The impurity barrier properties of thin reoxidized nitrided oxide (ONO) gate dielectrics formed by rapid thermal processing to boron penetration resulting from p+ poly gate processing is investigated. Measurements comparing the threshold voltage instability of capacitors fabricated with BF2 implanted poly gates subjected to various post-gate thermal cycles have been performed. The ONO gate dielectrics are found to be an excellent impurity barrier to boron diffusion, even in the presence of fluorine. The extent of the nitridation is also found to affect the diffusion barrier properties, with the highest temperature nitridations forming the best barriers. Reoxidation of the nitrided films reduces the barrier properties somewhat, but remarkable improvement is still observed over SiO2.