Design of PM helicon arrays

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Design of PM helicon arrays

1. Optimization of the discharge tube
2. Design of the permanent magnets
3. Design of a multi-tube array
4. Design and construction of a test chamber
5. Antennas and the RF distribution system
6. Experimental results
7. Design of a compact module
8. Ideas for further improvements to be tested

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A commercial helicon etcher (PMT MØRI)
It required two heavy electromagnets with
opposite currents.
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Previous experiment with 7 tubes
The stubby tube
It required a large electromagnet
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Plasmas merged density is uniform
High density and uniformity were achieved
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Optimization of discharge tube HELIC code

Radial profiles are arbitrary, but B and n must
be uniform axially.
HELIC gives not only the wave fields but also R,
the loading resistance.
D. Arnush, Phys. Plasmas 7, 3042 (2000).
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The HELIC user interface
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The Low-field Peak
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Mechanism of the Low Field Peak
Basic helicon relations
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The peak is sensitive to the density profile
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The peak depends on the boundary condition
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The peak depends on distance from endplate
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The peak depends on the type of antenna
Single loop m 0, bidirectional HH
(half-wavelength helical) m 1,
undirectional Nagoya Type III m 1,
bidirectional
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Typical scan of Rp vs n, B
Each point requires solving a 4th order
differential equation gt100 times. A typical scan
takes 3 hours on a PC.
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Matrices for optimizing discharge tube
Vary the tube length and diameter
Vary the RF frequency
Vary the endplate conductivity
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Vary the pressure and frequency
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Vary H (endplate distance) for 3 diam
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Vary diam for H 2 at 100G
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Vary the frequency
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Not much variation with pressure
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Vary the endplate material
Initially, it seems that the conducting endplate
is better. However, it is because the phase
reversal at the endplate has changed, and the
tube length has to be 1/4 wavelength longer to
get constructive interference. By changing H,
almost the same R can be achieved.
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Relation of R to plasma density
Rp ltlt Rc
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Relation of R to plasma density
Rp gt Rc
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Final design
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The New Stubby tube
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Design of PM helicon arrays

1. Optimization of the discharge tube
2. Design of the permanent magnets
3. Design of a multi-tube array
4. Design and construction of a test chamber
5. Antennas and the RF distribution system
6. Experimental results
7. Design of a compact module
8. Ideas for further improvements to be tested

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Characteristics of permanent magnet rings
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The B-field of annular PMs
The field reverses at a stagnation point very
close to the magnet.
Plasma created inside the rings follows the field
lines and cannot be ejected.
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Optimization of magnet geometry

actual


Result Field strength ? magnet volume Spacing
improves uniformity slightly
actual
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The field of 4 stacked magnet rings
The internal and external fields at various
radii. The individual rings can be seen at large
radii.
Calibration of the calculated field with a
gaussmeter.
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For the designed tube, B 60G is good
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Proof of principle on 3 diam tube
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Radial density profiles at Z1 and Z2
Upper probe
Lower probe
x 1010 cm-3
Proof of principle discharge in the external
field gives much more plasma downstream.
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The final design for 2 tubes
Material NdFeB Bmax 12 kG Attractive force
between two magnets 2 cm apart 516 Newtons 53
kg
The magnets are dangerous!
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Wooden frame for safe storage
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Single tube, final configuration
Radial Bz profiles at various distances below the
magnet.
Discharge tube
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Design of PM helicon arrays

1. Optimization of the discharge tube
2. Design of the permanent magnets
3. Design of a multi-tube array
4. Design and construction of a test chamber
5. Antennas and the RF distribution system
6. Experimental results
7. Design of a compact module
8. Ideas for further improvements to be tested

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Design of array
Radial density profiles at Z1 7.4 cm and
Z2 17.6 cm below discharge.
The density at Z2 is summed over nearest tubes.
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Computed uniformity n(x) for various y
Half-way between rows
1/4-way between rows

Directly under a row
Beyond both rows
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A tube spacing of 7 is chosen
For a single row, a distance L 17.5 cm between
two tubes gives less than 2 ripple in density.
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Design of PM helicon arrays

1. Optimization of the discharge tube
2. Design of the permanent magnets
3. Design of a multi-tube array
4. Design and construction of a test chamber
5. Antennas and the RF distribution system
6. Experimental results
7. Design of a compact module
8. Ideas for further improvements to be tested

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An 8-tube linear test array
Top view
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Possible applications

• Web coaters
• Flat panel displays
• Solar cells
• Optical coatings

A web coater
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The array source is vertically compact
Side view
Probe ports
The magnets can be made in two pieces so that
they hold each other on an aluminum sheet. Once
placed, the magnets cannot easily be moved, so
for testing we use a wooden support.
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The wooden magnet frame is used in testing
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Water and RF connections
These will be shown in detail later
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An 8-tube staggered array in operation
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Design of PM helicon arrays

1. Optimization of the discharge tube
2. Design of the permanent magnets
3. Design of a multi-tube array
4. Design and construction of a test chamber
5. Antennas and the RF distribution system
6. Experimental results
7. Design of a compact module
8. Ideas for further improvements to be tested

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Antennas
The antennas are m 0 loops made of three turns
of 1/8 diam copper tubing.
The reason for m 0 is that m 1 antennas are
too long, and much of the plasma is lost by
radial diffusion before exiting the tube. The
antenna must be close to the exit aperture and be
tightly wound onto the tube.
The helicon wave pattern for m 0 is a peculiar
one but theory is straightforward. The wave
changes from pure electromagnetic to pure
electrostatic in each half cycle.
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The RF system
The critical elements are the junction box and
the transmission lines.
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Antenna connections (1)
The antennas must be connected in parallel with
cables of equal length. The first trial was to
use standard RG/8U cables and N connectors. These
arced and overheated.
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Reason that RF connectors dont work

High voltages and currents occur when there is NO
PLASMA that is, before breakdown or when the
plasma disrupts. Then, if the RF power on a tube
is set for 400W, the peak-to-peak voltage can be
5kV, and the rms current can be 12A. Once the
discharge is on, the RF power goes into the
plasma rather than into the cables and connectors.
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Antenna connections (2)
In the second try, all connections were solidly
soldered, and RG/393 teflon-insulated cable was
used. This method works for CW operation in
experiment but may be marginal for industrial
use.
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Connections (3) a rectangular transmission line
For w gtgt h, we find
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Impedance for various pipe diameters
For Z0 50W, h 3/4, but exactly 50W is not
necessary
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Transmission line construction (1)
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Transmission line construction (2)
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Transmission line construction (3)
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Transmission line (4) water connections
No high voltage is applied along a water line.
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Pictures (1)
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Pictures (2)
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Pictures (3)
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Design of the matching circuit
Standard circuit
Alternate circuit
Analytic formulas from Chen. The important part
is that the impedance changes with cable length.
F.F. Chen, Capacitor tuning circuits for
inductive loads, UCLA Rept. PPG-1401
(unpublished) (1992) F.F. Chen, Helicon Plasma
Sources, in "High Density Plasma Sources", ed. by
Oleg A. Popov (Noyes Publications, Park Ridge,
NJ), Chap. 1 (1995)
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Adapted to N tubes in parallel
The problem with array sources is that the cable
lengths cannot be short. The match circuit
cannot be close to all the tubes.
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C1, C2 for N8, L 0.8mH, Z1 110 cm, Z2 90
cm
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Variation with the number of tubes N
Note that it is not possible to match to 1 or 2
tubes with the same length cables used for 8
tubes.
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Design of PM helicon arrays

1. Optimization of the discharge tube
2. Design of the permanent magnets
3. Design of a multi-tube array
4. Design and construction of a test chamber
5. Antennas and the RF distribution system
6. Experimental results
7. Design of a compact module
8. Ideas for further improvements to be tested

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Experimental layout
Staggered configuration
Compact configuration
Four probe positions
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Effect of B-field strength (magnet height D)
Variation of loading resistance with D
Variation of density with D
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Variation with RF power and Ar pressure
Variation of density argon pressure
Variation of density with RF power
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Density jump inside the tube
compared with theory for various circuit
resistances Rc
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Deployment of movable probe array
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An linear array of 15 probes
H. Torreblanca, Multitube helicon source with
permanent magnets, Thesis, UCLA (2008).
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Density profiles across the chamber (1)
Staggered configuration, 3kW Side Langmuir probe
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Density profiles across the chamber (2)
Compact configuration, 3kW Side Langmuir probe
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Density profiles across the chamber (3)
Staggered configuration, 3kW Bottom probe array
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Density profiles across the chamber (4)
Compact configuration, 3kW Bottom probe array
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Density profiles along the chamber (1)
Staggered configuration, 3kW Bottom probe array
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Density profiles along the chamber (2)
Compact configuration, 3kW Bottom probe array
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Calibration of the collector array
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Design of PM helicon arrays

1. Optimization of the discharge tube
2. Design of the permanent magnets
3. Design of a multi-tube array
4. Design and construction of a test chamber
5. Antennas and the RF distribution system
6. Experimental results
7. Design of a compact module
8. Ideas for further improvements to be tested

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A compact, 8-tube module
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Stacked modules for large-area coverage
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Match circuit fits on top of module
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Design of PM helicon arrays

1. Optimization of the discharge tube
2. Design of the permanent magnets
3. Design of a multi-tube array
4. Design and construction of a test chamber
5. Antennas and the RF distribution system
6. Experimental results
7. Design of a compact module
8. Ideas for further improvements to be tested

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Ferrites for better coupling
The RF energy outside the antenna is wasted.
Perhaps it can be captured with a ferrite cover.
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Untested ideas
One-piece ceramic tube
Ferrite transformer coupling
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Varying the magnet shapes and spacings

Varying the ID and OD of PMs shows that B depends
mainly on total volume of magnet.


This shows that not much uniformity is lost if
the magnet spacing is zero
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