The Scanning Electron Microscope (SEM) is a very
useful tool for many aspects of science. A biologist might use an SEM
to study the tiniest micro-structures of an insect, the geologist
might use it to find out what chemicals are present in a rock
specimen, and the automobile engineer might use it to find tiny
imperfections in a car part. SEM images are created using electrons
instead of the photons of light we use to see the world around us. Photons
also help us view the microscopic world, as for example, in the
familiar light microscope. Electrons are more useful than photons
because they have a shorter wavelength. This permits magnified
imaging of a specimen by electrons up to 200 times greater
than with the light microscope. The theoretical limit of magnification for
a light microscope is about 2,000X whereas the best SEM can
magnify and resolve a specimen greater than 400,000X!!!
The most common imaging process begins by bombarding a specially
prepared specimen with a narrow beam of electrons and scanning that
beam back and forth across the surface of the specimen. When these
electrons hit the specimen they "kick out" electrons that were on the
surface of the specimen. The "kicked out" or secondary electrons are
collected by a special detector and processed to display the magnified
image on a TV-tube monitor for direct observation. The image can be
captured on photographic film as a permanent record, or digitally
processed for analysis and storage. The beam is focussed by a lens made
from an electromagnet, and the specimen is typically magnified by changing
the ratio between the display screen and the area of the specimen scanned.
You can think of the above process as if you were running on the top
of a 10-story building and dropping a series of tennis balls, the
electron beam, into a big basket of more tennis balls, the specimen.
When the tennis balls you drop land in the basket, tennis balls in the
basket are "kicked out" by the collision, secondary electrons. If you
had a friend on the ground catching the balls that came from the
basket, he or she would be like the detector.
The secondary electrons are not the only particles generated by
beam and specimen interaction. Backscattered electrons, X-rays, and
Auger electrons are also produced and can be used to image specimens
for special kinds of observations.
Our high resolution Hitachi scanning electron microscope is equipped with
an EDAX X-ray analysis system which includes an energy dispersive X-ray
spectrometer and permits the detection of elements down to boron. In concert
they provide a powerful tool for correlating the microstructures in a
specimen with its elemental composition.