Strategies and techniques for fabricating MEMS bistable thermal actuators.

Abstract

Bistable elements are beginning to appear in the field of MEMS as they allow engineers to design sensors and actuators which require no electrical power and possess mechanical memory. This research focuses on the development of novel strategies and techniques for fabricating MEMS bistable structures to serve as no electrical power thermal actuators. Two parallel strategies were explored for the design and fabrication of the critical bistable element. Both strategies involved an extensive material study on candidate thin film materials to determine their temperature coefficient of expansion and as-deposited internal stress properties. Materials investigated included titanium tungsten, Invar, silicon nitride and amorphous silicon deposited using either sputtering or PECVD. Deposition parameters were experimentally determined to produce tensile, compressive and stress-free films. A full set of graphs are presented. To address the 3D MEMS topology challenge required for bistability, this research explored two different strategies for fabricating 3D non-planar hemispherical dome structures using minimal processing steps. The first approach used the thermal/chemical reflow of resist, along with traditional binary lithography with a single photomask. Specific thermal/chemical reflow conditions were experimentally developed to produce hemispherical dome over a wide range. The second approach introduced a novel maskless procedure for fabricating the dome using grayscale lithography. After evaluating the above results, it was decided to use engineered compressive stress in released thin film sandwiches to form the 3D dome structures required for bistable actuation. Three different types of released multi-layer diaphragms were studied: 1) oxide-polyimide diaphragms, 2) oxide-aluminum diaphragms, and 3) oxide-aluminum-polyimide diaphragms.