Blunting and wear of AFM tips during dynamic lithography

Mechanical scanning probe lithography using AFM tips has been applied to the fabrication of nanostructures on material surfaces. Particularly, the dynamic lithography under the AFM tapping mode is promising in fabricating the surface nanostructures at the width of around 20 nm. However, tip blunting and gradual atom-by-atom wear occurred during the process although the tip tapping motion has largely reduced the tip-surface interaction force and time. This study aims to investigate the blunting rate and mechanism of such AFM tips. It was found that the maximum contact stress was in the range of 0.7 to 1.0 GPa when a silicon AFM tip (initial tip radius under 20 nm) was tapped on a PMMA thin-film surface. And the contact stress was mainly attributed to the tip radius, which was the key factor to limit machinability and machining efficiency of the dynamic lithography. Furthermore, the mechanism of the tip blunting was through its plastic deformation and tip wear. A stress analysis indicated that it was the large stress gradient that contributed to the high density of wrinkles in the tips, and that ninety percent of the blunting of a tip were caused by the plastic deformation, leading to a significant increase of the tip radius.