Electron Interactions With SF6

Abstract

Sulfur hexafluoride (SF6) is commonly used as a gaseous dielectric and as a plasma etching gas. In this work, the state of knowledge on electron-interaction cross sections and electron-swarm parameters in SF6 is comprehensively reviewed and critically assessed. Cross sections are presented and discussed for the following scattering processes: total electron scattering; differential elastic; elastic integral; elastic momentum; total vibrational; total and partial ionization; total dissociative and nondissociative electron attachment; and dissociation into neutrals. Coefficients for electron-impact ionization, effective ionization, electron attachment, electron drift, and electron diffusion are also reviewed and assessed. In addition, complementary information on the electronic and molecular structure of the SF6 molecule and on electron detachment and ion transport in parent SF6 gas is provided that allows a better understanding of the nature of the cross sections and swarm parameters. The assessed data are used to deduce cross sections and coefficients for which there exist no direct measurements at the present time. The present work on electron interactions with the SF6 molecule reveals a rather simple picture which can be summarized as follows: (1) Elastic electron scattering is the most significant electron scattering process over the electron energy range from ∼0.01 to ∼1000 eV. (2) Below 15 eV the most distinct inelastic energy-loss process is vibrational excitation—direct dipole excitation involving the ν3 mode and indirect vibrational excitation via negative ion states involving the ν1 mode. (3) Below ∼0.1 eV electron attachment forming SF6− is the most dominant interaction (along with elastic scattering). Above this energy, the cross sections for dissociative electron attachment forming fragment anions [principally SFx− (x=3, 4, and 5) and F−] are appreciable, with the room temperature total electron attachment cross section dominated by the formation of SF5− between ∼0.3 and 1.5 eV and by the formation of F− beyond ∼2.0 eV. (4) Above ∼16 eV dissociative ionization becomes significant, generating principally SFx+ (x=1, 3, 4, and 5) and F+ positive-ion fragments which, together with elastic electron scattering, makes up most of the total electron scattering cross section. (5) Electron-impact dissociation into neutral fragments SFx (x=1, 2, and 3) and F occurs above ∼15 eV, with cross section values potentially exceeding those for ionization for electron energies near 20 eV. (6) The total electron scattering cross section exhibits distinct structure due to negative-ion resonances near 0.0, 2.5, 7.0, and 11.9 eV. The most significant data needs are for direct measurements of vibrational excitation cross sections, for cross sections for electron-impact dissociation into neutral fragments, and for the momentum transfer cross section at low energies.