Damping Analysis in Si Torsional Micromirrors

Abstract

Micromirrors are extensively used in MEMS/NEMS based actuators and accordingly the design of superior performance mirror structures are prime requisite in MEMS/NEMS industry. Thermal and viscous damping of a dynamic vibrating micromirror are important factors degrading its performance by inducing energy dissipation. The primary sources of energy losses are viscosity and thermal conduction near the walls of the structure in the acoustic boundary layer. Hence it is necessary to accurately assess the bounds of energy dissipation owing to thermal and viscous damping. In this paper, the depths of the thermal and viscous boundary layers corresponding to thermal conduction and viscous drag at different eigen frequencies are investigated. Both the temperature and pressure distributions surrounding the vibrating micromirror are considered for the analysis. The penetration depths communicate the extent of thermal and viscous boundary layers and clearly indicate the extent of energy loss. In micromirrors, the thickness of boundary layer is a critical parameter since the dissipated energy is mainly distributed in it. The air domain surrounding the micromirror is modelled using thermoacoustics of COMSOL Multiphysics software along with the eigen frequency analysis. In the present study, the thickness of the boundary layers and quality factor are analysed for a micromirror vibrating in torsional mode. The thermal and viscous penetration depths decrease with eigen frequency and hence to develop high quality devices with low damping, the micromirror are verified to be operated at higher frequencies.