Cyclic fatigue and creep of electroformed micromachines

Abstract

Resonant micromachines have been used to make accelerometers, angular rate sensors, voltage controlled oscillators, pressure, flow and chemical sensors. Resonant microsystems are in constant motion and so present new challenges in the area of reliability. A variety of materials have been employed to fabricate resonant device including semiconductors, quartz and electroformed metals. Material changes due to fatigue can occur in metallic structures under cyclic load. For many materials, there exists a fatigue limit below which a cyclic load will not lead to failure. Fatigue limits on the microlevel have not been verified. This paper will explore the correlation between macro and micro fatigue limits. Creep is another metallurgical effect, which is understood at the macro level, but has not been studied extensively in microsystems. Sensor output shift can be affected by a variety of factors, including traditional material creep. Electrostatic attraction and gas adsorption/desorption from microcavity surfaces have been found to be sources of parametric drift in this study. Charge build-up at the surface of the CMOS nitride passivation layer has been found to lead to stiction under extended high temperature operational life test conditions. This paper will illustrate how proper circuit and micromachine design, material selection and processing can allow electroformed materials to be used, without creep or cyclic fatigue affecting the performance of the resonant microsystem.