Method for operating a multi-beam particle microscope in an autofocus determination mode and multi-beam particle microscope

Abstract

A method for operating a multi-beam particle microscope working with a multiplicity of charged particle beams is disclosed that is particularly useful when inspecting locally charging objects. The method comprises operating the multi-beam particle microscope in an inspection mode of operation and in an autofocus determination mode. A projection path particle optics is operated in the autofocus determination mode in a different manner than in the inspection mode. In the autofocus inspection mode just one of the individual particle beams in the projection path is selected to provide a signal for an autofocus routine. The specific way of selecting or filtering of said individual particle beam guarantees that a signal generated on the basis of said beam is free of any crosstalk.

Description

[0001] Method for operating a multi-beam particle microscope in an autofocus determination mode and multi-beam particle microscope

[0002] Field of the invention

[0003] The invention relates to multiple particle beam system in general and to multi-beam particle microscopes operating with a plurality of individual charged particle beams in particular. Specifically, the invention relates to a method for operating a multi-beam particle microscope in an autofocus determination mode and multi-beam particle microscope.

[0004] Prior art

[0005] With the ongoing development of ever smaller and ever more complex microstructures such as semiconductor components, there is a need to further develop and optimize planar production techniques and inspection systems for producing and inspecting small dimensions of the microstructures. For instance, the development and production of the semiconductor components require monitoring of the design of test wafers, and the planar production techniques require process optimization for reliable production with high throughput. Moreover, there have been recent demands for an analysis of semiconductor wafers for reverse engineering and for a customized, individual configuration of semiconductor components. Therefore, there is a need for inspection means which can be used with high throughput to examine the microstructures on wafers with high accuracy.

[0006] Typical silicon wafers used in the production of semiconductor devices have diameters of up to 300 mm. Each wafer is divided into 30 to 60 repeating regions ("dies") with a size of up to 800 mm2. A semiconductor apparatus comprises multiple semiconductor structures, which are produced in layers on a surface of the wafer by planar integration techniques.

[0007] Semiconductor wafers typically have a plane surface on account of the production processes. The structure dimension of the integrated semiconductor structures in this case extends from a few pm to the critical dimensions (CD) of a few nanometres, with the structure dimensions becoming even smaller in the near future; the expectation is that in future the structure dimensions or critical dimensions (CD) will correspond to the 3 nm, 2 nm or even smaller process nodes of the International Technology Roadmap for Semiconductors (ITRS). In the case of the aforementioned small structure dimensions, defects of the order of the critical dimensions must be identified quickly over a very large area. For multipleapplications, the specification requirement regarding the accuracy of a measurement provided by inspection equipment is even higher, for example by a factor of two or one order of magnitude. For instance, a width of a semiconductor feature must be measured with an accuracy of better than 1 nm, for example 0.3 nm or even less, and a relative position of semiconductor structures must be determined with an overlay accuracy of better than 1 nm, for example 0.3 nm or even less.

[0008] The MSEM, a multi-beam scanning electron microscope, is a relatively new development in the field of charged particle systems (“charged particle microscopes”, CPMs). For instance, a multi-beam scanning electron microscope is disclosed in US 7244949 B2 and in US 2019 / 0355544 A1. In the case of a multi-beam electron microscope or MSEM, a sample is irradiated simultaneously by a plurality of individual electron beams arranged in a field or raster. For instance, 4 to 10000 individual electron beams can be provided as primary radiation, with each individual electron beam being separated from an adjacent individual electron beam by a pitch of 1 to 200 micrometres. For example, an MSEM has approximately 100 separate individual electron beams (“beamlets”), which for instance are arranged in a hexagonal raster, with the individual electron beams being separated by a pitch of approximately 10 pm. The plurality of individual charged particle beams (primary beams) are focused on a surface of a sample to be examined by way of a common objective lens. For example, the sample can be a semiconductor wafer which is secured to a wafer holder mounted on a movable stage. When the wafer surface is illuminated by the primary individual charged particle beams, interaction products, for example secondary electrons or backscattered electrons, emanate from the surface of the wafer. Their start points correspond to those locations on the sample on which the plurality of individual primary particle beams are focused in each case. The amount and the energy of the interaction products depend on the material composition and the topography of the wafer surface. The interaction products form multiple secondary individual particle beams (secondary beams), which are collected by the common objective lens and, after passing through a projection imaging system of the multi-beam inspection system, are incident on a detector arranged in a detection plane. The detector comprises multiple detection regions, each of which comprises several detection pixels, and the detector captures an intensity distribution for each of the secondary individual particle beams. An image field of 100 pm x 100 pm, for example, is obtained in the process.

[0009] The state-of-the-art multi-beam electron microscope comprises a sequence of electrostatic and magnetic elements. At least some of the electrostatic and magnetic elements can be set in order to adapt the focus position and the stigmation of the plurality of individual charged particle beams. The state-of-the-art multi-beam system with charged particles moreovercomprises at least one crossover plane of the primary or the secondary individual charged particle beams. Moreover, the state-of-the-art system comprises detection systems in order to facilitate settings and / or alignments. The state-of-the-art multi-beam particle microscope comprises at least one beam deflector (“deflection scanner”) for collective scanning of a region of the sample surface by means of the plurality of primary individual particle beams in order to obtain an image field of the sample surface.

[0010] What is known as a beam splitter (or alternatively beam separator or beam divider) is used to separate the particle-optical beam path of the primary beams from the particle-optical beam path of the secondary beams. In this case, separation is implemented by means of special arrangements of magnetic fields and / or electrostatic fields, for example by means of a Wien filter.

[0011] In the case of scanning electron microscopes for wafer inspection, it is desirable to keep the imaging conditions stable such that the imaging can be carried out with great reliability and high repeatability. The throughput depends on a plurality of parameters, for example the speed of the stage and of the realignment at new measurement sites, and the area measured per unit of capture time. The latter is determined, inter alia, by the dwell time on a pixel, the pixel size and the number of individual particle beams. Additionally, time-consuming image postprocessing may be required for a multi-beam electron microscope; by way of example, the signal generated from charged particles by the detection system of the multibeam system must be digitally corrected before the image field from a plurality of image subfields or partial images is put together ("stitching").

[0012] Here, the grid positions of the individual particle beams on the sample surface can deviate from the ideal grid position in a plane arrangement. The resolution of the multi-beam electron microscope can be different for each of the individual particle beams and can depend on the individual position of the individual particle beam in the field of individual particle beams, and consequently can depend on the specific grid position of said individual particle beams.

[0013] Conventional systems of charged particle beam systems are stretched to their limits with increasing demands on resolution and throughput.

[0014] One approach for improving precision and resolution lies in the use of a so-called autofocus. Here, while scanning the sample surface, the current relative focal position of the individual electron beams is ascertained continuously ("on-the-fly") in view of the sample surface / object plane and an appropriate correction of the relative focal position is undertaken. By way ofexample, the focusing of the individual particle beams is adapted for each image field. By way of example, this procedure is based on a model of the sample or the assumption that the sample properties do not change much from image field to image field such that prediction values for improved focusing can be ascertained by extrapolation or interpolation. Another approach is for example recording a focus series to determine an optimum focus position.

[0015] Various methods for recording a focus series of an object or reference object and various methods for evaluating the image series ascertained thereby are already known from the prior art. DE 102021 200799 B3 discloses various methods for an improved focusing, which can also take account of field inclination in addition to field curvature. The disclosure of DE 102021 200799 B3 is fully incorporated in the present patent application by reference. The described method for ascertaining a global focus location or optimal focal plane is based on the capture of contrast measures in different focus or Z-positions. A method for determining a contrast measure can be e.g. one of the following methods: a spectral method, an image contrast, a histogram method, an edge filter, a method of relative spread or blur, or a gradient method. In this case, different methods can also be combined with one another.

[0016] In very general terms, it is the case that many autofocus methods use image sharpness algorithms (e.g. CNR) to ascertain the optimal focusing. These image sharpness algorithms in fact ascertain the positions of a minimal beam waist of the individual particle beams very precisely and comprehensibly. In addition to the aforementioned image sharpness algorithms, there are also other algorithms, based on other criteria, for finding a best "focus position". In this context, a contrast gradient method is an important method. While image sharpness methods direct their optimization to positions with minimal beam waist ("circle of least confusion"), contrast gradient methods optimize the Gauss plane ("Gauss optimum"). However, sometimes these elaborated algorithms for determining an optimum focus position fail. This failure occurs relatively more often when locally charging objects are inspected.

[0017] Charging of an object is localized at the illumination area, causing local distortion in an electrostatic field above the surface of the object. This effects the projection beam path (secondary beam path). Beam path variations due to local charging of a sample influences the capability to determine a best focus position, delivers faulty results when individual charged particle beams in the projection path overlap (crosstalk) or when some beams are clipped at apertures in the projection system.

[0018] DE 102018124044 B3 discloses a method for determining the occurrence of cross talk. A specific detection unit is used that comprises a CCD camera with a high local resolution, butwith a slow read-out speed. It is disclosed that the occurrence of crosstalk is not timely constant, but varies during an inspection task.

[0019] WO 2024 / 125816 A, WO 2024 / 165235 A1 and WO 2024 / 153792 A1 disclose methods for avoiding or compensating a charging of samples or objects.

[0020] DE 102020125534 B3 is directed to a multiple particle beam microscope and an associated method with a fast autofocus around an adjustable working distance.

[0021] However, no solutions for improving an autofocus routine based on a focus series when inspecting locally charging objects are disclosed.

[0022] Description of the invention

[0023] It is an object of the invention to provide a multiple particle beam system with an improved resolution. Particularly, it is an object of the present invention to provide an improved method for operating a multi-beam particle microscope in an autofocus determination mode allowing for an improved resolution during inspection tasks.

[0024] The object is achieved by the subject matter of the independent claims. Advantageous embodiments of the invention are evident from the dependent claims.

[0025] The present patent application claims the priority of the German patent application No. 10 2025 101 460.4 filed on 16 January 2025, the disclosure of which in the full scope thereof is incorporated in the present patent application by reference.

[0026] The present invention is based on two fundamental findings with respect to focus series carried out on locally charging objects:

[0027] Firstly, it is not necessary to detect the full multiplicity of charged individual particle beams with a detection unit for generating image data to be analyzed with an autofocus algorithm. Instead, it has turned out that generating just one individual image with a respectively assigned single secondary individual particle beam is sufficient for realizing an autofocus routine.

[0028] Secondly, measures can be undertaken in the projection path of a multi-beam charged particle microscope to filter or cut-out a crosstalk free area or part of said single secondaryindividual particle beam and thereby to generate a crosstalk free signal beam. An individual image can be generated based exclusively on data related to said crosstalk free signal beam. With a thus-generated individual image, the above-mentioned algorithms for determining the best focus position do not fail because of crosstalk any more.

[0029] It is noted that a respective measure / filter or cut-out in the primary or illumination path is not that successful for improving a stability / failure rate of autofocus routines. The reason is that the particle optical setting in the illumination path should be kept unchanged and stable except for any focus variation due to carrying out the focus series as such. Furthermore, a blocking of all first individual particle beams except for a cross-talk free area of exactly one individual particle beam in the illumination path would be more expensive to implement. Still, in principle, a respective solution in the illumination path for eliminating crosstalk as a source for failure in autofocus routines could be implemented in an analogous manner.

[0030] According to a first aspect of the invention, the invention is directed to a method for operating a multi-beam particle microscope, including the following steps:

[0031] operating the multi-beam particle microscope in an inspection mode, comprising the following steps:

[0032] irradiating via an illumination path an object with a multiplicity of charged first individual particle beams, wherein each charged first individual particle beam irradiates a separate individual field region of the object in a scanning fashion; collecting charged second individual particle beams which emerge or emanate from the object on account of the first individual particle beams;

[0033] projecting via a projection path the second individual particle beams onto detection regions of a detection unit in such a way that the second individual particle beams emerging or emanating from two different individual field regions are projected onto different detection regions, wherein a projection path particle optics is operated in an inspection mode of operation; and

[0034] generating individual images of each of the individual field regions on the basis of data which are obtained or have been obtained by means of signals from each of the detection regions;

[0035] switching from the inspection mode to an autofocus determination mode; and

[0036] operating the multi-beam particle microscope in the autofocus determination mode and carrying out a focus series by varying a focus setting in the illumination path, comprising the following steps for each step of the focus series:irradiating via the illumination path the object with the multiplicity of charged first individual particle beams, wherein each charged first individual particle beam irradiates a separate individual field region of the object in a scanning fashion; collecting the charged second individual particle beams which emerge or emanate from the object on account of the first individual particle beams;

[0037] projecting via the projection path exactly one of the multiplicity of second individual particle beams fully or partly onto a detection region of the detection unit serving as an autofocus detection region, wherein the projection path particle optics is operated in an autofocus mode of operation which is different from the inspection mode of operation;

[0038] generating an individual image on the basis of data which are obtained or have been obtained by means of signals from the autofocus detection region, said individual image serving as autofocus determination image; and

[0039] analyzing the autofocus determination image by applying an image analysis algorithm for determining an optimum focus setting in the illumination path.

[0040] Operating a multi-beam particle microscope in an inspection mode is in principle known. It is noted that in the projection step second individual particle beams emerging or emanating from two different individual field regions are projected onto different detection regions. In other words, there is a 1:1 correlation between individual field regions on the one hand and detection regions on the other hand. The projection is furthermore a focussed projection. However, this does not mean that the projection process is 100% free of any crosstalk between different second individual particle beams. Still, any crosstalk still occurring is often negligible and does not meaningfully deteriorate an image quality. Still further, crosstalk during an image inspection can be suppressed by several measures as known in the art.

[0041] However, it has been found that this basic finding concerning negligible or suppressed remaining crosstalk when operating the multi-beam particle microscope in the inspection mode does not hold when operating a multi-beam particle microscope in an autofocus determination mode and carrying out a focus series by varying a focus setting in the illumination path, in particular when the object to be inspected is a locally charging object. Then, remaining crosstalk is the main reason that known autofocus determination algorithms can fail.

[0042] According to the present invention, operating the multi-beam particle microscope in the autofocus determination mode comprises projecting via the projection path exactly one of the multiplicity of second individual particle beams fully or partly onto a detection region of thedetection unit serving as an autofocus detection region, wherein the projection path particle optics is operated in an autofocus mode of operation which is different from the inspection mode of operation. Here, the term “exactly one” indicates that either no crosstalk at all or no relevant crosstalk arises when projecting the exactly one of the multiplicity of second individual particle beams on the autofocus detection region. “Fully projecting” means in the present context that the whole beam which is a selected one of the multiplicity of second individual particle beams is projected onto the autofocus detection region. “Partly projecting” means that just a part, but still a part of the selected one of the multiplicity of second individual particle beams is projected on the autofocus detection region. Furthermore, the autofocus detection region is therefore illuminated by the selected beam or just by part of the selected beam. The illumination of the autofocus detection region can be completely and thus the full sensitive area of the autofocus detection region can be illuminated. Alternatively, it is possible that just a part of the sensitive area of the autofocus detection region is illuminated by the selected beam.

[0043] It is furthermore noted that the projecting step when operating the multi-beam particle microscope in the autofocus determination mode comprises a focused projection. In other words, the projection is as focused as possible; there is no willful defocusing in the projection path.

[0044] In order to achieve the projection conditions as described above, the projection path particle optics is set, accordingly. This setting is a special setting and the projection path particle optics is operated in the autofocus mode of operation which is different from the inspection mode of operation. Therefore, at least one setting of one particle optical element in the projection path differs from the respective setting in the inspection mode of operation.

[0045] According to a preferred embodiment of the present invention, projecting the exactly one of the multiplicity of second individual particle beams fully or partly onto the autofocus detection region comprises projecting exclusively a crosstalk free part of said second individual particle beam on the autofocus detection region, said crosstalk free part of said second individual particle beam serving as a signal beam. The term “signal beam” indicates the fact that this signal beam is used for generating the autofocus determination image. Since crosstalk is eliminated in this embodiment, crosstalk between neighboring beams in the projection path cannot make an autofocus determination algorithm fail.

[0046] According to a preferred embodiment of the invention, in the autofocus mode of operation any crosstalk of the second individual particle beams with the second individual particlebeam that is used for generating the autofocus determination image (the signal beam) is excluded as a matter of principle. In other words, the setting in the projection path is such that the occurrence of crosstalk can be excluded as a matter of principle. This contributes to the success of autofocus determination according to the present invention.

[0047] According to a preferred embodiment, the switching step from the inspection mode to the autofocus determination mode comprises increasing a magnification in the projection path without changing a pitch between neighboring detection regions and / or without changing a size of the detection regions and / or without changing a position of or without re-assigning the detection regions. In other words, preferably, there is no further change in any setting of the detection unit. The detection unit still is used as it is used in the inspection step. This significantly facilitates a realization of the inventive method.

[0048] According to a preferred embodiment of the invention, the magnification in the projection path is increased at least by a factor of 1,05, in particular at least by a factor of 1,50 or 2,00. When changing a magnification in the projection path, a spot size of each second individual particle beam in a detection plane is increased. This increase in general generates more overlap in a halo region of the beams with one another. However, the distance between neighboring second individual particle beams is also increased. Thereby it becomes possible to generate a region in one of the beams that is 100% free of crosstalk as a matter of principle. Preferably, according to the present embodiment, the complete sensitive area of a detection region and in particular the autofocus detection region becomes exclusively illuminated by exactly one beam of the multiplicity of second individual particle beams and is thus completely free of any crosstalk. Only the data from this autofocus detection region is used for generating the autofocus determination image.

[0049] According to a preferred embodiment of the present invention, the method further comprises the following steps:

[0050] Using a centrally arranged detection region of the detection unit as the autofocus detection region; and / or

[0051] using a centrally arranged second individual particle beam in a raster arrangement of the second individual particle beams for generating a signal beam or for generating the signal beam imaged on the autofocus detection region.

[0052] According to this embodiment, a symmetry of the raster arrangement is kept during the magnification process. A data analysis based on a centrally arranged detection region and / or a centrally arranged second individual particle beam in the raster arrangement facilitates autofocus data determination and analysis. It is, however, also possible to use a differentdetection region of the detection unit and / or a different one of the multiplicity of second individual particle beams for generating the signal beam. In theory, it is also possible to realize the specific projection conditions according to the present invention not just for one of the multiplicity of second individual particle beams, but for more than one of the multiplicity of second individual particle beams. However, then, the projection conditions concerning each of these beams has to be fulfilled for each of these beams exactly. However, normally, just using exactly one of the multiplicity of second individual particle beams for generating a signal beam for the autofocus determination algorithm is already sufficient which facilitates the entire process.

[0053] According to a preferred embodiment of the present invention, during carrying out the autofocus determination mode with the increased magnification no further dynamic alignment of the projection path particle optics is carried out. In other words, when carrying out a focus series, there is no need for a further dynamic alignment of the projection path particle optics. This further facilitates the operation of the multi-beam particle microscope in the autofocus determination mode. It is noted that a dynamic alignment of the projection path particle optics has just to be carried out in the normal inspection mode of the multi-beam particle microscope.

[0054] According to a preferred embodiment of the invention, the method comprises the following steps in the autofocus determination mode:

[0055] Providing a setting to the projection path particle optics in such a way that an intermediate image of the second individual particle beams is generated in an aperture plane; deflecting the second individual particle beams in the aperture plane in such a way that only one of the second individual particle beams is not cut off completely by the aperture, but all other second individual particle beams are cut off completely by the aperture, a part of that only one second individual particle beam that is not cut off completely defining a signal beam; and

[0056] imaging the signal beam on the autofocus detection region.

[0057] According to this embodiment of the invention, the general idea for generating a signal beam that is entirely free of any crosstalk is realized by filtering a crosstalk free part of one of the beams and in other words to cut off any signals stemming from all other beams. The aperture is used for this cutting purpose. It is clear that according to this approach it is a necessary requirement that the second individual particle beams are well separated from one another for the cutting off process. Therefore, the intermediate image of the second individual particle beams is generated in the aperture plane. Here, focus spots are formed in the apertureplane. Nevertheless, it is clear that - if any remaining crosstalk occurs in a detection plane -there is also a respective crosstalk in the intermediate image plane. Therefore, a cut off line between the selected second individual particle beam generating a signal beam on the one hand and a neighboring second individual particle beam being completely cut off by the aperture on the other hand does not run in the middle between the two focus spots. Instead, the cut off line is closer to the focus spot of the selected second individual particle beam to get rid of a beam region that is deteriorated by crosstalk.

[0058] The aperture that is used for the cutting off process can in principle be any aperture provided in the projection path. However, a very elegant and simple solution is to apply a contrast stop as the cutting off aperture. In the plane of a contrast stop, a common crossover of all second individual particle beams is formed in the normal inspection mode. In the autofocus determination mode the setting of the projection path particle optics can be set in a different way and in such a way that in the contrast stop plane there is no longer a common crossover of all second individual particle beams, but an intermediate image of the second individual particle beams is positioned in said plane.

[0059] According to a preferred embodiment of the invention, the method further comprises deflecting the signal beam between the aperture plane and a detection unit for imaging the signal beam on the autofocus detection region. This measure can counterbalance the deflection that was necessary for cutting off purposes in the aperture plane.

[0060] According to a further preferred embodiment of the invention providing a setting to the projection path particle optics comprises increasing an excitation of a lens system arranged between the object and the aperture plane and decreasing an excitation of a lens system arranged between the aperture plane and the detection unit. By this measure it is still possible that the signal beam impinges on the autofocus detection region in a focused way. Furthermore, the suggested setting of the projection path particle optics is realizable very easily. It is noted that each of the mentioned lens systems can comprise one or more particle optical lenses. The lenses can be magnetic lenses and / or electrostatic lenses.

[0061] According to a further preferred embodiment of the invention, the autofocus determination mode further comprises the following step:

[0062] Providing a setting to the projection path particle optics in such a way that a raster arrangement of the multiplicity of second individual particle beams is imaged on the detection unit in such a way that only a crosstalk free area of an outermost second individual particle beam arranged in a first direction of the raster arrangement is imaged on an outermostdetection region in the detection unit arranged in a second direction opposing the first direction.

[0063] This embodiment is once again very easy to realize. It can be realized just by providing appropriate deflections of the raster arrangement. The described embodiment relies on the fact that an outermost part of a second individual particle beam arranged in an outer shell of the raster arrangement is free of any crosstalk as a matter of principle. The described deflection of the entire raster arrangement ensures that only this crosstalk free area is imaged on the detection unit and thus on the autofocus detection region. All other second individual particle beams may nevertheless impinge in a detection plane, but they will not interfere with the signal generated by a detection on the autofocus detection region.

[0064] According to a preferred embodiment of the invention, the method further comprises providing a setting to the projection path particle optics comprising deflecting the multiplicity of second individual particle beams in the second direction. This measure can be the only measure that is necessary for realizing the above-described embodiment of the invention.

[0065] According to a further preferred embodiment of the invention, the detection unit comprises a combination of a particle detection system and a light detection system, and a detection region comprises a light entrance surface of a light detector. The light detector can for example be an avalanche photodiode or any other kind of light detector. It is noted that in a system combination of a particle detection system and a light detection system there is a mapping between a particle detection plane and a light detection plane. The considerations with respect to the present invention hold for both a particle detection plane and a light detection plane. It is noted that a system combination of a particle detection system and a light detection system can pose more restraints on a signal evaluation by the detection unit than a mere particle detection system normally does. One of the constraints is that a pitch between adjacent light detectors can normally be not changed and a size of a light entrance surface of a light detector is normally fixed. However, the method according to the present invention is so easy and flexible that it can be carried out also with a specific detection unit comprising a combination of a particle detection system and a light detection system.

[0066] According to a preferred embodiment of the invention, the object is a locally charging object. The power of the method for operating a multi-beam particle microscope becomes fully visible under this circumstance. Locally charging objects provide much more challenges with respect to the topic of crosstalk than non-charging objects. Furthermore, a local charge in the object plane changes between different locations and therefore crosstalk occurring or not occurring at different areas or regions on the object is also a varying condition. Still,according to the present invention, stable autofocus determination using image analysis algorithms is possible.

[0067] According to a further preferred embodiment of the invention, the method further comprises the following step:

[0068] Determining the optimum focus setting in the illumination path based on the analysis of the autofocus determination images;

[0069] switching from the autofocus determination mode to the inspection mode; and

[0070] operating the multi-beam particle microscope in the inspection mode with the determined optimum focus setting in the illumination path.

[0071] The thus determined optimum focus setting can thus be used when further operating the multi-beam particle microscope in the inspection mode again. According to an embodiment, a setting of the particle optical unit in the projection path is adapted after determining the optimum focus setting in the illumination path. Then, preferably, an optimum setting in the projection path is determined and respectively set.

[0072] According to a preferred embodiment of the invention, determining the optimum focus setting in the illumination path comprises determining a set of optimum focus settings for all first individual particle beams. This is possible because the shape of the image shell in the object plane is normally known.

[0073] According to a preferred embodiment of the invention, determining the set of optimum focus settings comprises an interpolation and / or an extrapolation. Additionally, or alternatively, determining the set of optimum focus settings comprises using prior knowledge about an image shell of the multiplicity of first individual particle beams when irradiating the object. In order words, an image filed curvature and / or image field inclination can be known a priori. This knowledge can be used to determine the optimum focus settings not only for the first individual particle beam corresponding (partly) to the signal beam, but to all first individual particle beams.

[0074] According to a further preferred embodiment of the invention, the method further comprises dynamically adjusting the projection path particle optics when operating the multi-beam particle microscope in the inspection mode. This is in principle already known from the art. However, this dynamical adjustment of the projection path particle optics is normally not necessary when operating the multi-beam particle microscope in the autofocus determination mode and thereafter switching back to the inspection mode.According to a second aspect of the present invention, the invention is directed to a multibeam particle microscope configured for carrying out the method as described in various embodiments above. This configuration can be realized for example by an appropriate programming of a control unit of the multi-beam particle microscope. It is noted that it is normally not necessary to amend or adapt the multi-beam particle microscope hardware for successfully carrying out the inventive method.

[0075] The embodiments and aspects of the invention as described above can be combined fully or in part with one another, as long as no technical contradictions occur.

[0076] The invention will be even better understood with reference to the following figures:

[0077] Fig. 1 : schematically shows an example of a multi-beam charged particle device; Fig. 2: schematically show an example of a detection unit;

[0078] Fig. 3: schematically shows another example of a detection unit;

[0079] Fig. 4: schematically illustrates a locally charging object;

[0080] Fig. 5: depicts a relation between charging and defocus in the illumination path;

[0081] Fig. 6: schematically illustrates a method for operating a multi-beam particle microscope;

[0082] Fig. 7: schematically illustrates method steps of an inspection mode;

[0083] Fig. 8: schematically illustrates method steps of an autofocus determination mode; Fig. 9: schematically illustrates aspects a setting of a projection path particle optics in the autofocus determination mode;

[0084] Fig. 10: schematically illustrates aspects of a setting of a projection path particle optics in the autofocus determination mode;

[0085] Fig. 11 : schematically illustrates aspects of a setting of a projection path particle optics in the autofocus determination mode;

[0086] Fig. 12: schematically illustrates a setting in a projection path particle optics in an inspection mode and in an autofocus determination mode; and Fig. 13: schematically illustrates aspects of a setting of a projection path particle optics in the autofocus determination mode.

[0087] Fig. 1 schematically shows a multiple particle beam system using the example of a multibeam particle microscope 1. The multi-beam particle microscope 1 comprises a beam generating apparatus 300 with a particle source 301, for instance an electron source. A divergent particle beam 309 is collimated by a sequence of condenser lenses 303.1 and 303.2 and incident on a multi-aperture arrangement 305. The multi-aperture arrangement305 comprises a plurality of multi-aperture plates 304, 306 and a field lens 307. A plurality of individual particle beams 3 or individual electron beams 3 are generated by the multiaperture arrangement 305. Midpoints of apertures in the multi-aperture plate arrangement 305 are arranged in a field which is imaged on a further field formed by beam spots 5 in the object plane 101. The pitch between the midpoints of apertures of a multi-aperture plate 306 can be for instance 5 pm, 100 pm and 200 pm. The diameters D of the apertures are smaller than the pitch of the midpoints of the apertures; examples of the diameters are 0.2 times, 0.4 times and 0.8 times the pitch between the midpoints of the apertures.

[0088] The multi-aperture arrangement 305 and the field lens 307 are configured to generate a multiplicity of focal points 323 of primary beams 3 in a raster arrangement on a surface 321. The surface 321 need not be a plane surface but rather can be a spherically curved surface in order to account for a field curvature of the subsequent particle-optical system.

[0089] The multi-beam particle microscope 1 further comprises a system of electromagnetic lenses 103 and an objective lens 102, which image the beam foci 323 from the intermediate image surface 321 in the object plane 101 with reduced size. In between, the first individual particle beams 3 pass through the beam switch 400 and a collective beam deflection system 500, by means of which the plurality of the first individual particle beams 3 are deflected during operation and the image field is scanned. The first individual particle beams 3 incident in the object plane 101 for example form a substantially regular field, wherein the pitch between adjacent incidence locations 5 can be 1 pm, 10 pm or 40 pm, for example. For instance, the field formed by the incidence locations 5 can have a rectangular or hexagonal symmetry.

[0090] The object 7 to be examined can be of any desired type, for instance a semiconductor wafer or a biological sample, and can comprise an arrangement of miniaturized elements or the like. The surface 15 of the object 7 is arranged in the object plane 101 of the objective lens 102. The objective lens 102 can comprise one or more electron-optical lenses. For instance, this can be a magnetic objective lens and / or an electrostatic objective lens.

[0091] The primary particles 3 incident on the object 7 generate interaction products, for example secondary electrons, backscattered electrons or primary particles which have experienced a reversal of movement for other reasons, and these interaction products emanate from the surface of the object 7 or from the first plane 101 or object plane 101. The interaction products emanating from the surface 15 of the object 7 are shaped by the objective lens 102 to form secondary particle beams 9. In the process, the secondary beams 9 pass through the beam switch 400 downstream of the objective lens 102 and are supplied to a projectionsystem 200. The projection system 200 comprises in the example shown an imaging system 205 with projection lenses 206, 208 and 210, a contrast stop 214 around a cross-over 212, a collective beam deflection device 222 (anti-scan) and a multi-particle detector 207. Incidence locations 25 of the second individual particle beams 9 on detection regions of the multiparticle detector 207 are located with a regular pitch in a third field. Exemplary values are 10 pm, 100 pm and 200 pm.

[0092] The multi-beam particle microscope 1 further comprises a computer system or a control unit 10, which in turn can have a single-part or multi-part design and which is designed both to control the individual particle-optical components of the multi-beam particle microscope 1 and to evaluate and analyse the signals obtained by the multi-detector 207 or, more generally, a detection unit 209.

[0093] The multi-beam particle microscope 1 is suited for carrying out the present invention.

[0094] Further information relating to such multi-beam particle beam systems or multi-beam particle microscopes 1 and component parts used therein, such as, for instance, particle sources, multi-aperture plate and lenses, can be obtained from the international patent applications WO 2005 / 024881 A2, WO 2007 / 028595 A2, WO 2007 / 028596 A 1, WO 2011 / 124352 A1 and WO 2007 / 060017 A2 and the German patent applications DE 102013016113 A1 and DE 102013014976 A1, the disclosure of which is fully incorporated in the present application by reference.

[0095] Figure 2 is a schematic illustration for elucidating a realization of the detector 209 by way of example; reference is initially made once again to the normal operating mode. In this case, the detection unit 209 comprises a scintillator plate 207 as particle detector, onto which scintillator plate the interaction products, for example secondary electron beams 9, are directed by a projection path particle optical unit. Said projection path particle optical unit comprises, if it is integrated into the multi-beam particle microscope from Fig. 1 , the particle optical components of the particle optical unit which shape the second individual particle beams 9, i.e. e.g. the objective lens 102, which direct the second individual particle beams 9 toward the detection unit 209, such as e.g. the beam switch 400, and which focus the second individual particle beams 9 on the surface of the scintillator plate 207, such as e.g. the lens 210. The second individual particle beams 9 are incident on the scintillator plate 207 at incidence locations 213. Even if the second individual particle beams 9 are focused on the surface of the scintillator plate 207, beam spots having diameters that are not arbitrarily smallare formed on the surface. The midpoints of the beam spots can be regarded as the incidence locations 213, which are arranged at the distance P2 (cf. Fig. 1) from one another.

[0096] The scintillator plate 207 contains a scintillator material, which is excited to emit photons by the incident particles of the second individual particle beams 9. Each of the incidence locations 213 thus forms a source of photons. Figure 2 illustrates just a single corresponding beam path 221 emanating from the incidence location 213 of the central electron beam of the five second individual particle beams 9 illustrated. The beam path 221 passes through a light optical unit 223, which comprises a first lens 225, a mirror 227, a second lens 229 and a third lens 231 in the example shown, and then impinges on a light receiving surface 235 (signal entrance surface 235) of a light detection system 237. The light receiving surface 235 is formed by an end face of an optical fiber 239, into which at least a portion of the photons is coupled and guided to a light detector 241. The light detector 241 can comprise e.g. a photomultiplier, an avalanche photodiode, a photodiode or other types of suitable light detectors. The light optical unit 223 is configured such that it optically images the surface of the scintillator plate 207 into a region 243 in which the light receiving surface 235 is arranged. On account of this optical imaging, optical images of the incidence locations 213 are generated in the region 243. In the region 243, a separate light receiving surface 235 of the light detection system 237 is provided for each of the incidence locations 213. Each of the further light receiving surfaces 235 (signal entrance surfaces 235) is formed by an end face of a light guide 239, which guides the light coupled into the end face to a light detector 241. On account of the optical imaging, a light receiving surface 235 is assigned to each of the incidence locations 213, wherein the light entering a respective light receiving surface 235 is detected by a separate light detector 241. The light detectors 241 output electrical signals via signal lines 245. Said electrical signals represent intensities of the second individual particle beams 9. Consequently, the locations on the surface of the scintillator plate 207 which are imaged onto the light receiving surfaces of light detectors 241 define different detection points or detection regions. On account of the projection path particle optical unit described above, interaction products, for example electrons, which emanate from two different individual field regions of an object 7 are also projected onto different detection regions of the scintillator plate 207. In the exemplary embodiment explained here, the light detectors 241 are arranged at a distance from the light receiving surfaces 235, onto which the light optical unit 223 images the scintillator plate 207, and the received light is guided to the light detectors 241 through optical fibers 239. However, it is also possible for the light detectors 241 to be arranged directly where the light optical unit generates the image of the scintillator plate and the light-sensitive surfaces of the light detectors thus form the light receiving surfaces.In this case, figure 2 merely schematically elucidates some details of the detection unit 209. It should still be pointed out at this juncture that by virtue of the scanning movement of the primary particle beams 3 over an object 7 or a sample, many points of the object 7 or sample are irradiated or scanned. In this case, each primary particle beam 3 sweeps wholly or partly over an individual field region of the object 7. In this case, each primary particle beam 3 is allocated a dedicated individual field region of the object 7. From these individual field regions of the object 7, interaction products, e.g. secondary electrons, then in turn emanate from the object 7. The interaction products are then projected onto the detection regions of the particle detector or onto the scintillator plate 207 in such a way that the interaction products emanating from two different individual field regions are projected onto different detection regions of the scintillator plate 207. Light signals are emitted by each detection region of the scintillator plate 207 upon incidence of the interaction products, e.g. secondary electrons, on said detection region, wherein the light signals emitted by each detection region are fed to a light detector 241 assigned to the respective detection region. In other words, the situation is that each primary particle beam 3 comprises (via the secondary beams 9) its own detection region on the scintillator 207 and also its own light detector 241 , which together form a detection channel in the normal inspection mode in the described example.

[0097] Figure 3 shows an alternative embodiment variant of a detection unit 209. In this variant, no optical fibers 239 are provided; instead, photons emanating from the scintillator plate 207, after the optical imaging, impinge directly on an array having light-sensitive detectors 241, for example an array comprising photomultipliers, photodiodes, or avalanche photodiodes.

[0098] Detection architectures other than the ones illustrated in figure 2 and figure 3 are also suitable for carrying out the method according to the invention for operating a multi-beam particle microscope 1 in an inspection mode and in a autofocus determination mode according to the invention. Reference is made, for example, to the method of DED (“direct electron detection”), which manages without light detectors and in which secondary electrons are directly converted into a current signal.

[0099] Figure 4 schematically illustrates a locally charging object and related effects on trajectories of charged first individual particle beams 3 and charged second individual particle beams 9: For grounds of comparison, figure 4a schematically illustrates a situation wherein the object 7 is not charged / not chargeable. The trajectories of the first individual particle beams 3 and the second individual particle beams 9 are well defined and clearly separated. In contrast thereto, figure 4b shows an object 7 with a locally charged area 110. The incoming firstindividual particle beams 3 are slightly influenced by the charging which is schematically indicated by the additional dotted lines. The positions of incidence in the object plane 101 slightly change. Furthermore, second individual particle beams 9 emerging or emanating from the object 7 on account of the first individual particle beams 3 are also influenced by the charging. The trajectories of the second individual particle beams 9 are thus slightly changed with respect to the situation depicted in figure 4a.

[0100] As already described above in the general part of the description, it was found that image analyses algorithms for determining an optimum focus setting in the illumination path showed a failure much more often when inspecting a locally charging object compared to a situation where the object is non-charging. Therefore, a possible reason for the failure was thus seen in the fact of the local charging.

[0101] Figure 5 depicts a relation between charging and defocus in the illumination path of a charged multi-beam particle microscope: The shown diagram is the result of ray tracing simulations with a homogenous charge up of an object 7. The results for different landing energies are shown (500 eV, 1500 eV and 3000 eV). The results indicate that a primary beam defocus introduced by local electric fields at the object due to object charge up are negligible unless the landing energy becomes small and the surface voltage of the object becomes large. However, a respective failure of the autofocus determination algorithms not only occurs with small landing energies and a large surface voltage of the object 7.

[0102] Therefore, it has been found that a varying focus position and therefore algorithm failure during caring out a focus series for the determination of the best focus in the primary path / illumination path is not the reason for the autofocus routine failure on charging objects 7. It is concluded that the routine failure is thus primarily caused by crosstalk / overlapping beams due to a charge distortion in the projection path / secondary path of the multi-beam particle microscope 1. This finding triggered the approach to look for a solution of the failure problem by avoiding crosstalk in the projection path of a multi-beam particle microscope as a matter of principle.

[0103] Figure 6 schematically illustrates a method for operating a multi-beam particle microscope 1. More concretely, figure 6 illustrates a switching between different modes of the multi-beam particle microscope 1: In a first method step S1 the multi-beam particle microscope 1 is operated in an inspection mode. This can be a normal mode of operation as in principle known in the art.In method step S2 switching from the inspection mode to an autofocus determination mode is carried out.

[0104] In a further method step S3 the multi-beam particle microscope 1 is operated in the autofocus determination mode and a focus series is carried out by varying a focus setting in the illumination path / primary path. The autofocus determination mode comprises measures for excluding or supressing any crosstalk in the projection path as a matter of principle which will be further described below.

[0105] In a method step S4 switching from the autofocus determination mode back to the inspection mode is carried out. Optionally, a realignment of the projection path is carried out after switching back to the inspection mode.

[0106] In a further method step S5 the multi-beam particle microscope is operated in an inspection mode, wherein for example further individual field regions of an object 7 are irradiated in a scanning fashion.

[0107] In a method step S6 switching from the inspection mode to the autofocus determination mode is carried out once again.

[0108] In a method step S7 once again the multi-beam particle microscope 1 is operated in the autofocus determination mode and another focus series is carried out by varying a focus setting in the illumination path.

[0109] Afterwords, within method step S8, it is switched back from the autofocus determination mode to the inspection mode with an optional realignment of the projection path and so on.

[0110] Figure 7 schematically illustrates method steps of the inspection mode: In a method step S70 an object 7 is irradiated via an illumination path with a multiplicity of charged first individual particle beams 3, wherein each charged first individual particle beam 3 irradiates a separate individual field region of the object 7 in a scanning fashion.

[0111] In a method step S71 charged second individual particle beams 9 which emerge or emanate from the object 7 on account of the first individual particle beams 3 are collected.

[0112] In a step S72 the second individual particle beams 9 are projected via a projection path onto detection regions of a detection unit 209 in such way that the second individual particlebeams 9 emerging or emanating from two different individual field regions are projected onto different detection regions, wherein a projection path particle optics 200 is operated in an inspection mode of operation.

[0113] In a method step S73 individual images of each of the individual field regions are generated on the bases of data which are obtained or have been obtained by means of signals from each of the detection regions.

[0114] Figure 8 schematically illustrates method steps of an autofocus determination mode according to the present invention: In a method step S80 the object 7 is irradiated via the illumination path with the multiplicity of charged first individual particle beams 3, wherein each charged first individual particle beam 3 irradiates a separate individual field region of the object 7 in a scanning fashion. The setting of the illumination path particle object is in principle the same as in the inspection mode, except for a slightly changed focus setting due to the purpose of carrying out the focus series. The focus setting can for example be changed by exciting an objective lens system 102.

[0115] In a method step S81 the charged second individual particle beams 9 which emerge or emanate from the object 7 on account of the first individual particle beams 3 are collected. This can be done for example by a corresponding extraction field generated for example by the objective lens system 102.

[0116] In a further method step S82 exactly one of the multiplicity of second individual particle beams 9 is fully or partly projected via the projection path onto a detection region of the detection unit 209 serving as an autofocus detection region. During this method step, the projection path particle optical optics 200 is operated in an autofocus mode of operation which is different from the inspection mode of operation. Preferably projecting the exactly one of the multiplicity of second individual particle beams 9 fully or partly onto the autofocus detection region comprises projecting exclusively a crosstalk free part of said second individual particle beam 9 on the autofocus detection region, said cross talk free part of the second individual particle beam 9 serving as a signal beam. Preferably, any crosstalk of the second individual particle beams 9 with the second individual particle beam 9 that is used for generating the autofocus determination image is excluded as a matter of principle.

[0117] In a method step S83 an individual image is generated on the basis of data which are obtained or have been obtained by means of signals from the autofocus detection region, said individual image serving as autofocus determination image.In a further method step S84 the autofocus determination image is analyzed by applying an image analysis algorithm for determining an optimum focus setting in the illumination path. The image analysis algorithm for determining an optimum focus setting in the illumination path can be any suitable image analysis algorithm as already known in the art. The algorithm can for example be based on the determination of a contrast measure. A contrast measure can be e. g. one of the following methods: A spectral method, an image contrast, a histogram method, an edge filter, a method of relative spread or blur, or a gradient method.

[0118] Furthermore, autofocus methods using image sharpness algorithms (e. g. CNR) can be used.

[0119] The autofocus determination images generated during each step of the focus series can be compared to one another using an image analysis algorithm in order to determine the best autofocus determination image and to determine an optimum focus setting in the illumination path based thereon.

[0120] Figure 9 schematically illustrates aspects of a setting of a projection path particle optics 200 in the autofocus determination mode. According to the depicted embodiment, a magnification in the projection path is increased in the autofocus determination mode compared to the inspection mode: Figure 9a shows the situation in the detection plane 290 in the inspection mode and figure 9b shows the situation in the detection plane 290 in the autofocus determination mode. The detection plane 290 can be a particle detection plane or a light detection plane as already described above. In the present example shown, the situation fits best to a light detection plane with a plurality of detection regions 250 clearly separated and spaced from one another. For ease of illustration, just eight different detection regions 250i to 250s are illustrated in figure 9a. When projecting the plurality of second individual particle beams 9 via the projection path onto the detection regions 250 second individual particle beams 9 emerging or emanating from two different individual field regions on the object 7 are projected onto different detection regions 250. However, this 1:1 relationship does not strictly hold, but some crosstalk occurs. This crosstalk is illustrated by the halos 2511 to 251s in figure 9a. It is noted that the occurrence of a halo does not mean that there is a willful defocusing when the second individual particle beams 9 impinge on the detection plane (or the respectively assigned light rays do). Instead, it holds true in principle that the beam diameter is, e.g., changed due to a local charging. It is well possible that a minimum beam diameter (beam waste) is still situated in the detection plane 290; however, this can also be differentIn the present example, the central detection region 2503 serves as autofocus detection region. However, it is apparent from figure 9a that it is not just the central beam that is incident onto the autofocus detection region 2503, but that surrounding beams also contribute to the signal generation in the autofocus detection region 2503. It is just a relatively small area indicated with “a” in figure 9 that is really crosstalk free. However, the detection region 2503 does not have any spatial resolution, but simply collects and adds all incoming signals. Therefore, with an arrangement as depicted in figure 9a an autofocus determination routine would fail due to crosstalk.

[0121] The situation is different as depicted in figure 9b: In the example shown, the magnification in the projection path is significantly increased, for example at least by a factor of 2. Increasing the magnification by a factor s increases the radius of the halo (becoming s*r) and increases the distance between neighboring beam spots (becoming s*p). However, also the crosstalk free area a is increased. In the example shown it is increased to such an extent that the whole entrance surface of a detection region 2503 is truly exclusively illuminated just by a part of a central beam 9. Therefore, by choosing an appropriate magnification it is possible to prepare an autofocus detection region (here: detection region 250s) that is as a matter of principle totally free of crosstalk. Therefore, when using exclusively a completely crosstalk free signal of the autofocus detection region 2503, an autofocus determination routine does no longer fail because of crosstalk even in the presence of local charges on an object 7.

[0122] It is noted that just increasing a magnification in the projection path in the described manner is sufficient for totally suppressing crosstalk. It is not necessary to change a pitch between neighboring detection regions and / or to change a size of a detection region and / or to change a position or to reassign the detection regions. Specifically, this holds also for combinations of a particle detection system with a light detection system wherein a detection region 250 comprises a light entrance surface of a light detector. However, the described method comprising the significantly increased magnification in the projection path can also be carried out with other types of detection systems as detection unit 209.

[0123] Figure 10 schematically illustrates aspects of a setting of a projection path particle optics 200 in the autofocus determination mode. According to the shown embodiment of the invention a filtering process or cut-off process is used in order to generate a signal beam 252 that is as a matter of principle completely free of crosstalk. According to this embodiment, a setting is provided to the projection path particle optics 200 in such a way that an intermediate image of the second individual particle beams 9 is generated in an aperture plane 263 which is in a depicted example the intermediate image plane 263. The hexagonal raster arrangementshows the focus spots 215 of the second individual particle beams 9 in the intermediate image. In the intermediate image the charged individual second particle beams 9 are rather well separated. Therefore, by deflecting the second individual particle beams 9 in a manner as depicted in figure 10b, an outermost beam can be separated by the aperture 260 from the remaining beams of the raster arrangement 262. In other words, the second individual particle beams 9 are deflected in the aperture plane 263 in such a way that only one of the second individual particle beams 9 is not cut-off completely by the aperture 260, but all other second individual particle beams 9 are cut-off completely by the aperture 260, a part of said only one second individual particle beam 9 that is no cut-off completely defining a signal beam 252. Said signal beam 252 is then imaged on the autofocus detection region.

[0124] It is clear that - if any crosstalk occurs in the detection plane 290 - there must also exist crosstalk in the intermediate image plane I the aperture plane 263. This fact is illustrated in figure 11 showing not only the focus spots 215 of the raster arrangement of second individual particle beams 9, but explicitly showing a halo 2511 and 2512 for two exemplary beams. One of these beams becomes the signal beam 252 when cutting-off the remaining beams. Figure 11b further illustrates that the cut-off of the signal beam also cuts-off I filters out all parts of the respective beam that comprises an area of crosstalk with a neighboring beam. This is in principle indicated by the curved cut on the right-hand side in figure 11b showing the part of the second individual particle beam generating the signal beam 252.

[0125] The principle of the embodiment depicted in figures 10 and 11 can be realized rather easily with a conventional multi-beam particle microscope: It is for example possible to provide a setting to the projection path particle optics 200 comprising increasing an excitation of a lens system arranged between the object 7 and the aperture plane 263 and decreasing an excitation of a lens system arranged between the aperture plane 263 and the detection unit 209. Furthermore, the signal beam 252 generated by the filtering process I the cut-off of other beams and therefore any crosstalk can be deflected between the aperture plane 263 and the detection unit 209 for imaging the signal beam 252 on the autofocus detection region. This can in principle be any detection region of the detection unit 209.

[0126] Figure 12 schematically illustrates a setting in a projection path particle optics 200 in an autofocus determination mode and for grounds of comparison in an inspection mode. The illustrated setting can be used for realizing the embodiments as described in figures 10 and 11. Figure 12a shows the setting of the projection path particle optics 200 in the inspection mode and figure 12b shows the setting in the autofocus determination mode. Depicted are in both cases three trajectories 9a, 9b and 9c. Second individual particle beams emerging oremanating from the object plane 101 of the object 7 are extracted by the objective lens system 102 and then pass through a sequence of projection lenses 206, 208 and 210 and are then projected onto the detection unit 209 comprising a detection plane 290. The detection plane 290 is in the present case realized by a particle detection system 207 comprising for example a scintillating plate, but the detection plane 290 also corresponds to a corresponding plane in which light entrance surfaces of light detectors are positioned. Other configurations of the detection unit 209 are of course also possible. Between the projection lens 206 and the projection lens 208 an aperture stop 260 is provided. In the inspection mode of operation this aperture stop 260 realizes a contrast stop 214 which is used for filtering the second individual particle beams 9 with respect to their starting angles from the object 7. Level with this aperture stop 260 all individual particle beams emerging vertically from the object cross the particle optical axis Z level with the aperture stop 260. The cross-over 212 is indicated by a dotted circle in figure 12a.

[0127] Fig. 12b shows a setting of the projection path particle optical 200 in the autofocus determination mode. In this mode, no cross-over plane or cross-over 212 is arranged level with the aperture stop 260. Instead, an intermediate image plane is arranged level with the aperture stop 260. This is schematically indicated by the focus spot 215 where the beam trajectories 9a and 9c emanating from the same location on the object cross each other. Therefore, level with the aperture stop 260 there exists an intermediate image plane 263. Deflecting the plurality of second individual particle beams 9 within plane 263 allows for cutting-off all second individual particle beams 9 except for a part of just one beam generating the signal beam 252.

[0128] In order to achieve a respective setting in the autofocus determination mode, an excitation of a projection lens 206 is increased compared to a setting in the inspection mode in the example shown. On the other hand, the excitations of the projection lenses 208 and 210 are decreased such that overall focusing on the detection unit 209 is still realized. In the present example, a double-deflection system with deflectors 281, 282 is used to cut-out the signal beam 252 which is then used for the determination of the best focus position in an autofocus determination routine. A remaining deflector 283 is used for deflecting the remaining signal beam 252 after cut-out onto one of the detection regions of the detection unit 209.

[0129] It is noted that other setting configurations of the projection path particle optics in a multibeam particle microscope 1 are also possible for realizing the invention.Figure 13 schematically illustrates aspects of a setting of a projection path particle optic 200 in the autofocus determination mode according to another embodiment of the present invention. According to the embodiment shown, a setting is provided to the projection path particle optics 200 in such a way that a raster arrangement of the multiplicity of second individual particle beams 9 is imaged on the detection unit 209 in such a way that only a crosstalk-free area of an outer most second individual particle beam arranged in a first direction of the raster arrangement is imaged on an outer most detection region 250s in the detection unit 209 arranged in a second direction opposing the first direction. In the embodiment shown in figure 13 the first direction is the direction to left and the second direction is the direction to the right. The outer most second individual particle beam is the very left beam with the halo 2511 on the left side of figure 13. It is noted that in principle there exists crosstalk with a neighboring beam having the halo 2512. However, there exists a crosstalk-free region of the very left beam which can be made use of when deflecting the originally left outer most beam with a halo 2511 not on the corresponding detection region 250i, but on the Tightest detection region 251s. The active and sensitive area of the detection region 251s serving as an autofocus detection region is completely free of crosstalk as a matter of principle.

[0130] It is noted that the method according to the invention works very well also in the presence of an object 7 that is a locally charging object 7.

[0131] According to another embodiment, the method according to the present invention further comprises a determining step of the optimum focus setting in the illumination path based on the analysis of the autofocus determination images. In a further method step switching from the autofocus determination mode to the inspection mode is carried out. Afterwords, the multi-beam particle microscope 1 can be operated in the inspection mode with the determined optimum focus setting in the illumination path.

[0132] Preferably, determining the optimum focus settings in the illumination path comprises determining a set of optimum focus settings for all first individual particle beams 3. According to an example, this determination comprises an interpolation and I or and extrapolation. Additionally, or alternatively, determining the set of optimum focus settings comprises using prior knowledge about an image shell of the multiplicity of the first individual particle beams 3 when irradiating the object 7.

[0133] According to a preferred embodiment, the method further comprises dynamically adjusting the projection path particle optics 200 when operating the multi-beam particle microscope 1in the inspection mode. It is noted that this dynamic adjustment is normally not necessary when operating the multi-beam particle microscope 1 in the autofocus determination mode.

[0134] A multi-beam particle microscope 1 as depicted in figure 1 can be configured for carrying out the method as described in further detail in various embodiments above.

[0135] The embodiments depicted in the figures are not meant to be limiting the extent of protection of the invention, but shall rather illustrate basic principles of the invention.

[0136] It is noted that a respective measure / filter or cut-out in the primary or illumination path is not as successful for improving a stability / failure rate of autofocus routines as the described solution in the projection path. The reason is that the particle optical setting in the illumination path should be kept unchanged and stable except for any focus variation due to carrying out the focus series as such. Furthermore, a blocking of all first individual particle beams 3 except for a cross-talk free area of exactly one individual particle beam (signal beam 252) in the illumination path would be more expensive to implement. Still, in principle, a respective solution in the illumination path for eliminating crosstalk as a source for failure in autofocus routines can be implemented in an analogous manner compared to the solution as further described / implemented in the projection path. The blocked first individual particle beams 3 could be directed into a beam dump or on an aperture stop, as for example disclosed in the international patent application No. PCT / EP2024 / 025340 filed on 12 December 2024 with priority of 19 December 2023, the disclosure of which in the full scope thereof being incorporated in the present patent application by reference.

[0137] A method for operating a multi-beam particle microscope working with a multiplicity of charged particle beams is disclosed that is particularly useful when inspecting locally charging objects. The method comprises operating the multi-beam particle microscopel in an inspection mode of operation and in an autofocus determination mode. A projection path particle optics 200 is operated in the autofocus determination mode in a different manner than in the inspection mode. In the autofocus inspection mode just one of the individual particle beams 9 in the projection path is selected to provide a signal for an autofocus routine. The specific way of selecting or filtering of said individual particle beam guarantees that a signal generated on the basis of said beam is free of any crosstalk.

[0138] List of reference signs

[0139] 1 Multi-beam particle microscopePrimary particle beams (first individual particle beams) Beam spots, incidence locations

[0140] Object, sample

[0141] Sample stage

[0142] Secondary particle beams (second individual particle beams) a trajectory

[0143] b trajectory

[0144] c trajectory

[0145] 0 Computer system, controller

[0146] 5 Surface

[0147] 01 Object plane

[0148] 02 Objective lens system

[0149] 03 Field lens system

[0150] 10 locally charged area

[0151] 05 Axis

[0152] 00 Projection system

[0153] 05 Imaging system

[0154] 06 Projection lens

[0155] 07 Scintillator plate

[0156] 08 Projection lens

[0157] 09 Detection unit

[0158] 10 Projection lens

[0159] 12 Cross-over

[0160] 13 Incidence location

[0161] 14 Contrast stop

[0162] 15 focus spot in intermediate image

[0163] 21 Optical beam path

[0164] 22 Collective beam deflection device (anti-scan)

[0165] 23 Light optical unit

[0166] 25 Lens

[0167] 27 Mirror

[0168] 29 Lens

[0169] 31 Lens

[0170] 35 Light receiving surface

[0171] 37 Light detection system

[0172] 39 Optical fiber, light guide

[0173] 41 Light detector43 Region for optical imaging of the scintillator surface 45 Line

[0174] 50 Detection region

[0175] 51 Halo

[0176] 52 Signal beam

[0177] 60 Beam stop

[0178] 61 Intermediate image plane

[0179] 62 raster arrangement of second individual particle beams 63 Intermediate image plane (projection path)

[0180] 81 Deflector

[0181] 82 Deflector

[0182] 83 Deflector

[0183] 90 Detection plane

[0184] 300 Beam generating apparatus

[0185] 301 Particle source

[0186] 303 Collimation lens system

[0187] 304 Filter plate

[0188] 305 Multi-aperture arrangement

[0189] 306 Micro-optics, multi-aperture plates

[0190] 307 Field lens

[0191] 308 Field lens system

[0192] 309 Diverging particle beam

[0193] 321 surface, intermediate image plane

[0194] 323 Beam foci

[0195] 400 Beam switch

[0196] 500 Collective beam deflection system

[0197] 600 Sample stage

[0198] P Pitch between detection regions

[0199] r Radius of halo or radius of irradiation area

[0200] a Cross-talk free area

[0201] s Magnification

Claims

1. Claims1. A method for operating a multi-beam particle microscope, including the following steps:operating the multi-beam particle microscope in an inspection mode, comprising the following steps:irradiating via an illumination path an object with a multiplicity of charged first individual particle beams, wherein each charged first individual particle beam irradiates a separate individual field region of the object in a scanning fashion;collecting charged second individual particle beams which emerge or emanate from the object on account of the first individual particle beams;projecting via a projection path the second individual particle beams onto detection regions of a detection unit in such a way that the second individual particle beams emerging or emanating from two different individual field regions are projected onto different detection regions, wherein a projection path particle optics is operated in an inspection mode of operation; andgenerating individual images of each of the individual field regions on the basis of data which are obtained or have been obtained by means of signals from each of the detection regions;switching from the inspection mode to an autofocus determination mode; and operating the multi-beam particle microscope in the autofocus determination mode and carrying out a focus series by varying a focus setting in the illumination path, comprising the following steps for each step of the focus series:irradiating via the illumination path the object with the multiplicity of charged first individual particle beams, wherein each charged first individual particle beam irradiates a separate individual field region of the object in a scanning fashion;collecting the charged second individual particle beams which emerge or emanate from the object on account of the first individual particle beams;projecting via the projection path exactly one of the multiplicity of second individual particle beams fully or partly onto a detection region of the detection unit serving as an autofocus detection region, wherein the projection path particle optics is operated in an autofocus mode of operation which is different from the inspection mode of operation; and generating an individual image on the basis of data which are obtained or have been obtained by means of signals from the autofocus detection region, said individual image serving as autofocus determination image; andanalyzing the autofocus determination image by applying an image analysis algorithm for determining an optimum focus setting in the illumination path.

2. The method according to claim 1,wherein projecting the exactly one of the multiplicity of second individual particle beams fully or partly onto the autofocus detection region comprises projecting exclusively a cross-talk free part of said second individual particle beam on the autofocus detection region, said cross-talk free part of said second individual particle beam serving as a signal beam.

3. The method according to any one of the preceding claims,wherein in the autofocus mode of operation any crosstalk of the second individual particle beams with the second individual particle beam that is used for generating the autofocus determination image is excluded as a matter of principle.

4. The method according to any one of the preceding claims,wherein the switching step from the inspection mode to the autofocus determination mode comprises increasing a magnification in the projection path without changing a pitch between neighboring detection regions and / or without changing a size of the detection regions and / or without changing a position of or re-assigning the detection regions.

5. The method according to the preceding claim,wherein the magnification in the projection path is increased at least by a factor of 1 ,05, in particular at least by a factor of 1 ,50 or 2,00.

6. The method according to any one of claims 4 to 5, further comprising the following step:using a centrally arranged detection region of the detection unit as the autofocus detection region; and / orusing a centrally arranged second individual particle beam in a raster arrangement of the second individual particle beams for generating a signal beam or for generating the signal beam imaged on the autofocus detection region.

7. The method according to any one of claims 4 to 6,wherein during carrying out the autofocus determination mode with the increased magnification no further dynamic alignment of the projection path particle optics is carried out.

8. The method according to any of claims 1 to 3, further comprising the following steps in the autofocus determination mode:providing a setting to the projection path particle optics in such a way that an intermediate image of the second individual particle beams is generated in an aperture plane; anddeflecting the second individual particle beams in the aperture plane in such a way that only one of the second individual particle beams is not cut off completely by the aperture, but all other second individual particle beams are cut off completely by the aperture, a part of said only one second individual particle beam that is not cut off completely defining a signal beam; andimaging the signal beam on the autofocus detection region.

9. The method according to the preceding claim, further comprising the following step:deflecting the signal beam between the aperture plane and the detection unit for imaging the signal beam on the autofocus detection region.

10. The method according to any one of claims 8 to 9,wherein providing a setting to the projection path particle optics comprises increasing an excitation of a lens system arranged between the object and the aperture plane and decreasing an excitation of a lens system arranged between the aperture plane and the detection unit.

11. The method according to any one of claims 1 to 3, further comprising the following steps in the autofocus determination mode:providing a setting to the projection path particle optics in such a way that a raster arrangement of the multiplicity of second individual particle beams is imaged on the detection unit in such a way that only a cross-talk free area of an outermost second individual particle beam arranged in a first direction of the raster arrangement is imaged on an outermost detection region in the detection unit arranged in a second direction opposing the first direction.

12. The method according to the preceding claim,wherein providing a setting to the projection path particle optics comprises deflecting the multiplicity of second individual particle beams in the second direction.

13. The method according to any one of the preceding claims,wherein the detection unit comprises a combination of a particle detection system and a light detection system, andwherein a detection region comprises a light entrance surface of a light detector.

14. The method according to any one of the preceding claims,wherein the object is a locally charging object.

15. The method according to any one of the preceding claims, further comprising the following steps:determining the optimum focus setting in the illumination path based on the analysis of the autofocus determination images;switching from the autofocus determination mode to the inspection mode; and operating the multi-beam particle microscope in the inspection mode with the determined optimum focus setting in the illumination path.

16. The method according to the preceding claim,wherein determining the optimum focus setting in the illumination path comprises determining a set of optimum focus settings for all first individual particle beams.

17. The method according to the preceding claim,wherein determining the set of optimum focus settings comprises an interpolation and / or an extrapolation; and / orwherein determining the set of optimum focus settings comprises using prior knowledge about an image shell of the multiplicity of first individual particle beams when irradiating the object.

18. The method according to the preceding claim, further comprising the following step:dynamically adjusting the projection path particle optics when operating the multibeam particle microscope in the inspection mode.

19. A multi-beam particle microscope configured for carrying out the method as claimed in any one of the preceding claims.

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