W. Richard Bowen and Nidal Hilal 4

1.2 THE ATOMIC FORCE MICROsCOPE 3
monitoring of the deflection of the cantilever, providing that the spring
constant of the lever arm is sufficiently calibrated.
In this chapter the basic principles of operation of an AFM will be presented,
outlining the most common imaging modes and describing the
acquisition of force distance measurements and techniques to calibrate
cantilever spring constants.
.2 The aTomIC forCe mICrosCope
In Figure 1.1 the basic set-up of a typical AFM is shown. Cantilevers
are commonly either V-shaped, as shown, or a rectangular, ‘diving
board’ shaped. The cantilever has at its free end a sharp tip, which acts
as the probe of interactions. This probe is most commonly in the form
of a square-based pyramid or a cylindrical cone. A few examples of different
configurations for levers and probes are shown in Figure 1.2.
Commercially manufactured probes and cantilevers are predominantly
of silicon nitride (the formula normally given for silicon nitride is Si 3N 4,
although the precise stoichiometry may vary depending on the manufacturing
process) or silicon (Si). Typically the upper surface of the cantilever,
opposite to the tip, is coated with a thin reflective surface, usually of
either gold (Au) or aluminium (Al).
The probe is brought into and out of contact with the sample surface
by the use of a piezocrystal upon which either the cantilever chip or the
surface itself is mounted, depending upon the particular system being
Photodetector
A B
Sample surface
C D
Light path
Probe
Light source
y
Chip
Cantilever
fIgure . Basic AFM set-up. A probe is mounted at the apex of a flexible Si or Si 3N 4
cantilever. The cantilever itself or the sample surface is mounted on a piezocrystal which
allows the position of the probe to be moved in relation to the surface. Deflection of the
cantilever is monitored by the change in the path of a beam of laser light deflected from the
upper side of the end of the cantilever by a photodetector. As the tip is brought into contact
with the sample surface, by the movement of the piezocrystal, its deflection is monitored.
This deflection can then be used to calculate interaction forces between probe and sample.
z
x