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Physics of Thin Films

PES 449 / PHYS 549


Chemical Vapor Deposition (CVD)

 Ohring Chapter 4 Sections 1 - 5


Overview

not all components are found in all CVD systems:

CVD schematic

Source gas

Reacts on substrate to deposit film


Types of CVD reactions

Pyrolysis - thermal decomposition
AB(g) ---> A(s) + B(g)

ex: Si deposition from Silane at 650 C

SiH4(g) ---> Si(s) + 2H2(g)

use to deposit: Al, Ti, Pb, Mo, Fe, Ni, B, Zr, C, Si, Ge, SiO2, Al2O3, MnO2, BN, Si3N4, GaN, Si1-xGex, . . .

Reduction

often using H2

AX(g) + H2(g) <===> A(s) + HX(g)

often lower temperature than pyrolysis

reversible => can use for cleaning too

ex: W deposition at 300 C

WF6(g) + 3H2(g) <===> W(s) + 6HF(g)

use to deposit: Al, Ti, Sn, Ta, Nb, Cr, Mo, Fe, B, Si, Ge, TaB, TiB2, SiO2, BP, Nb3Ge, Si1-xGex, . . .

Oxidation

often using O2

AX(g) + O2(g) ---> AO(s) + [O]X(g)

ex: SiO2 deposition from silane and oxygen at 450 C (lower temp than thermal oxidation)

SiH4(g) + O2(g) ---> SiO2(s) + 2H2(g)

use to deposit: Al2O3, TiO2, Ta2O5, SnO2, ZnO, . . .

Compound formation

often using amonia or water vapor

AX(g) + NH3(g) ---> AN(s) + HX(g)

AX(g) + H2O(g) ---> AO(s) + HX(g)

ex: deposit wear resistant film (BN) at 1100 C

BF3(g) + NH3(g) ---> BN(s) + 3HF(g)

use to deposit: TiN, TaN, AlN, SiC, Al2O3, In2O3, SnO2, SiO2, . . .

Disproportionation

compunds involving elements with multiple valence states

2AB(g) <===> A(s) + AB2(g)

ex: CVD Ge deposition

use to deposit: Al, C, Ge, Si, III-V compounds, . . .

Reversible Transfer

ex: CVD GaAs deposition

use to deposit: GaInAs, AlGaAs, InP, FeSi2, . . .


Thermodynamics of CVD

  • identify possible reactions
  • ignores rate information
  • not strictly correct in flowing system (no equilibrium)
  • Ellingham plots can be useful
    • Handout XCl plots
      • MoCl5, ReCl3 and AsCl3 are all reduced by H to metals and HCl
      • Ni, Fe, and Co chlorides reduce at intermediate temperatures
      • SiCl4 reduces at high temperature
      • many other metal chlorides are too stable
      • CrCl2 is close - but not quite
        • try to adjust partial pressures to force a reaction
        • adjust GCrCl2 by changing PCrCl2 / PCl2
          • G = Go + RT ln(PCrCl2/ PCl2)
        • can deposit metal from chloride if Go(MCl) - Go(HCl) < 10kcal
        • so need PCrCl2 / PCl2 = 1000 at 1400 K


CVD Film Growth Steps

once reaction is identified, consider the process in detail:
  • source: production of appropriate gas
  • transport of gas to substrate
  • deposition of film:
    • adsorption of gas on substrate
    • reaction on substrate
  • transport of "waste" products away from substrate

 CVD Sources

  • types of sources
    • gasses (easiest)
    • volatile liquids
    • sublimable solids
    • combination
  • materials should be
    • stable at room temperature
    • sufficiently volatile
      • high enough partial pressure to get good growth rates
    • reaction temperature < melting point of substrate
    • produce desired element on substrate with easily removable by-products
    • low toxicity

Substrates

need to consider
  • adsorption
  • surface reactions

ex: WF6 deposits on Si but not on SiO2

Growth of films

depends on
  • transport of gas to surface
  • adsorption of gas on substrate
  • reaction rates on substrate
  • transport of products away from substrate

 
Mass transport in gas

goals
  • deliver gas uniformly to substrate (uniform films)
  • optimize flow for maximum deposition rate

    Two flow regimes

  • Molecular flow
    • diffusion in gas
      • D ~ T3/2 / P from Kinetic Theory of Gasses
      • reduce Pressure for higher D and higher deposition rate
  • Viscous flow
    • low flow rates produces laminar flow (desired)
    • high flow rates produces turbulent flow (avoid)

    laminar flow: simple case: flow past a plate

    flow past a plate showing stagnant layer

    near plate velocity = 0 ==> stagnant layer

    • diffuse gas through stagnant layer to surface

    mass transport depends on

    • fundamental parameters

      experimental parameters

      reactant concentration

      pressure

      diffusivity

      gas velocity

      boundary layer thickness

      temperature distribution

      reactor geometry

      gas properties (viscosity . . .)

     Simple model (Grove, 1967)

    • AB(g) ---> A(s) + B(g)
      • flow model to surface

        F1 = flux to surface

        F2 = flux consumed in film

        CG = concentration of AB in gas

        CS = concentration of AB at surface

      F1 = hG (CG - CS)

      • where hG = gas diffusion rate constant

      F2 = kS CS

      • where kG = surface rate constant

      in steady state: F1 = F2 = F

      • equation for flux

        growth rate of film is proportional to F

      NOTE: Two rate-limiting cases

      • mass transfer limited
        • small hG
        • growth controlled by transfer to substrate
        • hG is not very temperature dependent
        • common limit at higher temperatures
      • surface reaction limited
        • small kS
        • growth controlled by processes on surface
          • adsorption
          • decomposition
          • surface migration
          • chemical reaction
          • desorption of products
        • kS is highly temperature dependent (increases with T)
        • common limit at lower temperatures
        • often preferred

    Stagnant layer model

    • assume transport depends only on diffusion across stagnant layer (mass transfer limited)

      stagnant layer model

      equations of stagnant layer

    variations along flow direction

    • Consider flow into and out of a volume (as in Chapter 1)

      apply boundary conditions

  • all gas reacts at substrate
    • C = 0 at y = 0
  • initial concentration is constant
    • C = Ci at x = 0
  • no flow out of the top
    • dC/dy = 0 at y = b

    SOLVE differential equation subject to these boundary conditions

    C(x, y) = a mess (see equation 4-41)

    ASSUME large flow rate or large chamber

    • vaveb >> D

      examine this solution:

  • proportional to Ci
  • at y = b, sin = 1
  • C decreases exponentially with x

    tricks to improve uniformity

    • tilt substrate into flow
    • increase T continuously along x
    • single wafer processing
      • often using a flow of gas normal to sample

    deposition depends on:

  • materials: gas, film
  • geometry (b, substrate orientation)
  • process conditions (Ci, pressure, velocity)
  • temperature and temperature distribution (D . . .)


Summary

advantages:
  • high growth rates possible
  • can deposit materials which are hard to evaporate
  • good reproducibility
  • can grow epitaxial films

    disadvantages

  • high temperatures
  • complex processes
  • toxic and corrosive gasses


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