To study the frequency dependence of the transition time, the DC stress signal of the conventional TDDS is replaced by an AC stress signal, see Figure 8.16.
The capture probability of the defect, that is the defects occupancy during the stress cycle, increases during the high cycles and decreases during the low cycles of the AC signal. From the two-state perspective, the capture probability only depends on the effective stress time , which is primarily determined by the duty cycle of the AC stress signal and the overall stress time . Thus the two-state model does not implicate any frequency dependency of the charge capture and emission times. However, by performing AC stress experiments defects have been found which show frequency dependent capture times, see Figure 8.17.
The defects A1 and A2 visible in the spectral maps do not show any frequency dependence because they have a capture time below , which is shorter than the AC stress pulse width. In contrast, the clusters for the defects A3 and A4 become fainter with increasing frequency. This can be explained with increased capture times at higher frequencies which leads to a reduced capture probability at the end of the AC stress cycle. Quite remarkably, the frequency dependence of the capture times can be nicely reproduced by the four-state NMP model, shown for defect A3 in Figure 8.18.
In general, experiments using high frequency AC signals exceeding have to be thoroughly designed as capacitive and inductive couplings may affect the measurements. Reference measurements have to be performed to guarantee the signal integrity of the configuration. Nonetheless, AC measurements using frequencies from several MHz up to GHz require special test structures. Recently, an on-chip oscilloscope has been proposed to study the frequency dependence of NBTI and PBTI of high-k transistors . Although PBTI was found to be frequency independent for , NBTI appears to be frequency dependent up to at least . The former is in agreement with the frequency dependent capture time of defect A3, see Figure 8.18, as this investigations were performed on a pMOSFET as well.« PreviousUpNext »Contents