A method includes forming a metal layer over a substrate; forming a dielectric layer over the metal layer; performing a plasma treatment to a first portion of the dielectric layer, such that a carbon concentration of the first portion of the dielectric layer is lower than a carbon concentration of a second portion of the dielectric layer; selectively forming an inhibitor over the first portion of the dielectric layer; and selectively forming a hard mask over portions of the metal layer that is uncovered by the inhibitor.