Bilayer and trilayer X-ray mirror coatings containing W, Pt, or Ir, in combination with C, C/Co, B4C, or B4C/Ni: X-ray reflectance, film stress, and temporal stability.

X-ray reflectance and film stress were measured for 12 bilayer and trilayer reflective interference coatings and compared with a single-layer Ir coating. The interference coatings comprise a base layer of W, Pt, or Ir, top layers of either C or B 4 C, and, in the case of the trilayer coatings, middle layers of either Co or Ni. The coatings were deposited by magnetron sputtering. Film stress was measured using the wafer curvature technique, while X-ray reflectance was measured at grazing incidence over the ∼0.1-10k e V energy band using synchrotron radiation. Re-measurements over a period of more than two years of both stress and X-ray reflectance were used to assess temporal stability. The X-ray reflectance of all 12 bilayer and trilayer coatings was found to be both stable over time and substantially higher than single-layer Ir over much of the energy range investigated, particularly below ∼4k e V, except near the B and C K-edges, and the Co and Ni L-edges, where we observe sharp, narrow drops in reflectance due to photo-absorption in layers containing these materials. Film stress was found to be substantially smaller than single-layer Ir in all cases as well; however, film stress was also found to change over time for all coatings (including the single-layer Ir coating). The effective area of future X-ray telescopes will be substantially higher if these high reflectance bilayer and/or trilayer coatings are used in place of single-layer coatings. Additionally, the smaller film stresses found in the bilayer and trilayer coatings relative to single-layer Ir will reduce coating-stress-driven mirror deformations. Nevertheless, as all the interference films studied here have stresses that are far from zero (albeit smaller than that of single-layer Ir), methods to mitigate such deformations must be developed in order to construct high-angular-resolution telescopes using thin mirror segments. Furthermore, unless film stress can be sufficiently stabilized over time, perhaps through thermal annealing, any such mitigation methods must also account for the temporal instability of film stress that was found in all coatings investigated here.

Pd/B4C/Y multilayer coatings for extreme ultraviolet applications near 10 nm wavelength.

A new extreme ultraviolet (EUV) multilayer coating has been developed comprising Pd and Y layers with thin B4C barrier layers at each interface, for normal incidence applications near 10 nm wavelength. Periodic, nonperiodic, and dual-stack coatings have been investigated and compared with similar structures comprising either Mo/Y or Pd/B4C bilayers. We find that Pd/B4C/Y multilayers provide higher reflectance than either Mo/Y or Pd/B4C, with much lower film stress than Pd/B4C. We have also investigated the performance of periodic multilayers comprising repetitions of Pd/Y, Ru/Y, or Ru/B4C/Y, as well as Pd/B4C multilayers deposited using reactive sputtering with an Ar:N2 gas mixture in order to reduce stress: these material combinations were all found to provide poor EUV performance. The temporal stability of a periodic Pd/B4C/Y multilayer stored in air was investigated over a period of 16 months, and a slight reduction in peak reflectance was observed. Periodic Pd/B4C/Y multilayers were also found to be thermally stable up to 100°C; at higher temperatures (200°C and 300°C) we observe a slight reduction in peak reflectance and a slight increase in multilayer period. High-resolution transmission electron microscopy and selected area diffraction of an as-deposited Pd/B4C/Y film indicates a fully amorphous structure, with interfaces that are both smoother and more abrupt than those observed in a comparable Pd/B4C multilayer in which the Pd layers are polycrystalline. The new Pd/B4C/Y multilayers are suitable for normal-incidence imaging and spectroscopy applications, including solar physics, plasma physics, high-brightness EUV light sources, and others.

Extreme ultraviolet multilayer for the FERMI@Elettra free electron laser beam transport system.

In this work we present the design of a Pd/B4C multilayer structure optimized for high reflectance at 6.67 nm. The structure has been deposited and also characterized along one year in order to investigate its temporal stability. This coating has been developed for the beam transport system of FERMI@Elettra Free Electron Laser: the use of an additional aperiodic capping layer on top of the structure combines the high reflectance with filter properties useful in rejecting the fundamental harmonic when the goal is to select the third FEL harmonic.

Capped Mo/Si multilayers with improved performance at 30.4 nm for future solar missions.

Novel capping layer structures have been deposited on periodic Mo/Si multilayers to optimize reflectance at 30.4 nm. Design, deposition and characterization of such coatings are presented. Most of the structures proposed show improved performance with respect to standard Mo/Si multilayers and are stable over time. Reflectance at 121.6 nm and in the visible spectral range have been also tested to explore the applicability of such coatings to the Multi Element Telescope for Imaging and Spectroscopy (METIS) instrument, a coronagraph being developed for the ESA Solar Orbiter platform.

Iridium/silicon multilayers for extreme ultraviolet applications in the 20-35 nm wavelength range.

We have developed an Ir/Si multilayer for extreme ultraviolet (EUV) applications. Normal incidence reflectance measurements of a prototype film tuned to 30 nm wavelength show superior performance relative to a conventional Mo/Si multilayer structure; we also find good stability over time. Transmission electron microscopy and electron dispersive x-ray spectroscopy have been used to examine the microstructure and interface properties of this system: we find amorphous Si layers and polycrystalline Ir layers, with asymmetric interlayer regions of mixed composition. Potential applications of Ir/Si multilayers include instrumentation for solar physics and laboratory EUV beam manipulation.

Realization and characterization of an XUV multilayer coating for attosecond pulses.

The experimental characterization of an aperiodic reflecting multilayer (ML) structure designed to reflect and compress attosecond pulses in the extreme ultraviolet spectral region is presented. The MLs are designed for the 75-105 eV spectral interval with suitable reflectance and phase behavior, in particular high total spectral reflectivity coupled with very wide bandwidth and spectral phase compensation. The experimental phase behavior of the multilayer has been obtained through electron photoemission signal using an innovative method that is presented and discussed in this paper. With this ML we have demonstrated pulse compression by reflection from 450 as to 130 as.

Performance, structure, and stability of SiC/Al multilayer films for extreme ultraviolet applications.

We report on the performance, structure and stability of periodic multilayer films containing silicon carbide (SiC) and aluminum (Al) layers designed for use as reflective coatings in the extreme ultraviolet (EUV). We find that SiC/Al multilayers prepared by magnetron sputtering have low stress, good temporal and thermal stability, and provide good performance in the EUV, particularly for applications requiring a narrow spectral bandpass, such as monochromatic solar imaging. Transmission electron microscopy reveals amorphous SiC layers and polycrystalline Al layers having a strong <111> texture, and relatively large roughness associated with the Al crystallites. Fits to EUV reflectance measurements also indicate large interface widths, consistent with the electron microscopy results. SiC/Al multilayers deposited by reactive sputtering with nitrogen comprise Al layers that are nearly amorphous and considerably smoother than films deposited nonreactively, but no improvements in EUV reflectance were obtained.

Extreme-ultraviolet multilayer coatings with high spectral purity for solar imaging.

Future solar experiments designed to perform solar plasma diagnostics will also be based on extreme-ultravilet observations. Multilayer (ML) optics are essential in this spectral region since these coatings have high reflectivity at normal incidence. Typically, the reflectivity curve of a ML coating has a small but finite bandwidth, and this can be a serious drawback when several spectral lines fall within the bandwidth. In fact, spectral lines emitted by different ion species can correspond to different plasma conditions. We present the design, realization, and characterization of an innovative ML structure with high reflectivity coupled with a strong rejection ratio for two adjacent spectral features. The key element is an optimized capping layer structure deposited on top of the ML that preserves the performance reflectance at the target wavelength and at the same time suppresses the reflectance at specific adjacent wavelengths. Application to the Fe xv3x10(6) K coronal emission line at 28.4 nm with rejection of the He ii Lyman-alpha line at 30.4 nm is presented.

Performance optimization of Si/Gd extreme ultraviolet multilayers.

We compare the performance, stability and microstructure of Si/Gd multilayers containing thin barrier layers of W, B(4)C, or SiN(x), and determine that multilayers containing 0.6 nm thick W barrier layers at each interface provide the best compromise between high peak reflectance in the extreme ultraviolet near lambda=60 nm and good stability upon heating. The Si/W/Gd films have sharper interfaces and also show vastly superior thermal stability relative to Si/Gd multilayers without barrier layers. We find that these structures have relatively small compressive film stresses, and show good temporal stability thus far. We measured a peak reflectance of 29.7% at lambda=62.5 nm, and a spectral bandpass of Deltalambda=9 nm (FWHM), for an optimized Si/W/Gd multilayer having a period d=32.0 nm.

Measurement of dysprosium optical constants in the 2-830 eV spectral range using a transmittance method, and compilation of the revised optical constants of lanthanum, terbium, neodymium, and gadolinium.

The optical constants beta, delta of the complex refractive index (ñ = 1-delta+ibeta) of Dy were obtained in the 2-830 eV energy range using a novel transmittance method. Si/W/Dy/W films were deposited by dc-magnetron sputtering on Si photodiode substrates, and the transmittance was characterized using synchrotron radiation. The extinction coefficients beta of Dy and the transmittance of a Si capping layer and two W interface barrier layers as functions of energy were solved simultaneously using a nonlinear optimization routine. The measured transmittances of the capping and barrier layers were primarily used as indicators for any flaws in the transmittance results. The dispersion coefficients delta of Dy were calculated using the Kramers-Kronig integral, and a complete set of beta values required for this integral was obtained by combining the present data with data from the literature. Sum rule tests on Dy show some deficiencies in the present data, which may be attributed to lower film density compared with the bulk value. Similar procedures were applied to previously measured transmittances of B4C/La, Si/Tb, Si/Nd, and Si/Gd films, where B4C or Si were used as capping layers on those reactive rare-earth films. The improved sets of transmittance values of B(4)C and Si capping layers were used as input in the optimization routine to solve for more accurate beta values of La, Tb, Nd, and Gd. The revised optical constants of these materials, tested for consistency with partial sum rules, are also reported.

High performance EUV multilayer structures insensitive to capping layer optical parameters.

We have designed and tested a-periodic multilayer structures containing protective capping layers in order to obtain improved stability with respect to any possible changes of the capping layer optical properties (due to oxidation and contamination, for example)-while simultaneously maximizing the EUV reflection efficiency for specific applications, and in particular for EUV lithography. Such coatings may be particularly useful in EUV lithographic apparatus, because they provide both high integrated photon flux and higher stability to the harsh operating environment, which can affect seriously the performance of the multilayer-coated projector system optics. In this work, an evolutive algorithm has been developed in order to design these a-periodic structures, which have been proven to have also the property of stable performance with respect to random layer thickness errors that might occur during coating deposition. Prototypes have been fabricated, and tested with EUV and X-ray reflectometry, and secondary electron spectroscopy. The experimental results clearly show improved performance of our new a-periodic coatings design compared with standard periodic multilayer structures.

Monochromatic mammography using scanning multilayer X-ray mirrors.

A prototype system for breast imaging using monochromatic X-rays has been developed using a scanning multilayer X-ray mirror in combination with a conventional mammography tube and an imaging detector. The X-ray mirror produces a monochromatic fan beam tuned near 19 keV, with an energy bandpass of approximately 1.5 keV. Rotating the mirror about the tube's focal spot in synchronization with the X-ray generator and detector enables the acquisition of monochromatic X-ray images over large areas. The X-ray mirror also can be rotated completely out of the beam so that conventional polychromatic images can be acquired using a K-edge filter, facilitating direct comparison between the two modes of operation. The system was used to image synthetic, tissue-equivalent breast phantoms in order to experimentally quantify the improvements in image quality and dose that can be realized using monochromatic radiation. Nine custom phantoms spanning a range of thicknesses and glandular/adipose ratios, each containing both glandular- and calcification-equivalent features, were used to measure contrast and signal-difference-to-noise ratio (SDNR). Mean glandular dose (MGD) was computed from measured entrance exposure, and a figure-of-merit (FOM) was computed as FOM = SDNR2/MGD in each case. Monochromatic MGD ranges from 0.606 to 0.134 of polychromatic MGD for images having comparable glandular SDNR, depending on breast thickness and glandularity; relative monochromatic dose decreases with increasing glandularity for all thicknesses. Monochromatic FOM values are higher than the corresponding polychromatic FOM values in all but one case. Additionally, the monochromatic contrast for glandular features is higher than the polychromatic contrast in all but one case as well. These results represent important steps toward the realization of clinically practical monochromatic X-ray breast imaging systems having lower dose and better image quality, including those for digital mammography, digital breast tomosynthesis, contrast-enhanced spectral mammography and other modalities, for safer, more accurate breast cancer detection, diagnosis and staging.

Laboratory-based X-ray reflectometer for multilayer characterization in the 15-150 keV energy band.

A laboratory-based X-ray reflectometer has been developed to measure the performance of hard X-ray multilayer coatings at their operational X-ray energies and incidence angles. The instrument uses a sealed-tube X-ray source with a tungsten anode that can operate up to 160 kV to provide usable radiation in the 15-150 keV energy band. Two sets of adjustable tungsten carbide slit assemblies, spaced 4.1 m apart, are used to produce a low-divergence white beam, typically set to 40 µm × 800 µm in size at the sample. Multilayer coatings under test are held flat using a vacuum chuck and are mounted at the center of a high-resolution goniometer used for precise angular positioning of the sample and detector; additionally, motorized linear stages provide both vertical and horizontal adjustments of the sample position relative to the incident beam. A CdTe energy-sensitive detector, located behind a third adjustable slit, is used in conjunction with pulse-shaping electronics and a multi-channel analyzer to capture both the incident and reflected spectra; the absolute reflectance of the coating under test is computed as the ratio of the two spectra. The instrument's design, construction, and operation are described in detail, and example results are presented obtained with both periodic, narrow-band and depth-graded, wide-band hard X-ray multilayer coatings.

Normal-incidence silicon-gadolinium multilayers for imaging at 63 nm wavelength.

Si/Gd multilayers designed as narrowband reflective coatings near 63 nm were developed. The highest peak reflectance of 26.2% at a 5 degrees incident angle was obtained at 62 nm, and the spectral bandwidth was 7.3 nm FWHM. The fits for x-ray and extreme ultraviolet reflectance data of Si/Gd multilayers indicate the possibility of silicide formation at the Si-Gd interfaces. B(4)C, W, and SiN were deposited as interface barrier layers to improve the reflectance of Si/Gd multilayers. More than an 8% increase in reflectance was observed from the interface-engineered Si/W/Gd and Si/B(4)C/Gd multilayers.

Aperiodic multilayers with enhanced reflectivity for extreme ultraviolet lithography.

We have developed novel aperiodic multilayers, covered by capping layers resistant to environmental attack, that offer superior performance for extreme ultraviolet lithography. We have designed these coatings using an optimization procedure based on an algorithm able to acquire domain knowledge inside the space of possible solutions. An integrated intensity increase of up to 2.18 times that obtained using standard periodic multilayers has been estimated. The aperiodic structures have minimal absorption in the topmost layers, which makes them especially insensitive to both the choice of capping layer material and any subsequent capping layer degradation due to oxidation or contamination. This property allows for the use of the most resilient capping layer materials available, thereby leading to a significantly improved lifetime. We have produced prototype capped aperiodic coatings and have measured their performance.

Atomic force microscopy characterization of Zerodur mirror substrates for the extreme ultraviolet telescopes aboard NASA's Solar Dynamics Observatory.

The high-spatial frequency roughness of a mirror operating at extreme ultraviolet (EUV) wavelengths is crucial for the reflective performance and is subject to very stringent specifications. To understand and predict mirror performance, precision metrology is required for measuring the surface roughness. Zerodur mirror substrates made by two different polishing vendors for a suite of EUV telescopes for solar physics were characterized by atomic force microscopy (AFM). The AFM measurements revealed features in the topography of each substrate that are associated with specific polishing techniques. Theoretical predictions of the mirror performance based on the AFM-measured high-spatial-frequency roughness are in good agreement with EUV reflectance measurements of the mirrors after multilayer coating.

SiC/Tb and Si/Tb multilayer coatings for extreme ultraviolet solar imaging.

Narrowband SiC/Tb and Si/Tb multilayers are fabricated with as much as a 23% normal-incidence reflectance near a 60 nm wavelength and spectral bandpass (FWHM) values of 9.4 and 6.5 nm, respectively. The structural properties of the films are investigated using extreme ultraviolet and x-ray reflectometry and transmission electron microscopy. Thermal stability is investigated in films annealed to as high as 300 degrees C. Because of their superior thermal stability, relatively high reflectance, and narrower spectral bandpass, Si/Tb multilayers are identified as optimal candidates for solar physics imaging applications, where the peak response can be tuned to important emission lines such as O v near 63.0 nm and Mg x near 61.0 nm. We describe our experimental procedures and results, discuss the implications of our findings, and outline prospects for improved performance.

Terbium-based extreme ultraviolet multilayers.

We have fabricated periodic multilayers that comprise either Si/Tb or SiC/Tb bilayers, designed to operate as narrowband reflective coatings near 60 nm wavelength in the extreme ultraviolet (EUV). We find peak reflectance values in excess of 20% near normal incidence. The spectral bandpass of the best Si/Tb multilayer was measured to be 6.5 nm full width at half-maximum (FWHM), while SiC/Tb multilayers have a more broad response, of order 9.4 nm FWHM. Transmission electron microscopy analysis of Si/Tb multilayers reveals polycrystalline Tb layers, amorphous Si layers, and relatively large asymmetric amorphous interlayers. Thermal annealing experiments indicate excellent stability to 100 degrees C (1 h) for Si/Tb. These new multilayer coatings have the potential for use in normal incidence instrumentation in a region of the EUV where efficient narrowband multilayers have not been available until now. In particular, reflective Si/Tb multilayers can be used for solar physics applications where the coatings can be tuned to important emission lines such as O V near 63.0 nm and Mg X near 61.0 nm.

Normal-incidence reflectance of optimized W /B(4) C x-ray multilayers in the range 1.4 nm < l > 2.4 nm.

We have fabricated W/B(4)C multilayers having periods in the range d = 0.8-1.2 nm and measured their soft-x-ray performance near normal incidence in the wavelength range 1.4

Experimental comparison of extreme-ultraviolet multilayers for solar physics.

We compare the reflectance and stability of multilayers comprising either Si/Mo, Si/Mo2C, Si/B4C, Si/C, or Si/SiC bilayers, designed for use as extreme-ultraviolet (EUV) reflective coatings. The films were deposited by using magnetron sputtering and characterized by both x-ray and EUV reflectometry. We find that the new Si/SiC multilayer offers the greatest spectral selectivity at the longer wavelengths, as well as the greatest thermal stability. We also describe the optimization of multilayers designed for the Solar-B EIS instrument. Finally, we compare experimental reflectance data with calculations and conclude that currently available optical constants cannot be used to adequately model the performance of many of these multilayers.