Residual-Gas-Induced Oxygen Accumulation as an Analytical Bias in XPS of Sputter-Treated Metal Surfaces

Trace oxygen-containing residual gases in X-ray photoelectron spectroscopy (XPS) chambers can adsorb on or react with sputter-treated metal surfaces, introducing uncertainty into oxygen-related XPS analysis. However, such residual-gas effects remain insufficiently clarified. In this work, ten representative elemental metal films were examined under identical XPS chamber conditions, and the primary comparison focused on seven films showing lower apparent initial oxygen levels after Ar⁺ sputtering. Time-resolved O 1s measurements revealed a common two-stage behavior, with an initial rapid increase followed by a slower approach to a quasi-steady state. The apparent transition to the slower stage was material-dependent; several metals entered this stage within the early measurement period, whereas Al showed a prolonged rapid accumulation stage of approximately 2000 s. Among the seven films, the early stage O 1s accumulation rates were relatively high for W, Mo, and Mn, intermediate for Al, and lower for Fe, Co, and Ni. Kinetic modeling further indicated two distinct oxygen-accumulation behaviors, with Avrami-type and pseudo-second-order (PSO) models providing the preferred descriptions. Measurable O 1s growth despite nearly unchanged metal core-level spectra indicates that metal-peak stability alone is insufficient to rule out the presence of ultrathin oxygen-containing overlayers. High-resolution O 1s fitting revealed that the accumulated oxygen was mainly present as chemically bound M-O and -OH species rather than only molecularly adsorbed H₂O. These findings improve the understanding of residual-gas interactions with sputter-treated metal surfaces and provide practical guidance for optimizing oxygen-related XPS measurement protocols to reduce analytical bias.