This work focuses on the design and implementation of a feedback control system on a parallel electrode plasma etching (PE) process with showerhead arrangement used to etch a 500 A amorphous silicon thin film on a 4 in wafer. The feedback control system consists of three spatially distributed proportional integral controllers that use measurements of the etching rate at three locations across the wafer to manipulate the inlet concentration of carbon tetrafluoride in the showerhead. The controller is implemented on a detailed fundamental model of the process, which accounts for diffusive and convective mass transfer, bulk and surface reactions, and non-uniform fluid flow and plasma electron density profiles, and is shown to reduce the etching rate nonuniformity from 30.2% to 3.8%.