Fe contamination has always been one of the most critical issues in the integrated circuit (IC) industry due to its catastrophic effect on device reliability and electrical characteristics. With complementary metal oxide semiconductor (CMOS) technology scaling down, this issue has been attracting more attention. In this paper, the impact of Fe impurity on the reliability of gate oxide integrity (GOI) in advanced CMOS technology is investigated. Intentional contamination of polysilicon gates was conducted in both boron- and phosphorus-doped devices. Failure analysis of the gate oxide was conducted with high-resolution transmission electron microscopy (HRTEM) and the energy dispersive X-ray (EDX) technique. The experimental results disclose that the properties of PMOS are much more sensitive to Fe contamination than those of NMOS. It is suggested that the reason for the above phenomena is that Fe precipitates at the PMOS gate/oxide interface but dissolves uniformly in the NMOS poly gate due to lower formation energy of the FeB pair (0.65 eV) in PMOS than that of the P4-Fe cluster (3.2 eV) in NMOS.