Auger Electron Spectroscopy (AES)


 Auger (pronounced oh - jyay) Electron Spectroscopy (AES) is an analytical method for extremely small volumes of materials. It is named after the man who first postulated the effect. In this process, the sample is bombarded with a beam of electrons as in SEM/EDX analysis. Occasionally, one of the incoming electrons will knock a core shell electron loose, a second electron will fall into its place, losing some energy (E1) in the process which in turn ejects a third electron from the atom with the energy E1 - its binding energy. This is the Auger process and that third electron is known as an Auger electron. The energy with which it leaves the atom is a function of only the energy levels of the 3 electons of the atom and so is characteristic of the particular set of interactions and the element. Thus, by knowing the energy of the Auger electron, one can determine from what element it was emitted.

The volume of interaction with the electron beam is far larger than the beam itself but the Auger electrons have very little energy left

  and so can only escape from the top 1 to 5 atomic layers of the sample. Since most of the lateral spreading of the electron beamtakes place at lower levels than this, the analysis volume is approximately the diameter of the electron beam times the escape depth. Since a field emission beam may be less about 100Å in diameter and the depth is typically about 10Å the total analysis volume may be as little as 100,000 cubic Å or about 100 cubic nm. By comparison, the analysis volume for X-rays under similar conditions would be about 1 cubic µm or 1,000,000,000 cubic nm.

 A conventional Auger system can be used to analyze particles that can be clearly seen at about 10KX magnification. Particles or regions smaller than that usually require a field emission Auger system.

This powerful analytical tool can be combined with ion beam sputtering to be even more useful. With this modification, the surface is analyzed, then removed with the ion beam and then the new surface is analyzed. By repeating this process, changes in the composition with depth can be determined with nm resolution through a thickness of about 1 µm or so.

Generally, the electrons involved in these transitions come from the core shells and so are relatively insensitive to the bonding state. For lighter elements, however, especially silicon and aluminum, some information about state is available. It is very limited and so Auger can not give much information about compounds.

Quantitative determinations by AES are made by measuring the peak-to-peak height of the derivative of the spectrum which is the number of electrons vs their energy. This means it is a function of the slopes of the peaks. By comparing the heights for all the elements present and multiplying by the sensitivity factors for each element quite good concentration information can be obtained provided only mixtures of pure elements or specific compounds with known sensitivity factors are involved. In the more real case where there may be multiple compounds of one element or one element may be present in a compound and in the pure state, the quantitation becomes less reliable because the slopes can be quite sensitive to chemical state.