Study on EUV emission properties of laser-produced plasma at ILE, Osaka
M. NakaiHiroaki NishimuraK. ShigemoriN. MiyanagaT. NorimatsuKeiji NagaiRyoji MatsuiTakehiro HibinoT. OkunoFarshad SohbatzadehY. TaoKazuhisa HashimotoMichiteru YamauraShinsuke FujiokaHideo NagatomoVasilii ZhakhovskiiKatsunobu NishiharaShigeaki UchidaYoshinori ShimadaHiroyuki FurukawaMasahiro NakatsukaYasukazu Izawa
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Abstract:
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.Keywords:
Extreme Ultraviolet Lithography
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Light sources based on laser plasmas using tin as target material are known to provide high conversion efficiency of laser power to emission in the 13.5 nm spectral region. In addition, laser plasmas produced from microscopic droplet targets enable the utilization of the mass-limited concept which minimizes the effect of target debris produced from the laser plasma interaction. By combining the mass-limited target concept and tin as the choice of target material, we are developing an extreme-ultraviolet (EUV) light source that can supply high power while remaining essentially debris-free. This source uses tin-doped microscopic droplet liquid targets that are generated at high-repetition rates (>30 kHz), which allows convenient upward power scaling when coupled with a high averaged-power laser. Detailed studies of the radiation from this source have been made using a precision Nd:YAG laser. Broad parametric studies of the conversion efficiency along with in-band spectroscopy of this EUV source have been performed. The parametric dependence of conversion efficiency is established based on measurements made by the Flying Circus diagnostic tool and a calibrated high-resolution flat-field spectrometer. These measurements have been independently confirmed by the Flying Circus 2 team.
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One key aspect in the drive to optimize the radiative output of a laser-produced plasma for extreme ultraviolet lithography is the radiation transport through the plasma. In tin-based plasmas, the radiation in the 2% bandwidth at 13.5 nm is predominantly due to 4d-4f and 4p-4d transitions from a range of tin ions (Sn7+ to Sn12+). The complexity of the configurations involved in these transitions is such that a line-by-line analysis is, computationally, extremely intensive. This work seeks to model the emission profiles of each ion by treating the transition arrays statistically, thus greatly simplifying radiation transport modeling. The results of the model are compared with experimental spectra from tin-based laser-produced plasmas.
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The use of dual laser pulses, a lithium plasma source produces unambiguously defined line emission at 13.5 nm with almost no off-band components, which minimize unnecessary heating of EUV optics. A plasma hydrodynamic calculation evaluated the plasma expansion time of 80 ns, within which the plasma density decreased to the critical density of the 1064-nm laser pulse.
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Extreme ultraviolet (EUV) emission in the 11–15 nm wavelength range from a thin liquid water jet target under illumination with a high repetition rate, high average power laser (Nd-YLF) has been studied. To find the optimum conversion efficiency of laser light into EUV radiation, different laser parameters were applied. The laser intensity was varied between 1011 and 1015 W/cm2, and pulse duration in the range from 30 ps to 3 ns. A maximum conversion efficiency of 0.12% in 2.2% bandwidth and 4π steradian at 13 nm was achieved at a repetion rate of 250 kHz, and a strong dependence of the conversion efficiency on both laser intensity and pulse duration was found.
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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.
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The primary requirement for transport of radiation through a material media is to have estimates for the absorption coefficients at the appropriate wavelengths. For many applications, the appropriate wavelengths are determined by the temperature of the material in question. In many other cases where the source of radiation is not characterized by the local temperature, one needs a versatile source of radiation continuously covering a wavelength interval appropriate for the problem at hand.
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The potential for coherent extreme ultra-violet (EUV) light in probing laser-produced plasmas is investigated. New results are presented to demonstrate that EUV radiation can be employed to measure heat penetration into solid targets from electrons using the signature of a change of opacity due to heating. We examine, in particular, the effects of hot electron heating of targets. In addition, phase variations after transmission through a laser-irradiated target change the subsequent propagation of the radiation, suggesting a simple diagnostic measuring the far-field footprint of coherent EUV radiation can be a useful measurement of the uniformity of target heating.
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