Selection of Preparation Techniques Based on Material Problems and TEM Analyses - Sample Preparation Topic for Transmission Electron Microscopy - page 176

Polymers are often mixed multiphase materials. In order to investigate mixed
polymer-metal or polymer-ceramic materials, etc., the classic ion bombardment
technique cannot be used, since it would degrade all the interfaces. Depending on
the material's hardness, if the second phase is a metal, semiconductor, ceramic, or
any other blend, it may be prepared using tripod polishing techniques for hard mate-
rials and ultramicrotomy or cryo-ultramicrotomy for soft materials. The focused ion
beam technique, either at room temperature or at low temperatures using cryo-FIB,
can yield good results. This latter technique is not yet fully developed for TEM
analyses on such materials, but it is still used for cryo-SEM observations.
6 Selection Criteria Based on Material Organization
6.1 Bulk Materials
In materials science, a bulk material does not present any particular difficulty
and can be thinned, depending on its properties, using electrochemical, chemical,
mechanical, or ionic thinning.
In biology, bulk material (tissue or cells) is usually hydrated and requires
substitution-infiltration-embedding or freezing before being prepared in a thin sec-
tion using ultramicrotomy or cryo-ultramicrotomy. In biology, there are also bulk
materials that are hard and nearly dry, e.g., bone, tooth, wood, and chitin. These
types of materials can either be thinned directly or following embedding if they are
porous.
6.2 Single-Layer or Multilayer Materials
In materials science and biology, self-supporting single-layer materials are observ-
able directly after being deposited on a TEM grid. In materials science, the materials
composed of thin layers or multiple layers must be analyzed over areas large enough
to be characterized. Depending on the material type, multilayer materials can be pre-
pared using mechanical methods such as tripod polishing, ultramicrotomy, or ion
thinning.
If the material is ductile, as with certain metals, ultramicrotomy can be used. In
other cases, the tripod polishing technique, ion milling, and FIB thinning are often
used. Tripod polishing is the technique of choice for semiconductor, ceramic, or
composite materials. It is used to prepare large observable areas (sometimes up to
a millimeter) and therefore provides information on several scales. In thin layer or
multilayer materials prepared using ion milling, the thin areas are smaller.
In biology, single or multilayer most often correspond to cell cultures. We then
come back to the problem of hydrated biological materials and to the ultramicro-
tomy or cryo-ultramicrotomy techniques, with the need to select the orientation of
the slice: longitudinal or cross-sectional plane.