This paper provides an overview on the current understanding of electromigration in metals. The discussion is first focused on studies in bulk metals and alloys. This part includes a thermodynamic formulation of electromigration, a kinetic analysis of the atomic processes and a review of the theory. In addition, experimental results in interstitial and substitutional systems are summarised. The second part of the paper reviews the studies in metallic thin films. This emerged as an important area of electromigration studies since the late 1960s when electromigration damage was found to cause failure of conductor lines in integrated circuits. The discussion will review first the basic nature of electromigration in thin films with emphasis on the role of grain boundaries in mass transport and damage formation. Then the materials issues of electromigration will be explored according to the scaling trends in VLSI technology. Finally, the recent results of electromigration in fine lines and device contacts of dimensions in the micrometre range are discussed.

https://iopscience.iop.org/article/10.1088/0034-4885/52/3/002/pdf

Electromigration in metals

Electromigration studies of large‐ and fine‐grain aluminum films showed that fine‐grain films were characterized by a mixed orientation and a grain size of ≈2 μ, and that large‐grain films had a highly preferred 98% 〈111〉 orientation and a grain size of ≈8 μ. Stripes fabricated from the large‐grained films had a consistently longer life, higher activation energy, and lower standard deviation of the life distribution than stripes fabricated from fine‐grained films. These results are interpreted in terms of the nature of flux divergences in the two films. Calculations give the magnitude of vacancy supersaturations at various structural divergences present in the films studied.

Electromigration Damage in Aluminum Film Conductors

Grain boundary self-diffusion in Cu polycrystals of different purity

The important integrated circuit failure mechanism of electromigration is reviewed. The basic physics of failure is introduced and the concepts of electromigration and thermally induced stresses are discussed. A concept of thermodynamics in a conducting material and the response to non-direct current will be presented.

Electromigration in integrated circuit conductors

One of the modes of electromigration damage is shown to be related to irregularities in the grain‐boundary structure of films. Based on the restriction of electromigration to grain boundaries, calculations are performed to determine the magnitude and distribution of excess vacancy concentrations at sites where electromigration flux divergencies may be present. The attainment of an equilibrium vacancy supersaturation (Δc/c0) is found to be practically instantaneous. The maximum supersaturations are less than 1 under all conditions used. This is found to be inadequate for void formation by vacancy condensation unless heterogeneous nucleating sites are present (e. g., oxide particles, boundary‐surface oxide intersection). The rate of observable hole formation is accounted for by either void growth at a grain‐boundary triple‐point junction or by grain‐boundary grooving, each being related to the electromigration flux. Grooving seems most likely in pure films with all boundaries having similar character (e. g....

Void Formation and Growth During Electromigration in Thin Films

Resistance monitoring has been used to follow structural changes during electromigration in aluminum films. Two stages of migration were found, the first corresponding to gross mass transport, the second to void growth and stripe failure. An activation energy for the first stage was determined to be 0.5–0.6 eV from change‐in‐rate, change‐in‐temperature tests, indicating boundary diffusion. Transmission electron microscopy showed voids existing in areas of thinned aluminum. Corollary work on aluminum stripes on NaCl substrates showed nonadhesion to be a strong contributor to void formation, suggesting the possibility that the thinned regions were caused by hot spots at sites of nonadhesion.

L. Berenbaum

Papers

Resistance monitoring and effects of nonadhesion during electromigration in aluminum films