The invention relates to integrated circuit fabrication, and more particularly to a metal gate structure. An exemplary structure for a CMOS semiconductor device comprises a substrate comprising an isolation region surrounding and separating a P-active region and an N-active region; a P-metal gate electrode over the P-active region and extending over the isolation region, wherein the P-metal gate electrode comprises a P-work function metal and an oxygen-containing TiN layer between the P-work function metal and substrate; and an N-metal gate electrode over the N-active region and extending over the isolation region, wherein the N-metal gate electrode comprises an N-work function metal and a nitrogen-rich TiN layer between the N-work function metal and substrate, wherein the nitrogen-rich TiN layer connects to the oxygen-containing TiN layer over the isolation region.