Biomedical Engineering Reference
In-Depth Information
(b)
40 nm
18
(a)
15
L
12
H
9
6
69 2
z piezo displacement (nm)
15
18
21
24
Fig. 4.10. Left: amplitude-distance curve showing jumping from a high amplitude state (H) to a low
amplitude state. Right: effect of this jumping on an image. The dashed lines in the left-hand figure
correspond to the set-points used in the three regions in the right images. With a set-point near the
discontinuity in amplitude-distance, unstable imaging will occur. Reproduced with permission
from
341
. Copyright 2000 by the American Physical Society.
set-point. This sort of situation will lead to instability in imaging. The origin of the
instability in the image on the right would be unclear without the amplitude-distance
curve. If such a feature is observed by the user, he should change the amplitude set-point to
a value with a unique solution, shown by the upper and lower segments in the image in
Figure 4.10. This example illustrates that sometimes scanning parameters can be adjusted
in one of two directions in order to improve imaging. In the case shown above, the best
solution might be to increase the set-point, rather than decrease it, as it will result in lower
tip wear.
As described previously, in IC-AFM there are normally four types of signal that may
be saved as images. There are signals of four types - the height (
zpiezo
voltage signal,
and
z
sensor, if available), amplitude (error signal), and phase signals. In addition, each
channel may be obtained in one of two directions, or in both. It is up to the user which
images to record and save. The height signals are the most important, as they are the only
signals with a meaningful
z
scale, and the only signals from which we can make useful
topographical measurements. It is not really necessary to collect signals in both direc-
tions, so only one height signal (typically the
z
sensor data, if it's available, otherwise the
z
piezo voltage) is normally collected. The amplitude signal can help in visualizing the
shape of the sample, and in spotting features for later measurement in the height images.
The phase image can serve a similar purpose, and in addition gives information about
heterogeneity of the sample (see Section 3.2.3.2 for more discussion on this). Thus, the
phase image can be highly useful on certain samples. It is rare that both forward and
backward images are needed, so typically three images will be collected, height, amp-
litude and phase, either in forward or backward directions. If this is the case, it is
important the user remembers to collect all images in the same direction, as forward
and backward images may not be perfectly aligned with each other. If the phase image is
