Process dependence of AC/DC PBTI in HKMG n-MOSFETs

It is well known that n-MOSFET aging under AC and DC Positive Bias Temperature Instability (PBTI) is strongly dependent on the adopted HK stack processes. In this work it is reported, for the first time, a detailed analysis of the nature of the PBTI degradation and recovery under DC and AC conditions by a novel stress and test methodology. Our observations over two HK processes (A & B) support the PBTI physical picture of two uncorrelated independent contributions to the PBTI damage. Namely, electron trap activation in pre-existing (before stress) process induced traps as well as electron traps generation in the HK bulk oxide. It is shown that the relative role of these two components to PBTI aging is strongly process dependent and fully explains the observed DC vs. AC PBTI process sensitivity. This finding challenges the generally expected dependence of AC/DC PBTI V_t shift ratio as function of AC pulse duty cycle. The root cause of such a striking AC and DC PBTI aging differences in these two processes and its implication to technology qualification are discussed in details.