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Cubic boron nitride film synthesis by reactive sputtering with pulsed RF substrate bias
Diamond and Related Materials 10 Ž2001. 1380᎐1384
Cubic boron nitride film synthesis by reactive sputtering with pulsed RF substrate bias M. WakatsuchiU , Y. Ueda, M. Nishikawa Graduate School of Engineering, Osaka Uni¨ ersity, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
Abstract Cubic boron nitride Žc-BN. films are synthesized by reactive sputtering with RF biased substrate in electron cyclotron resonance ŽECR. plasmas. In this study, continuous working ŽCW. and pulsed RF bias modes are applied for deposition and the results are compared. In the CW mode, c-BN is synthesized above a certain threshold ion energy and this energy changes with the ion to boron-atom flux ratio to the substrate. An experimental study is made in the pulsed bias mode under various conditions: the pulse period and the duty ratio. As a result in the case of the pulsed bias mode, higher ion energy is required than that in the CW mode for c-BN synthesis. The boundary between c-BN and h-BN formation regions is found on the diagram expressed as the threshold ion energy vs. ion to boron atom fluence ratio. Stress measurement is also carried out for all obtained samples and a strong correlation between the c-BN content and the compressive stress in both substrate bias modes is found. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Cubic boron nitride; Ion bombardment; RF; Stress
1. Introduction Cubic boron nitride Žc-BN. films have been widely studied by many researchers because of these unique features: their hardness second only to diamond w1x, the widest band gap Ž; 6.4 eV. of group III-V compounds w2x, properties as a semiconductor both as n-type and p-type w3,4x. However, c-BN films are much behind in practical use in contrast to diamond and other III-V nitrides Že.g. GaN and AlN.. The main reason why practical applications of the c-BN film are limited is that the c-BN film is generally poor in adhesion to a substrate owing to large internal stress w5,6x. Therefore, a method to reduce stress is necessary and effective
U
Corresponding author. Tel.: q81-6-6877-5111; fax: q81-6-68797867. E-mail address: [email protected] ŽM. Wakatsuchi..
processes must be developed in order to put various kinds of c-BN applications into practice. As one of the effective processes, an intermediate layer formation with no c-BN material is proposed as a stress buffer w7,8x. Recently, high energy Nq ion implantation to as-deposited c-BN films has been reported to be effective in stress reduction and delamination prevention w9x. We have studied c-BN synthesis by reactive sputtering in electron cyclotron resonance ŽECR. plasmas. The purpose of this study is to create a new multilayered structure of c-BN films by depositing layers with and without ion impact effect alternately. This alternate ion impact is achieved by pulse modulation of RF substrate bias. The effects of this method on BN phase and adhesion are investigated. In this paper, it will conclude with discussions on c-BN formation conditions for continuous working ŽCW. and pulsed bias cases.
0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 0 . 0 0 5 5 3 - 7
M. Wakatsuchi et al. r Diamond and Related Materials 10 (2001) 1380᎐1384
1381
2. Experimental apparatus and procedure Reactive sputtering with a pure boron target in ECR plasmas is employed for preparation of BN film samples. Since details of the experimental setup are given elsewhere w7x, we give only a brief explanation about sample preparation and evaluation methods. The deposition apparatus is a conventional ECR plasma source composed of a cylindrical chamber Ž15 cm in diameter., two magnet coils, a microwave generator with a frequency of 2.45 GHz and a vacuum pumping system. A pure-boron target disk Ž2 inch in diameter. is placed near the ECR zone and d.c.-biased to be sputtered by ArrN2 mixed ECR plasmas. A single-crystal silicon Ž111. wafer as a substrate Ž1 = 1cm2 . is mounted on a sample holder with its surface parallel to the target surface at a distance of 5 cm from the target. All the samples are deposited at a temperature of 500⬚C. As many researchers have pointed out, ion bombardment plays an essential role to synthesize c-BN phase w10,11x. In this method, ion bombardment is enhanced and controlled by CW and pulsed RF Ž13.56 MHz. substrate bias. The averaged ion bombardment energy Ei is estimated as Ei s eŽ Vp y Vs ., where Vp and Vs are the plasma space potential measured by the Langmuir probe method and the substrate bias, respectively. Fourier-transform infrared spectroscopy ŽFT-IR. is used for phase identification of BN films. Stress measurement is made by means of the bending beam method w12x which measures substrate bending profiles before and after deposition.
Fig. 1. Typical FT-IR absorption spectra of BN films.
BN films with almost single c-BN phase Ž Fc ; 90%. are successfully synthesized. These films, however, show delamination within a few hours after exposure to the humid atmosphere. Fig. 2 shows the aspect of delamination proceeding. With delamination proceeding, a peak-shift of the TO mode Žsp 3 . is often observed in the FT-IR analysis. This shift is also shown in Fig. 2. The peak position of the sample without delamination Žas-deposited sample. is located at 1091 cmy1 and the peak shifts toward lower wavenumbers with delamina-
3. Experimental results 3.1. Continuous working RF bias mode In the CW mode, BN film samples prepared under several deposition conditions are investigated. Fig. 1 shows typical IR transmission spectra of the samples. Here, the deposition condition is expressed by experimental parameters of the ion to boron-atom flux ratio to the substrate ⌫ir⌫B and the ion bombardment energy Ei . The ion flux and the boron-atom flux are estimated from the ion saturation current of the Langmuir probe and the deposition rate of the sample prepared without substrate bias, respectively. Experimental results show that the c-BN phase is synthesized above a certain threshold ion-energy and the threshold decreases with increase in the flux ratio ⌫ir⌫B w7x. However, the c-BN content Fc evaluated by the relative IR absorbance of the sp 3 TO mode Žc-BN. and the sp 2 stretching mode Žh-BN. also tends to decrease with increase in ⌫ir⌫B . We have confirmed that some of sp 3 dominant BN samples are c-BN polycrystalline films by transmission electron diffraction w7x.
Fig. 2. Peak shift of sp 3 TO mode with delamination proceeding.
M. Wakatsuchi et al. r Diamond and Related Materials 10 (2001) 1380᎐1384
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Fig. 3. Schematic image of pulsed RF wave.
tion proceeding. For the completely delaminated sample, the peak position is at 1058 cmy1 which is the same wavenumber for bulk c-BN in atmospheric pressure w13x. Since this TO mode peak-shift is related to internal stress of c-BN films w14,15x, this peak-shift behavior suggests that internal stress is relieved in the process of the delamination w16x. 3.2. Pulsed RF bias mode Pulsed RF bias mode experiments are carried out for BN film deposition. Fig. 3 shows the schematic of a pulsed RF wave. There are two controllable parameters in the pulsed mode: pulse period ŽT . and duty ratio ŽTon rT .. First, pulse period effects on the c-BN content and the compressive stress of the samples are investigated under the fixed duty ratio of 50%. Fig. 4 shows changes in the c-BN content and the compressive stress with respect to the pulse period. The pulse period is varied from 0.02 to 10 s. Under the assump-
Fig. 5. Ion energy dependences of the c-BN content and the compressive stress under the duty ratio of Ža. 50, Žb. 30 and Žc. 10%. Circles, squares and triangles denote the pulse period of 10, 1 and 0.5 s Ž0.6 s for 50%., respectively.
Fig. 4. Relations between Ža. the c-BN content Žb. the compressive stress and the pulse period. Circles, squares, and triangles denote the ion energy of 204, 279 and 329 eV, respectively.
tion that the film grows epitaxially, the growth time of h-BN monolayer-sheet is to 2.68 s Žthe deposition rate of 0.125 nmrs. which is included in the range of pulse periods in our experiment. The ion bombardment energy is set at 204, 279 and 329 eV. In the CW bias mode, almost single-phase c-BN film is obtained at the ion energy of Ei s 204 eV and film hardly grows at Ei s 329 eV because of resputtering on the substrate. For the pulsed bias experiment, c-BN films grow even at Ei s 329 eV in all pulse periods. At Ei s 204 eV, however, c-BN does not grow for longer pulse periods than T s 0.2 s. Similarly at Ei s 279 eV, c-BN does not grow for longer pulse periods than T s 0.6 s. From these results, the pulse period range required for c-BN synthesis expands with increase in the ion energy. With regard to the compressive stress, large stress values Žtypically 5 ; 6 GPa. are obtained for samples with c-BN as the dominant phase Ž Fc ; 80%.. This tendency is common to the result in the CW bias mode and the c-BN dominant samples show delamination after exposure to the atmosphere. Next, duty-ratio effects are investigated under the conditions of the duty ratio of 50, 30 and 10%. Fig. 5 shows ion energy dependence of the c-BN content and the compressive stress under the duty ratio of Ža. 50, Žb. 30, and Žc. 10%. The pulse period is set at 0.5 Ž0.6 s
M. Wakatsuchi et al. r Diamond and Related Materials 10 (2001) 1380᎐1384
for only 50%., 1 and 10 s. Though slight differences for each pulse period are seen, the threshold energy for c-BN synthesis increases with decrease of the duty ratio. In addition, the maximum c-BN content decreases with decrease of the duty ratio. Particularly, this behavior is remarkable for the duty ratio of 10%; the maximum c-BN content of samples obtained under this duty ratio is only 48% even at the ion energy of 779 eV. On the other hand, the compressive stress of BN samples clearly has a relation to the c-BN content. Ion energy dependence of the c-BN content and the compressive stress show similar tendency regardless of the duty ratio and the pulse period. This fact implies that there is a close correlation between the compressive stress and c-BN formation mechanisms.
4. Discussion The above-described results of the two bias modes and their correlation look very complicated because of the existence of many parameters. We tried to express the c-BN formation condition with parameters of ⌫ir⌫B and Ei for the CW bias mode w7x. In the pulsed bias mode, however, the effective ion flux in the bias-off phase is different from that in the bias-on phase. Accordingly, we express here the deposition condition with the ion to boron-atom fluence ratio ⌽ ir⌽ B which is a time-integrated value of the flux ratio ⌫ir⌫B . Fig. 6 shows the relation between the threshold ion energy for c-BN synthesis and the fluence ratio. In this diagram, c-BN is synthesized at just above the boundary line Ždashed line.. By using ⌽ ir⌽ B instead of ⌫ir⌫B , the deposition condition is expressed synthetically and a consistent tendency of the threshold for c-BN synthesis is seen between the CW mode and the pulsed mode. The meaning of the boundary is not clear yet and further investigations are necessary with consideration of c-BN formation mechanisms.
Fig. 6. Deposition condition plotted as the threshold ion energy for c-BN synthesis vs. the ion to boron-atom fluence ratio. Circles, squares and triangles denote the CW mode, the pulse period of 1 s and that of 10 s, respectively.
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Fig. 7. Relation between the c-BN content and the compressive stress. Closed and open circles denote the CW mode and pulsed mode, respectively.
As regards c-BN formation mechanisms, the compressive stress seems to contribute c-BN synthesis. In fact, delamination after air-exposure occurs also for high-stressed samples prepared in the pulsed mode similarly to those in the CW mode. Furthermore, samples with c-BN as the dominant phase have large compressive stress, which is common both in the CW and pulsed modes. Fig. 7 clearly shows the relation between the c-BN content and the compressive stress. Samples with low c-BN content Žalmost with h-BN phase only. have low stress, while those with high c-BN content Ž Fc s 80 ; 90%. have large stress which ranges from 4 to 6 GPa regardless of the bias modes: pulsed or CW. According to the compressive stress induced c-BN formation model w17x, c-BN is synthesized above a certain threshold stress which is determined by the theory of thermodynamics. The h-BN ª c-BN dynamic transition boundary is 2.3 GPa at 500⬚C on the extraporated boundary line by Bundy-Wentorf on the phase py T diagram of BN, however, some researchers pointed out the Bundy-Wentorf’s boundary shows the deviation from experimental results and calculations by Solozhenko w18,19x. Apparently, our result appears to agree with the thermodynamic equilibrium at 500⬚C. However, according to the phase diagram of BN, the stress threshold must decrease with decrease in temperature. Generally, c-BN hardly grows at low temperature Ž; 200⬚C. w20x, which appears to contradict the model. In addition c-BN is synthesized under the condition that the total stress is reduced by a buffer layer between top c-BN layer and a substrate w7x. Therefore, expanded considerations including atomic-scale ion bombardment process for c-BN formation mechanisms are needed.
5. Conclusion Pulsed RF substrate bias is applied for c-BN film
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M. Wakatsuchi et al. r Diamond and Related Materials 10 (2001) 1380᎐1384
synthesis by reactive sputtering with ECR plasmas. Under the duty ratio of 50%, c-BN is hard to grow at a large pulse-period and low ion-energy. With a decrease in the duty ratio, the threshold ion energy for c-BN synthesis increases and the maximum c-BN content tends to decrease These results form a boundary for c-BN formation condition on the diagram as the ionenergy vs. the ion to boron-atom fluence ratio. Further investigations on c-BN formation mechanisms involving stress contribution are necessary to clarify the meaning of the boundary. References w1x C.F. Gardinier, Ceram. Bull. 67 Ž1988. 1006. w2x L. Vel, G. Demazeau, J. Etourneau, Mater. Sci. Eng. B 10 Ž1991. 149. w3x O. Mishima, K. Era, J. Tanaka, S. Yamaoka, Appl. Phys. Lett. 53 Ž1988. 962. w4x R.H. Wentorf, Jr., J. Chem. Phys. 36 Ž1962. 1990. w5x S. Gimeno, J.C. Mufloz, A. Lousa, Thin Solid Films 317 Ž1998. 376. w6x I.-H. Kim, S.-H. Kim, K.-B. Kim, Vac. Sci. Technol. A 16 Ž1998. 2295.
w7x M. Wakatsuchi, Y. Takaba, K. Kanai, Y Ueda, M. Nishikawa, Jpn. J. Appl. Phys. 38 Ž1999. 4515. w8x S. Nishiyama, E. Takahashi, Y. Iwamoto, A. Ebe, N. Kuratani, K. Ogata, Thin Solid Films 281r282 Ž1996. 327. w9x H.-G. Boyen, P. Widmayer, D. Schwertberger, N. Deyneka, P. Ziemann, Appl. Phys. Lett. 76 Ž2000. 709. w10x M. Sokolowski, J. Cryst. Growth 46 Ž1979. 136. w11x C. Weissmantel, J. Vac. Sci. Technol. 18 Ž1981. 179. w12x G.G. Stoney, Proc. R. Soc. A 82 Ž1909. 172. w13x P.J. Gielisse, S.S. Mitra, J.N. Plendl et al., Phys. Rev. 155 Ž1967. 1039. w14x J.A. Sanjurjo, E. Lopei-Curz, P. Vogel, M. Cardona, Phys. Rev. B 28 Ž1983. 4579. w15x R. Tommer, H. Mfiller, A. Cardona, P. Vogel, Phys. Rev. B 21 Ž1980. 4869. w16x S. Ulrich, J. Schwan, W. Donner, H. Ehrhardt, Diamond Relat. Mater. 5 Ž1996. 548. w17x D.R. McKenzie, W.D. McFall, W.G. Sainty, C.A. Davis, R.E. Collins, Diamond Related. Mater. 2 Ž1993. 970. w18x V.L. Solozhenko, in: J. H. Edgar ŽEd.., Properties of group III nitrides, INSPEC Publication, London, 1994, p. 43. w19x J. von der Gonna, H.J. Meurer, G. Nover, T. Peun, D. ¨ Schnonbohm, G. Will, Mater. Lett. 33 Ž1998. 321. ¨ w20x D.J. Kester, R. Messier, J. Appl. Phys. 72 Ž1992. 504.
Cubic boron nitride film synthesis by reactive sputtering with pulsed RF substrate bias M. WakatsuchiU , Y. Ueda, M. Nishikawa Graduate School of Engineering, Osaka Uni¨ ersity, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
Abstract Cubic boron nitride Žc-BN. films are synthesized by reactive sputtering with RF biased substrate in electron cyclotron resonance ŽECR. plasmas. In this study, continuous working ŽCW. and pulsed RF bias modes are applied for deposition and the results are compared. In the CW mode, c-BN is synthesized above a certain threshold ion energy and this energy changes with the ion to boron-atom flux ratio to the substrate. An experimental study is made in the pulsed bias mode under various conditions: the pulse period and the duty ratio. As a result in the case of the pulsed bias mode, higher ion energy is required than that in the CW mode for c-BN synthesis. The boundary between c-BN and h-BN formation regions is found on the diagram expressed as the threshold ion energy vs. ion to boron atom fluence ratio. Stress measurement is also carried out for all obtained samples and a strong correlation between the c-BN content and the compressive stress in both substrate bias modes is found. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Cubic boron nitride; Ion bombardment; RF; Stress
1. Introduction Cubic boron nitride Žc-BN. films have been widely studied by many researchers because of these unique features: their hardness second only to diamond w1x, the widest band gap Ž; 6.4 eV. of group III-V compounds w2x, properties as a semiconductor both as n-type and p-type w3,4x. However, c-BN films are much behind in practical use in contrast to diamond and other III-V nitrides Že.g. GaN and AlN.. The main reason why practical applications of the c-BN film are limited is that the c-BN film is generally poor in adhesion to a substrate owing to large internal stress w5,6x. Therefore, a method to reduce stress is necessary and effective
U
Corresponding author. Tel.: q81-6-6877-5111; fax: q81-6-68797867. E-mail address: [email protected] ŽM. Wakatsuchi..
processes must be developed in order to put various kinds of c-BN applications into practice. As one of the effective processes, an intermediate layer formation with no c-BN material is proposed as a stress buffer w7,8x. Recently, high energy Nq ion implantation to as-deposited c-BN films has been reported to be effective in stress reduction and delamination prevention w9x. We have studied c-BN synthesis by reactive sputtering in electron cyclotron resonance ŽECR. plasmas. The purpose of this study is to create a new multilayered structure of c-BN films by depositing layers with and without ion impact effect alternately. This alternate ion impact is achieved by pulse modulation of RF substrate bias. The effects of this method on BN phase and adhesion are investigated. In this paper, it will conclude with discussions on c-BN formation conditions for continuous working ŽCW. and pulsed bias cases.
0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 0 . 0 0 5 5 3 - 7
M. Wakatsuchi et al. r Diamond and Related Materials 10 (2001) 1380᎐1384
1381
2. Experimental apparatus and procedure Reactive sputtering with a pure boron target in ECR plasmas is employed for preparation of BN film samples. Since details of the experimental setup are given elsewhere w7x, we give only a brief explanation about sample preparation and evaluation methods. The deposition apparatus is a conventional ECR plasma source composed of a cylindrical chamber Ž15 cm in diameter., two magnet coils, a microwave generator with a frequency of 2.45 GHz and a vacuum pumping system. A pure-boron target disk Ž2 inch in diameter. is placed near the ECR zone and d.c.-biased to be sputtered by ArrN2 mixed ECR plasmas. A single-crystal silicon Ž111. wafer as a substrate Ž1 = 1cm2 . is mounted on a sample holder with its surface parallel to the target surface at a distance of 5 cm from the target. All the samples are deposited at a temperature of 500⬚C. As many researchers have pointed out, ion bombardment plays an essential role to synthesize c-BN phase w10,11x. In this method, ion bombardment is enhanced and controlled by CW and pulsed RF Ž13.56 MHz. substrate bias. The averaged ion bombardment energy Ei is estimated as Ei s eŽ Vp y Vs ., where Vp and Vs are the plasma space potential measured by the Langmuir probe method and the substrate bias, respectively. Fourier-transform infrared spectroscopy ŽFT-IR. is used for phase identification of BN films. Stress measurement is made by means of the bending beam method w12x which measures substrate bending profiles before and after deposition.
Fig. 1. Typical FT-IR absorption spectra of BN films.
BN films with almost single c-BN phase Ž Fc ; 90%. are successfully synthesized. These films, however, show delamination within a few hours after exposure to the humid atmosphere. Fig. 2 shows the aspect of delamination proceeding. With delamination proceeding, a peak-shift of the TO mode Žsp 3 . is often observed in the FT-IR analysis. This shift is also shown in Fig. 2. The peak position of the sample without delamination Žas-deposited sample. is located at 1091 cmy1 and the peak shifts toward lower wavenumbers with delamina-
3. Experimental results 3.1. Continuous working RF bias mode In the CW mode, BN film samples prepared under several deposition conditions are investigated. Fig. 1 shows typical IR transmission spectra of the samples. Here, the deposition condition is expressed by experimental parameters of the ion to boron-atom flux ratio to the substrate ⌫ir⌫B and the ion bombardment energy Ei . The ion flux and the boron-atom flux are estimated from the ion saturation current of the Langmuir probe and the deposition rate of the sample prepared without substrate bias, respectively. Experimental results show that the c-BN phase is synthesized above a certain threshold ion-energy and the threshold decreases with increase in the flux ratio ⌫ir⌫B w7x. However, the c-BN content Fc evaluated by the relative IR absorbance of the sp 3 TO mode Žc-BN. and the sp 2 stretching mode Žh-BN. also tends to decrease with increase in ⌫ir⌫B . We have confirmed that some of sp 3 dominant BN samples are c-BN polycrystalline films by transmission electron diffraction w7x.
Fig. 2. Peak shift of sp 3 TO mode with delamination proceeding.
M. Wakatsuchi et al. r Diamond and Related Materials 10 (2001) 1380᎐1384
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Fig. 3. Schematic image of pulsed RF wave.
tion proceeding. For the completely delaminated sample, the peak position is at 1058 cmy1 which is the same wavenumber for bulk c-BN in atmospheric pressure w13x. Since this TO mode peak-shift is related to internal stress of c-BN films w14,15x, this peak-shift behavior suggests that internal stress is relieved in the process of the delamination w16x. 3.2. Pulsed RF bias mode Pulsed RF bias mode experiments are carried out for BN film deposition. Fig. 3 shows the schematic of a pulsed RF wave. There are two controllable parameters in the pulsed mode: pulse period ŽT . and duty ratio ŽTon rT .. First, pulse period effects on the c-BN content and the compressive stress of the samples are investigated under the fixed duty ratio of 50%. Fig. 4 shows changes in the c-BN content and the compressive stress with respect to the pulse period. The pulse period is varied from 0.02 to 10 s. Under the assump-
Fig. 5. Ion energy dependences of the c-BN content and the compressive stress under the duty ratio of Ža. 50, Žb. 30 and Žc. 10%. Circles, squares and triangles denote the pulse period of 10, 1 and 0.5 s Ž0.6 s for 50%., respectively.
Fig. 4. Relations between Ža. the c-BN content Žb. the compressive stress and the pulse period. Circles, squares, and triangles denote the ion energy of 204, 279 and 329 eV, respectively.
tion that the film grows epitaxially, the growth time of h-BN monolayer-sheet is to 2.68 s Žthe deposition rate of 0.125 nmrs. which is included in the range of pulse periods in our experiment. The ion bombardment energy is set at 204, 279 and 329 eV. In the CW bias mode, almost single-phase c-BN film is obtained at the ion energy of Ei s 204 eV and film hardly grows at Ei s 329 eV because of resputtering on the substrate. For the pulsed bias experiment, c-BN films grow even at Ei s 329 eV in all pulse periods. At Ei s 204 eV, however, c-BN does not grow for longer pulse periods than T s 0.2 s. Similarly at Ei s 279 eV, c-BN does not grow for longer pulse periods than T s 0.6 s. From these results, the pulse period range required for c-BN synthesis expands with increase in the ion energy. With regard to the compressive stress, large stress values Žtypically 5 ; 6 GPa. are obtained for samples with c-BN as the dominant phase Ž Fc ; 80%.. This tendency is common to the result in the CW bias mode and the c-BN dominant samples show delamination after exposure to the atmosphere. Next, duty-ratio effects are investigated under the conditions of the duty ratio of 50, 30 and 10%. Fig. 5 shows ion energy dependence of the c-BN content and the compressive stress under the duty ratio of Ža. 50, Žb. 30, and Žc. 10%. The pulse period is set at 0.5 Ž0.6 s
M. Wakatsuchi et al. r Diamond and Related Materials 10 (2001) 1380᎐1384
for only 50%., 1 and 10 s. Though slight differences for each pulse period are seen, the threshold energy for c-BN synthesis increases with decrease of the duty ratio. In addition, the maximum c-BN content decreases with decrease of the duty ratio. Particularly, this behavior is remarkable for the duty ratio of 10%; the maximum c-BN content of samples obtained under this duty ratio is only 48% even at the ion energy of 779 eV. On the other hand, the compressive stress of BN samples clearly has a relation to the c-BN content. Ion energy dependence of the c-BN content and the compressive stress show similar tendency regardless of the duty ratio and the pulse period. This fact implies that there is a close correlation between the compressive stress and c-BN formation mechanisms.
4. Discussion The above-described results of the two bias modes and their correlation look very complicated because of the existence of many parameters. We tried to express the c-BN formation condition with parameters of ⌫ir⌫B and Ei for the CW bias mode w7x. In the pulsed bias mode, however, the effective ion flux in the bias-off phase is different from that in the bias-on phase. Accordingly, we express here the deposition condition with the ion to boron-atom fluence ratio ⌽ ir⌽ B which is a time-integrated value of the flux ratio ⌫ir⌫B . Fig. 6 shows the relation between the threshold ion energy for c-BN synthesis and the fluence ratio. In this diagram, c-BN is synthesized at just above the boundary line Ždashed line.. By using ⌽ ir⌽ B instead of ⌫ir⌫B , the deposition condition is expressed synthetically and a consistent tendency of the threshold for c-BN synthesis is seen between the CW mode and the pulsed mode. The meaning of the boundary is not clear yet and further investigations are necessary with consideration of c-BN formation mechanisms.
Fig. 6. Deposition condition plotted as the threshold ion energy for c-BN synthesis vs. the ion to boron-atom fluence ratio. Circles, squares and triangles denote the CW mode, the pulse period of 1 s and that of 10 s, respectively.
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Fig. 7. Relation between the c-BN content and the compressive stress. Closed and open circles denote the CW mode and pulsed mode, respectively.
As regards c-BN formation mechanisms, the compressive stress seems to contribute c-BN synthesis. In fact, delamination after air-exposure occurs also for high-stressed samples prepared in the pulsed mode similarly to those in the CW mode. Furthermore, samples with c-BN as the dominant phase have large compressive stress, which is common both in the CW and pulsed modes. Fig. 7 clearly shows the relation between the c-BN content and the compressive stress. Samples with low c-BN content Žalmost with h-BN phase only. have low stress, while those with high c-BN content Ž Fc s 80 ; 90%. have large stress which ranges from 4 to 6 GPa regardless of the bias modes: pulsed or CW. According to the compressive stress induced c-BN formation model w17x, c-BN is synthesized above a certain threshold stress which is determined by the theory of thermodynamics. The h-BN ª c-BN dynamic transition boundary is 2.3 GPa at 500⬚C on the extraporated boundary line by Bundy-Wentorf on the phase py T diagram of BN, however, some researchers pointed out the Bundy-Wentorf’s boundary shows the deviation from experimental results and calculations by Solozhenko w18,19x. Apparently, our result appears to agree with the thermodynamic equilibrium at 500⬚C. However, according to the phase diagram of BN, the stress threshold must decrease with decrease in temperature. Generally, c-BN hardly grows at low temperature Ž; 200⬚C. w20x, which appears to contradict the model. In addition c-BN is synthesized under the condition that the total stress is reduced by a buffer layer between top c-BN layer and a substrate w7x. Therefore, expanded considerations including atomic-scale ion bombardment process for c-BN formation mechanisms are needed.
5. Conclusion Pulsed RF substrate bias is applied for c-BN film
1384
M. Wakatsuchi et al. r Diamond and Related Materials 10 (2001) 1380᎐1384
synthesis by reactive sputtering with ECR plasmas. Under the duty ratio of 50%, c-BN is hard to grow at a large pulse-period and low ion-energy. With a decrease in the duty ratio, the threshold ion energy for c-BN synthesis increases and the maximum c-BN content tends to decrease These results form a boundary for c-BN formation condition on the diagram as the ionenergy vs. the ion to boron-atom fluence ratio. Further investigations on c-BN formation mechanisms involving stress contribution are necessary to clarify the meaning of the boundary. References w1x C.F. Gardinier, Ceram. Bull. 67 Ž1988. 1006. w2x L. Vel, G. Demazeau, J. Etourneau, Mater. Sci. Eng. B 10 Ž1991. 149. w3x O. Mishima, K. Era, J. Tanaka, S. Yamaoka, Appl. Phys. Lett. 53 Ž1988. 962. w4x R.H. Wentorf, Jr., J. Chem. Phys. 36 Ž1962. 1990. w5x S. Gimeno, J.C. Mufloz, A. Lousa, Thin Solid Films 317 Ž1998. 376. w6x I.-H. Kim, S.-H. Kim, K.-B. Kim, Vac. Sci. Technol. A 16 Ž1998. 2295.
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