Failure of microelectromechanical systems subjected to impulse loads

Understanding the mechanical response of microelectromechanical systems (MEMS) under impulse loading is a prerequisite for improved design criteria and device survivability under severe loading conditions. Microscale pad structures of three different heights manufactured by the Sandia SUMMiT™ IV process were tested at extreme accelerations generated by using a pulsed laser-loading set-up. The devices were subjected to impulsive loads of 40 ns in duration, which is of the order of wave transit times in the MEMS substrate and devices. The stresses in the device substrate were obtained with the aid of Michelson interferometry, while scanning electron microscopy was used to investigate device failure. The experimental findings were supported by a one-dimensional wave propagation analysis that explored the correlation between failure magnitude and device height. As predicted by the one-dimensional analysis, taller structures were more prone to failure compared with shorter ones for the same loading conditions. Different failure modes, such as delamination of multi-layered structures and material failure (fracture), occurred verifying that dynamic loading could lead to failure of MEMS devices. In order to gain detailed information on the stress state in the pads, three-dimensional finite element simulations were preformed focusing specifically at stress concentrations generated by the device geometry. The simulations accurately predicted the location of failure recorded in the experiments although it was seen that the details of failure initiation and progression were highly dependent on geometry.