An AFM probe is sensitive to changes in the physical properties at a sample's surface. The physical properties include the chemical composition, the hardness, and the adhesion. Images of surface physical properties measured with the AFM are qualitative and show changes in the physical property. The difficulties in getting quantitative values for physical properties are derived from the inability to absolutely control the probe geometry and chemical properties of an AFM probe.

As illustrated in Figure 4-17, the cantilever in an AFM can twist or rotate as it is scanned across a surface. The amount of motion is in some way related to the differences in the chemical/physical properties of a surface. The chemical/physical property could be the nano-roughness or it could be differences in chemical composition.

FIGURE 4-17 Lateral forces on the tip of an AFM cantilever can cause the cantilever to bend vertically (lower right), bend horizontally (lower left), or twist (upper).

Typically, the force constant coefficient for twisting in the lateral direction is much less than the force constant coefficient for bending. Thus, the cantilever tends to twist while scanning. Because the physical dimensions of the cantilever are not well characterized in commercially available cantilevers, the force constant coefficients must be calculated for each cantilever. Calculations are derived from the physical dimensions and the cantilever's material.