Transcript Document 7703752

Plasmas are essential for making
computer chips
SHEATH
For instance, for etching 60 nm features
Etch rate is greatly enhanced by ions
UCLA
Why use radiofrequency power?
•
•
•
•
DC discharges: low density, internal electrodes
Microwave (ECR), 2.45 GHz: Large B-field, expensive
plumbing, limited flexibility, energetic electrons.
RF (0.4 - 30 MHz): No internal electrodes needed,
inexpensive power, years of development and
experience.
VLF (30 - 300MHz): relatively undeveloped, short skin
depth, finite-wavelength effects, circuit losses
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Types of RF plasma sources
•
•
•
•
History: old RIE parallel plate etchers
Inductively coupled plasmas (ICPs)
Dual frequency capacitively coupled plasmas (CCPs)
Helicon wave sources (HWS) and hybrids
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Schematic of a capacitive discharge
Gas inlet
Main RF
Powered electrode
Wafer
Sheath
Plasma
Chuck
Sheath
Grounded electrode
He coolant
Bias RF
Gas outlet
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The GEC Reference
Cell
In the early days of plasma
processing, the Gaseous
Electronics Conference
standardized a capacitive
discharge for 4-inch wafers, so
that measurements by different
groups could be compared.
Brake et al., Phys. Plasmas 6, 2307 (1999)
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Problems with the original RIE discharge
•
•
•
•
•
The electrodes have to be inside the vacuum
Changing the power changes both the density and
the sheath drop
Particulates tend to form and be trapped
Densities are low relative to the power used
In general, not enough adjustments for controlling
the ion and electron distributions and the
plasma uniformity
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Dual-frequency CCPs are better
W. Tsai et al., JVSTB 14, 3276 (1996)
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One advantage of a capacitive discharge
GAS INLETS
GLASS SUBSTRATE
RF
HOLES
Fast and uniform gas feed for depositing amorphous silicon on very
large glass substrates for displays (Applied Komatsu)
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Types of RF plasma sources
•
•
•
•
Old RIE parallel plate etcher (GEC reference cell)
Inductively coupled plasmas (ICPs)
New dual frequency capacitively coupled plasmas (CCPs)
Helicon wave sources (HWS)
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Inductive coupling: The original TCP patent
US Patent 4,948,458, Ogle, Lam Research, 1990
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The Lam TCP (Transformer Coupled Plasma)
Simulation by Mark Kushner
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Top and side antenna types
US Patent 4,948,458, Fairbairn, AMAT, 1993
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Applied Materials' DPS (Decoupled Plasma Source)
US Patent 4,948,458, Fairbairn, AMAT, 1993
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Other antennas in AMAT patent
US Patent 4,948,458, Fairbairn, AMAT, 1993
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30
30
20
20
10
10
z (cm)
z (cm)
B-field pattern comparison (1)
0
0
-10
-10
-20
-20
-30
-30
30 r (cm)
-30
-30
-20
-10
0
10
Horizontal strips
20
-20
-10
0
10
20
30
Vertical strips
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r (cm)
30
30
20
20
10
10
z (cm)
z (cm)
B-field pattern comparison (2)
0
0
-10
-10
-20
-20
-30
-30
r (cm)
-30
-30
-20
-10
0
10
3 close coils
20
30
-20
-10
0
10
20
30
2 separate coils
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r (cm)
B-field pattern comparison (3)
20
30
15
20
10
10
z (cm)
z (cm)
5
0
-10
0
-5
-10
-20
-15
-30
-30
-20
-10
0
10
Lam type
20
30 -20
r (cm)
-20
-15
-10
-5
0
5
10
15
20
AMAT type
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r (cm)
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Antenna elements near axis are not necessary!
In MEMs etcher by Plasma-Therm
(now Unaxis), density is uniform well
outside skin depth
z (cm)
12
10
10
-3
cm )
3 mTorr, 1.9 MHz
n (10
0
How do ICPs really work?
Prf(W)
8
800
240
200
6
4
2
0
-5
0
5
10
r (cm)
15
20
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UCLA experimental chamber
10 cm
30 cm
35.7 cm
21 cm
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In the plane of the antenna, the density peaks well
outside the classical skin layer
3
n (10
11
-3
cm )
2
KTe (eV)
1
RF Bz field
skin depth
0
Data by John Evans
0
5
r (cm)
10
15
Nonlinear effects have been observed
Collisionless power
absorption
(Godyak et al., Phys. Rev.
Lett. 80, 3264 (1998)
UCLA
Second harmonic
currents
Smolyakov et al., Phys.
Plasmas 10, 2108 (2003)
Ponderomotive
force
Godyak et al., Plasma
Sources Sci. Technol.
10, 459 (2001)
Anomalous skin effect (thermal motions)
skin wall
antenna
x
J
o
x
J
x
B
x
x
E.g., Kolobov and Economou, Plasma Sources Sci. Technol. 6, R1 (1997).
Most references neglect collisions and curvature.
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Electron trajectories are greatly affected
by the nonlinear Lorentz force
without FL
with FL
RF phase
(degrees)
0
180
360
540
720
900
1080
1260
Skin depth
m

dv
 e E  v  B
dt
FL

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Without FL, electrons are fast only in skin
80
with V x B
no V x B
E (eV)
60
40
20
Argon ionization threshold
0
0
360
720
Phase (degrees)
1080
1440
Reason: The radial FL causes electrons to bounce off
the sheath at more than a glancing angle.
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Electrons spend more time near center
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Density profile in four sectors of equal area
1.2
Relative density
1.0
0.8
0.6
0.4
0.2
Points are data from Slide 5
0.0
0
5
r (cm)
10
15
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A more compact ICP is possible
Gas inlet ring
Silicon gas sieve plate
Silicon wall slabs
Antenna
Antenna
PLASMA
ESC
Points are data from Slide 5
Inverted showe
Disadvantages of stove-top antennas
•
•
•
•
•
Skin depth limits RF field penetration. Density falls
rapidly away from antenna
If wafer is close to antenna, its coil structure is seen
Large coils have transmission line effects
Capacitive coupling at high-voltage ends of antenna
Less than optimal use of RF energy
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Magnetic field above coil is wasted
30
20
z (cm)
10
0
-10
-20
-30
-30
-20
-10
0
10
20
30
r (cm)
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Coupling can be improved with magnetic cover
H
H
H
H=J
B=mH
H
B
B
E
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Four configurations tested
Meziani, Colpo, and Rossi, Plasma Sources Science and Technology 10, 276 (2001)
The dielectric is inside the vacuum
Meziani, Colpo, and Rossi, Plasma Sources Science and Technology 10, 276 (2001)
Iron improves both RF field and uniformity
(Meziani et al.)
7
40
700 W
6
5
20
Ji (mA/cm )
4
2
Br (Gauss)
Argon
30 mtorr, 600 W
700 W
3
100 W
2
100 W
1
10
2 turn coil
2 turn coil + mag. pole
Spiral
MaPE
w.o. magnetic pole (C1)
w. magnetic pole (C2)
0
0
1
2
3
4
5
6
7
8
9
10 11 12
Irms (A)
Vacuum chamber
2
0
2
4
Serpentine coil embe
a magnetic cor
6
r (cm)
5
di
Magnetic
material
750 x720 mm
X (cm)
420
mm
2 loops in //
3 loops in //
2 serpentines
in //
110
mm
10
Multiple gas injection
Y (cm)
1 2 3 4 5 6 7 8 9
8
800 x 800 mm
Magnets are used in Korea (G.Y. Yeom)
SungKyunKwan Univ. Korea
Both RF field and density are increased
11
1.2x10
Antenna type=serpentine(7m)
Operating pressure=15mTorr
700
Without magnetic fields
With magnetic fields
11
1.0x10
3
)
600
10
8.0x10
Ni (Ion Density /cm
Vrms (Volts)
500
400
300
10
6.0x10
10
4.0x10
200
No multipolar magnetic fields
With multipolar magnetic fields
10
2.0x10
100
400
0
500
1000
1500
Input power (W)
2000
600
800
1000
1200
1400
1600
1800
2000
2200
RF power(Watts)
SungKyunKwan Univ. Korea
Serpentine antennas
(suggested by Lieberman)
Magnets
Plasma Application
Modeling Group
POSTECH
Density uniformity in two directions
600
550
1000W RF Input power
1500W RF Input power
2000W RF Input power
-6
Ion Saturation Current (10 A)
500
450
400
350
300
250
200
150
100
0
10
20
30
40
Position, Parallel to the antenna (cm)
500
1000W RF Input Power
1500W RF Input Power
2000W RF Input Power
400
-6
Ion Saturation Current(10 A)
450
350
300
250
200
150
100
50
-30
-20
-10
0
10
Probe Position (cm)
20
30
G.Y. Yeom, SKK Univ., Korea
Effect of wire spacing on density
7.2cm
10.2cm
1E
+0
11
1E
+0
11
1E
+0
11
8E
+0
10
6E
+0
10
4E
+0
10
2E
+0
10
7.8cm
9cm
Park, Cho, Lee, Lee, and Yeom,
IEEE Trans. Plasma Sci. 31, 628 (2003)
11.4cm
0
13.2cm
Plasma Application
Modeling Group
POSTECH
Types of RF plasma sources
•
•
•
•
Old RIE parallel plate etcher (GEC reference cell)
Inductively coupled plasmas (ICPs)
New dual frequency capacitively coupled plasmas (CCPs)
Helicon wave sources (HWS)
UCLA
A LAM Exelan oxide etcher
Plasma Application
Modeling Group
POSTECH
A dual-frequency CCP
Thin gap. Unequal areas to increase sheath drop on wafer
Low frequency
controls ion
motions and
sheath drop
27
MHz
2
MHz
High frequency
controls plasma
density
UCLA
Most of volume is sheath
Large electrode
E
Sheath
PLASMA
Sheath
E
Small electrode
•
•
Electrons are emitted by secondary emission
Ionization mean free path is shorter than sheath thickness
• Ionization occurs in sheath, and electrons are
•
accelerated into the plasma ( - mode)
Why there is less oxide damage is not yet known
UCLA
The density increases with frequency squared
(a)
(b)
Debye length
1,0
70
60
50
40
30
(c)
20
Sheah width (cm)
Plasma density (peak value), (10
16
-3
m )
Density
0,8
0,6
(d)
0,4
10
0,2
0
0
20
40
Frequency (MHz)
60
0
20
40
Frequency (MHz)
60
Reason: The rf power is  I2R, where I is the electron current
escaping through the sheath. Since one bunch of electrons is
let through in each rf cycle, <Irf> is proportional to .
Plasma Application
Modeling Group
POSTECH
Effect of frequency on plasma density profiles
45 mTorr 27 MHz 800 V
45 mTorr 13.56 MHz 800 V
17
17
10
13.56 MHz
-3
16
Concentration (m )
-3
Concentration (m )
10
10
15
10
14
10
16
10
27 MHz
15
10
14
10
13
13
10
10
0,030
0,035
0,040
0,030
0,045
0,035
0,040
r (m)
r (m)
45 mTorr 40 MHz 800 V
45 mTorr 60 MHz 800 V
0,045
17
10
17
Concentration (m )
40 MHz
16
10
-3
-3
Concentration (m )
10
15
10
14
10
13
10
60 MHz
16
10
15
10
14
10
13
10
0,030
0,035
0,040
r (m)
0,045
0,030
0,035
0,040
0,045
r (m)
Plasma Application
Modeling Group
POSTECH
IEDF at Wall – Pressure Variation
10 mTorr
20 mTorr
30 mTorr
50 mTorr
Plasma Application
Modeling Group
POSTECH
Effect of frequency on IEDF at the smaller electrode
LHS 45 mTorr 13 MHz 800 V
(a)
13.56 MHz
0,012
LHS 45 mTorr 27 MHz 800 V
0,016
IEDF ( 1/Ntotal N /)
IEDF ( 1/Ntotal N /)
0,016
0,008
0,004
27 MHz
(b)
0,012
0,008
0,004
0,000
0,000
0
100
200
300
400
500
600
0
100
Ion energy (eV)
0,016
400
500
600
LHS 45 mTorr 60 MHz 800 V
-1
IEDF ( 1/Ntotal N /) (eV )
IEDF ( 1/Ntotal N /)
300
Ion energy (eV)
LHS 45 mTorr 40 MHz 800 V
0,016
200
40 MHz
0,012
(c)
(d)
60 MHz
0,012
0,008
0,008
0,004
0,004
0,000
0,000
0
100
200
300
400
Ion energy (eV)
500
600
0
100
200
300
400
500
600
Ion energy (eV)
Plasma Application
Modeling Group
POSTECH
Effect of increasing low-frequency drive
Electron distribution changes
from Druyvesteyn to biMaxwellian. (Secondary
electron emission neglected,)
KTe falls, density rises. The
decrease of KTe is an
unexplained collisionless
effect.
(H.C. Kim and J.K. Lee, PRL)
Plasma Application
Modeling Group
POSTECH
Effect of increasing low-frequency drive
The RF E-field rises at the
plates and falls at the
midplane.
Now add secondary
electrons. They greatly
increase density as they get
accelerated in the sheath.
(H.C. Kim and J.K. Lee, PRL)
Plasma Application
Modeling Group
POSTECH
Types of RF plasma sources
•
•
•
•
Old RIE parallel plate etcher (GEC reference cell)
Inductively coupled plasmas (ICPs)
New dual frequency capacitively coupled plasmas (CCPs)
Helicon wave sources (HWS)
UCLA
A helicon source requires a DC magnetic field..
U. Wisconsin
...and is based on launching a circularly
polarized wave in the plasma
(a)
_
+
_
B
k
+
_
+
+
_
+
_
+
(b)
(c)
_
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Where helicons lie on the CMA diagram
6
ci
5
log ( c/ )
4
lower hybrid
pi
 cc)
3
2
f = 13.56 MHz

-3
1
n = 1E12 cm
B = 100 G
c
1/2
0
-1
3.0
3.5
4.0
4.5
log ( p2/ 2)
5.0
5.5
6.0
UCLA
Helicons are bounded whistler (R-) waves
2
2
2
2

/


/

c k
p
p
  2 1


2
2
1  c cos /   p /  1
c cos / 

2 2
c cos  /  1
cos  k z / k :
c 2k 2
2
 2p  k
 2
 c k z
2

 p 1  ne2 m
 nem0
k

 0 m0 
2
k z c c
k z  0m eB
kz B
n
ktot  v
B
Basic helicon relation
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A Nagoya Type III antenna converts
inductive to electrostatic coupling
_
+
_
B
k
+
_
+
This is an m = 1 antenna (E ~ cos m)
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The antenna can be twisted to match the
helicon's helical waveform
B, k
B
Nagoya III Antenna
(a)
Right Helical Antenna
(b)
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The R-wave propagates to the right, and
the L-wave to the left (for this antenna helicity)
B
(a)
+
--
--
+
--
+
+
--
(b)
(c)
But the L-wave is very weak, and this antenna is unidirectional
UCLA
Some advantages of helicon sources
•
•
•
•
•
Much higher density at given power than ICPs
Density peak occurs downstream, away from the
antenna and in the processing chamber
Magnetic field can be adjusted for uniform density for
each processing chemistry and pressure
KTe is low at the substrate, allowing for greater range
of sheath drops
High ionization permits low pressure, few collisions in
sheath, and no particulate formation
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Axial density and temperature profiles
Density increases greatly as
B-field is added.
The density peak is detached
from the source.
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Two commercial helicon reactors
The PMT (Trikon) MØRI source
The Boswell source
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The Coil Current Ratio shapes the plasma
The MØRI source
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How do helicon source really work?
A cyclotron (TG) wave at the surface rapidly
damps the RF energy
14
5000
40G
(b)
12
4000
J z (arb . units )
10
P(r)
3000
2000
8
11
n (1 0 )
6
4 .2
4
4 .2
1000
no T G
2
0
0.00
0
0.01
0.02
0.03
r (m)
0.04
Typical radial deposition
profile
0.05
-6
-4
-2
0
r (c m )
2
4
Direct detection of the TG peak in the
RF current
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6
The m = +1 mode is much stronger than m = -1
+1
=1
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The L-mode pattern is narrower than the R
m = – 1, L- mode
m = + 1, R-mode
This may be the reason the R-mode is more strongly excited at the edge
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There are actually 2 types of helicon discharges
The Big Blue Mode
The Low Field Peak
B > 800G, n > 1013 cm-3
Due to an neutral depletion
instability
No important application yet
Low density, low B-field
Ideal for plasma processing
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Reflection from end causes the L.F. peak
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Computed plasma loading with endplate
3.0
-3
n (cm )
2E +11
2.5
4E +11
6E +11
R (ohm s)
2.0
The low-field peak shifts
to higher B-field with
higher density.
8E +11
1E +12
1.5
1.0
0.5
0.0
0
50
100
150
200
B (G )
6
d
5 cm
5
The peak depends on the
distance from antenna to
endplate, and disappears
in an infinite plasma.
10 cm
No bdy
R (ohm s)
4
3
2
1
0
0
50
100
150
B (G )
200
250
300
UCLA
ICMP: a hybrid ICP with magnetic field
•
•
•
Planar antenna, as in an
ICP
Diverging field lines
intersect walls
Field shape adjustable
and affects the results
Slobodan, Virko,
Kirichenko, and Shamrai
(Kiev, Ukraine)
Resonances due to axial modes are seen
The discharge stops at a critical B-field. The
resonances are broadened by increasing pressure
Slobodan, Virko, Kirichenko, and Shamrai (Kiev, Ukraine)
Computed distribution of power absorption
The density at each peak
increases linearly with magnetic
field.
Computed deposition profile for
one mode shows radial and
axial resonances.
Slobodan, Virko, Kirichenko, and Shamrai (Kiev, Ukraine)
An array of helicon sources to cover a large area
U CL A
DC MAGNET COIL
3"
18"
PERMANENT MAGNETS
ROTATING PROBE ARRAY
UCLA
A 7-tube array
PROBE
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A 7-tube array gives
good uniformity and high density
Power scan at z = 7 cm, 5 mT A, 20 G, 13.56 MHz,
2.0
ARGON
P(kW)
1.5
N (10
12
-3
cm )
3.0
2.5
2.0
1.5
1.0
1.0
0.5
7-tube m=0 array
0.0
0
5
10
R (cm)
15
20
25
30
UCLA
2-D density scans show no m = 6 asymmetry
20
3% contours
10
0
-10
-20
-20
-10
0
10
UCLA
20
There are also linear helicon antennas
Rusty Jewett's serpentine helicon antenna
UCLA
A large-area source with serpentine antennas
To cover a large area, long antennas can be placed on the top
of this chamber. This permits insertion of large apertures for
fast pumping of gaseous products. (R. Jewett)
UCLA
Helicons in rectangular geometry
Waveforms in 3D with plane boundary conditions
(Thesis, R. Jewett)
UCLA
Helicon tools have been modeled
MØRI tool:
Kinder and Kushner, JVSTA 19, 76 (2001)
TG mode is seen
Power deposition
Bose, Govindan, and Meyyappan, IEEE
Trans. Plasma Sci. 31, 464 (2003)
Plasma density
What next for RF sources?
•
Control of KTe, species production, ion velocities
— Electron filtering, pulsed plasmas, gas feed and
pumping, additive gases to absorb electron groups,
shaped bias voltage, electronegative optimization. etc.
•
•
•
Understanding and eliminating oxide damage
Large area sources for FPDs, not wafers
Eventual widespread adoption of helicon sources
UCLA