The grain orientations at a large number of electromigration failure sites in interconnects were measured, and were compared to the orientations at points away from failure sites. Electron back-scattering diffraction analysis revealed a preferred in-plane orientation at the failure sites. The origin of a 3-fold (rather than 6-fold) symmetry which was observed in (111) pole figures for failure-site grains was considered. Ignoring the electron flow direction, the interconnect had a mirror-plane (2-fold) symmetry perpendicular to the length. When combined with the 3-fold symmetry of the lattice, 6-fold symmetry was to be expected. However, because the (111) planes which corresponded to the (111) poles at 70.5º were not exactly perpendicular to the interconnect length direction, the mirror symmetry was broken when considering the direction of electron flow and a 3-fold symmetry of the preferred orientation was to be expected. The particular 3-fold symmetry which was found revealed that there was a preference for the failure-site grains to have an orientation such that the in-plane projection of one of the (111) poles at 70.5º lay parallel, instead of anti-parallel, to the electron flow direction. This grain orientation made it easier for voids on the upper side of an interconnect to spread across the full thickness and cause failure. On the other hand, a grain orientation in which the in-plane projection of a (111) pole lay anti-parallel to the electron flow direction facilitated the spreading of voids from the lower side of an interconnect. Thus, the observed 3-fold symmetry revealed a preference for upper-side void-failure.

Crystallographic Study of Electromigration Failure Sites in Submicron Al(Cu) Interconnects. X.Chu, J.A.Prybyla, S.K.Theiss, M.A.Marcus: Applied Physics Letters, 1999, 75[24], 3790-2