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V.
Measurement Procedure
This chapter will lead you through your first measurement in the different
AFM modes available.
V.1.
Operation Modes
Different operation modes are possible. The two fundamental ones are the
Contact Mode - with and without feedback - and the Non-Contact Mode (also
intermittent or modulation mode).
V.1.a.
Contact Mode without Feedback (constant height)
In contact mode, the tip is brought in contact with the sample. The sample
is scanned without any feedback control of the z scanner, i.e. topographic
variations are directly translated into a deflection of the cantilever. The
photo detector signal is acquired during scanning, which can be directly
converted into a height signal by using the formula:
V
dV
nm
V
dV
dz
246
2
Here,
V is the peak-to-peak amplitude of the interferogram; dz is the
change in height corresponding to a change dV in detector voltage.
Important note: This mode is only suitable for samples with a maximum
height corrugation of
/4! Otherwise, the contrast may be inverted.
The contact mode without feedback, also called const.-height mode, is the
easiest and fastest way to record an AFM image, because it is not depending
on z feedback. Please note that the image topography is inverted if the
working point within the interference signal is set to a negative slope.
For contact mode scans, cantilevers with a small stiffness (< 5 N/m) are used
in order to keep the forces between tip and sample small.
