Adhesion and friction forces in microelectromechanical systems: mechanisms, measurement, surface modification techniques, and adhesion theory

Microscopic devices capable of performing sensing, actuation, and control tasks, known as microelectromechanical systems (MEMS), are expected to lead to new technologies with profound impact on science and engineering. The multidisciplinary nature of these miniaturized devices has necessitated the integration of basic knowledge of various mechanical, electrical, chemical, and thermal phenomena encountered at the microscale, and the discovery of novel methods for fabricating versatile micromachines. As the growth of MEMS accelerates, reliability and long-term durability issues are expected to assume even greater importance. In view of the low stiffness and large surface-to-volume ratio of micromachine devices, high interfacial attractive (adhesion) forces often lead to permanent surface attachment, a phenomenon known as stiction. Representative examples where surface interaction influences micromachine functionality are presented to elucidate the importance of adhesion and friction forces at the microdevice level, followed by a description of special microstructures for static and dynamic friction testing under conditions typical of MEMS devices. Surface texturing, fabrication of stand-off microfeatures, and alteration of the surface chemical behavior by adsorption of low surface energy self-assembled monolayers are shown to be effective surface modification techniques for controlling adhesion and friction at MEMS interfaces. Simulation results from a generalized adhesion theory are used to interpret the contribution of capillary, van der Waals, and electrostatic attractive forces to the total interfacial force. The theory is based on the surface topography description by fractal geometry and uses elastic-plastic constitutive models for the deformation of asperity microcontacts derived from finite element simulation results. Analytical results demonstrate the profound effect of surface roughness on the magnitude of adhesion and repulsive forces at polysilicon interfaces.