When is lowered towards the stress voltage, as required in the OFIT
technique,
extracted from the rising and falling pulse edges start to deviate,
introducing a hysteresis. The hysteresis is only visible for larger pulse
amplitudes, indicating degradation (marked with
and
) due to
stress. While the impact of the oxide traps visible during medium
appears to be fully recoverable, the component causing the hysteresis is not.
This can be seen in Fig. 5.13 and Fig. 5.14, where
increases during
subsequent measurements performed on the same device. We attribute this
hysteresis to the creation of additional interface states due to NBTI stress at
[78]. Starting at
there is nearly no stress. The deeper
the device is stressed into inversion the larger the hysteresis becomes,
resulting in an increased offset for the next pulse. The total hysteresis
at a certain stress level hence not only consists of the hysteresis of the
momentary charge pumping measurement but depends on the previous
measurements3 .
As displayed in the inset in Fig. 5.13, the very first pulses are almost free of
stress (no hysteresis, ) and hence the deviation of
from
is
entirely due to oxide traps. Only a negligible amount of interface states
are created by the measurement process. The hysteresis-free area will be
discussed in more detail in the next section.
When the experiment is repeated at a lower frequency (see bottom of
Fig. 5.14), one finds that the interface state contribution can be scaled to the
reference frequency (
) [44]. This is compatible with the fact that
the stress duration is practically independent of frequency. On the other hand,
the recoverable oxide trap contribution to
depends on frequency,
consistent with the idea that the lower the frequency (corresponding
to more time per pulse) the more oxide traps can contribute to
[95].
Finally, at a low temperature, displayed at the top of Fig. 5.14, practically no hysteresis is introduced (no NBTI stress) and also the oxide trap contribution is reduced, consistent with the idea that these traps are due to a thermally activated tunneling mechanism [98] rather than elastic (and thus temperature-independent) hole tunneling [94].