design of pm helicon arrays
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Design of PM helicon arrays
UCLA
1. Optimization of the discharge tube
2. Design of the permanent magnets
3. Design of a multi-tube array
4. Design and construction of a test chamber
5. Antennas and the RF distribution system
6. Experimental results
7. Design of a compact module
8. Ideas for further improvements to be tested
A commercial helicon etcher (PMT MØRI)
It required two heavy electromagnets with opposite currents.
Previous experiment with 7 tubes
UCLA
ROTATING PROBE ARRAY
PERMANENT MAGNETS
3"
DC MAGNET COIL
18"
54 mm
2.4 mm
6.4 mm10 cm
2.5
cm
The “stubby” tube
It required a large electromagnet
Plasmas merged; density is uniform
UCLA
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
High density and uniformity were achieved
A c o m p u t a t i o n a l p r o g r a m f o rH E L I C O N s a n d I C P s
V e r s i o n 1 . 0
A u t h o r : D o n a l d A r n u s h , w i t h F r a n c i s F . C h e nP r o g r a m m e r s : R i c k L a n , R a v a s h E l i a s s i
U C L AA
Optimization of discharge tube: HELIC code
UCLAD. Arnush, Phys. Plasmas 7, 3042 (2000).
Radial profiles are arbitrary, but B and n must be uniform axially.
HELIC gives not only the wave fields but also R, the loading resistance.
a
b
c
AntennaLc
a b
h
Loop antenna
Helical antenna
B0
The HELIC user interface
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The Low-field Peak
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DENSITY vs. MAGNETIC FIELD4-cm tube, uniform field
0.0
0.5
1.0
1.5
2.0
0 20 40 60 80 100 120B (G)
n (1
012
cm
-3)
Mechanism of the Low Field Peak
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0 1,r
z
ne nk
k B a k B
Basic helicon relations
The peak is sensitive to the density profile
0
1
2
3
4
5
6
0 50 100 150 200 250 300
B (G)
R (
ohm
s)
Uniform
Flat with rolloff
Parabolic
The peak depends on the boundary condition
0
1
2
3
4
5
6
7
8
0 50 100 150 200 250 300
B (G)
R (
ohm
s)
Insulating
Conducting
The peak depends on distance from endplate
0
1
2
3
4
5
6
0 50 100 150 200 250 300
B (G)
R (
ohm
s)
5 cm
10 cm
No bdy
d
The peak depends on the type of antenna
0
1
2
3
4
5
6
0 50 100 150 200 250 300
B (G)
R (
ohm
s)Loop, d = 10 cm
HH10, d = 10 cm
Nagoya III
Single loop: m = 0, bidirectionalHH (half-wavelength helical): m = 1, undirectionalNagoya Type III: m = 1, bidirectional
Typical scan of Rp vs n, B
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0.0
0.5
1.0
1.5
2.0
2.5
1E+11 1E+12 1E+13n (cm-3)
R (
ohm
s)
1000464215100462210
B (G) d = 3", H = 2", 13.56MHz
Each point requires solving a 4th order differential equation >100 times.A typical scan takes ~ 3 hours on a PC.
Matrices for optimizing discharge tube
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Vary the tube length and diameter Vary the RF frequency
Vary the pressure and frequency
Vary the endplate conductivity
Vary H (endplate distance) for 3” diam
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0.0
0.5
1.0
1.5
2.0
2.5
1E+11 1E+12 1E+13n (cm-3)
R (
ohm
s)
H = 3 in.
H = 2 in.
H = 1 in.
100G, d = 3", 13.56 MHz
Vary diam for H = 2” at 100G
UCLA
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 (
ohm
s)
d = 4 in.
d = 3 in.
d = 2 in.
100G, H = 2", 13.56 MHzTube diameter
Vary the frequency
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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 (
ohm
s)
f = 27.12 MHz
f = 13.56 MHz
f = 2 MHz
D = 3", L = 2", 100G
Not much variation with pressure
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0.0
0.5
1.0
1.5
2.0
2.5
1E+11 1E+12 1E+13n (cm-3)
R (
ohm
s)
1 mTorr
3 mTorr10 mTorr
D = 2", H = 1.5", 100G
Vary the endplate material
UCLA
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)
Conducting
Insulating
H = 2", D = 2", B = 80G
0.0
0.2
0.4
0.6
0.8
1E+11 1E+12 1E+13n (cm-3)
R (
ohm
s)
H = 1.5 in., conducting
H = 3.0 in., insulating
Initially, it seems that the conducting endplate is better. However, it is because the phase reversal at the endplate has changed, and the tube length has to be ~1/4 wavelength longer to get constructive interference. By changing H, almost the same R can be achieved.
Relation of R to plasma density
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pin rf
p c
RP P
R R
7 1/ ½ 1.0 10 secsdN dt Snc n
c i eW E W W 101.1 10 WattsoutP n
Rp << Rc
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 G
63 G
40 G
25 G
16 G10 G
Relation of R to plasma density
UCLA
pin rf
p c
RP P
R R
7 1/ ½ 1.0 10 secsdN dt Snc n
c i eW E W W 101.1 10 WattsoutP n
10
100
1000
1E+11 1E+12 1E+13n (cm-3)
Pin
(W
)1000
500
200
100
Loss
Prf (W)
Rp > Rc
Final design
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2.120.25
4.00
2.25
Lesker QF50 cover plate
Lesker QF50 centering ringParker 2-330 Viton O-ring
Corning #237580 tubing, 63.5 mm OD, 4.8 mm wall
Parker 2-239 Viton O-ring
Dimensions are in inches
The “New Stubby” tube
UCLA
Design of PM helicon arrays
UCLA
1. Optimization of the discharge tube
2. Design of the permanent magnets
3. Design of a multi-tube array
4. Design and construction of a test chamber
5. Antennas and the RF distribution system
6. Experimental results
7. Design of a compact module
8. Ideas for further improvements to be tested
Characteristics of permanent magnet rings
UCLA
Internal field
External field
The B-field of annular PMs
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The field reverses at a stagnation point very close to the magnet.
Plasma created inside the rings follows the field lines and cannot be ejected.
Optimization of magnet geometry
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Result: Field strength magnet volumeSpacing improves uniformity slightly
actual
actual
The field of 4 stacked magnet rings
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-400
-300
-200
-100
0
100
200
-20 -15 -10 -5 0 5 10 15 20z (cm)
Bz
(G)
0.0
2.5
4.0
r (cm)
0
50
100
150
200
250
300
-20 -15 -10 -5 0 5 10 15 20z (cm)
|Bz|
(G
)
Calc.
Expt.
The internal and external fields at various radii. The individual rings can be seen at large radii.
Calibration of the calculated field with a gaussmeter.
For the designed tube, B ~ 60G is good
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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
Proof of principle on 3” diam tube
UCLA
Gate Valve
To Turbo Pump
34 cm
36 cm
D
Z1
Z2
0
20
40
60
80
100
120
5 10 15 20 25 30 35 40z (cm)
Bz
(G)
B (0)231928
3338
D (cm)
-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
Radial density profiles at Z1 and Z2
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0
2
4
6
8
10
-5 0 5 10 15 20r (cm)
n (
101
0cm
-3)
Z1, 40
Z1, 35
Z1, 30
Z1, 21
Z1, 1
D (cm)500W, 1mTorr
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
Upper probe Lower probex 1010 cm-3
Proof of principle: discharge in the external field gives much more plasma downstream.
The final design for 2” tubes
UCLA
12.7 cm
7.6 cm
PLASMA
Material: NdFeB
Bmax = 12 kG
Attractive force between two magnets 2 cm apart:
516 Newtons = 53 kg
The magnets are dangerous!
Wooden frame for safe storage
UCLA
Single tube, final configuration
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0
50
100
150
200
250
300
350
-4 -2 0 2 4r (cm)
Bz
(G)
56789101214
z (cm from midplane of magnet)
Radial Bz profiles at various distances below the magnet.
Discharge tube
Design of PM helicon arrays
UCLA
1. Optimization of the discharge tube
2. Design of the permanent magnets
3. Design of a multi-tube array
4. Design and construction of a test chamber
5. Antennas and the RF distribution system
6. Experimental results
7. Design of a compact module
8. Ideas for further improvements to be tested
Design of array
UCLA
The density at Z2 is summed over nearest tubes.
0
2
4
6
8
10
-5 0 5 10 15 20r (cm)
n (
10
11cm
-3)
Z1, single magnet
Z1, with neighbors
Z2, single magnet
Z2, with neighbors Radial density profilesat Z1 = 7.4 cm and Z2 = 17.6 cm below discharge.
Computed uniformity n(x) for various y
0
1
2
3
4
5
6
7
8
-30 -20 -10 0 10 20 30x (cm)
n (
10
11 c
m-3
)
Sum
Row1
Row2
Avg: all y
y = 0
(a)
0
1
2
3
4
5
6
7
8
-30 -20 -10 0 10 20 30x (cm)
n (
10
11 c
m-3
)
Sum
Row1
Row2
Avg: all y
y = 7.5
(b)
0
1
2
3
4
5
6
7
8
-30 -20 -10 0 10 20 30x (cm)
n (
10
11 c
m-3
)
Sum
Row1
Row2
Avg: all y
y = 15
(c)
0
1
2
3
4
5
6
-30 -20 -10 0 10 20 30x (cm)
n (
10
11 c
m-3
)
Sum
Row1
Row2
Avg: all y
y = 22.5
(d)
Half-way between rows 1/4-way between rows
Directly under a row Beyond both rows
A tube spacing of 7” is chosen
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0
2
4
6
8
10
12
15 20 25 30L (cm)
Rip
ple
(+
/- %
)
L = 17.5 cm gives < 2% ripple at 17 cm below source
For a single row, a distance L = 17.5 cm between two tubes gives less than 2% ripple in density.
Design of PM helicon arrays
UCLA
1. Optimization of the discharge tube
2. Design of the permanent magnets
3. Design of a multi-tube array
4. Design and construction of a test chamber
5. Antennas and the RF distribution system
6. Experimental results
7. Design of a compact module
8. Ideas for further improvements to be tested
An 8-tube linear test array
UCLA
165 cm
53.3 cm
17.8
17.8
17.8
17.8 cm73.7 cm
8.9 cm
x
y
Top view
Possible applications
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• Web coaters• Flat panel displays• Solar cells• Optical coatings
A web coater
The array source is vertically compact
UCLA
The magnets can be made in two pieces so that they hold each other on an aluminum sheet.
Once placed, the magnets cannot easily be moved, so for testing we use a wooden support.
165 cm
30 cm
15 cm
Side view
Probe ports
The wooden magnet frame is used in testing
UCLA
64"
21"
3.5"
12"
Wooden magnet support
2.00
5.00
3.75
2.5
24 ea. 2.5 x 5.5 x 1/2"4 ea. 2.5 x 64 x ¾”
1 ea. 21 x 64 x 1/2”16 ea. 1/4” diam dowels
Water and RF connections
UCLA
These will be shown in detail later
An 8-tube staggered array in operation
UCLA
Design of PM helicon arrays
UCLA
1. Optimization of the discharge tube
2. Design of the permanent magnets
3. Design of a multi-tube array
4. Design and construction of a test chamber
5. Antennas and the RF distribution system
6. Experimental results
7. Design of a compact module
8. Ideas for further improvements to be tested
Antennas
UCLA
The antennas are m = 0 loops made of three turns of 1/8” diam copper tubing.
The reason for m = 0 is that m = 1 antennas are too long, and much of the plasma is lost by radial diffusion before exiting the tube. The antenna must be close to the exit aperture and be tightly wound onto the tube.
The helicon wave pattern for m = 0 is a peculiar one but theory is straightforward. The wave changes from pure electromagnetic to pure electrostatic in each half cycle.
The RF system
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The critical elements are the junction box and the transmission lines.
Antenna connections (1)
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The antennas must be connected in parallel with cables of equal length.
The first trial was to use standard RG/8U cables and N connectors.
These arced and overheated.
Reason that RF connectors don’t work
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0
1000
2000
3000
4000
5000
6000
0 50 100 150 200 250 300 350 400Watts
Vol
ts p
-p
V p-p
Fit
0
2
4
6
8
10
12
14
0 50 100 150 200 250 300 350 400Watts
Am
ps r
ms
I rms
Fit
High voltages and currents occur when there is NO PLASMA; that is, before breakdown or when the plasma disrupts. Then, if the RF power on a tube is set for 400W, the peak-to-peak voltage can be 5kV, and the rms current can be 12A. Once the discharge is on, the RF power goes into the plasma rather than into the cables and connectors.
Antenna connections (2)
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In the second try, all connections were solidly soldered, and RG/393 teflon-insulated cable was used. This method works for CW operation in experiment
but may be marginal for industrial use.
Connections (3): a rectangular transmission line
UCLA
.25
0.70
ground
ground
HV RF
water in
water out
water at HV
1 1 10
000
cosh ( / ) cosh ( / ) 377 cosh ( / )
44 4r r r
h d h d h dZ
c
0 30ln(2 / )Z h d
For w >> h, we find
Impedance for various pipe diameters
UCLA
0
20
40
60
80
100
120
140
0 1 2 3 4 5 6h (in.)
Z0 (W
)0.25
0.375
0.5
0.625
0.75
0.875
1.0050W
d (in.)
For Z0 = 50W, h ~ 3/4”, but exactly 50W is not necessary
Transmission line construction (1)
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Transmission line construction (2)
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10.50
3.50
4.00
24.50
3.50
14.00
17.50
1.50
60.00
Transmission line construction (3)
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0.25
1.25
Transmission line (4): water connections
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TOP SHEET
BOTTOM SHEET
14.00 14.0022.00
14.00 14.0022.00
7.007.00 7.007.507.507.007.007.00
28.50
1.50
1.50
8.50
8.50
No high voltage is applied along a water line.
Pictures (1)
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Pictures (2)
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Pictures (3)
UCLA
Design of the matching circuitUCLA
T 2 R2 X 2 B2 R(T 2 R)
C1 R
BC2
1
T 2(X B)
Z ZL cos(kz) j sin(kz)
cos(kz) jZL sin(kz)
“Standard” circuit “Alternate” circuit
Analytic formulas from Chen. The important part is that the impedance changes with cable length.
F.F. Chen, Capacitor tuning circuits for inductive loads, UCLA Rept. PPG-1401 (unpublished) (1992); F.F. Chen, Helicon Plasma Sources, in "High Density Plasma Sources", ed. by Oleg A. Popov (Noyes Publications, Park Ridge, NJ), Chap. 1 (1995)
Adapted to N tubes in parallel
UCLA
R, L
R, L
R, L
R, L
PS
N loads
Z2 - short cables
Distributor
Z1Z2
Z1 - long cable
C1C2
Matching ckt. 50W
The problem with array sources is that the cable lengths cannot be short. The match circuit cannot be close to all the tubes.
C1, C2 for N=8, L = 0.8H, Z1 = 110 cm, Z2 = 90 cm
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0
200
400
600
800
1000
1200
0 50 100 150 200Z1 (cm)
C (
pF)
C1(S)
C2(S)
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
1800
2000
0.5 1 1.5 2 2.5 3 3.5 4 4.5 5R (ohms)
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)
Variation with the number of tubes N
UCLA
0
200
400
600
800
1000
1200
1400
1600
1800
2000
0 0.5 1 1.5 2 2.5L (uH)
C1
(pF
)
8
6
4
2
1
N
0
100
200
300
400
500
600
700
800
0 0.2 0.4 0.6 0.8 1 1.2 1.4
L (uH)
C2
(pF
)
8
6
4
2
1
N
Note that it is not possible to match to 1 or 2 tubes with the same length cables used for 8 tubes.
Design of PM helicon arrays
UCLA
1. Optimization of the discharge tube
2. Design of the permanent magnets
3. Design of a multi-tube array
4. Design and construction of a test chamber
5. Antennas and the RF distribution system
6. Experimental results
7. Design of a compact module
8. Ideas for further improvements to be tested
Experimental layout
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Staggered configuration
Compact configuration
Four probe positions
Effect of B-field strength (magnet height D)
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0
5
10
15
20
4 5 6 7 8 9 10 11
D (in)
n (
10
11 c
m-3
)
3kW2kW
Staggered, 20mTorrZ1 level, x=0", y=3.5"
0
1
2
3
4
4 5 6 7 8 9 10 11
D (in)
R(W
)
2kW3kW
Staggered, 20mTorr
Variation of density with D
Variation of loading resistance with D
Variation with RF power and Ar pressure
UCLA
0
5
10
15
20
25
30
0 20 40 60 80 100 120 140 160 180 200 220
P (mTorr)
n (1
01
1 c
m-3
)
3kW2kW
StaggeredZ1 level, x=0", y=3.5", D=7"
0
5
10
15
20
25
1.6 2 2.4 2.8 3.2 3.6 4 4.4 4.8 5.2
PRF (kW)
n (
10
11 c
m-3
)
Z1 levelZ2 level
Staggered, 20mTorr x=0", y=3.5", D=7"
Variation of density with RF power
Variation of density argon pressure
Density jump inside the tube
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0.01
0.1
1
10
100
0 50 100 150 200 250 300 350 400
Power per tube (W)
n (
10
11 c
m-3
)
0.10
0.50
1.00
Meas.
Rc (W)
compared with theory for various circuit resistances Rc
Deployment of movable probe array
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An linear array of 15 probes
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0.375Ó
19.75Ó
4.0"0.25Ó
H. Torreblanca, Multitube helicon source with permanent magnets, Thesis, UCLA (2008).
Density profiles across the chamber (1)
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0
4
8
12
16
20
-8 -6 -4 -2 0 2 4 6 8y (in)
n (1
011 c
m-3
)
Z1, x = 0Z1, x = 3.5Z2, x = 0Z2, x = 3.5
.
Staggered3kW, D=7", 20mTorr 0 3.5
Staggered configuration, 3kW
Side Langmuir probe
UCLA
0
2
4
6
8
10
-8 -6 -4 -2 0 2 4 6 8y (in)
n (1
011 cm
-3)
Z1, x = 0Z1, x = 3.5Z2, x = 0Z2, x = 3.5
Compact 3kW, D=7", 20mTorr
0 3.5
Compact configuration, 3kW
Side Langmuir probe
Density profiles across the chamber (2)
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0
1
2
3
4
5
-8 -6 -4 -2 0 2 4 6 8y(in)
n (1
011 c
m-
- 70714
x (in)Staggered3kW, D=7", 20mTorr
Density profiles across the chamber (3)
0 7-7 14
Staggered configuration, 3kW
Bottom probe array
UCLA
Density profiles across the chamber (4)
Compact configuration, 3kW
Bottom probe array
0
2
4
6
8
10
-8 -6 -4 -2 0 2 4 6 8
y(in)
n (1
011 c
m-
-70714
Compact3kW, D=10", 20mTorr
x (in)0 7-7 14
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0
1
2
3
4
5
-8 -6 -4 -2 0 2 4 6 8 10 12 14 16x(in)
n (
10
11 c
m-
-3.503.5
Staggered, 3kW, D=7", 20mTorr y (in)
Density profiles along the chamber (1)
Staggered configuration, 3kW
Bottom probe array
UCLA
0
1
2
3
4
5
-8 -6 -4 -2 0 2 4 6 8 10 12 14 16x(in)
n (
10
11 c
m-
-3.503.5
Staggered, 3kW, D=7", 20mTorr y (in)
Density profiles along the chamber (2)
Compact configuration, 3kW
Bottom probe array
Calibration of the collector array
UCLA
0
1
2
3
4
5
6
7
8
9
10
-8 -6 -4 -2 0 2 4 6 8y (in)
n (1
011 cm
-3)
LP, x = 0LP, x = 3.5LP over Al, x = 0LP over Al, x = 3.5ICC, x=0ICC, x=3.5
Compact, 2kW D=10", 20mTorr
0
1
2
3
4
5
6
7
8
9
10
-8 -6 -4 -2 0 2 4 6 8y (in)
n (1
011 cm
-3)
LP, x = 0LP, x = 3.5LP over Al, x = 0LP over Al, x = 3.5ICC, x = 0ICC, x = 3.5
Compact, 3kW, D=10", 20mTorr
0
1
2
3
4
5
-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7
y(in)
LP/(
LP o
ver
Al)
x=0x=3.5
0
1
2
3
4
5
-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7
y(in)
LP/IC
C
x=0x=3.5
Design of PM helicon arrays
UCLA
1. Optimization of the discharge tube
2. Design of the permanent magnets
3. Design of a multi-tube array
4. Design and construction of a test chamber
5. Antennas and the RF distribution system
6. Experimental results
7. Design of a compact module
8. Ideas for further improvements to be tested
A compact, 8-tube module
UCLA
28.00
7.00 7.00 14.00
TOP VIEW OF ONE MODULE END VIEW
5.50
Stacked modules for large-area coverage
UCLA
Match circuit fits on top of module
UCLA
Design of PM helicon arrays
UCLA
1. Optimization of the discharge tube
2. Design of the permanent magnets
3. Design of a multi-tube array
4. Design and construction of a test chamber
5. Antennas and the RF distribution system
6. Experimental results
7. Design of a compact module
8. Ideas for further improvements to be tested
Ferrites for better coupling
UCLA
The RF energy outside the antenna is wasted.
Perhaps it can be captured with a ferrite cover.
Untested ideas
UCLA
One-piece ceramic tube
Ferrite transformer coupling
Varying the magnet shapes and spacings
UCLA
Varying the ID and OD of PMs shows that B depends mainly on total volume of magnet.
This shows that not much uniformity is lost if the magnet spacing is zero
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