Mechanisms of Electromigration Damage in Metallic Thin Films
Interaction between conducting electrons and thermally diffusing metal atoms causes a net drift of the atoms in the direction of electron flow. In thin films, where joule heating is minimized by substrate cooling and the temperature is relatively low, this “electromigration” has been shown to be confined mainly to grain boundaries, and, in some cases, surfaces. Because of certain irregularities in the boundary network, the flux of atoms is nonuniform, leading to divergencies and localized depletion. Depletion leads to the formation of grain boundary holes by either void nucleation and growth or by accelerated grain boundary grooving. Decrease in the damage rate is achieved by control of atomic mobility in grain boundaries and surfaces. A particularly effective method of controlling mobility is to introduce a solute species which migrates to appropriate defect sites and effectively inhibits motion of host atoms. The solute, however, being in preferred diffusion paths, is susceptible to electromigration and becomes locally depleted. Restriction of solute depletion is critical for reduction of gross damage by loss of solvent.
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