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Permanent-magnethelicon 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
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Helicons are RF plasmas in a magnetic field
UCLA
Prf % 200 16% 400 21% 600 35% 800 38%
1000 42%
Density increase over ICP
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Helicon sources usually use heavy magnets
UCLA
The MØRI source of Plasma Materials Technologies, Inc.
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UCLA
Size of the MØRI source
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UCLA
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 cmhigh and 5 cm in diameter
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UCLA
How did we get here?
Lc
a b
h
Loop antennaHelical antenna
B0
D. Arnush, Phys. Plasmas 7, 3042 (2000).
First, the HELIC code of Arnush
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Examples of HELIC results: Pr , Pz
UCLA
0
1000
2000
3000
4000
5000
0.000 0.005 0.010 0.015 0.020 0.025r (m)
P(r
) (a
rb.)
2.5E+114.0E+116.3E+111.0E+12
n (cm-3)
0
1
2
3
4
5
6
0 5 10 15 20 25 30z (cm)
P(z
)
1.6E+122.5E+124.0E+126.3E+121.0E+13
13 MHz, 80G
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Most important, RnB: power deposition
UCLA
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RnB results organized in matrices
UCLA
100W
300W
500W
D = 7.4" D = 11" D = 13"
Folder Folder Folder
1 mTorr, ei = -1D = 0
Folder
Folder Folder Folder
FolderD13W100 D0W100
Folder
D7W100Folder
D11W100Folder
D0W500
D7W300 D11W300 D13W300 D0W300
D7W500 D11W500 D13W500Folder
conductor
insulator
1 mTorr 10 mTorr
Folder FolderP1cond P10cond
D = 0, 300W
Folder FolderP1ins P10ins
Matrix A: B-field Matrix C: pressure
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
UCLA
1. Diameter: 2 inches
2. Height: 2 inches
3. Aluminum top
4. Material: quartz
5. “Skirt” to prevent eddy currents canceling the antenna current
5.1 cm
10 cm
5 cmANTENNA
GAS INLET (optional)
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)
UCLA
R, L
R, L
R, L
R, L
PS
N loads
Z2 - short cables
Distributor
Z1Z2
Z1 - long cable
Matching ckt.(standard) 50W
C1C2
Matching ckt.(alternate)
C1C2
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Design of 50W matching circuit (2)
UCLA
Let the load be RL + jXL :
R and X have first to be transformed by cable length
k = R0C, R0 = 50W
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Forbidden regions of L, R, and Z (N = 8)
UCLA
0
500
1000
1500
2000
0.5 1.5 2.5 3.5 4.5R (ohms)
C (p
F)
C1(S)C2(S)
0
200
400
600
800
1000
1200
1400
1600
0 50 100 150 200Z2 (cm)
C (p
F)
C1(S)C2(S)
0
200
400
600
800
0 0.5 1L (mH)
C2
(pF)
86421
N
0
500
1000
1500
2000
0 0.5 1 1.5 2 2.5L (mH)
C1
(pF)
86421
N
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Instead, we can use annular PMs
UCLA
-16
-14
-12
-10
-8
-6
-4
-2
0
2
4
6
8
-10 -8 -6 -4 -2 0 2 4 6 8 10
NdFeB ring magnet3” ID, 5” OD, 1” high
Stagnation point
Put plasma here
or here, to adjust B-field
The far-field is fairly uniform
PM Permanent magnet
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Experimental chamber
UCLAGate Valve
To Turbo Pump
PUMP BAFFLE AND GROUND PLANE
WALL MAGNETS
Port 1
Port 3
Port 2
38
46
6.8
16.9
27.2
19 cm
Langmuir probes at three ports
The magnet height is set for optimum B-field
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UCLA
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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UCLA
Typical scan of “Low-field peak”
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UCLA
Density of ejected plasma
0
1
2
3
4
5
-25 -20 -15 -10 -5 0 5 10 15 20 25r (cm)
n (1
011
/cm
3 )
n11, 65Gn11, 280G
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
-20 -10 0 10 20r (cm)
n (1
011
cm
-3)
6.816.927.2
62 G, 400Wcm below source
Gate Valve
To Turbo Pump
PUMP BAFFLE AND GROUND PLANE
WALL MAGNETS
Port 1
Port 3
Port 2
38
46
6.8
16.9
27.2
19 cm
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Vertical probe for inside plasma
UCLA
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0
2
4
6
8
10
12
14
16
0 2 4 6 8 10 12z (cm)
n 11
Jan. 4 w. Mk2Jan. 5 w. ESPion
Bottom of tube
400W, 62G
Horiz. Probe point
0
4
8
12
16
0 2 4 6 8 10 12z (cm)
n 11
400W, 13.56 MHz, 15 mTorr
Axial density profile inside the tube
Density inside the tube is low (<1013 cm-3) because plasma is efficiently ejected.
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UCLA
Downstream density: 6 x 1011 cm-3
0
1
2
3
4
5
6
7
0 200 400 600 800 1000 1200Prf (W)
n (1
011 c
m-3
)
HeliconICP
15 mTorrPort 2
Prf % 200 16% 400 21% 600 35% 800 38% 1000 42%
Density increaseover ICP
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UCLA
Port 2 density is higher at 13 MHz
0
1
2
3
4
5
6
7
8
9
0 200 400 600 800 1000 1200Prf (W)
n (1
011 c
m-3
) 13 MHz
27 MHz
Port 2
This was a surprise and is contrary to theory.
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Cause and location of the “double layer”
1/20½, /s sn n e
1/2 2( / ) ½ ½is s e is is ev c KT M W Mv KT
20 0 0/ / ( / )B B n n r r
F.F. Chen, Phys. Plasmas 13, 034502 (2006)
n
xs x
ne = ni = n
PLASMA
SHEATH
ni
ne
+
ns
PRESHEATH
v = cs
0 , where e /e en n e V KT Maxwellian electrons
Bohm sheath criterion
1/40/ 1.28r r e
A sheath must form here
Single layer forms where r has increased 28%
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UCLA
Where a diffuse “double layer” would occur
0
50
100
150
200
250
5 10 15 20 25 30z (cm)
B (G
)
Approx. location of "double layer"
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Ion energy distribution by Impedans SEMion
UCLA
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IEDFs vs. Prf in Port 1
UCLA
0E+00
5E-07
1E-06
2E-06
2E-06
0 5 10 15 20 25 30V
IED
F
600W500W400W400W repeat300W200W100W
Vs
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Conditions at Port 1 (r = 0) at 400W
UCLA
0
1
2
3
4
5
6
-20 -10 0 10 20r (cm)
n (1
011 c
m-3
)
Port 1400W
Te (eV)
n11
0
1
2
3
4
5
6
7
-100 -80 -60 -40 -20 0 20V
I2 (mA
)2
Ii squaredIi squared (OML)
0.01
0.1
1
10
100
-5 0 5 10 15 20V
I e (m
A)
IeIe(fit)Ie (0)
Te eVs iVs n11
2.14 15.9 27 4.21
Plasma potential
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Mach probe (in Port 2)
UCLA
0.50
1 ln 0.75ln
( 1.34)
flow up up
s down down
v I Ic K I I
K
Hutchinson:
Result:
0.14f
s
vc
0.14f
s
vc
This seems very low. It should be much higher in Port 1
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helicon ARRAY sources
SUBSTRATE
TO PUMP
DISTRIBUTOR
MEDUSA MEDUSA 1
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The Medusa 2 large-area array
UCLA
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An array source for roll-to-roll processing
UCLA
165 cm
30 cm
15 cm
Probe ports
Aluminum sheetHeight can be adjusted electrically if desired
The source requires only 6” of vertical space above the process chamber. Two of 8 tubes are shown.
Z1Z2
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65"
21"
7"
7"
7"
7"29"
3.5"
12"
7"3/4” aluminum
Endplates: gas feed and probe port at each end.
Tubes set in deeply (½ inch)
1/2" aluminum
Top view of Medusa 2
Possible positions shown for 8 tubes.
Substrate motion
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Two arrangements of the array
Staggered array
Covers large area uniformly for substrates moving in the y-direction
165 cm
53.3 cm
17.8
17.8
17.8
35.6 cm
x
y
165 cm
53.3 cm
17.8
17.8
17.8
x
yTop view 17.8 Compact array
Gives higher density, but uniformity suffers from
end effects.
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Operation with cables and wooden magnet tray
It’s best to have at least 3200W (400W per tube) to get all tubes lit equally.
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Details of distributor and discharge tube
UCLA
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
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Operation with rectangular transmission line
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Radial profile at Z2 across rows
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
011 c
m-3
) nKTe
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Density profiles with staggered array
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
)
-3.503.5
Staggered, 2kW, D=7", 20mTorr
y (in.)
Argon
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Density profiles with compact array
Compact configuration, 3kW
Bottom probe array
0
2
4
6
8
10
-8 -6 -4 -2 0 2 4 6 8 10 12 14 16x (in.)
n (1
011 c
m-3
)
3.5-03.5
Compact, 3kW, D=7", 20mTorr
y (in)
Data by Humberto Torreblanca, Ph.D. thesis, UCLA, 2008.
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Plans for an 8-tube array for round substrates
UCLANo center tube is necessary!