Chemical Vapour Deposition (CVD).pdf

cvd

2. ⚫ChemicalVapourDeposition(CVD)isachemical
processusedtoproducehighpurity,highperformance
solidmaterials.
⚫InatypicalCVDprocess,thesubstrateisexposedto
oneormorevolatileprecursorswhichreactand
decomposeonthesubstratesurface toproducethe
desireddeposit.
⚫Duringthisprocess,volatileby-productsarealso
produced,whichareremovedbygasflowthroughthe
reactionchamber.

3. Transportof
reactantsby
forced
convection to
the
deposition
region
Transportof
reactantsby
diffusion from
themaingas
streamtothe
substrate
surface.
Adsorption
ofreactants
inthewafer
(substrate)
surface.
Chemical
decomposition
andother
surface
reactionstake
place.
Desorption
ofby-
products
fromthe
surface
Transportof
by-products
bydiffusion
Transportof
by-products by
forced
convection
awayfromthe
deposition
region.

5. ⚫CVD’sareclassifiedintotwotypesonthebasisof
OperatingPressure.
1.AtmosphericPressureCVD
2.LowPressureCVD
⚫PlasmaEnhancedCVD
⚫PhotochemicalVapour Deposition
⚫ThermalCVD

6. CASE1:HIGHTEMPERATURE
ThisprocessisusedtodepositSiliconandcompound
filmsorhardmetallurgicalcoatingslikeTitanium
CarbideandTitaniumNitride.
CASE2:LOWTEMPERATURE
ManyinsulatingfilmlayerssuchasSilicondioxideneed
tobedepositedatlowtemperaturesforeffective
deposition.

7. ⚫Aluminiumoxidefilmsaredepositedbythismethod
fromaluminiumtrichloride,argonandoxygengas
mixturesattemperaturesrangingfrom800-1000
degreeCelsius
⚫Thefilmshavelowchlorinecontent,whichcontinue
todecreasewithincreasingtemperature.
⚫Analysisofthefilmgrowthrateonthesubstrates
revealedthat,thegrowthtakesplaceonlybydiffusion
from800to950degreeCelsiusandonlybygasphase
reactionat1000degreeCelsius.

9. ⚫Filmthicknessuniformitycannotbemaintained.
⚫Largenumberofpinholedefectscanoccur.
⚫Wafer(Substrate)throughputislowduetolow
depositionrate.
⚫Thedepositsgetcontaminatedveryeasilysinceittakes
placeatatmosphericpressure.
⚫Maintainingstochiometryisextremelydifficult.

10. ⚫ThedepositionofSiliconcarbidethinfilmis
performedusinglowpressureCVDofDichlorosilane/
Acetylene/Hydrogen reactionsystem.
⚫TheSiliconcarbidefilmdepositedatthreedifferent
temperatureshasthreedifferentproperties.
1023K AMORPHOUS
1073K MICROCRYSTALLINE
1173K PREFERENTIALLY
ORIENTED

11. ⚫Thistechniquepermitseitherhorizontalorvertical
loadingofthewafersintothefurnaceand
accommodatesalargenumberofwafersforprocessing.
⚫Theprocessresultsinthedepositionofcompoundswith
excellentpurityanduniformity.
⚫Howeverthetechniquerequireshighertemperatures
andthedepositionrateislow.

12. ⚫Plasma-enhancedchemicalvapordeposition(PECVD)
isaprocessusedtodepositthinfilmsfromagasstate
(vapor)toasolidstateonasubstrate.
⚫Chemicalreactionsareinvolvedintheprocess,which
occuraftercreationofaplasmaofthereactinggases.
⚫TheplasmaisgenerallycreatedbyRF(AC)frequency
orDCdischargebetweentwoelectrodes,thespace
betweenwhichisfilledwiththereactinggases.
⚫ThehelpinghandofthePlasmahelpsinincreasing
thefilmqualityatlowtemperatureandpressure.

13. ⚫PECVDuseselectricalenergywhichistransferred to
thegasmixture.
⚫Thistransformsthegasmixtureintoreactiveradicals,
ions,neutralatomsandmolecules,andotherhighly
excitedspecies.
⚫Theseatomicandmolecularfragmentsinteractwitha
substrateand,dependingonthenatureofthese
interactions,eitheretchingordepositionprocesses
occuratthesubstrate.
⚫SomeofthedesirablepropertiesofPECVDfilmsare
goodadhesion,lowpinholedensityanduniformity.

15. ⚫REINBERGTYPEREACTOR(DIRECT):
Reactants,by-products,substratesandplasmaarein
thesamespace.
Capacitive-coupled Radio Frequencyplasma.
Rotatingsubstratesarepresent.
⚫DOWNSTREAM REACTOR(INDIRECT):
Plasmaisgeneratedinaseparatechamberandis
pumpedintothedepositionchamber.
Allowsbettercontrolofpurityandfilmqualitywhen
comparedtotheDirecttype.

16. ⚫Althinfilmsaredepositedviaphotochemicalvapour
depositiononcatalyticlayersofTi,TiO
2,andPd,using
dimethylaluminumhydride.
⚫Depositioniscarriedoutatlowgaspressurestoinduce
asurfacereactionbasedonadsorptionandsubsequent
decompositionofadsorbates.
⚫OfthesethreelayersTiissoeffectiveasacatalystthat
theAlfilmsarethermallydepositedevenatalow
substratetemperatureof60°Cwithagrowthrateof0.5
nm/min.

17. ⚫TheUVlightgeneratedbyadeuteriumlamphelped
increasethegrowthrates.Ontheotherhand,Alcould
bedepositedonTiO
2layersonlyunderirradiationata
substratetemperatureof120°CIttakesseveralminutes
tocovertheTiO
2surfacewithAlandinitiatetheAl
film'sgrowth.
⚫Here,theUVlightinhibitedtheAlgrowthonthe
surface,whereasthefilmsaredepositedthermally.
⚫X-rayphotoelectronspectroscopyshowedthe
formationofaphotolyticproductionofthe
adsorbate,whichactspresumablyasacenterthat
inhibitsfurtherAlgrowth.

18. ⚫InthermalCVDprocess,temperaturesashighas2000
degreeCelsiusisneededtodepositthecompounds.
⚫Therearetwobasictypesof reactorsforthermalCVD.
1.Hotwallreactor
2.Coldwallreactor
Ahotwallreactorisanisothermalsurfaceintowhich
thesubstratesareplaced.Sincethewholechamberis
heated,precisetemperaturecontrolcanbeachieved
bydesigningthefurnaceaccordingly.

19. ⚫Adisadvantageofthehotwallconfigurationisthat
depositionoccursonthewallsofthechamberaswell
asonthesubstrate.
⚫Asaconsequence,hotwallreactorsmustbefrequently
cleanedinordertoreducecontaminationof
substrates.
⚫Inacoldwallreactor,onlythesubstrateisheated.
⚫Thedepositiontakesplaceontheareaofthehighest
temperature,sinceCVDreactionsaregenerally
endothermic.

20. ⚫Thedepositionisonlyonthesubstrateincoldwall
reactors,andthereforecontaminationofparticlesis
reducedconsiderably.
⚫However,hotwallreactorshavehigherthroughput
sincethedesignscaneasilyaccommodatemultiple
wafer(substrate)configurations.

21. ⚫Variableshapedsurfaces,givenreasonableaccessto
thecoatingpowdersorgases,suchasscrewthreads,
blindholesorchannelsorrecesses,canbecoated
evenlywithoutbuild-uponedges.
⚫Versatile–anyelementorcompoundcanbedeposited.
⚫HighPuritycanbeobtained.
⚫HighDensity–nearly100%oftheoreticalvalue.
⚫MaterialFormationwellbelowthemeltingpoint
⚫Economicalinproduction,sincemanypartscanbe
coatedatthesametime.

22. ⚫CVDhasapplicationsacrossawiderangeofindustries
suchas:
⚫Coatings–Coatingsforavarietyof applicationssuchas
wearresistance,corrosionresistance,hightemperature
protection,erosionprotectionandcombinationsthereof.
⚫Semiconductorsandrelateddevices–Integratedcircuits,
sensorsandoptoelectronicdevices
⚫Densestructuralparts–CVDcanbeusedtoproduce
componentsthataredifficultoruneconomicaltoproduce
usingconventional fabricationtechniques.Denseparts
producedviaCVDaregenerallythinwalledandmaybe
depositedontoamandrelorformer.

23. ⚫OpticalFibres–Fortelecommunications.
⚫Composites–PreformscanbeinfiltratedusingCVD
techniquestoproduceceramicmatrixcompositessuch
ascarbon-carbon,carbon-siliconcarbideandsilicon
carbide-siliconcarbidecomposites.Thisprocessis
sometimescalledchemicalvapourinfiltrationorCVI.
⚫Powderproduction–Productionofnovelpowdersand
fibres
⚫Catalysts
⚫Nanomachines