Cr/Sc/Mo multilayer for condenser optics in water window microscopes E C m T a b a A R R 1 A A K M S W R S 1 s a h h s o r m t o a C K c a s X u l h 0 ARTICLE IN PRESSG Model LSPEC 46622; No of Pages[.]
Trang 1Journal of Electron Spectroscopy and Related Phenomena xxx (2016) xxx–xxx
ContentslistsavailableatScienceDirect
j o ur na l ho me p a g e :w w w e l s e v i e r c o m / l o c a t e / e l s p e c
microscopes
Tadashi Hatanoa,∗, Takeo Ejimaa, Toshihide Tsurub
a IMRAM, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
b Faculty of Education, Art and Science, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata 990-8560, Japan
a r t i c l e i n f o
Article history:
Received 6 August 2016
Received in revised form
13 December 2016
Accepted 28 December 2016
Available online xxx
Keywords:
Multilayer
Soft X-ray
Water window
Reflectance
Sputtering deposition
a b s t r a c t
Wedevelopedahigh-reflectancewide-reflection-bandmultilayerforapplicationincondenseropticsin microscopesworkinginthewaterwindowsoftX-rayregion.Grazingincidence20periodCr/Sc mul-tilayerssuffereddamagewhentheyweredepositedontoroidalsubstrates, possiblybecauseofthe compressivestressofthethickSclayerspresentinthem.Toavoidsuchdamages,weminimizedthe
SclayerthicknessbyusingaCr/Sc/Mo10tri-layerstructure.Theresultingmultilayerhadaperiodic thicknessof9.8nmtoreflect310eVsoftX-rayatanangleofincidenceof77.2◦.Themultilayerswere successfullydepositedonSiwafersandconcaveBK7substratesbyionbeamsputteringwithnodamage Thepeakheight,angularacceptance,andspectralwidthofthemeasuredreflectanceoftheCr/Sc/Mo10 tri-layerwere27.4%,1.5◦,and35eV,respectively,showingthatitwassuitableforcondenseropticsof broadbandhigh-ZplasmasoftX-raysources
©2017TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense
1 Introduction
LaserproducedplasmashaveemergedaspowerfulsoftX-ray
sources.Inparticular,highlychargedionsofhigh-Zelementssuch
asAu,Pb,andBiradiatebroadbandsoftX-rays[1].Ontheother
hand,mirrors,gratings,polarizers,andotherX-rayopticalelements
havebeenupgradedwithmultilayercoatings[2–4].Inourprevious
study,agrazingincidencecondensermirrorwithamagnification
of 1 and a spotsize of∼1mm workingin thecarbon window
regionbelowtheK-absorptionedgeofCwasdeveloped[5].The
mirrorsurfacewascoatedwithaCo/C multilayerwithaperiod
thicknessof10.9nm.Thiscondensermirrorcouldbeusedin
vari-oustypesofmicroscopessuchastheimaging-andcontact-types,
and in otherexperiments inthe carbon windowregion.Above
CK-edge,microscopyin thewaterwindowregionbetweenthe
K-absorptionedgesofCandOenablestheobservationofliving
cellsata10nmorderresolution.Inordertorealizethepractical
applicationofBiplasmasourcesforwaterwindowmicroscopes,
shorterperiodgrazingincidencemultilayersreflecting310eVsoft
X-raysarestudied.SinceahighlychargedBiplasmaradiationis
unresolvedtransitionarrays (UTAs)andnota discreteemission
line,itsilluminationpowerdependsonthereflectionbandwidth
∗ Corresponding author.
E-mail address: hatano@tagen.tohoku.ac.jp (T Hatano).
ofthecondensermirroraswellaspeakreflectanceand numer-icalaperture.Thenumericalapertureis0.2iftheentireareaof
a100mm×40mmsubstrateissuccessfullycoated[5].Therefore, thegoalofthepresentstudyistorealizeahigh-reflectanceand wide-reflection-bandmultilayer
First, we designed and fabricated multilayers by a conven-tionalmethod.Asdescribedinthenextsection,theconventionally designedmultilayerssufferedfromdamages.Thisledustodevelop
anewstackedtri-layermodelinordertoobtainbetterresults
2 Fabrication of Cr/Sc multilayers of conventional model
Fordesigningthemultilayersreflecting310eVsoftX-rays,the opticalconstantsofseveralmaterialswereplottedinFig.1.Scis
aquiteexcellentmaterialbecauseofitssmallk.ThesmallkofCr andthelargedifferenceinnbetweenScandCrmakeCrthesecond suitablecandidateforhighreflectancemultilayers.Innormal inci-dencegeometry,short-periodhigh-reflectanceCr/Scmultilayers havebeenreported[6],anda normalincidencecondenser mir-rorhasactuallybeenfabricatedwithsuchCr/Scmultilayers[7], althoughtheirperiodnumberwaslargeandconsequentlytheir reflectionbandwasnarrow.UnderBragg’scondition,theperiod thicknessofamultilayerinagrazingincidencegeometrybecome thickerininverseproportiontocosineoftheangleofincidence Assumingawavelengthof4nmandanangleofincidenceof77.2◦,
aCr/Scmultilayerwithaperiodthicknessof9.9nmwasobtained
http://dx.doi.org/10.1016/j.elspec.2016.12.010
0368-2048/© 2017 The Authors Published by Elsevier B.V This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ).
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Fig 1.Optical constants of various materials at 310 eV.
ThelayerstructurewasdeterminedtobeaCr(3.7nm)/Sc(6.2nm)
20bi-layer.Apeakreflectanceof32.2%wasexpectedfors-polarized
lightwhentheRMSinterfaceroughnesswas0.5nm,asshownby
thedashedlineinFig.2
First, a multilayer with this design was deposited on a Si
wafer.Anionbeamsputteringdepositionapparatuswasused.The
reflectancewasmeasuredatthereflectometrybeamlineBL-11Dof
thePhotonFactory[8,9].Asatisfyingresultwasobtainedwitha
peakreflectanceof23.5%.Then,multilayerswiththesamedesign
weredepositedontoroidalfusedsilicaglasssubstrates.Inthiscase,
justafterthedeposition,themultilayercoatinggotdamagedand
thedamagegrewinafinelacepattern.Twoweekslater,thewhole
Fig 2. Calculated reflectance of Cr/Sc/Mo 10 tri-layer, Cr/Sc 20 bi-layer, and Cr/Sc
10 bi-layer.
multilayercameoffthesubstrate.Thisdamageisattributedtothe compressivestressofthethickSclayerspresentinthese multilay-ers
3 Design of three-material multilayer
Inadditiontothestackedbi-layermodel,stackedtri-or more-layermodelsincludingthinabsorbinglayershavebeenproposedto achievehighreflectanceatlowperiodicnumbers[10].For310eV softX-ray,wechoseMobecausetheopticalconstantsofCr,Sc, andMoformalargetriangle,whichindicatesarapidgrowthinthe reflectance
Inorder todeterminethelayerthicknesses,a layer-by-layer designmethod[11]wasappliedtotheproposedstackedtri-layer model.Athinnerlimitof1.4nmwasassumedbecauseanisland structure appeared during the early stage of the Mo thin film growthuntilthethicknessofthefilmreached1.4nm[12].Thefirst
CrlayerontheSiO2orSisubstratewas5.0nmthick.ThefirstSc andMo,andthesecondCrlayershadalmostthesamethickness,Sc (3.3nm)/Mo(2.7nm)/Cr(3.7nm).Inthesubsequenttri-layersthe mosttransparentScandtheleasttransparentMolayersbecame thickerandthinner,respectively.Fromthefifthtri-layeronwards, thethicknessesofthelayersbecameconstant,andthereflectance wassaturatedbythetenthtri-layer.Thelastsixtri-layerswereCr (3.6nm)/Sc(4.8nm)/Mo(1.4nm)andthetoplayerwasCr(3.3nm) Thus,theproposedmultilayerhadadepth-gradeddistributionof individuallayerthicknessesaboutanominalopticalperiodto opti-mizeperformance.ThetotalSclayerthicknessintheCr/Sc/Mo10 tri-layerwasreduced by63% compared tothatin theCr/Sc 20 bi-layer
ThecalculatedreflectanceoftheproposedCr/Sc/Mo10tri-layer was32.3%atthepeakasshown bythesolidlineinFig.2.The RMS interface roughnesswas assumed to be0.5nm.This peak reflectancewascomparabletothatoftheCr/Sc20bi-layerandthe reflectionbandwidthwaslargerthanthatoftheCr/Sc10bi-layer
4 Deposition of Cr/Sc/Mo multilayers
Cr (3.6nm)/Sc (4.8nm), Cr (3.6nm)/Mo (1.4nm), and Sc (4.8nm)/Mo(1.4nm)multilayersweredepositedonSiwafersby ionbeamsputteringattentativedepositionratesofCr,Sc,andMo Allthesemultilayersarebi-layerstacksoftheproposedtri-layer stack.TheirperiodthicknessesweremeasuredbysmallangleX-ray reflectometrytocorrectthedepositionrates.Althoughadecrease
inthelayerthicknessdue totheformationof interfaciallayers causessomeerrorinthedepositionrates,thisprocedurecancels outthe occurrenceof sucherrors andmultilayers withdesired thicknessescanbedeposited.AthicknesscontrolledCr/Sc/Mo mul-tilayerwasdepositedonaSiwafer
To test the multilayer deposition on concave substrates for anydamage,weusedaconcavesubstrate,butsubstituted spher-icalBK7substrateswitharadiusofcurvature of50mmforthe toroidalcondensersubstrateof R=620mmand =31.7mm[5] Theposition-dependentdepositionratewasmeasuredby carry-ingoutthedepositionwithsmallSiwafersheldonacylindrical surfacewitharadiusofcurvatureof50mmandthencarryingout smallangleX-rayreflectometry.WedepositedtheCr/Sc/Mo10 tri-layerandCr/Sc20bi-layerwithlaterallyuniformthicknesseson concaveBK7substratesusingourprogrammableshuttersystem [13].Nodamagewasobservedonthesurfacesofthesemultilayers Thesubstratesurfacewasobservedbeforeandafterdepositionbya microscopicinterferometer(VertScan,RyokasystemsInc.)to ana-lyzetheroughnessoftheBK7substratesandthatofthemultilayer surfaces,respectively.TheirRMSroughnesswas1.33and1.12nm, respectively.Itwasfoundthattheroughnessofthesubstratewas
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Fig 3. Measured reflectance of the Cr/Sc/Mo 10 tri-layer deposited on a Si wafer.
dominantandthemultilayerdidnotincreasetheroughness.These
roughnessvalues will reducethe reflectanceof themultilayers
depositedontheconcavesubstratetohalfofthatofthemultilayers
depositedontheflatsubstrates
ThereflectanceofthemultilayerswasmeasuredatBL-11Dof
thePhotonFactory.Theangulardispersionofthereflectanceofa
Cr/Sc/Mo10tri-layeronaSiwaferat310eVisshowninFig.3
Thepeakreflectancewas27.4%.Thereflectionbandwidth(angular
acceptance)was1.5◦.Thespectralreflectanceatanangleof
inci-denceof77.2◦wasalsomeasuredandthespectralbandwidthwas
foundtobe35eV,whichmatchesthatofaUTAofhighlycharged
Biplasma
ThereflectanceoftheCr/Sc/Mo10tri-layerandCr/Sc20bi-layer
depositedonconcavesubstrateswasalsomeasured.Sincethe
ele-vationanglefromthecentertotheedgebecome15◦onconcave
substrateswitharadiusofcurvatureof50mmandadiameterof
substratesizeof50mm,the12.8◦grazingincidenceontothecenter
isobstructedbytheedge.Therefore,thereflectancemeasurements
aroundanangleofincidenceof77.2◦werecarriedoutataradial
coordinateofr=20mm.Forthis,alaterallyuniformthickness
depo-sitionwasrequired.TheinletsofFig.4showfrontandsideviews
ofthereflectionmeasurementset-up.Theconcavesubstrateswere
rolledby23.6◦.TheresultsareshowninFig.4.Thereflectanceofthe
Cr/Sc/Mo10tri-layerdepositedonaconcavesubstratewas
approx-imately0.54 timeslowerthanthat oftheCr/Sc/Mo 10tri-layer
depositedonaflatsubstrate(Fig.3).Thisisinlinewiththe
rough-nessofthesesubstrates.TheCr/Sc20bi-layershowedapeakheight
sameasthatoftheCr/Sc/Mo10 tri-layerandnarrowerangular
acceptance
6 Conclusions
Wedesignedandfabricatedamultilayerreflectorforgrazing
incidencecondenseropticsinmicroscopesoperatingat310eVsoft
Fig 4.Measured reflectance of Cr/Sc/Mo 10 tri-layer and Cr/Sc 20 bi-layer deposited
on concave BK7 substrates.
X-ray The multilayer had a Cr/Sc/Mo 10 tri-layer structure Si wafersandconcaveBK7substrateswereusedforthedeposition
of multilayers No damage was observed The reflection mea-surementswereperformedatBL-11DofthePhotonFactory.The measuredpeakreflectanceofthemultilayerwas27.4%,whilethe angularacceptancewas1.5◦.Thespectralwidthwas35eV,whichis suitableforcondenseropticsofwide-bandhigh-Zplasmasources
Acknowledgments
Reflectancemeasurementswereperformedundertheapproval
ofthePhotonFactoryProgramAdvisoryCommittee(ProposalNo 2015G667).ThisworkwaspartlysupportedbyCAMSProject2015, IMRAM,TohokuUniversity
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