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.     

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.     

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.     

Mo/Si and Mo/Be multilayer thin films on Zerodur substrates for extreme-ultraviolet lithography

Multilayer-coated Zerodur optics are expected to play a pivotal role in an extreme-ultraviolet (EUV) lithography tool. Zerodur is a multiphase, multicomponent material that is a much more complicated substrate than commonly used single-crystal Si or fused-silica substrates. We investigate the effect of Zerodur substrates on the performance of high-EUV reflectance Mo/Si and Mo/Be multilayer thin films. For Mo/Si the EUV reflectance had a nearly linear dependence on substrate roughness for roughness values of 0.06-0.36 nm rms, and the FWHM of the reflectance curves (spectral bandwidth) was essentially constant over this range. For Mo/Be the EUV reflectance was observed to decrease more steeply than Mo/Si for roughness values greater than approximately 0.2-0.3 nm. Little difference was observed in the EUV reflectivity of multilayer thin films deposited on different substrates as long as the substrate roughness values were similar.     

Molybdenum-silicon aperiodic multilayer broadband polarizer for 13-30 nm wavelength range

Aperiodic molybdenum/silicon (Mo/Si) multilayer designed as a broadband polarizer with reflectivity over a wide wavelength range of 13-30 nm at a fixed incidence angle of 45° was developed by a numerical method. The multilayer was prepared using direct current magnetron sputtering. The reflectivity and polarizing properties were characterized using synchrotron radiation. In 13-30 nm wavelength region, the measured s-reflectivity (R_s) with standard deviation is 13.5%±3.1%, p-reflectivity (R_p) with standard deviation is 1.4%±0.5%. The mean of polarization degree is 81.2%. This broadband multilayer polarizer can be used in extreme ultraviolet polarization measurements, and will greatly simplify experimental arrangements.     

Quasi index matching for minimum reflectance at a dielectric-conductor interface for obliquely incident p- and s-polarized light

Conditions for reducing the reflectance of a dielectric-conductor interface for p- and s-polarized light to a minimum at any angle of incidence phi are determined. The refractive indices of a transparent immersion medium (liquid) that achieve minimum reflectance at normal incidence, phi=0, and at phi=45 degrees are independent of polarization. These indices provide sufficient data to determine the real and imaginary parts of the complex refractive index of an absorbing substrate. Reflection at a dielectric-Au interface at 500 nm wavelength is considered as an example. It is shown that the lowest possible reflectance is attained for p-polarized light at phi=45 degrees and that the associated p-reflection phase shift is also minimum at that angle. For phi> or =65 degrees the lowest reflectance of p-polarized light occurs when the ambient is vacuum or air. However, this lowest reflectance at the air-Au interface is not a true minimum in a mathematical sense.     

Carbon contamination analysis and its effect on extreme ultra violet mask imaging performance using coherent scattering microscopy/in-situ accelerated contamination system

The coherent scattering microscopy/in-situ accelerated contamination system (CSM/ICS) is a developmental metrology tool designed to analyze the impact of carbon contamination on the imaging performance. It was installed at 11B EUVL beam-line of the Pohang Accelerator Laboratory (PAL). Monochromatized 13.5 nm wavelength beam with Mo/Si multilayer mirrors and zirconium filters was used. The CSM/ICS is composed of the CSM for measuring imaging properties and the ICS for implementing acceleration of carbon contamination. The CSM has been proposed as an actinic inspection technique that records the coherent diffraction pattern from the EUV mask and reconstructs its aerial image using a phase retrieval algorithm. To improve the CSM measurement accuracy, optical and electrical noises of main chamber were minimized. The background noise level measured by CCD camera was approximately 8.5 counts (3 sigma) when the EUV beam was off. Actinic CD measurement repeatability was <1 A (3 sigma) at 17.5 nm line and space pattern. The influence of carbon contamination on the imaging properties can be analyzed by transferring EUV mask to CSM imaging center position after executing carbon contamination without a fine alignment system. We also installed photodiode and ellipsometry for in-situ reflectivity and thickness measurement. This paper describes optical design and system performance observed during the first phase of integration, including CSM imaging performance and carbon contamination analysis results.     

Design of multilayer extreme-ultraviolet mirrors for enhanced reflectivity

We show numerically that the reflectivity of multilayer extreme-UV (EUV) mirrors tuned for the 11-14-nm spectral region, for which the two-component, Mo/Be and Mo/Si multilayer systems with constant layer thickness are commonly used, can be enhanced significantly when we incorporate additional materials within the stack. The reflectivity performance of the quarter-wavelength multilayers can be enhanced further by global optimization procedures with which the layer thicknesses are varied for optimum performance. By incorporating additional materials of differing complex refractive indices-e.g., Rh, Ru, Sr, Pd, and RbCl-in various regions of the stack, we observed peak reflectivity enhancements of as much as ~5% for a single reflector compared with standard unoptimized stacks. We show that, in an EUV optical system with nine near-normal-incidence mirror surfaces, the optical throughput may be increased by a factor as great as 2. We also show that protective capping layers, in addition to protecting the mirrors from environmental attack, may serve to improve the reflectivity characteristics.     

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.     

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.     

Design and fabrication of an integrated polarized light guide for liquid-crystal-display illumination

An integrated polarized light guide was designed and fabricated for use as a liquid-crystal backlight with emphasis on uniformity of the light and conversion of p-polarized to s-polarized light. Two different micro-optical structures were fabricated both on the top and the bottom surfaces of the light guide. On the top surface, a subwavelength grating separates s-polarized and p-polarized light to achieve a polarization-conversion efficiency of 69%. A 1.7 gain factor of polarization efficiency was obtained to increase the utility of light for liquid-crystal illumination.     

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.     

High reflectivity multilayer for He-II radiation at 30.4 nm

SiC/Mg and B(4)C/Mo/Si multilayers were designed for He-II radiation at 30.4 nm. These multilayers were prepared by use of a direct current magnetron sputtering system and measured at the National Synchrotron Radiation Laboratory, China. The measured reflectivities were 38.0% for the SiC/Mg multilayer at an incident angle of 12 deg and 32.5% for the B(4)C/Mo/Si multilayer at 5 deg, respectively. A dual-function multilayer mirror was also designed by use of the aperiodic SiC/Mg multilayer. Annealing experiments were performed to investigate the thermal stability of the SiC/Mg multilayer. The interface of the SiC/Mg multilayer before and after annealing was studied by electron-induced x-ray emission spectra, which evidences the absence of thermal reaction products at the interfaces after annealing.     

Mo/Si multilayers for EUV lithography by ion beam sputter deposition

Mo/Si multilayers for applications in extreme ultraviolet (EUV) lithography have been prepared on Si wafer substrates using ion beam deposition. The multilayers were characterised by transmission electron microscopy, secondary ion mass spectroscopy, atomic force microscopy, photoelectron spectroscopy, X-ray reflectometry at grazing incidence, and EUV-reflectivity measurements at nearly normal incidence. The surface and the interfaces of the multilayers are rather smooth with only small roughness. The material properties of the layers are characterised by some intermixing and silicide formation at the Mo-Si interfaces and a polycrystalline grain structure of the Mo layers, which is in agreement with prior studies. Appearance of multilayer diffraction spots, well-resolved Kiessig fringes and other diffraction evidence indicate very good coherence of the wave fields and in this manner a good reproducibility of the multilayer period of 6.7 nm. Normal incidence peak reflectivities of 64-65% in the EUV spectral range were routinely obtained at 13.4 nm wavelength. This reflectivity value and the formation of an EUV standing wave field are confirmed using photoelectron spectroscopy, and an application for defect particle analysis is proposed. The obtained results are discussed in comparison to literature data of multilayers prepared by other deposition techniques and considering new attempts of interface engineering.     

Polarization and incidence angle-dependent transmittance of transparent nickel electrodes with various thicknesses

The polarization and incidence angle-dependent transmittance of thin nickel film with various thicknesses deposited on glass substrates was first investigated by using a modified UV-Vis spectrometer. The thin nickel films showed relatively high uniform transmittance over a wide range of wavelengths, 300-1100 nm. The thickness-dependent dielectric and optical constants extracted from the experimental transmittance are significantly distinct from those of the thick nickel film. In particular, the p-polarized light transmittance largely increases with larger incidence angle, but the s-polarized light transmittance behavior is opposite from that of p-polarized light. The difference of the polarization-dependent transmittance increases parabolically with the incidence angle.     

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.     

Mo/Si multilayer-coated ruled blazed gratings for the soft-x-ray region

Two Mo/Si multilayer-coated blazed gratings have been fabricated for operation at soft-x-ray wavelengths above the Si L edge, λ ≥ 12.4 nm, at (near) normal incidence. The sawtooth profile of the grating structure was mechanically ruled into a 200-nm Au film that was deposited onto a plane glass substrate. To smooth the rough Au surface and to prevent interdiffusion of the Au film with the upper Mo/Si multilayer, a carbon film was evaporated onto the Au grating surface of one of the gratings before the deposition of the multilayer coating. We matched the multilayer grating, working on blaze in the third diffraction order, in which an absolute diffraction efficiency of 3.4% at a wavelength of 14 nm was measured, whereas only 1.1% was achieved for a similar grating (without a carbon interlayer). These efficiencies are higher than those obtained for other ruled blazed gratings reported in the literature. As a result of the multilayer and grating periodicity, the wavelength of diffraction can be tuned bya rotation of the grating, which is important for application in a soft-x-ray monochromator.     

Design and manufacture of all-dielectric nonpolarizing beam splitters

Past research on all-dielectric nonpolarizing beam splitters is reviewed. It is shown that, for a 50-nm spectral region, it is possible to design and manufacture a two-material nonpolarizing plate beam splitter for use at an angle of 45 degrees (with a measured rms reflectance of 0.50 +/- 0.01 for both s- and p-polarized incident light).     

Optical characterization of a compact multilayer-mirror polarimeter in the extreme-ultraviolet range

A molybdenum-silicon (Mo/Si) multilayer-mirror (MLM) polarimeter has been constructed and used to analyze the extreme-ultraviolet (EUV) emission from excited HeI and HeII states following electron impact on He for wavelengths ranging from approximately 256 to 584 A. A ratio of reflectivities for s- and p-polarized light, R(s):R(p) approximately , and a resolving power of lambda/Dlambda approximately 6 at 304 A were obtained. These characteristics and the use of a VYNS (a copolymer material composed of 90% vinyl chloride and 10% vinyl acetate) spectral filter were sufficient to allow a detailed polarization study of the first two members of the Lyman series of He(+) at wavelengths of 304 A (HeII p --> s) and 256 A (HeII p --> s) for impact-electron energies ranging from threshold to 1500 eV. The MLM has also been used as a single flat-surface mirror polarimeter for the analysis of longer-wavelength radiation (517 to 584 A) from the (snp) (1) P (o) --> (1s(2)) (1) S series of neutral He with R(s)/R(p) approximately. Although MLM polarimeters were previously used for EUV measurements with bright photon sources such as those provided by synchrotron facilities, the results presented clearly demonstrate the feasibility of such devices with lower-intensity electron and ion impact sources. The compact design of the apparatus makes it suitable as a portable measurement and calibration device.     

Extreme-ultraviolet polarization and filtering with gold transmission gratings

The polarization and transmission characteristics of freestanding gold transmission gratings, with 200-nm periods, for extreme-ultraviolet (EUV) radiation (1 < 200 nm) have been measured. We find that EUV transmission through the gratings is dominated by the waveguide characteristics of the gratings and that polarization efficiencies of 90% for wavelengths of 121.6 nm are achievable. Both the EUV polarization and transmission properties are in good agreement with a complete vector, numerical solution of Maxwell's equations. The fraction of open area to total area of the grating has been measured using a 10-keV proton beam and was found to be in good agreement with the microscopic slit and wire dimensions that were obtained by scanning electron microscopy. The use of these gratings for particle measurements in the presence of intense EUV radiation is briefly discussed.