Ion heating and density production in helicon sources near the lower hybrid frequency

We report measurements of electron density and perpendicular ion temperatures in an argon helicon plasma for five different rf antennas: a Nagoya type III antenna, a `Boswell' saddle coil antenna, a 19 cm long m = + 1 helical antenna, a 30 cm long m = + 1 helical antenna, and a 19 cm long m = + 1 helical antenna with narrow straps. The general properties of the source as a function of rf power and neutral pressure are reviewed and detailed measurements of electron density, electron temperature and ion temperature as a function of magnetic field strength and rf frequency are presented. The experimental results clearly indicate that for all antennas, the electron density is maximized when the rf frequency is close to and just above the lower hybrid frequency. The ion temperature is maximized when the rf frequency is less than 70% of the lower hybrid frequency. Ion temperatures in excess of 1 eV for 750 W of input power have been observed. These results suggest that the mechanisms responsible for coupling energy into the ions and electrons are distinct and therefore helicon sources can be configured to maximize electron density without simultaneously maximizing the perpendicular ion temperature. Enhanced ion heating is not a desirable feature of plasma sources intended for use in plasma etching, thus operational regimes that yield high plasma densities without increased ion heating might be of interest to industry.