A semiconductor device is provided. The semiconductor device includes a semiconductor device comprising: a first stacked field effect transistor (FET) structure in a first device area, the first stacked FET structure comprising a first pull down (PD) transistor, and a first pull up (PU) transistor disposed over the first PD transistor, a first metal gate that is shared by the first PD transistor and the first PU transistor; and an oxygen blocking layer provided on the first metal gate.