Investigation of the influence of ramped voltage stress on intrinsic tbd of MOS gate oxides

Abstract

This study investigates the influence of a pre-stressing ramped voltage stress prior to a constant voltage stress on the time to breakdown. Constant voltage stress and combined ramped/constant voltage stress measurements were performed on six MOS gate oxide thicknesses. The time to breakdown distributions were compared and an increase of the time to breakdown for pre-stressed oxides was observed in some cases. A further analysis of the current-time characteristics gave conclusions about the trapping properties of the oxide. It was found that the initial positive charge build up in the oxide is an important indicator for degradation which must be considered for highly accelerated reliability measurements on pre-stressed oxides. Since common understanding of oxide breakdown and models for breakdown mechanisms cannot describe all of the experimental results a qualitative model is proposed.