A gas supply unit is disclosed. An exemplary gas supply unit includes an upper plate provided with a plurality of injection holes; and a divider plate constructed and arranged against the upper plate to guide a flow of gas from the injection holes; wherein one of the plurality of injection holes is a center injection hole and the other than said one of the plurality of injection holes are arranged concentrically around the center injection hole as outer injection holes; and wherein the divider plate is provided with a center through hole fluidly communicating with the center injection hole and is provided with a plurality of protrusions extending towards the upper plate thereby creating a plurality of zones, each of the zones fluidly communicating with one of the outer injection holes.