The physics of XPS are shown in this graph. Click to view image enlarged.

What is XPS?

X-ray photoelectron spectroscopy (XPS), also known as ESCA, an abbreviation for Electron Spectroscopy for Chemical Analysis, provides quantitative compositional information from the top 10 atomic layers of a sample surface for the elements lithium to uranium. Furthermore, information regarding the chemical states of any elements present can also be obtained.

XPS is used to measure elements that contaminate a surface, uniformity of elemental composition across the top surface (aka, line profiling or mapping), and uniformity of elemental composition as a function of ion beam etching (aka, depth profiling).

XPS can also be used to analyze the change in surface chemistry of a material after chemical or physical treatments such as: leaching, reduction, fracture, cutting or scraping in air or UHV to expose the bulk chemistry, ion beam etching to clean off some of the surface contamination, exposure to heat to study the changes due to heating, exposure to reactive gases or solutions, exposure to ion beam implant, exposure to UV light, etc... 

How measurements are obtained

A sample is irradiated with a beam of monochromatic soft X-rays. Photoelectron emission results from the atoms in the specimen. The kinetic energies of these electrons relates to the atom and orbital from which they originated. The distribution of kinetic energies from a sample is then measured directly by the electron spectrometer.

Atomic orbitals from atoms of the same element in different chemical environments are found to possess slightly different (but measurable) binding energies. These "chemical shifts" arise because of the variations in electrostatic screening experienced by core electrons as the valence and conduction electrons are drawn towards or away from the specific atom. Differences in oxidation state, molecular environment and co-ordination number all provide different chemical shifts.

Photoelectron binding energy shifts are, therefore, the principal source of chemical information. It should be noted that these shifts can be very small and can only be detected using a high performance instrument with suitable software such as our Kratos Axis Ultra DLD instrument equipped with a monochromated Al Ka x-ray source and hemispherical analyzer.