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tetramethysilane PECVD: From process diagnostic and modeling to a-Si:C:H hard coating composition
Diamond & Related Materials 16 (2007) 1259 – 1263 www.elsevier.com/locate/diamond
Argon/tetramethysilane PECVD: From process diagnostic and modeling to a-Si:C:H hard coating composition A. Soum-Glaude a,b,⁎, L. Thomas a,b , A. Dollet a , P. Ségur c , M.C. Bordage c a
c
PROMES-CNRS, Tecnosud, Rambla de la Thermodynamique, 66100 Perpignan, France b Université de Perpignan Via Domitia, 52 Av Paul Alduy, 66100 Perpignan, France CPAT-CNRS, Université Paul Sabatier, 118 Route de Narbonne, 31062 Toulouse cedex, France Available online 19 December 2006
Abstract Amorphous hydrogenated silicon carbide films were grown from capacitively coupled low frequency argon–tetramethylsilane plasmas. Mass spectrometry, optical emission spectroscopy and electrostatic probe were used to identify electron properties and chemical species under various deposition conditions. Film microstructure and mechanical properties were also characterized. The main goal of this analysis was to better understand the relationships between process parameters, gas-phase composition, film microstructure and tribological properties. A preliminary modeling of the plasma reactor was also carried out. First simulation results confirmed the trends from gas-phase analyses. © 2006 Elsevier B.V. All rights reserved. Keywords: PECVD; Plasma diagnosis; Plasma modeling; Si-doped DLC; Hard coatings
1. Introduction Silicon-doped diamond-like carbon (DLC) films are promising for applications requiring high hardness, low friction and wear and good thermal stability [1–4]. These amorphous films can be prepared in plasma-assisted chemical vapor deposition (PACVD) processes from various precursors [5], mainly from silane– hydrocarbons mixtures but also from organosilicon molecules containing Si–C bonds [6]. Organosilicon plasma chemistry is complex and its consequences on film growth are not yet well understood; for this reason, experimental diagnostic of organosilicon plasmas is of particular interest. Optical emission spectroscopy (OES) and electrostatic probe (EP) measurement techniques provide local information without any disturbance of the sampled region. Mass spectrometry (MS) allows identification of most chemical species produced in the plasma and reacting with the growing film. In this work, argon–tetramethylsilane (TMS) mixtures were used to deposit Si-doped DLC films (a-Si:C:H) on steel substrates. OES, MS and EP measurements were performed and compared to a first simulation of the plasma ⁎ Corresponding author. PROMES-CNRS, Tecnosud, Rambla de la Thermodynamique, 66100 Perpignan, France. Tel.: +33 468 68 22 53; fax: +33 468 68 22 13. E-mail address: [email protected] (A. Soum-Glaude). 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.11.003
reactor in order to gain insight into the deposition mechanisms. Local parameters obtained from diagnostic and simulation (chemical species concentrations, etc) can be correlated to several key features of the film such as deposition rate, composition, microstructure and mechanical properties. 2. Experimental a-Si:C:H coatings were grown on steel substrates, capacitively coupled to a low frequency generator (50 kHz). In this device (described elsewhere [7]), DC bias (Vdc) and applied power are coupled and substrates are independently heated. The cathode area is 38.5 cm2, leading in this work to a power density at the cathode varying from 0 to 2.78 W/cm2. The total gas flow varies from 8 to 34 NL/h. The precursor molar fraction is in the 7%–12% range. The operating pressure is fixed at 93 Pa (0.7 Torr). OES measurements were performed during deposition, perpendicularly to the discharge axis, by means of an optical fiber equipped with an infinite focus optics, moveable in (r,z) directions. Mass spectrometry (SXP Elite quadrupole, VG Instruments) was used to detect the most important species produced in the process. At floating potential, the MS sampling device (hole diameter 30 μm) was plunged in the plasma at a fixed position close to the cathode to minimize its influence on the
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Fig. 1. Typical MS histogram of an argon/TMS low frequency plasma.
discharge. To quantify electron concentration (Ne) and temperature (Te) in the plasma, EP measurements (Smartprobe, Scientific System) were carried out on the symmetry axis of the discharge close to the substrate holder. a-Si:C:H films were characterized by Energy Dispersive Spectroscopy (EDS), Fourier Transformed InfraRed (FTIR) and X-Ray Photoelectron (XPS) spectroscopies. Film hardness was measured by using an instrumented nanoindenter. A micro-scratch tester under constant load gave access to alternate friction coefficients (μ) against steel.
through abstraction of methyl groups by electron impacts. The Si(CH3)3 signal increases with increasing power up to 20– 30 W, then decreases to the benefit of Si(CH3)2 and SiCH3. The breaking of Si–C bonds in the two latter species is confirmed by concomitant variations of the CH3 signal versus power (Fig. 2b). A low signal attributed to CH2 radical (m / q = 14 g mol− 1) is also observed at high plasma power. Different elementary processes can lead to its formation: electron impact on CH3, CH4 or more complex chemical species such as C2H4. The signal obtained at m / q = 16 g mol− 1 is ascribed to CH4 since it cannot result from the fragmentation of any molecule or radical produced in the gas mixture. Reactions such as CH3 + H or CH2 + H2 are efficient to produce methane. The high density of H2 as well as the detection of a low signal at m / q = 1 g mol− 1, the intensity of which increases with increasing power, are consistent with these reactions. The H signal cannot be ascribed to dissociative ionization of H2 in the MS ionizer, since electron energy in the ionizer is only 17 eV whereas the energy threshold of the elementary process e− + H2 → H + H+ + 2e− is 17.3 eV [10]. OES measurements performed on the Hα line (λ = 656.2 nm) for various power conditions (not shown) confirm MS observations of the increase of H density with increasing plasma power. The Hα line profile can be decomposed in three Gaussian components corresponding to three energetic H populations [11]. Two components, NC (narrow) and BC (broad), are attributed to molecular hydrogen dissociation through direct
3. Results and discussion 3.1. Plasma diagnostics Neutral species produced in Ar–TMS plasma were identified by MS. Fig. 1 mass histogram illustrates the chemical complexity of the plasma. Atoms and simple radicals such as H, H2 (m / q = 1 and 2 g mol− 1), CHx (CH2 and CH3 at m / q = 14 and 15 g mol− 1) are produced from the breaking of Si–C and C–H bonds. Heavier chemical species are observable at m / q close to 28 g mol− 1, corresponding to C2Hy = 2,4,6 molecules partly issued from the recombination of CHx fragments. Three groups of species containing Si–C and Si–H bonds can be identified around m / q = 73 g mol− 1 (Si(CH3)3), 5 g mol− 1 (Si(CH3)2) and 43 g mol− 1 (SiCH3). TMS dissociation can roughly be interpreted in terms of CH3 group abstraction [8] and H production. TMS was detected with electron energy equal to 20 eV in the MS ionizer, as its direct ionization cross section is low [9]. The other species were recorded at 16 eV (SiCxHy) or 17 eV (H, H2, CHx, C2Hy). Fragmentation of parent molecules in the mass spectrometer is then limited and the corresponding signal is enhanced. The influence of plasma power, TMS and total flow rates on MS signals was recorded. Whatever the process conditions the TMS signal (m / q = 88 g mol− 1) decreases rapidly with increasing discharge power. Complete TMS dissociation is observed for plasma power above 15 W. Fig. 2a describes the evolution of Si(CH3)x = 1,2,3 signals when varying plasma power. Changes in relative intensities partly reflect the gradual dissociation of the initial reactant molecule
Fig. 2. MS signal evolutions for Si(CH3)x (a), CHx, H, H2 (b) versus plasma power.
A. Soum-Glaude et al. / Diamond & Related Materials 16 (2007) 1259–1263
Fig. 3. Evolutions of Ne and Te versus Vdc and plasma power in pure argon.
electron collision and ro-vibrational pumping by multiple electron collisions. They are observed in Ar/H2 plasma mixtures and correspond to the production of H atoms from H2 (detected by MS). The third component, IC (intermediate), is specific to plasma deposition and is the most important one. It is attributed to dissociation of TMS and other heavy reaction products. MS signals corresponding to these products increase with increasing the power applied to the discharge (increased Ne). Electron density Ne (number of electrons in the plasma per volume unit) and electron temperature Te (electron kinetic energy in eV =3/2 k Te) were obtained from EP measurements. Fig. 3 shows the first plots of Ne and Te versus power and DC bias in pure argon plasma, where two electron sources (hence two regimes) classically compete. Below a Vdc amplitude of 100–150 V, the energy of ions impinging the cathode is low, Ne ranges from 2 to 4 108 cm− 3 whereas Te is close to 1 eV. Since the plasma potential remains low (3 to 10 V) in all experiments, the maximum energy attain-
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able by ions is then Eions =q · (Vp – Vdc) # q Vdc. At low DC bias, electrons are mainly produced in the plasma volume (α electrons). As Vdc amplitude increases, the efficiency of ion bombardment increases thus enhancing the production of energetic secondary electrons (γ) emitted from the powered electrode (Te # 4 eV for Vdc =−130 V). Finally, as power is increased, electron density increases up to 2–3.109 cm− 3 and electrons are thermalized (Te # 2 eV) through collision effects in the discharge volume. The DC bias ranges from −100 to −200 V then corresponds to the change from a α regime to a γ regime of the plasma. EP measurements on Te were comforted by OES on argon lines (using the method of Zambrano et al. [12]). OES in pure argon shows the same decrease of Te from 2 to 1.2 eV when |Vdc| is increased. Moreover, for a fixed power, electron temperature increases when TMS is injected in the discharge (from 1.2 to 1.9 eV respectively for TMS fraction increasing from 0 to 12% in argon at Vdc =−200 V). This result could mean that TMS dissociation arises from ro-vibrational excitations from ground to dissociative levels, rather than from direct dissociative electron collisions. Analysis of electron energy distribution functions (EEDF) should confirm this hypothesis. 3.2. Plasma reactor simulation To our knowledge, simulation of plasma-assisted deposition of amorphous silicon carbide from tetramethylsilane has never been attempted to date. The difficulty mainly arises from the lack of data related to electron–TMS collision cross sections and chemical reactions mechanisms. A one-dimensional self-consistent model was used for modeling the electrical discharge by solving Poisson equation and transport equations for charged particles along the vertical axis (z). The model is based on local electric field assumption. Drift-diffusion approximation was used for solving transport equations for charged particles. Emission of secondary electrons from the cathode was considered through a constant emission
Table 1 Simplified reaction mechanism considered in the modeling Gas-phase reactions Electron impact dissociation Si(CH3)4 + e− → Si(CH3)3 + CH3 + e− Reactions involving organosilicon species Si(CH3)3 + CH3(+M) → Si(CH3)4(+M) Si(CH3)3 + CH3(+M) → Si(CH3)2CH2 + CH4(+M) Si(CH3)3 + Si(CH3)3 →Si(CH3)2CH2 + HSi(CH3)3 Si(CH3)3 + C2H4 → (CH3)3SiCH2CH2 Reactions involving hydrocarbon species CH3 + CH3 (+M) → C2H6(+ M) CH3 + CH3(+M) → CH2 + CH4(+M) CH2 + CH2 → C2H4 CH2 + CH2 → C2H2 + H2 CH3 + C2H6 → CH4 + C2H5 CH3 + C2H5 → C3H8 CH3 + C2H5 → C2H6 + CH2
Surface reactions
Contribution of reacting species to film growth (%)
Si(CH3)3→Si(s) + C(s) + 4.5 H2
54.7
Si(CH3)2CH2 → Si(s) + C(s) + C2H6 + H2 C2H5 → 2 C(s) + 2.5 H2 CH3 → C(s) + 1.5 H2 CH2 → C(s) + H2 C2H4 → 2 C(s) + 2 H2 C2H2 → 2 C(s) + H2
7.08
22.1 8.08 5.03 2.82 0.035
Contributions (molar %) are calculated for the following conditions: P = 93 Pa, T = 623 K, Vexc = 150 V, Rf power = 18 W, total flow rate = 0.034 Nm3h− 1 TMS molar fraction = 3%. (s): solid species.
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coefficient (γ). More details on the electrical discharge modeling can be found in Ref. [13]. Data for modeling argon discharges are abundant in the literature. Electron impact ionization cross sections of TMS were proposed [9,14] but cross sections for momentum transfer, electron attachment and TMS molecule excitation/dissociation are not yet available. Huber et al. [15] reported a value of 7.45 eV for the electronic excitation threshold, but the cross section has not been presented. From the experimental data on TMS ionization by electron impact, the corresponding excitation process can reasonably be assigned to dissociation into Si(CH3)3 + CH3. From the resolution of the Boltzmann equation, electron–molecule cross sections were adjusted in order to obtain the best fit between calculated and experimental transport parameters recently measured by Yoshida et al. [16] in pure TMS. Rate coefficients from McGinnis et al. [14] were considered for ion-molecule reactions. 2D reactor simulation including coupled momentum, heat and species transfer was performed by using the CFD code FLUENT 6.2. The time-averaged profile of TMS dissociation frequency across the discharge (obtained from the corresponding modeling) was used as input data for 2D steady-state reactor simulation. A rough extrapolation of the 1D dissociation frequency profile to 2 dimensions (r,z) was performed by assuming a parabolic profile in the radial direction (r) with zero production at the reactor wall and maximum production on the symmetry axis (r = 0). A simplified chemical scheme was derived from literature data and from estimated values. Recombination reactions between primary electron impact dissociation products (Si(CH3)3 and CH3) have been reported in the literature [17] producing the Si(CH3)2CH2 species. The gas-phase and surface reactions selected for performing FLUENT simulations are reported in Table 1. At this preliminary stage, most rate coefficients were roughly estimated. Saturated molecular species in their ground state were considered unreactive towards the surface, except C2H2 and C2H4 for which sticking coefficients of 2.10− 3 were used [18]. Sticking coefficients of one were assumed for other radical species. First simulation results produced deposition rates much lower than experimental ones. The time-averaged dissociation frequency was increased and adjusted in order to reproduce the experimental deposition rate profiles. This essential increase of the frequency amounts to accounting for ro-vibrational excitation of the TMS molecule in the discharge, as experimental measurements have suggested (see above). As shown in Table 1, Si(CH3)2CH2 and Si(CH3)3 are important species that may significantly contribute to film growth. Other species such as CH2, CH3, C2H2 and C2H4 have also a rather important contribution (Table 1). These findings are in agreement with MS observations and suggest that the above hydrocarbon and organosilicon species probably play a key role in the deposition process. Finally, simulation shows that large amounts of H2 are produced in the gas phase, but H2 production arises almost only from surface desorption, in agreement with OES measurements. 3.3. Film characterization In a previous work [7], the chemical composition of a-Si:C:H films was analyzed for various gas residence times and DC bias
values in order to highlight the effect of neutral species and energetic ions impinging on the surface during growth. FTIR measurements showed that the relative importance of Si–H (2100 cm− 1) and C–H (2800–3000 cm− 1) absorption bands compared to Si–C band (760 cm− 1) decreases when power or gas residence time in the reactor increases, indicating a limitation of H content in the films. This result is in agreement with MS and OES measurements, suggesting that H atoms created in the plasma recombine in the gas phase to produce molecular hydrogen, which cannot be incorporated in the material. As shown by MS measurements, recombination of CHx radicals produced by TMS dissociation partly leads to the formation of C2Hx species. As C2Hx molecules and radicals are involved in the formation of C–C bonds in the films when increasing plasma power or gas residence time, C–C bonds in the film can be favored to the detriment of Si–C ones. Indeed, FTIR spectra showed a decrease of the Si–C absorption band accompanied by an increase of the Si–(CH2) n–Si one (998 cm− 1) [7]. These results are consistent with a decrease of the Si/C ratio observed in EDS (from 1.6 to 0.6). XPS measurements also showed the presence of Si–CHx bonds in the films. A peak located near 16 eV (C2s) is observed on the corresponding valence bands. Film hardness, elastic modulus and friction coefficient (μ) against steel were measured at various plasma powers. Hardness (H) and Young modulus (E) of the films are maximized for Vdc ranging from − 100 to − 250 V (21 b H b 30 GPa, 140 b E b 170 GPa). The calculated ratio H3/E2, related to the resistance of coatings to plastic deformation [19], is also maximized meaning that the mechanical response of the coatings will be highly elastic under load. The best friction coefficients (μ # 0.16–0.18) are obtained at high H3/E2 ratio, when ion bombardment is combined to long gas residence times in the reactor. The corresponding a-Si:C:H coatings would then be good candidates for low friction under high loads. 4. Conclusions Preliminary results of diagnostic and simulation of low frequency argon–tetramethylsilane plasmas were presented. Mass spectrometry, optical emission spectroscopy and Langmuir probe measurements showed that organosilicon species such as Si(CH3)3 and hydrocarbon species such as CH3, CH2, C2H2, C2H4 are likely to be key deposition species influencing a-Si:C:H film microstructure and mechanical properties. Plasma reactor simulation results confirmed these experimental trends. The reaction mechanism used in the simulations will be improved and additional electrical probe and MS measurements will be performed to further confirm these preliminary results. References [1] K. Oguri, T. Arai, Surf. Coat. Technol. 47 (1991) 710. [2] W.C. Vassell, A.K. Gangopadhyay, T.J. Potter, M.A. Tamor, M.J. Rokosz, ASM J. Mater. Eng. Perform. 6 (1997) 426. [3] A. Erdemir, C. Donnet, J. Phys., D, Appl. Phys. 39 (2006) R311. [4] K. Vercammen, H. Haefke, Y. Gerbig, A. Van Hulsel, E. Pflüger, J. Meneve, Surf. Coat. Technol. 133 (2000) 466.
A. Soum-Glaude et al. / Diamond & Related Materials 16 (2007) 1259–1263 [5] D.S. Kim, Y.H. Lee, Thin Solid Films 283 (1996) 109. [6] M. Ducarroir, Ann. Chim. Sci. Matér. 23 (1998). [7] A. Soum-Glaude, L. Thomas, E. Tomasella, Surf. Coat. Technol. 200 (22–23) (2006) 6425. [8] J.L. Jauberteau, I. Jauberteau, J. Aubreton, Int. J. Mass Spectrom. 228 (1) (2003) 49. [9] R. Basner, R. Foest, M. Schmidt, F. Sigeneger, Int. J. Mass Spectrom. 153 (1996) 65. [10] H.M. Rosenstock, K. Draxl, B.W. Steiner, J. Phys. Chem. Ref. Data 6 (1977). [11] L. Thomas, E. Tomasella, J.M. Badie, R. Berjoan, M. Ducarroir, Chem. Vapor Depos. 3 (9) (2003) 130.
[12] [13] [14] [15] [16] [17] [18] [19]
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G. Zambrano, H. Riasco, P. Prieto, Surf. Coat Technol. 172 (2003) 144. C. Kamphan, Ph Thesis, Université Paul Sabatier, Toulouse (France), 2004. S. McGinnis, K. Riehl, P.D. Haaland, Chem. Phys. Lett. 232 (1995) 99. V. Huber, K.R. Asmis, A.-C. Sergenton, M. Allan, S. Grimme, J. Phys. Chem., A 102 (1998) 3254. K. Yoshida, S. Mori, Y. Kishimoto, H. Ohuchi, H. Hasegawa, M. Shimozuma, H. Tagashira, J. Phys., D, Appl. Phys. 38 (2005) 1918. N. Herlin, M. Lefebvre, M. Péalat, J. Perrin, J. Phys. Chem. 96 (1992) 7063. A. Dollet, S. de Persis, M. Pons, M. Matecki, Surf. Coat. Technol. 177–178 (2004) 382. C. Charitidis, S. Logothetidis, P. Douka, Diamond Relat. Mater. 8 (1999) 558.
Argon/tetramethysilane PECVD: From process diagnostic and modeling to a-Si:C:H hard coating composition A. Soum-Glaude a,b,⁎, L. Thomas a,b , A. Dollet a , P. Ségur c , M.C. Bordage c a
c
PROMES-CNRS, Tecnosud, Rambla de la Thermodynamique, 66100 Perpignan, France b Université de Perpignan Via Domitia, 52 Av Paul Alduy, 66100 Perpignan, France CPAT-CNRS, Université Paul Sabatier, 118 Route de Narbonne, 31062 Toulouse cedex, France Available online 19 December 2006
Abstract Amorphous hydrogenated silicon carbide films were grown from capacitively coupled low frequency argon–tetramethylsilane plasmas. Mass spectrometry, optical emission spectroscopy and electrostatic probe were used to identify electron properties and chemical species under various deposition conditions. Film microstructure and mechanical properties were also characterized. The main goal of this analysis was to better understand the relationships between process parameters, gas-phase composition, film microstructure and tribological properties. A preliminary modeling of the plasma reactor was also carried out. First simulation results confirmed the trends from gas-phase analyses. © 2006 Elsevier B.V. All rights reserved. Keywords: PECVD; Plasma diagnosis; Plasma modeling; Si-doped DLC; Hard coatings
1. Introduction Silicon-doped diamond-like carbon (DLC) films are promising for applications requiring high hardness, low friction and wear and good thermal stability [1–4]. These amorphous films can be prepared in plasma-assisted chemical vapor deposition (PACVD) processes from various precursors [5], mainly from silane– hydrocarbons mixtures but also from organosilicon molecules containing Si–C bonds [6]. Organosilicon plasma chemistry is complex and its consequences on film growth are not yet well understood; for this reason, experimental diagnostic of organosilicon plasmas is of particular interest. Optical emission spectroscopy (OES) and electrostatic probe (EP) measurement techniques provide local information without any disturbance of the sampled region. Mass spectrometry (MS) allows identification of most chemical species produced in the plasma and reacting with the growing film. In this work, argon–tetramethylsilane (TMS) mixtures were used to deposit Si-doped DLC films (a-Si:C:H) on steel substrates. OES, MS and EP measurements were performed and compared to a first simulation of the plasma ⁎ Corresponding author. PROMES-CNRS, Tecnosud, Rambla de la Thermodynamique, 66100 Perpignan, France. Tel.: +33 468 68 22 53; fax: +33 468 68 22 13. E-mail address: [email protected] (A. Soum-Glaude). 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.11.003
reactor in order to gain insight into the deposition mechanisms. Local parameters obtained from diagnostic and simulation (chemical species concentrations, etc) can be correlated to several key features of the film such as deposition rate, composition, microstructure and mechanical properties. 2. Experimental a-Si:C:H coatings were grown on steel substrates, capacitively coupled to a low frequency generator (50 kHz). In this device (described elsewhere [7]), DC bias (Vdc) and applied power are coupled and substrates are independently heated. The cathode area is 38.5 cm2, leading in this work to a power density at the cathode varying from 0 to 2.78 W/cm2. The total gas flow varies from 8 to 34 NL/h. The precursor molar fraction is in the 7%–12% range. The operating pressure is fixed at 93 Pa (0.7 Torr). OES measurements were performed during deposition, perpendicularly to the discharge axis, by means of an optical fiber equipped with an infinite focus optics, moveable in (r,z) directions. Mass spectrometry (SXP Elite quadrupole, VG Instruments) was used to detect the most important species produced in the process. At floating potential, the MS sampling device (hole diameter 30 μm) was plunged in the plasma at a fixed position close to the cathode to minimize its influence on the
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A. Soum-Glaude et al. / Diamond & Related Materials 16 (2007) 1259–1263
Fig. 1. Typical MS histogram of an argon/TMS low frequency plasma.
discharge. To quantify electron concentration (Ne) and temperature (Te) in the plasma, EP measurements (Smartprobe, Scientific System) were carried out on the symmetry axis of the discharge close to the substrate holder. a-Si:C:H films were characterized by Energy Dispersive Spectroscopy (EDS), Fourier Transformed InfraRed (FTIR) and X-Ray Photoelectron (XPS) spectroscopies. Film hardness was measured by using an instrumented nanoindenter. A micro-scratch tester under constant load gave access to alternate friction coefficients (μ) against steel.
through abstraction of methyl groups by electron impacts. The Si(CH3)3 signal increases with increasing power up to 20– 30 W, then decreases to the benefit of Si(CH3)2 and SiCH3. The breaking of Si–C bonds in the two latter species is confirmed by concomitant variations of the CH3 signal versus power (Fig. 2b). A low signal attributed to CH2 radical (m / q = 14 g mol− 1) is also observed at high plasma power. Different elementary processes can lead to its formation: electron impact on CH3, CH4 or more complex chemical species such as C2H4. The signal obtained at m / q = 16 g mol− 1 is ascribed to CH4 since it cannot result from the fragmentation of any molecule or radical produced in the gas mixture. Reactions such as CH3 + H or CH2 + H2 are efficient to produce methane. The high density of H2 as well as the detection of a low signal at m / q = 1 g mol− 1, the intensity of which increases with increasing power, are consistent with these reactions. The H signal cannot be ascribed to dissociative ionization of H2 in the MS ionizer, since electron energy in the ionizer is only 17 eV whereas the energy threshold of the elementary process e− + H2 → H + H+ + 2e− is 17.3 eV [10]. OES measurements performed on the Hα line (λ = 656.2 nm) for various power conditions (not shown) confirm MS observations of the increase of H density with increasing plasma power. The Hα line profile can be decomposed in three Gaussian components corresponding to three energetic H populations [11]. Two components, NC (narrow) and BC (broad), are attributed to molecular hydrogen dissociation through direct
3. Results and discussion 3.1. Plasma diagnostics Neutral species produced in Ar–TMS plasma were identified by MS. Fig. 1 mass histogram illustrates the chemical complexity of the plasma. Atoms and simple radicals such as H, H2 (m / q = 1 and 2 g mol− 1), CHx (CH2 and CH3 at m / q = 14 and 15 g mol− 1) are produced from the breaking of Si–C and C–H bonds. Heavier chemical species are observable at m / q close to 28 g mol− 1, corresponding to C2Hy = 2,4,6 molecules partly issued from the recombination of CHx fragments. Three groups of species containing Si–C and Si–H bonds can be identified around m / q = 73 g mol− 1 (Si(CH3)3), 5 g mol− 1 (Si(CH3)2) and 43 g mol− 1 (SiCH3). TMS dissociation can roughly be interpreted in terms of CH3 group abstraction [8] and H production. TMS was detected with electron energy equal to 20 eV in the MS ionizer, as its direct ionization cross section is low [9]. The other species were recorded at 16 eV (SiCxHy) or 17 eV (H, H2, CHx, C2Hy). Fragmentation of parent molecules in the mass spectrometer is then limited and the corresponding signal is enhanced. The influence of plasma power, TMS and total flow rates on MS signals was recorded. Whatever the process conditions the TMS signal (m / q = 88 g mol− 1) decreases rapidly with increasing discharge power. Complete TMS dissociation is observed for plasma power above 15 W. Fig. 2a describes the evolution of Si(CH3)x = 1,2,3 signals when varying plasma power. Changes in relative intensities partly reflect the gradual dissociation of the initial reactant molecule
Fig. 2. MS signal evolutions for Si(CH3)x (a), CHx, H, H2 (b) versus plasma power.
A. Soum-Glaude et al. / Diamond & Related Materials 16 (2007) 1259–1263
Fig. 3. Evolutions of Ne and Te versus Vdc and plasma power in pure argon.
electron collision and ro-vibrational pumping by multiple electron collisions. They are observed in Ar/H2 plasma mixtures and correspond to the production of H atoms from H2 (detected by MS). The third component, IC (intermediate), is specific to plasma deposition and is the most important one. It is attributed to dissociation of TMS and other heavy reaction products. MS signals corresponding to these products increase with increasing the power applied to the discharge (increased Ne). Electron density Ne (number of electrons in the plasma per volume unit) and electron temperature Te (electron kinetic energy in eV =3/2 k Te) were obtained from EP measurements. Fig. 3 shows the first plots of Ne and Te versus power and DC bias in pure argon plasma, where two electron sources (hence two regimes) classically compete. Below a Vdc amplitude of 100–150 V, the energy of ions impinging the cathode is low, Ne ranges from 2 to 4 108 cm− 3 whereas Te is close to 1 eV. Since the plasma potential remains low (3 to 10 V) in all experiments, the maximum energy attain-
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able by ions is then Eions =q · (Vp – Vdc) # q Vdc. At low DC bias, electrons are mainly produced in the plasma volume (α electrons). As Vdc amplitude increases, the efficiency of ion bombardment increases thus enhancing the production of energetic secondary electrons (γ) emitted from the powered electrode (Te # 4 eV for Vdc =−130 V). Finally, as power is increased, electron density increases up to 2–3.109 cm− 3 and electrons are thermalized (Te # 2 eV) through collision effects in the discharge volume. The DC bias ranges from −100 to −200 V then corresponds to the change from a α regime to a γ regime of the plasma. EP measurements on Te were comforted by OES on argon lines (using the method of Zambrano et al. [12]). OES in pure argon shows the same decrease of Te from 2 to 1.2 eV when |Vdc| is increased. Moreover, for a fixed power, electron temperature increases when TMS is injected in the discharge (from 1.2 to 1.9 eV respectively for TMS fraction increasing from 0 to 12% in argon at Vdc =−200 V). This result could mean that TMS dissociation arises from ro-vibrational excitations from ground to dissociative levels, rather than from direct dissociative electron collisions. Analysis of electron energy distribution functions (EEDF) should confirm this hypothesis. 3.2. Plasma reactor simulation To our knowledge, simulation of plasma-assisted deposition of amorphous silicon carbide from tetramethylsilane has never been attempted to date. The difficulty mainly arises from the lack of data related to electron–TMS collision cross sections and chemical reactions mechanisms. A one-dimensional self-consistent model was used for modeling the electrical discharge by solving Poisson equation and transport equations for charged particles along the vertical axis (z). The model is based on local electric field assumption. Drift-diffusion approximation was used for solving transport equations for charged particles. Emission of secondary electrons from the cathode was considered through a constant emission
Table 1 Simplified reaction mechanism considered in the modeling Gas-phase reactions Electron impact dissociation Si(CH3)4 + e− → Si(CH3)3 + CH3 + e− Reactions involving organosilicon species Si(CH3)3 + CH3(+M) → Si(CH3)4(+M) Si(CH3)3 + CH3(+M) → Si(CH3)2CH2 + CH4(+M) Si(CH3)3 + Si(CH3)3 →Si(CH3)2CH2 + HSi(CH3)3 Si(CH3)3 + C2H4 → (CH3)3SiCH2CH2 Reactions involving hydrocarbon species CH3 + CH3 (+M) → C2H6(+ M) CH3 + CH3(+M) → CH2 + CH4(+M) CH2 + CH2 → C2H4 CH2 + CH2 → C2H2 + H2 CH3 + C2H6 → CH4 + C2H5 CH3 + C2H5 → C3H8 CH3 + C2H5 → C2H6 + CH2
Surface reactions
Contribution of reacting species to film growth (%)
Si(CH3)3→Si(s) + C(s) + 4.5 H2
54.7
Si(CH3)2CH2 → Si(s) + C(s) + C2H6 + H2 C2H5 → 2 C(s) + 2.5 H2 CH3 → C(s) + 1.5 H2 CH2 → C(s) + H2 C2H4 → 2 C(s) + 2 H2 C2H2 → 2 C(s) + H2
7.08
22.1 8.08 5.03 2.82 0.035
Contributions (molar %) are calculated for the following conditions: P = 93 Pa, T = 623 K, Vexc = 150 V, Rf power = 18 W, total flow rate = 0.034 Nm3h− 1 TMS molar fraction = 3%. (s): solid species.
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coefficient (γ). More details on the electrical discharge modeling can be found in Ref. [13]. Data for modeling argon discharges are abundant in the literature. Electron impact ionization cross sections of TMS were proposed [9,14] but cross sections for momentum transfer, electron attachment and TMS molecule excitation/dissociation are not yet available. Huber et al. [15] reported a value of 7.45 eV for the electronic excitation threshold, but the cross section has not been presented. From the experimental data on TMS ionization by electron impact, the corresponding excitation process can reasonably be assigned to dissociation into Si(CH3)3 + CH3. From the resolution of the Boltzmann equation, electron–molecule cross sections were adjusted in order to obtain the best fit between calculated and experimental transport parameters recently measured by Yoshida et al. [16] in pure TMS. Rate coefficients from McGinnis et al. [14] were considered for ion-molecule reactions. 2D reactor simulation including coupled momentum, heat and species transfer was performed by using the CFD code FLUENT 6.2. The time-averaged profile of TMS dissociation frequency across the discharge (obtained from the corresponding modeling) was used as input data for 2D steady-state reactor simulation. A rough extrapolation of the 1D dissociation frequency profile to 2 dimensions (r,z) was performed by assuming a parabolic profile in the radial direction (r) with zero production at the reactor wall and maximum production on the symmetry axis (r = 0). A simplified chemical scheme was derived from literature data and from estimated values. Recombination reactions between primary electron impact dissociation products (Si(CH3)3 and CH3) have been reported in the literature [17] producing the Si(CH3)2CH2 species. The gas-phase and surface reactions selected for performing FLUENT simulations are reported in Table 1. At this preliminary stage, most rate coefficients were roughly estimated. Saturated molecular species in their ground state were considered unreactive towards the surface, except C2H2 and C2H4 for which sticking coefficients of 2.10− 3 were used [18]. Sticking coefficients of one were assumed for other radical species. First simulation results produced deposition rates much lower than experimental ones. The time-averaged dissociation frequency was increased and adjusted in order to reproduce the experimental deposition rate profiles. This essential increase of the frequency amounts to accounting for ro-vibrational excitation of the TMS molecule in the discharge, as experimental measurements have suggested (see above). As shown in Table 1, Si(CH3)2CH2 and Si(CH3)3 are important species that may significantly contribute to film growth. Other species such as CH2, CH3, C2H2 and C2H4 have also a rather important contribution (Table 1). These findings are in agreement with MS observations and suggest that the above hydrocarbon and organosilicon species probably play a key role in the deposition process. Finally, simulation shows that large amounts of H2 are produced in the gas phase, but H2 production arises almost only from surface desorption, in agreement with OES measurements. 3.3. Film characterization In a previous work [7], the chemical composition of a-Si:C:H films was analyzed for various gas residence times and DC bias
values in order to highlight the effect of neutral species and energetic ions impinging on the surface during growth. FTIR measurements showed that the relative importance of Si–H (2100 cm− 1) and C–H (2800–3000 cm− 1) absorption bands compared to Si–C band (760 cm− 1) decreases when power or gas residence time in the reactor increases, indicating a limitation of H content in the films. This result is in agreement with MS and OES measurements, suggesting that H atoms created in the plasma recombine in the gas phase to produce molecular hydrogen, which cannot be incorporated in the material. As shown by MS measurements, recombination of CHx radicals produced by TMS dissociation partly leads to the formation of C2Hx species. As C2Hx molecules and radicals are involved in the formation of C–C bonds in the films when increasing plasma power or gas residence time, C–C bonds in the film can be favored to the detriment of Si–C ones. Indeed, FTIR spectra showed a decrease of the Si–C absorption band accompanied by an increase of the Si–(CH2) n–Si one (998 cm− 1) [7]. These results are consistent with a decrease of the Si/C ratio observed in EDS (from 1.6 to 0.6). XPS measurements also showed the presence of Si–CHx bonds in the films. A peak located near 16 eV (C2s) is observed on the corresponding valence bands. Film hardness, elastic modulus and friction coefficient (μ) against steel were measured at various plasma powers. Hardness (H) and Young modulus (E) of the films are maximized for Vdc ranging from − 100 to − 250 V (21 b H b 30 GPa, 140 b E b 170 GPa). The calculated ratio H3/E2, related to the resistance of coatings to plastic deformation [19], is also maximized meaning that the mechanical response of the coatings will be highly elastic under load. The best friction coefficients (μ # 0.16–0.18) are obtained at high H3/E2 ratio, when ion bombardment is combined to long gas residence times in the reactor. The corresponding a-Si:C:H coatings would then be good candidates for low friction under high loads. 4. Conclusions Preliminary results of diagnostic and simulation of low frequency argon–tetramethylsilane plasmas were presented. Mass spectrometry, optical emission spectroscopy and Langmuir probe measurements showed that organosilicon species such as Si(CH3)3 and hydrocarbon species such as CH3, CH2, C2H2, C2H4 are likely to be key deposition species influencing a-Si:C:H film microstructure and mechanical properties. Plasma reactor simulation results confirmed these experimental trends. The reaction mechanism used in the simulations will be improved and additional electrical probe and MS measurements will be performed to further confirm these preliminary results. References [1] K. Oguri, T. Arai, Surf. Coat. Technol. 47 (1991) 710. [2] W.C. Vassell, A.K. Gangopadhyay, T.J. Potter, M.A. Tamor, M.J. Rokosz, ASM J. Mater. Eng. Perform. 6 (1997) 426. [3] A. Erdemir, C. Donnet, J. Phys., D, Appl. Phys. 39 (2006) R311. [4] K. Vercammen, H. Haefke, Y. Gerbig, A. Van Hulsel, E. Pflüger, J. Meneve, Surf. Coat. Technol. 133 (2000) 466.
A. Soum-Glaude et al. / Diamond & Related Materials 16 (2007) 1259–1263 [5] D.S. Kim, Y.H. Lee, Thin Solid Films 283 (1996) 109. [6] M. Ducarroir, Ann. Chim. Sci. Matér. 23 (1998). [7] A. Soum-Glaude, L. Thomas, E. Tomasella, Surf. Coat. Technol. 200 (22–23) (2006) 6425. [8] J.L. Jauberteau, I. Jauberteau, J. Aubreton, Int. J. Mass Spectrom. 228 (1) (2003) 49. [9] R. Basner, R. Foest, M. Schmidt, F. Sigeneger, Int. J. Mass Spectrom. 153 (1996) 65. [10] H.M. Rosenstock, K. Draxl, B.W. Steiner, J. Phys. Chem. Ref. Data 6 (1977). [11] L. Thomas, E. Tomasella, J.M. Badie, R. Berjoan, M. Ducarroir, Chem. Vapor Depos. 3 (9) (2003) 130.
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