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. Electromigration comparison tests were carried out at 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-0.5%Sn, Cu-1.0%Sn and Cu-2wt%Sn alloys were 0.95, 1.25 and 1.14eV, respectively. The measured critical lengths for the 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.

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