An electrothermally-actuated bistable MEMS relay for power applications

Abstract

This thesis first develops a bistable mechanism that does not rely on internal stress or hinges for its bistability, which is then combined with transient electrothermal actuation and contact structure to develop a MEMS relay for power switching. The relay components, fabricated by a through etch of a silicon wafer using deep reactive ion etch (DRIE), move laterally in the plane of the wafer. The synthesis, analysis, design, fabrication and testing of the relay are all described in this thesis. The bistable mechanism comprises two parallel cosine curved clamped-clamped beams that are also clamped together at their centers. If designed properly it exhibits a second stable deflected shape that is nearly a mirror image of its as-fabricated shape mirrored through the centerline connecting its clamped ends. Both theoretical and finite-element analyses are applied to the modeling, design and optimization of the force-displacement characteristic of the curved beam mechanism. The analyses results agree well with experimental measurement after accounting for fabrication variations. If fabricated properly, the mechanism is mechanically robust; some mechanisms have been switched between their bistable states one million times without sign of failure. The bistable mechanism is combined with two contacts to form a crossbar relay. The contact structure is designed with flat shape and additional compliance; and proper metalization process is developed for its DRIE etched sidewalls; both contributing to a more reliable and low resistance contact. Because of bistability this relay requires no actuation power in either its on or off states. Therefore, transient electrothermal actuators to switch its states are designed, partially with a mechanical model developed for cantilevers subjected to both lateral and axial forces.

(cont.) These actuators typically require 1-ms 50-V pulses to switch the relay. In the off state, the relay contacts stand off more than 200 V. In the on state the best relay contacts exhibit a total resistance of 60 mfQ and a current carrying capacity of 3 A. The relay can switch at a maximum 5 Hz rate. These characteristics make it a good candidate for power protection applications.