In Contact AFM mode, also known as repulsive mode, an AFM tip makes soft "physical contact" with the sample. The tip is attached to the end of a cantilever with a low spring constant, lower than the effective spring constant holding the atoms of the sample together.

The slope of the van der Waals curve is very steep in the repulsive or contact regime. As a result, the repulsive van der Waals force balances almost any force that attempts to push the atoms closer together. In AFM this means that when the cantilever pushes the tip against the sample, the cantilever bends rather than forcing the tip atoms closer to the sample atoms. Even if you design a very stiff cantilever to exert large forces on the sample, the interatomic separation between the tip and sample atoms is unlikely to decrease much. Instead, the sample surface is likely to deform

In addition to the repulsive van der Waals force described above, two other forces are generally present during contact AFM operation: a capillary force exerted by the thin water layer often present in an ambient environment, and the force exerted by the cantilever itself. The capillary force arises when water wicks its way around the tip, applying a strong attractive force (about 10-8N) that holds the tip in contact with the surface.

As long as the tip is in contact with the sample, the capillary force should be constant because the distance between the tip and the sample is virtually incompressible. It is also assumed that the water layer is reasonably homogeneous. The variable force in contact AFM is the force exerted by the cantilever. The total force that the tip exerts on the sample is the sum of the capillary plus cantilever forces, and must be balanced by the repulsive van der Waals force for contact AFM. The magnitude of the total force exerted on the sample varies from 10-8 (with the cantilever pulling away from the sample almost as hard as the water is pulling down the tip - see Force vs. distance curves ), to the more typical operating range of 10-7 to 10-6N.

Many different methods have been employed to detect the deflection of the cantilever caused by the interaction of the sample with the probe. Tunneling: The first AFM used tunneling to detect the bending of the tip. An STM tip is placed on the top of a metal cantilever and the feedback response is used to maintain a constant tunneling current. Capacitance: A plate is placed above the AFM cantilever, which acts as the other plate of a capacitor. The capacitance between the two plates reflects the deflection of the cantilever. Interferometry: A laser beam is split with one part reaching the detector directly while the other part is focussed on the back of the cantilever, gets reflected and then reaches the detector. The coherent beams travel different paths and produce an interference pattern, which changesas the cantilever moves up and down. Laser diode feedback: The light from a single-mode laser is reflected off the cantilever back into the laser cavity, thus causing a change in the gain of the laser. Piezoresistivity: This method uses no external system to monitor deflection. A silicon cantilever, with a resistor network embedded into it, upon bending responds with a resistivity change due to stress on the crystal. Optical deflection: A laser is focussed on to the back of the cantilever and reflected onto a split photo detector. The differences in the voltage responses of the two halves changes as the cantilever deflection changes.

Most AFMs currently on the market detect the position of the cantilever with optical techniques. In the most common scheme, shown in Figure 9, a laser beam bounces off the back of the cantilever onto a position-sensitive photodetector (PSPD). As the cantilever bends, the position of the laser beam on the detector shifts. The PSPD itself can measure displacements of light as small as 10Å. The ratio of the path length between the cantilever and the detector to the length of the cantilever itself produces a mechanical amplification. As a result, the system can detect sub-angstrom vertical movement of the cantilever tip.

Figure 9. The beam-bounce detection scheme.

In constant-force mode, the speed of scanning is limited by the response time of the feedback circuit, but the total force exerted on the sample by the tip is well controlled. Constant-force mode is generally preferred for most applications. Constant-height mode is often used for taking atomic-scale images of atomically flat surfaces, where the cantilever deflections and thus variations in applied force are small. Constant-height mode is also essential for recording real-time images of changing surfaces, where high scan speed is essential.

Once the AFM has detected the cantilever deflection, it can generate the topographic data set by operating in one of two modes - constant-height or constant-force mode. In constant-height mode, the spatial variation of the cantilever deflection can be used directly to generate the topographic data set because the height of the scanner is fixed as it scans. In constant-force mode, the deflection of the cantilever can be used as input to a feedback circuit that moves the scanner up and down in z, responding to the topography by keeping the cantilever deflection constant. In this case, the image is generated from the scanner's motion. With the cantilever deflection held constant, the total force applied to the sample is constant.