The effect of current direction on the electromigration in short-lines with reservoirs

We have conducted electromigration tests on Cu/low-k standard short-line structures as well as those with varying numbers of reservoirs. We found that the presence and number of reservoirs as well as current direction relative to the reservoir led to markedly different electromigration performance in terms of lifetime and distribution shape. That is, in the case of a reservoir by the anode (or electron sink) line-end, lifetimes were similar or slightly better than the In the case of a reservoir by the cathode (or electron source) line-end, lifetimes were extremely longer than the long-line case or even immortal. Further, the distribution of the cathode reservoir structure exhibited a roll-over shape, in which the first fails corresponded to small void sizes and the later fails corresponded to larger void sizes. We believe these observations are related to the differences in stress development during electromigration, which will be constrained at the line-end at which the reservoir is present, and has been verified through simulation. Finally, we propose a method for assigning an equivalent length to these complex short-line configurations which indicates a ldquostrengthrdquo of the short-line benefit which would be equivalent to that of a standard short-line.