Correlation of lifetimes from CVS and RVS using the 1/E-model for thermally grown oxides on polysilicon

Accelerated stress measurements such as constant voltage stress (CVS) and ramped voltage stress (RVS) are commonly used in industry for the evaluation of oxide lifetimes. The main advantage of RVS over CVS is the short measurement time. Therefore, RVS is widely used, especially, in short dielectric screens and, lifetimes are extrapolated from the RVS measurement results. For this lifetime extrapolation a correlation between RVS data and CVS lifetimes is assumed. This correlation between CVS and RVS results is investigated for six oxides which had been thermally grown from polysilicon. CVS and RVS measurements were performed over a wide range of bias conditions and the measurement results were directly compared. This comparison showed that RVS increases oxide lifetimes. The current-time characteristics were studied in order to find the cause of the increased RVS lifetimes. They indicated lower currents for RVS than for CVS at equal bias voltage levels. Further measurements were carried out to study the effect of a RVS prior to CVS. Findings from these measurements with pre-stressed oxides confirmed the RVS lifetime increase which had been seen earlier. The increase in RVS lifetimes is critical for the prediction of oxide lifetimes at operating voltage. This increase has to be taken into account when lifetimes are predicted from RVS results. If it is not oxide lifetimes will be overestimated.