Transcript Section 5: Thin Film Deposition Jaeger Chapter 6 Part 2: Chemical Methods

Section 5: Thin Film Deposition
Part 2: Chemical Methods
Jaeger Chapter 6
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Chemical Vapor Deposition (CVD)
source
chemical reaction
film
substrate
More conformal deposition vs. PVD
t
Shown here
is 100%
conformal
deposition
t
step
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LPCVD Examples
(a )SiO2
gas
solid
SiH 4  O2 SiO2  2 H 2 
gas
350oC-500oC
(b)PSG : phospho silicate glass.P2O5  SiO2 
4 PH 3  5O2 2 P2O5  6 H 2
SiH 4  O2 SiO2  2 H 2 
350oC-500oC
(c)TEOS :tetraethylene orthosilicate.
SiOC2 H 5 4  SiO2  C X H Y OZ 
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LPCVD Examples
( d )Si3 N 4
3SiH 4  NH 3 Si3 N 4  12 H 2
(e)Poly  Si
SiH 4 
Si  2 H 2
600 o C
( f )W
WF6  3H 2  W  6 HF 
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CVD Mechanisms
reactant
5
1
surface
diffusion
2
3
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
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4
stagnant
gas layer
CVD Deposition Rate [Grove Model]
film
D
Si
F1
k s  ko e
F3

F1 =
F3 =

 hG
 E
kT
 = thickness of stagnant layer
D [ CG - CS] / 
kS CS
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F1  F3
Grove model of CVD (cont’d)
1
F3 
 CG
1 1

hG k s
Film growth rate = F3 / N
N = atomic density
of deposited film
dx F3
   contant with time
dt N
Note: This result is exactly the same as the Deal-Grove model
or thermal oxidation with oxide thickness =0
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Deposition Rate versus Temp
[log scale] Rate
gas transport limited R  T
3
2
surface-reaction
limited
0
1/T
high T
low
T
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Growth Rate Dependence on Flow Velocity
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LPCVD Features
(1) More conformal deposition,
if T is uniform
Wafer
topography
(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
Furnace tube
wafers
(2) Mass depletion problem
in
more
less
out
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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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