A unified model to determine the energy partitioning between target and plasma in nanosecond laser ablation of silicon

Abstract

In semiconductor industry, pulsed nanosecond lasers are widely applied for the separation of silicon wafers. Here, the high intensities employed activate a cascade of complex multi-physical and multi-phase mechanisms, which finally result in the formation of a laser induced plasma, shielding the target from the incoming laser beam. Such induced plasma plume, by preventing the laser to effectively reach the target, reduces the overall efficiency and controllability of the ablation process. Modelling can be a useful tool in the optimization of industrial laser applications, allowing a deeper understanding of the way the laser energy distributes between target and induced plasma. Nevertheless, the highly multi-physical character of laser ablation poses serious challenges on the implementation of the various mechanisms underlying the process within a common modelling framework. A novel strategy is here proposed in order to simulate in a simplified, yet physically consistent way, a typical industrial application as laser ablation of silicon wafers. Reasonable agreement with experimental findings is obtained. Three fundamental mechanisms have been identified as the main factors influencing the accuracy of the numerical predictions: the transition from evaporative to volumetric mass removal occurring at critical temperature, the collisional and radiative processes underlying the initial plasma formation stage and the increased impact of the liquid ejection mechanism when a sub-millimeter laser footprint is used.