1

Non-invasiveScanningRamanSpectroscopyandTomographyforGraphene

MembraneCharacterization

StefanWagner1,ThomasDieing2,AlbaCenteno3,AmaiaZurutuza3,AndersonD.Smith4,MikaelÖstling4,SatenderKataria1,MaxC.Lemme1

1DepartmentofElectricalEngineeringandComputerScience,UniversityofSiegen,Hölderlinstrasse3,57076Siegen,Germany2WITecWissenschaftlicheInstrumenteundTechnologieGmbH,Lise-Meitner-Str.6,89081Ulm,Germany3GrapheneaS.A.,AvenidadeTolosa,76,20018-SanSebastián,Spain4SchoolofInformationandCommunicationTechnology,KTHRoyalInstituteofTechnology,E229,16440Kista,Sweden

Correspondingauthor:max.lemme@uni-siegen.de

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Abstract

Graphene has extraordinary mechanical and electronic properties, making it a promising

material formembranebasednanoelectromechanical systems (NEMS).Here, chemical-vapor-

deposited graphene is transferred onto target substrates to suspend it over cavities and

trenches forpressure-sensorapplications.Thedevelopmentof suchdevices requires suitable

metrology methods, i.e., large-scale characterization techniques, to confirm and analyze

successfulgraphenetransferwithintactsuspendedgraphenemembranes.Weproposefastand

noninvasiveRamanspectroscopymappingtodistinguishbetweenfreestandingandsubstrate-

supportedgraphene,utilizingthedifferentstrainanddopinglevels.Thetechniqueisexpanded

tocombinetwo-dimensionalareascanswithcross-sectionalRamanspectroscopy,resulting in

three-dimensionalRaman tomographyofmembrane-basedgrapheneNEMS.Thepotentialof

Raman tomography for in-line monitoring is further demonstrated with a methodology for

automated data analysis to spatially resolve the material composition in micrometer-scale

integrated devices, including free-standing and substrate-supported graphene. Raman

tomographymaybeappliedtodevicescomposedofothertwo-dimensionalmaterialsaswellas

siliconmicro-andnanoelectromechanicalsystems.

Keywords: Raman spectroscopy, Raman tomography, suspended graphene, non-invasive,

strain,doping,nanoelectromechanicalsystems,NEMS,MEMS,2Dmaterials

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Since its discovery, graphene has received substantial attention due to its exceptional

electrical and mechanical properties. It is the thinnest and one of the strongest materials

known with a thickness of 0.34nm (one atomic layer) and a Young’s modulus possibly

exceeding 1TPa.1 These properties make it possible to suspend this intrinsically nanoscale

materialacrossseveralhundredsofmicrometersdistance.2Furthermore,itisstretchableupto

20%ofitslength3andimpermeabletogases,includingHelium.4Inaddition,ithasahighcarrier

mobility5,6 and exhibits a piezoresistive effect i.e. the resistance changes when mechanical

stressisapplied,exhibitingagaugefactoroftheorderof3.7Thesepropertiessuggestagreat

varietyofpossibleapplications, inparticular inthefieldofmicro-andnanoelectromechanical

systems (MEMS/NEMS). Demonstrations reported in literature include graphene cantilever

beams for nano-switches,8mechanical graphene radio-frequency (RF) switches,9 desalination

usingnanoporousgraphenemembranes10orhighlyscalablegraphenepressuresensors.7,11All

oftheseapplicationsrequiresuspended(singleormultiple layer)grapheneovereitherclosed

cavities or open trenches. Mechanically exfoliated graphene has been used extensively to

investigate fundamental material properties and to demonstrate prototype devices.12–16

However,mechanically exfoliated flakes, reaching typical average sizes of only a few square

micrometers, are further distributed randomly on the substrate with a varying number of

layers. Even though precise placement of such graphene flakes over cavities and holes is

feasible,itrequirescontinuousmonitoringofthetransferprocessunderanopticalmicroscope.

Thismakes theprocess very cumbersome,andonlyprototypedevices canbedemonstrated.

Large-scalefabricationofNEMSdeviceswithhighyieldobviouslyrequiressuspendedgraphene

regions over large-areas. Chemical-vapor deposition (CVD) of graphene on catalytic metallic

substratesandsubsequenttransfer,however, isapromisingtechnique,17evenforsuspended

graphenefilms.18Here,thegraphenesizeisonlylimitedbythegrowthsubstrate.19

Copper (Cu) is typicallyusedasa substrate togrowgrapheneas it is compatiblewith silicon

technologyandrelatively inexpensive, inparticular if it isreused.20Thegraphenecanthenbe

transferredonlarge-scale,openingupthepotentialwafer-scaleprocessingofgraphene-based

devices.19,21Regardlessofthegrowthandtransfermethods,thepresenceofgrapheneonthe

device substrates is most commonly confirmed by Raman spectroscopy, which is a fast and

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potentially non-invasive analytical method for graphene and other twodimensional (2D)

materials.22Ramanspectroscopycanprovideawiderangeofinformationaboutgraphenesuch

as the number of layers22, stacking order23, quality22, defects24,25, doping24,26,27, thermal

properties28,29andstrain.30,31Thesecanbemeasuredandanalyzed fromsubstratesupported

graphene(SSG)22aswellassuspended,free-standinggraphene(FSG)32,whichisotherwisenot

easy to detect due to its thinness and its high broad-band transparency.33 Most Raman

measurements are obtained by recording single spectra at a few points of interest in the

graphene. For large-scale characterization, this acquisition process can be cumbersome and

mayyieldrandomresults.Therefore,large-areascansarepreferredoversinglespectraopening

up the possibility to analyze a continuous area where each pixel of the resulting image

corresponds to one single spectrum. Gajewski et al.34 and Suk et al.17 demonstrated high

resolutionRamanspectroscopytoinvestigatestrainonFSGversusSSGusingsmallareamaps.

With wide-field Raman spectroscopy it is possible tomap large areas with short acquisition

times.35 These systems, however, need one or multiple filters for predefining the desired

Ramanpeaksandarethereforenotabletorecordcompletespectra.Furthermore,thex,yand

z-resolutionisinthemicrometerrange35incomparisontothesub-micrometerresolutionofthe

conventional Raman systems. Three-dimensional (3D) Raman tomography is a very useful

feature of Raman spectroscopy where several line or area scans are recorded at different

depths (z-positions) through a material. This technique has been used for observing

composition of biological samples36–38, lattice defects39 and phase transitions40 in

semiconductorsandqualityofsyntheticdiamonds41inthepast.

Here,wedemonstratea fast,noninvasive,and large-areaconfocalRamanspectroscopy

and tomography method that distinguishes SSG and FSG rapidly and on a large-scale. The

methodcanpotentiallybeintegratedintoin-lineprocessingforrapid,large-scalemetrology.It

utilizes single spectra, 2D area mapping, cross-sectional scans as well as 3D mapping of

graphene.Bycombiningthearealmappingdatawiththedepthinformation,atrue3Dimageof

a specific volume can be reconstructed, and the in-plane continuity and homogeneity of

graphene with a resolution of 300 nm in lateral and 900 nm in vertical dimensions can be

evaluated. Furthermore, we present 3D Raman tomographic scans of FSG areas and their

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surroundings.Withthistechnique,onecaninvestigatethesurfaceandtheinsideofamaterial,

i.e.,materiallayers,differentphases,possibleinclusions,andcavities,whichotherwisecannot

beretrievedinanondestructivewaywithconventionalanalyticaltechniques.Inthiswork,we

applythismethodasanefficientwaytoscreenwhethergraphenemembranesaredamagedor

destroyedduring device fabrication. In principle, themethod canbe extended to investigate

otherelectrondevices,particularlythoseincludingRamanactive2Dmaterials.

Test structures for FSG membranes were fabricated on Si substrates with 1.6 μm of

thermallygrownsilicondioxide(SiO2,Figure1a).Cavitieswerethenetched1.4μmdeepinto

theSiO2withareactiveionetching(RIE)process(Figure1b).Thetestareascontainedtightly

spaced cavities including round holes with sizes between 3 and 15 μm diameter as well as

squareandrectangulargeometrieswithsimilarsizes.Finally,CVDgraphenegrownoncopper

foils was transferred with an optimized transfer process (Figure 1c). Figure 1d,e shows

micrographsoftheteststructureareasonthechipaftergraphenetransfer.

RamanexperimentswereconductedusingaWITecapyronconfocalRamanmicroscope

equippedwitha532nmlasercoupledtothemicroscopeusingasingle-modeopticalfiber.For

the high reproducibility of the experiments, the laser powerwas determined using a power

meter integrated in the optical path and controlled by the TruePower functionality of the

apyron microscope. The microscope was connected to a 600 mm ultrahigh-throughput

spectrometer (UHTS 600) using a photonic fiber. A 300 g/mm grating was used for the

dispersionandanEMCCDcameraforthedetectionofthescatteredlight.Forbestverticaland

lateral resolution aswell as for best collection efficiency, a 100× objectivewith a numerical

aperture(NA)of0.9waschosenforthemeasurements.Itshouldbenotedthatwehaveuseda

low-resolution 300 g/mm grating in combination with the 600 mm spectrometer to obtain

moreintense signalswith a pixel resolutionof 2.2 cm−1@2800 relative cm−1. This, in turn,

allowedustodecreasethescantimessignificantlyandtoquicklyidentifytheFSGregionsover

large-areas.However,afterquick identificationofsuspendedgraphenemembranesusingour

methodology,onecanuseahigh-resolutiongrating like1800g/mmorhighertoobtainmore

precisequantitativeinformationaboutdesiredRamanpeakpositionsandpeakwidthsatthose

specificlocations.Thesampleswerepositionedeitherusingamotorizedstepperstageforthe

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large-area imagesorusingapiezoelectricstageforthehigh-resolution images.Thezposition

wascontrolledbyasteppermotorstagewith10nmsinglestepaccuracy.Aschematicimageof

theRamansetupisshowninFigure2a.

TypicalRamanspectraofgrapheneshowthreecharacteristicfeatures,namelytheDpeak

at ∼1350 cm-1, the Gpeak at ∼1580 cm-1 and the 2Dpeak at ∼2700 cm-1.22 The intensity,

position, and full width at halfmaximum (fwhm) of these peaks depend on the quality, the

number of layers, the defect density of graphene,22 doping24,26,27 and strain.29,30 Also, the

incidentlaserpowercanhavesignificanteffectsongrapheneandcauseirreversibledamageto

the graphene structure.42,43 Since the threshold for eachmaterial, each device configuration,

andeachRamansystemsetupisdifferent,theeffectofthelaserpowerhasbeendetermined

before the actual measurements. The limit of the non-invasive regime was established by

analyzingthe2DpeakofSSGandFSG,asthisisthemostintensepeakforsinglelayergraphene

and can be easily observed evenwith very low laser powers. Furthermore, it is a very good

indicationforthepresenceofgraphene.22Thelaserpowerwasvariedoverawiderangefrom

0.1mW to 10mWand single Raman spectra of SSG and FSGon the Si/SiO2 substrateswere

recorded.ThepeakpositionandFWHMof the2Dpeakareplottedasa functionof the laser

powerininFigure2b.

Asignificantredshift(shifttowardlowerenergies)ofthe2Dpeakswasobservedaswell

asanincreaseinthefwhmforbothSSGandFSGsamplesforlaserpowersabove1mW.Atlow

ormoderatelaserpowers,areversiblephotoinduceddopingofSSGtakesplace,whichresults

inareversible2Dpeakshift.44However,forhigherlaserpowers,thereisasignificantriseinthe

sampletemperature,whichcanresult inanannealingeffectcausinganonreversibleshiftand

broadeningofthe2Dpeak.42ItisinthisregimethatRamanmicroscopybecomesinvasive,asit

modifiesthepropertiesofthegrapheneduringthemeasurements.Therefore,alaserpowerof

1 mW was used for all the Raman measurements in the present study to remain in the

noninvasiveregime.Thedifferenceinthe2DpeakpositionsbetweenSSGandFSGcanbedue

tothesubstrateinfluence,i.e,strain45anddopingconditions.34,46,47

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TheopticalimagesinFigure3a,bprovideanoverviewoftheentirechipandthemeasuredtest

area.Figure3bshowsahighdensityofcavitieswithdifferentsizesandgeometriesinwhichthe

goalwastoidentifyintactmembranes.Suchafastscanisusefultodeterminewhereamore-

elaborate and time consuming Raman tomography scan makes sense or is necessary or if

interestingresultscanbeexpected.Tocompensateforslighttiltsofthesamplerelativetothe

scanning stage, a tilted plane was defined using the system’s software and the sample was

scannedalongthisplane,keepingthesampleinfocusatalltimes.Afastlarge-areaRamanscan

of530×530μm2wasconductedat20msperspectrumontheteststructurearearesultingina

total scan time of 13 min. 200 × 200 spectra were taken on this area, and an image was

constructed inwhicheachpixel is representedbyone single spectrum.The resulting spectra

werefilteredforcosmicraysandbaseline-corrected.Finally,theintensityofthe2Dpeakwas

extractedfromeachacquiredspectrumandplottedasamapinFigure3c.TheareaswithFSG,

indicatedbyahighintensityofthe2Dpeak,caneasilybeidentifiedfromthisRamanimageand

appear as bright yellow areas. The analysis of the 2D peak region thus provides a clear

distinction between covered and noncovered cavities. This was confirmed with a high-

resolutionRamanareascanofa200×150μm2area(Figure3d).

ThescanningRamantechniqueprovidesadditionalinformationthatcanbeextractedafterdata

acquisition.Wescanneda35×32μm2areawithfourcavitiesin0.5μmwidestepsand100ms

integrationtimeperpixel.Thecavities’locationonthechipisindicatedwithawhitesquarein

Figure3c.TheresultingintensitymapisplottedinFigure4aaftertheremovalofthecosmicray

andbackgroundsignals.ThespectrashowninFigure4bwerederivedbyclusteranalysis48and

spectral de-mixing48, a software algorithm that automatically detects similar regions in the

spectral data that allows to objectively identify different materials (clusters) in a Raman

spectrum (Figure 4b). The k-means cluster analysis is described in detail in section S2 of the

Supporting Information.Using these spectra, abasis analysiswasperformedon the full data

set,revealingtheareasinwhichthedifferentspectracanbefound.Thespectraandtheimages

in Figure 4 are color-coded and thus serve to determine and assign the presence of FSG

(yellow),SSG(turquoise),amorphouscarbon(magenta),andsilicon(blue).Furthermore,avery

lowDpeakcanbeseenindicatingaratherhighqualityofthegraphenewithonlyfewdefects.A

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blackbackgroundcolordisplaystheabsenceofanyofthefilteredmaterials(i.e.,airorsilicon

dioxide).Fromtheresulting image,onecandeterminethat twoof thecavities (cavitiesno.3

and 4) are completely covered, one is partly covered (no. 1), and the fourth (no. 2) is not

coveredatall.

In addition to this area scan andmaterial classification, we performed depth scans in the z

direction along a 35 μm line across cavity 2 and 3, indicated by the green line in Figure 4a.

Thesescansstartedwiththefocalpointabovethesamplesurfaceand,afterfinishingeachline

scan,thefocalpointofthelaserwasautomaticallyadjustedinthezdirectionsothatitscanned

thesamplestep-by-step intothematerial.ThemaximumzresolutionoftheRamansystemis

900nm,andatotalof24linescanswithastepof300nminthezdirectionwererecordedwith

anintegrationtimeof500msperpixel.Themaximumtheoreticallypossiblezresolutionfora

greenlaserwascalculatedtobebetween630and680nm.49Inaddition,oversamplingcanbe

used to improve the image quality, whichwas done in case of the cross-sectional scan. For

materials with different Raman fingerprints, a detectable chemical contrast peak-by-peak

analysis is possible. Further improvements in resolution down to a maximum of 200 nm in

lateraland600nminthezdirectioncanbeaccomplishedbyusinghigh-NAobjectives,e.g.,oil

immersionobjectives in combinationwithadifferent laser. Thecross-sectional image (Figure

4c)wasreconstructedfromthesemeasurementsandshowstheSSGturquoiseontopoftheSi

(blue).TheFSGinyellowoncavity3isalsoclearlyvisibleincomparisontocavity2,whichisnot

coveredwithagraphenemembrane.Inparticular,thethicknessofgrapheneisnotdisplayedto

scaledue to the limited z resolution.At the topof the cavity, an areaof amorphous carbon

(magenta) at the periphery of the cavities can be seen, indicated by a broad peak with

overlapping D andG peakswith no signal of the 2D peak. Such overlapping or broad peaks

around 1300−1700 cm−1 are characteristic of amorphous or polymeric carbon products.50 In

thepresentcase,thereisastronglikelihoodofpolymericresidueformationfromtheRIEcavity

etchprocesswithCHF4gas,which is known tocreateaCxFy layeron the substrate surface,51

hencetheobservedRamanfeature.AriseintheDpeakcanbeobservedintheFSG(yellow),

SSG (turquoise), and the amorphous carbon (magenta) spectra. It can be attributed to the

formationofamorphouscarbonduringthe450°Cannealingprocessthatwasusedtoremove

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PMMA after transfer. The D peak in the FSG spectrum seems to have lower intensity in

comparisontothespectraobtainedinotherregions.Thisisduetothefactthatthepresented

spectraarenotnormalized,andthe2DpeakforFSGismuchmoreintense,asdiscussedearlier,

thussuppressingtheDpeak intensity.The1.6μmthickSiO2layer isdisplayedasablackgap

betweenthegrapheneandtheSi.NotethattheSilayeratthebottomofthecavitiesappears

tobeshifted inthezdirectioncomparedtothatbelowtheSiO2 inthecrosssectional images

(Figure4c),eventhoughthelayerisflatinrealityandthecavitiesdonotreachintotheSilayer.

This is an artifact due to different optical paths of the laser in both cases: the residual SiO2

thicknessofapproximately200nminthecavitiesismuchthinnerthanthatontherestofthe

substrate(1.6μm).TheSiO2isopticallytransparentforthewavelengthof532nm.Duringthe

measurementontheSi, thebeamisfocusedatapoint located insidetheSi layer.Therefore,

theincidentandtheoutgoinglightpassingthroughtheSiO2layerarerefractedattheinterfaces

between air and SiO2 and at the interface between SiO2 and Si. As a result, the focal point

appears closer to thedetector than it actually is. Thiseffectbecomes stronger for increasing

SiO2thickness,whichresultsintheSilayerbeingdisplayedatdifferentz-positions.Forknown

beam paths, device designs, or structures, such artifacts can be removed by including an

appropriatealgorithmintotheanalyzingsoftware.

A 3D tomography image of the graphenemembrane test structure was reconstructed

fromeightareascansatdifferentzpositionswith300nmstepwidth(Figure4d).Thetotalscan

timeisapproximately2.5h,aseightimagesof18×18μm2with3600singlespectraand300ms

integrationtimeeachweretakentoachieveahigh-resolutionimage.Differentmaterialswere

again color-coded according to Figure 4b. The resulting 3D image (Figure 4e) allows us to

analyzetheentiredeviceincludingmaterialsinsidethesubstrate.Forexample,Figure4eclearly

showsthatthegraphene is indeedsuspendedacrossthecavityanddoesnotextend intothe

cavity(withinthelimitedzresolution).Furthermore,nographeneispresentatthebottomof

theholes.Theringofamorphouscarbon(magenta) ismainlydetectedatthetopedgeofthe

cavityandisveryslightlyvisibleintheareawherethesidewallsmeetthebottomofthecavity

butnotalongthesidewallsandthebottom.

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One or several layers of the 3D-image stack can be analyzed in depth by extracting

detailedmaterial informationatdifferent locations in the3Dspace. In caseof thegraphene,

thedopingandstrainconditionsinFSGandSSGareofgreatinterestandcanbedeterminedby

choosing the second image from the stack show in Figure 4d. Figure 5a shows the

representative Raman spectra for suspended and nonsuspended regions, with positions

indicatedinFigure5bcorrespondingtothegray,purple,andpinkcrosses.AlowerDpeakand

anincreaseinGand2DpeakintensityfromSSG(grayandpinkspectra)toFSG(purplespectra)

canbeobserved.Areamaps inFigure5b−eprovideadditionaldetailed informationregarding

thedistributionofthedisplayedvalues,whichcannotbeobtainedwithsinglespectrarecorded

on justa fewplacesacross thesample.Figure5bshowsaclear increase in2Dpeak intensity

fromSSGtoFSG.Inaddition,themapofthepeakposition(Figure5c)andfwhm(Figure5d)of

the 2D peak have been reconstructed using a Lorentzian fit of that peak. Black pixels in the

imagescorrespondtozerovaluesandindicatespectrainwhichnosolutionfortheautomated

fitting could be found. Finally, themap of the 2D overG peak intensity ratio is displayed in

Figure5e.ThelowDpeak,thehighandnarrow2Dpeak,andahigher2Dintensityaswellasa

higher intensity ratio of 2D and G peaks in the case of FSG indicate different graphene

properties thanSSG.22,23,52 Further analysisof theavailabledata shows that the2Dpeaksare

blue-shifted(shiftstowardhigherenergies)byapproximately15cm−1intheSSG.Thisshiftcan

alsobeobservedinthelaser-power-dependentRamanmeasurements(Figure2).Theobserved

decreaseinI2D/IG,duetoadecreasein2DpeakintensityinSSG(Figure5e),canbeattributed

toholedoping.34,46,47Theblueshiftofthe2DpeakinSSGresultsfromtheSiO2substrates,which

is known to result in p-doping due to trapped charges in the oxide53 and ambientmolecular

doping/humidity54.FSGovercavitiesislikelytohavebuilt-intensilestrainduetothegeometry

of membranes, i.e. slight sagging of the membranes or strain which was introduced during

processing.45 In contrast, thermal annealing is known to induce compressive strain in SSGon

SiO2 due to thermally inducedmechanical transformations.55 Even though both SSG and FSG

show spectral inhomogeneity across the regions, FSG exhibits a clear contrast to the

surrounding supported regions. Therefore, different doping and strain levels are likely

responsiblefortheobserveddistinctRamancharacteristicsofFSGandSSG.

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A similar device to the one discussed in Figures 4 and 5was investigatedwith Raman

tomography.Here,thecavitieswerecoveredwithpolytetrafloerethylen(PTFE),andtheresults

areshowninFigureS1.Here,thePTFElayersonthesidewallsclearlyshowupatthesidewalls

downtothebottomofthecavity.Thisimagedemonstratesthatalsounderneaththegraphene

membrane,i.e.,chemicalinformationcanberetrievednoninvasivelyfromtheinsideofdevices.

ThisextendstheapplicabilityoftheRamantomographymethodtographenecoveredchemical

microreactors56andmicro-andnanobubblescoveredbygraphene57,58ormicrofluidicsystems.

Si-baseddevicescouldbeinvestigatedwithlaserswithhigherwavelengthssuchas785or1064

nm for higher penetration, albeit with the disadvantage of decreased resolution. Here,

applicationsinsiliconMEMScanbeenvisioned,todeterminenoninvasivelythefullreleaseof

MEMSsystemsorthestatusofintegratedandcoveredpartslikemembranesorthesupporting

structures.Anotherapplicationexamplecouldbeverticalheterostructureslikegraphene-based

hot electron transistors,59 even though in all cases, the limited z-resolution prevents the

determination of exact thicknesses and locations of very thin layers in the devices. To

determinethese,destructivemethodssuchascross-sectionalSEMorTEMstillhavetobeused.

In summary, we have demonstrated a noninvasive, large-area, and relatively rapid

metrologymethodtodeterminethepresenceoffree-standingCVDgrapheneandtodistinguish

itclearly fromgraphenesupportedonasubstratebasedonconfocalRamanmicroscopy.The

noninvasivelimitofthelaserpowerwasdeterminedinawidepowerrange.Specificregionsof

a test chip were analyzed by single-spectrum, high-resolution 2D areamapping.We further

introducedanovelmethodologytoreconstruct3Dtomographicimagesofsolid-statedevices,

whichareauniquewaytogainnondestructiveinsightwithfairlyhighspatialresolutionofthe

structure,material,compound,andotherphenomena.Thesteppermotorandpiezoscannerin

the Raman system allow the correlation of large-area-scale data with high-resolution

information.Thepresent study suggests, inprinciple, in-line integrationofanoninvasiveand

noncontact metrology method for graphene as well as other devices composed of Raman-

activematerials.Thiswouldbeofhighinterestforfutureapplicationsasitprovidesameansof

qualitycontrolduringfabrication,whichcouldbeautomatedtoinvestigatehundredsofpoints

ofinterestacrossachip.Themethodissuitabletoprovidechemicalandstructuralinformation

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notonlyof thesurfacebutalso insidedevicesandthusenablesthenoninvasiveanalysisofa

widerangeofapplications,includingsiliconMEMSandmicrofluidics.

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Methods

Theteststructureswerefabricatedonap-dopedsiliconsubstratewitha1.6μmthermaloxide

layer. The structuresare cavitieswithdiametersbetween3and20μmwith circular, square,

and rectangular geometry. They were etched 1.4 μm deep into the SiO2 layer using a

photoresistmaskandanAr-andCHF3-basedreactive-ion-etching(RIE)processusing200mW

and 40 mTorr for a vertical etch profile. The monolayer graphene samples were grown by

chemicalvapordeposition(CVD)ina4in.coldwalledreactor(AixtronBM)usingcopper(Cu)foil

as the growth substrate. Cu surface pretreatment and high-temperature annealing were

performedbeforethegraphenegrowth.Thegraphenesynthesiswascarriedoutusingmethane

asthecarbonsource.Afterthesynthesis,asemidryproprietarytransferprocessusingPMMA

as the supporting layerwasused to suspend themonolayer grapheneonto the cavities. The

supportingpolymerwasremovedbyannealingthesampleat450°Cfor2hinN2atmosphere.

Finally,thegraphenesampleswereannealedatatemperatureof450°Ctominimizeresidues

formedbyprocessing.

Acknowledgements

Funding from the German Federal Ministry of Education and Research (BMBF, NanoGraM,

03XP0006), theEuropeanResearchCouncil (ERC, InteGraDe,3017311), theGermanResearch

Foundation (DFG LE 2440/1-2) and he Spanish Ministry of Economy and Competitiveness

(MINECO, NanoGraM, CDTI IDI-20150827 and SPRI IG-2015/0001026) is gratefully

acknowledged.

Supportinginformation

- Raman 3D-image stack and tomography image of a second device fabricated using a

PTFElayertocoatthesidewallsofthecavity

- Descriptionofthek-meansclusteranalysismethodusedinthesoftwaretoanalyzethe

Ramandata

14

Figure1:Cross-sectionalschematicofthefabricationprocessforteststructures:(a)substrate;

(b)RIEcavityetch;(c)graphenetransferontoteststructurestocoverthecavities.(d)Optical

micrographoftheteststructures;(e)zoomed-inareaoftheredsquaremarkedin(d)indicating

thecross-sectionalviewofthefabricationprocess.

15

Figure2:(a)Typicalbeampathofanα300/apyronsystem;(b)Ramanpeakpositionandthefull

width half-maximum (fwhm) of the 2D peak as a function of laser power for FSG and SSG.

Values for FSG are higher than for SSG due to substrate influence. The dashed vertical line

indicatesthelimitforthenoninvasiveregimeforcharacterizinggraphene.

16

Figure3: (a)Stitchedwhite-light imageofthecompletechip; (b)stitchedwhite-light imageof

the test structures; (c) Raman image of the test structures displaying 2D peak intensity; (d)

zoomed-inimageofthelarge-areamap(lightbluesquare)showingthe2Dpeakintensityforan

areawithcoveredanduncoveredcavities.

17

Figure4:(a)2DRamanscanoffourcavities(1,2,3,and4)fromtheteststructuresindicatedby

awhitesquareinFigure3c;(b)demixedspectrashowingFSG(yellow),SSGonSiO2(turquoise),

amorphouscarbon(magenta),andsilicon(darkblue);(c)cross-sectionofcavities2and3taken

atthegreenlineindicatedinpanel(a)withagraysquare;(d)sequenceimagesofcavity4for

the3Dscantakenatdifferentz-positions; (e)3Dtomography imageofcavity4reconstructed

fromthedata(graysquareinpanel(a)).

18

Figure5: (a)SinglespectraofthesurroundingSSG(blackandpink)andtheFSGinthemiddle

(purple);(b)intensitymapofthe2Dpeakwithcrossesmarkingthesinglespectrainpanel(a);

(c)2Dpeakposition;(d)thefwhmofthe2Dpeakand(e)I2D/IGratio.

19

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