Multilayer-coated laminar grating with 16% normal-incidence efficiency in the 150-A wavelength region

We characterized a laminar grating with a Mo/Si multilayer coating by using synchrotron radiation and atomic force microscopy. The grating substrate had 2400 grooves/mm, 40-A groove depth, and 2080-A groove width. The microroughness of the grating substrate was 5 A rms. The multilayer coating was optimized to have peak normal-incidence reflectance at a wavelength near 150 A. For an angle of incidence of 10 degrees the peak grating efficiency was 16.3% in the +1 order and 15.0% in the -1 order. The efficiency in the zero order was lower by a factor of 40 owing to the excellent matching of the groove depth and groove width to the wavelength of the incident radiation. By dividing the grating efficiencies by the measured reflectance of the multilayer coating, we obtained inferred groove efficiencies of 34% and 32% in the +1 and -1 orders, respectively.     

Investigation of the properties of an ion-etched plane laminar holographic grating

We have measured the efficiency over the range 125-225 A of a bare ion-etched plane laminar holographic grating made of fused silica and with 1000 grooves/mm. The measured efficiency of each order oscillates with wavelength because of constructive and destructive interference between radiation diffracted from the lands and the grooves. We measured the grating groove profile with an atomic force microscope, and the resulting groove depth of 434 ? 6 A agrees well with the values determined independently from the oscillatory behavior of the efficiency measurements. Grating efficiency in the +1 order peaked at values of 0.027%, 0.011%, and 0.005% at wavelengths of 191, 157, and 132 A, respectively; and the derived groove efficiencies are 27%, 25%, and 27%. The irregular shape at the land-groove edges dominates the large grating roughness of 23-45-A rms, but even regions far from the edges have a roughness of 10-18-A rms. The average groove profile was used to model the grating efficiency, and the resulting wavelengths predicted for different order maxima and minima agree well with measured wavelengths, although the calculated efficiencies are greater than the measured results by 10-20%.     

High-efficiency MoRu-Be multilayer-coated gratings operating near normal incidence in the 11.1-12.0-nm wavelength range

MoRu-Be multilayer coatings were applied to two diffraction gratings for the purpose of enhancing their normal-incidence efficiency in the 11.1-12.0-nm wavelength range. The grating substrates were replicas of a holographic master grating that had a blazed groove profile with 2400 grooves/mm and a 2-m radius of curvature. The relatively low average microroughness (0.8 nm) of the grating surfaces contributed to the relatively high groove efficiency of the grating substrates and the reflectance of the MoRu-Be multilayer coatings. The peak efficiency, measured with synchrotron radiation, was 10.4% in the second diffraction order at a wavelength of 11.37 nm.     

Thin-film interference effects on the efficiency of a normal-incidence grating in the 100-350-A wavelength region

Thin-film interference effects were observed in the normal-incidence efficiency of a 2400-groove/mm replica grating. The efficiency was measured in the 100-350-A wavelength range and had an oscillatory behavior that resulted from the presence of a thin SiO(2) coating. The thicknesses of the SiO(2) and the underlying oxidized aluminum layers were inferred from computer modeling of the zero-order efficiency. The efficiencies in the diffracted orders were calculated with the modified integral approach and accounting for the multilayer coating and the groove profile derived from atomic force microscopy. The calculated and measured efficiencies were in good agreement.     

Efficiency and polarization performance of a multilayer-coated laminar grating in the 6.5-6.9-nm wavelength region

We applied a Mo/B4C multilayer coating to a laminar holographic grating with 2400 grooves/mm and a 1-m radius of curvature. By use of synchrotron soft x rays the multilayer-coated grating was evaluated to have diffraction efficiencies of 3.1% and 0.017% for s- and p-polarized radiation, respectively, at a 6.7-nm wavelength at a 45.35 degrees grazing angle of incidence in the +1 (inside) grating order. Thus the polarizance was estimated to be 98.9% at least. The zero-order peak was suppressed by the destructive interference caused by the groove profile.     

Efficiency calibration of the first multilayer-coated holographic ion-etched flight grating for a sounding rocket high-resolution spectrometer

We have fabricated the four flight gratings for a sounding rocket high-resolution spectrometer using a holographic ion-etching technique. The gratings are spherical (4000-mm radius of curvature), large (160 mm x 90 mm), and have a laminar groove profile of high density (3600 grooves/mm). They have been coated with a high-reflectance multilayer of Mo/Si. Using an atomic force microscope, we examined the surface characteristics of the first grating before and after multilayer coating. The average roughness is approximately 3 A rms after coating. Using synchrotron radiation, we completed an efficiency calibration map over the wavelength range 225-245 A. At an angle of incidence of 5 degrees and a wavelength of 234 A, the average efficiency in the first inside order is 10.4 +/- 0.5%, and the derived groove efficiency is 34.8 +/- 1.6%. These values exceed all previously published results for a high-density grating.     

Efficiencies of master, replica, and multilayer gratings for the soft-x-ray-extreme-ultraviolet range: modeling based on the modified integral method and comparisons with measurements

The near-normal-incidence efficiencies of a 2400-groove/mm holographic master grating, a replica grating, and a multilayer grating are modeled in the soft-x-ray-extreme-ultraviolet (EUV) regions and are compared with efficiencies that are measured with synchrotron radiation. The efficiencies are calculated by the computer program PCGrate, which is based on a rigorous modified integral method. The theory of our integral method is described both for monolayer and multilayer gratings designated for the soft-x-ray-EUV-wavelength range. The calculations account for the groove profile as determined from atomic force microscopy with a depth scaling in the case of the multilayer grating and an average random microroughness (0.7 nm) for the short wavelengths. The refractive indices of the grating substrate and coatings have been taken from different sources because of the wide range of the wavelengths (4.5-50 nm). The measured peak absolute efficiency of 10.4% in the second diffraction order at a wavelength of 11.4 nm is achieved for the multilayer grating and is in good agreement with a computed value of approximately 11.5%. Rigorous modeling of the efficiencies of three similar gratings is in good overall agreement with the measured efficiency over a wide wavelength region. Additional calculations have indicated that relatively high normal incidence efficiency (of at least several percent) and large angular dispersion in the higher orders can be achieved in the 4.5-10.5-nm range by application of various multilayer coatings.     

Replication of a holographic ion-etched spherical blazed grating for use at extreme-ultraviolet wavelengths: efficiency

Using synchrotron radiation, we have measured the efficiency at an angle of incidence of 10 degrees of a holographic ion-etched spherical blazed grating and three of its fourth-generation replicas. The measured efficiency profile of replicas 1 and 3 prior to multilayer coating oscillated from thin-film interference produced by the replicas' Al/Al2O3/SiO2 structure. A Mo2C/Si multilayer coating was applied to the master grating and replicas 1 and 2. After coating, the maximum grating efficiency occurred in the -2nd order and the maximum values were 12.4% at 143.8 angstroms for the master and 11.6% at 145.2 angstroms for replicas 1 and 2. On the basis of measurements obtained after coating, the derived groove efficiency was 22.2% for the master, 19.4% for replica 1, and 19.3% for replica 2. The groove efficiency of the uncoated replica 3 was 24.3% at 142.5 angstroms. We find that the replicas are reasonably faithful copies of the ion-etched master, and models based on measured atomic force microscope groove profiles are in general agreement with measured results. However, subtle issues remain regarding the widths of the peak order profile and the location of its maximum wavelength.     

Normal-incidence efficiencies of 4800-grooves/mm-ruled replica gratings with multilayer and gold coatings in the 125-325-A wavelength region

The normal-incidence efficiencies of two 4800-grooves/mm ruled replica gratings, one with a dual-bandpass molybdenum/silicon multilayer coating and the other with a gold coating, were measured by use of synchrotron radiation in the 125-325-A wavelength region. The peak reflectance of the multilayer coating was 22% in the first Bragg order near 235 A and 28% in the second Bragg order near 126 A. The peak efficiency of the multilayer grating was 2.6% in the first diffraction order near 225 A and 0.3% in the second diffraction order near 125 A. The efficiencies of the multilayer grating were much higher than the corresponding efficiencies of the gold grating. The characterization of the surfaces of the gratings by atomic force microscopy indicated rms microroughness values in the 5-18-mum(-1) frequency range of 12-20 A for the multilayer grating and 22-32 A for the gold grating. Both gratings had bumpy surface features larger than the nominal groove height. The rather large surface roughness and groove irregularities had a detrimental effect on the grating efficiencies.     

Efficiency of a grazing-incidence off-plane grating in the soft-x-ray region

Efficiency measurements of a grazing-incidence diffraction grating in the off-plane mount were performed using polarized synchrotron radiation. The grating had 5000 grooves/mm, an effective blaze angle of 14 degrees, and was gold coated. The efficiencies in the two polarization orientations (TM and TE) were measured in the 1.5-5.0 nm wavelength range and were compared with the efficiencies calculated using the PCGrate-SX code. The TM and TE efficiencies differ, offering the possibility of performing unique science studies of astrophysical, solar, and laboratory sources by exploiting the polarization sensitivity of the off-plane grating.     

Skylab 3600 groove/mm replica grating with a scandium-silicon multilayer coating and high normal-incidence efficiency at 38-nm wavelength

A Sc-Si multilayer coating was applied to a replica of the 3600 groove/mm grating, developed for the SO82A spectroheliograph that flew on the Skylab mission, for the purpose of enhancing the normal-incidence efficiency in the extreme-ultraviolet region. The efficiency, measured at an angle of incidence of 6 degrees with synchrotron radiation, had a maximum value of 7.2% at a wavelength of 38 nm and was a factor of 3 higher than the efficiency of the gold-coated Skylab grating. The measured efficiency of the Sc-Si grating was in good agreement with the efficiency calculated by use of the modified integral method.     

Polarization and efficiency of a concave multilayer grating in the 135-250-Å region and in normal-incidence and SeyaNamioka mounts

A molybdenum/silicon multilayer coating was applied to a holographic ion-etched blazed grating substrate that had 2400 grooves/mm and a radius of curvature of 2.2 m. Scanning probe microscopy yielded the same surface microroughness (5 ? rms) before and after deposition of the multilayer. The efficiency and polarization performance of the grating was measured by synchrotron radiation in the 135-250-? wavelength region. In the second grating order and the second Bragg order of the multilayer coating, the peak normal-incidence efficiency was 7.5% at a wavelength of 147 ?, representing a groove efficiency of 27%. At an angle of incidence of 35°, the polarization performance of the grating was 95%-100% in the 210-250-? wavelength region. In a Seya-Namioka spectrometer mount at an angle of incidence of 30°-40°, the grating is a nearly perfect polarizing optical element in the wavelength bands between 125 and 300 ?, which are covered by the multilayer coating.     

Replication of a holographic ion-etched spherical blazed grating for use at extreme-ultraviolet wavelengths: topography

We have measured the topography of a holographic ion-etched spherical blazed grating and three of its replicas using an atomic force microscope. The master grating had a roughness of less than 5 angstroms rms, a blaze angle of 2.5 degrees, and an antiblaze angle of 3.3 degrees. Thus the groove profile was more triangular than sawtooth. We find that the replication process did not significantly change the master grating. Moreover, we find no significant difference in roughness, blaze angle, or antiblaze angle between the master and its replicas before or after multilayer coating. However, bumps were observed on the gratings after coating, the cause of which is not understood. Although widespread, they occupy a relatively small fraction of the total area.     

Profiles of a high-aspect-ratio grating determined by spectroscopic scatterometry and atomic-force microscopy

The new and fast scatterometry method called optical diffraction microscopy is compared with atomic-force microscopy by use of cross-section scanning-electron microscope images as references. The sample is a high-aspect-ratio grating with a period of approximately 1000 nm. To allow the atomic-force microscope to track all parts of the grating profile, the grating is investigated at different tilt angles. The measured quantities of the profile include sidewall angle gamma (approximately 90 degrees), groove height h (approximately 2000 nm), and degree of filling f (approximately 40%). The two methods, which respond to quite different material properties, give consistent results within standard uncertainties of u(gamma)相似文献   

Near-normal-incidence extreme-ultraviolet efficiency of a flat crystalline anisotropically etched blazed grating

We have measured the extreme-ultraviolet (EUV) efficiency at an angle of incidence of 10 degrees of a flat crystalline anisotropically etched blazed grating. The measured efficiencies are high for uncoated gratings and agree well with a calculated model derived from a reasonable estimate of the groove profile. The highest groove efficiencies derived from the measurements are 48.8% at 19.07 nm and 64.1% at 16.53 nm for the -2 and -3 orders, respectively, which are comparable to the best values obtained yet from a holographic ion-etched blazed grating. This presents opportunities to instrument designs for high-resolution EUV spectroscopy in astrophysics where high efficiency in high orders is desirable.     

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 ).     

表面和亚表面特性对多层膜光栅成像对比度的影响

基于Si/SiO2材料对制备出名义节距为50 nm的多层膜光栅,重点分析了多层膜光栅研磨抛光过程中的亚表面损伤和湿法刻蚀均匀性问题。并利用原子力显微镜（AFM）和透射电子显微镜（TEM）对多层膜的截面粗糙度和刻蚀光栅结果进行了测量和分析。测量结果显示:多层膜光栅制备过程中的截面粗糙度降低和刻蚀均匀性的提高,有助于TEM测量获得均一的高成像对比度多层膜光栅图像。     

Groove profile modification of blazed gratings by dip coating with hardenable liquids

We modify groove profile of various blazed gratings with groove densities as great as 3600 lines/mm by dip coating with hardenable liquids with the aim of reducing the blaze angle. The groove profiles resulting from coatings with different layer thickness are measured by atomic force microscopy. A highly reproducible blaze angle reduction to as high as a factor of 6 is achieved with mechanically ruled as well as ion-beam-etched holographic blazed gratings. Blaze angles, to as small as 0.7 deg, which are required for vacuum-UV and soft-x-ray applications but can hardly be formed with sufficient groove profile accuracy by direct ruling, are realized with this coating technique.     

Effective diffraction gratings via acidic etching of thermally poled glass

Relief diffraction gratings are formed via acidic chemical etching of a periodically poled soda-lime glass. The thermal poling under 1000 V DC is performed at 325 °C using a thermally stable glassy-carbon anodic electrode with periodic grooves, the depth of the grooves being of ∼650 nm. Poling-induced modification of the glass results in deepening the glass anodic surface in the regions under the ribs of the anodic electrode due to volume relaxation and in increasing chemical durability of these regions in acidic media comparatively to the virgin glass. Chemical etching of the poled glass in NH₄F:8H₂O solution allows additional to the thermal poling shaping of the glass surface via faster dissolution of unpoled/less poled glass regions. The morphology of the glass surface before and after the etching is characterized with atomic force and scanning electron microscopy. About 30 min etching provides the formation of ∼0.9 μm in height relief diffraction gratings with the diffraction efficiency close to the theoretically achievable ∼30% for multi-order diffraction. In vivo measuring of the diffraction efficiency in the course of the etching allows precise fabrication of the gratings.     

Comparison of mechanically ruled versus holographically varied line-spacing gratings for a soft-x-ray flat-field spectrograph

Recent progress in the design of aspheric wave-front recording systems has permitted the manufacture of holographic gratings with highly variable groove densities that are suitable for flat-field spectrographs. A holographic grating thus recorded was processed to produce a laminar profile by use of reactive-ion etching. Measurements are reported of the absolute diffraction efficiency of this grating and of a comparable mechanically ruled grating. It is found that the holographic grating is much more effective in suppressing the higher orders. The spectral resolution was determined by use of a carbon Kalpha x-ray generator and a spectrograph with an imaging detector. The spectral resolution of the holographic grating was approximately 3 times worse than that of the ruled grating.