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Surface impurities can have a significant influence on hydrogen uptake of materials. Examples such as the hydrogen spillover effect demonstrate that even very small surface impurity quantities can lead to order-of-magnitude changes in the total amount of hydrogen taken up by a material. In this work, we report the first experimental demonstration of promoted deuterium uptake in Ru thin films by Sn. Deuterium plasma exposures were carried out for Ru-capped targets covered by Sn up to a few atoms in thickness. After the exposure, the residual Sn content and the deuterium retention were measured to quantify the Sn etching and the deuterium uptake, respectively. By increasing the amount of Sn from zero to one atomic layer on Ru, we found after the exposure that the Sn content stays unchanged while the deuterium uptake rate severely increases with the Sn content by 2-3 orders of magnitude. These results can be understood by simulations using a reaction-diffusion model with multiple surface species and the lateral surface migration of deuterium. By contrast, as the as-deposited Sn content goes above one atomic layer, Sn removal takes place, and the deuterium uptake rate decreases with the as-deposited Sn content. Possible explanations are proposed by considering the interplay between Sn etching and deuterium uptake. In all, this work provides insights into interactions between multiple surface species in relation to plasma-induced hydrogen uptake. By further development, this could eventually lead to a potential mitigation method to circumvent the promoted hydrogen uptake in Ru-capped films.

We compare the performance of a hydrogen radical probe to historic data determined via two-photon absorption laser induced fluorescence using a comparable cascaded arc source under similar operating conditions. This probe has dual heat flux sensors (DHFS) each coated with materials with different catalytic properties for hydrogen atoms. In the ideal situation, the hydrogen radical flux can be deduced based on the difference between the heat loads measured by these two sensors. The influence of DHFS temperature on the performance was also assessed. The experimental results showed measurement errors of <10% could be obtained regardless of the probe temperature during plasma exposures. To convert heat fluxes into atomic fluxes, we calibrated the difference of the recombination coefficients using a vacuum ultraviolet absorption technique, which is more reliable than modeled values based on assumptions or scattered values reported in literature. As a result, we measured the hydrogen plasma and radical parameters at various settings using both a double Langmuir probe and the DHFS. The typical atom flux in the 1022 m−2s−1 range was in good agreement with those obtained using optical techniques. We also observed that the ion and atom fluxes are both sensitive to the background gas pressure. These findings validate application of the DHFS to the cascaded arc source, and could pave the way for optimization of the source performance in the plasma material processing experiments.

Blister formation has been an emerging research topic for extreme ultraviolet (EUV) mirrors exposed to hydrogen plasmas. Similar to plasma-facing materials in nuclear fusion reactors, it has been reported that blister formation in EUV mirrors is initiated by hydrogen uptake due to hydrogen ion or atom bombardment. However, the research so far has focused on Mo/Si multilayers exposed to only hydrogen ions or atoms, while the EUV mirror typically has a Ru capping layer facing hydrogen plasmas. We present experimental work to measure plasma-induced hydrogen uptake of Ru films. We bombarded our designed Ru-capped target with a low-temperature deuterium plasma and measured the deuterium retention using elastic recoil detection. Contrary to ion-driven deuterium uptake, the deuterium uptake rate of the Ru film had no dependence on the deuterium ion flux or energy after a period of plasma exposure. A reaction-diffusion model has been built to calculate the time evolution of deuterium retention, which well fits the experimental data. Based on this model, we conclude that the surface composition of the Ru film is the limiting factor for the deuterium uptake, which is seriously weakened when the surface is covered by Ru oxide. After the Ru oxide is reduced by the plasma, the uptake rate is predominantly driven by the deuterium surface coverage on metallic Ru. Our model also indicates that at the deuterium-populated Ru surface, deuterium has a low absorption barrier to penetrate the surface, which is supported by previously reported computational work.