Professor N Cheung, U.C. BerkeleyLecture 2EE143 F20101Microstructure of Electronic MaterialsAmorphousmaterialsSingle-CrystalMaterialProfessor N Cheung, U.C. BerkeleyLecture 2EE143 F20102The Si AtomThe Si CrystalHigh-performance semiconductor devices require defect-free crystals“diamond” structureProfessor N Cheung, U.C. BerkeleyLecture 2EE143 F20103Unit cell:View in <100>directionCrystallographic PlanesView in <110>directionView in <111>directionSi lattice constant =5.431Å 5 x 1022atoms/cm3Professor N Cheung, U.C. BerkeleyLecture 2EE143 F20104Miller IndicesCrystallographic NotationNotation Interpretation( h k l )crystal plane{ h k l }equivalent planes[ h k l ]crystal direction< h k l >equivalent directionsh: inverse x-interceptk: inverse y-interceptl: inverse z-intercept(Intercept values are in multiples of the lattice constant;h, k and l are reduced to 3 integers having the same ratio.)Professor N Cheung, U.C. BerkeleyLecture 2EE143 F20105-+Top ofvalence bandBottom ofconduction bandelectronholeEnergy gap=1.12 eVCarrier Concentrations of Intrinsic (undoped) Sin (electron conc)= p (hole conc)= niProfessor N Cheung, U.C. BerkeleyLecture 2EE143 F20106Donors: P, As, SbAcceptors: B, Al, Ga, InDopants in SiBy substituting a Si atom with a special impurity atom (Column Vor Column III element), a conduction electron or hole is created.Professor N Cheung, U.C. BerkeleyLecture 2EE143 F20107n-type SemiconductorIf ND>> NA(such that ND– NA 10 ni):ADNNn ADiNNnp2andNote n >> p+=ND/cm3NA/cm3n-typeProfessor N Cheung, U.C. BerkeleyLecture 2EE143 F20108p-type SemiconductorIf NA>> ND(such that NA– ND 10 ni):Note p >> n+=ND/cm3NA/cm3p-typeDANNp DAiNNnn2andProfessor N Cheung, U.C. BerkeleyLecture 2EE143 F20109Resistivity Range of MaterialsAdding parts/billionto parts/thousandof “dopants” to pureSi can changeresistivity by8 orders ofmagnitude !Si withdopantsSiO2, Si3N4Note:1 -m = 100 -cmProfessor N Cheung, U.C. BerkeleyLecture 2EE143 F2010Principle of Monolithic Process Integration* A sequence of Additive and Subtractivesteps with lateral patterningProcessingStepsExample: CMOS Integrated CircuitSi waferProfessor N Cheung, U.C. BerkeleyLecture 2EE143 F201011Crystal PullingCrystal IngotsShaping and Polishing300 mm waferCzochralski Crystal GrowthProfessor N Cheung, U.C. BerkeleyLecture 2EE143 F201012Professor N Cheung, U.C. BerkeleyLecture 2EE143 F201013Professor N Cheung, U.C. BerkeleyLecture 2EE143 F2010Maximum impurity of starting Si wafer is equivalent to1 mg of sugar dissolved in an Olympic-size swimming pool..Maximum impurity of starting Si wafer is equivalent to1 mg of sugar dissolved in an Olympic-size swimming pool..99.999999999 % (so99.999999999 % (so--called “eleven nines” ) !!called “eleven nines” ) !!Purity of Starting IC Si WaferPurity of Starting IC Si WaferProfessor N Cheung, U.C. BerkeleyLecture 2EE143 F2010Metallurgic Grade Silicon (MG): Si 90-99%, US$ 1–2.5/kgSolar Grade Silicon (SG)*: Si 99.99–99.999%, US$ 30–40/kgElectronic Grade Silicon (EG): Si > 99.9999%, over US$ 60/kgSolar Cell Grade SiliconFor reference onlyProfessor N Cheung, U.C. BerkeleyLecture 2EE143 F201016glass platePhotolithographyPositive ResistRegion exposed to light willbe dissolved in developmentsolution.chromiumProcessing TemperatureAmbientProfessor N Cheung, U.C. BerkeleyLecture 2EE143 F2010Example : Deep UV PhotolithographySequence:(1)Surface Prime, (2) Coat, (3) Prebake,(4) Expose, (5) Post Exposure bake,(6) Develop, (7) Hard BakeProfessor N Cheung, U.C. BerkeleyLecture 2EE143 F2010Example :Deep UV Photolithography(continued)Professor N Cheung, U.C. BerkeleyLecture 2EE143 F2010Example :Deep UVPhotolithography(continued)*All bakingsub-stepsare similarbut with differenttemperature and timeProfessor N Cheung, U.C. BerkeleyLecture 2EE143 F201020EtchingIsotropic(e.g. Wet Etching)Anisotropic(e.g. Reactive Ion Etching RIE )Processing TemperatureAmbientPattern resist maskEtching thin filmEtching completedRemove resist maskProfessor N Cheung, U.C. BerkeleyLecture 2EE143 F201021SiO2SisolutionHFSiEtching SelectivityExample: HF solution etches SiO2but not Si* A high etching selectivity is usually desiredProfessor N Cheung, U.C. BerkeleyLecture 2EE143 F201022“Anisotropic “ Wet Etching of Si CrystalsEtchants : KOH or EDP (Ethylene-Diamine_Pyrocatechol)Top viewCross-sectionEffect of differentmask openingEtching stopsEtching continues(100) SisubstrateProfessor N Cheung, U.C. BerkeleyLecture 2EE143 F201023Thermal Oxidation• O2(or H2O) diffuses through SiO2and reacts with Si at the interfaceto form more SiO2.• 1 m of SiO2formed consumes0.44 m of Si substrate.• Thin oxide growth (e.g. gate oxide)- use O2. Dry oxidation• Thick oxide growth (e.g. fieldoxide) - use H2O. Wet oxidationOxide (Xox)thicknessOxidation time(t)ttSi + O2 SiO2Si + 2 H2O SiO2+ 2H2Processing Temperature900-1100oCProfessor N Cheung, U.C. BerkeleyLecture 2EE143 F201024Uneven surface topography with windowoxidationSi1stoxidation2ndoxidationSiPattern oxide windowby litho and etchSiO2SiNote uneven Si surfaceafter window oxidationSiO2SiSiO2SiSiO2Realistic topographywith 2-dimensional effectProfessor N Cheung, U.C. BerkeleyLecture 2EE143 F201025Thermal OxidizationnitrideSiSi3N4O2Sipad oxide~100 Asilicon nitrideas oxidation maskLocal Oxidation“LOCOSProcess”SiO2Professor N Cheung, U.C. BerkeleyLecture 2EE143 F201026Ion ImplantationIon Energy~1 keV to200 keVProcessing TemperatureRoom temp duringimplantation.After implantation,a 900oC-1000oCanneal step is needed to:1) activate dopants2) restore Si crystallinitytypically used to introduce dopants into semiconductorsProfessor N Cheung, U.C. BerkeleyLecture 2EE143 F201027Diffusion TasDKinTempTEnergyActivationQConstantDiffusionDeDDkTQ0• To introduce dopants into semiconductors [ Predeposition]• To spread out the dopant profile [ Drive-in]Processing Temperature850-1150oCProfessor N Cheung, U.C. BerkeleyLecture 2EE143 F201028Predeposition• Si surface concentrationmaintained at constant Cs(solid-solubility) during predep.• Dose of dopantincorporation (in #/cm2)=DtCs2Professor N Cheung, U.C. BerkeleyLecture 2EE143 F201029Predeposition and Drive-in•Drive-in meansremoving dopant supplyafter Predep step andanneal at hightemperature•Half-gaussian depthprofile after long drive-in.•Dopant dose conservedduring drive-in.•Diffusion distance DtPredep
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