Embodiments of the disclosure generally provide a method and apparatus for processing a substrate in a vacuum process chamber. In one embodiment a vacuum process chamber is provided that includes a chamber body and lid disposed on the chamber body. A blocker plate is coupled to the lid and bounds a staging plenum therewith. A gas distribution plate is coupled to the lid. The gas distribution plate separates a main plenum defined between the gas distribution plate and the blocker plate from a process volume defined within the chamber body. The gas distribution plate and the blocker plate define a spacing gradient therebetween which influences mixing of gases within the main plenum.