In this conference, we will report about new EUV research activity in Kyushu-Univ. in Japan. We have been planning EUV Exposure Research Center for support material development (Fig.1) and also new high power EUV source investigation program has been on going. It’s trigger is donation of 30kW CO2 driver laser system from Gigaphoton Inc. in 2022. Also In this paper we will discuss about the Sn plasma dynamics which dominate the EUV emission by using Thomson Scattering (TS) measurement4)(Fig.3). Recent TS results have revealed whole profiles of electron temperature and ion density in the EUV sources. These results mention that there is still sufficient potential to increase EUV output power and conversion efficiency in near future. This conceptual investing encourage us to improve EUV Light Source performance.
We demonstrated a water-window (WW) soft x-ray (SXR) source using a regenerative liquid bismuth target irradiated by a solid-state laser. A tank filled with the solid Bi particles was heated by a band heater to make liquid Bi, and then it was pushed out from the nozzle by the nitrogen gas. A 1064-nm Nd:YAG laser with a pulse duration of 150 ps and a repetition rate of 10 Hz was irradiated to the liquid Bi target. We observed the time-integrated spectrum of SXR with a wavelength shorter than 6 nm using a spectroscopy and energy spectrum of the suprathermal ions emitted from the WWSXR source using a Faraday cup. The number of photons was observed to be about 1 ×1013
photons/nm·sr·pulse at a peak wavelength of 3.9 and 4.2 nm, and 0.4 ×1013 photons/nm·sr·pulse at a peak wavelength of 2.4 nm. The total number of photons emitted in 2.3 – 4.4 nm was about 1×1013 photons/sr·pulse. Suprathermal ions were also emitted with a maximum energy of 140 keV from the hot, dense Bi plasma. These results have the potential to use a short wavelength light source for next-generation lithography systems.
We have demonstrated the spatial separation of extreme ultraviolet (EUV) emission and energetic tin (Sn) ions as debris from a laser-produced plasma (LPP) with a double-laser-pulse irradiation scheme based on a plasma X-ray laser configuration. We used two Nd:YAG lasers operating at a wavelength of 1064 nm for pre- and main pulses. The pre-pulse at a laser intensity of 2×108 W/cm2 was irradiated to the Sn solid planar target to produce a pre-plasma. After 20 ns from pre-pulse irradiation, the main pulse at a laser intensity of 2×1011 W/cm2 was irradiated in a tangential direction to the pre-plasma. We observed the angular distributions of EUV emission and energetic ions using an X-ray diode and a Faraday cup. The EUV was slightly emitted toward the main pulse. On the other hand, energetic ions were emitted toward the prepulse side. Under similar conditions, charge-separated energetic Sn ions were measured using an electrostatic energy analyzer (ESA), and the maximum kinetic energy and maximum valence were reduced.
Recently progress of LPP EUV light source is remarkable. Ten years ago, power level is only several 10 W level. At present 250W power level is realized in semiconductor mass production factories1) by ASML. On the other hand, pioneer of this Unique technologies including; combination of pulsed CO2 laser and Sn droplets, dual wavelength pico second laser pulses for shooting and debris mitigation by magnetic field have been applied by Gigaphoton2). They have demonstrated high average power >300W EUV power with CO2 laser more than 27kW at output power in cooperation with Gigaphoton and Mitsubishi Electric3). In near future more higher power (>600W) EUV source is required to fit High NA (>0.55) lithography of semiconductor industry.
In this paper we will discuss about the Sn plasma dynamics which dominate the EUV emission by using Tomson scattering (TS) measurement4. Recent TS results have revealed whole profiles of electron temperature and ion density in the EUV sources. These results mention that there is still sufficient potential to increase EUV output power and conversion efficiency in near future.
We evaluated the charge-separated energy spectra of the suprathermal highly charged gadolinium (Gd) ions as debris from a laser-produced plasma (LPP). Laser pulses with pulse durations of 6 ns and 150 ps were irradiated to a solid planar Gd target. Charge-separated suprathermal Gd ions from an LPP were measured using an electrostatic energy analyzer (ESA). The maximum ionic charge state was q = 16, and the maximum energy was about 30 keV (q = 16) at the pulse duration of 150 ps under the laser intensity of IL = 2 × 1012 W/cm2. At the pulse duration of 6 ns under the same laser intensity of IL = 2 × 1012 W/cm2 by a control of a laser pulse energy and a focal spot diameter, the maximum ionic charge state was q = 15, and the maximum energy was 15 keV (q = 10), approximately half of that in the case of the pulse duration of 150 ps.
We demonstrate a supercontinuum vector beam analysis based on interactions between angular-variant polarization and birefringence in the solid state. The supercontinuum vector beam analysis provides both the two-dimensional distribution of the birefringence and the wavelength properties, from a single-shot image. The angular resolutions of the retardance and azimuthal angle are evaluated. The retardances were determined at 650 nm to be 92.9° ± 9.3° and 87.1° ± 18.7°, respectively. The spatial resolution of 60 μm and the resolution of the retardance was 12 mrad.
We evaluated the charge-separated spectra of highly charged suprathermal gadolinium (Gd) ions from a 1064-nm, nanosecond laser-produced plasma (LPP) extreme ultraviolet (EUV) source developed for beyond EUV (BEUV) lithography. The charge distribution of these suprathermal ions emitted from a solid planar Gd target was measured by an electrostatic energy analyzer (ESA). The maximum ionic charge state was observed to be Z = 16 and to possess a maximum energy of about 15 keV at the optimum laser intensity of 2 x 1012 W/cm2 to produce the efficient 6.7-nm EUV emission. This evaluation provides important information essential for the development of debris mitigation schemes in the BEUV source for next generation lithography.
The interactions of short pulse lasers with matter are interesting subjects not only in applications such as surface fabrication but also in physical phenomena for study. Optical short pulse lasers have abilities to occur the ablation phenomena accompanying the creation of high temperature, high pressure, and excited states of electrons. The picosecond soft x-ray laser (SXRL) pulse also has ability to occur the ablation. The SXRL having the wavelength of 13.9 nm and duration of 7 ps is one of attractive x-ray source for ablation study, because the ablation threshold obtained with the focused SXRL pulse is much smaller than those obtained with other lasers having longer durations and/or longer wavelengths. The low ablation threshold of a material for the SXRL beam has a possibility of efficient nanometer scale surface machining by an ablation. The ablation study will lead to the physical research and the direct surface machining. In addition, the wavelength of the SXRL is very close to the wavelength of the extreme ultraviolet (EUV) lithography system (λ = 13.5 nm). In the presentation, we report on development of the soft x-ray laser irradiation system. The irradiation system has an intensity monitor based on the Mo/Si multilayer beam splitter. This intensity monitor provides the irradiation energy onto sample surface. The SXRL has an ability to confirm the ablation threshold and to examine the damage property of EUV optical elements, which have the same specifications of those in the EUV lithography. And more, it is possible to evaluate the doses for sensitivity of resists.
Relations among absorption of pump beam, quantum defect and thermal load were investigated for pump wavelengths of
968 nm and near 940 nm in two independent, real-time measurement experiments complemented with thermal
distribution simulation. Saturation of absorption at 969 nm pumping for non-lasing operational regime, which affects
temperature rise and exists independently of the thin disk type, disk head construction, pump power and pump beam
diameter is reported. Disk temperature dependence of absorption, quantum defect and disk geometry and large difference
between absorption, disk temperature and o-o efficiency at both pump wavelengths are discussed.
The effect of sub-ns duration and sub-mJ energy laser pulse on 13.5 nm extreme ultraviolet (EUV) source diameter and conversion efficiency has been investigated. It was demonstrated that an in-band EUV source diameter as low as 18 μm has been produced due to short scale length of the picoseconds duration laser plasma. Such EUV source is suitable for high brightness and high repetition rate metrology applications.
We evaluate the EUV emission and the spatial distributions of the plasma parameters by use of the two-dimensional (2-D) radiation hydrodynamic simulation in the microplasma high-brightness EUV source. The expected EUV source size, which is attributed to the expanding microplasma by the hydrodynamic motion, was evaluated to be 15 μm at the laser pulse duration of 150 ps [full width at half-maximum (FWHM)]. The numerical simulation suggests that the high brightness EUV source should be produced by use of a dot target based microplasma with the source diameter less than 20 μm. The emission at 13.5 nm was attributed to Sn charge states between Sn7+ and Sn12+ with the UTA spectral structure.
In this work we present results on the influence of laser pulse duration and irradiating power density
on the conversion efficiency (CE) and the ion energy of gadolinium (Gd) laser produced plasmas.
Three lasers were used with 10 ns, 150 ps and 140 fs pulse durations. By varying the lasers output
energies, experiments could be carried out for a power density range of 1011 - 1015 W/cm2. A
maximum CE of 0.4% was achieved within a 0.6% bandwidth in 2π steradians using the picosecond
laser. A faraday cup was used to calculate ion yield and time of flight measurements of each laser.
The picosecond laser also showed a reduction in the ion time of flight measurements compared with
the nanosecond pulse.
We demonstrate a table-top strong band emission water window source based on laser-produced high-Z plasmas.
Resonance emission from multiply charged ions merges to produce intense unresolved transition arrays in the 2 to 4
nm region, extending below the carbon K edge (4.37 nm). Arrays resulting from n = 4-n = 4 transitions are overlaid
with n = 4-n = 5 emission and shift to shorter wavelength with increasing atomic number. Under spectral analysis a
guideline for microscope construction design for single-shot live cell imaging is proposed based on the use of a
bismuth plasma source, coupled with multilayer mirror optics.
We demonstrate an efficient extreme ultraviolet (EUV) source for operation at λ = 6.7 nm by optimizing the optical
thickness of gadolinium (Gd) plasmas. Using low initial density Gd targets and dual laser pulse irradiation, we
observed a maximum EUV conversion efficiency (CE) of 0.54% for 0.6% bandwidth (BW) (1.8% for 2%BW),
which is 1.6 times larger than the 0.33% (0.6%BW) CE produced from a solid density target. Enhancement of the
EUV CE by use of a low-density plasma is attributed to the reduction of self-absorption effects.
We have observed guiding of 1016-W/cm2 ultrashort laser pulses in a discharge-produced plasma channel in an
argon-gas-filled 1-cm long alumina (Al2O3) capillary. An optimum timing of the laser pulse injection was around
150 ns for the discharge with a peak current of 200 A at the discharge time 150 ns. One-dimensional
magnetrohydrodynamic (MHD) code was used to evaluate the average degree of ionization of Ar in the preformed
plasma channel. In the preformed plasma the averaged ion charge state of Ar3+ was obtained from the observed
visible emission spectra and from the MHD simulation. The spectrum of the laser pulse in the preformed by the
discharge argon plasma channel was not changed and was well reproduced in the particle-in-cell (PIC) simulation.
We characterize the emission spectra of a potassium plasma and its temporal behavior at 39 nm. To understanding
the potassium spectral behavior without contamination effect, we use a laser-produced plasma to control the plasma
parameters by changing the laser intensity and wavelength. Potassium ions produced strong broadband emission
around 40 nm ranging from K3+ to K5+ ions at a time-averaged electron temperature of about 12 eV. Emission at 39
nm is caused during the recombining phase and it was reproduced by hydrodynamic simulation, which accounted for
atomic processes. As the emission spectral behavior of the laser-produced potassium plasma XUV source is similar
to that of the hollow cathode-mode discharge-produced plasma spectrum, it indicates that the emission from the
discharge-produced plasma occurs in a region of high electron density close to 1020 cm-3.
We have demonstrated a laser-produced plasma extreme ultraviolet source operating in the 6.5-6.7 nm region based
on rare-earth targets of Gd and Tb coupled with a Mo/B4C multilayer mirror. Multiply charged ions produce strong
resonance emission lines, which combine to yield an intense unresolved transition array. The spectra of these
resonant lines around 6.7 nm suggest that the in-band emission increases with increased plasma volume by
suppressing the plasma hydrodynamic expansion loss at an electron temperature of about 50 eV, resulting in
maximized emission. We also have investigated the dependence of the spectral behavior and conversion efficiencies
of rare-earth plasma extreme ultraviolet sources with peak emission at 6.7 nm on laser wavelength and the initial
target density. The maximum conversion efficiency was 1.3% at a laser intensity of 1.6 × 1012 W/cm2 at an operating
wavelength of 1064 nm, when self-absorption was reduced by use of a low initial density target.
An engineering prototype high average power 13.5-nm source has been shipped to semiconductor facilities to permit
the commencement of high volume production at a 100 W power level in 2011. In this source, UTA (unresolved
transition array) emission of highly ionized Sn is optimized for high conversion efficiency and full recovery of the
injected fuel is realized through ion deflection in a magnetic field. By use of a low-density target, satellite emission
is suppressed and full ionization attained with short pulse CO2 laser irradiation. The UTA is scalable to shorter
wavelengths, and Gd is shown to have similar conversion efficiency to Sn (13.5 nm) at a higher plasma temperature,
with a narrow spectrum centered at 6.7 nm, where a 70% reflectivity mirror is anticipated. Optimization of short
pulse CO2 laser irradiation is studied, and further extension of the same method is discussed, to realize 100 W
average power down to a wavelength of 3 nm.
Strong beams of coherent radiation are essential to induce nonlinear excitation phenomena in biology and material
sciences. Optical-field-induced ionization by an ultrashort laser pulse produces ultrabroadband bursts of radiation
with photon energies ranging from radio-wave at the microsecond timescale to x-ray at the attosecond timescale. As
the laser pulse drives an ultrafast-discharge with high current it induces nonlinear spectral conversion in a few
femtoseconds and generates terahertz electromagnetic waves. Broadband terahertz generation has been reported in
air and rare gases. If the radiation frequency depends on the electron plasma density, it should vary with the laser
pulse intensity, and the kind and density of the gas. However, the peak radiation frequencies reported are almost
independent of those parameters. From the laser-gas interaction point of view, the terahertz generation mechanism is
not enough understood. We demonstrate a frequency-tuning scheme that uses the laser pulse duration to control the
ultrafast-discharge current timescale, yielding a terahertz energy of 0.1 μJ and a conversion efficiency of 10-4 by use
of the homemade power supply with 60-A discharge current at 1 kHz. We also propose a simple physical model to
explain the generation of terahertz radiation with the laser propagation in an ultrafast-discharge.
When plasmas are instantaneously created around an electromagnetic wave, frequency of the wave up-converted to
the frequency, which depends on the plasma frequency. This phenomenon is called as the flash-ionization predicted
by S. C. Wilks et al [1]. The theory requires not only the plasma creation in time much shorter than an oscillation
period of the electromagnetic wave but also plasma length much longer than a wavelength of it. We have
demonstrated the proof of principle experiment using the interaction between a terahertz wave and plasmas created
by an ultra short laser pulse, which ensures the plasma creation time-scale much shorter than a period of
electromagnetic source wave and plasma length longer than a wavelength of the wave. We observed frequency upconversion
from 0.35 THz to 3.3 THz by the irradiance of the Ti:sapphire laser in ZnSe crystal.
We have reported a discharge-produced plasma extreme ultraviolet source based on a pure potassium vapor.
Potassium ions produced strong broadband emission around 40 nm with a bandwidth of 8 nm [full width at halfmaximum
(FWHM)]. The current-voltage characteristics of discharge suggest that the source operates in a hollow
cathode mode. By comparison with atomic structure calculations, the broadband emission is found to be primarily
due to 3d-3p transitions in potassium ions ranging from K 2+ to K4+.
Electromagnetically induced transparency (EIT) is a well-known quantum phenomena that electromagnetic wave
controls the refractive index of medium. It enables us to create a passband for low frequency electromagnetic wave
in a dense plasma even if the plasma is opaque for the electromagnetic wave. This technique can be used to prove the
ion acoustic wave because the ion acoustic frequency is lower than the plasma frequency. We have investigated a
feasibility of electromagnetic radiation at THz region corresponding to the ion acoustic frequency from a dense
plasma. We confirmed that the passband is created at about 7.5 THz corresponding to the ion acoustic frequency in
the plasma (1021 cm-3) with a Ti:Sapphire laser (800 nm, 1017 W/cm2). The estimated radiation power is around 1
MW, which is expected to be useful for nonlinear THz science and applications.
We reported the production of a plasma channel in a capillary discharge-produced plasma. Plasma parameters of its
channel were observed by use of both a laser interferometer and a hydrogen plasma spectrum. A time-resolved
electron temperature was measured, and its maximum temperature of 3 eV with electron densities of the order of
1017 cm-3 was observed at a discharge time of 150 ns and a maximum discharge current of 400 A. Intense laser pulse
was guided over many vacuum Rayleigh lengths using its channel.
Pulse compression through filamentation in a free-space argon gas-filled cell has been demonstrated by use of the
high energy laser pulse. Compression and splitting of the optical laser pulse due to multiple filamentation in an argon
gas-filled cell were observed. A 130-fs pulse was compressed to less than 60 fs (full width at half-maximum) with
the output energy of 16 mJ at the argon gas pressure of 25 kPa.
We report on the demonstration of a dc to ac radiation converter (DARC) source as a THz emitter. THz radiation is
generated through the spatiotemporal change in electron plasma density induced by a relativistic ionization front
propagating in a ZnSe crystal. The central emission frequency is 1.2 THz with a bandwidth of 0.7 THz [full width at
half-maximum (FWHM)]. The central frequency is attributed to production of the electron plasma density of the
order of 1015 cm-3 and is in good agreement with the expected theoretical value.
We demonstrated enhancement of extreme ultraviolet (EUV) emission at 13.5 nm from a lithium plasma by use of dual laser pulses. A single laser pulse produced a lithium plasma condition for the EUV emission far beyond its optimum. Utilization of dual laser pulses, however, enhanced the EUV emission energy, and its maximum in-band EUV conversion efficiency (CE) in a measured solid angle was observed to be 2% at a delay time between 20 and 50 ns.
We observed significant amplification of the argon excimer laser emission at 126 nm initiated by femtosecond high-intensity laser-produced electrons. By introducing an optical feedback with a vacuum ultraviolet multilayer mirror, the small signal gain coefficient of 2.3 cm-1 was evaluated at 126 nm.
We demonstrated a debris-free, efficient laser-produced plasma extreme ultraviolet (EUV) source by use of a regenerative liquid microjet target containing tin-dioxide (SnO2) nano-particles. By using a low SnO2 concentration (6%) solution and dual laser pulses for the plasma control, we observed the EUV conversion efficiency of 1.2% with undetectable debris.
Bio-active hydroxyapatite (HAp) coatings were deposited by the pulsed laser deposition method using a KrF excimer laser. We changed the sintered temperature of HAp ceramics target and successfully deposited the poly-crystalline HAp coating layer at room temperature.
Characteristics of suprathermal ions and neutral particles from a laser-produced tin plasma by use of a colloidal
microjet target containing tin dioxide (SnO2) nanoparticles were investigated. Suprathermal ion emissions were
reduced by producing a low-density preplasma. Simultaneously, the maximum conversion efficiency of 1.2% at 13.5
nm with a bandwidth of 2% and a solid angle of 2&pgr; sr was observed. Neutral particles, however, were not suppressed
under the optimum laser-plasma conditions.
We demonstrated a debris-free, efficient laser-produced plasma extreme ultraviolet (EUV) source by use of a regenerative liquid microjet target containing tin-dioxide (SnO2) nano-particles. By using a low SnO2 concentration (6%) solution and dual laser pulses for the plasma control, we observed the EUV conversion efficiency of 1.2% with undetectable debris.
The Ar2* excimer production kinetics were initiated by electrons produced in the optical-field-induced ionization (OFI) process. The use of an Ar-filled hollow fiber extended an OFI plasma length up to 30 cm. By use of a 126-nm mirror, the maximum one-pass gain coefficient at 126 nm was observed to be 0.58% cm-1 at 10 atm Ar.
Extreme ultraviolet (EUV) radiation at the wavelength of around 13nm waws observed from a laser-produced plasma using continuous water-jet. Strong dependence of the conversion efficiency (CE) on the laser focal spot size and jet diameter was observed. The EUV CE at a given laser spot size and jet diameter was further enhanced using double laser pulses, where a pre-pulse was used for initial heating of the plasma.
Characteristics of extreme ultraviolet (EUV) light at the wavelength between 5 to 20 nm from rare gas cryogenic targets irradiated with a nano-second laser pulse are studied. Spatial distribution of the EUV light and the ion current was measured and found to vary as cos2θ and cos6θ, respectively, where θ is an angle to the target normal. Energetic ions were detected, which had a velocity of the order of 106 cm/s. According to the observed cos2θ spatial distribution, the spatially-integrated total EUV output energy for the xenon cryogenic target was evaluated to be 1.5 mJ/pulse, leading to the conversion efficiency of 0.2%/pulse.
We have used a pair of newly constructed electrodes to improve the discharge stability and electrical input power. The electrode shape was designed so that the discharge width became narrower, which lead to the increase of the input power density by 22%. As a result, the maximum output energy increased from 150 to 200 μJ at 147.8 nm. The pulse duration of 250 ns (FWHM) became shorter compared to the previous result (400 ns). This long pulse operation indicated the laser oscillation in an afterglow mode. The laser beam shape was circular with a beam divergence of 2.5 mrad. Because of the long pulse duration, this beam shape reflected on a cavity mode (multi-mode) as a result of the optical feedback. A small signal gain coefficient increased almost linearly with the increase of the discharge voltage. The maximum gain coefficient at 147.8 nm was 3.5%cm-1 at 31 kV.
We have observed Ar2* emission at 126 nm by use of a high intensity laser pulse as an excitation source. Kinetic analysis revealed that high-intensity laser-produced electrons via optical field induced ionization (OFI) process initiated the Ar2* production kinetics. Ar2* production kinetics initiated by OFI electrons was mainly governed by the three-body association process, which was analogous to the case of electron beam excitation. The use of a hollow fiber controlled propagation characteristics of a high intensity laser pulse in a high-pressure gas, leading to the increase of the excimer emission intensity
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