Introduction to Sources of Information

• Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics X-ray Photoelectron ~$600 (2 CCMR copies, 1 copy on reserve in Engineering Library) • Surface Analysis, Briggs & Grant, ~$300 (1 CCMR copy) Spectroscopy (XPS) • XPS of Polymers Database, ~$600 (1 CCMR copy on CD) • UK Surface Analysis forum, www.uksaf.org • XPS Short Courses (John Grant), www.surfaceanalysis.org • Sources of Information • Principles of XPS and Auger • E-mail list-serv [email protected] • How to prepare samples for XPS – Subscribe at http://lists.ccmr.cornell.edu • Instrumentation, X rays, Photoelectron detection – CCMR system updates, announcements, questions, etc. • Data acquisition • Sources for IMFP: – Quantitative and Qualitative analyses – Quases-IMFP-TPP2M software (10.6MB) free download at – Spin-orbit splitting, Plasmons, Shake-up, etc. www.quases.com – Sample charge control – NIST program IMFPWIN (1 CCMR copy) – Overlayer effects – Ion sputtering

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Introduction to Surface Analysis X-ray Photoelectron The Study of the Outer-Most Layers of Materials (<100 Å). Spectroscopy (XPS) • Electron • Ion Spectroscopies Spectroscopies • Sources of Information SIMS: Secondary Ion • Principles of XPS and Auger XPS: X-ray Mass Spectrometry Photoelectron • How to prepare samples for XPS Spectroscopy • Instrumentation, X rays, Photoelectron detection SNMS: Sputtered Neutral Mass • Data acquisition AES: Auger Electron Spectrometry – Quantitative and Qualitative analyses Spectroscopy – Spin-orbit splitting, Plasmons, Shake-up, etc. ISS: Ion Scattering Spectroscopy – Sample charge control EELS: Electron Energy Loss – Overlayer effects Spectroscopy – Ion sputtering 8/18/2010 3 8/18/2010 4

Comparison of Sensitivities X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy H Ne Co Zn Zr Sn Nd Yb Hg Th for Chemical Analysis (ESCA) is a widely AES and XPS used technique to investigate the chemical 1% composition of surfaces.

5E19 RBS X-ray Photoelectron spectroscopy, PIXE based on the photoelectric effect,1,2 was

1ppm 5E16 developed in the mid-1960’s by Kai Siegbahn and his research group at the SIMS University of Uppsala, Sweden.3

1. H. Hertz, Ann. Physik 31,983 (1887). 1ppb 5E13 0 20 40 60 80 100 2. A. Einstein, Ann. Physik 17,132 (1905). 1921 Nobel Prize in Physics. 8/18/2010 ATOMIC NUMBER 5 8/18/2010 3. K. Siegbahn, Et. Al.,Nova Acta Regiae Soc.Sci., Ser. IV, Vol. 20 (1967). 6 1981 Nobel Prize in Physics. The Photoelectric Process Photoionization Cross Section

Ejected Photoelectron Incident X-ray • Scofield cross-sections are proportional rate of Free Electron XPS spectral lines are identified emitted photoelectrons Level by the shell from which the • Typically the C 1s Conduction Band electron was ejected (1s, 2s, 2p, transition is given a value etc.). Fermi of 1, sometimes F 1s Level  The ejected photoelectron has • Peaks are created with Valence Band kinetic energy: areas proportional to KE=hv-BE-  Scofield cross-sections 2p L2,L3  XPS typically uses relation 2s L1  BE=hv-KE-   Kinetic energy of the exciting x- ray must be known 1s K  Work function,, of the detector is known and constant  Each pathway has a 8/18/2010 photoionization cross-section 7 8/18/2010 8

Elemental XPS Spectrum Auger Relation of Core Hole

Incident X-ray or Emitted Auger Electron electron

Free Electron L electron falls to fill core level Level vacancy (step 1). Conduction Band  KLL Auger electron emitted to Fermi conserve energy released in Level step 1. Valence Band  The kinetic energy of the emitted Auger electron is: 2p L2,L3 KE=E(K)-E(L2)-E(L3). 2s L1  A 3-step process which often makes Auger peaks more difficult to characterize than 1s K XPS peaks

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Auger Spectrum X-ray Photoelectron Spectroscopy Small Area Detection

•Auger utilizes an electron beam to Monochromatic X-ray Beam Electrons are extracted scan the sample surface only from a narrow solid •Electron beams can be focused to angle. much smaller spot sizes (~5 nm) than x-rays •Electrons from the beam are collected along with photoemitted X-ray penetration electrons depth ~1mm. Electrons can be ~10 nm •Typically the derivative spectrum is excited in this entire volume. ~1 mm2 used to quantify peak intensities •Derivative peaks can vary greatly depending on the broadness of the signal peak.

8/18/2010 11 8/18/2010 SSI system: X-ray spot 150 to 1000 microns 12 Inelastic Mean Free Path (IMFP or ) Inelastic Mean Free •IMFP is the average distance an electron travels before it undergoes Path (IMFP or ) for: an inelastic collision (and therefore loses energy and can become part of the XPS background) •Electron elastic scattering is neglected •IMFP depends on: Nickel •The kinetic energy of the electron •The material in which it is traveling •Similar to, but not to be confused with Effective Attenuation Length (EAL) which tries to account for elastic scattering effects •IMFP is usually denoted by  •IMFP generally larger for softer materials like polymers (up to 10 nm) •IMFP for metals typically 1-3 nm Sources for IMFP: Polymer .Quases-IMFP-TPP2M software (10.6MB) free download at www.quases.com .NIST program IMFPWIN (can obtain copy from me) .Online IMFP Grapher at www.lasurface.com

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Mean Escape Depth (MED) Information Depth (ID) or Analysis Depth •Mean escape depth is defined as the average depth with respect to the surface normal, from which electrons escape •Information depth can be identified as the sample thickness •MED =  cos() where  is the angle with respect to the surface normal from which a specified percentage (95% or 99%, e.g.) of the •At high angles, elastic scattering of electrons may be significant detected signal originates •The 95% ID corresponds to 3 if elastic scattering effects are neglected  =75°  = •Practical information depth is 3cos() 0° Less Surface More Surface Sensitive, Sensitive greater electron  in the SSI system is 55°, unless using an angled stage. escape depth cos(55)= 0.573

CCMR system has detector 55° from surface-normal of a horizontal sample Tilt stages are used for Angle-resolved analyses cos(55) = 0.573 8/18/2010 15 8/18/2010 16

Samples Introduction to X-ray Photoelectron Spectroscopy •Ideal sample: (XPS) •UHV compatible, nothing with high vapor pressure •Very clean, will discuss sample handling •Conductive, metals or metal thin films on conducting • Sources of Information substrate • Principles of XPS and Auger •Flat, polished surface (deposited on silicon substrate, • How to prepare samples for XPS e.g.) • Instrumentation, X rays, Photoelectron detection •About 1cm x 1cm square or larger • Data acquisition •Things to consider: – Quantitative and Qualitative analyses •Do you need the sample back? – Spin-orbit splitting, Plasmons, Shake-up, etc. •Can it be broken or modified for mounting? – Sample charge control •Maximum sample size ~100 mm wide and ~50 mm tall – Overlayer effects – Ion sputtering

8/18/2010 17 8/18/2010 18 Insulating Samples: Sample Charging Insulating Samples: Charge Neutralization

- Ejected Photoelectron Incident X-ray

 Photoemission of electrons leaves the sample with a net positive charge  The positive charge makes it more difficult for electrons to escape the surface  This results in lower kinetic- energy photoelectrons and shifts peaks to higher binding energies.  Non-uniform charging of the surface can lead to peak Grid aids in keeping electric broadening field uniform +

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Types of Surfaces Surface Contact

Contamination layer Ideal Surface

Surface Deposited • Use non-magnetic, ultra- Microstructure Thin Film clean tweezers to handle the sample • Try not to touch the surface to be analyzed Laterally •Any dust generated can end Inhomogeneous- Rough Surface- up on the sample surface Emitted intensity May get shadowing after going into vacuum May vary with effects Orientation 8/18/2010 21 8/18/2010 22

Use of Gloves Aluminum Foil UHV oil-free Aluminum foil

• Plastic ziploc bags and Aluminum foil often has an oil film on it to prevent Reynolds Aluminum foil 24oct06b_2.dat Data Set 2 d sticking Total Acquisition Time 17.067 (mins) (1000.0 (ms) x 1 x 1024) Source: Al 3 •If you must handle the x 10

40 sample directly, use of Name Pos. FWHM Area At%

Ca 2p 345.36 5.754 3965.5 0.497 C 1s C 1s 282.64 3.377 119061.1 75.683 silicone-based, powder- 35 O 1s 530.58 3.746 77774.8 16.873 Si 2p 99.38 3.170 3300.3 2.568 Al 2p 70.96 3.682 3699.3 4.379 free gloves is 30 recommended In Reynolds wrap: 25 • Aluminum signal is much lower due to a O 1s

thicker hydrocarbon layer CPS 20 •Silicon peaks could be due to silicone- based mineral oil 15

• Background at high BE indicates 10 Ca 2p presence of overlayer

5 Si 2p 8/18/2010 23 8/18/2010 24 Al 2p 1000 800 600 400 200 0 Binding Energy (eV) CasaXPS (T his string can be edited in CasaXPS.DEF/PrintFootNote.txt) Sample Handling Sample Handling

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Sample Drop-off Introduction to

.Drop off samples X-ray Photoelectron .My office, Clark D21 .In the dessicator outside of D21B. Fill out a drop-off Spectroscopy (XPS) sheet AND email me to let me know it is there. You can your ID entered into the D21 door lock. .Better if you know what scan regions you need, • Sources of Information should talk with me if you don’t. • Principles of XPS and Auger .View system schedule online • How to prepare samples for XPS .CCMR Coral • Instrumentation, X rays, Photoelectron .Surface Analysis, XPS detection .Do NOT schedule time for yourself • Data acquisition .This is only a tentative schedule and may be offset – Quantitative and Qualitative analyses due to longer runs, system breakdowns, maintenance, – Spin-orbit splitting, Plasmons, Shake-up, etc. etc. I will try to update often. – Sample charge control – Overlayer effects – Ion sputtering

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Instrumentation for XPS Instrumentation for XPS

Surface analysis by XPS requires irradiating a solid in an Ultra-high Vacuum (UHV) chamber with monoenergetic soft X- rays and analyzing the energies of the emitted electrons.

8/18/2010 29 8/18/2010 30 Why UHV for Surface Analysis? Schematic of SSI system .Electron gun (10 keV Analyzer .Anode (aluminum) produces characteristic x-rays Pressure .Crystal Monochromator Degree of Vacuum Torr focuses x-rays and reduces 102 x-ray energy width Low Vacuum .Sample must be at focus of 10-1  Remove adsorbed gases from both the monochromator and Medium Vacuum the sample. collection lens 10-4  Eliminate adsorption of .Collection lens collects High Vacuum contaminants on the sample. photoelectrons -8 .Detector measures 10  Prevent arcing and high Ultra-High Vacuum voltage breakdown. incidence of photoelectrons 10-11  Increase the mean free path for .Some systems scan the electrons, ions and photons. anode to scan the sample. This system moves the

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SSI system Anode: X-ray Source

.X-rays are produced by hitting a metal anode with high-energy electrons (5-15keV) .>99.9% of this energy is dissipated as heat, therefore anode cooling is critical

.AlKa x rays have an overall line width of ~0.85eV

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Anode: X-ray Source Non-Monochromated vs. Monochromated X-rays

.Mg, Al, and Cu are . Non-monochromated x- . Monochromators common XPS rays contain typical cut the anodes Bremsstrahlung radiation characteristic x-ray line .Mg has a lower x- .Peak width ~0.85 eV to ~0.3 eV ray output than Al .X-rays scatter throughout .Focus beam onto .Al Ka x-rays can chamber, creating sample probe to larger BE’s photoelectrons on all .Insulating samples than Mg surfaces. These require an electron E(Al-Mg) = 233 eV photoelectrons help to flood gun to neutralize neutralize insulating charge build-up samples .Greater sample heating may occur 8/18/2010 35 8/18/2010 36 Monochromated vs. non-Monochromated X-rays XPS Analyzers

Non-monochromated Bremsstrahlung radiation creates a higher background

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Photoelectron Detection Hemispherical Analyzer

•Analyzer resolution is typically 1% of the pass energy •In order to get 0.3eV resolution, need a pass energy of 30 eV •SSI uses fixed pass energy. A retarding lens at the input of the detector enables this. •Some other systems vary the pass energy.

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SSI Analyzer Resolution Analyzer Transmission Calibration

SSI SSI instrument is Resolution# only calibrated for 1 2 High Res pass energy 150eV 3 4 Low Res (Resolution 4)

8/18/2010 41 8/18/2010 42 Peak Widths Pass Energy Calibration Photon width typically ~0.3 eV for monochromated x-rays

Binding Energy (eV)

•SSI system uses Au x-ray type Al Ka Mg Ka Monochrom 4f7/2 peak and the Cu Natural peak width -ated Al Ka 2p3/2 peak to calibrate Peak the energy scale Analyzer/detector width Au 4f 7/2 83.95 83.95 83.96(84) •Calibration will be (0.25 to 1.5 eV) performed bi-weekly •Typical drift between calibrations is <~0.1eV Cu 2p 3/2 932.63 932.62 932.62 .At high resolution (i.e. low pass energy), Ea is typically the smallest value

.Ep typically ~0.3 eV for monochromated x-rays Ag3d 5/2 ------368.21 .Possible to calculate/estimate x-ray decay lifetime from En if this is the largest contribution to peakwidth

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SSI Instrument Parameters SSI Instrument Parameters

.Manufacturer: Surface Science Instruments (SSI) . Spot Sizes: 150, 300, 600, 1000 microns .Model: X-Probe (SSX-100) . Energy Resolution (Au4f) 0.8 to 1.8 eV .X-ray kV and mA emission: 10kV, 1.5 to 22.5 mA (spot-size dependent) . Pass Energies: .X-ray Energy: 1486.6 eV (8.3393 Å) .150 V for Resolution 4 setting (must use for calibrated analysis) .Analyzer Type: 180-degree hemispherical .100 V for Resolution 3 setting .Binding energy range: -50 to 1100 eV . 50 V for Resolution 2 setting (most common for high-resolution analysis) .System Base Pressure: < 10-9 Torr . 25 V for Resolution 1 setting .Normal Operating Pressure: 1.6 x 10-9 Torr . Detector type: SSI Position Sensitive Detector, resistive anode, .Angle of X-ray incidence: a = 71° (relative to sample normal) 40mm x 40mm, electronically defined as 128 active channels with maximum .Standard Electron emission angle:  = 55° (relative to sample normal) count rate of 1,000,000 .Angle between X-ray and Analyzer axes  = 71 ° (fixed, non-variable) Typical Information given: Sample was analyzed using a … Surface Science Instruments SSX-100 … with operating pressure < 2x10-9 Torr … monochromatic AlKa x rays at 1486.6 eV. … Photoelectrons were collected at an angle of 55-degrees from the surface normal … hemispherical analyzer with pass energy of … 8/18/2010 45 8/18/2010 46

Basis for User Fees Academic machine time currently $30/hour Technician time $85/hour Introduction to External Labs Cornell-subsidized academic rates •A new system can •Typical sample costs: X-ray Photoelectron easily cost $500k • ~0.5 to 2 hours per analysis spot, depends on: ($25k/year depreciated •surface cleanliness/roughness •count rates over 20 years) • 1/2 hour tech time ($85/hr) + 1 hour machine time, Spectroscopy (XPS) •Can charge >$300/hr includes: machine time •Sample prep and setup • Sources of Information •Can charge for all time •Pump-down time that your sample is in the •Assistance with data analysis, etc. • Principles of XPS and Auger system, including pump- • Requires over $30k/year to run this system in • How to prepare samples for XPS down time •Maintenance • Instrumentation, X rays, Photoelectron detection •You work on their •Upgrades • Data acquisition •Personnel costs schedule. Can take •User fees and usage by appropriate groups justifies – Quantitative and Qualitative analyses weeks to get your data having/keeping the system. Rates can be adjusted at any – Spin-orbit splitting, Plasmons, Shake-up, or sample back. time etc. – Sample charge control – Overlayer effects – Ion sputtering 8/18/2010 47 8/18/2010 48 Relative Sensitivity Factor (RSF) Quantitative Analysis

•Typically use the peak with Ipeak a nelectrons (E) scofieldTdetector(E) largest RSF value in more simply calculations

Ipeak a natoms RSFScofield •Can use survey scan data or peak scan data to •Ipeak is also referred to as a relative sensitivity factor (RSF) or atomic calculate atomic%, if taken sensitivity factor(ASF) at Resolution4 •nelectrons is the electron population •If doublet peaks are close •scofield is the Scofield cross-section together, use combined •Tdetector(E) is the transmission function of the detector at peak RSF values. energy E •RSF •RSF values are somewhat well- • Au 4f7/2 = 9.58 defined and vary between • Au 4f5/2 = 7.54 instrument brands •Au 4f = 17.12 = sum of both Au 4f peaks 8/18/2010 49 8/18/2010 50

Detection Limits Background Subtraction Accurate • Typical detection limit is 0.1 to 1 atomic % • Background is produced by inelastic scattering of photoelectrons • Factors that affect the detection limit: •RSF •Signal-to-noise ratio in the spectrum •Time to acquire data Precise •Energy resolution of the analyzer •Peak overlap issues •Light elements may have fewer peaks •Interference from other XPS and Auger peaks •Overlayer effecting the information depth. Accurate •Background choice & precise

•Quantification accuracy is about 10-20%, so a 50-50 alloy may be seen as a 50% (+/- 5 to 10%) •Shirley background assumes the background is proportional to the # of •Detection precision is excellent and generally very repeatable electrons with kinetic energies higher than the peak energy. Generally better than a straight line for metal peaks. •Linear background is determined by the endpoints. Generally works well for polymers and insulators 8/18/2010 51 8/18/2010 52

Tougaard Background Subtraction Artifacts

Tougaard utilizes whole survey spectrum to create background •X-ray damage. A sample that undergoes a material change due to x- ray exposure or heat.

•Ghost peaks. •Impurity elements in the x-ray source •A dual anode source can have up to 2% cross-talk (2% of x-rays coming from the other anode)

•Charging of poorly conducting samples •Charging may vary over time or a sample may take a long time to equilibrate •Causes a shift in measured peak energies •Peak broadening •If only parts of a sample charge, peaks from those areas will shift

8/18/2010 53 8/18/2010 54 Sample Damage Due to Irradiation Spin-Orbit Splitting

• Can perform multiple scans of the sample over time to check for degradation or damage

•Peak doublets can make analysis trickier due to: Gold 4f5/2 and •Making background choice 4f7/2 peaks more difficult •Greater likelihood of interference with other peaks or artifacts

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Overlayer Effects Overlayer Thickness

• t is overlayer thickness •  is the electron attenuation length in the film • Io and Is are peak intensities from film and Hill, JM et. al., Chem. Phys. Lett. 1976; 44: 225 substrate, respectively

• so and ss are film/substrate sensitivity factors

? • Can measure layers covering film or substrate, including organic layers a) Copper thin film on gold ? • Instrument independent b) Heterogeneous structure • Works for large and small film thicknesses c) Buried thick copper layer ? between gold ? • Assumes Io and Is originate from similar d) Copper substrate beneath gold photoelectron energies • Thickogram studies account for differing photoelectron energies

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Thickogram Surface Plasmon Effects

• Calculate intensity ratios (A) •Photoemitted electrons can interact with surface plasmons and generate • Calculate KE ratios (B) resonance at integer multiples of the plasmon frequency A • Plot and draw a line to calculate •This interaction reduces the primary peak intensity and is distributed to the C thickness (C) plasmon peaks • dotted line denotes thicknesses that are difficult to measure in practice •Seen typically in metals or materials with free electrons (low-signal to noise)

B • Eo and Es are KE (not BE) of film and substrate peaks

• Applicable to a wide range of KE above ~500eV • Applicable for emission angles up to ~60° from surface normal • 45° emission angles minimize errors due to elastic scattering and surface roughness

8/18/2010 59 8/18/2010 60 Chemical Shifts Chemical Shifts- Electronegativity Effects

• There is a redistribution of charge of the outer electrons when a chemical bond is formed •This results in a shift in binding energies of core electrons •Consult Briggs/Grant textbook for more info • Chemical shift (eV) is calculating/estimating shifts proportional to the summation of nearest neighbor interactions •Many chemical shifts listed in • There are several the XPS handbook electronegativity scales (Pauling, e.g.) • Double-bonds have twice contribution of single bonds 8/18/2010 61 8/18/2010 62

Shake-up Sputter/ion Cleaning •Utilize as a last resort or necessity Shake up generally due to electron •Sputtering a sample surface can remove impurities interaction with ringed-Carbon •Depth profiling can be very informative and can produce A LOT structures of data •Depth profiling can cause •Sample damage •Surface roughening due to varying sputtering rates of elements •Implantation of sputtering gas

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