Three-layer polysilicon surface

Surface micromachining does not usually require two-sided processing of the silicon, rendering it often more CMOS-compatible. A structural material is patterned over the top of a sacrificial material. Subsequently, the sacrificial material is etched, leaving the anchored structural material free to move. Capacitance is used almost exclusively as the transduction technique with surface micromachined devices. An example of a three-layer polysilicon surface micromachined accelerometer is shown in Fig. 7.1.12f. 

Figure 1.7 PolyMUMPS three-layer polysilicon surface micromacliming process offered by MEMSCAP. Polysilicon and oxide layers are deposited and patterned in a cyclic process, with anneal steps of the doped saciilicial oxide between polysilicon depositions. PolyO is an electrical layer that is not released. Polyl and Poly2 are structural layers that can be released. The deposition and patterning steps shown here result in a polysiUcon wheel defined in Polyl that is constrained by a hub defined in Poly2. Dimples defined in POLYl keep the wheel from becoming stuck to the PolyO layer. (Reprinted with permission from MEMSCAP Eic.) See color plate section.

In the case of in-use stiction, it is hypothesized that moisture from the environment (relative humidity) comes in contact with the MEMS structural surfaces. If, during operation, these structures come in contact, the moisture can cause a temporary bond that, like release stiction, can then become permanent with time. To reduce in-use stiction, three basic techniques have been attempted. The first is to use a hermetic seal around the microstructure to eliminate the possibility of moisture encountering the structure. Secondly, the use of techniques to minimize the work of adhesion has been employed. Specifically, Houston et al. have used ammonium fluoride to reduce the work of adhesion on surface micromachined structures [59, 60]. Lastly, various coatings and/or surface treatments have been used on the microstructure to eliminate the chance of contact between two surfaces that have the prevalence to stick (e.g., polysilicon and silicon, each material with a native oxide). The University of California, Berkeley has pioneered techniques of using self-assembled layer monolayer coatings to minimize in-use stiction [18, 25, 59, 61]. Also, other researchers have used fluorocarbon coatings to minimize the in-use stiction [62-64]. 

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