1Professor N Cheung, U.C. BerkeleyLecture 7EE143 S06Ion ImplantationxBlocking maskSi+C(x)as-implantdepth profileConcentration Profile versus Depth is a single-peak functionDepth xEqual-ConcentrationcontoursReminder: During implantation, temperature is ambient. However, post-implant annealing step (>900oC) is required to anneal out defects.Reminder: During implantation, temperature is ambient. However, post-implant annealing step (>900oC) is required to anneal out defects.y2Professor N Cheung, U.C. BerkeleyLecture 7EE143 S06Advantages of Ion Implantation• Precise control of dose and depth profile• Low-temp. process (can use photoresist as mask)• Wide selection of masking materialse.g. photoresist, oxide, poly-Si, metal• Less sensitive to surface cleaning procedures• Excellent lateral dose uniformity (< 1% variation across 12” wafer)n+n+Application example: self-aligned MOSFET source/drain regionsSiO2p-SiAs+As+As+Poly Si Gate3Professor N Cheung, U.C. BerkeleyLecture 7EE143 S06Monte Carlo Simulation of 50keV Boron implanted into SiYou can download this program from http://www.srim.org/index.htm4Professor N Cheung, U.C. BerkeleyLecture 7EE143 S06[Conc] = # of atoms/cm3[dose] = # of atoms/cm2[Conc] = # of atoms/cm3[dose] = # of atoms/cm2Depth x in cmdose ()φ=∞∫Cxdx0C(x) in #/cm3(1) Range and profile shape depends on the ion energy(for a particular ion/substrate combination)(2) Height (i.e. Concentration) of profile depends on the implantation dose5Professor N Cheung, U.C. BerkeleyLecture 7EE143 S06Mask layer thickness can block ion penetrationThin maskThickMaskphotoresistSiO2 ,Si3N4 ,or othersCompleteblockingIncompleteBlockingSUBSTRATENo blocking6Professor N Cheung, U.C. BerkeleyLecture 7EE143 S06Ion Implantere.g. AsH3As+, AsH+, H+, AsH2+Magnetic Mass separationIonsourceTranslationalwafer holdermotion.As+AcceleratorColumnAccelerator Voltage: 1-200kVDose ~ 1011-1016/cm2Accuracy of dose: <0.5%Uniformity<1% for 8” wafer$3-4M/implanterion beam (stationary)waferspinning waferholder~60 wafers/hour7Professor N Cheung, U.C. BerkeleyLecture 7EE143 S068Professor N Cheung, U.C. BerkeleyLecture 7EE143 S06Eaton HE3 High Energy Implanter, showing the ion beam hitting the 300mm wafer end-station.9Professor N Cheung, U.C. BerkeleyLecture 7EE143 S06Implantation Dose[]2cm#AreaScanningBeamIontimeImplantqampsinCurrentBeamIon=×=ΦOver-scanning of beam across wafer is common. In general , Implant area > Wafer areaOver-scanning of beam across wafer is common. In general , Implant area > Wafer areaFor singly charged ions (e.g. As+)Dose10Professor N Cheung, U.C. BerkeleyLecture 7EE143 S06ASecondaryelectron effecteliminatedeFaradaycupions++VPractical Implantation Dosimetry+ bias applied to Faraday Cup to collectall secondary electrons.Cup current = Ion current* (Charge collected by integrating cup current ) / (cup area) = doseWafer holder wheelaperture for dose monitoring11Professor N Cheung, U.C. BerkeleyLecture 7EE143 S06Dose [#/area] : looking downward, how many fishper unit area for ALL depths ?Concentration [#/volume] :Looking at a particular location, how many fish per unit volume ?Meaning of Dose and Concentration12Professor N Cheung, U.C. BerkeleyLecture 7EE143 S06Ion Implantation Energy Loss MechanismsSi++SiSiee++ElectronicstoppingNuclearstoppingCrystalline Si substrate damaged by collisionElectronic excitation creates heat13Professor N Cheung, U.C. BerkeleyLecture 7EE143 S06Light ions/at higher energy more electronic stoppingHeavier ions/at lower energy more nuclear stoppingEXAMPLES Implanting into Si:Ion Energy Loss CharacteristicsH+B+As+Electronic stoppingdominatesElectronic stoppingdominatesNuclear stoppingdominates14Professor N Cheung, U.C. BerkeleyLecture 7EE143 S06Stopping MechanismsE1(keV) E2(keV)B into Si 3 17P into Si 17 140As into Si 73 80015Professor N Cheung, U.C. BerkeleyLecture 7EE143 S06SubstrateLess crystallinedamageSe > SnMore crystallinedamage at end of range Sn> SeSurfacex ~ RpA+Eo= incidentkineticenergySeE ~ 0SnE=EoSeSnDepth xSn≡ dE/dx|nSe≡ dE/dx|e16Professor N Cheung, U.C. BerkeleyLecture 7EE143 S0617Professor N Cheung, U.C. BerkeleyLecture 7EE143 S06Gaussian Approximation of One-Dimensional Implant Depth ProfileC(x)Cp0.61 Cp∆RpRpx=0Depth x()()()straggleallongitudinRrangeprojectedReCxCppR2Rxp2p2p=∆=⋅=∆−−yxUniform implantation at all lateral positionsNote: This is for no masking18Professor N Cheung, U.C. BerkeleyLecture 7EE143 S06Rp and ∆Rp values are given in tables or charts e.g. see pp. 113 of JaegerProjected Range and StraggleNote: this means 0.02 µm.19Professor N Cheung, U.C. BerkeleyLecture 7EE143 S06Rp and ∆Rp values from Monte Carlo simulation[see 143 Reader for other ions](both theoretical & expt values are well known for Si substrate)10 10 0 10 0 010 010 0 010 0 0 0∆Rp=185.34201 +6.5308 E -0.01745 E2 +2.098e-5 E3 -8.884e-9 E4Rp=51.051+32.60883 E -0.03837 E2 +3.758e-5 E3 -1.433e-8 E4∆RpRpB11 into SiProjected Range & Straggle in AngstromIon Energy E in keV20Professor N Cheung, U.C. BerkeleyLecture 7EE143 S06()()[]ppRCdxxCdxxC∆πφ⋅⋅=≈=∫∫∞+∞−∞20xppRC∆πφ⋅=∴2pR∆φ4.0≅GaussianUsing Gaussian Approximation:+∞−∞negligibleDose-Concentration Relationship Dose =Cp21Professor N Cheung, U.C. BerkeleyLecture 7EE143 S06Junction Depth, xjxC(x) xjCBxj1xj2Deep Implant()for C(x) is used : japproxGaussianIfxxC==C(x) CBShallowImplantCB= Bulk ConcentrationCpexp [ - ( xj-Rp) 2/ 2(∆Rp) 2] = CBwe can solve for xj .x22Professor N Cheung, U.C. BerkeleyLecture 7EE143 S06Sheet Resistance RSof Implanted LayersxC(x) log scalexjCBµ10171019µnTotal doping concµpp-sub (CB)n() ()[]∫−µ⋅=jx0BSdxCxCxq1RExample:n-type dopants implantedinto p-type substratex =0x =xjx•Needs numerical integrationto get Rs value23Professor N Cheung, U.C. BerkeleyLecture 7EE143 S06Approximate Value for RS()[]⇒→ ≅≅=∫RqCxdxqRqRohmsxssj1110µµφµφIf C(x) >>CBfor most depth x of interest and use approximation: µ(x) ~ constantuse the µ for the highestdoping region which carriesmost of the currentThis expression assumes ALLimplanted dopants are 100%electrically activatedor ohm/square24Professor N Cheung, U.C. BerkeleyLecture 7EE143 S06Example Calculations200 keV Phosphorus is implanted into a p-Si ( CB= 1016/cm3) with a dose of 1013/cm2. From graphs or tables , Rp =0.254 µm , ∆Rp=0.0775µm(a) Find peak concentrationCp = (0.4 x
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