Pra.Ma.

Usage of Scanning Force Microscopy with oscillating cantilever was firstly anticipated by Binnig [1]. Earlier experimental realizations of scanning with oscillated cantilever was realized in works [2,3]. It was demonstrated influence of the force gradients on the cantilever frequency shift and possibility of non-contact scanning sample surface. It must be noted also that Durig studied frequency shift of oscillating cantilever under influence of STM tip [4].

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. Corresponding mode of Scanning Force Microscope operation (Intermittent Contact mode or Semicontact mode) is in common practice.
'Feeling" the contact repulsive forces under the scanning leads to the additional phase shift of cantilever oscillations relatively piezodriver oscillations. This phase shift depends on the material characteristics. Registration the phase shift under scanning (Phase Contrast Imaging mode) is very useful for nanostructured and geterogeneous materials. Similarly to Contact Error mode Semicontact Error mode can be employed for finding minor irregularities on large areas.
The Semicontac mode can be characterized by some advantages in comparison with dc Contact mode. First of all, in this mode the force of pressure of the cantilever onto the surface is less, that allows to work with softer and easy to damage materials such as polymers and bioorganics. The semicontact mode is also more sensitive to the interaction with the surface that gives a possibility to investigate some characteristics of the surface - distribution of magnetic and electric domains, elasticity and viscosity of the surface.

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).