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.
The conventional deflection-mode atomic force microscope operates by optically monitoring the slope near the end of a microcantilever in contact with the sample surface. This signal is usually interpreted as a measure of height change. Lateral forces from friction, surface geometry, or inclination of the cantilever to the surface also affect the slope due to cantilever buckling. We calculate the deflection of a hollow triangular model cantilever subject to both lateral and normal forces. The measured response of the servo circuit to an inclined, loaded cantilever is then determined. This shows (1) errors are always present in height measurements of structures on inhomogeneous surfaces; (2) the sensitivity to buckling can be reduced by repositioning the laser; (3) friction measurements can be accurately made by scanning in two directions and applying the proper calibration.
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