A gaseous mixture is deposited onto a substrate surface using a showerhead. A first plenum of the showerhead has a plurality of channels fluidicly coupled with an interior of a processing chamber. A second plenum gas flows through a plurality of tubes extending from a second plenum of the showerhead through the channels into the interior of the processing chamber. The diameter of the tubes is smaller than the diameter of the channels such that a first plenum gas flows into the interior of the processing chamber through a space defined between the outer surface of the tubes and the surface of the channels. The length and diameter of the tubes determine the level of distribution and the molar ratio of the first gas and the second gas in the gaseous mixture that is deposited on the surface of the substrate.