A grid assembly for injecting process gas to a chamber. The grid assembly including a gas inlet for delivering the process gas to the grid assembly, a plurality of nozzles extending vertically through at least a portion of the grid assembly, and a plurality of layers in a vertical stacked arrangement. The plurality of layers including a top layer including one or more internal gas injection channels configured to receive process gas from the gas inlet, a bottom layer including a plurality of internal gas injection channels having one or more injection apertures configured to deliver the process gas about a horizontal plane to one or more of the plurality of nozzles, and one or more sublayers disposed between the top layer and the bottom layer, each of the one or more sublayers including an increasing number of internal gas injection channels as the one or more sublayers advance from the top layer to the bottom layer. Plasma processing apparatuses and method of processing workpiece are also provided.