Design and fabrication of a polysilicon micro mirror with a flat surface by gold metallization

A new method to obtain the flat surface in spite of the stress unbalance of the structure is proposed. The curvature of a mirror surface is due to the stress unbalance of the structure. The polysilicon structure which has largely used as micro mirror has a compressive residual stress and a negative stress gradient in general. Therefore, the surface of the micro mirror which is consists of polysilicon has a convex curvature. The net bending moment with respect to neutral surface by stress distribution determines the curvature eventually and can be removed by optimal deposition of the mirror metal which has slightly tensile stress. The concept to obtain a flat surface is shown.