Hydrophobic effect of surface patterning on Si surface

Adhesion and friction force between flat Si tip and nano-patterned Si surface have been investigated by atomic force microscopy (AFM) in air and high vacuum conditions (10−5 Torr) at room temperature (RT) and at 180 °C. The pattern consists of parallel grooves realized on a Si(0 0 1) single crystal by focused ion beam (FIB) milling. The FIB technique was also used to modify a commercial AFM Si probe in order to obtain a flat tip, suitable for tribological characterization of patterned surfaces, since it presents an extended contact area. In high vacuum the coefficient of friction (CoF) on the pristine surface is about 64% lower than in ambient condition, related to the presence of a water meniscus. The situation is completely different on the patterned surface where CoF does not change appreciably in the different analyzed conditions. The adhesive force related to the presence of a water layer is lower on the patterned surface with respect to the pristine surface. These findings suggest that patterning induces a hydrophobic character to the Si surface.