High-throughput multi-beam scanning microscope with hexagonal raster of beamlets

Abstract

A multi-beam charged particle system and a method of operating a multi-beam charged particle system with moving stage is provided. With the method and system configured to execute the method, a high throughput of an image acquisition is provided.

Description

[0001] High-throughput Multi-beam scanning microscope with hexagonal raster of beamlets

[0002] Cross-reference to related applications

[0003] This application claims the priority of German patent application 10 2024 209 281.9, the complete content of which is incorporated by reference herein.

[0004] Field of the invention

[0005] The disclosure relates to a method of image acquisition with a multi-beam charged particle microscope utilizing a scanning or moving stage during imaging.

[0006] Background of the invention

[0007] WO 2005 / 024881 A2 discloses an electron microscope system which operates with a multiplicity of electron beamlets for the parallel scanning of an object to be inspected with a bundle of electron beamlets. The bundle of primary charged particle beamlets is generated by directing a primary charged particle beam onto a multi-beam forming unit, comprising at least one multi-aperture plate, which has a multiplicity of openings. A portion of the electrons of the electron beam is incident onto the multi-aperture plate and is absorbed there, and another portion of the beam transmits the openings of the multi-aperture plate and thereby, in the beam path downstream of each opening, an electron beamlets is formed whose cross section is defined by the cross section of the respective opening. The plurality of primary charged particle beamlets are focused by an objective lens on a surface of a sample and trigger secondary electrons or backscattered electrons to emanate as secondary electron beamlets from the sample, which are collected and imaged onto a detector. Each of the secondary beamlets is incident onto a separate detector element or group of detector elements, so that the secondary electron intensities detected therewith provide information relating to the surface of the sample at the location where the corresponding primary beamlet is incident onto the sample. The bundle of primary beamlets is scanned systematically over the surface of the sample and an electron microscopic image of the sample is generated. For high throughput applications with a plurality of beamlets, image acquisition or direct write with a scanning stage have been proposed. These methods are typically called "slanted scan", where the raster of beamlets is slightly rotated with respect to a linear moving direction of a stage. The method of slanted scan has been successfully used with rectangular arrays. However, for multi-beam microscopes with multiple beamlets and a common imaging optical system, a hexagonal raster arrangement of beamlets is preferred. In prior art, it is assumed that a slanted scan is not possible with a fully hexagonal raster arrangement of beamlets. For example, for image acquisition of brain tissue with a multi-beam charged particle microscope, typically a scanning stage has been used in combination with a hexagonal raster of 91 beamlets, where not all 91 beamlets have been used for image acquisition. Similar, in US2023182440 AA, a slanted scan is proposed with a hexagonal arrangement, where not all beamlets are used for scanning image acquisition with a scanning stage. US2023182440 AA propose a complex arrangement or limitation of beamlets within a hexagonal raster and thereby limits throughput and flexibility of a multi-beam scanning microscope.

[0008] Therefore, it is still a need for improvement of the throughput of a scanning image acquisition with a scanning stage and with multiple beamlets, which are arranged in a hexagonal raster arrangement. It is still a need for improvement of a flexibility of a scanning image acquisition apparatus, capable for image acquisition with a scanning stage or purely scanning operation of the multiple beamlets for image acquisition.

[0009] Description of the invention

[0010] A multi-beam charged particle imaging system and an improved method of operation of a multi-beam charged particle beam system is provided. The improved method of operation allows an image acquisition with increased throughput.

[0011] According to the embodiments, a method of wafer inspection with a multi-beam inspection tool with a continuously moving stage utilizes with a plurality of charge particle beamlets with low or even no loss of throughput. To achieve the large throughput, a plurality of charge particle beamlets is arranged in a hexagonal raster of beamlets within a plurality of S hexagon-shaped shells, and wherein the hexagonal raster of beamlets is enclosed by an enclosing hexagon. The full number of beamlets within a hexagonal raster is given by J = 3 * S * (S-l) + 1. A step SI according to the method is comprising selecting a number N of beamlets within one (lateral) period of beamlets of the hexagonal raster of beamlets, and selecting a multiplexing factor K. A step S2 according to the method is comprising determining and adjusting a rotation angle a between a raster coordinate system xr, yr of the hexagonal raster of beamlets and a scanning coordinate system xs, ys, and comprising determining a scanning deflection width ps and a movement velocity vs of the continuously moving stage. A step S3 according to the method is comprising determining a scanning pattern EO, the scanning pattern covering at least the scanning deflection width ps. A step S4 according to the method is comprising an image acquisition by executing a movement of a stage of the multi-beam inspection tool along a movement direction ys with the velocity vs and synchronously executing the scan pattern EO and synchronously collecting image data within a plurality of image stripes of a surface of an object mounted to the stage. According to the method, the rotation angle a is determined according to a = arctan [K / (N * 3)] .

[0012] First, the number N of beamlets within on (lateral) period can by any number between 2 and the number J of beamlets in the hexagonal raster. In an example, the number N of beamlets is determined according to N = 2*S - 1. With the determination of rotation angle a and number N according to the equations, an image acquisition of a selected and predetermined surface area of a wafer is acquired without gaps and without wasting throughput by unnecessary beamlets, and thereby all J beamlets within the hexagonal raster of beamlets contribute to the image formation of a surface. Thereby, wafer inspection at maximum throughput is achieved.

[0013] In an example, the method is further comprising a step S5. During step S2, a displacement step of displacement step size dxs between a wafer support table and the hexagonal raster of beamlets is executed in a direction perpendicular to the movement direction ys. The displacement step size dxs is determined such that two adjacent plurality of image stripes are contributing to an image area of a surface of an object without a gap in between. The displacement step size dxs is given by dxs = J * ps with the number J of beamlets. In an example, the displacement step of displacement step size dxs is executed by a long-stroke actuator of the stage.

[0014] In an example, the method comprises executing a step S6, comprising an image acquisition by executing a movement of a stage of the multi-beam inspection tool along the movement direction ys with the velocity vs and synchronously executing the scan pattern EO and collecting image data within a plurality of image stripes, wherein step S6 has an opposing movement direction with respect to the movement direction of step S4. In an example, either step S4, step S6 or a combination of both is iteratively repeated until an image of a desired surface area of a wafer if acquired.

[0015] In an example, step S2 is further comprising selecting an interlacing factor L > 1, and wherein the scanning deflection width ps is determined according to ps = [ p / N * cos a] / L. With the interlacing factor, a scanning width is reduced and thereby a scanning induced image error such as a scanning induced distortion is reduced by a factor LA3.

[0016] With selection of an interlaced imaging method with L > 1, the method is further comprising a step S7. Step 7 is comprising executing a lateral interlacing displacement step ddx between the wafer support table and the raster of beamlets perpendicular to the movement direction ys by a lateral interlacing displacement step ddx = ps / L. In an example, the lateral interlacing displacement step ddx is executed by a long-stroke actuator of the stage.

[0017] The example of the method with L > 1 is further comprising at least one of a repeating of step S4 or step S6. For example, the method is comprising iteratively repeating the step S7 and an image acquisition step S4 or S6 at least L times each.

[0018] For high-throughput performance of an inspection task with a moving stage, it is essential to maintain the rotation angle a as specified above. In an example, the method is therefore further comprising a step S8 of determining a deviation angle 0 between the target movement direction ys and the real movement direction of the wafer support table and a step of compensating an effect of the deviation angle 0. In an example, compensating the effect of the deviation angle 0 comprises modifying the scan pattern EO into a modified scan pattern EO.S. In an example, compensating an effect of the deviation angle 0 comprises executing a synchronous movement of the wafer support base in a direction perpendicular to the target movement direction ys with a velocity vsx ~ si n(0). In an example, both compensation methods are combined.

[0019] In an example, the method therefore comprises no image multiplexing and a multiplexing factor K is selected to K = 1. In an example, a charge provided during a dwell time with a single charged particle beamlet is reduced, for example to reduce a charging effect or a damage to the wafer surface. In an example, the multiplexing factor K is selected K > 1 with K being an odd integer with K = 3,5,7....

[0020] In an alternative example, the multiplexing factor K is selected K > 1 with K being an even integer with K = 2,4,6, ... In the alternative example, the method is further comprising a step S8 comprising a filtering a single row or beamlets from the hexagonal raster of beamlets. For example, the filtering of a single row of beamlets is performed by positioning a movable field stop in an intermediate image surface of the multi-beam charged particle beam system. The filtering the single, outermost row of beamlets slightly reduces the throughput by S / J. For example, with S = 6, the throughput loss is about 6.6%. With S = 11, the throughput is reduced to below 3.4%, which is still considerably less compared to the loss of 20% or more of the prior art.

[0021] In an example, the method is further comprising a step S10. Step S10 is comprising executing a step displacement ASS of the wafer support base in a direction perpendicular to the target movement direction ys. For example, with a modified scan pattern EO.S, a scanning range of a scanning deflector might be exceeded. This can be compensated by inserting a step displacement ASS of the wafer support base at certain time intervals, when a scanning range of a scanning deflector is close to be exceeded.

[0022] In an example, the method is further comprising a step Sil. Step 11 is comprising executing during a first time interval Tl a synchronous movement of the wafer support base in a direction parallel to the target moving direction ys with a velocity opposite to the movement velocity vs of the stage, and, after each time interval Tl, executing a step movement parallel to the target moving direction ys of the wafer. Thereby, a movement velocity of the wafer surface with respect to the hexagonal pattern of beamlets can be reduced during intervals Tl, during which more complex scanning patterns can be executed. For example, a movement between the wafer surface with respect to the hexagonal pattern of beamlets is completely stopped during intervals Tl despite continuous movement of the wafer stage in movement direction ys.

[0023] In an example, the method is further comprising a step S12, which is comprising a stitching of the image data of the plurality of image stripes collected during iterative execution of steps S4 or S6. In an example, step S12 is further comprising a multiplexing of the image data of the plurality of image stripes collected during iterative execution of steps S4 or S6.

[0024] In an example, a method of wafer inspection with a multi-beam inspection tool with a plurality of charge particle beamlets arranged in a hexagonal raster of beamlets is comprising selecting an even number N* of beamlets within one period of beamlets of the hexagonal raster of beamlets and selecting a multiplexing factor K with K >= 2. The method is further comprising an image acquisition of an image stripe by executing a movement of a stage of the multi-beam inspection tool along a movement direction ys with a velocity vs, wherein the image acquisition is comprising collecting a first image data of the image stripe with a first beamlet and a second image data of the image stripe with a second beamlet. In an example, the method is comprising determining a voltage contrast image by computing the difference between the second image data and the first image data.

[0025] In an example, the method is further comprising determining and adjusting a rotation angle a between a raster coordinate system xr, yr of the hexagonal raster of beamlets and a scanning coordinate system xs, ys, and comprising determining a scanning deflection width sw and a movement velocity vs such that a first image stripe acquired by a first beamlet at least partially overlaps with a second image stripe acquired by a second beamlet.

[0026] With a multi-beam inspection tool with a plurality of charge particle beamlets arranged in a hexagonal raster within an enclosing hexagon, a number N of beamlets within one repetitive period of beamlets is an odd number. For example, during selecting an even number N* of beamlets within one period of beamlets, at least one redundant beamlet within each period of beamlets is selected. In an example, the method is comprising filtering of redundant beamlets from the hexagonal raster of beamlets. In an example, the step of filtering of redundant beamlets is comprising a filtering a single row of beamlets from the hexagonal raster of beamlets.

[0027] A multi-beam charged particle beam system for wafer inspection is therefore comprising: a wafer stage configured for continuous movement at a velocity vs in movement direction ys during image acquisition, and a control unit with a memory and a processor. The memory is configured for storing software instructions, which - when executed by the processor - are causing the multi-beam charged particle beam system for executing a method as described above. In an example, the stage is comprising a long-stroke actuator configured for continuous movement of a wafer support table in the movement direction ys. In an example, the stage is comprising a rotation actuator configured for adjusting angle a.

[0028] In an example, the stage is comprising a short-stroke actuator configured for continuous or stepwise movement of a wafer support table in the movement direction ys or perpendicular to the movement direction ys. In an example, the multi-beam charged particle beam system is further comprising a movable field stop in an intermediate image surface of the multibeam charged particle beam system.

[0029] With the multi-beam charged particle imaging system and the improved method of operation of a multi-beam charged particle beam system, an image acquisition with low aberrations, high resolution and increased throughput is enabled. Low aberrations and high resolution are achieved by utilizing a plurality of beamlets within a hexagonal raster arrangement, adapted to an aperture size of an objective lens of the multi-beam system. The dense package of beamlets withing the hexagonal raster arrangement contributes to the increase of throughput. Next, by selecting angle a accordingly, not beamlet must be sacrifices and all image information from all beamlets can be used for image acquisition. Thereby, maximum throughput is achieved, including higher throughput compared to solutions of the prior art. Highest throughput is of the essence for inline use within a fabrication environment of semiconductor fabrication.

[0030] Embodiments of the present disclosure will be explained in more detail with reference to drawings, in which:

[0031] Figure 1 is a schematic sectional view of a multi-beam charged particle system according to an embodiment

[0032] Figure 2 illustrates an example of a hexagonal raster of beamlets and methods of image acquisition according to the prior art.

[0033] Figure 3 illustrates an example of a determination of a rotation of a hexagonal raster configured for a slanted scanning operation with a sample stage Figure 4 illustrates examples of scan patterns for image acquisition with a moving stage

[0034] Figure 5 illustrates a further example of a more complex scan pattern for image acquisition with a moving stage

[0035] Figure 6 illustrates an image acquisition with an interlacing of image stripes

[0036] Figure 7 illustrates an image acquisition with an image multiplexing for charge reduction

[0037] Figure 8 illustrates an example of a determination of a rotation of a filtered hexagonal raster configured for a slanted scanning operation

[0038] Figure 9 illustrates a multi-beam charged particle beam system configured for slanted scanning operation with a sample stage.

[0039] Figure 10 illustrates a modified scan pattern for compensation of an angle error

[0040] Figure 11 illustrates a perpendicular movement of a wafer support table compensation of an angle error

[0041] Figure 12 illustrates a multi-beam charged particle beam system configured for a filtered hexagonal raster for filtering a single row of beamlets

[0042] Figure 13 illustrates an effect of a step-wise movement of a wafer support table perpendicular to a continuous movement of the wafer stage.

[0043] Figure 14 illustrates an effect of a continuous and a step-wise movement of a wafer support table parallel to a continuous movement of the wafer stage.

[0044] Figure 15 illustrates an example of a method of operation of a multi-beam charged particle beam system

[0045] In the exemplary embodiments of the invention described below, components similar in function and structure are indicated as far as possible by similar or identical reference numerals. Some array elements, for example the plurality of primary charged particle beamlets, are identified by a reference number. Depending on the context, the same reference number may also identify a single element out of 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).

[0046] Figure 1 is a schematic illustration of a multi-beam charged particle imaging system 1 (in short also multi-beam system 1) according to an embodiment. The multi-beam system 1 uses a plurality of charged particle beams for forming an image of an object 7. The multi-beam system 1 generates a plurality of J primary particle beams 3 which strike the object 7 to be examined in order to generate interaction products, e.g. secondary electrons, which emanate from the object 7 and are subsequently detected. The multi-beam system 1 is of the scanning electron microscope (SEM) type, which uses a plurality of primary electron beams 3 which are incident on a surface of the object 7 at a plurality of locations and generate there a plurality of primary electron beam focus spots 5. The object 7 to be examined can be of any desired type, e.g., a semiconductor wafer or a semiconductor mask, and can comprise an arrangement of miniaturized elements. The surface 25 of the object 7 is arranged in an object plane 101 of an objective lens 102 of an object irradiation unit 100.

[0047] A diameter of the minimal beam spots or focus spots 5 shaped in the object plane 101 can be small. Exemplary values of this diameter are below four nanometers (nm), for example three nm or less. The focusing of the primary charged particle beamlets 3 for shaping the focus spots 5 is carried out by the objective lens system 102. In this case, the objective lens system 102 can comprise a magnetic immersion lens. Further examples of focusing means are described in the US patent application US 2023 / 245852 AA, the entire content of which is herewithin incorporated in the disclosure. For sake of simplicity, only three primary beamlets 3.1, 3.2 and 3.3 with corresponding focus points 5.1, 5.2 and 5.3 are shown in figure 1. Exemplary values of the pitch P between the incidence locations of beam spots 5 are 1 micrometer, 10 micrometers, or more, for example 40 micrometers.

[0048] The primary particles 3 striking the object 7 generate interaction products, e.g. secondary electrons, back-scattered electrons, which emanate from the surface of the object 7. The interaction products emanating from the surface of the object 7 are shaped by the objective lens 102 to form secondary electron beamlets 9. For sake of simplicity, through the disclosure, all the interaction products are collectively described as secondary electrons, forming secondary electron beamlets 9 (here: secondary electron beamlets 9.1, 9.2 and 9.3). The multi-beam system 1 provides a detection beam path 13 for guiding the plurality of secondary particle beamlets 9 to a secondary electron imaging system or detection unit 200. The secondary electron imaging system 200 comprises several electron-optical lenses 205.1 to 205.5 for directing the secondary particle beams 9 towards a spatially resolving particle detector 600. The detector 600 is arranged in the image plane 225. The detector 600 is comprising a plurality of detection elements. Detection elements can for example be diodes such as PMDs, or CMOS detection elements, provided with electron-to-light conversion elements, or can be formed as direct electron detection elements.

[0049] In an example, the detector 600 comprises an electron-to-light conversion element, such as a scintillator plate, by which secondary electrons are converted into light, and a plurality of light detection elements. The combination of the electron-to-light conversion element and the plurality of light detection elements hereby form together a plurality of electron detection elements. The detector or image sensor 600 can further comprise a relay optical system for imaging and guiding the photons generated by the electron to photon conversion unit at the secondary charged particle image spots 15 on dedicated photon detection elements, such as a plurality of photomultipliers or avalanche photodiodes (not shown). Such an image sensor is disclosed in US 9,536,702, which is hereby incorporated by reference.

[0050] The imaging with the secondary electron imaging system 200 is strongly magnifying such that both the raster pitch of the primary beams on the wafer surface and the size and shape of focal points 5 of the primary beamlets 3 are imaged in much magnified fashion. By way of example, a magnification is between lOx and lOOx such that one nm on the wafer surface is imaged enlarged to between 10 nm and 100 nm. By way of example, a magnification is between lOOx and 300x such that one nm on the wafer surface is imaged enlarged to between 100 nm and 300 nm. In an example, an image field of a multi-beam system with for example 100 pm diameter is enlarged to approximately 30 mm.

[0051] The primary particle beams 3 are generated in a beam generation apparatus 300 comprising at least one charged particle emitter 301, at least one collimation lens 303, a multi-aperture arrangement 305 and a first field lens 331 and a second field lens 333. The charged particle emitter 301 is connected to a voltage supply for providing an emitter voltage VK to the emitter 301 and generates at least one diverging charged particle beam 309, which is at least substantially collimated by the at least one collimation lens 303, and which illuminates the multi-aperture arrangement 305. The multi-aperture arrangement 305 comprises at least one first multi-aperture or filter plate 304, which has a plurality of J openings formed therein in a first raster arrangement. Particles of the illuminating particle beam 309 pass through the J apertures or openings of the first multi-aperture plate 304 and form the plurality J of primary beamlets 3. Particles of the illuminating beam 309 which strike the first aperture plate 304 are absorbed by the latter and do not contribute to the formation of the primary beamlets 3. A multi-aperture arrangement 305 usually has at least a further multi-aperture plate 306, for example a lens array, a stigmator array or an array of deflection elements. In this example, the particle beam 309 is perfectly collimated by collimation lens 303. However, it is also possible to design the multi-aperture arrangement 305 for a diverging or converging incident particle beam 309.

[0052] Together with the field lens 331 and a second field lens 333, the multi-aperture arrangement 305 focuses each of the primary beamlets 3 in such a way that focal points are formed in an intermediate image surface 321. Alternatively, the beam foci and the intermediate image surface 321 can be virtual. The intermediate image surface 321 can be curved and tilted to pre-compensate a field curvature and image plane tilt of the charged particle imaging system arranged downstream of the intermediate image surface 321.

[0053] The at least one field lens 103 and the objective lens 102 provide a first imaging particle optical unit for imaging the surface 321 onto the object plane 101 such that a second raster configuration of focus spots 5 of the primary beamlets is formed there. The plurality of primary charged particle beamlets 3 form a crossover point 108, in the vicinity of which a first scanning deflector 110 is arranged. The first scanning deflector 110 is used to deflect the plurality of primary beamlets 3 collectively and synchronously such that the plurality of focus spots 5 are moved simultaneously over the surface 25 of the object 7. The first scanning deflector 110 is driven by a scanning control unit 860 such that in an inspection mode of operation, a plurality of two-dimensional image data of the surface 25 is acquired. Additionally, the multi-beam system 1 can comprise further static deflectors and multipole elements 112 configured to adjust the position and beam shapes of the plurality of the primary beamlets 3. The objective lens 102 and the projection lenses 205 provide a secondary electron imaging system 200 for imaging the object plane 101 onto the detection plane 225. The objective lens 102 is thus a lens or a lens system that is part of both the first and the second particle optical unit, while the field lenses 103, 331 and 333 belong only to the first particle optical unit 100, and the projection lenses 205 belongs only to the secondary electron imaging system 200. With combined action of field lenses 103, 331 and 333 and objective lens 102, a rotation of the raster of the plurality of primary beamlets 3 can be adjusted to a desired rotation angle of the raster of the plurality of primary beamlets 3 with respect to a scanning movement direction of the stage 500.

[0054] A beam divider 400 is arranged in the beam path of the first particle optical unit 100 between the field lens 103 and the objective lens system 102. The beam divider 400 is also part of the second optical unit in the beam path between the objective lens system 102 and the projection lenses 205.

[0055] The secondary electron imaging system 200 comprises the second, collective beam deflector 222 which is arranged in the vicinity of a crossover plane or pupil plane 21a of the secondary electron beamlets 9. The second, collective beam deflector 222 is operated synchronously with the first beam deflector 110 and compensates during use a beam deflection of the secondary electron beamlets 9 such that the focus points 15 of the secondary beamlets 9 remain at constant position on the detection plane 225. Thereby, each focus points 15 of each individual secondary beamlet 9 is kept within the area of a set of detection elements, which is assigned to the individual secondary beamlet 9.

[0056] Together with the objective lens 102, the lenses 205 serve to focus the secondary beams 9 on the spatially resolving detector 600 and, in the process, compensate the imaging scale and the twist of the plurality of secondary electron beamlets 9 as a result of a magnetic lens such that a third raster arrangement of the focal points 15 of the plurality of secondary electron beamlets 9 remains constant on the detector plane 225. The electron-optical lenses 205.1 to 205.5 are shown as magneto-optical elements but are not limited to magneto-optical elements and can comprise also electro-static lens elements or stigmators. The secondary electron imaging system 200 further comprises an exchangeable contrast aperture 284a, 284b, mounted on an aperture filter module 214 at a pupil plane 21 of the secondary electron imaging system 200. With an exchange mechanism (not shown), different aperture stops 284a or 284b can be positioned in the second pupil plane 21b and aligned with respect to the optical axis 2105 of the secondary electron imaging system 200.

[0057] Further information relating to such multi-beam particle beam systems and components used therein, such as, for instance, particle sources, multi-aperture plate and lenses, can be obtained from the international patent applications WO 2005 / 024881, WO 2007 / 028595, WO 2007 / 028596, WO 2011 / 124352 and WO 2007 / 060017 and the German patent applications having the publication numbers DE 10 2013 016 113 Al and DE 10 2013 014 976 Al, the disclosure of which in the full scope thereof is incorporated by reference in the present application.

[0058] The multi-beam charged particle imaging system 1 furthermore comprises a control unit 800 configured both for controlling the individual particle optical components of the multiple particle beam system and for evaluating and analyzing the signals obtained by the detector 600. In this case, the control or controller unit 800 can be constructed from a plurality of individual electronic components, devices and computers. By way of example, the control unit 800 comprises a control operation processor 880, a control module 840 for the control of the electron-optical elements of the secondary electron imaging system 200 and a control module 830 for the control of the electron-optical elements of the primary beamlet generation unit 100. For example, primary beamlet control module 830 is configured for an adjustment of a rotation angle or the raster of primary beamlets 3 with respect to a scanning movement direction of the stage 500. The control unit 800 further comprises a stage control module 850 for positioning and moving the sample surface 25 or sample 7 by stage 500 within the object plane 101. The control unit 800 further comprises a control module 503 to adjust a sample voltage VS, and for supplying the sample voltage VS to the sample 7.

[0059] Further, the control unit 800 comprises the scanning control module 860. During an inspection mode of operation, a plurality of focus points 15 of secondary electron beamlets is formed in the detection plane 225, and a plurality of signals is recorded during scanning operation of the primary beamlets 3 over the surface 25 of the sample 7. The detector 600 comprises a plurality of sets of detection elements with one set of detection elements for each secondary electron beamlet 9. During use, each set of detection elements is configured to record the intensity signal of the assigned secondary electron beamlet 9. The plurality of intensity signals for the plurality of secondary electron beamlets 9 is transferred to the image data acquisition unit 810, where the image data is processed and stored in memory 890. The setup of the secondary electron optical imaging system 200, the detector 600, and the assignment of the sets of detection elements to the focus spots 15 of the secondary electron beamlets 9 is initially determined and stored in the memory 890 of the control unit 800 of the multi-beam charged particle imaging system 1.

[0060] According to the example of figure 1, the multi-beam charged particle imaging system 1 further comprises a retractable monitoring system 230, which can be inserted into the secondary electron beam path in front of the detection plane 225. The monitoring system 230 comprises further imaging elements and a high-resolution detector. The monitoring system 230 is connected to a monitoring control unit 820.

[0061] The plurality of focus spots 5 of the primary beamlets 3 form a regular raster arrangement of incidence locations, which are formed in the object plane 101. For a large throughput, a large number J of beamlets 3 is desired. With the single beam path with a single objective lens 102, the number of beamlets and the throughput is limited by a circular shape of the aperture of the objective lens 102. A large number J of beamlets 3 can be achieved with a hexagonal arrangement or raster of beamlets 3, wherein the plurality of beamlets 3 is arranged on a hexagonal grid within an enclosing hexagon. The number J of primary beamlets 3 may be seven, 19, 37, or more. In practice, the number J of beamlets 3, and hence the number of incidence locations or focus spots 5, can be chosen to be significantly greater, such as, for example, J = 91, J = 127 or J = 469. Generally, the number J of beamlets 3 is given by

[0062] (1) J = 3 * S * (S-l) + 1.

[0063] With S being the number of hexagonal "rings" or "shells" within the hexagonal patten or raster. An example with S = 5 is illustrated in Figure 2a. The plurality of focus points 5 is arranged in a hexagonal pattern 251 within an enclosing hexagon 254. It comprises S = 5 rings or shells 253.1 to 253.5, each ring forming a hexagon. The outermost hexagon 253.5 is identical to the enclosing hexagon 254. The hexagonal raster pattern 251 is arranged in a raster coordinate system with coordinates xr, yr.

[0064] Figure 2b illustrates examples of an image acquisition with the plurality of primary charged particle beamlets 3 according to the prior art. In a first example, the scanning operation control module 860 is configured to provide during use a scanning signal to scanning deflector 110. Thereby, each primary charged particle beamlet 3 is deflected by the collective multibeam raster scanner 110 such that the corresponding focus spot 5.i is scanned over an image patch 245. i of a single beamlet (Figure 2a). Each image patch 245. i has a diameter AP of for example 8pm to 10pm, which is corresponding to the pitch p of beamlets 3. The scanning operation comprises a scanning of a plurality of parallel image scanning lines 241 along scanning direction 143.1 for image acquisition. At the end of each image scanning line 241, each beamlet 3 is moved back to the starting position of a next scanning line, which is also called "flyback" 243. During image acquisition along image scanning lines 241, the scanning operation is controlled to achieve a dwell time dT of about 20ns or 50 ns at each image point, with for example 8000 images points per image scanning line 241. The time for flyback 243 can be much shorter, for example 20ns in total. Figure 2c shows the parallel operation of a plurality of primary charged particle beamlets 3 to acquire an image of a surface area segment of a wafer surface, consisting of a plurality of image patches 245 arranged in a pattern like the raster of beamlets 251. The method of scanning image acquisition according to the prior art has the disadvantage of discontinuously accelerating and decelerating of the wafer stage 500. Thereby, a throughput is limited during image acquisition. Throughput is not only limited by the time to move the wafer from a first to a second inspection position, but also by the time it takes the stage 500 to come to a complete standstill (also called the ring-down time).

[0065] To overcome the disadvantage of the image acquisition by image scanning in xr- and yr- coordinates (see figure 2 for raster coordinates), an image acquisition with a scanning stage has been proposed, whereby stage 500 is moved with constant velocity in approximately yr- direction and scanning operation of charged particle beamlets is performed only in xr- direction. Such methods are known in image acquisition and direct-write lithography with rectangular raster of beamlets. The rectangular raster of beamlets is slightly rotated with respect to the movement direction yr of the stage, and the method is referred to as "slanted scan". However, according to prior art it was believed that a slanted scan cannot be applied with an hexagonal arrangement of beamlets and utilizing all beamlets within a hexagonal arrangement of beamlets. For example, in "Advances in high-resolution, high-throughput whole mouse brain volume electron microscopy", presented by S. MIKULA, S. K. MIKULA, M. MUELLER, N. NEEF, J. TISLER, J. TRITTHARDT, W. DENK, at the Neuroscience in October 2015, not all J = 91 beamlets of the hexagonal raster have been used during image acquisition with a scanning stage, and thus the throughput of a multi-beam system could not completely be exploited. An example is illustrated in Figure 2d with a hexagonal raster 251 comprising 61 beamlets in five hexagonal shells 253.1 to 253.5. A group of 16 beamlets (encircled by rhombus 251b) of the 61 beamlets is redundant and is not required for image acquisition. Thereby, throughput is reduced by (S-1)A2 / J. For example, the throughput is significantly reduced by more than 25%. For example, beamlet 5.j and beamlet 5.i cover an overlapping scanning beam path 259 during scanning movement of the stage 500. A similar selection of a limited number of beamlets within a hexagonal raster and a significant loss of throughput or more than 25% is disclosed in US 2023 0282440 Al. With the limited number of usable beamlets for imaging, the throughput is significantly reduced, and the loss of throughput easily outweighs the gain in reduction of time for stage alignment and ring-down time required for image acquisition with steady stage. The throughput of an image acquisition is, measured in area per second. In addition, any double exposure with charged particle beamlets might deteriorate the image result or even cause damage to a surface of an object of interest, e.g. by e-beam induced deposition of a residual gas on the surface 15 of the object 7.

[0066] According to the first embodiment, a method of surface inspection with high throughput is provided with continuous movement of a wafer surface 25 in a target direction ys. The method according to the first embodiment utilizes a slanted scan but does not sacrifice throughput. An example according to the first embodiment is illustrated in figure 3. The illustration in figure 3a is limited to J = 61 beamlets arranged in S = 5 shells 253.1 to 253.5 with J = 61 focus points 5, which are arranged in a hexagon of hexagonal raster points 251. However, throughout the disclosure, but the raster is not limited to for example S = 5 shells 253.1 to 253.5 with J = 61 beamlets. Instead, the embodiments and examples can easily be extended to any number S of shells, comprising for example larger numbers L of beamlets within the hexagonal arrangement, for example comprising J = 91 beamlets, J = 127, J = 169, J = 217, J = 271, J = 331, or even more beamlets, like J = 397 or J = 469 or more.

[0067] In a step SI of the method according to the first embodiment, a number N of beamlets in a period 255 of beamlets is derived. The number N corresponds to the number of beamlets which cover one repetitive image stripe during image acquisition with the moving stage. The number N can be any number with N <= J, but some numbers are preferred over others. In a first example of step SI, the number of beamlets of one period 255 is determined according to (2) N = 2S - 1 with S given by number of shells 253. In the example of figure 3a with S = 5 and five shells or hexagons 253.1 to 253.5, one period 255 is formed by N = 9 beamlets. The first example is illustrated in Figure 3a.

[0068] In a step S2, the rotation angle a of the raster for slanted scanning is determined according to

[0069] (3) a. = arctan [K / ( I * / V)] with N given by the number of beamlets of one period 255 and factor K. Factor K is described below in the third embodiment, and throughout the first or second embodiment below, K = 1. In step S2, the slant angle a between target movement direction of the continuously moving stage 500 and the raster 251 is adjusted.

[0070] The rotation angle a corresponds to the angle between coordinate system xr, yr of the raster 251 and movement direction of the moving stage 500 in direction ys. The rotation of the raster coordinate system xr, yr into scanning coordinate system xs, ys is illustrated in Figure 3b. In the example, the rotation angle is determined to be al = 3.6705°. As described above, the rotation angle a of the raster coordinate system xr, yr with respect to the scanning coordinate system xs, ys can be adjusted by at least one of the objective lens 102, the field lenses 103 and field lens 333, or a combined action thereof.

[0071] If the number N of beamlets is selected according to equation (2) with K = 1, each scanning path 259 is covered by a single beam let, for example scanning path 259. i is covered by beamlet

[0072] 3.1 with focus point 5.i. Figure 3c illustrates the projected scanning paths 259 of one period

[0073] 255.1 during movement of the stage in direction ys, wherein each is built by a beamlet 3 with corresponding focus point 5. A i-th beamlet 3.i, with focus point 5.i contributes to the first projected path 259. i of a first period 255.1. The k-th beamlet 3.k, with focus point 5.k contributes to the first projected path 259. k of a next period of equidistant scanning beam paths 259.

[0074] In the first example, the scanning width sw is selected to be corresponding to the distance ps between each projected scanning path 259, which is given by

[0075] (4) ps = [ p / N * cos a] with the pitch p of the hexagonal raster 251. In an example with S = 6, and a typical pitch of p = 10pm, J = 91 and N = 11, a = 3°, it follows ps = 0.91pm.

[0076] In a step S3, a scanning pattern EO of a scanning deflection of the plurality of primary charged particle beamlets 3 is determined.

[0077] According to an example of step S3, the scanning pattern EO is selected such that the raster of beamlets is scanned in xs-direction over the full width sw = ps. Thus, a scanning pattern EO is determined wherein in scanning operation by scanning deflector 110 is achieved in xs- direction, which is rotated with respect to the xr-direction by slant angle a (see raster coordinate system xr, yr illustrated in figure 2b). The scanning deflection must not necessarily be exactly in xs-direction but is selected at least almost or approximately perpendicular to the movement direction ys of the stage. The scanning pattern may consist of a repetition of scanning line 241 by scanning deflection in xs-direction for image acquisition over a complete width sw = ps and fly-back 243 in (negative) xs-direction (see Figure 3d).

[0078] In a step S4, an image acquisition of a first plurality of J image stripes 257 is initiated by initiating a continuous movement of stage 500 in ys-direction and synchronized scanning deflection of the plurality of primary charged particle beamlets 3 according to the selected scan pattern EO. Figure 3e illustrates three image stripes 257.1 to 257.3 of three adjacent focus points 5.1 to 5.3 during collective scanning of all beamlets parallel to xs-direction during movement of the stage parallel to ys-direction. Thereby, by each single beamlet, image stripes 257 of width ps are covered during movement of stage 500 in direction of ys. The movement velocity vs of stage 500 is determined from the number of pixels along on scanning line of length ps. A typical number of pixels or dwell points, respectively, with dwell time dT per x- scan leads to an image time TSX per scanning line width or pitch sw in x-direction of

[0079] (5) TSX = dT * sw / dx = dT * NX with NX being the number of pixels in one scanning line width sw. The movement velocity vs of the stage 500 in target movement direction ys is thus given by

[0080] (6) vs = dy / TSX with pixel raster dy in y-direction. With dy = dx, the movement velocity is given by

[0081] (7) vs = dxA2 / (sw * dT) For example, with a dwell time of dT = 20ns corresponding to a scanning frequency of 50MHz, and a raster resolution of 1 nm, the movement velocity for stage 500 is determined to vs = 5.51E-5 m / s.

[0082] Since the beamlets 3 are arranged in hexagonal raster 251 and limited to an enclosing hexagon 254, some beamlets 3 are missing for a full and equidistant coverage of a surface area with projected scanning paths 259, and an interlacing of scanning operations is required for a full coverage of larger surface areas. In step SA full coverage of a surface area of for example a wafer surface 25 is achieved by interlacing the first scanning paths 259.1 with a first raster position 251.1 and second or further scanning paths 259.2 at second or further raster positions 259.2.

[0083] In a step S5, after an image acquisition of a first or generally previous plurality of J image stripes 257 is completed, movement of stage 500 in direction ys is stopped and wafer surface 25 is moved in a direction perpendicular to movement direction ys - here direction xs - by a step size dxs. The step displacement dxs in xs-direction is given by

[0084] (8) dxs = J * ps.

[0085] The step movement by dxs perpendicular to movement direction ys must not necessarily be performed by the wafer stage 500. As is explained in examples below, a step movement dxs can also be achieved by deflection scanner 110 or other static deflection elements of the multi-beam charged particle beam system 1.

[0086] In a step S6, an image acquisition similar to step S4 is repeated. Figure 3f illustrates the projected scanning paths 259.1 during movement of the stage in direction ys. After moving stage 500 along the movement direction ys, stage 500 is moved in xs-direction by step dxs such that missing scanning paths within the first scanning paths 259.1 are filled by scanning paths 259.2 (for example along scanning path 259.2m). Each alternating movement direction of stage 500 along ys can be in opposite direction, as indicated in figure 3f. After each image acquisition steps S4 and S6, a step S5 is repeated and an image of a predetermined surface area of a surface 25 of a wafer 7 is obtained.

[0087] The displacement step dxs in xs-direction perpendicular to the scanning direction ys is further illustrated in figure 3g at an example with S = 3, a = 6.5868° and J = 19. In this example, the lateral displacement step is dxs = 37.7pm; in the example with J = 91, the lateral displacement step is dxs = 82.6pm.

[0088] In an example of step S3, a scanning operation of the collective deflection scanner of the multibeam charged particle beam system 1 is adjusted to a modified scan pattern EO.M. The example is illustrated in Figure 4. Figure 4a shows the effective scanning pattern EO of image acquisition lines 241a within a scanning image stripe 257a during continuous movement of the wafer surface 25 via stage 500 in movement direction ys with a scanning deflection EO similar to figures 3d and 3e. During time TSX = NX * dT with th number of image pixels across the image stripe 257 in xs-direction, the wafer surface 25 moves in direction of ys by distance dys = TSX * vs. This effectively leads to image acquisition of image pixels along tilted lines 241.1a or 241.2a. According to the first example, scanning deflection by collective deflection scanner 110 is adjusted to achieve a scanning deflection pattern EO.b (see Figure 4b) with an additional deflection component -l*dys in movement direction ys per scanning time TSX. With this deflection scanning pattern EO.b being not perpendicular to movement direction ys, movement of wafer surface 25 is compensated and image acquisition is achieved at imaging lines perpendicular to movement direction ys.

[0089] During scanning operation, typically a scanning deflection of the plurality of charged particle beamlets 3 is not achieved in a step-wise manner, but rather in more continuous deflection of the plurality of charged particle beamlets 3. Thus, each focus spot 5 on the wafer surface 25 is slightly moving during a dwell time dT, and the residual image pixel from which information is obtained is of slightly elliptical shape. According to a further example, this deviation from a circular shape is at least partially compensated by determining a further modified scanning pattern EO.p during step S3. An example is illustrated in Figure 5. Figure 5a shows the modified scanning pattern 145 ("EO.p"). During continuous movement of the wafer surface 25, the scan pattern EO.p (label 145) is repeated until a plurality of image stripes 257 is achieved. Scanning image acquisition with the scanning pattern 145 of Figure 5a is achieved by scanning deflecting each primary charged particle beamlet 3 with scanning deflector 110 rallel to movement direction ys and thus forming a plurality of scanning mage nes 241. i parallel to movement direction ys. After the scanning image line 241. i, the plurality of focus spots 5 of charged particle beamlets 3 is moved by flyback 243. s to the starting position of the next, subsequent scanning image line 241. i+1. The starting position of the next, subsequent scanning image line 241. i+1 is shifted opposite to the scanning direction ys by a distance proportional to the movement velocity vs divided by NX. In the example of Figure 5, the number of scanning lines or pixels NX across an image stripe 257 in xs-direction is NX = 10. After the NX-th scanning line 241. NX is scanned, each focus spot 5 of each beamlet 3 is deflected by beam deflector 110 to the starting position of a subsequent scan pattern EO.p along fly-back 243.1. Figure 5b illustrates the scanning image acquisition lines 241 within the repeated scan patterns 145. i-1, 145. i and 145. i+1 during image acquisition in direction parallel to a continuous movement of the wafer surface 25.

[0090] A second embodiment is illustrated in Figure 6a. The method according to the second embodiment forms along similar steps SI to S6 and it is referred to the steps of the method according to the first embodiment. According to the second embodiment, scanning of each beamlet by scanning pattern EO with particle scanner 110 in x-direction is reduced. For example, after each image acquisition with scanning lines 259.1 with raster 251.1 at a first lateral position with respect to the continuously moving stage 500, either the raster of beamlets 251 or stage 500 is moved by displacement step ddx in x-direction, and a subsequent set of adjacent scanning lines 259.2 with raster 251.2 at a second or subsequent lateral position with respect to the continuously moving stage 500 is acquired. Thereby, a plurality of image acquisitions is performed, and the plurality of image stripes is interlaced. A displacement step ddx perpendicular to movement direction of the continuously moving stage 500 can be very small, for example ddx = lnm, ddx = 1.5nm, or ddx = 2nm, such that no scanning of beamlets 3 in x-direction with the scanning deflector 110 is needed, and step S3 can be omitted. In another example, only the scanning width of a scanning pattern EO is reduced to a fraction of the pitch ps, for example by selecting a factor L > 1 in equation for the modified scanning width sw:

[0091] (9) sw = ps / L = [ p / N * cos a] / L

[0092] In another example, only the scanning width sw of a scanning pattern EO is reduced to a fraction of the pitch ps for example by L = 10, and a small displacement step ddx is selected accordingly. Generally, a sequence of image stripes 257 of scanning width sw is obtained by movement of stage in ys during image scanning with scanning deflector 110 over scanning width sw in direction perpendicular to the movement direction ys. After acquisition of each image stripe 257, a small displacement step ddx in x-direction by sw is performed. The factor L is also called interlacing factor L.

[0093] A method according to the second embodiment is comprising the steps of the first embodiment. The method according to the second embodiment is further comprising selecting an interlacing factor L during step SI. A method according to the second embodiment further comprises a step S7. In step 7, after an image acquisition of a first or generally previous plurality of J image stripes 257 of scanning width sw with L > 1 is completed, movement of stage 500 in direction ys is stopped and a position of the wafer surface 25 with respect to the raster of beamlets 251 is changed in direction xs perpendicular to movement direction ys by a step size ddx. The step displacement ddx in xs-direction is given by

[0094] (10) ddx = sw.

[0095] An example with L = 2 is illustrated in figure 6b. A subsequent image stripe 257 of scanning width sw is obtained by movement of stage in ys during image scanning with scanning deflector 110 over scanning width sw in direction perpendicular to the movement direction ys. The operation is repeated L times, with L any number between L = 1 and L = ps / dx, with dx being the pixel spacing. The first embodiment with completely electron-optically scanning beamlets 3 to cover the distance sw between center paths of single beamlets 259 constitutes an example with L = 1, and an example without any scanning deflection of beamlets constitutes an example with ddx = dx, or L = ps / dx, with for example L ~ 454 in case of J = 91 beamlets and dx = 2nm.

[0096] By reducing the scanning width to sw = ps / L, the movement velocity vs of moving stage 500 according to eq. (7) is increased. With increased scanning velocity vs, or reduced scanning width sw, a charge deposition per surface area is reduced and a net surface charge of an area on for example a wafer surface is reduced. Thereby, also an electric field generated by a surface charge during image scanning is reduced. Furthermore, the reduced accumulated surface charges within smaller stipes of reduced scanning width sw may dissipate more quickly.

[0097] By reducing the scanning width to sw with L > 2 with the collective scanning deflector 110, scanning-induced distortion is reduced. Scanning induced distortion is typically of third order. In an example, a scanning induced distortion after full width scanning of width ps with L = 1 is in the order of lOnm. By reduction of the scanning width by L = 3, the scanning induced distortion can be reduced by a factor of more than 20, for example by a factor of 25, for example by a factor of LA3. Thereby, scanning induced distortion is reduced to be below 0.5nm, for example to be below 0.3nm.

[0098] The displacement steps ddx of step S7 can either be achieved by collective deflector 110 of the multi-beam charged particle beam system 1, or by displacement of the wafer stage 500 by a small displacement step ddx perpendicular to the movement direction ys. Small displacement steps ddx can for example be achieved with a short-stroke high precision stage component. As is explained in examples below, a step movement ddx can also be achieved by deflection scanner 110 or other static deflection elements of the multi-beam charged particle beam system 1.

[0099] Displacement steps S7 and image acquisition steps S4 or S6 are repeated L times until a complete set of image stripes is acquired similar to figure 3e. Thereafter, a step S5 is executed. The steps S4 to S7 are repeated until an inspection image of a desired surface area of a wafer surface 25 is acquired.

[0100] In a third embodiment, a further improvement of a scanning image acquisition with a continuously moving stage 500 is achieved. One specific issue of an image acquisition can be charging effects induced by the charges of the beamlets during exposure of a wafer surface 25 during dwell time dT. As one method to overcome or minimize surface charging, a multiplexing of image acquisitions is proposed. An example of the third embodiment is illustrated at figure 7. According to equation (2), N is an odd number and a division by two does not give an integer. However, in some examples with N = 9, 15, 21,... in one period, N is not a prime number and can be divided by a multiplexing factor K of K = three, five, seven or even more. Multiplexing factor K is introduced above in equation (2). With K = 3, it is thus possible to achieve a larger rotation angle a. In the example of figure 7, with N = 9 and K = 3, rotation angle a is determined according to eq. (3) by a = 10.89°. As a consequence, in each projected beam path 259 of a first beamlet is overlapping with two further projected beam paths 259 of two further beamlets such that an image acquisition of each surface position on a wafer 7 is achieved by superposing the imaging results of K = 3 beamlets and with K being an odd number K = 1, 3, 5,.... According to this method of image multiplexing with more than one beamlet contributing to an image acquisition of an image of a surface segment of a wafer 7, the individual charge of a primary beamlet at each position can be reduced by either reducing the dwell time dT or the beam current. Thereby, an effect of a charging of a surface with each beamlet is reduced. A method according to the third embodiment is comprising the steps of the first embodiment. The method according to the third embodiment is further comprising selecting a multiplexing factor K during step SI.

[0101] The image acquisition method according to the first example of the third embodiment however allows only for superposition of odd numbers of beamlets with K = 3, 5, 7,.... In figure 8, a second example of the image acquisition with a moving stage 500 is illustrated. According to the method, a row 261 within the raster 251 of primary charged particle beamlets 3 is blocked. Thereby the number of beamlets N in one period is reduced by one, and an even number N* of beamlets 3 in one period is achieved with

[0102] (11) N* = 2S - 2

[0103] Figure 8a illustrates an example with S = 5, N* = 8, K = 1 and a4 = 4.1278° according to equation (3). With N* being an even number, it is possible to select even numbers for multiplexing factor K as well, for example K = 2. With for example a multiplexing factor K = 2, high throughput can be maintained, while the effect of charging is reduced. An example of multiplexing factor K = 2 is shown in Figure 8b, with an angle a4 = 16.1021°. The angle a4 can be derived according to a modification of equation (3). Generally, rotation angles a can be predetermined for a set of multiplexing factors K and stored for example in a look-up table.

[0104] A method according to the second example further comprises a step S8 of blocking a row 261 within the raster 251 of primary charged particle beamlets 3.

[0105] Generally, according to the third embodiment with a multiplexing factor K = 2 or more, for example K = 3, surface positions on the surface 25 of an object such as a wafer 7 are exposed at least twice by at least two charged particle beamlets of the plurality of primary charged particle beamlets 3. Thereby, with a first exposure by a first beamlet, a charging of a surface position is achieved while a first image is acquired. The subsequent second exposure with the second beamlet is configured to acquire a second image of a pre-charged surface area. Thereby, voltage contrast imaging can be performed. For example, differences between first and second images are computed. Thereby, a defect sensitivity for certain types of defects is increased. For example, second images can be compared to reference images and for example buried defects, which generate to a change in a voltage contrast, can be detected.

[0106] According to the third embodiment, a multiplexing or voltage contrast imaging method is achieved by a selection of K > 1 and selection of the corresponding rotation angle a between raster coordinates and movement direction of the stage. Image multiplexing is, however, not limited thereto. In an example, a pre-charging or image multiplexing is achieved by selection of a scanning width sw of the scanning deflection in direction xs travers to the movement direction of the stage in ys which is exceeding the distance ps according to eq. (4). For example, with an interlacing factor L selected to be smaller as 1 (generally L < 1), for example L = 0.5, a scanning width sw according to eq. (9) exceeds the pitch ps between to projected beampaths 259 by a factor of 2, and each surface area is image scanned twice, including a first image scan by a first beamlet and a second image scan by a second beamlet.

[0107] In the example of the voltage contrast imaging with an even number of images, for example with multiplexing factor K = 2, for example a row 261 of redundant primary charged particle beamlets 3 is blocked. Blocking can be achieved by a field stop or individual beam deflectors. Such blocking of individual beamlets is not limited to blocking a complete row like row 261. Other examples of blocking redundant beamlets are possible as well, such that an even number N* is obtained. In another example, however, the redundant primary charged particle beamlets 3 are not blocked but they are used as additional means, for example for additionally pre-charging the sample surface, or for monitoring means.

[0108] According to the third embodiment, a method of wafer inspection is comprising an image acquisition of each image stripe by executing a movement of a stage of the multi-beam inspection tool along a movement direction, wherein the image acquisition is comprising collecting a first image data of a image stripe (257) with a first beamlet and a second image data of the image stripe (257) with a second beamlet (for image stripe 257 reference is made to figure 3). Thereby, a voltage contrast image can be computed by the difference between the second image data and the first image data.

[0109] Figure 9 illustrates an example of a multi-beam charged particle beam system 1, capable of image acquisition with a moving stage 500. The multi-beam charged particle beam column 2 comprises the object irradiation unit 100 and the detection unit 200, combined by the beam divider 400. Only one beam path of charged particles is illustrated in figure 9. The multi-beam charged particle beam column 2 is rigidly mounted on a frame 925. The frame 925 is mounted via damping pedestals 913 to a baseplate 907, which is rigidly installed by adjustable installation pedestals 931 to the floor 901 within a laboratory or fabrication environment. The stage 500 is mounted via fast and long-stroke actuators and bearings 157 to a support base 909, which is connected to the baseplate 907. The stage 500 is configured for precision adjustment in at least three degrees of freedom, including positioning in x, y and rotation around the z-axis. The stage 500 can be configured with more degrees of freedom, including position adjustment in z-direction and adjustment of tilt angles via rotation around x- and y- axis. The stage 500 is configured with fast actuators and bearings 157 with long stroke or long range to move the stage 500 along ys-direction and thereby to cover large areas on a wafer 7 during image acquisition. A diameter of a wafer can be for example 300mm, thus for a complete scanning image acquisition of a wafer surface, a movement range of the stage 500 in ys-direction can be at least 300mm. Connected to the stage 500 is a wafer support base 151. In an example, wafer support base 151 can also be considered as part of the stage 500. The wafer support base 151 is connected to long-stroke stage 500 via actuators and guiding means 153, which allow a fast movement of the wafer support base 151 with short stroke. For example, wafer stage 500 moves the wafer 7 continuously with velocity vs in ys-direction with long stroke actuators 157, and short-stroke actuators and bearings 153 are configured for step movement of wafer 7 perpendicular to ys-direction by ddx or dxs during method steps S5 and S7. Both movements and positioning accuracy is controlled by position sensor 1021, for example comprising at least one Laser interferometer 1027. Positioning by short-stroke actuators 153 can further be controlled by embedded sensors (not shown). Short-stroke actuators 153 can be piezo-actuators, and a precision support or bearings can comprise for example flexible elements such as monolithically formed parallel spring connections with level support, allowing a control of tiny movements by below one nm with high precision up to step movements of several pm, for example 10pm or more, for example even 100pm.

[0110] In the example of figure 9, the frame 925 and stage 500 is encapsuled by a vacuum chamber 903, which is connected to multi-beam charged particle beam column 2. Control unit 800 comprises a control unit 850 for stage movement and adjustment and is supported by feedback-control loop with control units 1023 in response to the signals generated by position sensor system 1021. A feed-back control loop is further configured to actively control the damping pedestals 913 such that the frame 925 and multi-beam charged particle beam column 2 are feed-back controlled to actively compensate vibrations below the Eigenfrequency of the frame 925 and multi-beam charged particle beam column 2.

[0111] During step S2 of a method according to the first to third embodiment, the target movement direction ys of the wafer surface 25 is determined and the wafer surface 25 or the raster 251 of beamlets 3 is rotated accordingly, such that the movement direction ys of stage 500 is rotated from the raster coordinate system xr, yr by angle a (see e.g. figure 3b). Typically, the precision adjustment of the angle a between raster coordinates xr, yr and the scanning movement direction ys comprise a combination of at least two magnetic lenses 103, by which a rotation of the raster of primary charged particle 3 can be controlled with high precision. Such magnetic lenses 103 in the primary beam path can be accompanied by at least two magnetic lenses 205.1 to 205.5 (see figure 1) in the detection unit 200 for compensation any change of the raster arrangement of secondary electron beamlets 9. Furthermore, the adjustment of movement direction ys can comprise a rotation by stage 500 around a direction normal to the wafer surface (i.e. the z-axis).

[0112] In an example of the embodiments, a residual error in the setup of rotation angle a between the movement direction ys with respect to the raster coordinates xr, yr is present. Therefore, a method according to the first to third embodiments can further comprise a step S9 for precision adjustment of the angle a between the movement direction ys with respect to the raster coordinates xr, yr.

[0113] In a first example, a precision adjustment of the scanning movement direction ys is achieved by continuously moving the stage 500 with long stroke actuators 157 in a direction slightly deviating from the target movement direction ys, and adjusting the scan pattern EO with the electron-optical deflector 110 of the multi-beam charged particle beam column 2. An example is illustrated in figure 10. Like in figure 6b, L = 2 is selected in the example. Figure 10a illustrates mismatch of the angle 0 of the scanning velocity direction vs' of the moving stage 500 with target scanning direction ys. During each scan of a scanning line in xs-direction, the wafer surface 25 is moved in xs-direction by ds = TSX * vs' * si n(0), with the scanning time TSX per line-scanning in xs-direction. To compensate the displacement ds, each line width of each scanning line 241.6 is reduced by ds. The mismatch is compensated by a modified scan pattern EO.S. A step S9 therefore comprises a determination of mismatch angle 0. A step S9 further comprises the determination of displacement ds and a configuration of each scanning line 241.6 in xs-direction with a length (ps - ds) and each fly-back 243.6 with a length ps. After each flyback 243.6, the starting point of the next scanning line 241.6i+l is shifted by ds in direction opposite to ds (see figure 10b). Figure 10c illustrates the effect of the adjusted scanning length to ps - ds on a scan pattern 145 on a surface of a wafer 7 after scanning in direction vs' deviating from the target scanning direction ys.

[0114] In a second example, a precision adjustment of the scanning direction ys with angle a relative to the xr-yr-coordinate system of the raster of beamlets 3 is achieved by continuously moving the stage 500 with long stroke actuators 157 in a direction slightly deviating from the target movement direction ys by parallel and continuous movement with the short stroke actuator 153 perpendicular to the long-stroke movement direction. Thereby, a precision adjustment of the scanning angle a can be obtained. An example is illustrated in Figure 11. A deviation angle 0 of the long-stroke movement of stage 500 in direction of the target scanning direction ys is compensated by a short-stroke movement velocity vsx in xs-direction or at least almost perpendicular to the target movement direction ys with vsx = vs' * si n (0) . A step S9 therefore comprises a triggering a continuous movement of the wafer surface 25 in a direction perpendicular to the target movement direction ys with velocity vsx.

[0115] Figure 12 illustrates an example of a multi-beam charged particle beam column 2. A masking blade or stop 351.1 is arranged at an intermediate image position 321 of the plurality of primary charged particle beamlets 3. For the other elements illustrated in figure 1 or figure 9, reference is made to the description thereof. The position of the at least one masking blade 351.1 can be adjusted with bearings and actuators of the stop unit 354 such that for example one row 261 of S primary charged particle beamlets 3 of the raster 251 is blocked. Such an arrangement allows a very flexible change between image acquisition methods and allows usage of all J beamlets within the hexagon of raster of beamlets 251 according to the first and second embodiment as well as a reduction by S beamlets to achieve a multiplexing imaging according to the third embodiment with K = 2. Other means for blocking individual beamlets 3 are for example an active deflector array 306 within the multi-aperture arrangement 305, by which individual beamlets 3 can be deflected into for example a beam dump (not shown).

[0116] Throughout the embodiments, a step time or settling time of a step wise motion of a long- range stage 500 is avoided by a first and continuous movement of a wafer surface 25 by a wafer stage 500 in a first movement direction. The target movement direction ys of the wafer surface 25 is rotated by angle a with respect to the yr-axis in raster coordinate system xr, yr. In an example, a precision adjustment of the rotation angle a of the target movement direction ys is achieved by a synchronized movement of the wafer surface 25 in a second movement direction perpendicular to the first movement direction, such that, as a result, the wafer surface 25 is moved in the target movement direction ys. In a second set of examples, a deviation of a movement direction by stage 500 from the target movement direction ys is compensated by a scanning pattern of the plurality of charged particle beamlets 3 by deflection scanner 110. In an even further example, a step-wise exposure pattern EO.p is selected by scanning deflector 110 and image acquisition in scanning lines 241 is achieved by scanning deflection and movement in ys-direction in parallel. In both latter examples, the scanning deflection by deflection scanner 110 can reach out of range of the deflection scanner 110.

[0117] In a fourth embodiment, a method of surface inspection is provided with continuous movement of a wafer surface 25 in a target direction ys by a long-stroke actuator of a stage 500 for scanning for example across a full wafer 7 with diameter of up to 300mm, and a superposed, stepwise movement with a short-stroke actuator 153 of about below few mm, for example 100pm. The settling time or step time of the small movement with short- stroke actuator 153 can be much less compared to a step time of the long-stroke actuator of a stage 500. For example, short- stroke actuator 153 can be directly coupled to wafer holder 151 (see figure 5) with considerably lower mass compared to a full-range stage 500 with six degrees of freedom.

[0118] In a first example, the stepwise movement with a short-stroke actuator 153 consists of a stand-still operation by short-stroke actuator 153, followed by a step-wise movement to a new position perpendicular to the movement direction ys of the long-stroke stage 500. An example is illustrated in Figure 13. In this example, a movement direction of the continuously moving long-stroke actuator 157 is deviating from the target movement direction ys by deviation angle 0 (see Figure 10 and description thereof), which is compensated by modified scanning pattern EO.s. After a sequence of applications of scanning pattern EO.S, the scanning lines 241.6 are incrementally shifted and the starting point is shifted by an accumulated scanning shift ASS. The method according to the fourth embodiment therefore comprises method steps of the first to third embodiments and one further method step S10. In method step S10, the position of the wafer surface 25 is displaced by short stroke actuators 153 in direction perpendicular to target movement direction ys. Thereby, the accumulated scanning shift ASS is compensated and the scanning pattern EO.S is reset to start for example at the center of the image field. Step S10 is for example executed when the accumulated scanning shift ASS reaches a predefined accumulated scanning shift. For example, the predefined accumulated scanning shift is given by the scanning range of the scanning deflector 110. In figure 13 illustrates a path 149 of a point on a wafer surface during execution, with four executions of step S10.

[0119] In a second example, stepwise movement with a short-stroke actuator 153 consists of a continuous movement of the wafer surface 25 by short-stroke actuator 153 in a direction parallel to the movement direction ys, with a movement velocity opposite to the movement by long-stroke actuator 157 of stage 500, followed by a step-wise movement parallel to the movement direction ys of the long-stroke actuator 157 stage 500. The example is illustrated in Figure 14. During a first time interval Tl, a point on a wafer surface 25 is kept at a constant position while movements of short stroke actuator 153 and long stroke actuator 157 cancel each other. During the first time interval Tl, for example, a scan pattern according EO.p (see figure 5 and the description thereof) is performed to acquire a first image patch with a beamlet 3. After acquiring the first image patch, short stroke actuator 153 jumps at time t2 to the start position of a second image patch and scan pattern EO.p is repeated to acquire the second image patch during a repetition of time interval Tl. Figure 14 illustrates in the upper part the velocities vs of long stroke actuator (LSA) and short stroke actuator (SSA) over time. In the lower part, the trajectory 149 of a point on the wafer surface 25 is illustrated. A method of the second example therefore comprises a step Sil. Step 11 comprises a step Sll.l with an actuation of short stroke actuators 153 to continuously move wafer support base 151 in direction opposite to movement direction ys and with movement velocity -vs opposite to the of the continuous movement with long stroke actuators 157. Stepp 11.1 is executed during an image acquisition step S4 or S6. Step 11 further comprises after certain time intervals Tl a step Sll.l, comprising a displacement step of wafer support base 151 in opposite direction to the continuous movement of step Sll.l. Thereby, a position of a wafer surface 25 is kept constant even during a continuous movement of a wafer stage 500 by long stroke actuators 157 over a long range of several 10 or 100mm.

[0120] According to examples of the embodiments, the stage 500 is continuously moved in a second, ys-direction while an image is acquired by scanning of the plurality of primary charged particle beamlets 3 with the collective multi-beam raster scanner 110 in a first, xs- direction. Stage movement and stage position is monitored and controlled by sensors known in the art, such as Laser interferometers, grating interferometers, confocal micro lens arrays, or similar.

[0121] During an image scanning operation step, the control unit 800 is configured to trigger the image sensor 600 to detect in predetermined time intervals a plurality of timely resolved intensity signals from the plurality of secondary electron beamlets 9, and the digital image of an image patch is accumulated and stitched together from all scan positions of the plurality of primary charged particle beamlets 3.

[0122] A method of wafer inspection with a multi-beam inspection tool with a moving stage and high throughput is illustrated in Figure 15.

[0123] A method of image acquisition with moving stage therefore comprises several steps SO to S12. Examples of steps SI to Sil are described an reference is made also to the description of steps SI to Sil in the embodiments.

[0124] Step SO is comprising loading a wafer 7 and registering a wafer coordinate system to the reference coordinate system of the multi-beam charged particle system 1. Step SO further comprises determining an inspection area on a wafer surface 25 and setting inspection parameters of the multi-beam charged particle system 1.

[0125] Step SI is comprising selecting a number N of beamlets within one period 255 of beamlets of the hexagonal raster of beamlets 251. Generally, N is selected between N = 1 und N = J-l, wherein J is the number of beamlets 3 in the hexagonal raster and given by eq. (1). In an example, N is selected according to eq. (2) with N = 2*S - 1, with S being the integer number of hexagonal "rings" within the hexagonal raster of beamlets 251.

[0126] Step SI further comprises determining a multiplexing factor K =1, 2, 3,..., and determining an interlacing factor L = 1, 2, 3, 4,..., wherein K and L are integer numbers. The default values of K and L can be K = L = 1. In an example, K is selected from the odd numbers only, such that K = 3, 5, 7, ....

[0127] In Step S2, a rotation angle a is determined by eq. (3) and the rotation angle a between the raster coordinate system xr, yr and the scanning coordinate system xs, ys is adjusted. A target movement direction of the wafer stage 500 is selected in direction of ys. The raster coordinate system xr, yr is known or measured within the reference coordinate system of the multi-beam charged particle system 1. In an example, raster coordinate system xr, yr and reference coordinate system of the multi-beam charged particle system 1 can be identical.

[0128] Adjustment of rotation angle a between the raster coordinate system xr, yr and the scanning coordinate system xs, ys is achieved by at least one of a rotating the wafer 7 by wafer stage 500, and by a rotation the raster 251 of beamlets by for example magnetic lenses 103 and 102 of the multi-beam charged particle system 1.

[0129] Step S2 further comprises determining the movement velocity vs according to eq. (6) or (7). Step S2 further comprises determining the scanning width or scanning deflection width sw according to equation (9).

[0130] Step S2 further comprises determining the displacement step size dxs according to equation (8).

[0131] In examples with L > 1, step S2 further comprises determining the interlacing step size ddx.

[0132] Step S3 comprises determining a scanning pattern EO. For examples of scanning patterns, it is referred to the example above. A scanning pattern EO may comprise a scanning deflection of the plurality of charged particle beamlets 3 in direction approximately perpendicular to the movement direction ys. A scanning pattern EO may comprise a scanning deflection of the plurality of charged particle beamlets 3 in direction approximately parallel to the movement direction ys. A scanning pattern EO may comprise a flyback 243. Steps S4 or steps S6 are comprising an image acquisition of a set of image stripes 257 - one for each beamlet 3 of the plurality of J beamlets 3. Steps S4 and S6 are comprising performing the stage movement along movement direction ys with velocity vs while executing the scan pattern EO and performing image acquisition. Step S4 and Step S6 may differ in the movement direction, for example step S4 with movement in positive ys- direction, and step S6 with movement in negative ys-direction.

[0133] After each step S4, and if interlacing factor L is selected to L > 1, a Step S7 is executed. Step S7 comprises executing a lateral interlacing displacement step ddx between the wafer support table 151 and the raster of beamlets 251 perpendicular to the movement direction ys. The lateral interlacing displacement step ddx can be executed for example by short stroke actuators 153 of the wafer stage 500. In an example, the lateral interlacing displacement step ddx can be achieved by a deflection offset of the raster of beamlets 251 by scanning deflector 110 or other deflectors within the multi-beam charged particle system 1. Steps S4 or S6 and S7 are repeated L times.

[0134] After repetition, a first plurality of image stripes 257, wherein the total number of image stripes is given by the product of J / K * L, is acquired, and the plurality of image stripes 257 is fully covering an area segment of a wafer surface. The required area segment for imaging is generally predetermined. One area segment can for example comprise 1mm x 1 mm or more, for example 1mm x 10mm, or even more, for example a complete die on a wafer or a complete wafer surface.

[0135] Step S5 comprises a displacement step of displacement step size dxs. Step S5 comprises executing a lateral "large" displacement step dxs between the wafer support table 151 and the raster of beamlets 251 perpendicular to the movement direction ys. A lateral "large" displacement step dxs can for example be executed by long stroke actuators 157 of the wafer stage 500.

[0136] Steps S4 to S7 are repeated until an inspection area determined in step SO is fully covered by the plurality of image stripes 257

[0137] In an example, the method further comprises optional Step S8. Step S8 comprises filtering a row or beamlets from the plurality of beamlets 3. Step S8 is for example executed if a multiplexing with an even number K = 2, 4, 6... is selected during step S3. A filtering of a row of beamlets can for example be performed by moving a field stop 351.1, 351.2 in an intermediate image surface 321 and blocking a row of beamlets.

[0138] During image acquisition in steps S4 and S6, a deviation angle 0 between the target movement direction ys and the real movement direction of a wafer surface 25 with velocity vector vs' may be inevitable or may be detected for example by monitoring system 230. Optional Step S9 is comprising a step of determining the deviation angle 0 and compensating an effect of the deviation angle 0. Compensating the deviation angle 0 comprises at least one of a modification of a scan pattern EO to a modified scan pattern EO.S and a synchronous movement of the wafer support base 151 in a direction approximately perpendicular to the target movement direction ys with a velocity vsx ~ si n( 0).

[0139] In an example, the method further comprises Step S10. Step S10 comprises executing at determined time intervals a step displacement ASS of the wafer support base 151 in a direction approximately perpendicular to the target movement direction ys. With the step displacement ASS, an exceeding of a scanning range of the deflection scanner 110 by a modified scan pattern EO.S is avoided.

[0140] In an example, the method further comprises Step Sil. Step Sil comprises executing during first time intervals Tl a synchronous movement of the wafer support base 151 in a direction parallel to the target moving direction ys with a velocity opposite to the movement velocity vs, and, after each time interval Tl, executing a step movement parallel to the target moving direction ys of stage 500.

[0141] Step S12 comprises collecting and stitching the image data collected during steps S4 and S6. In case of multiplexing factor K > 1, step S12 may further comprising an image processing method such as for example superposing of image data or difference image computation.

[0142] The method steps do not need to be executed in the order of their numbering; certain method steps may be iteratively repeated or may be executed in parallel. For example, Step S12 can be initiated during performing the first execution of an image acquisition step S4 or S6. An inspection method according to an example may also comprise image acquisition step S4 or step S6 with movement direction of the wafer stage 500 in one direction only, including a step back during step S5 to the starting point of a subsequent image acquisition step S4 or step S6. Various modification and combinations of the method steps, embodiments, and examples are possible, and the description of the examples shall not limit the scope of the method of wafer inspection with a multi-beam inspection tool with a moving stage and with high throughput.

[0143] A list of reference numbers is provided:

[0144] 1 multi-beamlet charged-particle system

[0145] 2 multi-beam charged particle beam column

[0146] 3 primary charged particle beamlets, or plurality of primary charged particle beamlets

[0147] 5 primary charged particle beam spot

[0148] 7 object, wafer or semiconductor mask

[0149] 9 secondary electron beamlet, forming the plurality of secondary electron beamlets

[0150] 11 primary beam path

[0151] 13 secondary electron beam path

[0152] 15 secondary charged particle image spot

[0153] 21 common pupil plane

[0154] 25 surface of object

[0155] 100 object irradiation unit

[0156] 101 object plane

[0157] 102 objective lens

[0158] 103 field lens

[0159] 108 beam cross over

[0160] 110 first scanning deflector

[0161] 112 electrostatic element

[0162] 143 beamlet scanning direction

[0163] 145 scan pattern 149 trajectory of wafer surface

[0164] 151 wafer support base

[0165] 153 short stroke actuators

[0166] 157 long stroke actuators

[0167] 200 detection unit

[0168] 205 electron-optical lens

[0169] 214 aperture filter module

[0170] 222 second deflector

[0171] 225 detection or image plane

[0172] 230 monitoring system

[0173] 241 scanning line

[0174] 243 fly-back

[0175] 245 image patch of single beamlet

[0176] 251 hexagonal raster of beamlets

[0177] 253 ring-segments of hexagonal raster

[0178] 254 enclosing hexagon of raster

[0179] 255 one period of raster points

[0180] 257 scanning image stripe

[0181] 259 center path of single beamlets

[0182] 261 blocked beamlets

[0183] 284 aperture filter

[0184] 300 charged-particle multi-beamlet generator

[0185] 301 charged particle source

[0186] 303 collimating lenses 304 filter plate

[0187] 305 multi-aperture arrangement

[0188] 306 multi-aperture plates

[0189] 309 primary electron beam

[0190] 321 intermediate image surface

[0191] 331 first field lens

[0192] 333 second field lens

[0193] 351 masking blade

[0194] 354 Stop unit

[0195] 400 beam divider

[0196] 500 sample stage

[0197] 503 Sample voltage supply

[0198] 600 image sensor or detector

[0199] 800 control unit

[0200] 810 imaging control module

[0201] 820 monitoring control unit

[0202] 830 primary beampath control module

[0203] 840 secondary beampath control module

[0204] 850 stage control module

[0205] 860 scanning operation control unit

[0206] 880 Control operation processor

[0207] 890 memory

[0208] 901 fab floor

[0209] 903 vacuum enclosure 907 base plate

[0210] 909 stage support base

[0211] 913 active damper

[0212] 921 bellow 925 frame

[0213] 931 mounting pedestal

[0214] 1021 position sensors

[0215] 1023 feedback control

[0216] 1027 laser beam 2105 optical axis of detection unit

Claims

ClaimsWhat is claimed is:

1. A method of wafer inspection with a multi-beam inspection tool (1) with a plurality of charge particle beamlets (3) arranged in a hexagonal raster of beamlets (251) within a plurality of S hexagon-shaped shells (253) comprising an enclosing hexagon (254), the method comprising- a step SI, comprising selecting a number N of beamlets (3) within one period (255) of beamlets of the hexagonal raster of beamlets (251), and selecting a multiplexing factor K,- a step S2, comprising determining and adjusting a rotation angle a between a raster coordinate system xr, yr of the hexagonal raster of beamlets (251) and a scanning coordinate system xs, ys, and comprising determining a scanning deflection width sw and a movement velocity vs,- a step S3, comprising determining a scanning pattern EO, the scanning pattern EO covering at least the scanning deflection width sw,- a step S4, comprising an image acquisition by executing a movement of a stage (500) of the multi-beam inspection tool (1) along a movement direction ys with the velocity vs and synchronously executing the scan pattern EO and collecting image data within a plurality of image stripes (257), wherein the rotation angle a is determined according to a = arctan [K / ( 3 * N)].

2. The method according to claim 1, wherein the number N of beamlets (3) is determined according to N = 2*S - 1.

3. The method according to any of the claims 1 or 2, further comprising a step S5, comprising executing a displacement step of displacement step size dxs between a wafer support table (151) and the hexagonal raster of beamlets (251) in a direction perpendicular to the movement direction ys, with dxs is given by dxs = J * ps with the number J of beamlets (3) given by J = 3 * S * (S-l) + 1.

4. The method according to claim 3, further comprising at least one of a repeating of step S4 or executing a step S6, comprising an image acquisition by executing a movement of a stage (500) of the multi-beam inspection tool (1) along the movement direction ys with the velocity vs and synchronously executing the scan pattern EO and collecting image data within a plurality of image stripes (257), wherein step S4 and step S6 have opposing movement directions.

5. The method according to any of the claims 1 to 4, wherein step S2 is further comprising selecting an interlacing factor L > 1, and wherein the scanning deflection width sw is determined according to sw = [ p / N * cos a] / L.

6. The method according to claim 5, further comprising a step S7, comprising executing a lateral interlacing displacement step ddx between the wafer support table (151) and the raster of beamlets (251) perpendicular to the movement direction ys by a lateral interlacing displacement step ddx = ps / L, and comprising at least one of a repeating of step S4 or executing a step S6, step S6 comprising an image acquisition by executing a movement of a stage (500) of the multi-beam inspection tool (1) along the movement direction ys with the velocity vs and synchronously executing the scan pattern EO and collecting image data within a plurality of image stripes (257), wherein step S4 and step S6 have opposing movement directions.

7. The method according to claim 6, comprising iteratively repeating the step S7 and an image acquisition step S4 or S6 at least L times.

8. The method according to any of the claims 1 to 7, further comprising a step S8 of determining a deviation angle 0 between the target movement direction ys and the real movement direction of the wafer support table (151) and compensating an effect of the deviation angle 0.

9. The method according to claim 8, wherein compensating an effect of the deviation angle 0 comprises modifying the scan pattern EO into a modified scan pattern EO.S.

10. The method according to claims 8 or 9, wherein compensating an effect of the deviation angle 0 comprises executing a synchronous movement of the wafer support base (151) in a direction perpendicular to the target movement direction ys with a velocity vsx ~ si n( 0) .

11. The method according to any of the claims 1 to 10, wherein the multiplexing factor K is selected K = 1.

12. The method according to any of the claims 1 to 10, wherein the multiplexing factor K is selected K > 1 with K being an odd integer with K = 3,5,7....

13. The method according to any of the claims 1 to 10, wherein the multiplexing factor K is selected K > 1 with K being an even integer with K = 2,4,6, the method further comprising a step S8 comprising a filtering a single row (261) of beamlets (3) from the hexagonal raster of beamlets (251).

14. The method according to claim 13, wherein the filtering of a single row (261) of beamlets (3) is performed by positioning a movable field stop (351.1, 351.2) in an intermediate image surface (321) of the multi-beam charged particle beam system (1).

15. The method according to any of the claims 1 to 14, further comprising a step S10, comprising executing a step displacement ASS of the wafer support base (151) in a direction perpendicular to the target movement direction ys.

16. The method according to any of the claims 1 to 15, further comprising a step Sil, comprising executing during a first time interval Tl a synchronous movement of the wafersupport base (151) in a direction parallel to the target moving direction ys with a velocity opposite to the movement velocity vs of the stage (500), and, after each time interval Tl, executing a step movement parallel to the target moving direction ys of the wafer (500).

17. The method according to any of the claims 3 to 16, further comprising a step S12, further comprising a stitching of the image data of the plurality of image stripes (257) collected during iterative execution of steps S4 or S6.

18. The method according to claim 17, wherein step S12 is further comprising a multiplexing of the image data of the plurality of image stripes (257) collected during iterative execution of steps S4 or S6.

19. The method according to any of the claims 3 to 14, wherein step S5 is comprising executing the displacement step of displacement step size dxs by a long-stroke actuator (157) of the stage (500).

20. The method according to any of the claims 6 to 19, wherein step S7 is comprising executing the lateral interlacing displacement step ddx by a long-stroke actuator (153) of the stage (500).

21. A multi-beam charged particle beam system (1) for wafer inspection, comprising:- a wafer stage (500) configured for continuous movement at a velocity vs in movement direction ys during image acquisition,- a control unit (800) with a memory (890) and a processor (880), the memory (890) configured for storing instructions when executed by the processor (880) causing the multibeam charged particle beam system (1) for executing a method according to any of the claims 1 to 20.

22. The multi-beam charged particle beam system (1) of claim 21, wherein the stage (500) is comprising a long-stroke actuator (157) configured for continuous movement of a wafer support table (151) in the movement direction ys.

23. The multi-beam charged particle beam system (1) of claim 22, wherein the stage (500) is comprising a short-stroke actuator (151) configured for continuous or stepwise movement of a wafer support table (151) in the movement direction ys or perpendicular to the movement direction ys.

24. The multi-beam charged particle beam system (1) of any of the claims 21 to 23, further comprising a movable field stop (351.1, 351.2) in an intermediate image surface (321) of the multi-beam charged particle beam system (1).

25. A method of wafer inspection with a multi-beam inspection tool (1) with a plurality of charge particle beamlets (3) arranged in a hexagonal raster of beamlets (251) within a plurality of S hexagon-shaped shells (253), the method comprising- selecting an even number N* of beamlets (3) within one period (255) of beamlets of the hexagonal raster of beamlets (251), and selecting a multiplexing factor K with K >= 2,- determining and adjusting a rotation angle a between a raster coordinate system xr, yr of the hexagonal raster of beamlets (251) and a scanning coordinate system xs, ys, and comprising determining a scanning deflection width sw and a movement velocity vs,- acquiring an image of an image stripe (257) by executing a movement of a stage (500) of the multi-beam inspection tool (1) along a movement direction ys with the velocity vs, the image acquisition comprising collecting a first image data of the image stripe (257) with a first beamlet (3) and a second image data of the image stripe (257) with a second beamlet (3).

26. The method according to claim 25, further comprising determining a voltage contrast image by computing the difference between the second image data and the first image data.

27. The method according to claim 25 or 26, further comprising filtering of redundant beamlets from the hexagonal raster of beamlets (251).

28. The method according to claim 27, comprising filtering a single row (261) of beamlets (3) from the hexagonal raster of beamlets (251).

Images