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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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