MW Discharges

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MW Discharges
(Chapter 10)
EE 403/503 Introduction to Plasma Processing
November 4, 2009
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Te,MW gt Te,RF Te, DC (5-15 eV) (1-2 eV)
0.915 GHz 2.45 GHz 300 GHz 33 cm 12.24
cm 1 mm
10 mTorr
Magnetized ECR
Unmagnetized ECR
At frequencies above 10 GHz --gt Water absorption
MW
Radiation interact with plasma as a dielectric
medium
Radiation interact with plasma as it interact
with individual electrons
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Description
0.915 GHz 2.45 GHz 300 GHz 33 cm 12.24
cm 1 mm
Funny
EM Applications
Information
Bibliography
Interesting
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Advantages

• Higher electron kinetic temperatures and lower
  pressures
• Higher fraction of ionization and dissociation
  than DC and low frequency discharges
• Lower voltages across the sheath -gt Less
  sputtering of the wall
• No electrodes -gt less contamination
• More stable over a wide range of background gas
  pressures relative to DC RF

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Applications
ECR for Microelectronics Plasma
Processing Fusion for initial, steady state and
high density plasmas Sources for Photons,
ions, free radicals, excited atoms and
dissociated neutral species Laser to pump laser
medium Continuous flow plasmas Chemical reactors
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Continuous Flow Non-resonant Microwave Plasma
(High electric field of radiation aligned with
the axis of the tube)
Reactants
Quartz Tube
Waveguide (atmospheric Pressure)
coolant
Plasma (low pressure, 0.1-5 KW)
Tapered Resonator
Product
Resonant Cavity R?0 d?0 Multimode Cavity
Rgtgt?0 dgtgt?0
(Remember 2.45 GHz -gt 12.24 cm)
Resonant or multimode cavity reactor
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2.45 GHz, 0.4-10 KW
Isolates the magnetron from the variable plasma
load. It functions like one way valve for
microwave power.
Minimizes the reflected power, while maximizing
the forward power absorbed by the load
Quartz Tube separates the working gas pressue
from the one within the waveguide
A variable load
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Cathode (heated filament)
Anode
B
X
Basic Magnetron Structure
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Electron Cyclotron Resonance (ECR) Plasma Reactor
Input Waveguide
Ceramic Window
Magnetic Resonance Coil
Resonance Surface
Plasma
Magnetic Nozzle Trim Coil
Target Material
B
B
B
B
B
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Power Coupling to the Plasma
(with B)
Energy transfer frequency
Low pressure
High pressure
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Optimum Power coupling to Plasma
(with B)
Pages 425-433
12
Immersed ECR System
Page 504
13
(No Transcript)
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Distributed ECR System
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Theoretical Model

Static background magnetic field B of radiation
is neglegibly small
E from radiation plus the effect of surrounding
plasma
Maxwell Equation
Propagation Equation
Attenuation constant
Wave number
Page 471
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Heald and Wharton Propagation Equation
Complex refractive index
Propagation Constant
Real refractive index How much the radiation is
slowed down in the medium relative to
propagation in vacuum
Attenuation index How much is the wave is
attenuated
1- Propagation in an unmagnetized plasma (P
472) 2- Propagation in a magnetized (B0) plasma
(P 473) a- ? angle between E and B0 (P. 474) b-
?0 right circularly polarized wave (P 475) c-
?0 left circularly polarized wave (P 475) d-
?90 B0E (P 476) e- ?90 B0B (P 476)
Pages 471-476
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Plasma
Microwave Horn
B0
waveguide
Network Analyzer
Pages 481-482
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0
?c20 MHz
-4
?c40 MHz
-8
Collision frequency
-12
Signal Attenuation (dB)
Example
-16
?ce4.0 GHz ?pe1.0 GHz Plasma Diameter 12
cm ?uh4.123 GHz
-20
Upper hybrid frequency
-24
?UH
Upper Hybrid Frequency
?ce
130 MHz
4.04
4.20
4.24
4.28
4.08
4.12
4.16
4.00
Wave Frequency ?, (GHz)
Page 479
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?c20 MHz
?c40 MHz
?UH
Page 479
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Microwave Breakdown of Gases
Free Space Wavelength
Ionization Potential of the Gas
Breakdown Electric Field
Electron Mean Free Path
Characteristic Diffusion Length
With five variables and two independent
dimensions (Length and voltage), three
dimensionless variables are sufficient to
describe the relationship implied by the above
equation
e.g.
The implication is that it requires a
three-dimensional surface to specify the RF and
microwave breakdown conditions in the gas.
Page 484