The principle of STM is remarkably simple. A small, atomically sharp tip is brought to within less than one nanometer of the surface (but not in actual contact). Applying a voltage (of the order of 1 to 2 V) across the tip and sample causes a quantum mechanical tunnelling current of a few nanoamps to flow. As this tunneling current is exponentially dependent on the tip-sample separation, any small change in that separation results in a large change in tunneling current.
STM images are built up by scanning the tip in a raster pattern across the surface under study. The tunnelling current is monitered as this happens. It is possible to build up an image directly using the value of this current. However, it is much more conventional to adjust the tip-sample separation in a feedback loop to maintain a constant tunnelling current. The "z-position" of the tip (i.e the amount it has been moved towards or away from the sample) is then used to build a topographic image of the surface. Figure 1 indicates the consequent motion of tip as it moves across the surface.
In practice the tip is mounted on piezoelectric tubes. The voltages to these tubes are adjusted to scan the tip across the surface on the nanometer scale, in a rastor pattern. The image is then simply a map of the voltage applied to control the tip z-position. Figure 2 shows an example of such an image, obtained from an Ag/Ge(111) surface.
Since the invention of STM other related techniques have been developed. These act in basically the same way, but the tunnelling current of STM is replaced by some other measured parameter. The most common probe microscopy after STM is atomic force microscopy (AFM). In this the role of tunnelling current is replaced by the van der Waals forces between the tip and sample. These forces are measured by mounting the tip on a flexable cantilever. The forces then cause a detectable deflection of the cantilever, and the tip "height" is adjusted to keep this constant. AFM has the advantage that insulators may be imaged.
The forces due to magnetic interactions between tip and sample may be used in a similar way in magnetic force microscopy and can give valuable information about the magnetic structure of the surface. Further variations on the theme are still being invented. Of course, several of these techniques may be combined, for example measuring the tunneling current whilst performing AFM with a metallic tip.
