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.

In femtosecond laser multi-pulse ablation, the ablated surface and the fluence irradiated on this surface are ever changing. Thus, the simple summarization of single pulse ablation is not an accurate shape forecast method and the influence of beam spot change must be considered. In this paper, a new simulation method is proposed to forecast the microstructure shape, in which fluence distribution, ablation directions, and ablation rates of each pulse on the ablated surface are determined according to the spatial propagation characteristics of laser and two logarithmic ablation models. Further, the microstructure shape is obtained by accumulating the effects of all pulses. Three kinds of ablation -on-focus ablation, off-focus convergent beam ablation, and divergent beam ablation -are investigated using the proposed method. Simulation results show that the influence of beam spot change on microstructure shape is obvious. In convergent beam ablation, the structure has a sharp bottom, and the sidewall changes from concave to convex with the increase of pulses. In contrast, the structure obtained by divergent beam ablation has a much higher draft-angle and flatter bottom. Onfocus ablation can be regarded as parallel beam ablation when ablations proceed within the Rayleigh range and the profile keeps to the parabola function. In experiments, microstructures are machined with three kinds of ablation in typical metals, and it is proved that the proposed structure-developing model is a valid shape forecast method when vaporization is the main ablation mechanism.

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.