The chemical composition and structure of the Si(3)N(4/)thermal SiO2 interface in silicon/oxide/nitride/oxide (SONO) structures were studied by using electron energy loss spectroscopy (EELS) and ellipsometric measurements. Both experiments show the existence of excess silicon at the Si3N4/fhermal SiO2 interface, in the form of Si-Si bonds in the Si-rich silicon oxynitride. Wet oxidation of the as-deposited Si3N4 has profound effects on the interlaces in SONO structure. Mechanisms responsible for these observations are proposed based on the chemical reactions during the synthesis of the SONO structure. Particularly, we propose that the Si-Si bonds are produced by replacing nitrogen with oxygen during the oxidation of Si3N4 These bonds should be the responsible candidates for the positive charge accumulation in re-oxidized nitrided oxide at the hot hole injection and ionizing radiation.