Adhesion and Friction Issues Associated With Reliable Operation of MEMS
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deposited thin films, is one of the core technological processes underlying MEMS. Surface microstructures have lateral dimensions of 50-500 /xm with thicknesses of 0.1-2.5 /im and are offset 0.1-2 /xm 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 lb shows that the opening of the sacrificial layer is terminated on the isolation layer. The microstructural thin film is then deposited and etched (Figure lc). Finally selective etching of the sacrificial layer creates the freestanding micromechanical structures such as the cantilever beam shown in cross section in Figure Id. The technique can be extended to make multiple-layer microstructures. Polysilicon microstructures are an important example of structures fabricated by a surface-micromachining process. The mechanical properties of silicon make it an excellent material for micromachining. With polysilicon as the structural material, the sacrificial layer is an oxide film, the isolation layer is typically silicon nitride, and the structural layer is polysilicon deposited by lowpressure chemical vapor deposition. The oxide film is etched by hydrofluoric acid, which etches silicon nitride relatively slowly and has a negligible effect on polysilicon.
Because surface microstructures have a large ratio of surface area to volume, they are particularly vulnerable to adhesion to the substrate or adjacent microstructures during the release process or later during use.810 This phenomenon is more generally called stiction because, in these microdevices, one is often dealing with a restoring force that has a tangential as well as vertical component. Because microactuators have surfaces in normal or sliding contact, friction and wear are also critical issues.11 Surface modifications are needed to reduce adhesion and friction in micromechanical structures. Microelectromechanicalsystem fabrication processes pose stringent constraints on surface coatings. The coatings must be compatible not only with the surface-microstructure fabrication and release process but also with the back-end processes of wafer dicing, die attachment, and final hermetic encapsulation. Major consideration must be given to the temperatures required for die attachment and hermetic encapsulation typically in the range of 350-425°C. These examples show the importance of understanding the surface properties of the materials used in MEMS technology and of manipulating the interfacial forces responsible for stiction in microdevices. In the next section, a review of various experimental designs will be presented that are used to quantify adhesion and friction in the length scale relevant to the MEMS technology. Then various recent advances in surface processes for reducing adhesion and friction in MEMS will be described. Finally some of the important scient
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