As with STM, a sharp tip scans across the sample surface, but in this case the tip is on the end of a small cantilever; the cantilever is deflected slightly as the tip interacts with the surface. A laser shines onto the cantilever and reflects onto an array of photodiodes, which are able to measure the deflection of the laser and so give the changes in height of the tip.
There are a number of different possible modes of operation, divided into contact modes and non-contact modes. In the simplest form of contact mode, the tip is simply drawn across the surface (often at a constant height), and the deflection of the tip is used to build up topographic image of the surface. However, this technique presents problems such as damage to the surface and the possibility of the tip sticking to the surface as a result of strong attractive forces.
In one common form of non-contact mode, called tapping mode, the piezodrive makes the cantilever oscillate near its resonant frequency. The amplitude of oscillation decreases as the tip comes close to the surface (as a result of electrostatic or Van der Waals forces for example), and this measurement is used to determine the height of the surface.
Unlike the STM technique, AFM has the advantage of not needing to be carried out in a vacuum; however, atomic resolution is much more difficult to achieve.
