Gas distribution assemblies are described including a first plate and a second plate. The first plate may define a plurality of first apertures, and the second plate may define a plurality of second apertures in a first region of the second plate and a plurality of third apertures in a second region of the second plate. The second apertures may align with the first apertures. An area defined by the second region may be less than an area defined by the first region. The second plate may be sealingly coupled with the first plate to define a volume between the first plate and the second plate. The volume may be fluidly accessible from the third apertures, and fluidly isolated from the first and second apertures.