Recent developments on high-power compact extreme ultraviolet (EUV or XUV) sources have enabled us to study the materials ablation induced by the irradiation of intense EUV light. The interactions of EUV light with matters along the ablation have potential advantage for the use in advanced materials processing. We have studied the charge states and their energy distributions in the EUV ablation plasma using an E´B mass-charge analyzer. The measurement was also conducted for conventional laser ablation plasma induced by a 1064 nm Nd:YAG laser. The results showed noticeable difference between EUV and laser ablations, where EUV ablation only showed singly charged Si ions, and laser ablation showed multiply charged ions up to Si3+. The energy spectra of multiply charged ions were distributed in higher energy range. It has been predicted that EUV ablation plasma have higher electron density close to the solid density and lower electron temperature at 1 to few eV, while laser ablation plasma generally has lower electron density around the critical density and higher electron temperature exceeding 10 eV mainly by inverse bremsstrahlung absorption at an irradiation intensity around 109 W/cm2 range. Thus, ionization did not proceed much and only singly charged ions were detected for EUV ablation and the electron impact ionization proceeded to have multiply charged ions for laser ablation. This study showed that the experimental results reflected the general characteristics of the ablation plasma, and showed possibilities of modeling of entire EUV ablation dynamics from expanding ions.
Fast neutrons, which are neutrons with energies greater than 1 MeV, are expected to be a source of nondestructive inspection for a large-size infrastructure such as a bridge girder because of their mean free path exceeding the meter. A neutron-imaging device with 10-ns time resolution can discriminate pulsed neutrons from X-rays via time of flight. For this purpose, we require a fast-response neutron imager with large aperture and high image resolution. A neutron-imaging device with time resolution of 10 ns and aperture size of 40 cm × 60 cm was developed. It was filled with fast response liquid scintillator [1] in an aluminum honeycomb plate, which converts neutrons to optical light images. The scintillation light images were relayed using an optical lens and detected using a fast response image intensified CCD. The detector was tested at an electron linear accelerator (LINAC) facility in Osaka University. A short X-ray pulse (30 ps pulse duration) was generated using LINAC, and X-ray radiograph images were obtained with a 10- ns exposure time duration. The radiograph images were well attenuated within 10-ns from the X-ray injection. A high energy X-ray image and a neutron radiograph image of a 30-cm thick concrete block with iron blocks located behind it were successfully observed. This promising technique could facilitate nondestructive inspection of large concrete constructions.
In the present paper, we overview fabrication methods to produce density-controlled tin and xenon targets for generating
extreme ultraviolet (EUV) light. The target can be classified as a mass-limited target. In the case of tin, EUV was
relatively monochromatic, and its conversion efficiency was higher than bulk tin. Using the nano-template method, the
cellular foam size was controlled by the template size. The density was 0.5 ~1.5 g/cm3. In the case of the 0.5 g/cm3
foam, its morphology was controlled by changing the ethanol content of the precursor tin solution. The morphology
difference was useful to control the angular distribution of EUV radiation. SnO2 nanofiber, which is oriented low-density
material, was fabricated continuously using a electrospinning method. The width and the shape of the fiber were
controlled by optimizing precursor solution. A transparent film with tin and SnO2 elliptic spheres were prepared using
liquid crystalline cellulose derivative. Low density xenon was prepared from liquid xenon using a swirl atomizer to
produce a density of 0.2 g/cm3.
EUV emission from spherical and planer targets were precisely characterized as an experimental database for use in
EUV source generation at high repetition rates. In the single-shot base experiments, conversion efficiency as high as
those for the plasma geometry has been demonstrated. The integrated experiment was made with 10 Hz plasma
generation.
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 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.
It is very effective for mass-limited tin-foil targets to adapt for the EUV source. Tin-foil targets in account of formation, size, and thickness have been developed for debris mitigation. The amount of ions from targets is 40 % decreased tin-foil targets of 1μm or 5μm thickness than tin-bulk targets. The ion velocity is one order of magnitude less than bulk targets. The EUV emission spectra of tin-foil are more narrowing than bulk targets. The targets supply for high repetition rate of 10 kHz is applied for a novel method. It is called "Punch-out" method. The flight of graphite foil that it is a test targets was succeed to observe by using a gated ICCD camera. The target velocity is achieved to be about 120 m/s. This value can be applied for targets supply with high repetition rate of 10 kHz.
Our institute has been investigating laser-produced tin plasma and EUV emission, and found the highest conversion efficiency of 3% at 13.5 nm in 2% bandwidth. In the present paper, we introduce fabrication methods of density-controlled tin targets to generate relatively monochromatic EUV with keeping similar conversion efficiency.
The first method is the nano-template method, where liquid tin solution was immersed into a polymer film with monodispersed size nanoparticle. The density can be controlled by tin concentration of the solution. The target can be classified into mass-limited target. We have shown a tendency of monochromatic EUV emission around 13.5 nm with decreasing of tin density. The intensity was higher than tin foil with bulk density. The tendency has a merit to mitigate heat effect of the first EUV mirror. The fabrication method has another merit to control not only density but also the poresize of tin oxide to be 100 nm ~ 10 mm. Recent experiments exhibited an EUV character depending on the poresize.
The second is liquid crystalline template method to obtain porous tin oxide. The precursor with tin oxide and cellulose provides mechanically stable and transparent film. The film has wavy sub-microstructure derived from microscopic liquid crystal domain structure. The method is simple and short duration for the hydrolysis reaction to solidify tin compound. This material has a merit of feasibility of fabrication, and was applied for rotation target for 10 Hz and 5 kHz laser repetition.
Extreme ultraviolet (EUV) emission from laser produced tin plasma was investigated for 1064, 532 and 266 nm laser wavelengths. The EUV conversion with tin target tends to be high for shorter laser wavelength and is optimized at 4-5x1010 W/cm2 for 1064 and 532 nm. The EUV emission exhibits laser wavelength dependence in terms of angular distribution and structures of emission spectra. It is found that spectra for 532 nm and 266 nm showed spectral dips at around 13.5 nm and these dips are well replicated in computer simulations. Both the angular distribution together with the spectral dips may suggest existence of opaque plasmas surrounding the EUV emission region.
Dimitri Batani, F. Strati, H. Stabile, M. Tomasini, C. Olivotto, Tara Desai, G. Lucchini, Michel Koenig, A. Benuzzi-Mounaix, Hiroaki Nishimura, Yoshihiro Ochi, Jiri Ullschmied, Jiri Skala, Bozena Kralikova, Miroslav Pfeifer, Christelle Kadlec, Tomas Mocek, A. Praeg, T. Hall, Paolo Milani, E. Barborini, P. Piseri
Experiments on the Equation of State (EOS) of Carbon were performed at the PALS and LULI laboratories. We used Carbon samples with two different value of initial density, in order to explore a wider region of the phase diagram. We obtain experimental data for carbon Hugoniot at Megabar pressures induced by laser-driven shock waves. The target rear side emissivity due to the shock unloading was recorded from (two-materials two-steps) targets (Al-C) with
space and time resolution. By applying the impedance mismatch method, a direct determination of relative EOS points was obtained. Experimental data are compared with previous experiments and with theoretical models. Our results indicate a higher compressibility of carbon at Megabar pressures compared to theoretical models.
Extreme ultraviolet (EUV) emission from laser produced plasma attracts much attention as a next generation lithography
source. The characterization of EUV emission has been carried out using GEKKO XII laser system. The twelve beams
irradiated tin or tin-oxide coated spherical targets uniformly and dependence of EUV spectra on laser intensity were
obtained with a transmission grating spectrometer and two grazing incidence spectrometers. The EUV Conversion
Efficiency (CE, the ratio of EUV energy at the wavelength of 13.5 nm with 2 % bandwidth to incident laser energy) was
measured using an absolutely calibrated EUV calorimeter. Optimum laser intensities for the highest conversion were
found to be 0.5- 1x1011 W/cm2 with CE of 3 %. The spectroscopic data indicate that shorter wavelength emission
increases at higher laser intensities due to excessive heating beyond optimum temperatures (20- 40 eV). The CE was
almost independent on the initial coating thickness down to 25 nm.
Extremely ultraviolet (EUV) light at around 13.5 nm of wavelength is the most probable candidate of the light source for lithography for semiconductors of next generation. We have been studying about the EUV light source from laser-produced plasma. Detailed understanding of the EUV plasma is required for developments of modeling with simulation codes. Several parameters should be experimentally measured to develop the important issues in the simulation codes. We focused on density profile, properties of EUV emission, and opacity of the laser-produced plasmas. We present re-cent experimental results on these basic properties of the laser-produced EUV plasmas.
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.
Here we discuss the results of the experiments performed using the Prague Asterix Laser System (PALS) of wavelength 0.44 μm (3ω of Iodine laser) and energy ≈ 250 J in 450 ps (FWHM). Two sets of experiments were carried out, firstly, generation of high quality shocks which were steady in time and uniform in space using Phase Zone Plates (PZP), to establish the scaling laws of shock pressure Vs. laser intensity for aluminum foil target of thickness 8 μm. Our results show a good agreement with the delocalized laser absorption model. Secondly, measurements of the Equation of State of carbon compressed by shocks at megabars of pressure have been realized. Equation of State were obtained for carbon using the impedance mismatch technique. Step targets allowed the simultaneous measurements of shock velocity in two different materials. Aluminum was used as a reference material and relative EOS data for carbon have been obtained up to ≈ 14 Mbar pressure.
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.
To demonstrate the feasibility of the fast ignitor scheme, the 100 TW Petawatt Module laser PWM was illuminated on an imploded spherical CD shell, which enhanced neutron yield ~106 for the first time. This is due to the fusion reactions between the energetic deuteron beam and cold dense deuterons. Without PWM, we have observed no neutrons above the detection threshold ~105. Proton rear emission and possible gigagauss scale magnetic fields from 60 TW laser illuminated plastic target are observed in a good agreement with a two-dimensional PIC simulation. The PWM is up graded to the world biggest Petawatt Laser (say PW laser). The out put is 1 Petawatt, which is used also for ultra-intense laser matter interactions above the intensity of 1020 W/cm2.
An imploded plasma core is irradiated by a 100 ps laser pulse in a model experiments of fast ignition. Additional laser pulses for drilling and heating are introduced co- axially with the laser beams for the implosion. The preformed imploded core is created by the 12 beams of 0.53 micrometers laser with the total energy of 800 J. The additional heating pluses contain 100 ps pulses separated by 300 ps at the wavelength of 1.06 micrometers with the total energy of 320J. The first pulse is intended for drilling the coronal pulses surrounding the core and the second is for addition heating of the core. We measured the imploded core additionally heated with 100 ps pulses.
IN order to directly observe low-mode implosion nonuniformities, especially of l equals 1, which prevents stable formation of a hot spark in the compressed core plasma at the final stage of the implosion, a series of direct-drive implosion experiments has been performed at the Gekko-XII glass laser facility by using gas-filled plastic- shell targets. Partially coherent light (PCL) was used as a drive laser to suppress middle- to higher-modes of the irradiation nonuniformity down to approximately 1 percent. A clear shift from the center of the chamber and a slight crosswise structure were seen in the time- and space- resolved shape of the shell in the accelerating phase with an x-ray framing camera and absorbed laser intensity were also estimated by using a rocket equation. In order to confirm the property of these results, separate experiments under similar laser conditions were performed by using Au- coated sapphire spheres as surrogate target. The x-ray intensity distribution on the circumference of the target in the XFCs image, which is strongly dependent on the drive nonuniformity in the accelerating shell resulted from the l equals 1 drive nonuniformity. In our experiments, the l equals 1 drive nonuniformity due to some reproducible factors was found to be of the order of approximately 10-20 percent.
X-ray spectroscopy is one of the most important diagnostics of laser-produced plasmas, finding application in diverse areas such as laser fusion, x-ray lasers, and novel experiments using shot-pulse lasers to probe chemical and biological phenomena on the femtosecond timescale. Depending on the aims of these experiments, either high resolution spectra combined with either spatial or time resolution, or monochromatic x-ray spectrometer was also used here in x-ray diagnostics of 4f yields 3d transitions in Nickel-like transitions of elements with atomic numbers between 70 and 74. The dependence of this x-ray emission on laser energy, spot size, and target materials provides information about ionization degree, electron temperature and density - important parameters for the population inversion of a Ni- like x-ray laser in the water window.
Indirect/direct-hybrid drive scheme to suppress the initial imprint of the laser irradiation nonuniformities has been proposed and investigated as a new drive scheme for inertial fusion. In direct drive inertial confinement fusion, initial imprinting of laser irradiation nonuniformity is considered to cause seeding of the perturbation on target surface in the very beginning of the irradiation which may be amplified by Rayleigh-Taylor instability in the acceleration phase of the implosion and be deleterious to efficient heating of the hot spark at the center of the compressed fuel core plasma. In indirect/direct-hybrid drive scheme, the target is first irradiated very uniformly with low-intensity soft x-ray prepulse from external sources apart from the target. Indirect x-ray pre-irradiation of the surface causes a pre- expansion layer of the plasma before the irradiation of the direct drive laser beam. When the drive beam comes later, the target has a substantial stand-off distance between the ablation front and the beam absorption region. Thus the thermal smoothing effect is expected to occur in this transport layer, and the initial imprint can be significantly reduced. We have demonstrated planar target experiments on the indirect/direct hybrid scheme and observed reduction of the initial imprint. Implosion experiments the indirect/direct hybrid drive spherical capsules with external x-ray sources has been started. Overall implosion was performed successfully.
The initial imprint of density perturbation due to spatial nonuniformity of laser intensity is one of the most important issues in laser fusion research. Several imprint mitigation scheme by means of soft x-ray radiation have been proposed to reduce the induced perturbation through the thermal conduction region. One of the schemes uses an external x-ray source prior to laser incidence to produce preformed plasma. Another has a low-density foam layer and high-Z material to heat the foam radiatively and make it uniform. We present the dynamics of these schemes and the perturbation growth with nonuniform laser from the results of 2D simulation using our integrated code.
KEYWORDS: Plasma, Optical spheres, Foam, X-rays, Wave propagation, Signal to noise ratio, Information operations, High power lasers, Radiography, Optical simulations
Laboratory simulation of the ejecta-ring collision of Supernova 1987A has been performed by using a high power laser syste. Pure hydrodynamic modeling was applied using the invariance of the Euler equations and the experiment was designed with 1D hydrodynamic simulations. Three laser beams of (lambda) equals 0.53 micrometers , at the intensity I equals 2 X 1014 W/cm2 irradiated a CH foil to generate a strong shock wave and subsequent dense plasma flow in a low density CHO foam including a solid sphere at its center. Shock wave propagation and ma sam flow with vortex ring-like structures were observed with gated x-ray radiography. Overall hydrodynamic behaviors are consistent with 1D and 2D hydrodynamic simulations. The collision dynamics is dominated by a complicated interplay of reflected shock, diffracted shock, transmitted shock waves, and plasma flow behind the main shock passage.
In the direct-drive scheme implosion of the inertial confinement fusion, the hot spark formation is critically affected by laser irradiation non-uniformities and subsequent hydrodynamic instabilities. Influence of the low- modal irradiation non-uniformities on the hot spark formation was investigated by means of the time- and space- resolved x-ray spectroscopic measurements. Experimental results were compared with post-processed hydro-code simulations by the aid of x-ray spectrum analysis code.
Reviewed are the progress in direct drive implosion researches with Gekko XII laser system. Precise observation of the growth rate of Rayleigh-Taylor instability and the suppression of imprints using indirect-direct hybrid implosion have been investigated. Theoretical and experimental researchers on the fast ignition scheme are also studied. Relativistic laser plasma interaction experiments with Peta-Watt Module and Gekko XII are also described. Finally, the future direction of the research including the development of solid state laser for fusion reactor is discussed.
The physics of laser-plasma interaction and x-ray generation by laser have been extensively investigated as close relevance laser fusion. Efficient conversion to x-rays and controllability of wavelength, pulse width, and geometrical size have given us the high potentiality of applications to wide scientific and technical fields. An efficient high average power laser with a good beam quality is also under development as the key technology for the applications.
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