Mechanism of Femtosecond Laser Ablation of Ruthenium Films based on Two-Temperature Model and Molecular Dynamics Simulation

Due to the widespread application of ruthenium (Ru) in free electron laser mirrors, it is of great significance to understand the performance under extreme conditions to optimize its properties. Therefore, this work focused on the damage dynamic behavior of the Ru film under femtosecond laser irradiation. The method combined two-temperature model (TTM) and molecular dynamics (MD) simulation was chosen, as for the suitable time and space scale, and the electron–lattice coupling temperature (ELT) and maximum deposition depth were used as the modeling basis to simulate the ablation and peeling of 50 nm Ru film on single crystal silicon (Si) substrate by femtosecond laser under 15 keV and 3 mrad conditions. The lattice structure of the Ru film was analyzed to observe the surface ablation and internal fracture evolution, which further explains the microscopic mechanism in the damage process from the perspective of stress wave propagation, revealing the intrinsic relationship between the structural response and material degradation on different scales. The results show that the ablation threshold of the Ru film irradiated by laser with pulse width of 100 fs is about 21.75896 J/cm2. Atomic force microscopy (AFM) shows that the surface roughness of the film is about 7.60 nm. Considering the actual situation, the process of laser irradiation of samples in the range of 2–8 roughness was simulated in order to explore the influence of roughness on the damage threshold. It was found that the larger the roughness is, the earlier the moment of damage occurred, and the lower its damage threshold is. In this work, the damage mechanism of the Ru film is studied on microscale, and the damage process is explored on time scale, which has theoretical guiding significance for the application of free electron laser mirror film materials.