Adapting the duration of exposure slots in multi-beam writers

Method for exposing a desired pattern on a target in a charged-particle lithography apparatus, using a particle beam of electrically charged particles, which is directed to and illuminates a pattern definition device comprising an aperture array composed of a plurality of blanking apertures through which said particle beam penetrates for writing said desired pattern by exposing a multitude of pixels within an image area on the target,

• wherein in the pattern definition device each of said plurality of blanking apertures is selectively adjustable to expose, in accordance with said desired pattern, a corresponding aperture image on the target during a respective exposure interval, wherein during a writing process of said desired pattern, a sequence of exposure intervals is made, wherein in each exposure interval the blanking apertures are imaged onto the target, thus generating a corresponding plurality of aperture images, wherein the position of aperture images is kept fixed relative to the target during an exposure interval, but between exposure intervals the position of aperture images is shifted over the target, thus exposing the multitude of pixels within said image area on the target, each exposure interval comprising an integer number of exposure slots of predetermined duration,
• wherein the aperture images have a nominal width, and the aperture images are mutually overlapping on the target so each pixel is exposed through a number of aperture images overlapping at the respective pixel, which, referring to the exposure slots of aperture images overlapping at the pixel as contributing exposure slots, results in an exposure of the respective pixel through an effective pixel exposure time which is the sum of durations of contributing exposure slots, wherein the method comprises adapting the duration of exposure slots according to following steps, performed respectively in relation to a group of all pixels within the image area which are exposed through the same set of contributing exposure slots:
• (i) determining a desired duration of the effective pixel exposure time for the pixels in the group of pixels, as a function of the time when the exposure of the pixels is done,
• (ii) determining contributing exposure slots for the pixels,
• (iii) calculating durations for the contributing exposure slots thus determined such that the sum of the durations over said contributing exposure slots is an actual effective exposure time which approximates said desired duration of the effective pixel exposure time,
• wherein the steps are repeated for varying groups of pixels, and
• in step (iii) the durations are calculated in accordance with a predetermined set of allowed durations, wherein at least one of the durations thus calculated is different from the other durations selected for said set of exposure slots.