Uniform and graded multilayers as x-ray optical elements

A detailed comparison of the performance of uniform and graded multilayers as soft x-ray monochromators and normal incidence collectors has been made. In particular, the responses of flat depth-, and laterally graded multilayers to Al K(alpha) radiation, lambda = 8.34 A, have been computed and compared with the corresponding uniform multilayer. Furthermore, the efficiency of graded and uniform multilayers as normal incidence x-ray collectors has been calculated in terms of effective areas for parabolic reflectors tuned to the O K(alpha) line, lambda = 23.7 A. Finally, the effective areas of four strong solar emission lines in the 30-60 A region have been computed for uniform multilayers. Normal incidence multilayer mirrors are well suited for spectroheliograph type of applications.     

Survey of Ti-, B-, and Y-based soft x-ray-extreme ultraviolet multilayer mirrors for the 2- to 12-nm wavelength region

We have performed an experimental investigation of Ti-, B(4)C-, B-, and Y-based multilayer mirrors for the soft x-ray?extreme ultraviolet (XUV) wavelength region between 2.0 and 12.0 nm. Eleven different material pairs were studied: Ti/Ni, Ti/Co, Ti/Cu, Ti/W, B(4)C/Pd, B/Mo, Y/Pd, Y/Ag, Y/Mo, Y/Nb, and Y/C. The multilayers were sputter deposited and were characterized with a number of techniques, including low-angle x-ray diffraction and normal incidence XUV reflectometry. Among the Ti-based multilayers the best results were obtained with Ti/W, with peak reflectances up to 5.2% at 2.79 nm at 61° from normal incidence. The B(4)C/Pd and B/Mo multilayer mirrors had near-normal incidence (5°) peak reflectances of 11.5% at 8.46 nm and 9.4% at 6.67 nm, respectively, whereas a Y/Mo multilayer mirror had a maximum peak reflectance of 25.6% at 11.30 nm at the same angle. The factors limiting the peak reflectance of these different multilayer mirrors are discussed.     

Interface engineered ultrashort period Cr-Ti multilayers as high reflectance mirrors and polarizers for soft x rays of lambda = 2.74 nm wavelength

Cr-Ti multilayers with ultrashort periods of 1.39-2.04 nm have been grown for the first time as highly reflective, soft-x-ray multilayer, near-normal incidence mirrors for transition radiation and Cherenkov radiation x-ray sources based on the Ti-2p absorption edge at E = 452 eV (lambda = 2.74 nm). Hard, as well as soft, x-ay reflectivity and transmission electron microscopy were used to characterize the nanostructure of the mirrors. To achieve minimal accumulated roughness, improved interface flatness, and to avoid intermixing at the interfaces, each individual layer was engineered by use of a two-stage ion assistance process during magnetron sputter deposition: The first 0.3 nm of each Ti and Cr layer was grown without ion assistance, and the remaining 0.39-0.72 nm of the layers were grown with high ion-neutral flux ratios phi (phiTi = 3.3, phiCr = 2.2) and a low energy Eion (ETi = 23.7 and ECr = 21.2), ion assistance. A maximum soft-x-ray reflectivity of R = 2.1% at near-normal incidence (approximately 78.8 degrees) was achieved for a multilayer mirror containing 100 bilayers with a modulation period of 1.379 nm and a layer thickness ratio of tau = 0.5. For a polarizing multilayer mirror with 150 bilayers designed for operation at the Brewster angle, 45 degrees, an extinction ratio, Rs/Rp, of 266 was achieved with an absolute reflectivity of R = 4.3%.     

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.     

X-ray imaging by angular raster scanning

The technology of x-ray W-Al multilayer mirrors with an angular reflection width of more than 0.4 degrees at a 1.54-A wavelength is developed. On this basis an x-ray scanner is constructed. We show experimentally the possibility of object-transfer imaging with a resolution of ~20 mum.     

Atomic scale interface engineering by modulated ion-assisted deposition applied to soft x-ray multilayer optics

Cr/Sc and Ni/V multilayers, intended as normal incidence soft x-ray mirrors and Brewster angle polarizers, have been synthesized by employing a novel modulated low-energy and high-flux ion assistance as a means of engineering the interfaces between the subnanometer layers on an atomic scale during magnetron sputter deposition. To reduce both roughness and intermixing, the ion energy was modulated within each layer. The flat and abrupt interfaces yielded soft x-ray mirrors with near-normal incidence reflectances of R = 20.7% at the Sc 2p absorption edge and R = 2.7% at the V 2p absorption edge. Multilayers optimized for the Brewster angle showed a reflectance of R = 26.7% and an extinction ratio of R(s)/R(p)=5450 for Cr/Sc and R = 10% and R(s)/R(p)=4190 for Ni/V. Transmission electron microscopy investigations showed an amorphous Cr/Sc structure with an accumulating high spatial frequency roughness. For Ni/V the initial growth mode is amorphous and then turns crystalline after approximately 1/3 of the total thickness, with an accumulating low spatial frequency roughness as a consequence. Elastic recoil detection analyses showed that N was the major impurity in both Cr/Sc and Ni/V with concentrations of 15 at. % and 9 at. %, respectively, but also O (3 at. % and 1.3 at. %) and C (0.5 at. % and 1.9 at. %) were present. Simulations of the possible normal incidence reflective properties in the soft x-ray range of 100-600 eV are given, predicting that reflectivities of more than 31% for Cr/Sc and 5.8% for Ni/V can be achieved if better control of the impurities and the deposition process is employed. The simulations also show that Cr/Sc is a good candidate for mirrors for the photon energies between the absorption edges of B (E = 188 eV) and Sc (E = 398.8 eV).     

Wolter type I x-ray focusing mirror using multilayer coatings

A multilayer coating is a useful addition to a mirror in the x-ray region and has been applied to normal incidence mirrors used with soft x rays. When a multilayer coating is used on grazing incidence optics, higher performance can be achieved than without it. Cr/Sc multilayers coated on a Wolter type I mirror substrate for a soft x-ray microscope are considered. The reflectivity and effective solid angle are calculated for Wolter type I mirrors with uniform and laterally graded multilayer coatings. The laterally graded multilayer mirror showed superior x-ray performance, and the multilayer tolerances were relaxed. This multilayer mirror could be especially useful in the soft x-ray microscope intended for biological applications.     

Carbon/Titanium multilayers as soft-x-ray mirrors for the water window

C/Ti multilayers with a period thickness of 2.1-2.7 nm were produced by electron-beam evaporation in ultrahigh vacuum as soft-x-ray mirrors in the water window (lambda = 2.3-4.4 nm). For smoothing the individual interfaces and thus enhancing the total reflectance, each layer was ion polished with an Ar(+) ion beam after deposition. For a multilayer of 85 bilayers, a reflectance of approximately 11% at an angle of incidence of 59 degrees (with respect to the surface normal) by use of s-polarized radiation at a wavelength of 2.77 nm was achieved.     

High-efficiency x-ray gratings with asymmetric-cut multilayers

We present the fabrication and analysis of efficient and highly dispersive gratings for the x-ray and extreme ultraviolet (EUV) regime. We show that an asymmetric-cut multilayer structure can act as a near-perfect blazed grating. The precision and high line density are achieved by layer deposition of materials, which can be controlled to the angstrom level. We demonstrate this in the EUV regime with two structures made by cutting and polishing magnetron-sputtered multilayer mirrors of over 2000 bilayers thick, each with a period of 6.88 nm. These were cut at angles of 2.9° and 7.8° to the surface. Within the 3% bandwidth rocking curve of the multilayer, the angular dispersion of the diffracted wave was in agreement with the grating equation for elements with 7250 and 19,700 line pairs/mm, respectively. The dependence of the measured efficiency was in excellent agreement with a formulation of dynamical diffraction theory for multilayered structures. At a wavelength of 13.2 nm, the efficiency of the first-order diffraction was over 95% of the reflectivity of the uncut multilayer. We predict that such structures should also be effective at shorter x-ray wavelengths. Both the Laue (transmitting) and Bragg (reflecting) geometries are incorporated in our formalism, which is applied to the analysis of multilayer Laue lenses and focusing and dispersing Bragg optics.     

Reflection-wavelength control method for layer-by-layer controlled x-ray multilayer mirrors

A reflection-wavelength control method for a layer-by-layer controlled x-ray multilayer mirror without interface roughness is proposed. The reflection wavelength of the multiperiodic mirror is found to be simply determined by a combination ratio of periodic layers. Multiperiodic x-ray mirrors with reflectance wavelengths at 3.374 nm (C VI 1s-2p) and 3.950 nm (Ca XVIII 3d-5f) are successfully designed.     

Extreme-ultraviolet Mo/Si multilayer mirrors deposited by radio-frequency-magnetron sputtering

Mo/Si multilayer mirrors with a high reflectance at normal incidence in the 130-135-? spectral region have been deposited by rf magnetron sputtering. A new and quick technique was used to calibrate the deposition rates. Characterization by transmission electron microscopy (TEM) and grazing-incidence x-ray diffraction (XRD) indicated good thickness control of the deposition process and low interface roughness. However, the TEM and, indirectly, the XRD reflectance measurements, indicated that the interfaces are asymmetric. A brief review discussing the origin of the modulation of the Bragg peak intensities in the XRD reflectance is given. An analytical formula was derived for periodic multilayers that describes the effect of asymmetric interfaces on the amplitude of the Bragg peak modulation. Theoretically, in XRD reflectance measurements, any asymmetry in the interdiffusion of the Mo-Si interfaces results in a decrease of the usual amplitude modulation of the Bragg peaks. Extreme-ultraviolet (XUV) reflectance measurements were also made with synchrotron radiation on a new high-resolution reflectometer. The near-normal-incidence peak reflectances measured at λ = 134 ? were ~ 59% for the best multilayer mirrors. Good fits to both XRD and XUV reflectance measurements have been obtained with a model that allows for interface asymmetry.     

Surface finish requirements for soft x-ray mirrors

We have examined the correlations between direct surface-finish metrology techniques and normalincidence, soft x-ray reflectance measurements of highly polished x-ray multilayer mirrors. We find that, to maintain high reflectance, the rms surface roughness of these mirrors must be less than ~ 1 ? over the range of spatial frequencies extending approximately from 1 to 100 μm(-1)1 (i.e., spatial wavelengths from 1 μm to 10 nm). This range of spatial frequencies is accessible directly only through scanning-probe metrology. Because the surface-finish Fourier spectrum of such highly polished mirrors is described approximately by an inverse power law (unlike a conventional surface), bandwidth-limited rms roughness values measured with instruments that are sensitive to only lower spatial frequencies (i.e., optical or stylus profileres) are generally uncorrelated with the soft x-ray reflectance and can lead to erroneous conclusions regarding the expected performance of substrates for x-ray mirrors.     

Wide-angle, high-extinction-ratio, infrared polarizing beam splitters using frustrated total internal reflection by an embedded centrosymmetric multilayer

A centrosymmetric multilayer stack of two transparent thin-film materials, which is embedded in a high-index prism, is designed to function as an efficient polarizer or polarizing beam splitter (PBS) under conditions of frustrated total internal reflection over an extended range of incidence angles. The S(LH)(k)LHL(HL)(k)S multilayer structure consists of a high-index center layer H sandwiched between two identical low-index films L and high-index-low-index bilayers repeated (k times) on both sides of the central trilayer maintaining the symmetry of the entire stack. For a given set of refractive indices, all possible solutions for the thicknesses of the layers that suppress the reflection of p-polarized light at a specified angle, and the associated reflectance of the system for the orthogonal s polarization, are determined. The angular and spectral sensitivities of polarizing multilayer stacks employing 3, 7, 11, 15, and 19 layers of BaF(2) and PbTe thin films embedded in a ZnS prism, operating at lambda=10.6 microm, are presented. The 15- and 19-layer stack designs achieve extinction ratios (ER) >30 dB in both reflection and transmission over a 46 degrees -56 degrees field of view inside the prism. Spectral analysis reveals additional discrete polarizing wavelengths other than the design wavelength (lambda=10.6 microm). The 11-, 15-, and 19-layer designs function as effective s-reflection polarizers (|R(s)|(2)>99%) over a 2-3 microm bandwidth. The effect of decreasing the refractive index contrast between the H and L layers on the resulting ER is also considered.     

Atomic-precision multilayer coating of the first set of optics for an extreme-ultraviolet lithography prototype system

We present our results of coating a first set of optical elements for an extreme-ultraviolet (EUV) lithography system. The optics were coated with Mo-Si multilayer mirrors by dc magnetron sputtering and characterized by synchrotron radiation. Near-normal incidence reflectances above 65% were achieved at 13.35 nm. The run-to-run reproducibility of the reflectance peak wavelength was maintained to within 0.4%, and the thickness uniformity (or gradient) was controlled to within +/-0.05% peak to valley, exceeding the prescribed specification. The deposition technique used for this study is an enabling technology for EUV lithography, making it possible to fabricate multilayer-coated optics to accuracies commensurate with atomic dimensions.     

Narrow-bandpass multilayer mirrors for an extreme-ultraviolet Doppler telescope

We have developed narrow-bandpass MoSi/Si multilayer mirrors for a Japanese sounding-rocket program. A high spectral resolution lambda/Dlambda exceeding 40 was achieved by a two-mirror telescope with a multilayer coating. The single telescope had two bandpasses in the extreme-UV range for detecting a coronal high-velocity flow; the wavelength at peak reflectance (hereafter peak wavelength) in one of the bandpasses was 210.2 A, situated on the blue side of the target's Fe xiv 211.3-A coronal emission line, and the peak wavelength in the other was 213.3 A on the red side. A high uniformity in a peak wavelength of less than 1 A was achieved over a primary (secondary) mirror surface 158 (96) mm in diameter. The ratio of the reflectance for the Fe xiv line at 211 A to that for an intense He ii line as a contaminant at 304 A in the telescope system became 2 x 10(5) owing to a wave trap consisting of a single Si layer on the MoSi/Si multilayer. The narrow-bandpass (~5-A) telescope was launched on 31 January 1998 by sounding rocket S520CN-22, and images of the whole-Sun corona at Fe xiv 211 A were successfully obtained.     

Grazing-incidence efficiencies in the 28-42-A wavelength region of replicas of the Skylab 3600-line/mm concave grating with multilayer and gold coatings

The efficiencies of replicas of the Skylab 3600-line/mm concave grating with multilayer and gold coatings were measured by using synchrotron radiation at an angle of incidence of 79 degrees and in the 28-42-A wavelength range. The blaze angle of the grating facets that faced the incident radiation was 3.1 degrees , and the average angle of the opposite facets was 6 degrees . For the gold grating, the -1 outside order had the highest efficiency of any diffracted order (excluding the zero order) over the entire wavelength range. Calculations of the grating efficiency indicated that the high efficiency in the -1 order resulted from the rather small angle (6 degrees ) of the facets opposite the incident radiation. For the multilayer grating, the efficiency in the on-blaze +2 inside order was enhanced in the 30-34-A wavelength region as a result of the high reflectance of the multilayer coating. The maximum efficiency in the +2 order occurred at the wavelength (32 A) corresponding to the peak of the reflectance of the multilayer coating on the facets facing the incident radiation. These results further demonstrate that a multilayer coating can be used to enhance the efficiency, in a selected wavelength range and in the on-blaze order, of a grating operating at a small grazing angle (11 degrees ).     

Embedded centrosymmetric multilayer stacks as complete-transmission quarter-wave and half-wave retarders under conditions of frustrated total internal reflection

A centrosymmetric multilayer stack of two transparent materials, which is embedded in a high-index prism, can function as a complete-transmission quarter-wave or half-wave retarder (QWR or HWR) under conditions of frustrated total internal reflection. The multilayer consists of a high-index center layer sandwiched between two identical low-index films with high-index-low-index bilayers repeated on both sides of the central trilayer, maintaining the symmetry of the entire stack and constituting a QWR (Delta(t)=90 degrees or 270 degrees ) or HWR (Delta(t)=180 degrees ) in transmission. A QWR design at wavelength lambda=1.55 microm is presented that employs an 11-layer stack of Si and SiO(2) thin films, which is embedded in a GaP cube prism. The intensity transmittances for the p and s polarizations remain >99% and Delta(t) deviates from 90 degrees by <+/-3 degrees over a 100 nm spectral bandwidth (1.5< or =lambda< or =1.6 microm), and by < or =+/-7 degrees over an internal field view of +/-1 degrees (incidence angle 44 degrees < or = phi(0)< or =46 degrees inside the prism). An HWR design at lambda=1.55 microm employs seven layers of Si and SiO(2) thin films embedded in a Si cube, has an average transmittance >93%, and Delta(t) that differs from 180 degrees by <+/-0.3 degrees over a 100 nm bandwidth (1.5< or =lambda< or =1.6 microm) and by <+/-17 degrees over an internal field view of +/-1 degree . The sensitivity of these devices to film-thickness errors is also considered.     

Liquid-phase broadband cavity-enhanced absorption spectroscopy measurements in a 2 mm cuvette

A novel implementation of broadband cavity enhanced absorption spectroscopy (BBCEAS) has been used to perform sensitive visible wavelength measurements on liquid-phase solutions in a 2 mm cuvette placed at normal incidence to the cavity mirrors. The overall experimental methodology was simple, low cost, and similar to conventional ultraviolet-visible absorption spectroscopy. The cavity was formed by two concave high reflectivity mirrors. Three mirror sets with nominal reflectivities (R) of R = 0.99, 0.9945, and 0.999 were used. The light source consisted of a high intensity red, green, blue, or white LED. The detector was a compact charge-coupled device (CCD) spectrograph. Measurements were made on the representative analytes, Ho(3+), and the dyes brilliant blue-R, sudan black, and coumarin 334 in appropriate solvents. Cavity enhancement factors (CEF) of up to 104 passes for the high reflectivity mirrors were obtained. The number of passes was limited by relatively high scattering and absorption losses in the cavity, of approximately 1 x 10(-2) per pass. Measurements over a wide wavelength range (420-670 nm) were also obtained in a single experiment with the white LED and the R = 0.99 mirror set for Ho(3+) and sudan black. The sensitivity of the experimental setup could be determined by calculating the minimum detectable change in the absorption coefficient alpha(min). The values ranged from 5.1 x 10(-5) to 1.2 x 10(-3) cm(-1). The limit of detection (LOD) for the strong absorber brilliant blue-R was 620 pM. A linear dynamic range of measurements of concentration over about two orders of magnitude was demonstrated. The overall sensitivity of the experimental setup compared very favorably with previous generally more experimentally complex and expensive liquid-phase cavity studies. Possible improvements to the technique and its applicability as an analytical tool are discussed.     

Analytical study of the principal angle used in optical experiments

An analytical method has been developed for directly calculating the principal angle theta(p) at which the phase difference between the two reflection coefficients is equal to 90 degrees and at which the ratio of /r(p)/ to /r(s)/ is equal or close to a minimum. The equations given here can be used in many optical applications. For example, ellipsometric data measured at an incidence angle theta(p) will have higher precision than data measured at other incidence angles. theta(p) is the principal angle. Instead of three principal angles, there is only one principal angle, which can be found in the region of 0 < or = theta(p) < or = 90 degrees for most metallic materials used in applications. Results show good agreement between the measured and the calculated spectra of delta(p) and rho(po).     

Extreme-ultraviolet-induced oxidation of Mo/Si multilayers

The extreme-ultraviolet (EUV)-induced oxidation of Mo/Si multilayer mirrors was characterized by several methods: EUV reflectivity, x-ray photoelectron spectroscopy, small-angle x-ray reflectometry, atomic force microscopy, and EUV scattering measurements. Based on the results of the different investigation techniques, an oxidation model was developed to explain the degradation of the mirrors under EUV radiation.