Practical Electron Microscopy and Database

The schematic illustrations in Figure 4944a show the convergent illumination configurations of various modes in TEMs. In the CTEM condition in Figure 4944a (a), the condenser mini-lens (CM lens) is strongly excited, and incident electrons are focused on the pre-focal point of the objective pre-field, resulting in a parallel illumination on a wide area on the specimen and providing highly coherent electron illumination. In the EDS condition in Figure 4944a (b), the CM lens is turned off and the incident electrons are focused on the specimen by the objective pre-field, resulting in a small-probe illumination. In this case, the illumination angle (α₁) is large so that high beam intensity is obtained for a small area in the analytical EDS method. In the NBD mode in Figure 4944a (c), a smaller condenser aperture is used to form a smaller illumination angle (α₂). Therefore, a small-diameter probe with relatively high coherence in the illumination is achieved. In the illumination condition in Figure 4944a (c), the illumination angle (α) with a constant probe size can be changed by changing the excitations of the condenser lenses and the CM lens to obtain the incident illumination to form ideal convergent beam electron diffraction (CBED) patterns.

(a)	(b)	(c)

Figure 4944a. Convergent illumination configurations: (a) CTEM mode, (b) EDS mode and (c) NBD mode.

In conventional TEM systems, the TEM system is operated at TEM imaging mode and the TEM specimen is illuminated by an almost parallel electron beam. The objective lens is used to form a magnified (intermediate) image of the illuminated area before intermediate and projector lenses. This formed image is further magnified by postspecimen lenses onto the image detector. More readings can be obtained at Difference of Imaging Geometries of TEM and STEM Systems.

In TEM operation, the electron beam at near crossover should be centered using the condenser X Y deflection coils (B1 and B2 double deflectors: see page1920) because an unevenly illuminated field of view will be induced when it is opened up if the beam is off center as shown in Figure 4944b.

Figure 4944b. Schematic illustration of uneven illumination if the beam at near crossover is not centered: (a) Beam crossover, (b) Beam is slightly opened, and (c) Beam is ‘widely’ opened.

The schematic illustration in Figure 4944c presents the position of illumination 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 4944c. Schematic illustration of the structure of typical TEM systems (e.g. JEM-2010F here).

Note that we need to recalibrate the condenser lens adjustment from the FilterControl software if the EEL specimen illumination changes when the spectrum offset is changed.