Comparison of silicon etching properties between F- negative ion and SF3+ positive ion-beam etchings

Etching properties of Si substrate by using negative-ion (F^- ) and positive-ion (SF₃⁺) beams were studied. These are dominant ions in pulse-modulated SF₆ plasma with a alternate bias. F^- ions were extracted from a RF plasma-sputter-type heavy negative ion source by using SF₆ gas as a ionized material. SF₃⁺ ion was also extracted from a SF₆ plasma of the source. In the experiment, p-Si substrates and silicon oxides covered with a gold-plated tungsten mesh (100 mesh/inch) were etched by the ion beam at an energy of 50-900 eV with a current density of 3-16 μA/cm². Both etching rates for these ions showed a linear dependence on ion velocity. Chemical sputtering is considered to be dominant process of etching in the low energy range. Threshold etching energies of Si by F^- was 4.8 eV and larger that 21 eV by SF₃⁺. This is because the molecule ion required dissociation energy. For one fluorine atom at the same velocity in these two ion species, F^- is considered to have an almost same ability as SF₃⁺