In a pattern definition device for use in a particle-beam processing apparatus a plurality of apertures (21) are arranged within a pattern definition field (pf) wherein the positions of the apertures (21) in the pattern definition field (pf) taken with respect to a direction (X, Y) perpendicular, or parallel, to the scanning direction are offset to each other by not only multiple integers of the effective width (w) of an aperture taken along said direction, but also multiple integers of an integer fraction of said effective width. The pattern definition field (pf) may be segmented into several domains (D) composed of a many staggered lines (pl) of apertures; along the direction perpendicular to the scanning direction, the apertures of a domain are offset to each other by multiple integers of the effective width (w), whereas the offsets of apertures of different domains are integer fractions of that width.