The Atomic Force Microscope
The Atomic Force Microscope (AFM) was not the first SPM technique to be invented, but it is the easiest to understand. In essence a very fine probe is used to read a surface. As the probe moves over the surface it encounters lumps and bumps, which cause it to move up and down. This movement is detected by the deflection of a laser beam that is reflected off the back of the probe - as shown right.

In practice this simple system would risk damage to both the probe and the surface being studied. The scanning tip would tend to plough through large bumps, but may not probe to the bottom of deep holes. To get round this the sample is mounted on a piezoelectric stage, which allows the sample to be adjusted up or down by as little as 0.01nm. As the AFM tip rides over the surface, electronics drive the stage to keep the deflection of the laser beam constant - so it stays at the same height throughout the process.

The reason AFM is so important is that it can probe surfaces in air, and even under water. Earlier imaging techniques were restricted to use in hard vacuum - and often samples had to be pre-treated with metallic coatings.AFM techniques have the ability to look at living cellular machinery!