A plasma treatment chamber comprises one or more sidewalls and a support surface within the sidewalls holds a workpiece. An array of individual gas injectors is distributed about the sidewalls. Pump ports are along the sidewalls to eject gas from the chamber. Aa etch rate uniformity of a material on the workpiece is controlled by: using the array gas injectors to inject one or more gas flows in across the workpiece; injecting a first gas flow from a first set of adjacent individual gas injectors to etch the materials on the workpiece; and simultaneously injecting a second gas flow from remaining gas injectors. The second gas flow either dilutes the first gas flow to reduce an area on the workpiece having a faster etch rate, or acts as an additional etchant to increase the etch rate in the area of the workpiece having the faster etch rate.