MEMS reliability in shock environments

Abstract

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...

Investigation of the response of microstructures under the combined effect of mechanical shock and electrostatic forces

“…Therefore, in the design of a MEMS device, both the electrostatic forces and the shock forces have to be taken into account, even if the microstructure undergoes small deflection and operates within a small range of the electrostatic force, to avoid this dynamic pull-in instability. This dynamic instability has been reported by Tanner et al [4] as a strange mode of failure, which is characterized by contacts and overlaps among the fingers and electrodes of the parallel-plate capacitors. Figure 5 illustrates the concept of this section.…”

“…Such highly dynamic loads may lead to various damage mechanisms, such as forming of cracks and chipping of small fragments. In MEMS, shock loads can cause microstructures, such as suspended microbeams, to hit the stationary electrodes underneath them causing stiction [3] and short circuit problems [4], and hence failure in the device's function. Unlike failure in large devices, failure in MEMS does not have to mean fracture of structures due to high stresses; it can occur through stiction and electric short circuits due to contacts between movable and stationary electrodes.…”

“…Tanner et al [4] tested MEMS microengines against shock pulses of various time durations and maximum amplitudes. The microengines employ comb-drive actuators, which are composed of folded springs, anchors and a series of comb fingers actuated by electrostatic forces.…”

“…The microengines employ comb-drive actuators, which are composed of folded springs, anchors and a series of comb fingers actuated by electrostatic forces. Tanner et al [4] observed a strange failure mode in the comb-drive actuators, where the comb fingers contact the ground plane resulting in electrical shorts. They calculated the maximum deflection of a comb finger at the shock level at which this strange failure mode was observed.…”

Reliability Model for MEMS Accelerometers

“…In [2][3], the MEMS material fatigue and aging under long-term cyclic loading has been discussed. In [4] [5], MEMS reliability under shock and vibration environments has been explored. In [6], efforts have been made to explore the physical mechanism of stiction in surface micromachining and its impact on MEMS reliability.…”