Material Consideration on Ta, Mo, Ru, and Os as Glue Layer for Ultra Large Scale Integration Cu Interconnects

The electromigration resistance of ultra-large scale integration (ULSI) Cu interconnects can be improved by inserting an adhesion promoter between Cu and the diffusion barrier. A metallurgical survey was accomplished to select the element having a good Cu adhesion property. For adoption as an interconnect material, it should have a low resistivity and should not react with Cu to avoid increasing the resistance of Cu interconnects. Ru, Os, Mo, W, and Ta satisfied the above conditions. The Cu adhesion property of these elements was estimated by the lattice misfit concept. The Cu adhesion property was experimentally examined and compared hcp elements (Ru and Os), which have a good matching interface with fcc Cu, with the bcc elements (Mo and Ta). Ru and Os, which had lower lattice misfit values, showed a better adhesion property than the bcc elements having higher lattice misfit values. Among these elements, Ru had the best Cu adhesion property and thus it can be an optimum glue layer element for Cu interconnects.