A method for forming a semiconductor structure is provided. The method includes forming a dielectric structure on a semiconductor substrate, introducing a first gas on the dielectric structure to form first conductive structures on the dielectric structure, and introducing a second gas on the first conductive structures and the dielectric structure. The second gas is different from the first gas. The method also includes introducing a third gas on the first conductive structures and the dielectric structure to form second conductive structures on the dielectric structure. The first gas and the third gas include the same metal.