In order to determine the susceptibility of our MEMS (MicroElectroMechartical Systems) devices to shock, tests were performed using haversine shock pulses with widths of 1 to 0.2 ms in the range from 500g to 40,000g. We chose a surface-rnicromachined microengine because it has all the components needed for evaluation: springs that flex, gears that are anchored, arrd clamps and spring stops to maintain alignment. The microengines, which were unpowered for the tests, performed quite well at most shock levels with a majority functioning after the impact.Debris from the die edges moved at levels greater than 4,000g causing shorts in the actuators and posing reliability concerns. The coupling agent used to prevent stiction in the MEMS release weakened the die-attach bond, which produced failures at 10,000g and above. At 20,000g we began to observe structural damage in some of the thin flexures and 2.5-micron diameter pin joints.We observed electrical failures caused by the movement of debris. Additionally, we observed a new failure mode where stationary comb fingers contact the ground plane resulting in electrical shorts. These new failures were observed in our control group indicating that they were not shock related. INTRODUC~ONReliability studies and predictions are becoming crucial to the success of MEMS as they reach commercialization. Cunningham et al. has addressed the issue of shock robustness in silicon microstructure [1]. They evaluated different microbeam designs and found that those with reduced stress distributions were more robust to the effects of shock. Brown et al. performed extensive experiments on' MEMS sensors, including shock, vibration, temperature cycling, and flight tests on artillery projectiles [2]. They saw promising results on automobile-grade accelerometers. However, sensors differ from microactuators in that they do not have rubbing surfaces. Surfaces in intimate contact during the environmental test maybe at risk. This was demonstrated in reports on humidity effects and wear [3, 4].Microacttrators are used to drive many different types of devices from gear trains to pop-up mirrors [5]. Microacttrators are typically complex with beams, comb fingers, linkages, gears, and springs. Each of these elements could be damaged by a shock impact. The objective of this study was to determine what elements, if any, of the microengine are susceptible to shock, with the understanding that the results could be applied to other MEMS actuators. EXPERIMENTAL APPROACHThis study used the electrostatically driven microactuator (microengine) developed at Sandia National Laboratories [6]. The rnicroengine consists of orthogonal linear comb drive actuators mechanically connected to a rotating gear as seen in Figure 1. By applying voltages, the linear displacement of the comb drives was transformed into circular motion. The X and Y linkage arms are connected to the gear via a pin joint. The gear rotates about a hub, which is anchored to the substrate.It was our intention to perform experiments with highe...

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Humidity is shown to be a strong factor in the wear of rubbing surfaces in polysilicon micromachines. We demonstrate that very low humidity can lead to very high wear without a significant change in reliability. We show that the volume of wear debris generated is a function of the humidity in an air environment. As the humidity decreases, the wear debris generated increases. For the higher humidity levels, the formation of surface hydroxides may act as a lubricant.The dominant failure mechanism has been identified as wear. The wear debris has been identified as amorphous oxidized silicon. Large slivers (approximately 1 micron in length) of debris observed at the low humidity level were also amorphous oxidized silicon. Using transmission electron microscopy (TEM), we observed that the wear debris forms spherical and rod-like shapes.We compared two surface treatment processes: a fluorinated silane chain, (FTS) and supercritical C 0 2 dried (SCCOJ. The microengines using the SCC02 process were found to be less reliable than those released with the U S process under two humidity levels.

A~STRACTExperiments have been performed on surface micromachined microengines driving load gears to determine the effect of the rotation frequency on median cycles to failure. We did observe a frequency dependence and have developed a model based on fundamental wear mechanisms and forces exhibited in resonant mechanical systems. Stressing loaded microengines caused observable wear in the rotating joints and in a few instances lead to fracture of the pin joint in the drive gear.

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