Electromigration-Induced Stress Interaction between Via and Polygranular Cluster

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Electromigration-Induced Stress Interaction between Via and Polygranular Cluster Young-Joon Park, In-Suk Choi* and Young-Chang Joo* Thin Film Technology Research Center, Korea Institute of Science and Technology, Seoul 130-650, Korea *Seoul National University, School of Materials Science and Engineering, Seoul 151-742, Korea ABSTRACT We have investigated the stress interaction between via and polygranular cluster in the pure Al line using 1-dimensional computer simulation. The conventional belief was that the fastest stress evolution at the via occurs when the polygranular cluster is just below (or above) the via. However, the electromigration induced stress at the via would be faster when a cluster is apart from via because the stress interaction between via and clusters may assist electromigration. We simulated the time that the via reaches a certain stress value as a function of the distance of the cluster. It gives a specific distance where the time was minimum (i.e the fastest stress evolution). We named the position as the Fastest Stress Enhancing Polygranular cluster Position (FaSEPP). As a function of the current density, the FaSEPP decreases. INTRODUCTION Electromigration is atomic diffusion driven by a momentum transfer from conducting electrons. With every new generation of integrated circuits, as interconnect linewidths have been reduced, electromigration from high current densities leads to concern about the interconnect reliability [1]. Electromigration-induced failures occur at the site of flux divergences. For example, void is nucleated where incoming flux is smaller than outgoing flux and hillock is formed in the other way. Accumulation or depletion due to these flux divergences generates compressive or tensile stress respectively [2]. When the maximum stress reaches a critical value, electromigration-induced damages occur. In order to make highly reliable integrated circuit, the detail understanding of the mechanism is necessary. However considering the technical difficulties in measuring electromigration-induced stress directly, computer simulations may be an alternative to understand electromigration effect inside interconnects. In this respect, several attempts have been made for computer simulations of electromigration [3-6]. There are two important sites of flux divergence, the line-ends and the cluster-ends. The line-ends are connected with the via made of tungsten or blocked by a diffusion barrier between the line-end and the via. Therefore the flux from the via to the cathode end of the line or from the

D8.11.1

Electron flow via Cathode end of the cluster

d

Anode end of the cluster

via

l Anode end of the line

Cathode end of the line

Figure 1 The schematic structure of the pure Al line used in this simulation. The total length of line was 100µm and the cluster length, l, was 20µm. The distance of cluster from the via, d, varied from 0 to 70 um. anode end of the line to the via, is zero because of materials discontinuity. Local variation of the grain structures causes another importa

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