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Q-Control
Principle
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. Therefore, 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 enlarging 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. By applying Q-Control, therefore, delicate
and highly sensitive surface structures that could not
be scanned with a standard scanning force microscope
can now be characterized with high resolution.
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(Right) Resonance curve of
a free oscillating silicon cantilever. By applying
Q-Control the effective quality factor can be increased
from about 450 to almost 20,000, i.e. by more than
one order of magnitude. In general a much smaller
increase of the effective quality factor is already
sufficient to ensure a stable operation of the SFM
in the net-attractive interaction regime. Note
that not just the shape of the amplitude curve was
modified by Q-Control, but that also the slope of
the phase signal increased. This of great relevance
when it comes to increasing the overall sensitivity
for magnetic or electrostatic fields.
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