Links in this section:

Introduction
Fundamentals of the TEM technique
Beam-sample interaction
The Analytical TEM
Detector Protection
Qualitative Analysis
Quantitative Analysis
Microanalysis Examples (1)
Microanalysis Examples (2)
Microanalysis Examples (3)
Summary

Fundamentals of the TEM technique

In a TEM, like an optical microscope, a beam is passed through a series of lenses to form a magnified image of a sample that has been inserted in the area of the objective lens (Figure 1). This image is viewed through projection onto a viewing screen. However, electron beams are easily scattered by air molecules and TEM columns must be kept under high vacuum. In addition, electrons cannot be focussed by glass lenses and electromagnetic lenses are used instead. Just as in their optical counterpart the machining of these lenses are critical, as aberrations can have a major effect on resolution. However, electromagnetic lenses do have the advantage that astigmatism can be corrected electronically and magnification can easily be changed by adjusting the lens current. Because the TEM is a multiple lens system, different analytical conditions can be obtained by adjustment of lens and alignment conditions. Computer control of contemporary microscopes helps significantly in the ease of operation of these complex instruments.

Resolution of TEMs is far superior to that of optical microscopes due to the fact that electrons are used for the source of illumination rather than visible or ultra-violet light. Optical microscopes are limited to a resolution in the order of 100nm whereas modern TEMs demonstrate resolutions approaching 0.1nm. This has proved extremely valuable in the examination of biological ultra-structures such as DNA and viruses, and the structure of materials such as grain boundary properties in metallic specimens, and failures in semiconductor devices.

Quite early in the development of TEMs it was observed that, due to their short wavelengths, crystalline materials diffract electrons. A parallel beam of electrons passing through a regular spaced crystal lattice in the sample holder of a TEM will form a diffraction pattern in the back focal plane of the objective lens. This can be projected onto a viewing screen or recorded on film for measurement. Study of these diffraction patterns helps explain the structure of materials.

By adding a set of scan coils to the electron optic column, a focussed electron beam can be scanned over a sample. The scanning transmission electron microscope (STEM) uses this facility to control the beam for microanalysis (e.g. X-ray mapping). STEM systems also include electron detectors for collecting images of electrons transmitted or scattered back from the sample.

TEMs have been equipped with elemental analysis capabilities since the 1960s. The earlier incarnation of this mode of analysis was the Electron Microscope Micro-Analyser or EMMA developed by AEI Instruments. This was unique in that primary analysis was performed by Wavelength Dispersive Spectrometry (WDS) rather than Energy Dispersive Spectrometry (EDS). Although the WD spectrometers gave better spectral resolution than EDS, collection efficiency was poor and spectrometer stability was a problem. Energy Dispersive Spectrometers were installed on Transmission Electron Microscopes in the early 1970s, and provided much better collection efficiency along with the ability to acquire a range of elements between Na and U simultaneously. This, in conjunction with improved ability to produce higher energy probes with spatial resolution down in the nanometer range, was instrumental in the formation of the first analytical TEM/STEMs.

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