the uniformity of a plasma distribution having a tendency to peak toward the axis of a processing chamber is improved by positioning a hollow body on the chamber axis with an open end facing the processing space . the hollow body controls the distribution of the plasma away from the center and allows plasma at the center . the geometry of the hollow body can be optimized to render the plasma uniform for given conditions . in combined deposition and etch processes , such as simultaneous and sequential etch and ipvd processes , the hollow body provides for a uniform plasma for etching while allowing deposition parameters to be optimized for deposition .