A Study on Multiple-Reflection and Rayleigh Absorption-Emission-Scattering Effects in Laser Micromachining

Abstract

Laser micromachining involves material removal induced by vaporization and hydrodynamic melt ejection. In particular, vaporization is directly related to micromachining efficiency because the laser radiation can be attenuated by the ablation plume. Various mathematical models for laser micromachining have been developed. Especially, multiple reflections models have been addressed by a number of researchers because the models are valid for laser micromachining simulations. In those multiple reflections models, however, there is no consideration for the plume phenomena. This research offers a novel multiple reflections model that accounts for the absorption and distribution of the laser radiation by the ablation plume during laser micromachining. Laser micromachining may be achieved through a photothermal process, a photochemical process, and a combined photothermal and photochemical process. It was reported that the laser ablation process for silicon might be dominated by the photothermal process when using a near ultraviolet frequency-tripled Nd:YAG laser. In this study, the laser micromachining of silicon was conducted using a near UV nanosecond pulsed Nd:YAG laser, where only the photothermal process was taken into consideration. In this research, a fundamental study of laser micromachining was performed with computational analysis and compared with experimental results. High-power Q-switched diode-pumped laser micromachining simulations were conducted on N-type silicon wafers. In the numerical model, the volume of fluid method was adopted to trace the free surface with the governing equations including continuity, momentum, and energy equation. The laser beam was considered as a surface heat flux with near-Gaussian distribution. The simulation model took into account the physical phenomena, taking place during micromachining of the silicon wafer, such as the effects of recoil pressure producing a narrow and deep profile, Fresnel absorption transferring the e...