Micro-nitrogen Single Crystal Silicon Power Devices

In recent years, nitrogen behavior in single crystal silicon has been intensively studied. In general, nitrogen is used as a protective gas or a carrier gas in manufacture processes of very large scale integration (VLSI). It is well known that nitrogen in silicon can suppress microdefects, and lock dislocations to increase wafers' mechanical strength. Nitrogen in Czochralski (CZ) silicon has been recently believed to improve the yield and performance of integrate circuits. However, many aspects of nitrogen in CZ silicon has still kept unclear. In this paper, process-induced defects in nitrogen doped CZ silicon (NCZ-Si) and their influence on power diode and switching transistor characteristics are investigated.In order to study the effect of nitrogen-doping on device performance, 3 and 4 inches NCZ-Si ingots grown in nitrogen atmosphere were used for manufacturing diode and transistor respectively. In each steps of the process, the wafers were checked by an optical microscope after the preferential etching of beveled samples for defects, and measured by a Fourier transition infrared spectrometer (FTIR) for interstitial oxygen (Oi) concentration. Finally, the device characteristics were tested to find its correlation with the process-induced defects. For comparison, the common CZ silicon (ACZ-Si) samples with the almost same oxygen concentration and thermal history which were grown in argon ambient were also used in our experiments.It was found that after the diode process, all supersaturation Oi in NCZ-Si samples was precipitated and the Oi concentration fell to the corresponding solubility concentration. On the contrary, only slight oxygen precipitates were formed in the ACZ-Si wafer. When the initial oxygen concentration in NCZ silicon is higher than 1018 cm"3 , high-density dislocations (~105cm~2) were formed in diode active region and resulted in reverse recovery time (Trr) decrease. It was found that nitrogen has no important influence on reverse breakdown voltage and forward voltage drop of diodes. It is concluded that during the diode process nitrogen strongly enhances oxygen precipitation. According to our results, it is suggested that nitrogen-doping will impair the Trr characteristics of diodes if the oxygen concentration is a little higher(>1018cm~3); if not, it has no important influence on diode characteristics.To achieve a heavily doped substrate which is necessary for switching transistor, a prolonged high temperature phosphorus diffusion(main diffusion) process was employed. It was found that similar density dislocations were formed in the high resistance region of both ACZ-Si and NCZ-Si samples in this process. Nevertheless, for ACZ-Si samples, the dislocation distribution was very heterogeneous, while it was homogeneous in NCZ-Si samples. It was also found that the dislocation density and distribution had never changed in the following transistor process. Accordingly, transistor storage time(Ts) were measured. It was demonstrated that, correlated with dislocation distribution, the NCZ-Si transistors' Ts were more homogeneously distributed than those of ACZ-Si transistors. It is concluded that nitrogen-doping can make the dislocation distribution more homogeneous, and so improve the distribution uniformity of the transistor storage time.