Biomedical Engineering Reference
In-Depth Information
However, with the scan calibration sensors operating in a closed-loop configuration,
zooming to a specific scan location requires no intermediate scans.
z
axis measurement
Correction of hysteresis and creep in the
z
axis is different from the correction in the
xy
axis. This is because the
xy
axis motions are predetermined and the
z
axis motion is non-
deterministic, and depends on the surface topography of the sample being scanned. It is not
possible to predict the surface topography, so closed-loop methods will not work. There-
fore, AFMs with
z
calibration sensors use an open-loop configuration for measuring
heights. In a
z
-sensored AFM, when accurate height data is required, the
z
-sensor signal
is used instead of the
z
voltage to directly measure the height signal. Typically, the AFM
software will allow the user to use either the
z
voltage signal (which has lower noise, and is
thus more precise), or the
z
sensor signal (which is more accurate).
2.2.1.4 Three-dimensional
x-y-z
scanner configurations
Piezoelectric ceramics must be configured so that they can move the probe, or sample, in
the
X
,
Y
and
Z
axes. There are a few standard configurations that are used in AFM
instruments. They are the tripod, the tube, and flexures (see Figure 2.13). Each of these
designs may be configured for more or less motion, depending on the application for
which the scanner is being used. It is also possible to create scanners that use a combin-
ation of any of the three basic designs. Currently, the tube scanner is the most widely used,
and is the scanner configuration present in
75% of AFMs in use. This type of scanner is
so widely used because it is very compact, allows very precise movements especially at
small scan ranges, but mainly because it is simple to fabricate. It is also particularly
convenient to engineer a probe-scanning AFM with a tube scanner, because there is a clear
optical path down the centre of the tube. However, it has some disadvantages; tube
scanners, due to their geometry are subject to a lot of non-linearity, particularly bow (an
example of the effect of scanner bow is shown in Section 6.2), when using the full range of
the scanner.
>
2.2.1.5 Scanners for fast AFM
In order to develop an AFM that is able to scan much faster than normal, the scanner must
be able to overcome the limitation of the traditional scanners, which is their low first
resonant frequency. A scanner with a higher resonant frequency will allow faster scanning
without the scanner going into resonance. Ando
et al
. have made significant progress in
this direction [3, 4, 34]. For example, a fast scanner has been constructed from piezoelec-
tric stacks, to achieve a high resonant frequency of 240 kHz versus 15 kHz for a typical
tube scanner [34]. An alternative technique is to use resonant scanners [35, 36]. This
means a very high scan rate can be used, but the scan rate is fixed. Typically these are
constructed from high resonant frequency flexure scanners, or can also be constructed with
tuning fork arrangements, although these are somewhat impractical for large samples [35].
Fast scanning AFM systems have been shown to achieve scanning as fast as 80 ms per
frame in intermittent contact mode in liquid [4], or even as fast as 1 ms in contact mode
(albeit without full feedback) [35], compared to
ca
. 100 seconds for a normal AFM.
However, in order to scan samples with significant topography, the greatest challenge is to
create a
z
-axis positioner whose response is fast enough to react to rapid changes in the
sample height, due to extremely fast
x-y
scanning over the sample.
