Electron Interactions With SF6

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Abstract
Sulfur hexafluoride ( SF 6 ) 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 SF 6 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 SF 6 molecule and on electron detachment and ion transport in parent SF 6 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 SF 6 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 ν 3 mode and indirect vibrational excitation via negative ion states involving the ν 1 mode. (3) Below ∼0.1 eV electron attachment forming SF 6 − is the most dominant interaction (along with elastic scattering). Above this energy, the cross sections for dissociative electron attachment forming fragment anions [principally SF x − ( x=3, 4, and 5) and F − ] are appreciable, with the room temperature total electron attachment cross section dominated by the formation of SF 5 − between ∼0.3 and 1.5 eV and by the formation of F − beyond ∼2.0 eV . (4) Above ∼16 eV dissociativeionization becomes significant, generating principally SF x + ( 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 SF x ( 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.

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