Set point - Tapping mode
In this mode of operation, set-point refers to the amplitude of oscillation of the cantilever at which the tip taps the sample surface. In this case lowering the amplitude set-point will increase the tapping force exerted on the sample, and increasing the amplitude set-point will reduce the tapping force. Similar constraints apply to this parameter for tapping mode as for contact mode. If the set-point is too high, then the tapping force is reduced to such a level that the tip may not track the surface properly and move in and out of feedback. This will result in poor image tracking, similar to that displayed in Figure 13. If the set-point is too low, and the tapping force is too high, then the tip may become damaged and artefacts will result. In general, the amplitude set-point will be 50 - 90 % of the cantilever free amplitude.
When changing these parameters it is important to remember that they can all affect each other. Therefore it is good practice to optimise each one and then repeat. For example
- Optimise set-point
- Optimise gains
- Optimise scan rate
Then repeat until maximum image quality is achieved.