Bradley University, Mechanical Engineering, Peoria, IL 61615
Microelectromechanical systems are used in a wide range of applications including microswitches, inertial sensors, and micromirrors. As microdevices are scaled down, surface forces can be on the same order of magnitude as external or inertial forces. These excessive forces impact the topography, surface charge, and reliability of the devices. Some common tribological issues include release adhesion, frictional failure, and wear. A custom microdevice was used to investigate the effect of hydrofluoric acid (HF) sacrificial layer release processing on the tribological properties of the sidewall surface. Specifically, the force of adhesion and surface roughness were measured as a function of processing time. To remove sacrificial SiO2 and allow for free standing structures, devices are submerged in aqueous HF. Due to an electrochemical potential difference between structural silicon and gold electrodes, HF can continue to affect silicon layers even after removing the sacrificial SiO2 as a result of a reduction/oxidation reaction and galvanic corrosion. Roughness was measured using white light interferometry and adhesion measurements were taken using an electrostatically driven microtribometer. Results demonstrate that devices exposed to hydrofluoric acid for a longer period of time have a higher roughness and a correspondingly lower adhesion force. This is attributed to an accelerated formation of porous silicon caused by the etch-induced galvanic corrosion. Experiments are interpreted with respect to the mechanism of galvanic corrosion and the principle nanoscale adhesive intermolecular components. This work was funded by the Bradley University Heuser Research Grant.
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