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We present experimental evidence of a thermally activated transformation of negative-bias-temperature-stressinduced transient hole trapping at preexisting oxide traps into more permanent trapped holes in the ultrathin oxynitride gate p-MOSFET. The transformation is also shown to correlate with the generation of stress-induced leakage current, indicating that it is one of the key mechanisms of bulk trap generation. A similar observation (reported elsewhere) applies to the HfO 2 gate p-MOSFET, implying that the observed hole-trap transformation is a common mechanism for bulk trap generation across different gate oxide technologies. The results further imply that preexisting oxide defects, usually deemed irrelevant to negative-bias temperature instability, have a definite role on long-term device parametric drifts.Index Terms-Bias-temperature instability, MOSFET, oxygen vacancy, oxynitride, stress-induced leakage current (SILC).

The evolution of the recoverable (R) component of negative-bias temperature instability (NBTI) is examined, as a function of the number of stress and relaxation cycles, for the SiON, HfSiON, and HfO 2 p-MOSFETs. At typical NBTI oxide fields (~7 MV/cm), a steady and substantial decrease of the R component in the case of the HfO 2 p-MOSFET is observed, while the R component of the SiON and HfSiON p-MOSFETs are found to remain constant. A decrease in the R component of the SiON and HfSiON p-MOSFETs is observed only at much higher oxide fields (> 10 MV/cm). Evidence shows that the decrease in the R component is due to a greater tendency for the hole traps in the HfO 2 to be transformed into a permanent form (P) under a given oxide field. The result therefore implies that, under typical NBTI oxide fields, the R and P components could share a common defect origin in the case of the HfO 2 p-MOSFET. On the other hand, the R and P components are likely to have originated from different defect precursors in the case of the SiON and HfSiON p-MOSFETs. The existence of different oxide fields at which the transformation of the R component into a permanent form occurs for different gate dielectrics implies that the nature of the defect precursors responsible for the R component is intrinsic to the gate dielectric material.

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