April 30, 2010 / Vol. 8, Supplement / CHINESE OPTICS LETTERS 163
Development of non-periodic multilayer in the EUV,
soft X-ray, and X-ray ranges
Zhanshan Wang (
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∗
, Jingtao Zhu (
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Zhong Zhang (
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), Fengli Wang (
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Fangfang Wang (
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), and Lingyan Chen (
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Institute of Precision Optical Engineering, Physics Department, Tongji University, Shanghai 200092, China
∗
E-mail: wangzs@tongji.edu.cn
Received November 25, 2009
A recent development of mirrors is reviewed in this letter. For some applications, such as the hard X-ray
telescope, polarization measurements in synchrotron radiation facilities, extreme ultraviolet (EUV) solar
observations, and dense plasma diagnostics in China, a series of non-periodic novel multilayers with special
performance are developed. X-ray supermirror, EUV broadband polarizer, EUV wide-angular mirror, and
double period Kirkpatrick-Baez (K-B) mirror are successfully designed by using different multilayer stack
structures.
OCIS codes: 310.0310, 340.0340, 350.1260.
doi: 10.3788/COL201008S1.0163.
Multilayer coatings have been widely us ed as key reflec-
tive elements in extreme ultraviolet (EUV), soft X-ray,
and X-ray ranges. In the early years, periodic multi-
layer coatings were utilized as reflective mirrors in as-
tronomical observation, X-ray lasers etc. at synchrotron
radiation facilities and dense plasma diagnostics, partic-
ularly in the soft X-ray region
[1−4]
. Except for normal
periodic multilayer coatings , ther e are some special re-
quirements which are not met by just using periodic
multilayer coatings in some applications, such as broad-
band width, wide angular, and multi-function mirrors. In
this regard, non-periodic multilayer coatings have been
developed for these cases. Development of thin film depo -
sition techniques has made it pos sible to fabricate highly
precise multilayers with lateral d-spacing variations nec-
essary to significantly improve the overall reflectivities of
these mirrors. Combining non-periodic multilayer coat-
ings with special substrates, X-ray optical elements with
beam-shaping properties have beco me possible. Today,
non-periodic multilayer coatings are increasing ly being
used. This paper summarizes the recent developments
in non-periodic multilayer coatings for applications in X-
ray telescope, EUV polarimetr y, X-ray microscope, and
dense plasma diagnostics.
The key to design non-periodic multilayer coatings is
the selection of merit functions, optimized algorithms,
and initial str ucture s. The selection of merit function
depends on the requirements of non-periodic multilayer
coatings. Different applications need different merit func-
tions. After the selection of merit function, optimized
algorithms need to be chosen
[5−8]
. Three algorithms,
simulated annealing algorithm, random sear ch algorithm,
and loca l optimization algorithm, are used. When the
local optimization algorithm is selected, the initial struc-
tures of multilayers are very important as they decide
how sa tisfac tory the obtained results are. Quarter wave
periodic multilayer and multilayer stacks with a va riety
of periods are often used during optimization. Moreover,
the initial solutions generated by an analytical expre ssion
can be used in the most of the time.
Multilayer coatings were deposited by magnetron sput-
tering in vacuum systems at a base pressure below
10
−4
Pa using Ar gas with purity of 99.9999% and a
sputtering gas pressure of typical 0.2 Pa. Multilayers
require the deposition of highly uniform thickness coat-
ing, so masks and varying rotation speed methods were
used. The structures of the periodic and non-periodic
multilayer coatings were characterized using X-ray re-
flectometry. The EUV and so ft X- ray characteristics
were performed with polarized radiation using the syn-
chrotron reflectometer at BESSY II. The roughnesses o f
the substrate and the multilayer coatings were measured
by using an atomic force microscope
[9]
.
Supermirrors can provide a broadband reflectivity com-
paring with periodic multilayers. There are two kinds of
supe rmirrors. One is the non-periodic multilayer which
has a broad energy band at a fixed grazing angle, the
other gives a broad angular band at a fixed X-r ay energy.
The supermirror is the key element for next-generation
hard X-ray telescope. For easy measurement, super-
mirrors working at Cu Kα line (8 keV) were designed.
Hence, most of o ur studies on supermirrors with a wide
angular range were conducted using the Cu Kα line.
For spe c ial requirements of the angular interva l at
8 keV, the simulated a nnealing algor ithm was used to
design a multilayer supermirror with two reflec tion bands
to check the feasibilities of the design and fabrication of
the supermirro r. Figure 1(a) shows the designed and
measured reflectivity curves of the W/C supermirror
as function of grazing angle. Figure 1(b) indicates the
thickness distribution of each layer in the supermirror
multilayer, which results in a big layer thickness oscilla-
tion along the depth of the multilayer structure. This
multilayer is prepared by using the direct current mag-
netron sputtering method. The difference between the
designed and measured supermirrors depends on the
achievable level of interface and roughness. Interfacial
roughness and diffusion result from various materia ls
and/or growth dependent mechanisms, including the
interlayer formation by diffusion or by mixing due to
1671-7694/2010/S10163-04
c
2010 Chinese Opt ics Letters