Effect Of Rapid Thermal Processing On Impurity Behaviour In Czochralski Silicon

Rapid thermal processing (RTP), as an agile manufacturing means, has been well employed in internal gettering (IG) process of Czochralski (Cz) silicon in the past decade. The essential reason for this is that the vacancies induced by high temperature RTP can enhance oxygen precipitation (OP) in Cz silicon and, moreover, the vacancy concentration profile can be well controlled by RTP. Although the effects of high temperature RTP on OP in lightly doped Cz silicon have been substantially clarified, whether they are applicable to heavily doped Cz silicon remains unclear. In addition, it has been experimentally found that the high temperature RTP under nitrogen ambient exhibits a stronger enhancement effect on OP in Cz silicon than that under argeon ambient. The reason for this result, however, is still an open question. In this thesis, the effects of high temperature RTP on OP in heavily arsenic (As)-doped Cz silicon have been investigated. Moreover, the nitrogen in-diffusion in silicon subjected to high temperature RTP under nitrogen ambient has been evaluated in order to better understand the enhancement effect on OP of the RTP under nitrogen ambient. Lastly, the effects of high temperature RTP under nitrogen ambient on the mechanical strength of Cz silicon have been tentatively investigated. Listed below are the primary results obtained in this thesis.(1) Oxygen precipitation (OP) behaviors in heavily As-doped Cz silicon wafers without and with the prior RTP at 1100-1250℃, subjected to the subsequent two-step anneals of 450,650 and 800℃/4 h+1000℃/16 h, have been investigated, respectively. It is found that the prior RTP can enhance OP in each two-step anneal. Such enhancement effect exhibits most significantly in the two-step anneal with the nucleation anneal at 800℃. However, in order to form the highest density of oxygen precipitates, the most desirable nucleation temperature is 650℃in both cases without and with prior RTP. The OP nucleation mechanisms operating in the three low temperatures and the effect of RTP on them have been tentatively discussed.(2) Comparative investigation on OP behaviors in conventional and nitrogen-codoped heavily As-doped silicon wafers without and with the prior RTP at 1250℃, subjected to the subsequent two-step anneals of 450,650 and 800℃/4 h+ 1000℃/16 h, has been carried out. It is clearly shown that nitrogen can enhance OP in heavily As-doped silicon in each case. In the case without the prior RTP, the nitrogen enhancement effect exhibits strongest at the nucleation temperature 650℃, and the most suitable OP nucleation temperature is 650℃in both kinds of samples. While, in the case with the prior RTP at 1250℃, the most suitable OP nucleation temperature is 650 and 800℃for the conventional and nitrogen-codoped samples, respectively. Moreover, the nitrogen enhancement effect exhibits most significantly at the nucleation temperature of 800℃.(3) The nitrogen in-diffusion in silicon wafers subjected to high temperature RTP under nitrogen ambient has been investigated. It is found that nitrogen atoms substantially diffuse into silicon during the RTP at 1150-1250℃and the in-diffused nitrogen concentration increases with the RTP temperature. After RTP at 1250℃for 90 s, the nitrogen concentrations reach 1×1016 and 5×1015 cm-3 at surface and a depth of 100μm in the silicon wafer, respectively. It is also shown that the nitrogen concentration in silicon wafer subjected to RTP at 1250℃increases with the annealing time in the range of 30-90 s. X-ray photon spectroscopy analysis shows that the violent irradiation from halogen lamps in RTP facilitates the nitridation of silicon surface. Simultaneously, the Si-N bonds dissociate during the high temperature process of RTP. In turn, the dissociated nitrogen atoms diffuse into silicon wafer. The above-mentioned results show that the high temperature RTP under nitrogen ambient injects a high concentration of nitrogen impurity in addition to a high concentration of vacancies. Therefore, it exhibits a stronger enhancement effect on OP than that under argon ambient.(4) The gliding behaviors of indentation-induced dislocations during the heat-treatment for the silicon wafers subjected to the prior RTP at 1150-1250℃under argon and nitrogen ambient, respectively, have been comparatively investigated. It is found that the prior RTP under argon ambient has no substantial effect on the gliding behavior of dislocation, while, that under nitrogen ambient significantly reduces the maximum gliding distance and gliding speed of dislocation. Such reduction becomes ever stronger with the RTP temperature. The aforementioned results indicate that the nitrogen impurity induced by the high temperature RTP under nitrogen ambient improve the mechanical strength of silicon wafer remarkably.