Atsushi Sunahara

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.

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.

A collective laser thomson scattering system for diagnostics of laser-produced plasmas for extreme ultraviolet light sources

To develop a diagnostic system for laser-produced plasmas for extreme ultraviolet (EUV) light sources, collective laser Thomson scattering (LTS) was applied to laser-produced carbon plasmas to measure plasma parameters such as electron density (ne) and electron temperature (Te). Plasmas having parameters necessary for an EUV light source (ne = 1024–1025 m-3, Te = 30–50 eV) were achieved, and these parameters were successfully evaluated by a pilot diagnostic system with errors below 10%. From these results, an LTS system for diagnostics of tin plasmas for real EUV light sources was designed.

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.

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.

Theoretical simulation of extreme UV radiation source for lithography

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.

Time-resolved two-dimensional profiles of electron density and temperature of laser-produced tin plasmas for extreme-ultraviolet lithography light sources

Time-resolved two-dimensional (2D) profiles of electron density (ne) and electron temperature (Te) of extreme ultraviolet (EUV) lithography light source plasmas were obtained from the ion components of collective Thomson scattering (CTS) spectra. The highest EUV conversion efficiency (CE) of 4% from double pulse lasers irradiating a Sn droplet was obtained by changing their delay time. The 2D-CTS results clarified that for the highest CE condition, a hollow-like density profile was formed, i.e., the high density region existed not on the central axis but in a part with a certain radius. The 2D profile of the in-band EUV emissivity (ηEUV) was theoretically calculated using the CTS results and atomic model (Hullac code), which reproduced a directly measured EUV image reasonably well. The CTS results strongly indicated the necessity of optimizing 2D plasma profiles to improve the CE in the future.

Magnetized fast isochoric laser heating for efficient creation of ultra-high-energy-density states

Fast isochoric heating of a pre-compressed plasma core with a high-intensity short-pulse laser is an attractive and alternative approach to create ultra-high-energy-density states like those found in inertial confinement fusion (ICF) ignition sparks. Laser-produced relativistic electron beam (REB) deposits a part of kinetic energy in the core, and then the heated region becomes the hot spark to trigger the ignition. However, due to the inherent large angular spread of the produced REB, only a small portion of the REB collides with the core. Here, we demonstrate a factor-of-two enhancement of laser-to-core energy coupling with the magnetized fast isochoric heating. The method employs a magnetic field of hundreds of Tesla that is applied to the transport region from the REB generation zone to the core which results in guiding the REB along the magnetic field lines to the core. This scheme may provide more efficient energy coupling compared to the conventional ICF scheme.It is desirable to deposit more energy in the dense plasma core to trigger the fusion ignition. Here the authors demonstrate enhanced energy coupling from laser to plasma core by using solid targets and guiding the transport of relativistic electron beam with external magnetic field.

Control of unsteady laser-produced plasma-flow with a multiple-coil magnetic nozzle

We report an experimental demonstration of controlling plasma flow direction with a magnetic nozzle consisting of multiple coils. Four coils are controlled separately to form an asymmetric magnetic field to change the direction of laser-produced plasma flow. The ablation plasma deforms the topology of the external magnetic field, forming a magnetic cavity inside and compressing the field outside. The compressed magnetic field pushes the plasma via the Lorentz force on a diamagnetic current: ju2009×u2009B in a certain direction, depending on the magnetic field configuration. Plasma and magnetic field structure formations depending on the initial magnetic field were simultaneously measured with a self-emission gated optical imager and B-dot probe, respectively, and the probe measurement clearly shows the difference of plasma expansion direction between symmetric and asymmetric initial magnetic fields. The combination of two-dimensional radiation hydrodynamic and three-dimensional hybrid simulations shows the control of the deflection angle with different number of coils, forming a plasma structure similar to that observed in the experiment.

Production of intense, pulsed, and point-like neutron source from deuterated plastic cavity by mono-directional kilo-joule laser irradiation

This paper reports an experimental investigation of a scheme to produce an intense, pulsed, point-like, and quasi-monoenergy neutron source. In this scheme, the inner wall of a deuterated plastic spherical cavity is mono-directionally irradiated by a 2.4u2009kJ laser beam through an open-tip gold cone inserted into the cavity. The whole inner wall of the cavity is illuminated by laser light owing to multiple laser reflections, and the laser-ablated plasma stagnates near the center of the cavity, at which a several keV hot spot is generated. Thermonuclear and beam D-D fusion reactions occur in the hot spot. We have demonstrated the neutron yield exceeding 107 neutrons per pulse from au2009<100u2009μm diameter hot spot with the deuterated plastic cavity and mono-directional GEKKO-XII laser irradiation.