Method and apparatus for processing a substrate with a beam of energetic particles. The beam is directed from a source through a rectangular aperture in a shield positioned between the source and substrate to a treatment zone in a plane of substrate movement. Features on the substrate are aligned parallel to a major dimension of the rectangular aperture and the substrate is moved orthogonally to the aperture's major dimension. The beam impinges the substrate through the aperture during movement. The substrate may be periodically rotated by approximately 180° to reorient the features relative to the major dimension of the rectangular aperture. The resulting treatment profile is symmetrical about the sides of the features oriented toward the major dimension of the rectangular aperture.