W. Richard Bowen and Nidal Hilal 4

4 1. BAsIC PRINCIPLEs OF ATOMIC FORCE MICROsCOPy
A B
C D
fIgure .2 Example of SEM images of different probes and cantilever types.
A: pyramidal probe; B: conical high aspect ratio probe for high resolution imaging; C: two
V-shaped cantilevers for contact mode imaging; D: chip with a series of beam-shaped levers
of different lengths. In this case the levers are tipless to allow mounting of particles of interest
for force measurements.
used (these two configurations are referred to as tip-scanning or surfacescanning,
respectively). Movement in this direction is conventionally
referred to as the z-axis. A beam of laser light is reflected from the reverse
(uppermost) side of the cantilever onto a position-sensitive photodetector.
Any deflection of the cantilever will produce a change in the position of
the laser spot on the photodetector, allowing changes to the deflection to
be monitored. The most common configuration for the photodetector is
that of a quadrant photodiode divided into four parts with a horizontal
and a vertical dividing line. If each section of the detector is labelled A
to D as shown in Figure 1.1, then the deflection signal is calculated by
the difference in signal detected by the A � B versus C � D quadrants.
Comparison of the signal strength detected by A � C versus B � D
will allow detection of lateral or torsional bending of the lever. Once
the probe is in contact with the surface, it can then be raster-scanned
across the surface to build up relative height information of topographic
features of the sample.