MEMS reliability in a vibration environment

Abstract

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...

ESD testing and combdrive snap-in in a MEMS tunable grating under shock and vibration

“…Reliability estimation of MEMS devices has been mainly done thorough extensive Electrostatic discharge (ESD) tests 1,2 , shock and vibration tests 3,4 , fatigue and creep tests 5,6 , and aging tests through rapid thermal cycling. These tests have so far mainly been used to qualify devices.…”

Vibration and shock reliability of MEMS: modeling and experimental validation

“…There have also been various attempts to study the shock and vibration reliability of particular MEMS devices at Sandia National Laboratories [14,15] and at other laboratories [16]. These methods provide a qualitative insight into the conditions that may cause failure but are not useful for making quantitative predictions, as they neither account for damping, nor for electrostatic spring softening.…”

Mechanical fatigue analysis of gold microbeams for RF-MEMS applications by pull-in voltage monitoring

“…Surface properties resisting to wear and stiction must be provided also. Some works present in the literature propose a failure model at the system level: examples are provided for complicated micro-engines [2,3], for wireless strain sensing systems with a high reliability package [4], for the thermo-mechanical reliability of technological packaging [5], etc.…”