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Optics Communications 281 (2008) 3032–3035

Two channel multilayer mirrors for astrophysics

Julien Gautier a,*, Franck Delmotte a, Marie Franc�oise Ravet a, Arnaud Jerome a,Franc�oise Bridou a, Francoise Varniere a, Frederic Auchere b

a Laboratoire Charles Fabry de l’Institut d’Optique, CNRS, Univ Paris-Sud, Campus Polytechnique, RD128, 91127 Palaiseau cedex, Franceb Institut d’Astrophysique Spatiale Centre, Scientifique Batiment, 91403 Orsay cedex, France

Received 10 October 2007; received in revised form 18 January 2008; accepted 31 January 2008

Abstract

A two-channel mirror reflecting both Fe-IX/X (k = 17.1 nm) and He-II (30.4 nm) resonance lines at near normal incidence has beendesigned, fabricated and characterized. These two passbands are often chosen in space instruments designed for the observation of thesolar corona. The mirror structure used for optimization is a superposition of two periodic multilayers with three components per period.It has been designed by using optimization software with an appropriate merit function. The theoretical reflectivity for both resonancelines can reach 0.25. It is shown that, by using a set of filters, one can select either the Fe-IX/X or the He-II channel. The spectralresponse of the two-channel mirror has been measured on synchrotron radiation source on a large wavelength range, from 12 nm to35 nm. Experimental reflectivity reaches 0.32 for the Fe-IX/X line and 0.19 for the He-II line.� 2008 Elsevier B.V. All rights reserved.

Keywords: Multilayers; EUV optics

1. Introduction

The wavelength region ranging from 15 nm to 40 nm isof particular importance for EUV astrophysics [1,2] (EUVimaging telescopes), high harmonic generation sources [3],X-ray laser sources [4] and applications. In order toimprove the reflectivity of multilayer mirrors in the near30 nm range, we have proposed a new periodic multilayerstructure with three components [5]. In this previous paper,we have studied theoretically and experimentally theincrease of reflectance generated by adding a third materialin a periodic multilayer structure for wavelengths in therange 20 nm to 40 nm. Simulations show that multilayersmade of B4C, Mo and Si materials present higher reflectiv-ity than two component multilayers (B4C/Si and Mo/Si) ifthey are deposited in an optimal order: B4C over Mo overSi. B4C/Mo/Si structures present a theoretical reflectanceof 42% at 32 nm which represents a great increase as com-

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doi:10.1016/j.optcom.2008.01.069

* Corresponding author.E-mail address: julien.gautier@ensta.fr (J. Gautier).

pared with B4C/Si or Mo/Si multilayers (with a theoreticalreflectance of 34% and 30%, respectively).

In this paper, we study theoretically and experimentallythe possibility of using three components multilayers toselect two wavelengths of interest for astrophysics the Fe-IX/X (k = 17.1 nm) and He-II (k = 30.4 nm) emissionlines. In part 2 we describe the interest of such multilayermirrors for astrophysics applications. Part 3 deals withthe design and optimization of the multilayer structure.Then, in part 4, we present and discuss the realizationand experimental performances of a dual channel mirror.

2. Interest of two channel multilayer mirrors for astrophysics

Two channel multilayer mirrors (i.e., mirror reflectingtwo channels at the same time) are of great interest forastrophysics because one can choose the channel just byusing an appropriate set of filters. In the case of Fe-IX/X(k = 17.1 nm) and He-II (k = 30.4 nm) emission lines, amagnesium filter can isolate the 30.4 nm peak, while an alu-minum/zirconium filter can select the 17.1 nm region.

Table 1Optimized reflectivity and peak position for different weighting factors

Weightingfactors(a,b,c)

R(17.1 nm)(%)

Peakposition1 (nm)

R(30.4 nm)(%)

Peakposition2 (nm)

R(33.5 nm)(%)

(1,1,1) 48.2 17.1 1 – 1(1,3,5) 21.7 17.1 8.7 27.4 1.6

J. Gautier et al. / Optics Communications 281 (2008) 3032–3035 3033

These two lines are commonly used in space solar physics.The 17.1 nm passband is centered on a set of emission linesof iron dominated by lines from Fe-IX and Fe-X. Theselines are emitted around 0.8–1 MK, a temperature typicalof that of the average quiet solar corona. The 30.4 nm pass-band is dominated by the Lyman resonance line of He-II.This line is formed at about 50 K at the interface betweenthe solar chromosphere and the corona. The combinationof these two passbands gives access to a wide variety ofsolar phenomenon. With our system the two solar linescould be observed by using the same coating. Existingspace instrumentation, like the EIT [6] on the SOHO space-craft, uses one telescope (one entrance pupil) per observedwavelength. A multi band coating permits the selection ofseveral spectral lines with a single entrance pupil, thereforereducing the amount of heat to be rejected out of theinstrument, resulting in a significant gain in mass.

3. Design of two channel multilayer mirrors

Previous work on two-channel mirrors in the EUVrange has already been reported by Ejima et al [7]. Theyhave studied a multilayer design allowing high reflectivityat two wavelengths (He-I k = 58.4 nm and He-IIk = 30.4 nm) [7]. Their structure consisted of one periodicmultilayer optimized for k = 30.4 nm where the upperlayerwas optimized to increase the surface reflectivity (metallicreflexion) at k = 58.4 nm. For the selection of Fe-IX andHe-II emission lines, the surface reflectivity is too low innormal incidence: we have to use Bragg reflexion from mul-tilayer for both lines.

We consider a structure based on the superposition oftwo periodic multilayers, named ML1 and ML2 (Fig. 1).With such a structure, the calibration of each period thick-ness is done independently by depositing multilayer ML1on one sample, then ML2 on another sample. Once ML1and ML2 are calibrated, the multilayer mirror is made bya successive deposition of ML1 and ML2 on the same sam-ple. As shown in Fig. 1, both multilayers ML1 and ML2consist of periodical B4C/Mo/Si stacks. We have alreadydemonstrated the interest of such structure to optimizebroadband mirrors for reflexion of attosecond pulses [8,9].

The number of periods, optimized by simulation, is 10 inML1 and 4 and ML2. Thickness of each material in multi-layers ML1 and ML2 have been optimized by using a homemade Matlab code with optimization tools. The optimiza-

Si substrate

periodic multilayer ML2

periodic multilayer ML1

SiMoB4C

4 ×

≈≈≈≈ Si

Mo

B4C

10 ×

Fig. 1. Design of two channel multilayer.

tion consists on the minimization of a merit function MF inthe form:

MF ¼ 1� ða� Rð17:1Þ þ b� Rð30:4Þ � c� Rð33:5ÞÞ ð1Þwhere R(k) is the mirror reflectivity at wavelength k(in nm)and a, b and c are weighting factors. The rejection of33.5 nm wavelength (third term in Eq. (1)) is needed in or-der to obtain a good spectral selectivity around 30.4 nmwavelength (selection of He-II line).

The reflectivity is calculated at normal incidence usingthe iterative method [10]. We make the hypothesis thatthe interfaces between adjacent layers are perfect; i.e., thereare no scattering losses due to interface imperfections orcompound formation. The optical constants of the threematerials in the structure were obtained from the Centerfor X-Ray Optics World Wide Web Server [11].

Some results of optimization for different weighting fac-tors values are shown in Table 1. In this table, we givereflectivity and the peak position for the two channelsand the reflectivity at 33.5 nm. As one can see, withoutweighting (a, b and c equal to one), there is only one per-cent reflectivity at 30.4 nm. It has been necessary toincrease the b weighting factor in the merit function. Bestresults, in term of reflectivity and peak position, areobtained by using values of a, b and c, respectively, equalto 3, 5 and 5. We can notice that the first peak is well cen-tered. The second peak position might be improved byusing a more sophisticated merit function.

The result of this thickness optimization is shown inTable 2. For the top multilayer (ML2), a thickness of0 nm was found for the molybdenum. Thus the optimalstructure is made of a B4C/Si periodic multilayer depositedon top of a B4C/Mo/Si multilayer. The theoretical reflectiv-ity of the line Fe-IX (k = 17.1 nm) is 0.37 and 0.26 for theHe-II line (k = 30.4 nm). As explained previously, in theinstrumental system the selection of Fe-IX or He-II linecan be realized by using filters. Different filter thicknesseshave been studied. Fig. 2 shows the theoretical reflectivity

(2,4,3) 38.7 17 25 31.6 2.8(3,5,5) 37.1 17 26.2 31.1 0.15

Table 2Optimized thicknesses of multilayer ML1 and ML2

ML1 ML2

Number of periods 10 4B4C thickness (nm) 1.3 5.2Mo thickness (nm) 3.5 0a-Si thickness (nm) 13 12.5

20 30 40

1E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1

Ref

lect

ivity

Wavelength (nm)

Mg filter thickness 0 nm 50 nm 100 nm 200 nm

20 30 401E-8

1E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1

1

Ref

lect

ivity

Wavelength (nm)

Zr filter thickness 0 nm 50 nm 100 nm 200 nm

Fig. 2. Multilayer theoretical reflectivity after transmission through a magnesium and zirconium filters.

10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 400.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Ref

lect

ivity

Wavelength (nm)

Fig. 3. Experimental (dots) and theoretical (line) reflectivities versuswavelegth for the two channels multilayer at 10� of incidence.

3034 J. Gautier et al. / Optics Communications 281 (2008) 3032–3035

of the stack after the transmission through a magnesium fil-ter (Fig. 2a) or a zirconium filter (Fig. 2b) in logarithmicscale. As one can see, 200 nm thickness of magnesiumselects the He-II line with a rejection rate of 71 for theFe-IX line. Fifty nanometre thickness of zirconium selectsthe Fe-IX with a rejection of 380 for the He-II line.

4. Experimental results

Multilayers are deposited using a Ion Beam Sputteringdeposition system described in details in Ref. [1]. For thisstudy, B4C, Mo and Si targets are mounted on a targetholder which rotates alternately in front of the substrate.The deposition rates and thickness layer are controlled byusing a cooled quartz balance. Multilayers are depositedon silicon polished substrate. Each periodic multilayerwas calibrated separately. After deposition, each periodicmultilayer structure is controlled by means of the graz-ing-incidence X-ray reflectometry at k = 0.154 nm [12].By fitting the experimental data we can deduce the periodof the multilayer, the thicknesses ratio, the average interfa-cial roughnesses and the material complex indices at0.154 nm. Synchrotron radiation measurements in thewavelength region 10–50 nm have been performed onBEAR beamline reflectometer at Trieste SincrotroneELETTRA [13]. Experimental reflectivity versus wave-length has been measured with an incidence angle of10�in S polarization. We used a normal incidence mono-chromator for low energy (less than 40 eV), and a1200 lines/mm grazing incidence monochromator forhigher energy with Si or Al filters. The energy resolution,E/DE, is above 2000. Fig. 3 shows the reflectivity versuswavelength for the optimized stack. The measured reflec-tivity reaches 0.32 for the Fe-IX line and 0.19 for the He-II line. The calculated curve has also been plotted. Wecan notice a quite good agreement between the theoreticaland experimental curves for short wavelengths. Neverthe-less differences appear for higher wavelengths certainlydue to interfacial silicide formation and to the differencebetween the deposited and the tabulated material indices.

5. Conclusion

In summary, we have studied the possibility to reflectand select two wavelengths of the emission solar line witha unique multilayer coating. It consists of the superpositionof two periodic B4C/Mo/Si multilayers with different per-iod thicknesses. Theoretical optimization of such a struc-ture can be done easily by simulation. Experimentalreflectivity values for Fe-IX line and He-II line reach,respectively, 0.32 and 0.19, close to the theoretical reflectiv-ity values (0.37 and 0.26, respectively). This multilayer mir-ror, combined with a set of filters, brings new solutions fornext space telescopes for EUV solar imaging. Indeed onlyby moving filter, one can observed two lines of the solaremission lines with a good selectivity.

Acknowledgements

The authors would like to thank Prof. Nannarone andA. Giglia for their assistance during synchrotron radiationmeasurements on BEAR beamline at ELETTRA. This re-

J. Gautier et al. / Optics Communications 281 (2008) 3032–3035 3035

search was supported by the Centre National de la Recher-che Scientifique program ‘‘Nouveaux materiaux-fon-ctionnalites nouvelles” and the Centre National d’EtudesSpatiale in the R & T program framework. All multilayerdepositions have been carried out on the deposition ma-chine of the CEMOX (Centrale d’elaboration et de metrol-ogie des optiques X) implemented at the Institut d’Optiqueby PraXO (Pole d’optique des Rayons X d’Orsay).

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