The reflow behavior of sputter-deposited Cu-Mn alloy into narrow trench lines formed in a SiO2 dielectric layer was investigated. At a substrate temperature of 350 °C, Cu-Mn completely filled a trench of 50nm width. The reflow was rate-controlled by mixed-mode diffusion. An interfacial layer was formed between Cu and SiO2 during this process, which was considered to improve wettability and enhance reflow filling. The reflowed Cu-Mn lines had a few grain boundaries, most of which are coherent twin boundaries.