Physical Vapor Deposition (PVD):
SPUTTER DEPOSITION
‹ We saw CVD
Gas phase reactants: Pg ≈ 1 mTorr to 1 atm.
Good step coverage, T > > RT
‹ …PECVD
Plasma enhanced surface diffusion without need for
elevated T
‹ We will see evaporation: Evaporate source material, Peq.vap. Pg ≤ 10−6 Torr
(another PVD)
Poor step coverage, alloy fractionation: ∆ Pvapor
‹ We will see Dry etching
Momentum transfer and chemical reaction from
plasma to remove surface species
‹ Now
sputter deposition. Noble or reactive gas P ≈ 10 -100
mTorr
What is a plasma?
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What is a plasma? A gas of ionized particles,
typically noble gas (e.g. Ar+ + e-)
P ≈ 10-100 m Torr
Ionization
Ionization
potential
potentialof
ofAr
Ar
≈≈16
16eV
eV
Ionization
event
cathode
-
v Ar +
anode
Ex
v e−
⊕
V ≈ 1 kV
Plasma is only self-sustaining over a range of pressures:
typically 1 or 10 mT > P > 100 mT.
To understand “Why this pressure range?”,
we need to understand
what goes on inside a plasma?
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2
First, what is molecular density at 10 mT?
Ideal gas: n = P/(kBT)
24
Log[n (#/m3)]
23
λ (cm)
10-3
λAr ≈ 3 cm
10-2
( λ[Ar + ] much less)
1 Atm=
10-1
0.1 MPa
≈14 lb/in2
0
22
21
10
1 Torr
n = 3.2 x 1020 m-3
10 mT
20
0
101
1
2
3
Log[P (N/m2)]
At 10 mT, molecular spacing ≈ n-1/3 = 0.15 microns.
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λ=
2.5 x 1025 m-3
25
k BT
2π d 2 P
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5
Is this = λ ?
3
Spacing between molecules ≈
n-1/3
= 0.15 microns.
z
z
z
z
z
z
z
λ=
z
λAr ≈ 3 cm
kB T
2π d 2 P
( λ[Ar + ] much less)
Thermal velocity of Ar, v ≈
3k B T
≈ 103 m/s
m Ar
What’s final velocity of ions at x = λ? E field accelerates Ar+, e- between collisions.
v 2f = v 20 + 2ax ≈ 2
Eq
λ
m
v Ar ≈ 4 ×10 5 m/s
v e − ≈ 2 ×10 7 m/s,
(only 0.1% to 1% of nAr are ions):
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z
z
4
z
Be clear about different velocities:
So we have:
low-vel. Ar+
Ionization
event
vkT ≈ 103 m/s
vAr+ ≈ 4 × 105 m/s
ve- ≈ 2 × 107 m/s
v Ar +
3k B T
m Ar
high vel. e-
cathode
-
v≈
anode
Ex
v e−
⊕
V ≈ 1 kV
The plasma is highly conducting due to electrons:
J = nqv x
Thus Je- >> JAr+
at
Eq λ
J e− = σE = nqv x ≈ nq ≈ nq
2m v
2
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σ e−
nq 2 λ ne 2τ
≈
=
2m v
2m
5
Plasma is self-sustaining only for 1 or 10 mT < P < 100 mT.
“Why this pressure range?”
Necessary conditions:
If pressure is too low, λ is large, too few collisions in plasma to sustain energy
1) λ < L
so collisions exchange energy
within plasma
λ=
k BT
2π d 2 P
If pressure is too high, λ is small, very little acceleration between collisions
2) EK > ionization potential of
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Ar+
1
mv 2f = 2ax ≈ Eqλ
2
6
Plasma is self-sustaining only for 1 or 10 mT < P < 100 mT.
λ=
k BT
2π d 2 P
Log[λ (m)]
0
1) λ < L ≈ 10 cm
V
1
mv 2f ≈ Eqλ ≈ λ (eV)
L
2
Self-sustained
plasma
100 V
EK (eV)
2
1000 V
λ ≈ 3 cm: EK ≈ 0.3 x V
Ne+ 22
Ar+ 16
1 Kr+ 13
2) EK > Ar+
-1
-2
0
-3
-1
-4
-2
10 mT: λ ≈ 3 cm
-5
-1
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0
1
2
2
Log[P (N/m )]
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3
4
5
-3
7
Cathode
What about Glow?
Ar+ impact
on cathode
=> Lots more
electrons, plus
sputtered atoms
E K ~ v e2−
Anode
Ex
v e−
v Ar +
x
¨
§
1.5eV
¦ glow
3 eV
cathode
anode
-
⊕
¦
E
e−
K
<
~ 1.5eV
Cathode dark space,
no action
v 2f = 2ax
Ionization
Faraday dark space
x
§
1.5 ~< E
e-s swept out
e−
K
<
~ 3eV
E K > 3eV
e- - induced
=> ionization,
optical excitation
High conductivity
plasma
of Ar ⇒ visible glow
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¨
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Inside a plasma
(D.C.
¨
3 eV
2
EK ~ ve −
1.5eV
or “cathode sputtering”)
§
¦ glow
Ionization
x
cathode
J e − , v e − >> J Ar + ,v Ar + ⇒
Surfaces in plasma charge negative,
attract Ar+ repel e∴ plasma ≈ 10 V positive
anode
-
⊕
Cathode
Target
relative to anode
Ar+
e-
Cathode sheath:
low ion density
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Anode
+ VD.C.
Target species
Substrate
Plasma
High conductivity
9
Which species, e- or Ar+ ,
is more likely to dislodge an atom at electrode ?
P ≈ 10-100 m Torr
cathode
Cathode
is “target”,
source material
At 1 keV,
-
v Ar +
anode
Ex
v e−
⊕
V ≈ 1 kV
Sputter removal (etching)
occurs here
vAr+ = ve/43
Momentum transfer:
pe
=
mv
PAr = MV = 1832mv/43 ≈ 43pe-
No surprise.
from ion implantation,
most energy transfer when:
i.e. incoming particle has mass
close to that of target.
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∆E = E1
4 M1 M 2
2
(M1 + M 2 )
10
Sputtering process
Ar+ impact, momentum transfer at cathode ⇒ 1) e- avalanche and
2) released target atoms, ions.
Atomic billiards
Elastic energy transfer
E2
θ
4M1 M2
E2
2
∝
cos
θ
E1 (M1 + M2 )2
E1
For e- hitting anode, substrate, M1 < < M2
E2 greatest for M1 ≅ M2
E 2 4M1
≈
M2
E1
(small)
But e- can give up its EK in inelastic collision:
1
2
me v e ⇒ ∆U
2
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Excitation of atom or ion
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Sputtering process:
ablation of target
P ≈ 10-100 m Torr
Cathode
is “target”,
source material
cathode
-
anode
v Ar +
Ex
v e−
cathode
-
⊕
V ≈ 1 kV
anode
Ar+
⊕
V ≈ 1 kV
Momentum transfer of Ar+
on cathode erodes cathode atoms
⇒ flux to anode, substrate.
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Mostly-neutral
source atoms
(lots of e’s around)
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Target material (cathode)
must be conductive
…or must use RF sputtering
(later)
12
How plasma results in deposition
5) Flux => deposition at anode:
Al, some Ar,
some impurities
1) Ar+ accelerated to cathode
cathode
2) Neutral
target species (Al)
kicked off;
3) Some Ar,
Ar+ and e- also.
anode
+
Ar+
Ar =Ar+ + e-
-
Oct. 17, 2005
Ar
Al
e-
4) e- may ionize
impurities
(e.g. O => O-)
Al
O+
⊕ V ≈ 1 kV
Al
Ar+
Al
6) Some physical
resputtering of film (Al) by Ar
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Sputtering rate of source material in target is key parameter.
Typically 0.1 - 3 target atoms released/Ar incident
Sputtering rates vary little from material to material.
Vapor pressure of source NOT important
(this differs greatly for different materials).
cathode
2) Neutral
target species (Al)
kicked off.
anode
+
Ar+
Al
Ar =Ar+ + e-
-
Ar
Al
⊕ V ≈ 1 kV
Al
+
Ar+
Al
6) Some physical
resputtering of Al by Ar
cos θ
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Sputtering yield
S = Sputtering yield =
# atoms, molecules from target
# incident ions
E1
⎡
⎛ E 2 ⎞⎤
E2
S = σ 0nA
× ⎢1+ ln⎜ ⎟ ⎥
4 E thresh ⎢⎣
⎝ E b ⎠ ⎥⎦
πd2
# / area
# excited in
each layer;
Eb
E2 >Et
Random walk
to surface;
E2
4 M1 M 2
2
∝
2 cos θ
E1 (M1 + M 2 )
E = surface
Ethresh= energy b
binding energy
to displace
interior atom
Oring, Fig. 3.18
φ
S
Sputter rate depends on angle of incidence,
relative masses, kinetic energy.
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Surface
binding energy
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90°
φ
15
Figure removed for copyright reasons.
Table 3-4 in Ohring, M. The Materials Science of Thin Films. 2nd
ed. Burlington, MA: Academic Press, 2001. ISBN: 0125249756.
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Figure removed for copyright reasons.
Figure 12.13 in Campbell, S. The Science and Engineering of
Microelectronic Fabrication. 2nd ed. New York, NY: Oxford University
Press, 2001. ISBN: 0195136055.
Sputter yield vs. ion energy, normal incidence
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Sputtering miscellany
Isotropic flux:
cos θ = normal component
of flux
Anisotropic flux:
cosn θ: more-narrowly
directed at surface
(surface roughness)
Poor step coverage.
Higher P
Lower P
Large target, small substrate
=> Good step coverage,
more uniform thickness
Moving substrates
Higher pressure
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λ=
=> shorter λ,
better step coverage
…but more trapped gas
kB T
2π d 2 P
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p
λ (cm)
10 mT 2
1 mT 20
18
Target composition vs. film composition
Sputtering removes outer layer of target;
can lead to problem with multi-component system, but only initially
Initial target composition A3B
A
Surface
composition
B
of target
If sputter yield A > B
A3B
A2.5B
A2B
time
Deposited film initially richer in A than target; film composition eventually correct.
A
B
Another approach:
Co-sputtering => composition control;
multiple targets & multiple guns.
Also can make sample “library”.
A2B
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AB
AB2
19
Varieties of sputtering experience
D.C. sputter deposition:
Only for conducting materials.
if DC sputtering were used for insulator. e.g. carbon, charge would accumulate
at each electrode and quench plasma within 1 - 10 mico-sec.
cathode
anode
t < 1 micro sec
Ar =Ar+ + eC
eWhat does 1 microsec
tell you about ion speed?
(>106 cm /sec)
‹
C
Ar
⊕ V ≈ 1 kV
O+
Ar+
C
Therefore, use RF plasma…
RF -sputter system is basically a capacitor with gas dielectric. Energy density ↑ as f ↑
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‹
RF plasma sputtering
Vplasma
ω p ~ 10 7 s −1
Plasma conducts at low f
Target
Thus, V = 0 in plasma
Substrate
e-
Plasma potential still V > 0 due to high velocity, (-) charging of surfaces.
Potential now symmetric.
Mobility of target species (concentration gradient )still => sputtering in 1/2 cycle:
2eV
v≤
≈ 7 ×10 4 m /s
M
Vplasma
Smaller target => higher field
V1 ⎛ A2 ⎞
=⎜ ⎟
V2 ⎝ A1 ⎠
Target
m
m ≈ 1,2
F.C.C. reserves: 13.56 MHz for sputtering
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+
Ar+
Ar+ +
Substrate
Some re-sputtering
of wafer
21
Varieties of sputtering experience
Vplasma
‹ RF Bias sputtering
Target
Negative wafer bias
enhances re-sputtering of film
V≈
-1 kV
+
Ar+
Vbias ≈
-100 V
Ar+ +
Substrate
Some re-sputtering
of wafer
Figure removed for copyright reasons.
Figure 3-24 in Ohring, 2001.
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Bias is one more handle
for process control.
Used in SiO2: denser,
fewer asperities.
Bias affects stress,
resistivity,
density,
dielectric constant…
22
Film stress in RF
sputter deposition
Cathode
Film
Species
Ar+
-Vbias
Better step coverage
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Varieties of sputtering experience
B
‹Magnetron sputtering use magnets
( )
ur
v ur
F = q v × B + Ex q
¦ vz Bx ⇒ Fy ⇒ vy
§ vy Bx ⇒ Fz ⇒ vz
mv
mv
r
=
F=
,
qB
r
2
N
S
Ions spiral around B field lines
S
N
N
S
Cathode
target
v ≈ 6 × 106 m/s for electron, r < 1 mm for 0.1 T
B field enhances time of e- in plasma
V+
⇒ more ionization, greater Ar+ density.
Ex
v0
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‹ Reactive sputter deposition:
Mix reactive gas with noble gas (Ar or Ne).
analogous to PECVD
Ti or TiN target
Ar +N2
Also useful for oxides:
other nitrides:
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TiN
SiOx, TiO, CrO…
SiN, FeN,…
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Figure removed for copyright reasons.
Figure 12.27 in Campbell, 2001.
Resistivity and composition of reactivity sputtered TiN vs. N2 flow.
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Thin film growth details (R < 1)
R
≡
1) Arrival rate,
physical
adsorption
3) Chemical
reaction
Rate of arrival
Diffusion rate
5) Growth
Anode
4) Nucleation
6) Bulk diffusion
2) Surface
diffusion
If R > 1, these processes have reduced probability
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‹ Sputtering metals
‹
Can use DC or RF
Sputter alloys, compounds Concern about different sputter yield S
e.g. Al
Si
But S does not vary as much as Peq.vap
which controls evaporation
S = 1.05 0.5
Sputter target T
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<<
Tevap
No diffusion,
High diffusion,
surface enriched in
composition change, species
low S components
distributed over entire source
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Figure removed for copyright reasons.
Table 3-7 in Ohring, 2001.
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