Section 5: Thin Film Deposition Jaeger Chapter 6 Chemical Vapor ...

Section 5: Thin Film Deposition
Part 2: Chemical Methods
Jaeger Chapter 6
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Chemical Vapor Deposition (CVD)
substrate
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source
More conformal deposition vs. PVD
Shown here
is 100%
conformal
deposition
t
t
chemical reaction
film
step

( a)
SiO
SiH
+ O →SiO
( b)
PSG : phospho
4PH3
+ 5O
→2P
O
SiH
Si
4
4
2
+ O →SiO
+ 2H
+ 2H
( OC H ) → SiO + C H O ↑
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↑
silicate
+ 6H
( c)
TEOS : tetraethylene
orthosilicate.
2
gas
gas 350 o C-500 o C
2
2
LPCVD Examples
5
2
4
( d ) Si
3SiH
( e)
Poly
(
SiH
f
WF
) W
+
N
+
solid
2
2
2
5
2
2
2
350 o C-500 o C
−
→
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↑
600 C
⎯ ⎯ →Si
3 H
NH
Si
X
→ Si
W
glass.
LPCVD Examples
6
4
3
4
4
o
2
3
2
3
+
+
Y
N
4
Z
2 H
6 HF
[ PO
+ SiO ]
2
↑
5
+ 12 H
2
2
2

1
2
surface
diffusion
CVD Mechanisms
reactant
substrate
1 = Diffusion of reactant to surface
2 = Absorption of reactant to surface
3 = Chemical reaction
4 = Desorption of gas by-products
5 = Outdiffusion of by-product gas
3
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Example Poly-Si Deposition
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5
4
stagnant
gas layer

F 1
F
F
CVD Deposition Rate [Grove Model]
1
3
=
=
F 3
δ
film
D [ C G -C S ] / δ
k S C S
Si
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G
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D
=
δ
k
s
=
h
k
G
o
e
−ΔE
kT
δ = thickness of stagnant layer
s
F =
Grove model of CVD (cont’d)
1
∴F
3 = ⋅CG
1 1
+
h k
Film growth rate = F 3 / N
dx F3
∴ = = contant with time
dt N
1
F
3
N = atomic density
of deposited film
Note: This result is exactly the same as the Deal-Grove model
or thermal oxidation with oxide thickness =0

[log scale] Rate
0
Deposition Rate versus Temp
high T
gas transport limited
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R∝T 3 2
surface-reaction
limited
low
T
1/T
Growth Rate Dependence on Flow Velocity

LPCVD Features
(1) More conformal deposition,
if T is uniform
(2) Inter-wafer and intra-wafer thickness uniformity less
sensitive to gas flow patterns. (i.e. wafer placement).
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Comments about LPCVD
(1) Sensitivity to gas flow pattern
(2) Mass depletion problem
in
more less
wafers
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Furnace tube
out
Wafer
topography

Plasma Enhanced CVD
• Ionized chemical species allows a lower process
temperature to be used.
• Film properties (e.g. mechanical stress) can be tailored
by controllable ion bombardment with substrate bias voltage.
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Atomic Layer Deposition
• The process involves two self-limiting half reactions that are repeated in cycles
• Unlike CVD, in ALD pulses of precursors are introduced in each cycle
• ALD is highly conformal and enables excellent thickness uniformity and control
down to nm-scale
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