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

December 2019
Micromachines 11(1):48

DOI:10.3390/mi11010048

Authors:

Harald Plank

Graz University of Technology

Robert Winkler

Graz University of Technology

Christian H Schwalb

GETec Microscopy GmbH

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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.

Feature sizes, 3D shape flexibility, and scanning probe microscopy (SPM) advantages: (a) shows a horizontally grown Pt-C FEBID tip, where the beam moves across the original tip edge (topdown in this image) with scan speeds around 30 nm/s. The resulting nanowires reveal constant widths in the sub-20 nm regime along the entire tip; (b) vertically grown 3D tip, composed of a four-legged base, which then converges to a tall single pillar. While the latter enables high aspect ratio measurements, the former increases the mechanical stability and the adhesion to the surface; (c) shows a FEBID modification (green arrow) of a Pt-Ir coated conductive-atomic force microscopy (C-AFM) tip (red arrow). The tip was also fully purified, revealing a tip radius below 10 nm, which strongly improves the lateral resolution capabilities, as shown in (d) by a direct comparison of FEBID (top) and the original Pt-Ir tip (bottom), performed on a nanogranular gold sample. The scan width is 1 μm, while both Z scales span across 14 nm, as indicated.

…

Atomic force microscopy (AFM) imaging with FEBID nano-pillars. (a) and (b) show tapping mode height images of SiN after 0.5 h and 7 h continuous operation, respectively, where the maintained image quality reflects the tip robustness. The Z-range is 3 nm in both high-resolution images. (c) shows a commercially available AFM tip, modified by a Pt-C nanopillar, revealing an end radius of ∼4 nm, as shown by a circle in the inset (scale bar is 200 nm). (d) shows a direct comparison of close-packed polystyrene spheres obtained with the aforementioned standard tip and the FEBID tip at the left and right, respectively (scale bar is 400 nm). Selected line scans along the white lines are shown below and clearly demonstrate the advantage of deeper profiling abilities via FEBID tips. (a) and (b) were adapted and reprinted from Chen et al., Nanotechnology 2006 [64], (c) and (d) were adapted and reprinted from Brown et al., Ultramicroscopy 2013 [28].

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High-speed AFM imaging of biological systems using FEBID modified Si tips. (a) Formation process of cartwheels and centrioles by the assembly of CrSAS-6 homodimers, which arrange due to their coiled-coil/globular head domains. (b) High-speed, high-resolution imaging using FEBID tips, which make it possible to follow the formation of a cartwheel. Scale bar is 50 nm; Z range, 6.3 nm. Adapted and reprinted from Nievergelt et al., Nat. Nanotechnol. 2018 [65].

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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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Sub-10 nm resolution magnetic force microscopy (MFM) imaging via FEBID super-tips. (a) shows a Co-based, FEBID super-tip with a metal content of ~60 at.% and end radii below 10 nm. Such tips are then used for imaging bit patterned media, as shown by the height and lift-mode phase images in (b) and (c), respectively. As evident, the MFM image (c) clearly reveals the written bits with lateral resolution in the sub-10 nm regime. Adapted and reprinted from Belova et al., Rev. Sci. Instrum. 2012 [111].

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

Micromachines2020,11,48;doi:10.3390/mi11010048www.mdpi.com/journal/micromachines

Review

FocusedElectronBeam‐Based3DNanoprinting

forScanningProbeMicroscopy:AReview

HaraldPlank

1,2,3,

*,RobertWinkler

,ChristianH.Schwalb

,JohannaHütner

,JasonD.Fowlkes

5,6

,

PhilipD.Rack

5,6

,IvoUtke

andMichaelHuth



ChristianDopplerLaboratoryforDirect–WriteFabricationof3DNano–Probes(DEFINE),InstituteofElectron

MicroscopyandNanoanalysis,GrazUniversityofTechnology,8010Graz,Austria;robertwinkler@felmi‐zfe.at

InstituteofElectronMicroscopyandNanoanalysis,GrazUniversityofTechnology,8010Graz,Austria

GrazCentreforElectronMicroscopy,8010Graz,Austria

GETecMicroscopyGmbH,1220Vienna,Austria;chris.schwalb@getec‐afm.com(C.H.S.);

johanna.huetner@getec‐afm.com(J.H.)

CenterforNanophaseMaterialsSciences,OakRidgeNationalLaboratory,OakRidge,TN37831,USA;

fowlkesjd@ornl.gov(J.D.F.);prack@utk.edu(P.D.R.)

MaterialsScienceandEngineering,TheUniversityofTennessee,Knoxville,Knoxville,TN37996,USA

MechanicsofMaterialsandNanostructuresLaboratory,Empa‐SwissFederalLaboratoriesforMaterials

ScienceandTechnology,Feuerwerkerstrasse39,3602Thun,Switzerland;ivo.utke@empa.ch

PhysicsInstitute,GoetheUniversityFrankfurt,60323FrankfurtamMain,Germany;

michael.huth@physik.uni‐frankfurt.de

*Correspondence:harald.plank@felmi‐zfe.at

Received:29November2019;Accepted:20December2019;Published:30December2019

Abstract: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.

Keywords:scanningprobemicroscopy;atomicforcemicroscopy;tipfabrication;nano‐printing;

focusedelectronbeam‐induceddeposition;3Dprinting;nano‐fabrication;additivemanufacturing



1.Introduction

Scanningprobemicroscopy(SPM)techniques,likeatomicforcemicroscopy(AFM),scanning

tunnelingmicroscopy(STM),ortip‐enhancedRamanscattering(TERS),havebecomeanintegralpart

ofsurfacecharacterizationinresearchanddevelopment.Duringthelastdecades,avarietyof

measurementmodeshavebeendeveloped,whichgobeyondsimplemorphologyassessmentto

characterizinglaterally‐resolvedsurfaceproperties,likeelectricalconductivity[1],temperature[2],

magnetization[3],surfacepotentials[4],chemicalmapping[5],andothers[6,7].Dedicatednano‐probes

arerequiredtorealizetheseadvancedSPMmodes,andarejustasimportantasadvancementsin

Micromachines2020,11,482of30

mechanicaldesignandelectroniccontrols.Theproperfabricationofsuchspecializedtips,however,is

challengingduetothehighdemands:tipsharpnessforhigh‐resolutionimaging,long‐termmechanical

durability,specificmaterialchemistryforfunction,andspecialshapesareonlyafewissuesthathave

tobeconsideredduringfabrication.

Amongthevariousnanofabricationmethods,focusedelectronbeam‐induceddeposition(FEBID)

isapromisingcandidatefortailoredmanufacturingofSPMtips.Inbrief,FEBIDdecomposessurface

physisorbed/chemisorbedprecursormoleculesundercontinuouselectronbeamexposure[8–10].

Traditionally,FEBIDhasbeenusedforthefabricationofprotectivecoatingsduringtransmission

electronmicroscopy(TEM)lamellapreparation[8,11],photomaskrepair[12–14],circuitediting[8],or

forelectricallycontactingnanowiresorcarbonnanotubes[15].Duringthelast15years,dedicated

FEBIDapplicationshavebeendemonstrated,suchasnanolithography[16,17],stress–strainsensors[18],

hallsensors[19],gassensors[20,21],andothers[8,22–24].Furthermore,opticalapplications,suchas

polarizationfilters[25],spiralphaseplatesforvortexbeams[26],andphotonic–magneticmetaatoms

[27]havebeendemonstrated.InthecontextofSPMnano‐probes,FEBIDiswellsuitedtodeposit

narrow,freestandingnanoscalepillarsusingthestationaryelectronbeammode(oftendenotedasquasi‐

1Dnano‐pillarinliterature).SPMtipswithahighaspectratioandatipapexradiusoflessthan10nm

[28–32]mayberoutinelyachievedusingthisapproach,asdiscussedinmoredetaillater.Inthelastfive

years,3D‐FEBIDhasexperiencedarenaissanceduetoanimprovedfundamentalunderstandingofthe

process.3D‐FEBIDreliesonthecontrolledlateralmovementoftheelectronbeamatslowscanspeeds,

intherangeofnanometerpermilliseconds[33].Thismodeenablesthefabricationoffreestanding,

inclinednanowires,orsegments,actingasfundamentalbuildingblocks,whichcanbecombinedinto

complex,mesh‐like3Dnanoarchitectures.Recentprogressinthis3Dnano‐printingmode[34]now

opensupnewpathwaysforSPMtipfabrication.

ThisarticlereviewstheliteraturerelevanttoSPMtipconceptsusing3D‐FEBIDtodemonstrate

currentcapabilitiesandtotriggerfurtherideasinthistechnologicallychallengingfield.Thearticle

beginswithanintroductionconcerningFEBIDandits3Dcapabilities,complementedbyanoverview

ofboundaryconditionsforthesubstrates.TheadvantagesofusingFEBIDforSPMnano‐probe

depositionarehighlightedtogetherwithalreadydemonstratedSPMexamples,complementedbya

shortdiscussionconcerningfurtheropportunitiesandremainingchallenges.Forabroaderoverviewof

potential3D‐FEBIDapplicationsbeyondSPMnano‐probes,werefertotheliterature[34,35].

2.TheTechnology

2.1.FocusedElectronBeam‐InducedDeposition(FEBID)

Imageacquisitionwithascanningelectronmicroscope(SEM)isaccompaniedbytheformationof

acarbonlayerintheexposedsurfacearea.Thelayerformsinresponsetotheelectronbeam‐induced

dissociationofhydrocarbonresidueseverpresentinthevacuumchamberofSEMs[36].These

contaminationlayersareusuallyunwanted;ontheotherhand,thislocalizeddepositionofmaterialcan

beusedforacontrolleddirect‐writeofnanostructuresbyscanningtheelectronbeamina

preprogrammedwayinthepresenceofasuitablyselectedadsorbedprecursor.Thistechnique,called

FEBID,hasevolvedintoapowerfulnanofabricationtechnology.Agasinjectionnozzleisusedto

constantlysupplyprecursormoleculesforcontinuousdeposition,anddependingontheprecursor

chemistry,otherelementsbeyondcarboncanbedeposited(seeFigure1b).Thenozzleofthegas

injectionsystem(GIS)isbroughtclosetothesubstratetoachievealocallyhighprecursorsurface

coverageinthebeamimpactregion,withoutexceedingthepressurelimitsoftheSEM.Inmostcases,

thegaseousprecursorphysisorbs,ratherthanchemisorbs,atthesurface,andthemoleculesdiffuse

until,afterashortresidencetime,theydesorbagain,leavingthesubstrateunaffected.However,inthe

physisorbedstate,theprecursorcanalsobedissociatedunderelectronirradiationbythefocused

electronbeam.Thiselectron‐induceddecompositionoftheprecursorresultsinvolatilefragments,

whichcanleavethebeamimpactregion,whilenon‐volatilefragmentssticktothesurface,forminga

soliddeposit.Byguidingthefocusedelectronbeamacrossthesurface,depositsinalldimensionalities,

fromquasi0‐dimensional(singledots[37]),1D(lines[38]),2D(thinfilms[39]),2.5D(pads[40]),bulky

Micromachines2020,11,483of30

3D[41],toverticalpillars[42],arefeasible.Beyondthat,true3D‐geometriescanbeattainedasdiscussed

inthefollowing,openingupcompletelynewpossibilities,thankstothemajorprogressachievedin

recentyears[34].



Figure1.Aspectsof3Dnano‐printingviafocusedelectronbeam‐induceddeposition(FEBID):(a)Feature

sizesofaPt–Cmulti‐podstructurein52°tiltedview.Thediameterofindividualnanowiresisroutinely

wellbelow100nm,while20nmcanbeachievedforspecialFEBIDconditions;(b)Materials:alarge

numberofprecursorsallowthedepositionofmaterialscontainingdifferentchemicalelements[43].

Whileforsome,thefabricationoffreestanding3Dobjectshasbeendemonstrated(green)invarious

studies(numbersintheleftcornerscorrespondtothenumberofarticlesfortherespectiveelement)[34],

thesuitabilityfor3D‐FEBIDofother(2D)‐FEBIDprecursors(yellow)isstillpending;(c)Pt–Cbased

FEBID3Dnano‐towersfabricatedontopofmineralwires,whichisextremelychallengingviaalternative

techniques.AdaptedandreprintedfromWinkleretal.,ACSAppl.Mater.Interfaces2017[44];(d)In

additiontostraightsegments,arbitrarilycurvednano‐wiresarepossible;(e)Fabricationofmultiple3D

geometrieswith640elementsoverseveralμm²inasingleprocessstep.

2.2.3D‐FEBID

MovingtheelectronbeamintheX/Yplanenormallyresultsindepositiononthesurface.Ifthe

beamscanningspeed(alsodenotedaspatterningvelocityorwritingspeed)isreducedtotheorderof

tensofnmperseconds,thedepositcanliftoffthesubstrateandformafreestandingnanowire.

Furthermore,thenanowire(segment)inclinationangledependsonthepatterningvelocity.Conversely,

iftheelectronbeamiskeptstationaryatonepoint,averticalpillarevolves.Bythat,verticalandtilted

nanowireswithatypicaldiameterbetween20and50nmcanbegrown(seeFigure1a).Segment

inclinationanglesrangingfromverticaltohorizontalarepossible,althoughreliablefabricationinthe

sub‐5°rangeforsegmentslongerthan1μmcanbechallenging.Sincepositionandexposuretime(dwell

time,DT)oftheelectronbeamintheX/Yplanecanbecontrolledaccurately,curvedwires(seeFigure

1d)andcomplexmulti‐branchgeometriescanbealsofabricated.Avarietyofprocessparameters(beam,

gas,patterning,temperature)influencethe3Dgrowth[33];themostimportantaretheprimarybeam

energy,beamcurrent,andthepatterningvelocity.Amoredetaileddiscussionwillbeprovidedinthe

respectivesectionsbeloworcanbefoundinliterature[33].

2.3.DemandsontheSubstrate

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Asmentionedintheprevioussection,FEBIDisatrueadditive,direct‐write3Dfabrication

technology,whichminimizesthedemandsonthesupportingsubstratematerialandsurfacetopology.

Thesubstratesurfaceneedonly(1)bevacuumcompatible,(2)provideatleastlowelectrical

conductivity,(3)withstandelectronexposuretoacertaindegree,and(4)bechemicallyinerttothe

precursormaterial.Demands(1)and(2)canbesoftenedbyusinganenvironmental‐SEM(ESEM),which

allowsfabricationinpressureconditionsuptoabout1mbar[15],orbythinconductivecoatingsto

preventelectricalcharging,respectively.Demand(3)canbemitigatedbytheapplicationoflow‐current

3D‐FEBID,withthedownsideoflongerfabricationtimes.Demand(4)isinherentlydictatedbythe

precursorchemistry,thereforebeinginvariable.However,formosttargetelements,alternative

precursorsareavailable.NotethatmanyFEBIDprecursorsarenon‐reactiveincombinationwithmost

surfacematerials,althoughbio‐materialsareoftenmoresensitive.WhenconsideringtheSPM

cantilever,mostmaterialsystemsarecompatiblewithallfourdemands,astheyaretypicallyfabricated

invacuumconditionsanyway.Conveniently,thesubstrateprecursorsurfacecoverageisindependent

ofthesurfacemorphology.Therefore,FEBIDcanbeperformedonanysubstratetopology/geometry

whichcanbeaccessedbytheelectronbeam[8,45].Forthisreason,FEBIDsupersedesalternative(3D)

nano‐fabricationtechniquesthatarelimitedtoflatoronlyslightlycurvedsubstrates.Figure1cshows

anexample,where3D‐FEBIDobjectshavebeenplacedonamineralwire,whichisextremely

challengingtoachievebyothernano‐fabricationtechnologies.Althoughofminorrelevanceinmost

cases,wewanttomentionprecursorsurfaceimpingementfluxaspectsinthiscontext.Whilethe

adsorptionofprecursormoleculestakesplaceonanymorphology,shadowingeffectscanoccurdueto

thedirectionalgasfluxcharacterfromthegasinjectionnozzle.Substratediffusionandnon‐directional

precursoradsorptionpartlycompensatefortheinhomogeneoussurfacecoverageintheshadowofa

morphologicalbarrier[46,47].Nevertheless,lowerdepositionratesarenotonlypresentbehindlarge

obstacles(e.g.,higherelectrodestructures),butalsothedeposititselfcancauseshadowingeffects

[48,49].Thus,aproperpatterningsequencehastobeconsideredtoreducenon‐homogeneousgrowth

rateeffectsduringdeposition[40].Also,edges,astypicallypresentwhenplacinga3D‐FEBIDtipatthe

endofacantilever,resultindifferentdepositionratescomparedtoaflatsubstrate.Unfortunately,

topographyvariationsand/orshadowingcandegradepredictabilityinthe3Dgrowthprocess.The

influenceoftheprecursorsurfacecoveragecanberelaxedbyoperatingintheso‐calledelectronlimited

workingregime,wheresufficientprecursorsurfacecoverageismaximizedandconstantatagiven

position,thuslesseningtheinfluenceoflocalcoveragevariations[33].Often,suchworkingconditions

aredifficulttoestablishandanoptimizationoftheGISalignmentand/ordesignoftheGISareneeded.

3.FeatureSizes

3.1.IntrinsicFEBIDFeatureShapes

Asdiscussedinthe“Technology”section,thefocusedelectronbeamdrivesprecursordissociation

atthesurface.Thefocusedelectronbeamdiameterthereforeplaysasignificantroleindictatingthe

spatialresolutionofthefinaldeposit.MostSEMsprovidebeamdiametersdowntoafewnanometers

full‐widthhalf‐maximum(FWHM).Consequently,thisshouldtheoreticallyallowforthefabricationof

featurediametersinthesamerange.However,localizednano‐synthesisismostefficientviasecondary

electrons[50],whicharegeneratednotonlybyprimarybutalsobyscatteredelectronsnearthesurface.

Hence,backscatteredelectrons(BSE)andforward‐scatteredelectrons(FSE)areinvolvedaswell,which

expandthespatialrangeofpotentialdissociationprocesses(see[51–56]fordetailedsimulationsof

FEBIDgrowth).Thisexplainswhyrealfeaturesizesareaboutanorderofmagnitudelargerthanthe

primarybeamdiameter.Theexactlyachievabledimensionsvarywiththeprimarybeamparameters

andtheprecursormaterial,bothimpactingtheelectronmeanfreepathsand,bythat,thestatistical

interactionvolumeinsolidmaterials.Togiveconcretenumbersfornanowiresproducedbytheoften

usedPt‐basedMe3CpMePt(IV)precursor(seeFigure1b),featuredimensionsmostlyrangebetween20

nmand60nmin3Dspace,butcangobelow5nmunderspecialconditions[8,57,58].Withsuchsmall

featuresizes,3D‐FEBIDexceedstheachievableresolutionofother3Dprintingtechniques[10].Together

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withthelowdemandsonthesubstrateandthehigharchitecturalflexibility,3D‐FEBIDfulfillsthehigh

demandsforcontrolledSPMtipfabricationfromamorphologicalpointofview.

3.2.SPMRelevantTailoring

ThelateralresolutionachievableinSPM‐basedmethodsisconceptuallyrelatedtothetipradiusat

theapex[59].Thefirststudiesgobacktotheearly1990s,whereHübneretal.[60]andSchiffmannetal.

[61]studiedtherelationshipbetweentipshapes(diameter,height,andconicalangle)andfabrication

parameters,simplyusingcarboncontaminantsinsidetheSEM,anddeliveredoneofthefirstproof‐of‐

principlesforFEBID‐basedAFMtipmodification.Meanwhile,thebasicunderstandingoftheprocess

hasstronglyimproved,whichallowsspecifictailoring.For3D‐FEBIDstructures,theachievabletip

resolutiondependsonseveralprocessparameters.Figure2showsatiltedSEMimageofanano‐pillar,

depositedusingMe2Au(acac)astheprecursor.Theoverallshapeofapillarcanbedividedintothree

distinctregions,asindicatedinFigure2:(1)thecylindricalshaftwithhighparallelism(<2°)if

fabricatedproperly,(2)thetopmostconicalregion,and(3)theapexregion[8].Inmanycases,asmall

narrowingatthebaseoccurs(seelowestpartsinFigure2),whichappearsformediumandhighbeam

currents.Thismorphologicalpeculiarityistheresultofanenhancedlateraldepositionrateduetothe

strongerprecursordiffusionfromthelargesubstratereservoirandpillardiameters,whichvariesover

alengthscalecomparabletotheprecursor‐specificaveragediffusionlength(~150nmfor

Me3CpMePt(IV)[48,51,62]).SuchwideningcanbeadvantageousforSPMapplicationsduetothe

improvedbondingasaconsequenceofthelargerinterfacearea.Thelengthoftheshaftcanbeprecisely

adjustedbythegrowthtime,whichallowsforheight‐to‐widthaspectratioshigherthan100.Thevertical

dimensionofthetopmostconicalregionscaleswiththeprimaryenergy[63],asitdirectlymimicsthe

upperpartoftheelectroninteractionvolumeinthedepositmaterial,whichstatisticallyformsbythe

energydependentmeanfreepathsofelectronsinmatter/scatteringevents.TherightgraphinFigure2

givesadirectcomparisonofthetopmostpillarregion,wherethesharperconicalshapeforhigher

primaryelectronenergiesisevident.Asmentionedatthebeginning,sub‐10nmapexradiicanbe

routinelyachievedviaFEBIDforlowbeamcurrents(<100pA)withoutanyfurthertreatments,as

exemplarilyshownbythetransmissionelectronmicroscopy(TEM)micrographinsettopleft.



Micromachines2020,11,486of30



Figure2.FEBIDnano‐pillarcharacteristics.Thetiltedscanningelectronmicroscope(SEM)imageatthe

leftshowsthetypicalnano‐pillarmorphology,whichcanbecategorizedintothreeverticalsections:a

slightlybroaderbasesectionclosetothesubstrate,acylindricalshaftregion,andthetopmostcone,

terminatedbytheapex.Thelattercanexhibittipradiidownto5nm,asshownbythetransmission

electronmicroscopy(TEM)insettopleft.Whiletheshaftlengthisproportionaltothegrowthtime,the

verticalexpansionoftheconicalregionscaleswiththeprimaryelectronenergyduetothevaryingmean

freepathsofelectrons,statisticallydescribedbytheinteractionvolume.Theverticalexpansionofthe

latterdecreaseswithlowerprimaryenergies,whichexplainsthehighercurvatureofthetipregion

(compare5keV/squareswith30keV/invertedtriangles).TheinsetsshowrepresentativeSEMimagesof

thetipregioninatitledview.

Inprinciple,FEBID‐based3Dtipdepositioncanbeperformedintwodifferentconfigurations.In

oneconfiguration,thetipismountedverticallyintheSEM(paralleltotheelectronbeamaxis)andthe

electronbeamisheldstationary;thedepositgrowsverticallyalongtheelectronbeamaxis.Inthe

alternativeconfiguration,thetipismountedsidewaysintheSEMandtheelectronbeamisscanned

alongtheAFMtipaxis;thedepositgrowslaterally.Ineitherconfiguration,theresultingdeposit

orientationwithrespecttotheAFMtipisthesame,buttheresolutionisdifferent.Thelatter,orlateral

scanningapproach,ismorefavorabletoachievethesmallesttipwidths.SuchatipisshowninFigure

3,withalengthof700nmandanalmostconstantwidthof18nm.Theminimumresolutionisachieved

usingthehighestpossibleprimaryelectronenergyontheSEMs(mostly30keV),whichreducesthe

elasticscatteringprobabilityduringbeamtransmissionthroughthethinnanowire.Consequently,the

numberofBSE,FSE,andrelatedsecondaryelectrons(SE‐IIandSE‐III,respectively)emittedfromthe

surfacearereducedandthebeamdiameteralone,broadenedbythesecondaryelectron(SE‐I)action

radius,determinesthedepositwidth.Thethicknessofthegrowingwire,paralleltotheincoming

electronbeam,canbecontrolledbythepatterningvelocityand,intheidealcase,approachesthesame

valuesasforthewidth[29].Tipsdepositedinthiswayareidealforpinholecharacterizationorimaging

ofhighaspectratiosidewalls.Theproblemwithsuchtips,however,istheirfragilecharacterdueto

theirsmallcross‐sectionalareaandacomparablesmallinterfaceareaatthepillar–tipinterface.The

verticalfabricationconfigurationcanbeofadvantageunderthesecircumstances.Forexample,whilea

singlepillarfabricatedbystationarybeamexposure(Figure2)stillhasasmallinterfacearea,3D‐FEBID

canbeusedasshowninFigure3btoovercomethisfragilityproblem.Thefour‐leggedstructure(Figure

3b)depositedby3D‐FEBIDprovidesamorestablebase,whilestillconvergingtotherequiredvertical

singlepillar.Whilethesinglepillariswell‐connectedtothemergingbranchesofthetetrapod,the

FEBID/sampleinterfaceareaisfour‐timeshigherthanforsinglepillars.Idealpillarheightsarebelow1

μmtominimizelateralflexingforreliableAFMoperation.Comparedtolaterallygrowntips(seeFigure

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3a),vertical3Dtipshavelargertipdiametersof~50nmbutareradiallysymmetrical,whileapexradii

aretypicallybelow10nm(Figure2).Asmentionedbefore,themainproblemwithbothtiptypesisthe

lateralflexduetothehighcarboncontents.This,however,canberemediedbypost‐growthprocessing,

asdiscussedinthechapter“MaterialsandFunctionalities”.



Figure3.Featuresizes,3Dshapeflexibility,andscanningprobemicroscopy(SPM)advantages:(a)

showsahorizontallygrownPt–CFEBIDtip,wherethebeammovesacrosstheoriginaltipedge(top‐

downinthisimage)withscanspeedsaround30nm/s.Theresultingnanowiresrevealconstantwidths

inthesub‐20nmregimealongtheentiretip;(b)verticallygrown3Dtip,composedofafour‐leggedbase,

whichthenconvergestoatallsinglepillar.Whilethelatterenableshighaspectratiomeasurements,the

formerincreasesthemechanicalstabilityandtheadhesiontothesurface;(c)showsaFEBIDmodification

(greenarrow)ofaPt–Ircoatedconductive‐atomicforcemicroscopy(C‐AFM)tip(redarrow).Thetip

wasalsofullypurified,revealingatipradiusbelow10nm,whichstronglyimprovesthelateral

resolutioncapabilities,asshownin(d)byadirectcomparisonofFEBID(top)andtheoriginalPt–Irtip

(bottom),performedonananogranulargoldsample.Thescanwidthis1μm,whilebothZscalesspan

across14nm,asindicated.

3.3.Applications

Theperformanceofsimplepillar‐basedtipmodificationsofSPMprobeshasbeendemonstrated

bydifferentgroups.Thefirstreportsgobackto1993,bySchiffmanetal.,whodemonstratedthesuperior

characteristicsofhighaspectratioFEBIDcarbontipscomparedtoconventional,pyramid‐likeAFMtips

[61].Morerecently,Chenetal.[64]showedtheimprovedlateralresolutioncapabilitiesofsuchtips

usingmica,copper,andSiNsamplesindryand,inparticular,inliquidconditions,whichmaintaintheir

qualityevenafter7hcontinuousmeasurements(seeFigure4a,b).UsingasimilarFEBIDtip

modificationapproach,Brownetal.couldclearlydemonstratetheadvantageofthehighaspectratio

characteristics,asshowninFigure4c[28].Inmoredetail,theyusedcloselypackedpolystyrenespheres

astestsamplesandcouldshowthatFEBIDtipswentabout200nmdeeperthancommercialtips,which

alsoimprovedthelateraldiameteranalysesforsuchchallengingsamplesurfaces(seeFigure4d).

AnotherveryimpressiveapplicationwasshownbyNievergeltetal.,whousedFEBID‐modifiedSitips

forhigh‐speedmeasurementsofbiologicalsystems[65].Figure5ashowstheassemblyprocessofcoiled‐

coil/globularheadCrSAS‐6homodimersinliquids.Thesystemendsupincentriolarcartwheel

arrangements,whichactasscaffoldsfortheformationofanewcentrioleviatherecruitmentof

additionalperipheralcomponents(shadedyellow).Figure5bshowsatime‐resolvedseriesofhigh‐

resolutionmeasurements,whichdirectlyrevealstheformationprocessofthefirstcartwheel.Inthat

study,theapplicationofFEBIDtipswasessentialfromanadhesionpointofview,whilehigh‐resolution

capabilitieswereindispensablyneededandsuccessfullydemonstrated(scalebarsare50nm).Further

applications,whichstronglybenefitfromtheresolutioncapabilitiesofFEBIDtips,arediscussedlater

inthefunctionalitycontext.

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

Figure4.Atomicforcemicroscopy(AFM)imagingwithFEBIDnano‐pillars.(a)and(b)showtapping

modeheightimagesofSiNafter0.5hand7hcontinuousoperation,respectively,wherethemaintained

imagequalityreflectsthetiprobustness.TheZ‐rangeis3nminbothhigh‐resolutionimages.(c)shows

acommerciallyavailableAFMtip,modifiedbyaPt–Cnanopillar,revealinganendradiusof∼4nm,as

shownbyacircleintheinset(scalebaris200nm).(d)showsadirectcomparisonofclose‐packed

polystyrenespheresobtainedwiththeaforementionedstandardtipandtheFEBIDtipattheleftand

right,respectively(scalebaris400nm).Selectedlinescansalongthewhitelinesareshownbelowand

clearlydemonstratetheadvantageofdeeperprofilingabilitiesviaFEBIDtips.(a)and(b)wereadapted

andreprintedfromChenetal.,Nanotechnology2006[64],(c)and(d)wereadaptedandreprintedfrom

Brownetal.,Ultramicroscopy2013[28].



Figure5.High‐speedAFMimagingofbiologicalsystemsusingFEBIDmodifiedSitips.(a)Formation

processofcartwheelsandcentriolesbytheassemblyofCrSAS‐6homodimers,whicharrangedueto

theircoiled‐coil/globularheaddomains.(b)High‐speed,high‐resolutionimagingusingFEBIDtips,

whichmakeitpossibletofollowtheformationofacartwheel.Scalebaris50 nm;Zrange,6.3 nm.

AdaptedandreprintedfromNievergeltetal.,Nat.Nanotechnol.2018[65].

3.4.Challenges

Theactualchallengesintermsoffeaturesizearemainlyrelatedtothecross‐sectionalshapesof

individuallyinclined3D‐FEBIDnanowires,especiallywhenelementsarenotparalleltotheincoming

electronbeam.AscanbeseeninFigure1a,freestandingwirescanrevealablade‐likecharacterwiththe

longerdimensionoftheverticalcross‐sectionbeingorientedalongtheelectronbeamaxis.Suchnon‐

circularshapesentailspatiallyinhomogeneousmechanicalstiffness,whichcanbecomedetrimental

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duringAFMmeasurements,wherethestructuretwistingorinhomogeneousbendingconvolutesthe

finalimage[66].Perfectlycircularshapesonlyariseforverticalpillarsdepositedviastationary

exposure,whileconversely,inclinedsegmentsrevealanon‐linearvariationofitsthickness‐to‐width

aspectratio.Thesenon‐symmetricalcross‐sectionsarehighlycorrelatedwiththeelectron–solid

interactionvolumeandthereforedependontheprimarybeamenergy,thedensityofthedeposited

materialandontheinclinationangle.Averygoodcompromise,however,istheapplicationoflow

primaryenergies(around5keVandless),leadingtothickness:widthaspectratiosaround1.2:1fora

wideinclinationanglerange.Thephysicsgoverningtheshapeoftheinclinedsegmentcross‐section,

andthescalingbehavior,arecurrentlyunderinvestigationandwillbedisseminatedinafuture

publication.Thelong‐termgoalbasedoncurrentresearchactivitiesisapatterning‐basedcompensation

thatenablesclosetocircularcross‐sectionalshapes,independentofprocessparametersandinclination

angles[67].Subsequently,thiscapabilitywillbeextendedtotunethecross‐sectionalshapesalongsingle

branches,whichwouldopenupentirelynewpossibilitiesforapplication‐specifictailoring(e.g.,

mechanical,magnetic,oropticalanisotropyin3Dspace).

4.MaterialsandFunctionalities

Apartfromfeaturesizeaspects,thefunctionalitybecomesrelevantwhenfocusingonspecialSPM

operationmodes,suchaselectrostaticforcemicroscopy(EFM),Kelvinforcemicroscopy(KFM),

magneticforcemicroscopy(MFM),orscanningthermalmicroscopy(SThM),tonameafew.

Consequently,theavailabilityofsuitableprecursormaterialsisofessentialimportance.Figure1bshows

theperiodictableandindicateselementsthathavesuccessfullybeenusedforFEBID(yellowandgreen)

and3D‐FEBID(green).Whileamoredetailedoverviewofmetallicandmagneticmaterialpropertiesof

FEBIDmaterialsisgivenbyHuthetal.[23]anddeTeresaetal.[24],here,welimitourdiscussionto

3D‐FEBIDstructures.

Withafewexceptions[8,24,68–74],FEBIDmaterialsnotoriouslysufferfromhighlevelsof

contaminants(usuallycarbon)duetoincompleteprecursormoleculedissociation.Thecontamination

accumulatesaspartiallydissociatedmolecularfragmentsbecomeincorporatedinthedeposit.The

precursorsareoftenorganometallicinnature,whichexplainsthehighcarbonlevelsobserved[23,75].

Themicrostructureofsuchdepositsrevealsananogranularcharacter,wheremetallicnano‐grains(~2–

5nm)areembeddedina(hydro‐)carbonmatrix,asevidentbydark‐andbright‐appearingregionsin

theTEMmicrographinFigure2.Contaminatedmicrostructuresreduceorevenentirelymaskthe

intendedfunctionality.Detailedstudiesconcerningcompositiontunabilityusingbothprimarybeam

parametersandpatterningstrategiesrevealedalimitedabilitytocontrolcontaminationlevels[76,77].

Thisissuewasaddressedinrecentyearsbythesuccessfulintroductionofseveralprotocolstotuneboth

chemicalpurityandfunctionalproperties[23,75].Forexample,thefunctionalpropertiescanbetuned

viapost‐growthelectronbeamexposure,denotedase‐beamcuring(EBC)[78].EBCinitiatestwo

processes:(1)thedissociationofincompletelydissociatedprecursormolecules,whichreleasetarget

atoms,leadingtoslightgrain‐growth[22,23,78–80];and(2)themodificationandcross‐linkingofthe

carbonmatrix(sp3tosp2)[21,29,80,81].Theformereffectallowsprecisetuningoftheelectrical

conductivity[78]andevenenablesacontrolledinsulator–metaltransition,asdemonstratedforPtund

Au[18,22,23,78].TheunderlyingprocessisthemodulationoftheCoulombbarrierduetoreducedgrain‐

to‐graindistances,whichenableschemical[39,82]orthermalsensing,asdiscussedlater.TheEBCcross‐

linkingeffecthasmechanicalimplications,astheYoungmoduluscanpreciselybetuned,as

demonstratedforPtbasedmaterials[21].Pleasenote,EBChasaminoreffectonthechemistrybuta

majoreffectonthe(electric)functionality,whichisextremelyuseful,asdiscussedlater.Inothercases,

materialpurityisofhighestimportance.Inrecentyears,differentapproacheshavebeendemonstrated,

includinggas‐/laser‐/temperature‐assistedapproachesduring[72,83–88]orafterdeposition[44,89–91].

Althoughdifferentinexecution,thecommonelementformostoftheseapproachesisthetendencyfor

morphologicaldisruptionduringcarbonremoval,whichbecomesevenmorechallengingfor

freestanding3Dnano‐architectures[44,84].Thisaspectisdiscussedinthe“Challenges”sectionbelow.

Inthefollowing,wereviewdifferentSPMapplications,whichtakeadvantageofbothitsmorphology

anditsmaterialproperties.

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4.1.Applications

4.1.1.GeneralAFM

Carbon‐containing3D‐FEBIDstructuresexhibithighelasticityduringforceload[29],which

originatesfromthehighcarboncontentofupto90at.%[75],equivalenttomorethan70vol.%.During

AFMoperation,thetipexperiencesforcesinverticalandlateraldirections,whichimmediatelyimplies

mechanicalpropertiesofsinglepillars,whichcanbeconsideredasfundamentalbuildingblocksfor

morecomplex3D‐FEBIDarchitectures.ThistopicisreviewedingreatdetailbyUtkeetal.inanother

articleofthisspecialissue,whichgivesacomprehensiveinsightintomechanicalproperties,andtheir

dependencyonfabricationandonpost‐growthtreatmentapproaches[92].Inbrief,FEBIDpillarsare

muchstifferalongthemainaxiscomparedtosituationswherelateralforcesbecomerelevant[66].As

thelatterdependsonthepillarlengthsaswell,single‐pillar‐likeFEBIDstructuresshouldbeaslongas

needed,butasshortaspossible.

EBChasbeendemonstratedtochangetheYoungmodulusfromabout14GPato80GPa(even

highervaluesarepossible[92]),withoutaffectingtheoverallmorphology,includingtheapexradii[21].

ThegreatadvantageofEBCistheminimallydisruptiveinfluence,evenonfragile3Dnanostructures,

whichmakesthisapproachalmoststraightforward.AlthoughthematerialgetsstifferbyEBC,therisk

ofbreakingattheinterfaceduringSPMoperationremainsandgetsevenhigher,asarisingforcesget

moreconcentratedtothoseregions[66].Thesolutionforthisproblemcanbemorecomplex3Ddesigns,

whereoneconceptualexampleisshowninFigure3b.Inarecentstudy,therelationshipbetween3D

designandtheresultantstiffnessinverticalandlateraldirectionwasexplored[66].Itwasfoundthat

thetri‐podandtetra‐podarrangementsstronglyincreasethestiffness—upto60N/mand5N/minthe

verticalandlateraldirection,respectively—whichcomesintotherelevantrangewhenaimingforSPM

applications.Thetetra‐podarrangementprovidedthebestoverallmechanicalpropertieswithradially

symmetricstiffnesses.EBCtreatmentfurtherincreasedthestiffnessupto200N/minthevertical

direction,whichissufficientforstableAFMoperation.Inthesamestudy,theadvantageous

implicationsofEBCtreatmentsonthewearresistanceofthetipapexcouldbeshown,whichfinally

allowedhigh‐speed(~80μm/s)andlong‐termAFMmeasurements(uptofourhourswere

demonstratedwithoutweareffects)[66].Importantly,thestudyconclusivelydemonstratedthe

suitabilityofFEBID‐based3DnanostructuresforAFMoperationwithhigh‐resolutioncapabilities.

Fromacommercialpointofview,FEBID‐basedtipmodificationhasalreadyfounditswayinto

commercialproductsbythecompanyNanoTools(Munich,Germany),whoprovideabroadvarietyof

shapes,dimensions,andfabricationangles,basedonFEBID’s3Dcapabilities[93].

4.1.2.Electric

Next,wefocusonelectrically‐basedSPMmodessuchasC‐AFM,EFM,andKFM.Theearliest

reportsontheapplicationofFEBIDnano‐pillarsaselectricallyconductivetipsgobacktotheearly1990s,

whereHübneretal.demonstratedthatevencarbontipscanbeusedforSTM[60].Shortlyafter,

Schössleretal.deliveredoneofthefirstreportsontheapplicationofametal‐organicprecursorforSTM

tipmodification.Inmoredetail,anAu‐basedprecursorwasusedforthefabricationoffieldemitters

withsub‐10nmapexradiiandanangularemissiondensityof0.2mA/sr,whichweresuccessfully

appliedasSTMtipsandasSTM‐basednano‐lithographytools[16].Thelatterapplicationwasalso

demonstratedbyWendeletal.,whousedFEBID‐based,ultrahard,amorphouscarbontipsfornano‐

lithography.Byacombinationofhardtappingandtriangularvoltagepulses,theycouldfabricate25

nmlines(FWHM)inphotoresist,whichfinallyallowedthefabricationof30nmmetalstructures[17].

Morerecently,Chenetal.modifiedcommerciallyavailable,Pt–IrcoatedconductiveAFMtipsby20nm

wideand250nmlong,Pt‐basedFEBIDpillars[94].ThetipswerethenusedforEFMandKFM

measurementsofSi–Gequantumrings,wheretheycouldrevealtheexistenceofacentralSi–Ge‐like

areawithinthequantumringsthemselves,asshowninFigure6a–d.Theimprovedlateralresolution

wasduetothesmallpillardiametersand,inparticular,duetothesmallapexradiiforFEBIDpillars

(sub10‐nm;seeFigure2a),whichisabout2–3timessmallerthancoatedSitips(nominallyspecified

with20–25nm).ThatsituationisshowninmoredetailinFigure3c,whichisatiltedSEMimageofa

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coatedtip(redarrow),furthermodifiedbyaPtFEBID‐nano‐pillar(greenarrow).Whilethemuch

smallerapexradiusforthelatterisimmediatelyevident,Figure3dshowsadirectcomparisonofAFM

heightimagesonanAunano‐particlesampleincontactmode.Theimprovedlateralresolution

achievablewithFEBIDtips(upper)comparedtoPt–Ircoatedtips(lower)reliesonthefactthatfully

purifiedFEBIDtipsdonothave/needaconductivecoating,whichmakestheminherentlysharper.

WhileChenetal.successfullydemonstratedtheEFM/KFMsuitabilityforFEBIDtipsevenintheiras‐

depositedstate,C‐AFMrequiresfullypurified(metallic)tipsforreliablemeasurements.Asmentioned

above,differentprotocolshavebeenintroduced,whichallowthechemicaltransferintocarbon‐free

FEBIDmaterials.Aparticularlygentleapproachistheelectronbeam‐assistedpurificationinanH2O

atmosphere(20–120Pa)atroomtemperature,whichhassuccessfullybeenappliedtoPt‐andAu‐based

FEBIDmaterials[44,83,90].Thechemicalpurificationapproachishighlyefficientandcreatesdefect‐

free,concerningnanoporesorcracks,structuresafterpurification.Figure6showsadirect,in‐scale

comparisonofaPt–Cnano‐pillarbefore(e)andafterfullpurification(f),whichshows:(1)thereduced

apexradius,(2)increasedPtgrainsizes,(3)densegrainpacking,and(4)themaintainedshapefidelity.

Complementaryscanningtransmissionelectronmicroscopy‐basedelectronenergylossspectroscopy

(STEM‐EELS)measurementsconfirmedthecarbonremovalunderidealpurificationconditions[44,90].



Figure6.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non‐

conductiveregionsofthick(redcircles)andnanometerthin(purplerings)impuritylayers.Image

SuchtipscanthenbeusedasC‐AFMtips,whichisexemplarilyshowninFigure6g,h,wherethe

surfacetopographyisrevealedforamulti‐layerstructure,asschematicallyshownatthebottom.While

theheightimage(g)takesadvantageofboththesharpapexandthesteepsidewallstoreachthebottom

atfinetrenches,(h)showsa3Dheightplotwiththecurrentsignalasoverlay.Asevident,thecorrelated

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measurementsarealmostdigital,revealingconductive(turquoise)andinsulating(black)regionsand

identifyingnanometerthicksurfaceimpurities(purplecircles).Apartfromresolutionaspects,thereis

anotheressentialadvantageoffullypurifiedFEBIDtipscomparedtocoatedAFMtips:becausetheyare

entirelymadeofpurematerial(e.g.,Pt),thedelaminationproblemofcoatedAFMtipsisentirelyabsent,

alsomakingsuchprobesmoredurable.SuchFEBID‐basedconductivetipswillthenfulfillthe

requirementsforapplicationinC‐AFM,KFM,andEFM,asalreadyshownconceptually,butcanalso

beusedforpiezoelectricforcemicroscopy(PFM)andscanningspreadingresistancemicroscopy

(SSRM).Notably,Nohetal.[96]andRobertsetal.[97]havepreviouslyusedFEBIDandfocusedelectron

beam‐inducedetching(FEBIE)forthefabricationofadvancedinsulatedPFMandSPMprobesforuse

inliquidenvironments.Inshortsummary,theparticularadvantagesofFEBIDtipsforelectrically‐based

AFMmodesarethesharpapex,highaspectratioshapesifneededandtheprovenwearresistances.

Beyondthat,FEBIDallowsforspecialdesigns,whichenablemorphologicaladaptionaccordingtothe

individualsituation,asdiscussedinthe“DesignFlexibility”sectionbelow.

4.1.3.Magnetic

Now,weturntomagnetictips,wherethematerialqualityiscriticallyimportanttoprovidethe

requiredsensitivityduringMFMmeasurements.Ironandcobaltarethemostpromisingcandidatesfor

precursormaterials,basedonFigure1b.As‐depositedFeand/orCoFEBIDpuritymustbehighforthe

ferromagneticpropertytoemerge,asdiscussedinareviewbydeTeresaetal.[24].Topurifymagnetic

materials,H2OorO2assistedapproachesarenotsuitable,becausesurfaceandinternaloxidationoccurs

[24].Hence,differentresearchgroupshaveexploredtheprocesswindowduringinitialFEBIDto

minimizeunwantedimpurities.ThefirststudiesinthatdirectionstemmedfromUtkeetal.andLauet

al.,whofabricatednano‐pillarsfromCo2(CO)8precursorforMFMtests[32,98,99].TheConano‐tips

werecomposedof34at.%Co,51at.%C,and14at.%Oandallowedalateralresolutioninthesame

rangeasforcommerciallyavailable,coatedmagnetictips(~40nm),butprovidedthehighaspectratio

advantages,asdiscussedbefore.Inthefollowingdecade,strongprogresswasmadeinthefabrication,

optimization,andapplicationofmagneticmaterialsingeneral,ascomprehensivelyreviewedbyde

Teresaetal.[24].Severalstudiesexploredthevariationinchemicalcompositionasafunctionofprimary

electronbeamconditions[100,101],patterningparameters[102],gasfluximplications[103],and

substratetemperatures[101,104].Cocontentsupto95at.%werefoundathigherbeamcurrents,

primaryenergies,andelevatedtemperatures,wheretheremainingC/Ofractionstemsfromthe

unavoidablesurfaceoxidation,oncethesampleisexposedtoambientconditionsafterprocessing.

Foriron‐basedmaterials,themostrelevantprecursorsareFe(CO)5andFe2(CO)9,firststudiedby

Takeguchietal.[105]andFuruyaetal.[106].InsimilarstudiesasthosereportedaboveforCo,primary

beamandpatterningparameters,H2Oco‐exposure,substratetemperatures,andpost‐growthannealing

stepswerestudiedforFe‐containingdeposits(seereference[24]foradetailedoverview).Particularly

interestingwasthestudybyTakeguchietal[105],inwhichFEBID’s3Dcapabilitieswereusedtostudy

therelationshipsbetweenmorphology,thermalpost‐treatments,andtheresultingmagneticproperties

[105].ThatstudywasoneofthefirstwhichappliedFEBID’spowerful3Dcapabilities,bymeansofrods,

dots,rings,triangles,andevenmorecomplexstructures,forfundamentalresearch.Aparticularly

importantstudywasdonebyRodriguezetal.[107],whoinvestigatedthecoercivefieldasafunctionof

widthandthicknessinFenanowires.Thefindingsclearlyrevealedthatnarrowtipsbelow50nmin

widthshouldbehighlybeneficialforMFMapplications,whilethethicknessisnotascritical.Asimilar

resultwasfoundforCo‐basednanowires,asreportedbyVenturietal.[108]andWolfetal.[109],who

studiedthemagneticinductionaswellasmagneticstrayfieldsasafunctionofthetipdistanceforfull

optimizationofMFMtipmorphologies.Designruleswereproposedasaresultofthesestudies,which

showedthatthetiplengthscanbeadaptedaccordingtotherequirements(pinholes,sidewalls,grooves),

whilethetipdiametershouldbeassmallaspossible.Gavagninetal.[42]confirmedthesefindings

experimentallyandalsodemonstratedtheimportanceoftherelativetiltanglebetweentipandsample,

whichhastobeperpendicularforthehighestlateralresolutionandmagneticsensitivity.Froman

applicationpointofviewforCo‐basedFEBIDstructures,Stilleretal.reportedthemodificationof

Akiyamatips[110],whileBelovaetal.demonstratedtheoptimizedfabricationofCo‐basedFEBID

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super‐tipsasshowninFigure7a.Themetalcontentofthosetipswasaround60at.%,andallowedlateral

resolutionsdown10nm,whichcanbeconsideredasthecurrentbenchmarkforFEBID‐basedMFMtips

[111].



Figure7.Sub‐10nmresolutionmagneticforcemicroscopy(MFM)imagingviaFEBIDsuper‐tips.(a)

showsaCo‐based,FEBIDsuper‐tipwithametalcontentof~60at.%andendradiibelow10nm.Such

tipsarethenusedforimagingbitpatternedmedia,asshownbytheheightandlift‐modephaseimages

in(b)and(c),respectively.Asevident,theMFMimage(c)clearlyrevealsthewrittenbitswithlateral

resolutioninthesub‐10nmregime.AdaptedandreprintedfromBelovaetal.,Rev.Sci.Instrum.2012

[111].

WhileFEBID‐basedCostructurescanrevealmetalcontentsupto95at.%rightafterdeposition,Fe‐

basednanowirestypicallyrangearound50at.%,whichistoolowtoexploittheirfullpotential.Jose‐

MariadeTeresaetal.demonstratedthatsuchFe‐basednanowirescanbetransformedintonominally

pure,highlycrystalline,andmorphologicallystablenano‐pillarsviathermalpost‐growthannealingat

temperaturesofatleast400°C[105,112].Thelatter,however,mightbechallengingforcantilever

platforms,whichcontainintegratedcomponents,suchasself‐sensingorresistiveelements.

AheteronuclearFe–Coprecursor(HCo3Fe(CO)12)wassynthesizedanddemonstratedtobefully

3D‐FEBIDcompatible,whichservesasanalternativetothehomonuclearparadigm[113–115].The

resultingdepositsrevealedaCo3Fecomposition,whichisnominallycarbon‐/oxygen‐freewhen

disregardingthethin,unavoidablesurfaceoxidationlayer.Thisprecursormaterialisanidealcandidate

forMFMtipmodificationowingtothelowvaporizationtemperatureandnon‐reactivecharacterwith

thesurface.Figure8ashowsanSEMsideviewofaself‐sensingcantileverwithapre‐existingtip,

modifiedbyaCo3Fenano‐pillar,showninmoredetailattheright.SuchtipsrevealtypicalFEBID

diametersandapexradiiwellbelow100nmand10nm,respectively,whichcanbeuseddirectlyafter

fabricationwithoutanyfurtherpost‐growthtreatment.Forbenchmarkpurposes,aPt/Comultilayer

systemwasstudied,asrepresentativelyshowninFigure8bbya3DheightscanwithanMFMphase

overlay.Theperpendicularmagneticanisotropyleadstoaclearmagneticphasecontrast,not

recognizableinthemorphology.Figure8cshowsthephasecontrastinmoredetail,usinganFe‐based

FEBIDnano‐pillar,fabricatedatoptimizedgrowthconditions.Individualfeaturesarewellresolved,

whiletypicalphaseshiftsrangearound5°foroptimizedliftheightsandamplitudes.Incontrast,Figure

8dshowsthesamesampleimagedbyaCo3Fesuper‐tipwithverysimilaroveralldimensions.Asclearly

evident,phaseshiftsaremorethanthree‐timeshigheratoptimizedliftheights(seescales),which

reducesthenoiseandincreasesthelateralresolution,whichwasfoundtobeclearlybelow20nm.

Togetherwiththefactthatgrowthtimesaresimilar,thesefirstattemptsshowthehighperformanceof

thisheteronuclearprecursor.Roundedupbytheuncomplicatedhandling,thismaterialhasenormous

potentialtorevolutionizeFEBID‐basedfabricationofmagnetictipsforhigh‐performanceMFM

operation.

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

Figure8.FEBID‐based,high‐performanceMFMtips.(a)showsatiltedSEMsideviewofapre‐

structured,self‐sensingcantilever,modifiedbyamagneticCo3Fetipvia3D‐FEBIDat20keV/13pA.Such

tipsrevealtypicaldiametersandapexradiiwellbelow100nmand10nm,respectively.(b)showsa3D

heightimagewiththeMFMsignalasoverlay,takenfromaCo/Ptmultilayerstructurewith

perpendicularmagneticanisotropy.(c)and(d)showMFMphaseimagesfromthesamesample,

however,acquiredviaFeandCo3FeFEBIDtips,withsimilarmorphologies.Acloserlookrevealsthat

MFMtipsfromCo3Ferevealmuchstrongerphasecontrastsbyafactorofmorethanthree,whilelateral

resolutionisalsoimprovedtothesub‐20nmregime.ImagesarecourtesyofGETecMicroscopy[95](a–

Apartfromtheseclassicalapplications,moreadvancedAFMoperationmodeshavebeen

demonstratedincombinationwithFEBIDtips.ApowerfulexpansiontoclassicalMFMmeasurements

viaphase‐shiftdetectionisferromagneticresonanceforcemicroscopy(FMRFM).Thisoperationmode

isbasedonthedetectionofthemagneticresonancewithamagneticsampleduetothemagneticforce

exertedonacantileverequippedwithasmallmagnetictip[116].Sangiaoetal.fabricatedsmallConano‐

spheresviaFEBIDontopofasoftAFMcantileverinaverycontrolledmanner,rangingfrom100nmto

500nm,asshowninFigure9a,b[117].ThesphericalshapeisadvantageousforFMRFMoperation,asit

minimizesthemagnetichysteresis.TheCocontentwasrevealedtoincreasewiththediameter(Figure

9c),whileaminimumdiameterof150nmwasfoundasthelowerlimitforFEBID–Cospheresfor

appropriatemagneticsensitivity[24],almosteliminatingunwantedremanenceeffects.Althoughthe

lateralresolutionduringFMRFMoperationcorrelateswiththesizeofthenanosphere,impressive

resultshavebeenshowninrecentyears[118–123].Figure9d,eshowsarepresentativeresult,inwhich

FMRFMwasusedfordefectdetection(inthiscase,morphologicallyinduced),whileclassicalMFM

operationisunabletoextractthatinformation(compare(d)with(e)).



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

Figure9.FEBID‐basedferromagneticresonanceforcemicroscopy(FMRFM).(a)and(b)showdifferently

sizedConano‐spheres(topdown),fabricatedontopofahigh‐resolutionAFMcantileverinthetop(a)

andsideviews(b),revealingaclosetosphericalmorphology.(c)showsthecobaltcontentofthebeads

asafunctionofthediameter,whichhastobecorrelatedwithitsmagneticproperties.Additional

measurementsrevealedaminimumdiameterof150nmtoexploitthefullperformance.(d)and(e)show

FMRFMandMFMmeasurements,respectively,usingaCo‐AFMcantilevermodifiedviaFEBID.These

measurementsuseanin‐planemagneticfield,whichalignsthesamplemagnetization.Amicrowave‐

frequencycurrentisthenintroducedperpendiculartothemagneticfield,butalsoin‐plane.Thelatter

decreasesthequasi‐staticcomponentofthemagnetization,whichimpactsthemagneto‐staticforce

betweenthesampleandtip.Oncethemagneticresonanceisfound,FMRFMrevealsdefectsinducedby

chemistryormorphology,asprovokedinthisexamplebyadifferentshapeofthecentralelementinthe

3×3array.ThisexplainswhythatpartisdarkinFMRFMmode(d).Incontrast,classicalMFM

measurementsareunabletodetectsuchdefects,asshownin(e).Althoughpowerfulinitsanalytical

capabilities,thelateralresolutioniscurrentlylimitedtoabout90nm,asthatdependsonthebead

diameter.(a–c)wereadaptedandreprintedfromSangiaoetal.,BeilsteinJ.Nanotechnol.2017[117].(d)

and(e)wereadaptedandreprintedfromChiaetal.,Appl.Phys.Lett.2012[118].

4.1.4.Thermal

Asmentionedbefore,EBChasaminorinfluenceondepositmorphologybutastrongimpacton

theelectricalfunction:thenano‐granularcompositionremains,whiletheelectricaltunneling

probabilitybetweenadjacentnano‐grainsincreases[23,78].Thelattereffectalsodependson

temperature,leadingtodecreasingresistancesathighertemperaturesduetothermallyassisted

tunneling.Bythat,themacroscopiccurrentthroughsuchbridgesprovidesinformationofthematerial’s

temperatureswithanegativetemperaturecoefficient(NTC)ofelectricresistance.Thisthermistor‐based

SThMconceptwasfirstshownbyEdingeretal.in2001[124,125],butnotfollowedinmoredetail,which

mightbeexplainedbythelimitedknowledgeaboutmaterialtuningandcontrolled3Dnano‐printing

capabilitiesatthattime.Sattelkowetal.recentlyrevisitedthe3Dthermistorconcept,optimized

mechanical/electricalaspects,anddemonstratedthermalprobing[66].AssummarizedinFigure10,the

authorsusedaself‐sensingAFMcantileverplatform(a),whichwaspre‐structuredwithtwoelectrodes,

coveringthepre‐existingtipaswell(b).FEBIDtetra‐podswereusedas3Dnano‐bridgesbetweenthose

twoelectrodes,asshownbyanoverview(b)andbyclose‐upsin(c)and(d).Onceincontactwiththe

sample,thetetra‐podsheatupandchangetheirelectricconductivityduetotheNTCbehavior.Because

ofthesmallactivevolumesintheFEBID‐createdcontactregion,theheatresponseisveryfastandallows

fordynamicsensingofatleast30ms/K,asshowninFigure10e.Thisfunctionalnano‐probenotonly

demonstratesAFMcompatibilitybutalsotheusefulnessofanano‐granularmaterial,amicrostructure

whichisoftenregardedasineffectiveconcerningrealapplications.Thishallmarkdemonstrationof

functionaltransductionwascomplementedbyanapex‐radiusbelow10nmandfastfabricationtimes

around10minutes,includingEBC.

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

Figure10.FEBID‐based3Dnano‐probesforlocalizedthermalsensing.(a)showstheself‐sensing

cantileverplatform(copyrightGETecMicroscopy,Austria[95]),wheresensingelementsandpre‐

structuredAuelectrodesareindicatedblueandyellow,respectively.(b)showsanSEMimageofthetip

region,inwhichthetruncated,pre‐existingtip,thesplitelectrodes,andthe3Dnano‐bridgeareevident.

Thelatterisshowninhighermagnificationinaside(c)andtopview(d),wherethe3Dnano‐bridgeis

showninred.Onceincontactwiththesamplesurface,thesmallactivevolumesofthe3Dstructureallow

afastresponse,asshowninadynamicresponsetestin(e)bytheredcurveincomparisontothereference

temperature(blue).Asensingratebetterthan30ms/Kwithanoiselevelbelow±0.5Kcouldbe

demonstrated,whichexhibitstheadvantagesof3D‐FEBIDnano‐structuresforadvancedAFMconcepts,

applicableonevenpre‐finishedmicro‐cantilever.AdaptedandreprintedfromSattelkowetal.,Appl.

Mater.Interfaces2019[66].

4.1.5.Optical

AfirstexampleofFEBID‐basedspectroscopywasreportedbyCastagneetal.in1999,who

modifiedSi–AFMprobesbycarbonsupertipswithvariablelengths(1.3–3.3μm),widths(130–300nm),

andapexradiidownto15nm[126].SuchprobeswerethenusedinacombinedAFM/photonscanning

tunnelingmicroscopetostudytheirnearfieldopticalbehavior.Thestudyrevealedthatevencarbon

tipsareconvenientfornearfieldopticalconversionofevanescentwavesinthenearinfra‐redrange.

Theauthorsalsostatedthatthelateralresolutionduringimagingmaybelimitedasaconsequenceof

theopticalpropertiesofcarbontipsandgavetheory‐basedrecommendationsforfurtheroptimization

towardshigh‐performanceopticalnearfieldnanoprobesforapplicationinscanningnearfieldoptical

microscopy(SNOM)orTERSmicroscopy.Shortlyafter,Sqallietal.focusedonthebeneficial

implicationsofsmallFEBID‐basedAunano‐dots/‐ellipsesonlightscatteringcapabilitiesduringSNOM

[127].Inmoredetail,lightfiber‐basedprobesweremodifiedbydifferentlyshapedFEBIDnano‐

structuresandstudiedbyacombinationoftheoryandexperiments.Theauthorscoulddemonstrate

thatsphericalstructureswith~60nmdiameterincreaseboththesignalandthecontrastinnearfield

transmissionmeasurements.Thestudyalsoshowedthatellipticalshapesallowtuningoftheresonant

wavelengthasaconsequenceoftherelativeorientationbetweenellipticnano‐structuresandthe

incomingpolarizedlight,whichdemonstratesFEBID’susefulnessduetoitsdesignflexibility.

AnimpressivestudyconcerningTERSmicroscopywasdonebyDeAngelisetal.,whointroduced

ahighlyadvancedTERStipconcept,basedonfocusedionbeam(FIB)andFEBID[128].Figure11shows

thetaperedwaveguide,fabricatedona100nmthickSi3N4AFMcantilever(a).First,FIBwasusedto

fabricatethephotoniccrystal,consistingofatriangularregionwithholeandseparationdistancesof160

nmand250nm,respectively(b).Forthecentralwaveguide,aPt‐based,3D‐FEBIDstructureactedas

scaffold,whichwascoveredwith30nmAganddown‐shapedviaFIB(c)toapexradiibetween2.5and

5nm(furtherprocessstepswereapplied,asdetailedinreference[128]).Suchprobeswerethenusedin

AFMoperation,wheretheincominglaseriscoupledviathebacksidethroughthephotonicelement,

whichlaunchessurfaceplasmon,furtherpropagatingalongtheAgtiptotheapex,wherethelocalized

plasmonresonanceoccurs(d).DetectionoftheRamansignalwasdoneintransmission.Figure11eand

fshowanAFM3Dheightimageofasub‐micrometerSinanocrystal/SiOxtrenchandthecorresponding

Ramanintensityalongtheredlines,respectively.Thevaryingintensityreflectsthecrystallinityofthe

material,whichcorrelateswellwiththewallregion(e),whichprovessub‐10nmresolution.While

FEBIDstructuresactedasscaffoldsinthiscase,recentprogressconcerningpurityandshapestability

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holdthepotentialtofabricatethecentraltipsolelyvia3D‐FEBID[44],whichwouldconsiderably

simplifytheentirefabricationprocess.



Figure11.FEBID/focusedionbeam(FIB)assisted,photonic/plasmonictip‐enhancedRamanscattering

(TERS)nano‐probe.(a)Overviewofa100nmthickSi3N4AFMcantilever,equippedwiththephotonic

andplasmonicelement,basedonFIBandFEBID,respectively.(b)showsacloseupinwhichtheFIB

holes(160nmdiameterand250nmdistance)areevident.ThecentralelementusesaPt‐based3D‐FEBID

pillarasscaffold,furthercoatedwithpureAgandmilleddownviaFIBtoapexradiiof5nm

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

Abstract and Figures