Typically, the scanners used for moving the probe relative to the sample in an AFM are constructed from piezoelectric materials. This is because piezo materials are readily available, easily fabricated in desirable shapes and cost effective. Scanners in an AFM may be constructed from other types of electromechanical devices such as voice coils. All that is important is that the electromechanical device have accurate positioning.

The most common type of piezoelectric materials in use for AFM scanners are constructed from amorphous PdBaTiO3. The ceramics may be “hard” or “soft”, depending on the formulation. Hard ceramics have smaller coefficients of expansion, but are more linear. Soft ceramic formulations
have more non-linearities and have greater expansion coefficients. After fabrication, piezoelectric ceramics are polarized. Polarization may be lost by elevating the piezos to a temperature about their critical temperature or by giving them an over voltage.

Electronically, piezos act as capacitors and store charges on their surface. Capacitances of ceramics may be as large as 100 μfarads. Once a charge is placed on the piezoceramic, the piezoceramic will stay charged until it is dissipated. Electronic circuits used for driving the piezoceramics in an AFM must be designed to drive large capacitive loads.

All piezoceramics have a natural resonance frequency that depends on the size and shape of the ceramic. Below the resonance frequency, the ceramic will follow an oscillating frequency; at resonance there is a 90 degree phase change, and above resonance there is a 180 degree phase change. To a great extent, the resonance frequencies of the piezoelectric ceramics limit the scan rates of atomic force microscopes.

Piezoelectric materials can be fabricated in several shapes such that they have more or less motion. As an example, a disk, as illustrated in Figure 2-8, gets longer and narrower when a voltage is applied. The ceramic changes geometry such that the volume is preserved during expansion. Another configuration for a piezoelectric ceramic is a tube, with electrodes on the inside and outside. This configuration gives a lot of motion, and is very rigid. Another configuration is the bimorph, constructed from two thin slabs of piezo material that are polarized in opposite directions. When a voltage is applied the ceramic expands in a parabolic fashion.