6.152J/3.155J
6.152J/3.155J 1
What does
have to do with
Microelectronic Processing?
Need to understand
Di l i i
i i i
l ili i iti i i
i l i s
Need to understand
i iti l i
ili li il
ls i iO
2
l)
l i iti
i il i
Electrical,mechanical properties depend on all of the above
chemical reactions
Gas diffusivity,
Solid-state
diffusion
Wed.,Sept,10,2003
Materials Science
fferences,meta s,ox des and sem conductors
Ox dat on rates,compound format on (GaAs)
So ub ty of mpur es n S
Chem ca react ons for CVD precursors,byproduct
Gas concentrat on (cr ca to CVD react on rates)
Surface mob ty (key to qua ty f m growth)
What contro nterfaces (e.g,S /A
crysta growth,and mpur es
How manage gra n growth,f m m crostructure
Atomic bonding,
Vacuum Technology and film growth
( FET)
Al
n
Diffusion
Resistor
Poly Si
Resistor
Al Al
P ly
P ly
p
-
Implant
poly-Gate p-MOS
-Si
++
Po
++
Po
All layers above n-type Si made by CVD except gate oxide and Al
6.152J/3.155J Wed.,Sept,10,2003 2
1
6.152J/3.155J
6.152J/3.155J 3
What will we cover in next few lectures?
Chemical vapor deposition (CVD)
i l
mi l i i il
al i l l i
Oxidation Wed,Sept 17
i l i i
l i i li
vacuum technology,… gas kinetics,
Diffusion and ion implantation
i Wed,Oct,1
Nov 5,12
li il iti i
Wed.,Sept,10,2003
Mon,Sept 15
Most w de y used method for growth of
cro-e ectron c grade sem conductor f ms,
so w de y used for meta s and ox des
Key advantage of S, stab e un form ox de
How contro ts growth,th ckness,qua ty
These processes take place in vacuum or controlled environment,
Therefore,need to understand
Mon,Sept 29
How sem conductor surfaces are doped
Physical vapor deposition (PVD)
Growth of qua ty f ms by sputter depos on or evaporat on
6.152J/3.155J 4
i i l
l l l llisi
“Movie”
d
d
l le
i i
i p d
2
=>
l
p d
2
Volume swept out by 1 molecule
lpd
2
i l V
2
)
L
velocity
“Snap shot”
n =
N
V
=
N
L
3
m 5 x 10
-26
kg
Wed.,Sept,10,2003
Gas K net cs and Vacuum Techno ogy
How far does a mo ecu e trave between co ons?
Mean free path ≡l
mo ecu
mpact parameter,scatter ng
cross sect on =
between collisions =
Cons der a vo ume of gas (e.g,N
number N,
2
6.152J/3.155J
6.152J/3.155J 5
Total volume of sample
L
3
= V N lpd
2
\l=
V
Npd
2
=
1
npd
2
(n =
N
V
)
More accurately l=
1
2 npd
2
Ideal gas,pV = Nk
B
T,
n=p / k
B
T =>
\l=
k
B
T
2pd
2
p
l
p d
2
Volume swept out by 1 molecule
between collisions = lpd
2
i i
p l (cm)
1 atm 10
-5
10
-2
1 mT 10
Wed.,Sept,10,2003
Gas K net cs
1 Torr
6.152J/3.155J 6
What is flux of atoms hitting surface per unit time?
area
# / vol,
v
x
J /
nv
x
2
related to pressure (elec,field)
v
x
,v
speed
P(v)
v
vms
v
v = vP
ú
(v)dv
Maxwell speed distribution,
P(v) = 4p
m
2pkT
è
í
˙
v
2
exp -
mv
2
2kT
è
í
˙
v
rms
=
3kT
m
v =
8kT
pm
,
v
x
=
2kT
pm
v
rms
≈ 500 m/s
v x =v /2
Wed.,Sept,10,2003
( # area time) =
Analogous to current density,
Calculating gas velocities
We need
3/ 2
3
6.152J/3.155J
6.152J/3.155J 7
So flux of atoms hitting surface per unit time
area
# / vol,
v
x
J
x
=
nv
x
2
=
n
2
2kT
pm
ideal
gas
p
2pmkT
= J
x
Di i l l i l
p =
E
kin
Vol
= n
mv
2
2
= Jmv
Numerically,Jx = 3.5 ￥10
22
p(Torr)
MT(g / mole?K)
(atoms /cm
2
sec)
This gives a flux 1 monolayer (ML) arriving per sec at 10
-6
Torr
l=
k
B
T
2pd
2
p
Compare,
Wed.,Sept,10,2003
mens ona ana ys s,(force/area = en/vo,):
Diffusivity
DG
D
0
exp -
è
DG
Recall for solids,D =í˙
kT?
Debye n 10
13
s
-1
For gas,no energy barrier,just collisions,
dC n
J = D @ D
gas gas
dx l
lv
x
D a
gas
2nv
x
(cm
2
/s)
2
kT
recall l=
2pd
2
p
v
\D
x
μ T
much weaker T-dep,than in so
gas
μT
lid
3/ 2
6.152J/3.155J Wed.,Sept,10,2003 8
4
6.152J/3.155J
6.152J/3.155J 9
L
Flow is viscous; p > 1 mT
Molecular,
p
What does this imply for pumping?
Pump power > viscosity;
must transport lg,# of molecules
must attract and hold molecules,
Knudsen N
0
≡
l
L
l
L
<1
l
L
>1
L
L
l=
k
B
T
2pd
2
p
Recall:
p l (cm)
1 atm 10
-5
10
-2
1 mT 10
Wed.,Sept,10,2003
= dimension of chamber or reactor
ballistic flow; < 1 mT
Knudson number
Pump efficiency critical;
1 Torr
6.152J/3.155J 10
Gas flow and pump speed
Gases are compressible unlike liquids…
so express flow as number of molecules/t,/t.
N μ pV Define throughput,Q,Q =
dN
dt
μ p
dV
dt
≡ pS
Q (std-p
I =
1
R
ê
á
ˉ
V
q
t
l
p
p
p
fi Q =Cp - p
p()
[Units of conductance
μ area/length]
Wed.,Sept,10,2003
not volume
[Units of ) => liters/min
or sccm]
Ohm’s aw:
pump
chamber
Conductance of vacuum component:
5
6.152J/3.155J
6.152J/3.155J Wed.,Sept,10,2003 11
Gas flow and pump speed
Effective pump speed,S,never exceeds
conductance of worst component or pump speed,S
p,
(p
p
is pressure
nearer pump)
But throughput,Q,
Q ≡ pS,p =
Q
S
p
p
=
Q
S
p
Conductance of vacuum component:
fi Q =Cp - p
p()
p
p
p
pump
chamber
Q =C
Q
s
-
Q
s
p
ê
á
á
ˉ
S
p
S
C
fi s =
cs
p
c + s
p
=
1
1
c
+
1
s
p
Vacuum technology,Generating low pressure
Two classes of vacuum pumps:
1) Molecules physically removed from chamber
a) mechanical pump
b) Turbo molecular pump
c) Oil diffusion pump
2) Molecules adsorbed on a surface,
or buried in a layer
a) Sputter/ion pump (with Ti sublimation)
b) Cryo pump
6.152J/3.155J Wed.,Sept,10,2003 12
6
(760 Torr
1 Torr
1 milli
6.152J/3.155J
6.152J/3.155J Wed.,Sept,10,2003 13
1) Molecules physically removed from chamber
a) Mechanical pump b) Oil diffusion pump c) Turbo molecular pump
Hot Si oil vaporized,
jetted toward fore pump,
momentum transfer to gas,
which is pumped out.
S = 12A L/s
Oil contamination,
Vibrations.
But pumps from
1 atm to mT.
S 2 x 10
4
L/s
1 atm
)
1 milliT
10
-6
T
10
-9
T
Rotating (25 krpm) vanes
impart momentum to gas,
pres’re incr’s away from chamber,
gas pumped by backing pump.
No oil,S = 10
3
L/s
6.152J/3.155J Wed.,Sept,10,2003 14
2) Molecules adsorbed on a surface,or buried in a layer
a) Sputter/ion pump (with Ti sublimation) b) Cryo pump
Gas is ionized
by hi-V,
ions spiral in B field,
embed in anode,
Coated by Ti.
No moving parts,
no oil.
S depends on
pump size and
S(H) >
>S(O,N,H
2
O)
B
v
1 atm
(760 Torr)
1 Torr
T
10
-6
T
10
-9
T
Very clean,
molecules condense
on cold (120 K) surfaces,
No moving parts,S 3A (cm
2
)L/s
7
6.152J/3.155J
PUMP SUMMARY
Two classes of vacuum pumps:
1) Molecules physically removed from chamber
a) mechanical pump Pumps from 1 atm; moving parts,oil
b) Turbo molecular pump Clean,pumps lg,M well,from 1 atm;
low pump speed,moving parts
c) Oil diffusion pump No moving parts; oil in vac
2) Molecules adsorbed on a surface,
or buried in a layer
a) Sputter/ion pump Clean,pumps reactants,no moving parts;
(with Ti sublimation) pumps from 10
-4
T,
b) Cryo pump Clean,no moving parts;
pumps from 10
-4
T,
6.152J/3.155J Wed.,Sept,10,2003 15
Vacuum technology,Deposition chambers
Standard vacuum,p >10
-6
Torr Ultrahigh vacuum,p >10
-11
Torr;
Glass or stainless steel,Stainless steel (bakeable);
usually diffusion pumped,Ion and/or turbo pumped
CVD,thermal evap,or sputter dep,thermal evap,Sputter deposition
=> polycrystalline films => better quality films,epitaxial
6.152J/3.155J Wed.,Sept,10,2003 16
8
6.152J/3.155J
Vacuum technology,Deposition chambers
Ultrahigh vacuum,p >10
-11
Torr;
Stainless steel (bakeable);
Ion and/or turbo pumped
thermal evap,Sputter deposition
=> better quality films,epitaxial
Baking a stainless-steel uhv system
(T up to 200 C for 10’s of hrs)
desorbs water vapor,organics
from chamber walls;
these are ion-pumped out;
pressure drops as T returns to RT.
6.152J/3.155J Wed.,Sept,10,2003 17
Thin film growth general
Bonds on 3 sides
More bonds
3 bonds with substrate
Arrival,sticking,
surface diffusion,
bonding
Bonds on 1 side
Rate of arrival
R ≡
Diffusion rate
Film growth competes with gas arrival,
1) R > 1 fi Non-equilibrium,fast growth,many misaligned islands form,
leading to defective (high-surface-en),polycrystalline film,columnar grains,
This 3-D growth is the Volmer-Weber mode; Can fi amorphous film,
2) R < 1 => Slower,more equilibrium,layer-by-layer growth,larger grains
(raise surface temperature to ↑ mobility fi↑ g.s,),If film and substrate
have same crystal structure,film may grow in perfect alignment with
substrate (“epitaxy”),This 2-D growth is the Frank-van der Merwe mode.
6.152J/3.155J Wed.,Sept,10,2003 18
9
6.152J/3.155J
Thin film growth details (R < 1)
1) Arri
phys
l
i l
i
i l
i
l i
Bu di i
R ≡
Rate of arrival
va rate,
ca
adsorpt on
3) Chem ca
react on
4) Nuc eat on
5) Growth
6) lk ffus on
Diffusion rate
2) Surface
diffusion
If R > 1,these processes have reduced probability
6.152J/3.155J Wed.,Sept,10,2003 19
Looking ahead…
Thin films made by a variety of means:
thermal vapor deposition (evaporation)
Physical vapor deposition
- for metals
(PVD)
sputter deposition
DC-magnetron- for metals
-RF for oxides
chemical vapor deposition
Chemical vapor deposition
- for metals,semiconductors
(CVD)
6.152J/3.155J Wed.,Sept,10,2003 20
10
6.152J/3.155J
6.152J/3.155J 21Wed.,Sept,10,2003
Thin film growth details (R < 1)
1) Arri
phys
l
i l
i
i l
i
l i
Bu di i
R ≡
Rate of arrival
va rate,
ca
adsorpt on
3) Chem ca
react on
4) Nuc eat on
5) Growth
6) lk ffus on
Diffusion rate
2) Surface
diffusion
If R > 1,these processes have reduced probability
6.152J/3.155J Wed.,Sept,10,2003 22
11
6.152J/3.155J 1
What does
have to do with
Microelectronic Processing?
Need to understand
Di l i i
i i i
l ili i iti i i
i l i s
Need to understand
i iti l i
ili li il
ls i iO
2
l)
l i iti
i il i
Electrical,mechanical properties depend on all of the above
chemical reactions
Gas diffusivity,
Solid-state
diffusion
Wed.,Sept,10,2003
Materials Science
fferences,meta s,ox des and sem conductors
Ox dat on rates,compound format on (GaAs)
So ub ty of mpur es n S
Chem ca react ons for CVD precursors,byproduct
Gas concentrat on (cr ca to CVD react on rates)
Surface mob ty (key to qua ty f m growth)
What contro nterfaces (e.g,S /A
crysta growth,and mpur es
How manage gra n growth,f m m crostructure
Atomic bonding,
Vacuum Technology and film growth
( FET)
Al
n
Diffusion
Resistor
Poly Si
Resistor
Al Al
P ly
P ly
p
-
Implant
poly-Gate p-MOS
-Si
++
Po
++
Po
All layers above n-type Si made by CVD except gate oxide and Al
6.152J/3.155J Wed.,Sept,10,2003 2
1
6.152J/3.155J
6.152J/3.155J 3
What will we cover in next few lectures?
Chemical vapor deposition (CVD)
i l
mi l i i il
al i l l i
Oxidation Wed,Sept 17
i l i i
l i i li
vacuum technology,… gas kinetics,
Diffusion and ion implantation
i Wed,Oct,1
Nov 5,12
li il iti i
Wed.,Sept,10,2003
Mon,Sept 15
Most w de y used method for growth of
cro-e ectron c grade sem conductor f ms,
so w de y used for meta s and ox des
Key advantage of S, stab e un form ox de
How contro ts growth,th ckness,qua ty
These processes take place in vacuum or controlled environment,
Therefore,need to understand
Mon,Sept 29
How sem conductor surfaces are doped
Physical vapor deposition (PVD)
Growth of qua ty f ms by sputter depos on or evaporat on
6.152J/3.155J 4
i i l
l l l llisi
“Movie”
d
d
l le
i i
i p d
2
=>
l
p d
2
Volume swept out by 1 molecule
lpd
2
i l V
2
)
L
velocity
“Snap shot”
n =
N
V
=
N
L
3
m 5 x 10
-26
kg
Wed.,Sept,10,2003
Gas K net cs and Vacuum Techno ogy
How far does a mo ecu e trave between co ons?
Mean free path ≡l
mo ecu
mpact parameter,scatter ng
cross sect on =
between collisions =
Cons der a vo ume of gas (e.g,N
number N,
2
6.152J/3.155J
6.152J/3.155J 5
Total volume of sample
L
3
= V N lpd
2
\l=
V
Npd
2
=
1
npd
2
(n =
N
V
)
More accurately l=
1
2 npd
2
Ideal gas,pV = Nk
B
T,
n=p / k
B
T =>
\l=
k
B
T
2pd
2
p
l
p d
2
Volume swept out by 1 molecule
between collisions = lpd
2
i i
p l (cm)
1 atm 10
-5
10
-2
1 mT 10
Wed.,Sept,10,2003
Gas K net cs
1 Torr
6.152J/3.155J 6
What is flux of atoms hitting surface per unit time?
area
# / vol,
v
x
J /
nv
x
2
related to pressure (elec,field)
v
x
,v
speed
P(v)
v
vms
v
v = vP
ú
(v)dv
Maxwell speed distribution,
P(v) = 4p
m
2pkT
è
í
˙
v
2
exp -
mv
2
2kT
è
í
˙
v
rms
=
3kT
m
v =
8kT
pm
,
v
x
=
2kT
pm
v
rms
≈ 500 m/s
v x =v /2
Wed.,Sept,10,2003
( # area time) =
Analogous to current density,
Calculating gas velocities
We need
3/ 2
3
6.152J/3.155J
6.152J/3.155J 7
So flux of atoms hitting surface per unit time
area
# / vol,
v
x
J
x
=
nv
x
2
=
n
2
2kT
pm
ideal
gas
p
2pmkT
= J
x
Di i l l i l
p =
E
kin
Vol
= n
mv
2
2
= Jmv
Numerically,Jx = 3.5 ￥10
22
p(Torr)
MT(g / mole?K)
(atoms /cm
2
sec)
This gives a flux 1 monolayer (ML) arriving per sec at 10
-6
Torr
l=
k
B
T
2pd
2
p
Compare,
Wed.,Sept,10,2003
mens ona ana ys s,(force/area = en/vo,):
Diffusivity
DG
D
0
exp -
è
DG
Recall for solids,D =í˙
kT?
Debye n 10
13
s
-1
For gas,no energy barrier,just collisions,
dC n
J = D @ D
gas gas
dx l
lv
x
D a
gas
2nv
x
(cm
2
/s)
2
kT
recall l=
2pd
2
p
v
\D
x
μ T
much weaker T-dep,than in so
gas
μT
lid
3/ 2
6.152J/3.155J Wed.,Sept,10,2003 8
4
6.152J/3.155J
6.152J/3.155J 9
L
Flow is viscous; p > 1 mT
Molecular,
p
What does this imply for pumping?
Pump power > viscosity;
must transport lg,# of molecules
must attract and hold molecules,
Knudsen N
0
≡
l
L
l
L
<1
l
L
>1
L
L
l=
k
B
T
2pd
2
p
Recall:
p l (cm)
1 atm 10
-5
10
-2
1 mT 10
Wed.,Sept,10,2003
= dimension of chamber or reactor
ballistic flow; < 1 mT
Knudson number
Pump efficiency critical;
1 Torr
6.152J/3.155J 10
Gas flow and pump speed
Gases are compressible unlike liquids…
so express flow as number of molecules/t,/t.
N μ pV Define throughput,Q,Q =
dN
dt
μ p
dV
dt
≡ pS
Q (std-p
I =
1
R
ê
á
ˉ
V
q
t
l
p
p
p
fi Q =Cp - p
p()
[Units of conductance
μ area/length]
Wed.,Sept,10,2003
not volume
[Units of ) => liters/min
or sccm]
Ohm’s aw:
pump
chamber
Conductance of vacuum component:
5
6.152J/3.155J
6.152J/3.155J Wed.,Sept,10,2003 11
Gas flow and pump speed
Effective pump speed,S,never exceeds
conductance of worst component or pump speed,S
p,
(p
p
is pressure
nearer pump)
But throughput,Q,
Q ≡ pS,p =
Q
S
p
p
=
Q
S
p
Conductance of vacuum component:
fi Q =Cp - p
p()
p
p
p
pump
chamber
Q =C
Q
s
-
Q
s
p
ê
á
á
ˉ
S
p
S
C
fi s =
cs
p
c + s
p
=
1
1
c
+
1
s
p
Vacuum technology,Generating low pressure
Two classes of vacuum pumps:
1) Molecules physically removed from chamber
a) mechanical pump
b) Turbo molecular pump
c) Oil diffusion pump
2) Molecules adsorbed on a surface,
or buried in a layer
a) Sputter/ion pump (with Ti sublimation)
b) Cryo pump
6.152J/3.155J Wed.,Sept,10,2003 12
6
(760 Torr
1 Torr
1 milli
6.152J/3.155J
6.152J/3.155J Wed.,Sept,10,2003 13
1) Molecules physically removed from chamber
a) Mechanical pump b) Oil diffusion pump c) Turbo molecular pump
Hot Si oil vaporized,
jetted toward fore pump,
momentum transfer to gas,
which is pumped out.
S = 12A L/s
Oil contamination,
Vibrations.
But pumps from
1 atm to mT.
S 2 x 10
4
L/s
1 atm
)
1 milliT
10
-6
T
10
-9
T
Rotating (25 krpm) vanes
impart momentum to gas,
pres’re incr’s away from chamber,
gas pumped by backing pump.
No oil,S = 10
3
L/s
6.152J/3.155J Wed.,Sept,10,2003 14
2) Molecules adsorbed on a surface,or buried in a layer
a) Sputter/ion pump (with Ti sublimation) b) Cryo pump
Gas is ionized
by hi-V,
ions spiral in B field,
embed in anode,
Coated by Ti.
No moving parts,
no oil.
S depends on
pump size and
S(H) >
>S(O,N,H
2
O)
B
v
1 atm
(760 Torr)
1 Torr
T
10
-6
T
10
-9
T
Very clean,
molecules condense
on cold (120 K) surfaces,
No moving parts,S 3A (cm
2
)L/s
7
6.152J/3.155J
PUMP SUMMARY
Two classes of vacuum pumps:
1) Molecules physically removed from chamber
a) mechanical pump Pumps from 1 atm; moving parts,oil
b) Turbo molecular pump Clean,pumps lg,M well,from 1 atm;
low pump speed,moving parts
c) Oil diffusion pump No moving parts; oil in vac
2) Molecules adsorbed on a surface,
or buried in a layer
a) Sputter/ion pump Clean,pumps reactants,no moving parts;
(with Ti sublimation) pumps from 10
-4
T,
b) Cryo pump Clean,no moving parts;
pumps from 10
-4
T,
6.152J/3.155J Wed.,Sept,10,2003 15
Vacuum technology,Deposition chambers
Standard vacuum,p >10
-6
Torr Ultrahigh vacuum,p >10
-11
Torr;
Glass or stainless steel,Stainless steel (bakeable);
usually diffusion pumped,Ion and/or turbo pumped
CVD,thermal evap,or sputter dep,thermal evap,Sputter deposition
=> polycrystalline films => better quality films,epitaxial
6.152J/3.155J Wed.,Sept,10,2003 16
8
6.152J/3.155J
Vacuum technology,Deposition chambers
Ultrahigh vacuum,p >10
-11
Torr;
Stainless steel (bakeable);
Ion and/or turbo pumped
thermal evap,Sputter deposition
=> better quality films,epitaxial
Baking a stainless-steel uhv system
(T up to 200 C for 10’s of hrs)
desorbs water vapor,organics
from chamber walls;
these are ion-pumped out;
pressure drops as T returns to RT.
6.152J/3.155J Wed.,Sept,10,2003 17
Thin film growth general
Bonds on 3 sides
More bonds
3 bonds with substrate
Arrival,sticking,
surface diffusion,
bonding
Bonds on 1 side
Rate of arrival
R ≡
Diffusion rate
Film growth competes with gas arrival,
1) R > 1 fi Non-equilibrium,fast growth,many misaligned islands form,
leading to defective (high-surface-en),polycrystalline film,columnar grains,
This 3-D growth is the Volmer-Weber mode; Can fi amorphous film,
2) R < 1 => Slower,more equilibrium,layer-by-layer growth,larger grains
(raise surface temperature to ↑ mobility fi↑ g.s,),If film and substrate
have same crystal structure,film may grow in perfect alignment with
substrate (“epitaxy”),This 2-D growth is the Frank-van der Merwe mode.
6.152J/3.155J Wed.,Sept,10,2003 18
9
6.152J/3.155J
Thin film growth details (R < 1)
1) Arri
phys
l
i l
i
i l
i
l i
Bu di i
R ≡
Rate of arrival
va rate,
ca
adsorpt on
3) Chem ca
react on
4) Nuc eat on
5) Growth
6) lk ffus on
Diffusion rate
2) Surface
diffusion
If R > 1,these processes have reduced probability
6.152J/3.155J Wed.,Sept,10,2003 19
Looking ahead…
Thin films made by a variety of means:
thermal vapor deposition (evaporation)
Physical vapor deposition
- for metals
(PVD)
sputter deposition
DC-magnetron- for metals
-RF for oxides
chemical vapor deposition
Chemical vapor deposition
- for metals,semiconductors
(CVD)
6.152J/3.155J Wed.,Sept,10,2003 20
10
6.152J/3.155J
6.152J/3.155J 21Wed.,Sept,10,2003
Thin film growth details (R < 1)
1) Arri
phys
l
i l
i
i l
i
l i
Bu di i
R ≡
Rate of arrival
va rate,
ca
adsorpt on
3) Chem ca
react on
4) Nuc eat on
5) Growth
6) lk ffus on
Diffusion rate
2) Surface
diffusion
If R > 1,these processes have reduced probability
6.152J/3.155J Wed.,Sept,10,2003 22
11
