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Formation process of Ni–N films by reactive sputtering at different substrate temperatures
Vacuum 59 (2000) 721}726
Formation process of Ni}N "lms by reactive sputtering at di!erent substrate temperatures M. Kawamura*, Y. Abe, K. Sasaki Department of Materials Science, Faculty of Engineering, Kitami Institute of Technology, Kitami, Hokkaido 090-8507, Japan
Abstract Ni}N thin "lms are prepared by reactive sputtering in an rf diode sputtering system. Deposition parameters, such as nitrogen #ow ratio, substrate temperature and substrate holder potential were changed and their e!ects were investigated. When the substrate was heated at 2203C, Ni and Ni N phases were observed by XRD. In the case of substrate temperature being room temperature (RT), Ni N phase also appeared and characteristic nitrogen incorporation into Ni lattice was seen. In addition, when the substrate holder was set to the #oat potential, nitrogen content of the "lm which is thought to be lower than the case of ground potential, caused stronger resputtering e!ect. Because it was observed that the formation phase and nitrogen content of the "lm depend on the substrate temperature, the nitrogenizing reaction on the deposit surface was considered to rule the "lm formation. This was also supported by the results on the target surface analysis after sputtering and plasma emission spectroscopy measurements. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Reactive sputtering is one of the most popular methods to prepare metal nitride "lms. Among various metal nitrides, nickel nitride is relatively less-common and its property and formation process have not been reported much. Several results have been published about the Ni}N "lm formation by reactive sputtering method so far. For example, Dorman and Sikkens prepared nickel nitride "lms by dc sputtering and obtained phases which changed from Ni to Ni N, Ni N, and also Ni N with increasing nitrogen partial pressure [1]. Takamori et al. also reported the formation of Ni N phase by rf sputtering, however, Ni N and Ni N were not appeared there [2]. But no studies have ever tried to make clear the e!ect of substrate temperature. * Corresponding author. Fax: #81-157-26-4973. E-mail address: [email protected] (M. Kawamura).
0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 3 3 9 - 0
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M. Kawamura et al. / Vacuum 59 (2000) 721}726
In the present work, we have investigated the change on crystal structure, nitrogen concentration and electrical resistivity of Ni}N "lms with nitrogen #ow ratios paying attention to the substrate temperature. The e!ect of the substrate holder potential has been also examined. To investigate the reaction mechanism of this system, surface analysis of the target and plasma emission spectroscopy were also carried out. Based on the results, the formation mechanism and the feature of the present reaction system were discussed.
2. Experimental Films with about 300 nm thickness were prepared by sputtering 4 N purity nickel target in Ar}N gas mixture using an rf diode sputtering system (SPF-210, ANELVA). The system was evacuated below 3.5;10\ Torr and then sputtering gases with various nitrogen #ow ratios (0}100%) were introduced into the system with the total gas #ow controlled at 0.35 ccm. The total gas pressure was mainly "xed at 8 mTorr during the sputtering. Corning C7059 glasses were used as substrates. The substrate temperature was either 2203C or water-cooled (RT). The rf power was 100 W. The substrate holder potential was set to either ground or #oat. The crystal structure of the "lms was investigated by X-ray di!raction (XRD). Electrical resistivity was measured by four-probe method. To determine the nitrogen concentration in the "lms, electron probe microanalysis (EPMA) was carried out using bulk Ni and Si N samples as reference. Plasma emission spectrum was also measured using a multi-channel spectrometer model S2000 (Ocean Optics Inc.).
3. Results and discussion 3.1. Characterization of Ni-N xlms Fig. 1 shows XRD patterns of the "lms deposited at RT by setting the substrate holder at ground potential. By sputtering the target in pure Ar gas, a single phase of Ni (fcc structure) with (111) preferential orientation is obtained. After introduction of nitrogen into the sputtering gas, the peak intensity becomes weak and its position shifts toward lower di!raction angle. This change will be given below in detail. After the pattern becomes almost amorphous in state at 20% N concentra tion of sputtering gas, the new peaks assigned to Ni N (hcp structure) appear at 30% N . Then, a peak which can be assigned to Ni N phase [1] is found at 100% N sputter gas case. We also con"rmed that the Ni N(110) peak intensity became strong when the "lm was prepared under a higher nitrogen pressure (12 mTorr). Di!erent formation phase change was observed on the substrate at 2203C, as shown in Fig. 2. Being dissimilar to the case of Fig. 1, Ni peak shift is not seen after the introduction of nitrogen. At 15% N , small peaks of Ni N phase begin to occur. With increasing N concentration, a coexist ence of metal and nitride phases was observed until a single phase of Ni N is obtained at 60% N . Compared to the result at RT, considerable di!erences are pointed out as follows: (1) di!raction peaks of Ni phase do not shift; (2) Ni N phase appears at lower nitrogen #ow ratio; (3) Ni N phase does not occur.
M. Kawamura et al. / Vacuum 59 (2000) 721}726
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Fig. 1. XRD patterns for Ni}N "lms deposited at RT.
Fig. 2. XRD patterns for Ni}N "lms deposited at 2203C.
When the substrate was electrically #oated, similar phase change was observed at each substrate temperature. But some di!erences were observed at higher nitrogen #ow ratio, compared to the case of ground potential. Concretely, Ni N phase occurred at 20% of nitrogen #ow ratio and its single phase was achieved at 70% at 2203C. And at RT, an amorphous pattern appeared at 30% and the peaks of Ni N phase were seen from 40%. When the substrate holder is #oat potential, the resulting negative bias should be larger than that in the case of ground potential. It can be thought with this consideration that resputtering of deposited materials occurs more strongly in the case of #oat potential, especially on nitrogen adatoms. Consequently, nitrogen atoms remain less in the deposited "lms even when the same nitrogen gas is fed. It leads to a delay of nitride phase formation. Fig. 3 shows the change of Ni lattice constant of the "lms calculated from the interplanar spacing of (111) for the "lm deposited at RT or (200) for that at 2203C. Because of the individual preferred orientation, the lattice constant could not be calculated from the uni"ed plane. At RT, Ni lattice
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Fig. 3. Ni lattice constant of the deposited "lm. The "lms were prepared with various nitrogen #ow ratios at the substrate temperature of RT and 2203C using a substrate holder kept at ground or #oat potential.
expands remarkably with the increase of nitrogen #ow ratio. This can be explained by nitrogen incorporation into the lattice. And the nitrogen content in the "lm deposited at ground potential is expected to be larger than that deposited at the #oat potential. At 2203C, on the other hand, the lattice constant does not increase with the nitrogen #ow, almost no deviation from that of bulk Ni (a "3.524 As ) is seen even at 20% N case. Nitrogen concentration of each "lm was determined by EPMA. As a result, we con"rmed that the "lms deposited at RT contained a certain amount of nitrogen before the occurrence of Ni N phase (6.6 at% and 22 at% for the "lms deposited under 10% and 30% of nitrogen #ow ratio, respectively). However, at 2203C, nitrogen was not detected until the nitride phase was formed. Then the nitrogen concentration in the "lm rose gradually accompanying the increase of nitride amount after the occurrence of nitride. Because Ni N single-phase "lms showed about 28 at% of nitrogen, stoichiometric composition of Ni N was con"rmed. The "lm deposited at RT under 100% of nitrogen #ow ratio had several percent higher concentration than that of the "lms deposited at 2203C. This result is consistent with the XRD data where a higher nitride (Ni N phase) was seen at RT case. Fig. 4 shows electrical resistivity change of the "lms. At 0% of nitrogen #ow ratio, both "lms deposited at two temperatures show very low resistivity (6.8 and 9.0 l) cm for 2203C and RT, respectively), being almost the same as that of bulk Ni (o"6.8 l) cm). It shows that the obtained Ni "lms have an excellent electrical property. After the introduction of nitrogen into the sputtering gas, the resistivity increased with the #ow ratio at the case of the substrate is RT. This can be explained by impurity scattering e!ect because of the nitrogen concentration of "lms. On the other hand, the resistivity did not change in the case of 2203C deposition until the nitride phase was formed. Then it rose gradually and reached about 45 l) cm for Ni N single phase. Though the Ni N single phase is obtained at both temperatures, the resistivity is quite di!erent. This seems to be caused by the di!erence of crystallinity, considering both XRD patterns shown in Figs. 1 and 2. 3.2. Reaction mechanism of this system The results mentioned above clearly show that there is a characteristic di!erence on the formation process, or nitrogen incorporation process of Ni}N "lms, depending on the substrate
M. Kawamura et al. / Vacuum 59 (2000) 721}726
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Fig. 4. Change of electrical resistivity of Ni}N "lms deposited at the substrate temperature of RT and 2203C.
temperatures. This kind of di!erence is not always seen on metal nitride formation process by reactive sputtering method. For example, in the system of nitrogen and Ti, which is a typical active metal, the e!ect of substrate temperature was seen on crystallinity and electrical resistivity but not on formation phases [3]. It is well known that the surface of active metal target easily becomes nitride where the nitrogen pressure is high above the threshold and nitride "lm forms on the substrate [4]. One of the evidences for the target poisoning is the abrupt decrease of the deposition rate. But in the present reaction system, the rate gradually decreased with increasing N concentra tion in sputter gas, and the rate at 100% N became about 60% of that at 0% N . We think the present Ni}N /Ar sputter system is considerably di!erent from other systems including active metal targets. An additional experiment was carried out to investigate the nitrogen concentration of Ni target surface by EPMA for thin "lms for examining whether the nitride layer exists at the target surface or not. Because it is hard to examine the sputtering target itself, Ni plate placed on the sputtering target was investigated. The plate was taken out after sputtering for 80 min, at 100% of nitrogen #ow ratio and 8 mTorr of gas pressure. By the analysis, nitrogen was not detected on the Ni plate sample. Plasma emission spectra in a range from 300 to 850 nm were acquired during the sputtering. The obtained peaks were assigned to argon, nitrogen and nickel according to the Refs. [5,6]. The peaks of nickel atoms are observed in every sputter gas condition from 0 to 100% N . It indicates that Ni atoms come from the target even at the sputtering in pure nitrogen. The spectra did not di!er between two substrate potentials and also at two substrate temperatures. This result may indicate that Ni target surface hardly forms its nitride during the sputtering process compared to other metals. This will be partly explained by the chemical inactiveness of Ni and nitrogen. Metals like as Ti, W, etc. form their nitride easily by chemisorption, however, Ni hardly chemisorbs nitrogen [7]. And this is because the heat of adsorption in the system of Ni}N is exceptionally small (42 kJ/mol) [8]. Fig. 5 shows emission intensity of N> ions (j"391 nm) as a function of N #ow ratio. This perfect linearity is very characteristic compared to the well-known TiN case [9], and this means that there is a very weak gettering e!ect in the present metal/gas combination. Even so, N> ions were incorporated into the deposited Ni "lms as soon as nitrogen was introduced into the chamber at RT of substrate temperature, as already shown in Fig. 1. At 2203C, however, the nitrogen incorporation into the Ni lattice was not seen, but Ni N phase occurs at lower N concentration
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M. Kawamura et al. / Vacuum 59 (2000) 721}726
Fig. 5. Plasma emission intensity of N> ions (j"391 nm) as a function of N #ow ratio.
sputtering gas. The results indicate that the reaction of Ni N lattice formation is promoted by the heating of the substrate.
4. Conclusion The formation process of Ni}N "lms by reactive sputtering was investigated. When the substrate was heated at 2203C, Ni and Ni N phases were observed. But at RT, Ni N phase also appeared and characteristic nitrogen incorporation into Ni lattice was seen. Consequently, substrate temperature a!ected the formation process very much. No evidence for the presence of nitride at target surface was obtained, which is consistent with the fact that Ni is relatively inactive against nitrogen. The results also show that Ni}N reactive sputtering system does not have a target poisoning phenomenon which is commonly observed in other reactive sputtering systems.
Acknowledgements The authors thank T. Noshiroya for technical assistance, Dr. M. Ueda and N. Miyazaki for measurement and fruitful discussion of EPMA.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Dorman GJWR, Sikkens S. Thin Solid Films 1983;105:251. Takamori T, Shih KK, Dove DB, Nywening RW, Re ME. J Appl Phys 1990;68:2192. Kawamura M, Abe Y, Yanagisawa H, Sasaki K. Thin Solid Films 1996;287:115. Kinbara A., Supattaringu-gensyo. Tokyo: Tokyo University Publications, 1991. p. 120 [in Japanese]. Zaidel' AN, Prokof'ev VK, Raiskii SM, Slavnyi VA, Shreider EYa. Tables of spectral lines. New York: IFI/Plenum Press, 1970. Pearse PWB, Gaydon AG. The identi"cation of molecular spectra. London: Chapman & Hall, 1976. Yoshida S., Hakumaku. Tokyo: Baifukan, 1990. 24[in Japanese]. Ehrlich G. Brit J Appl Phys 1964;15:349. Francombe MH, Vossen JL. In: Physics of Thin Films. San Diego: Academic Press Inc, 1989. p. 6}10.
Formation process of Ni}N "lms by reactive sputtering at di!erent substrate temperatures M. Kawamura*, Y. Abe, K. Sasaki Department of Materials Science, Faculty of Engineering, Kitami Institute of Technology, Kitami, Hokkaido 090-8507, Japan
Abstract Ni}N thin "lms are prepared by reactive sputtering in an rf diode sputtering system. Deposition parameters, such as nitrogen #ow ratio, substrate temperature and substrate holder potential were changed and their e!ects were investigated. When the substrate was heated at 2203C, Ni and Ni N phases were observed by XRD. In the case of substrate temperature being room temperature (RT), Ni N phase also appeared and characteristic nitrogen incorporation into Ni lattice was seen. In addition, when the substrate holder was set to the #oat potential, nitrogen content of the "lm which is thought to be lower than the case of ground potential, caused stronger resputtering e!ect. Because it was observed that the formation phase and nitrogen content of the "lm depend on the substrate temperature, the nitrogenizing reaction on the deposit surface was considered to rule the "lm formation. This was also supported by the results on the target surface analysis after sputtering and plasma emission spectroscopy measurements. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Reactive sputtering is one of the most popular methods to prepare metal nitride "lms. Among various metal nitrides, nickel nitride is relatively less-common and its property and formation process have not been reported much. Several results have been published about the Ni}N "lm formation by reactive sputtering method so far. For example, Dorman and Sikkens prepared nickel nitride "lms by dc sputtering and obtained phases which changed from Ni to Ni N, Ni N, and also Ni N with increasing nitrogen partial pressure [1]. Takamori et al. also reported the formation of Ni N phase by rf sputtering, however, Ni N and Ni N were not appeared there [2]. But no studies have ever tried to make clear the e!ect of substrate temperature. * Corresponding author. Fax: #81-157-26-4973. E-mail address: [email protected] (M. Kawamura).
0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 3 3 9 - 0
722
M. Kawamura et al. / Vacuum 59 (2000) 721}726
In the present work, we have investigated the change on crystal structure, nitrogen concentration and electrical resistivity of Ni}N "lms with nitrogen #ow ratios paying attention to the substrate temperature. The e!ect of the substrate holder potential has been also examined. To investigate the reaction mechanism of this system, surface analysis of the target and plasma emission spectroscopy were also carried out. Based on the results, the formation mechanism and the feature of the present reaction system were discussed.
2. Experimental Films with about 300 nm thickness were prepared by sputtering 4 N purity nickel target in Ar}N gas mixture using an rf diode sputtering system (SPF-210, ANELVA). The system was evacuated below 3.5;10\ Torr and then sputtering gases with various nitrogen #ow ratios (0}100%) were introduced into the system with the total gas #ow controlled at 0.35 ccm. The total gas pressure was mainly "xed at 8 mTorr during the sputtering. Corning C7059 glasses were used as substrates. The substrate temperature was either 2203C or water-cooled (RT). The rf power was 100 W. The substrate holder potential was set to either ground or #oat. The crystal structure of the "lms was investigated by X-ray di!raction (XRD). Electrical resistivity was measured by four-probe method. To determine the nitrogen concentration in the "lms, electron probe microanalysis (EPMA) was carried out using bulk Ni and Si N samples as reference. Plasma emission spectrum was also measured using a multi-channel spectrometer model S2000 (Ocean Optics Inc.).
3. Results and discussion 3.1. Characterization of Ni-N xlms Fig. 1 shows XRD patterns of the "lms deposited at RT by setting the substrate holder at ground potential. By sputtering the target in pure Ar gas, a single phase of Ni (fcc structure) with (111) preferential orientation is obtained. After introduction of nitrogen into the sputtering gas, the peak intensity becomes weak and its position shifts toward lower di!raction angle. This change will be given below in detail. After the pattern becomes almost amorphous in state at 20% N concentra tion of sputtering gas, the new peaks assigned to Ni N (hcp structure) appear at 30% N . Then, a peak which can be assigned to Ni N phase [1] is found at 100% N sputter gas case. We also con"rmed that the Ni N(110) peak intensity became strong when the "lm was prepared under a higher nitrogen pressure (12 mTorr). Di!erent formation phase change was observed on the substrate at 2203C, as shown in Fig. 2. Being dissimilar to the case of Fig. 1, Ni peak shift is not seen after the introduction of nitrogen. At 15% N , small peaks of Ni N phase begin to occur. With increasing N concentration, a coexist ence of metal and nitride phases was observed until a single phase of Ni N is obtained at 60% N . Compared to the result at RT, considerable di!erences are pointed out as follows: (1) di!raction peaks of Ni phase do not shift; (2) Ni N phase appears at lower nitrogen #ow ratio; (3) Ni N phase does not occur.
M. Kawamura et al. / Vacuum 59 (2000) 721}726
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Fig. 1. XRD patterns for Ni}N "lms deposited at RT.
Fig. 2. XRD patterns for Ni}N "lms deposited at 2203C.
When the substrate was electrically #oated, similar phase change was observed at each substrate temperature. But some di!erences were observed at higher nitrogen #ow ratio, compared to the case of ground potential. Concretely, Ni N phase occurred at 20% of nitrogen #ow ratio and its single phase was achieved at 70% at 2203C. And at RT, an amorphous pattern appeared at 30% and the peaks of Ni N phase were seen from 40%. When the substrate holder is #oat potential, the resulting negative bias should be larger than that in the case of ground potential. It can be thought with this consideration that resputtering of deposited materials occurs more strongly in the case of #oat potential, especially on nitrogen adatoms. Consequently, nitrogen atoms remain less in the deposited "lms even when the same nitrogen gas is fed. It leads to a delay of nitride phase formation. Fig. 3 shows the change of Ni lattice constant of the "lms calculated from the interplanar spacing of (111) for the "lm deposited at RT or (200) for that at 2203C. Because of the individual preferred orientation, the lattice constant could not be calculated from the uni"ed plane. At RT, Ni lattice
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Fig. 3. Ni lattice constant of the deposited "lm. The "lms were prepared with various nitrogen #ow ratios at the substrate temperature of RT and 2203C using a substrate holder kept at ground or #oat potential.
expands remarkably with the increase of nitrogen #ow ratio. This can be explained by nitrogen incorporation into the lattice. And the nitrogen content in the "lm deposited at ground potential is expected to be larger than that deposited at the #oat potential. At 2203C, on the other hand, the lattice constant does not increase with the nitrogen #ow, almost no deviation from that of bulk Ni (a "3.524 As ) is seen even at 20% N case. Nitrogen concentration of each "lm was determined by EPMA. As a result, we con"rmed that the "lms deposited at RT contained a certain amount of nitrogen before the occurrence of Ni N phase (6.6 at% and 22 at% for the "lms deposited under 10% and 30% of nitrogen #ow ratio, respectively). However, at 2203C, nitrogen was not detected until the nitride phase was formed. Then the nitrogen concentration in the "lm rose gradually accompanying the increase of nitride amount after the occurrence of nitride. Because Ni N single-phase "lms showed about 28 at% of nitrogen, stoichiometric composition of Ni N was con"rmed. The "lm deposited at RT under 100% of nitrogen #ow ratio had several percent higher concentration than that of the "lms deposited at 2203C. This result is consistent with the XRD data where a higher nitride (Ni N phase) was seen at RT case. Fig. 4 shows electrical resistivity change of the "lms. At 0% of nitrogen #ow ratio, both "lms deposited at two temperatures show very low resistivity (6.8 and 9.0 l) cm for 2203C and RT, respectively), being almost the same as that of bulk Ni (o"6.8 l) cm). It shows that the obtained Ni "lms have an excellent electrical property. After the introduction of nitrogen into the sputtering gas, the resistivity increased with the #ow ratio at the case of the substrate is RT. This can be explained by impurity scattering e!ect because of the nitrogen concentration of "lms. On the other hand, the resistivity did not change in the case of 2203C deposition until the nitride phase was formed. Then it rose gradually and reached about 45 l) cm for Ni N single phase. Though the Ni N single phase is obtained at both temperatures, the resistivity is quite di!erent. This seems to be caused by the di!erence of crystallinity, considering both XRD patterns shown in Figs. 1 and 2. 3.2. Reaction mechanism of this system The results mentioned above clearly show that there is a characteristic di!erence on the formation process, or nitrogen incorporation process of Ni}N "lms, depending on the substrate
M. Kawamura et al. / Vacuum 59 (2000) 721}726
725
Fig. 4. Change of electrical resistivity of Ni}N "lms deposited at the substrate temperature of RT and 2203C.
temperatures. This kind of di!erence is not always seen on metal nitride formation process by reactive sputtering method. For example, in the system of nitrogen and Ti, which is a typical active metal, the e!ect of substrate temperature was seen on crystallinity and electrical resistivity but not on formation phases [3]. It is well known that the surface of active metal target easily becomes nitride where the nitrogen pressure is high above the threshold and nitride "lm forms on the substrate [4]. One of the evidences for the target poisoning is the abrupt decrease of the deposition rate. But in the present reaction system, the rate gradually decreased with increasing N concentra tion in sputter gas, and the rate at 100% N became about 60% of that at 0% N . We think the present Ni}N /Ar sputter system is considerably di!erent from other systems including active metal targets. An additional experiment was carried out to investigate the nitrogen concentration of Ni target surface by EPMA for thin "lms for examining whether the nitride layer exists at the target surface or not. Because it is hard to examine the sputtering target itself, Ni plate placed on the sputtering target was investigated. The plate was taken out after sputtering for 80 min, at 100% of nitrogen #ow ratio and 8 mTorr of gas pressure. By the analysis, nitrogen was not detected on the Ni plate sample. Plasma emission spectra in a range from 300 to 850 nm were acquired during the sputtering. The obtained peaks were assigned to argon, nitrogen and nickel according to the Refs. [5,6]. The peaks of nickel atoms are observed in every sputter gas condition from 0 to 100% N . It indicates that Ni atoms come from the target even at the sputtering in pure nitrogen. The spectra did not di!er between two substrate potentials and also at two substrate temperatures. This result may indicate that Ni target surface hardly forms its nitride during the sputtering process compared to other metals. This will be partly explained by the chemical inactiveness of Ni and nitrogen. Metals like as Ti, W, etc. form their nitride easily by chemisorption, however, Ni hardly chemisorbs nitrogen [7]. And this is because the heat of adsorption in the system of Ni}N is exceptionally small (42 kJ/mol) [8]. Fig. 5 shows emission intensity of N> ions (j"391 nm) as a function of N #ow ratio. This perfect linearity is very characteristic compared to the well-known TiN case [9], and this means that there is a very weak gettering e!ect in the present metal/gas combination. Even so, N> ions were incorporated into the deposited Ni "lms as soon as nitrogen was introduced into the chamber at RT of substrate temperature, as already shown in Fig. 1. At 2203C, however, the nitrogen incorporation into the Ni lattice was not seen, but Ni N phase occurs at lower N concentration
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Fig. 5. Plasma emission intensity of N> ions (j"391 nm) as a function of N #ow ratio.
sputtering gas. The results indicate that the reaction of Ni N lattice formation is promoted by the heating of the substrate.
4. Conclusion The formation process of Ni}N "lms by reactive sputtering was investigated. When the substrate was heated at 2203C, Ni and Ni N phases were observed. But at RT, Ni N phase also appeared and characteristic nitrogen incorporation into Ni lattice was seen. Consequently, substrate temperature a!ected the formation process very much. No evidence for the presence of nitride at target surface was obtained, which is consistent with the fact that Ni is relatively inactive against nitrogen. The results also show that Ni}N reactive sputtering system does not have a target poisoning phenomenon which is commonly observed in other reactive sputtering systems.
Acknowledgements The authors thank T. Noshiroya for technical assistance, Dr. M. Ueda and N. Miyazaki for measurement and fruitful discussion of EPMA.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Dorman GJWR, Sikkens S. Thin Solid Films 1983;105:251. Takamori T, Shih KK, Dove DB, Nywening RW, Re ME. J Appl Phys 1990;68:2192. Kawamura M, Abe Y, Yanagisawa H, Sasaki K. Thin Solid Films 1996;287:115. Kinbara A., Supattaringu-gensyo. Tokyo: Tokyo University Publications, 1991. p. 120 [in Japanese]. Zaidel' AN, Prokof'ev VK, Raiskii SM, Slavnyi VA, Shreider EYa. Tables of spectral lines. New York: IFI/Plenum Press, 1970. Pearse PWB, Gaydon AG. The identi"cation of molecular spectra. London: Chapman & Hall, 1976. Yoshida S., Hakumaku. Tokyo: Baifukan, 1990. 24[in Japanese]. Ehrlich G. Brit J Appl Phys 1964;15:349. Francombe MH, Vossen JL. In: Physics of Thin Films. San Diego: Academic Press Inc, 1989. p. 6}10.