EE 403503

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EE 403/503 Introduction to Plasma Processing
DC Discharge I
October 21, 2009
2
d
Plasma

Anode
Cathode
A
V ?
I ?
3
V-I Characteristic of DC Discharges
1000 V
Background Ionization
Glow Discharge
Arc Discharge
Townsend Discharge
Glow to Arc Transition
Breakdown Voltage
500
Abnormal Glow
Thermal
Normal Glow
Non- Thermal
0
1
10-8
10-4
104A
10-6
10-2
102
10-10
4
1000 V
Background Ionization
500
0
10-8
1
104A
10-10
10-4
- e and A are generated by cosmic rays and other
forms of background radiation - No excitation
take place - Absolutely dark - Non self
substaining discharge
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• Collection of all electrons and ions that are
  produced by external radiation.
• A dark current discharge emits very little light

0
10-8
1
104A
10-10
10-4
Electrons pick up sufficient energy in a mean
free path to make ionizing collisions with
neutrals. (e?eE gt ?i )
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ne
nedne
Anode
Cathode
x
x
xdx
Ionization Coefficient (probable of ionizing
collision per meter)
Assuming E is constant No Space Charge
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First Townsend Coefficient (?)
? Mean Free Path ? 1/? is number mean free
path per unit length.
?x Mean free path that meets the minimum
requirement for ionization.
Fraction of ?x among all the ?.
Number of ?x that lead to ionization
Depends on Te and Type of Gas
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First Townsend Coefficient (?)
Effective Ionization Potential
9
First Townsend Coefficient (?)
1000
100
He
?/p Ion Pairs/Meter-Torr
H2
N2
10
Ne
Ar
1
103
104
105
E/P Volts/Meter-Torr
Cobine 1958 See Page 247
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Anode
Neutral Atom
Cathode
Ion

Electron


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Anode
Cathode
Je, 0
1st generation
Anode
Cathode
Je, 0
Anode
Cathode
2nd Generation
Anode
Cathode
12
Anode
Cathode
3rd generation
Anode
Cathode
nth generation
Anode
Cathode
13
Geometric Series
If
Secondary Emission Coefficient
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First Townsend Coefficient (ionization by
electron collision)

?? is secondary Townsend coefficient ionization
rate by ion collisions
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Breakdown Voltage
VB
0
10-8
1
104A
10-10
10-4
Townsend breakdown criterion
VE x d
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Almost Constant
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VBV
104
Ar
H
103
He
102
-1
1
10
100
1000
Pd Torr-cm
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Minimum Breakdown Condition
Breakdown above 100 Torr
Vb 3000 d 1.35 kV for dry atmospheric
air Eb 30 kV/cm
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Corona Discharge
Outer Electrode
High Voltage Power Supply
Inner Electrode
Active Volume
For dry Air for parallel electrodes
kV
kV/m