Multiphysics simulation of electrostatically actuated micromirrors in viscous medium

Electrostatically, comb-driven actuated MEMS (Micro-Electro-Mechanical Systems) micromirrors are of particular interest for a broad range of light manipulation applications like display, imaging, and telecommunication ones. They can meet the high-resolution, small size, low-power consumption and high-scanning speed requirements in such demanding applications. These resonating apparatuses are surrounded by air, which modifies the dynamics of the system. Study of these modifications and developing relevant simulation tools are important needs for design optimization. The rotational resonance frequency of a micromirror is investigated in this thesis by a numerical modeling capable of taking account of the presence of residual stress in the device and fluid-structural interaction (FSI) between air and the micromirror interface. Viscous dissipation at the narrow gap between combfingers, as the dominant loss mechanism for the air-packaged micromirror, has been studied. First, an analytical model for simplified geometry and small oscillations has been provided, and the necessity of including slip boundary condition in the simulations has been investigated. Then, the detailed air flow between combfingers in the large angle oscillations has been modeled by a three-dimensional (3-D) computational-fluid-dynamics (CFD) simulation. Due to the mesh deformation, a constant remeshing method has been adopted. Also, the drag damping at the surface of the tilting micromirror is simulated. The quality factor corresponding to each dissipation mechanism has been obtained, and the overall value compared well with the available experimental results.