6.7 Conclusions

Ultra-fast short-time NBTI stress and relaxation measurements from the to the seconds regime using different temperatures, stress voltages, and oxide thicknesses have been performed. A large dataset is examined here using well defined extraction parameters (, , and ). Amongst them the reference time is identified as the most crucial one. It can be seen that depending on the range used for the data extraction () the reference time is also changed. While a settled gate pulse, i.e. a small , does not contain the full degradation and relaxation data and may therefore indicate a wrong distribution of time constants, too broad limits of may produce spurious relaxation transients due to a limited resolution of smaller than . Comparing the different gate voltage criteria taken for the OTF routine yields that choosing a rather large reflects the completely different fast- measurement method best.

In the initial degradation phase, which is often explained by elastic hole trapping, the data can be well fit by a logarithmic time dependence [15, 12, 42]. As this log-dependence is considerably distorted during long-term measurements, alternatively a power-law using an exponent considerably smaller () than generally observed during long-time stress () can be used. However, the main disadvantage of the power-law is that the fit is ill defined for up to medium stress conditions. Only high temperatures and/or high show the aforementioned small .

Moreover, the extracted activation energy of about is compatible with the values typically obtained during long-time stress [106]. The temperature and voltage dependencies of stress and relaxation rule out elastic and thus temperature-independent hole tunneling as being responsible for short-time NBTI degradation as proposed by [94, 104]. A possible explanation could involve an inelastic tunneling process [98].