Surface-limited deuterium uptake of Ru films under plasma exposure

Abstract

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.

Promotion of Plasma-Induced Deuterium Uptake in Ruthenium Films by Monolayer-Thick Tin Layers

“…Based on these experimental results, we assume a pure Ru(0001) surface and define 1 monolayer (ML) as an overlayer with the atomic density of the Ru surface, which equals 1.74 × 10 19 atoms/m 2 according to the measured areal density of the Ru film. The RuO 2 layer on the surface was approximately 1 ML in thickness as confirmed in our previous work in ref .…”

“…The Ru layer was a polycrystalline film with a dominant (0001) orientation. The areal densities of the Ru and Ti films were (2.9 ± 0.23) × 10 21 and (1.75 ± 0.14) × 10 21 atoms/m 2 , respectively, according to Rutherford backscattering spectroscopy (RBS) measurements as described in detail in ref . If we assume a Ru density of 12.2 g/cm 3 and a Ti density of 4.54 g/cm 3 , the Ru and Ti films had thicknesses of 39.9 ± 3.2 and 30.6 ± 2.4 nm, respectively.…”

“…We used RBS to determine the Sn content on the target before and after plasma exposure, and we used ERD to measure the amount of deuterium in the target after plasma exposure. The details of the measurement techniques are described in ref 12, and here we give a brief summary as well as the measurement strategy for each target. The experiments were carried out at the DIFFER ion beam facility (IBF), where we used a 4 He + beam at 2.4 MeV, and a fraction of the incident beam was backscattered by a chopper for relative ion dose Deuterium retention in the Sn-covered and Sn-free targets exposed to the deuterium plasma at an ion energy of 1 eV.…”

“…31,34 The deuterium flux from the RuO 2 to Ru surface is also excluded in the model due to strong deuterium desorption from the RuO 2 surface 28 and scarcity of empty sites on the Ru surface. 12 Depth-wise and lateral deuterium fluxes constitute the model for calculation of nine concentrations in the system. Based on Ru oxide removal and conservation of deuterium atoms, we have the following differential equations where the parameters are divided into fixed parameters in Tables 1 and 2, and free parameters in Table 3.…”

“…Based on Ru oxide removal and conservation of deuterium atoms, we have the following differential equations where the fluxes are defined as where the parameters are divided into fixed parameters in Tables and , and free parameters in Table . Since the Ru-capped targets in this work are provided by the same supplier as that for our previous work, we use the parameters determined using our previous model as fixed parameters in this model as shown in Table . For the Sn-covered surface, we neglect the desorption process and assume a sticking coefficient of unity for deuterium atoms, since hydrogen radicals and Sn atoms lead to exothermic and spontaneous formation of stannane. , By keeping all of the fixed parameters constant and varying the free ones, we fit the D retention with this model for all of the exposure conditions in Figure .…”