Improved beam control for a multi-beam scanning particle imaging system

Abstract

A method of operating a control circuitry (800) of a multi-beam scanning particle imaging system (1) is provided. The multi-beam scanning particle imaging system (1) comprises a micro-optics arrangement (305) arranged to split an incoming aggregate particle beam (309) into multiple particle beams (3). The multi-beam scanning particle imaging system (1) is configured to jointly scan at least a set of the multiple particle beams across an object plane (101). The method comprises obtaining one or more sensor signals of one or more sensors arranged along a particle beam path of the multi-beam scanning particle imaging system (1), based on the one or more sensor signals, determining one or more characteristics of the aggregate particle beam (309) at the micro-optics arrangement (305), and, based on the one or more characteristics of the aggregate particle beam (309), controlling one or more particle optics elements (302) of the multi-beam scanning particle imaging system (1).

Description

[0001] Description

[0002] Improved beam control for a multi-beam scanning particle imaging system

[0003] Field of the invention

[0004] Various examples of the disclosure generally relate to the field of multi-beam scanning particle imaging systems, in particular to a method of operating a control circuitry of a multi-beam scanning particle imaging system for improving beam control. The present disclosure furthermore relates to a multi-beam scanning particle imaging system comprising a control circuitry implementing the method.

[0005] Background

[0006] Scanning Particle Microscopes (SPMs) are instruments used for imaging and analyzing the surfaces of samples at the nanoscale level. They employ a focused beam of particles, such as electrons or ions, to probe the surface of the sample. A widely used implementation is the scanning electron microscope (SEM). SPMs offer unique capabilities and advantages in terms of resolution, contrast, and sample preparation. Key Components of SPMs are a particle source, lenses, a sample chamber, a scanning system and detectors. The particle source generates a beam of particles for scanning the sample. SPMs can use various particles as probes, including electrons, ions, or even neutral atoms or molecules. Electromagnetic or electrostatic lenses focus and direct the particle beam onto the sample surface, allowing for high-resolution imaging and analysis. The sample chamber is maintained under vacuum to prevent particle scattering and ensure optimal imaging conditions. The scanning system includes electromagnetic coils or plates for scanning the particle beam across the sample surface in a raster pattern, enabling imaging and analysis. Different detectors capture signals generated by interactions between the particle beam and the sample, providing information about surface morphology, composition, and other properties.

[0007] Multi-beam particle microscopes are an advancement in particle microscopy technology. These instruments utilize multiple particle beams simultaneously to scan and analyze the sample surface. By employing parallel imaging, multi-beam particle microscopes offer significant improvements in imaging speed, throughput, and efficiency compared to singlebeam instruments.

[0008] For example, a multi-beam scanning electron microscope (MSEM) is disclosed in US 7 244 949 B2 and US 2019 / 0355544 A1. In an MSEM, a sample is simultaneously irradiated with a plurality of individual electron beams arranged in a field or grid. For example, some tens or hundreds of individual electron beams can be provided as primary radiation, with each individual electron beam separated from an adjacent individual electron beam by a pitch of 1 to 200 micrometers. For example, an MSEM has about 100 separate individual electron beams ("beamlets") arranged, for example, in a hexagonal grid, with the individual electron beams separated by a pitch of about 10 micrometers. The plurality of charged individual particle beams (primary beamlets) are each individually focused on a surface of a sample to be examined by means of a common large-field optical system including, inter alia, a common objective lens. For example, the sample may be a semiconductor wafer mounted on a wafer holder mounted on a movable stage. As the wafer surface is illuminated with the charged primary beamlets, interaction products, such as secondary electrons or backscattered electrons are emitted from the wafer surface. Their respective starting points correspond to the locations on the sample where the multiple primary particle beams are focused. The number and energy of the interaction products depend on, among other things, the material composition and the topography of the wafer surface. The interaction products form a plurality of secondary individual particle beams (secondary beamlets) which are collected by the common objective lens and incident on a detector arranged in a detection plane as a result of a projection imaging system of the multi-beam inspection system. The detector comprises a plurality of detection regions, each of which comprises a plurality of detection pixels, and the detector captures an intensity distribution for each of the secondary individual particle beams. An image field of, for example, 100 micrometers by 100 micrometers may be obtained.

[0009] Especially in multi-beam particle scanning microscopes, the generation of particle beams is critical for accurate imaging. Aspects to consider when generating particle beams include beam intensity and beam positioning in the sample plane. The particle beams may vary for example after changing the particle source or during operation of the microscope, for example due to aging of the particle source.

[0010] For example, DE 10 2021 118 561 B4 relates to a method of operating a multi-beam particle microscope. A beam current is measured. Measuring of the beam current comprises a current measurement on a multi-aperture array during an image acquisition process at selected positions. A deviation of the measured beam current from a nominal beam current is determined. The determined deviation is decomposed into a drift component and a high frequency component. The high-frequency component of the beam current is controlled by means of a first beam current control means and / or an effect of the high-frequency component on an image quality of the multi-beam particle microscope is compensated by means other than the first beam current control means. Optionally, the drift component of the beam current can be controlled by means of a second beam current control means.

[0011] Likewise, WO 2023 / 001401 relates to a multi-beam particle microscope with improved beam current stability. Fig. 6 schematically illustrates a quadrant detector: a multi-aperture plate is structured into five separate regions 351, 352, 353, 354 and 367 that are isolated from one another. Excess electrons are measured. The inner region 367 comprises all openings in the multi-aperture array. Therefore, the inner region 367 is not disturbed at all by any structuring or by separately provided detectors. This ensures a very good beam current quality of the generated individual particle beams. The outer region is subdivided into the four quadrants 351, 352, 353 and 354. The quadrants 351 and 353 have an area that is identical in size. The same holds for the larger areas of regions 352 and 354. If the beam cone of the illuminating particle beam impinges centered onto the multi-aperture array, the signals generated by the measurement of regions 351 and 353 shall indicate the same signal strength. The same holds for signals generated by measurements on the regions 352 and 354. In the different scenario when the beam cone of the illuminating particle beam is shifted into one direction, the signal generated by each quadrant 351, 352, 353 and 354 shows a variation that allows for identifying the direction of the shift.

[0012] Summary

[0013] Accordingly, a need exists for improved techniques of generating beams in a multi-beam particle microscope. In particular, the techniques should also be usable during operation of the microscope, i.e., also in-situ during an analysis of a sample.

[0014] This need is met by the features of the independent claims. The features of the dependent claims define embodiments. A method of operating a control circuitry of a multi-beam scanning particle imaging system is provided. The multi-beam scanning particle imaging system comprises a micro-optics arrangement arranged to split an incoming aggregate particle beam into multiple particle beams. Each beam of the multiple particle beams may be also referred to as a beamlet. The multi-beam scanning particle imaging system is configured to jointly scan at least a set of the multiple particle beams across an object plane. For example, the multi-beam scanning particle imaging system may be a multi-beam scanning electron microscope. The incoming aggregate particle beam may be an aggregate electron beam and the multiple particle beams may be multiple electron beams. At least a set of the multiple electron beams may be used for jointly scanning the object plane. In other examples, the particles may comprise ions, e.g., helium ions. According to the method, one or more sensor signals of one or more sensors are obtained. The one or more sensors are arranged along a particle beam path of the multi-beam scanning particle imaging system. Based on the one or more sensor signals, one or more characteristics of the aggregate particle beam at the micro-optics arrangement are determined. It is to be noticed that, although the sensors may not be positioned at the micro-optics arrangement, nevertheless, characteristics of the aggregate particle beam at the micro-optics arrangement are determined based on the sensor signals. Based on the one or more characteristics of the aggregate particle beam, one or more particle optics elements of the multi-beam scanning particle imaging system are controlled.

[0015] For example, the one or more characteristics may be indicative of a tilt angle of the aggregate particle beam with reference to an optical axis of a beam generation apparatus of the multi-beam scanning particle imaging system at a reference position along the beam path. The reference position may be at the point of ingress of the aggregate particle beam at the micro-optics arrangement. In some examples, the reference point may be in short distance upstream to the micro-optics arrangement. However, the characteristics determined for the aggregate particle beam may have a close relationship to the characteristics of the multiple particle beams that are generated from the aggregate particle beam at the micro-optics arrangement.

[0016] For example, a collimated and telecentric aggregate particle beam may be assumed. The aggregate particle beam comprises a large number of particles, for example electrons, each having its own trajectory. Each trajectory may have a tilt angle with respect to an optical axis of a beam generation apparatus of the multi-beam scanning particle imaging system. The optical axis may be defined as an axis perpendicular to a central aperture of a multi-aperture plate of the micro-optics arrangement. The aggregate particle beam may be considered as one global beam. A global beam tilt may be defined as a combined consideration of the trajectories of the particles of this global beam. For example, the global beam tilt may be defined as an angle by which a medium angle of all or some of the trajectories of the particles deviates from the optical axis. If only some of the trajectories of the particles are considered, the considered particles should be selected uniformly over the cross section of the aggregate particle beam. The one or more characteristics may comprise the global beam tilt at a reference position along the beam path.

[0017] Additionally or as an alternative, a local beam may be defined as a combined consideration of the particles that constitute one of the micro particle beams or as a combined consideration of the particles that are spatially close within a vicinity to each other in the order of a size of an aperture of a multi-aperture plate of the micro-optics arrangement. A local beam tilt may be defined for each such local beam as a combined consideration of the trajectories of the particles of this local beam. For example, the local beam tilt may be defined as an angle by which a medium angle of all or some of the trajectories of the particles deviates from the optical axis. If only some of the trajectories of the particles are considered, the considered particles should be selected uniformly over the cross section of the local beam. The one or more characteristics may comprise a corresponding local beam tilt for each local beam at a reference position along the beam path. The one or more characteristics may comprise additionally a deviation of the angles of the local beams with respect to each other.

[0018] For example, a plurality of local beams may be considered. The plurality of local beams may comprise some of the multiple particle beams generated at the micro-optics arrangement. For example, the micro-optics arrangement may have 91 apertures in a hexagonal arrangement. For example, six local beams generated by the outermost apertures and one local beam generated by the central aperture may be analyzed with sensors arranged downstream the micro-optics arrangement and corresponding local beam tilts may be determined. A global beam tilt may be determined based on the seven local beam tilts, for example as an average tilt angle. The one or more particle optics elements may be adjusted or controlled such that predefined requirements regarding the global beam tilt, the local beam tilts and / or the local beam angles with respect to each other are achieved or at least approximately achieved. For example, requirements may define that the global beam tilt is below 10 mrad, that each local beam tilt is below 10 mrad, and that a maximum deviation between local beam angles of neighboring local beams is below 5 mrad.

[0019] Although in the examples discussed above, a collimated and telecentric aggregate particle beam is assumed, the examples and embodiments discussed herein may also be applied to divergent or convergent aggregate particle beams which may also have a medium angle of propagation and angles for each single trajectory to which corresponding restrictions may be applied, for example a maximum deviation of the medium angle of propagation from the optical axis.

[0020] For example, the one or more particle optics elements are arranged upstream of the micro-optics arrangement along the particle beam path. The one or more particle optics elements may comprise for example electrical or magnetic lenses, for example a condenser lens system, and / or a deflector system. With the particle optics elements upstream of the micro-optics arrangement, a change in the particle optics elements may have a direct effect on the aggregate particle beam at the micro-optics arrangement and may be detected by the one or more sensors. Thus, a direct feedback loop may be implemented.

[0021] In some examples, the micro-optics arrangement may comprise the one or more particle optics elements. For example, the one or more particle optics elements may be configured to selectively act upon different ones of the multiple particle beams, for example by lenses and / or deflectors for each particle beam of the multiple particle beams. In some examples, the micro-optics arrangement may comprise lenses and / or deflectors acting on the multiple particle beams as a whole. By use of a suitable model considering lenses and / or deflectors for each particle beam of the multiple particle beams or for the entirety of the multiple particle beams, the one or more characteristics of the aggregate particle beam at the micro-optics arrangement may be determined based on sensor signals obtained by the one or more sensors, in particular by sensors downstream of the micro-optics arrangement.

[0022] In various examples, a load lock of the multi-beam scanning particle imaging system may be controlled to load a sample into a vacuum chamber of the multi-beam scanning particle imaging system. The one or more sensor signals may be obtained upon loading the sample into the vacuum chamber. Furthermore, a sample stage of the multi-beam scanning particle imaging system may be controlled to place the sample in the object plane and the one or more sensor signals are obtained upon the sample being placed in the object plane.

[0023] This means that, based on the one or more sensor signals, the determination of the characteristics of the aggregate particle beam and the control of the particle optics elements may be performed with the sample in the vacuum chamber, i.e. , in-situ.

[0024] According to various examples, the micro-optics arrangement comprises at least two multi-aperture plates arranged in parallel and spaced apart along the particle beam path.

[0025] The one or more sensors may comprise for each particle beam of a further set of the multiple particle beams an assigned particle detection sensor arranged downstream of the micro-optics arrangement along the particle beam path. For example, the further set of multiple particle beams may comprise some outermost apertures of the micro-optics arrangement, for example six outermost apertures of a hexagonal micro-optics arrangement. The further set off multiple particle beams may comprise further apertures, for example a central aperture of the micro-optics arrangement. For determining the one or more characteristics of the aggregate particle beam at the micro-optics arrangement, for each particle beam of the further set of the multiple particle beams a corresponding electrical current may be determined based on a sensor signal from the assigned particle detection sensor as the characteristics of the aggregate particle beam at the micro-optics arrangement.

[0026] In some examples, controlling the one or more particle optics elements comprises varying control parameters of the particle optics elements for minimizing the variation of the electrical currents.

[0027] In some examples, controlling the one or more particle optics elements comprises varying control parameters of the particle optics elements for maximizing the electrical currents.

[0028] According to various examples, the apertures of one of the at least two multi-aperture plates are aligned to apertures of another one of the at least two multi-aperture plates in a direction along the particle beam path. For example, the at least two multi-aperture plates may comprise a first aperture plate and a second aperture plate. At least some or all of the apertures of the first multi-aperture plate may be arranged in a same pattern as some or all of the apertures of the second multi-aperture plate. For example, the first and the second multi-aperture plates may be arranged such that at least some of the apertures of the first multi-aperture plate are aligned with corresponding apertures of the second multi-aperture plate. As a result, corresponding apertures of the first and second multi-aperture plates are arranged along an axis that is parallel to the optical axis of the beam generation apparatus. A particle beam of the multiple particle beams passing the corresponding apertures of the first and second multi-aperture plates may have a maximum intensity when being aligned to the optical axis and may have a reduced intensity when being not aligned to the optical axis. The control parameters of the particle optics elements may be varied to achieve an overall maximum intensity of the monitored particle beams, i.e., the particle beams of the further set of the multiple particle beams. In some examples, the control parameters of the particle optics elements may be varied to achieve a uniform intensity of the monitored particle beams. The above optimization criteria, i.e., overall maximum intensity and uniformity of the intensity, may be combined, for example by assigning a corresponding score to each optimization criteria and combining the scores by weighing. Further criteria may be considered, for example upper and lower intensity limits for each monitored particle beam.

[0029] According to various examples, a position of a first multi-aperture plate of the at least two multi-aperture plates is varied with respect to a second of the at least two multi-aperture plates in a direction perpendicular to the particle beam path. For example, corresponding apertures of the first and second multi-aperture plates are misaligned. The misalignment may be a fraction of a diameter of the apertures. A distance between the first and second multi-aperture plates may be small, e.g., less than a distance between two neighboring apertures in the plates. In this situation, a particle beam which is generated by the first multi-aperture plate and aligned to the optical axis will be absorbed partly by the second multi-aperture plate. However, a particle beam which is misaligned with respect to the optical axis but aligned to the misalignment of the first and second multi-aperture plates, may pass through the corresponding apertures of the first and second multi-aperture plates with no or no significant loss of intensity. As the misalignment between the first and second multi-aperture plates is intentionally invoked, a direction and / or degree of misalignment of the monitored particle beam may be determined and the particle optics elements may be adjusted accordingly. In various examples, a position of one of the at least two multi-aperture plates, i.e. a first multi-aperture plate, is varied with respect to another one of the at least two multi-aperture plates, i.e. a second multi-aperture plate, in a direction along the particle beam path.

[0030] Thus, for example, a distance between corresponding apertures of the first and second multi-aperture plates may be varied. As a result, a particle beam which is generate by the first multi-aperture plates and is not aligned to the optical axis will be partly absorbed by the second multi-aperture plate. The amount of absorption depends on the distance between the first and second multi-aperture plates. The larger the distance, the higher the absorption may be. However, a particle beam which is aligned with respect to the optical axis may pass through the corresponding apertures of the first and second multi-aperture plates with no or no significant loss of intensity independent of the distance between the first and second multi-aperture plates. The particle optics elements may be adjusted such that the absorption is minimized.

[0031] According to various examples, each of the one or more the sensors comprises at least one of a Faraday cup and a quadrant detector. For example, for each of the further set of the multiple particle beams a corresponding sensor may be provided, for example a corresponding Faraday cup or a corresponding quadrant detector. In some examples, a corresponding sensor may be movable and the further set of the multiple particle beams may be monitored one after the other. The quadrant detector may be considered as a particle current sensor, similar to the Faraday cup, that has a spatial resolution and a small area which is assumed to cover the area on in which the particle beamlet is expected to arrive when the particle optics elements are varied. Effects of the variation of the particle optics elements on the direction of each particle beamlet may be determined and considered.

[0032] The one or more sensors may comprise particle detection sensors in a detector system of the multi-beam scanning particle imaging system. The multi-beam scanning particle imaging system may be in a mirror mode in which at least the further set of the multiple particle beams is reflected across the object plane. In a usual operating mode of the multibeam scanning particle imaging system, the sample or the sample carrier are set to a high voltage with a polarity opposite to the polarity of the particles of the multiple particle beams. As a result, the particles from the beam generation apparatus (so-called primary particles, for example primary electrons) are slowed down and hit the sample. When the primary particle beam interacts with the sample surface, it causes ionization and excitation. Some of the particles (e.g. electrons) from the sample are knocked out due to this interaction. These knocked-out electrons are so called secondary particles (e.g. secondary electrons) that are accelerated by the high voltage and may be detected in the detector system of the multi-beam scanning particle imaging system. In the mirror mode, the high voltage at the sample or sample carrier is increased such that the primary particles (e.g. primary electrons) do not reach the surface of the sample and are reflected into the detector system of the multi-beam scanning particle imaging system. In the detector system, a position and intensity of each beamlet of the multiple particle beams may be detected and this may be considered for adjusting the control parameters of the particle optics elements. It is to notice that due to the mirror mode the secondary particles are not affected by the sample such that the sample can be maintained in the multi-beam scanning particle imaging system while adjusting the control parameters of the particle optics elements.

[0033] According to various examples, the further set of the multiple particle beams are adjustments beams. I.e. , the further set of multiple particle beams may not be used for scanning the sample, but only for adjustment purposes. Each of the at least two multiaperture plates may have inner apertures and outer apertures positioned in an outer portion surrounding the inner apertures. The inner apertures split the incoming aggregate particle beam into the set of multiple particle beams and the outer apertures split the incoming aggregate particle beam into the further set of multiple particle beams. In other words, the inner apertures split the incoming aggregate particle beam into the beamlets used for scanning the sample, and the outer apertures split the incoming aggregate particle beam into the adjustment beamlets. Accordingly, the sensors may be arranged to detect sensor signals based on the adjustment beamlets and may not affect the scanning beamlets. As a result, the multi-beam scanning particle imaging system can be continuously adjusted while scanning the sample. Changes in the multi-beam scanning particle imaging system, for example due to aging of the particle source, can be compensated in-situ.

[0034] In further examples, at least one of the at least two multi-aperture plates is positioned between a first one of the one or more particle optics elements and a second one of the one or more particle optics elements. For example, in addition to the above first and second multi-aperture plates, a third multi-aperture plate may be arranged between the first one and the second one of the particle optics elements. For example, the first one of the particle optics elements may comprise a first lens and a first deflector and the second one of the particle optics elements may comprise a second lens and a second deflector. The aggregate particle beam may be affected by the first lens and first deflector. The first lens may provide a first partial focusing and / or collimation of the aggregate particle beam. Then, the aggregate particle beam is split by the third multi-aperture plate into multiple particle beamlets which are then commonly focused and / or a collimated by the second lens and optionally aligned by the second deflector. The multiple particle beamlets may then be directed through the first and second multi-aperture plates as discussed above. This arrangement can increase the effect by increasing the distance between the multiaperture plates. In addition, installation space may be more easily available between the particle optics elements. Depending on how early the aggregated particle beam is split by the third multi-aperture plate, the effect of the Coulomb interaction can be reduced as the current can be reduced.

[0035] According to various examples, the one or more sensors comprise a diffraction element positioned in a particle beam path of at least one of the multiple particle beams. The diffraction element may comprise for example a diffraction crystal, in particular a monocrystal, or a diffraction grating. The one or more sensors may also comprise for each particle beam of the at least one of the multiple particle beams an assigned two- dimensional particle detection sensor arranged downstream of the micro-optics arrangement along the particle beam path. For determining the one or more characteristics of the aggregate particle beam at the micro-optics arrangement, a corresponding diffraction pattern is determined for each particle beam of the at least one of the multiple particle beams based on a sensor signal from the assigned two- dimensional particle detection sensor. The diffraction patterns may be used to characterize the aggregate particle beam at the micro-optics arrangement. When the diffraction element is aligned to the optical axis, e.g. a (main) axis of the crystal structure is aligned to the optical axis, a symmetry of the diffraction pattern may depend on the angle of incidence of the particle beam. Symmetry of the particle beam may be determined based on information from the two-dimensional particle detection sensor. The two-dimensional particle detection sensor may be positioned between the micro-optics arrangement and the object plane. As discussed above, adjustment beamlets may be provided. For each adjustment beamlet a corresponding diffraction element and two- dimensional particle detection sensor may be provided.

[0036] For example, the one or more particle optics elements may be controlled by varying control parameters of the particle optics elements for maximizing symmetry of the diffraction pattern. In some examples, the one or more particle optics elements may be controlled by varying control parameters of the particle optics elements for achieving a predefined diffraction pattern or for achieving a predefined orientation of the diffraction pattern.

[0037] According to some examples, the two-dimensional particle detection sensor comprises particle detection sensors in a detector system of the multi-beam scanning particle imaging system with the multi-beam scanning particle imaging system being in a mirror mode reflecting the at least one of the multiple particle beams across the object plane. One or more diffraction elements may be provided in a beam path of one, several or all beamlets downstream of the micro-optics arrangement. The one or more diffraction elements may be moved into the beam path while the sample is already arranged in the multi-beam scanning particle imaging system. After adjustment of the particle optics elements, the one or more diffraction elements may be removed from the beam path and the sample may be scanned. The diffraction elements may comprise for example a diffraction crystal, in particular a mono-crystal, or a diffraction grating.

[0038] In various examples, the micro-optics arrangement comprises a multi-aperture plate comprising at an upstream side of the plate one or more pyramid-shaped elements. The one or more sensors may comprise a current sensor measuring a current resulting from particles of the aggregate particle beam absorbed by the multi-aperture plate. The multiaperture plate as well as the pyramid-shaped elements may be made of an electrically conductive material and a current induced by a focused and deflected aggregate particle beam may be measured. Based on the oblique surfaces of the pyramid-shaped elements, positions of the tip and the base of each pyramid-shaped element may be determined. Determining the one or more characteristics of the aggregate particle beam at the microoptics arrangement comprises focusing the aggregate particle beam on the plane of the multi-aperture plate and scanning the one or more pyramid-shaped elements. For each of the one or more pyramid-shaped elements a corresponding offset of a tip of the pyramidshaped element with respect to a center of the base of the pyramid-shaped element is determined as the characteristics of the aggregate particle beam at the micro-optics arrangement. It may be assumed that focusing the aggregate particle beam and deflecting the aggregate particle beam in a plane perpendicular to the optical axis does not change the tilt of the aggregate particle beam. Therefore, an alignment of the optical axis of the beam generation apparatus and the aggregate particle beam propagation direction may be achieved when the tip of a pyramid-shaped element is detected to be at the center of the base of the same pyramid-shaped element as determined by the focused and deflected aggregate particle beam.

[0039] Controlling the one or more particle optics elements may comprise varying control parameters of the particle optics elements for minimizing the one or more offsets, i.e. achieving that the point of detection of the tip of the pyramid-shaped element corresponds to the center of the detected base of the pyramid-shaped element. The pyramid-shaped elements may be arranged outside the aperture arrangement on the multi-aperture plate of the micro-optics arrangement so that the pyramid-shaped elements do not affect the scanning of the sample. Scanning of the sample may be performed when setting the particle optics elements in the scanning configuration, e.g. with the aggregate particle beam in a collimated and telecentric configuration.

[0040] According to various examples, the micro-optics arrangement comprises at least a multiaperture plate and a multi-beam multi-pole alignment element arranged in parallel and spaced apart along the particle beam path. The multi-beam multi-pole alignment element may be positioned downstream of the multi-aperture plate along the particle beam path. The multi-beam multi-pole alignment element may comprise for each particle beam (i.e. for each beamlet) of a further set of the multiple particle beams at least one pair of opposing poles. The multi-beam multi-pole alignment element may comprise for each beamlet four pairs of opposing poles arranged along edges of an octagon. By applying voltages to each pole, the multi-beam multi-pole alignment element may be configurable to act as a deflector or stigmator or both on the corresponding beamlet. A stigmator is a component that may reduce astigmatism of the beamlet.

[0041] The one or more sensors may comprise for each beamlet an assigned particle detection sensor arranged downstream of the micro-optics arrangement along the particle beam path. Determining the one or more characteristics of the aggregate particle beam at the micro-optics arrangement may comprise applying a first excitation pattern to the at least one pair of opposing poles of each of the further set of the multiple particle beams, and applying a second excitation pattern to the at least one pair of opposing poles of each of the further set of the multiple particle beams. The second excitation pattern may differ from the first excitation pattern. For each particle beamlet an offset between a point of ingress of the beamlet at the assigned particle detection sensor while applying the first excitation pattern and a point of ingress of the particle beam at the assigned particle detection sensor while applying the second excitation pattern may be determined as the characteristics of the aggregate particle beam at the micro-optics arrangement.

[0042] It is assumed that the multi-beam multi-pole alignment element is aligned to the multiaperture plate along the optical axis of the beam generation apparatus. This means that each pair of poles of the multi-beam multi-pole alignment element is aligned to a corresponding aperture of the multi-aperture plate, i.e. a center of the aperture is aligned to a center between the pair of poles along the optical axis. Thus, when the multi-beam multi-pole alignment element is acting as a stigmator, the cross-sectional shape of the beamlet generated by the aperture of the multi-aperture plate may be modified, for example from a circular shape to an elliptical shape or from one elliptical shape to another elliptical shape, but the center of the beamlet does not move in the plane perpendicular to the optical axis as long as the beamlet has no tilt with respect to the optical axis. However, the situation changes when the beamlet has a tilt with respect to the optical axis. The beam tilt causes an additional beam shift when the multi-beam multi-pole alignment element acts as a stigmator, because the beamlet will be slightly deflected when the beam is traveling oblique through the pair of poles. In particular, when using four pairs of poles, i.e. a so-called 8-pole, a tilt in any direction will result in a beam shift of the beamlet. By applying different excitation patterns, different beam shifts are generated when the beamlet is not perfectly aligned to the optical axis. However, when the beamlet is aligned to the optical axis, no beam shift is generated, no matter which (stigmator) excitation pattern is applied.

[0043] Therefore, an alignment of the optical axis of the beam generation apparatus and the beam propagation direction may be achieved when no beam shift can be detected with varying excitation patterns at the multi-beam multi-pole alignment element, i.e. there is no offset between the points of ingress of the beamlet at the assigned particle detection sensor although the excitation patterns are varied.

[0044] For example, the multi-beam multi-pole alignment element may comprise at least two pairs of opposing poles for each particle beam of the further set of the multiple particle beams. When the first excitation pattern is applied, a same potential is applied to all poles. When applying the second excitation pattern, a first potential is applied to a first pair of opposing poles of the at least two pairs of opposing poles, and a second potential is applied to a second pair of opposing poles of the at least two pairs of opposing poles. The second potential is different from the first potential. Each of the one or more the sensors may comprise a Faraday cup or a quadrant detector. For example, for each of the further set of the multiple particle beams a corresponding sensor, for example a corresponding Faraday cup or a corresponding quadrant detector, may be provided. In some examples, the corresponding sensor may be movable, e.g. by linear translators, and the further set of the multiple particle beams may be monitored one after the other.

[0045] In some examples, the one or more sensors may comprise particle detection sensors in a detector system of the multi-beam scanning particle imaging system. The multi-beam scanning particle imaging system may be in a mirror mode as discussed above.

[0046] Various examples relate to a method that further includes removing the micro-optics arrangement from particle beam path. As a result, the aggregate particle beam is not split up into the multiple particle beams. A first edge element in a first plane perpendicular to an optical axis of a beam generation apparatus of the multi-beam scanning particle imaging system is moved from a position radially outside of the aggregate particle beam into the aggregate particle beam. The first plane is located between the particle optics elements and the object plane. Thus, an increasing portion of the aggregate particle beam is absorbed at the first edge element during the movement. While moving the first edge element, a first course of an electrical current of a portion of the aggregate particle beam not being absorbed by the first edge element is detected with one or more sensors. The one or more sensors may be the sensors described above used for obtaining the sensor signals for determining the characteristics of the aggregate particle beam as described above with the micro-optics arrangement in place. However, the one or more sensors may be different sensors specifically arranged for detecting the electrical current of the portion of the aggregate particle beam not being absorbed by the first edge element.

[0047] Next, the first edge element is removed from the aggregate particle beam, and a second edge element in a second plane perpendicular to the optical axis is moved from a position radially outside of the aggregate particle beam into the aggregate particle beam, thus absorbing an increasing portion of the aggregate particle beam at the second edge element. The second plane is different from the first plane and located between the particle optics elements and the object plane. A direction of movement of the first edge in the first plane may be parallel to a direction of movement of the second edge in the second plane. While moving the second edge element, a second course of an electrical current of a portion of the aggregate particle beam not being absorbed by the second edge element is detected with the one or more sensors. Based on the first course and second course, at least one of an offset value and a width difference value is determined. The offset value indicates an offset between a center of the aggregate particle beam at the first plane and a center of the aggregate particle beam at the second plane. The width difference value indicates of a difference between a width of the aggregate particle beam at the first plane and a width of the aggregate particle beam at the second plane.

[0048] Control parameters of the particle optics elements may be varied for minimizing at least one of the offset value and the width difference value.

[0049] For example, the first edge element, e.g. a sharp edge, is pushed step by step into the aggregate particle beam and the unblocked beam current is measured for each step. The profile in this first plane can be extracted from the measured beam current signal (e.g. as an integrated signal and using deconvolution). In particular, the center position and the width of the profile in the plane may be obtained. Extended to two different planes along the beam path in the z-direction (i.e., the direction of the optical axis of the beam generation apparatus), the combination of the two center positions and the two widths can be used to calculate the tilt of the beam. Also, the opening angle of the aggregate particle beam, i.e. the collimation, may be calculated. The tilt and collimation can then be adjusted, e.g. iteratively, via the control parameters of the one or more particle optics elements, including for example lenses and deflectors.

[0050] The one or more sensors may comprise a Faraday cup positioned between the first plane and the object plane, and between the second plane and the object plane.

[0051] The one or more sensors may comprise particle detection sensors in a detector system of the multi-beam scanning particle imaging system with the multi-beam scanning particle imaging system being in a mirror mode reflecting the portion of the aggregate electronbeam not being absorbed by the first or second edge elements across the object plane.

[0052] According to further examples, the one or more sensors comprise a magnetic coil and a Faraday cup. The magnetic coil is positioned between the first plane and the object plane, and between the second plane and the object plane. The magnetic coil is configured to deflect the aggregate particle beam to the Faraday cup positioned outside the optical axis. The coil can be electrically controlled and does not generate particles. No mechanical movement is required, in particular the Faraday cup does not need to be mechanically moved into the particle beam.

[0053] According to various examples, the method further comprises removing the micro-optics arrangement from the particle beam path and arranging a single-aperture plate in a first plane perpendicular to an optical axis of a beam generation apparatus of the multi-beam scanning particle imaging system with a center of the single aperture aligned to the optical axis. A size of the single aperture corresponds to a size of the cross section of the aggregate beam at the first plane. Then an edge element is moved in a second plane perpendicular to the optical axis into the aggregate particle beam that passed the single aperture. The second plane is downstream of the first plane. Depending on the position of the edge element, a part of the aggregate particle beam is absorbed at the edge element. Downstream of the edge element one or more sensors are arranged for determining an electrical current resulting from the part of the aggregate particle beam that is not absorbed at the edge element. While moving the edge element, a course of an electrical current of the portion of the aggregate particle beam not being absorbed by the edge element is detect with the one or more sensors. A width and / or position of the aggregate particle beam at the second plane is determined based on the course of the electrical current. A non-tilted and collimated particle beam may exhibit a width and position at the second plane that corresponds to the width and position of the single aperture in the first plane. A tilted particle beam may have a different position in the second plane than in the first plane, and a not-collimated particle beam may have the different width in the second plane than in the first plane.

[0054] Based on the determined course of the electrical current, a tilt and / or collimation of the aggregate particle beam may be corrected by adjusting the control parameters of the particle optics elements.

[0055] The one or more sensors may comprise a Faraday cup positioned between the second plane and the object plane.

[0056] According to various examples, the aggregate particle beam has a known non-uniform intensity profile in a cross section in a plane perpendicular to the optical axis of the beam generation apparatus of the multi-beam scanning particle imaging system. The following steps may be performed. The micro-optics arrangement may be removed from particle beam path. A single-aperture plate may be arranged in a first plane perpendicular to the optical axis with a center of a single aperture of the single-aperture plate aligned to the optical axis. The first plane may correspond essentially to the plane in which the microoptics arrangement was positioned. A size of the single aperture is smaller than a size of the cross section of the aggregate particle beam at the first plane. For example, the size of the single aperture may correspond to an outer diameter of the arrangement of apertures in a multi-aperture plate of the micro-optics arrangement. Then, a two- dimensional intensity course of the electrical current generated by a portion of the aggregate particle beam that has not been absorbed at the single-aperture plate is determined with a two-dimensional sensor arranged in a second plane perpendicular to the optical axis downstream of the first plane. An offset between an expected position of the known non-uniform intensity profile and a position of the corresponding intensity profile determined in the two-dimensional intensity course may be determined.

[0057] The control parameters of the particle optics elements may be varied for minimizing the offset.

[0058] Further, a difference between a size of the portion of the aggregate particle beam that has not been absorbed at the single-aperture plate in the two-dimensional intensity course and a size of the single-aperture plate may be determined. The control parameters of the particle optics elements may be varied for minimizing the difference.

[0059] As in the previous example, the following example relies on the assumption that the aggregate particle beam has a known non-uniform intensity profile in a cross section in a plane perpendicular to an optical axis of a beam generation apparatus of the multi-beam scanning particle imaging system. However, the micro-optics arrangement is not removed from the particle beam path so that the aggregate particle beam is split into multiple electron beams. A two-dimensional intensity course of the electrical current generated by the multiple particle beams is determined with a two-dimensional sensor arranged in a plane perpendicular to the optical axis downstream of the micro-optics arrangement. Based on this, an offset between an expected position of the known non-uniform intensity profile and a position of a corresponding intensity profile determined in the two- dimensional intensity course may be determined.

[0060] Control parameters of the particle optics elements may be varied for minimizing the offset. Furthermore, a difference between a size of a particle beam of the multiple particle beams in the two-dimensional intensity course and a size of a corresponding aperture in the micro-optics arrangement that generated the particle beam may be determined and the control parameters of the particle optics elements may be varied for minimizing the difference.

[0061] A further example includes removing the micro-optics arrangement from particle beam path and arranging a diffraction element in a first plane perpendicular to an optical axis of the beam generation apparatus of the multi-beam scanning particle imaging system. The diffraction element may comprise for example a diffraction crystal, in particular a monocrystal, or a diffraction grating. For example, the diffraction element may be arranged instead of the micro-optics arrangement in the beam path. A two-dimensional intensity course of the electrical current generated by a diffraction pattern of the aggregate particle beam passing through the diffraction element is determined with a two-dimensional sensor arranged in a second plane perpendicular to the optical axis downstream of the first plane. The diffraction element may be arranged so that an axis of the diffraction element, for example a main axis of a mono-crystal, is aligned with the optical axis. Symmetry of the diffraction pattern may depend on an angle of inclination of the aggregate particle beam. When the angle of incidence of the aggregate particle beam is parallel to the axis of the diffraction element, the diffraction pattern may be symmetrical with respect to the center of the aggregate particle beam.

[0062] Control parameters of the particle optics elements may be varied for optimizing symmetry of the diffraction pattern.

[0063] According to various examples, the one or more sensors comprise particle detection sensors in a detector system of the multi-beam scanning particle imaging system. For determining the one or more characteristics of the aggregate particle beam at the microoptics arrangement, for each particle beam of at least some of the multiple particle beams pitches between a point of ingress of the particle beam and points of ingress of neighboring particle beams are determined in an image of the object plane generated based on sensor signals from the particle detection sensors. Further, a variation of the pitches is determined as the characteristics of the aggregate particle beam at the microoptics arrangement. Control parameters of the particle optics elements may be varied for minimizing the variation of the pitches. Further examples relate to a computer program product including program code for executing the above-described exemplary methods. For example, method steps of the above methods may be performed by a control circuitry or processing device of the multibeam scanning particle imaging system.

[0064] According to various examples, a control circuitry of a multi-beam scanning particle imaging system is provided. The multi-beam scanning particle imaging system comprises a micro-optics arrangement arranged to split an incoming aggregate particle beam into multiple particle beams. The multi-beam scanning particle imaging system may be configured to jointly scan at least a set of the multiple particle beams across an object plane. The control circuitry may be configured to obtain one or more sensor signals of one or more sensors arranged along a particle beam path of the multi-beam scanning particle imaging system. Furthermore, the control circuitry may be configured to determine one or more characteristics of the aggregate particle beam at the micro-optics arrangement based on the one or more sensor signals, and to control one or more particle optics elements of the multi-beam scanning particle imaging system based on the one or more characteristics of the aggregate particle beam.

[0065] The control circuitry may be configured to perform any of the above described exemplary methods.

[0066] A system comprises the above control circuitry and the multi-beam scanning particle imaging system.

[0067] Brief description of the drawings

[0068] FIG. 1 schematically illustrates a multi-beam scanning particle imaging system, e.g. a multi-beam scanning electron microscope (MSEM), according to various examples.

[0069] FIG. 2 schematically illustrates generation of beamlets and a beam path according to various examples.

[0070] FIG. 3 schematically illustrates a global beam tilt and local beam tilts according to various examples. FIG. 4 schematically illustrates a micro-optics arrangement including uniformity sensors according to various examples.

[0071] FIG. 5 schematically illustrates details of a micro-optics arrangement according to various examples.

[0072] FIG. 6 schematically illustrates fields of view of an MSEM according to various examples.

[0073] FIG. 7 schematically illustrates an arrangement of two multi-aperture plates and corresponding beam paths according to various examples.

[0074] FIG. 8 schematically illustrates an arrangement of two multi-aperture plates and corresponding beam paths according to further examples.

[0075] FIG. 9 schematically illustrates an arrangement of two multi-aperture plates and corresponding beam paths according to further examples.

[0076] FIG. 10 schematically illustrates an arrangement of two multi-aperture plates and corresponding beam paths according to further examples.

[0077] FIG. 11 schematically illustrates an arrangement of apertures in a multi-aperture plate according to various examples.

[0078] FIG. 12 schematically illustrates an arrangement of apertures in a multi-aperture plate according to further examples.

[0079] FIG. 13 schematically illustrates an arrangement of apertures in a multi-aperture plate according to further examples.

[0080] FIG. 14 schematically illustrates an arrangement of two multi-aperture plates and corresponding beam paths according to various examples.

[0081] FIG. 15 schematically illustrates an arrangement of two multi-aperture plates and corresponding beam paths according to further examples. FIG. 16 schematically illustrates a multi-aperture plate which is arranged upstream of a micro-optics arrangement according to various examples.

[0082] FIG. 17 schematically illustrates a configuration with a multi-aperture plate arranged upstream of a micro-optics arrangement according to various examples.

[0083] FIG. 18 schematically illustrates techniques for controlling particle optics elements including quadrant detectors according to various examples.

[0084] FIG. 19 schematically illustrates a quadrant detector according to various examples.

[0085] FIG. 20 schematically illustrates an image of beamlets captured at a detector system in a mirror mode according to various examples.

[0086] FIG. 21 schematically illustrates an image of beamlets captured at a detector system in a mirror mode according to further examples.

[0087] FIG. 22 schematically illustrates techniques for controlling particle optics elements including a diffraction element according to various examples.

[0088] FIG. 23 schematically illustrates a micro-optics arrangement comprising a multi-aperture plate comprising pyramid-shaped elements according to various examples.

[0089] FIG. 24 schematically illustrates a top view of a multi-aperture plate comprising pyramidshaped elements according to various examples.

[0090] FIG. 25 schematically illustrates particle beams incident on a pyramid-shaped element according to various examples.

[0091] FIG. 26 schematically illustrates detection of a pyramid-shaped element with a non-tilted particle beam according to various examples.

[0092] FIG. 27 schematically illustrates detection of a pyramid-shaped element with a tilted particle beam according to various examples. FIG. 28 schematically illustrates techniques for controlling particle optics elements including a multi-pole alignment element according to various examples.

[0093] FIG. 29 schematically illustrates a multi-pole alignment element acting as deflector according to various examples.

[0094] FIG. 30 schematically illustrates a multi-pole alignment element acting as stigmator on a non-tilted beamlet according to various examples.

[0095] FIG. 31 schematically illustrates a multi-pole alignment element acting as stigmator on a tilted beamlet according to various examples.

[0096] FIG. 32 schematically illustrates techniques involving two movable edges for controlling particle optics elements according to various examples.

[0097] FIG. 33 schematically illustrates a beam generation apparatus including two movable edges according to various examples.

[0098] FIG. 34 schematically illustrates evaluation of current signal profiles generated by two movable edges according to various examples.

[0099] FIG. 35 schematically illustrates an estimation of a tilt angle resolution according to various examples.

[0100] FIG. 36 schematically illustrates an estimation of a collimation accuracy according to various examples.

[0101] FIG. 37 schematically illustrates an implementation for edge techniques according to various examples.

[0102] FIG. 38 schematically illustrates an implementation for edge techniques according to further examples.

[0103] FIG. 39 schematically illustrates techniques for controlling particle optics elements utilizing a known non-uniform intensity profile of a particle beam according to various examples. FIG. 40 schematically illustrates the techniques of FIG. 39 utilizing a coil for deflecting multiple particle beamlets according to various examples.

[0104] FIG. 41 schematically illustrates determination of a two-dimensional intensity course by using the techniques of FIG. 39 according to various examples.

[0105] FIG. 42 schematically illustrates determination of tilt and divergence of a particle beam based on an intensity course according to various examples.

[0106] FIG. 43 schematically illustrates techniques for controlling particle optics elements utilizing a known non-uniform intensity profile of a particle beam according to various examples, wherein a two-dimensional intensity course is determined.

[0107] FIG. 44 schematically illustrates determination of tilt and divergence of a particle beam based on an intensity course according to various examples.

[0108] FIG. 45 schematically illustrates calibration techniques based on diffraction elements according to various examples.

[0109] FIG. 46 schematically illustrates determination of a diffraction pattern according to various examples.

[0110] FIG. 47 schematically illustrates a symmetric diffraction pattern of a non-tilted particle beam according to various examples.

[0111] FIG. 48 schematically illustrates an asymmetric diffraction pattern of a tilted particle beam according to various examples.

[0112] FIG. 49 schematically illustrates a diffraction pattern for a non-tilted divergent particle beam according to various examples.

[0113] FIG. 50 schematically illustrates a diffraction pattern for a non-tilted collimated particle beam according to various examples.

[0114] FIG. 51 schematically illustrates a diffraction pattern for a non-tilted convergent particle beam according to various examples. FIG. 52 schematically illustrates a diffraction pattern including higher order illumination spots for a non-tilted particle beam according to various examples.

[0115] FIG. 53 schematically illustrates illumination spot diameters of a diffraction pattern over a collimation angle according to various examples.

[0116] FIG. 54 shows a flowchart of a method of operating a control circuitry of a multi-beam scanning particle imaging system according to various examples.

[0117] Detailed description

[0118] Some examples of the present disclosure generally provide for a plurality of circuits or other electrical devices. All references to the circuits and other electrical devices and the functionality provided by each are not intended to be limited to encompassing only what is illustrated and described herein. While particular labels may be assigned to the various circuits or other electrical devices disclosed, such labels are not intended to limit the scope of operation for the circuits and the other electrical devices. Such circuits and other electrical devices may be combined with each other and / or separated in any manner based on the particular type of electrical implementation that is desired. It is recognized that any circuit or other electrical device disclosed herein may include any number of microcontrollers, a graphics processor unit (GPU), integrated circuits, memory devices (e.g., FLASH, random access memory (RAM), read only memory (ROM), electrically programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM), or other suitable variants thereof), and software which co-act with one another to perform operation(s) disclosed herein. In addition, any one or more of the electrical devices may be configured to execute a program code that is embodied in a non-transitory computer readable medium programmed to perform any number of the functions as disclosed.

[0119] In the following, embodiments of the invention will be described in detail with reference to the accompanying drawings. It is to be understood that the following description of embodiments is not to be taken in a limiting sense. The scope of the invention is not intended to be limited by the embodiments described hereinafter or by the drawings, which are taken to be illustrative only. The drawings are to be regarded as being schematic representations and elements illustrated in the drawings are not necessarily shown to scale. Rather, the various elements are represented such that their function and general purpose become apparent to a person skilled in the art. Any connection or coupling between functional blocks, devices, components, or other physical or functional units shown in the drawings or described herein may also be implemented by an indirect connection or coupling. A coupling between components may also be established over a wireless connection. Functional blocks may be implemented in hardware, firmware, software, or a combination thereof.

[0120] 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 or the array elements. E.g., each primary charged particle beamlet 3.1 , 3.2, 3.3 is one of the plurality of primary charged particle beamlets 3. Same reference numbers in the various drawings referred to similar or identical components.

[0121] Hereinafter, techniques of operating and controlling a multi-beam charged particle raster scanning microscope will be disclosed. Specifically, techniques will be explained for MSEMs, but other types of multi-beam charged particle raster scanning microscope may be employed as well.

[0122] In a multi-beam charged particle raster scanning microscope, multiple images are contemporaneously acquired. I.e., multiple charged particle beams are scanned in sync and contemporaneously, yielding multiple images in multiple Fields of View (FOVs). Therefore, the amount of imaging data acquired per time unit is significantly higher if compared to a single-beam charged particle raster scanning microscope.

[0123] In this description, the terms upstream and downstream will be used as follows. In a multibeam charged particle raster scanning microscope, a particle beam is generated at a particle source and directed to a sample and from the sample to a detector. This direction of the particle beam is called downstream. The opposite direction is called upstream. If two components are placed along the above particle beam path, a first component is upstream of a second component if the first component is closer to the particle source than the second component. Consequently, in this case, the second component is downstream of the first component. FIG. 1 is a schematic illustration of an MSEM 1. Further information relating to such MSEMs and components used therein, such as, for instance, particle sources, multiaperture plate and lenses, can be obtained from the international patent applications WO 2005 / 024881 , WO 2007 / 028595, WO 2007 / 028596, WO 2011 / 124352, WO 2007 / 060017, WO 2023 / 001401 , WO 2023 / 001402, WO 2023 / 016678, WO 2023 / 247067 and WO 2024 / 008329, and the German patent applications having the publication numbers DE 10 2013 016 113 A1 , DE 102013 014 976 A1, DE 10 2021 118 561 B4 the disclosure of which in the full scope thereof is incorporated by reference in the present application.

[0124] The MSEM 1 uses a plurality of charged particle beams (also referred to as beamlets) for imaging a sample 7 on a sample stage 500. The MSEM 1 generates a plurality of J primary beamlets 3.1, 3.2, 3.3 which strike the sample 7 to generate interaction products, e.g., secondary electrons, which emanate from the sample 7, form secondary beamlets 9.1 , 9.2, 9.3, and are subsequently detected.

[0125] Each one of the primary and secondary beamlets 3.1 , 3.2, 3.3, 9.1 , 9.2, 9.3 is formed and guided by a respective imaging subsystem of the MSEM 1. Each imaging subsystem is associated with a respective field of view (FOV). Images acquired by a respective imaging subsystem depict the respective FOV. The multiple FOVs are arranged in a spatial pattern to thereby define a composite FOV.

[0126] The primary beamlets 3.1 , 3.2, 3.3 are formed by electrons which are incident on a surface 25 of the sample 7 at a plurality of locations and generate a plurality of primary electron beam focus spots 5.1 5.2, 5.3 that are spatially separated from one another.

[0127] The sample 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.

[0128] The surface 25 of the sample 7 is arranged in a sample plane 101 (also referred to as object plane 101) of an objective lens system 102 of a first particle optical unit 100 (also referred to as illumination system 100).

[0129] A diameter of the minimal beam spots or focus spots 5.1 5.2, 5.3 shaped in the sample plane 101 can be small. Exemplary values of this diameter are below four nanometers, for example three nm or less. The focusing of the primary beamlets 3.1 , 3.2, 3.3 for shaping the focus spots 5.1 5.2, 5.3 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 German patent DE 102020125534 B3, the entire content of which is herewith incorporated in the disclosure.

[0130] The number J of primary beamlets 3.1 , 3.2 and 3.3 (i.e., the number of FOVs) may be five, 25, 90 to 100, or more (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 FIG 1).

[0131] In practice, the number of beamlets J, and hence the number of FOVs, can be chosen to be significantly greater, such as, for example, J = 10 x 10, J = 20 x 30 or J = 100 x 100. Exemplary values of the pitch between the incidence locations and FOVs are 1 micrometer, 10 micrometers, or more, for example 40 micrometers.

[0132] For example, each FOV may have a hexagonal shape. The number J of primary beamlets 3.1 , 3.2 and 3.3 (i.e., the number of FOVs) may be 91 and the composite FOV may also have a hexagonal shape.

[0133] The number of primary and secondary beamlets J defines the number of FOVs. Each imaging subsystem has a respective FOV. The respective FOV is defined by scanning the respective pair of primary and secondary beamlets (e.g., beamlets 3.1 and 9.1) over the sample 7 in the respective FOV.

[0134] The primary beamlets 3.1 , 3.2, 3.3 striking the sample 7 generate interaction products, e.g., secondary electrons, back-scattered electrons, which emanate from the surface of the sample 7, or primary particles that have experienced a reversal of movement for other reasons. The interaction products emanating from the surface 25 of the sample 7 are shaped by the objective lens system 102 to form the secondary beamlets 9.1 , 9.2, 9.3. Secondary electrons included in the secondary beamlets 9.1 , 9.2, 9.3 are used for imaging.

[0135] The MSEM 1 provides a detection beam path for guiding the plurality of secondary beamlets 9.1, 9.2, 9.3 to a secondary electron imaging system 200. The secondary electron imaging system 200 includes several electron-optical lenses 205.1 to 205.5 for directing the secondary beamlets 9.1, 9.2, 9.3 towards a spatially resolving detector system 600. The imaging with the secondary electron imaging system 200 is strongly magnifying such that both the pattern of the primary beamlets on the wafer surface and the size and shape of focus points of the primary beamlets are imaged in much magnified fashion. By way of example, a scale factor I magnification is between 100x 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 device with for example 100 pm diameter is enlarged to approximately 30 mm.

[0136] The primary beamlets 3.1 , 3.2, 3.3 are generated in a beam generation apparatus 300 comprising at least one particle source 301 (e.g., an electron source), at least one collimation lens 303, a micro-optics arrangement 305, a first field lens 331 and a second field lens 333. The particle source 301 generates at least one diverging particle beam 309, which is at least substantially collimated by the at least one collimation lens 303, and which illuminates the micro-optics arrangement 305. The micro-optics arrangement 305 includes a first multi-aperture plate 304 (also referred to as filter plate or multi-hole aperture plate), 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 3.1, 3.2, 3.3. Particles of the illuminating particle 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 3.1, 3.2, 3.3. The micro-optics 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.

[0137] Together with the first field lens 331 and the second field lens 333, the micro-optics arrangement 305 focuses each of the primary beamlets 3.1 , 3.2, 3.3 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 to pre-compensate a field curvature of the imaging system arranged downstream of the intermediate image surface 321.

[0138] At least one field lens 103 and the objective lens system 102 provide a first imaging particle optical unit for imaging the surface 321 , in which the beam foci are formed, onto the sample plane 101 such that a second pattern of focus spots 5.1 5.2, 5.3 of the primary beamlets 3.1, 3.2, 3.3 is formed there. Typically, the surface 25 of the sample 7 is arranged in the sample plane 101 , and the focus spots 5.1 5.2, 5.3 are correspondingly formed on the sample surface 25. The plurality of primary beamlets 3.1 , 3.2, 3.3 form a crossover point 108, in the vicinity of which a first deflection scanner 110 is arranged. The first deflection scanner 110 is used to deflect the plurality of primary beamlets 3.1 , 3.2, 3.3 collectively and synchronously such that the plurality of focus spots 5.1 5.2, 5.3 are moved contemporaneously over the surface 25 of the sample 7. Raster scanning is implemented, thereby imaging the sample 7. The first deflection scanner 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 is acquired. Additionally, the MSEM 1 can include further static deflectors configured to adjust the position of the plurality of the primary beamlets 3.1, 3.2, 3.3.

[0139] The objective lens system 102 and the projection lenses 205 provide the secondary electron imaging system 200 for imaging the sample plane 101 onto an imaging plane 225. The objective lens system 102 is thus a lens or a lens system that is part of both the first particle optical units 100 and the secondary electron imaging system 200, while the field lenses 103, 331 and 333 belong only to the first particle optical unit 100, and the projection lenses 205 belong only to the secondary electron imaging system 200.

[0140] 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 secondary electron imaging system 200 in the beam path between the objective lens system 102 and the projection lenses 205.

[0141] The first deflection scanner 110 is arranged in a primary electron beam path or in a joint electron beam path. In the example shown in FIG. 1 , the secondary beamlets 9.1 , 9.2, 9.3 are transmitted during use the first deflection scanner 110 in opposite direction and the scanning movement of the secondary beamlets 9.1 , 9.2, 9.3 is partially compensated. The secondary electrons have typically a different kinetic energy compared to the primary electrons. Therefore, the scanning movement of the moving irradiation positions is only partially compensated. To fully compensate the scanning movement of the secondary beamlets 9.1, 9.2, 9.3, a collective beam deflector 222 is arranged in the secondary electron beam path.

[0142] The secondary electron imaging system 200 includes the second, collective beam deflector 222 which is arranged in the vicinity of a crossover point of the secondary beamlets 9.1, 9.2, 9.3. The second, collective beam deflector 222 is operated synchronously with the first deflection scanner 110 and compensates during use a beam deflection of the secondary beamlets 9.1 , 9.2, 9.3 such that centers 15 of the beamlets 9 remain at constant position on the imaging plane 225. Thereby, each secondary beamlet 9 is kept within the area of a set of detection elements, which is assigned to the individual secondary beamlet 9.

[0143] The secondary electron imaging system 200 includes the electron-optical lenses 205.1 to 205.5 to adjust a focus plane of the secondary beamlets 9.1, 9.2, 9.3. A defocus can be applied. The electron-optical lenses 205.1 to 205.5 can thus implement corrective elements to correct the focus plane. 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. With the electron-optical lenses 205.1 to 205.5, the secondary beamlets 9.1 , 9.2, 9.3 can be focused into the imaging plane 225 of the secondary electron imaging system 200.

[0144] The secondary electron imaging system 200 can include a plurality of further corrective elements, for example at least one of a multi-aperture array element, a deflector or an exchangeable aperture stop. Together with the objective lens system 102, the lenses serve to focus the secondary beamlets 9.1, 9.2, 9.3 on the spatially resolving detector system 600 and, in the process, allow to correct or compensate the magnification and rotation of the pattern of the secondary beamlets 9.1 , 9.2, 9.3 in the imaging plane 225. Thereby, the pattern of the plurality of secondary beamlets 9.1 , 9.2, 9.3 can stabilized. For example, a first and second magnetic lenses 205.4 and 205.5 (as further examples of corrective elements) are designed in reversed order to one another and have oppositely directed magnetic fields. A Larmor rotation of the secondary beamlets 9.1 , 9.2, 9.3 can be compensated by suitably applying control signals to (drive) the magnetic lenses 205.4 and 205.5.

[0145] The MSEM 1 is furthermore associated with a processing device 800 (also referred to as control circuitry) 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 system 600. The processing device 800 can be separated from the MSEM 1 or can be part of the MSEM 1. For example, the processing device 800 can be configured to acquire pairs of test images and then evaluate the test images to determine values of one or more imaging parameters. In this case, the control or processing device 800 can be constructed from a plurality of individual electronic computers or electronic components. By way of example, the processing device 800 includes a control processor 880, a control module 840 for the control of the electro-optical elements of the secondary electron imaging system 200, and a control module 830 for the control of the electro- optical elements of the primary beamlet generation unit. The processing device 800 is further connected to a control module 503 for supplying a voltage to the sample 7, said voltage also being referred to as extraction voltage. Thereby, during use, an extraction field is generated between the objective lens system 102 and the surface 25 of the sample 7. During use, the extraction field decelerates the primary charged particles of the primary beamlets 3.1, 3.2, 3.3 before the object surface 25 is reached and generates an additional focusing effect on the plurality of primary beamlets 3.1 , 3.2, 3.3. At the same time, the extraction field serves during use to accelerate the secondary particles out of the surface 25 of the sample 7.

[0146] Further, the processing device 800 includes the scanning control unit 860 for the raster scanning.

[0147] The detector system 600 includes a plurality of sets of detection elements with one set of detection elements for each secondary beamlet 9. During use, each set of detection elements is configured to record the intensity signal of the assigned secondary beamlet 9. The plurality of intensity signals for the plurality of secondary beamlets 9.1, 9.2, 9.3 is transferred to an image data acquisition unit 810, where the image data is processed and stored in memory 890. Accordingly, multiple images are acquired, one for each imaging subsystem. These multiple images (or an aggregated image determined based on images of respective sequences) can be combined to a composite image having a composite FOV.

[0148] Accurate generation of the primary beamlets 3.1 , 3.2, 3.3 is an important factor for obtaining accurate images. For example, beamlets generated by the micro-optics arrangement 305 may be required to be equidistant, parallel, collimated and / or telecentric, i.e. the plurality of beamlets shall illuminate the sample 7 at equidistant raster positions, telecentric and perpendicular to the sample surface 25 and sample plane 101 , respectively.

[0149] It has been found that illumination conditions of the illuminating particle beam 309 affects accuracy of the beamlets 3.1 , 3.2, 3.3. Uniformity and telecentricity of illumination at the micro-optics arrangement 305 may have great impact on the accuracy of the beamlets 3.1 , 3.2, 3.3.

[0150] FIG. 2 schematically illustrates the generation of beamlets 3 at the micro-optics arrangement 305. The particle source 301 generates a divergent particle beam 309 which is collimated and optionally deflected by particle optics elements 302. The particle optics elements 302 may include for example one or more collimation lenses 303 and / or one or more alignment deflectors 313. The collimated beam 309 strikes the micro-optics arrangement 305. The micro-optics arrangement 305 generates the beamlets 3 which are then projected on the sample surface 25 by means of the field lenses 103, 331, 333 and the objective lens system 102. The collimated beam 309 includes particles, e.g., electrons, that form the beamlets 3. Therefore, the collimated beam 309 may be considered as an incoming aggregate particle beam which characteristics directly affect characteristics of the beamlets 3. Electrons of the collimated beam 309 that do not form the beamlets 3 are absorbed at the micro-optics arrangement 305.

[0151] Accurate adjustment of the collimated beam 309 may be required. However, illumination conditions of the illuminating particle beam 309 may vary upon modifications at the particle source 301 and due to aging in operation. Influencing parameters include the position of the particle source 301 with respect to an optical axis (dashed line in FIG. 2) of the beam generation apparatus 300, axis and tilt of the collimation lens system 303 and a stigmator which may optionally be present upstream of the micro-optics arrangement 305.

[0152] Furthermore, it has been found that in a collimated particle beam, individual trajectories of particles may have a different direction than a direction of the collimated particle beam as a whole. FIG. 3 illustrates this. The overall macroscopic direction of the collimated (aggregate) particle beam 309 is indicated by arrow 310. However, individual particle trajectories 311 may have slightly different directions. Therefore, there may be different objectives to be achieved for optimizing the characteristics of the collimated beam 309 striking the micro-optics arrangement 305. One objective may be to optimize the collimated aggregate particle beam 309 so that it strikes the micro-optics arrangement 305 perpendicularly, i.e. at 90° with respect to the surface of the multi-aperture plate 304, with a minimum tilt, e.g. with a tilt of less than 10 mrad. A further objective may be to optimize the individual particle trajectories 311 so that they each strike the micro-optics arrangement 305 perpendicularly, i.e. at 90° with respect to the surface of the multiaperture plate 304 with a minimum tilt, for example with a tilt of less than 10 mrad, and more preferably with a tilt of less than 5 mrad or 1 mrad. A reference for the tilt angle may be an optical axis perpendicular to the surface of the multi-aperture plate 304 through a central aperture thereof.

[0153] The uniformity of the intensity of the aggregate particle beam 309 may affect the intensity of the beamlets 3. For a uniform beam current intensity of the individual beamlets 3, the emission characteristic of the particle source 301 may be important, more specifically, a uniformity of the emission characteristic over the entire emission angle used. When relatively large emission angles are used, the emission characteristic of the particle source 301 , e.g. of a thermal field emission source, is not uniform throughout.

[0154] Accordingly, the irradiance at the first multi-aperture plate 304 is no longer uniform throughout, and there are relatively large variations in the current intensities in different individual beamlets 3. However, in the case of a multi-beam scanning particle imaging system, it may be a system requirement that there be only a small variation in current intensities between the various beamlets, typically less than a few percent or even less than one percent, so that all individual image fields of the multi-image field are scanned with an equivalent number of particles or electrons. For example, this is a prerequisite for obtaining images with approximately the same brightness. The achievable resolution of the individual images may also depend on the beamlet current.

[0155] FIG. 4 illustrates a micro-optics arrangement 305 including uniformity sensors. A particle beam 309 is generated by the particle source 301, such as an electron source, and collimated by collimation lenses 303. Alignment deflectors 313 may be arranged downstream of the collimation lenses 303. As a result, the collimated particle beam 309 strikes the first multi-aperture plate 304. The multi-aperture plate 304 may have a plurality of apertures 85 forming beamlets 3 having a cross-section corresponding to the apertures 85. The multi-aperture plate 304 is followed by the further multi-aperture plate 306 having a lens array, i.e. , apertures having electrodes 81. The apertures of the further multiaperture plate 306 are aligned with the apertures 85 of the multi-aperture plate 304. A plurality of uniformity sensors 160 may be provided on the surface of the multi-aperture plate 304 facing the particle source 301. The uniformity sensors 160 may be provided at locations between the apertures 85 and outside the apertures 85. The uniformity sensors 160 measure the uniformity of the incident collimated particle beam 309. The uniformity sensors 160 may be coupled to the control unit 830 to adjust the intensity, for example, by adjusting the collimating lenses 303. An angle of incidence j on the first multi-aperture plate 304 is expected to be 90°, i.e., the particle trajectories should be parallel to the z-axis perpendicular to the surface of the multi-aperture plate 304 in the x / y-plane. As discussed above in connection with FIG. 3, an average trajectory 310 as well as the individual particle trajectories 311 should have small tilt angles with respect to the z-axis at each lateral position on the multi-aperture plate 304. A tilt angle deviation of the angle of incidence j of 90° on the multi-aperture plate 304 has the effect that the beamlets 3 pass the electrostatic lens fields in the further multi-aperture plate 306 in an off-axis position with respect to the local lens axis (z, z', z"), so that the beamlets 3 are deflected, which leads to deviations from the ideal raster position. This effect can be at least partially compensated for by components downstream of the micro-optics arrangement 305.

[0156] According to FIG. 5, an exemplary micro-optics arrangement 305 comprises, in the z- direction of propagating particles, a sequence of five multi-aperture plates 304 and 306.1 to 306.4 and a global condenser lens 308. Each multi-aperture plate 304 and 306.1 to 306.4 comprises a plurality of apertures 85 spaced at the same lateral pitch P1 in each plate, and each plate is aligned to generate and shape a plurality of primary charged particle beamlets 3. The plurality of multi-aperture plates 304 and 306.1 to 306.4 and the global lens electrode 308 are spaced apart by spacers 83.1 to 83.5. The micro-optics arrangement 305 is shown in cross section (x, z) with only four apertures 85 in each multiaperture plate shown.

[0157] On the entrance side, each aperture 85 has a circular shape. Parts of the collimated incident electron beam 309 pass through the apertures 85 to form the plurality of primary charged particle beamlets 3. The first multi-aperture plate 304 is covered with a metal layer 99 for stopping and absorbing the incident electron beam 309 at the periphery of the plurality of apertures 85. The metal layer 99 is formed of, for example, aluminum or gold and is connected to a large capacitance, for example, ground (0V). In use, a large portion of the incident electrons from the electron beam 309 is absorbed in the absorbing layer 99 and an electric current corresponding to the number of absorbed electrons is generated.

[0158] The bulk material of the aperture plate 304 is made of a conductive material, for example doped silicon, and is connected to ground level. The multi-aperture plate 306.1 comprises for example four or eight of electrostatic elements for each of the plurality of apertures 85, for example electrodes 81 , to individually deflect each of the plurality of beamlets 3.

[0159] The multi-aperture plate 306.2 may have the function of a micro-lens array comprising a plurality of ring electrodes 81 , each ring electrode being set to an individually defined potential so that the focus positions of the plurality of primary beamlets 3 are independently adjusted in the intermediate image surface 321.

[0160] The multi-aperture plate 306.3 is a ground electrode plate. The multi-aperture plate 306.3 is made of conductive material, for example doped silicon, and is connected to ground level (0V).

[0161] The multi-aperture plate 306.4 may act as a terminating multi-aperture plate and comprises a plurality of ring electrodes 81 arranged around each or the plurality of terminating apertures 94 of the multi-aperture plate 306.4. Each of the ring electrodes 81 is individually connected to the control unit 830, which is configured to provide, in use, a plurality of individual voltages to the ring electrodes 81 for an individual and independent manipulation of the penetration depth of the electrostatic field from a ring electrode 82 of the electrostatic condenser lens 308 into the terminating apertures 94.

[0162] The tilt of the beamlets 3 may be measured by beam position distortion and adjusted at a particular reference working point. As illustrated in FIG. 6, in the sample plane 101 each beamlet 3 may be configured to scan a corresponding field of view (FoV) 610. The field of views may be arranged with a pitch 620 corresponding, for example, to the distance from the center of one FoV 610 to the center of another neighboring FoV 610. A scan reference object having pre-calibrated positions of reference markers may be placed in the sample plane 101 , and positions of the beamlets are measured. Pitch variation can be determined on the reference object.

[0163] The adjustment target may be to achieve the minimum pitch variation, e.g., via the tilt of the condenser illumination. However, this adjustment target does not inherently include minimum tilt and parallel illumination. A tilt error may be compensated for by another alignment error, and errors may be introduced, such as aberrations on the beamlets, resulting in a loss of resolution or a variation in resolution across the beams. Therefore, the following examples are intended to adjust and optimize the aggregate particle beam at the micro-optics arrangement, in particular to optimize the tilt and collimation of the aggregate particle beam at the micro-optics arrangement.

[0164] In principle, one or more sensor signals may be obtained from one or more sensors disposed along a particle beam path of the multi-beam scanning particle imaging system, and one or more characteristics of the aggregate particle beam at the micro-optics arrangement may be determined based on the one or more sensor signals. Based on the one or more characteristics of the aggregate particle beam, one or more particle optics elements 302 of the multi-beam scanning particle imaging system are controlled. In other words, sensor information is obtained indicating tilt and / or collimation deviations of the aggregate particle beam 309 as it strikes the micro-optics arrangement 305. For example, the characteristics of the aggregate particle beam 309 may include a global beam tilt, e.g., a tilt of the average trajectory 310 of the aggregate particle beam 309, at a reference position along the beam path, e.g., at the first aperture plate 304. In addition or alternatively, the characteristics of the aggregate particle beam 309 may include a local beam tilt, e.g., tilts of the individual particle trajectories 311 , at a reference position along the beam path, e.g., at the first aperture plate 304.

[0165] The particle optics elements 302 may be controlled to compensate for these deviations. The particle optics elements 302 may be located upstream of the micro-optics arrangement 305 along the particle beam path, for example the collimation lens 303. The particle optics elements 302 may be included in the micro-optics arrangement 305, for example the condenser lens of 308 discussed above in connection with FIG. 5, or collimation lenses arranged between components of the micro-optics arrangement, as will be described in more detail in connection with FIG. 17.

[0166] The particle optics elements 302 may act on the aggregate particle beam 309 as a whole, for example the collimation lens 303 and / or the alignment deflectors 313. Additionally or alternatively, the particle optics elements 302 may selectively act upon different ones of the multiple particle beamlets, for example the condenser lens 308 in connection with the ring electrodes 81 arranged in the multi-aperture plate 306.4.

[0167] It is important to note that the techniques described herein may be performed in-situ, i.e. , while the MSEM is in operation, e.g., while scanning a sample, or at least when the MSEM is ready to scan a sample. For example, a load lock of the MSEM may be controlled to load a sample 7 into a vacuum chamber of the MSEM 1, e.g., the sample stage 500 is controlled to place the sample 7 in the sample plane 101. The sensor signals can be obtained with the sample 7 loaded into the vacuum chamber, i.e. the sample is placed in the sample plane 101.

[0168] For example, an additional multi-aperture plate is included in the micro-optics arrangement 305. As shown in FIG. 7, in addition to the first multi-aperture plate 304, an additional multi-aperture plate 350 is disposed in parallel and downstream of the first multi-aperture plate 304. The additional multi-aperture plate 350 may be disposed with an offset with respect to the first multi-aperture plate 304. The offset may be in the x- and / or y-direction, i.e., perpendicular to the z-direction. The offset may be less than a diameter of an aperture 85, e.g., 10% to 30% of the diameter of the aperture 85. However, when viewed in the z-direction, the apertures of the first multi-aperture plate 304 and the apertures of the additional multi-aperture plate 350 partially overlap. The arrangement of the apertures of the additional multi-aperture plate 350 is substantially the same as the arrangement of the apertures of the first multi-aperture plate 304, i.e., a pitch and size of the apertures are substantially the same.

[0169] As further shown in FIG. 7, due to the offset between the first and additional multi-aperture plates 304, 350, the intensity of the beamlets 3 is reduced when the particles of each of the beamlets 3 are collimated and strike the first multi-aperture plate 304 without tilting, i.e., the beamlet arrives at a right angle of incidence with respect to the surface of the first multi-aperture plate 304. However, as shown in FIG. 8, a tilted beamlet 3.3 can pass through the corresponding apertures in the first and additional multi-aperture plates 304, 350 with less or no loss of intensity compared to the non-tilted beamlet 3.1 .

[0170] The intensities of at least some of the beamlets generated by the first and additional multiaperture plates 305, 350 are monitored. Sensors for monitoring the intensities of the beamlets may be disposed downstream of the micro-optics arrangement 305. The sensors may determine an electrical current for each monitored beamlet. The monitoring of the beamlets and the corresponding sensors will be discussed below in connection with FIGs. 16 through 18.

[0171] Characteristics of the aggregate particle beam 309 may be determined based on monitoring the beamlets 3. As discussed in connection with FIG. 3, the aggregate particle beam 309 may have local beam tilts resulting in tilted beamlets, such as beamlet 3.3. To compensate for the local beam tilts, the particle optics elements 302 may be varied to minimize variation in the electrical currents of the monitored beamlets.

[0172] FIGS. 9 and 10 illustrate a further exemplary implementation with an additional multiaperture plate 350 in the micro-optics arrangement 305. In addition to the first multiaperture plate 304, the additional multi-aperture plate 350 is arranged parallel to and downstream of the first multi-aperture plate 304. An arrangement of apertures of the additional multi-aperture plate 350 substantially corresponds to the arrangement of apertures of the first multi-aperture plate 304. The additional multi-aperture plate 350 may be arranged in alignment with the first multi-aperture plate 304, i.e. , centers of the apertures of the additional multi-aperture plate 350 are aligned with corresponding centers of the apertures of the first multi-aperture plate 304 along the optical axis.

[0173] As shown in FIG. 9, due to the alignment of the first and additional multi-aperture plates 304, 350, beamlets generated at the first multi-aperture plate 304 and propagating parallel to the optical axis, i.e., beamlets having no tilt, are substantially not affected by the additional multi-aperture plate 350. However, when the aggregate particle beam 309 has a global tilt or a local tilt, all or some of the beamlets have a tilt with respect to the optical axis and are therefore affected by the additional multi-aperture plate 350, as shown in FIG. 10. The intensity of the tilted beamlets is reduced compared to the non-tilted beamlets, as can be seen from the widths of the beamlets 3 in FIGs. 9 and 10.

[0174] The intensities of at least some of the beamlets generated by the first and additional multiaperture plates 305, 350 are monitored. Sensors for monitoring the intensities of the beamlets may be disposed downstream of the micro-optics arrangement 305. The sensors may determine an electrical current for each monitored beamlet. The monitoring of the beamlets and the corresponding sensors will be discussed below in connection with FIGs. 16 through 18.

[0175] Characteristics of the aggregate particle beam 309 may be determined based on the monitoring of the beamlets. To compensate for global and / or local beam tilt, the particle optics elements 302 may be varied to maximize the electrical currents of the monitored beamlets.

[0176] The techniques described above in connection with FIGs. 7 and 8 may be combined with the techniques described in connection with FIGs. 9 and 10. For example, the additional multi-aperture plate 350 may be movable under control of the control module 830 such that its position with respect to the first multi-aperture plate 304 may be varied in a direction perpendicular to the optical axis. As a result, an offset between the first and additional multi-aperture plates 304 and 350 may be configured as discussed in FIGs. 7 and 8, and subsequently an alignment between the first and additional multi-aperture plates 304 and 350 may be configured as discussed in FIGs. 9 and 10 may be configured. The particle optics elements 302 may be varied to minimize variation in the measured electrical currents and to maximize the measured electrical currents. A corresponding weighing of the two adjustment targets may be considered. Optimization may be performed iteratively by repeatedly applying the above configurations.

[0177] In some examples, only some beamlets may be used to determine the characteristics of the aggregate particle beam 309 and to adjust the particle optics elements 302. Hereafter, these beamlets will be referred to as adjustment beamlets, while the beamlets used to scan the sample 7 will be referred to as scanning beamlets. For example, adjustment beamlets may be located on a circumference outside of the scanning beamlets. FIG. 11 shows an exemplary arrangement of apertures in the first multi-aperture plate 304, and FIG. 12 shows an exemplary arrangement of apertures in the additional multi-aperture plate 350.

[0178] The first multi-aperture plate 304 provides seven apertures 1101-1107 for generating the scanning beamlets and 12 apertures 1110-1121 for generating the adjustment beamlets. It should be noted that the apertures for generating the adjustment beamlets can also be used for generating the scanning beamlets, for example, after the particle optics elements 302 have been adjusted. The number of apertures for generating the scanning and adjustment beamlets is only an example and may be selected as appropriate.

[0179] The additional multi-aperture plate 350 provides a corresponding arrangement of apertures. As shown in FIG. 12, the additional multi-aperture plate 350 provides seven apertures 1201-1207 for passing the scanning beamlets and 12 apertures 1210-1221 for passing the adjustment beamlets. The apertures 1201-1207 for passing the scanning beamlets have a larger diameter than the corresponding apertures 1101-1107, so that the scanning beamlets are not affected by the additional multi-aperture plate 350 even if the scanning beamlets are not perfectly aligned with the optical axis and / or collimated. However, the apertures 1210-1221 for the adjustment beamlets have substantially the same diameter as the apertures 1110-1121 for generating the adjustment beamlets. Generally, although not shown in FIGs. 11 and 12, in order to achieve higher sensitivity on the adjustment beamlets, the adjustment beamlets may have a smaller diameter than the scanning beamlets, i.e., apertures 1110-1121 and 1210-1221 may have a smaller diameter than apertures 1101-1107.

[0180] In another example, as shown in FIG. 13, the first multi-aperture plate 304 may provide 91 apertures (without reference signs) for generating the scanning beamlets and six apertures 1301-1306 for generating the adjustment beamlets. The apertures 1301-1306 may be located in an outer peripheral region with respect to the apertures for the scanning beamlets. As shown, the apertures 1301-1306 may be disposed on the major axis of the hexagonal arrangement of the apertures for the scanning beamlets. When scanning the intensity of the adjustment beamlets with the detector system 600, as described in more detail below, the detector system 600 may provide corresponding detectors for the adjustment beamlets at these designated positions.

[0181] FIGs. 14 and 15 illustrate the effect of the quality of collimation on the arrangement of the aligned first and additional multi-aperture plates 304 and 350. In FIG. 14, the aggregate particle beam 309 is perfectly collimated and has no local tilt. As a result, the beamlets 3 generated at the first multi-aperture plate 304 pass through the apertures of the additional multi-aperture plate 350 without being affected. The intensities of the beamlets 3 are the same and have a high value. In contrast, in FIG. 15, the aggregate particle beam 309 is not perfectly collimated. As a result, at least some of the beamlets 3 generated at the first multi-aperture plate 304 are partially blocked at the additional multi-aperture plate 350. In particular, the intensity of the outer peripheral beamlets 3.1 and 3.6 may be significantly reduced. The beamlets 3.1 and 3.6 may be configured as adjustment beamlets as described above in connection with FIGs. 11-13.

[0182] The intensity of the beamlets, particularly the intensity of the adjustment beamlets, may be obtained by various sensors.

[0183] For example, the detector system 600 may be used to detect the intensity of the beamlets. With reference to FIG. 1 , when scanning the sample 7, the sample 7 or the sample stage 500 may generally be supplied with a high voltage such that particles, e.g. primary electrons, are decelerated before reaching the sample surface 25 and secondary particles are accelerated. The voltage applied to the sample 7 or the sample stage 500 can be increased so that the electrons no longer reach the sample surface 25 and are reflected into the beam divider 400 and guided into the secondary electron imaging system 200, where they finally reach the detector system 600. The increased voltage configuration is referred to as the mirror mode. In the mirror mode, the intensity of the beamlets may not be affected by the sample 7 such that the detector system 600 can directly provide reliable intensity information.

[0184] Therefore, in the above techniques, the intensity of the beamlets can be obtained in the mirror mode by using the detector system 600 as an intensity sensor. As discussed above, the detector system 600 may provide detectors for receiving and measuring the adjustment beamlets, particularly the peripheral adjustment beamlets produced by apertures 1110-1121 in FIG. 11 and 1301-1306 in FIG. 13.

[0185] In addition or alternatively, Faraday cups may be used to determine the intensity of the beamlets, particularly the adjustment beamlets. The Faraday cups may be fixedly provided for the adjustment beamlets or may be movable within the first particle optical unit 100 such that they can be moved into the beamlet path and removed from the beamlet path after adjustment. The Faraday cups may be located downstream of the micro-optics arrangement 305 and upstream of the field lens 103 or upstream of the second field lens 333.

[0186] In addition or alternatively, quadrant detectors may be used to determine the intensity of the beamlets, particularly the adjustment beamlets. The quadrant detectors may also provide beamlet position information. The quadrant detectors may be fixedly provided for the adjustment beamlets or may be movable within the first particle optical unit 100 such that they may be moved into the beamlet path and removed from the beamlet path after adjustment. The quadrant detectors may be located downstream of the micro-optics arrangement 305 and upstream of the field lens 103 or upstream of the second field lens 333. The quadrant detectors may be integrated into the micro-optics arrangement, for example, downstream of the global condenser lens 308.

[0187] In the above examples, the additional multi-aperture plate 350 is disposed downstream of the first multi-aperture plate 304. However, in various examples, the additional multiaperture plate may be disposed upstream of the first multi-aperture plate 304. FIG. 16 illustrates an additional multi-aperture plate 371 disposed upstream of the micro-optics arrangement 305. The additional multi-aperture plate 371 provides peripheral apertures for generating adjustment beamlets 3.1 and 3.7, and central apertures for generating scanning beamlets 3.2 to 3.6. The apertures for generating the adjustment beamlets may be smaller in diameter than the apertures for generating the scanning beamlets. In fact, the scanning beamlets 3.2 to 3.6 are formed at the first multi-aperture plate 304.

[0188] Faraday cups 1601 and 1602 are provided as sensors for measuring the intensity 11 , I2 of the adjustment beamlets 3.1 and 3.7. As discussed above, the particle optics elements 302, such as collimation lenses 303 and alignment deflectors 313, can be adjusted to maximize the intensity and uniformity of the adjustment beamlets 3.1 and 3.7. Additionally or alternatively, quadrant detectors may be provided as sensors for measuring the intensity of the adjustment beamlets 3.1 and 3.7. The quadrant detectors may also provide position information of the adjustment beamlets 3.1 and 3.7. Alternatively, the intensity of the adjustment beamlets 3.1 and 3.7 may be measured by the detector system 600 in mirror mode.

[0189] FIG. 17 illustrates another example configuration in which the additional multi-aperture plate 371 is positioned upstream of the micro-optics arrangement 305. In this example, the additional multi-aperture plate 371 is disposed within the particle optics elements 302. The particle optics elements 302 may include collimation lenses 303.1 to 303.4 and alignment deflectors 313.1 and 313.2. The additional multi-aperture plate 371 may be positioned downstream of the collimation lenses 303.1 and 303.2 and upstream of the collimation lenses 303.3 and 303.4. Consequently, in the illustrated configuration, the additional multi-aperture plate 371 may be positioned downstream of the alignment detector 313.1 and upstream of the alignment detector 313.2.

[0190] The additional multi-aperture plate 371 provides peripheral apertures for generating adjustment beamlets 3.1 and 3.4 and central apertures for generating scanning beamlets 3.2 and 3.3. The apertures for generating the adjustment beamlets may have a smaller diameter than the apertures for generating the scanning beamlets. The adjustment and scanning beamlets thus generated are further collimated and deflected by the collimation lenses 303.3 and 303.4 and the alignment deflector 313.2. In effect, the scanning beamlets 3.2 and 3.3 are formed at the first multi-aperture plate of the micro-optics arrangement 305. As discussed above in connection with FIG. 16, Faraday cups may be provided as sensors for measuring the intensity of the adjustment beamlets 3.1 and 3.4. The particle optics elements 302, i.e. collimation lenses 303 and alignment deflectors 313, may be adjusted to maximize the intensity and uniformity of the adjustment beamlets 3.1 and 3.4. Additionally or alternatively, quadrant detectors may be provided as sensors for measuring the intensity of the adjustment beamlets 3.1 and 3.4. The quadrant detectors may also provide position information of the adjustment beamlets 3.1 and 3.4.

[0191] Alternatively, the intensity of the adjustment beamlets 3.1 and 3.4 may be measured by the detector system 600 in mirror mode.

[0192] FIG. 18 illustrates another exemplary technique for controlling the particle optics elements 302. The micro-optics arrangement 305 may comprise apertures for generating scanning beamlets 3.2 to 3.5 and additional apertures for generating adjustment beamlets 3.1 and 3.6. The adjustment beamlets may be arranged in an area outside the arrangement of the scanning beamlets, i.e. the adjustment beamlets 3.1 and 3.6 are peripheral beamlets with respect to the scanning beamlets 3.2 to 3.5. More than the illustrated two adjustment beamlets 3.1 and 3.6 may be provided in the micro-optics arrangement 305, for example six adjustment beamlets as shown in FIG. 13. The apertures may be provided, for example, in the first multi-aperture plate 304 and corresponding apertures may be provided in the further multi-aperture plate 306 and a condenser lens plate 307. A corresponding quadrant detector 1801 , 1802 may be provided for each adjustment beamlet.

[0193] FIG. 19 shows the quadrant detector 1802 in more detail. The following details regarding the quadrant detector 1802 apply to quadrant detectors associated with the other adjustment beamlets in the same manner. The quadrant detector 1802 can have four detector fields, and each detector field can provide a corresponding intensity signal depending on the amount of electrons incident on the detector field. Depending on the position where the adjustment beamlet 3.6 strikes the quadrant detector 1802, one detector field may indicate a greater intensity than another detector field. When the center of the quadrant detector 1802 is aligned with the center of the aperture of the adjustment beamlet 3.6 in the micro-optical arrangement 305, a deviation of the beamlet 3.6 from the center of the quadrant detector 1802 in the plane perpendicular to the optical axis can be determined. Such a deviation indicates a tilt of the beamlet 3.6 due to a global or local tilt in the aggregate particle beam 309. The particle optics elements 302 may be controlled to minimize the deviations determined at the quadrant detectors. In various examples, the distance between the micro-optics arrangement 305 and the particle optics elements 302 may be varied, i.e. , the micro-optics arrangement 305 may be moved along the optical axis, e.g., by means of an adjustment device. A center of a beamlet can be detected at two different positions of the micro-optics arrangement 305 along the optical axis. If the particle beam 309 has no tilt, the center should be at substantially the same position for both arrangements. If the center deviates, the tilt may be determined by the geometry of the array and / or the particle optics elements may be controlled to minimize the deviation of the center.

[0194] In particular, by using the additional multi-aperture plate 350, 371 described above in alignment with the first multi-aperture plate 304, even a small cropping of a tilted beamlet may be detectable at the detector system 600. Here, cropping means that a portion of the beamlet is cut off, i.e., absorbed, at the additional multi-aperture plate 350, 371. For example, in the mirror mode, a beamlet may be detected at the detector system 600 as having a diameter of 30 pm. With the additional multi-aperture plate 350, 371 in the beam path, a tilt of the beamlet results in a cropping of the image of the beamlet at the detector system 600. A cropping of the order of 1 pm or less may be detectable at the detector system 600.

[0195] In addition, a tilt of a beamlet at the micro-optics arrangement 305 may result in a drift of the image of the beamlet at the detector system 600. FIG. 20 shows an image of beamlets 3 captured at the detector system 600 in the mirror mode with an illumination current of 570 pA. FIG. 21 shows an image of beamlets 3 captured at the detector system 600 in the mirror mode with an illumination current of 50 pA. As shown, there is a drift between image positions of corresponding beamlets, for example, between an image position of beamlet 3.1 in FIG. 20 and an image position of beamlet 3.1 in FIG. 21. This may indicate that the beamlet 3.1 has a tilt at the micro-optics arrangement 305. The particle optics elements 302 may be controlled such that the drift of all or at least some of the images of the beamlets 3 at the detector system 600 is minimized.

[0196] In connection with FIG. 22, another exemplary technique for controlling the particle optics elements 302 is illustrated. In the micro-optics arrangement 305, a diffraction element (in transmission) is positioned in at least some of the beamlet paths. The diffraction element may include, for example, a diffraction crystal, particularly a mono-crystal, or a diffraction grating. For example, additional adjustment beamlets may be provided as described above, and in each beamlet path of the adjustment beamlets a corresponding diffraction element is positioned. The diffraction elements may be located in a separate plate or may be integrated into one of the plates of the micro-optics arrangement, such as the condenser lens plate 307. For each particle beam provided with the diffraction element, a two-dimensional particle detection sensor 2201 , 2202 is associated and located downstream of the micro-optics arrangement 302 along the particle beam path. When a particle beamlet passes through the diffraction element, a corresponding diffraction pattern 2203, 2204 is generated. The diffraction patterns 2203, 2204 are detected by the associated two-dimensional particle detection sensors 2201 , 2202. Depending on the type of diffraction element, the diffraction pattern may comprise a central illumination spot (zero order diffraction) and several peripheral illumination spots (higher order diffraction) surrounding the central illumination spot, see also FIGs. 47-52. A symmetry of the peripheral illumination spots may depend on an angle of inclination of the beamlet with respect to the structure of the diffraction element. The structure of the diffraction element may be aligned with the optical axis of the illumination system 100 of the MSEM 1 . Any tilt of the beamlet will cause an asymmetry in the spot intensity of the diffraction pattern. The diffraction pattern can be scanned with the two-dimensional particle detection sensors 2201 , 2202. The control parameters of the particle optics elements 302 can be controlled to maximize the symmetry in the spot intensity of the diffraction pattern.

[0197] A further exemplary technique for controlling the particle optics elements 302 will be described in connection with FIGs. 23 to 27.

[0198] FIG. 23 illustrates a micro-optics arrangement 305 including pyramidal elements. The micro-optics arrangement 305 comprises a multi-aperture plate including, at an upstream side of the plate, one or more pyramid-shaped elements 2301 . For example, the pyramidshaped elements 2301 may be provided on an upper side of the first aperture plate 304. As shown in more detail in FIG. 24, four pyramid-shaped elements 2301 are provided on the surface of the first aperture plate 304 in a peripheral region surrounding the arrangement of apertures 85.

[0199] As shown in FIG. 23, the particle optics elements 302, in particular the collimation lenses 303 and the alignment deflectors 313, are controlled so that the aggregate particle beam 309 is focused on the plane of the first multi-aperture plate 304 and scans at least the one or more pyramid-shaped elements 2301. For example, a current sensor may be provided at the first aperture plate 304 for measuring a current resulting from particles of the aggregate particle beam 309 absorbed by the multi-aperture plate 304. A pattern of the current determined during scanning may correspond to a topography of the surface of the first aperture plate 304 and the pyramid-shaped elements 2301 as scanned by the focused particle beam 309. Based on this scanning, a corresponding offset of a tip of the pyramid-shaped element 2301 with respect to a center of the base of the pyramid-shaped element 2301 as seen by the scanning beam is determined for each pyramid-shaped element 2301. This offset characterizes the aggregate particle beam at the micro-optics arrangement, in particular a global tilt of the aggregate particle beam 309.

[0200] More specifically, as shown in FIG. 25, a non-tilted aggregate particle beam 309 may scan the pyramid-shaped element 2301 as indicated by the solid arrows. A scan pattern may identify a tip 2501 of the pyramid-shaped element 2301 , as well as edges and corners of the base of the pyramid-shaped element 2301. As shown in FIG. 26, in a projection as scanned by the non-tilted particle beam 309, the tip 2501 is detected at a center of the base 2502. However, when the aggregate particle beam 309 is tilted, the pyramidal element 2301 is scanned as indicated by the dashed arrows in FIG. 25. As a result, the scanning pattern identifies the tip 2501 outside the center of the base 2502, as shown in FIG. 27.

[0201] The control parameters of the particle optics elements 302 can be varied to minimize the offset of the tip 2501 from the base 2502 for each of the pyramid-shaped elements 2301 . After scanning the pyramidal elements 2301 and adjusting the control parameters of the particle optics elements 302, the collimation lenses 303 are defocused so that the aggregate particle beam 309 does not focus on the surface of the first aperture plate 304, but provides a collimated beam.

[0202] FIGs. 28 to 31 illustrate a further exemplary technique for controlling parameters of the particle optics elements 302. As shown in FIG. 28, the micro-optics arrangement of 305 comprises at least a first multi-aperture plate 304 and a further multi-aperture plate 306.1 including a multi-pole alignment element, i.e. the further multi-aperture plate 306.1 includes a multi-pole alignment element 2801 in at least some of its apertures. The multipole alignment element 2801 may be an 8-pole alignment element which may be used as a stigmator and / or deflector. The eight components of the alignment element 2801 may be arranged in an octagon as shown in FIG. 29, and two on each side of this octagon may be supplied with a specific electrical potential indicated by signed numbers in FIGs. 29 to 31. A positive number may refer to a positive potential and a negative number may refer to a negative potential. The number indicates the magnitude of the electrical potential.

[0203] FIG. 29 illustrates the functioning of the alignment element 2801 as a deflector. With the applied electrical potentials shown in FIG. 29, a beamlet entering the alignment element

[0204] 2801 at a central position 2901 and not tilted as indicated by arrow 2802 in FIG. 28 is deflected to the left such that it exits the alignment element 2801 at an off-center position 2902.

[0205] FIG. 30 illustrates the functioning of the alignment element 2801 as a stigmator. With the applied electrical potentials indicated in FIG. 30, astigmatism of a beamlet entering the alignment element 2801 at a central position 2901 and not tilted as indicated by arrow

[0206] 2802 in FIG. 28 can be corrected with respect to the axes 3001. The beamlet is not deflected so that it exits the alignment element 2801 in the same central position in which it entered the alignment element 2801 , as indicated by the intersection with the dashed axis.

[0207] FIG. 31 illustrates the functioning of the alignment element 2801 as a stigmator in conjunction with a tilted beamlet. With the applied electrical potentials shown in FIG. 31 , astigmatism of a beamlet entering the alignment element 2801 at the central position 2901 and tilted as indicated by arrow 2804 in FIG. 28 can be corrected with respect to the axes 3101. However, the beamlet is also deflected so that it exits the alignment element 2801 in an off-center position as indicated by the intersection with the dashed axis.

[0208] This may be used to detect and compensate for tilted illumination at the first aperture plate 304. For example, a deflection of each or some of the beamlets generated by the first multi-aperture plate 304 may be determined at the detector system 600 as the stigmation excitation is varied in the mirror mode. A corresponding tilt indication may be determined for each or at least some of the beamlets generated at the first multi-aperture plate 304. Characteristics of the aggregate particle beam 309 may be determined based on the tilt indications. The particle optics elements 302 may be controlled to minimize the tilt indications overall, or to keep the tilt indications within the predefined limits.

[0209] For example, a first excitation pattern may be applied to each 8-pole alignment element 2801 of each beamlet to act as a stigmator. For example, the same electrical potential may be applied to each opposing pole pair of an 8-pole alignment element, but different pole pairs are supplied with different electrical potentials. At the detector system 600, an incidence position is determined for each beamlet. A second excitation pattern may then be applied to each 8-pole alignment element 2801 of each beamlet to act as a stigmator. However, the second excitation pattern is different from the first excitation pattern. The second excitation pattern may also be a zero excitation pattern, that is, no potentials are applied to the poles in the second excitation pattern. Again, at the system detector 600, an incidence position is determined for each beamlet. For each beamlet, an offset is determined between the incidence position when the first excitation pattern is applied and an incidence position when the second excitation pattern is applied. These offsets may characterize the aggregate particle beam 309 at the micro-optics arrangement 305, in particular the local and global tilt. Minimizing the offsets may be used as a target function for controlling the particle optics elements 302.

[0210] In various examples, the incidence positions may be determined using Faraday cups or quadrant detectors located downstream of the micro-optics arrangement 305.

[0211] The techniques described above may require or benefit from general calibrations, such as general alignment of the particle beam 309 along the optical axis. Such calibrations may not be performed in-situ. For example, specific adjustment devices may be inserted into the illumination system 100 and / or the micro-optics arrangement 305 may be removed from the illumination system 100.

[0212] So-called knife-edge techniques may be used to measure the profile of the particle beam 309. For example, a sharp edge may be pushed step by step into the particle beam 309 in an x / y plane and the unblocked beam current may be measured for each step. The profile in that plane may be extracted from the measured beam current, e.g., by integration or deconvolution, particularly the center position and width of the profile in the plane.

[0213] These techniques can be extended to two planes at different z-positions, and the combination of the two center positions and widths can be used to calculate the tilt of the beam and / or the opening angle of the beam, i.e., the collimation. The tilt and collimation can then be adjusted iteratively using the particle optics elements of 302, in particular collimation lenses 303 and / or alignment deflectors 313.

[0214] The principle is illustrated in FIG. 32. After the micro-optics arrangement 305 is removed from the path of the particle beam 309, a first edge element 3202 is moved in a first plane perpendicular to an optical axis 3204 of the beam generation apparatus 300 from a position radially outside the aggregate particle beam 309 into the aggregate particle beam 309. Thus, an increasing portion of the aggregate particle beam 309 is absorbed at the first edge element 3202. The first plane is between the particle optics elements 302 and the object plane 101 . While the first edge element 3202 is moved, a first course 3206 of an electrical current of a portion of the aggregate particle beam 309 that is not absorbed by the first edge element 3202 is detected with a detector 3208. After the first course 3206 is obtained, the first edge element 3202 is removed from the particle beam 309. Next, a second edge element 3210 is moved in a second plane perpendicular to the optical axis 3204 from a position radially outside the particle beam 309 into the particle beam 309. Thus, as the second edge element 3210 advances, an increasing portion of the particle beam 309 is absorbed at the second edge element 3210. The second plane is between the first plane and the detector 3208. As the second edge element 3210 moves, a second course 3212 of an electrical current of a portion of the aggregate particle beam that is not absorbed by the second edge element 3210 is detected. Based on the first course 3206 and the second course 3212, an offset value indicative of an offset between a center of the aggregate particle beam at the first plane and a center of the aggregate particle beam at the second plane can be calculated. This offset value is related to a mean tilt angle 3214 of the particle beam 309 with respect to the optical axis 3204. Additionally or alternatively, a width difference value may be determined based on the first course 3206 and the second course 3212. The width difference indicates a difference between a width of the particle beam 309 at the first plane and a width of the particle beam 309 at the second plane. The width difference relates to the collimation 3216 of the particle beam 309. A small width difference or no width difference indicates perfect or near perfect collimation. Based on the offset value and the width difference, the particle optics elements 302 can be adjusted to reduce the tilt and optimize the collimation.

[0215] FIG. 33 illustrates exemplary implementation details for the knife edge techniques. A (not shown) single aperture plate may be provided to restrict the cross section of the particle beam 309. A first linear translator 3302 may be provided to drive the first edge 3202. A second linear translator 3304 may be provided for driving the second edge 3210. A further linear translator 3306 may be provided for driving the detector 3208. The detector 3208 may be a Faraday cup providing an integrated current signal. As can be seen, the microoptics arrangement 305 is removed from the path of the particle beam 309. In further examples, the detector 3208 of FIG. 32 may be implemented by the detector system 600. The MSEM 1 may be operated in the mirror mode reflecting the portion of the particle beam 309 that is not absorbed by the first or second edges 3202, 3210 across the sample plane 101 to the detector system 600.

[0216] FIG. 34 shows the current profile as an integrated current signal 3402 determined while moving the first edge 3202 into the particle beam 309, and the current profile as an integrated current signal 3404 determined while moving the second edge 3202 into the particle beam 309. By deconvolution of the integrated current signal 3402 and the integrated current signal 3404, extracted profiles 3406 and 3408, respectively, can be obtained. The extracted profiles 3406 and 3408 indicate a width and position of the particle beam 309 in the direction of movement of the first and second edges in the first plane and the second plane, respectively. The resulting arrangement of the particle beam 309 with respect to the optical axis can be determined based on the extracted profiles 3406 and 3408. As can be seen in the illustration to the right of FIG. 34, the tilt and collimation of the particle beam 309 may be determined.

[0217] The above knife edge measurement may be performed in different transverse dimensions (e.g., x and y directions) to obtain a complete profile of the particle beam 309. In general, the measurement accuracy may depend on the step size of the linear translators, the measurement accuracy of the current detector 3208, and the evaluation software.

[0218] The accuracy of the tilt and collimation measurements can be estimated as follows. The mechanical error in the position of the first and second edges can be assumed to be at most 1 pm. A distance d between the first and second planes can be 20 cm. A position accuracy of the first and second translators by corresponding encoders can be assumed to be 1 nm. A measurement resolution of the profile (maximum) by convoluted curve fitting can be 100 nm.

[0219] Therefore, for the tilt angle, a positioning inaccuracy dt of 1.1 pm can be assumed for each profile. The tilt angle resolution can be calculated to be 0.011 mrad based on the distance d = 20 cm and twice the positioning inaccuracy 2 x dt = 1.1 pm as shown in FIG. 35.

[0220] Similarly, for the collimation, an inaccuracy of the Full Width at Half Maximum (FWHM) of de = 2 x 100 nm in total can be assumed. Based on the distance d = 20 cm and the FWHM inaccuracy de, the opening angle resolution can be calculated to be 0.001 mrad as shown in FIG. 36.

[0221] FIG. 37 shows another exemplary implementation of the knife edge techniques. As in FIG. 33, the first linear translator 3302 may be provided to drive the first edge 3202 and the second linear translator 3304 may be provided to drive the second edge 3210. However, in this exemplary implementation, the detector is not provided along the optical axis of the beam generation apparatus 300, but instead a coil 3702 is provided to deflect the particle beam 309 to a side wall of the beam generation apparatus 300. A deflection angle may be about 90°. A detector 3704 may be provided at the side wall of the beam generating apparatus 300. The detector 3704 may be a Faraday cup or any other suitable detector, such as a quadrant detector. For clarity, the particle optics elements 302 have been omitted from FIG. 37. However, even in this implementation, the micro-optics arrangement 305 may be removed from the path of the particle beam 309.

[0222] In general, for deflecting the particle beam to a side wall of the beam generation apparatus 300, not only the coil described above may be used, but also a mirror-like plane may be used. The mirror-like plane may be disposed at an angle of 45 degrees with respect to the optical axis of the beam generation apparatus 300. The mirror-like plane may be an electrically conductive plate with an opposing electrical field. Particles, such as electrons, are slowed down and experience a reversal of motion in the direction perpendicular to the plate surface. The angle of reflection is therefore equal to the angle of incidence. This allows the measurement path to be extended, thereby increasing sensitivity.

[0223] FIG. 38 shows another exemplary implementation of the knife-edge techniques. The micro-optics arrangement is removed from the particle beam path and, for example, instead of the micro-optics arrangement, a single-aperture plate 3802 is arranged in a first plane perpendicular to the optical axis of the beam generation apparatus 300. The first plane may substantially correspond to the plane in which the micro-optics arrangement was arranged before being removed. A center of the single aperture of the single-aperture plate 3802 is aligned with the optical axis. A size of the single aperture corresponds to a size of the cross-section of the aggregate particle beam 309 at the first plane. An edge element 3804 is movably disposed in a second plane perpendicular to the optical axis, such that the edge element 3804 can be moved into the aggregate particle beam 309 to such an extent that it can completely block the aggregate particle beam 309. The second plane is different from the first plane and is positioned downstream of the first plane. The edge element 3804 may be moved continuously or incrementally in the second plane from outside the path of the aggregate particle beam 309 into the aggregate particle beam 309, thereby absorbing an increasing portion of the aggregate particle beam 309 at the edge element 3804. While moving the edge element 3804, e.g., in an x-direction, a course of an electrical current l(x) of a portion of the aggregate particle beam not absorbed by the edge element 3804 is detected by a detector 3806. A width of the aggregate particle beam 309 at the second plane is determined based on the course of the electrical current. When the aggregate particle beam 309 is not tilted with respect to the optical axis, the width of the aggregate particle beam 309 at the second plane (as determined by the movement of the edge element 3804) substantially corresponds to the size of the single aperture. When the aggregate particle beam 309 is tilted as shown in FIG. 38, the width of the aggregate particle beam 309 at the second plane is smaller.

[0224] Control parameters of the particle optics elements 302 may be varied to align the determined width with the size of the single aperture.

[0225] The detector 3806 may comprise a Faraday cup positioned between the second plane and the object plane.

[0226] According to various examples, the aggregate particle beam 309 may have a known non- uniform intensity profile in a cross-section in a plane perpendicular to the optical axis of the beam generation apparatus 300. For example, the aggregate particle beam 309 may have an intensity distribution similar to a normal distribution or a Gaussian distribution with an intensity maximum at the center of the beam cross-section.

[0227] For example, as shown in FIG. 39, a multi-aperture plate 3902 may be provided in the path of the particle beam 309. The multi-aperture plate 3902 may be one of the multiaperture plates of the micro-optics arrangement 305 that is also used to scan a sample in an operation of the MSEM 1. In some examples, the micro-optics arrangement 305 may be removed. The multi-aperture plate 3902 may be a multi-aperture plate or grating that is specifically designed for calibrating the beam generation apparatus 300, i.e., with known sizes of apertures and known spacing between apertures. A specially designed multiaperture plate may have fewer and larger apertures than the multi-aperture plates of the micro-optics array 305. When the micro-optics arrangement 305 is used as the multiaperture plate 3904, optical lenses and deflectors in the micro-optics arrangement 305 may be disabled, i.e. , turned off. When the multi-aperture plate 3902 is irradiated with the particle beam 309, multiple particle beamlets 3906 are generated in the downstream direction. The irradiated multi-aperture plate 3902 may have to be conductive and grounded.

[0228] A two-dimensional detector 3904 is disposed in a plane perpendicular to the optical axis and downstream of the multi-aperture plate 3902 at a predetermined distance, e.g., 20 cm. Both the multi-aperture plate 3902 and the detector 3904 may be aligned with the optical axis. In particular, the multi-aperture plate 3902 and the detector 3904 may have no tilt with respect to the optical axis.

[0229] When a known object such as the multi-aperture plate 3902 is irradiated with the particle beam 309 having a known profile and imaged on the spatially resolved detector 3904, the collimation can be determined from the width of the shadow cast and the tilt can be determined from the shift in the position of the known profile.

[0230] FIG. 40 shows another exemplary implementation of the techniques described above. As in FIG. 39, the multi-aperture plate 3902 may be provided in the path of the particle beam 309. However, in this exemplary implementation, the detector is not provided along the optical axis of the beam generation device 300. Instead, a coil 4002 is provided that deflects the multiple particle beamlets 3906 toward a side wall of the beam generation apparatus 300. A deflection angle may be about 90°. The two-dimensional detector 3904 may be provided on the side wall of the beam generation apparatus 300.

[0231] As illustrated in FIG. 41 , a two-dimensional intensity course 4102 of the electrical current generated by the multiple particle beamlets 3906 may be determined with the two- dimensional detector 3904.

[0232] The tilt and divergence of the particle beam 309 may be determined based on the course 4102 of the electrical current, as illustrated in FIG. 42.

[0233] On the left side of FIG. 42, the path of the particle beam 309 and the generated multiple particle beamlets 3906 is schematically illustrated with the intensity distribution 4202 of the particle beam 309 at the multi-aperture plate 3902. In the center of FIG. 42, the intensity 4204 measured at the two-dimensional detector 3904 is shown. In the shaded areas, the intensity is substantially zero. Based on the intensity 4204, the beam position at the two-dimensional detector 3904 can be reconstructed. Specifically, a deviation (offset) of the maximum from the center of the multi-aperture plate 3902 provides the tilt 4206 of the particle beam 309 with respect to the optical axis 4208. The measured width of the profile sections (corresponding to a diameter or size of a beamlet at the two-dimensional detector 3904) compared to the known aperture diameter provides the divergence of the beamlets. The use of a grating may also allow determination of the emittance of a beamlet.

[0234] Control parameters of the particle optics 302 may be varied to minimize the deviation (offset) between the determined position of the maximum of the beam intensity at the two- dimensional detector 3904 and the center of the multi-aperture plate 3902 and / or to minimize the divergence of the beamlets.

[0235] The measurement accuracy of tilt and collimation can be estimated as follows. Assuming that the center of the aperture is located on the optical axis with an accuracy of 1 pm and the diameter of the aperture is determined with an accuracy of 1 pm. The positional accuracy of the two-dimensional detector is 1 pm and the accuracy of the maximum position in the profile is 1 pm. Thus, the sum of the possible deviations is 3 pm. With a distance between the two planes of d = 20 cm, the accuracy of the tilt resolution is 0.011 mrad and the accuracy of the opening angle resolution is 0.02 mrad.

[0236] Another example in which the aggregate particle beam 309 is assumed to have a known non-uniform intensity profile is shown in FIGs. 43 and 44. The micro-optics arrangement may be removed from the particle beam path, and a single-aperture plate 4302 may be provided in the path of the particle beam 309. The single-aperture plate 4302 may be arranged in a first plane perpendicular to the optical axis 4304, wherein a center of a single aperture of the single-aperture plate 4302 is aligned with the optical axis 4304 of the beam generation apparatus 300. A size of the single aperture is smaller than a size of the cross-section of the aggregate particle beam 309 in the first plane. Thus, a cropped particle beam 4308 is generated by the single-aperture plate 4302 based on the particle beam 309. The single-aperture plate 4302 may need to be conductive and grounded.

[0237] A two-dimensional detector 4310 is disposed in a second plane perpendicular to the optical axis 4304 and downstream of the single-aperture plate 4302 at a predetermined distance, e.g., 20 cm. Both the single-aperture plate 4302 and the detector 4310 may be aligned with the optical axis 4304. In particular, both the single-aperture plate 4302 and the detector 4310 may have no tilt with respect to the optical axis 4304.

[0238] When a known object such as the single-aperture plate 4302 is irradiated with the particle beam 309 having a known profile and imaged on the spatially resolved detector 4310, collimation can be determined from the width of the shadow cast and tilt can be determined from the shift in position of the known profile.

[0239] As further illustrated in FIG. 43, a two-dimensional intensity course 4306 of the electrical current generated by the cropped particle beam 4308 may be determined with the two- dimensional detector 4304.

[0240] The tilt and divergence of the particle beam 309 may be determined based on the course 4306 of the electrical current, as shown in FIG. 44.

[0241] On the left side of FIG. 44, the course of the particle beam 309 and the generated cropped particle beam 4308 is schematically illustrated with the intensity distribution 4402 of the particle beam 309 at the single-aperture plate 4302. In the center of FIG. 44, the intensity 4404 measured at the two-dimensional detector 4310 is shown. In the shaded (cropped) areas, the intensity is essentially zero. Based on the intensity 4404, the beam position at the two-dimensional detector 4310 can be reconstructed. In particular, a deviation (offset) of the maximum from the center of the single-aperture plate 4302 provides the tilt 4406 of the particle beam 309 with respect to the optical axis 4304. The measured width of the profile (corresponding to a diameter or size of the cropped particle beam 4308 at the two-dimensional detector 4310) compared to the known single aperture diameter provides the divergence of the particle beam 309.

[0242] Control parameters of the particle optics 302 may be varied to minimize the deviation (offset) between the determined position of the maximum of the beam intensity at the two- dimensional detector 4310 and the center of the single-aperture plate 4302 and / or to minimize the divergence of the beam.

[0243] Further exemplary calibration techniques may be based on diffraction elements. As shown in FIG. 45, the micro-optics arrangement may be removed from the beam generation apparatus 300 and a diffraction element 4502 may be disposed in a first plane perpendicular to the optical axis of the beam generation apparatus of 300. The diffraction element 4502 may comprise, for example, a diffraction crystal, in particular a single crystal, or a diffraction grating. The first plane may substantially correspond to the plane in which the micro-optics arrangement was located prior to being removed. The diffraction element 4502 may be inserted into the beam generation apparatus 300 by means of a linear translator 4504. A two-dimensional detector 4506 may be disposed in a second plane perpendicular to the optical axis of the beam generation apparatus 300. The second plane may be located downstream of the first plane, for example, at a distance of about 30 cm. The two-dimensional detector 4506 may be configured to determine a two- dimensional intensity course generated at the two-dimensional detector 4506 by a diffraction pattern of the aggregate particle beam passing through the diffraction element 4502.

[0244] Diffraction in transmission at the diffraction element 4502 may provide direct access to tilt and collimation of the particle beam 309. The symmetry of spot intensity of the diffraction pattern is extremely sensitive to the angle of incidence. It is not the exact pattern that is important, but the symmetry of the intensities. The intensities of a diffraction order are the same when the tilt disappears. In addition, the sharpness of the diffraction spots may depend directly on the collimation of the particle beam. The diameter of the diffraction spot is minimized when the beam is parallel. Control parameters of the particle optics elements 302 may be varied to optimize the symmetry of the diffraction pattern and / or the sharpness and size of the diffraction spots.

[0245] The diffraction pattern may include a central illumination spot 4508 (zero order diffraction) and multiple peripheral illumination spots 4510 (higher order diffraction) surrounding the central illumination spot.

[0246] FIG. 46 schematically illustrates diffraction at a diffraction crystal 4502 with a tilted particle beam 309.

[0247] FIGs. 47 to 51 illustrate diffraction patterns produced by a particle beam 309 passing through a diffraction crystal.

[0248] FIG. 47 illustrates a diffraction pattern for a perfectly aligned particle beam 309, i.e. , a non-tilted particle beam 309. The higher order illumination spots 4510 are symmetrically arranged around the zero order illumination spot 4508. FIG. 48 shows a diffraction pattern for a tilted particle beam 309. Some of the higher order illumination spots 4510, i.e., the illumination spots on the left, are substantially larger than the illumination spots on the right.

[0249] FIGs. 49 to 51 illustrate a diffraction pattern for a non-tilted particle beam 309. The higher order illumination spots 4510 are symmetrically arranged around the zero order illumination spot 4508. However, compared to the diffraction pattern illustrated in FIG. 50, where the higher order illumination spots 4510 are also symmetrically arranged around the zero order illumination spot 4508, the elimination spots in FIGs. 49 and 51 have a larger diameter than the elimination spots in FIG. 50. The reason for this larger diameter is that the particle beam 309 is either divergent, as in FIG. 49, or convergent, as in FIG. 51 . In contrast, the particle beam 309 of FIG. 50 is collimated. The control parameters of the particle optics elements 302 can be adjusted to achieve the diffraction pattern of FIG. 50.

[0250] In the examples above, zero order and first order diffraction spots were considered. However, higher order diffraction spots can also be considered. The first diffraction orders with Miller indices (1 , + / -1 , + / -1) and (1 , + / -3, + / -3) should have high intensity. At a detector distance of 30 cm, they lie at a radius of 3.4 mm and 8.5 mm, see Fig. 52. The above example values may apply to a diffraction crystal composed of silicon, for example. Other values may apply to other materials. The two-dimensional detector 4506 should be at least this size.

[0251] Regarding the accuracy of collimation over spot size, the following can be estimated. The diffraction spot diameter at 0.1° defocus is approximately 500 pm. The sharpest possible diffraction spot depends on crystal quality, detector resolution, and source emittance. Assuming that the sharpest possible diffraction spot is 50 pm in diameter (with a detector resolution of 5 pm), the collimation accuracy is about 0.01°. FIG. 53 shows the spot diameter on the detector 4506 at a distance of 30 cm over the collimation angle.

[0252] FIG. 54 is a flowchart of a method 5400 according to various examples. The method of FIG. 54 is for use in a processing device associated with an MSEM such as the MSEM 1 illustrated in FIG. 1. For instance, the method of FIG. 54 can be executed by the processing device 800 illustrated in FIG. 1. The method of FIG. 54 can be executed using parallel processing distributed amongst multiple compute units. In FIG. 54, optional method steps are indicated by dashed boxes. Prior to scanning a sample, a general calibration of the MSEM 1 , in particular of the beam generation apparatus 300, may optionally be performed. In optional method step 5402, the micro-optics arrangement 305 may be removed from the path of the particle beam 309, and calibrations as discussed above in connection with FIGs. 32 to 53 may be performed in step 5404. Upon completion of the calibration, the micro-optics arrangement 305 may be moved into the path of the particle beam 309 in step 5406.

[0253] To scan a sample, the prepared sample may be loaded into the MSEM 1. In step 5408, a load lock of the MSEM 1 is controlled to load the sample 7 into a vacuum chamber of the MSEM 1. In step 5410, the sample stage 500 may be controlled to place the sample 7 in the object plane 101.

[0254] In step 5412, one or more sensor signals are obtained from one or more sensors located along a path of the particle beam 309. Based on the one or more sensor signals, in step 5414, one or more characteristics of the aggregate particle beam 309 at the micro-optics arrangement 305 are determined. Based on the one or more characteristics of the aggregate particle beam 309, one or more particle optics elements 302 are controlled in step 5416.

[0255] Obtaining the sensor signals, determining the characteristics of the aggregate particle beam 309, and controlling the particle optics elements 302 may be performed as described in the above examples in connection with FIGs. 1 to 31.

[0256] It should be noted that steps 5412 to 5416 may be performed while the sample 7 is placed on the sample stage 500 within the MSEM 1. Further, at least some of steps 5412 to 5416 may be performed in parallel with scanning the sample 7, such that the particle optics elements 302 may be adjusted in-situ. In particular, steps 5412 to 5616 may be performed repeatedly, for example while scanning the sample, to ensure adjustment of the particle optics elements of 302, i.e. to improve the tilt, collimation and / or telecentricity of the aggregate particle beam 309 at the micro-optics arrangement 305.

Claims

Claims1. A method of operating a control circuitry (800) of a multi-beam scanning particle imaging system (1), the multi-beam scanning particle imaging system (1) comprising a micro-optics arrangement (305) arranged to split an incoming aggregate particle beam (309) into multiple particle beams (3), the multi-beam scanning particle imaging system (1) being configured to jointly scan at least a set of the multiple particle beams across an object plane (101), wherein the method comprises:- obtaining one or more sensor signals of one or more sensors arranged along a particle beam path of the multi-beam scanning particle imaging system (1),- based on the one or more sensor signals, determining one or more characteristics of the aggregate particle beam (309) at the micro-optics arrangement (305), wherein the one or more characteristics comprise a tilt angle of the aggregate particle beam (309) with reference to an optical axis of a beam generation apparatus (300) of the multi-beam scanning particle imaging system (1) at a reference position along the beam path,- based on the one or more characteristics of the aggregate particle beam (309), controlling one or more particle optics elements (302) of the multi-beam scanning particle imaging system (1).

2. The method of claim 1 , wherein the method further comprises:- controlling a load lock of the multi-beam scanning particle imaging system (1) to load a sample (7) into a vacuum chamber of the multi-beam scanning particle imaging system (1), wherein the one or more sensor signals are obtained upon loading the sample (7) into the vacuum chamber.

3. The method of claim 2, wherein the method further comprises:- controlling a sample stage (500) of the multi-beam scanning particle imaging system (1) to place the sample (7) in the object plane (101), wherein the one or more sensor signals are obtained upon the sample (7) being placed in the object plane (101).

4. The method of any one of the preceding claims, wherein the one or more particle optics elements (302) are arranged upstream of the micro-optics arrangement (305) along the particle beam path.

5. The method of any one of the preceding claims, wherein the micro-optics arrangement (305) comprises the one or more particle optics elements (302).

6. The method of any one of the preceding claims, wherein the one or more particle optics elements (302) are configured to selectively act upon different ones of the multiple particle beams (3).

7. The method of any one of the preceding claims, wherein the one or more characteristics comprise a global beam tilt at a reference position along the beam path.

8. The method of any one of the preceding claims, wherein the one or more characteristics comprise a local beam tilt at a reference position along the beam path.

9. The method of any one of the preceding claims, wherein the micro-optics arrangement (305) comprises at least two multi-aperture plates (304, 350) arranged in parallel and spaced apart along the particle beam path, wherein the one or more sensors comprise for each particle beam (3) of a further set of the multiple particle beams an assigned particle detection sensor arranged downstream of the micro-optics arrangement (305) along the particle beam path, wherein determining the one or more characteristics of the aggregate particle beam at the micro-optics arrangement (305) comprises:- determining for each particle beam (3) of the further set of the multiple particle beams a corresponding electrical current based on a sensor signal from the assigned particle detection sensor as the characteristics of the aggregate particle beam (309) at the micro-optics arrangement (305).

10. The method of claim 9, wherein controlling the one or more particle optics elements (302) comprises varying control parameters of the particle optics elements (302) for minimizing the variation of the electrical currents.11 . The method of claim 9 or claim 10, wherein controlling the one or more particle optics elements (302) comprises varying control parameters of the particle optics elements (302) for maximizing the electrical currents.

12. The method of any one of claims 9-11 , wherein the apertures of one (304) of the at least two multi-aperture plates (304, 350) are aligned to apertures of another one (350) of the at least two multi-aperture plates (304, 350) in a direction along the particle beam path.

13. The method of any one of claims 9-12, further comprising: varying a position of one (304) of the at least two multi-aperture plates (304, 350) with respect to another one (350) of the at least two multi-aperture plates (304, 350) in a direction perpendicular to the particle beam path.

14. The method of any one of claims 9-13, further comprising: varying a position of one (304) of the at least two multi-aperture plates (304, 350) with respect to another one (350) of the at least two multi-aperture plates (304, 350) in a direction along the particle beam path.

15. The method of any one of claims 9-14, wherein each of the one or more the sensors comprises at least one of a Faraday cup and a quadrant detector.

16. The method of any one of claims 9-15, wherein the one or more sensors comprise particle detection sensors in a detector system (600) of the multi-beam scanning particle imaging system (1) with the multi-beam scanning particle imaging system (1) being in a mirror mode reflecting at least the further set of the multiple particle beams (3) across the object plane (101).

17. The method of any one of claims 9-16, wherein the further set of the multiple particle beams are adjustments beams (3), wherein each of the at least two multi-aperture plates (304, 350) have inner apertures and outer apertures positioned in an outer portion surrounding the inner apertures, wherein the inner apertures split the incoming aggregate particle beam into the set of multiple particle beams and the outer apertures split the incoming aggregate particle beam into the further set of multiple particle beams.

18. The method of any one of claims 9-17, wherein at least one (371) of the at least two multi-aperture plates (304, 371) is positioned between a first one of the one or more particle optics elements (302) and a second one of the one or more particle optics elements (302).

19. The method of any one of claims 1-8, wherein the one or more sensors comprise a diffraction element positioned in a particle beam path of at least one of the multiple particle beams and for each particle beam of the at least one of the multiple particle beams an assigned two-dimensional particle detection sensor (2201 , 2202) arranged downstream of the micro-optics arrangement (305) along the particle beam path, wherein determining the one or more characteristics of the aggregate particle beam (309) at the micro-optics arrangement (305) comprises:- determining for each particle beam of the at least one of the multiple particle beams a corresponding diffraction pattern (2203, 2204) based on a sensor signal from the assigned two-dimensional particle detection sensor (2201 , 2202) as the characteristics of the aggregate particle beam (309) at the micro-optics arrangement (305).

20. The method of claim 19, wherein controlling the one or more particle optics elements (302) comprises varying control parameters of the particle optics elements (302) for maximizing symmetry of the diffraction pattern (2203, 2204).

21. The method of claim 19 or claim 20, wherein the two-dimensional particle detection sensor (2201 , 2202) is positioned between the micro-optics arrangement (302) and the object plane (101).

22. The method of any one of claims 19-21 , wherein the two-dimensional particle detection sensor comprises particle detection sensors in a detector system (600) of the multi-beam scanning particle imaging system (1) with the multi-beam scanning particle imaging system (1) being in a mirror mode reflecting the at least one of the multiple particle beams across the object plane (101).

23. The method of any one of claims 1-8,wherein the micro-optics arrangement (305) comprises a multi-aperture plate (304) comprising at an upstream side of the plate one or more pyramid-shaped elements (2301), wherein the one or more sensors comprise a current sensor measuring a current resulting from particles of the aggregate particle beam (309) absorbed by the multiaperture plate (304), wherein determining the one or more characteristics of the aggregate particle beam (309) at the micro-optics arrangement (305) comprises:- focusing the aggregate particle beam (309) on the plane of the multi-aperture plate(304) and scanning the one or more pyramid-shaped elements (2301),- determining for each of the one or more pyramid-shaped elements (2301) a corresponding offset of a tip (2501) of the pyramid-shaped element (2301) with respect to a center of the base (2502) of the pyramid-shaped element (2301) as the characteristics of the aggregate particle beam (309)at the micro-optics arrangement(305).

24. The method of claim 23, wherein controlling the one or more particle optics elements (302) comprises varying control parameters of the particle optics elements (302) for minimizing the one or more offsets.

25. The method of any one of claims 1-8, wherein the micro-optics arrangement (305) comprises at least a multi-aperture plate (304) and a multi-beam multi-pole alignment element (2801) arranged in parallel and spaced apart along the particle beam path, wherein the multi-beam multi-pole alignment element (2801) is positioned downstream of the multi-aperture plate (304) along the particle beam path, wherein the multi-beam multi-pole alignment element (2801) comprises for each particle beam of a further set of the multiple particle beams at least one pair of opposing poles, wherein the one or more sensors comprise for each particle beam of the further set of the multiple particle beams an assigned particle detection sensor arranged downstream of the micro-optics arrangement (305) along the particle beam path, wherein determining the one or more characteristics of the aggregate particle beam (309) at the micro-optics arrangement (305) comprises:- applying a first excitation pattern to the at least one pair of opposing poles of each of the further set of the multiple particle beams,- applying a second excitation pattern to the at least one pair of opposing poles of each of the further set of the multiple particle beams, the second excitation pattern being different from the first excitation pattern,- determining for each particle beam of the further set of the multiple particle beams an offset between a point of ingress of the particle beam at the assigned particle detection sensor while applying the first excitation pattern and a point of ingress of the particle beam at the assigned particle detection sensor while applying the second excitation pattern as the characteristics of the aggregate particle beam (309) at the micro-optics arrangement (305).

26. The method of claim 25, wherein controlling the one or more particle optics elements (302) comprises varying control parameters of the particle optics elements (302) for minimizing the offset.

27. The method of claim 25 or claim 26, wherein the multi-beam multi-pole alignment element (2801) comprises for each particle beam of the further set of the multiple particle beams at least two pairs of opposing poles, wherein applying the first excitation pattern is applying a same potential to all poles, and wherein applying the second excitation pattern is applying a first potential to a first pair of opposing poles of the least two pairs of opposing poles and applying a second potential to a second pair of opposing poles of the least two pairs of opposing poles, the second potential being different from the first potential.

28. The method of any one of claims 25-27, wherein the particle detection sensor comprises at least one of a Faraday cup and a quadrant detector.

29. The method of any one of claims 25-28, wherein the particle detection sensor comprises particle detection sensors in a detector system (600) of the multi-beam scanning particle imaging system (1) with the multi-beam scanning particle imaging system (1) being in a mirror mode reflecting at least the further set of the multiple particle beams across the object plane (101).

30. The method of any one of the preceding claims, further comprising:- removing the micro-optics arrangement (305) from particle beam path,- moving a first edge element (3202) in a first plane perpendicular to an optical axis of a beam generation apparatus (300) of the multi-beam scanning particle imaging system (1) from a position radially outside of the aggregate particle beam (309) into the aggregate particle beam (309), thus absorbing an increasing portion of the aggregate particle beam (309) at the first edge element (3202), the first plane being positioned between the particle optics elements (302) and the object plane (101),- while moving the first edge element (3202), detecting with one or more sensors a first course (3206) of an electrical current of a portion of the aggregate particle beam (309) not being absorbed by the first edge element (3202),- removing the first edge element (3202) from the aggregate particle beam (309),- moving a second edge element (3210) in a second plane perpendicular to the optical axis from a position radially outside of the aggregate particle beam (309) into the aggregate particle beam (309), thus absorbing an increasing portion of the aggregate particle beam (309) at the second edge element (3210), the second plane being different from the first plane and positioned between the particle optics elements (302) and the object plane (101),- while moving the second edge element (3210), detecting with the one or more sensors a second course (3212) of an electrical current of a portion of the aggregate particle beam (309) not being absorbed by the second edge element (3210),- determining, based on the first course (3206) and second course (3212), at least one of an offset value indicative of an offset between a center of the aggregate particle beam (309) at the first plane and a center of the aggregate particle beam (309) at the second plane, and a width difference value indicative of a difference between a width of the aggregate particle beam (309) at the first plane and a width of the aggregate particle beam (309) at the second plane.31 . The method of claim 30, further comprising:- varying control parameters of the particle optics elements (302) for minimizing at least one of the offset value and the width difference value.

32. The method of claim 30 or claim 31 , wherein the one or more sensors comprise a Faraday cup positioned between the first plane and the object plane (101), and between the second plane and the object plane (101).

33. The method of any one of claims 30-32, wherein the one or more sensors comprise particle detection sensors in a detector system (600) of the multi-beam scanning particle imaging system (1) with the multi-beam scanning particle imaging system (1) being in a mirror mode reflecting the portion of the aggregate particle beam (309) not being absorbed by the first or second edge elements (3202, 3210) across the object plane (101).

34. The method of any one of claims 30-33, wherein the one or more sensors comprise a magnetic coil (3702) and a Faraday cup (3704), wherein the magnetic coil (3702) is positioned between the first plane and the object plane (101), and between the second plane and the object plane (101), wherein the magnetic coil (3702) is configured to deflect the aggregate particle beam (309) to the Faraday cup (3704) positioned outside the optical axis.

35. The method of any one of the preceding claims, further comprising:- removing the micro-optics arrangement (302) from particle beam path,- arranging a single-aperture plate (3802) in a first plane perpendicular to an optical axis of a beam generation apparatus (300) of the multi-beam scanning particle imaging system (1) with a center of the single aperture aligned to the optical axis, wherein a size of the single aperture corresponds a size of the cross section of the aggregate particle beam (309) at the first plane,- moving an edge element (3804) in a second plane perpendicular to the optical axis, thus absorbing an increasing portion of the aggregate particle beam (309) at the edge element (3804), the second plane being different from the first plane and positioned downstream of the first plane,- while moving the edge element (3804), detecting with the one or more sensors a course of an electrical current of a portion of the aggregate particle beam (309) not being absorbed by the edge element (3804),- determining a width of the aggregate particle beam (309) at the second plane based on the course of the electrical current.

36. The method of claim 35, further comprising:- varying control parameters of the particle optics elements (302) for aligning the determined width to the size of the single aperture.

37. The method of claim 35 or claim 36, wherein the one or more sensors comprise a Faraday cup positioned between the second plane and the object plane.

38. The method of any one of the preceding claims, wherein the aggregate particle beam (309) has a known non-uniform intensity profile in a cross section in a plane perpendicular to an optical axis of a beam generation apparatus (300) of the multi-beam scanning particle imaging system (1), the method further comprising:- removing the micro-optics arrangement (302) from particle beam path,- arranging a single-aperture plate (4302) in a first plane perpendicular to the optical axis with a center of a single aperture of the single-aperture plate aligned to the optical axis, wherein a size of the single aperture is smaller than a size of the cross section of the aggregate particle beam at the first plane,- determining with a two-dimensional sensor (4310) arranged in a second plane perpendicular to the optical axis downstream of the first plane a two-dimensional intensity course (4306) of the electrical current generated by a portion of the aggregate particle beam (309) that has not been absorbed at the single-aperture plate (4302),- determining an offset between an expected position of the known non-uniform intensity profile and a position of a corresponding intensity profile determined in the two-dimensional intensity course (4306).

39. The method of claim 38, further comprising:- varying control parameters of the particle optics elements (302) for minimizing the offset.

40. The method of claim 38 or claim 39, further comprising:- determining a difference between a size of the portion of the aggregate particle beam (309) that has not been absorbed at the single-aperture plate (4302) in the two- dimensional intensity course (4306) and a size of the single-aperture plate (4302),- varying control parameters of the particle optics elements (302) for minimizing the difference.41 . The method of any one of the preceding claims,wherein the aggregate particle beam (309) has a known non-uniform intensity profile in a cross section in a plane perpendicular to an optical axis of a beam generation apparatus (300) of the multi-beam scanning particle imaging system (1), the method further comprising:- determining with a two-dimensional sensor (3904) arranged in a plane perpendicular to the optical axis downstream of the micro-optics arrangement (302) a two- dimensional intensity course (4102) of the electrical current generated by the multiple particle beams,- determining an offset between an expected position of the known non-uniform intensity profile and a position of a corresponding intensity profile determined in the two-dimensional intensity course (4102).

42. The method of claim 41 , further comprising:- varying control parameters of the particle optics elements (302) for minimizing the offset.

43. The method of claim 42, further comprising:- determining a difference between a size of a particle beam of the multiple particle beams in the two-dimensional intensity course (4102) and a size of a corresponding aperture in the micro-optics arrangement (302) that generated the particle beam,- varying control parameters of the particle optics elements (302) for minimizing the difference.

44. The method of any one of the preceding claims, the method further comprising:- removing the micro-optics arrangement (302) from particle beam path,- arranging a diffraction element (4502) in a first plane perpendicular to an optical axis of a beam generation apparatus (300) of the multi-beam scanning particle imaging system (1),- determining with a two-dimensional sensor (4506) arranged in a second plane perpendicular to the optical axis downstream of the first plane a two-dimensional intensity course of the electrical current generated by a diffraction pattern of the aggregate particle beam (309) passing through the diffraction element (4502).

45. The method of claim 44, further comprising:- varying control parameters of the particle optics elements (302) for optimizing symmetry of the diffraction pattern.

46. A computer program product including program code for executing the method of any one of the preceding claims.

47. A control circuitry (800) of a multi-beam scanning particle imaging system (1), the multi-beam scanning particle imaging system (1) comprising a micro-optics arrangement (305) arranged to split an incoming aggregate particle beam (309) into multiple particle beams (3), the multi-beam scanning particle imaging system (1) being configured to jointly scan at least a set of the multiple particle beams (3) across an object plane (101), wherein the control circuitry (800) is configured to- obtain one or more sensor signals of one or more sensors arranged along a particle beam path of the multi-beam scanning particle imaging system (1),- based on the one or more sensor signals, determine one or more characteristics of the aggregate particle beam (309) at the micro-optics arrangement (302), and- based on the one or more characteristics of the aggregate particle beam (309), control one or more particle optics elements (302) of the multi-beam scanning particle imaging system (1).

48. The control circuitry of claim 47, wherein the control circuitry (800) is configured to perform the method of any one of claims 1 to 45.

49. A system, comprising:- the control circuitry (800) of claim 47 or 48, and- the multi-beam scanning particle imaging system (1).

Images