One of the key performance factors of the laser machining of materials is the efficiency. The extension of the depth of field increases the machining rate, especially on non-flat surfaces. Previous work from our group successfully implemented a novel, vari-focal liquid lens (TAG lens) for the ultrafast z-scanning in laser micro-machining. It was showed experimentally and theoretically that the micro-machining efficiency of silicon and polyimide can be improved over a range of defocus distance. In this presentation, we present a numerical simulation of laser thermal ablation with ultrafast z-scanning using COMSOL Multiphysics. The model includes absorption of laser radiation, heat transfer in solid, deformed geometry for materials removal, and random sampling of focal positions. This simulation not only shows the transient response of the laser-material interaction, but accounts for some of the complexities simplified by the theoretical model. Therefore, it exhibits better estimation of the ablation rate in a real system. In addition, to demonstrate the improved machining efficiency on non-flat surfaces, we design an experiment of laser machining on roughened silicon to compare the z-scanning machining system with the conventional machining system. The numerical model is shown to be consistent with the experiment result.
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