a 1.2mev, 100ma proton implanter - how can i...
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A 1.2MeV, 100mA Proton Implanter GeoffRyding,TedSmick,BillPark,JoeGillespie,PaulEide,
TakaoSakase,KanOta,RonHorner,DrewArnold
Abstract Ahighenergy,highcurrentprotonimplanterhasbeendevelopedforproducingthinlayersofc‐Si
crystallinesiliconwiththicknessesintherangeof5‐20µm.Theimplanterdescribedisdesignedtooperateatenergiesbetween0.4‐1.2MeVwithprotonbeamcurrentsupto100mA.ThissystemcanalsobeusedtoexfoliateothermaterialssuchasGaAsandSiC.
Introduction Thereisanincreasingdemandforrenewableenergyusingphotovoltaictechnology.Inparticular,
photovoltaiccellsarecommonlyfabricatedoncrystallinesiliconwaferswhichareconventionallyproducedbyslicinganingotofsinglecrystalsilicon.Thismethodofproducingphotovoltaiccellsaccountsforapproximately80%ofthepresentworldwideproductionofsolarcells.Unfortunately50%
ormoreofthesiliconislostasaresultofthekerfconsumedintheslicingprocess.Furthermore,theminimumwaferthicknessthatcanbeproducedbysawingorslicingis≥115µm,whichismuchthickerthanneededforoptimumphotovoltaicdevices(1).Thinnersiliconlaminacanbemadebyimplanting
hydrogenionstothedesireddepthbelowthesurfaceofthesiliconwafer.Afterasuitablenumberofionshavebeenimplanted,athinlayerofmicrobubblesisformedandonsubsequentheatingofthewafer,thetoplayerexfoliatestoproduceathinlayerofsiliconwithathicknesspreciselyequaltothe
depthoftheoriginalimplantedlayer(2).Usingthistechnique,asmanyas10ormorelayersofsiliconcanbepeeledfromastandardsiliconwafer,eachhavingtheidealthicknessforthefabricationofanefficientphotovoltaiccell.Inthiswaythecostofthesinglecrystalsiliconmaterial,whichisgenerallythe
dominantcostcomponentofthefinishedcell,canbereducedby80%.Anaddedbenefitisthefactthatthecellsmadefromthethinlaminaareflexible.
Inthelast3yearswehaveinvestigatedthepotentialadvantagesoflaminabasedphotovoltaiccellsindetail(1).EarlyworkwasbasedonanairinsulatedDCacceleratoroperatingwitharibbonbeamof
protonsintheenergyrangeof350‐420keVandbeamcurrentsof10‐75mA(3).Criticalprocessstepsincludinglighttrappingschemes,andmetalcontacttechniqueshavebeendevelopedusingthe4.5µmlaminaproducedbythismachine.Asthelaminathicknessincreasesthecellefficiencycanincreasebut
thecostofincreasingtheenergyandoperatingvoltageoftheacceleratoralsoincreases.Theimplanterdescribedhereoperatesroutinelyatenergiesbetween0.4‐1.2MVandbeamcurrentsupto100mA.Thesebasicparametersrepresentanoptimumcompromisebetweencellperformanceandtotal
implanteroperatingcost.
Anionimplantersuitableforsolarcellproductionofthetypedescribedhasthreeprimaryrequirements:anaccelerationschemewhichwillacceleratetheionstothevelocityrequiredforpenetrationtothe
appropriatedepthbelowthesurface,abeamcurrentintensitythatresultsintherequiredsystemproductivityandhighspeedwaferhandling.
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Thenovelimplanterdescribedoperateswithprotoncurrentsupto100mAandenergiesupto1.2MeVandthewaferhandlingnon‐productivetimeis<8%ofthetotalcycletime.
System Architecture and performance targets Thehighbeampowerrequirementsoutlinedpresenttwoprimarychallenges.Firstthe120kWionbeam
mustbegeneratedwithreasonablepowerefficiencyandsecondtheprocesschambermustbecapableofabsorbingthispowerwithoutexcessiveheatingofthetargetwafers.
ThechallengeshavebeenmetbyadoptingthesystemarchitectureshowninFigure1whichidentifiesthemajorcomponentsoftheimplanter.Thereare4basictypesofionacceleratorstructuresthatcanbe
usedtoaccelerateionstotheMeVenergyrange:linearhighfrequency(LINAC),circularhighfrequency(Cyclotron,synchrotron),RFQ(RFquadrupoleaccelerators)andDC(SingleendedorTandem).Thepowerconversionefficiencyofthesesystemscanbedefinedastheratiooffinalbeampowertototal
inputpower.InthecaseofRFtypeacceleratorsthepowerefficiencyisgenerallylow(≤20%)asaresultoflossesintheamplifierandresonantcavitycomponents.ForthisreasonaDCapproachwasadoptedusingstandardcommercialhighvoltagepowersupplies
Figure 1 System architecture
Theionsource,theaccelerationtubeandthehighvoltagegeneratorarecontainedwithinapressurevesselwithadiameterof2.1mandalengthof4.1m.ThevesselcontainscompressedSF6gasatapressureintherange50‐100psitofacilitateoperationatvoltagesupto1.2MV.Theionbeamwhich
emergesfromthisbeamgenerationsystemisfocusedbyamagneticquadrupolelensanddirectedintoamagneticscannersystemwhichdeflectsandscansthebeaminthehorizontalplaneasshown.The
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scannedbeamisthendeflectedandanalyzedbyamagneticdipolemagnetwhichservestofilteroutunwantedionbeamsanddirectthecollimatedbeamontotheinsideofalargediameterrotating
processdrum.Thepseudo‐squaresiliconwafersaremountedontheinsideofthedrumduringimplantandbetweenimplants,highspeedrobotsareusedtoloadandunloadthesewafersthroughavacuumload‐locksystem.
ThebasicperformancetargetsforthesystemaresummarizedinTable1.
Table 1 Implanter design targets
LaminaThickness 5‐20micronsLaminaProductionrate 150/hourSystemEnergyrequirements <2kWhrs/laminaIonBeamEnergy 0.4‐1.2MeVProtonbeamcurrent 100mAPowerefficiency(Beampower/totalpower) >40%Machineutilization >90%
Voltage Generator MosthighpowerDCionacceleratorshaveincorporatedsomeformofcascade‐rectifiervoltagemultiplierbasedontheconceptsofJohnCockroftandErnestWaltontogeneratethehighpotentials
requiredtoacceleratetheions(4).Theoutputofthegeneratorisconnectedtotheionsourceanddeterminesthefinalenergyoftheions.Ionsextractedfromthesourcepassintoanaccelerationtubeconsistingofseveralelectrodesandtheappropriatebiasvoltagesfortheseelectrodesisgenerally
derivedfrompotentialdividerresistorsconnectedtotheoutputofthehighvoltagegenerator.Asthebeamcurrentinsuchasystemisincreased,severaldifficultiescanbeencountered.ThetwoprimarychallengesaresummarizedinTable2.
Table 2 Fundamental challenges for high current, high voltage DC accelerators
1.Highvoltagegenerationandpowerefficiency
Sincetheoutputpowermustflowthrougheachmultiplierstageinseries,theavailableoutputpower,thevoltageregulationandthesystemefficiencydeclinesasmorestagesareadded.
2.Productionofastableelectrostaticaccelerationfield
Smallbeamcurrentsinterceptingtheaccelerationelectrodescanresultinlargeperturbationsoftheacceleratingfieldwhichinturnleadstobreakdownandavalancheeffects.Theseinterceptedbeamsmayoriginateasaresultofsmallaberrationsorchargeexchangewithresidualgasmoleculesinthevacuumsystem.Attemptstominimizetheseeffectsbydecreasingthevalueofresistivedividersresultsinwastedpowerandfurtherlossofefficiency.
Wehavedevelopedanovelpowersupplysystemwhichovercomesthetwoprimarychallengesoutlinedabove.ThesystemconceptisshownschematicallyinFigure2.
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Figure 2 Schematic of the high voltage generator concept
Asystemisshownschematicallywithfiveelectricallyisolatedalternatorsmountedonacommon
insulatingdriveshaftwhichinturnisdrivenbyacommondrivemotor.Eachalternatorprovidesanelectricallyisolatedsourceofpowerforitscorrespondinghighvoltagepowersupply.Asshown,thesepowersuppliesareconnectedinserieswiththefinaloutputattachedtotheionsourceandeach
intermediateoutputisconnectedtothecorrespondingelectrodeoftheaccelerationtube.ThemainadvantagesofthisarchitecturearesummarizedinTable3.
Table 3 Advantages of the system architecture
1 Theelectrostaticfieldintheacceleratortubeispreciselycontrolledbytheregulatedoutputsettingsoftheindividualpowersupplieswhicharecapableofregulatingvoltageoverawiderangeofcurrentloads.Theinfluenceofspuriousbeamstrikeorscatteredbeamandsecondaryparticlesisvirtuallyeliminated.
2 Fieldgradientsalongthetubecanbeeasilycontrolledtogiveoptimumionopticalpropertiessimplybyadjustingtheoutputsettingsofeachindividualpowersupply.
3 Individualsectionsoftheaccelerationtubecanbe‘voltageconditioned’withoutinfluencefromotherregionsofthetube.
4 Themaximumvoltageandcurrentcanbescaledbyselectingtheappropriatenumberofpowersuppliesandalternators.
5 Standardcommercialpowersuppliescanbeused
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Thehighpowerimplanteritselfhasbeenconstructedusingthreehighvoltagegeneratorassembliesinsidethehighpressurecontainmenttank.Eachgeneratorstackconsistsofa100HP,3‐phaseAC
inductionmotorwithanefficiencyof94%,five12kVA,permanentmagnet,3‐phaseACalternatorsoperatingat80%efficiencyandfiveseriesCockroft‐Waltonpowersupplies(5)operatingatvoltagesupto80kV,currentsupto125mAandefficienciesof85%.
OneofthesegeneratorassembliesisshowninFigure3.
Figure 3 High voltage generator assembly
AphotographofthecompletehighvoltagegeneratorsectionofthemachineisshowninFigure4.
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Figure 4 The high voltage generator
Inthisconfigurationthereare15isolatedalternatorsand15independenthighvoltagepowersupplies
connectedinserieswhichresultinamaximumcombinedcapabilityof1.2MVand125mA.
Ion Source and acceleration tube optics Theprotonbeamisdevelopedusingahighcurrentelectroncyclotronresonance(ECR)microwaveionsourcecoupleddirectlyintoaconventionalelectrostaticaccelerationcolumn.Figure5showsaschematicofthesourceandcolumn.
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Figure 5 Schematic of ion source and acceleration column
ThesourceconsistsofthreesolenoidelectromagnetssurroundingacircularplasmachamberfedwithH2
gas.Theplasmachamberisexcitedbyupto1kWof2.45GHzmicrowavesfedthroughathreelayervacuumwindow(6)intothechamber.Athreestubtunerandridgedtaperedwaveguidematchtheimpedanceofthemicrowavesystemtotheplasma.Thesourcearchitectureisbasedonpreviousion
sourcesdevelopedatChalkRiver(7)andLosAlamos(8).ThesourcecomponentsaredisplayedontheleftsideofFigure5.Themagneticfieldisadjustedsuchthatthefieldisslightlyaboveresonanceatthevacuumwindow,taperingtothe875Gresonantfieldtowardstheexitofthechamber.Theboundary
elementcodeLorentz‐3D(9)wasusedtomodelthesolenoidalfieldsandoptimizethedesigntoachievethedesiredfieldshape.
Figure 6 PBGUNS simulation of the beam extraction for a 70kV, 100mA beam
ThebeamisextractedataDCvoltageofupto80kVatcurrentsupto125mA(powersupplylimited)usingatetrodeextractiongeometrywithelectrostaticsuppressionofbackstreamingelectrons.The2D
codePBGUNS(10)wasusedtomodeltheextractiongeometryandoptimizethebeamemittanceandcurrenttobefedintotheaccelerationcolumn.AscreenshotfromPBGUNSisshowninFigure6.
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TheECRsourceprovidesmanyadvantagesoverconventionalionsourcesincludingahighprotonfractionof90%ormore,highcurrentdensityof200mA/cm2andabove,lowgasloadtothevacuum
system,andlongperiodsbetweenserviceswithnofilamentstowearout.Thehighprotonfractionallowsthebeamtobeinjecteddirectlyintotheaccelerationcolumnwithoutpre‐accelerationmagneticanalysis,greatlysimplifyingtheterminaldesign.
Theaccelerationcolumnconsistsof14accelerationstages(the15thbeingthesourceitself),each
capableofacceleratingthebeamupto80keVpersection.Sinceeachstageisdrivenbyanindependenthighvoltagepowersupply,thevoltageoneachelectrodecanbeeasilyadjustedtooptimizethetuneofthesystemandalsoaidintheinitialhighvoltageconditioningofthecolumn.Simulationofthecolumn
opticswereagainstudiedusingLorentz‐3D.Figure7showsaraytracingsimulationoftheaccelerationcolumnincludingspacechargeeffectfora1.2MeV,100mAprotonbeam.
Figure 7 Beam trajectories simulation for a 1.2 MeV, 100mA proton beam using Lorentz‐3D
Thecolumnhasconsiderablevacuumpumpingareathroughoutitslengthwithameasuredconductanceinexcessof2000L/secforhydrogen.Thishighconductance,combinedwiththelowgasloadfromtheECRsourceallowstheentiresystemtobevacuumpumpedbyturbo‐molecularpumpsatground
potential.EliminatingvacuumpumpsintheSF6environmentatterminalpotentialgreatlysimplifiestheoverallsystem.Aluminaceramicbushingsareusedtofurtherreducethegasloadtothesystemcomparedwithepoxyresinbushings.
Thecentralportionofeachelectrodeismadeoftitaniumwitha100mmaperturetominimizebeam
strikeandprovidegoodpumping.Overlappingstainlesssteelstressringswithembeddedsamariumcobaltpermanentmagnetssurroundthecentralapertureandblockthelineofsightbetweenthebeamandthebushingstopreventcoatingandeventualshortingofthebushing.Theembeddedmagnets
suppressbackstreamingelectronsinthetubebylimitingthetotalenergygainedbyanelectrontooneortwogaps.Thisminimizesthebremsstrahlungradiationaswellastheloadonthehighvoltagesupplies.Themagneticfieldsrotate90degreesateachstagewithmagneticadjustmentsmadeatthe
entranceandexitsuchthatthebeamcenterlineremainsclosetothecolumnaxisthroughoutitslengthandexitsoncenterandparalleltotheaxis.Electrostaticsuppressionispresentattheexitofthecolumn
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tofurthersuppresselectronsenteringthecolumnandprovideafieldfreeregionbeyondthecolumnforbeamneutralization.
Beam Scan System Themagneticscansystemperformstwokeyfunctions.Bymagneticallydeflectingthebeamthrough
approximately80degrees,unwantedspeciesandlowenergycontaminantsareremovedfromthebeampriortoimplant.Second,byincorporatingahighspeedoscillatingscannermagnet,thebeamisrepeatedlysweptacrossthewafersinacontrolledfashion,providingveryhighdoseuniformityforthe
implant(11).Asetofmagneticquadrupolescontroltheshapeofthebeamspotonwafertominimizeoverscan,instantaneousheating,andchanneling.ThemagneticscanningsystemisshownschematicallyinFigure8.
Figure 8 Magnetic scanning system
Toavoidlargeinstantaneoustemperaturevariationsonthewafercausedbytheveryhighpowerbeam(upto135kWonwafer),thescanspeedcanbeincreasedupto2kHz,almostanorderofmagnitude
fasterthantypicalmagneticallyscannedimplanters..
Process chamber and wafer drum Thesiliconexfoliationprocessrequiresanimplanttemperatureintherange30‐140°Cwithatemperaturevariationacrossthewaferoflessthan20°C.Controlofwafertemperatureinthisrangeisa
formidablechallengewhenbeampowersupto120kWareused.Inordertoreducebeamheatingeffects,thewellknowntechniqueofbeampowersharinghasbeenadoptedbyimplantingthewafersin
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largebatches.Ineachbatch60wafersaremountedontheinsideofa3.1mdiameter,watercooleddrum.Thefacetsofthedrumonwhichthewafersaremountedarecoatedwithacompliantpolymerof
suitablethermalconductivityandthewafersarepressedagainstthismaterialbythecentrifugalforcesgeneratedwhenthedrumisrotated.Thetotalareaexposedtothescannedbeamisapproximately1.5E4cm²andinordertomaintainatemperaturerisebelow120°C,athermalcontactbetweenthe
waferandthedrumofapproximately70mW/cm²°Kisrequired.Thedrumcanrotateaboutitsaxisatspeedsupto400rpmwhichprovidesradialaccelerationsofthewaferupto280Gs.Theresultingclampingforceofthewaferagainsttheelastomericcoatingresultsintherequiredimplanttemperature.
ThedrumandthesystemwhichuniformlyscansanddeflectsthebeamontotheinsideofthedrumisshownschematicallyinFigure1.AndaphotographofthedrumitselfisshowninFigure9.
Figure 9 Photograph of the wafer drum
Onecomplicationofthisarrangementisthefactthattheangleatwhichthebeamstrikeseachwafervariesfromtheedgeofthewafertothecenterofthewaferasaconsequenceofitsrotationasthe
drumrotates.Thiseffect(12)canresultinundesirablechannelingoftheionsdownaxialorplanarchannelsofthesinglecrystalsiliconandcanoccurinbothrotatingdiskandrotatingdrumprocessstationswhenevertheincidentionbeamaxisisnotparalleltotheaxisifrotation.Itisoftenreferredto
asthe‘coneangle’effect.Inthesystemdescribedthevariationinangleisapproximately±3°andby
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selectingtheappropriateaveragetiltandtwistangleofthewaferswithrespecttothebeam,allaxialandplanarchannelingeffectshavebeenavoided.
Wafer handling Aftereachwaferbatchhasbeenimplantedtotherequireddoseforsuccessfulexfoliation,thewafers
mustbeunloadedfromthedrumandreplacedbyanewbatchofwafersinreadinessforthenextimplant.Duringthisexchangesequencetheionbeamisnotbeingusedanditisthereforeverydesirabletominimizethetimetakenforanywaferhandling.
Tominimizewaferexchangetimesallwaferhandlingtasksaredoneinparallel.Thesystemshownin
Figure10isusedtotransferimplantedwafersfromthedrumandintoastackingelevatorlocatedinavacuumloadlock.Itconsistsofapick/placerobotmechanismandaconveyorbeltsystemasshown.Asecondsystemconsistingofidenticalcomponentsisusedforthereverseprocesswherebyun‐implanted
wafersfromasecondstackingelevatorlocatedinthevacuumloadlockaretransferredbytheconveyorsystemtoasecondpick/placerobotwhichplacesthemonthedrum.Withthisarrangement,onesystemisdedicatedtounloadingimplantedwafersfromthedrumandanothersystemisdedicatedto
loadingun‐implantedwafersontothedrum.Forthishighlyparallelarchitecturetofunctionproperlyallactionsmustbeexecutedwithhighprecision.On‐the‐flyconveyortrajectorymodificationtechniquesandactivewaferalignmentmechanismsareutilizedtocorrectanypositionalvariabilityofthewafers.
Sophisticatedstate‐machinealgorithmsareimplementedsothatanactionstartsassoonastherightconditionsaremet.Atthebeginningofeachaction,sensorsareutilizedtoensurethesoftwarestate
matchesthehardware.Alltheaxesareservo‐controlled;thereforeanydeviationfromthedesignedtrajectoryisimmediatelydetectedandreported.Attheendofeachaction,sensorsareutilizedtoconfirmsuccessfulcompletion.Asaresult,allaxesworkharmoniouslyinparallelwiththestate‐
machinegeneratingtheoptimumsequenceinreal‐time.
Finiteelementanalysis,modalanalysis,andkinematic/dynamicsimulationsoftwarehavebeenusedextensivelytooptimizethehardwareforhighspeedautomation.Complexcalculationshavebeendonetodevelopadvancedhighspeedmotioncontrolprofilesforallaxes.
Theresultisahighspeedwaferhandlingsystemwhichreliablyunloadsandloads60pseudo‐square
wafersfromtheimplantdrumandtransfersthemintothevacuumloadlockin90seconds.
Thestackingelevatorsintheloadlockareinturnunloadedandloadedbyanexternal6axisrobot(13)whichcanbeprogrammedtointerfacewithavarietyoffactorydeliverysystems.
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Figure 10 The three main wafer transfer mechanisms of the drum load or unload system
Performance results Theimplanterdescribedhasbeensuccessfullyoperatedwith45mAprotonbeamsatenergiesupto800keV.ThesystemisnowbeingshippedtotheTwinCreeksLaminarbasedsolarfactoryinSenatobia,MS.
Conclusion AnovelacceleratorarchitecturehasbeendemonstratedwhichiscapableofproducinghighpowerprotonbeamswithMeVenergiesandcurrentsupto100mA.Thesystemhasbeentestedusingalarge
area,highspeedprocessstationwhichiscapableofmaintainingwafertemperaturesbelow140°Catbeampowersupto120kW.Thisimplanternowprovidesapracticalanduniquesolutionforthefabricationoflowcost,flexible,singlecrystalsiliconsolarcellswithefficiencies≥16%.
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