h h Zone plate # FZ P 1 0.25 μ m 50 μ m 220 mm # FZ P 2 Outermost zone widt Central stop diameter (Gold wire core diameter ) Primary focal lengt at 12.4 keV ( λ = 1Å ) Number of zone Zone plate diameter Zone plate thickness 50 80 μ m ~ 20 μ m 0.1 μ m 50 μ m 68 mm 50 70 μ m ~ 40 μ m Table I. Zone plate parameters (Cu / Al system) C h a r a c t e r i z a t i o n o f S p u t t e r e d - s l i c e d F r e s n e l Z o n e P l a t e a t B L 2 0 X U : H i g h - R e s o l u t i o n H a r d X - r a y M i c r o b e a m E x p e r i m e n t s 50 μ m 5 μ m ∆ d n = 0.25 μ m 2 μ m 50 μ m ∆ d n = 0.1 μ m (a ) (b ) Fig. 1. SE M micrographs s howing cros s s ections of Cu/Al c oncentric m ulilayers on Au wire core. Fresnel zone plate with an outermos t zone widths of (a ) 0.25 μ m and (b ) 0.1 μ m. smooth radius Figure 1 T able I having X -ray microprobe/microscopy, which has b een e xtensively developed in the soft X-ray domain, is now being extended to higher photon energies ( > 8 keV). This extension w ill promote t he various domai ns of basic science and technology , such as t he observation and c haracterization of th i cke r material s, including medical-biological samples and i ndustrial materials. T he success of high s patia l resolution studies of materials of sub-micron size is due to t he X -ray brilliance combined with th e availability of various micro-focusing optics. Th e expected applications of t he microprobe in hard/high-energy X -ray r egions are microscopy , microanalysis, micro-spectroscopy and micro - diffraction. T he Fresnel z one plate ( FZP) f abricated by lithography technique r ealizes t he highest s patia l resolution in t he soft X -ray domain, it is not thic k enough to be us ed in t he hard X -ray domain domain (aspect ratio ( height/width) : around 8:1) . Compared with t he FZP m ade by lithography technique t he s puttered-sliced FZP ( ss-FZP ) ca n be m ade thick enough with no aspect rati o limitation and is prov en t o work even at quit e high X-ray energies (over 100 keV) [1,2]. The ss-FZP composed of alternating transparent (Al) and opaque (Cu) layers (total 50 ~ 100 layers ) was f abricated by physical v apor deposition (d c planar m agnetron s puttering) on a fine gold wire core with a surface and having a of 25 μ m at a rotation speed of 15 ~ 50 rpm [3]. A fte r deposition, t he wire sample was s liced normal to t he wire axis and i ts thickness was adjusted to 20 ~ 40 μ m by mechanical polishing. H ere, two types of ss-FZP with outermost zone w idt hs of 0.25 μ m ( # FZP1) and 0. 1 μ m ( # FZP 2) were fabricated. s hows SEM micrographs of t hese tw o FZPs. T he parameters are give n in . T hey were c haracterized using k nife- edge scanning method (former) and the scanning microscopic method with a test pattern (latter). Th e experiment was performed at t he end station of beamlin e BL20X U (hutch # 2). This beamline is a unique beamlin e, a 248 m-long beam pat h equipped with an in-vacuum undulator source and 111 80 m monochromator Si 111 46 m 248 m 30 m 170 m slit for E c 30 ke V Ion chambe r FZ P OS A 220 mm Ion chambe r slit for E > 30 ke V slit Ion chambe r ( Liq. nitrogen cooling) (7 keV ~ 37 ke V) # h u t c h 2 SPring- 8 BL20X U ( M e d i c a l A p p l i c a t i o n h u t c h ) CCD knife edge, test pattern sample # h u t c h 1 OS A : Order Sorting Aperture FZP : Fresnel Zone Plat e Hard X-ray Focusing (BL20X U) Fig. 3. Schematics of optical system of Fresnel zone plate evaluation and scanning X-ray microscopy . Fig. 2 Fig. 3 –1. 5 –1.0 –0. 5 0 0. 5 1 .0 1. 5 Distance ( μ m) FWHM = 0.3 μ m E = 12.4 ke V Horizontal Scan Intensity (arb. unit) Fig. 2. Focused beam profile measured by a knife-edge horizontal scan at 12.4 keV. The full-line curve is the numerical derivative of the raw intensity data. double-crystal monochromator covering the energy range 8 keV ~ 37.7 keV. T he monochromator wa s placed 46 m downstream from the source point. A liquid-nitrogen cooling system is employed for th e monochromator. Firs t, we performed a characterization of # FZP1. The X-ray energy was chosen to be 12.4 keV. A quadrant s lit ( 50 μ m in horizontal width) wa s installed in the beamline 200 m upstream from th e FZP to c reat e a stable pseudo light s ource, and kn ife-edge scanning was performed in transmission geometry. T he minimum focusing size obtained was 0. 3- μ m full width at half maximum (FWHM) fo r the horizontal direction as shown in , and th e focal length obtained was 220 mm. A schematic view of the experimental set-up is shown in . T he diffraction limit of the first order focus of th e FZP, 1.22 d n (where d n is t he outermost z one width: 0.25 μ m ) is 0.3 μ m. T he focused beam size determined by the geometrical optics, 0.06 μ m, is smaller t han t he diffraction-limited resolution of # FZP1. T he focusing size obtained here (0.3 μ m) agrees well with the theoretical limit of the FZP wit h outermost z one width of 0.25 μ m. Diffraction efficiency for the first-order light was estimated by comparing the incident beam intensity through th e order sorting aperture (OSA) and the total intensit y of t he focused beam t hrough t he O SA. Th e observed efficiency of approximately 15% agrees well with that of the calculated one at 12.4 keV. Secondly, a scanning microscopy experiment was c onducted on # FZP2. E mploying scanning 112 0.1 μ 0.2 μ – 4 – 3 – 2 – 1 0 1 2 3 4 Intensity (arb. unit) Distance ( μ m) Resolution Test Pattern (Scanning Microscope ) Fig. 4. Scanning microscopy image of a test pattern . Scanning step: 0.025 μ m. X-ray wavelength: 0.82 Å. Fig. 4 Nagao Kamijo Kansai Medical Universit y E-mail: fe2n-kmj y@ asahi-net.or.jp microscopy to create image s of the test pattern wit h fine structures is one method that may be used fo r m easuring t he focal beam size [1]. T he X-ra y energy was chosen to be 15 keV. A quadrant slit ( 100 μ m × 100 μ m) was installed in t he beamlin e 200 m upstream from the FZP . T he designed outermost zone width of # FZP 2 used here is 0.1 μ m. T hus , t he diffraction limit of t he first-order focus of t he FZP is 0.12 μ m. Th e scanning experiment was c onducted using a test pattern made of 0.5 μ m-thick tantalum with seve n periodic steps of 0.1 and 0.2 μ m line-and-spac e deposited on an Si 3 N 4 membrane. T he result of the scanning image is shown in . H ere, th e transmitted intensity was detected using an io n c hamber. T he fine pattern of 0. 1- μ m wi de wa s clearly resolved in the measured image. Therefore, the r esolution l im it of the microscope is estimate d to be 0.1 ~ 0. 2 μ m, which is close to the diffraction- limited resolution of # FZP2. T he total flux of th e microbeam obtained is ~1 0 9 photons s -1 . In conclusion, ss-FZP s have now been proven to be a genuine X -ray focusing element. It is possible to work in a wide X-ray energy range (8 ~ 100 ke V) , even though their numerical aperture are quite small (in the order of 10 -4 ). References [1] Y. Suzuki et al . , Nucl . Instrum . Meth . A 467-468 (2001) 951. [2] N. Kamijo, Y. Suzuki, M. Awaji, A. Takeuchi, H. Takano, T. Ninomiya, S. Tamura and M.Yasumoto , J. Synchrotron Rad. 9 (2002) 182. [3] S. Tamura et al . , J. Synchrotron Rad. 9 (2002) 154. 113
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