A surface micromachined offset-drive method to extend the electrostatic travel range

Abstract

A surface micromachined offset-drive mechanism is demonstrated to address the pull-in problem of electrostatic actuation in microelectromechanical systems (MEMS). Similar to the vertical comb drive method, our offset-drive method uses fabricated electrodes that are laterally offset rather than directly opposite to each other. As a consequence, the electrodes are never engaged and are driven mostly by fringing fields. An offset electrode capacitance model is proposed for analysis. Simulations show that the proposed offset scheme may require lower operating voltages than some competing methods, and a full travel range can be achieved without pull-in. It is also found that drive performance is influenced by the structure's lateral gap. Lateral gaps smaller than a critical value are still susceptible to the pull-in problem. For devices on a planar semiconductor substrate, this method is electrically equivalent to the series capacitor mechanism due to the high substrate dielectric constant. We demonstrate that the substrate may be etched to improve the performance of this offset-drive method. An offset-drive test structure consisting of an aluminum beam suspended across two posts is designed and fabricated, with an etched substrate underneath. By measuring the maximum beam deflection, the offset-drive method is characterized experimentally.