On the side instability of comb-fingers in MEMS electrostatic devices

This article reports a side electromechanical instability problem of slender comb-fingers in MEMS electrostatic comb-drive actuators. This instability is induced by two adjacent comb-fingers being excessively bent to contact each other. In this work, based on the assumed shaped energy method, a one degree-of-freedom model was derived to deal with such side pull-in phenomenon of individual fingers. A numerical iterative scheme was introduced to solve for the deflections of fingers from the theoretical model and to predict the side pull-in voltages. FEM simulations were conducted for comparison with the theoretical analyses. Theoretical and simulation results agreed well with each other in the deflections of comb-fingers and pull-in voltages. It was found that in the finger array, each finger exhibits distinct feature; inner fingers show negligible deformation while sided finger are strongly deflected by the transverse electrostatic force. In particular, the first and last fingers show the largest deflections and thus have the highest tendencies to contact adjacent fingers. Moreover, by widening these two fingers, the deflection of every finger can be suppressed. This suggests that the side instability of the comb-fingers is dominated by the first and last fingers. Using the proposed model, the deflection of every individual finger can be calculated and a more reliable design of comb-drive actuator can be achieved.