Alkoxyl monolayers as anti-stiction coatings in Si-based MEMS devices

Abstract

Reducing surface energy is key to the success of many microelectromechanical systems (MEMS). In this report we present a strategy for the efficient assembly of alkoxyl monolayers onto a silicon surface to control surface energy. This is achieved by an all-liquid process in which the hydrogen terminated silicon surface resulting from aqueous HF etching is coated with a close-packed alkoxyl monolayer. The adhesion to silicon surface is reduced by a factor of 40 by the monolayer coating and friction coefficient of the coated surface is only 10-2. These coatings are successfully implemented in a model MEMS structure: cantilever beam array (CBA). Release-stiction is eliminated for polycrystalline silicon beams with a thickness of only 2 μm but with lengths up to 2 mm. Electrostatic actuation of coated beams in a controlled environment shows that the monolayer coating prevents in-use stiction at relative humidity as high as 90%.