Release stiction

Release stiction has been recognized as a problem since the late 1980s, especially for surface micromachined structures. During drying, surface tension from the liquid-vapor interface causes a downward force on the structural layer. If the layer touches the substrate, it is prone to stick onto the surface. It is hypothesized that etch products and/or contaminants in the rinse water can then precipitate out of solution during drying and cause a bond that is stronger (e.g., a chemical bond)... 

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. ... 

This process-induced water can accumulate underneath the released microstructures and can lead to in-process stiction. To prevent this, the wafer is heated so that the excess water evaporates [23]. But selection of the temperature is critical, because water is not only a product of the reaction but also an initiator for hydrolyzing the Si02 network. The reaction does not start or is slowed down if water is depleted too much. Thus, there is a processing window limited on the low-temperature end by the onset of stiction and on the high temperature end by reaction kinetics (Fig. 5.3.9). 

Alley L, Cuan GJ, Howe RT, Komvopoulos K (1992) The effect of release-etch processing on surface microstructure stiction. In Proceedings of the technical digest IEEE solid-state sensor and actuator workshop, Hilton Head, June 1992, pp 202-207... 

One of the issues which often arise with flexible structures is the contact between the released structures and the substrate, which can provide a large area for stiction to take place. Use of supercritical drying or sublimation has been successful to overcome such problems [1]. 

Figure 1.10 Stiction that can occur during the sacrificial release etch, (a) Before sacrificial etching, the sacrificial oxide is below the mechanical layer, (b) After the chip is removed from the etch bath it begins to dry and the remaining fluid forms a bridge between the substrate and the mechanical layer, (c) Capillary forces from the meniscus of the fluid exert a downward force on the cantilever and cause it to come into contact with the substrate, (d) The surface forces, such as Van der Waals attraction, that dominate at the microscale cause the cantilever to become stuck to the substrate. (Reprinted with permission from lOP Publishing Ltd.) [15].

A final approach to avoid stiction effects is to apply a self-assembled monolayer (SAM) that is hydrophobic as a part of the release process. This process forms a hydrophobic Teflon-like coating on the contacting surfaces that reduces the capillary forces decreasing stiction effects [2]. 

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