[Application] A boundary between a plasma and a plasma sheath is controlled such that a portion of the shape is not parallel to a plane defined by a front surface of the workpiece facing the plasma. Ions in the plasma are directed toward the workpiece. These ions can either seal pores or clean a material from a structure on the workpiece. This structure may, for example, have multiple sidewalls. A process that both cleans a material and seals pores in the structure may be performed.