AES Instrumentation: Primary Electron Sources

Three kinds of primary electron sources are in common use in Auger electron spectrometers.

1. A tungsten cathode source consists of a wire filament bent in the shape of a hairpin. The filament operates at ~2700 K by resistive heating. The tungsten cathodes are widely used because they are both reliable and inexpensive. Lateral resolution is limited because the tungsten cathode current densities are only about 1.75 A/cm2.
2. Lanthanum hexaboride (LaB₆) cathodes provide higher current densities because LaB₆ has a lower work function and greater emissivity than tungsten. At 2000 K, current densities of ~100 A/cm2 are available. Higher current densities provide narrower electron beams useful for analyzing smaller features.
3. Field Emission electron sources consist of very sharp tungsten points at which electrical fields can be >10E7 V/cm. At these fields, electrons tunnel directly through the barrier and leave the emitter with near zero work functions. Field emission guns provide the brightest beams (1E3 to 1E6 A/cm2). However, the low work functions are only obtained with extremely clean tips. A single atom on the tip can increase the work function and reduce electron emission. Ultrahigh vacuum and continuous heating (~2000 K) keep the tip clean and the electron beam stable. Electron beams as narrow as 10 nm provide Auger analysis of small features.

Auger instruments have primary electron beam columns similar to electron microscopes. The columns may include both electrostatic and magnetic lenses for beam focusing as well as quadrupole deflectors for beam steering and octopole lenses for beam shaping.

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