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High-quality cBN thin films prepared by plasma chemical vapor deposition with time-dependent biasing technique
Thin Solid Films 407 (2002) 67–71
High-quality cBN thin films prepared by plasma chemical vapor deposition with time-dependent biasing technique Hangsheng Yang*, Chihiro Iwamoto, Toyonobu Yoshida Department of Materials Engineering, Faculty of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-Ku, Tokyo 113-8656, Japan
Abstract Cubic boron nitride thin films were deposited on silicon (111) wafers by inductively coupled plasma-enhanced chemical vapor deposition. The influences of Ar flow rate and the time-dependent substrate biasing condition on the composition and the transition layer’s thickness of cBN films were systematically investigated. By using the time-dependent biasing technique, with decreasing the substrate bias voltage gradually from sputtering mode to a final appropriate value for cBN deposition mode, 600 nm thick high quality and stoichiometric cBN films consisting of more than 98% cubic phase were successfully deposited with the deposition rate of 1.5 nmys. The transition layer consisted of an amorphous layer and a turbostratic boron nitride layer that could be reduced less than 10 nm, which proved that, by using proper deposition technique, high-quality cBN films similar to those prepared by physical vapor deposition methods can be prepared even by chemical vapor deposition. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Cubic boron nitride; Chemical vapor deposition; Time-dependent biasing technique; High-resolution transmission electron microscopy
1. Introduction Cubic boron nitride (cBN) thin films possess excellent physical and chemical properties, such as ultrahigh hardness second to only diamond, inertness against iron and oxygen even at high temperature, as well as the possibility of use as n- and p-type doped semiconductors w1x, which have significant potential technological applications in cutting tools, electronic devices, etc. In the past two decades, a variety of chemical vapor deposition (CVD) and physical vapor deposition (PVD) techniques have been used to prepare cBN films with the assistance of ion bombardment w2–9x. A review of vapor phase deposition of cBN thin film has been provided by Yoshida w10x. Under optimized conditions, most of the cBN films prepared were found to be a three-layered structure w3,7x: an initial amorphous boron nitride (aBN) layer on a substrate, followed by a highly oriented turbostratic boron nitride (tBN) layer with its c-axis parallel to the substrate surface, and a final cBN layer. Due to the existence of the initial sp2-bonded BN transition layer and some tBN mixed in the cBN layer *Corresponding author. Tel.: q81-3-5841-7099; fax: q81-3-58417099. E-mail address: [email protected] (H. Yang).
w11x, the purity of cBN films is low, making them inappropriate for electronic applications. In particular, cBN films prepared by CVD techniques were reported to have lower cubic phase content than those prepared by PVD techniques because of the relatively thick initial layer w12x. To date, only PVD techniques were found to be able to deposit cBN films with an initial sp2-bonded BN transition layer of less than 20 nm thickness and cBN content higher than 90%. The inductively coupled plasma-enhanced chemical vapor deposition (ICP-CVD) technique is one of the cBN thin film deposition techniques w7,8x. Using this technique, cBN films of 80% purity were successfully prepared w7x. In this study, in order to increase the quality of cBN films deposited by the ICP-CVD technique, a time-dependent biasing technique (TDBT) was developed. TDBT is a technique in which a relatively high minus substrate bias voltage is applied initially when B2H6 (dilute in He) gas is introduced into a deposition chamber. Then, the substrate bias voltage is continuously and gradually reduced to a final appropriate value for film deposition. Using this TDBT, we were able to deposit boron nitride films with more than 98% cubic phase and an initial sp2-bonded BN transition
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 0 0 1 4 - 7
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layer less than 10 nm thickness under optimized deposition parameters. 2. Experimental details The apparatus for cBN film deposition was described elsewhere w7x. The deposition parameters were as follows: 28=28=0.5 mm3 mirror-polished Si (111) wafers were used as substrates, and the reactor chamber was pre-evacuated to approximately 4=10 y6 torr and then backfilled with the desired gases. Before film deposition, H2 plasma and N2 plasma were generated for 10 min and 30 min, respectively, to clean the substrate surface and the chamber walls. Film deposition was carried out for 5–30 min and an ICP was generated with 13.56 MHz–7 kW power input. After the pre-treatments, the plasma was generated with mixture gases of 2.7–16 sccm Ar and 1 sccm N2, and the initial substrate bias voltage was set at y150 V during the introduction of 10% B2H6 (He). The flow rate of 10% B2H6 (He) was increased from 1 to 10 sccm in 3 min, and finally the substrate bias voltage was reduced continuously to a final appropriate value in 3 min for film deposition. The substrate bias voltage was controlled by an auxiliary RF power supply from y150 V to q60 V, and the plasma potential was approximately 90 V. All gases were electronic grade and controlled by mass flow controllers. The working pressure, being monitored by a Shulz gauge calibrated for N2, was kept at 1 mtorr throughout the deposition. The substrate temperature was approximately 300 8C in all experiments. After deposition, the samples were cooled to room temperature under argon atmosphere without any post-treatment. Samples were characterized ex situ mainly by using an FTIR spectrometer (JASCO FTyIR-700) with the transmission mode at normal incidence, and the standard analyzing area was 200=200 mm2. The IR absorption is measured from original spectra without any treatment. The maximum height intensity was measured to quantitatively determine the cBN and sp2-bonded BN (the baseline was determined by standard base line method). Nano-structures of cBN films were investigated by highresolution transmission electron microscopy (JEM-4000 FXII and JEM ARM 1250, the accelerating voltage is 400 kV and 1250 kV, respectively). Cross-sectional HRTEM samples were prepared by the standard method of mechanical polishing the samples to approximately 100 mm, followed by dimple grinding to approximately 25 mm, and finally, Ar ion polishing to a thickness sufficient for TEM observation. Position-resolved X-ray photoelectron spectroscopy (XPS) measurements were carried out with a ShimadzuyKratos AXIS-HS equipment to investigate film composition.
Fig. 1. The FTIR absorptions of cBN films deposited at different Ar flow rates. Films were deposited at bias y20 V using TDBT, N2 and 10% B2H6 (He) flow rates were 1 and 10 sccm, respectively. The ‘sp2-bonded BN 1380’ is the IR absorption of sp2-bonded BN at peak near 1380 cmy1; the ‘cBN 1080’ is the IR absorption of cBN at peak near 1080 cmy1.
3. Results 3.1. Effect of Ar flow rate on cBN film deposition Fig. 1 shows the FTIR absorbance of cBN films deposited at different Ar flow rates with the final deposition bias voltage set at y20 V using TDBT. Even at a low Ar flow rate of 2.7 sccm, the FTIR spectra showed relatively strong absorption of cBN near 1080 cmy1, which indicates that the deposited films contain a cubic phase. However, the absorptions of sp2-bonded BN near 1380 and 780 cmy1 were also strong, suggesting that the films are possibly mixtures of cBN and tBN. Peaks near 1380 cmy1 and 780 cmy1 decrease abruptly with the increase in Ar flow rate up to approximately 10 sccm. A further increase of the Ar flow rate has little effect on sp2-bonded BN absorptions, and the slight decrease of the peaks of tBN is assumed to be due to the decrease of the thickness of the initial sp2bonded transition layer. From FTIR observation, we found that by using TDBT, when N2 and 10% B2H6 (He) flow rates were 1 and 10 sccm, respectively, the optimized Ar flow rate for depositing films with high cBN content was approximately 16 sccm at 300 8C. We did not further increase the Ar flow rate because the maximum flow of mass flow controller for Ar was 16 sccm, and if the gas flow rate were further increased, the deposition pressure could not be controlled at 1 mtorr. 3.2. Effect of substrate bias voltage on cBN film deposition The substrate bias voltage also has a strong effect on cBN film deposition. At an Ar flow rate of 16 sccm, the final substrate bias voltage window for cBN deposition was approximately from 0 V to y85 V, which
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Fig. 2. The FTIR absorptions of sp2-bonded BN in cBN films deposited at different substrate bias voltages. The reactant gases flow rates of Ar, N2 and 10% B2H6 (He) were 16, 1 and 10 sccm, respectively. The ‘sp2-bonded BN 1380’ is the IR absorption of sp2-bonded BN at peak near 1380 cmy1; the ‘cBN 1080’ is the IR absorption of cBN at peak near 1080 cmy1.
was slightly wider than the window reported by Ichiki et al. using the same apparatus w7,8x. Fig. 2 shows the relationship between sp2-bonded boron nitride and cBN IR absorbances in cBN films and the applied substrate bias voltage. A minimum sp2-bonded BN absorption was found at the final substrate bias voltage near y20 V. Therefore, in our case, when the flow rates of N2 and 10% B2H6 (He) were 1 and 10 sccm, respectively, the optimal deposition conditions were a substrate bias voltage of y20 V and an Ar flow rate of 16 sccm when TDBT was used. Fig. 3 shows the FTIR spectrum of a film deposited at the optimal conditions. A strong peak near 1080 cmy1 with a very weak peak at approximately 1380 cmy1 could be observed. The peak near 780 cmy1 was almost negligible, which indicates that the film consisted of high cBN content. Based on data from Ichiki et al.’s of the FTIR absorbances of cBN and tBN w7x, the film contained more than 98% cubic phase wdefined as I1080 y(I1080qI1380), the absorption coefficients of cBN and sp2-bonded BN are 23 000 cmy1 and 30 000 cmy1, respectivelyx w7x. To the authors’ knowledge, it is the purest cBN film prepared to date. The total thickness of the film in Fig. 3 was estimated to be approximately 460 nm, including approximately 8 nm of the initial transition sp2-bonded BN layer. The deposition rate was calculated to be approximately 1.5 nmy s. Even if no additional treatment was applied for the enhancement of film adhesion, the film still adhered on the substrate in vacuum. In fact, film of more than 600 nm thickness could be deposited without peeling off in vacuum. However, it peeled off from the substrate within 1 h after it was taken out of the vacuum chamber, due to compressive stress and humid air w13x. 3.3. HRTEM observation of cBN films Fig. 4 shows the cross-sectional image of the interface between BN film and Si substrate of a cBN film
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Fig. 3. The FTIR spectrum of a high content cBN film deposited at optimal conditions. Film was deposited for 10 min at bias y20 V using TDBT, and Ar, N2 and 10% B2H6 (He) flow rates were 16, 1 and 10 sccm, respectively.
deposited at a substrate bias of y10 V, and 16 sccm, 1 sccm and 10 sccm Ar, N2 and 10% B2H6 (He), respectively. The three-layered structure was clearly observed, with a very thin aBN layer of approximately 4 nm thick, followed by a texture tBN layer of approximately 6 nm thick with its c-axis nearly parallel to the substrate surface, and the upper layer that consisted of pure cBN. In the cubic layer of film, there were many twins and stacking faults, and the characteristic of every third cBN (111) plane closely matching that of the alternative tBN (0002) plane was observed, which is in agreement with previous results w8,14x. The lattice fringe of tBN was approximately 0.34 nm, which was slightly wider than
Fig. 4. Cross-sectional HRTEM image of the interface between Si substrate and a cBN film deposited at bias of y10 V using TDBT. Ar, N2 and 10% B2H6 (He) flow rates were 16, 1 and 10 sccm, respectively. The total thickness of initial transition layer is approximately 10 nm, the lattice fringe of tBN was approximately 0.34 nm; in the upper cBN layer, lattice fringe of cBN (111) was 0.21 nm and rich in twins and stacking faults.
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sp2-bonded BN transition layer to date for films deposited by CVD. Finally, the thin hBN surface layer on top of the cBN films, as reported by Park et al. w15x, was not observed in all of our HRTEM observations. 3.4. XPS measurements Fig. 6 shows the XPS spectrum of the cBN film in Fig. 4. The bottom curve is measured from the film surface, and the bonding energies of 191 eV for B (1s) and 398 eV for N (1s) levels are in agreement with those previously reported w8,16x. The peaks for C (1s) and O (1s) near 280 eV and 530 eV were also observed. However, these peaks were eliminated by Ar ion etching for 30 s. The upper curve is measured after Ar ion etching for 30 s, peaks of C (1s) and O (1s) were obviously eliminated. Therefore, C and O are adsorption species after the film was taken out of the deposition chamber. The ByN ratio calculated from the area ratio of B (1s) to N (1s) gave a stoichiometric value of 1.0, which proved that the film was stoichiometric cBN film. The p plasmon loss peaks near the main B (1s) and N (1s) peaks were not observed in the spectrum, indicating that the surface consists of a pure cubic phase, in agreement with the results of HRTEM observation. 4. Discussion
Fig. 5. Cross-sectional HRTEM image of a whole cBN film deposited at bias of y40 V using TDBT. Film was deposited at bias y40 V using TDBT, N2 and 10% B2H6 (He) flow rates were 1 and 10 sccm, respectively. Above a very thin initial layer (10 nm) is almost pure cBN layer of approximately 45 nm thickness.
the standard hBN value of 0.33 nm, due to dislocations or distortions, which were frequently observed in the tBN layer. According to FTIR measurement w7x, the total sp2-bonded BN layer thickness in the cBN film of Fig. 4 is approximately 11 nm, which is in good agreement with the result of HRTEM observation (4 nm aBNq6 nm tBN). Therefore, it can be deduced that the upper layer is an almost pure cubic phase. Fig. 5 shows another HRTEM image of a whole cBN film deposited at a bias voltage of y40 V with TDBT. Above an approximately 10 nm-thick initial sp2-bonded BN transition layer is a 45 nm thick pure cBN layer. The film thickness calculated from FTIR absorbance was 11 nm for the sp2-bonded transition layer and 50 nm for the cBN layer; these values were also in good agreement with those of HRTEM observation. Therefore, both HRTEM observation and FTIR measurement proved that, with the assistance of TDBT, cBN thin films deposited by ICP-CVD could reduce the initial sp2-bonded BN layer thickness to less than 10 nm, which, to the authors’ knowledge, is the thinnest initial
It is now well accepted that cBN film deposition requires ion bombardment, and there is a bombardment ion momentum or energy window for the nucleation of cBN w2,9x. Later works proved that the bombardment ion energy or momentum needed for sustaining the growth of cBN is slightly lower than that necessary for cBN nucleation w17,18x. In this study, TDBT was performed for film deposition; thus, the initial minus
Fig. 6. XPS spectra of the cBN film in Fig. 4. The bottom curve is the spectrum measured at the cBN surface, and the upper curve is the spectrum measured after 30 sec Ar ion etching. After 30 sec etching, the peaks of C and O were successfully eliminated.
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substrate bias voltage was rather high (y150 V). In the present study, below a substrate bias voltage of y150 V, no film could be deposited because of the Ar ion sputtering effect. Thus, the influence of the introduction of reactant gases into the chamber was totally eliminated. Then, the substrate bias voltage was reduced continuously, and at a certain substrate bias voltage, the nucleation of cBN occurred. As a result, the initial layer is thin. If the film were deposited under the condition optimized for cBN nucleation, films would be rich in hBN as shown in Fig. 2, at a deposition substrate bias voltage of y80 V. Therefore, the substrate bias should be reduced to a final value appropriate for pure cBN upper layer growth, as suggested by Amagi et al. w18x. In this paper, the optimal final bias was found to be approximately y20 V, and further growth of pure cBN was possible, which revealed the fact that the optimized substrate bias for sustaining cBN growth is much lower than that for cBN nucleation. Regarding film deposition techniques, both CVD and PVD were successfully used to prepare cBN films. However, to date, all studies using the CVD techniques reported lower cBN content and a thicker initial sp2bonded layer w12x. In this study, however, we found that by using TDBT, CVD can be used to prepare cBN films with high cBN content and a thin initial sp2-bonded BN layer, similar to the PVD technique. 5. Conclusions In summary, we developed a deposition technique called TDBT for low-pressure ICP-CVD. Under the optimized deposition parameters, we deposited for the first time high-quality stoichiometric cBN films with more than 98% cubic phase and a thin initial sp2-bonded BN transition layer of approximately 8 nm thick. Moreover, cBN films of 600 nm thick were deposited without peeling off in vacuum. The effect of TDBT was also discussed and our results strongly indicate that cBN
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growth and nucleation should be treated separately. Further reduction of the sp2-bonded content is in progress. Acknowledgments This paper was financially supported by a grant-inAid for Scientific Research (A) (Grant No. 13355028), Ministry of Education, Culture, Sports, Science and Technology, Japan. References w1x L. Vel, G. Demazeau, J. Mater. Sci. Eng. B 10 (1991) 149. w2x K. Inagawa, K. Watanabe, H. Ohsone, K. Saitoh, A. Itoh, J. Vac. Sci. Technol. A 5 (1987) 2696. w3x D.J. Kester, K.S. Ailey, R.F. Davis, K.L. More, J. Mater. Res. 8 (1993) 1213. w4x D.R. Mckenzie, W.D. McFall, W.G. Sainty, C.A. Davis, R.E. Collins, Diamond Relat. Mater. 2 (1993) 970. w5x M. Mieno, T. Yoshida, Jpn. J. Appl. Phys. 29 (1990) L1175. w6x H. Hofsass, C. Ronning, U. Griesmeier, M. Gross, S. Reinke, M. Kuhr, Appl. Phys. Lett. 67 (1995) 46. w7x T. Ichiki, T. Mosose, T Yoshida, J. Appl. Phys. 75 (1994) 1330. w8x T. Ichiki, T. Yoshida, Appl. Phys. Lett. 64 (1994) 851. w9x Y. Yamada, Y Tatebayashi, O. Tsuda, T. Yoshida, Thin Solid Films 259 (1997) 137. w10x T. Yoshida, Diamond Films Technol. 7 (1997) 87. w11x W.L. Zhou, Y. Ikuhara, M. Murakawa, S. Watanabe, T. Suziki, Appl. Phys. Lett. 66 (1995) 2049. w12x P.B. Mirkarimi, K.F. McCarty, D.L. Medlin, Mater. Sci. Eng. Report 21 (1997) 47. w13x G.F. Cardinale, P.B. Mirkarimi, K.F. McCarty, et al., Thin Solid Films 253 (1995) 153. w14x D.V. Shtansky, O. Tsuda, Y. Ikuhara, T. Yoshida, Acta Mater. 48 (2000) 3749. w15x K.S. Park, D.Y. Lee, K.J. Kim, D.W. Moon, J. Vac. Sci. Technol. 15 (1997) 1041. w16x R. Trehan, Y. Lifshitz, J.W. Rabalais, J. Vac. Sci. Technol. A8 (1990) 4026. w17x P.B. Mirkarimi, D.L. Madlin, K.F. McCarty, et al., J. Appl. Phys. 82 (1997) 1617. w18x S. Amagi, D. Takahashi, T. Yoshida, Appl. Phys. Lett. 70 (1997) 946.
High-quality cBN thin films prepared by plasma chemical vapor deposition with time-dependent biasing technique Hangsheng Yang*, Chihiro Iwamoto, Toyonobu Yoshida Department of Materials Engineering, Faculty of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-Ku, Tokyo 113-8656, Japan
Abstract Cubic boron nitride thin films were deposited on silicon (111) wafers by inductively coupled plasma-enhanced chemical vapor deposition. The influences of Ar flow rate and the time-dependent substrate biasing condition on the composition and the transition layer’s thickness of cBN films were systematically investigated. By using the time-dependent biasing technique, with decreasing the substrate bias voltage gradually from sputtering mode to a final appropriate value for cBN deposition mode, 600 nm thick high quality and stoichiometric cBN films consisting of more than 98% cubic phase were successfully deposited with the deposition rate of 1.5 nmys. The transition layer consisted of an amorphous layer and a turbostratic boron nitride layer that could be reduced less than 10 nm, which proved that, by using proper deposition technique, high-quality cBN films similar to those prepared by physical vapor deposition methods can be prepared even by chemical vapor deposition. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Cubic boron nitride; Chemical vapor deposition; Time-dependent biasing technique; High-resolution transmission electron microscopy
1. Introduction Cubic boron nitride (cBN) thin films possess excellent physical and chemical properties, such as ultrahigh hardness second to only diamond, inertness against iron and oxygen even at high temperature, as well as the possibility of use as n- and p-type doped semiconductors w1x, which have significant potential technological applications in cutting tools, electronic devices, etc. In the past two decades, a variety of chemical vapor deposition (CVD) and physical vapor deposition (PVD) techniques have been used to prepare cBN films with the assistance of ion bombardment w2–9x. A review of vapor phase deposition of cBN thin film has been provided by Yoshida w10x. Under optimized conditions, most of the cBN films prepared were found to be a three-layered structure w3,7x: an initial amorphous boron nitride (aBN) layer on a substrate, followed by a highly oriented turbostratic boron nitride (tBN) layer with its c-axis parallel to the substrate surface, and a final cBN layer. Due to the existence of the initial sp2-bonded BN transition layer and some tBN mixed in the cBN layer *Corresponding author. Tel.: q81-3-5841-7099; fax: q81-3-58417099. E-mail address: [email protected] (H. Yang).
w11x, the purity of cBN films is low, making them inappropriate for electronic applications. In particular, cBN films prepared by CVD techniques were reported to have lower cubic phase content than those prepared by PVD techniques because of the relatively thick initial layer w12x. To date, only PVD techniques were found to be able to deposit cBN films with an initial sp2-bonded BN transition layer of less than 20 nm thickness and cBN content higher than 90%. The inductively coupled plasma-enhanced chemical vapor deposition (ICP-CVD) technique is one of the cBN thin film deposition techniques w7,8x. Using this technique, cBN films of 80% purity were successfully prepared w7x. In this study, in order to increase the quality of cBN films deposited by the ICP-CVD technique, a time-dependent biasing technique (TDBT) was developed. TDBT is a technique in which a relatively high minus substrate bias voltage is applied initially when B2H6 (dilute in He) gas is introduced into a deposition chamber. Then, the substrate bias voltage is continuously and gradually reduced to a final appropriate value for film deposition. Using this TDBT, we were able to deposit boron nitride films with more than 98% cubic phase and an initial sp2-bonded BN transition
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layer less than 10 nm thickness under optimized deposition parameters. 2. Experimental details The apparatus for cBN film deposition was described elsewhere w7x. The deposition parameters were as follows: 28=28=0.5 mm3 mirror-polished Si (111) wafers were used as substrates, and the reactor chamber was pre-evacuated to approximately 4=10 y6 torr and then backfilled with the desired gases. Before film deposition, H2 plasma and N2 plasma were generated for 10 min and 30 min, respectively, to clean the substrate surface and the chamber walls. Film deposition was carried out for 5–30 min and an ICP was generated with 13.56 MHz–7 kW power input. After the pre-treatments, the plasma was generated with mixture gases of 2.7–16 sccm Ar and 1 sccm N2, and the initial substrate bias voltage was set at y150 V during the introduction of 10% B2H6 (He). The flow rate of 10% B2H6 (He) was increased from 1 to 10 sccm in 3 min, and finally the substrate bias voltage was reduced continuously to a final appropriate value in 3 min for film deposition. The substrate bias voltage was controlled by an auxiliary RF power supply from y150 V to q60 V, and the plasma potential was approximately 90 V. All gases were electronic grade and controlled by mass flow controllers. The working pressure, being monitored by a Shulz gauge calibrated for N2, was kept at 1 mtorr throughout the deposition. The substrate temperature was approximately 300 8C in all experiments. After deposition, the samples were cooled to room temperature under argon atmosphere without any post-treatment. Samples were characterized ex situ mainly by using an FTIR spectrometer (JASCO FTyIR-700) with the transmission mode at normal incidence, and the standard analyzing area was 200=200 mm2. The IR absorption is measured from original spectra without any treatment. The maximum height intensity was measured to quantitatively determine the cBN and sp2-bonded BN (the baseline was determined by standard base line method). Nano-structures of cBN films were investigated by highresolution transmission electron microscopy (JEM-4000 FXII and JEM ARM 1250, the accelerating voltage is 400 kV and 1250 kV, respectively). Cross-sectional HRTEM samples were prepared by the standard method of mechanical polishing the samples to approximately 100 mm, followed by dimple grinding to approximately 25 mm, and finally, Ar ion polishing to a thickness sufficient for TEM observation. Position-resolved X-ray photoelectron spectroscopy (XPS) measurements were carried out with a ShimadzuyKratos AXIS-HS equipment to investigate film composition.
Fig. 1. The FTIR absorptions of cBN films deposited at different Ar flow rates. Films were deposited at bias y20 V using TDBT, N2 and 10% B2H6 (He) flow rates were 1 and 10 sccm, respectively. The ‘sp2-bonded BN 1380’ is the IR absorption of sp2-bonded BN at peak near 1380 cmy1; the ‘cBN 1080’ is the IR absorption of cBN at peak near 1080 cmy1.
3. Results 3.1. Effect of Ar flow rate on cBN film deposition Fig. 1 shows the FTIR absorbance of cBN films deposited at different Ar flow rates with the final deposition bias voltage set at y20 V using TDBT. Even at a low Ar flow rate of 2.7 sccm, the FTIR spectra showed relatively strong absorption of cBN near 1080 cmy1, which indicates that the deposited films contain a cubic phase. However, the absorptions of sp2-bonded BN near 1380 and 780 cmy1 were also strong, suggesting that the films are possibly mixtures of cBN and tBN. Peaks near 1380 cmy1 and 780 cmy1 decrease abruptly with the increase in Ar flow rate up to approximately 10 sccm. A further increase of the Ar flow rate has little effect on sp2-bonded BN absorptions, and the slight decrease of the peaks of tBN is assumed to be due to the decrease of the thickness of the initial sp2bonded transition layer. From FTIR observation, we found that by using TDBT, when N2 and 10% B2H6 (He) flow rates were 1 and 10 sccm, respectively, the optimized Ar flow rate for depositing films with high cBN content was approximately 16 sccm at 300 8C. We did not further increase the Ar flow rate because the maximum flow of mass flow controller for Ar was 16 sccm, and if the gas flow rate were further increased, the deposition pressure could not be controlled at 1 mtorr. 3.2. Effect of substrate bias voltage on cBN film deposition The substrate bias voltage also has a strong effect on cBN film deposition. At an Ar flow rate of 16 sccm, the final substrate bias voltage window for cBN deposition was approximately from 0 V to y85 V, which
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Fig. 2. The FTIR absorptions of sp2-bonded BN in cBN films deposited at different substrate bias voltages. The reactant gases flow rates of Ar, N2 and 10% B2H6 (He) were 16, 1 and 10 sccm, respectively. The ‘sp2-bonded BN 1380’ is the IR absorption of sp2-bonded BN at peak near 1380 cmy1; the ‘cBN 1080’ is the IR absorption of cBN at peak near 1080 cmy1.
was slightly wider than the window reported by Ichiki et al. using the same apparatus w7,8x. Fig. 2 shows the relationship between sp2-bonded boron nitride and cBN IR absorbances in cBN films and the applied substrate bias voltage. A minimum sp2-bonded BN absorption was found at the final substrate bias voltage near y20 V. Therefore, in our case, when the flow rates of N2 and 10% B2H6 (He) were 1 and 10 sccm, respectively, the optimal deposition conditions were a substrate bias voltage of y20 V and an Ar flow rate of 16 sccm when TDBT was used. Fig. 3 shows the FTIR spectrum of a film deposited at the optimal conditions. A strong peak near 1080 cmy1 with a very weak peak at approximately 1380 cmy1 could be observed. The peak near 780 cmy1 was almost negligible, which indicates that the film consisted of high cBN content. Based on data from Ichiki et al.’s of the FTIR absorbances of cBN and tBN w7x, the film contained more than 98% cubic phase wdefined as I1080 y(I1080qI1380), the absorption coefficients of cBN and sp2-bonded BN are 23 000 cmy1 and 30 000 cmy1, respectivelyx w7x. To the authors’ knowledge, it is the purest cBN film prepared to date. The total thickness of the film in Fig. 3 was estimated to be approximately 460 nm, including approximately 8 nm of the initial transition sp2-bonded BN layer. The deposition rate was calculated to be approximately 1.5 nmy s. Even if no additional treatment was applied for the enhancement of film adhesion, the film still adhered on the substrate in vacuum. In fact, film of more than 600 nm thickness could be deposited without peeling off in vacuum. However, it peeled off from the substrate within 1 h after it was taken out of the vacuum chamber, due to compressive stress and humid air w13x. 3.3. HRTEM observation of cBN films Fig. 4 shows the cross-sectional image of the interface between BN film and Si substrate of a cBN film
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Fig. 3. The FTIR spectrum of a high content cBN film deposited at optimal conditions. Film was deposited for 10 min at bias y20 V using TDBT, and Ar, N2 and 10% B2H6 (He) flow rates were 16, 1 and 10 sccm, respectively.
deposited at a substrate bias of y10 V, and 16 sccm, 1 sccm and 10 sccm Ar, N2 and 10% B2H6 (He), respectively. The three-layered structure was clearly observed, with a very thin aBN layer of approximately 4 nm thick, followed by a texture tBN layer of approximately 6 nm thick with its c-axis nearly parallel to the substrate surface, and the upper layer that consisted of pure cBN. In the cubic layer of film, there were many twins and stacking faults, and the characteristic of every third cBN (111) plane closely matching that of the alternative tBN (0002) plane was observed, which is in agreement with previous results w8,14x. The lattice fringe of tBN was approximately 0.34 nm, which was slightly wider than
Fig. 4. Cross-sectional HRTEM image of the interface between Si substrate and a cBN film deposited at bias of y10 V using TDBT. Ar, N2 and 10% B2H6 (He) flow rates were 16, 1 and 10 sccm, respectively. The total thickness of initial transition layer is approximately 10 nm, the lattice fringe of tBN was approximately 0.34 nm; in the upper cBN layer, lattice fringe of cBN (111) was 0.21 nm and rich in twins and stacking faults.
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sp2-bonded BN transition layer to date for films deposited by CVD. Finally, the thin hBN surface layer on top of the cBN films, as reported by Park et al. w15x, was not observed in all of our HRTEM observations. 3.4. XPS measurements Fig. 6 shows the XPS spectrum of the cBN film in Fig. 4. The bottom curve is measured from the film surface, and the bonding energies of 191 eV for B (1s) and 398 eV for N (1s) levels are in agreement with those previously reported w8,16x. The peaks for C (1s) and O (1s) near 280 eV and 530 eV were also observed. However, these peaks were eliminated by Ar ion etching for 30 s. The upper curve is measured after Ar ion etching for 30 s, peaks of C (1s) and O (1s) were obviously eliminated. Therefore, C and O are adsorption species after the film was taken out of the deposition chamber. The ByN ratio calculated from the area ratio of B (1s) to N (1s) gave a stoichiometric value of 1.0, which proved that the film was stoichiometric cBN film. The p plasmon loss peaks near the main B (1s) and N (1s) peaks were not observed in the spectrum, indicating that the surface consists of a pure cubic phase, in agreement with the results of HRTEM observation. 4. Discussion
Fig. 5. Cross-sectional HRTEM image of a whole cBN film deposited at bias of y40 V using TDBT. Film was deposited at bias y40 V using TDBT, N2 and 10% B2H6 (He) flow rates were 1 and 10 sccm, respectively. Above a very thin initial layer (10 nm) is almost pure cBN layer of approximately 45 nm thickness.
the standard hBN value of 0.33 nm, due to dislocations or distortions, which were frequently observed in the tBN layer. According to FTIR measurement w7x, the total sp2-bonded BN layer thickness in the cBN film of Fig. 4 is approximately 11 nm, which is in good agreement with the result of HRTEM observation (4 nm aBNq6 nm tBN). Therefore, it can be deduced that the upper layer is an almost pure cubic phase. Fig. 5 shows another HRTEM image of a whole cBN film deposited at a bias voltage of y40 V with TDBT. Above an approximately 10 nm-thick initial sp2-bonded BN transition layer is a 45 nm thick pure cBN layer. The film thickness calculated from FTIR absorbance was 11 nm for the sp2-bonded transition layer and 50 nm for the cBN layer; these values were also in good agreement with those of HRTEM observation. Therefore, both HRTEM observation and FTIR measurement proved that, with the assistance of TDBT, cBN thin films deposited by ICP-CVD could reduce the initial sp2-bonded BN layer thickness to less than 10 nm, which, to the authors’ knowledge, is the thinnest initial
It is now well accepted that cBN film deposition requires ion bombardment, and there is a bombardment ion momentum or energy window for the nucleation of cBN w2,9x. Later works proved that the bombardment ion energy or momentum needed for sustaining the growth of cBN is slightly lower than that necessary for cBN nucleation w17,18x. In this study, TDBT was performed for film deposition; thus, the initial minus
Fig. 6. XPS spectra of the cBN film in Fig. 4. The bottom curve is the spectrum measured at the cBN surface, and the upper curve is the spectrum measured after 30 sec Ar ion etching. After 30 sec etching, the peaks of C and O were successfully eliminated.
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substrate bias voltage was rather high (y150 V). In the present study, below a substrate bias voltage of y150 V, no film could be deposited because of the Ar ion sputtering effect. Thus, the influence of the introduction of reactant gases into the chamber was totally eliminated. Then, the substrate bias voltage was reduced continuously, and at a certain substrate bias voltage, the nucleation of cBN occurred. As a result, the initial layer is thin. If the film were deposited under the condition optimized for cBN nucleation, films would be rich in hBN as shown in Fig. 2, at a deposition substrate bias voltage of y80 V. Therefore, the substrate bias should be reduced to a final value appropriate for pure cBN upper layer growth, as suggested by Amagi et al. w18x. In this paper, the optimal final bias was found to be approximately y20 V, and further growth of pure cBN was possible, which revealed the fact that the optimized substrate bias for sustaining cBN growth is much lower than that for cBN nucleation. Regarding film deposition techniques, both CVD and PVD were successfully used to prepare cBN films. However, to date, all studies using the CVD techniques reported lower cBN content and a thicker initial sp2bonded layer w12x. In this study, however, we found that by using TDBT, CVD can be used to prepare cBN films with high cBN content and a thin initial sp2-bonded BN layer, similar to the PVD technique. 5. Conclusions In summary, we developed a deposition technique called TDBT for low-pressure ICP-CVD. Under the optimized deposition parameters, we deposited for the first time high-quality stoichiometric cBN films with more than 98% cubic phase and a thin initial sp2-bonded BN transition layer of approximately 8 nm thick. Moreover, cBN films of 600 nm thick were deposited without peeling off in vacuum. The effect of TDBT was also discussed and our results strongly indicate that cBN
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growth and nucleation should be treated separately. Further reduction of the sp2-bonded content is in progress. Acknowledgments This paper was financially supported by a grant-inAid for Scientific Research (A) (Grant No. 13355028), Ministry of Education, Culture, Sports, Science and Technology, Japan. References w1x L. Vel, G. Demazeau, J. Mater. Sci. Eng. B 10 (1991) 149. w2x K. Inagawa, K. Watanabe, H. Ohsone, K. Saitoh, A. Itoh, J. Vac. Sci. Technol. A 5 (1987) 2696. w3x D.J. Kester, K.S. Ailey, R.F. Davis, K.L. More, J. Mater. Res. 8 (1993) 1213. w4x D.R. Mckenzie, W.D. McFall, W.G. Sainty, C.A. Davis, R.E. Collins, Diamond Relat. Mater. 2 (1993) 970. w5x M. Mieno, T. Yoshida, Jpn. J. Appl. Phys. 29 (1990) L1175. w6x H. Hofsass, C. Ronning, U. Griesmeier, M. Gross, S. Reinke, M. Kuhr, Appl. Phys. Lett. 67 (1995) 46. w7x T. Ichiki, T. Mosose, T Yoshida, J. Appl. Phys. 75 (1994) 1330. w8x T. Ichiki, T. Yoshida, Appl. Phys. Lett. 64 (1994) 851. w9x Y. Yamada, Y Tatebayashi, O. Tsuda, T. Yoshida, Thin Solid Films 259 (1997) 137. w10x T. Yoshida, Diamond Films Technol. 7 (1997) 87. w11x W.L. Zhou, Y. Ikuhara, M. Murakawa, S. Watanabe, T. Suziki, Appl. Phys. Lett. 66 (1995) 2049. w12x P.B. Mirkarimi, K.F. McCarty, D.L. Medlin, Mater. Sci. Eng. Report 21 (1997) 47. w13x G.F. Cardinale, P.B. Mirkarimi, K.F. McCarty, et al., Thin Solid Films 253 (1995) 153. w14x D.V. Shtansky, O. Tsuda, Y. Ikuhara, T. Yoshida, Acta Mater. 48 (2000) 3749. w15x K.S. Park, D.Y. Lee, K.J. Kim, D.W. Moon, J. Vac. Sci. Technol. 15 (1997) 1041. w16x R. Trehan, Y. Lifshitz, J.W. Rabalais, J. Vac. Sci. Technol. A8 (1990) 4026. w17x P.B. Mirkarimi, D.L. Madlin, K.F. McCarty, et al., J. Appl. Phys. 82 (1997) 1617. w18x S. Amagi, D. Takahashi, T. Yoshida, Appl. Phys. Lett. 70 (1997) 946.