Multibeam system and multibeam generating unit with reduced susceptibility to drift and damage - Patents.com

Abstract

A multi-beam generating unit of a multi-beam charged particle imaging system is disclosed that has reduced susceptibility to drift and extended lifetime. Drift due to X-ray irradiation and thermal load is minimized by at least one combination of shielding elements, cooling members, or improved methods for operating the active multi-aperture element. The lifetime is further improved by annealing methods of the active multi-aperture element or the microelectronic device that forms, for example, the voltage supply unit.

Description

[Technical field]

[0001] The present disclosure relates to a multi-beam generating unit having an array of microlenses or multipole elements for a multi-beam charged particle imaging system. [Background technology]

[0002] WO 2005 / 024881 discloses an electron microscope system operating with a plurality of electron beamlets for parallel scanning of an object to be examined by the electron beamlets. The beamlets of the multibeam charged particle microscope are generated in a multibeam generating unit. The electron beamlets are generated by directing a primary electron beam onto the multibeam generating unit. The multibeam generating unit comprises a first multiaperture element having a plurality of apertures. A portion of the electrons of the electron beam enters the multiaperture element and is absorbed there, and another portion of the beam transmits through the apertures of the multiaperture element, so that downstream of each aperture an electron beamlet is formed whose cross section is defined by the cross section of the aperture. Furthermore, a suitably selected electric field provided in the beam path upstream and / or downstream of the multiaperture element causes each aperture in the multiaperture element to act as a lens on the electron beamlets passing through the aperture, so that each of said electron beamlets is focused into a surface present at a certain distance from the multiaperture element. The surface on which the electron beamlets are focused is imaged by downstream optics onto the surface of the object or sample to be inspected. The primary electron beamlets trigger secondary or backscattered electrons to emerge from the object as secondary electron beamlets, which are collected and imaged onto a detector. Each of the secondary beamlets is incident on a separate detector element, so that the secondary electron intensity detected thereby provides information related to the sample at the location where the corresponding primary beamlet is incident on the sample. A number of primary beamlets are systematically scanned across the surface of the sample, and an electron microscope image of the sample is generated as is typical for scanning electron microscopes. The resolution of a scanning electron microscope is limited by the focal diameter of the primary beamlets incident on the object. As a result, in a multi-beam electron microscope, all beamlets form the same small focal spot on the object.

[0003] The systems and methods illustrated in great detail in WO 2005 / 024881 on the example of electrons are generally very well applicable to charged particles.The invention therefore aims to propose a charged particle beam system which operates with multiple charged particle beams and can be used to achieve higher imaging performances, such as better resolution and a narrower range of resolution for each beamlet of the multiple beamlets.

[0004] Multibeam charged particle microscopes generally use both micro-optical array elements and macroscopic elements in a charged particle projection system. The multibeam generating unit comprises elements for splitting, partially absorbing and influencing the beam of charged particles. As a result, a plurality of beamlets of charged particles in a predefined raster configuration are generated. The multibeam generating unit comprises a first multi-aperture element, a further multi-aperture element and micro-optical elements such as micro-optical deflection elements or multipole array elements.

[0005] The multibeam generating unit includes a first multi-aperture element with a number of apertures. Primary electron beamlets impinge on the first multi-aperture element, a part of the electrons passing through the apertures and forming a number of beamlets. However, a large part is absorbed at the surface of the first multi-aperture element. On the other hand, a number of primary charged particle beamlets are formed from the electrons passing through the apertures. For this task, an array optical element, for example a multipole or multistigmator array or a lens array, is arranged downstream of the first multi-aperture element. The array optical element is provided with a number of apertures, each of which is provided with at least one electrode or coil for influencing each primary electron beamlet individually or in concert. During their operation, it has been observed that the multistigmator array or the lens array undergoes a drift in the performance of the array optical element.

[0006] There can be several reasons for the drift of the performance of the array optics. One particular reason is the drift of the driver of the array optics. For example, a multistigmator array of several, about 100 beamlets, can have 8 or 12 electrodes per beamlet, totaling more than 1000 electrodes, which need to be individually controlled by several voltage supply units, which can be formed by microelectronic devices. Thus, the control architecture of the multiple multipole electrodes of the array optics includes several microelectronic devices in parallel. The microelectronic devices provide several predetermined voltages for the electrodes or predetermined currents for the coils. The drift of the voltage or current can result from thermal drift, charging effects of the microelectronic devices, or local damage generated, for example, by X-ray radiation. The drift can be irreversible damage and can lead to failure of the driver of the electrodes of the multipole elements.

[0007] The first cause of drift in microelectronic devices can be scattering or absorption of primary electrons: a small fraction of the impinging electrons are absorbed or scattered within an aperture, for example in an electrode, leading to charging effects and local changes in voltage.

[0008] The second reason for drift may be secondary radiation generated by primary electrons absorbed in the first multi-aperture element. Secondary radiation includes secondary electrons and electromagnetic radiation including X-rays or gamma radiation. Typically, a voltage supply unit or a microelectronic device for controlling the array optical element is placed near or around the array optical element and may be penetrated by or absorb the secondary radiation. It has been observed that the secondary radiation creates charging effects and ultimately damage to the microelectronic device.

[0009] A third reason for drift may be the power driving the individual microelectronic devices, which can result in high thermal loads.

[0010] The secondary radiation is absorbed only to a small extent in the absorbing layer or the bulk material of the first multi-aperture element. In US Patent Application Publication No. 2019 / 0051494, it is proposed to add an additional second multi-aperture element. However, a second multi-aperture element of sufficient thickness needs to have multiple apertures for the multiple primary charged beamlets generated by the first multi-aperture element. A thick plate increases the amount of scattered charged particles and has a negative effect of the multiple primary beamlets generated in the first multi-aperture element. In modern designs of beam generators with a large number of primary beamlets, multiple multi-aperture elements cannot be given a thickness sufficient to adequately block all the x-rays generated. In addition, the secondary radiation is not directed and is generated in all directions including the multiple apertures. Thus, x-rays may pass through the second multi-aperture element and still cause charging effects on the first multi-aperture element and the electron optical elements downstream of the second multi-aperture element. Additionally, drift in microelectronic devices may also be produced for other reasons mentioned above.

[0011] US Patent Application Publication No. 2017 / 0133194 discloses a particle beam system comprising a particle source, a first multi-aperture plate with a plurality of openings through which the particle beam is formed downstream, a second multi-aperture plate with a plurality of openings penetrated by the particle beam, an aperture plate with an opening penetrated by all particles that also penetrate the openings of the first and second multi-aperture plates, a third multi-aperture plate with a plurality of openings penetrated by the particle beam and with a plurality of field generators providing a dipole or quadrupole field of the beam, respectively, and a controller for providing potentials to the multi-aperture plate and the aperture plate such that the second openings in the second multi-aperture plate act as lenses for the particle beam 3, respectively, and provide adjustable excitation to the field generators. The controller itself is provided outside the vacuum envelope of the system. The controller is connected via a data link to electronic circuitry for generating adjustable voltages provided to the field generators or electrodes. The data link is routed through a seal in the vacuum envelope, so the electronics are located inside the vacuum chamber and are not shielded against secondary radiation, for example against X-rays generated inside the vacuum chamber.

[0012] It is therefore an object of the present invention to provide an improved beam generator for a multi-beam charged particle system with reduced effects of drift, such as thermal drift of charging effects. A further object of the present invention is to provide an improved driver for an improved beam generator for a multi-beam charged particle system with reduced effects of drift or damage. A further object of the present invention is to provide an improved method of operating an improved beam generator for a multi-beam charged particle system with reduced effects of drift, such as thermal drift of charging effects. A still further object of the present invention is to provide improved shielding of secondary radiation, such as x-rays. Summary of the Invention

[0013] The object of the present invention is solved by an improved architecture of a multibeam generating unit of a multibeam charged particle imaging system with reduced susceptibility to drift and extended lifetime. The improved architecture minimizes drift due to X-ray irradiation and thermal loads by a combination of at least one member of the group including shielding elements, cooling members or improved methods for operating active multi-aperture elements. The lifetime is further improved by an annealing method of the active multi-aperture elements or of the microelectronic devices forming, for example, the voltage supply units.

[0014] This patent application claims the priority of German Patent Application Publication No. 10 2022 201 005.1, which has a filing date of January 31, 2022, the entire content of which is incorporated by reference into this patent application.

[0015] In a first embodiment, an improved multi-beam system having a plurality of J primary charged particle beamlets is provided, configured for an improved method of operating an active multi-aperture element. The improved multi-beam system comprises at least one active multi-aperture element and a control unit configured to control the active multi-aperture element. The active multi-aperture element comprises a plurality of J apertures arranged in a raster configuration, the apertures configured to transmit a first plurality of J primary charged particle beamlets through the active multi-aperture element during use. The raster configuration may be a hexagonal or rectangular raster of J apertures, or the apertures may be arranged in a series of circular rings. The active multi-aperture element further comprises a plurality of electrodes, with at least one electrode arranged around each of the apertures. According to a first embodiment, the plurality of electrodes includes at least a first electrode group and a second electrode group. The improved multi-beam system further comprises a first voltage source unit configured to provide a plurality of voltages to the first electrode group during use, and a second voltage source unit configured to provide a plurality of voltages to the second electrode group during use. The first voltage source unit and the second voltage source unit may be part of an active multi-aperture element. The first voltage source unit and the second voltage source unit are connected to a control unit. The control unit is configured to control the plurality of voltages provided by the first voltage source unit and the second voltage source unit during use to the first electrode group and the second electrode group. The control may be achieved, for example, via an image quality monitor or a voltage drift monitor. In the latter example, the improved multi-beam system further comprises a monitoring device connected to at least the first voltage source unit. The monitoring device may monitor drift of the first voltage source unit during use.

[0016] The control unit is further configured to compensate, in use, for drift of at least the first voltage source unit. In one example, the control unit is configured to compensate, in use, for drift of the first voltage source unit by a compensation control signal provided to the second voltage source unit.

[0017] The first electrode group and the second electrode group can be disposed at different angular segments of the raster configuration of the plurality J of apertures. In an alternative example, the first electrode group and the second electrode group can be disposed at different radial segments of the raster configuration.

[0018] The active multi-aperture element can be a microlens array having a single ring electrode at each of a plurality of apertures. The active multi-aperture element can also be a multipole array comprising a plurality of multipole elements having K electrodes disposed around each of a plurality of apertures, where the number of electrodes K of each multipole element is 2, 4, 6, 8 or 12.

[0019] According to a further example of the first embodiment, the improved multi-beam system further comprises a shielding member. The shielding member is arranged and configured to shield the first voltage source unit and the second voltage source unit from secondary radiation. The secondary radiation may be X-ray radiation or secondary electrons, which cause charging effects or damage in the voltage source unit. In one example, the first shielding member is arranged at the beam inlet side of the primary multi-beamlet forming unit and configured with a large aperture. The charged particle beam from the particle beam source is filtered by the large aperture to generate a beam having a diameter and shape of a raster configuration. Thereby, secondary radiation generated in the multi-aperture plate of the primary multi-beamlet forming unit is reduced. Furthermore, due to the large aperture, the first shielding member can be configured with a material composition of sufficient thickness including a high density material so that the secondary radiation generated in the first shielding member cannot penetrate the primary multi-beamlet forming unit below.

[0020] In one example, the active multi-aperture element comprises an outer or peripheral zone in which at least a first voltage source unit and a second voltage source unit are disposed, and an inner or membrane zone having a plurality of apertures J. In this example, a second shielding member can be provided between the outer or peripheral zone and the membrane zone.

[0021] In a further example, at least a first voltage source unit is disposed in a space between the plurality of J openings, and the shielding member is formed as a cap covering the first voltage source unit.

[0022] According to a further example of the first embodiment, an active multi-aperture element of a multi-beam system is provided. The active multi-aperture element comprises a base plate having an inner membrane zone with a plurality of J apertures arranged in a raster configuration, whereby the active multi-aperture element is configured to transmit a plurality of J primary charged particle beamlets during use. The active multi-aperture element further comprises a plurality of J multipole elements, each multipole element comprising one aperture of the plurality of J apertures, each multipole element comprising K electrodes, each multipole element configured to affect one of the primary charged particle beamlets.

[0023] The active multi-aperture element further comprises a plurality of L voltage source units, the plurality of L voltage source units being arranged on the base plate. According to this example, each of the K electrodes of one multipole element is connected to only one of the plurality of L voltage source units. For example, the first plurality of J multipole elements includes at least a first multipole element group and a second multipole element group. The electrodes of the first multipole element group are connected to the first voltage source unit, and the second multipole element group is connected to the second voltage source unit. The first multipole element group and the second multipole element group can be arranged in different annular or ring sections of the raster configuration or in different angular sections of the raster configuration. An example active multi-aperture element includes a second plurality J of multipole elements disposed downstream of the first plurality of multipole elements, each multipole element comprising a plurality K2 of electrodes, with each of the K2 electrodes of a multipole element of the second plurality J of multipole elements being connected to only one of the plurality L of voltage source units. In a further example, each of the K1 electrodes of a multipole element of the first plurality J of multipole elements and each of the K2 electrodes of a corresponding multipole element of the second plurality J of multipole elements are connected to the same voltage source unit.

[0024] In a second embodiment, an improved multi-beam system is provided having at least one shielding or cooling member. The primary multi-beamlet forming unit of the multi-beam system comprises an active multi-aperture element. The active multi-aperture element comprises a plurality of J apertures arranged in a raster configuration, configured to transmit a first plurality J of primary charged particle beamlets through the active multi-aperture element in use. The active multi-aperture element further comprises a plurality of electrodes, with at least one electrode arranged around each of the apertures, and at least a first voltage source unit configured to provide a plurality of voltages to an electrode group of the plurality of electrodes. The primary multi-beamlet forming unit of the multi-beam system according to the second embodiment further comprises a shielding member provided to shield at least the secondary radiation from impinging on the first voltage source unit.

[0025] In one example, the first shielding member is disposed at the beam entrance side of the primary multi-beamlet forming unit and configured with a large aperture, which allows the first shielding member to be configured with a material composition of sufficient thickness including a high density material such that the secondary radiation generated within the first shielding member cannot penetrate into the primary multi-beamlet forming unit below.

[0026] In one example, the active multi-aperture element comprises an outer or peripheral zone in which at least a first voltage source unit and a second voltage source unit are arranged, and an inner or membrane zone having a plurality of J apertures. In this example, a second shielding member can be provided between the outer or peripheral zone and the membrane zone. According to this example, the first voltage source unit is arranged adjacent to the plurality of J apertures, and the second shielding member is arranged between the plurality of J apertures and the first voltage source unit. Such a second shielding member can be elongated parallel to the propagation direction of the primary charged particle beamlet.

[0027] The primary multi-beamlet forming unit according to the second embodiment may further comprise a cooling member configured to reduce thermal drift of the first voltage supply unit. The cooling member may be connected to a heat sink outside the vacuum chamber of the multi-beam system.

[0028] The shielding member may be provided in contact with the first voltage source unit. In such an example, the shielding member may be the same as the cooling member. In one example, the first voltage source unit is disposed between the plurality of J openings, and the shielding member may be attached to the first voltage source unit and configured as a cap or platelet covering the first voltage source unit.

[0029] According to one example, the shielding member comprises a material from a first material group comprising molybdenum, ruthenium, rhodium, palladium or silver. Such a shielding member is preferably configured with a thickness D of more than 1 mm, thereby allowing for more than 80% absorption of secondary radiation.

[0030] According to a further example, the shielding member comprises a material of a second group of materials including tungsten, rhenium, osmium, iridium, platinum, gold or lead. Such a shielding member can be provided with a smaller thickness, for example about 100 μm or more, thereby absorbing more than 80% of the secondary radiation. In both examples, the shielding member can be connected to ground level, thus preventing any charging of the shielding layer.

[0031] In a third embodiment of the present invention, a method for extending the lifetime of an active multi-aperture element is provided. According to the third embodiment, such a method for operating a multi-beam array element comprises: a) performing a series of test tasks to monitor the voltage drift or imaging performance of an active multi-aperture element; b) triggering an annealing step of the active multi-aperture element if the voltage drift or image performance exceeds a predetermined threshold.

[0032] The annealing step comprises at least one of treating the voltage supply unit of the active multi-aperture element with a pulse of voltage VG or thermal annealing at a temperature above 200°C, preferably above 250°C.

[0033] The annealing step may further comprise at least one of treating the membrane zone of the active multi-aperture element with a pulse of voltage VG, or a thermal annealing at a temperature above 250° C., or a low energy plasma treatment.

[0034] The secondary radiation can induce local charging effects in the membrane zone or voltage source unit. The pulsing of the voltage VG reduces the local charging effects. Local damage can be generated by X-ray radiation, for example at the interface between the silicon layer and the silicon oxide layer or at structures in the membrane zone or voltage source unit. At least part of the local damage can be repaired by a thermal or plasma annealing step. However, even if the annealing steps are repeated, the damage will accumulate. Therefore, the method can further monitor the dose of the secondary radiation, the number or frequency of the annealing steps, and predict the end of life of the voltage source unit or the active multi-aperture element.

[0035] In a fourth embodiment, an improved method of operating an active multi-aperture element is provided. The method of operating an active multi-aperture element of a multi-beam charged particle microscope comprises: a) determining, in a calibration step, a plurality of digital control signals for controlling at least a first voltage source unit and a second voltage source unit, the first voltage source unit and the second voltage source unit being configured to provide a plurality of voltages to at least a first electrode group and a second electrode group of a plurality of electrodes of an active multi-aperture element, in one example, the electrode or electrodes forming a plurality of multi-pole elements; b) performing successive inspection tasks; c) monitoring the imaging performance by an image quality monitor of the multi-beam charged particle microscope or monitoring the voltage drift of at least the first voltage supply unit; d) triggering a voltage correction step if the imaging performance or voltage drift exceeds a predefined threshold; e) determining a set of compensated digital control signals for controlling at least a second voltage supply unit during the voltage compensation step; and f) providing a compensating digital control signal to at least a second voltage source unit.

[0036] According to the method, the drift in the first voltage source unit is compensated by providing a corrected signal to the second voltage source unit. Thus, the drift of the first voltage source unit is compensated by providing a corrected voltage by the second voltage source unit. According to a fourth embodiment, a set of compensation digital control signals is determined according to at least the voltage drift of the first voltage source unit.

[0037] According to the solution provided by the embodiments of the present invention or any combination thereof, drift of the voltage source unit is effectively reduced by the shielding or cooling member, whereby damage to the multi-aperture element and the voltage source unit by X-ray radiation is minimized. The multi-beam system can further be provided with a voltage monitor. According to the improved operating method and the multi-beam system configured to perform the improved method, the effect of voltage drift of the voltage source unit can be reduced, thereby enabling a longer use of the active multi-aperture plate.

[0038] The above-mentioned annealing method can at least partially reduce or repair local charging and local defects, and can extend the life of the multi-aperture element or voltage supply unit, thus increasing the uptime of the multi-beam charged particle microscope and reducing service or maintenance, including replacement of expensive parts.

[0039] In the examples of the present disclosure and claims, an array of electrostatic elements, such as electrostatic microlenses or electrostatic multipole elements, is described, for which a driving voltage is provided by at least one voltage source unit. However, the active multi-aperture elements can also be configured as magneto-mechanical elements having coils instead of electrodes. In these equivalent examples, the driving current is provided by at least one current source unit, which can comprise, for example, an ASIC or other equivalent microelectronic device. Thus, the coils, driving current or current source units are equivalent means to the electrodes, driving voltage or voltage source units, and the present invention can be easily applied to magneto-mechanical array elements.

[0040] It should be understood that the present invention is not limited to the present embodiments and examples, but also includes combinations and variations of the present embodiments and examples.

[0041] The embodiments of the present disclosure will now be described in more detail with reference to the drawings. [Brief description of the drawings]

[0042] [Figure 1] 1 is a schematic cross-sectional view of a multi-beam charged particle system for wafer inspection according to a first embodiment. [Diagram 2] FIG. 3 shows a multi-beam forming unit 305 with an improved driving architecture. [Diagram 3] FIG. 3 is a top view of the multipole array element 306.2. [Figure 4] FIG. 4 illustrates a section of the multipole array element 306.2 of FIG. [Diagram 5] FIG. 3 shows a section of an alternative multipole array element 306.2. [Figure 6] FIG. 3 is a top view of a further example of a multipole array element 306.2. [Figure 7] FIG. 3 shows a multi-beam forming unit 305 having a shielding member 93. [Figure 8] FIG. 3 shows a further example of a multipole array element 390. [Figure 9] FIG. 1 illustrates a method for operating a multipole array element with extended lifetime. [Figure 10] FIG. 13 illustrates spot aberration induced by global voltage offset of individual voltage supply units. [Figure 11] FIG. 1 illustrates a method for operating a multipole array element with reduced effects of voltage drift. [Figure 12] FIG. 13 shows a multi-aperture element formed as a microlens array comprising multiple ring electrodes. [Figure 13] FIG. 13 shows the vacuum section of the primary multi-beamlet formation unit 305 undergoing thermal or plasma annealing. [Figure 14] FIG. 13 shows a further example of a multi-beam forming unit 305 having a shielding member 93. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] In the exemplary embodiments of the invention described below, functionally and structurally similar components are indicated, wherever possible, by similar or identical reference signs. The multibeam raster unit in this example is described in an illumination beam path in which the charged particles propagate in the positive z-direction, the z-direction pointing downwards. However, the multibeam raster unit can also be applied in an imaging beam path in which the secondary charged particle beamlets propagate in the negative z-direction in the coordinate system of FIG. 1. Still, successive multi-aperture elements are arranged one after the other in the propagation direction of the transmitted charged particle beam or beamlet. The beam entrance side or top side is understood to be the first surface or side of the element in the direction of the transmitted charged particle beam or beamlet, and the bottom side or beam exit side is understood to be the last surface or side of the element in the direction of the transmitted charged particle beam or beamlet.

[0044] The schematic representation of FIG. 1 shows the basic features and functions of a multi-beam charged particle microscope system 1 according to an embodiment of the present invention. It should be noted that the symbols used in the drawings have been selected to symbolize their respective functionality. The type of system shown is that of a multi-beam scanning electron microscope (MSEM or Multi-SEM) that uses multiple primary electron beamlets 3 to generate multiple primary charged particle beam spots 5 on a surface 25 of an object 7, such as a wafer, whose upper surface 25 is located in the object plane 101 of an objective lens 102. For simplicity, only five primary charged particle beamlets 3 and five primary charged particle beam spots 5 are shown. The features and functions of the multi-beamlet charged particle microscope system 1 can be implemented using electrons or other types of primary charged particles, such as ions and in particular helium ions. Further details of the microscope system 1 are given in DE 102020209833.6, filed 5 August 2020, which is incorporated herein in its entirety by reference.

[0045] The microscope system 1 comprises an object illumination unit 100, a detection unit 200 and a beam splitter unit 400 for separating a secondary charged particle beam path 11 from a primary charged particle beam path 13. The object illumination unit 100 comprises a charged particle multi-beam generator 300 for generating a plurality of primary charged particle beamlets 3 and is adapted to focus the plurality of primary charged particle beamlets 3 in an object plane 101, where a surface 25 of a wafer 7 is positioned by a sample stage 500.

[0046] The primary beam generator 300 generates a number of primary charged particle beamlet spots 311 in an intermediate image plane 321, which is typically a spherically curved surface. The positions of the multiple focal points (311) of the multiple primary charged particle beamlets (3) are generated and adjusted in the intermediate image plane (321) by the multibeam generating unit (305) to pre-compensate for the field curvature and field tilt of elements of the object illumination unit (100) downstream of the multibeam generating unit 305.

[0047] The primary beamlet generator 300 includes a source 301 of primary charged particles, such as electrons. The primary charged particle source 301 emits a diverging primary charged particle beam, which is collimated by at least one collimating lens 303 to form a collimated or parallel primary charged particle beam 309. The collimating lens 303 is typically composed of one or more electrostatic or magnetic lenses, or a combination of electrostatic and magnetic lenses. The collimated primary charged particle beam is incident on a primary multi-beam forming unit 305. The multi-beam forming unit 305 essentially comprises a first multi-aperture element or filter plate 304 illuminated by the collimated primary charged particle beam 309. The first multi-aperture element or filter plate 304 comprises a plurality of apertures, which are in a raster configuration for generating a plurality of primary charged particle beamlets 3 generated by the transmission of the collimated primary charged particle beam 309 through the plurality of apertures. The multi-beamlet forming unit 305 in this example comprises two active multi-aperture elements 306.1-306.2, which are located downstream of the first multi-aperture or filter plate 304 with respect to the direction of movement of the electrons in the beam 309. For example, the first active multi-aperture element 306.1 has the function of a microlens array with a number of ring electrodes, each of which is set to a defined potential such that the focal positions of the multiple primary beamlets 3 are adjusted in the intermediate image plane 321. The second active multi-aperture element 306.2 is configured as a multipole array deflector, for example with two, four or eight electrostatic elements for each of the multiple apertures, for example to deflect each of the multiple beamlets individually. The multi-beamlet forming unit 305 according to some embodiments is configured with a terminal multi-aperture element (310). The multi-beamlet forming unit 305 is further configured with an adjacent electrostatic field lens 307, which in some examples is combined with the multi-beamlet forming unit 305. Together with an optional second field lens 308, the multiple primary charged particle beamlets (3) are focused in or close to the intermediate image plane 321.

[0048] In or close to the intermediate image plane 321, a further active multi-aperture element configured as a beam steering multi-aperture element 390 having a plurality of apertures with electrostatic elements, e.g. deflectors, can be arranged to individually steer the propagation direction of each of the plurality of charged particle beamlets 3. The apertures of the beam steering multi-aperture element 390 are configured to have a larger diameter to allow the passage of the plurality of primary charged particle beamlets 3 even if the focal spots 311 of the primary charged particle beamlets 3 are located on the curved intermediate image plane 321. The primary charged particle source 301, the active multi-aperture element 306 and the beam steering multi-aperture element 390 are controlled by a primary beamlet control module 830 connected to the control unit 800.

[0049] A number of focal points of the primary charged particle beamlets 3 passing through the intermediate image plane 321 are imaged by the field lens group 103 and the objective lens 102 into the image plane 101, where the surface 25 of the wafer 7 is located. A decelerating electrostatic field is generated between the objective lens 102 and the wafer surface by applying a voltage to the wafer by the specimen voltage supply (503). The object illumination system 100 further comprises a focusing multi-beam raster scanner 110 close to the first beam crossover 108, capable of deflecting the multiple charged particle beamlets 3 in a direction perpendicular to the propagation direction of the charged particle beamlets. The propagation direction of the primary beamlets throughout this example is in the positive z-direction. The objective lens 102 and the focusing multi-beam raster scanner 110 are centered on the optical axis 105 of the multi-beam charged particle system 1, which is perpendicular to the wafer surface 25. The multiple primary charged particle beamlets 3 forming multiple beam spots 5 arranged in a raster configuration are scanned synchronously across the wafer surface 25. In one example, the raster configuration of the focal spots 5 of the plurality N of primary charged particles 3 is a hexagonal raster of about 100 or more primary charged particle beamlets 3, for example, N=91, N=100, or N=about 300 beamlets. The primary beam spots 5 have a distance of about 6 μm to 15 μm and a diameter below 5 nm, for example, 3 nm, 2 nm, or even less. In one example, the beam spot size is about 1.5 nm and the distance between two adjacent beam spots is 8 μm. At each scanning position of each of the plurality of primary beam spots 5, a plurality of secondary electrons are respectively generated to form a plurality of secondary electron beamlets 9 in the same raster configuration as the primary beam spots 5. The intensity of the secondary charged particle beamlets 9 generated at each beam spot 5 depends on the intensity of the impinging primary charged particle beamlets 3 illuminating the corresponding spot 5, the material composition and topology of the object 7 under the beam spot 5, and the charging condition of the sample at the beam spot 5. The secondary charged particle beamlets 9 are accelerated by an electrostatic field generated by a specimen charging unit 503 between the specimen 7 and the objective lens 102 .The secondary charged particle beamlets 9 are accelerated by an electrostatic field between the objective lens 102 and the wafer surface 25, collected by the objective lens 102, and pass through the first focusing multi-beam raster scanner 110 in the opposite direction to the primary beamlets 3. The secondary beamlets 9 are scan-deflected by the first focusing multi-beam raster scanner 110. The secondary charged particle beamlets 9 are then guided by the beam splitter unit 400 to follow the secondary beam path 11 of the detection unit 200. The secondary electron beamlets 9 are traveling in the opposite direction to the primary charged particle beamlets 3, and the beam splitter unit 400 is configured to separate the secondary beam path 11 from the primary beam path 13, typically by a magnetic field or a combination of a magnetic field and an electrostatic field. Optionally, additional magnetic correction elements 420 are present in the primary and secondary beam paths.

[0050] The microscope system 1 comprises a vacuum chamber 31 for maintaining a vacuum environment for the charged particle beam. The vacuum chamber 31 is shown very diagrammatically. Parts of the vacuum chamber 31 may be formed by a beam tube around a number of primary or secondary charged particle beamlets. Other functional elements of the charged particle optics may be located outside the vacuum chamber 31.

[0051] The detection unit 200 images the secondary electron beamlets 9 onto an image sensor 207, where they form a number of secondary charged particle image spots 15. The detector or image sensor 207 includes a number of detection pixels or individual detectors. For each of the multiple secondary charged particle beam spots 15, the intensity is detected separately, and the material composition of the wafer surface 25 is detected with high resolution of a large image patch of the wafer with high throughput. For example, by a raster of 10x10 beamlets with an 8 μm pitch, an image patch of about 88 μm x 88 μm is generated in one image scan by the focused multi-beam raster scanner 110, with an image resolution of, for example, 2 nm or less. The image patch is sampled by half the beam spot size, and therefore has a pixel count of 8000 pixels per image line of each beamlet, so that an image patch generated by 100 beamlets contains 6.4 gigapixels. Digital image data is collected by the control unit 800. Details of digital image data collection and processing, e.g. using parallel processing, are described in DE 102019000470.1 and US Pat. No. 9,536,702, which are incorporated herein by reference.

[0052] The projection system 205 further comprises at least a second focusing raster scanner 222, which is connected to a scanning and imaging control unit 820. The control unit 800 and the imaging control unit 820 are configured to compensate for residual errors in the positions of the multiple focal points 15 of the multiple secondary electron beamlets 9, such that the positions of the multiple secondary electron focal spots 15 are held constant at the image sensor 207.

[0053] The projection system 205 of the detection unit 200 comprises further electrostatic or magnetic lenses 208, 209, 210 and a second crossover 212 of the multiple secondary electron beamlets 9, where an aperture 214 is located. In one example, the aperture 214 further comprises a detector (not shown) connected to an imaging control unit 820. The imaging control unit 820 is further connected to at least one electrostatic lens 206 and a third deflection unit 218. The projection system 205 may further comprise at least a first multi-aperture corrector 220 having an aperture and an electrode for individually influencing each of the multiple secondary electron beamlets 9, connected to the control unit 800 or the imaging control unit 820, and optional further active elements 216.

[0054] The image sensor 207 is configured with an array of sensing areas in a pattern that matches the raster arrangement of the secondary electron beamlets 9 focused onto the image sensor 207 by the projection lens 205. This allows each individual secondary electron beamlet to be detected independently of the other secondary electron beamlets incident on the image sensor 207. The image sensor can also serve as an image quality monitor for the multi-beam charged particle microscope 1. The image sensor 207 shown in FIG. 1 can be an electron-sensitive detector array, such as a CMOS or CCD sensor. Such an electron-sensitive detector array can include an electron-photon conversion unit, such as a scintillator element or an array of scintillator elements. In another embodiment, the image sensor 207 can be configured as an electron-photon conversion unit or scintillator plate arranged in the focal plane of the multiple secondary electron particle image spots 15. In this embodiment, the image sensor 207 may further comprise relay optics for imaging and guiding the photons generated by the electron-to-photon conversion unit at the secondary charged particle image spot 15 on a dedicated photon detection element, such as a plurality of photomultiplier tubes or avalanche photodiodes (not shown). Such an image sensor is disclosed in U.S. Pat. No. 9,536,702, cited above and incorporated by reference. In one example, the relay optics further comprises a beam splitter for splitting and guiding the light to a first slow photodetector and a second fast photodetector. The second fast photodetector is constituted by an array of photodiodes, such as avalanche photodiodes, that are fast enough to resolve the image signals of the multiple secondary electron beamlets 9 according to the scanning speed of the multiple primary charged particle beamlets 3. The first slow photodetector is preferably a CMOS or CCD sensor that provides a high-resolution sensor data signal that serves as an image quality monitor for the multi-beam charged particle microscope 1.

[0055] While acquiring an image patch by scanning the multiple primary charged particle beamlets 3, the stage 500 is preferably not moved, and after acquiring an image patch, the stage 500 is moved to the next image patch to be acquired. In an alternative embodiment, the stage 500 is moved continuously in a second direction while an image is acquired by scanning the multiple primary charged particle beamlets 3 in a first direction by the focused multi-beam raster scanner 110. The stage movements and stage positions are monitored and controlled by sensors known in the art, such as laser interferometers, grating interferometers, confocal microlens arrays, etc.

[0056] According to an embodiment of the present invention, a plurality of electrical signals are generated, converted into digital image data and processed by the control unit 800. During image scanning, the control unit 800 is configured to trigger the image sensor 207 to detect a plurality of time-resolved intensity signals from the plurality of secondary electron beamlets 9 at predetermined time intervals, and digital images of the image patches from all scanning positions of the plurality of primary charged particle beamlets 3 are accumulated and stitched together.

[0057] The multi-beam generating unit 305 is described, for example, in U.S. Patent Application No. 16 / 277.572, U.S. Patent Application Publication No. 2019 / 0259575, and U.S. Patent Application No. 16 / 266.842, filed February 4, 2019, both of which are incorporated by reference herein. Further details of the multi-beam generating unit 305, which is insensitive to fabrication errors and scattering, are disclosed in PCT / EP2021 / 025095, filed September 3, 2021, which is incorporated by reference herein.

[0058] To enhance the performance of the multi-beam charged particle microscope during use, each of the multiple charged particle beamlets is individually controlled, for example by individual focus correction by multiple individually controlled ring electrodes of the microlens array 306.1 or multiple individually controlled electrodes of the stigmator or multipole array element 306.2. The individual control of the voltages of the multiple electrodes is provided by a programmable control element. A first embodiment of the present invention of an improved control architecture for generating and controlling multiple voltages is shown in FIG. 2. A primary multi-beamlet forming unit 305 comprising multi-aperture elements 304, 306 and 310, each having a membrane arranged in parallel in the membrane zone 199 and a support structure in the support zone 197, is mounted on a carrier or support substrate 271 with the additional function of a support substrate. In this example, the primary multi-beamlet forming unit 305 comprises three active multi-aperture elements 306, including a microlens array element 306.1 and a multipole array element 306.2, each comprising multiple electrodes arranged in each aperture 85. The further active multi-aperture element 306.3 can be, for example, a second multipole array element. At least two ASICs 261.1 and 261.2 are mounted on the support substrate 271. For example, the ASICs form a voltage supply unit for the electrodes of the active multi-aperture elements. ASICs or voltage supply units may be provided for the ring electrodes of the microlens array 306.1 and the electrodes of the multipole array element 306.2. The electrodes are connected to the voltage supply unit 261 by low-voltage wiring connections 257.1 and 257.2. The ASICs are controlled by digital signals via digital signal lines 267.1 and 267.2 and receive power from a power supply (not shown) by low-voltage supply lines 269.1 and 269.2. In this example, only J=25 apertures (85) in the membrane zone 199 and only two ASICs 261.1 and 261.2 are shown. The multiple J of apertures 85 can be larger, for example J=91, J=100 or more, for example J=1000.The number of electrodes of each multipole element can be, for example, 6, 8 or 12 electrodes in each aperture, and the total number of electrodes can easily exceed 500, for example, more than 700, or even more, for example J=1000, with the multipole array element having up to 12000 electrodes. In addition, individual voltages are provided to the J ring electrodes of the active multi-aperture element 306.1 forming the microlens array. Thus, the number of ASICs can also be significantly greater than two, for example, 6, 8 or even up to about 100 ASICs may be required. The ASICs form, for example, a number of digital-to-analog converters (DACs) for providing analog voltages to the electrodes, one DAC for each electrode. In one example, a multibeam raster unit 305 of J=91 beamlets requires at least 728 individual DACs and 728 individual wiring connections 257 to the 728 electrodes.

[0059] FIG. 2a shows a cross section through an example of a primary multi-beamlet forming unit 305 according to the first embodiment. The primary multi-beamlet forming unit 305 comprises a filter plate 304 and several active multi-aperture elements 306.1-306.3 with different functions. The primary multi-beamlet forming unit 305 further comprises a terminal multi-aperture element 310. At the beam entrance or top side 74 a primary electron beam 309 enters. A plurality of primary electron beamlets 3 exit the primary multi-beamlet forming unit 305 at the bottom side or beam exit side 76. Each of the multi-aperture elements 304, 306 and 310 comprises an inner membrane zone 199 with a plurality of apertures 85 arranged in a raster configuration of a plurality of primary electron beamlets 3, generated by the primary multi-beamlet forming unit 305. Each multi-aperture element 304, 306 and 310 further comprises a support zone 197, by which the multi-aperture elements are aligned and mounted relative to each other. The primary multi-beamlet forming unit 305 further comprises a carrier element 271 on which the support zone 197 of at least one multi-aperture element is attached. A number of voltage supply units including ASICs 262.1 and 262.2 are arranged on the carrier element. Microelectronic devices such as the ASIC devices 262.1 and 262.2 generate and provide a number of voltages required for a number of electrodes of the active multi-aperture array elements.

[0060] FIG. 2b shows a top view in the positive z-direction of the primary multi-beamlet forming unit 305. Identical elements are labeled with identical reference symbols. To control the electrodes individually, the electrodes are connected to microelectronic devices such as ASIC devices 262.1 and 262.2 by wire connections 257. Additional wiring for shielding and absorbing layers or sensors can be provided. High voltage is provided by an external controller using connection 251 to the multi-aperture element or connection 253 to the ASIC 261. High voltage connections 251 and 253 can be electrically shielded by a coaxial shielding casing 255. The electrodes require drive voltages that differ by several orders of magnitude, for example between 10V and up to 1 kV. For example, the multi-lens array 306.1 requires J voltage wires to provide about 200V to each of the ring electrodes of the lens array (not shown). The multipole array 306.2 for beam correction and deflection requires, for example, J×8 voltage wires to provide on the order of a few volts with very low noise. Voltages are provided by ASICs 261.1 and 261.2 (only two shown) located within the vacuum, with a digital interface to an array control unit 840. The array control unit 840 is an element of the primary beam path control module 830, which is configured to control the active multi-aperture elements 306.1-306.3.

[0061] Routing of signals and voltage supplies is obtained via UHV flanges (not shown). In one example, the ASICs or voltage supply units 261.1 and 261.2 are further connected to a voltage drift monitor 835 that controls at least a representative voltage or voltage control output 263.1 and 263.2 of each of the ASICs. The voltage drift monitor 835 is connected to an array control unit 840. The array control unit 840 is configured to evaluate the voltage control outputs 263.1 and 263.2 and is configured to compensate for drift of the first voltage supply unit or ASIC 261.1, for example, by calculating a digital control signal provided to the first voltage supply unit 261.1 or the second voltage supply unit 261.2. The array control unit 840 is further connected to an image performance sensor 860, which provides an input to the array control unit 840 to determine digital control signals provided to ASICs 261.1 and 261.2 via digital signal lines 267.1 and 267.2. For example, the astigmatism of individual beamlets is determined, and a corresponding correction for the astigmatism is derived to compensate for the astigmatism by the multipole elements corresponding to the individual beamlets. The architecture given above allows for precise and reliable control of the multipole array elements. Further details are provided below.

[0062] FIG. 3 shows an example of an active multi-aperture element formed as a multipole array 306.2 with a plurality of J apertures 85 arranged in a hexagonal raster configuration. The raster configuration is rotated about the xy axis. Thereby, the rotation of the magneto-dynamic objective lens 102 and other magnetic lenses is pre-compensated. In each of the plurality of apertures, eight electrodes 81 are arranged to form a multipole element 87. In this example, only J=61 apertures with J=61 multipole elements 87 are shown. The multipole array 306.1 is organized into eight sections 273.1-273.8 by section boundaries 275. Each section includes an electrode group 83.1-83.8. The section boundaries are shown in FIG. 4 in the example of a multipole element 87 in aperture 85.59, where electrodes 81.1-81.4 and 81.8 are members of a third electrode group 83.3, and electrodes 81.5-81.2 are members of a fourth electrode group 83.4. For simplicity, only a single aperture and a single multipole element 87 are shown. In one example, the electrodes 81 in the section 273 form a first electrode group connected to a first voltage source unit 261.3. The electrodes in the section 261.4 form a second electrode group connected to a voltage source unit 261.4. Thus, by applying a local compensating voltage offset to the electrodes of the aperture 85.59 of the second electrode group of the section 273.4, the global drift in the voltage source unit 261.3 can be compensated. Thereby, the relative voltage difference between the eight electrodes 81.1-81.8 is maintained and the multipole element 87 operates in an expected and predetermined manner. Fig. 5 shows another example in which the sections are selected such that the electrodes of each multipole element in the aperture 85.59, in this example the multipole element 87, belong to and are connected to only one voltage source unit 261.3. In this example, the global drift of the voltage source unit 261.3 does not affect the relative voltage differences between the eight electrodes 81.1-81.8 and the multipole element 87 operates in the expected and predetermined manner. However, the voltage source unit 261 may also have local voltage drifts and may be composed of multiple microelectronic subunits, such as a multicore architecture. In another example, the sections are not formed as angular sections.

[0063] According to the example of Fig. 5, the multibeam array element 306.2 of the multibeam system (1) comprises an inner membrane zone 199 having a number of J apertures 85 arranged in a raster configuration, which is configured to transmit a number of J primary charged particle beamlets 3 during use. The multibeam array element 306.2 further comprises a number of J multipole elements (87), each multipole element (87) comprising one aperture (85) of the number of J apertures (85), each multipole element comprising a number of K1 electrodes (81.1-81.K), each multipole element (87) being configured to influence one of the primary charged particle beamlets (3). The multibeam array element 306.2 further comprises a number of L voltage supply units, each of the K electrodes (81.1-81.K) of one multipole element (87) being connected to only one of the number of L voltage supply units 261. In one example, a number L of voltage supply units (261) are arranged on a base plate (271) around a raster arrangement of a number J of apertures (85).

[0064] An example with annular sections 273.1-273.5 is shown in Fig. 6. The choice of annular sections is advantageous when one task of the multipole array element 306.2 is to contribute to the focusing ability of the primary multi-beamlet forming unit 305 as well as to compensate for the field curvature and field tilt of the multi-beam charged particle microscope system 1. By allocating the multipole elements 87 to different ring sections, the corresponding voltage source units of the corresponding electrode groups are driven by similar powers. The first electrode group of the first section 273.1 is driven by a larger voltage, for example, to achieve a stronger focusing ability, while the fifth electrode group of the fifth section 273.5 is driven by a lower voltage on average. The fifth electrode group is therefore less affected by, for example, the thermal drift of the first voltage source unit 261.1 of the first electrode group.

[0065] The performance of the multipole array element 306.2 is sensitive to drifts of the voltage source unit 261. The voltage source unit 261 may exhibit voltage drifts over time or may accumulate different voltage offsets over time. Furthermore, inaccurate voltages generated for the multipole array element 306.2 may be the result of local or global damage of the voltage source unit 261. Secondary radiation, such as X-ray radiation, may be one source of different offset voltages or damage. According to a second embodiment of the invention, the effect of secondary radiation is reduced by a shielding member. An example is shown in FIG. 7. In FIG. 7, a thick absorber or first shielding member 93.1 is provided at the entrance side 74 of the filter plate 304. The first shielding member 93.1 has an aperture 91 that is configured to restrict the primary beam 309 to the area required to illuminate the raster of apertures 85. In one example, the aperture 91 has a conical shape with an increasing diameter in the z-direction. Thereby, scattering of the primary electron beam 309 inside the aperture 91 is minimized. The cross section of the aperture 91 can be configured according to the area of ​​the raster configuration, for example by a hexagonal shaped area with a diameter of about 1 mm or more. Thus, the first shielding member 93.1 with a large aperture 91 can be given a large thickness. Thereby, the secondary radiation 901.1 generated upstream of the first shielding plate 93.1 can be effectively prevented from reaching the voltage supply unit 261, reducing voltage drift or damage. Furthermore, the transmission of the secondary radiation 901.2 generated in the first shielding member 93.1 is reduced. The first shielding member 93.1 is configured by a highly conductive material and is connected to the ground level. Thereby, charging, including local surface charges, of the first shielding member 93.1 is avoided.

[0066] The aperture 91, whose size and area are arranged according to the raster of the apertures 85, effectively reduces the beam diameter of the primary charged particle beam 309.1 to a filtered primary beam 309.2, whose size and area are according to the raster of the apertures 85. Thereby, the number of absorbed electrons or the generation of secondary radiation in the filter plate 304 is reduced to a minimum value. The first multi-aperture element or filter plate 304 comprises an absorbing layer made of a dense and highly conductive material. The absorbing layer is connected to the ground level. Most of the primary charged particles of the filtered beam 309.2 are absorbed and the corresponding charge is dissipated to the ground level. However, some primary electrons are still scattered in the filter plate 304 and some secondary radiation 901.3 may still be generated in the filter plate 304. The secondary radiation 901.3, such as X-rays or secondary electrons, may be emitted in all directions. In particular, X-rays may penetrate the thin film zone 199 and the support element 197 of the active multi-aperture element 306, affect the voltage source unit 261, and cause damage or charge drift. In one example, further damage or drift of the voltage source unit 261 is avoided. In this example, the primary multi-beamlet forming unit 305 is provided with a second shielding member 93.2, which is arranged between the multi-aperture elements 304, 306 and 310 and the voltage source element 261. Thereby, the secondary radiation 901.3 generated in the multi-aperture elements 304, 306 and 310 can be effectively prevented from reaching the voltage source unit 261, and the voltage drift or damage of the voltage source unit 261 is reduced. In one example, the shielding member 93.2 is arranged around the multi-aperture elements 304, 306 and 310 and surrounds the multi-aperture elements 304, 306 and 310.

[0067] The primary multi-beamlet forming unit 305 may be configured with at least a first shielding member 93.1, a second shielding member 93.2 or a combination of both. In a further example shown in FIG. 7, still further damage or drift of the voltage source unit 261 is avoided. In this example, the primary multi-beamlet forming unit 305 is provided with a third shielding member 93.3, which is arranged below the multi-aperture elements 304, 306 and 310 and the voltage source element 261. Thereby, the secondary radiation 901.4 generated by the electron-optical elements downstream of the primary multi-beamlet forming unit 305 can be effectively prevented from reaching the voltage source unit 261, and the voltage drift or damage of the voltage source unit 261 is further reduced. In FIG. 7, only one voltage source unit 261 is shown, but as explained above, several voltage source units 261.1-261.N can be arranged around the active multi-aperture array elements 306.1-306.3.

[0068] Secondary radiation in the form of X-ray radiation 901 may generate space charge or local charging effects in semiconductors. Charging effects may accumulate over time and affect the performance of microelectronic devices such as transistors or the capacity of DACs. Thus, charging effects are a source of voltage drift. X-ray radiation 901 may further be absorbed and heat is generated. Changes in the operating temperature of the voltage source unit 261 are a further source of voltage drift. However, even with the shielding member 93, the operating temperature of the voltage source unit 261 is generally influenced by its operating conditions, such as, for example, the current required to charge the electrodes to reach the required voltage. Since the voltage source unit 261 is located inside the vacuum chamber, cooling via heat convection is not possible. Thus, in one example, the voltage source unit 261 in FIG. 7 is physically connected to a cooling member 97. The cooling member 97 may be connected to a heat sink outside the vacuum chamber 31 (see FIG. 1, not shown in FIG. 7). Thereby, the temperature of the voltage source unit 261 may be controlled and the voltage drift due to temperature may be minimized. In one example, the cooling member 97 and the shielding member 93 are identical and formed as a single member.

[0069] The effect of the shielding element 93 is typically described by the absorption coefficient μ of the secondary radiation in question. Attenuation is typically described by the Beer-Lambert law using the following equation: I = I0exp(-μD) where D is the thickness. The measured absorption coefficient μ of the X-ray energy spectrum produced by 30 keV electron irradiation is, for example, 1.1 / mm for aluminum, 14.2 / mm for iron, and 17.5 / mm for copper. However, the generated X-ray spectrum depends on the electron energy and, for example, on the material composition of the filter plate 304. In general, it is advantageous to utilize high density paramagnetic or diamagnetic materials, such as, for example, materials of the first group consisting of molybdenum, ruthenium, rhodium, palladium or silver (element numbers 42, 44-47), or materials of the second group consisting of materials such as tungsten, rhenium, osmium, iridium, platinum, gold or lead (element numbers 74-79 and 82). On the other hand, other materials, such as, for example, silicon or aluminum, are not suitable for blocking X-ray radiation. Typically, the multi-aperture element is formed by microstructuring silicon or silicon compounds. With typical thicknesses of the multi-aperture elements 304, 306 and 310 below 200 μm, only less than 10% of the secondary radiation is absorbed in each multi-aperture element. Even a thick coating of gold, for example 5 μm thick, would still transmit more than 70% of the secondary radiation. Thus, even a thick coating or film is not sufficient to effectively shield the secondary radiation. Such a thick coating would also cause stress bending or deformation of the membrane zone and is therefore not possible.

[0070] According to the second embodiment, the attenuation of the secondary radiation and thus the prevention of charging effects or damage of the voltage source unit is achieved by the shielding elements 93.1-93.3. For example, the first shielding element 93.1 with a large aperture 91 with a diameter of about 1.1 mm and an area according to the raster of the primary beamlets 3 can be made of a sufficiently absorbing and highly conductive material with a sufficient thickness. For example, a sufficient attenuation of the secondary radiation to a ratio of 10E-5 or less is achieved by a shielding element with a thickness of about D=1 mm made from one of the materials of the first material group or the second material group. The second shielding element 93.2 is arranged between the multi-aperture elements (304, 306 and 310) and the voltage source unit 261. A sufficient attenuation of the secondary radiation to a ratio of 10E-5 or less is achieved by a second shielding element 93.2 with a thickness of about D=1 mm made from one of the materials of the first material group or the second material group.

[0071] The shielding member 93 can also be provided by a thick support layer, for example 2 mm thick, made for example from aluminum or silicon, which can be provided with a layer of the second material group having a thickness of about 200 μm, or a layer of the first material group having a thickness of about 300 μm. Of course, other thicker materials are also possible, such as copper or zirconium, or any combination thereof. To increase the conductivity and reduce the surface charge, the shielding member 93 can also be provided with a conductive surface coating, for example by a layer made from copper, gold or lead.

[0072] FIG. 8 shows further examples of shielding members 93.4. In some applications, the spacing between the individual apertures 85 or multi-aperture elements is large enough to allow a microelectronic device, such as the voltage source unit 261, to be placed or directly structured between the apertures. In FIG. 1, an example is given with a multipole array element 390 configured as a beam steering multi-aperture element. With such an element, the telecentricity properties of multiple primary charged particle beamlets can be corrected. In such an example, the shielding member 93.4 made from a material of the second material group allows the shielding member 93.4 to be placed as a covering plate of a cap directly above each voltage source unit. With a material of the second group and a shielding member 93.4 thickness of about 100 μm or more, a suppression of more than 99% is achieved. FIG. 8a shows an xy cross section of a multipole array 390 with multiple apertures, including the apertures labeled 85.1 to 85.8. Between the openings 85.i, a number of small voltage supply units 261.1-261.7 formed as microelectronic devices are arranged, which act as DACs for providing corresponding voltages to the electrodes 81 of the multipole element 87. A number of wiring connections 257 between the voltage supply units 261.i and the electrodes 81 are shown. The further inventive signal lines 267 and voltage lines 269 of FIG. 2 are not shown. FIG. 8b shows a cross section along the line AB with the two openings 85.5 and 85.8. The electrodes 81.54 and 81.88 of the third group are connected to the voltage supply unit 261.3 by connections 257.34 and 257.38. The electrode 81.58 is of the second group, which is connected to the voltage supply unit 261.2 via the line 257.28. The voltage supply unit 261.3 is covered by a shielding member 93.4, which is formed as a cap or plate made of a material of the second group, e.g. tungsten, platinum or lead, having a thickness of about 100 μm or more, and which is configured to cover the voltage supply unit 261.3 from the entrance side of the primary charged particle beamlets 3.5 and 3.8.Thereby, the secondary radiation 901.3 from the filter plate 304 (see FIG. 7) mentioned above is absorbed by more than 99%, minimizing the damage of the charging of the voltage supply unit 261.3. Of course, a further shielding member can be placed on the back side to avoid damage from secondary radiation 901.4 generated downstream of the multipole array 390.

[0073] Fig. 14 shows a further example of the shielding member 93.4. Fig. 14 is similar to Fig. 7, and reference is made to Fig. 7 for its description. As in the example of Fig. 7, the voltage source units 261 are arranged around the multi-aperture elements 304, 306 and 310. In the example of Fig. 14, the shielding member 93.4 is provided to cover the voltage source units 261 as a covering plate or cap directly above each voltage source unit 261. Each cap can cover one or more voltage source units 261, and each cap can have an extension in the z-direction to provide shielding between the primary charged particle beamlets 3 and to mount the cap on a carrier or support substrate 271. Thereby, direct contact between the voltage source units 261 and the shielding member 93.4 can be avoided.

[0074] Throughout these examples, the substrate 271 is shown as a single support substrate for supporting the voltage source unit 261 and at least one multi-aperture plate 306. Of course, it is also possible, for example, that the multi-aperture plate 306 is mounted on a first support substrate 271 and the voltage source unit 261 is mounted on a second support substrate that is mechanically separated from the first support substrate 271 and electrically connected to the first support substrate by a flexible connection similar to the wire connection 257.

[0075] According to the second embodiment example, secondary radiation-induced damage or charge accumulation of the microelectronic circuit of the voltage source unit is reduced to a minimum value. Thereby, the contribution to the voltage drift is significantly reduced. According to the third embodiment of the present invention, a further reduction of the voltage drift induced by charge accumulation is provided. The functioning of the microelectronic semiconductor structure in the presence of secondary radiation, such as secondary electron radiation and X-rays, is affected by charging effects, including, for example, positive space charge accumulation in silicon oxide or surface charging effects at interfaces. Some of the charging effects can be reduced or balanced by applying a pulse of negative voltage -VG. Thus, according to the first embodiment of the third embodiment, a method is provided for balancing or restoring the charging effects of the voltage source unit by applying a pulse of negative voltage -VG to the voltage source unit. However, some charging effects and damages are not reversible by applying a voltage pulse. However, some of these charging effects or damages are reversible by heating the microelectronic device to a temperature above 250° C. Many charging effects or local damages of the voltage source unit can be reduced or completely annealed by such heating for several minutes. According to a second example of the third embodiment, the multi-aperture element 304, 306, 310 or 390 or the voltage supply unit 261 are treated by voltage pulses and a thermal annealing process to reduce charging effects or damage. Thermal annealing can be achieved by a resistive heater or by external heating, for example by IR laser irradiation. Figure 9 shows the method. In a first step M, a series of inspection tasks are performed by the multi-beam charged particle microscope 1 according to the first embodiment. In step D, the control unit interrupts the measurement and optionally starts step A of the annealing procedure. After completing the annealing procedure, the control unit triggers the continuation of the measurement with the next inspection task. The interruption of the inspection task in step D can be triggered by various parameters. According to a first example, the imaging performance indicators of the multi-beam charged particle microscope 1 are monitored.If a deviation in beam quality due to a malfunction of the active multi-aperture element 306 is detected, a trigger signal is generated to start the annealing course A. According to a second example, a representative voltage output is measured by the voltage drift monitor 835 (see FIG. 2). If the voltage drift exceeds a predefined threshold, a trigger signal is generated to start the annealing course A. According to a third example, the trigger signal is generated by a model-based control. An estimated voltage drift or an accumulated exposure dose over time is calculated, and a trigger signal is generated when the estimated voltage drift or the accumulated exposure dose over time exceeds a predefined threshold. The annealing procedure according to step A can include a negative voltage pulse applied to the voltage source unit 261 or a thermal annealing, or a combination of both.

[0076] The method according to the third embodiment can extend the life of the voltage source unit. In optional step C, the success of the annealing process is judged. With the increase in the duration of use or the dose of irradiation of the voltage source unit, irreversible damage accumulates, gradually reducing the performance of the voltage source unit over time. After reaching a certain damage level, even after repeated annealing, a replacement step R can be triggered to replace the multi-beam forming unit 305 or the active multi-aperture element 390.

[0077] The multipole array according to this embodiment can comprise more than 488 electrodes (see FIG. 3). By increasing the number of beamlets J and making the correction of each individual beamlet more precise, for example by a multipole element 87 with 12 electrodes, the number of electrodes supplied with individual voltages can be increased to well over 1000 for a single multipole array 306.2. In advanced multibeam charged particle microscopes, more than one multipole array may be required. In one example, two multipole arrays are required to achieve deflection and angular correction of multiple primary beamlets. In a further example, four or five multipole arrays are required to achieve individual focusing capability of each beamlet. Thus, a very large number of individual voltages must be generated, controlled and provided for the multiple electrodes. According to a fourth embodiment, a method is provided for operating multiple electrodes of an active multi-aperture element, such as a multipole array or a microlens array, by at least two voltage supply units. In one example, an array of octopoles is controlled using multiple voltage source units, for example ASICs forming an array of DACs each with 128 DAC channels as an example. Due to routing constraints, some octopoles are controlled by more than one ASIC (see Fig. 3 and Fig. 4). In such an example, the voltage offset or voltage drift of the first ASIC affects the beam quality or beam deflection. An example is shown in Fig. 10. Fig. 10 shows the effect of a global voltage offset of one voltage source as can be measured by an image quality monitor of the multi-beam charged particle system 1. In this example, the voltage source unit 261.4 of the electrode in the section 83.4 has an offset. Since all electrodes are given a constant global voltage offset, the beam spot 5.n with all electrodes of the corresponding multipole element supplied by the same voltage source unit 261.4 does not show any deviation. However, the focal spot 5.k or 5.m with a corresponding multipole element, which is supplied with voltages by at least two different voltage supply units, exhibits astigmatic behavior and / or deflection due to the voltage offset.From the position and the effect of the deflection or beam shape of the spots 5.1...J, the offset voltage of the voltage supply unit 261 can be determined. The primary electron spots 5.1...J can be measured directly in a mirror imaging method or the beam aberrations can be determined from image acquisition of a calibration test sample.

[0078] According to the example of FIG. 4, the first electrode group 81.1-81.4 and 81.8 are controlled by the first voltage source unit 261.3 and the second electrode group 81.5-81.7 are controlled by the second voltage source unit 261.4. In this example, the voltage offset of the first voltage source unit 261.3 introduces a beam tilt. The beam tilt can be compensated by an equivalent voltage offset of the electrodes 81.5-81.7 of the second electrode group. The method is not limited to only two voltage source units and two corresponding aperture groups. A multi-beam forming unit may require six, eight, ten or even more voltage source units. An example is also shown in FIG. 8, where smaller voltage source units with fewer DAC channels are used. Here, for example, with a 10x10 aperture array, 50 voltage source units with 50 electrode groups may be utilized.

[0079] The method according to the fourth embodiment is described in more detail in Fig. 11. In a first step CM, digital control signals for controlling the voltage source units are determined. The determination can be performed, for example, during a calibration step of the multi-beam charged particle microscope 1, and the imaging performance is optimized by optimized control parameters of the multi-beam charged particle microscope 1, including optimized digital control signals for controlling the voltage source units for the operation of the active multi-aperture element 306, including the multipole array 306.2 or 390. During the implementation of successive inspection tasks in step M, the performance is repeatedly monitored in step PM. If the performance parameters exceed a predefined threshold, a voltage correction step VC is triggered. In a first example, the monitoring step is performed by an image quality monitor. The image quality monitor monitors the imaging performance. In particular, if a reduction in imaging performance is detected for beamlets corresponding to multipole elements 87 that are supplied with voltage by at least two voltage source units, a voltage correction step is triggered. In a second example, the monitoring step utilizes a signal generated by a voltage drift monitor 835 (see Fig. 2). For example, if the representative voltage generated by the first voltage source unit deviates from the predetermined voltage by more than a predetermined threshold, a voltage correction step is triggered, for example a constant voltage offset of the electrodes of the second multipole element group is determined and generated. The predetermined threshold can be, for example, 1V, 0.5V or even less. The corresponding digital control signal for controlling the second voltage source unit is then modified to achieve a compensation digital control signal. Thereby, it is achieved that the second voltage source unit applies a constant voltage offset value to the required electrodes of the second electrode group. Thereby, the imaging performance of the multibeam charged particle microscope 1 is maintained even if the individual voltage source units are subject to different voltage drifts.

[0080] FIG. 12 shows an example of an active multi-aperture element 306.1 with a number of ring electrodes 81 which in use form a number of microlenses. In this example the ring electrodes are grouped into four sections 83.1-83.4. The voltage supplied in use is configured to adjust the focal position of the number of primary beamlets 3 in the curved intermediate image plane 321. Thereby the field curvature of the object illumination unit 100 is compensated. A voltage is provided to each group of sections or electrodes by a separate voltage supply unit 261.1-261.4. A typical voltage is 100V, but larger voltages of up to 200V are possible. The voltage supply units of the microlenses are therefore even more sensitive to thermal drifts. Now, for example, if the individual voltage source unit 261.3 suffers from a drift, for example due to X-ray radiation or thermal drift, the focal spot 311 or the primary charged particle beamlet 3 corresponding to the section 83.3 will have a certain offset from the intermediate image plane 321 and will be out of focus in the object plane 101 of the multi-beam charged particle microscope 1. The voltage drift of the voltage source unit 261.3 can be detected either by the voltage monitor 835 or by the imaging quality monitor. The beamlet corresponding to the section 83.3 provides a lower resolution during imaging, and this resolution can be determined, for example, by the image quality monitor. For example, the voltage drift of the voltage source unit 261.3 can be compensated by providing a modified control signal to the voltage source unit 261.3 or to the voltage source units 261.1, 261.2 and 261.4 by the method described in the fourth embodiment. Thereby, the focusing of the multiple primary beamlets in a single focal plane is maintained. If this focus plane deviates from the image plane 101 , the focus plane can be adjusted by further lens elements of the multi-beam charged particle beam system 1 .

[0081] Due to X-ray radiation, not only the voltage source unit or ASIC 261 but also the active multi-aperture element 306 of the primary multi-beamlet forming unit 305 may accumulate local damage, including local charging effects. The membrane zone 199 of the multi-aperture element is formed, for example, by doped silicon with features made from an insulating layer and silicon dioxide. The interconnection between the electrodes and the voltage source unit or ASIC 261 may be formed by a metal layer. Thus, for example, X-ray radiation may cause local inter-surface defects between silicon and silicon dioxide, which may be the cause of local charging effects, which affect the electric field generated by the electrodes and, therefore, the performance of the active element. At least a large part of the defects or local charging effects can be annealed by thermal or plasma annealing. Figure 13 shows a further example of an annealing operation. Figure 13 is similar to Figure 7, and reference is made to Figure 7 for its description. In addition to FIG. 7, the shielding plates 93.1 and 93.3 further operate as vacuum valves with slides 281.1 and 281.2. In FIG. 13, both valves are shown in the open position, but both valves can be closed by the movable slides 281.1 and 281.2. Thereby, the closure of the primary multi-beamlet forming unit 305 and the isolated vacuum compartment can be generated. The required annealing temperature of about 250° C. or more can be achieved, for example, via infrared irradiation by the irradiation device 285. In one example, the annealing is achieved by a low-energy gas plasma, such as, for example, a hydrogen plasma or a nitrogen plasma, at about 0.1 mbar. The plasma can be operated by the plasma generator 283, for example, with a frequency of 13.56 MHz and a low power of about 10-20 W. By either or both methods, local defects can be at least partially restored and the life of the multi-aperture element can be extended.

[0082] According to the solution provided by the embodiment of the present invention, the drift of the voltage source unit 261 can be effectively reduced by the shielding member 93, by the cooling member 97 or by an improved operation method of the voltage source unit 261, including the optional application of the voltage monitor 835. It is also possible to implement a combination of the shielding member 93 and the cooling member 97 with an improved operation method of the active multi-aperture element. Thereby, the effect of the voltage drift can be reduced and damage to the multi-aperture element and the voltage source unit by X-ray radiation is minimized. Thereby, for example, the life of the voltage source unit or the active multi-aperture element can be extended. By the above-mentioned annealing method, local charging and local defects can be at least partially reduced or restored and the life of the multi-aperture element or the voltage source unit can be extended. Thus, the uptime of the multi-beam charged particle microscope is increased and service or maintenance, including replacement of expensive parts, is reduced. [Explanation of symbols]

[0083] 1. Multi-beamlet charged particle microscope system 3 Primary charged particle beamlet, or multiple primary charged particle beamlets 5 Primary charged particle beam spot 7 Objects or Water 9 Secondary electron beamlet forming multiple secondary electron beamlets 11 Secondary electron beam path 13 Primary beam path 15 Secondary charged particle image spot 25 Wafer surface 31 Vacuum Chamber 74 Beam entrance or upper side 76 Bottom side or beam exit side 81 Multipolar electrode 83 Electrode Group 85 Aperture 87 Multipole Element 91 Large Opening 93 Shielding material 97 Cooling Materials 100 Object Irradiation Unit 101 Objective surface 102 Objective Lens 103 Field Lens Group 105 Optical axis of multi-beamlet charged particle microscope system 108 First Beam Crossover 110 Condensing multi-beam raster scanner 197 Support Zone 199 Membrane Zone 200 detection units 205 Projection System 206 Electrostatic Lens 207 Image Sensor 208 Imaging Lens 209 Imaging Lens 210 Imaging Lens 212 The Second Crossover 214 Aperture Filter 216 Active Elements 218 Third Deflection System 220 Multi-aperture corrector 222 Second Deflection System 251 High Voltage Wiring Connection 253 Ground wire 255 Coaxial Shielding and Insulation 257 Low Voltage Wiring Connection 261 Voltage Source Unit 263 Voltage Control Output 267 Digital Signal Line 269 ​​Low Voltage Supply Line 271 Career Elements 273 classification 275 Section Boundary 281 Vacuum valve slider 283 Plasma Generator 285 Infrared Source 300 Charged Particle Multi-Beamlet Generator 301 Charged Particle Source 303 Collimating Lens 304 Filter Plate 305 Primary Multi-Beamlet Formation Unit 306 Active Multi-Aperture Element 307 First Field Lens 308 Second Field Lens 309 Primary Electron Beam 310 Termination Multi-Aperture Element 311 Primary electron beamlet spot 321 Intermediate image plane 390 Active or Beam Steering Multi-Aperture Elements 400 Beam splitter unit 420 Correction element 500 Sample Stage 503 Specimen voltage supply source 800 Control Unit 820 Imaging control module 830 Primary Beam Path Control Module 835 Voltage Drift Monitor 840 Array Control Unit 860 Image performance sensor 901 Secondary radiation

Claims

1. A multi-beam system (1) having an active multi-aperture element (306, 390) and a control unit (840) configured to control the active multi-aperture element (306, 390), the active multi-aperture element (306, 390) comprising: a plurality of J apertures (85) arranged in a raster configuration, configured, in use, to transmit a first plurality J of primary charged particle beamlets (3) through said active multi-aperture element (306, 390); a plurality of electrodes (81) comprising at least one electrode (81) arranged around each of said openings (85), said plurality of electrodes (81) comprising a first group of electrodes (83.1) and a second group of electrodes (83.2); a first voltage supply unit (261.1) configured, in use, to provide a plurality of voltages to said first group of electrodes (83.1); a second voltage supply unit (261.2) configured, in use, to provide a plurality of voltages to said second electrode group (83.2); Equipped with said first voltage supply unit (261.1) and said second voltage supply unit (261.2) are connected to said control unit (840); the control unit (840) is configured, in use, to control a plurality of voltages provided by the first voltage source unit (261.1) and the second voltage source unit (261.2) to the first electrode group (83.1) and the second electrode group (83.2); A multi-beam system (1), wherein the control unit (840) is further configured to compensate, during use, for drift of the first voltage supply unit (261.1).

2. 2. The multi-beam system (1) of claim 1, wherein the control unit (840) is configured, during use, to compensate for the drift of the first voltage source unit (261.1) by means of a compensation control signal provided to the second voltage source unit (261.2).

3. 3. The multi-beam system (1) according to claim 1 or 2, wherein the first electrode group (83.1) and the second electrode group (83.2) are arranged in different angular sections (273.1-273.8) of the raster arrangement.

4. 3. The multi-beam system (1) according to claim 1 or 2, wherein the first electrode group (83.1) and the second electrode group (83.2) are arranged in different radial sections (273.1-273.5) of the raster arrangement.

5. The multi-beam system (1) according to any one of claims 1 to 4, further comprising a monitoring device (835) connected to at least the first voltage supply unit (261.1) and configured to monitor a drift of the first voltage supply unit (261.1).

6. The multi-beam system (1) according to any one of claims 1 to 5, wherein the active multi-aperture element (306, 390) is a microlens array (306.1) having a ring electrode (81) at each of the plurality of apertures 85.

7. The multibeam system (1) according to any one of claims 1 to 5, wherein the active multi-aperture element (306, 390) is a multipole array (306.2) comprising a plurality of multipole elements (87) having K electrodes (81.1 to 81.K) in each of the plurality of apertures 85, the number K of electrodes of each multipole element (87) being 2, 4, 6, 8 or 12.

8. The multi-beam system (1) according to any one of claims 1 to 7, further comprising a shielding member (93) provided for shielding the first voltage supply unit (261.1) and the second voltage supply unit (261.2) from secondary radiation.

9. 9. The multi-beam system (1) of claim 8, wherein the shielding member (93) is provided between the first voltage source unit (261.1) and the second voltage source unit (261.2) of the active multi-aperture element (306, 390) and a membrane zone (199), and comprises the plurality of J apertures (85).

10. 9. The multi-beam system (1) according to claim 8, wherein the first voltage supply unit (261.1) is arranged between the plurality of J openings (85), and the shielding member (93.4) ​​is formed as a cap covering the first voltage supply unit (261.1).

11. A primary multi-beamlet formation unit (305) for a multi-beam system (1), comprising: an active multi-aperture element (306) comprising a plurality of J apertures (85) arranged in a raster configuration, the plurality of J apertures (85) being configured, in use, to transmit a first plurality of J primary charged particle beamlets (3) through said active multi-aperture element (306, 390); a plurality of electrodes (81) comprising at least one electrode (81) arranged around each of said openings (85); at least a first voltage supply unit (261) configured to provide a plurality of voltages to a group of electrodes (83) of said plurality of electrodes (81); a shielding element (93) provided for shielding at least the first voltage supply unit (261) from secondary radiation; a primary multi-beamlet forming unit (305) comprising:

12. The primary multi-beamlet forming unit (305) of claim 11, wherein the first voltage source unit (261) is positioned adjacent to the plurality of J apertures (85), and the shielding member (93) is positioned between the plurality of J apertures (85) and the first voltage source unit (261).

13. The primary multi-beamlet forming unit (305) according to claim 12, wherein the shielding member (93) is elongated parallel to a propagation direction of the primary charged particle beamlets (3).

14. The primary multi-beamlet forming unit (305) according to claim 11, wherein the shielding member (93) is provided in contact with the first voltage supply unit (261).

15. The primary multi-beamlet forming unit (305) of claim 14, wherein the at least a first voltage supply unit (261) is disposed between the plurality of J apertures (85).

16. The primary multi-beamlet forming unit (305) according to any one of claims 11 to 15, further comprising a cooling member (97) configured to reduce thermal drift of the first voltage supply unit (261).

17. The primary multi-beamlet forming unit (305) according to claim 16, wherein the shielding member (93) and / or the cooling member (9) are connected to a heat sink.

18. The primary multi-beamlet forming unit (305) according to any one of claims 11 to 17, wherein the shielding member (93) comprises a material from a first material group comprising molybdenum, ruthenium, rhodium, palladium or silver.

19. The primary multi-beamlet forming unit (305) of claim 18, wherein the shielding member (93) has a thickness D greater than 1 mm.

20. The primary multi-beamlet forming unit (305) of any one of claims 11 to 17, wherein the shielding member (93) comprises a material from a second material group including tungsten, rhenium, osmium, iridium, platinum, gold or lead.

21. The primary multi-beamlet forming unit (305) of claim 20, wherein the shielding member (93) has a thickness D greater than 100 μm.

22. The primary multi-beamlet forming unit (305) according to any of claims 11 to 21, wherein the shielding member (93) is connected to a ground level.

23. 1. A method of operating an active multi-aperture element (306.2, 390) with increased lifetime, comprising: - performing a series of test tasks to monitor the voltage drift or imaging performance of said active multi-aperture element (306); - triggering an annealing step of said active multi-aperture element (306) if the voltage drift or image performance exceeds a predefined threshold; A method comprising:

24. 24. The method of claim 23, wherein the annealing step comprises at least one of treating the active multi-aperture element (306) with a pulse of a voltage V, a thermal annealing at a temperature above 200° C., for example 250° C., or a low-energy plasma treatment.

25. 25. The method according to claim 23 or 24, wherein the annealing step comprises at least one of a treatment of a voltage supply unit (261) of the active multi-aperture element (306) with a pulse of a voltage V, or a thermal annealing at a temperature above 200° C., for example 250° C.

26. The method according to any one of claims 23 to 25, further comprising the step of predicting an end of life of the voltage supply unit (261) or the active multi-aperture element (306).

27. A method of operating an active multi-aperture element (306.2, 390) of a multi-beam charged particle microscope (1), comprising: - determining, in a calibration step, a number of digital control signals for controlling at least a first and a second voltage source unit (261, 261.3, 261.4) configured to provide a number of voltages to at least a first and a second electrode group (83, 83.1, 83.2) of the plurality of electrodes (81.1-81.K) of the plurality of multipole elements (87); - performing successive inspection tasks; - monitoring the imaging performance of said multi-beam charged particle microscope (1) or the voltage drift of at least said first voltage supply unit (261.3); - triggering a voltage correction step if said imaging performance or said voltage drift exceeds a predefined threshold; - determining a set of compensated digital control signals for controlling at least said second voltage supply unit (261.4); providing said compensation digital control signal to at least said second voltage supply unit (261.4); A method comprising:

28. 28. The method of claim 27, wherein the set of compensated digital control signals is determined according to the voltage drift of the at least a first voltage supply unit (261.3).

29. An active multi-aperture element (306.2, 306.3, 390) for a multi-beam system (1), comprising: - a base plate (271), an inner membrane zone (199) having a number J of apertures (85) arranged in a raster configuration, the number J being configured to transmit, in use, a number J of primary charged particle beamlets (3); a plurality of J multipole elements (87), each arranged multipole element (87) comprising one aperture (85) of said plurality of J apertures (85), each multipole element (87) comprising a plurality of K electrodes (81.1 to 81.K), each multipole element (87) being configured to influence one of said primary charged particle beamlets (3); a plurality of L voltage source units (261), each of the K electrodes (81.1 to 81.K) of one multipole element (87) being connected to only one of the plurality of L voltage source units (261); Equipped with an active multi-aperture element (306.2, 306.3, 390), in which said plurality L of voltage supply units (261) are arranged on said base plate (271);

30. 30. The active multi-aperture element (306.2, 306.3, 390) of claim 29, wherein the first plurality J of multipole elements includes at least a first multipole element group and a second multipole element group, the electrodes of the first multipole element group being connected to a first voltage source unit and the second multipole element group being connected to a second voltage source unit.

31. 31. The active multi-aperture element (306.2, 306.3, 390) of claim 30, wherein the first group of multipole elements and the second group of multipole elements are arranged in different annular sections of the raster configuration.

32. An active multi-aperture element (306.2, 306.3, 390) according to claims 29 to 31, further comprising a second plurality J of multipole elements arranged downstream of the first plurality of multipole elements, each multipole element comprising a plurality K2 of electrodes, each of the K2 electrodes of one multipole element of the second plurality J of multipole elements being connected to only one of the plurality L of voltage source units.

33. 33. The active multi-aperture element (306.2, 306.3, 390) of claim 32, wherein each of the K1 electrodes of a multipole element of the first plurality of J multipole elements and each of the K2 electrodes of a multipole element of the second plurality of J multipole elements are connected to the same voltage supply unit.

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