IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Microelectromechanical Systems (MEMS) Thursday Sessions
       Session MM-ThM

Paper MM-ThM9
Stability of Alkylsilane Monolayer Films in Humid Environments

Thursday, November 1, 2001, 11:00 am, Room 130

Session: Characterization of MEMS Materials
Presenter: T.M. Mayer, Sandia National Laboratories
Authors: T.M. Mayer, Sandia National Laboratories
H.I. Kim, Sandia National Laboratories
M.G. Hankins, Sandia National Laboratories
M.P. de Boer, Sandia National Laboratories
Correspondent: Click to Email
Alkylsilane monolayer films on SiO@sub 2@ are used to prevent adhesion in micromechanical (MEMS) devices. We have studied the stability of these films in humid environments, where degradation may lead to loss of hydrophobic character, water adsorption, and adhesion of MEMS components by capillary condensation. In this work we study silane monolayer films with both fluorocarbon and hydrocarbon side chains, deposited by both solution and chemical vapor deposition methods. In-situ ellipsometry and interfacial force microscopy measurements examine water vapor adsorption and its effect on adhesion and friction. Ex-situ atomic force microscopy and x-ray reflectivity measurements examine the morphology and density of the films before and after exposure. We find that chemical binding of the film to the surface is critical for its stability. Silanol films are not strongly bound to the surface and exhibit substantial water adsorption. This is accompanied by an irreversible increase in friction when probed with a similarly functionalized tip. In the presence of high humidity at room temperature, the silanol film restructures to form small droplets on the surface, leading to increased adhesion in cantilever beam MEMS test structures. In contrast, silanol films that have been annealed to react with surface hydroxyls are strongly bound to the surface and display negligible water adsorption, no effect on adhesion or friction, and no surface restructuring after exposure to high humidity (>80% RH) for short periods (10 hr) at room temperature. Stability of these films after more severe exposure (longer times at higher temperature), mechanisms of degradation, and long-term effects on the performance and reliability of MEMS devices will be addressed. Sandia is a multiprogram laboratory operated by Sandia Corp., a Lockheed Martin Company, for the U. S. Dept. of Energy under contract DE-AC04-94AL85000.