Characterization of electromigration in semiconductor device interconnects using microscopic techniques

Electromigration is an important failure mechanism which affects the functionality and lifetime of integrated circuits. In this study I used the techniques of optical microscopy, atomic force microscopy and scanning electron microscopy. From earlier studies the origin of higher gray scale values in interconnect lines observed using optical microscopy was not clear. From Atomic Force Microscopic (AFM) studies, we found higher gray scale values are due to voids. Metal pileup (hillocks) and depletion (voids) are also observed by AFM. We observed a strong correlation between brightness values using optical microscopy and height profiles (obtained by AFM) of an interconnect line. The addition of relatively small amounts of copper has been previously shown to increase device interconnect lifetimes. Through the use of a Scanning Electron Microscope (SEM) with Energy Dispersive Spectroscopic (EDS) capabilities we have measured the copper concentration as a function of position along interconnects after failure. Most of the damage was observed in the forms of voids, hillocks and extrusions at the cathode side, but not all of the damage. We found relatively higher copper content towards the anode end. We also observed peaks in copper concentration in the middle of the interconnect line which indicates a blocking of copper diffusion and creates sites for interconnect failures. Some models proposed by others have been discussed to understand the behavior of copper in electromigration. Most of our results are in good agreement with the models that discuss some sort of copper blocking mechanism. The differences between wide and narrow interconnect lines were observed. We attribute our results to the different grain structure in wide vs. narrow interconnect lines.