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Home , Plasma polymerization

diagnostic and analysis for the by low inductively-coupled plasma

Z. LI , X. GILLON, M. DIALLO, L. HOUSSIAU, J. -J. PIREAUX

University of Namur, FUNDP. Centre de Recherche en Physique de la Matière et du Rayonnement (PMR-LISE) Rue de Bruxelles, 61, B-5000 Namur, Belgium

Abstract : Optical emission spectroscopy has been employed to investigate the species formed in the during styrene plasma polymerization, while Fourier Transform infrared spectroscopy was used to characterize the deposited materials. The optical emission spectrum shows the presence of CH and other species in the plasma and the IR spectrum evidences the aromatic and aliphatic CH bands of the deposited . The information from the two spectroscopies is used to investigate the process of pulsed plasma polymerization.

Keyword : plasma polymerization, , optical emission spectroscopy, Fourier Transform infrared spectroscopy

1 Introduction Hereafter, we present our experimental setup and analyse the optical emission spectra in the plasma and the Pulsed plasma polymerization processes at low infrared spectra of the deposited polymer surface. temperature are of great interest for many applications, but highly depend on the discharge parameters. The 2 Experimental apparatus desired surface composition and structure of a plasma 2.1 Plasma reactor deposited polymer requires the control of the flow rate, the duty-cycle and the discharge power among The plasma reactor consists of three main parts: the other variable parameters such as the geometry of the planar source (ICP-P 200, JE PlasmaConsult GmbH, German), the gas injection system and the vacuum system, the reactivity of the starting monomer, the chamber (Fig. 1) frequency of the excitation signal and the temperature of the substrate. Therefore, pulsed plasma polymerization offers more advantages than other conventional chemical polymerization processes. Plasma is a mixture of free radicals, , , excited , metastables, energetic photons and neutral molecules. Radicals generated in a plasma can combine with other reactive species. A fundamental knowledge of the complex reaction processes in a plasma and an effective control of the types and relative ratios of free radicals and the other species are essential for a successful plasma organic synthesis. Diagnostics in a plasma deposition process are relatively difficult because practically all physical parts that are in contact with the plasma, interact with it and get coated. Therefore, it is necessary to develop improved diagnostic methods or instruments to get reliable results. In this paper, the plasma process is studied by optical emission spectroscopy (OES) in order to analyse the light Fig. 1 Schematic of the apparatus used in this work. emitted during the process. Specific surface properties of the deposited material, such as polymer chemical The ICP-P 200 generator is an inductively coupled composition, hydrophobic or hydrophilic characteristics, 13.56MHz RF plasma source with a matching unit for the roughness, and cross linking are also required. Deposited efficient production of a high density, low temperature polymer films were therefore characterized by FTIR plasma. By using a planar, multi-turn spiral coil antenna, (Fourier Ttransform infrared spectroscopy). the RF field is coupled through a dielectric window (quartz) into the plasma chamber. The source can be operated in the 0.25-100Pa pressure range, its power can be varied from a few Watts up to 1200W, and supplied in After the substrate is fixed on the sample holders placed the continuous wave mode or in a pulsed mode with a 7cm below the window of the plasma source, the system variable pulse rate and duty-cycle. is evacuated to a base pressure of 10Pa. Plasma The reactive gas is injected through a 20-cm-diameter polymerization is carried out by introducing the heated in-house made shower ring, which is comprised of a 6- styrene monomer vapour into the plasma chamber mm-diameter stainless steel pipe with 24 holes (1mm in through a leak valve and by adjusting the pressure, which diameter) drilled on the inside and regularly distributed is tuned by adjusting the monomer flow in order to keep a around the ring, thus allowing a homogeneous gas constant pressure. The polymerization is performed in a distribution in the chamber. The ring is located inside the pulsed mode with 50% duty cycle at the frequency of vessel and is in near contact with the walls. 10Hz. The plasma duration is 4 minutes at a work The plasma chamber consists of a 30-cm-diameter pressure of 25Pa. grounded stainless steel vessel connected to a pumping 3 Results and discussion system (Varian primary pump and V-300Ht turbo molecular pump). The sample holder is mounted on an Chemical bonds of the organic monomer are quickly electrically insulated manipulator connected to an broken in the plasma and reactive species are formed electrical feedthrough so that a bias voltage can be under the impact of the generated electrons and of free applied to the sample holder if necessary. The distance radicals. Light emissions in the discharge are acquired as shown in Fig. 2. between the sample holder and the quartz window of the plasma source can be adjusted from 7cm to 17cm. The 20000 pressure in the reaction chamber is measured with a Hα Hγ Baratron gauge. More details are given in [1] 16000 Hβ C H Benzene For the purpose of plasma diagnostics, the chamber is 6 6 2 2 CH:A ∆-X Π H equipped with different view-ports allowing plasma 12000 2 C :A 3Π -X 3Π analysis by means of optical emission spectroscopy ( 2 g u

3 3 + and quartz windows) and also by mass spectroscopy. 3 3 Fulcher α : H d Π -a Σ 8000 C :A Π -X Π 2 u g 2 g u 2.2 Optical spectroscopy Intensity (A.U) C The light emitted by the plasma is transmitted to a 4000 750mm focal length monochromator (Princeton 0 Instruments SP2756A, f/9.7) through an optical fibre 200 300 400 500 600 700 800 900 positioned on the window of the reaction chamber, which Wavelength (nm) probes the plasma located between the shower ring and the substrate (Fig. 1). The monochromator is equipped Fig. 2 Emission spectrum of the styrene plasma at 28Pa with 2400, 1800, and 600grooves/mm gratings that are and 180W. blazed at 240nm, 500nm, and 300nm respectively. The In spite of the large amount of spectroscopic knowledge 2D detector is a CCD one with 1234x400 pixels available for organic materials, a number of transitions (Princeton Instrument, E2V CCD36-10). This apparatus are not clearly identified and thus the exact energy values allows us to study emission lines in the 200nm to 1100nm of the excited ro-vibrational levels are not exactly known. range. The spectra are acquired by a PC loaded the Molecular species are identified by comparing observed WinSpec 3.1 acquisition software. Under standard spectral lines with known data [2][3], and atomic lines are acquisition conditions, the resolution is 0.05nm with 1200 compared to the data files from Kurucz [4]. The grooves/mm grating. The optical fibre (25-degree solid assignment of the identified molecular bands and atomic angle, 19brins of 200µm) is capable to acquire the optical lines are shown in Table 1. emission especially in the UV-VIS range. Table 1 The species identified in the styrene plasma. The chemical component and the bonding states of Species Spectral transition Theoretical styrene polymer films on the substrate were characterized system υ′→υ″ wavelength (nm) 1 1 by Fourier Transform infrared spectroscopy (FTIR, BIO- C6H6 A B2u -X A1g 260.3, 266.7, 274.1 RAD 60A). The spectral resolution is 4cm -1 and each (benzene) CH A2∆–X2Π (0,0) 431 absorption spectrum is acquired over 60 scans to improve (2,2) 432.4 3 3 the signal to noise radio. C2 A Πg–X Πu (0,0) 516.5 (1,1) 512.9 2.3 Surface treatment and plasma polymerization (2,2) 509.8 Before being placed in the plasma reactor, (6,7) 541.3 (5,4) 467.8 substrates are cleaned in two successive ultrasonic baths (4,3) 468.4 of acetone and isopropanol. The substrates are then dried (3,2) 469.8 with gas. (2,1) 471.5 (1,0) 473.7 the A 2∆-X2Π transition. As can be seen in Fig. 4 a 3Π 3Σ+ 2 2 H2 d u-a g 570-650 simulation of the CH (A ∆-X Π) transition was done by Flucher α employing LIFBASE software [6] and by supposing that H2 725.4, 752.5, 719.6 Atomic line the vibrational and rotational temperatures are the same at Hβ, Hγ ,Hα 434.5, 486.1, 656.2 3000K. This temperature may be close to the gas-kinetic H 865.5 temperature in the plasma. The H γ at 434nm appears in C p8p-p3d, 889.9 2p5s-2p3p 887.3 the molecular band, and the carbon atomic line at 432nm p6p-s2p3 432.2 yields a strong intensity meaning this line cannot be simulated accurately. Other parameters, such as It is clearly seen that the styrene plasma contains the temperature and obtained from the following species: C6H6, CH, C, C 2, H, and H 2. It is analysis of the OES, can be used to describe the difficult to identify the polyatomic bands, and properties of inductively coupled plasma. We still need to their identification is in progress. Among them, the most pursue this study. intense molecular band observed is the ro-vibration transition (0-0) at 430.6nm in the A2∆-X2Π of the CH signature, and its higher vibrational level (2, 2) at 432nm. The most intense atomic line is found as H α at 656nm. Compared with the emission of CH, the C2 emission intensity is weak. It seems that the C-C or C=C bond of the monomer is broken and forms the CH radical. Then by dehydrogenation and by recombination, the C 2 species is formed. It is evident that the H is formed by the breaking of the C-H bond (bond energy 4.28eV). Two types of C=C bond in styrene can be found, aromatic (bond energy 2.7eV) and (bond energy 6.4eV). The aromatic C=C bond in styrene monomer is more easily broken under the impact electron [5]. In our Fig. 4 Simulation for CH molecular band A2∆-X2Π at experiments, the C 6H6 and CH emissions appear always transition (0, 0) and (2, 2) for the spectrum at 180W and in the styrene plasma. 25Pa. 1000 Most requires energy greater than 10eV, 800 150W however the dissociation of a molecule requires much less 600 energy. It seems noteworthy that the dehydrogenation of 400 an organic molecule requires very little energy in 200 comparison with the ionization energy. The scission of 0 bonds occurs with a far greater frequency than the

Intensity 1000 formation of ions [5]. 180W 800 FTIR measurements were carried out to investigate the 600 chemical structure of the styrene polymer deposited on 400 the substrate. Such a spectrum of polystyrene on Si is 200 shown in Fig. 5. It is quite similar to the one observed by 0 200 300 400 500 600 700 800 900 [7]. The main absorption bands have been identified. Wavelength (nm) Fig. 3 The effect of power on OES of styrene plasma. On the other hand, the emission intensity of the different species varies with the plasma parameters. An example of this intensity variation as a function of power is given in Fig. 3. The influence of the plasma parameters will be discussed elsewhere; however, we can say that a higher power may cause more dehydrogenation and more fragmentations in the plasma. CH bands are readily produced during an electrical discharge when carbon and are present. They exhibit a rotational structure with different vibration levels, characteristic of a diatomic hydride. This is due to

0.24 and the radical recombination allows for the formation of polymer-like film. Combining OES of the gas phase, CH Aromatic mono-substitution FTIR and XPS of the plasma polymerized films, we plan 0.22 to study the influence of different plasma parameters on the fragments generated in plasma gas and on the 0.20 Methylene C-H bend composition and the structure of the deposited film.

CH aromatic 0.18 C=C aromatic Acknowledgments Absorbance CH aliphatic We thanks by the Interuniversity Attraction Poles

0.16 } programme (Belgium science policy, Contract P6-08) for } financial support.

0.14 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm -1 ) Reference

1. Haïdopoulos, M., et al. Morphology of polystyrene Fig. 5 FTIR absorbance spectrum of polystyrene on the films deposited by RF plasma. 2007, Vol. 228 Pt 2, pp. Si substrate (25Pa, 180W and deposition of 4 minutes). 227–239. The band assignments observed in the spectrum are 2. Herzberg, Gerhard. Molecular spectra and molecular summarized in Table 2. The bands near 3000cm -1 divide structure III. Electronic spectra and electronic structure of into two categories. Bands above 3000cm -1 correspond to polyatomic molecules. s.l. : D. Van Nostrand Company, absorption by aromatic CH stretching modes in the phenyl INC, 1966. -1 rings. Below 3000cm the absorption is due to CH 2 single 3. Peach, G. Unified theories of the pressure broadening bonds. The band at 1453cm -1 is a methylene CH bend. and shift of spectral lines: I general formulation for The main fingerprint of polystyrene at 702cm -1 multipole. J. Phys. B: At. Mol. Phys. 1984, Vol. 17, p. corresponds to the mono-substituted aromatic ring. 2599. -1 Another band at 611cm is due to a CH 3 vibration. As 4. Kurucz, R. L. Retzko et al found [7], the backbone of RF plasma http://www.cfa.harvard.edu/amp/ampdata/kurucz23/sekur. polymer should be cross-linked. html. Kurucz Atomic Line Database . Table 2 The observed spectral modes of the deposited 5. Yasuda, H. Plasma polymerization. s.l. : Academic styrene polymer on Si substrate. Press, Inc., 1985. ISBN 0-12-768760-2. 6. Luque, Jorge and Crosley, David R. LIFBASE: Wavenumber (cm -1) Assignment Database and spectral simulation program 3000-3120 Aromatic =C-H stretch( symmetric and asymmetric modes) 7. Retzko, I., and al. Chemical analysis of plasma- 2967 Methyl C-H stretch, asymmetric polymerized film: The application of X-ray photoelectron 2925, 2927 Methylene C-H asymmetrical stretching spectroscopy (XPS), X-ray absorption spectroscopy 1600, 1495 In-plane phenyl ring bending mode (NEXAFS) and fourier transferm infrared spectroscopy (FTIR). Journal of Electron Spectroscopy. 2001, Vol. 121, 1453 Methylene C-H bend pp. 111-129. 1491 C=C aromatic 1375 Methyl C-H bend

1029 Ring in phase CH bend

738 H2C=C=CH 2 702 Aromatic ring out of plan

611 CH 3

4 Conclusion From the optical emission spectra, we find that the species formed during the pulsed plasma are essentially composed of benzene and CH emission. The IR spectrum of the deposited material evidences the existence of aromatic and aliphatic species. With the chosen plasma parameters, the styrene molecule is broken