Innovative approach to high stroke electrostatic actuators

Abstract

The electrostatic actuation is still a preferred principle in modern microelectromechanical systems (MEMS). Based on the Coulomb attraction between two different point charges, the electrostatic actuation is a surface effect and thus volume-independent. In addition, the efficiency of electrostatic actuation increases with a decreasing gap size between the electrodes. The relatively simple morphology of an electrostatic actuator allows low-cost wafer-level fabrication, making it a versatile and convenient principle for MEMS actuators. Although the electrostatic principle seems only to be applicable for actuators in micrometer scale, this paper presents the successful upscaling of an electrostatic actuator to the millimeter scale, still utilizing the major advantages, including wafer-level fabrication, of the said principle and providing a high stroke with low actuation voltage. For such an actuator, we replace the common silicon with the non-conducting OrmoComp, a UV-curable hybrid polymer, suitable for wafer-level fabrication. With a significantly lower elastic modulus, only a fraction of actuation voltage is necessary for a similar deflection. The electrodes are realized with additional coatings of thin metal layers. To achieve a high stroke, while maintaining a relatively low voltage, the actuator design is based on a redesigned zipper actuator. With our developed fabrication process, we are able to create a highly displaced out-of-plane actuator, while almost eliminating the initial gap between the electrodes. Experimental results of a wafer-level fabricated zipper actuator show an out-of-plane motion up to 470 μm at an actuation voltage of 374 V.