A candid comparison of the SWEAT technique and the conventional test procedure for electromigration study in sub-half micron ULSI interconnects

The thicknesses of the various layers forming the TiN-AlCu-TiN stack were varied and the impact on reliability was studied using the SWEAT technique (Root and Turner, Int. Reliability Physics Symp., 1985) along with the conventional electromigration procedure for 0.35 μm SWEAT structures and interconnects respectively. The SWEAT results indicated that the lifetime correlates well with the AlCu thickness, while the TiN thicknesses were relatively unimportant. The conventional test procedure indicated that the lifetime correlates well with the bottom TiN thickness or even the sum of the bottom and top TiN thicknesses, while the AlCu thickness was relatively less important. It is understood that when the AlCu layer opens up during the electromigration test, the TiN acts as a shunt to prolong conductor life and that a thicker TiN layer does this better, provided that the current density is not extremely high. The relatively high current density associated with the SWEAT procedure does not allow for this mechanism, and the lifetime is almost entirely dependent on the AlCu properties. Therefore, the conventional test procedure is good for reliability estimation, while process monitoring (especially AlCu properties) may be done using the SWEAT procedure