Laser-induced Gas Plasma Research Articles

Oriented design of wafer-level stacking structures to enhance reliability of laser debonding of thin devices

The laser debonding process in advanced packaging has become a key technology for the debonding of ultra-thin wafers. However, improving the reliability of debonding and efficient removal of adhesive residues from thin and brittle devices remains challenging. Here, we fabricated bonding pairs formed by inorganic responsive layer, thermal buffer layer, and adhesive layer based on a structural design-oriented strategy. Experimental results showed that the thinned device wafers did not show any damage after laser debonding, and the residual adhesive film could be easily torn off. The response layer (a-Si:H) with high temperature resistance and corrosion resistance has strong photosensitivity under the action of ultraviolet laser with a wavelength of 355 nm, and rapidly decomposes to release H2 and polysilicon nanoparticles. The thermal buffer layer is primarily intended to block the heat flow and compensate internal stress. The adhesive layer acts both as an adhesive during temporary bonding and as a barrier to laser-induced gas shock waves and plasma etching of device layers during debonding. This rational structural design contributes to improves the reliability of the laser debonding process and shows great potential for application in the next generation of thin device debonding.

Terahertz Radiation from Two-Color Laser-Induced Gas Plasma Filament Under a Wide Range of Pressure

Terahertz Radiation from Two-Color Laser-Induced Gas Plasma Filament Under a Wide Range of Pressure

Scaling up and parametric characterization of two-color air plasma terahertz source

The plasma produced by frequency mixing of an ultrafast pulsed laser and its second-harmonic field in air is a promising, table-top source of terahertz radiation. To maximize the generation efficiency of such a source, multi-parameter optimization is essential. A parametric study involving various external controls is presented here. Starting from the optimization of phase and polarization of the second-harmonic field, the effect of laser intensity on the terahertz flux is reported. Moreover, the effect of external focusing conditions, such as focal length of the lens and effect of laser beam aperture, is explored in detail. Contrary to the previous reports, we find that with the increase of laser intensity, terahertz (THz) flux increases with more conversion efficiency; this is attributed to the multiple ionization of the air, leading to stacked release of contributory electrons. Exploration of beam convergence reveals that relatively weaker focusing leads to increased terahertz production compared to sharp focusing conditions. Moreover, temporal characterization of the emitted terahertz pulse conducted using electro-optic sampling provides pulse duration of 368 fs with corresponding frequency spectra up to 3.6 THz. The maximum electric field of the terahertz pulse is estimated to be 1.12 MV cm−1 in our experimental conditions. Results of 2D particle-in-cell simulations are also presented with the objective of validating the role of transient photocurrent mechanism in THz generation from two-color laser-induced gas plasma. We believe the broadband, bright THz source presented here will be useful in the imaging of bigger samples as well as nonlinear spectroscopy.

Control of the terahertz spectra generated from laser induced plasma

Broadband terahertz (THz) emission generated from laser induced gas plasma provides an effective tool for studying nonlinear spectrum, imaging and remote sensing. Recently, the contribution of plasma oscillation to the THz emission was revealed from the nitrogen molecules pumped by intense two-color laser pulses. Plasma oscillation contributes only to the THz emission at relatively low plasma density due to negligible plasma absorption. More generally, with the THz emission generated from the ionizing gaseous medium, the surrounding plasma is expected to play an important role in the generation process. For the THz radiation from laser filament, the plasma region is extended in the laser propagation direction, and the effect of surrounding plasma on the emitted THz spectrum needs studying. In this work, we investigate the relation between pump power and filament length from THz spectrum emitted by air filament driven by two-color laser pulse. The time domain spectrum of THz field is recorded by an electro-optic (EO) sampling technique. In our experiments, significant frequency shifts are observed as the pump power and the filament length increase, and we find that the center frequency of the THz radiation is shifted towards longer wavelength, which is the so called red-shift of the THz spectrum. This red-shift is independent of THz radiation angle. The observations are explained by the plasma absorption inside the air filament. Our theoretical model is based on three mechanisms: the ionization-induced photocurrent, the plasma current oscillation and the plasma absorption. We coherently add up all the local THz fields inside the air filament, and simultaneously consider the plasma absorption induced correction of the THz spectrum. The simulation well reproduces the experimental observation. The skin depth decreases as the plasma density increases, thus the plasma absorption dominates the red-shift process. If the skin depth is larger than the filament length, the plasma oscillation contributes to the THz spectrum dominantly, and thus leading to the blue-shift of THz spectrum. Our results indicate that for the extended filament length or higher plasma density, the combining effect of photocurrent, plasma oscillation and absorption, results in the observed low-frequency broadband THz spectrum. Our study offers a method of coherently controlling the broadband THz spectrum.

Terahertz emission from laser-driven gas plasmas: a plasmonic point of view

We disclose an unanticipated link between plasmonics and nonlinear frequency down-conversion in laser-induced gas-plasmas. For two-color femtosecond pump pulses, a plasmonic resonance is shown to broaden the terahertz emission spectra significantly. We identify the resonance as a leaky mode, which contributes to the emission spectra whenever electrons are excited along a direction where the plasma size is smaller than the plasma wavelength. As a direct consequence, such resonances can be controlled by changing the polarization properties of elliptically-shaped driving laser pulses. Both, experimental results and 3D Maxwell consistent simulations confirm that a significant terahertz pulse shortening and spectral broadening can be achieved by exploiting the transverse driving laser beam shape as an additional degree of freedom.

Pulsed CO2 laser-induced gas plasma spectroscopy based on single beam splitting for trace metal analysis on a material surface

ABSTRACTLaser-induced gas plasma spectroscopy based on pulsed CO2 laser beam splitting has been applied to the problem of trace film analysis on the silicon surface. In this study, 2.1 J of laser energy (70% of the laser beam) was focused at a 10-degree incidence on a metal mesh attached to a sample surface containing trace metal elements in order to produce a gas plasma. The remaining part of the laser beam (approximately 30% or 0.9 J) was employed to vaporize a film which had been deposited on the material by focusing the laser beam 3 cm under the surface. In this scheme, the vaporized metal film moves into the gas plasma region, in which the dissociation and excitation takes place. Our measurements show that the detection of Cr on the silicon surface can be made with high sensitivity. The limit of detection of Cr in the silicon material was approximately 7.5 × 1012 atom/cm2.

Improved Ignition Efficiency in Method for Laser-induced Noble Gas Plasma Light Source

Improved Ignition Efficiency in Method for Laser-induced Noble Gas Plasma Light Source

Probing electron-atom collision dynamics in gas plasma by high-order harmonic spectroscopy.

Plasma is a complex system involving diverse collisional processes and interactions, such as electron-impact excitation, ionization, recombination, etc. One of the most important methods for studying the properties and dynamics of plasma is to analyze the radiations from plasma. Here, we demonstrate the high-order harmonic (HH) spectroscopy for probing the complex electron-atom collision (EAC) dynamics in a laser-induced gas plasma. These measurements were carried out by using an elliptically polarized pump and a time-delayed linearly polarized probe. The HH spectra from argon and krypton plasmas were recorded by scanning the time delay up to hundreds of picoseconds. We found that the delay-dependent HH yield contains three distinct regions, i.e., the first enhancement, the subsequent suppression, and the final restoration regions. A qualitative analysis shows that these features are clear signatures of the EAC processes and interactions involved in the delay-dependent HH spectroscopy.

Contribution of photocurrent mechanism and influence of plasma length in THz generation by two-color laser induced plasma

The contribution of the photocurrent mechanism has been investigated in terahertz (THz) generation in laser-induced gas plasma. This study was done through comparing the THz amplitude for two phase differences φ=0, π/2 between the fundamental pulse and its second harmonic. Our particle-in-cell simulation result showed that this mechanism played the most important role in the two-color regime with a low-frequency range. We also investigated the effect of the plasma channel length on generated THz amplitude, and it was found that THz amplitude would depend significantly on the plasma channel length.

Resonant Effects in Terahertz Generation with Laser-Induced Gas Plasmas

Resonant Effects in Terahertz Generation with Laser-Induced Gas Plasmas

Metal-Assisted Laser-Induced Gas Plasma for the Direct Analysis of Powder Using Pulse CO2 Laser

Analysis of powder samples available in small quantities has been carried out using metal-assisted gas plasma by utilizing a transversely excited atmospheric (TEA) CO2 laser. The powder was homogeneously mixed with Si grease, and the mixed powder was painted on a metal subtarget. When a TEA CO2 laser was directly focused on the metal subtarget at atmospheric pressure of He gas, a high-temperature He gas plasma was induced. It is assumed that the powder particles were vaporized to be effectively atomized and excited in the gas plasma region. This method has been employed in the rapid analyses of elements in organic and inorganic powder samples present in small quantities. Detection of trace elements of Cr and Pb has been successfully made by using the supplement powder and loam soil, respectively. The detection limits of Pb in loam soil were approximately 20 mg/kg.

Theory of terahertz emission from femtosecond-laser-induced microplasmas.

We present a theoretical investigation of terahertz (THz) generation in laser-induced gas plasmas. The work is strongly motivated by recent experimental results on microplasmas, but our general findings are not limited to such a configuration. The electrons and ions are created by tunnel ionization of neutral atoms, and the resulting plasma is heated by collisions. Electrons are driven by electromagnetic, convective, and diffusive sources and produce a macroscopic current which is responsible for THz emission. The model naturally includes both ionization current and transition-Cherenkov mechanisms for THz emission, which are usually investigated separately in the literature. The latter mechanism is shown to dominate for single-color multicycle laser pulses, where the observed THz radiation originates from longitudinal electron currents. However, we find that the often discussed oscillations at the plasma frequency do not contribute to the THz emission spectrum. In order to predict the scaling of the conversion efficiency with pulse energy and focusing conditions, we propose a simplified description that is in excellent agreement with rigorous particle-in-cell simulations.

Generation of gas-plasma flows by laser ablation of photopolymerizable compositions

The results of studying the effectiveness of using photopolymerizable compositions (PPC) in various-purpose laser-plasma facilities with regard to the transportation and fine dosing of the working medium, as well as providing a long service life and reliability, are presented. The characteristics of the pulsed and continuous laser radiation curing of PPC and mixtures thereof are discussed in terms of the spectral-energetic characteristics of the incident radiation. The thresholds of nanosecond laser ablation in the UV and near-IR regions, specific ejected mass, momentum coupling coefficient, and efficiency of conversion of laser radiation energy into the kinetic energy of the laser-induced gas-plasma ablation jet for PPC in the liquid and solid states. The highest efficiency of generation of the gas-plasma ablation jet was achieved for the laser irradiation of initially liquid PPC at a wavelength of 355 nm, a wavelength optimal for the photopolymerization of PPC.

Thermophysical and gas-dynamic characteristics of laser-induced gas-plasma flows under femtosecond laser ablation of titanium in vacuum

We report the results of experimental investigation of thermophysical and gas-dynamic characteristics of the gas-plasma flows induced by ultrashort laser pulse irradiation (the radiation wavelength of a titanium target in vacuum . The use of combined interferometric technique and complex experimental data processing allowed us to estimate the momentum coupling coefficient , the efficiency of laser energy conversion to the kinetic energy of the gas-plasma flow (65 % – 85 %), the spatiotemporal distributions of the particle density and velocity , the static and total pressure and temperature in the flow. Our data are compared with published data obtained by other methods.

Analysis of sodium aerosol using transversely excited atmospheric CO2 laser-induced gas plasma spectroscopy

Taking advantages of the special characteristics of a transversely excited atmospheric (TEA) CO2 laser, the analysis of sodium aerosol has been successfully conducted by using laser-induced gas plasma spectroscopy (LIGPS) method. In this study, the sodium aerosol was deposited on a nickel metal plate; the metal plate functions as a subtarget to initiate a gas plasma. When a pulsed TEA CO2 laser was focused on the metal surface, a large-volume and high-temperature gas plasma was induced. The fine particles of sodium then entered into the gas plasma region to be dissociated and excited. By using this technique, a semi quantitative analysis of sodium aerosol was made. The detection limit of sodium was approximately 200 ppb.

Experimental investigation of the dynamics of laser-induced gas-plasma flows under femtosecond laser ablation of copper in vacuum

Thermophysical and gasdynamic characteristics of gas-plasma flows induced by ultrashort laser pulses interacting with a thinfilm copper target in vacuum were studied experimentally. Using combined laser interferometry and complex processing of experimental data, we estimated the momentum coupling coefficient and the efficiency of laserenergy conversion to kinetic energy, spatiotemporal distributions of the number density and velocities of particles, pressure, and temperature in the gas-plasma flow. We provide comparative analysis of presented data with those found in the literature, which were obtained by other meth� ods.

Direct evidence of mismatching effect on H emission in laser-induced atmospheric helium gas plasma

A time-resolved orthogonal double pulse laser-induced breakdown spectroscopy (LIBS) with helium surrounding gas is developed for the explicit demonstration of time mismatch between the passage of fast moving impurity hydrogen atoms and the formation of thermal shock wave plasma generated by the relatively slow moving major host atoms of much greater masses ablated from the same sample. Although this so-called “mismatching effect” has been consistently shown to be responsible for the gas pressure induced intensity diminution of hydrogen emission in a number of LIBS measurements using different ambient gases, its explicit demonstration has yet to be reported. The previously reported helium assisted excitation process has made possible the use of surrounding helium gas in our experimental set-up for showing that the ablated hydrogen atoms indeed move faster than the simultaneously ablated much heavier major host atoms as signaled by the earlier H emission in the helium plasma generated by a separate laser prior to the laser ablation. This conclusion is further substantiated by the observed dominant distribution of H atoms in the forward cone-shaped target plasma.

Polarization property of the THz wave generated from a two-color laser-induced gas plasma

The recent experiments show that the polarization of the terahertz (THz) wave generated from two-color laser-induced gas plasma is a linear polarization, and the polarization angle of the THz wave can be controlled by the optical delay of the two laser beams. In this paper, the polarization property of the THz wave is studied theoretically via a classical and a quantum model, respectively. An analytical expression is obtained to reflect the relationship between the polarization angle of the THz wave and the relative phase of the two-color lasers.

Theoretical research on terahertz air-breakdown coherent detection with the transient photocurrent model

The physical mechanism for sensing broadband terahertz (THz) wave via using femtosecond (fs) laser induced gas plasma without any local accessory near the plasma, i.e. THz air breakdown coherent detection, is systemically investigated by utilizing the transient photocurrent model. Previous observed results, such as conversion from incoherent to coherent detection, can be numerically obtained. Further calculations and analysis show that it is through modification of the gas ionization process, and not acceleration of freed electrons or through a four-wave-mixing (FWM) process, that the THz waveforms can be encoded into the detected second harmonic (SH) signals.

A comprehensive study of H emission in a TEA CO2 laser-induced helium gas plasma for highly sensitive analysis of hydrogen in metal samples

Our previous work on an innovative method of hydrogen (H) analysis using the specific characteristics of a TEA CO2 laser, “selective detection method of H”, has been improved to realize a high H sensitive analysis with a detection limit of several µg/g. For this purpose, first, we clarified the origin of the H emission disturbance coming from H2O molecules; namely, we showed that most of the H emission came from H2O on the metal surface and not from H2O existing in the surrounding gas when we formed a laser-induced gas plasma. Second, the difference in the emission characteristics between the H emission from H2O on the metal surface and H emission from inside in sample was studied to determine the optimum gating time of the optical multi-channel analyzer (OMA). Third, the gas plasma was totally covered by fresh helium gas using a big pipe (5 mm in diameter) and by flowing a high amount of He (10 l/min). Also, we demonstrated that our methods could potentially be applied to H analysis in steel samples, where an H analysis with a sensitivity of less than 1 µg/g is required without employing a heating process, by removing H2O on the sample surface with the aid of defocused TEA CO2 laser irradiation. Thus, we stress that our method can be used for a highly sensitive, in-situ analysis of H for metal samples.