Analysis of damping optimization through perforations in proof-mass of SOI capacitive accelerometer

Abstract

MEMS capacitive accelerometers are ubiquitously used in wide-ranging applications. Different applications require a trade-off between design parameters to realize either high sensitivity or precision or wide-dynamic range or speed of response. Planar MEMS structures for sensors usually have a large area compared to thickness or gap. In such structures, squeeze film damping properties of the gas (or air) in the narrow gap significantly affect the dynamic performance of the device. Schemes to reduce the damping effect normally include perforations in the structure to reduce path-lengths of air movement in the narrow gap. But perforations in the structure decrease the mass of the structure leading to a reduction in sensitivity. Therefore, the structural design requires selection of perforation parameters that can provide an optimal trade-off between sensitivity and damping coefficient. This paper discusses our studies through numerical computation when using different configurations of perforations on a typical SOI-based capacitive square accelerometer structure with 1 µm air gap. Both static analysis and analysis at first resonant frequency were carried-out on a range of structures to characterize sensitivity and damping coefficients. The ratio of perforation size versus perforation pitch, η, is used as a basis for sensitivity normalization and studies were carried out to compute damping coefficients for structures with different values of η and count of perforations. Studies reveal a reduction in damping coefficient by 90% to 97% for the η range 0.3 < η < 0.55. The corresponding reduction in effective change in capacitance of the device is limited to the range of 10–25%.