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

摘要

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 threshold or critical length, 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 Al for a deviation of the length dependence of the drift rate from the Blech equation in near-threshold interconnects. New analytical expressions for the drift velocity in the near-threshold 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 viscosity are in agreement with values measured independently for Al/TiN interfacial diffusion. Corroborating microstructural evidence showed that the hillocks grow by a wedge-shaped 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-threshold regime results in a critical product of 670±120 A/cm, well below the value obtained from the inverse linear extrapolation predicted by Blech.