Gas Damping in Vibrating MEMS Structures

This chapter deals with gas damping in vibrating MEMS structures. MEMS structures are surrounded usually by gas, for example, air, and when they move or vibrate, the structures interact with the surrounding gas. The most dominant effect of the gas is the damping force acting on the vibrating structure, but also spring forces and inertial forces may be important for the operation of the MEMS device. MEMS structures have air gap heights and the thin film makes it necessary to consider the gas rarefaction effects in the gaps. Higher frequencies are discussed briefly in the chapter. In narrow gaps, the number of gas molecules is small, and the continuum mechanics cannot anymore explain the gas flow. The increasing vibration frequency breaks down the pure viscous flow assumption and makes the damping depend on frequency. The compressibility is an out-of-plane phenomenon, similar to the flow-through perforations. The ratio of the compressible force and the viscous damping force is an important relation. At relatively high frequencies, the wavelength in the gas might become comparable with the surface dimensions. There are various commercial tools available for numerical simulation of the gas forces in MEMS systems. N-S solver is a very powerful tool in analyzing gas damping phenomena in MEMS structures. The use of the Reynolds solver is a very tempting alternative for squeeze-film problems. The DSMC is a powerful method for simulating high Knudsen number rarefied flows in complex geometries. Automated tools are available in building system-level components from FEM simulations.