Understanding Thin Film Laser Ablation: The Role of the Effective Penetration Depth and the Film Thickness

In recent years, several applications for the laser ablation of thin metal films from the glass substrate side have been studied.In this case, the laser pulse transmits through the glass substrate, the metal film absorbs the pulse, and the energy is confined at the molybdenum-glass interface. The ablation mechanism is called confined laser ablation. It was observed that the confined laser ablation of 430 nm thin molybdenum films with ultrashort laser pulses leads to the lift-off of intact disks.The ablation efficiency can be increased by a factor of 10 compared to the direct ablation from the film side.The goal of the study is to investigate, if a confined ablation,which enables this high ablation efficiency, can be achieved also at different pulse durations and film thicknesses.For this purpose, the pulse duration is varied between 20 ns and 330 ns at a constant film thickness of 430 nm, andthe film thickness is varied between 10 nm and 5000 nm at a constant pulse duration of 10 ps. The single pulse ablation threshold fluences are determinedfor the film and glass substrate side irradiation. The determined threshold fluences are finally compared to calculated melting and evaporation threshold fluences.The results suggest that the ablation mechanism for glass side ablation is based mainly on melt dynamics, if the effective penetration depth of the laser pulse is in the order of the film thickness. A high efficient confined ablation can be observed, if the effective penetration depth is smaller than the film thickness.