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

18
Chapter 1
Figure 1-14. Scanning electron microscope at RCA Laboratories (Zwyorkin et al., 1942)
using electrostatic lenses and vacuum-tube electronics (as in the amplifier on left of picture).
An image was produced on the facsimile machine visible on the right-hand side of the picture.
This material is used by permission of John Wiley & Sons, Inc.
Further SEM development occurred after the Second World War, when
Charles Oatley and colleagues began a research and construction program in
the Engineering Department at Cambridge University. Their first SEM
images were obtained in 1951, and a commercial model (built by the AEI
Company) was delivered to the Pulp and Paper Research Institute of Canada
in 1958.
Sustained commercial production was initiated by the Cambridge
Instrument Company in 1965, and there are now about a dozen SEM
manufacturers worldwide. Figure 1-15 shows one example of a modern
instrument. Image information is stored in a computer that controls the SEM,
and the image appears on the display monitor of the computer.
A modern SEM provides an image resolution typically between 1 nm and
10 nm, not as good as the TEM but much superior to the light microscope. In
addition, SEM images have a relatively large depth of focus: specimen
features that are displaced from the plane of focus appear almost sharply infocus.
As we shall see, this characteristic results from the fact that electrons
in the SEM (or the TEM) travel very close to the optic axis, a requirement
for obtaining good image resolution.