Sputter磁控濺射概述

NANO-MASTER,INC.NANO-MASTERSputter磁控濺射沉積技術(shù)介紹吳運(yùn)祥2017年8月7日

NANO-MASTER,INC.概述Sputterdepositionisawidelyusedtechniquetodepositthin?lmsonsubstrates.Thetechniqueisbasedonionbombardmentofasourcematerial,thetarget.Ionbombardmentresultsinavaporduetoapurelyphysicalprocess,i.e.thesputteringofthetargetmaterial.Hence,thistechniqueispartoftheclassofphysicalvapordepositiontechniques,whichincludesthermalevaporationandpulsedlaserdeposition.Themostcommonapproachforgrowingthin?lmsbysputterdepositionistheuseofamagnetronsourceinwhichpositiveionspresentintheplasmaofamagneticallyenhancedglowdischargebombardthetarget.Thetargetcanbepoweredindifferentways,rangingfromdirectcurrent(DC)forconductivetargetstoradiofrequency(RF)fornon-conductivetargets,toavarietyofdifferentwaysofapplyingcurrentand/orvoltagepulsestothetarget.

NANO-MASTER,INC.概述Sincesputteringisapurelyphysicalprocess,addingchemistryto,forexample,depositacompoundlayermustbedoneadhocthroughtheadditionofareactivegastotheplasma,i.e.reactivesputtering.Theundesirablereactionofthereactivegaswiththetargetmaterialresultsinanon-linearbehaviorofthedepositionparametersasafunctionofthereactivegas?ow.Tomodelthisbehavior,the?uxesofthevariousspeciestowardthetargetmustbedetermined.However,equallyimportantarethe?uxesofspeciesincidentatthesubstratebecausetheynotonlyin?uencethereactivesputterdepositionprocess,butalsocontrolthegrowthofthedesired?lm.Indeed,themicrostructureofmagnetronsputter-deposited?lmsisde?nedbytheidentityoftheparticlesarrivingatthesubstrate,their?uxes,andtheenergyperparticle.

NANO-MASTER,INC.濺射沉積有多流行？Onewaytocomparesputterdepositionwithotherdepositiontechniquesistocounttherelativenumberofscienti?cpublicationsandpatentspublishedeachyearforeachdepositiontechnique.Inordertoprovideabaselinefortherateofincreaseinpublicationsingeneral,we?rstdeterminethenumberofpublicationsperyearthatrefertothecombinationofkeywords‘gold’OR‘silver’OR‘copper’ontheWebofSciences.Similarly,abaselineforpublishedpatentsiseasilyfoundbycountingthenumberofpatentspublishedannuallybyenteringthesearchterm‘the’intheDelphionDatabase.Figure5.1(a)isplotoftherelativenumberofscienti?cpapersperyearperdepositiontechnique.Figure5.1(b)providesthesameinformationforpatents.Forthedifferentphysicalvapordeposition(PVD)techniques,magnetronsputteringisclearlyusedextensivelyinthescienti?ccommunity,andcompeteswithpulsedlaserdeposition(PLD)asthemostimportantdepositiontechnique.Fromthesearchinthepatentdatabase,itcanbeconcludedthatsputterdepositionisstillthemostpopulartechnique.

NANO-MASTER,INC.濺射沉積有多流行？

NANO-MASTER,INC.濺射沉積有多流行？Thistechniqueisappliedinbothresearchlaboratoriesandindustrialplantstodepositawidevarietyofmaterials.Wefocusonkeyaspectsofsputterdeposition.Describingthephysicsbehindthesputterprocess,i.e.theinteractionbetweentheionandthetarget,isa?rstpriority.However,sincemanyreviewarticlesareavailable,onlytheessentialpointsarediscussedhere.Then,abasicsystemdesignisdescribed.Itshouldbenotedthatweexcludeionbeamsources,whichhavebeenwellreviewedintheliterature,owingtolimitationsassociatedwithscalabilityandpowersupplyoptions.

NANO-MASTER,INC.濺射沉積有多流行？Sputterdepositionisalsousedtodepositcompound?lmsbyaddingareactivegastothedischarge.This,however,greatlyincreasesthecomplexityofthedepositionprocess,andexplainstheongoinginterestinacademiatoinvestigatethistechniqueofwhichseveralaspectsarenotcompletelyunderstood.Sputteredtargetatomsareejectedwithsubstantialkineticenergy,oftheorderoforlargerthanbondenergies,andhencecansigni?cantlyaffect?lmgrowthkineticsandmicrostructure.Thus,energylossmechanismsduringtransportinthegasphaseareimportant.

NANO-MASTER,INC.什么是濺射？Sputteringistheejectionofatomsbythebombardmentofasolidorliquidtargetbyenergeticparticles,mostlyions.Itresultsfromcollisionsbetweentheincidentenergeticparticles,and/orresultantrecoilatoms,withsurfaceatoms.AmeasureoftheremovalrateofsurfaceatomsisthesputteryieldY,de?nedastheratiobetweenthenumberofsputter-ejectedatomsandthenumberofincidentprojectiles.Excellentreviewarticlesonsputteringareavailableintheliterature,andonlytheessentialfeaturesarediscussedhere.Basedonthelargeamountofexperimentalandcalculateddataasafunctionofionandtargetmaterial,severaltrendsareapparent.Foragivenionmassandtarget,Yexhibitsamaximumasafunctionofionenergyaswellasaminimum(threshold)energy.AnexampleisshowninFigure5.2forAr+bombardmentofCu.

NANO-MASTER,INC.什么是濺射？

NANO-MASTER,INC.什么是濺射？Whencomparingthesputteryieldoftargetmaterialsbombardedbyagivenionatconstantenergy,onenoticesatrendrelatedtothepositionoftheelementintheperiodictable(seeFigure5.3andthefollowingdiscussion).Severalauthorshavederivedequationsdescribingthesputteryieldasafunctionofenergyandprojectile–targetcombinations.P.Sigmundisthefatherofthesetheories.Hiswork‘TheoryofsputteringI.Sputteryieldofamorphousandpolycrystallinetargets’,publishedinPhysicalReview,isabenchmarkinthis?eld.AccordingtothetheoryofSigmund,thesputteryieldnearthreshold,i.e.atlowionenergy,isgivenby

NANO-MASTER,INC.什么是濺射？

NANO-MASTER,INC.什么是濺射？withEtheenergyoftheprojectile,andM1andM2themassesoftheprojectileandthetargetatom(inamu).Usisthesurfacebindingenergyandαadimensionlessparameterdependingonthemassratioandtheionenergy.Atlowenergy,andmassratiosM2/M1lowerthan1,αisoftheorderof0.2.Thisequationcanbeunderstoodasfollows.Anincomingiontransfersitsmomentumtothetargetatomswhichexplainstheterm4M1M2/(M1+M2)2withamaximumwhenM1=M2.Tosputteranatomfromthetarget,momentumtransferfromtheion-inducedcollisionmustovercomethesurfacebarrier,givenbythesurfacebindingenergyUs.

NANO-MASTER,INC.什么是濺射？Therefore,wecanexpectaninverselyproportionalrelationshipbetweentheyieldandthesurfacebindingenergy.BasedonEq.(5.1),wecanexpectthatfortheenergyrangeofinterestforsputterdeposition,thesputteryieldwillvarylinearlywiththeionenergy(seeFigure5.2).ThebehaviorofthesputteryieldovertheperiodictablecanalsobeunderstoodfromEq.(5.1),becausethesputteryieldisde?nedbymomentumtransferandsurfacebindingenergy.However,differencesinatomicdensityamongdifferentmaterialsalsoaffectYthroughvariationsintherange(depth)ofmomentumtransfer.

NANO-MASTER,INC.什么是濺射？InadditiontothetheoryofSigmund,heuristicapproachesbasedonsemi-empiricalequations,andsimulations(foranoverviewonsputteryieldsimulations)arealsoavailable.Commonlyusedsemi-empiricalformulaeforthecalculationofthesputteryieldweredevelopedbyYamamuraetal.Theequationsarevalidforthebombardmentofmonoatomicsolidsbyprojectilesatnormalincident,andthesputteryieldY(E)isgivenbywithE,M1,M2,andUsasde?nedforEq.(5.1).Us,thesurfacebindingenergy,isintimatelyconnectedwith,andexplainsthepresenceofthethresholdenergy(Eth)forsputtering(Table5.1).Theothersymbolsarede?nedinTable5.1.

NANO-MASTER,INC.什么是濺射？

NANO-MASTER,INC.什么是濺射？AlthoughtheseequationsprovideavalueforthesputteryieldYasafunctionofionenergyandmaterialchoice,theyarenotinstructiveinexplainingthesputteringprocessindetail.Hence,someauthorshavedevelopedsimpler,andmoretransparent,modelsofthesputteringprocess.AnexcellentexampleistheworkofMahanetal.,inwhichthesputteryieldY(E)isderivedbasedonthefollowingassumptions.Theeffectivenumberofrecoilingtargetatomscreatedperincidentionismultipliedbytheprobabilitythattherecoiliscloseenoughtothesurfacetoescapeandbytheprobabilitythattherecoilsaretravelingtowardthesurface,or

NANO-MASTER,INC.什么是濺射？withEtheprojectileenergyandEavgtheaverageenergyoftherecoils.TheratioE/Eavggivestheaveragenumberofrecoils.TheratiobetweentheprojectedrangeoftherecoilsRprandtheprojectedrangeoftheprojectileRppgivestheprobabilitythattherecoilsarecloseenoughtothesurfacetoescape.Finally,theterm{1/4}istheaverageprobabilitythattherecoilsaremovingtowardthesurface.Usingapproximations,theaveragerecoilenergyandtheprojectedrangecanbecalculatedstraightforwardlygivinginsightintothesputteringprocess.Agoodexampleofthisisthecalculationofthethresholdenergy.Thephysicsbehindthethresholdenergyisthattherecoilatomhasinsuf?cientenergytoovercomethesurfaceenergybarrierUswhenitsaverageenergyEavgisequaltoorlowerthanthesurfacebarrierenergy.Theaveragerecoilenergyinthissimpli?edmodeliscalculatedas

NANO-MASTER,INC.什么是濺射？andthethresholdenergyE=EthisthereforefoundbysubstitutingUsforEavg,andobtainingEth=2.72Us/γ,whichissimilartotheformulaeproposedbyYamamuraetal.(seeTable5.1).Anotherapproachtoobtainavalueforthesputteryieldistosimulatetheoverallsputteringprocess.Themostcommonlyusedsimulationpackageisthewell-knownSRIMcodedevelopedbyZiegleretal.andcompletelydescribedintheirbook.Figures5.2and5.3weresimulatedusingthiscode.Allfeaturesdiscussedinthepreviousparagraphsarepresentinboth?gures.SRIMisastaticsimulationcode.Thatis,itdoesnotaccountfortargetchangesduetotheionbombardmentitself.Toaccountforthesechanges,Molleretal.developedadynamicversionofSRIM,i.e.TRIDYN.

NANO-MASTER,INC.什么是濺射？Allapproachesalsoprovidetheangularandenergydistributionofthesputteredparticles.Thompsonshowedthatwithinacertain(low)ionenergyrange,theenergydistributionfollowstheexpressionwhichisbasedontheassumptionofaplanarsurfacebarrierforsputteredparticles.ThisexpressiongivesapeakatUs/2.FalconeestimatedtheaverageenergyofsputteredparticlesēasSubstitutingreasonablevaluesintoEq.(5.6)showsthattheenergyofthesputteredparticlesisatleastoneorderofmagnitudehigherthanthecorrespondingthermalevaporationenergyforthesameparticle?ux(Figure5.4).

NANO-MASTER,INC.什么是濺射？Indeed,at1000Kthethermalenergyisonlyoftheorderof0.1eV.WithasurfacebindingenergyofafeweV(forCuitis3.5eV),themaximumintheenergydistributionoccursat1.8eV,andusingtheSRIMcalculatedthresholdenergy,theaverageenergyis15.1eV.Ifthepressureduringdepositionislowenough,thesputteredparticlesinthegasphaseareballisticandcanreachthesubstratewithfewornocollisionsinthegasphase.Inallofthiswork,oneimportantconceptisnotaddressedatall,i.e.thesputteryieldofcompoundmaterials.TheYamamuraformulaecanbeappliedtomulticomponentmaterialssuchascompoundsandalloysusingweightedaveragevaluesforZ2,M2,andUs.

NANO-MASTER,INC.什么是濺射？

NANO-MASTER,INC.什么是濺射？Surfacebindingenergies,inparticular,aredif?culttoobtainforoxidesandnitrides.Formetals,thesurfacebindingenergyisgenerallysetequaltothevaporizationenthalpy,butthisapproachisnotapplicabletooxides,nitrides,sul?des,etc.Inthecontextofthepreferentialsputteringofoxygenfromoxidesduringdepthpro?lingofoxidethin?lmsforanalyticalapproaches,modelshavebeenproposedtoestimateavalueofthesurfacebindingenergyofthemetalandtheoxygenatoms.Usingthesevalues,themodi?cationofthesurfacecompositionbyionbombardmentiscalculatedandcomparedwithexperimentalvalues.Resultstypicalforthiskindofstudy,includingamodelforthesurfacebindingenergy,arepublishedbyMalherbeetal.

NANO-MASTER,INC.什么是濺射？Somesimulationcodes,e.g.TRIDYN,useadifferentapproach,wheretheeffectivesurfacebindingenergiesofOandMarechosentobedependentontheactualsurfacecompositionbyuseofamatrixmethod.ThematrixelementsofsurfacebindingenergiesareSBVO–O,SBVO–M,SBVM–O,andSBVM–M.TheseelementsareevaluatedbytheformulaewherenandmdependonthestoichiometryoftheoxideMnOm.Us,Misthemetalsurfacebindingenergy,ΔHfdenotestheformationenthalpypermoleculeofthecompound,and

ΔHdissdenotesthedissociationenergyoftheoxygenmolecule.

NANO-MASTER,INC.什么是濺射？IftheconcentrationsofOandMatthesurfaceareCOandCM,respectively,weobtainthesurfacebindingenergyofMandOinthefollowingway:

NANO-MASTER,INC.高能粒子如何產(chǎn)生？Sputteringisinitiatedbythebombardmentofenergeticparticlesatthetarget.Theseenergeticparticlesaregenerallyions.Twoapproachescanbefollowedtoproduceionsandsputterthetargetmaterials.The?rstisquitestraightforwardbyusinganionsourcewhichisaimedtowardthetarget.Collectingthesputteredparticlesonasubstrateenablesthedepositionofathin?lm.However,ionbeamsputteringisnotwidelyusedforindustriallarge-scaleapplications.Ionsgunsaremoreoftenutilizedinsurfaceanalyticaltechniquessuchassecondaryionmassspectrometry(SIMS)ortobombardthesubstrateduringthin?lmdeposition.

NANO-MASTER,INC.高能粒子如何產(chǎn)生？Anothersourceofionsisaplasma.Byapplyingahighnegativevoltagetothecathode,i.e.thetarget,positivelychargedionsareattractedfromtheplasmatowardthetarget.Theionsgainenergyintheelectric?eldandbombardthetargetwithsuf?cientenergytoinitiatesputtering.Sputteringwas?rstdiscussedintheliteraturebyW.R.Grovein1852,whousedthiskindofset-up.Agoodstartingpointtodiscussplasma-basedsputterdepositionisusingthesimplestexperimentalarrangement.Thatis,acathodeandananodearepositionedopposedtoeachotherinavacuumchamber.Typically,thevacuumchamberispumpedbyacombinationofturbomolecularandrotarypumps,althoughadiffusionpumpisstilloftenused.Afterpumpingtoabasepressureoftheorderof1×10?4Paorlower,anoblegas(usuallyargon)isintroducedintothevacuumchamber,reachingapressurebetween1and10Pa.

NANO-MASTER,INC.高能粒子如何產(chǎn)生？Whenahighvoltagedifferenceintherangeof2000Visappliedbetweencathodeandanode,aglowdischargeisignited.Itisnotinthescopeofthischapteronsputterdepositiontodescribealldetailsrelatedtoaglowdischarge,butdiscussingafewcanbeinstructive.Tode?neaglowdischarge,onecanfollowthecurrent–voltage(I–V)characteristics.Itisimportanttorealizethatthesecharacteristicsdependalsoonthepressureandtheseparationbetweencathodeandanode.Themaincharacteristicsofthedischarge,suchasbreakdownvoltage,I–Vcharacteristics,andstructureofthedischarge,dependonthegeometryoftheelectrodes(cathodeandanode)andvacuumvessel,thegas(es)used,andtheelectrodematerial.TheI–VcharacteristicsofsuchadischargeareillustratedinFigure5.5forawiderangeofcurrents.

NANO-MASTER,INC.高能粒子如何產(chǎn)生？

NANO-MASTER,INC.高能粒子如何產(chǎn)生？Threegeneralregionscanbeidenti?edinthe?gure,thedarkdischargeregion,theglowdischarge,andthearcdischarge.TheelectriccircuitofthedischargegapalsoincludesanexternalohmicresistanceR.Inthiscase,Ohm’slawforthecircuitcanbewrittenaswhereEMFistheelectromotiveforceandVisthevoltageofthegasdischarge.Equation(5.9)isusuallyreferredtoastheloadline,andisalsoshowninFigure5.5.IntersectionoftheI–Vcharacteristicandtheloadlinegivestheactualvalueofcurrentandvoltageinadischarge.Byadjustingtheballastresistorinthecircuitdiagram,wecansweepoutanI–Vcharacteristicthatishighlynon-linearandshowsthethreegeneralregions.Eachoftheseregionsencompassesmanyinterestingphenomena.

NANO-MASTER,INC.高能粒子如何產(chǎn)生？Letus?rstfocusonthedarkdischargeregime,betweenAandEinFigure5.5.Thenamereferstothefactthatthedischargeremainsinvisibletotheeye,i.e.thereisnovisiblelightemittedexceptforthecoronadischargeandthebreakdownitself.ThechangeintheI–Vcharacteristiccanbeunderstoodfromadescriptionoftheresponsiblephysicalprocesses.BetweenAandB,theionsandelectronsformedbythebackgroundionizationmovetowardtheelectrodesduetotheappliedelectrical?eld,producingaweakelectricalcurrent.Increasingtheappliedvoltageresultsinabettercollectionef?ciency,i.e.alargerfractionoftheproducedionsandelectronswillreachtheelectrodes.Atasuf?cienthighvoltage,thecurrentwillsaturatebecauseallproducedelectronsandionsreachtheelectrodes.Hence,intheregionbetweenBandC,thecurrentremainsconstantwithincreasingvoltage.

NANO-MASTER,INC.高能粒子如何產(chǎn)生？Someradiationcounters,e.g.aGeiger–Mullercounter,makeuseofthefactthatthemeasuredcurrentwilldependlinearlyonthestrengthoftheradiationsource.Whenthevoltageacrossthelow-pressuredischargetubeisincreasedfurther,onenoticesastrongincreaseincurrent(seeregionsC–E).Hence,moreelectronsandionsmustbeproduced.Theoriginofthecurrentincreaseisfoundintheimpactionizationofatomsbytheoriginalelectronsacceleratedacrosstheelectric?eld.Hence,anavalancheofelectronandionproductionwillfollow,leadingtoastrongincreaseincurrent.ThisregioniscalledtheTownsenddischarge.Coronadischarges(D–E)occurinTownsenddarkdischarges,priortoelectricalbreakdown,inregionsofhighelectric?eldnearsharppoints,edges,orwires.Ifthecoronacurrentsarehighenough,coronadischargesaretechnically‘glowdischarges’andvisibletotheeye.

NANO-MASTER,INC.高能粒子如何產(chǎn)生？Forlowcurrents,theentirecoronaisdark.Coronadischargesareoftenappliedtotreatthesurfaceofpolymersandtorenderthesurfacemore‘a(chǎn)ctive’bybreakingatomicbonds.Finally,atsuf?cientlyhighelectrical?elds,breakdownwilloccurduetoadditionofsecondaryelectronsemittedfromthecathodeasaresultofionandphotonimpact(seebelow).Atthebreakdownpotential(pointE),thecurrentmayincreasesigni?cantly,andisusuallylimitedbytheinternalresistanceofthepowersupplyconnectedbetweentheplates.Iftheinternalresistanceofthepowersupplyisveryhigh,thedischargetubecannotdrawenoughcurrenttobreakdownthegas,andthetubewillremaininthecoronaregimewithsmallcoronapoints(brushdischarges)presentontheelectrodes.Iftheinternalresistanceislower,thenthegaswillbreakdownattheindicatedvoltageandmoveintothenormaldischargeregime(regionF–G).

NANO-MASTER,INC.高能粒子如何產(chǎn)生？ThebreakdownvoltageforaparticulargasandelectrodematerialdependsontheproductofthepressurePandthedistancedbetweentheelectrodes,asexpressedinPaschen’slaw:withAandBconstants,andγtheelectronemissionyieldinducedbyphotonandionbombardment.TheconstantsAandBdependonthechosengasandde?netheTownsendionizationcoef?cient.Thislattercoef?cientgivestheelectronproductionperunitlength,orthemultiplicationoftheelectronsperunitlengthalongtheelectric?eld.

NANO-MASTER,INC.高能粒子如何產(chǎn)生？Incontrasttothedarkdischargeregime,theplasmaintheregionF–Gisluminousinthevisible,andhenceitiscalledaglowdischarge.Theexcitationofthegasatomsbyelectronimpactformstheoriginofthegasglow.Theplasmadensityisnowsuf?cientlyhighthattheelectric?eldbetweentheelectrodesbecomesdistortedfromitsoriginalcon?gurationintheTownsenddischarge.Alongthedischarge,onecannotice,especiallyatlowpressure,asequenceofdarkandbrightlayers.Theselayershavespecialnames.ClosetothecathodeisadarklayerknownastheAstondarkspace,followedbyathinlayerofthecathodeglow.Thisbrightlayeristhenfollowedbythecathodedarkspace.Sharplyseparatedfromthislatterregionisthenegativeglow.Theluminosityofthenegativeglowdecreasestowardtheanode,becomingtheFaradaydarkspace.AftertheFaradaydarkspaceisthepositivecolumn.

NANO-MASTER,INC.高能粒子如何產(chǎn)生？Attheanodeside,thepositivecolumngoesoverintotheanodedarkspacefollowedbyanarrowanodeglow.AschematicoverviewisshowninFigure5.6.Mostofthevoltagedropbetweencathodeandanodeoccursbetweenthecathodeandthenegativeglow.Thelengthofthecathodefallregionor‘thedarkspace’fromthecathodetotheboundaryofthenegativeglowistypicallyafewcentimeters.So,withinthisregionmostpowerisdissipatedandonenoticesastrongvoltagedrop.Thevoltagedropsoveradistancewhichisgenerallynotexactlyequaltothewidthofthedarkspace.Todistinguishbetweenthetworegions,onereferstothe?rstregionasthecathodesheath.Whentheglowdischargecoversonlyapartofthecathode,thedischargeisinthenormalglowdischargemode.

NANO-MASTER,INC.高能粒子如何產(chǎn)生？

NANO-MASTER,INC.高能粒子如何產(chǎn)生？Inthisregime,thecurrentdensityattheelectrodesisindependentofthedischargevoltageandhencebyincreasingthecurrent,thepartcoveredbytheplasmaincreasesatconstantdischargevoltage(seeregionF–G).FrompointGon,theplasmacompletelycoversthecathodesurfaceandwithincreasingdischargecurrent,thedischargevoltageincreases.Inthisregime,theabnormaldischargeregion,sputterdepositionistypicallyperformed.AtpointH,theelectrodesbecomesuf?cientlyhotthatthecathodenowemitselectronsthermionically.IftheDCpowersupplyhasasuf?cientlylowinternalresistance,thedischargewillundergoatransitionfromglowtoarc.Astheenergyofthearrivingfastneutralsandionsde?nesthesputteryield,itisinterestingtostudythetypicalionenergydistributioninaglowdischarge.

NANO-MASTER,INC.高能粒子如何產(chǎn)生？Indeed,theerosionspeedofthetargetwillbede?nedbythe?uxofionsandfastneutralsbombardingthecathodeandthesputteryieldwhichdependsontheenergyandmassofthesespecies.AtthetypicalpressuresofaDCargonglowdischarge,theAr+ionshaveasmallmeanfreepath.Thismeansthatthedistancetheiontravelsbeforeitmakesacollision,oritceasestobeanion,isshort.Theshortmeanfreepathisdueprimarilytothefollowingeffect.WhenanAr+ionpassescloseenough(afew?)toanAratom,quantummechanicaltunnelingoccurs.So,theionbecomesagainanatom,whiletheatomfromwhichtheelectronisremovednowbecomesanion.Thisprocessiscalledasymmetricchargeexchangecollisionbecausetheresultisstillanionandanatom.

NANO-MASTER,INC.高能粒子如何產(chǎn)生？However,theionwhichhasalreadygainedenergybyaccelerationintheelectric?eldwithinthecathoderegionbecomesaneutralAratomwiththesameenergy,whiletheatombecomesanionwiththelowenergyoftheatom.Thisnewlycreatedionisnowacceleratedbythevoltagegradientinthecathoderegion.ThefastneutralAratomcannotgainenergyanymore,andowingtothehighpressurewillloseenergybycollisionwithotheratoms.So,evenwiththestrongelectric?eld,de?nedbythehighdischargevoltageandthecathodesheaththickness,thenumberofhighenergyionsarrivi