IN-PLANE TEXTURING IN

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IN-PLANE TEXTURING IN

SPUTTERED FILMS

J.F.

WHITACRE*,

S.M.

YALISOVE

and

J.C.

BILELLO

University

_of

Michigan,Department

_of

MaterialsScienceand Engineering,2300 Hayward Street, AnnArbor, MI48109, USA

(Received8October1999)

Filmsconsisting ofMo, Cr,andTahaveall been foundtodisplaywell-defined biaxial textureswhen grown undercertain conditions.Awell-definedout-of-planetextureevolves

withinthefirst 100nmof the film, followed by theevolutionofapreferred crystal-lographicorientation inthe plane of thefilm.Theseeffect werestudiedusingX-ray pole-figure analysis, scanning electron microscopy(SEM),transmissionelectron microscopy

(TEM),transmissionelectrondiffraction(TED),and highresolutiongrazingincidence

X-rayscattering(GIXS).Ithas been found that in-planetextureevolvesonly when there is,onaverage, oblique adatom fluxincident ontothe substrate.Further, the type of

out-of-planetexturecan be controlled by altering the deposition conditions. Parameters

including cathode-to-substrate distance, deposition rate, average angle of adatom

inci-dence, and sputter gas pressure, have been shownto determinethetypeout-of-plane texture, as well as therateofin-planetextureevolution.Thestudiesconducted have shown thatit ispossibleto createand control biaxially textured films and multilayers made of a variety ofmaterials.Arecentmodelwhichdescribes thisphenomenais discussed.

Keywords: Sputtered films;Texture;Multilayers;X-ray; TEM;SEM

INTRODUCTION

Thin metallic films are frequently used in applications such as

inte-grated circuit interconnects

(Muraka, 1993),

protective coatings

(Windischmann, 1992;Adamsetal.,

1993),

magnetic recording media

(Katoaka

etal., 1993; Kiefand Egelhoff

Jr.,

1993; Kimetal.,

1993),

and superconducting multilayers

(Mattson,

1997; Bauer etal.,

1997).

* Corresponding author.

Thetexturingin these filmsis acritical property,andtheabilityto

con-trol the development ofboth out-of-plane and in-planetexture is of

interest. Relevantstudieshave focused on theeffects of energeticion

beams (Bradley et al.,

1986),

sputter gas pressure

(Je

et al.,

1997),

deposition geometry (Hagemeyer etal., 1993; Karpenkoet al., 1994;

Bilelloetal., 1995;

Harper

etal., 1997; Rodriguez-Navarroetal.,

1998),

surface energetics(Knuytetal., 1995; ThompsonandCarel,

1995),

and

impurity concentration

(van

de Waterbeemd and van Oosterhout,

1967)

haveontexturing. Thedevelopmentofin-planetexturehas been

attributedtoanumber ofmechanisms.Theexistenceof oblique adatom

flux has been found to benecessary in somecases (Karpenko et al.,

1994;

Harper

etal.,1997;

Bauer

etal.,

1997).

Othersusedanoff-axis ion beamtopreferentiallysputter films duringgrowth, thereby encouraging

the formation of in-plane texture(Bradley etal.,

1986).

Still another

investigation hasshown that filmsgrownwithoutanyaverageoblique influencedonotdevelop any in-plane texturing

(Malhotra

etal.,

1996).

Deposition geometryplays a critical role in in-plane texturing, while

the role of deposition kinetics and energetics are not currently well

understood.

Belowisareviewof work donetoincreaseunderstandingoftexture

evolution in thin sputteredfilms.

A

particularsetofdeposition condi-tions whichleadtothe formationofastrongbiaxialtextureinsputtered

films of

Mo, Cr, CrN,

and

Ta

will be described. Once the basic

phe-nomena of in-plane texturing was well documented, further

experi-mentsweredonetoprovidebetter understanding of the role ofadatom

kineticsduringgrowth. This wasaccomplishedby altering the sputter

gas pressure, chamber geometry, deposition rate, and growth

tem-perature during film growth. Experimental results will be presented

and discussed in light ofa model which describes in-plane texturing

andisbased on twosimplephysicalmechanisms.

DESCRIPTIONOF EXPERIMENTS

Deposition Chamber

All filmswere deposited in a DentonTM _cryo-pumped

_DC

_magnetron

sputter chamber (Whitacre et al.,

1998). A

base pressure ofat most

varied from 2 to 20mTorr. The substrates, as received test-grade Si

(100)

wafers, were mounted on a platen which rotated beneath the sputter targetat a rateof20rpm. This setupallowed formultiple films

tobegrownatonce.Thesubstrateswereexposedto afull 180 rangeof

flux in the plane parallel to their direction ofrotation.

In

the plane

normaltorotationdirection,however, theangular flux rangeismuch

less.Thisconfiguration providesabreak in the symmetry of the

depo-sition: thereis, on average,more oblique fluxin one plane over any

other. Figure showsaschematicof thedeposition chamber.

Analysis Techniques

Microstructureand film texturingwerestudiedusing scanning electron

microscopy

(SEM),

transmission electron microscopy

(TEM),

trans-missionelectrondiffraction

(TED), X-ray

pole figure analysis, and high

resolution

X-ray

diffraction (using the symmetric

GIXS

geometry).

An

Hitachi S-800 field emission SEM was used with its accelerating

potential at 7kV and its sample distance set to 5 mm, allowing for resolved imagestobe recorded atmagnifications of20,000-40,000x.

Further analyses used a JEOL 2000

TEM

(accelerating voltage=

200

kV).

Plan-viewsampleswerecreatedusinganHFetching technique

(Whitacre

etal.,

1998),

whilecross sectionsamplesweremadeby tripod polishing. TEDwas performed usingthe JEOL 2000to qualitatively

analyze texturing. Alldiffraction patterns were recorded from a defined

areaof equalsize ineachofthe samples usingaselectedareaaperture.

Pole figu.re analysis wasdone using both a Rigaku reflection pole

figure apparatus which used the Schultz geometry, and an inelTM

(It) ca

e

pl

sutrate/./7/,

U

rotation direction (b)

]

cathode average

ou v //

rotationection

FIGURE Schematic showing deposition chamber geometry: (a) overview, and

(b) representation showing how the average flux vector changes angle ofincidence

position sensitive detector attached to a Huber four-circle

diffracto-meter(Karpenkoetal., 1994;Malhotraetal.,

1998).

scanscollectedin thesymmetric grazingincidence

X-ray

scattering

(GIXS)

geometry allowed for thedegreeof in-plane texturing offilms

tobe studied preciselyatvaryingdepths (Karpenko,

1996).

RESULTS AND DISCUSSION Texture Evolution in

Mo

Films

Theevolutionof both in-plane andout-of-planetextureswasobserved

in Mo films grown at a rate of

_34nm/min

under the conditions

described above (Karpenko etal., 1994; Bilello et al.,

1995).

Texture

evolution may be seen in the

(110)

pole figures and corresponding

TEM/TED

analysisshown in Fig. 2.

By

thetimethe film reachesathicknessof 200 nrn,awell-defined

out-of-planetexture,

(110)

in thiscase, is observed.

As

thefilms become

thicker, both the out-of-planeand in-plane texturesbecome stronger.

The 2_ttmthick filmdisplaysa "timesrandom" value of 16: this film is 16

times more organized crystallographically than a randomly oriented

sample(Karpenkoetal.,

1994).

The

_TEM/TED

analysis supports the

pole figure data: as the film becomes thicker,the electron diffraction

(a)

FIGURE2 (a)Pole figures showing theevolutionoftextureinMofilmsfor thick-nessesofa:200run,b:500run,c: 1000runand d:2000run.Thenumber inthe upper righthand comer of eachpole figureisthe "times random value.(b) TEM/TED

ana-lysis for thesamefilms.The directionof grain elongation in the thickerfilms is

patterns showan increase in localized diffraction consistent with the

existenceof in-planetexture.There is alsoanincrease ingrainsize inthe

planeof the film; the grains becomeelongatedin the directionnormalto

substratemotion.

By

comparingthe electron diffraction patternstothe

brightfield

TEM

images,itcanbededucedthatthe grainsareelongated

in their

(100)

direction and arenarrowestin the

(110)

direction. The

surfacemorphologyof the 2_tmthick film isshown in the

SEM, AFM,

and cross section

TEM

imagesofFig.3. Thegrainsarefacetedandhave

somevoid formation betweenthem,asattested tobythe

TEM

image.

X-ray

analysis, scans in the symmetric

GIXS

geometry, ofMo

filmsof different thicknesses show theevolution of in-plane texturing

(Karpenko,

1996).

Figure 4 has this data. The full-width at halfo

maximum of the recorded peaks isinversely related to the degree of in-plane texturing; thediffractedintensityatany

_b

isproportional to

the number of grainswiththat particular in-planeorientation. Thistype

of analysis allows for precise comparison of the degree of in-plane texturing in various films.

Chromium

Crfilms grownunder nominally the sameconditionsasthe Mo films

describedabove alsodisplayedin-planetexturing. Figure 5 shows the

FIGURE 3 Comparative images showing the surface morphology ofa2pro thick

MoFilm:(a)plan-viewSEM, (b) AFM (theprofileplotis a cross sectiontaken from the area indicated by thewhiteline), and(c) cross sectionbright-fieldTEM (figure adapted fromWhitacreetal.,1998).

104

1000

0 60 20 B0 240 300 360

Phi(’)

FIGURE4 Plot showing the detecteddiffractedintensityinforatexturedMofilm

oriented inthe(110) Braggconditionusingbscansinthe symmetricGIXSgeometry.

Foreach film, theX-raypenetration depthwas100 nm.Thethickerthe film, the higher

thedegree oftextureobservedinthetop 100 run(figureadapted from Karpenko,1996).

90 90 90 f/-

-

_counts 135/./’- "",>.4,5 135:... 135./

....,

.45

/.

-.

::,,(....,,

700

,.---_

--..,..

,.>

/

:S

(a) 270 _(b) 270 _(el 270

5mTorr 7 mTorr I0mTorr

FIGURE 5 Plan-viewSEMimages and corresponding(110) polefigures for 1.5pm

thickCrfilms grownwithArsputter gas pressures of(a) 5mTorr, (b)7mTorr, and

(c) 10mTorr. The 5mTorr film has a strong(111) out-of-plane texture anda

well-defined in-plane texture. The in-plane texture is lessdefined in the 7 and 10mTorr

surface morphology and

(110)

pole figures from a

Cr

film grown at roomtemperaturewithacathode height of 9.5cm,a rateof 20

_rim/

minandthree different

Ar

pressures: 5, 7 and 10mTorr. Thepole

fig-ures show thatin-planetextureoccursin varying degreesin Crfilms.

Texturingdependenceonsputter gaspressurewillbediscussed in alater

section.Though the grain shape andsymmetryof the

Cr

films arenot

identical tothose observedin Mo,the sameeffects and evident: grain

elongationin theplaneofgrowthnormaltothe direction of substrate

motionduringgrowth,and faceted surface morphology.

Tantalum

Figure 6 shows the surfacemorphology andcorresponding

(002)

and

(202)

pole figures ofa3_Im thick (tetragonalphase) Ta film grown

under theconditionsdescribed in the experimental section. The

micro-structure differs dramatically from that observed in the

Mo

and Cr

films; the surface is not faceted, but instead consists of elongated

rounded features.

As

inthe other cases,however,thedirectionof

fea-tureelongation isnormalto the direction ofsubstratemotion during

deposition. The pole figures show that thefilmhasastrong

(002)

out-of-planetexture.The

(202)

polefigure shows that there isasignificant in-planetexture,thoughitis not asstrongasthatobserved in films such

asMoor

Or.

Chromium Nitride

In-planetexturing has been observed inCrN (Zhao,

1998).

The

depo-sitionconditions leadingtotheseeffectsareidenticaltothosedescribed

FIGURE6 Plan-view SEM images and corresponding pole figures for 3pin thick

Ta film. The pole figuresindicate a strong (002) out-of-plane texture and some

in-planetexture. Thedirection ofsurface feature elongationisnormaltothedirection of

above,except thatnowthereactivesputteringisused.

A

dependenceof

nitrogencontent ontexturingwasobserved.

Texture Control

The type ofout-of-planetexture has been foundto depend upon the

cathode height during deposition, the growth rate (Karpenko etal.,

1994; Bilello et al.,

1995),

and sputter gas pressure (Windischmann,

1992).

Figure 7 shows SEM and corresponding

(110)

pole figuresfor 2.5_{tm Mo}filmsgrownwithacathodeheightof 9.5 and 12cm. If the

cathode height is 9.5 cm, there is a strong 111 out-of-plane texture

component, while the film grown with the cathode height at 12cm

showsonly

(110)

typeout-of-planetexture.These filmsweregrownat,

nominally, thesamerate.If theratesarechanged, however,wealsosee adifference inthetextureorientation

(Karpenko

etal.,

1994).

Thereis

a difference between 2_tmthick Mo films grown at different growth

rates: films grown at 68

nm/min

displays a strong

(111)

type

out-of-plane orientation, while similar films grown at 34

nm/min

shows a

strong

(110)

out-of-planetexture.

Figure 5 showspolefigures fromCrfilmsgrownatthree different

Ar

pressures: 5, 7,and 10mTorr.The type ofout-of-planetexturedepends

upon thesputtergaspressure. The filmgrownat 5mTorrhas a

(111)

out-of-planetexture, while the filmgrownat10

mTorr

displaysa

₍₁₁₂₎

out-of-plane texture. The 7mTorr film has a mix of the two types ofout-of-planetexturing. The filmdepositedusing 5mTorrof

Ar

dis-playsahigherdegree of in-planetexturethanthefilmsgrown under 7

and 10mTorrofAr.

FIGURE 7 Plan-view SEM images and corresponding (110) pole figures for 2tm

thick Mo films grown with cathode-to-platen distances of(a) 11ern and (b) 7cm.

The smallercathodeto substratedistance provided a mixed(111)/(110) out-of-plane

FIGURE8 Plan-viewSEMimages and corresponding(110)pole figures for 1.5ttm

thick Crfilms grownat 290Cwith Arpressures of(a) 10mTorr, and (b)3mTorr.

Thereisless in-plane organization for the film grownat10mTorr. Bothfdms display

astrong(111)out-of-planetexture.

Temperature

Effects

Chromium filmsweredepositedatatemperatureof-,_{290C usingtwo}

different sputteringgas

(Ar)

pressures. The cathode height was 9.5m

and a shieldedfilamentheater affixed totheceilingof the deposition

chamber was used. Figure 8 shows plan-view SEM and

(110)

pole

figuresoftwo2_Ixmfilmsgrownusing 10 and 3

mTorr

of

Ar.

Several differences frompreviousgrowthsareevident.The grainsize

issignificantlysmallerthanfilmsgrownunder the same conditionsat

room temperature. The grains are still faceted, though they are no

longerelongatedintheplane of growth.Thepolefigures show that each film hasa

₍₁₁₁₎

out-of-planetexture, butonlythe filmgrownusing the

lower

Ar

pressure of3

mTorr

displaysanyin-plane texturing.

Growthonto RoughSurfaces

Mo films

_400tm

thick were deposited onto well-defined roughened

surfaces

(Whitacre

et al.,

1998).

This study found that if the rough

features were largeenough to limitthe angular rangeof adatom flux

duringgrowth thereis a measurable decreasein the rate of in-plane texturing

(Whitacre

etal.,

1998). By

slightlymanipulatingthe geometry of thedeposition, substrateroughnesslimitsthefilmstexturing.

Discussion

It is evident that in-plane texture develops in thin sputtered films if thereis:

(1)

onaverage, oblique adatomflux,and

(2)

sufficientadatom

If the substrate restsdirectlybeneath thecathode during growth,no

in-plane texture develops, though a strong out-of-plane texture and

faceted surface

morphology

exists

(Malhotra

etal.,

1996).

If, however,

the substrateiskepteitheratastationary off-axis location

(Malhotra

etal.,

1996)

or isrotated beneaththecathode,biaxial texture iscreated

inthe film.

It

has been shownthat if thesubstrate surfaceisroughened

such that the averageangleof oblique adatom fluxislimited, therateof in-plane texturing decreases(Whitacreetal.,

1998).

Inall of thesecases,

an out-of-plane texture develops first, followed by the evolution of in-planetexture asgrainscompeteduringgrowth.

A

model has been proposed which describes in-plane texturing

(Karpenkoetal.,

1997). It

isassumed that in-plane texturingis aresult

ofthe combination oftwophysical mechanisms:

(1)

anisotropicsurface

transport, and

(2)

atomistic shadowing effects whichoccurwith oblique

adatomincidence.Anisotropic surfacetransport causesgrainstogrow

faster along a particular crystallographic direction in the plane of

growth. Forexample, Mo grainsgrowmostrapidly along their

(100)

axisand slowest in the

(110)

direction. Thisresults ingrainswhichare

elongated

(to

increasingdegreesasthe filmsthicken) alongtheir

(100)

direction.

If there is minimal surface diffusion, atomic shadowing occurs.

Adatomspileuponexisting grains creating significantvoidformation

between those grains

(Dirks

and

Learns, 1977).

Ifthe adatoms are

obliquely incident onto the substrate, this shadowing effectwill also

causethe adatom pileuptobemoreefficient in one in-plane direction overany other.

In

particular, adatomswill contributemostefficientlyto

in-plane formationinthedirectionnormaltothe projection of the flux

vector ontotheplaneof the substrate.Thisisthein-planedirectionof

highestadatomcaptureefficiency.Minimumcapture efficiencyis inthe

directionparalleltothe projection of the incoming fluxvector.When

both shadowing and anisotropic surface transport conditions exist

concurrently with oblique adatom flux, those grains whose crystal-lographic fast in-plane growth direction corresponds to the highest

adatom capture efficiency direction will compete favorably during

deposition. Eventually, onlythosegrainsthatnucleatedwith theirfast crystallographic growthdirectionalignedwiththe directionof highest

expressed mathematically and compared with

X-ray

data collected

from filmswasin-planetexturing(Karpenko,

1996).

Ifthismodelis correct, an increase in eithersurfacetemperatureor

adatom kinetic energy should affect the in-plane texturing rate by

altering anisotropic surfacediffusionandthe effectivenessof the shad-owingmechanism.If thefilmsurfaceisheated, surfacediffusionshould

dominatetheshadowingeffect and in-planetextureshould evolvemore

slowly.This isobservedin theCrfilmsstudied,where the filmgrownat

290Cin 10mTorrof

Ar

has almostnoin-plane texturingcompared

toasimilar filmgrownat roomtemperature.

An

increase inadatomkineticenergy should promotesurface

diffu-sionand workagainsttheshadowingeffect.

By

lowering the

Ar

pres-sure,the averageadatom kinetic energy increases(Meyeretal.,

1981).

Experimentally,different

(or

mixed) out-of-planetexturingisobserved.

However,

there isnotasignificantdifference inthedegree of texturing

between

Cr

filmsusing different

Ar

pressures. This effectiscurrently underconsideration andwillbe addressedinalaterpublication.

CONCLUSIONS

For

sputteredthinfilmstodevelopabiaxial texture,theremustbe,on

average,someoff-axisoroblique geometrical influenceonthegrowing

film to allow for grain competition during growth. This paper has

addressed how thepresence ofoblique adatom fluxduring DC

mag-netronsputteringleadstothedevelopment of in-planetexture.

Once

the

general phenomena was characterized in a variety of films, further

studies weredonetoexplore therole ofsurfaceenergeticsandadatom

kineticsduringgrowth. Ithasbeenshown thatin-plane grain

compe-tition is sensitive to surface diffusion; films develop in-plane texture

moreslowly atelevated temperatures. Ithas also been shown the an

increase in adatomkinetic energy, while beingenoughto control the

type ofout-of-planetexture, isnotenoughtodramatically affettherate

of in-plane texturing. The relevance of these results to a model

which describes the evolution of in-plane texturingwasdiscussed.

By

addressingquestions raised, a newlevel ofunderstandingofin-plane

Acknowledgments

This work wassupported under

ARO

Army

contractnumbers

DAAH

04-95-1-0120 and

DAAG

55-98-1-0382. Diffraction data collected at

SSRL beamline7-2, fundedbytheUS

DoE.

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