TappingMode AFM
10
Scanning Probe Microscope Training Notebook
Rev. F
4.0
TappingMode AFM
Figure 4.0a
Feedback Loop Electronics
TappingMode AFM operates by scanning a tip attached to the end of an oscillating cantilever
across the sample surface.
The cantilever is oscillated at or slightly below its resonance frequency with an amplitude ranging
typically from 20nm to 100nm. The tip lightly “taps” on the sample surface during scanning,
contacting the surface at the bottom of its swing.
The feedback loop maintains a constant oscillation amplitude by maintaining a constant RMS of
the oscillation signal acquired by the split photodiode detector.
The vertical position of the scanner at each (x,y) data point in order to maintain a constant
“setpoint” amplitude is stored by the computer to form the topographic image of the sample
surface. By maintaining a constant oscillation amplitude, a constant tip-sample interaction is
maintained during imaging.
Operation can take place in ambient and liquid environments. In liquid, the oscillation need not be
at the cantilever resonance.
When imaging in air, the typical amplitude of the oscillation allows the tip to contact the surface
through the adsorbed fluid layer without getting stuck.