High-speed laser remote sensing of defects inside a concrete specimen was demonstrated. In the proposed measurement setup, high-power laser pulses irradiated a concrete surface to generate vibration that can be detected by an optical interferometer, which was constructed using photorefractive crystal. The laser-based remote sensing system achieved inspection speeds of 25 Hz. The predominant frequency of a mock-up defect that was embedded in a concrete specimen was measured. The inspection result was identical to that obtained using a conventional hammering method.
Koichi Kasuya, S. Ozawa, T. Norimatsu, H. Azechi, K. Mima, S. Nakai, S. Suzuki, B. Budner, W. Mroz, N. Kasuya, W. Kasuya, Kei. Kasuya, Y. Izawa, H. Furukawa, Y. Shimada, T. Yamanaka, M. Nakai, K. Nagai, K. Yokoyama, K. Ezato, M. Enoeda, M. Akiba, A. Prokopiuk
The most recent fundamental research results to investigate surface erosions of nuclear fusion candidate
chamber materials are described in short. We used a commercial surface profiler with a red semiconductor
laser. Various material surfaces ablated and eroded by a rather short pulse electron beam and a short pulse
ArF laser light were measured with this surface profiler and the associated three-dimensional analysis
software. Threshold input levels for various sample surface erosions with electron and laser beams were
clearly decided for the first time with our new method in this article. After the above fundamental results were
gathered, the methods to inspect inner surface conditions of nuclear fusion reactor chambers were newly
proposed with various kinds of laser displacement sensors. The first one is the erosion monitor with the above
profiler, and the second one is the laser induced ultrasonic wave detection method to inspect deeper surface
layers than the first one.
We propose a new scheme for high conversion efficiency from laser energy to 13.5 nm extreme ultra violet emission
within 2 % band width, a double pulse laser irradiation scheme with a tin droplet target. We consider two-color lasers, a
Nd:YAG laser with 1.06 µm in wavelength as a prepulse and a carbon dioxide laser with 10.6 µm in wavelength for a
main pulse. We show the possibility of obtaining a CE of 5 - 7 % using a benchmarked radiation hydro code. We have
experimentally tested the new scheme and observed increase of CE greater than 4 %. We show many additional
advantages of the new scheme, such as reduction of neutral debris, energy reduction of debris ions, and decrease of out
of band emission. We also discuss debris problems, such as ion sputtering using newly developed MD simulations, ion
mitigation by a newly designed magnetic coil using 3-PIC simulations and tin cleaning experiments.
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.
For EUV lithography the generation of clean and efficient light source and the high-power laser technology are key issues. Theoretical understanding with modeling and simulation of laser-produced EUV source based on detailed experimental database gives us the prediction of optimal plasma conditions and their suitable laser conditions for different target materials (tin, xenon and lithium). With keeping etendue limit the optimal plasma size is determined by an appropriate optical depth which can be controlled by the combination of laser wavelength and pulse width. The most promising candidate is tin (Sn) plasma heated by Nd:YAG laser with a pulse width of a few ns. Therefore the generation technology of clean Sn plasma is a current important subject to be resolved for practical use. For this purpose we have examined the feasibility of laser-driven rocket-like injection of extremely mass-limited Sn or SnO2 (punched-out target) with a speed exceeding 100m/s. Such a mass-limited low-density target is most preferable for substantial reduction of ion energy compared with usual bulk target. For high average power EUV generation we are developing a laser system which is CW laser diode pumped Nd:YAG ceramic laser (master oscillator and power amplifier system) operating at 5-10 kHz repetition rate. The design of practical laser for EUV source is being carried out based on the recent performance of >1 kW output power.
It is very effective for mass-limited tin-foil targets to adapt for the EUV source. Tin-foil targets in account of formation, size, and thickness have been developed for debris mitigation. The amount of ions from targets is 40 % decreased tin-foil targets of 1μm or 5μm thickness than tin-bulk targets. The ion velocity is one order of magnitude less than bulk targets. The EUV emission spectra of tin-foil are more narrowing than bulk targets. The targets supply for high repetition rate of 10 kHz is applied for a novel method. It is called "Punch-out" method. The flight of graphite foil that it is a test targets was succeed to observe by using a gated ICCD camera. The target velocity is achieved to be about 120 m/s. This value can be applied for targets supply with high repetition rate of 10 kHz.
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.
Extreme ultraviolet (EUV) emission from laser produced tin plasma was investigated for 1064, 532 and 266 nm laser wavelengths. The EUV conversion with tin target tends to be high for shorter laser wavelength and is optimized at 4-5x1010 W/cm2 for 1064 and 532 nm. The EUV emission exhibits laser wavelength dependence in terms of angular distribution and structures of emission spectra. It is found that spectra for 532 nm and 266 nm showed spectral dips at around 13.5 nm and these dips are well replicated in computer simulations. Both the angular distribution together with the spectral dips may suggest existence of opaque plasmas surrounding the EUV emission region.
Extremely ultraviolet (EUV) light at around 13.5 nm of wavelength is the most probable candidate of the light source for lithography for semiconductors of next generation. We have been studying about the EUV light source from laser-produced plasma. Detailed understanding of the EUV plasma is required for developments of modeling with simulation codes. Several parameters should be experimentally measured to develop the important issues in the simulation codes. We focused on density profile, properties of EUV emission, and opacity of the laser-produced plasmas. We present re-cent experimental results on these basic properties of the laser-produced EUV 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.
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.
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.
In order to increase the plasma production efficiency in the laser triggered lightening experiment, it is proposed that a hyper-velocity micro-particle flow produced by a chemical explosion shall be used along the laser beam path. A preliminary experiment to verify the proposal has successfully been done in the laboratory using 100 J 80 nsec CO2 laser pulse.
Generation of a long laser-plasma channel capable of triggering and guiding an electrical discharge is a crucial issue for laser-triggering protection system. We make a long plasma channel to increase the probability of triggered lightning by laser. To produce a long laser plasma channel, we propose da new technique called hybrid plasma channel method which combines weakly and strongly ionized plasma channels to maximize laser-energy efficiency of discharge guiding. We investigate the characteristics of the hybrid plasma channels to maximize laser-energy efficiency of discharge guiding. We investigate the characteristics of the hybrid plasma channel method through several laboratory experiments. The weakly ionized channel was generated by UV laser pulses in air. As the number density of electrons in weakly ionized channel is proportional to 1.1 power of laser intensity, nitrogen and oxygen molecules can not attributed to the source of ionized plasma. It is suggested that dissociation process of impurities in air whose density is 1011 - 1012 cm-3 plays an important role in plasma formation and leader triggering effect. The 50 percent flashover voltage using the hybrid plasma channel method is lower than that without the weakly ionized plasma channel. It was also found that higher repetition rate of the plasma generation on lowers the V50 furthermore.
Laboratory and field experiments for laser triggered lightning were performed in order to induce so called triggered lightning, in which an electric leader initiates from tall objects on the ground to thunderclouds, using a plasma channel produced by CO2 lasers and Nd lasers. Significant milestone was achieved in technological developments and verification of a scientific feasibility of the techniques such as determination of laser trigger timing which lead to laser triggered natural lightning. In the laboratory experiments technologies to produced long plasma channels and characteristics of discharge process induced by plasma channels were extensively investigated. In the field experiments developed are an automatic laser trigger system using an RF burst emission by a preliminary breakdown, methods to measure lightning path, thundercloud movement and field strength, and to make a continuous plasma channel at the top of the lightning tower which acts effectively for leader initiation.
A series of field experiments for laser triggered lightning have been carried out targeting the winter thunderstorms in Fukui, Japan. A combination of a 2 kJ CO2 laser system, a 600 J Nd-glass laser and a 4th harmonics 100 mJ YAG laser was used to produce a plasma channel which are effective to trigger and guide an electrical leader from the lightning tower. A new scheme of autotriggering laser system by preliminary breakdown (PB) has been developed. The PB triggering system demonstrated its capability of irradiating the laser at the right timing for triggering the leader. On February 11th 1997, in the field experiment, the electric leader was initiated by the laser plasma using the PB triggering system for the first time. A number of diagnostics including UHF interferometers, a tower current monitor, a capacitive antennas verified the initiation of leader triggered by the laser plasma both in time and space.
A series of laboratory experiments has been conducted to investigate the initiating and guiding effects of laser plasma channel on electrical discharge. It was confirmed that the plasma channels have strong guiding effects and reduce the required electrical field strength for electrical discharges to occur. A field experimental site targeting natural lightning is being prepared to develop the thunder storm monitoring system and to test the laser and optical systems against various weather conditions. The results from the laboratory experiments and laser transmission in snowy conditions as well as attempt of initiating electrical leader will be discussed.
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