Practical Electron Microscopy and Database

Spherical Aberration (C_s) correction was first introduced by Rose [1,2] However, due to the need for high mechanical precision and lens current stability, as well as for computer control, Cs correctors had not been achieved until the end of the 20th century [3, 4]. The first practical aberration corrector was installed on a test bench to verify the applicability of C_s correction [12]. The demonstration setup on a modified SEM included a C_s corrector entirely incorporated into a specimen chamber and a CCD camera coupled with a scintillator underneath the specimen chamber. The electron probe was focused on the scintillator, and scanned circularly with a series of diameters so that the ray displacements induced by the aberration corrector could be observed.

In scanning TEM, the resolution-limiting factor is the size of the focused electron probe. As shown in Equation [4954d], the beam size can be reduced by minimizing C_s. With use of Cs correctors, it was demonstrated that probe size below 0.1 nm can be achieved [5 - 8]. An important advantage of C_s correctors is that larger acceptance angles can be used that increase the probe current considerably. In the TEM, C_s correctors allow for a much more straightforward interpretation of high-resolution images. Furthermore, a negative Cs value of objective lens has been applied in materials studies. For instance, this technique simplified the detection of light atoms [9] and enabled to image all types of atomic columns in the dielectric SrTiO₃ and the superconductor YBa₂Cu₃O₇ [10].

Spherical aberration correctors combine multipoles and rotational symmetry lenses. The example in Figure 4945 shows a TEM system facilitated with a C₃/C₅ corrector for spherical aberration corrections. Note that different from most commercial TEM systems (see schematic diagram of TEM systems), this TEM system without phosphor screen has the incident electrons emitted from the bottom of the systems and the detectors on the top.

	【*】 Detectors: HAADF, BF, EELS 【*】 QOCM: Quadrupole/octupole module called quadrupole– octupole coupling module 【*】 Four round projector lenses: PL1, PL2, PL3, and PL4. 【*】 OL: Objective lens 【*】 QLM: Quadrupole lens module 【*】 C3/C5 corrector: quadrupole–octupole C3/C5 corrector 【*】 Three round condenser lenses: CL1, CL2, and CL3 【*】 CFEG: Cold field emission gun
Figure 4945. (a) The TEM column and (b) Schematic cross-section of the column. [11]

[1] Rose H. 1990. Outline of a spherically corrected semiaplanatic medium-voltage transmission electron-microscope. Optik 85:19–24
[2] Rose H. 1999. Prospects for realizing a sub-Å sub-eV resolution EFTEM. Ultramicroscopy 78:13–25
[3] Haider M, Rose H, Uhlemann S, Schwan E, Kabius B, Urban K. 1998. A spherical-aberration-corrected 200 kV transmission electron microscope. Ultramicroscopy 75:53–60
[4] Krivanek OL, Dellby N, Lupini AR. 1999. Towards sub-Å electron beams. Ultramicroscopy 78:1–11
[5] Haider M, Uhlemann S, Zach J. 2000. Upper limits for the residual aberrations of a high-resolution aberration-corrected STEM. Ultramicroscopy 81:163–75
[6] Krivanek O, Nellist P, Dellby N, Murfitt M, Szilagyi Z. 2003. Towards sub-0.5 Å electron beams. Ultramicroscopy 96:229–37
[7] Batson P. 2003. Aberration correction results in theIBMSTEMinstrument. Ultramicroscopy 96:239–49
[8] Nellist P, Chisholm M, Dellby N, Krivanek O, Murfitt M, et al. 2004. Direct sub-Angstrom imaging of a crystal lattice. Science 305:1741
[9] Urban K, Kabius B, Haider N, Rose H. 1999. A way to higher resolution: spherical-aberration correction in a 200 kV transmission electron microscope. J. Electron Microsc. 48:821–26
[10] Jia C. L., Lentzen M., Urban K., 2003. Atomic-Resolution Imaging of Oxygen in Perovskite Ceramics, Science 299:870-873
[11] Krivanek OL, Corbin GJ, Dellby N, Elston BF, Keyse RJ, Murfitt MF, Own CS, Szilagyi ZS, Woodruff JW. An electron microscope for the aberration-corrected era, Ultramicroscopy, 108 (2008) 179–195.
[12] Haider, M., Braunshausen, G. & Schwan E. (1995). Correction of the spherical aberration of a 200 kV TEM by means of a Hexapole-corrector. Optik 99, 167–179.