Electromigration failure of contacts and vias in sub-micron integrated circuit metallizations

Abstract

Electromigration failure of contacts to silicon and intermetal contacts (vias) in metallizations used in submicron IC technologies occurs by the formation of voids in the Al alloy layer. As feature sizes are reduced the amount of Al that needs to be removed from a contact/via to cause failure will decrease, so that the reliability of these structures is of primary importance for future metallization development. Accurate estimation of contact/via reliability requires an understanding of the physical processes governing failure. We have carried out an investigation of voiding failure of contact/vias with a variety of metallizations. In this paper, it is shown that voiding failure is identical in both contacts and vias and is independent of the structure (i.e. with or without W - plugs). Void formation is dominated by the presence of interfaces between dissimilar materials (e.g. Al W , Al TiN ) and is relatively insensitive to the conductor microstructure. At constant current, the failure times of contacts decrease rapidly with decreasing feature size. Failure kinetics of contacts and vias are consistent with a void formation mechanism that is controlled by the growth of voids in the Al layer. The rate of void formation is determined by the drift velocity of the Al layer, and the contact geometry. The void growth model of failure can be used to understand the effects of contact structure (e.g. W - plug versus sputtered Al), and of feature size reduction on reliability. An accurate description of the failure kinetics is possible only with a consideration of mechanical stress generation by electromigration, and its effect on the drift velocity. Consequently a current density — length product for electromigration failure exists. For a given conductor length there is a current density below which failure will not occur. In general, the failure time, tf, exhibits a current density dependence of the form tf ∝ 1 (j − jc) , where jc is the critical current density. Above jc, failure times vary inversely with the current density in the structure, tf ∝ 1 j . This dependence differs from the inverse square dependence, tf ∝ 1 j2 , commonly observed for failure of long stripes due to void formation at random locations associated with the conductor microstructure. We discuss contact failure distributions and show that the dispersion of failure times varies with temperature and current density.