Evaluation of SOI Substrates by Positron Annihilation

The annihilation characteristics of positrons in Si-on-insulator (SOI) wafers were studied using a monoenergetic positron beam. From measurements of Doppler broadening spectra of the annihilation radiation as a function of incident positron energy, the line-shape parameter S of the buried oxide (BOX) layers fabricated by the separation-by-implanted-oxygen process was found to be smaller than that of a SiO2 film fabricated by conventional thermal oxidation. This was attributed to the suppression of the positronium formation in the BOX layer due to the trapping of positrons by defects; the charge state of the defects was estimated to be negative. Positrons implanted into the superficial Si layer annihilated from the trapped state by the defects. These defects were considered to be related to oxygen atoms, and the concentration of such oxygen-related defects was high at the region close to the SOI/BOX interface.