Paper EM-WeA1
Interdiffusion Characterization of Selective Chemical Vapor Deposition Cobalt Cap and Copper

Wednesday, October 21, 2015, 2:20 pm, Room 210E

Electromigration (EM) failure is always one of the key challenges of BEOL Cu interconnects. The continuous shrink of Cu feature size leads to higher current densities, which lower EM lifetimes. The interface between Copper and dielectric cap has been identified as the key diffusion path for copper atoms, and the adhesion of the interface is critical to EM performance. Different methods developed to improve the adhesion between copper and dielectric cap interface. One method of improving the adhesion has been the use of alloy seed, CuMn or CuAl. Mn (Al) will segregate and bond chemically to the copper and dielectric cap interface during dielectric cap deposition which can improve the adhesion of the interface and suppress Cu atoms migration along the interface under current stress. An alternate method of improving the adhesion is to have a self-aligned CoWP Cap or Selective CVD Co Cap on top of Cu which was developed for more stringent EM requirement of advanced nodes, such as 32nm, 20nm and beyond. Compare to self-aligned CoWP plating process, selective CVD Co Cap has higher selectivity and more compatible with porous ultra low k film. In this paper, we focus on interdiffusion characterization of selective CVD Co cap and copper. Co/Cu interdiffusion of cobalt films with different precursors are thoroughly evaluated and compared. Nitrogen content in Co film which enriches the grain boundaries due to low solubility in Co was identified as the key knob to control Co/Cu interdiffusion. Less nitrogen in Co film results in more copper diffusion. Better EM results on 20nm groundrule test structures demonstrated with higher nitrogen in Co film. Thermal anneal with H2 gas was found to be able to reduce nitrogen content in Co film which result in more Cu diffusion. Secondary Ion Mass Spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS) and Electron Energy Loss Spectroscopy (EELS)/ Energy Dispersive X-ray Analysis (EDX) line scan were used for Cu diffusion characterization which clearly show Cu signal on top of Co film. A novel analytical technique of SIMS has been developed to characterize nitrogen content in Co film that CoN was selected as nitrogen detection molecular since nitrogen has some interaction with Co within the film.