ap-6-11.dvi

Acta Polytechnica Vol. 51 No. 6/2011

Czech Contribution to Athena

R. Hudec, L. Ṕına, V. Marš́ıková, A. Inneman, M. Skulinová, M. Mı́ka

Abstract

We describe the recent status of the Czech contribution to the ESA Athena space mission, with emphasis on the
development of new technologies and test samples of X-ray mirrors with precise surfaces, based on new materials, and
their applications in space. In addition, alternativeX-rayoptical arrangements are investigated, suchasKirkpatrick-Baez
systems.

1 Introduction
Design and development of X-ray optics have a long
tradition in the Czech Republic (e.g. Hudec et al.,
1991, 1999, 2000, 2001, Inneman et al. 1999, 2000).
A range of related technologies have been exploited
and investigated, including technologies for future
large light-weight X-ray telescopes.
Future large space X-ray telescopes (such as

Athena, consideredbyESA[16]) requireprecise light-
weight X-ray optics based on numerous thin reflect-
ing shells. Novel approaches and advanced technolo-
gies need to be developed and exploited. In this
paper, we refer to Czech efforts in connection with
Athena, focusing on the results of test X-ray mirror
shells produced by glass thermal forming (GTF) and
by shaping Si wafers. Both glass foils and Si wafers
are commercially available, have excellent surfacemi-
croroughnessofa few0.1nm,and lowweight (thevol-
ume density is 2.5 g ·cm−3 for glass and 2.3 g ·cm−3
for Si). Technologies need to be exploited for shaping
these substrates to achieve the requiredpreciseX-ray
optic geometries without degrading the fine surface
microroughness.
Although glass and, more recently, silicon wafers

have been considered the most promising materials
for future advanced large aperture X-ray telescopes,
other alternative materials are also worth further
study, such as amorphous metals and glassy carbon
(Marsch et al., 1997). In order to achieve sub-arsec
angular resolutions, the principles of active optics
need tobeadopted. TheAthenaX-rayobservatory is
a newX-ray telescope of theEuropeanSpaceAgency
(ESA). This project supersedes IXO (International
X-rayObservatory)aswell asNASA’sConstellation-
X concept and also ESA’s XEUS mission concept
(White et al., 2009). The spacecraft configuration
for the Athena study is a mission featuring two large
X-ray telecopes, each with an optical bench with a
focal length of approx. 11.5 m and a suite of focal
plane instruments. The Athena mission concept was
presented and discussed in detail at the 1st Athena

science meeting held atMPE inGarching, Germany,
in June 2011 [16].

2 Czech contribution to
Athena-related studies

At the moment, the Czech contribution to Athena
concentrates on: (1) participating in defining sci-
entific goals, justification and project preparation,
(2) participating in the design and development of
mirror technologies. The first author of this paper
was delegated as a member of the Athena Telescope
Working Group. In mirror development, we focus
on supporting ESA ESTEC micropore silicon tech-
nology design and also on designing and developing
alternative background technologies and designs, as
discussed in greater detail below. Originally, these
technologieswere studied for theESA/NASA/JAXA
IXO project, but we believe that at least some of
them might be valuable for Athena.

3 The glass foil option

Glass science and technology has a long tradition in
the Czech Republic. At the same time, glass tech-
nology is one of the most promising technologies for
producing mirrors for Athena and/or similar space
telescopes, as thevolumedensityof glass ismore than
3 times less than the volumedensity of electroformed
nickel layers. Glass foils can be used as flats, or may
be shaped or thermally slumped to achieve the re-
quired geometry. Thermal forming of glass is not a
new technology. It has been used in various sectors
of the glass industry and in glass art, as well as in
the production of Cherenkov mirrors. However, the
application of this technology in X-ray optics is re-
lated to the need to improve accuracy significantly
andminimize errors. As the first step, small (various
sizes typically less than 100 × 100 mm) glass sam-
ples of various typesprovidedbyvariousmanufactur-
ers were used and thermally shaped. The geometry

Acta Polytechnica Vol. 51 No. 6/2011

was either flat or curved (cylindrical or parabolic).
The project continued with larger samples (up to
300 × 300 mm) and further profiles. Recent efforts
have focused on optimizing the relevant parameters
of both glass material and substrates, as well as the
parameters of the slumping process.
Various approaches have been investigated. We

note that these are not quite identical with efforts
by other teams (e.g. Zhang et al., 2010, Ghigo
et al., 2010). The glass samples were thermally
formed at Rigaku, Prague, and also at the Institute
ofChemicalTechnology inPrague. For large samples
(300×300mm), facilities at theOpticalDevelopment
Workshop in Turnov were used. The strategy is to
develop a technology suitable for inexpensive mass
production of thin X-ray optics shells, i.e., to avoid
expensivemandrels and techniques that are not suit-
able for mass production, or that are too expensive.
Numerous glass samples havebeen shapedand tested
in order to find the optimal parameters. The shapes
and profiles of both mandrels, as well as the result-
ing glass replicas, havebeen carefullymeasuredusing
metrological devices. The results show that the qual-
ity of the thermal glass replica can be significantly
improved by optimizing the material and improving
the design of the mandrel, by modifying the thermal
forming process, as well as by optimizing the tem-
perature. After themodifications and improvements,
some of them significant, we obtained the resulting
deviation of the thermally formed glass foil from the
ideal designed profile to be less than 1 μm (peak to
valley value) in the best case. This value is, however,
strongly dependent on the exact temperature, so we
believe that further improvements are still possible.
The fine originalmicroroughness (typically better

than 1 nm) of the original float glass foil was found
not to be degraded by the thermal forming process.
Wenote thatourapproach in thermalglass forming is
different from the approaches used by other authors.
Recent efforts have been devoted to optimizing the
whole process, using and comparingdifferent forming
strategies etc., as the final goal is to further improve
the forming accuracy to less than 0.1 μmvalues. For
the near future, we plan to continue these efforts
together with investigations of computer-controlled
forming of glass foils (according to the principles of
active optics).

4 The silicon wafer option

Silicon is a relatively light material and already dur-
ing the manufacturing process it is lapped and pol-
ished (either onone side or onboth sides) to veryfine
smoothness (better than a few 0.1 nm) and thickness
homogeneity (of the order of 1 μm). Another obvious
option, recently considered as one of most promising
for high-precisionX-ray optics for Athena, is the use

of X-ray optics based on commercially available sili-
con wafers manufactured mainly for the purposes of
the semiconductor industry.
The main advantages of the application of Si

wafers in space X-ray optics are (i) the volume den-
sity, which is more than 3 times lower than the elec-
troformed nickel used in the past for galvanoplas-
tic replication of multiply nested X-ray mirrors, and
slightly less than the alternative approach of glass
foils, (ii) very high thickness homogeneity, typically
less than 1 μm over 100 mm, and (iii) very small
surfacemicroroughness either on one side or on both
sides (typically of the order of a few 0.1 nm or even
less). Silicon wafers were expected to be used in the
ESA XEUS and IXO projects, and are still under
consideration for the Athena project. The recent
baseline optics for the Athena X-ray telescope de-
sign is, like XEUS and IXO, based on X-Ray High
Precision Pore Optics (X-HPO), a technology cur-
rently under development with ESA funding (RD-
Opt, RD-HPO), with a view to achieving large effec-
tive areas with low mass, reduced telescope length,
high stiffness, and a monolithic structure, favoured
for handling the thermal environment and for simpli-
fying the alignment process (Bavdaz et al. 2010). In
addition, due to the higher packing density and the
associated shorter mirrors required, conical approx-
imation to the Wolter-I geometry becomes possible.
X-HPO optics is based on ribbed Si wafers stacked
together. The Si wafers for achieving the conical ap-
proximation are formed by stacking a large number
of plates together using a mandrel. The typical size
of the Si wafers is 10×10 cm.
There are also alternative X-ray optics arrange-

ments with the use of Si wafers. In this paper, we
refer to the development of an alternative design of
innovative precise X-ray optics based on Si wafers.
Our approach is basedon two steps, namely (i) devel-
oping dedicated Si wafers with properties optimized
for use in space X-ray telescopes, and (ii) precisely
shaping the wafers to the optical surfaces (Figure 4).
Stacking to achieve nested arrays is performed af-
ter the wafers have been shaped. In this approach,
Multi FoilOptics (MFO) is thus created from shaped
Si wafers. Formore details onMFO, seeHudec et al.
(2005).
This alternative approach does not require the Si

wafers to have a ribbed surface, so problems with
transferring any deviation, stress, and/or inaccuracy
from one wafer to the neighbouring plates or even to
thewhole stackedassemblywill be avoided. However,
suitable technologies for precise stacking of optically
formed wafers to a multiple array have to be devel-
oped.
The Si wafers available on the market are de-

signed for use mainly in the semiconductor industry.
It is obvious that the requirements of this industry

Acta Polytechnica Vol. 51 No. 6/2011

are not the same as the requirements of precise space
X-ray optics. Si wafers are a monocrystal (single
crystal) with some specifics, and this must also be
taken into account. Moreover, Si wafers are fragile,
and it is very difficult to bend and/or shape them
precisely for the thicknesses required for X-ray tele-
scopes, i.e. around 0.3–1.0mm. An exception is thin
Si wafers below 0.1 mm in thickness. However, these
can be hardly used in this type of X-ray optics be-
cause of diffraction limits. Also, while their thickness
homogeneity is mostly perfect, the same is not true
in the case of commercially available wafers for their
flatness (note that we refer here to the deviation of
the upper surface of a free-standingSiwafer from the
ideal plane, while in the semiconductor community
flatness is usually represented by a set of parame-
ters).
In order to achieve the very high accuracy re-

quired by ESA for future large space X-ray tele-
scopes like Athena, the parameters of the Si wafers
need to be optimized (for application in X-ray op-
tics) at the production stage. For this purpose we
have established and developed a multidisciplinary
working group including specialists from the devel-
opment department of the Si wafer industry with
the goal to design and manufacture Si wafers with
improved parameters (mostly flatness) optimized for
application in X-ray telescopes. It should be noted
that the manufacture of silicon wafers is a compli-
cated process with numerous technological steps and
with many free parameters that can be modified and
optimized to achieve optimal performance. This can
also be useful for further improving the quality of
X-HPO optics. As we are dealing with high-quality
X-ray imaging, the smoothness of the reflecting sur-
face is important. The standard microroughness of
commercially available Si wafers (we have used the
products of ON Semiconductor, Czech Republic) is
of the order of 0.1 nm, as confirmed by several inde-

pendent measurements using various techniques in-
cluding the Atomic Force Microscope (AFM). This
is related to the method of chemical polishing used
in themanufacture of Si wafers. Themicroroughness
of Si wafers exceeds the microroughness of glass foils
andmost other alternativemirrormaterials and sub-
strates. The flatness (in the sense of the deviation of
the upper surface of a free-standing Si wafer from a
plane) of commercially available Si wafers was how-
ever found not to be optimal for use in high-quality
(order of arcsec angular resolutions) X-ray optics.
Most Si wafers show deviations from the plane of
the order of a few tens of microns. After modify-
ing the technological process during Si wafer manu-
facture, we were able to reduce this value to just a
few microns. Also, the thickness homogeneity was
improved. In collaboration with the manufacturer,
further steps are planned to improve the flatness (de-
viation from an ideal plane) and the thickness homo-
geneity of Si wafers. These and planned improve-
ments introduced at the Si wafer manufacture stage
can also be applied for other designs of Si wafer op-
tics including X-HPO, and can play a crucial role in
the Athena project.
The X-ray optics design for Athena is based on

theWolter 1 arrangement, and hence requires curved
surfaces. However, due to the material properties of
monocrystalline Si, Si wafers (except very thin ones)
are extremely difficult to shape. It is obvious thatwe
have to overcomethis problem in order to achieve the
fine accuracy and stability required for future large
X-ray telescopes. The final goal is to provide opti-
cally shapedSiwaferswithno or little internal stress.
Threedifferent alternative technologies for shapingSi
wafers have been designed and tested to achieve pre-
cise optical surfaces (Figure 1). The samples shaped
and tested were typically 100 to 150 mm large, typ-
ically 0.6 to 1.3 mm thick, and were bent to either
cylindrical or parabolic test surfaces.

Fig. 1: Example of precise silicon wafer shaping. After deposition of poly-Si (thickness 1436 nm at temperature 615◦C)
and for wafer thickness 507 μm, a warp of 110 mm (R = 25.6 m) was achieved. Left: Wafer deformation map. Right:
Warp profile perpendicular to the facet

Acta Polytechnica Vol. 51 No. 6/2011

The development described here is based on a
scientific approach, and hence the large number of
samples formed with different parameters must be
precisely measured and investigated in detail. Es-
pecially precise metrology and measurements play a
crucial role in this type of experiment. The samples
of bentwaferswith the investigatedtechnologieshave
been measured, including Taylor-Hobsonmechanical
and STILL optical profilometry, aswell as optical in-
terferometry (ZYGO) and AFM (Atomic Force Mis-
croscope) analyses. It has been confirmed that all
these three technologies do not degrade the intrin-
sic fine microroughness of the wafer. While the two
physical/chemical technologies exploited give peak-
to-valley (PV) deviations (of the real surface of the
sample compared with the ideal optical surface) of
less than 1 to 2 μm over the 150 mm sample length
as preliminary values, the deviations of the first ther-
mally bent sample are larger, of the order of 10 μm.
Taking into account that the applied temperatures,
as well as other parameters, were not optimized for
this first sample, we anticipate that the PV (peak
to valley) value can be further reduced down to the
order of 1 μm and perhaps even below. Fine ad-
justments of the parameters can also further improve
the accuracy of the results for the other two tech-
niques.

5 The Kirkpatrick-Baez
option

Although Wolter systems are generally well-known
and widely used, Hans Wolter was not the first to
propose X-ray imaging systems based on reflection
of X-rays. In fact, the first grazing incidence system
to form a real image was proposed by Kirkpatrick
and Baez in 1948. This system consists of a set of
two orthogonal parabolas of translation (Figure 2).
The first reflection focuses to a line, which is focused
by the second surface to a point. This was necessary
to avoid the extreme astigmatism sufferedby a single
mirror, but it still was not free of geometric aberra-
tions. Nevertheless, the system is attractive because
it is easy to construct the reflecting surfaces. These
surfaces can be produced as flat plates and then me-
chanically bent to the required curvature. In order
to increase the aperture, a number of mirrors can be
nested together, but it shouldbenoted that this nest-
ing introduces additional aberrations. This configu-
ration is used mostly in experiments not requiring
a large collecting area (solar, laboratory). The ap-
plications in X-ray astronomy and astrophysics were
limited in the past, despite initial success on sound-
ing rockets (e.g. Gorenstein et al., 1978). Neverthe-
less, largemodules ofKirkpatrick-Baez (KB)mirrors

based on float glass have also been suggested for stel-
lar X-ray experiments (LAMAR experiment in KB
configuration designed for the Shuttle experiment,
e.g. Fabricant et al. 1988).

Fig. 2: Multi Foil Optics (MFO) in the Kirkpatrick-Baez
(K-B) arrangement

Superior silicon substrates have recently become
available that make it possible to consider design-
ing large KB modules with this novel material. As
mentioned above, Si wafers are difficult to shape, es-
pecially to small radii. To overcome this difficulty,
anotherX-ray optics arrangement can be considered,
namely theKirkpatrick-Baez (KB) system. Then the
curvature radii aremuch larger, of the order of a few
km,while the imagingperformance is similar. For the
same effective area, however, the focal length of the
KB system is about twice as large as the focal length
of the Wolter system. Nevertheless, KB systems rep-
resent a promising alternative to the classicalWolter
systems in future large space X-ray telescopes.
A very important factor is the ease (and hence

the reduced cost) of constructing highly segmented
modules basedonmultiply nested thin reflecting sub-
strates, in comparisonwith theWolter design. While
e.g. the Wolter design for Athena requires the sub-
strates to be precisely formed with curvatures as
small as 0.25m, the alternativeKBarrangementuses
almost flat or only slightly bent sheets. Hence the
feasibility of constructing a KB module with the re-
quired5 arcsecFWHMatanaffordable cost is higher
than for the Wolter arrangement.
Advanced KB telescopes are based on the Multi

FoilOptics (MFO)approach(X-raygrazing incidence
imagingopticsbasedonnumerousthin reflecting sub-
strates/foils). The distinction between MFO and
other optics using packed or nested mirrors is that
MFO is based on numerous very thin (typically less
than0.1mm) substrates. TheMFOKBtestmodules
were recently designedand constructed atRigaku In-
novativeTechnologiesEurope (RITE) inPrague, and
2 modules were tested in full aperture X-ray tests in
the test facilityof theUniversityofBoulder,withpre-
liminary results of FWHM 26 arcsec for a full stack
of 24 standard Si plates at 5 keV, and even better for
glass foils and 1D imaging (Figures 3, 4).

Acta Polytechnica Vol. 51 No. 6/2011

Fig. 3: Left: Test K-B modules assembled at Rigaku RITE in Prague, Right: The X-ray test facility for full aperture
X-ray tests at the University of Colorado in Boulder

Fig. 4: Themeasurement results of theK-B1D testmodulewith glass foils, full aperture tests at 5 keV at theUniversity
of Colorado at Boulder. The estimated FWHM of 1D focus is 4 arcsec

In our opinion, the use of KB design instead of
Wolter in Athena might help to retain high perfor-
mance (such as effective area) even in the event of a
reduced budget for Athena.

6 Conclusion
Suitable technologies for future largeX-raytelescopes
require extensive researchwork. Twopromising tech-
nologies suitable for future large-aperture and fine
resolution X-ray telescopes, such as Athena, were
exploited and investigated in detail, namely Glass
Thermal Forming and Si wafer bending. In both
cases, promising results have been achieved, with
peak-to-valley deviations of the final profiles from
the ideal profiles being of the order of 1 μm in the
best cases, with space for further essential improve-

ments and optimization. In the Czech Republic, an
interdisciplinary team with 10 members is cooperat-
ing closely with experienced specialists, including re-
searchers from a large company producing Si wafers.
Si wafers have been successfully bent to the desired
geometry by three different techniques. In the best
cases, the accuracy achieved for a 150mmSiwafer is
1–2 μm for deviation from the ideal optical surface.
Experiments are continuing in an attempt to further
improve the forming accuracy.
As an alternative, we have investigated the KB

option for Athena, with quite promising results jus-
tifying further efforts in this direction. A major ad-
vantage of KB is its low cost, as there is no need for
bending to small curvatures and/or for theuse of (ex-
pensive) mandrels. Less expensive optics can main-
tain a high effective area even on a reduced budget.

Acta Polytechnica Vol. 51 No. 6/2011

Acknowledgement

We acknowledge the support provided by the Grant
Agency of the Academy of Sciences of the CzechRe-
public, grant IAAX01220701, by theMinistry of Ed-
ucation and Youth of the Czech Republic, projects
ME918, ME09028 and ME09004. The investigations
related to the ESA IXO (now Athena) project were
supported by ESA PECS Project No. 98038. We
also acknowledge collaboration with Drs. J. Sik and
M. Lorenc from ON Semiconductor Czech Republic,
and with the team of Prof. Webster Cash from the
University of Colorado at Boulder for X-ray tests of
KB modules in their X-ray facility.

References

[1] Hudec, R., Valnicek, B., Cervencl, J., et al.:
SPIE, 1343, 162, 1991.

[2] Hudec, R., Pina, L., Inneman, A.: SPIE, 3766,
62, 1999.

[3] Hudec, R., Pina, L., Inneman, A.: SPIE, 4012,
422, 2000.

[4] Hudec, R., Inneman, A., Pina, L.: In Lobster-
Eye: Novel X-ray Telescopes for the 21st Cen-
tury, New Century of X-ray Astronomy, ASP
Conf. Proc., 251, 542, 2001.

[5] Hudec, R., Pina, L., Inneman, A. et al.: SPIE,
5900, 276, 2005.

[6] Inneman, A., Hudec, R., Pina, L., Goren-
stein, P.: SPIE, 3766, 72, 1999.

[7] Inneman, A., Hudec, R., Pina, L.: SPIE, 4138,
94, 2000.

[8] Kirkpatrick, P., Baez, A. V.: J. Opt. Soc. Am.
38, 766, 1948.

[9] Marsch,H., et al.: Introduction to Carbon Tech-
nologies, University of Alicante, 1997.

[10] White, N. E., Hornschemeier, A. E.: Bulletin
of the American Astronomical Society, Vol. 41,
2009, p. 388.

[11] White, N.E., Parmar,A., Kunieda,H.: Interna-
tional X-ray Observatory Team, 2009, Bulletin

of the American Astronomical Society, Vol. 41,
p. 357.

[12] http://ixo.gsfc.nasa.gov

[13] Bavdaz, M., et al.: Proceedings of the SPIE,
Vol. 7732, 2010, p. 77321E–77321E-9.

[14] Zhang, W. W., et al.: Proceedings of the SPIE,
Vol. 7732, 2010, p. 77321G–77321G-8.

[15] Ghigo, M., et al.: Proceedings of the SPIE,
Vol. 7732, 2010, p. 77320C–77320C-12.

[16] http://www.mpe.mpg.de/athena/
home.php?lang=en

[17] Gorenstein, P., et al.: Astrophys. J. 224, 718,
1978.

[18] Fabricant, D. G., et al.: Applied Optics, 27,
No. 8, 1456, 1988.

René Hudec
E-mail: rhudec@asu.cas.cz
Astronomical Institute
Academy of Sciences of the Czech Republic
251 65 Ondřejov, Czech Republic
Faculty of Electrical Engineering
Czech Technical University in Prague
Technická 2, 166 27 Prague, Czech Republic

Ladislav Ṕına
Faculty of Nuclear Engineering
Czech Technical University in Prague
Břehová 78/7, 110 00 Prague, Czech Republic

Veronika Marš́ıková
Adolf Inneman
Rigaku Innovative Technologies Europe, s.r.o.
Novodvorská 994, 142 21, Prague 4, Czech Republic

Michaela Skulinová
Astronomical Institute
Academy of Sciences of the Czech Republic
251 65 Ondřejov, Czech Republic

Martin Mı́ka
Institute of Chemical Technology
Technická 5, 166 28 Prague, Czech Republic