In order to investigate the reflow characteristics of copper, copper was deposited on hole and trench patterns by metal organic chemical vapor deposition and it was annealed in nitrogen and oxygen ambients with the annealing temperatures ranging from 350 to550 °C.Upon annealing in an oxygen ambient at higher than 450 °C, copper was reflowed into the trench patterns whose line- width and aspect ratio were 0.2 μm and 4:1, respectively. Copper oxide was found with a thickness of less than a fifth of the total film thickness. The resistivity of the copper film increased when reflow occurred. It is thought that the reflow of copper in an oxygen ambient takes place because of enhanced surface diffusion.