Pra.Ma.

In [2] was demonstrated also possibility of materials sensing under abrupt decreasing of cantilever oscillation amplitude. Possibility of scanning sample surface not only in attractive but also in repulsive forces was demonstrated in [4]. Relatively small shift of oscillating frequency with sensing repulsive forces means that contact of cantilever tip with sample surface under oscillation is not constant. Only during small part of oscillating period the tip "feels" contact repulsive force. Especially it concerns to oscillations with relatively high amplitudes. Scanning sample surface with cantilever oscillated in this manner is not non-contact, but intermittent contact.

When probe tip under vibrating touches the sample surface it experiences not only repulsive but also adhesive, capillary and some other forces. So interaction of the tip with sample surface results not only in frequency but in the phase shift as well. If the sample surface is not homogeneous the same must be phase shift under scanning sample surface. Distribution of the phase shift under scanning over the sample surface reflects distribution of the sample material characteristics. Such mode of operation when phase shift under scanning is registered (Phase Contrast Imaging mode) is very useful for materials investigation.
Phase Imaging mode gives valuable information for a wide range of applications, in some cases giving contrast where none was anticipated from the material properties. This mode is used, for example, for biological objects, specimens with magnetic and electric properties and for a lot of other samples.

References

1. US Pat. 4724318.
2. J. Appl. Phys. 61, 4723 (1987).
3. Appl. Phys. Lett. 53, 2400 (1988).
4. Phys. Rev. Lett. 57, 2403 (1986).