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RF Plasma Sources and How to Use Helicons Francis F. Chen Professor Emeritus, UCLA Semes Co., Ltd.,...
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Transcript of RF Plasma Sources and How to Use Helicons Francis F. Chen Professor Emeritus, UCLA Semes Co., Ltd.,...
RF Plasma Sources
and
How to Use HeliconsFrancis F. Chen
Professor Emeritus, UCLA
Semes Co., Ltd., Chungnam, Korea, February 15, 2012
Plasma is necessary for etching
UCLA
SHEATH
Three kinds of RF discharges
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• Capacitively Coupled Plasmas (CCPs), formerly called Reactive Ion Etchers (RIEs)
• Inductively Coupled Plasmas (ICPs)
• Helicon Wave Sources (HWS)
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Capacitive Discharges (CCPs)
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Schematic of a capacitive discharge
Plasma
Sheath
Sheath
Gas inlet
Gas outlet
Main RF
He coolant
Chuck
Bias RF
Powered electrode
Wafer
Grounded electrode
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Sheaths keep a plasma neutral
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UCLA
Sheaths are very thin
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De
e
T eV
n 7 4
1018 3.( )
( )mmnumerically,
11 3 17 310 cm (10 m ), 4 eVe en T Let
Then 50 mD
Debye sheaths are approximately 5D thick
5 200 m = 0.2mmD
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The Child-Langmuir Law
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The Debye (normal) sheath has a voltage drop of about 5KTe.
If additional voltage is applied, a CL sheath forms with only ions.
ions and electrons
ions only
quasi-neutral
+ +++
d V 3/4
Most of the volume is sheath
UCLA
• Electrons are emitted by secondary emission ( and modes)
• Ionization mean free path is shorter than sheath thickness ()
• Ionization occurs in sheath, and electrons are accelerated
into the plasma (gamma mode)
• Why there is less oxide damage is not yet known
Large electrode
Small electrode
PLASMA
Sheath
Sheath
E
E
Effect of frequency on plasma density profiles
0,030 0,035 0,040 0,045
1013
1014
1015
1016
1017
45 mTorr 13.56 MHz 800 VC
on
cen
trat
ion
(m
-3)
r (m)
13.56 MHz
0,030 0,035 0,040 0,045
1013
1014
1015
1016
1017
45 mTorr 27 MHz 800 V
Co
nce
ntr
atio
n (
m-3)
r (m)
27 MHz
0,030 0,035 0,040 0,045
1013
1014
1015
1016
1017
45 mTorr 40 MHz 800 V
Co
nce
ntr
atio
n (
m-3)
r (m)
40 MHz
0,030 0,035 0,040 0,045
1013
1014
1015
1016
1017
45 mTorr 60 MHz 800 V
Co
nce
ntr
atio
n (
m-3)
r (m)
60 MHz
Plasma ApplicationModeling GroupPOSTECH
Problems with the original CCP discharges
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• 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, too few knobs to turn to control the ion and electron distributions and the plasma uniformity
Ion velocity distribution can be adjusted by applying low frequencies to substrate
UCLA
27 MHz
2 MHz
Thin gap. Unequal areas to increase sheath drop on wafer
High frequency controls plasma density
Low frequency controls ion motions and sheath drop
A LAM Exelan oxide etcher
Plasma ApplicationModeling GroupPOSTECH
CCPs are great for large gas feed
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GAS INLETS
HOLESRFGLASS SUBSTRATE
Fast and uniform gas feed for depositing amorphous silicon on very large glass substrates for displays (Applied Komatsu)
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Inductively Coupled Plasmas (ICPs)
Inductively Coupled Plasmas (ICPs)
The antenna can be on top or on the side
TCP (Transformer Coupled Plasma)PlasmaTherm etcher
Or it can be a combination
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Applied Materials patent
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RF B-field pattern comparison
-30
-20
-10
0
10
20
30
-30 -20 -10 0 10 20 30
Lam type AMAT type
-20
-15
-10
-5
0
5
10
15
20
-20 -15 -10 -5 0 5 10 15 20
Simulation of the plasma in an ICP
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Early AMAT system
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The electrostatic chuck is an essential part
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How can the RF energy get inside?
0
2
4
6
8
10
12
-5 0 5 10 15 20r (cm)
n (1
010
cm
-3)
800
240
200
Prf(W)3 mTorr, 1.9 MHz
The Plasma-Therm etcher
The density is high where RF is low!
UCLA
0
1
2
3
0 5 10 15r (cm)
n (1011 cm-3)
KTe (eV)
RF Bz field skin depth
Explanation 1: electron path with Lorentz force
0
180
360
540
720
900
1080
1260
RF phase(degrees)
Skin depth
with FL
without FL
dm e
d t
vE v B
F L UCLA
Electrons spend more time near center
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Explanation 2: Sheaths at endplates
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HIGH DENSITY
LOWER DENSITY
SHEATH
B
+
+
e
e
ION DIFFUSION+
-
E
(b)
1
2
The Simon short-circuit effect at endplates causes an electric field to
develop that drives the ions inwards, and the electrons can “follow”
even across magnetic fields.
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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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Proposed enhancements of ICPs
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Coupling can be improved with a magnetic cover
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H H
H H
B B
E
H = J
B = H
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The ferrite is inside the vacuum
Meziani, Colpo, and Rossi, Plasma Sources Science and Technology 10, 276 (2001)
Fluxtrol F improves both RF field and uniformity(Meziani et al.)
0 1 2 3 4 5 6 7 8 9 10 11 120
1
2
3
4
5
6
7
700 W
100 W
700 W
100 W
w.o. magnetic pole (C1) w. magnetic pole (C2)
Br (
Ga
uss)
Irms
(A) 0 2 4 6 8 102
10
20
40Argon30 mtorr, 600 W
2 turn coil 2 turn coil + mag. pole Spiral MaPE
Ji (
mA
/cm
2 )
r (cm)
Magnetic material
2 loops in //
2 serpentines in //
3 loops in //
1 2 3 4 5 6 7 8 9
X (cm)
Y (cm)
110 mm 800 x 800 mm
750 x720 mm
Magnets are used in Korea (G.Y. Yeom)
SungKyunKwan Univ. KoreaSungKyunKwan Univ. Korea
Both RF field and density are increased
4006008001000120014001600180020002200
2.0x1010
4.0x1010
6.0x1010
8.0x1010
1.0x1011
1.2x1011
Without magnetic fields With magnetic fields
Ni (Ion Density /cm
3 )
RF power(Watts)
0 500 1000 1500 2000
100
200
300
400
500
600
700 Antenna type=serpentine(7m)Operating pressure=15mTorr
Vrm
s (V
olts
)
Input power (W)
No multipolar magnetic fields With multipolar magnetic fields
SungKyunKwan Univ. KoreaSungKyunKwan Univ. Korea
Serpentine antennas(suggested by Lieberman)
Plasma ApplicationModeling GroupPOSTECH
Magnets
Density uniformity in two directions
0 10 20 30 40100
150
200
250
300
350
400
450
500
550
600
1000W RF Input power 1500W RF Input power 2000W RF Input power
Ion
Satu
ratio
n C
urr
ent (1
0-6A
)
Position, Parallel to the antenna (cm)
-30 -20 -10 0 10 20 3050
100
150
200
250
300
350
400
450
500
Ion S
atu
ration
Curr
ent(
10
-6A
)
Probe Position (cm)
1000W RF Input Power 1500W RF Input Power 2000W RF Input Power
G.Y. Yeom, SKK Univ., Korea
Effect of wire spacing on density (calc.)
7.2cm
7.8cm
9cm
10.2cm
11.4cm
13.2cm
0
2 E + 0 1 0
4 E + 0 1 0
6 E + 0 1 0
8 E + 0 1 0
1 E + 0 1 1
1 E + 0 1 1
1 E + 0 1 1
Plasma ApplicationModeling GroupPOSTECH
Park, Cho, Lee, Lee, and Yeom, IEEE Trans. Plasma Sci. 31, 628 (2003)
UCLA
Helicon Wave Sources (HWS)
--
+ --
+
--
+ --
+
(a)
(b)
(c)
B
In helicon sources, an antenna launches wavesin a dc magnetic field
The RF field of these helical waves ionizes the gas.The ionization efficiency is much higher than in ICPs.
Why are helicon dischargessuch efficient ionizers?
Trivelpiece-Gould mode
Helicon mode
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 1 2 3 4
r (cm)
P(r
) (
/ cm
2 )
Parabolic Profile: R = 1.47 Ohms
Square Profile: R = 1.71 Ohms
The helicon wave couples to an edge
cyclotron mode, which is rapidly
absorbed.
A helicon discharge at Wisconsin
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A commercial helicon etcher (PMT MØRI)
It required two heavy electromagnets with opposite currents.
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Replace heavy electromagnet with small permanent magnet
and
Design of an array source with small tubes
A long antenna requires a long tube, and plasma goes to wall before it gets out.
An m = 0 loop antenna can generate plasma near the exit aperture. Note the “skirt” that minimizes eddy currents in the flange.
Antenna must be short so that a short tube can be used
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The low-field peak: constructive interference
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1E+11 1E+12 1E+13n (cm-3)
R (
oh
ms)
100.0
63.1
39.8
25.1
15.8
10.0
B(G) L=2", 1mTorr, conducting
Low-field peak
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1E+11 1E+12 1E+13n (cm-3)
R (
oh
ms)
100.0
63.1
39.8
25.1
15.8
10.0
B(G) L=2", 1mTorr, conducting
Low-field peak
R is the plasma resistance, which determines the RF power absorbed by the plasma,
The tube length is designed to maximize rf energy absorption
Final design of the discharge tube
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5.1 cm
10 cm
5 cmANTENNA
GAS INLET (optional)
Use the far-field of the magnet
12.7 cm
7.6 cm
PLASMA
NdFeB material, 3”x 5”x1” thickBmax = 12 kG
-16
-14
-12
-10
-8
-6
-4
-2
0
2
4
6
8
-10 -8 -6 -4 -2 0 2 4 6 8 10
0
50
100
150
200
250
300
0 2 4 6 8 10 12z (in.)
Bz (G
)
0.0
0.52
0.92
r (in.)
D
The magnet position sets the B-field magnitude
UCLA5.1 cm
10 cm
5 cmANTENNA
Nd Permanent Magnet
7.6 cm
12.7 cm
A quartz tube with 3-turn antenna
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An 8-tube staggered array in operation
UCLAUCLA
The magnet tray
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The magnets are dangerous!
Array fed by a 50 water-cooled transmission line
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Density profiles along the chamber
Staggered configuration, 2kW
Bottom probe array
0
1
2
3
4
5
-8 -6 -4 -2 0 2 4 6 8 10 12 14 16x (in.)
n (1
011 c
m-3
)
-3.5
0
3.5
Staggered, 2kW, D=7", 20mTorr
y (in.)
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Density profiles along the chamber
Compact configuration, 3kW
Bottom probe array
0
2
4
6
8
10
-8 -6 -4 -2 0 2 4 6 8 10 12 14 16
x (in.)
n (
10
11 c
m-3
)
3.5-03.5
Compact, 3kW, D=7", 20mTorr
y (in)
Data by Humberto Torreblanca, Ph.D. thesis, UCLA, 2008.
Conclusion
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• Helicon sources can produce high densities over large areas.
• Permanent-magnet helicon arrays are cheap and compact.
• Their design was made possible by extensive theory.
• To penetrate the industry, other gases must be tried.
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THE END
Thank you