Takashi Iizuka

Comparison Among Multilayer Soft X-Ray Mirrors Fabricated By Electron Beam, Dc-, Rf-Magnetron Sputtering And Ion Beam Sputtering Deposition

Multilayer Mo/Si mirrors have been fabricated by various physical vapor deposition methods. The best mirror fabricated by RF-magnetron sputtering showed a reflectivity of 57.8% at an incident angle of 25° and at a wavelength of 12.66 nm with synchrotron radiation reflectometer. The characteristics of fabricated multilayer mirrors have been measured using transmission electron microscope for surface and cross-sectional micrographs, electron diffraction for crystalline nature, small-angle x-ray diffractometer and synchrotron radiation reflectometer for reflectivity. Particularly, the dependencies of deposition parameters of Ar pressure and input power in RF-magnetron sputtering and substrate temperature in electron beam deposition in ultra high vacuum, have been investigated. The crystalization of Mo layers is clearly admitted for the mirrors by DC-and RF-magnetron sputtering. Surface roughness is minimum for the mirrors by RF-magnetron sputtering and ion beam sputtering. A possible reason of low reflectivity for the mirrors by ion beam sputtering is discussed from the resluts of additional analysis.

Suppression of columnar-structure formation in Mo-Si layered synthetic microstructures

We have previously reported that columnar-structures were formed in electron beam (EB) deposited and DCmagnetron sputtered Mo-Si layered synthetic microstructures (LSMs). The columnar structures reduced x-ray reflectivity by roughing layer interfaces of the LSMs. We here investigated the conditions to suppress columnar structure formation, by varying the substrate temperature (T9) in EB deposition and the argon pressure (PAT ) in D C- and RF-sputtering. In the EB deposited LSMs, the columnar structure disappeared and almost uniform LSMs were obtamed both at T□ 400°C and T9< -155°C. In the DC- and RF-sputtered LSMs, the columnar structure formation was suppressed by lowering PAr. The measured x-ray reflectivity of the LSMs increased according to the suppression of the columnar structure formation.

Fabrication and evaluation of Cr-C multilayer mirrors for soft x rays

We have selected Cr and C as a material pair of multilayer (ML) mirrors for soft x rays at the wavelength near 5 nm. The Cr-C ML structures have successfully been fabricated with rf- magnetron sputtering. The ML structures have been characterized with transmission electron microscopy (TEM), x-ray diffraction, and reflectivity measurement by SR light. From the observation of TEM images, Cr-C MLs had uniform and less-defective layered structures with the d-spacing down to 2.4 nm. Reflectivities at normal and grazing incidence were remarkably decreased with decreasing the ML d-spacing. Normal incidence reflectivity at 5 nm was as high as 7%. The regularity of the d-spacing of the MLs was evaluated with a moire image that was formed by putting a reference stripe pattern on the cross-sectional TEM micrograph. Compared to Ni-C MLs, Cr-C is a preferable material combination for x-ray mirrors for shorter wavelength and normal incidence.

Radiation damage of multilayer mirrors for soft x-ray lasers

We have successfully demonstrated double pass enhancement of amplified spontaneous emission of soft x rays, 23.2 and 23.6 nm of 3p - 3s transitions in Ne-like Ge, using an x-ray multilayer mirror. In this paper, we report on the fabrication of the mirror and analysis of its damage suffered during the experiments. The mirror used was a Mo-Si multilayer mirror with the reflectivity of 35% at the wavelength of 23.6 nm, deposited by an rf-sputtering system. In the damaged area of the mirror, only the multilayer was locally evaporated and the bare substrate underneath appeared. The size of the damaged area corresponded to the aperture size. We carried out the simulation on the spatial and temporal distribution of the mirror temperature during the experiment. Assuming that thermal x rays enter the mirror with the largest amount of energy among all the fluxes at the early stage of the enhancement, the result of the simulation can explain the damage feature and the temporal profile of the intensity of the amplified spontaneous emissions.