(or zero) there will be no modulated signal at w2. A feedback electronic circuit is used to maintain the potential of the surface such that Vp = Vs.

FIGURE 4-28 Apparatus used for measuring surface potential with an AFM. The mechanical vibration frequency w1 is not presented in this figure.

Scanning Kelvin Probe Microscopy is a quantitative technique and measures the absolute potential of surface. In contrast, EFM (see Section 4.2.1) measures electric fields and is qualitative. Additionally, because SKPM uses the vibrating mode to track topography, there is less force placed on the surface and the integrity of the probe stays intact. However, SKPM is a more costly technique to implement because it requires a lock-in amplifier and PID controller.

Figure 4-29 illustrates the use of an KPM for imaging the surface of a phase recorded optical media. The topography image does not show the bits on the surface, however, the bits are very apparent in the potential image. Both the topography and potential image are measured simultaneously.

FIGURE 4-29 Topography (left) and Potential (right) image of a CD-R/W sample.