Estimation of the mechanical stiffness constant of MEMS-based parallel-plate micro-actuators

The fabrication and testing of a parallel plate MEMS electrostatic micro-actuator is reported. The device consists of stationary bulk silicon and movable membrane chips with three spring-like x-beam configurations fabricated from a silicon on insulator (SOI) wafer. A SU-8 photoresist layer was deposited on the stationary chip to act as a spacer since its thickness determines the electrostatic force that can be applied. This in turn has an effect on the displacement of the micro-actuator. We investigated the effects of the applied voltage on the displacement of movable x-beam membranes for different arm designs with similar surface areas. We achieved maximum stable displacements of 8.75 mu m and 9.89 mu m for spacer thicknesses of 28 mu m and 33 mu m at 95 VDC and 128 VDC, respectively, for a serpentine arm design. Beyond these voltages, we found the displacement of the micro-actuator tended to be non-uniform and unstable. We also estimated the mechanical stiffness constants of our x-beam designs from the snap-in conditions. Our estimates for various spacer thicknesses were within 5% of one another.