Through silicon via (TSV) is one of the most important technologies in 3-dimensional wafer/chip stacking. However, there are some issues related to defect-free TSV filling and Cu pumping. In this study, a defect-free TSV filling was achieved using iodide ions. The working mechanism of iodide ions in TSV filling was systemically investigated by electrochemical measurements; It was found that the formation of CuI on the electrode surface is a key process for inhibiting Cu ion reduction. This inhibition effect of iodide ions enables defect-free TSV filling. Furthermore, the study of microstructure of the filled Cu revealed that the electrochemical reduction of CuI during the TSV filling forms small Cu grains in TSV. The small Cu grains cause Cu pumping in subsequent processes, which damages semiconductor devices. We achieved a defect-free TSV filling with enlarged Cu grains using a two-step filling method that effectively promotes the direct reduction of Cu ions rather than electrochemical reduction of CuI. (C) 2018 Elsevier Ltd. All rights reserved.