Sho Amano

Isochronous storage ring of the New SUBARU project

The aims of the New SUBARU project are to promote industrial applications in the VUV and soft X-ray region and to develop research and development towards new light sources. The main facility of the New SUBARU project is the 1.5 GeV electron storage ring which is under construction at the SPring-8 site in Harima Science Garden City, Japan. The storage ring is quasi-isochronous and has variable momentum dispersion for the deep study of beam dynamics in very short bunches.

Rotating cryogenic drum supplying solid Xe target to generate extreme ultraviolet radiation

In order to generate high-average-power extreme ultraviolet radiation around 13.5nm from laser-produced plasma, a fast rotating cryogenic drum system which can continuously supply a solid Xe target for high-repetition laser pulses has been developed. It was successfully operated at a rotating speed of 1000rpm and an up-down speed of 3mm∕s. The growth rate of the solid Xe layer is kept at a constant speed (for example, 18μm∕s) with a constant rate of Xe input flow, independent of the rotating speed. It has been demonstrated that wipers play an important role in recovering the laser craters with a recovery rate of 150μm∕s. These performances allow laser illuminations on the solid Xe target with a pulse repetition rate of 10kHz without overlapping laser craters.

X-ray generation in cryogenic targets irradiated by 1 μm pulse laser

Soft x-ray spectral radiations from Xe, H2O, and CO2 cryogenic targets irradiated by a 1 μm neodymium doped YAG-slab laser at pulse widths of 12–20 ns and at laser intensities of 5×1010–1012 W/cm2 have been observed. These targets radiate soft x-rays in a wavelength range of 10–13 nm which is useful for projection microlithography. We have found a strong x-ray spectral peak at λ=10.8 nm with a Xe cryogenic target. The measured x-ray conversion efficiency with the Xe target was 0.8%/sr(λ=10.8±0.27 nm) at a laser intensity of 1×1012 W/cm2. This was ten times or more efficient than that with H2O and CO2 targets.

Laser wavelength and spot diameter dependence of extreme ultraviolet conversion efficiency in ω, 2ω, and 3ω Nd:YAG laser-produced plasmas

Conversion efficiency and spectra of extreme ultraviolet radiations from a cryogenic planar solid xenon target were investigated as a function of laser wavelength (ω, 2ω, and 3ω Nd:YAG) and the laser focus spot size (50–700μm) at the intensity 1010–5×1012W∕cm2. The conversion efficiency increased with laser intensity and reached the maximum value at about 1011W∕cm2 for all colors. It was found that an edge effect appears more strongly at the ω-laser case, indicating more lateral energy loss, while it appears only weakly for higher harmonics. Shorter-wavelength lasers generated significant conversion efficiencies even at lower laser energies; that is, with smaller laser spots. As the wavelength decreased from ω, 2ω, and 3ω, a spectral hump appeared in the extreme ultraviolet band around 13.5nm region, while the spectral intensity at 10.8nm drastically decreased. High-energy photon generation in the tail of 10.8nm peak was found to be strongly suppressed at shorter-wavelength laser (3ω), while the conversio...

Extreme ultraviolet source using a forced recombination process in lithium plasma generated by a pulsed laser

An extreme ultraviolet source having a tamper has been studied. This target scheme recombines forcedly lithium ions by low temperature electrons from the tamper, converting Li3+ rapidly to excited Li2+ which emit intense 1s-2p Lyman α emissions at 13.5nm. A strong 13.5nm emission appeared at 20–30ns after the time of laser peak within a small space volume near the tamper. The authors obtained an enhancement of extreme ultraviolet conversion efficiency by a factor of about 2 with the tamper against that of a target without the tamper at the same laser irradiation condition.

Effect of Laser-Plasma X-Ray Irradiation on Crystallization of Amorphous Silicon Film by Excimer Laser Annealing

The effect of laser plasma soft X-ray (LPX) irradiation on crystallization by excimer laser annealing (ELA) was investigated at low ELA energy densities. The crystalline fraction at energy densities of 50 and 60 mJ/cm2 for LPX followed by ELA is nearly equal to that at 80 to 100 mJ/cm2 for the ELA method with non-LPX irradiation. The results obtained indicate that LPX irradiation before ELA reduces the critical energy density for the start of crystallization. The combined method of LPX irradiation and ELA will enable us to realize a low-temperature process for ELA crystallization.

High average and high peak brightness slab laser

A high average and high peak brightness Nd:YAG MOPA laser system composed of a laser-diode-pumped Nd:YAG master oscillator, flash-lamp-pumped slab power amplifiers and a phase conjugated mirror was developed. The system demonstrates an average output power of 235 W at a repetition rate of 320 Hz and a peak power of 30 MW at a pulse duration of 24 ns with M/sup 2/=1.5. Both an average brightness of 7/spl times/10/sup 9/ W/cm/sup 2//spl middot/sr and a peak brightness of 1/spl times/10/sup 15/ W/cm2/spl middot/sr are achieved simultaneously. The system design rules that we confirmed suggest that by replacing lamp pumping in the amplifier with laser-diode pumping, an average output power of /spl sim/1 kW can be obtained at /spl sim/1 kHz with a higher average brightness of /spl sim/3/spl times/10/sup 10/ W/cm/sup 2//spl middot/sr and a higher peak brightness of /spl sim/3/spl times/10/sup 15/ W/cm/sup 2//spl middot/sr.

Soft x-ray emissions from laser plasma of cryogenic mixture targets

Soft x-ray spectral radiation from Xe–CO2 mixture cryogenic targets with a Xe mole fraction of 0% (pure CO2) to 100% (pure Xe) irradiated by a 1 μm pulse laser at a laser intensity IL of 0.3–1.0×1012 W/cm2 has been observed. The x-ray conversion efficiency per Xe mole fraction was found to have maximum values at the Xe fractions of 10% for IL=6.0×1011 W/cm2 and 5% for IL=1.2×1012 W/cm2, which were about five and ten times as large as that in a pure Xe target, respectively. The x-ray conversion efficiencies in the cryogenic mixture targets at Xe fractions of 10%–40% were about 1.3 %/sr/nm for λ=10.8 nm and IL=1012 W/cm2, which was as high as that in a pure Xe cryogenic target. The physical mechanism behind this enhanced emission is discussed.

Effect of Laser Plasma X-ray Irradiation on Nucleation in Amorphous Silicon Film

The effects of laser plasma X-ray (LPX) irradiation on amorphous silicon (a-Si) film structure and the density of states around the edge of the valence and conduction bands were investigated for low-temperature crystallization. An LPX with a photon energy of 115 eV was generated by a Nd:YAG excitation laser using solid Xe. The Raman spectra of a-Si films were not changed by LPX irradiation. However, the optical transmittance of a-Si film was increased by LPX irradiation at approximately 570 nm. This phenomenon relates to the movement of Si atoms to the quasi-nuclei phase in a-Si film.

Soft x-ray spectra from laser heated lithium targets

The extreme ultraviolet emission characteristics of laser-produced lithium plasma are experimentally and theoretically investigated. Taking into account the target oxidation, the ion fractional populations are analyzed and the averaged plasma parameters are obtained. Theoretical results show an insignificant influence of oxygen on the temperature dependence of the lithium ion fractional number. A comparison between the theoretical spectrum for stationary and homogenous plasma and experiments shows a reasonable agreement.