The invention relates to a method and a device for modifying in a spatially periodic manner at least in some regions a surface of a substrate (24), said surface being disposed on a sample plane (P) for which end different regions (211, 221, 231, 232) of the substrate surface are acted upon successively with a spatially periodic illumination pattern of an energy density above a processing threshold of the substrate surface, where the illumination pattern is generated by diffraction of an input beam (10) and superimposition of resulting, diffracted sub-beams (12, 14) by means of a grid interferometer (100), and where, in order to select the substrate surface region to be illuminated in each case (211, 221, 231, 232), the input beam (10) is deviated by means of a beam-deviating unit (16) arranged upstream of the grid interferometer (100).