Investigation of temporary stiction in poly-SiGe micromirror arrays

Stiction, permanent but also temporary, remains one of the most critical reliability concerns for micro-electro-mechanical systems (MEMS). In this paper, we present an investigation of temporary stiction of standard poly-SiGe MEMS. This is done by analyzing the hysteresis in the displacement-versus-voltage characteristics of electrostatically actuated micromirrors. The anomalous widening of the “pull-in window”, caused by stiction-induced delay of the pull-out, is used as an indication of the amount of mirror-to-surface stiction. Differences in the pull-out voltage of micromirrors within an array indicate varying levels of stiction from mirror to mirror. It is shown that a 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) self-assembled monolayer (SAM) can reduce this in-use stiction. Analytical and finite element methods are used to quantify the stiction forces. Also the impact of the relative humidity and of a vacuum thermal treatment is studied. The results show that humidity at the surface is the main cause of the stiction in these devices.