The influence of thermal diffusion on laser ablation of metal films

Single-shot ablation thresholds of nickel and gold films in the thickness range from 50 nm to 7 m have been measured for 14 ns laser pulses at 248 nm, using photoacoustic shock wave detection in air. The metal films were deposited on fused silica substrates. The ablation threshold was found to increase linearly with film thickness up to the thermal diffusion length of the film. Beyond this point it remains independent of film thickness. The proportionality between threshold fluence and thickness allows the prediction of ablation thresholds of metal films from the knowledge of their optical properties, evaporation enthalpies and thermal diffusivities. Physically it proves that ablation is driven by the energy density determined by the thermal diffusion length. A simple thermodynamic model describes the data well. Thermal diffusivities, an essential input for this model, were measured using the technique of transient thermal gratings. In addition, the substrate dependence of the ablation threshold was investigated for 150 nm Ni films.