The Effect of Cu Addition to Al-Si Interconnects on Stress Induced Open-Circuit Failures

Stress induced open-circuit failures in Al-Si and Al-Si-Cu interconnects are examined in detail. The failure rate increases, as a P-SiN film gets thicker, as an interconnect becomes narrower and thinner and as the mean grain size of an Al alloy becomes larger. In failure analyses, two kinds of voids in the interconnects are observed. One is the slit-like void which is a very thin crack perpendicular to the Al lines. Open failure sites are always observed at slit-like voids. The other is a wedge shaped void which is formed along a grain boundary at the edge of the interconnects. The addition of Cu as low as 0.1%, substantially depresses the open failures. However, further Cu addition causes little change in the failure rate. The failures are suppressed not by Cu precipitates but by Cu atoms dissolved in Al grains. The wedge shaped void and the slit-like void are formed mainly by grain boundary diffusion and lattice diffusion of vacancies, respectively. In Al-Si-Cu alloys, dissolved Cu atoms are bonded to vacancies and suppress the lattice diffusion, resulting in enhancement of the lifetime.