Developing a manufacturable process for the deposition of thick polysilicon films for micromachined devices

Current techniques for manufacturing integrated Micro-Electrical-Mechanical Systems (iMEMS/sup (R)/) place significant limitations on the time and temperature of anneal during fabrication. This is due to the polysilicon film used for the MEMS structure being deposited after the formation of the P and N junctions for the BiMOS devices. Critical requirements for the polysilicon micro-machined structure include low stress gradient through the thickness of the film and tensile residual stress. The film must have uniform stress through the thickness of the film to provide flat polysilicon beams after release. Residual tensile stress is beneficial for structural stability. A clear understanding of the deposition conditions that affect the residual stress, stress gradient and grain structure of these thick polysilicon films is necessary to provide a robust manufacturable process in high volume production. This presentation outlines how the deposition and anneal conditions effect grain formation, residual stress and stress gradients in polysilicon films. Deposition conditions that provide large grain films, small grain films and mixed phase films are evaluated. The affect of incorporating oxygen and phosphine gases during polysilicon deposition is investigated. A physical model describing grain formation is presented. A final process selection based on process robustness, process integration and device performance is presented.