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The electron gun in electron microscopes generates electrons and provides the first coherent crossover of electron beam (see schematic diagram). In the electron guns of conventional electron microscopes, resistance heating is produced in the gun filaments and electrons are boiled off the tip by thermionic emission.

Figure 1975a shows the structure of the electron probe-forming system in STEM mode in JEOL JEM-2010F TEMs. A setup, consisting of electrostatic gun lens and twin condenser lens system, controls the de-magnification of the Schottky field emission source. Compared to the cold field emitter, the Schottky electron source has a much larger emission area. Each element of the electron source can be assumed to emit electrons incoherently and thus results in an incoherent broadening of the probe [1]. A large de-magnification factor between the source and probe reduces this probe-broadening effect. In addition, the beam current decreases due to the probe-forming aperture. A cross-over is formed between the two condenser lenses (C1 and C2) by employing near-maximum excitation in the C1 lens, resulting in a large source de-magnification. The C2 lens and the gun lens are used to tune the probe coherence further by setting the probe size. In the STEM mode, both condenser and objective mini-lenses are tuned off.

The schematic illustration in Figure 1975b presents the position of electron gun part in typical TEM systems.

1. Electron gun 2. Wehnelt unit 3. Anode 4. Electron gun second beam delector coil 5. Anode chamber isolation valve 6. 1st condenser lens coil 7. Condenser polepiece 8. 3rd condenser lens coil 9. Condenser aperture assembly 10. Specimen chamber 11. Goniometer 12. specimen holder 13. Stigmator screening cylinder 14. Objective lens coil 15. Objective lens liner tube 16. Field limiting aperture 17. Intermediate lens stigmator 18. Intermediate polepiece 19. Intermediate lens linear tube 20. Projector lens beam deflector coil 21. Projector upper polepiece 22. Projector lower polepiece 23. Binoculars 24. Viewing chamber 25. Viewing window 26. Dispensing magazine 27. Receiving magazine 28. Camera chamber 29. Lift arm 30. HT cable to high voltage tank 31. Anode chamber, or called acceleration tube 32. Gas inlet 33. Electron gun 1st beam deflector coil 34. Condenser lens stigmator coil 35. Spot alignment coil 36. Condenser lens 1st beam deflector coil 37. Condenser lens 2nd beam deflector coil 38. Condenser minilens (CM) lens coil 39. Stage heater 40. Objective polepiece 41. Objective lens stigmator coil 42. 1st image shift coil 43. Objective minilens (OM) lens coil 44. 2nd image shift coil 45. 1st intermediate lens coil 46. 2nd intermediate lens coil 47. 3rd intermediate lens coil 48. Projector lens coil 49. Viewing chamber isolation valve 50. High resolution diffraction chamber 51. Small screen 52. Large screen

Figure 1975b. Schematic illustration of the structure of typical TEM systems (e.g. JEM-2010F here).

For TEM analysis, the size of electron source must be large to achieve a large area of illumination with sufficient electron density. From this point of view, thermionic sources are more than capable of meeting these criteria. However, this relatively large source size in addition to a large electron energy spread means that thermionic sources are not applicable to scanning probe microscopy and EELS microanalysis.

A field emission gun (FEG) is needed for electron holography since the spatial coherence with a conventional electron gun extremely limits the resolution. Note that even though for a TEM with a conventional electron gun, a smaller illumination aperture can be used to increase the degree of coherence, the smaller aperture still cannot practically be used for holography measurement because of the smaller coherent beam current I_coh.

Ion pumps are often directly added to the specimen stage (especially for analytical EMs) or gun chambers of EMs to enhance the vacuum in these important regions. As shown in Figure 1975c (a), in most modern TEMs, the electron gun, top lenses, and specimen chamber are maintained at ultra-high vacuum by an ion pump, while the viewing screens and photographic chamber are maintained at a lower vacuum, which is referred to as high vacuum, by either a diffusion pump or a turbomolecular pump. This vacuum level is backed by a mechanical (rotary) pump. However, some TEMs have lower vacuum in the specimen chamber as shown in Figure 1975c (b).

[1] J.M. Cowley, Image contrast in transmission scanning electron microscopy, Appl. Phys. Lett. 15 (1969) 58-60.