendency to form cyclic structures. This will mean that the polymer will become more radiation-resistant, since Mahoney noted that the majority of polymers, which are radiation-resistant have aromatic constituents [6], due to the higher stability (protection of resonance [6]) of aromatic compounds. This stability issue has also been mentioned by

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Briggs and Hearn [18] (bearing in mind, that increased aromaticity tends to mean an increase in cross-linking, thus reduced sputtering yields).

Table 3.2: Characteristic positive secondary ions of the repeat unit of damaged PMMA, + + observed using C60 and Arn (n = 250, 500, 1000 and 2000) cluster ion beams. The structures are obtained from [12].

m/z m/z Chemical Secondary ion structure calculated observed formula + 91.054 91.053 C7H7

+ 105.070 105.071 C8H9

+ 115.054 115.061 C9H7

+ 121.101 121.089 C9H13

+ 128.062 128.059 C10H8

+ 165.070 165.071 C13H9

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Figure 3.3: Positive secondary ion mass spectra of PMMA films, obtained after an ion dose of 5 × 1012 ions/cm2 was delivered, using primary ion beams, of: (a) 40 keV and (b) + 20 keV C60 at room temperature.

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Figure 3.4: Positive secondary ion mass spectra of PMMA films, obtained after an ion 12 2 + dose of 5 × 10 ions/cm was delivered, using cluster ion beams, of: (a) Ar250 and (b) + Ar500 , working at 20 keV incident energy and room temperature.

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Figure 3.5: Positive secondary ion mass spectra of PMMA films, obtained after an ion 12 2 + dose of 5 × 10 ions/cm was delivered, using two cluster ion beams, of: (a) Ar1000 and + (b) Ar2000 , working at 20 keV incident energy and room temperature.

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3.3.1.2. Secondary Ion Yields at the Pseudo-Steady-State Regions Secondary ion yields of molecular mass peaks are defined as the number of detected secondary ions divided by the number of primary ions (as mentioned in Chapter 1). Here, + + secondary ion yields obtained with C60 are approximately twice that of Ar cluster ion beams. This is because of a reduction in the energy per atom (eV/atom) when there is an increasing of the cluster size of ion beam, as shown in Figure 3.6. This is in agreement + with previous studies that showed C60 ion beams provide greater secondary ion yields, in + + + + + comparison with ion beams ranging from Ga [19], Au [2], Ar , Xe and SF5 [3].

Furthermore, the enhancement of secondary ion yields, when moving from 20 to 40 keV + + C60 , is more pronounced with the m/z 126 (C7H10O2) (Figure 3.6). For instance, the + + secondary ion yields of m/z 126 (C7H10O2) obtained with C60 beam at 40 keV is 20% higher than that obtained with the 20 keV, while for m/z 186 the improvement of the yields + + + + is about 5%. For Ar cluster beams (Ar250 , Ar500 , Ar1000 ) most secondary ion yields (at + m/z 126 and m/z 186 fragments) are, in general, slightly smaller than that of the Ar2000 + cluster beam. Interestingly, the highest ion yields are obtained with Ar2000 cluster beam, which has the lowest E/n. Note that the fragmentation in the spectra is slightly different + + + (Figures 3.4 and 3.5), the smaller Ar cluster beams (Ar250 , Ar500 , Ar1000 ) generated more low mass fragments, this may explain why their high mass ion yields is reduced + compared to the Ar2000 .

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Figure 3.6: Positive secondary ion yields of PMMA fragments (m/z 126, m/z 139 and m/z + + + 186) with various ion beams, including 40 keV C60 , 20 keV C60 and 20 keV Ar250 , + + + 11 2 Ar500 , Ar1000 and Ar2000 , using primary ion dose of 5 × 10 ions/cm . The ion yields were determined from an average of three stages from the pseudo-steady-state regions; all experiments were performed at room temperature (25 °C). The accumulated ion dose at each stage was: 2.71 × 1013, 5.42 × 1012 and 7.59 × 1012 ions/cm 2,respectively, at 40 keV + 13 13 13 2 + C60 ; 1.47 × 10 , 1.78 × 10 and 2.03 × 10 ions/cm respectively, at 20 keV C60 ; 4.24 13 13 13 2 + 13 13 × 10 , 1.85 ×10 and 2.61 × 10 ions/cm at 20 keV Ar250 ; 1.44 × 10 , 1.90 × 10 13 2 + 13 13 and 2.28 × 10 ions/cm , respectively, at 20 keV Ar500 ; 1.31 × 10 , 1.69 × 10 and 2.05 13 2 + 12 12 × 10 ions/cm ,respectively, at 20 keV Ar1000 ; and 5.91 × 10 , × 9.32 10 and 1.29 × 13 2 + 10 ions/cm ,respectively, at 20 keV Ar2000 . 3.3.1.3. Depth Profiling Secondary ion yields of PMMA fragments (m/z 126, m/z 139 and m/z 186), chosen at random, follow, roughly, the same behaviour, during depth profiling investigations, (Figure 3.7). As mentioned in Chapter 1, section 1.7, three stages (regions) exist: an initial transient, a plateau (a steady-state or “pseudo-steady-state”), and a final region, reaching the interface e.g. between PMMA and the silicon (secondary ion yields of Si are not + + shown). The three regions display with C60 and Ar cluster beams.

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Figure 3.7: Depth profiles of PMMA fragment secondary ion yields (m/z 126, m/z 139 and + m/z 186) are plotted as a function of increasing primary ion dose, using a 20 keV C60 . These profiles were performed at room temperature (25 °C), on a 34 nm thick, PMMA film.

Calculations Sputtering Yields The sputtering yield volume is the average amount of material ejected from the bombardment of the sample per incident ion [20]. Therefore, to determine the sputtering yields, the volume of material removed by the ion beam and the ion dose required to reach the interface (PMMA/Si) must be known. The volume of material sputtered can be calculated by using equation 3.1

푉 = 퐿 × 푊 × 푑 Equation 3.1 where: V is the volume of the sputtering material in nm3; L (nm), W (nm) stand for the length and width of the raster area and d (nm) for the thickness of the removed film. The number of primary ions per area unit or the ion fluence, is calculated using the equation 3.2

퐼flux = 퐼Dose × 퐴Raster area Equation 3.2 2 2 Iflux is the ion fluence in ion/s; and IDose (ion/cm ), ARaster area (cm ) stands for the interface ion dose at 50% of the decreased secondary ion signal of the steady state, (Figure 3.8a) and the raster area, respectively. Thus, the volume-sputtering yield can be obtained by: 109

푉 푌 = Equation 3.3 퐼flux where: Y is the sputtering yield in nm3/ion.

Furthermore, as it will be seen in chapter 6, when it comes to comparing different cluster types e.g. Arn and C60, finding the yield/nucleon is more beneficial in showing the trend of change of the yield (per nucleon) with that of the energy (per nucleon). Therefore, if V = W D L, where W (in meters) and D (in meters), i.e. the width and length of the rastered area, and having the dose per unit area (in m2), then we have:

푉 푌 = 2 Equation 3.4 (퐼푑표푠푒/m ) 푊 퐿 푊 퐿 퐷 푌 = 2 Equation 3.5 (퐼푑표푠푒/m ) 푊 퐿 퐷 푌 = 2 Equation 3.6 (퐼푑표푠푒/m )

This is a relation between the yield and depth and Idose (with units of cubic-meters per ion). However, using clusters of size n, then this will change to:

퐷 푌 = 2 Equation 3.7 (퐼푑표푠푒/m ) 푛 and by dividing by the atomic weight of each atom in the projectiles, one will get the yield per nucleon.

Depth Resolution The depth resolution is an important factor, to evaluate the quality of the depth profile. By definition, it corresponds to the distance (interface width) between 84% and 16% or between 16% and 84% change in the signal of the secondary ion intensity [6].

To obtain the depth resolution, the secondary ion intensity of m/z 186 was normalized by its last value of the pseudo-steady-state. Then, the ion dose was converted to a depth scale by this equation:

퐼 퐷푒푝푡ℎ = 푑 × 0 Equation 3.8 퐼Dose where: d, I0 stand for thickness of the film (nm) (measured by DektakXT) and initial ion dose (ion/cm2), respectively. As the signal fluctuates during the profile the interface width is obtained from the data points closest to the 84% and 16% values of the normalized secondary ion intensity (Figure 3.8b).

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Figure 3.8: Normalized secondary ion intensity of the fragment ion of PMMA, m/z 186, + 2 using a 20 keV C60 , plotted as a function of: (a) ion dose (ions/cm ) and (b) depth (nm). The horizontal red line (a) indicates the interface ion dose after decreasing the secondary ion signal to 50% of its steady-state. The vertical blue lines (b) show the interface width from 81.28% to 12.89%, and the grey box shows the depth resolution (Δz).

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+ 3.3.1.3.1. Using 20 and 40 keV C60 Cluster Ion Beams ToF-SIMS depth profiles of the secondary ion yield fragments of PMMA (m/z 186), + + (C9H14O4) , bombarded with 20 and 40 keV C60 , are depicted in Figure 3.9. The characteristic ion yield decays rapidly followed by a pseudo-steady-state region, where the signal stabilises with increasing ion dose. Further increase in the ion dose leads to the + interface between PMMA and the Si substrate (Si6 signal m/z 168 not shown in this Figure 3.9). As seen from Figure 3.9, there is a slightly variation between the depth + + profiles obtained with the 20 and that of the 40 keV C60 . The 20 keV C60 reaches the interface between PMMA film and the substrate at a higher total ion dose than that of the + 40 keV C60 (the same film thickness), Table 3.3. This is in agreement with previous + studies on PMMA, using SF5 polyatomic primary ion beams, working under different, + lower, energies (5 and 8 keV) [21]. The secondary ion signal is more stable with the C60 + (20 and 40 keV) in comparison with SF5 (5 keV) [22], [23]. However, the most stable + secondary ion signal intensities, of PMMA, were obtained with a 10 keV C60 beam [24].

The interface dose, sputtering yields and depth resolutions obtained from the depth + profiles of PMMA are summarized in Table 3.3. The sputtering yield produced with C60 at 40 keV is approximately twice that of the 20 keV beam. Additionally, the 20 keV beam produces yields which are nearly twice that of earlier PMMA studies, where the yields were obtained from a 10 keV beam, (Table 3.3) [25]. The yields obtained from PMMA, 3 3 + with 5 keV (32.4 ± 4.1 nm /ion) and 8 keV (47.8 ± 0.9 nm /ion) SF5 [21], were smaller + than that produced with C60 . This might be due to the chemical composition of the cluster beam [9].

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Figure 3.9: Depth profiles of secondary ion yields of m/z 186 fragment of PMMA, plotted + as a function of increasing C60 primary ion doses, working at: a) 20 keV and b) 40 keV. These profiles were performed at room temperature (25 °C), on a 34 nm ± 0.1 nm thick, PMMA films.

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+ The better (smallest) depth resolution was obtained with 20 keV C60 when compared to that obtained with 40 keV. This is even better than that of a previous study of PMMA film, whose film was 25.8 nm thick, and was deposited on an Au substrate with the same cluster beam and energy [26]. In that study, the sputtering yield (30.59 nm3/ion) is lower than the one obtained from the current work. This can be attributed to the difference in the molecular weight of PMMA (i.e. different lengths of the chain of the polymer, since more weight can sometimes affect sputtering yield volume/PI, see Figure 1.13) or the substrate on which the films were deposited.

Regardless of the fragment ion of the repeat unit of PMMA, the sputtering yields and depth resolutions were the same (Figure 3.7), as expected. In agreement with existing literature, sputtering yields strongly depend on the energy of the beam used, and lower energies will likely lead to better depth resolutions.

Table 3.3: Summary of PMMA interface doses, sputtering yields and depth resolutions, + using 10, 20 and 40 keV C60 primary ion beams, obtained at room temperature.

+ C60 ion Interface Sputtering Depth Ratio of Reference beam dose × 1013 yields resolution sputtering energy (ions/cm2) (nm3/ion) (nm) yields 20 keV keV/10 keV and 40 keV/20 keV + C60

10 --- 38 ± 5 -- 2.3 [25] 2.2 20 3.88 ± 0.004 87.4 ± 12.24 12.5 ± 1.75 work 40 1.79 ± 0.00 189.9 ± 26.59 17.2 ± 2.41 work

+ 3.3.1.3.2. Using 20 keV Arn Cluster Ion Beams (n = 250-2000) Figures 3.10 and 3.11 shows depth profiles of the m/z 186 fragment ion yields of PMMA film (34 nm) as a function of increasing Ar cluster ion dose (at 20 keV). The obtained profiles were roughly the same shape for all of the four different sizes of the Ar cluster ion doses. They showed that as the cluster ion doses increase, it is still possible to + reach the interface (the ion yields of m/z 186 and the Si6 , m/z 168). The interface ion dose decrease as the size of the Ar cluster ion beams increases (see Table 3.4). Also, the span of the pseudo-steady-state region decreases as the size of the cluster of the Ar beam increases. + This could be attributed to the fact that smaller clusters have more energy per atom (Ar250

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+ + + (80 eV/atom), Ar500 (40 eV/atom), Ar1000 (20 eV/atom) and Ar2000 (10 eV/atom), and consequently, the energetic primary particles would damage the sample

The secondary ion yields of fragments were enhanced before reaching the interface + + + between PMMA and Si substrate, with Ar500 , Ar1000 and Ar2000 ion beams. The enhancement was more apparent with increasing the size of Ar cluster ion beams. The increase of the secondary ion yields before reaching the interface may be due to a weaker bonding of PMMA to substrates (as the sample itself became thinner), in comparison with the intermolecular-bonding of the PMMA sample itself. Again, as mentioned earlier, the key reason is that of the energy of the bombarding particle (i.e. cluster size). If its energy is lost to sputtering, alone, and not to causing a damage effect on the sample, then the yield will be larger, since the whole energy is now dedicated to cause sputtering. There could be another reason behind this phenomenon of sudden rise and dip in the yield, at the end of the pseudo-steady-state region. It could be that the sample layers are acting like a cushioning bed, which dissipates the energy of the bombarding primary ions; a similar action to what a normal bed does when a person lies on it, as the weight of the person is distributed across many springs of the bed. As each top layer (those facing the bombarding primary ions) desorbs and the sample becomes thinner, then that cushioning effect is reduced and the impact of the primary ions penetrates deeper and more so, on the substrate. Now, due to the change in the atomic structure from PMMA to Si, the substrate starts to absorb more of the impacting energy of the primary ions with no cushioning effect of the sample top layers, thus reflecting the impact energy. This means that more energy is now available to cause more secondary ion yield, hence the rise in the yield at the end of the pseudo-steady-state region. Moreover, there is a possibility that the sudden rise in the yield is the result of resonance occurring, hence the big increase of the yield.

This enhancement has been previously observed in a variety of organic materials, including Irganox [27], cholesterol [28] and fullerene [29]. As seen in Figures 3.10 and + 3.11 the secondary ion yields of Si6 have remained, roughly constant with increasing ion dose of Ar cluster beam, for the duration of the steady-state stage of the graph. The yield then increased as the bombardment reached the interface when the m/z 186 PMMA ion + + signals decreased. However, the Si6 ion yields did not increase with Ar2000 cluster ion beams. Indeed, at this cluster beam size, the energy per atom was not enough to sputter the silicon substrate, easily. Thus, the sputtering yields of Si were low. For this reason, only the PMMA signals were used to determine the interface position and the interface width.

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The PMMA interface doses, sputtering yields and depth resolutions, extracted from the depth profiles in Figures 3.10 and 3.11 (as explained in section 3.3.1.3), for different sizes of Ar cluster ion beams are summarized in Table 3.4. The sputtering yields are plotted as a function of increasing the cluster size of Ar-cluster ion beams, at a constant energy, as illustrated in Figure 3.12.

In general, the sputtering yields increase with increased number of atoms in an Ar+ + + cluster. The yield generated by the Ar2000 beam is 41% higher than that of the Ar250 + beam. Actually, the enhancement in sputtering yields, of Ar2000 , is nearly 23% more than + + that of Ar1000 , and this in turn is enhanced by about 6%, in comparison with Ar500 , and + this is even more enhanced by about 9%, in comparison with Ar250 . This is attributed to + the Ar2000 beam having the lowest energy per atom (10 eV/atom) so the penetration depth of the cluster is lower and more energy is available in the surface layers for sputtering.

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Figure 3.10: Depth profiles of the m/z 186 fragment ions of the PMMA secondary ion + yields, are plotted as a function of primary ion doses, working with (a) Ar250 and (b) + Ar500 , cluster ion dose, at 20 keV. These profiles were performed at room temperature, on a 34 nm thick, PMMA film.

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Figure 3.11: Depth profiles of the m/z 186 fragment ions of the PMMA secondary ion + yields, are plotted as a function of primary ion doses, working with: (a) Ar1000 and (b) + Ar2000 cluster ion dose, at 20 keV. These profiles were performed at room temperature, on a 34 nm thick, PMMA film.

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+ Figure 3.12: The sputtering yields of PMMA are plotted as a function of the size of Arn cluster ion beams (n = 250-2000), at room temperature with an incident energy of 20 keV. Table 3.4: Summary of results of PMMA interface doses, sputtering yields and depth + resolutions, with different sizes of the Arn cluster ion beams (n = 250-2000) at an incident energy of 20 keV, performed at room temperature, with their corresponding standard deviations.

Cluster ion beam Interface dose Sputtering yields Depth resolution size (ions/cm2) (nm3/ion) (nm) × 1013 + Ar250 5.71 ± 0.008 59.44 ± 8.32 10.47 ± 0.46 + Ar500 5.24 ± 0.003 64.74 ± 9.06 10.22 ± 0.25 + Ar1000 4.95 ± 0.002 68.53 ± 9.59 7.50 ± 0.11 + Ar2000 4.04 ± 0.13 83.98 ± 11.76 6.54 ± 0.12

Results of PMMA in this study roughly have the same trend as those in other studies of polystyrene (PS), investigated by Rading and co-workers [10] at 20 keV (from 40 to 10 eV/atom) and at 10 keV (from 20 to 5 eV/atom), when employing different sizes of Ar+ + + cluster ion (Ar500 to Ar2000 ). However, those authors also noticed a decrease in the + sputtering yields at higher cluster sizes than Ar2000 [20]. From their results of PS and from this current work of PMMA, it seems that that the maximum sputtering yields is about the + Ar2000 mark.

+ At a constant cluster size (e.g. Ar2000 ), the sputtering yields increased linearly with increasing total energy (from 10 to 20 keV). Similar results were observed with Irganox 119

1010 [30], leucine [31] and spiro-TTB [32]. However, at a constant energy (e.g. 20 keV), the sputtering yields decreased with an increase in the size of the clusters [33]. Figure 3.13 shows the trends of the sputtering yields; results are taken from current work and a previous study on PMMA and PS (as explain above).

Figure 3.13: Combining PMMA sputtering yields (in the current work with 20 keV and a previous study with 10 keV [33]), and PS yields (in a previous study with energies from 10 + to 20 keV [10]), are plotted as a function of the size of Arn cluster ion sizes (n = 250 to 5000). The sputtering yield increases when the energy per atom in Ar cluster beams increases using a 10 keV. However, it decreases with increasing energy per atom using a 20 keV (Figure 3.14). The yields are nearly the same at 3.33 eV/atom and 80 eV/atom (59 nm3/ion). Also, the yields are nearly the same at 6.66 eV/atom and 20 eV/atom (69 nm3/ion). The highest yield is at 10 eV/atom, and the lowest is at 2 eV/atom. Since the way of calculating the sputtering yields depend on the thicknesses of the samples, as well as the fact that the other study had a rastered area that is different to our study, compounded by the other fact that the other study was investigating under a dual beam mode, all these factors may well had an effect on the similarities of the sputtering yields at quite different E/n values.

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Figure 3.14: Sputtering yields of PMMA are plotted as a function of the energy per atom. + + + A constant energy was 10 keV, with Ar1500 , Ar3000 and Ar5000 cluster ions (from a + + previous study) [33]. A constant energy was 20 keV, with Ar250 to Ar2000 cluster ions (from the current work). Improvements in depth resolutions were observed gradually with increasing the size of Ar cluster ions, at a constant energy (Table 3.4 and Figure 3.15). This variation of depth resolution with increasing the size of Ar-cluster ion beams is displayed in Figure 3.15. The + + best depth resolution is achieved with Ar2000 . Depth resolutions achieved with Ar250 and + + + Ar500 are similar, but from Ar500 to Ar1000 it significantly improves. This improvement + + + + from Ar500 to Ar1000 is higher than that from Ar1000 to Ar2000 . This improvement of depth resolution with the increase of the cluster size was observed by Shard and co-workers when they investigated Irganox, at a constant energy (12 keV) [30].

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Figure 3.15: Depth resolution of PMMA is plotted as a function of Ar cluster ion beam size (n = 250-2000), at room temperature, with a 20 keV beam. 3.3.2. Influence of Sample Cooling on the Depth Profiles of PMMA + 3.3.2.1. Using 20 and 40 keV C60 Cluster Ion Beams Depth profiles of PMMA at room, cryogenic temperatures and again at room + temperature after being cooled, with C60 cluster ions (20 and 40 keV), are presented in Figure 3.16.

+ As can be seen from Figure 3.16, that in both 20 and 40 keV C60 primary ion beams, the increases in the secondary ion yields of the fragment ions of m/z 186, before reaching + the interface between PMMA and Si substrate (Si6 data not shown), become more pronounced with decreasing temperatures (from -100 to -170 °C). This (increasing yields effect) was not observed at room temperature; in both cases, before and after the cooling of the sample. From this data, it seems that this effect becomes more pronounced at low cryogenic temperature.

It is also noticed from that the depth profiles of m/z 186 ion signals (Figure 3.16a), that the primary ion dose (ions/cm2) required to reach the interface between the film and the Si + substrate, increases when decreasing temperatures, with the 20 keV C60 ion beams, implying a reduced sputter yield at low temperature. Obtained data, shown in the Appendix together with details about the craters and the chronology of the conducted experiments,

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tend to give rise to a possibility that some cross-linking (see the cross-linking section), had occurred, which probably had been induced due to some ion-bombardments effect (a chemical effect that could affect bonds between molecules, thus producing free radicals and eventually cross-linking) and/or due to purely the effect of the physical bombardments of these ions. For instance, the interface dose at -125 °C is 19% higher than the one + obtained at room temperature. All the interface region values, with 20 and 40 keV C60 are summarized in Table 3.5.

The values of the plateau signals, after the initial transient regions (the decreasing signals), in which the ion signals of m/z 186 remain nearly constant with increasing ion dose, are obtained at room temperature, before and after cooling process of the films, at + incident energies of 20 and 40 keV C60 .

The behaviour of the m/z 186 ion yields at room temperature, before any cooling of the films occurs, is different to that which is observed after cooling process (returning back to room temperature). In fact, the yields at the pseudo-steady-state after cooling of the samples are higher than that before the cooling of the samples. This may be attributed to some restructuring of the material that is occurring during the cooling process that is incompletely reversed after cooling is switched off. There is another possibility there could well be some H2O molecules that are still in the analysis chamber, which will be deposited on the sample, due to condensation (in the cooling stage of the investigation [34]–[36].

The sputtering yields of PMMA at and below room temperatures that are obtained from + 20 and 40 keV C60 beams are reported in Table 3.5 and their ratios, in Table 3.6.

+ In general, the 40 keV C60 beam produces higher sputtering yields than that obtained from the 20 keV beam, at all temperatures. At both energies (20 and 40 keV), the sputtering yields decrease with decreasing the temperature of the films and then stabilise after -125 °C (Figure 3.17). However, the decrease of the yields is slightly more + pronounced at 40 keV C60 . For instance, when the temperature decreases from 25 °C to - 100 °C, the yield is reduced by 4% when using the 20 keV beam, while it decreases by 10%, when the 40 keV beam is used (Table 3.5). Similar results (a decrease of the sputtering yields with the temperature of the sample (from 25 °C to -100 °C)) were + observed with polyatomic primary ion beams, SF5 , at 5 and 8 keV energies, as a previous PMMA study has shown [21]. The following reason is thought to be behind this finding, when cooling of the films occurs, the cross-linking between the PMMA molecules decreases, with consequent topography change that supresses the sputtering yields [6].

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After cooling of films, the yields of PMMA are enhanced, in both low and high energy per atom, in comparison with the yields at room temperature, before the cooling was carried out (Figure 3.18). For example, the sputtering yields at room temperature, after the cooling was carried out, is 13% and 5% higher than that before the process of cooling the + sample with 20 and 40 keV C60 , respectively (Table 3.5).

+ The depth resolution of PMMA is summarized in Table 3.5 for 20 and 40 keV C60 . In + contrast to the sputtering yields, depth resolutions obtained with the 20 keV C60 beam is better than that resulting from the 40 keV beam, at all temperatures (Figure 3.17 and 3.18).

+ A worse depth resolution at a higher energy per atom (the C60 investigation stage) was expected because of the interlayer mixing and increasing in beam-induced topography formation. While sputtering yields decrease, the corresponding depth resolution improves, as temperature decreases. Note that, a further decrease in temperature, below -125 °C, does not affect depth resolution. The enhancement in depth resolution with cooling is more + prominent with higher energy per atom in the C60 beams investigations. For instance, the resolution improves by nearly 31%, by using 20 keV, while using a 40 keV beam, it is about 40%, as the temperature decreases from 25 °C (before cooling) to -125 °C , for both energies (Table 3.5). These improvements of depth resolutions were observed by Mahoney + et al. when investigating PMMA with 5 keV SF5 , with temperature ranging from 25 to • 75 °C [37]. It is thought that the reasons behind the changes in depth resolutions are the same that was mentioned earlier for the changes in sputtering yields.

The depth resolution improves at 25 °C after the cooling as compared to before the + cooling of the PMMA film, at both energies of C60 beams. However, the improvement of the depth resolution at room temperature from before to after the cooling of the sample is higher with 40 keV (Figure 3.18). As with the sputter yield measurements, there appears to be changes in the sample that persist after the cooling is switched off. This is probably due to cross-linking effects, as explained in the cross-linking section 1.6.

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Table 3.5: Summary of the results of PMMA interface dose, sputtering yields and depth + resolutions, at different temperatures, using 20 and 40 keV C60 , with their standard deviations.

Cluster ion Temperature (°C) Interface dose Sputtering Depth + 2 beam (C60 ) (ions/cm ) yields resolution × 1013 (nm3/ion) (nm) 20 keV 25 (before cooling) 3.88 ± 0.004 87.44 ± 12.24 12.5 ± 0.11 •100 4.04 ± 0.004 84.11 ± 11.78 11.54 ± 0.24

•125 4.62 ± 0.022 73.44 ± 10.28 8.57 ± 0.12 •170 4.43 ± 0.029 76.56 ± 10.72 8.46 ± 0.22 25 (after cooling) 3.37 ± 0.018 100.65 ± 14.09 10.74 ± 0.27 40 keV 25 (before cooling) 1.789 ± 0.00 189.81 ± 26.57 17.15 ± 0.59 •100 1.98 ± 0.007 171.22 ± 23.97 15.03 ± 0.55 •125 2.33 ± 0.007 145.93 ± 20.43 10.35 ± 0.14 •170 2.28 ± 0.006 148.84 ± 20.84 10.44 ± 0.16 25 (after cooling) 1.70 ± 0.005 199.93 ± 27.99 12.72 ± 0.00

Table 3.6: Ratio of the sputtering yields, of PMMA (before and after cooling the PMMA + films) and below room temperatures (°C), using 20 and 40 keV C60 . Temperature (°C) Ratio of the 25 (before •100 •125 •170 25 (after sputtering cooling) cooling) yields 40 keV/20 2.17 2.04 1.99 1.94 1.99 + keV C60

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Figure 3.16: Depth profiles of the m/z 186 fragment ions of the PMMA secondary ion + yields are plotted as a function of increasing C60 primary ion dose at: a) 20 keV and b) 40 keV. The profiles were performed at room temperature, 25 °C, (before and after the cooling process of the films) and at temperatures (from -100 to cryogenic -170 °C), on a 34 nm ± 0.1 nm thick, PMMA films.

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Figure 3.17: The sputtering yields of PMMA are plotted as a function of temperatures + (°C), using 20 and 40 keV C60 . Symbols (*) and (^) refer to before and after the cooling of the PMMA film, respectively.

Figure 3.18: Depth resolution of PMMA is plotted as a function of temperatures (°C) with + 20 and 40 keV C60 Cluster ion beams. Symbols (*) and (^) refer to before and after the cooling of the PMMA film, respectively.

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Secondary ion yield of m/z 186, at the pseudo-steady-state regions, is enhanced with 40 + keV beam as compared to 20 keV C60 beam, at all sample temperatures (Figure 3.19). The highest ion yield was obtained at 25 °C, after the cooling of the PMMA films and the lowest yield was detected at -170 °C, in both energies of the investigation. However, a different behaviour of secondary ion yields of m/z 186 is noticeable, for both 20 and 40 + keV C60 beams, during the cooling of the sample. From 25 to -100 °C, the ion yield increases, then it decreases with a further decrease in temperature, when the sample is + being bombarded with the 40 keV C60 beam. However, in the case of the 20 keV, the yields decrease when the sample is cooled from room temperature to -100 °C. Additionally, a further decrease of the temperature has no effect on the ion yields (the variation of yields is within the experimental error bars).

Figure 3.19: A comparison of secondary ion yields of the m/z 186 PMMA film fragment ions at room temperature (before and after the cooling of the samples of PMMA) and below room temperatures, averaged from three selected layers within the pseudo-steady- + 13 13 state region with an accumulated 20 keV C60 primary ion dose of 1.26 × 10 , 1.47 × 10 13 2 + 12 and 1.68 × 10 ions/cm and an accumulated 40 keV C60 primary ion dose of ~ 4 × 10 , 7 ×1012, and 9 × 1012 ions/cm2 (Symbols (*) and (^) indicate before and after cooling the films, respectively).

+ + 3.3.2.2. Using 20 keV Ar500 and Ar2000 Cluster Ion Beams The effect of the film temperature on the depth profiles of PMMA, with different argon cluster sizes is shown in Figure 3.20. The pseudo-steady-state region of secondary ion

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yields of the fragment (m/z 186) looks more stable at cryogenic temperatures (-50 and -150 °C) as compared to that at room temperature (before and after the cooling process of the + sample), with Ar500 . However, the profiles are unchanged at temperatures from -50 to - 150 °C, as seen in Figure 3.20a.

+ With Ar2000 , the secondary ion yields increase before reaching the interface, at all temperatures, Figure 3.20b. The reason for this enhancement is the same one that was mentioned in section 3.3.1.3.2 (Ar cluster ion beams). Also with this cluster beam, the ion dose necessary to reach the interface increases with decreasing the temperature. It seems that the film etching process involves longer time with decreasing temperatures. Temperature affects the sputtering yield and depth resolution.

At room temperature, after already going through the cooling process of the PMMA film, the m/z 186 ion yields were observed to be lower, in comparison with that before the + cooling, with both cluster beam sizes. This is in contrast with C60 where the yield, after the sample was cooled, is higher than before the cooling (Figure 3.20 and Figure 3.16). It may well be that the surface of the films is contaminated, for example with water ice, after the process of cooling the samples was carried out. As C60 has more energy per atom than + Ar500-2000 clusters, it will penetrate deeper into the subsurface of the sample, and since the + sample has suffered cross-linking processes then the higher energy C60 ions are able to + break these extra bonds more than the Arn ones, which are unable to do such breakages. + + This means that C60 ions produce more fragments, much more than Arn ions are able to do at room temperature, after cooling occurs.

Interface dose, sputtering yields and depth resolutions obtained from PMMA depth + + profiling, with Ar500 and Ar2000 cluster ion beams, at a constant energy (20 keV), are + summarized in Table 3.7. At all temperatures, the sputtering yields produced with Ar2000 + are higher than that obtained with Ar500 (Figure 3.21). This may be attributed to the fact + that Ar2000 has low energy per atom (10 eV/atom) and therefore experiences shallow penetration and has deposited more ion-induced damage near the surface. For both cluster sizes, the yields decrease with decreasing temperature of the film of the sample. However, + the decrease is more pronounced when working with Ar2000 . For instance, from 25 °C (before the cooling process being carried out) to -150 °C, the yields decrease by 17% with + + Ar2000 , in comparison with Ar500 , were its value is only 2%.

+ With Ar500 , after cooling the PMMA film, the yields improve on returning to room + temperature. However, with Ar2000 , a decrease of the sputtering yields was observed. The

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+ trend of the sputtering yields obtained with C60 and Ar cluster ion beams are similar when cooling the films (Figure 3.17 and Figure 3.21). The yields fall the most significantly with + C60 at 40 keV. In fact, when the temperature is decreasing from 25 °C to -125 °C, the + yields decrease by 23% and 16%, with C60 (at 40 and 20 keV, respectively), and fall by + + 17% and 2%, with Ar2000 and Ar500 , respectively. However, when comparing the sputtering yield before and after cooling the sample a difference is noticeable. In fact, with + + + C60 and Ar500 the yields increase after cooling, whereas it decrease with Ar2000 .

Table 3.7: Summary of the results of PMMA interface dose, sputtering yields and depth + + resolutions, using different temperatures, with 20 keV Ar500 and Ar2000 cluster ion beams, with their respective standard deviations.

Cluster ion Temperature Interface dose Sputtering Depth beam size (°C) × 1013 yields resolution (ions/cm2) (nm3/ion) (nm) + Ar500 25 (before 5.22 ± 0.001 64.58 ± 9.04 12.10 ± 0.48 cooling) -50 5.37 ± 0.003 63.27 ± 8.86 9.49 ± 0.71 -150 5.38 ± 0.001 63.12 ± 8.84 9.49 ± 0.53 25 (after 4.82 ± 0.020 70.46 ± 9.86 10.18 ± 0.89 cooling) + Ar2000 25 (before 3.83 ± 0.003 88.71 ± 12.42 6.61 ± 0.10 cooling) -50 4.13± 0.001 82.29 ± 11.52 5.49 ± 0.23 -150 4.59 ± 0.001 73.96 ± 10.35 4.75 ± 0.22 25 (after 4.16 ± 0.276 81.82 ± 11.45 6.39 ± 0.43 cooling)

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Figure 3.20: Depth profiles of PMMA fragment, m/z 186; secondary ion yields plotted as a + + function of primary ion dose: a) Ar500 and b) Ar2000 cluster ion beams, at 20 keV. The profiles were performed at room temperature, 25 °C, (before and after the cooling process of the ilms), and at cryogenic temperatures (-50 and -150 °C), on a 34 nm PMMA film.

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Figure 3.21: The sputtering yields of PMMA are plotted as a function of temperatures + + (°C), with Ar500 and Ar2000 cluster ion beams, at an incident energy of 20 keV. Symbols (*) and (^) refer to before and after cooling of the PMMA film, respectively.

Figure 3.22 shows depth resolutions of PMMA, at different temperatures, with 40 + + eV/atom and 10 eV/atom, in Ar500 and Ar2000 cluster ion beams, respectively. The best depth resolution is obtained with low energy per atom (10 eV/atom), in comparison with that of the high energy per atom (40 eV/atom), at all temperatures of the sample. Depth resolution decreases with decreasing temperature of the sample, for both energies. With + Ar500 , the resolution obtained at -150 ºC is 22% higher than that of room temperature (before the process of cooling the sample was carried out). Note that depth resolutions + obtained at -50 and -150 ºC looks very similar. With Ar2000 , the improvement in depth resolution from 25 °C (before cooling of the film) to that at -150 °C is about 28% and from that at -50 °C to that at -150 ºC is around 13%. The improvement of the depth resolution when decreasing the temperature of the sample is consistent with what have been observed + with C60 . This is probably due to the cross-linking effects that have already been suggested as a probable cause for such behaviour. It can be seen, from Figure 3.21, that the depth resolution improves with lower energy per atom . However, the improvement in depth resolution when decreasing the temperature is less pronounced with Ar cluster ion beam.

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The resolution has a similar improvement at room temperature, after cooling of the PMMA film, with low energy per atom, however, with high energy per atom, it becomes better after cooling, in comparison with before cooling of the films. The obtained values are in good agreement with the previous room temperature measurements, with Ar cluster ion beams, at room temperature, Figure 3.15.

Secondary ion yields of m/z 186, at different sample temperatures, are displayed in + Figure 3.23. The ion yields of m/z 186 are enhanced with Ar2000 cluster ion beam, in + comparison with the yields obtained when using an Ar500 cluster ion beam, at all temperatures of the sample except that at -150 °C, where they are within experimental error of each other. With both sizes of Ar cluster ion beams, the greater PMMA ion yields are found at room temperature, before the cooling of the sample in comparison with the + value obtained after cooling of the PMMA film. With Ar2000 beams, the steady-state ion + yields are reduced with a decrease in the temperature. Conversely, with Ar500 beams, the ion yields are unchanged within experimental uncertainty with decreasing temperatures; however, they are slightly lower at room temperature, when measured before and after the cooling process. The trend of the ion yields with Ar cluster ion beams differs from that of + C60 ion beams during cooling in respect of whether the measurements are taken before

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and after cooling the sample at room temperature, as shown in Figures 3.19 and 3.23. Note + + the ion yields display no clear trend when cooling the sample with C60 and Ar500 both at 20 keV. Also, the ion yields, when the samples return to room temperature after cooling, + are greater than before cooling with C60 at both energies; while with Ar clusters the variations of the yields are within the experimental uncertainty.

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3.4. Summary and Conclusions Positive ion SIMS analysis of depth profiles of PMMA was investigated with different cluster ion beams, at room temperature and cryogenic temperatures. The best depth profiles + + were achieved at -125 and -170 ºC, with C60 beams and -150 ºC, with Ar2000 beams. At + low temperatures of -125 and -170 ºC, with C60 beams, depth profiles required greater + interface ion dose. With Ar2000 beams, a high interface dose was observed at -150 ºC, but + at room temperature the Ar2000 beams required a low interface dose in comparison with beams of low ion cluster sizes. The sputtering yields decreased with decreasing + + + temperatures, with C60 and Ar2000 beams, but with the Ar500 beam, the yields produced + similar values at and below room temperature. With C60 beams, the sputtering yields are greater with high energy per atom than low energy per atom, however, with Ar cluster ion beams, they are greater with low energy per atom. This could be attributed to the energy + deposition of Ar cluster ions and C60 ions on the sample surface, in that the higher energy + C60 ions are able to penetrate deeper in the sample, thus causing more breakages of bonds. + This is not the case with Arn where it only is able to sputter the surface layer molecules.

The best depth resolutions were observed with low energy per atom and low + + + temperatures (at -125 ºC, using C60 and -50 ºC, using Ar500 and Ar2000 ). It was also + + noticed that the resolution did not change at -170 ºC, using C60 and -150 ºC, using Ar500 , + but when Ar2000 was used, at -150 ºC, it offered a small improvement. + Secondary ion yields (e.g. of m/z 186) generated, at room temperature, using C60 beams, at the pseudo-steady-state region are approximately 2 times higher than that + + obtained with Ar250 and Ar2000 beams (Figure 3.6). When the sample was cooled, the ion + yields decreases when the temperature is at -170 ºC with the 40 keV C60 beams, and at - + 100 ºC with the 20 keV C60 beam, in comparison with results at room temperature. At + cryogenic temperatures, and using Ar2000 , the ion yield decreased, however, it roughly + stayed the same (taking errors into consideration) when Ar500 was used, in comparison to room temperature. After cooling (return to room temperature) the PMMA films, the ion + + + yields were high with C60 and low with Ar500 and Ar2000 beams as compared to before cooling at room temperature. In general, the low energy per atom generated the best depth profiles and depth resolutions, while higher energies per atom created better sputtering yields and secondary + ion yields, when C60 beams were used. The low energy per atom, in Ar cluster beams, generated the highest sputtering yields, depth resolution and secondary ion yields. This is attributed to ion beam-induced-damage, deposited near the surface and the decreased

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topography formation. At low temperatures, the depth profiles and depth resolution + improve, but sputtering and secondary ion yields decrease with both cluster beams (C60 and Ar+ cluster beams). The reason for this may be related to the physical and chemical changes of PMMA, which are changing when temperature is decreasing, from room temperature to cryogenic temperatures, since cross-linking is being introduced during cooling. All data, their errors, and their propagation is furnished and explained in the Appendix.

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3.5. References [1] N. Nieuwjaer, C. Poleunis, A. Delcorte, and P. Bertrand, “Depth profiling of polymer samples using Ga+ and C 60+ ion beams,” Surf. Interface Anal., vol. 41, no. 1, pp. 6–10, 2009. [2] A. M. Piwowar, J. C. Vickerman, and J. Wiley, “The role of molecular weight on the ToF-SIMS spectra of PMMA using Au+ and C60+ primary ions †,” Surf. Interface Anal, vol. 42, no. 8, pp. 1387–1392, 2010. [3] F. Kotter and A. Benninghoven, “Secondary ion emission from polymer surfaces under Ar+ , Xe+ and SF5+ ion bombardment,” Appl. Surf. Sci., vol. 133, no. 1–2, pp. 47–57, 1998. [4] S. Ninomiya, K. Ichiki, H. Yamada, Y. Nakata, T. Seki, and J. Matsuo, “The effect of incident energy on molecular depth profiling of polymers with large Ar cluster ion beams,” surf. Interface Anal, vol. 43, no. 1–2, pp. 221–224, 2011. [5] J. S.Hammond, “Comparison of SIMS and MALDI for Mass Spectrometric Imaging,” in Imaging Mass Spectrometry Protocols for Mass Microscopy, M. Setou, Ed. Tokyo: Springer Japan, 2010, pp. 235–257. [6] C. M. Mahoney, “Cluster secondary ion mass spectrometry of polymers and related materials.,” Mass Spectrom. Rev., vol. 29, no. 2, pp. 247–93, Jan. 2010. [7] A. Delcorte, B. J. Garrison, and K. Hamraoui, “Dynamics of Molecular Impacts on Soft Materials : From Fullerenes to Organic Nanodrops,” Anal. Chem., vol. 81, no. 16, pp. 6676–6686, 2009. [8] A. Delcorte and M. Debongnie, “Macromolecular sample sputtering by large Ar and CH4 clusters: elucidating chain size and projectile effects with molecular dynamics,” J. Phys. Chem. C, vol. 119, no. 46, pp. 25868–25879, 2015. [9] K. Shen, W. Abdreas, and W. Nicholas, “Molecular Depth Profiling with Argon Gas Cluster Ion Beams,” J. Phys.Chem. C, vol. 119, no. 16, pp. 15316–15324, 2015. [10] D. Rading, R. Moellers, H. Cramer, and E. Niehuis, “Dual beam depth profiling of polymer materials : comparison of C60 and Ar cluster ion beams for sputtering,” surf. Interface Anal, vol. 45, no. 1, pp. 171–174, 2013. [11] C. M. Mahoney and W. Andreas, “Molecular depth profiling with cluster ion beams,” in Cluter Secondary Ion Mass Spectrometry Principles and Applications, 1st ed., D. M. Desiderio, N. M. M. Nibbering, and J. A. Loo, Eds. Canada: John Wiley & Sons. Inc, 2013, pp. 117–205. [12] G. J. Leggett and J. C. Vickerman, “Effects of damage during the SIMS analysis of poly (vinyl chloride) and poly (methyl methacrylate),” Appl. Surf. Sci., vol. 55, no. 2–3, pp. 105–115, 1992. [13] M. S. Wagner, “Degradation of poly (acrylates) under SF5+ primary ion bombardment studied using time-of-flight secondary ion mass spectrometry . 1 . Effect of main chain and pendant methyl groups,” surf. Interface Anal, vol. 36, no. 1, pp. 42–52, 2004. [14] I. Briggs, David and Fletcher, “Qualitative interpretation of spctra,” in TOF-SIMS: Material Analysis by Mass Spectrometry, 2nd ed., Chichester: IM Publication LLP and SurfaceSpectra Limited, 2013, pp. 417–448. [15] M. S. Wagner, “Impact Energy Dependence of SF5+ -Induced Damage in Poly (methyl methacrylate ) Studied Using Time-of-Flight Secondary Ion Mass Spectrometry,” Anal. Chem., vol. 76, no. 5, pp. 1264–1272, 2004. [16] A. M. Leeson, M. R. Alexander, R. D. Short, D. Briggs, and M. J. Hearn, “Secondary Ion Mass Spectrometry of Polymers : a ToF SIMS Study of Monodispersed PMMA,” surf. Interface Anal, vol. 25, no. 4, pp. 261–274, 1997. [17] D. Briggs, I. W. Fletcher, and N. M. Gon, “Positive secondary ion mass spectrum of poly ( methyl methacrylate ): a high mass resolution ToF-SIMS study †,” surf. Interface Anal, vol. 29, no. 5, pp. 303–309, 2000. 137

[18] D. Briggs and M. J. Hearn, “Interaction of ion beams with polymers, with particular reference to SIMS,” Vacuum, vol. 36, no. 11–12, pp. 1005–1010, 1986. [19] N. Nieuwjaer, C. Poleunis, A. Delcorte, and P. Bertrand, “Depth profiling of polymer samples using ion beams Ga+ and C60+,” surf. Interface Anal, vol. 41, no. 1, pp. 6–10, 2009. [20] L. Houssiau and N. Mine, “Molecular depth profiling of polymers with very low energy reactive ions,” Surf. Interface Anal., vol. 42, no. 8, pp. 1402–1408, 2010. [21] C. M. Mahoney, J. G. Kushmerick, and K. L. Steffens, “Investigation of Damage Mechanisms in PMMA during ToF-SIMS Depth Profiling with 5 and 8 keV SF5+ Primary Ions,” J. Phys. Chem. C, vol. 114, pp. 14510–14519, 2010. [22] C. M. Mahoney, A. J. Fahey, G. Gillen, C. Xu, and J. D. Batteas, “Temperature- controlled depth profiling in polymeric materials using cluster secondary ion mass spectrometry (SIMS),” Appl. Surf. Sci., vol. 252, no. 19, pp. 6502–6505, 2006. [23] C. M. Mahoney, A. J. Fahey, and G. Gillen, “Temperature-Controlled Depth Profiling of Poly ( methyl methacrylate ) Using Cluster Secondary Ion Mass Spectrometry . 1 . Investigation of Depth Profile Characteristics,” Anal. Chem., vol. 79, no. 3, pp. 828–836, 2007. [24] H.-Y. Liao, M.-H. Tsai, W.-L. Kao, D.-Y. Kuo, and J.-J. Shyue, “Effects of the temperature and beam parameters on depth profiles in X-ray photoelectron spectrometry and secondary ion mass spectrometry under C60+–Ar+ cosputtering,” Anal. Chim. Acta, vol. 852, pp. 129–136, 2014. [25] R. Möllers, N. Tuccitto, V. Torrisi, E. Niehuis, and A. Licciardello, “Chemical effects in C60 irradiation of polymers,” Appl. Surf. Sci., vol. 252, no. 19, pp. 6509– 6512, 2006. [26] C. Szakal, S. Sun, A. Wucher, and N. Winograd, “C60+ molecular depth profiling of a model polymer,” Appl. Surf. Sci., vol. 231–232, pp. 183–185, 2004. [27] T. B. Angerer, P. Blenkinsopp, and J. S. Fletcher, “High energy gas cluster ions for organic and biological analysis by time-of-flight secondary ion mass spectrometry,” Int. J. Mass Spectrom., vol. 377, pp. 591–598, 2015. [28] P. D. Rakowska, M. P. Seah, J. Vorng, R. Havelund, and I. S. Gilmore, “Determination of the sputtering yield of cholesterol using Arn+ and C60+(+) cluster ions,” Analyst, vol. 141, pp. 4893–4901, 2016. [29] T. Mouhib et al., “Organic depth profiling of C60 and C60/phthalocyanine layers using argon clusters,” Surf. Interface Anal., vol. 45, no. 1, pp. 163–166, 2013. [30] J. L. S. Lee, S. Ninomiya, J. Matsuo, I. S. Gilmore, M. P. Seah, and A. G. Shard, “Organic Depth Profiling of a Nanostructured Delta Layer Reference Material Using Large Argon Cluster Ions,” Anal. Chem., vol. 82, no. 1, pp. 98–105, 2010. [31] K. Ichiki et al., “High sputtering yields of organic compounds by large gas cluster ions,” Appl. Surf. Sci., vol. 255, no. 4, pp. 1148–1150, 2008. [32] E. Niehuis, “Depth profiling in organic electronics,” in TOF-SIMS: Material Analysis by Mass Spectrometry, 2nd ed., Chichester: IM Publication LLP and SurfaceSpectra Limited, 2013, pp. 637–660. [33] V. Cristaudo, C. Poleunis, B. Czerwinski, and A. Delcorte, “Ar cluster sputtering of polymers: Effects of cluster size and molecular weights,” Surf. Interface Anal., vol. 46, no. S1, pp. 79–82, 2014. [34] A. Piwowar, J. Fletcher, N. Lockyer, and J. Vickerman, “Investigating the effect of temperature on depth profiles of biological material using ToF-SIMS,” surf. Interface Anal, vol. 1–2, no. March 2010, pp. 207–210, 2011. [35] A. M. Piwowar, J. S. Fletcher, J. Kordys, N. P. Lockyer, N. Winograd, and J. C. Vickerman, “Effects of cryogenic sample analysis on molecular depth profiles with TOF-secondary ion mass spectrometry,” Anal. Chem., vol. 82, no. 19, pp. 8291– 8299, 2010.

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[36] C. M. Mahoney, “Surface Analysis of Organic Materials with Polyatomic Primary Ion Source,” in Cluster Secondary Ion Mass Spectrometry Principle and Applications, D. M. Desiderio, N. N. M. M, and J. A. Loo, Eds. Canada: John Wiley & Sons. Inc, 2013, pp. 77–116. [37] C. M. Mahoney, A. J. Fahey, G. Gillen, Chang Xu, and J. D. Batteas, “Temperature- Controlled Depth Profiling of Poly ( methyl methacrylate ) Using Cluster Secondary Ion Mass Spectrometry . 2 . Investigation of Depth Profile Characteristics,” Anal. Chem., vol. 79, no. 3, pp. 837–845, 2007.

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Chapter 4: Molecular Depth Profiling of Organic Semiconductors with Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) Using + + Cluster Ion Beams (20 and 40 keV C60 and 20 keV Ar250-2000 ) at Different Temperatures

4.1. Introduction Organic semiconductors are mainly composed of carbon atoms bonded with hydrogen, nitrogen, sulphur, and/or oxygen. Organic materials are utilised to create different organic electronic devices such as OLEDs [1], OPVCs [2] and OFETs [3], [4]. Organic electronics consist of various small organic molecules arranged as films. The thicknesses of the films are typically in the range of 10 nm to 100 nm [5]. The quality of the interfaces between these layers is important. These materials (whether single layers or bi-(multi)layers films + deposited on substrates) are difficult to depth profiles with cluster beams such as C60 because of containing aromatic hydrocarbons, and these are very stable molecules, as mentioned earlier, and they tend to not make fragments to sputter away. However, this beam has displayed successful depth profiling for a wide range of organic materials (non- aromatic hydrocarbons; such as biological, polymers) with SIMS. In recent years, noble argon cluster beams have been shown successful depth profiles for organic semiconductors with SIMS such as fullerene [6], rubrene [7] (for single-layers) and OLEDs [8] (for multilayers).

In this chapter, the interface between the silicon and two widely used organic molecules in organic electronics, namely 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS- pentacene) and an amorphous poly(dimethyl-triarylamine) (PTAA), have been investigated as a single-layer (as a reference) via ToF-SIMS. These compounds consist of many benzene rings (see Figure 4.1).

The purpose of this study is to determine the effect of cluster ion beams and sample temperatures on organic depth profiling, comparing the sputtering yields and depth resolution between organic/inorganic and secondary ion yields.

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Figure 4.1: Chemical structures of (a) PTAA [9] and (b) TIPS-pentacene organic semiconductor samples. 4.2. Experimental Section 4.2.1. Sample Preparation 4.2.1.1. PTAA PTAA (made in-house) was dissolved in anisole at 2.65% w/v and filtered through a 0.45 µm PTFE filter using a syringe. A silicon wafer (1 × 1 cm2, purchased from IDB, Technologies, Ltd, UK) was sonicated in methanol, acetone, and isopropanol (15 minutes in each solvent, separately), rinsed with isopropanol, and blown dry with a nitrogen gun.

Then, the wafer was subjected to 20 minutes of UV/O3 treatment to remove any organic contaminants. Uniform films were obtained by spin-coating 75 µl of the PTAA solution at 3000 rpm for 2 minutes. The film thicknesses were measured by a DektakXT Stylus

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profilometer (Bruker) with a 3 mg stylus force in three areas of the film. The average thickness was 51.5 nm with a low standard deviation (0.8 nm).

4.2.1.2. TIPS-pentacene A silicon wafer (1 × 1 cm2 purchased from IDB, Technologies, Ltd, UK) was cleaned and UV/O3 treated as mentioned above (see section 4.2.1.1). TIPS-pentacene (purchased from Sigma-Aldrich) was evaporated onto the cleaned silicon wafers, heated to 30 °C (using an Edward Auto 306 Thermal Evaporator) at 8 × 10-7 mbar and a deposition rate of 0.05 nm s-1 to produce 50 nm thin films. However, the thicknesses obtained by AFM (in tapping mode) and DektakXT were different from the thickness displayed by a quartz crystal microbalance (QCM) in the evaporator (an evaporation chamber). It seems that there is some non-uniformity in the thicknesses of the films. A photo was taken to show the different colourings of the surface of a sample, thus indicating the aforementioned thicknesses issue (the photo is shown in the appendix, as in Figure 7.5).

4.2.2. ToF-SIMS Analysis The analysis of PTAA and TIPS-pentacene was carried out on a J105 3D Chemical Imager ToF-SIMS instrument as described in the instrumentation chapter (Chapter 2).

PTAA and TIPS-pentacene experiments were analysed at room temperature with (25 + + °C) with 20 and 40 keV C60 cluster beams and four different sizes of Ar cluster beams + + + + (Ar250 , Ar500 , Ar1000 , and Ar2000 ) with 20 keV. The parameters used within these experiments were a sputtering area of 200 × 200 µm2 with Ar+ clusters; 350 × 350 µm2 + 2 with C60 for PTAA; 350 × 350 µm for TIPS-pentacene; and 16 × 16 pixels for both compounds. However, the beam current was a possible source of variability in the measured depth profiles; hence, it should be measured before and after running each 11 2 + 12 experiment. The average primary ion dose was 5 × 10 ions/cm with C60 , 1.64 × 10 ions/cm2 with Ar+ clusters for PTAA, and 5 × 1011 ions/cm2 for TIPS-pentacene. In addition, these organic samples were analysed at low temperatures ranging from -50 to - + 150 °C. The PTAA cooling was analysed with 20 keV C60 , and analysis of the TIPS- + + pentacene cooling was done with Ar500 and Ar2000 . Furthermore, the TIPS-pentacene films were analysed at room temperature again after cooling samples (return to room + + temperature during 24 hours) with Ar500 and Ar2000 .

The low-mass range for mass spectra was acquired from 30 to 900 Da in a positive-ion mode. The spectra below (sections 4.3.1 and 4.4.1) were selected at the ion dose of 5 × 12 2 + 10 ions/cm (which was at the tenth analysis layer of the film with C60 , the third layer 142

with Ar+ clusters for PTAA, and the tenth layer for TIPS-pentacene for both beams) at room temperature.

Sputtering yields were measured for PTAA (using DektakXT, as mentioned above to calibrate the film thickness) and for TIPS-pentacene (using AFM, as mentioned below, and DektakXT as mentioned in the Appendix), thus their film thicknesses will be known, as well as, the dose to reach the interfacial region, and the size of the ion beam raster areas, as described in more detail in Chapter 3. The interface widths (depth resolution) were determined from the decrease in PTAA and TIPS-pentacene molecular secondary ions, falling in value from 86% to 16%.

4.3. Analysis of the PTAA Sample at room Temperature 4.3.1. Results and Discussion 4.3.1.1. Secondary Ion Mass Spectra of PTAA

+ Spectra of the PTAA sample obtained using different cluster ion beams, namely C60 + + + + (20 keV and 40 keV), Ar250 Ar500 , Ar1000 , and Ar2000 (20 keV), using ToF-SIMS (a + positive-ion mode) are displayed in Figures 4.2, 4.3, and 4.5. Spectra obtained using C60 sputtering at both energies revealed many fragmentations of the repeat unit of PTAA. In + contrast, spectra obtained with Arn sputtering revealed fewer fragmentations. Clean spectra appear using a larger Ar+ cluster size probably due to the low energy per atom in the cluster, and this seems to be attributed to the inability of the lower energy per atom particles to cause more fragments than those of the higher energy ones. Vickerman et al. + + employed Arn sputtering using different sizes of cluster ions, as compared to C60 sputtering at the same energy (20 keV) for angiotensin III (standard sample) in a positive- + + ion mode. Spectra of angiotensin III obtained with Ar500 and Ar2000 beams showed + reduced fragmentation and decreased background chemical species as compared to C60 + + beams. Furthermore, spectra with the same cluster size for both sputtering (C60 and Ar60 ), in which the energy per atom is the same, 333.33 eV/atom, presented very similar results (probably because of similar damage accumulation) [10]. Matsuo et al. also studied small biomolecular samples including arginine [11], [12] and tri-peptide Gly-Gly-Gly samples + + + [12] with Ar ion beams (Ar monoatomic and Arn clusters) in a positive-ion mode. Also, spectra of arginine reveal decreased fragmentation with decreasing energy per Ar atom from 8000 eV/atom to 26.67 eV/atom and 5.33eV/atom when increasing the size of Ar+ + + + ions from Ar , Ar300 , and Ar1500 , respectively, at a constant energy (10 keV), and a molecular ion of arginine was enhanced as compared to the Ar+ monomer beam [12]. Furthermore, spectra of tri-peptide Gly-Gly-Gly samples showed the same trends with 143

+ + + + different Ar ion sizes (Ar , Ar1000 , and Ar1750 ), a 3 keV incident energy, and the energy per Ar atom of 8000 eV/atom, 3eV/atom, and 1.7 eV/atom, respectively [12].

Molecular ions and fragmentation of the PTAA sample were calculated, and the observed m/z from the ToF-SIMS analysis are listed in Table 4.1.

Table 4.1: Molecular ion and fragmentation calculated and observed m/z from ToF-SIMS analysis of PTAA samples in a positive-ion mode.

Calculated m/z Observed m/z Molecular ion and fragmentation

+ 194.097 194.086 [M-C6H5] + 254.096 254.069 [M-CH3-2H] 268.112 268.080 [M-3H]+ 270.128 270.103 [M-H]+

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Figure 4.2: Positive secondary ion mass spectra of PTAA films were obtained after an ion 12 2 + dose of 5 × 10 ions/cm with C60 primary ion beams of energies: (a) 40 keV and (b) 20 keV at room temperature. The ion dose used to accumulate the spectra was 5 × 1011 ions/cm2.

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146

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4.3.1.2. Depth Profiles of PTAA Sample at Room Temperature + 4.3.1.2.1. Using 20 and 40 keV C60 Cluster Ion Beams The depth profiles of PTAA films on silicon substrates are illustrated in Figure 4.6, + + + + + where the secondary ion yields of [M-CH3-2H] , [M-3H] , and [M-H] and C11 from C60 projectiles are plotted as a function of increasing the primary ion dose, using 20 keV (Figure 4.6a) and 40 keV (Figure 4.6b) beams. The signals of secondary ion yields of PTAA fall dramatically until reaching the value of zero with an increasing ion dose. The steady-state region and the interface region could not be recognised. Therefore, the interface between PTAA and Si was not reached in both energies. It is thought that the decreasing ion signals are as a result of damage accumulation and cross-linking of PTAA + samples as well as C60 ion deposition in the crater bottom (the inserts spectra in Figure 4.6 show the presence of carbon in the crater) that prevent the Si ions from reaching the interface. These results are similar to the ion signals of PS and PC when they have been + bombarded using 20 keV C60 projectiles at 48° incidence [13] and even similar to rubrene + compound results, using 10 keV C60 projectiles [7].

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Figure 4.6: Secondary ion yields, of depth profiles of the [M-H]+ m/z 270 molecular ions and m/z 254 and m/z 168 fragment ions (related to the repeat unit of the PTAA sample) + + + and C11 from C60 projectiles, are plotted as a function of increasing C60 primary ion dose using: a) 20 and b) 40 keV. These profiles were obtained at room temperature of a 51.5 nm ± 0.8 nm PTAA film. The insert is the spectra of the last layer shown carbon deposition. 149

+ 4.3.1.2.2. Using 20 keV Arn Cluster Ion Beams (n = 250-2000) The impact of the Ar+ cluster beam size on secondary ion yields of PTAA films on a Si + + substrate is presented in Figures 4.7 and 4.8. For all the cluster sizes (Ar250 , Ar500 , + + Ar1000 , and Ar2000 ), depth profiles display the familiar three regions of signal responses (see section 1.6 for more details). The molecular ion and fragment ion yield decays at the + + + transient regions are very sharp with a small size of the Ar cluster beams (Ar250 , Ar500 , + + and Ar1000 ) in comparison with Ar2000 cluster beams. Therefore, the molecular damage in PTAA films is higher and results in more ion-induced damage accumulation at the transient regions using smaller Ar+ cluster beams at the same total energy. The behaviour of secondary ion yields of the pseudo-steady-state region of the PTAA and before reaching the interface is similar to that observed for the secondary ion yields of PMMA (Figures 3.10 and 3.11).

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151

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The depth profile characteristics, the ion dose required to reach the interface, the sputtering yields, and depth resolution were extracted. The sputtering yields obtained using + + + Ar250 , Ar500 , and Ar1000 cluster ion beams were similar because the interface dose has similar values (Figure 4.9 and Table 4.2). However, the sputtering yields were enhanced + using an Ar2000 cluster ion beam because the ion dose needed to reach the interface is lower than that of the smaller sizes of Ar+ cluster ions. This is in contrast to the obtained sputtering yields of PMMA samples, which were approximately three times greater (Table 3.4). It is thought that when PTAA is bombarded with ion beams, cross-linking may occur because it is composed of aromatic components, but PMMA degraded probably because of the main-chain scission processes.

+ + + Depth resolution for Ar250 , Ar500 , and Ar1000 was not possible to obtain because the interface width between its two end-points was difficult to identify, due to heavy noise that was present in the profiles (Figures 4.7 and 4.8a). However, the interface width was + identified with Ar2000 projectiles, thus depth resolution was possible to obtain using this + + Ar cluster size. Although a numerical value was only calculated for the Ar2000 beam, depth profiles of PTAA, confirms that it has the sharpest interface, since the curve drops + sharply (compared with the other Arn ), thus having a very narrow spread; therefore, it has the best depth resolution (Figure 4.8b and Table 4.3). In comparison, depth resolution of + PMMA using Ar2000 cluster beams is better than that of the PTAA. This result may reflect the difference in their structures; PMMA consists of two methyl groups and an ester group, however, PTAA consists of three aromatic components (benzene rings) linked, with two methyl groups substituted in one benzene ring and nitrogen between two rings (Figure 4.1). Thus, the physical and chemical properties of these materials are determined by their chemical structures.

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Figure 4.9: Sputtering yields of PTAA are plotted as a function of Ar+ cluster ion beam size (n = 250-2000) at room temperature, using 20 keV. Table 4.2: Summary of results of PTAA interface dose, sputtering yields, and depth + resolution with different sizes of Arn cluster ion beams (n = 250-2000) at an incident energy of 20 keV at room temperature with standard deviations.

Cluster ion beam Interface dose × Sputtering yield Depth resolution size (20 keV) 1013 (ions/cm3) (nm3/ion) (nm) + Ar250 2.45 ± 0.17 21.07 ± 2.95 - + Ar500 2.63 ± 0.07 19.58 ± 2.74 - + Ar1000 2.48 ± 0.05 20.80 ± 2.91 - + Ar2000 1.66 ± 0.02 31.00 ± 4.34 16.04 ± 0.096

Figure 4.10 shows the secondary ion yields of the pseudo-steady-state region of PTAA samples as a function of eV/atom. The deprotonated [M-H]+ and [M-3H]+ ions, and the + fragment ion, [M-CH3-2H] , yields, increase when the energy per atom decreases. With high energy per atom, the deprotonated and fragment ions have roughly similar yields, + with [M-CH3-2H] yield staying the same. However, at 10 eV/atom (the lowest energy + + used), the deprotonated [M-H] ions have higher yields than the fragment [M-CH3-2H] and [M-3H]+ ions. The reason for this behaviour could be attributed to a depletion of charges, by the high energy particles, on the surface of the semi-conductor material, PTAA. These charges could be retained locally (since PTAA is a semi-conductor, which doesn’t allow charges to be highly mobile).

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Figure 4.10: Average (three selected layers from two areas) of secondary ion yields of PTAA samples in the pseudo-steady-state region at room temperature are shown as a + 14 function of eV/atom with an accumulated: Ar250 primary ion doses of 1.10 × 10 , 1.16 × 14 14 2 + 14 14 10 , and 1.22 × 10 ions/cm ; Ar500 ion doses of 1.19 ×10 , 1.25 × 10 , and 1.32 × 14 2 + 13 13 13 2 10 ions/cm ; Ar1000 ion doses of 9.29 ×10 , 9.94 ×10 , and 1.06 × 10 ions/cm ; and + 13 13 13 2 Ar2000 ion doses of 4.83 × 10 , 5.40 × 10 , and 5.96 × 10 ions/cm , all using an incident energy of 20 keV. 4.4. Analysis of the TIPS-pentacene Sample at Room Temperature 4.4.1. Secondary Ion Spectra of TIPS-pentacene Spectra of TIPS-pentacene obtained of the mass range (m/z = 30-900) using 20 and 40 + + + + keV C60 cluster ion beams demonstrate molecular ion signals [M] , [M+H] , [M+2H] , and [M+3H]+, as shown in Figures 4.11 and 4.12. The intensities of the molecular ion + 11 12 signals are decreased as the C60 primary ion dose increase from 5 × 10 to 5 × 10 ions/cm2. It is thought that this decreasing in signals is attributed to the accumulative + damage produced by C60 ion beams. Furthermore, one notices that fragmentations from m/z 280 to m/z 350 have higher intense peaks in comparison with molecular ion peaks. Using 20 keV Ar+ cluster ion beams, as shown in Figures 4.13 and 4.14, spectra of TIPS- pentacene show intense peaks of molecular ions [M]+ and [M+H]+, and fragmentations of + m/z 280 to m/z 350 decrease as compared to C60 . Moreover, TIPS-pentacene spectra using + + + Ar cluster beams display two peaks of [M-C4H9Si ] and [M-C3H7] , however, these peaks

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+ + + do not appear in spectra obtained using C60 ion beams. Furthermore, Ar1000 and Ar2000 + cluster ion beams reveal a new peak, [M+C10H9Si] , at a high mass range. Fragmentations below m/z 500 decrease with increasing sizes of Ar+ cluster ion beams because of low energy per atom in a cluster size as recognized in spectra of PTAA samples, as shown in section 4.3.1. Spectra obtained from Ar+ cluster ion beams using an ion dose of 5 × 1012 ions/cm2 confirm that the molecular ion signals are still appearing from depth profiles; in Figures 4.16 and 4.17, the molecular ion signals are the highest using an ion dose of 5 × 1011 ions/cm2 (the initial layer). Table 4.3 shows calculated and observed m/z of molecular ions and fragmentations of TIPS-pentacene from spectra of ToF-SIMS analysis.

Table 4.3: Calculated and observed m/z from spectra of ToF-SIMS analysis of molecular and fragmentation ions of TIPS-pentacene in a positive-ion mode.

Calculated m/z Observed m/z Molecular ion and fragmentation + 553.329 553.322 [M-C4H9Si] + 595.323 595.316 [M-C3H7] 638.376 638.358 [M]+ 639.384 639.362 [M+H]+ 640.392 640.341 [M+2H]+ 641.400 641.354 [M+3H]+ + 795.424 795.421 [M+C10H9Si]

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Figure 4.11: Positive secondary ion mass spectra of TIPS-pentacene films obtained 11 12 2 + after an ion dose of: (a) 5 × 10 and (b) 5 × 10 ions/cm , using 20 keV C60 primary ion beams at room temperature.

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Figure 4.12: Positive secondary ion mass spectra of TIPS-pentacene films obtained after 11 12 2 + an ion dose of: (a) 5 × 10 and (b) 5 × 10 ions/cm , using 40 keV C60 primary ion beams at room temperature.

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Figure 4.13: Positive secondary ion mass spectra of TIPS-pentacene films obtained after 12 2 + + an ion dose of 5 × 10 ions/cm , using Ar cluster ion beams, (a) Ar250 and (b) Ar500 , at a 20 keV incident energy and room temperature. The ion dose used to accumulate the spectra was 5 × 1011 ions/cm2.

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Figure 4.14: Positive secondary ion mass spectra of TIPS-pentacene films obtained after 12 2 + + an ion dose of 5 × 10 ions/cm with Ar cluster ion beams, (a) Ar1000 and (b) Ar2000 , at a 20 keV incident energy and room temperature. The ion dose used to accumulate the spectra was 5 × 1011 ions/cm2.

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4.4.2. Depth Profiles of TIPS-pentacene Samples at Room Temperature + 4.4.2.1. Using 20 and 40 keV C60 Cluster Ion Beams

+ + + The [M] secondary ion yields of TIPS-pentacene and C11 of the C60 projectiles are plotted as a function of primary ion dose, as shown in Figure 4.15. As for PTAA, using + C60 cluster beams, using different energies, fails to achieve satisfactory depth profiles of TIPS-pentacene (Figure 4.15). This is attributed to benzene rings in the sample structures, which aid the cross-linking of the polymer chain, the reason which has been explained earlier.

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+ 4.4.2.2. Using 20 keV Arn Cluster Ion Beams (n = 250-2000) Depth profiles of TIPS-pentacene with different sizes of Ar+ cluster ion beams at 20 keV are shown in Figures 4.16 and 4.17. The usual three regions were observed after the sputtering process: a surface transient region, a pseudo-steady-state region, and an interfacial region. Different Ar+ cluster ion doses were successfully used to reach the + + interface location between the [M] and the Si6 (m/z 168) (Table 4.4). The pseudo-steady- state region decreases in width in the depth profile with increasing the size of Ar+ cluster ion beams at the same energy (20 keV). In addition, the yields of [M]+ fell sharply with the increasing size of Ar+ cluster ion beams, leading to better depth resolution (Table 4.4).

The ion yields of [M]+ (m/z 638) enhance before reaching the interface between the TIPS-pentacene and the Si substrate. The improvement of the ion yields grows significantly with the increasing size of Ar+ cluster ion beams before reaching the interface.

Cluster ion beam Interface dose × Sputtering yields Depth resolution size (20 keV) 1013 (ions/cm2) (nm3/ion) (nm) + Ar250 2.98 ± 0.028 141.09 ± 19.75 15.67 ± 0.94 + Ar500 2.70 ±0.007 155.84 ± 21.82 11.01 ± 0.43 + Ar1000 2.44 ± 0.035 172.50 ± 24.15 9.35 ± 0.59 + Ar2000 1.75 ± 0.035 240.74 ± 33.70 7.09 ± 0.45

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Figure 4.16: Average (twice repeated) depth profiles of [M]+ m/z 638 of the TIPS- + pentacene secondary ion yields and that of secondary ion yields of Si6 m/z 168 are plotted + as a function of increasing cluster ion dose using 20 keV beams of: (a) Ar250 and (b) + Ar500 . These profiles were obtained at room temperature of a 42 nm ± 4 nm TIPS- pentacene film.

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Figure 4.17: Average (twice repeated) depth profiles of [M]+ m/z 638 of the TIPS- + pentacene secondary ion yields and that of Si6 m/z 168 are plotted as a function of + + increasing cluster ion dose using 20 keV beams of: (a) Ar1000 and (b) Ar2000 . These profiles were obtained at room temperature of a 42 nm ± 4 nm TIPS-pentacene film. The TIPS-pentacene sputtering yields significantly increased with increasing number of atoms in an Ar+ cluster, using the same energy (Figure 4.18 and Table 4.4). The sputtering 165

+ + yields generated with Ar2000 were 72% higher than those of Ar250 , while the enhancement + + + + of the sputtering yields was nearly 11% from Ar250 to Ar500 , and from Ar250 to Ar1000 , it is about 23%. The same trend is observed in PMMA with a variety of Ar+ cluster sizes at room temperature (Figure 3.12). However, higher yields are generated of TIPS-pentacene (Table 4.4) than of PMMA (Table 3.4), using the same size of Ar+ cluster beams.

Figure 4.18: The sputtering yields of TIPS-pentacene are plotted as a function of Ar+ cluster ion beam size (n = 250-2000) at room temperature using 20 keV. The TIPS-pentacene depth resolution improved with increasing size of Ar+ cluster ion beams at room temperature (Table 4.4 and Figure 4.19). This variation in depth resolution as a function of the Ar+ cluster ion beam size is illustrated in Figure 4.19. The best + (smallest) depth resolution was observed with Ar2000 beams. The improvement in depth + + + + + resolution from Ar250 to Ar500 is higher than that from Ar500 to Ar1000 and from Ar1000 + to Ar2000 . The same trend is observed in PMMA (Figure 3.15). PMMA depth resolution is better (Table 3.4) than that of TIPS-pentacene, using the same size of Ar+ cluster ion beams.

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Figure 4.19: Depth resolution of TIPS-pentacene is plotted as a function of the size ofAr+ cluster ion beams (n = 250-2000) at room temperature using 20 keV. Figure 4.20 shows the secondary ion yields of TIPS-pentacene in the pseudo-steady- state region as a function of eV/atom in Ar+ cluster ion beams, using a 20 keV energy. The ion yields of [M]+, [M+H]+, and [M+2H]+ decreased with an increase in the energy per + + atom in the Ar clusters. However, the ion yields of the fragments, [M-C4H9Si] and [M- + C3H7] , are independent of the increase in the energy per atom, as shown in Figure 4.20. In general, the ion yields are enhanced at low energy per atom (10 eV/atom), but the enhancement of the ion yields drops when using high energy per atom (80 eV/atom), regardless of the fragment ions. In addition, the molecular [M]+ displays the highest + + + secondary ion yields in comparison with other ions ([M+H] , [M+2H] , [M-C4 H9Si] , and + [M-C3H7] ).

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+ + + + Figure 4.20: Secondary ion yields of [M] , [M+H] , [M+2H] , [M-C4H9Si] , and [M- + C3H7] , of the pseudo-steady-state region of TIPS-pentacene, averaged of three selected layers at room temperature are plotted as a function of eV/atom with an accumulated: + 13 13 13 2 + Ar250 primary ion doses of 1.05 × 10 , 1.21 × 10 , and 1.38 × 10 ions/cm ; an Ar500 12 13 13 2 + ion doses of 8.45 ×10 , 1.00 × 10 , and 1.16 × 10 ions/cm ; an Ar1000 ion doses of 12 12 13 2 + 12 12 8.13 ×10 , 9.76 ×10 , and 1.94 × 10 ions/cm ; and Ar2000 4.31 × 10 , 5.93 × 10 , and 7.55 × 1012 ions/cm2, using an incident energy of 20 keV. 4.4.3. Influence of Sample Cooling on Depth Profiles of PTAA samples + 4.4.3.1. Using 20 keV C60 Cluster Ion Beams Depth profiles of PTAA components at low temperatures (-50 and -150 °C) are shown in Figure 4.21, where the ion yields of the PTAA molecular ion and fragmentations and + + + C11 from C60 beams are plotted as a function of increasing 20 keV C60 primary ion dose. The ion yields at cryogenic temperatures behave as those obtained at room temperature, as shown above in Figure 4.6. The yields decay so rapidly with increasing ion dose at both temperatures, and therefore, the steady-state region and the interfacial region are not + distinctively showing. The same behaviour was observed of PS with SF5 beams. PS depth profiles of the steady-state region obtained at various temperatures (at, below, and above room temperature) were lacking constant ion signals, at -125 to 150 °C [14]. Also, rubrene compound was observed to be not effect by reducing temperature (-150 °C) [7].

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Figure 4.21: Secondary ion yields are plotted as a function of increasing primary ion dose + + at 20 keV C60 during depth profiles for molecular ion [M-H] m/z 270 and fragments m/z + 254 and m/z 286 (related to the repeat unit of the PTAA sample); and C11 m/z 132 from C60 projectiles. These profiles were obtained at cryogenic temperatures: (a) -50 °C and (b) -150 °C, of a 51.5 nm ± 0.8 nm PTAA film. 169

4.4.4. Influence of Sample Cooling on Depth Profiles of TIPS-pentacene + + 4.4.4.1. Using 20 keV Ar500 and Ar2000 Cluster Ion Beams The depth profiles obtained from TIPS-pentacene investigation at room and cryogenic temperatures show the three regions with Ar+ cluster ion beams (Figure 4.22). Secondary + ion yields at low temperature (-50 °C and -150 °C) with Ar2000 cluster beams present an ion-dose-independent and more persistent steady-state region in compared with room temperature (25 °C) before and after cooling (to return to room temperature) the TIPS- pentacene film. Therefore, the interface dose required to reach the Si increases at low temperatures compared to the sample at room temperature. The differences in the interfaces may be attributed to some restructuring of the material that is occurring during the cooling, see the note mentioned in chapter 3 (section 3.3.2.1) about a possible reason for this possible restructuring. At lower temperatures within the time-frame of depth profiling, the cross-linking is suppressed [15]. For this reason, the pseudo-steady-state yield at low temperature is independent of ion dose and more persistent. No major + variation in the pseudo-steady-state span is observed with Ar500 beams. In addition, for both sizes of cluster ions, the usual enhancement of the secondary ion yields before reaching the interface (at room temperature) is not observed at cryogenic temperatures. However, at room temperature, samples show a better depth resolution (Table 4.5 and Figure 4.24). The absence of the enhancement of the secondary ion yields before reaching the interface at cryogenic temperatures may be due to the strong adhesion between the film and Si.

At room temperature, before and after the cooling of the TIPS-pentacene films, the + + behaviour of secondary ion yields is different between Ar500 and Ar2000 beams in terms of + the doses required to reach the interface region. In the case of the Ar500 cluster ion beams, the interface of the sample is reached at a higher ion dose before the application of cooling, + whereas with Ar2000 , the lower ion dose is required as compared to after cooling (Table 4.5).

The behaviour of depth profiles of TIPS-pentacene ion yields is different from that of + + + PMMA (Figure 3.20) using Ar500 and Ar2000 at cryogenic temperature. Using Ar500 ion beams, the ion yields of PMMA at room temperature (before cooling of the film) and below room temperature are similar, but the ion yields of TIPS-pentacene decreased at low temperature in comparison with that at room temperature (before cooling). In contrast to TIPS-pentacene, the increase in the ion yields before reaching the interface region does not + occur in PMMA using Ar500 (before and after cooling the film). 170

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The interface dose, sputtering yields, and depth resolutions extracted from the depth profile of TIPS-pentacene at different temperatures are summarized in Table 4.5. As shown in + + Figure 4.23, the yields with Ar2000 are higher than that with Ar500 ion beams at all temperatures, probably because of its the lowest energy per atom. For both Ar+ cluster ion beams, the sputtering yields decrease with decreasing temperature (from room temperature + to -50 °C and -150 °C). The reduction of the yields using Ar2000 (10 eV/atom) is nearly + nine times higher than that of the Ar500 (40 eV/atom) cluster ion beams (Figure 4.23). In fact, sputtering yields depend on the ionisation potentials and the atomic volume of the material [16], [17], [18]. Decreasing the temperature will increase the binding energy, therefore, the sputtering yields decrease. After cooling was carried out, the yields of TIPS- + pentacene were enhanced when measured at room temperature again using Ar500 beams, + but when using Ar2000 beams they decreased in comparison with that was measured before the cooling process. The same trends have been observed of PMMA sputtering yields. However, TIPS-pentacene sputtering yields are higher than that of the PMMA yields.

Figure 4.23: The sputtering yields of TIPS-pentacene are plotted as a function of + + temperature (°C) using Ar500 and Ar2000 cluster ion beams at an incident energy of 20 keV (*) before and (^) after the cooling the TIPS pentacene film.

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The temperature effect on depth resolution in TIPS-pentacene depth profiles with 40 + + and 10 eV per atom, respectively, in Ar500 and Ar2000 clusters is displayed in Figure 4.24. Changes in the resolution appear at low temperatures. The depth resolution degraded when cooling the sample (in comparison with room temperature) for both low and high energy per atom in Ar+ clusters. The behaviour of TIPS-pentacene resolution at cryogenic temperatures is in contrast with that of the PMMA (Figure 3.22 and Figure 4.24). For + + example, at low temperatures, PMMA resolution with Ar500 and with Ar2000 is 2-4 times + better than that of TIPS-pentacene. At room temperature with Ar500 cluster beams, the resolution is slightly improved in TIPS-pentacene in comparison with that of the PMMA resolution before cooling the films, but after cooling, the resolution is similar. However, + with Ar2000 cluster beams, the enhancement of PMMA resolution is better than that of the TIPS-pentacene before and after cooling of the films. A note mentioned, in chapter 3 (section 3.3.2.1), about a possible reason for such behaviour, is noteworthy.

Figure 4.24: Depth resolution of TIPS-pentacene is plotted as a function of temperatures + + (°C) using Ar500 and Ar2000 cluster ion beams at an incident energy of 20 keV. Symbols (*) and (^) refer to before and after the cooling TIPS-pentacene film, respectively. The TIPS-pentacene sputtering yields and depth resolutions presented in this section were calculated using the average film thickness obtained with an atomic force microscope (AFM). Because of the non-uniformity of the TIPS-pentacene film, DektakXT was used to measure the thicknesses of the craters created by the impact of the Ar+ cluster beams (see

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the appendix in Figure 7.6 and Table 7.4). The results (sputtering yields and depth resolutions) calculated with film thicknesses from DektakXT and AFM have the same trend. However, the values obtained are very different. In fact, when measuring the film thickness with DektakXT, the tip removed some materials even when using the lowest tip force. AFM did not scratch the film, but because of the field of view (100 µm) the cantilever was unable to reach the middle of craters (350 µm × 350 µm). Consequently, the absolute values for sputtering yields and depth resolutions from DektakXT are uncertain, as reflected by estimated uncertainties, but conclusions regarding trends are still valid.

Table 4.5: Summary of results of TIPS-pentacene interface dose, sputtering yields, and depth resolution obtained of TIPS-pentacene films at different temperatures using 20 keV + + Ar500 and Ar2000 cluster ion beams with standard deviations. Measurements of the film thickness (44 nm) were obtained by AFM.

Cluster ion Temperature (°C) Interface dose Sputtering Depth beam size × 1013 yields resolution (nm) (ions/cm2) (nm3/ion) + Ar500 25 (before cooling) 5.65 ± 0.11 76.39 ± 10.69 10.40 ± 0.54

-50 6.38 ± 0.58 69.25 ± 9.70 16.27 ± 1.78 -150 6.62 ± 0.65 66.74 ± 9.34 15.97 ± 2.03 25 (after cooling) 4.99 ± 0.27 88.28 ± 12.36 10.83 ± 1.73 + Ar2000 25 (before cooling) 2.60 ± 0.01 168.91 ± 23.65 9.75 ± 2.93 -50 4.11 ± 0.497 108.92 ± 15.25 19.46 ± 1.83 -150 5.15 ± 0.08 85.50 ± 11.97 12.50 ± 1.67 25 (after cooling) 3.49 ± 0.14 126.18 ± 17.67 10.36 ± 2.28

As shown in Figure 4.25, the secondary ion yields of TIPS-pentacene [M]+ deteriorated with decreasing temperatures from 25 °C (before cooling the film) to -150 °C, no matter the size of the Ar+ cluster beams. At cryogenic temperatures (from -50 °C to -150 °C), the ion yields obtained with high and low energy per Ar atom in clusters were similar. Interestingly, the ion yields at room temperature before cooling the film are enhanced in comparison with those obtained at cryogenic temperatures. With low energy per atom in Ar+ clusters, the increase of yields is nearly threefold, whereas that of high energy per atom has not much changed. It can also be noted that after the cooling process of the sample, the ion yields at room temperature decreased in comparison with those obtained before the cooling.

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Figure 4.25: Average of secondary ion yields of three layers of the pseudo-steady-state + 13 region using: a 20 keV Ar500 (10 eV/atom) primary cluster ion dose of 1.96 × 10 2 + 13 2 (ions/cm ) and a 20 keV Ar2000 (40 eV/atom) primary ion dose of 1.30 × 10 (ions/cm ) for molecular ion [M]+ m/z 638 TIPS-pentacene as a function of temperature (at and below room temperature). Symbols (*) and (^) indicate before and after cooling the TIPS- pentacene film, respectively.

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4.5. Summary and Conclusions Positive-ion spectra of PTAA and TIPS-pentacene show less fragmentation using 20 keV Ar+ cluster ion beams, especially with those of low energy per atom, as in the case of + + Ar2000 beams, in comparison with the C60 projectiles that composed of high energy per atom (using 20 and 40 keV energies).

Both PTAA and TIPS-pentacene depth profiles showed a significant amount of signal decay associated with beam-induced sample damage and carbon deposition is observed + when employing C60 ion beams under energies of both 20 and 40 keV. In addition, low temperatures reveal that the constant ion yields of PTAA do not allow in depth profiling because of carbon deposition. For TIPS-pentacene, the experiments were not investigated any further.

Depth profiles of PTAA with Ar+ primary beams of different sizes using 20 keV at room temperature were investigated. However, the generated sputtering yields were of + + similar values with different sizes of Ar cluster beams, except for Ar2000 ion beams. Therefore, this sample film was not investigated at cryogenic temperatures because there was too much noise in the depth profile results, and it was a difficult material for the ion beams to penetrate the sample, so measurements of sputtering yields would be very low and depth resolution measurement would be very difficult. It was noted that the depth resolution at room temperature was difficult to identify from the interface width from 84% + + to 16% (excluding the Ar2000 beam). Secondary ion yields of [M-H] are enhanced with low energy per atom and reduced with high energy per atom in Ar+ cluster beams in + + comparison with [M-CH3-H2] and [M-3H] .

Depth profiles of TIPS-pentacene, produced a pseudo-steady-state region, with regard to secondary ion yield, throughout desorption of materials of Ar+ primary cluster beams ions, using 20 keV energy, and at both room temperature and low temperatures. A wider span of this region was observed at low temperatures, accompanied with, a broader interface Secondary ion yields of the [M]+ molecular ion, and the [M+H]+ and [M+2H]+ + protonated ions are the highest when low energy per atom of the Ar2000 cluster ion beam, were used at room temperatures.

In conclusion, ToF-SIMS analysis was utilised to determine the effect of different primary cluster ion beams of different characteristics and different sample temperatures, on organic-semiconductor for depth profiling purpose.

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At room temperature, from depth profiling the interfacial region, sputtering yields and + + depth resolution measurements were obtained, using Ar cluster beams; however C60 beams did not produce successful results. Again, secondary ion yields of molecular ions and fragment ions of the pseudo-steady-state region were obtained, for the Ar+ beams, and + the Ar2000 cluster beams provided the highest sputtering yields and the best depth resolution, probably, due to their low energy (per atom). Additionally, improvement of the sputtering yields secondary ion yields of molecular and fragment ions, and better depth resolutions, were observed, for TIPS-pentacene, than those for PTAA.

For TIPS-pentacene, at room temperature, the better depth resolution and highest + + sputtering yields were obtained, using Ar500 and Ar2000 . Moreover, the yields were the highest at room temperature as compared to cryogenic temperatures, when using high and + low energy per atom. Furthermore, they are higher with low energy per atom in Ar2000 + cluster beams in comparison with high energy per atom in Ar500 cluster beams.

All data, their errors, and their propagation is furnished and explained in the Appendix.

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4.6. References [1] S.-C. Lo and P. L. Burn, “Development of dendrimers: macromolecules for use in organic light-emitting diodes and solar cells,” Chem. Rev., vol. 107, no. 4, pp. 1097– 1116, 2007. [2] B. R. Saunders and M. L. Turner, “Nanoparticle--polymer photovoltaic cells,” Adv. Colloid Interface Sci., vol. 138, no. 1, pp. 1–23, 2008. [3] H. Yoo et al., “Self-Assembled, Millimeter-Sized TIPS-Pentacene Spherulites Grown on Partially Crosslinked Polymer Gate Dielectric,” Adv. Funct. Mater., vol. 25, no. 24, pp. 3658–3665, 2015. [4] V. Raghuwanshi, D. Bharti, and S. P. Tiwari, “Flexible organic field-effect transistors with TIPS-Pentacene crystals exhibiting high electrical stability upon bending,” Org. Electron., vol. 31, pp. 177–182, 2016. [5] E. Niehuis, “Depth profiling in organic electronics,” in TOF-SIMS: Material Analysis by Mass Spectrometry, 2nd ed., Chichester: IM Publication LLP and SurfaceSpectra Limited, 2013, pp. 637–660. [6] T. Mouhib et al., “Organic depth profiling of C60 and C60/phthalocyanine layers using argon clusters,” Surf. Interface Anal., vol. 45, no. 1, pp. 163–166, 2013. [7] R. J. Thompson et al., “Revealing surface oxidation on the organic semi-conducting single crystal rubrene with time of flight secondary ion mass spectroscopy,” Phys. Chem. Chem. Phys., vol. 15, no. 14, pp. 5202–5207, 2013. [8] S. Ninomiya et al., “Analysis of organic semiconductor multilayers with Ar cluster secondary ion mass spectrometry,” Surf. Interface Anal., vol. 43, no. 1–2, pp. 95–98, 2011. [9] R. S. Sprick, M. Hoyos, O. Navarro, and M. L. Turner, “Synthesis of poly (triarylamine) s by C--N coupling catalyzed by (N-heterocyclic carbene)-palladium complexes,” React. Funct. Polym., vol. 72, no. 5, pp. 337–340, 2012. [10] S. Rabbani, A. M. Barber, J. S. Fletcher, N. P. Lockyer, and J. C. Vickerman, “TOF- SIMS with argon gas cluster ion beams: A comparison with C 60+,” Anal. Chem., vol. 83, no. 10, pp. 3793–3800, 2011. [11] S. Ninomiya, Y. Nakata, K. Ichiki, T. Seki, T. Aoki, and J. Matsuo, “Measurements of secondary ions emitted from organic compounds bombarded with large gas cluster ions,” Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms, vol. 256, no. 1, pp. 493–496, 2007. [12] S. Ninomiya et al., “A fragment-free ionization technique for organic mass spectrometry with large Ar cluster ions,” Appl. Surf. Sci., vol. 255, no. 4, pp. 1588– 1590, 2008. [13] S. Iida, T. Miyayama, N. Sanada, M. Suzuki, G. L. Fisher, and S. R. Bryan, “Optimizing C60 incidence angle for polymer depth profiling by ToF-SIMS,” Surf. Interface Anal, vol. 43, no. 1–2, pp. 214–216, 2011. [14] C. M. Mahoney, A. J. Fahey, G. Gillen, C. Xu, and J. D. Batteas, “Temperature- controlled depth profiling in polymeric materials using cluster secondary ion mass spectrometry (SIMS),” Appl. Surf. Sci., vol. 252, no. 19, pp. 6502–6505, 2006.

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[15] M. and A. W. Christine M, “Molecular depth profiling with cluster ion beams,” in Cluter Secondary Ion Mass Spectrometry Principles and Applications, 1st ed., D. M. Desiderio, N. M. M. Nibbering, and J. A. Loo, Eds. Canada: John Wiley & Sons. Inc, 2013, pp. 117–205. [16] M. P. Seah, R. Havelund, and I. S. Gilmore, “Systematic Temperature Effects in the Argon Cluster Ion Sputter Depth Profiling of Organic Materials Using Secondary Ion Mass Spectrometry,” J. Am. Soc. Mass Spectrom., vol. 27, no. 8, pp. 1411–1418, 2016. [17] M. P. Seah, “Universal Equation for Argon Gas Cluster Sputtering Yields,” J. Phys. Chem. C, vol. 117, no. 24, pp. 12622–12632, 2013. [18] I. Yamada, “Materials processing by gas cluster ion beams,” Mater. Sci. Eng. R Reports, vol. 34, no. 6, pp. 231–295, 2001.

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Chapter 5: Investigation of the Interface between Bi-layered Organic Materials (Semiconductor and Insulating Materials) using Molecular Depth Profiling ToF-SIMS Cluster Ion Beams

5.1. Introduction Devices such as OLEDs, OPVs and OFETs contain active materials in the form of organic semiconductors. OLEDs are built of organic semiconducting materials in a cascading multi-layer manner: the hole injection layer, the hole transport layer, the emission layer, the electron transporting layer and the electron injection layer [1]. However, OPVs have only a donor (hole transport materials) and an acceptor (electron transport materials) layer [2].

Both OLEDs and OPVs have a cathode (e.g. Al) and an anode (e.g. ITO covered a glass substrate) at the top and bottom of the organic semiconductors, respectively for the injection/collection of the charge carriers. As for OFETs, they consist of an insulator polymer and organic semiconductor used as gate dielectric and active layer, respectively, and three corresponding electrodes (gate, source and drain) [3]. OFETs can take four different architectures as depicted in Figure 5.1. The quality of the interface between the conductor (cathode, anode, source, drain and gate electrodes) and organic materials, and that between organic materials is very important to be of high performance in such devices (OLEDs, OPVs and OFETs).

Figure 5.1: The structures of OFET devices: (a) top-gate/top-contact, (b) top-gate/bottom- contact (c) bottom-gate/top-contact (d) bottom-gate/bottom-contact [4]. S and D refer to source and drain, respectively. 180

With this in mind, the quality of these interfaces has been investigated using SIMS. Ar- GCIBs have been used to depth profile a diversity of organic layers that are used in OLEDs

[5], [6] and OPVs [7], [8]. A model of OFETs, in the form of pentacene/parylene/SiO2/Si films, was depth profiled using SIMS in a dual beam mode (pulsed Bi+ primary ion beam at 25 keV was used for analysis and 500 eV Cs+ for sputtering (ION-ToF-SIMS)) [9]. Also, TIPS-pentacene/PAMS [10] and QBS/PVN [11] blended films were investigated with Cs sputtering ions (ToF-SIMS local caesium sputtering ion and CAMECA IMS-6f magnetic sector SIMS (15 keV Cs+), respectively).

The interface between the conductor (Si) and dielectric (PMMA) or semiconductor (active layers as PTAA or TIPS-pentacene) layers has already been investigated in chapters 3 and 4. Herein, the interface between two organic compounds (dielectric and active layers) was depth profiled using ToF-SIMS, to determine the effect of the primary ion beam choice on the measurement of the organic interface. Four models of OFETs were created from organic bi-layer films consisting of TIPS-pentacene or PTAA and insulating PMMA on silicon substrates (source and drain electrodes were excluded). These films + + were bombarded with C60 and Arn cluster beams (n = 250-2000) at 20 keV.

5.2. Experimental Section 5.2.1. Samples Preparation Two bi-layers films with PMMA and TIPS-pentacene were obtained as in the following steps. For the first bi-layer (TIPS-pentacene/PMMA/Si), a 34 nm PMMA film was deposited onto a clean silicon substrate via a spin-coating process (the procedure is detailed in section 3.2.1). Then, a 50 nm film (determined by QCM) of TIPS-pentacene was thermally evaporated on the PMMA (see section 4.2.1.2). For the second bi-layer (PMMA/TIPS-pentacene/Si), PMMA was spin-coated on thermally evaporated TIPS- pentacene film (the same deposition procedure as for TIPS-pentacene/PMMA/Si).

Two other bi-layers films were obtained with PTAA and PMMA. A clean silicon substrate was first coated with PMMA (34 nm film), followed by a spin-coating process of PTAA (section 4.2.1.1) on top of the PMMA film or vice versa. This procedure was intended to produce either PTAA/PMMA/Si or PMMA/PTAA/Si bi-layers films. The four bilayer structures are illustrated in Figure 5.2. Note that the order of deposition of semiconductors was alternated to study what effect this alteration has on their depth profiles.

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Figure 5.2: Illustration of the four bi-layer films structures deposited on Si substrates. 5.2.2. ToF-SIMS Analysis The four bi-layer structures developed in section 5.2.1 were investigated using a J105 3D Chemical Imager. Three bi-layer films (TIPS-pentacene/PMMA/Si, PTAA/PMMA/Si + and PMMA/PTAA/Si) were investigated with C60 beam using 20 keV energy. TIPS- + pentacene/PMMA/Si was depth profiled with four different sizes of Ar-GCIBs (Ar250 , + + + + Ar500 , Ar1000 and Ar2000 ). PMMA/TIPS-pentacene/Si was bombarded with Ar250 and + + Ar1000 beams. PTAA/PMMA/Si and PMMA/PTAA/Si were bombarded with Ar250 and + Ar2000 . The bombardments were carried out with the same condition, parameters, as those mentioned in section 3.2.1.2, expect for the bilayer films of PTAA/PMMA/Si, which had + + a raster area of 200 µm × 200 µm when they were depth profiled using Ar250 and Ar2000 .

5.3. Results and Discussion 5.3.1. TIPS-pentacene/PMMA/Si Bi-layer Films Bi-layer films of TIPS-pentacene evaporated on PMMA on silicon substrates were depth profiled with different beams and different cluster ion sizes. Secondary ion yields as a function of primary ion doses using different sizes of Ar+ cluster ion are depicted in Figures 5.3 and 5.4. The Ar cluster size influences the behaviour of the secondary ion + yields. With Ar250 cluster beam, the ion yield of TIPS-pentacene decays sharply until it disappears completely. Consequently, the pseudo-steady-state region is indistinguishable (Figure 5.3a). This behaviour is in contrast with what have been observed before in the single-layer of TIPS-pentacene. In fact, with the single-layer (TIPS-pentacene/Si) a stabilisation of the ion yields was observed after an initial transient region with increasing ion dose, and therefore an interface ion dose was identified from the depth profile (Figure 4.16a). It must be noted that for the single-layer, TIPS-pentacene was thermally evaporated on Si while, but in this part of the investigation, the film was grown on PMMA. The discrepancy in the behaviour between the single-layer and bi-layer could be attributed to

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the different support layer that is used, which can lead to having a different structure of TIPS-pentacene, because of different growth mechanism. As the secondary ion yield signal of TIPS-pentacene starts decreasing, the signal of PMMA starts appearing (Figure 5.3a). Then the ion yield of PMMA increases with increasing primary ion dose until the ion yield + of Si6 from the substrate is revealed. However, as for TIPS-pentacene the pseudo-steady- + state region in depth profiling of PMMA was not generated with Ar250 beam. + + + With Ar500 , Ar1000 and Ar2000 cluster beams, the three familiar regions are noticeable, + + as shown in Figure 5.3b and 5.4. Increasing the size of Ar cluster beam from Ar500 , to + + Ar1000 and then to Ar2000 improves the plateau of the secondary ion yields of the TIPS- pentacene. In fact, the pseudo-steady-state becomes more stable. Interestingly, with beams + + + of large Ar cluster ions (Ar500 , Ar1000 and Ar2000 ), the enhancement of the secondary ion yields before reaching the interface, observed in the single-layer (TIPS-pentacene/Si), was not noticeable in the bi-layer scenario. This could be attributed to the bonding of TIPS- pentacene to PMMA being stronger than its bonding with Si, or a difference in the energy deposition mechanism in the underlying material (PMMA or Si), which effects the sputter yield at the interface region. As mentioned earlier, when the yield of TIPS-pentacene disappears that of PMMA appears. The interfaces between TIPS-pentacene and PMMA were observable when using + + + the Ar500 , Ar1000 and Ar2000 ion beams. Note that the ion dose required to reach the interface between TIPS-pentacene and PMMA decreases with increasing the size of the Ar cluster beams (Table 5.2). As for the ion yields of PMMA, they initially increase with an increase of primary ion dose. Then, they stabilised as a plateau at the pseudo-steady-state region, followed by the interfacial region. Finally, the yields of PMMA decrease and the signals of Si starts showing. The enhancement of the secondary ion yields of PMMA before reaching the + + interface with Ar1000 and Ar2000 is consistent with that of the single-layer PMMA (Figure 3.11).

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Figure 5.3: Depth profiles of the bi-layer films of TIPS-pentacene on PMMA, on silicon + + + substrates. Secondary ion yields of [M] m/z 638, [C9H14O4] m/z 186 and Si6 m/z 168 + + are plotted as a function of increasing cluster ion dose of: (a) Ar250 and (b) Ar500 , using 20 keV energy beams.

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Figure 5.4: Depth profiles of bi-layer films of TIPS-pentacene on PMMA, on silicon + + + substrates. Secondary ion yields of [M] m/z 638, [C9H14O4] m/z 186 and Si6 m/z 168 + + are plotted as a function of increasing cluster ion dose of: (a) Ar1000 and (b) Ar2000 , using 20 keV energy beams. 185

Sputtering yields of the bi-layer films of TIPS-pentacene were presumed to be equal to that of the single-layer one (Table 4.4). This presumption is based on a previous study of bi-layers using ToF-SIMS. Mouhib and co-workers demonstrated that the sputtering yields of PVP and P4VP, when coated on PMMA (PVP/PMMA/Si and P4VP/PMMA/Si), were the same as the single-layer on Si (PVP/Si and P4VP/Si). In this study, the samples were + + sputtered with C60 using a 15 keV energy beam and were analysed with Ga using a 12 keV energy beam, in a positive-ion mode [12]. In addition, bi-layer films of fullerene (C60), which were thermally evaporated on tin phthalocyanine (SnPc) evaporated on silicon substrates, have shown very similar sputtering yields to the single-layer of C60 when + + sputtered with Ar1000 using 10 keV energy beams, in a positive-ion mode (Bi3 at 30 keV for analysis) [8].

The depth resolution of TIPS-pentacene/PMMA on Si bi-layer films was calculated by using equation 5.1.

∆푧 = 푌 × ∆퐼interface width Equation 5.1 where Δz is the depth resolution; Y the sputtering yields and ΔI the difference of ion dose across the interface width. Note the sputtering yields are obtained from the single-layer of TIPS-pentacene (Table 4.4). The interface width between TIPS-pentacene and PMMA, bi- layer, is the difference when the TIPS-pentacene ion signal decreases in value from 84% to 16%.

+ The depth resolutions of TIPS-pentacene/PMMA, using four different sizes of Arn cluster beams, are summarized in Table 5.1. The depth resolution improves when + + increasing the size of the Arn cluster ion beam (Figure 5.5). For instance, from Ar250 to + Ar2000 the depth resolution improves by 50%. The trend of the depth resolution obtained + from bi-layer samples (as Arn cluster ion beam size increased) is similar to that of the single-layer of TIPS-pentacene. One notices that the obtained depth resolutions of the bi- layer films sputtering are two times higher than that of the single-layer TIPS-pentacene (Figure 5.5). In the bi-layer film, the top layer could be implanted into the lower layer by the collision processes during the sputtering process. This mixing of molecules at the interface of the organic layers (PMMA and TIPS-pentacene), which is not possible between TIPS-pentacene and the Si substrate in the case of the single layer, may well be the reason behind better depth resolution of the single-layer. It is worth mentioning that this probably does fall under the remit of diffusion, since the latter involves continuous movement of particles from a high density region to another of low particle density. In our

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investigation, this seems to be the case, since the trend in the data in Figure 5.5 is the same as that of single-layer TIPS-pentacene.

Table 5.1: Summary of results of TIPS-pentacene (bi-layer film of TIPS- pentacene/PMMA/Si) interface doses, sputtering yields and depth resolutions with different + Arn cluster ion beams (n = 250-2000) using an incident energy of 20 keV. Cluster ion beam size Interface dose × 1013 Depth resolution (nm) (ions/cm2) + Ar250 ------29.78 ± 1.36 + Ar500 3.90 ± 0.808 22.67 ± 0.60 + Ar1000 3.42 ± 0.121 18.37 ± 0.36 + Ar2000 2.48 ± 0.024 15.03 ± 1.00

Figure 5.5: Depth resolution of the interface: between TIPS-pentacene and PMMA (Bi- layer in red) and between TIPS-pentacene and Si (Single-layer in black) was plotted as a + function of cluster ion size of Arn (n = 250-2000), at room temperature using a 20 keV energy beam. Secondary ion yields of the TIPS-pentacene deposited onto PMMA at the pseudo- steady-state region as a function of eV/atom in Ar+ cluster beams using a 20 keV energy beams are displayed in Figure 5.6. There is a reduction in ion yields of [M]+, [M+H]+ and [M+2H]+ observed as energy per atom in the Ar+ clusters increased. Conversely, the yields + + of the fragments ions of [M-C4 H9Si] and [M-C3H7] are unchanged with energy per atom. + In general, the ion yields are enhanced with a decreased energy per atom, of Arn cluster beam, with the exception of fragment ions. The highest secondary ion yields were 187

+ + + observed with molecular [M] , and the lowest with [M-C4H9Si] and [M-C3H7] ions. It is noted that the secondary ion yields of bi-layer films of TIPS-pentacene are lower within a factor of 2 of those of the single-layer films (Figure 4.20). The relative yields of the different ions are very similar and the change in the absolute measured yields could be attributed to a slight change in instrument sensitivity between the two experiments.

+ The bi-layer TIPS-pentacene/PMMA/Si was investigated using C60 cluster ion beams + + + + of 20 keV energy. Unlike the Arn cluster beams (Ar500 , Ar1000 and Ar2000 ), the ion yields of TIPS-pentacene decrease sharply and carbon deposition on PMMA was observed (Figure 5.7). Therefore, the interface between TIPS-pentacene and PMMA was not distinctive. This is consistent with the behaviour of single-layer TIPS-pentacene (Figure 4.15a).

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Figure 5.7: Depth profiles of TIPS-pentacene on PMMA, bi-layer films, on silicon + + + substrates. Secondary ion yields of [M] m/z 638, [C9H14O4] m/z 186 and C11 m/z 132, + are plotted as a function of increasing 20 keV C60 cluster ion dose. 5.3.2. PMMA/TIPS-pentacene/Si Bi-layer Films In the previous section, the interface between TIPS-pentacene and PMMA was + identified using Ar500-2000 cluster ion beams, in the TIPS-pentacene/PMMA/Si bi-layer investigation. In this section, the results obtained by depth profiling of PMMA/TIPS- + + pentacene/Si, using Ar250 and Ar1000 cluster ion beams, are discussed. Initially, with both + + + cluster beam (Ar250 and Ar1000 ), the PMMA fragment ion yield (C9H14O4) decreases sharply with increasing the primary ion dose of Ar+ cluster ion beams (Figure 5.8). This is + followed by a pseudo-steady-state. For Ar1000 , an enhancement of the secondary ion yields was observed after the pseudo-steady-state as observed in the single-layer of PMMA on Si substrate. However, the interface between PMMA and Si was detected instead of that between the PMMA and TIPS-pentacene for both Ar+ cluster beams. This could be due to a thinner TIPS-pentacene film, keeping in mind that PMMA was dissolved in DCB. In addition, TIPS-pentacene can be dissolved in DCB. So, spin-coating PMMA (in DCB) on top of TIPS-pentacene might lead to a reduction of the thickness of the film of TIPS- pentacene, or a desolvation of the TIPS-pentacene layer and production of a mixed- composition, single-layer. As a suggestion of future work, it is thought that it would be beneficial if more attention is taken in choosing the solvent in which PMMA will be dissolved. 189

Figure 5.8: Depth profiles of PMMA spin-coated on an evaporated TIPS-pentacene, bi- + + layer films, on silicon substrates. Secondary ion yields of [M] m/z 638, [C9H14O4] m/z + + 186 and Si6 m/z 168, are plotted as a function of increasing cluster ion dose of: (a) Ar250 + and (b) Ar1000 , all using 20 keV energy beams.

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5.3.3. PTAA/PMMA/Si Bi-layer Films In the previous sections it was seen that Ar+ cluster beams succeed in depth profiling of small molecular (TIPS-pentacene) evaporated on an insulator (PMMA). Herein, + + PTAA/PMMA/Si bi-layer films were investigated with cluster ion beams C60 , Ar250 and + Ar2000 .

Secondary ion yields of the depth profile of PTAA and PMMA (PTAA was spin-coated + + on PMMA) on silicon using 20 keV Ar250 and C60 decrease until they completely disappear (Figures 5.9a and 5.10). Consequently, the interface between two organic layers does not show distinctly. This is in contrast with the behaviour of secondary ion yields + + when depth profiling the single-layer PMMA with both cluster beams (C60 and Ar250 ) was carried out, earlier, as shown in Figures 3.9a and 3.10a. Note as for the single-layer of + PTAA/Si, C60 beams falls to depth profiling PTAA on PMMA/Si (Figure 4.6a and + Figures 5.9). With Ar250 beams the secondary ion yields of the single-layer PTAA, have shown distinctively, three regions (an initial, a pseudo-steady-state and an interfacial region) as depicted in Figure 4.7a.

At this stage of the study, larger Ar cluster ion sizes were used. Increasing the sizes of Ar cluster lead to an improvement of the shape of the secondary ion yields of PTAA, while that of PMMA are unchanged (see in appendix for the graph of secondary ion yields + + + obtained with Ar500 and Ar1000 in Figure 7.7a and b). On using Ar2000 beams (Figure 5.9b), secondary ion yields of PTAA in these films produced the three regions distinctively, as was the case for the single-layer PTAA on Si (Figure 4.9b). The PMMA signal is too noisy to identify the interface between PTAA and PMMA.

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Figure 5.9: Depth profiles of PTAA on PMMA, bi-layer films, on silicon substrates. + + + Secondary ion yields of [C9H14O4] m/z 186, [M-H] m/z 270 and Si6 m/z 168, are plotted + + as a function of increasing cluster ion dose of: (a) Ar250 and (b) Ar2000 , using 20 keV beams. 192

Figure 5.10: Depth profiles of PMMA on PTAA, bi-layer films, on silicon substrates. + + + Secondary ion yields of [M-H] m/z 270, [C9H14O4] m/z 186 and C11 m/z 132, are + plotted as a function of increasing C60 cluster ion dose using 20 keV energy beams. 5.3.4. PMMA/PTAA/Si Bi-layer Films At this juncture of the study, the order of the layers of the previous investigation of the bi-layer films was altered, whereby PMMA became on top of PTAA on Si. This structure + + + was depth profiled using C60 , Ar250 and Ar2000 primary cluster ion using 20 keV beams. + + Figure 5.11 shows the progression of the secondary ion yields of [C9H14O4] , [M-H] and + + Si9 as a function of primary ion dose of C60 using a 20 keV beam. The yields of PMMA and PTAA follow the same trend, as the ion dose increases. Both signals decay, pass through the pseudo-steady-state and fall at which point where the ion yield of Si reveals itself. Therefore, the interface between two polymers was not successfully identified. Interestingly, in contrast to the single-layer of PTAA, in this bi-layer system no carbon + deposition has been identified, when depth profiling, using C60 beams. This is evidence + that PMMA mixed up with PTAA during spin-coating. As C60 successfully depth profiled PMMA, the mixing may promote the removal of PTAA without carbon deposition.

+ + When bombarding PMMA/PTAA/Si with Ar250 and Ar2000 beams, the ion yields of PMMA and PTAA show the same trend. These results roughly echo those results that were + + obtained using C60 beams. In addition, when Ar cluster ion size increases (from Ar250 to 193

+ Ar2000 ) the ion yields are enhanced at the end of the pseudo-steady-state for both PMMA and PTAA. This is consistent with the results obtained from the single-layers of PMMA and PTAA, when investigated using the same size of Ar cluster ion beams. It is noted that the intensity of PTAA and PMMA yields from the bi-layer system are at least 50% lower than those of the single-layers of these materials. Moreover, the PMMA ion yields are + + enhanced more, when using C60 , than those of the PTAA, in comparison with using Ar250 + and Ar2000 cluster ion beams.

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5.4. Conclusions Bi-layer films of TIPS-pentacene/PMMA/Si, PMMA/TIPS-pentacene/Si, + PTAA/PMMA/Si and PMMA/PTAA/Si were depth profiled using 20 keV beams of C60 + + + + and different sizes of Ar cluster ions (e.g. Ar250 , Ar500 , Ar1000 and Ar2000 ). Interface + detection, between the organic layers also attempted. It has been found that by using C60 beams it was not possible to investigate organic conducting materials (TIPS-pentacene and PTAA), probably because of the deposition of carbon during sputtering. Ar cluster ion beams were successfully used in depth profiling the bi-layer TIPS-pentacene/PMMA/Si; + and the bi-layer of PTAA/PMMA/Si using Ar2000 cluster ion beams. For TIPS- pentacene/PMMA/Si, the interface was clearly identified, while the signals were too noisy during the PTAA/PMMA/Si investigation. The size of Ar cluster ions influences the stability of the pseudo-steady-state, the depth resolution and the interface dose of TIPS- pentacene/PMMA/Si bi-layer films. In fact, the pseudo-steady-state becomes more stable, + + the depth resolution and secondary ion yields (excluding [M-C4H9Si] and [M-C3H7] ions) improve, and the interface dose decreases, in all investigations using larger cluster sizes. Similar improvement in the stability of the pseudo-steady-state was perceived for PTAA/PMMA/Si, when Ar cluster size was increased. When insulator (PMMA) was on top of the conducting material (TIPS-pentacene and PTAA), by using the Ar cluster beams, it was not possible to identify the interface between organic materials. For PMMA/TIPS- pentacene/Si, the interface between PMMA and Si was observed rather than that between PMMA and TIPS-pentacene. This is probably due to a thinner TIPS-pentacene film, or mixing of the organic layer during the sample preparation. In regards to PMMA/PTAA/Si, the yields of these two materials show the same behaviour when the cluster ion dose increased. This suggests that bi-layer film PMMA/PTAA/Si is exhibiting characteristics of + a blend of two materials. Also, when C60 beams were employed to depth profiles this bi- layer, no carbon deposition has been observed which is, in contrast to, what have been found in the study of single-layer PTAA.

Generally, the results of the bi-layer film of TIPS-pentacene/PMMA/Si demonstrated that Ar cluster beams are useful in analysing depth profiles of organic materials used in electronic devices used in OFETs. This conclusion is in line with previous studies of organic materials used in OLEDs and OPVs. The other bi-layer structures require rigorous deposition of the bi-layers (e.g. a better choice of the solvent and deposition techniques and conditions) prior to SIMS investigation.

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5.5. References [1] Y. Li, J.-Y. Liu, Y.-D. Zhao, and Y.-C. Cao, “Recent advancements of high efficient donor-acceptor type blue small molecule applied for OLEDs,” Mater. Today, vol. 20, no. 5, pp. 258–266, 2017. [2] Y. Ie and Y. Aso, “Development of donor-acceptor copolymers based on dioxocycloalkene-annelated thiophenes as acceptor units for organic photovoltaic materials,” Polym. J., vol. 49, no. 1, pp. 13–22, 2017. [3] Y. Jiang, Y. Guo, and Y. Liu, “Engineering of Amorphous Polymeric Insulators for Organic Field-Effect Transistors,” Adv. Electron. Mater., vol. 3, no. 11, pp. 1–12, 2017. [4] W. H. Lee and Y. D. Park, “Organic semiconductor/insulator polymer blends for high-performance organic transistors,” Polymers (Basel)., vol. 6, no. 4, pp. 1057– 1073, 2014. [5] S. Ninomiya et al., “Analysis of organic semiconductor multilayers with Ar cluster secondary ion mass spectrometry,” Surf. Interface Anal., vol. 43, no. 1–2, pp. 95–98, 2011. [6] S. Ninomiya1 et al., “Molecular depth profiling of multilayer structures of organic semiconductor materials by secondary ion mass spectrometry with large argon cluster ion beams,” Rapid Commun. mass Spectrom., vol. 23, no. 20, pp. 3264– 3268, 2009. [7] T. Mouhib et al., “Molecular depth profiling of organic photovoltaic heterojunction layers by ToF-SIMS: comparative evaluation of three sputtering beams.,” Analyst, vol. 138, no. 22, pp. 6801–10, 2013. [8] T. Mouhib et al., “Organic depth profiling of C60 and C60/phthalocyanine layers using argon clusters,” Surf. Interface Anal., vol. 45, no. 1, pp. 163–166, 2013. [9] A. Vincze and J. Jakaboviˇ, “Surface and interface properties of thin pentacene and parylene layers,” Cent.Eur. J. Phys, vol. 7, no. 2, pp. 270–278, 2009. [10] T. Ohe, M. Kuribayashi, A. Tsuboi, K. Satori, M. Itabashi, and K. Nomoto, “Organic thin-film transistors with phase separation of polymer-blend small- molecule semiconductors: dependence on molecular weight and types of polymer,” Appl. Phys. express, vol. 2, no. 12, p. 121502, 2009. [11] M. Kang et al., “Ambipolar Small-Molecule: Polymer Blend Semiconductors for Solution-Processable Organic Field-Effect Transistors,” J. Am. Chem. Soc., vol. 28, no. 5, pp. 1256–1260, 2016. [12] T. Mouhib, A. Delcorte, C. Poleunis, and P. Bertrand, “C60 molecular depth profiling of bilayered polymer films using ToF-SIMS,” surf. Interface Anal, vol. 43, no. 1–2, pp. 175–178, 2011.

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Chapter 6 Conclusions and Future Work The main goal of this study was to investigate depth profiling of organic films of the insulating (PMMA) and semi-conducting materials (PTAA and TIPS-pentacene) utilising + + ToF-SIMS. The results under different polyatomic ion beams (C60 and Arn ) and at different temperatures were compared. The data obtained from single-component films served as a reference for the examination of bi-layers of PMMA, PTAA and TIPS- pentacene (the second main goal) which were chosen for their wide usage in OFETs.

6.1. Conclusions

+ Single-layer of PMMA, PTAA and TIPS-pentacene were analysed with C60 (at 20 and + + + + 40 keV), and Ar-GCIBs (Ar250 , Ar500 , Ar1000 and Ar2000 ) of 20 keV energies.

6.1.1. Sputtering Yields + At room temperature, investigating PMMA, using C60 beams produced higher sputtering yields than the Ar-GCIBs. For the GCIBs, increasing cluster sizes resulted in an improvement of the sputter yield. Comparing sputter yields (Y) as a function of E/n is convenient from a practical sense but from a fundamental point of view the yield per nucleon (Y/nuc) is often used as a more appropriate measure of the sputtering efficiency of each projectile. Yield per atom (Y/n) is an analogous parameter used in the ‘universal’ sputtering plot of Seah, but for projectiles of different chemistry the yield per nucleon is a more appropriate representation to consider. Replotting the combined PMAA sputter yield data set in this form gives a straight-line relationship in a log-log plot (Figure 6.1). Interestingly, no threshold E/nuc for PMMA sputtering was observed in our data. Simulations of other polymers systems by Delcorte et al. suggest a threshold at ~0.1-1 eV/nuc [1], [2]. Our lowest value (for Ar2000) is 0.25 eV/nuc which might be just above threshold for this material.

+ In comparing the results of C60 and Ar-GCIBs, if one calculates the difference between sputtering yields of higher energy per atom beams to the lower ones for each ion, one finds + that the difference is more pronounced in the case of the C60 . The difference in sputtering + 3 yield obtained for the C60 beams was 103 nm /ion, while that for the Ar-GCIBs was 23 nm3/ion. This is expected result, for the higher energy delivery system has more ability to pass its energy to more constituents of the sample. The difference was ~ 400% (Table 3.3 and Table 3.4).

By working out the sputtering yield per nucleon of the investigated clusters (Ar250, + Ar500, Ar1000, and Ar2000, at 20 keV and C60 at both 20 keV and 40 keV, as well as 10 keV 198

obtained from a previous study [3] (for PMMA), a logarithmic (for both axes) graph has been drawn (Figure 6.1). Corresponding plots have been generated for PTAA, and TIPS- pentacene, using the available GCIB data.

3 PMMA 2.5

1.5

log(Y/nuc) 0.5

0 -1 -0.5 0 0.5 1 1.5 2 -0.5 log(E/nuc)

1.4 TIPS-pentacene 1.2

0.8

0.6

0.4 log(Y/nuc) 0.2

0 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 log(E/nuc)

0.5 PTAA 0.4 0.3 0.2 0.1 0 -0.8 -0.6 -0.4 -0.2 -0.1 0 0.2 0.4 -0.2 log(Y/nuc) -0.3 -0.4 -0.5 -0.6 log(E/nuc)

Figure 6.1: Sputtering yield per nucleon, for PMMA (top), TIPS-pentacene (middle) and PTAA (bottom), vs. energy per nucleon.

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It can be seen that sputtering yields (y-axis) against energy per nucleon (x-axis), form a linear relationship, for all materials. This could be interpreted as:

For PMMA,

log(Y/n) = m log(E/n) + constant (C) Equation 6.1 where m is the slope of the line and hence it is calculated from the graph. C is the intercept with the log(Y/n) axis, and hence it is calculated from the graph. Let it be a value of log(q), where q is the Y/n value at which the straight line of the graph intercepts the y-axis, and is a non-zero number.

Y = eC nuc(1-m) Em Equation 6.2 From the graph of PMMA data, and using MS-Excel functions (instructing Excel to assume a straight-line relationship), the slope of the graph was worked out as (1.04) and the intercept was worked out as (0.55).

+ From equation 6.2, using C60 for PMMA produces higher yields than the lower energy + + per atom Arn investigations. This is because although the energy of the C60 beam (40 + + keV) is twice that of the 20 keV energy beam of Arn , the n value (n = 2000) for Ar2000 is + more than 30 times that of the C60 one (n = 60). The same can be said about the TIPS- pentacene graph and PTAA, though, with different values for the intercept and slope.

The straight-line logarithmic equation, as has been suggested above, is consistent with + Seah's Universal Equation, which he obtained for Arn ions alone. In this study, the two ion + + beams, Arn (n =250, 500, 1000, 200) and C60 were amalgamated into one graph, which has correlated the data into a near linearity between energy per nucleon and its corresponding sputtering yield. The amalgamation had the benefit of confirming the suggested linearity of Seah's equation, and that energy per nucleon is the main variable that affects sputtering. As for the link between the yield and the mean sputtered fragment size (and that it is low for organic materials, which are used in this study), this was not investigated in this study, and the same goes the anticipated threshold energy (in Seah's equation), due to lack of resources, namely time.

+ Investigating sputtering yields using C60 for PTAA and TIPS-pentacene was not possible, because the interface was not apparent. This is probably due to carbon deposition on the films during the sputtering process. This seems to be a recurring phenomena with materials that have aromatic hydrocarbons in their molecules, as observed by other studies

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[3], [4]. Aromatic bonds are very stable, as explained in chapter one. Furthermore, this could be attributed to the existence of triple carbon bonds, in the case of TIPS-pentacene.

As for PTAA, investigating its sputtering yields, using Ar-GCIBs of various cluster + + sizes (from Ar250 (80 eV/atom) to Ar1000 (20 eV/atom), produced no effect on its depth profiling characteristics in terms of the shape of the profile, but the sputtering yields + improved by 48% when Ar2000 was used.

For TIPS-pentacene, as with PMMA, increasing cluster size of Ar-GCIBs meant an improvement in its sputtering yield. The sputtering yields of TIPS-pentacene were about 200-300% greater than those obtained of PMMA, which in turn, were about 200-300% greater than those of PTAA, for all cluster sizes of Ar-GCIB. The differences in characteristics, between the TIPS-pentacene and both PMMA and PTAA, may be attributed to the triple bond in the structure of TIPS-pentacene. Moreover, the higher molecular weight of TIPS-pentacene might have some effect. Additionally, polymer chains experience more bonds, van der Waals interactions, along the length of their chains compared with non-polymers, namely TIPS-pentacene.

+ For PMMA, using C60 at 40 keV and 20 keV produced decreasing sputtering yields as o + the sample was cooled to -125 C temperatures. A similar response was found using Ar500 + (40 eV/atom) and Ar2000 (10 eV/atom), for both PMMA and TIPS-pentacene, albeit at a different rate. As mentioned above, since very low room temperature sputtering yield was + obtained for PTAA (even for Ar2000 beams) it was thought that it would be unproductive to investigate PTAA further.

After cooling was switched off and samples had returned to room temperature + (overnight), PMMA was depth profiled again with C60 (20 keV and 40 keV), producing an enhancement of sputtering yields, in comparison with results obtained before cooling. + This result, together with the Ar500 investigations regarding PMMA and TIPS-pentacene, need to be investigated again to be sure of its validity. In reproducing it, one could see if the effects of structural changes of the materials have any role in producing such an enhancement.

6.1.2 Secondary Ion Yields at the pseudo-steady-state region At room temperature, PMMA produced the highest secondary ion yields using 40 keV + + C60 (667 eV/atom), in comparison with all other investigated Ar-GCIBs (20 keV C60 and

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+ 20 keV Arn ), bearing in mind that PTAA and TIPS-pentacene did not produce a pseudo- + steady-state region, using C60 . More importantly, for all investigated materials it was noticed that when using Ar- + + + + GCIBs (Ar250 , Ar500 , Ar1000 and Ar2000 ) of 20 keV energy, there were enhancements of secondary ion yield signals just before the corresponding interface (i.e. at the end of the pseudo-steady-state region). This phenomenon is accompanied by a reduction in the ion dose required to reach the interface so appears to be a sputter yield enhancement effect, which can be indirectly measured by observing the increased ion yield near the interface. , For all investigated materials, increasing cluster size of the beams meant an improvement in secondary ion yields; albeit with PTAA there were large errors. The secondary ion yields of PMMA and PTAA were similar, but they were almost an order of magnitude lower than that obtained from TIPS-pentacene samples, for all Ar-GCIB sizes. A possible reason for this behaviour could be as follows: although the data obtained for each material were obtained on the same day investigation (i.e. using different ion clusters in the same day, for one type of material), the different materials’ data were obtained on different days. This may have some influence on the investigations. Moreover, as mentioned above, the triple bond effects and molecular weights could be reasons behind this. Furthermore, different patterns of ionisation of the investigated material, during their bombardment by the primary ions, might have some role. During the cooling of the samples, there was an enhancement of the secondary ion + yields just before reaching the interface region when analysing PMMA with C60 . The reason for which is not entirely clear, possibly due to some kind of resonance that is affecting the selvedge layer; it could be tested by changing the angle of incidence of the primary ions; more research is needed in this regard.

The secondary ion yields, at the pseudo-steady-state region, for PMMA, using the 20 + keV C60 beam, were reduced during cooling the sample to -100 °C. A further decrease in the temperature produced no discernible effect on the secondary ion yields, as the + variations were within the experimental error. As for the 40 keV C60 beam, it produced an increase (at -100 °C) then a drop on further cooling. This suggests that the effect of sample cooling on secondary ion yield is influenced by the energy of the projectile, as the 40 keV + + C60 beam has double the energy than that of the 20 keV C60 beam, hence is more able to cause breakages of bonds. However, both beams then reach a stage where they cannot cause more breakages, hence the levelling of the ion yields.

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For PMMA and TIPS-pentacene, the previously mentioned enhancement before the + + + interfaces (in C60 beams) was showing again, for Ar500 and Ar2000 sputtering. As cooling was carried out there was a decrease in secondary ion yields with temperature. However, + for PMMA, using Ar500 produced a slight increase, but the result is not conclusive, due to the large error bars.

As with sputter yield measurements, cooling appears to have a lasting effect of secondary ion yields, after samples have returned to room temperatures. For PMMA, its + secondary ion yields showed an enhancement when C60 (20 keV and 40 keV) were used, in comparison with results obtained before cooling. However, for both PMMA and TIPS- + + pentacene, using Ar500 and Ar2000 , it was noticed that they produce less secondary ion yields after a cooling experiment (see Figures 3.19, 3.23 and 4.25).

6.1.3 Depth Resolution + In investigating single-layers of PMMA, PTAA and TIPS-pentacene with C60 (at 20 and 40 keV), at room temperature, PTAA and TIPS-pentacene did not show interfaces (see + above for possible reasons). As for PMMA, the 20 keV C60 beam produced a slightly better depth resolution (about 5 nm improvement) than at 40 keV.

+ + + + Increasing cluster size of the Ar-GCIBs (Ar250 , Ar500 , Ar1000 and Ar2000 , of 20 keV), for PMMA and TIPS-pentacene, caused an improvement in depth resolution. With PTAA, + the only investigation that produced results was that of the Ar2000 beam . It has produced a measurable good depth resolution, while no results were obtained for the other cluster beams, due to large signal fluctuations.

Cooling the sample of PMMA produced better depth resolution values, levelling off after -125 °C. Using Ar-GCIB, with lower energy per atom, produced further improvement in the depth resolution, in comparison with the higher energy per atom beams

+ At room temperature using C60 , after cooling was switched off an enhancement of depth resolution with PMMA was noticed, in comparison with results obtained before + cooling. In general, for all temperatures, PMMA, using C60 at 40 keV produced worse + depth resolutions than the C60 at 20 keV beams.

In comparing the depth resolutions, at room temperature (i.e. before and after cooling), + the higher energy per atom scenarios using Ar500 clusters improved depth resolutions, + whereas, the lower energy per atom, Ar2000 beam, produced roughly constant depth

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resolutions, taking errors into consideration. This suggests temperature effects depth resolutions in different ways for different cluster beams.

In general, in sputtering PMMA with Ar-GCIBs, it was observed that the higher sputtering yields and the better depth resolutions were obtained when lower energy per atom were used, for each of the investigated temperatures of the sample. This agrees with published literature (see section 3.3.2.1 for a possible reason for such behaviour).

No significant trends regarding depth resolutions were observed in the TIPS-pentacene samples due to the large errors (possibly due to the inhomogeneous film) that made the variation in depth resolutions with temperatures indiscernible.

Having explored a number of fundamental sputtering characteristics using single-layer films, attention turned to bi-layers as mimics of organic electronic devices. This provided the opportunity to explore the ability of measure the interface between organic layers.

+ + + + In TIPS-pentacene/PMMA/Si bi-layer films, using Arn GCIBs (Ar250 , Ar500 , Ar1000 + and Ar2000 ), the interface between TIPS-pentacene and PMMA was successfully identified. Interestingly, using Ar-GIBs did not produce the enhancement of secondary ion yields that was observed just before reaching the interface in single-layer (TIPS- pentacene/Si) samples. This could be attributed to a probable stronger bonding of TIPS- pentacene to PMMA than to Si (giving more credit to a possible resonance issue, mentioned earlier). As with the single-layer samples, depth resolutions obtained from the bi-layers improved with increasing cluster size of the Ar-GCIBs. It was also noticed that + the obtained depth resolutions, when sputtering the bi-layer films with Arn GClBs were double those obtained of the single-layer TIPS-pentacene, possibly due to interlayer + mixing effects. Again as with the single-layer, the investigation of this bi-layer, using C60 , did not produce a successful result, possibly due to carbon deposition on TIPS-pentacene during the sputtering process. This is something to be investigated in future work, together with the above single-layer problem.

With the PMMA/TIPS-pentacene/Si sample, the obtained secondary ion yields profile + + using Ar250 and Ar1000 ion beams suggested only a single-layer PMMA on Si substrate, thus showing only the existence of PMMA/Si interface. TIPS-pentacene ion yield from this sample was extremely low, below the value of the Si substrate yield. This could be attributed to the thinner (or non-existent) TIPS-pentacene film, which causes the interface between PMMA and TIPS-pentacene to be undetectable. During the preparation of the PMMA/TIPS-pentacene/Si layer, a solution of PMMA (in DCB) was used on top of the 204

TIPS-pentacene layer in attempt to create a TIPS-pentacene/PMMA bi-layer. It seems that this DCB might have dissolved the whole of the TIPS-pentacene layer and might have made a mixture with some of the PMMA layer. During the spinning process to produce the PMMA layer, some of that mixture mixture have escaped, leaving a mainly thinner PMMA layer. This would manifest itself in the non-existent TIPS-pentacene signal, and a noted increase in PMMA signals, in comparison with the single-layer scenarios, for both primary beams. This secondary ion yield increase is evident for both the pseudo-steady-state region signal and the interface signal.

+ + Depth profiling of PTAA/PMMA/Si, using C60 and Ar250 beams, have displayed a + decreasing secondary ion yields for both PTAA and PMMA. With Ar2000 beams, three secondary ion yield regions of PTAA appeared. However, determining the interface between PTAA and PMMA was difficult because the signals were too noisy. The interface between the materials in PTAA/PMMA/Si and PMMA/PTAA/Si was not discernible under the conditions used in this study.

+ + Using Ar-GCIBs of increasing cluster size (from Ar250 to Ar2000 ) the corresponding secondary ion yields for PMMA/PTAA/Si bi-layers were enhanced at the end of the pseudo-steady-state, for both PMMA and PTAA. This is consistent with the results obtained from the single-layers of PMMA and PTAA when investigated using the same + clusters. Interestingly, when depth profiling this bi-layer system with C60 beams, the profiling was successful, suggesting that no carbon deposition occurred, in contrast to the single-layer PTAA case. The different profiling shapes for PMMA/PTAA/Si and PTAA/PMMA/Si suggest that during spin-coating some mixing between PMMA and PTAA occurred. This probable mixing could have caused the removal of PTAA, and prevented carbon deposition. This is indicated by the similarity of secondary ion yields + + profiles between the two beams Ar250 and C60 . Interfaces between the materials in PMMA/TIPS-pentacene/Si, PTAA/PMMA/Si, and PMMA/PTAA/Si, were not discernible. Therefore, depth resolutions of these systems were not calculated.

6.2 Future Work There have been a few suggested future studies mentioned throughout this thesis. However, below are some further suggestions that need addressing.

 In this study, H2O-GCIB and CO2-GCIBs were supposed to have been used alongside + the Ar-GCIBs and C60 ones, to explore the effect of chemical bonding in the very large projectile clusters. The use of molecular clusters for depth profiling has not previously 205

been systematically studied. Due to lack of experimental resources, as pointed out in Chapter 1, it was not possible to perform these studies within the timescale of this

project. As pointed out in literature, H2O-GCIB features an increased secondary ion yield and less fragmentation [5], [6], [7], and therefore could be beneficial in making

comparisons with existing work. CO2 clusters are very novel for SIMS studies and little work has been published to-date. One study does suggest increased secondary ion yields [8]. Both GCIBs could also be used to investigate the effects of sample temperature changes, on sputtering, depth profiling, and making comparison with sputtering yields and depth resolutions, using different cluster sizes beams at the constant energy of 20 keV. Again, the benefits of such comparative studies are to see if projectile/sample parameters could produce similar results, thus confirming such parameters as universal, within the context and scope of relevant materials.  To investigate the effects of low temperature sample temperatures, as well as above room temperature, on OFET materials that are bi-layered (TIPS-pentacene/PMMA/Si, PMMA/TIPS-pentacene/Si, PTAA/PMMA/Si and PMMA/PTAA/Si). However, one needs to ascertain the effect of the preparation methods, used in creating bi-layered samples.  The non-appearance of the interface in the TIPS-pentacene sample needs further investigation concerning the thickness measurement of the sample surface. First of all, the samples were of inhomogeneous thicknesses. Moreover, using DektakXT profilometry method has a draw-back in that it etches the sample surface further. In this study, measuring thicknesses using AFM was limited to a field of view of ~ 100 µm, whereas the craters sizes were ~ 350 µm × 350 µm. Therefore, a wider field of view AFM is needed, so that it is possible to measure right across the sample, or at least to the middle of the crater. The difficulty in measuring the sample thickness, and uniformity of thickness was a major source of experimental uncertainty in these studies.  To investigate single-layered PMMA and TIPS-pentacene, above room temperature, as well as investigating more low-temperature values e.g. 0 °C, -25 °C, -50 °C, and so on. This is in order to have a more continuous data set, from low temperature to higher than room temperatures, for improving our understanding of this parameter in different sample materials. Moreover, this would show if the anticipated results would follow patterns obtained for other materials, such as NPB and Irganox 1010.

 To investigate larger clusters and/or lower energies, to look for possible sputtering thresholds.

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 The enhancement in sputtering yields, after cooling was switched off and samples + + returned to room temperature (for PMMA using C60 and TIPS-pentacene using Ar500 ) needs to be reproduced again, for confirmation purposes.  To ascertain the effect of topography changes, due to the cooling process that this research conducted. AFM could help in exploring this issue. It would be interesting to link any topography changes with the resulting depth resolution.  As mentioned in Chapter 1, one needs to address the issue of the type of the polymer being investigated, i.e. whether it is atactic, syndiotactic, and isotactic, as each might

have its own Tg and/or Tm.  The resonance possibility needs to be investigated. For example, changing the angle of bombardment might indicate such effect. Another way is to turn the sample in different angles to test such suggestion, as well as the crystallinity effects.

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6.3 References [1] A. Delcorte, B. J. Garrison, and K. Hamraoui, “Dynamics of Molecular Impacts on Soft Materials : From Fullerenes to Organic Nanodrops,” Anal. Chem., vol. 81, no. 16, pp. 6676–6686, 2009. [2] A. Delcorte and M. Debongnie, “Macromolecular sample sputtering by large Ar and CH4 clusters: elucidating chain size and projectile effects with molecular dynamics,” J. Phys. Chem. C, vol. 119, no. 46, pp. 25868–25879, 2015. [3] R. Möllers, N. Tuccitto, V. Torrisi, E. Niehuis, and A. Licciardello, “Chemical effects in C60 irradiation of polymers,” Appl. Surf. Sci., vol. 252, no. 19, pp. 6509– 6512, 2006. [4] N. Nieuwjaer, C. Poleunis, A. Delcorte, and P. Bertrand, “Depth profiling of polymer samples using ion beams Ga+ and C60+,” surf. Interface Anal, vol. 41, no. 1, pp. 6–10, 2009. [5] S. Iida, T. Miyayama, N. Sanada, M. Suzuki, G. L. Fisher, and S. R. Bryan, “Optimizing C60 incidence angle for polymer depth profiling by ToF-SIMS,” Surf. Interface Anal., vol. 43, no. 1–2, pp. 214–216, 2011. [6] S. S. (née Rabbani), A. Barber, I. B. Razo, J. S. Fletcher, N. P. Lockyer, and J. C. Vickerman, “Prospect of increasing secondary ion yields in ToF-SIMS using water cluster primary ion beams,” Surf. Interface Anal., vol. 46, no. S1, pp. 51–53, 2014. [7] S. Sheraz Née Rabbani, A. Barber, J. S. Fletcher, N. P. Lockyer, and J. C. Vickerman, “Enhancing secondary ion yields in time of flight-secondary ion mass spectrometry using water cluster primary beams,” Anal. Chem., vol. 85, no. 12, pp. 5654–5658, 2013. [8] I. B. Razo, S. Sheraz, A. Henderson, N. P. Lockyer, and J. C. Vickerman, “Mass spectrometric imaging of brain tissue by time-of-flight secondary ion mass spectrometry-How do polyatomic primary beams C60+, Ar2000+, water-doped Ar2000+ and (H2O) 6000+ compare?,” Rapid Commun. Mass Spectrom., vol. 29, no. 20, p. 1851, 2015. [9] H. Tian, D. Maci\każek, Z. Postawa, B. J. Garrison, and N. Winograd, “CO2 cluster ion beam, an alternative projectile for secondary ion mass spectrometry,” J. Am. Soc. Mass Spectrom., vol. 27, no. 9, pp. 1476–1482, 2016.

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Chapter 7 Appendix Calibration Details: Calibration of the J105 instrument was carried out twice over the span of the investigations. This involved the assignment of known peaks in the mass spectra, using standard materials , indium, histidine and PMMAA ToF-SIMS spectrum was obtained for each material in turn, in time-spectra format (i.e. in the Time-Of-Flight, of the recorded ions), instead of the usual m/z ratio-spectrum. A software calibration routine was performed whereby known signals in the time spectra were assigned to known m/z values. + + + The m/z values used were as follows: for indium, In , 114.904, In2 , 229.808, and In3 , + + 344.712; for histidine, C6H9N3O2 , 155.070 and its fragment C5H8N3 , 110.072; for PMMA, fragments at m/z 126.068, 139.076, and 186.089. Once calibrated the instrument maintained a constant mass accuracy (5 ppm) as long as the buncher or post-buncher optics were not retuned. The sample topography and primary cluster flight time has no influence on the mass calibration due to the design of the instrument.

Synthesis of PTAA:

Figure 7.1: Synthesis of PTAA. Reagents and conditions: Pd2(dba)3, KOtBu, toluene, 105 °C, 22 h. Turner’s group has made it [9].

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How the experiments were performed and where the craters are in respect to each other:

For PMMA Sample:

Figure 7.2: Bombarded areas, represented as squares, signify different time/day of experimental investigations. Coloured squares, size of which is 350 µm × 350 µm per represent analysis craters, due to the bombardment of the PMMA sample film, blue, sitting on a Si substrate (size 1 cm × 1 cm), with cluster ion beams, of: (a) 20 keV and (b) 40 keV, energies. The colours, detailed in the Table 7(a) below, signify different sample temperatures, in the experimental investigations, starting, as shown, from the bottom-right of the sample, going upwards, and finishing at the top left-hand of the sample. Different Si substrates were used for each cluster ion beam.

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Table 7.1: Different temperatures and/or days of experimental investigations for the PMMA films, together with their corresponding cluster ion beam energies (20 keV and 40 + keV C60 beams), as well as, the repetition number of the experiments. Energy (keV) 20 40 Day 13/03/2017 11/04/2017 Temperature (°C) Room temperature Room temperature Repeat 3 3 Types of crater red red Day 22/03/2017 11/04/2017 Temperature (°C) -170 -170 Repeat 3 3 Types of crater orange orange Day 28/03/2017 11/04/2017 Temperature (°C) -100 -125 Repeat 3 3 Types of crater grey purple Day 31/03/2017 12/04/2017 Temperature (°C) -125 -100 Repeat 3 3 Types of crater purple grey Day 01/04/2017 13/04/2017 Temperature (°C) Room temperature Room temperature Repeat 3 3 Types of crater green green

Figure 7.3: Bombarded areas, represented as squares, signify different time/day of experimental investigations. Coloured squares, size of which is 350 µm × 350 µm per represent analysis craters, due to the bombardment of the PMMA sample film, blue, sitting + + on a Si substrate (size 1 cm × 1 cm), with 20 keV Ar500 and Ar2000 cluster ion beams. The colours, detailed in the Table 7(b) below, signify different sample temperatures, in the experimental investigations, starting, as shown, from the bottom-right of the sample, going upwards, and finishing at the top left-hand of the sample. 211

Table 7.2: Different temperatures and/or days of experimental investigations for the + + PMMA film, together with their corresponding 20 keV Ar500 and Ar2000 cluster ion beams), as well as, the repetition number of experiments

+ + Polyatomic beams Ar500 Ar2000 Day 02/05/2017 02/05/2017 Temperature (°C) Room temperature Room temperature Repeat 3 3 Types of crater red red Day 03/05/2017 03/05/2017 Temperature (°C) -50 -50 Repeat 3 3 Types of crater orange orange Day 04/05/2017 04/05/2017 Temperature (°C) -150 -150 Repeat 3 3 Types of crater yellow yellow Day 05/05/2017 05/05/2017 Temperature (°C) Room temperature Room temperature Repeat 3 3 Types of crater purple purple

For TIPS-pentacene Sample:

Figure 7.4: Bombarded areas, represented as squares, signify different time/day of experimental investigations. Coloured squares, size of which is 350 µm × 350 µm per represent analysis craters, due to the bombardment of the TIPS-pentacene films, orange, + + sitting on a Si substrate (size 1 cm × 1 cm), with 20 keV Ar500 and Ar2000 cluster ion beams. The colours, detailed in the Table 7(c) below, signify different temperatures, in the experimental investigations, starting, as shown, from the bottom-right of the sample, going upwards, and finishing at the top left-hand of the sample.

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The following is a photo taken for TIPS-pentacene sample. It is not a very clear one, though. However, with some effort, one could see the etchings that bombardments have cause on the surface of the sample.

Figure 7.5: Image of the TIPS-pentacene film. Table 7.3: Different temperatures and/or days of experimental investigations for the TIPS- + + pentacene film, together with their corresponding 20 keV Ar500 and Ar2000 cluster ion beams), as well as, the repetition number of experiments

+ + Polyatomic beams Ar500 Ar2000 Day 19/06/2017 20/06/2017 Temperature (°C) Room temperature Room temperature Repeat 3 2 Types of crater red Red Day 21/06/2017 22/06/2017 Temperature (°C) -50 -50 Repeat 2 2 Types of crater orange orange Day 23/06/2017 22/06/2017 Temperature (°C) -150 -150 Repeat 2 3 Types of crater purple purple Day 25/06/2017 25/06/2017 Temperature (°C) Room temperature Room temperature Repeat 3 2 Types of crater green green

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The results, below, are for TIPS-pentacene, show when the measurements of the films were taken for depth calculation of craters, using DektakXT, which created these craters by the bombardment of primary ion beams.

Table 7.4: Summary of TIPS-pentacene results for interface dose, sputtering yields, and + + depth resolution at different temperatures under 20 keV Ar500 and Ar2000 cluster ion beams, with standard deviation. These measurements of the crater depth were obtained by DektakXT.

Cluster Temperature Depth (nm) Interface Sputtering Depth ion beam (°C) dose × 1013 size (ions/cm2) yields resolution

(nm3/ion) (nm)

+ Ar500 25 (before 135.78 ± 10.8 5.65 ± 0.11 240.18 ± 33.63 32.09 ± 1.66 cooling) -50 112.88 ± 7.77 6.38 ± 0.58 168.56 ± 23.60 41.65 ± 7.23

-150 74.44 ± 7.29 6.62 ± 0.65 112.25 ± 15.72 26.92 ±

25 (after 104.78 ± 5.66 4.99 ± 0.27 216.50 ± 30.31 25.55 ± 2.67 cooling) + Ar2000 25 (before 102.35 ± 16.66 2.60 ± 0.01 391.98 ± 54.88 23.92 ± 18.05 cooling) -50 90.05 ± 10.59 4.11 ± 0.50 220.83 ± 30.92 39.98 ± 0.06

-150 92.71 ± 4.25 5.15 ± 0.08 178.67 ± 25.01 27.91 ± 5

25 (after 101.63 ± 16.50 3.49 ± 0.14 290.56 ± 40.68 15.80 ± 0.44 cooling)

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Figure 7.6: The sputtering yields (a) and Depth resolution (b) of TIPS-pentacene plotted + + as a function of temperature (°C) with Ar500 and Ar2000 cluster ion beams at an incident energy of 20 keV. Symbols (*) and (^) indicate before and after cooling the TIPS-pentacene film, respectively. 215

+ + Depth profiles of PTAA/PMMA/Si bi-layer film, using Ar500 and Ar1000 are presented below:

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Experimental method for DektakXT mesurements: Figure 7.8 shows how the measurements of the thicknesses of the PTAA film by DektakXT were obtained. Three lines (which were made by scratching the films with a needle) across the film were scanned, each, three times, at different positions along each line. The profiles are shown. After adjusting DektakXT for levelling of the sample, two built-in cursors were used to specify the areas across which a depth is to be calculated, i.e. the thickness of the film (51.5 ± 0.8 nm). The DektakXT has a reported vertical resolution of 0.1nm. The same procedure was carried out for the PMMA samples, when measuring its thickness by DektakXT. However, data for the investigated samples are not available to show.

The thicknesses and the subsequent quality of the various samples of the single layered TIPS-pentacene, as well as, the bi-layers of PMMA, PTAA and TIPS-pentacene were noted throughout the thesis as having a mixing problem, with the substrate and/or another layer. It was beyond the capability of the DektakXT to measure any sample mixing between molecular layers as it is purely a physical measurement of the total film thickness.

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Figure 7.8: Measurements of the thicknesses of the PTAA film by DektakXT. The white lines represent scratches made on the samples. The red boxes represent the areas where DektakXT was made to inspect their depths. The measurements of the depths are presented on the right, in three sub-figures, where the red tracings represent the DektakXT data as a function of the span of each area.

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The uncertainty budget for errors and their propagation: An uncertainty budget including the major sources of errors was compiled using a standard template provided by the US National Institute of Standards, on their web-site, the link for which is: https://www.nist.gov/document/uncertaintybudgettabletemplate16jan2013xlsx.

The measurements and other factors considered in this error analysis are contained in the following spreadsheets, together with their estimated magnitude and distribution. Type

A errors were determined based on repeat measurements e.g. ion current, and assumed to be normally distributed. Type B errors were based on estimates or specifications and assumed to have a rectangular distribution. The calculation applies a coverage factor or 2

(95% confidence) and 1.7 respectively to Type A and Type B errors.

The uncertainty in all ion beam currents was assumed to be 10%, based on repeated measurements taken throughout the experiments. The C60 current was generally more stable than the GCIB current. The error in the crater dimensions was based on the standard

350 × 350 m2 crater area. Multiple DektakXT profiles across craters were used to estimate the uncertainty in the crater depth. As a first approximation, these errors should suffice for these types of experiments.

Relative standard deviations for each important parameter (current, crater volume etc.) were then combined in an appropriate manner to give the final estimated uncertainty in the derived data values e.g. sputter yield. This involved taking the square root of the sum of the squares of the expanded uncertainty values. For plotting the graphs, the logarithmic plot of PMMA needed a slightly different approach regarding its errors. The relative errors of the yield (14%) were multiplied by (1/ln10) (which came to 6%) so that the errors could be plotted in the PMMA graph, [1].

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Primary Ion current Uncertainty Component Estimated Units degrees of Estimated Unc Type (A, B) Probability Divisor Std Unc (A) Relative Description Uncertainty freedom in Distribution Contribution (%) Measurement Units (A)

repeatability 5 pA 10 5E-12 A Normal, 2s 2 2.5E-12 0.903614458 resolution 1 pA 1 1E-12 B Rectangular 1.732051 5.7735E-13 0.048192771 calibration 1 pA 1 1E-12 B Rectangular 1.732051 5.7735E-13 0.048192771

Min Degrees of Freedom ν 1 Effective Degrees of Freedom νeff 11 Combined Uncertainty, uc 2.62996E-12 Coverage factor, k, uses effective degrees of freedom 2.25 Expanded Uncertainty, U 5.9174E-12 Expanded Uncertainty, U, Rounded to 2 Significant Digits 0.0000000000059 A

Sputter crater width/length Uncertainty Component Estimated Units degrees of Estimated Unc in Type (A, B) Probability Divisor Std Unc (A) Relative Description Uncertainty freedom Measurement Distribution Contribution (%) Units (nm)

repeatability(xy) 5 um 10 5000 A Normal, 2s 2 2500 0.903614458 resolution (xy) 1 um 1 1000 B Rectangular 1.732051 577.3502692 0.048192771 calibration (xy) 1 um 1 1000 B Rectangular 1.732051 577.3502692 0.048192771

Min Degrees of Freedom ν 1 Effective Degrees of Freedom νeff 11 Combined Uncertainty, uc 2629.95564 Coverage factor, k, uses effective degrees of freedom 2.25 Expanded Uncertainty, U 5917.400189 Expanded Uncertainty, U, Rounded to 2 Significant Digits 5,900 nm

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Sputter crater depth Uncertainty Component Estimated Units degrees of Estimated Unc in Type (A, B) Probability Divisor Std Unc (A) Relative Description Uncertaint freedom Measurement Distributio Contribution (%) y Units (nm) n

repeatability (z) 1 nm 10 1 A Normal, 2s 2 0.5 0.08813161 resolution (z) 0.1 nm 1 0.1 B Rectangular 1.732051 0.05773502 0.001175088 7 calibration (z) 1 nm 1 1 B Rectangular 1.732051 0.57735026 0.117508813 9 reproducibility in film thickness 3 nm 10 3 A Normal, 2s 2 1.5 0.793184489

Min Degrees of Freedom ν 1 Effective Degrees of Freedom νeff 12 Combined Uncertainty, uc 1.68424068 Coverage factor, k, uses effective degrees of freedom 2.23 3.75585671 Expanded Uncertainty, U 5 Expanded Uncertainty, U, Rounded to 2 Significant Digits 3.8 nm

Propagation of random errors

expanded relative uncertainty standard measured quantity measured value (k=2) deviation

crater width 350 um 6 um 1.70% crater width 350 um 6 um 1.70% crater depth 50 nm 4 nm 8%

Crater volume SQRT[(1.7)2+(1.7)2+(8.0)2] 8%

Primary ion current 50 pA 6 pA 12% Number of primary ions 12%

Sputter Yield = Crater volume/Primary ions SQRT[(8.0)2+(12.0)2] 14%

% rel(Vol(err))=sqr{rel(width(err)2)+rel(length(err)2)+rel(depth(err)2)}= 8.40% 8.4 14.42

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References:

[1] E. M. Stuve, “Estimating and Plotting Logarithmic Error Bars,” 2004. [Online]. Available: http://faculty.washington.edu/stuve/uwess/log_error.pdf. [Accessed: 03-Jun-2018].

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