Patent Number: 
Section: claims

1. A multi-beamlet multi-column particle-optical system comprising a plurality of particle-optical multi-beamlet columns,the plurality of particle-optical multi-beamlet columns being disposed in an array for simultaneously exposing a same substrate;with each particle optical column having an optical axis and comprising:a beamlet generating arrangement comprising at least one multi-aperture plate having a plurality of apertures for generating a pattern of multiple beamlets of charged particles, andan electrostatic lens arrangement downstream of the beamlet generating arrangement, the electrostatic lens arrangement comprising at least one electrode element,the at least one electrode element having an aperture allowing the generated multiple beamlets of charged particles to pass through, the aperture being defined by an inner peripheral edge facing the optical axis, the aperture having a centre and a predetermined shape in a plane orthogonal to the optical axis;wherein in at least one of the plurality of charged particle columns, the predetermined shape of the aperture of the at least one electrode element is a non-circular shape with at least one of a protrusion and an indentation from an ideal circle about the centre of the aperture, andwherein a first distance between a point on the inner peripheral edge of the aperture disposed closest to the centre of the aperture is at least about 5% smaller than a second distance between a point on the inner peripheral edge of the aperture disposed furthest away from the centre of the aperture. 2. The particle-optical system according to claim 1, wherein the electrode element comprises an annular inner electrode member having an inner peripheral edge and wherein the aperture is formed by the inner peripheral edge. 3. The particle-optical system according to claim 1, wherein the electrostatic lens arrangement of the at least one of the plurality of charged particle multi-beamlet columns comprises two or more electrode elements which are disposed coaxially and spaced apart in the direction of the optical axis. 4. The particle-optical system according to claim 3, wherein the apertures of the at least two electrode elements have substantially the same non-circular shape. 5. The particle-optical system according to claim 4, wherein the aperture of a first electrode element of the at least two electrode elements has an area that is by at least 5% larger than an area of the aperture of a second electrode element of the at least two electrode elements. 6. The particle-optical system according to claim 1, the plurality of columns comprising a first group of columns comprising at least one column, wherein the aperture of the at least one electrode element of the electrostatic arrangement of each column of the first group of columns has a first shape, and further comprising a second group of columns comprising at least one column, wherein the aperture of the at least one electrode element of the electrostatic arrangement of each column of the second group of columns has a second shape that is different from the first shape. 7. The particle-optical system according to claim 6, whereinthe first shape is different from the second shape with respect to at least one of a number of indentations, a number of protrusions, a shape of a protrusion, a shape of an indentation, a size of a protrusion, a size of an indentation, a symmetry of the shape and any combination thereof. 8. The particle-optical system according to claim 6, whereineach column of the first group of columns is surrounded by a first configuration of neighbouring columns and each column of the second group of columns is surrounded by a second configuration of neighbouring columns, and wherein the first configuration is different from the second configuration. 9. The particle-optical system according to claim 8, wherein the first configuration differs from the second configuration with respect to at least one of a number of neighbouring columns disposed closest to the respective column, a number of neighbouring columns disposed second closest to the respective column, a symmetry of the configuration of neighbouring columns and any combination thereof. 10. The particle-optical system according to claim 1, wherein the non-circular shape comprises a shape having one, two or four indentations extending from an ideal circle towards the centre of the aperture, the ideal circle having a radius equal to the second distance. 11. The particle-optical system according to claim 1, wherein each column has at least one closest neighbouring column and at least one second closest neighbouring column, and wherein a number of indentations in the shape of the aperture of the at least one electrode element of at least one column is equal to a number of second closest columns around the at least one column, with the indentations extending from the ideal circle towards the centre of the aperture, the ideal circle having a radius equal to the second distance. 12. The particle-optical system according to claim 1, wherein the particle-optical multi-beam columns are arranged in a rectangular array of N rows 1 to N and M lines 1 to M orthogonal to the rows,wherein a third group of columns is comprised of columns disposed in line 1, rows 2 to N−1, and line M rows 2 to N−1, and in row 1, lines 2 to M−1 and row N, lines 2 to N−1, and wherein the apertures of the at least one electrode element of the electrostatic arrangement of each column of the third group of columns have a same third shape. 13. The particle-optical system according to claim 1, wherein the particle-optical multi-beam columns are arranged in a rectangular array of N rows 1 to N and M lines 1 to M orthogonal to the rows,wherein a fourth group of columns is comprised of columns disposed in line 1, row 1, in line 1, row N, in line M, row 1 and in line M row N,and wherein the apertures of the at least one electrode element of the electrostatic arrangement of each column of the fourth group of columns have a same fourth shape. 14. The particle-optical system according to claim 1, wherein the particle-optical multi-beam columns are arranged in a rectangular array of N rows 1 to N and M lines 1 to M orthogonal to the rows,wherein a fifth group of columns is comprised of columns disposed in lines 2 to M−1 in respective rows 2 to N−1,and wherein the apertures of the at least one electrode element of the electrostatic arrangement of each column of the fifth group of columns have a same fifth shape. 15. The particle-optical system according to claim 1, wherein the at least one electrode elements of the electrostatic arrangements of neighbouring columns have a same distance from a substrate plane and are arranged on a mounting structure extending substantially in a plane orthogonal to the optical axes of the neighbouring columns. 16. The particle-optical system according to claim 1, further comprising a first and a second mounting structure,wherein the optical axes of columns disposed adjacent to one another are arranged in parallel,wherein each of the adjacent columns comprises two or more electrode elements with a first electrode element and at least a second electrode element being arranged coaxially and spaced apart in the direction of the optical axis of the column,wherein the first electrode elements of the adjacent columns have a same first distance from a substrate plane and are arranged on the first mounting structure,wherein the second electrode elements of the adjacent columns have a same second distance from the substrate plane and are arranged on the second mounting structure, the first and second mounting structures being arranged parallel to one another in a plane orthogonal to the optical axes of the columns. 17. The particle-optical system according to claim 16, wherein the first and second mounting structures are spaced apart by electrically insulating spacer elements. 18. The particle-optical system according to claim 3, wherein the electrode elements further comprise a substantially cylindrical shielding member, the substantially cylindrical shielding member having a radius equal to or greater than the second distance. 19. The particle-optical system according to claim 1, wherein the aperture is shaped and arranged such as to provide multi-pole correction for electrostatic fields generated within the electrode arrangement. 20. The particle-optical system according to claim 1, wherein each beamlet generating arrangement comprises a charged particle source for generating a beam of charged particles, and a beam patterning structure downstream of the charged particle source, the beam patterning structure comprising at least the multi-aperture plate and being configured to blank out at least a portion of the charged particle beam such that a pattern of multiple beamlets is formed downstream of the patterning structure. 21. The particle-optical system according to claim 1, wherein the centre of the at least one aperture of the electrode element of the electrostatic lens arrangement of a first column is disposed at least 50 mm from the centre of the at least one aperture of the electrode element of the electrostatic lens arrangement of a second column, with the second column being arranged closest to the first column in the array of multi-beam charged particle columns. 22. The particle-optical system according to claim 1, wherein the shape of the aperture is asymmetric with respect to the optical axis of the respective multi-beam particle-optical column. 23. The particle-optical system according to claim 1, wherein the centre of the aperture is disposed on the optical axis of the respective multi-beamlet particle-optical column. 24. A method of exposing a substrate by multi-beam multi-column exposure, comprising:generating a plurality of multiple beamlet patterns by a respective plurality of multi-beamlet particle-optical columns of the particle-optical system according to claim 1 and directing the plurality of multiple beam let patterns towards a substrate to be exposed;generating electrostatic fields by applying electric potentials to the at least one electrode elements of the electrostatic lens arrangements of the plurality of multi-beamlet particle-optical columns,transmitting the multiple beamlet patterns through respective apertures of the electrode elements of the electrostatic lens arrangements of the plurality of multi-beamlet particle-optical columns.