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Production and characterization of hydrogenated amorphous carbon thin films deposited in methane plasmas diluted by noble gases
Materials Science and Engineering B 112 (2004) 101–105 www.elsevier.com/locate/mseb
Production and characterization of hydrogenated amorphous carbon thin films deposited in methane plasmas diluted by noble gases G. Capote, F.L. Freire Jr.* Departamento de Fı´sica, Pontifı´cia Universidade Cato´lica do Rio de Janeiro, Rua Marqueˆs de Sa˜o Vicente, 225-Ga´vea, Post-Office Box 38071, 22452-970, Rio de Janeiro, RJ, Brazil
Abstract The dilution effects of the precursor methane atmosphere by three noble gases (Ar, Ne and He) on the mechanical properties and the microstructure of hydrogenated amorphous carbon films deposited by rf-PECVD were studied. The chemical composition and atomic density of the films were determined by ion beam analysis. The film microstructure was probed by means of Raman spectroscopy. The internal stress was determined through the measurement of the changing of the substrate curvature by a profilometer, while nanoindentation experiments provided the film hardness. The results show that the precursor atmosphere dilution by different noble gases did not induce substantial modifications in the microstructure or in the mechanical properties of the films. On the other hand, the composition, the microstructure and the mechanical properties of the films are strongly dependent on the self-bias voltage. The results confirm the importance of the ion bombardment during film growth on the mechanical properties of the films. # 2004 Elsevier B.V. All rights reserved. Keywords: PECVD; Amorphous hydrogenated carbon; Methane; Noble gases
1. Introduction Research and development of nanostructured materials with improved, tailor-designed properties is a fundamental need for the growth and advance of automotive, aerospace, biomedical and electronic industries among others. Plasma synthesis of coatings is a powerful and versatile way to obtain such materials. Among them, the family of amorphous hydrogenated carbon (a-C:H) coatings stands out due to its properties: high hardness, chemical inertness, low friction and high wear resistance, and also due to the possibility to tune these properties by specific settings of the plasma conditions and deposition technique [1]. Amorphous hydrogenated carbon films are mostly obtained by plasma decomposition of a hydrocarbon-containing precursor atmosphere. In a-C:H films deposited by methane plasma decomposition, the structure is composed of sp2 hybridized clusters interconnected by sp3 hybridized carbon atoms. It is usually accepted that surface chemio* Corresponding author. Tel.: +55 21 3114 1272; fax: +55 21 3114 1040. E-mail address: [email protected] (F.L. Freire Jr.). 0921-5107/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.05.012
sorption of carbon carrying nutral radicals is the main channel for film growth [2]. The addition of noble gases to the hydrocarbon precursor atmosphere is expected to increase the ratio of ion to neutral radicals on the surface of the growing film without changing the H/C ratio of the gas mixture. This is in fact a powerful way to investigate the effect of ion bombardment on the structural arrangement and properties of a-C:H films. Recently, a comparison between the effects of different noble gases dilution of CH4 atmospheres was carried out [3,4]. Both works investigated the structure, mechanical and optical properties of the films, though the investigation by Sun et. al. was restricted to methane rich atmospheres only, i.e., noble gas dilution down to 50% [3]. The effects of argon dilution up to 98% were also recently investigated [5,6]. Nevertheless, in spite of intense experimental and theoretical work it is not yet fully understood which mechanisms and plasma species are responsible for the film deposition process [2,7]. In this work, a-C:H films were deposited by plasma enhanced chemical vapor deposition (PECVD) using three noble gases (Ar, Ne and He) methane gas mixtures. The effect of ion bombardment on the structural arrangement,
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G. Capote, F.L. Freire Jr. / Materials Science and Engineering B 112 (2004) 101–105
chemical composition and mechanical properties by means of noble gas dilution of the hydrocarbon precursor atmosphere was investigated.
3. Results and discussions Fig. 1 shows the deposition rate as function of the selfbias voltage for CH4 partial pressures of 2 and 100% and for three noble gases used. The deposition rate measured for films deposited from pure methane atmosphere is one order of magnitude higher than the deposition rate determined for films deposited from noble gas-diluted atmosphere, as expected when one compares the carbon atom concentration in the plasma atmosphere. The deposition rates obtained for the three noble gases used were quite similar. For all deposition conditions an increase in the deposition rate for
Deposition rate (nm/min)
The a-C: H films were deposited by PECVD from pure methane, CH4–Ar, CH4–Ne and CH4–He mixtures employing an asymmetrical capacitively-coupled deposition system. Silicon substrates were mounted on a water-cooled 7.5-cm diameter copper cathode fed by a rf (13.56 MHz) power supply. The films were deposited with a total pressure of 13 Pa and total incoming gas flux of 50 sccm up to a thickness of about 400 nm. Two CH4 partial pressures were employed, 2 and 100%, and the self-bias voltage (Vb) was varied between 50 and 500 V by adjusting the r.f power. The rf power increased from 1 to 60 W. The chemical composition was determined by ion beam analysis (IBA): Rutherford backscattering spectrometry (RBS) and elastic recoil detection analysis (ERDA) employing a 4-MV Van de Graaff accelerator KN-4000 from High Voltage Engineering Corp. The atomic density was inferred by combining the areal atomic density provided by IBA and the thickness obtained by stylus profilometry. The atomic arrangement of the films was probed by Raman scattering spectroscopy. The former was performed with a Renishaw 2000 system using an Ar+-ion laser (l = 488 nm) in backscattering geometry. The laser power on the sample was ~0.6 mW and the laser spot has 1 mm of diameter. All measurements were carried out in air at room temperature. The stress determination was made by measuring the curvature of the substrates by means of stylus profilometry and by applying Stoney’s equation, as described in detail elsewhere [8]. The hardness of the films was measured employing a nanoindenter with loads in the range of 200– 800 mN. The film hardness was obtained according to the Oliver and Pharr method [9]. The final values presented in this work correspond to the average of 8 indentations carried out in different spots for penetration depths in the range of 20–30 nm that are shallower than 10% of the samples thickness.
CH4- Ar CH4- Ne CH4- He CH4
30
20
10
0 0
100
200
300
400
500
Vb (-V) Fig. 1. Deposition rates of a-C:H films as a function of self-bias voltage. The lines are only to guide the eyes.
higher Vb values was observed and it can be explained by the higher power input needed to achieve the desired self-bias voltage and the consequent more efficient plasma dissociation. Also, the contribution to an increased generation of adsorption sites on the film surface due to the more energetic ion bombardment should be taken into account. Moreover, differences due to physical sputtering appear to be secondary since the results obtained for the three different noble gases are nearly the same. The hydrogen content and atomic density of the all films as function of the self-bias voltage are presented in Figs. 2 and 3, respectively. Films deposited at low Vb present higher hydrogen contents and lower atomic densities, while for 40
Hydrogen content (at.%)
2. Experimental procedures
40
CH4- Ar CH4- Ne CH4- He CH4
35
30
25
20
15 0
100
200
300
400
500
Vb (-V) Fig. 2. Hydrogen content measured by ERDA as a function of self-bias voltage for a-C:H films. The lines are only to guide the eyes.
G. Capote, F.L. Freire Jr. / Materials Science and Engineering B 112 (2004) 101–105
23
1.4
1.2
CH4- Ar CH4- Ne CH4- He CH4
1.0
Raman intensity (arbitrary units)
D
3
Atomic Density (10 atoms/cm )
1.6
200
300
400
500
98% Ne + 2% CH4
98% He + 2% CH4
100% CH4
Vb (-V) Fig. 3. Atomic density as a function of the self-bias voltages for a-C:H films. The lines are only to guide the eyes.
higher Vb values the density increases and hydrogen content decreases. Both density and hydrogen content tend to constant values for jVbj > 250 V. Within the experimental accuracy, the hydrogen content and atomic density of the films are not strongly dependent on the noble gas dilution. Hydrogen content in a-C:H films deposited in noble gasesCH4 is systematic lower than that in pure CH4. This result can be attributed to a more effective preferential hydrogen sputtering under noble gas bombardment. The decrease of the hydrogen content for higher self-bias voltages was previously reported for a-C:H films deposited in both pure [10–12] and noble gases diluted [4] methane atmospheres. In which concerns the film density, despite the large experimental uncertainties, the density of films deposited in Ar–CH4 mixtures are systematically higher than the values obtained from other gas mixtures. Simple kinematics considerations show that the momentum transferred in binary collisions with carbon atoms is nearly the same for helium and argon atoms. Despite the higher ionization energy for helium that probably results in a higher degree of plasma ionization, argon bombardment seems to be more efficient in order to produce a more compact film microstructure. Raman spectra obtained from a-C:H films deposited with Vb = 350 V for pure methane and gas mixtures with He, Ne, and Ar are presented in Fig. 4, while the self-bias dependence was illustrated in Fig. 5. In this figure, the Raman spectra obtained from films deposited in a precursor atmosphere with 98% of argon are shown. The spectrum from the film deposited with Vb = 50 V is typical of a polymer-like a-C:H film, presenting a high luminescence background which, in fact, did not permit a reliable analysis of the D and G bands. The other spectra are typical of diamond-like arrangements [1,10,12]. Two overlapping
1500
2000 -1
Raman shift (cm ) Fig. 4. Selected Raman spectra obtained from a-C:H films deposited with Vb = 350 V for CH4 partial pressures of 2 and 100% (Ar, Ne and He mixtures were indicated). The arrows indicate the position of the D and G bands.
bands, known as the D and G bands, dominate them. While the G band, that appears approximately 1545 cm1, is associated with the optically allowed E2 g zone center of crystalline graphite, the D band, that appears approximately 1390 cm1, is originated in the relaxation of the full D6 h symmetry of finite graphite crystallites that allows many forbidden modes to show Raman activity [13]. After subtraction of the luminescence background, the spectra can be fitted using two Gaussian lines. The ID/IG intensity ratios, G band peak position and G bandwidth, obtained from the
Raman intensity (arbitrary units)
100
G
98% Ar + 2% CH4
1000
0
103
-50 V
G D -200 V
-350 V -500 V
1000
1500
2000 -1
Raman shift (cm ) Fig. 5. Raman spectra obtained from films deposited with a CH4 partial pressure of 2% and 98% Ar for several self-bias voltages. The self-bias voltages are quoted in the figure. The arrows indicate the position of the D and G bands.
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25
0.9
CH4 - Ar CH4 - Ne CH4 - He CH4
0.6
-1
ωG (cm )
1555 1550 1545
-1
ΓG (cm )
150
15
10
CH4 - Ar CH4 - Ne CH4 - He CH4
145 140 135
5
130 100
200
300
400
500
Vb (-V) Fig. 6. ID/IG intensity ratio, G band peak position and G bandwidth as function of the self-bias voltage for a-C:H films deposited from different precursor atmospheres. The lines are only to guide the eyes.
fitting parameters are plotted in Fig. 6. All the curves present the same behavior for all precursor atmosphere used. The increase of ID/IG ratio, together with the shift of the G band peak towards higher frequencies, accompanied by a reduction of the bandwidth, is usually interpreted in terms of an increase of the graphitic domains, either in number or in size [14]. The results suggest a progressive graphitization of the a-C:H films upon self-bias increase. 3.5
Compressive Stress (GPa)
20
Hardness (GPa)
ID/IG
1.2
3.0
2.5
0
100
200
300
400
500
Vb (-V) Fig. 8. Hardness as a function of the self-bias voltage for a-C:H films deposited with CH4 partial pressures of 2 and 100%. The lines are only to guide the eyes.
Figs. 7 and 8 present the results of the mechanical properties characterization. The internal compressive stress is presented in Fig. 7. The low values of hardness and stress measured for films deposited at low self-bias voltage can be attributed to the polymeric character of the films. The internal stress has a maximum at around 200 V for films deposited in all CH4-noble gases atmospheres, while it appears at Vb = 350 V for films deposited from pure methane atmosphere. Similar behavior was observed for the hardness measurements, shown in Fig. 8. However, in the case of films deposited in pure methane atmosphere the maximum is again displaced to higher self-bias values. The existence of this maximum in a-C:H films deposited by PECVD was observed before [4,10,15] and was explained by the subplantation model [16]. The fact that the position of the maximum is dependent on the precursor gas used can be understood in terms of the different ion assistance regimes during film growth.
2.0
4. Summary and conclusions 1.5 CH4 - Ar CH4 - Ne CH4 - He CH4
1.0
0.5 0
100
200
300
400
500
Vb (-V) Fig. 7. Compressive internal stress as a function of the self-bias voltage for a-C:H films deposited with CH4 partial pressures of 2 and 100%. The lines are only to guide the eyes.
A systematic investigation of the effects of the dilution of the CH4 precursor atmosphere by Ar, Ne and He was carried out in this work. The obtained results point out to the fact that the plasma modifications due to methane dilution by noble gases did not result in substantial modifications on the film microstructure or mechanical properties. Only minor differences, such as, a smaller incorporation of hydrogen in the film microstructure and small density changes were observed for films deposited from highly diluted atmospheres. In addition, if there is a change of the film building block in conditions of high noble gases dilution, as sug-
G. Capote, F.L. Freire Jr. / Materials Science and Engineering B 112 (2004) 101–105
gested for the case of methane diluted in argon [7], it does not affect the final film properties. The results obtained from the series of films deposited with different self-bias voltages clearly show that the composition, microstructure and mechanical properties of the films are strongly dependent on the energy of the incoming ions. When we plot the internal stress and hardness as functions of the self-bias voltage, there are differences in the position of the maximum observed from films deposited from pure methane atmospheres and from diluted atmospheres. These results confirm the importance of the ion bombardment during film growth on the mechanical properties of the films and can be explained on basis of two different bombardment regimes and can be, at least qualitatively, described by the subplantation model.
Acknowledgements The authors are very grateful for experimental work by A.R. Zanatta, L.G. Jacobsohn and M.E.H. da Costa. This work is partially supported by the Brazilian agencies: CNPq, CAPES and FAPERJ.
105
References [1] J. Robertson, Prog. Solid State Chem. 21 (1991) 199. [2] W. Jacob, Thin Solid Films 326 (1999) 1. [3] Z. Sun, C.H. Lin, Y.L. Lee, J.R. Shi, B.K. Tay, X. Shi, J. Appl. Phys. 87 (2000) 8122. [4] E. Tomasella, C. Meunier, S. Mikhailov, Surf. Coat. Technol. 141 (2001) 286. [5] L. Valentini, J.M. Kenny, G. Mariotto, P. Tosi, G. Carlotti, G. Socino, L. Lozzi, S. Santuci, J. Vac. Sci. Technol. A 19 (2001) 1611. [6] L.G. Jacobsonh, G. Capote, N.C. Cruz, A.R. Zanatta, F.L. Freire Jr., Thin Solid Films 419 (2002) 46. [7] C. Riccardi, R. Barni, M. Fontanesi, P. Tosi, Chem. Phys. Lett. 329 (2000) 66. [8] L.G. Jacobsohn, F.L. Freire Jr., J. Vac. Sci. Technol. A 17 (1999) 2841. [9] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [10] M.A. Tamor, W.C. Vassell, J. Appl. Phys. 76 (1994) 3823. [11] J.W. Zou, K. Reichelt, K. Schmidt, B. Dischler, J. Appl. Phys. 65 (1989) 3914. [12] L.G. Jacobsohn, D.F. Franceschini, M.E.H. Maia da Costa, F.L. Freire Jr., J. Vac. Sci. Technol. A 18 (2000) 2230. [13] F. Tuinstra, J.F. Koening, J. Chem. Phys. 53 (1970) 1126. [14] O. Dillon, J. Woollam, V. Katkanant, Phys. Rev. B 29 (1984) 3482. [15] R.G. Lacerda, F.C. Marques, F.L. Freire Jr., Diam. Rel. Mater. 8 (1999) 495. [16] Y. Lifshitz, S.R. Kasi, J.W. Rabalais, W. Eckstein, Phys. Rev. B 41 (1990) 10468.
Production and characterization of hydrogenated amorphous carbon thin films deposited in methane plasmas diluted by noble gases G. Capote, F.L. Freire Jr.* Departamento de Fı´sica, Pontifı´cia Universidade Cato´lica do Rio de Janeiro, Rua Marqueˆs de Sa˜o Vicente, 225-Ga´vea, Post-Office Box 38071, 22452-970, Rio de Janeiro, RJ, Brazil
Abstract The dilution effects of the precursor methane atmosphere by three noble gases (Ar, Ne and He) on the mechanical properties and the microstructure of hydrogenated amorphous carbon films deposited by rf-PECVD were studied. The chemical composition and atomic density of the films were determined by ion beam analysis. The film microstructure was probed by means of Raman spectroscopy. The internal stress was determined through the measurement of the changing of the substrate curvature by a profilometer, while nanoindentation experiments provided the film hardness. The results show that the precursor atmosphere dilution by different noble gases did not induce substantial modifications in the microstructure or in the mechanical properties of the films. On the other hand, the composition, the microstructure and the mechanical properties of the films are strongly dependent on the self-bias voltage. The results confirm the importance of the ion bombardment during film growth on the mechanical properties of the films. # 2004 Elsevier B.V. All rights reserved. Keywords: PECVD; Amorphous hydrogenated carbon; Methane; Noble gases
1. Introduction Research and development of nanostructured materials with improved, tailor-designed properties is a fundamental need for the growth and advance of automotive, aerospace, biomedical and electronic industries among others. Plasma synthesis of coatings is a powerful and versatile way to obtain such materials. Among them, the family of amorphous hydrogenated carbon (a-C:H) coatings stands out due to its properties: high hardness, chemical inertness, low friction and high wear resistance, and also due to the possibility to tune these properties by specific settings of the plasma conditions and deposition technique [1]. Amorphous hydrogenated carbon films are mostly obtained by plasma decomposition of a hydrocarbon-containing precursor atmosphere. In a-C:H films deposited by methane plasma decomposition, the structure is composed of sp2 hybridized clusters interconnected by sp3 hybridized carbon atoms. It is usually accepted that surface chemio* Corresponding author. Tel.: +55 21 3114 1272; fax: +55 21 3114 1040. E-mail address: [email protected] (F.L. Freire Jr.). 0921-5107/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.05.012
sorption of carbon carrying nutral radicals is the main channel for film growth [2]. The addition of noble gases to the hydrocarbon precursor atmosphere is expected to increase the ratio of ion to neutral radicals on the surface of the growing film without changing the H/C ratio of the gas mixture. This is in fact a powerful way to investigate the effect of ion bombardment on the structural arrangement and properties of a-C:H films. Recently, a comparison between the effects of different noble gases dilution of CH4 atmospheres was carried out [3,4]. Both works investigated the structure, mechanical and optical properties of the films, though the investigation by Sun et. al. was restricted to methane rich atmospheres only, i.e., noble gas dilution down to 50% [3]. The effects of argon dilution up to 98% were also recently investigated [5,6]. Nevertheless, in spite of intense experimental and theoretical work it is not yet fully understood which mechanisms and plasma species are responsible for the film deposition process [2,7]. In this work, a-C:H films were deposited by plasma enhanced chemical vapor deposition (PECVD) using three noble gases (Ar, Ne and He) methane gas mixtures. The effect of ion bombardment on the structural arrangement,
102
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chemical composition and mechanical properties by means of noble gas dilution of the hydrocarbon precursor atmosphere was investigated.
3. Results and discussions Fig. 1 shows the deposition rate as function of the selfbias voltage for CH4 partial pressures of 2 and 100% and for three noble gases used. The deposition rate measured for films deposited from pure methane atmosphere is one order of magnitude higher than the deposition rate determined for films deposited from noble gas-diluted atmosphere, as expected when one compares the carbon atom concentration in the plasma atmosphere. The deposition rates obtained for the three noble gases used were quite similar. For all deposition conditions an increase in the deposition rate for
Deposition rate (nm/min)
The a-C: H films were deposited by PECVD from pure methane, CH4–Ar, CH4–Ne and CH4–He mixtures employing an asymmetrical capacitively-coupled deposition system. Silicon substrates were mounted on a water-cooled 7.5-cm diameter copper cathode fed by a rf (13.56 MHz) power supply. The films were deposited with a total pressure of 13 Pa and total incoming gas flux of 50 sccm up to a thickness of about 400 nm. Two CH4 partial pressures were employed, 2 and 100%, and the self-bias voltage (Vb) was varied between 50 and 500 V by adjusting the r.f power. The rf power increased from 1 to 60 W. The chemical composition was determined by ion beam analysis (IBA): Rutherford backscattering spectrometry (RBS) and elastic recoil detection analysis (ERDA) employing a 4-MV Van de Graaff accelerator KN-4000 from High Voltage Engineering Corp. The atomic density was inferred by combining the areal atomic density provided by IBA and the thickness obtained by stylus profilometry. The atomic arrangement of the films was probed by Raman scattering spectroscopy. The former was performed with a Renishaw 2000 system using an Ar+-ion laser (l = 488 nm) in backscattering geometry. The laser power on the sample was ~0.6 mW and the laser spot has 1 mm of diameter. All measurements were carried out in air at room temperature. The stress determination was made by measuring the curvature of the substrates by means of stylus profilometry and by applying Stoney’s equation, as described in detail elsewhere [8]. The hardness of the films was measured employing a nanoindenter with loads in the range of 200– 800 mN. The film hardness was obtained according to the Oliver and Pharr method [9]. The final values presented in this work correspond to the average of 8 indentations carried out in different spots for penetration depths in the range of 20–30 nm that are shallower than 10% of the samples thickness.
CH4- Ar CH4- Ne CH4- He CH4
30
20
10
0 0
100
200
300
400
500
Vb (-V) Fig. 1. Deposition rates of a-C:H films as a function of self-bias voltage. The lines are only to guide the eyes.
higher Vb values was observed and it can be explained by the higher power input needed to achieve the desired self-bias voltage and the consequent more efficient plasma dissociation. Also, the contribution to an increased generation of adsorption sites on the film surface due to the more energetic ion bombardment should be taken into account. Moreover, differences due to physical sputtering appear to be secondary since the results obtained for the three different noble gases are nearly the same. The hydrogen content and atomic density of the all films as function of the self-bias voltage are presented in Figs. 2 and 3, respectively. Films deposited at low Vb present higher hydrogen contents and lower atomic densities, while for 40
Hydrogen content (at.%)
2. Experimental procedures
40
CH4- Ar CH4- Ne CH4- He CH4
35
30
25
20
15 0
100
200
300
400
500
Vb (-V) Fig. 2. Hydrogen content measured by ERDA as a function of self-bias voltage for a-C:H films. The lines are only to guide the eyes.
G. Capote, F.L. Freire Jr. / Materials Science and Engineering B 112 (2004) 101–105
23
1.4
1.2
CH4- Ar CH4- Ne CH4- He CH4
1.0
Raman intensity (arbitrary units)
D
3
Atomic Density (10 atoms/cm )
1.6
200
300
400
500
98% Ne + 2% CH4
98% He + 2% CH4
100% CH4
Vb (-V) Fig. 3. Atomic density as a function of the self-bias voltages for a-C:H films. The lines are only to guide the eyes.
higher Vb values the density increases and hydrogen content decreases. Both density and hydrogen content tend to constant values for jVbj > 250 V. Within the experimental accuracy, the hydrogen content and atomic density of the films are not strongly dependent on the noble gas dilution. Hydrogen content in a-C:H films deposited in noble gasesCH4 is systematic lower than that in pure CH4. This result can be attributed to a more effective preferential hydrogen sputtering under noble gas bombardment. The decrease of the hydrogen content for higher self-bias voltages was previously reported for a-C:H films deposited in both pure [10–12] and noble gases diluted [4] methane atmospheres. In which concerns the film density, despite the large experimental uncertainties, the density of films deposited in Ar–CH4 mixtures are systematically higher than the values obtained from other gas mixtures. Simple kinematics considerations show that the momentum transferred in binary collisions with carbon atoms is nearly the same for helium and argon atoms. Despite the higher ionization energy for helium that probably results in a higher degree of plasma ionization, argon bombardment seems to be more efficient in order to produce a more compact film microstructure. Raman spectra obtained from a-C:H films deposited with Vb = 350 V for pure methane and gas mixtures with He, Ne, and Ar are presented in Fig. 4, while the self-bias dependence was illustrated in Fig. 5. In this figure, the Raman spectra obtained from films deposited in a precursor atmosphere with 98% of argon are shown. The spectrum from the film deposited with Vb = 50 V is typical of a polymer-like a-C:H film, presenting a high luminescence background which, in fact, did not permit a reliable analysis of the D and G bands. The other spectra are typical of diamond-like arrangements [1,10,12]. Two overlapping
1500
2000 -1
Raman shift (cm ) Fig. 4. Selected Raman spectra obtained from a-C:H films deposited with Vb = 350 V for CH4 partial pressures of 2 and 100% (Ar, Ne and He mixtures were indicated). The arrows indicate the position of the D and G bands.
bands, known as the D and G bands, dominate them. While the G band, that appears approximately 1545 cm1, is associated with the optically allowed E2 g zone center of crystalline graphite, the D band, that appears approximately 1390 cm1, is originated in the relaxation of the full D6 h symmetry of finite graphite crystallites that allows many forbidden modes to show Raman activity [13]. After subtraction of the luminescence background, the spectra can be fitted using two Gaussian lines. The ID/IG intensity ratios, G band peak position and G bandwidth, obtained from the
Raman intensity (arbitrary units)
100
G
98% Ar + 2% CH4
1000
0
103
-50 V
G D -200 V
-350 V -500 V
1000
1500
2000 -1
Raman shift (cm ) Fig. 5. Raman spectra obtained from films deposited with a CH4 partial pressure of 2% and 98% Ar for several self-bias voltages. The self-bias voltages are quoted in the figure. The arrows indicate the position of the D and G bands.
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25
0.9
CH4 - Ar CH4 - Ne CH4 - He CH4
0.6
-1
ωG (cm )
1555 1550 1545
-1
ΓG (cm )
150
15
10
CH4 - Ar CH4 - Ne CH4 - He CH4
145 140 135
5
130 100
200
300
400
500
Vb (-V) Fig. 6. ID/IG intensity ratio, G band peak position and G bandwidth as function of the self-bias voltage for a-C:H films deposited from different precursor atmospheres. The lines are only to guide the eyes.
fitting parameters are plotted in Fig. 6. All the curves present the same behavior for all precursor atmosphere used. The increase of ID/IG ratio, together with the shift of the G band peak towards higher frequencies, accompanied by a reduction of the bandwidth, is usually interpreted in terms of an increase of the graphitic domains, either in number or in size [14]. The results suggest a progressive graphitization of the a-C:H films upon self-bias increase. 3.5
Compressive Stress (GPa)
20
Hardness (GPa)
ID/IG
1.2
3.0
2.5
0
100
200
300
400
500
Vb (-V) Fig. 8. Hardness as a function of the self-bias voltage for a-C:H films deposited with CH4 partial pressures of 2 and 100%. The lines are only to guide the eyes.
Figs. 7 and 8 present the results of the mechanical properties characterization. The internal compressive stress is presented in Fig. 7. The low values of hardness and stress measured for films deposited at low self-bias voltage can be attributed to the polymeric character of the films. The internal stress has a maximum at around 200 V for films deposited in all CH4-noble gases atmospheres, while it appears at Vb = 350 V for films deposited from pure methane atmosphere. Similar behavior was observed for the hardness measurements, shown in Fig. 8. However, in the case of films deposited in pure methane atmosphere the maximum is again displaced to higher self-bias values. The existence of this maximum in a-C:H films deposited by PECVD was observed before [4,10,15] and was explained by the subplantation model [16]. The fact that the position of the maximum is dependent on the precursor gas used can be understood in terms of the different ion assistance regimes during film growth.
2.0
4. Summary and conclusions 1.5 CH4 - Ar CH4 - Ne CH4 - He CH4
1.0
0.5 0
100
200
300
400
500
Vb (-V) Fig. 7. Compressive internal stress as a function of the self-bias voltage for a-C:H films deposited with CH4 partial pressures of 2 and 100%. The lines are only to guide the eyes.
A systematic investigation of the effects of the dilution of the CH4 precursor atmosphere by Ar, Ne and He was carried out in this work. The obtained results point out to the fact that the plasma modifications due to methane dilution by noble gases did not result in substantial modifications on the film microstructure or mechanical properties. Only minor differences, such as, a smaller incorporation of hydrogen in the film microstructure and small density changes were observed for films deposited from highly diluted atmospheres. In addition, if there is a change of the film building block in conditions of high noble gases dilution, as sug-
G. Capote, F.L. Freire Jr. / Materials Science and Engineering B 112 (2004) 101–105
gested for the case of methane diluted in argon [7], it does not affect the final film properties. The results obtained from the series of films deposited with different self-bias voltages clearly show that the composition, microstructure and mechanical properties of the films are strongly dependent on the energy of the incoming ions. When we plot the internal stress and hardness as functions of the self-bias voltage, there are differences in the position of the maximum observed from films deposited from pure methane atmospheres and from diluted atmospheres. These results confirm the importance of the ion bombardment during film growth on the mechanical properties of the films and can be explained on basis of two different bombardment regimes and can be, at least qualitatively, described by the subplantation model.
Acknowledgements The authors are very grateful for experimental work by A.R. Zanatta, L.G. Jacobsohn and M.E.H. da Costa. This work is partially supported by the Brazilian agencies: CNPq, CAPES and FAPERJ.
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References [1] J. Robertson, Prog. Solid State Chem. 21 (1991) 199. [2] W. Jacob, Thin Solid Films 326 (1999) 1. [3] Z. Sun, C.H. Lin, Y.L. Lee, J.R. Shi, B.K. Tay, X. Shi, J. Appl. Phys. 87 (2000) 8122. [4] E. Tomasella, C. Meunier, S. Mikhailov, Surf. Coat. Technol. 141 (2001) 286. [5] L. Valentini, J.M. Kenny, G. Mariotto, P. Tosi, G. Carlotti, G. Socino, L. Lozzi, S. Santuci, J. Vac. Sci. Technol. A 19 (2001) 1611. [6] L.G. Jacobsonh, G. Capote, N.C. Cruz, A.R. Zanatta, F.L. Freire Jr., Thin Solid Films 419 (2002) 46. [7] C. Riccardi, R. Barni, M. Fontanesi, P. Tosi, Chem. Phys. Lett. 329 (2000) 66. [8] L.G. Jacobsohn, F.L. Freire Jr., J. Vac. Sci. Technol. A 17 (1999) 2841. [9] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [10] M.A. Tamor, W.C. Vassell, J. Appl. Phys. 76 (1994) 3823. [11] J.W. Zou, K. Reichelt, K. Schmidt, B. Dischler, J. Appl. Phys. 65 (1989) 3914. [12] L.G. Jacobsohn, D.F. Franceschini, M.E.H. Maia da Costa, F.L. Freire Jr., J. Vac. Sci. Technol. A 18 (2000) 2230. [13] F. Tuinstra, J.F. Koening, J. Chem. Phys. 53 (1970) 1126. [14] O. Dillon, J. Woollam, V. Katkanant, Phys. Rev. B 29 (1984) 3482. [15] R.G. Lacerda, F.C. Marques, F.L. Freire Jr., Diam. Rel. Mater. 8 (1999) 495. [16] Y. Lifshitz, S.R. Kasi, J.W. Rabalais, W. Eckstein, Phys. Rev. B 41 (1990) 10468.