Ultrashort pulse selective laser ablation of multi-layer thin film systems

An experimental study on selective single pulse ablation of a SiO2/Si3N4-multilayer thin film system on fused silica substrate using ultrashort pulsed laser radiation with wavelengths of 343 nm, 515 nm and 1030 nm is presented. The effect of a NiCr metal interlayer and the influence of its thickness in the range of 0.5 - 3 nm on the ablation behavior are investigated. Clean delamination can be achieved in thin film systems including a metal layer. A decreasing ablation threshold with decreasing laser wavelength is observed for the dielectric materials SiO2 and Si3N4. In contrast, the ablation threshold of the metal layer shows a negligible influence of the wavelength. However, a significant influence of the metal layer thickness on the ablation threshold is found, causing a decrease of the threshold fluence from 0.81 J/cm2 for a NiCr layer thickness of 0.5 nm to 0.09 J/cm2 for 3 nm thickness using IR laser radiation. The quality regarding the precision of ablation crater edges depends on the ablation mechanism. In the case of direct ablation due to absorption in the upper layer, a thermal effect is observed in the form of an ablation crater edge characterized by ridges of molten material. If there is a layer with a higher ablation threshold on top of the thin film stack, precise ablation with clean crater edges is caused by confined ablation due to absorption in the underlying layers.