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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 Bragg’s Law, expressed as
  • 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:

Module Structure

The module contains four sections;

Introduction

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 / (1-(v/c)2)

(1)

E = h/mv

(2)

V = (m-me)c2

(3)

Transmission Electron Microscope

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 )]

(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:

1/f = 1/u + 1/v

(5)

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.

Scanning Electron Microscope

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.

Scanning Transmission Electron Microscope

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

 

The University of Liverpool
Copyright of The University of Liverpool 2000