A trench in an inter-layer dielectric formed on a semiconductor substrate is defined by a bottom and sidewalls. A copper barrier lines the trench with a copper-growth-promoting liner over the barrier. The trench has bulk copper filling it, and includes voids in the copper. The copper with voids is removed, including from the sidewalls, leaving a void-free copper portion at the bottom. Immersion in an electroless copper bath promotes upward growth of copper on top of the void-free copper portion without inward sidewall copper growth, resulting in a void-free copper fill of the trench.