Biomedical Engineering Reference
In-Depth Information
0.0
m
m
10.0
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m
Fixed microscope structure
20 nm
Scanning motion
0 nm
0.0
m
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10.0
m
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Fig. 6.10. Effect of scanner bow. Left: with tube scanners fixed at one end, the trajectory of the
probe is curved. Right: the result is an apparent curvature in the height of measured samples,
although the height change is small over a large area.
6.2.4 Edge overshoot in the Z axis
Hysteresis in the piezoelectric ceramic that moves the cantilever in the perpendicular
motion to the surface can cause edge overshoot. Hysteresis is an inherent property of
piezoelectric materials, and means that forward and backward movements are not exactly
equivalent. The effect in the
Z
axis affects the AFM's ability to trace accurately over step
profiles. This problem is most often observed when imaging microfabricated structures
such as patterned Si wafers or compact disks, but may be observed in any sample with
sharp-edged features. The effect can cause the images to appear visually better because the
edges appear sharper. However, a line profile of the image structure shows errors, as
shown in Figure 6.11.
Edge overshoot cannot be avoided by the user. It will only occur on microscopes
without a
z
axis calibration sensor, however. In cases where this occurs step height
measurements should only use the unaffected (flat) portion of the feature profile.
6.2.5 Scanner creep
Creep in piezoelectrics gives rise to the phenomenon that when an instantaneous voltage is
applied to the piezoelectric and maintained, the response of the material does not follow
exactly the applied voltage, but instead continues to move in the same direction as the
initial offset, even when the voltage is no longer changing. This is illustrated in
Figure 6.12. The practical effect of this is that when the user translates the scanning
position on the sample, moves the probe to the start of a new scan, or zooms into a previous
scan (all of which are done by rapidly changing the voltage applied to the piezoelectric),
distortion occurs in the image. The duration of this effect is limited, and eventually it
disappears. An example of this distortion ('scanner drift') is shown in Figure 6.12.
This artefact can be removed by simply waiting for the piezo position to stabilize. One
way is to make an initial scan in any new region, before recording a second scan free of
