Patent Number: 
Section: claims

1. A method of generating a two-level pattern for lithographic processing by multiple beamlets, the method comprising:providing a pattern in vector format;converting the vector format pattern into a pattern in pixmap format; andforming a two-level pattern by application of error diffusion on the pixmap format pattern. 2. The method of claim 1, wherein the pixmap comprises an array of pixel cells, and wherein a multi-level value is assigned to each pixel cell. 3. The method of claim 2, wherein providing multi-level values to pixel cells is based on relative coverage of the vector-format pattern by the respective pixel cell. 4. The method of claim 2, wherein providing multi-level values to pixel cells is based on dose level values of the vector format pattern. 5. The method of claim 1, wherein the vector-format pattern is formed by two-level values. 6. The method of claim 1, wherein application of error diffusion includes:dividing the array of pixels in portions, each portion being assigned to be patterned by a different beamlet;determining an error diffusion parameter value for each portion;assigning a two-level value to the pixel cells within each portion using said error diffusion parameter value as determined. 7. The method of claim 6, wherein determining the error diffusion parameter value is based on beamlet current measurements. 8. The method of claim 6, wherein said error diffusion parameter value is a threshold value, and wherein said assigning a two-level value to the pixel cells within a portion is based on comparison with the threshold value determined for said portion. 9. The method of claim 6, wherein said error diffusion parameter is a value representing the higher level of the two-level value. 10. The method of claim 1, wherein said error diffusion is a type of one-dimensional, 1D, error diffusion. 11. The method of claim 1, wherein said error diffusion is a type of two-dimensional, 2D, error diffusion. 12. The method of claim 11, wherein the 2D-error diffusion uses a Floyd-Steinberg kernel. 13. The method of claim 2, wherein the application of error diffusion is further restricted by disallowing diffusion towards one or more pixel cells that fulfill a no-shift condition. 14. The method of claim 13, wherein the no-shift condition is that a multi-level value assigned to said one or more pixels is equal to or below a further threshold value. 15. The method of claim 14, wherein said further threshold value equals zero. 16. The method of claim 13, wherein the no-shift condition is that said one or more pixels are located outside a feature. 17. A computer readable medium for performing, when executed by a processor, the method of generating a rasterized two-level pattern as defined by claim 1. 18. A pattern generator comprising:an input for receiving a pattern in vector format;a processing unit for performing the method of generating a two-level pattern for lithographic processing according to claim 1; andan output for supplying the two-level pattern. 19. The pattern generator of claim 18, further comprising a memory for storing a pattern in pixmap format, the memory being communicatively coupled to the processing unit. 20. A charged particle multi-beamlet system for exposing a target using a plurality of beamlets, the system comprising:a beamlet modulation system for modulating the plurality of beamlets so as to form an exposure pattern;a projection system for projecting the modulated beamlets on to the surface of the target;a deflector array for deflecting the plurality of beamlets in a first direction;a substrate support member for supporting the target to be exposed;a control unit arranged to coordinate relative movement between the substrate support member and the plurality of beamlets in a second direction and movement of the group of beamlets in the first direction such that the target can be exposed in accordance with an array of pixel cells;wherein the charged-particle multi-beamlet system further comprises a beamlet pattern generator of claim 18. 21. The system of claim 20, wherein the projection system comprises an array of projection lens systems. 22. The system of claim 21, wherein the plurality of beamlets is arranged in groups of beamlets, and each projection lens system corresponds with a group of beamlets. 23. The system of claim 21, wherein the deflector array comprises a plurality of deflectors, each deflector being arranged to deflect a corresponding group of beamlets. 24. Lithographic system comprising:a preprocessing unit;a charged particle multi-beamlet system for exposing a target using a plurality of beamlets in accordance with a two-level pattern;wherein the preprocessing unit comprises a beamlet pattern generator of claim 18. 25. The lithographic system of claim 24, wherein the charged particle multi-beamlet system comprises:a beamlet modulation system for modulating the plurality of beamlets so as to form an exposure pattern;a projection system for projecting the modulated beamlets on to the surface of the target;a deflector array for deflecting the plurality of beamlets in a first direction;a substrate support member for supporting the target to be exposed;a control unit arranged to coordinate relative movement between the substrate support member and the plurality of beamlets in a second direction and movement of the group of beamlets in the first direction such that the target can be exposed in accordance with an array of pixel cells. 26. The system of claim 25, wherein the projection system comprises an array of projection lens systems. 27. The system of claim 26, wherein the plurality of beamlets is arranged in groups of beamlets, and each projection lens system corresponds with a group of beamlets. 28. The system of claim 26, wherein the deflector array comprises a plurality of deflectors, each deflector being arranged to deflect a corresponding group of beamlets.