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.
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.
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.