Performance of a permanent-magnet helicon source at 27 and 13 MHz

Francis F. Chen
2012 Physics of Plasmas  
A small helicon source is used to create dense plasma and inject it into a large chamber. A permanent magnet (PM) is used for the dc magnetic field (Bfield), making the system very simple and compact. Though theory predicts that better antenna coupling will occur at 27.12 MHz, it was found that 13.56 MHz surprisingly gives even higher density due to practical effects not included in theory. Complete density n and electron temperature T e profiles are measured at three distances below the
more » ... es below the source. The plasma inside the source is also measured with a special probe, even under the antenna. The density there is lower than expected because the plasma created is immediately ejected, filling the experimental chamber. The advantage of helicons over ICPs (Inductively Coupled Plasmas, with no B-field) increases with RF power. At high B-fields, edge ionization by the Trivelpiece-Gould mode can be seen. These results are useful for design of multiple-tube, large-area helicon sources for plasma etching and deposition because problems are encountered which cannot be foreseen by theory alone. I. Background Helicon discharges are known to be good sources of dense plasma for industrial applications, but they normally require a large, heavy electromagnet and its power supply. This disadvantage has been overcome by the invention 1 of permanent-magnet helicon discharges using the remote, reverse field of small annular magnets in combination with the Low-Field Peak 2 in density caused by constructive interference of the reflected backward wave. This effect causes a useful increase in density occurring at a density that depends on the magnetic field and the length of the discharge tube. An array of eight small tubes, built several years ago 3 , successfully produced plasmas of density in the 10 11 cm -3 range, uniform over 56 cm width. This experiment demonstrated that a simple, inexpensive helicon arrays can cover large substrates with uniform plasma for roll-to-roll processing. In the present work, one of the helicon sources in the array is studied in detail in a cylindrically symmetric system to see if PM helicons can used for other applications such as spacecraft thrusters or optical coatings. It has been shown 4 that helicon sources can produce an interesting amount of thrust for that purpose, but experiments so far have used large electromagnets for the DC field, and these may be incompatible with the weight limits of spacecraft. The use of PM helicons for thrusters has already been investigated extensively by Takahashi et al. 5, 6, 7 Their configuration of PMs is quite different from ours and does not use annular magnets. For thruster applications, that work is much more advanced than ours, since the ion energy distributions were measured in detail. The present paper deals instead with the physics of PM helicon discharges and how their design for ejecting plasma has unexpected considerations. Comparisons between 13.56 and 27.12 MHz frequencies and between B = 0 and B > 0 operation are made. II. Apparatus
doi:10.1063/1.4754580 fatcat:6pyitw3ignbvxavoxjaakm3y2a