Electromigration Induced Damage and Structure Change in Cr-Al/Cu and Al/Cu Interconnection Lines

The mechanisms of electromigration open failure in quartz passivated Al-Cu interconnection lines were studied by interrupting the stress test at selected intervals and photographically recording the damage. In this way a semicontinuous record of the progressive damage build up to and including open failure has been obtained for a variety of stress conditions, passivation thickness, flat lines and these going over steep topography and interconnection lines with a thin Cr underlay. It is demonstrated that the predominant mechanism that controls open failure and its variation within the confines of the above variables is the dynamic behavior of voids and their interaction with the physical and geometrical structure of the line, which act as obstacles to void motion. It is further shown that during stress the physical structure of the line i.e. precipitate particles and grain size is also coarsening which in turn provides more effective obstacles to void motion. Open failure is caused by mobile void/obstacle interactions. With no passivation on the line, failure times are significantly reduced and dynamic void behavior is no longer present i.e. voids grow in place until an open occurs. Stable underlayers such as Cr, markedly increase the lifetime by temporarily supporting the current enabling both the dynamic healing process to more effectively come into play or supporting the void until it can move downstream into the larger bond pad.