Stress-profile characterization and test-structure analysis of single and double ion-implanted LPCVD polycrystalline silicon

A very low stress gradient across the polysilicon layers is required for the fabrication of large micromechanical structures based on surface-micromachining technologies. In this work the residual stress and the stress gradient of 2 μm thick LPCVD polysilicon layers are presented as a function of deposition, doping and annealing conditions. Low stress 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 measurements. The effects of the polysilicon stress gradient on surface-micromachined accelerometers are analysed. Polysilicon layers with low tensile stress and a stress gradient lower than 5 MPa μm-1 are required for the fabrication of surface-micromachined z-accelerometers.