Binary Cu-alloy layers for Cu-interconnections reliability improvement

This paper discusses applications of thin dilute Cu-alloy films in Cu-interconnections to reduce Cu electromigration (EM) and improve physical-vapor-deposited (PVD) Cu seed integrity. PVD CuSn, CuIn and CuZr films are studied with in-film concentration of Sn, In and Zr from 0.3 to 1.2 at.%. With either of the alloy films, significant reduction of thin Cu film agglomeration on the TaN barrier is observed, suggesting suppression of Cu mobility along the Cu/barrier interface, which thus reduces EM. After annealing, the dopant redistribution rate is found in the order of Sn>In>Zr. The bulk resistivity differences between as-deposited Cu-alloy and pure Cu films are 3.6, 1.1 and 18 μΩ-cm for CuSn, CuIn and CuZr, respectively (normalized to 1 at.%). The resistance decreases 30-50% after annealing due to the dopant rejection from the grains. CuSn (0.3 at.%) is discussed as a specific example to correlate film properties with wafer-level electrical and EM results. When annealed at 350°C, Cu-interconnect lines with 500 Å (nominal thickness) CuSn films have similar metal line resistance to that of pure Cu. EM mean-time-to-failure (MTTF) improves with increasing CuSn film thickness. A 1 kÅ CuSn film improves EM by a factor of two.