Adhesion and Friction Issues Associated With Reliable Operation of MEMS

Abstract

A growing interest exists in developing technologies that use silicon and other electronic materials as mechanical materials. Using standard processes of the integrated-circuit industry, researchers have successfully fabricated miniature mechanical components (micromachines) such as membranes, gears, motors, pumps, and valves. The integration of miniaturized mechanical components with microelectronic components has spawned a new technology known as microelectromechanical systems (MEMS). It promises to extend the benefits of microelectronic fabrication to sensing and actuating functions. Early applications of this technology include the digital mirror display, which has of the order of 106 aluminum thin-film micromirrors fabricated on top of a complementary-metal-oxide-semiconductor static random-access-memory integrated circuit. Other applications include integrated accelerometers for tasks such as air-bag deployment.A number of fabrication techniques have been developed for this technology and have been reviewed elsewhere. In this review, I focus on surface-micromachining technology and adhesion and friction problems in surface-micromachined polycrystalline silicon (polysilicon) structures, though many of the principles discussed will also apply both to single-crystalline silicon and nonsilicon-based structures. Surface micromachining, defined as the fabrication of micromechanical structures from deposited thin films, is one of the core technological processes underlying MEMS. Surface microstructures have lateral dimensions of 50-500 μm with thicknesses of 0.1–2.5 μm and are offset 0.1–2 μm from the substrate. The basic steps in a surface-micromachining process appear in Figure 1. First the substrate is typically coated with an isolation layer (Figure la) that protects it during subsequent etching steps. A sacrificial layer is then deposited on the substrate and patterned. For simplicity, Figure 1b shows that the opening of the sacrificial layer is terminated on the isolation layer. The microstructural thin film is then deposited and etched (Figure 1c). Finally selective etching of the sacrificial layer creates the freestanding micromechanical structures such as the cantilever beam shown in cross section in Figure 1d. The technique can be extended to make multiple-layer microstructures.