A compact, quasi-monochromatic laser-plasma EUV source based on a double-stream gas-puff target at 13.8 nm wavelength

A compact, high-repetition table-top EUV source, based on a gas-puff target, is presented. This source was developed in our group and is capable of emitting quasi-monochromatic radiation at 13.8 nm wavelength with the inverse relative bandwidth of 140 and pulse energies up to ～1.3 μJ/pulse at 10-Hz repetition rate. The source is debris-free, operates near the lithographic wavelengths and offers the energy density of ～0.4 mJ/cm² in each EUV pulse. These three features make the source attractive for lithographic experiments. The timing optimization of the source, its energy per pulse, spectral and spatial distributions are pointed out in detail as well as the enhancement in energy density achievable with a multi-layer collector mirror compared to a planar mirror of equal reflectivity.     

Low-energy ion emission from a xenon gas-puff laser-plasma X-ray source

We have measured low-energy ion emission from a gas-puff laser-plasma X-ray source. The ions may cause the degradation of the condenser mirror of the extreme ultra-violet projection lithography system. A 0.7 J in 8 ns Nd:YAG laser at 1.06 μm was focused onto the xenon gas-puff target with an intensity of ∼10¹² W/cm². The silicon (111) plates, placed at a distance of 32 mm from the laser-interaction region, were exposed with the xenon ions. The average ion energy was measured to be less than 50 eV with a Faraday-cup detector placed close to the silicon plates. The xenon deposition occurred in the silicon plates with a depth of less than 40 nm. The deposition density was measured with a quadrupole secondary ion mass spectrometer to be 10²¹ /cm³ after 1500 laser shots. The energy-conversion efficiency from the laser energy into the ions is ∼0.1%/4 π sr/shot. For the lithography system, if we can remove such ion bombardment completely using novel techniques such as electro-magnetic devices or gas flow curtain techniques, the lifetime of the condenser mirror will be extended significantly. Received: 20 November 2000 / Published online: 9 February 2001     

Characterization of a laser-plasma extreme-ultraviolet source using a rotating cryogenic Xe target

A laser-plasma source for extreme-ultraviolet (EUV) light that uses a rotating cryogenic solid-state Xe target has been characterized. We focused on parameters at the wavelength of 13.5 nm with 2% bandwidth required for an EUV lithography source and investigated improvements of the conversion efficiency (CE). With the drum rotating, there was an increase in CE and less fast ions compared with the case for the drum at rest. It is considered that the Xe gas on the target surface can produce optimal-scale plasma, and satellite emission lines in Xe plasma effectively increase the EUV intensity, and the ion number is decreased by the gas curtain effect. The dependence of CE on the laser wavelength, laser energy and intensity also studied. As a result, the maximum CE was 0.9% at 13.5 nm with 2% bandwidth under the optimal condition. By continuous irradiation of a Nd:YAG slab laser at a repetition rate of 320 Hz and an average power of 110 W, the target continuously generated EUV light with an average power of 1 W at 13.5 nm with 2% bandwidth. The achieved performances provide valuable information for the design of a future EUV lithography source.     

Strong temperature effect on X-ray photo-etching of polytetrafluoroethylene using a 10 Hz laser-plasma radiation source based on a gas puff target

The first results of experiments on direct photo-etching of heated PTFE using a 10 Hz X-ray source based on a laser-irradiated gas puff target are presented. X-ray radiation in the wavelength range from 6 to 20 nm was produced as a result of irradiation of a double-stream gas puff target with Nd:YAG laser pulses of energy 0.8 J and time duration 3 ns. The resulting X-ray pulses with energy of about 100–200 mJ were used to irradiate samples of PTFE to create microstructures by direct photo-etching. Strong enhancement of the photo-etching process was observed for samples heated up to 300 °C. PACS 52.38.Ph; 81.65.Cf; 61.82.Pv     

Computational optimization analysis of a gas-jet target in a laser-plasma short-wave radiation source

A computational method is developed for optimizing the xenon gas jet used as the target in a laserplasma short-wave radiation source. The method is based on numerical hydrodynamic simulation of the jet flowing from the nozzle into vacuum, followed by computation of the optimization criterion describing the observed intensity of plasma glow. The application of this method permits an unambiguous and objective choice of optical experimental geometries and flow conditions; as a result, the radiation yield can be increased by several times. The calculated results are compared with available experimental data.     

Characterization and optimization of the laser-produced plasma EUV source at 13.5 nm based on a double-stream Xe/He gas puff target

The paper describes a debris-free, efficient laser-produced plasma source emitting EUV radiation. The source is based on a double-stream Xe/He gas-puff. Its properties and spectroscopic signatures are characterized and discussed. The spatio-spectral features of the EUV emission are investigated. We show a large body of results related to the intensity and brightness of the EUV emission, its spatial, temporal, and angular behavior and the effect of the repetition rate as well. A conversion efficiency of laser energy into EUV in-band energy at 13.5 nm of 0.42% has been gained. The electron temperature and electron density of the source were estimated by means of a novel method using the FLY code. The experimental data and the Hullac code calculations are compared and discussed. The source is well suited for EUV metrology purposes. The potential of the source for application in EUV lithography was earlier demonstrated in the optical characterization of Mo/Si multi-layer mirrors and photo-etching of polymers.     

Efficient micromachining of poly(vinylidene fluoride) using a laser-plasma EUV source

In this paper an efficient micromachining of poly(vinylidene fluoride) (PVDF) by direct photo-etching with a laser-plasma EUV (extreme ultraviolet) source was demonstrated for the first time. Mass spectroscopy was employed to investigate the ablation products and revealed emission of numerous molecular species of C-containing fragments of the polymer chain. Chemical surface changes after irradiation were investigated using X-ray photoelectron spectroscopy (XPS). The XPS spectra obtained for PVDF samples, irradiated with low and high EUV fluence, indicate significant differences between chemical structures in near-surface layers. It was shown that irradiation with low fluence results in defluorination and thus carbon enrichment of the polymer in near-surface layer. In contrary, irradiation with high fluence leads to intense material ablation and hardly modifies the chemical structure of the remaining material.     

Single-optical-element soft-x-ray interferometry with a laser-plasma x-ray source

We report on a compact interferometer for the water-window soft-x-ray range that is suitable for operation with laser-plasma sources. The interferometer consists of a single diffractive optical element that focuses impinging x rays to two focal spots. The light from these two secondary sources forms the interference pattern. The interferometer was operated with a liquid-nitrogen jet laser-plasma source at lambda=2.88 nm. Scalar wave-field propagation was used to simulate the interference pattern, showing good correspondence between theoretical and experimental results. The diffractive optical element can simultaneously be used as an imaging optic, and we demonstrate soft-x-ray microscopy with interferometric contrast enhancement of a phase object.     

基于小型加速器的编码中子源成像研究

编码中子源成像可以在对中子注量率影响不大的情况下大大提高成像的准直比, 从而提高成像质量.北京大学开展了基于小型加速器的编码中子源成像技术研究工作. 不同于已有的基于反应堆的小面积编码板的研究工作, 北京大学建立了基于小型加速器的大面积编码板的编码中子源成像实验平台, 并对加速器中子源上的实验方法和数据处理进行了探索, 对比了重建算法, 获得了初步的重建照片.研究工作表明, 编码中子源成像技术可用于加速器中子源, 但重建图像质量仍须提高. 关键词： 加速器中子源 中子成像 编码源成像 图像重建     

Design of a new compact THz source based on Smith-Purcell radiation

In recent years, people are dedicated to the research work of finding compact THz sources with high emission power. Smith-Purcell radiation is qualified for the possibility of coherent enhancement due to the effect of FEL mechanism. The compact experiment device is expected to produce hundreds mW level THz ray. The electron beam with good quality is provided under the optimized design of the electron gun. Besides, the grating is designed as an oscillator without any external feedbacks. While the beam passes through the grating surface, the beam bunching will be strong and the second harmonics enhancement will be evident, as is seen from the simulation results.     

Physical and chemical modifications of PET surface using a laser-plasma EUV source

Extreme ultraviolet (EUV) radiation is the electromagnetic radiation ranging from vacuum ultraviolet to soft X-rays. A single EUV photon carries enough energy to ionize any atom or molecule. The penetration depth of the radiation in any material is very short, ranging from tens to hundreds nanometers. Intense EUV pulses can remove material from the surface or modify its morphology or/and chemical structure. In this work, the radiation from a laser-plasma EUV source based on a double-stream gas-puff target was used for surface modification of polyethylene terephthalate (PET). The PET samples were irradiated with the EUV pulses emitted from krypton plasma and focused with a gold-plated ellipsoidal collector. The spectrum of the focused radiation covered the wavelength range from 9 to 70 nm. The PET samples were irradiated for 1 s–2 min at a 10-Hz repetition rate. Surface morphology of polymer samples after irradiation was investigated using a scanning electron microscope. Changes in chemical surface structure of the irradiated samples were investigated using an X-ray photoelectron spectroscopy. Different kinds of surface microstructures were obtained depending on the EUV fluence in a single pulse and the total EUV fluence. XPS measurements also revealed a modification of the chemical structure.     

Design of a new compact THz source based on Smith-Purcell radiation

In recent years, people are dedicated to the research work of finding compact THz sources with high emission power. Smith-Purcell radiation is qualified for the possibility of coherent enhancement due to the effect of FEL mechanism. The compact experiment device is expected to produce hundreds mW level THz ray. The electron beam with good quality is provided under the optimized design of the electron gun. Besides, the grating is designed as an oscillator without any external feedbacks. While the beam passes through the grating surface, the beam bunching will be strong and the second harmonics enhancement will be evident, as is seen from the simulation results.     

X-ray source based on a compact electron linear accelerator with a thin anode

An experiment confirming the possibility of extracting a narrow energy band from a wide spectrum produced by a compact electron linear accelerator with a thin transmission anode is described. A flat crystal of pyrolytic graphite was used as a monochromator.     

Quantitative analysis of thin samples by differential absorption imaging using a laser-plasma soft X-ray source

We consider the application of the differential absorption imaging technique in the soft X-ray region. The surface-density maps of test samples containing bromine, resulting from the application of the differential absorption imaging technique, are presented. Images of the samples were obtained in an X-ray monochromatic projection imaging scheme based on a spherically bent crystal using line emission close to the $L_2$ edge of bromine from a microplasma produced by focusing a nanosecond laser on the surface of an aluminum target.     

EUV emission from solids illuminated with a laser-plasma EUV source

In this paper a possibility of performing fluorescence experiments in extreme ultraviolet (EUV) using a laboratory EUV source was demonstrated. Usually such measurements are performed using third-generation synchrotrons because of extremely low probability of fluorescence in this wavelength range. In this work a 10 Hz laser-plasma EUV source based on a gas puff target was used. Fluorescent radiation from Al and Si was registered. Despite relatively low spectral resolution, some differences in Si spectra coming from different kind of samples were revealed.     

High-resolution gamma-ray radiography produced by a laser-plasma driven electron source

An electron beam from a laser-plasma accelerator is converted into a gamma-ray source using bremsstrahlung radiation in a dense material. The gamma-ray beam has a pointlike source size because it is generated by a high quality electron beam with a small source size and a low divergence. Using this gamma-ray source, the radiography of complex and dense objects with submillimeter resolution is performed. It is the first evidence of a gamma-ray source size of a few hundreds micrometers produced with laser-driven accelerators. This size is consistent with results from Monte Carlo simulations.     

Evaluation of lab-scale EUV microscopy using a table-top laser source

High brightness Extreme Ultraviolet (EUV) sources for laboratory operation are needed in nano-fabrication and actinic (“at-wavelength”) inspection of the masks for high volume manufacturing in next generation lithography. Laser-plasma EUV sources have the required compactness and power scalability to achieve the demanding requirements. However, the incoherent emission lacks the brightness for single-shot high contrast imaging. On the other hand, fully coherent sources are considered to be unsuitable for full-field sample illumination and prone to speckles. We evaluate the capabilities of a lab-scale amplified-spontaneous-emission (ASE) EUV laser source to combine brightness and high quality imaging with full-field imaging, along with rapid acquisition and compactness.     

Sequential confocal and multiphoton laser scanning microscopy using a single photonic crystal fibre based light source

A novel approach that simplifies the laser source requirements for confocal and multiphoton laser scanning (CLSM and MPLSM) using the novel dispersion properties of photonic crystal fibre (PCF) is reported. By tuning the fs-pulsed Ti:Sapphire laser to the zero dispersion wavelength of the PCF, the spectral and temporal properties of the source are largely unaffected and hence this source can easily be used for MPLSM. Conversely, by tuning the Ti:Sapphire laser emission wavelength by less than 10 nm to anomalously pump the PCF, the resultant white-light supercontinuum source can perform CLSM. Sequential CLSM and MPLSM of a double-labelled guinea pig detrusor (smooth muscle layer) specimen is described. PACS 87.64.Tt; 87.64.Vv; 42.65.Ky     

Buried target detection based on time reversal focusing with a probe source

Buried target detection under the background of strong reverberation in shallow water is a complicated problem. As the target is buried, the echo of the active sonar is very weak and the echo-to-reverberation ratio (ERR) is quite low. In the paper, the technique of time reversal (TR) with a probe source is discussed to detect a buried target. By TR transmission, the sound wave is focused at the target and the ensonification acoustic energy at the target is maximized. By reception focusing, the echo received by each sensor is added coherently and the waveform of the transmitted signal is recovered. Finally, the matched filtering is used to detect the target and estimate the target range. The waveguide experiment provides a practical implementation guideline to apply TR to buried target detection.