Adhesion micromachine surfaces

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]. 

The release techniques discussed here do not prevent adhesion from occurring during micromachine operation. Microstructure surfaces may come into contact unintentionally through acceleration or electrostatic forces, or intentionally in applications where surfaces impact or shear against each other. When adhesive attractions exceed restoring forces, surfaces permanently adhere to each other causing device failure—a phenomenon known as in-use stiction. ... 

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