Investigation of adhesion during operation of MEMS cantilevers

Reliability of MEMS is a major concern for the commercialization of laboratory prototypes. Surface adhesion or stiction strongly affects the reliability of MEMS devices which have sliding or rubbing contacts. Determination of adhesion energies, adhesion forces, and pull-off forces are important for predicting stiction in MEMS. We present an experimental technique to estimate the pull-off forces for MEMS surfaces. Polysilicon microcantilevers were electrostatically actuated using gradually varying voltages. A hysteresis was observed in the voltage at which the tip of the cantilevers made and broke contact with the substrate. Pull-off forces were estimated from the hysteresis in the voltage values using a strain energy formulation. The pull-off forces for microcantilevers dried out of isopropyl alcohol and repaired using laser irradiation were estimated to be in the range of 45-121 nN. The role of adhered length, variable external loading, and actuating signal on in-use stiction is also investigated. From our experimental results, we demonstrate an empirical approach to predict in-use stiction of microcantilevers.

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