Response of Mems Devices under Shock Loads

There is strong experimental evidence for the existence of strange modes of failure of MEMS devices under shock. Such failures have not been explained with conventional models of MEMS. These failures are characterized by overlaps between moving microstructures and stationary electrodes, which cause electrical shorts. This work presents a model and simulation of MEMS devices under the combination of shock loads and electric actuation, which will shed the light on the influence of these forces on the pull-in instability. Our results indicate that the reported strange failures can be attributed to early dynamic pull-in instability. The results show that the combination of a shock load and an electric actuation makes the instability threshold much lower than the threshold predicted considering the effect of shock alone or electric actuation alone. Several results are presented showing the response of MEMS devices due to half-sine pulse, triangle pulse, and rectangular pulse shock loads of various durations and strengths. The effects of linear viscous damping and incompressible squeeze-film damping are also investigated. INTRODUCTION AND BACKGROUND The technology of microelectromechanical systems (MEMS) is now rapidly maturing and many MEMS devices are ready for marketing. Currently, the commercialization of MEMS is a major focus for engineers. One of the most critical issues affecting the commercialization of MEMS devices is their reliability under mechanical shock and impact. MEMS can be exposed to shock during fabrication, deployment, and operation. Examples of such conditions are dynamic loading in space applications and harsh environments in military applications [1]. Further, a crucial criterion for automotive and industrial applications is the survivability of portable devices containing MEMS when dropped on hard surfaces [2], which can induce significant shock loads. Such highly dynamic loads may lead to various damage mechanisms, such as forming of cracks and chipping of small fragments. Hence, there are increasing demands to improve the design of MEMS to withstand shock loads. MEMS devices typically employ capacitive sensing and/or actuation, in which one plate or electrode is actuated electrically and its motion is detected by capacitive changes. Electric actuation is the most used and preferred method of excitation and detection in MEMS for its simplicity, high efficiency, and low power consumption. There are numerous examples of MEMS devices, which rely on electric excitation and detection, such as comb-drive actuators, resonant microsensors, and RF MEMS switches. In this method, the driving load is simply the attractive force between two electrodes of a capacitor. The DC component applies an electrostatic force on the structure, thereby deflecting it to a new equilibrium position, while the AC component vibrates the structure around this equilibrium position. The combined electric load has an upper limit beyond which the mechanical restoring force of the structure can no longer resist its opposing electric force, thereby leading to the collapse of the structure. This structural instability phenomenon is known as `pull-in’. A key aim in the design of many MEMS devices is to tune the electric load away from the pull-in instability, in order to avoid failure of the device. Many studies have addressed the pull-in phenomenon and presented tools to predict its occurrence so that designers can avoid it [3-6]. However, these studies typically account for the