The effect of water on friction of MEMS

Water plays a significant role in the performance of micro electro mechanical systems (MEMS). A special apparatus was employed to investigate the adhesive friction attributed to water at low coverages, i.e., in the nanometer range, where friction and adhesion are a function of the water layer thickness. In addition, the history of the sample surface also plays a significant role. The friction forces associated with hydrophobic samples are negligibly affected by humidity changes, whereas those of hydrophilic samples show a strong dependence. Sample coverage and the friction force are also influenced by the sample temperature. High forces were measured for high humidities at low sample temperatures, for hydrophilic silicon. In contrast, hydrophobic samples show an increase of the friction force with increasing temperature. Experiments performed under high vacuum demonstrated that decreasing the water layer thickness by desorption decreases the friction force with several sub-minima and sub-maxima. The friction signal is accompanied by sudden fluctuations. For submonolayer coverage the friction force starts to increase.