Elements of a Transmission Electron Microscope

Not only does the electron gun of an electron microscope emit electrons into the vacuum and accelerate them between cathode and anode, but it is also required to produce an electron beam of high brightness and high temporal and spatial coherence. The conv

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Not only does the electron gun of an electron microscope emit electrons into the vacuum and accelerate them between cathode and anode, but it is also required to produce an electron beam of high brightness and high temporal and spatial coherence. The conventional thermionic emission from a tungsten wire is limited in temporal coherence by an energy spread of the emitted electrons of the order of a few electronvolts and in spatial coherence by the gun brightness. Schottky-emission and field-emission guns are newer alternatives, for which the energy spread is less and the gun brightness higher. The condenser-lens system of the microscope controls the specimen illumination, which ranges from uniform illumination of a large area at low magnification, through a stronger focusing for high magnification, to the production of an electron probe of the order of a few nanometres or even less than a nanometre in diameter for scanning transmission electron microscopy or for microanalytical methods. The useful specimen thickness depends on the operation mode used and the information desired. Specimen manipulation methods inside the microscope are of increasing interest but are restricted by the size of the specimen and by the free space inside the polepiece system of the objective lens. The different imaging modes of a TEM can be described by ray diagrams,

as in light optics, which can also be used to evaluate the depth of focus or to establish a theorem of reciprocity between conventional and scanning transmission electron microscopy. Electron prism spectrometers or imaging filter lenses allow electron energy-loss spectra (EELS) to be recorded and various operating modes of electron spectroscopic imaging (ESI) and diffraction (ESD) to be used. Observation of the image on a fluorescent screen and image recording on photographic emulsions can be replaced by techniques that allow digital, parallel and quantitative recording of the image intensity.

L. Reimer, Transmission Electron Microscopy © Springer-Verlag Berlin Heidelberg 1997

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4. Elements of a Transmission Electron Microscope

4.1 Electron Guns 4.1.1 Physics of Electron Emission Thermionic Emission. The conduction electrons in metals or compounds have to overcome the work function ¢w if they are to be emitted from the cathode into the vacuum. Figure 4.1 shows the dependence of potential energy on a coordinate z normal to the surface. The potential energy V(z) of an electron in front of a conducting surface at a distance z larger than the atomic diameter can be calculated by considering the effect of a mirror charge with opposite sign behind the surface; with an electric field E, the potential energy V = -eiEiz is superposed on that of the mirror charge, giving

e2

1

V(z) = w and cf>w - lJ.cf>w for thermionic or Schottky emission or can tunnel through the barrier of width w for field emission

4.1 Electron Guns

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Table 4.1. Parameters of thermionic, Schottky and field emission cathodes at E = 100 keV Characteristic parameters: Cathode temperature Tc Work function

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