Biomedical Engineering Reference
In-Depth Information
0
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Fig. 6.8. Effect of
x-y
non-linearity in AFM images. Left: example of an AFM image of a test
sample (TGX01, see Appendix A) when scanned without correctly linearizing the AFM scanner.
Right: linearized AFM image of the same sample. The spacing of the squares at the top, bottom, left
and right sides should be all the same distance apart. Images courtesy of Mikromasch.
to produce, and gives rapid and very precise response under most circumstances. How-
ever, most AFM scanners do introduce some artefacts into the images obtained, the tube
scanner more than most. The artefacts described in this section all occur with piezoelec-
tric tube scanners. Many of them are avoided when using a linearized scanner (see
Chapter 2).
6.2.1 X-Y calibration/linearity
All atomic force microscopes must be calibrated in the
X-Y
axis so that the images and
measurements obtained are accurate. The motion of the scanners should also be linear so
that the distances measured from the images are accurate. Due to the non-linearity of
piezoelectric scanners, without correction, the features on an image will typically appear
smaller on one side of the image than on the other, see Figure 6.8. Once the scanner is
properly linearized, it is also critical that the scanner be calibrated. In other words, it is
possible for the scanner to be linear but not calibrated. If the calibration is incorrect, then
the
X-Y
values measured from line profiles will be incorrect.
A common method for correcting the problems of
X-Y
non-linearity and calibration is to
add calibration sensors to the
X-Y
piezoelectric scanners. These sensors can be used to
correct the linearity and the calibration in real time; often, such a system is described as
having linearized scanners. If these are not available, and non-linearity is detected in
images, then the instrument should be re-linearized according to the manufacturer's
instructions. Typically this is carried out with a test grid as illustrated above, and in
Appendix A. Note that non-linearity at just one edge of the image could be due to other
effects; see the other sections in this chapter.
6.2.2
z calibration and linearity
Height measurements in an AFM require that the piezoelectric ceramics in the
Z
axis of the
microscope are also both linear and calibrated. Usually the microscope is calibrated at only
