FIGURE 4-26 Apparatus used for AFM FE-RAM testing.

The ferroelectric or piezoelectric response of a film is measured by monitoring the motion of the cantilever when a bias is applied to the film with the apparatus illustrated in Figure 4-26. A butterfly curve corresponding to the electrical excitation of the ferro/piezo film (Figure 4-27) is measured. The spatial resolution of the measurement is proportional to the probe diameter.

FIGURE 4-27 Butterfly curve of ferroelectric sample measured with an AFM (green).

The force between an electrically conductive probe and a sample is given by Equation 4-5. The force will be zero if the potential of the probe (Vp) and the sample (Vs) are equal. In the Kelvin probe mode, the probe is scanned across a surface, typically in vibrating mode, and a feedback loop is used to keep the voltage between the probe and surface equal.

The feedback loop used to control the potential between the probe and surface is zero and is illustrated in Figure 4-28. The mechanical vibration mode is w1 and the probe potential is vibrated at another frequency w2. The signal at the photodetector is modulated at both frequencies; w1 is generated from mechanics, and w2 is generated from varying electric potential. When the force between the probe and surface is minimized