Connecting water outgassing with desorption energetics of molybdenum and technical vacuum materials

Achieving and maintaining ultraclean vacuum environments is critical for contamination-sensitive processes such as semiconductor manufacturing, where residual gases directly affect performance and reproducibility. During pump-down, H2O outgassing from ambient-exposed technical materials often dominates the gas load, yet energetics-resolved studies of H2O release from these materials remain limited compared with ideal, single-crystal surfaces. In this work, ultrahigh-vacuum temperature-programmed desorption (TPD) was used to investigate H2O outgassing from Mo, 304 stainless steel, 6061 aluminum, Macor, and Si3N4 following exposure to 50% relative humidity (RH) at 20 °C. TPD spectra quantified retained H2O and enabled the extraction of apparent desorption-energy distributions. Coupling these distributions to a surface “sojourn-time” model translated TPD-derived energetics into effective release timescales, identifying which energetic states dominate gas load at a given temperature and how elevated bakeout temperatures accelerate H2O desorption. Molybdenum desorbed 1.5–3.7 monolayer equivalents of H2O over a broad range (∼50–650 °C) for exposure times from 15 min to 480 h with recurring desorption features embedded within a broad ∼90–260 kJ/mol apparent desorption-energy envelope. X-ray and ultraviolet photoelectron spectroscopy measurements showed that ambient exposure forms a hydroxylated, MoO3-rich surface that progressively dehydrates and reduces to a metallic state during heating. Applying the same protocol to other materials revealed strongly material-dependent H2O retention. After 20 h at 50% RH, Mo, 304 SS, and Si3N4 released ∼1–3 monolayers of H2O, whereas 6061 aluminum and Macor released substantially larger amounts at lower temperatures (<200 °C). These measured inventories and desorption-energy distributions provide a quantitative basis for comparing vacuum materials and optimizing pump-down and bakeout strategies.