Electromigration threshold of thin-film conductors

The connection between the mechanical properties of a thin film conductor and its electromigration threshold (jl)th, where j is the current density and l is the conductor length is analysed. It is assumed that electrodiffusion can lead to elastic strain of the conductor (electrodiffusional stresses). As is shown for aluminium, the electromigration threshold value is determined by the level of compressive stresses at which the electrodiffusional stresses intensively relax via a dislocation glide (at T ≲ 470 K) or high-temperature dislocation creep (at T ≳ 470 K). Expressions are proposed for estimation of (jl)th for thin film conductors made of various materials. It is assumed that the value of the electromigration threshold is determined by the parameter μ(Z∗ϱ), where μ is the shear modulus, Z∗ the effective charge, ϱ the resistivity of a thin film material. In descending order of (jl)th, metals may be arranged into five groups: noble metals (gold, copper, aluminium, silver); IIB group metals (cadmium, zinc); refractory metals (molybdenum, tantalum, tungsten, chromium, titanium); platinum metals (nickel, iridium, rhodium, palladium, platinum) and soft low-melting point metals (indium, tin, lead). Among d-elements (jl)th decreases with the increase of the group number in each period of the periodic table. To increase the electromigration resistance of the conductors of integrated circuit metallization, the dispersion hardened alloys based on metals with a high value of μ(Z∗ϱ), for example, aluminium, gold, silver or copper should be used.

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