Wet-etch release process for silicon-micromachined structures using polystyrene microspheres for improved yield

One of the final steps in fabricating microelectromechanical devices often involves a liquid-etch release process. Capillary forces during the liquid evaporation stage after the wet-etch process can pull two surfaces together resulting in adhesion of suspended microstructures to the supporting substrate. This release-related adhesion can greatly reduce yields. In this paper, we present a wet-etch release method that uses polystyrene microspheres in the final rinse liquid. The polystyrene microspheres act as physical barriers between the substrate and suspended microstructures during the final liquid evaporation phase. A plasma-ashing process is utilized to completely remove the polystyrene microspheres from the microstructure surfaces. Using this process, release yields >90% were achieved, as compared to yields of 20-50% when the polystyrene microspheres were not included. This release process is inexpensive, easy to implement, and is effective for both single-crystal and polycrystalline silicon MEMS devices.