A method of reducing surface roughness of a resist feature disposed on a substrate includes generating a plasma having a plasma sheath and ions therein. A shape of the boundary between the plasma and plasma sheath is modified using a plasma sheath modifier, so that a portion of the boundary facing the substrate is not parallel to a plane defined by the substrate. During a first exposure, the resist feature is exposed to electromagnetic radiation having a desired wavelength and the ions are accelerated across the boundary having the modified shape toward the resist features over an angular range.