Mechanism of single-shot damage of Ru thin films irradiated by femtosecond extreme UV free-electron laser

Abstract

Ruthenium is a perspective material to be used for XUV mirrors at free-electron laser facilities. Yet, it is still poorly studied in the context of ultrafast laser-matter interaction. In this work, we present single-shot damage studies of thin Ru films irradiated by femtosecond XUV free-electron laser pulses at FLASH. Ex-situ analysis of the damaged spots, performed by different types of microscopy, shows that the weakest detected damage is surface roughening. For higher fluences we observe ablation of Ru. Combined simulations using Monte-Carlo code XCASCADE(3D) and the two-temperature model reveal that the damage mechanism is photomechanical spallation, similar to the case of irradiating the target with optical lasers. The analogy with the optical damage studies enables us to explain the observed damage morphologies.

Damage accumulation in thin ruthenium films induced by repetitive exposure to femtosecond XUV pulses below the single-shot ablation threshold

“…If we assume that there is a damage threshold in the range of 20-25 mJ/cm 2 it will lead to the conclusion that with 209 pulses, the Ru film was exposed to a larger number of pulses that can cause damage than in the 1600 pulses case, explaining the larger damage with 209 pulses. In our recent investigations [7] we showed that the nature of single-shot XUV-induced ablation of Ru is photomechanical spallation in the stress confinement regime. The latter means that heating of the lattice is faster than the acoustic relaxation time, which means that heating occurs at almost isochoric conditions.…”

“…It was shown in damage studies of metals induced with optical lasers [18][19][20] that this situation leads to the generation of large thermo-induced stresses and, as a result, to spallation of the top part of a metal. The single-shot spallation threshold at 20 • grazing incidence was measured to be Fspall=64+/-13 mJ/cm 2 of absorbed fluence, while the melting threshold was calculated to be Fmelt=13 mJ/cm 2 [7]. It is known from literature [18], that spallation starts with nucleation of subsurface voids or cavities in a melted layer of irradiated metal, created as a result of propagation of a tensile stress wave.…”

“…Since Ru has a high reflection coefficient for the extreme ultraviolet and soft X-ray wavelength ranges, and has a low oxidation rate, it is a very attractive material for reflective coatings. This work is a continuation of a series of studies of optics durability exposed to FEL radiation below the single-shot damage threshold published here [6] and a detailed analysis of the nature of single shot damage discussed here [7].…”

Modeling of XUV-induced damage in Ru films: the role of model parameters

“…The relatively high value of its critical angle (27°at 92 eV photon energy) allows operation in a wide range of grazing incidence angles. In our recent study [13], we investigated femtosecond XUV-induced single-shot damage of a 50 nm thick Ru film on a Si substrate. The experiment was performed at Free-electron LASer in Hamburg (FLASH) [1].…”

“…Experimental analysis of the damaged morphologies, together with simulations of photoabsorption and ultrafast evolution of electron and lattice temperatures, showed that the nature of single-shot damage is photomechanical spallation, a phenomenon that was previously studied in the case of irradiation of solids with optical [14][15][16][17][18][19] and XUV [20][21][22][23] lasers. The spallation threshold for Ru was measured to be F spal 200 40 mJ∕cm 2 of incident fluence [13].…”