Coherent Illumination in EMs
- Practical Electron Microscopy and Database -
- An Online Book -  

This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.


The most promising coherent electron sources for electron microscopes (EMs) have been found in ultrasharp nanotips [1–3] in field emission electron guns due to the very high coherence and significant brightness. The important, common structural feature of their brightest field emitters is that they end with a single atom at the apex of a nanoprotrusion. The resulting characteristics are high coherence, strong focusing, sharp energy spectra, and high degeneracy [4] that significantly improve the spatial and temporal coherence properties. The use of ultrasharp nanotips in the EMs significantly improves the resolution and magnification, and allows for the simultaneous imaging of a larger area.

For a perfectly coherent illumination in TEM, the resulting phase contrast transfer function continues to oscillate between +1 and -1 even at very large spatial frequencies.

The schematic illustrations in Figure 3697 show the convergent illumination configurations of various modes in TEMs. In the CTEM condition in Figure 3697 (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 3697 (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 3697 (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 3697 (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.

CTEM mode EDS mode 2685a

Figure 3697. Convergent illumination configurations: (a) CTEM mode, (b) EDS mode and (c) NBD mode.

Fresnel fringes are produced along the edges of TEM objects when the objects are out of focus under a coherent illumination. For instance, the edge between the carbon film and vacuum, or the aperture edges in the TEM present such fringes.



[1] M. Rezeq, J. Pitters, R.A. Wolkow, J. Chem. Phys. 124 (2006) 204716.
[2] V.T. Binh, N. Garcia, Ultramicroscopy 42–44 (1992) 80.
[3] K. Nagaoka, H. Fujii, K. Matsuda, M. Komaki, Y. Murata, C. Oshima, T. Sakurai, Appl. Surf. Sci. 182 (2001) 12.
[4] V.T. Binh, S.T. Purcell, Appl. Surf. Sci. 111 (1997) 157.



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