Mechanical stress or atomic density distribution in passivated metal lines is induced by electromigration, and plays an important role in the mechanism of electromigration damage. Recently, a governing parameter for electromigration damage in passivated polycrystalline lines, atomic flux divergence (AFDgen*), was formulated by adding the effect of the atomic density gradient to the governing parameter for electromigration damage in unpassivated polycrystalline lines, AFDgen. The latter has already been utilized to construct a prediction method for electromigration failure in unpassivated lines, as the first step toward the development of a practical method. In this article, a prediction method for electromigration failure in a passivated polycrystalline line is proposed using AFDgen*. It is shown that the proposed method is able to predict the failure location as well as the lifetime of the passivated polycrystalline lines. Both the lifetime and the failure location in a passivated polycrystalline line are predicted by means of numerical simulation of the failure process covering the building up of atomic density distribution, void initiation, void growth, and ultimately—line failure. The usefulness of this method is verified by an experiment where two passivated straight lines with different lengths are treated.

A governing parameter, called AFDgen*, which reflects electromigration damage in a polycrystalline line covered with a passivation layer is proposed. The formulation is based on the parameter AFDgen previously introduced in our studies. With the help of AFDgen we can calculate the atomic flux divergence due to electromigration by considering two-dimensional distributions of current density and temperature and also by simply considering the microstructure of polycrystalline lines and bamboo lines. AFDgen has been identified as a governing parameter for electromigration damage in unpassivated polycrystalline lines and bamboo lines through experimental verification. As the first step in the development of a practical and universal prediction method for electromigration damage, we treated metal lines not covered with a passivation layer. On the other hand, metal lines used in packaged silicon integrated circuits are covered with passivation. Electromigration induces a mechanical stress (atomic density) gradient in such lines. This gradient plays an important role in the mechanism of electromigration damage. The new parameter proposed here, AFDgen*, includes the effect of the atomic density gradient. We develop also an AFDgen*-based method for determination of film characteristics. This method is applied to both covered and uncovered metal lines made of the same Al film. The film characteristics of both line types are obtained experimentally. Based on a discussion about the validity of the obtained characteristic constants, we were finally able to conclude that the AFDgen* parameter and the proposed method for deriving film characteristics are useful.

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With the scaling down process of microcircuits in semiconductor devices, the density of electric current in interconnecting metal lines increases, and the temperature of the device itself rises. Electromigration is a phenomenon that metallic atoms constructing the line are transported by electron wind. The damage induced by electromigration appears as the formation of voids and hillocks. The growth of voids in the metal lines ultimately results in electrical discontinuity. Our research group has attempted to identify a governing parameter for electromigration damage in metal lines, in order to clarify the electromigration failure and to contribute to circuit design. The governing parameter is formulated based on the divergence of the atomic flux by electromigration, and is denoted by AFD. The prediction method for the electromigration failure has been developed by using AFD. The AFDbased method makes it possible to predict the lifetime and failure site in universal and accurate way. In the actual devices, the metal lines used in the integrated circuit products are covered with a passivation layer, and the ends of the line are connected with large pads or vias for current input and output. Also, the microstructure of metal line distinguishes the so-called bamboo structured line from polycrystalline line depending on the size of metallic grains relative to the line width. Considering the damage mechanisms depending on such line structure, our research group has made a series of studies on the development of the prediction method. This article is dedicated to make a survey of some recent achievements for realizing a reliable circuit design against electromigration failure.