Characterization of zone plate properties using monochromatic synchrotron radiation in the 2 to 20 nm wavelength range.

A zone plate composed of Mo zones having 4 mm outermost zone diameter, 100 nm outermost zone width, and supported on a silicon nitride membrane was characterized using monochromatic synchrotron radiation in the 2 to 20 nm wavelength range. The zero and first order efficiencies were measured and compared to ab initio calculations that account for the optical properties of the materials, the width and shape of the zones, and multiple-layer thin-film effects. It is shown that the thicknesses of the Mo zones and the membrane and the ratio of the zone width to zone period can be independently determined from the measured diffraction efficiencies in the zero and first orders and that the computational code can be used to reliably design zone plates that are optimized for applications such as solar irradiance monitors in the extreme ultraviolet region.

Radiometry and metrology of a phase zone plate measured by extreme ultraviolet synchrotron radiation.

The diffraction efficiency, focal length, and other radiometric and metrology properties of a phase zone plate were measured by using monochromatic synchrotron radiation in the 7-18.5 nm wavelength range. The zone plate was composed of molybdenum zones having a 4 mm outer diameter and 70 nm nominal thickness and supported on a 100 nm thick silicon nitride membrane. The diffraction efficiency was enhanced by the phase shift of the radiation passing through the zones. The measured first-order efficiency was in good agreement with the calculated efficiency. The properties of the zone plate, particularly the small variation of the efficiency with off-axis angle, make it suitable for use in a radiometer to accurately measure the absolutely calibrated extreme ultraviolet emission from the Sun.

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-resolution imaging spectrometer for recording absolutely calibrated far ultraviolet spectra from laser-produced plasmas.

An imaging spectrometer was designed and fabricated for recording far ultraviolet spectra from laser-produced plasmas with wavelengths as short as 155 nm. The spectrometer implements a Cassegrain telescope and two gratings in a tandem Wadsworth optical configuration that provides diffraction limited resolution. Spectral images were recorded from plasmas produced by the irradiation of various target materials by intense KrF laser radiation with 248 nm wavelength. Two pairs of high-resolution gratings can be selected for the coverage of two wavebands, one grating pair with 1800 grooves/mm and covering approximately 155-175 nm and another grating pair with 1200 grooves/mm covering 230-260 nm. The latter waveband includes the 248 nm KrF laser wavelength, and the former waveband includes the wavelength of the two-plasmon decay instability at 23 the KrF laser wavelength (165 nm). The detection media consist of a complementary metal oxide semiconductor imager, photostimulable phosphor image plates, and a linear array of 1 mm(2) square silicon photodiodes with 0.4 ns rise time. The telescope mirrors, spectrometer gratings, and 1 mm(2) photodiode were calibrated using synchrotron radiation, and this enables the measurement of the absolute emission from the laser-produced plasmas with temporal, spatial, and spectral resolutions. The spectrometer is capable of measuring absolute spectral emissions at 165 nm wavelength as small as 5x10(-7) J/nm from a plasma source area of 0.37 mm(2) and with 0.4 ns time resolution.

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.

Subnanometer-scale measurements of the interaction of ultrafast soft x-ray free-electron-laser pulses with matter.

At the recently built FLASH x-ray free-electron laser, we studied the reflectivity of Si/C multilayers with fluxes up to 3 x 10(14) W/cm2. Even though the nanostructures were ultimately completely destroyed, we found that they maintained their integrity and reflectance characteristics during the 25-fs-long pulse, with no evidence for any structural changes over lengths greater than 3 A. This experiment demonstrates that with intense ultrafast pulses, structural damage does not occur during the pulse, giving credence to the concept of diffraction imaging of single macromolecules.

Highly sensitive visible-blind extreme ultraviolet Ni/4H-SiC Schottky photodiodes with large detection area.

Ni/4H-SiC Schottky photodiodes of 5 mm x 5 mm area have been fabricated and characterized. The photodiodes show less than 0.1 pA dark current at -4 V and an ideality factor of 1.06. A quantum efficiency (QE) between 3 and 400 nm has been calibrated and compared with Si photodiodes optimized for extreme ultraviolet (EUV) detection. In the EUV region, the QE of SiC detectors increases from 0.14 electrons/photon at 120 nm to 30 electrons/photon at 3 nm. The mean energy of electron-hole pair generation of 4H-SiC estimated from the spectral QE is found to be 7.9 eV.

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.

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.

Response of a SiC photodiode to extreme ultraviolet through visible radiation.

The responsivity of a type 6H-SiC photodiode in the 1.5-400 nm wavelength range was measured using synchrotron radiation. The responsivity was 0.20 A/W at 270 nm and was less than 0.10 A/W in the extreme ultraviolet (EUV) region. The responsivity was calculated using a proven optical model that accounted for the reflection and absorption of the incident radiation and the variation of the charge collection efficiency (CCE) with depth into the device. The CCE was determined from the responsivity measured in the 200-400 nm wavelength range. By use of this CCE and the effective pair creation energy (7.2 eV) determined from x-ray absorption measurements, the EUV responsivity was accurately modeled with no free parameters. The measured visible-light sensitivity, although low compared with that of a silicon photodiode, was surprisingly high for this wide bandgap semiconductor.

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.

Structural characterization and lifetime stability of Mo/Y extreme-ultraviolet multilayer mirrors.

We have observed a dramatic dependence of the extreme ultraviolet (EUV) reflectivity of Mo/Y multilayers on the oxygen content of yttrium. This is explained as being due to a change in the microstructure and an increase in roughness of the yttrium layers and not just to an increase in absorption owing to the amount of oxygen within the yttrium layers. We found that the best reflectivity of 38.4% was achieved with an oxygen content of 25%, which was reduced to 32.6% and 29.6% for multilayers manufactured from oxygen-free yttrium and 39%-oxygen yttrium, respectively. These results highlight the importance of including experimentally determined optical constants as well as interface roughness in multilayer calculations. In addition, the lifetime stability of Mo/Y multilayers with different capping layers was monitored for 1 year. The molybdenum- and palladium-capped samples exhibited low surface roughness and approximately 4% relative reflectivity loss in 1 year. The relative reflectivity loss of the yttrium-capped sample (yttrium with 39% oxygen) was approximately 8%. However, the reflectivity loss in all three capping layers occurred within the first 100 days after the deposition, and the reflectivity remained stable afterward.

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.

Mo/B4C/Si multilayer-coated photodiode with polarization sensitivity at an extreme-ultraviolet wavelength of 13.5 nm.

A silicon photodiode coated with an interface-engineered Mo/Si multilayer was developed as a polarization sensitive detector. The Mo/B4C/Si multilayer was designed to reflect 13.5-nm extreme-ultraviolet (EUV) radiation at an incident angle of 45 degrees, at which the maximum polarization sensitivity occurs. The sensitivity of this specially coated photodiode and its polarization responses were determined by measurement of the reflectance and transmittance of the multilayer coating with synchrotron radiation. The Mo/B4C/Si multilayer was found to reflect 69.9% of the s-polarized radiation and only 2.4% of the p-polarized radiation, thus transmitting approximately 0.2% s-polarized radiation and 8.4% p-polarized radiation at a 13.5-nm wavelength and a 45 degrees angle of incidence. A polarization ratio, (Tp - Ts)/(Tp + Ts), of 95% was achieved with sufficiently high sensitivity from this photodiode. This result demonstrates the high polarization sensitivity and the usefulness of multilayer-coated photodiodes as novel EUV polarimeters.

Normal-incidence efficiencies of multilayer-coated laminar gratings for the extreme-ultraviolet imaging spectrometer on the solar-B mission.

The normal-incidence efficiencies of two laminar gratings and the reflectances of two parabolic mirrors with matching multilayer coatings were measured by monochromatic synchrotron radiation and were compared with modeling calculations. These optics were developed for the Extreme-Ultraviolet Imaging Spectrometer to be launched on the Japanese Solar-B mission. Each optic has two sectors coated with Mo/Si multilayers that reflect the 17-21-nm and 25-29-nm wave bands at normal incidence. The measured peak grating efficiencies are in the 8%-12% range and are in good agreement with efficiency calculations that account for the effects of groove profile and the microroughness as determined by atomic force microscopy.

Measurement of extreme-ultraviolet attenuation edges of magnesium, tin, and indium filters.

We determined the energies of the Mg L2,3, Sn N4,5, and In N4,5 attenuation edges by measuring the transmission of high-resolution synchrotron radiation through thin filters. The Al L2,3 and Si L3 edges observed in the first and higher diffraction orders from the monochromator were used as energy fiducials. For each attenuation edge, the onset of attenuation with increasing energy and the inflection point of the attenuation curve were measured. The measured energy values were compared with previously determined attenuation edge energies and with electron binding energies. The measured energies of the inflection points are Mg L2 (49.89 +/- 0.02 eV), Mg L3 (49.58 +/- 0.02 eV), Sn N4 (25.00 +/- 0.02 eV), Sn N5 (23.97 +/- 0.02 eV), In N4 (17.66 +/- 0.02 eV), and In N5 (16.70 +/- 0.02 eV).