Comparison of femtosecond laser-induced front- and rear-side ablation of films

Femtosecond laser rear-side ablation, in which the laser beam first passes through the substrate, then irradiates on the film at the substrate–film interface, can be used to fabricate films on a transparent substrate. In this method, the material removal mechanism is different from that in front-side ablation, and the ablation occurs in an enclosed space. The different material removal process in rear-side laser ablation will bring about some unique ablation results. In this paper, the material removal mechanisms and ablation results of Cr film in rear-side ablation are investigated by comparing with those in front-side ablation. It is found that rear-side ablation can achieve smaller feature size than front-side ablation, especially when the laser fluence is small. However, owing to its different material breaking and ejecting process, the rear-side ablation tends to have worse bottom morphology and irregular edge morphology. In addition, the microstructures in the two ablations have opposite cross-sectional shapes and the ripple period in rear-side ablation is larger than that in front-side ablation.