All microscopes need a source of electrons to illuminate the specimen. Fortunately, electron sources are plentiful, but to get the best images and other signals out of our expensive microscope, we need to use the best available source. There are stringent requirements for the beam of electrons and these are best met by only two types of source: thermionic and field-emission sources. Thermionic sources are either tungsten filaments or lanthanum hexaboride (LaB6) crystals, and field emitters are fine tungsten needles. In this chapter we’ll first explain briefly the physics of these two emission processes because then you’ll understand why we operate the sources in certain ways. Next we’ll tell you the characteristics we need from our electron beam. Then we’ll compare the three sources and show you that no one source is best for all aspects of TEM, but all three have their roles. Finally, we’ll explain ways to check that a particular source meets your specification.
KeywordsBeam Size Spatial Coherency Electron Source Energy Spread Source Size
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- Broers, A.N. (1974) SEM 1974 (Ed. O. Johari), p. 9, IITRI Chicago.Google Scholar
- Joy, D.C. (1973) SEM 1973 (Ed. O. Johari), p. 743, IITRI Chicago.Google Scholar
- Reimer, L. (1993) Transmission Electron Microscopy; Physics of Image Formation and Microanalysis, 3rd edition, Springer-Verlag, New York.Google Scholar
- Veneklasen, L.H. (1975) Optik 36, 410.Google Scholar