Incorporation of a small amount of a secondary metal element such as tin, indium or aluminum into copper interconnects structures has been shown to improve the device reliability and electromigration. In this study, the incorporation of tin during copper damascene plating was investigated at conditions that allow void-free filling. The study focused on the effects of the additive package used, the current density, and the feature size on the tin incorporation. The Sn concentration in Cu was found to increase with the non-metallic impurity levels. Because the plating chemistry and plating current have strong impacts on the incorporation of the non-metallic impurities, they were also found to change the Sn incorporation. For example, Cu films plated at a lower current density, which showed higher non-metallic impurity levels, were also found to incorporate more Sn, opposite to the expectation from the more negative reversible potential of Sn. In addition, the Sn incorporation is also higher in narrow lines as compared with overburden films which, correlate well to findings for non-metallic impurity levels. By combining a plating chemistry which results in high non-metallic impurity levels and a low current density, 60nm wide Cu lines with up to about 0.1% Sn were plated, a 10-fold increase from the overburden film.