Materials Science on CD-ROM User Guide
Introduction to Electron Microscopes
Version 2.1
Peter Goodhew, University of Liverpool
Abigail Callanan, MATTER
General Comments
This module is one of several devoted to microscopy. It particularly treats the common
components of electron microscopes themselves. No attempt is made to cover image contrast,
diffraction, analysis or the many image modes available in most modern microscopes. These
topics are covered in companion modules.
Assumed Pre-knowledge
This is an introductory module which assumes little pre-knowledge. It explores some of
the capabilities and components of three types of microscope; The scanning electron
microscope (SEM), the transmission electron microscope (TEM) and the scanning transmission
electron microscope (STEM). Electron microscopes have several components in common and
this module concentrates on these common topics.
It would be helpful to understand, before embarking on this module:
- The behaviour of an electron in electric and magnetic fields.
- The action of a thin lens, the ray diagrams associated with a convex lens and the
concept of the focal length of a lens.
- The essence of diffraction theory, at least as far as Braggs Law, expressed as
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nl = 2d sinq
- The generation of characteristic X-rays by the scattering of high energy electrons.
Related MATTER modules which refer to these topics include:
The module contains four sections;
This section starts with a picture of each type of microscope. Clicking on each picture
leads to a brief description on the use of that microscope, with a few typical
micrographs. Additional information on high energy electrons includes data for a range of
electron energies (1 to 1000keV) and the following equations:
m = me / Ö
(1-(v/c)2)
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(1) |
E = h/mv
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(2) |
V = (m-me)c2
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(3) |
Overview
An interactive microscope column reveals many of the components and controls as the
mouse pointer is moved over it. Double clicking on each component takes you to the
relevant pages in the module.
Electron Gun
In this section the two most common types of electron gun are shown. The field
emission gun is described and there is a simulation of a thermionic triode gun,
including instructions on reaching filament saturation. The filament current and bias
values are on an arbitrary scale, as is usual on the controls of a real TEM.
Lens system
In this section the general electromagnetic lens is described, with emphasis on the
image rotation which it produces. Additional information on the effect of an
electromagnetic field on an electron is available, where the Lorentz equation is
introduced:
F = e [E + ( v x B )]
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(4) |
There is a simulation of the motion of an electron in a magnetic field and in both
electric and magnetic fields.
The relationship between object and image for a convex lens is treated by means of an
animation in which the object distance can be freely altered. Virtual images are also
simulated, magnification is explained and the thin lens equation is introduced:
The use of such a lens to de-magnify an electron beam is also demonstrated.
The double condenser lens system which is used in most EM illumination systems
is dealt with over several screens. The concepts of "underfocus" and
"overfocus" are introduced via ray diagrams and beam convergence is
defined. The idea of a reduced beam convergence in the overfocused condition is
illustrated and the condenser aperture is introduced. There is then a full simulation of a
double condenser system with readout of beam diameter, spot size and convergence angle.
In a similar treatment the objective lens and its aperture are dealt with. The
idea that a diffraction pattern is formed at the back focal plane of such a lens in then
introduced. The effect of the aperture on the appearance of the diffraction pattern is
simulated, and the effect of tilting the beam is shown.
Finally the projector system is shown. Since the lenses are essentially acting
in the same way as has already been seen for the condenser and objective lenses, the
emphasis here is ion the combination of several lenses of modest magnification to produce
ultimate magnifications which can be large. The fact that individual lenses may be
switched off in some imaging conditions is also dealt with.
Vacuum system
This simple page shows three types of pump - rotary, diffusion and sputter ion - and
gives brief details of their performance.
Camera and Display
This simple animation shows the procedure for taking a photograph in the TEM. It is
supported by a ray diagram which shows how a large depth of field is created.
Eucentric Goniometer
This complex simulation shows the interaction of the various tilts and translations
which can be transmitted to the specimen in a TEM. The effect of moving the specimen away
from its eucentric position is shown - the imaged region then moves across the field of
view.
Overview
An animation of the whole microscope is shown. The relationship between the key
components can be seen, and the scanning process, leading to a serial image, is animated.
The CRT trace is notional and does not represent the actual variation of intensity across
a single line of the image in this version.
Electron Gun
The three pages in this section are the same as those used in the TEM section (see
above), since the electron gun principles are identical.
Lens system
This section contains simulations related to the general magnetic lens, the condenser
system and its aperture (as described above), but omits the objective lens details which
are covered in the equivalent TEM section.
Everhart-Thornley Detector
This simulation shows the most commonly used secondary electron detector and
illustrates the curved path of secondaries towards the +ve biased detector, giving a high
collection efficiency. The lower collection efficiency of an un-biased detector is shown
for comparison.
Scanning System
The three most common modes of scanning are animated on this page, and are shown with
an example of the images they produce. Conventional scanning is compared with a rocked
beam, and with the diffraction pattern which can be collected when the beam is held
stationary.
The three screens in this section illustrate the basic elements of a dedicated STEM.
The inverted column typical of a dedicated instrument is shown, with simple
"secondary" and "transmitted" detectors which can be selected.
The second screen makes the point that there is potentially a large experimental volume
available after the specimen, and that this is regularly used for sensitive analysis
techniques such as PEELS. In the final screen a couple of typical spectra are shown.
This section will link to the MATTER module "Analysis in the Electron
Microscope" which will be released in 1997.
Bibliography
For further study the following texts are recommended:
Goodhew, P.J. and Humphreys, F.J., Electron Microscopy and Analysis, 2nd
Edition, Taylor & Francis, 1988 Order!
Williams, D.B. and Carter, C.B., Transmission Electron Microscopy: A Textbook
for Materials Science, Plenum Press 1996
Hirsch, P.B., Howie, A., Nicholson, R.B., Pashley, D.W. and Whelan, M., Electron
Microscopy of Thin Crystals, Butterworths 1965
Thomas, G. and Goringe, M.J., Transmission Electron Microscopy of Materials,
Wiley- Interscience 1979
Goldstein, J.I., Scanning Electron Microscopy and X-ray Microanalysis, Plenum
1981
Chescoe, D. and Goodhew, P.J., The Operation of Transmission and Scanning Electron
Microscopes, Oxford University Press/Royal Microscopical Society 1990
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