It was recalled that one of the most important aspects of electromigration failure was the current density dependence of the failure time. Many studies had been made of widths that were greater than 1, and the consensus indicated that a current density exponent of 2 indicated a microstructure-related failure of the metallization. There was very little available data for feature sizes that were close to 0.5; especially for metallizations that consisted of Al alloys that were clad with refractory layers. Here, an investigation was made of the current density dependence of the microstructure-related failure of narrow (0.6) conductors with multi-layer TiN refractory layers. This dependence was then correlated with void formation due to electromigration. It was shown that the current density dependence of microstructural failure in such narrow conductors differed significantly from that which was observed at widths of more than 1. At a width of 0.6, an exponent which was as low as 1 was found; as compared with an exponent of 2 at a width of 1.2 for the microstructure-related failure of long multi-layer stripes. The number of voids which formed during accelerated testing, and their average nucleation rate, appeared to be independent of the current density in 0.6 stripes; contrary to the results for wider stripes. It was suggested that the growth of voids governed the failure kinetics of narrow stripes.

A.S.Oates: Applied Physics Letters, 1995, 66[12], 1475-7