Biomedical Engineering Reference
In-Depth Information
Fig. 2.29. Examples of contact and non-contact probes. Left: a typical v-shaped contact-mode
cantilever. The whole probe is made from silicon nitride (Si
3
N
4
), and has an integrated square
pyramidal probe tip. Right: a probe designed for oscillating modes such as non-contact AFM. The
cantilever is usually rectangular (or a modified rectangle shape like this one). The whole probe is
made from silicon, and is much stiffer and more prone to breaking than the contact probe; however it
has a sharper tip.
had a probe etched in the silicon at the end. Early in the evolution of AFM it was discovered
that the best AFM probes could be constructed from MEMs technology. There are two
materials commonly used for AFM cantilevers: silicon nitride (Si
3
N
4
) and silicon (Si).
Si
3
N
4
is used for creating probes that have very low force constants. The thin films used
for creating Si
3
N
4
probes must have very low stress so the cantilevers don't bend naturally
from the stress. Practically, most Si
3
N
4
films have some residual stress and in fact,
cantilevers made with Si
3
N
4
tend to have curvature along their primary axis.
Cantilevers fabricated from silicon tend to have less residual stress than Si
3
N
4
and so
tend not to suffer from bending. However, the Si probes that are fabricated at the end of the
cantilever can be brittle and can be more likely to chip when they contact a surface. Most
of the probes used in optical lever-based AFM force sensors are constructed from Si.
2.5.2 Contact versus oscillating mode probes
Cantilevers for optical lever-based AFM can be operated in two basic topography modes;
contact (static) mode and oscillating modes, see Section 3.1. The cantilevers used for contact
mode have force constants that are typically much less than 1 N/m and are fabricated from
either silicon or silicon nitride. On the other hand, oscillating mode cantilevers are usually
fabricated from silicon and have force constants that are greater than 10 N/m. Examples are
given in Figure 2.29. There are also a large number of other probes available differentiated
by differing tip geometries (for example many examples of probes with 'sharpened' and
high-aspect-ratio tips are available), cantilever force constants, and coatings. Non-
topographic modes are commonly carried out with these speciality cantilevers, see below
for more about such cantilevers and Section 3.2 for details of their applications.
The rectangular cantilevers used as AFM force sensors have the same mechanical
properties as all cantilevered beams. They have a vertical force constant and resonant
frequency given by Equation 2.5. Additionally a cantilever has torsional and lateral
bending force constants given by Equations 2.6 and 2.7. The calculations of the corre-
sponding equations for v-shaped levers is considerably more complex, see [44].
