Reducing the effects of adhesion and friction in microelectromechanical systems (MEMSs) through surface roughening: Comparison between theory and experiments
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Due to the large surface-to-volume ratios and the low loads encountered in microelectromechanical systems (MEMSs), the surface forces become important and may lead to permanent adhesion and high friction between near contacting and contacting surfaces. The effect of these forces can be reduced through surface texturing (roughening) at the contact interface. Moreover, modifying the distribution of the contacting surface asperities so that it becomes positively asymmetric (unbalance between the peak and valley heights) and as peaky as possible (making slender asperities) reduces these forces even further. In the current study, the effects of these parameters, i.e., roughness, asymmetry, and peakiness, on reducing the adhesion and friction in polycrystalline silicon (also referred as polysilicon) MEMS surfaces, were theoretically and experimentally investigated. Polysilicon films with different levels of roughness, asymmetry, and peakiness were fabricated. The roughness characteristics of these films were used in a continuum-based micromechanics model to predict the level of adhesion and friction in actual MEMS devices. Experiments were also conducted to evaluate the adhesion pull-off force and friction coefficient associated with these films. It is found, both experimentally and theoretically, that the adhesion pull-off force and friction coefficient can be reduced by an order of magnitude by increasing the roughness, asymmetry, and peakiness of the contacting surfaces under low external normal forces. Stick-slip behavior, which may be indicative of the presence of adhesive forces, also reduces considerably with the increase of these parameters. Lastly, good agreement was found between simulations and experimental results. Thus, 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 environmental conditions. 2005 American Institute of Physics.
