3.4 The atomic force microscope
The most commonly used scanning probe microscope is the AFM – the atomic force microscope. It works in a way much more similar to the gramophone stylus, but instead of detecting the movement of the probe tip electromagnetically, it usually does so optically. As the probe tip is drawn across the sample, a laser beam is reflected off the cantilever on which the tip is mounted. A position-sensitive optical detector picks up the deflection of the beam, converting the angle of bending into a voltage signal. A topographic image of the sample can be generated directly from this signal. Alternatively, the height of the cantilever can be continually adjusted to maintain the cantilever deflection at a constant angle, and the control signal that varies the height is used to generate the image of the scanned surface.