A method includes forming a first metal line, forming a dielectric layer, with the first metal line being in the dielectric layer, and etching back the first metal line to form a trench in the dielectric layer. A lower part of the first metal line remains under the trench. The method further includes filling a photo sensitive material in the trench, and performing a photolithography process to pattern the photo sensitive material. A via opening is formed in the dielectric layer and the photo sensitive material. A second metal line and a via are formed, wherein the via is formed in the via opening, and the second metal line is over and joined to the via.

A method for treating a semiconductor substrate containing a refractory metal and copper, the method including: an oxidation treatment step of forming copper oxide on a surface of the copper; and a step of removing the refractory metal after the oxidation treatment step.