Artifacts; Tip Shape Issues - JPK instruments nanowizard afm Handbook

7. Artifacts

An ideal AFM image is an accurate representation of a sample surface. Every part of
an image that differs from the sample surface is an artifact. As with any analytical
technique, scanning probe microscopy is not free of artifacts, so the microscopist must
be able to recognize them to interpret his images properly. There are several sources
of artifacts in AFM.

7.1 Tip shape issues

tip
The shape of the AFM tip can have a drastic effect on the images that are
acquired. This is one area where having reproducible probes is an advantage, if
scan
sample
the tip shape is well characterized, so that the images can be better interpreted,
and the obvious artifacts identified.
The following scheme gives an impression of how the tip shape can influence the
image of a given feature on the sample. The feature taken here as an example is a
perfect rectangular step on the surface. None of the tips shown produce an exact
image of the feature. The image is always some combination of the tip shape and
the true surface topography. The sharpest, narrowest tip produces the most
accurate representation of the surface.
A practical example is shown on the right. The AFM image shows a 3D view of a
red blood cell with protrustions on the surface. In fact, the rim of the cell is rather
steep and not shaped like a ramp as displayed in this image. The apparent ramp
shape is caused by the edges of the pyramidal shaped cantilever.
Two parameters commonly used to model tip geometry are a cone angle of the
main pyramid that forms the tip, and an equivalent radius of the tip end. The
images of small sharp features on the surface are dominated by the tip radius,
while the images of larger ones are dominated by the cone shape of the tip. The
cone angle of the tip also has an effect on the images of depressions in the
surface, changing the apparent side angles and sometimes even preventing the tip
reaching the bottom of the depression. Regions with shallow features and a
gradient that changes gently are reproduced well by the tip, however.
The relationship between the observed width W of a feature and the diameter of
the probe tip can be calculated for an idealized tip shape, such as the one shown
here.
= − −
2 2 2
x R ( R d )
2
W
R >> = =
d W 8 dR d
For , and
8 R
For R = 10 nm and d = 5 nm, the observed width would be W = 20 nm
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JPK Instruments NanoWizard Handbook Version 2.2a 37