Yuki Nakai

Pure-tin microdroplets irradiated with double laser pulses for efficient and minimum-mass extreme-ultraviolet light source production

Laser-driven expansion of pure-tin microdroplets was demonstrated to produce an efficient and low-debris extreme-ultraviolet (EUV) light source. The pre-expansion is indispensable for resolving the considerable mismatch between the optimal laser spot diameter (∼300μm) and the diameter (∼20μm) of microdroplets containing the minimum-mass Sn fuel for generating the required EUV radiant energy (∼10mJ/pulse). Explosive expansion of microdroplets was attained by using a laser prepulse, whose intensity was at least 3×1011W∕cm2. The expanded microdroplet was irradiated with a CO2 laser pulse to generate EUV light. A combination of low density and long-scale length of the expanded microdroplet leads to a higher EUV energy conversion efficiency (4%) than that (2.5%) obtained from planar Sn targets irradiated by a single CO2 laser pulse. This scheme can be used to produce a practical EUV light source system.

Absolute evaluation of out-of-band radiation from laser-produced tin plasmas for extreme ultraviolet lithography

Out-of-band (OOB) radiation (at wavelengths longer than 130nm) from an extreme ultraviolet (EUV) light source reduces the precision of lithography. The energy of the OOB radiation from laser-produced Sn plasmas were measured by using an absolutely calibrated transmission grating spectrometer equipped with a charge-coupled device. The dependence of the OOB radiant energy on the mass and size of the tin fuel was clarified. The dominant source of the OOB radiation is peripheral heating around the laser spot via electron thermal conduction and radiation from the high-temperature EUV emission region.

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.

Development of Magnetic Separation System for Powder Separation

The interfusion of impurities such as metallic wear debris has been a problem in the manufacturing process of foods, medicines, and industrial products. Gravity separation system and membrane separation system have been widely used for powder separation, however, sufficient separation rate was never achieved. Magnetic separation is a promising method to separate magnetic particles efficiently. Magnetic separation system under wet process is used conventionally, but it has some demerits such as necessity of drying treatment after separation and difficulty of running the system in the cold region. Thus, magnetic separation under dry process is prospective as an alternative method. In this paper, we developed High Gradient Magnetic Separation (HGMS) system under dry process. The present major problem of dry HGMS system is the blockage of magnetic filter caused by particle coagulation or deposition. To solve this problem, we developed the apparatus in which arrangement of magnetic filter is variable. Using this apparatus, we optimized the filter arrangement and achieved high separation efficiency of impurities from the mixed powder (0.1 wt% content of impurities) without blocking of filters.

High Gradient Magnetic Separation of Pneumatic Conveyed Powder Products

The interfusion of ferromagnetic impurities such as metallic wear debris has been one of the problems in the manufacturing process of powder products. To remove such impurities, we developed high gradient magnetic separation system (HGMS) under dry process which has many advantages against conventional technology under wet process. The major problem of dry HGMS system was the blockage of magnetic filter caused by particle aggregation or deposition. In our previous study, we succeeded to reduce the blockage under dry condition by using prismatic-shaped filter whose slope was steeper than repose angle of the powder samples. In order to actualize the magnetic separation without blockage, we introduced the mechanism of pneumatic conveyance system of powders. The powder samples are dispersed adequately, and high separation efficiency can be obtained with developed system.

Optimum laser-produced plasma for extreme ultraviolet light source

The optimum plasma conditions of extreme ultraviolet (EUV) light source for lithography were experimentally clarified that is optically thin and minimum mass Sn plasma generated from a limited size target. Sn plasma is quite opaque for EUV light of 13.5 nm in wavelength, therefore 13.5 nm light emitted from deep within a Sn plasma is strongly absorbed, thus optically thin plasma production is essential for efficient EUV generation. Contamination of EUV optics caused by debris emanated from laser irradiated Sn targets is a serious problem in the Sn based EUV source system. Target residue around the laser spot is the dominant source of neutral debris, which can be reduced with supplying the minimum mass target containing the minimum number of Sn atoms required for sufficient EUV generation. Spectral purity of generated EUV light is an important requirement to expose clear mask image on a photo-resist film without chromatism. Out-of-band radiation in the vacuum ultraviolet range is dominantly radiated from the laser spot peripheral. Target size must be equal to the laser spot size to suppress the out-of-band radiation.

Comparative and quantitative study of neutral debris emanated from tin plasmas produced by neodymium-doped yttrium-aluminum-garnet and carbon dioxide laser pulses

Amount of neutral debris emanated from extreme ultraviolet light source must be minimized to maximize its lifetime. Emanation of neutral atomic debris was experimentally investigated using laser-induced-fluorescence technique for carbon dioxide (CO2, 10.6 μm in wavelength) and Nd-doped yttrium-aluminum-garnet (Nd:YAG, 1.064 μm) lasers irradiated tin foils. Total number of neutral atomic debris from CO2 laser-irradiated tin foils was 1/100 times smaller than that from Nd:YAG irradiated ones. Competitiveness of CO2 laser was revealed in terms of debris mitigation.

Characterization of extreme ultraviolet emission from tin- droplets irradiated with Nd:YAG laser plasmas

EUV emission from spherical and planer targets were precisely characterized as an experimental database for use in EUV source generation at high repetition rates. In the single-shot base experiments, conversion efficiency as high as those for the plasma geometry has been demonstrated. The integrated experiment was made with 10 Hz plasma generation.

Characterization of extreme ultraviolet emission from tin-droplets irradiated with Nd:YAG laser plasmas

EUV emission from spherical and planer targets were precisely characterized as an experimental database for use in EUV source generation at high repetition rates. In the single-shot base experiments, conversion efficiency as high as those for the plasma geometry has been demonstrated. The integrated experiment was made with 10 Hz plasma generation, obtained conversion efficiency is low mainly due to unstable positioning of the droplets.