EE 143: Microfabrication TechnologyLecture Module 3:Film DepositionCTN 2/11/10Copyright @ 2010 Regents of the University of California at BerkeleyEE 143: Microfabrication Technology LecM 3 C. Nguyen 2/14/10 1EE 143Microfabrication TechnologySpring 2010Prof. Clark T.-C. NguyenDept. of Electrical Engineering & Computer SciencesUniversity of California at BerkeleyBerkeley, CA 94720Lecture Module 3: Film DepositionEE 143: Microfabrication Technology LecM 3 C. Nguyen 2/14/10 2Thin Film DepositionEE 143: Microfabrication TechnologyLecture Module 3:Film DepositionCTN 2/11/10Copyright @ 2010 Regents of the University of California at BerkeleyEE 143: Microfabrication Technology LecM 3 C. Nguyen 2/14/10 3Thin Film Deposition• Methods for film deposition:ª Evaporationª Sputter depositionª Chemical vapor deposition (CVD)ª Plasma enhanced chemical vapor deposition (PECVD)ª Epitaxyª Electroplatingª Atomic layer deposition (ALD)Evaporation:• Heat a metal (Al,Au) to the point of vaporization• Evaporate to form a thin film covering the surface of the Si wafer• Done under vacuum for better control of film compositionEE 143: Microfabrication Technology LecM 3 C. Nguyen 2/14/10 4Evaporation2Pd 2kT path freemean πλ==Filament Evaporation System: VacuumPumpwafer+-W filamentAl staplesk = Boltzmann ConstantT = temperatureP = pressured = diameter of gas molecule1. Pump down to vacuum → reduces film contamination and allows better thickness control2. Heat W filament → melt Al, wet filament3. Raise temperature →evaporate AlEE 143: Microfabrication TechnologyLecture Module 3:Film DepositionCTN 2/11/10Copyright @ 2010 Regents of the University of California at BerkeleyEE 143: Microfabrication Technology LecM 3 C. Nguyen 2/14/10 5Evaporation (cont.)• λ can be ~60m for a 4Å particle at 10-4Pa (-0.75 μTorr)ª thus, at 0.75 μTorr, get straight line path from Al staple filament to waferProblem: Shadowing & Step CoverageGet an openSourceSourceProblem: line of sight depositionSolns: i. Rotate water during evaporationii. Etch more gradual sidewallsBetter Solution: forget evaporation → sputter deposit the film!EE 143: Microfabrication Technology LecM 3 C. Nguyen 2/14/10 6Sputter Deposition• Use an energetic plasma to dislodge atoms from a material target, allowing the atoms to settle on the wafer surface VacuumPumpwaferNot as low a vacuum as evaporation (~100 Pa)(750 mTorr)Ar+Ar+plasmaTarget (Al, SiO2, Si2N4, ZnO, Ti, …)EE 143: Microfabrication TechnologyLecture Module 3:Film DepositionCTN 2/11/10Copyright @ 2010 Regents of the University of California at BerkeleyEE 143: Microfabrication Technology LecM 3 C. Nguyen 2/14/10 7Sputter Deposition Process• Step-by-step procedure:1. Pump down to vacuum2. Flow gas (e.g., Ar)3. Fire up plasma (create Ar+ ions) → apply dc-bias (or RF for non-conductive targets)4. Ar+ ions bombard target (dislodge atoms)5. Atoms make their way to the wafer in a more random fashion, since at this higher pressure, λ ~60μm for a 4Åparticle; plus, the target is much bigger• Result: better step coverage!Torr 0075012.0atmTorr 760 atm 109.8 Pa 1 Pa) 100(~6-=⎟⎠⎞⎜⎝⎛×=→750 mTorrEE 143: Microfabrication Technology LecM 3 C. Nguyen 2/14/10 8Problems With Sputtering1. Get some Ar in the film2. Substrate can heat upª up to ~350oC, causing nonuniformity across the waferª but it still is more uniform than evaporation!3. Stress can be controlled by changing parameters (e.g., flow rate, plasma power) from pass to pass, but repeatability is an issue• Solution: use Chemical Vapor Deposition (CVD)EE 143: Microfabrication TechnologyLecture Module 3:Film DepositionCTN 2/11/10Copyright @ 2010 Regents of the University of California at BerkeleyEE 143: Microfabrication Technology LecM 3 C. Nguyen 2/14/10 9Chemical Vapor Deposition (CVD)• Even better conformity than sputtering• Form thin films on the surface of the substrate by thermal decomposition and/or reaction of gaseous compoundsª Desired material is deposited directly from the gas phase onto the surface of the substrateª Can be performed at pressures for which λ (i.e., the mean free path) for gas molecules is smallª This, combined with relatively high temperature leads toª Types of films: polysilicon, SiO2, silicon nitride, SiGe, Tungsten (W), Molybdenum (M), Tantalum (Ta), Titanium (Ti), …Excellent Conformal Step Coverage!EE 143: Microfabrication Technology LecM 3 C. Nguyen 2/14/10 10The CVD ProcessWaferGas Flow – Gas Stream(a)Energy required to drive reactions supplied by several methods: Thermal (i.e., heat), photons, electrons (i.e., plasma)(b)(c)(d)(e) (e)(d)Reactant gas (+ inert diluting gases) are introduced into the reaction chamberGas species move to the substrateReactants adsorb onto the substrateReaction by-products desorbed from surfaceAtoms migrate and react chemically to form filmsThis determines the ultimate conformality of the film (i.e., determines step coverage)EE 143: Microfabrication TechnologyLecture Module 3:Film DepositionCTN 2/11/10Copyright @ 2010 Regents of the University of California at BerkeleyEE 143: Microfabrication Technology LecM 3 C. Nguyen 2/14/10 11The CVD Process (cont.)Step-by-Step CVD Sequence:a) Reactant gases (+ inert diluting gases) are introduced into reaction chamberb) Gas species move to the substratec) Reactants adsorbed onto the substrated) Atoms migrate and react chemically to form filmsThis determines to a large extent whether or not a film is conformal (i.e. better step coverage)e) Reaction by-products desorbed and removed from reaction chamberNot Conformal Conformallow Tnot enough adatom migrationHigh TPlenty of adatom migrationSurfaceprocessesGas phaseprocessesEE 143: Microfabrication Technology LecM 3 C. Nguyen 2/14/10 12CVD ModelinggNSimplified Schematic:gJsJsNδsurfaceNg= conc. of reactant molecules in the gas streamNs= conc. of reactant molecules at the surfaceJs= flux of gas molecules at the surfaceJg= flux of molecules diffusing in from the gas stream·const.] ratereaction surface [ s== kNkJsss()()sggsgggNNhNNDJ −=−⎟⎟⎠⎞⎜⎜⎝⎛=δGoverning Equations:Effective diffusion const. for the gas moleculeVapor phase mass-transfer coefficientEE 143: Microfabrication TechnologyLecture Module 3:Film DepositionCTN 2/11/10Copyright @ 2010 Regents of the University of California at BerkeleyEE 143: Microfabrication Technology LecM 3 C. Nguyen 2/14/10
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