In this study, the change in residual stress in electroplated copper thin-film interconnections during thermal cycling was investigated from a view point of their initial crystallinity. The crystallinity is mainly dominated by the seed layer material for electroplating because of the lattice mismatch between the seed layer material and copper. By applying a ruthenium seed layer, which is effective for decreasing the lattice mismatch, the crystallinity of electroplated copper thin films was improved and their stability was very high during annealing up to 200°C. In addition, the amplitude of residual stress in the interconnection formed on the ruthenium seed layer decreased drastically during thermal cycling. Therefore, it is very important to improve the crystallinity of the interconnection for assuring the high thermal stability of 3D modules.