Surface engineering and microtribology for microelectromechanical systems

Abstract

Micromachines, particularly surface micromachines, often include smooth and chemically active surfaces. Because the kinetic energies, start-up forces and torques involved in their operation, and hence available to overcome retarding forces, are necessarily small, surface effects will be critical whenever micromachine contact occurs, whether unintentionally or as part of normal operation. Consequently, basic knowledge of the surface topography characteristics and microscale tribological phenomena arising at micromachine interfaces is of paramount importance to the reliability and robustness of microelectromechanical systems (MEMS). In view of the very small masses and tight tolerances, smooth surfaces, and light loads, characterization of chemomechanical surface interactions must be performed at the microscale. Microtribology is an emerging field dealing with friction and wear phenomena occurring at micrometer scales. Recent developments in this field have begun to lend valuable insight into surface interaction and material property characterization on scales relevant to MEMS. In this publication, the emphasis is on the analysis of various surface micromechanisms, such as solid bridging, liquid meniscus formation, van der Waals force, and electrostatic charging, and the significance of surface roughness and material properties. Experimental and theoretical results for the stiffness of silicon micromachines needed to overcome stiction are compared for different surface roughnesses. The fabrication, basic operation features, and measuring capabilities of microstructures designed specifically to perform microscale tribotesting under controlled conditions are presented. The efficacy of different surface engineering techniques, such as formation of standoff bumps on one of the countersurfaces, roughening (texturing), and surface chemistry modification (e.g. self-assembled monolayers), to reduce high adhesion forces at MEMS interfaces is interpreted in light of analytical and experimental results. It is demonstrated that surface engineering and modification of physicochemical surface properties are effective means of enhancing the reliability and performance of microsystems.