AN INTRODUCTION TO THIN FILMSDr. Jeevan Kumar PadartiThisarticle provides a basicknowledgeinthinfilms andwas writtenkeepinginviewofunder graduate and postgraduate students who arehaving thinfilms intheir curriculum. This lecture notes contains method of nucleation,types of growth, sputtering process, methods of growthwhich includesvacuum evaporation,flash evaporation andchemicalvapourdeposition. Methodsfor measurement of thickness of thinfilmsarealsoincluded.Finally electric and optical properties of these films are illustrated.1. Introduction:Thinfilm isabranchthatdealswith very thinstructurallayers of differentmaterials. In recent years, thin film science has been grownworld wideintoamajorresearch area. Theimportanceofthinfilm coatingleads breakthrough inmicro electronics, optics and nanotechnology/1/. Thin films withthickness rangingfrom onetoseveralmicronsarevery essential for thermal barrier coatings and to protectmaterialsfrom thermalandatmospheric influences/2/.Thefilm scienceandtechnologyplays an important role in hightechindustries. Thin film technologyhasbeendevelopedprimarily forthe needoftheintegratedcircuitindustry. Thedemand fordevelopmentof smaller and smallerdeviceswith higherspeedespeciallyin new generationofintegratedcircuitsrequires advancedmaterialsand newprocessing techniques suitable for future giga scale integration (GSI) technology. Inthis regard, physicsandtechnologyofthinfilmscanplay animportant roletoachievethisgoal. The productionofthinfilmsfordevicepurposes has been developedoverthepast40years.Thin filmsasatwodimensionalsystem areofgreat importance to many real-worldproblems.Their materialcosts are verysmallascomparedtothe corresponding bulkmaterialandthey perform the samefunction when it comes to surface processes.Thus,knowledge and determination ofthenature, functionsandnewpropertiesof thin filmscan be used for the developmentof new technologies like solar cells/3/, sensors/4/, optical applications/5/, electronic engineering/6/ferroelectrics/7/etc.

2. Thin film nucleation and growth:Athinfilmdoesnotgrowasperfectslabs of bulk like materials. When compared with the bulkmaterial,physical propertiesofthin filmson substratemaystronglydiffer, dependingespecially on development ofmorphology and structure. Features like grainsize,shape,orientationandother are determined to a large extent at an early stage of nucleationandgrowthandcanbeinfluenced by depositionconditions/8/.Theindividual atomicprocess which determinefilm growthinitinitialstageisshownin fig.1.Fig.1Thesubstrateatomsaregiveninopencirclesandfilm atomsareshownindarkcircles.Condensation ofnewmaterial fromthegasphasecanbedescribed by animpinging rate. The numberof particlesper cm2persecond is given byr=P (2MKT0)-1/2 .. (1).Where Pisthevapour pressure,Mthemolecular weight, KBoltzmanConstant andT0is the temperature.Onceaparticleiscondensedfrom avapour phase,itmightimmediatelyre-evaporateoritmay diffusealong the surface. This diffusionprocess mightleadtoabsorptionparticularlyatspecialsites, edgesorotherdefectsorthediffusionparticlesmay re-evaporate. In all these processes characteristic activation energy has to be overcome. i.e., the numberofparticlesbeingable toparticipateina particularprocess is given by the expression

Edis KT

V e (2)Where visthe desorptionrateandEdesisthe activationenergy forthe desorption. The corresponding activation energy for the absorptionanddiffusiondepends upontheatomic details of the particularprocess.Fig.2Besidetheabsorptionofspecialdefectsitesandsurfacediffusion, nucleationmorethanone absorption particlemight occur,asmightfurther film growthbyadditionofparticlestoanalready formedisland. Theislands formationisshownin thefig.2.Inordertoobtainasmoothfilm surface during the growth, sufficient high surface mobility of the diffusing species and elevatedtemperatures areneeded.In thermodynamicallyequilibrium, condensationand re-evaporationsare equaland hence nonetgrowthis result.Asthereisacrystal growth,itmust beclearlyisa non-equilibrium process/8/.Mainlytherearethreetypes ofthinfilm growth phenomenais observed.1. Layer-by-layergrowth2. Island growthand3.layer-plus-island growthLayer-by-layergrowth:The layer-by-layer process of growthof thinfilmisshowninthefigure.3.Inthis method theinteraction betweenthe substrateandlayer atomsisstrongerthan betweenneighboring layer atoms.First a layer of layer atomsis formed on a substrate substance. Lateran a second layer is formed on the first layer and completes the growthonthefirstlayer.Athirdlayerformson thesecondlayerandsoon.Eachnewlayerstarts togrow only when the lasthas beencompleted.

Fig.3This layer-by-layer growth results1.Highcrystal quality2.Filmatomsare more strongly bound to each otherandfast diffusion.Island growth:IslandgrowthisshowninFig.4. If the interactionbetweenthe neighboringfilm atoms exceeds the over layer substrate interaction leadstoislandgrowth. Inthiscaseanisland deports always near a multilayer conglomerate of absorptionatoms.The first few atomiclayers usually grow as islands of a depositing material centeredonnucleationsite.Theislandgrowsuntil they touch each other (percolationthreshold).This percolationthresholdtypically occurred between5to 10nm.Thethinfilm propertiesaround percolation threshold and belowpercolationthresholdwillbe very different frombulk properties.

Fig.4The film atoms in islands growth are morestronglyboundtoeachother thanthesubstrate withslow diffusion.Layer-plus-island growth:The layer-plus-island growth is aninteresting one and is shown in Fig.5. After formation of one orseveralcompletelayers,firstisland formationcan occurs. A 3D island growsover the top of the firstfull layer.In thiscase a latticemismatch between the substrate and the depositedfilmmay occur. It may not occurin epitoxialgrowth.Fig.5

Effect of grainsize:The grain size of the thin film formed on substratedependson deposition rate and Temperature/9/.Dependenceofgrainsizeon variousfactorsis shown inthefigure.6.Grain size increases with substrate temperature uptocertaintemperature andthenbecomes constant Fig.6 (a).Grain size ismore for the thicksample thanthinsample. Uptoentertain temperature,thegrainsizedidnotshow any difference on thethicknessof thesample.Thegrainsizewithdepositrateisshowninfig.6(b).Grainsizeisconstant uptocertaindeposit rate. On further increasing the deposit rate, grain sizedecreases.Fig.6Grainsizeversusannealingtemperature in showninthe fig.6(c).Asthetemperature goingondecreasingthegrainsizealsodecreases.Agraphbetweengrainsizeandthefilm thickness atconstanttemperatureisshownin fig.6(d).On increasingthe grainsize, the thickness of the filmincreases.3. Sputtering process:Sputteringaphysicalprocesswherebyatoms inasolidtargetmaterialisejectedintoa gas phasedueto bombardment ofmaterialby energeticions. This processiscommonly used for thin filmdeposition, as showninthe fig.7.Whenanionstrikesthetargetcluster ofclosed packed atoms, inthe first collision, pushestheatom deeperintothecluster. Subsequent collisionsbetweentheatoms can

Fig.7resultin someof theatoms nearthesurfacebeing ejectedawayfromthe cluster.The numberof atoms ejectedfromthesurface perincident particle is called sputter yield. This is an importantparametertomeasure theefficiency of thesputteringprocess.The ionsfor the sputteringprocess are supplied byplasma that is induced in sputtering equipment onan ionor electron accelerator. Mostly organionsare used forthesputtering process.3.1. Sputteryield:It isdefined asthe mean number of atomsremoved fromthesurfaceofthesolidper incident ion. It is given bySputteringyield,S=(Atoms removed/incident ions)Sputteringis called by the interaction of incidentparticles with the target surface atoms. The sputter yieldwill influenced by thefallowing factors1. Energyif incidentparticle2.Target material3. Incident angleof particleand4.Crystal structure of the target surface.Sputteryieldcanbemeasured by the fallowing methods:1.Weight loss ofthetarget2. Decreaseof target thickness3. Collectionof sputteringmaterial etc.

3.2 Sputteryield- ionenergy:A graph between sputter yield(S) and ionenergy (E)is shownin the Fig.8.

Plasmaissentbetweentwo plates.One plate is made with the source called target, and kept negativepotential.Thetargetismade withthematerial,ofwhichthethinfilmhas to be prepared.

SE0 EFig. 8Thereisnosputteryielduptocertainenergy andafterthat sputteryieldincreasesand becomes maximum between 10Kev and100Kev.Afterthatsputteryield decreases.E0is thethresholds energyforthe sputtering.Sputter yieldismaximum athighenergies.Itdecreases at very highenergies(>100Kev)because the ionsloosemuchoftheirenergy farbelowthe surface.Thesputteringyieldveryless depends on the temperatureof the target surface.

Fig.9The plasmapositiveionAr+strikesthetarget bytheforce ofattractionandknockingthe atomloose,thetargetatomthenlandonthewafer(substrate).Theprocessiscontinuedandmoreand moreatomsaredepositonthe substrate to forma thinfilm.4.Thin film preparations:Thinfilmscanbepreparedbydepositing thematerialsonaninsulatingsubstratewith0

3.3Advantagesof sputtering:

coating to a thick ness from few0

A to

Sputteringiswellemerges ascleaner,more flexible andcontrollablemeansof deposition thanothermeans.Themainadvantages ofthe sputtering are:1.The ability of transferof material fromthe solid unheated source inthe absenceofPossibly reactive crucibleor container.2.Withsuitable precautionsthe source cathode compositionispreservedinthe growingfilmand3. Thistechnique is havinghighadvantageof depositionofmaterialoversubstratetoform thin film.Asimpleexperimentaltechniqueisshownin thefigure.Fig.9.Generallyargongasis usedforthesputtering process,andiskeptbetweenthetwoelectrodes,whichiskeptinavacuumchamber.Whenahighenergyelectronisstrikestheargonatom, it is ionizedforminganAr+and anelectron.By using the high energy field, low pressure plasma is formedbetween theplates.

100A.Manymethodsforthinfilmthickness deposits exit. Here depicting some of thewidelyusedmethodsforthepreparationofthin films.4.1Vacuumevaporation:This method is usedto prepare the thin films of varietyofsubstancesina highevacuated chamber in which the material is heated by electricmeansas shown in the figure.10.

Fig.10

Avacuum pumpisconnectedtoa chambertobe evacuated. Insidethechamber maskedwaferissuspendedbyasupporting

N=[(N V

)2

2]1

.(4)

standandmaterial tobeevaporatedis kept onanothersupport.Heatingcoils arearrangedto heat the material.

Fromequation1, N=P VKTSubstitutinginequation 2,

Duetoelectricalheating,thematerial is radiated in straight lines in all the directionsfromthesourceanddepositonthewafer.Thisprocessiscontinueduntilathin

l =[(P )2KTl = KT/([P2

2]12]1

.(5)

film of requires size is deposited on the wafer.Whentheprocessiscompleted,the vacuumisreleasedandthewaferisremoved, thisprocessleavesathinuniformfilmonthe depositionmaterial onall parts of the wafer. This vacuum evaporated technique is mostsuitable for deposition of highly reactive

tItisobservedthat,ifthevapourpressureisloweredthemorewillbethe collision distancel.IfthevalueofpressurePisverysmall,l is very long and even collision may not takes place(lisgreaterthanthe dimensionsofthe container.).If P is very small, the value ofKT

materialsuchasaluminum,whichisdifficult toworkwithair.Themethodiscleanand

2

isnearlybecomes5.Orl=5/P(in2

allowsa bettercontact betweenthelayer of depositedmaterialandthesubstratesurface. Inaddition, because,evaporationtravelsin straightlines,very preciouspatternis produced.Kinetic theoryof gas andemission condition:

cm.).Hence in high evacuating chambers,pressureisveryminuteand lis veryhigh.At equilibrium condition, the vapour pressure of the gas molecules is alsosaturated, andthe numberofmolecules,n,striking the surfaceofasubstrateperunit areaperunittime is givenas

The deposition condition of thin

n=P/(

2MKT)

(6)

filmisbetterunderstoodbythekinetictheory ofthe gas/10/. Whenasolidsubstanceis meltedorheatedinvacuum,form vapour atoms ormolecules enclosed in a space and createa vapourpressure. Atasteadystateof evaporation, thesemoleculeswill have equilibriumvaporpressure.

As 1/(2K)=3.513x1022n=3.513x1022P/ MT .(7) WheremisthemolecularmassandMisthe grammolarweight.Thecorrespondingmassevaluatedpercm2 per sec.isgiven by theequation

PV =NKTor

G=5.833x10-2P

M(in gmcm2/sec.)... (8)T

P=( N)KT .. (3)VWhereNisthetotalnumberofvapouratoms

Similarly,thecorrespondingsaturatedvapour pressure

ormolecules,Vthevolumeofthechamber, T the absolute temperature and K is the

P=17.44G

T .(9)M

Boltzmann constant. These molecules oratomscancolloidwithoneanotherinamean

Thevolume of the vapour

time andletlistheaveragedistanceof travelbeforesufferingacollisionwithother.

V=3638

T . (10)M

Then

According to the above equations,it is clear

thattherateofdepositionisproportionaltoP

thesurfaceofthecylinderandcanbe

and

(M/T). It increases with pressure P

approximately betweenthe point sourceand surface source

andasellas M butdecreases with T.i.e., at high substrate temperature there will bedecreaseintherateofdeposition.It is assumedthat thegaseous molecules incidentonthe substrate will condense to builda film, but this isnot truein therealcase.All impingingmoleculesdonot condense toformthe depositlayers, some are reflectedback in to thegaseous state ands somecorrection factor has to beinducedandthisis known as the stickingcoefficient whichhasa maximumvalue of1,for all the molecules are assume to condenseon the substrate. Distribution of deposit:

Expression for material deposition:The amount ofthe material deposition onthe substratedependsnotonly onthe natureof the sourcebutalsoonthe inclination of the waferstream() andon thesubstrate normaldirection.Substrate x

from heatersourcewillspreadinallthe directionsbut thevelocitydistributiondepends onthenature ofthesource,whicharebroadly classified in tothreegroups.Point source:If the source is tiny sphere comparingtothedistancefrom areceiving substrate is called a point source. The emitting vapourstreamfromthepointsourcewillbe

hsource

rFig.11

having same velocity distribution in all the directions.Surfacesource:Iftheemittedvapour molecule velocities are directional, thenthe source is calledasasurfacesource.The velocityof moleculesismaximumalong thesource-normal directionbutitdecreaseswithincreasing the angle of inclination of the direction with source-normal. The velocity distribution fallows the cosine law and decreases with

Let us now consider the amount ofmaterial(dm)that will emit throughthe solid angle(dw)ofasourceintheform ofappoint surfacesource.Ifm isthetotalamount evaporated,thedepositedamount isdirectly proportional tototalmass ofthe evaporation(m), solid angle (dw)and thecosineoftheangleofincidence()tothenormalbetweenthe sourceand substrate.i.e., dm m dw Cosdm mdwCos

increasingof.If

=0,thesurfacesource

dm=(mdwCos)/4

resembles a point source except it is not aspherical.Cylindrical source:Theemission of vapourstreamwill befrom

Theaboveequationisvalidforthesurface source.Ifthesourceisapointsource,=0and hence, dm= mdw(11)4

Let dAisthe unitareaof thesubstrateata distancer fromthesource alongthe vapour direction. In this case, the amount of the material deposit alsodependson.Hencefor the surface source:m

If0mt =4r2dFor thesurfacesource:If= 0,

cos (19)

dm=

4r

coscosdA (12)

t = m cos ..(20)

Dm=

If=0,m cosdA4r2

(13)

4r2dIf0,m

For apoint source:

t = cos cos (21)

= 0,then

4r2dNowletusconsiderthethicknessvariationof

m

Dm= cosdA4r2For = 0,

.(14)

depositat different points on horizontalplane PQRS,whichisatadistanceofhfrom the source.Let point A andBareatthedistancesofh and r from the source respectively. If we

Dm=

m dA4r2

.(15)

considerapointsourceandlett0 andtarethe thicknessof thefilmsat two points.

If =0,and

= 0,then

Then,onsubstituting r =hinequation (19)

Dm=

m dA4r2

.. (16)

t0=

m 14r2d h2

(22)

Thesubstratereceivesamaximumamountof deposits.Hence,positionofthesubstratewith

Similarly we can getm 1

respecttothesourceaswellasthenatureofthe source, considerably effect the filmthickness. Considering the point source and surface

t1=4dh2

[1+(

x)2]2h

(23)

source, let isexamining the thicknessofthe

Dividing equation (23) by equation (22),3

film.

The density d of the filmmaterial,D=dm/V .. (17)

t1/ t0 =[1+(

x)2]2h

..(24)

Where V is the volume of the film whichisequaltot dA, wheretisthethickness of the film.

where cos=h/rForsurfacesource:wecanwritetheratioas fallows,

D= dm/(t dA)or dA= dm/(d.t) For a point source:

t0=

m 14d h2

and

Dm=dA

m dA, substituting for4r2

(25)m 1

Dm= Or t=

m4r2m4r2d

dm,d.t

(18)

t1=

4d

(h2 + x2)2

(26)

Dividing equation (26) by equation (25) one canget2

boatandlimitedamountof chargefedata timetotheboat,there will benotimefor contributiontobuildupthedifferentvapourpressure.Thismethodofdepositioniscalled

x 2t1/t0=(1+ )h2

..(27)

flashevaporation.Thechargeisinapowderformisfedfroma

where cos=cos

=h/r.

reservoirtotheheaterboatthroughacute. The cute is made vibration which is connectedtoavibrator. The vibrator is adjusted such that only small amount of

B. Flash evoperation:Thismethod isgenerally adopted when the materialhas atendency to decompose or dissociateduring evaporation.Fig.12Inthis process,a smallamountof chargeinpowderform isfedatatimetoa whiter hotboatmadewithtungstenor molybdenum,sothat instantaneousevaporation ofthetotalchargetakesplacewithout remains any residue. This process is showninthe fig.12. Becauseof thehigh temperatureofthe

chargefalls onthe boat; sothatitcan evaporatecompletelybythetimeof next charge falls onit.Bythismethodaconstant compositionfilmcanbeproduced.5.Thinfilmthicknessmeasurements:Thickness playsanimportant roleinthefilm properties unlike a bulk material. The film properties are thickness dependent. The applicationofthinfilmsin opticaldevices like interference filters anti-reflection coatings etc. are highly dependents on film thickness.Inall thicknessmeasurements, it is generally assumed that the films are homogeneous and more or less uniformlydeposited on the substrate, and henceassumedto havemeanthickness t.Film thicknessmeasurement techniquesare basedondifferentprinciples suchasmassdifference,light absorption interferenceeffects etc.5.1 Massmethod:Thismethoddepends onincreaseof massofafilmduetodepositionoffilmand isshowninthefigure. Theincreaseinfilm weightcan be measuredby asuitablemicro balance.This balance is made up of quartz and depositsareformedonapanwhich is suspended from the end of the beam by means of aquartz fibre.

Theotherendofthebeam carryinga counterpoise.Apointeroramirrorisattached tothebeam ofthebalance.Withincreasingof weightduring the deposition, thepointermoves andits displacementcanbemeasuredbya travelingmicroscope.Ifmirrorisfixedin the placeof thepointer,with depositionof themass mirror rotatesand therotation canbe measured

Fig.13bylampandscalearrangement.Beforedoing theexperimentthebalanceisstandardizedwithknown weights.Letm isthemassdeposited duetothe formation of thin film, V the volume of the film formed,thendensityofthe materialofthethin film,d=m/V ..(28) IfAisthesurfaceareaifthefilmandtisthe thickness,V=t.A ..(29) Then density canbe givenbytheexpressiond= m/t.A ort=m/d.A ..(30)

plateNthefrequencyconstantdependson nature ofthecrystalandtisthethicknessofthethin film.Forquartzcutat35020,thevalueofNis1670mm.kc/s.On depositingthe material on quartz substrate,thefrequency goingonchanges.The change offrequencydf =-Cft dfilm .(32)Wheredfilmisthedensityofthefilmdeposited,Cfisthesensitivityofthemassdetermination.Fromthe above equation,t = df/( Cf dfilm) .(33)Intheaboveexperiment,thetemperatureofthequartzcrystal canaffect the frequencyof oscillations. Whiledepositingthe film onquartz, thetemperatureincreases.Toavoidthiseffect,crystaliscooledbeforetakingthemeasurements.The sensitivity ofthe thicknessmeasurement by this methodisabout10-8g.cm-25.3 Opticalmethod (photometric method):There are several thickness measuring methods which depends on optical properties such as reflection, transmission, interference etc. The more commonly usedmethodis photometric method, bases on transmittanceoflight/12/.Thismethod depends on changein transmittance oflightat normalincidence.IfIois theintensityincidentlightnormallyonafilmand I is the intensity of the transmittedlight, then,

By knowing the massm,film area A and the density of the material d,the thickness of the filmcanbemeasured.

I =etI0

.(34)

5.2CrystalOscillator Method:

Wheretisthethicknessofthefilmand istheabsorptioncoefficient(Lambertslaw).Thisratio of I/I0is called transmittance T. Thent

Thismethod wasinvestigatedbySuerbrey/11/. Inthis method, thicknessmeasurement depends on oscillationsof quarts crystal whenexcited,

T=eTakinglogarithms both thesides, logT=-t

.(35)..(36)

andthefrequencyoftheoscillationsdepends on thickness by the equationF= v/2t=N/t ..(31) Wherefisthefrequencyofthecrystalisthe velocity of the transverse wave normal tol

That is, a transmittance versus filmthicknessgraphon asemi-log scalewillbeastraight line. Hence, if one canmeasure the transmittance,filmthicknesscanbemeasured.Thismethodcan be usedforthefilmwhichcan not absorbthe light.Whenlightfalls onthe thinfilmwhich

is coated ona substrate, bothtransmittance T and reflectance R will showmaximaand minima, and vice versa. The position of maximaandminimafor both transmittanceand reflectancecanberecordedinaspectrograph as afunctionofwavelength.Letthemaximum

used,bthedisplacement of thefringe at thestep anda is the fringewidth.

andtheminimumoccursatwavelengths 1and2inthe transmissioncase, then12

2 nf t =

or1 2

12

t =2nf(1 2)

..(37)

Where nf is the refractive index of thefilm. Fromtheaboveequation,thethicknessofthe filmcanbemeasured.5.4Interferometry-method:Film thicknesscanbemeasuredaccurately by interference fringes usingmultiple beam interferometers.A film,thickness of whichisto be determined,depositedonflat surface,soasto leavesharpedgebetweenthe filmanduncoated region ofthe substrate. AmicroscopicSlide can be used as a substrate. This complete substrate and the filmare coated with silver coating for a good reflection. It is having asharpstepatthefilmedge.Anotherflatglassslideknownasareferenceplate,coatedwith Agasinthecaseoffilm plate,isthenplaced overthespecimenasshown inthefig.14 (b).A monochromaticparallel beanoflight isallowed tofall normally on these plates. The arrangementforthisisshowninthefigure14 (a), which is the similar arrangement as that of thewedgemethodtofindathicknessof the wire. The reflectedlightisthenobserved throughthe microscope.Asetofsharpfringes perpendicular tothe stepwithequal displacementswillbe observedas shown inthe figure 14(b). Thethicknesscanbemeasured by the equation,bT = (38)2aWheretheis thewavelength of the light

Fig.14Thesharpnessof thefringedepends onthe reflectivityof the metal coating, airgap etc.Themetal coating on the two plates should besame.Thin thick ness of the thin filmcan be measured0up tothe orderof 30- 20,000A.6. Chemical Vapour Deposition (CVD):Chemical vapour depositionis aversatile process suitableformanufacturingofcoatings,powders, fibers etc.Thistechnology isnow an essential factor inthemanufacturingof semiconductors and other electroniccomponents, optical, optoelectronic/13/ etc. Chemical vapour depositionmay bedefinedasa solid ona heated surfacefroma chemicalreaction in thevapour phase.It belongstotheclassof vapour-transfer process whichis atomistic in nature that is the depositionspecies are the atoms ormolecules or the combination of these.The processcanbedefined in general formas fallows:Thedeposition of film from gaseous phase by thermal decomposition or chemical reactionon substratesurfacesathigh temperatureisknown aschemical vapour deposition process orvapourplating.This techniqueis used forthe preparing various

inorganicandorganiccompounds.The basic principle involves decompositionsofvapour phasespecies and their subsequent deposition onsubstrates or reactions amongthe vapourspeciesinaneutral atmosphere. Sometimesacarriergas alsoused either to control rate of the reaction or to preventundesired reactions attheprevailing elevatedtemperatures.Some of common reactions for formation of filmsasgiven below:AB A+B AB+CD AC+BD

Fig.16areusedfortheformationoffilmdepositon the surface of the wafer (Substrate). The

2AB2(AB)n

AB4+AnA +nB

remaining waste gas is pump out by an exhaustpump,afterwastetreatment.6.1.thermal decomposition:

WhereA,B,C,Darethe differentconstituents andnis aninteger.Therector fortheCVDis shownas fallows:

Manycompounds whenheatedtohigh temperatures decompose resultsinthe formationofsolidaswellas agaseous phase. When an organic compound such as tetra ethylene orthosilicate which is inliquidform isheatedtoatemperature,adielectricfilm of SiO2 isformed.Si(OC2H5)4 SiO2+4C2H4+ 2H2O (On decomposition)Sometimesoxygencanalso beintroducedin tothereactortocontrolthecharacteristicofthe film.

Fig.15Inthisreactor,thereshouldbeagasinflowand an out flow should be provided for wasteproducts.Thepressurizedgasshouldbesent to deposition chamber through a gas metering fromgascabinet.Gasmeteringdeviceisused tocontrolgas flowintothechamber. Gas chamber contains a substrate at the bottom whichisprovidedwitha heater(Fig.16).The plasmaor the incoming stream linegas decomposeandtheatomsormoleculeswhich

Siliconorgermaniumwhenheated to700-9000C atapressureof10Torr,it decomposeto formSi and Ge films.SiH4 Si+ 2H2GeH4 Ge+ 2H2Silicon film can also be prepared by thereduction of tetrachloride silicone vapour withan nalkalimetal hydride inthe presence of organic solvent tetra ethylene glycoldimethyl etherSiCl4+ LiAl5 SiH4+ LiCl + AlCl3SiH4 Si +2H2The reaction for the preparation of carbonfilmfor carbon resistors can be prepared by

passingbenzene vapours toa temperature of500- 10000C.C6H6 C + Gaseousproducts6.2Vapour phasereaction:Siliconoxide(SiO2) filmscanbeproducedby the reaction of SiCl4 vapour with carbon dioxide inthepresenceofhydrogen.SiCl4+ CO2+H2 SiO2+ C Cl4+ H2SnO2 filmoftenusedinmetaloxideresistors canbepreparedfrom SnCl4vapourinpresence of water and HCl vapour.SnCl4+2H2O+ HCl SnO2+4HCl +HCl6.3Vapour transportation method:Inthismethod,vapours oftworeacting constituentsare passed oversubstrate keptat high temperature region when the reaction takesplacetoform thedesiredfilm.These techniquesaregenerally used for the preparations ofverythickfilmsandevenflat shaped crystals of several mm sizes.FormationofCdSandCdSe bypassingCdand Sor Sevapourthroughthefurnace keptathigh temperaturecanbe achieved.Cd+S CdS Cd+Se CdSeIn this technique, their will be three temperaturezones,twoforvaporization of two constituent elements and bringthem together to reactat a thirdzonetoformdirectproduct.6.4 Disproportionalmethod:Thisdependsonthedifferenceinthestability ofpolyvalent metalcompounds attwo temperatures.BothSiI2and GeI2arestable atlower temperatures whereasSiI2andGeI2at comparatively high temperatures. By preparing SiI2 vapourathightemperatures(11000C)and passingthemthroughatube keptatlower temperature(9000C),areaction cycle will

takesplace asfallows:SiI4+Si(at11000C) 2SiI22SiI2(at9000C) Si+SiI4SiI4thusformedcan againbeconnectedto SiI2by recycling. For theformation of thegermanium filmthetwo temperatureregions should be2000C and 3000Conly whichare much lower than thesiliconiodides.6.5ApplicationsofCVD:CVDhasapplicationsacrossawiderangeif industries.Coatings: Coatings for a variety of applicationssuchas wear resistance,corrosion resistance, hightemperature protection,erosion protection etc.Semiconductorsandrelated devices:CVDcanbe used to produce integrated circuits, sensors, and opto electricdevices.Dens structure parts:CVD canalsobe usedto producecomponents that aredifficult orun- commercial toproduce usingconventional fabrication techniques.Optical fibers:For telecommunications.Composites:thesetechniquesare usedto reproduceceramiccompositessuchas carbon- carbon, carbon siliconcarbides siliconcarbides- silicon carbide composites. This process sometimes calledas chemical vapour infiltration.Powder production: This technique is used to produce thenovel powders.Thesetechniquesarealsousedincatalystsandnanomechines.7. Electrical properties of thin films:In the case of thinmetallic films, its resistively hasbeenfound to bemuch highertothat of correspondingbulk material,and it decreases with increasing film thickness and attains a valueafter approaching the bulk and then becomes constantas shown inthe fig.17.

filmfromwhichtheelectronsscatteringinall thedirections.Thereare threeregions of scattering 1)between0and1,theelectrons will not able to travel their mean free path (mfp)l`b ,andhencetheystrikethesurface.InA region(2)between1and2,theelectronswill travel their full mfp lband in region (3)between2and,theelectronsonceagainhit the surface asmfp is less thanlb.t

ConductivityagainstthicknesstFig.17Thin films are classified as continuous and

Theratiooftheconductivityofthefilmf tothe bulklbis givenas fallows:f lf =

discontinuous media. If the thin film iscontinuous with out any break or gap in betweentheadjacent regionsandhassome crystallinestructurethat of bulkmaterial,is calledcontinuous film,if not,itiscalled discontinuous film.

b lb3t= +4lb

1t2lb

log

lb t

7.1.Conductionin continuous metallic film:Ifthethicknessofthefilmisattheorderofthe

tIf = k, then one can write the equationaslb

meanfreepathofelectron,thentheconduction

f3kk1= + log

. (39)

electrons willbemorefrequentlyscatteredby two surface boundaries. Consequently, its conductivitydecreases.Thisdecrease of conductivityduetoreductionofthematerial size is calledsize effect.

b 4 2 kAccordingto Sondheimer, the relation between conductivity of filmandbulkis givenas,fortlb and k 1,

f3k1= [log ]

(40)

Filmsurface

b 4 k

For thickfilms,tlb,and k 1,

t f

b = [1+3 ]1

b f 8k

Filmsurface

3 3l=1+ =1+

b 8k 8t

Fig.18Theconductivityofthinfilm

f dependson

On expanding inpower seriesandneglecting thehigher orderterms,

thethicknessofthefilmt.Thescatteringof

f 3=t[1 l]

(41)

electronscanbe explained asfallows:Let us consider anarbitrarypoint withinthe

b 8

It is observedthatbis always greater thanf . Let P is the fraction of electrons is secularly scattered from two surfaces and the rest are diffusedwithcompletelossoftheirenergy.Then,onecan write

csNaRbf.d

b f

=1+3 (1p)8k

.. (42)K

From theequation(42),itcanbeobservedthat, when p=1,b =f ,thatistheconductivityof thefilm isindependentofthethickness, and same asthatof thebulk..If p 1, or nearly equal tozero,1

dFig.20

f =b 3

A graph betweenf.t

versus where t is the

1+ Theslopeofthegraphgivesgivesthevalue8k

f =b

11+3lb 8t

.(43)

off .7.2. Temperature coefficient of resistivity:

Thevaluesoff decreaseswithdecreasingthef

Thetemperaturecoefficientofresistivitycanbedefinedastheratioofthechangeofthe resistivelyperunitriseofthetemperatureto

thickness. A graph betweenb

against k is

its initial resistivity.

showninthefigure.19,fordifferentvaluesofp.

If b

istheresistivelyofthebulk,

dbis

These experimental curves show a reasonableagreement with predicted values.

the change in resistivity in dT rise of temperature, thenTemperaturecoefficientofresistanceofthe bulk

P=0f

P=1

(TCR)b= (44)

1 db b dT

=b

b Similarly, the temperature coefficient ofresistancefor thefilmcan bewrittenas

(TCR)

=1 dt =

(45)

f dT tDividingtheequation(45)bytheequation(44)

kFig.19

f =

b .df . dT

Thicknessofthefilm,fordifferentelementsis

b f

dT db

shownin thefigure.20.

Forasmallriseoftemperature,thechangeinbulk resistivity and the change in film resistivity is almost equal and hence,

f=

b =f

(46)

8. Optical behavior of thin films:

b f bThisratioofTCRofafilmtoitsbulkisproportional totheratio of their conductivities.7.3.Conductivity and activation energyAthinfilm generallyexhibitsa highresistivity. Conductioncanbetakesplaceinhighelectricfields fortheelectrons to overcomethepotentialbarrier betweenthe islands.The variation of resistance withtemperaturecan be represented as

Optical propertiesoffilmshave beenstudied extensively because of their applications in variousopticalandelectro-opticaldeviancesand ithasbeen foundthatthereis oftenaconsiderable deviationofopticalparametersfrom thatofbulk material.8.1.Reflection,transmission, absorptionand energy gap:Whenabeamoflightpassesthroughathinfilm, it ispartiallyreflected, transmittedandabsorbed. LetR,AandTarethereflected,absorbedand

E R=ATeKT

..(46)

transmittedlightandincident light, then

I0 istheintensity0fthe

Where

A0,theconstantsforaparticularmetalfilm

I0 =R+A+T

.. (47)

and Eis theactivationenergyfortheconduction.The resistance of such a film decreases with

Reflectancecanbedefinesastheintensityoflight reflected tothe intensity of incident light.

temperature.Inthehighfields,theresistanceofthefilmlinearly dependent onthe field.

If Ir

istheintensityofthereflectedlight,the

Thevalueofconductivity versus1/T,whereTis the temperature inKelvin for the thin filmscoated on Pyrexsubstrateatdifferentthicknessisshowninthe

reflectanceR =

Ir (48)I0

figure.21.

Absorbance can be defined as the ratio of intensity of the light absorbed to the incident

light.If

Ia istheintensityoflightabsorbed,the

Log

Decreasingd

absorbance,A=Ia I0

.. (49)

Transmittancecan be definesasthe ratioof intensity oflight transmittedtothe intensity of incident light.

IfIt

istheintensityoflighttransmitted,then,the

1/TFig.21

the transmittance,T =It I0

.. (50)

Itisinterestingto notethattheactivationenergy also dependsonthefilm thickness.Onincreasingthefilm thickness, the activation energy decreases continuously, and becomes constant after certain thickness. Beyondthat, the activation energy and conductivity behaves like a bulkmaterial.

Addingtheequations48, 49and 50,R+A+T=(Ir +Ia +It)/I0=1 .(51) Forperfectlynon-absorbingmaterial,Ia =0and hence, R + T=1.

Whenamonochromaticbeam oflightincident ona medium ata normalincidence,therelationbetween R,n andKare givenas(n1)2 +k2R= .. (52)(n+1)2 +k2

Theresultingabsorptionspectrum is however,iscontinuous ofintenseabsorptionat shorter wavelength.A plot between energy versus wave vector k isshown inthefigure.23.

And

(1R)2etT= (53)1R2t

Where isthecoefficientofabsorption,tisthe thicknessofthefilmthroughwhichthelightpassesand k is the extinctionconstant.Anabsorptionspectrum ofathinfilmmaterialis showninthe fig.22.Theabsorptionoflightbythe filmmay takesplacebroadly by twoprocesses:

E Eg

direct

indirect

log

hFig.22

kFig.23Thetransitionissaidto be direct whenthe conduction band minimum and the valence bandmaximum occursatthesamewavelength. The absorption edge may occur at h=EgwhereEg,theminimumwidthoftheforbiddenenergy band ofthematerial.Ifthe minimum oftheconductionbandandthe maximumofthevalencebanddonotcoincide,

1. Byraisingtheelectronsfromvalencebandtotheconduction band and2.By exciting the lattice vibrations of the materialbyphotonenergy, orbyboth processes.The lattice provides the information of bond length, effective charges and lattice vibrational

the transition is saidto be indirect.Thus the electronic transitions betweenthevalenceandconductionbands can beadirectorindirect.Thetransitionprobablyrelated withabsorptioncoefficient asp

frequencies. Electronic bond structure can be

=A(hEg)

..(54)

studied by process1.Opticalmethodsprovide very simpleway of findingthe bandgapcomparingto electrical methods.Evenintheabsenceofanythermalenergyat0Konlypossibleabsorptionthattcantakesplace is whentheincidentradiationhassufficientenergytoexcite valence electrons across forbidden energyband gapin to theconduction band.

Where p is called transition probability. A graphforlogand loghis shown in fig.24. Allthegraphsarestraightlinesandtheslopeof the graph gives the value of p. The p has discretevalueslike,1,3/1,2..Letusnowconsiderthecaseofmetals wheretheabsorptionisprimarilyduetofreecharge carriers leadingto theinterband

transitions involving nochangeof energy. Then the absorptioncoefficientisgiven as

Refractionand transmission by single film:

=4

Ne2

.. (55)

Let us consider a thin film PQ, RS on their boundaries,letalightisincidentatAwithan

ncm*(1+22)Wherenis therefractive index,c thevelocity of light,Nfreechargeconcentration, relaxation

angleof0.Apartofthelightisreflectedand the rest is transmitted through the film andemerges out from other side of the film of

time andm* is the reducedmass. If22 1, then

thicknessd1

.Let

1 istheangleofrefraction.

=4

Ne2

TherayatCcanalsobereflectedintothesamemedium and suffers number of reflections as

ncm*22

shownin thefig.25.

=4

Ne2

ncm*22cAs=On substituting andsimplification,2 2

=Ne

c3m*n

.. (56)

whereis thewavelength of the lightused.Agraph between and2 is shown inthe fig.24.2Fig.24

Fig.25Combinationof parallelreflected beams representedby 1,2,3,4thetransmittedbeamof lightrayscam berepresentedby1,2,3,4. These raysmayleadto constructive or destructive interference depending on optical pathlength traveledbythe rays, beforetheircombination. Let DBis perpendiculartoAB,thenthepath difference ofthetworays

It shows that and2 is linearlydependent. Thisgenerally happensin visibleandinfrared

= (AC+CD)-AM=2n1d1cos1

.. (57)

region .Thelinearity generally observed formetals andsemimetallic substances.

Where

d1 idthethicknessofthemedium(thin

film)andn1 is the refractive index. The pointD

AndMwillbeinthesamephase.Thenthe optical pathdifference,

The length of the sample will reduce causing increasingof its resistance. The change inthe

= 2n1d1cos1

(58)

electricalresistanceduetoapplied magnetic

Whereis thewavelength of the lightused.Nowifweconsiderallthereflected rays1,2,3, there will be a constant phase

fieldwithreferenceto zeromagneticfieldscandefineas magneto-resistance.The magneto-resistanceof amaterial canbedefinedasthechangein resistively due

difference1

betweenthesuccessiveraysdue

tothe applied magnetic field totheresistively

toextraopticalpathtraversedbythemediumin thefilm.Andis given by

at zero field.If H

and0

are theresistivities

2

of a substanceat fieldH and field zero, then

1 =

n1d1cos1

The magneto resistance

The summation of the amplitudes of the reflected rays

H 0 = =0 0

(61)

r= a1r1+a2r2 +........= a1r1

As the resistivity is the reciprocal of conductivity and let and arethe

H 0

Where

a1r1,a2r2.Aretheamplitudesof

conductivities at field H andzero respectively,

the reflectedwaves1, 2,3..

Magnetoresistance

The total reflectance or the reflectivity is

given by the equation,

= 0 H

(62)

2 2 0

2 r+r+2rrcos2R=r =1 2 1 2 1

..(59)

Whenmagnetic fieldissmall, the magneto

1+r1 r2

+2r1r2cos21

resistance isproportional tothe squareof the

Similarly, the transmittance T is given by

field,and hence,

2 2 2

2 nttT =t =1 1 2

=AH

..(63)

n 1+2rrcos2 +r2r2

0Where is isknown as coefficientof magneto

9. Magneto resistance:

.(60)

resistivity. Forhigher fields, the magneto resistancefallows thequadratic law

The application of magnetic field alters the

=

AH22

..(64)

electricalresistance ofa thin film.Thischange in the resistanceof materialdue to the applicationof themagnetic fieldprovides informationregardingthe shapeoftheFermi surfaceof the thinfilm.When electrons are moving in a

0 1+CHWherec=2andis the electron mobility. If H=0,equation (64)convertstoequation (63)In the caseof transverse magneticfield, the coefficientof magneto resistance

substance, and a magnetic field is applied perpendiculartothecurrentdirection,aHall

A=( 0 )22ne

. (65)

field will be built up to counterbalance theLorentzforce.Alltheseelectronswillnothave samevelocity.AsaresulttheHallEffectcancompensate the Lorentz force and the net

Where nis thenumberof electronsper unit volumeand eis the chargeofthe electron. Substituting the value ofA in equation (63)

current inthe directionis zero. Fast electrons whichdeviateone way orother undergo more collisions. As theresult theirmeanfree along

= ( 0)2H20 2ne

..(66)

Kohler showedthat the magneto resistance depends on temperature, magnetic field and even onsamplepurityand geometrical consideration.Accordingto him,

less than, the magnetoresistance is independent of its dimensions (Reverse in the caseof HallEffect).Themagnetoresistanceof thinfilmdecreaseswithdecreasingofparticle

=f (H )0 0

..(67)

size/14/10. Sheet Resistance:

F is a function depending on geometricalconsideration and field direction. TheHdependenceofmagnetoresistanceon is0shown in the figure .26.at different temperatures.3

Sheetresistanceisthemeasureofresistanceof thin films that hasuniformthickness.For a rectangular three dimensional conductor, the resistance of a conductor isdirectlyproportionaltoitslengthandinverselyproportional to the areaofcross section.IfListhelengthandAistheareaof crosssection,resistanceL

2 R or A1 L

0 R=A

.(69)

H0

where istheresistivityofthematerial.IfWisthewidthandtisthethicknessofthethin film, then we can write,R=LWt

(1)78K (2)195K (3)219KFig.26

R= LtW

Wilson-Summerfieldtheorygivestherelation between resistivity inthe magnetic field H as

R=R LsW

.(70)

H =0

(1+0.2732H2)Or

Where

RsiscalledsheetresistanceandcanbeL

H=(1+0.2732H2)0

.(68)

definesasresistivityperunitthickness.AsWisadimensionallessquantity,theunitofsheet

It indicates that, the magneto resistance is astrong function of mobility. Higher theH

resistance must be ohm. To differentiate from resistance,theunitofsheetresistance is considered as ohm/square. If L=W, for a

mobility, higher the ratio of0

. This

square,R= Rs

forasquarelamina,thesheet

parameterdependsongeometryofthesample.H

resistance is equal to the resistance of thelamina.

Forarectangularsample,

variesonlength0

Measurement:

tobreadthratio(l/b).ForasmallervalueofH

Sheetresistancecanbemeasuredbyfour-point probemethod.Ageometricalcorrectionfactor

(l/b),0

isalarger one.Ifthe(l/b)value is

(CF) is usually required to convert the

factoraccountsaccordingtothesamplesize, shape andspacing.The sheet resistance measured by the four- probemethod

Applications:It is usually usedtoevaluatethe out come of semiconductor doping,metal deposition and resistivepasteprinting.Thisapplicationcanalso

R =Vx(CF)s I

..(71)

be used in screen printing and hybrid micro circuits.

Where Visthe measured DC voltage across the voltageprobe andIistheDCcurrentpassing throughthetwocurrentprobes.Thevalueof CFforsamplesof various sizesandshapes can befound inthe referencebooks.Thesheetresistanceofathinfilm with differentdopingconcentrationsis showninthe fig.27.Fig.27Onincreasingthedoping concentration,the sheetresistancedecreases continuously.The value of sheet resistance is less for n-type dopingthanp-typedopingforthe samedoping concentration.

References:1.J.A.Venables, Rep.Prog.Phys.47 (1984)3992.C.Ratsch, J.A.Venables.Jour.Vac.Science&Tech.A21 (2003) S963.J.Poortmansand V. Arkipov,thin filmsolar cells, John Wiley & sonspublications(2006)4.Joachim.P et al Sensors4(2004)1565.Mereno, etal.Opticsletters30 (2005)9146. H.Okimura. 17(1980)53597.Rezamoazzami, IEEE tras. Elct. dev.39 (1992) 20448.Thinfilmprocesses: ElsevierPublishing,19919.M.YamaguchinandTNagotoma Jpn.Jour.Phys. 37(1998)516610.Hand book of thinfilmdeposition processes and techniques. Krishna sheshan, William AndrewPublishing,2002.11.G.Sauerbrey,ZeitschriftfrPhysik,155 (1959) 206-22212. B. A. Nikitinet.al. Measurement techniques,30 (1987)19613.Hideki Matsumura et.al, Thin Solid Films,501(2006) 58.14.B. P. ZoteV,Russian physics journal,18(1975)692

3

=

f b

b

t

0

2

2

0 1 2 1 1 2