It was recalled that, upon passing an electrical current through metallic conductor lines, the resultant drift velocity was generally taken to decrease linearly with inverse line-length. A central parameter in this theory was the threshold length; the interconnect length at which the electromigration flux completely vanished due to a counteracting mechano-diffusion flux. Experimental evidence from drift experiments on unpassivated polycrystalline pure Al provided for a deviation, from the above theory, for the length dependence of the drift rate in near-threshold interconnects. New analytical expressions were found, for the drift velocity in the near-threshold regime, by assuming diffusional creep to be the plastic flow mode involved in electromigration-induced hillocking. The diffusivities derived from the creep viscosity were in agreement with values measured independently for Al/TiN interfacial diffusion. Microstructural evidence showed that the hillocks grew via a wedge-shaped tilt of the original line, uniformly over its width. The new expression had a marked effect upon the determination of the critical length. Extrapolation of the new parabolic dependence of drift velocity upon line length in the near-threshold regime resulted in a critical product of 670A/cm. This was well below the value obtained from the usual inverse linear extrapolation.

Plasticity of Electromigration-Induced Hillocking and its Effect on the Critical Length. J.Proost, L.Delaey, J.D’Haen, K.Maex: Journal of Applied Physics, 2002, 91[11], 9108-15