The present disclosure generally provides methods of providing at least metastable radical molecular species and/or radical atomic species to a processing volume of a process chamber during an electronic device fabrication process, and apparatus related thereto. In one embodiment, the apparatus is a gas injection assembly disposed between a remote plasma source and a process chamber. The gas injection assembly includes a body, a dielectric liner disposed in the body that defines a gas mixing volume, a first flange to couple the gas injection assembly to a process chamber, and a second flange to couple the gas injection assembly to the remote plasma source. The gas injection assembly further includes one or more gas injection ports formed through the body and the liner.