Fully in-situ formation of Hf-based metal-oxide-nitride-oxide-silicon (MONOS) structures utilizing electron-cyclotron-resonance (ECR) plasma sputtering was investigated for the first time. It was found that the MONOS structures with in-situ formed HfN_0.5 gate electrode showed faster programing, high injection efficiency and larger flat-band voltage (V_FB) shift of 2.5 V compared to the MONOS structure with ex-situ formed Al gate electrode under programing voltage of 10 V. The retention characteristics of MONOS structure with in-situ formed HfN_0.5 gate electrode were also superior to the MONOS structure with ex-situ formed Al gate electrode. The degradation of retention for V_FB shift after 60 min of programing was 40% for the MONOS structure with ex-situ formed Al gate electrode, while it was 16% for the MONOS structure with in-situ formed HfN gate electrode.