We investigated the effects of leveler and suppressor contents together on crystal structures of narrow 50 nm width Cu interconnects. Lowering both suppressor and leveler contents led to a bigger decrease in grain size just after plating and bigger increase in grain size after annealing than when just suppressor or leveler content was decreased. It is reported that the finer the grain size is, the larger the grain boundary energy is. We considered that grain size decrease after electroplating was due to nucleation enhancement of Cu by the decreased suppressor or leveler content, and by both decreased suppressor and leveler contents. Furthermore, by the combination of high purity electrolyte (9N) with optimized suppressor and leveler contents, the mean grain size after annealing was increased by about 25% over that of the conventional electroplating conditions, leading to realization of narrow Cu wires with low resistivity. These results were considered to be due to the fact that the very small grain boundaries with much smaller amounts of impurities, obtained by plating using 9N electrolyte with optimized additive contents, enhanced grain growth substantially during annealing. Electric resistivity under 3 mu Omega center dot cm can be expected based on the fitted experimental curve. (c) 2019 The Electrochemical Society.