Flow control features of CVD chambers

A gas distribution assembly, comprising:

• an annular body comprising:
• an annular ring having an inner annular wall positioned at an inner diameter, an outer wall positioned at an outer diameter, an upper surface, and a bottom surface;
• an upper recess formed into the upper surface;
• a lip formed radially outward of the inner annular wall;
• and
• a seat formed into the inner annular wall;
• an upper plate positioned in the upper recess, comprising:
• a disk-shaped body having a plurality of first apertures formed therethrough, wherein the first apertures extend beyond a surface of the body thereby forming raised cylindrical bodies;
• and
• a bottom plate positioned on the seat, comprising:
• a disk-shaped body having a plurality of second apertures formed therethrough which aligns with the first apertures, the second apertures arranged in a twelve sides polygonal pattern;
• and
• a plurality of third apertures formed through the bottom plate, the third apertures having a diameter less than a diameter of the second apertures, each of the third apertures disposed between at least two second aperture, the bottom plate sealingly coupled to the upper plate in a manner that fluidly isolates the plurality of first and second apertures from the plurality of third apertures.