Practical Electron Microscopy and Database

The schematic illustration in Figure 4264a presents the position of image-forming lens system in typical (S)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 4264a. Schematic illustration of the structure of typical TEM systems (e.g. JEM-2010F here).

There are two methods normally used for spherical aberration correction at high accelerating voltages in TEM and STEM: i) Using magnetic hexapoles with transfer round lenses [1] in both TEM and STEM system; ii) Combining four magnetic quadrupoles and at least three octupoles in STEM system [2] due to its off-axis aberrations. Image-forming C_s-corrector is also called TEM C_s-corrector. Some examples of aberration corrections in TEM imaging mode are shown in page4109.

Figure 4264b shows an example of microscopes with the C_s (spherical aberration)-correctors for both probe-and image-forming systems. The image-forming C_s-corrector is mounted between the objective lens and the intermediate lens, and the probe-forming C_s-corrector is between the condenser lens and the condenser mini-lens. The height of each corrector is 25 cm.

Figure 4264b. JEM-2200FS TEM/STEM with probe-and image-forming C_s-correctors. Adapted from [3].

Figure 4264c shows the photograph of a LVEM (low-voltage electron microscope) with delta C_s correctors operated in the range 30 - 60 kV. The CFEG stands for cold field emission gun installed on the top of the column. The delta correctors for STEM and TEM are integrated for both probe- and image-forming systems. An Enfina spectrometer is installed at the bottom of the column for EELS-based measurements.

Figure 4264c. The photograph of a LVEM with delta C_s correctors operated in the range 30 - 60 kV. [4]

Figure 4264d shows the full view of of the practical double-sextupole C_s correctors for STEM and for TEM, respectively. The positions of the correctors with respect to the front and back focal planes for STEM and TEM, respectively, are also indicated.

Figure 4264e shows the schematic comparison of Zemlin (diffractogram)-tableau characteristics for the axial aberrations up to third orders. First-order aberration (e.g. defocus and twofold astigmatism, A₁) shows the elliptical distortion even without electron beam tilting because the impact of first-order aberrations does not depend on the tilt angle. For the aberrations of higher orders (n≥2), such as second-order axial coma B₂, three-fold astigmatism A₂, third-order spherical aberration C₃ (>0), third-order star aberration S₃ and four-fold astigmatism A₃, there is no elliptical distortion observable for the un-tilted case. In these cases, only at illumination tilts the characteristic distortion due to the aberrations becomes discernible. Note that the higher-order aberrations have equal symmetries to the ones in Figure 4264e as discussed in page3740.

[1] Haider, M., Braunshausen, G. and Schwan, E. 1995. Correction of the spherical aberration of a 200 kV TEM by means of a hexapole-corrector, Optik, 99, 167–179.
[2] Krivanek, O. L., Dellby, N., and Lupin, A. R. 1999. Towards sub-Å electron beams, Ultramicroscopy, 78, 1–11.
[3] Hidetaka Sawada, Takeshi Tomita, Mikio Naruse, Toshikazu Honda, Paul Hambridge, Peter Hartel, Maximilian Haider, Crispin Hetherington, Ron Doole, Angus Kirkland, John Hutchison, John Titchmarsh and David Cockayne, Experimental evaluation of a spherical aberration-corrected TEM and STEM, J Electron Microsc (Tokyo) (2005) 54 (2): 119-121. doi: 10.1093/jmicro/dfi001.
[4] Takeo Sasaki, Hidetaka Sawada, Fumio Hosokawa, Yuji Kohno, Takeshi Tomita, Toshikatsu Kaneyama, Yukihito Kondo, Koji Kimoto, Yuta Sato, and Kazu Suenaga, Performance of low-voltage STEM/TEM with delta corrector and cold field emission gun, Journal of Electron Microscopy 59(Supplement): S7–S13 (2010).