Chemical Vapor Deposition (CVD)
Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer…
or metal film growth : high vacuum, single element…
CVD:
toxic, corrosive gas flowing through valves,
T
… Contrast with
up to 1000°C, multiple simultaneous reactions,
gas dynamics, dead layers… whose idea was it?
All layers above poly-Si made by CVD, except gate oxide and aluminum
Mon., Sept. 15, 2003
1
CVD
reactors
Control
module
Four
reaction
chambers
(similar to those
for Si oxidation)
Control T,
gas mixture,
pressure,
flow rate
Mon., Sept. 15, 2003
2
CVD is film growth from vapor/gas phase via chemical reactions
in gas and on substrate:
homogeneous nucleation),
e.g. SiH4 (g) Æ Si (s) + 2H2 (g)
Do not want Si to nucleate above substrate (
but on substrate surface (heterogeneous nucleation).
Twall
Reactor
Transport
of precursors
across
dead layer to
substrate
Pyrolysis: thermal
Susceptor
film
T sub> Twall
Chemical reaction:
Decomposed species
bond to substrate
decomposition
at substrate
More details…
by-products
Removal of
Mon., Sept. 15, 2003
3
CVD Processes
8
1 Bulk
Bulk transport
transport
of byproduct
Reactant
molecule
7 Diffusion of
Transport
Carrier gas
2
across bndry
4
(g) byproduct
(Maintain hi p,
layer
Decomposition
slow reaction)
6 Desorption
3 Adsorption
5
J1 µDgDC
Reaction with film
J2 ~ kiCi
Surface diffusion
Mon., Sept. 15, 2003
4
Gas transport
J1 µDgDC
Transport
across
boundary
layer
2
Knudsen NK ≡
l
L
<1
L
Viscous flow
Dgas ª
lvx
2
Mon., Sept. 15, 2003
5
Revisit gas J1 = hg(Cg - Cs)
dC
D (Cg - Cs)
J1 = D
=
dx
d(x)
dynamics:
Boundary layer
Layer thickness, d(x)
lv x
(unlike solid)
And we saw gas diffusivity D =
2
z
u
gas vel: u0
boundary layer
Cg
d (x)
d (x)
u = 0
s
Cs
wafer
wafer
x
x = L
hx
Fluid dynamics:
d(x) =
r = mass density, h = viscosity
ru0
L
1
h
2 L
Reynolds #: Re = r u0L
d =
Ú d(x)dx = 23 L ru0L
≡
3 Re
ease of gas flow
h
L 0
D
3 D
Æ
Re
So: hg = d
2 L
Mon., Sept. 15, 2003
6
Several processes in series
Simplify CVD to 2 steps:
Bou