Abstract: A new method of forming a tantalum carbide nitride diffusion barrier layer having optimized nitrogen concentration for improved thermal stability is described. A contact region is provided in a substrate. A via is opened through an insulating layer to the contact region. A tantalum carbide nitride barrier layer is deposited within the via wherein the tantalum carbide nitride layer has an optimized nitrogen content of between about 17% and 24% by atomic percentage. A layer of copper is deposited overlying the tantalum carbide nitride barrier layer to complete copper metallization in the fabrication of an integrated circuit device.