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esign, deposition, and characterization ofultilayer coatings for the Ultraviolet andisible-Light Coronagraphic Imager

aria-Guglielmina Pelizzo, Daniele Gardiol, Piergiorgio Nicolosi,lessandro Patelli, and Valentino Rigato

The Ultraviolet and Visible-Light Coronagraphic Imager is the baseline coronagraph for the EuropeanSpace Agency’s payload Solar Orbiter, a solar mission whose launch in 2011 is expected. To prove thefeasibility of its innovative design, a sounding rocket version of the same instrument has been approvedby NASA. One of the main technological challenges of the instrument is the achievement of multilayeroptical coatings with suitable properties. We describe the design, fabrication, and characterization ofsuch coatings. © 2004 Optical Society of America

OCIS codes: 120.4640, 310.1860, 350.1260.

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. Introduction

he Solar Orbiter �SOLO� mission1 is dedicated totudies of the Sun and the heliosphere. It was re-ently selected by the European Space Agency for aaunch scheduled for 2011. One of its instruments,art of the baseline payload for the observation of theolar corona, is the Ultraviolet and Visible-light Coro-agraphic Imager �UVCI�.2 The UVCI will obtainimultaneous images of the full Sun corona from 1.2o 2.6 solar radii at He II Lyman-� �304 � and H I

yman-� �1216 Å� lines to provide what is hoped to behe first determination of a He–H abundance map ofhe corona. These data will be fundamental for un-erstanding of the origin and dynamics of the solarind and the effects of streamers, coronal holes, and

oronal mass ejections. To complete the set of nec-ssary data, electron density will be derived by

M.-G. Pelizzo �pelizzo@dei.unipd.it�, P. Nicolosi, and A. Patellire with the Dipartimento di Ingegneria dell’Informazione and thestituto Nazionale per la Fisica della Materia—Laboratory for Ul-raviolet and X-Ray Optical Research, via Gradenigo, 6B - 35131adua, Italy. D. Gardiol is with the Istituto Nazionale distrofisica—Osservatorio Astronomico di Torino, Italy, Strada Os-ervatorio, 20 10025 Pino Torinese, Italy. V. Rigato is with thestituto Nazionale di Fisica Nucleare—Laboratori Nazionali diegnaro, Strada Romea, 4 - 35020 Legnaro �Padua�, Italy.Received 23 April 2003; revised manuscript received 11 Novem-

er 2003; accepted 15 January 2004.0003-6935�04�132661-09$15.00�0© 2004 Optical Society of America

isible-light images of the K-F corona analyzedhrough a polarimetric channel.

To establish a proof of concept for UVCI innova-ive design, the U.S. Naval Research Laboratorynd the Istituto Nazionale di Astrofisica–sservatorio Astronomico di Torino, in collabora-

ion with the Universities of Firenze, Padova, andavia, have proposed a sounding rocket version ofhe same instrument3 as part of the Helium Reso-ance Scattering in Corona and Heliosphere �HER-CHEL� experiment, whose payload includes aecond instrument, the Extreme-Ultraviolet Im-ger Telescope �EIT�, dedicated to Sun disk obser-ations. The HERSCHEL experiment wasecently approved by NASA for a launch scheduledn late 2005 or early 2006. Moreover, the Italian

inistry for the University and Research recentlypproved and financed a ground-based version ofhe UVCI.

The optical design of the UVCI–HERSCHEL in-trument consists of an off-axis Gregorian telescopemaging the corona at 1.4 to 3 solar radii. As isnown, common optical coatings provide negligibleeflectivity at 304 Å in normal-incidence configura-ions. Therefore, multilayer coatings are needed.4his kind of coating has already been used success-

ully for other space experiments, such as in theIT–Solar and Heliospheric Observatory5 that was

aunched by the European Space Agency and NASAn 1995. In this case the telescope is capable ofbserving the solar disk at several extreme-UVavelengths of important ions �He II and Fe IX–X,

1 May 2004 � Vol. 43, No. 13 � APPLIED OPTICS 2661

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e XII, and Fe XV�; the telescope mirrors had beenivided into four quadrants, each of them coveredith a Mo–Si multilayer optimized at a differentavelength.The multilayer coatings of the UVCI have to pro-

ide high reflectivity at three spectral regions ofnterest simultaneously, 304- and 1216-Å lines and

visible-light broadband �4500–6000 Å�, becausehe three spectral channels will share a commonptical path. Together with the optical design,his requirement represents one of the main inno-ative aspects of the coronagraph and is the reasonhy a prototype is needed. The wavelength selec-

ion is made by means of filters placed in front of theetector. The UVCI–HERSCHEL prototype willerify the capability of the optical coatings to satisfyhe scientific requirements. Moreover, a crucialbjective is to prove the capability of the design toeject visible light coming from the solar disk and toeduce the noise that is due to visible scattered lightnside the instrument, which again depends on theuality of the optical coatings, and in particular onheir superficial roughness.

The optical design and the coating requirementsre described in Section 2. In Section 3 the design ofhe multilayer coatings is described and their contri-ution to the effective area of the HERSCHEL instru-ent is estimated. In Section 4 we describe the

abrication of preliminary samples through sputter-ng deposition and their characterization through nu-lear and optical techniques. In Section 5 theeflectivity of the samples at various spectral wave-engths is presented. Our conclusions are discussedn Section 6.

. UVCI–HERSCHEL Coating Requirements

he UVCI–HERSCHEL optical design is shown inig. 1. It shares with the UVCI–SOLO the sameptical concept, but here dimensions and distancesetween the optical elements are rescaled for obser-ation from the Earth �1 astronomical unit �AU��ather than from the SOLO spacecraft, which during

Fig. 1. Optical design of a single HERSCHEL–UVCI channel.

Table 1. Multilayer Coating Requirements, T

Parameter Target Requirem

R at 304 Å �one reflection� 30%FWHM at 304-Å peak �two reflections� 15 ÅR at 1216 Å �one reflection� 25%R in the visible �one reflection� 30%

aStray-light rejection: contribution of forbidden lines at �170, 2

662 APPLIED OPTICS � Vol. 43, No. 13 � 1 May 2004

ts orbits about the Sun spans a variable distance0.2–0.5 AU�. It is assumed that the rescaling doesot affect the stray-light rejection properties of theptical design in a critical way. The telescope is anff-axis externally occulted Gregorian. The lightrom the solar disk is blocked on the front aperturey an external occulter �EO�. The disk light thatasses through the annular front aperture is re-ected back through the front aperture by a Sun-isk rejection mirror �MO� placed behind thexternal occulter. This mirror has an entrance ap-rture �EA� that is entirely in the shadow of the EO.he EA allows the telescope’s primary mirror �M1�

o collect the coronal light going through the EA.O is a slightly tilted spherical mirror whose focal

ength is equal to the distance between the EO andO. M1 creates real images of the diffracting

dges of the EO and the EA. The image of the EO’sdges is blocked by an internal occulter �IO�,hereas that of the EA is dumped into a Lyot traplaced around and behind the secondary mirrorM2�. In this way mirror M2 focuses only the coro-al light on the detector plane. The UV lightasses through an interference filter and is directlymaged. The visible light is reflected by the filter,asses through a polarimeter �not shown in Fig. 1�,nd then is imaged. Because of the short durationf the rocket flight, the instrument is split into twinhannels, channel He II for observation at 304 Å andhannel H I for observation at 1216 Å. The twoelescope mirrors of the He II channel are coveredith multilayer coatings, whereas those of the H I

hannel are coated with standard Al–MgF2 to en-ance 1216-Å reflectivity �for scientific purposes�.n both channels a suitable optical element �a mir-or for the He II channel and an interference filteror the H I channel�, placed in front of the detector,elects the UV line and reflects the visible light intohe polarimeter assembly.

The UVCI–SOLO will host a single optical channel,hich must be capable of observation at the three

cientific wavelengths with the same optical ele-ents. Therefore, multilayer coatings for theVCI–SOLO must be designed not only to enhance

eflectivity at 304 Å �which is the most critical wave-ength� but also to provide an adequate reflectivityevel at 1216 Å and in the visible band. Scienceequirements fix this level at 25% at 1216 Å and at30% in the visible spectral region.The same requirement applies then to the multi-

ayer coatings for the He II channel of the UVCI–

tical Calculations, and Experimental Valuesa

Theoretical Calculation Experimental Value

27% 25 � 1% �samples A, B�18 Å �19 Å �samples A, B�27% 27 � 2% �samples B, C�40% 40 � 2% �sample C�

284, 296, 321, and 315–317 Å is less than 10�2 �two reflectrons�.

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ERSCHEL instrument as a consequence of itseing a proof of principle for the UVCI–SOLO. Be-ides having high reflectivity in the three spectralanges of scientific interest, multilayer coatingsave to fulfill other requirements:

• They must be selective to suppress unwantedadiation near the 304-Å line at the focal plane; inarticular, strong emission lines at 284, 296, and 321and a doublet at 315–317 Å must be filtered out.• Multilayer second-order peaks must prevent re-

ection of other emission lines, such as the iron linesocated from 170 to 200 Å.

In Table 1 the basic scientific requirements of theultilayers are summarized.

. HERSCHEL Coating Design

he first step in multilayer design is the choice ofaterials for multilayer couples, which must have

ow absorption and high optical contrast. Ofourse, materials must also be compatible from ahemical point of view, and diffusion and compoundormation at interfaces must be minimized. Theest coupled material candidates for multilayersith high, 304-Å reflectivity are Si–C, Mo–Si, C–Al,nd B4C–Si �Ref. 6�; the last two have the highesteak reflectivity and the narrowest full width atalf-maximum �FWHM; Fig. 2�. Moreover, a thin

ayer of MgF2 deposited onto the last aluminumayer of the C–Al multilayer could guarantee pro-ection from oxidation and an increased reflectivityoth at 1216 Å and in the visible spectral region.owever, long-term coating stability is fundamen-

al for the success of a space mission, and onlyo–Si multilayers, whose performance was proved

n space on board the Solar and Heliospheric Ob-ervatory, have been tested over long times.herefore in this preliminary phase we have con-entrated on standard Mo–Si multilayer deposition.To optimize the deposition parameters, an anal-

sis of the optical design must be performed. Inarticular, as the multilayer spectral position de-ends on the angle of incidence �AOI� of light raysnto the optics, a map of the AOI on the two mirrors

ig. 2. Simulations of multilayer reflectivity for different coupledaterials and optimized for 304-Å high reflectivity: Mo–Si �solid

urve�, B4C–Si �dashed curve�, C–Al �dotted–dashed curve�.

ust be drawn. This map will allow the structuref the multilayer to be defined and the FWHM of theultilayer reflectivity curve to be evaluated for

ach point in the field of view �FOV�. The Sun’sorona from 1.4 to 3 solar radii is imaged onto theetector. Rays that come from different points inhe FOV follow different optical paths inside thenstrument, and therefore they are reflected by mir-ors M1 and M2 at different reflection angles. It ishen necessary to calculate the AOIs for differentoints in the FOV that are representative of theOV itself and for a number of rays to provide auitable sampling of the entrance pupil. For pur-oses of this research we considered 24 points in theOV geometrically disposed as 3 series of 8 pointsqually spaced in the azimuthal coordinate by 45°t three heights from the center of the Sun, i.e., 1.5,.2, and 3.0 solar radii. For each of the points inhe FOV that we considered, a square grid ofqually spaced rays at the level of the pupil wasuilt, and the AOI of each ray onto the two mirrorurfaces was evaluated. Data that correspond toays falling onto one of the two bulk mirrors ortopped by the inner or outer occulter were dis-arded. Results of the calculation for 2.2 solar ra-ii are shown in Figs. 3 and 4. The figures showight maps of AOIs onto either mirror M1 or mirror2 at the radial height shown.In each mirror map one can see a contour plot of the

OI onto the mirror surface labeled in degrees, theoundary of the nominal bulk mirror �a circle�, andhe rays for which the calculation was made �thosehat reach the focal plane, shown as small filled cir-les within the mirror edge�.

ig. 3. Eight contour plots of angle of incidence at surfaces ofirror M1 for rays coming from different point sources in the FOV

ymmetrically distributed at a distance of 2.2 solar radii �FOVources are marked by shaded areas on the picture of the Sun’sorona�. Incident points of rays on the mirror surface are markedy single filled circles.

1 May 2004 � Vol. 43, No. 13 � APPLIED OPTICS 2663

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From analysis of the AOI data we determined thatirror M1 shows a relatively higher AOI variability

han does M2 and that the variability of the AOI as aunction of position on the mirror is dominated by a

onotonic trend along the off-axis coordinate,hereas other variations can be regarded as second-

rder modifications.We initially considered standard, nongraded pe-

iodic Mo–Si multilayer coatings. For each mirrore optimized the Mo–Si multilayer coating param-

ters �period d and gamma ratio dMo�d� to have the04-Å peak efficiency at the two median values ofngle of incidence on M1 �9°� and M2 �4°�, respec-ively; multilayer parameters are listed in Table 2.heoretically, rays that fall onto mirrors M1 and2 at different AOIs cause a spectral shift and a

hange in the peak reflectivity value of the multi-ayer reflectivity curve. To quantify the effect athe focal plane we determined the distribution m�,��N �relative number of rays with incidence angleson M1 and � on M2 over N total rays reaching the

ig. 4. Eight contour plots of angle of incidence at surfaces ofirror M2 for rays coming from different point sources in the FOV

ymmetrically distributed at a distance of 2.2 solar radii �FOVources are marked by shaded areas on the picture of the Sun’sorona�. Incident points of rays on the mirror surface are markedy single filled circles.

Table 2. Multilayer-Coating Parameters of Mirrors M1 and M2

Parameter Mirror M1 Mirror M2

Couple materials Mo–Si Mo–SiPeriod 166 Å 164 ÅdMo�d 0.18 0.19Top layer

a-SiOx �10 Å �10 Åa-Si �5 Å �5 Å

664 APPLIED OPTICS � Vol. 43, No. 13 � 1 May 2004

ocal plane�. Then, using this distribution, weound the M1–M2 reflectivity curve associated withach point source in the FOV according to the fol-owing formula:

R� �

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here m�i, �j� is the fraction of rays reflected byirror M1 at an angle i that has a �j incidence

ngle on mirror M2, r1� is the reflectivity of M1 forncidence angle i, and r2� is the reflectivity of M2or incidence angle �j. The reflectivity of each mul-ilayer mirror was calculated with IMD software.7

These results will provide an estimate of the ex-ected flux at the focal point for each point on theOV. An example of expected flux for a pointource at 2.2 solar radii and azimuthal angle 90° ishown in Fig. 5: reflectivity curve R� is comparedith the nominal curve �i.e., the curve obtained for�4°, 9°��N � 1� shown by the dashed curve. Vari-

tion of the curve’s shape and peak position is neg-igible �values are given in the figure�. Thisonclusion is valid for each point source in the grid.t guarantees a uniform response over the FOV ofhe multilayer coatings; therefore a standard peri-dic multilayer can be accepted and there is no

ig. 5. Reflectivity R� versus wavelength for a point source at 2.2olar radii and 90° of azimuth. The nominal curve is shown as aashed curve for comparison �see text�.

ig. 6. Mirror M1 multilayer reflectivity curve superimposedpon the solar disk’s emission spectrum.

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ecessity to consider graded multilayer designs forurther optimization. To verify the rejection prop-rties at the unwanted wavelength we comparedhe reflectivity curves of the multilayer with theolar emission spectrum8,9 �see Fig. 6�. Data re-ated to the solar disk are easily available, and,ecause they represent a worst case with respect tohe coronal emission spectrum, we can consider es-ablishing them as an upper limit. It can be seenhat the second-order multilayer peak does notverlap iron-ion spectral lines at 170–200 Å, and,herefore, no significant contribution from theseines to the 304-Å reflected radiation can be ex-ected. In Fig. 7 the combined effect of doubleeflection on mirrors M1 and M2 is shown. Theesultant spectrum shows that the rejection of un-anted radiation from such adjacent lines as Si IX,96 Å, and Mg VIII, 315 Å, is efficiently achieved.esults of this theoretical analysis are summarized

n Table 1, with the target requirements given foromparison.

. Deposition of Samples

reliminary samples of multilayers optimized for 304were deposited onto Si �100� wafer substrates.10,11

hey consisted of 15 periods of Mo–Si bilayers. Weade the top layers of the samples by growing an15-Å additional silicon on top of the last silicon

ayer. An unknown part of the top layer naturallyxidizes when that layer is exposed to the environ-ent, forming a SiOx protective layer. The multi-

ayers were produced by rf magnetron sputtering in3.56-MHz excited argon plasma. The depositionhamber measured 110 cm � 72 cm � 65 cm and wasvacuated by a turbomolecular pump. One of theain problems during deposition was to limit the

mount of oxygen in the chamber to prevent oxida-ion of the deposited materials. By sputtering tita-ium onto the walls before deposition we furthereduced the oxygen content in the chamber’s residualas and lowered the base vacuum to 6 � 10�3 Pa.he ultimate pressure was almost entirely due toydrogen gas �as tested with a residual gas analyzer�,nd it was enough to prevent the oxygen contamina-ion in the deposited films from reaching 1 at. %, as

ig. 7. Combined effect of R� efficiency and the solar disk’s spec-rum.

onfirmed by nuclear techniques analysis. This wasrue also when multilayer deposition was performedt a low rate �0.5 Å�s�. The operating pressure ofhe argon gas was 5.0 � 102 Pa. The magnetronputter sources were 2 in.- ��5-cm-� diameter disks.he rf power applied was 60 W for molybdenum and50 W for silicon. Plasma parameters were charac-erized by cylindrical Langmuir probe measurementsuited for rf plasma.12 The plasma potential waseasured to be �25 V, and the argon-ion–atom ratioas of the order of 2. The growth of the molybde-um layers was assisted by bombardment by high-nergy neutral argon reflected by the target surface.his flux can be evaluated to be of the same order as

he ion flux but is characterized by a higher meannergy of �50 eV.Two substrates can be deposited at the same time.

hey are placed on a sample holder at a distance of6.5 cm from the target and parallel to the targeturface. The sample holder rotates 180° to allow theubstrates to face alternately the molybdenum andilicon targets. It can be biased from �100 to �100such that ions and electrons that are present in the

lasma can be accelerated toward or repulsed fromhe substrate. This property has been used to studyhe effect of ion bombardment on roughness at layernterfaces.13

The multilayer structure was characterized by nu-lear and optical techniques. We used ion-beamechniques to analyze the multilayer composition todentify contaminants and control the total amount of

o and Si deposited. These analyses were carriedut with HVEC 2.5-MeV and CN 7.0-MeV Van deraaff accelerators. We used conventional-particle Rutherford backscattering spectroscopy at

on energies of 1.8–2.2 MeV at scattering angles of70° and 160° and various tilt angles to investigateilicon and molybdenum content and profiles �Fig. 8�.e used elastic recoil detection analysis with a 2.2-eV �-particle beam at a 30° scattering angle and a

5° incidence angle at the sample surface to deter-

ig. 8. �-2.0-MeV Rutherford backscattering spectroscopy mea-ured and simulated spectrum of a sample optimized for high,0.4-nm efficiency � � 20°, � � 55°�. The multilayered structures highlighted.

1 May 2004 � Vol. 43, No. 13 � APPLIED OPTICS 2665

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ine the hydrogen concentration. We determinedarbon and oxygen content by using �d, p� nucleareactions at 900 keV. The oxygen and carbon con-entrations were less than 1 at. %; hydrogen was be-ow the elastic recoil analysis detection limit �1 at%�.

Furthermore, multilayer structures were charac-erized by x-ray and ion-beam techniques. X-ray re-ectance �XRR� measurements were carried out athe University of Brescia with a Bruker D8 Advanceiffractometer equipped with a Gobel mirror andith angular resolution better than 0.01° and accu-

acy of 0.001°. The XRR measurements, with.154-nm K� copper radiation, showed the presencef a highly regular structure. Experimental dataere fitted by IMD software to recover structuralarameters. In Fig. 9, XRR measurements and fitsf two samples �A and B� are shown. The period andhe ratio were 164.5 Å and 0.20, respectively, forample A and 164.6 Å and 0.21 for sample B, match-ng the nominal requirements for mirror M2 listed inable 2.

ig. 9. XRR and relative fits of two multilayer samples ��a� sam-le A, �b� sample B� in which structural parameters match mirror2’s nominal parameters.

ig. 10. Schematic of the vacuum facility used for –2 reflectiv-ty measurement at 304 Å.

666 APPLIED OPTICS � Vol. 43, No. 13 � 1 May 2004

. Characterization of Samples’ Reflectivity

he ultimate multilayer performance can be deter-ined only through accurate measurements in the

pectral ranges in which the mirrors are used. Toerform those measurements we used suitable exper-mental setups. Figure 10 is a schematic of thexperimental setup that we used to perform measure-ents at 304 Å.14 At this spectral wavelength, no

aboratory sources with spectral bands large enough toover the whole efficient bandwidth of the multilayerre available. Therefore in-band characterizationan be performed only at synchrotron facility beamines, which provide a spectrally continual sources.nfortunately, access to synchrotron facility beam

ines is often limited in time, and therefore the abilityo perform preliminary characterization in the labo-atory is fundamental. To achieve this goal we per-ormed angular measurements at 304-Å fixedavelength. Even if this characterization cannot

eplace in-band spectral characterization, a theoret-cal fit of the experimental data can provide impor-ant information, such as multilayer structuralarameters d and , an estimation of the interfaceoughness, and the peak reflectivity value. In-bandfficiency can then be recovered through simulationy use of that information in combination with pa-ameters derived by XRR analysis.

Our experimental setup is equipped with aollow-cathode source filled with helium gas, whichrovides a spectrally narrow wavelength at 304 Å.avelength selection is performed by a grazing-

ncidence grating monochromator. Selected radia-ion is sent into a test chamber, where the samplend the detector are located. The dispersive sys-em consists of a spherical grating monochromatorhat is mounted upon a 1200-line�mm gold-coated-m radius grating working in an off-Rowlandrazing-incidence configuration. The wavelengths selected by rotation of the grating about an axisangent to its vertex and parallel to the grooves.he reflectivity test chamber is equipped with aotorized system that permits the rotation of both

he sample and the detector in a –2 configuration.he detector is a channel electron multiplier oper-ted in photon-counting regime. The source wasperated at a pressure of �3 � 102 Pa and a currentf 100 mA; the entrance and exit slits were adjustedo maintain an incident flux of �100,000 cycles�s onhe sample. The spectral resolution of the systems limited by the intrinsic width of the spectral linemitted by the source, which, in the present case,as been estimated to be negligible. By takingnly photonic noise into account we evaluated theelative error in the measurement to be of the orderf 1%. Furthermore, the statistical spread of inde-endent measurements on the same sample re-ulted in a maximum deviation of 5%, which can bettributed mainly to nonuniform detector quantumfficiencies. Measurements of two structurallyimilar samples �A and B� optimized for high reflec-ivity at 304 Å are shown in Fig. 11, where a fit of

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he reflectivity curve of sample A is also shown.e simulated multilayer sample B in-band effi-

iency by using parameters recovered by both XRRnd angular reflectivity data fits and compared itith the efficiency curve of a nominal multilayer

tructure with sharp interfaces �Fig. 12�. Experi-ental samples show a peak reflectivity of �25%;

he nominal value is 27%.A schematic of the apparatus that we used toeasure the reflectivity at 1216 Å is shown in Fig.

3.15 It consists of an extreme-UV source coupledith a Johnson–Onaka monochromator and a test

ig. 12. In-band reflectivity of sample B recovered by simulation.ashed curve, nominal reflectivity curve shown for comparison.

ig. 13. Schematic of the vacuum facility used for –2 reflectiv-ty measurement at 1216 Å.

hamber. In this case the monochromator isounted upon a 1-m-radius 600-line�mm platinum-

oated grating that works in a normal-incidenceonfiguration. As a source we used a Hamamatsueuterium lamp emitting the deuterium I Lyman-�ine at 1216 Å. A manual rotation stage facilitateselection of the incidence angle on the sample. In-ident and reflected beams were acquired by a chan-el electron multiplier detector, and the reflectivityurves were related to the ratio of the two signals.he reflectivity of samples at 1216 Å strongly de-ends on the thickness of the native oxide layer onop of the multilayer. Two samples were measured:ample B was exposed to air only for a couple of hoursnd sample C was measured after long exposure toir such that the oxidation process could be consid-red to be kinetically in equilibrium. Experimentalata and relative fits are shown in Fig. 14. From theata fit we estimated a five-oxide-layer thickness ofample B �assuming that all oxides are SiO2� and a0-Å oxide layer thickness of sample C that is well ingreement with that reported by Hunter,16 who esti-ated a 10-Å silicon oxide layer thickness after at-ospheric exposure. Sample B was measured again

fter a month. The measurements were substan-ially identical to those obtained for sample C, indi-ating that 27% reflectivity is what we should assumeo be realistic for the UVCI instrument’s optics.part from this specific case, this measurement tech-ique has been found quite useful for determining thehickness of generic thin oxide layers and studyingheir evolution in time.

ig. 15. Schematic of the apparatus used for reflectivity measure-ent in the visible spectral range.

ig. 11. Measured reflectivity at 304 Å and relative fits of sample�crosses� and sample B �triangles� as functions of incidence angle

n the samples.

ig. 14. Measured reflectivity at 1216 Å and fits of sample Btriangles� and sample C �diamonds� as functions of incidence an-le on the samples.

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Measurements of sample C were performed in theisible spectral range by use of the setup sketchedn Fig. 15. A halogen lamp in front of a pinhole andcollimating lens were placed upon a rotating arm.fter reflection on the multilayer surface, the beamassed through a condensing lens, which focused itnto the entrance slit of a Czerny–Turner spectro-raph. A spectral interval of 50 nm could be ac-uired by a photodiode array. To acquire theirect beam we rotated the arm on which the sourceas mounted and translated the sample out of the

ptical path. The background signal that was dueo both electronic noise and room light was sub-racted from both direct and reflected radiation ac-uisitions, and reflectivity was deduced by the ratiof the two. The incidence angle on the sample waset at 35° because of mechanical constraints. Inhis case, because SiOx is transparent to visibleadiation, various layers of molybdenum and siliconan contribute to the reflectivity. Results and sim-lation are shown in Fig. 16, and the results are inood agreement with the multilayer structure al-eady used for fitting the measurements performedn the other spectral wavebands.

. Conclusions

he UVCI is a novel space coronagraph instrumentedicated to observation of solar corona imaging.o obtain simultaneous measurement at differentpectral ranges we coated the telescope mirrorsith multilayers that are required to perform overlarge spectral band, ranging from the soft-x-ray

egion to the visible. A detailed and systematicnalysis of the instrument design was performed toptimize the multilayer parameters and to achievehe required performance uniformly over the entireOV. The criteria used can be considered a stan-ard procedure for the optimization of multilayeroatings for large-FOV telescopes. Multilayeroating samples were deposited by rf magnetronputtering, and structural parameters were inves-igated by various nuclear and optical techniques.tructural parameters were found to match theominal requirements. The reflectivity of samplesas measured in all the spectral regions by use of

everal setups and measurement techniques. Re-

668 APPLIED OPTICS � Vol. 43, No. 13 � 1 May 2004

ectivity at 1216 Å was found to depend on oxida-ion of the last layer, a factor that strongly affectsultilayer performance in the soft-x-ray region

lso; therefore, reflectivity measurement at thisavelength can be used for monitoring multilayerging. The UVCI instrument requirements can beonsidered fully satisfied at a 304-Å spectral wave-ength. Areas in which we are striving for im-rovement are increasing 1216-Å reflectivity andchieving a smaller FWHM, which may be possibleith different two-layer materials Al–C multilayersre now under study.

The authors thank Ester Antonucci, Italian princi-al investigator of the UVCI instrument, and Elzaontempi of the University of Brescia for performing

he XRR reflectivity measurements. This researchas supported by the Italian Space Agency under

ontract ASI I�R�113�02 and by the University ofadova.

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