Biomedical Engineering Reference
In-Depth Information
Fig. 4.7. Image of a test/calibration sample showing the effect of changing the gain settings during
scanning. The height image is shown on the left, and the shaded height is on the right, which shows
the fine details of the effects of the gain settings more clearly.
0
m
m
8
m
m
0
m
m
1
m
m
0
m
0
m
m
m
8
m
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1
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m
Fig. 4.8. Example of zooming to feature. Selecting a feature of interest in the left image gives the
image at right.
4.2.5 Other signals and measuring LFM images
When scanning is optimized the user may choose which signal images to save. The user
will always want to save a height image, whether from
z
scanner voltage or
z
sensor data,
as it is the only signal with a fully calibrated
z
scale, from which the user can make height
measurements. The error signal (cantilever vertical deflection signal) image can be useful
to appreciate quickly the shape of the sample, as well as to spot areas where the height
signal is invalid (areas of large or unchanging error signal). Some researchers publish
the error signal; often it is a simple way to display features at different heights in the same
image. The lateral deflection signal may or may not be useful. On many samples, the
lateral deflection signal will show no more details than the vertical deflection signal. This
is because lateral twisting of AFM cantilevers is much less sensitive than vertical bending
[77]. However, if there is a requirement to record the lateral deflection (for example, if
friction contrast is expected in the sample), it is simply a matter of selecting the signal to
be saved. Unlike the other channels, for which forwards and backwards signals should be
equivalent, it can be worthwhile recording both forward and backwards lateral deflection
