Gas distribution apparatuses for improving mixing uniformity

A processing chamber comprising:

• a chamber body having a top wall, bottom wall and at least one sidewall defining a processing volume;
• a mixing plate adjacent a back plate of a showerhead, the mixing plate having a back surface and a front surface defining a thickness of the mixing plate, the mixing plate having a mixing channel comprising a top portion and a bottom portion defining a mixing channel length, the top portion having a first inner diameter and the bottom portion having a second inner diameter not equal to the first inner diameter, the mixing plate including at least two gas inlets in fluid communication with the top portion of the mixing channel, a mixer disposed within the thickness of the mixing plate in the top portion of the mixing channel, the mixer having a top plate and a mixer stem extending from the top plate, the mixer stem comprising a top and a bottom defining a mixer stem length, the top of the mixer stem connected to the top plate and extending into the top portion of the mixing channel, the mixer including a plurality of blades positioned along the mixer stem length and the top plate including at least one top inlet extending through the thickness of the top plate into the mixing channel;
• and
• a substrate support spaced a distance from a front plate of the showerhead.