Adhesion and contact modeling and experiments in microelectromechanical systems including roughness effects

The intermolecular adhesive forces in microelectromechanical systems (MEMS) applications are very significant and can hinder normal operation of sensors and actuators as well as micro-engines where catastrophic adhesion and high friction could be promoted. It has been experimentally shown that surface texturing (roughening) decreases the effect of these forces. In this paper, a model that predicts the effects of roughness, on the adhesion and contact forces in MEMS interfaces is presented. The three key parameters used to characterize the roughness, the asymmetry and the flatness of a surface topography are the root-mean-square roughness (RMS), skewness and kurtosis, respectively. It is predicted that surfaces with high RMS, high kurtosis and positive skewness exhibit lower adhesion and are thus less prone to collapsing when they come into contact or near contact. Moreover, polysilicon films with different levels of roughness, asymmetry and peakiness (sharpness) were fabricated. Experiments were conducted to evaluate the adhesive pull-off forces associated with these films. The roughness characteristics of these films were also used in the model to predict the adhesive pull-off forces. Good agreement was obtained between the theoretical and experimental results. Such a model could be used to determine the critical characteristics of a microstructure prior to fabrication to prevent adhesion and lower friction in terms of surface roughness, mechanical properties and environment.