The footprint of a reactive atom plasma processing tool can be modified using an aperture device. A flow of reactive gas can be injected into the center of an annular plasma. An aperture can be positioned relative to the plasma such that the effective footprint of the plasma can be changed without adjusting the gas flows or swapping out the tool. Once the aperture and tool are in position relative to a workpiece, reactive atom plasma processing can be used to modify the surface of the workpiece, such as to etch, smooth, polish, clean, and/or deposit material onto the workpiece. If necessary, a coolant can be circulated about the aperture. Multiple apertures can also be used to provide a variety of footprint sizes and/or shapes. This description is not intended to be a complete description of, or limit the scope of, the invention. Other features, aspects, and objects of the invention can be obtained from a review of the specification, the figures, and the claims.