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In the place of the light
beam of a normal microscope electron beams are used, which are focussed
with the help of elctromagnetical fields. One differentiates between two
principal procedures:
1. TEM - transmission electron
microcopy. Transmission electron microscopes require the preparation of
films so thin that they are transparent to a beam of electrons with energies
of roughly 200 kiloelectron volts. This means the film must have a thickness
of only one, or a few, hundred nanometres (10-9 metre). Films
of lighter elements, such as aluminum, can be thicker, while films of heavier
elements, such as gold, must be thinner.
2. SEM - scanning electron
microscopy. A scanning electron microscope uses a narrow beam of
electrons (often of about 40 kiloelectron volts) that scans the surface
of a sample and forms a corresponding image from the backscattered electrons
or secondary electrons. No special surface preparation is necessary, and,
since the depth of focus in an SEM is much greater than in an optical microscope,
quite irregular surfaces, such as fractures, can be studied successfully.
Useful magnifications range from 100 to 20,000. |
Electron microscopy
