The Impact of Nitrogen on the Frequency Dependence of Negative-Bias Temperature Instability

Negative-bias temperature instability (NBTI) of the ultra-thin oxynitride gate p-MOSFET is studied as a function of frequency under unipolar ac gate stress. Device threshold voltage shift |DeltaV_t| is shown to exhibit an inverse power-law dependence on frequency, i.e. |DeltaV_t| prop f^gamma, where the exponent gamma ~ 0.042 for p-MOSFETs (A) with ~ 1.2 at. % nitrogen concentration [N] at the Si-SiO₂ interface. The exponent y is observed to decrease with increased [N] (gamma ~ 0.017 for p-MOSFETs (B) with [N] ~ 4.2 at. %), indicating a much weaker NBTI frequency dependence in more heavily nitrided ultra-thin gate p-MOSFETs. Analysis shows that the weaker frequency dependence is due to the increased generation and locking-in of nitrogen-related deep-level hole traps, which suppress the recovery of the p-MOSFET. The findings reveal important implications of the nitrogen-driven NBTI mechanism on high-frequency circuit operation.