Cavitation Bubble Dynamics during Laser Wet Etching of Transparent Sapphire Substrates by 1064 nm Laser Irradiation

Laser induced backside wet etching (LIBWE) has shown to be a promising method to process transparent sapphire substrates. However, the etching rates of the conventional LIBWE are not high enough. Our recent experiments indicated that decreasing the gap between the sapphire and the confined plate to a certain value can enhance the etching rates and improve the surface quality. In this paper, we conduct the numerical simulation and the high speed photography imaging to explore the mechanism of the enhanced etching rate processed by 1064 nm pulsed laser in CuSO4 solution. The volume-of-fluid method (VOF) is used to compute the growth and collapse of the laser induced cavitation bubbles being attached to the rear sapphire surface and the flow field distribution in the gap between the sapphire and the confined plate. The cavitation bubble's dynamics at different gap heights are observed by high speed photography technique. Both the numerical and experimental data indicate that with the decreasing gap height, the bubble lifetime increases and the flow field distribution is favorable to enhance the liquid generating photochemical deposition on the rear sapphire surface. Furthermore, the absorption to laser increases, which leads to the increasing of the etching rate. The faster liquid convection at smaller gap height is conducive to heat dissipation around the processed zone, which is good to improve the surface quality.