A chemical deposition apparatus having a chemical deposition chamber formed within the chemical isolation chamber includes a gas seal. The chemical deposition chamber includes a showerhead module having a faceplate with gas inlets to deliver reactor chemistries to a wafer cavity for processing a semiconductor substrate. The showerhead module includes primary exhaust gas outlets to remove reaction gas chemistries and inert gases from the wafer cavity. An inert gas feed delivers seal gas which flows radially inwardly at least partly through a gap between a step of the showerhead module and the pedestal module to form a gas seal. Secondary exhaust gas outlets withdraw at least some of the inert gas flowing through the gap to provide a high Peclet number.