Elements of a transmission electron microscope

Abstract

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 electron volts 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 nanometers or even less than a nanometer 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 energy filters 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. © 2008 Springer Science+Business Media, LLC All rights reserved.