To enable Cu fill of through-Si vias (TSV) with a high aspect ratio (diameter 3 μm, depth 50 μm), the electroless deposition of a Cu seed on Co liner material was investigated. The reducing agent glyoxylic acid showed anodic oxidation on Co, which did not appear for the case of formaldehyde. From electrochemical analysis, an optimized bath composition caused limited Co corrosion during the electroless Cu nucleation phase. The concentration ratio of the complexing agent (ethylenediaminetetraacetic acid) and copper was found to strongly impact the liner corrosion; in case free complexing agent is present in the bath, the Co corrosion was assisted by complexation and replacement reactions. A continuous seed layer could be deposited in the entire TSV, which enabled the filling by electrochemical deposition (ECD). The substantially thinner total copper overburden generated for this combination of a wet-chemical seed deposition and an ECD fill process contributes to a cost reduction of its chemical mechanical polishing (CMP).