As ULSI dimensions continue to shrink, the production requirements become more stringent. For the 0.25um process and its sequential generations, metal line width approaches 0.3um or less and the pitch is less than 0.35um. Therefore, it is a challenge to etch the metal lines. Yield loss analysis of 0.25um and 0.18um products showed only TiN residues. These were from the barrier layer of the TiN/Al(Cu)/TiN layered metal structure, and caused an electrical short and resulted in a low yield. The study focuses on the diagnosis of failure mechanism based on the SEM views of the sites having electrical shorts. Based on the topography of the leakage sites, we propose that excess Cu migrated to the bottom of the Al(Cu)/TiN interface. Large ..-Al2Cu precipitates grew and obstructed the general etching process, and formed TiN residues. In addition the..-precipitates were lifted off in the subsequent PR wet stripping process. Meanwhile, Al voids were also formed at the side wall of metal lines neighboring the ..-precipitates during the processes. We examined the short/open-yield-structure (a pattern of Wafer Accept Test) after metal patterning but before PR wet stripping. The etching residue was detected by SEM and analyzed with EDS. The residue contained Cu and was most likely the well-known theta phase Al2Cu. An optimized recipe for interconnections formation was devised and thermal treatments were implemented following the metal depositing process to improve the yield. With SEM observation, we revealed the failure mechanism of Al (Cu) interconnects with spacing failure induced by..-Al2Cu precipitates and significantly contributed to the sequential correction actions.