Biomedical Engineering Reference
In-Depth Information
Fig. 2.7. The difference between sample-scanning (left) and probe-scanning (right) microscopes. In
a sample scanning AFM the sample is mounted on an x-y-z scanner and the force sensor remains
fixed. In the probe scanning AFM the sample remains fixed and the probe is scanned. The advantage
of a probe scanning AFM is that it can scan larger samples.
In this topic, the motion control mechanisms of the AFM stage capable of moving
several millimetres or greater (the coarse movement controls) are designated X , Y and Z .
The motion control mechanisms that are used for moving small distances (the x , y and z
scanners) are designated x , y and z .
The design of all AFM instruments can be divided into two different configurations as
illustrated in Figure 2.7. In the first configuration (left) the sample is scanned and the force
sensor is held in one place. In the second configuration, the sample is held fixed and the
probe is scanned. In general all AFMs can be divided into such sample-scanning or probe-
scanning microscopes. For sample-scanning AFMs, the mass of the sample is included in
the feedback loop, reducing the size of sample that may be probed, as well as practical
limits on the sample's dimensions. The advantage of the probe scanning (also known as
tip-scanning) microscope is that it can be used on any size of sample. In addition, because
there is nothing underneath the scanning probe except the sample, it is simple to add
accessories to this type of microscope. For example, a liquid cell is easier to use with a
probe scanning microscope, and they are easier to integrate with additional optical options,
for example to irradiate the sample from the side while scanning, or to mount the entire
AFM in an optical microscope. However, the construction of a probe scanning microscope
is much more difficult, as the whole tip-optical-lever assembly must be moved while
scanning, and care must be taken not to introduce further vibrations from the scanning
mechanism into the probe. A sample scanning AFM design is rather simpler, but some-
what limits sample size.
2.2.1
x-y-z scanners
Typically, the scanners used for moving the probe relative to the sample in an AFM are
constructed from piezoelectric materials. This is because such piezoelectric materials are
readily available, easily fabricated in desirable shapes, and cost effective. However,
 
 
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