5.4 Conclusions

In this chapter the impact of mixed NBTI/HC stress conditions on pMOSFET characteristics was studied with a focus on the recoverable component of degradation. It was found that recovery of large-area devices after different stress conditions can clearly deviate from the behavior expected from a simple electrostatic model. This behavior is strong evidence that fewer defects contribute to recovery than would have been expected. Interestingly, the study of the impact of mixed NBTI/HC stress on the behavior of single oxide defects showed that source-side defects can remain neutral after mixed NBTI/HC stress and do not contribute to recovery although they are charged after homogeneous NBTI stress. Defect characteristics like and distort and, as a result, if a defect captures a charge carrier it emits it immediately afterwards still at stress conditions. Consequently, the defect remains neutral after mixed NBTI/HC stress. This effect is determined by non-equilibrium processes triggered by hot carriers with sufficient energies and depends on the detailed defect configuration. By replacing the conventional equilibrium distribution in the charge trapping model with a thorough carrier transport treatment which also considers secondary majority carriers in the channel, recovery after different stress condition can be modeled properly.

Furthermore, recovery after homogeneous NBTI stress depends on the “history" of the device. Especially if stress and recovery cycles with mixed NBTI/HC stress are performed, recovery after homogeneous NBTI stress is considerably reduced with respect to the cumulative mixed NBTI/HC stress time. Measurements at a single defect level show that this reduction is due to changes of the defect step heights, which is most probably attributed to a distortion of the electrostatic surrounding due to the activation or deactivation of other defects in the vicinity of the observed defect. On the other hand, with a much greater impact on the reduction of recovery, some defects simply disappear from the measurements after numerous cycles of mixed NBTI/HC stress. As a result, recovery after a subsequent homogeneous NBTI stress can be reduced down to zero. Such a deactivation of oxide defects has previously been attributed to defect volatility, a repeated dis- and reappearance of oxide defects. However, a continuous reduction of recovery as well as a seemingly “permanent" deactivation of some of the defects are evidence that a permanent deactivation might play a role here.

The results in this chapter show clearly that NBTI and HCD are not completely independent degradation mechanisms. The fact that effects like II, which are typically associated with HCD, affect the recoverable component of degradation, which is typically associated with the recoverable component of NBTI, leads to the conclusion that degradation mechanisms can be coupled.