W. Richard Bowen and Nidal Hilal 4

8 1. BAsIC PRINCIPLEs OF ATOMIC FORCE MICROsCOPy
shape of the probe is of fundamental importance in determining those
interactions. In addition to the radius of curvature at the apex of the
probe, the geometry of all of the parts of the probe which can interact
with the sample are of great importance, particularly when imaging or
performing indentation measurements.
When imaging a sample surface, features of greatly varying geometry
may be encountered. The ability to resolve these features depends
upon both the sharpness of the probe tip and the aspect ratio of the
probe. First, the probe sample contact area is a limit to AFM resolution.
This is dependent not just upon the sharpness of the probe tip,
but also upon the force with which the probe presses into the surface
and the consequent mechanical deformations induced in the probe
and sample. As the sample is deformed, the probe tip will become
indented into the sample and a greater part of the probe surface will be in
direct contact with the sample. Conversely if it presses against a hard
sample with sufficient force, the probe itself may become deformed,
similarly contributing to an increased interaction area. This interaction
will be dependent upon the shape of the probe as well as purely on the
radius of curvature found at the apex. Features present upon the surface
which are smaller in size than the contact area will be unable to be
successfully resolved.
When asperities, which are sharper than the probe, on the surface are
encountered, the image of the feature which is obtained will be based
more upon the shape of the probe than upon the surface feature [51, 52]
due to convolution effects. This is a problem potentially arising in all
forms of SPM. This may be commonly observed when low aspect ratio
probes are scanned over surfaces with high aspect ratio asperities. In
effect the probe is imaged by the surface. A similar effect may be seen
when the probe encounters a step edge on the sample, which is steeper
than the side of the probe. In this case a broadening effect will occur
where the feature under observation will appear to be wider than it actually
is. These convolution effects serve to produce images which rather
than being true representations of sample topography represent a composite
of the topographies of both sample and probe. The finer the tip
and higher the aspect ratio of the probe, will provide images which are
a truer representation of the real topography of the sample.
.4 ImagIng modes
There are many different imaging modes available for the AFM, providing
a range of different information about the sample surfaces being
examined. However, for simplicity the most common modes will be
considered here. In Figure 1.3 the force regimes under which the main