Influence of Fringing Fields on Parallel Plate Capacitance for Capacitive MEMS Accelerometers

In this paper we present the influence of using basic capacitance formula of parallel plate structure. Such structure is used in typical MEMS accelerometer that have dimensions significantly affecting the precision of calculations. It is caused by electric field that occurs also outside the overlapping area of plates. Therefore, the real capacitance is larger than the calculated one. Moreover, comb-drive structures contains two capacitor of different gap sizes wherein the electric field distribution is not the same as for parallel plate structure. Thus, the knowledge about the amount of underestimation is crucial when estimating the device performance. The parallel plate structure in comb-drive is simulated in order to find the influence of geometrical dimensions of the structure on the underestimation caused by basic capacitance formula. Finger length, width, thickness and gap distance between fingers is analyzed. The results are presented as function of real to calculated capacitances ratio on all input parameters separately. Moreover, such dependencies are approximated with simple analytical functions. In simulations Comsol environment is used.