1. A method for forming a pattern on a surface of a target by means of a beam of electrically charged particles, the method comprising: producing said particle beam; directing said particle beam through a pattern definition means producing a patterned particle beam comprising one or more beamlets; projecting said patterned particle beam by means of an optical column comprising at least one controllable deflection means onto said target surface to form, at a nominal location on the target, a beam image comprising the image of defining structures in the pattern definition means, moving, during a sequence of exposure steps, said beam image to different locations, respectively, by changing the nominal location of the beam image with respect to the target between exposure steps, and introducing, during an exposure step, a global point spread function uniform to all beamlets by varying the actual location of the beam image around the respective nominal location by means of said at least one controllable deflection means, through a set of locations comprising a distribution of locations within the image plane centering around a mean location coinciding with the nominal location, wherein said set of locations is generated using a location generator comprising a noise generator or other random generator, said location generator producing a sequence of locations in a plane centered around the origin of the plane, and said sequence is superposed to the current nominal position.