Surface Treatments for Modifying the Tribological Behavior of Microsystems (Keynote)

The surfaces of microelectromechanical systems (MEMS) that have impacting and sliding contacts are frequently modified by coatings to improve the tribological behavior of the structural material. In silicon microsystems, chemisorbed hydrocarbon or fluorocarbon monolayers are commonly used to modify the surface energy of the oxidized silicon to resist water adsorption and adhesion. Thin conformal solid films can also be employed to improve the wear resistance of the structural material. The requirements of coatings for microsystems will be discussed in terms of tribological behavior, processing, operation and aging characteristics. Although contacting surfaces of polycrystalline silicon have arithmetic roughness of less than 50 nm, contact forces below 1 mN prescribe that real contact occurs at only a few asperities. Defects in the coatings, and interfaces that are deeply buried present opportunities for adsorption of reactive species and degradation of the coating over time. An understanding of the mechanical and chemical degradation mechanisms of tribological coatings is of primary importance for the reliable operation of microsystems. At the same time, relating composition and structure of the contacting surfaces to the tribological behavior is quite challenging due to the limited size and number of real contact spots. A variety of surface treatments for silicon microsystems have been investigated, including chemisorbed monolayers, vapor phase lubricants and hard coatings. The tribological behavior of polycrystalline silicon containing chemisorbed monolayers is dramatically influenced by water vapor in the environment during fabrication processes, storage or operation. Surface analytical tools and friction measurements using micromachined tribometers to understand the impact of coating degradation on the tribological behavior of microsystems are illustrated.