At Laser-Laboratorium Gottingen different laser-plasma sources were tested, which are going to be used for characterization of optical components and sensoric devices in the wavelength region from 11 to 13nm. In all cases EUV radiation is generated by focussing a Q-switched Nd:YAG laser into a gas puff target. By the use of xenon or oxygen as target gas, broadband as well as narrowband EUV radiation is obtained, respectively. Different types of valves and nozzles were tested in order to optimize the emitted radiation with respect to maximum EUV intensities, small source diameters and pointing stability. The investigation of these crucial source parameters was performed with specially designed EUV pinhole cameras, utilizing evaluation algorithms developed for standardized laser beam characterization. In addition, a rotatable pinhole camera was developed which allows spatially and angular resolved monitoring of the soft X-ray emission characteristics. With the help of this camera a strong angular dependence of the EUV intensity was found. The results were compared with fluorescence measurements for visualization of the target gas jet. To explain these results a theoretical model was developed, including the absorptance of the EUV radiation in the surrounding target gas.
This paper describes a simple method for formation of quasi-monochromatic soft x-ray radiation from laser-produced plasmas. The method uses a special combination of laser target materials and x-ray filters. Targets with middle atomic numbers Za~10-25 are chosen, so hydrogen- and helium-like ions ([H]- and [He]-like ions) are excited in the laser-produced plasmas. The x-ray K-absorption filters isolate only a few lines in the spectrum: resonance lines and corresponding satellites of [H]- and [He]-like ions. These lines occupy a narrow spectral band λ/δλ=10-100. The contribution of continuum radiation out of the separated spectral band is studied theoretically and experimentally. It is shown that the continuum radiation contribution does not exceed 20% of the line radiation intensity. The method of formation of quasi-monochromatic soft x-ray radiation is used for reflectometry (measurements of crystal reflectivity, calibration of crystal spectrometers and x-ray detectors) and for radiometry (absolute radiatin yield measurements) of laser-produced plasmas.
A spectroscopic investigation was made of a debris-free soft X-ray radiation source driven by the pulses of a solid-state laser (0.4 J, 6 ns, 1.08 microns) focused in a pulsed xenon jet. Source images at a wavelength of 180 Å were obtained using a concave soft X-ray multilayer mirror. To obtain space-resolved source emission spectra above 125 Å, advantage was taken of a stigmatic high-transmission broadband diffraction spectrograph. The spectrograph comprised a large-aperture transmission diffraction grating (1000 or 5000 lines/mm) and a novel aperiodic focusing normal-incidence multilayer mirror possessing a uniform reflectivity in the 125 -250 Å range. The yield of soft X-ray radiation was determined with the aid of a fast absolute-calibrated X-ray AXUV-5 photodiode. The photoabsorption in the peripheral gas-jet regions was found to play a significant role in the soft X-ray yield. Numerical model simulations were performed to elucidate the plasma dynamics.
The soft X-ray spectrograph was also employed to study the interaction of the pulsed gas jet with the incident stream of the plasma produced by laser irradiation of a ~1-cm distant solid target. The soft X-ray spectra arising from the interaction were attributed to the charge exchange of multiply charged plasma ions with gas jet atoms.
In this contribution we describe a laser plasma source for Extreme Ultraviolet Lithography (EUVL) based on a Xe-cluster target. Although Xe-clusters as target systems for EUVL are known for some time, no attempts have been made for a systematic study of the influence of the laser parameters on the EUV-emission at a well defined Xe-aggregation.
The MBI burst mode laser used offers some unique features: Within one burst (duration 800 μs) the repetition rate of single laser pulses can be adjusted between 30 and 1000 kHz. The average power per burst is about 5 kW at the maximum energy of 4 J/burst. The pulse duration of a single pulse can be adjusted from the ps- to ns-range.
We have examined the EUV-emission from the Xe-cluster target within one burst of the laser as a function of single pulse intensity and repetition rate. Based on the measured EUV-spectra the conversion efficiency at 13.4 nm wavelength in dependence on pulse duration in the range from 30 ps to 3 ns were estimated.
X-ray spectra of Cu plasmas at the focus of a four-beam, solid-state diode-pumped laser have been recorded. This laser-plasma X-ray source is being developed for JMAR's lithography systems aimed at high- performance semiconductor integrated circuits. The unique simultaneous overlay of the four sub-nanosecond laser beams at 300 Hertz produces a bright, point-plasma X-ray source. PIN diode measurements of the X-ray output indicate that the conversion efficiency (ratio of X-ray emission energy into 2π steradians to incident laser energy) was approximately 9 percent with average X-ray power yields of greater than 10 Watts. Spectra were recorded on calibrated Kodak DEF film in a curved-crystal spectrograph. A KAP crystal (2d = 26.6 Angstroms) was used to disperse the 900 eV to 3000 eV spectral energies onto the film. Preliminary examination of the films indicated the existence of Cu and Cu XX ionization states. Additional spectra as a function of laser input power were also recorded to investigate potential changes in X-ray yields. These
films are currently being analyzed. The analysis of the spectra provide absolute line and continuum intensities, and total X-ray output in the measured spectral range.
The fabrication and the fundamental study of a repetitive nanosecond x-ray generator having a sealed field emission x-ray tube is described. A compact Marx generator storing 12 Joules directly drives a field emission tube with voltage pulses > 380 kV and with < 4 nanosecond risetime from an equivalent generator-impedance of 52 Ω. A numerical model is used in which the x-ray tube's cathode width and anode-cathode gap (AK) spacing are permitted to change with time while electron flow between the cathode and anode is space-charge-limited (SCL) and nonrelativistic. Coupling this model to an equivalent circuit representation of the Marx generator, which includes the voltage variation of the BaTiO3 Marx capacitors, an estimation of the cathode current, anode-cathode potential and the x-ray spectrum was obtained and compared with measured values.
Absolute intensities of spectra in a dense-plasma-focus (DPF) source have been recorded and analyzed. This DPF source has been identified as one of the more promising sources for X-ray lithography. The source, developed by Science Research Laboratory, Inc., is currently undergoing testing and further development at BAE Systems, Inc. The DPF operates at 60 Hz and produces an average output pulse of ~5 J of X rays into 4π steradians in a continuous operation mode. In all runs, there was an initial number of pulses, typically between 30 to 40, during which the X-ray output increased and the DPF appeared to be undergoing a conditioning process, and after which a "steady-state" mode was achieved where the average X-ray power was relatively constant. Each spectral run was exposed to ~600 J of output, as
measured by the PIN. The X-ray spectral region between 0.8 and 3 keV was recorded on Kodak DEF film in a potassium acid phthalate (KAP) convex curved-crystal spectrograph. The source emits neon line radiation from Ne IX and Ne X ionization stages in the 900 to 1300 eV region, suitable for lithographic exposures of photoresist. Two helium-like neon lines contribute more than 50% of the total energy. From continuum shape, plasma temperatures were found to be approximately 170-200 eV. The absolute, integrated spectral outputs were verified to within 30% by comparison with measurements by a PIN detector and a radiachromic X-ray dosimeter.
A short pulse X-ray generation experiment was performed by laser-Compton scattering through interaction between a 3 ps, 14 MeV electron beam and 100 fs, 85 mJ laser photons in a 90° scattering configuration. The observed X-ray intensity was typically 30,000 photons/pulse and roughly matched the theoretically expected intensity. The X-ray energy and pulse duration were estimated theoretically to be 2.3 keV and 280 fs from the observed electron and laser beam parameters. The pulse to pulse fluctuation of the X-ray output was measured as 25% (rms) during 30-minute operation. The fluctuation was expressed as a function of the fluctuation of the timing between the electron and laser beams. The measured fluctuation of the X-rays was approximately consistent with that caused by the fluctuation of each beam. We are improving the system to achieve the photon energy to 15 keV and the intensity to 10 million photons/pulse by increasing the electron beam energy to 40 MeV and the laser pulse energy to 1 J/pulse, and to stabilize the X-ray intensity to be better than 3% by improving the timing fluctuation between beams to be less than 300 fs. The amplitude fluctuation of each electron and laser beams are improved to be less than 1% to contribute to the achievement of the targeted 3% X-ray fluctuation.
A compact, high-repetition rate, ultrashort-pulse laser-driven hard-x-ray source based on the combination of a femtosecond laser system with an x-ray diode is demonstrated. A comparison with available laser-plasma hard-x-ray sources is presented. Hard-x-ray fluxes exceeding 1010 photons/s (emitted in 4π) are realized at a repetition rate of 250 kHz. Numerical modeling is performed which proves that picosecond and sub-picosecond hard-x-ray pulses can be produced with this source.
Choosing the best performing x-ray optic for a specific x-ray fluorescence application is tricky since x-ray fluorescence requirements can vary from extremely small x-ray beams for spatially resolved measurements to intense, highly monochromatic beams for locating trace amounts of impurities in samples. Additionally, due to the wide variety of commercially available x-ray optics to choose from, each providing different outputs, one has to decide which optic suits one's application requirements best. Two such optics, a doubly curved crystal (DCC) optic and a monolithic polycapillary focusing optic, were examined for use in micro-beam x-ray fluorescence and low level impurity detection. In these two measurements, intense output beams are needed. With the two optics examined, the average CuKa x-ray intensities were 6 x 107 photons/sec-μm2 for the polycapillary optic and 8 x 105 photons/sec-μm2 for the DCC optic from a 20W sealed-tube, 120μm diameter Cu source. Thus, with an extremely low power source, the polycapillary output intensity was almost 100 times more than from the DCC optic. Because the spot sizes from the two optics were different, a better intensity comparison is insertion gain, which showed the polycapillary optic had an 8 times higher insertion gain than the DCC optic. In addition to intensity, lowering the minimum detectable limit (MDL) in x-ray fluorescence measurements requires highly monochromatic x-ray beams. Of the two optics examined, the DCC optic (with a 0.37mm Ni filter) produced an x-ray beam that would detect about a 20% lower impurity concentration than the polycapillary optic (with a 0.44mm Ni filter). In addition, the DCC optic's MDL can be improved, since this optic produces a highly monochromatic beam by diffraction, eliminating the need for a filter. On the other hand, the polycapillary optic transmits polychromatic x rays, requiring a filter to create a monochromatic beam from the polycapillary output, thereby reducing the characteristic line intensity. Thus we found the
X rays emitted over a large angular range from conventional, laboratory-based sources can be transformed into a parallel beam or focused onto a small sample area to give efficient utilization of small sources for powder diffraction. For optimal system design, it is important that source parameters be well characterized. Source to window distance, spot size, intensity, and uniformity were measured for an Oxford Ultrabright Molybdenum source. Two polycapillary optics, a weakly focusing, and a collimating, optic were also characterized in detail. Measurements of x-ray diffraction data have been assessed for silicon and organic powders, and agree well with parameters predicted from the source and lens characterization.
Two-dimensional multilayer optics have been widely used for enhancing and monochromatizing x-ray beams for various diffraction applications. However, when they are applied to Mo Kα radiation, the performance suffers from the fact that multilayer optics can use only a very small portion of the source. This is because the rocking curve of a multilayer becomes narrower at higher energy. Comparing to the optics for the Cu Kα radiation, the throughput of the optics for Mo Kα depends heavily on how small the source is. Based on the theoretical ray-tracing study, we have designed, fabricated and tested a system combining a microfocusing x-ray source and a side-by-side multilayer optic. Initial test results agree well with the theoretical expectation. The flux gain for a sample smaller than 100 micrometers is about 9 fold compared to a system composing of 2 kW sealed tube and graphite monochromator. The resolution of a diffractometer can also be improved by configuring the optical path. This paper will discuss the system design, detailed comparison between this system and a sealed tube-graphite monochromator based system, and possible applications such as small molecule diffraction system. A theoretical comparison to a rotating anode based system will also be discussed.
The potential use of polycapillary optics could provide extraordinarily high spatial resolution imaging of radioactive sources for a new generation of gamma cameras is being investigated. A series of images from 125I brachytherapy seeds in Lucite phantoms display resolution better than 0.1 mm and signal to noise ratios in excess of a factor of 10. Such "cellular" level resolution might allow early stage location of prostate tumors, and be used to study their size, shape, and rate of growth. Even before being developed into the compact size needed for transrectal investigation of prostate cancer, imaging detectors using such high spatial resolution polycapillary angular filters may be valuable for small animal model studies and other research.
This paper is about the beam divergences of polycapillary optics. The definitions and measurement methods of polycapillary optics beam local divergence and global divergence are given. Factors like source spot size, optic input focal distance etc. for determining the beam local divergences are analyzed. Some simulation and experimental results for the polycapillary optics, which are used for X-ray diffraction and X-ray lithography, will also be presented.
We report on recent developments of innovative grazing incidence X-ray optics for laboratory applications. These efforts focus on the developments of grazing incidence micromirrors with ellipsoidal and paraboloidal profiles and apertures well below 1 mm, as well as on the developments of innovative arrangements and approaches such as X-ray collimators based on the Lobster Eye X-ray lenses. The applications of these elements will be indicated and discussed. Moreover, we will discuss the innovative alternative technologies for X-ray optics elements in study and-or under considerations
Novel X-ray sources and applications may be achieved with a combination of gateable electron micro-sources and tailored electron targets. A simple, broad area X-ray source can be constructed in a biplanar geometry, one side consisting of a low atomic number, X-ray transmissive substrate with an array of emitters, and the other side a simple metallic target. The simple metallic target can be replaced with a composite target in which different areas of the surface are coated with different X-ray emitting metals. The resulting X-ray spectrum will be the composite of spectra of all the irradiated metals. The spectrum is selected with sets of separately gated field emitter arrays, each registered to a respective anode metal. Low voltage electronics control the gate array selections. With the further addition of electron focusing within the tube and an external X-ray detector, the device becomes capable of imaging the composition of the electron target. By augmenting the device with a sample handling capability, whereby the sample is put in the location of the target, this instrument then becomes capable of X-ray compositional analysis in a manner analogous to an electron microprobe or SEM with EDX attachments. Such instruments can be miniaturized and used for automated analysis systems. The potential for low power, automated analysis by small, unmanned, distributed systems could augment the capabilities already present with high power laboratory instrumentation. An important technical issue on which the practicality of these developments depends, is the robustness of gated field emitter sources. Recent progress in this area is described.