Characterization of Accelerated Fatigue in Thick Epi-Polysilicon Vacuum Encapsulated MEMS Resonators

Fatigue in thick (> 20 μm), epitaxially deposited polysilicon MEMS is characterized from 25°C and up to 250°C in an environment free of oxygen and humidity. This work is the first to report fatigue initiated in the native epi-polysilicon, free from any contributions due to oxide or environmental conditions. Fatigue was most prominent above stresses of 1.5 GPa at all temperatures. Resonant frequency shift is used to measure crack growth, and elevated temperatures increased the total frequency drift and frequency drift rates. Since the epi-polysilicon surfaces roughen due to grain boundary migration during fabrication, we conclude that epi-polysilicon fatigue can be attributed to a subcritical cracking mechanism that arises from surface roughness. Furthermore, the measured increases in the critical crack length before failure suggest that the fracture toughness increases at the elevated temperatures.

I. Introduction

Fatigue is broadly defined as cyclic mechanical loading causing progressive fracture over time, occurring at loads below the static strength of the material [1]. Fatigue is a common failure mechanism at the macro-scale particularly in the automotive, aerospace, and energy industries where there are dynamic loading conditions on structural materials, typically metals. Ductile materials, such as metals, are prone to fatigue, due in most part to intrinsic mechanisms that cause damage ahead of the crack tip. For example, dislocation motion causes blunting and re-sharpening of the crack tip in ductile materials [2].