Surface & Coatings Technology 201 (2007) 5512 – 5517 www.elsevier.com/locate/surfcoat
Physico-chemical properties of plasma-polymerized tetravinylsilane V. Cech a,⁎, J. Studynka a , N. Conte b , V. Perina c a
Institute of Materials Chemistry, Brno University of Technology Purkynova 118, CZ-612 00 Brno, Czech Republic b CSM-Instruments, Rue de la Gare 4, CH-2034 Peseux, Switzerland c Nuclear Physics Institute, Academy of Sciences, CZ-250 68 Rez near Prague, Czech Republic Available online 30 August 2006
Abstract Plasma polymer films of tetravinylsilane were deposited on silicon wafers using an RF glow discharge operated in pulsed mode. Microscopic and spectroscopic techniques revealed that physico-chemical properties of plasma polymer depend on the effective power used if the flow rate was constant. The deposition rate (8–165 nm min− 1) and surface roughness (2.0–5.8 nm) of films varied with the RF power. An organic/inorganic character (C/Si ratio) of films and a content of vinyl groups could be controlled by the effective power. The refractive index of films increased with power from 1.63 (0.05 W) to 1.75 (10 W) at a wavelength of 633 nm and the extinction coefficient rose sharply with the power from 0.09 (0.05 W) to 0.37 (10 W) at a wavelength of 240 nm. The elastic modulus and hardness of plasma polymer could be varied from 11 GPa (0.05 W) to 30 GPa (10 W) and from 1.4 to 5.9 GPa, respectively. © 2006 Elsevier B.V. All rights reserved. PACS: 68.37.Ps; 78.30.-j; 81.15.Gh; 82.80.Yc Keywords: [B] Nano-indentation; Atomic force microscopy (AFM); Ellipsometry; Infrared spectroscopy; Rutherford backscattering spectroscopy; [C] PE CVD
1. Introduction Plasma-Enhanced Chemical Vapor Deposition (PE CVD) is a suitable technique for preparation of thin films on a basis of organosilicones [1,2]. The technique enables reproducible deposition of plasma polymer films with desired physicochemical properties [3] by changing the deposition conditions (power, monomer flowrate, pressure). Such a film of controllable organic/inorganic character could be used as a compatible interlayer for multicomponent materials such as nanocomposites and fiber-reinforced polymer composites. The thin film (interlayer) has to possess functional groups for chemical bonding to both the materials, e.g. an inorganic reinforcement and polymer matrix, and suitable elastic modulus. We found out [4] that the elastic modulus of plasma-polymerized (pp-) VTES films can be controlled by RF power using vinyltriethoxysilane (VTES) as a monomer. The plasma polymer exhibited good adhesion to the glass substrate through siloxane bonding. However, the double bond in vinyl group was split during the plasma process even if a very low power was used and thus plasma polymer was deficient
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of vinyl species, i.e. the functional group intended for adhesion bonding with polyester resin (polymer matrix). Tetravinylsilane (TVS) was used as a monomer for deposition of pp-TVS films for the first time. The helical coupling system [5] was applied to deposit single films using an RF glow discharge operated in pulsed mode. Conditions for a stable discharge and an influence of the deposition conditions on the vinyl content in pp-TVS films were observed in our study. Deposited films were analyzed extensively using microscopic and spectroscopic techniques in order to characterize surface morphology, elemental composition and chemical structure of plasma polymer. Ellipsometric measurements were carried out and evaluated data gave information on the film thickness, deposition rate, and optical constants corresponding to the film. Selected mechanical properties of single films were determined from nanoindentation measurements. 2. Experimental details Plasma-polymerized tetravinylsilane (pp-TVS) films were prepared by PE CVD employing an RF helical coupling system [5], operated at pulsed regime, on IR transparent silicon wafers (0.8 × 10 × 10 mm3; Terosil Co. Czech). Tetravinylsilane, Si– (CH_CH2)4 (TVS, purity 97%, Sigma Aldrich), was used as the
V. Cech et al. / Surface & Coatings Technology 201 (2007) 5512–5517 Table 1 Deposition conditions tested for preparation of pp-TVS films using a stable plasma Frequency
13.56 MHz
ton, toff Period Duty cycle Effective power Effective power density Basic pressure Process gas pressure Monomer vapor flow rate
1 ms, 1–999 ms 2–1000 ms 0.1–50% 0.05–10 W 8 × 10− 4–1 × 10− 1 Wcm− 3 2 × 10− 3 Pa 0.1–4.4 Pa 0.06–1.2 sccm
monomer. The effective power (Peff) of pulsed plasma was controlled by changing the ratio of the time when plasma was switched on (ton) to the time when plasma was switched off (toff), Peff = ton/T × Ptotal, where the period was defined as T = ton + toff, Ptotal = 50 W, and the duty cycle, DC, was defined as DC = ton/ T × 100%. The silicon wafer was pretreated by O2 plasma (5 sccm, 4 Pa, 25 W) for 10 min in order to improve the film adhesion and then the wafer was stored in a load lock. Employing a mechanical manipulator, the pretreated wafer was placed into the plasma zone after plasma reached the steady state, which was monitored by mass spectroscopy. Pulsed plasma was operated at conditions given in Table 1. The monomer flow rate was set for 0.06 sccm and increased by about one tenth up to 1.2 sccm. The stability of glow discharge was observed by phototransistor, which was sensitive for a region of visible wavelengths (300 nm–850 nm). The output voltage of the transistor was dependent on the intensity of radiation from the whole visible region. We observed an increase of the voltage measured by oscilloscope if the radiation of glow discharge increased. Thus it was possible to check optical stability of the discharge. Neutral plasma species drawn out from the glow discharge were investigated by a quadrupole mass spectrometer HAL 500 by Hidden Analytical. Measurements were made with a step of 0.01 m/z. An atomic force microscope ACCUREX II L by TopoMetrix was used to characterize surface morphology of deposited films. RMS roughness was computed from an area of 5 μm × 5 μm. A phase-modulated spectroscopic ellipsometer UVISEL (Jobin–Yvon) was employed to determine the film thickness and optical properties of pp-TVS films. Range of measurements was 300–830 nm with a step of 3 nm, angle of incidence was 70°, spot size 100 μm × 300 μm, and integration time was set for 200 ms. A single layer model usingclassical oscillator “ dispersion for the polymer film was employed to fit recorded data. The dispersion and the thickness of the film were simultaneously fitted. The elemental composition of thin films was studied by conventional and resonant Rutherford Backscattering Spectrometry (RBS) and Elastic Recoil Detection Analysis (ERDA) methods using Van de Graaf generator with a linear electrostatic accelerator. The RBS spectra were evaluated by computer code GISA 3 [6] and the ERDA ones by SIMNRA code [7] both using cross-section values from SigmaBase. A systematic error of evaluated concentrations was up to 5 at.%.
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Chemical structure of plasma polymer was investigated using a NICOLET IMPACT 400 Fourier transform infrared (FTIR) spectrophotometer. Measuring range was from 400 to 4000 cm− 1 with a resolution of 0.96 cm− 1. We employed a Nano Hardness Tester (NHT) by CMS Instruments in order to carry out nanoindentation tests. The Young's modulus and hardness of films were determined from unload-displacement curves measured at 12% of the film thickness. The loading/unloading mode was linear, the approach speed was 750 nm min− 1, the loading and unloading rate was the same and set for 0.3 mN/min. The indenter type was a diamond tip of Berkovich (B-H22). 3. Results and discussion 3.1. Stability of RF glow discharge We investigated stability of glow discharge with respect to the deposition conditions given in Table 1 at the beginning of our study. Both time parameters ton and toff were set for 1 ms
Fig. 1. Optical response of discharge to the supplied power for different pulsed regimes. Time dependence of excitation pulses (above) and corresponding optical response of discharge (below) are given for ton:toff = 1 ms:1 ms (a) and 1 ms:4 ms (b).
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conditions were used to deposit a set of plasma polymer films, which were analyzed with respect to their physico-chemical properties. 3.2. Mass spectroscopy Mass spectroscopy was employed for a cleaning procedure of deposition chamber in order to check residual gases and reach the basic pressure. An investigation of neutral plasma species drawn out from the glow discharge was another task of the technique. Fragmentation of TVS molecule into smaller species in plasma zone is the first step of deposition process. Mass spectra measured for pulsed plasma burning at different effective power and compared with that without discharge are shown in Fig. 2(a). The normalized band area for selected species was plotted as a function of the mass for different power in Fig. 2(b). An assignment of measured peaks was summarized in Table 2. More and more vinyl groups were split from the central silicon atom of TVS molecule, while a number of vinyl derivatives (m/z = 25–27) and hydrogen molecules increased, when the effective power was enlarged. However, pulsed plasma contained a certain number of vinyl groups bonded to silicon atom even if the effective power was set for 10 W, see Fig. 2. 3.3. Ellipsometry
Fig. 2. (a) Comparison among mass spectra corresponding to pulsed plasma of tetravinylsilane operated at different power and the spectrum without discharge, (b) band area for selected species as a function of effective power.
and toff was prolonged step by step up to 999 ms, while changing the flow rate and the corresponding process pressure. The phototransistor was used in order to observe the optical response of discharge to the supplied power. The case of ton: toff = 1 ms:1 ms is depicted on the left in Fig. 1. The upper graph shows a time dependence of incoming power pulses and the bottom one represents an optical response of plasma. It was evident that the plasma burning did not correspond to every excitation pulse but every other excitation pulse did not cause ignition of plasma. A deficiency in monomer gas in plasma chamber seemed to be the reason of this phenomenon. The monomer was burned up during excitation pulse, the process pressure decreased, and time of 1 ms (toff) was insufficient to fill up the chamber by new monomer. The disturbance was eliminated when toff was prolonged up to 4 ms and then the plasma burned with every excitation pulse as shown on right in Fig. 1. Thus the maximum power of 10 W could be used. We found out that a flow rate of 0.45 sccm (process pressure 1.5 Pa) was suitable for a stable glow discharge burning at a wide range of the effective power, i.e. 0.05–10 W. The above deposition
Ellipsometric measurements were performed in order to determine the film thickness and optical constants, i.e. the refractive index and extinction coefficient, of pp-TVS films. The mean deposition rate was calculated from the film thickness and deposition time. A power dependence of the deposition rate was plotted in Fig. 3 for ton set for 1 ms and toff varied from 999 ms (0.05 W) to 4 ms (10 W). The mean deposition rate increased from 8 nm min− 1 (0.05 W) up to a sharp maximum at 165 nm min− 1 (2.5 W, ton:toff = 1:19), which was followed by a descent down to 86 nm min− 1 (10 W). Dispersions of the refractive index and extinction coefficient for pp-TVS films deposited at different effective power are given in Fig. 4. The results correspond to pp-TVS films with a thickness about 400 nm deposited at the same monomer flowrate of 0.45 sccm but different power. The value of refractive index at the wavelength of 633 nm increased with power from 1.63 (0.05 W) to 1.75 (10 W). There is a growth of
Table 2 Plasma species assigned to mass spectra m/z
Plasma species
2 24 25 26 27 28 40 53–58 80–84 106–110
H2 C_C –C_CH HC_CH H2C_CH H2C_CH2 Si–C Si–HC_CH2 and its derivatives Si–(HC_CH2)2 and its derivatives Si–(HC_CH2)3 and its derivatives
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Fig. 3. Mean deposition rate as a function of effective power for pp-TVS film.
the refractive index and extinction coefficient in the ultraviolet region. A different shape of dispersion in ultraviolet region for the refractive index corresponding to a film deposited at higher power (10 W) may be caused by higher absorption in UV region. The extinction coefficient rose sharply with the power from 0.09 (0.05 W) to 0.37 (10 W) at the wavelength of 240 nm.
Fig. 5. Surface morphology of pp-TVS films deposited at different effective power, (a) 0.1 W, 1:499, (b) 10 W, 1:4.
3.4. Atomic force microscopy The surface morphology of pp-TVS films was determined by AFM. Microphotos in Fig. 5 show a difference in surface morphology between the films deposited at lower (0.1 W) and higher (10 W) power. Surface of films was relatively flat but the RMS roughness varied with power from 2.0 nm (0.1 W) to 5.8 nm (10 W), see Fig. 6. There is a break in the plotted
Fig. 4. Dispersions of refractive index (a) and extinction coefficient (b) for ppTVS films deposited at different power.
Fig. 6. RMS roughness of pp-TVS films dependent on effective power used for film deposition.
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V. Cech et al. / Surface & Coatings Technology 201 (2007) 5512–5517 Table 3 Assignment of IR absorption bands Adsorption band
Wavenumber [cm− 1]
Assignment
A B C D E F G H I J K L M
3650–3200 3000–2800 2122 1714 1591 1461 1412 1255 1100–1000 1015 959 845 732
O–H stretching CH2, CH3 stretching Si–H stretching C_O stretching C_C stretching in vinyl CH2 scissoring CH2 deformation in vinyl CH2 wagging in Si–CH2–R Si–O–C stretching _CH wagging in vinyl _CH2 wagging in vinyl Si–H bending Si–C stretching
Fig. 7. Elemental composition of pp-TVS films deposited at different power was evaluated from RBS and ERDA spectra.
dependence at a power of 2.5 W, which corresponds to the maximum of deposition rate. Therefore, an abrupt increase in surface roughness could be related to a rise in ablation process [8] that caused a descent of deposition rate (Fig. 3). 3.5. Elemental composition and chemical structure of films The pp-TVS films of a film thickness about 1 μm were used to characterize chemical properties of the plasma polymer. Elemental composition of films was determined from RBS and ERDA spectra, see Section 2. A dependence of atomic concentrations on the effective power is given in Fig. 7. The concentration of hydrogen atoms was about 56 at.% and did not vary with the effective power. The same trend or a slight decrease was observed for the concentration of silicon atoms in plasma polymer deposited at enhanced power. A mean concentration of silicon atoms was about 5 at.%. However, the oxygen concentration decreased rapidly at the expense of carbon atoms with increased power. The concentration of oxygen atoms descended from 13 at.% (0.05 W) to 1 at.% (10 W), while carbon concentration rose from 27 at.% (0.05 W)
Fig. 8. Infrared spectra of pp-TVS films prepared at different effective power.
to 42 at.% (10 W). The C/Si ratio indicated great changes in organic/inorganic character of plasma polymer with enhanced power (Fig. 7). The film deposited at a power of 10 W was hydrogenated amorphous carbon containing a small amount of silicon-carbide phase. TVS molecule does not contain any oxygen and thus, the oxygen atoms incorporated in plasma polymer could have their origin in pretreatment procedure using O2 plasma. Most likely, the residual oxygen was desorbed from the wall of plasma chamber during the deposition process and was incorporated in plasma polymer. We estimated the flow rate of desorbed oxygen at b 0.05 sccm from the pressure increase in separate deposition chamber. The results on elemental composition revealed that the residual oxygen was effectively built in plasma polymer only if a lower power (b 5 W) was used. The lower power the higher oxygen concentration in pp-TVS film. Typical infrared spectra of pp-TVS films deposited at different power are given in Fig. 8. The assignment of IR absorption bands using Refs. [9,10] was summarized in Table 3. One of the dominant peaks, the absorption band B, was assigned to CH2, CH3 stretching vibrations. However, both the species cannot be distinguished with respect to symmetric character of the band B. The intensity and area of absorption bands A, D, and I, corresponding to species such as OH, C_O, and Si–O–C, respectively, descended with increased power and
Fig. 9. Power dependence of mechanical properties for pp-TVS films.
V. Cech et al. / Surface & Coatings Technology 201 (2007) 5512–5517
the trend was in good agreement with a descent of oxygen concentration in plasma polymer revealed by RBS measurements. A decrease of bands C, H, L, and M assigned to species containing silicon atoms was observed as well and the trend corresponded to an increase of C/Si ratio and a decrease of silicon concentration with enhanced power. An occurrence of vinyl groups in plasma polymer was evident from IR spectra corresponding to pp-TVS films deposited at lower power (≤2.5 W). Quantity of vinyl groups decreased with enhanced power as a descent of bands E, J, and especially G, K indicated. 3.6. Nanoindentation Load-displacement curves were measured at a contact depth of 50 nm for all the films with the film thickness about 400 nm. Five indents were produced on each sample with spacing between the indentations of 5 μm. Nanoindentation measurements were evaluated by Oliver and Pharr method [11] in order to determine the Young's modulus and hardness of pp-TVS films. The Poisson's ratio used for plasma polymer was 0.3. The mean modulus and hardness were calculated from five values for each film. The results on mechanical properties of pp-TVS films are summarized in a form of power dependences in Fig. 9. The modulus increased almost 3 times and the hardness increased almost 4 times with enhanced power. The increase of modulus could be related to a higher crosslinking and/or an alteration of chemical structure with increasing organic character of plasma polymer, when the effective power was enhanced. 4. Conclusions Plasma polymer films of tetravinylsilane were deposited on silicon wafers by plasma-enhanced chemical vapor deposition. The plasma was monitored by mass spectroscopy and photodiode with respect to the duty cycle (0.1–50%) and period (2–1000 ms) of pulsed regime, monomer flow rate (0.06–1.2 sccm), pressure (0.1–4.4 Pa), and effective power (0.05–10 W). Pp-TVS films deposited at a flow rate of 0.45 sccm (process pressure 1.5 Pa), and an effective power of 0.05–10 W were analyzed by microscopic and spectroscopic techniques extensively to evaluate surface morphology (AFM), elemental composition and chemical structure (RBS, ERDA, FTIR). A
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phase-modulated spectroscopic ellipsometer was employed to determine the film thickness (mean deposition rate) and optical constants (refractive index, extinction coefficient) of deposited films. The refractive index increased by 7.4% at the wavelength of 633 nm when the power was enhanced from 0.05 to 10 W. The extinction coefficient in ultraviolet region (240 nm) rose sharply by 4 times with enhanced power (0.05–10 W). The results revealed that an organic/inorganic character (C/Si ratio) of films and a content of vinyl groups could be controlled by the effective power. Nanoindentation measurements enabled us to evaluate the elastic modulus and hardness of films prepared at different power. The elastic modulus and hardness could be varied from 11 GPa (0.05 W) to 30 GPa (10 W) and from 1.4 to 5.9 GPa for an increased power, respectively. Acknowledgements This work was supported in part by the Czech Ministry of Education (contracts COST 527.110 and 1P05OC087) and the Czech Science Foundation (contract 104/03/0236). References [1] A.M. Wrobel, M.R. Wertheimer, in: R. d'Agostino (Ed.), Plasma Deposition, Treatment, and Etching of Polymers, Academic Press, New York, 1990, p. 163. [2] Y. Segui, in: R. d'Agostino, P. Favia, F. Fracassi (Eds.), Proceedings of NATO ASI Plasma Processing of Polymers, Acquafredda di Maratea, Kluwer Academic Publ., 1997, p. 305. [3] V. Cech, J. Vanek, J. Zemek, L. Zajickova, R. Prikryl, Variety of functional interlayers utilizable for polymer composites, Proc. 11th Int. Conf. on Composites/NanoEngineering, August 8–14, 2004, Marriott Hilton-Head Beach and Golf Resort, South Carolina, USA, pp. 2. [4] V. Cech, J. Vanek, A.A. Goruppa, F.R. Jones, J. Mater. Sci. 40 (2005) 5099. [5] V. Cech, R. Prikryl, R. Balkova, J. Vanek, A. Grycova, J. Adhes. Sci. Technol. 17 (2003) 1299. [6] J. Saarilahti, E. Rauhala, Nucl. Instrum. Methods Phys. Res. B64 (734) (1992). [7] M. Mayer, SIMNRA User's Guide, Forschungscentrum JulichInst. fur Plasmaphysik, 1998. [8] H. Yasuda, Plasma Polymerization, Academic Press, Orlando, 1985. [9] D. Lin-Vein, N.B. Colthup, W.G. Fateley, J.G. Grasselli, The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules, Academic Press, San Diego, 1991. [10] V.P. Tolstoy, I.V. Chernyshova, V.A. Skryshevsky, Handbook of Infrared Spectroscopy of Ultrathin Films, John Wiley and Sons, New Jersey, 2003. [11] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564.
