The electromigration damage in polycrystalline AlSiCu lines caused by dc current densities of the order of magnitude 10~(10) Am~(-2) at 500 K has been investigated by long-time high-resolution ac noise measurements over a wide range of temperatures. A multiple-probe setup allowed us to monitor the electrical resistance increase along the line and to show that sudden enhancements of the noise power and the resistance occurred in the same line segment. The peak in the distribution function N(E) of the activation energies E for the defect motion responsible for l/f noise was found to shift from E_0=0.7 eV with Gaussian width #DELTA#E=0.1 eV in the undamaged samples to E_o=1.0eV and #DELTA#E=0.2 eV in the damaged samples. The latter values are compatilbe with the Cu diffusivity along intermediate-misfit grain boundaries in Al, indicating that the damage process depends not only on the composition but also on the microstructure of the lines. It is proposed that electromigration causes Cu atoms to drift towards and get trapped in grain boundaries, that their thermally activated motion in the boundaries contributes substantially to the l/f noise, and that eventually the Cu atoms assemble on the anode side of the sample. The dependence of the diffusivity on the grain-boundary mismatch may cause flux divergencies at boundary junctions and thus lead to void formation.
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