Since the interaction between solid materials and their surrounding media, whether gaseous or liquid, occurs at the surface, analytical techniques capable of providing information from the interaction region are fundamental in understanding the processes that are occurring. X-ray Photoelectron Spectroscopy (XPS) is one such technique, and is capable of analysing both conducting and insulating materials.
During analysis, the surface is irradiated by soft X-rays and the energy of the emitted photoelectrons measured. These energies are typically less than 2000 eV; the electrons interact strongly with materials and only those emitted from the top few atomic layers can escape from the surface without energy loss. The energy of these electrons is determined by the atomic number of the emitting element, and is sensitive to changes in the number of electrons in the valence band, so that surface chemical state information is obtained.
XPS is now a mature technique, the first commercial instruments having become available as long ago as 1969, and judged by the number of scientific publications, it is not only the most popular of the surface analytical techniques, but also the fastest growing. This is due to the fact that it provides relatively easily quantified surface chemical state information. This trend is likely to continue, as instruments capable of operation without expert guidance move from the laboratory to the production line, where they can assist in quality control. Furthermore, developments in data processing have for the first time enabled quantitative surface chemical state images to be produced.
In spectroscopic mode, improvements in instrumentation resulting in increased energy resolution and sensitivity have led to analysis from smaller areas. The minimum analysis area is now of the order of 10 µm diameter, and so some means of locating this region was required, which led to the development of imaging XPS. The first imaging instrument, the ESCASCOPE became available in 1990, and allowed single energy images to be used to locate the region for selected area analysis. Current instruments acquire images with a spatial resolution of about 3 µm either by scanning a focused X-ray probe over the surface, or by maintaining the spatial orientation of the photoelectrons whilst energy filtering them, known as parallel imaging. The latter is the preferred mode of operation, since it allows faster acquisition and minimises sample damage by exposure to X-rays.