Crystallographic Study of Electromigration Failure Sites In Submicron Al(Cu) Interconnects

Crystallographic grain orientations at a large number of electromigration failures sites in Al(Cu) interconnects have been measured and compared to the orientations away from failure sites and in witness lines (those in which no current passed). For the first time a preferred orientation is identified for the failure-site grains. A unique aspect of this experiment was the use of a very large sample set of 750 runners. The samples consisted of 50-line testers at three different linewidths (0.8, 0.5, and 0.3 microns), with three different Cu concentrations (0, 0.5, and 0.9 wt%Cu), and two different initial Cu distributions (quenched into solution and precipitated). All samples had been electromigration stressed to failure at a temperature of 225C and a current density of 2MA/cm sup 2. Electron Back Scattered Diffraction (EBSD) was used to determine the crystallographic orientation of the grains adjacent to failure sites. Statistical analysis indicates a preferred in-plane azimuthal orientation of these grains in addition to the near-111 texture. The failure site grains are aligned such that the projection of one of the {111} crystallographic directions lies along the interconnect length direction. These results are significant in that crystallographic orientation has long been speculated to be important in determining the location and dynamics of electromigration failure. In fact, our results are broadly consistent with a previous proposal that due to a severely low diffusivity, a (111) plane (near-)perpendicular to an interconnect may prevent a void from migrating past it, causing the void instead to spread across the linewidth and thickness (i.e., across the (111) plane) resulting in failure[Chu, Bauer, and Mullins, APL, 70, 194 (1997)]. Finally, the dependence of our results on linewidth, Cu-concentration, and initial Cu distribution will be presented.