A semiconductor device and its manufacturing method are presented. The manufacturing method includes providing a substrate structure; forming a first metal layer on the substrate structure; forming a second metal layer on the first metal layer; forming a first oxide layer on the second metal layer at a first temperature; and conducting the remaining manufacturing processes including thermal processes at a second temperature that is higher than the first temperature. This method reduces the concentration of the first metal diffused into the surface of the second metal layer during the thermal processes, thus reducing the amount of the oxide of the first metal formed on the surface of the second metal layer. Therefore, it is beneficial to the establishment of metal wire connections.