MSE 550 CVD

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MSE 550 - CVD
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Chemical Vapor Deposition

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• Chemical vapor deposition (CVD) is a widely used
  method for depositing thin films of a large
  variety of materials. Applications of CVD range
  from the fabrication of microelectronic devices
  to the deposition of protective coatings. In a
  typical CVD process, reactant gases (often
  diluted in a carrier gas) at room temperature
  enter the reaction chamber. The gas mixture is
  heated as it approaches the deposition surface,
  heated radiatively or placed upon a heated
  substrate. Depending on the process and operating
  conditions, the reactant gases may undergo
  homogeneous chemical reactions in the vapor phase
  before striking the surface. Near the surface
  thermal, momentum, and chemical concentration
  boundary layers form as the gas stream heats,
  slows down due to viscous drag, and the chemical
  composition changes. Heterogeneous reactions of
  the source gases or reactive intermediate species
  (formed from homogeneous pyrolysis) occur at the
  deposition surface forming the deposited
  material. Gaseous reaction by-products are then
  transported out of the reaction chamber.

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What is CVD?

• Thin film formation from vapor phase reactants.
  Deposited films range from metals to
  semiconductors to insulators.
• An essential process step in the manufacturing of
  microelectronic devices. High temperatures and
  low pressures are the most common process
  conditions, but are not necessary.
• All CVD involves using an energy source to break
  reactant gases into reactive species for
  deposition.

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Chemical Vapor Deposition CVD is the formation
of a film on a surface from a volatile precursor
(vapor or gas), as a consequence of one or more
chemical reactions which change the state of the
precursor. Many different films can be deposited
elements and compounds, crystalline,
polycrystalline, and amorphous. Most films can be
deposited from several different precursor
systems. Plasma discharges can be used to help
things along, or the substrate and/or the gas can
be heated or cooled. Different deposition
techniques, process conditions, and treatment
after deposition produce films with
differing characteristics, suitable for different
applications. Each film has an optimal set of
characterization techniques. In every case, CVD
processes must provide a volatile precursor
containing the constituents of the film
transport that precursor to the deposition
surface encourage or avoid reactions in the gas
phase encourage surface reactions that form the
film and do it rapidly, reproducibly, and
uniformly for industrial applications.
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• 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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Examples of CVD

• Metals/Conductors - W, Al, Cu, doped poly-Si
• Insulators (dielectrics) - BPSG, Si3N4, SiO2
• Semiconductors - Si, Ge, InP, GaAsP
• Silicides - TiSi2, WSi2
• Barriers TiN, TaN

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• Types of CVD reactions
• Pyrolysis - thermal decomposition
• AB(g) ---gt A(s) B(g)
• ex Si deposition from Silane at 650 C
• SiH4(g) ---gt 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) ltgt A(s) HX(g)
• often lower temperature than pyrolysis
• reversible gt can use for cleaning too
• ex W deposition at 300 C
• WF6(g) 3H2(g) ltgt 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) ---gt AO(s) OX(g)
• ex SiO2 deposition from silane and oxygen at 450
  C (lower temp than thermal oxidation)
• SiH4(g) O2(g) ---gt SiO2(s) 2H2(g)

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• Compound formation
• often using ammonia or water vapor
• AX(g) NH3(g) ---gt AN(s) HX(g)
• AX(g) H2O(g) ---gt AO(s) HX(g)
• ex deposit wear resistant film (BN) at 1100 C
• BF3(g) NH3(g) ---gt BN(s) 3HF(g)
• use to deposit TiN, TaN, AlN, SiC, Al2O3, In2O3,
  SnO2, SiO2, . . .
• Disproportionation
• compounds involving elements with multiple
  valence states
• 2AB(g) ltgt A(s) AB2(g)

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• use to deposit Al, C, Ge, Si, III-V compounds, .
  . .
• Reversible Transfer
• ex use to deposit GaInAs, AlGaAs, InP, FeSi2, .
  . .

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• 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
near plate velocity 0 gt stagnant layer
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• diffuse gas through stagnant layer to surface
• mass transport depends on

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Generic CVD Steps

• Five steps occur during all CVD processes
• Reactants pumped through reactor.
• Reactants diffuse across boundary layer to
  surface.
• Reactants adsorb on surface (adatoms).
• Surface reactions pos. formation of islands or
  clusters.
• Pos. surface diffusion of adatoms.
• Diffusion of by-products away from surface.

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• Simple model (Grove, 1967)
• AB(g) ---gt A(s) B(g)
• 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

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• 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

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variations along flow direction
Consider flow into and out of a volume (as in
Chapter 1)
apply boundary conditions
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• 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 gtgt D¹
• 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

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Variables Affecting Steps

• Diffusion of reactants to by products away from
  surface.
• Gas flow rates, pressure, reactor configuration
• boundary layer formation
• Temperature
• diffusion is a mildly activated process
• Surface reaction and surface diffusion
• Temperature (both are affected exponentially by
  temperature)
• Surface interactions (reactive and overall
  sticking coefficient of a species)
• Rate of incoming reactants and out going
  by-products

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Molecule/Surface Interactions

• Physisorption
• No electron transfer occurs between adatom
  surface
• Binding energies 0.1 eV
• Essentially condensation
• Substrate/adsorbate independent
• Doesnt occur at temperatures much above the
  boiling point of the adsorbate
• Chemisorption
• Electronic interactions between surface
  adspecies
• Binding energies 1 eV
• This is a chemical reaction
• Substrate/adsorbate selective

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Nucleation and Growth

• Initial formation of clusters (nuclei)
• Can grow or shrink until a critical size is
  reached
• Above critical size, increasing nuclei size
  lowers the overall surface energy
• Clusters can grow at the expense of
  sub-critically sized clusters
• Clusters impinge and grow together to form
  continuous film (coalescence)

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Limiting Cases

• Assuming steps occur sequentially, then the
  slowest step determines the deposition rate.
• Reactions
• Mass Transport

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Surface Reactions

• Reaction rate strong function of temperature
• Reactant species must first be adsorbed on
  substrate (wafer) for deposition to take place.
• After adsorption, the reactive species can remain
  fixed at the surface site or migrate along the
  surface. This is temperature dependent and
  affects step coverage.

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Mass Transport Limited Depositions

• Processes are run in laminar flow regime
• Re?UL/? lt 2100
• Boundary layer forms next to wafer surfaces
• Due to viscous forces in the gas
• Reactants must diffuse across the boundary layer
  to reach wafer surface
• Boundary layer thickness decreases with
  increasing gas velocity
• Rate increases with decreasing boundary layer
• Maximizes when reaction becomes reaction rate
  limited

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Why Epi?

• Bipolar devices
• Add a lightly doped layer on a heavily doped
  substrate increases breakdown voltage of the
  collector/substrate junction
• Deposited over patterned heavily doped areas
  called buried layers
• CMOS
• No SiOx precipitates, unlike substrates
• Less junction leakage due to defects
• Smoother surface leads to better gate oxide

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Silicon Precursors

• SiCl4- Silicon tetrachloride (1150-1250 C)
• SiHCl3-Tricholorosilane (1110-1150 C)
• SiH2Cl2-Dichlorosilane (1020-1120 C)
• SiH4- Silane (650-900 C)
• Gas phase nucleation a problem
• Si2H6- Disilane (400-600 C)
• Pyrogenic and toxic

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Doping an Epi Layer

• Add dopant gas to deposition reaction
• PH3, AsH3, B2H6
• Added in ppm levels in H2
• Outdiffusion from doped substrate
• High deposition temperature drives diffusion out
  of highly doped substrates into growing epi films
  
• Auto doping
• Deposition temperature evaporates dopants from
  substrates or from chamber walls
• Dopants are then incorporated in growing film

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• Types of pressure dependent CVD processes
• Atmospheric Pressure CVD (APCVD)
• Sub-atmospheric CVD (SACVD)
• Low Pressure CVD (LPCVD)
• In general, APCVD is controlled by the rate of
  reactant transport to the wafer, LPCVD is
  controlled by the reaction rate

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LPCVD

• Lower Pressure
• Increases diffusivity by about 1000x
• Increases boundary layer thickness by (?P) 0.5
  (factor of about 25 for 1 torr vs. 760 torr)
• Results in a net mass transport increase by more
  than an order of magnitude
• Reaction rate limited
• Temperature control critical

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Depletion Effects

• Batch reactors prone to reactant depletion
• Due to deposition on wafers nearer the gas
  entrance
• Increase gas flow
• Reduces fraction of reactant depleted
• Distributed feed
• Adds fresh reactant along length of reactor
• Temperature gradient
• Increase temperature along length to compensate
  for lower reactant concentration
• Can significantly affect film properties (grain
  size, stress, etc.

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PECVD

• Film growth involves surface reactions with
  incoming gas radical molecules.
• Most radicals are electrically neutral so their
  transport to the wafer surface is by gas phase
  diffusion.
• Radical concentration gradient drives the
  diffusion process.

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PECVD Process Control Parameters

• RF power, frequency, and bias
• Gas composition and flow rate
• System pressure and reactant partial pressure
• Deposition time
• Temperature

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