Cantilever tuning - only for tapping and non-contact modes
Once the laser is aligned onto the cantilever and the photodiode is centred, then the cantilever can be tuned. This means that the resonant frequency of the cantilever can be determined. This is almost always an automated process in the AFM software, and all the operator has to input is the frequency range to scan over and the target amplitude of the cantilever. In most tapping mode tip holders there is a small piezo-ceramic element that is located where the AFM chip is held securely in the tip holder. This piezo can be driven by a signal and made vibrate. This then vibrates the chip which then vibrates the cantilever. The resonant frequency of the cantilever can then be found by scanning through a range of frequencies and monitoring the amplitude of oscillation of the cantilever on the photodiode. When the peak with the highest amplitude is detected within the frequency range scanned then the AFM computer will assume this is the resonant peak for the cantilever. It is therefore important to scan though the correct frequency range as harmonics may be detected if the incorrect frequency range is used.
Manufacturers for AFM probes will supply them with nominal resonant frequencies or a range of frequencies which are a good guide to use. Figure 18 is an example of the type of software interface for cantilever tuning that many AFMs use. A number of parameters are able to monitored and/or adjusted but the main ones are start frequency, end frequency and target amplitude.