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Negative bias temperature instability (NBTI) in plasma-nitrided oxide affected by the nitrogen incorporation of a TiN diffusion barrier layer of gate stack is first proposed. In the standard 50-nm DRAM technology, three different groups including high, medium, and low N2 flow were deposited by sputtering to investigate in this paper. The high N2 flow group was with higher drive current and higher transconductance, but worse NBTI performance in pMOSFETs than that of the low N2 flow group. Boron (B) concentration at the bottom of p+ polycrystalline silicon (poly-Si) for the high N2 flow group is 5.4% higher than that for the low N2 flow group, which is verified by the secondary ion mass spectroscopy depth profile. A vertical electric field across gate nitrided oxide enhanced by high B concentration in the interface of p+ poly-Si/nitrided oxide resulted in the degradation of NBTI and time-dependent dielectric breakdown of the high N2 flow group. However, process optimization still needs to be studied between device performance and reliability of pMOSFETs in future generation.