Abstract: In a gas distribution showerhead assembly, a circular path splitting manifold having an axis of symmetry comprises a gas supply inlet, an enclosure with plural gas outlets extending through the enclosure, and a plurality of channels stacked along the axis of symmetry, and comprising plural paths of equal lengths between the inlet and respective ones of the plural outlets. A showerhead open to the plural outlets comprises a showerhead floor and a plurality of gas injection holes extending axially through the floor of the showerhead.