Normal-incidence Multilayer Mirrors Research Articles

High-irradiance illumination system for transmission extreme ultraviolet microscopy

A high-irradiance illumination system with normal-incidence multilayer mirrors is developed. The system consists of two concave Mo/Si multilayer mirrors with aspherical substrates that provide critical illumination by imaging a laser-produced plasma (LPP) light source at a wavelength of 13.6 nm with a magnification of almost unity. By effectively correcting spherical aberration, a raytracing computation indicates that the irradiance on the sample can be increased by about four times compared to that for a conventional system with spherical mirrors. We experimentally confirmed the enhancement by combining a LPP light source with aspherical mirrors fabricated by magneto rheological finishing.

Broadband normal-incidence mirrors for a range of 111–138 Å based on an a-periodic Mo/Be multilayer structure

Broadband Mo/Be multilayer structures were designed for maximum uniform normal-incidence reflectivity in a broad range of 111–138 Å, which lies near and beyond the L2,3 absorption edge of Si. A comparison was made of the capabilities of two classes of aperiodic structures and of so-called “stack” structures, which are composed of several periodic structures with different periods stacked one over the other. Six-stack Mo/Be 80-layer structures were synthesized on concave (R = 1 m) superpolished fused silica substrates. Their absolute reflectivity was measured at 13% – 14% in the 111–138 Å optimization range using a laboratory reflectometer with a laser-plasma radiation source. The normal-incidence reflection spectra of the mirrors were recorded in the configuration of a transmission-grating spectrograph using the slowly varying quasicontinuum of a laser-driven tungsten plasma. Comparing the reflectivity data with the reflection spectra recorded with a CCD permitted estimating a decrease in the detector responsivity beyond the Si L-edge. The broadband normal-incidence multilayer mirrors facilitate the development of a high-resolution imaging spectrograph covering a usable range about the Si L-edge to characterize, for instance, the L-edge fine structures and chemical states. These mirrors will also find use in imaging solar instruments with a high spectral resolution operating aboard a spacecraft and in laboratory instruments for plasma diagnostics.

Conception of broadband stigmatic high-resolution spectrometers for the soft X-ray range

We formulate an approach to the development of stigmatic high-resolution spectral instruments for the soft X-ray range , which is based on the combined operation of normal-incidence multilayer mirrors (including broadband aperiodic ones) and grazing-incidence reflection gratings with nonequidistant grooves (so-called VLS gratings). A concave multilayer mirror serves to produce a slightly astigmatic image of the radiation source (for instance, an entrance slit), and the diffraction grating produces a set of its dispersed stigmatic spectral images. The width of the operating spectral region is determined by the aperiodic structure of the multilayer mirror and may range up to an octave in wavelength.

Aperiodic normal-incidence antimony-based multilayer mirrors in the 8 — 13-nm spectral range

The optical properties of several materials were analysed from the standpoint of fabrication of broadband normal-incidence multilayer mirrors possessing maximal uniform reflectivity in the 8–13-nm range. By solving the inverse problem of multilayer optics we show that aperiodic Sb/(B4C, Sc, Si) multilayer structures optimised for maximum uniform reflectivity in the 8–13-nm range are able to afford a normal-incidence reflectivity R∼10 % throughout this range. The best results are exhibited by the pair Sb/B4C, for which the average reflection coefficient amounts to about 13%. The dependence of optimisation result on the programmable limitation on the minimal layer thickness in the multilayer structure was numerically investigated. An empirical rule was established whereby setting the lower bound for a layer thickness at a level ∼λmin/4 (in this case, λmin = 8 nm) does not result in an appreciable lowering of attainable uniform reflectivity.

High-reflectivity Cr∕Sc multilayer condenser for compact soft x-ray microscopy

The condenser is a critical component in compact water-window x-ray microscopes as it influences the exposure time via its efficiency and the resolution via its numerical aperture. Normal-incidence multilayer mirrors can reach large geometrical collection efficiencies and match the numerical aperture of the zone plate but require advanced processing for high total reflectivity. In the present article we demonstrate large-diameter normal-incidence spherical Cr∕Sc multilayer condensers with high and uniform reflectivity. Dc-magnetron sputtering was used to deposit 300 bilayers of Cr∕Sc with a predetermined d-spacing matching the λ=3.374nm operating wavelength on spherical substrates. The mirrors show a uniform reflectivity of ∼3% over the full 58mm diameter condenser area. With these mirrors an improvement in exposure time by a factor of 10 was achieved, thereby improving the performance of the compact x-ray microscope significantly.

Measurements of the xenon density in a pulsed jet from absorption of monochromatic soft X-rays

The density distribution in a pulsed xenon gas jet emanating through cylindrical and conic (supersonic) nozzles in vacuum was investigated experimentally and theoretically for different stagnation pressures and at different instants of time. The measurements were made from absorption of monochromatic soft X-rays (SXRs) (λ=13.6 nm) produced by a nanosecond laser-plasma source. Focusing normal-incidence multilayer mirrors were employed for the monochromatisation of soft X-rays and the production of transmission jet images (X-ray absorption patterns). Numerical simulations were made of the transient outflow of xenon in vacuum taking cluster production into account. The measured absolute density values and the density distribution agree nicely with the simulations.

Recent advances in ion-assisted growth of Cr/Sc multilayer X-ray mirrors for the water window

Cr/Sc multilayer X-ray mirrors intended for normal incidence reflection in the water window wavelength range, λ =[2.4–4.4 nm] , have been grown by ion-assisted sputter deposition and characterized using soft and hard X-ray reflectivity. By extracting low-energy ions, with energies, E ion, ranging from 9 to 113 eV and with ion-to-metal flux ratios, Φ , between 0.76 and 23.1, from the sputtering plasma to the growing film, the nano-structure of the multilayer interfaces could be modified. A significantly increased soft X-ray reflectivity, using λ=3.374 nm, for Cr/Sc multilayers with layer thicknesses in the range 0.4–2.8 nm, was obtained when high ion-to-metal flux ratios, Φ Cr=7.1 and Φ Sc=23.1, and low energy ions, E ion=9 eV, were used. An experimental reflectivity of 5.5% was obtained at 76° for a multilayer with 400 bi-layers. Simulations of the reflectivity data showed that the interface widths are <0.425 nm. It could be concluded that roughness of low spatial frequency is reduced at lower ion energies than the high spatial frequency which was eliminated at the expense of intermixing at the interfaces at higher ion energies. The predicted performance of normal incidence multilayer mirrors grown at optimum conditions and designed for λ=3.374 and 3.115 nm indicates possible reflectivities of 6.5% and 14%, respectively.

Design and performance of a laser-plasma-based compact soft x-ray microscope

We describe the design of a user-friendly compact water-window x-ray microscope. The microscope is based on a λ=3.37 nm liquid-jet-target laser-plasma source in combination with a normal-incidence multilayer condenser mirror and high-resolution diffractive optics for the imaging. With its high mechanical and thermal stability, the instrument demonstrates enhanced resolution and potential for compact x-ray imaging with the quality of synchrotron-based microscopes. Furthermore, a new sample handling system, computer control, and other improvements facilitate application-oriented x-ray microscopy outside the synchrotron laboratory.

C/Si multilayer mirrors for the 25–30-nm wavelength region

We report a new material combination, C/Si, for normal-incidence multilayer mirrors in the wavelength region 25-30 nm. The multilayers, fabricated by ion-beam-sputtering deposition, were characterized by near-normal-incidence reflectance measurements by using a discharge source and a grazing-incidence monochromator. The highest measured near-normal-incidence reflectance was R = 23% (25.6 nm), R = 20% (28.3 nm), R = 25% (30.4 nm) at incident angles of 10 degrees , 12 degrees , and 4 degrees , respectively. The multilayers were also characterized by transmission electron microscopy, which revealed sharp layer interfaces and low interfacial roughness.

EUV Astrophysics with ALEXIS: The Wide View

The Array of Low Energy X-ray Imaging Sensors (ALEXIS) satellite is Los Alamos’ pathfinding small space mission achieving low cost and rapid development time for its technology demonstration and science goals. The ALEXIS satellite contains the ALEXIS telescope array, which consists of six EUV/ultrasoft X-ray telescopes utilizing normal incidence multilayer mirrors, microchannel plate detectors, and thin UV rejecting filters. Each telescope is tuned to a relatively narrow bandpass centered at either 130, 171, or 186 angstroms. Each telescope has a 33° field-of-view, and a resolution of ~ 0.25°. With each 50 s rotation of the satellite, the telescopes scan most of the anti-solar hemisphere of the sky. The spacecraft is controlled exclusively from a ground station located at Los Alamos.This paper discusses the characteristics and performance of the ALEXIS telescopes and the results from the mission in spite of the damage incurred to the spacecraft at launch.

Normal Incidence Multilayer Mirrors for Soft X-Rays.

We review first the state of the art fabrication techniques for normal incidence multilayer mirrors, and then discuss the problems such as columnar structure, thermal stability, mechanical stress, focusing properties of multilayer mirrors, film thickness distribution, and finally the applications of multilayer mirrors for the x-ray lasers and x-ray projection lithography, using mainly the data experimentally obtained.

Thermal Stability of Normal Incidence Multilayer Mirrors for the X-Ray Wavelength near CarbonK-Edge

Annealing effects in the short-period multilayers Cr3C2/C, TiC/C, Cr3C2/(B + C), and CrB2/C were studied in a wide temperature range ∼200–1200°C by x-ray scattering and cross-sectional electron microscopy. It was shown that the thermodynamic equilibrium of the layer materials at their interfaces and stabilization of layer structure by impurities and heat treatment are effective approaches to short-period multilayers with enhanced thermal stability of their structure and optical properties.

Thermal Stability of Normal Incidence Multilayer Mirrors for the X-Ray Wavelength near Carbon K-Edge

Annealing effects in the short-period multilayers Cr3C2/C, TiC/C, Cr3C2/(B + C), and CrB2/C were studied in a wide temperature range ~200-1200°C by x-ray scattering and cross-sectional electron microscopy. It was shown that the thermodynamic equilibrium of the layer materials at their interfaces and stabilization of layer structure by impurities and heat treatment are effective approaches to short-period multilayers with enhanced thermal stability of their structure and optical properties.

Imaging of laser-irradiated targets at a wavelength of 33.8 Å using a normal-incidence multilayer mirror

The morphology of the 33.8 Å emission from laser-irradiated targets was studied using a concave mirror with a W/B4C multilayer coating. The mirror had peak normal-incidence reflectance of 1.8% at a wavelength of 33.8 Å. The emissions from a variety of targets were imaged on film with a spatial resolution of 30 μm in the target plane. Radiatively heated, low-density plastic and silica foams, x-ray laser targets, and a gas-filled enclosure were imaged. Several targets were simultaneously imaged at wavelengths of 33.8 and 130 Å using two normal-incidence microscopes.

Diffraction-limited large X-ray optics

It is shown that the technologies required to produce large normal-incidence multilayer x-ray mirrors with diffraction-limited resolution are now available. Applications of these mirrors in x-ray astronomy and x-ray lithography are discussed.

Diffraction-Limited Large X-Ray Optics

It is shown that the technologies required to produce large normal-incidence multilayer x-ray mirrors with diffraction-limited resolution are now available. Applications of these mirrors in x-ray astronomy and x-ray lithography are discussed.

Schwarzschild microscope for carbon Kα radiation

A Schwarzschild objective (magnification, ×32; numerical aperture, 0.2), which has a 0.1-µm resolution within 30 µm of the object height, was designed and fabricated. We have developed new normalincidence multilayer mirrors for carbon Kα radiation (wavelength, 44.8 Å), NiCr (80-20 wt. %)/C multilayers (thickness period, 22.5 Å; number of layers, 50), which are deposited by ion-beam sputtering with the thickness distribution corrected by deposition masks. Magnified images were taken on photographic film with the Schwarzschild objective by using an electron impact carbon Kα radiation source, and a resolution of < 0.5 µm was confirmed.

Fabrication and investigation of imaging normal-incidence multilayer mirrors with a narrow-band reflection in the range λ ≃ 4.5 nm

Soft X-ray spherical normal-incidence mirrors (D = 60 mm, r = 2000 mm) with metal-carbon multilayer coatings, which provide a narrow-band reflection in the spectral range λ ∼ 4.5 nm, have been synthesized using the pulsed laser deposition technique. The peak reflectivity of the multilayers reaches 13%, and the wavelength-to-bandwidth ratio λ/Δλ ∼ 80. The spectral characteristics of the mirrors and their aperture uniformity have been evaluated using a broadband laser-plasma XUV radiation source. Discussed briefly are the applications to the investigation of complex spectra of broad-band sources (the Sun, laser-produced plasma, etc.) employing telescopes for X-ray astronomy and stigmatic high-resolution laboratory spectrometers.

Soft X-ray submicron imaging experiments with nanosecond exposure

A high throughput soft X-ray optical system was designed, constructed and tested for obtaining magnified images of nonradiating objects at wavelengths λ=175–200 Å. The optical system consists of normal incidence multilayer mirrors including a condenser and Schwarzschild objective with 10× magnification and it was applied together with laser produced plasma as an X-ray source. The magnified images with 0.7 μm resolution were produced in one shot exposure at 0.8-1 J of incident laser energy. These results show the potential for development of table-top soft X-ray microscopes with 1 nanosecond exposure and submicron spatial resolution.

Multilayer mirrors and filters for imaging the earth's inner magnetosphere

We present guidelines for the performance of multilayer mirrors and filters for imaging the earth's inner magnetosphere by isolating the 304-Å He⁺ emission. Our analysis shows that the combination of filters and multilayer mirrors for an inner magnetosphere imager must attenuate the 1216-Å Lyman α emission by six orders of magnitude and the 584-, 834-, and 1026-Å background emissions by three orders of magnitude relative to the 304-Å signal. Multilayer mirrors with the highest possible reflectance at 304 Å are needed to preserve as much of the low-intensity 304-Å signal as possible. The design of multilayer mirrors is discussed, and the results of an exhaustive computer search for the best multilayer mirror design for 304-Å radiation are presented and analyzed. The theoretical performance of these new multilayer mirror designs is significantly better than that of designs currently used for imaging the solar 304-Å emission. Promising materials for normal-incidence multilayer mirrors for 304-Å radiation are B₄C, Be, Si, or SiC used with Al, Mg, or Mg2Si. Promising materials for a filter for this application are Te passivated with Si, or a combination of Al or Si with B or C.