X-ray Photoelectron Spectroscopy (XPS) is important for determining the surface chemistry and structure of materials and coatings. Mobile phone displays and medical devices, for example, need functional and protective coatings to work. Reducing thickness can cut cost dramatically, but doing so increases the risk of problems such as holes and contaminants that cause failure. Better understanding of coating chemistry and thickness will promote better design and manufacturing decisions.
For 50 years, UK-based Kratos Analytical Ltd has led the world in XPS and development of new instruments. 95% of instruments are exported, where they are used by coatings and advanced materials companies to research new products and spot defects and contaminants that may compromise material performance. Kratos has recently developed a new XPS instrument – a High Energy Ag L alpha X-ray source – which can measure at twice the depth of normal XPS. However, the current calibration and measurement infrastructure is not designed for XPS at these energies, making it hard to be confident in what exactly is being measured.
XPS works by firing X-rays at a surface, causing electrons to be emitted, which are then measured by a detector. The electron binding energy is different for each element, so XPS can identify which elements exist within a material. The electron kinetic energy is the difference between the X-ray energy and the binding energy, so higher energy X-rays mean faster electrons. Traditionally XPS has an X-ray energy of around 1500 electron volts, allowing electrons to escape from only 5-10nm into a material’s surface. Kratos’ new X-ray source doubles the X-ray energy and increases the electron escape depth.
The new instrument could identify the presence of elements deeper in the sample, but could not quantify them with sufficient certainty. Kratos approached NPL to explore methods to determine exactly how they could turn data from their new instrument into meaningful information.