Chapter 5
Pulsed BTI Measurements

In the previous chapter various BTI stress tests were performed using the measurement-stress-measurement (MSM) and the on-the-fly (OTF) technique. Special attention was given to the fitting of the measurement data onto a universal relaxation law, yielding a separation of the degradation into a recoverable and a poorly recoverable or permanent component. Data gathered at different temperatures and stress voltages were found to follow a universal relaxation law. Interestingly both stress polarities, i.e. NBTI and PBTI stress on a pMOS, always resulted in a negative shift of the threshold voltage. Unfortunately, PBTI had been rarely discussed in literature until Liu et al. monitored a positive shift of the threshold voltage due to PBTI-stressed pMOS-devices [24], which contradicts the results presented by Grasser et al. [30].

Figure 5.1: pMOSFET monitored under of NBTI stress followed by of relaxation. While is measured by the FPM (open squares), is measured by OFIT (solid circles). The fast pulsed -characteristics reveal a negative shift of for NBTI, while during PBTI a positive shift is visible (Fig. 5.2). At the end of the recovery phase the curve is scaled to match the value of . According to Liu et al. the difference between two curves (shown in the inset) yields the amount of contributing oxide traps. Data is taken from [24].

Figure 5.2: pMOSFET monitored under of PBTI stress followed by of relaxation. While is measured by the FPM (open squares), is measured by OFIT (solid circles). In contrast to Fig. 5.1 the fast pulsed -characteristics reveal a positive shift of for PBTI. At the end of the recovery phase the curve is scaled to match the value of . According to Liu et al. the difference between two curves (shown in the inset) yields the amount of contributing oxide traps. Data is taken from [24].

One reason of this discrepancy might be the fact that Grasser et al. used the OTF and the eMSM technique (cf. Chapter 2.3 and 2.1.3), while the two measurement techniques used in [24] are both based on the application of fast gate pulses: The newly developed on-the-fly fast charge pumping (OFIT) technique and the fast pulsed -characteristics have been discussed in Chapter 2.5 and 2.2.1. The measurement results obtained by those two pulsed setups are only at a first glance interpreted in a correct way, as the -curve obtained by OFIT is simply scaled to align the -curve at the end of the recovery phase in [24]. Based on this alignment scheme depicted in Fig. 5.1 and Fig. 5.2, Liu et al. stated a fast oxide trap component () corresponding to the difference of , which is shown in the insets of Fig. 5.1 and Fig. 5.2. Compared to that, the interface states are considered to recover only slowly. It was furthermore concluded that the fast oxide traps are responsible for the predominant part of -degradation in the fast pulsed -characteristics only, since their influence during a DC measurement is drastically reduced due to the measurement delay. Consequently, this makes the interface states dominate the DC regime.

When taking a closer look at the pulsed -characteristics of Fig. 5.1 and Fig. 5.2, a surprisingly huge offset of about between the reference value and the first measurement point after of stress can be detected. As this already accounts for more than of the total degradation built up after of stress, the high initial seems to be at least questionable.

In order to determine to what extent interface states and oxide charges really contribute to the measurement signal, a more detailed study of the fast pulsed and the OFIT technique, besides further measurements is needed. Especially the measurement delay of the setup in combination with its accuracy is of particular interest here.

 5.1 Pulsed -Characteristics
 5.2 Further Data Extraction Options
  5.2.1 Determination of the Fitting Region
  5.2.2 Impact of the Pulse Amplitude
  5.2.3 Varying Pulse Rise/Fall Times
  5.2.4 Consequences
 5.3 Experimental Identification of Defects
 5.4 OFIT versus CP
 5.5 Analysis of the OFIT Technique
  5.5.1 Dependence on Gate Voltage Low-Level
  5.5.2 Hysteresis due to Stress
 5.6 Extrapolation of Oxide Trap Contribution
 5.7 Simulation of the Charge Pumping Current
 5.8 Results
 5.9 Conclusion