Fatigue of directly wafer-bonded silicon under static and cyclic loading

Fatigue tests were performed to investigate the reliability properties of wafer-bonded single crystalline silicon exposed to static or cyclic mechanical loading. A distinct decrease of strength with increasing load duration or cycle number was found which limited the lifetime of mechanically stressed wafer-bonded components. The occurrence of fatigue is related to siloxane bonds in the bonded interface between the silicon wafers. It was shown that fatigue is absent either if siloxane bonds are not present in the bonded interface or if local areas of pure silicon bonds were formed between the silicon wafers. As a consequence of this fatigue phenomenon, loaded wafer-bonded silicon sensors and actuators may suddenly fail during application. Therefore, appropriate fracture mechanical techniques were developed to predict either the time-to-failure or the cycles-to-failure for a given loading situation. These concepts allow for the optimization of the device layout with respect to long-term reliability, which can reduce the development time and costs.