Chapter 4 Ion implant

Chapter4,IonimplantationProcess,所谓“离子注入”，简单地讲，就是先使待掺杂的原子（或分子）电离，再加速到一定能量使之“注入”到晶体中，然后经过退火使杂质激活，达到掺杂的目的。,离子注入,IonImplantation,*ConcentrationProfileversusDepthisasingle-peakfunction,Reminder:Duringimplantation,temperatureisambient.However,post-implantannealingstep(900)isrequiredtoannealoutdefects.,AdvantagesofIonImplantation,PrecisecontrolofdoseanddepthprofileLcess(canusephotoresistasmask)Wideselectionofmaskingmaterialse.g.photoresist,oxide,poly-Si,metalLesssensitivetosurfacecleaningproceduresExcellentlateraldoseuniformity(Waferarea,ImplantationDose,PracticalImplantationDosimetry,Secondaryelectroneffecteliminated,*(Chargecollectedbyintegratingcupcurrent)/(cuparea)=dose,IonImplantationEnergyLossMechanisms,Nuclearstopping,Electronicstopping,a.Electronicstopping,b.Nuclearstopping,Zischargenumber，Mismass(neutronsplusprotons)。,Sn=2.810-15eVcm2(ZiZt/Z1/3)(Mi/(Mi+Mt)whereZ=(Zi2/3+Zt2/3)3/2OnceSn(E)andSe(E)areknown,thenRPcanbecalculated。,EnergyLossandIonProperties,Lightions/athigherenergymoreelectronicstoppingHeavierions/atlowerenergymorenuclearstopping,Rp=projectedrange,=longitudinalstraggle,Eo=incidentkineticenergy,StoppingMechanisms,ChooseGaussianfunctionasapproximation,GaussianApproximationofImplantProfile,Rp=projectedrange,=longitudinalstraggle,UsingGaussianApproximation:,Dose-ConcentrationRelationship,Gaussian,JunctionDepth,xj,SheetResistanceofImplantedLayers,Approximations,ThisexpressionassumesALLimplanteddopantsarefullyactivated,(1)IfC(x)CBformostxofinterestandapproximate(x)constant,usetheforthehighestdopingregionwhichcarriesmostofthecurrent,TheRPofP，AsandAn,TheRPofBandGa,ProjectedRangeandStraggle,TheRPofimpurityinSiO2,TheRPofimpurityinphotoresist,ExampleCalculations,a)Findpeakconcentration,(b)Findjunctiondepths,(c)Findsheetresistance,200keVPhosphorusisimplantedintoap-Si(CB=1016/cm3)withadoseof1013/cm2.Fromgraphsortables,Rp=0.254m,Rp=0.0775m,Fromthemobilitycurveforelectrons(usingpeakconcasimpurityconc),n=350cm2/V-sec,1.一个器件需要注入硼，峰值在0.3um处，峰值浓度为1E17cm-3，求此工艺需要的注入能量和剂量。如果衬底是N型材料，衬底浓度为1E15cm-3，求注入后的结深。,Rp=0.3um，根据图5.9可知，能量为90keV，Rp0.068um。,1)ImplantProfiledependsonlyonincidentionmomentum,NOTonchargestate.,2)Chargecarriedbyionswillbeneutralizedbychargesinthesubstrateafterimplantation.,3)n,p,Nd+,Na-chargesinsemiconductorsarecausedbythechemistryoftheimplanteddopants,andareNOTrelatedtochargescarriedbytheions.,KineticEnergyofMultipleChargedIons,acceleratingvoltage+=xkV,MolecularIonImplantation,Molecularionwilldissociateimmediatelyintoatomiccomponentsafterenteringasolid.,Allatomiccomponentswillhavesamevelocityafterdissociation?.,Proof,Bindingenergyofmolecule(severaleV)isnegligiblecomparedwithimplantationenergy(manykeV),Theonlywaytosatisfyboth1and2is:vBF2=vB=vF,MolecularImplantationforShallowJunctions,ForconventionalbeamlineimplantersBeamcurrentIasacceleratorvoltage,ImplantationDamage,(1)RestoreSicrystallinity.,(2)PutdopantsintoSisubstitutionalsitesforelectricalactivation.,退火1)目的:a)消除晶格损伤;b)激活注入杂质.2)退火条件:离子注入时，由碰撞引起能量传递，当传递的能量大于晶格结合能时，晶格就会受损伤。存在一个阈值剂量th.Dth时，晶格完全损伤，衬底表面呈无定型状态。th值的大小与注入能量，注入物质，靶材料及注入过程中的衬底温度有关。,Interstitial-Vacancyrecombination,：Thecarrierconc.isdominatedbypointdefects.T=pointdefects=carrierconc.。：500-600C,thediffusivityofthedefectisraisedtocauseagglomerationandformationofextendeddefects.Athigherboronthedefectdamageinfluenceismorelager。：Athightemperature，theextendeddefectsareannealedoutandactivecarrierconc.approachestheimplantdose。,a)AnnelingofB,DopantActivation,*SheetReistanceislimitedbydopantsolidsolubility*ShallowerjunctionswillhavehigherRs,IonChanneling,“LuckyIons”,Scatteredionsfallintootherchannelingdirections:deeperpenetration,PreventionofChannelingbyimplantingway,Atypicaltiltangleis7。andatwistangleofabout30。whenthewaferisimplanted.,PreventionofChannelingbyPre-amorphization,Step1,HighdoseSi+implantationtocovertsurfacelayerintoamorphousSi,Step2,Implantationofdesireddopantintoamorphoussurfacelayer,Disadvantage:Oneadditionalhigh-doseimplantationstep,浅结的形成困难的原因及办法在亚微米CMOS器件中，源/漏要求浅结。硼的扩散速率比砷快，形成浅的P+N结有困难。注入以后快速热退火是常用的技术之一。用硼制作浅结的困难有：硼很轻，注入时的RP大，一般不希望使入射能量小於10kev,采用分子注入（BF2)的办法解决；沟道效应：硼被偏转注入时，进入主晶轴的概率较高-产生沟道效应，解决的办法是：注硼前先注较重的原子使衬底非晶化，流行注锗；高温退火时出现异常扩散，即有退火瞬态效应，拖尾区最为明显。假如MOS器件的特征尺寸为0.1m，那么源/漏的结深接近0.05m，注入能量再低也有困难。常