The present disclosure relates to a semiconductor processing apparatus. The processing chamber includes a chamber body and lid defining an interior volume, a substrate support disposed in the interior volume and a showerhead assembly disposed between the lid and the substrate support. The showerhead assembly includes a faceplate configured to deliver a process gas to a processing region defined between the showerhead assembly and the substrate support and an underplate positioned above the faceplate, defining a first plenum between the lid and the underplate, the having multiple zones, wherein each zone has a plurality of openings that are configured to pass an amount of inert gas from the first plenum into a second plenum defined between the faceplate and the underplate, in fluid communication with the plurality of openings of each zone such that the inert gas mixes with the process gas before exiting the showerhead assembly.