Hard X-Ray Microprobe
with Sputtered-Sliced Fresnel Zone Plate
at the TRISTAN Main Ring in KEK
Y. Suzuki1, N. Kamijo2, S. Tamura2, K. Handa3,
A. Takeuchi4, S. Yamamoto5, H. Sugiyama5
1
Advanced Research Laboratory, Hitachi Ltd., Hatoyame, Saitama 350-03, Japan
E-mail: suzuki@harl.hitachi.co.jp
2
Osaka National Research Institute, Ikeda, Osaka 563, Japan
3
Ritsumeikan University, Kusatsu, Siga 525, Japan
4
Tsukuba University, Tsukuba, Ibaraki 305, Japan
5
Photon Factory KEK, Tsukuba, Ibaraki 305, Japan
Abstract. Hard X-ray microbeam with zone plate optics has been tested at the
MR-BW-TL beamline on the TRISTAN main-ring of the KEK. A sputteredsliced Fresnel zone plate with Au core and Ag/C multi-layer is used as an
X-ray focusing device. The outermost zone width of the zone plate is 0.35
µm. A focused spot size of about 0.5 µm has been achieved at an X-ray
energy of 8.54 keV.
1 Introduction
Many types of X-ray focusing devices are developed to generate micro-focus X-ray
beam in hard X-ray region. Total-reflection mirrors [1], multilayer mirror optics [2],
Fresnel zone plates [3-6], and Bragg-Fresnel lenses [7] are used in synchrotron
radiation facilities, and sub-micrometer spot size has already been achieved. Nominal
spatial resolution (spot size) of the microfocus beam is determined mainly by the
performance of X-ray focusing element. However, flux density of a focused beam is
limited by the brilliance of X-ray source because of emittance conservation
(Liouville’s theorem) on beam transportation. Therefore, a high brilliance X-ray
source is indispensable for practical X-ray microprobes. We have tried to generate a
hard X-ray microbeam with sub-micrometer spot size at the TRISTAN main-ring testbeamline. In this paper, we describe the results of an X-ray focusing test in which a
sputtered-sliced Fresnel zone plate was used.
2 Experimental Setup
2.1 X-Ray Source and Beamline
The TRISTAN main-ring at the National Laboratory for High Energy Physics (KEK)
is an electron-positron colliding ring constructed for experiments on elementary
particles. Its circumference is 3 km; the maximum beam energy is 31 GeV. The ring
was modified to operate as a high-brilliance synchrotron light source in the summer of
1995, and operation has been done only for three months in the fall of the year 1995.
A planar type 120 pole undulator with magnetic period of 4.5 cm and maximum Kvalue of 1.1 was installed in the straight section of the TRISTAN mainring. The
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Y. Suzuki et al.
storage ring was operated at 8 GeV in multi-bunch mode for the microbeam
experiment. The stored electron beam current was 2-15 mA during the experiment.
Calculated source brilliance of the undulator radiation is 2 x 1018 photons/
s/mrad2/mm2/0.1% bandwidth at 10 mA stored current. This value is close to that of
other third generation synchrotron light sources (ESRF and APS). Details of the
storage ring and undulator will be described elsewhere [8].
Fig. 1. The optical system
A schematic diagram of the experimental set up is shown in Fig. 1. Undulator
radiation passing through double graphite heat-absorbers (each 0.1 mm thick) and
double beryllium windows (each 0.2 mm thick) is monochromatized with a liquidnitrogen cooled Si 220 double crystal monochromator. The X-ray energy is fixed at
8.54 keV. The K-value of the undulator is tuned by changing the magnet gap as the
output flux from the monochromator reaches maximum. The third beryllium window
(0.2 mm thick) located between the first crystal and second crystal of the
monochromator separates the vacuum from atmospheric pressure. Finally the
monochromatic X-ray beam impinges on an X-ray focusing device in air. The distance
between the light source and the experimental station is about 100 m. A cross slit is
placed between the first crystal and second crystal of the monochromator. The cross
slit is used as a pseudo-point source for the microbeam experiment, and a demagnified
image of the cross slit is generated at the focal point to generate a microfocus X-ray
beam. The distance between the cross slit and the X-ray focusing device is about 16
m. Details of the beamline and monochromator system will be published elsewhere by
other authors [9].
2.2 X-Ray Focusing Optics
A sputtered-sliced FZP fabricated by Kamijo and Tamura [4, 5] is used as the X-ray
focusing device. The FZP consists of alternating multilayer zones constructed by
magnetron sputtering. Fifty Ag/C concentric multilayer structures are deposited onto a
Au wire substrate of 47 µm diameter, and the film thickness (zone width) of the first
inner layer is set to 0.4 µm, and 0.25 µm for the outermost layer. The diameter of the
FZP is 80 µm under these conditions. After deposition, the wire sample is sliced
Hard X-Ray Microprobe with Sputtered-Sliced Fresnel Zone Plate
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normal to the wire axis onto a plate. Finally, the FZP is thinned by mechanical
polishing. When the thickness of the FZP is more than 10 µm, the FZP operates as an
amplitude-modulating zone plate, and the efficiency of the first order diffraction is not
higher than 1/π2 (∼ 10 %). When the FZP is thinned to less than 10 µm, the zone plate
becomes a phase-modulating zone plate, and the theoretical limitation of the firstorder diffraction efficiency increases to 40 %. The thickness of the FZP used in this
experiment is estimated to be about 8 - 9 µm. The diffraction efficiency is measured
by comparing the flux density of X-rays incident on the FZP with the intensity of the
focused beam. The measured efficiency of the first-order diffraction is about 10 % at
an X-ray energy of 8 keV. The FZP has a central stop zone (47 µm-diameter gold
core). Taking the annular aperture of the FZP into account, the intrinsic diffraction
efficiency achieved by the zoned area is estimated to be about 16 %. This value is
larger than the theoretical limitation of the first-order diffraction efficiency for the
amplitude-contrast FZP. Therefore this FZP is considered to be a phase-shifted FZP.
The measured focal length is about 146 mm at an X-ray energy of 8.54 keV. In this
experiment a demagnified image of the X-ray source is formed by the FZP, to
generate a micro-focus X-ray beam. When the cross slit placed between the first
crystal and second crystal of the monochromator is used as a point-like X-ray source,
magnification (M) is defined by the equation M = f/L, where f (= 146 mm) is the focal
length of the FZP and L (= 16 m) is the distance between source and FZP. The slit
width used in this experiment is estimated to be about 50 µm. When the slit width is
50 µm, a focused spot size of 0.5 µm can be derived by geometrical optics. An order
sorting aperture (OSA), a 20 µm-diameter pin-hole made of 0.2 mm-thick Ta plate, is
used for selecting the first-order diffraction.
The FZP, bonded onto a graphite plate (1 mm thick), is mounted onto a
manipulator. A cross slit (0.14 mm x 0.14 mm) is placed just in front of the FZP to
reduce background noise. The intensity of the focused X-ray beam is measured with
an ionization chamber, or a NaI scintillation counter. Focused beam profiles are
measured by knife-edge scan. The knife-edge, which is actually a gold wire of diamter
50 µm, is scanned with a translation stage driven by a stepping motor. The minimum
step of the scanner is 0.025 µm. Line-spread-function of the optical system is derived
from numerical differential of the measured knife-edge scan profiles.
3 Results
The cross slit between the first and second crystal of the monochromator is used as a
pseudo-point source for the X-ray focusing experiment. The focused beam profile
measured by knife-edge scanning is shown in Fig. 2a. A spot size (spatial resolution)
of about 0.5 µm in full-width at half-maximum (FWHM) was achieved. This spot size
is close to the diffraction limited resolution of the FZP (0.3 µm). Therefore, the FZP
used in this experiment is considered to be very near to that of the ideal zone plate.
The FZP was not a practical device, in the bending magnet beamline of the Photon
Factory 2.5 GeV storage ring, because of low efficiency and small numerical aperture.
However, by using the high brilliance undulator radiation from the TRISTAN mainring, a practical hard X-ray microprobe has been achieved. When the demagnified
image of the undulator source point was formed by the FZP, the focused beam size
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Y. Suzuki et al.
measured by edge-scanning was 1.4 µm in FWHM as shown in Fig. 2b, and the flux
density of the focused X-ray beam measured with the ionization chamber was about
1 x 106 photons/s/mm2/10mA stored current. This beam intensity is sufficient for
applying to the scanning microscopy experiment.
Fig. 2. Focused beam profiles measured by knife-edge scan.
(a) A slit between the 1st crystal and the 2nd crystal is used as a pseudo-point source.
(b) SR souce is demagnifies without the slit. λ = 1.45 Å, horizontal focus
Acknowledgements
Authors would like to express much appreciation to all the members of the TRISTAN
super light facility and the accelerator group of the TRISTAN for modification and
operation of the TRISTAN main-ring as a synchrotron light source. They are also
grateful to Dr. S. Aoki for his valuable advice. This study has been performed under
the approval of the TRISTAN-MR Committee (Proposal No. MR95A).
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