Chemical Vapor Deposition (CVD)
Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer…
clean and simple
or metal film growth :
… Contrast with
high vacuum, single element… clean and simple
CVD:
toxic, corrosive gas flowing through valves,
T up to 1000°C, multiple, simultaneous chemical reactions,
gas dynamics, dead layers…
whose idea was it?
Insulator
x, ,
InsulatorSi0
Si0
x
wet
growth
wet growth
Figure removed for copyright reasons.
Figure 4-1 in Ohring, M. The Materials Science of Thin Films. 2nd ed.
Burlington, MA: Academic Press, 2001. ISBN: 0125249756.
Poly
PolySiSielectrode:
electrode:
0.3
0.3- -0.4
0.4µm
µmCVD
CVD
Gate
Gateoxide:
oxide:
500Å
x, ,
500ÅSi0
Si0
x
dry
drygrowth
growth
CVD is the single most widely used deposition method in IC manufacture
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Wed., Oct. 12, 2005
1
CVD
reactors
Four
reaction
chambers
(similar to those
for Si oxidation)
Control
module
Control T,
gas mixture,
pressure,
flow rate
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Wed., Oct. 12, 2005
2
CVD is film growth from vapor/gas phase
via chemical reactions in gas and on substrate:
e.g. SiH4 (g) → Si (s) + 2H2 (g)
Do not want Si to nucleate above substrate (homogeneous nucleation),
but on substrate surface (heterogeneous nucleation).
Twall
Reactor
Transport
of precursors
across
dead layer to
substrate
Removal of
by-products
Susceptor
film
T sub> Twall
Pyrolysis: thermal
decomposition
at substrate
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Wed., Oct. 12, 2005
Chemical reaction:
Decomposed species
bond to substrate
More details…
3
CVD Processes
8
1
Bulk transport
of by-product
Bulk
transport
Reactant
molecule
Carrier gas
(Maintain hi p,
slow reaction)
J1 ∝ Dg ∆C
7
2 Transport
across bndry
layer
3
Diffusion of (g)
by-product
4
Decomposition
5
Adsorption
6 Desorption
Reaction with film
J 2 ~ k iCi
Surface diffusion
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Wed., Oct. 12, 2005
4
Hi vel, low P
Gas transport
Low vel, hi P
Laminar flow across plate:
2
Hi vel, low py
Transport across
“boundary layer”
Figure removed for copyright reasons.
Low vel, hi py…
…shorter λ
Figure 4-7a in Ohring, 2001.
J1 ∝ Dg ∆C
kB T
λ=
2π d 2 P
Knudsen NK ≡
λ
L
Viscous flow
Dimension
Dimensionout
outofof
screen
screenmust
mustalso
also
be
considered
be considered
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<1
Figure removed for copyright reasons.
L
Figure 4-7b in Ohring, 2001.
Dgas ≈
λv x
2
Laminar flow pipe.
Wed., Oct. 12, 2005
Conductance ∝ A/L
5
J1 = −hg (Cg − Cs )
dC
D
Revisit gas
J1 = −D
=−
Cg − Cs )
(
dynamics:
dx
δ (x)
Boundary layer
Layer thickness, δ(x)
D=
And we saw gas diffusivity
λ vx
(unlike solid)
z
u
2
boundary layer
gas vel: u0
δ (x)
Cg
Cs
us = 0
wafer
1
δ =
L
x=L
x
wafer
Fluid dynamics:
δ (x)
δ (x ) =
ηx
ρ u0
ρ = mass density, η = viscosity
η
2 L
2
∫ δ (x )dx = 3 L ρ u L ≡ 3 Re
0
0
L
uL
Reynolds #: Re = ρ 0
η
ease of
gas flow
3D
So: hg = δ → 2 L Re
D
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Wed., Oct. 12, 2005
6
Where
δ (x ) =
ηx
ρ u0
does come from?
z
u
Diffusion Eq.:
C
C
∂C
∂ 2C
=D 2 ⇒ ≈D 2
t
δ
∂t
∂z
Cg
δ (x)
Cs
wafer
x=L
x
δ = Dt
η
D(cm /s) =
ρ
2
σ
η ⎛ m2 ⎞
−2
η = (Nsm ),
ε&
ρ ⎜⎝ s ⎟⎠
Analogous to diffusion length
t = x / u0
δ =
η= viscosity,
Re = ρ
ρ = mass density
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η x
ρ u0
Wed., Oct. 12, 2005
Reynolds #:
ease of
gas flow
u0 L
η
7
Several processes in series
Simplify CVD to 2 steps:
Boundary
layer
AB
J1 =
Dg
δ
∆C
J2
B
A
J 2 = ksCs
Sticking coefficient γAB,
0 ≤ γAB ≤ 1
AB bounces
off surface
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Good
adhesion
Reaction rate constant, ks
…no sold-state diffusion here,
reaction occurs at surface.
Let’s analyze, solve for J2…
Wed., Oct. 12, 2005
8
Two main CVD
process:
J1 =
Dg
δ
∆C
A
B
J1 = hg(Cg − Cs)
Boundary
layer
B
J2
A
J 2 = k sCs
In steady state:
J1 = J2,
hg ( Cg - Cs ) = ksCs
Electrical analogy:
hg
Cg ,
Cs =
hg + k s
J 2 = k sCs =
hg k s
hg + k s
J1 = J2,
R = R1+R2
G = 1/R= G1G2 /(G1+G2)
Two processes in series; slowest one limits film growth
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Wed., Oct. 12, 2005
9
Cg
Two main CVD
process:
J1 =
Dg
δ
∆C
A
B
J1 = hg(Cg − Cs)
Boundary
layer
B
J2
A
J 2 = k sCs
J 2 = k sCs =
⎛ # ⎞
⎟
Film growth rate ≡ v = J⎜
⎝ area − t ⎠
hg k s
hg + k s
Cg
Cg N f
=
v=
1 1
hg + k s N f
+
hg k s
hg k s C g
1
,
⎛ # ⎞
Nf⎜ ⎟
⎝ vol ⎠
v (thickness/time)
Slower process controls growth
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10
Two main CVD
process:
A
B
J1 = hg (Cg − Cs)
Boundary
layer
J2
B
A
J 2 = k sCs
Cg N f
v=
1 1
+
hg ks
Examine these 2 limits of growth: hg
limited or ks limited…
Reaction limited growth,
Transport limited growth,
ks<< hg:
hg << ks:
3λ v xCg Re
3DCg
hg Cg
Re =
→
v=
4LN f
2LN f
Nf
∆G
−
k sCg Cg
k 0e kT
=
v=
Nf
Nf
ease of gas flow
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11
Transport limited growth :
Reaction limited growth :
3λvxCg Re
hgCg 3DCg
Re =
→
v=
4LN f
2LN f
Nf
Most CVD is done in this limit
∆G
−
k sC g
Cg
v=
=
k 0e kT
Nf
Nf
∆G = free energy change in reaction
where gas dynamics,
reactor design are important.
(∆G ≅ ∆H for gas
becasue gas reaction no ∆S)
Remedy for boundary layer
J2
B
A
Susceptor, 3o -10o
More uniform ug, Cg ⇒
uniform film growth rate , v
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Choice of reactants and
temperature are critical
Wed., Oct. 12, 2005
12
CVD FILM GROWTH
GAS TRANSPORT-LIMITED
3λ vx Cg
v=
Re
4N f L
λ=
kBT
,
2
2π d Pg
REACTION-RATE LIMITED
v=
2k B T
,
vx =
πm
Nf
−
k 0e
∆G
kT
∆G = ∆H - T∆S
Re ~ u0
(∆G ≅ ∆H for gas → no ∆S for
gas reaction)
1
=
Pg k B T
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Nf
=
Cg
∆G = free energy change in reaction
Cg
v ∝T
k sCg
1
− ∆H
2
v~e
u0
Wed., Oct. 12, 2005
kT
13
Transport
limited
v ∝T
ln (v)
1
2
high T
u0
low T
gas − vel,
Most CVD is transport-limited.
ln (v)
Slow, layer-by-layer growth,
epitaxy. Requires high T, low
pressure, low gas viscosity.
Chamber design, gas dynamics
control process.
To reduce nucleation of
products in gas phase, use low
partial pressure (LPCVD).
u0
− ∆H
v~e
Rate:
T 1/2
⇒ ∆H
Arrhenius-like
1/T
1000K
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Reaction
limited
Wed., Oct. 12, 2005
400K
T
14
kT
Review CVD
We saw…
CVD is film growth from vapor/gas phase via chemical reactions
gas and at substrate:
e.g. SiH4 (g) → Si (s) + 2H2 (g)
in
Twall
Reactor
Transport
of precursors
across
dead layer to
substrate
3.155J/.152J
Removal of
by-products
Susceptor
film
T sub> Twall
Pyrolysis:
Chemical reaction:
thermal decomposition
at substrate
Decomposed species
bond to substrate
Wed., Oct. 12, 2005
15
Reaction
limited
Gas transport
limited
ln (v)
v ∝T
1/ 2
u0
low T
Transport-limited CVD.
Chamber design, gas dynamics
control film growth.
Non uniform film growth.
ln (v)
Slow, layer-by-layer growth,
epitaxy, require high T,
low pressure,
λ/L = NK >> 1.
That puts you in the
Reaction-limited regime
u0
T 1/2
− ∆H
v~e
Rate:
Arrhenius-like
⇒ ∆H
1/T
1000K
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high T
Wed., Oct. 12, 2005
400K
T
16
kT
Some CVD reactions
Silane pyrolysis
Silane oxidation
(450°C)
SiH4 (g) + O2(g) → SiO2 (s) + 2H2 (g)
(heat induced reaction)
SiH4 (g) → Si (s) + 2H2 (g) ( 650°C)
(by LPCVD for gate oxide)
This ⇒ poor Si at 1 atm, so use low pressure
rate
Si - tetrachloride reduction
Probability
4 and
ProbabilityofofSiCl
SiCl
4 and
2H2 meeting goes as
2H meeting goes as
cc1*c*c222 2
1 2
v
c1c2 2
Poly Si
SiCl4 (g) + 2H2 (g) →
Si (s) + 4HCl (g) (1200°C)
SiCl4
H2
(Si-tetrachloride actually much more complex than this;
PSiCl 4
PH 2
Crystalline
etch
8 different compounds are formed, detected by RGA)
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17
Some CVD reactions (cont.)
Doping
Phosphine
Diborane
2PH3 (g) →2P (s) + 3H2 (g)
B2H6 (g) →2B (s) + 3H2 (g)
Si-nitride compound formation
3 SiCl2H2 (g) + 4NH3 (g) → Si3N4 (s) + 6H2(g) + 6HCl (g) (750° C)
(dichloro-silane)
rate
c1c2 2
GaAs growth
Trimethyl Ga (TMG) reduction
SiCl4
(CH3)3 Ga + H2 → Ga (s) + 3CH4
Arsene
Or
2AsH3 → 2As (s) + 3H2
H2
Least abundant element
on surface limits growth
velocity
⎯⎯
⎯
⎯⎯
→ 6 GaAs (s) + 6 HCl g
As4 (g) + As2 (g) + 6 GaCl (g) + 3 H2 (g)←
⎯
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750°C
850°C
Wed., Oct. 12, 2005
18
How can you select process parameters to get
desired product and growth characteristics?
Consider:
1)
SiH4 (g) → SiH2 (g) + H2 (g) Three unknown pressures
Ptot = PSiH4 + PH2 + PSiH2
Total pressure = ∑ partial Ps
…still have 2 unknown Ps
PSiH2 + PSiH4
Si=>
H
2) Conservation of atoms
4PSiH4 + 2PSiH 2 + 2PH 2
…still have 1 unknown P
= const
PPmolecule ∝∝NNmolecule
molecule
molecule
3) “Equilibrium constant”, K (cf. Law of mass action)
K≡
PH 2 ⋅ PSiH2
PSiH4
∆G
−
kT
rate
c1c2 2
= K0e
SiCl4
And similarly for
each reaction.
H2
These 3 equations provide a starting place for growth parameters.
(Many equations for real systems; done on computer)
Do a run, analyze results, …tweak process.
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Where does
∆G
−
PH 2 ⋅ PSiH2
K≡
= K0e kT come from?
PSiH4
Consider “mass action” for dice…
Consider “mass action” for electrons and holes:
N-type semiconductor
Intrinsic semiconductor
Conduction
band
n
ni
EF
p
pi
Valence
band
Recombination
2
probability set
i
i
by energy gap and
number of each species
n = n pi
n
n = np
2
i
p
K indicates a bias at equilibrium in the reaction
toward the products(different molecular species)
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Donor
levels
EF
Wed., Oct. 12, 2005
More free electrons
=> more recombination,
fewer holes (Eg same)
K PSiH4 = PH2 ⋅ PSiH2
20
Exercise
Assume reaction: AB → A + B
Ptot = 1 atm, T = 1000 K,
←
K = 1.8 × 109 Torr × exp ( - 2 eV / kB T ) = 0.153
Assume PA ≈ PB find PAB
Solution:
K=
PAPB
= 0.153
PAB
and Ptot = PA + PB + PAB ,
PA ≈ PB. ∴ 760 Torr = 2PA + PAB
PA2 = 0.153 PAB = 0.153 (760 - 2 PA), quadratic → PA = 10.8 Torr = PB ,
so PAB = 738 Torr
2
PA + 0.306PA − 0.153 × 760 = 0
Small value of K, 0.153 Torr, implies that at equilibrium,
the product of the right-hand side partial pressures
Is but 15% of the reactant (left-hand-side) partial pressure;
the reaction may not produce much in equilibrium. What if you change T?
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21
Atmospheric Pressure CVD: APCVD
(little used today, but illustrative)
High P, small λ => slow mass transport, large reaction rates;
film growth limited by mass transfer, boundary layer;
(quality of APCVD Si from silane is poor, better for dielectrics).
Example:
SiH4 + 2O2 → SiO2 + 2H2OT = 240 - 450°C
Done in N2 ambient (low partial pressure of active gas, reduces film growth rate)
add 4 - 12% PH3 to make silica flow, planarize.
ln v
Transport ltd APCVD
low T,
Figure removed for copyright reasons.
reaction rate
Figure 9-4a in Plummer, J., M. Deal, and P. Griffin. Silicon VLSI Technology:
limited
Fundamentals, Practice, and Modeling. Upper Saddle River, NJ: Prentice Hall,
2000. ISBN: 0130850373.
Note
Noteinduction
inductionheating
heatingofof
substrate;
substrate;cold
coldwalls
walls
1 /T
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22
Low Pressure CVD (LPCVD) for dielectrics and semiconductors
λ
Equilibrium not achieved at low P where
L
(molecular flow, few collisions).
= Kn > 1
kB T
λ=
2π d 2 P
lower P => higher Dg, hg; this improves transport reduces boundary layer,
extends reaction-controlled regime (which is where you operate)
ks term
Hot
Hotwalls
walls
hg at 1 Torr
ln v
LPCVD
hg at 760 Torr
Figure removed for copyright reasons.
Figure 9-4b in Plummer et al., 2000.
Transport
limited
Reaction limited
1 /T
3.155J/.152J
Conversely:
Conversely:ififyou
youwanted
wantedtotowork
workinin
transport limit at low T, you must
transport limit at low T, you must
reduce gas flow
reduce gas flow
Wed., Oct. 12, 2005
23
Low Pressure CVD (LPCVD) for dielectrics and semiconductors
If hot-wall reactor
⇒ uniform T distribution but surface
of reactor gets coated. So system
must be dedicated to 1 species to
avoid contamination.
If cold-wall reactor
Reduce reaction rate,
reduce deposition on
surfaces. For epi Si.
All poly-Si is done by hot-walled LPCVD;
good for low pin-hole SiO2, conformality
Figure removed for copyright reasons.
Figure 9-4b in Plummer et al., 2000.
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Wed., Oct. 12, 2005
24
Low Pressure CVD (LPCVD) for dielectrics and semiconductors
In such non-equilibrium, large λ, reactant-starved cases,
growth rate can be controlled by reaction kinetics
LPCVD is kinetically throttled; transport-rate controls film growth
Silane pyrolysis
SiH4 (g) → Si(s) + 2H2 (g)
T = 575 - 650°C
10 - 100 nm/min
(APCVD is at equilibrium; transport limited)
LPCVD
Requires no carrier gas
Fewer gas-phase reactions, fewer particulates
Eliminates boundary-layer problem
Lower p => larger Dg, extends reaction limited regime
Good conformal growth (unlike sputtering or other PVD methods which
are more directional)
Strong T dependence to reaction growth rate.
Easier to control T with hot-walled furnace.
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25
R.F. Plasma-enhanced CVD (PECVD) for dielectric
MOS metallization: avoid contact interaction betw. Al & Si, SiO2, ∴T < 450°C
At low T, surface diffusion is slow,
must supply kinetic energy for surface diffusion.
Plasma provides that energy…and enhances step coverage.
What is a plasma? Ionized noble gas, accelerated by AC (RF) or DC voltage,
collides with active species in gas and at surface, importing Ekin
Metal CVD
Step coverage is important for electric
contacts.
oxide
WF6 + 3H2 → W + 6 HF
∆G ≈ 70 kJ / mole (0.73 eV/atom)
oxide
semi
below 400°C
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26
Figure removed for copyright reasons.
Table 4-1 in Ohring, 2001.
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Wed., Oct. 12, 2005
27
When a system undergoes a process in which
only heat energy, dQ, and pV work occur at constant T,
B
B
∫ dG = RT ∫ d ln( p) ⇒ G
A
dp
dp
dG = NkB T
or RT
p
p
Vdp
dQ = TdS
B
then dG = Vdp
⎛ dG ⎞ pB
exp⎜−
⎟∝
⎝ RT ⎠ pA
− GA = RT ln(pB / pA )
A
Define this ratio pB/pA, as K
For multiple reagents:
K≡
PH 2 ⋅ PSiH2
PSiH4
−
= K0e
∆G
kT
Entropy of mixing, dSmix = −RN i ln(N i ) , leads to similar result
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Wed., Oct. 12, 2005
28