Shumin Zhang

Broadband Mo∕Si multilayer transmission phase retarders for the extreme ultraviolet

Experimental results on aperiodic broadband transmission molybdenum/silicon multilayer phase retarders for the extreme ultraviolet range are presented. The broadband phase retarders were designed using a numerical method and made using direct current magnetron sputtering on silicon nitride membrane. The polarization properties of these aperiodic transmission phase retarders have been investigated using the soft x-ray polarimeter at BESSY-II. The measured phase shift was about 42° in the wavelength range of 13.8–15.5nm, and the corresponding s-component transmission (Ts) decreased from 6% to 2% with increasing wavelength.

Complete polarization analysis of extreme ultraviolet radiation with a broadband phase retarder and analyzer

The polarization state of the BESSY UE56/1-PGM beamline radiation in the broad wavelength range of 12.7–15.5nm was measured using a molybdenum/silicon transmission phase retarder and a reflection analyzer with aperiodic multilayer interference structures, which can broaden the spectral response of these optical elements. The characteristics of the circular polarized undulator radiation, as well as the polarization properties of the two polarizing elements, were determined by a complete polarization analysis. Furthermore, the polarization of the radiation as a function of the undulator shift setting was also measured at the wavelength of 13.1nm by use of the broadband phase retarder-analyzer pair.

Extreme ultraviolet broadband Mo/Y multilayer analyzers

Broadband extreme ultraviolet molybdenum/yttrium aperiodic multilayer analyzers were designed for polarization experiments in 8.5–11.7nm wavelength range. The multilayer analyzers were made using direct current magnetron sputtering and characterized using the soft x-ray polarimeter at BESSY-II facility. Measured s reflectivities at the Brewster angle are 5.5% for a multilayer designed for 8.5–10.1nm wavelength range and 6.1% for one designed for 9.1–11.7nm. The multilayers also exhibit high polarization degree up to 98.79%. In addition, the multilayer was also measured over 38°–52° angular range at the fixed wavelength of 10.2nm and the mean s reflectivity is 6.2%.

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.

Current research activities in the field of multilayer for EUV, soft x-ray and x-rays in IPOE

The present status of studies on EUV, soft x-ray and x-ray multilayer in the Institute of Precision Optical Engineering (IPOE) is briefly reviewed. With the aim of realizing a Mach-Zender interferometer working at 13.9nm, we have developed a semitransparent beam splitter with multilayer deposited on the back side of a silicon nitride membrane. On the basis of the experimental optical properties of the beam splitter, design has been performed to define the multilayer structure that provides the highest product of reflectivity and transmission. Optimized Mo/Si multilayer has been successfully deposited on the back side of a silicon nitride membrane by use of the magnetron sputtering. Measurements by means of a reflectometer in Beijing Synchrotron Radiation Facility at 13.9nm and at an angle of 7.2° provide a reflectivity of 20% and a transmission of 22%. Such a beam splitter has been used for X-ray Mach-Zender interferometer at 13.9nm. The broadband multilayer analyzer in the range between 12.4nm and 20nm is designed, and made which can deviate the Quasi-Brewsters angle several degree and show very high polarization. The main feature of our design approach is the use of an analytical solution as a starting point for direct computer search, and the desired results can be given in a reasonable time. The method can be applied in different spectral range for suitable material combination. Supermirrors with broad angular band working at different wavelength such as Cu Kα line are designed, manufactured and measured. The results show that the performance of the supermirrors is in agreement with designed data.

Extreme ultraviolet reflective multilayers at 30.4nm

Mutilayers are important optical elements and widely used for extreme ultraviolet astronomical observation. For selecting the emission line multilayers should have high reflectivity and narrow spectral bandwidth. In this paper, six different multilayers including Si/C, Si/B4C, Si/Mo/B4C, Si/SiC, Mg/SiC and Mo/Si were designed for normal incidence angle of 5° at He-IIemission line(λ=30.4 nm). These multilayers have been fabricated using a direct current magnetron sputtering system. The period of multilayers were measured by X-ray diffractometer(XRD) and the reflectivities were measured on National Synchrotron Radiation Facility in Hefei, China. Then the reflectivities and the spectral bandwidth of these multilayers were compared respectively. It shows that the spectral bandwidth of multilayers of low Z materials is narrower than that of the normal Mo/Si multiayer, the reflectivity of Si/Mo/B4C multilayer is higher than that of Si-based multilayers of two kinds of materials. And Mg/SiC multilayer has the highest reflectivity of 43.81% and the narrowest spectral bandwidth of 1.44nm, which proves that Mg/SiC multilayer is more potential for selecting the emission line in extreme ultraviolet solar physics.

High Reflectivity Multilayer Mirror for He-II Radiation at 30.4nm in Solar Physics Application

The SiC/Mg and B4C/Mo/Si multilayers were fabricated for He-II radiation at 30.4nm. The measured reflectivitier were 38.0% for SiC/Mg multilayer at incident angle 12 degree, and 32.5% for B4C/Mo/Si multilayer at 5 degree, respectively.

Combination of Surface Characterization Techniques for Analyzing the Roughness of the Substrate

The extreme ultraviolet (EUV) molybdenum/silicon (Mo/Si) multilayers were fabricated using the direct current magnetron sputtering method on different preparation substrate: (a) wiped method, (b) untreated and (c) ultrasonic cleaning method. To meet the requirement of comprehensive characterization of morphology of substrates for EUV multilayer, a suitable combination of different measuring techniques, such as atomic force microscopy (AFM), x-ray diffractometer (XRD) rocking curve and synchrotron radiation (SR) reflectance, were chosen. It is shown that the reflectance of x-ray multilayers is sensitive to the roughness and cleanness of the substrate from the SR measurement results, and the maximum reflectance of 68.6% at 13.5nm was obtained using the ultrasonic cleaning method. It is demonstrated on the analysis according to the experimental results that the combination of surface characterization techniques using the AFM, x-ray scattering technique and SR characterization can be used for investigating the detail topography information of the substrate.

Design of soft x-ray multilayer polarizing elements

The status of polarizing optical elements for the soft x-ray range is reviewed. The criteria for optimum polarization of soft x-ray reflective analyzers, transmission polarizers and phase retarders based on multilayer interference structures are presented. Following these principles, soft x-ray polarizing optical elements have been designed. Designs of broad angular and wavelength range reflective analyzers based on a combination of analytical and numerical methods are also discussed.

Design and fabrication of W/Si multilayer mirrors for the K-edge of Ti

The periodic mulitlayer with high reflectivity and small full width at half maximum (FWHM) in hard x-ray range, while the reflectivity of non-periodic multilayer decreases and FWHM increase. Therefore, the optimum coating design must be found out as a compromise between the requirements for the reflectivity and the FWHM. We have used purely numerical techniques to design broad angular multilayer mirror in angle intervals (2.9°-3.1°), which is starting from an appropriate periodic multilayer structure. In our method, the risk of local minimization of the merit function disappears, because we refined the desired depth-distribution of the period using a direct numerical algorithm and the analytical solution as a starting point for computer calculation. The plateau reflectivity can be obtained in a few minutes. The main feature of our approach is the use of an analytical solution as a starting point for direct computer search, and the desired results can be given in a reasonable time. This technique is able to design almost any given reflectivity spectrum both energy- and angle-dependent and in a reasonable amount of time. The periodic and non-periodic W/Si multilayer for grazing incidence multilayer mirrors at the K-edge of Ti (0.275 nm) were both designed and fabricated by high vacuum DC magnetron sputtering coater model JGP560C6, and the multilayer films were characterized by X-ray reflectivity measurements on a laboratory x-ray diffractometer(XRD) and the atomic force microscope (AFM). We find good agreement of the changing trend of surface roughness between the simulation of XRD and measurement of AFM.