Low-Stress Epitaxial Polysilicon Process for Micromirror Devices

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This paper reports key process technologies for the application of epitaxial polysilicon (epi-poly-Si) to scanning micromirrors, including low-stress epi-poly-Si deposition, the chemical mechanical polishing (CMP) of epi-poly-Si and the fabrication of trench isolations in the epi-poly-Si film. Epi-poly-Si thicker than 20 µm was deposited at a rate of 300 nm/min using an atmospheric pressure epitaxial reactor. The residual film stress was 57.2±1.4 MPa (compressive), and the stress gradient was 1.41±0.84 MPa/µm (more tensile toward the surface) for an as-deposited 21 µm thick film. The average surface roughness Ra was initially 202±48 nm, but improved to 10 nm by 10 min CMP. The stress gradient after CMP was as small as -0.30±0.1 MPa/µm. This leads to only negligible bending of cantilevers, e.g. 0.5 µm for a 1000 µm long cantilever. Isolation trenches refilled with poly-Si was fabricated in epi-poly-Si. A stepwise design to minimize voids in the refilled trench was demonstrated.