Development of techniques to monitor and control minority carrier lifetime in silicon to improve yields in a university fab

Measurement of minority carrier lifetime (τ), either indirectly via pn diode leakage current or directly from MOS capacitors, is used as a monitor to detect and correct contamination in an university fab. Diodes fabricated utilizing an internal gettering process exhibited improvements in leakage current levels about the wafer center when compared with diodes fabricated sans gettering, indicating the possibility of contamination by outdiffusion of impurities from the tube walls. Surface charge analysis (SCA) of an oxidized silicon wafer detected a 30% decrease in τ over a 6 hr N₂ anneal in the tube used for gettering. The same experiment conducted in a tube contaminated with Zn yielded a 45% drop in lifetime. Both gas sources and RCA clean chemistry are being investigated as possible contamination sources of the getter tube