The impacts of interfacial layer (IFL) thickness and crystallinity of HfO2/IFL bi-layer on electrical properties were clarified using synchrotron radiation photoemission spectroscopy (SRPES) and electrical measurements of nFETs (HfSix/HfO2) and pFETs (Ru/HfO 2) including BTI. It was found that crystallization of HfO 2 causes significant degradation in electron mobility and PBTI, whereas the impacts on hole mobility and NBTI are negligible. The SRPES measurement revealed that the crystallization temperature depends on HfO 2 thickness. We also found that the IFL thickness is the dominant factor for both electron mobility and PBTI. Therefore, a careful optimization of the HfO2/IFL bi-layer is indispensable. We proposed a novel technique for controlling the bi-layer thickness and demonstrated dual metal CMOS devices with high mobility and high reliability even by a post high-k process lower than 500°C for the very first time.