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In FESEM: In a field emission (FE) scanning electron microscope no heating but a so-called "cold" source is employed. An extremely thin and sharp tungsten needle (tip diameter 10-7 -10 -8 m) functions as a cathode in front of a primary and secondary anode. The electrons are accelarated in the direction of the colom by a voltage gradient (see example for a traditional SEM in the table here below). The voltage between cathode and anode is in the order of magnitude of 0.5 to 30 KV. Because the electron beam produced by the FE source is about 1000 times smaller than in a standard microscope, the image quality is markedly better. As field emission necessitates an extreme vacuum (10-8 Torr) in the column of the microscope, a device is preswent that regurlarly decontaminates the electron source by a current flash. In contrast to a conventional tungsten filament, a FE tip last theoretically for a lifetime, provided the vacuum is maintained stable.
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