Ultrathin TaSiC amorphous films prepared by magnetron cosputtering using TaSi2 and C targets on Si(100), in a sandwiched scheme Si(100)/TaSiC(5 nm)/Cu, were evaluated for barrier performance in copper metallization. Optimizing carbon content maximizes thermal stability of the films as depicted by sheet-resistance, x-ray diffraction, and transmission electron microscopy examination. The stability temperatures of 700 °C (24 at. % C) and 750 °C (34 at. % C) have been systematically verified and discussed. Since Ta, Si, and C are compatible with integrated circuit (IC) processing, the TaSiC films are readily applicable for sub-65-nm IC production.