Investigation of Optical Flatness of Stretched Membrane Drum-Type Micromirror

Realizing an optically flat and lightweight high-speed scanning micromirror still remains as a challenging problem. In this paper, we propose a drum-type lightweight micromirror that is capable of providing high scanning speed and retaining optical flatness, simultaneously. The fabrication technique and surface deformation analysis of the proposed micromirror are described. The drum-type design is realized using a stretched polycrystalline (poly-) Si membrane across a rigid crystalline (c-) Si ring. The tensile stress in the poly-Si membrane is 300-400 MPa that keeps the membrane flat. At the static condition, the total peak-to-valley surface heights in the center poly-Si membrane of the designed five distinct shapes of micromirrors are varied from 16 to 29 nm. The stress concentration at mirror edge is investigated by the amount of surface distortion which is less than 40 nm (i.e., one-tenth wavelength of the blue light). The maximum total peak-to-valley surface height is about 50 nm and the dominant profile is at the connecting part between the c-Si ring and the poly-Si membrane. The amount of dynamic deformation in the poly-Si membrane is less than 40 nm. This satisfies the optical flatness compared to the wavelength of blue light.