Relationship between interfacial adhesion and electromigration in Cu metallization

Abstract

Electromigration in Cu conductors differs from that in Al alloys. In Cu, it is almost exclusively via interfacial or surface diffusion whereas for Al alloys, diffusion at the temperatures of interest is primarily through the grain boundaries. It has been argued interfaces are such rapid pathways because of the poor adhesion between Cu metal and the adjacent materials from which the interface is composed. Accordingly, this study focuses on relating the electromigration induced drift velocity and the adhesion to Cu of upper surface cap materials. In addition, traditional electromigration lifetime studies were performed to correlate the results of the drift velocity measurements to device reliability. The results indicate that the adhesion of the Cu/cap interface can be directly correlated to the electromigration lifetime of copper conductors. Samples were prepared by standard dual damascene proc-sses with electrodeposited Cu and SiLK Semiconductor Dielectric, Trademark of the Dow Chemical Company. The liner material (Ta) wa!i the same in all cases. The cap layer above the metal lines was varied. Samples were prepared with caps composed of Silicon Nitride (SIN), BloK'" (a nitrided form of Sic or NSiC), Sic and an electrolessly deposited film of CoWP. (BLoK is a trademark of Applied Materials Corp.) For the drift velocity measirements, the manufacturing process was intempted after only one level of dielectric was applied so that in-situ SEM observations could be made. Two additional drift velocity samples were made to test the importance of surface preparation. IOne was a control that was identical to an already deposited BloK'" sample and the second had the Cu clean step removed before BloK'" deposition to ensure a contaminated interface. The samples prepared for electromigration lifetime experiments were covered with silicon dioxide to preclude damage due to oxidation during testing perfonned in air. Blanket metal strips of identical composition and thermal history were prepared for adhesion studies.