Biomedical Engineering Reference
In-Depth Information
Fig. 2.36. Example data showing the effect of temperature variations on positioning accuracy. Left:
while heating, the position of the probe on the sample will drift considerably: Right: at elevated
temperatures, once stabilized, the probe will not drift.
When heating the sample, to avoid damage to the scanner the most important thing is to
thermally isolate the scanner from the hot sample, this is typically done by use of a spacing
material with very high thermal insulation, and high stiffness (i.e. a ceramic such as
MACOR
1
- a ceramic with very low thermal conductivity and a low thermal expansion
coefficient).
As a sample is heated or cooled, there will be considerable thermal drift in the AFM
stage in the
X-Y
and
Z
directions. The thermal drift can only be minimized by making the
stage as symmetrical as possible and using low drift materials. Usually there is drift as the
stage temperature is raised, and the drift stops when the desired temperature is reached and
the stage temperature is stabilized. Data illustrating this is shown in Figure 2.36.
Cooling samples in an AFM stage has another potential problem. Condensation occurs
on the components that are being cooled. When cooling a sample in an AFM stage, it is
recommended that the entire stage be maintained in a dry environment such as in a glove
box. If going to very low temperatures, the ideal situation is to scan in a vacuum. As such
temperatures are generally used in physics applications this is rather common. Another
approach is to image in liquid nitrogen vapour at ambient pressure [89]. At cryogenic
temperatures, AFM becomes very sensitive, due to the lack of thermal excitation from air
molecules on the cantilever, and also reduced sample movement, and some unique
experiments may be carried out. Examples include biological molecules too delicate to
measure in ambient conditions [90, 91], measurement of frozen liquids [92] and even
single atom hopping from sample to tip [1]. While the design and use of AFMs to be
inserted in vacuum and operated at cryogenic temperatures is outside the scope of this
book, the interested reader is directed to the references [90, 93].
