So far the recoverable component of BTI has been successfully explained using the four-state NMP model. For simplicity, the DW model had been used to describe . Although the latter explains the limited amount of experimental data available at that time, it does not rely on a correct physical mechanism. In order to provide a accurate description of the permanent threshold voltage shift the HR model has been introduced. The HR model relies on chemical reactions between H atoms with defects and hydrogen transport through the oxide. These mechanisms are supported by DFT calculations.
At the beginning is studied on a large-area pMOSFET subjected to zero volt at all four terminals of the transistor, . Subsequently, sweeps are recorded and extracted. Quite remarkably, a drift of was found over several days, see Figure 13.7.
At higher temperatures gets more pronounced, a consequence of newly created defects. Note, during fabrication a forming gas anneal process step with a duration of typical at is performed. Compared to the time scale of defects determining P, several days up to months, see Figure 13.7, the annealing step is an order of magnitude smaller. Furthermore, earlier publications assumed that simply to be baked “away” at higher . This notion has to be carefully reconsidered, since also at zero bias a significant amount of is accumulated, albeit on a very large time scale.
Figure 13.8 shows the characteristics of extracted from our first long exploratory time experiment.
A single large-area pMOSFET has been probed for sixty days. The stress bias was switched between and while the device temperature was changed to , and . After each stress cycle the temperature is switched back to where the sweeps are recorded. From each voltage sweep cycle a new is obtained. Figure 13.8 clearly shows a large increase in during the stress phases with . In contrast, when is applied, changes only slightly. The characteristics of recorded over several month can be nicely explained by the HR model.
A second long-term experiment has been performed on a virgin pMOSFET with dimensions . This time the stress bias of is applied during cycles. Again, is extracted from 20 sweeps performed at . Figure 13.9 shows the characteristics of recorded from measurements during 90 days.
Quite remarkably, although the device is heavily stressed during phase D, the permanent component decreases. This degradation reversal behavior is due to defects with an energy level close to the valence band edge which are annealed in phase B and can not be recharged again during phase D. Except for the degradation reversal, which requires closer inspection, the HR model again is able to explain the experimental data very well.« PreviousUpNext »Contents