Laser-plasma X-ray Source Research Articles

Ray-tracing simulations of a bent crystal X-ray optics for imaging using laser–plasma X-ray sources

Ray-tracing simulations of an optical X-ray system based on a spherically bent crystal operating in Bragg configuration for monochromatic projection imaging of thin samples are presented, obtained using a code developed for that purpose. The code is particularly suited for characterizing experimental arrangements routinely used with laser-produced plasma X-ray sources. In particular, the spatial resolution of the imaging system was investigated and a careful study of the complex pattern of the X-ray backlighting beam was performed.

Soft laser-plasma X-ray source for differential absorption imaging of tracing elements in thin samples

The differential imaging technique is particularly suitable for the detection of small concentrations of contrasts agents for biological and medical applications in samples using X-ray radiography. In this paper, we present an application of this technique using a laser-plasma soft X-ray source combined with a bent crystal. Using a Fresnel plate as a test object, we were able to obtain spatial resolutions of the order of a few tens of microns. The use of our configuration to perform differential imaging of a test-sample at the L2 edge of Br at 1,596 eV is finally demonstrated.

Micromachining of organic polymers by direct photo-etching using a laser plasma X-ray source

The first experiments on direct photo-etching of organic polymers using a laser plasma X-ray source based on a laser-irradiated gas puff target are presented. High-intensity nanosecond pulses of soft X-ray radiation in the wavelength range from about 1 to 8 nm were produced by irradiation of the xenon/helium double-stream gas puff target with laser pulses from The Prague Asterix Laser System (PALS). The resulting X-ray pulses were used to irradiate samples from organic polymers and form microstructures by X-ray direct photo-etching. The results show relatively high efficiency of X-ray direct photo-etching that could be useful for micromachining of organic polymers.

Performance Optimization of a High-Repetition-Rate KrF Laser Plasma X-Ray Source for Microlithography.

In order to develop a high-intensity laser plasma x-ray source appropriate for industrial application of x-ray lithography, experiments have been carried out using a high-repetition-rate (up to 40 Hz) excimer laser (249 nm, 300 mJ) with a power density of 2 × 1013 W/ cm2 in the laser focus. In this study emphasis is given to remedying specific problems inherent in operating the laser plasma x-ray source at high repetition rates and in its prolonged operation. Two different methods of minimizing the production of target debris are investigated. First, the use of helium as a quenching gas results in a reduction of the amount of atomic debris particles by more than two orders of magnitude with negligible x-ray absorption. Second, a tape target as opposed to a solid target reduces the production of larger debris particles by a further factor of 100. Remaining debris is stopped by an aluminized plastic or beryllium filter used to avoid exposure of the resist by plasma ultraviolet radiation. The x-ray source has been used to image x-ray transmission mask structures down to 0.3 μm onto general purpose x-ray photo-resist. Results have been analyzed with SEM. The x-ray emission spectrum of the repetitive laser plasmas created from an iron target has been recorded and the conversion efficiency of the laser light into x-rays that contribute to exposure of the resist was measured to be 0.3% over 2π sr.

A laser plasma X-ray source for the analysis of wafer surfaces by grazing emission X-ray fluorescence spectrometry

A wavelength-dispersive X-ray fluorescence spectrometer incorporating a novel X-ray source with which X-rays are generated by means of light pulses from a powerful femtosecond laser is described. The instrument has been designed in order to explore whether laser-induced X-rays can replace conventional X-ray sources in the examination of wafer surfaces for traces of contaminant elements by X-ray fluorescence spectrometry. The laser plasma source developed employs as target an ultra-thin (10–20 μm) silicon foil situated parallel to and a few mm above the wafer surface to be analyzed. The Si-K α radiation emitted from the laser plasma cloud produced on the surface of the target as a result of the interaction of the femtosecond laser pulse with the silicon foil is used for the excitation of aluminium atoms on the surface of the sample wafer.

Micromachining of organic polymers by direct photo-etching using a laser plasma X-ray source

The first experiments on direct photo-etching of organic polymers using a laser plasma X-ray source based on a laser-irradiated gas puff target are presented. High-intensity nanosecond pulses of soft X-ray radiation in the wavelength range from about 1 to 8 nm were produced by irradiation of the xenon/helium double-stream gas puff target with laser pulses from The Prague Asterix Laser System (PALS). The resulting X-ray pulses were used to irradiate samples from organic polymers and form microstructures by X-ray direct photo-etching. The results show relatively high efficiency of X-ray direct photo-etching that could be useful for micromachining of organic polymers.

Control of Micro- and Nano-Structure in Ultralow-Density Hydrocarbon Foam

The present report describes ultralow density (2~10 mg/cm3) hydrocarbon plastic (CH2) foams with a controllable micro- or nano-meter-sized lamella structure. These foams are prepared from a poly(4-methyl-1-pentene) gel with hexanol and butanol derivatives by extraction using supercritical fluid CO2. Their intended use is in laser-plasma experiments and as a laser-plasma X-ray source. Their microstructure approached to nanometer and was varied using different alcohols. The finest ultralow density foam was prepared from 2-butanol gel.

Structure of solvated Fe(CO)5: XANES and EXAFS measurements using ultrafast laser plasma and conventional X-ray sources

The structures of solvated iron pentacarbonyl, Fe(CO)5, have been measured by X-ray absorption near edge structure and extended X-ray absorption fine structure spectroscopy. Cyclohexane and benzene have been used as solvents. The results were analyzed in conjunction with theoretical molecular structures based on density functional theory calculations. The calculations as well as previously published infrared absorption data showed that between 60% and over 90% of iron pentacarbonyl molecules have C2v symmetry in aromatic solvents at room temperature. Typically, one solvent molecule interacts with the iron atom trans to the apical CO-ligand. This publication presents, to our knowledge, the first X-ray absorption fine structure measurements of chemical solutions ever performed with a laser plasma X-ray source. The results were found to be in good agreement with measurements using a micro-focus X-ray tube as well as with theoretical X-ray absorption spectra of solvated Fe(CO)5.

Brief History of Targets for High-Rep-Rate Laser-Plasma X-Ray Source-Focused on Approaches Using Xe Cryogenic Targets-

Historical review of high-rep-rate laser-plasma x-ray source is presented with being focused on approaches using Xe cryogenic targets and related issues. Recent achievements on EUV output power in international competitions and collaborations by research groups are summarized.

Ultrafast broadband laser plasma X-ray source for femtosecond time-resolved EXAFS

Key issues for the generation of broadband thermal X-ray spectrum using femtosecond laser-produced plasmas are discussed. Experiments realized at high intensity (3 × 10 18 W/cm 2 ) with INRS low and high repetition rate ultrafast laser systems show that intense broadband emission can be generated with a controlled spectrum suitable for backlighting and absorption experiments. The duration of this emission is a few times the laser pulse duration and high average X-ray power can be achieved. Sub-picosecond extended X-ray absorption fine structure experiments can now be designed with the suitable probing samples and geometry.

Conceptual Design of Soft X-ray Microscopy for Live Biological Samples

This study describes the conceptual design of a soft x-ray microscope system based on a laserbased source for biomedical application with high resolution ( ${\leq}$ 50nm). The laboratory scale soft x-ray microscope consists of high power laser plasma x-ray source and grazing incidence mirrors with high reflectivity. The laser plasma source used for developing this system employs Q-switched Nd-YAG pulsed laser. The laser beam is focused on a tantalum (Ta) target. The Wolter type I mirror was used as condenser optics for sample illumination and as objective mirror for focusing on a detector. The fabrication of the Wolter type I mirror was direct internal cutting using ultraprecision DTM. A hydrated biological specimen was put between the two silicon wafers, the center of which was $Si_3N_4$ windows of 100㎚ thickness. The main issues in the future development work are to make a stable, reliable and reproducible x-ray microscope system.

Capillary nozzles for liquid-jet laser-plasma x-ray sources

We describe a method to fabricate tapered glass nozzles suitable for liquid-jet-target generation in laser-plasma soft x-ray and extreme ultraviolet sources. In the method, a tapered nozzle is formed as an integral part of a flexible capillary glass tubing. The method makes use of inert materials, extending the possible choice of target liquids compared to current nozzles. It also provides flexibility as regards nozzle diameter and pressure, thereby allowing optimization of the target size and extending the range of applicability for the liquid-jet-target laser plasmas.

Table-top X-ray microscopy : Sources, optics and applications

We have developed the first operative compact sub-visible-resolution x-ray microscope for the water-window region (λ = 2.3 - 4A nm). The microscope is based on a 100 Hz liquid-jet-target laser-plasma x-ray source, normal-incidence multilayer condenser optics, diffractive zone plate optics and CCD detection. In the present article we emphasize the system's aspects and summarize the recent progress on the components, all aiming at the reduction of the exposure time of a few seconds, i.e., similar to bending-magnet based microscopes. This primarily includes improved laser-plasma source, improved condenser optics using Cr/Sc multilayers, and improved image handling capability using wavelet algorithms. Such compact short-exposure time microscopes would significantly increase the applicability of the technology.

Improved liquid jet laser-plasma source for X-ray microscopy

We increase the x-ray flux from a liquid-jet laser-plasma x-ray source by optimizing the target geometry. A new nozzle fabrication method allows us to produce stable microscopic liquid jets with a wide range of diameters. The improved x-ray flux is demonstrated by optimizing the diameter of an ethanol liquid-jet for our 3 ns, □ =532 nm Nd:YAG laser and measuring the flux at the □=3.37 nm C VI emission line. Preliminary data suggest that the x-ray flux can be increased by more than a factor of 4 compared to previous experiments. The goal is to significantly reduce the exposure time of our laser-plasma-based compact x-ray microscope by improving the source.

Time and space-resolved measurement of a gas-puff laser-plasma x-ray source

The dynamic interaction processes between a nano-second laser pulse and a gas-puff target, such as those of plasma formation, laser heating, and x-ray emission, have been investigated quantitatively. Time and space-resolved x-ray and optical measurement techniques were used in order to investigate time-resolved laser absorption and subsequent x-ray generation. Efficient absorption of the incident laser energy into the gas-puff target of 17%, 12%, 38%, and 91% for neon, argon, krypton, and xenon, respectively, was shown experimentally. It was found that the laser absorption starts and, simultaneously, soft x-ray emission occurs. The soft x-ray lasts much longer than the laser pulse due to the recombination. Temporal evolution of the soft x-ray emission region was analyzed by comparing the experimental results to the results of the model calculation, in which the laser light propagation through a gas-puff plasma was taken into account.

A laser-plasma source for CCD calibration in the soft X-ray range

A CCD calibration system based on a laser-plasma X-ray source has been developed. The system is particularly suited for CCD calibration in the soft X-ray spectral range and allows the user to continuously tune in energy the incident radiation. This last issue is of great interest when studying the X-ray response of Si detectors near the K-edge. We report on the use of this facility for calibration of a back-illuminated CCD for single-photon spectroscopy of laser-plasma emission.

Is the increased relative biological effectiveness of high LET particles due to spatial or temporal effects? Characterization and OER in V79-4 cells

The efficiency of producing biological damage varies with radiation quality. Conventional explanations rely on spatial differences in the radiation track structure; generally however there are also very large temporal differences in delivery of the radiation at the cellular level. High-LET radiation normally deposits substantial amounts of energy by individual heavily ionizing tracks on a timescale of the order of picoseconds. By contrast each low-LET radiation track deposits a small amount of energy. Many of these tracks, distributed over the whole cell, are required to deliver an equivalent dose to a high-LET track and they are usually delivered over much longer timescales (typically seconds) during which chemical, biochemical and biological processes are occurring.In this paper the design, characterization and initial application of a high-brightness, laser-plasma ultrasoft x-ray source is described. This has been used to investigate the importance of the temporal differences by irradiating mammalian cells with an energy deposition with spatial properties of low-LET radiation and temporal properties similar to high-LET radiation. The present system delivers a typical dose, to the incident surface of the cells, of 0.12 Gy per pulse delivered in <10 ps. The capabilities of the x-ray source were tested by determining the survival of V79-4 hamster cells irradiated with picosecond pulses of ultrasoft x-rays under aerobic and anaerobic conditions, which were found to be consistent with previously published non pulsed data with x-rays of similar energy. These results support the expectation that the disappearance of an oxygen effect for high-LET radiation particles is due to their spatial properties rather than the very short timescale of each particle traversal. For other effects, particularly non-targeted phenomena such as induced genomic instability, expectations may be less clear cut.

Projecting picosecond lattice dynamics through x-ray topography

A method for time-resolved x-ray diffraction studies has been demonstrated. As a test case, coherent acoustic phonon propagation into crystalline InSb is observed using a laser plasma x-ray source. An extended x-ray topogram of the semiconductor’s surface was projected onto a high spatial resolution x-ray detector and acoustic phonons were excited by rapidly heating the crystal’s surface with a femtosecond laser pulse. A correlation between the spatial position on the x-ray detector and the time of arrival of the laser pulse was encoded into the experimental geometry by tilting the incident laser pulse with an optical grating. This approach enabled a temporal window of 200 ps to be sampled in a single topogram, thereby negating the disadvantages of pulse-to-pulse fluctuations in the intensity and spectrum of the laser-plasma source.

Liquid-jet laser-plasma X-ray sources for microscopy and lithography

We review the development of compact laser-plasma soft x-ray sources based on microscopic liquid-drop or liquid-jet targets. It is shown that such sources provide practically debris-free, high-flux operation at water-window and EUV wavelengths. This regenerative and solid-density target system holds promise for the generation of high-average power using high-repetition-rate lasers. Application of the source to compact x-ray microscopy, multi layer-op tics characterization and EUV lithography is briefly discussed.

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.