Comparison of silicon etching properties between F/sup -/ negative ion and SF/sub 3//sup +/ positive ion-beam etchings

Etching properties of Si substrate by using negative-ion (F/sup -/) and positive-ion (SF/sub 3//sup +/) beams were studied. These are dominant ions in pulse-modulated SF/sub 6/ plasma with a alternate bias. F/sup -/ ions were extracted from a RF plasma-sputter-type heavy negative ion source by using SF/sub 6/ gas as a ionized material. SF/sub 3//sup +/ ion was also extracted from a SF/sub 6/ 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 /spl mu/A/cm/sup 2/. 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/sup -/ was 4.8 eV and larger that 21 eV by SF/sub 3//sup +/. This is because the molecule ion required dissociation energy. For one fluorine atom at the same velocity in these two ion species, F/sup -/ is considered to have an almost same ability as SF/sub 3//sup +/.