Biomedical Engineering Reference
In-Depth Information
low-frequency size), to take account of the frequency shift as the tip approaches the
sample. Selecting the wrong frequency (such as one at a higher frequency than the
amplitude maximum) may allow imaging, but will usually give very poor images and
possibly image artefacts. Having selected the operating frequency, the amplitude of the
driving piezo oscillation is adjusted to give the desired oscillation amplitude of the
cantilever. The oscillation amplitude, like the cantilever deflection, is normally shown
only in terms of the photodetector output (e.g. rms amplitude in volts), so the desired
signal varies from one instrument to another, but as discussed in Chapter 3, amplitudes in
intermittent-contact AFM can vary from 1 to 100 nm [108]. In most AFM systems, an
amplitude set-point is then chosen. For contact AFM, the set-point is a deflection value,
which means that increasing the set-point leads to greater forces between the tip and the
sample. However, for intermittent-contact mode, feedback is based on a
decrease
in
amplitude, so a lower set-point means a greater tip-sample interaction force. For example,
if the free oscillation amplitude,
A
0
is 1.0 V, the user might choose 0.9 V as a conservative
set-point, meaning the free amplitude will be allowed to decrease by 10% during approach
at which point the system will go into feedback. Thus, a value of lower than 0.9 V would
mean a greater force of interaction, and vice versa. Now, unlike contact-mode probes, IC-
AFM probes are highly stiff, and so they are less prone to thermal noise and bending, and
thus oscillation amplitude is highly stable. This should mean that false-engage is less of a
problem. However, long-range forces between tip and sample do usually affect the
oscillation slightly when operating in air. So, the user must once again be careful to
avoid false-engage, so that it might be necessary to use a lower set-point than 90%. It is
useful, again, to observe the error signal (the amplitude) as the tip approaches the sample,
to help diagnose false engage. Once the oscillation frequency and amplitude set-point are
chosen, an approach may be made. Due to the change in resonant frequency as the probe
approaches the sample, it is sometimes helpful to withdraw the probe a little after a
successful approach, and re-optimize the operating frequency.
Having approached successfully, scanning and optimization of gains are very similar to
contact mode. So the procedure described in Section 4.2.3 can be used. The first-time user
is reminded that the imaging mechanism for IC-AFM is completely different from that of
contact-mode AFM (see Chapter 3). This means that optimal imaging parameters will
usually be completely different for contact and IC-AFM, even on the same sample, and
using similar probes. One aspect to be aware of is that the response of the probe to large
topographic changes is rather slow in IC-AFM compared to contact mode, meaning
scanning may need to be carried out more slowly. If the tip does not properly track the
surface, either the speed may be decreased, the gains increased, or the amplitude set-point
decreased. An example of the effect of scanning too quickly is shown in Chapter 6. Note
that unlike in contact mode, it's not really possible to make force-distance curves in
IC-AFM mode. One reason for this is that the cantilever is so stiff that trying to do this
would apply a very large pressure to the tip of the probe, and damage it. However, often
AFM systems do allow the user to obtain amplitude-distance curves. An example showing
the utility of this is shown in Figure 4.10. Amplitude-distance curves have also found use
in measuring long-distance forces on the tip, for example in MFM [340].
As described by Garcia
et al
. [341, 342], this sort of curve can serve a useful diagnostic
purpose. As shown in Figure 4.10, it is possible to observe non-ideal curves, i.e. curves
where there is more than one possible tip-sample distance at a particular amplitude
