The achievable instrumental performance of a STEM is mainly determined by the size and shape of the incident electron probe. The most important optical factor in achieving the optimum probe profile is the radius of the probe-forming aperture, which determines the convergence semi-angle of the illumination. Small deviations from this optimum can degrade both the resolution and interpretability of the image contrast.
Furthermore, in general, the spatial resolution attained in an STEM image is determined by the probe size and current as well as by the beam–sample interaction. Therefore, the diffraction limit on the attainable spatial resolution, when imaging with an illumination cone of half-angle αgeom, can be given by,
Physically, dgeom is the full-width at half-maximum (FWHM) of the resultant probe due to geometric aberrations.
With small electron probes, e.g. ≤ 0.2 nm, STEM can obtain atomic resolution information from crystalline specimens and their defects [6 - 8]. Aberration-corrected STEM instruments now routinely achieve a resolution of better than 0.1 nm [1 - 5].
The spatial resolutions of EELS/EFTEM can be slightly worse than those of HAADF and TEM imagings primarily due to the specimen drifts during the slower acquisitions of the spectra or elemental mapping.
For the TEM configuration with a top-entry type EDS detector, the detector is placed above the objective lens in a TEM system with a high viewing angle (e.g. 70 °) to a horizontal specimen. Since the detector takes X-ray signals from the top of the objective lens, a large bore polepiece for the objective lens is needed. The use of such polepieces degrades the spatial resolution as well as the probe size.
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