Biomedical Engineering Reference
In-Depth Information
Fig. 2.15. Schematic diagram of the optical lever sensor. In an optical lever, as the end of the
cantilever bends the position of the laser spot on the detector changes. As the cantilever - detector
distance
D
cd
is large, a small movement of the cantilever causes a large change in the laser spot
position at the detector.
The cantilever in the optical lever AFM is typically fabricated with a MEMS process.
The cantilevers are small, generally between 50 and 300 microns long, 20-60 microns
wide, and between 0.2 and 1 micron thick. Section 2.5 has a more detailed discussion of
the cantilevers and probes used in an AFM. The optical lever AFM force sensor
requires alignment each time the probe is changed. Typically, alignment is accom-
plished by first positioning the laser beam onto the cantilever, and then confirming that
the light is reflected onto the centre of the photodetector by looking at the photode-
tector signal. This alignment procedure is rather time-consuming, and is not always
fully reproducible; small changes in the laser alignment can affect the force-sensitivity
of the system. The alignment procedure is one of the disadvantages of the optical lever
system. A procedure for optical alignment is given in Section 4.2. The laser can also
give rise to image artefacts as shown in Section 6.6. In the ideal optical AFM design,
theprobewouldhavea90
angle with respect to the surface. Practically, however, this
is not possible because of the constraint of the mechanism that holds the probe in place.
This requires that there be an angle between the probe/cantilever and the surface, to
ensure that only the tip of the probe touches the sample. This angle is usually between
5
8
8
8
. Such angles can also cause artefacts in the images. Some probes are
available with a counter-angle built into the geometry, i.e. the tip is mounted onto
the cantilever at
ca
.12
and 15
so that it can approach the sample at an angle close to the
perpendicular. The optical lever sensor is by far the most widely used force sensor for
AFMs. The following sections cover the design and implementation of optical lever
force sensors.
8
2.2.2.2 Integrating optical lever force sensors and scanners
The first AFM designs scanned the sample and kept the probe stationary. This sample-
scanning design is optimal for only limited types of sample. To create tip-scanning AFMs
it is necessary to design AFM scanners where the
x-y
-
z
scanner is integrated with the
