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Sidewall Adhesion and Sliding Contact Behavior of Polycrystalline Silicon Microdevices Operated in High Vacuum

The reliability and performance of contact-mode microelectromechanical systems (MEMS) depend strongly on the tribological properties of contact interfaces. Knowledge of the dominant friction and wear mechanisms at submicrometer length scales is therefore of paramount importance to the design of MEMS devices with contact interfaces. The objective of this study was to examine changes in the adhesion behavior and morphology of sliding sidewall surfaces of polycrystalline silicon MEMS devices operated in high vacuum (~105 torr) and under low apparent contact pressures (0.1-18 kPa) and correlate these changes to the operation lifetime. Sidewall adhesion increased with applied contact pressure. Typically, a twofold to fourfold increase in side wall adhesion was measured upon cessation of the device operation (typically, ~106 sliding cycles) due to the increase of the static friction force above the restoring force available by the device. Scanning electron microscopy (SEM) revealed very small amounts of ultrafine wear debris (10-140 nm) on the sidewall surfaces of about half of the tested devices, without discernible changes in the surface topography. Cross-sectional transmission electron microscopy (TEM) showed that sliding did not cause the removal of the silicon oxide film (5-13 nm in average thickness) from the sidewall surfaces. Atomic force microscopy (AFM) indicated that sliding contact was confined at the top of a few elevated ridges on the sidewall surfaces, resulting in nanoscale wear that smoothened locally the surfaces. SEM, TEM, and AFM results of this study show that the tribological properties of contact-mode MEMS devices operating in high vacuum are controlled by only a few nanoscopic contacts, which depend on the local nanotopography of the interacting surfaces.