Atomic Force Microscopy: Feedback operation

The AFM uses a piezoelectric element to position and scan the sample with high resolution. A tube-shaped piezoelectric ceramic that has a high stability is used in most SPMs. Application of voltage results in the stretching or bending of the piezoelectric tube, allowing it to move in all three dimensions and to position the cantilever probe with very high precision. For example, by applying a voltage to one of the two electrodes (xy-axis) the tube scanner expands and tilts away from a center position (xy-origin). A corresponding negative voltage applied to the same electrode causes the tube scanner contract, resulting in movements on the xy-plane relative to the origin. The magnitude of the movement depends on the type of piezoelectric ceramic, the shape of the element, and the applied voltage.

Feedback control is used for many common applications such as thermostats that are used to maintain a particular temperature in buildings and autopilot commonly used in airplanes. In the AFM, a feedback loop is used to keep the force acting on the tip in a fixed relationship with the surface while a scan is performed . The feedback loop consists of the piezoelectric tube scanner, the cantilever and tip, the sample, and the feedback circuit. The feedback circuit consists of proportional and integral gain controls and provides an immediate response to scanning parameter changes. A computer program acts as a compensation network that monitors the cantilever deflection and attempts to keep it at a constant level.

4-Contact Mode (Static Mode)