Surface roughness has a significant affect on adhesion. We used a single-asperity model to describe a smooth tip in contact with a rough surface and predicted that an optimal size of asperity will yield a minimum of adhesion. Experimentally, adhesive forces on silicon wafers with varying roughness were measured using AFM cantilevers with varying tip radii. It was found that minima do exist, and for all tip radii, the adhesion falls significantly for roughness greater than 1–2 nm and drops at higher roughness for larger tips. In addition to RMS roughness, the roughness exponent is another important parameter for the characterization of rough surfaces and its affect on adhesion was also investigated. We developed computer programs to simulate a set of fractal rough surfaces with differing roughness exponents. The adhesive forces between an AFM tip and the fractal surfaces were calculated and the adhesion was seen to decrease as the roughness exponent increases. This work should help minimize MEMS stiction and progress the understanding of nanoscale contact mechanics.

Liu, D.-L., Martin, J., & Burnham, N. A. (2008). Optimum Roughness for Minimum Adhesion. STLE/ASME 2008 International Joint Tribology Conference, 593–595.