Biomedical Engineering Reference
In-Depth Information
Scanning Tunnelling Microscope:
In the original AFM built in 1985 a scanning
tunnelling microscope tip was use to measure
the motion of a cantilever [19]. Although this
technique was viable, implementation and
operation were very difficult.
Interferometer:
A Michelson interferometer can be adapted to
measure the deflection of a cantilever in an
AFM [37]. Although very sensitive, the
interferometer was not successful because of
fringe hopping. That is, the probe could jump
between interference fringes while scanning.
Crystal oscillator:
A piezoelectric crystal such
as quartz can be used to measure the force
between a probe and a surface [38]. If the
probe mounted on the crystal is vibrated and
positioned close to a surface, the interaction of
the probe and surface will cause a change in
the vibration. This change is proportional to
force.
Piezo-resistive cantilevers:
A cantilever can be fabricated that has a small
piezo-resistive element in it that changes
resistanceif the cantilever bends [39, 40].
This type of sensor is viable, but very difficult
to manufacture in appropriate quantities.
Fig. 2.14. Different force sensors employed in AFM designs.
The principle of the optical lever is shown in Figure 2.15. The lever consists of a laser
focused to a spot on the back of a reflective cantilever; the beam is then reflected onto a split
photodetector, which measures the position of the laser spot. In an analogous way to a
mechanical lever, the optical lever magnifies a small movement of the cantilever, to create
a large movement at the photodiode. The chief advantage of this system is that it is highly
sensitive to very small movements of the cantilever, and it is quite simple to build [23, 24, 41].
2.2.2.1 Optical lever sensors
The design for an optical lever AFM sensor is illustrated in Figure 2.15. A laser beam is
reflected by the back side of a reflective cantilever onto a four-segment photodetector. If a
probe, mounted on the front side of the cantilever, interacts with the surface the reflected
light path will change. The force is then measured by monitoring the change in light
detected by the four quadrants of the photodetector.
