Materials selection and design of microelectrothermal bimaterial actuators

Abstract

A common form of MEMS actuator is a thermally actuated bimaterial, which is easy to fabricate by surface micromachining and permits out of plane actuation, which is otherwise difficult to achieve. This paper presents an analytical framework for the design of such microelectrothermal bimaterial actuators. Mechanics relationships for a cantilever bimaterial strip subjected to a uniform temperature were applied to obtain expressions for performance metrics for the actuator, i.e., maximum work/volume, blocked (force) moment, and free-end (displacement) slope. Results from finite-element analysis and closed form relations agree well to within 1%. The optimal performance for a given pair of materials and the corresponding thickness ratio were determined. Contours of equal performance corresponding to commonly used substrates (e.g., Si, SiO2) were plotted in the domain of governing material properties (thermal expansion coefficient and Young's modulus) to identify candidate materials for further development. These results and the accompanying methodology provide a rational basis for comparing the suitability of "standard" materials for microelectrothermal actuators, as well as identifying materials that might be suitable for further research