Electron Transport, Ionization, and Attachment Coefficients in C₂F₄ and C₂F₄ /AR Mixtures

Abstract

In the use of perfluorocyclobutane (c-C₄F₈) as a plasma processing gas for silicon dioxide etching, perfluoroethylene (C₂F₄) is produced as a by-product of electron and photon impact on c-C₄F₈, and also as a product of thermal decomposition of c-C₄F₈. Consequently, dissociative processes can make C₂F₄ a significant gas constituent. Therefore, in order to model c-C₄F₈ plasmas, it is important to know the electron transport, ionization, and attachment coefficients of C₂F₄ in order to understand fully the chemical and physical processes occurring in the discharge. This knowledge may also be of relevance to gaseous dielectrics since C₂F₄ may be produced in SF₆ circuit breakers by vaporization of polytetrafluoroethylene insulators.¹ Measurements of electron transport, ionization, and attachment coefficients are reported in this paper for pure C₂F₄ gas. In addition, measurements of the electron drift velocity and the effective ionization coefficient as functions of the density-reduced electric field E/N are reported for mixtures of C₂F₄ with Ar, which may be useful in efforts to obtain electron collision cross sections for C₂F₄ using Boltzmann codes. The effect of Penning ionization on the measured effective ionization coefficients in C₂F₄/Ar mixtures is also investigated.