ROLES OF TI-INTERMETALLIC COMPOUND LAYERS ON THE ELECTROMIGRATION RESISTANCE OF AL-CU INTERCONNECTING STRIPES

Various suggestions for the origin of the beneficial effect of TiAl3 layers on the electromigration resistance of Al-Cu interconnect stripes have been evaluated experimentally. The leading candidate for this effect, the shunting of current through the intermetallic compound layer that spans opens (voids) in the Al-Cu layer, cannot account for the correlation found in the present investigation between the median time to failure and the median rate of increase of electrical resistance in the Al-Cu layer of different multilayer interconnect configurations. When the TiAl3 layer lies above the Al-Cu conducting layer, the major enhancement of median lifetime to failure is an effect the TiAl3 layer has on the "electromigration resistance" of the Al-Cu layer. This effect arises from the overlayer acting to decrease the rate of mass transport in the Al-Cu layer rather than from any effect on the grain size or texture of the Al-Cu layer. It is believed that the TiAl3 overlayer acts to produce triaxiality in the local stress fields developed at mass sinks and sources. This triaxiality of the stress field should act to increase the local yield strengths. A model analysis shows that the yield strengths required to reduce the mass flux by the observed amount are reasonable for a near triaxial state of stress. Only when the TiAl3 layer is below the Al-Cu layer is it possible that shunting is the cause of the enhancement in the median time to failure relative to that for a single Al-Cu interconnect layer.