The performance of ultrafine wires in the back end of the line (BEOL) is degraded by the persistent polygranular microstructure in copper which introduces more diffusion pathways for copper atoms and which leads to faster electromigration failure times. To improve interconnect reliability, Co-containing capping layers have been used to reduce surface diffusion. Applying the same logic to other interfaces, alloying solutes have been proposed to slow down grain boundary diffusion by increasing the activation energy for atomic motion, but the mechanisms are not well understood and there are many conflicting reports as to their efficacy. One challenge for improving interconnect performance through alloying is a lack of information regarding segregation interactions at grain boundaries and interfaces when minute concentrations are introduced into the copper lattice. Historically, solute was expected to pin GBs, increase resistivity, and reduce diffusivity by GB “stuffing”. More recent studies on GB interface states called ‘complexions’ suggest a more complicated relationship, which can explain these results as well as cases where segregation increases mobility or enhance diffusion. To apply complexion analysis in technologically relevant alloy systems, we are investigating dilute copper alloys created by co-electrodeposition or nanolaminate fabrication using a microfluidic device with separate inputs for solvent and solute. Alloying copper with cobalt may offer a means for stabilizing grain boundaries against electromigration void formation in advanced interconnects. Here we present a means for co-depositing dilute copper alloys, using Co and Ag as the solutes of interest. Microstructure and compositional analysis are presented.
Initial work in the co-deposition of dilute copper alloys has yielded insights into the plating requirements, microstructure and composition of the subsequent films. Additional refinements to the process and analysis are being pursued for comparison to alloys deposited in our 300 mm processing line. Less Co diffusion is observed than expected from literature values of the diffusivity, even under fairly aggressive conditions (500°C for 5 hours). Microstructural analysis with concurrent SIMS testing is used to describe these results in terms of the recrystallization of the alloy. In particular, we discuss whether or not the presence of the alloying element influences the final microstructure of the film and provide a mechanistic explanation for these observations.