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C–SiC–Si gradient films formed on silicon by ion beam assisted deposition at room temperature
Surface and Coatings Technology 128᎐129 Ž2000. 274᎐279
C᎐SiC᎐Si gradient films formed on silicon by ion beam assisted deposition at room temperature K. Volz a,U , M. Kiuchi b, M. Okumurab, W. Ensinger a a
Philipps-Uni¨ ersity of Marburg, Materials Science Center,Hans-Meerwein Str., 35032 Marburg, Germany b Osaka National Research Institute, Ikeda, Osaka 563, Japan
Abstract Ion beam assisted deposition ŽIBAD. of carbon under medium energy Ž35 keV. Ar ion bombardment onto a silicon target results in the formation of silicon carbide. The formation of a-C᎐SiC᎐Si gradient films is dependent on the ionratom arrival ratio Ž IrA.. All films are deposited at room temperature. The gradient layers formed are examined for their composition using RBS. Depending on the IrA ratios, mixed interfaces of different widths, where silicon carbide exists, are formed. The Si᎐C bonding in the mixed region is proven by XPS. The film formed on top of some samples contains amorphous carbon regions as shown by Raman spectroscopy. The surface of the layers grown is rather smooth with a roughness of several nanometres. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Ion beam assisted deposition; Silicon carbide; Amorphons carbon; Gradient films
1. Introduction Ion beam assisted deposition of compound films with non-mass-separated broad beam ion sources has mainly focused on nitrogen or oxygen ion bombardment, because on one hand the resulting nitrides and oxides are of technological interest, on the other hand the use of oxygen or nitrogen as feed gas for an ion source is feasible and in so far favourable as it leads to a mono-elemental ion beam. Thus, only the desired two partners of a binary compound such as a metal nitride with evaporation of the metal and implantation of nitrogen ions are involved w1x. This is the main reason why only a comparatively small amount of work has dealt with carbides. For carbide formation, the carbon has to be delivered as atoms from a second evaporator, or from the residual gas when reactive atoms are used
U
Corresponding author. Tel.: q49-6421-2881139; fax: q49-64212822124. E-mail address: [email protected] ŽK. Volz..
w2x, or from an ion source which emits only metal ions, such as a MEVVA. Only very few papers can be found in literature, where SiC films are formed by IBAD or similar methods. Riviere ` et al. w3x have shown SiC formation during sputtering of a SiC target on a heated Si substrate under high energy Ž160 keV. Xe ion bombardment. Other work mainly focuses on two step processes: a carbon film w4x or a SirC multilayer w5x is evaporated prior to irradiation with high energy Ž) 100 keV. ions. A possibility of forming thin carbide films in situ is ion beam assisted deposition under medium energy rare gas ion bombardment when ion beam mixing effects are used. In this case, carbon is evaporated under rare gas ion bombardment and forms a thin compound film by ion beam mixing by the penetrating rare gas ions. In the present paper, the formation of an a-CrSiC gradient film by means of this technique is discussed. Silicon carbide is a very hard and both mechanically and chemically stable material, which may act as a wear and corrosion protection film. Its formation as a thin film is therefore of technological interest.
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 6 0 4 - 6
K. Volz et al. r Surface and Coatings Technology 128᎐129 (2000) 274᎐279
2. Experimental
Carbon was evaporated by electron beam evaporation using a system capable of providing 10-keV electrons and currents of up to 1 A. The condensation rate was measured with a quartz crystal monitor, which was
275
shielded against the ion beam. The evaporation rate was varied between 0.05 and 0.5 nmrs. As substrate materials Ž100. silicon wafers have been used. The condensing carbon film was bombarded with argon ions, which were delivered from a bucket type ion source. The acceleration voltage was kept at 35 kV. The atom incidence angle was 45⬚, the ions impinged
Fig. 1. Ža. 3.7-MeV He 2q RBS spectra of a silicon wafer, on which carbon has been evaporated under argon ion bombardment Ž IrA s 0.5.. For comparison the spectrum of a virgin silicon wafer is also given. Žb. Carbon depth profiles in dependance on the ionratom arrival ratio. The profiles have been obtained from RUMP fits.
276
K. Volz et al. r Surface and Coatings Technology 128᎐129 (2000) 274᎐279
perpendicular to the target surface. The ion bombardment intensity was determined by charge collection in a shielded Fraday cup. The apparatus has been described in detail elsewhere w6x. The argon ion current density was 90 Arcm2 for all samples, in order to exclude any heating effect of the ion beam. Hence, the ionratom arrival ratios were between 0.1 and 1 in this study. The base pressure in the vacuum system was 10y5 Pa, the working pressure 2 = 10y3 Pa, mainly from argon gas release from the ion source. The samples were mounted on a water-cooled rotatable copper sample holder. The temperature of the samples was measured by a thermocouple clamped tightly to the target holder. It remained in all cases below 100⬚C. The elemental composition of the samples was measured by Rutherford backscattering spectrometry using 3.7 MeV He 2q ions. The enhancement of the carbon signal for the energy used has been taken into account for the RUMP w7x simulations of the RBS spectra. The analysis beam impinged at an angle of 10⬚ with respect to the surface normal and the backscattered particles were detected at an angle of 170⬚. The chemical bonding in the films formed was determined by XPS measurements using Mg K ␣ radiation for excitation. Sputtering with 3-keV Ar ions has been used to obtain depth information. Raman measurements were made to detect carbon homonuclear bonds present in the films. Atomic force microscopic measurements were performed in contact mode to characterise the surface of the films grown.
3. Results and discussion Fig. 1a exemplarily shows a 3.7-MeV He 2q RBS spectrum of a silicon wafer, on which a carbon film has been evaporated under argon ion bombardment. For comparison a silicon reference spectrum is given. The signals of the substrate, the evaporated carbon and the implanted argon can be clearly observed. This indicates that argon did not diffuse out completely and is incorporated into the growing film. RUMP simulations of the RBS spectra give a maximum argon content of up to 16 at.% throughout the mixing zone and the carbon top layer. In Fig. 1b the carbon depth profiles for samples treated with different ionratom arrival ratios are depicted. These profiles have been obtained from RUMP simulations of the RBS spectra. A carboncontaining film in the near surface region has been formed for all ionratom arrival ratios. With increasing irradiation intensity the peak area decreases, indicating a more severe sputtering for films formed with high IrA ratios. But even for the highest IrA used in this study, a carbon containing film has formed. For ionratom arrival ratios of 0.5 and below, a carbon rich film has formed on the silicon surfaces. For IrA s 1 a nearly stoichiometric ŽCrSi ratio 1. film has been formed due to ion beam mixing of the evaporated carbon with the silicon substrate by the impinging argon ions. The flanks of the carbon depth profiles to the substrate do not have the same slope for all samples, indicating that the mixing effect of carbon into silicon and vice versa depends on the IrA ratio.
Fig. 2. Width of the mixing zone, where recoil implantation of carbon into the silicon substrate and vice versa has been taking place. As a measure for the intermixed zone the width of the region, where the carbon concentration is between 30 and 70 at.%, has been taken.
K. Volz et al. r Surface and Coatings Technology 128᎐129 (2000) 274᎐279
277
Fig. 3. Series of Si 2p XPS spectra of the sample formed with IrA s 0.25. The spectra are taken at different sputter times, i.e. at different sample depths. For comparison the theoretical positions of the Si᎐Si and Si᎐C bonds are given. As a guide, the maxima of the XPS peaks are connected by lines.
A more quantitative description of this mixing behaviour is given in Fig. 2. Here the width of the region, where the carbon concentration is in the range between 30 and 70 at.% is taken as a measure for the efficiency of the ion beam induced mixing effect. It can be seen that for low IrA ratios carbon deposition takes place so fast that a large amount of the deposited carbon does not undergo any mixing. A carbon layer builds up on top of a shallow intermixed zone. This behaviour can be observed up to ionratom arrival ratios of approximately 0.5. Between IrA s 0 and 0.5 the width of the intermixed zone, where the stoichiometry is close
to the one of silicon carbide, increases in width with increasing ionratom arrival ratio. For IrA values of 0.5 and above a saturation behaviour can be observed. The mixed zone does not significantly increase in width anymore. This indicates that now an equilibrium between atoms sputtered from the surface by the impinging ion beam and mixed silicon and carbon atoms is established. The evaporated carbon atoms and the silicon atoms from the substrate which are mixed by the argon ions produced an intermixed zone, which has ᎏ for the conditions used in this study Žion energy, ion mass, etc.. ᎏ a maximum width of approximately
Fig. 4. Raman spectrum of the sample treated with IrA s 0.5. The wave number region, where the most dominant features of C᎐C bonds are located, are shown. For comparison the theoretical positions of the carbon G- and D-lines are given.
278
K. Volz et al. r Surface and Coatings Technology 128᎐129 (2000) 274᎐279
70 nm Žassuming the atomic density of SiC.. Hence, from this compositional analysis follows that comparatively thick silicon carbide layers can be grown by evaporating carbon on a silicon target under argon ion irradiation. As the deposition has been carried out at very low temperature, the formation of crystalline silicon carbide is not expected. However, the formation of the Si᎐C bonds has to be proven in order to conclude the formation of the carbide and not of a mixed region without formation of heteroatomar bonds. XPS is an analysis technique, which yields very sensitively information on the chemical bonding. This is also true for the Si᎐C system, especially when the chemical shift of the Si 2p core level peak is considered. Charge transfer from Si to the more electronegative C leads to a shift of the Si 2p level to higher binding energies as silicon carbide bonds are formed. Values for the different bonds are: Si᎐Si: 98.8᎐99.5 eV; Si᎐C: 100.1᎐101.0 eV w8x. Fig. 3 shows a series of Si 2p XPS spectra from the sample formed with the ionratom arrival ratio of 0.25. Depth information can be obtained from these spectra, as between the recording of the single spectra the samples have been sputter etched to gain information on the bonding also in greater depths. To guide the eyes, the positions of the maxima of the peaks are connected by lines. The positions of Si᎐Si and Si᎐C bonds are given for comparison. Deep in the sample Žsputter times 13᎐23 min. the maximum of the Si 2p peak is observed at 99.1 eV, a value typical for Si᎐Si bonds. As one proceeds further to the surface, the position of the peak maximum shifts to higher binding energy values Žsputtering time 5᎐12 min.. This indicates that in this region a mixture of Si᎐Si and Si᎐C bonds is present. For sputtering times between 1 and 3 min, the peak position saturates at a value, which is typical for Si᎐C bonds. This indicates that in this region the complete amount of silicon present in the sample is bonded to carbon. Hence, the XPS analysis has shown that indeed silicon carbide has been formed, as Si᎐C bonding has been proven. In the region, where no more homoatomar silicon bonds have been observed, the carbon content in the samples is approximately 60 at.%. This shows that for lower C concentrations homoatomar Si bonds are possible. Information on the C᎐C bonding is difficult to obtain from XPS analysis as the C 1s peak always contains a contamination distribution. Therefore Raman spectroscopy has been applied, which gives very sensitive information on homoatomar carbon bonding. In Fig. 4 the Raman spectrum of the sample formed with IrA s 0.5 is depicted. A broad feature can be observed at approximately 1500 cmy1 . The theoretical positions of the carbon G- Žsingle crystalline graphite. and D- Ždisorder induced line. lines
Fig. 5. AFM pictures of samples treated with different IrA ratios. Ža. IrA s 0.25; Žb. IrA s 1.The mean square roughness of the samples is: Ža. 8 nm, Žb. 11 nm.
are also given. In the spectra shown here, not two separate features are observed, but a broad peak appears at wave numbers in between the theoretical ones of crystalline and disordered graphite. This broad line is characteristic of the amorphous regime. It has been observed previously in all ion-implanted graphite and diamond, which turned amorphous upon ion irradiation w9x. It therefore can be concluded that homoatomar carbon bonds are present in the samples but that the carbon is in the amorphous state due to high fluence ion bombardment and the low preparation temperature.
K. Volz et al. r Surface and Coatings Technology 128᎐129 (2000) 274᎐279
Information on the surface morphology of the samples has been obtained by AFM. In Fig. 5 two AFM pictures for different IrA ratios are shown wŽa.: IrA s 0.25; Žb.: IrA s 1x. For the lower irradiation intensity, the mean square roughness of the surface is 8 nm, for IrA s 1, the mean square roughness increases to 11 nm. This is due to a more intense ion bombardment, which causes a more pronounced sputtering. The structures, which can be observed at the surface are bigger for the higher IrA ratio. This can be due to the fact that at IrA s 1 a carbide film is located on top of the sample, whereas for IrA s 0.25 an amorphous carbon film is located on top of the wafer. These two different materials may differ in their sputter properties and therefore different structures can evolve under ion bombardment.
4. Summary It has been shown that carbon evaporation onto a silicon target under Ar ion irradiation results in the formation of silicon carbide. The width of the region where the carbide is formed depends on the ionratom arrival ratio. For the conditions applied, up to 70-nm thick carbide films can be formed. The Si᎐C bonding in the intermixed region has been proven by XPS analysis. It has been shown by Raman spectroscopy that homoatomar carbon bonds exist in films containing more
279
carbon than silicon. Depending on the IrA ratio, which can also be varied during film growth, gradient films composed of carbon and silicon carbide regions can be formed at room temperature. These films are amorphous. It would also be possible to evaporate silicon on any material, e.g. metals, prior to the IBAD process and to form a CrSiC gradient film on top of this metal after this step, which can be used as a corrosion and wear resistant coating. These types of coatings are under investigation at present. References w1x F.A. Smidt, Int. Mater. Rev. 35 Ž2. Ž1990. 6. w2x W. Ensinger, A. Schroer, ¨ Surf. Coat. Technol. 103-104 Ž1998. 168. w3x J.P. Riviere, ` M. Zaytouni, J. Delafond, Surf. Coat. Technol. 84 Ž1996. 376. w4x T. Kimura, Y. Tatebe, A. Kawamura, S. Yugo, Y. Adachi, J. Appl. Phys. 24 Ž12. Ž1985. 1712. w5x J.P. Riviere, ` M. Zayatouni, M.F. Denanot, J. Allain, Mater. Sci. Eng. B29 Ž1995. 105. w6x T. Sato, K. Ohata, N. Asahi et al., Nucl. Instrum. Methods Phys. Res. B 19-20 Ž1987. 644. w7x L.R. Doolittle, Nucl. Instrum. Methods Phys. Res. B 9 Ž1985. 344. w8x C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Handbook of X-Ray Photoelectron Spectroscopy; Perkin-Elmer Corporation, Physical Electronics Division; Eden Prairie, MN, USA, 1979. w9x J.E. Smith Jr., M.H. Brodsky, B.L. Crowder, M.I. Nathan, J. Non-Cryst. Solids 8᎐10 Ž1972. 179.
C᎐SiC᎐Si gradient films formed on silicon by ion beam assisted deposition at room temperature K. Volz a,U , M. Kiuchi b, M. Okumurab, W. Ensinger a a
Philipps-Uni¨ ersity of Marburg, Materials Science Center,Hans-Meerwein Str., 35032 Marburg, Germany b Osaka National Research Institute, Ikeda, Osaka 563, Japan
Abstract Ion beam assisted deposition ŽIBAD. of carbon under medium energy Ž35 keV. Ar ion bombardment onto a silicon target results in the formation of silicon carbide. The formation of a-C᎐SiC᎐Si gradient films is dependent on the ionratom arrival ratio Ž IrA.. All films are deposited at room temperature. The gradient layers formed are examined for their composition using RBS. Depending on the IrA ratios, mixed interfaces of different widths, where silicon carbide exists, are formed. The Si᎐C bonding in the mixed region is proven by XPS. The film formed on top of some samples contains amorphous carbon regions as shown by Raman spectroscopy. The surface of the layers grown is rather smooth with a roughness of several nanometres. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Ion beam assisted deposition; Silicon carbide; Amorphons carbon; Gradient films
1. Introduction Ion beam assisted deposition of compound films with non-mass-separated broad beam ion sources has mainly focused on nitrogen or oxygen ion bombardment, because on one hand the resulting nitrides and oxides are of technological interest, on the other hand the use of oxygen or nitrogen as feed gas for an ion source is feasible and in so far favourable as it leads to a mono-elemental ion beam. Thus, only the desired two partners of a binary compound such as a metal nitride with evaporation of the metal and implantation of nitrogen ions are involved w1x. This is the main reason why only a comparatively small amount of work has dealt with carbides. For carbide formation, the carbon has to be delivered as atoms from a second evaporator, or from the residual gas when reactive atoms are used
U
Corresponding author. Tel.: q49-6421-2881139; fax: q49-64212822124. E-mail address: [email protected] ŽK. Volz..
w2x, or from an ion source which emits only metal ions, such as a MEVVA. Only very few papers can be found in literature, where SiC films are formed by IBAD or similar methods. Riviere ` et al. w3x have shown SiC formation during sputtering of a SiC target on a heated Si substrate under high energy Ž160 keV. Xe ion bombardment. Other work mainly focuses on two step processes: a carbon film w4x or a SirC multilayer w5x is evaporated prior to irradiation with high energy Ž) 100 keV. ions. A possibility of forming thin carbide films in situ is ion beam assisted deposition under medium energy rare gas ion bombardment when ion beam mixing effects are used. In this case, carbon is evaporated under rare gas ion bombardment and forms a thin compound film by ion beam mixing by the penetrating rare gas ions. In the present paper, the formation of an a-CrSiC gradient film by means of this technique is discussed. Silicon carbide is a very hard and both mechanically and chemically stable material, which may act as a wear and corrosion protection film. Its formation as a thin film is therefore of technological interest.
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 6 0 4 - 6
K. Volz et al. r Surface and Coatings Technology 128᎐129 (2000) 274᎐279
2. Experimental
Carbon was evaporated by electron beam evaporation using a system capable of providing 10-keV electrons and currents of up to 1 A. The condensation rate was measured with a quartz crystal monitor, which was
275
shielded against the ion beam. The evaporation rate was varied between 0.05 and 0.5 nmrs. As substrate materials Ž100. silicon wafers have been used. The condensing carbon film was bombarded with argon ions, which were delivered from a bucket type ion source. The acceleration voltage was kept at 35 kV. The atom incidence angle was 45⬚, the ions impinged
Fig. 1. Ža. 3.7-MeV He 2q RBS spectra of a silicon wafer, on which carbon has been evaporated under argon ion bombardment Ž IrA s 0.5.. For comparison the spectrum of a virgin silicon wafer is also given. Žb. Carbon depth profiles in dependance on the ionratom arrival ratio. The profiles have been obtained from RUMP fits.
276
K. Volz et al. r Surface and Coatings Technology 128᎐129 (2000) 274᎐279
perpendicular to the target surface. The ion bombardment intensity was determined by charge collection in a shielded Fraday cup. The apparatus has been described in detail elsewhere w6x. The argon ion current density was 90 Arcm2 for all samples, in order to exclude any heating effect of the ion beam. Hence, the ionratom arrival ratios were between 0.1 and 1 in this study. The base pressure in the vacuum system was 10y5 Pa, the working pressure 2 = 10y3 Pa, mainly from argon gas release from the ion source. The samples were mounted on a water-cooled rotatable copper sample holder. The temperature of the samples was measured by a thermocouple clamped tightly to the target holder. It remained in all cases below 100⬚C. The elemental composition of the samples was measured by Rutherford backscattering spectrometry using 3.7 MeV He 2q ions. The enhancement of the carbon signal for the energy used has been taken into account for the RUMP w7x simulations of the RBS spectra. The analysis beam impinged at an angle of 10⬚ with respect to the surface normal and the backscattered particles were detected at an angle of 170⬚. The chemical bonding in the films formed was determined by XPS measurements using Mg K ␣ radiation for excitation. Sputtering with 3-keV Ar ions has been used to obtain depth information. Raman measurements were made to detect carbon homonuclear bonds present in the films. Atomic force microscopic measurements were performed in contact mode to characterise the surface of the films grown.
3. Results and discussion Fig. 1a exemplarily shows a 3.7-MeV He 2q RBS spectrum of a silicon wafer, on which a carbon film has been evaporated under argon ion bombardment. For comparison a silicon reference spectrum is given. The signals of the substrate, the evaporated carbon and the implanted argon can be clearly observed. This indicates that argon did not diffuse out completely and is incorporated into the growing film. RUMP simulations of the RBS spectra give a maximum argon content of up to 16 at.% throughout the mixing zone and the carbon top layer. In Fig. 1b the carbon depth profiles for samples treated with different ionratom arrival ratios are depicted. These profiles have been obtained from RUMP simulations of the RBS spectra. A carboncontaining film in the near surface region has been formed for all ionratom arrival ratios. With increasing irradiation intensity the peak area decreases, indicating a more severe sputtering for films formed with high IrA ratios. But even for the highest IrA used in this study, a carbon containing film has formed. For ionratom arrival ratios of 0.5 and below, a carbon rich film has formed on the silicon surfaces. For IrA s 1 a nearly stoichiometric ŽCrSi ratio 1. film has been formed due to ion beam mixing of the evaporated carbon with the silicon substrate by the impinging argon ions. The flanks of the carbon depth profiles to the substrate do not have the same slope for all samples, indicating that the mixing effect of carbon into silicon and vice versa depends on the IrA ratio.
Fig. 2. Width of the mixing zone, where recoil implantation of carbon into the silicon substrate and vice versa has been taking place. As a measure for the intermixed zone the width of the region, where the carbon concentration is between 30 and 70 at.%, has been taken.
K. Volz et al. r Surface and Coatings Technology 128᎐129 (2000) 274᎐279
277
Fig. 3. Series of Si 2p XPS spectra of the sample formed with IrA s 0.25. The spectra are taken at different sputter times, i.e. at different sample depths. For comparison the theoretical positions of the Si᎐Si and Si᎐C bonds are given. As a guide, the maxima of the XPS peaks are connected by lines.
A more quantitative description of this mixing behaviour is given in Fig. 2. Here the width of the region, where the carbon concentration is in the range between 30 and 70 at.% is taken as a measure for the efficiency of the ion beam induced mixing effect. It can be seen that for low IrA ratios carbon deposition takes place so fast that a large amount of the deposited carbon does not undergo any mixing. A carbon layer builds up on top of a shallow intermixed zone. This behaviour can be observed up to ionratom arrival ratios of approximately 0.5. Between IrA s 0 and 0.5 the width of the intermixed zone, where the stoichiometry is close
to the one of silicon carbide, increases in width with increasing ionratom arrival ratio. For IrA values of 0.5 and above a saturation behaviour can be observed. The mixed zone does not significantly increase in width anymore. This indicates that now an equilibrium between atoms sputtered from the surface by the impinging ion beam and mixed silicon and carbon atoms is established. The evaporated carbon atoms and the silicon atoms from the substrate which are mixed by the argon ions produced an intermixed zone, which has ᎏ for the conditions used in this study Žion energy, ion mass, etc.. ᎏ a maximum width of approximately
Fig. 4. Raman spectrum of the sample treated with IrA s 0.5. The wave number region, where the most dominant features of C᎐C bonds are located, are shown. For comparison the theoretical positions of the carbon G- and D-lines are given.
278
K. Volz et al. r Surface and Coatings Technology 128᎐129 (2000) 274᎐279
70 nm Žassuming the atomic density of SiC.. Hence, from this compositional analysis follows that comparatively thick silicon carbide layers can be grown by evaporating carbon on a silicon target under argon ion irradiation. As the deposition has been carried out at very low temperature, the formation of crystalline silicon carbide is not expected. However, the formation of the Si᎐C bonds has to be proven in order to conclude the formation of the carbide and not of a mixed region without formation of heteroatomar bonds. XPS is an analysis technique, which yields very sensitively information on the chemical bonding. This is also true for the Si᎐C system, especially when the chemical shift of the Si 2p core level peak is considered. Charge transfer from Si to the more electronegative C leads to a shift of the Si 2p level to higher binding energies as silicon carbide bonds are formed. Values for the different bonds are: Si᎐Si: 98.8᎐99.5 eV; Si᎐C: 100.1᎐101.0 eV w8x. Fig. 3 shows a series of Si 2p XPS spectra from the sample formed with the ionratom arrival ratio of 0.25. Depth information can be obtained from these spectra, as between the recording of the single spectra the samples have been sputter etched to gain information on the bonding also in greater depths. To guide the eyes, the positions of the maxima of the peaks are connected by lines. The positions of Si᎐Si and Si᎐C bonds are given for comparison. Deep in the sample Žsputter times 13᎐23 min. the maximum of the Si 2p peak is observed at 99.1 eV, a value typical for Si᎐Si bonds. As one proceeds further to the surface, the position of the peak maximum shifts to higher binding energy values Žsputtering time 5᎐12 min.. This indicates that in this region a mixture of Si᎐Si and Si᎐C bonds is present. For sputtering times between 1 and 3 min, the peak position saturates at a value, which is typical for Si᎐C bonds. This indicates that in this region the complete amount of silicon present in the sample is bonded to carbon. Hence, the XPS analysis has shown that indeed silicon carbide has been formed, as Si᎐C bonding has been proven. In the region, where no more homoatomar silicon bonds have been observed, the carbon content in the samples is approximately 60 at.%. This shows that for lower C concentrations homoatomar Si bonds are possible. Information on the C᎐C bonding is difficult to obtain from XPS analysis as the C 1s peak always contains a contamination distribution. Therefore Raman spectroscopy has been applied, which gives very sensitive information on homoatomar carbon bonding. In Fig. 4 the Raman spectrum of the sample formed with IrA s 0.5 is depicted. A broad feature can be observed at approximately 1500 cmy1 . The theoretical positions of the carbon G- Žsingle crystalline graphite. and D- Ždisorder induced line. lines
Fig. 5. AFM pictures of samples treated with different IrA ratios. Ža. IrA s 0.25; Žb. IrA s 1.The mean square roughness of the samples is: Ža. 8 nm, Žb. 11 nm.
are also given. In the spectra shown here, not two separate features are observed, but a broad peak appears at wave numbers in between the theoretical ones of crystalline and disordered graphite. This broad line is characteristic of the amorphous regime. It has been observed previously in all ion-implanted graphite and diamond, which turned amorphous upon ion irradiation w9x. It therefore can be concluded that homoatomar carbon bonds are present in the samples but that the carbon is in the amorphous state due to high fluence ion bombardment and the low preparation temperature.
K. Volz et al. r Surface and Coatings Technology 128᎐129 (2000) 274᎐279
Information on the surface morphology of the samples has been obtained by AFM. In Fig. 5 two AFM pictures for different IrA ratios are shown wŽa.: IrA s 0.25; Žb.: IrA s 1x. For the lower irradiation intensity, the mean square roughness of the surface is 8 nm, for IrA s 1, the mean square roughness increases to 11 nm. This is due to a more intense ion bombardment, which causes a more pronounced sputtering. The structures, which can be observed at the surface are bigger for the higher IrA ratio. This can be due to the fact that at IrA s 1 a carbide film is located on top of the sample, whereas for IrA s 0.25 an amorphous carbon film is located on top of the wafer. These two different materials may differ in their sputter properties and therefore different structures can evolve under ion bombardment.
4. Summary It has been shown that carbon evaporation onto a silicon target under Ar ion irradiation results in the formation of silicon carbide. The width of the region where the carbide is formed depends on the ionratom arrival ratio. For the conditions applied, up to 70-nm thick carbide films can be formed. The Si᎐C bonding in the intermixed region has been proven by XPS analysis. It has been shown by Raman spectroscopy that homoatomar carbon bonds exist in films containing more
279
carbon than silicon. Depending on the IrA ratio, which can also be varied during film growth, gradient films composed of carbon and silicon carbide regions can be formed at room temperature. These films are amorphous. It would also be possible to evaporate silicon on any material, e.g. metals, prior to the IBAD process and to form a CrSiC gradient film on top of this metal after this step, which can be used as a corrosion and wear resistant coating. These types of coatings are under investigation at present. References w1x F.A. Smidt, Int. Mater. Rev. 35 Ž2. Ž1990. 6. w2x W. Ensinger, A. Schroer, ¨ Surf. Coat. Technol. 103-104 Ž1998. 168. w3x J.P. Riviere, ` M. Zaytouni, J. Delafond, Surf. Coat. Technol. 84 Ž1996. 376. w4x T. Kimura, Y. Tatebe, A. Kawamura, S. Yugo, Y. Adachi, J. Appl. Phys. 24 Ž12. Ž1985. 1712. w5x J.P. Riviere, ` M. Zayatouni, M.F. Denanot, J. Allain, Mater. Sci. Eng. B29 Ž1995. 105. w6x T. Sato, K. Ohata, N. Asahi et al., Nucl. Instrum. Methods Phys. Res. B 19-20 Ž1987. 644. w7x L.R. Doolittle, Nucl. Instrum. Methods Phys. Res. B 9 Ž1985. 344. w8x C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Handbook of X-Ray Photoelectron Spectroscopy; Perkin-Elmer Corporation, Physical Electronics Division; Eden Prairie, MN, USA, 1979. w9x J.E. Smith Jr., M.H. Brodsky, B.L. Crowder, M.I. Nathan, J. Non-Cryst. Solids 8᎐10 Ž1972. 179.