Michiteru Yamaura

Characterization of extreme ultraviolet emission using the fourth harmonic of a Nd:YAG laser

Characterization of an extreme ultraviolet (EUV) emission from laser-produced tin plasma was investigated for 266 and 1064nm laser wavelengths. The EUV emission exhibits a laser-wavelength dependence in terms of angular distribution and structures of emission spectra. Angular distributions expressed in a form of I(θ)∝cosαθ became α=1.3 and 0.5, respectively, for 266 and 1064nm laser wavelength. It is found that spectra from 266nm laser plasma show dips at around 13.5nm that had been well replicated in computer simulations. Both angular distribution and spectral structure at 13.5nm suggest the existence of an opaque plasma region in front of the EUV source plasma generated by 266nm radiation.

Angular distribution control of extreme ultraviolet radiation from laser-produced plasma by manipulating the nanostructure of low-density SnO2 targets

We have found that the divergence of a relatively monochromatic extreme ultraviolet (EUV) emission from a laser-produced plasma can be manipulated by changing the target morphology which is a porous low-density tin oxide (SnO2) structure. The fundamental light of a Nd-YAG laser was irradiated on the target with laser intensity of ∼1011W∕cm2 and pulse duration of 10ns. The nanostructure and density of the targets were tuned by a combination of colloidal polymer template and sol-gel processes [Gu, Nagai, Norimatsu, Fujioka, Nishimura, Nishihara, Miyanaga, and Izawa, Chem. Mater. 17, 1115 (2005)], which has a merit in large-scale preparation. When the target has an open cell nanostructure, the EUV emission directed predominantly along target normal, while a closed cell target exhibited divergent emission. The angular distribution may be affected by the orientation of the microstructured initial target, and this phenomenon can be applied to wavefront control of EUV emission.

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.

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.

Improvement of the atmospheric discharge laser-triggered ability using multiple pulses from a kilohertz KrF laser

The potential ability of lasers to control lightning can be improved by using a train of pulses with submillisecond separations. Laser-triggered experiments in a small-scale (10‐mm gap) atmospheric discharge facility show that the triggering is dramatically enhanced when a five-pulse train of sub-Joule energy is used instead of a single pulse. This effect increases rapidly as the pulse interval is reduced. It appears that at a submillisecond pulse interval, sufficient positive and negative ions survive in subsequent pulses, thus enabling easy deionization. Hence, significant plasma buildup occurs from one pulse to the next. However, this persistence of ions would appear to imply that the rate of recombination (effectively a charge transfer between ions) is considerably lower than previously believed.

Lifetime evaluation of weakly ionized plasma channel by accumulation effect of charged particles by means of laser absorption

The density of weakly ionized plasma and the lifetime for laser-triggered lightning are improved by using a KrF excimer laser with a high repetition rate of the order of kilohertz. We have studied how to create a weakly ionized plasma channel with the plasma density of 1013 cm−3 and the long lifetime of the order of milliseconds in order to trigger and guide lightning discharge efficiently. The density of a weakly ionized plasma channel is 1013 cm−3 and the plasma channel has a long lifetime of the order of milliseconds, which were achieved by utilizing the accumulation effect of charged particles generated by the high-repetition-rate laser. Moreover, it is clarified that the dominant mechanism of decay of plasma generated the high-repetition-rate laser was ion-ion recombination between N+ and O−.

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.

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.

Evaluation of weakly ionized plasma channel by accumulation effect of charged particles using high-repetition-rate KrF excimer laser

The triggering ability of the laser-triggered lightning method is improved by using a KrF excimer laser with a high-repetition-rate of kHz order. The density of a weakly ionized plasma channel is enhanced by the accumulation effect of charged particles generated by the high-repetition-rate laser. The accumulation effect of charged particles and the enhanced triggered discharge were confirmed in the case of a laser repetition rate of 1 kHz.

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.