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Berkeley ELENG 143 - Lecture Notes

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1Professor N Cheung, U.C. BerkeleyLecture 8EE143 F051) Implant Profile depends only on incident ion momentum, NOT on charge stateA+AA-Same implantprofileSamemomentumNote : Kinetic Energy = (momentum)2/2M2) Charge carried by ions will be neutralized by charges in the substrate after implantation. 3) n, p, Nd+, Na- charges in semiconductors are caused by the chemistry of the implanted dopants, and are NOT related to charges carried by the ions.2Professor N Cheung, U.C. BerkeleyLecture 8EE143 F05Kinetic Energy of Multiple Charged Ionsaccelerating voltage = x kVB+P+As+Kinetic Energy = x · keVB+++B++Kinetic Energy = 2x · keVKinetic Energy = 3x · keVSinglychargedDoublychargedTriplycharged3Professor N Cheung, U.C. BerkeleyLecture 8EE143 F05Molecular Ion ImplantationBF2+Kinetic Energy = x keVBFFB has 11 amuF has 19 amuaccelerating voltage= x kV-+%2019191111....21..21..222=++≈⋅=⋅===+BFofEKBofEKvmFofEKvmBofEKvvvVelocityBFBBFFBMolecular ion will dissociate immediatelyinto atomic components after entering a solid.All atomic componentswill have same velocityafter dissociation.SolidSurface4Professor N Cheung, U.C. BerkeleyLecture 8EE143 F05FBFBFFSame velocity V = VBF(move together as a molecule)V = VFB''afterAll Atomic Components have same Velocity…Binding energy of molecule (~ several eV) is negligible compared with implantation energy (many keV). 12mBvB2 + 2(12mFvF2) = 12mBv′B2 + 2(12mFv′F2) [1] (mB + 2mF)vB = mBv'B + 2mFv'F [2] The only way to satisfy both [1] and [2] is : v′B = v′F = vB = vF.5Professor N Cheung, U.C. BerkeleyLecture 8EE143 F05For conventional beamline implantersBeam current I ↓ as accelerator voltage ↓B+I (B+) ↓ as voltage < 5kVBF2+I (BF2+) can still be high with 25kV accelerating voltage but the effective B implantation energy is ~5 keVMolecular Implantation for Shallow Junctions* For ultra-shallow junction which needs ~1keV B+ energy,B10H14(+) at ~10keV is proposed6Professor N Cheung, U.C. BerkeleyLecture 8EE143 F05(1) Restore Si crystallinity.(2) Put dopants into Si substitutional sitesfor electrical activationAfter implantation, we need an annealing step.A typical ~900oC, 30min will:Implantation Damage7Professor N Cheung, U.C. BerkeleyLecture 8EE143 F05Implantation Damage8Professor N Cheung, U.C. BerkeleyLecture 8EE143 F05Amount and type of Crystalline Damage9Professor N Cheung, U.C. BerkeleyLecture 8EE143 F05Solid Epitaxial “Growth” through the Implant Damaged Region10Professor N Cheung, U.C. BerkeleyLecture 8EE143 F05Solid Epitaxial “Growth” through the Implant Damaged Region – cont.11Professor N Cheung, U.C. BerkeleyLecture 8EE143 F05Interstitial-Vacancy recombinationAgglomerationof secondary defectsSecondary defectsAnnealed outDopant Activation Versus Annealing Temp12Professor N Cheung, U.C. BerkeleyLecture 8EE143 F05* Sheet Resistance is limited by dopant solid solubility* Shallower junctions will have higher RSDopant Activation13Professor N Cheung, U.C. BerkeleyLecture 8EE143 F05Ion ChannelingTo minimize channeling, we tilt wafer by 7owith respect to ion beam.To minimize channeling, we tilt wafer by 7owith respect to ion beam.random scattering pathdeeper penetrationSi CrystalRandom componentchanneledcomponentC(x)xAxial ChannelingPlanar ChannelingRandom14Professor N Cheung, U.C. BerkeleyLecture 8EE143 F05“Lucky Ions”xC(x) xjCBWithchanneling tail xj’Some scattered ions fall into other channeling directions, causing deeper penetrationSome scattered ions fall into other channeling directions, causing deeper penetration7oNoChanneling15Professor N Cheung, U.C. BerkeleyLecture 8EE143 F05Si+1 E15/cm2Si crystalDisadvantage : Needs an additional high-dose implantation stepDisadvantage : Needs an additional high-dose implantation stepB+Si crystalAmorphous SiStep 1High dose Si+implantation to covertsurface layer intoamorphous SiStep 2Implantation ofdesired dopantinto amorphous surface layerPrevention of Channeling by Pre-amorphization16Professor N Cheung, U.C. BerkeleyLecture 8EE143 F05Transverse (or Lateral) Straggle (∆Rtor ∆ R⊥) ∆Rt∆Rt∆Rp>1∆Rt∆Rp17Professor N Cheung, U.C. BerkeleyLecture 8EE143 F05yMaskC(y) at x=Rpx = RpImplanted speciehas lateral distribution,larger than mask opening Implanted speciehas lateral distribution,larger than mask opening xyHigher concentrationLowerconcentrationLateral Scattering Causes Feature Enlargement18Professor N Cheung, U.C. BerkeleyLecture 8EE143 F05() ()()()()∆+−∆−⋅⋅=⋅⋅=∫+−∆−−ttaaRyyRayerfcRayerfcxCKdyexCKyxCt22,222'masky=-ay=+axA 2-D formulation of Implantation Profile19Professor N Cheung, U.C. BerkeleyLecture 8EE143 F05()() ()( ) () () ()[]() ()For a i e no maskCxy CxC x y C x K erfc erfcCx Cx KK→∞=∴=⋅⋅ −∞-+∞=⋅⋅∴=..,,212A 2-D formulation of Implantaion Profile – cont.20Professor N Cheung, U.C. BerkeleyLecture 8EE143 F05(details of erfcfunction alsocovered in Diffusion sectionof EE143 Reader)[Curves also given in Jaeger]Semi-log Plotsof Gaussian and erfc functions21Professor N Cheung, U.C. BerkeleyLecture 8EE143 F05Transmission Factor of Implantation MaskWhat fraction of dose gets into Si substrate?x=0 x=dC(x)Mask material (e.g. photoresist)Si substrateC(x)Mask material with d=∞x=0 x=d-22Professor N Cheung, U.C. BerkeleyLecture 8EE143 F05Transmitted Fraction() ()()()()TCxdxCxdxerfcdRRerfc x e dyCx dCx Rdppyxp=−=−=−==∫∫∫∞−−000412212102∆πRule of thumb Good masking thickness:dR Rpp=+43∆.~are values of for ions intothe masking


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