3.5.2 Non-contact (tapping) mode
If the probe is taken further away from the sample, into the region beyond the maximum on the force-separation curve of Figure 7, it can be operated in non-contact mode. Typically, the separation required is of the order of 10 nm. The attractive force is much smaller than the repulsive force of the contact mode, and its gradient with respect to separation is also much lower. To make matters worse, in order to prevent the probe tip being drawn into contact mode by the attractive force, a higher-stiffness cantilever is used. These two factors make the deflection signal from the cantilever much smaller than in contact mode. In order to improve the signal-to-noise ratio, the cantilever is often operated in a different way: using a piezoelectric actuator near its root, it is excited into oscillation in the vertical direction very close to, but slightly above, its natural resonant frequency. Typically, this will be a frequency of a couple of hundred kilohertz, with an amplitude of a small number of nanometres. At the separations used in non-contact mode scanning, the attractive force between tip and sample increases as the tip approaches the sample. The effect of this on the oscillating probe is to cause it to linger a little longer when it is near the sample – it's as though the spring softens a little on this part of the cycle. This spring softening lowers the resonant frequency of oscillation of the cantilever, and because we are operating near resonance this leads to marked changes in the amplitude of vibration. This method is sometimes called ‘tapping mode’.
Thus, by monitoring the vibration of the cantilever (either its frequency, its amplitude, or its phase relative to the driving signal fed into the piezoelectric actuator), and using the z-drive on the microscope to maintain this signal at a constant value, the average separation of the tip and the sample can be kept constant, too. The z-drive signal then provides the data for the topographic image that is generated. Non-contact mode has a much lower resolution than contact mode (3 nm lateral, 0.1 nm vertical are typical), but it has the advantage of imposing very tiny forces on the sample, and it is therefore suitable for imaging biological molecules, which are easily deformed or damaged.