Thermo- and Electromechanical Behavior of Thin-Film Micro and Nanostructures

Abstract

Applications using thin-film micro- and nanomechanical structures for actuation and sensing require the coupling of energy between various physical domains. This chapter focuses on two important couplings: thermomechanics and electromechanics. Thermomechanical phenomena is considered in Sect. 54.1, where we describe broad aspects of the deformation characteristics and stress states that arise when dealing with a large class of thin-film microstructures. These include the origin of stresses in multilayer films and their qualitative evolution through processing and release from the substrate. A basic framework is described for the analysis of the thermomechanics of multilayer films, emphasizing the linear response. Issues of geometric and material nonlinearity are then taken up, and equal emphasis is put on the generality of the analysis approach and specific applications. As much as possible, we show comparisons between theoretical predictions and companion experimental results. A common use of electromechanics in microsystems involves the application of an electric potential between two electrodes where one is fixed and the other is connected to a deformable elastic structure. The electric potential produces an electric field and an associated electrostatic force that deforms the structure, and in turn alters the electrostatic force, resulting in fully-coupled nonlinear behavior. At some point an instability can occur where the deformable structure snaps into contact with the fixed electrode. This phenomena, called pull-in, is often used for switching applications. In Sect. 54.2 we describe the basic electromechanical phenomena using a parallel-plate electrostatic actuator as a reference. We discuss many important phenomena including pull-in, external forcing, stabilization, time response, the effects of dielectric charging, and breakdown of gases in small gaps. We address these phenomena for a wide range of micromechanical structures including cantilevered beams and plates, torsionally suspended plates, and zipper actuators with curved electrodes.