Quantitative Auger and XPS Analysis of Thin Films
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ergy levels: an empty core level (Ei) and two higher levels (£2 and E3). In this internal process, an electron from one of the higher levels fills the core level while the other is ejected with the balance of the energy. So the kinetic energy E of the ejected electron (called an Auger electron after its discoverer) is approximately given by = E,-E2-E3,
(1)
where the binding energies are taken to be positive numbers. The empty core level is typically created by bombarding the specimen with an electron beam of several keV However, illuminating the specimen with x-rays (as used for XPS) also ejects core-level electrons and gives rise to Auger electrons. The creation of a photoelectron is comparatively straightforward. A photon of energy hv can interact with an electron, imparting to it all of its energy. The energy of this ejected photoelectron is the difference between the photon energy and the binding energy of the electron BE, E = hv- BE.
(2)
The surface-sensitive nature of AES and XPS is a result of the short inelastic mean free path (IMFP) of the emitted electrons. For example, the IMFP of a 50 eV electron in a solid is =5 A and increases slowly with kinetic energy approximately as VE. 5 Only those electrons which have not scattered inelastically emerge from the sample with the characteristic energy of the element that emitted them. Since most elements have electrons with binding energies (and therefore ejected Auger electron energies £) in the 50-1,500 eV range, they have short IMFPs and high surface sensitivity. In XPS, one can assure low photoelectron energies by using low photon
energies (see Equation 2). In the laboratory, the most commonly used x-ray sources for XPS are Mg K ^ (1,253.6 eV) and Al Kou (1,486.6 eV), giving rise to photoelectron energies in the desired range. Using a tunable source, such as synchrotron radiation, provides the advantage that one can choose the best energy for each study. Figure la illustrates a typical Auger experiment. A 3-5 keV electron beam is used to excite the specimen and the ejected electrons are energy analyzed. The primary beam penetrates deep (~several /am) below the surface, creating a plume of backscattered electrons. As a result, there is a significant flux of backscattered electrons emerging from the surface in the vicinity of the primary electron beam. Those that are elastically scattered, or have lost little energy, contribute to the excitation of the atoms near the surface. Thus the intensity of Auger electrons has two contributions: electrons from atoms excited by the primary beam and those from atoms excited by backscattered electrons. This can be a particularly important issue for thin-film specimens, since the near-surface often is a different material than the bulk (substrate) below. An advantage of using an electron beam for the excitation is that the technology for focusing and scanning the beam is well developed. As a result, it is possible to perform scanning Auger to obtain a surface chemical map with submicron resolution. For small diameter beam
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