As illustrated in Figure 4-8, the cantilever can be excited with a piezoelectric ceramic. The cantilevers have natural resonant frequency ω_o given by:

At the resonance frequency, there is a 90 degree phase shift. When the probe tip interacts with a surface, the resonance frequency shifts to a lower value, and there is a corresponding change in the phase. When scanning in the vibrating modes, a constant relationship is maintained by the feedback electronics, which keeps either the phase shift or amplitude constant at a given frequency, while scanning.

FIGURE 4-8 Top: A drive piezo causes the cantilever to resonate. Left: At resonance (fr) there is a large increase in the vibration amplitude of the cantilever. Right: At resonance there is a 90 degree phase shift in the cantilevers motion.

As mentioned in Section 4.0, there is a "contamination" layer on surfaces in ambient air with a thickness between 1 and 50 nanometers. The probe surface interaction forces are governed by the capillary forces between the probe and the contamination layer. The probe may be vibrated in three separated regimes as it is scanned across the surface, see Figure 4-9.

In the first regime, the probe is vibrated across the surface of the contamination layer. The vibration amplitude must be very small and a very stiff probe must be used. The images of the surface contamination layer are typically very "cloudy" and appear to have low resolution. This is because the contamination fills in the nanostructures at the surface.