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 .