Investigation Of Oxygen Precipitation And Internal Gettering In Czochralski Silicon

One primary reason for the application of Czochralski (CZ) silicon in the fabrication of integrated circuits is that CZ silicon possesses internal gettering (IG) capability related to oxygen precipitation and its induced defects. During the process of device fabrication, the oxygen precipitates and their induced secondary defects in the bulk of silicon wafer can act as the effective gettering sites for the heavy metals. Therefore, with the ever-decreasing feature size of integrated circuits, the IG capability of CZ silicon wafer is increasingly important for improving the manufacturing yield of ultra-large-scale integrated (ULSI) circuits. In this dissertation, the effects of annealing schemes, annealing conditions, nitrogen-doping as well as the neutron-irradiation on oxygen precipitation and IG in CZ silicon have been investigated. Listed below are the significant results obtained in this dissertation.The effects of pre-annealing at low temperature on oxygen precipitation in CZ silicon during the subsequent low temperature annealing or Lo-Hi two-step annealing have been investigated. It was found that the pre-annealing at low temperature (300~750℃), especially the ramping annealing from 300℃ exhibited strong enhancement on oxygen precipitation. It is considered that the molecular oxygen (O2i) formed at low temperatures is a fast diffuser, which can enhance the diffusion of oxygen and therefore the nucleation of oxygen precipitation. In nitrogen-doped CZ (NCZ) silicon, the nitrogen can also enhance the nucleation of oxygen precipitation by combining with oxygen atoms to form N2On at low temperatures;furthermore, the O2i might facilitate the formation of N2On. A novel IG process based on ramping anneal from a low temperature has been developed, which can reduce the annealing time and therefore the thermal budget.It was revealed that the prior ramping anneal from a low temperature could affectthe morphologies of oxygen precipitate generated by the subsequent Lo-Hi two-step annealing. It was verified that the prior ramping anneal could result in the generation of sphere-like oxygen precipitates with high density but small size, while most of the oxygen precipitates in the samples without the prior ramping anneal were of platelets in large size.The effect of annealing atmosphere on oxygen precipitation in CZ siliconsubjected to the conventional Hi-Lo-Hi three-step IG treatment has been investigated. It was found that the high temperature annealing in various atmospheres resulted in almost identical oxygen out-diffusion length but different distribution of oxygen precipitates in the silicon wafers. It was confirmed that the high-temperature annealing in oxidizing atmosphere or steam-mixed atmosphere induced a high concentration of silicon self-interstitials to retard oxygen precipitation, while the annealing in nitrogen atmosphere induced a high concentration of vacancies and a certain content of nitrogen impurity to enhance oxygen precipitation. Accordingly, in comparison with the sample annealed in Ar atmosphere, much fewer oxygen precipitates and wider denuded zone (DZ) were observed in the samples with high-temperature annealing in O2 or steam-mixed atmosphere, whereas, much denser oxygen precipitates and narrower DZ were observed in the samples with high-temperature annealing in N2 atmosphere.The effects of nitrogen-doping on oxygen precipitation and IG in CZ silicon during the Hi-Lo-Hi three-step annealing have been investigated. After the Hi-Lo-Hi three-step annealing, it was found that an M-like oxygen precipitate profile existed in the cross section of NCZ silicon wafer;however, it was not the case in the conventional CZ silicon wafers. It was believed that the nitrogen atoms could combine with oxygen atoms to form N2On complexes at low temperature, and combined with oxygen atoms and vacancies to form N2V2Om complexes at high temperature. Both the complexes could act as the nuclei to enhance oxygen precipitation thus influencing the distribution of oxygen precipitates across the silicon wafer. Furthermore, the diffusivity of nitrogen in silicon was much higher than that of oxygen, so the concentration of nitrogen in the DZ was too low to enhance oxygen precipitation, thus resulting in a substantially DZ of oxygen precipitates.It was confirmed that the "magic denude zones" (MDZs) could be established in CZ and NCZ silicon wafers by the rapid thermal processing (RTP) followed by a Lo-Hi two-step annealing. The oxygen in the near-surface region of the silicon wafer diffused out sufficiently during the MDZ formation process, so the MDZs were stable and could survive in the subsequent rigorous annealing. It was further confirmed that the nitrogen-doping had little effect on the formation of MDZ in NCZ silicon, however, during the RTP treatment, the nitrogen atoms could combine with vacancies to form N2V2, which restrained the out-diffusion of vacancies and pair-annihilation reaction of vacancies and silicon interstitials. So the vacancy concentration profile inNCZ silicon after RTP treatment was uniform and thus resulting in the uniform distribution of oxygen precipitates formed in the subsequent Lo-Hi annealing, which was not the case in the conventional CZ silicon.The effect of neutron irradiation on the formation of thermal donors has been studied. It was found that some irradiation-induced defects introduced acceptor levels, which could affect the Fourier transform infrared (FTIR) spectra of the thermal donors measured at liquid helium temperature, i.e. some individual IR absorption lines related to singly ionized donors were appeared in the FTIR spectra of the neutron-irradiated silicon but not in those of the non-irradiated silicon. Furthermore, it was proved that VOn defects could facilitate the formation of thermal donors, which were suggested to be oxygen precipitates embryos at low temperature, so the VOn could enhance the formation of oxygen precipitates during the annealing at low temperature.It was verified that the vacancies introduced by the neutron-irradiation could enhanced the nucleation and growth of oxygen precipitates in CZ silicon, but had little effect on the out-diffusion of oxygen at high temperature. The Ostwald ripening of oxygen precipitates in neutron-irradiated silicon was observed in our experiment, which was due to the vacancy enhancement of the formation of platelet-like oxygen precipitates that were readily dissolved by the high temperature treatment. In the neutron-irradiated NCZ silicon, both the nitrogen and vacancy can enhance oxygen precipitation, but the enhancement effect of nitrogen operates at temperatures above 900℃, while that of vacancy operates mainly in temperature range of 700~1000℃.