Effect of Residual Stress on the Mechanical Behaviour of Microswitches at Pull‐In

Abstract

Abstract: Out‐of‐plane bending tests on microswitches are performed and some numerical models proposed by the authors are applied in this study. It is known that, when electric field excites microcantilever and double clamped microbeam bending out of the wafer plane, very often residual strain and stress, respectively, occur. Microswitch mechanical behaviour is consequently affected. Pull‐in voltage, at which the electrostatic force prevails on the elastic restoring force, significantly changes. To effectively model this microsystem initial strain and stress must be evaluated. In particular, initial curvature due to the microfabrication process has to be introduced into the numerical model of microcantilever, while, in double clamped microbeams, the pre‐loading tensile stress has to be added. Geometrical non‐linearity sometimes occurs, for large displacement in cantilevers and because of the axial‐flexural coupling in microbridges. In this paper the finite element method (FEM) approaches proposed by the authors are applied to include both the geometrical and electromechanical non‐linearities and to evaluate the residual stress in microswitches. A reverse analysis is implemented. Axial pre‐stress is computed by means of the measured value of pull‐in voltage. Some preliminary criteria to correlate the residual stress occurring and the microfabrication process are even drawn. Microdevice location on the wafer plane and residual effects of microfabrication are found interdepending, although a statistic analysis of the whole problem could not yet be completed.