Deoksuk Jang

Correlation between particle removal and shock-wave dynamics in the laser shock cleaning process

It has been shown that the laser shock cleaning (LSC) method is effective for eliminating micron- and submicron-scale particulates from solid surfaces. In the LSC process, a high-power laser pulse induces optical breakdown of the ambient gas close to the solid surface to be cleaned and the subsequently-created shock wave followed by a high-speed flow stream detaches the particles. Therefore, there should be a strong correlation between the dynamics of the shock wave and the cleaning performance. In this work, experimental analyses are conducted to measure the cleaning performance using micron-sized alumina particles attached to a silicon surface. The experimental data showing the particle-removal performance are compared with the results of the dynamics of the laser-induced shock waves, leading to a simple model for particle removal by the LSC scheme in the continuum-flow regime.

Enhanced efficiency of laser shock cleaning process by geometrical confinement of laser-induced plasma

Surface cleaning based on the laser-induced breakdown of gas and subsequent plasma and shock wave generation can remove small particles from solid surfaces. Accordingly, several studies were performed to expand the cleaning capability of the process. In this work, the cleaning process using laser-induced plasma (LIP) under geometrical confinement is analyzed both theoretically and experimentally. Two-dimensional numerical analysis is conducted to examine the behavior of the LIP shock wave under geometrical confinement for several geometries. As a result of the analysis, we propose a simple and practical method to amplify the intensity of laser-induced shock. In the proposed method, a flat quartz plate placed close to the focal point of the laser pulse confines the expansion of the LIP, allowing the plasma to expand only in one direction. As a consequence of the plasma confinement, the intensity of the shock wave produced is increased significantly. Experiments demonstrate that the enhanced shock wave can remove smaller particles from the surface better than the existing process.

Enhancement of airborne shock wave by laser-induced breakdown of liquid column in laser shock cleaning

In laser shock cleaning (LSC), the shock wave is generated by laser-induced breakdown of the ambient gas. The shock wave intensity has thus been a factor limiting the performance of the LSC process. In this work, a novel method of amplifying a laser-induced plasma–generated shock wave by the breakdown of a liquid column is proposed and analyzed. When the laser beam is focused on a microscale liquid column, a shock wave having a significantly amplified intensity compared to that generated by air breakdown alone can be generated in air. Therefore, substantially amplified cleaning force can be obtained. The dynamics of a shock wave induced by a Q-switched Nd:YAG laser was analyzed by laser flash shadowgraphy. The peak pressure of the laser-induced shock wave was approximately two times greater than that of air breakdown at the same laser fluence. The proposed method of shock wave generation is expected to be useful in various applications of laser shock processing, including surface cleaning.

Surface processing technique based on opto-hydrodynamic phenomena occurring in laser-induced breakdown of a microdroplet

We report the development of a surface processing technique based on the optical breakdown of a microdroplet and subsequent ejection of a pulsed microjet. The microjet was sufficiently fast to remove nanoparticles from surfaces and erode most materials. The small volume of the droplet enabled precise and selective treatment of surfaces. When the jet was impinged onto a laser spot focused by the droplet, ablation rates substantially larger than those in conventional pulsed laser ablation were obtained with significantly reduced thermal effects. The jet could remove 20 nm particles and an oxide layer from solid surfaces by hydrodynamic impact only.

Effect of liquid environment on laser-induced backside wet etching of fused silica

In laser-induced backside wet etching (LIBWE), the liquid absorbent indirectly heats the transparent material, causing explosive phase change and cavitation. Accordingly, the hydrodynamics of the absorbing liquid, including the size of the liquid chamber, is strongly related to the ablation process. Because the hydrodynamics is dependent on the elastic deformation of the sample, the sample thickness also affects the performance of LIBWE. In this work, experimental analyses were performed to elucidate the hydrodynamics in LIBWE and the effect on the etch rate by varying the liquid chamber size and sample thickness. A KrF excimer laser was used to ablate fused silica samples in toluene and the etch rate was quantified using a scanning profilometer. Laser flash shadowgraphy and photodeflection probing techniques were employed for in situ measurement of the laser-induced hydrodynamics and displacement of the sample, respectively, with a time resolution of approximately nanoseconds. To directly observe the eff...

Generation of metal nanoparticles by laser ablation of metal microparticles and plume dynamics

This paper describes the process of nanoparticle synthesis by laser ablation of consolidated microparticles, focusing on the dynamics of ablation plume. We have generated nanoparticles by high-power pulsed laser ablation of Al and Cu microparticles using a Q-switched Nd:YAG laser (wavelength 355 nm, FWHM 10 ns, fluence 0.8~2.0 J/cm2). Microparticles of mean diameter 18 ~ 80 micrometers are ablated in the ambient air. The generated nanoparticles are collected on a glass substrate and the scanning electron micrographs of the samples are examined for characterizing the particles. The effect of laser fluence and collector position on the distribution of particle size is investigated. Optical diagnostics and numerical simulations are conducted to study the flow field and plume dynamics. The dynamics of ablation plume and shock wave is analyzed by monitoring the photoacoustic probe-beam deflection signal. Nanosecond time-resolved images of the ablation process are also obtained by laser flash shadowgraphy. Based on the results of experiment and numerical simulation, discussions are made on the dynamics of ablation plume.

Enhancement of cleaning efficiency by geometrical confinement of plasma expansion in the laser shock cleaning process for nanoscale contaminant removal

It has been shown that the laser shock cleaning (LSC) process is effective for removing nanoscale particles from solid surfaces and thus has various potential applications in microelectronic manufacturing. In this work, we propose a simple method to amplify the shock wave intensity generated by laser-induced breakdown (LIB) of air. The suggested scheme employs a plane shock wave reflector which confines the plasma expansion in one direction. As the half of the LIB-induced shock wave is reflected by the reflector, the intensity of the shock wave propagating in the opposite direction is increased significantly. Accordingly, the enhanced shock wave can remove smaller particles from the surface than the existing LSC process. The LSC process under geometrical confinement is analyzed both theoretically and experimentally. Numerical computation of the plasma/shock behavior shows about two times pressure amplification for the plane geometry. Experiments confirm that the shock wave intensity is enlarged by the effect of geometrical confinement of the plasma and shock wave. The result of cleaning tests using polystyrene particles demonstrates that the particle removal efficiency increases by the effect of geometrical confinement.

Experimental Analysis of Bubble Dynamics Induced by Pulsed-Laser Heating of Absorbing Liquid

The bubble dynamics induced by direct laser heating is experimentally analyzed as a first step to assess the technical feasibility of laser-based ink-jet technology. To understand the interaction between laser light and ink, the absorption spectrum is measured for various ink colors and concentrations. The hydrodynamics of laser-generated bubbles is examined by the laser-flash photography. When an Ar ion laser pulse (wavelength 488 nm) with an output power up to 600 mW is incident on the ink solution through a transparent window, a hemispherical bubble with a diameter up to can be formed with a lifetime in a few tens of microsecond depending on the laser power and the focal-spot size. Parametric study has been performed to reveal the effect of laser pulse width, output power, ink concentration, and color on the bubble dynamics. The results show that the bubble generated by a laser pulse is largely similar to that produced by a thin-film heater. Consequently, the present work demonstrates the feasibility of developing a laser-actuated droplet generation mechanism for applications in ink-jet print heads. Furthermore, the results of this work indicate that the droplet generation frequency is likely to be further increased by optimizing the process parameters.

Role of Liquid Vaporization in Liquid-Assisted Laser Cleaning

Liquid-assisted cleaning technology utilizing a nanosecond laser pulse is effective for removing submicron particulates from a variety of solid substrates. In the technique, saturated vapor is condensed on a solid surface to form a thin liquid film and the film is evaporated explosively by laser heating. The present work studies the role of liquid-film evaporation in the cleaning process. First, optical interferometry is employed for in-situ monitoring the displacement of the laser-irradiated sample in the cleaning process. The experiments are performed for estimating the recoil force exerted on the target with and without liquid deposition. Secondly, time-resolved visualization and optical reflectance probing are also conducted for monitoring the phase-change kinetics and plume dynamics in vaporization of thin liquid layers. Discussions are made on the effect of liquid-film thickness and dynamics of plume and acoustic wave. The results confirm that cleaning force is generated when the bubble nuclei initially grow in the strongly superheated liquid.

Physical mechanisms of liquid-assisted laser cleaning

Liquid-assisted cleaning technology utilizing a nanosecond laser pulse is effective for removing submicron particulates from a variety of solid substrates. In the technique, saturated vapor is condensed on the solid surface to form a thin liquid film and the film is evaporated explosively by laser heating. The present work studies the role of liquid-film evaporation in the cleaning process. First, optical interferometry is employed for in-situ monitoring the displacement of the laser-irradiated sample in the cleaning process. The experiments are performed for estimating the recoil force exerted on the target with and without liquid deposition. Secondly, time-resolved visualization and optical reflectance probing are also conducted for monitoring the phase-change kinetics and plume dynamics in vaporization of thin liquid layers. Discussions are made on the effect of liquid-film thickness and dynamics of plume and acoustic wave. The results confirm that cleaning force is generated when the bubble nuclei initially grow in the superheated liquid.