In Situ Study of Al2Cu Precipitate Evolution During Zlectromigration in Submicron Al Interconnects

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Abstract
We have studied the behavior of Al2Cu precipitates during electromigration in submicron runners using in situ plan view TEM. Al (0.5 wt. % Cu) runners of widths between 0.3 and 1.0 μ were stressed at temperatures between 200 and 275° C and a current density of 2×106A/cm2. A greater number of precipitates nucleate per unit length in narrower runners (0.3 and 0.5 μ wide) than in wider runners (0.8 and 1.0 μ wide), although their average size is smaller in the narrower runners. The distribution of precipitates along the length of the runners is skewed towards the anode ends. Their number decreases and the skewing of their distribution increases with increasing temperature and linewidth. Precipitates are three times more likely to be found “downstream” (with respect to the electron flow) of a polygranular region and “upstream” of a blocking grain than the reverse. After their initial nucleation and growth, precipitates either decline in size or continue to grow during electromigration stressing depending on their position in the line and on the local grain structure. Grain boundaries profoundly influence the speed and direction of growth; however, the direction of growth cannot be predicted based on grain boundary inclinations alone. Precipitates tend to stay in one place and grow (or shrink) rather than migrate, in contrast to the behavior of voids which tend to migrate at higher temperatures. When precipitate migration occurs, it is either in association with void migration or by the replacement of small Al grains. Small precipitates remain compact as voids pass over or under them.