6.2.3 Gate Voltage Criteria

In this section it is demonstrated that, in general, fast NBTI measurements have to be taken with a grain of salt. This is largely due to difficulties with synchronization between the stimulus and the actual measurement. So even when the experiment is free of systematic synchronization errors, i.e. switching of the gate voltage and recording of I
 D   start at the same time, the finite settling time of real signals makes ex-post time zero adjustments necessary. Hence, the time evolution of the actual waveform has to be checked carefully [18]. It turned out that the pulse length is around 0.3%  longer than originally set by the pulse generator. This factor has to be accounted for and the real stress times tstr   of the sequences need to be extracted using the applied gate pulse. As shown in Fig. 6.7 the pulse is affected by the transient behavior and a possible overshoot due to the non-instantaneous switching between V
 G,rel   , which is applied in-between the pulses, and V
 G,str   . Therefore, after the transition regime, a steady state value of VG,str   is determined and set as VG,strref   (usually taken at tstr∕2  ). Then an error criterion, i.e. |(VG,str − VG,strref)∕VG,strref| ≤ ϵ  is employed. Since noise is apparent in all three sequences, ϵ  has to be chosen large enough to not disrupt the pulse, usually in the range of ϵ ≈ 0.3%  . Starting at tstr∕2  and moving as well to lower (to the beginning of the pulse) and higher (to the end of the pulse) times sets new borders of our accepted stress time tstr   .


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Figure 6.7: The main graph is enlarged to make the transient and the overshoot of different stress pulses visible which are shown in the inset. This is due to the limited switching speed of the pulse generator when moving from VG,rel   to VG,str   and back. The employed error criterion |(VG,str − V   ref)∕V    ref| ≤ ϵ
          G,str     G,str  is displayed for ϵ = 0.3%  . The first (last) proper values of the pulse for each sequence are marked by circles (squares). The noise is apparent in all three sequences and limits ϵ  to extremely small values.



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Figure 6.8: The extracted change in ID   for different values of ϵ  of the relative and absolute truncation criterion is depicted. The logarithmic dependence for longer relaxation times is indicated, too.



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Figure 6.9: The main graph is enlarged to make the transient and the overshoot visible. The bounds due to both ‘ϵabs   -criterion’ and the ‘ϵrel   -criterion’ are displayed for ϵ = 0.3%  , with the first points of the relaxation pulse (after tstr = 1ms  ) marked by circles.


The treatment of the relaxation phase is more complex. It is argued that the noise level is the same during stress and relaxation (the DSO continuously records, using the same settings), and the settling time of the pulse generator in theory is equal regardless if switching from VG,rel   to VG,str   or vice versa occurred. The criterion for the relaxation phase could then be established as ‘all points extending to both sides of t = 2tstr   that fulfill |(VG − VG,relref)∕VG,strref| ≤ ϵ  ’. This effectively uses the same absolute allowed deviation from VG,relref   as was used during determination of the stress phase, hence this method will be referred to as the ‘ϵabs   -criterion’. On the other hand, the relative error in ID   (and hence in ΔVTH   ) that would erroneously be attributed to NBTI is given by the relative deviation of VG   , asking for a criterion |(VG − V    ref)∕V   ref| ≤ ϵ
       G,rel     G,rel  . This method, which is tighter by a factor of |V    ∕V    | ≈ 7
  G,str  G,rel  , is referred to as the ‘ϵ
 rel   -criterion’. Both methods were investigated thoroughly, and the relative method was chosen.