Reliability in a maskless lithography system

A maskless multi-beamlet lithography system for transferring a pattern onto a surface of a target, comprising:

• at least one beamlet optical unit for generating a plurality of beamlets;
• at least one measuring unit for measuring a property of each beamlet;
• at least one control unit operationally coupled to said measuring unit for determining those beamlets of said plurality of beamlets which have a value for the property outside a predefined range of values for said property as invalid beamlets, and those beamlets of said plurality of beamlets which have a value for the property inside the predefined range of values for said property as valid beamlets wherein the control unit is arranged for switching off or permanently blanking said invalid beamlets, thus preventing invalid beamlets from reaching said surface;
• at least a stage for during a first and a second scan of a first row, moving the target with respect to the rest of the system along the first row in a scanning direction (S1);
• and
• at least one actuator for inducing a shift of said surface and at least part of said lithography system with respect to one another, wherein said shift being performed in a shifting direction (S2) substantially transverse to said scanning direction (S1) over a distance, such that the positions that were covered by invalid beamlets are in a second scan covered by replacement beamlets wherein said control unit is arranged for:
• during a first scan of a first row, moving the target with respect to the rest of the system along the first row in a scanning direction (S1) and, during said movement, writing said pattern onto said surface by using said valid beamlets of said plurality of beamlets, wherein a part of said pattern at positions of said invalid beamlets is not correctly written;
• subsequently shifting said surface and at least part of said lithography system with respect to one another, wherein the magnitude of said shift is such that each invalid beamlet is replaced by a replacement beamlet, wherein said replacement beamlet is one of said valid beamlets of said plurality of beamlets, wherein said shift being performed in a shifting direction (S2) substantially transverse to said scanning direction (S1) over a distance, such that the positions that were covered by invalid beamlets are in a second scan covered by replacement beamlets;
• during the second scan of said first row, moving the target with respect to the rest of the system along said first row of said first scan, and, during said movement, original, unmodified pattern data of said invalid beamlets for said first scan is used for controlling said replacement beamlets during said second scan.