Electromigration in Cu thin films with Sn and Al cross strips

Electromigration in Cu thin films is studied in a cross-strip configuration. Cu lines with isolated areas of Cu(Al) or Cu(Sn) are tested between 250 and 390 °C with the following results. The hillock and void marker motion indicates that Sn moves in the direction of electron flow. The marker polarity indicates that it decreases the grain boundary electromigration of Cu, in agreement with previous studies. This study also finds evidence of active surface migration in Cu. During tests in forming gas, hillocks and voids form adjacent to a native Al2O3 layer at all temperatures, indicating the likelihood that Cu migrates faster through the Cu free surface than the interface between the surface layer of Al2O3 and Cu(Al). Active surface migration in Cu thin films is also evidenced by the growth of hillocks with highly developed facets, most of which are attached to the underlying film by narrow necks.