Electrostatic zipping actuators and their applications to MEMS

Abstract

Electrostatic actuation is the most common and well-developed method of generating motion on the micro scale. To overcome the challenge of providing both high force and large displacement, electrostatic zipping actuators have been developed and applied to various devices. As device thicknesses increase, however, conventional laterally- moving zipping actuators become less practical due to their high pull-in voltages caused by their minimum achievable electrode gaps. This thesis presents a fundamental improvement of the laterally-moving electrostatic zipping actuator. Its major contributions are: 1) a compliant starting zone is introduced into the fixed electrode to significantly reduce the pull-in voltage of the zipping electrode; 2) numerical and analytical methods are developed to solve general zipping actuator problems; 3) optimization is performed to minimize the effort required to actuate the zipping electrode and its load; and 4) the improved zipping actuators are designed into a relay to illustrate their use and performance. To design a cross-bar micro relay, two zipping actuators are combined with a curved bistable switch beam and two contacts.

(cont.) The micro relay is monolithically fabricated in silicon using deep reactive ion etching to move laterally in the wafer plane. Both actuators provided up to 10 mN of actuation force over their 80 [mu]m of stroke at 140 V, and toggle the bistable relay at a maximum rate of 160 Hz. Pullin voltage, actuation voltage and force-displacement measurements of the actuators and switch beam confirm theoretical expectations based on numerical, analytical and finite element analyses, after accounting for fabrication variations. The shortest pulse required to switch the relay is 400 [mu]s, and the time taken for the actuator to close the relay was approximately 3 ms. The relay was operated at 100 Hz for over 120 hours through more than 40 million cycles without any observed stiction or fracture fatigue. To achieve low contact resistance for a laterally-moving micro relay, wet anisotropically etched silicon [111] planes are developed to form relay contact surfaces that offer flat wiping surfaces and ease of thick metalization. Experimental contacts are fabricated and their average contact resistance is measured to be [approx.] 50 m[omega].

(cont.) A process plan is also proposed to combine the [111] plane contacts with the prior zipping actuators and the switch beam to build a micro relay with low contact resistance for power protection applications. The compliant starting zone concept can also be applied to vertically-moving MEMS devices. A MEMS valve is also designed using a zipping actuator having com- pliant starting zones. As another application of the zipping mechanism, a nonlinear spring is also presented and analyzed.