Hydrogen incorporation and release from nonevaporable getter coatings based on oxygen-free Pd/Ti thin films

Abstract

Oxygen-free Pd/Ti thin films have been developed as novel nonevaporable getter coatings for ultrahigh vacuum (UHV) chamber walls, which can maintain high pumping speed even after many cycles of air-vent and only require activation at a relatively low temperature of 133°C. Here, the authors clarify the hydrogen (H) incorporation and release properties of such oxygen-free Pd/Ti thin films using scanning transmission electron microscopy (STEM), energy dispersive x-ray spectroscopy (EDS), and H depth profiling by 1H(15N,αγ)12C nuclear reaction analysis (NRA). EDS mapping and STEM show that the layers of Pd (6 nm) and Ti (14 nm) prepared by sequential UHV deposition on a stainless steel 304L substrate are clearly separated and that the Ti is completely covered by Pd, which prevents oxidation of Ti. NRA reveals that H accumulates preferentially in the Ti layer, where the H-content increases almost linearly with the H2 dosage. Quantitative analysis indicates that TiH1.92 is produced in the Ti layer after an exposure of 4×104 Langmuir, where the H uptake is still not saturated. Upon heating in UHV, H starts to desorb above 170°C and the release rate increases above 200°C. Repeated cycles of H incorporation at room temperature (20°C) and release at about 240°C are possible. Although the H incorporation rate changes during the initial absorption/desorption, it stabilizes after a few (∼3) incorporation/release cycles.