Chemical vapor deposition of fluoroalkylsilane monolayer films for adhesion control in microelectromechanical systems

Abstract

We have developed a new process for applying a hydrophobic, low adhesion energy coating to microelectromechanical (MEMS) devices. Monolayer films are synthesized from tridecafluoro-1,1,2,2- tetrahydrooctyltrichlorosilane (FOTS) and water vapor in a low-pressure chemical vapor deposition process at room temperature. Film thickness is self-limiting by virtue of the inability of precursors to stick to the fluorocarbon surface of the film once it has formed. We have measured film densities of ∼3 molecules nm2 and film thickness of ∼1 nm. Films are hydrophobic, with a water contact angle >110°. We have also incorporated an in situ downstream microwave plasma cleaning process, which provides a clean, reproducible oxide surface prior to film deposition. Adhesion tests on coated and uncoated MEMS test structures demonstrate superior performance of the FOTS coatings. Cleaned, uncoated cantilever beam structures exhibit high adhesion energies in a high humidity environment. An adhesion energy of 100 mJ m−2 is observed after exposure to >90% relative humidity. Fluoroalkylsilane coated beams exhibit negligible adhesion at low humidity and ⩽20 μJ m−2 adhesion energy at >90% relative humidity. No obvious film degradation was observed for films exposed to >90% relative humidity at room temperature for ∼24 h.