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Applications of Multilayer Optics
Zhanshan Wang*, Jingtao Zhu, Baozhong Mu, Zhong Zhang, Fengli Wang, Jing Xu,
and Lingyan Chen
Institute of Precision Optical Engineering (IPOE), Physics Department, Tongji University, Shanghai 200092, China
Elsevier use only: Received date here; revised date here; accepted date here
Abstract
Recent Chinese applications of multilayer optics in plasma diagnostics were reviewed in this paper. For some applications
such as plasma diagnostics, Extreme ultraviolet (EUV) astronomical observations in China, we developed a series of
multilayer optics with special performance. Double periodic Kirkpatrick-Baez (K-B) mirrors with high reflectivity at 8keV
and 4.75keV, double bi-functional mirror with high reflectivity at 30.4nm and very low reflectivity at 58.4nm and large
parabolic periodic multilayer optics working at 19.5nm for testing the performance of an EUV telescope were successfully
designed, fabricated and characterized by using different multilayer stack structures. © 2001 Elsevier Science. All rights
reserved
Keywords: multilayer optics; reflectivity; magnetron sputtering; EUV; K-B microscope
1. Introduction
Multilayer optics are widely used as key reflective
elements in EUV lithography, astronomical
observations, X-ray lasers, applications at
synchrotron radiation facilities and plasma
diagnostics, in Extreme ultraviolet (EUV), soft X-ray
and X-ray ranges for more than thirty years[1-4].
Except normal periodic multilayer optics, there are
some special requirements which are not meet by
using periodic multilayer coatings in some of
applications, such as bi-function optic, etc. special
multilayer optics have been developed in these cases.
Progress of thin-film deposition techniques made it
possible to fabricate multilayers with lateral dspacing variations of the high precision necessary to
modify the performance of these mirrors
significantly. Combining the special multilayer optics
with figured substrates, EUV, soft X-ray and X-ray
optical elements with beam-tailoring properties
became possible. Today, multilayer optics with
special property is gradually used with a further
increasing tendency.
In China, there are some special requirements for
using multilayer coatings in EUV, soft X-ray and Xray region. This paper summarizes our recent
developments of periodic multilayer optics in
applications of dense plasma diagnostics and EUV
———
*
Corresponding author. Tel.: +86-21-65984652; fax: +86-21--65986323; e-mail: [email protected].
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astronomical observations. In the first section we
introduce the design, fabrication and characterization
of multilayer optics. In the second section, some
examples of multilayer optics with special
performance for applications in EUV, soft X-ray and
X-ray range were described.
2. Design and Experimental
Keys to design of multilayer optics with special
performance include how to select merit functions,
optimized algorithms, and initial structures. The
choice of merit function depends on the requirements
of multilayer optics. Different applications need
different merit functions. After selection of merit
function, optimized algorithms need to choose [5-8].
There are three algorithms we used. They are
simulated annealing algorithm, random search
algorithm and local optimization algorithm. When the
local optimization algorithm is selected, initial
structures of multilayers is very important and decide
to how good the results obtained. Quarter wave
periodic multilayer and multilayer stacks with a
variety of periods are often used during the
optimization. The initial solutions generated by
analytical expression can be used in most of time.
The multilayer optics were deposited by
magnetron sputtering in vacuum systems at base
pressure below 110-4 Pa, using Ar gas with purity
99.9999%, and typical sputtering gas pressure of 0.2
Pa. For depositing multilayers required high
thickness uniformity, masks and varying rotation
speed methods were used. The structures of the
periodic and non-periodic multilayer coatings were
characterized using X-ray reflectometry. The EUV
and soft X-ray characterization was performed at the
synchrotron reflectometer. The roughness of
substrate and multilayer optics was measured by
using an atomic force microscope. Experimental
details can be found in previous publications [9].
3. Results and Discussions
3.1. X-ray K-B microscope system by using double
periodic multilayers
K-B configuration consists of two perpendicular
concave spherical mirrors in tandem. Rays from
object point are reflected by the first mirror
(tangential mirror here) and form a tangential line
while not a point. Similarly, rays via the second
mirror (sagittal mirror here) form a sagittal line. Thus
the perpendicular structure plays an important role to
overcome the strong astigmatism. K-B microscope is
often used in dense plasma diagnostics [10]. There is
a strong requirement for developing a K-B
microscope working at 4.75keV to investigate the
evolvement of laser produced plasma with time. The
performance of K-B microscope is dependent on the
quality of concave spherical mirrors which should
have accurate figure, low roughness and high
reflectivity and accuracy of system alignment.
Alignment of K-B microscope is generally based on
repeating X-ray at wavelength imaging experiments
if the very high performance wanted to be obtained.
This operation has to be done in vacuum for soft Xray K-B microscope working at 4.75keV because of
the absorption of 4.75keV soft X-ray in air, so the
experiments are very complex and expensive. The
alignment can be done in air by using 8keV Cu K
line and use in the vacuum for imaging working at
4.75keV if the K-B microscope works at 4.75keV
and 8keV at the same time in order to simplify the
alignment and save money. A double-periodic
multilayer optics was proposed for above purpose,
which consists of two part stacks with different
periodic thicknesses. The top stack reflects the soft
X-rays at 4.75keV for imaging, while the bottom
stack is designed at X-rays at 8keV, Cu K line for
alignment in the air. Therefore, it’s available for Xrays of two different energies to be reflected at the
same grazing angle. It is possible to design the top
and bottom stacks individually and then put them
together. We developed the double period multilayer
which includes W/B4C top layers and W/B4C bottom
layers. Shown as Fig. 1, this multilayer has high
reflectivity at grazing angle of 1.2° both for 4.75keV
and 8keV rays. Thus we can realize the alignment of
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4.75keV K-B microscope by imaging experiments at
8keV in air condition. Because the FWHM of 8 keV
is narrower than that of 4.75keV, the alignment
accuracy will be adequate enough. A one
dimensional advanced K-B microscope with two
spherical mirrors in one direction working at 4.75keV
was set up firstly, shown as Fig. 2, whose imaging
performance was better than that who has one
spherical mirror in one direction because the
spherical and coma aberration were partly corrected.
The imaging alignments results at 8keV were shown
as Fig 3 (a). Then, we marked the best object point
with a small ball. In Shenguang laser facility, we
should just make the target at the same point with the
marked ball. Finally, we got the imaging results of
1500l/inch mesh at 4.75keV (Ti Kα) (shown in figure
3 (b)). In summary, double periodic multilayer
provides a very practical method for the alignment of
soft X-ray grazing system in vacuum.
Fig.1. Reflectivity of double periodic W/B4C multilayer designed
both for 4.75keV and 8keV
periodic multilayer optics to solve the difficulty of
alignment in the vacuum at 4.75keV. The
experimental image of 2000lines/inch golden mesh
was shown in Figure 4 and a series of images of a
dense plasma were gotten which produced by hitting
eight laser beams with 300J pulse energy and 2ns
time width along four directions perpendicular to
each other to a cylindrical target with 260 micron in
diameter and thickness of chamber of 20 micron. The
laser beam to produce the Ti plasma has width of
100ps and 800J energy per pulse. The different time
dense plasma imaging during its evolvement could be
obtained by changing the backlit laser time relative to
the laser starting time who produced the dense
plasma.
(a)
(b)
Fig. 3. Imaging experimental results of X-ray KB microscope.
(a) Imaging at 8keV in air condition. (b) Imaging at 4.75keV
in vacuum.
Fig. 2. Schematic of one dimensional advanced K-B microscope
After the successful fabrication of one
dimensional advanced K-B microscope, we were
asked to make another two dimensional K-B
microscope working at 4.75keV to cast a dense
plasma backlit image at the detector for diagnostics
of its evolvement. We adopted the same double
Fig. 4. Two dimensional imaging experimental results of Xray KB microscope working at 4.75keV
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3.2. Extreme ultraviolet Mg/SiC aperiodic multilayer
with high reflectivity at 30.4 nm and low reflectivity
at 58.4 nm
The secondary phase of the Chinese Lunar
Exploration Program is scheduled to be launched
around 2013, which includes an Extreme Ultraviolet
Imager (EUVI). The EUVI is designed to observe the
earth’s plasmasphere by imaging the He+ 30.4 nm
emission from the lunar orbit. However, a bright
background radiation from the earth’s ionosphere,
which is a He 58.4 nm emission, must be blocked
[11]. Mg/SiC is a new material pair in the extreme
ultraviolet (EUV) region [12], which shows good
performance in the 25-35 nm region [13]. A novel
Mg/SiC nonperiodic multilayer, which exhibited HR
at 30.4 nm and LR at 58.4 nm, was designed,
fabricated and characterized.
The Mg/SiC nonperiodic multilayer was designed.
Substrates were pieces taken from 100 orientation
polished single-crystal silicon wafers. The optical
constants of magnesium, silicon carbide, and silicon
between 10 nm and 41.3 nm were from the Center for
X-ray Optics [14], while the optical constants
between 41.3 nm and 70 nm were from Ref.
[15,16,17]. The incident angle was 5°. The layer
number was limited to 60 because of absorption.
periodic multilayer exhibits a reflectivity of 58.7% at
30.4 nm and 2.2% at 58.4 nm.
In Figure 6, at an incident angle of 5°, the
reflectivity of the nonperiodic multilayer at
wavelengths of 30.4 nm and 58.4 nm is 36.7% and
2.0% respectively, while the lowest reflectivity is
0.024% at 63.0 nm. In comparison, a periodic
multilayer was fabricated and measured, whose
reflectivity is 34.4% at 30.4 nm and 1.8% at 58.4 nm.
Since the Mg/SiC nonperiodic multilayer
exhibited the lowest reflectivity at 63.0 nm rather
than 58.4 nm, the refractive indices of magnesium
and silicon carbide used in the design were assumed
to be inaccurate.
Fig. 6. Measured reflectivity curve of fabricated Mg/SiC
aperiodic multilayer (solid squares) and periodic multilayer
(hollow circles) at an incident angle of 5°
3.3. Parabolic periodic multilayer optics with large
area working at 19.5nm
Fig. 5. Calculated reflectivity curve of designed Mg/SiC
nonperiodic multilayer (solid squares) and periodic multilayer
(hollow circles) at an incident angle of 5°
A calculated reflectivity curve of a designed
Mg/SiC nonperiodic multilayer and a periodic
multilayer is shown in Figure 5. At an incident angle
of 5°, the nonperiodic multilayer has a reflectivity of
54.1% at 30.4 nm and 0.1% at 58.4 nm, while the
In recent years, the development of multilayer
technology has enabled the construction of
instrumentation and led to a number of successful
missions including Solar and Heliospheric
Observatory/Extreme ultraviolet Imaging Telescope
(SOHO/EIT) and Transition Region and Coronal
Explorer (TRACE) [18,19,20]. At the wavelengths
longer than the Si L-absorption edge near 12.4nm,
Mo/Si was widely used in the wavelength of 13~20
nm wavelength region for its high stability and high
reflectivity. The accurate deposition of high reflective
and uniformity on ultra-smooth polished parabolic
substrates is one of the major challengers for
construct a collimator to test the performance of solar
telescope working at 19.5nm. the Mo/Si multilayer
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mirror with the size of 230mm was investigated for
wavelength of 19.5nm (Fe-XII).
Figure 7 shows the X-ray reflective (XRR) curves
measured by XRD. 18 positions were measured alone
the diameter direction of the mirror with a spacing of
10mm. For each measured spot, the period thickness
can be calculated from the X-ray reflective peaks
according to the amended Bragg formula [11].The
period thicknesses of multilayer can be calculated for
each measured position spot. The mean period
thickness is 10.30nm, in agreement with the design
one.
Fig. 7. X-Ray reflectivity curves along the diameter of
200mm measured with spacing of 10mm
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calculated uniformity is within ±0.3% in diameter of
200mm.
Finally, the reflectivity is measured by the
reflectometer on beam line U26 at National
Synchrotron Radiation Laboratory (NSRL) in Hefei,
China. Reflectivities were measured at the fixed
incident angel of 5 degree using wavelength-scanning
mode. Along the diameter of 200mm, the reflectivity
is measured with spacing of 10mm. Figure 9 shows
all the measured curves. The consistency of the
reflectivity curves also indicates a good uniformity.
The measured peak reflectivity at 19.5nm is
(42±2)%.
Fig. 9. The reflectivity curves measured on beamline U26
at NSRL
4. Summary
Fig. 7. Normalization lateral layer thickness distribution
along the diameter of 200mm, each data point is calculated
from the measured period thickness shown in Figure 7
Figure 8 shows the normalization lateral layer
thickness distribution along the diameter of 200mm.
The uniformity (ΔD/D0) is normalized by the period
thickness (D0) at center spot (X=0mm). The
Some multilayer optics for different applications
in EUV, soft X-ray and X-ray wavelength region
were studied. All the multilayers were deposited by
using DC magnetron sputtering and characterized by
grazing incidence x-ray reflectometry analysis and
synchrotron radiation. A series of multilayer optics
were developed to meet the special requirement of
some experiments in China, such as dense plasma
diagnostics and EUV astronomical observations. The
double periodic multilayer mirrors are shown to
become essential optical elements in K-B microscope
system for dense plasma diagnostics. It will be
convenient to align the K-B microscope system in air,
not in vacuum chamber. A Mg/SiC nonperiodic
multilayer for the EUV with high reflectivity at 30.4
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nm and low reflectivity near 58.4 nm has been
demonstrated. This kind of nonperiodic multilayer is
expected to be applied to the EUV Imager in the
second phase of the Chinese Lunar Exploration
Program. For the purpose of testing the performance
of EUV telescope gotten extreme ultraviolet imaging
of solar corona by selecting Fe-XII emission line at
wavelength of 19.5nm, Mo/Si multilayer mirror was
deposited on large parabolic substrate with diameter
of 230mm with a fairly good uniformity. The
uniformity of lateral layer thickness distribution is
within ±0.3% in diameter of 200mm, measured by Xray diffractometer. The measured peak reflectivity is
42% at wavelength of 19.5nm.
Acknowledgments
This work is supported by the National Natural
Science Foundation of China (Grant No. 10825521,
10675091, 10675092, and 10876023), High-tech 863
program (Grant No. 2006AA12Z139) and by the
Shanghai Committee of Science and Technology,
China (Grant No. 09XD1404000, 07DZ22302,
09ZR1434300, 0952nm06900). The authors thank
Professor Alan Michette in King’s College London,
Dr. Mike MacDonald and Mark Roper in STFC
Daresbury Laboratory and Dr. Franz Schäfers and
Andreas Gaupp at BESSY-II for their kindly help in
discussion and measurement.
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