Materials selection for microfabricated electrostatic actuators

Microfabricated electrostatic actuators are employed in a wide variety of microelectromechanical systems (MEMS) for applications ranging from relays and switches to valves and displays. The rapid expansion of the set of materials available to MEMS designers motivates the need for a systematic and rational approach toward the selection of materials for electrostatic actuators. We apply the Ashby methodology to accomplish such a selection. The primary performance and reliability metrics considered are the actuation voltage, speed of actuation, stroke (or displacement), actuation force, stored energy, electrical resistivity, mechanical quality factor, and resistance to fracture, fatigue, shock, and stiction. The materials properties governing these parameters are the Young’s modulus, density, fracture strength, intrinsic residual stress, resistivity, and intrinsic material damping. Materials indices are formulated by appropriate combination of these properties and a graphical procedure for materials selection is presented. Our analysis suggests that diamond, alumina, silicon carbide, silicon nitride, and silicon are excellent candidates for high-speed, high-force actuators; polymers for large-displacement, low actuation-voltage devices; and aluminum for low-electrical resistivity, low actuation-voltage and high-speed actuators. The properties of composite actuator structures are briefly discussed.