Thesis-PDF - IAP/TU Wien

1 µN) between a probe and the sample surface. Soon it became evident that
it was also a convenient tool for obtaining topological information of a surface
by rastering across it. In the recent years probe–sample interactions were also
increasingly used for quantitative analysis of mechanical, electronic, magnetic,
biological, and chemical sample properties. The AFM can map this information
with atomic resolution to the sample surface and in contrast to STM is not limited
to conductive samples. ([34], [8])
3.2 Principles of AFM
The Atomic Force Microscope (AFM) measures the forces between a probe, the
AFM cantilever tip, and the surface. In order to obtain a 3D image [z(x, y)]
of the surface, multiple scans [z(x)] displaced laterally from each other in the y
direction are acquired. The cantilever is rastered across the surface by means of
piezoelectric drives. This can be done either by dragging the tip over the surface
or by oscillating it just above the surface.
Figure 3.1: Generalized scheme of an Atomic Force Microscope (AFM)
with optical beam sensing. Image adapted from [35].
The movement of the cantilever probe can be sensed by either tunneling,
capacitive, or optical detectors. Optical beam deflection methods are often preferred
because of their sensitivity, reliability and ease of implementation ([36],
[37]). Advantages are a large working distance, insensitivity to distance changes
and capability to measure angular changes (i.e. related to friction forces) when
using a four-quadrant optical detector. A laser beam is deflected from the top side
of the cantilever onto a position-sensitive photodetector (see Fig. 3.1). Because
of the relatively large laser travel distance (in the cm range or more) the slightest
skewness or bending of the cantilever or change of resonance frequency while
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