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The effects of plasma nitridation and fluorine incorporation on the components of negative-bias temperature instability (NBTI) in p-type MOSFETs with plasma-nitrided SiON gates were investigated. To clarify these effects, NBTI-induced threshold-voltage shift was separated into two components: one for generation of traps at the SiON/Si-substrate interface and one for positive charges within the SiON bulk. It was found that the proportions of the interface and bulk components can be controlled with the plasma nitridation method: The bulk component was increased by radio-frequency plasma nitridation, while the interface component was dominant in the case of electron-cyclotron-resonance plasma nitridation. Lowering the nitrogen concentration near the SiON/Si-substrate interface decreased the interface component. Lowering the nitrogen concentration near the poly-Si/SiON interface did not decrease NBTI, while it decreased positive oxide charges in the as-fabricated MOSFETs. Furthermore, it was demonstrated that the fluorine incorporation decreases the interface component in plasma-nitrided SiON gates, while it does not decrease the bulk component.