RAPID THERMAL-PROCESSING OF PIEZORESISTIVE POLYCRYSTALLINE SILICON FILMS - AN INNOVATIVE TECHNOLOGY FOR LOW-COST PRESSURE SENSOR FABRICATION

Rapid thermal processing is evaluated as a low cost and flexible singl e wafer technology to develop SOI pressure sensors, allowing the fabri cation of polycrystalline piezoresistors on thermally grown silicon di oxide with a turnaround time of a few minutes. The growth kinetics and the microstructure of polysilicon films obtained by rapid thermal che mical vapour deposition using an argon-silane gas mixture are investig ated as a function of the process pressure and temperature. Polysilico n films deposited at 5 mbar and at temperatures lower than 750 degrees C exhibit high compressive stresses and grain sizes of about 30 nm du e to a high level of oxygen contamination. At 1 mbar the lower oxygen contamination enables to deposit well crystallized films, with residua l tensile stresses comparable to those of classical low pressure chemi cal vapour deposition polysilicon and with grain sizes reaching 55 nm. Longitudinal gauge factors of boron-implanted piezoresistors patterne d on polysilicon films deposited at 5 mbar and 720 degrees C show a ma ximum of 20-22 in the 2x10(19)-4x10(19) cm(-3) doping range. Longitudi nal gauge factors of 25-32 are measured for piezoresistors made from b oron-diffused polysilicon deposited at 5 mbar and 850 degrees C, thus illustrating the flexible capabilities of rapid thermal processing for the silicon microsystem technology.