Biomedical Engineering Reference
In-Depth Information
Fig. 6.5. Example of double-tip imaging. Left: an image of vesicles measured with a dirty tip. Right:
DNA molecules measured with a broken tip, each molecule has a false 'twin' next to it. Centre: a
badly broken and contaminated tip which produced double-tip images like these.
6.1.3 Probe-sample angle
When scanning large features, artefacts can be introduced by having a large angle between
the probe and the sample, as illustrated in Figure 6.6. Ideally, the AFM probe should be
perpendicular to the sample surface.
Solving this problem is achieved by adjusting the angle between the probe and the
sample so that they are perpendicular. Often, a set of three adjustment screws on the
microscope allows the user to adjust this angle. In many microscopes the probe is designed
to be at a 12
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angle with respect to the sample, and some probes are designed with this
angle in mind, i.e. such that when the cantilever substrate is at 12
to the sample, the probe
will be perpendicular to it. Some AFMs do not have mechanical adjustments to control the
probe-sample angle. In this case, the sample must be adjusted to correct the probe-sample
angle.
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6.1.4 Side-wall/probe imaging
Certain samples with extremely high-aspect-ratio features are very difficult to image
correctly, and they can interact with the probe in such a way that the image contains
repeating images of the probe, or of the side-walls of the probe. Examples of features that
produce side-wall images are spherical micro-organisms, spherical particles or red blood
cells, with their typical doughnut-like shape, images of which often are great on the top of
the cell, but it's not possible to image the sides of the cell, and images of the probe side-
Fig. 6.6. Illustration of probe-sample angle problems. With the probe at an angle to the sample,
distortions are introduced, and sample features appear asymmetric.
