Highly-Selective Laser Ablation for Thin-Film Electronics: Overcoming Variations Due to Minute Optical Path Length Differences in Plastic Substrates

Roll-to-roll production of thin organic and large-area electronic (TOLAE) devices often involves a 2-step process per functional layer: an un-pattered continuous deposition of the film and subsequent structuring process by for instance laser ablation. Thin film organic devices should be protected using ultra-barrier films. To perform laser ablation of functional layers on top of such barrier films, in particular that of transparent electrodes, an unprecedented selectivity of laser ablation is required to completely remove the layers without damaging the thin-film barrier layers underneath. When performing highly-selective laser ablation of indium tin oxide (ITO) on top of silicon nitride (SiN) barrier layers, we observed the emergence of visible large-scale patterns due to local variations in ablation quality. Our investigations using a very sensitive Raman spectroscopy setup show that the ablation variations originate from minute optical path length differences present in the heat-stabilized plastic substrates and locally consequential destructive or constructive interference occurs between the reflected light from the back of the substrate and incoming laser beam. By performing laser ablation under an angle such that the reflected and incoming laser beam do not spatially overlap, highly-selective uniform laser ablation can be performed, even for two transparent overlapping layers.