Atomic chlorine concentration measurements in a plasma etching reactor. II. A simple predictive model

Abstract

Atomic chlorine concentrations in Cl2 plasmas have been measured using the technique of infrared absorption spectroscopy. A wide range of plasma conditions were examined, including power input densities of 0.05–0.5 W/cm−3, pressures of 200–800 mTorr, and excitation frequencies of 0.1–13.5 MHz. A simple kinetic model accurately predicts atomic chlorine concentrations as a function of discharge parameters. The primary source of atomic chlorine was assumed to be an electron-impact dissociation of Cl2, and the reaction rate coefficient was assumed to be independent of plasma conditions. The electron density was estimated from a plasma impedance analysis. The depletion of atomic chlorine was successfully modeled assuming second-order recombination on the electrode surfaces; depletion by gas-gas recombination and by flow from the reactor were insignificant over the range of plasma conditions studied. Because the diffusion of atomic chlorine is fast relative to the formation and depletion rates, the atomic chlorine concentration is fairly independent of interelectrode position. Atomic chlorine concentrations were accurately represented by the equation [Cl]=3.5×108 [e−]0.5P0.5, where [Cl] and [e−] are in cm−3 and P is in mTorr.