Working Distance in SEM
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Working distance is a distance between the specimen and the lower pole piece in SEM system. The final lens, focusing an electron beam on specimen, is operated with a relatively long working distance so that secondary electrons (SE) are easily collected with a lateral detector.

Figure 4586a shows schematics of the electron beam emerging from the final aperture in the objective lens and striking the specimen. A relatively large aperture and short working distance (W) in Figure 4586a (a) create a large convergence angle and thus a shallow depth-of-field. A small aperture diameter and long working distance (W) in Figure 4586a (b) create a small convergence angle and thus a large depth-of-field.

Schematics of the electron beam emerging from the final aperture in the objective lens and striking the specimen

Figure 4586a. Schematics of the electron beam emerging from the final aperture in the objective lens and striking the specimen: (a) Small depth-of-field mode and (b) Large depth-of-field mode.

Figure 4586b shows the schematic illustration of EBSD setup in a SEM system. The working distance for EBSD is roughly between 8 and 45 mm.

Objective Polepiece in EMs

Figure 4586b. The schematic illustration of EBSD setup in a SEM system.

For EDS measurements, the distance between the specimen and detector is critical, especially at long working distances in SEMs. The take-off angle is very large at such a working distance, therefore it is possible that only weak or no X-rays enter the detector. There is always an optimum where the maximum X-rays can be measured. Each microscope has a different geometry, and thus searching the optimum position of the detector is necessary at the installation of the EDS system.

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