Electron microscopy
Contamination and Cleaning of Electron Guns in EMs
- Practical Electron Microscopy and Database -
- An Online Book -
Microanalysis | EM Book                                                                                   https://www.globalsino.com/EM/        

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


Cold field-emission (CFE) gun emitter is normally made of tungsten with the surface of the (310) plane, working at room temperature without heating. Contamination of residual gases on the surface of the emitter generates emission noise. For the same reason, the cold emission guns are not very stable. For instance, some emission current from a freshly cleaned tip only takes 40 min to decay to 50%. [1] Therefore, regular maintenance, the so-called flashing process, is often needed.

For thermal FEGs (field emission guns), electrons are emitted through a reduced potential barrier (reduced by an applied electric field) even when heating the emitter to a lower temperature than the thermal electron emission temperature of 1600 - 1800 K. This phenomenon is also called the Schottky effect. Comparing to cold FEGs, thermal FEGs have disadvantage of large energy spread (0.6 -0.8 eV) because of heating the emitter. However, they have smaller emission noise and produce a stable emission current without gun flashing because there is no adsorption of contamination on the emitters.

The most common phenomena in high-voltage TEMs is discharging, for instance, discharging often occurs when the high voltage is turned on. Actually, this discharge cleans the inner surface of the electron gun even though it can also induce damage to the insulators.







[1] Krivanek OL, Corbin GJ, Dellby N, Elston BF, Keyse RJ, Murfitt MF, Own CS, Szilagyi ZS, Woodruff JW. An electron microscope for the aberration-corrected era, Ultramicroscopy, 108 (2008) 179–195.



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