Contact mode is typically used for scanning hard samples and when a resolution of greater than 50 nanometers is required. The cantilevers used for contact mode may be constructed from silicon or silicon nitride. Resonant frequencies of contact mode cantilevers are typically around 50 KHz and the force constants are below 1 N/m. Figure 4-6 illustrates a few of the many examples of contact mode AFM images:

FIGURE 4-6 Contact mode images: Left: Bits on a compact disk. Center: Image of a metal surface. Right: Nano-particles on a surface.

In order to make more sensitive measurements requiring better signal/ noise ratios in scientific instruments, it is common to modulate the signal being measured and use phase or amplitude detection circuits. Use of modulated techniques shifts the measurement to a higher frequency regime where there is less than 1/f noise. Such techniques were developed for the AFM soon after it was invented.

In order to make the S/N ratio higher, and thus be able to measure lower forces with the AFM, the probe is vibrated as it is scanned across a surface. As shown in Figure 4-7, the probe is vibrated in and out of surface potential. The modulated signal can then be processed with a phase or amplitude demodulator.

FIGURE 4-7 Left: Potential diagram showing the motion of the probe in vibrating mode. Right: The probe vibrates as it scans across a surface.