Development and analysis of the vertical capacitive accelerometer

The microelectromechanical systems (MEMS) accelerometer, also referred to as the vertical capacitive torsional accelerometer (TXL), estimates the acceleration by detecting the changes in capacitance. We optimize five design variables of the MEMS accelerometer by considering the robustness of the design and maximizing the sensitivity of TXL, while avoiding the pull-in effect. TXL is manufactured using the reverse surface micromachining (RSM). After fabricating, we verify the optimization of the design by measuring the capacitance and the eigenfrequency of TXL with and without electrical forces. The first eigenfrequency of a design and a product without electrical forces has 8.72% difference in frequency. In the simulation for the first eigenfrequency of TXL considering electrical force, the pull-in (Potential shorting of the electrodes) does not appear between 0 V and the computed critical voltage. It occurs after applying a voltage higher than the critical voltage of TXL with an air gap of 2 μm. To explain these differences, we study the effects of the undercut in the etching holes, using the three dimensional finite element method with multiphysics. An important observation in this analysis is that the air gap is designed to be 2 μm, but the undercut in the etching hole makes this air gap to be about 4 μm.