A study was made of a technique which used large interconnect arrays, together with a Wheatstone bridge. Three types of structure, with various numbers of Ti/TiN/Al(Cu)/TiN-based interconnects, were used; starting with a small unit having 5 lines in parallel. A serial arrangement permitted the testing of interconnect arrays having 480 possible failure links. A Wheatstone bridge-type wiring which used 4 large arrays in each device permitted the simultaneous testing of 1920 interconnects. Together with a statistical deconvolution, to the single interconnect level, the results indicated that the electromigration failure mechanism obeyed a perfect log-normal behavior; down to the 4σ level. A total of more than 75000 interconnects was tested at temperatures ranging from 155 to 200C. None of the samples exhibited early failure. The activation energy of the relevant electromigration mechanism (the Cu incubation time) was deduced to be 1.08eV. It was suggested that the interface diffusion of Cu along the Al(Cu) sidewalls, and along the top and bottom refractory layers (coupled with grain-boundary diffusion within the interconnects), contributed to the Cu incubation mechanism.

Statistical Analysis of Early Failures in Electromigration. M.Gall, C.Capasso, D.Jawarani, R.Hernandez, H.Kawasaki, P.S.Ho: Journal of Applied Physics, 2001, 90[2], 732-40