Distance ( μ m) 0 -2 -1 0 1 2 3 FWHM ~ 0.6 μ m Vertical Intensity (arb. units) Fig. 2. Focused beam profile measured by edge-scan. X-ray wavelength: 1.4 Å, f ∼ 158 mm, pinhole X-ray source (20 μ m in diameter). Fig. 1. Schematic diagram of experimental setup. 68 X-RAY MICROBEAM AND SCANNING MICROSCOPY WITH MULTILAYER ZONE PLATE Many types of X-ray focusing devices has been developed to generate micro-focused X-ray beams in the hard X-ray region, and sub- μ m spot size has already been achieved. Among those types of optical devices, a sputtered-sliced Fresnel zone plate (FZP) has the advantage of a narrow zone width and thickness; i.e. , a high aspect ratio of the zone structure. Therefore, the sputtered-sliced FZP is considered to be useful for focusing high energy X-rays. Evaluation of multilayer FZPs in the X-ray regions shorter than 1.6 Å are now in progress at SPring-8 [1]. Experiment have been performed at beamline BL47XU . A schematic diagram of the experimental setup is shown in Fig. 1. Undulator radiation is monochromatized through Si 111 double crystal monochromator, and focused by a multilayer FZP. The FZP consists of alternating multilayer zones constructed with magnetron sputtering. Fifty Al/Cu concentric multilayer structures are deposited onto a Au wire substrate with a diameter 47 μ m [2-5]. The gold core of the FZP acts as a central beam stop for the FZP with adequate thickness. We have tested two kinds of FZPs with outermost zone w i d t h s o f 0 . 2 5 μ m a n d 0 . 1 5 μ m . A f t e r t h e deposition, the wire sample is sliced and thinned down to about 20 - 40 μ m. The measured focal length of the FZP with outermost zone width of 0.25 μ m and 0.15 μ m were found to be about 158 mm at 1.4 Å and 234 mm at 0.45 Å, respectively. The focused beam profiles measured by edge- scanning for the FZP with an outermost zone width of 0.25 μ mare shown in Fig. 2. The thickness of the FZP was estimated to be about 20 μ m. The full-width at half maximum of the focused beam was about 0.6 μ m at 1.4 Å. This focused beam was generated by using pinhole X-ray source (about 20 μ m in diameter) placed 9 m upstream from the FZP. Therefore, the focused beam profile was nearly symmetrical in the vertical and horizontal directions. The present 20 μ m thick Cu/Al FZP was expected to operate as a phase- modulation FZP in this X-ray wavelength region, and the efficiency was expected to be higher than 10%, which is the theoretical limit of amplitude- modulated FZP. The actually measured diffraction efficiency at 1.4 Å was about 25%. The focused beam profile for the FZP with an outermost zone of 0.15 μ m is shown in Fig. 3. A focused beam size of 0.7 μ m was achieved in the vertical direction. In this experiment the focus spot was generated by demagnification of the undulator light source. Therefore , the horizontal spot size (about 4.7 μ m ) is determined by the source size and the magnification. The FZP was estimated to Monochromator X-rays from undulator FZP f Si 111 OSA (20 μ m) Focus point ~ 47 m from source point Fig. 5. Focused spot image taken by image sensor. X-ray wavelength: 0.15 Å (82.7 keV), f ∼ 690 mm. Fig. 4. Scanning X-ray microscopic image. Sample : #1500 gold mesh (1500 lines/inch) Image size : 101 × 101 pixel Pixel size : 0.5 μ m × 0.5 μ m X-ray wavelength : 0.45 Å Integration time : 1 s/pixel Au L-fluorescent X-ray yield. Fig. 3. Focused beam profile measured by edge-scan. X-ray wavelength: 0.45 Å (27.8 keV), f ∼ 234 mm. 69 References [1] Y. Suzuki, M. Awaji, Y. Kohmura, A. Takeuchi, N . K a m i j o , S . T a m u r a , a n d K . H a n d a , 6 t h International Conference on X-ray Microscopy Proc., to be published. [2] Y. Suzuki et al. , X-ray Microscopy and Spectromicroscopy III (1998) 117. [3] Y. Suzuki et al. , J. Synchrotron Rad. 4 (1997) 60. [4] N. Kamijo et al. , Rev. Sci. Instrum. 68 (1997) 14. [5] S. Tamura et al. , Mat. Res. Soc. Proc. 524 (1998) 31. be about 40 μ m thick, with a diffraction efficiency of 15% measured at 0.45 Å. Therefore, this FZP is also considered to be a phase-modulation FZP. The total intensity of the focused beam was found to be 10 10 photons/s under the focusing conditions shown in Fig. 3. This level of flux of the focused beam should be sufficient for most applications in fluorescent X-ray scanning microscopy. The results of a preliminary experiment in scanning microscopy is shown in Fig. 4. The test sample was a gold mesh, and the Au L-fluorescent X-rays were detected with a scintillation counter. The efficiency of the 40 μ m thick Al/Cu multilayer FZP was expected to be highest at 0.15 Å. The focussed spot image at 0.15 Å taken by the image sensor is shown in Fig. 5. The focal spot appears to be in the center of the image. The spot size was less than 1 pixel (6 μ m) wide vertically, and 2 pixels wide (12 μ m) horizontal. Although the measured spot size was limited by the spatial resolution of the detector, it was confirmed that the sputtered-sliced FZP with adequate thickness can function as a focusing element for X-rays as short as 0.15 Å. Distance ( μ m) Intensity (arb. units) 0 -6 -4 -2 0 2 4 6 Vertical FWHM ~ 0.7 μ m 25 μ m Yoshio Suzuki SPring-8 / JASRI E-mail: yoshio@spring8.or.jp 100 μ m
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