Biomedical Engineering Reference
In-Depth Information
Fig. 2.16. Designs for tip-scanning AFMS with optical lever sensors. Left: the laser is scanned with
the cantilever. Right: the laser is fixed and the cantilever is scanned, a lens keeps the laser light
focused on the cantilever.
optical lever AFM force sensor. The simplest approach to integrating the x-y - z scanner
would be to mount the optical lever sensor at the end of the scanner. This is not feasible
because the z piezo is not responsive enough to move the entire light lever up and down as
the probe is scanned across the surface. Such an AFM would be too slow to be practical.
Two methods are employed for creating a combined optical lever AFM scanner with an
x-y - z scanner.
In the first configuration, illustrated schematically on the left of Figure 2.16, the laser
and photodetector are scanned in the X-Y axis, and the probe is mounted at the end of the Z
piezoelectric. In this design the z piezo is part of the optical lever optics. This means that as
the probe is moved up and down in the Z direction the light path changes. However, it can
be shown geometrically that the Z motion of the cantilever has a minimal effect on the
operation of the AFM optical lever AFM sensor. In this design, commonly the x-y scanner
would be a flexure scanner, and the z scanner a simple piezo stack. Also illustrated in
Figure 2.16 is the other approach that is commonly used. The laser is held fixed and a lens
is used to focus the laser light onto the scanning cantilever. As the lens moves back and
forth in the X-Y plane, the laser light stays focused on the cantilever. The photodetector
must be then mounted on the x-y translator.
2.2.3 Coarse
movement - probe-sample approach
One of the major challenges in AFM design is making a motion control system that
permits the approach of the probe to the surface before scanning. This must be done such
that the probe does not crash into the surface and break. An analogous engineering
challenge would be to fly from the earth to the moon in 60 seconds and stop 38 meters
from the surface without overshooting or crashing.
Z
 
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