| Microelectromechanical systems (MEMS) are used in a wide range of applications including sensors, actuators, biomedical devices, projection systems, and photovoltaic structures. Advances in MEMS have increased the demand for more reliable microstructures. The reliability and performance of many MEMS devices depends strongly on the tribological properties of contact interface. Knowledge of the origins and evolution of the surface forces is therefore important to the design of MEMS devices with contact interfaces. In this dissertation, special microdevices fabricated by surface micromachining were used to characterize interfacial phenomena encountered under typical operation conditions of MEMS devices. Dynamic impact and sliding contact experiments were performed to determine the critical parameters of device lifetime. The evolution of static adhesion force and friction force was examined under controlled loading and environmental conditions. A criterion of micromachine failure due to excessive interfacial adhesion (stiction) was formulated based on the observed experimental trend. Surface modification was examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM), and the observations were compared to the evolution of critical interfacial properties. The change of the interfacial force was explained in the context of nanoscale surface modification effects resulting in both physical and chemical surface changes. The deposition of self-assembled monolayer (SAM) films is one of the most promising surface chemical treatments for preventing micromachine stiction during release and operation. The tribological properties of micromachine sidewall surfaces coated with a conformal FOTS film were also investigated in the context of goniometry measurements, adhesion and static friction tests, and X-ray photoelectron spectroscopy (XPS) results. This study extends the understanding of the effect of interfacial phenomena on the operation of contact-mode MEMS devices, and the obtained results are valuable to the improvement of MEMS reliability. |
