Chemical Vapor Deposition (CVD)
Processes,gift of SiO
2
- 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,SiH
4
(g)? Si (s) + 2H
2
(g)
Do not want Si to nucleate above substrate (
but on substrate surface (heterogeneous nucleation).
T
wall
Reactor
Transport
of precursors
across
dead layer to
substrate
Pyrolysis,thermal
Susceptor
film
T
sub
> T
wall
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
J
1
μD
g
DC
Reaction with film
J
2
~ k
i
C
i
Surface diffusion
Mon.,Sept,15,2003
4
Gas transport
J
1
μD
g
DC
Transport
across
boundary
layer
2
Knudsen N
K
≡
l
L
<1
L
Viscous flow
D
gas
a
lv
x
2
Mon.,Sept,15,2003
5
Revisit gas
J
1
= h
g
(
C
g
- C
s
)
dC D
(
C
g
- C
s
)
J
1
= 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,u
0
boundary layer
C
g
d (x)
d (x) u = 0
s
C
s
wafer
wafer
x
x = L
hx
Fluid dynamics,d(x) =
r = mass density,h = viscosity
ru
0
L
1 h 2 L Reynolds #,Re = r
u
0
L
d =
ú
d(x)dx =
2
3
L
ru
0
L
≡
3Re
ease of gas flow
h
L
0
D 3 D
Re
So:
h
g
=
d 2 L
Mon.,Sept,15,2003 6
Several processes in series
Simplify CVD to 2 steps,
Boundary
AB
layer
D
g
J
1
=
d
DC
J
2
B
A
J
2
= kC
ss
Reaction rate constant,k
Sticking coefficient g
AB
,
s
…as in oxidation,but no
0 ≤ g
AB
≤ 1
sold-state diffusion here,
reaction occurs at surface,
AB bounces Good
off surface adhesion
Let’s analyze,solve for J
2
…
Mon.,Sept,15,2003 7
J
1
= J
2
,
h
g
( C
g
- C
s
k
s
C
s
J
2
= k
s
C
s
=
h
g
k
s
h
g
+ k
s
C
g
C
s
=
h
g
h
g
+ k
s
C
g
,
Boundary
layer
J
2
= k
s
C
s
B
A
AB
J
2
J
1
= h
g
C
g
- C
s
( )
process:
J
1
=
D
g
d
DC
In steady state,
) =
Two main CVD
J
1
= J
2
,
1
+R
2
1
G
2
/(G
1
+G
2
)
Electrical analogy,
R = R
G = 1/R= G
Two processes in series; slowest one limits film growth
Mon.,Sept,15,2003 8
Boundary
layer
J
2
= k
s
C
s
B
A
AB
J
2
J
1
= h
g
C
g
- C
s
()
Two main CVD
process:
J
1
=
D
g
d
DC
J
2
= k
s
C
s
=
h
g
k
s
h
g
+ k
s
C
g
≡ v = J
#
area - t
ê
á
ˉ
1
N
#
vol
ê
á
ˉ
,v =
h
g
k
s
h
g
+ k
s
C
g
N
f
=
C
g
N
f
1
h
g
+
1
k
s
Film growth rate
Slower process controls growth
Mon.,Sept,15,2003 9
Boundary
layer
J
2
= k
s
C
s
B
A
AB
J
2
J
1
= h
g
C
g
- C
s
()
Two main CVD
process:
Examine these 2 limits of growth,
h or k limited…
g s
Transport limited growth,
Reaction limited growth,
k << h,
v =
C
g
N
f
1
h
g
+
1
k
s
s g
h << k,
g s
g
hC 3DC kT
gxg
3lv C Re
v =
k
s
C
g
=
C
k
0
e
-
DG
v =
gg
Re =
N
f
2LN
f
4LN
f
N
f
N
f
ease of gas flow
Mon.,Sept,15,2003 10
Transport limited growth, Reaction limited growth,
hC 3DC 3lv C Re
kC C
-
DG
gxg
g
kT
v =
gg
Re =
v =
sg
= k
0
e
N
f
2LN
f
4LN
f
N
f
N
f
Most CVD is done in this limit
where gas dynamics,
G = free energy change in reaction
reactor design are important.
(?G @?H for gas
becasue gas reaction no?S)
3
o
-10
o
B
A
J
2
Remedy for boundary layer
Susceptor,
More uniform u
g
,C fi Choice of reactants and
g
uniform film growth rate,v
temperature are critical
Mon.,Sept,15,2003 11
CVD FILM GROWTH
GAS TRANSPORT-LIMITED
REACTION-RATE LIMITED
3lv C
DG
v =
xg
kC C
-
g
kT
4N
f
L
N
v =
sg
= k
0
e
f
N
f
2k
B
T
v =,
G = free energy change in reaction
x
pm
k
B
T
Re
l =,
2pd
2
P
g
(?G @?H for gas? no?S for
gas reaction)
Re ~ u
0
1
=
k
B
T
v ~ e
-
DH
kT
v μT
1
2
u
0
g
P
g
C
Mon.,Sept,15,2003 12
Transport
ln (v)
high T
limited
1
2
v μTu
0
low T
Reaction
limited
gas - vel,
u
0
-
DH
Rate:
v ~ e
kT
Most CVD is transport-
limited,Slow,layer-by-layer
ln (v)
growth,epitaxy,Requires
T
1/2
high T,low pressure,low gas
viscosity,Chamber design,
fi DH
gas dynamics control
process.
Arrhenius-like
To reduce nucleation of
1 / T
products in gas phase,use
T
1000K 400K
low partial pressure (LPCVD),
Mon.,Sept,15,2003 13
Review CVD
We saw…
CVD is film growth from vapor/gas phase via chemical reactions
Mon.,Sept,15,2003 14
4
(g)?
2
(g)
Pyrolysis:
at substrate
film
Susceptor
Reactor
T
wall
T
sub
> T
wall
Transport
across
substrate
by-products
:
bond to substrate
in gas and at substrate,
e.g,SiH Si (s) + 2H
thermal decomposition
of precursors
dead layer to
Removal of
Chemical reaction
Decomposed species
ln (v
v μT u
0
1/2
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,
l/L = N
K
>> 1,
That puts you in the
limited
Rate:
v ~ e
-
DH
kT
high T
low T
u
0
)
Reaction
limited
Arrhenius-like
H
T
1/2
Gas transport
fi D
Reaction-limited regime
1 / T
T
1000K 400K
Mon.,Sept,15,2003 15
Some CVD reactions
Silane pyrolysis
(heat induced reaction)
SiH
4
(g)? Si (s) + 2H
2
(g) ( 650°C)
This fi poor Si at 1 atm,so use low pressure
Silane oxidation (450°C)
SiH
4
(g) + O
2
(g)? SiO
2
(s) + 2H
2
(g)
(by LPCVD for gate oxide)
v
Si - tetrachloride reduction
SiCl
4
(g) + 2H
2
(g)?
Si (s) + 4HCl (g) (1200°C)
Crystalline
P
SiCl
4
P
H
2
Poly Si
(Si-tetra…actually much more complex than this;
etch
8 different compounds are formed,detected by RGA)
Mon.,Sept,15,2003 16
Some CVD reactions (cont.)
Doping
Phosphine
Diborane
2PH
3
(g)?2P (s) + 3H
2
(g)
B
2
H
6
(g)?2B (s) + 3H
2
(g)
GaAs growth
Trimethyl Ga (TMG) reduction
(CH
3
)
3
Ga + H
2
Ga (s) + 3CH
4
Least abundant
element on surface
Arsene 2AsH
3
2As (s) + 3H
2
limits growth velocity
750°C
6 GaAs (s) + 6 HCl gOr As
4
(g) + As
2
(g) + 6 GaCl (g) + 3 H
2
(g)
¨?
850°C
Si-nitride compound formation
3 SiCl
2
H
2
(g) + 4NH
3
(g)? Si
3
N
4
(s) + 6H
2
(g) + 6HCl (g) (750° C)
Mon.,Sept,15,2003 17
How can you select process parameters to get
desired product and growth characteristics?
Consider,SiH
4
(g)? SiH
2
(g) + H
2
(g) Three unknown pressures
1) Total pressure =? partial Ps
P
tot
= P
SiH
4
+ P
H
2
+ P
SiH
2
…still have 2 unknown Ps
Si
2) Conservation of ratio =>
H
P
SiH
2
+ P
SiH
4
4P
SiH
4
+ 2P
SiH
2
+ 2P
H
2
= const
…still have 1 unknown P
3),Equilibrium constant”,K (cf,Law of mass action)
K ≡
P
H
2
P
SiH
2
P
SiH
4
= K
0
e
-
DG
kT
=?H for gas
And similarly for
each reaction,
These equations provide a starting place for growth parameters,(Many
eqs,for real systems; done on computer) Do a run,analyze results,
tweak process,
Mon.,Sept,15,2003 18
-
Where does K ≡
P
H
2
P
SiH
2
= K
0
e
D
kT
G
come from?
P
SiH
4 Consider,mass action” for class groups…
Consider,mass action” for electrons and holes,
Intrinsic semiconductor
N-type semiconductor
Conduction
n
i
n
band
E
F
Donor
levels
E
F
p
i
p
Valence
band
n
Recombination
2 2
n
probability set
n
i
= n
i
p
i i
= np
by energy gap and
More free electrons
number of each species
p
=> more recombination,
fewer holes (E
g
same)
P
H
2
P
SiH
2
= KP
SiH
4
K indicates a bbias at equilibrium in the reaction
toward the products(different molecular species)
Mon.,Sept,15,2003 19
Exercise
Assume reaction,AB? A + B P
tot
= 1 atm,T = 1000 K,
¨
K = 1.8 ￥ 10
9
Torr ￥ exp ( - 2 eV / k
B
T )
Assume P
A
≈ P
B
find P
AB
Solution,K =
P
A
P
B
and at 1000 Kelvin,K = 0.153 Torr,
P
AB
and P
tot
= P
A
+ P
B
+ P
AB,
P
A
≈ P
B.
\ 760 T = 2P
A
+ P
AB
P
A
2
= 0.153 P
AB
= 0.153 (760 - 2 P
A
)? P
A
= P
B
= 10.9 Torr,P
AB
= 738 Torr
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 lower T?
Mon.,Sept,15,2003 20
Atmospheric Pressure CVD,APCVD
(little used today,but illustrative)
High P,small l => 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,
SiH
4
+ 2O
2
SiO
2
+ 2H
2
O T = 240 - 450°C
Done in N
2
ambient (llow partial pressure of active gas,reduces reaction rate)
Mon.,Sept,15,2003
add 4 - 12% PH
3
low T,
limited
ln v
Transprt ltd APCVD
T
to make silica flow,planarize.
reaction rate
1/
21
Low Pressure CVD (LPCVD) for dielectrics and semiconductors
Equilibrium not achieved at low P where
l
= K >1
n
L
(molecular flow,few collisions),
l =
k
B
T
2pd
2
P
lower P => higher D
g
,h
g
improves transport
reduces boundary layer,
Mon.,Sept,15,2003 22
F.9.13
LPCVD
h
g
ln v
Transport
limited
Reaction limited
T
h
g
k
s
term
extends reaction-limited regime
at 1 Torr
1/
at 760 Torr
Low Pressure CVD (LPCVD) for dielectrics and semiconductors
Hot wall reactor
fi uniform T distribution but
surface of reactor gets coated,
So system must be dedicated to
1 species to avoid contamination.
Cold wall reactor
Reduce reaction rate,
deposition on
surfaces,For epi Si,
All poly-Si is done by hot-walled LPCVD;
good for low pin-hole SiO2
,conformality
Mon.,Sept,15,2003
23
Low Pressure CVD (LPCVD) for dielectrics and semiconductors
In such non-equilibrium,large l cases,
growth rate is reaction limited,
Low P LPCVD kinetically controlled,reaction-rate limited,
Silane pyrolysis SiH
4
(g)? Si (s) + 2H
2
(g) ( 575 - 650°C)
10 - 100 nm/min
(Atm,P APCVD equilibrium,transport ltd.)
LPCVD
+ requires no carrier gas
+ fewer gas-phase reactions,fewer particulates
+ eliminates boundary layer problem
+ lower P => higher D
g
,extends reaction-limited regime
+ good conformal growth (unlike sputtering or other PVD methods
which are directional)
- strong temperature dependence to growth rate
+ easier to control T with hot-wall furnace
Mon.,Sept,15,2003 24
R.F,Plasma-enhanced CVD (PECVD) for dielectric
MOS metallization,avoid contact interaction betw,Al & Si,SiO
2
,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 E
kin
Metal CVD
Step coverage is important for electric
contacts,
oxide
WF
6
+ 3H
2
W + 6 HF
oxide
DG ≈ 70 kJ / mole (0.73 eV/atom)
semi
below 400°C
Mon.,Sept,15,2003 25
Processes,gift of SiO
2
- 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,SiH
4
(g)? Si (s) + 2H
2
(g)
Do not want Si to nucleate above substrate (
but on substrate surface (heterogeneous nucleation).
T
wall
Reactor
Transport
of precursors
across
dead layer to
substrate
Pyrolysis,thermal
Susceptor
film
T
sub
> T
wall
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
J
1
μD
g
DC
Reaction with film
J
2
~ k
i
C
i
Surface diffusion
Mon.,Sept,15,2003
4
Gas transport
J
1
μD
g
DC
Transport
across
boundary
layer
2
Knudsen N
K
≡
l
L
<1
L
Viscous flow
D
gas
a
lv
x
2
Mon.,Sept,15,2003
5
Revisit gas
J
1
= h
g
(
C
g
- C
s
)
dC D
(
C
g
- C
s
)
J
1
= 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,u
0
boundary layer
C
g
d (x)
d (x) u = 0
s
C
s
wafer
wafer
x
x = L
hx
Fluid dynamics,d(x) =
r = mass density,h = viscosity
ru
0
L
1 h 2 L Reynolds #,Re = r
u
0
L
d =
ú
d(x)dx =
2
3
L
ru
0
L
≡
3Re
ease of gas flow
h
L
0
D 3 D
Re
So:
h
g
=
d 2 L
Mon.,Sept,15,2003 6
Several processes in series
Simplify CVD to 2 steps,
Boundary
AB
layer
D
g
J
1
=
d
DC
J
2
B
A
J
2
= kC
ss
Reaction rate constant,k
Sticking coefficient g
AB
,
s
…as in oxidation,but no
0 ≤ g
AB
≤ 1
sold-state diffusion here,
reaction occurs at surface,
AB bounces Good
off surface adhesion
Let’s analyze,solve for J
2
…
Mon.,Sept,15,2003 7
J
1
= J
2
,
h
g
( C
g
- C
s
k
s
C
s
J
2
= k
s
C
s
=
h
g
k
s
h
g
+ k
s
C
g
C
s
=
h
g
h
g
+ k
s
C
g
,
Boundary
layer
J
2
= k
s
C
s
B
A
AB
J
2
J
1
= h
g
C
g
- C
s
( )
process:
J
1
=
D
g
d
DC
In steady state,
) =
Two main CVD
J
1
= J
2
,
1
+R
2
1
G
2
/(G
1
+G
2
)
Electrical analogy,
R = R
G = 1/R= G
Two processes in series; slowest one limits film growth
Mon.,Sept,15,2003 8
Boundary
layer
J
2
= k
s
C
s
B
A
AB
J
2
J
1
= h
g
C
g
- C
s
()
Two main CVD
process:
J
1
=
D
g
d
DC
J
2
= k
s
C
s
=
h
g
k
s
h
g
+ k
s
C
g
≡ v = J
#
area - t
ê
á
ˉ
1
N
#
vol
ê
á
ˉ
,v =
h
g
k
s
h
g
+ k
s
C
g
N
f
=
C
g
N
f
1
h
g
+
1
k
s
Film growth rate
Slower process controls growth
Mon.,Sept,15,2003 9
Boundary
layer
J
2
= k
s
C
s
B
A
AB
J
2
J
1
= h
g
C
g
- C
s
()
Two main CVD
process:
Examine these 2 limits of growth,
h or k limited…
g s
Transport limited growth,
Reaction limited growth,
k << h,
v =
C
g
N
f
1
h
g
+
1
k
s
s g
h << k,
g s
g
hC 3DC kT
gxg
3lv C Re
v =
k
s
C
g
=
C
k
0
e
-
DG
v =
gg
Re =
N
f
2LN
f
4LN
f
N
f
N
f
ease of gas flow
Mon.,Sept,15,2003 10
Transport limited growth, Reaction limited growth,
hC 3DC 3lv C Re
kC C
-
DG
gxg
g
kT
v =
gg
Re =
v =
sg
= k
0
e
N
f
2LN
f
4LN
f
N
f
N
f
Most CVD is done in this limit
where gas dynamics,
G = free energy change in reaction
reactor design are important.
(?G @?H for gas
becasue gas reaction no?S)
3
o
-10
o
B
A
J
2
Remedy for boundary layer
Susceptor,
More uniform u
g
,C fi Choice of reactants and
g
uniform film growth rate,v
temperature are critical
Mon.,Sept,15,2003 11
CVD FILM GROWTH
GAS TRANSPORT-LIMITED
REACTION-RATE LIMITED
3lv C
DG
v =
xg
kC C
-
g
kT
4N
f
L
N
v =
sg
= k
0
e
f
N
f
2k
B
T
v =,
G = free energy change in reaction
x
pm
k
B
T
Re
l =,
2pd
2
P
g
(?G @?H for gas? no?S for
gas reaction)
Re ~ u
0
1
=
k
B
T
v ~ e
-
DH
kT
v μT
1
2
u
0
g
P
g
C
Mon.,Sept,15,2003 12
Transport
ln (v)
high T
limited
1
2
v μTu
0
low T
Reaction
limited
gas - vel,
u
0
-
DH
Rate:
v ~ e
kT
Most CVD is transport-
limited,Slow,layer-by-layer
ln (v)
growth,epitaxy,Requires
T
1/2
high T,low pressure,low gas
viscosity,Chamber design,
fi DH
gas dynamics control
process.
Arrhenius-like
To reduce nucleation of
1 / T
products in gas phase,use
T
1000K 400K
low partial pressure (LPCVD),
Mon.,Sept,15,2003 13
Review CVD
We saw…
CVD is film growth from vapor/gas phase via chemical reactions
Mon.,Sept,15,2003 14
4
(g)?
2
(g)
Pyrolysis:
at substrate
film
Susceptor
Reactor
T
wall
T
sub
> T
wall
Transport
across
substrate
by-products
:
bond to substrate
in gas and at substrate,
e.g,SiH Si (s) + 2H
thermal decomposition
of precursors
dead layer to
Removal of
Chemical reaction
Decomposed species
ln (v
v μT u
0
1/2
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,
l/L = N
K
>> 1,
That puts you in the
limited
Rate:
v ~ e
-
DH
kT
high T
low T
u
0
)
Reaction
limited
Arrhenius-like
H
T
1/2
Gas transport
fi D
Reaction-limited regime
1 / T
T
1000K 400K
Mon.,Sept,15,2003 15
Some CVD reactions
Silane pyrolysis
(heat induced reaction)
SiH
4
(g)? Si (s) + 2H
2
(g) ( 650°C)
This fi poor Si at 1 atm,so use low pressure
Silane oxidation (450°C)
SiH
4
(g) + O
2
(g)? SiO
2
(s) + 2H
2
(g)
(by LPCVD for gate oxide)
v
Si - tetrachloride reduction
SiCl
4
(g) + 2H
2
(g)?
Si (s) + 4HCl (g) (1200°C)
Crystalline
P
SiCl
4
P
H
2
Poly Si
(Si-tetra…actually much more complex than this;
etch
8 different compounds are formed,detected by RGA)
Mon.,Sept,15,2003 16
Some CVD reactions (cont.)
Doping
Phosphine
Diborane
2PH
3
(g)?2P (s) + 3H
2
(g)
B
2
H
6
(g)?2B (s) + 3H
2
(g)
GaAs growth
Trimethyl Ga (TMG) reduction
(CH
3
)
3
Ga + H
2
Ga (s) + 3CH
4
Least abundant
element on surface
Arsene 2AsH
3
2As (s) + 3H
2
limits growth velocity
750°C
6 GaAs (s) + 6 HCl gOr As
4
(g) + As
2
(g) + 6 GaCl (g) + 3 H
2
(g)
¨?
850°C
Si-nitride compound formation
3 SiCl
2
H
2
(g) + 4NH
3
(g)? Si
3
N
4
(s) + 6H
2
(g) + 6HCl (g) (750° C)
Mon.,Sept,15,2003 17
How can you select process parameters to get
desired product and growth characteristics?
Consider,SiH
4
(g)? SiH
2
(g) + H
2
(g) Three unknown pressures
1) Total pressure =? partial Ps
P
tot
= P
SiH
4
+ P
H
2
+ P
SiH
2
…still have 2 unknown Ps
Si
2) Conservation of ratio =>
H
P
SiH
2
+ P
SiH
4
4P
SiH
4
+ 2P
SiH
2
+ 2P
H
2
= const
…still have 1 unknown P
3),Equilibrium constant”,K (cf,Law of mass action)
K ≡
P
H
2
P
SiH
2
P
SiH
4
= K
0
e
-
DG
kT
=?H for gas
And similarly for
each reaction,
These equations provide a starting place for growth parameters,(Many
eqs,for real systems; done on computer) Do a run,analyze results,
tweak process,
Mon.,Sept,15,2003 18
-
Where does K ≡
P
H
2
P
SiH
2
= K
0
e
D
kT
G
come from?
P
SiH
4 Consider,mass action” for class groups…
Consider,mass action” for electrons and holes,
Intrinsic semiconductor
N-type semiconductor
Conduction
n
i
n
band
E
F
Donor
levels
E
F
p
i
p
Valence
band
n
Recombination
2 2
n
probability set
n
i
= n
i
p
i i
= np
by energy gap and
More free electrons
number of each species
p
=> more recombination,
fewer holes (E
g
same)
P
H
2
P
SiH
2
= KP
SiH
4
K indicates a bbias at equilibrium in the reaction
toward the products(different molecular species)
Mon.,Sept,15,2003 19
Exercise
Assume reaction,AB? A + B P
tot
= 1 atm,T = 1000 K,
¨
K = 1.8 ￥ 10
9
Torr ￥ exp ( - 2 eV / k
B
T )
Assume P
A
≈ P
B
find P
AB
Solution,K =
P
A
P
B
and at 1000 Kelvin,K = 0.153 Torr,
P
AB
and P
tot
= P
A
+ P
B
+ P
AB,
P
A
≈ P
B.
\ 760 T = 2P
A
+ P
AB
P
A
2
= 0.153 P
AB
= 0.153 (760 - 2 P
A
)? P
A
= P
B
= 10.9 Torr,P
AB
= 738 Torr
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 lower T?
Mon.,Sept,15,2003 20
Atmospheric Pressure CVD,APCVD
(little used today,but illustrative)
High P,small l => 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,
SiH
4
+ 2O
2
SiO
2
+ 2H
2
O T = 240 - 450°C
Done in N
2
ambient (llow partial pressure of active gas,reduces reaction rate)
Mon.,Sept,15,2003
add 4 - 12% PH
3
low T,
limited
ln v
Transprt ltd APCVD
T
to make silica flow,planarize.
reaction rate
1/
21
Low Pressure CVD (LPCVD) for dielectrics and semiconductors
Equilibrium not achieved at low P where
l
= K >1
n
L
(molecular flow,few collisions),
l =
k
B
T
2pd
2
P
lower P => higher D
g
,h
g
improves transport
reduces boundary layer,
Mon.,Sept,15,2003 22
F.9.13
LPCVD
h
g
ln v
Transport
limited
Reaction limited
T
h
g
k
s
term
extends reaction-limited regime
at 1 Torr
1/
at 760 Torr
Low Pressure CVD (LPCVD) for dielectrics and semiconductors
Hot wall reactor
fi uniform T distribution but
surface of reactor gets coated,
So system must be dedicated to
1 species to avoid contamination.
Cold wall reactor
Reduce reaction rate,
deposition on
surfaces,For epi Si,
All poly-Si is done by hot-walled LPCVD;
good for low pin-hole SiO2
,conformality
Mon.,Sept,15,2003
23
Low Pressure CVD (LPCVD) for dielectrics and semiconductors
In such non-equilibrium,large l cases,
growth rate is reaction limited,
Low P LPCVD kinetically controlled,reaction-rate limited,
Silane pyrolysis SiH
4
(g)? Si (s) + 2H
2
(g) ( 575 - 650°C)
10 - 100 nm/min
(Atm,P APCVD equilibrium,transport ltd.)
LPCVD
+ requires no carrier gas
+ fewer gas-phase reactions,fewer particulates
+ eliminates boundary layer problem
+ lower P => higher D
g
,extends reaction-limited regime
+ good conformal growth (unlike sputtering or other PVD methods
which are directional)
- strong temperature dependence to growth rate
+ easier to control T with hot-wall furnace
Mon.,Sept,15,2003 24
R.F,Plasma-enhanced CVD (PECVD) for dielectric
MOS metallization,avoid contact interaction betw,Al & Si,SiO
2
,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 E
kin
Metal CVD
Step coverage is important for electric
contacts,
oxide
WF
6
+ 3H
2
W + 6 HF
oxide
DG ≈ 70 kJ / mole (0.73 eV/atom)
semi
below 400°C
Mon.,Sept,15,2003 25
