Effects Of Rapid Thermal Processing On Oxygen Precipitation And Internal Gettering In Czochralski Silicon

The ever-smaller feature size of integrated circuit imposes on increasingly stringent requirements on the defect control and internal gettering(IG)capability of Czochralski(CZ)silicon wafers.Under the circumstance,the IG process based on oxygen precipitation in CZ silicon wafers has been continuously improved.In recent years,the MEMC company in America,a leading silicon supplier,has presented a patented IG process based on rapid thermal process(RTP),which is believed to be a milestone in the defect engineering of silicon wafers.Such a process is not only of technological importance but also arouses a fundamental issue on the effect of RTP on oxygen precipitation in CZ silicon.Despite the great progress made in the research on this issue,the exact effects of RTP on oxygen precipitation in different kinds of CZ silicon wafers subjected to various thermal cycles have not substantially clarified.In this dissertation,the oxygen precipitation behaviors in CZ silicon wafers subjected to different RTPs and subsequent thermal anneals and,moreover,the RTP-based IG processes have been detailedly investigated.In the following,the primary results achieved herein are listed.The effects of prior RTP at high temperatures on the oxygen behaviors in CZ and NCZ silicon wafers subjected to low-high(L-H)two-step anneal were investigated.It was shown that:(1).The RTP-induced vacancies in CZ silicon enhanced the nucleation for oxygen precipitation most significantly at 800℃,while,for NCZ silicon,the vacancies coact with the nitrogen atoms to enhance the nucleation for oxygen precipitation most significantly in the temperature range of 800～1000℃.(2). At temperatures above 900℃,the nitrogen atoms are superior to the vacancies in terms of the enhancement of nucleation for oxygen precipitation and,moreover,the co-existing of nitrogen atoms and vacancies in CZ silicon will more significantly enhance the nucleation for oxygen precipitation.In view of the different effects of RTP-induced vacancies on the oxygen precipitation behaviors in CZ and NCZ silicon wafers,it is believed that the RTP-based IG process for NCZ silicon wafer should be somewhat different from that for CZ silicon wafer,that is,the one for NCZ silicon wafer is RTP at 1250℃followed by the ramping anneal from 800 to 1000℃with a rate of 1℃/min and then with a 16 h isothermal anneal;while,that for CZ silicon wafer is RTP at 1250℃followed by 800℃/4 h + 1000℃/16 h anneal. The influences of temperature and cooling rate of RTP under N₂ ambient on oxygen precipitation and formation of DZ in CZ silicon wafers subjected to the subsequent L-H two-step anneal were investigated,as a result,the IG process based on the RTP under N₂ ambient for CZ silicon wafers was proposed.In comparison with the RTP under Ar ambient,the RTP under Ar ambient at the lower temperatures could lead to high density of oxygen precipitates generated in CZ silicon wafer subjected to the subsequent L-H two-step anneal.Moreover,with a low cooling rate of RTP,a DZ could be formed in the near-surface region with CZ silicon wafer.With an appropriate cooling rate,the RTP at lower temperatures led to a wider DZ but a lower density of bulk microdefects(BMDs).Accordingly,the RTP under N₂ ambient at appropriate temperatures and with desirable cooling rates could result in a width of DZ and an appropriate density of BMDs in CZ silicon wafers subjected to the subsequent L-H two-step anneal.This result reclaims the formerly widespread accepted viewpoint that the RTP under N₂ ambient cannot be applied to the IG process for CZ silicon wafers.The effect of two consecutive RTP under different ambients on oxygen precipitation and formation of DZ during the subsequent thermal cycles for the lightly and heavily boron-doped CZ silicon wafers were investigated.Regarding the lightly boron-doped CZ silicon wafers,if the first-step RTP was performed under Ar ambient, then the oxygen precipitation and formation of DZ were determined by the second-step RTP ambient;while,if the first-step RTP was performed under N₂ or O₂ ambient,then high density of BMDs and DZ could be formed during the L-H two-step anneal subsequent to the RTP under any ambient,but the width of DZ were different. As for the heavily boron-doped CZ silicon wafers,if with only one-step RTP under Ar ambient,then a high density of BMDs was formed but DZ was not generated after the subsequent L-H two-step anneal;however,if the RTP ambient was changed as O₂,the density of BMDs formed by the subsequent L-H two-step anneal was quite low.Therefore,for the heavily boron-doped CZ silicon wafers,making a trade-off between the above two cases,a width of DZ and a high density of BMDs could be formed by the L-H two-step anneal subsequent to the two-step RTP consecutively performed under Ar and O2 ambients.The formation of BMDs and DZ in the silicon nitride film coated CZ silicon wafers subjected to the high temperature RTP followed with L-H two-step anneal was investigated.With the same L-H two-step anneal,the amount of precipitated oxygen (â–³[O_i])in the silicon nitride film coated CZ silicon wafers with a prior RTP at 1200℃was comparable to that in the silicon wafers with a prior RTP at 1250℃and, moreover,a width of DZ was generated in the silicon nitride film coated CZ silicon wafers.It is preliminarily believed that the silicon-nitrogen bonds within the silicon nitride film were broken by the RTP and,moreover,the released nitrogen atoms diffused into the silicon wafers.Furthermore,due to the coaction of in-diffused nitrogen atoms and the vacancies induced by the RTP,oxygen precipitation in silicon wafers was significantly enhanced.Oxygen precipitation during the 1000℃anneal subsequent to the nucleation anneal at 800℃by the RTP or conventional furnace anneal(CFA)for the lightly and heavily boron-doped CZ silicon wafers were respectively investigated.Regarding the lightly boron-doped CZ silicon wafers,the RTP at 800℃for 1 h and the CFA at 800℃for 4 h led to the comparativeâ–³[O_i],indicating that the optical radiation of RTP enhanced the nucleation of oxygen precipitates most likely due to the enhanced oxygen diffusion.As for the heavily boron-doped CZ silicon wafer,compared with the CFA, the RTP not only enhanced the nucleation of oxygen precipitates but also altered the cross-sectional distribution of BMDs formed in the subsequent 1000℃anneal. Concretely speaking,the nucleation by the CFA led to quite uniform distribution of BMDs across the silicon wafer,whereas,the nucleation by the RTP resulted in not uniform cross-sectional distribution of BMDs,that is,a large number of oxygen precipitates and dislocation loops as well as large-sized stacking faults formed in the near-surface region of silicon wafer,while,a large number of oxygen precipitates and a small amount of staking faults as well as large-sized oxygen precipitates accompanied with dislocation loops were generated in the bulk region.The dissolution of oxygen precipitates in CZ silicon by the high temperature RTP and CFA and the regrowth of oxygen precipitates during the subsequent CFA were investigated.In terms of the increase in[O_i],the effect of RTP for a short period of time on the dissolution of oxygen precipitates could be equivalent to that of CFA for a long time.On the other hand,the regrowth of oxygen precipitates after the RTP was quite different from that after the CFA.For the dissolution of oxygen precipitates by the RTP,due to the very short period of thermal cycle,even for the small-sized oxygen precipitates,they were not dissolved completely.Therefore,for the regrowth of oxygen precipitates,the undissolved small and large sized oxygen precipitates simultaneously acted as the nuclei,thus leading to increased number of BMDs.While, for the dissolution of oxygen precipitates by the CFA,the small-sized oxygen precipitates were substantially dissolved.Thus,during the regrowth of oxygen precipitates,only the residual large sized oxygen precipitates acted as the nuclei. Consequently,the resulting BMDs kept nearly the same density as those after the dissolution by CFA.