The effects of dielectric overcoating on electromigration in aluminum interconnections

The phenomena of electromigration is self-diffusion effect, and has an associated activation energy. Although the activation energy for bulk self-diffusion in aluminum is 1.48 eV, experimentally measured energies for films range approximately from 0.5 to 1.2 eV. This lower observed energy may be or has been ascribed by investigators to crystalline imperfections in and on the surface of the aluminum film. Of the three contributions to the effective activation energy, i.e., the surface, grain boundary, and bulk component, the surface component is here investigated. In light of the requirements for LSI and multilevel integrated circuitry, i.e., narrower, thinner metallizafion implying a larger surface to bulk atom ratio, correlation is made between surface treatment and effective activation energy of the conductor. Surface treatment is accomplished through vapor plating of dielectric overcoatings of P2O5-SiO2and Al2O3-SiO2glass. A study is made of the effects of dielectric overcoating as a function of aluminum thickness. It is seen that surface treatment is effective for ahtminum less than 5000 Å thick. It is further pointed out that for aluminum thickness greater than 6000 Å, surface passivation will have no effect with respect to electromigration. It has been shown that the type of aluminum deposition technique and deposition and post-deposition heat treatment is critical in obtaining the maximum bulk component of the activation energy. The dielectric overcoating, in combination with optimal bulk properties, increases the mean time between failure of the aluminum stripes by one to two orders of magnitude at high current densities and elevated ambients, than that found in many conventional integrated circuits.