Bragg–Fresnel Optics
V. V. Aristov
Institute of Microelectronics Technology, and High Purity Materials RAS
Chernogolovka, Moscow Region, 142432 Russia
E-mail: aristo@ipmt-hpm.ac.ru
1 Introduction
Bragg-Fresnel optics (BFO) has been developed in IMT RAS for more than 10 years
already [1]. The operation of BFO is based on the combination of two kinds of
diffraction: Bragg diffraction of hard X-ray radiation on a perfect single crystal or a
multilayer interference mirror and Fresnel diffraction on an artificially created
microstructure. At present, BFO is the only instrument of X-radiation focusing in a
wide range (up to 100 keV) with a space resolution to 0.1 micron. BFO is used in a
number of techniques of X-ray diagnostics and as the basis for the development of
new instruments. Bragg-Fresnel lenses successfully tested with use any of synchrotron
radiation sources in Russia (Novosibirsk, VEPP-2M), Germany (Hamburg,
HASILAB; Berlin, DESY, DORIS), Japan (Tsukuba, Photon Factory), and France
(Grenoble, ESRF; Paris, LURE).
Table 1. Working parameters of Bragg-Fresnel lenses
Characteristics
Linear lens
Circular lens
crystals
Elliptical lens
(multilayers)
Yes
Yes
Yes
continuous energy
2–100 keV
discrete energy
2–100 keV
continuous energy
0.1–10 keV
Spectral band ∆E/E
10-4
10-5– 10-6
10-3
Minimum resolution
0.2 µm
0.2 µm
0.2 µm
Diffraction efficiency
to 40%
to 40%
to 40%
300
105
105
1000
106
106
Possibility of using
various energy values
Range
Energy gain in focusing
from a 100µm aperture
to a spot of:
0.3 µm
0.1 µm
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V.V. Aristov
2 Fields of Application (Under Developing Today)
1.
2.
3.
4.
5.
6.
7.
8.
X-ray microprobe in the energy range up to 100 keV with a submicron space
resolution.
Local double and triple crystal diffractometry.
Low-angle scattering for structure analysis.
X-ray microscopy.
X-ray holography.
Phase-contrast microscopy and microtomography.
X-ray scanning microscopy.
X-ray lithography.
(a)
(b)
Fig. 1. Examples of Bragg-Fresnel lenses on silicon
(a) Linear lens, (b) Circular lens
Bragg-Fresnel Optics
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3 X-Ray Scanning Microscopy on the Base of BFL1
Space scanning is due to the X-ray diffraction on an ultrasonic superlattice. Changes
in the wavelength of the superlattice give rise to the changes in the space position of
X-ray diffraction sattelites. Two-dimensional scanning is achieved by using two
ultrasonic superlattices that scan on X-ray beam in space in two mutually
perpendicular directions.
Specifications of today X-ray scanning microscope: the range of scanning with an
electric switching is 300x200 µm2, mechanical scanning 1x1 mm2.
Fig. 2. Schematic drawing of an X-ray scanning microscope (two dimensional scanning is
achieved by using two ultrasound supergratings which scan an X-ray beam in space
on two mutually perpendicular plate) [2]
4 Future of BFO
In the period of more than 10 years, lenses have been developed, fabricated and are
now operating. What can be the directions of further development of Bragg-Fresnel
optics and what improvements should be made? We think that today approaches have
been marked and technologies are being developed which allow us to increase the
resolution to 0.1–0.05 nm and to decrease the band pass by an order of magnitude and
1
The work is done in collaboration with Laboratoire de Cristallographie at CNRS (Grenoble).
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V.V. Aristov
to increase the diffraction efficiency by 1.5–2 times. It also seems expedient to use
composite lenses with 1, 2, 3, ... orders of diffraction, to enlarge the set of crystals and
reflections (for crystal lenses). Moreover, no investigations have yet been made on the
possibility of using asymmetric diffraction geometry, transmittance (Laue) diffraction,
and some other interesting problems. Let us now consider the ideas and technological
possibilities which would permit achieving the parameters mentioned above.
5 Electron-Beam Lithography
A technology has been developed which allows a three-dimensional correction of the
proximity effect, with allowance for the resist development. This technology provides
the possibilities for drawing and fabrication of complicated structures where large size
and fine elements are alternated (Fig. 3.) and possibilities to fabricate kinoform zone
plates as well. Moreover, for lens reproducing a special technology was developed
including electroplating of metals of resist relief and sequential imprinting in soft
materials (Fig. 4.). This technology is of special importance for X-ray optics.
Fig. 3. Map of the world made by e-beam lithography (Al on GaAs)
Bragg-Fresnel Optics
Fig. 4. (a) Kinoform structure in Si,
(b) Zone plates (diameter 500 µm) in resist (b1),
Cu metal replica from resist (b2),
printing by Cu-replica in polymer (b3)
Fig. 5. Counter map of the world. Line size near to 5 nm [4]
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V.V. Aristov
6 Plasmochemical Etching
For Bragg-Fresnel lenses on crystals, this technique is of crucial importance because
structures with linear parameters from several microns to 0.1 µm are to be etched to
the same depth 2 - 3 µm on one and the same substrate. We have coped with this
problem (Fig. 6.), and an appropriate procedure is now being tested [5].
Fig. 6. Results of etching in Si crystal depth of profile equal to (a) - 4 µm, (b) -2 µm
7 Composite Lenses
Of other technological methods, mention should be made of ion implantation, MBE
for growing gradient layers, and creation of lenses on super mirrors. However, all
these methods raise a challenging problem of increasing the band pass. In the case of
the diffraction of a plane wave on a gradient grating, the extension of the spectral
range is an experimental fact. In the case of a spherical wave, a gradient grating
operates as a one-dimensional Fresnel zone plate with its own focus distance [1].
Composite lenses afford an improvement of the lens reflection ability. This is a
well known fact, but we consider the possibility of using the second and fourth orders
of diffraction from the viewpoint of specific features of the technology of lens
fabrication. This possibility can be realized if the zone sizes are 1:3 instead of 1:1. At
the same sizes of a zone plate, the intensity gain is 1.5-2 times at a decreasing
background [6].
8 Conclusion
I would like to emphasize that lenses on crystals and on multilayer mirrors are, at
present, only the beginning of the way. Bragg-Fresnel diffraction has great poten-
Bragg-Fresnel Optics
IV - 33
tialities related to the possibility of varying different parameters. But the discussion of
these potentialities is beyond the scope of this paper.
In conclusion, I would like to thank my colleagues from the institute for their
efficient work in this hard time for Russian sciences. My thanks are due to the Organizing Committee of XRM-96 for the financial support which made the participation
in the work of the conference possible for workers of IMT RAS.
References
1
2
3
4
5
6
7
8
V.V. Aristov and A.I. Erko, X-ray Microscopy IV, Proceedings of the 4-th
International Conference Chernogolovka, Russia, September 20-24, 1993,
Institute of Microelectronics Technology, Chernogolovka, Russia (1994).
See papers in this book.
S.V. Dubonos, B.N. Gaifullin, V.N. Matveev, H.F. Raith, A.A. Svintsov, and
S.I. Zaitsev, J. Vac. Sci Technol. B 13 (6) Nov/Dec (1995) 25-26.
V.V. Aristov, S.V. Dubonos, R.Ya. Dyachenko, B.N. Gaifullin, H.F. Raith,
A.A. Svintsov, and S.I. Zaitsev, Microelectron. Eng. 27 (1995) 195.
Nano World Picture, Microscopy and analysis 34 (1995) 51.
V.A. Yunkin, D. Fischer, and E. Voges,. Reactive ion etching of silicon submicron-sized trenches in SF6/Cl2F3 plasma, Microelectronic Engineering 27 (1995)
463.
E.V. Shulakov, V.V. Aristov, Surface (Russian) N 3-4 (1966) 53-59.
E.V. Shulakov, V.V. Aristov, Surface (Russian) N 3-4 (1996) 60-68.