SPM Fundamentals for the MultiMode
Review of TappingMode AFM
Rev. B
MultiMode SPM Instruction Manual
43
2.5
Review of TappingMode AFM
2.5.1 General Operating Concepts
One advantage of TappingMode AFM is an absence of frictional forces which exert torque on the
cantilever. Unlike traditional contact AFM, the feedback loop keeps a vibrating cantilever at a
constant amplitude, rather than keeping a cantilever at a constant de
fl
ection. The tip on the
cantilever is modulated through mechanical excitation at its resonance. A laser beam is re
fl
ected off
of a microfabricated cantilever, onto a mirror, then re
fl
ected onto a photodiode array. The laser spot
oscillates vertically across the array as a result of the vibrating cantilever. The signal from the
photodiodes is recti
fi
ed, then lowpass
fi
ltered into a DC voltage (
RMS Ampl.
). The magnitude of
the RMS amplitude is proportional to the amount of cantilever motion.
The feedback system compares the RMS amplitude to the setpoint voltage. The two voltages are
kept equal by controlling the amount of cantilever movement. The sample surface is in close
proximity to the cantilever. The distance is such that the tip touches the surface only at the lowest
point of its oscillation. The RMS voltage is reduced to the setpoint voltage by the feedback loop
moving the tip into the sample. The sample restricts the cantilever movement until the desired RMS
voltage is reached. The damping of the cantilever is held constant by moving the tip in Z as it is
simultaneously translated in X and Y.
The engagement of TappingMode AFM requires that the setpoint voltage be smaller than the RMS
voltage, which is set automatically by the software. The tip is lowered until the RMS reaches the
setpoint (see
and
for explanation).