Macromolecule formation in low density CF4 plasmas: The influence of H2

Abstract

High molecular weight fluorocarbon species are regarded as important contributors to the nucleation of films and particulates in fluorocarbon plasmas. The chemical reaction mechanisms by which fluorocarbon macromolecules form within a plasma are generally unknown. To elucidate these mechanisms, experiments were conducted in a rf capacitively coupled discharge in a Gaseous Electronics Conference reference cell. The relationships between macromolecule growth and plasma pressure, power, flow rate, and the fraction of H2 in the CF4 gas feed are identified. Macromolecule growth was found to increase with increased pressure and rf power, and decreased flow rate. A set of electron-induced dissociation and radical-recombination reactions are simulated using Chemkin–Aurora, a commercially available plasma chemistry model, and are in good agreement with the experimental results of macromolecule growth. We show that a primary mechanism by which fluorocarbon macromolecules form in a plasma occurs by electron-induced dissociation of a fluoroalkane to produce a fluoroalkyl radical and a fluorine atom, followed by a three-body radical–radical recombination reaction with CF3. Hydrogen is shown to have a profound effect on this reaction sequence by reducing the gas phase atomic fluorine concentration through the formation of HF which in turn increases the CF3 concentration available to participate in the macromolecule growth process. At moderate levels of hydrogen in the feed gas (<20%), macromolecule growth is directly correlated with the fraction of hydrogen in the feed gas. At high concentrations of hydrogen, hydrofluorocarbon and hydrocarbon growth occurs in the plasma at the expense of fluorocarbon macromolecule growth. The conditions under which the formation of these species occurs is consistent with observations in the literature of dramatic reductions in silicon dioxide etching rate. The transition between the formation of fluorocarbon macromolecules and hydrocarbon species in a CF4/H2 plasma is shown to be fundamental to understanding the growth process of each class of species within the plasma.