Degradation and subsequent healing by electromigration in Al-1 wt % Si thin films

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Abstract
Electrical resistance of Al-1 wt % Si thin-film conductors has been measured as a function of time t, temperature, and current polarity in order to investigate both generation and recovery of (microstructural) damage caused by electromigration. The fractional change of electrical resistance ΔR/R is characterized by three distinct stages: (i) undetectable ΔR/R during an incubation period τ; (ii) linear increase of ΔR/R with t−τ; and (iii) abrupt decrease of ΔR/R when polarity is reversed, followed by gradual resumption of the previous linear increase. Examination of the conductor surface during these three stages by scanning electron microscopy reveals: (i) undetectable microstructural damage; (ii) generation of (first) holes and (then) hillocks; and (iii) recovery followed by further generation of microstructural damage. Results are interpreted by (i) generation of stress σ in grain boundaries; (ii) formation of holes when σ exceeds a critical tensile stress σ+c and hillocks when σ exceeds a critical compressive stress σ−c (‖σ+c‖ < ‖σ−c‖), and (iii) interchange of tensile and compressive stress by polarity reversal. The last stage, in fact, represents superposition of a continuation of the linear increase (degradation) of ΔR/R due to the applied current and an exponential decrease (healing) of ΔR/R, characterized by τ, due to stress relaxation. In general, damage and subsequent healing by electromigration involve a delicate balance between applied current, time, and spatial distribution of (elastic) tensile and compressive stress, (anelastic) formation of holes, and (plastic) formation of hillocks, as dictated by the concomitant microstructure.