Abstract: The present disclosure provides a method of manufacturing a semiconductor device. The method includes: forming a transistor region in a substrate; forming a gate dielectric layer over the transistor region; forming a diffusion-blocking layer over the gate dielectric layer; forming a first portion of a work function layer over the diffusion-blocking layer; forming a second portion of the work function layer over the first portion of the work function layer; forming a plurality of barrier elements on or under a top surface of the second portion of the work function layer; and forming a gate electrode over the work function layer, wherein the plurality of barrier elements block oxygen from diffusing into the work function layer during the formation of the gate electrode.