Chemical Vapor Deposition (CVD)
References :
D.M. Dobkin and M.K. Zuraw,
Principles of Chemical Vapor
Deposition (Kluwer Academic
Publishers, 2003)
M. L. Hitchman and K. F. Jensen,
Chemical Vapor Deposition
(Academic Press, 1993)
M. Ohring, The Materials Science of
Thin Films (Academic Press, 1992)
M.A. Herman, W. Richter and H.
Sitter, Epitaxy: Physical Principles
and Technical Implementation
(Springer, 2004)
Chemical Vapor Deposition (CVD)
Thin Film Deposition
PVD
CVD
• CVD : Film species are
supplied in the form of a
precursor gas
Chemical Vapor Deposition (CVD)
Horizontal:
Barrel:
Pancake:
From Ohring, Fig. 4-13, p. 178
CVD Chemistry
• Heterogeneous and homogeneous
reactions
From Herman, Fig. 8.3, p. 173
From Sze,
Fig. 19, p.
323
Chemical Vapor Deposition (CVD)
• All CVD systems consist of three
steps:
1) gas transport into the chamber
and to the substrate
2) chemical reactions forming the
film
aA(g) + bB(g) → cC(s) + dD(g)
3) removal of reaction byproducts
from the chamber
Why CVD ?
• Advantages:
• No crucible interactions
• CVD is more conformal
compared to PVD methods
which are line-of-sight
• No alloy fractionation as
with thermal methods
CVD Applications
• CVD used to produce poly-Si,
SiO2, and SiN in MOSFETs
From Ohring, Fig. 4-1, p. 148
CVD Systems
CVD
AP-CVD
VPE
LP-CVD
PE-CVD
AP-CVD
• Viscous flow produces boundary
layer at surfaces due to friction
From Jaeger, Fig. 6.9, p. 121
AP-CVD Systems
• Viscous flow makes it difficult to
achieve uniform film growth on a
large number of stacked wafers in a
reactor
• Reactions are “mass transfer limited”
requiring flat lying wafers
From Ohring, Fig. 4-13,
p. 178
LP-CVD Systems
• ~ 10 mT - 1 T
• Uses low pressures to enhance
diffusion and mean free path of gas
molecules toward the substrates
LP-CVD Systems
• Produces faster growth rates,
more uniform deposition, and
more conformal deposition
• Wafers can be stacked closer
together to achieve higher
throughput
From Ohring, Fig. 4-14, p. 180
CVD Chemistry
• CVD requires that a volatile
compound be found for the
precursors
From Dobkin, Table 5-6, p. 133
Poly-Si CVD
• LPCVD at 600 - 650 °C :
SiH4(g) → Si(s) + 2H2(g)
SiO2 CVD
• 300 - 500 °C :
SiH4(g) + O2(g) → SiO2(s) +
2H2(g)
• 900°C :
SiCl2H2(g) + 2N2O(g) →
SiO2(s) + 2N2(g) + 2HCl(g)
• 700 °C :
Si(C2H5O)4 + 12O2 → SiO2 +
8CO2 + 10H2O
SiN CVD
• APCVD at 700 – 900°C :
3SiH4(g) + 4NH3(g) →
Si3N4(s) + 12H2(g)
• LPCVD at 700-800 °C :
3SiCl2H2(g) + 4NH3(g) →
Si3N4(s) + 6HCl(g) + 6H2(g)
W CVD
• 250-500 °C:
WF6(g) + 3H2(g) → W(s) +
6HF(g)
• W on Si at < 200 °C:
6WF6(g) + 3Si(s) → 2W(s) +
3SiF4(g)
• LPCVD at 800°C :
2MCl5(g) + 5H2(g) → 2M(s)
+ 10HCl(g)
where M = Mo, Ta, or Ti
Vapor Phase Epitaxy (VPE)
• Epitaxy: a single crystal
substrate acts as a template for
a film of identical or related
crystal structure
Si VPE
• Sources include:
silicon tetrachloride (SiCl4)
dichlorosilane (SiH2Cl2),
trichlorosilane (SiHCl3)
silane (SiH4)
• 1200 °C: SiCl4(g) + 2H2 → Si(s) +
4HCl(g)
• 650 °C: SiH4(g) → Si(s) + 2H2(g)
• Doping:
p-type: biborane (B2H6)
n-type: arsine (AsH3) or
phosphine (PH3)
GaAs VPE
From Grovenor, Fig. 3.27, p. 167
GaAs VPE
• Source zone :
GaCl3(g) may be produced by passing HCl
over Ga :
6HCl(g) + 2Ga(s) → 2GaCl3(g) + 3H2(g)
• Decomposition zone :
As4(g) produced by decomposition of
AsH3(g) :
4AsH3 → As4(g) + 6H2(g)
• Deposition zone :
As4(g) + 4GaCl3(g) + 6H2(g) → 4GaAs(s) +
12HCl(g)
MOVPE = OMVPE = MOCVD
• VPE using metalorganic species
• It is most commonly used for
deposition of III-V compounds
From Ohring, Fig. 4-18, p. 187
MOCVD
From Herman, Fig. 8.14 & 8.15, p. 180
Metalorganics
From Herman, Fig. 8.21, p. 187
MOCVD
From Hitchman, Appendix 6.3, p. 383
MOCVD
From Ohring, Table 4-5, p. 187
MOCVD
From Hitchman, Appendix 6.2, p. 382
MOCVD
AsH3(g) + Ga(CH3)3(g) → GaAs(s) + 3CH4(g)
TMG
From Herman, p. 192
MOCVD
From Herman, Fig. 8.23, p. 190
MOCVD
From Herman, Fig. 8.17, p. 182
MOCVD
From Ohring, Table 4-6, p. 189
CVD Films and Coatings
From Ohring, Table 4-1, p. 154
CVD Kinetics
• Viscous flow produces boundary
layer at surfaces due to friction
From Ohring, Fig. 4-7, p. 163
CVD Kinetics
From Ohring, Fig. 4-8(a), p. 168
J(x,y) = C(x,y)v - DC(x,y)
Boundary conditions:
1. Chemical reaction is complete at the
surface  C = 0 when y = 0
2. No net diffusion at the top of the
reactor (gas molecules are reflected)
 dC/dy = 0 when y = b
3. Input source gas concentration is Ci
 C = Ci at x = 0
CVD Kinetics
From Ohring, Fig. 4-8(a), p. 168
C(x,y) = (4Ci/p)sin(py/2b) exp (-p2Dx/4vb2)
flux of gas toward surface (cm-2s-1)
= J(x) = -D C(x,y)/y at y = 0
J(x) decreases along the direction x
growth rate, R = J(x) / film atom density
CVD Kinetics
• Growth rate declines along xdirection
• Correct by increasing growth
temperature along x-direction or
tilting the substrates towards the gas
flow
From Ohring, Fig. 4-8(b), p. 168
AP-CVD
From Jaeger, Fig. 6.9, p. 121
CVD Kinetics
• Flux of gas molecules at the substrate
surface:
Js = ksNs
ks = surface reaction rate constant (first order
kinetics)
Ns = concentration of reactants above the
surface
• Flux of gas molecules diffusing from gas
stream:
Jg = hg (Ng – Ns)
hg = mass transfer coefficient
Ng = gas concentration in the vapor
CVD Kinetics
• At steady-state,
Js = Jg
• Growth rate,
R = Js/N = [ kshg / (ks + hg) ] (Ng/N)
N = film atomic density
CVD Kinetics
• ks = surface reaction rate constant
• hg = mass transfer coefficient
• ks >> hg
mass-transfer-limited growth
r = hgNg/N
r is temperature-insensitive
• hg >> ks
surface-reaction-limited growth
r = ksNs/N is temperature sensitive
CVD Kinetics
• Desired growth regime is in Tinsensitive part of curve
Mass-transfer-limited
Surface-reaction-limited
From Jaeger, Fig. 6.10, p. 122
PE-CVD
• In conventional CVD chemical
reactions are controlled by
thermal energy provided by
heating the substrate
• The thermal energy provides the
energy necessary to break bonds
• In PE-CVD, a plasma is used to
decompose the gas molecules for
film deposition
From Dobkin, Table 6-3, p 172
PE-CVD
PECVD (N plasma) :
2SiH4 + N2(g) → 2SiNH(s) + 3H2(g)
PECVD (Ar plasma) :
SiH4(g) + NH3(g) → SiNH(s) + 3H2(g)
PE-CVD
Conventional
CVD
Thermal Energy
High Substrate
Temperature
Small Substrates
(for Uniform
Heating)
PE-CVD
Plasma
Energy
(electron-atom
collisions)
Low Substrate
Temperature
(Ts < 300 °C)
Large Areas
PE-CVD
Evaporation
PE-CVD
Alloys Fractionate No Fractionation
Crucible
No Crucible
Interactions
Line-of-Sight
Not Line-of-Sight
Sputtering
Limited
Composition
Control
Line-of-Sight
PE-CVD
Excellent
Composition
Control
Not Line-of-Sight
PE-CVD
PE-CVD
Capacitively
Coupled
Electron
Cyclotron
Resonance
(ECR)
Inductively
Coupled
PE-CVD
from Hitchman,
Fig. 7.2, p. 392
ECR Plasma
• Electrons in a B-field move in a
circular path with the Larmor
frequency:
w = eB/m
• An em field at the Larmor frequency
will be in phase with the electron
motion and add energy to the electron
on each orbit
From Dobkin, Fig. 6-11, p 161
ECR Plasma
• This effect is known as electron
cyclotron resonance (ECR)
• Normally w = 2.45 GHz and B =
875 Gauss
From Ohring, Fig. 4-16, p. 184
ECR Plasma
• Electrons are trapped by the field
lines
• Increased ionization (10-2 –
10-1)
• Lower pressures (~10-4 – 102 Torr)
• Greater plasma densities
(~1012 cm-3)
• Lower substrate
temperatures (< 300 °C)
Afterglow/Remote Plasma
afterglow
metastable
neutrals
plasma
decaying
plasma
substrate
additional
gas input
Inductive-Coupled Plasma (ICP)
• Another method of exciting a plasma is
to use inductive methods rather
capacitive methods
• A time varying magnetic field from a
solenoid will create a time varying
electric field
• This em field can be used to excite
electrons and sustain the plasma
From Dobkin, Fig. 6-13, p. 163