ArticlePDF AvailableLiterature Review

Focused Electron Beam-Based 3D Nanoprinting for Scanning Probe Microscopy: A Review

Authors:
  • GETec Microscopy GmbH

Abstract and Figures

Scanning probe microscopy (SPM) has become an essential surface characterization technique in research and development. By concept, SPM performance crucially depends on the quality of the nano-probe element, in particular, the apex radius. Now, with the development of advanced SPM modes beyond morphology mapping, new challenges have emerged regarding the design, morphology, function, and reliability of nano-probes. To tackle these challenges, versatile fabrication methods for precise nano-fabrication are needed. Aside from well-established technologies for SPM nano-probe fabrication, focused electron beam-induced deposition (FEBID) has become increasingly relevant in recent years, with the demonstration of controlled 3D nanoscale deposition and tailored deposit chemistry. Moreover, FEBID is compatible with practically any given surface morphology. In this review article, we introduce the technology, with a focus on the most relevant demands (shapes, feature size, materials and functionalities, substrate demands, and scalability), discuss the opportunities and challenges, and rationalize how those can be useful for advanced SPM applications. As will be shown, FEBID is an ideal tool for fabrication / modification and rapid prototyping of SPM-tipswith the potential to scale up industrially relevant manufacturing.
FEBID-based electric AFM modes. (a-d) show height (left) and Kelvin force microscopy (KFM) (right) images of SiGe quantum ring, performed with commercial, Pt-Ir coated AFM tips (upper row) and Pt-C nano-pillar modified tips (lower row). While the resolution improvement is evident by the more circular feature shapes, the KFM images reveal many more details, as representatively indicated by the blue and white rings. Adapted and reprinted from Chen et al., IEEE 2012 [94]. (e,f) show TEM micrographs of the tip region of a Pt-based FEBID nano-pillar after deposition (e) and after full purification using e-beam assisted carbon removal in H2O atmospheres at room temperature [44,90]. As evident, the pillar gets smaller in width, which entails a slight reduction of the apex radius in the sub-10 nm regime. The larger Pt crystals and the dense packing are evident, while the carbon-free character was confirmed by scanning transmission electron microscopy-based electron energy loss spectroscopy (STEM-EELS) measurements. Such highly conductive tips are then used for C-AFM measurements, as representatively shown in (g) and (h). The scheme below shows the layer setup, consisting of Au paths separated by Al2O3 lines on Si. The advantage of the high aspect ratio pillar is an accurate edge profiling in the height image (g), while the current signal (skin overlay in (h)) allows for the identification of nonconductive regions of thick (red circles) and nanometer thin (purple rings) impurity layers. Image copyright GETec Microscopy 2019 [95].
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Micromachines2020,11,48;doi:10.3390/mi11010048www.mdpi.com/journal/micromachines
Review
FocusedElectronBeamBased3DNanoprinting
forScanningProbeMicroscopy:AReview
HaraldPlank
1,2,3,
*,RobertWinkler
1
,ChristianH.Schwalb
4
,JohannaHütner
4
,JasonD.Fowlkes
5,6
,
PhilipD.Rack
5,6
,IvoUtke
7
andMichaelHuth
8
1
ChristianDopplerLaboratoryforDirect–WriteFabricationof3DNano–Probes(DEFINE),InstituteofElectron
MicroscopyandNanoanalysis,GrazUniversityofTechnology,8010Graz,Austria;robertwinkler@felmizfe.at
2
InstituteofElectronMicroscopyandNanoanalysis,GrazUniversityofTechnology,8010Graz,Austria
3
GrazCentreforElectronMicroscopy,8010Graz,Austria
4
GETecMicroscopyGmbH,1220Vienna,Austria;chris.schwalb@getecafm.com(C.H.S.);
johanna.huetner@getecafm.com(J.H.)
5
CenterforNanophaseMaterialsSciences,OakRidgeNationalLaboratory,OakRidge,TN37831,USA;
fowlkesjd@ornl.gov(J.D.F.);prack@utk.edu(P.D.R.)
6
MaterialsScienceandEngineering,TheUniversityofTennessee,Knoxville,Knoxville,TN37996,USA
7
MechanicsofMaterialsandNanostructuresLaboratory,EmpaSwissFederalLaboratoriesforMaterials
ScienceandTechnology,Feuerwerkerstrasse39,3602Thun,Switzerland;ivo.utke@empa.ch
8
PhysicsInstitute,GoetheUniversityFrankfurt,60323FrankfurtamMain,Germany;
michael.huth@physik.unifrankfurt.de
*Correspondence:harald.plank@felmizfe.at
Received:29November2019;Accepted:20December2019;Published:30December2019
Abstract:Scanningprobemicroscopy(SPM)hasbecomeanessentialsurfacecharacterization
techniqueinresearchanddevelopment.Byconcept,SPMperformancecruciallydependsonthe
qualityofthenanoprobeelement,inparticular,theapexradius.Now,withthedevelopmentof
advancedSPMmodesbeyondmorphologymapping,newchallengeshaveemergedregardingthe
design,morphology,function,andreliabilityofnanoprobes.Totacklethesechallenges,versatile
fabricationmethodsforprecisenanofabricationareneeded.Asidefromwellestablishedtechnologies
forSPMnanoprobefabrication,focusedelectronbeaminduceddeposition(FEBID)hasbecome
increasinglyrelevantinrecentyears,withthedemonstrationofcontrolled3Dnanoscaledeposition
andtailoreddepositchemistry.Moreover,FEBIDiscompatiblewithpracticallyanygivensurface
morphology.Inthisreviewarticle,weintroducethetechnology,withafocusonthemostrelevant
demands(shapes,featuresize,materialsandfunctionalities,substratedemands,andscalability),
discusstheopportunitiesandchallenges,andrationalizehowthosecanbeusefulforadvancedSPM
applications.Aswillbeshown,FEBIDisanidealtoolforfabrication/modificationandrapid
prototypingofSPMtipswiththepotentialtoscaleupindustriallyrelevantmanufacturing.
Keywords:scanningprobemicroscopy;atomicforcemicroscopy;tipfabrication;nanoprinting;
focusedelectronbeaminduceddeposition;3Dprinting;nanofabrication;additivemanufacturing
1.Introduction
Scanningprobemicroscopy(SPM)techniques,likeatomicforcemicroscopy(AFM),scanning
tunnelingmicroscopy(STM),ortipenhancedRamanscattering(TERS),havebecomeanintegralpart
ofsurfacecharacterizationinresearchanddevelopment.Duringthelastdecades,avarietyof
measurementmodeshavebeendeveloped,whichgobeyondsimplemorphologyassessmentto
characterizinglaterallyresolvedsurfaceproperties,likeelectricalconductivity[1],temperature[2],
magnetization[3],surfacepotentials[4],chemicalmapping[5],andothers[6,7].Dedicatednanoprobes
arerequiredtorealizetheseadvancedSPMmodes,andarejustasimportantasadvancementsin
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mechanicaldesignandelectroniccontrols.Theproperfabricationofsuchspecializedtips,however,is
challengingduetothehighdemands:tipsharpnessforhighresolutionimaging,longtermmechanical
durability,specificmaterialchemistryforfunction,andspecialshapesareonlyafewissuesthathave
tobeconsideredduringfabrication.
Amongthevariousnanofabricationmethods,focusedelectronbeaminduceddeposition(FEBID)
isapromisingcandidatefortailoredmanufacturingofSPMtips.Inbrief,FEBIDdecomposessurface
physisorbed/chemisorbedprecursormoleculesundercontinuouselectronbeamexposure[8–10].
Traditionally,FEBIDhasbeenusedforthefabricationofprotectivecoatingsduringtransmission
electronmicroscopy(TEM)lamellapreparation[8,11],photomaskrepair[12–14],circuitediting[8],or
forelectricallycontactingnanowiresorcarbonnanotubes[15].Duringthelast15years,dedicated
FEBIDapplicationshavebeendemonstrated,suchasnanolithography[16,17],stress–strainsensors[18],
hallsensors[19],gassensors[20,21],andothers[8,22–24].Furthermore,opticalapplications,suchas
polarizationfilters[25],spiralphaseplatesforvortexbeams[26],andphotonic–magneticmetaatoms
[27]havebeendemonstrated.InthecontextofSPMnanoprobes,FEBIDiswellsuitedtodeposit
narrow,freestandingnanoscalepillarsusingthestationaryelectronbeammode(oftendenotedasquasi
1Dnanopillarinliterature).SPMtipswithahighaspectratioandatipapexradiusoflessthan10nm
[28–32]mayberoutinelyachievedusingthisapproach,asdiscussedinmoredetaillater.Inthelastfive
years,3DFEBIDhasexperiencedarenaissanceduetoanimprovedfundamentalunderstandingofthe
process.3DFEBIDreliesonthecontrolledlateralmovementoftheelectronbeamatslowscanspeeds,
intherangeofnanometerpermilliseconds[33].Thismodeenablesthefabricationoffreestanding,
inclinednanowires,orsegments,actingasfundamentalbuildingblocks,whichcanbecombinedinto
complex,meshlike3Dnanoarchitectures.Recentprogressinthis3Dnanoprintingmode[34]now
opensupnewpathwaysforSPMtipfabrication.
ThisarticlereviewstheliteraturerelevanttoSPMtipconceptsusing3DFEBIDtodemonstrate
currentcapabilitiesandtotriggerfurtherideasinthistechnologicallychallengingfield.Thearticle
beginswithanintroductionconcerningFEBIDandits3Dcapabilities,complementedbyanoverview
ofboundaryconditionsforthesubstrates.TheadvantagesofusingFEBIDforSPMnanoprobe
depositionarehighlightedtogetherwithalreadydemonstratedSPMexamples,complementedbya
shortdiscussionconcerningfurtheropportunitiesandremainingchallenges.Forabroaderoverviewof
potential3DFEBIDapplicationsbeyondSPMnanoprobes,werefertotheliterature[34,35].
2.TheTechnology
2.1.FocusedElectronBeamInducedDeposition(FEBID)
Imageacquisitionwithascanningelectronmicroscope(SEM)isaccompaniedbytheformationof
acarbonlayerintheexposedsurfacearea.Thelayerformsinresponsetotheelectronbeaminduced
dissociationofhydrocarbonresidueseverpresentinthevacuumchamberofSEMs[36].These
contaminationlayersareusuallyunwanted;ontheotherhand,thislocalizeddepositionofmaterialcan
beusedforacontrolleddirectwriteofnanostructuresbyscanningtheelectronbeamina
preprogrammedwayinthepresenceofasuitablyselectedadsorbedprecursor.Thistechnique,called
FEBID,hasevolvedintoapowerfulnanofabricationtechnology.Agasinjectionnozzleisusedto
constantlysupplyprecursormoleculesforcontinuousdeposition,anddependingontheprecursor
chemistry,otherelementsbeyondcarboncanbedeposited(seeFigure1b).Thenozzleofthegas
injectionsystem(GIS)isbroughtclosetothesubstratetoachievealocallyhighprecursorsurface
coverageinthebeamimpactregion,withoutexceedingthepressurelimitsoftheSEM.Inmostcases,
thegaseousprecursorphysisorbs,ratherthanchemisorbs,atthesurface,andthemoleculesdiffuse
until,afterashortresidencetime,theydesorbagain,leavingthesubstrateunaffected.However,inthe
physisorbedstate,theprecursorcanalsobedissociatedunderelectronirradiationbythefocused
electronbeam.Thiselectroninduceddecompositionoftheprecursorresultsinvolatilefragments,
whichcanleavethebeamimpactregion,whilenonvolatilefragmentssticktothesurface,forminga
soliddeposit.Byguidingthefocusedelectronbeamacrossthesurface,depositsinalldimensionalities,
fromquasi0dimensional(singledots[37]),1D(lines[38]),2D(thinfilms[39]),2.5D(pads[40]),bulky
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3D[41],toverticalpillars[42],arefeasible.Beyondthat,true3Dgeometriescanbeattainedasdiscussed
inthefollowing,openingupcompletelynewpossibilities,thankstothemajorprogressachievedin
recentyears[34].
Figure1.Aspectsof3Dnanoprintingviafocusedelectronbeaminduceddeposition(FEBID):(a)Feature
sizesofaPt–Cmultipodstructurein52°tiltedview.Thediameterofindividualnanowiresisroutinely
wellbelow100nm,while20nmcanbeachievedforspecialFEBIDconditions;(b)Materials:alarge
numberofprecursorsallowthedepositionofmaterialscontainingdifferentchemicalelements[43].
Whileforsome,thefabricationoffreestanding3Dobjectshasbeendemonstrated(green)invarious
studies(numbersintheleftcornerscorrespondtothenumberofarticlesfortherespectiveelement)[34],
thesuitabilityfor3DFEBIDofother(2D)FEBIDprecursors(yellow)isstillpending;(c)Pt–Cbased
FEBID3Dnanotowersfabricatedontopofmineralwires,whichisextremelychallengingviaalternative
techniques.AdaptedandreprintedfromWinkleretal.,ACSAppl.Mater.Interfaces2017[44];(d)In
additiontostraightsegments,arbitrarilycurvednanowiresarepossible;(e)Fabricationofmultiple3D
geometrieswith640elementsoverseveralμinasingleprocessstep.
2.2.3DFEBID
MovingtheelectronbeamintheX/Yplanenormallyresultsindepositiononthesurface.Ifthe
beamscanningspeed(alsodenotedaspatterningvelocityorwritingspeed)isreducedtotheorderof
tensofnmperseconds,thedepositcanliftoffthesubstrateandformafreestandingnanowire.
Furthermore,thenanowire(segment)inclinationangledependsonthepatterningvelocity.Conversely,
iftheelectronbeamiskeptstationaryatonepoint,averticalpillarevolves.Bythat,verticalandtilted
nanowireswithatypicaldiameterbetween20and50nmcanbegrown(seeFigure1a).Segment
inclinationanglesrangingfromverticaltohorizontalarepossible,althoughreliablefabricationinthe
subrangeforsegmentslongerthan1μmcanbechallenging.Sincepositionandexposuretime(dwell
time,DT)oftheelectronbeamintheX/Yplanecanbecontrolledaccurately,curvedwires(seeFigure
1d)andcomplexmultibranchgeometriescanbealsofabricated.Avarietyofprocessparameters(beam,
gas,patterning,temperature)influencethe3Dgrowth[33];themostimportantaretheprimarybeam
energy,beamcurrent,andthepatterningvelocity.Amoredetaileddiscussionwillbeprovidedinthe
respectivesectionsbeloworcanbefoundinliterature[33].
2.3.DemandsontheSubstrate
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Asmentionedintheprevioussection,FEBIDisatrueadditive,directwrite3Dfabrication
technology,whichminimizesthedemandsonthesupportingsubstratematerialandsurfacetopology.
Thesubstratesurfaceneedonly(1)bevacuumcompatible,(2)provideatleastlowelectrical
conductivity,(3)withstandelectronexposuretoacertaindegree,and(4)bechemicallyinerttothe
precursormaterial.Demands(1)and(2)canbesoftenedbyusinganenvironmentalSEM(ESEM),which
allowsfabricationinpressureconditionsuptoabout1mbar[15],orbythinconductivecoatingsto
preventelectricalcharging,respectively.Demand(3)canbemitigatedbytheapplicationoflowcurrent
3DFEBID,withthedownsideoflongerfabricationtimes.Demand(4)isinherentlydictatedbythe
precursorchemistry,thereforebeinginvariable.However,formosttargetelements,alternative
precursorsareavailable.NotethatmanyFEBIDprecursorsarenonreactiveincombinationwithmost
surfacematerials,althoughbiomaterialsareoftenmoresensitive.WhenconsideringtheSPM
cantilever,mostmaterialsystemsarecompatiblewithallfourdemands,astheyaretypicallyfabricated
invacuumconditionsanyway.Conveniently,thesubstrateprecursorsurfacecoverageisindependent
ofthesurfacemorphology.Therefore,FEBIDcanbeperformedonanysubstratetopology/geometry
whichcanbeaccessedbytheelectronbeam[8,45].Forthisreason,FEBIDsupersedesalternative(3D)
nanofabricationtechniquesthatarelimitedtoflatoronlyslightlycurvedsubstrates.Figure1cshows
anexample,where3DFEBIDobjectshavebeenplacedonamineralwire,whichisextremely
challengingtoachievebyothernanofabricationtechnologies.Althoughofminorrelevanceinmost
cases,wewanttomentionprecursorsurfaceimpingementfluxaspectsinthiscontext.Whilethe
adsorptionofprecursormoleculestakesplaceonanymorphology,shadowingeffectscanoccurdueto
thedirectionalgasfluxcharacterfromthegasinjectionnozzle.Substratediffusionandnondirectional
precursoradsorptionpartlycompensatefortheinhomogeneoussurfacecoverageintheshadowofa
morphologicalbarrier[46,47].Nevertheless,lowerdepositionratesarenotonlypresentbehindlarge
obstacles(e.g.,higherelectrodestructures),butalsothedeposititselfcancauseshadowingeffects
[48,49].Thus,aproperpatterningsequencehastobeconsideredtoreducenonhomogeneousgrowth
rateeffectsduringdeposition[40].Also,edges,astypicallypresentwhenplacinga3DFEBIDtipatthe
endofacantilever,resultindifferentdepositionratescomparedtoaflatsubstrate.Unfortunately,
topographyvariationsand/orshadowingcandegradepredictabilityinthe3Dgrowthprocess.The
influenceoftheprecursorsurfacecoveragecanberelaxedbyoperatinginthesocalledelectronlimited
workingregime,wheresufficientprecursorsurfacecoverageismaximizedandconstantatagiven
position,thuslesseningtheinfluenceoflocalcoveragevariations[33].Often,suchworkingconditions
aredifficulttoestablishandanoptimizationoftheGISalignmentand/ordesignoftheGISareneeded.
3.FeatureSizes
3.1.IntrinsicFEBIDFeatureShapes
Asdiscussedinthe“Technology”section,thefocusedelectronbeamdrivesprecursordissociation
atthesurface.Thefocusedelectronbeamdiameterthereforeplaysasignificantroleindictatingthe
spatialresolutionofthefinaldeposit.MostSEMsprovidebeamdiametersdowntoafewnanometers
fullwidthhalfmaximum(FWHM).Consequently,thisshouldtheoreticallyallowforthefabricationof
featurediametersinthesamerange.However,localizednanosynthesisismostefficientviasecondary
electrons[50],whicharegeneratednotonlybyprimarybutalsobyscatteredelectronsnearthesurface.
Hence,backscatteredelectrons(BSE)andforwardscatteredelectrons(FSE)areinvolvedaswell,which
expandthespatialrangeofpotentialdissociationprocesses(see[51–56]fordetailedsimulationsof
FEBIDgrowth).Thisexplainswhyrealfeaturesizesareaboutanorderofmagnitudelargerthanthe
primarybeamdiameter.Theexactlyachievabledimensionsvarywiththeprimarybeamparameters
andtheprecursormaterial,bothimpactingtheelectronmeanfreepathsand,bythat,thestatistical
interactionvolumeinsolidmaterials.Togiveconcretenumbersfornanowiresproducedbytheoften
usedPtbasedMe3CpMePt(IV)precursor(seeFigure1b),featuredimensionsmostlyrangebetween20
nmand60nmin3Dspace,butcangobelow5nmunderspecialconditions[8,57,58].Withsuchsmall
featuresizes,3DFEBIDexceedstheachievableresolutionofother3Dprintingtechniques[10].Together
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withthelowdemandsonthesubstrateandthehigharchitecturalflexibility,3DFEBIDfulfillsthehigh
demandsforcontrolledSPMtipfabricationfromamorphologicalpointofview.
3.2.SPMRelevantTailoring
ThelateralresolutionachievableinSPMbasedmethodsisconceptuallyrelatedtothetipradiusat
theapex[59].Thefirststudiesgobacktotheearly1990s,whereHübneretal.[60]andSchiffmannetal.
[61]studiedtherelationshipbetweentipshapes(diameter,height,andconicalangle)andfabrication
parameters,simplyusingcarboncontaminantsinsidetheSEM,anddeliveredoneofthefirstproofof
principlesforFEBIDbasedAFMtipmodification.Meanwhile,thebasicunderstandingoftheprocess
hasstronglyimproved,whichallowsspecifictailoring.For3DFEBIDstructures,theachievabletip
resolutiondependsonseveralprocessparameters.Figure2showsatiltedSEMimageofananopillar,
depositedusingMe2Au(acac)astheprecursor.Theoverallshapeofapillarcanbedividedintothree
distinctregions,asindicatedinFigure2:(1)thecylindricalshaftwithhighparallelism(<2°)if
fabricatedproperly,(2)thetopmostconicalregion,and(3)theapexregion[8].Inmanycases,asmall
narrowingatthebaseoccurs(seelowestpartsinFigure2),whichappearsformediumandhighbeam
currents.Thismorphologicalpeculiarityistheresultofanenhancedlateraldepositionrateduetothe
strongerprecursordiffusionfromthelargesubstratereservoirandpillardiameters,whichvariesover
alengthscalecomparabletotheprecursorspecificaveragediffusionlength(~150nmfor
Me3CpMePt(IV)[48,51,62]).SuchwideningcanbeadvantageousforSPMapplicationsduetothe
improvedbondingasaconsequenceofthelargerinterfacearea.Thelengthoftheshaftcanbeprecisely
adjustedbythegrowthtime,whichallowsforheighttowidthaspectratioshigherthan100.Thevertical
dimensionofthetopmostconicalregionscaleswiththeprimaryenergy[63],asitdirectlymimicsthe
upperpartoftheelectroninteractionvolumeinthedepositmaterial,whichstatisticallyformsbythe
energydependentmeanfreepathsofelectronsinmatter/scatteringevents.TherightgraphinFigure2
givesadirectcomparisonofthetopmostpillarregion,wherethesharperconicalshapeforhigher
primaryelectronenergiesisevident.Asmentionedatthebeginning,sub10nmapexradiicanbe
routinelyachievedviaFEBIDforlowbeamcurrents(<100pA)withoutanyfurthertreatments,as
exemplarilyshownbythetransmissionelectronmicroscopy(TEM)micrographinsettopleft.
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Figure2.FEBIDnanopillarcharacteristics.Thetiltedscanningelectronmicroscope(SEM)imageatthe
leftshowsthetypicalnanopillarmorphology,whichcanbecategorizedintothreeverticalsections:a
slightlybroaderbasesectionclosetothesubstrate,acylindricalshaftregion,andthetopmostcone,
terminatedbytheapex.Thelattercanexhibittipradiidownto5nm,asshownbythetransmission
electronmicroscopy(TEM)insettopleft.Whiletheshaftlengthisproportionaltothegrowthtime,the
verticalexpansionoftheconicalregionscaleswiththeprimaryelectronenergyduetothevaryingmean
freepathsofelectrons,statisticallydescribedbytheinteractionvolume.Theverticalexpansionofthe
latterdecreaseswithlowerprimaryenergies,whichexplainsthehighercurvatureofthetipregion
(compare5keV/squareswith30keV/invertedtriangles).TheinsetsshowrepresentativeSEMimagesof
thetipregioninatitledview.
Inprinciple,FEBIDbased3Dtipdepositioncanbeperformedintwodifferentconfigurations.In
oneconfiguration,thetipismountedverticallyintheSEM(paralleltotheelectronbeamaxis)andthe
electronbeamisheldstationary;thedepositgrowsverticallyalongtheelectronbeamaxis.Inthe
alternativeconfiguration,thetipismountedsidewaysintheSEMandtheelectronbeamisscanned
alongtheAFMtipaxis;thedepositgrowslaterally.Ineitherconfiguration,theresultingdeposit
orientationwithrespecttotheAFMtipisthesame,buttheresolutionisdifferent.Thelatter,orlateral
scanningapproach,ismorefavorabletoachievethesmallesttipwidths.SuchatipisshowninFigure
3,withalengthof700nmandanalmostconstantwidthof18nm.Theminimumresolutionisachieved
usingthehighestpossibleprimaryelectronenergyontheSEMs(mostly30keV),whichreducesthe
elasticscatteringprobabilityduringbeamtransmissionthroughthethinnanowire.Consequently,the
numberofBSE,FSE,andrelatedsecondaryelectrons(SEIIandSEIII,respectively)emittedfromthe
surfacearereducedandthebeamdiameteralone,broadenedbythesecondaryelectron(SEI)action
radius,determinesthedepositwidth.Thethicknessofthegrowingwire,paralleltotheincoming
electronbeam,canbecontrolledbythepatterningvelocityand,intheidealcase,approachesthesame
valuesasforthewidth[29].Tipsdepositedinthiswayareidealforpinholecharacterizationorimaging
ofhighaspectratiosidewalls.Theproblemwithsuchtips,however,istheirfragilecharacterdueto
theirsmallcrosssectionalareaandacomparablesmallinterfaceareaatthepillar–tipinterface.The
verticalfabricationconfigurationcanbeofadvantageunderthesecircumstances.Forexample,whilea
singlepillarfabricatedbystationarybeamexposure(Figure2)stillhasasmallinterfacearea,3DFEBID
canbeusedasshowninFigure3btoovercomethisfragilityproblem.Thefourleggedstructure(Figure
3b)depositedby3DFEBIDprovidesamorestablebase,whilestillconvergingtotherequiredvertical
singlepillar.Whilethesinglepillariswellconnectedtothemergingbranchesofthetetrapod,the
FEBID/sampleinterfaceareaisfourtimeshigherthanforsinglepillars.Idealpillarheightsarebelow1
μmtominimizelateralflexingforreliableAFMoperation.Comparedtolaterallygrowntips(seeFigure
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3a),vertical3Dtipshavelargertipdiametersof~50nmbutareradiallysymmetrical,whileapexradii
aretypicallybelow10nm(Figure2).Asmentionedbefore,themainproblemwithbothtiptypesisthe
lateralflexduetothehighcarboncontents.This,however,canberemediedbypostgrowthprocessing,
asdiscussedinthechapter“MaterialsandFunctionalities”.
Figure3.Featuresizes,3Dshapeflexibility,andscanningprobemicroscopy(SPM)advantages:(a)
showsahorizontallygrownPt–CFEBIDtip,wherethebeammovesacrosstheoriginaltipedge(top
downinthisimage)withscanspeedsaround30nm/s.Theresultingnanowiresrevealconstantwidths
inthesub20nmregimealongtheentiretip;(b)verticallygrown3Dtip,composedofafourleggedbase,
whichthenconvergestoatallsinglepillar.Whilethelatterenableshighaspectratiomeasurements,the
formerincreasesthemechanicalstabilityandtheadhesiontothesurface;(c)showsaFEBIDmodification
(greenarrow)ofaPt–Ircoatedconductiveatomicforcemicroscopy(CAFM)tip(redarrow).Thetip
wasalsofullypurified,revealingatipradiusbelow10nm,whichstronglyimprovesthelateral
resolutioncapabilities,asshownin(d)byadirectcomparisonofFEBID(top)andtheoriginalPt–Irtip
(bottom),performedonananogranulargoldsample.Thescanwidthis1μm,whilebothZscalesspan
across14nm,asindicated.
3.3.Applications
TheperformanceofsimplepillarbasedtipmodificationsofSPMprobeshasbeendemonstrated
bydifferentgroups.Thefirstreportsgobackto1993,bySchiffmanetal.,whodemonstratedthesuperior
characteristicsofhighaspectratioFEBIDcarbontipscomparedtoconventional,pyramidlikeAFMtips
[61].Morerecently,Chenetal.[64]showedtheimprovedlateralresolutioncapabilitiesofsuchtips
usingmica,copper,andSiNsamplesindryand,inparticular,inliquidconditions,whichmaintaintheir
qualityevenafter7hcontinuousmeasurements(seeFigure4a,b).UsingasimilarFEBIDtip
modificationapproach,Brownetal.couldclearlydemonstratetheadvantageofthehighaspectratio
characteristics,asshowninFigure4c[28].Inmoredetail,theyusedcloselypackedpolystyrenespheres
astestsamplesandcouldshowthatFEBIDtipswentabout200nmdeeperthancommercialtips,which
alsoimprovedthelateraldiameteranalysesforsuchchallengingsamplesurfaces(seeFigure4d).
AnotherveryimpressiveapplicationwasshownbyNievergeltetal.,whousedFEBIDmodifiedSitips
forhighspeedmeasurementsofbiologicalsystems[65].Figure5ashowstheassemblyprocessofcoiled
coil/globularheadCrSAS6homodimersinliquids.Thesystemendsupincentriolarcartwheel
arrangements,whichactasscaffoldsfortheformationofanewcentrioleviatherecruitmentof
additionalperipheralcomponents(shadedyellow).Figure5bshowsatimeresolvedseriesofhigh
resolutionmeasurements,whichdirectlyrevealstheformationprocessofthefirstcartwheel.Inthat
study,theapplicationofFEBIDtipswasessentialfromanadhesionpointofview,whilehighresolution
capabilitieswereindispensablyneededandsuccessfullydemonstrated(scalebarsare50nm).Further
applications,whichstronglybenefitfromtheresolutioncapabilitiesofFEBIDtips,arediscussedlater
inthefunctionalitycontext.
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Figure4.Atomicforcemicroscopy(AFM)imagingwithFEBIDnanopillars.(a)and(b)showtapping
modeheightimagesofSiNafter0.5hand7hcontinuousoperation,respectively,wherethemaintained
imagequalityreflectsthetiprobustness.TheZrangeis3nminbothhighresolutionimages.(c)shows
acommerciallyavailableAFMtip,modifiedbyaPt–Cnanopillar,revealinganendradiusof4nm,as
shownbyacircleintheinset(scalebaris200nm).(d)showsadirectcomparisonofclosepacked
polystyrenespheresobtainedwiththeaforementionedstandardtipandtheFEBIDtipattheleftand
right,respectively(scalebaris400nm).Selectedlinescansalongthewhitelinesareshownbelowand
clearlydemonstratetheadvantageofdeeperprofilingabilitiesviaFEBIDtips.(a)and(b)wereadapted
andreprintedfromChenetal.,Nanotechnology2006[64],(c)and(d)wereadaptedandreprintedfrom
Brownetal.,Ultramicroscopy2013[28].
Figure5.HighspeedAFMimagingofbiologicalsystemsusingFEBIDmodifiedSitips.(a)Formation
processofcartwheelsandcentriolesbytheassemblyofCrSAS6homodimers,whicharrangedueto
theircoiledcoil/globularheaddomains.(b)Highspeed,highresolutionimagingusingFEBIDtips,
whichmakeitpossibletofollowtheformationofacartwheel.Scalebaris50nm;Zrange,6.3nm.
AdaptedandreprintedfromNievergeltetal.,Nat.Nanotechnol.2018[65].
3.4.Challenges
Theactualchallengesintermsoffeaturesizearemainlyrelatedtothecrosssectionalshapesof
individuallyinclined3DFEBIDnanowires,especiallywhenelementsarenotparalleltotheincoming
electronbeam.AscanbeseeninFigure1a,freestandingwirescanrevealabladelikecharacterwiththe
longerdimensionoftheverticalcrosssectionbeingorientedalongtheelectronbeamaxis.Suchnon
circularshapesentailspatiallyinhomogeneousmechanicalstiffness,whichcanbecomedetrimental
Micromachines2020,11,489of30
duringAFMmeasurements,wherethestructuretwistingorinhomogeneousbendingconvolutesthe
finalimage[66].Perfectlycircularshapesonlyariseforverticalpillarsdepositedviastationary
exposure,whileconversely,inclinedsegmentsrevealanonlinearvariationofitsthicknesstowidth
aspectratio.Thesenonsymmetricalcrosssectionsarehighlycorrelatedwiththeelectron–solid
interactionvolumeandthereforedependontheprimarybeamenergy,thedensityofthedeposited
materialandontheinclinationangle.Averygoodcompromise,however,istheapplicationoflow
primaryenergies(around5keVandless),leadingtothickness:widthaspectratiosaround1.2:1fora
wideinclinationanglerange.Thephysicsgoverningtheshapeoftheinclinedsegmentcrosssection,
andthescalingbehavior,arecurrentlyunderinvestigationandwillbedisseminatedinafuture
publication.Thelongtermgoalbasedoncurrentresearchactivitiesisapatterningbasedcompensation
thatenablesclosetocircularcrosssectionalshapes,independentofprocessparametersandinclination
angles[67].Subsequently,thiscapabilitywillbeextendedtotunethecrosssectionalshapesalongsingle
branches,whichwouldopenupentirelynewpossibilitiesforapplicationspecifictailoring(e.g.,
mechanical,magnetic,oropticalanisotropyin3Dspace).
4.MaterialsandFunctionalities
Apartfromfeaturesizeaspects,thefunctionalitybecomesrelevantwhenfocusingonspecialSPM
operationmodes,suchaselectrostaticforcemicroscopy(EFM),Kelvinforcemicroscopy(KFM),
magneticforcemicroscopy(MFM),orscanningthermalmicroscopy(SThM),tonameafew.
Consequently,theavailabilityofsuitableprecursormaterialsisofessentialimportance.Figure1bshows
theperiodictableandindicateselementsthathavesuccessfullybeenusedforFEBID(yellowandgreen)
and3DFEBID(green).Whileamoredetailedoverviewofmetallicandmagneticmaterialpropertiesof
FEBIDmaterialsisgivenbyHuthetal.[23]anddeTeresaetal.[24],here,welimitourdiscussionto
3DFEBIDstructures.
Withafewexceptions[8,24,68–74],FEBIDmaterialsnotoriouslysufferfromhighlevelsof
contaminants(usuallycarbon)duetoincompleteprecursormoleculedissociation.Thecontamination
accumulatesaspartiallydissociatedmolecularfragmentsbecomeincorporatedinthedeposit.The
precursorsareoftenorganometallicinnature,whichexplainsthehighcarbonlevelsobserved[23,75].
Themicrostructureofsuchdepositsrevealsananogranularcharacter,wheremetallicnanograins(~2–
5nm)areembeddedina(hydro)carbonmatrix,asevidentbydark‐andbrightappearingregionsin
theTEMmicrographinFigure2.Contaminatedmicrostructuresreduceorevenentirelymaskthe
intendedfunctionality.Detailedstudiesconcerningcompositiontunabilityusingbothprimarybeam
parametersandpatterningstrategiesrevealedalimitedabilitytocontrolcontaminationlevels[76,77].
Thisissuewasaddressedinrecentyearsbythesuccessfulintroductionofseveralprotocolstotuneboth
chemicalpurityandfunctionalproperties[23,75].Forexample,thefunctionalpropertiescanbetuned
viapostgrowthelectronbeamexposure,denotedasebeamcuring(EBC)[78].EBCinitiatestwo
processes:(1)thedissociationofincompletelydissociatedprecursormolecules,whichreleasetarget
atoms,leadingtoslightgraingrowth[22,23,78–80];and(2)themodificationandcrosslinkingofthe
carbonmatrix(sp3tosp2)[21,29,80,81].Theformereffectallowsprecisetuningoftheelectrical
conductivity[78]andevenenablesacontrolledinsulator–metaltransition,asdemonstratedforPtund
Au[18,22,23,78].TheunderlyingprocessisthemodulationoftheCoulombbarrierduetoreducedgrain
tograindistances,whichenableschemical[39,82]orthermalsensing,asdiscussedlater.TheEBCcross
linkingeffecthasmechanicalimplications,astheYoungmoduluscanpreciselybetuned,as
demonstratedforPtbasedmaterials[21].Pleasenote,EBChasaminoreffectonthechemistrybuta
majoreffectonthe(electric)functionality,whichisextremelyuseful,asdiscussedlater.Inothercases,
materialpurityisofhighestimportance.Inrecentyears,differentapproacheshavebeendemonstrated,
includinggas/laser/temperatureassistedapproachesduring[72,83–88]orafterdeposition[44,89–91].
Althoughdifferentinexecution,thecommonelementformostoftheseapproachesisthetendencyfor
morphologicaldisruptionduringcarbonremoval,whichbecomesevenmorechallengingfor
freestanding3Dnanoarchitectures[44,84].Thisaspectisdiscussedinthe“Challenges”sectionbelow.
Inthefollowing,wereviewdifferentSPMapplications,whichtakeadvantageofbothitsmorphology
anditsmaterialproperties.
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4.1.Applications
4.1.1.GeneralAFM
Carboncontaining3DFEBIDstructuresexhibithighelasticityduringforceload[29],which
originatesfromthehighcarboncontentofupto90at.%[75],equivalenttomorethan70vol.%.During
AFMoperation,thetipexperiencesforcesinverticalandlateraldirections,whichimmediatelyimplies
mechanicalpropertiesofsinglepillars,whichcanbeconsideredasfundamentalbuildingblocksfor
morecomplex3DFEBIDarchitectures.ThistopicisreviewedingreatdetailbyUtkeetal.inanother
articleofthisspecialissue,whichgivesacomprehensiveinsightintomechanicalproperties,andtheir
dependencyonfabricationandonpostgrowthtreatmentapproaches[92].Inbrief,FEBIDpillarsare
muchstifferalongthemainaxiscomparedtosituationswherelateralforcesbecomerelevant[66].As
thelatterdependsonthepillarlengthsaswell,singlepillarlikeFEBIDstructuresshouldbeaslongas
needed,butasshortaspossible.
EBChasbeendemonstratedtochangetheYoungmodulusfromabout14GPato80GPa(even
highervaluesarepossible[92]),withoutaffectingtheoverallmorphology,includingtheapexradii[21].
ThegreatadvantageofEBCistheminimallydisruptiveinfluence,evenonfragile3Dnanostructures,
whichmakesthisapproachalmoststraightforward.AlthoughthematerialgetsstifferbyEBC,therisk
ofbreakingattheinterfaceduringSPMoperationremainsandgetsevenhigher,asarisingforcesget
moreconcentratedtothoseregions[66].Thesolutionforthisproblemcanbemorecomplex3Ddesigns,
whereoneconceptualexampleisshowninFigure3b.Inarecentstudy,therelationshipbetween3D
designandtheresultantstiffnessinverticalandlateraldirectionwasexplored[66].Itwasfoundthat
thetripodandtetrapodarrangementsstronglyincreasethestiffness—upto60N/mand5N/minthe
verticalandlateraldirection,respectively—whichcomesintotherelevantrangewhenaimingforSPM
applications.Thetetrapodarrangementprovidedthebestoverallmechanicalpropertieswithradially
symmetricstiffnesses.EBCtreatmentfurtherincreasedthestiffnessupto200N/minthevertical
direction,whichissufficientforstableAFMoperation.Inthesamestudy,theadvantageous
implicationsofEBCtreatmentsonthewearresistanceofthetipapexcouldbeshown,whichfinally
allowedhighspeed(~80μm/s)andlongtermAFMmeasurements(uptofourhourswere
demonstratedwithoutweareffects)[66].Importantly,thestudyconclusivelydemonstratedthe
suitabilityofFEBIDbased3DnanostructuresforAFMoperationwithhighresolutioncapabilities.
Fromacommercialpointofview,FEBIDbasedtipmodificationhasalreadyfounditswayinto
commercialproductsbythecompanyNanoTools(Munich,Germany),whoprovideabroadvarietyof
shapes,dimensions,andfabricationangles,basedonFEBID’s3Dcapabilities[93].
4.1.2.Electric
Next,wefocusonelectricallybasedSPMmodessuchasCAFM,EFM,andKFM.Theearliest
reportsontheapplicationofFEBIDnanopillarsaselectricallyconductivetipsgobacktotheearly1990s,
whereHübneretal.demonstratedthatevencarbontipscanbeusedforSTM[60].Shortlyafter,
Schössleretal.deliveredoneofthefirstreportsontheapplicationofametalorganicprecursorforSTM
tipmodification.Inmoredetail,anAubasedprecursorwasusedforthefabricationoffieldemitters
withsub10nmapexradiiandanangularemissiondensityof0.2mA/sr,whichweresuccessfully
appliedasSTMtipsandasSTMbasednanolithographytools[16].Thelatterapplicationwasalso
demonstratedbyWendeletal.,whousedFEBIDbased,ultrahard,amorphouscarbontipsfornano
lithography.Byacombinationofhardtappingandtriangularvoltagepulses,theycouldfabricate25
nmlines(FWHM)inphotoresist,whichfinallyallowedthefabricationof30nmmetalstructures[17].
Morerecently,Chenetal.modifiedcommerciallyavailable,Pt–IrcoatedconductiveAFMtipsby20nm
wideand250nmlong,PtbasedFEBIDpillars[94].ThetipswerethenusedforEFMandKFM
measurementsofSi–Gequantumrings,wheretheycouldrevealtheexistenceofacentralSi–Gelike
areawithinthequantumringsthemselves,asshowninFigure6a–d.Theimprovedlateralresolution
wasduetothesmallpillardiametersand,inparticular,duetothesmallapexradiiforFEBIDpillars
(sub10nm;seeFigure2a),whichisabout2–3timessmallerthancoatedSitips(nominallyspecified
with20–25nm).ThatsituationisshowninmoredetailinFigure3c,whichisatiltedSEMimageofa
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coatedtip(redarrow),furthermodifiedbyaPtFEBIDnanopillar(greenarrow).Whilethemuch
smallerapexradiusforthelatterisimmediatelyevident,Figure3dshowsadirectcomparisonofAFM
heightimagesonanAunanoparticlesampleincontactmode.Theimprovedlateralresolution
achievablewithFEBIDtips(upper)comparedtoPt–Ircoatedtips(lower)reliesonthefactthatfully
purifiedFEBIDtipsdonothave/needaconductivecoating,whichmakestheminherentlysharper.
WhileChenetal.successfullydemonstratedtheEFM/KFMsuitabilityforFEBIDtipsevenintheiras
depositedstate,CAFMrequiresfullypurified(metallic)tipsforreliablemeasurements.Asmentioned
above,differentprotocolshavebeenintroduced,whichallowthechemicaltransferintocarbonfree
FEBIDmaterials.AparticularlygentleapproachistheelectronbeamassistedpurificationinanH2O
atmosphere(20–120Pa)atroomtemperature,whichhassuccessfullybeenappliedtoPt‐andAubased
FEBIDmaterials[44,83,90].Thechemicalpurificationapproachishighlyefficientandcreatesdefect
free,concerningnanoporesorcracks,structuresafterpurification.Figure6showsadirect,inscale
comparisonofaPt–Cnanopillarbefore(e)andafterfullpurification(f),whichshows:(1)thereduced
apexradius,(2)increasedPtgrainsizes,(3)densegrainpacking,and(4)themaintainedshapefidelity.
Complementaryscanningtransmissionelectronmicroscopybasedelectronenergylossspectroscopy
(STEMEELS)measurementsconfirmedthecarbonremovalunderidealpurificationconditions[44,90].
Figure6.FEBIDbasedelectricAFMmodes.(ad)showheight(left)andKelvinforcemicroscopy(KFM)
(right)imagesofSiGequantumring,performedwithcommercial,Pt–IrcoatedAFMtips(upperrow)
andPt–Cnanopillarmodifiedtips(lowerrow).Whiletheresolutionimprovementisevidentbythe
morecircularfeatureshapes,theKFMimagesrevealmanymoredetails,asrepresentativelyindicated
bytheblueandwhiterings.AdaptedandreprintedfromChenetal.,IEEE2012[94].(e,f)showTEM
micrographsofthetipregionofaPtbasedFEBIDnanopillarafterdeposition(e)andafterfull
purificationusingebeamassistedcarbonremovalinH2Oatmospheresatroomtemperature[44,90].As
evident,thepillargetssmallerinwidth,whichentailsaslightreductionoftheapexradiusinthesub10
nmregime.ThelargerPtcrystalsandthedensepackingareevident,whilethecarbonfreecharacterwas
confirmedbyscanningtransmissionelectronmicroscopybasedelectronenergylossspectroscopy
(STEMEELS)measurements.SuchhighlyconductivetipsarethenusedforCAFMmeasurements,as
representativelyshownin(g)and(h).Theschemebelowshowsthelayersetup,consistingofAupaths
separatedbyAl2O3linesonSi.Theadvantageofthehighaspectratiopillarisanaccurateedgeprofiling
intheheightimage(g),whilethecurrentsignal(skinoverlayin(h))allowsfortheidentificationofnon
conductiveregionsofthick(redcircles)andnanometerthin(purplerings)impuritylayers.Image
copyrightGETecMicroscopy2019[95].
SuchtipscanthenbeusedasCAFMtips,whichisexemplarilyshowninFigure6g,h,wherethe
surfacetopographyisrevealedforamultilayerstructure,asschematicallyshownatthebottom.While
theheightimage(g)takesadvantageofboththesharpapexandthesteepsidewallstoreachthebottom
atfinetrenches,(h)showsa3Dheightplotwiththecurrentsignalasoverlay.Asevident,thecorrelated
Micromachines2020,11,4812of30
measurementsarealmostdigital,revealingconductive(turquoise)andinsulating(black)regionsand
identifyingnanometerthicksurfaceimpurities(purplecircles).Apartfromresolutionaspects,thereis
anotheressentialadvantageoffullypurifiedFEBIDtipscomparedtocoatedAFMtips:becausetheyare
entirelymadeofpurematerial(e.g.,Pt),thedelaminationproblemofcoatedAFMtipsisentirelyabsent,
alsomakingsuchprobesmoredurable.SuchFEBIDbasedconductivetipswillthenfulfillthe
requirementsforapplicationinCAFM,KFM,andEFM,asalreadyshownconceptually,butcanalso
beusedforpiezoelectricforcemicroscopy(PFM)andscanningspreadingresistancemicroscopy
(SSRM).Notably,Nohetal.[96]andRobertsetal.[97]havepreviouslyusedFEBIDandfocusedelectron
beaminducedetching(FEBIE)forthefabricationofadvancedinsulatedPFMandSPMprobesforuse
inliquidenvironments.Inshortsummary,theparticularadvantagesofFEBIDtipsforelectricallybased
AFMmodesarethesharpapex,highaspectratioshapesifneededandtheprovenwearresistances.
Beyondthat,FEBIDallowsforspecialdesigns,whichenablemorphologicaladaptionaccordingtothe
individualsituation,asdiscussedinthe“DesignFlexibility”sectionbelow.
4.1.3.Magnetic
Now,weturntomagnetictips,wherethematerialqualityiscriticallyimportanttoprovidethe
requiredsensitivityduringMFMmeasurements.Ironandcobaltarethemostpromisingcandidatesfor
precursormaterials,basedonFigure1b.AsdepositedFeand/orCoFEBIDpuritymustbehighforthe
ferromagneticpropertytoemerge,asdiscussedinareviewbydeTeresaetal.[24].Topurifymagnetic
materials,H2OorO2assistedapproachesarenotsuitable,becausesurfaceandinternaloxidationoccurs
[24].Hence,differentresearchgroupshaveexploredtheprocesswindowduringinitialFEBIDto
minimizeunwantedimpurities.ThefirststudiesinthatdirectionstemmedfromUtkeetal.andLauet
al.,whofabricatednanopillarsfromCo2(CO)8precursorforMFMtests[32,98,99].TheConanotips
werecomposedof34at.%Co,51at.%C,and14at.%Oandallowedalateralresolutioninthesame
rangeasforcommerciallyavailable,coatedmagnetictips(~40nm),butprovidedthehighaspectratio
advantages,asdiscussedbefore.Inthefollowingdecade,strongprogresswasmadeinthefabrication,
optimization,andapplicationofmagneticmaterialsingeneral,ascomprehensivelyreviewedbyde
Teresaetal.[24].Severalstudiesexploredthevariationinchemicalcompositionasafunctionofprimary
electronbeamconditions[100,101],patterningparameters[102],gasfluximplications[103],and
substratetemperatures[101,104].Cocontentsupto95at.%werefoundathigherbeamcurrents,
primaryenergies,andelevatedtemperatures,wheretheremainingC/Ofractionstemsfromthe
unavoidablesurfaceoxidation,oncethesampleisexposedtoambientconditionsafterprocessing.
Forironbasedmaterials,themostrelevantprecursorsareFe(CO)5andFe2(CO)9,firststudiedby
Takeguchietal.[105]andFuruyaetal.[106].InsimilarstudiesasthosereportedaboveforCo,primary
beamandpatterningparameters,H2Ocoexposure,substratetemperatures,andpostgrowthannealing
stepswerestudiedforFecontainingdeposits(seereference[24]foradetailedoverview).Particularly
interestingwasthestudybyTakeguchietal[105],inwhichFEBID’s3Dcapabilitieswereusedtostudy
therelationshipsbetweenmorphology,thermalposttreatments,andtheresultingmagneticproperties
[105].ThatstudywasoneofthefirstwhichappliedFEBID’spowerful3Dcapabilities,bymeansofrods,
dots,rings,triangles,andevenmorecomplexstructures,forfundamentalresearch.Aparticularly
importantstudywasdonebyRodriguezetal.[107],whoinvestigatedthecoercivefieldasafunctionof
widthandthicknessinFenanowires.Thefindingsclearlyrevealedthatnarrowtipsbelow50nmin
widthshouldbehighlybeneficialforMFMapplications,whilethethicknessisnotascritical.Asimilar
resultwasfoundforCobasednanowires,asreportedbyVenturietal.[108]andWolfetal.[109],who
studiedthemagneticinductionaswellasmagneticstrayfieldsasafunctionofthetipdistanceforfull
optimizationofMFMtipmorphologies.Designruleswereproposedasaresultofthesestudies,which
showedthatthetiplengthscanbeadaptedaccordingtotherequirements(pinholes,sidewalls,grooves),
whilethetipdiametershouldbeassmallaspossible.Gavagninetal.[42]confirmedthesefindings
experimentallyandalsodemonstratedtheimportanceoftherelativetiltanglebetweentipandsample,
whichhastobeperpendicularforthehighestlateralresolutionandmagneticsensitivity.Froman
applicationpointofviewforCobasedFEBIDstructures,Stilleretal.reportedthemodificationof
Akiyamatips[110],whileBelovaetal.demonstratedtheoptimizedfabricationofCobasedFEBID
Micromachines2020,11,4813of30
supertipsasshowninFigure7a.Themetalcontentofthosetipswasaround60at.%,andallowedlateral
resolutionsdown10nm,whichcanbeconsideredasthecurrentbenchmarkforFEBIDbasedMFMtips
[111].
Figure7.Sub10nmresolutionmagneticforcemicroscopy(MFM)imagingviaFEBIDsupertips.(a)
showsaCobased,FEBIDsupertipwithametalcontentof~60at.%andendradiibelow10nm.Such
tipsarethenusedforimagingbitpatternedmedia,asshownbytheheightandliftmodephaseimages
in(b)and(c),respectively.Asevident,theMFMimage(c)clearlyrevealsthewrittenbitswithlateral
resolutioninthesub10nmregime.AdaptedandreprintedfromBelovaetal.,Rev.Sci.Instrum.2012
[111].
WhileFEBIDbasedCostructurescanrevealmetalcontentsupto95at.%rightafterdeposition,Fe
basednanowirestypicallyrangearound50at.%,whichistoolowtoexploittheirfullpotential.Jose
MariadeTeresaetal.demonstratedthatsuchFebasednanowirescanbetransformedintonominally
pure,highlycrystalline,andmorphologicallystablenanopillarsviathermalpostgrowthannealingat
temperaturesofatleast400°C[105,112].Thelatter,however,mightbechallengingforcantilever
platforms,whichcontainintegratedcomponents,suchasselfsensingorresistiveelements.
AheteronuclearFe–Coprecursor(HCo3Fe(CO)12)wassynthesizedanddemonstratedtobefully
3DFEBIDcompatible,whichservesasanalternativetothehomonuclearparadigm[113–115].The
resultingdepositsrevealedaCo3Fecomposition,whichisnominallycarbon/oxygenfreewhen
disregardingthethin,unavoidablesurfaceoxidationlayer.Thisprecursormaterialisanidealcandidate
forMFMtipmodificationowingtothelowvaporizationtemperatureandnonreactivecharacterwith
thesurface.Figure8ashowsanSEMsideviewofaselfsensingcantileverwithapreexistingtip,
modifiedbyaCo3Fenanopillar,showninmoredetailattheright.SuchtipsrevealtypicalFEBID
diametersandapexradiiwellbelow100nmand10nm,respectively,whichcanbeuseddirectlyafter
fabricationwithoutanyfurtherpostgrowthtreatment.Forbenchmarkpurposes,aPt/Comultilayer
systemwasstudied,asrepresentativelyshowninFigure8bbya3DheightscanwithanMFMphase
overlay.Theperpendicularmagneticanisotropyleadstoaclearmagneticphasecontrast,not
recognizableinthemorphology.Figure8cshowsthephasecontrastinmoredetail,usinganFebased
FEBIDnanopillar,fabricatedatoptimizedgrowthconditions.Individualfeaturesarewellresolved,
whiletypicalphaseshiftsrangearoundforoptimizedliftheightsandamplitudes.Incontrast,Figure
8dshowsthesamesampleimagedbyaCo3Fesupertipwithverysimilaroveralldimensions.Asclearly
evident,phaseshiftsaremorethanthreetimeshigheratoptimizedliftheights(seescales),which
reducesthenoiseandincreasesthelateralresolution,whichwasfoundtobeclearlybelow20nm.
Togetherwiththefactthatgrowthtimesaresimilar,thesefirstattemptsshowthehighperformanceof
thisheteronuclearprecursor.Roundedupbytheuncomplicatedhandling,thismaterialhasenormous
potentialtorevolutionizeFEBIDbasedfabricationofmagnetictipsforhighperformanceMFM
operation.
Micromachines2020,11,4814of30
Figure8.FEBIDbased,highperformanceMFMtips.(a)showsatiltedSEMsideviewofapre
structured,selfsensingcantilever,modifiedbyamagneticCo3Fetipvia3DFEBIDat20keV/13pA.Such
tipsrevealtypicaldiametersandapexradiiwellbelow100nmand10nm,respectively.(b)showsa3D
heightimagewiththeMFMsignalasoverlay,takenfromaCo/Ptmultilayerstructurewith
perpendicularmagneticanisotropy.(c)and(d)showMFMphaseimagesfromthesamesample,
however,acquiredviaFeandCo3FeFEBIDtips,withsimilarmorphologies.Acloserlookrevealsthat
MFMtipsfromCo3Ferevealmuchstrongerphasecontrastsbyafactorofmorethanthree,whilelateral
resolutionisalsoimprovedtothesub20nmregime.ImagesarecourtesyofGETecMicroscopy[95](a
c)andMichaelHuth(d).Copyright2019.
Apartfromtheseclassicalapplications,moreadvancedAFMoperationmodeshavebeen
demonstratedincombinationwithFEBIDtips.ApowerfulexpansiontoclassicalMFMmeasurements
viaphaseshiftdetectionisferromagneticresonanceforcemicroscopy(FMRFM).Thisoperationmode
isbasedonthedetectionofthemagneticresonancewithamagneticsampleduetothemagneticforce
exertedonacantileverequippedwithasmallmagnetictip[116].Sangiaoetal.fabricatedsmallConano
spheresviaFEBIDontopofasoftAFMcantileverinaverycontrolledmanner,rangingfrom100nmto
500nm,asshowninFigure9a,b[117].ThesphericalshapeisadvantageousforFMRFMoperation,asit
minimizesthemagnetichysteresis.TheCocontentwasrevealedtoincreasewiththediameter(Figure
9c),whileaminimumdiameterof150nmwasfoundasthelowerlimitforFEBID–Cospheresfor
appropriatemagneticsensitivity[24],almosteliminatingunwantedremanenceeffects.Althoughthe
lateralresolutionduringFMRFMoperationcorrelateswiththesizeofthenanosphere,impressive
resultshavebeenshowninrecentyears[118–123].Figure9d,eshowsarepresentativeresult,inwhich
FMRFMwasusedfordefectdetection(inthiscase,morphologicallyinduced),whileclassicalMFM
operationisunabletoextractthatinformation(compare(d)with(e)).
Micromachines2020,11,4815of30
Figure9.FEBIDbasedferromagneticresonanceforcemicroscopy(FMRFM).(a)and(b)showdifferently
sizedConanospheres(topdown),fabricatedontopofahighresolutionAFMcantileverinthetop(a)
andsideviews(b),revealingaclosetosphericalmorphology.(c)showsthecobaltcontentofthebeads
asafunctionofthediameter,whichhastobecorrelatedwithitsmagneticproperties.Additional
measurementsrevealedaminimumdiameterof150nmtoexploitthefullperformance.(d)and(e)show
FMRFMandMFMmeasurements,respectively,usingaCoAFMcantilevermodifiedviaFEBID.These
measurementsuseaninplanemagneticfield,whichalignsthesamplemagnetization.Amicrowave
frequencycurrentisthenintroducedperpendiculartothemagneticfield,butalsoinplane.Thelatter
decreasesthequasistaticcomponentofthemagnetization,whichimpactsthemagnetostaticforce
betweenthesampleandtip.Oncethemagneticresonanceisfound,FMRFMrevealsdefectsinducedby
chemistryormorphology,asprovokedinthisexamplebyadifferentshapeofthecentralelementinthe
3×3array.ThisexplainswhythatpartisdarkinFMRFMmode(d).Incontrast,classicalMFM
measurementsareunabletodetectsuchdefects,asshownin(e).Althoughpowerfulinitsanalytical
capabilities,thelateralresolutioniscurrentlylimitedtoabout90nm,asthatdependsonthebead
diameter.(ac)wereadaptedandreprintedfromSangiaoetal.,BeilsteinJ.Nanotechnol.2017[117].(d)
and(e)wereadaptedandreprintedfromChiaetal.,Appl.Phys.Lett.2012[118].
4.1.4.Thermal
Asmentionedbefore,EBChasaminorinfluenceondepositmorphologybutastrongimpacton
theelectricalfunction:thenanogranularcompositionremains,whiletheelectricaltunneling
probabilitybetweenadjacentnanograinsincreases[23,78].Thelattereffectalsodependson
temperature,leadingtodecreasingresistancesathighertemperaturesduetothermallyassisted
tunneling.Bythat,themacroscopiccurrentthroughsuchbridgesprovidesinformationofthematerial’s
temperatureswithanegativetemperaturecoefficient(NTC)ofelectricresistance.Thisthermistorbased
SThMconceptwasfirstshownbyEdingeretal.in2001[124,125],butnotfollowedinmoredetail,which
mightbeexplainedbythelimitedknowledgeaboutmaterialtuningandcontrolled3Dnanoprinting
capabilitiesatthattime.Sattelkowetal.recentlyrevisitedthe3Dthermistorconcept,optimized
mechanical/electricalaspects,anddemonstratedthermalprobing[66].AssummarizedinFigure10,the
authorsusedaselfsensingAFMcantileverplatform(a),whichwasprestructuredwithtwoelectrodes,
coveringthepreexistingtipaswell(b).FEBIDtetrapodswereusedas3Dnanobridgesbetweenthose
twoelectrodes,asshownbyanoverview(b)andbycloseupsin(c)and(d).Onceincontactwiththe
sample,thetetrapodsheatupandchangetheirelectricconductivityduetotheNTCbehavior.Because
ofthesmallactivevolumesintheFEBIDcreatedcontactregion,theheatresponseisveryfastandallows
fordynamicsensingofatleast30ms/K,asshowninFigure10e.Thisfunctionalnanoprobenotonly
demonstratesAFMcompatibilitybutalsotheusefulnessofananogranularmaterial,amicrostructure
whichisoftenregardedasineffectiveconcerningrealapplications.Thishallmarkdemonstrationof
functionaltransductionwascomplementedbyanapexradiusbelow10nmandfastfabricationtimes
around10minutes,includingEBC.
Micromachines2020,11,4816of30
Figure10.FEBIDbased3Dnanoprobesforlocalizedthermalsensing.(a)showstheselfsensing
cantileverplatform(copyrightGETecMicroscopy,Austria[95]),wheresensingelementsandpre
structuredAuelectrodesareindicatedblueandyellow,respectively.(b)showsanSEMimageofthetip
region,inwhichthetruncated,preexistingtip,thesplitelectrodes,andthe3Dnanobridgeareevident.
Thelatterisshowninhighermagnificationinaside(c)andtopview(d),wherethe3Dnanobridgeis
showninred.Onceincontactwiththesamplesurface,thesmallactivevolumesofthe3Dstructureallow
afastresponse,asshowninadynamicresponsetestin(e)bytheredcurveincomparisontothereference
temperature(blue).Asensingratebetterthan30ms/Kwithanoiselevelbelow±0.5Kcouldbe
demonstrated,whichexhibitstheadvantagesof3DFEBIDnanostructuresforadvancedAFMconcepts,
applicableonevenprefinishedmicrocantilever.AdaptedandreprintedfromSattelkowetal.,Appl.
Mater.Interfaces2019[66].
4.1.5.Optical
AfirstexampleofFEBIDbasedspectroscopywasreportedbyCastagneetal.in1999,who
modifiedSi–AFMprobesbycarbonsupertipswithvariablelengths(1.3–3.3μm),widths(130–300nm),
andapexradiidownto15nm[126].SuchprobeswerethenusedinacombinedAFM/photonscanning
tunnelingmicroscopetostudytheirnearfieldopticalbehavior.Thestudyrevealedthatevencarbon
tipsareconvenientfornearfieldopticalconversionofevanescentwavesinthenearinfraredrange.
Theauthorsalsostatedthatthelateralresolutionduringimagingmaybelimitedasaconsequenceof
theopticalpropertiesofcarbontipsandgavetheorybasedrecommendationsforfurtheroptimization
towardshighperformanceopticalnearfieldnanoprobesforapplicationinscanningnearfieldoptical
microscopy(SNOM)orTERSmicroscopy.Shortlyafter,Sqallietal.focusedonthebeneficial
implicationsofsmallFEBIDbasedAunanodots/ellipsesonlightscatteringcapabilitiesduringSNOM
[127].Inmoredetail,lightfiberbasedprobesweremodifiedbydifferentlyshapedFEBIDnano
structuresandstudiedbyacombinationoftheoryandexperiments.Theauthorscoulddemonstrate
thatsphericalstructureswith~60nmdiameterincreaseboththesignalandthecontrastinnearfield
transmissionmeasurements.Thestudyalsoshowedthatellipticalshapesallowtuningoftheresonant
wavelengthasaconsequenceoftherelativeorientationbetweenellipticnanostructuresandthe
incomingpolarizedlight,whichdemonstratesFEBID’susefulnessduetoitsdesignflexibility.
AnimpressivestudyconcerningTERSmicroscopywasdonebyDeAngelisetal.,whointroduced
ahighlyadvancedTERStipconcept,basedonfocusedionbeam(FIB)andFEBID[128].Figure11shows
thetaperedwaveguide,fabricatedona100nmthickSi3N4AFMcantilever(a).First,FIBwasusedto
fabricatethephotoniccrystal,consistingofatriangularregionwithholeandseparationdistancesof160
nmand250nm,respectively(b).Forthecentralwaveguide,aPtbased,3DFEBIDstructureactedas
scaffold,whichwascoveredwith30nmAganddownshapedviaFIB(c)toapexradiibetween2.5and
5nm(furtherprocessstepswereapplied,asdetailedinreference[128]).Suchprobeswerethenusedin
AFMoperation,wheretheincominglaseriscoupledviathebacksidethroughthephotonicelement,
whichlaunchessurfaceplasmon,furtherpropagatingalongtheAgtiptotheapex,wherethelocalized
plasmonresonanceoccurs(d).DetectionoftheRamansignalwasdoneintransmission.Figure11eand
fshowanAFM3DheightimageofasubmicrometerSinanocrystal/SiOxtrenchandthecorresponding
Ramanintensityalongtheredlines,respectively.Thevaryingintensityreflectsthecrystallinityofthe
material,whichcorrelateswellwiththewallregion(e),whichprovessub10nmresolution.While
FEBIDstructuresactedasscaffoldsinthiscase,recentprogressconcerningpurityandshapestability
Micromachines2020,11,4817of30
holdthepotentialtofabricatethecentraltipsolelyvia3DFEBID[44],whichwouldconsiderably
simplifytheentirefabricationprocess.
Figure11.FEBID/focusedionbeam(FIB)assisted,photonic/plasmonictipenhancedRamanscattering
(TERS)nanoprobe.(a)Overviewofa100nmthickSi3N4AFMcantilever,equippedwiththephotonic
andplasmonicelement,basedonFIBandFEBID,respectively.(b)showsacloseupinwhichtheFIB
holes(160nmdiameterand250nmdistance)areevident.ThecentralelementusesaPtbased3DFEBID
pillarasscaffold,furthercoatedwithpureAgandmilleddownviaFIBtoapexradiiof5nm