Enhanced depth resolution in positron analysis of ion irradiated SiO2 films

We report the observation of defects following 1.7 MeV Si+ through-implantation of SiO2 films thermally grown on Si(100) substrates. Films were irradiated to fluences of 1012, 1013, and 1014 ions/cm2, and analyzed using a variable energy positron beam. Enhanced depth resolution was achieved via iterative chemical etching and measurement, for the sample irradiated to 1012 Si+/cm2, and for an unirradiated control sample. The positron annihilation “S parameter” is reduced in the implanted SiO2, due to a reduction in the formation of positronium (the atom-like positron–electron bound pair), and is increased in the Si substrate, due to the creation of vacancy-type defects. The concentration of implant-induced defects was found to be very nearly constant as a function of depth in the SiO2 film, indicating a significant contribution of both the electronic and nuclear stopping of the implanted Si+ ions to the production of positron-trapping defects. We find that the maximum overlayer thickness which allows simultaneous detection of defects in the substrate (i.e., through-oxide probing of substrate defects) is ∼600 nm.