3.155J/6.152J
6.152J/3.155J Nov,5,2003 1
Physical Vapor Deposition (PVD):
SPUTTER DEPOSITION
We will see evaporation:
(another PVD)
Evaporate source material,P
eq.vap.
P
g
￡10
-6
Torr
Poor step coverage,alloy fractionation,D P
vapor
u
Now sputter deposition,Noble or reactive gas P § 10 mTorru
We saw CVD
Gas phase reactants,P
g
§ 1 mTorr to 1 atm.
Good step coverage,T > > RT
u
…PECVD
Plasma enhanced surface diffusion without need for
elevated T
u
…Dry etching Momentum transfer from plasma to remove surface speciesu
What is a plasma?
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Initiate ionization (breakdown) with spark V
DC
> > V
bkdn
fi ions (e.g,Ar
+
+e
-
)
ions < < 1% of atoms,
Self sustaining plasma in V > V
bkdn
What goes on inside a plasma?
P § 10-100 m Torr
cathode anode

V § 1kV
-
E
x
v
e
-
v
Ar
+
Ionization
event
What is a plasma? A gas of ionized particles,conducting at low freq.
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012345
Log[P (N/m
2
)]
25
24
Log[n (#/m
3
)]
23
22
21
20
1 Atm=
0.1 MPa
§14 lb/in
2
2.5 x 10
25
m
-3
1Torr
10 mT
n =3.2x10
20
m
-3
Ideal gas,n = P/(k
B
T)
Spacing between molecules § n
-1/3
= 0.15 microns,Is this = l?
What is molecular density at 10 mT?
6.152J/3.155J Nov,5,2003 4
l
l
l
l
l l
l
l
l
l
l
0.1% to 1% of n
Ar
are ions,E field accelerates Ar
+
,e
-
between collisions.
v
f
2
= v
0
2
+ 2ax a 2
Eq
m
l
l
Ar
a 3cm
( l[Ar
+
]muchless)
l=
k
B
T
2pd
2
P
Spacing between molecules § n
-1/3
= 0.15 microns.
Is this = l?No!
v
e
-
a 2 ￥10
9
cm
s
,v
Ar
a10
7
cm
s
J = nqv
x
cathode anode

V § 1kV
-
E
x
v
e
-
v
Ar
+
Ionization
event
And n
e-
>> n
Ar+
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Which species,e
-
or Ar
+
,
is more likely to dislodge an atom at electrode?
P § 10-100 m Torr
cathode anode

V § 1kV
-
E
x
v
e
-
v
Ar
+
E
kin
§ 1 keV =
= (1/2) M
Ar
V
Ar
2
= 916 m
e
V
Ar
2
= (1/2) m
er
v
e
2
= (1/2) m
e
v
e
2
\ V
Ar
= v
e
/43
Momentum transfer?
P
Ar
= MV = 1832mv/43
p
e
= mv
No surprise.
From ion implantation,most energy transfer when:
i.e,incoming particle has mass
close to that of target.
DE = E
1
4M
1
M
2
M
1
+ M
2
()
2
Cathode
is,target”,
source material
6.152J/3.155J Nov,5,2003 6
Sputtering process
Ar
+
impact,momentum transfer at cathode fi e
-
avalanche and released target atoms,ions.
For e
-
hitting anode,substrate,M
1
< < M
2
E
2
E
1
a
4M
1
M
2
(small)
But e
-
can give up all its E
K
in inelastic collision:
1
2
m
e
v
e
2
fi DU
Excitation of atom or ion
Elastic energy transfer
E
2
E
1
μ
4M
1
M
2
M
1
+ M
2
()
2
cos
2
q
E
2
greatest for M
1
@ M
2q
E
1
E
2
Atomic billiards
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P § 10-100 m Torr
cathode anode

V § 1kV
-
E
x
v
e
-
v
Ar
+
Cathode
is,target”,
source material
cathode anode

V § 1kV
-
Mostly-neutral
source atoms
Target material (cathode)
must be conductive
…or use RF sputtering
(later)
Momentum transfer of Ar
+
on cathode erodes cathode atoms
fi flux to anode,substrate.
Sputtering process
Ar
+
6.152J/3.155J Nov,5,2003 8
v
e
-
v
Ar
+
E
x
Cathode Anode
x
Ar
+
impact
on cathode
fimostly
electrons
x
1.5eV
E
K
~ v
e
-
2
3eV
Ionization
glow
v
f
2
= 2ax
E
K
e
-
<1.5eV
~
Cathode dark space,
no action
1.5 < E
K
e
-
< 3eV
~~
e
-
- induced
optical excitation
of Ar fi visible glow
cathode anode

-
Inside a plasma
E
K
> 3eV
fi ionization,
High conductivity
plasma
Faraday dark space
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x
1.5eV
E
K
~ v
e
-
2
3eV
Ionization
glow
J
e
-,v
e
- >> J
Ar
+,v
Ar
+ fi
\ plasma § 10 V positive
relative to anode
cathode anode

-
Inside a plasma
(D.C,or cathode sputtering)
Cathode sheath:
low ion density
Plasma
High conductivity
Cathode
Anode
e
-
Ar
+
+ V
D.C.
Film species
Target
Substrate
6.152J/3.155J Nov,5,2003 10
cathode
anode

V § 1kV
-
Al
Ar
Al
+ Ar
+
+ Ar
+
1) Ar
+
accelerated to cathode
Al
Ar =Ar
+
+e
-
e
-
2) Neutral
target species (Al)
kicked off;
3) some neutral
Ar and e
-
also.
4) e
-
may ionize
impurities
(e.g,O => O
-
)
O
-
5) Deposited at anode:
Al,some Ar,
some impurities
6) Some physical
resputtering of Al by Ar
Al
How plasma results in deposition
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cathode
anode

V § 1kV
-
Al
Ar
Al
+ Ar
+
+ Ar
+
Al
Ar =Ar
+
+e
-
2) Neutral
target species (Al)
kicked off.
6) Some physical
resputtering of Al by ArAl
Sputtering rate of source material in target is key,Typically 0.1 - 3 atoms/Ar
Sputtering rates vary little from material to material,
Vapor pressure or source NOT important
(this differs greatly for different materials).
cos q
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S = Sputtering yield =
# atoms,molecules from target
# incident ions
S =s
0
n
A
E
2
4 E
thresh
￥,.,ln
E
v
E
b
ê
á
ˉ
è
í
˙
# excited in
each layer
# / area
p d
2
Random walk
to surface;
E
b
= binding energy
Sputtering yield
q
S
90°
Oring,Fig,3.18,
Table 3-4
q
Sputter rate depends on angle of incidence,
relative masses,kinetic energy.
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A
argon
=40amu
A
Ti
=48amu
DE = E
1
4M
1
M
2
M
1
+ M
2
()
2
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Sputtering miscellany
cos q = normal component
of flux
Large target,small substrate
=> good step coverage,
more uniform thickness
Moving substrates
cos
n
q,more-narrowly
directed at surface
Higher pressure => shorter l,
better step coverage
l=
k
B
T
2pd
2
P
p l (cm)
10 mT 2
1 mT 20
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Sputtering removes outer layer of target => more uniform composition
(problem with multicomponent system only initially)
Target composition vs,film composition
A
3
B
A
2.5
B
A
2
B
If sputter yield A > B
time
Surface
composition
of target
A
B
Initial target composition A
2
B (e.g.Si
2
W)
Co-sputtering => composition control,
sample library
A
2
B AB AB
2
A B
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D.C,sputter deposition,Only for conducting materials.
if DC sputtering were used for insulator,e.g,carbon,charge would accumulate
at each electode and quench plasma within 1 - 10 mico-sec.
Varieties of sputtering experience
+ Ar
+
cathode
anode

V § 1kV
-
C
Ar
C
t < 1 micro sec
C
Ar =Ar
+
+e
-
e
-
O
-
u Therefore,use RF plasma…
RF -sputter system is basically a capacitor with gas dielectric,Energy density B with f B
6.152J/3.155J Nov,5,2003 18
u
RF plasma sputtering
Target
Substrate
V
plasma
Plasma potential still > 0
due to high e
-
velocity,Potential now symmetric.
Mobility of target species still sufficient to => deposition in 1/2 cycle:
v ￡
2eV
M
a 7 ￥10
4
m /s
Target
Substrate
V
plasma
Smaller target => higher field
+ Ar
+
Ar
+
+
Some re-sputtering
of wafer
V
1
V
2
=
A
2
A
1
ê
á
ˉ
m
m § 1,2
Plasma conducts at low f
Thus,V = 0 in plasma
w
p
~10
7
s
-1
F.C.C,reserves,13.56 MHz for sputtering
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u Bias sputtering
Negative wafer bias
enhances re-sputtering of film
Varieties of sputtering experience
Target
Substrate
V
plasma
+ Ar
+
V
bias
§
-100 V
Ar
+
+
Some re-sputtering
of wafer
Bias is one more handle
for process control.
Used in SiO
2
,denser,
fewer asperities.
Bias affects stress,
resistivity,
density,
dielectric constant…
Resitivity of Ta films vs.
substrate bias voltage.
Similar results hold for W,
Ni,Au,Cr.
300 nm
160 nm
V§
-1 kV
6.152J/3.155J Nov,5,2003 20
Cathode
-V
bias
Film stress in RF
sputter deposition
Film
Species
Better step coverage
Ar
+
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B
uMagnetron sputtering use magnets
v
F = q
v
v ￥
v
B()+ E
x
q
F =
mv
2
r
,
r =
mv
qB
v
z
B
x
fi F
y
fi v
y
v
y
B
x
fi F
z
fi v
z
Cathode
target
S
N S
N
N
S
B field enhances time of e
-
in plasma
fi more ionization,greater Ar
+
density.
V
+
E
x
v
0
Varieties of sputtering experience
v § 6 ￥ 10
6
m/s for electron,r < 1 mm for 0.1 T
Ions spiral around B field lines
6.152J/3.155J Nov,5,2003 22
Reactive sputter deposition,Mix reactive gas with noble gas (Ar or Ne).
analogous to PECVD
u
TiN
Ti or TiN target
Ar +N
2
Also useful for oxides,SiO
x
,TiO,CrO…
other nitrides,SiN,FeN,…
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Thin film growth details (R < 1)
1) Arrival rate,
physical
adsorption
2) Surface
diffusion
3) Chemical
reaction
4) Nucleation
5) Growth
6) Bulk diffusion
R >
Rate of arrival
Diffusion rate
If R > 1,these processes have reduced probability
Anode
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Sputtering metals
Sputter alloys,compounds Concern about different sputter yield S
But S does not vary as much as P
eq.vap
which
controls evaporation
e.g,Al Si
S = 1.05 0.5
Can use DC or RF
Sputter target T << T
evap
No diffusion,
surface enriched in
low S components
High diffusion,
composition change,species
distributed over entire source
u
u