Designs to improve polysilicon micromirror surface topology

Abstract
Many applications for surface micromachined micromirrors require optically flat mirror surfaces. Holographic data storage and optical beam steering for display applications need reflective surfaces free of phase distortion to reduce the signal noise level, minimize cross-talk, and keep the system in focus. Typical micromirror designs may have unintentional embossing on the mirror surface resulting from the top surface conforming to the pattern of underlying layers. The bottom layers may have been shaped to form electrodes to electrostatically move the mirror. The mirror surface may also sag as a result of an imbalance between the stresses in the metal layer and the underlying polysilicon layer. This paper presents a study of the surface of a baseline micromirror, and shows the results of applying smart designs to minimize surface topology defects. The micromirrors presented in this paper were fabricated through the multi-user MEMS processes and categorized with an interferometric microscope.