Quantitative comparison of three calibration techniques for the lateral force microscope

The quantitative use of atomic force microscopes in lateral mode for friction measurements has been limited by uncertainty about reliable calibration techniques. This article describes a comparison of three methods that have been proposed for the calibration of the lateral sensitivity of atomic force microscopes: (a) one based on movement of the photodiode assembly, (b) one based on the slope of the friction-loop while the contacting surfaces are in static contact, and (c) one based on a comparison of the lateral force signal on a surface with changing slopes of known orientation. All three methods gave comparable results thereby confirming their robust nature, and also confirming the validity of atomic force microscope methods for lateral force measurement. However, (b) indicated that for the commercial instrument used here, the lateral signal sensitivity is load dependent. A simple extension to (a) revealed the nature of this dependence: a misalignment of the four-quadrant photodiode detection system with respect to the alignment of the reflected beam path resulting in a coupling of the normal and lateral signals. The result is that the lateral signal does not scale directly with friction, requiring that for friction versus applied load studies, lateral signal calibration be performed across the full range of applied loads of interest. To a greater or lesser extent, this shortcoming will be evident in a wide range of commercial instruments. All three methods studied here have special advantages: (a) provides the most complete information about the detector response, (b) provides a calibration method when friction characteristics of colloid probes of a wide range of possible materials are of interest, and (c) has the advantage of also providing information about the state of the tip geometry.