Ryoji Matsui

Monochromatic imaging and angular distribution measurements of extreme ultraviolet light from laser-produced Sn and SnO2 plasmas

Properties of extreme ultraviolet (EUV) emission from laser-produced Sn and SnO2 plasmas were investigated. EUV emission images were taken with a monochromatic imager for 13.5nm with 4% bandwidth. It was found that the EUV emitting plasma is not formed symmetrically with respect to the target surface normal but extends toward laser incident axis. This result is consistent with the angular distribution of EUV emission peaked toward the direction nearly perpendicular to the laser axis. The asymmetric plasma can be attributed to the interaction of a long laser pulse with expanding plasma along the path of laser incidence.

Kα spectroscopy to study energy transport in ultrahigh-intensity laser produced plasmas

Abstract K α line emission from partially ionized material was studied to diagnose energy transport in ultrahigh-intensity laser produced plasmas. Dependence of the emission intensity on initial target conductivity was investigated by using a plastic target (C 2 H 3 Cl coated with CH) and a metallic target (Al coated with Mg) irradiated with a 1-TW Ti:sapphire laser pulse of 132 fs at 2×10 17 W/cm 2 . Depth of the heated region was deduced from Cl or Al He- α line intensities given as a function of overcoat thickness. No significant difference was observed between the two types of targets. Density–depth product of the hot region was typically 0.1 mg/cm 2 for both targets and the temperature of hot electrons measured by an electron spectrometer was 50 keV . The experimental results were analyzed with an integration calculation with a one-dimensional (1D) relativistic Fokker–Planck code, a 1D radiation hydrodynamics (rad-hydro) simulation, and an atomic–kinetic model developed for ionization-shifted Cl K α lines. The observed transport features are discussed.

Properties of EUV emissions from laser-produced tin plasmas

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.

Numerical study of Kα emission from partially ionized chlorine

Abstract Population kinetics and spectral synthesis codes have been developed to analyze Kα emission from partially ionized chlorine in hydrocarbon plasmas irradiated with high-intensity ultra-short laser pulses. The population kinetics processes are calculated using a bi-Maxwellian electron temperature distribution composed of fast and cold electrons. The fast electrons dominantly contribute to ionize the K-shell bound electrons. i.e., inner-shell ionization, while the cold electrons produce ionization from the outer-shell electrons. The resultant Kα emission provides a distinct spectral signature for each charge state. Also included in the calculation are the opacity effects in the framework of an escape probability method. It is shown that the Kα emission is saturated at a plasma thickness of more than a few microns. Further, we find that unresolved satellite lines overlap significantly with the corresponding parent Kα lines of the next charge state, and make a large contribution to composite spectrum. Finally, time-dependent properties of the Kα emission are also discussed briefly.

Estimation of emission efficiency for 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.

Study on EUV emission properties of laser-produced plasma at ILE, Osaka

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.

X-ray spectroscopy on energy transport and deposition in ultra-intensity laser produced plasmas

Observation of line radiation emanating from inner shell transitions in partially ionized high Z material is suggested as a diagnostic method for heating of dense plasma with an ultra-short high-intensity laser pulse. A new atomic-kinetics code specified for the analysis of the ionization-shift Kα lines was developed to derive electron temperature of the bulk plasma heated by hot electrons. Chlorine Kα to Cl15+ Heα lines from a solid planar target were observed with a 1 TW laser system at ILE Osaka. Comparison of the experimental results with the code prediction infers a ∼100 eV temperature plasma on the target surface with a depth of about 0.6 μm at 1.5×1017 W/cm2. Dependence of the temperature on laser intensity was found to be very weak.

Progress in understanding of laser-produced plasmas for EUV source

Our research aims at the physical understanding of laser-produced plasma (LPP) as an EUV source for lithography. To provide a database of EUV source this comprehensive research includes high performance laser development, target fabrication, plasma experiments, theory and simulations. We will report present progress of each subject, particularly recent experimental results of Sn and SnO/sub 2/ targets and comparison with computer simulations.