Method of manufacturing semiconductor device having metal gates

A method of manufacturing a semiconductor device having metal gates, comprising:

• providing a substrate, wherein a first conductive type transistor and a second conductive type transistor are disposed on the substrate, and the first conductive type transistor comprises a first sacrifice gate and the second conductive type transistor comprises a second sacrifice gate;
• removing the first sacrifice gate of the first conductive type transistor to form a first trench;
• forming a first metal layer lining a sidewall and a bottom of the first trench without filling up the first trench;
• removing the second sacrifice gate of the second conductive type transistor to form a second trench;
• forming a second metal layer in the first trench and the second trench;
• and
• forming a third metal layer on the second metal layer wherein the third metal layer completely fills the first trench and the second trench.