Hajime Tanuma

Plasma physics and radiation hydrodynamics in developing an extreme ultraviolet light source for lithography

Extreme ultraviolet(EUV)radiation from laser-produced plasma (LPP) has been thoroughly studied for application in mass production of next-generation semiconductor devices. One critical issue for the realization of an LPP-EUV light source for lithography is the conversion efficiency (CE) from incident laser power to EUVradiation of 13.5-nm wavelength (within 2% bandwidth). Another issue is solving the problem of damage caused when debris reaches an EUV collecting mirror. Here, we present an improved power balance model, which can be used for the optimization of laser and target conditions to obtain high CE. An integrated numerical simulation code has been developed for the target design. The code agrees well with experimental results not only for CE but also for detailed EUV spectral structure. We propose a two-pulse irradiation scheme for high CE, and reduced ion debris using a carbon dioxide laser and a droplet or a punch-out target. Using our benchmarked numerical simulation code, we find a possibility to obtain CE up to 6–7%, which is more than twice that achieved to date. We discuss the reduction of ion energy within the two-pulse irradiation scheme. The mitigation of energetic ions by a magnetic field is also discussed, and we conclude that no serious instability occurs due to large ion gyroradius.

Modeling of radiative properties of Sn plasmas for extreme-ultraviolet source

Atomic processes in Sn plasmas are investigated for application to extreme-ultraviolet (EUV) light sources used in microlithography. We develop a full collisional radiative (CR) model of Sn plasmas based on calculated atomic data using Hebrew University Lawrence Livermore Atomic Code (HULLAC). Resonance and satellite lines from singly and multiply excited states of Sn ions, which contribute significantly to the EUV emission, are identified and included in the model through a systematic investigation of their effect on the emission spectra. The wavelengths of the 4d−4f+4p−4d transitions of Sn5+ to Sn13+ are investigated, because of their importance for determining the conversion efficiency of the EUV source, in conjunction with the effect of configuration interaction in the calculation of atomic structure. Calculated emission spectra are compared with those of charge exchange spectroscopy and of laser produced plasma EUV sources. The comparison is also carried out for the opacity of a radiatively heated Sn...

Analysis of EUV spectra of Sn XIX-XXII observed in low-density plasmas in the Large Helical Device

We have measured extreme ultraviolet (EUV) spectra from highly charged tin ions in low-density and high-temperature plasmas produced in the Large Helical Device at the National Institute for Fusion Science. The EUV spectra emitted after injection of a tin pellet into a hydrogen plasma were monitored by a grazing incidence spectrometer whose wavelength resolution is about 0.01 nm. Two different types of spectral feature were measured in the 13–17 nm region depending on whether the discharge was stably sustained or underwent radiation collapse. The measured EUV spectra were analyzed by considering the difference in dominant charge states observed in the two types of spectra. Apart from the complex quasi-continuum structure around 13.5 nm, several strong lines occurring at wavelengths longer than 14 nm were found to originate from the transitions between excited states of Sn XXI and Sn XXII by comparison with the other experimental data including the charge exchange collisions experiments. Most of the strong spectral lines which were not identified as Sn XXI or Sn XXII were assigned to the resonance transitions of Sn XIX and Sn XX from comparison with the results of theoretical calculations.

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.

Properties of Kilonovae from Dynamical and Post-merger Ejecta of Neutron Star Mergers

Ejected material from neutron star mergers give rise to electromagnetic emission powered by radioactive decays of r-process nuclei, which is so called kilonova or macronova. While properties of the emission are largely affected by opacities in the ejected material, available atomic data for r-process elements are still limited. We perform atomic structure calculations for r-process elements: Se (Z=34), Ru (Z=44), Te (Z=52), Ba (Z=56), Nd (Z=60), and Er (Z=68). We confirm that the opacities from bound-bound transitions of open f-shell, Lanthanide elements (Nd and Er) are higher than those of the other elements over a wide wavelength range. The opacities of open s-shell (Ba), p-shell (Se and Te), and d-shell (Ru) elements are lower than those of open f-shell elements and their transitions are concentrated in the ultraviolet wavelengths. We show that the optical brightness can be different by >2 mag depending on the element abundances in the ejecta such that post-merger, Lanthanide-free ejecta produce brighter and bluer optical emission. Such blue emission from post-merger ejecta can be observed from the polar directions if the mass of the preceding dynamical ejecta in these regions is small. For the ejecta mass of 0.01 Msun, observed magnitudes of the blue emission will reach 21.0 mag (100 Mpc) and 22.5 mag (200 Mpc) in g and r bands within a few days after the merger, which are detectable with 1m or 2m-class telescopes.

EUV emission spectra in collisions of multiply charged Sn ions with He and Xe

Extreme ultraviolet emission spectra of multiply charged Sn ions were measured in the wavelength range 5–38 nm, following electron capture into the excited states of slow Snq + (q = 5–21) ions passing through He and Xe gas targets. Identification of the transitions was carried out by comparison with calculations using the Hebrew University Livermore Laboratory Atomic physics Code. The target and the charge-state dependences of 4d–nl (nl = 4f, 5p and 5f) and 4p–4d transitions were observed.

Formation of Helium Cluster Ions N 2Hen+, COHen+, and O 2Hen+ (n ≤ 13) in Cooled Helium Gas

Helium cluster ions have been observed when diatomic molecular ions, N 2 + , CO + , and O 2 + , are injected into a drift tube filled with helium gas cooled by liquid helium. The structures of cluster ions and three-body association rates have been discussed. Each size distribution of cluster ions X He n + ( X = N 2 , CO, and O 2 , n ≤13) in mass spectra strongly depends on the electric field strength in the drift tube. A common magic number n = 12 is observed for these three clusters, which corresponds to the icosahedral structure. Another magic number n = 2 is observed only for O 2 He n + , which is similar to O 2 Ar 2 + reported as a very stable cluster ion. For each injected ion with helium atoms, the rate coefficients at 4.4 K are estimated roughly by the extrapolation method.

The mobility of N2+ and CO+ in helium gas at 4.35 K

The ion mobilities of N2+ and CO+ in He have been measured using a liquid-helium-cooled, selected ion drift tube at 4.35 K. Mobility reaches a maximum at an effective temperature of between 400 K and 600 K, where it is in reasonable agreement with the room temperature measurements of other experimenters. Below 100 K there is a region in which the mobilities are constant, N2+ close to the polarization limit and CO+ somewhat lower, possibly due to the occurrence of collisionally induced, rotational excitation. At very low effective temperature though, below 10 K, the mobilities decrease sharply, below the polarization limit. This may be further evidence for the existence of orbiting resonance such as that suggested previously by Kojima et al. for He+ in He.

EUV spectroscopy of Xe ions from the large helical device at the National Institute for Fusion Science for stable plasmas and plasmas undergoing radiation collapse

We have observed EUV spectra of xenon ions from the large helical device (LHD) at the National Institute for Fusion Science in Toki in the wavelength range of 10–17 nm using a high resolution SOXMOS spectrometer. A small quantity of xenon gas was injected into the large helical device. In some cases, the plasma evolution was stable and a steady discharge was obtained for several seconds, but sometimes the plasma underwent radiation collapse and rapid cooling and in this situation the EUV yield was significantly increased. Investigation of the spectra showed that during the heating phase and in a stable plasma, the emission was dominated by ions with open 4s and 4p subshells, while during radiation collapse, the spectra were dominated by lines from species with open 4d subshells. From a comparison of these spectra with theoretical data from atomic structure calculations and also with charge state specific data generated at Tokyo Metropolitan University, it was possible to make tentative assignments of the strongest lines arising from 4d–4f and 4p–4d transitions in Xe XVII and XVIII.

A cryogenic electrostatic storage ring project at RIKEN

A new, compact electrostatic storage ring for atomic and molecular physics is planned to be built at RIKEN, Japan. The new ring will be cooled down below 10 K to store rotationally cold molecular ions for a long period.