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

We report measurements of electron density and perpendicular ion temperatur es 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 rn = +1 heli cal antenna with narrow straps. The general properties of the source as a f unction of rf power and neutral pressure are reviewed and detailed measurem ents of electron density, electron temperature and ion temperature as a fun ction of magnetic field strength and rf frequency are presented. The experi mental results clearly indicate that for all antennas, the electron density is maximized when the rf frequency is close to and just above the lower hy brid frequency. The ion temperature is maximized when the rf frequency is l ess 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 e lectron density without simultaneously maximizing the perpendicular ion tem perature. Enhanced ion heating is not a desirable feature of plasma sources intended for use in plasma etching, thus operational regimes that yield hi gh plasma densities without increased ion heating might be of interest to i ndustry.