Dry stiction of micro structures - Theory and experiment

It is well known that compliant freestanding microstructures often stick to the underlying substrate due to capillary pull during wet processing. Hence, it is generally believed that dry processing, which involves only gases, avoids stiction. Contrary to this expectation, here we show experimentally that stiction may also occur during dry processing. We investigate both experimentally and theoretically possible origins of the force that brings the microstructures into contact leading to stiction in a dry environment. The study suggests that aerodynamic drag is the primary force that is responsible for dry stiction. It comes into play during venting or purging with nitrogen the vacuum chambers (reactive ion etcher (RIE)) with compliant microstructures. During the venting process, gases rush into the chamber through an inlet setting up local flows inside the chamber, and subjecting the freestanding microstructures to aerodynamic drag. If the structures are close to one another, such as two parallel structural beams anchored at the ends (studied in detail in this paper), the beam facing the flow shields the downstream beam from the flow. Thus, the upstream beam experiences a larger drag compared to the shielded one, and the gap between them reduces. Smaller is the gap, higher is the shielding effect. The upstream beam may contact the downstream one, and depending on the surface properties of the beams (such as pure aluminium surface prior to oxidation), they may stick to one another through a zipping action. Dry stiction may thus be avoided by slowing down the venting or purging process of RIE chambers with compliant microstructures.