Electron optics of STEM

Not shown in Fig. 3.1 are other analytical detectors normally found in STEM, because of lack of space. The principal omission is an X-ray detector that analyses X-rays emitted by the specimen, others include detectors designed to collect cathodoluminescence, specimen current. Auger electrons, etc. The lack of physical space around the specimen is often the deciding factor on which detectors are employed on a particular machine. In general, dedicated STEM has room to employ more detectors. In these respects, STEM is very much about detection systems, and not so much about the probe formation. 

Having said that, the electron-optics of STEM is dedicated to the production of a small probe. Before we review the probe formation, it is profitable to ask what determines the useful probe size for a realistic sample with finite thickness Part of this question can be answered through an understanding of the physics of electron scattering in solids, and its effect on the beam broadening. 

Monte Carlo calculations [3.1] give the best impression of the beam scattering which controls the interaction volume between the fast electron beam and the 

d is the diameter within which 90% of the beam is to be found, Z and A are the atomic number and mass respectively of the element, Eo is the primary beam energy in kilovolts, and p is the sample density in grams per cubic centimeter. This produces beam sizes for 100 keV incident electrons as shown in Table 3.1, with most data taken from [3.2] except for the new estimate for the oxide superconductor YBa2Cu307 (YBCO). The latter is achieved by substituting Z with the composition averaged y/ Z ) and replacing A with the composition averaged (A), from cross-sectional considerations. 

After the probe size, the current within it is another very important parameter of the STEM. A small probe with a small electron flux may be good on the specification tables of the manufacturers brochures but is useless in practice. Because electromagnetic lenses suffer from large aberrations, only a small cone of illumination along the central optical axis is useful for information transfer. The important parameter regarding the current in the probe is then 

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