AES Theory: Auger Electron Spectroscopy

Auger electron spectroscopy (AES) identifies elemental compositions of surfaces by measuring the energies of Auger electrons. An Auger spectrum plots a function of electron signal intensity versus electron energy. The Auger energies fall between secondary electron energies on the low end and backscattered electron energies on the high end. Those backscattered electrons that recoil with 100 % of their primary energy form the elastic peak.

The terms secondary and backscattered are sometimes defined in the operational terminology of scanning electron microscopy (SEM). The true secondary electrons have energy less than ~50 eV. They can be detected with the SEM secondary electron detector biased at +50 to +200 V. All electrons with too much energy to be trapped in the secondary electron detector fall in the backscattered category.

The Auger electrons start with narrow energy distributions, but they soon lose energy as they pass through materials. Auger electrons fail to emerge with their characteristic energies if they start from deeper than about 1 to 5 nm into the surface. Thus, Auger analysis is surface specific. Auger electrons that escape from deeper in the sample contribute loss tails to the spectrum background. The secondary and backscattered electrons have broad energy distributions that tail into the Auger region. The sum of these interfering signals is much greater than the Auger signals themselves. Auger display algorithms use differentiation to enhance the signal relative to the interferences.

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