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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MicroE!ectroMechanicrd Systems (MEMS) were subjected to a vibration environment that had a peak acceleration of 120g and spanned frequencies from 20 to 2000 Hz. The device chosen for this test was a surface-micromachined microengine because it possesses many elements (springs, gears, rubbing surfaces) that may be susceptible to vibration. The microengines were unpowered during the test. We observed 2 vibration-related failures and 3 electrical failures out of 22 microengines tested. Surprisingly, the electrical failures also arose in four microengines in our control group indicating that they were not vibration related. Failure analysis revealed that the electrical failures were due to shorting of stationary comb fingers to the ground plane. INTRODUCnON An element of the success of MicroElectroMechanical Systems (MEMS) as they reach commercialization depends on reliability studies and predictions. MEMS are typically classified as sensors or actuators. Brown et al. performed extensive experiments on MEMS acceleration sensors including shock, vibration, temperature cycling, and flight tests on artillery projectiles [1]. He 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 may be at risk. This was demonstrated in reports on humidity effects and wear [2, 3]. Microactuators are used to drive many different types of devices from gear trains to pop-up mirrors [4]. During vibration experiments by Lee et al. [5, 6], reflected optical patterns from a clamped micromirror were monitored and were determined to be error free over a range of frequencies from 200 Hz to 10 kHz. They claim no effect from vibration of the clamped mirror on this scale. But what happens when an actuator is not clamped and is free to move? Vibration causes motion in the actuator promoting the surfaces to rub and thus mimics normal operation. In addition, vibration perpendicular to the normal operation direction will impact guides or constraints. Both of these effects can generate wear debris leading to failure. One of the first experiments [7] to show wear as a dominant failure mechanism during operation ran polysilicon mi-croturbines [8] and gears at rotational speeds up to 600,000 rpm. A focused air jet directed at the turbine induced the rotation. Previous experiments [9] on the lifetime of the Sandia-designed surface-micromachined rnicroengine [10] investigating frequency dependence revealed wear as the dominant failure mechanism. We subjected our MEMS actuator to vibration. The microengine has springs that flex, guides that can be impacted, and surfaces that rub maldng it a good candidate for vibration studies. The resonant frequency of the microengine (about 1500 Hz) is in the range of our system-requirement frequencies, which may be of concern. The objective was to determine any susceptibility of the microengine to vibration with the understanding that the results would apply to a broader ran...

Electrostatic discharge (ESD) and electrical overstress (EOS) damage of Micro-Electro-MechanicalSystems (MEMS) Fig. 1 [4].

Two major problems associated with Si-based MEMS (MicroE1ectroMechanical Systems) devices are stiction and wear. Surface modifications are needed to reduce both adhesion and friction in rnicromechanical structures to solve these problems. In this paper, we will present a CVD (Chemical Vapor Deposition) process that selectively coats MEMS devices with tungsten and significantly enhances device durability. Tungsten CVD is used in the integrated-circuit industry, which makes this approach manufacturable. This selective deposition process results in a very conformal coating and can potentially address both stiction and wear problems confronting MEMS processing. The selective deposition of tungsten is accomplished through the silicon reduction of WFc. The self-limiting nature of the process ensures consistent process control. The tungsten is deposited after the removal of the sacrificial oxides to minimize stress and process integration problems. The tungsten coating adheres well and is hard and conducting, which enhances performance for numerous devices. Furthermore, since the deposited tungsten infiltrates under adhered silicon parts and the volume of W deposited is less than the amount of Si consumed, it appears to be possible to release adhered parts that are contacted over small areas such as dimples. The wear resistance of tungsten coated parts has been shown to be significantly improved by rnicroengine test structures.