generating the motion is not linear and has hysteresis. Such an error appears as a displacement in the F/D curve when the probe is in the repulsive region of the curve.

FIGURE 4-36 The force distance curve is measured by moving the probe at the end of a cantilever toward a surface (red and black traces at the top). The letters A-F indicate the position of the probe as it moves toward and away from the surface.

A nano-indent in a surface can be generated by physically forcing the probe into a surface, causing surface material to be dislodged. These "loading curves" can be created only if the spring constant of the AFM cantilever is stiff enough, and the probe material hard enough. Figure 4-37 shows a nano-indent created by a silicon probe in the surface of PMMA.

FIGURE 4-37 Nano-indentation made with an AFM probe in PMMA surface

There are two problems that make it difficult to get quantitative data when nano-indenting with an AFM. The first is that the probe geometry is not well known or at best poorly defined. Secondly, as seen in Figure 4-38, the probe sample angle is constantly changing as the nano-indent process is being made.