DTIC ADA441085: Modeling and Optimization for Epitaxial Growth: Transport and Growth Studies

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T R R
ECHNICAL ESEARCH EPORT
Modeling and Optimization for Epitaxial Growth: Transport and
Growth Studies
by Andrew J. Newman, P.S. Krishnaprasad
CDCSS T.R. 99-2
(ISR T.R. 99-19)
+ CENTER FOR DYNAMICS
C D
- AND CONTROL OF
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Modeling and Optimization for Epitaxial Growth: Transport and Growth
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Modeling and Optimization for Epitaxial Growth:
Transport and Growth Studies
AndrewJ. Newman (cid:0) P.S.Krishnaprasad
InstituteforSystemsResearchand InstituteforSystemsResearchand
ElectricalEngineeringDepartment ElectricalEngineeringDepartment
UniversityofMaryland UniversityofMaryland
CollegePark,MD20742 CollegePark,MD20742
[email protected] [email protected]
OriginalVersion: January10,1999
CurrentVersion: March17,1999
Abstract
This report details the objectives, methodologies, and results for Phase II of the project, “Modeling and Op-
timizationfor Epitaxial Growth” (see [21] for Phase I report). This project is a joint effort between the Institute
forSystemsResearch(ISR)andNorthropGrummanCorporation’sElectronicSensorsandSystemsSector(ESSS),
Baltimore,MD.Theoverallobjectiveistoimprovemanufacturingeffectivenessforepitaxialgrowthofsiliconand
silicon-germanium(Si-Ge)thinfilmsonasiliconwafer. GrowthtakesplaceintheASMEpsilon-1chemicalvapor
deposition(CVD)reactor,aproductiontoolcurrentlyinuseatESSS.PhaseIIprojectresultsincludedevelopmentof
anewcomprehensiveprocess-equipmentmodelcapableofpredictinggasflow,heattransfer,speciestransport,and
chemicalmechanismsinthereactorunderavarietyofprocessconditionsandequipmentsettings.Applicationsofthe
modelincludepredictionandcontrolofdepositionrateandthicknessuniformity;studyingsensitivityofdeposition
ratetoprocesssettingssuchastemperature,pressure,andflowrates;andreducingtheuseofconsumablesviapurge
flowoptimization. Theimplicationsofvarioussimulationresultsarediscussedintermsofhowtheycanbeusedto
reducecostsandimprove productquality, e.g., thicknessuniformityofthin films. Wedemonstratethatachieving
deposition uniformity requires some degree of temperature non-uniformityto compensate for the effects of other
phenomenasuchasreactantdepletion, gasheatingandgasphasereactions,thermaldiffusionofspecies,andflow
patterns.
(cid:1) ThisresearchwassupportedbygrantsfromtheNorthropGrummanFoundation,theNationalScienceFoundation’s EngineeringResearch
CentersProgram: NSFDCDR8803012andNSFGrantEEC-9527576,andtheArmyResearchOfficeundertheODDR&EMURI97Program
GrantNo.DAAG55-97-1-0114.
Contents
1 Introduction 1
2 ProjectSummary 2
2.1 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.2 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.3 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.4 SummaryofPhaseIResults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.5 SummaryofPhaseIIResults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.6 Follow-OnPlans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 EquipmentandProduct 4
3.1 ProcessChamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2 ProductandConsumables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.3 ProcessConditionsandRecipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.4 EquipmentSettings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.5 OperatingStructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4 Process-EquipmentModel 8
4.1 Motivation:UniformityCaseStudy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2 Process–EquipmentState . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.3 ModelingApproach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.4 ReactorGeometryandFiniteVolumeMesh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.5 TransportPhenomena . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.5.1 Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.5.2 GasPhaseTransport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.5.3 BoundaryConditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.5.4 MaterialProperties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.6 ChemicalMechanismsforGrowth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.7 UnmodeledPhenomenaandEquipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5 ResultsandApplications 20
5.1 DepositionRatePrediction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5.1.1 WaferTemperatureSensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5.1.2 ChamberPressureSensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
5.1.3 FlowRateSensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.1.4 CarrierGasSensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
5.2 DepositionUniformityPrediction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
5.3 ProcessChamberTransportPhenomenaPrediction . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
5.4 PurgeFlowOptimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
5.5 InjectorFlowAnalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
6 Conclusions 32
7 Acknowledgements 33
i
1 Introduction
This report details the objectives, methodologies, and results for Phase II (covering March 1998 through February
1999) of the project, “Modelingand Optimization for Epitaxial Growth” (see [21] for Phase I report). This project
is a joint effort between the Institute for Systems Research (ISR) and Northrop Grumman Corporation’sElectronic
SensorsandSystemsSector(ESSS),Baltimore,MD.Theoverallobjectiveistoimprovemanufacturingeffectiveness
for epitaxial growth of silicon and silicon-germanium (Si-Ge) thin films on a silicon wafer. Growth takes place in
the ASM Epsilon-1 chemical vapor deposition (CVD) reactor, a production tool currently in use at ESSS. Phase
II project results include developmentof a new comprehensiveprocess-equipment model capable of predicting gas
flow, heat transfer, species transport, and chemical mechanisms in the reactor under a variety of process conditions
and equipment settings. Applications of the model include prediction and control of deposition rate and thickness
uniformity; studying sensitivity of deposition rate to process settings such as temperature, pressure, and flow rates;
andreducingtheuseofconsumablesviapurgeflowoptimization. Theimplicationsofvarioussimulationresultsare
discussedintermsofhowtheycanbeusedtoreducecostsandimproveproductquality,e.g.,thicknessuniformityof
thinfilms. Wedemonstratethatachievingdepositionuniformityrequiressomedegreeoftemperaturenon-uniformity
to compensate for the effects of other phenomena such as reactant depletion, gas heating and gas phase reactions,
thermaldiffusionofspecies,andflowpatterns.
TheEpsilon-1isasingle-waferlamp-heatedCVDreactormanufacturedbyASM,Inc.,Phoenix,AZ.Thereactor
isusedfordepositinglayersofepitaxialSi-Ge,epitaxialsilicon(epi-Si),andpolycrystallinesilicon(poly-Si). Initial
physical-mathematicalmodelsforgrowthofpolycrystallinesiliconintheEpsilon-1arepresentedin[21]. Theinitial
modelsfocusonthermallyactivatedgrowthofpoly-SiintheEpsilon-1.Theyincorporateheattransferwithinthewafer
andbetweenwafer,chamberwalls,andlamps;andsurfacedepositionchemicalkinetics. Experimentalvalidationof
lamp heating models and experimentaldeterminationof chemical kineticsparameters are also presented. However,
theinitialmodelssufferfromsomedeficiencieswhichwenowdiscuss.
ThemodelingoffundamentalaspectsofCVDinvolvesbothchemicalkineticsandtransportphenomena.Depend-
ingonthespecificprocessandoperatingconditions,itisoftenassumedthatoneormorefactorshassignificantlymore
influencethanallothersoverdepositionproduct.Inthosecases,thefactorsthatareconsideredlessimportantareoften
completelyormostlyneglected.
Thistypeofsimplificationwasadoptedintheinitialmodels,whichfocusedonlowtemperatureepitaxy.Inthelow
temperatureregime,growthrateislimitedbysurfacereactionphenomenaratherthanbymasstransportphenomena.
Furthermore,surfacereactionphenomenasuchasadsorptionanddesorptionofreactantspeciesarestronglydependent
ontemperature. Forthisreason,thelowtemperatureregimeissaidtobethermallydriven(kineticallylimited). This
motivated an approach in which initial models consisted only of a simplified conjugate heat transfer model and an
Arrhenius growth law. Dynamics of transport phenomena were neglected. Inlet conditions for gas phase species
concentrationsandtemperaturewereassumedtoholdthroughouttheprocesschamber.Chambergeometryplayedno
role.
However, as we show in this report, the simplifications made for the initial models seriously compromise their
usefulnesstowardstudyinguniformityissuesforthinfilmgrowthintheEpsilon-1.Itturnsoutthatchambergeometry
andavarietyofcomplexphenomenaplayarole,includingdepletionofreactants,non-uniformgasheating,gasphase
chemistry,thermaldiffusion,andgasflowpatterns. Thisnecessitatestheincorporationofdetailedmodelsforthree-
dimensionaleffectsofgasflow,gasphaseheattransfer,andtransportofchemicalspecies,inadditiontothepreviously
modeled heat transfer phenomena. Furthermore, a more accurate model for surface reaction chemical kinetics is
requiredtoincorporatereactiveintermediariesproducedinthegasphase. Finally,itiscrucialthatthemodelsreflect
thecouplingamongthesevariousphenomenaintheprocesschamber.
Deficiencies in the initial models have been addressed by development of a comprehensive process-equipment
modelwhichaccountsforthemechanismsandfactorsdescribedabove. Themodelprovidesaplatformforstudying
theeffectofequipmentsettingsandprocessconditionsondepositionproductcharacteristics,e.g.,depositionrateand
thicknessuniformity.
This report is organized as follows. Section 2 serves as a stand-alone executive project summary and general
overviewofresults. Section3providesdetailsoftheequipmentandprocessesonwhichthemodelsare based. The
comprehensiveprocess-equipmentmodel is presentedin Section 4. Results and applications of the modeling effort
arecontainedinSection5. WegivesomeconcludingremarksinSection6.
1
2 Project Summary
Thissectionservesasastand-aloneexecutivesummary. Itsummarizesprojectobjectivesandresultsfromtheview-
pointofbenefitstosemiconductormanufacturingatNorthropGrumman.
2.1 Objectives
Asstatedearlier,theoverallobjectiveoftheprojectistoimprovemanufacturingeffectivenessforgrowthofthinfilms
intheEpsilon-1reactor. Theoverallgoalisaccomplishedviaimprovementofproductquality,increasedflexibilityof
operation,andreductionofmanufacturingcosts.Thedescriptionofprojectobjectivesandprocessdetailsthatfollows
isbasedondiscussionswithanddemonstrationsbyNorthropGrummanparticipants[23].
Within the scope of this phase of the project, product quality is determined solely by deposition thickness uni-
formity. Other factors, such as film composition and resistivity uniformity, are important quality measures but not
consideredhere. Thicknessvariationsof5%arecurrentlyacceptableformostapplications,althoughthereisnoguar-
anteethatsuchaspecificationwillremainstable. Currently,variationsintherangeof2%areroutinelyachievedwith
theEpsilon-1. Improvedresultsarealwaysdesirable.
TheEpsilon-1iscapableofoperationinseveralregimesforpressure,temperature,andflowrates,anddeposition
viainjectionofseveraltypesofprecursorandcarriergases.Predictionofdepositionratesandotherfilmcharacteristics
foragivencombinationofprocessconditionsiskeytotakingadvantageofthemachine’sflexibility.Themanufacturer
provides some predictive guidance and data. However, there is the desire for manufacturing “off-the-curve,” i.e.,
operatinginregimesandproducingfilmswithcharacteristicsthatdonotappearinmanufacturerprovidedinformation.
Wehaveidentifiedandfocusedonseveralmanufacturingcostsforwhichreductionwouldbeadvantageous. Per-
formance of devices at high frequency is difficult to predict based on properties of the product and manufacturing
parameters. Toachieveaproductwiththedesiredproperties,NorthropGrummanoperateswithathreetofourmonth
manufacturingcyclefollowedbyalongtestingcycle. Itcantakeuptotwoyearstoconvergeonthedesiredproduct.
Each manufacturing cycle requires an initial period of experimentation in which the necessary equipment settings
andprocessconditionsaredetermined. Onceparametersaredetermined,andthecustomerissatisfied,theprocessis
certified,andparametersareusuallynotchangedforseveralyearsinordertoprovidethecustomerwithaconsistent
product.Itispossible,however,fordriftofequipmentcharacteristicsovertimetocausesdegradation,necessitatingad-
ditionalexperimentation.Inaddition,thechambertubeisperiodicallycleanedandreplaced,requiringare-calibration
ofprocesssettings.Theresultisthatthevarioustrial-and-errorstepshaveasignificantimpactontime-to-manufacture
andotherproductioncosts.
Othercostconcernsincludeoperationalintegrityand“down-time”ofequipment,andtheuseofconsumablessuch
asprocessgases. Itisclearthatreductionsinexperimentalsteps,equipmentfailure,andgasconsumptionwillhavea
beneficialimpactonmanufacturingcosts.
2.2 Methodology
Projectobjectivesareaccomplishedby
gaininganunderstandingoftheprocessesandequipmentviaphysicalandmathematicalmodeling,and
(cid:2)
usingtheresultingvalidatedmodelsforoptimizationofprocessconditionsandequipmentsettings.
(cid:2)
ThemodelingeffortincludesdevelopmentofphysicsbasedmodelsforfundamentalCVDphenomena,experimen-
taldeterminationofmodelingparameters,andexperimentalvalidationofmodelpredictions. Processandequipment
models are used to develop software and analytical tools for “off-line” prediction of deposition results, conducting
“what-if”experiments,anddeterminingoperatingparametersforimprovedperformanceandproductquality.
2.3 History
Theproject“ModelingandOptimizationforEpitaxialGrowth”wasinitiatedinDecember1996,underanagreement
betweenNorthropGrummanESSSandtheISR.PhaseIoftheprojectroughlyencompassedtheperiodJanuary1997
throughSeptember1997. FinalPhaseIresultswerepresentedtoNorthropGrummanparticipantsinAugust1997,at
theSEMATECHAdvancedEquipment/ProcessControlSymposium[20]in September1997,andpublishedin[21].
2
Phase II of the project began in March 1998. A formal presentation of results to Northrop Grumman participants
was made in October 1998. This report, which includes some additional results, and revisionsto the October 1998
presentation,waswrittenduringthesubsequentperiod.
2.4 SummaryofPhaseIResults
TherearetwomaincomponentsofthePhaseIeffort[21]: physicsbasedmodelingofpoly-SigrowthintheEpsilon-1
andacomparativestudyofmethodsforreducingthecomplexityofthosemodelssotheyareeasiertouseforsimulation
andcontrolpurposes.
Theinitial modelingeffortproduceda detailedanalysisof physicalandchemicalmechanismsfor growthin the
Epsilon-1,alongwithanoverallmodelingframeworkwhichidentifiedandintegratedthenecessarymodelcomponents
suchasmacroscopiclevelheattransferandmicroscopiclevelmorphologyevolution. Then,simplifiedphysicsbased
modelsforpoly-SidepositionintheEpsilon-1weredeveloped.Thesemodelspredictthetimeevolutionfor
heat transfer within the wafer and among wafer, heat lamps, and chamber walls (temperature distributions in
(cid:2)
solids),and
reactionrateatthewafersurfaceviachemicalkinetics.
(cid:2)
Theinitialwaferthermalmodelaccountsforeffectsofconductionwithinthesolidwafer,convectivelossestothe
gasphase,andradiativelossestotheambient.Radiativeheattransferfromlampstowaferismodeledbydetermining
spatialprofilesofradiantheatfluxintensityforindividuallampgroupsandlampzones.Theseradiantintensityprofiles
werecomputedanalyticallyusingviewfactormethodsandthenvalidatedusingdatafrompoly-Sigrowthexperiments
usingtheEpsilon-1reactor.
TheinitialchemicalkineticsmodelisanArrheniusrelationshipforsurfacereactionrateversustemperature. The
Arrhenius parameters, activation energy and pre-exponential constant, were determined via poly-Si growth experi-
mentsusingtheEpsilon-1reactor.
The initial models were used to predict growth rates and thickness profiles for several test process recipes. It
shouldbeemphasizedthatthepredictivecapabilityoftheinitialmodelswaslimitedtothelowtemperaturethermally
activatedregime.Theydonotaccountfortheeffectsofgasflowpatterns,gasphaseheattransfer,gasphasechemical
reactions,andotherphenomenaaffectingtherateandspatialdistributionoftransportofchemicalspeciestothewafer
surface.
DuetothecomplexityandhighcomputationaldemandsofCVDmodels,weinvestigatedtheuseofmodelreduc-
tiontechniquestoreducethemodelcomplexity,leadingtofastersimulationandfacilitatingtheuseofstandardcontrol
andoptimizationstrategies. Thecomparativestudyofmodelreductionmethodsexaminedtheproperorthogonalde-
composition(POD)andbalancedrealizationapproachestoreducingmodelcomplexity. Thesemethodswereapplied
tothe dynamicmodelsforheattransferinvolvingthewaferandlampsintheEpsilon-1. Numericalsimulationsand
subsequentanalysisrevealedthatboththePODandbalancingmethodsproduceastatespacebasisthat allowsfora
significantreductioninmodeldimensionality(seealso[22]).
2.5 SummaryofPhaseIIResults
We developeda comprehensiveprocess-equipmentmodel, incorporatingthree-dimensionaleffectsof gasflow, heat
transfer, species transport, and gas phase and surface chemical mechanisms. The process-equipment model is im-
proved over initial models, in that it is based on more accurate information providedby the manufacturer, and has
the capabilityto takeinto accountmorephenomenasuspectedto havea significantinfluenceon depositionrateand
uniformity.
Theprocess-equipmentmodelprovidesatoolforpredictionofdepositionrateandotherprocessvariablesgiven
aset ofprocessconditionsandequipmentsettings. ThesepredictionsallowNorthropGrummanto simulategrowth
experimentsinadvance,narrowparameterchoices,andperformfeweractualexperiments.Conditionsandsettingscan
beoptimizedoff-line,takingintoaccountsimulationresultsandsensitivityanalysisforpressure,temperature,andflow
rates. Theeffectofadjustmentstowafertemperatureset-point,chamberpressure,sourcegasflowrate,thermocouple
offsets,andinjectorsettingscanbepredictedandtunedoff-line.
We validated the predictive capability of the model over a range of operating conditions by comparing simula-
tionresultswithexperimentaldata. We usedsimulationstopredicttherelationshipbetweensilicongrowthrateand
wafer temperature, chamber pressure, source gas flow rate, and choice of carrier gas. We determined an Arrhenius
3
relationship for temperature sensitivity of silicon growth rate, with activation energy the same for simulations and
experiments. Simulationresultsindicatethatgrowthrateincreaseswiththelogarithm(base10)ofchamberpressure,
inagreementwithknownrelationships.Wefoundapowerlawrelationshipconnectingpoly-Sigrowthratewithsilane
flowrateattheinlet. Finally,wedemonstratedthatsubstitutionofnitrogenforhydrogenasthecarriergasresultsina
significantlyincreaseddepositionrate.
Weshowinthisreportthatauniformtemperaturedistributiononthewafersurfaceresultsinanon-uniformdepo-
sitionthicknessprofile.Thus,itisapparentthatachievingdepositionuniformityrequiressomedegreeoftemperature
non-uniformitytocompensatefortheeffectsofotherphenomena.Thistemperaturenon-uniformityisprovidedbythe
thermocoupleoffsets.Thefactorsthatinfluencedepositionrateanduniformityarestudiedwiththegoalofformulating
strategiesforthesettingofthermocoupleoffsets.
Simulationresultsshowthatconsumptionofprocessgasescanbereducedbydecreasingthepurgegasflowfrom
7slmto5slmandpossiblyfurtherwithoutcompromisingtheabilityofthepurgegastopreventback-sidedeposition.
2.6 Follow-OnPlans
ExistinggasinjectionequipmentintheEpsilon-1willbereplacedinthenearfuturewithnewgasinjectors. Thenew
equipmentwillprovidemoredegreesoffreedomforadjustingthegasflowprofileintheprocesschamber.Themodels
shouldberevisedtoreflectthenewequipment,andtheeffectofvaryinginjectoropeningsizesshouldbeinvestigated
viasimulationsandexperiments.
ItisclearthatthesettingofthermocoupleoffsetshasastronginfluenceondepositionuniformityintheEpsilon-1.
Anexperimentalstudyisnecessarytoinvestigatetheeffectofeachindividualoffsetonwafertemperaturedistribution
andgrowthrateuniformity.
NorthropGrummanhasobservedthatabuild-upofdepositedmaterialleavesacoatingonthequartzshelves[23].
The effect on deposition uniformity is unclear, although it has not yet caused a problem in that regard. Its main
damageistothestructuralintegrityofthechamber. Inoneinstance,mechanicalstressesduetofilmbuild-uponthe
quartzcausedthequartztocrack,destroyingthechamber. Forthisreason,thequartzchamberiscleanedorreplaced
periodically.Inaddition,theflowguidehasbeenusedtodirecttheflowsothatthebuild-upoccursinalessdamaging
location.
Inordertopreventunwantedbuild-upoffilms,anetchcleanstepusingHClisperformedpriortodepositionsteps.
Duringthisstep,HClispumpedintothechamberalongwithH purgegases.Itispossiblethattheflowofpurgegases
2
throughthering-shelfgapforcestheetchgasawayfromcertainlocationsontheshelfnearthegap,thuspreventing
etchinginthoselocations. Areductioninpurgegasflowratemayalleviatethisoccurrence. Theeffectofpurgegas
flowontheHCletchcleanofthechambershouldbestudiedviasimulationwiththegoalofreducingthebuildupof
depositsonchamberwalls.
ModelsforSi-Gechemicalmechanismsareneededtoapplytheprocess-equipmentmodeltoSi-Gegrowth. ISR
participantsarereceivingassistancefromagovernmentlaboratory[19]towarddeterminingchemicalkineticsparam-
etersforgrowthofSi-Ge.
NorthropGrummanisinterestedinthinfilmgrowthonpatternedwafers.Itisnecessarytostudytheeffectofflow
and thermal conditions on film morphology for patterned wafers, and to determine the relationships among pattern
pitch,thicknessuniformity,andprocessconditions.
3 Equipment and Product
TheASMEpsilon-1reactorisaradiantlyheated,gasinjected,singlewaferprocessingsystemforCVDofdopedor
undopedepitaxiallayersona6inchdiametersemiconductorwafer. Accurateanddetaileddataregardingthereactor
geometry,materials,andoperationwasfurnishedbythemanufacturer,ASMAmerica,Inc.,duringanApril1998visit
bytheauthorstotheirheadquartersandmanufacturingfacilityinPhoenix,AZ.
3.1 ProcessChamber
Figure1showsacross-sectionalviewoftheASMEpsilon-1processchamberandlampassembly. Figure2showsa
top-downviewofthewaferlevelapparatus. Diagramsofotherviewsofthereactorarefoundin[21]. Theinletand
outletsidesofthereactorarereferredtoasthefrontandrear,respectively.
4
Figure1:Cross-sectionalviewoftheASMEpsilon-1processchamberandlampassembly.Processgasesflowthrough
uppersection(abovewafer)from inlet(left)to outlet(right). Purgegasesflowinlowersection(belowwafer). The
upperlamparrayilluminatesthetopsurfaceofthewafer. Thelowerlamparrayandspotlampsilluminatethebottom
ofthesusceptor.
Deposition takes place in the process chamber, which is a horizontally oriented quartz tube of lenticular shape,
i.e., a cross-sectionalview looking into the chamber front shows a flat bottom, short verticalsides, and curvedtop.
Processgasesarepumpedintothechamberthroughtheinletflange,flowhorizontallythroughthechamberoverthe
wafersurface,andarepumpedoutthroughtheexhaustflangeviapneumaticactuators.Anoptionalflowguidecanbe
usedtoforcetheinletflowawayfromthechamberroofandtowardthewafer. Theinletflangeisdesignedtocreatea
specializedsonicflowwithpossibleswirlingandmixingpropertiesasthegasenterstheprocesschamber.Ithasbeen
observedbythemanufacturerthattheinletflangedesignaidsinachievingdepositionuniformity[18].
The6inchdiameterwaferrestsonsmallquartzpinsattachedtothepocketofarotatingsusceptorthatissurrounded
bythesusceptorring. Thesusceptorandringareconstructedofgraphitecoatedwithsilicon-carbide. Thechamberis
dividedbythesusceptor,ring, anda quartzshelfintoupperandlowersections. Thingapsbetweenthequartzshelf
and the ring, and between the ring and susceptor, allow gas to flow between upper and lower chamber sections. In
addition,diffusionofspeciesfromuppertolowerandvisa-versacanoccurduetoconcentrationandthermalgradients.
Forthisreason, purgegasesare pumpedintothe lowerregionthroughthe susceptorrotationshaft andapurgeinlet
inthe frontwall. Thepurgeflowpreventstheprocessgasesfromescapingtothe lowersection, whichcanresultin
unwanteddepositionontheback-sideofthesusceptor.
The wafer and chamber are heated by upper and lower arrays of linear tungsten-halogen lamps, and four spot
lampsdirectedatthecenterofthesusceptor(see[21]fordetailsandanalysis). Heatradiationisintensifiedbygold
coated reflectorssurrounding the process chamberon all sides. Four thermocouplesmeasure the temperature at the
center,front, rear, andsideofthe susceptor. Wenotethat thewhilethe centerthermocoupleislocatedatthecenter
of the susceptor, the other three thermocouples are located at the front, rear, and side of the ring that surrounds the
susceptor.Thus,susceptortemperatureismeasuredonlyapproximatelyatpointsotherthanthecenter.
The quartz chamber and lamp-house are cooled by air flow. All components are contained in a stainless steel
enclosure.
3.2 ProductandConsumables
Northrop Grumman uses the ASM Epsilon-1 reactor to deposit thin films of epitaxial Si-Ge, epi-Si, and poly-Si.
Epitaxiallayersaredepositedonsiliconwafersthatareeitherbareorcoveredwithapatternedlayerofsilicondioxide
(SiO ). Poly-Siisdepositedonalayerofsilicondioxide.
2
Themajorsourcegasesusedtodepositepi-Silayerscommerciallyare(see,e.g.,[24,28])
(cid:2) silane(SiH4)atlowtemperatures((cid:3) 1000C);and
silicontetrachloride(SiCl ),dichlorosilane(SiH Cl ),andtrichlorosilane(SiHCl )athighertemperatures.
(cid:2) 4 2 2 3
5
Gap (cid:5)Wafer Susceptor (cid:4)Ring Quartz Shelf
(cid:7)Front (cid:4)Rear
(cid:6)Thermocouples
Figure2: OverheadviewoftheEpsilon-1atwaferlevel. Gasflowsfromfront(upstream)torear(downstream). The
quartzshelvesareconnectedto thequartzchamberwallstoform acontiguousbody. Thegraphitecoatedsusceptor
ringfitsintoaspacewithinthequartzstructure,leavingathingapbetweenringandshelfonallsides. Thegraphite
coated susceptor fits into the ring structure, and is supported and rotated by special apparatus located through and
under the lowerchamber section. Thereis also agap, although muchsmaller, between the ringand susceptor. The
waferrestsonsmallquartzpinsattachedtothesusceptor.Thecenterthermocouplemeasurestemperatureatthecenter
ofthesusceptor. Thefront,rear,andsidethermocouplesmeasuretemperatureatthreelocationsinthering.
Thisprojectdealswithlowtemperaturegrowth,forwhichNorthropGrummanusessilaneasthesourcegas.Germane
(GeH )isaddedtothemixtureforgrowthofSi-Ge. Thecarriergasiseitherhydrogen(H )ornitrogen(N ). Dopant
4 2 2
precursorssuchasarsine(ArH )canalsobeaddedtothemixture.
3
Thereisadirectrelationshipbetweengrowthofpoly-Siandepi-Si.Theprocessingstepsareidentical,andgrowth
rates are virtually identical [23]. Experiments are performed at Northrop Grumman by depositing poly-Si rather
than epi-Si, because they have tools for measuring poly-Si films (e.g., nanospec, ellipsometer) but not for epi-Si
(e.g.,SIMS).(Itisthepresenceoftheoxideboundarythatallowsforeasiermeasurementofpoly-Si.) Thus,poly-Si
experimentsallowforrapidprocessevaluation. Forthisreason,NorthropGrummansometimesperformsepi-Siand
poly-Siexperimentsinparallel,withmeasurementstakenforthepoly-Sifilms.
Inlightoftheabove,forpurposesofthisreport,weperformedexperimentsandsimulationsforgrowthofpoly-Si.
Theprocessgasesusedwereamixtureof2%SiH dilutedinH asthesiliconsource,togetherwith20slmH asthe
4 2 2
carrier,exceptasnotedotherwise.
Specificationsforthefinalproduct(thinfilm)includecharacteristicssuchaschemicalcomposition,filmthickness,
dopantconcentration,crystalstructure,resistivity,andpossiblyotherfactors. Bothaggregateandspatiallydistributed
quantitiesareimportant.Usually,spatialuniformityisspecifiedbyavariationtoleranceacrossthewafersurface,e.g.,
5%allowablenon-uniformity.
Ingeneral,aggregatecharacteristicssuchasaveragegrowthratearedeterminedbyprocessconditionsfortemper-
ature,pressure,andflowratesassetbytheuserinprocessrecipes. Thespatialdistributionsofthevariousproperties,
andhenceuniformity,aremainlycontrolledbyequipmentsettingssuchasthermocoupleoffsetsandinjectoropening
sizes. Thesetwomethodsofequipmentandprocesscontrolaredescribedbelow.
3.3 ProcessConditionsandRecipes
Inordertoachievethedesiredaggregatecharacteristics,theprocessengineerdesignsstep-by-steprecipes. Eachstep
performs a particular task such as etch, bake, purge, or deposit, for a specified amount of time. We consider only
thedepositionstepshere. Theprocessengineerspecifiesthechoiceofsource,carrier,andpurgegases,set-pointsfor
temperature, pressure, and flow rates, and time duration, for each deposition step. We refer to these specifications
asrecipeinputs. TheyareprogrammedintotheEpsilon-1microprocessorandcontrolledautomaticallyin–situ. For
example,PIDcontrollersandMFCsregulatethethermocoupletemperaturesandinletflowratesaroundtheirrespective
set-points.
Si-Ge films are deposited at a temperature of 675 C. This falls within the low temperature regime which is
roughly 600–800 C. At low temperature, surface reactions are thermally activated and controlled by deposition ki-
6