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

TEM Specimens and Images 123
of a semiconductor material. This instrument also gives an electron or ionbeam
image of the surface of the specimen, allowing the ion beam to be
positioned and scanned along a line in order to cut a slice of material at a
precise location. In this way, it is possible to prepare a TEM specimen that
represents a cross section through a particular component; see Fig. 4-21.
Instead of thinning a bulk material, TEM specimens are sometimes built
up in thickness by thin-film deposition. Typically, the material is placed in
a tungsten “boat” that is electrically heated in vacuum, causing the material
to evaporate and then condense onto a substrate; see Fig. 4.20h. If the
substrate is soluble (for example, alkali halides dissolve in water), the film is
floated onto the liquid surface and captured on a TEM grid, similar to thin
sections of biological tissue. Vacuum-deposited thin films are used in the
electronics and optical industries but are usually on insoluble substrates such
as silicon or glass. For TEM examination, the substrate must be thinned
down, for example by dimple grinding or jet thinning from its back surface.
Figure 4-21. Cross-sectional TEM image showing a polycrystalline tungsten “via” that makes
electrical connection between adjacent layers of aluminum in a multilayer integrated circuit.
From such images, the thickness and grain structure of each layer can be determined. From
Zhang (2000), by courtesy of Elsevier Ltd.