What’s special about helicon discharges? Helicon waves are whistler waves confined to a cylinder. Helicon discharges are made by exciting these waves.

The boundary has a large effect on the ionization efficiency The H mode peaks at the center, but its currents or charges at the boundary mode-couples to an electron cyclotron wave (TG mode) at the edge. The TG wave is electrostatic and travels slowly inward, efficiently depositing the RF energy into the electrons.

k1k1 k2k2 k0k0 k 0 = helicon wave, k 1 = ion acoustic wave k 2 = Trivelpiece-Gould mode This was verified experimentally. The deposition occurs via parametric instability

UCLA As P rf is raised, the sidebands get larger due to the growth of the LF wave. Krämer et al. detected the ion wave B. Lorenz, M. Krämer, V.L. Selenin, and Yu.M. Aliev, Plasma Sources Sci.Technol. 14, 623 (2005)

Thus, helicon research links several disciplines 1. Low-temperature plasma physics 2. Space physics (whistler waves) 3. Magnetic fusion (B-field, RF power, Bohm diffusion) 4. Laser fusion (parametric instabilities) A helicon discharge in a straight cylinder can produce densities up to cm 3 with only 1-2 kW.

How can we use this dense source? This is a commercial helicon source made by PMT, Inc. and successfully used to etch semiconductor wafers. It required two large and heavy electromagnets and their power supplies. Computer chips are now etched with simpler sources without a DC B-field. New applications require larger area coverage.

Possible uses of large-area plasma processing Roll-to-roll plastic sheets Smart windows OLED displays Solar cells, mass productionSolar cells, advanced

Distributed helicon source: proof of principle Achieved n > 1.7 x cm -3, uniform to  3%, but large magnet is required. F.F. Chen, J.D. Evans, and G.R. Tynan, Plasma Sources Sci. Technol. 10, 236 (2001)

The problem with small magnets A small solenoidField lines diverge too rapidly Annular permanent magnets have same problem

However, the external field can be used Note that the stagnation point is very close to the magnet Place plasma in the external field, and eject downwards

External field Internal field The bottom curve is when the tube is INSIDE the magnet PM helicons: proof of principle

Evolution of a multi-tube PM helicon source 1.Antenna design 2.Discharge tube geometry 3.Permanent magnets 4.RF circuitry Next: construction and testing of Medusa 2 Medusa Medusa 1

Helicon m = 1 antennas Only the RH polarized wave is strongly excited Nagoya Type III antenna: symmetric, so RH wave is driven in both directions. RH helical antenna: RH wave is driven only in the direction matching the antenna’s helicity. This antenna has the highest coupling efficiency

Why we use an m = 0 antenna A long antenna requires a long tube, and plasma goes to wall before it gets out. An m = 0 loop antenna can generate plasma near the exit aperture. Note the “skirt” that minimizes eddy currents in the flange. Now we have to design the diameter and length of the tube.

The low-field peak: an essential feature The peak occurs when the backward wave is reflected to interfere constructively with the forward wave. R is the plasma resistance, which determines the RF power absorbed by the plasma,

Designing the tube geometry Adjust a, H, and  RF so that n and B are in desired range.

This is done with the HELIC code D. Arnush, Phys. Plasmas 7, 3042 (2000). L c is made very large to simulate injection into a processing chamber. The code computes the wave fields and the plasma loading resistance R p vs. n and B

Choose a peak at low B, mid cm -3 density Low-field peak

Typical R(n,B) curves at the low-field peak Vary the B-fieldVary the tube length Vary the tube diameterVary the RF frequency

Final tube design for MHz Material: Pyrex or quartz With aluminum top

Reason for maximizing R p : circuit loss R c R c = 1.0  R c = 0.1 

Magnet design for G Vary the outside diameter Vary the vertical spacing

Final magnet design NdFeB material, 3”x 5”x1” thick B max = 12 kG

RF circuitry For equal power distribution, the sources are connected in parallel with equal cable lengths. The problem is that the cable lengths, therefore, cannot be short. The length Z2 and the antenna inductance L are the most critical.

C1, C2 for N=8, L = 0.8  H, Z1 = 110 cm, Z2 = 90 cm (unless varied) Allowable values of C1, C2 in match circuit There is an upper limit to each antenna’s inductance L. The range of Z2 can be restrictive for large arrays

Current and voltage in CW operation Coax connectors cannot take the startup voltage or the CW current. All joints have to be soldered or have large contact area. Junction boxConnection to water- cooled antenna

A low-R c, 50-  cooled, rectangular transmission line

Layout of 8-tube test module, Medusa 2 Compact configurationStaggered configuration The spacing is determined from the single-tube density profiles to give 2% uniformity

Side view Probe ports Aluminum sheet Adjustable height The source requires only 6” of vertical space above the process chamber Z1 Z2

Wooden frame for safety and movability

Medusa 2 in operation at 3 kW CW

Radial profile between tubes at Z2

UCLA ” Compact configuration, 3kW Side Langmuir probe Density profiles across the chamber << 4” below tubes << 7” below tubes

UCLA Density profiles across the chamber ” Staggered configuration, 3kW Bottom probe array

Density profiles along the chamber Staggered configuration, 2kW Bottom probe array

UCLA Density profiles along the chamber Compact configuration, 3kW Bottom probe array Data by Humberto Torreblanca, Ph.D. thesis, UCLA, 2008.

CONCLUSIONS  We’ve found a sweet spot where the tube, the antenna, the magnet, and the matching circuit can all work together.  There’s a large step between laboratory physics and a practical device.