The effect of carbon on oxygen precipitation in high carbon CZ silicon crystals

Mat. Res. B u l l . , Vol. 18, pp. 1437-1441, 1983. Printed in the USA. 0025-5408/83 $3.00 + .00 Copyright (e) 1983 Pergamon P r e s s Ltd.

THE EFFECTOF CARBONON OXYGENPRECIPITATION IN HIGH CARBONCZ SILICON CRYSTALS C.Y. Kung, L. Forbes*, and J.D. Peng Fairchild Camera and Instrument Corporation Silicon Materials Division Healdsburg, CA 95448 *Presently at Department of Electrical Engineering and Computer Science, Oregon State University Corvallis, OR 97331

(Received March 21, 1983; Refereed)

ABSTRACT TO c l a r i f y the role of carbon impurities in the formation of oxygen precipitates, the behavior of CZ silicon wafers with varying carbon concentrations from 0.2 ppm to 2 ppm were studied under different heat treatment conditions. I t is found that the rate of reduction in i n t e r s t i t i a l oxygen is not a function of carbon concentration as reported previously for both the cases of me~i~umand low temperature annealing. Under medium temperature (I050 C) annealing no carbon reduction was detected, even though the reduction in i n t e r s t i t i a l oxygen can be very large, while with low temperature (750°C) annealing, oxygen reduction is always associated with the carbon reduction, but is not dependent on carbon concentration ~lone. A heterogenous precipitation model is presented to explain the observed phenomena. INTRODUCTION I t is generally believed that heterogenous nucleation mechanisms are domina~in the formation of oxygen precipitates for both medium temperature and low temperature annealing. Attention has recently been given to the effect of carbon on the nucleation of these precipitates. Recent experiments on oxygen precipitation show rather clearly that high carbon concentrations can be very helpful in the nucleation process (]-3). However, some other groups asserted that the correlation between carbon impurities and oxygen precipitation was very weak when the Bnnealing temperature was about lO00°C (4-5). In this research, carbon atom was found to play a more complex role in oxygen precipitation than has previously been reported. A model is presented to explain the observed phenomena of carbon drop in the case of low temperature annealing. 1437

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EXPERIMENTAL PROCEDURE The sample used in this study were boron doped, P-type, 76mmdiameter 500~m thickness Czochralski (CZ) Si wafers of ( I l l ) orientation, dislocation free and not backdamaged, with a resistivity of 1.5 - 3.0 ohm-cm. Wafers were taken from four different sections of the ingot, A,B,C, and D, with oxygen concentrations varying from 34 ppm (A section,seed portion) to 29 ppm (D section, tail portion) and carbon concentration varying from O.2ppm (A section) to 2.2 ppm ( D section). The samples were isothermally annealed in a flowing N2 environment at 750oc for X hours (X being from 0 to lO0 h), and isothermally at 1050oc for Y hours (Y being from 0 to lO0 h) and in the low to medium two-step annealing procedure at 750°C for 8 h and at 1050°C for Y hours (Y being from 0 to 40 h). The oxygen and carbon concentrations of as received and the annealed wafers were determined using a Nicolet MXI Fourier transform infrared spectrometer, which is based on the 1979 ASTMprocedure or standards. To compensate for the resolution limit of the FTIR instrument and the variation of the measured position on each sample tested, the determinations of the reduction of carbon or oxygen concentrations have been done carefully and double checked. For the wafers from the seed portion (with C~O.3ppm), the carbon concentration always dropped to less than 0.05 ppm, when a drop occurred. For the higher carbon concentration wafers (tail portion), the determination of the occurrences of carbon drops depends on either the carbon concentration being reduced to one-half of its original value or dropping by an amount larger than 0.5 ppm, the carbon concentration never came back after later annealing stages. A series of tests have been repeated for a set of (IO0) wafers (6), and the results are the same. RESULTS AND DISCUSSION Low and Medium Temperature Isothermal Anneals Fig. 1 shows the residual oxygen r a t i o vs. anneal time f o r both medium and low temperature isothermal anneals for wafers taken from four d i f f e r e n t portions of the ingot. In the case of low temperature anneals, a l l observed oxygen reductions were accompanied by a reduction in the carbon concentration, while with medium temperature anneals no carbon drop was detected, even though the oxygen concentration may have been reduced to a very low value. The oxygen reduction r a t i o versus carbon concentration for both medium and low temperature isothermal anneals are shown in Fig. 2. The oxygen reduction ratio decreases as carbon concentration increases. This assessment is different from previous results which indicated that at low temperature oxygen reductions are increased as carbon concentration increases (3). Again, Fig. 2 shows that a reduction of carbon occurs only with low temperature anneals and is independent of the i n i t i a l carbon concentration. In view of the low temperature anneal results shown in Fig. 1 one can conclude that carbon atoms have played a direct role in the formation of oxygen precipitates, since a reduction in the carbon concentration has been found to be associated with every reduction in the oxygen concentration with low temperature anneals. I t is evident that carbon atoms associated heterogeneous nucleation dominates the type of precipitation process at low temperature and that carbon atoms have played an active role. However the reduction of oxygen is not proportional to the carbon concentrations which indicates that carbon atoms did not act alone in the heterogeneous oxygen precipitation process with low temperature annealing. For the medium temperature anneals, our results show that the seed portion has a much faster reduction rate than the tail portions. However, no carbon reduction has been found in the case of medium temperature anneals.

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Two-Step Anneals. The residual oxygen ratio for two-step anneals is also shown In Fig. i", the band of scattered data(from seed portion to tail portion) is much narrower in comparison to the Band for isothermal anneals. Differences in the oxygen drops between the seed and tail portion seem to be reduced by the low temperature ( 750°C ) pre-anneals. Carbon concentrations were also found to be reduced by the low to medium two-step anneals. The differences in oxygen precipitation behavior between two-step anneals and medium temperature isothermal anneals seem to depend on the role of carbon atoms. A study of the FTIR spectra of the foxygen precipitates has also shown this. (7) For the 750°C pre-annealed wafers, oxygen concentrations dropped much faster than with 1050oc isothermal heat treatments. Fig. l shows that for the tail portion, the reduction of oxygen for the wafers under 750oc/ 8h + 1050°C/ Xh is even faster than that for wafers with 1050°C/ (8+X)h. Carbon concentration drops were found on the pre-annealed ( 750°C 8h) wafers, but not on the wafers which were heat treated at 1050°C only. For medium to low two-step annealed wafers, no carbon concentration drops were found. Figures 3 (a) and 3~b) show the subtractive FTIR spectra of a B section wafer annealed at 1050uc 40h and of a two-step annealed 750oc 8h + 1050°C 40h wafer respectively. The vertical axis is the relative absorbance in Log scale. The amount of oxygen reduction was about the same for both wafers. However the isothermally anoealed wafer clearly shows an additional absorption in the area around 1230 cm- l . A very clear difference is seen by subtracting spectra (a) from spectra (b) as shown in Fig. 3(c). ApparentlX, spectra 3(a) has a larger absorption coefficient at 1230 cm-' and I125 cm- i which are a t t r i buted to the SiO2 precipitates while spectra 3(b) shows a larger absorption in the I030 to llO0 cm-z range which is generated by SiOx ( x < 2) (8). The I

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differences on FTIR Spectra indicate that the percentage combination of SiO2 and SiOx ( x<2 ) precipitate in these two wafers are different (8). I t is suggested that large numbers of carbon atoms and Si i n t e r s t i t t a l s have been involved in the SiOx precipitation with low to medium two-step annealing. Figures 4(a) and 4(b) show Wright solution etched bulk defects in the specimens, generated by isothermal 1050oc annealing and low to medium two-step annealing, defect features and densities are also different although the oxygen reductions are the same. The difference of carbon reduction behavior between low temperature ( low to medium two-step) anneal and medium temperature ( medium to low two-step ) anneal also indicates that the precipitation kinetics between low temperature and medium temperature are different. The role of carbon atoms in oxygen precipitation at low temperature annealing need further studies. Here, a simple model is suggested to explain the carbon reduction behavior at low temperature annealing. I t is very possible taht vacancies and s e l f - i n t e r s t i t i a l s coexisted in thermal e~uilibrium ( l o c a l l y ) in Si at hlgh temperature (9, lO ). During crystal cooling , the carbon atoms diffused into vacancy sites and formed a relatively stable Si i n t e r s t i t i a l s and carbon atoms complexes. ( Carbon atoms are at substitutional sites). After longer 750°C anneals, such complexes absorbed oxygen atoms and grow to a larger precipitates, causing the carbon atoms to lose their identity as the substitutional impurity and become undetectable by the FTIR spectrometer; a reduction of carbon is therefore noticed. However, at higher anneal. ing temperature ( 1050°C ), such silicon-carbon complexes are dissolved and no carbon reduction is noticed during the subsequent 750oc annealing.

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Fig. 4 Bulk defect features induced after 8 hours at 105OOCwith and (b) 8 h pre-annealing at 750°C.

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CONCLUSIONS In presenting our conclusions, some important phenomena which are related to the behavior of carbon are summarized as follows: I. With low temperature anneals, oxygen reduction has always been accompanied by carbon reduction. 2. Isothermal medium temperature anneals and medium to low two-step anneals showed no carbon reduction, no matter how much the oxygen concentration had been reduced. 3. For a given high carbon ingot the oxygen reduction rate does not show any correlation to the i n i t i a l carbonconcentration. Seed portion wafers have always shown a high reduction rate, where as center .portion and near t a l l portion wafers do not. Our results suggest that there are two d i s t i n c t l y different mechanisms of oxygen precipitation involved in the 1050oC anneals and in the 7SOOC-I050oc anneals, with a carbon associated heterogeneous process in the case of the l a t t e r two-step low to medium temperature anneals. REFERENCES l . R. F. Pinizzotto and H. F. Schaake in: Defects tn SemiConductor, J. Narayan and T. Y. Tan eds. (Materials Research Society, North Holland, N. Y.,1981) pp. 387-392. 2. S. Kishino, Y. Matsushtta, M. Kanamori and T. Lizuka, Jap. J. Appl. Phys., 21, 1 (lgSZ). 3. Y. Matsushita, J. of Crystal Growth, 56, 516 (1982). 4. R. A. Craven in: Semiconductor Silico~-~, H. R. Huff, R. J. Kriegler and T. Takeishi eds. (iE1ectrochem. Soc., Pennington, N. J.,Ig81) pp.254-271. 5. N. Inoue, K. Wada and J. Osaka, ibid, pp. 281-293. 6. J. D. Peng and C.Y. Kung, internal report. 7. L. Forbes, C. Y. Kung and J. Peng, Materials Research Society Symposium, Boston (1982). 8. K. Tempelhoff, F. Spiegelberg, R. Gleichmann and D.Wruck, Phys. Stat. Sol. (a), 56, 213 (1979). 9. U. Gosele and T.Y. Tan, Defects in SemiConductor I I , S. Mahajan and J. W. Corbett eds. (.Materials Research Society, North' Holland, N. Y., 1983) PP. 45-59. IO.T.Y. Tan, F. Morehead and U. Gosele, Abstract 281, p.432, The Electrochem. Soc. Extended Abstracts, San Francisco, CA 1983.