Effect of Ti insertion between Cu and TiN layers on electromigration reliability in Cu/(Ti)/TiN/Ti layered damascene interconnects

The effect of Ti insertion between Cu and TiN layers on electromigration in Cu/(Ti)/TiN/Ti layered damascene interconnects was investigated. Ti insertion enhanced the wetting property of Cu to underlayer TiN and increased the sheet resistance of the layered film when annealed. This resistance change was caused by diffusion of Ti into the Cu film and resulting in reduction in cross-sectional area of the Cu film. The insertion of thinner Ti with air-exposure before Cu deposition was found to be effective in obtaining a lower sheet resistance for the composite Cu film and for the line resistance of a Cu damascene structure. The air-exposure treatment did not influence the via resistance. The Cu damascene interconnects with Ti insertion have up to 100 times longer electromigration lifetime than those without Ti insertion. Microstructures of the Cu film such as grain size, its distribution and texture were almost the same in samples with and without Ti insertion. The improvement of interface quality between Cu and underlayer, which would have an impact on the diffusivity of Cu atoms at the interface, and the existence of Ti in Cu play an important role in improving electromigration resistance.