5.2.2 Impact of the Pulse Amplitude

Unfortunately, the different FPM-setups for NBTI and PBTI featuring different pulse amplitudes do not give comparable qualitative results for degradation and relaxation (cf. Fig. 5.7). During an FPM-pulse, always the same number of data points per is recorded to guarantee a constant resolution of pulse amplitude per time, i.e. a constant slope. However, -extraction using (5.1) strongly depends on the data range of the gate pulse. The usable range was already determined to be less than wide for NBTI stress, but is unavoidably even more limited for PBTI stress. That is because it is important to avoid any NBTI stress during a PBTI-FPM, since the interplay between NBTI and PBTI is not yet understood and therefore not distinguishable at the present day. In fact, the readout pulse during PBTI mainly covers the accumulation regime and just records the onset of inversion; it is not allowed further towards more negative bias. This yields a very limited range of down to for the -extraction during PBTI.

Unfortunately, the noisy measurement data in combination with the fitting algorithm results in an oscillating stress curve, depicted clearly in Fig. 5.10. Applying the -extraction heavily reduces this oscillation as the current criterion in the linear regime () is less sensitive to a change of the slope of the -characteristics. For NBTI, the strongly differing degradation values after as well as the differing slopes of the degradation curves for the manual fitting, the -, and the -extraction can be explained by the different mobility degradation of the corresponding . As the extraction scheme of is at least comparable for the manual and the -extraction, because both are extracted in the linear drain current regime, the corresponding degradation and relaxation is also similar.

Figure 5.10: Fast pulsed -measurements after [24, 51]. In order to prevent any effects caused by NBTI stress, the possible pulse amplitude during PBTI stress is limited. Hence fitting these pulses yields very noisy output in contrast to NBTI, whose pulses are not strictly constrained. These different limits of the usable pulse amplitude make the FPM routine less applicable to PBTI stress compared to NBTI stress.