The effect of humidity on the reliability of a surface micromachined microengine

Humidity is shown to be a strong factor in the wear of rubbing surfaces in polysilicon micromachines. We demonstrate that very low humidity can lead to very high wear without a significant change in reliability. We show that the volume of wear debris generated is a function of the humidity in an air environment. As the humidity decreases, the wear debris generated increases. For the higher humidity levels, the formation of surface hydroxides may act as a lubricant. The dominant failure mechanism has been identified as wear. The wear debris has been identified as amorphous oxidized silicon. Large slivers (approximately 1 μm in length) of debris observed at the low humidity level were also amorphous oxidized silicon. Using transmission electron microscopy (TEM), we observed that the wear debris forms spherical and rod-like shapes. We compared two surface treatment processes: a fluorinated silane chain (FTS) process and supercritical CO₂ dried (SCCO₂) process. The microengines using the SCCO₂ process were found to be less reliable than those released with the FTS process under two humidity levels