Towards a deeper understanding of the formation of friction-induced hillocks on monocrystalline silicon

Friction-induced hillocks can be produced on monocrystalline silicon by scratching under given conditions. Results show that the height of these hillocks increases with the applied normal load or number of scratching cycles, but decreases with the sliding velocity. Transmission electron microscope (TEM) and energy dispersive x-ray (EDX) analysis show that the hillock contains a thin superficial oxidation layer and a thick disturbed (amorphous and deformed) layer in the subsurface. Although the formation of the silicon hillock is the combined results of mechanical interaction and tribochemical reaction, the mechanical interaction plays a more dominant role. Further analysis indicates that the formation of hillock is mostly induced by the amorphization of crystal silicon during scratching. Low sliding speed is found to facilitate the formation of a thick amorphization layer under the same loading condition. Since the friction-induced hillock is the initial surface damage on the nanoscale, the results will shed new light on understanding and controlling the nanowear process of silicon in micro/nanoelectromechanical systems.