Analysis of Plasma/Microflow Induced by Optical Breakdown of a Liquid Droplet and Applications

Abstract

In this work, a novel surface processing technique entitled “Droplet Opto-hydrodynamic Processing” (DOP) is introduced. The process is based on the laser-induced breakdown (LIB) of a micro liquid droplet and subsequent ejection of a high-speed micro liquid jet via an explosive vaporization process. The speed of the microjet (~1600 m/s) is high enough to ablate a variety of materials with substantially large removal rates and remove nanoscale particles from solid surfaces. In particular, the combined effect of the hydrodynamic impact of the jet and the partially transmitted (refocused) laser pulse can form microstructures with negligible thermal effects. Furthermore, the small volume of the liquid droplet (~10 nanoliter per pulse) enables precise and selective processing of surfaces. This dissertation focuses on analyzing hydrodynamics of the laser-generated high-speed liquid jet and developing its applications, i.e., micromachining and surface treatment.First, the hydrodynamics of the micro liquid jet induced by LIB of a free-falling droplet using a Q-switched Nd:YAG laser is analyzed. Time-resolved visualization and beam intensity calculation in a microdroplet are conducted for understanding the dynamics and mechanism of liquid jet formation. Effects of process parameters, including incident laser energy, position of droplet, and droplet size, on the hydrodynamics were examined. Mechanism of various hydrodynamics due to plasma formation variation was explained by the estimation of beam intensity distribution through the droplet. As controlling the process parameters, converged or diverged liquid jet was produced.For a micromachining application, collimated high-speed micro liquid jets with speeds up to 900 m/s, produced by inducing the optical breakdown of a microdroplet and optimized for the process, were employed. The high-pressure plasma generated by a focused laser pulse formed a high-speed microflow from the droplet that, together with the transmitted portion of the laser pulse, could be used to micromachine target surfaces. In the process, materials are removed by the combined effects of a laser pulse (F = ~100 J/cm2) and a high-speed pulsed liquid jet (V = ~900 m/s) ejected from the microdroplet. The opto-hydrodynamic phenomena occurring during this process and the interaction of the hybrid laser/liquid jet with various materials, including copper, aluminum, stainless steel, alumina, and boron nitride, are investigated experimentally. The results show that the laser/liquid jet can remove the materials with substantially increased removal rates and reduced thermal side effects compared with the conventional pulsed laser ablation process. Visualization of the process reveals that the materials are partially ablated and melted by the laser pulse during the early stage of the process and that the molten material is subsequently eliminated by the hydrodynamic impact of the liquid jet.In the surface treatment application, it is demonstrated that the laser-generated liquid jet can remove particles as small as 10 nm from Si wafers, adhesive ablation debris produced by the conventional laser machining, and thin copper oxide layer on the copper substrate. In the process, the process parameters, including the incident laser energy and the position of the droplet relative to the laser focus, were optimized to maximize the cleaning power. As a result, laser-induced breakdown of a microdroplet produces a radially spreading liquid jet with speeds up to 1600 m/s. Impingement of the liquid jet with atomized droplets on the contaminated substrate removes the nanoparticles, Al2O3 particles 10-50 nm in diameter, and piles of ablation debris under the hydrodynamic drag forces. Impingement of the liquid jet, in addition, tears off the peripheral rim structure from ablation site of Si wafer. It is also shown that thin copper oxide layer (Cu2O and CuO) is mechanically removed by the hydrodynamic impact. The proposed cleaning process is expected to be useful for selectively cleaning local areas with minimal exposure to water.