A method for forming a semiconductor device is provided. First, a dielectric layer is provided on a substrate, wherein a first recess and a second recess are formed in the dielectric layer. After a mask layer is filled into the first recess and the second recess, the mask layer in the second recess is removed away, thereby forming a patterned mask layer. Subsequently, a nitride treatment is performed to remove unwanted residue of the mask layer in the second recess.