A two-step method of filling copper into a high-aspect ratio via or dual-damascene structure. The first step sputters at a low temperature of no more than 100° C. and with at least portions of high wafer bias, thereby filling a lower half of the hole. The initial copper sputtering is preferably performed through multiple cycles of low-level and high-level pedestal bias to deposit copper on exposed corners and to sputter resulting overhangs from the corners while depositing deep in the hole. The second step may include either electrochemical plating or sputtering performed at a higher temperature, e.g., at least 200° C. and with lower wafer bias to complete the hole filling. In another aspect of the invention, diffusion promoting gas such as hydrogen is added to the copper sputter plasma.