Paper PS2-ThM10
Characterization of an Expanding Chlorine Plasma Produced by an Electromagnetic Surface-Wave

Thursday, November 12, 2009, 11:00 am, Room B2

Plasmas produced by propagating surface waves have attracted attention because of their long and stable plasma columns without accompanying guiding structures. This is because the electric field supporting the discharge is provided by a traveling wave that carries the power away from the applicator, guided by the plasma column and the dielectric tube enclosing it. In long, narrow plasmas the wave-to-plasma power transfer is usually assumed to occur locally such that the axial density profile is determined by the wave attenuation coefficient. As a result the electron density, n_e, decreases in a quasi-linear manner along the plasma column in the direction of the wave propagation down to the critical density for surface wave propagation where the plasma decays abruptly (= expansion region) . At low pressures, however, the plasma tends to expand well beyond this critical point such that the description of the axial density distribution in terms of the local approximation is no longer valid. We investigated the influence of gas pressure on the spatial structure of a high-density chlorine plasma produced in a 6 mm, inside diameter, quartz tube by a propagating 2450 MHz surface wave. The axial variation of the electron density was determined from the spatial phase characteristics of the wave and the 828.0 nm emission line of Xe inserted as a tracer. As expected, n_e decreased linearly with axial position from the wave launcher, except in an expansion region near the end of the plasma column where the decrease of n_e was more abrupt. The thickness of this expansion region decreased with increasing pressure, going from about 8 cm at 5 mTorr to less than 1 cm at 100 mTorr. The Cl₂ percent dissociation obtained from the calibrated Cl₂(306 nm)-to-Xe emission ratio remained fairly constant except in the expansion region where it decreased sharply. For example, at 5 mTorr, the Cl₂ percent dissociation was 95 % near the wave launcher and 15 % at 2 cm from the end of the plasma column. While the expansion region showed a decrease in the electron density and Cl₂ percent dissociation, no noticeable change in the electron energy distribution function (EEDF) was observed. For all pressures and axial positions, the EEDF determined by trace-rare-gas-optical-emission-spectroscopy remained Maxwellian. The electron temperature (T_e) was fairly independent of the axial position, going from ~12 eV at 5 mTorr to ~2 eV at 100 mTorr. The high T_e values are due to a combination of high gas temperatures (T_g = 463 K at 5 mTorr and 635 K at 100 mTorr, measured by N₂ C->B emission rotational spectra) and small tube bore ( 0.6 cm ), and are in good agreement with a global model.