Bias-Temperature Instability in the High-k/Metal Gate Stacks

Besides the negative-bias temperature (NBTI) effect that is seen in the conventional polysilicon on SiO 2 or SiON gate stack, the high-k/metal gate stack also exhibits positive-bias temperature instability (PBTI) which has an adverse impact on the operation of the n-MOSFET. Significant threshold-voltage V t shift but negligible transconductance g m degradation was observed after PBTI stress. On the other hand, both a substantial shift in V t and degradation of g m were observed after NBTI stress. On this basis, some studies have concluded that PBTI and NBTI are due to different physical mechanisms. The lack of interface degradation in the case of PBTI may be understood in terms of the absence of the PBTI effect in the SiO 2 dielectric. However, the role of charge trapping in the high-k dielectric remains poorly appreciated and is often viewed as resulting from the transient " filling/emptying " of pre-existing oxide defects which may not have any impact on long term device degradation. In this talk, we will present experimental results from our recent dynamic BTI studies on the HfO 2 /TiN gate stack. In terms of charge trapping and oxide defect generation in the high-k oxide, dynamic PBTI and NBTI exhibit both similar and different experimental features:  Similarity – Hole trapping and electron trapping are observed to evolve gradually, when the test device is being " cycled " numerous times between stress and relaxation, from a transient state (i.e. the trapped charge could be re-emitted within the relaxation period chosen) into a more permanent state (i.e. the trapped charge could no longer be re-emitted within the relaxation period chosen). The observed evolution implies that the some of the trap levels might have shifted from shallow to deep, thus " locking " in the trapped charges. The result also implies that trap properties are continuously changing and may not be presumed as static and described by fixed capture/ emission time constants.  Difference – When part of the transient hole trapping is changed into a more permanent state, generation of stress induced leakage current (SILC) can be clearly observed. However, no SILC generation is observed in the case of the dynamic PBTI when part of the transient electron trapping has evolved into a more permanent form. To probe the reason(s) behind the similarity and difference observed, ab-initio simulation was performed on an amorphous HfO 2 supercell using VASP, focusing first …