EFFECT OF POST-PATTERN ANNEALING ON THE GRAIN-STRUCTURE AND RELIABILITY OF AL-BASED INTERCONNECTS

The possibility is addressed of improving the electromigration resista nce of Al and Al-Cu thin-film conductors with ''quasi-bamboo'' structu res by post-pattern anneals that decrease the maximum polygranular seg ment length. Pure Al, Al-2Cu, and Al-2Cu-1Si lines were patterned and annealed at temperatures high enough to stimulate grain growth. Approp riate anneals led to predominantly bamboo structures with short polygr anular segments. These grain structures had a high median time to fail ure with a relatively low deviation of the time to failure. Metallogra phic analyses showed that polygranular segment length was a dominant f actor in determining the failure site. Post-pattern annealing promotes a preferential shortening of the relatively long polygranular segment s that cause early failures. However, even after annealing, failure oc curred at the longest residual polygranular segments, even when these were significantly shorter than the ''Blech length'' under the test co nditions. Statistical analysis of the failure of alloy lines revealed a simple exponential relation between the failure time and the longest polygranular segment length within. line, which is functionally ident ical to that previously found for lines tested in the as-patterned con dition.