Beam defining devices used for synchrotron X-ray and neutron scattering techniques have both common and different aspects. For instance, a common feature is that Bragg diffraction by single crystals is used to define the energy and the energy spread of beams and that similar focusing schemes apply to both neutron and X-ray optics. The scattering amplitudes of many materials are of the same order of magnitude for X-rays and neutrons giving rise to similar performances of the optical devices. On the other hand, the generally low absorption for neutrons compared to that for X-rays of the same wavelength lead to specific differences between both techniques. Besides these intrinsic interaction properties of the two radiations with matter, the properties of the sources are important for the design of beam optics. While neutron scattering is severely flux-limited and thus needs a big source size and beam divergences up to a degree, modern synchrotron X-ray sources are by many orders of magnitude smaller and emit low-divergence beams of very high intensity. As a consequence of this, mosaic crystals are used for neutron monochromatization and perfect crystals are more suitable for synchrotron radiation beams. However, for particular techniques perfect crystals are required also for neutron experiments and mosaic crystals can provide high X-ray fluxes for synchrotron radiation experiments. The problem of radiation heating which is important for synchrotron is inexistent for neutron devices.

This paper gives an overview of soft x-ray monochromators for synchrotron radiation. Showing some typical examples for different energy regions and applications, general design criteria are explained, and specific problems, limitations and future developments using undulator radiation from third generation storage rings are addressed. For a more complete information the reader is referred to various recent review articles."

We have used W/Si multilayers and multilayer/KAP crystal combinations in a double crystal monochromator beamline at BESSY. The throughput in the energy region from 200 to 1600 eV was drastically increased as compared with previous experiments and thus the performance compares favourably with other monochromators using conventional optics in this energy region. XAS spectra of the 2p edges of 3d transition metals are shown. No x-ray induced damage of the multilayers could be detected using Auger depth profile analysis after one week of exposure to the synchrotron beam. AES profiles distinguished up to the first 30 layers. Our XAS results open up the possibility of constructing multielement high resolution soft x-ray monochromators as well as high intensity broadband monochromators.

A simple two-mirror monochromator based on multilayer films has been developed for use with synchrotron radiation. Measurements of the performance of this monochromator and the characterization of a typical component mirror are described.

The theory of the reflecting properties of multilayer neutron monochromators is reviewed. It is shown that both the Bragg reflection and the mirror reflection can be understood, at least qualitatively, within the familiar kinematical theory of neutron scattering. For a detailed quantitative understanding of these phenomena, including such questions as the optimum number of bilayers, the effect of the multiple reflection that is neglected in the kinematical theory must be taken properly into account. The way in which this can be done using the dynamical theory of neutron scattering is indicated.

The neutron reflectivity spectra of Ni-aimultilayers has been measured as a function of angle of incidence and neutron wavelength in the range 1-7Å. The measurements are discussed in terms of the fabrication method employed and compared with the corresponding X-ray reflectivity spectra.

A review is given of the techniques to produce multilayer coatings for soft X-ray reflection. Current work of the improvements of the energy resolution of these systems is described. The state of the art in the production of imaging objectives is reviewed.

One of the most important considerations in the design and manufacture of multilayer mirrors is material compatibility. Ideally, the materials selected should not chemically react or interdiffuse thereby avoiding the formation of second phases, enhancing the smoothness of the deposited layer interface and hence enhancing the reflectivity of the optic. It is also possible to increase the reflectivity by decreasing the thickness of the more absorbing layers and increasing the thickness of the less absorbing layers, for any given periodicity. The optimum thickness of each layer, however, is rigorously related to the specific materials as well as the x-ray wavelength being used. The transparency of the spacing (low absorption) layers can thus be augmented by materials of low absorption that are thermodynamically stable and which deposit in a smooth continuous fashion. Whereas the use of boron as opposed to carbon as a spacing material provides lower absorption, it is typically inferior in providing as low an interfacial roughness in multilayer form. For example, in rhenium-tungsten/carbon multilayers, produced by evaporation, a typical rough-ness of <0.3 nm is obtained versus 0.7 nm for rhenium-tungsten/boron multilayers. Our initial findings with B₄C layers indicate deposits which show layer smoothness exceeding those containing carbon with the advantage of greater stability, lower absorption and therefore greater reflectivity. The manufacture, microstructure and reflectivity of W/B₄C multilayers will be the focus of this presentation.

Molecular Beam Epitaxy (MBE) is a very powerful deposition technique, capable of growing multilayer materials with atomically sharp interfaces.1 However, to date MBE techniques have not been applied to the deposition of multilayer coatings for soft x-ray applications. One reason is that almost all MBE systems in present use are designed with thermal evaporation sources and 800 °C substrate heaters for the growth of GaAs-based semiconductors. However, many of the materials needed for the fabrication of x-ray optical elements require the ability to evaporate high melting point materials such as silicon, molybdenum, tungsten, etc., as well the capability to reach substrate temperatures -1000°C during growth to obtain epitaxy. We describe here the design of a new MBE system in our laboratory, which includes a 1000 0C substrate heater and two electron beam evaporators, as well as the usual evaporation (Knudsen) cells. While conventional UHV electron beam evaporation techniques can deposit refractory materials, MBE also has the advantage of in situ monitoring during growth, using Reflected High and Low Energy Electron Diffraction (RHEED and LEED), for layer thickness control at the atomic level. Initial results on XUV optical coatings using this MBE system are presented.

We describe a new facility for characterization of the XUV optical performance of multilayer reflectors. Samples are loaded into an ultra-high vacuum, computer-controlled reflectometer consisting of in-vacuum stepper motor translation and rotation stages with relative positioning accuracies of 1µm and 0.1 mdeg, respectively. The reflectometer is used with a laboratory light source which provides collimated, monochromatic radiation from approximately 1800 Å to < 8 Å. The reflectometer is also easily transported for use with synchrotron radiation. Reflectance measurements, using a variety of detectors, can be made from grazing incidence to near-normal incidence, and the reflectometer can be rotated from horizontal to vertical while under vacuum in order to perform polarization-sensitive measurements. In addition to characterizing multilayer reflectors fabricated in our laboratory, the apparatus is well suited for the determination of optical constants using the reflectance versus incidence angle method.

A 1D Position Sensitive Detector is well-suited to the test of the short period X-ray multilayer produced for X-ray fluorescence analysis systems and for synchrotron beam lines optics. Because it gives directly the incident and reflected angular beam distributions the PSD avoids the time-consuming scanning mode necessary for the classical detectors. In such tests classical X-ray tubes have been used. Some examples of reflectivity tests for multilayer mirrors with periods ranging from 113Å to 24Å are presented. A way is indicated to improve the 0.03° angular resolution which presently limits our reflectivity measurements possibility.

We have observed that carbon tungsten multilayers smooth substrate surface defects at atomic scale. We have made experiments to understand what is the process responsible of this effect. We have realized silicon tungsten multilayer and carbon single layer to smooth different rough substrate (Li F, frosted Float-Glass). Grazing X-ray reflection (1.54 Å) and soft X-ray reflectivity (44.7 Å) measurements have been used to characterize roughness before and after deposit and in-situ kinetic ellipsometry to understand interfaces formation. Carbon appears to be the determinant factor of the smoothing process.

The reflectivities of plane Pt-Si multilayer mirrors of various d-spacings and layer thickness ratios have been measured as a function of angle at wavelengths within the soft X-ray and EUV regions. The measured performance is compared with theory and the effect of heat treating the mirrors interpreted in terms of Pt film agglomeration. The imaging characteristics of a concave Pt-Si multilayer mirror are presented.

In this paper results are presented of the deposition and characterization of Ni - C and NiSi - C multilayer coatings consisting of a large number of layers (typically 475; d = 22.3 Å) containing ultra thin metal layers (4 Å). During deposition, information about changes in the roughness of the top interface is obtained using an in situ X-ray reflectometer. Afterwards the coatings have been characterized using CuKα diffraction; the resolutions and relative reflectivities have been determined. The influence of the presence of a hydrogen atmosphere during deposition and the effect of mixing Si with Ni have been studied. Finally we report on a Ni - C multilayer coating consisting of 815 layers, having a resolution λ/Δλ of 275.

A new modular set grazing goniometer, using commercial components, for characterization of thin films or stacks is described.

The present status of studies on soft X-ray multilayers in Japan is briefly reviewed. This includes the design concepts, optical constants of substrates and thin films, fabrication techniques, evaluation methods, and some applications.

For realizing high-reflection multilayer mirrors, Mo-Si multilayers of 60 layer pairs with layer thicknesses of d_Si ≈ 7.3 nm and d_Mo - 4.0 nm are deposited by magnetron sputter-ing on super-polished Si (100) substrates with rms surface roughness of several tenth of a nm. A typical reflectance value obtained by synchrotron radiation reflectometer using p-polarization radiation was 36 % at an incident angle 10^o and a wavelength of λ = 21.0 nm. The structure and crystalline nature of these multilayers have been investigated by X-ray diffraction both at small and wide diffraction angles, transmission electron microscopy for surface and cross-sectional views, and selected area electron diffraction. These results suggest that the Si layers are amorphous whereas the Mo layers are crystalline with [110] axis oriented normal to the substrate. The grain sizes of Mo layers have approximately of the similar size as the film thickness along the direction of the film depth and approxi-mately 10 nm along the film surface. The interfaces also show several tenth to one nm rms roughness probably due to interlayer diffusion or to duplication of the substrate surface roughness. Using the results investigated in this paper, a microscopic structure of Mo-Si multilayer mirror is proposed.

Multilayer Mo/Si mirrors have been fabricated by various physical vapor deposition methods. The best mirror fabricated by RF-magnetron sputtering showed a reflectivity of 57.8% at an incident angle of 25° and at a wavelength of 12.66 nm with synchrotron radiation reflectometer. The characteristics of fabricated multilayer mirrors have been measured using transmission electron microscope for surface and cross-sectional micrographs, electron diffraction for crystalline nature, small-angle x-ray diffractometer and synchrotron radiation reflectometer for reflectivity. Particularly, the dependencies of deposition parameters of Ar pressure and input power in RF-magnetron sputtering and substrate temperature in electron beam deposition in ultra high vacuum, have been investigated. The crystalization of Mo layers is clearly admitted for the mirrors by DC-and RF-magnetron sputtering. Surface roughness is minimum for the mirrors by RF-magnetron sputtering and ion beam sputtering. A possible reason of low reflectivity for the mirrors by ion beam sputtering is discussed from the resluts of additional analysis.

A review is presented of current theoretical topics relating to the modelling of X-reflectivity from multilayer structures. The topics comprise: (1) Fresnel theory and its limitations; (2) atomic plane iterated diffraction; (3) sinusoidal layering and optical Bloch waves; (4) imperfect layers.

A new design method for soft X-ray multilayers is presented. The method utilizes a recurrence formula for the complex amplitude reflectance of a multilayer and its graphical representation in the Gaussian plane. The design procedure gives a clear insight into the evolution of the complex amplitude reflectance of a multilayer at every stage of the deposition as the multilayer builds up. The thickness of each layer is optimized layer by layer so as to attain the highest possible reflectance for a given set of coating materials with the minimum number of layers. Some examples are given to demonstrate the usefulness of the present method in the design of multilayer elements for soft X-rays.

The matricial formalism is adopted to treat the influence of the interfacial roughness. It is shown that the matrix characterizing the interface is modified in such a way that the Fresnel reflection coefficient is decreased and the Fresnel transmission coefficient is en-hanced by appropriate factors. These coefficients are formally similar to the so-called Debye-Waller factor, but they take account of the effective wavenumber of each medium. It results that a rough interface is in a way, equivalent to a homogeneous transition layer. Examples of characterization using measurements performed at 0.154 nm are given.

We present measurements of X-ray scattering from various substrates, from W/Si and W/C multilayers deposited on ordinary and superpolished substrates and diffraction measurements from these multilayers. We find no significant change in scatter after deposition of the multilayer in the cases with ordinary substrates, but there is some evidence of surface smoothing in one case with superpolished fused quartz as substrate. The measurements show that multilayers deposited on conventional Si-substrates have a mosaicity which is correlated with the scattering measurements. This correlation, however, is not found for multilayers on supersmooth substrates. Reflectivity-and Darwin curve measurements show that a well defined d-spacing is obtained in all cases. There is, however, an interesting difference between multilayers on supersmooth substrates and on ordinary substrates which cannot be explained by conven-tional imperfections.

Interfacial and substrate roughness can greatly reduce the performance of multilayer optical coatings for soft x-rays. Due to the short wavelength of soft x-ray light, multilayers with roughness on the order of a few Angstroms are required to achieve adequate performance. In the present work, roughness measurements have been made on uncoated silicon wafers and float glass using a WYKO TOPO-3D phase shifting interferometer. The silicon wafers are found to be slightly smoother than the float glass. The effects of different cleaning methods and of the deposition of silicon "buffer layers", on substrate roughness also have been examined. An acid cleaning method is presented that gives more consistent results than detergent cleaning. Healing of the roughness due to sputtered silicon buffer layers was not observed on the length scale probed by the WYKO. Sputtered multilayers have been characterized with both the WYKO interferometer and low-angle x-ray diffraction to yield information about the roughness of the top surface and of the interfaces of the multilayer. Simulations based on these measurements show the need for extremely smooth layers for certain mirrors. Preliminary results on film growth in our recently acquired Molecular Beam Epitaxy (MBE) system are also presented. Although a MBE acid cleaning method greatly increases the roughness of the substrates, Reflection High Energy Electron Diffraction (RHEED) shows this cleaning method is not necessary to achieve epitaxial growth.

The roughness of interfacial surfaces of multilayer coatings contributes significantly to the degradation of normal incidence reflectivity for the XUV wavelengths. Sub-nanometer rms roughness of the interfacial surfaces is required to appreciably reduce interfacial scattering of XUV radiation. The goal of the present work is to measure the amount of surface roughness healing which can be achieved by the deposition of thin silicon films, using dc triode sputtering, upon candidate substrates for figured XUV optics. The substrates used in the present work are float glass, semiconductor grade silicon wafers, and diamond turned aluminum flats. The surface roughness profiles are measured with a WYKO TOPO-3D phase shifting interferometer and a Scanning Tunneling Microscope (STM). This paper describes initial rms roughness measurements and autocovariance calculations for the above-mentioned substrates.

For metal-coated mirrors, the Fresnel reflection coefficient becomes significant at X-ray and XUV wavelengths only when the angle of incidence approaches ninety degrees. Hence, grazing incidence mirrors have traditionally been utilized in order to obtain adequate reflectance for X-ray and XUV imaging applications. Although very large optical surfaces are required to achieve modest collecting areas, one benefit of grazing incidence mirrors is that the image degradation effects of optical surface irregularities diminish as the cosine of the incident angle. Recent advances in multilayer optical coating technology have made the use of normal incidence X-ray/XUV mirrors feasible from a sensitivity or total reflectance standpoint for many applications of interest; however, the effects of optical fabrication errors upon image quality is not as well understood. By applying Fourier techniques and linear systems theory, we have derived an analytic expression for a generalized surface transfer function that includes the effects of optical fabrication errors over the entire range of relevant spatial frequencies. The Fourier transform of this transfer function yields the image distribution or point spread function (PSF) from which encircled energy characteristics or other image quality criteria can be predicted. This transfer function characterization allows parametric studies and sensitivity analyses to be performed which provide insight into the relative importance of surface irregularities in the various spatial frequency domains as a function of wavelength for both normal and grazing incidence configurations.

We present results of multilayer research conducted in our laboratory in early 1988. This work includes fabrication of imaging multilayer optics operating at normal incidence in the extreme ultraviolet, improvements in spatial uniformity of multilayers and in precision of achieving a selected multilayer period. An image obtained with multilayer optics on a recent solar rocket flight is also presented.

In soft x-ray fluorescence yield near edge spectroscopy (FYNES) experiments it is desirable to reduce the ratio of scattered relative to fluorescent radiation emerging from the sample. Just above an absorption edge the fluorescent and elastically scattered photons have similar energies and cannot be electronically discriminated in our proportional counter detector. For low concentration (10 ppm) samples a large detector area maximizes collected light but allows scattered radiation over a large solid angle to contribute. One approach to improve photon collection in FYNES employs a focusing multilayer mirror analyzer. This device would intercept a significant fraction of solid angle around the sampling point. Photons would arrive at near-normal incidence, be energy analyzed, reflected, and focused onto the detector element. The problem of a multilayer's low reflectivity may be counterbalanced by its large area, ability to concentrate flux and, particularly, its ability to discriminate between fluorescent and scattered light. Model multilayer mirror devices have been obtained and we have undertaken an examination of their soft x-ray performance at the National Synchrotron Light Source. The mirrors were energy calibrated and found to possess unwanted azimuthal grading of 2d spacing. In fluorescence yield tests a multilayer mirror used as an analyzer exhibited filtering behavior at special angles which utilized only small regions of the mirror surface. It is expected that improvements in mirror fabrication or reevaluation of experimental geometry will make multilayer mirror analysis the procedure of choice for low background, low concentration FYNES studies.

Multilayer reflection filters were designed for use in photo-CVD experiments with soft X-rays of ≈100 eV. Mo/Si (27 layers) and Rh/Si (21 layers) filters were fabricated on superpolished CVD-SiC substrates by means of ion-beam sputtering. Their spectral reflectances were measured at 10°-55° angles of incidence over a photon energy range of 65-115 eV. A peak reflectance of 43% and a FWIIM of ≈8 eV were found at ≈49° angle of incidence for soft X-rays of ≈100 eV. The filters were then used as a dispersive element in photo-CVD experiments at the Photon Factory. It was found that the filters could withstand strong undispersed synchrotron radiation for many hours, retaining a reflectance of ≈28%, and that they were able to deliver a narrow-band photon flux of ≈10¹⁵ photons/s at ≈100 eV into the photo-reaction chamber.

The objective of this investigation was the design and analysis of an imaging soft x-ray multilayer microscope with a spatial resolution of 0.1 microns. Since the optical aberrations of grazing incidence mirrors are large, and since x-ray scattering effects can severely degrade performance, an x-ray microscope using normal incidence multilayer optics has been designed. Recently, a normal incidence multilayer Cassegrain x-ray telescope was flown and produced high spatial resolution images of the Sun with extremely low x-ray scattering. These images clearly indicate that aplanatic imaging soft x-ray/XUV microscopes should be achievable using normal incidence multilayer optics technology. The microscope we have chosen to investigate is of the Schwarzschild configuration, which consists of two concentric spherical mirrors with their radii of curvature chosen such that the third-order spherical aberration, coma, and astigmatism are minimized. A ray tracing analysis of the optical system has been performed which indicates that diffraction-limited performance can be expected for an object height of 0.2 mm. Plans for fabrication of this imaging soft x-ray microscope will also be discussed.

We have made and tested several sliced multilayer structures that can function as transmissive x-ray optical elements (diffraction gratings, zone plates and phase gratings) at 8-keV. Both amplitude-modulating and phase-modulating devices have been produced. We show data taken in the beam of a 8 keV x-ray source that demonstrate diffraction and focusing. The diffraction patterns produced by the multilayer devices closely match theoretical predictions. Such transmissive optics have the potential for a wide application to high-resolution microscope and spectrometer systems.

We here present a new, simple and experimentally unambiguous method of assessing the reflectivity of concave X-ray multilayer mirrors. Preliminary results indicating reflectivities ≈80% of the calculated value are shown for mirrors of 100mm radius of curvature fabricated for use in a 236Å XUV laser scheme.

The design, fabrication, and preliminary characterization and test results for reflection Fabry-Perot etalon structures operating with soft X-rays is described. The etalons were designed for maximum performance near normal incidence and for 125 Å radiation, close to the silicon L absorption edge. The structures were fabricated with a dc sputtering technique and consist of two molybdenum/silicon multilayers separated by a silicon spacer. The samples have been characterized by several complimentary methods and reflectivity measurements have been made using synchrotron radiation. The reflectance has a sharp maximum (27%) for 124 Å light incident at 10^o from normal. Interference effects between the two reflectors, through the spacer, are clearly demonstrated.

During the last ten years, optics conception in the X and X-UV spectral ranges has made great strides. The use of metallic multilayers as artificial interferential reflectors enabled the design of new optics : flat or curved mirrors, polarizers and most recently soft Xray beamsplitters. These optical components possess a large transmission coefficient as well as significant reflection power. The first experimental results obtained, with carbon/tungsten beamsplitter at 1,000 eV are presented here. These results are in agreement with theoretical calculations and allow applications on the interferometry and X-ray laser cavity design grounds to be considered.