Selection of Preparation Techniques Based on Material Problems and TEM Analyses - Sample Preparation Topic for Transmission Electron Microscopy - page 175

5 Selection Criteria Based on Material Type
Metallic materials : Electrochemical and chemical preparations yield the best results.
When the material is a single-phase material, electrolytic thinning is the most appro-
priate. If it is a multiphase material, chemical thinning is still possible, since a
solution can be found for the different phases that results in a common dissolution.
Otherwise, classic ion milling or FIB thinning is used.
Semiconductors : Electrochemical and chemical preparations yield the best
results, but are applicable only if the material is composed of a single element such
as silicon and more rarely for a multiphase semiconductors, for which it will be
difficult to find a suitable solution to dissolve the different components. Mechanical
thinning using tripod polishing, classic ion thinning, or FIB thinning is generally
used.
Ceramics and minerals : For these two types of materials, the thinning techniques
most often used are mechanical (crushing, tripod polishing, and ultramicrotomy)
and ionic (bombardment and FIB). With the exception of some conductive ceram-
ics, these materials are all insulators and electrochemical preparations do not apply.
However, chemical thinning may be possible if the ceramic is a single phase and
there is a known etchant.
Pol ymers : The preparation of a single-phase or multiphase polymer material
can be performed using ultramicrotomy and cryo-ultramicrotomy, or possibly using
gentle tripod mechanical thinning or cryo-FIB.
Biological materials : These materials are multiphase, hydrated, and unstable.
Preliminary preparations for stabilizing the structure and increasing the hardness of
the material are indispensable before the use of the ultracryomicrotomy technique.
These prior chemical preparations make it possible to observe only dehydrated and
embedded structures.
Conversely, cryo-techniques are used to study morphology in hydrated
phases.
For biological materials such as viruses, bacteria, and isolated macromolecules,
their structure is often observed using preparations of deposits of isolated particles at
room temperature or in cryogenic mode (fine particle dispersion or frozen hydrated
film).
Immunolabeling (cytochemistry) techniques are used for localizing proteins
having a particular chemical function.
Composite or mixed-composite material : These are multiphase materials con-
taining at least two crystalline and/or amorphous phases. This is the case with types
of concrete, for example. For such a microstructure to be tied to the formation mech-
anisms and its properties, it must be observed while keeping the different phases
together in the thin slice.
This type of material is difficult to prepare using classic ion bombardment, which
creates much damage and deteriorates the interfaces. The best results are obtained
using a soft mechanical tripod polishing technique, ultramicrotomy, or the focused
ion beam (FIB) technique. These last two techniques help to keep the different
phases together within the thin slice.