Reliability of single-crystal aluminum lines and its limitation

Abstract

The quantitative electromigration (EM) lifetime estimation for single‐crystal aluminum (Al) lines has been carried out for the first time. The lifetime limiting factor that degrades the excellent endurance against EM for single‐crystal Al lines has turned out to be the temperature difference between the line and the pad due to Joule heating by high current density. No voids were observed in the middle part of the line in accelerated test patterns for 15 hours at 283 °C even at a current density of 3.5×107 A/cm2. On the other hand, hillocks and/or extrusions were always observed in the line near the positive pad and voids in the negative pad. The voids, initially grown in the line near the negative pad, migrated and accumulated in the negative pad. Al atom flux divergence due to a large temperature difference near the line‐pad junction by Joule heating as large as 100 °C was the origin of the void and hillock formation. The Al atom flux in the line was calculated from the accumulated void volume with the temperature distribution taken into account. The obtained diffusion coefficient for Al atoms were well explained from the activation energy of 1.28 eV. It has also been found that a violent electrical resistance oscillation take place when the initially grown voids in the line migrate into the negative pad. The elapsed time until this oscillation started was defined as the lifetime of the single‐crystal Al lines. The void volume fractions near the line‐pad junction that initiated void migration into the negative pad were independent of temperature. By using this void volume fraction, the lifetime was calculated at various current densities and temperatures. It has been clarified that the temperature difference between the line and the pad reduce the lifetime seriously. It is concluded that this temperature difference should be restrained as low as possible when single‐crystal Al lines are operated at a current density above 1×106 A/cm2 for the guaranteed 10 years.