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

124 Chapter 4
The procedures outlined in Fig. 4-20 yield plan-view images, in which
the sample is viewed perpendicular to its original surface. For some
purposes, a cross-sectional image is needed, for example to show the growth
mechanism of a film. The required specimen is made by using a diamond
wheel to cut the film (on its substrate) into several thin strips, which are then
turned through 90 degrees about their long axis and glued together by epoxy
cement. Dimple grinding followed by ion milling then produces a specimen
that is thin at the center, resulting in a TEM image of the substrate and film
seen in cross section.
Cross-sectional specimens are invaluable for analyzing problems that
arise in the manufacture of integrated circuits on a silicon chip. As device
sizes shrink, it becomes necessary to make use of the high spatial resolution
of the TEM for this kind of failure analysis. The problem of producing a
cross section containing a specific component is solved by using a FIB
machine, in which Ga+ ions are focused (by electrostatic lenses) into a beam
as small as 10 nm in diameter. The machine produces a scanned image of the
surface of the chip, allowing the ion beam to be precisely positioned and
then scanned in a line in order to cut into the silicon on either side of the
component. This leaves a slice only 100 nm in thickness, which is then lifted
out and viewed in cross section in the TEM, as in Fig. 4-21.