A plasma processing apparatus includes a process chamber, a platen positioned in the process chamber for supporting a workpiece, a source configured to generate a plasma in the process chamber having a plasma sheath adjacent to the front surface of the workpiece, and an insulating modifier. The insulting modifier is configured to control a shape of a boundary between the plasma and the plasma sheath so a portion of the shape of the boundary is not parallel to a plane defined by a front surface of the workpiece facing the plasma. Controlling the shape of the boundary between the plasma and the plasma sheath enables a large range of incident angles of particles striking the workpiece to be achieved.