Evolution of oxide charge trapping under bias temperature stressing

Abstract

Transient oxide-charge trapping and detrapping, commonly regarded as a parasitic effect in the interpretation of dynamic bias-temperature stress (BTS) data, may play an important role on the long term reliability of the gate oxide as revealed by recent studies on the SiON and HfO2 gate dielectrics. Specifically, it is found that transient charge trapping (one which relaxes upon removal of the applied electrical stress) is transformed into more permanent trapped charge when the applied electrical cum thermal stress exceeds a certain threshold. Below the threshold, cyclical transient charge trapping and detrapping behavior is observed. The observations imply that the oxide structure may be modified by the applied stress, making it susceptible to permanent defect generation. In addition, it is found that when the transformation of hole trapping occurs under negative-bias temperature stress, a correlated increase of the gate current is always observed, which points to the transformation process being the origin for bulk oxide trap generation. However, when the transformation of electron trapping occurs under positive-bias temperature stress, an increase of the gate current is not always observed. From ab initio simulation, we show that an intrinsic oxide defect – the oxygen vacancy-interstitial (VO−Oi) – could consistently explain the experimental observations. An interesting feature of the VO−Oi defect is that it can exists in various metastable configurations with the interstitial oxygen Oi in different positions around the vacancy VO, corresponding to different trap energy states in the oxide bandgap. This characteristic is able to account for the BTS induced generation of deep-level trapped charges as well as transformation of transient (or shallow) to permanent (or deep) charge trapping.