Characterization of atomic force microscope tips by adhesion force measurements

The resolution limit in an atomic force microscope image usually is attributed to the finite radius of the contacting probe. Here, it is shown that this assumption is valid only when adhesion forces are minimal. Relative to the tip-imposed geometrical limit, the resolution and contrast in AFM images can be degraded by increasing adhesion forces. The large adhesion forces observed for some tips at low humidity conditions are shown to be due to tip contamination or poorly formed tip apexes. Methods to determine and to reduce the extent of tip contamination are described. Cleaning carried out using UV-ozone or oxygen-plasma etching were found to significantly reduce the minimum adhesion force.