Laser-induced Plasma Shock Waves Research Articles

Real time NDE of laser shock processing with time-of-flight of laser induced plasma shock wave in air by acoustic emission sensor

Acoustic emission sensor is used to research the time-of-flight of the shock wave induced by laser-plasma in air for real time nondestructive evaluation (NDE) of laser shock processing. The time-of-flight of the shock wave propagating from the source to the sensor declines nonlinearly and similarly at the different distances for different laser energies. The velocity of the shock wave at the distance of 30 mm increases faster than that of the distance of 35 mm. The relationship between the laser energy and the distance is almost linearly when the signal with distortion is measured by acoustic emission sensor. Finally, Taylor solution is used to analyze the experimental results, and the empirical formula between the energy of the shock wave and the laser energy is established, which will provide a theoretical basis for real time NDE of laser shock processing.

LASER PROPULSION FOR TRANSPORT IN WATER ENVIRONMENT

Problems that cumber the development of the laser propulsion in atmosphere and vacuum are discussed. Based on the theory of interaction between high-intensity laser and materials, such as air and water, it is proved that transport in a water environment can be impulsed by laser. The process of laser propulsion in water is investigated theoretically and numerically. It shows that not only the laser induced plasma shock wave can be used, but also the laser-induced bubble oscillation shock waves and the pressure induced by the collapsing bubble can be used. Many experimental results show that the theory and the numerical results are valid.

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.

Mechanism of laser-induced plasma shock wave evolution in air

A theoretical model is proposed to describe the mechanism of laser-induced plasma shock wave evolution in air. To verify the validity of the theoretical model, an optical beam deflection technique is employed to track the plasma shock wave evolution process. The theoretical model and the experimental signals are found to be in good agreement with each other. It is shown that the laser-induced plasma shock wave undergoes formation, increase and decay processes; the increase and the decay processes of the laser-induced plasma shock wave result from the overlapping of the compression wave and the rarefaction wave, respectively. In addition, the laser-induced plasma shock wave speed and pressure distributions, both a function of distance, are presented.

采用光偏转法研究激光等离子体冲击波

Laser probe beam deflection technique is used for the analysis of laser-induced plasma shock waves in air and distilled water. The temporal and spatial variations of the parameters on shock fronts are studied as functions of focal lens position and laser energy. The influences of the characteristics of media are investigated on the well-designed experimental setup. It is found that the shock wave in distilled water attenuates to an acoustic wave faster than in air under the same laser energy. Good agreement is obtained between our experimental results and those attained with other techniques. This technique is versatile, economic, and simple to implement, being a promising diagnostic tool for pulsed laser processing.

Adhesion force change on multilayer EUVL mask due to laser induced plasma shock wave

Change in adhesion force between a borosilicate glass microsphere and 40 Al 2O 3/TaN/Ru/MoSi pairs on a silicon wafer used as a multilayer extreme ultraviolet lithography mask stack were characterized by force–distance spectroscopy after cleaning Al 2O 3 layers using a laser induced plasma (LIP) shock wave. The adhesion force of the Al 2O 3 surface decreased at a higher laser energy and a lower gap distance above a threshold gap distance without changes in surface roughness. Frictional electrostatic repulsion, triboelectricity, was identified as the cause of lower adhesion forces on Al 2O 3 surface due to the high velocity and pressure of the LIP shock waves. The adhesion force decreased by increasing the number of exposures of LIP shock waves to the substrate.

Numerical Simulation of Laser Shock Cleaning Process for Micro-Scale Particle Removal

Recent studies have demonstrated that micro/nano-scale particles can be removed effectively from solid surfaces by laser-induced plasma shockwaves. This work proposes a two-dimensional theoretical model for numerical simulation of the hydrodynamics and particle behavior in the laser shock cleaning process. The spatial variation of the cleaning efficiency on the surface and the pressure/temperature/velocity fields are analyzed by numerical simulation. Furthermore, the model predicts the trajectory of the detached particles after the shockwave passage. Parametric studies are conducted to examine the effect of such process parameters as the laser pulse energy and the spot size on the cleaning process, with discussion of optimal cleaning conditions.

Underwater pressure amplification of laser-induced plasma shock waves for particle removal applications

Underwater amplification of laser-induced plasma (LIP)-generated transient pressure waves using shock tubes is introduced and demonstrated. Previously, it has been shown that LIP for noncontact particle removal is possible on the sub-100-nm level. This is now enhanced through shock tube utilization in a medium such as water by substantially increasing shock wave pressure for the same pulse energy. A shock tube constrains the volume and changes the propagation direction of the expanding plasma core by focusing a pulsed-laser beam inside a tube with a blind end, thus increasing the wave front pressure generated. Current amplification approach can reduce radiation exposure of the substrate from the shock wave because of the increased distance from the LIP core to the substrate provided by the increased pressure per unit pulse energy. For the same pulsed laser, with the aid of a shock tube, substantial levels of pressure amplitude amplification (8.95) and maximum pressure (6.48MPa) are observed and reported.

Particle saltation removal in laser-induced plasma shockwave cleaning

A new particle saltation removal model in laser-induced plasma shockwave cleaning is presented due to the drawbacks in other particle removal models. Considering the elastic energy stored in the adhered particle during shockwave–particle interaction, the threshold of the particle elastic deformation height, leading to the particle removal, is obtained. With taking the collision between gas molecules in plasma shockwave rear and adhered particle into account, the threshold of the plasma shockwave Mach number is also obtained which is a requirement to the shockwave strength. It is found that the smaller particle needs stronger shockwave than the larger particle. The gas pressure distribution characteristics in the plasma shockwave rear offer a saltation surrounding to the particle, and it is another requirement to the shockwave strength for a successful particle saltation. Finally, particle-cleaning experiment using laser-induced plasma shockwave proves the correctness of the particle saltation model.

Thermal loading of laser induced plasma shockwaves on thin films in nanoparticle removal

Damage concerns, such as substrate/film material alterations, damage, and delamination of thin films, have become a central problem in sub-100 nm particle removal applications. In the laser induced plasma (LIP) removal technique both LIP shockwave and radiation heating are potential sources of thermomechanical damage. The specific objective of current study is to conduct a computational investigation of the LIP shockwave effect on the thermoelastic response of a thin chromium (Cr) film deposited on a quartz substrate and to identify the conditions leading to the onset of plastic film deformations. The experimentally characterized shockwave pressure and temperature (approximated from gas dynamic relations) were prescribed as boundary conditions in the computational analysis. From the shockwave arrival times for different travel distances, the shockwave radius as well as the velocity were obtained as a function of the shockwave propagation time. Radial (and circumferential) stresses, caused by thermal expansion of the Cr film, were most dominant and, hence, of damage concern. It is determined that the resultant temperature rise utilizing a 1064 nm Nd:yttrium-aluminum-garnet (YAG) laser (450 mJ) due to the film-shockwave interactions was not sufficiently high to initiate film and/or substrate damage. No material alteration/damage of the Cr film is predicted due to the temperature and pressure of LIP shockwaves at the firing distance of 2 mm, with a high strain rate gain factor of two (minimum), though damage was observed experimentally for 1064 nm Nd:YAG laser at the pulse energy of 370 mJ. Reported results indicate that the leading cause of observed thin film damage during nanoparticle removal is almost certainly radiation heating from the LIP core.

Nanoparticle Removal Using Laser-Induced Plasma Shock Waves

Removal of sub-100 nm particles from substrates such as wafers and photo masks is an essential requirement in semiconductor, microelectronics, and nanotechnology applications. The proposed laser-induced plasma (LIP) based approach is an effective technique for removal of sub-100 nm particles, as the minimum tolerable particle on the substrates shrinks to sub-100 nm levels with each technological node. In the current study, our progress in sub-100 nm particle removal is reviewed, and the results of the kinetic theory simulations conducted to understand the dynamics of the gas molecule-nanoparticle interactions excited by the shock front are discussed. It is shown from the simulations and experiments that particles as small as sub-100 nm can be successfully detached. To explain possible mechanisms for the nanoparticle detachment in nanoscale, the concepts of rolling resistance moment and rocking motion are utilized as novel detachment mechanisms. The pressure experiments illustrate that the peak pressure levels achieved with the LIP shock wave fields are below damage thresholds of most substrate materials. The potential of the proposed approach as a practical noncontact, dry, fast, and damage-free method for removal of sub-100 nm particles is discussed.

Selective removal of 10–40-nm particles from silicon wafers using laser-induced plasma shockwaves

Current and projected nanoparticle cleaning requirements, especially in semiconductor and nano-manufacturing, necessitate a technique that is not only capable of removing sub-100 nm particles, but also is damage-free and able to perform localized (selective) area cleaning of a relatively small number of particles. A particle cleaning technique based on laser-induced plasma (LIP) shockwaves has been considered and investigated in recent years as a prospective approach. In the current study, the removal of polystyrene latex (PSL) nanoparticles in the diameter range of 10–40 nm from silicon substrates is demonstrated using shockwaves generated by the expansion of LIP for the first time. The effectiveness and practical uses of the LIP technique in the sub-100-nm range are also discussed.

Pressure amplification of laser induced plasma shockwaves with shock tubes for nanoparticle removal

A method using shock tubes for amplifying the dynamic pressure of Laser Induced Plasma (LIP) shockwaves for removing sub-100-nm nanoparticles is introduced and demonstrated. The higher the amplitude of the pressure generated, the smaller the particles that can be removed and, thus the more useful for a variety of applications. Constraining the expansion of the LIP core with a shock tube is a non-contact approach to increase pressure amplitude by an order of magnitude for removal of particles without damaging the substrate through shockwave and LIP radiation heating effects. Heat transfer to the substrate increases as the distance from the LIP core to the substrate (gap distance d) decreases. LIP shockwaves in air, however, demonstrate a pressure decrease on an order of magnitude per every 5 mm in the reported experimental set-up. The shock tube technique allows for higher pressures at distances significantly further from the core of LIP. In the current investigation, the effect of a set of shock tubes to amplify the transient pressure of the LIP-generated shockwave fronts has been studied to evaluate their pressure amplification performances. The effectiveness of a shock tube is quantified in terms of its pressure amplification factor. Through experimental data from several shock tube geometries examined, pressure amplification factors of 11.00 have been experimentally verified which is a ratio of shock-tube-generated transient pressure of 523 kPa to in-air LIP transient pressure of 47.5 kPa at the same gap distance d = 10 mm. The potential advantages of shock tubes as an amplification approach are discussed.

Molecular-level mechanisms of nanoparticle detachment in laser-induced plasma shock waves

Detachment and detachment mechanisms of nanoparticles from flat surfaces subjected to shock waves are investigated by employing molecular gas dynamic simulations using the direct simulation Monte Carlo method and experimental transient pressure data. Two mechanisms for nanoparticle detachment based on rolling moment resistance of the adhesion bond and the elastic restitution effect are introduced. As a result of present simulations, it is computationally demonstrated that the pulsed laser-induced shock waves can generate sufficient rolling moments to detach sub-100-nm particles and initiate removal. The transient moment exerted on a 60nm polystyrene latex particle on a silicon substrate is presented and discussed.

Nanoparticle Detachment Using Shock Waves

The fundamentals of nanoparticle detachment at the sub-100nm level using pulsed laser-induced plasma (LIP) shock waves are investigated in the current study. Two detachment mechanisms based on rolling resistance moment and rolling by resonant frequency excitation are identified as possible detachment mechanisms for nanoparticles. The gas molecule-nanoparticle interactions are studied using the direct simulation Monte Carlo method to gain knowledge about the nature of the detachment forces and moments acting on a nanoparticle in the LIP shock wave field. The discrete nature of the gas molecules colliding with the particle on the sub-100 nm length scale is linked to the stochastic transient moment experienced by the particle. Both experimental and computational findings of the current study indicate that nanoparticle detachment at the sub-100 nm level is possible by LIP shock waves.

Quantitative Analysis of Liquid by Quick Freezing Into Ice Using Nd-YAG Laser-Induced Atmospheric Plasma

A new approach of quantitative analysis of liquid sample using laser ablation technique was developed. The liquid was immediately freezed using the mixture of dry ice and alcohol in weight ratio of 95% : 5%. As a result, an increase of the repulsion force from the sample surface will enable the generation of the laser-induced shock wave plasma which was difficult to carry out on liquid surface. The ice sample was then irradiated using Nd-YAG laser operated in its fundamental wavelength. In order to increase the signal to background ratio and to obtain a sharp atomic line spectra, helium gas was used instead of air. Dynamic characterization of the spatially integrated time profile of the Cu I 521.8 nm, Cu I 510.5 nm and H α lines shows a shock excitation stage and cooling stage which is corresponded to our shock wave model even when the plasma was generated under atmospheric gas pressure. Further study of the time profile averaged temperature of the atmospheric plasma also shows an increase of temperature during the shock excitation stage followed by diminution of temperature during the cooling stage. An application of this technique was then applied to quantitative analysis of several liquid samples. A linear calibration curve which intercept at 0 point was obtained for all of the elements investigated in this study such as sodium, potassium, lithium, copper, silver, lead and aluminum. A detection limit of around 1 ppm was found for the above element. This new technique will contribute to a great extent of laser atomic emission spectrochemical analysis for liquid samples.

Hydrogen emission by Nd-YAG laser-induced shock wave plasma and its application to the quantitative analysis of zircalloy

An experiment was carried out to demonstrate the detection of a hydrogen emission line, HI656.2nm (Hα), in a plasma induced by a Q-switched Nd-YAG (YAG, yttrium aluminium garnet) laser in a low pressure gas on various types of samples, such as zinc, a glass slide, and a zircalloy tube. Contribution by surface water could be suppressed by a laser cleaning treatment and the resulting calibration curve obtained for zircalloy tube samples doped with various concentrations of hydrogen (0, 200, 540, and 960) suggest potential applications to the quantitative analysis of hydrogen. A study of the dynamic process represented by the time profiles of the hydrogen emission, in comparison with those for zinc atomic emission, revealed a specific feature that is related to the small mass of hydrogen. This specific feature can be explained by the shock wave excitation mechanism in terms of new hypothetical process, namely, a mismatch between the movement of ablated hydrogen atoms and the formation of the shock wave.

Shock-wave propagation and cavitation bubble oscillation by Nd:YAG laser ablation of a metal in water.

A highly sensitive fiber-optic sensor based on optical beam deflection is applied for investigating the propagation of a laser-induced plasma shock wave, the oscillation of a cavitation bubble diameter, and the development of a bubble-collapse-induced shock wave when a Nd:YAG laser pulse is focused upon an aluminum surface in water. By the sequence of experimental waveforms detected at different distances, the attenuation properties of the plasma shock wave and of the bubble-collapse-induced shock wave are obtained. Besides, based on characteristic signals, both the maximum and the minimum bubble radii at each oscillation cycle are determined, as are the corresponding oscillating periods.

Water Analysis by Laser-Induced Shock Wave Plasma Spectroscopy Using Re-crystallized KBr Powder Confined in a Cylindrical Tube

It has been demonstrated that shock wave plasma is produced even for a powder sample when a Q-sw Nd:YAG laser beam was focused at a reduced pressure on the surface of powder confined in a small vertical hole. KBr powder was dissolved in the water to be analyzed, and the water was boiled. The re-crystallized KBr, which consisted of trapped impurities present in the water, was contained in the hole to be irradiated by the laser pulse at low pressure.

Experimental Study on the Sensitive Emission Lines Intensities of Metal Samples Using Laser Ablation Technique and Its Comparison to Arc Discharge Technique

An experimental study has been carried out to measure the sensitive emission lines intensities of several metal samples (copper, zinc, silver, gold, gallium, nickel, silicone and iron) using laser ablation technique conducted in low pressure surrounding gas by means of Laser Induced Shock Wave Plasma Spectroscopy (LISPS) and in atmospheric pressure region using Laser Induced Breakdown Spectroscopy (LIBS). In both cases the Nd-YAG laser was operated at its fundamental wavelength of 1.064 nm with pulse duration of 8 ns and its intensity tightly focused on the metal samples in helium or air as an ambient gas. The laser energy was fixed at approximately 100 mJ using a set of neutral density filters placed tilted in front of the laser output window. The result of the intensity measurements showed a good agreement which those obtained using arc discharge technique as shown in Massachusetts Institute of Technology Wavelength Table. Further evaluation of these results on the basis of standard deviation leads to the conclusion that LISPS is more favorable for quantitative analysis compared to LIBS. It was further shown that replacing air by helium gas at low pressure improve to some extent the LISPS reproducibility and sensitivity.