introducing at least one process gas into a reaction chamber through a showerhead located above a substrate to be coated;

wherein the showerhead comprises:

a gas inlet feeding into a plenum comprising an upper plate, an outer sidewall, and a lower shower plate having holes therein;

a domed internal baffle plate located within the plenum at a mouth of the gas inlet, the domed internal baffle plate comprising a lip and a domed portion; the domed internal baffle plate comprises a plurality of perforations;

wherein the perforations consist of:

a first zone of perforations consisting of a central hole and a first plurality of holes distributed in a first circle around the central hole having a diameter that is about ⅙ to about ⅕ of a diameter of the domed portion of the domed internal baffle plate;

a second zone of perforations consisting of a second plurality of holes distributed in a second circle around the first circle and having a diameter that is about ¼ to about ½ of the diameter of the domed portion of the domed internal baffle plate; and

a third zone of perforations consisting of a third plurality of holes distributed in a third circle around the second circle and having a diameter that is about ½ to about ¾ of the diameter of the domed portion of the domed internal baffle plate.