Optically Flat Micromirror Using Stretched Membrane with Crystallization-Induced Stress

The flat and light-weighted micromirror is realized using the tense poly-Si film across a rigid c-Si drum. The tensile stress of ~600 MPa is obtained using the crystallization of a-Si film. Compared to the research carried out by Nee et al., the initial film has the purer amorphous phase and generates larger stress. The mirror satisfies the better optical flatness <lambda/10 for the visible light. The peak-to-valley distance of the micromirror is ~20 nm. The membrane profile is found to depend on the process sequence relating to the timing of the annealing. The mirror shape is stable against the high temperature.