What’s special about helicon discharges?

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

3. 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.

4. The deposition occurs via parametric instability k2 k1 k0 k0 = helicon wave, k1 = ion acoustic wave k2 = Trivelpiece-Gould mode This was verified experimentally.

5. 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) As Prfis raised, the sidebands get larger due to the growth of the LF wave. UCLA

6. 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 1014 cm3 with only 1-2 kW.

7. 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.

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

9. Distributed helicon source: proof of principle Achieved n > 1.7 x 1012 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)

10. The problem with small magnets Annular permanent magnets have same problem A small solenoid Field lines diverge too rapidly

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

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

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

14. 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

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

16. 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,

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

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

19. Low-field peak Choose a peak at low B, mid 1012 cm-3 density

20. Typical R(n,B) curves at the low-field peak Vary the B-field Vary the tube length Vary the tube diameter Vary the RF frequency

21. Final tube design for 13.56 MHz Material: Pyrex or quartz With aluminum top

22. Rc = 1.0 W Rc = 0.1 W Reason for maximizing Rp:circuit loss Rc

23. Magnet design for 60-100 G Vary the outside diameter Vary the vertical spacing

24. Final magnet design NdFeB material, 3”x 5”x1” thick Bmax = 12 kG

25. 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.

26. 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 C1, C2 for N=8, L = 0.8mH, Z1 = 110 cm, Z2 = 90 cm (unless varied)

27. 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 box Connection to water-cooled antenna

28. A low-Rc, 50-W, cooled, rectangular transmission line

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

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

31. Wooden frame for safety and movability

32. Medusa 2 in operation at 3 kW CW

33. Radial profile between tubes at Z2

34. Density profiles across the chamber Compact configuration, 3kW Side Langmuir probe 0 3.5” << 4” below tubes << 7” below tubes UCLA

35. Density profiles across the chamber Staggered configuration, 3kW Bottom probe array -7 0 7 14” UCLA

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

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

38. 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.