A method for manufacturing a dual work function semiconductor device and the device made thereof are disclosed. In one aspect, a method includes providing a gate dielectric layer over a semiconductor substrate. The method further includes forming a metal layer over the gate dielectric layer. The method further includes forming a layer of gate filling material over the metal layer. The method further includes patterning the gate dielectric layer, the metal layer and the gate filling layer to form a first and a second gate stack. The method further includes removing the gate filling material only from the second gate stack thereby exposing the underlying metal layer. The method further includes converting the exposed metal layer into an metal oxide layer. The method further includes reforming the second gate stack with another gate filling material.