Friction-induced hillocks on monocrystalline silicon in atmosphere and in vacuum

Using an atomic force microscope, the friction-induced hillocks of 0.5–7.1 nm in height were observed on Si(1 0 0) surface under both atmosphere and vacuum conditions. Larger hillocks can be produced by either higher loads or more scratch cycles. Detailed surface analyses on the hillocks revealed that the thickness of oxidation layer formed during scratching was 1.2 nm in vacuum and 1.5 nm in atmosphere, which was much smaller than the height (3.5 nm) of the detected hillocks. Further analysis indicated that oxidation in scratching only led to very small increase in height, i.e. <23% of the total height of the detected hillocks. TEM observations demonstrated that the hillocks are largely amorphous structured. Therefore, the generation of friction-induced hillocks was the coupled results of oxidation reaction and mechanical interaction; the mechanical interaction, which might be most likely through the shear deformation of amorphous superficial layer in silicon, should play a dominant role. These findings will shed new light on the subtle mechanism of nanowear and provide possibilities for preventing the wear failure of MEMS/NEMS.