Biomedical Engineering Reference
In-Depth Information
Fig. 2.13. Configurations of common AFM scanners. Top: a tube scanner is configured so that it
moves in the
x-y
-
z
axes. Four electrodes on the outside are used for the
x-y
axis motion, and the
inner electrode is used for the
z
axis motion. Middle: a flexure scanner operates by pushing on a
flexure with a piezoelectric which then causes the stage to move. There is a gain in the motion
given by the ratio of
L
2
/
L
1
. A one-dimensional flexure is shown for clarity, typically flexure
scanners are set-up to scan in the
x-y
axes. Bottom: the simplest three-dimensional scanner, the
tripod scanner.
2.2.2 Force sensors
The force sensor in an AFMmust be able to measure very low forces. This is because, for a
very sharp probe to be used, a low applied force is required so that the pressure (force/area)
can be low enough so that the probe is not broken.
A number of different force sensors have been tested and demonstrated to work with an
AFM. Some of these force sensor designs are illustrated in Figure 2.14. The use of an
optical lever (sometimes known as a light lever), used routinely for measuring minute
motions in scientific instrumentation, was first demonstrated in an AFM in 1988 [23].
With the advent of microfabricated cantilevers the optical lever AFM became the most
widely used design for the force sensor in an AFM, and today, nearly all AFMs employ
optical lever force sensors.
