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.

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.

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 conversion efficiency at 13.5nm was as large as that at ω. This indicates that a Xe[XI] ion-rich plasma have been efficiently produced in the ablation plasma by using 3ω laser without overheating the underdense plasma responsible for extreme ultraviolet emission.

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.

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