An electron microscopy study of wear in polysilicon microelectromechanical systems in ambient air

Wear is a critical factor in determining the durability of microelectromechanical systems (MEMS). While the reliability of polysilicon MEMS has received extensive attention, the mechanisms responsible for this failure mode at the microscale have yet to be conclusively determined. We have used on-chip polycrystalline silicon side-wall friction MEMS specimens to study active mechanisms during sliding wear in ambient air. Worn parts were examined by analytical scanning and transmission electron microscopy, while local temperature changes were monitored using advanced infrared microscopy. Observations show that small amorphous debris particles (∼50-100 nm) are removed by fracture through the silicon grains (∼500 nm) and are oxidized during this process. Agglomeration of such debris particles into larger clusters also occurs. Some of these debris particles/clusters create plowing tracks on the beam surface. A nano-crystalline surface layer (∼20-200 nm), with higher oxygen content, forms during wear at and below regions of the worn surface; its formation is likely aided by high local stresses. No evidence of dislocation plasticity or of extreme local temperature increases was found, ruling out the possibility of high temperature-assisted wear mechanisms.