We develop a new simulation code for the optimization of efficiency of the laser pumped plasma (LPP) extreme ultra violet (EUV) light source, which is applicable to the pre-plasma formation from a tin droplet target irradiated by a pre-pulse laser. We investigate algorithms of reorganization of the mesh for the Lagrangian hydrodynamics simulation. We also investigate the model of the liquid to gas phase transition to calculate the dynamics of particle formation through the laser ablation.
Double-clade crystalline fiber waveguide (CFW) has been produced by using adhesive-free bond (AFB®) technology. The waveguide consists of a 1 at.% Yb:YAG core, un-doped YAG inner cladding and ceramic spinel outer cladding. It is a direct analog of the conventional double-clad glass fiber laser in the crystal domain. Signal gain of 45 or 16.5 dB has been measured in a preliminary master oscillator power amplifier (MOPA) experiment. Due to the high laser gain and the weak Fresnel reflection at the uncoated waveguide ends, the CFW even starts self-lasing above a certain pump power. Laser output power of 4 W in the backward propagation direction has been measured for input pump power of 44 W. After considering the same amount of forward propagated laser power, the laser efficiency to the absorbed pump power is estimated to be about 44%. In principle, CFW can have extremely large single mode area for high efficiency and high power laser applications. So far, Single mode area < 6700 μm2 has been demonstrated in Er:YAG CFWs.
We are developing a high-repetition and high-average-power Nd:YAG laser amplifier pumped by fiber coupled LDs in order to apply to laser machining of carbon composite materials such as CFRP (Carbon Fiber Reinforced Plastic). Final target is 1.5 kW output at wavelength of 1064 nm by 8 kW LD pumping. The amplifier consists of several Nd:YAG ceramic thin discs on a non-doped ceramic YAG block. At first, we measured wave-front distortion and small-signal gain of a prototype amplifier whose target is more than 500 W output.
Koichi Kasuya, S. Ozawa, T. Norimatsu, H. Azechi, K. Mima, S. Nakai, S. Suzuki, B. Budner, W. Mroz, N. Kasuya, W. Kasuya, Kei. Kasuya, Y. Izawa, H. Furukawa, Y. Shimada, T. Yamanaka, M. Nakai, K. Nagai, K. Yokoyama, K. Ezato, M. Enoeda, M. Akiba, A. Prokopiuk
The most recent fundamental research results to investigate surface erosions of nuclear fusion candidate
chamber materials are described in short. We used a commercial surface profiler with a red semiconductor
laser. Various material surfaces ablated and eroded by a rather short pulse electron beam and a short pulse
ArF laser light were measured with this surface profiler and the associated three-dimensional analysis
software. Threshold input levels for various sample surface erosions with electron and laser beams were
clearly decided for the first time with our new method in this article. After the above fundamental results were
gathered, the methods to inspect inner surface conditions of nuclear fusion reactor chambers were newly
proposed with various kinds of laser displacement sensors. The first one is the erosion monitor with the above
profiler, and the second one is the laser induced ultrasonic wave detection method to inspect deeper surface
layers than the first one.
We show an atomic model of Sn for the EUV sources. We show an improvement of the model in terms of the
selection of energy levels and correction of the wavelength of the emission lines including resonance and satellite
lines of combined 4d-4f and 4p-4d transition arrays. Calculated spectrum agrees well with experiments, showing
that the present model is useful both for theoretical investigation of the optimum conditions the EUV sources,
and for the analysis of experimental spectrum. A modeling method to estimate the initial spatial profile of the
discharge path for the analysis of laser-assisted discharge pumped plasma sources is also proposed.
We investigate characteristic feature of the atomic radiation from tin plasmas, which allow one to obtain high power EUV emission at λ=13.5nm efficiently. We develop a collisional radiative model of tin ions to calculate steady-state and time dependent ion abundance, level population, and coefficients of radiative transfer of the plasma. The model, which is based atomic data calculated using the Hullac code is refined both theoretically and experimentally. Calculation of the spectral emissivity and opacity are carried out over a wide range of plasma density and temperature, and pumping conditions to obtain high conversion efficiency are discussed.
We propose a new scheme for high conversion efficiency from laser energy to 13.5 nm extreme ultra violet emission
within 2 % band width, a double pulse laser irradiation scheme with a tin droplet target. We consider two-color lasers, a
Nd:YAG laser with 1.06 µm in wavelength as a prepulse and a carbon dioxide laser with 10.6 µm in wavelength for a
main pulse. We show the possibility of obtaining a CE of 5 - 7 % using a benchmarked radiation hydro code. We have
experimentally tested the new scheme and observed increase of CE greater than 4 %. We show many additional
advantages of the new scheme, such as reduction of neutral debris, energy reduction of debris ions, and decrease of out
of band emission. We also discuss debris problems, such as ion sputtering using newly developed MD simulations, ion
mitigation by a newly designed magnetic coil using 3-PIC simulations and tin cleaning experiments.
Laser-produced Sn plasma is an efficient extreme ultraviolet (EUV) light source, however the highest risk in the Sn-based EUV light source is contamination of the first EUV collection mirror caused by debris emitted from the Sn plasma. Minimum mass target is a key term associated with relaxation of the mirror contamination problem. For design of the optimum minimum mass Sn target, opacity effects on the EUV emission from the laser-produced Sn plasma should be considered. Optically thinner plasma produced by shorter laser pulse emits 13.5 nm light more efficiently; 2.0% of conversion efficiency was experimentally attained with drive laser of 2.2 ns in pulse duration, 1.0 × 1011 W/cm2 in intensity, and 1.064 μm in wavelength. Under the optimum laser conditions, the minimum mass required for sufficient EUV emission, which is also affected by the opacity, is equal to the product of the ablation thickness and the required laser spot size. Emission properties of ionized and neutral debris from laser-produced minimum mass Sn plasmas have been measured with particle diagnostics and spectroscopic method. The higher energy ions have higher charge states, and those are emitted from outer region of expanding plasmas. Feasibility of the minimum mass target has been demonstrated to reduce neutral particle generation for the first time. In the proof-of-principle experiments, EUV emission from a punch-out target is found to be comparable to that from a static target, and expansion energy of ion debris was drastically reduced with the use of the punch-out target.
For EUV lithography the generation of clean and efficient light source and the high-power laser technology are key issues. Theoretical understanding with modeling and simulation of laser-produced EUV source based on detailed experimental database gives us the prediction of optimal plasma conditions and their suitable laser conditions for different target materials (tin, xenon and lithium). With keeping etendue limit the optimal plasma size is determined by an appropriate optical depth which can be controlled by the combination of laser wavelength and pulse width. The most promising candidate is tin (Sn) plasma heated by Nd:YAG laser with a pulse width of a few ns. Therefore the generation technology of clean Sn plasma is a current important subject to be resolved for practical use. For this purpose we have examined the feasibility of laser-driven rocket-like injection of extremely mass-limited Sn or SnO2 (punched-out target) with a speed exceeding 100m/s. Such a mass-limited low-density target is most preferable for substantial reduction of ion energy compared with usual bulk target. For high average power EUV generation we are developing a laser system which is CW laser diode pumped Nd:YAG ceramic laser (master oscillator and power amplifier system) operating at 5-10 kHz repetition rate. The design of practical laser for EUV source is being carried out based on the recent performance of >1 kW output power.
We have been developing a high average-power laser system for science and industry applications that can generate an output of 20 J per pulse at 10-Hz operation. Water-cooled Nd:glass zig-zag slab is pumped with 803-nm AlGaAs laser-diode modules. To efficiently extract energy from the laser medium, the laser beam alternately passes through dual zig-zag slab amplifier modules. Twin LD modules equipped on each slab amplifier module pump the laser medium with a peak power density of 2.5 kW/cm2. In high power laser system, thermal load in the laser medium causes serious thermal effects. We arranged cladding glasses on the top and bottom of the laser slab to reduce thermal effects.
We have developed an integrated Laser Ablation Fluid Radiation simulation Code (LAFRAC) to estimate the behaviors of highly energetic ions and neutral particles from LPP EUV light sources, and estimated recombination and charge transfer processes between the particles from laser-produced Xe EUV light sources. We clarify that charge transfer effects greatly affect on the number density of neutral particles, especially high energy (more than roughly 500 eV) neutral particles.
Properties of laser-produced tin (Sn) plasmas were experimentally investigated for application to the Extreme Ultra-Violet (EUV) lithography. Optical thickness of the Sn plasmas affects strongly to EUV energy, efficiency, and spectrum. Opacity structure of uniform Sn plasma was measured with a temporally resolved EUV spectrograph coupled with EUV backlighting technique. Dependence of the EUV conversion efficiency and spectra on Sn target thickness were studied, and the experimental results indicate that control of optical thickness of the Sn plasma is essential to obtain high EUV conversion efficiency and narrow spectrum. The optical thickness is able to be controlled by changing initial density of targets: EUV emission from low-density targets has narrow spectrum peaked at 13.5 nm. The narrowing is attributed to reduction of satellite emission and opacity broadening in the plasma. Furthermore, ion debris emitted from the Sn plasma were measured using a charge collector and a Thomson parabola ion analyzer. Measured ablation thickness of the Sn target is between 30 and 50 nm for the laser intensity of 1.0 x 1011 W/cm2 (1.064 μm of wavelength and 10 ns of pulse duration), and the required minimum thickness for sufficient EUV emission is found to be about 30 nm under the same condition. Thus almost all debris emitted from the 30 nm-thick mass-limited Sn targets are ions, which can be screened out by an electro-magnetic shield. It is found that not only the EUV generation but also ion debris are affected by the Sn target thickness.
Integrated laser ablation simulation code includes phase transition from liquid to neutral gas to partially ionized plasma, detail laser absorption processes, equation of state, hydrodynamics, and radiation transport, is developed to describe ablation phenomena with phase transition and properties of emission plasmas. For an application of this simulation code, we perform simulations on optimization of laser produced plasmas for extreme ultra violet (EUV) light sources. Because of very low laser intensities (from 1010 W/cm2 to 1011 W/cm2) compared with that in laser fusion cases, it is necessary to include phase transition effects into ablation radiation hydrodynamics code.
Extreme Ultra Violet (EUV) light source produced by laser irradiation emits not only the desired EUV light of
13 ~ 14 nm (about 90 eV) but also shorter x-rays. For example, emissions around 4 ~ 8 nm (about 150 ~ 300 eV)
and 1 ~ 2.5 nm (about 0.5 ~ 1.2 keV) are experimentally observed from Sn and/or SnO2 plasmas. These
emissions are correspond to the N-shell and M-shell transitions, respectively. From the view point of energy
balance and efficiency, these transitions should be suppressed. However, they may, to some extent, contribute
to provide the 5p and 4f levels with electrons which eventually emit the EUV light and enhance the intensity.
To know well about radiative properties and kinematic of the whole plasma, atomic population kinetics and
spectral synthesis codes have been developed. These codes can estimate the atomic population with nl-scheme
and spectral shapes of the EUV light. Radiation hydrodynamic simulation have been proceeding in this analysis.
Finally, the laser intensity dependence of the conversion efficiency calculated by these codes agrees with that of
the corresponding experimental results.
A possible design window for extreme ultraviolet (EUV) radiation source has been introduced, which is needed for
its realistic use for next generation lithography. For this goal, we have prepared a set of numerical simulation codes to
estimate the conversion efficiency from laser energy to radiation energy with a wavelength of 13.5 nm with 2 %
bandwidth, which includes atomic structure, opacity and emissibity and hydro dynamics codes. The simulation explains
well the observed conversion efficiency dependence of incident power using GEKKO XII laser system as well as spectral
shapes. It is found that the conversion efficiency into 13.5 nm at 2% bandwidth has its maximum of a few percent at the
laser intensity 1-2 x 1011 W/cm2.
A new research project on extreme ultraviolet (EUV) source development has just been started at the Institute of Laser Engineering, Osaka University. The main task of this project is to find a scientific basis for generating efficient, high-quality, high power EUV plasma source for semiconductor industry. A set of experimental data is to be provided to develop a detailed atomic model included in computer code through experiments using GEKKO-XII high power laser and smaller but high-repetitive lasers. Optimum conditions for efficient EUV generation will be investigated by changing properties of lasers and targets. As the first step of the experiments, spherical solid tin and tin-oxide targets were illuminated uniformly with twelve beams from the GEKKO XII. It has been confirmed that maximum conversion efficiency into 13.5 nm EUV light is achieved at illumination intensity less than 2 x 1011 W/cm2. No significant difference is found between laser wavelengths of one μm and a half μm. Density structure of the laser-irradiated surface of a planar tin target has beem measured experimentally at 1012 W/cm2 to show formation of double ablation structure with density plateau by thermal radiation transport. An opacity experiment has just been initiated.
High power Nd:YAG laser beam with wavelength 1.064micrometers , pulse-width 20 and 100 pico-second was focused onto Zn coated target; and separated by beam splitter and delivered via optical fibers. Some key problems of ultra short pulsed high power laser beam transporting in optical fiber were involved and solved; laser ablation effect by laser irradiation and after optical fiber transportation were comparatively studied. Ablation rate was defined as ablated volume to laser fluence and used as a description of ablation ability. The relationship of ablation rate with laser fluence was plotted and optimum condition was found. Corresponding to the optimal condition, ablation effect in cases of 20 and 100 pico- second pulse-width, 90 degree(s) and 20 degree(s) laser incident angles, using spherical lens and cylindrical lens for laser beam focusing was compared respectively. It was shown that if other conditions unchanged, used spherical lens and cylindrical lens, ablation rate was almost same, but it was high as pulse-width equal to 100ps or laser incident angle equal to 90 degree(s) than as 20ps or 20 degree(s) respectively. In addition, laser beam over 5.6 watt, transporting rate near 90 per was output at the end of the optical fiber. Using the laser was this output power, the same thickness Zn coating was completely removed. Basing on this experiment, the possibility of laser surface ablation cleaning under complicated circumstance was confirmed.
Generation of thrust by laser propulsion has gained reality and much attention due to the recent development of high average-power lasers and demonstrations of sizeable object launching. Generating thrust requires a large amount of energy or high average power, but the question is how it is provided. This study deals with effectiveness of highly repetitive ultrashort laser pulses on generating high momentum coupling coefficient, Cm in vacuum condition. Two laser parameters, pulse width and repetition rate, have been studied in terms of the enhancement of Cm. It was found that with pico- and femtosecond pulse ablation, higher Cm is generated compared with longer pulse duration although it is on the same scaling as longer pulse duration. With high repetition rate pulses (80 MHz), more than one order of magnitude enhancements in Cm have been observed compared with single pulse interaction.
A 3D analysis code of thermal birefringence in solid-state lasers was developed. Basic equations include thermal conduction, absorption of laser energy, thermal stress and thermal birefringence. Relative phase shift induced by thermal effects are measured and found to be in good agreement with quantitative simulation results. Edge effects of thermal birefringence are quantitatively estimated by 3D simulation. Those are unable to be estimated by 2D analysis.
Investigations on laser cleaning for decontaminated surface have been performed by simulations and experiments. Basic equations of simulations are the equation of thermal conduction, the absorption of laser energy, boundary conditions of melting and evaporation. Simulations have bee performed for the case of 2-layers, basic material is SUS304 and surface pollutant is Fe3O4. Simulation results were in good agreements with experiments at the wide range of laser fluence.
Using the theories of photon count statistic and test data of the ultraweak photon emission from biological system, the biophoton field's spectra distribution properties were studied in this paper. An experimental setup for testing UPE in different spectral region were designed. The test data proved UPE of living biological system exists in wide spectra region from UV-visible to IR. Using the test data, we can obtain important conclusions that is UPE almost has nothing to do with wavelength. The conclusion has important significance for proving the bio-photon coherence. In the end of this paper, the medical applications of UPE in 21st century were discussed simply.
We have proposed and designed a diode-pumped solid-state laser (DPSSL) driver for inertial fusion energy (IFE) which consists of water-cooled zig-zag path Nd:glass slab amplifiers. A driver module has 10 kJ total output energy at 351 nm and operates at 12 Hz with 10.4 percent overall efficiency. The laser driver producing 4 MJ blue output for IFE will consists of 400 modules. To confirm the design, we are developing a small scale DPSSL module of 10J X 10Hz laser output at 1053 nm.
The progress of implosion physics research and relevant technologies enable us to examine technical and economical feasibility, and to plan the realistic strategy to the commercial power plant. The most important key issue for IFE is the driver technology. The development of the laser fusion driver is opening new industrial technologies based on the photon processes and new fields of high energy physics.
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