non-invasive scanning raman spectroscopy and …raman experiments were conducted using a witec...
TRANSCRIPT
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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:[email protected]
2
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
12
notonlyof thesurfacebutalso insidedevicesandthusenablesthenoninvasiveanalysisofa
widerangeofapplications,includingsiliconMEMSandmicrofluidics.
13
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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