 |  |  | |  | |  | |  | |  |
AES is a surface sensitive, non-destructive technique for identifying the elements in the first few atomic layers (~1 to 5 nanometres) on a specimen surface and is able to provide quantitative data on the detected elements. High-energy electron beam bombardment of the surface results in the emission of Auger electrons at characteristic discrete energies. Combined with inert gas ion sputtering, AES can be used to obtain depth composition profiles. The technique can be used to map the distribution of elements present on a specimen surface (spatial resolution of 0.5 mm). The technique is capable of detecting all elements with the exception of hydrogen and helium. Spectrometers can be fitted with a facility for in-situ testing of bonded joints and other specimen configurations, which are rapidly analysed under high vacuum conditions. Ultra-high vacuum conditions (1 x 10–10 Pa) are required to prevent contamination and oxidation of the fracture surfaces.
The technique is not particularly suited to examination of polymers (i.e. insulating materials) due to the possibility of beam damage and electrical charging of the sample, which can complicate data interpretation. The technique provides limited information on oxidation states. AES is a useful and complementary technique to XPS as it has better spatial resolution (smaller spot size is possible), but exhibits more sample damage and is less suited to determining chemical bonding or oxidation state.
This technique uses the inelastic scattering of low energy electrons in order to measure the vibrational spectra of surface species (electron-analogue of Raman spectroscopy). Since the technique employs low energy electrons, it is necessarily restricted to use in ultra-high vacuum environments. However, the use of low energy electrons ensures that it is a surface specific technique. The energy loss of a beam of electrons of fixed incident energy is analysed. This method has high sensitivity but requires flat, preferably conducting samples and has lower resolution than IRS techniques.
Secondary Ion Mass Spectrometry (SIMS)
In SIMS, the surface is bombarded with a beam of high-energy ions resulting in the ejection of molecular fragments, atoms and ions from the surface, which are subsequently analysed (traditionally only the positive ions). It is capable of providing surface elemental analysis and depth concentration profiles on areas from several mm to sub micron. SIMS can detect all elements and isotopes including hydrogen and hydrogenated compounds with very high sensitivity (parts per billion). It is, however, not readily amenable to quantitative analysis (complex and requires reference standards). The depth resolution is under 200 nanometres and areas of up to 500 mm can be studied. Whilst SIMS is suitable for all materials, flat sample surfaces are required to give the best spatial and depth resolution. There are a number of different variants of the technique including static SIMS (sub-mono-layer elemental analysis), dynamic SIMS (depth composition profiles) and imaging SIMS (elemental mapping) analysis.
RBS is one of a number of ion scattering techniques, which provide different information on an elemental level, depending on the primary ion energy (typically 1–3 MeV) and the scattered ion that is detected. A beam of positive helium ions (He+) is directed at the target surface and the ions, which are scattered by the sample nuclei, are measured and analysed. RBS is a non-destructive technique since the erosion and the radiation degradation of the sample material by the particle impact is negligible. As a result RBS is used for the quantitative, non-destructive compositional depth profiling and thickness measurements on thin films. The depth distribution of constituents can be reconstructed with a depth resolution of 10 to 20 nm. This method can probe several thousand atomic layers and is ideal for surface analysis up to 2 mm depth.
Next: Chromatographic Analysis