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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/HfO2) including BTI. It was found that crystallization of HfO2 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 HfO2 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 500degC for the very first time.