Description of physical Phenomenon
The Auger effect was initially discovered in the 1923 by Lise Meitner and later attributed to Pierre Victor Auger in 1925. By 1968, the utilization of this technique was properly demonstrated for the analysis of specimens, specifically, for the identification of the chemical composition at the outer surfaces of solids [ 1 ]. Due to some limitations on the elemental sensitivity and charging of the specimen, AES is generally best suited for materials that are good conductors such as metals and alloys. Reasonable analyses of other materials such as glass, oxides and organic compounds have also been attained. Since the process occurs by the interaction of at least 3 electrons, the technique is limited to atomic values greater than 3.
The physical phenomenon occurring in the generation of Auger electrons is triggered by the bombardment of the specimen with a high energy particle such as a neutral atom, electron, x-ray photon or ion. Nonetheless highly energized electrons are preferred due to the fact that these particles can be manipulated into following specific trajectories by the use of magnetic fields. This has proven to be an advantage since it provides directionality control of the bombarding beam.
Once the high energy electron impinges on the specimen, ionization of some atoms occurs. Essentially, this means that an electron has been ejected from a lower orbital or shell of the atom leaving a core vacancy. At this point the atom becomes ionized or excited and in the immediate process of relaxation that follows, an electron from an upper orbital will migrate to fill the core hole left by the initially ejected electron. The change in energy necessary for the reconfiguration allows yet another electron to become sufficiently energized to escape from the same or another level. This electron is the Auger electron. Figure 1 is an animation depicting a simplified schematic of the actual process.
Figure 1. Auger electron process
The detection of the Auger electron is performed taking advantage of the fact that energies exhibited by these particles are characteristics of both the levels from which they escape and the element exhibiting this specific shell configuration. For this reason, AES is of particular importance in the analysis of surface phenomenon of materials as Auger electrons are generally emitted from depths in the range from 0.5 - 3 nm.
Application of AES
Bone response to implants produced by electron beam melting [ 2 ].
A group from the University of Gothenburg in Sweden carried out the surface characterization of electron beam melted titanium alloy implants. The alloy utilized in the fabrication of the implants was Ti-6Al-4V and the study utilized a PHI 660 scanning microprobe. By selecting 4 regions in a total of six implant specimens, they were able to analyze the average surface composition without any visible damage. The regions selected for analysis in each implant are shown in Figure 2 below.
Figure 2. Ti-6Al-4V EBM fabricated implants for bone ingrowth. Reproduced from [ 2 ]
The results of this analysis demonstrated that all samples exhibited a similar composition at the surface with only localized and minimal variations in the content of iron (Fe) for two of the specimens. Among the major constituents in the analyzed samples, carbon (C), titanium (Ti) and oxygen (O) where observed followed by other elements such as aluminum (Al) and Fe.
The AES study concluded that, due to a fairly large amount of oxygen at the surface, the EBM manufactured implants exhibited an increased oxide thickness which is of very practical importance for the proper bone growth and further integration of the implants to the living organism.
[ 1 ] ASM Handbook, Volume 10, Materials Characterization
[ 2 ] Thomsen, P., et al., Electron Beam-Melted, Free-Form-Fabricated Titanium Alloy Implants: Material Surface Characterization and Early Bone Response in Rabbits, Journal of Biomedical Materials Research Part B: Applied Biomaterials, 2008.