Electron Beam Convergence in TEM
- Practical Electron Microscopy and Database -
- An Online Book -

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

 

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Information limit depends on the damping envelope incorporating partial temporal coherence due to chromatic aberration, but not partial spatial coherence due to beam convergence.

The beam convergence can be reduced using a smaller illuminating aperture in combination with the brightness of a field emission gun. Moreover, with the introduction of a Cs-corrector [1], the dependence of spatial coherence on beam convergence reduces significantly and may even be eliminated completely at Cs = zero. In that case, the point resolution is equal to the information limit.

Spatial coherency originates from the fact that the illumination is never perfectly parallel and is actually slightly convergent at the best imaging condition, which can be described as having a distribution of different illumination tilts. The effect of illumination tilting on the phase contrast transfer function (PCTF) is to cause a phase shift about the y-axis and, in the same way as for the temporal coherency, the PCTF is averaged over this tilt distribution.

The condenser lens system provides variable probe-convergence angles in STEM mode and adjustable parallel illumination in TEM mode.

In addition to the specimen itself, the X-ray generation process is also affected by the probe size, current, and convergence angle. Fortunately, elemental concentration quantification can be done with reasonable accuracy by comparing the peak intensities with k-factors in EDS spectra.

 

 

[1] H. Rose, Outline of a spherically corrected semiaplanatic medium-voltage transmission electron microscope, Optik, 85 (1990) 19–24.

 

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