Nanotribology and nanomechanics of MEMS devices

MEMS devices are made of single-crystal silicon, LPCVD polysilicon films and other ceramic films. Very little is understood about tribology and mechanical characterization of these materials on micro- to nanoscales. Atomic force microscopy/friction force microscopy (AFM/FFM) techniques are increasingly used for tribological studies of engineering surfaces at scales, ranging from atomic and molecular to microscales. These techniques have been used to study surface roughness, adhesion, friction, scratching/wear, indentation, and boundary lubrication of bulk and treated silicon. Commonly measured roughness parameters are found to be scale dependent, requiring the need of scale-independent fractal parameters to characterize surface roughness. Measurements of micro-scale friction and macroscale friction show that friction values both on micro- and macroscales of all samples are about the same. The microscale values are lower than the macrovalues as there is less ploughing contribution in microscale measurements. Local variation in microscale friction is found to correspond to the local slope. Directionality in the friction is observed on microscale which results from the surface preparation and anisotropy in surface roughness. Microscratching/microwear and nanoindentation studies indicate that coated/treated silicon is superior to bare silicon. Chemically bonded lubricants appear to be suitable for MEMS devices. Finally, ultrasmooth surfaces under extremely lightly loaded conditions are required for ultralow friction and near-zero wear.