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.

Thermal damage can be a major problem in laser machining thin films. This paper proposes a laser rear-side ablation technique that incorporates a sacrificial layer to absorb excess energy, thereby lessening the level of thermal damage created in the laser processing of metallic films. The sacrificial layer is placed between the quartz substrate and the metallic film. Experiments were performed on the laser machining of holes in a palladium film using phenol formaldehyde resin as the sacrificial layer in which the pulse energy and sacrificial layer thickness were varied. The experimental results validate the proposed approach in that the judicious choice of the values of the pulse energy and the thickness of the sacrificial layer lead to the creation of high-quality holes that do not display the effects of thermal damage. Thus, the proposed technique can be considered to be a promising method for the laser machining of metal films.