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Deposition of Si-DLC films with high hardness, low stress and high deposition rates
Surface and Coatings Technology 133᎐134 Ž2000. 247᎐252
Deposition of Si-DLC films with high hardness, low stress and high deposition rates J.C. Damasceno a , S.S. Camargo Jr a,U , F.L. Freire Jr b, R. Carius c a
Engenharia Metalurgica e de Materiais-Uni¨ ersidade Federal do Rio de Janeiro, Cx. Postal 68505, Rio de Janeiro, RJ, CEP 21945-970, ´ Brazil b ´ Uni¨ ersidade Catolica Departamento de Fısica-Pontificia ´ ´ do Rio de Janeiro, Cx. Postal 38071, Rio de Janeiro, RJ, CEP 22454-970, Brazil c ISI-IPV, Forschungszentrum Julich D-52425, Germany ¨ GmbH, Postfach 1913, Julich, ¨
Abstract In this work silicon-incorporated diamond-like carbon ŽSi-DLC. films were produced by plasma enhanced chemical vapor deposition ŽPECVD. from gaseous mixtures of CH 4 and SiH 4 . A study of the influence of self-bias and gas composition on the mechanical and structural properties of the films was carried out. Results show that films deposited at high self-bias present high deposition rates, low stress and surprisingly high hardness. Increasing silane concentration in the gas phase leads to an enhancement of the observed effects. Compositional and structural characterization show that deposition at high bias leads to increased sp 2 character and rather low silicon contents. Increasing the silane content in the plasma leads to an increase in the sp 3 fraction of the films, and yields a further reduction of stress with almost no effect upon hardness. In this way, the possibility of producing films with high hardness Ž) 20 GPa., low stress Ž; 0.5 GPa. and high deposition rates Ž) 40 nmrmin. has been demonstrated. This result is thought to be very important from the point of view of technological applications. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Diamond-like carbon; Silicon; Plasma chemical vapor deposition ŽCVD.
1. Introduction Silicon-incorporated diamond-like carbon films ŽSiDLC. have been attracting increasing interest of researchers since they have a great potential for solving some of the major drawbacks of pure DLC films. For this reason, a considerable amount of work on these films has been carried out in the last 3᎐4 years. Indeed, Si-DLC films present: reduced residual internal stress w1,2x; high deposition rates w3x; good adhesion to most substrates, including various metal alloys, steels and glasses w4᎐6x; very high hardness w7,8,5x; improved thermal stability w9᎐11x; reduced hydrogen loss and graphitization w12,13x; low friction coefficients independent of relative humidity w14᎐16x; resistance to oxidation, moisU
Corresponding author.
ture and corrosion w2,17,18x; and low wettability w19x. This remarkable collection of properties makes this material a promising candidate for a large number of technological applications as metallurgical and protective coatings. In fact, Si-DLC films have already been tested as wear-resistant tribological coatings for highstress power applications, which included car engines, transmission gears and manufacturing tools, with good results w17x. Among the different deposition techniques, the most widely used has been plasma enhanced chemical vapor deposition ŽPECVD.. Various source gases have been used, such as: methane ŽCH 4 . and silane ŽSiH 4 . w1,15x; CH 4 , SiH 4 and Ar w4x; CH 4 , SiH 4 and He w3x; benzene ŽC 6 H 6 . and H 2-diluted SiH 4 w16x; tetramethylsilane ŽTMS. w18x; CH 4 and TMS w20x; acetylene ŽC 2 H 2 . and TMS w5,19x; CH 4 , TMS, Ar and H 2 w21x; and CH 4 , silicon tetrachloride ŽSiCl 4 ., Ar and H 2 w14x. Despite
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 9 3 2 - 4
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J.C. Damasceno et al. r Surface and Coatings Technology 133᎐134 (2000) 247᎐252
the various deposition conditions used by different laboratories, a general finding is that silicon contributes to the stabilization of the carbon sp 3 phase, with beneficial effects on the properties of the material w1,7,14,15,20x. However, in spite of all the work carried out there has been no report up to now of a systematic study of the influence of deposition conditions on the structure and properties of these films. In this work we report on the influence of variation in the cathode self-bias and gas mixture composition on the properties of Si-DLC films deposited by PECVD from gaseous mixtures of SiH 4 and CH 4 , and show that is possible to produce hard films Ž) 20 GPa. with low internal stress Ž; 0.5 GPa. at high deposition rates Ž) 40 nmrmin. at high values of self-bias.
2. Experimental Silicon-incorporated hydrogenated amorphous carbon films ŽSi-DLC. were deposited from gaseous mixtures of methane and silane onto crystalline silicon substrates placed on the cathode of a conventional radio frequency Ž13.56 MHz. parallel-plate glow discharge reactor. A gas flow of approximately 10 sccm, with silane contents in the range 0᎐20 vol.% were fed into the reactor through mass flow controllers. During all deposition runs the electrode distance and chamber pressure Žmonitored by a capacitance manometer. were kept fixed at 2.5 cm and 2.0 Pa, respectively. Films with a thickness between 1 and 2 m were produced at different self-bias voltages Ž VB . from y50 up to y1100 V. It must be noted that no temperature control of the substrate was carried out during deposition. Therefore, substantial heating of the substrate surface may occur as a result of its interaction with the plasma, mainly in case of the depositions with high power densities, in contrast to our previous work, where the substrate temperature was found to be limited to approximately 100⬚C at VB s y200 V. The film composition was determined by ion beam analysis. A 4-MeV van de Graaff accelerator provided Heq beams. For Rutherford backscattering spectrometry ŽRBS. measurements, we used a 2-MeV beam, with the detector positioned at 165⬚ with respect to the incident ions. Hydrogen content of the samples was determined by elastic recoil detection analysis ŽERDA., with a probe of 2.2 MeV Heq incident on the sample at an angle of 75⬚ with respect to the surface normal. Detection of recoil protons was performed at an angle of 30⬚. A 10-m Mylar foil covered the detector in order to prevent the detection of forward-scattered alpha particles. Raman spectra were recorded in a backscattering geometry using the 488-nm line of a Coherent Innova 100 argon laser for excitation, a Spex
Fig. 1. Deposition rate as a function of negative self-bias voltage of Si-DLC films prepared with silane concentrations in the gas of 0.2 vol.% Žsquares. and 2.0 vol.% Žcircles..
1404 double monochromator for dispersion, and a liquid nitrogen-cooled CCD camera for measuring the single-shot spectra. Infrared spectra were measured in a vacuum Fourier transform spectrometer Bruker IFS 66V. Residual internal stress was obtained by the substrate bending method using a Dektak IIA stylus profilometer, which was also used for the thickness measurements. All films presented compressive stresses. Indentation of the samples was carried out with a Vickers diamond micro-indenter with a 0.25-N load for approximately 20 s, keeping the indentation depth smaller than 20% of the sample thickness. Hardness was obtained from the measurement of the indentation diagonals on an optical microscope using the differential interference contrast ŽDIC. technique. In all cases hardness was calculated from the average of a series of 20 different indentations.
3. Results and discussion The deposition rate of Si-DLC films is strongly increased when the RF power density dissipated in the plasma is increased in a similar manner to that observed for pure a-C:H films. As shown in Fig. 1, the deposition rate as a function of the resulting cathode self-bias potential presents an increase of one order of magnitude when the self-bias voltage is increased from y50 to y1100 V. In addition, comparison of the results obtained for films produced with two different silane
J.C. Damasceno et al. r Surface and Coatings Technology 133᎐134 (2000) 247᎐252
concentrations in the plasma, e.g. 0.2 and 2.0 vol.%, shows that the deposition rate can be further increased by silane addition to the discharge. For instance, films deposited with 0.2 vol.% SiH 4 present a maximum deposition rate of approximately 28 nmrmin, whereas with 2.0 vol.% SiH 4 , a deposition rate of 35 nmrmin was achieved at VB s y1100 V and under the specific deposition conditions employed. The residual internal stress of Si-DLC films depends strongly on the cathode self-bias potential employed. In the low self-bias range, the internal stress is increased when the self-bias voltage increases, attains a maximum, and then decreases monotonically for high values of self-bias, as shown in Fig. 2a. Comparing the stress values obtained for VB s y200 and y1100 V, an internal stress reduction larger than three-fold was achieved for films deposited with 0.2 vol.% SiH 4 . Furthermore, increasing the silane content in the reactor resulted in an overall decrease in stress. In this way, films with residual internal stresses lower than 1.0 GPa could be obtained. The behavior observed for film hardness as a function of the cathode self-bias was rather remarkable. As is evident in Fig. 2b, a monotonic increase of hardness can be observed when the self-bias is increased from y50 up to y1100 V. Even for high values of self-bias voltage, no reduction in the hardness values was noted, in contrast to what is generally observed for pure a-C:H. Also, hardness values for films deposited with 2.0 vol.% silane are systematically higher than those of films deposited with lower silane contents. At this point, it is important to mention that the
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influence of the substrate hardness, as well as the limited thickness of the films and elastic recovery after indentation, on the present results are expected to be small, since hardness values measured by nanoindentation on some of our samples fell within 10᎐15% of the present values. Accordingly, other authors have also obtained a good agreement between hardness values measured by nano- and Vickers indentation for films with similar thickness and within the same range of values Ž20᎐25 GPa. w22x. In this way, one can guarantee the reliability of these results and conclude that Si-DLC films with high hardness, low internal stress and high deposition rates can be produced at high values of self-bias. The behavior observed for the deposition rate and internal stress as a function self-bias are similar to that observed years ago for pure a-C:H films deposited from CH 4 w23x. It is important to emphasize, however, that in the present case, the addition of silane to the discharge enhanced the deposition rate and contributed to an overall stress relief. These facts have been already observed in our laboratory for films deposited at VB s y200 V w1x. However, in the case of pure a-C:H films, hardness is low for the low bias material Žpolymer-like films., achieves a maximum for the so-called diamondlike films that are deposited at an intermediate value of bias, and decreases strongly in the case of films deposited at high bias Žgraphite-like films.. In contrast to this, hardness values for the present films show no evidence of reduction at high self-bias but, on the contrary, are slightly increased. Moreover, films de-
Fig. 2. Ža. Residual internal stress and Žb. Vickers micro hardness as a function of negative self-bias voltage of Si-DLC films prepared with silane concentrations in the gas of 0.2 vol.% Žsquares. and 2.0 vol.% Žcircles..
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Fig. 3. Atomic composition as a function of negative self-bias voltage of Si-DLC films prepared with 2.0 vol.% silane in the plasma.
posited with a larger concentration of SiH 4 in the plasma showed even higher hardness values. Recently, Lacerda et al. reported on the deposition of hard a-C:H films with low stress at high values of self-bias w24x. They could obtain films with a hardness of 14 GPa, stress of 0.5 GPa and deposition rate of 14 nmrmin by methane gas decomposition in a RF sputtering system, without the addition of any impurities to the films. However, similar to pure a-C:H, their maximum hardness values Ž; 25 GPa. were obtained at VB s y200 V. Those authors attributed their results to the different geometry of their deposition system. Although a similar effect may play an important role here, our results clearly show that the addition of silane to the discharge results in a further enhancement of the observed effects, improving the material properties. In order to try to understand the above-described behavior, compositional and structural characterization of the samples was carried out. Fig. 3 shows the atomic composition as a function of self-bias obtained from RBS and ERDA measurements of samples deposited with 2.0 vol.% SiH 4 in the discharge. It is evident that carbon content of the films is increased as the self-bias increases, whereas silicon content is correspondingly decreased. Whereas at low self-bias voltages the silicon content of the films is much larger than in the gas phase, at high values of bias the silicon concentration in the samples tends to reach a value similar to the gas phase. Also, the hydrogen concentration of the films is somewhat reduced by the self-bias increase. An equivalent behavior was obtained for the composition of films
deposited with 0.2 vol.% silane in the gas, although in this case the silicon contents were approximately one order of magnitude smaller. The observed variation in carbon and silicon contents of the films as a result of increasing self-bias can be understood based on the larger dissociation crosssections of silane compared to methane molecules and their dependence on the RF power density in the discharge. Therefore, at low RF power densities Žlow self-bias voltages. the dissociation rate of silane molecules is much higher than that of methane, leading to a relatively larger density of Si-related radicals in the plasma than C-related ones, thus enhancing silicon incorporation into the films. As the RF power is increased Žincreased self-bias ., the dissociation rate of methane molecules increases substantially, leading to higher carbon contents in the samples. Consistent with this picture, estimates based on the results presented above show that the deposition rate of silicon atoms is essentially independent of the self-bias in the investigated range, whereas the deposition rate of carbon atoms present a ten-fold increase when self-bias increases from VB s y50 to y1100 V. In this way, films with very low silicon concentrations are obtained at the highest values of bias. The hydrogen content of the samples, on the other hand, is thought to be controlled by a different mechanism, namely particle bombardment, which is strongly enhanced by the self-bias increase. However, samples deposited with self-bias voltages from y400 up to y1100 V present similar hydrogen contents, in contrast to this simple picture. A deeper investigation is still needed in order to clarify this issue. It must be noted that the present results show that it is possible to obtain films with high hardness, low stress and high deposition rate with very low silicon contents. As already pointed out, a substantial stress decrease can be obtained with the incorporation of just a few at.% of silicon atoms w1x. These results suggest that on the improvement of the film properties as a result of silane addition to the discharge, changes in plasma andror film growth conditions may play an important role. Raman investigation of Si-DLC films has shown that an increase in the self-bias voltage has a strong effect on the structure of the material. It is evident from the spectra shown in Fig. 4a that when the self-bias voltage is increased from y200 to y1100 V, the G-peak is shifted from approximately 1500 to 1570 cmy1 . Correspondingly, the relative intensity of the D-peak is also greatly increased. This behavior has been extensively observed by other authors in the case of pure a-C:H films, and is indicative of an increased sp 2 character. Although in the present case the compositional changes induced by the self-bias variation may also somewhat affect the Raman spectra, the observed effect on both
J.C. Damasceno et al. r Surface and Coatings Technology 133᎐134 (2000) 247᎐252
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Fig. 4. Ža. Raman and Žb. infrared spectra of Si-DLC films prepared with 2.0 vol.% silane in the plasma and different negative self-bias voltages.
peak position and intensity is much larger than expected from the observed variation in the carbon content of the films w9,20x. Therefore, one can conclude that the Raman spectra provide strong evidence of an increased sp 2 character of the films deposited at high bias. On the other hand, the comparison between the Raman spectra obtained for films deposited with two different silane concentrations showed that increasing silane concentration in the plasma enhances the sp 3 character of the films, in accordance with previous results w1,15,20x. Infrared analysis of the films has led to consistent conclusions about their bonding properties. Fig. 4b shows the spectra obtained in the range of the C᎐H stretching mode Ž2700᎐3100 cmy1 .. It is evident that a strong reduction in the peaks in the region around 2900 cmy1 ᎏ which are generally attributed to hydrogen bonded to sp 3 carbon ᎏ occurs as a consequence of increasing the self-bias voltage. The observed decrease is much larger than expected from a simple reduction in the hydrogen content. On the other hand, in the region above 3000 cmy1 ᎏ which is related to hydrogen bonded to sp 2 carbon ᎏ a significant reduction in the absorbance is not observed, suggesting an increased sp 2 fraction of the films deposited at high bias, in accordance with the Raman results. Again, the analysis of infrared spectra of films deposited with different silane concentrations has shown that the effect of silane addition to the discharge is to increase the sp 3 character of the films. Thus, one can conclude that films obtained at high values of self-bias present good mechanical properties
and an increased sp 2 character. The addition of larger silane contents to the discharge, on the other hand, increases the sp 3 character Žreduces sp 2 . and contributes to a further improvement in the mechanical properties. This puzzling situation can be understood in two different ways: Ži. one may admit that the material structure is such that sp 2 carbon atoms also contribute to the mechanical properties, as the results of Lacerda et al. for a-C:H films seem to indicate w22,24x; or, more plausibly, Žii. one must consider that the mechanical properties are not only dependent on the sp 2rsp 3 fraction, but also on the detailed material bonding properties, in particular on the hydrogenation of the sp 3 matrix. In this way, films deposited at high bias can present higher hardness due to their reduced density of Žsp 3 . C᎐H bonds, as the results in Fig. 4b suggest, which would imply a more rigid network. Still, a deeper analysis of this issue will be the subject of a future publication. In order to investigate to what extent the properties of Si-DLC could be improved by silicon addition, a series of depositions with varying gas composition was carried out at high values of self-bias. Fig. 5 shows the effect of increasing silane content in the plasma on the hardness of films produced at y800 V. Although some variation in the hardness can be observed, with somewhat higher values obtained for silane contents in the range 2᎐5 vol.%, all films present a hardness higher than 20 GPa. The insert in Fig. 5 shows that almost stress-free films can be obtained in this way, since the residual stress of the films is strongly reduced to approximately 0.5 GPa. The corresponding maximum de-
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In this way, it has been demonstrated that Si-DLC films with high hardness Ž) 20 GPa., low stress Ž; 0.5 GPa. and high deposition rates Ž) 40 nmrmin. can be deposited by PECVD.
Acknowledgements This work was supported by the Finep and CNPq Brazilian agencies, the CNPqrDLR International Cooperation Program and DLRrBMBF ŽGermany.. References
Fig. 5. Vickers micro hardness as a function of silane content in the plasma for Si-DLC films deposited at a self-bias of y800 V. The insert shows the results for the residual internal stress of the same films.
position rate achieved with 20 vol.% silane is larger than 40 nmrmin. This is a very important result from the point of view of technological applications, since it demonstrates the possibility of producing hard Si-DLC films with low stress and a high deposition rate.
4. Conclusions A study of the influence of deposition parameters, namely self-bias and gas composition, on the properties of Si-DLC films deposited by PECVD from gaseous mixtures of silane and methane has been carried out. Films deposited at high self-bias show high deposition rates, low stress and surprisingly high hardness. Increasing silane concentration in the gas phase leads to an enhancement of the observed effects. Compositional and structural characterization of the films showed that Si-DLC films deposited at high values of bias show a silicon content similar to the silicon concentration in the gas phase, and an increased sp 2 character. Increasing silane contents up to 20 vol.% yielded a further reduction in stress, with almost no effect on hardness.
w1x A.L. Baia Neto, R.A. Santos, F.L. Freire, Jr et al., Thin Solid Films 293 Ž1997. 206. w2x A.K. Gangopadhyay, P.A. Willermet, M.A. Tamor, W.C. Vassell, Tribol. Int. 30 Ž1997. 9. w3x L. Jiang, X. Chen, X. Wang, L. Xu, F. Stubhan, K.-H. Merkel, Thin Solid Films 352 Ž1999. 97. w4x W.-J. Wu, M.-H. Hon, Thin Solid Films 345 Ž1999. 200. w5x J. Michler, M. Tobler, E. Blank, Diamond Relat. Mater. 8 Ž1999. 510. w6x A.K. Gangopadhyay, P.A. Willermet, W.C. Vassell, M.A. Tamor, Tribol. Int. 30 Ž1997. 19. w7x C. De Martino, F. Demichelis, A. Tagliaferro, Diamond Relat. Mater. 3 Ž1994. 547. w8x C. De Martino, G. Fusco, G. Mina et al., Diamond Relat. Mater. 6 Ž1997. 559. w9x S.S. Camargo Jr, A.L. Baia Neto, R.A. Santos, F.L. Freire Jr, R. Carius, F. Finger, Diamond Relat. Mater. 7 Ž1998. 1155. w10x S.S. Camargo Jr, R.A. Santos, A.L. Baia Neto, R. Carius, F. Finger, Thin Solid Films 332 Ž1998. 130. w11x W.-J. Wu, M.-H. Hon, Surf. Coat. Technol. 111 Ž1999. 134. w12x S.S. Camargo Jr., R.A. Santos, W. Beyer, Diamond Relat. Mater., in press. w13x A.L. Baia Neto, S.S. Camargo Jr, R. Carius, F. Finger, W. Beyer, Surf. Coat. Technol. 120r121 Ž1999. 395. w14x K. Oguri, T. Arai, Thin Solid Films 208 Ž1992. 158. w15x J. Smeets, J. Meneve, R. Jacobs, L. Eersels, E. Dekempeneer, J. Phys. IV ŽColoque C3. Ž1993. 503. w16x M.-G. Kim, K.-R. Lee, K.Y. Eun, Surf. Coat. Technol. 112 Ž1999. 204. w17x W.C. Vassel, A.K. Gangopadhyay, T.J. Potter, M.A. Tamor, M.J. Rokosz, J. Mater. Eng. Perf. 6 Ž1997. 426. w18x C.-P. Klages, A. Dietz, T. Hoing, R. Thyen, A. Weber, P. ¨ Willich, Surf. Coat. Technol. 80 Ž1996. 121. w19x M. Gischke, A. Hieke, F. Morgenweck, H. Dimigen, Diamond Relat. Mater. 7 Ž1998. 454. w20x X. Zhang, W.H. Weber, W.C. Vassell, T.J. Potter, M.A. Tamor, J. Appl. Phys. 83 Ž1998. 2820. w21x T. Michler, M. Grischke, K. Bewilogua, A. Hieke, Surf. Coat. Technol. 111 Ž1999. 41. w22x F.C. Marques, R.G. Lacerda, M.M. de Lima Jr, J. Vilcarromero, Thin Solid Films 343r344 Ž1999. 222. w23x M.A. Tamor, W.C. Vasell, K.R. Carduner, Appl. Phys. Lett. 58 Ž1991. 592. w24x R.G. Lacerda, F.C. Marques, Appl. Phys. Lett. 73 Ž1998. 617.
Deposition of Si-DLC films with high hardness, low stress and high deposition rates J.C. Damasceno a , S.S. Camargo Jr a,U , F.L. Freire Jr b, R. Carius c a
Engenharia Metalurgica e de Materiais-Uni¨ ersidade Federal do Rio de Janeiro, Cx. Postal 68505, Rio de Janeiro, RJ, CEP 21945-970, ´ Brazil b ´ Uni¨ ersidade Catolica Departamento de Fısica-Pontificia ´ ´ do Rio de Janeiro, Cx. Postal 38071, Rio de Janeiro, RJ, CEP 22454-970, Brazil c ISI-IPV, Forschungszentrum Julich D-52425, Germany ¨ GmbH, Postfach 1913, Julich, ¨
Abstract In this work silicon-incorporated diamond-like carbon ŽSi-DLC. films were produced by plasma enhanced chemical vapor deposition ŽPECVD. from gaseous mixtures of CH 4 and SiH 4 . A study of the influence of self-bias and gas composition on the mechanical and structural properties of the films was carried out. Results show that films deposited at high self-bias present high deposition rates, low stress and surprisingly high hardness. Increasing silane concentration in the gas phase leads to an enhancement of the observed effects. Compositional and structural characterization show that deposition at high bias leads to increased sp 2 character and rather low silicon contents. Increasing the silane content in the plasma leads to an increase in the sp 3 fraction of the films, and yields a further reduction of stress with almost no effect upon hardness. In this way, the possibility of producing films with high hardness Ž) 20 GPa., low stress Ž; 0.5 GPa. and high deposition rates Ž) 40 nmrmin. has been demonstrated. This result is thought to be very important from the point of view of technological applications. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Diamond-like carbon; Silicon; Plasma chemical vapor deposition ŽCVD.
1. Introduction Silicon-incorporated diamond-like carbon films ŽSiDLC. have been attracting increasing interest of researchers since they have a great potential for solving some of the major drawbacks of pure DLC films. For this reason, a considerable amount of work on these films has been carried out in the last 3᎐4 years. Indeed, Si-DLC films present: reduced residual internal stress w1,2x; high deposition rates w3x; good adhesion to most substrates, including various metal alloys, steels and glasses w4᎐6x; very high hardness w7,8,5x; improved thermal stability w9᎐11x; reduced hydrogen loss and graphitization w12,13x; low friction coefficients independent of relative humidity w14᎐16x; resistance to oxidation, moisU
Corresponding author.
ture and corrosion w2,17,18x; and low wettability w19x. This remarkable collection of properties makes this material a promising candidate for a large number of technological applications as metallurgical and protective coatings. In fact, Si-DLC films have already been tested as wear-resistant tribological coatings for highstress power applications, which included car engines, transmission gears and manufacturing tools, with good results w17x. Among the different deposition techniques, the most widely used has been plasma enhanced chemical vapor deposition ŽPECVD.. Various source gases have been used, such as: methane ŽCH 4 . and silane ŽSiH 4 . w1,15x; CH 4 , SiH 4 and Ar w4x; CH 4 , SiH 4 and He w3x; benzene ŽC 6 H 6 . and H 2-diluted SiH 4 w16x; tetramethylsilane ŽTMS. w18x; CH 4 and TMS w20x; acetylene ŽC 2 H 2 . and TMS w5,19x; CH 4 , TMS, Ar and H 2 w21x; and CH 4 , silicon tetrachloride ŽSiCl 4 ., Ar and H 2 w14x. Despite
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 9 3 2 - 4
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J.C. Damasceno et al. r Surface and Coatings Technology 133᎐134 (2000) 247᎐252
the various deposition conditions used by different laboratories, a general finding is that silicon contributes to the stabilization of the carbon sp 3 phase, with beneficial effects on the properties of the material w1,7,14,15,20x. However, in spite of all the work carried out there has been no report up to now of a systematic study of the influence of deposition conditions on the structure and properties of these films. In this work we report on the influence of variation in the cathode self-bias and gas mixture composition on the properties of Si-DLC films deposited by PECVD from gaseous mixtures of SiH 4 and CH 4 , and show that is possible to produce hard films Ž) 20 GPa. with low internal stress Ž; 0.5 GPa. at high deposition rates Ž) 40 nmrmin. at high values of self-bias.
2. Experimental Silicon-incorporated hydrogenated amorphous carbon films ŽSi-DLC. were deposited from gaseous mixtures of methane and silane onto crystalline silicon substrates placed on the cathode of a conventional radio frequency Ž13.56 MHz. parallel-plate glow discharge reactor. A gas flow of approximately 10 sccm, with silane contents in the range 0᎐20 vol.% were fed into the reactor through mass flow controllers. During all deposition runs the electrode distance and chamber pressure Žmonitored by a capacitance manometer. were kept fixed at 2.5 cm and 2.0 Pa, respectively. Films with a thickness between 1 and 2 m were produced at different self-bias voltages Ž VB . from y50 up to y1100 V. It must be noted that no temperature control of the substrate was carried out during deposition. Therefore, substantial heating of the substrate surface may occur as a result of its interaction with the plasma, mainly in case of the depositions with high power densities, in contrast to our previous work, where the substrate temperature was found to be limited to approximately 100⬚C at VB s y200 V. The film composition was determined by ion beam analysis. A 4-MeV van de Graaff accelerator provided Heq beams. For Rutherford backscattering spectrometry ŽRBS. measurements, we used a 2-MeV beam, with the detector positioned at 165⬚ with respect to the incident ions. Hydrogen content of the samples was determined by elastic recoil detection analysis ŽERDA., with a probe of 2.2 MeV Heq incident on the sample at an angle of 75⬚ with respect to the surface normal. Detection of recoil protons was performed at an angle of 30⬚. A 10-m Mylar foil covered the detector in order to prevent the detection of forward-scattered alpha particles. Raman spectra were recorded in a backscattering geometry using the 488-nm line of a Coherent Innova 100 argon laser for excitation, a Spex
Fig. 1. Deposition rate as a function of negative self-bias voltage of Si-DLC films prepared with silane concentrations in the gas of 0.2 vol.% Žsquares. and 2.0 vol.% Žcircles..
1404 double monochromator for dispersion, and a liquid nitrogen-cooled CCD camera for measuring the single-shot spectra. Infrared spectra were measured in a vacuum Fourier transform spectrometer Bruker IFS 66V. Residual internal stress was obtained by the substrate bending method using a Dektak IIA stylus profilometer, which was also used for the thickness measurements. All films presented compressive stresses. Indentation of the samples was carried out with a Vickers diamond micro-indenter with a 0.25-N load for approximately 20 s, keeping the indentation depth smaller than 20% of the sample thickness. Hardness was obtained from the measurement of the indentation diagonals on an optical microscope using the differential interference contrast ŽDIC. technique. In all cases hardness was calculated from the average of a series of 20 different indentations.
3. Results and discussion The deposition rate of Si-DLC films is strongly increased when the RF power density dissipated in the plasma is increased in a similar manner to that observed for pure a-C:H films. As shown in Fig. 1, the deposition rate as a function of the resulting cathode self-bias potential presents an increase of one order of magnitude when the self-bias voltage is increased from y50 to y1100 V. In addition, comparison of the results obtained for films produced with two different silane
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concentrations in the plasma, e.g. 0.2 and 2.0 vol.%, shows that the deposition rate can be further increased by silane addition to the discharge. For instance, films deposited with 0.2 vol.% SiH 4 present a maximum deposition rate of approximately 28 nmrmin, whereas with 2.0 vol.% SiH 4 , a deposition rate of 35 nmrmin was achieved at VB s y1100 V and under the specific deposition conditions employed. The residual internal stress of Si-DLC films depends strongly on the cathode self-bias potential employed. In the low self-bias range, the internal stress is increased when the self-bias voltage increases, attains a maximum, and then decreases monotonically for high values of self-bias, as shown in Fig. 2a. Comparing the stress values obtained for VB s y200 and y1100 V, an internal stress reduction larger than three-fold was achieved for films deposited with 0.2 vol.% SiH 4 . Furthermore, increasing the silane content in the reactor resulted in an overall decrease in stress. In this way, films with residual internal stresses lower than 1.0 GPa could be obtained. The behavior observed for film hardness as a function of the cathode self-bias was rather remarkable. As is evident in Fig. 2b, a monotonic increase of hardness can be observed when the self-bias is increased from y50 up to y1100 V. Even for high values of self-bias voltage, no reduction in the hardness values was noted, in contrast to what is generally observed for pure a-C:H. Also, hardness values for films deposited with 2.0 vol.% silane are systematically higher than those of films deposited with lower silane contents. At this point, it is important to mention that the
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influence of the substrate hardness, as well as the limited thickness of the films and elastic recovery after indentation, on the present results are expected to be small, since hardness values measured by nanoindentation on some of our samples fell within 10᎐15% of the present values. Accordingly, other authors have also obtained a good agreement between hardness values measured by nano- and Vickers indentation for films with similar thickness and within the same range of values Ž20᎐25 GPa. w22x. In this way, one can guarantee the reliability of these results and conclude that Si-DLC films with high hardness, low internal stress and high deposition rates can be produced at high values of self-bias. The behavior observed for the deposition rate and internal stress as a function self-bias are similar to that observed years ago for pure a-C:H films deposited from CH 4 w23x. It is important to emphasize, however, that in the present case, the addition of silane to the discharge enhanced the deposition rate and contributed to an overall stress relief. These facts have been already observed in our laboratory for films deposited at VB s y200 V w1x. However, in the case of pure a-C:H films, hardness is low for the low bias material Žpolymer-like films., achieves a maximum for the so-called diamondlike films that are deposited at an intermediate value of bias, and decreases strongly in the case of films deposited at high bias Žgraphite-like films.. In contrast to this, hardness values for the present films show no evidence of reduction at high self-bias but, on the contrary, are slightly increased. Moreover, films de-
Fig. 2. Ža. Residual internal stress and Žb. Vickers micro hardness as a function of negative self-bias voltage of Si-DLC films prepared with silane concentrations in the gas of 0.2 vol.% Žsquares. and 2.0 vol.% Žcircles..
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Fig. 3. Atomic composition as a function of negative self-bias voltage of Si-DLC films prepared with 2.0 vol.% silane in the plasma.
posited with a larger concentration of SiH 4 in the plasma showed even higher hardness values. Recently, Lacerda et al. reported on the deposition of hard a-C:H films with low stress at high values of self-bias w24x. They could obtain films with a hardness of 14 GPa, stress of 0.5 GPa and deposition rate of 14 nmrmin by methane gas decomposition in a RF sputtering system, without the addition of any impurities to the films. However, similar to pure a-C:H, their maximum hardness values Ž; 25 GPa. were obtained at VB s y200 V. Those authors attributed their results to the different geometry of their deposition system. Although a similar effect may play an important role here, our results clearly show that the addition of silane to the discharge results in a further enhancement of the observed effects, improving the material properties. In order to try to understand the above-described behavior, compositional and structural characterization of the samples was carried out. Fig. 3 shows the atomic composition as a function of self-bias obtained from RBS and ERDA measurements of samples deposited with 2.0 vol.% SiH 4 in the discharge. It is evident that carbon content of the films is increased as the self-bias increases, whereas silicon content is correspondingly decreased. Whereas at low self-bias voltages the silicon content of the films is much larger than in the gas phase, at high values of bias the silicon concentration in the samples tends to reach a value similar to the gas phase. Also, the hydrogen concentration of the films is somewhat reduced by the self-bias increase. An equivalent behavior was obtained for the composition of films
deposited with 0.2 vol.% silane in the gas, although in this case the silicon contents were approximately one order of magnitude smaller. The observed variation in carbon and silicon contents of the films as a result of increasing self-bias can be understood based on the larger dissociation crosssections of silane compared to methane molecules and their dependence on the RF power density in the discharge. Therefore, at low RF power densities Žlow self-bias voltages. the dissociation rate of silane molecules is much higher than that of methane, leading to a relatively larger density of Si-related radicals in the plasma than C-related ones, thus enhancing silicon incorporation into the films. As the RF power is increased Žincreased self-bias ., the dissociation rate of methane molecules increases substantially, leading to higher carbon contents in the samples. Consistent with this picture, estimates based on the results presented above show that the deposition rate of silicon atoms is essentially independent of the self-bias in the investigated range, whereas the deposition rate of carbon atoms present a ten-fold increase when self-bias increases from VB s y50 to y1100 V. In this way, films with very low silicon concentrations are obtained at the highest values of bias. The hydrogen content of the samples, on the other hand, is thought to be controlled by a different mechanism, namely particle bombardment, which is strongly enhanced by the self-bias increase. However, samples deposited with self-bias voltages from y400 up to y1100 V present similar hydrogen contents, in contrast to this simple picture. A deeper investigation is still needed in order to clarify this issue. It must be noted that the present results show that it is possible to obtain films with high hardness, low stress and high deposition rate with very low silicon contents. As already pointed out, a substantial stress decrease can be obtained with the incorporation of just a few at.% of silicon atoms w1x. These results suggest that on the improvement of the film properties as a result of silane addition to the discharge, changes in plasma andror film growth conditions may play an important role. Raman investigation of Si-DLC films has shown that an increase in the self-bias voltage has a strong effect on the structure of the material. It is evident from the spectra shown in Fig. 4a that when the self-bias voltage is increased from y200 to y1100 V, the G-peak is shifted from approximately 1500 to 1570 cmy1 . Correspondingly, the relative intensity of the D-peak is also greatly increased. This behavior has been extensively observed by other authors in the case of pure a-C:H films, and is indicative of an increased sp 2 character. Although in the present case the compositional changes induced by the self-bias variation may also somewhat affect the Raman spectra, the observed effect on both
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Fig. 4. Ža. Raman and Žb. infrared spectra of Si-DLC films prepared with 2.0 vol.% silane in the plasma and different negative self-bias voltages.
peak position and intensity is much larger than expected from the observed variation in the carbon content of the films w9,20x. Therefore, one can conclude that the Raman spectra provide strong evidence of an increased sp 2 character of the films deposited at high bias. On the other hand, the comparison between the Raman spectra obtained for films deposited with two different silane concentrations showed that increasing silane concentration in the plasma enhances the sp 3 character of the films, in accordance with previous results w1,15,20x. Infrared analysis of the films has led to consistent conclusions about their bonding properties. Fig. 4b shows the spectra obtained in the range of the C᎐H stretching mode Ž2700᎐3100 cmy1 .. It is evident that a strong reduction in the peaks in the region around 2900 cmy1 ᎏ which are generally attributed to hydrogen bonded to sp 3 carbon ᎏ occurs as a consequence of increasing the self-bias voltage. The observed decrease is much larger than expected from a simple reduction in the hydrogen content. On the other hand, in the region above 3000 cmy1 ᎏ which is related to hydrogen bonded to sp 2 carbon ᎏ a significant reduction in the absorbance is not observed, suggesting an increased sp 2 fraction of the films deposited at high bias, in accordance with the Raman results. Again, the analysis of infrared spectra of films deposited with different silane concentrations has shown that the effect of silane addition to the discharge is to increase the sp 3 character of the films. Thus, one can conclude that films obtained at high values of self-bias present good mechanical properties
and an increased sp 2 character. The addition of larger silane contents to the discharge, on the other hand, increases the sp 3 character Žreduces sp 2 . and contributes to a further improvement in the mechanical properties. This puzzling situation can be understood in two different ways: Ži. one may admit that the material structure is such that sp 2 carbon atoms also contribute to the mechanical properties, as the results of Lacerda et al. for a-C:H films seem to indicate w22,24x; or, more plausibly, Žii. one must consider that the mechanical properties are not only dependent on the sp 2rsp 3 fraction, but also on the detailed material bonding properties, in particular on the hydrogenation of the sp 3 matrix. In this way, films deposited at high bias can present higher hardness due to their reduced density of Žsp 3 . C᎐H bonds, as the results in Fig. 4b suggest, which would imply a more rigid network. Still, a deeper analysis of this issue will be the subject of a future publication. In order to investigate to what extent the properties of Si-DLC could be improved by silicon addition, a series of depositions with varying gas composition was carried out at high values of self-bias. Fig. 5 shows the effect of increasing silane content in the plasma on the hardness of films produced at y800 V. Although some variation in the hardness can be observed, with somewhat higher values obtained for silane contents in the range 2᎐5 vol.%, all films present a hardness higher than 20 GPa. The insert in Fig. 5 shows that almost stress-free films can be obtained in this way, since the residual stress of the films is strongly reduced to approximately 0.5 GPa. The corresponding maximum de-
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In this way, it has been demonstrated that Si-DLC films with high hardness Ž) 20 GPa., low stress Ž; 0.5 GPa. and high deposition rates Ž) 40 nmrmin. can be deposited by PECVD.
Acknowledgements This work was supported by the Finep and CNPq Brazilian agencies, the CNPqrDLR International Cooperation Program and DLRrBMBF ŽGermany.. References
Fig. 5. Vickers micro hardness as a function of silane content in the plasma for Si-DLC films deposited at a self-bias of y800 V. The insert shows the results for the residual internal stress of the same films.
position rate achieved with 20 vol.% silane is larger than 40 nmrmin. This is a very important result from the point of view of technological applications, since it demonstrates the possibility of producing hard Si-DLC films with low stress and a high deposition rate.
4. Conclusions A study of the influence of deposition parameters, namely self-bias and gas composition, on the properties of Si-DLC films deposited by PECVD from gaseous mixtures of silane and methane has been carried out. Films deposited at high self-bias show high deposition rates, low stress and surprisingly high hardness. Increasing silane concentration in the gas phase leads to an enhancement of the observed effects. Compositional and structural characterization of the films showed that Si-DLC films deposited at high values of bias show a silicon content similar to the silicon concentration in the gas phase, and an increased sp 2 character. Increasing silane contents up to 20 vol.% yielded a further reduction in stress, with almost no effect on hardness.
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