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:

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, Si_1-xGe_x, . . .

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, Si_1-xGe_x, . . .

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:

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

Reversible Transfer

ex:
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 ÆG_CrCl2 by changing P_CrCl2 / P_Cl2

ÆG = ÆG^o + RT ln(P_CrCl2/ P_Cl2)

can deposit metal from chloride if ÆG^o(MCl) - ÆG^o(HCl) < 10kcal

so need P_CrCl2 / P_Cl2 = 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 ~ T^3/2 / 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 ==> 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)
- F₁ = flux to surface
  
  F₂ = flux consumed in film
  
  C_G = concentration of AB in gas
  
  C_S = concentration of AB at surface
F₁ = h_G (C_G - C_S)
- where h_G = gas diffusion rate constant
F₂ = k_S C_S
- where k_G = surface rate constant
in steady state: F₁ = F₂ = F
- growth rate of film is proportional to F
NOTE: Two rate-limiting cases
- mass transfer limited
  - small h_G
  - growth controlled by transfer to substrate
  - h_G is not very temperature dependent
  - common limit at higher temperatures
- surface reaction limited
  - small k_S
  - growth controlled by processes on surface
    - adsorption
    - decomposition
    - surface migration
    - chemical reaction
    - desorption of products
  - k_S 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)

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 = C_i 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
- v_aveb >> D¹
  
  examine this solution:
proportional to C_i
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 (C_i, 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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