Transcript Slide 1

LANGMUIR PROBES IN THE INTENSE RF
ENVIRONMENT INSIDE A HELICON
DISCHARGE
Francis F. Chen, UCLA
Gaseous Electronics Conference, Austin TX, Tuesday, October 23, 2012
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The permanent-magnet helicon source
LANGMUIR PROBE
The discharge tube
is 5 cm in diam
and 5 cm high
PERMANENT
MAGNET
HEIGHT
ADJUSTMENT
GAS FEED
COMPENSATION
ELECTRODE
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The Langmuir probe
VERTICAL PROBE 110126
SW = spot weld, SJ = slip joint, SS = soft solder, SG = superglue
SW
C
SJ
0.64
L1
L2
SG
ss
SG
22 cm
Compensation Electrode (CE)
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Distortion caused by RF pickup
0.00010
0.12
Negative current
Negative current
0.10
0.08
0.06
0.04
0.00005
0.00000
-0.00005
0.02
-0.00010
0.00
-20
0
20
40
60
80
Probe voltage
Electron current is greatly distorted.
This is new: residence time at
cos(wt) ~ 0 is taken into account.
-100
-80
-60
-40
-20
0
20
Probe voltage
Saturation ion current is
not affected.
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The simple Langmuir formula is valid!
1/2
2  eV p 
Ii  Ap ne


  M 
Ii2  V p
,
I e  nevthe e
(V p Vs )/ KTe
10
0.60
Ii squared
Ii squared (OML)
0.50
Ie
Ie(fit)
Ie (0)
1
Ie (mA)
0.30
2
I (mA)
2
0.40
0.1
0.20
0.01
0.10
0.00
-100
0.001
-80
-60
V
-40
-20
0
This gives n without knowing Te
-4
-2
0
2
V
4
6
8
10
This gives Te and VS after
subtracting ion current fit
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The art of ion subtraction
1000
160
Ii squared
Ii squared (OML)
140
100
Ie (mA)
80
2
I (mA)
2
120
100
Ie
Ie(fit)
Ie (raw)
10
60
1
40
20
0.1
0
-100
-80
-60
V
-40
-20
-10
0
180
-5
0
5
10
15
20
5
10
15
20
Vp
1000
Ie
Ie(fit)
Ie (raw)
Ii squared
Ii squared (OML)
160
140
100
Ie (mA)
2
I (mA)
2
120
100
80
10
60
1
40
20
0.1
0
-100
-80
-60
V
-40
-20
0
-10
-5
0
Vp
Electron distribution functions cannot be trusted.
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RF amplitude inside discharge
15 mTorr, 280G
Volts peak-to-peak
50
40
30
20
With top plate
10
Probe at antenna
Probe at center
Probe at top
0
0
200
400
600
800
RF power (W)
1000
1200
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False Te’s without Compensation Electrode
50
100
Ii squared
Ii(OML)
10
I (mA)
30
1
2
I (mA)
2
40
Ie
Ie(fit)
Ie (0)
T1 = 8.22 eV
20
0.1
10
0.01
0
-100
-80
-60
V
-40
-20
0
100
-15
-10
-5
0
5
10
15
20
V
100
Ie
Ie(fit)
Ie (0)
Ie
Ie(fit)
Ie (0)
10
I (mA)
10
I (mA)
-20
20
1
1
T2 = 4.65 eV
T3 = 2.97 eV
0.1
0.1
0.01
0.01
-20
-15
-10
-5
0
V
5
10
15
20
-20
-15
-10
-5
0
5
10
15
20
V
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Importance of a large C.E.
100
100
1
27.12 MHz
400W
15 mTorr
0.1
File 110531_15
Ie
Ie(fit)
Ie (0)
10
I (mA)
10
I (mA)
File 110531_24
Ie
Ie(fit)
Ie (0)
1
27.12 MHz
400W
15 mTorr
0.1
Te=8.17 eV
Te=3.57 eV
Vs=44.9 V
Vs=33.1 V
0.01
0.01
-5
0
5
10
15
V
20
25
30
35
-5
0
5
10
15
20
25
30
35
V
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Sample data
60
16
n11
KTe
-3
40
3.0
11
30
2.5
20
2.0
n (10
8
4
10
0
3.5
1.5
13.56 MHz, 400W
0
0
2
4
6
8
10
z (cm)
Density scan along axis
12
KTe (eV)
cm )
50
12
n11
4.0
400W, 13.56 MHz, 15 mTorr
1.0
0
20
40
60
p (mTorr)
80
100
Pressure scan of n and Te
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Density saturation inside discharge
25
9
13.56 MHz, 15 mTorr
-3
cm )
7
-3
cm )
20
11
15
n (10
11
n (10
Port 2
8
10
13 MHz
6
5
4
27 MHz
3
2
5
1
0
0
0
200
400
600
Prf (W)
800
1000
1200
Power scan at center of discharge
0
200
400
600
Prf (W)
800
1000
1200
Power scan 17 cm below discharge
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Electron emission at high Vp
60
20
50
0
18
40
n11
50
0
20
n11
50
0
16
50
SA = 25
30
14
20
12
10
10
0
0
20
Vmax
40
60
0
80
20
Vmax
40
60
80
Same data, w. Vmax=70 point
+100V
Emission adds to ion current in
subsequent pulses
-100V
25 msec/div
Hiden ESPion
Scan Average
SA = 4 here
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Conclusions
• Probes can be used even under the antenna
• The compensation electrode has to be large enough
• Spuriously high KTe otherwise
• KTe is Maxwellian if ion current is subtracted right
• Non-Maxwellian EEDFs cannot be trusted
• Fast sweeps are needed to avoid electron emission
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Title here
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