In scanning force microscopy often dynamic modes are used, in which in contrast to static modes the micromechanical probe is interacting with the sample surface just temporary. Thereby in principle it is possible to reduce the forces exerted by the probing tip on the sample. The Q-Control module is a modification or hardware add-on to scanning force microscopes that comprises an additional feedback circuit. It allows to increase the effective quality factor of the dynamic system. As a results a further significant reduction of the interaction forces between the probe and the sample can be achieved.

Various applications show that many highly sensitive surface structures, in particular ultrathin organic layers or biological samples in liquids, can only be non-destructively imaged and characterized by using such an active feedback circuit. Furthermore, Q-Control often allows to improve the maximum achievable resolution in topographical measurements and to increase the overall sensitivity for magnetic or electrostatic fields.

When operating a scanning force microscope in a dynamic mode the oscillation of the cantilever is influenced by non-linear interaction forces between the probing tip and the surface. In principle the instantaneous forces exerted on the sample while scanning the surface can be either repulsive or attractive. Experimental findings and corresponding computer simulations of the tapping mode show that by choosing appropriate system parameters the scanning force microscope can continuously be operated in the regime of net-attractive interaction forces. Thereby the risk of modifying the sample surface by the probing tip is minimized. However, in most cases the range in which the system parameters have to be adjusted is rather narrow and therefore a stable operation of the scanning force microscope in this interaction regime is difficult to achieve.

With the help of the Q-Control module it is possible to reduce the damping of the dynamic system, i.e. to increase the effective quality factor of the oscillating cantilever and thereby to enlarge the regime of net-attractive interaction forces. This method allows the user to minimize the forces exerted by the probing tip on the sample surface. Therefore by applying Q-Control delicate and highly sensitive surface structures that could not be scanned with a standard scanning force microscope can now be characterized with high resolution.

In a liquid medium the oscillation of a cantilever is strongly affected by hydrodynamic damping. This leads to quality factors in the single digit range and a loss in force sensitivity. The Q-Control technique as a countermeasure allows increasing the effective quality factor up to three orders of magnitude in liquids.