Electromigration in nanometer Al-Cu interconnect lines

Nanometer Al-Cu interconnect lines were fabricated and stress-tested to access their electromigration properties. Lifetimes up to thousands of hours under a stress current density of 5 x 10 exp 6 A/cm exp 2 at 250 deg C were observed. However, these nanometer lines appeared to suffer early failures that might have resulted from microstructural factors. Several unique electromigration features of these narrow lines were observed. Anomalous bumps formed in all groups of test lines. The bumps were morphologically different from hillocks observed in other electromigration testing of larger dimensions. These bumps were expansions of line segments, and had spheroidal shapes with smooth surfaces and larger diameters than the lines. Narrowing of segments in the lines was also observed. Introduction of triangular transition segments that make a gradual width change from contact patterns to the narrow lines helped to increase the reliability of the pad-to-line transitions.