excerpt from Microcosmos by Jeremy Burgess, Michael Marten and Rosemary Taylor
© Copyright 1987, Cambridge University Press, reproduced by permission.
General DesignElectron microscopes are of two basic types - transmission electron microscopes (TEMs) and scanning electron microscopes (SEMs). The difference between them lies in the way the electron beam is controlled to form the final image. To begin with, however, we examine the general features which all electron microscopes have in common.
The microscope consists of three separate but interacting parts. First, there is a system of vacuum pumps which removes the air from the space inside the microscope tube or 'column'. A good vacuum is necessary to allow the electrons to move unimpeded down the column (which may be a meter in length) and to minimize contamination of the specimen resulting from interactions between the electron beam and residual gas molecules.
The second part of the electron microscope is the power supply. The lenses in an electron microscope are electromagnetic - they consist of a coil of wire through which a current flows, producing a magnetic field. Functions such as brightness, focus and magnification are achieved by altering the levels of electric current in the various lenses. The control of these currents must be very precise, and they must remain stable once set. This is one function of the power supply. Another is to provide the high voltage necessary to accelerate the electrons towards the specimen. In a conventional TEM this requires a stable voltage of 40,000-100,000 volts. In the SEM, the voltage can usually be varied between about 500 and 40,000 volts. The voltage determines the wavelength of the electrons in the beam, and it must be held constant to about 1 part in 100,000 or better if chromatic aberration is to be acceptably low.
The third part of the microscope is the column itself. This consists of a series of electromagnetic lenses stacked on top of one another. The coil of wire that forms each lens is shielded from external magnetic fields and cooled by refrigerated water At the top of the column is the chamber housing the source of the electrons, the 'electron gun'. This is a heated tungsten wire with its associated electrodes.
The specimen chamber in a TEM is about half way down the column. In an SEM it is at the base of the column. The specimen is always inserted through an air lock to prevent loss of vacuum in the column.
Image recording is quite different in the two classes of microscope. In the TEM, sensitive material (film or photographic plates) is introduced into the base of the column and directly exposed to the electrons in the vacuum. In the SEM, the visible image is produced on a cathode ray tube screen which may be remote from the microscope itself, and it is recorded by conventional photography using a camera focused on the screen.
Optical lenses work because rays of light are bent as they pass between media of different refractive index. Lenses for light microscopy consist of many pieces of glass differing both in refractive index and shape; by altering these two variables, the lens designer is able to overcome aberrations and produce a lens of chosen characteristics. Electron lenses do not work in this way. As the electrons pass through an electron lens, they do not encounter physical in homogeneities corresponding to air-to-glass interfaces. They travel through a vacuum. However, they do encounter, and are influenced by, the magnetic field within the core of the lens.
Several important design features result from the use of electromagnetic fields to refract the electron beam. The most obvious is that the focal length of the lens is related to the strength of the magnetic field, and hence to the size of the current flowing in the coil of the lens. This means that functions such as focus and magnification are controlled simply by altering electric currents; they do not require physical movements or the exchange of objectives, as they do in the case of a light microscope.
Another consequence is that electron lenses can only be made as positive elements - converging lenses, in optical terms. This imposes a great restraint on the performance of the microscope, since aberrations cannot be corrected by the manufacture of 'compound' lenses. In practice this means that electron lenses always operate at very small apertures; as a result, the best resolution of an electron microscope, although a vast improvement on the light microscope, is very limited compared to the potential resolution which could be achieved with high-aperture lenses working at the same wavelength.
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Page authored by Paul Perkes and the ACEPT W3 Group
Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287-1504
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