Current commercial SPM scanners are made from piezoelectric material that expands and contracts proportionally to an applied voltage. Scanners have a voltage range for each axis, with the expansion/contraction dependent on the polarity of the voltage.
The scanner can manipulate samples/probes precisely in three dimensions because its construction combines independently operated piezo electrodes for X, Y, and Z into a single tube. In Bottom Scanning AFM the sample moves relative to the tip. In Tip Scanning AFM the tip moves relative to the sample. AC voltages applied to the different electrodes produce a scanning raster motion in X and Y.
Each scanner responds differently to applied voltage because of the differences in the material properties and dimensions of each piezoelectric element. Sensitivity is a measure of this response, a ratio of how far the piezo extends or contracts per applied volt. The relationship of displacement vs. force is nonlinear. This is because piezo scanners are more sensitive at the end of travel than the beginning. Thus, opposite scans will behave differently and display hysteresis effects.
Applying a nonlinear voltage in real-time based on a prior calibration routine produces a linear scan in X and Y in both scan directions. If not corrected through calibration, piezo nonlinearity and hysteresis can cause SPM image distortion.
Piezoelectric materials' sensitivity decreases exponentially with operation time. Most of the change occurs at the beginning of the scanner's life. As such, scanners need to be calibrated often in early life, but may not require calibration as often as the scanner ages.
Drift of the piezo displacement may occur with large changes in X and Y offsets. When a large offset is performed a DC voltage is applied to the scanner to move the requested offset distance. The majority of the offset distance occurs quickly, but the remaining movement is slow. This slow portion of the travel is known as piezo creep.
In images, creep causes distortion in the direction of the offset. Usually creep will settle out within the first scan if the X, Y offset is relatively small.
With tube scanning instruments; X, Y displacement is accomplished by moving the end of the scanner in X, Y arcs. In most systems, the Z displacement inherent in swinging an arc is compensated for by translating the Z piezo. Without this compensation a bow of the image may result (depressed middle, raised sides). Many systems further compensate for bow by using flattening algorithms.