A method of forming an integrated circuit structure includes providing a substrate; forming a metal feature over the substrate; forming a dielectric layer over the metal feature; and forming an opening in the dielectric layer. At least a portion of the metal feature is exposed through the opening. An oxide layer is accordingly formed on an exposed portion of the metal feature. The method further includes, in a production tool having a vacuum environment, performing an oxide-removal process to remove the oxide layer. Between the step of forming the opening and the oxide-removal process, no additional oxide-removal process is performed to the metal feature outside the production tool. The method further includes, in the production tool, forming a diffusion barrier layer in the opening, and forming a seed layer on the diffusion barrier layer.