Charging and discharging of oxide defects in reliability issues

Negative bias temperature instability (NBTI) has been recognized as a major reliability challenge years ago but a detailed microscopic understanding of the mechanisms governing NBTI is still missing. It appears to be very well established that depassivation of dangling bonds at the interface combined with diffusion of hydrogen may play a crucial role in this context. Some authors have argued that charging defects via tunneling also constitutes an additional contribution to this phenomenon. Conventionally, tunneling levels are believed to remain at fixed positions within the oxide bandgap regardless whether they are occupied or not. From a theoretical point of view, defect energy levels shift within the silicon dioxide bandgap due to charging or discharging. As a result, defect levels for tunneling into and out of a trap have to be distinguished. On the basis of this understanding of trapping, defects can be categorized as fixed charges, switching oxide charges, interface traps or other types of defects. In this study, we conduct an investigation on the energetics and the tunneling dynamics of a series of individual defects suspected to contribute to NBTI and deduce their behavior.