Development of a Sessile Drop Method Concerning ... - IOPscience

Jpn. J. Appl. Phys. Vol. 38 (1999) pp. 1847–1851 Part 1, No. 4A, April 1999 c °1999 Publication Board, Japanese Journal of Applied Physics

Development of a Sessile Drop Method Concerning Czochralski Si Crystal Growth Susumu S AKAI ∗ , Xinming H UANG, Yasunori O KANO1 and Keigo H OSHIKAWA Faculty of Education, Shinshu University, Nishinagano, Nagano 380-8544, Japan of Materials Science and Chemical Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8561, Japan

1 Department

(Received December 7, 1998; accepted for publication January 8, 1999)

A sessile drop method for the measurement of the oxygen dissolution rate from silica glass to silicon melt proposed previously has been further developed. The main error in the measurement was the weight loss of the silica plate itself because of the reaction between the silica plate and the carbon crucible. A pyrolytic boron nitride (PBN) plate was placed between the silica plate and carbon crucible, and results showed that the error was reduced effectively using the protective PBN plate. As an application of the sessile drop method, the effect of OH content in different silica materials on the oxygen dissolution rate was also investigated. There was no evident difference in the dissolution rate from the different kinds of silica materials with different OH concentrations. KEYWORDS: Czochralski silicon crystal growth, silicon melt, silica glass, oxygen dissolution rate, sessile drop, oxygen transportation

1. Introduction The Czochralski (CZ) method has been widely used for growing silicon single crystals for substrates for large-scale integrated circuits (LSI). As a silica crucible is generally used that gradually dissolves in the silicon melt during the growth process, some oxygen is incorporated into the silicon crystal. It is well known that the dissolved oxygen atoms in the crystal increase the mechanical strength of silicon wafers,1) and oxygen precipitates in the bulk crystal perform site for gettering metal impurities.2) On the other hand, high density of oxygen precipitates causes degradation of the silicon wafer. Therefore, it is very important to control the process of oxygen transportation during CZ silicon crystal growth. Hoshikawa et al.3) reported that four processes of oxygen transportation are recognized in CZ silicon crystal growth: (1) oxygen dissolution from silica glass to silicon melt, (2) transportation in the melt, (3) evaporation from the surface of the melt, and (4) incorporation from the melt into the crystal. All oxygen atoms are transported through the silicon melt, only a small amount are incorporated into the silicon crystal, and most of them evaporate from the surface of the melt. In order to understand the mechanism of oxygen transportation in CZ silicon crystal growth, it is necessary to investigate every oxygen transportation process quantitatively. Many investigators have measured the oxygen dissolution rate from silica glass to silicon melt.4–8) Huang et al. proposed a new sessile drop method for measuring the oxygen dissolution rate.8) Compared to the oxygen dissolution rate obtained using the conventional measurement method,6) a much larger value has been obtained using the sessile drop method. In their study, the experimental accuracy of the sessile drop method was also analyzed, and the main error in the measurement of the oxygen dissolution rate was the weight loss of the silica plate itself. However, the reason for the weight loss of the silica plate was unclear, although an air leak was suggested. In this paper, the sessile drop method proposed previously has been further developed. The factors by which the experimental accuracy is affected have been analyzed in detail and solution is proposed. The effect of drop size, Ar pressure and ∗ Permanent

address: Department of Materials Science and Chemical Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8561, Japan.

temperature on the dissolution rate has also been investigated. Furthermore, as an application of the sessile drop method, the effect of OH content in different kinds of silica materials on the oxygen dissolution rate was investigated. 2. Experimental Figure 1 shows a schematic diagram of the assembly of the sessile drop sample. A commercial high purity silicon block with a weight ranging from 20 to 170 mg was placed on a silica plate 6 × 6 × 1 (mm3 ) or 10 × 10 × 6 (mm3 ) in size. A sessile drop was obtained after melting the silicon. A furnace with a carbon heater was used for melting the silicon. The heater was 70 mm in diameter and 130 mm in height and produced a maximum temperature of approximately 1500◦ C. The heater was externally covered by a carbon felt heat shield. A Pt-Rh B-type thermocouple was incorporated between the heater and the heat shield to control the temperature. A carbon crucible capping a carbon plate, which had a 20-mmdiameter hole for observation, was placed in the center of the heater. The carbon cap allowed uniform distribution of heat to the sample and protected the sample from the influence of gas convection. The carbon crucible was 55 mm in diameter, 54 mm in height and 4 mm thick. The silicon sample was contained in the carbon crucible. Immediately below the carbon crucible was another B-type thermocouple for measuring the temperature of the sample. All of these were mounted in a water-cooled stainless steel chamber in which Ar gas flowed at a rate of approximately 2 `/min to protect against oxidation. The purity of the Ar gas was 99.9995%. The Ar pressure in the chamber could be controlled from 10−2 Torr to 7600 Torr using a pressure controller. The accuracy of the pressure measurement was estimated to be ±3%. In order to investigate the effect of the reaction between the silica plate and the carbon crucible on the oxygen dissolution rate, (i) no protective plate, (ii) a SiC plate and (iii) a PBN plate were set between the silica plate and the carbon crucible. The furnace was heated to a temperature in the range of 1430 to 1500◦ C at a heating rate of 10◦ C/min. The thermocouple for measuring the temperature of the sample was recalibrated by measuring the melting point of silicon during the melting process.8) The temperature was increased to a desired value and held for 300 min, then the heating power was switched off to quench the silicon sample. The accuracy in the temperature measurement was estimated to be ±2◦ C.

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Fig. 1. A schematic diagram of the assembly of the sessile drop sample.

Weight variation with time was obtained by measuring the weight of the sample before and after the melting experiment. The oxygen dissolution rate in the sample was calculated from the weight variation. The weight of the sample was measured using an electronic balance with an accuracy of 0.1 mg manufactured by the A&D Company. Because the shape of the silicon drop changed after solidification, it was difficult to measure the area of the contact interface between the silicon melt and the silica plate; thus the area was calculated using Laplace’s equation with a suitable surface tension value.9) 3. Results and Discussion 3.1 Accuracy of measurement The oxygen dissolution rate was calculated from the weight change of the sample by considering that the weight change was due to evaporation of SiO, and the oxygen content in the SiO was derived from the dissolution of SiO2 . The oxygen dissolution rate is defined as the number of oxygen atoms passing through a unit contact area between the silicon melt and the silica plate per unit time. The main error in the measurement of the oxygen dissolution rate was the weight loss of the silica plate itself. It was about ±5% in the case of a 100 mg drop sample at 1480◦ C even after the following correction.8) A blank silica plate (no silicon melt on it) of the same size was measured simultaneously as a reference. The net weight change that resulted from the reaction between the silicon melt and the silica glass was determined to be the difference in weight between the drop sample and the blank silica plate. The error in measuring the weight loss of the blank silica plate remained in the final result of the oxygen dissolution rate. It was found that the error increased with increasing temperature, decreasing pressure and decreasing size of the drop sample. For example, the error was approximately ±10% for a 50 mg drop sample at 1500◦ C under 5 Torr in this work. The main reason for the weight loss of the blank silica plate was thought to be the reaction between the silica plate and the carbon crucible after other factors such as leakage of the chamber were suppressed. If the weight loss of the silica plate is suppressed sufficiently and if it is much less than the weight change of the drop sample, the error in the measurement of the weight loss of the blank silica plate would not significantly affect the final accuracy of measuring the oxygen dissolution rate. In order to investigate the effect of the reaction between the silica

Fig. 2. Weight change rate of the blank silica plates treated at 1450◦ C for 300 min.

plate and the carbon crucible on the measurement of the oxygen dissolution rate, a SiC plate or a PBN plate was placed between the silica plate and the carbon crucible as shown in Fig. 1. Figure 2 shows the weight change rates (weight loss per unit surface area) of the blank silica plates treated at 1450◦ C for 300 min. The weight loss was reduced considerably using the SiC plate or the PBN plate as a protective plate, and the best result was obtained using the PBN plate. The error in the measurement of oxygen dissolution rate for a 50 mg drop sample at 1450◦ C and 5 Torr was only about ±2% using the PBN plate. The accuracy of the measurement can evidently be increased by suppressing the weight loss of the blank silica plate and using a protective PBN plate. The weight change of the silica plate in this experiment was considered to be mainly from the deoxidization of the silica plate by the reaction shown in eq. (1):10) C + SiO2 = SiO + CO.

(1)

The PBN plate was efficient because it prevented the reaction. There was, however, slight weight loss from the silica plate even if the PBN plate was used. The reasons were as follows. (a) Air still leaked from the ambient atmosphere into the chamber, and CO gas was generated by the reaction between the oxygen in the air and the carbon heater and shield in the furnace. The reaction between the CO gas and the silica plate (SiO2 ) resulted in the weight change. (b) The Ar gas contained some oxygen and CO gas was generated by the reaction between the oxygen and the carbon in the furnace. (c) The silica plate sublimated at high temperature and low pressure. In this experiment, the leak rate in the chamber was 2.9 × 10−5 Pa·m3 /s and the oxygen content in the Ar gas was less than 0.2 ppm. If all the oxygen atoms from scenarios (a) and (b) reacted with the carbon in the furnace, generated CO gas, and all the CO gas continued to react with SiO2 , it can be estimated that the weight loss of the silica plate would be about 5.6 mg, which is more than the whole weight loss of the blank silica plate. The difference between the estimated and the practical weight loss can be explained by considering that the CO gas and the SiO2 did not completely react. It can be expected that sublimation of the silica plate would not be the main reason for the weight change of the blank silica plate

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by considering the above estimation. The units of the ordinate in Fig. 2 are mg/cm2 . As discussed above, the weight loss of the blank silica plate was due to reactions occurring at the surface of the silica plate. Thus the weight change rate defined as the weight loss per unit surface area was used in the rectification for measuring the oxygen dissolution rate. Although the weight loss of the silica plate could not be offset completely, a rectification in the measurement of the oxygen dissolution rate was successfully carried out using the protective PBN plate. 3.2 Influence of drop size The effect of the contact area between the silica plate and the silicon melt on the oxygen dissolution rate is shown in Fig. 3. The contact area was changed by increasing or decreasing the drop size. The smaller the drop size, the larger the oxygen dissolution rate. The result implied that the oxygen transportation process in the silicon melt was facilitated due to a decrease in the drop size. However, under these experimental conditions, the effect of oxygen transportation in the silicon melt on the dissolution process could not be reduced completely because the dissolution rate still showed a tendency to increase in spite of a considerable decrease in the drop size as shown in Fig. 3. 3.3 Influence of temperature and Ar pressure The dependence of the oxygen dissolution rate on the Ar pressure for some 50 mg drop samples at different temperatures is shown in Fig. 4. It was clear that the oxygen dissolution rate was larger at a higher temperature and under a lower Ar pressure. The oxygen dissolution rate should be independent of the Ar pressure if an intrinsic oxygen dissolution rate was obtained. However, the results in Fig. 4 show that there was still some dependence of the oxygen dissolution rate on pressure. It was also noted that the temperature coefficient of the oxygen dissolution rate increased with decreasing of Ar pressure. It is possible that the value of the oxygen dissolution rate obtained here was affected by evaporation from the surface of the silicon melt. At the surface of the silicon melt, the Chapman-Enskog ex-

Fig. 3. Effect of the contact area between the silica plate and the silicon melt on the oxygen dissolution rate of 50 mg drop samples at 20 Torr at different temperatures.

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pression indicates that the diffusion coefficient, D, of the SiO is proportional to the term of T 1.5 /P as shown in eq. (2).11) The diffusion coefficient, D, can be regarded as the efficiency of the oxygen transportation at the surface. DAB =

0.0026T 1.5 0.5 2 P MAB σAB ÄD

(2)

where DAB is the diffusion coefficient of SiO in Ar gas, T the temperature, P the pressure, MAB the effective mass, σAB the collision diameter, and ÄD the collision integral. From eq. (2), the diffusion coefficient increases when temperature increases and/or pressure decreases. If evaporation from the surface of the melt affects the dissolution process directly, the temperature and pressure dependence of the dissolution rate in Fig. 4 can be explained by eq. (2). In the same way, the different temperature coefficients of the oxygen dissolution rate obtained under different Ar pressures can be explained, because the term ∂ D/∂ T is proportional to the term T 0.5 /P according to eq. (2). Based on the results shown in Fig. 3 and Fig. 4, oxygen dissolution in the sessile drop samples under these experimental conditions was still affected by evaporation at the surface of the silicon melt and the transportation process in the silicon melt, and an exact intrinsic oxygen dissolution rate has not yet been obtained. 3.4 Comparison with literature Figure 5 shows some typical results for the oxygen dissolution rate obtained from some 100 mg sessile drop samples under 20 Torr. Some data are also plotted for comparison.4–7, 12) The experiments for measuring the oxygen dissolution rate reported by Chaney and Varker,4, 5) Hirata and Hoshikawa6) and Abe et al.7) are summarized as follows. Some silica rods or plates were immersed in molten silicon held in a silica crucible, which was generally used in CZ silicon single-crystal growth. The sample were immersed for a certain time, and the silica rods or plates were pulled out of the melt. The oxygen dissolution rates were obtained by measuring the decrease in diameter of the silica rods and the weight change of the silica plates before and after the experiments. This kind of measurement is called the conventional measurement method. On the other hand, Huang et al.12) used a small silica crucible 10 mm in diameter and 10 mm in height and melted 1 g

Fig. 4. Dependence of the oxygen dissolution rate on Ar pressure for 50 mg drop samples at different temperatures.

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Fig. 6. Effect of OH content in different silica materials on the oxygen dissolution rate obtained using 50 mg drop samples at 20 Torr of Ar gas. Fig. 5. Some typical results for the oxygen dissolution rate obtained from 100 mg sessile drop samples under 20 Torr. Some data from ref. 4,5, and 6 at 10 Torr, ref. 7 at 760 Torr and ref. 12 at 20 Torr are also presented for comparison.

of silicon in the crucible. The sample was kept at the desired temperature for 300 min and quenched. The oxygen dissolution rate was calculated from the weight change of the sample before and after the melting experiment. This measurement is referred to as a small crucible measurement. Using the small crucible measurement, it was found that the oxygen dissolution rate was about 2 times larger than that obtained from the conventional measurement method. The effect of oxygen transportation in the silicon melt was reduced by decreasing the volume of the melt using the small crucible. Furthermore, using the sessile drop method, the oxygen dissolution rate was about 3 times larger than that obtained from the small crucible measurement. From these results, two benefits may be inferred for the sessile drop method. One is that oxygen transportation in the silicon melt became much faster by decreasing the volume of the melt. Another is that the surface area of the silicon melt became much larger than the area of the contact interface between the silica plate and the silicon melt, thereby evaporation is enhanced. An approximate dissolution rate limited process can be obtained using the sessile drop method compared to the other measurement methods. The sessile drop method is probably the best one to measure the oxygen dissolution rate. Incidentally, the oxygen transportation phenomena in the silicon drop is similar to that at the triple junction (ambient gas, silicon melt and silica crucible) in CZ silicon crystal growth, where the oxygen dissolution rate is also very fast. 3.5 Influence of silica materials with different OH concentrations A silica crucible is used to hold the silicon melt, and it dissolves considerably into the silicon melt during CZ silicon crystal growth. Many kinds of silica materials may be used for producing the crucible, and especially the impurities in the various silica materials are different. It is, therefore, very important to investigate the effect of the different kinds of silica materials on oxygen dissolution. Ikari et al.13) investigated the formation of the interfacial phase between silica glass and silicon melt and focused their

attention on the effect of OH content in the silica glass. It was reported that the surface density of the interfacial phase increased with increasing OH concentration in the silica glass. The formation of the interfacial phase is one of the most critical problems in growing large and long ingots, because the interfacial phase grows during crystal growth and may release silica particles into the silicon melt. This is suspected to be one of the causes of polycrystallization. On the other hand, it has been reported that the surface density of the interfacial phase depended on the oxygen dissolution rate.12) The faster the oxygen dissolution rate, the less the surface density of the interfacial phase, and the interfacial phase finally disappeared when the dissolution rate was sufficiently fast. It is, therefore, necessary to investigate the effect of OH content of silica materials on the oxygen dissolution rate from silica glass to silicon melt. Figure 6 shows the effect of OH content in different kinds of silica plates on the oxygen dissolution rate obtained using 50 mg drop samples. The oxygen dissolution rate was almost independent of the OH concentration in the silica materials. The result is explained as follows. The OH concentration in the silica plates used in this experiment was less than 1000 ppm. If SiO2 molecules including the OH group are dissolving, it is necessary to dissolve about 280 molecules of SiO2 to dissolve one OH group. The value of 280 is obtained by considering that the molecular weight of SiO2 is 60 and that of OH is 17. In other words, the effect of the OH group on the oxygen dissolution rate would be only about 0.36%. Even if the OH group enhances the dissolution rate of its neighboring molecules of SiO2 and the number is estimated to be 5, the effect of each OH group would not exceed 2%. It can then be understood why there is no difference in the dissolution rate for the different kinds of silica materials with different OH concentrations under these experimental conditions. In practical CZ silicon crystal growth, there is a large amount of silicon melt in the silica crucible, and the oxygen transportation in the silicon melt is the governing process for the whole oxygen transportation and the dissolution process is not. It can be inferred that the different kinds of silica materials with different OH concentrations do not affect the oxygen dissolution rate in practical CZ silicon crystal growth.

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4. Conclusions A sessile drop method for the measurement of the oxygen dissolution rate from silica glass to silicon melt proposed previously has been further developed. The error in the measurement of the oxygen dissolution rate with the sessile drop method is due to the weight loss of the silica plate itself. The main reason for the weight loss is the reaction between the silica plate and the carbon crucible after suppressing air leaks. The reaction may be prevented by placing a PBN plate between the silica plate and the carbon crucible, whereby the error is reduced effectively. An intrinsic oxygen dissolution rate has not yet been obtained under these experimental conditions because oxygen transportation in the silicon melt and evaporation from the surface of the melt can not be completely ignored. However, using the sessile drop method, a much larger value of the oxygen dissolution rate has been obtained compared to that obtained by the conventional measurement method or the small crucible measurement. It can be concluded, therefore, that an approximate dissolution rate limited process has been obtained using the sessile drop method. The sessile drop method is a proper one of all measurement methods reported to date to measure the oxygen dissolution rate and its dependence on temperature, impurities and other factors. As an application of the sessile drop method, the effect of OH content in different kinds of silica materials on the oxygen dissolution rate was also investigated. There was no evidence for differences in the dissolution rate from different silica materials with different OH concentrations ranging from 0 to 800 ppm. It can be concluded, therefore, that the different

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values of OH concentration in silica crucibles do not affect the oxygen dissolution rate in practical CZ silicon crystal growth. Acknowledgments This work was supported by 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 MeltCrystal Interface”. We also wish to acknowledge H.Watanabe for supplying the silica materials.

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