Multi-beam charged particle microscope design with mirror for field curvature correction

Abstract

A multi-beam charged particle system (1) with reduced field curvature is provided. The multi-beam charged particle system comprises a charged particle mirror element (700) for compensating for the field curvature of the charged particle imaging element (102, 103.1, 103.2). The charged particle mirror element may be configured to create, during use, a curved shaped virtual reflective surface (1321) for reflecting primary charged particles (3.1-3.3). The method can be applied to a multi-beam charged particle system with high requirements on beam uniformity and throughput.

Description

Technical Field

[0001] The present invention relates to a multi-beam charged particle microscope design having mirrors as design components for correcting field curvature. Background Art

[0002] WO 2005 / 024881 A2 discloses an electron microscope system, which operates using multiple electron beamlets (Beamlets) that use a beam of electron beamlets to scan the object to be inspected in parallel. The primary charged particle beamlet of this beam is generated by directing the primary charged particle beam to a multi-beam forming unit, which includes at least one porous plate with multiple openings. A portion of the electrons of the electron beam are incident on the porous plate and absorbed there, and another portion of the electron beam passes through the openings of the porous plate, thereby forming an electron beamlet in the beam path downstream of each opening, and its profile is defined by the profile of the opening. In addition, a properly selected electric field provided in the beam path upstream and / or downstream of the porous plate causes each opening in the porous plate to act as a lens on the electron beamlet passing through the opening, so that each of the electron beamlets is focused and forms a focal spot in a surface at a certain distance from the porous plate. The surface forming the focus of the electron beamlet is imaged by a downstream optical device onto the surface of the object or sample to be inspected. The primary charged particle beamlet triggers secondary electrons or backscattered electrons as secondary electron beamlets emitted from the object, which are collected and imaged onto a detector. Each secondary beamlet is incident on an individual detector element so that the secondary electron intensity detected by it provides information related to the sample at the position where the corresponding primary beamlet is incident on the sample. A beam of primary beamlets is systematically scanned over the sample surface and an electron microscope image of the sample is produced in the usual manner of a scanning electron microscope. The resolution of a scanning electron microscope is limited by the focal diameter of the primary beamlets incident on the object. Therefore, in a multi-beam electron microscope, all beamlets should form the same small focal spot on the object.

[0003] Multi-beam microscopes for wafer inspection form multiple focal spots of multiple primary charged particle beamlets on the wafer surface. The imaging lens generates a field curvature that causes the multiple primary focal spots to deviate from the flat wafer surface. As a result, the field curvature causes large deviations in the focal spot size on the wafer surface. As the throughput requirements of inspection tasks increase and the number of charged particle beamlets increases accordingly, the field size also increases and increases the deviation due to field curvature.

[0004] In the prior art, it has been considered to compensate for the field curvature by forming at least one micro-optical element as an integral part of the multi-beam forming unit. The porous plate with electrodes is usually formed by layer deposition and etching techniques and forms a stack of different layers. For larger strokes, higher voltages must be supplied to the electrostatic lenses. The non-uniformity of the layer deposition and the leakage of the electric field lead to non-uniform electron-optical properties of the electrostatic elements on the porous plate. In the porous stack, the optical performance is usually limited. In the multi-beam forming unit, it is difficult to achieve a sufficient stroke to individually change the focus position of each primary charged particle beamlet with high precision as required by the wafer inspection task. Summary of the invention

[0005] Accordingly, an object of the present invention is to provide a charged particle beam system that operates with multiple charged particle beams and can be used to achieve higher imaging performance, such as better resolution of each beamlet in multiple beamlets and a narrower resolution range. Another object of the present invention is to provide a multi-beam charged particle beam system with reduced field curvature.

[0006] US 2014 / 0 158 902 A1 discloses a particle optical configuration for a multi-beam system, but does not disclose a charged particle reflector element.

[0007] US2011 / 0 291 021A1 relates to a single beam system and discloses an apparatus for reflective electron beam lithography. The apparatus comprises an electron source, a patterned electron reflector generator structure, a stage, a reduction electron lens and an ExB separator. The ExB separator is configured to bend the electron beam towards the trajectory of the dynamic pattern generator structure. The patterned electron reflector structure is configured to reflect a selected portion of the electron beam to form a patterned electron beam. The ExB separator is further configured to allow the patterned electron beam to pass directly 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 herein has a straight projection axis and significantly reduces the electron beam path by a factor of three to one (compared to existing apparatuses using magnetic prisms).

[0008] Description of the invention

[0009] The object of the invention is achieved by the independent claims. The dependent claims relate to advantageous embodiments.

[0010] This application claims priority to German Patent Application No. 10 2022 206937.4 filed on July 7, 2022, the disclosure of which is incorporated by reference into the present application for its entirety.

[0011] The object of the present invention is achieved by a multi-beam charged particle system with reduced field curvature. The multi-beam charged particle system with reduced field curvature comprises design means for compensating for the field curvature aberration introduced by the charged particle imaging element.

[0012] 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 planar object. The imaging system comprises at least one lens element, the at least one lens element comprising an objective lens and at least one field lens. The imaging system further comprises a charged particle reflector element, the reflector element being configured to compensate for the field curvature of the plurality of lens elements during use. The charged particle reflector element contributes to the generation of the field curvature of the imaging system (whose sign is opposite to the field curvature generated by the at least one lens element). Therefore, due to the design with the charged particle reflector element, the effect of the at least one lens element on the field curvature can be compensated. Due to the reduced field curvature, the focusing deviation of the primary charged particle beamlet from the object plane of the plane is minimized, and a predetermined resolution requirement can be achieved for a larger field size or a larger number of primary charged particle beamlets. Therefore, a larger field size with a larger number of primary charged particle beamlets can be used for inspection tasks and the output of the inspection tasks is improved.

[0013] The multi-beam charged particle system further comprises a control unit. The control unit is configured to provide at least one voltage to the charged particle reflector element during use. The charged particle reflector element and the voltage are configured to generate a reflective lens field having a virtual reflective surface with positive optical power or focusing optical power during use. The primary charged particles are decelerated and returned or reflected at the virtual reflective surface of the reflective lens field.

[0014] In one example, the charged particle reflector element comprises at least three electrodes, the electrodes comprising at least one first and one second annular electrode and a surface electrode. The electrodes are connected to a control unit, and the control unit is configured to provide a first voltage U1 to the first annular electrode, a second voltage U2 to the second annular electrode, and a reflector voltage Um to the reflector electrode during use. The voltage is configured to generate a reflective lens field of a virtual reflective surface with a positive optical focal length or a focusing optical focal length during use. In one example, the surface electrode has a curved shape. In one example, the surface electrode is a segmented electrode containing a plurality of N electrode segments, and wherein the control unit is further configured to provide a plurality of reflector voltages Um1 to UmN to the plurality of N electrode segments during use. In one example, the charged particle reflector element comprises a third electrode connected to the control unit, and the control unit is further configured to provide a third voltage U3 to the third annular electrode during use. The examples can also be combined. Due to the examples, a virtual reflective surface with a curved shape having a positive optical focal length or a focusing optical focal length is formed. A charged particle beamlet is reflected at the virtual reflective surface.

[0015] In one example, the charged particle reflector element is arranged in a plane where the plurality of primary charged particle beamlets at least partially overlap. In one example, the charged particle reflector element is arranged in a pupil plane. In an alternative example, the charged particle reflector element is arranged near an intermediate field plane, where a plurality of focal spots of a primary charged particle beamlet are formed. In one example, the charged particle reflector element comprises a plurality of porous plates having a plurality of holes, which are configured to individually receive and reflect each individual primary charged particle beamlet of a plurality of primary charged particle beamlets. In one example, at least one porous plate is configured with a plurality of electrodes, and the control unit is configured to provide an individual voltage to each electrode for individually controlling the reflection position of each individual primary charged particle beamlet. Therefore, a virtual reflection surface of a curved shape of a primary charged particle beamlet is obtained, and each primary charged particle beamlet is reflected at the virtual reflection surface. However, the porous plate configured to generate a virtual reflection surface of a curved shape is not limited to the above example, for example, at least one porous plate comprises a plurality of holes of different diameters.

[0016] According to one example, a multi-beam charged particle system further includes a secondary electron beam splitter or beam splitter, which is configured to guide secondary beamlets generated at the focal spots of multiple primary beamlets at the surface of a planar object to a detector. The multi-beam charged particle system further includes a secondary electron imaging system, which includes multiple charged particle optical elements for forming focal spots of secondary electron beamlets on the detector plane. The secondary electron beam splitter includes a separator segment for separating the beam path of primary charged particles from the beam path of secondary electrons. In one example, the secondary electron beam splitter further includes at least one first segment configured in the beam path of primary charged particles, and at least one second segment configured in the beam path of secondary electrons, the first segment and the second segment being configured to compensate for dispersion and further aberrations of the separator segment.

[0017] According to a first specific embodiment of the present invention, the charged particle reflector element is configured for normal incidence of the plurality of primary beamlets, so that the reflected primary beamlets propagate in a direction parallel to the primary beamlets before incident on the charged particle reflector element. The plurality of primary charged particle beamlets form a first path from the multi-beam forming unit to the charged particle reflector element, and form a second path of the primary charged particle beamlets after reflection from the charged particle reflector element in the direction of at least one objective lens. In the first specific embodiment, near the charged particle reflector element, the first beam path and the second beam path are at least partially parallel to each other.

[0018] According to a first specific embodiment, the multi-beam charged particle system further comprises a primary charged particle beam splitter for guiding the primary charged particle beamlets from the multi-beam forming unit to the charged particle reflector element along a first beam path. The primary charged particle beam splitter is further configured to guide the primary charged particle beamlets along a second beam path after the primary charged particle beamlets are reflected from the charged particle reflector element in the direction of at least one objective lens. In one example, the primary charged particle beam splitter comprises a splitter segment for separating the first beam path from the second beam path, at least one first segment configured in the first beam path, and at least one second segment configured in the second beam path, the first segment and the second segment being configured to compensate for dispersion and further aberrations of the splitter segment.

[0019] According to an example, the primary charged particle beam splitter and the secondary electron beam splitter are formed as an integrated unit.

[0020] According to a second specific embodiment of the present invention, the charged particle reflector element is configured for oblique incidence. According to the second specific embodiment, the multi-beam charged particle system is configured to form a first path from the multi-beam forming unit to the charged particle reflector element, and to form a second path of a primary charged particle beamlet after reflection from the charged particle reflector element in the direction of at least one objective lens, wherein the first and second paths are arranged at an angle exceeding 10° (e.g. 15° or 20°). In an example, the charged particle reflector element according to the second specific embodiment has an elliptical cross-section.

[0021] According to a third embodiment of the present invention, the multi-beam charged particle system further comprises a second charged particle mirror element, the first and second mirror elements being configured to jointly compensate for field curvature of the plurality of lens elements during use.

[0022] According to an example of a specific embodiment, the charged particle mirror element is further configured to compensate for further imaging aberrations during use. In addition to field curvature, other aberrations can be compensated using the charged particle mirror element. Such further aberrations may be field dependent or field invariant. An example is compensation of axial chromatic aberration or dispersion of the plurality of beamlets, or compensation of field dependent astigmatism or coma.

[0023] In a further specific embodiment, a method for operating a multi-beam charged particle system for variable compensation of field curvature is provided. The field 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 provided by the control unit for driving the charged particle mirror element are configured to variably compensate for the field curvature and optionally other aberrations.

[0024] By each of the specific embodiments or examples of the present invention, a multi-beam charged particle beam system with reduced field curvature is provided. Thus, the present invention allows wafer inspection with higher precision and less variation in the focal spot size of the focal spots disposed on the wafer surface in the object plane.

[0025] It should be understood that the present invention is not limited to the specific embodiments and examples described, but also includes combinations and variations of the specific embodiments and examples described. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Specific embodiments of the present invention will be described in more detail with reference to the accompanying drawings, in which:

[0027] Figure 1 is a schematic cross-sectional view of a multi-beam charged particle system of the prior art;

[0028] Figure 2 A multi-beam charged particle system with a charged particle mirror element is schematically depicted;

[0029] Figure 3 Draw the field curvature effect of the prior art system;

[0030] Figure 4 An example of a first specific embodiment according to the present invention is shown;

[0031] Figure 5 illustrates the effect of a field curvature-free design of a system according to a first embodiment;

[0032] FIG6 illustrates some examples of charged particle mirror elements;

[0033] Figure 7 An example of a charged particle mirror element using a plurality of porous plates is shown;

[0034] Figure 8 Draw with Figure 7 An example of a multi-beam charged particle system having a charged particle mirror element;

[0035] Fig. 9 depicts the components of a charged particle beam splitter;

[0036] Fig.10 A second specific embodiment of the present invention is shown;

[0037] FIG. 11 illustrates a charged particle mirror element according to a second embodiment of the present invention;

[0038] Fig.12 Another example of the second embodiment of the present invention is shown;

[0039] FIG13 schematically illustrates another variation of the present invention;

[0040] Fig.14 A method of operating a multi-beam charged particle system with a mirror corrector is shown. DETAILED DESCRIPTION

[0041] In the exemplary embodiments of the present invention described below, components having similar functions and structures are indicated by similar or identical reference numerals as much as possible.

[0042] Some array elements, such as a plurality of primary charged particle beamlets are identified by reference numerals. Depending on the context, the same reference numeral may also identify a single element or an array element. Each primary charged particle beamlet (3.1, 3.2, 3.3) is one of a plurality of primary charged particle beamlets (3).

[0043] Figure 1The schematic diagram of a multi-beam charged particle system 1 shows the basic features and functions. It should be noted that the symbols used in the figure are selected to symbolize their respective functions. The type of system shown is a multi-beam scanning electron microscope, which uses multiple primary charged particle beamlets 3 to produce multiple primary charged particle beam spots 5 on the surface 25 of an object 7, such as a wafer or a mask substrate, the surface 25 of the top of which is located 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-beam charged particle system 1 can be implemented using electrons or other types of primary charged particles (such as ions, especially helium ions). More details of the microscope system 1 are provided in the international patent application PCT / EP2021 / 066255 filed on June 16, 2021, which is incorporated into the present application for reference in its entirety.

[0044] The system 1 comprises an object irradiation unit 100 and a detection unit 200, and a secondary electron beam splitter or beam splitter unit 400 for separating a secondary charged particle beam path 11 from a 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 adapted to focus the plurality of primary charged particle beamlets 3 on an object plane 101, wherein a surface 25 of an object 7 is positioned by a sample carrier 500.

[0045] The primary beam generator 300 generates a plurality of primary charged particle beamlet spots in the intermediate image surface 321. The primary beamlet generator 300 comprises at least one primary charged particle source 301, such as electrons. The at least one primary charged particle source 301 emits a divergent primary charged particle beam, which is collimated by at least one collimating lens 303 to form a collimated or parallel primary charged particle beam 309. The collimating lens 303 is generally composed of one or more electrostatic or magnetic lenses, or a combination of electrostatic and magnetic lenses. The collimated primary charged particle beam 309 is incident on a primary multi-beam forming unit 305. The multi-beam forming unit 305 basically comprises a first porous plate or filter plate 304, which is irradiated by the collimated primary charged particle beam 309. The first porous plate or filter plate 304 comprises a plurality of holes in a grating configuration for generating a plurality of primary charged particle beamlets 3, which are generated by transmitting a collimated primary charged particle beam 309 through the plurality of holes. The multi-beamlet forming unit 305 comprises at least one further porous plate 306, which is located downstream of the first porous plate or filter plate 304 with respect to the direction of movement of the electrons in the primary charged particle beam 309. For example, the second porous plate 306 comprises, for example, four or eight electrostatic elements for each of the plurality of holes, for example for individually deflecting each of the plurality of beamlets. The multi-beamlet forming unit 305 according to some specific embodiments is configured with an end porous plate (307). The multi-beamlet forming unit 305 is further configured with an adjacent electrostatic field lens 308.1, which in some instances is integrated in the multi-beamlet forming unit 305. Together with the second field lens 308.2, the plurality of primary charged particle beamlets 3 are focused in or near the intermediate image surface 321. Each of the primary charged particle source 301 and the active porous plate 306 is controlled by a control unit 800 .

[0046] The multiple focal spots of the primary charged particle beamlets 3 passing through the intermediate image surface 321 are imaged into the object plane 101 by the field lens group 103 and the objective lens 102, and the surface 25 of the object 7 is also located in the object plane 101. A voltage is applied to the object by the sample voltage supply unit 503, and a deceleration electrostatic field is generated between the objective lens 102 and the surface 25 of the object. The object irradiation system 100 further includes a collective multi-beam raster scanner 110 located near the beam intersection 108, through which the multiple charged particle beamlets 3 can be deflected in a direction perpendicular to the propagation direction of the charged particle beamlets. The propagation direction of the primary beamlets in the example is the positive z direction. The objective lens 102 and the collective multi-beam raster scanner 110 are centered on the optical axis (not shown) of the multi-beam charged particle system 1, which is perpendicular to the wafer surface 25. The multiple primary charged particle beamlets 3 forming the multiple beam spots 5 configured in a raster structure are synchronously scanned on the wafer surface 25. In one example, the grating structure of the focal spot 5 of the plurality of J primary charged particles 3 is a hexagonal grating of about one hundred or more primary charged particle beamlets 3, for example, J=91, J=100, or J is about 300 or more beamlets. The distance of the primary beam spot 5 is about 6μm to 45μm, and the diameter is less than 5nm, for example, 3nm, 2nm or even smaller. In one example, the beam spot size is about 1.5nm, and the distance between two adjacent beam spots is 8μm. At each scanning position of each of the plurality of primary beam spots 5, a plurality of secondary electrons are generated respectively to form a plurality of secondary electron beamlets 9 having the same grating structure 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 impacting primary charged particle beamlet 3 irradiating the corresponding beam spot 5, the material composition and morphology 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 the electrostatic field between the objective lens 102 and the object surface 25, collected by the objective lens 102, and pass through the collective multi-beam raster scanner 110 in the opposite direction to the primary beamlets 3. The plurality of secondary beamlets 9 are deflected by the first collective multi-beam raster scanner 110 for scanning. The plurality of secondary charged particle beamlets 9 are then guided by a secondary electron beam splitter or beam splitter unit 400 to follow the secondary beam path 11 of the detection unit 200. The plurality of secondary beamlets 9 travel in the opposite direction to the primary charged particle beamlets 3, and the beam splitter unit 400 is typically configured to separate the secondary beam path 11 from the primary beam path 13 by a magnetic field or a combination of a magnetic field and an electrostatic field.

[0047] The detection unit 200 images the secondary electron beamlets 9 onto the image sensor 600 to form a plurality of secondary charged particle image spots 15 on the image sensor. The detector or image sensor 600 includes a plurality of detector pixels or individual detectors. For each of the plurality of secondary charged particle beam spots 15, the intensity is detected separately, and the characteristics of the object surface 25 are detected with high resolution for a large image block of the object 7. For example, for a grating of 10×10 beamlets with a pitch of 8 μm, a single image scan using the collective multi-beam raster scanner 110 produces an image block of approximately 88 μm×88 μm, with an image resolution of, for example, 2 nm or less. The image block is sampled using half the beam spot size, so the number of pixels per image line for each beamlet is 8000 pixels, so that the image block produced by 100 beamlets contains 6.4 giga (billion) pixels. The digital image data is collected by the control unit 800. Details regarding digital image data collection and processing (e.g., using parallel processing) are described in International Patent Application WO 2020151904 A2 and U.S. Patent No. US 9.536.702, the contents of which are incorporated by reference into this application for reference.

[0048] The detection unit 200 further comprises at least one collective raster scanner 222, which is connected to a scanning and imaging control unit 860. The scanning control unit 860 is configured to compensate for residual differences in the positions of the multiple focal spots 15 of the multiple secondary electron beamlets 9, so that the positions of the multiple secondary electron focal spots 15 at the image sensor 600 remain constant.

[0049] The detection unit 200 further comprises electrostatic or magnetic lenses 205.1 to 205.5 and a second intersection of the plurality of secondary electron beamlets 9, at which an aperture 214 is located. The detection unit 200 can further comprise at least one first multi-aperture corrector 216 having apertures and electrodes for influencing each of the plurality of secondary electron beamlets 9 individually.

[0050] The image sensor 600 is configured as an array of sensing areas whose pattern is compatible with the raster configuration of the secondary electron beamlets 9 focused onto the image sensor 600 by the projection lens 205. This enables detection of each individual secondary electron beamlet independently of other secondary electron beamlets incident on the image sensor 600. Figure 1The image sensor 600 shown in the figure can be an electron-sensitive detector array, such as a CMOS or CCD sensor. This electron-sensitive detector array can include an electron-to-photon conversion unit, such as a scintillator element or a scintillator element array. In another embodiment, the image sensor 600 can be configured as an electron-to-photon conversion unit or a scintillator plate, which is configured in the focal plane of multiple secondary electron particle image spots 15. In this embodiment, the image sensor 600 can further include a relay optical system for imaging at the secondary charged particle image spots 15 on dedicated photon detection elements (such as multiple photomultiplier tubes or avalanche photodiodes (not shown)) and guiding the photons generated by the electron-to-photon conversion unit. This image sensor is disclosed in US 9,536,702, which is cited above and incorporated into this application by reference.

[0051] It is preferred that the stage 500 is not moved during acquisition of an image block by scanning a plurality of primary charged particle beamlets 3, and after acquisition of an image block, the stage 500 is moved to the next image block to be acquired. In an alternative embodiment, the stage 500 is continuously moved in the second direction while acquiring an image by scanning a plurality of primary charged particle beamlets 3 in the first direction with 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 a laser interferometer, a grating interferometer, a confocal microlens array, etc.

[0052] During image scanning, the control unit 800 is configured to trigger the image sensor 600 to detect multiple time-resolved intensity signals from multiple secondary electron beamlets 9 at predetermined time intervals, and a digital image of the image block is accumulated and stitched together from all scanning positions of the multiple primary charged particle beamlets 3.

[0053] The multi-beam generating unit 305 is explained, for example, in US2019 / 0259575 and US10741355 B1, both of which are incorporated by reference into the present application for reference. In the prior art, the positions of the multiple focal points of the multiple primary charged particle beamlets (3) in the intermediate image surface (321) are adjusted by the multi-beam generating unit (305). According to the prior art, the multi-beam generating unit 305 is the only component for pre-compensating the field curvature of the optical elements of the object irradiation unit (100) located downstream of the multi-beam generating unit 305. The amount of field curvature is adjusted according to the driving parameters of the object irradiation unit 100, such as the focusing ability of the objective lens 102, or the electrostatic field generated by the voltage provided between the objective lens 102 and the object surface 25 by the sampling voltage supply unit 503, both of which are the main sources of the amount of field curvature. However, compensating the field curvature with the multi-beam generating unit 305 alone is limited, so an alternative solution or an additional solution is needed to compensate the field curvature. This alternative or additional solution is provided by the present invention described in the following examples and specific embodiments. Figure 2 The present invention is schematically illustrated with simplified examples. Figure 2 A multi-beam charged particle system 1 is shown, which has a charged particle source 301, at least one collimating lens 303 and a primary multi-beamlet forming unit 305. Herein, only three primary charged particle beamlets 3.1 to 3.3 are shown, but their number may be larger, for example more than 100, 300 or even more. The system 1 further comprises a plurality of lens elements, which include 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 a surface 25 of a planar object 7. The system 1 further comprises a charged particle mirror element (700), which is configured to compensate for the field curvature of the plurality of lens elements 102, 103.1, 103.2 during use. The charged particle reflector element (700) is configured to reflect multiple primary charged particle beamlets 3, and thereby divide the primary beam path 13 into a first primary beam path segment 13.1 between the multi-beam forming unit 305 and the charged particle reflector element 700, and a second primary beam path segment 13.2 after the primary charged particle beamlet 3 is reflected from the charged particle reflector element 700 in the direction of the objective lens 102.

[0054] The system 1 further comprises a control unit 800. The control unit is configured to provide a plurality of voltages to the charged particle mirror element 700 during use. The charged particle mirror element 700 and the voltages are configured to generate a reflective lens field having a virtual reflective surface during use. More details of the reflective lens field will be described below. Figure 1 The system 1 further comprises a secondary electron beam splitter or beam splitter 400 configured to direct the secondary beamlets generated at the focal spot of the plurality of primary beamlets at the surface of the planar object along the secondary electron beam path 11 to the detector 600 .

[0055] Figure 3 FIG. 4 illustrates the field curvature effect according to the prior art. Figure 3 A plurality of primary beamlets 3.1 to 3.5 are shown, each having its beam waists 74.1 to 74.5 on a spherical curved surface 43 of radius R. The minimum focal spot size 74.1 to 74.5 is identical to the beam waist of the beamlets 3.1 to 3.5. Due to the tilt of the image plane, the center of the spherical surface 43 can also deviate from the optical symmetry axis 105. If a flat object 7 such as a mask substrate or a wafer is placed in the image plane 101, the primary charged particle focal spot 5 (which is the same origin as the secondary electrons on the wafer surface 25) will be very different and include a focal spot 5.1 with a minimum spot size, a focal spot 5.2 with an average spot size and a focal spot 5.3 with a maximum spot size (see Figure 3 b). The spot size variation results in a different resolution for each secondary electron beamlet 9. However, for mask writing or wafer inspection tasks, there are strict requirements on the uniformity of the resolution, for example the maximum deviation of the resolution provided by each secondary electron beamlet 9 should not exceed, for example, 10% or even less, for example 7% or only 5% of the resolution requirement. The resolution requirement itself can, for example, be 2nm or less, for example 1.5nm. Therefore, field curvature is one of the driven aberrations, which limits the performance of the multi-beam charged particle system 1 for imaging tasks, such as multi-beam direct writing of microstructures on masks or wafer inspection tasks.

[0056] Figure 4 The first embodiment of the present invention is shown. The multi-beam charged particle system 1 according to the first embodiment comprises a charged particle beam source 301 and two collimating lenses 303 for forming a primary charged particle beam 309. The multi-beam forming unit 305 forms a plurality of primary charged particle beamlets 3. Figure 1 The example of the embodiment of the present invention describes more details about the multi-beamlet forming unit 305. The system 1 further comprises a charged particle mirror element 700, having a mirror electrode 1315 and a plurality of further electrodes 1317, which are configured to form a virtual reflection surface 1321 during use for reflecting incident primary charged particle beamlets 3 (only three beamlets 3.1 to 3.3 are shown). The mirror electrode 1315 and the plurality of further electrodes 1317 are connected to a control unit 800, which provides a predetermined voltage to the mirror electrode 1315 and the plurality of further electrodes 1317.

[0057] The system 1 further comprises a primary beam splitter 460 for guiding the primary charged particle beamlets 3 from the multi-beam forming unit 305 along the first beam path 13.1 to the charged particle mirror element 700 and configured to guide the primary charged particle beamlets 3 along the second beam path 13.2 after being reflected from the charged particle mirror element 700 in the direction of the at least one objective lens 102. The primary beam splitter 460 of this example comprises a splitter segment 460.1 for separating the first beam path 13.1 from the second beam path 13.2, a first segment 460.2 arranged in the first beam path 13.1, and a second segment 460.3 arranged in the second beam path 13.2, the first segment 460.2 and the second segment 460.3 being configured to compensate for dispersion and further aberrations of the splitter segment 460.1. The system 1 further comprises a common path field lens 1328 in the common beam path between the separator section 460.1 and the charged particle reflector element 700. The system 1 further comprises at least one first field lens 103.1 in the first beam path 13.1 and at least one second field lens 103.2 in the second beam path 13.2. Figure 4 For other components, refer to Figure 1 and its description.

[0058] Figure 5 The effect of the invention is illustrated. Compensation of the field curvature by the charged particle mirror element 700 provides a system 1 without field curvature, wherein all points or beam waists 74.1 to 74.5 with minimal aliasing are formed in the object plane 101. On the flat surface 25 of the object 7, all focal spots 5.ij have the same minimum diameter. In practice, there may be some smaller diameter variations due to residual aberrations, which are below a predetermined critical value. To further compensate for the residual diameter variations of the focal spots, the multi-beamlet forming unit 305 also has a multipole array or lens array 306 for compensating for residual aberrations. In the absence of field curvature, the number of beamlets J can be increased and a higher throughput of the system 1 can be achieved.

[0059] FIG6 illustrates some examples of charged particle mirror elements 700. FIG6a shows a first example of a charged particle mirror element 700, which includes three annular electrodes 1317.1 to 1317.3 and one continuous mirror electrode 1315 having a cup shape with a flat bottom. By means of a control unit 800, a plurality of voltages U1 to U3 and Um are provided to the electrodes 1317.1 to 1317.3 and 1315, such that incident primary charged particles along, for example, a charged particle path 1311 are decelerated and eventually reflected at a virtual reflection surface 1321. The geometry of the electrodes 1317 and 1315 and the voltages U1 to U3 and Um are selected to generate equipotential lines 1341 with electric field lines 1342 (i.e., dashed lines), such that a virtual reflection or mirror surface 1321 is formed. The virtual reflection or mirror surface 1321 corresponds to an equipotential surface, at which the primary charged particles are decelerated to a kinetic energy of 0 kV. Therefore, at the virtual reflecting surface 1321, the primary charged particle is reflected and accelerated by the electric field, so that the reflected primary charged particle travels in a substantially opposite direction along the path 1311. The voltage is selected according to |U1|>|U2|>|U3|>|Um|, where |U1| is approximately equal to the kinetic energy of the primary charged particle. |U2| is selected within the range of |U1| / 5<|U2|<|U1| / 3. |U3| is selected within the range of |U2| / 4<|U3|<|U2| / 2. |Um| is selected as a positive voltage, where Um~-1x U3. Therefore, the primary charged particle is decelerated at the virtual reflecting mirror surface 1321 to reach zero kinetic energy, and is accelerated in the opposite direction to reach the target kinetic energy of the primary charged particle again after reflection. Figure 6b shows an equivalent example with a curved reflector electrode 1315. 6c and 6d show an equivalent example with a segmented mirror electrode 1315 comprising a plurality of annular segments 1315.1 to 1315.4 to which different mirror voltages Um1 to Um4 are provided to produce a virtual mirror surface 1321 during use. For simplicity, only annular segments are shown, but the number of annular segments may be greater. It should also be understood that this example is not limited to annular segments, other shapes are also possible, such as hexagonal or elliptical segments or grating segments with appropriate voltages Um provided to form a curved virtual reflective surface 1321.

[0060] The charged particle reflector element 700 and the voltages provided to the electrodes of the charged particle reflector element 700 by the control unit 800 can be further configured to change the field curvature compensation. Since the control unit 800 provides different voltages to the electrodes of the charged particle reflector element 700, the curvature of the virtual reflection surface can be adjusted. After a setting change of the multi-beamlet charged particle beam system 1, for example, after a change in the sampling voltage provided by the sampling voltage supply unit 503, or after a change in the magnification by the objective lens 102, for example, a change in the field curvature compensation is required.

[0061] Not only can the field curvature be compensated by the reflector element 700, but also other aberrations of the multi-beam charged particle system 1 can be compensated. An example is the compensation of axial chromatic aberration or dispersion of multiple beamlets. The virtual reflection surface 1321 corresponds to an equipotential surface, which is suitable for reflecting primary charged particles (3) of a specific first kinetic energy. For other or second kinetic energies, reflection occurs at different equipotential surfaces, which are suitable for reflecting primary charged particles (3) of a specific second kinetic energy. By appropriately designing the electrodes 1317 including the reflector electrodes 1315, a curved virtual reflection surface 1321 of a suitable shape and sequence for the kinetic energy distribution of the primary charged particles can be configured. Therefore, imaging aberrations corresponding to changes in the kinetic energy of the primary charged particles are provided.

[0062] Figure 7 Another example of a charged particle reflector element 700 is depicted. In this example, the charged particle reflector element 700 is configured as a reflector array, one reflector for each primary charged particle beamlet, wherein five trajectories 1311.1 to 1311.5 of the primary charged particle beamlets are shown. The reflector array is formed by porous plates 1327.1 to 1327.3 and a reflector porous plate 1325. In one example, each hole of at least one of the porous plates 1327.1 to 1327.3 and 1325 has an annular electrode. For example, the porous plate 1327.2 includes J holes for a plurality of J primary charged particle beamlets, each hole having an annular electrode. A voltage U2i is provided to the i-th annular electrode of the i-th hole. Using the voltages U1i to U3i and Umi, the return point is adjusted according to the virtual reflector surface 1321 for each primary charged particle beamlet.

[0063] In one example, according to Figure 7 The charged particle mirror element 700 is further configured to compensate for another imaging aberration during use. For example, at least one of the multi-aperture plates 1327.1 to 1327.3 comprises a multipole electrode, rather than a ring electrode arranged around each aperture, which is configured to correct the aberration of each beamlet individually. An example is compensation for field-dependent astigmatism or coma aberration.

[0064] Figure 8 An example of a multi-beam charged particle system 1 according to the first embodiment is shown, which has a configuration as shown in FIG. Figure 7 The charged particle reflector element 700 of the reflector array can further include a field lens electrode 1318. Figure 8 For further information, refer to Figure 4 and its description.

[0065] Fig. 9An example of a primary beam splitter 460 is shown. The primary beam splitter 460 comprises a splitter segment 460.1 for separating a first beam path 13.1 from a second beam path 13.2, and a first segment 460.2 arranged in the first beam path 13.1 and a second segment 460.3 arranged in the second beam path 13.2, the first segment 460.2 and the second segment 460.3 being arranged to compensate for dispersion and further aberrations of the splitter segment 460.1. In the direction of the first beam path 13.1, a charged particle mirror element 700 is arranged and configured to reflect primary charged particles travelling along the first beam path 13.1 into the second beam path 13.2. The second segment 460.2 comprises three magnetic sectors 460.3a to 460.3c and an electrostatic element 460.3d. All elements are connected to a control unit 800 which is configured to provide corresponding voltages and currents to the elements of the primary beam splitter 460 and the charged particle mirror element 700 .

[0066] The example according to the first embodiment is configured for reflection at normal incidence at the charged particle mirror element 700. Therefore, the example of the first embodiment requires 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, the primary beam splitter 460 is not required and reflection at oblique incidence is used at the charged particle mirror element 700. Fig.10 A second specific embodiment of the present invention is shown. Like elements are marked with like reference numerals and are referred to in detail in the drawings. Figure 4 and its description. Figure 4 ,according to Fig.10 The example does not have the primary beam splitter 460 according to the first specific embodiment. Instead, the system 1 is configured for an oblique incidence angle 87 relative to the normal 705 of the charged particle mirror element 700. The primary charged particles traveling along the first beam path 13.1 are reflected at the virtual mirror surface 1321 of the charged particle mirror element 700 into the second beam path at the same oblique angle 87 relative to the mirror normal 705. To separate the first beam path 13.1 and the second beam path 13.2, the oblique angle 87 is selected to be more than 5°, for example 10°. The first beam path 13.1 and the second beam path 13.2 are configured at an angle twice as large as the oblique incidence angle 87. Therefore, the first beam path 13.1 and the second beam path 13.2 are configured at an angle of more than 10°, preferably 15° or even 20°. Fig.11b An example of a charged particle mirror element 700 is shown, which is configured for oblique incidence at an angle 87 relative to the mirror normal 705. The mirror 700 is configured similarly to the mirror 700 of FIG. 6, but with Fig.11bThus, the astigmatism generated when the incident light is obliquely incident at an angle 87 at the circular reflector is compensated. An example of a circular reflector 700 configured for normal incidence is shown in Fig.11a .

[0067] Fig.12 Another example according to the second specific embodiment is shown. The same elements are marked with the same reference numerals and are referred to as Fig.10 The system 1 is configured to generate a first intermediate image surface 321.1 in a first beam path 13.1 upstream of a charged particle mirror element 700. The charged particle mirror element 700 is placed in a pupil plane, and a second intermediate image surface 321.2 is formed in a second beam path 13.2 downstream of the charged particle mirror element 700. By the action of the charged particle mirror element 700, the second intermediate image surface 321.2 is formed in a curved shape, which is configured to compensate for the field curvature of the optical elements downstream of the second intermediate image surface 321.2 (including the objective lens 102 and at least one field lens 103.3).

[0068] Fig.13a Another variation according to the first embodiment is shown. Here, the primary beam splitter 460 and the beam splitter unit 400 are arranged adjacent to each other. Fig.13b In one example, the primary beam splitter 460 and the beam splitter unit 400 are formed into one unit 480 . Fig.13c A third embodiment is shown which is similar to the second embodiment. Here, a first charged particle reflector element 700.1 and a second charged particle reflector element 700.2 are arranged in the main beam path 13, which comprises a first beam path segment 13.1, a second beam path segment 13.2 and a third beam path segment 13.3. Due to the two reflecting charged particle reflector elements 700.1 and 700.2, the compensation of the field curvature can be further improved.

[0069] Due to the mirror element 700 according to a specific embodiment of the present invention, the field curvature is compensated and the focusing deviations of the focal spots 5 of the plurality of beamlets 3 no longer limit the number or the field size of the beamlets J. Therefore, due to the mirror element 700, a larger number of beamlets J is possible, for example J>300, J>1000 or even J>10000. In order to provide sufficient charged particle beam current to each beamlet 3, several charged particle beam sources can also be used in one multi-beam charged particle beam system 1. Fig.13b An example is shown in FIG. 3 with two charged particle beam sources 301 . 1 and 301 . 2 and two focusing lens systems 303 . 1 and 303 . 2 provided upstream of a multi-beam forming unit 305 .

[0070] Fig.14The method for operating a multi-beam charged particle system 1 having at least one first charged particle reflector element 700 is illustrated. In an initial step S, parameter settings of the multi-beam charged particle system 1 are selected. The parameters of the multi-beam charged particle system 1 include, for example, settings of the objective lens 102, the impact energy (Landing energy) of the primary charged particles selected by the voltage provided by the sample voltage supply unit 503, or settings of the field lens 103 for compensating for the rotation of the plurality of primary beamlets 3. The parameter settings in step S may result in a surface 43 on which a focal spot 5 with a minimum spot diameter is formed (see Figure 3 ) field curvature changes.

[0071] In a compensation step C, a specific radius R of the field curvature for the parameter setting selected in step S is determined. The field curvature radii R for a plurality of parameter settings can be stored in a memory of the control unit 800 or can be calculated, for example, from the drive currents supplied to the lens elements 103 and 102 according to a predetermined mathematical lens model. For a specific radius R of the field curvature, control signals for a plurality of drive voltages U and Um for the charged particle mirror element 700 are calculated by the control unit 800. The plurality of drive voltages U and Um comprise, for example, voltages U1 to U3 for the annular electrodes 1317.1 to 1317.3 and at least one voltage Um for at least one segment of the mirror electrode 1315. In another example, for at least one porous plate 1327 (see FIGS. 6 and 7 ), the plurality of drive voltages U and Um are calculated. Figure 7 ), the driving voltage includes a plurality of driving voltages U1i, i=1...J (where J is the number of primary beamlets). In addition, other control signals and voltages for compensating for other aberrations of the charged particle reflector element 700 can be generated by the control unit 800 and provided to the charged particle reflector element 700. Of course, it is known from the prior art systems that other elements of the multi-beam charged particle beam system 1 are also driven and controlled by the control unit 800.

[0072] In an application step A, an application of the multi-beam charged particle system 1 is performed. This application may be, for example, performing an inspection task of a semiconductor wafer, or performing a multi-beam lithography task, such as patterning of a semiconductor mask. A metrology step M is triggered by the control unit 800 to control the performance of the multi-beam charged particle system 1. Thus, in the repetition of the method starting from step C, the performance of the charged particle mirror element 700 is controlled and ultimately adjusted.

[0073] The metrology step M can also be performed before or during the execution of the tasks in step A, and the correction of the charged particle mirror element 700 for field curvature and other aberrations can be monitored during the application of step A. Typically, during the monitoring step M, the performance and residual aberrations of the multi-beam charged particle beam system (1) are determined. Based on the residual aberrations, the control step can be triggered, and a drive voltage configured to compensate for the correction of the residual aberrations can be determined and provided to at least one electrode of the charged particle mirror element (700).

[0074] Due to the operating method of the multi-beam charged particle system 1 and the configuration of the charged particle mirror element 700, variable compensation of field curvature can be achieved. The field curvature depends on the parameter setting of the multi-beam charged particle system 1, and the design of the charged particle mirror element 700 and the voltage provided by the control unit 800 for driving the charged particle mirror element 700 are configured to variably compensate for field curvature and selectively other aberrations.

[0075] Features of specific embodiments improve the performance of the multi-beam charged particle system 1 to achieve a higher resolution of less than 5nm, preferably less than 3nm, more preferably less than 2nm or even less than 1nm. These improvements are particularly related to further developing a multi-beam charged particle system with a larger number of multiple primary beamlets (such as more than 100 beamlets, more than 300 beamlets, more than 1000 beamlets or even more than 10,000 beamlets). These improvements are particularly related to conventional applications of multi-beam charged particle systems, such as in mask writing applications or semiconductor inspection and inspection, which require high reliability, high reproducibility and low machine-to-machine deviation. Due to the features described in the specific embodiments and combinations thereof, each of the multiple beamlets has, for example, a beamlet diameter in the range from 2nm to 2.1nm, an average resolution of 2.05nm, and the resolution range achieved by the features and methods of the specific embodiments is less than 0.15%, preferably 0.1%, and even more preferably 0.05% of the average resolution.

[0076] The present invention is further described by the following items:

[0077] Item 1: A multi-beam charged particle system (1), comprising:

[0078] An object irradiation unit (100), wherein the object irradiation unit (100) comprises:

[0079] - a charged particle beam source (301);

[0080] - a multi-beam forming unit (305) for forming a plurality of primary beamlets (3, 3.1, 3.2, 3.3);

[0081] - a plurality of lens elements (102, 103, 103.1, 103.2, 103.3), comprising 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);

[0082] - a charged particle mirror element (700) configured to compensate for field curvature of the plurality of lens elements (102, 103, 103.1, 103.2, 103.3) during use.

[0083] Item 2: The multi-beam charged particle system (1) as described in Item 1, further comprising:

[0084] - a control unit (800), and wherein

[0085] The control unit (800) is configured to provide multiple voltages (U1, U2, U3, Um) to the charged particle reflector element (700) during use, wherein the charged particle reflector element (700) and the voltages (U1, U2, U3, Um) are configured to generate a reflective lens field distribution having a virtual reflective surface (1321) during use.

[0086] Item 3: A multi-beam charged particle system (1) as described in Item 2, wherein the charged particle reflector element (700) comprises at least three electrodes (1317.1, 1317.2, 1315) connected to the control unit (800), the electrodes comprising at least a first and a second annular electrode (1317.1, 1317.2) and a reflector electrode (1315), and wherein the control unit (800) is configured to provide a first voltage U1 to the first annular electrode (1317.1), a second voltage U2 to the second annular electrode (1317.2), and a reflector voltage Um to the reflector electrode (1315) during use, and the electrodes and the voltages are configured to generate the virtual reflecting surface (1321) during use.

[0087] Item 4: The multi-beam charged particle system (1) as described in Item 3, wherein the reflector electrode (1315) has a curved shape.

[0088] Item 5: A multi-beam charged particle system (1) as described in Item 3 or Item 4, wherein the reflector electrode (1315) is a segmented electrode comprising a plurality of N electrode segments (1315.1 to 1315.N), and wherein the control unit (800) is further configured to provide a plurality of reflector voltages Um1 to UmN to the plurality of N electrode segments (1315.1 to 1315.N) during use.

[0089] Item 6: A multi-beam charged particle system (1) as described in any one of Items 3 to 5, wherein the charged particle reflector element (700) includes a third electrode (1317.3) connected to the control unit (800), and the control unit (800) is further configured to provide a third voltage U3 to the third annular electrode (1317.3) during use.

[0090] Item 7: The multi-beam charged particle system (1) as described in any one of Items 3 to 6, wherein the charged particle reflector element (700) is arranged in a plane where the multiple primary charged particle beamlets (3) at least partially overlap.

[0091] Item 8: The multi-beam charged particle system (1) as described in any one of Items 3 to 6, wherein the charged particle reflector element (700) is arranged near an intermediate field plane where a plurality of focal spots are formed.

[0092] Item 9: A multi-beam charged particle beam system (1) as described in Item 8, wherein the charged particle reflector element (700) comprises a plurality of porous plates (1327.1, 1327.2, 1327.3, 1325) having a plurality of holes, which are configured to individually receive and reflect each individual primary charged particle beamlet (3) of the plurality of primary charged particle beamlets (3).

[0093] Item 10: A multi-beam charged particle system (1) as described in any one of Items 8 to 9, wherein the charged particle reflector element (700) is configured for normal incidence of the multiple primary beamlets (3) so that the reflected primary beamlets propagate approximately in a direction parallel to the incident primary beamlets.

[0094] Item 11: The multi-beam charged particle system (1) according to Item 10, further comprising:

[0095] - a primary charged particle beam splitter (460), used to guide the primary charged particle beamlets (3) from the multi-beam forming unit (305) to the charged particle reflector element (700) along a first beam path (13.1), and configured to guide the primary charged particle beamlets (3) in the direction of the at least one objective lens (102) along a second beam path (13.2) after reflection from the charged particle reflector element (700).

[0096] Item 12: The multi-beam charged particle system (1) of Item 10, wherein the primary charged particle beam splitter (460) comprises:

[0097] - a separator section (460.1) for separating the first beam path (13.1) from the second beam path (13.2);

[0098] - at least one first section (460.2) arranged in the first beam path (13.1);

[0099] - at least one second section (460.3) which, in the second beam path (13.2), the first section and the second section (460.2, 460.3) are configured to compensate for the dispersion and further aberrations of the separator section (460.1).

[0100] Item 13: A multi-beam charged particle system (1) as described in any one of Items 1 to 7, wherein the charged particle reflector element (700) is configured to tilt the incident angle (87) to form a first path (13.1) from the multi-beam forming unit (305) to the charged particle reflector element (700), and after reflection from the charged particle reflector element (700), form a second path (13.2) of the primary charged particle beamlet (3) in the direction of the at least one objective lens (102), wherein the first and second paths (13.1, 13.2) are arranged at an angle of more than 15° relative to each other.

[0101] Item 14: The multi-beam charged particle system (1) as described in Item 13, wherein the charged particle reflector element (700) has an elliptical cross-section.

[0102] Item 15: The multi-beam charged particle system (1) as described in any one of Items 1 to 14, further comprising:

[0103] - a secondary electron beam splitter (400) configured to direct secondary beamlets (9) generated at the focal spot (5) of the plurality of primary beamlets (3) at the surface (25) of the planar object (7) to a detector (700).

[0104] Item 16: The multi-beam charged particle system (1) as described in Item 15, further comprising a secondary electron imaging system (200), wherein the secondary electron imaging system comprises a plurality of lens elements (205.1, 205.2, 205.3, 205.4, 205.5).

[0105] Item 17: The multi-beam charged particle system (1) of Item 15 or 16, wherein the primary charged particle beam splitter (460) and the secondary electron beam splitter (400) are formed as an integrated unit (480).

[0106] Item 18: A multi-beam charged particle system (1) as described in any one of Items 1 to 17, further comprising a second charged particle reflector element (700.2), the first and second reflector elements (700.1, 700.2) being configured to compensate for the field curvature of the multiple lens elements (103.1, 103.2, 103.3, 102) during use.

[0107] Item 19: A multi-beam charged particle system (1) as described in any one of Items 1 to 18, wherein the charged particle reflector element (700) is further configured to compensate for further imaging aberrations of at least one of the primary beamlets (3, 3.1, 3.2, 3.3) during use.

[0108] Item 20: A method of operating a multi-beam charged particle system (1) including a charged particle mirror element (700), comprising:

[0109] Step S: selecting parameter settings of the multi-beam charged particle system (1);

[0110] Step C: determining the field curvature of the multi-beam charged particle system (1) with the selected parameter setting; determining at least a driving voltage configured to compensate for the field curvature; and providing the driving voltage to at least one electrode of the charged particle reflector element (700);

[0111] Step A: Execute the application of the multi-beam charged particle system (1).

[0112] Item 21: The method as described in Item 20 further comprises a step M of monitoring the performance of the multi-beam charged particle system (1) and determining residual aberrations.

[0113] Item 22: The method of Item 21, further comprising the step of determining at least one corrected drive voltage configured to compensate for the residual aberration, and providing the corrected drive voltage to at least one electrode of the charged particle mirror element (700).

[0114] Item 23: The method of any one of Items 20 to 22, wherein the application is one of a wafer inspection or a mask writing task.

[0115] The present invention is not limited to the above-mentioned specific embodiments or projects. The specific embodiments or examples can be combined with each other in whole or in part, and may have various changes and modifications. Although several improvements are described in the example of a multi-beam charged particle system for inspection, these improvements are not limited thereto and are also applicable to other multi-beam charged particle systems, such as a multi-beam lithography system for mask writing applications.

[0116] In all embodiments, electrons are generally understood as charged particles. Although some embodiments are explained using electrons as an example, it should not be limited to electrons and can also be perfectly applicable to all types of charged particles, such as, for example, helium or neon ions.

[0117] A list of reference numerals is provided below

[0118] 1 Multi-beamlet charged particle system

[0119] 3 Primary charged particle beamlet

[0120] 5 Primary charged particle beam spot

[0121] 7 Objects

[0122] 9 Secondary Electron Beamlets

[0123] 11 Secondary electron beam path

[0124] 13 Primary charged particle beam path

[0125] 15 Secondary electron image spot

[0126] 25 Surface of an object

[0127] 43 Spherical surface

[0128] 74 Waist

[0129] 87 Oblique incident angle

[0130] 100 Object irradiation unit

[0131] 101 Object Plane

[0132] 102 Objective lens

[0133] 103 Field lens

[0134] 105 Optical axis of multi-beamlet charged particle system 1

[0135] 108 First beam intersection point

[0136] 110 Collective multi-beam raster scanner

[0137] 200 Detection Units

[0138] 205 lens element

[0139] 214 hole filter

[0140] 216 Active components

[0141] 222 Second Deflection System

[0142] 300 Charged Particle Multi-Beamlet Generator

[0143] 301 Charged Particle Source

[0144] 303 Collimating lens

[0145] 304 filter plate

[0146] 305 One-shot multi-beam forming unit

[0147] 306 Multi-hole Plate

[0148] 307 end porous plate

[0149] 308 Field Lens

[0150] 309 Primary Electron Beam

[0151] 321 Intermediate Image Surface

[0152] 400 Beam Splitter Unit

[0153] 460 Primary beam splitter

[0154] 480 Integrated primary and secondary charged particle separator unit

[0155] 500 Sample stage

[0156] 503 Sample Voltage Supply Unit

[0157] 600 Image Sensor

[0158] 700 Charged Particle Mirror Element

[0159] 705 Normal axis of charged particle reflector element

[0160] 800 Control Unit

[0161] 860 Scanning Control Unit

[0162] 1311 Charged particle beam path or trajectory

[0163] 1315 Reflector Electrode

[0164] 1317 Electrode

[0165] 1318 Field lens electrode

[0166] 1321 Virtual Reflection Surface

[0167] 1328 Common Path Field Lens

[0168] 1341 Equipotential Line

[0169] 1342 Electric Field Lines

Claims

1. A multi-beam charged particle system (1), comprising: An irradiation unit (100), wherein the object irradiation unit (100) comprises: - 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 field curvature of the plurality of lens elements (102, 103, 103.1, 103.2, 103.3) during use.

2. The multi-beam charged particle system (1) according to claim 1, comprising: - a control unit (800), and wherein The control unit (800) is configured to provide multiple voltages (U1, U2, U3, Um) to the charged particle reflector element (700) during use, wherein the charged particle reflector element (700) and the voltages (U1, U2, U3, Um) are configured to generate a reflective lens field distribution having a virtual reflective surface (1321) during use.

3. A multi-beam charged particle system (1) as described in claim 2, wherein the charged particle reflector element (700) includes at least three electrodes (1317.1, 1317.2, 1315) connected to the control unit (800), the electrodes include at least a first and a second annular electrode (1317.1, 1317.2) and a reflector electrode (1315), and wherein the control unit (800) is configured to provide a first voltage U1 to the first annular electrode (1317.1), a second voltage U2 to the second annular electrode (1317.2), and a reflector voltage Um to the reflector electrode (1315) during use, and the electrodes and the voltages are configured to generate the virtual reflecting surface (1321) during use.

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

5. A multi-beam charged particle system (1) as described in claim 3 or 4, wherein the reflector electrode (1315) is a segmented electrode, which comprises a plurality of N electrode segments (1315.1 to 1315.N), and wherein the control unit (800) is further configured to provide a plurality of reflector voltages Um1 to UmN to the plurality of N electrode segments (1315.1 to 1315.N) during use.

6. A multi-beam charged particle system (1) as described in any one of claims 3 to 5, wherein the charged particle reflector element (700) includes a third annular electrode (1317.3) connected to the control unit (800), and the control unit (800) is further configured to provide a third voltage U3 to the third annular electrode (1317.3) during use.

7. The multi-beam charged particle system (1) as claimed in any one of claims 3 to 6, wherein the charged particle reflector element (700) is arranged in a plane where the multiple primary charged particle beamlets (3) at least partially overlap.

8. The multi-beam charged particle system (1) as claimed in any one of claims 3 to 6, wherein the charged particle reflector element (700) is arranged near an intermediate field plane where a plurality of focal spots are formed.

9. A multi-beam charged particle system (1) as described in claim 8, wherein the charged particle reflector element (700) comprises a plurality of porous plates (1327.1, 1327.2, 1327.3, 1325) having a plurality of holes, which are configured to individually receive and reflect each individual primary charged particle beamlet (3) of the plurality of primary charged particle beamlets (3).

10. A multi-beam charged particle system (1) as described in any one of claims 8 to 9, wherein the charged particle reflector element (700) is configured for normal incidence of the multiple primary beamlets (3) so that the reflected primary beamlets propagate approximately in a direction parallel to the incident primary beamlets.

11. The multi-beam charged particle system (1) as claimed in claim 10, further comprising: - a primary charged particle beam splitter (460), used to guide the primary charged particle beamlets (3) from the multi-beam forming unit (305) to the charged particle reflector element (700) along a first beam path (13.1), and configured to guide the primary charged particle beamlets (3) in the direction of the at least one objective lens (102) along a second beam path (13.2) after reflection from the charged particle reflector element (700).

12. The multi-beam charged particle system (1) of claim 10, wherein the primary charged particle beam splitter (460) comprises: - a separator section (460.1) for separating the first beam path (13.1) from the second beam path (13.2); - at least one first section (460.2) arranged in the first beam path (13.1); - at least one second segment (460.3) which, in the second beam path (13.2), the first segment and the second segment (460.2, 460.3) are configured to compensate for the dispersion and further aberrations of the separator segment (460.1).

13. A multi-beam charged particle system (1) as described in any one of claims 1 to 7, wherein the charged particle reflector element (700) is configured for an oblique incident angle (87) to form a first path (13.1) from the multi-beam forming unit (305) to the charged particle reflector element (700), and after reflection from the charged particle reflector element (700), to form a second path (13.2) of the primary charged particle beamlet (3) in the direction of the at least one objective lens (102), wherein the first path (13.1) and the second path (13.2) are arranged at an angle exceeding 15° relative to each other.

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

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

16. The multi-beam charged particle system (1) as described in claim 15 further comprises a secondary electron imaging system (200), which comprises a plurality of lens elements (205.1, 205.2, 205.3, 205.4, 205.5).

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

18. A multi-beam charged particle system (1) as described in any one of claims 1 to 17, further comprising a second charged particle reflector element (700.2), the first and second reflector elements (700.1, 700.2) being configured to compensate for the field curvature of the multiple lens elements (103.1, 103.2, 103.3, 102) during use.

19. A multi-beam charged particle system (1) as described in any one of claims 1 to 18, wherein the charged particle reflector element (700) is further configured to compensate for further 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 field curvature of the multi-beam charged particle system (1) with selected parameter settings, determining at least one driving voltage configured to compensate for the field curvature, and providing the driving voltage to at least one electrode of the charged particle reflector element (700); Step A: Execute the application of the multi-beam charged particle system (1).

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

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

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

Images