Effects of Vapor Environment and Counter-Surface Chemistry on Tribochemical Wear of Silicon Wafers

The effects of counter-surface chemistry and adsorption of water and alcohol from the environment on the tribological responses of silicon surfaces were investigated using atomic force microscopy. When scratching with SiO₂ tips at contact pressures below the hardness of the materials, changing the environment yielded drastically different wear behaviors. In humid air, the adsorbed water molecules facilitated wear of the surface and material removal. In N₂ environment, there was subsurface deformation but no wear, so the surface protruded outward in the rubbing region. In the ethanol vapor condition, the adsorbed alcohol molecules acted as a lubricant and prevented any discernible changes to the surface even at contact pressures above 1 GPa. These results extend upon previous studies of vapor-phase alcohol lubrication using even more protective longer-chain alcohols where failure was observed at much lower contact pressures in macroscale tests, probably due to high-pressure asperity contacts. Thus, the chemical environment can govern the response of silicon to mechanical rubbing. Rubbing with a diamond tip, however, yielded protrusions in all three environments, showing that the chemistry of the counter-surface also contributes to the tribological response; in this case, diamond is not tribochemically reactive toward Si surface. The protrusion formed by the diamond tip in ethanol vapor was only ~20 % the height of the one in humid air, even though the measured friction coefficients (and so the applied shear forces) were similar. These results clearly show that the surface chemistry at the tribological interface can substantially alter both the wear and subsurface damage processes.