FIGURE 2-15 Top: A PZT tube is configured such that it moves in the x-y-z axis. Four electrodes on the outside are used for the x-y axis motion, and the inner electrode is used for the z axis motion. Bottom: A flexure scanner operates by pushing on a flexure with a PZT. In this design, the ceramic pushes against the flexure which then causes the stage to move. There is a gain in the motion given by the ratio of L₂/L₁.

The force sensor in an AFM must be able to measure very low forces. This is because if a small probe is used the pressure, force/area, must be small so that the probe is not broken. A number of different force sensors have been tested and demonstrated to work with an AFM. Several of the force sensor designs are illustrated in Figure 2-16.

The light lever, used routinely for measuring minute motions in scientific instrumentation, was first demonstrated in 1988. With the advent of microfabricated cantilevers the Light Lever AFM (LL-AFM) became the most widely used design for the force sensor in an AFM.

The most widely used force sensor for AFM's is the light lever sensor. However, crystal sensors are rapidly gaining acceptance. The following sections cover the design and implementation of light lever and crystal force sensors.