Chemical-Reviews-ALD-Overview-Jan-2010

.2010,110,111–131111

AtomicLayerDeposition:AnOverview

*

DepartmentofChemistryandBiochemistryandDepartmentofChemicalandBiologicalEngineering,UniversityofColorado,

Boulder,Colorado80309

ReceivedFebruary12,2009

Contents

uction

2

O

3

ALDasaModelALDSystem

landPlasmaorRadical-EnhancedALD

lALD

orRadical-EnhancedALD

rsforALD

LDUsingThermalChemistry

silaneEliminationChemistry

tionChemistry

enReductionChemistry

tionandGrowthduringALD

xideALDonH-Si(100)

LDonOxideSurfaces

2

O

3

ALDonCarbonNanotubesand

GrapheneSurfaces

peratureALD

2

O

3

ALDandOtherMetalOxideALD

ticSiO

2

ALD

olymers

ighAspectRatioStructures

articles

anolaminatesandAlloys

rMLD

cPolymers

Organic-InorganicPolymers

onalTopics

alALDBehaviorandtheALDWindow

-SelectiveALDforSpatialPatterning

hericPressureALD

iologicalTemplates

mergingAreas

sions

ledgments

nces

111

112

113

113

114

115

116

116

117

117

118

118

118

119

119

119

120

121

122

123

124

125

125

126

127

127

127

127

128

128

128

129

129

isProfessorintheDepartmentofChemistryand

BiochemistryandDepartmentofChemicalandBiologicalEngineeringat

ChemistryfromYaleUniversity(1977)istryfrom

theUniversityofCaliforniaatBerkeley(1983).Priortohisappointments

attheUniversityofColoradoatBoulder,wasaBantrell

PostdoctoralFellowatCaltech(1983-4)andanAssistantProfessorin

theDepartmentofChemistryatStanfordUniversity(1984-1991).Dr.

GeorgeisaFellowoftheAmericanVacuumSociety(2000)andaFellow

oftheAmericanPhysicalSociety(1997).Hehasalsoreceivedthe

AmericanChemicalSocietyColoradoSectionAward(2004),R&D100

AwardforParticle-ALD(2004),NSFPresidentialYoungInvestigatorAward

(1988-1993),oundationFellowship(1988).Dr.

George’sresearchinterestsareintheareasofsurfacechemistry,thin

filmgrowth,rrentlydirectinga

researcheffortfocusingonatomiclayerdeposition(ALD)andmolecular

layerdeposition(MLD).Thisresearchisexaminingnewsurfacechemistries

forALDandMLDgrowth,measuringthinfilmgrowthrates,and

characterizingthepropertiesofthinfiservedasChairof

thefirstAmericanVacuumSociety(AVS)TopicalConferenceonAtomic

LayerDeposition(ALD2001)heldinMonterey,teaches

isacofounder

ofALDNanoSolutions,Inc.,astartupcompanythatisworkingto

commercializeALDtechnology.

uction

Atomiclayerdeposition(ALD)hasemergedasan

importanttechniquefordepositingthinfilmsforavariety

nductorprocessinghasbeenoneof

themainmotivationsfortherecentdevelopmentofALD.

TheInternationalTechnologyRoadmapforSemiconductors

(ITRS)hasincludedALDforhighdielectricconstantgate

oxidesintheMOSFETstructureandforcopperdiffusion

barriersinbackendinterconnects.

1

Inaddition,ALDhasmet

*E-mailaddress:@.

challengingrequirementsinotherareasincludingthedeposi-

tionofhighqualitydielectricstofabricatetrenchcapacitors

forDRAM.

2

Miniaturizationinthesemiconductorindustryhasledto

therequirementforatomiclevelcontrolofthinfilm

urizationhasproducedveryhighaspect

rthin

filmtechniquecanapproachtheconformalityachievedby

essityforcontinuous

andpinhole-freefilmsinsemiconductordeviceshasdriven

pplicationswithsimilar

demandingrequirementsoutsideofthesemiconductorin-

dustryarelowelectronleakagedielectricsformagneticread/

writeheads

3

anddiffusionbarriercoatingswithlowgas

permeability.

4

10.1021/cr900056b2010AmericanChemicalSociety

PublishedonWeb11/30/2009

112ChemicalReviews,2010,Vol.110,No.1

ticrepresentationofALDusingself-limiting

surfacechemistryandanABbinaryreactionsequence.(Reprinted

ght1996AmericanChemical

Society.)

ALDisabletomeettheneedsforatomiclayercontrol

andconformaldepositionusingsequential,self-limiting

aticshowingthesequential,self-

limitingsurfacereactionsduringALDisdisplayedinFigure

1.

5

MostALDprocessesarebasedonbinaryreaction

sequenceswheretwosurfacereactionsoccuranddeposita

binarycompoundfiethereareonlyafinitenumber

ofsurfacesites,thereactionscanonlydepositafinitenumber

ofthetwosurfacereactionsis

self-limiting,thenthetworeactionsmayproceedina

sequentialfashiontodepositathinfilmwithatomiclevel

control.

TheadvantagesofALDareprecisethicknesscontrolat

theÅf-limitingaspect

ofALDleadstoexcellentstepcoverageandconformal

rfaceareas

willreactbeforeothersurfaceareasbecauseofdifferent

precursorgasflr,theprecursorswilladsorb

andsubsequentlydesorbfromthesurfaceareaswherethe

cursorswillthen

proceedtoreactwithotherunreactedsurfaceareasand

produceaveryconformaldeposition.

Theself-limitingnatureofthesurfacereactionsalso

producesanonstatisticaldepositionbecausetherandomness

oftheprecursorfl

aresult,ALDfilmsremainextremelysmoothandconformal

totheoriginalsubstratebecausethereactionsaredrivento

completionduringeveryreactioncycle.

6

Becausenosurface

sitesareleftbehindduringfilmgrowth,thefilmstendtobe

ctorisextremely

importantforthedepositionofexcellentdielectricfilms.

7

ALDprocessingisalsoextendibletoverylargesubstrates

precursorsaregasphasemolecules,andtheyfillallspace

independentofsubstrategeometryanddonotrequireline-

nlylimitedbythesizeof

processisalsodominated

ethesurfacereactionsare

performedsequentially,thetwogasphasereactantsarenot

parationofthetwo

reactionslimitspossiblegasphasereactionsthatcanform

particlesthatcoulddepositonthesurfacetoproducegranular

films.

Theuseoftheterm“ALD”datesbackapproximatelyto

o2000,thetermatomiclayerepitaxy(ALE)

wasincommonuse.

8-13

Othertermshavebeenusedto

describe

14

ALD,includingbinaryreactionsequencechemis-

tryandmolecularlayerepitaxy.

15

ThetransitionfromALE

George

toALDoccurredasaresultofthefactthatmostfilmsgrown

usingsequential,self-limitingsurfacereactionswerenot

er,amorphous

filmsweremostpreferredfordielectricanddiffusionbarrier

uently,theuseofALDgrewinprefer-

enceandnowdominateswiththepractitionersinthefield.

ThehistoryofALEandALDdatesbacktothe1970sin

ginalpioneerofALEwasTuomoSuntola,

whodemonstratedsomeofthefirstALEprocessesasearly

asAugust/September1974.

16

ThefirstALEsystemdevel-

opedwasZnS.

16

ThefirstALEpatentemergedin1977.

17

ThefirstliteraturepaperonALEappearedin1980inThin

SolidFilms.

18

ThefirstapplicationofALEwaselectrolu-

firstpublicdisplayofanALEdevice

wasanelectroluminescentdisplaythatoperatedinthe

firstcommercial

ALEreactorwastheF-120soldbyMicrochemistryin1988.

ThefirstofaseriesofALEmeetingswasheldin1990and

firstofaseriesofyearlyALD

meetingswasheldin2001andhascontinuedthroughthe

presentdate.

Manyearlierreviews

-21

haveaddressedthebasicsofALE

orALD.

5,8,11,12,19

Manypreviousreviewshaveconsidered

theapplicationofALE

27

orALDtomicroelectronicsand

nanotechnology.

19,22-

Theintentofthispresentreviewis

d,thisreview

isfocusedonanoverviewofkeyconceptsandnewdirections

iconductorroadmapiscomingtoanend

inafewyearsbecauseofthelimitsofthecurrentelectronic

tinuedprogress,thefutureforelectronic

materialswillembraceasyetundefil

almostcertainlybepartofthenewparadigmsbecauseofits

abilitytocontroldepositionontheatomicscaleandtodeposit

conformallyonveryhighaspectratiostructures.

2

O

3

ALDasaModelALDSystem

TheALDofAl

earlierextensive

2

O

3

hasdevelopedasamodelALDsystem.

AnreviewbyPuurunenhaspreviously

discussedthedetailsofAl

sectionwillonlymentionthe

2

O

highlights

3

ALD.

20

Consequently,this

ofAl

usuallyperformedusingtrimethylaluminum

2

O

ALDis

3

(TMA)

2

O

3

andH

2

firstreportsofAl

2

O

3

ALDusingTMAand

H

2

Odatebacktothelate1980sandearly1990s.

28,29

More

recentworkinthesemiconductorindustryisusingTMA

andozoneforAl

onAl

2

O

3

ALD.

30,31

Thisreviewwillconcentrate

2

O

3

ALDusingTMAandH

surfacechemistryduringAl

2

O.

The

2

O

3

ALDcanbedescribed

as

5,14,32

(A)AlOH*+Al(CH

3

)

3

fAlOAl(CH

3

)

2

*+CH

4

(1)

(B)AlCH

3

*+H

2

OfAlOH*+CH

4

(2)

growthoccursduringalternatingexposurestoTMA

2

O

ALD

3

andH

reactions

2

are

2

O

very

3

ALDisamodelsystembecausethesurface

effindriver

fortheefficientreactionsistheformationofaverystrong

rallreactionforAl

2

O

3

ALDis

2Al(CH

3

)

3

+3H

2

OfAl

2

O

3

+3CH

4

∆H)-376kcal(3)

AtomicLayerDeposition

onpathandpredictedenergeticsforreactionsof

Al(CH

3

)

3

ontheAl-OH*surfacesitecalculatedusingthe

Al(OAl(OH)

2

)

OH

2

-ucturesareshownusingthe

Al(OH

2

)-clusterforclarity.(Reprintedwithpermissionfrom

ght2002AmericanInstituteofPhysics.)

Thisreactionhasanextremelyhighreactionenthalpy.

33

This

isoneofthehighestreactionenthalpiesencounteredforany

ALDreaction.

ThepotentialenergysurfacesduringAl

2

O

densityfunctionaltheory(DFT).

3

ALD

34

have

beenmodeledusingThese

calculationsshowthatAl(CH

3

)

3

existsinadeepprecursor

wellcomplexedtoAlOH*speciespriortoitssurface

reaction,asshowninFigure2.

34

Likewise,thecalculations

showthatH

2

Oisalsoinadeepprecursorwellcomplexed

toAlCH

3

*

complexesresultfromstrongLewisacid-baseinteractions

ghtheseprecursorwellshavenotbeen

experimentallyobserved,theymaybefairlygeneralfor

variousclassesofALDreactions.

ThesurfacechemistryofAl

2

O

3

ALDhasbeenconfirmed

byinsituFTIRstudies.

32,35,36

TheFTIRdifferencespectra

clearlyshowthelossofAlOH*speciesandconcurrentgain

ofAlCH

3

*se,the

lossofAlCH

3

*speciesandtheconcurrentgainofAlOH*

speciesisobservedduringtheH

2

phase

reactionproductsduringAl

2

O

3

ALDhavealsobeen

37,38

identi-

fi

Al(CH

3

)

3

andD

2

Oasthereactants,CH

asexpected

3

Dwasobservedas

themainreactionproduct,fromthesurface

chemistryforAl

2

O

3

ALD.

37

Byrepeatingthesurfacereactions,Al

ABcycles.

2

O

3

14,39

growthis

extremelylinearwiththenumberofVarious

techniques,suchasspectroscopicellipsometryandquartz

crystalmicrobalance(QCM)measurements,havecharacter-

izedthegrowthpercycleduringAl

2

O

3

l

measured

14,39

Al

The

2

O

resulting

3

ALDgrowthratesare1.1-1.2ÅperAB

the

2

O

underlying

3

ALDfilmsaresmoothand

son

nanoparticlesshowexcellentconformalityofAl

films.

35,40,41

Investigationsonhighaspectratiotrench

2

O

3

ALD

sub-

stratesalsorevealsuperbconformality,asillustratedbythe

cross-sectionalscanningelectronmicroscopy(SEM)image

inFigure3.

42

OneofthehallmarksofALDisself-limitingsurface

f-limitingsurfacereactionsduringAl

2

O

ALDhavebeenobservedbyinsituFTIR

32,35

andQCM

39

3

investigations

14

aswellasbyspectroscopicellipsometry

ctantexposuresrequiredforthesurface

ChemicalReviews,2010,Vol.110,No.1113

-sectionalSEMimageofanAl

of300nmonaSiwaferwitha

2

O

trench

3

ALDfilmwith

athicknessstructure.

(ght1999JohnWiley

&Sons.)

reactionstoreachcompletionrevealthatthereactivesticking

coefficientsduringAl

2

O

3

ALDaremuchlessthanunity.

Basedonrequiredexposuretimes,thereactivesticking

coefficients

14

areintherangeof∼10

-3

-10

-4

duringAl

ALD.

2

O

3

ThegrowthperoneALDcycleisalsomuchsmallerthan

oneAl

2

O

3

“monolayer”.Thegrowthratesof1.1-1.2Åper

ABcyclecanbecomparedwiththethicknessofoneAl

“monolayer”.Thismonolayerthicknessisestimatedusing

2

O

3

thedensityof3.0g/cm

3

forAl

°

2

O

3

ALDfilmsgrownat177

units

C.

43

Basedonthisdensity,thenumberdensityof“Al

isF)1.77×10

22

Alnumber

2

O

3

”

2

O

3

units/cm

3

.The

6.8

of

Al

2

O

3

unitspersquarecentimeterisequaltoF

2/3

)×

10

14

F

-1/3

cm

-2

)3.8

.Likewise,

Å.Thegrowth

themonolayer

perABcycle

thickness

of1.1-

is

1.2

equal

Åper

to

ABcycleismuchlessthanthisestimateofthemonolayer

thickness.

ThedisagreementbetweengrowthperABcycleandthe

monolayerthicknessisnotsurprisingbecauseALDgrowth

surfacechemistryisnotrequiredtoalwaysyielda“mono-

layer”relation

betweenALDgrowthandsurfacechemistryisclearly

illustratedbythetemperature-dependenceofAl

wthperABcycle

2

O

decreases

3

ALD

progressivelywithtemperaturebetween177and300°

decreaseresultsfromtheprogressivelossofAlOH*and

AlCH

3

*surfacespeciesathighertemperatures.

14,32

Thecontinuousandpinhole-freenatureofAl

superbelectricalproperties.

2

O

isrevealedbytheir

3

ALD

films

Current-voltagecurvesforvariousAl

onn-Si(100)revealelectrical

2

O

3

ALDfilmthick-

nesses

7

behaviorthatisvery

similartothatofthermalSiO

2

fi

haveadielectricconstantof∼7anddisplayvery

2

O

3

ALDfilms

lowelectron

leakage.

7

Increasesinthecurrentdensityversusapplied

potentialoccurasaresultofFowler-Nordheimtunneling.

Thischaracteristicisconsistentwiththeabsenceofany

defectsorpinholesintheAl

Al

2

O

3

ALDfixcellent

propertieshaveenabled

2

O

3

ALDfilmstoserve

44-46

asgate

oxidesandtopassivatesemiconductorsurfaces.

landPlasmaorRadical-EnhancedALD

lALD

ALDiscloselyrelatedtochemicalvapordeposition

(CVD)basedonbinaryreactionssuchasA+BfProduct.

114ChemicalReviews,2010,Vol.110,No.1

ForCVDusingbinaryreactions,theAandBreactantsare

presentatthesametimeandformtheproductfilmcontinu-

,thesubstrateisexposedto

theAandBreactantsindividuallyandtheproductfilmis

ic

recipeforALDistofindaCVDprocessbasedonabinary

reactionandthentoapplytheAandBreactantsseparately

binaryreactionsequence.

TherearemanyexamplesofALDresultingfrombinary

esforTiO

binaryCVDreactions

2

andZnOare

basedonthefollowingandtheir

correspondingreactionenthalpies:

33

TiO

2

ALD:TiCl

4

+2H

2

OfTiO

2

+4HCl

∆H)-16kcal(4)

ZnOALD:Zn(CH

2

CH

3

)

2

+H

2

OfZnO+2C

2

H

6

∆H)-70kcal(5)

TheseALDsystemsyieldagrowthperABcycleof∼0.4Å

from150to600°CforTiO

2

ALD

47

and2.2-2.5Åfrom

100to160°CforZnOALD.

48,49

TheseALDchemistries

havenegativeheatsofreactionandarerobustALDreactions.

Thesereactionsoccurspontaneouslyatvarioustemperatures

andwillbereferredtoasthermalbecausetheycanbe

performedwithouttheaidofplasmaorradicalassistance.

AsurveyofdevelopedALDprocessesrevealsthatmost

thermalALDsystemsarebinarycompoundsbasedonbinary

reactantCVD.

20,21

ThemostcommonthermalALDsystems

arebinarymetaloxidessuchasAl

2

O

3

,TiO

commonthermalALD

2

,ZnO,ZrO

systemsare

2

,

HfO

binary

2

,andTa

metal

2

O

nitrides

5

.Other

suchasTiN,TaN,andW

systemsalsoexistforsulfidessuchasZnS

2

l

ALDandCdS

andphosphidessuchasGaPandInP.

orRadical-EnhancedALD

Thereisalsoaneedforsingle-elementALDmaterials,

suchasmetalsandsemiconductors,thatcanbedeposited

forsomenotable

exceptionsdiscussedinsection5,thesingle-elementfilms

ofmetalsandsemiconductorsareverydifficulttodeposit

ately,thesesingle-

elementscanbedepositedusingplasmaorradical-enhanced

ALD.

22

Theradicalsorotherenergeticspeciesintheplasma

helptoinducereactionsthatarenotpossibleusingjust

sourcescanbeusedtogenerate

hydrogenradicalsthatreducethemetalorsemiconductor

enradicalscanalsobeproducedusinga

hottungstenfieformetalALDusingmetal

reactantsandhydrogenradicalsisshowninFigure4.

Hydrogenradical-enhancedALDwasfirstdemonstrated

forTiALD

50

usingaH

2

sanotherALD

systemthathasbeenstudied

51

extensivelyusinghydrogen

radicalsfromH

2

ctantsforTaALDare

TaCl

ALD

5

andhydrogenradicals.

51

Thesurfacechemistryfor

Tacanbeexpressedas

(A)Ta*+TaCl

5

fTaTaCl

5

*(6)

(B)TaCl

5

*+5H·fTa*+5HCl(7)

TaCl

5

isfiuently,the

hydrogenradicalsreducetheTaatomsandremovethe

ghthegrowthpercycle

George

ticdiagramofhydrogenradical-enhancedALD

usingametalreactantandhydrogenradicals.

duringTaALDisonly0.08ÅperABcycle,theTaALD

filmshaveexcellentfilmresistivitiesandshowgoodCu

barrierproperties.

51

Thesmallgrowthpercycleisattributed

tosterichindrancecausedbythelargeTaCl

XRDalsoindicatesthattheTa

5

admolecule

filmis

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