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Contact
AFM
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The
AFM (Atomic Force Microscope) is an instrument that is used to study the
surface structure of a sample by measuring the force between atoms. At the
lower end of the Z scanner, there is a cantilever of very tiny dimensions: 125
micron long, 30 micron wide and 1 micron thick, which is manufactured by means
of micro-machining techniques. At the free end of the cantilever, there is a
very sharp cone-shaped or pyramid-shaped tip. As the distance between the
atoms at this tip and the atoms on the surface of the sample becomes shorter,
these two sets of atoms will interact with each other. As shown in Figure 1,
when the distance between the tip and the surface atoms becomes very short,
the interaction force is repulsive due to electrostatic repulsion, and when
the distance gets relatively longer, the interatomic force becomes attractive
due to the long-range van der Waals forces.
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1. Relation between the force and the distance between atoms
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interatomic force between atoms can bend or deflect the cantilever, and the
amount of the deflection will cause a change in the reflection angle of the
laser beam that is bounced off the upper surface of the cantilever. This
change in laser path will in turn be detected by the PSPD (Position Sensitive
Photo Detector), thus enabling the computer to generate a map of the surface
topography. In contact mode AFM the probe makes soft contact with the sample
surface, and the study of the samples topography is then conducted by
utilizing the repulsive force that is exerted vertically between the sample
and the probe tip. Even though the interatomic repulsive force in this case is
merely 1~10 nN, the spring constant of the cantilever is also sufficiently
small (less than 1 N/m), thus allowing the cantilever to react very
sensitively to very minute forces. The AFM is able to detect even the
slightest amount of a cantilevers deflection as it moves across a sample
surface. Therefore, when the cantilever scans a convex area of a sample, it
will deflect upward, and when it scans a concave area, it will deflect
downward. This probe deflection will be used as a feedback loop input that is
sent to an actuator (z-piezo). In order to produce an image of the surface
topography, the z-piezo will maintain the same cantilever deflection by
keeping a constant distance between the probe and the sample.
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| Basic
contact & DFM |
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| Lateral
force measurement |
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| F-D Spectroscopy |
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