1. Basic Concepts in Reliability Design - nl3prc

[2] Designing for Reliability22), 23), 24)3.5.5 Stress MigrationWhen IC scaling reached the point where the width of the Al metal became less than 2 µm, Al openfailures due to extended exposure to a high-temperature environment were reported.This failure mechanism has been called stress migration and is considered to be caused by Al atomsmigrating to relieve stress generated in the Al metal due to the difference between the thermalexpansion coefficients of the Al metal and the passivation film of the interlayer insulator. It is knownthat this depends on the Al metal material, its deposition method and residual stress. 21)Countermeasures, such as the addition of Cu to the Al metal, the use of barrier metal under the Almetal, and reducing passivation film stress, are taken to minimize this effect.Acceleration of stress migration failure due to temperature does not occur uniformly because itdepends on a combined mechanism of void diffusion and stress relief. However, it is known that theapparent activation energy at temperatures below 125°C is 0.7 eV for Al-Si and 0.9 eV for Al-Si-Cu.2-29