Electrical and microstructural investigation of polysilicon emitter contacts for high-performance bipolar VLSI

Key electrical characteristics of polysilicon emitter contacts in bipolar transistors, such as contact resistance and recombination velocity, are extremely sensitive to the microstructure of the polysilicon/single-crystal silicon interface. In this study, we correlated the microstructural and electrical characteristics of this interface by performing cross-sectional transmission electron microscopy (XTEM) on actual transistors on the same chip where ring-oscillator speeds were measured. The base current and emitter resistance of the fastest devices approached values typical of single-crystal silicon emitters. Interpretation of these electrical data and of the SIMS impurity profile indicates that significant restructuring of the polysilicon/single-crystal interface had taken place. This conclusion was indeed confirmed by the XTEM results. Although the low-current performance was degraded because of higher junction capacitances, the high-current switching speed was improved because of the minimal emitter contact resistance. Since the current gain was sufficiently high and very uniform, it is concluded from this work that minimization of both junction depth and contact resistance is the most important design consideration for high-performance submicron transistors, rather than maximization of the gain enhancement of the polysilicon/single-crystal interface.