Permanent-magnet
helicon sources for
etching, coating, and thrust
Francis F. Chen, UCLA
Low Temperature Plasma Teleseminar, June 14, 2013; originally prepared for (but not given at):
2013 Workshop on Radiofrequency Discharges, La Badine, La Presqu’ile de Giens, France, 29-31 May 2013
Helicons are RF plasmas in a magnetic field
Density increase over ICP
Prf
200
400
600
800
1000
%
16%
21%
35%
38%
42%
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Helicon sources usually use heavy magnets
The MØRI source of Plasma Materials Technologies, Inc.
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Size of the MØRI source
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The source has been simplified to this
This uses a commercially
available 2 x 4 x 0.5 inch
NeFeB magnet
The discharge is 5 cm
high and 5 cm in diameter
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How did we get here?
First, the HELIC code of Arnush
Loop antenna
Helical antenna
h
B0
a
b
Lc
D. Arnush, Phys. Plasmas 7, 3042 (2000).
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Examples of HELIC results: Pr , Pz
5000
-3
n (cm )
2.5E+11
4.0E+11
6.3E+11
1.0E+12
P(r) (arb.)
4000
3000
2000
1000
0
0.000
0.005
0.010
0.015
r (m)
0.020
0.025
6
13 MHz, 80G
5
1.6E+12
2.5E+12
4.0E+12
6.3E+12
1.0E+13
P(z)
4
3
2
1
0
0
5
10
15
20
25
30
z (cm)
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Most important, RnB: power deposition
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RnB results organized in matrices
Matrix A: B-field
D = 7.4"
100W
300W
500W
Folder
D7W100
Folder
D7W300
Folder
D7W500
1 mTorr, ei = -1
D = 11"
D = 13"
Folder
D11W100
Folder
D11W300
Folder
D11W500
Folder
D13W100
Folder
D13W300
Folder
D13W500
Matrix C: pressure
D = 0, 300W
D=0
1 mTorr
10 mTorr
conductor
Folder
P1cond
Folder
P10cond
insulator
Folder
P1ins
Folder
P10ins
Folder
D0W100
Folder
D0W300
Folder
D0W500
Similar matrices for tube size determined the tube design.
After that, there are matices for the experimental results.
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Optimized discharge tube: 5 x 5 cm
GAS INLET
(optional)
1. Diameter: 2 inches
2. Height: 2 inches
3. Aluminum top
5 cm
ANTENNA
4. Material: quartz
5. “Skirt” to prevent eddy currents
canceling the antenna current
5.1 cm
10 cm
The antenna is a simple loop, 3 turns for 13 MHz, 1 turn for 27 MHz.
The antenna must be as close to the exit aperture as possible.
Antenna: 1/8” diam tube, water-cooled
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Design of matching circuit (1)
N loads
R, L
Z2 - short
cables
R, L
Z2
R, L
R, L
Z1 - long
cable
Z1
Matching ckt.
(standard)
C2
C1
50W
PS
Distributor
Matching ckt.
(alternate)
C2
C1
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Design of 50W matching circuit (2)
Let the load be RL + jXL :
R and X have first to be transformed by cable length
k = wR0C, R0 = 50W
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Forbidden regions of L, R, and Z (N = 8)
2000
800
N
N
8
6
4
2
1
1000
200
0
0
0.5
1
L (mH)
1.5
2
0
2.5
0.5
1
L (mH)
1600
2000
C1(S)
C2(S)
1400
1200
C1(S)
C2(S)
1500
1000
C (pF)
C (pF)
400
500
0
8
6
4
2
1
600
C2 (pF)
C1 (pF)
1500
800
1000
600
500
400
200
0
0
0
50
100
Z2 (cm)
150
200
0.5
1.5
2.5
R (ohms)
3.5
4.5
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Instead, we can use annular PMs
8
PM  Permanent magnet
6
4
2
NdFeB ring magnet
3” ID, 5” OD, 1” high
0
-2
Stagnation point
The far-field is fairly uniform
-4
-6
Put plasma here
-8
-10
or here, to adjust B-field
-12
-14
-16
-10
-8
-6
-4
-2
0
2
4
6
8
10
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Experimental chamber
19 cm
The magnet height is
set for optimum B-field
6.8
Port 1
16.9
WALL MAGNETS
27.2
Port 2
38
Langmuir probes
at three ports
Port 3
46
PUMP BAFFLE AND GROUND PLANE
Gate Valve
To Turbo Pump
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Optimization of the B-field
Peak density in Port 2 vs. B and Prf @ 27 MHz
30-60 G is best
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Typical scan of “Low-field peak”
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Density of ejected plasma
5
n11, 65G
n11, 280G
19 cm
3
n (10 /cm )
4
11
3
6.8
2
Port 1
16.9
1
WALL MAGNETS
27.2
Port 2
0
38
-25
-20
-15
-10
-5
0
5
10
15
20
25
r (cm)
Port 3
46
4.5
3.0
2.5
n (10
11
-3
PUMP BAFFLE AND GROUND PLANE
6.8
16.9
27.2
3.5
cm )
62 G, 400W
cm below source
4.0
2.0
1.5
1.0
Gate Valve
0.5
0.0
-20
-10
0
r (cm)
10
20
To Turbo Pump
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Vertical probe for inside plasma
LANGMUIR PROBE
PERMANENT
MAGNET
HEIGHT
ADJUSTMENT
GAS FEED
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Axial density profile inside the tube
16
400W, 13.56 MHz, 15 mTorr
16
Jan. 4 w. Mk2
Jan. 5 w. ESPion
14
12
10
Horiz. Probe point
n11
n11
12
8
8
Bottom of tube
6
4
4
400W, 62G
2
0
0
0
2
4
6
z (cm)
8
10
12
0
2
4
6
z (cm)
8
10
Density inside the tube is low (<1013 cm-3) because plasma is
efficiently ejected.
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12
Downstream density: 6 x 1011 cm-3
7
15 mTorr
Port 2
Helicon
ICP
5
-3
cm )
6
4
n (10
11
Density increase
over ICP
3
2
1
0
0
200
400
600
Prf (W)
800
1000
1200
Prf
200
400
600
800
1000
%
16%
21%
35%
38%
42%
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Port 2 density is higher at 13 MHz
9
Port 2
8
n (10
11
-3
cm )
7
13 MHz
6
5
4
27 MHz
3
2
1
0
0
200
400
600
Prf (W)
800
1000
1200
This was a surprise and is contrary to theory.
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Cause and location of the “double layer”
F.F. Chen, Phys. Plasmas 13, 034502 (2006)
ne = ni = n
ns
ni
n
PRESHEATH
ne
v = cs
PLASMA
+
SHEATH
xs
x
ne  n0e , where    eV /KTe
vis  cs  ( KTe / M )1/2  Wis  ½Mvis2  ½KTe
 s  ½, ns / n0  e1/2 B / B0  n / n0  (r0 / r )2
r / r0  e1/4  1.28
Maxwellian electrons
Bohm sheath criterion
A sheath must form here
Single layer forms where r has increased 28%
Where a diffuse “double layer” would occur
250
B (G)
200
150
100
Approx. location of "double layer"
50
0
5
10
15
20
25
30
z (cm)
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Ion energy distribution by Impedans SEMion
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IEDFs vs. Prf in Port 1
2E-06
600W
500W
400W
400W repeat
300W
200W
100W
Vs
IEDF
2E-06
1E-06
5E-07
0E+00
0
5
10
V
15
20
25
30
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Conditions at Port 1 (r = 0) at 400W
7
100
6
Ii squared
Ii squared (OML)
10
Ie (mA)
2
3
1
2
0.1
1
0
0.01
-100
-80
-60
V
-40
-20
0
20
-5
0
5
V
10
15
20
6
Port 1
400W
5
-3
3
cm )
4
11
n11
n (10
I (mA)
4
2
5
Ie
Ie(fit)
Ie (0)
2
Te
eVs
iVs
n11
2.14
15.9
27
4.21
Te (eV)
Plasma potential
1
0
-20
-10
0
r (cm)
10
20
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Mach probe (in Port 2)
0.50
Hutchinson:
v flow
vf
cs
Result:
vf
cs
 0.14
cs
 I up 
1  Iup 
 ln 
  0.75ln 

K  I down 
I
 down 
( K  1.34)
 0.14
This seems very low. It should be much higher in Port 1
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helicon ARRAY sources
DISTRIBUTOR
SUBSTRATE
TO PUMP
MEDUSA
MEDUSA 1
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The Medusa 2 large-area array
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An array source for roll-to-roll processing
165 cm
Height can be adjusted electrically if desired
Aluminum sheet
15 cm
30 cm
Z1
Probe ports
Z2
The source requires only 6” of vertical space above the
process chamber. Two of 8 tubes are shown.
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Top view of Medusa 2
3.5"
29"
7"
7"
21"
7"
7"
65"
Possible positions shown
for 8 tubes.
Tubes set in deeply (½ inch)
7"
3/4” aluminum
12"
1/2" aluminum
Endplates: gas feed and
probe port at each end.
Substrate
motion
Two arrangements of the array
35.6 cm
y
17.8
Staggered array
x
53.3 cm
17.8
17.8
Covers large area
uniformly for substrates
moving in the y-direction
165 cm
Top view
53.3 cm
17.8
y
x
17.8
17.8
17.8
165 cm
Compact array
Gives higher density, but
uniformity suffers from
end effects.
Operation with cables and wooden magnet tray
It’s best to have at least 3200W (400W per tube)
to get all tubes lit equally.
Details of distributor and discharge tube
The top gas feed did not improve
operation.
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A rectangular 50W transmission line
50-W line with ¼” diam Cu pipe for cooled center conductor
Operation with rectangular transmission line
Radial profile at Z2 across rows
3.5
3
n
KTe
-3
cm )
2.5
n (10
11
2
1.5
1
0.5
0
-25
-20
-15
-10
-5
0
r (cm)
5
10
15
20
25
Density profiles with staggered array
Staggered configuration, 2kW
Bottom probe array
5
Staggered, 2kW,
D=7", 20mTorr
Argon
n (1011 cm-3)
4
y (in.)
-3.5
0
3.5
3
2
1
0
-8
-6
-4
-2
0
2
4
6
x (in.)
8
10
12
14
16
Density profiles with compact array
Compact configuration, 3kW
Bottom probe array
10
Compact, 3kW,
D=7", 20mTorr
y (in)
3.50
3.5
n (1011 cm -3)
8
6
Data by Humberto
Torreblanca, Ph.D.
thesis, UCLA, 2008.
4
2
0
-8
-6
-4
-2
0
2
4
6
x (in.)
8
10
12
14
16
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Plans for an 8-tube array for round substrates
Distributor
RG 393 teflon coax cable to each antenna.
No center
is necessary!
Antennas aretube
water-cooled
in pairs, so that
Antennas are 1/8" OD copper tubing.
water enters and leaves the system at ground
potential.
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