Apparatus and method for forming thin layers on a substrate are provided. A processing chamber has a gas delivery assembly that comprises a lid with a cap portion and a covering member that together define an expanding channel at a central portion of the lid, the covering member having a tapered bottom surface extending from the expanding channel to a peripheral portion of the covering member. Gas conduits are coupled to the expanding channel and positioned at an angle from a center of the expanding channel to form a circular gas flow through the expanding channel, The bottom surface of the chamber lid is shaped and sized to substantially cover the substrate receiving surface. One or more valves are coupled to the passageway, and one or more gas sources are coupled to each valve. A choke is disposed on the chamber lid adjacent a perimeter of the tapered bottom surface.