Patent Number: 
Section: claims

1. Multiple beam charged particle optical system comprising:a charged particle source for generating a plurality of charged particle beamlets,charged particle optics for directing the charged particle beamlets from the charged particle source towards a target, wherein each charged particle beamlet defines a beamlet centre line,said charged particle optics comprising one or more electrostatic lens arrays, each comprising two or more array electrodes for generating a plurality of electrostatic lenslets, wherein each lenslet is arranged for focusing a corresponding charged particle beamlet, and wherein each lenslet defines a lenslet optical axis, characterized in thatat least one of said one or more electrostatic lens arrays comprises one or more off-axis electrostatic lenslets, wherein the beamlet centre line of the corresponding charged particle beamlet passes through said off-axis electrostatic lenslet at a distance from its lenslet optical axis. 2. Multiple beam charged particle optical system comprising:a charged particle source for generating a plurality of charged particle beamlets,charged particle optics for directing the charged particle beamlets from the charged particle source towards a target, wherein each charged particle beamlet defines a beamlet centre line,said charged particle optics comprising one or more electrostatic lens arrays, each comprising two or more array electrodes for generating a plurality of electrostatic lenslets, wherein each lenslet is arranged for focusing a corresponding charged particle beamlet, and wherein each lenslet defines a lenslet optical axis,wherein at least one of said one or more electrostatic lens arrays comprises a stack comprising a first and second array electrode provided with a first and second lens hole array, respectively, wherein the lens holes of the first and second lens hole array are aligned with each other,wherein at least one of said one or more electrostatic lens arrays comprises one or more off-axis electrostatic lenslets, each off-axis lenslet comprising a first lens hole of said first lens hole array and an opposite second lens hole of said second lens hole array, andwherein the beamlet centre line of the corresponding charged particle beamlet passes through said off-axis electrostatic lenslet at a distance from a centre of said off-axis electrostatic lenslet. 3. Multiple beam charged particle optical system according to claim 1 or 2, wherein each one or a subset of said one or more off-axis electrostatic lenslets is individually adjusted and/or arranged for providing an individual correction, or desired deflection or position of each corresponding charged particle beamlet. 4. Multiple beam charged particle optical system according to claim 1 or 2, the charged particle optics comprising a central axis, wherein the plurality of lenslets of the at least one of said one or more electrostatic lens arrays is substantially symmetrically arranged around said central axis. 5. Multiple beam charged particle optical system according to claim 4, wherein the distance between the optical axis of each off-axis electrostatic lenslet and the beamlet centre line of the corresponding charged particle beamlet is a function of the radial distance between said off-axis electrostatic lenslet and the central axis. 6. Multiple beam charged particle optical system according to claim 5, wherein the distance between the optical axis of each off-axis electrostatic lenslet and the beamlet centre line of the corresponding charged particle beamlet is proportional to the radial distance between said off-axis electrostatic lenslet and the central axis. 7. Multiple beam charged particle optical system according to claim 5, wherein the distance between the optical axis of each off-axis electrostatic lenslet and the beamlet centre line of the corresponding charged particle beamlet is proportional to the radial distance between said off-axis electrostatic lenslet and the central axis to the third power. 8. Multiple beam charged particle optical system according to claim 3, wherein each off-axis electrostatic lenslet is shifted with respect to the beamlet centre line of the corresponding charged particle beamlet in substantially the same direction. 9. Multiple beam charged particle optical system according to claim 4, wherein each off-axis electrostatic lenslet is shifted with respect to the beamlet centre line of the corresponding charged particle beamlet in a substantially radial direction with respect to said central axis. 10. Multiple beam charged particle optical system according to claim 4, wherein each off-axis electrostatic lenslet is shifted with respect to the beamlet centre line of the corresponding charged particle beamlet in a substantially tangential direction with respect to said central axis. 11. Multiple beam charged particle optical system according to claim 3, wherein each off-axis electrostatic lenslet of a quadrant of the at least one of said one or more electrostatic lens arrays is shifted with respect to the beamlet centre line of the corresponding charged particle beamlet in substantially the same direction. 12. Multiple beam charged particle optical system according to claim 1 or 2, wherein each off-axis electrostatic lenslet is shifted in substantially the same direction and wherein the distance between the optical axis of each off-axis electrostatic lenslets and the beamlet centre line of the corresponding charged particle beamlet is substantially constant. 13. Multiple beam charged particle optical system according to claim 1 or 2, wherein an off-axis electrostatic lenslet is embedded in, set in front of or behind an on-axis electrostatic lenslet of said charged particle optics. 14. Multiple beam charged particle optical system according to claim 2, wherein the stack comprises a third array electrode provided with a third lens hole array, wherein the lens holes of the first, second and third lens hole arrays are aligned with each other,wherein a lenslet axis of said electrostatic lens runs through the centers of said first and second lens hole,wherein each off-axis lenslet comprises a third lens hole of said third lens hole array, and wherein the center of said third lens hole is arranged at a distance from said lenslet axis. 15. Multiple beam charged particle optical system according to claim 14, wherein the second array electrode is arranged between the first and third array electrode. 16. Multiple beam charged particle optical system according to claim 14, wherein the third array electrode is arranged between the first and second array electrode. 17. Multiple beam charged particle optical system according to claim 14, the charged particle optics comprising a central axis, wherein the plurality of lenslets of the at least one of said one or more electrostatic lens arrays are substantially symmetrically arranged around said central axis. 18. Multiple beam charged particle optical system according to claim 17, wherein the distance between the center of said third lens hole and said lenslet axis is a function of the radial distance between said off-axis electrostatic lenslet and the central axis. 19. Multiple beam charged particle optical system according to claim 17, wherein the distance between the center of said third lens hole and said lenslet axis is proportional to the radial distance between said off-axis electrostatic lenslet and the central axis. 20. Multiple beam charged particle optical system according to claim 17, wherein the distance between the center of said third lens hole and said lenslet axis is proportional to the radial distance between said off-axis electrostatic lenslet and the central axis to the third power. 21. Multiple beam charged particle optical system according to claim 14, wherein the third lens hole of each off-axis electrostatic lenslet is shifted with respect to the beamlet centre line of the corresponding charged particle beamlet in substantially the same direction. 22. Multiple beam charged particle optical system according to claim 17, wherein the third lens hole of each off-axis electrostatic lenslet is shifted with respect to the beamlet centre line of the corresponding charged particle beamlet in a substantially radial direction with respect to said central axis. 23. Multiple beam charged particle optical system according to claim 17, wherein the third lens hole of each off-axis electrostatic lenslet is shifted with respect to the beamlet centre line of the corresponding charged particle beamlet in a substantially tangential direction with respect to said central axis. 24. Multiple beam charged particle optical system according to claim 14, wherein the third lens hole of each off-axis electrostatic lenslet of a quadrant of the at least one of said one or more electrostatic lens arrays is shifted with respect to the beamlet centre line of the corresponding charged particle beamlet in substantially the same direction. 25. Multiple beam charged particle optical system according to claim 14, wherein the third lens hole of each off-axis electrostatic lenslet is shifted in substantially the same direction, and wherein the distance between the center of said third lens hole and said lenslet axis is substantially constant. 26. Multiple beam charged particle optical system according to claim 14, wherein the stack comprises a fourth and fifth array electrode provided with a fourth and fifth lens hole array, respectively, wherein the lens holes of the first, second, third, fourth and fifth lens hole array are aligned with each other,wherein the lenslet axis of said electrostatic lens runs through the centers of said aligned first, second, fourth and fifth lens holes, andwherein the third lens hole array is arranged in between the first and second lens hole array on the one side and the fourth and fifth lens hole array on the other side. 27. Multiple beam charged particle optical system according to claim 14, wherein said electrostatic lens array comprises an array of substantially parallel lens axes. 28. Multiple beam charged particle optical system according to claim 17, wherein the third lens hole array is rotated around the central axis with respect to the first and/or second lens hole array. 29. Multiple beam charged particle optical system according to claim 1 or 2, wherein the system further comprisesa sensor for accurately determining a deviation between an actual position and a desired position of the target, anda controller for adjusting a potential of said off-axis lenslet or said third array electrode based on the determined deviation. 30. Maskless charged particle beam lithography assembly comprising a multiple beam charged particle optical system according to claim 1 or 2. 31. Electrostatic lens array comprising one or more off-axis electrostatic lenslets, for use in a multiple beam charged particle optical system according to claim 1 or 2. 32. Use of a multiple beam charged particle optical system according to claim 1 or 2, for generating a plurality of charged particle beamlets and for directing the charged particle beamlets from the charged particle source towards a target.