Tsuyoshi Ando

Optimum laser pulse duration for efficient extreme ultraviolet light generation from laser-produced tin plasmas

Conversion efficiencies (CEs) from incident laser energy to 13.5nm light within a 2% bandwidth were measured with changing laser pulse durations for laser-produced tin plasmas. Experimental results indicate that the optimum pulse duration is determined by two parameters: one is the optical depth of tin plasma for 13.5nm light and the other is laser absorption rate in 13.5nm emission-dominant region. The maximum CE of 2.2% is obtained with pulse duration of 2.3ns.

Low-density tin targets for efficient extreme ultraviolet light emission from laser-produced plasmas

Influence of initial density of tin (Sn) targets has been quantitatively investigated for efficient extreme ultraviolet light emission from laser-produced plasmas. With a decrease in the initial density, conversion efficiency (CE) from incident laser energy to output 13.5nm light energy in a 2% bandwidth increases; 2.2% of the peak CE was attained with use of 7% low-density SnO2 targets (0.49g∕cm3) irradiated with a Nd:YAG laser, of which wavelength, pulse duration, and intensity are, respectively, 1.064μm, 10ns, and 5×1010W∕cm2. The peak CE is 1.7 times higher than that obtained with the use of solid density Sn targets. Experimental results may be attributed to the influence of the initial density and/or microstructure of the targets on expansion dynamics of the plasmas.

Conversion efficiency of extreme ultraviolet radiation in laser-produced plasmas

A simple analytical model is presented for the conversion of laser beam energy into extreme ultraviolet radiation. The model is compared with experimental results to show good agreements under different conditions of the laser wavelength λL, the laser intensity SL, the pulse duration tL, and the target atomic number Z0. It turns out that relatively high conversion efficiencies are obtained when the Planck optical thickness of the plasma is τ≈0.3–0.5, which is attained under an optimized combination of SL and tL once λL is fixed. The λL scaling on the conversion efficiency is derived.

Neutral Debris Mitigation in Laser Produced Extreme Ultraviolet Light Source by the Use of Minimum-Mass Tin Target

Neutral tin (Sn) atoms expanding from laser-produced Sn plasmas, called neutral atomic Sn debris, were characterized for application to the extreme ultraviolet (EUV) lithography. Laser-induced-fluorescence (LIF) technique was used to observe spatial distribution and temporal evolution of the neutral atomic debris. Dependence of LIF intensity on number of the neutral atomic Sn debris was calibrated by coupling with 13.5-nm EUV light backlight technique. Dominant source of the neutral atomic debris was found to be periphery of the laser spot heated thermally by high-temperature EUV source plasma. Sufficient conversion efficiency from driver laser energy to 13.5-nm light one was obtained with a very thin Sn dot coated on a glass substrate. With the use of such a minimum-mass Sn target, total amount of the neutral atomic debris can be reduced down to 1% of that from bulk Sn target.

Properties of EUV and particle generations from laser-irradiated solid- and low-density tin targets

Properties of laser-produced tin (Sn) plasmas were experimentally investigated for application to the Extreme Ultra-Violet (EUV) lithography. Optical thickness of the Sn plasmas affects strongly to EUV energy, efficiency, and spectrum. Opacity structure of uniform Sn plasma was measured with a temporally resolved EUV spectrograph coupled with EUV backlighting technique. Dependence of the EUV conversion efficiency and spectra on Sn target thickness were studied, and the experimental results indicate that control of optical thickness of the Sn plasma is essential to obtain high EUV conversion efficiency and narrow spectrum. The optical thickness is able to be controlled by changing initial density of targets: EUV emission from low-density targets has narrow spectrum peaked at 13.5 nm. The narrowing is attributed to reduction of satellite emission and opacity broadening in the plasma. Furthermore, ion debris emitted from the Sn plasma were measured using a charge collector and a Thomson parabola ion analyzer. Measured ablation thickness of the Sn target is between 30 and 50 nm for the laser intensity of 1.0 x 1011 W/cm2 (1.064 μm of wavelength and 10 ns of pulse duration), and the required minimum thickness for sufficient EUV emission is found to be about 30 nm under the same condition. Thus almost all debris emitted from the 30 nm-thick mass-limited Sn targets are ions, which can be screened out by an electro-magnetic shield. It is found that not only the EUV generation but also ion debris are affected by the Sn target thickness.

The de Haas-van Alphen effect in a Kondo system

Abstract A theory of the Kondo effect on the de Haas-van Alphen oscillations is presented. Results of our calculations may explain some characteristics left unsolved in the experiments on dilute Zn Mn and Cu Cr alloys.

Fine Structures of Laser-Driven Punched-Out Tin Fuels Observed with Extreme Ultraviolet Backlight Imaging

The laser-driven punched-out scheme, which has been proposed for supplying minimum-mass tin fuels at a high repetition rate, was investigated for use in an extreme ultraviolet (EUV) light source. The density profile of punched-out tin fuel is a critical parameter for validating applicability of this scheme. An EUV backlight technique was developed to measure the density profile and fine structure. Experimental results reveal that a punched-out target does not remain its initial thin shape, but expands into a rarefied gas and liquid and small particles under the present target and laser conditions.

Energy spectra and charge states of debris emitted from laser-produced minimum mass tin plasmas

Laser-produced Sn plasma is an efficient extreme ultraviolet (EUV) light source, however the highest risk in the Sn-based EUV light source is contamination of the first EUV collection mirror caused by debris emitted from the Sn plasma. Minimum mass target is a key term associated with relaxation of the mirror contamination problem. For design of the optimum minimum mass Sn target, opacity effects on the EUV emission from the laser-produced Sn plasma should be considered. Optically thinner plasma produced by shorter laser pulse emits 13.5 nm light more efficiently; 2.0% of conversion efficiency was experimentally attained with drive laser of 2.2 ns in pulse duration, 1.0 × 1011 W/cm2 in intensity, and 1.064 μm in wavelength. Under the optimum laser conditions, the minimum mass required for sufficient EUV emission, which is also affected by the opacity, is equal to the product of the ablation thickness and the required laser spot size. Emission properties of ionized and neutral debris from laser-produced minimum mass Sn plasmas have been measured with particle diagnostics and spectroscopic method. The higher energy ions have higher charge states, and those are emitted from outer region of expanding plasmas. Feasibility of the minimum mass target has been demonstrated to reduce neutral particle generation for the first time. In the proof-of-principle experiments, EUV emission from a punch-out target is found to be comparable to that from a static target, and expansion energy of ion debris was drastically reduced with the use of the punch-out target.

Structural Characterization of High-Quality ZnS Epitaxial Layers Grown on GaAs Substrates by Low-Pressure Metalorganic Chemical Vapor Deposition

Structural properties of ZnS epitaxial layers grown on GaAs (001) substrates by low-pressure metalorganic chemical vapor deposition have been studied by high-resolution X-ray diffraction, transmission electron microscopy (TEM) and atomic force microscopy (AFM) measurements. The full width at half maximum (FWHM) of the ZnS (004) diffraction curves decreased with increasing layer thickness and the value determined for an 8-µm-thick ZnS layer was as narrow as 30 arcsec. The decrease in FWHM indicated the improvement of the crystalline uniformity of the ZnS epitaxial layer. Cross-sectional TEM measurement enabled us to observe a large number of stacking faults and microtwins in the vicinity of the interface between ZnS and GaAs. In addition, the decrease in the stacking-fault density with increasing layer thickness was observed, and the stacking-fault density for a 6-µm-thick ZnS surface was estimated to be about 5×107 cm-2. On the other hand, a large number of rhombus-shaped holes were observed on the ZnS surface. An increase in the size and a decrease in the density of these holes were observed with increasing layer thickness. Hence, the root-mean-square value of ZnS surface roughness increased with increasing layer thickness. Cross-sectional AFM measurement indicated that the rhombus-shaped holes on the ZnS surface had a reverse-pyramidal shape formed by four symmetric {113} facets.

Extreme Ultraviolet Emission from Laser-Irradiated Low-Density Xe Targets

The effect of the initial density of xenon (Xe) targets on extreme ultraviolet (EUV) emission has been investigated. With decreasing initial density, it was found that the spectral width around 11 nm becomes broad, and the intensity of 13.5 nm emission increases. The maximum conversion efficiency (CE) for solid Xe targets is approximately 0.6% at a 2×1011 W/cm2 of laser intensity, whereas that for the low-density targets is approximately 0.7% at a 4×1011 W/cm2. The spectral broadening with decreasing initial density can be attributed to the change in optical thickness of laser-produced plasmas. The enhancement of CE is attributed to optically thick plasma formation.