Compressive Overload Effects on the Resonance Frequency of Monocrystalline Silicon Microbeams and Implications for Fatigue Mechanisms

Abstract

Previous work demonstrated the marked effect of load ratio on the fatigue life of both mono- and poly-crystalline silicon thin films, with decreasing fatigue lives for more negative load ratios. In this paper, we investigated the role of a single compressive load on the stiffness (and therefore flaw population) of a monocrystalline Si microbeam. Specifically, we measured the resonance frequency (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ~ 40 kHz) of a microresonator with ppm-level resolution before and after applying a compressive stress ranging from 1.5 to 5.5 GPa. The resolution in J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> measurements is enough to capture the formation of subcritical penny-shaped cracks (lateral dimensions tens of nanometers), or the extension of existing cracks. No significant decrease in f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> was measured within the resolution of the technique. This result indicates that the mechanism known for the fatigue of notched ceramics under cyclic compressive loads at the bulk scale, relying on the formation of cracks upon the first compressive cycle, does not apply to the fatigue of silicon thin films. Instead of a decrease in J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> , the results highlighted an increase in J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> in some specimens, which is interpreted as compression-induced crack healing (as opposed to the well documented thermally induced healing). A review of the proposed fatigue mechanisms for Si thin films is also provided.