Alkyltrichlorosilane-based self-assembled monolayer films for stiction reduction in silicon micromachines

We have investigated the potential of self-assembled monolayer (SAM) coatings for the purpose of adhesion reduction in microelectromechanical systems (MEMS). Two types of SAM coatings, derived from the precursor molecules octadecyltrichlorosilane [CH₃ (CH₂)₁₇SiCl₃, OTS] and 1H,1H,2H,2H-perfluorodecyltrichlorosilane [CF₃(CF₂)₇(CH₂)₂ SiCl₃, FDTS], were applied to polycrystalline silicon microstructures in a liquid-based process. Due to the hydrophobicity of these coatings, the water capillary forces responsible for the phenomenon known as release-related stiction are eliminated, and SAM-coated cantilever beams 2 μm thick, 2 μm above the substrate, and up to 2 mm in length emerge dry and free standing when removed from the final water rinse. The effects of SAM coating on adhesion encountered during device operation, termed in-use stiction, were characterized using arrays of cantilever beams of varying lengths. Structures made with a polycrystalline silicon of 3-nm rms roughness gave apparent works of adhesion of 30 and 8 μJ/m² for the OTS and FDTS SAM coatings, respectively, in comparison to 56 mJ/m² for standard oxide-coated structures. These results demonstrate that OTS coating reduces adhesion by more than three orders of magnitude over the conventional process and that the fluorinated SAM can lessen it further by four times. With regard to thermal stability, both SAM coatings can withstand heat treatment for 5 min at 450°C in an N₂ ambient. In air, the OTS film begins to degrade at 150°C while the fluorinated coating remains intact up to 400°C. Therefore, both types of SAM coatings are compatible with several MEMS packaging techniques, with the FDTS monolayers exhibiting superior stiction and thermal stability properties to those derived from OTS. Furthermore, the FDTS formation does not require any chlorinated solvents such as carbon tetrachloride, which has been banned from industrial use, making the latter coating an industrially viable antistiction treatment