X-ray Photoelectron Spectroscopy

I’ve used a Thermo Scientific KAlpha XPS system, which uses a monochromatic Al Kα X-ray source (fancy talk for a very precise X-ray beam) and operates at an ultra-low pressure of about 10^-8 mbar to keep things clean. The system can detect elements down to 0.1- 0.2 atomic percent, so it’s super sensitive. We ran two types of scans: survey spectra at a pass energy of 200 eV to get a broad picture of all the elements present, and high-resolution spectra at 50 eV to zoom in on specific elements like Al, Au, Co, Cr, Fe, Mo, Nb, Ni, Ta, W, oxygen, and even nitrogen.

To get a sense of how the composition changed deeper into the film, we did something called depth profiling. This involves gently etching away layers of the material with an argon ion beam (set at 500 eV with a medium current) and taking XPS measurements at each step. For both the as-deposited films (before any heating) and the annealed ones, we used a sequence of etches. This let us probe down to analyze the through thickness compositional variations, which we confirmed with the system’s software. We recorded high-resolution spectra at the surface and after each etch to track how the elements and their chemical states changed with depth. All the data was crunched using Thermo Advantage software, which helped us make sense of the results.