Local Melting during Electromigration in Cu Conductor Lines

We have observed abrupt, reversible resistance changes during electromigration (EM) testing of damascene Cu conductor lines. The tests were conducted at temperatures of 300°C or 350°C and with current densities from 0.6 × 106 A/cm2 to 1.6 × 106 A/cm2. In most cases, an incubation period with negligible resistance increase was followed by a period of continuous, gradual resistance increase, attributed to formation and growth of voids in the conductor line. If the direction of current was reversed, the resistances of the Cu lines decreased, due to the refilling of voids by the back flowing atoms. With further EM, the resistance curves showed spikelike features, with a sudden resistance increase followed by a resistance decrease, often to values close to those before the start of EM. In other cases, no resistance decrease occurred, and the line failed. We present resistance data, microstructural observations, and thermal calculations that suggest that the resistance decrease results from sudden, local Joule heating and melting of conductor line segments, and from voids being partially filled by the back-flowing liquid Cu, which then solidifies. In some cases, line failure results from liquid Cu erupting through the top surface passivation layer, rather than flowing back to the voids.