Similarly to standard AFM (Fig. 17.1), friction force microscopy (FFM) is based on the relative motion of a sharp tip on a solid surface. This motion is realized by a scanner formed by piezoelectric elements, which moves the surface perpendicularly to the tip with a certain time periodicity. The scanner can be also extended or retracted in order to vary the normal force FN between tip and surface. This force is responsible for the deflection of the microcantilever supporting the tip. If FN increases while scanning due to local variations of the surface height, the scanner is retracted by a feedback loop. If FN decreases, the surface is brought closer to the tip by extending the scanner. In such a way, the surface topography can be reconstructed line by line from the vertical deformation of the scanner. An accurate control of the imaging process is made possible by a light beam, which is reflected from the rear of the cantilever into a photodetector. When the bending of the cantilever changes, the light spot on the detector moves up or down. This causes a variation of the photocurrent corresponding to the value of FN to be controlled.

The scan motion is also accompanied by friction. A tangential force F with the opposite direction to the scan velocity v hinders the sliding motion. The force F causes the torsion of the cantilever, and can be recorded simultaneously with the topography if the photodetector can measure not only the normal deflection but also the torsion of the lever while scanning. In practice this is made possible by a four-quadrants photodetector, which converts the photocurrent corresponding to the lateral force into a voltage VL. Note that the friction also causes lateral bending of the cantilever, but this effect is modest if the thickness of the lever is much less than its width.

The first atomic friction measurements by Mate et al. [206] were actually based on the deflection of a tungsten wire.