Abstract

Bias temperature instability (BTI) and hot-carrier degradation (HCD) are among the most important reliability issues, which affect the performance of metal-oxide-semicon-ductor field-effect transistors (MOSFETs). Both are typically studied in an idealized setting. In particular, for BTI studies no voltage is applied to the drain, leading to laterally homogeneous degradation. With increasing drain bias, the degradation becomes more and more inhomogeneous and the contribution of HCD to the total degradation increases. Even though it is well understood that this mixed BTI/hot-carrier (HC) degradation corresponds to what actually happens in real circuits, there is only a limited number of studies available on the impact of the mixed stress conditions.

In this thesis, the problem of mixed negative bias temperature instability (NBTI)/hot-carrier (HC) stress conditions on SiON pMOSFET characteristics is discussed. This contains on the one hand a comparison of commonly used measurement methods for the threshold voltage shift: the single point measurement of the drain current and the gate voltage. It is shown that in the case of mixed NBTI/HC stress both methods provide different results for the shift, which has to be considered when modeling the degradation. On the other hand, the focus is on the contribution of single defects to the recoverable component of the threshold voltage degradation. Quite remarkably, mixed NBTI/HC stress affects the recoverable component considerably due to two effects.

First, the contribution of oxide defects to recovery after mixed NBTI/HC stress is suppressed independently of their lateral position. As an explanation, from an electrostatic point of view, recovery after mixed NBTI/HC stress is mainly attributed to charge carrier emissions by oxide defects near the source, which have been charged during stress. However, the experimental characterization of recovery after different stress conditions clearly suggests that even defects located in the vicinity of the source can remain uncharged after mixed NBTI/HC stress and thus do not contribute to the recovery signal although they are fully charged after homogeneous NBTI stress. As a consequence, recovery of the threshold voltage shift can be negligibly small after certain stress conditions. This leads to the conclusion that a simple electrostatic model neither describes the behavior of degradation during mixed NBTI/HC stress nor the recovery afterwards properly. Only if secondary generated carriers triggered by impact ionization and the carrier distribution functions are correctly considered, agreement with experimental data is obtained.

Second, the experimental data collected during this work shows that the contribution of oxide defects to the recoverable component depends strongly on the device “history". The experimental characterization shows that mixed NBTI/HC stress anneals a considerable number of oxide defects and thus dramatically reduces recovery after all kinds of stress conditions, homogeneous NBTI and mixed NBTI/HC. In this context, volatility as a possible mechanism responsible for such a reducution is discussed.

As a conclusion, both degradation mechanisms, NBTI and HCD have an impact on each other. This impact depends on both, stress conditions which trigger physical mechanisms associated with NBTI as well as HCD and previoiusly applied stress or in other words the “history".