W. Richard Bowen and Nidal Hilal 4

6 1. BAsIC PRINCIPLEs OF ATOMIC FORCE MICROsCOPy
.3 CanTIlevers and probes
Depending upon the uses required and the forces which may act upon
them, cantilevers may be chosen from a large range available. Most microcantilevers
used in AFMs are produced from monolithic Si 3N 4 or Si using
micromachining techniques developed in the semi-conductor industry [9,
15–20]. These two materials are used extensively due to the high suitability
of their mechanical properties such as high yield strengths and elastic moduli
[21–24]. Applications where high forces are experienced require stiff
levers with probes resistant to deformation. On the contrary, where low
forces are experienced or when samples are soft and easily deformed, levers
with low force constants are required for both the increased force sensitivity
at low forces and avoiding deforming or damaging samples. In addition to
these silicon-based cantilevers and probes, the literature describes a number
of other materials which have been utilised in the production of AFM
levers. These include the production of diamond tips integrated into silicon
levers [25, 26] or fabricated diamond levers [27], particularly useful where
the probe tip may be subject to very high pressures at the apex, such as during
nano-indentation measurements; quartz ‘tuning fork’ levers with polymeric
tips for dynamic imaging modes [28]; metal wires, such as tungsten,
as levers [4, 29, 30]; as well as more exotic materials [31, 32]. In addition,
ultrasharp probes with very high aspect ratios can be manufactured by the
growing of Si ‘whiskers’ on the apex of the probe to improve the resolution
of samples with rough surfaces due to their relatively high aspect ratio [33,
34]. In addition, a large amount of work is being undertaken to investigate
the attachment of carbon nanotubes to the apex of cantilever tips to act as
ultrasharp probes. Carbon nanotubes have diameters typically in the range
of 1–20 nm and a very high aspect ratio making them ideal, providing that
they can be attached in a robust and predictable manner [35–39].
Another topic which must be considered when undertaking AFM
measurements is the presence of contaminants on the probe, particularly
at the tip. Commercially bought probes are placed on a sticky polymer
surface, such as polydimethylsiloxane (PDMS), in plastic boxes and as a
result tend to be coated in a hydrophobic contaminant layer [40]. In addition
to this, once removed from packaging, the probes are likely to suffer
exposure to airborne organic contaminants with the likelihood of exposure
increasing over time. This can have a significant effect on imaging
resolution, which depends to a great extent on the radius of curvature
of the probe tip being as small as possible. Even the presence of a small
amount of contamination could increase this significantly. It has also been
observed that the presence of a contamination layer can increase the measured
adhesion values between a probe and surface under ambient conditions
[41]. The resultant increased forces between probe and sample will