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 single wafer technology to develop SOI pressure sensors, allowing the fabrication of polycrystalline piezoresistors on thermally grown silicon dioxide with a turnaround time of a few minutes. The growth kinetics and the microstructure of polysilicon films obtained by rapid thermal chemical vapour deposition using an argon-silane gas mixture are investigated as a function of the process pressure and temperature. Polysilicon films deposited at 5 mbar and at temperatures lower than 750 °C exhibit high compressive stresses and grain sizes of about 30 nm due to a high level of oxygen contamination. At 1 mbar the lower oxygen contamination enables to deposit well crystallized films, with residual tensile stresses comparable to those of classical low pressure chemical vapour deposition polysilicon and with grain sizes reaching 55 nm. Longitudinal gauge factors of boron-implanted piezoresistors patterned on polysilicon films deposited at 5 mbar and 720 °C show a maximum of 20–22 in the 2×10 19-4×10 19 cm −3 doping range. Longitudinal gauge factors of 25–32 are measured for piezoresistors made from boron-diffused polysilicon deposited at 5 mbar and 850 °C, thus illustrating the flexible capabilities of rapid thermal processing for the silicon microsystem technology.