The effect of thin interfacial oxides on the electrical characteristics of silicon bipolar devices

Abstract

The effect of thin interfacial oxides on the impurity diffusion from polysilicon to the silicon substrate has been studied in detail. Polysilicon films were deposited on the silicon substrate in two different process conditions to control the thickness of interfacial oxides. Results show that the presence of about 1-nm-thick oxides retarded the impurity diffusion by about 10 nm and an increase of the sheet resistance of about 10 percent has been observed. Bipolar devices, which are sensitive to the impurity profiles, were fabricated with identical processing apart from the polysilicon deposition conditions. A detailed analysis of their electrical characteristics shows the difference of collector current components and hence the increase of current gain by about two times. These results indicate that the effect of interfacial oxides on the impurity profile is expressed by the segregation coefficient m , which is the ratio of C si /C polySi at the interface. The sensitivity of m for the device characteristics was calculated by a process-device simulator, and it is demonstrated that the current gain is a strong function of m for shallow emitters.