Electron interactions with SF6

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-inter action cross sections and electron-swarm parameters in SF6 is comprehensive ly reviewed and critically assessed. Cross sections are presented and discu ssed for the following scattering processes: total electron scattering; dif ferential elastic; elastic integral; elastic momentum; total vibrational; t otal and partial ionization; total dissociative and nondissociative electro n attachment; and dissociation into neutrals. Coefficients for electron-imp act ionization, effective ionization, electron attachment, electron drift, and electron diffusion are also reviewed and assessed. In addition, complem entary information on the electronic and molecular structure of the SF6 mol ecule and on electron detachment and ion transport in parent SF6 gas is pro vided that allows a better understanding of the nature of the cross section s and swarm parameters. The assessed data are used to deduce cross sections and coefficients for which there exist no direct measurements at the prese nt time. The present work on electron interactions with the SF6 molecule re veals a rather simple picture which can be summarized as follows: (1) Elast ic electron scattering is the most significant electron scattering process over the electron energy range from similar to 0.01 to similar to 1000 eV. (2) Below 15 eV the most distinct inelastic energy-loss process is vibratio nal excitation-direct dipole excitation involving the nu(3) mode and indire ct vibrational excitation via negative ion states involving the vl mode. (3 ) Below similar to 0.1 eV electron attachment forming SF6- is the most domi nant interaction (along with elastic scattering). Above this energy, the cr oss sections for dissociative electron attachment forming fragment anions [ principally SFx- (x=3, 4, and 5) and F-] are appreciable, with the room tem perature total electron attachment cross section dominated by the formation of SF5- between similar to 0.3 and 1.5 eV and by the formation of F- beyon d similar to 2.0 eV. (4) Above similar to 16 eV dissociative ionization bec omes significant, generating principally SFx+ (x=1, 3, 4, and 5) and F+ pos itive-ion fragments which, together with elastic electron scattering, makes up most of the total electron scattering cross section. (5) Electron-impac t dissociation into neutral fragments SFx (x=1, 2, and 3) and F occurs abov e similar to 15 eV, with cross section values potentially exceeding those f or ionization for electron energies near 20 eV. (6) The total electron scat tering cross section exhibits distinct structure due to negative-ion resona nces near 0.0, 2.5, 7.0, and 11.9 eV. The most significant data needs are f or direct measurements of vibrational excitation cross sections, for cross sections for electron-impact dissociation into neutral fragments, and for t he momentum transfer cross section at low energies. (C) 2000 American Insti tute of Physics. [S0047-2689(00)00204-X].