Laser-induced electron dynamics and surface modification in ruthenium thin films

We performed the experimental and theoretical study of the heating and damaging of ruthenium thin films induced by femtosecond laser irradiation. We present the results of an optical pump-probe thermoreflectance experiment with rotating sample allowing to significantly reduce heat accumulation in irradiated spot. We show the evolution of surface morphology from growth of a heat-induced oxide layer at low and intermediate laser fluences to cracking and grooving at high fluences. Theoretical analysis of thermoreflectance in our pump-probe experiment allows us to relate behavior of hot electrons in ruthenium to the Fermi smearing mechanism. This conclusion invites more research on Fermi smearing of transition metals. The analysis of heating is performed with the two-temperature modeling and molecular dynamics simulation, results of which demonstrate that the calculated single-shot melting threshold is higher than experimental damage threshold. We suggest that the onset of Ru film damage is caused by the heat-induced stresses that lead to cracking of the Ru film. Such damage accumulates during repetitive exposure to light.