Multi-beam system and multi-beam forming unit having low sensitivity to secondary emission

Abstract

A multi-beam charged particle beam system and a method for operating the multi-beam charged particle beam system with higher precision are provided. This improvement relates to a multi-beam forming unit that has lower sensitivity to secondary electrons, scattered charged particles, and x-ray radiation. Thereby, a plurality of primary charged particle beamlets can be generated with higher precision and a longer lifespan of the multi-beam forming unit. The present system and method are applicable to improved inspection of samples, such as wafer or mask inspection.

Description

Technical Field

[0001] The present invention relates to a multi-beam forming unit having a microlens or an array of multipole elements of a multi-beam charged particle imaging system.

Background Art

[0002] WO 2005 / 024881 discloses an electron microscope system operating with a plurality of electron beamlets for parallel scanning of an object to be inspected. In a multi-beam generation unit, a plurality of beamlets for a multi-beam charged particle microscope are generated. The plurality of electron beamlets are generated by sending a primary electron beam towards the multi-beam generation unit. The multi-beam forming unit includes a first multi-aperture element or a filter plate having a number of apertures. A part of the electrons of the electron beam is incident on the filter plate and absorbed there, and another part of the beam passes through the aperture openings of the filter plate. Electron beamlets whose cross-sections are defined by the cross-sections of the aperture openings are formed. Further, the multi-beam forming unit further includes a lens for each electron beamlet, a deflection plate, or a multi-aperture array element functioning as an aberration correction device. Downstream of the multi-beam generation unit, further electron optical elements are arranged for forming a focal spot of the electron beamlets on the surface of the object or sample. The primary electron beamlets induce secondary electrons or backscattered electrons to diverge from the object as secondary electron beamlets, which are collected and imaged on a detector. Each of the secondary beamlets is incident on a separate detector element such that the secondary electron intensity detected by the detector element provides information about the sample at the location where the corresponding primary beamlet is incident on the sample. The plurality of primary beamlets are regularly scanned over the surface of the sample, and a microscopic image of the sample is generated as in a scanning electron microscope. The resolution of the scanning electron microscope is limited by the focal diameter of the primary beamlets incident on the object. Therefore, in the multi-beam electron microscopy method, it is necessary for all beamlets to form the same minute focus on the object.

[0003] The systems and methods disclosed in detail in the electronic embodiments in International Publication No. 2005 / 024881 are generally known to be very well applicable to charged particles. Correspondingly, the present invention has an object to propose a charged particle beam system that operates with a plurality of charged particle beams and can be used to achieve higher imaging performance such as better resolution and a narrower resolution range for each of a plurality of beamlets.

[0004] A multi-beam charged particle microscope generally uses both a micro-optical array element and a macro element in a charged particle projection system. The multi-beam generation unit includes elements for splitting, partially absorbing, and influencing a beam of charged particles. As a result, a plurality of beamlets of charged particles in a predefined raster configuration are generated. The multi-beam generation unit includes micro-optical elements such as a filter plate and further multi-aperture elements such as a micro-optical deflection element or a multipole array element. These array optical elements, such as a multipole or a multi-aberration correction device array or a lens array, are arranged downstream of the filter plate. The array optical element includes a plurality of apertures each having at least one electrode or coil that individually or in combination influences each primary electron beamlet. The control architecture of the plurality of electrodes or coils of the array optical element includes several microelectronic devices in parallel. The microelectronic devices supply a plurality of predetermined voltages or currents to the plurality of electrodes or coils. During operation, it has been observed that the performance of the array optical element is subject to drift. There can be several reasons for the drift in the performance of the array optical element. Specific reasons may be residual charge and local damage that may occur during the use of the array optical element.

[0005] The first cause of drift can be scattered or absorbed primary electrons. The second cause of drift can be secondary electrons generated, for example, in a filter plate. The third cause is secondary emission, including X-ray emission. These electrons or secondary emissions can be absorbed or scattered at the aperture of the array optical element, which can cause charging effects and local changes in the surface potential of the array optical element. It has also been observed that secondary emission can cause damage to the array optical element. Other effects can be, for example, electron-induced contamination of the aperture of the array optical element.

[0006] Therefore, an object of the present invention is to provide an improved multi-beam generator for a multi-beam charged particle system that has low sensitivity to charging effects and damage.

[0007] International Publication No. 2021 / 180365 discloses certain improvements to multi-beam raster units such as multi-beam generation units and multi-beam deflection units of a multi-beam charged particle microscope. These improvements include the design, manufacture, and adjustment of a multi-beam raster unit including an aperture of a specific shape and dimensions. These improvements enable more precise multi-beam generation and multi-beam deflection or aberration correction.

[0008] WO 2005 / 024881 discloses a particle optical system. The particle optical system includes a charged particle source for generating a beam of charged particles, and a multi-aperture plate disposed in the beam path of the beam of charged particles. The multi-aperture plate has a plurality of apertures formed therein in a predetermined first array pattern. Downstream of the multi-aperture plate, a plurality of charged particle beamlets are formed from the beam of charged particles, and a plurality of beam spots are formed on the image plane of the device by the plurality of beamlets. The plurality of beam spots are arranged in a second array pattern. The multi-aperture plate and a particle optical element for manipulating the beam of charged particles and / or the plurality of beamlets are included. The first array pattern has a first pattern regularity in a first direction, the second array pattern has a second pattern regularity in a second direction that is electro-optically corresponding to the first direction, and the second regularity is higher than the first regularity.

[0009] US 2003 / 0209673 A1 relates to an electron optical system array having a plurality of electron lenses. The electron optical system array includes an upper electrode, a central electrode, and a lower electrode disposed along the paths of a plurality of charged particle beams. The upper electrode, the central electrode, and the lower electrode have a plurality of apertures on the paths of the plurality of charged particle beams. An upper shield electrode is interposed between the upper electrode and the central electrode and has a plurality of shields corresponding to the respective paths of the charged particle beams. A lower shield electrode is interposed between the lower electrode and the central electrode and has a plurality of shields corresponding to the respective paths of the charged particle beams. SUMMARY OF THE INVENTION PROBLEM TO BE SOLVED BY THE INVENTION

[0010] The problem of the present invention is solved by an improved architecture of a charged particle multi-beamlet generator of a multi-beam charged particle imaging system that has low sensitivity to scattered charged particles, charging and damage by secondary electrons and secondary emissions.

[0011] This application claims the priority of German Patent Application No. 102022206314.7 filed on June 23, 2022, and the disclosure in its entirety is incorporated herein by reference.

[0012] The above problems are solved by the present invention as described in the claims and various embodiments of the present invention. The present invention provides an improved multi-beam charged particle imaging system and an improved operating method for such a system. The multi-beam charged particle imaging system includes a charged particle multi-beamlet generator for generating a plurality of primary charged particle beamlets. The multi-beam charged particle imaging system includes an object irradiation system for focusing a plurality of primary charged particle beamlets on the surface of an object at a plurality of irradiation positions. In use, at each irradiation position on the surface of the object, secondary charged particles are generated, from which a plurality of secondary beamlets are formed. The multi-beam charged particle imaging system includes a secondary electron imaging system for focusing a plurality of secondary beamlets and for forming a plurality of focal points of the secondary beamlets on the image plane. The multi-beam charged particle imaging system further includes a detector disposed on the image plane.

[0013] This improved multi-beam system includes at least one active array optical element and a control unit configured to control the active array optical element. The active array optical element includes a plurality J of apertures arranged in a raster configuration configured to transmit a first plurality J of primary charged particle beamlets through the active array optical element in use. The raster configuration can be a hexagonal or rectangular raster of J apertures, or the apertures can be arranged on a series of circular rings. The active array optical element further includes a plurality of electrodes including at least one electrode disposed at the periphery of each of the apertures. The active array optical element can be a microlens array having a single ring electrode in each of the plurality of apertures. The active array optical element can also be a multipole array including a plurality of multipole elements having a plurality K of electrodes disposed at the periphery of each of the plurality of apertures, and the number K of electrodes of each multipole element is 2, 4, 6, 8 or 12.

Means for Solving the Problem

[0014] In the first embodiment, the improved multi-beam system includes an improved charged particle multi-beamlet generator having a shielding multi-aperture plate. The shielding multi-aperture plate is arranged and configured to shield scattered charged particles and secondary emissions from incident on the active array optical element. The secondary emissions can be x-ray emissions or secondary electrons that cause charging effects or damage in the active array optical element. In one example, the shielding multi-aperture plate is arranged on the beam incident side of the active array optical element.

[0015] According to the first embodiment, the improved charged particle multi-beamlet generator is configured to generate a plurality J of primary charged particle beamlets during use. The charged particle multi-beamlet generator (300) according to the present invention includes a multi-beam forming unit (305) having several array optical elements, including a filter plate (304), a shielding multi-aperture plate (306), and a first active array optical element (307) arranged in the propagation direction of the primary charged particles. The filter plate (304) includes a plurality of first apertures (85.1) each having a first diameter D1 for generating a plurality of primary charged particle beamlets (3) during use. During use, a primary charged particle beam (309) is incident on the filter plate (304). Most of the primary charged particles are absorbed by the filter plate (304), and the primary charged particles passing through the first apertures (85.1) form the primary charged particle beamlets (3). The first active array optical element (307) includes a plurality of third apertures (85.3) each having a third diameter D3. In the vicinity of each third aperture (85.3), at least one electrode (81, 82) is arranged to be configured to individually affect the primary charged particle beamlets (3) during use. For example, the first active array optical element (307) can include a plurality of ring electrodes (82) configured to individually generate an electrostatic field for focusing each of the primary charged particle beamlets (3). For example, the first active array optical element (307) can include a plurality of multipole electrodes (81) configured to independently generate an electrostatic field for deflecting, focusing, or correcting aberrations of each of the primary charged particle beamlets (3).

[0016] During use, some primary charged particles are scattered in the filter plate (304) to generate secondary electrons. According to the first embodiment of the present invention, the acceptance angle or capture angle α for the scattered charged particles or secondary electrons is reduced, and the charging or damage of the first active array optical element (307) and any further downstream array optical elements is reduced. With the limited acceptance angle or capture angle α, the amount of scattered charged particles or secondary electrons that can enter the multi-aperture stack portion below the filter plate (304) is reduced. The reduction of the acceptance angle α is realized by the shielding multi-aperture plate (306) and the geometry of the arrangement of the shielding multi-aperture plate (306) within the multi-beamlet generator forming unit (305). The shielding multi-aperture plate (306) is disposed between the filter plate (304) and the first active array optical element (307). The shielding multi-aperture plate (306) includes a plurality of second apertures (85.2), each having a second diameter D2.

[0017] The filter plate (304) has a first thickness L1. L1 is preferably kept at a low thickness of less than 20 μm, for example L1 <= 10 μm. The shielding multi-aperture plate (306) has a third thickness L3 and is arranged at a distance L2 downstream of the filter plate (304). In one embodiment, the distance L2, the thickness L1, and the second diameter D2 define an opening angle α, where tan(α) = D2 / (L1 + L2). In one embodiment, the distance L2, the thicknesses L1 and L3, and the second diameter D2 define an opening angle α, where tan(α) = D2 / (L1 + L2 + L3). According to the first embodiment, the distance L2, the thickness L1 or L3, and the second diameter D2 are selected such that a reduced opening angle with tan(α) < 0.3, preferably tan(α) < 0.25, is formed. The opening angle α is reduced, for example, by an increased distance L2 such as L2 > 70 μm, for example L2 = 70 μm, L2 = 80 μM or L2 = 100 μm. The opening angle α is reduced by an increased thickness L3 such as L3 > 70 μm, for example L3 = 100 μm or L3 = 120 μm. The opening angle α is reduced by a reduced diameter D2 such as D2 < 1.3 × D1, for example D2 <= 40 μm, for example D2 = 38 μm. In one embodiment, at least one of the thickness L3 or the distance L2 is increased and the diameter D2 is reduced. In one embodiment, the sum of the thickness and the distance (L1 + L2 + L3) is selected to exceed 130 μm, for example (L1 + L2 + L3) = 150 μm. In one embodiment, the second diameter D2 is selected according to 1.1 × D1 < D2 < 1.3 × D1.

[0018] The third aperture (85.3) has a third diameter D3, and D2 is selected to be smaller than D3. For example, D3 > 1.6×D1, but D3 may be further selected by D3 >= 1.8×D1. Thereby, the active array optical element (307) is physically shielded by the shielding multi-aperture plate (306), and the number of secondary electrons or scattered primary charged particles incident on the active array optical element (307) is further substantially reduced. For example, D2 is selected in the range of 0.625×D3 <= D2 <= 1.3×D1. For example, D2 = 1.3×D1 and D3 = 1.8×D1. For example, D2 = 1.2×D1 and D3 = 1.7×D1. For example, D2 = 1.1×D1 and D3 = 1.6×D1. For example, by achieving a ratio of about D2 / D3 <= 0.75, the electrodes (81, 82) of the active array optical element (307) are shielded by the shielding multi-aperture plate (306), and the number of secondary electrons or scattered primary charged particles incident on the electrodes (81, 82) is substantially reduced.

[0019] The shielding multi-aperture plate (306) can further include at least an absorption layer or a metal layer (361) covering the beam incident side of the shielding multi-aperture plate (306). According to one embodiment, the absorption layer or the metal layer (361) includes a material from the group of materials including molybdenum, ruthenium, rhodium, palladium or silver, tungsten, rhenium, osmium, iridium, platinum, or gold. Such a layer having a thickness of about 1 to 2 μm can provide sufficient stopping power for scattered charged particles and secondary electrons. The thickness of such a layer can be selected to be thicker so that X-ray radiation is also substantially shielded, for example, more than 10 μm or even more than 20 μm.

[0020] The shielding multi-aperture plate (306) can further be configured for improved reduction of scattered charged particles and secondary electrons. In a first embodiment, in each of the second apertures (85.2) of the shielding multi-aperture plate (306), at least one baffle (369) is formed for absorbing scattered charged particles and secondary electrons. In a second embodiment, the shielding multi-aperture plate (306) is formed by a two-layer structure, and each of the apertures (85.2) with diameter D2 is formed in a first layer having a thickness L3.1 < L3. Thereby, the scattering of charged particles or electrons inside the second aperture (85.2) is reduced. In a third embodiment, each of the second apertures (85.2) of the shielding multi-aperture plate (306) is formed in a conical shape (365) such that the minimum aperture diameter D2 is formed on the bottom side or the beam emission side of the shielding multi-aperture plate (306). The absorption layer (361) can also cover the conical aperture opening (85.2). In a fourth embodiment, each of the second apertures of the shielding multi-aperture plate is formed in a conical shape such that the minimum aperture diameter D2 is formed on the uppermost side or the beam incident side of the shielding multi-aperture plate. Thereby, the surface area where contamination may increase is reduced.

[0021] In one embodiment, the primary multi-beamlet forming unit (305) further includes an additional multi-aperture plate formed as an absorption plate (371) between the filter plate (304) and the shielding multi-aperture plate (306). In one embodiment, the diameter D4 of the fourth aperture (85.4) of the absorption plate (371, 371.1) is selected to be between the first diameter D1 and the third diameter D3, for example, D4 is selected in the range of 1.1×D1 < D4 <= D3.

[0022] The primary multi-beamlet forming unit (305) can include an additional multi-aperture plate including a further active array optical multi-aperture plate and a termination multi-aperture plate (310).

[0023] Due to the improvement according to the first embodiment, the lifespan of the primary multi-beamlet forming unit is extended. Also, the charging or damage of the active array optical element (307) caused by scattered charged particles and secondary electrons is reduced. The influence of scattered charged particles and secondary electrons is further reduced according to the second embodiment of the present invention. According to the second embodiment of the present invention, the charged particle multi-beamlet generator (300) includes a multi-beam forming unit (305) having a filter plate (304) and a first array optical element (307). The charged particle multi-beamlet generator (300) further includes a first absorption plate (371, 371.1) having a plurality of fourth apertures (85.4) with a diameter D4. The first absorption plate (371, 371.1) is disposed upstream of the filter plate (304). The filter plate (304) includes a plurality of first apertures (85.1) each having a first diameter D1 for generating a plurality of primary charged particle beamlets (3) during use. In the first example, the diameter D4 of the fourth aperture (85.4) is selected to be larger than the diameter D1 of the first aperture (85.1). During use, the primary charged particle beam (309) first enters the first absorption plate (371, 371.1). Most of the primary charged particles are absorbed by the first absorption plate (371, 371.1), and the primary charged particles passing through the fourth aperture (85.4) form pre-shaped charged particle beamlets (312). The pre-shaped charged particle beamlets (312) enter the filter plate (304). A part of the pre-shaped charged particle beamlets (312) is absorbed by the filter plate (304), and the primary charged particles passing through the first aperture (85.1) form primary charged particle beamlets (3). The first active array optical element (307) includes a plurality of third apertures (85.3) each having a third diameter D3. At least one electrode (81, 82) is disposed near each third aperture (85.3), and each electrode (81, 82) is configured to individually affect the primary charged particle beamlets (3) during use. For example, the first active array optical element (307) can include a plurality of ring electrodes (82) configured to independently generate an electrostatic field for focusing each of the primary charged particle beamlets (3).For example, the first active array optical element (307) can include a plurality of multipole electrodes (81) configured to independently generate an electrostatic field for deflecting, focusing, or correcting aberrations of each of the primary charged particle beamlets (3).

[0024] The number of scattered charged particles and secondary electrons downstream of the filter plate (304) is substantially reduced by the absorber plate (371, 371.1). In one embodiment, the diameter D4 of the fourth aperture (85.4) of the absorber plate (371, 371.1) is selected to be between the first diameter D1 and the third diameter D3, for example, D4 is selected to be in the range of 1.1×D1 < D4 <= D3.

[0025] The x-ray induced damage of the multi-aperture stack located under the absorber plate is further reduced by coating the upper surface, bottom surface, or all surfaces of the absorber plate (371, 371.1) with a thick metal layer (such as gold) that substantially shields x-ray radiation, or by using an all-metal absorber plate (371, 371.1) that substantially shields x-ray radiation. For example, the absorber plate (371, 371.1) can be coated with a conductive layer that is, for example, 10 μm or even thicker than 20 μm. For example, the absorber plate (371, 371.1) can be made of a suitable metal or metal alloy such as gold, tungsten, or a gold alloy. In one embodiment, the shielding multi-aperture plate (306) includes a material from the group of materials including molybdenum, ruthenium, rhodium, palladium, or silver, tungsten, ruthenium, osmium, iridium, platinum, or gold. In one embodiment, the conductive layer is formed from a material or material composition having a low atomic mass number. This reduces the generation of secondary electrons. For example, the conductive layer (361) is a metal layer, a graphite layer, or a doped semiconductor layer. Suitable materials having a low atomic mass number are aluminum, manganese, copper, or silver.

[0026] In one embodiment, the primary multi-beamlet forming unit (305) further includes a shielding multi-aperture plate (306) having a second aperture (85.2) with a second diameter D2, the shielding multi-aperture plate (306) has a third thickness L3, and 1.1×D1 < D2 <= 1.5×D1. In one embodiment, the primary multi-beamlet forming unit (305) includes a second absorption plate (371.2) between the filter plate (304) and the shielding multi-aperture plate (306). In one embodiment, the diameter D5 of the fifth aperture (85.5) of the absorption plate (371, 371.1) is selected to be between the first diameter D1 and the third diameter D3, for example, D5 is selected in the range of 1.1×D1 < D5 <= D3. The primary multi-beamlet forming unit (305) can include additional multi-aperture plates, including an additional active array optical multi-aperture plate and a terminating multi-aperture plate (310).

[0027] Inside each of the shielding multi-aperture plate (306), the first or second absorption plate (371, 371.1), each aperture (85.2, 85.4, 85.5) can be provided with at least one baffle (369). Each aperture can be formed in a conical shape (365) such that the minimum aperture diameter is formed on the bottom side or the beam exit side of the respective aperture (85.2, 85.4, 85.5). In another embodiment, each aperture can be formed in a conical shape (365) such that the minimum aperture diameter is formed on the upper side or the beam incident side of the respective aperture (85.2, 85.4, 85.5). For example, the absorption plate (371, 371.1, 371.2) can be provided with at least one metal layer (361) on the upper side or the beam incident side of the absorption plate (371, 371.1, 371.2).

[0028] The charged particle multi-beamlet generator (300) according to the second embodiment further includes a supply source (301) of primary charged particles and collimating lenses (303.1, 303.2) disposed upstream of the filter plate (304). According to the first example of the second embodiment, the absorption plate (371, 371.1) is disposed in the beam path of the collimated primary charged particle beam (309) generated by the collimating lenses (303.1, 303.1). For example, the absorption plate (371, 371.1) is disposed between the collimating lenses (303.1, 303.2) and the filter plate (304).

[0029] According to the second example of the second embodiment, the absorption plate (371, 371.1) is disposed within the divergent primary charged particle beam (309) between the supply source (301) and the collimating lenses (301.1, 301.2). The charged particle multi-beamlet generator (300) further includes collection lenses (315.1, 315.2) disposed between the supply source (301) of primary charged particles and the absorption plate (371, 371.1). The absorption plate (371) has a plurality of fourth apertures (85.4) arranged at a first pitch P1 and configured to generate a plurality of preformed beamlets (312) at a first pitch P during use. The first aperture (85.1) of the filter plate (304) is arranged at a second pitch P2. The charged particle multi-beamlet generator (300) is configured to supply a first control signal to the first collection lenses (315.1, 315.2) during use to adjust the flow of the plurality of primary charged particle beamlets (3), to match the pitch P1 of the plurality of preformed beamlets (312) with the second pitch P2 of the filter plate (304), and to form a bundle of parallel preformed beamlets (312). The charged particle multi-beamlet generator (300) further includes a control unit (830) configured to supply a second control signal to the collimator lenses (303.1, 303.2) to adjust the propagation angles of the plurality of preformed beamlets (312). Similarly, the absorption plate (371, 371.1) can be disposed within the converging primary charged particle beam (309) between the supply source (301) and the collimating lenses (303.1, 303.2).

[0030] According to one embodiment, the absorber plate (371, 371.1) includes at least a layer containing a material from a group of materials including molybdenum, ruthenium, rhodium, palladium or silver, tungsten, ruthenium, osmium, iridium, platinum, or gold. For example, a thickness of about 50 μm to 300 μm can provide sufficient stopping power for scattered charged particles and X-rays. In one embodiment, the absorber plate (371, 371.1) comprises a conductive layer formed of a material or material composition having a low atomic mass number. This reduces the generation of secondary electrons. For example, the conductive layer (361) is a metal layer, a graphite layer, or a doped semiconductor layer. Suitable metals with a low atomic mass number are aluminum, manganese, copper or silver.

[0031] With at least one absorber plate (371, 371.1) according to the second embodiment, the influence of scattered primary charged particles and secondary electrons in the primary multi-beamlet forming unit (305) is further reduced. Also, the influence of other secondary emissions such as X-ray radiation can be further reduced, and the lifespan of the primary multi-beamlet forming unit (305) is extended. Also, the charging or damage of the active array optical element (307) due to X-rays, scattered charged particles and secondary electrons is reduced.

[0032] The third embodiment of the present invention provides a system and method configured to reduce the influence of scattered primary charged particles and secondary electrons. According to the third embodiment, an improved method of operating the multi-beam forming unit (305) of the multi-beam charged particle microscope (1) is provided. The method includes generating and supplying a plurality of voltages to the elements of the multi-beam forming unit (305) configured to reduce the influence of secondary electrons.

[0033] The plurality of voltages includes a first voltage U1 supplied to the filter plate (304). The filter plate (304) includes a plurality of first apertures (85.1) for generating or transmitting a plurality of primary charged particle beamlets (3) during use. The plurality of voltages includes a plurality of individual fourth voltages U4 to a plurality of electrodes (81, 82), the electrodes (81, 82) being arranged in the vicinity of a plurality of third apertures (85.3) of the active array optical element (307), and each electrode (81, 82) being configured to generate an electric field for focusing, deflecting or shaping one of the primary charged particle beamlets (3). The plurality of voltages includes a second voltage U2 supplied to the shielding multi-aperture plate (306) arranged between the filter plate (304) and the active array optical element (307). The plurality of voltages including the second voltage U2 is adjusted to realize a repulsive or attractive force on the second electrode (353) arising from the primary charged particle beam at the intersection (317) with the filter plate (304). Generally, the plurality of voltages U0, U1, U2, U3, U4 and U5 are selected to form a potential distribution G along the propagation direction of the primary charged particle beamlet (3) such that at least one of the following three cases occurs. In case A, a potential barrier (in the form of a potential step A1 or a potential maximum A2) is formed along the z-axis to prevent low-energy secondary or scattered electrons from reaching the active array optical element (307). In case B, the potential distribution is adjusted such that the beam-forming aperture or the filter plate (304) is within a potential sink B1 formed along the z-axis. In this situation, secondary or scattered electrons emitted at the energy minimum of the sink cannot exit the potential sink along the z-axis and are effectively prevented from further penetrating into the elements downstream of the filter plate (304). In case C, the voltages U0 and U1 are selected such that during use, an electric field is formed between the absorber plate 371 and the filter plate 304 that attracts secondary or scattered electrons in the negative z-direction away from the active array optical element (307).

[0034] In the first embodiment, the first voltage U1 supplied to the filter plate (304) is selected to create a sink for secondary electrons. For example, U1 is selected to be a voltage greater than U2. In the second embodiment, the second voltage U2 supplied to the shielding aperture plate (306) is selected to create a barrier for secondary electrons. For example, U2 is selected to be smaller than U1, for example, U2 is selected to be negative with respect to the ground level U0, and U2 = U0 - US. The primary charged particles typically have a large kinetic energy of EK = about 5 - 35 keV and correspond to a voltage difference of UE = 5 - 35 kV. To substantially block secondary electrons, a shielding voltage US of less than 0.3% of UE is sufficient. In one embodiment, the shielding voltage US is about 100 V. Such a low voltage US < 0.3% × UE substantially reduces secondary electrons, while the primary charged particles are hardly affected. In one embodiment, the shielding voltage US is about 10 V. Even with such a low voltage US < 0.03% × UE, secondary electrons are substantially reduced, while the primary charged particles are hardly affected.

[0035] The present invention will be better understood with reference to the accompanying drawings.

Brief Description of the Drawings

[0036]

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Mode for Carrying Out the Invention

[0037] Hereinafter, even if not explicitly mentioned in the text, the same reference numerals indicate the same features.

[0038] FIG. 1 is a schematic diagram of a multi-beam charged particle imaging system 1 (also abbreviated as multi-beam system 1) according to an embodiment of the present invention. The multi-beam system 1 uses a plurality of charged particle beams to form an image of an object 7. The multi-beam system 1 generates a plurality J of primary particle beams 3 that strike the inspection object 7 in order to generate interaction products, such as secondary electrons, that are emitted from the object 7 and then detected. The multi-beam system 1 is a scanning electron microscope (SEM) type that uses a plurality of primary electron beams 3 that are incident on the surface of the object 7 at a plurality of locations and generate a plurality of spatially separated primary electron beam focus spots 5 there. The inspection object 7 can be of any desired type, such as a semiconductor wafer or a semiconductor mask, and may include the arrangement of micro elements. The surface 25 of the object 7 is disposed on the object plane 101 of the objective lens 102 of the irradiation system 100.

[0039] The diameter of the minimum beam spot or focal spot 5 formed on the object surface 101 can be made very small. Examples of values for this diameter are less than 5 nanometers, for example 4 nm or 3 nm or less. The focusing of the primary charged particle beamlets 3 for forming the focal spot 5 is performed by the objective lens system 102. In this case, the objective lens system 102 can include a magnetic immersion lens. Further examples of focusing means are described in German Patent No. 102020125534B3, the entire content of which is incorporated into the present disclosure.

[0040] A plurality of focal spots 5 of the primary beam form a regular raster arrangement of the incident locations formed on the object surface 101. The number J of primary beamlets can be 5, 25 or more. In practice, the number J of primary beamlets, and thus the number of incident locations or focal spots 5, can be selected to be significantly larger numbers, for example, J = 10×10, J = 20×30, or J = 100×100. Examples of values for the pitch P between the incident locations are 1 micrometer, 10 micrometers, or beyond that, for example 40 micrometers. For simplicity, in FIG. 1, only three primary beamlets 3.1, 3.2 and 3.3 are shown together with the corresponding foci 5.1, 5.2 and 5.3.

[0041] The primary particles of the primary beamlets 3 hitting the object 7 produce interaction products, such as secondary electrons, backscattered electrons or primary particles whose movement has been reversed for other reasons, emitted from the surface of the object 7. The interaction products are emitted from the surface of the object 7 and shaped by the objective lens 102 to form a secondary electron beamlet 9. For simplicity, throughout the present disclosure, all interaction products are collectively referred to as secondary electrons forming the secondary electron beamlet 9.

[0042] The multi-beam system 1 provides a detection beam path for guiding a plurality of secondary particle beamlets 9 to a secondary electron imaging system 200. The secondary electron imaging system 200 includes several electron optical lenses 205.1 to 205.5 for directing the secondary particle beam 9 towards a spatial resolution particle detector 600. The detector 600 is arranged on an image plane 225. The detector 600 includes a plurality of detection elements. The detection elements can be, for example, diodes such as PMDs, or CMOS detection elements equipped with electron-optical conversion elements, or can be formed as direct electron detection elements. In one embodiment, the detector 600 includes an electron-optical conversion element such as a scintillator plate where secondary electrons are converted into light, and a plurality of light detection elements.

[0043] Imaging by the secondary electron imaging system 200 is strongly magnified such that both the raster pitch of the primary beam on the wafer surface and the size and shape of the focus of the primary beam are greatly enlarged for imaging. For example, the magnification is between 100× and 300× such that 1 nm on the wafer surface is enlarged and imaged between 100 nm and 300 nm. In the process, for example, the image field of a multi-beam system with a diameter of 100 μm is enlarged to about 30 mm.

[0044] The primary particle beam 3 is generated in a beam generation device 300 including at least one particle source 301 (e.g., an electron source), at least one collimation lens 303, a primary multi-beamlet forming unit 305, a first field lens 308.1, and a second field lens 308.2. The particle source 301 generates at least one divergent particle beam 309, which is at least substantially collimated by at least one collimation lens 303 and illuminates the primary multi-beamlet forming unit 305. The primary multi-beamlet forming unit 305 includes at least one first multi-aperture plate or filter plate 304 having a plurality J of apertures formed in a first raster arrangement therein. The particles of the illuminating particle beam 309 pass through the J apertures or openings of the filter plate 304 to form a plurality J of primary beamlets 3. The particles of the illuminating beam 309 hitting the filter plate 304 are absorbed by the filter plate 304 and do not contribute to the formation of the primary beamlets 3. The primary multi-beamlet forming unit 305 typically has at least a further active multi-particle plate 307, such as a lens array, an aberration correction device array, or a deflection element array.

[0045] Together with the field lens 308.1 and the second field lens 308.2, the primary multi-beamlet forming unit 305 focuses each of the primary beamlets 3 so that a focus is formed on the intermediate image plane 321. Alternatively, the beam focus and the intermediate image plane 321 can be virtual. The intermediate image plane 321 can be curved so as to pre-correct the field curvature of an imaging system arranged downstream of the intermediate image plane 321.

[0046] At least one field lens 103 and the objective lens 102 provide a first imaging particle optical unit for imaging the surface 321 where the focal spot 5 of the primary beamlet is formed onto the object surface 101. Typically, the surface 25 of the object 7 is disposed on the object surface 101, and the focal spot 5 is formed on the object surface 25 correspondingly. A plurality of primary beamlets 3 form an intersection 108, and a first scanning deflector plate 110 is disposed in the vicinity thereof. The first scanning deflector plate 110 is used to deflect a plurality of primary beamlets 3 collectively and synchronously so that a plurality of focal spots 5 are simultaneously moved over the surface 25 of the object 7. The first scanning deflector plate 110 is driven by a scanning control unit 860 so that a plurality of two-dimensional image data of the surface 25 are acquired in the inspection operation mode. Further, the multi-beam system 1 can include additional electrostatic deflector plates configured to adjust the positions of the plurality of primary beamlets 3.

[0047] The objective lens 102 and the projection lens 205 provide a secondary electron imaging system 200 for imaging the object surface 101 onto the detection surface 225. Thus, the objective lens 102 is a lens or a lens system that is part of both the first and second particle optical units, while the field lenses 103, 307, and 308 belong only to the first particle optical unit 100, and the projection lens 205 belongs only to the second electron imaging system 200.

[0048] A beam splitter 400 is disposed in the beam path of the first particle optical unit 100 between the field lens 103 and the objective lens system. The beam splitter 400 is also part of a second optical unit in the beam path between the objective lens system 102 and the projection lens 205.

[0049] A first deflection scanner 110 is disposed on a primary electron beam path or an integrated electron beam path. In the embodiment shown in FIG. 1, during use, the secondary electron beamlet 9 passes through the first deflection scanner 110 in the opposite direction, and the scanning movement of the secondary beamlet 9 is partially compensated. Secondary electrons typically have a different kinetic energy compared to primary electrons. Therefore, the scanning movement of the moving irradiation position is only partially compensated. In order to completely compensate for the scanning movement of the secondary electron beamlet 9, a secondary deflection scanner 222 is disposed on the secondary electron beam path. The secondary electron imaging system 200 includes a second focusing beam deflector 222 disposed near the intersection of the secondary electron beamlets 9. The second focusing beam deflector 222 is operated in synchronization with the first beam deflector 110 and compensates for the beam deflection of the secondary electron beamlet 9 so that the focal spot 15 of the secondary beamlet 9 remains at a fixed position on the detection surface 225 during use.

[0050] The secondary electron imaging system 200 includes electron optical lenses 205.1 to 205.5 for adjusting the focal plane of the focal spot 15 of the secondary electron beamlet 9. The electron optical lenses 205.1 to 205.5 are shown as magneto-optical elements, but are not limited to magneto-optical elements and may also include electrostatic lens elements or aberration correction devices. The electron optical lenses 205.1 to 205.5 can focus the focal spot 15 of the secondary electron beamlet 9 onto the image plane 225 of the secondary electron imaging system 200. The secondary electron imaging system 200 includes at least one of a plurality of additional components, such as a multi-aperture array element, a deflection plate, or an exchangeable aperture diaphragm. Together with the objective lens 102, these lenses focus the secondary beam 9 onto the spatial detector 600, and in the process, compensate for the imaging scale and twist of the plurality of secondary electron beamlets 9 as a result of the magnetic lens so that the third raster arrangement of the foci 15 of the plurality of secondary electron beamlets 9 maintains a constant state on the detector surface 225. For example, the first and second magnetic lenses 205.4 and 205.4 are designed in reverse order to each other and have opposite magnetic fields. By appropriately driving the magnetic lenses 205.4 and 205.5, the Larmor rotation of the secondary electron beamlet 9 can be compensated. The secondary electron imaging system 200 has additional correction elements available, such as a multi-aperture plate 216.

[0051] Further information regarding such multi-beam particle beam systems and the components used therein, such as particle sources, multi-aperture plates, and lenses, can be obtained from International Publications Nos. WO 2005 / 024881, WO 2007 / 028595, WO 2007 / 028596, WO 2011 / 124352, and WO 2007 / 060017, as well as German Patent Applications Nos. DE 102013016113 A1 and DE 102013014976 A1, the entire disclosures of which are incorporated herein by reference.

[0052] The multi-beam charged particle imaging system 1 further includes a control system 800 configured for both controlling the individual particle optical components of the multi-particle beam system and evaluating and analyzing the signals obtained by the detector 600. In this case, the control or controller system 800 can be composed of a plurality of individual computers or components. For example, the control unit 800 includes a control processor 880 and a control module 840 for controlling the electron optical elements of the secondary imaging system 200 and the objective irradiation system 100. The control unit 800 is further connected to a control module 503 for supplying a voltage to the sample 7, and the voltage is also called the extraction voltage. Thereby, during use, an extraction field is generated between the objective 102 and the surface 25 of the object 7. During use, the extraction field decelerates the primary charged particles of the primary beamlets 3 before reaching the sample surface 25 and produces an additional focusing effect on the plurality of primary beamlets 3. At the same time, the extraction field also functions to accelerate the secondary particles from the surface 25 of the object 7 during use.

[0053] Also, the control unit 800 includes a scanning control module 860. During the inspection operation mode, a plurality of foci 15 of the secondary electron beamlets are formed on the detection surface 225, and a plurality of signals are recorded during the scanning operation of the primary beamlets 3 on the surface 25 of the sample 7. According to the present invention, the detector 600 includes a plurality of sets of detection elements, one set of detection elements for each secondary electron beamlet 9. During use, each set of detection elements is configured to record the intensity signal of the assigned secondary electron beamlet 9. The plurality of intensity signals of the plurality of secondary electron beamlets 9 are transferred to the image data acquisition unit 810, where the image data is processed and stored in the memory 890. The settings of the components of the multi-beam charged particle imaging system 1 are initially determined and stored in the memory 890 of the control unit 800 of the multi-beam charged particle imaging system 1.

[0054] The multi-beam charged particle imaging system 1 according to the first embodiment includes a charged particle multi-beamlet generator 300 having a primary multi-beamlet forming unit 305 for generating a plurality of primary charged particle beamlets 3. The primary multi-beamlet forming unit 305 is shown in FIG. 2. The primary multi-beamlet forming unit 305 includes a filter plate 304, a second multi-aperture plate 306, an active array optical element 307, and a terminal multi-aperture plate 310 in the direction in which the primary charged particles of the primary charged particle beam 309 propagate. Each multi-aperture plate can further include a thicker support section 333. The filter plate 403 is covered by a conductive coating 99 configured to absorb most of the incident charged particles from the primary charged particle beam 309. Through a plurality of first apertures 85.1, a plurality of primary charged particle beamlets 3.1 to 3.4 are formed. Each of the multi-aperture plates 304, 306, 307, and 310 includes a plurality of apertures 85.1 to 94. In FIG. 2, only one aperture 85.1 of the filter plate 304 and one aperture 94 of the terminal multi-aperture plate 310 are labeled. The plurality of apertures 85.1 to 94 of each of the multi-aperture plates 304, 306, 307, and 310 are provided at a pitch P2 within the same raster arrangement, forming a parallel sequence of apertures such that the collimated particle beamlets 3.1 to 3.4 can propagate straight through each of the sequences of apertures. The primary multi-beamlet forming unit 305 further includes at least a first capacitor electrode 308.1 and a plurality of spacers 83.1 to 83.3 and 86. The capacitor lens electrode 308.1 is configured to generate an electrostatic immersion lens field 92 that partially penetrates the terminal aperture 94 of the terminal multi-aperture plate 310 during use.

[0055] The active array optical element 307 and the termination multi-aperture plate 310 include a plurality of electrodes 81 and 82 configured to individually affect each of the primary charged particle beamlets 3.1 to 3.4. Thereby, a focus of the primary charged particle beamlets 3.1 to 3.4 is formed at an intermediate image plane 321 which can typically be curved and have an inclined component 323. Further details regarding the primary multi-beamlet forming unit 305 are disclosed in German Patent Application No. 102021208700.0 filed on August 10, 2021, which is incorporated herein by reference.

[0056] Figure 3 shows in more detail the filter plate 304 and the interaction between the filter plate 304 and the primary charged particle beam 309. The primary charged particles pass through the first aperture 85.1 to form a primary charged particle beamlet 3.1, or enter the absorption coating 99 of the filter plate 304 at a plurality of intersections 317 (only a few are shown in Figure 3). Basically, different interaction products can occur at each of the intersections 317. In the first example, x-ray radiation (also called bremsstrahlung) 351 is generated. This is shown at intersection 317.1. In the second example, secondary electrons are generated, which can leave the absorption coating to form a secondary electron beamlet 353. In addition to the secondary electrons and x-rays, some of the primary electrons may pass through the filter plate 304 to form a transmitted electron beam 355 despite the above. There is also a possibility that the primary charged particles scatter at contaminant particles at a scattering point 319 inside the aperture 85.1, generating a scattered electron beamlet 359. Thus, downstream of the filter plate 304, there may be not only primary charged particles forming the primary charged particle beamlet 3.1, but also secondary radiation 351, secondary electrons 353, scattered charged particles 359 and transmitted scattered charged particles 355. This undesirable radiation and undesirable charged particles cause undesirable charging, contamination and damage of the optical element in the vicinity of the filter plate 304.

[0057] Figure 4 shows the geometric characteristics of a typical prior art primary multi-beamlet forming unit 305. The filter plate 304 has an absorption layer 99, an electrode layer 98, and a plurality of apertures 85.1 with diameter D1. The filter plate (304) has a first thickness L1. L1 is preferably reduced to a thin thickness of less than 20 μm, for example L1 <= 10 μm. The filter plate 304 can also have a thickness L1.1 thinned at each of the plurality of apertures 85.1. This reduces the generation of scattered charged particles 359 inside the aperture 85.1. However, reducing the thickness increases the ratio of transmitted charged particles 355. The second multi-aperture plate 306 has a second aperture 85.2 with a larger diameter D2 > D1 and a thickness L3. The filter plate 304 and the second multi-aperture plate 306 are arranged with a gap of thickness L2 therebetween. The active array optical element 307 has a third aperture 85.3 and a thickness L5. The third aperture 85.3 has a diameter D3 < D2. The second multi-aperture plate 306 and the active array optical element 307 are arranged with a gap of thickness L2 therebetween. At each of the third apertures 85.3, at least one electrode 81.1 - 81.8 is arranged, for example eight electrodes 81.8 - 81.8 are arranged (however, only two electrodes 81.1 and 81.5 can be seen in the cross-section of Figure 5), whereby the primary charged particle beamlet 3.1 can be affected during use. Each electrode (81) is connected to a control unit (800) configured to supply a voltage to each electrode (81) to individually affect each primary charged particle beamlet (3) during use. For example, the first active array optical element (307) can include a plurality of ring electrodes (82) configured to independently generate an electrostatic field for focusing each primary charged particle beamlet (3). For example, the first active array optical element (307) can include a plurality of multipole electrodes (81) configured to independently generate an electrostatic field for deflecting, focusing, or correcting aberrations of each primary charged particle beamlet (3). The second multi-aperture plate 306, or the electrode layer 98 of the filter plate, can be configured as opposing electrodes forming, for example, an Einzel lens.In the design of the prior art, the second diameter D2 of the second multi-aperture plate 306 is selected to be relatively large in order to avoid the negative influence on the primary beamlet 3.1, and the third diameter D3 is selected to be relatively small (D3 < D2) with respect to the second diameter D2. Thereby, during use, the voltage supplied to the electrodes 81.1 to 81.8, which is necessary to generate an electrostatic field for affecting the primary beamlet 3, is reduced. From the edge of the first aperture 85.1, an opening angle α of approximately tan(α) = D2 / (L1 + L2 + L3) is formed. The large opening angle α of the prior art may cause undesirable radiation and undesirable charged particles to enter the active array optical element 307, resulting in a parasitic effect on the electrostatic field generated by the electrodes 81.1 to 81.8, thereby causing deterioration of the focal spot generated by the primary multi-beamlet forming unit 305. For example, secondary electrons 353 may adhere to the surface of the active array optical element 307, which may generate an additional electric field. Such parasitic charges accumulate over time and gradually degrade the performance of the primary multi-beamlet forming unit 305.

[0058] Figure 5 shows the first embodiment of the present invention. According to the first example, the distance L2, the thicknesses L1 and L3, and the second diameter D2 are selected such that a reduced opening angle is formed with tan(α) < 0.3, preferably tan(α) < 0.25. The reduction of the opening angle is achieved by the selection of the aperture diameter D2 with respect to D3 such that D2 < D3. In one example, D2 is selected in the range between 1.1×D1 and 1.3×D1. The third aperture (85.3) has a third diameter D3, and D2 is selected to be smaller than D3. For example, D3 > 1.6×D1, but D3 can be further selected with D3 >= 1.8×D1. Thereby, the active array optical element (307) is physically shielded by the second multi-aperture plate (306), and the number of secondary electrons or scattered primary charged particles incident on the active array optical element (307) is reduced. Therefore, according to the present invention, the secondary multi-aperture plate (306) is also referred to as the shielding multi-aperture plate (306). For example, D2 is selected in the range of 0.625×D3 <= D2 <= 1.3×D1. For example, D2 = 1.3×D1 and D3 = 1.8×D1. For example, D2 = 1.2×D1 and D3 = 1.7×D1. For example, D2 = 1.1×D1 and D3 = 1.6×D1. For example, by realizing a ratio of approximately D2 / D3 <= 0.75, the electrodes (81, 82) of the active array optical element (307) are shielded by the shielding multi-aperture plate (306), and the number of secondary electrons or scattered primary charged particles incident on the electrodes (81, 82) is substantially reduced. The opening angle α is reduced, for example, by a reduced diameter D2 with D2 < 1.3×D1, for example D2 <= 40μm. The reduced diameter D2 of the shielding multi-aperture plate (306) realizes the reduction of the opening angle α, and more transmitted charged particles 355 are absorbed by the shielding multi-aperture plate (306).

[0059] The opening angle α is further reduced, for example, by an increased distance such as L2 > 70 μm, for example L2 = 100 μm. An example of the increased distance L2 is shown in FIG. 6. Thereby, the probability that scattered or transmitted charged particles 359, 355 and secondary electrons 353.1, 355.2 are absorbed in the shielding multi-aperture plate (306) without causing any harm becomes even higher. The opening angle α is further reduced, for example, by an increased thickness L3 such as L3 > 70 μm, for example L3 = 120 μm.

[0060] In one embodiment, at least one of the thickness or distance L2 or L3 is increased and the diameter D2 is reduced. In one embodiment, the sum of the thickness and distance (L1 + L2 + L3) is selected to exceed 130 μm, for example (L1 + L2 + L3) = 150 μm. For example, to reduce the generation of scattered charged particles 359, the thickness of the filter plate 304 is selected to be L1 = 10 μm, and the distances are selected to be L2 = 20 μm and L3 = 120 μm. For example, the thickness of the filter plate 304 is selected to be L1 = 10 μm, and the distances are selected to be L2 = 70 μm and L3 = 70 μm.

[0061] The shielding multi-aperture plate (306) can further include at least an absorption layer or a metal layer (361) covering the beam incident side of the shielding multi-aperture plate (306). According to one embodiment, the absorption layer or the metal layer (361) includes a material from a group of materials including molybdenum, ruthenium, rhodium, palladium or silver, tungsten, rhenium, osmium, iridium, platinum, or gold. For example, such a layer with a thickness of about 1 to 2 μm can provide sufficient stopping power against scattered charged particles 359 and secondary electrons 353. In a further embodiment, the shielding multi-aperture plate (306) can further include at least an absorption layer or a metal layer (361) covering the beam exit side of the shielding multi-aperture plate (306). In a further embodiment, the shielding multi-aperture plate (306) is made of a material from a group including an absorbent, such as molybdenum, ruthenium, rhodium, palladium or silver, tungsten, rhenium, osmium, iridium, platinum, or gold, or an alloy thereof.

[0062] According to a further example of the first embodiment, the shielding multi-aperture plate (306) can be further configured for improved reduction of scattered charged particles 359 and secondary electrons 353. Some means for reducing scattered charged particles 359 and secondary electrons 353 are shown in FIG. 7. In the first example shown in FIG. 7a, in each of the second apertures (85.2) of the shielding multi-aperture plate (306), at least one baffle (369) is formed for absorbing scattered charged particles and secondary electrons. A plurality of baffles 369 can be formed, for example, by overlapping a plurality of metal layers 361 and a separation layer 363 with each other. The conductive metal layer 361 can be connected to the ground level. In the second example shown in FIG. 7b, the shielding multi-aperture plate (306) is formed in a two-layer structure, and each of the apertures (85.2) with diameter D2 is formed in a first layer having a thickness L3.1 < L3. Thereby, the scattering of charged particles or electrons inside the second aperture (85.2) is reduced. In the second example, the diameter D2 of the second aperture 85.2 with a reduced thickness can be made even smaller, for example, D1 < D2 < 1.15×D1. In the third example shown in FIG. 7c, each of the second apertures (85.2) of the shielding multi-aperture plate (306) is formed in a conical shape (365) such that the minimum aperture diameter D2 is formed on the bottom side or the beam emission side of the shielding multi-aperture plate (306). The absorption layer (361) can also cover the conical aperture opening (85.2). Thereby, by having the minimum diameter D2 on the bottom side or the beam emission side of the shielding multi-aperture plate (306), the opening angle α is further reduced to an extremely small value. Thereby, the protection or shielding of the active array optical element (307) by the shielding multi-aperture plate (306) is improved.

[0063] In the fourth embodiment (shown in FIG. 7d), each of the second apertures of the shielding multi-aperture plate is formed in a conical shape such that the minimum aperture diameter D2 is formed on the upper side or the beam incident side of the shielding multi-aperture plate (306). Thereby, similar to the second embodiment shown in FIG. 7b, the cross-sectional area of the aperture where there are scattered charged particles (359) or secondary electrons (353) is reduced, and the surface contamination induced by the scattered charged particles (359) or secondary electrons (353) is reduced. In the embodiments according to FIG. 7b or FIG. 7d, the aperture angle α is formed such that approximately tan(α)=D2 / (L1 + L2) or tan(α)=D2 / (L1 + L2 + L3.1). In order to keep the aperture angle small, a larger distance L2 or a smaller diameter D2 may be required in these embodiments.

[0064] FIG. 8 shows another embodiment according to the first embodiment. In one embodiment, the primary multi-beamlet forming unit (305) further includes an additional multi-aperture plate formed as an absorption plate (371) between the filter plate (304) and the shielding multi-aperture plate (306). The diameter D4 of the fourth aperture (85.4) of the absorption plate (371, 371.1) is selected to be between the first diameter D1 and the third diameter D3, for example, D4 is selected in the range of 1.1×D1 < D4 <= D3. The thickness of the absorption plate (371) is LX. Details of the absorption plate (370) will be described in detail in the following second embodiment.

[0065] The primary multi-beamlet forming unit (305) can include an additional multi-aperture plate including an additional active array optical multi-aperture plate and a terminal multi-aperture plate (310).

[0066] According to the first embodiment of the present invention, the capture angle or aperture angle α of the transmitted scattered charged particles 335, 359 or secondary electrons 353 is reduced, the charging or damage or contamination of the first active array optical element (307) is reduced, and the life of the primary multi-beamlet forming unit 305 is extended. Due to the aperture angle α,

[0067] By the improvement according to the second embodiment of the present invention, the influence of scattered charged particles and secondary electrons is further reduced. An example according to the second embodiment of the present invention is shown in FIG. 9. FIG. 9 shows a multi-beam forming unit (305) having a filter plate (304) and a first active array optical element (307). The filter plate (304) includes a plurality of first apertures (85.1) each having a first diameter D1 for generating a plurality of primary charged particle beamlets (3) during use. The first active array optical element (307) includes a plurality of third apertures (85.3) each having a third diameter D3 similar to the first active array optical element (307) described in the first embodiment of the present invention. The charged particle multi-beamlet generator (300) further includes a first absorption multi-aperture plate (371) having a plurality of fourth apertures (85.4) with a diameter D4. In the example of FIG. 9, the first absorption multi-aperture plate (371) (abbreviated as absorption plate 371) is disposed upstream of the filter plate (304). The diameter D4 of the fourth aperture (85.4) is selected to be larger than the diameter D1 of the first aperture (85.1). During use, the primary charged particle beam (309) first enters the first absorption plate (371). Most of the primary charged particles are absorbed by the first absorption plate (371), and the primary charged particles passing through the fourth aperture (85.4) form a pre-shaped charged particle beamlet (312). The pre-shaped charged particle beamlet (312) enters the filter plate (304). A part of the pre-shaped charged particle beamlet (312) is absorbed by the filter plate (304), and the primary charged particles passing through the first aperture (85.1) form a primary charged particle beamlet (3).

[0068] In one example, the diameter D4 of the fourth aperture (85.4) of the absorption plate (371) is selected between the first diameter D1 and the third diameter D3, for example, D4 is selected within the range of 1.1×D1 < D4 <= D3. The absorption plate (371) reduces the number of charged particles incident on the filter plate 304, and instead, transmitted charged particles 355 and secondary electrons 353 are generated in the absorption plate 371. Therefore, the number of transmitted charged particles 355 and secondary electrons 353 downstream of the filter plate (304) is substantially reduced.

[0069] In the embodiment according to FIG. 9, the primary multi-beamlet forming unit (305) includes a shielding multi-aperture plate (306) having a second aperture of a second diameter D2. The shielding multi-aperture plate (306) has a third thickness L3, and 1.1×D1 < D2 <= 1.5×D1. In one embodiment, the shielding multi-aperture plate (306) within the primary multi-beamlet forming unit (305) is configured for reducing the opening angle α and is arranged according to the first embodiment of the present invention.

[0070] A first example of a charged particle multi-beamlet generator (300) according to a second embodiment is shown in FIG. 10. The charged particle multi-beamlet generator (300) includes a source (301) of primary charged particles and collimating lenses (303.1, 303.2) arranged upstream of the filter plate (304). The collimating lenses (303.1, 303.2) can be formed as a pair of magnetic lenses 303.1 and 303.2 and can include further electrostatic elements such as a deflection element 313. An absorption plate (371) is arranged in the parallel beam path of the collimated primary charged particle beam (309) downstream of the collimating lenses (303.1, 303.2). For example, the absorption plate (371) is arranged between the collimating lenses (303.1, 303.2) and the filter plate (304). The collimated primary charged particle beam (309) has a diameter D1 at the incident side of the absorption plate (371). The absorption plate (371) generates a plurality of pre-formed beamlets (312) as parallel beamlets during use, and the parallel beamlets are incident perpendicularly to the filter plate 304. Both the absorption plate 371 and the filter plate 304 have a plurality of apertures with the same pitch P2.

[0071] A second example of the second embodiment is shown in FIG. 11. An absorber plate (371) is disposed in a diverging primary charged particle beam (309) between a source (301) and collimating lenses (303.1, 303.2). The charged particle multi-beamlet generator (300) of this example further includes a collection lens or lens pair (315.1, 315.2) disposed between a source (301) of primary charged particles and the absorber plate (371). The absorber plate (371) has a plurality of fourth apertures (85.4) arranged at a first pitch P1, configured to generate a plurality of preformed beamlets (312) having a first pitch P1 in use. A first aperture (85.1) of the filter plate (304) is arranged at a second pitch P2. The charged particle multi-beamlet generator (300) further includes a control unit (830) configured to supply a first control signal to the first collection lens (315.1, 315.2) to adjust the flow of a plurality of primary charged particle beamlets (3) in use. Thereby, the collection angle of the primary charged particles collected from the source (301) is adjusted by the variable focal length of the first collection lens (315.1, 315.2). The collection lens pair (315.1, 315.2) can further include a first multipole element (313.1) for adjusting the average propagation direction and position of the primary charged particle beam (309) on the incident side of the absorber plate 371. The control unit (830) is configured to supply a second control signal to the collimator lenses (303.1, 303.2) to match the pitch P1 of the plurality of preformed beamlets (312) with the second pitch P2 of the filter plate (304) and to adjust the propagation angles of the plurality of preformed beamlets (312) to form a bundle of parallel preformed beamlets (312). Thereby, the bundle of parallel preformed beamlets (312) is adjusted parallel and perpendicular to the filter plate, and the primary charged particle beamlet 3 can pass through a plurality of apertures in the primary multi-beamlet forming unit (305).

[0072] The control unit (830) can be part of the charged particle multi-beamlet generator (300) and can be connected to the control unit 800 of the multi-beamlet charged particle microscope system or can be part of the control unit 800 of the multi-beamlet charged particle microscope system.

[0073] In the embodiment shown in FIG. 12, the primary multi-beamlet forming unit (305) includes the first absorber plate (371.1) according to the above embodiment, and the second absorber plate (371.2) between the filter plate (304) and the shielding multi-aperture plate (306). In one embodiment, the diameter D5 of the fifth aperture (85.5) of the absorber plate (371, 371.1) is selected between the first diameter D1 and the third diameter D3. For example, D5 is selected in the range of 1.1×D1 < D5 <= D3. The primary multi-beamlet forming unit (305) can include additional multi-aperture plates, including an additional active array optical multi-aperture plate and a terminal multi-aperture plate (310).

[0074] Inside each of the shielding multi-aperture plate (306), the first or second absorber plate (371, 371.1, 371.2), each aperture (85.2, 85.4, 85.5) can be provided with at least one baffle (369). Each of the apertures can be formed in a conical shape (365) such that the minimum aperture diameter is formed on the beam incident side or the emission side of each respective aperture (85.2, 85.4, 85.5). At least one of the absorber plates (371, 371.1, 371.2), the shielding plate (306), and the filter plate (304) can be provided with at least one conductive layer (361, 99) on the beam incident side or the emission side of the plate (371, 371.1, 731.2, 304, 306).

[0075] According to one embodiment, the absorption multi-aperture plate (371) includes a material from a group of materials including molybdenum, ruthenium, rhodium, palladium or silver, tungsten, rhenium, osmium, iridium, platinum, or gold. In one embodiment, the thickness LX can be such that LX > 20 μm, for example LX = 30 μm. The greater thickness LX compared to the thickness L1 of the filter plate increases the blocking ability to prevent the transmission of charged particles 355 and improves the extinction of x-rays generated at the intersection with the primary charged particles. For example, a thickness of about 50 μm to 300 μm can provide a substantially complete blocking ability exceeding 95% of the X-rays 351. For example, a 60 μm layer of gold absorbs more than 95% of the x-rays. Further reduced transmission of x-rays is achieved, for example, by an absorption plate (371) made of tungsten. The absorption plate (371) can further include an absorption layer (361) for absorbing electrons or other charged particles. The absorption layer (99, 361) can be formed, for example, as a doped silicon or silicon oxide or graphite layer. Thereby, the backscattering of charged particles is reduced.

[0076] With at least one absorption plate (371, 371.1, 371.2) according to the second embodiment, the influence of scattered primary charged particles and secondary electrons in the primary multi-beamlet forming unit (305) is further reduced. Also, the influence of other secondary emissions such as x-ray radiation can be further reduced, and the lifetime of the primary multi-beamlet forming unit (305) is extended. Also, the charging or damage of the active array optical element (307) due to X-rays, scattered charged particles, and secondary electrons is reduced.

[0077] The third embodiment of the present invention provides a further system and method, particularly configured to reduce the influence of secondary electrons. The secondary electrons 353 generated in the filter plate 304 or the absorber plate 371 typically have a much lower kinetic energy compared to the primary charged particles. According to the third embodiment, an improved method of operating the multi-beam forming unit (305) of the multi-beam charged particle microscope (1) and the multi-beam charged particle microscope (1) is provided, which is provided with further means for reducing the charging or damage of the active array optical element 307 of the primary multi-beamlet forming unit (305). The primary multi-beamlet forming unit (305) according to the third embodiment is shown in FIG. 13. The primary multi-beamlet forming unit (305) includes a filter plate 304, a second, or shielding multi-aperture plate 306, and an active array optical element (307). The active array optical element (307) includes a first, or conductive layer 391, a separation layer 392, an electrode layer 393 including a plurality of electrodes (81, 82), a second separation layer 394, and a base layer 395.

[0078] The filter plate (304) includes a plurality of first apertures (85.1) for generating or transmitting a plurality of primary charged particle beamlets (3) during use. The filter plate can include a conductive absorption coating 99. The active array optical element (307) includes a plurality of electrodes (81, 82) disposed in the vicinity of a plurality of third apertures (85.3) of the active array optical element (307), and each electrode (81, 82) is configured to generate an electric field for individually focusing, deflecting, or shaping one of the primary charged particle beamlets (3). The second multi-aperture plate 306 can be an arranged and configured shielding multi-aperture plate 306 for reducing the opening angle α according to the first embodiment. The primary multi-beamlet forming unit (305) according to the third embodiment can further include the absorption plate 371 of the second embodiment of the present invention. All elements are separated from each other and electrically connected to the control unit 830. The control unit (830) is configured to supply a plurality of voltages to the elements of the multi-beam forming unit (305) to reduce the influence of secondary electrons according to the third embodiment.

[0079] The method includes the step of generating a plurality of voltages U4 by the control unit 803 and supplying them to the electrodes (81, 82) of the active array optical element (307). The voltages U4 are determined, for example, during the calibration of the multi-beamlet charged particle microscope system (1) and are determined, generated, and supplied by the control unit 830 based on the settings of the charged particle multi-beamlet generator (305) stored in the memory of the control unit 800. The method further includes the step of generating a plurality of voltages by the control unit 830 and supplying them to the elements of the multi-beam forming unit (305). The plurality of voltages includes a first voltage U1 supplied to the filter plate (304). The plurality of voltages includes a plurality of individual fourth voltages U4 supplied to the plurality of electrodes (81, 82). The plurality of voltages includes a second voltage U2 supplied to a second, or shielding multi-aperture plate (306), arranged between the filter plate (304) and the active array optical element (307). For example, the second voltage U2 is adjusted to obtain a repulsive or attractive force for secondary electrons (353) generated from the primary charged particle beam (309) at the intersection (317) with the filter plate (304). The magnitude of the first voltage U1 or the second voltage U2 is adjusted, for example, to match the range of the kinetic energy of the secondary electrons 353 generated at the filter plate 304. Thus, the magnitude of the first voltage U1 or the second voltage U2 is much smaller than the kinetic energy of the primary charged particles corresponding to the voltage difference between the anode of the source 301 and the filter plate 304. The voltage U1 or U2 is selected and configured to form a potential energy sink whose depth is adjusted to match the range of the energy of the secondary electrons 353. The primary charged particles typically have a large kinetic energy of EK = approximately 5 - 35 keV, corresponding to a voltage difference of UE = 5 - 35 kV. The secondary electrons typically have a kinetic energy of less than 100 eV, for example less than 50 eV.

[0080] Generally, the plurality of voltages U0, U1, U2, U3, U4, and U5 are selected such that at least one of the following three examples occurs. Three cases A, B, and C are shown in FIG. 14. In each case, the potential energy G corresponding to the voltages U0, U1, U2, U3, U4, and U5 is shown. The voltages U0, U1, U2, U3, U4, and U5 required for the generation of the potentials G0, G1, G2, G3, G4, and G5 are proportional to the negative values of the potential by U~-G.

[0081] In the lower part of FIG. 14, the multi-aperture plate of the multi-beam forming unit is schematically shown. Refer to FIG. 13 and its description. The potentials G0, G1, G2, G3, G4, and G5 along the z-axis passing through the apertures correspond to the sequence of the multi-aperture plate, and the corresponding voltages U0, U1, U2, U3, U4, and U5 are supplied by the control unit 830.

[0082] In example A, a potential barrier (in the form of potential step A1 or potential maximum A2) is formed along the z-axis to prevent low-energy secondary or scattered electrons from reaching the active array optical element (307).

[0083] In example B, the potential landscape is adjusted such that the beam-forming aperture or filter plate 304 is in a potential sink B1 formed along the z-axis. In this situation, secondary or scattered electrons emitted at the energy minimum of the sink cannot exit the potential sink along the z-axis and are substantially prevented from further penetrating into the elements downstream of the filter plate 304, including the active array optical element (307). An example is shown in FIG. 15. First, the first positive voltage U1 supplied to the filter plate (304) is selected to create a potential sink B1 for secondary electrons. For example, U1 is selected to exceed the voltage U2 supplied to the second multi-aperture plate 306, and U1 = U2 + US, where US is less than 100V.

[0084] In the second embodiment shown in FIG. 16, a second voltage U2 supplied to the second or shielding multi-aperture plate (306) is selected to generate a sink B2 for secondary electrons (see potential G in FIG. 14). For example, U2 is selected to exceed a third voltage U3 supplied to the conductive first layer 391 of the active array optical element, and U2 = U3 + US, where US is less than 100V. In one embodiment, both U1 and U2 exceed U3. To effectively block secondary electrons 353, a shielding voltage US of less than 0.3% of UE is sufficient in both embodiments. In one embodiment, the shielding voltage US is about 100V. Such a low voltage US < 0.3% × UE substantially reduces secondary electrons, while primary charged particles are only slightly affected. Voltages U0, U3, and U5 can be set to the ground level. In one embodiment, U1 and U2 are set to the ground level, and U3 is set to a negative voltage between -100V and 0V. The ground level can be any reference level of the charged particle microscope.

[0085] In Example C, voltages U0 and U1 are selected such that, during use, an electric field that draws secondary or scattered electrons in the negative z-direction away from the active array optical element (307) is formed between the absorber plate 371 and the filter plate 304 (FIG. 17).

[0086] It should be understood that combinations of A, B, and C are also possible. It should be understood that in some embodiments, the absorber plate 371 or the second multi-aperture plate 306 may be omitted.

[0087] Considering the small distances L2 and L4 between the multi-aperture plates, the amount of electric field interaction caused by the voltage difference between the filter 305, the second aperture plate 306, and the first layer 391 of the active array optical element (307) is extremely small, and thus has no significant effect on primary charged particles at high kinetic energies. On the other hand, low-speed secondary electrons are substantially prevented from reaching the active array optical element (307). This substantially eliminates one factor of charging and damage to the active array optical element (307).

[0088] In the embodiments and claims of the present disclosure, an array of electrostatic elements such as an electrostatic microlens or an electrostatic multipole element driven by at least one voltage source unit is described. However, the active multi-aperture element can also be configured as a magnetodynamic element having coils instead of electrodes. In these equivalent embodiments, a drive current is supplied instead of the voltage U4 by at least one current source unit that can be, for example, an ASIC or other equivalent microelectronic device. Therefore, the coil, drive current, or current source unit is an equivalent means of the electrode, drive voltage, or voltage source unit, and the present invention can be easily applied to magnetodynamic array elements as well.

[0089] The present invention will be further described by the following clauses.

[0090] Clause 1: A method of operating a multi-beam forming unit (305) of a multi-beam charged particle microscope (1), comprising: - generating a primary charged particle beam (309); - supplying a first voltage U1 to a filter plate (304), the filter plate (304) including a plurality of apertures (85) for forming and transmitting a plurality of primary charged particle beamlets (3) from the primary charged particle beam (309) during use, and the primary charged particle beam (309) generating secondary electrons (353) at an intersection (317) of primary electrons of the primary charged particle beam (309) with the filter plate (304), or - supplying a plurality of individual fourth voltages U4 to a plurality of electrodes (81, 82) of an active array optical element (307), the electrodes being arranged in the vicinity of the plurality of apertures (85) of the active array optical element (307) and each being configured to focus, deflect, or shape one of the primary charged particle beamlets (3). - Supplying at least one of a second voltage U2 to a second multi-aperture plate (306) disposed between a filter plate (304) and an active array optical element (307), or an absorber voltage U0 to an absorber plate (371) disposed upstream of the filter plate (304) in the propagation direction of a primary charged particle beam (309); - Adjusting at least one of the absorber voltage U0, the first voltage U1, or the second voltage U2 to prevent secondary electrons (353) from intersecting the active array optical element (307) by realizing potential barriers (A1, A2) or potential sinks (B1, B2, C) for the secondary electrons (353). A method.

[0091] Clause 1: The method according to Clause 1, wherein the second voltage U2 is adjusted to be smaller than the first voltage U1, thereby realizing a potential barrier (A1, A2) or a potential sink (B1) for secondary electrons (353) upstream of the second multi-aperture plate (306). A method.

[0092] Clause 3: The method according to Clause 1, wherein the second voltage U2 is adjusted to be larger than the first voltage U1, thereby realizing a potential sink (B2) for secondary electrons (353) in the vicinity of the second multi-aperture plate (306). A method.

[0093] Clause 4: The method according to any one of Clauses 1 to 3, wherein the absorber voltage U0 is adjusted to be larger than the first voltage U1, thereby realizing a potential sink (C) for secondary electrons (353) upstream of the filter plate (304). A method.

[0094] Clause 5: The method according to any one of Clauses 1 to 4, wherein at least one of the absorber voltage U0, the first voltage U1, or the second voltage U2 is supplied at a ground level or a reference level. A method.

[0095] Clause 6: A method according to any one of Clauses 1 to 5, further comprising the step of supplying a third voltage U3 to the first layer (391) of the active array optical element (307), wherein the third voltage U3 is equal to either the first voltage U1 or the second voltage U2.

[0096] Clause 7: A method according to any one of Clauses 1 to 6, further comprising the step of supplying a fifth voltage U5 to the fifth layer (395) of the active array optical element (307), wherein the fifth voltage U5 is equal to either the first voltage U1 or the second voltage U2.

[0097] Clause 8: A multi-beam charged particle microscope (1), - A charged particle source (301) and at least one condenser lens (303.1, 303.2) for generating a primary charged particle beam (309), - And a multi-beam forming unit (305), - The multi-beam forming unit (305) being, - In use, a filter plate (304) having a plurality of apertures (85) for forming a plurality of primary charged particle beamlets (3) from the primary charged particle beam (309), - An active array optical element (307) having a plurality of electrodes (81, 82), these electrodes being arranged in the vicinity of the plurality of apertures (85) of the active array optical element (307), and each electrode being configured to focus, deflect or shape one of the primary charged particle beamlets (3), the active array optical element (307), - At least one of a second multi-aperture plate (306) arranged between the filter plate (304) and the active array optical element (307) or an absorption plate (371) arranged in the propagation direction of the primary charged particle beam (309) upstream of the filter plate (304), - In use, a control unit (830) configured to execute any one of the methods according to Clauses 1 to 7, a multi-beam charged particle microscope (1).

[0098] Clause 9: A multi-beam charged particle microscope (1), comprising: - A charged particle source (301) and at least one condenser lens (303.1, 303.2) for generating and forming a primary charged particle beam (309); - A multi-beam forming unit (305); - The multi-beam forming unit (305) includes: - A filter plate (304) having a plurality of apertures (85.1) with a diameter D1 for forming a plurality of primary charged particle beamlets (3) during use; - An active array optical element (307) having a plurality of apertures (85.3) with a third diameter D3, which is configured to individually focus, deflect, or shape at least one of the primary charged particle beamlets (3) during use; - At least one second multi-aperture plate (306) having apertures (85.2) with a second diameter D2, disposed between the filter plate (304) and the active array optical element (307), or an absorption plate (371) having a plurality of apertures (85.4) with a diameter D4, the absorption plate (371) being disposed upstream of the filter plate (304) in the propagation direction of the primary charged particle beam (309); - A control unit (830) configured to adjust and supply at least one of a first voltage U1 to the filter plate (304), a second voltage U2 to the second multi-aperture plate (306), or an absorption plate voltage U0 to the absorption plate (371) during use, in order to prevent secondary electrons (353) generated at the filter plate (304) during use from intersecting the active array optical element (307) by realizing a potential barrier (A1, A2) or a potential sink (B1, B2, C) for the secondary electrons (353) upstream of the active array optical element (307).

[0099] Clause 10: The multi-beam charged particle microscope (1) according to Clause 9, wherein: - The filter plate (304) has a first thickness L1 < 20 μm, preferably L1 <= 10 μm, - The second multi-aperture plate (306) is arranged at a distance L2 from the filter plate (304), and the distance L2, the thickness L1, and the second diameter D2 are selected according to D2 / (L1 + L2) <= 0.3, preferably < 0.25, whereby the shielding of secondary electrons (353) or scattered primary electrons (355, 359) generated in the filter plate (304) during use is improved, the multi-beam charged particle microscope (1) according to clause 9.

[0100] Clause 11: The multi-beam charged particle microscope (1) according to clause 9 or 10, - The filter plate (304) has a first thickness L1 < 20 μm, preferably L1 <= 10 μm, - The second multi-aperture plate (306) is arranged at a distance L2 from the filter plate (304), has a third thickness L3, and the distance L2, the thicknesses L2 and L1, and the second diameter D2 are selected according to D2 / (L1 + L2 + L3) <= 0.3, preferably 0.25, the multi-beam charged particle microscope (1).

[0101] Clause 12: The multi-beam charged particle microscope (1) according to any one of clauses 9 to 11, wherein D2 and D3 are selected according to D3 >= D2 > D1, the multi-beam charged particle microscope (1).

[0102] Clause 13: The multi-beam charged particle microscope (1) according to clause 12, wherein D2 is selected according to 1.1×D1 < D2 < 1.3×D1, the multi-beam charged particle microscope (1).

[0103] Clause 14: The multi-beam charged particle microscope (1) according to any one of clauses 10 to 13, wherein (L1 + L2 + L3) > 130 μm, preferably > 150 μm, the multi-beam charged particle microscope (1).

[0104] Clause 15: The multi-beam charged particle microscope (1) according to any one of Clauses 9 to 14, wherein at least one baffle (369) is formed inside each of the apertures (85.2) of the second multi-aperture plate (306).

[0105] Clause 16: The multi-beam charged particle microscope (1) according to any one of Clauses 9 to 14, wherein the aperture (85.2) with diameter D2 is formed with a thickness L3.1 < L3 inside each of the apertures (85.2) of the second multi-aperture plate (306).

[0106] Clause 17: The multi-beam charged particle microscope (1) according to any one of Clauses 9 to 14, wherein each of the apertures (85.2) of the second multi-aperture plate (306) is formed in a conical shape (365).

[0107] Clause 18: The multi-beam charged particle microscope (1) according to Clause 17, wherein the minimum aperture diameter D2 is formed on the bottom side or the beam emission side of the second multi-aperture plate (306).

[0108] Clause 19: The multi-beam charged particle microscope (1) according to Clause 17, wherein the minimum aperture diameter D2 is formed on the upper side or the beam incident side of the second multi-aperture plate (306).

[0109] Clause 20: The multi-beam charged particle microscope (1) according to any one of Clauses 9 to 19, wherein the second multi-aperture plate (306) is provided with at least one metal layer (361) on the upper side or the beam incident side.

[0110] Clause 21: The multi-beam charged particle microscope (1) according to any one of Clauses 9 to 20, wherein D4 is within the range of 1.1×D1 < D4 < D3.

[0111] Clause 22: The multi-beam charged particle microscope (1) according to any one of Clauses 9 to 21, further comprising a second absorption plate (371.2), wherein the second absorption plate (371.2) is disposed between the filter plate (304) and the shielding multi-aperture plate (306).

[0112] Clause 23: The multi-beam charged particle microscope (1) according to any one of Clauses 9 to 22, - comprising a first collection lens (315.1, 315.2), - an absorption plate (371) having a plurality of apertures (85.4) is arranged at a first pitch P1 and is configured to generate a plurality of pre-formed beamlets (312) at the first pitch P1 during use, and is disposed between the first collection lens (315) and at least one collimator lens (303.1, 303.2), - the apertures (85.1) of the filter plate (304) are arranged at a second pitch P2 different from the first pitch P1, - a control unit (830) is configured to supply a first control signal to the first collection lens (315.1, 315.2) to adjust the flow of a plurality of primary charged particle beamlets (3) during use, to match the pitch P1 of the plurality of pre-formed beamlets (312) with the second pitch of the filter plate (304), and to supply a second control signal to at least one collimator lens (303.1, 303.2) to adjust the propagation angles of the plurality of pre-formed beamlets (312) to form parallel pre-formed beamlets (312).

[0113] Clause 24: A multi-beam forming unit (305), - a filter plate (304) having an aperture (85.1) with a first diameter D1, the filter plate (304) having a first thickness L1, - A shielding multi-aperture plate (306) having an aperture (85.2) with a second diameter D2, the shielding multi-aperture plate (306) being disposed at a distance L2 from the filter plate (304) and having a third thickness L3, - An active array optical element (307) having a plurality of electrodes (81, 82) disposed in the vicinity of a plurality of apertures (85.3) with a third diameter D3, the active array optical element (307) being disposed downstream of the shielding multi-aperture plate (306), A multi-beam forming unit (305) in which the distance L2, the thickness L1, and the second diameter D2 satisfy the requirement D2 / (L1 + L2) <= 0.3, preferably < 0.25.

[0114] Clause 25: The multi-beam forming unit (305) according to clause 24, wherein the distance L2, the thicknesses L1 and L3, and the second diameter D2 are selected according to D2 / (L1 + L2) <= 0.3, preferably 0.25.

[0115] Clause 26: The multi-beam forming unit (305) according to clause 24 or 25, wherein D3 is selected according to D3 >= D2 > D1.

[0116] Clause 27: The multi-beam forming unit (305) according to any one of clauses 24 to 26, wherein D2 is selected according to 1.1×D1 < D2 < 1.3×D1.

[0117] Clause 28: The multi-beam forming unit (305) according to any one of clauses 24 to 27, wherein (L1 + L2 + L3) > 130 μm, preferably 150 μm.

[0118] Clause 29: The multi-beam forming unit (305) according to any one of clauses 24 to 28, wherein at least one baffle (369) is formed inside each of the apertures (85.2) of the shielding multi-aperture plate (306).

[0119] Clause 30: The multi-beam forming unit (305) described in any one of Clauses 24 to 29, wherein within each of the apertures (85.2) of the shielding multi-aperture plate (306), an aperture with a diameter D2 is formed with a thickness L3.1 < L3. The multi-beam forming unit (305).

[0120] Clause 31: The multi-beam forming unit (305) described in any one of Clauses 24 to 30, wherein within each of the apertures (85) of the shielding multi-aperture plate (306), the aperture is formed in a conical shape (365). The multi-beam forming unit (305).

[0121] Clause 32: The multi-beam forming unit (305) described in Clause 31, wherein the minimum aperture diameter D2 is formed on the bottom side or the beam emission side of the shielding multi-aperture plate (306). The multi-beam forming unit (305).

[0122] Clause 33: The multi-beam forming unit (305) described in Clause 31, wherein the minimum aperture diameter D2 is formed on the upper side or the beam incident side of the shielding multi-aperture plate (306). The multi-beam forming unit (305).

[0123] Clause 34: The multi-beam forming unit (305) described in any one of Clauses 24 to 33, wherein the shielding multi-aperture plate (306) includes at least one conductive layer (361) on the upper side or the beam incident side of the shielding multi-aperture plate (306). The multi-beam forming unit (305).

[0124] Clause 35: The multi-beam forming unit (305) described in Clause 34, wherein at least one conductive layer (361) is one of a metal layer, a graphite layer, or a doped semiconductor layer. The multi-beam forming unit (305).

[0125] Clause 36: A multi-beam forming unit (305) according to any one of Clauses 24 to 35, wherein the shielding multi-aperture plate (306) contains a material from the group of materials containing molybdenum, ruthenium, rhodium, palladium or silver, tungsten, ruthenium, osmium, iridium, platinum, or gold.

[0126] Clause 37: A multi-beam forming unit (305) according to any one of Clauses 24 to 36, further comprising a first absorber plate (371, 371.1) having a plurality of apertures (85.4) with a diameter D4, where 1.1×D1 < D4 < D3.

[0127] Clause 38: A multi-beam forming unit (305) according to Clause 37, wherein the first absorber plate (371, 371.1) is disposed between the filter plate (304) and the shielding multi-aperture plate (306).

[0128] Clause 39: A multi-beam forming unit (305) according to Clause 37, wherein the first absorber plate (371, 371.1) is disposed upstream of the filter plate (304).

[0129] Clause 40: A multi-beam forming unit (305) according to any one of Clauses 24 to 39, further comprising a second absorber plate (371.2), wherein the second absorber plate (371.2) is disposed between the filter plate (304) and the shielding multi-aperture plate (306).

[0130] Clause 41: A multi-beam forming unit (305), - a filter plate (304) having apertures (85.1) with a first diameter D1, the filter plate (304) having a first thickness L1, and - A shielding multi-aperture plate (306) having an aperture (85.2) with a second diameter D2, the shielding multi-aperture plate (306) having a third thickness L3, and - An active array optical element (307) having a plurality of electrodes (81, 82) disposed in the vicinity of a plurality of apertures (85.3) with a diameter D3, the active array optical element (307) having a thickness L5, and including A multi-beam forming unit (305) in which 1.1×D1 < D2 <= 1.3×D1.

[0131] Clause 42: The multi-beam forming unit (305) according to Clause 41, wherein a plurality of baffles (369) are formed inside each of the apertures (85.2) of the shielding multi-aperture plate (306).

[0132] Clause 43: The multi-beam forming unit (305) according to Clause 41 or 42, wherein an aperture with a diameter D2 is formed with a thickness L3.1 < L3 inside each of the apertures (85.2) of the shielding multi-aperture plate (306).

[0133] Clause 44: The multi-beam forming unit (305) according to Clause 41, wherein each of the apertures (85.2) of the shielding multi-aperture plate (306) is formed in a conical shape (365).

[0134] Clause 45: The multi-beam forming unit (305) according to Clause 44, wherein the minimum aperture diameter D2 is formed on the bottom side or the beam emission side of the shielding multi-aperture plate (306).

[0135] Clause 46: The multi-beam forming unit (305) according to Clause 44, wherein the minimum aperture diameter D2 is formed on the upper side or the beam incident side of the shielding multi-aperture plate (306).

[0136] Clause 47: The multi-beam forming unit (305) described in any one of Clauses 41 to 46, wherein the shielding multi-aperture plate (306) includes at least one conductive layer (361) on the upper side or the beam incident side of the shielding multi-aperture plate (306).

[0137] Clause 48: The multi-beam forming unit (305) described in Clause 47, wherein at least one conductive layer (361) is one of a metal layer, a graphite layer, or a doped semiconductor layer.

[0138] Clause 49: The multi-beam forming unit (305) described in any one of Clauses 41 to 46, wherein the shielding multi-aperture plate (306) includes a material from the group of materials including molybdenum, ruthenium, rhodium, palladium, or silver, tungsten, rhenium, osmium, iridium, platinum, or gold.

[0139] Clause 50: The multi-beam forming unit (305) described in any one of Clauses 41 to 49, further including a first absorption plate (371, 371.1) having a plurality of apertures (85.4) with a diameter D4, where 1.1×D1 < D4 < D3.

[0140] Clause 51: The multi-beam forming unit (305) described in Clause 50, wherein the first absorption plate (371, 371.1) is disposed between the filter plate (304) and the shielding multi-aperture plate (306).

[0141] Clause 52: The multi-beam forming unit (305) described in Clause 50, wherein the first absorption plate (371, 371.1) is disposed upstream of the filter plate (304).

[0142] Clause 53: The multi-beam forming unit (305) according to Clause 52, further comprising a second absorption plate (371.2), wherein the second absorption plate (371.2) is disposed between the filter plate (304) and the shielding multi-aperture plate (306).

[0143] Clause 54: A multi-beam forming unit (305), - A filter plate (304) having an aperture (85.1) with a first diameter D1, the filter plate (304) having a first thickness L1, and - An active array optical element (307) having a plurality of electrodes (81, 82) disposed in the vicinity of a plurality of apertures (85.3) with a diameter D3, the active array optical element (307) having a thickness L5, and - A first absorption plate (371, 371.1) having a plurality of apertures (85) with a diameter D4, where 1.1×D1 < D4 < D3, and comprising the multi-beam forming unit (305).

[0144] Clause 55: The multi-beam forming unit (305) according to Clause 54, wherein the first absorption plate (371, 371.1) is disposed between the filter plate (304) and the first array optical element (306.2).

[0145] Clause 56: The multi-beam forming unit (305) according to Clause 54, wherein the first absorption plate (371, 371.1) is disposed upstream of the filter plate (304).

[0146] Clause 57: The multi-beam forming unit (305) according to any one of Clauses 54 to 56, wherein a plurality of baffles (369) are formed in at least one aperture of the apertures (85.4) of the first absorption plate (371, 371.1).

[0147] Clause 58: The multi-beam forming unit (305) described in any one of Clauses 54 to 57, wherein within at least one of the apertures (85.4) of the first absorber plate (371, 371.1), an aperture having a diameter D4 is formed with a thickness LX.1 < LX. The multi-beam forming unit (305).

[0148] Clause 59: The multi-beam forming unit (305) described in any one of Clauses 54 to 68, wherein within at least one of the apertures (85.4) of the first absorber plate (371, 371.1), the aperture is formed in a conical shape (365). The multi-beam forming unit (305).

[0149] Clause 60: The multi-beam forming unit (305) described in Clause 59, wherein the minimum aperture diameter D4 is formed on the bottom side or the beam emission side of the first absorber plate (371, 371.1). The multi-beam forming unit (305).

[0150] Clause 61: The multi-beam forming unit (305) described in Clause 59, wherein the minimum aperture diameter D4 is formed on the bottom side or the beam emission side of the first absorber plate (371, 371.1). The multi-beam forming unit (305).

[0151] Clause 62: The multi-beam forming unit (305) described in any one of Clauses 54 to 61, wherein the first absorber plate (371, 371.1) includes at least one conductive layer (361) on the upper side or the beam incident side of the first absorber plate (371, 371.1). The multi-beam forming unit (305).

[0152] Clause 63: The multi-beam forming unit (305) described in Clause 62, wherein at least one conductive layer (361) is one of a metal layer, a graphite layer, or a doped semiconductor layer. The multi-beam forming unit (305).

[0153] Clause 64: The multi-beam forming unit (305) described in any one of Clauses 54 to 63, wherein the first absorption plate (371, 371.1) contains a material from the group of materials including molybdenum, ruthenium, rhodium, palladium or silver, tungsten, rhenium, osmium, iridium, platinum, or gold, the multi-beam forming unit (305).

[0154] Clause 65: The multi-beam forming unit (305) described in Clauses 56 to 64, further including a second absorption plate (371.2), the second absorption plate (371.2) being disposed between the filter plate (304) and the first array optical element (306.2), the multi-beam forming unit (305).

[0155] Clause 66: A multi-beam charged particle microscope (1) including the multi-beam forming unit (305) described in any one of Clauses 24 to 65.

[0156] Clause 67: A multi-beam generation unit (300) for generating a plurality of primary charged particle beamlets (3), - a source (301) of charged particles, - a first collection lens (315) for collecting and forming a primary charged particle beam (309), - an absorption plate (371) having a plurality of apertures (85.4) arranged at a first pitch P1 and configured to generate a plurality of pre-formed beamlets (312) having the first pitch P1 in use, - a collimator lens (303), - a multi-beam forming unit (305) having a filter plate (304) with an aperture (85.1) arranged at a second pitch P2, - and a control unit (830). - The control unit (830) is configured to supply a first control signal to the first collection lens (315) to adjust the flow of a plurality of primary charged particle beamlets (3) during use, to match the pitch P1 of the plurality of preformed beamlets (312) with the second pitch P2 of the filter plate (304), and to supply a second control signal to the collimator lens (303) to adjust the propagation angles of the plurality of preformed beamlets (312) to form parallel preformed beamlets (312). A multi-beam generation unit (300).

[0157] Clause 68: A multi-beam charged particle microscope (1) including the multi-beam generation unit (300) described in Clause 67.

[0158] Clause 69: A multi-beam charged particle microscope (1) further including the multi-beam formation unit (305) described in any one of Clauses 24 to 65.

[0159] As long as no technical contradiction occurs as a result, the described embodiments and examples of the present invention can be combined with each other in whole or in part. Further, the present invention is not limited to specific embodiments, examples, and combinations thereof, and modifications of the embodiments are also possible. For example, the material composition and structure of the shielding plate (306) or the absorption plate (371) are also applicable to the filter plate (304), and the filter plate can be made of a metal or a metal composition containing at least one metal selected from the group of molybdenum, ruthenium, rhodium, palladium or silver, tungsten, rhenium, osmium, iridium, platinum, or gold. This improves the stopping power against x-rays. The cover layer (99) or the filter plate (304) can be formed from a conductive material with a low atomic mass number, for example, a graphite layer or a highly doped semiconductor layer, or a metal layer containing a metal with a low atomic mass number, such as aluminum, manganese, copper or silver. This reduces the generation of secondary electrons.

[0160] In principle, the wafer is referred to as the object, but the present invention is also applicable to other objects such as those used in semiconductor manufacturing. For example, the object can be a mask, such as a mask for EUV lithography, instead of a semiconductor wafer. Unlike a semiconductor wafer, such a mask is generally rectangular and has a relatively large thickness. However, the present invention is not limited to objects used in semiconductor manufacturing and is also applicable to general objects, including, for example, mineral probes or tissues. The present invention is further described with an example of a multi-beam system having a plurality of primary electron beamlets, but other charged particles, such as helium ions, can also be used.

Explanation of Signs

[0161] 1 Multi-beamlet charged particle microscope system 3 Primary charged particle beamlets 5 Focus spot of the primary charged particle beamlets 7 Object 9 Secondary electron beamlets 15 Focus spot of the secondary electron beamlets 25 Wafer surface 81 Multipole electrode 82 Annular electrode 83 Spacer 85 Aperture 86 Spacer 92 Electrostatic field (equipotential lines) 94 Terminal aperture 98 Layer of conductive material 99 Absorbing and conductive layer 100 Object irradiation unit 101 Object plane 102 Objective lens 103 Field lens group 108 First beam intersection 110 Collective multi-beam raster scanner 200 Secondary electron imaging system 205 Electron optical element 216 Multi-aperture correction plate 222 Second deflection system 225 Image plane 261 Voltage supply line 300 Charged particle multi-beamlet generator 301 Charged particle source 303 Collimating lens 304 Filter plate 305 Primary multi-beamlet forming unit 306 Mask multi-aperture plate 307 Active array optical element 308 Field lens 309 Primary electron beam 310 Terminal multi-aperture plate 311 Focus spot of primary charged particle beamlet 312 Pre-formed beamlet 313 Beam deflector plate 315 Collection lens 317 Intersection 319 Scattering point 321 Intermediate image plane 323 Intermediate image plane tilt component 351 X-ray emission 353 Secondary electrons 355 Transmitted charged particles 359 Scattered charged particles 361 Conductive layer 363 Separation layer 365 Inclined edge 367 Bulk structure 369 Baffle 371 Absorption plate 391 First layer 392 Second layer 393 Electrode layer 394 Fourth layer 395 Fifth layer 400 Beam splitter unit 500 Sample stage 503 Sample voltage source 600 Detector 800 Control unit 810 Imaging control module 830 Multi-beam Formation Control Unit 860 Scanning Control Unit 880 Control Processor 890 Memory

Claims

1. A method for operating a multi-beam forming unit (305) of a multi-beam charged particle microscope (1), comprising: generating a primary charged particle beam (309); supplying a first voltage U1 to a filter plate (304), wherein the filter plate (304) includes a plurality of apertures (85) for forming and transmitting a plurality of primary charged particle beamlets (3) from the primary charged particle beam (309) during use, and the primary charged particle beam (309) generates secondary electrons (353) at an intersection (317) of primary electrons of the primary charged particle beam (309) with the filter plate (304), or supplying a plurality of individual fourth voltages U4 to a plurality of electrodes (81, 82) of an active array optical element (307), wherein the electrodes are arranged in the vicinity of a plurality of apertures (85) of the active array optical element (307) and each is configured to focus, deflect or shape one of the primary charged beamlets (3); supplying at least one of a second voltage U2 to a second multi-aperture plate (306) disposed between the filter plate (304) and the active array optical element (307), or an absorption plate voltage U0 to an absorption plate (371) disposed upstream of the filter plate (304) in the propagation direction of the primary charged particle beam (309); adjusting at least one of the absorption plate voltage U0, the first voltage U1, or the second voltage U2 to prevent the secondary electrons (353) from intersecting the active array optical element (307) by realizing a potential barrier (A1, A2) or a potential sink (B1, B2, C) for the secondary electrons (353); and a method comprising the steps of

2. The method according to claim 1, wherein the second voltage U2 is adjusted to be smaller than the first voltage U1, thereby realizing a potential barrier (A1, A2) or a potential sink (B1) for the secondary electrons (353) upstream of the second multi-aperture plate (306).

3. The method according to claim 1, wherein the second voltage U2 is adjusted to be greater than the first voltage U1, thereby realizing a potential sink (B2) for the secondary electrons (353) in the vicinity of the second multi-aperture plate (306).

4. The method according to any one of claims 1 to 3, wherein the absorber plate voltage U0 is adjusted to be greater than the first voltage U1, thereby realizing a potential sink (C) for the secondary electrons (353) upstream of the filter plate (304).

5. The method according to any one of claims 1 to 4, wherein at least one of the absorber plate voltage U0, the first voltage U1, or the second voltage U2 is supplied at a ground level or a reference level.

6. The method according to any one of claims 1 to 5, further comprising the step of supplying a third voltage U3 to a first layer (391) of the active array optical element (307), wherein the third voltage U3 is equal to either the first voltage U1 or the second voltage U2.

7. The method according to any one of claims 1 to 6, further comprising the step of supplying a fifth voltage U5 to a fifth layer (395) of the active array optical element (307), wherein the fifth voltage U5 is equal to either the first voltage U1 or the second voltage U2.

8. A multi-beam charged particle microscope (1), a charged particle source (301) for generating a primary charged particle beam (309) and at least one condenser lens (303.1, 303.2), and a multi-beam forming unit (305), wherein the multi-beam forming unit (305) in use, a filter plate (304) having a plurality of apertures (85) for forming a plurality of primary charged particle beamlets (3) from the primary charged particle beam (309), an active array optical element (307) having a plurality of electrodes (81, 82), the electrodes being arranged in the vicinity of the plurality of apertures (85) of the active array optical element (307), and each electrode being configured to focus, deflect, or shape one of the primary charged particle beamlets (3), the active array optical element (307). At least one of a second multi-aperture plate (306) disposed between the filter plate (304) and the active array optical element (307), or an absorption plate (371) disposed upstream of the filter plate (304) in the propagation direction of the primary charged particle beam (309), A control unit (830) configured to execute any of the methods according to claims 1 to 7 during use A multi-beam charged particle microscope (1) comprising.

9. A multi-beam charged particle microscope (1), A charged particle source (301) and at least one condenser lens (303.1, 303.2) for generating and forming a primary charged particle beam (309), A multi-beam forming unit (305) Including The multi-beam forming unit (305) During use, a filter plate (304) having a plurality of apertures (85.1) with a diameter D1 for forming a plurality of primary charged particle beamlets (3), An active array optical element (307) having a plurality of apertures (85.3) with a third diameter D3, which is configured to individually focus, deflect or shape at least one of the primary charged particle beamlets (3) during use. An active array optical element (307), At least one second multi-aperture plate (306) having apertures (85.2) with a second diameter D2 disposed between the filter plate (304) and the active array optical element (307), or a plurality of apertures (85.4) with a diameter D4 An absorption plate (371) disposed upstream of the filter plate (304) in the propagation direction of the primary charged particle beam (309), By realizing a potential barrier (A1, A2) or a potential sink (B1, B2, C) for secondary electrons (353) upstream of the active array optical element (307), the secondary electrons (353) generated during use in the filter plate (304) are prevented from intersecting the active array optical element (307). During use, at least one of a first voltage U1 to the filter plate (304), a second voltage U2 to the second multi-aperture plate (306), or an absorption plate voltage U0 to the absorption plate (371) is adjusted and supplied. A control unit (830) configured to A multi-beam charged particle microscope (1) comprising.

10. The filter plate (304) has a first thickness L1 < 20 μm, preferably L1 <= 10 μm, The second multi-aperture plate (306) is arranged at a distance L2 from the filter plate (304), and the distance L2, the thickness L1, and the second diameter D2 are selected according to D2 / (L1 + L2) <= 0.3, preferably < 0.25, thereby improving the shielding of secondary electrons (353) or scattered primary electrons (355, 359) generated in the filter plate (304) during use. The multi-beam charged particle microscope (1) according to claim 9.

11. The filter plate (304) has a first thickness L1 < 20 μm, preferably L1 <= 10 μm, The second multi-aperture plate (306) is arranged at a distance L2 from the filter plate (304) and has a third thickness L3. The distance L2, the thicknesses L1 and L3, and the second diameter D2 are selected according to D2 / (L1 + L2 + L3) <= 0.3, preferably 0.

25. The multi-beam charged particle microscope (1) according to claim 9 or 10.

12. The multi-beam charged particle microscope (1) according to any one of claims 9 to 11, wherein D2 and D3 are selected according to D3 >= D2 > D1.

13. The multi-beam charged particle microscope (1) according to claim 12, wherein D2 is selected according to 1.1 × D1 < D2 < 1.3 × D1.

14. The multi-beam charged particle microscope (1) according to any one of claims 10 to 13, wherein (L1 + L2 + L3) > 130 μm, preferably > 150 μm.

15. The multi-beam charged particle microscope (1) according to any one of claims 9 to 14, wherein at least one baffle (369) is formed inside each of the apertures (85.2) of the second multi-aperture plate (306).

16. The multi-beam charged particle microscope (1) according to any one of claims 9 to 14, wherein inside each of the apertures (85.2) of the second multi-aperture plate (306), an aperture (85.2) with a diameter D2 is formed with a thickness L3.1 < L3.

17. The multi-beam charged particle microscope (1) according to any one of claims 9 to 14, wherein each of the apertures (85.2) of the second multi-aperture plate (306) is formed in a conical shape (365).

18. The multi-beam charged particle microscope (1) according to claim 17, wherein the minimum aperture diameter D2 is formed on the bottom side or the beam emission side of the second multi-aperture plate (306).

19. The multi-beam charged particle microscope (1) according to claim 17, wherein the minimum aperture diameter D2 is formed on the upper side or the beam incident side of the second multi-aperture plate (306).

20. The multi-beam charged particle microscope (1) according to any one of claims 9 to 19, wherein the second multi-aperture plate (306) includes at least one metal layer (361) on the upper side or the beam incident side.

21. The multi-beam charged particle microscope (1) according to any one of claims 9 to 20, wherein D4 is within the range of 1.1×D1 < D4 < D3.

22. The multi-beam charged particle microscope (1) according to any one of claims 9 to 21, further comprising a second absorption plate (371.2), wherein the second absorption plate (371.2) is disposed between the filter plate (304) and the shielding multi-aperture plate (306).

23. Including a first collection lens (315.1, 315.2), The absorption plate (371) having a plurality of apertures (85.4) is arranged at a first pitch P1 and is configured to generate a plurality of pre-formed beamlets (312) at the first pitch P1 during use, and is disposed between the first collection lens (315) and the at least one collimator lens (303.1, 303.2), The apertures (85.1) of the filter plate (304) are arranged at a second pitch P2 different from the first pitch P1, The control unit (830) is configured to supply a first control signal to the first collection lens (315.1, 315.2) to adjust the flow of the plurality of primary charged particle beamlets (3) during use, to match the pitch P1 of the plurality of pre-formed beamlets (312) with the second pitch of the filter plate (304), and to supply a second control signal to the at least one collimator lens (303.1, 303.2) to adjust the propagation angle of the plurality of pre-formed beamlets (312) to form parallel pre-formed beamlets (312). The multi-beam charged particle microscope (1) according to any one of claims 9 to 22.

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