Force Sensors
The construction of an atomic force microscope requires a force sensor to measure
the forces between a small probe and the surface being imaged. A common type of force
sensor utilizes the relationship between the motion of a cantilever and the applied
force. The relationship, given by Hook's law is:
F = - K * d
K is a constant and depends on the material and dimensions of the cantilever. D is the
motion of the cantilever. For a cantilever made of silicon that has dimensions of:
L = 100 microns
W = 20 microns
T = 1 micron
The force constant, K, is approximately 1 newton/meter. Thus if the cantilever is moved
by 1 nm, a force of 1 nanonewton is required.
Measuring the motion of the cantilever is possible with the "light lever" method. In
the light lever method, light is reflected from the back side of the cantilever into
a photo-detector. See figure 3.
Figure 3: The light lever sensor uses a laser beam to monitor the deflection of
the cantilever. When the cantilever moves up and down, the light beam moves across
the surface of the photo-detector.
The motion of the cantilever is then directly proportional to the output of the
photo-detector. Motions as small as 1 nm are routinely measured with the "light lever"
method in atomic force microscopes.
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