In Contact Mode AFM (or repulsive mode) the tip makes soft physical contact with the sample as it scans the surface to monitor changes in the cantilever deflection with a split photodiode detector. The tip attached to the cantilever contacts the surface through the adsorbed fluid layer on the sample surface. The constant force of the scanner tracing the tip across the sample (or the sample under the tip) causes the cantilever to bend with the changes in topography.
Topographic data is generated in one of two modes: constant-height or constant-force mode.
In constant-height mode direct use of the spatial variation of the cantilever deflection is used to generate the topographic data set. In this case, the height of the scanner is fixed as it scans. It is most often used when speed is essential (recording real-time images of changing surfaces) or where cantilever deflections are small (atomic scale images of atomically flat surfaces).
In constant-force mode the scanner is moved up and down in response to the topography to keep the cantilever deflection (and therefore the total applied force) constant. The image is generated from the scanner's motion. Response time of the feedback circuit limits the speed of scanning, but the total force exerted on the sample by the tip is well controlled. This mode is preferred for most applications.