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Lecture 12.0

Lecture 12.0. Deposition. Materials Deposited. Dielectrics SiO2, BSG Metals W, Cu, Al Semiconductors Poly silicon (doped) Barrier Layers Nitrides (TaN, TiN), Silicides (WSi 2 , TaSi 2 , CoSi, MoSi 2 ). Deposition Methods. Growth of an oxidation layer Spin on Layer

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Lecture 12.0

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  1. Lecture 12.0 Deposition

  2. Materials Deposited • Dielectrics • SiO2, BSG • Metals • W, Cu, Al • Semiconductors • Poly silicon (doped) • Barrier Layers • Nitrides (TaN, TiN), Silicides (WSi2, TaSi2, CoSi, MoSi2)

  3. Deposition Methods • Growth of an oxidation layer • Spin on Layer • Chemical Vapor Deposition (CVD) • Heat = decomposition T of gasses • Plasma enhanced CVD (lower T process) • Physical Deposition • Vapor Deposition • Sputtering

  4. Critical Issues • Adherence of the layer • Chemical Compatibility • Electro Migration • Inter diffusion during subsequent processing • Strong function of Processing • Even Deposition at all wafer locations

  5. CVD of Si3N4 - Implantation mask • 3 SiH2Cl2 + 4 NH3Si3N4 + 6 HCl + 6 H2 • 780C, vacuum • Carrier gas with NH3 / SiH2Cl2 >>1 • Stack of wafer into furnace • Higher temperature at exit to compensate for gas conversion losses • Add gases • Stop after layer is thick enough

  6. CVD of Poly Si – Gate conductor • SiH4Si + 2 H2 • 620C, vacuum • N2 Carrier gas with SiH4 and dopant precursor • Stack of wafer into furnace • Higher temperature at exit to compensate for gas conversion losses • Add gases • Stop after layer is thick enough

  7. CVD of SiO2 – Dielectric • Si0C2H5 +O2SiO2 + 2 H2 • 400C, vacuum • He carrier gas with vaporized(or atomized) Si0C2H5and O2 and B(CH3)3 and/or P(CH3)3 dopants for BSG and BPSG • Stack of wafer into furnace • Higher temperature at exit to compensate for gas conversion losses • Add gases • Stop after layer is thick enough

  8. CVD of W – Metal plugs • 3H2+WF6 W + 6HF • T>800C, vacuum • He carrier gas with WF6 • Side Reactions at lower temperatures • Oxide etching reactions • 2H2+2WF6+3SiO2 3SiF4 + 2WO2 + 2H2O • SiO2 + 4HF  2H2O +SiF4 • Stack of wafer into furnace • Higher temperature at exit to compensate for gas conversion losses • Add gases • Stop after layer is thick enough

  9. Chemical Equilibrium

  10. CVD Reactor • Wafers in Carriage (Quartz) • Gasses enter • Pumped out via vacuum system • Plug Flow Reactor Vacuum

  11. CVD Reactor • Macroscopic Analysis • Plug flow reactor • Microscopic Analysis • Surface Reaction • Film Growth Rate

  12. Macroscopic Analysis • Plug Flow Reactor (PFR) • Like a Catalytic PFR Reactor • FAo= Reactant Molar Flow Rate • X = conversion • rA=Reaction rate = f(CA)=kCA • Ci=Concentration of Species, i. • Θi= Initial molar ratio for species i to reactant, A. • νi= stoichiometeric coefficient • ε = change in number of moles

  13. Combined Effects Contours = Concentration

  14. Reactor Length Effects SiH2Cl2(g) + 2 N2O(g) SiO2(s)+ 2 N2(g)+2 HCl(g) How to solve? Higher T at exit!

  15. Deposition Rate over the Radius CAs r Thiele Modulus Φ1=(2kRw/DABx)1/2

  16. Radial Effects This is bad!!!

  17. Combined Length and Radial Effects Wafer 10 Wafer 20

  18. CVD Reactor • External Convective Diffusion • Either reactants or products • Internal Diffusion in Wafer Stack • Either reactants or products • Adsorption • Surface Reaction • Desorption

  19. Microscopic Analysis -Reaction Steps • Adsorption • A(g)+SA*S • rAD=kAD (PACv-CA*S/KAD) • Surface Reaction-1 • A*S+SS*S + C*S • rS=kS(CvCA*S - Cv CC*S/KS) • Surface Reaction-2 • A*S+B*SS*S+C*S+P(g) • rS=kS(CA*SCB*S - Cv CC*SPP/KS) • Desorption: C*S<----> C(g) +S • rD=kD(CC*S-PCCv/KD) • Any can be rate determining! Others in Equilib. • Write in terms of gas pressures, total site conc.

  20. Rate Limiting Steps • Adsorption • rA=rAD= kADCt (PA- PC /Ke)/(1+KAPA+PC/KD+KIPI) • Surface Reaction • (see next slide) • Desorption • rA=rD=kDCt(PA - PC/Ke)/(1+KAPA+PC/KD+KIPI)

  21. Surface Reactions

  22. Deposition of Ge Ishii, H. and Takahashik Y., J. Electrochem. Soc. 135,1539(1988).

  23. Silicon Deposition • Overall Reaction • SiH4 Si(s) + 2H2 • Two Step Reaction Mechanism • SiH4 SiH2(ads)+ H2 • SiH2 (ads)  Si(s) + H2 • Rate=kadsCt PSiH4/(1+Ks PSiH4) • Kads Ct = 2.7 x 10-12 mol/(cm2 s Pa) • Ks=0.73 Pa-1

  24. Silicon Epitaxy vs. Poly Si • Substrate has Similar Crystal Structure and lattice spacing • Homo epitaxy Si on Si • Hetero epitaxy GaAs on Si • Must have latice match • Substrate cut as specific angle to assure latice match • Probability of adatoms getting together to form stable nuclei or islands is lower that the probability of adatoms migrating to a step for incorporation into crystal lattice. • Decrease temp. • Low PSiH4 • Miss Orientation angle

  25. Surface Diffusion

  26. Monocrystal vs. Polycrystalline PSiH4=? torr

  27. Dislocation Density • Epitaxial Film • Activation Energy of Dislocation • 3.5 eV

  28. Physical Vapor Deposition • Evaporation from Crystal • Deposition of Wall

  29. Physical Deposition - Sputtering • Plasma is used • Ion (Ar+) accelerated into a target material • Target material is vaporized • Target Flux  Ion Flux* Sputtering Yield • Diffuses from target to wafer • Deposits on cold surface of wafer

  30. DC Plasma • Glow Discharge

  31. RF Plasma Sputtering for Deposition and for Etching RF + DC field

  32. Target Al Cu TiW TiN Gas Argon Deposited Layer Al Cu TiW TiN Poly Crystalline Columnar Structure Sputtering Chemistries

  33. Deposition Rate • Sputtering Yield, S • S=α(E1/2-Eth1/2) • Deposition Rate  • Ion current into Target *Sputtering Yield • Fundamental Charge

  34. Sheath RF Plasma Plasma rf Sheath • Electrons dominate in the Plasma • Plasma Potential, Vp=0.5(Va+Vdc) • Va = applied voltage amplitude (rf) • Ions Dominate in the Sheath • Sheath Potential, Vsp=Vp-Vdc • Reference Voltage is ground such that Vdc is negative

  35. Floating Potential • Sheath surrounds object • Floating potential, Vf • kBTe=eV • due to the accelerating Voltage

  36. Plasma Chemistry • Dissociation leading to reactive neutrals • e + H2 H + H + e • e + SiH4  SiH2 + H2 + e • e + CF4  CF3 + F + e • Reaction rate depends upon electron density • Most Probable reaction depends on lowest dissociation energy.

  37. Plasma Chemistry • Ionization leading to ion • e + CF4  CF3- + F • e + SiH4  SiH3+ + H + 2e • Reaction depend upon electron density

  38. Plasma Chemistry • Electrons have more energy • Concentration of electrons is ~108 to1012 1/cc • Ions and neutrals have 1/100 lower energy than electrons • Concentration of neutrals is 1000x the concentration of ions

  39. Oxygen Plasma • Reactive Species • O2+eO2+ + 2e • O2+e2O + e • O + e  O- • O2+ + e  2O

  40. Plasma Chemistry • Reactions occur at the Chip Surface • Catalytic Reaction Mechanisms • Adsorption • Surface Reaction • Desorption • e.g. Langmuir-Hinshelwood Mechanism

  41. Plasma Transport Equations • Flux, J

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