THE INFLUENCE OF THERMAL-DIFFUSION ON LASER-ABLATION OF METAL-FILMS

Single-shot ablation thresholds of nickel and gold films in the thickn ess range from 50 nm to 7 mu m have been measured for 14ns laser pulse s at 248 nm, using photoacoustic shock wave detection in air. The meta l films were deposited on fused silica substrates. The ablation thresh old was found to increase linearly with film thickness up to the therm al diffusion length of the film. Beyond this point it remains independ ent of film thickness. The proportionality between threshold fluence a nd thickness allows the prediction of ablation thresholds of metal fil ms from the knowledge of their optical properties, evaporation enthalp ies and thermal diffusivities. Physically it proves that ablation is d riven by the energy density determined by the thermal diffusion length . A simple thermodynamic model describes the data well. Thermal diffus ivities, an essential input for this model, were measured using the te chnique of transient thermal gratings. In addition, the substrate depe ndence of the ablation threshold was investigated for 150 nm Ni films.