A Lateral-Drive Method to Address Pull-in Failure in MEMS

Electrostatic drive is the most common actuation method in MEMS due to its low power, simplicity and ability to generate large forces. However, an intrinsic pull-in failure in electrostatic actuation limits its workable range to only a part of the initial electrode gap. This dissertation investigates an effective lateral-drive method to solve this pull-in problem by building electrodes laterally offset rather than directly opposite to each other. An offset electrode capacitor model is proposed and results in a closed-form capacitance formula for theoretical analysis. Calculations show that laterally-driven beams may require a smaller operating voltage than do some other methods, and cover nearly the full travel range. Furthermore, the driving performance is affected by the structure's lateral gap. Lateral gap less 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 away to improve the performance. A laterally-driven test structure consisting of a fixed-fixed aluminum beam and two lateral substrate electrodes is designed and fabricated, with etched substrate underneath. By measuring the maximum beam deflection with interferometric profiler, the lateral-driven method is characterized experimentally. Misalignment in process degrades the performance. A self-aligned process is developed for fabricating a tunable laterally-driven Fabry-Perot interferometer.