Physics. SiO Vapor Pressure in an SiO2 Glass/Si Melt/SiO. Gas Equilibrium System. To cite this article: Xinming Huang et al 1999 Jpn. J. Appl. Phys. 38 L1153.
Jpn. J. Appl. Phys. Vol. 38 (1999) pp. L 1153–L 1155 Part 2, No. 10B, 15 October 1999 c °1999 Publication Board, Japanese Journal of Applied Physics
SiO Vapor Pressure in an SiO2 Glass/Si Melt/SiO Gas Equilibrium System Xinming H UANG, Kazutaka T ERASHIMA1 and Keigo H OSHIKAWA Faculty of Education, Shinshu University, Nishinagano, Nagano 380-8544, Japan Institute of Technology, 1-1-25 Tsujido-nishikaigan, Fujisawa, Kanagawa 251-0046, Japan
1 Shonan
(Received June 24, 1999; accepted for publication August 20, 1999)
SiO vapor pressure concerning Czochralski (CZ) Si crystal growth has been measured successfully by an SiO2 glass/Si melt/SiO gas equilibrium system. A Si sample was sealed in a silica ampoule after being evacuated, and the ampoule was heated to a certain temperature ranging from 1450◦ C to 1540◦ C in an Ar atmosphere. SiO vapor pressure was determined from the balance of pressure between inside and outside the ampoule. The result shows that SiO vapor pressure increases with increasing temperature. It is also found that the SiO vapor pressure shifts slightly from that calculated from thermodynamic data, especially at higher temperatures. KEYWORDS: CZ-Si growth, vapor pressure, SiO gas, Si melt, SiO2 glass
It is well known that most single silicon crystals used for large scale integrated circuit (LSI) fabrication are grown by the Czochralski (CZ) method, in which the most important impurity, oxygen, is incorporated into the silicon crystal. It was reported that almost all the oxygen dissolved from a silica crucible evaporates from the surface of the silicon melt.1) To understand the oxygen transportation process during the CZSi growth to control the quality of the silicon crystal, many investigations have been carried out including an investigation of the oxygen evaporation process occurring at the surface of the silicon melt.2–4) Some overall numerical analyses of oxygen transportation were also carried out in some of them the oxygen evaporation rate was assumed to be finite5) and in others it was assumed to be infinite.6, 7) However, in all of these analyses, the oxygen evaporation process was analyzed with a flux rate of SiO transference. Generally, the evaporation rate of SiO gas (flux rate) cannot be obtained in a practical CZ-Si growth therefore, some data measured using other apparatus were used in the analyses. Unfortunately, the flux rate of SiO evaporation depends on experimental conditions, such as the flow rate of Ar gas near the surface of the Si melt, which is difficult to measure precisely. Therefore, it is easily presumed that there must be much error in such an analysis. Another method for the numerical analysis is to define the evaporation rate as being proportional to the difference between SiO equilibrium vapor pressure and SiO partial pressure near the Si melt in a practical CZ-Si crystal growth system. The latter is a parameter which will be calculated in the numerical analysis, and the former is a material constant which should not depend on experimental apparatus. However, almost no experimental data on the SiO vapor pressure, especially in an SiO2 glass/Si melt/SiO gas equilibrium system concerning CZ-Si crystal growth has been published, except a theoretical result calculated from thermodynamic data was given by Carlberg.8) There are many experimental methods to measure vapor pressures of different materials,9) but none of them can be used directly for the present measurement of SiO vapor pressure because of the high temperature and reactivity of Si melt. An ampoule method is proposed in this work and the measured results will be described. About 1 g of commercial high purity Si was charged in a silica ampoule 50 mm in length, 13 mm in inner diameter and 15 mm in outer diameter. Figure 1 shows the cross section of the ampoule. The ampoule was evacuated to less than 10−6 Torr and the open side was welded with a silica plug. In
Fig. 1. Cross section of the ampoule sample for SiO vapor pressure measurement.
the vertical diameter direction of the ampoule, a 2 mm thickness silica plate was welded together with the wall of the ampoule. The function of the silica plate is to prevent the upper wall of the ampoule from drooping. The inside of the ampoule is thus separated into two regions. Two silicon blocks were charged in the two regions prior to welding. In order to keep the pressure in both regions equal, a hole was opened in the silica plate before the ampoule preparation. A furnace with a high-purity carbon heater was used for the measurement. The carbon heater was 210 mm in diameter and 250 mm in height. It could supply a high temperature of up to 1570◦ C. The heater was externally covered by a carbon heat shield. A Pt-Rh thermocouple was positioned between the heater and the heat shield to control the temperature. Immediately below the ampoule, another Pt-Rh thermocouple was set for measuring the temperature of the sample. All of these were mounted in a water-cooled stainless steel chamber. The whole measurement process was carried out under an Ar atmosphere with an Ar flow rate of 5 l/min. The Ar pressure was controlled to set values throughout the process. The
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accuracy of the pressure measurement was estimated to be ±1 Torr. Temperature distribution around the ampoule sample was approximately ±1◦ C and the temperature shift during the measurement was within ±1◦ C. Therefore, the accuracy of the temperature measurement was estimated to be ±2◦ C. The sample was heated at a rate of 15◦ C/min and the temperature was maintained for more than 20 h. The shape of the ampoule was observed from a view window at the top surface of the water-cooled stainless steel chamber during the heat treatment. In order to determine the equilibrium condition of the pressures between inside and outside of the ampoule, the Ar pressure in the chamber was adjusted according to the deformation of the ampoule. The pressure was maintained for more than 5 h after each adjustment. In particular, the deformation of the ampoule progressed very slowly near the equilibrium condition, and each pressure was maintained for more than 10 h to ensure measurement accuracy. On the other hand, the silica ampoule is denitrified after being maintained at high temperatures for a long time and silica glass becomes much harder if crystallization occurs. In this case, the ampoule must be changed. In other words, the sample must be changed for a new one after about 20 h of measurement, even though an equilibrium condition of pressure has not been found at one desired temperature. Figure 2 shows pictures of the ampoule: (a) an ampoule sample before heat treatment; (b) under balance condition of pressures between inside and outside the ampoule; (c) outside pressure higher than that inside; (d) inside pressure higher than that outside. The vapor pressure of SiO was determined when that the pressure inside the ampoule was balanced with the pressure outside the ampoule, and the pressures inside the ampoule and outside the ampoule were considered the same. This assumption regarding the balance condition would not result in an experimental error greater than 2% by considering the present sample assembly and the surface tension of the silica glass to
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be approximately 300 dyne/cm.10) SiO vapor pressures measured at various temperatures are shown in Fig. 3. A theoretical result calculated by Carlberg8) is also shown for comparison. As shown in Fig. 3, the SiO vapor pressure increased with increasing temperature, which was the same tendency as that of the result reported by Carlberg. The accuracy of the measurement also depended on the temperature. The error evidently decreased with increasing temperature. It was almost impossible to measure the vapor pressure near the melting point of silicon. There was almost no deformation of the ampoule sample when the temperature
Fig. 3. SiO vapor pressures at different temperatures.
Fig. 2. Typical pictures of the ampoule sample before measurement and during measurement (at 1500◦ C). (a) Before heat treatment; (b) under equilibrium condition of pressures between inside and outside the ampoule (outside pressure: 23 Torr); (c) outside pressure higher than that inside (outside pressure: 30 Torr); (d) inside pressure higher than the outside (outside pressure: 15 Torr).
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was lower than 1450◦ C, although the ambient Ar pressure was changed by more than 10 Torr and the conditions were maintained for more than 20 h. The experimental error was too large to obtain a reliable result. The problem was probably due to the hardening of the silica glass. It is well known that silica glass becomes much harder when temperature is decreased. There are some error bars in the measured data in Fig. 3, which were determined by considering that it was extremely difficult to identify the deformation of the ampoule in the pressure range within the error bar. It is noted that the measured result shifts slightly from the theoretical result, especially in the higher temperature region. The reason can be explained as follows. An interfacial phase will form between the silica glass and silicon melt after reaction at high temperatures, which has been reported by many researchers.11–14) The presence of the interfacial phase decreases oxygen solulibity in the silicon melt.11, 13) It can be assumed that the decrease of oxygen concentration (solubility) in the silicon melt due to the interfacial phase caused the SiO pressure to decrease in the present experimental system. On the other hand, the theoretical result on SiO vapor pressure was calculated using thermodynamic data related to the reaction between SiO2 and the silicon melt, and the presence of the interfacial phase was not taken into account.8) Therefore, it is easy to understand the disparity between the two sets of results. The main results of this study are summarized as follows. An ampoule method has been used for the first time to measure the SiO vapor pressure in an SiO2 glass/Si melt/SiO gas equilibrium system with regard to Czochralski (CZ) Si crystal growth. The result shows that SiO vapor pressure increases with increasing temperature. It is also found that the SiO vapor pressure shifted slightly from that calculated from ther-
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modynamic data, especially at higher temperatures. The reason for the shift is explained by considering that an interfacial phase forms at the interface of the SiO2 glass/Si melt. Acknowledgments This work was supported by the JSPS Research for the Future Program in the Area of Atomic Scale Surface and Interface Dynamics under the Project of “Dynamic Behavior of Silicon Atoms, Lattice Defects and Impurities near Silicon Melt-Crystal Interface”. We also wish to acknowledge Professor N. Imaishi of Kyushyu University for discussions.
1) K. Hoshikawa, H. Hirata, H. Nakanishi and K. Ikuta: Semiconductor Silicon, eds. H. R. Huff, R. J. Kriegler and Y. Takeshi (The Electrochem. Soc., Pennington, 1981) p. 101. 2) X. Huang, K. Terashima, H. Sasaki, E. Tokizaki, Y. Anzai and S. Kimura: Jpn. J. Appl. Phys. 33 (1994) 1717. 3) X. Huang, K. Terashima, E. Tokizaki, S. Kimura and E. Whitby: Jpn. J. Appl. Phys. 33 (1994) 3808. 4) X. Huang, K. Terashima, K. Izunome and S. Kimura: Jpn. J. Appl. Phys. 33 (1994) L902. 5) S. Togawa, X. Huang, K. Izunome, K. Terashima and S. Kimura: J. Cryst. Growth 148 (1995) 70. 6) K. Kakimoto, K. W. Yi and M. Eguchi: J. Cryst. Growth 165 (1996) 238. 7) K. W. Yi, K. Kakimoto, M. Eguchi and H. Noguchi: J. Cryst. Growth 163 (1996) 358. 8) T. Carlberg: J. Electrochem. Soc. 133 (1986) 1940. 9) O. Kubaschewski and C. B. Alcock: Metallurgical Thermochemistry (Pergamon Press, Oxford, 1979). 10) W. D. Kingery: J. Am. Ceram. Soc. 42 (1959) 6. 11) U. Ekhult and T. Carlberg: J. Electrochem. Soc. 139 (1989) 551. 12) U. Ekhult and T. Carlberg: J. Electrochem. Soc. 139 (1989) 3809. 13) X. Huang, K. Terashima, H. Sasaki, E. Tokizaki and S. Kimura: Jpn. J. Appl. Phys. 32 (1993) 3671. 14) X. Huang, K. Terashima, Y. Anzai, E. Tokizaki, H. Sasaki and S. Kimura: Appl. Phys. Lett. 64 (1994) 2261.
