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companies. Any rights not expressly granted herein are reserved. ATTENTION: Specifications and technical data are subject
to change without notice.
Figure 15
: Schematic drawing of the attoAFM I operated in contact mode without feedback.
V.1.b.
Contact Mode with Feedback on (constant force)
The tip is first brought into real contact with the sample surface. In this
mode, however, a feedback mechanism is employed to keep the deflection
signal at a constant value by varying the voltage of the z-scanner. This will
maintain a constant force between tip and sample. The feedback setpoint
should be set to the point of maximum slope in the interference signal
(working point), i.e. to the middle between two interference extrema. The
main measurement signal in this mode is the voltage applied to the z
scanner, which can be translated into height data by using the z piezo
calibration value (usually this is automatically done by the controller
software). The error signal is the detector signal.
This mode is suggested for samples with height corrugations >
/4!
Since a feedback always causes low pass filtering, the measurements in
constant force mode are usually slower but less noisy than constant height
images.
V.1.c.
Non-Contact Mode
In this mode, the cantilever is excited at its resonance frequency by an AC
voltage applied to the dither piezo. Correspondingly, the photo-detected AC
signal at the resonance frequency reflects the oscillation amplitude of the
cantilever. As the cantilever approaches the sample, this vibration amplitude
decreases rapidly with diminishing tip-sample distance. Note that the
oscillation amplitude is already reduced due to long-range forces (such as
van der Waals- or electrostatic/magnetic forces), i.e. t
he tip is not
necessarily physically touching the sample.
At this point, a oscillation amplitude called ‘set level’ is defined,
corresponding to a given force between sample and cantilever. The vibration
amplitude of the cantilever serves as the input to a feedback loop which
maintains the cantilever oscillation at the set level by adjusting the voltage
on the z-scanner (amplitude feedback). Alternatively, the amplitude and the
