Improved spiral loop antenna for inductively coupled plasma sources

The authors have developed and characterized a metal enamel-coated spiral loop antenna that is useful as an inductively coupled plasma source. It has enhanced plasma confinement due to multidipole magnetic cusp fields on the boundary, and is very efficient. Operation is possible at low neutral pressures, Ar, He, N₂ discharges at P_o > 6 × 10^-5 Torr, which a range of background magnetic field from 0-150 Gauss. In order to learn how to optimize the source uniformity for plasma processing applications, an attempt is being made to isolate the various features, such as gas choice, confinement mechanisms, and RF frequency. For instance, data for different working gases show that for zero background magnetic field N₂ plasmas are more radially uniform that Ar plasmas, whereas the opposite is true with B > 20 Gauss. The former case may be due to the high density of excited states in N₂, whereas the latter is probably a gyro radius effect. The plasma edge profile was examined as the line cusp density was varied from 0 to 14 per source chamber. Line cups seem to enhance the overall confinement (density), but minimally influence the edge radius shape, and overall radial uniformity.