William R. Hunter

Effect of oxygen atom bombardment on the reflectance of silicon carbide mirrors in the extreme ultraviolet region

Chemical-vapor-deposited silicon carbide mirrors were exposed to bombardment by 8-km/s oxygen atoms that simulated the effects of exposure in low Earth orbit for periods up to 7.5 yr. The reflectances of four mirrors were measured before and after exposure at five wavelengths (58.4, 73.6, 104.8, 121.6, and 161 nm) and at 11 angles of incidence from 5 degrees to 80 degrees . The oxygen exposure reduced the normal-incidence reflectances by factors of 1.5-4.5 in the wavelength rate of 58.4-121.6 nm but had no effect on the visual appearance. The optical constants and the thicknesses of the thin surface layers present on the SiC substrates were determined from the reflectance measurements. This analysis indicated that before exposure the surface layers were composed of SiO(x) (where x is ~ 1.5) with thicknesses of 0.8-1.8 nm. After exposure to 8-km/s oxygen atoms, the surface layers were composed of SiO(x) with thicknesses of 3.5-4.5 nm. There were no systematic differences between the measured reflectances after simulated space ex osures of 1.5, 4.5, and 7.5 years. This implied that most of the growth in thickness of the SiO(x). layers occurred early in the exposure and stabilized at thicknesses of 3.5-4.5 nm. The optical results were consistent with x-ray photoelectron spectroscopy of the four mirrors after oxygen exposure.

Proposed multilayer-grating designs for the Astrophysical Plasmadynamic Explorer (APEX): an EUV high-resolution spectroscopic SMEX

APEX is a proposed mission for a Small Explorer (SMEX) satellite. The instrument is a suite of 8 near-normal incidence EUV spectrometers and is the outgrowth of 17 years of research at NRL on multilayer coatings and holographic ion-etched diffraction gratings. A prototype spectrometer has been flown successfully on a sounding rocket. We have examined different multilayer and gratings designs and produced a configuration optimized for the proposed science. APEX will achieve a peak effective area of at least 30-50 cm2 in the range 90-275 Å with resolution ~10,000, significant improvements on Chandra and EUVE.

Effect of oxygen atom bombardment on the reflectance of SiC mirrors in the extreme-ultraviolet region

Chemical vapor deposited (CVD) silicon carbide mirrors were exposed to bombardment by 8 km/s (5.2 eV) oxygen atoms that simulated exposure in low earth orbit for periods up to 7.5 years. The reflectances of four mirrors were measured before and after exposure at 584, 736, 1048, 1216, and 1610 angstroms and at eleven angles of incidence ranging from 5 degree(s) to 80 degree(s). The oxygen exposure reduced the normal incidence reflectances by factors of 1.5 to 4.5 in the VUV but had no effect on the visual appearance. The optical constants and thicknesses of the surface layers present on the SiC substrates were determined from reflectance measurements. This analysis indicated that before exposure the surface layers were composed of SiOx (where x approximately equals 1.5) with thicknesses of 8 - 18 angstroms. After exposure the thicknesses had increased to 35 - 45 angstrom. There were no systematic differences in the reflectances after simulated space exposures of 1.5, 4.5, and 7.5 years. This implied that most of the growth in thickness of the SiOx layers occurred early in the exposure and stabilized at thicknesses of 35 - 45 angstroms. The optical results were consistent with x-ray photoelectron spectroscopy of the four mirrors after oxygen exposure.

High-efficiency holographic ion-etched gratings with multilayer coatings and operating on-blaze at normal incidence in the 125- to 300-A range

Multilayer coatings have been applied to holographic diffraction gratings that have ion-etched blazed groove profiles. For each grating, the multilayer coating was designed to match the blaze angle so that the grating had high efficiency near normal incidence and in a wavelength region of choice in the 125-300 angstrom range. The grating efficiencies as functions of wavelength and incidence angle were measured using synchrotron radiation and were compared to the expected results. Normal-incidence efficiencies up to 13 percent were achieved at a wavelength of 128 angstrom. The performance characteristics of multilayer holographic and ruled replica gratings were compared and were related to the shape and roughness of the groove profiles as determined by atomic force microscopy.

X-ray and XUV imaging and spectroscopy of dense plasmas using multilayer optics

High-reflectance multilayer mirrors and gratings have been developed and implemented in the x-ray and XUV regions. The imaging and spectroscopic instruments have high throughput and can be positioned at a large distance from the radiation source where damage from the plasma debris and the radiation flux does not occur.

Performance of multilayer-coated gratings for the extreme-ultraviolet imaging spectrometer (EIS) for the Solar-B mission

The measured efficiencies of two flight gratings and the reflectances of two flight mirrors developed for the Extreme-Ultraviolet Imaging Spectrometer (EIS) for the Japanese Solar-B mission are presented. Each optic has two sectors with Mo/Si multilayers that refelct the 17 - 21 nm and 25 - 29 nm wavebands at normal incidence. The efficiencies that were measured using monochromatic synchrotron radiation are in good agreement with the calculated efficiencies.

Normal-incidence efficiencies of 4800-grooves/mm-ruled replica gratings with multilayer and gold coatings in the 125–325-Å 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-microm(-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.

Fabrication and synchrotron measurements of Be-based multilayer x-ray mirrors

A first attempt to fabricate high reflectivity W/Be and Mo/Be multilayer mirror coatings for near normal incidence and grazing angle applications is reported. Forty layers each of W and Be were laid down alternatively on super polished fused silica substrates by sputter deposition with layer thicknesses of 16 angstroms and 40 angstroms, respectively and a 80 angstroms thick Be capping layer. Soft x-ray reflectivity measurements were carried out using the NRL reflectometer at the Brookhaven National Synchronous Light Source. Measurements at two grazing angles and photon energies yielded the following peak reflectivities: 14.3% (at 14.68 degree(s) and 439 eV) and 7.5% (at 20.2 degree(s) and 307 eV). Model peak calculations taking into account interlay roughness indicate layer roughness of 5.4 angstroms (rms) and a BeO thickness of 70 angstroms in the Be capping layer. In an attempt to reduce interlayer roughness and/or intermixing by reducing the bombardment of high energy Ar neutrals on the growing layers, experiments are underway to replace Ar as the sputter gas with Xe for the W/Be multilayer coatings.