Multi-beam charged particle microscope design with a mirror for image plane curvature correction

Abstract

A multi-beam charged particle system (1) with reduced image plane curvature is provided. This multi-beam charged particle system includes a charged particle mirror element (700) for compensating the image plane curvature of the charged particle imaging elements (102, 103.1, 103.2). This charged particle mirror element is configured to generate a curved virtual reflection surface (1321) for reflecting primary charged particles (3.1 to 3.3) during use. The present invention can be applied to applications of multi-beam charged particle systems that require higher requirements for beam uniformity and throughput.

Description

Technical Field

[0001] The present disclosure relates to a multi-beam charged particle microscope design including a mirror as a design means for correcting field curvature.

Background Art

[0002] WO2005 / 024881 discloses an electron microscope system that operates using a plurality of electron beamlets to scan an object to be inspected in parallel with an electron beamlet bundle. This primary charged particle beamlet bundle is generated by guiding a primary charged particle beam onto a multi-beam forming unit including at least one multi-aperture plate having a plurality of apertures. A portion of the electrons of the electron beam is incident on the multi-aperture plate and absorbed there, and another portion of the beam passes through the apertures of the multi-aperture plate, thereby forming electron beamlets in the beam path downstream of each aperture, the cross-section of which is defined by the cross-section of the aperture. Further, by appropriately selected electric fields provided in the beam path upstream and / or downstream of the multi-aperture plate, each aperture of the multi-aperture plate acts as a lens for the electron beamlet passing through the aperture, and as a result, each of the electron beamlets is focused, and a focal spot is formed on a surface at a distance from the multi-aperture plate. The surface on which the focus of the electron beamlet is formed is imaged onto the surface of an object or sample to be inspected by downstream optical components. The primary charged particle beamlet emits secondary electrons or backscattered electrons as a secondary electron beamlet from the object. The secondary electron beamlets are collected and imaged onto a detector. Each of the secondary beamlets is incident on a separate detector element, and as a result, the secondary electron intensity detected by the detector element provides information about the sample at the position where the corresponding primary beamlet is incident on the sample. The primary beamlet bundle is scanned systematically across the surface of the sample, and an electron microscope image of the sample is generated in the same manner as for a scanning electron microscope. The resolution of the scanning electron microscope is limited by the focal diameter of the primary beamlets incident on the object. Therefore, in the multi-beam electron microscopy method, all beamlets should form the same small focal spot on the object.

[0003] A multi-beam microscope for wafer inspection forms a plurality of focal spots of a plurality of primary charged particle beamlets on the wafer surface. The imaging lens generates field curvature, which leads to a deviation of a plurality of primary focal points from the flat wafer surface. Therefore, the field curvature leads to a large deviation in the focal spot size on the wafer surface. Due to the increasing requirements for the throughput of inspection tasks and the corresponding increase in the number of charged particle beamlets, the field of view size also increases, and the deviation due to field curvature also increases.

[0004] In the prior art, compensating for field curvature by at least one micro-optical element formed as an integral part of the multi-beam forming unit has been considered. A multi-aperture plate with electrodes is usually formed by layer deposition and etching techniques, and a stack of different layers is formed. To increase the stroke, a higher voltage must be applied by an electrostatic lens. The inhomogeneity of layer deposition and the leakage of the electric field lead to inhomogeneous electron-optical properties of the electrostatic elements across the multi-aperture plate. In a multi-aperture stack, the optical performance is usually limited. It is difficult to reach a stroke sufficient to individually vary the focal position of each primary charged particle beamlet within the multi-beam forming unit with the high accuracy required for wafer inspection tasks.

[0005] Correspondingly, the present invention is a charged particle beam system that operates using a large number of charged particle beams, and has an object of providing a charged particle beam system that can be used to achieve higher imaging performance, for example, better resolution and a narrower resolution range for each beamlet of a plurality of beamlets. A further object of the present invention is to provide a multi-beam charged particle beam system with reduced field curvature.

[0006] U.S. Patent Application Publication No. 2014 / 0158902 discloses a particle optical arrangement for a multi-beam system. A charged particle mirror element is not disclosed.

[0007] U.S. Patent Application Publication No. 2011 / 0291021 relates to a single-beam system and discloses an apparatus for reflective electron beam lithography. This apparatus includes an electron source, a patterned electron reflector generator structure, a stage, a reduction electron lens, and an E×B separator. The E×B separator is configured to bend the trajectory of the electron beam towards the dynamic pattern generator structure. The patterned electron reflector structure is configured to reflect a selected portion of the electron beam so as to form a patterned electron beam. The E×B separator is further configured to enable the patterned electron beam to pass straight towards the reduction electron lens. The reduction electron lens is configured to reduce the patterned electron beam and project the reduced patterned electron beam onto a target substrate. The apparatus disclosed in this specification has a straight projection axis and significantly shortens the electron beam path at a magnification of 3:1 (compared to prior art apparatuses using magnetic prisms).

SUMMARY OF THE INVENTION

[0008] The object of the present invention is solved by the independent claims. The dependent claims are directed to advantageous embodiments.

[0009] This application claims the priority of German Patent Application No. 102022206937.4, filed on July 7, 2022. The disclosure of this German patent application is incorporated herein by reference in its entirety.

[0010] The object of the present invention is solved by a multi-beam charged particle system with reduced image curvature. This multi-beam charged particle system with reduced image curvature is provided with design means for compensating for the image curvature aberration introduced by the charged particle imaging element.

[0011] According to the present invention, a multi-beam charged particle system comprises a charged particle beam source for generating a primary charged particle beam, a multi-beam forming unit for forming a plurality of primary charged particle beamlets from the primary charged particle beam, and an imaging system for forming a plurality of focal spots of the plurality of primary charged particle beamlets on the surface of a flat object. The imaging system comprises at least one lens element including an objective lens and at least one field lens. The imaging system further comprises a charged particle mirror element configured to compensate for the image plane curvature of the plurality of lens elements during use. The charged particle mirror element contributes to the image plane curvature of the imaging system with an opposite sign to the contribution of at least one lens element to the image plane curvature. Thereby, with a design comprising a charged particle mirror element, it is possible to compensate for the contribution of at least one lens element to the image plane curvature. By reducing the image plane curvature, the focal deviation of the primary charged particle beamlets from the flat object plane is minimized, and a predetermined resolution requirement for a larger field size or a larger number of primary charged particle beamlets can be achieved. Thereby, a larger number or a larger field size including primary charged particle beamlets can be utilized for an inspection task, and the throughput of the inspection task is improved.

[0012] The multi-beam charged particle system further comprises a control unit. The control unit is configured to apply at least one voltage to the charged particle mirror element during use. A charged particle mirror element and a voltage configured to generate a reflecting lens field of view having a virtual reflecting surface of positive or collecting power during use. The primary charged particles are decelerated and returned or reflected by the virtual reflecting surface of the reflecting lens field of view.

[0013] In one example, the charged particle mirror element comprises at least three electrodes, and the at least three electrodes include at least one of a first and a second ring-shaped electrode and a surface electrode. The electrodes are connected to a control unit, and the control unit is configured to apply a first voltage U1 to the first ring-shaped electrode, a second voltage U2 to the second ring-shaped electrode, and a mirror voltage Um to the mirror electrode during use. The voltages are configured to generate a reflection lens field having a virtual reflection surface with positive or collecting power during use. In one example, the surface electrode has a curved shape. In one example, the surface electrode is a segmented electrode including a plurality of N electrode segments, and the control unit is further configured to apply a plurality of mirror voltages Um1 to UmN to the plurality of N electrode segments during use. In one example, the charged particle mirror element comprises a third electrode connected to the control unit, and the control unit is further configured to apply a third voltage U3 to the third ring-shaped electrode during use. These examples can also be combined. By these examples, a curved virtual reflection surface having positive or collecting power is formed. The primary charged particle beamlet is reflected by the virtual reflection surface.

[0014] In one example, the charged particle mirror element is arranged in a plane where a plurality of primary charged particle beamlets overlap at least partially. In one example, the charged particle mirror element is arranged in the pupil plane. In an alternative example, the charged particle mirror element is arranged near an intermediate field plane where a plurality of focal spots of the primary charged particle beamlets are formed. In one example, the charged particle mirror element comprises a plurality of multi-aperture plates having a plurality of apertures, and the plurality of multi-aperture plates are each configured to receive and reflect each individual primary charged particle beamlet of the plurality of primary charged particle beamlets. In one example, at least one multi-aperture plate is configured to have a plurality of electrodes, and the control unit is configured to apply an individual voltage to each electrode for individual control of the reflection position of each individual primary charged particle beamlet. Thereby, a curved virtual reflection surface for the primary charged particle beamlet is achieved, and each primary charged particle beamlet is reflected by the virtual reflection surface. However, the multi-aperture plate configured to generate a curved virtual reflection surface is not limited to the above example. For example, at least one multi-aperture plate comprises a plurality of apertures with different diameters.

[0015] According to one example, the multi-beam charged particle system further comprises a secondary electron beam splitter or beam splitter configured to direct secondary beamlets generated at the focal spots of a plurality of primary beamlets on the surface of a flat object to a detector. The multi-beam charged particle system further comprises a secondary electron imaging system comprising a plurality of charged particle optical elements for forming focal spots of the secondary electron beamlets on the detector plane. The secondary electron beam splitter comprises a splitter segment for splitting the beam path of the primary charged particles from the beam path of the secondary electrons. In one example, the secondary electron beam splitter further comprises at least one first segment arranged in the beam path of the primary charged particles and at least one second segment in the beam path of the secondary electrons, and the first and second segments are configured to compensate for the dispersion and additional aberrations of the splitter segment.

[0016] According to the first embodiment of the present invention, a charged particle mirror element is configured such that, for perpendicular incidence of a plurality of primary beamlets, the reflected primary beamlets propagate in a direction parallel to the primary beamlets before entering the charged particle mirror element. The plurality of primary charged particle beamlets form a first path from the multi-beam forming unit to the charged particle mirror element, and form a second path from the charged particle mirror element of the reflected primary charged particle beamlets in the direction of at least one objective lens. In the first embodiment, near the charged particle mirror element, the first beam path and the second beam path are at least partially parallel to each other.

[0017] According to the first embodiment, the multi-beam charged particle system further includes a primary charged particle beam splitter for guiding the primary charged particle beamlets from the multi-beam forming unit to the charged particle mirror element along the first beam path. The primary charged particle beam splitter is further configured to guide the primary charged particle beamlets in the direction of at least one objective lens from the charged particle mirror element along the second beam path after reflection. In one example, the primary charged particle beam splitter includes a splitter segment for splitting the first beam path from the second beam path, at least one first segment disposed within the first beam path, and at least one second segment within the second beam path, and the first and second segments are configured to compensate for the dispersion and additional aberrations of the splitter segment.

[0018] According to one example, the primary charged particle beam splitter and the secondary electron beam splitter are formed as one integrated unit.

[0019] According to a second embodiment of the present invention, the charged particle mirror element is configured for oblique incidence. According to the second embodiment, the multi-beam charged particle system is configured to form a first path from the multi-beam forming unit to the charged particle mirror element and a second path from the charged particle mirror element of the primary charged particle beamlets after reflection in the direction of at least one objective lens, and the first path and the second path are arranged at an angle exceeding 10°, for example 15° or 20°. In one example, the charged particle mirror element according to the second embodiment has an elliptical cross-section.

[0020] According to a third embodiment of the present invention, the multi-beam charged particle system further comprises a second charged particle mirror element, and the first and second mirror elements are configured to jointly compensate for the image plane curvature of a plurality of lens elements during use.

[0021] According to an example of one embodiment, the charged particle mirror element is further configured to compensate for additional imaging aberrations during use. After the image plane curvature, other aberrations can be compensated using the charged particle mirror element. Such additional aberrations can be field-dependent or field-invariant. One example is the compensation of the axial chromatic aberration or dispersion of a plurality of beamlets, or the compensation of field-dependent spherical aberration or coma aberration.

[0022] In an additional embodiment, a method of operating a multi-beam charged particle system for variable compensation of image plane curvature is provided. The image plane curvature depends on the parameter settings of the multi-beam charged particle system, and the design of the charged particle mirror element and the voltage applied by the control unit to drive the charged particle mirror element are configured to variably compensate for the image plane curvature and optionally other aberrations.

[0023] Each of the embodiments or examples of the present invention provides a multi-beam charged particle beam system with reduced image plane curvature. Accordingly, the present invention enables the inspection of wafers with higher accuracy and lower variation in the focus spot size of the focus spots on the wafer surface placed in the object plane.

[0024] It should be understood that the present invention is not limited to the embodiments and examples only, but also includes combinations and modifications of the embodiments and examples.

[0025] Next, embodiments of the present disclosure will be described in more detail with reference to the drawings.

Brief Description of the Drawings

[0026]

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DETAILED DESCRIPTION OF THE INVENTION

[0027] In the exemplary embodiments of the present invention described below, components having similar functions and structures are denoted by similar or identical reference numerals as much as possible.

[0028] Some array elements, for example, a plurality of primary charged particle beamlets are identified by reference numerals. Depending on the context, the same reference numeral may identify a single element among the array elements. Each primary charged particle beamlet (3.1, 3.2, 3.3) is one of the plurality of primary charged particle beamlets (3).

[0029] The schematic diagram of FIG. 1 shows the basic features and functions of the multi-beam charged particle system 1. It should be noted that the symbols used in this figure are selected to symbolize their respective functions. The type of system shown is a multi-beam scanning electron microscope that uses a plurality of primary charged particle beamlets 3 to generate a plurality of primary charged particle beam spots 5 on the surface 25 of an object 7, such as a wafer or a mask substrate, whose top surface 25 is placed in the object plane 101 of the objective lens 102. For simplicity, only three primary charged particle beamlets 3.1 to 3.3 and three primary charged particle beam spots 5.1 to 5.3 are shown. The features and functions of the multi-beamlet charged particle system 1 can be implemented using electrons or other types of primary charged particles such as ions, especially helium ions. Additional details of the microscope system 1 are described in the international patent application PCT / EP2021 / 066255 filed on June 16, 2021, which is hereby incorporated by reference in its entirety.

[0030] The system 1 comprises an object irradiation unit (100) and a detection unit 200, as well as a secondary electron beam splitter or beam splitter unit 400 for separating the secondary charged particle beam path 11 from the primary charged particle beam path 13. The object irradiation unit 100 comprises a charged particle multi-beam generator 300 for generating a plurality of primary charged particle beamlets 3 and is adapted to focus the plurality of primary charged particle beamlets 3 onto the object plane 101 on which the surface 25 of the object 7 is arranged by the sample stage 500.

[0031] The primary beam generator 300 generates a plurality of primary charged particle beamlet spots on the intermediate image plane 321. The primary beamlet generator 300 comprises at least one source 301 of primary charged particles, such as electrons. The at least one primary charged particle source 301 emits a divergent primary charged particle beam, and the emitted divergent primary charged particle beam is collimated by at least one collimating lens 303 to form a collimated or parallel primary charged particle beam 309. The collimating lens 303 generally consists of one or more electrostatic or magnetic lenses, or is formed by a combination of an electrostatic lens and a magnetic lens. The collimated primary charged particle beam 309 is incident on the primary multi-beam forming unit 305. The multi-beam forming unit 305 basically comprises a first multi-aperture plate or filter plate 304 illuminated by the collimated primary charged particle beam 309. The first multi-aperture plate or filter plate 304 comprises a plurality of apertures in a raster configuration for generating a plurality of primary charged particle beamlets 3, and the plurality of primary charged particle beamlets 3 are generated by the collimated primary charged particle beam 309 passing through the plurality of apertures. The multi-beamlet forming unit 305 comprises at least one additional multi-aperture plate 306, and the at least one additional multi-aperture plate 306 is placed downstream of the first multi-aperture plate or filter plate 304 with respect to the direction of movement of the electrons in the beam 309. For example, the second multi-aperture plate 306 comprises, for each of the plurality of apertures, for example four or eight electrostatic elements for individually deflecting each of the plurality of beamlets. The multi-beamlet forming unit 305 according to some embodiments is configured to comprise a terminal multi-aperture plate (307). The multi-beamlet forming unit 305 is further configured to comprise an adjacent electrostatic field lens 308.1, and in some examples, the adjacent electrostatic field lens 308.1 is coupled within the multi-beamlet forming unit 305. In combination with the second field lens 308.2, the plurality of primary charged particle beamlets 3 are focused at or near the intermediate image plane 321.Each of the primary charged particle source 301 and the active multi-aperture plate 306 is controlled by the control unit 800.

[0032] The plurality of focal points of the primary charged particle beamlets 3 passing through the intermediate image plane 321 are imaged on the object plane 101 where the surface 25 of the object 7 is disposed by the field lens group 103 and the objective lens 102. By applying a voltage to the object by the sample voltage source 503, a decelerating electrostatic field is generated between the objective lens 102 and the object surface 25. The object irradiation system 100 further includes a collective multi-beam raster scanner 110 near the beam intersection 108, and the collective multi-beam raster scanner 110 can deflect the plurality of charged particle beamlets 3 in a direction perpendicular to the propagation direction of the charged particle beamlets. Throughout these examples, the propagation direction of the primary beamlets is the positive z direction. The objective lens 102 and the collective multi-beam raster scanner 110 have their centers on the optical axis (not shown) of the multi-beam charged particle system 1 perpendicular to the wafer surface 25. The plurality of primary charged particle beamlets 3 forming the plurality of beam spots 5 arranged in a raster configuration are synchronously scanned across the wafer surface 25. In one example, the raster configuration of the focal spots 5 of the plurality of J primary charged particles 3 is a hexagonal raster of about 100 or more primary charged particle beamlets 3, for example, J = 91, J = 100, or about 300 or more J beamlets. The primary beam spots 5 have a distance of about 6 μm to 45 μm and a diameter of less than 5 nm, for example, less than 3 nm, less than 2 nm or even smaller. In one example, the beam spot size is about 1.5 nm and the distance between two adjacent beam spots is 8 μm. A plurality of secondary electrons are generated at each scan position of each primary beam spot 5 of the plurality of primary beam spots 5, and they respectively form a plurality of secondary electron beamlets 9 having the same raster configuration as the primary beam spot 5. The intensity of the secondary charged particle beamlet 9 generated at each beam spot 5 depends on the intensity of the incident primary charged particle beamlet 3 illuminating the corresponding spot 5, the material composition and topography of the object 7 under the beam spot 5, and the charging condition of the sample at the beam spot 5.The plurality of secondary charged particle beamlets 9 are accelerated by an electrostatic field between the objective lens 102 and the object surface 25, collected by the objective lens 102, and pass through the first collective multi-beam raster scanner 110 in a direction opposite to that of the primary beamlet 3. The plurality of secondary beamlets 9 are scanned and deflected by the first collective multi-beam raster scanner 110. The plurality of secondary charged particle beamlets 9 then are guided by a secondary electron beam splitter or beam splitter unit 400 and follow the secondary beam path 11 of the detection unit 200. The plurality of secondary electron beamlets 9 propagate in a direction opposite to that of the primary charged particle beamlet 3, and the beam splitter unit 400 is configured to separate the secondary beam path 11 from the primary beam path 13, typically by a magnetic field or a combination of a magnetic field and an electrostatic field.

[0033] The detection unit 200 forms an image of the secondary electron beamlets 9 on the image sensor 600, thereby forming a plurality of secondary charged particle image spots 15 thereon. The detector or image sensor 600 comprises a plurality of detector pixels or individual detectors. The intensity is detected separately for each of the plurality of secondary charged particle beam spots 15 of the plurality of secondary charged particle beam spots 15, and the characteristics of the object surface 25 are detected with high resolution and high throughput for a large image patch of the object 7. For example, in a raster of 10×10 beamlets with a pitch of 8 μm, an image patch of approximately 88 μm×88 μm is generated in one image scan using the collective multi-beam raster scanner 110, which has an image resolution of, for example, 2 nm or less. The image patch is sampled at a size half of the beam spot size, and thus is sampled in such a manner that an image patch generated by 100 beamlets contains 6.4 gigapixels with 8000 pixels per image line per beamlet. The digital image data is collected by the control unit 800. Details of the collection and processing of the digital image data, for example, details of the collection and processing of the digital image data using parallel processing, are described in International Patent Application WO2020151904 and U.S. Patent No. 9,536,702, which are incorporated herein by reference.

[0034] The detection unit 200 further includes at least one second population raster scanner 222 connected to the scanning and imaging control unit 860. The scanning control unit 860 is configured to compensate for the residual difference in the positions of the plurality of focal points 15 of the plurality of secondary electron beamlets 9 in such a manner that the positions of the plurality of secondary electron focal spots 15 are kept constant at the image sensor 600.

[0035] The detection unit 200 further includes electrostatic or magnetic lenses 205.1 to 205.5 and a second intersection of the plurality of secondary electron beamlets 9, in which an aperture 214 is placed. The detection unit 200 can further include at least one first multi-aperture corrector 216 having apertures and electrodes for individually affecting each of the plurality of secondary electron beamlets 9.

[0036] The image sensor 600 is composed of an array of sensing areas in a pattern that matches the raster arrangement of the secondary electron beamlets 9 focused onto the image sensor 600 by the projection lens 205. This enables the detection of each individual secondary electron beamlet independently of other secondary electron beamlets incident on the image sensor 600. The image sensor 600 shown in FIG. 1 can be an electronic sensitive detector array such as a CMOS or CCD sensor. Such an electronic sensitive detector array can include an electron-photon conversion unit such as a scintillator element or an array of scintillator elements. In another embodiment, the image sensor 600 can be configured as an electron-photon conversion unit or a scintillator plate disposed on the focal plane of the plurality of secondary electron particle image spots 15. In this embodiment, the image sensor 600 can further include a relay optical system for imaging the photons generated by the electron-photon conversion unit at the secondary charged particle image spot 15 and guiding them onto dedicated photon detection elements such as a plurality of photomultiplier tubes or avalanche photodiodes (not shown). Such an image sensor is disclosed in U.S. Patent No. 9,536,702 incorporated by reference above.

[0037] While acquiring an image patch by scanning a plurality of primary charged particle beamlets 3, it is preferable not to move the stage 500. After acquiring the image patch, the stage 500 is moved to the next image patch to be acquired. In an alternative embodiment, the stage 500 is continuously moved in a second direction, during which an image is acquired by scanning a plurality of primary charged particle beamlets 3 in a first direction using a collective multi-beam raster scanner 110. The stage movement and stage position are monitored and controlled by sensors known in the art, such as laser interferometers, grating interferometers, confocal microlens arrays, and the like.

[0038] During image scanning, the control unit 800 is configured to operate the image sensor 600 to detect a plurality of intensity signals timely decomposed from a plurality of secondary electron beamlets 9 at predetermined time intervals, and a digital image of the image patch from all scanning positions of the plurality of primary charged particle beamlets 3 is accumulated and stitched together.

[0039] The multi-beam generation unit 305 is described, for example, in U.S. Patent Application Publication No. 2019 / 0259575 and U.S. Patent No. 10741355(B1), both of which are incorporated herein by reference. In the prior art, the positions of the plurality of focal points of the plurality of primary charged particle beamlets (3) are adjusted by the multi-beam generation unit (305) at the intermediate image plane (321). According to the prior art, the multi-beam generation unit 305 is the only means for pre-compensating the image plane curvature of the optical elements of the object irradiation unit (100) downstream of the multi-beam generation unit 305. The amount of image plane curvature is adjusted according to the driving parameters of the object irradiation unit 100, for example, the focusing power of the objective lens 102, which is the main cause of image plane curvature together, or the driving parameters of the object irradiation unit 100 regarding the electrostatic field generated between the objective lens 102 and the object surface 25 by the voltage supplied by the sample voltage source (503). However, the compensation of image plane curvature using only the multi-beam generation unit 305 is limited, and therefore alternative or additional solutions for compensating image plane curvature are sought. Such alternative or additional solutions are provided by the inventions described in the following examples and embodiments. FIG. 2 schematically shows a simplified example of the present invention. FIG. 2 shows a multi-beam charged particle system 1 comprising a charged particle source 301, at least one collimating lens 303, and a primary multi-beamlet forming unit 305. Here, only three primary charged particle beamlets 3.1 to 3.3 are shown, but this number can be larger, for example, more than 100, more than 300, or even larger. The system 1 further comprises a plurality of lens elements including at least one objective lens 102 and field lenses 103.1, 103.2 for forming a plurality of focal spots 5 of the plurality of primary beamlets 3 on the surface of a flat object 7. The system 1 further comprises a charged particle mirror element (700), and the mirror element (700) is configured to compensate for the image plane curvature of the plurality of lens elements 102, 103.1 and 103.2 during use.The charged particle mirror element (700) is configured to reflect a plurality of primary charged particle beamlets 3, and thus to separate the primary beam path 13 into a first primary beam path segment 13.1 from the multi-beam forming unit 305 to the charged particle mirror element 700 and a second primary beam path segment 13.2 from the charged particle mirror element 700 of the reflected primary charged particle beamlets 3 in the direction of the objective lens 102.

[0040] The system 1 further comprises a control unit 800. This control unit is configured to apply a plurality of voltages to the charged particle mirror element 700 during use. The charged particle mirror element 700 and these voltages are configured to generate a reflection lens field having a virtual reflection surface during use. Details of the reflection lens field will be further shown later. The system according to FIG. 1 further comprises a secondary electron beam splitter or beam splitter 400 configured to direct the secondary beamlets generated at the focal spots of the plurality of primary beamlets on the surface of a flat object along the secondary electron beam path 11 to the detector 600.

[0041] Figure 3 shows the influence of image plane curvature according to the prior art. Figure 3 shows a plurality of primary beamlets 3.1 to 3.5, each beamlet having its beam waist 74.1 to 74.5 on a spherical surface 43 of radius R. The minimum focus spot sizes 74.1 to 74.5 are the same as the beam waists of the beamlets 3.1 to 3.5. Due to the inclination of the image plane, the center of the sphere 43 can be further eccentric from the optical axis 105. When a flat object 7 such as a mask substrate or a wafer is placed on the image plane 101, the primary charged particle focus spot 5, which is the same as the origin of secondary electrons on the wafer surface 25, is very different, including a focus spot 5.1 having the minimum spot size, a focus spot 5.2 having the average spot size, and a focus spot 5.3 having the maximum spot size (see Figure 3b). The variation in spot size leads to different resolutions for each secondary electron beamlet 9. However, for mask writing or wafer inspection tasks, there are strict requirements for the uniformity of resolution. For example, the resolution provided by each secondary electron beamlet 9 should not exceed a maximum deviation, for example, 10% of the resolution requirement or even smaller, for example, 7% or only 5%. The resolution requirement itself can be, for example, 2 nm or less, for example, 1.5 nm. Therefore, image plane curvature is one of the driving aberrations that limit the performance of the multi-beam charged particle system 1 for imaging tasks such as multi-beam direct writing of microscopic structures onto a mask or wafer inspection tasks.

[0042] FIG. 4 shows a first embodiment of the present invention. The multi-beam charged particle system 1 according to the first embodiment includes a charged particle beam source 301 for forming a primary charged particle beam 309 and two collimating lenses 303. The multi-beamlet forming unit 305 forms a plurality of primary charged particle beamlets 3. More details regarding the multi-beamlet forming unit 305 have been described above with respect to the example of FIG. 1. The system 1 further includes a charged particle mirror element 700 including a mirror electrode 1315 and a plurality of additional electrodes 1317, which is configured to form a virtual reflection surface 1321 for reflecting the incident primary charged particle beamlets 3 (only three beamlets 3.1 to 3.3 are shown) during use. The mirror electrode 1315 and the plurality of additional electrodes 1317 are connected to a control unit 800, and the control unit 800 applies a predetermined voltage to the mirror electrode 1315 and the plurality of additional electrodes 1317.

[0043] System 1 further comprises a primary beam splitter 460 for guiding a primary charged particle beamlet 3 from a multi-beam forming unit 305 along a first beam path 13.1 to a charged particle mirror element 700, and configured to guide the primary charged particle beamlet 3 after reflection along a second beam path 13.2 from the charged particle mirror element 700 in the direction of at least one objective lens 102. The primary beam splitter 460 in this example includes a splitter segment 460.1 for splitting the first beam path 13.1 from the second beam path 13.2, a first segment 460.2 disposed within the first beam path 13.1, and a second segment 460.3 within the second beam path 13.2, and the first and second segments 460.2 and 460.3 are configured to compensate for the dispersion and additional aberrations of the splitter segment 460.1. System 1 further comprises a common path field lens 1328 within a common beam path between the splitter segment 460.1 and the charged particle mirror element 700. System 1 further comprises at least one first field lens 103.1 within the first beam path 13.1 and at least one second field lens 103.2 within the second beam path 13.2. For additional components in FIG. 4, refer to FIG. 1 and its description.

[0044] FIG. 5 shows the effect of the present invention. By compensating for the field curvature by the charged particle mirror element 700, a system 1 without field curvature is provided, and all minimum confusion points or beam waists 74.1 to 74.5 are formed within the object plane 101. All focus spots 5.ij have the same minimum diameter on the flat surface 25 of the object 7. In practice, there may be some variations in diameter due to residual aberrations, but these variations are smaller than a predetermined threshold. To further compensate for the residual diameter variations of the focus spots, a multi-pole array or lens array 306 for compensating for the residual aberrations can be further provided in the multi-beamlet forming unit 305. Without field curvature, the number J of beamlets can be increased, and a higher throughput of the system 1 can be achieved.

[0045] Figure 6 shows some examples of the charged particle mirror element 700. Figure 6a shows a first example including a charged particle mirror element 700 comprising three ring electrodes 1317.1 to 1317.3 and one continuous mirror electrode 1315 having a flat-bottomed cup shape. A plurality of voltages U1 to U3 and Um are applied to the electrodes 1317.1 to 1317.3 and 1315 by the control unit 800 such that the incident primary charged particles, for example, the incident primary charged particles along the charged particle path 1311, are decelerated and finally reflected by the virtual mirror surface 1321. The shape dimensions of the electrodes 1317 and 1315 and the voltages U1 to U3 and Um are selected to generate equipotential lines 1341 accompanied by electric field lines 1342 (i.e., dotted lines) so that a virtual reflection surface or virtual mirror surface 1321 is formed. The virtual reflection surface or virtual mirror surface 1321 corresponds to the equipotential surface at which the primary charged particles are decelerated to a kinetic energy of 0 kV. Thus, the primary charged particles are reflected by the virtual mirror surface 1321 and accelerated by the electric field in such a manner that the reflected primary charged particles move in substantially the opposite direction along the path 1311. The voltages are selected according to |U1| > |U2| > |U3| > |Um|, where |U1| is approximately equal to the kinetic energy of the primary charged particles, and |U2| is selected in the range of |U1| / 5 < |U2| < |U1| / 3. |U3| is selected in the range of |U2| / 4 < |U3| < |U2| / 2. |Um| is selected to be a positive voltage of Um ≒ -1 × U3. Thereby, the primary charged particles are decelerated to reach zero kinetic energy at the virtual mirror surface 1321 and then accelerated in the opposite direction to reach again the target kinetic energy of the primary charged particles after reflection. Figure 6b shows an equivalent example having a curved mirror electrode 1315. Figures 6c and 6d show equivalent examples having a segmented mirror electrode 1315 including a plurality of ring-shaped segments 1315.1 to 1315.4 to which different mirror voltages Um1 to Um4 are applied during use to generate the virtual mirror surface 1321. For simplicity, only four ring segments are shown, but the number or ring segments can be larger.This example is not limited to ring-shaped segments, and it is also understood that segments of other shapes, such as hexagonal or elliptical segments or a raster of segments suitable for forming a curved virtual reflecting surface 1321, can also be used, to which an appropriate voltage Um is applied.

[0046] The charged particle mirror element 700 and the control unit 800 can be further configured to change the compensation of the image plane curvature by applying a voltage to the electrodes of the charged particle mirror element 700. By applying different voltages to the electrodes of the charged particle mirror element 700 by the control unit 800, the curvature of the virtual reflecting surface can be adjusted. The change in the image plane curvature compensation may be required, for example, after a change in the settings of the multi-beamlet charged particle system 1, after a change in the sample voltage applied by, for example, the sample voltage supply unit 503, or after a change in magnification by the objective lens 102.

[0047] The mirror element 700 can compensate not only for the image plane curvature but also for other aberrations of the multi-beam charged particle system 1. One example is the compensation of the axial chromatic aberration or dispersion of a plurality of beamlets. The virtual reflecting surface 1321 corresponds to an equipotential surface suitable for reflecting primary charged particles (3) of a specific first kinetic energy. For other kinetic energies or a second kinetic energy, reflection occurs at a different equipotential surface suitable for reflecting primary charged particles (3) of the specific second kinetic energy. By appropriately designing the electrode 1317 including the mirror electrode 1315, an appropriate shape and sequence of the curved virtual reflecting surface 1321 with respect to the distribution of the kinetic energy of the primary charged particles can be configured. Thereby, an imaging aberration corresponding to the dispersion of the kinetic energy of the primary charged particles is provided.

[0048] Figure 7 shows another example of a charged particle mirror element 700. In this example, the charged particle mirror element 700 is configured as an array of mirrors, each with one mirror per primary charged particle beamlet, and five trajectories 1311.1 to 1311.5 of primary charged particle beamlets are shown. This mirror array is formed by multi-aperture plates 1327.1 to 1327.3 and a mirror multi-aperture plate 1325. In one example, a ring electrode is provided for each opening of at least one of the multi-aperture plates 1327.1 to 1327.3 and 1325. For example, the multi-aperture plate 1327.2 has J openings for J primary charged particle beamlets, and a ring electrode is provided for each opening. A voltage U2i is applied to the i-th ring electrode of the i-th opening. Voltages U1i to U3i and Umi adjust the return point according to the virtual mirror plane 1321 for each primary charged particle beamlet.

[0049] In one example, the charged particle mirror element 700 according to FIG. 7 is further configured to compensate for additional imaging aberrations during use. For example, at least one of the multi-aperture plates 1327.1 to 1327.3 is provided with multipole electrodes configured to individually correct the aberration of each beamlet, instead of the ring electrodes arranged around each opening. One example is the compensation of field-dependent spherical aberration or coma aberration.

[0050] Figure 8 shows an example of a multi-beam charged particle system 1 according to a first embodiment including a charged particle mirror element 700 configured as the mirror array of FIG. 7. The charged particle mirror element 700 can further include a field lens electrode 1318. For additional components in FIG. 8, refer to FIG. 4 and its description.

[0051] FIG. 9 shows an example of a primary beam splitter 460. The primary beam splitter 460 includes a splitter segment 460.1 for splitting a first beam path 13.1 from a second beam path 13.2, a first segment 460.2 disposed within the first beam path 13.1, and a second segment 460.3 within the second beam path 13.2, and the first and second segments 460.2 and 460.3 are configured to compensate for the dispersion and additional aberrations of the splitter segment 460.1. In the direction of the first beam path 13.1, the charged particle mirror element 700 is arranged and configured to reflect primary charged particles moving along the first beam path 13.1 into the second beam path 13.2. The second segment 460.2 includes three magnetic sectors 460.3a - 460.3c and an electrostatic element 460.3d. All elements are connected to a control unit 800, and the control unit 800 is configured to supply voltages and currents corresponding to the elements of the primary beam splitter 460 and the charged particle mirror element 700.

[0052] These examples according to the first embodiment are configured for reflection of normal incidence in the charged particle mirror element 700. Therefore, these examples of the first embodiment require a primary beam splitter 460 for beam path separation between the first beam path 13.1 and the second beam path 13.2. In the second embodiment, no primary beam splitter 460 is required, and reflection at an oblique incidence is used in the charged particle mirror element 700. FIG. 10 shows a second embodiment of the present invention. The same elements are labeled with the same reference numerals, see FIG. 4 and its description. Contrary to FIG. 4, the example according to FIG. 10 does not have the primary beam splitter 460 according to the first embodiment. Instead, the system 1 is configured for an oblique angle of incidence 87 with respect to the perpendicular 705 of the charged particle mirror element 700. The primary charged particles moving along the first beam path 13.1 are reflected at the virtual mirror surface 1321 of the charged particle mirror element 700 at the same oblique angle 87 with respect to the mirror perpendicular 705 into the second beam path. For separation of the first beam path 13.1 and the second beam path 13.2, the oblique angle 87 is selected to be greater than 5°, for example greater than 10°. The first path 13.1 and the second path 13.2 are arranged at an angle that is twice the oblique angle of incidence 87. Therefore, the first path 13.1 and the second path 13.2 are arranged at an angle greater than 10°, preferably at an angle of 15° or rather 20°. FIG. 11b shows an example of a charged particle mirror element 700 configured for oblique incidence at an angle 87 with respect to the mirror perpendicular 705. The mirror 700 is configured in the same way as the mirror 700 in FIG. 6, but has an elliptical cross section shown at the bottom of FIG. 11b. Thereby, the aberration generated at an oblique incidence at an angle 87 in the circular mirror is compensated. An example with a circular mirror 700 configured for normal incidence is shown in FIG. 11a.

[0053] FIG. 12 shows another example according to the second embodiment. The same elements are labeled with the same reference numerals, and reference is made to FIG. 10 and its description. The system 1 is configured to generate a first intermediate image plane 321.1 in a first beam path 13.1 upstream of the charged particle mirror element 700. The charged particle mirror element 700 is placed in the pupil plane, and a second intermediate image plane 321.2 is formed in a second beam path 13.2 downstream of the charged particle mirror element 700. By the operation of the charged particle mirror element 700, the second intermediate image plane 321.2 is formed in a curved shape configured to compensate for the image plane curvature of the optical elements including the objective lens 102 and at least one field lens 103.3 downstream of the second intermediate image plane 321.2.

[0054] FIG. 13a shows an additional modification of the first embodiment. Here, the primary beam splitter 460 and the beam splitter unit 400 are arranged adjacent to each other. In the example of FIG. 13b, the primary beam splitter 460 and the beam splitter unit 400 are formed as one unit 480. FIG. 13c shows a third embodiment similar to the second embodiment. Here, a first charged particle mirror element 700.1 and a second charged particle mirror element 700.2 are arranged in the primary beam path 13, and the primary beam path 13 includes a first beam path segment 13.1, a second beam path segment 13.2, and a third beam path segment 13.3. The compensation of the image plane curvature can be further improved by the two reflective charged particle mirror elements 700.1 and 700.2.

[0055] With the mirror element 700 according to an embodiment of the invention, the image plane curvature is compensated, and the focal deviation of the focal spots 5 of the plurality of beamlets 3 no longer limits the number J of beamlets or the field of view size. Thus, with the mirror element 700, a larger number J of beamlets, for example J>300, J>1000, or even J>10000, is possible. To supply each beamlet 3 with a sufficient charged particle beam current, it is also possible to use several charged particle beam sources within one multi-beam charged particle system 1. An example where two charged particle sources 301.1 and 301.2 with two condenser lens systems 303.1 and 303.2 are provided upstream of the multi-beam forming unit 305 is shown in FIG. 13b.

[0056] FIG. 14 shows a method of operating a multi-beam charged particle system 1 comprising at least one first charged particle mirror element 700. In a first step S, a setting of the parameters of the multi-beam charged particle system 1 is selected. The parameters of the multi-beam charged particle system 1 include, for example, the setting of the objective lens 102, the incident energy of the primary charged particles selected by the voltage applied by the sample voltage source 503, or the setting of the field lens 103 for compensating the rotation of the plurality of primary beamlets 3. The setting of the parameters in step S may in turn induce a change in the image plane curvature of the surface 43 (see FIG. 3) on which the focal spot 5 of the minimum spot diameter is formed.

[0057] In compensation step C, a specific radius R of field curvature for the selected parameter setting of step S is determined. The radii R of field curvature for a plurality of parameter settings can be stored in the memory of control unit 800 or calculated. For example, it can be calculated according to a predetermined mathematical lens model from the drive currents supplied to lens elements 103 and 102. For a specific radius R of field curvature, control signals for a plurality of drive voltages U and Um of charged particle mirror element 700 are calculated by control unit 800. The plurality of drive voltages U and Um include, for example, voltages U1 to U3 for ring electrodes 1317.1 to 1317.3 and at least one voltage Um for at least one segment of mirror electrode 1315. In another example, the drive voltage includes a plurality of drive voltages U1i for at least one multi-aperture plate 1327 where i = 1... J (J is the number of primary beamlets) (see the examples in FIGS. 6 and 7). Further, other control signals and voltages for compensating other aberrations by charged particle mirror element 700 can be generated by control unit 800 and applied to charged particle mirror element 700. Of course, as is known from prior art systems, other elements of multi-beam charged particle system 1 are also driven and controlled by control unit 800.

[0058] In application step A, the application of multi-beam charged particle system 1 is executed. Such an application can be the execution of an inspection task, for example, an inspection task of a semiconductor wafer, or the execution of a multi-beam lithography task such as patterning of a semiconductor mask. In order to control the performance of multi-beam charged particle system 1, measurement step M is started by control unit 800. Thereby, in the repetition of this method starting from step C, the performance of charged particle mirror element 700 is controlled and finally adjusted.

[0059] The measurement step M can be carried out before or during the execution of the task of step A, and during the application step A, it is possible to monitor the correction of the image plane curvature and other aberrations by the charged particle mirror element 700. Generally, during the monitoring step M, the performance and residual aberrations of the multi-beam charged particle system (1) are determined. Based on the residual aberrations, a control step can be initiated, a corrected drive voltage configured to compensate for the residual aberrations can be determined, and applied to at least one electrode of the charged particle mirror element (700).

[0060] Depending on the operating method of the multi-beam charged particle system 1 and the configuration of the charged particle mirror element 700, variable compensation of the image plane curvature becomes possible. The image plane curvature depends on the parameter settings of the multi-beam charged particle system 1, and the design of the charged particle mirror element 700 and the voltage applied by the control unit 800 to drive the charged particle mirror element 700 are configured to variably compensate for the image plane curvature and optionally other aberrations.

[0061] The features of the embodiments improve the performance of the multi-beam charged particle system 1 for achieving a higher resolution of less than 5 nm, preferably less than 3 nm, more preferably less than 2 nm or even less than 1 nm. These improvements have a particular relevance for the further development of multi-beam charged particle systems using a larger number of primary beamlets, such as more than 100 beamlets, more than 300 beamlets, more than 1000 beamlets or even more than 10000 beamlets. These improvements have a particular relevance for routine applications of multi-beam charged particle systems, for example, in mask writing applications or semiconductor inspection and review where high reliability, high reproducibility and low inter-machine deviation are required. By the features and their combinations described in the embodiments, each beamlet of the plurality of beamlets is provided with a beamlet diameter in the range of 2 nm to 2.1 nm with an average resolution of 2.05 nm. The range of resolution achieved by the features and methods of the embodiments is less than 0.15% of the average resolution, preferably 0.1%, even more preferably 0.05%.

[0062] The present invention will be further described by the following clauses.

[0063] Clause 1: Comprising an object irradiation unit (100), the object irradiation unit (100) being - a charged particle beam source (301), and - a multi-beam forming unit (305) for forming a plurality of primary beamlets (3, 3.1, 3.2, 3.3), and - a plurality of lens elements (102, 103, 103.1, 103.2, 103.3) including at least one objective lens (102) for forming a plurality of focal spots (5, 5.1, 5.2, 5.3) of the plurality of primary beamlets (3, 3.1, 3.2, 3.3) in the image plane (101), and - a charged particle mirror element (700) configured to compensate for the field curvature of the image plane of the plurality of lens elements (102, 103, 103.1, 103.2, 103.3) during use A multi-beam charged particle system (1) comprising.

[0064] Clause 2: - a control unit (800) further comprising, the control unit (800) being configured to apply a plurality of voltages (U1, U2, U3, Um) to the charged particle mirror element (700) during use, the charged particle mirror element (700) and the voltages (U1, U2, U3, Um) being configured to generate a reflection lens field distribution having a virtual reflection surface (1321) during use, The multi-beam charged particle system (1) according to Clause 1.

[0065] Clause 3: The charged particle mirror element (700) comprises at least three electrodes (1317.1, 1317.2, 1315), the at least three electrodes (1317.1, 1317.2, 1315) including at least one first and second ring-shaped electrode (1317.1, 1317.2) and a mirror electrode (1315) connected to a control unit (800), the control unit (800) being configured to apply a first voltage U1 to the first ring-shaped electrode (1317.1), a second voltage U2 to the second ring-shaped electrode (1317.2), and a mirror voltage Um to the mirror electrode (1315) during use, the electrodes and voltages being configured to generate a virtual reflection plane (1321) during use, the multi-beam charged particle system (1) according to clause 2.

[0066] Clause 4: The multi-beam charged particle system (1) according to clause 3, wherein the mirror electrode (1315) has a curved shape.

[0067] Clause 5: The multi-beam charged particle system (1) according to clause 3 or clause 4, wherein the mirror electrode (1315) is a segmented electrode including a plurality of N electrode segments (1315.1 to 1315.N), and the control unit (800) is further configured to apply a plurality of mirror voltages Um1 to UmN to the plurality of N electrode segments (1315.1 to 1315.N) during use.

[0068] Clause 6: The multi-beam charged particle system (1) according to any one of clauses 3 to 5, wherein the charged particle mirror element (700) comprises a third electrode (1317.3) connected to the control unit (800), and the control unit (800) is further configured to apply a third voltage U3 to the third ring-shaped electrode (1317.3) during use.

[0069] Clause 7: The multi-beam charged particle system (1) according to any one of clauses 3 to 6, wherein the charged particle mirror element (700) is arranged in a plane in which a plurality of primary charged particle beamlets (3) at least partially overlap.

[0070] Clause 8: The multi-beam charged particle system (1) according to any one of Clauses 3 to 6, wherein a charged particle mirror element (700) is arranged near an intermediate field plane where a plurality of focal spots are formed.

[0071] Clause 9: The multi-beam charged particle system (1) according to Clause 8, wherein the charged particle mirror element (700) comprises a plurality of multi-aperture plates (1327.1, 1327.2, 1327.3, 1325) having a plurality of apertures, and the plurality of multi-aperture plates (1327.1, 1327.2, 1327.3, 1325) are each configured to receive and reflect each individual one of the plurality of primary charged particle beamlets (3).

[0072] Clause 10: The multi-beam charged particle system (1) according to any one of Clauses 8 to 9, wherein the charged particle mirror element (700) is configured such that, for normal incidence of the plurality of primary beamlets (3), the reflected primary beamlets propagate in a direction substantially parallel to the incident primary beamlets.

[0073] Clause 11: - A primary charged particle beam splitter (460) for guiding the primary charged particle beamlets (3) from the multi-beam forming unit (305) to the charged particle mirror element (700) along a first beam path (13.1), and configured to guide the primary charged particle beamlets (3) from the charged particle mirror element (700) along a second beam path (13.2) in the direction of at least one objective lens (102) after reflection. The multi-beam charged particle system (1) according to Clause 10, further comprising

[0074] Clause 12: The primary charged particle beam splitter (460) - A splitter segment (460.1) for splitting the first beam path (13.1) from the second beam path (13.2), and - At least one first segment (460.2) arranged within the first beam path (13.1). - including at least one second segment (460.3) within the second beam path (13.2), the first and second segments (460.2, 460.3) being configured to compensate for the dispersion and additional aberrations of the splitter segment (460.1), The multi-beam charged particle system (1) according to clause 10.

[0075] Clause 13: The charged particle mirror element (700) forms a first path (13.1) from the multi-beam forming unit (305) to the charged particle mirror element (700) for an oblique angle of incidence (87), and forms a second path (13.2) from the charged particle mirror element (700) of the primary charged particle beamlet (3) after reflection in the direction of at least one objective lens (102), the first path and the second path (13.1, 13.2) being arranged at an angle exceeding 15° with respect to each other, the multi-beam charged particle system (1) according to any one of clauses 1 to 7.

[0076] Clause 14: The charged particle mirror element (700) has an elliptical cross-section, the multi-beam charged particle system (1) according to clause 13.

[0077] Clause 15: - A secondary electron beam splitter (400) configured to direct a secondary beamlet (9) generated at the focal spot (5) of a plurality of primary beamlets (3) on the surface (25) of a flat object (7) to a detector (700) The multi-beam charged particle system (1) according to any one of clauses 1 to 14, further comprising.

[0078] Clause 16: The multi-beam charged particle system (1) according to clause 15, further comprising a secondary electron imaging system (200) comprising a plurality of lens elements (205.1, 205.2, 205.3, 205.4, 205.5).

[0079] Clause 17: The multi-beam charged particle system (1) according to clause 15 or 16, wherein a primary charged particle beam splitter (460) and a secondary electron beam splitter (400) are formed as one integrated unit (480).

[0080] Clause 18: The multi-beam charged particle system (1) according to any one of clauses 1 to 17, further comprising a second charged particle mirror element (700.2), wherein the first and second mirror elements (700.1, 700.2) are configured to compensate for the image plane curvature of a plurality of lens elements (103.1, 103.2, 103.3, 102) during use.

[0081] Clause 19: The multi-beam charged particle system (1) according to any one of clauses 1 to 18, wherein the charged particle mirror element (700) is further configured to compensate for additional imaging aberrations of at least one of the primary beamlets (3, 3.1, 3.2, 3.3) during use.

[0082] Clause 20: A method of operating a multi-beam charged particle system (1) comprising a charged particle mirror element (700), the method comprising: Step S: Selecting a parameter setting of the multi-beam charged particle system (1); Step C: Determining the image plane curvature of the multi-beam charged particle system (1) having the selected parameter setting, determining at least one drive voltage configured to compensate for the image plane curvature, and applying the drive voltage to at least one electrode of the charged particle mirror element (700); Step A: Performing the application of the multi-beam charged particle system (1). and including.

[0083] Clause 21: The method according to clause 20, further comprising step M of monitoring the performance of the multi-beam charged particle system (1) and determining residual aberrations.

[0084] Clause 22: The method according to clause 21, further comprising the step of determining at least one corrected drive voltage configured to compensate for residual aberration and applying the corrected drive voltage to at least one electrode of the charged particle mirror element (700).

[0085] Clause 23: The method according to any one of clauses 20 to 22, wherein the application is one of a wafer inspection or a mask writing task.

[0086] The present invention is not limited only to the embodiments or clauses described above. The whole or parts of the embodiments or examples can be combined with each other, and numerous modifications and changes are possible. Although some improvements have been described in the example of the multi-beam charged particle system for inspection, the improvements are not limited only to those, and are also applicable to other multi-beam charged particle systems such as multi-beam lithography systems, for example, multi-beam lithography systems for mask writing applications.

[0087] Throughout the embodiments, electrons are generally understood to be charged particles. Although some embodiments have been described with examples of electrons, the embodiments are not limited only to electrons, and are applicable to all kinds of charged particles such as, for example, helium or neon ions.

Explanation of reference numerals

[0088] 1 Multi-beamlet charged particle system 3 Primary charged particle beamlets 5 Primary charged particle beam spots 7 Object 9 Secondary electron beamlets 11 Secondary electron beam paths 13 Primary charged particle beam paths 15 Secondary electron image spots 25 Surface of the object 43 Spherically curved surface 74 Beam waist 87 Oblique incident angle 100 Object irradiation unit 101 Image plane 102 Objective lens 103 Field lens 105 Optical axis of the multi-beamlet charged particle system 1 108 First beam intersection 110 Collective multi-beam raster scanner 200 Detection unit 205 Lens element 214 Aperture filter 216 Active element 222 Second deflection system 300 Charged particle multi-beamlet generator 301 Charged particle source 303 Collimating lens 304 Filter plate 305 Primary multi-beamlet forming unit 306 Multi-aperture plate 307 Terminal multi-aperture plate 308 Field lens 309 Primary electron beam 321 Intermediate image plane 400 Beam splitter unit 460 Primary beam splitter 480 Integrated primary and secondary charged particle splitter unit 500 Sample stage 503 Sample voltage source 600 Image sensor 700 Charged particle mirror element 705 Vertical axis with respect to the charged particle mirror element 800 Control unit 860 Scanning control unit 1311 Charged particle beam path or trajectory 1315 Mirror electrode 1317 Electrode 1318 Field lens electrode 1321 Virtual reflection surface 1328 Common path field lens 1341 Equipotential line 1342 Electric field line

Claims

1. An object irradiation unit (100) comprising, wherein the object irradiation unit (100) is - a charged particle beam source (301); - a multi-beam forming unit (305) for forming a plurality of primary beamlets (3, 3.1, 3.2, 3.3); - a plurality of lens elements (102, 103, 103.1, 103.2, 103.3) including at least one objective lens (102) for forming a plurality of focal spots (5, 5.1, 5.2, 5.3) of the plurality of primary beamlets (3, 3.1, 3.2, 3.3) in an image plane (101); - a charged particle mirror element (700) configured to compensate for the field curvature of the plurality of lens elements (102, 103, 103.1, 103.2, 103.3) during use A multi-beam charged particle system (1).

2. - A control unit (800) further comprising, wherein the control unit (800) is configured to apply a plurality of voltages (U1, U2, U3, Um) to the charged particle mirror element (700) during use, and the charged particle mirror element (700) and the voltages (U1, U2, U3, Um) are configured to generate a reflection lens field distribution having a virtual reflection surface (1321) during use, The multi-beam charged particle system (1) according to claim 1.

3. The charged particle mirror element (700) comprises at least three electrodes (1317.1, 1317.2, 1315), and the at least three electrodes (1317.1, 1317.2, 1315) include at least one first and second ring-shaped electrodes (1317.1, 1317.2) and a mirror electrode (1315) connected to the control unit (800). The control unit (800) is configured to apply a first voltage U1 to the first ring-shaped electrode (1317.1), a second voltage U2 to the second ring-shaped electrode (1317.2), and a mirror voltage Um to the mirror electrode (1315) during use, and the electrodes and the voltages are configured to generate the virtual reflection surface (1321) during use. The multi-beam charged particle system (1) according to claim 2.

4. The multi-beam charged particle system (1) according to claim 3, wherein the mirror electrode (1315) has a curved shape.

5. The mirror electrode (1315) is a segmented electrode including a plurality of N electrode segments (1315.1 to 1315.N), and the control unit (800) is further configured to apply a plurality of mirror voltages Um1 to UmN to the plurality of N electrode segments (1315.1 to 1315.N) during use. The multi-beam charged particle system (1) according to claim 3 or claim 4.

6. The charged particle mirror element (700) includes a third ring-shaped electrode (1317.3) connected to the control unit (800), and the control unit (800) is further configured to apply a third voltage U3 to the third ring-shaped electrode (1317.3) during use. The multi-beam charged particle system (1) according to any one of claims 3 to 5.

7. The charged particle mirror element (700) is arranged in a plane in which the plurality of primary charged particle beamlets (3) at least partially overlap. The multi-beam charged particle system (1) according to any one of claims 3 to 6.

8. The charged particle mirror element (700) is arranged near an intermediate field plane where a plurality of focal spots are formed. The multi-beam charged particle system (1) according to any one of claims 3 to 6.

9. The charged particle mirror element (700) includes a plurality of multi-aperture plates (1327.1, 1327.2, 1327.3, 1325) having a plurality of apertures, and the plurality of multi-aperture plates (1327.1, 1327.2, 1327.3, 1325) are individually configured to receive and reflect each of the plurality of primary charged particle beamlets (3) of the plurality of primary charged particle beamlets (3). The multi-beam charged particle system (1) according to claim 8.

10. The charged particle mirror element (700) is configured such that, for the perpendicular incidence of the plurality of primary beamlets (3), the reflected primary beamlets propagate in a direction substantially parallel to the incident primary beamlets. The multi-beam charged particle system (1) according to any one of claims 8 to 9.

11. - A primary charged particle beam splitter (460) for guiding the primary charged particle beamlet (3) from the multi-beam forming unit (305) to the charged particle mirror element (700) along a first beam path (13.1), and configured to guide the primary charged particle beamlet (3) from the charged particle mirror element (700) in the direction of the at least one objective lens (102) along a second beam path (13.2) after reflection, said primary charged particle beam splitter (460) The multi-beam charged particle system (1) according to claim 10, further comprising

12. wherein the primary charged particle beam splitter (460) - A splitter segment (460.1) for splitting the first beam path (13.1) from the second beam path (13.2), - At least one first segment (460.2) disposed within the first beam path (13.1), and - At least one second segment (460.3) within the second beam path (13.2), wherein the first and second segments (460.2, 460.3) are configured to compensate for the dispersion and additional aberrations of the splitter segment (460.1), The multi-beam charged particle system (1) according to claim 10.

13. The charged particle mirror element (700) forms a first path (13.1) from the multi-beam forming unit (305) to the charged particle mirror element (700) for an oblique angle of incidence (87), and forms a second path (13.2) from the charged particle mirror element (700) of the reflected primary charged particle beamlet (3) in the direction of the at least one objective lens (102), and the first path and the second path (13.1, 13.2) are arranged at an angle exceeding 15° with respect to each other. The multi-beam charged particle system (1) according to any one of claims 1 to 7.

14. The multi-beam charged particle system (1) according to claim 13, wherein the charged particle mirror element (700) has an elliptical cross-section.

15. - A secondary electron beam splitter (400) configured to guide the secondary beamlet (9) generated at the focal spot (5) of the plurality of primary beamlets (3) on the surface (25) of the flat object (7) to a detector (700) The multi-beam charged particle system (1) according to any one of claims 1 to 14, further comprising

16. The multi-beam charged particle system (1) according to claim 15, further comprising a secondary electron imaging system (200) comprising a plurality of lens elements (205.1, 205.2, 205.3, 205.4, 205.5).

17. The multi-beam charged particle system (1) according to claim 15 or 16, wherein the primary charged particle beam splitter (460) and the secondary electron beam splitter (400) are formed as one integrated unit (480).

18. The multi-beam charged particle system (1) according to any one of claims 1 to 17, further comprising a second charged particle mirror element (700.2), wherein the first and second mirror elements (700.1, 700.2) are configured to compensate for the image plane curvature of the plurality of lens elements (103.1, 103.2, 103.3, 102) during use.

19. The multi-beam charged particle system (1) according to any one of claims 1 to 18, wherein the charged particle mirror element (700) is further configured to compensate for additional imaging aberrations of at least one of the primary beamlets (3, 3.1, 3.2, 3.3) during use.

20. A method of operating a multi-beam charged particle system (1) comprising a charged particle mirror element (700), comprising: Step S: selecting parameter settings of the multi-beam charged particle system (1); Step C: determining the image plane curvature of the multi-beam charged particle system (1) having the selected parameter settings, determining at least one drive voltage configured to compensate for the image plane curvature, and applying the drive voltage to at least one electrode of the charged particle mirror element (700); Step A: performing the use of the multi-beam charged particle system (1) The method comprising.

21. The method according to claim 20, further comprising step M of monitoring the performance of the multi-beam charged particle system (1) and determining residual aberrations.

22. The method according to claim 21, further comprising determining at least one corrected drive voltage configured to compensate for the residual aberrations and applying the corrected drive voltage to the at least one electrode of the charged particle mirror element (700).

23. The method according to any one of claims 20 to 22, wherein the use is one of wafer inspection or mask writing task.

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