Dynamic Measurement of Gas Damping Effects in MEMS

The study of fundamental structural dynamic properties is critical for improving Micro-electromechanical System (MEMS) design and functionality. A new facility for measuring the structural dynamics of microsystem parts with a base-excitation technique is described and experimental results from a study of the effect of gas damping on the structural dynamics of MEMS are presented. The new facility includes a piezo-actuator to provide stepped-sine excitation to the bases of the test part and a laser Doppler velocimeter coupled to a microscope to sense velocity response. An environmental chamber enables examination of the effects of air pressure from atmospheric to near vacuum. Eliminating any resonances of the facility and any unintended excitation in the frequency range of interest were particularly important in this design. One of the tests that has been accomplished with the new facility is an experimental study of gas damping in MEMS. Gas damping has a disproportionately large effect on microsystems because of their large surface areas and their relatively small gaps from the substrate. This paper presents experimental results from measurements of micro-cantilever beams vibrating in ambient pressures from atmospheric down to 0.4 Pa. The results from these tests illustrate the strong dependence of energy dissipation on geometry and gas pressure. Effective gas damping at atmospheric pressure is shown to be as much as three orders of magnitude greater than structural damping. Some nonlinearity was evident in the test results at lower pressures where damping was reduced. The results from these tests will be used for future model validation.