Hydrogen gas diffusion influences cavity pressure of encapsulated inertial sensors

Abstract

MEMS inertial sensors are oscillatory electro-mechanical devices that detect relative motion in an inertial frame. Frictional losses in these systems, expressed as a quality factor Q, directly have bearing on their absolute measurement accuracy, and hence on the duration of an acceptably accurate response, for example during blind navigation. One of the sources of loss of quality factor is gas friction, which is reduced by operating such a device in an evacuated cavity. Because of the space constraints of many applications, inertial sensor packaging is kept slender and as close as possible to a device's functional parts. Through accelerated environmental tests, our measurements suggest that hydrogen gas molecules can diffuse out of the sensor cavity and change the quality factor. We propose a novel wafer-cutting methodology which allows to quantify diffusion paths through all micromechanical layers of a wafer, for the first time on wafer level, by facilitating different routes from the inner cavity to the ambient. We used the measurements of several hundred gyroscopes to validate a simulation model and protocol, so that cavity pressure changes over time can be numerically predicted. This is the first report on the systematic characterization of gas diffusion from micromachined cavities considering the influence of wafer dicing. • Hydrogen gas molecules can diffuse out of MEMS gyroscopes and change the Q factor. • Only the SiO2 layers of the MEMS wafer stack are relevant for hydrogen diffusion. • Gas diffusion impacts long-term stability of damping atmosphere of inertial sensors. • SiO2 layer interruption increases longterm stability of MEMS sensor cavity pressure.