Biomedical Engineering Reference
In-Depth Information
3
t
l
E
k
ver
¼
w
4
ð
2
:
7
Þ
where:
K
ver
¼
vertical force constant
K
tor
¼
torsional force constant
K
lat
¼
lateral force constant
3
w
l
E
k
lat
¼
t
4
ð
2
:
8
Þ
E
¼
Young's modulus
G
¼
modulus of rigidity
w
¼
cantilever width
1
t
3
G
k
tor
¼
w
3
l
ð
2
:
9
Þ
l
¼
cantilever length
2
t
2
H
þ
t
¼
cantilever thickness
2.5.3 Control of tip shape
Horizontal resolution with an AFM can often be improved with sharper probes. There are
many techniques available for sharpening AFM probes. Because the sharpness and
reproducibility of manufactured probes is one of the limiting factors on the quality of
AFM results, new types of probes are under constant development. Examples of this
include composite probes such as mixed silicon/silicon nitride probes, and probes termin-
ating in carbon nanotubes. However, it remains a major challenge to produce probes with a
reproducible tip radius below 10 nm at a reasonable cost.
•Si
3
N
4
probes are sharpened by adding an extra process step that changes the shape of
the pit that the Si
3
N
4
film is deposited on. However, this technique often gives double
tips which can cause substantial artefacts in images. Another option is an additional
oxidation/etching process after the probe is manufactured.
• Si probes can be sharpened by chemical etching, ion milling or by adding a carbon
nanotube (see Figure 2.30). Each of these techniques can create a sharper probe, but
also add to the price of fabricating the probe.
One method for controlling the geometry of the probe on an AFM cantilever is to mount a
sphere at the end of the cantilever. The sphere can be mounted directly on a cantilever that
does not have a probe. The sphere may also be mounted at the end of a 'plateau probe', or a
probe that does not have sharp tip, but instead has a flat plateau at the end of the tip.
Probe damage
The quality of an AFM image is critically dependent on the shape of the probe used for
measuring an image. The AFM probe can be severely damaged by tip approach. Handling
Fig. 2.30. SEM images of different types of sharpened silicon probes. A: a standard silicon
oscillating mode probe. B: 'super-sharp silicon' probe sharpened with an electrochemical etch. C:
'high-aspect-ratio' probe sharpened with ion milling. Right: probe modified with carbon nanotube.
Images A-C reproduced with kind permission from NanoWorld AG.
