Recent advances on electromigration in very-large-scale-integration of interconnects

Abstract

Today, the price of building a factory to produce submicron size electronic devices on 300 mm Si wafers is over billions of dollars. In processing a 300 mm Si wafer, over half of the production cost comes from fabricating the very-large-scale-integration of the interconnectmetallization. The most serious and persistent reliability problem in interconnectmetallization is electromigration. In the past 40 years, the microelectronic industry has used Al as the on-chip conductor. Due to miniaturization, however, a better conductor is needed in terms of resistance–capacitance delay, electromigration resistance, and cost of production. The industry has turned to Cu as the on-chip conductor, so the question of electromigration in Cumetallization must be examined. On the basis of what we have learned from the use of Al in devices, we review here what is current with respect to electromigration in Cu. In addition, the system of interconnects on an advanced device includes flip chip solder joints, which now tend to become weak links in the system due to, surprisingly, electromigration. In this review, we compare the electromigration in Al, Cu, and solder on the basis of the ratio of their melting point to the device operating temperature of 100 ° C . Accordingly, grain boundarydiffusion, surface diffusion, and lattice diffusion dominate, respectively, the electromigration in Al, Cu, and solder. In turn, the effects of microstructure, solute, and stress on electromigration in Al, Cu, and solder are different. The stress induced by electromigration in Cu/low-kinterconnects will be a very serious issue since the low-k dielectric (with a value of k around 2) tends to be weak mechanically. In a multilevel interconnect, a electromigration force due to current crowding, acting normal to current flow, has been proposed to explain why many electromigration induced damages occur away from the high current density region. In mean-time-to-failure analysis, the time taken to nucleate a void is found to be much longer than the growth of the void in Al and solder interconnects. This is not the case for Cuinterconnects for the nucleation of a void on a surface. On accelerated tests of electromigration in Cuinterconnects, the results gathered above 300 ° C will be misleading since the mass transport will have a large contribution of grain boundarydiffusion, which is irrelevant to electromigration failure in real devices induced by surface diffusion.