FIGURE 2-36 Comparison of line profiles measured with a dull probe (Left) and a sharp probe (Right). The profile made with the sharp probe shows the bottom of the trench, whereas the dull probe cannot reach the bottom of the trench. The sharp probe is unable to reach the bottom corners of the trench.

Materials

In principle, AFM cantilevers can be fabricated from any material that can be fabricated into a spring like cantilever. The first AFM cantilevers were fabricated from tungsten wire and had a probe etched in the silicon at the end. Early in the evolution of AFM it was discovered that the best AFM probes could be constructed from MEMs technology. There are two materials commonly used for AFM cantilevers; SiN and Si.

SiN is used for creating probes that have very low force constants. The thin films used for creating SiN probes must have very low stress so the cantilevers don’t bend naturally from the stress. Practically, most SiN films have some residual stress and in fact, cantilevers made with SiN tend to have curvature along their primary axis.

Cantilevers fabricated from Silicon tend to have less residual stress than SiN and tend not to suffer from bending. However, the Si probes that are fabricated at the end of the cantilever can be brittle and tend to chip if they contact a surface. Most of the cantilever/probes used in LL-AFM force sensors are constructed from Si.

Two basic geometries are used for AFM cantilevers, rectangular and triangular. The two primary shapes for probes are pyramidal and conical. Typically SiN probes are pyramidal and Si probes are conical, see Figure 2-37.

AFM cantilevers were initially fabricated from SiN in a triangular shape. Because of the cantilevers bending, Si became the preferred material.