Characterization of a low-pressure chlorine plasma column sustained by propagating surface waves using phase-sensitive microwave interferometry and trace-rare-gas optical emission spectroscopy

Abstract

Phase-sensitive microwave interferometry and trace-rare-gas optical emission spectroscopy were used to measure the line-integrated electron density, ne, and electron temperature, Te, in a high-density chlorine plasma sustained in a quartz discharge tube (inner diameter = 6 mm) by an electromagnetic surface wave at 2.45 GHz. For pressures in the 0.1–1 Torr range, ne decreased nearly linearly along the tube’s z-axis down to the critical density for surface wave propagation, where the plasma decayed abruptly. At lower pressures (< 50 mTorr), however, the plasma extended well beyond this critical point, after which ne decreased quasiexponentially toward the end of the plasma column. The length of this expansion region increased with decreasing pressure, going from ∼8 cm at 5 mTorr to ∼1 cm at 50 mTorr. Te was nearly independent of the axial position in the main plasma region and strongly decreased in the expansion region at lower pressures. The Cl2 percent dissociation, τD, obtained from the calibrated Cl2 (306 nm)-to-Xe (828 nm) emission ratio, displayed behavior similar to that of ne and Te. For example, at 5 mTorr, τD was close to 100% near the wave launcher and ∼70% at 0.5 cm from the end of the plasma column.