Electron microscopy
 
Vacuum of Electron Source/Gun
- Practical Electron Microscopy and Database -
- An Online Book -
Microanalysis | EM Book                                                                                   http://www.globalsino.com/EM/        

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The tungsten (W) hairpin filaments are less sensitive to the vacuum level than the LaB6 and field emission sources. The W filaments are normally stable at vacuums between 10-6 and ~10-3 Torr, while LaB6 filaments are stable only when the vacuum is better than 10-6 Torr. The thermal field emission sources require a vacuum of the order of 10-7 Torr for stable operation and the cold field emission sources are ~10-10 Torr. In general, the better the vacuum, the greater the stability.

Mostly, microelectronics industry has the need of the cleanest microscope columns. For instance, after the semiconductor wafers, containing numerous microelectronic devices, are observed with SEMs, various fabrication processes will continue. Therefore, the wafers cannot be contaminated during SEM observation. The vacuum configuration of the microscopes for this application would probably have a scroll pump used to rough out and to back a turbomolecular pump for the specimen chamber. In addition, ion getter pumps are typically attached to the electron electron gun and the upper portion of the column. In this case, the three types of pumps can provide the cleanest SEM system. A table on the comparison of various electron sources, including their vacuums, is presented in page1409.

 

 

 

 

 

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