Biomedical Engineering Reference
In-Depth Information
order of 5 nm or less) can be considerably more demanding. To obtain very high
resolution a large number of factors must be optimized.
The probe tip must be clean and particularly sharp. Even amongst probes rated as extra
sharp, a large variation in actual tip radius is likely to be found as discussed in Chapter 2.
For demanding applications, several tips could be tried, or a tip-check sample can be used.
When all else fails, attempting to scan a well-known sample (especially one of the probe
sharpness characterization samples) can often help to diagnose problems. Typically, if
great results on such a tip-checker sample cannot be obtained, they won't be found from
the sample of interest either. A list of samples suitable to characterize AFM probes is
included in Appendix A.
The sample must be well fixed to the substrate, which should not be moving. The
instrument must be at thermal equilibrium, and without drift. Sample drift is fairly easy to
spot, and an illustrative example is given in Section 6.6.4. Sometimes the method used to
fix the sample its substrate can be at fault, and a more rigid mounting (such as gluing with
an epoxy adhesive) can help. Thermal drift characteristics can sometimes be helped by
removing sources of illumination, which can heat the sample environment. Often, thermal
drift is reduced with time, so leaving the instrument set up, with the laser aligned, the tip
close to the sample, or in feedback with it, and the oscillation (if used) at the correct
frequency, for 30 minutes to an hour, can reduce drift considerably.
Sources of external noise and the vibration isolation must be optimized. When
scanning very flat samples at very high resolution, noise in the image that was previ-
ously invisible can often be seen in the image. In this case, the user must simply remove
all possible sources of noise, such as lights or electronic equipment that are not required,
and ensure the vibration isolation is fully functional, and uncompromised (e.g. by a
mechanical connection from the stage to an un-isolated surface).
Finally, scanning parameters must be optimized. For very small scans it is possible to
scan very quickly, as usually the feedback system does not have great changes in
z
-height
to cope with. In addition, it is usually necessary to scan very quickly to overcome even
small amounts of sample drift when imaging very small areas. For example, to obtain
'atomic lattice' resolution, with a scan size of ca. 10 nm, it is common to scan at about 60
lines per second. For high-resolution images, the perfect feedback is often found by
making many tiny changes to the gains to reach the ideal imaging conditions.
Force-distance curves are measured by monitoring the deflection of the cantilever as it
approaches, touches, and withdraws from the sample. By default, therefore, they are
measured in contact mode. Parameters to be selected by the user will include the
x
and
y
positions at which the curve is to be recorded, data density, movement speed (acquisition
rate), maximum allowed deflection (force), length of the curve and more. If the area of
interest is located in a particular region, it can be useful to image the sample before
measuring curves. However, under some circumstances, such as when the cantilever is
very stiff, or has been modified with a layer of molecules, it is not convenient to do
imaging and force spectroscopy at the same time, especially imaging in contact mode
which can damage a sensitive layer on the tip. So AFM software often has a separate mode
