3.6 Design considerations for AFM probes
AFMs have proved so useful in so many areas of science and engineering that they are now to be found in most universities and many manufacturing companies. The making of probes for these instruments is no longer a cottage industry, partly because of the sheer numbers involved, but also because of the need for consistency of performance from probe to probe. This requires precise shaping, both of the tip itself and of the cantilever on which it is mounted. The quality of image obtained from an AFM is greatly dependent both on the probe tip's shape and the elastic properties of the cantilever. For the cantilever, it is its lateral and vertical stiffness, its resonant frequency, and the quality (or ‘Q’) of that resonance that matter.
Resonance is described in more detail in Section 6 Vibrations and resonance.
For tapping-mode and frictional-mode atomic force microscopy, the preferred shape of the cantilever is just a rectangular or trapezoidal prism. This is because this shape has simple and easy-to-analyse mechanical resonant behaviour, and has low torsional stiffness, enabling a high sensitivity to lateral forces on the probe tip. Figure 10 shows a typical example.
In contrast, for the highest-resolution images, obtained in the so-called contact mode, the cantilever needs to have as much stiffness as possible in all directions except for vertical motion, where its stiffness needs to be sufficiently low to allow the force it exerts on the sample to be accurately controlled. For this, the optimum solution turns out to be the triangular double cantilever. Figure 11 shows one of these.