Microstructural Aspects of Interconnect Failure

The range of microstructural effects on thin film and interconnect properties is briefly described, and the improvement of interconnect reliability with increased strength is reviewed. We show that the strengthening effect of dispersed second phases depends on their resistance to coarsening during thermal treatments. The rapid coarsening of theta phases during annealing and accelerated electromigration testing is reviewed, leading to a discussion of metallurgical factors which determine the coarsening behavior. We describe alloy systems expected to have reduced coarsening rates. We suggest that the recently reported increased reliability of Al-Sc interconnects is due to finely dispersed coherent phases which are particularly resistant to coarsening. The range of electromigration failure morphologies is illustrated with particular emphasis on transgranular slit failures. The failures are discussed in terms of diffusion pathways and models for failure.