Optical emission spectroscopy and Langmuir probe diagnostics of CH3F/O2 inductively coupled plasmas

Abstract

A CH3F:O2 (50%:50%) inductively coupled discharge, sustained in a compact plasma reactor, was investigated as a function of power (20–400 W) and pressure (9–30 mTorr), using optical emission spectroscopy and Langmuir probe measurements. The electron density increased linearly with power but only weakly with pressure. The effective electron temperature was nearly independent of power and pressure. The gas temperature, obtained from the rotational structure of N2 (C → B) optical emission, increased from 400 to 900 K as a function of inductive mode power between 75 and 400 W at 10 mTorr. For constant feed gas flow, the absolute H, F, and O atom densities, estimated by optical emission rare gas actinometry, increased linearly with power. The absolute number density ratios H/Ar, F/Ar, and O/Ar, increased, decreased, and remained constant, respectively, with pressure. The H-atom density was estimated to be 5.4 × 1013 cm−3 at 400 W and 10 mTorr (gas temperature = 900 K), implying a high degree of dissociation of the CH3F feedstock gas. The F and O atom number densities were much lower (8.3 × 1012 cm−3 and 5.9 × 1012 cm−3, respectively) for the same conditions, suggesting that most of the fluorine and oxygen is contained in reaction products HF, CO, CO2, H2O, and OH. The relative number densities of HF, CO, and CO2 were observed to first rapidly increase with power, and then reach a plateau or decay slightly at higher power. Reaction mechanisms were proposed to explain the observed behavior of the number density of F and HF vs. power and pressure.