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Helicon sources are ICPs with a DC B0
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
Adjust a, H, and RF 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
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
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. 50
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
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
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
An linear array of 15 probes
UCLA
0.375Ó
19.75Ó
4.0"0.25Ó
H. Torreblanca, Multitube helicon source with permanent magnets, Thesis, UCLA (2008).
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.
Hydrogen RnB scans for 13.56 MHz
0.00
0.10
0.20
0.30
0.40
0.50
0.60
1E+10 1E+11 1E+12n (cm-3)
R (
oh
ms)
20
40
60
80
B (G)H = 1.0 in. conducting
13.56 MHz
0.0
0.1
0.2
0.3
0.4
0.5
0.6
1E+10 1E+11 1E+12n (cm-3)
R (
oh
ms)
20
40
60
80
B (G)H = 1.5 in. conducting
13.56 MHz
0.0
0.1
0.2
0.3
0.4
0.5
0.6
1E+10 1E+11 1E+12n (cm-3)
R (
oh
ms)
20
40
60
80
H = 1.5 in. insulatingB (G)
13.56 MHz
0.0
0.1
0.2
0.3
0.4
0.5
0.6
1E+10 1E+11 1E+12n (cm-3)
R (
oh
ms)
5
10
15
20
B (G) H = 2.0 in. conducting
13.56 MHz
No stable solution for hydrogen. Here, H is distance from antenna to endplate.
Hydrogen helicons in Medusa 2 tube
0
2
4
6
8
10
12
14
0 20 40 60 80 100
B-field (G)
Lo
we
r h
ybrid
fre
qu
en
cy (
MH
z)
Hydrogen
Argon
13.56 MHzn = 1E12 cm-3
z
nk
k B
The lower hybrid frequency LH) is 6.5 times higher for H than for Ar and is not << (RF). To neglect ion motions, need to have (RF) >> (LH). Need to decrease B to have lower (LH), but low B means bad coupling, like ICPs. Since k is same if we keep 2” diam tube, we have to increase (RF) and change n and kz.
2 2LH c ci
Meaning of the lower hybrid frequency
The exact lower hybrid frequency LH is given by
where p is the ion plasma frequency.
The last term is negligible except at very low density, so LH is proportional to B/M.
In simple helicons, is >> LH and c, so the ions cannot move with the RF. When LH approaches RF, the ions will move and contribute to the helicon current. Scime et al. have seen increased ion temperatures when ~ LH, but HELIC does not show any great effect there. At LH, the ion and electron orbits to B look like this:
The blue line is the ion cyclotron orbit, which has been distorted by the LH wave. The red line is the orbit of the electron guiding-center E x B drift. The cyclotron orbits of the electrons is too small to see.
2 2
1 1 1
c cLH p
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1E+11 1E+12 1E+13n (cm-3)
R (
oh
ms)
10
30
50
70
90
H = 1.0" conductingB (G)
27.12 MHz
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1E+11 1E+12 1E+13n (cm-3)
R (
oh
ms)
20
40
60
80
H = 1.5" conducting
B (G)
27.12 MHz
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1E+11 1E+12 1E+13n (cm-3)
R (
oh
ms)
20
40
60
80
B (G)
H = 1.5 in. insulating
27.12 MHz
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1E+11 1E+12 1E+13n (cm-3)
R (
oh
ms)
20
40
60
80
100
H = 3.0 in. conducting27.12 MHz
B (G)
There are stable solutions, but n has to be high, requiring LOTS of power.
Hydrogen RnB scans for 27.12 MHz
Compare hydrogen at 27.12 MHz with argon at 13.56 MHz
to get an idea of how the discharges behave in the standard 2” diam tube
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
1E+11 1E+12 1E+13n (cm-3)
R (
ohm
s)
75
50
25
Argon, 13.56 MHzH = 2"
B (G)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1E+11 1E+12 1E+13n (cm-3)
R (
ohm
s)
75
50
25
Hydrogen, 27.12 MHzH = 2"
B (G)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
1E+11 1E+12 1E+13n (cm-3)
R (
ohm
s)
100
75
50
Argon, 13.56 MHzH = 3"
B (G)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1E+11 1E+12 1E+13n (cm-3)
R (
ohm
s)
125
100
75
Hydrogen, 27.12 MHzH = 3"
B (G)
H is essentially the tube length
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1E+11 1E+12 1E+13n (cm-3)
R (
ohm
s)
100
80
60
40
20
Argon, 13.56 MHzB (G)
0
1000
2000
3000
4000
0.000 0.005 0.010 0.015 0.020 0.025r (m)
P(r
) (a
rb.)
100G, 1.6E1240 G, 6.3E11
Argon @ 13.56 MHz
0
1
2
3
4
-1.00 -0.95 -0.90 -0.85z(m)
P(z
) (a
rb.)
100G, 1.6E12
40 G, 6.3E11
Argon @ 13.56
How does the power deposition look in normal Ar discharges?
Here P(z) and P(r) are the power deposition profiles in z and r, and P(k) is the power spectrum. The cases are at two low-field peaks, and the spectrum is almost a pure mode. The dashed line is the location of the antenna.
0.000
0.004
0.008
0.012
0.016
0.020
0.024
0.028
0 25 50 75 100k (m-1)
P(k
) (a
rb.)
100G, 1.6E12
40 G, 6.3E11
50G, 3E11
Argon @ 13.56
Hydrogen, 50G, 3E11 @ 27.12 MHz
*
*
0
200
400
600
800
1000
1200
1400
0.000 0.005 0.010 0.015 0.020 0.025r (m)
P(r
) (a
rb.)
Hydrogen
ArgonR = 0.564R = 0.397
0
1
2
3
4
-1.00 -0.95 -0.90 -0.85z(m)
P(z
) (a
rb.)
Hydrogen
Argon
0.000
0.002
0.004
0.006
0.008
0.010
0 20 40 60 80 100k(m-1)
P(k
) (a
rb.)
Argon
Hydrogen
This compares the profiles for argon and hydrogen in the same 2 x 2” tube and at the same conditions: B = 50G and n = 3 x 1011 cm-3. However, f = 13.56 MHz for argon and 27.12 MHz for hydrogen.
Compare similar H and Ar discharges
0
2000
4000
6000
8000
0 0.005 0.01 0.015 0.02 0.025r (m)
P(r
)
1.5", conduct.
3.5", insul.
140G, 1.3E12H (in.), endplate
0
0.01
0.02
0.03
0.04
0.05
0 20 40 60 80 100 120 140k (m-1)
P(k
)
1.5", conduct.
3.5", insul.
140G, 1.3E12 H (in.), endplate
Both are near density peak, but conducting case has pure mode.
Power deposition profiles for two very different cases
P(r) is dominated by the TG mode and does not vary much.
P(z) peaks near the antenna (dashed line in each case). High P near endplate is not good, since plasma created there is lost fast.
The k-spectrum is pure for H = 1.5” but has other modes for H = 3.5”, as seen by the wiggles in the RnB curve on the last page.0
1
2
3
4
5
6
7
-1.00 -0.95 -0.90 -0.85 -0.80z (m)
P(z
)
1.5", conduct.
3.5", insul.
140G, 1.3E12
H (in.), endplate
R = 1.41
R = 1.67
0
1
2
3
4
5
6
-1.00 -0.95 -0.90 -0.85 -0.80z (m)
|Ez|
(z)
H = 1.5"
H = 3"
140G, 1.4E12, conducting
R = 1.67
R = 0.87
140G, 1.3E12, conducting
0
1
2
3
4
5
6
7
-1.00 -0.95 -0.90 -0.85 -0.80z (m)
P(z
)
H = 1.5 in.
H = 3 in.
140G, 1.3E12, conducting
140G, 1.4E12, conducting R = 1.67
R = 0.87
Comparison of waves in 1.5 in. and 3 in. long tubes
The short tube has higher P(z), but it is high near the endplate. The electric field |Ez|, however, fits properly , whereas it is too short for the 3” tube. The maximum of Ez at the endplate causes strong reflection, which gives a higher low-field peak. Thus, the short tube is better even though a lot of useless ionization occurs near the endplate. This shows that computing Ez may be the best way to fit the tube length to the half-wavelength of the helicon wave and optimize the loading.
Comparison of 3 optimized systems of different diameters
For hydrogen at 27.12 MHz
Tube: 2” diam, 1.5” highMagnet: 3 x 5”, 2” high
Tube: 3” diam, 2” highMagnet: 4 x 6”, 2” high
Tube: 6” diam, 3” highMagnet: 7 x 10”, 4” high
Note: antenna inductance has to be adjusted
Generation of a “double layer”
20
0 0
rB n
B n r
0 , where -e /e en n e V KT
1/ 2 1/4
0 0
, and thus 1.28n r
e en r
The Bohm velocity is reached when = ½, and sheath forms
Medusa source adapted to VASIMR
9 11
5
D
16
The optimized 9-cm diam source is shown with dimensions in cm, together with a NdFeB magnet designed for 400G at the antenna. D is the distance from the midplane of the magnet to the midplane of the antenna. The magnet is made in two pieces supported by a non-ferrous metal plate. The B-field can be adjusted by changing D either by hand or remotely with a motor.
A stronger B-field for higher density
Layout of magnet and tube for 600G operation,showing a gas feed line and a DC bias supply.
A small diam source with for testinghigh-field operation
5 11
3
D
6.6
L
A 5-cm diam helicon tube and a 600-G magnet designed for a small overall system diameter.
Conclusion on spacecraft thrusters
• “Ambipolar” sources can eject ions with automatic space-charge neutralization.
• Helicon sources can generate ions efficiently.
• Permanent magnets can reduce the complexity of helicon sources.
• However, for the fields and densities considered for the VASIMR project, the magnet may be too large to be practical.