Electromigration Failure in A1 Thin Films under Constant and Reversed DC Powering

In order to understand better the mechanism of failure due to electromigration in thin metallic films, life tests have been carried out under constant and reversed DC current, by following the changes of specimen resistance till failure is reached. The inversion of the current is chosen at times which are a decreasing fraction of the specimen life in DC. While under DC current, the resistance of the specimen increases continuously up to failure; when the current is reversed, the resistance decreases to some extent in the first period after inversion, and then increases again, thus showing the partial reversibility of the effect. A large increase in life is observed on specimens in which the current is alternatingly inverted. A model of electromigration damage due mainly to vacancy condensation is presented, in which the behavior of resistance change is derived from the kinetics of void formation. This model is in very good agreement with the experimental results. However, the partial reversibility of the effect corresponding to the small decrease in resistance upon current inversion, suggests that some change also occurs in the shape of the specimen due to mass transport and polarization associated with the impurities present at some internal sites, such as grain boundaries.