W. Richard Bowen and Nidal Hilal 4

Force
1.2 THE ATOMIC FORCE MICROsCOPE 5
Intermittent
contact mode
Contact mode
Distance
Repulsion
Non-contact mode
Attraction
fIgure .3 Diagram illustrating the force regimes under which each of the three most
common AFM imaging modes operate. Contact mode operation is in the repulsive force
regime, where the probe is pressed against the sample surface, causing an upwards deflection
of the cantilever. Non-contact mode interrogates the long-range forces experienced
prior to actual contact with the surface. With intermittent contact, or tapping mode, the
probe is oscillated close to the surface where it repeatedly comes into and out of contact
with the surface.
This ‘optical lever’ method to detect deflection of the cantilever is the
method primarily in use currently [2, 3]. However, the original design
for the AFM used an STM piggy-backed onto the upper side of the AFM
as a deflection sensor [1]. Whilst this allowed extremely accurate determination
of the deflection, it also could detect deflection only within a
very small range, which is insufficient for most purposes. In addition,
other methods have been trialled in the past for this purpose including
the measurement of optical interferometry effects [4] as well as fabricating
levers to be able to detect deflection through a piezoresistance-based
mechanism [5–14].
Scanners are available in different configurations, depending upon
the particular AFM employed, or the purpose for which it is required.
Tube scanners consist of a hollow tube made of piezoceramic material.
Depending on how electrical current is applied, the tube may extend in
the z-direction or be caused to flex in either the x- or y-direction to facilitate
scanning. Alternatively scanners may consist of separate piezocrystals
for each movement direction. Such a configuration removes certain
non-linearity problems, which may occur in the simpler tube scanners.
In many commercially available AFMs, especially in older models, the
movement in the x- and y-directions may be achieved by the movement
of the sample rather than by the movement of the probe.