An apparatus for processing a substrate is provided. The apparatus comprises a processing chamber and a showerhead. The showerhead is in the processing chamber and has a plurality of first holes with a first size in a first zone of the showerhead, a plurality of second holes with a second hole size in a second zone of the showerhead, and a plurality of third holes with a third hole size in a third zone of the showerhead. The first hole size is different from the second hole size. The first zone is surrounded by the second zone. An area of the first zone is larger than an area of the second zone. The first hole size is different from the third hole size. The first zone is surrounded by the third zone, and an area of the first zone is larger than an area of the third zone.