Semiconductor device having metal gate and manufacturing method thereof

A method of manufacturing a semiconductor device having metal gate comprising:

• providing a substrate having a first transistor and a second transistor formed thereon, the first transistor having a first gate trench formed therein, the second transistor having a second gate trench formed therein, and an opening width of the second gate trench is larger than an opening width of the first gate trench;
• forming a first work function metal layer in the first gate trench;
• forming a sacrificial masking layer in the first gate trench and the second gate trench;
• forming a patterned photoresist covering the second transistor and exposing the sacrificial masking layer in the first gate trench on the substrate;
• removing a portion of the sacrificial masking layer to expose a portion of the first work function metal layer;
• and
• removing the exposed the first work function metal layer to form a U-shaped work function metal layer.