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What’s special about helicon discharges?
-1
0
1
2
3
4
5
6
3.0 3.5 4.0 4.5 5.0 5.5 6.0
log (wp2/w2)
log
(c/
)
f = 13.56 MHz
n = 1E12 cm-3
B = 100 G
lower hybrid
Wci
wc
(wcW
c)1/2
Wpi
Density
B-f
ield
Helicon waves are whistler waves confined to a cylinder.Helicon discharges are made by exciting these waves.
The boundary has a large effect on theionization efficiency
Trivelpiece-Gould mode
Helicon mode
The H mode peaks at the center, but its currents or charges at the boundary mode-couples to an electron cyclotron wave (TG mode) at the edge. The TG wave is electrostatic and travels slowly inward, efficiently depositing the RF energy into the electrons.
k1k2
k0
k0 = helicon wave, k1 = ion acoustic wavek2 = Trivelpiece-Gould mode
This was verified experimentally.
0 1 2
0 1 2
0 1 2 1 20,
w w w
k k k
k k k k k
The deposition occurs via parametric instability
UCLA
As Prf is raised, the sidebands get larger due to
the growth of the LF wave.
Krämer et al. detected the ion wave B. Lorenz, M. Krämer, V.L. Selenin, and Yu.M. Aliev, Plasma Sources Sci.Technol. 14, 623 (2005)
Thus, helicon research links several disciplines
1. Low-temperature plasma physics
2. Space physics (whistler waves)
3. Magnetic fusion (B-field, RF power, Bohm diffusion)
4. Laser fusion (parametric instabilities)
A helicon discharge in a straight cylinder can produce densities up to 1014 cm3 with only 1-2 kW.
How can we use this dense source?
This is a commercial helicon source made by PMT, Inc. and successfully used to etch semiconductor wafers. It required two large and heavy electromagnets and their power supplies.
Computer chips are now etched with simpler sources without a DC B-field.
New applications require larger area coverage.
Possible uses of large-area plasma processing
Roll-to-roll plastic sheets
Smart windowsOLED displays
Solar cells, mass production Solar cells, advanced
Distributed helicon source: proof of principle
ROTATING PROBE ARRAY
PERMANENT MAGNETS
3"
DC MAGNET COIL
18"
Power scan at z = 7 cm, 5 mT A, 20 G, 13.56 MHz,
0.0
0.5
1.0
1.5
2.0
0 5 10 15 20 25 30R (cm)
N (
101
2 cm
-3) 3.0
2.5
2.0
1.5
1.0
P(kW)
7-tube m=0 array
ARGON
PROBE
Achieved n > 1.7 x 1012 cm-3, uniform to 3%, but large magnet is required.F.F. Chen, J.D. Evans, and G.R. Tynan, Plasma Sources Sci. Technol. 10, 236 (2001)
The problem with small magnets
-10
0
10
20
30
z (c
m)
QUARTZ TUBE
PVC PIPE
ANTENNA
MAGNET WINDING
7 cm
5 cm
13 cm
BNC connector
5 mm
17 mm
1 cm
1 cm
10 cm
Internal field
External field
Internal field
External fieldA small solenoid Field lines diverge
too rapidly
Annular permanent magnets have same
problem
However, the external field can be used
Note that the stagnation point is very close to the magnet
Place plasma in the external field, and eject downwards
Internal field
External field
Internal field
External field
Gate Valve
To Turbo Pump
34 cm
36 cm
D
Z1
Z2
-300
-250
-200
-150
-100
-50
0
50
100
150
0 5 10 15 20 25 30
z (cm)
Bz
(G)
Calculated
Measured
External field
Internal field
0
1
2
3
4
5
6
7
-5 0 5 10 15 20r (cm)
n (
101
0cm
-3)
Z2, 40
Z2, 35
Z2, 30
Z2, 21
Z2, 1
D (cm)
500W, 1 mTorr
The bottom curve is when the tube is INSIDE the magnet
PM helicons: proof of principle
Evolution of a multi-tube PM helicon source
1. Antenna design
2. Discharge tube geometry
3. Permanent magnets
4. RF circuitry
Next: construction and testing of Medusa 2
Medusa Medusa 1
Helicon m = 1 antennas
--
+
E
B
Only the RH polarized wave is strongly excited
Nagoya Type III antenna:symmetric, so RH wave is driven in both directions.
RH helical antenna:RH wave is driven only in the direction matching the antenna’s helicity.
This antenna has the highest coupling efficiency
Why we use an m = 0 antenna
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.
Now we have to design the diameter and length of the tube.
The low-field peak: an essential feature
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
The peak occurs when the backward wave is reflected to interfere constructively with the forward wave.
R is the plasma resistance, which determines the RF power absorbed by the plasma,
Designing the tube geometry
H
2a
CONDUCTING ORINSULATING ENDPLATE
1
Z
n
a k B
w
Adjust a, H, and wRF so that n and B are in desired range.
This is done with the HELIC codeD. Arnush, Phys. Plasmas 7, 3042 (2000).
a
b
c
Distant conducting shell
antenna
plasma
Lc
a b
h
Loop antenna
Helical antenna
B0
Lc is made very large to simulate injection into a processing chamber.
The code computes the wave fields and the plasma loading resistance Rp vs. n and B
Choose a peak at low B, mid 1012 cm-3 density
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
0.5
1.0
1.5
2.0
2.5
1E+11 1E+12 1E+13n (cm-3)
R (
oh
ms)
1000
464
215
100
46
22
10
B (G) d = 3", H = 2", 13.56MHz
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
1E+11 1E+12 1E+13n (cm-3)
R (
oh
ms)
d = 4 in.
d = 3 in.
d = 2 in.
100G, H = 2", 13.56 MHzTube diameter
0.0
0.5
1.0
1.5
2.0
2.5
1E+11 1E+12 1E+13n (cm-3)
R (
oh
ms)
H = 3 in.
H = 2 in.
H = 1 in.
100G, d = 3", 13.56 MHz
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
1E+11 1E+12 1E+13n (cm-3)
R (
oh
ms)
f = 27.12 MHz
f = 13.56 MHz
f = 2 MHz
Typical R(n,B) curves at the low-field peak
Vary the B-field Vary the tube length
Vary the tube diameter Vary the RF frequency
Final tube design for 13.56 MHz
5.1 cm
10 cm
5 cmANTENNA
GAS INLET (optional)
Material: Pyrex or quartzWith aluminum top
Reason for maximizing Rp: circuit loss Rc
pin rf
p c
RP P
R R
: pp c in rf p
c
RR R P P R
R
:p c in rfR R P P
10
100
1000
1E+11 1E+12 1E+13n0 (cm-3)
Pin
(W
)
1000
500
200
100
Loss
Prf (W)
No helicon ignition
Unstable equilibrium
Stable equilibrium
Rc = 1.0 W
10
100
1000
1E+11 1E+12 1E+13n0 (cm-3)
Pin
(W
)
1000
500
200
100
Loss
Prf (W)
Stable equilibria
Rc = 0.1 W
Magnet design for 60-100 G
Vary the outside diameter
Vary the vertical spacing
Final magnet design
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
RF circuitry
R, L
R, L
R, L
R, L
PS
N loads
Z2 - short cables
Distributor
Z1Z2
Z1 - long cable
C1C2
Matching ckt. 50W
For equal power distribution, the sources are connected in parallel with equal cable lengths. The problem is that the cable lengths, therefore, cannot be short.
The length Z2 and the antenna inductance L are the most critical.
C1, C2 for N=8, L = 0.8H, Z1 = 110 cm, Z2 = 90 cm(unless varied)
0
200
400
600
800
1000
1200
1400
1600
0 50 100 150 200Z2 (cm)
C (
pF)
C1(S)
C2(S)
0
200
400
600
800
1000
1200
1400
1600
0 0.5 1 1.5 2 2.5 3L (uH)
C (
pF)
C1(S)
C2(S)
Allowable values of C1, C2 in match circuit
There is an upper limit to each antenna’s inductance L.
The range of Z2 can be restrictive for large arrays
Current and voltage in CW operation
Coax connectors cannot take the startup voltage or the CW current. All joints have to be soldered or have large contact area.
Junction box Connection to water-cooled antenna
A low-Rc, 50-Wcooled, rectangular transmission line
Layout of 8-tube test module, Medusa 2
165 cm
53.3 cm
17.8
17.8
17.8
17.8 cm73.7 cm
8.9 cm
x
y
Compact configurationStaggered configuration
The spacing is determined from the single-tube density profiles to give 2% uniformity
Side view
165 cm
30 cm
15 cm
Probe ports
Aluminum sheet
Adjustable height
The source requires only 6” of vertical space above the process chamber
Z1
Z2
Wooden frame for safety and movability
Medusa 2 in operation at 3 kW CW
Radial profile between tubes at Z2
0
0.5
1
1.5
2
2.5
3
3.5
-25 -20 -15 -10 -5 0 5 10 15 20 25r (cm)
n (1
01
1 c
m-3
) n
KTe
UCLA
0 3.5”
Compact configuration, 3kW
Side Langmuir probe
Density profiles across the chamber
<< 4” below tubes
<< 7” below tubes
0
2
4
6
8
10
-8 -6 -4 -2 0 2 4 6 8y (in)
n (1
01
1cm
-3)
Z1, x = 0
Z1, x = 3.5
Z2, x = 0
Z2, x = 3.5
Compact 3kW, D=7", 20mTorr
UCLA
Density profiles across the chamber
0 7-7 14”
Staggered configuration, 3kW
Bottom probe array
0
1
2
3
4
5
-8 -6 -4 -2 0 2 4 6 8y (in.)
n (
10
11 c
m-3
)
-7
07
14
x (in.)Staggered3kW, D=7",
20mTorr
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.)
UCLA
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.
CONCLUSIONS
We’ve found a sweet spot where the tube, the antenna, the magnet, and the matching circuit can all work together.
There’s a large step between laboratory physics and a practical device.