1. Multitube Helicon Source with Permanent Magnets
Dissertation Defense by
Humberto Torreblanca
Adviser: Francis F. Chen
Jan 10th, 2008

2. Table of Contents
Plasma Processing and Distributed Sources
Helicon Waves
Proof of Principle: Plasma Injection
HELIC Calculations
Medusa 2: Discharge Tube and Processing Chamber
Medusa 2: Permanent Magnets
Medusa 2: Power Distribution and RF Matching
Diagnostic: Langmuir probe theory
Measurements
Conclusions

3. Plasma Processing and Distributed Sources
MOTIVATION
Old system, not optimized, use of bulky dc magnet coil.
Industry needs:
Arbitrary large area coverage.
Plasma density:
high (> )
uniform (< 5%)
Reliable, simple, compact, inexpensive.
We offer:
Why permanents magnets?
Use of permanent magnets (PMs).
Array of single helicon sources.
Optimization of helicon source.
No power or cooling system.
Compact.
Don't break down.

4. Plasma Processing and Distributed Sources
Helicon Waves
Proof of Principle: Plasma Injection
HELIC Calculations
Medusa 2: Discharge Tube and Processing Chamber
Medusa 2: Permanent Magnets
Medusa 2: Power Distribution and RF Matching
Diagnostic: Langmuir probe theory
Measurements
Conclusions

5. AT LOW B-FIELDS IS POSSIBLE TO HAVE
Helicon Waves
AT LOW B-FIELDS IS POSSIBLE TO HAVE
HIGH DENSITIES
low B-field density peak
F.F. Chen et al., Plasma Phys. Control. Fusion 39 (1997) A411–A420

6. Plasma Processing and Distributed Sources
Helicon Waves
Proof of Principle: Plasma Injection
HELIC Calculations
Medusa 2: Discharge Tube and Processing Chamber
Medusa 2: Permanent Magnets
Medusa 2: Power Distribution and RF Matching
Diagnostic: Langmuir probe theory
Measurements
Conclusions

7. Proof of Principle: Plasma Injection (1/2)
USING MAGNETS FAR-FIELD REGION
➞ PLASMA CAN BE INJECTED DOWNSTREAM
Two B-field regions: inner region and far-field region (past null point).
Inner B-field varies considerable with radius.
Far-field region is homogeneous with radius and extends downstream.

8. Proof of Principle: Plasma Injection (2/2)
HIGHER DENSITIES ARE ACHIEVED
WITH LOW B-FIELD (PM DISTANCE D INCREASED)
4” under the source (Z1 port)
7” under the source (Z2 port)
PM far-field region D >>1 cm.
Density is dramatically increased.

9. Plasma Processing and Distributed Sources
Helicon Waves
Proof of Principle: Plasma Injection
HELIC Calculations
Medusa 2: Discharge Tube and Processing Chamber
Medusa 2: Permanent Magnets
Medusa 2: Power Distribution and RF Matching
Diagnostic: Langmuir probe theory
Measurements
Conclusions

10. HELIC Calculations (1/4)
PLASMA LOADING (Rp) >> CIRCUIT LOSSES (Rc)
➞ MAXIMUM RF POWER TRANSFER
OPTIMIZATION of the system: CALCULATION of plasma loading Rp
Rp = Rp (magnetic field, tube geometry, RF frequency, pressure, endplate material)

11. HELIC Calculations (2/4)
CALCULATIONS OF PLASMA LOADING Rp = Rp(...?)
Tube dimensions: length vs. diameter
2" diam
3" diam
4" diam
2" long
Conditions
13.56 MHz, 3mTorr, 2.5 eV
4" long
6" long
Frequency vs. diameter
2" diam
3" diam
4" diam
2 MHz
Conditions
1.5" long, 2" diam., 2.5 eV
13.56 MHz
27.12 MHz
Pressure vs. frequency
1 mTorr
3 mTorr
10 mTorr
2 MHz
Conditions
1.5" long, 2" diam., 2.5 eV
13.56 MHz
27.12 MHz
Boundary condition
2” long
4” long
6” long
metal
Conditions:
2” diam, 1 mTorr, 2.5 eV
insulator
Using HELIC code by D. Arnush

12. HELIC Calculations (3/4)
PLASMA LOADING Rp vs. DENSITY
for different magnetic fields
for different RF frequencies
Low B-fields can give high densities and satisfy the condition: Rp >> Rc (~0.1Ω)
fRF = MHz give good enough loading and it's easier to match.

13. HELIC Calculations (4/4)
HELIC CODE CONCLUSIONS
Plasma loading Rp can be increased using:
Low B-fields (< 100 G).
Small tube (2" diam. x 2" length).
fRF = MHz.
Loop antenna.
Metal endplate.

14. Plasma Processing and Distributed Sources
Helicon Waves
Proof of Principle: Plasma Injection
HELIC Calculations
Medusa 2: Discharge Tube and Processing Chamber
Medusa 2: Permanent Magnets
Medusa 2: Power Distribution and RF Matching
Diagnostic: Langmuir probe theory
Measurements
Conclusions

15. Discharge Tube and Processing Chamber (1/5)
SCHEMATIC OF THE TEST CHAMBER WITH TWO POSSIBLE TUBES' CONFIGURATION
Staggered Configuration
Compact Configuration 15

16. Discharge Tube and Processing Chamber (2/5)
DESIGN OF CHAMBER BASED ON A SINGLE TUBE DENSITY PROFILE
Staggered Configuration
Compact Configuration

17. Discharge Tube and Processing Chamber (3/5)
DENSITY PROFILE CALCULATIONS ACROSS THE CHAMBER FOR BOTH CONFIGURATIONS
Staggered Configuration
Compact Configuration

18. Discharge Tube and Processing Chamber (4/5)
DENSITY PROFILE CALCULATIONS ALONG THE CHAMBER FOR BOTH CONFIGURATIONS
Staggered Configuration
Compact Configuration

19. Discharge Tube and Processing Chamber (5/5)
TEST CHAMBER IN STAGGERED TUBE CONFIGURATION
multitube helicon source with permanent magnets
discharge tube
magnets' tray

20. Plasma Processing and Distributed Sources
Helicon Waves
Proof of Principle: Plasma Injection
HELIC Calculations
Medusa 2: Discharge Tube and Processing Chamber
Medusa 2: Permanent Magnets
Medusa 2: Power Distribution and RF Matching
Diagnostic: Langmuir probe theory
Measurements
Conclusions

21. TOROIDAL PERMANENT MAGNETS
Axial magnetic field for ceramic and Neodymium Iron Boron (NdFeb) magnets
5" OD x 3" ID x 1" thickness NdFeb magnets.
The external B-field (far-field) is fairly homogeneous in radius.
Magnetic field lines with two possible locations for the discharge tube

22. Plasma Processing and Distributed Sources
Helicon Waves
Proof of Principle: Plasma Injection
HELIC Calculations
Medusa 2: Discharge Tube and Processing Chamber
Medusa 2: Permanent Magnets
Medusa 2: Power Distribution and RF Matching
Diagnostic: Langmuir probe theory
Measurements
Conclusions

23. Power Distribution and RF Matching (1/2)
RF POWER DISTRIBUTION
Coaxial cables
More circuit resistance (Rc)
Water-cooling of antennas only.
As shows, not as compact as transmission line.
Equal power distribution to all loads.
Transmission line
Less circuit resistance (Rc).
Water-cooling of all the electric system.
Compact.
Uneven power distribution to all loads.

24. Power Distribution and RF Matching (2/2)
MULTILOAD RF MATCHING WAS ACHIEVED WITH A SINGLE MATCHING NETWORK
Antenna + plasma: inductive - resistive load.
RF power supply: 50 Ω.
Standard configuration (smaller capacitance values).
Variable capacitors ( pF).
Sensitive to:
Number of loads.
Cable length (Z1 & Z2).
Antenna inductance.

25. Plasma Processing and Distributed Sources
Helicon Waves
Proof of Principle: Plasma Injection
HELIC Calculations
Medusa 2: Discharge Tube and Processing Chamber
Medusa 2: Permanent Magnets
Medusa 2: Power Distribution and RF Matching
Diagnostic: Langmuir Probe & Ion Current Collectors
Measurements
Conclusions

26. Langmuir Probe & Ion Current Collectors
DIAGNOSTIC
RF compensated probe
OML theory
Ion current collectors 26

27. Plasma Processing and Distributed Sources
Helicon Waves
Proof of Principle: Plasma Injection
HELIC Calculations
Medusa 2: Discharge Tube and Processing Chamber
Medusa 2: Permanent Magnets
Medusa 2: Power Distribution and RF Matching
Diagnostic: Langmuir Probe & Ion Current Collectors
Measurements
Conclusions

28. MEASUREMENTS SETUP Measurements (1/7) Z1 level: 4" below sources.
Along chamber at:
y = 0": in between rows of tubes.
y = ± 3.5": underneath tubes.
Across chamber at:
x = 0": underneath tubes.
x = 3.5": in between column of tubes.

29. Measurements (2/7) Experimental parameters
Tubes' configuration: staggered and compact.
PM distance D, RF power, pressure.
Measurements
density across chamber: n(y)
density along chamber: n(x)
2D profile: n(x,y)
Langmuir Probes (LP)
Staggered configuration
Fixed parameters
Variable quantities
D = 7"
p = 20 mTorr
PRF = 2 & 3 kW
Z1 & Z2 level
x = 0", y = 3.5"
Plasma loading as a function of PMs distance D: Rp(D)
Plasma density as a function of PMs distance D: n(D)
Plasma density as a function of RF power: n(PRF)
Plasma density as a function of pressure p: n(p)
Langmuir Probes (LP)
Staggered configuration
Compact configuration
2 kW
D = 7", 20 mTorr
n(y)|x = 0", n(y)|x = 3.5"
Z1 & Z2 levels
3 kW
Ion Current Collectors (ICC)
Staggered configuration
Compact configuration
2 kW
D = 7", 20 mTorr
n(y)|x = 0", n(y)|x = 3.5"
Z2 levels
3 kW

30. DENSITY AS A FUNCTION OF RF POWER
Measurements (3/7)
DENSITY AS A FUNCTION OF RF POWER
Density jump from low to high powers
Linear dependence for high powers
Sudden jump from ICP mode (weak glow) to helicon mode (bright glow) in one tube at a time.
Tube with slightly better antenna coupling or matching.
Critical power varies with the circuit resistance Rc.
Higher densities for distances closer to the sources (Z1 level = 4", Z2 level = 7").

31. DENSITY ACROSS THE CHAMBER n(y)
Measurements (4/7)
DENSITY ACROSS THE CHAMBER n(y)
Staggered configuration (L = 14")
Compact configuration (L = 7")
Staggered configuration → Discreteness of sources.
Compact configuration → No discreteness of sources. More uniform.
Compact configuration gives ~ 4 times more density than Staggered configuration.
At 4 kW increment by a factor of 2.4 → Staggered at mid and Compact at low

32. CALCULATED vs. MEASURED DENSITY VALUES
Measurements (5/7)
CALCULATED vs. MEASURED DENSITY VALUES
Staggered configuration (L = 14")
Compact configuration (L = 7")
The shape of the measured values for both configurations agrees with the calculated ones.
The magnitude differs by a factor of 4.25 for the staggered and by 3.3 for the compact configuration. (Extrapolation from 2 to 4 kW gives a factor of 2.4).
Due to the limited number of tubes the plasma uniformity is seen only between central tubes (0" < x < 7") for the compact configuration. However, the staggered configuration shows that this area can be extended.

33. CALCULATED vs. MEASURED VALUES FOR MAGNITUDE
Measurements (6/7)
CALCULATED vs. MEASURED VALUES FOR MAGNITUDE
in between rows of tubes y = 0"
underneath rows of tubes y = 3.5"
The closer the tubes (compact configuration) → the higher the density.
Density in the high 1011 cm-3 (almost 1012 cm-3).

34. CALCULATED vs. MEASURED VALUES FOR UNIFORMITY
Measurements (7/7)
CALCULATED vs. MEASURED VALUES FOR UNIFORMITY
in between rows of tubes y = 0"
underneath rows of tubes y = 3.5"
The closer the tubes (compact configuration) → the less the ripple.
Ripple is less than 4%.

35. Plasma Processing and Distributed Sources
Helicon Waves
Proof of Principle: Plasma Injection
HELIC Calculations
Medusa 2: Discharge Tube and Processing Chamber
Medusa 2: Permanent Magnets
Medusa 2: Power Distribution and RF Matching
Diagnostic: Langmuir Probe & Ion Current Collectors
Measurements
Conclusions

36. Conclusions Use of PM far-field region to inject plasma downstream.
Optimization of the system → increasing Rp by using:
small tube (2" by 2"), MHz RF frequency, loop antenna, metal endplate.
The shorter the distance between tubes:
the higher the plasma density.
the less the ripple density → the more density uniformity.
the closer to the tubes these characteristics are achievable.
Any number of single helicon sources (tube + antenna + PM) can be arranged to produce uniform (< 3%) high-density plasmas over an arbitrary area.

37. The End.
THANK YOU