Method of adapting focal settings and system thereof

Description

[0001] Method of adapting focal settings and system thereof

[0002] TECHNICAL FIELD

[0003] The present invention relates to a method of adapting focal settings, in particular a method of adapting focal settings for charged particle imaging using a multi-beam system, and a multibeam system configured to perform the method of adapting focal settings.

[0004] BACKGROUND

[0005] A scanning electron microscope (SEM) produces images by focusing a beam of electrons onto a sample surface. The electron beam including primary electrons is scanned across the sample, and the resulting signal, often generated by secondary electrons, is detected.

[0006] A multi-beam scanning electron microscope (MSEM) utilizes multiple electron beams to contemporaneously scan and image a sample, allowing for higher throughput and faster data acquisition compared to traditional single-beam SEMs.

[0007] In electron microscopy, achieving and maintaining sharp focus and minimal distortion is critical for obtaining clear, high-resolution images. Autofocus (AF) in electron microscopy is the automatic adjustment of the focus to ensure that the electron beam is correctly focused on the sample surface. This is especially important in MSEM, as each beamlet may need individual focusing due to differences in focal lengths across the array. Autostigmation (AS) addresses astigmatism, an aberration where the electron beam shape becomes elliptical rather than circular, causing image distortion. Astigmatism is corrected by adjusting stigmators to ensure that the beam remains symmetrical, allowing for uniform and high- quality imaging.

[0008] Focus adjustments for each beam in some implementations are achieved through focus maps, which ensure consistent focus across the entire array, enhancing image quality and resolution. Focus maps model the necessary focus settings by measuring values at specific support points, typically located outside the region of interest. These settings are then interpolated across the region of interest, eliminating the need for individual focus measurements at each point. Moreover, also extrapolation beyond the focus map region may be of interest. These maps account for the generally non-planar nature of large-scale sample surfaces. A similar method may be applied to other focal settings, e.g. stigmation.

[0009] Over time, electron microscopes tend to drift, meaning that focal settings determined at time t0may gradually diverge from their initial value, leading to a focal setting error between the predictions, e.g. provided by a focal setting map, and the actual requirements. Adjusting the focal setting map may be achieved through autofocus and autostigmation steps during the acquisition using additional support points within the region of interest. However, such additional support points may interfere with the imaging, as the illumination of the region of interest contaminates the surface, complicating accurate data interpretation. Moreover, performing additional autofocus and autostigmation steps during the acquisition takes time and reduces throughput.

[0010] Accordingly, there is a need for a method to determine a focal setting error and adapt focal settings during a measurement to improve image resolution while maintaining high throughput and low contamination.

[0011] SUMMARY

[0012] The problem is solved by a method of adapting focal settings according to claim 1 and a multi-beam system according to claim 18. The dependent claims define further embodiments.

[0013] The method of adapting focal settings comprises providing a focal setting map of a region of interest of a sample comprising a set of focal settings, acquiring an image by scanning a subregion of the region of interest with a plurality of charged particle beams with focal settings provided by the focal setting map, determining a focal setting error and updating the focal setting map based on the focal setting error.

[0014] The focal setting map may be obtained by measuring focal values at specific support points, e.g. around a region of interest. For example, focal values may be determined by analyzing image contrast, edge sharpness and / or focal distance. Based on the focal values, focal settings may be interpolated within the region of interest. Also, extrapolation beyond the region of interest may be possible. For example, focal values may comprise focus and / or stigmation values. Focal settings may for example comprise settings of at least one component of the multi-beam system that may be suitable for correcting aberrations, e.g. by focusing and / or stigmation. For example, focal settings may serve for controlling the at least one component, which may be configured to, for example, change a distance and / or position of a charged particle beam focal point and / or adjust a form of a charged particle beam. Such components may, for example, include lenses, e.g. objective lenses, collimation lenses or other particle-optical lenses, stigmators, field generators and / or deflection elements. The region of interest refers to a specific area or surface of the sample that is being targeted for investigation. The subregion of the region of interest may be at least part of the region of interest, i.e. may also correspond to the whole region of interest, or may correspond to only a part thereof. In some embodiments, the region of interest may comprise a plurality of subregions, wherein, in some embodiments, the subregions may overlap. In other embodiments, the region of interest may comprise only one subregion, wherein the subregion may be equal to the region of interest.

[0015] The image of the subregion of the region of interest may be acquired by the plurality of charged particle beams, wherein each individual charged particle beam may scan at least part of the subregion, e.g. a respective field of view (FOV), such that the image of the subregion comprises a plurality of FOVs. The multiple FOVs (mFOV) may be arranged in a spatial pattern to thereby define a composite FOV, such that the image of the subregion may also be referred to as composite FOV or mFOV.

[0016] In some embodiments, the plurality of charged particle beams may comprise a plurality of electron beams. In other embodiments, the plurality of charged particle beams may comprise a plurality of ion beams.

[0017] Focal setting errors may serve as a measure of how much focal settings predicted by the focal setting map deviate from focal settings that may actually be required to maintain high resolution.

[0018] In some embodiments, the focal setting error may be determined based on the image of the subregion and at least one reference image. In some embodiments, the at least one reference image may be acquired by scanning at least part of the subregion with at least part of the plurality of charged particle beams.

[0019] The at least one reference image may be acquired with focal settings that differ from the focal settings provided by the focal setting map.

[0020] In some embodiments, only one reference image may be acquired. In other embodiments, a plurality of reference images may be acquired, wherein each reference image of the plurality of reference images may be acquired with different focal settings.

[0021] To obtain focal settings that differ from the focal settings provided by the focal setting map, for example, a defocus may be applied. Ideally, a charged particle beam focal point may converge precisely on the sample surface. In a defocus operation, the charged particle beam focal point may be set above or below the sample surface, which may be achieved through altering settings of, for example, objective lenses and / or deflection elements. In another example, focal settings may be changed by changing stigmator settings and / or field generator settings. In some embodiments, when acquiring a plurality of reference images, settings may be changed in incremental steps.

[0022] By intentionally changing focal settings for the at least one reference image, an image with specific focal values may be anticipated. A difference between the actual focal values of the at least one reference image and the anticipated focal values may serve as a measure for the focal setting error.

[0023] The at least one reference image may be acquired by scanning only part of the subregion using only part of the plurality of charged particle beams, such that the at least one reference image comprises fewer FOVs than the image of the subregion. In another embodiment, the at least one reference image may be acquired by scanning at least part of the subregion with a single charged particle beam of the plurality of charged particle beams, such that the reference image comprises a single FOV. In yet another embodiment, the at least one reference image may be acquired by scanning the subregion using the plurality of charged particle beams, such that the at least one reference image comprises the same FOVs as the image of the subregion.

[0024] During charged particle imaging, the sample may become subject to contamination, i.e. deposition of unwanted materials on the sample surface such as carbonaceous material or residues present in the sample chamber, which may appear as a dark, hazy layer in the image and may obscure fine details. This may lead to reduced resolution or poor image quality, hampering correct interpretation of data. Therefore, in some embodiments, it may be preferable to minimize beam exposure or scanning duration to the essential minimum. Moreover, scanning only at least part of the subregion for reference reduces measurement time, such that throughput may be maintained. In some embodiments, the at least part of the subregion represented in the at least one reference image may be at least ten times smaller than the subregion. For example, the at least one reference image may be at least 100 times smaller than the subregion. In other embodiments, the at least one reference image may comprise the whole subregion such that the at least one reference image and the image of the subregion are of the same size or comprise the same number of FOVs.

[0025] The focal setting error may be determined based on the part of the subregion included in both the image of the subregion and the at least one reference image. In some embodiments, the at least one reference image with different focal settings and / or focus position may be acquired subsequent to the acquiring of the image of the subregion. By acquiring the reference image subsequent to the acquiring of the image of the subregion, i.e. performing a post-image scan, image data may not be subject to contamination.

[0026] In other embodiments, e.g. if contamination is not an issue, the at least one reference image may be acquired prior to the acquiring of the image of the subregion.

[0027] In some embodiments, the focal setting error may be determined based on the focal setting map and the image of the subregion. For example, reference beam values may be calculated based solely on the image of the subregion. Such reference beam values may, for example, be calculated based on data representing properties of the multiple charged particle beams, e.g. beam position, beam shape and / or aberration thereof. The reference values may be determined using a gradient method, Fourier analysis, or autocorrelation. In some embodiments, the focal setting error may be determined based on the reference beam values and beam values provided by the focal setting map. This way, no further reference image may be required. Further details are provided in US 10 388487 B2, filed on August 20, 2019, which is hereby fully incorporated by reference.

[0028] The determining a focal setting error may comprise analyzing sensor and / or system model data. Integrating data from various sources may be effective to determine focal settings. Additional working distance sensors, such as interferometers or charge-coupled device (CCD) sensors, may provide real-time, independent data that reflects the current state of the sample and system. A working distance sensor, for example, may measure the sample’s height or variations in surface topography, enabling adjustments based on detected surface changes. Similarly, a projector CCD image, which captures the projected beam’s characteristics, may reveal focus-related discrepancies directly within the imaging plane.

[0029] In addition to real-time sensor inputs, data from predictive models may facilitate adjustments that account for gradual changes over time, such as system drift. Drift models, which may track the known drift rate of the system in nanometers per second (nm / sec), provide timebased predictions of how much the system’s optimal focus setting is expected to shift. These models may be statistical or deterministic and may be developed using historical data or known properties of the system, such as temperature-induced expansion or contraction of components. In some embodiments, the focal setting error may be determined based on a combination of data obtained from reference image analysis, sensor data and / or data obtained from predictive models. For example, the data from various sources may be combined or integrated into a dataset to determine a focal setting error.

[0030] In some embodiments, the focal setting map may be updated each time after acquiring an image of a subregion, before the plurality of charged particle beams may be directed to another subregion of the region of interest. In other embodiments, the focal setting map may only be updated after a predetermined number of acquired images. In other embodiments, the focal setting map may only be updated if the focal setting error exceeds a predetermined threshold. Therefore, in some embodiments, the number of images acquired before the focal setting map is updated may vary. This way, the time to attain focus may be reduced and thus throughput may be further improved and / or contamination may be further reduced.

[0031] In some embodiments, the method may further comprise determining a drift rate of the focal setting error. In some embodiments the method may further comprise predicting exceedance of a predetermined threshold based on the drift rate, wherein the focal setting map is updated if the focal setting error exceeds the predetermined threshold. Thereby, the focal setting map may be updated before the focal setting error exceeds a critical value such that a high image resolution may be maintained. For example, based on the drift rate, the time another correction step is needed may be calculated and / or a number of image acquisitions prior to updating the focal setting map may be determined based thereon. The number of image acquisitions until updating the focal setting map may change during the acquisition.

[0032] In some embodiments, the focal setting map may be updated using a Kalman filter, wherein the Kalman filter may be based on a weighted average of the focal setting error. The focal setting error may be determined for each FOV, wherein each focal setting error may be subject to measurement uncertainty. The Kalman filter may update the focal setting map based on averaging the focal setting errors across these FOVs, taking the measurement uncertainties into account.

[0033] In other embodiments, the focal setting map may be updated using a moving average filter, wherein the moving average filter may operate by averaging focal setting errors obtained from a predetermined number of focal setting error determinations. For example, focal setting errors may be determined for a number of FOVs, and the moving average filter may update the focal setting map based on the average across these FOVs. In another example, a focal setting error may be determined for each individual charged particle beam, and the moving average filter may update the focal setting map based on the focal setting errors for each FOV associated with a particular charged particle beam.

[0034] The multi-beam system comprises a plurality of charged particle beams, a sample stage, a processing device comprising a memory and a control processor, wherein the processing device is configured to provide a focal setting map of a region of interest of a sample comprising a set of focal settings, control the multi-beam system to acquire an image by scanning a subregion of the region of interest with the plurality of charged particle beams with focal settings provided by the focal setting map, determine a focal setting error and update the focal setting map based on the focal setting error.

[0035] BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG. 1 schematically illustrates one embodiment of a multi-beam system.

[0037] FIG. 2 illustrates a method according to an embodiment.

[0038] FIG. 3 illustrates a region of interest of a sample comprising a plurality of subregions.

[0039] FIG. 4 (including subfigures 4A to 40) illustrates an example of an imaging condition.

[0040] FIG. 5 illustrates a subregion comprising a plurality of FOVs.

[0041] FIG. 6 illustrates an absolute focal setting error over a series of images acquired with and without adapted focal settings.

[0042] DETAILED DESCRIPTION

[0043] In the following, embodiments 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 application is not intended to be limited by the embodiments described hereinafter or by the drawings, which are taken to be illustrative only.

[0044] The drawings are to be regarded as being schematic representation and elements illustrated in the drawings are not necessarily shown to scale. Rather, the various elements are represented such as a function in general purpose becomes 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. Functional blocks may be implemented in hardware, firmware, software or a combination thereof.

[0045] Throughout the figures and the description, same reference numbers are used to describe same features or components.

[0046] Hereinafter, techniques will be primarily explained in the context of an MSEM. However, techniques disclosed herein can be equally applicable to other types of multi-beam charged particle microscopes, e.g., multi-beam ion microscope.

[0047] FIG. 1 is a schematic illustration of an MSEM 1 , according to one embodiment of a multibeam system. Further information relating to such MSEMs and components used therein, such as, for instance, particle sources, multi-aperture plate and lenses, can be obtained from the international patent applications WO 2005 / 024881 , WO 2007 / 028595, WO 2007 / 028596, WO 2011 / 124352 and WO 2007 / 060017 and the German patent applications DE 102013 016 113 A1 and DE 102013 014 976 A1, the disclosure of which in the full scope thereof is incorporated by reference in the present application.

[0048] The MSEM 1 uses a plurality of charged particle electron beams (also referred to as beamlet or simply beam) for imaging a sample 7. The MSEM 1 generates a plurality J of primary beams 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 beams 9.1 , 9.2, 9.3, and are subsequently detected.

[0049] Each one of the primary and secondary beams 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.

[0050] The primary beams 3.1, 3.2, 3.3 are formed by electrons which are incident on a surface 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.

[0051] The sample 7 to be examined can be placed on a sample support table 505 and can be of any desired type, e.g., a semiconductor wafer or a semiconductor mask, and can comprise an arrangement of miniaturized elements.

[0052] The surface of the sample 7 is arranged in a sample plane 101 of an objective lens system 102 of a first particle optical unit 100 (also referred to as illumination system). 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 ten nanometers, for example four nm or less. The focusing of the primary beams 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 102020 125 534 B3, the entire content of which is herewith incorporated in the disclosure.

[0053] The plurality J of primary beams 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 beams 3.1, 3.2 and 3.3 with corresponding focus points 5.1 , 5.2 and 5.3 are shown in FIG 1).

[0054] In practice, the plurality of beams 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, J = 100 x 100, or J = 512 x 512. The beams can furthermore be arranged in any shape, for instance hexagonal. Exemplary values of the pitch between the incidence locations and FOVs are 1 micrometer, 10 micrometers, or more, for example 40 micrometers.

[0055] The number of primary and secondary beams 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 beams (e.g., beams 3.1 and 9.1) over the sample 7 in the respective FOV.

[0056] The primary beams 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 of the sample 7 are shaped by the objective lens system 102 to form the secondary beams 9.1 , 9.2, 9.3. Secondary electrons included in the secondary beams 9.1 , 9.2, 9.3 are used for imaging.

[0057] The MSEM 1 , at a detection side, provides a detection beam path for guiding the plurality of secondary beams 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 beams 9.1, 9.2, 9.3 towards a spatially resolving detector system 600.

[0058] The imaging with the secondary electron imaging system 200 is strongly magnifying such that both the pattern of the primary beams on the wafer surface and the size and shape of focal points of the primary beams 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.

[0059] The primary beams 3.1, 3.2, 3.3 are generated, at the illumination side, in an imaging vacuum chamber 300 comprising a particle source 301 , at least one collimation lens 303, a multi-aperture arrangement 305 (which is generally optional) and 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 multi-aperture arrangement 305. The particle source 301 includes a photocathode. The photocathode may be held by a frame. The particle source 301 operates based on photoemission. For each primary beam 3.1 , 3.2, 3.2, a respective laser beam (not shown in FIG. 1) is incident on the photocathode.

[0060] The multi-aperture arrangement 305 includes a multi-aperture plate (MAP) 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 MAP 304 and form the plurality J of primary beams 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 beams 3.1, 3.2, 3.3. A multi-aperture arrangement 305 sometimes has at least a further MAP 306, 310 that may include beam deflection means, for example a lens array, a stigmator array, or an array of deflection elements. Such beam deflection means may individually deflect each of the multiple primary beams 3.1 , 3.2, 3.3.

[0061] As a general rule, using a multi-aperture arrangement such as the multi-aperture arrangement 305 is optional. In some scenarios, the MSEM 1 may not include a multiaperture arrangement in a beam path of the primary beams 3.1, 3.2, 3.3. In particular, the use of a multi-aperture arrangement such as the multi-aperture arrangement 305 limits the degrees of freedom with which the pattern of primary beams 3.1 , 3.2, 3.3 can be adjusted. For instance, individual beam positions of the beam pattern may not be shifted relative to each other, because otherwise the beams cannot pass through the multi-aperture arrangement. On the other hand, various disclosed techniques benefit from the degrees of freedom offered by an extended photocathode surface across which the incident laser beams can be freely repositioned, thereby also resulting in a repositioning of the generated primary beams 3.1 , 3.2, 3.3. Accordingly, various techniques are based on the finding that it can be beneficial to tailor beam characteristics of the primary beams 3.1 , 3.2, 3.3 an optical domain rather than an electron domain. In other words, by using one or more adjustable optical elements for each of the incident laser beams, the properties of the laser beams can be adjusted, thereby also adjusting the properties of the primary electron beams 3.1 , 3.2, 3.3. The degrees of freedom available for adjusting the primary beams 3.1, 3.2, 3.3 by adjusting the laser beams in the optical domain are significantly higher than reference implementations which use multi-aperture arrangement such as the multi-aperture arrangement 305.

[0062] A first field lens 308.1 and a second field lens 308.2 focus each of the primary beams 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 aberration of the imaging system arranged downstream of the intermediate image surface 321.

[0063] The 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 beams 3.1, 3.2, 3.3 is formed there. Typically, the surface of the sample 7 is arranged in the sample plane 101 , and the focal spots 5.1 , 5.2, 5.3 are correspondingly formed on the object surface 25. The plurality of primary beams 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 beams 3.1, 3.2, 3.3 collectively and synchronously such that the plurality of focus spots 5.1 , 5.2, 5.3 are scanned jointly and 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 beams 3.1 , 3.2, 3.3.

[0064] The objective lens system 102 and the projection lenses 205 provide a 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 and the second particle optical unit, while the field lenses 103, 308.1 and 308.2 belong only to the first particle optical unit 100, and the projection lenses 205 belongs only to the secondary electron imaging system 200. A beam divider 400 is arranged in the beam path of the first particle optical unit 100 between the field lens 103 and the objective lens system 102. The beam divider 400 is also part of the second optical unit in the beam path between the objective lens system 102 and the projection lenses 205.

[0065] 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 beams 9.1 , 9.2, 9.3 transmit during use the first deflection scanner 110 in opposite direction and the scanning movement of the secondary beams 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 beams 9.1,

[0066] 9.2, 9.3, the collective beam deflector 222 is arranged in the secondary electron beam path.

[0067] 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 beams 9.1 , 9.2,

[0068] 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 beams 9.1, 9.2, 9.3 such that spots 15 of the beams 9 remain at constant position on the imaging plane 225. Thereby, each secondary beam 9 is kept within the area of a set of detection elements, which is assigned to the individual secondary beam 9.

[0069] The secondary electron imaging system 200 includes electron-optical lenses 205.1 to 205.5 to adjust a focus plane of the secondary beams 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 electrostatic lens elements or stigmators. With the electron-optical lenses 205.1 to 205.5, the secondary beams 9.1, 9.2, 9.3 can be focused into the imaging plane 225 of the secondary electron imaging system 200.

[0070] 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 beams 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 beams 9.1 , 9.2, 9.3 in the imaging plane 225. Thereby, the pattern of the plurality of secondary beams 9.1 , 9.2, 9.3 can be 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 beams 9.1 , 9.2, 9.3 can be compensated by suitably applying control signals to (driving) the magnetic lenses 205.4 and 205.5. The secondary electron imaging system 200 - in the illustrated example - includes further corrective elements, specifically a multi-aperture plate 216.

[0071] The MSEM 1 furthermore is associated with a processing device 800 configured both for controlling the individual particle optical components of the multiple particle beam system and for evaluating and analyzing the signals obtained by the detector 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. Generally, processing device 800 may be implemented in hardware, firmware, software or combinations thereof, and may include one or more processors, application-specific integrated circuits, microcontrollers or other programmable or hardwired entities capable of performing the functions described herein. 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 beam 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 of the sample 7. During use, the extraction field decelerates the primary electrons of the primary beams 3.1, 3.2, 3.3 before the object surface is reached and generates an additional focusing effect on the plurality of primary beams 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 of the sample 7. Further, the processing device 800 includes the scanning control unit 860 for the raster scanning.

[0072] The processing device, e.g. processing device 800 comprising memory 890 and control processor 880 is configured to provide a focal setting map of a region of interest of a sample, e.g. sample 7, comprising a set of focal settings. For example, the set of focal settings may comprise settings of at least one component suitable for correcting aberrations, e.g. focus and / or stigmation. The processing device is further configured to control the multi-beam system, e.g. MBSEM 1, to acquire an image by scanning a subregion of the region of interest with the plurality of charged particle beams with focal settings provided by the focal setting map, determine a focal setting error, and update the focal setting map based on the focal setting error. In particular, processing device 800 may be configured, e.g. programmed, to implement the methods and techniques described further below.

[0073] The detector system 600 includes a plurality of sets of detection elements with one set of detection elements for each secondary beam 9, for providing strongly magnified images for each FOV. During use, each set of detection elements is configured to record the intensity signal of the assigned secondary beam 9. The plurality of intensity signals for the plurality of secondary beams 9.1 , 9.2, 9.3 is transferred to the 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.

[0074] Figure 2 illustrates a method according to some embodiments. At a step S1 , the method comprises providing a focal setting map of a region of interest of a sample.

[0075] As an example, Figure 3 illustrates an area 501 of a sample, e.g. sample 7, comprising a region of interest 500 comprising a plurality of subregions SR.i. The focal setting map of the region of interest 500 may be generated by measuring focal values at a set of support points 50, for example using conventional autofocus and / or autostigmation techniques. In some embodiments, the focus, stigmation and / or other focal values, e.g. focal distance, measured across the set of support points 50 are used to detect any focal discrepancies caused by for example sample tilt, uneven surfaces and / or varying thickness. The support points 50 may be located outside the region of interest 500, such that pre-illumination may not contaminate the region of interest 500. In some embodiments, the support points 50 may also be located within the region of interest 500. Then, from the measured focal values at the support points 50, focal settings for the region of interest 500 may be interpolated or extrapolated, i.e. focal settings are predicted and provided by the focal setting map for each subregion SR.i of the region of interest 500. Interpolation methods may include linear, polynomial or spline interpolation. Extrapolation methods may include analytic continuation.

[0076] Step S2 of Figure 2 comprises acquiring an image by scanning a subregion of the region of interest with a plurality of charged particle beams with focal settings provided by the focal setting map. As an example, Figure 4 illustrates a scanning operation of the plurality of primary charged particle beamlets 3 during an image acquisition of the subregion SR. The scanning operation control module 860 is configured to provide during use a scanning signal to scanning deflector 110. Thereby, each primary charged particle beamlet 3 is deflected by the collective multi-beam raster scanner 110 such that the corresponding focus spot 5.i is scanned over at least part of the subregion SR, e.g. over one FOV 245. i, which may also be referred to as image patch (Figure 4a). Each FOV 245. i has a diameter AP of for example 1 pm to 60 pm. The scanning operation comprises a scanning of a plurality of parallel image scanning lines 241 along scanning direction 143.1 for image acquisition. At the end of each image scanning line 241 , each beamlet 3 is moved back to the starting position of a next scanning line, which is also called “flyback” 243. During image acquisition along image scanning lines 241 , the scanning operation is controlled to achieve a predetermined dwell time at each image point, with for example 8000 images points per image scanning line 241. The time for flyback 243 can be much shorter than a line aquisition, for example 40 ns, 1 ps or 10 ps in total. Figure 4b shows the parallel operation of a plurality of primary charged particle beamlets 3 to acquire an image of the subregion SR of a region of interest, e.g. region of interest 500, consisting of a plurality of FOVs 245. i. The image of the subregion SR may also be referred to as composite FOV or multi FOV (mFOV) as it may be composed of a plurality of FOVs.

[0077] According to an example, the scanning operation control module 860 is configured to change the first scanning direction 143.1 into a second scanning direction 143.2 (Figure 4c). In the example, the second scanning direction 143.2 is opposite to the first scanning direction 143.1 , but other scanning directions, for example a second scanning direction inclined by an angle, are possible as well.

[0078] After image acquisition, in a step S3 of Figure 2, a focal setting error is determined. Focal setting errors may serve as a measure of how much focal settings predicted by the focal setting map deviate from focal settings that may actually be required.

[0079] In some embodiments, the focal setting error may be determined based on the image of the subregion and at least one reference image. As an example, Figure 5 illustrates the image of the subregion SR comprising a plurality of FOVs 245. i. The at least one reference image may, for example, be acquired by scanning only part of the subregion using only part of the plurality of charged particle beams. For example, the at least one reference image may be acquired by scanning a plurality of FOVs, such as FOVs 245.1 , 245.2, and 245.3, forming reference image 700. In other embodiments, the at least one reference image may comprise a single FOV. In some embodiments, the at least one reference image 700 may be acquired subsequent to the acquiring of the image of the subregion SR. In other embodiments, the at least one reference image 700 may be acquired prior to the acquiring of the image of the subregion SR.

[0080] In some embodiments, the at least part of the subregion represented in the at least one reference image, e.g. reference image 700, may be at least ten times smaller than the subregion SR. For example, the at least one reference image may be at least 100 times smaller than the subregion SR. In other embodiments, the reference image may comprise the whole subregion SR.

[0081] The at least one reference image may be acquired with at least part of the plurality of charged particle beams with focal settings that differ from the focal settings provided by the focal setting map. In some embodiments, only one reference image may be acquired. In other embodiments, a plurality of reference images may be acquired, wherein each reference image of the plurality of reference images may be acquired with different focal settings.

[0082] To obtain focal settings that differ from the focal settings provided by the focal setting map, for example, a defocus may be applied. Ideally, a charged particle beam focal point may converge precisely on the sample surface. In a defocus operation, the charged particle beam focal point may be set above or below the sample surface, which may be achieved through altering settings of, for example, objective lenses and / or deflection elements. In another example, focal settings may be changed by changing stigmator settings and / or field generator settings. In some embodiments, when acquiring a plurality of reference images, settings may be changed in incremental steps.

[0083] The focal setting error may be determined based on the part of the subregion included in both the image of the subregion and the at least one reference image. For example, the focal setting error of the subregion SR may be determined based on FOVs 245.1, 245.2 and 245.3 included in both the subregion SR and the reference image 700. By intentionally changing focal settings for the at least one reference image, an image with specific focal values may be anticipated. For example, the image of the subregion provides focal values of FOVs 245.1 , 245.2 and 245.3 determined using focal settings provided by the focal setting map. By changing the focal settings, changes in focal values of the FOVs 245.1, 245.2, and 245.3 may be anticipated. By analyzing the reference image 700 comprising FOVs 245.1 , 245.2 and 245.3, actual focal values may be determined. The actual focal values may be compared to the anticipated focal values and a focal setting error may be determined based on a difference between the actual focal values and the anticipated focal values. In some embodiments, the determining a focal setting error may comprise analyzing sensor and / or system model data. Additional working distance sensors, such as interferometers or charge-coupled device (CCD) sensors, may provide real-time, independent data that reflects the current state of the sample and system. A working distance sensor, for example, may measure the sample’s height or variations in surface topography, enabling adjustments based on detected surface changes. Similarly, a projector CCD image, which captures the projected beam’s characteristics, may reveal focus-related discrepancies directly within the imaging plane.

[0084] In addition to real-time sensor inputs, data from predictive models may facilitate adjustments that account for gradual changes over time, such as system drift. Drift models, which may track the known drift rate of the system in nanometers per second (nm / sec), may provide time-based predictions of how much the system’s optimal focus setting is expected to shift. These models may be developed using historical data or known properties of the system, such as temperature-induced expansion or contraction of components.

[0085] Updating the focal setting map takes time, which may increase throughput time or reduce throughput. Therefore, it may not be necessary to update the focal setting map after each image acquisition and / or determination of focal setting errors. At a step S4 of Figure 2, it may be determined whether the focal setting error exceeds a predetermined threshold and whether the focal setting map may need to be updated.

[0086] If, after image acquisition of, for example, subregion SR1, the focal setting error does not exceed a predetermined threshold, the plurality of charged particle beams may be directed to another subregion of the region of interest, for example, subregion SR2, to acquire another image by scanning the another subregion of the region of interest with a plurality of charged particle beams according to step S2 of Figure 2. In this case, the focal settings may be provided by the focal setting map as previously determined.

[0087] The focal setting error as determined in step S3 may be stored, e.g. in memory 890, for further analysis. For example, in some embodiments, the method may further comprise determining a drift rate of the focal setting error. In some embodiments the method may further comprise predicting exceedance of a predetermined threshold based on the drift rate, wherein the focal setting map is updated if the focal setting error exceeds the predetermined threshold. Thereby, the focal setting map may be updated before the focal setting error exceeds a critical value such that a high image resolution may be maintained. If the focal setting error does exceed a predetermined threshold, the focal setting map may be updated based on the focal setting error, as illustrated in Figure 2 step S5.

[0088] In some embodiments, the focal setting map may be updated using a Kalman filter, wherein the Kalman filter may be based on a weighted average of the focal setting error. The focal setting error may be determined for each FOV 245. i, wherein each focal setting error may be subject to measurement uncertainty. The Kalman filter may update the focal setting map based on averaging the focal setting errors across these FOVs, taking the measurement uncertainties into account.

[0089] In other embodiments, the focal setting map may be updated using a moving average filter, wherein the moving average filter may operate by averaging focal setting errors obtained from a predetermined number of focal setting error determinations. For example, focal setting errors may be determined for a number of FOVs, and the moving average filter may update the focal setting map based on the average across these FOVs 245.1 , 245.2 and 245.3. In another example, a focal setting error may be determined for each individual charged particle beam, and the moving average filter may update the focal setting map based on the focal setting errors for each FOV associated with a particular charged particle beam, e.g. beam 5.1.

[0090] Subsequently to the updating of the focal setting map, the plurality of charged particle beams may be directed to another subregion of the region of interest to acquire another image by scanning the another subregion of the region of interest with a plurality of charged particle beams with focal settings provided by the focal setting map according to step S2 of Figure 2, wherein the focal setting map may now be an updated focal setting map.

[0091] Figure 6 illustrates the effect of the method. Plot A and plot B represent absolute focal setting errors for images acquired using a MBSEM. Plot A (circles) represent absolute focal errors of images acquired with focal settings provided by the focal setting map, wherein the focal setting has not been updated throughout the measurement. With increasing number of image acquisitions, the absolute focal setting error increases such that a drift becomes apparent. Plot B (squares) represent absolute focal errors of images acquired with focal settings provided by the focal setting map, wherein the focal setting map has been updated throughout the measurement based on the focal setting error and using a Kalman filter as detailed above, such that the absolute focal setting error is limited in comparison to Plot A.

Claims

Claims1 . A method of adapting focal settings, comprising- providing a focal setting map of a region of interest (500) of a sample comprising a set of focal settings,- acquiring an image by scanning a subregion (SR) of the region of interest (500) with a plurality of charged particle beams (3) with focal settings provided by the focal setting map,- determining a focal setting error,- updating the focal setting map based on the focal setting error.

2. The method of claim 1 , wherein the set of focal settings comprises focus and / or stigmation values.

3. The method of claim 1 or 2, wherein the focal setting error is determined based on the image of the subregion (SR) and at least one reference image (700), wherein the at least one reference image (700) is acquired by scanning at least part of the subregion with at least part of the plurality of charged particle beams (3) with focal settings that differ from the focal settings provided by the focal setting map.

4. The method of claim 3, wherein the at least one reference image (700) is acquired by scanning at least part of the subregion with a single charged particle beam of the plurality of charged particle beams (3) with focal settings that differ from the focal settings provided by the focal setting map.

5. The method of claim 3 or 4, wherein the at least part of the subregion represented in the at least one reference image (700) is at least ten times smaller than the subregion.

6. The method of any one of claims 3 to 5, wherein the focal setting error is determined based on the part of the subregion included in both the image of the subregion and the at least one reference image.

7. The method of any one of claims 3 to 6, wherein the at least one reference image (700) is acquired subsequent to the acquiring of the image of the subregion (SR).

8. The method of any one of claims 3 to 6, wherein the at least one reference image (700) is acquired prior to the acquiring of the image of the subregion (SR).

9. The method of claim 1 or 2, wherein the focal setting error is determined based on the focal setting map and the image of the subregion (SR).

10. The method of any one of claims 1 to 9, wherein determining a focal setting error comprises analyzing sensor and / or system model data.

11. The method of any one of claims 1 to 10, wherein the focal setting map is updated only if the focal setting error exceeds a predetermined threshold.

12. The method of any one of claims 1 to 11 , further comprising determining a drift rate of the focal setting error.

13. The method of claim 12, further comprising predicting exceedance of a predetermined threshold based on the drift rate, wherein the focal setting map is updated if the focal setting error exceeds the predetermined threshold.

14. The method of any one of claims 1 to 13, wherein the focal setting map is updated using aKalman filter.

15. The method of claim 14, wherein the Kalman filter is based on a weighted average of the focal setting error.

16. The method of any one of claims 1 to 13, wherein the focal setting map is updated using a moving average filter.

17. The method of claim 16, wherein the moving average filter operates by averaging focal setting errors obtained from a predetermined number of focal setting error determinations.

18. A multi-beam system (1) comprising,- a plurality of charged particle beams (3),- a sample stage,- a processing device (800) comprising a memory (890) and a control processor (880), wherein the processing device (800) is configured to- provide a focal setting map of a region of interest (500) of a sample comprising a set of focal settings,- control the multi-beam-system (1) to acquire an image by scanning a subregion (SR) of the region of interest (500) with the plurality of charged particle beams (3) with focal settings provided by the focal setting map,- determine a focal setting error, - update the focal setting map based on the focal setting error.

19. The multi-beam system (1) of claim 18, wherein the multi-beam system (1) is configured to perform the method of any one of claims 1 to 17.

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