Post-plasma gas injection in barrier

A plasma processing apparatus is provided. The plasma processing apparatus includes: a plasma chamber; a processing chamber; a plasma source; and a barrier comprising a plurality of apertures, the barrier configured to allow neutral radicals to flow from the plasma chamber into the processing chamber via the apertures, the barrier comprising: a gas delivery system defining a channel extending in a circumferential direction of the barrier, the gas delivery system includes a wall dividing the channel into a first channel and a second channel, an inlet positioned in the first channel in a radial direction of the barrier offset from a circumferential center, and a plurality of outlets in fluid communication with the inlet via the channel. The gas delivery system is configured to inject gas from the plurality of outlets into the neutral radicals. The wall defines a plurality of openings spaced apart in a circumferential direction to provide fluid communication between the first channel and the second channel. Gas