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Initially, AFM probes were made by gluing a diamond shard to a
cantilever cut out of metallic foil or by tapering Fe, Ni or W wire.
This tedious preparation has been later substituted by batch production
of the probes using semiconductor technologies. In first commercial
AFM probes the cantilever and tip consisted of a thin Si3N4 film
on a glass substrate. The tip has a shape of square pyramid with
the nominal radius of curvature at the tip apex ~ 20 nm. According
to preparation technology these probes can be made thin, which defines
relatively small spring constants in the 0.01 - 0.6 N/m range. These
probes are regularly used for imaging in the contact mode and applied
to soft samples.
The tip shape and radius at the apex are important parameters that
define the range of applications and the value of the probe. Large
surface corrugations limits lateral image resolution substantially
and brings a tip shape into the play. For imaging of critical dimensions
structures such as deep and narrow trenches, one should use specially
etched probes (for example, with a FIB technology) or those made
of carbon nanotubes or Hi-RES-W spikes. High-resolution imaging
of flat samples depends primarily on the tip apex.
Monolithic silicon probes, which are etched from a Si wafer, are
most appropriate for ambient and vacuum AFM studies. Their cantilevers
have a rectangular shape with following parameters: width - 30-60
mm, length - 100-400 mm, thickness 2-8 mm. Spring constants of the
commercial probes vary in the 0.1 N/m to 600 N/m range. Typical
dimensions of Si tips are: height 8-10 mm, opening angle of ca.
30 degrees, apex radius 10 nm. They have a pyramidal shape, which
in ideal cases should be triangular near the apex.
Si probes are sharper than Si3N4 ones yet they have limitations
in stiffness when imaging of soft samples is of interest. An appropriate
solution for high-resolution imaging of such objects can be obtained
by making hybrid probes consisting of Si3N4 cantilevers and Si tips.
Such probes are unfortunately rare.
In addition to the sharp probes, which are applied for high-resolution
imaging, there is a need in the probes with large apex dimensions.
The LS probes with rounded apex shape with diameter in the 50-100
nm range are in demand for nanomechanical measurements and also
for low-wear imaging.
Characterization of AFM probes is rather important issue because
variations in the tip shape and apex size are not uncommon. There
are direct and indirect ways of characterization of the probes.
Scanning electron microscopy (SEM) and transmission electron microscopy
(TEM) provide a direct visualization of the tip shape and apex dimensions.
The indirect experimental procedure is based on imaging special
test structures such as nanoporous Al. The analysis of the images
obtained on such test samples helps to judge about shank of the
probe and apex size. You should use the test samples with extreme
caution by making probe evaluation in the low-force regime to avoid
tip damage.
Al backside coating improves the reflection of the laser beam.
But in some cases, a researcher sacrifices reflectivity in order
to avoid a possible bending of the cantilever in experiments at
different temperatures. There is also a chance that coating of the
cantilever backside brings some additional material to the probe
apex thus making it duller.
For measurements of electric or magnetic properties of samples
the cantilever coatings play most essential role. These studies
require coated probes with different stiffness as well as with apex
of various sizes. Most of the Si probes from our catalog can be
purchased with the coatings.
Further reading
Contact modes
Oscillatory modes
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