Plasticity of electromigration-induced hillocking and its effect on the critical length

Abstract

When passing electrical current through metallic conductor lines in integrated circuits, the resulting drift velocity is generally taken to decrease linearly with the inverse line length, following the work of Blech [I. A. Blech, J. Appl. Phys. 47, 1203 (1976); erratum J. Appl. Phys. 48, 2648 (1977)]. A central parameter in Blech's theory is the treshold or critical lenght, defined as the interconnect length at which the electromigration flux completely vanishes due to a counteracting mechanodiffusion flux. We provide experimental evidence from drift experiments on unpassivated, polycrystalline pure AI for a deviation of the lenght dependence of the drift rate from the Blech equation in near-treshold interconnects. New analytical expressions for the drift velocity in the near-treshold regime, taking into account diffusional creep as the plastic flow mode involved in electromigration-induced hillocking, have been validated by the experimental data. The diffusivities derived from the creep viscovity are in agreement with values measured independently for AI/TiN interfacial diffusion. Corroborating microstructural evidence showed that the hillocks grow by a wedgeshaped tilt of the original line uniformly over its width. The new expressions have a significant impact on the determination of the critical length, extrapolation of the newly predicted parabolic dependence of the drift velocity on line length in the near-treshold regime results in a critical product 670±120 A/cm, well below the value obtained from the inverse linear extrapolation predicted by Blech.