Integrated circuit having plural transistors with work function metal gate structures

An integrated circuit having plural transistors with different threshold voltages, comprising:

• a substrate;
• a first transistor with a first metal gate disposed on the substrate, wherein the first metal gate comprises a first bottom barrier layer, a first work function metal (WFM) layer and a first metal layer;
• a second transistor with a second metal gate disposed on the substrate, wherein the second metal gate comprises a second bottom barrier layer, a second WFM layer and a second metal layer;
• and
• a third transistor with a third metal gate disposed on the substrate, wherein the third metal gate comprises a third bottom barrier layer, a third WFM layer and a third metal layer, wherein the first transistor, the second transistor and the third transistor have the same conductive type, and a nitrogen concentration of the first bottom barrier layer>a nitrogen concentration of the second bottom barrier layer>a nitrogen concentration of the third bottom barrier layer.