A high-resolution  in situ  scanning electron microscopic study was made of drift velocity test structures which were used to simulate a Cu-line/W-via structure in multi-level interconnects. Electromigration comparison tests were carried out at temperatures ranging from 250 to 450C, using current densities of 5 x 105 to 2.1 x 106A/cm2. Under these conditions, the measured electromigration activation energies for Cu, Cu-0.5%Sn, Cu-1.0%Sn and Cu-2wt%Sn alloys were 0.73, 0.95, 1.25 and 1.14eV, respectively. The measured critical lengths for the Cu and Cu-Sn alloys were of the order of 2.5 at a current density of 2.1 x 106A/cm2. The observed average drift velocity of Cu mass transport in Cu-Sn alloys changed with depletion of the Sn atoms, which were also found to move in the direction of the electron current. Eventually, the Cu mass transport rate reached a value that was comparable to that in pure Cu test stripes. The measured resistivities of the Cu-0.5%Sn and Cu-1wt%Sn Van der Pauw test structures were 2.4 and 2.9 cm, respectively. When compared with Cu and Al-Cu interconnects, the Cu-Sn alloys exhibited a higher electromigration activation energy, a good resistance to hillock and void formation and a comparable resistivity. It was concluded that Cu-Sn alloys were potentially good candidates for interconnect applications where a high current density and a good electromigration resistance were required.

K.L.Lee, C.K.Hu, K.N.Tu: Journal of Applied Physics, 1995, 78[7], 4428-37