The disclosed techniques relate to methods and apparatus for etching a substrate. A plate assembly divides a reaction chamber into a lower and upper sub-chamber. The plate assembly includes an upper and lower plate having apertures therethrough. When the apertures in the upper and lower plates are aligned, ions and neutral species may travel through the plate assembly into the lower sub-chamber. When the apertures are not aligned, ions are prevented from passing through the assembly while neutral species are much less affected. Thus, the ratio of ion flux:neutral flux may be tuned by controlling the amount of area over which the apertures are aligned. In certain embodiments, one plate of the plate assembly is implemented as a series of concentric, independently movable injection control rings. Further, in some embodiments, the upper sub-chamber is implemented as a series of concentric plasma zones separated by walls of insulating material.