Maskless particle-beam system for exposing a pattern on a substrate

A method for forming a pattern on a surface of a substrate by means of a beam of electrically charged particles, comprising:

• producing said particle beam;
• directing said particle beam through a pattern definition means and producing a number of beamlets using an aperture array of the pattern defining means having a plurality of regularly arranged apertures of identical shape defining the shape of said beamlets, and using a blanking means of the pattern defining means to switch off the passage of selected beamlets, the others forming, as a whole, a patterned particle beam;
• projecting said patterned particle beam onto said substrate surface to form an image of those apertures which are switched on, each aperture corresponding to an image element on the substrate surface during a uniform exposure time;
• and
• providing a relative motion between the substrate and the patterned particle beam according to the movement of the beamlets over a sequence of adjacent image elements on the substrate surface, resulting in an effective motion of the patterned particle beam over the substrate surface and exposing different image elements at different exposure times,
• wherein, in the aperture array, the apertures are arranged in aperture fields whose area is covered by the apertures only to a fraction 1/N, where N is an integer of at least 4, and
• wherein a subset of the apertures is switched on during a fraction of the exposure time and switched off for the remaining fraction of the exposure time, and the total time of exposure of image elements corresponding to said apertures causes an exposure dose of said image elements to be lower than a threshold value of development, said threshold value being lower than or equal to the value corresponding to a full exposure as caused by an aperture switched on during the full exposure time.