STRESS-PROFILE CHARACTERIZATION AND TEST-STRUCTURE ANALYSIS OF SINGLEAND DOUBLE ION-IMPLANTED LPCVD POLYCRYSTALLINE SILICON

A very low stress gradient across the polysilicon layers is required f or the fabrication of large micromechanical structures based on surfac e-micromachining technologies. In this work the residual stress and th e stress gradient of 2 mu m thick LPCVD polysilicon layers are present ed as a function of deposition, doping and annealing conditions, Low s tress gradients are obtained by optimizing the doping profile using a two-step deposition and implantation process. The results obtained by mechanical test structures are corroborated by micro Raman measurement s. The effects of the polysilicon stress gradient on surface-micromach ined accelerometers are analysed. Polysilicon layers with low tensile stress and a stress gradient lower than 5 MPa mu m(-1) are required fo r the fabrication of surface-micromachined z-accelerometers.