Auger Electron Spectroscopy

Auger electrons are produced whenever incident radiation--photons, electrons, ions, or neutral atoms—interacts with an atom with an energy exceeding that necessary to remove an inner-shell electron (K, L, M, . . .) from the atom. This interaction, or scattering process, leaves the atom in an excited state with a core hole, that is, a missing inner-shell electron. These excited atoms are unstable, and de-excitation occurs immediately. The energy thus released is either emitted as a photon (x-ray fluorescence) or given to another outer shell electron, which is emitted if the energy is great enough (called the Auger transition).

In Auger electron spectroscopy (AES), the sample to be analyzed is bombarded with electrons. If an incident electron (Fig. 4a) strikes a K-shell (core-level) electron with sufficient energy to free it (Fig. 4b), the atom is left in a singly ionized state with a core-level electron vacancy. If the atom is near the surface, both the incident electron and the core-level electron are emitted from the sample as backscattered electrons, with energies below approximately 25 eV. The singly ionized atom undergoes electron rearrangement to achieve a more stable energy configuration, where an outer shell electron fills the core level vacancy (Fig. 4c).

Fig. 4 Representative electron configuration and Auger transition

These knockout events occur within a 0 to 3 nm region from the surface, which allows compositional analysis of the surface for all elements except hydrogen and helium. The probability for Auger emission exceeds that for x-ray emission as atomic number decreases. This is one of the reasons why Auger analysis has some advantages for light-element analysis. As with the characteristic x-ray emission, the energy of the Auger electrons is different for each element; therefore, analysis of Auger energies yields information on chemical identity. In addition, as with energy loss electrons, the Auger energy levels sometimes shift when an atom becomes oxidized, nitrided, and so on; therefore, information on the chemical state of the surface atoms can sometimes be obtained from Auger analysis.

The instrumentation typically used in AES includes an electron gun for primary electron excitation of the sample, an electron spectrometer for energy analysis of secondary electrons, a secondary electron detector for secondary electron imaging, a stage for sample manipulation, and an ion gun for sputter removal of atoms from the sample surface.

The electron spectrometer is usually the central component of an AES system. Various types of analyzers are in use and include sector analyzers, retarding field analyzers, hemispherical analyzers, and cylindrical mirror analyzers. For electron-excited AES, the cylindrical mirror analyzer generally offers superior signal-to-noise performance, which is associated with its high point transmission. Consequently, it remains the most prevalent spectrometer.

Detection sensitivity for most elements (except hydrogen and helium) is from 0.1 to 1.0 at%. The accuracy of quantitative analysis is limited to ±30% of the element present when calculated using published elemental sensitivity factors. Better quantification (±10%) is possible by using standards that closely resemble the sample.

Samples can be solid metal, ceramic, and organic materials with relatively low vapor pressures (<10-8 torr at room temperature). Higher vapor pressure materials can be handled by sample cooling. Similarly, many liquid samples can be handled by sample cooling or by applying a thin film onto a conductive substrate.

Sample size can be individual powder particles as small as 1 /'m in diameter. The maximum sample size depends on the specific instrument.

Interpretation of Auger spectra, particularly spectra of irregular surfaces such as powders or porous structures, is difficult due to the number of variables present, including surface contamination and roughness, dissociation effects of chemical species due to the incident electron beam itself (beam damage), and differences in sputtering rates of different chemical species. Thus, conversion of peak heights into atomic concentrations presumes an accuracy that may not exist. However, even as a qualitative or semiquantitative tool, Auger electron spectroscopy/scanning Auger microscopy has already proved invaluable in the solution of many problems.

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