A flat and lightweight micromirror with a drum design is realized using a tense polycrystalline (poly-) Si membrane across a rigid crystalline (c-) Si ring. The tensile stress of ∼600 MPa is obtained using the crystallization of amorphous (a-) Si film. The tension keeps the membrane flat. Compared to the previous research, the initial film has a purer amorphous phase, and a larger stress is obtained. In addition to the design dependence, the membrane flatness is found to depend on the process sequence relating to the timing of the crystallization. A peak-to-valley surface deformation of ∼20 nm is realized in a 500 μm diameter mirror satisfying the optical flatness of <λ/10 for visible light. Since the crystallization-induced stress is temperature independent, the mirror flatness is stable against temperature even with a metal coating for higher reflectivity.
- Optical flatness
ASJC Scopus subject areas
- Atomic and Molecular Physics, and Optics