Physical Principles of Electron Microscopy: An Introduction to TEM ...

122 Chapter 4
All of the above methods involve the application of a mechanical force to
achieve thinning. Unfortunately, for all but the most well-behaved materials
(e.g., biological tissue, layer materials, silicon), the specimen becomes
extremely fragile and cannot be mechanically thinned below about 1 �m. In
addition, the mechanical forces involved may leave a damaged surface layer,
containing a high density of defects such as dislocations. This damage is
undesirable if the TEM is being used to study the original defect structure of
the specimen. Therefore, some non-mechanical method is commonly used
for the final thinning.
One such method is chemical thinning, in which a chemical solution
dissolves the original surface and reduces the specimen thickness to a value
suitable for TEM imaging. In the simplest case, a thin piece of material is
floated onto the surface of a chemical solution that attacks its lower surface;
the sample is retrieved (e.g, by picking up by a TEM grid held in tweezers)
before it dissolves completely. More commonly, a jet of chemical solution is
directed at one or both surfaces of a thin disk. As soon as a small hole forms
in the center (detected by the transmission of a light beam), the polishing
solution is replaced by rinse water. If the procedure is successful, regions of
the specimen surrounding the hole are thin enough for TEM examination.
Alternatively, electrochemical thinning is carried out with a direct
current flowing between the specimen (at a negative potential) and a positive
electrode, immersed in a chemical solution. In the original window-frame
method (Fig. 4-20f), the specimen is in the form of a thin sheet (1 cm or
more in height and width) whose four edges are previously painted with
protective lacquer to prevent erosion at the edge. When partially immersed
in the electrolytic solution, thinning is most rapid at the liquid/air interface,
which perforates first. Small pieces of foil are cut adjacent to the perforated
edge and mounted on a TEM grid. Nowadays, electrochemical thinning is
usually done using the jet-thinning geometry (Fig. 4-20e), by applying a dc
voltage between the specimen and jet electrodes. Recipes for the solutions
used in chemical and electrochemical thinning are given in Goodhew (1985).
When thinning metals, glycerin is sometimes added to the solution to make
the liquid more viscous, helping to give the thinned specimen a polished
(microscopically smooth) surface.
Increasingly, ion-beam thinning is used for the final thinning stage,
particularly of materials that are chemically inert. Following mechanical
thinning (if necessary), a 3mm-diameter thin disk of the material is placed in
a vacuum system, where it is bombarded by argon ions produced by a gas
discharge within an ion gun. These ions transfer energy to surface atoms and
remove the material by the process of sputtering; see Fig. 4-20g. A focused
ion beam (FIB) machine uses a beam of gallium ions to cut thin slices, often