Impact of High-Thermal Budget Anneals on Polysilicon as a Micromechanical Material

With the goal of facilitating the development of surface micromachined polysilicon MEMS with postprocessed on-chip circuitry, we have evaluated the impact of a 1200 C 16-h anneal upon chemical-vapor-deposited (CVD) polysilicon under a variety of processing conditions. The results show that undoped polysilicon has a final stress of +10–20 MPa even when the films are vastly different as deposited and that phosphorus doping introduces a compressive trend that is evident only after the long anneal. X-ray diffraction, transmission-electronmicroscopy (TEM), and atomic-force-microscopy (AFM) studies of the polysilicon are used to analyze the grain orientations, grain sizes, and surface roughness of the material. The effect of the long anneal on residual stress in wet thermal and CVD oxides is also presented. Overall, the results indicate that the thermal budgets of conservative circuit processes can be accommodated within the fabrication sequence of surface-micromachined polysilicon microstructures. [266]