Improved objective lens design for multi-beam charged particle beam system

Abstract

A multi-beam charged particle system with an improved lens design and an improved lens design for a multi-beam charged particle system is provided, which is configured for maintaining a high resolution at high throughput. The improved lens design can be applied for high throughput wafer inspection with multi-beam charged particle system.

Description

[0001] lens design for multi-beam beam

[0002] Field of invention

[0003] The disclosure relates to a single column, multi-beam charged particle beam system with a single imaging objective lens system.

[0004] WO 2005 / 024881 A2 discloses an electron microscope system which operates with a multiplicity of electron beamlets for the parallel scanning of an object or sample to be inspected with a bundle of electron beamlets. The bundle of primary electron beamlets is generated by directing a primary charged particle beam onto a multi-beam forming unit, comprising at least one multi-aperture plate, which has a multiplicity of openings. One portion of the electrons of the electron beam is incident onto the multi-aperture plate and is absorbed there, and other portions of the beam transmit the plurality of openings of the multi-aperture plate. Thereby, in the beam path downstream of each opening, a plurality of primary electron beamlets are formed whose cross section is defined by the cross section of the corresponding openings. The plurality of primary electron beamlets are focused by an objective lens on a surface of a sample. Interaction volumes of the primary electrons within the sample are formed at the plurality of focus points. Secondary electrons or backscattered electrons are emanated from the interaction volumes. Thereby, a plurality of secondary electron beamlets is emitted from the sample, collected, and imaged onto a detector. Each of the secondary beamlets is incident onto separate detector elements, so that the secondary electron intensities detected therewith provide information relating to the surface of the sample at the location where the corresponding primary beamlet is incident onto the sample. The bundle of primary electron beamlets is scanned systematically over the surface of the sample and an electron microscopic image of the sample is generated. Alternatively, the sample is continuously moved by a sample stage and a scanning image is acquired.

[0005] Semiconductor inspection task require a high resolution and a high throughput. Furthermore, different inspection tasks of semiconductor samples require different landing energies of the primary electrons. The resolution of the electron optical imaging depends on the beam spot shapes and sizes of the primary electron beamlets at the surface of a sample. Furthermore, imaging properties shall be isotropic and independent from the individual beamlet. It has turned out that some requirements of semiconductor inspection tasks are in contradiction with conventional electron-optical designs of the objective lens of a multi-beam electron beam system. For example, a high throughput is achieved by a large beam current of the plurality of beamlets. A large beam current, however, increases the interaction of electrons especially at the beam cross-over and thereby increases a beam spot size or deteriorates the shapes of the beam spots. Therefore, resolution is reduced at high throughput with large beam current. For example, in US 10,062,541 BB, a magnetic lens with a radial gap, short focal length and allegedly high resolution is provided. However, such radial gap lenses of the prior art introduce a strong magnetic field close to a sample surface. The strong magnetic field comes along with a large variation in inclination angles of the primary beamlets at the sample surface. Therefore, imaging properties of solutions disclosed in the US 10,062,541 BB are not isotropic and depend on each individual beamlet. In US 11,417,495 B2, means are provided for reducing the variation in inclination angles of the primary beamlets. However, the system according to US 11,417,495 B2 requires a huge calibration effort, if for example a landing energy of primary electrons is changed. In addition, the strong magnetic field is not only responsible for a large variation in inclination angles of the primary beamlets, but also responsible for a large variation of the starting angles of the secondary electrons, emanating from the sample surface. For example, the magnetic field at the sample causes a variation of the inclination angles of the emitted secondary electron beamlets. The different beamlets will thus not overlap in a joint cross-over in the secondary projection path anymore. Such a joint cross-over is essential for momentum or energy filtering of the secondary electrons by means of an aperture in the joint cross-over. Absent of a joint cross-over, momentum or energy filtering of the secondary electrons by means of an aperture is not possible in the cross-over, which leads to beam-to-beam variations of the detector signal.

[0006] It is therefore a task of the invention to provide an objective lens with provides high and isotropic resolution in the illumination as well as in the projection path. It is a further task of the invention to provide an objective lens with provides high and isotropic resolution together with high throughput. It is a further task of the invention to provide an objective lens with provides high and isotropic resolution combined with a variability of the landing energy without the need of elaborative system calibration.

[0007] Intermediate document DE 10 2023 119 451 Al discloses a multi-beam particle system with an electrostatic booster lens arranged level with a cross-over of the primary particle beams. The electrostatic booster lens increases the kinetic energy of the primary particle beams only section-wise in the region of the cross-over, which means that the kinetic energy is significantly increased and immediately afterwards significantly decreased again in the region of the cross-over.

[0008] US 2020 / 0161079 Al, US 2020 / 0243296 Al, US 2023 / 0066086 Al and US 2023 / 0245852 Al provide further background art.

[0009] Description of the invention

[0010] The invention provides a multi-beam charged particle system with an objective lens for a high throughput and high imaging resolution. The invention provides a multi-beam charged particle system with an objective lens capable of isotropic imaging conditions for each of the plurality of beamlets.

[0011] The present patent application claims the priority of the German patent application No. 10 2024211 546.0 filed on 03 December 2024, the disclosure of which in the full scope thereof is incorporated in the present patent application by reference.

[0012] The multi-beam charged particle beam system according to the embodiments is configured for irradiation of a surface of an object or sample with a plurality of primary charged particle beamlets. For example, the multi-beam charged particle beam system is comprising a charged-particle multi-beamlet generator, comprising at least one charged particle source and multi-beam forming unit for generating the plurality of primary charged particle beamlets. The multi-beam charged particle beam system is further comprising an object irradiation unit, comprising at least one field lens and an objective lens for forming a plurality of focus spots of the plurality of primary charged particle beamlets in an image plane. The multi-beam charged particle beam system is further comprising a stage for positioning the surface of the sample in or at least close to the image plane. The multi-beam charged particle beam system is further comprising a beam tube for guiding the plurality of beamlets at a kinetic energy U0. According to a first embodiment, the objective lens is formed during use by a beam exit opening of the beam tube, which forms a first electrode. The beam tube is connected to a first voltage supply for providing during use a first voltage U1 to beam exit opening of the beam tube. The objective lens further comprises a coil, an upper pole shoe and a lower pole shoe. During use, a current is provided to the coil to generate a magnetic field. The upper and lower pole shoes are forming an axial gap of width AG for forming an axially confined magnetic field. The lower pole shoe is electrically isolated from the upper pole shoe and connected to a second voltage supply for providing during use a second voltage U2 to the lower pole shoe. The objective lens further comprises a decelerating electrode arranged in propagation direction of the primary charged particles downstream of the lower pole shoe. The decelerating electrode is arranged between the lower pole shoe and the image plane. The decelerating electrode is connected to a third voltage supply for providing during use a third voltage U3 to the decelerating electrode. The objective lens is further formed by an electrical field generated by sample voltage supply, which is connected to the stage and configured for providing during use a fourth voltage U4 to the sample. According to the first embodiment, the beam exit opening, the lower pole shoe, the electrode, the image plane are configured and the first to fourth voltages U1 to U4 are selected to generate during use a maximum field strength Emax within an axial extension MzE with MzE > 3mm, for example MzE > 5mm, for example MzE > 10mm. With this extended maximum of maximum field strength Emax, wherein the electrical field is not deviating more than 10% from the maximum field strength Emax over an axial distance MzE exceeding for example 3mm, an efficient deceleration of primary electrons is achieved over a shorter distance, such that a focal length of the objective lens is reduced. Thereby, aberrations are reduced, and a throughput can be increased without resolution loss. The extended maximum of maximum field strength Emax further acts on secondary electrons as well and reduced thereby an imaging aberration of a secondary electron imaging as well.

[0013] In addition, with the axial gap, forming an axially confined magnetic field lens, the magnetic field at the image plane is reduced and a tilt angle of primary beamlets is reduced. For example, the axial gap with a width AG is given by AG <= 15mm, for example AG <= 12mm, for example AG about 10mm. Furthermore, with the reduced magnetic field at the image plane, a tilt angle of secondary electrons is reduced.

[0014] In an example, the plurality of primary charged particle beamlets exit the beam exit opening of the beam tube at a kinetic energy U0 of U0 >= 30keV. With higher kinetic energies achievable with the objective lens, for example U0 = 60keV, or U0 = lOOkeV, a cross-talk within a cross over of the plurality of primary beamlets is reduced, and a throughput can be increased without resolution loss. In an example, even with higher kinetic energies, an axial distance zl between the beam exit opening of the beam tube and the image plane is given by zl [mm] <= U0 / 10 (keV / mm). With this reduced distance zl, the beam exit opening of the beam tube is closer to the image plane, and for example a beam deflector can be arranged closer to a beam cross over or pupil plane. Thereby, aberrations are reduced, and resolution is increased.

[0015] The multi-beam charged particle beam system is further comprising a control unit configured for generating and providing during use first to fourth voltages Ul, U2, U3, U4, with U3 < U2 < Ul, and with U4 = U3 + / - lkV. With the selection of first to fourth voltages, and the design of electrodes and pole shoes, an extended maximum of maximum field strength Emax is achieved. According to a second embodiment, the upper pole shoe is further connected to a fifth voltage supply for providing during use a boost voltage Ub to the upper pole shoe, wherein Ub > Ul. For example, Ub is selected within a range of 150% * UO / e | <= Ub <= 1160% * UO / e | , with e being the elementary charge of a primary charged particle. For example, U0 = 30keV and Ub = 30kV. For example, U0 = 60 keV and Ub = 30kV. The upper pole shoe comprises and extended pole shoe tube with axial length Lb with Lb selected between 14mm <= Lb <= 18mm. With the upper pole shoe forming a booster electrode, cross-talk within a beam cross-over is reduced and thus, aberrations are reduced and a throughput can be increased without resolution loss.

[0016] According to a third embodiment, the multi-beam charged particle beam system is further comprising a shielding electrode arranged between the decelerating electrode and the image plane. The shielding electrode is arranged in a circumference of a confined area centered around the optical axis. The shielding electrode is connected to a sixth voltage supply for providing during use the shielding electrode with a shielding voltage Us with Us < U4. For example, the shielding voltage Us is approximately given by Us = U4 - 100V.

[0017] According to a further aspect of the embodiments, a multi-beam charged particle beam system for irradiation of a surface of a sample with a plurality of primary charged particle beamlets is comprising an objective lens for forming a plurality of focus spots of the plurality of primary charged particle beamlets in an image plane, wherein the objective lens is comprising a coil, an upper pole shoe and a lower pole shoe, both forming an axial gap of width AG for forming an axially confined magnetic field. The lower pole shoe is electrically isolated from the upper pole shoe and connected to a second voltage supply for providing during use a second voltage U2 to the lower pole shoe. The upper pole shoe is connected to a fifth voltage supply for providing during use a boost voltage Ub to the upper pole shoe. For example, Ub is selected within a range of 160% * UO / e | < Ub < 1160% * UO / e | , with e being the elementary charge of a primary charged particle. In an example, the plurality of primary charged particle beamlets exit a beam exit opening of a beam tube arranged upstream of the upper pole shoe segment at a kinetic energy U0 of U0 >= 30keV, for example U0 = 30keV, U0 = 60keV, or U0 = lOOkeV, and the kinetic energy of the primary charged particle beamlets is increased during passage of the primary charged particle beamlets through the upper pole shoe segment at boost voltage Ub > Ul, wherein U1 is the voltage provided to the beam tube. In an example, the axial gap with width AG is given between 10mm <= AG <= 20mm.

[0018] In an example, an electrode is attached in electrical contact to the upper pole shoe. The electrode has an opening for passing the plurality of charged particle beamlets. The electrode is made from material with low magnetic permeability such that the magnetic field lens generated during use is confined between the edge of the upper pole shoe segment and the lower pole shoe segment but not influenced by the electrode. In an example, the electrode is further connected, for example via the upper pole shoe segment, to a microampere meter for measuring during use an electron beam current. For example, a faraday cup is formed into the electrode, and the multi-beam charged particle beam system is configured for deflecting the plurality of primary charged particle beamlets into the faraday cup for beam current measurement.

[0019] According to a further aspect, the upper pole shoe comprises an extended pole shoe tube for passing the plurality of primary charged particle beamlets. The extended pole shoe can have an axial length Lb selected between 8mm <= Lb <= 18mm. The kinetic energy of the primary charged particles is increased during passage of the primary charged particle beamlets through the extended pole shoe tube. In an example, the extended pole shoe tube is formed by the opening in the electrode. The electrode can be arranged between the upper pole shoe segment and the lower pole shoe segment. In this example, the axial gap with width AG between upper and lower pole shoe segments can be larger, for example between 15mm <= AG <= 20mm. In an example, the electrode is arranged between the beam tube and the upper pole shoe segment. In this example, the axial gap with width AG between upper and lower pole shoe segments can be smaller, for example between 10mm <= AG <= 15mm. In an example, a first electrode can be attached in electrical contact to the upper pole shoe segment between the upper pole shoe segment and the lower pole shoe segment, and a second first electrode can be attached in electrical contact to the upper pole shoe segment between the beam tube and the upper pole shoe segment. Thereby, a length Lb of the extended pole shoe can be increased.

[0020] According to a further aspect, an inner radius r4 of an edge of the upper pole shoe segment can be larger than the inner radius r2 of the lower pole shoe segment. With r4 > r2, a radial gap RG is formed in addition to the axial gap AG. The radial gap allows an extension of the magnetic field lens in direction of the beam tube. Thereby, no immersion lens is formed and the magnetic field at the sample is kept very small.

[0021] A multi-beam charged particle beam system according to the disclosure is comprising a control unit. The control unit is configured for generating and providing during use at least one of a voltage selected from the voltages Ul, U2, U3, U4, Us, Ub. In an example, a multi- beam charged particle beam system is further comprising a beam divider unit for separating a secondary electron beam path from a primary beam path. In an example, the multi-beam charged particle beam system is further comprising a detection unit with an image sensor for imaging and detecting a plurality of secondary electron beamlets. In an example, the detection unit comprises an aperture stop arranged in a pupil plane of the detection unit, and wherein the objective lens is configured such that the plurality of secondary electron beamlets form a cross-over at the pupil plane. With the improved objective lens, secondary electron beamlets form an overlapping cross-over at the aperture stop, and an isotropic imaging is achieved for each of the plurality of secondary beamlets at a high throughput.

[0022] According to a further embodiment, a method of operating a multi-beam charged particle beam system is provided. The method is comprising a step of positioning a surface of a sample in an image plane of the multi-beam charged particle beam system, and a step of generating and transmitting a plurality of primary charged particle beamlets through a beam exit opening of a beam tube at a kinetic energy U0 of U0 >= 30keV. The method is further comprising a step of generating and providing, by a control unit, a current I to a coil to generate a magnetic lens field, and a step of generating and providing, by a control unit, at least four voltages comprising first to fourth voltage Ul, U2, U3, U4 to elements forming electrodes comprising a lower pole shoe, a beam tube, a decelerating electrode arrange between the lower pole shoe and the image plane, and a wafer support table with U3 < U2 < Ul, and with U4 = U3 + / - lkV, to provide a decelerating electrical field E to decelerate the plurality of primary charged particle beamlets to a landing energy LE. In an example, the at least four voltages are selected to generate a maximum field strength Emax with 3kV / mm <= Emax <= 8kV / mm. The maximum field strength Emax is generated with an axial extension

[0023] MzE with MzE > 3mm, for example MzE > 5mm, for example MzE > 10mm. Within the axial extension MzE, the electrical field E does not deviate from the maximum field strength Emax bey more than 10%. In an example, the method is further comprising a step of generating and providing a fifth voltage Ub with Ub > U1 to an upper pole shoe. For example, Ub is selected in a range 150% * UO / e | <= Ub <= 1160% * UO / e | , with e being an elementary charge of a primary charged particle.

[0024] By the embodiments or examples of the invention, a multi-beam charged particle beam system and a method of operating a multi-beam charged particle beam system with high and isotropic resolution in the primary irradiation and secondary detection path is provided. The high and isotropic resolution is further provided for a higher throughput. The embodiments or examples of the invention address objects by taking into account several intertwined considerations and provide at least one, for example two or three solutions selected from a group of solutions including a) Providing a shorter focal length of an objective lens, to generally reduce aberrations. b) Providing or allowing a higher kinetic energy of charged particles at a beam cross-over, thereby to reduce a cross-talk during cross-over. For example, including a booster electrode formed for example by the upper pole shoe segment or attached to the upper pole shoe segment of a magnetic lens. c) Providing and configuring electrodes to generate a decelerating electrical field with a large extension of a maximum field strength. Thereby, a more efficient deceleration of primary charged particles is enabled over a shorter distance MzE. Thereby, desired landing energies LE over a large range can be achieved despite higher kinetic energies at a beam cross-over or a shorter focal length of the objective lens. d) Limiting a magnetic field strength at a surface of a sample to reduce any beam-tilt, including reducing a beam tilt of secondary electron beamlets emitted from the surface of the sample. Beam tilts are induced by magnetic fields at different tilt angles, thus with a limited magnetic field strength at the surface of a sample, tilts are reduced and an isotropic and equal resolution is achieved for each of a plurality of beamlets. e) Limiting a maximum field strength Emax of the electrical field, to avoid arcing or sparkovers. f) Providing a shielding electrode for avoiding arcing or damaging of the sample in the circumference of a confined area centered around the optical axis.

[0025] The invention allows therefore a direct write or wafer inspection with higher precision and with a higher throughput. It will be understood that the invention is not limited to the embodiments and examples but comprises also combinations and variations of the embodiments and examples.

[0026] Brief description of the drawings

[0027] Embodiments of the present disclosure will be explained in more detail with reference to drawings, in which:

[0028] Fig. 1 is a schematic sectional view of a multi-beam charged particle beam system 1 according to a first embodiment

[0029] Fig. 2 illustrates some details of a multi-beam charged particle beam system 1 according to the first embodiment

[0030] Fig. 3 illustrates a detector 600 with an optical relay system

[0031] Figs. 4a, 4b illustrate aspects of an improved objective lens 102 according to the first embodiment Fig. 5a-c illustrates kinetic energy, electrical and magnetic field along an optical axis 1105 of an improved objective lens 102

[0032] Fig. 6 illustrates aspects of an improved objective lens 102 according to a second embodiment

[0033] Fig. 7a, b illustrates kinetic energy, electrical and magnetic field along an optical axis 1105 of an improved objective lens 102

[0034] Figs. 8a, 8b illustrate aspects of an improved objective lenses 102 according to a third embodiment

[0035] Fig. 9 illustrates a further example of an improved objective lens 102 according to the third embodiment

[0036] Figs. 10a, b,c illustrate further aspects of an improved objective lens 102 according to an embodiment

[0037] Figs. 11a, b illustrate two further examples of an improved objective lens 102

[0038] Description of Exemplary Embodiments

[0039] In the exemplary embodiments of the invention described below, components similar in function and structure are indicated as far as possible by similar or identical reference numerals.

[0040] Some array elements, for example the plurality of primary charged particle beamlets, are identified by a reference number. Depending on the context, the same reference number may also identify a single element or the array elements. For example, each primary charged particle beamlet (3.1, 3.2, 3.3) is one beamlet of the plurality of primary charged particle beamlets (3).

[0041] The schematic representation of figure 1 illustrates basic features and functions of a multibeam charged-particle system 1 according to a first embodiment. It is to be noted that the symbols used in the figure have been chosen to symbolize their respective functionality. The type of system shown is that of a multi-beam scanning electron microscope using a plurality of primary charged particle beamlets 3 for generating a plurality of primary charged particle beam spots 5 on a surface 25 of a sample 7, such as a wafer or mask substrate located with a top surface 25 in an image plane 101 of an improved objective lens 102. More details of the improved objective lens 102 are illustrated below. For simplicity, only three primary charged particle beamlets 3.1 to 3.3 and three primary charged particle beam spots 5.1 to 5.3 are shown, but there can be more beamlets, for example the number J of beamlets can be more than J = 60, more than J = 90, for example J = 397, J = 547, or even more, for example J = 919. The features and functions of multi-beamlet charged-particle system 1 can be implemented using electrons or other types of primary charged particles such as ions, in particular Helium ions. Further details of the microscope system 1 are provided in International Patent application WO 2022 / 262970 Al, which is hereby fully incorporated by reference.

[0042] The multi-beam charged particle beam system 1 comprises an object irradiation unit 100 and a detection unit 200 and a secondary electron beam divider or beam splitter unit 400 for separating the secondary charged-particle beam path 13 from the primary charged-particle beam path 11. The object irradiation unit 100 comprises a charged-particle multi-beam generator 300 for generating the plurality of primary charged-particle beamlets 3 and is adapted to focus the plurality of primary charged-particle beamlets 3 in the image plane 101, in which the surface 25 of a sample or wafer 7 is positioned by a sample stage 500.

[0043] The primary beam generator 300 produces a plurality of intermediate focus spots of the primary charged particle beamlets 3 in an intermediate image surface 321. The primary beamlet generator 300 comprises at least one source 301 of primary charged particles, for example electrons. The at least one primary charged particle source 301 emits a diverging primary charged particle beam 309, which is collimated by at least one collimating lens 303. The collimating lens 303 usually consists of one or more electrostatic or magnetic lenses, or by a combination of electrostatic and magnetic lenses. The collimated primary charged particle beam 309 is incident on the multi-beam forming unit 305. A multi-beam forming unit 305 is for example explained in US 2019 / 0259575, and in US 10.741.355 Bl, both hereby incorporated by reference. The multi-beam forming unit 305 basically comprises a first multi-aperture plate or filter plate 304 illuminated by the primary charged particle beam 309. The first multi-aperture plate or filter plate 304 comprises a plurality of apertures in a raster configuration for generation of the plurality of primary charged particle beamlets 3, which are generated by transmission of parts of the primary charged particle beam 309 through the plurality of apertures. The multi-beamlet forming unit 305 comprises at least one further multi-aperture plate 306, which is located, with respect to the direction of movement of the electrons in beam 309, downstream of the first multi-aperture or filter plate 304. For example, a second multi-aperture plate 306 comprises for example four, eight or more electrostatic elements for each of the plurality of apertures, for example to deflect each of the plurality of beamlets 3 individually. The multi-beam forming unit 305 is further configured with an adjacent electrostatic field lens 331, which can be combined in the multi- beam forming unit 305. Together with a second field lens 333, each of the plurality of primary charged particle beamlets 3 is focused in or in proximity of the intermediate image surface 321. The primary charged-particle source 301 and each of the active multi-aperture plates 306 are controlled by control unit 830.

[0044] After passing the intermediate image surface 321, the plurality of focus points of primary charged particle beamlets 3 is imaged by field lens group 103 and objective lens 102 into the image plane 101, in which the surface 25 of the sample 7 is positioned. A decelerating electrostatic field is generated between the objective lens 102 and the object surface 25 by application of a voltage to the object by the sample voltage supply 503. With the decelerating electrostatic field generated by sample voltage supply 503, a landing energy EL of primary electrons is adjusted to for example 10keV, 2keV, IkeV, below 800eV, below 500 eV, below 300eV or even less, for example lOOeV. Figure 2 illustrates further details of an improved objective lens 102 and the decelerating electrostatic field. Same reference numbers are used, and reference is made also to the description of figure 1. A beam tube 151 is provided downstream of the multi-aperture arrangement 305. In the example of figure 2, the beam tube 151 is comprising several beam tube segments 151.1 to 151.5. The beam tube 151 being connected to a voltage supply with the first or tube voltage Ul. From the entrance of a beam tube 151, the plurality of primary charged particle beamlets 3 propagate along the primary beam path 11 at a constant kinetic energy ET until the exit opening 153 of the beam tube 151. The kinetic energy ET of the primary charged particle beamlets 3 during passing the beam tube 151 is for example 20keV, 30keV, 60keV, or more.

[0045] The plurality of primary charged particle beamlets 3 are imaged and focus points 5.o and 5.i are formed in an image plane 101 by field lenses 333 and 103 (see figure 1), and by the improved objective lens 102. The improved objective lens 102 is of the type of a magnetic lens with a coil 161 and a pole shoe 163 with a lower pole shoe segment 165, forming an axial gap for the magnetic field. A current I is provided during use to the coil 161 to generate the focusing magnetic field (not shown). The axial gap is further illustrated down below. With the axial gap, the magnetic field is constrained within a volume above the surface 25 of the sample?, such that no immersion lens is formed. At the surface 25 of the sample 7, primary as well as secondary beamlets are therefore almost free of any inclination angles generated by the magnetic field of objective lens 102.

[0046] According to the embodiment, at least the lower pole shoe segment 165 is connected to a voltage supply and a second voltage U2 is provided to the pole shoe 163. The lower pole shoe segment 165 forms an electrode and forms a potential difference given by the voltage difference U2 - Ul. Below the lower pole shoe segment 165, a decelerating electrode 133 can be provided, connected to a voltage supply for providing a voltage U3 to the electrode. In the example shown, the decelerating electrode 133 is provided as separate electrode. Via sample voltage supple 503, a sample voltage U4 is provided by sample voltage supply 503 to a sample mounting platform for holding and contacting during use a wafer 7. The voltage differences between Ul, U2, U3 and U4 are responsible for the generation of an electric field 137, illustrated in figure 2 with the equipotential lines of the electric field 137. According to the voltage difference between Ul, U2, U3 and U4, the electrical field 137 is generated, which is almost parallel to the propagation direction of the primary charged particle beamlets 3 and generates a decelerating force to the primary charged particles. The sample voltage U4 is adjusted such that the third kinetic energy or landing energy EL of the primary electrons is adjusted in a range below lOkeV, below 2keV, IkeV, 800eV, below 300eV or even below lOOeV. The electrical field 137 forms a decelerating field to reduce the kinetic energy of the primary charged particle beamlets 3 before impinging on the sample surface 25 arranged in the image plane 101, such that a high resolution is achieved. The electrical field 137 forms in parallel an extraction field for extracting and accelerating secondary electrons from the wafer 7. The electrical field 137 above the surface 25 of the sample 7 is therefore also called the extraction field 137.

[0047] The object irradiation system 100 of the multi-beam charged particle beam system 1, shown in Figure 1 and 2, further comprises a collective multi-beam raster scanner 110 in proximity of a beam cross over 108 by which the plurality of charged particle beamlets 3 can be deflected in scanning direction perpendicular to the propagation direction of the charged particle beamlets. The propagation direction of the primary beamlets throughout the examples is in positive z-direction. Objective lens 102 and collective multi-beam raster scanner 110 are centered at an optical axis (not shown) of the multi-beam charged-particle system 1. The plurality of primary charged particle beamlets 3, forming the plurality of beam spots 5 arranged in a raster configuration, is scanned synchronously over the wafer surface 25. The primary beam spots 5 have a distance about 6pm to 45pm and a diameter of below 5nm, for example 3nm. In an example, the beam spot size is about 3nm, and the distance between two adjacent beam spots is 8pm. At each scan position of each of the plurality of primary beam spots 5, a plurality of secondary electrons is generated, respectively, forming the plurality of secondary electron beamlets in the same raster configuration as the primary beam spots 5. The intensity of secondary charged particle beamlets, including beamlets 9.o and 9.i depends on the intensity of the impinging primary charged particle beamlet 3, the material compositions, the topography of the sample 7 under the beam spot 5, and the charging condition of the sample at the beam spot 5. The secondary charged particle beamlets are accelerated by the same electrostatic field 137 between objective lens 102 and object surface 25 and are collected by objective lens 102 and pass the first collective multibeam raster scanner 110 in opposite direction to the primary beamlets 3. The plurality of secondary beamlets is scanning deflected by the first collective multi-beam raster scanner 110. The plurality of secondary charged particle beamlets is then guided by the beam splitter unit 400 to follow the secondary beam path 13 to the detection unit 200. The beam divider 400 comprises a first beam divider segment 400.1 arranged in the primary beam path 11, and third beam divider segment 400.3 arranged in the secondary electron beam path 13, and a second beam divider segment 400.2 arranged in both primary and secondary electron beam path 11 and 13 and configured for dividing secondary electron beamlets 9 from the primary charged particles.

[0048] Detection unit 200 images the secondary electron beamlets 9 onto the surface 225 of an image sensor 600 to form there a plurality of secondary charged particle image spots 15. Detection unit 200 further comprises at least a second raster scanner 222, which is connected to scanning control unit 860 (see figure 1). Scanning control unit 860 is configured to compensate for a difference in the scanning deflection power of the first scanning deflector 110 in the common beam path and the second scanning deflector 222, such that the positions of the plurality secondary electron focus spots 15 are kept constant at image sensor 600. In the example, the detection unit 200 further comprises several beam tube segments 151.1 to 151.5. The detection unit 200 comprises first to third electron optical lenses 205.1 to 205.3, first and second electro-optical lens elements 211.1 and 211.2, first compensator 220, and at least two stigmators 264.1 and 264.2. A pupil or aperture stop 284 is arranged within a common pupil plane 258 between two tube segments 151.4 and 151.5 and for example mounted on a stage for adjustment or exchange. The pupil plane 258 corresponds to a joint cross-over of the secondary electron beamlets 9. With the pupil or aperture stop 284, a momentum or energy filtering is obtained. Such a momentum or energy filtering is for example described in WO 2024 / 078739 Al, which is hereby fully incorporated by reference. The detection unit 200 further comprises the image sensor 600, which is connected to imaging control module 810, configured to receive image data during scanning operation. The first to third electron optical lenses 205.1 to 205.3, the first and second electro-optical lens elements 211.1 and 211.2, the first compensator 220, and the stigmators 264.1 and 264.2 are connected to secondary beam-path control module 840.

[0049] The detector or image sensor 600 comprises a plurality of detector pixels or individual detectors. For each of the plurality of secondary charged particle beam spots 15, the intensity is detected separately, and the property of the object surface 25 is detected with high resolution for a large image patch of the sample 7 with high throughput. For example, with a raster of 10 x 10 beamlets with 8pm pitch, an image patch of approximately 88pm x 88pm is generated with one image scan with collective multi-beam raster scanner 110. The image patch is sampled with a pixel spacing of for example lnm, thus with a pixel number of 8000 pixels per image line for each beamlet, such that the image patch generated by 100 beamlets comprises 6.4 gigapixel. The digital image data is collected by control unit 800. Details of the digital image data collection and processing, using for example parallel processing, are described in international patent application WO 2020 / 151904 A2 and in US- Patent US 9.536.702, which are hereby incorporated by reference.

[0050] The image sensor 600 is configured by an array of sensing areas in a pattern compatible to the raster arrangement of the secondary electron beamlets focused by the detection unit 200 onto the image sensor 600. This enables a detection of each individual secondary electron beamlet independent from the other secondary electron beamlets incident on the image sensor 600. The image sensor 600 illustrated in figures 1 or 2 can be an electron sensitive detector array such as a CMOS or a CCD sensor. Such an electron sensitive detector array can comprise an electron-to-photon conversion unit, such as a scintillator element or an array of scintillator elements. In another embodiment, the image sensor 600 can be configured as electron-to-photon conversion unit or scintillator plate arranged in the focal plane 225 of the plurality of secondary electron particle image spots 15. An example is shown in figure 3. The image sensor 600 of figure 3 further comprises a relay optical system comprising collection lenses 605 and zoom lens 611 for imaging and guiding the photons generated by the electron-to-photon conversion unit 602 at the secondary charged particle image spots 15 on dedicated photon detection elements 623, such as a plurality of photomultipliers or avalanche photodiodes. Such an image sensor is disclosed in US 9,536,702, which is cited above and incorporated by reference. The image sensor 600 is further configured with an optionally extractable monitoring system 230, comprising a beam divider mirror 237, an imaging lens 235 and a CMOS sensor 232 with high resolution. The image sensor 600 may further comprise a plurality of light guiding fibers 615, configured to collect and guide photons to the dedicated photon detection elements 623. Light guiding fibers 615 can be mounted to a carrier 617 configured for lateral and rotational adjustment of the plurality of light guiding fibers 615.

[0051] During an acquisition of an image patch by scanning the plurality of primary charged particle beamlets 3, stage 500 is preferably not moved, and after the acquisition of an image patch, the stage 500 is moved to the next image patch to be acquired. In an alternative implementation, stage 500 is continuously moved in a second direction while an image is acquired by scanning of the plurality of primary charged particle beamlets 3 with the collective multi-beam raster scanner 110 in a first direction. Stage movement and stage position are monitored and controlled by sensors known in the art, such as Laser interferometers, grating interferometers, confocal micro lens arrays, or similar.

[0052] During an image scan, control unit 800 is configured to trigger the image sensor 600 to detect in predetermined time intervals a plurality of timely resolved intensity signals from the plurality of secondary electron beamlets, and the digital image of an image patch is accumulated and stitched together from all scan positions of the plurality of primary charged particle beamlets 3.

[0053] The control unit 800 of the multi-beamlet charged-particle system 1 further comprises an- imaging control module 810, configured to receive the data streams from the image sensor 600 and to generate during operation a digital image of the surface of the sample 7; a secondary beam-path control module 840, configured to control the detection unit 200; a primary beam-path control module 830, configured to control the elements of the object irradiation unit 100; a stage control module 850, configured to control the stage positioning and alignment, and including control of the sample voltage supply unit 503; a scanning operation control module 860, configured to control a scanning operation by the first collective multi-beam raster scanner 110 and the second deflection system 222; a control operation processor unit 880, configured to execute inspection tasks of samples, and configured to control the modules 810, 820, 830, 840, 850, 860 and a memory 890 for storing software, instructions and image data. The control operation processor unit 880 is further connected to an interface IX for exchange of data, instructions, software or user interaction.

[0054] It has been realized that systems according to the prior art have certain performance constraints for a combination of a high throughput and a high, isotropic resolution. Resolution is primarily limited by the size and shape of the beam spots 5. An isotropic resolution is achieved when resolution does not depend on the orientation of a sample 7. Furthermore, resolution provided by all J beamlets must be the same or at least not deviate for example by more than 10% of the resolution itself. Typical values for resolution are 4nm, 3.5nm, 3nm, 2.5nm or less.

[0055] Generally, a high throughput is required for wafer inspection. High resolution is for example achieved with larger currents of each beamlet 3 or a larger number J of beamlets or a combination of both. The requirement of high resolution, however, puts a constraint on the total beam current of all primary charged particle beamlets 3. All primary beamlets 3 intersect at the first beam cross-over 108 and all primary charged particles interact at the first beam cross-over 108, which results in a blurring of the beam spots 5. This blurring increases with the total beam current and therefore limits beam currents and thus throughput at a given resolution requirement.

[0056] In addition, some wafer inspection tasks require large landing energies. For example, high landing energies of LE = 10kV or more can be desired. A desired landing energy LE is for example predetermined at representative test samples.

[0057] Figure 4a illustrates further details of an improved objective lens 102 according to the first embodiment. Same reference numbers are used as within figures 1 and 2, and reference is made to the description above. The beam tube 151 reaches into the objective lens 102 until beam exit opening 153. Beam tube 151 is electrically connected to a first voltage supply 173.1 and beam tube 151 is set during use to a first voltage Ul. Close to the beam exit opening 153 of the beam tube 151, collective multi-beam raster scanner 110 and magnetooptical multipole deflection or correction elements 112 are arranged. The improved objective lens 102 comprises a pole shoe 163 comprising a first, upper pole shoe segment 167 and a second, lower pole shoe segment 165. Upper pole shoe segment 167 and lower pole shoe segment are electrically separated by an isolating gap 171. The isolation gap 171 can be formed by isolating material between the two pole shoe segments 165 and 167, or by formation of an air or vacuum gap. Lower pole shoe 165 is connected to a second voltage supply 173.2, and a voltage U2 is provided to the lower pole shoe 165, which thereby forms an electrode. Both pole shoes 165 and 167 form a gap 168, wherein the gap 168 is parallel to the optical axis 1105, or forms only a small angle with the optical axis 1105 (see also figure 4b). The magnetic lens is therefore sometimes also called axial gap lens. Downstream of the lower pole shoe 165 (in propagation direction of the primary charged particles), a decelerating electrode 133 is arranged. Decelerating electrode 133 is connected to a third voltage supply 173.3 and set during use to a third potential or voltage U3. Sample voltage U4 is provided to the sample 7 by a fourth or sample voltage supply 503. In total, during use, four voltages Ul, U2, U3 and U4 are provided to the beam tube 151, the lower pole shoe 165, the decelerating electrode 133 and the sample 7 via wafer stage 500. Three potential differences dUl = U2- Ul, dU2 = U3 - U2 and dU3 = U4 - U3 are adjusted by providing four different voltages Ul to U4. The electrode formed by lower pole shoe 165 enables a larger axial extension MzE of a maximum electrical field strength Emax of the decelerating electrical field E and thus a shorter length zE of the decelerating electrical field E (for illustration, see figure 5 below). For example, in good approximation, the axial extension

[0058] MzE of a maximum electrical field strength Emax is given by

[0059] MzE = (UO-LE) / Emax. For example, with a kinetic energy U0 of about 30keV, and a landing energy of LE < 1 keV, and Emax = 6kV / mm, the axial extension MzE of the maximum field strength Emax is about 5mm. Generally, the axial extension MzE of the maximum field strength Emax is about MzE = 3mm to MzE = 5mm for U0 = 30keV, and MzE = 8mm to MzE = 10mm for U0 » 30keV.

[0060] Thereby, the length of the beam tube 151 can be increased and the distance zl between the beam exit opening 153 of the beam tube 151 and the image plane 101 is reduced (see Figure 4b). For example, distance zl is reduced to zl < 10mm (for example for U0 = 30keV), or zl is reduced to below zl < 20mm for U0 = 60keV. Generally, axial distance zl is reduced to zl [mm] <= | U01 / 10 (keV / mm).

[0061] The geometrical parameters of the objective lens 102 are illustrated in figure 4b. Only half of the cross-sections of the radially symmetrical arrangement of elements is illustrated. The parameter values are illustrated in a simplified manner and not illustrated by scale. The beam exit opening 153 of beam tube 151 has an inner radius rl with a distance zl to the image plane 101, in which during use the wafer surface 25 is arranged. The upper pole shoe 167 has an inner radius r4 at a distance z4 to the image plane 101. The lower pole shoe 165 has an inner radius r2 at a distance z2 to the image plane 101. The decelerating electrode 133 has an inner radius r3 at a distance z3 to the image plane 101. The inner radii rl to r3 are selected between 1.5mm and 6mm, for example rl = r2 = r3 = 3.5mm. However, the radii must not be identical, but can differ up to 1 or 2 mm. The fourth radius r4 is selected between 1.5mm and 20mm, for example r4 = 3.5mm or r4 = 6mm. Between upper and lower pole shoes 167 and 165, an axial gap of width AG ~ | z4-z21 is formed. Some examples of further design values are given in Table 1. With the improved objective lens 102, the primary kinetic energy U0 of the primary charged particles within the beam tube 151 can be selected in a wide range between UO = 2OkeV and UO = lOOkeV. Table 1 gives exemplary design values for UO = 3OkeV and UO = 6OkeV for LE = 3keV.

[0062] Table 1: Exemplary design values of an improved objective lens 102 For example, the first to fourth voltages Ul, U2, U3, U4 are selected according to U3 < U2 < Ul, and with U4 = U3 + / - lkV. For example, U2 is selected in a range between 55% * U3 > U2 > 65% * U3. For example, for UO = 40keV, U2 = -21 kV and U3 = -34.5 kV are selected. For example, for UO = 70keV, U2 = -43kV and U3 = -66.5kV are selected. For example, U2 is approximately selected in a range 40% * UO / e < | U21 < 55% * UO / e. With a small axial gap AG of the magnetic lens with for example about AG = 10mm, for example AG <= 12mm, or for example AG <= 15mm, the magnetic field is constrained within T1 a small region and a magnetic field Bp at the surface 25 of the sample 7 is reduced to lmT, below 1.5mT, below 1.7mT, below 2mT, below 3mT or below 3.5mT. Thereby, an isotropy of an image acquisition is improved. Especially, secondary electrons are not substantially rotated by the magnetic field B and emanated normal to sample surface 25. The constraint of magnetic field B allows a reduction of the focal length fi. For example, a short focal length fi below for example fi < 20mm or fi < 25mm is enabled. Thereby, aberrations are reduced.

[0063] As a result of the reduced distance zl, the additional electrode, here formed by lower pole shoe 165 at voltage U2, and the reduction in axial gap AG of the improved objective lens 102, the primary electrons keep their kinetic energy U0 as far as possible and are decelerated to the landing energy LE over a shorter distance zE closer to the image plane 101. The deceleration distance zE defines the distance from the image plane 101, over which primary electrons are decelerated from the primary kinetic energy U0 to the landing energy LE. Examples of reduced deceleration distances zE are zE below 17mm at U0 = 60keV, zE below 9mm at U0 = 30keV, or even less, for example zE = 8mm. The shorter distance zE is achieved by the two electrodes at voltage U2 and U3, whereby the electrical field E achieves a broad maximum of axial extension MzE with maximum field strength Emax. Furthermore, by the constraint of the maximum field strength Emax between the two electrodes at voltage U2 and U3, a low electrical field strength Ep is achieved at the surface 25 of the sample 7. For example, U3 is approximately given by U3 = Ep * | zl-z31 . For example, U2 is approximately given by U2 = U3 + Emax * | z2-z31 . Kinetic energy UE, electrical field strength E and magnetic field strength B are illustrated in Figure 5a to 5c. Figure 5a illustrates the almost constant deceleration of primary electrons from kinetic energy U0 down to the landing energy LE, which is achieved by the extended region of length MzE of maximum electrical field strength Emax. Figure 5b shows the large extension of the maximum Emax of the electrical field E, generated by the additional electrode formed by the lower pole shoe segment 165 at distance z2. Emax is adjusted between 3 kV / mm and 8 kV / mm, for example Emax = 6 kV / mm. Ep is adjusted between 1 kV / mm and 3 kV / mm, for example Ep = 1.5 kV / mm. Figure 5c shows the constraint of the magnetic field B around a maximum at distance zB to the image plane (at z = 0). Again, distances in Figure 5 are not shown to scale.

[0064] With the reduced distance zE, the kinetic energy during transmission of the beam cross-over is increased and Columb interaction is reduced. The improved objective lens 102 further allows higher kinetic energies of primary electrons, for example a kinetic energy U0 = 60keV, U0 = 70keV, or more. Thereby, a Coulomb interaction at the beam cross-over 108 is even further reduced and beam blurring and aberrations are further reduced. The improved objective lens 102 further enables a constraint of the magnetic field B with a maximum field strength at distance zB to the image plane (101, here at z=0), and a very low magnetic immersion field Bp at the surface 25 of the sample 7.

[0065] Figure 6 illustrates a second embodiment. Same reference numbers are used as within figures 1, 2 and 4, and reference is made to the description above. According to the second embodiment, the upper pole shoe 167 comprises an extended pole shoe tube 169 with axial length Lb, which is arranged in propagation direction of the primary electrons after the beam exit opening 153 of the beam tube 151. The extended pole shoe tube 169 encloses the plurality of beamlets and an accelerating voltage Ub is provided by a fifth voltage supply 173.5 to the extended pole shoe tube 169. Extended pole shoe tube 169 is arranged close to the beam intersection or cross-over 108, where the plurality of primary electron beamlets 3 intersect each other. With the high voltage provided to extended pole shoe tube 169 of for example Ub = 30kV - Ul, primary electrons are accelerated and a transmission time at cross over is reduced, thereby an interaction between primary electrons is reduced and a beam blurring is reduced. Thereby, a resolution is increased. For example, with a kinetic energy of 30keV within the beam tube 151, primary electrons are accelerated by Ub to 60keV and interaction between primary electrons is reduced approximately by a factor of two. The extended pole shoe tube 169 acts thus as a booster electrode. A booster electrode is also described in PCT / EP2024 / 025211, filed on 18.07.2024, which is incorporated here within by reference.

[0066] With the extended pole shoe tube 169 or booster electrode, the need for a high kinetic energy of primary (and secondary) electrons throughout the multi-beam charged particle beam system is reduced, even at higher throughput. The high kinetic energy of primary electrons is limited to the region of beam cross over 108 and thus other electron-optical elements can be operated at lower voltages corresponding to lower kinetic energies. Table 2 summarizes some design parameters of the improved objective lens 102 with a maximum kinetic energy Umax = 60keV at the beam cross over 108 and a landing energy of LE = 1.5keV. The boost voltage Ub is for example selected in a range corresponding between 60% of the kinetic energy UO / e and 160% of the kinetic energy UO / e. It is understood that kinetic energy in keV corresponds to potential difference or voltage multiplied by elementary charge e. The elementary charge depends on the type of charged particles, for example electrons.

[0067] Table 2: Exemplary design values of an improved objective lens 102 according to the second embodiment

[0068] According to an example, the second voltage U2 provided to the lower pole piece 165 is U2 = OV. Thereby, it is possible to keep coil 161 at low voltage. Generally, according to an example, U2 is set to the mean value of Ub and U3 with distances set accordingly, thereby maintaining a linear change of the potential and a wide maximum along the optical axis 1105 of the decelerating electrical field 137 for decelerating the primary electrons.

[0069] In an example, a variation of the accelerating voltage Ub provided to the extended pole shoe tube 169 allows for a large range of landing energies LE, with which primary electrons reach the surface 25 of a wafer 7. Thereby, a yield of secondary electrons can be improved or a charging behavior of the surface 15 of a wafer can be influenced. Figure 7 illustrates kinetic energies UE (figure 7a) of primary electrons and the electrical field strength E (figure 7b) through the improved objective lens 102 according to the second embodiment. In the region around cross over 108 approximately at distance z4 to the image plane 101 (at z = 0), a large maximum of kinetic energy Umax is achieved. Between extended pole shoe tube 169 and decelerating electrode 133, a maximum of electrical field E is formed at Emax, where the maximum has a large extension of about | z4-z31 of about 9 to 11mm, for example 10mm along the optical axis 1105. Distance zE is here about 10mm to 14mm, for example 11.5mm

[0070] As described above, the decelerating electrical field 137 acts in parallel as an extraction and accelerating field for secondary electrons. Especially in the presence of charging wafers 7, with for example isolated capacities close to the surface 25, during electron beam inspection, electrons may be accumulated close to the surface 25. In certain examples, discharging of those locally confined charges have been observed, for example sparks. Such sparks may cause damage to sample 7 under inspection.

[0071] Figure 8a illustrates an example of an improved objective lens according to a third embodiment. Same reference numbers are used as in figures 1,2,4, and 6, and reference is made to the description above. The improved objective lens 102 further comprises a further electrode or shielding electrode 181. By a sixth voltage supply 173.6, the shielding electrode 181 is provided with a shielding voltage Us = U4 - 100V. With the shielding electrode 181, the decelerating electrical field 137 is constrained in a confined area 183 centered around the optical axis 1105. Thereby, discharging of locally confined charges, such as forming of sparks, is suppressed. The shielding electrode 181 therefore is set slightly above the potential of the wafer surface 25, thereby repelling electrons from or within the wafer 7. An example is illustrated in figure 8b, illustrating the repelling electrical field Es surrounding the extraction field Ep. The extraction field Ep is exerting an extracting and accelerating force Fe, while the repelling electrical field Es is exerting a repelling or suppressing force Fs to negative charges which might be accumulated in local capacitances or on a surface of an isolator 67. Figure 9 illustrates a further example of an improved objective lens 102 according to a third embodiment. Here, the shielding electrode 181 is arranged between the decelerating electrode 133 and surface 25 of wafer 7 and confined area 183 is even more reduced with an extension limited approximately by the radius of the decelerating electrode 133 with inner radius r3 of for example r3 < 5mm, or r3 <= 6mm.

[0072] Figure 10a illustrates a further example of an improved objective lens 102. Same reference numbers are used as in Figures 4, 6, 8 and 9, and reference is made to the description above. In the example of figure 10a, the electrode 133 and the lower pole shoe segment 165 are attached together or made from one piece, and identical voltages U2 and U3 are provided by voltage supply 173.3. For example, the pole shoe segment 165 can be made of a material with high magnetic permeability, and electrode 133 can be made of a conducting metal of low magnetic permeability. Thereby, a deceleration of the primary electrons in propagation direction z can be achieved in similar manner as illustrated in figure 7a. Thereby, the distance zl between exit of beam tube 153 and surface 25 of sample 7 can even further been reduced. Thereby, a focal length of the magnetic field lens exerted by pole shoe 163 can be further reduced.

[0073] Figure 10b illustrates a further example of an improved objective lens 102. Same reference numbers are used as in Figures 4, 6, 8, 9 and 10a, and reference is made to the description above. In the example of figure 10b, an electrode 175 is attached by mechanical engagement to the upper pole shoe segment 167. The electrode 175 is provided with an opening for passing the plurality of primary charged particle beamlets. The electrode 175 therefore forms at least a part of the extended pole shoe tube 169. The upper pole shoe segment 167 is made of a material of high magnetic permeability (such as soft iron, or nonferrous Nickel alloys), and the electrode 175 is made of a material with of low magnetic permeability, forming the electrode 175 of length Lb for forming the pole show tube 169. Thereby, the definition of the magnetic lens field generated by pole shoe 163 is separated from the definition of the electrostatic booster lens. The magnetic lens field is defined by the magnetic field defining edge 179 of the upper pole shoe segment 167, wherein the electrostatic field is defined by the inner shape or opening of the booster electrode 175. Both elements 177 and 175 are made of conductive material with different magnetic permeability, and upper pole piece 167 and electrode 175 are formed monolithically by mechanical engagement. Thereby, high potential Ub for accelerating electrons by voltage Ub can be applied via the pole shoe segment 167 to the electrode 175. The larger extension Lb of the pole tube is achieved without a negative impact to the confinement of the magnetic lens field for focusing the plurality of primary electron beamlets 3. Thereby, a focal length of the magnetic lens can be configured independently of the design of the booster electrode 175.

[0074] Figure 10c illustrates a further aspect of the improved objective lens 102 of figure 10b. Same reference numbers are used as in 10b, and reference is made to the description above. In Figure 10c, only the elements of high magnetic permeability 165 and 167 are shown. As within all the figures, the pole shoe segments 165 and 167 are formed as rotationally symmetrically segments around the optical axis 1105. The magnetic lens field is confined by the lower edge 179a of the upper pole shoe segment 167 and upper edge 179b of the lower pole shoe segment 165. In this example, an axial gap AG is formed as well as a radial gap RG = r4 - r2. The radial gap RG, however, is formed such that the radius R4 of the edge 179a of the upper pole shoe segment 167 exceeds the radius r2 of the lower pole shoe segment 165 (with r4 > r2), such that no immersion lens is formed and the magnetic field at the surface 25 of the sample 7 is still reduced. For example, the magnetic field Bp at the surface 25 of the sample 7 is reduced below 3.5mT, below 2mT, for example lmT. With the "inverted" radial gap lens design (with r4 > r2), the distance zB can be increased (see figure 5c). Thereby, for example, an electrostatic field of the booster electrode 175 and the magnetic field lens generated by pole shows 163 can be adjusted independently in their axial position.

[0075] Figure 11a illustrates a further aspect of the improved objective lens 102 of figure 10. Again, only half of the cross-section of the radially symmetrical arrangement of elements is illustrated. Here, a circular groove 187 forming a faraday cup is integrated into the electrically conducting booster electrode 175, which is attached to the upper pole piece segment 167. By beam deflector 110, the plurality of primary charged particle beamlets 3 (only one shown) can be deflected into the faraday cup 187 and a beam current can be measured. With measurement of current I by micro-Ampere-meter 185, the electron beam current is detected during use, for example during use with booster voltage Ub applied to the electrode 175 via voltage supply 173.5. Thereby, a beam current can be measured and for example, an electron source 301 can be adjusted.

[0076] Figure lib illustrates a further example of the improved objective lens 102 of figure 11a. Here, electrode 175 is attached to the magnetic part 177 of upper pole shoe segment 167 such that the electrode 175 is arranged during use between the upper pole shoe segment

[0077] 167 and the lower pole shoe segment 165. Thus, electrode 175 is arranged in propagation direction of the primary electrons downstream of the upper pole shoe segment 167.

[0078] Thereby, and axial gap GP between upper and lower pole shoe segments 167, 165 can be increased. In the example of figure 11a, on the other hand, electrode 175 is arranged in propagation direction of the primary electrons upstream of the upper pole shoe segment 167. In both examples, electrode 175 and upper pole shoe segment 167 are attached via positive mechanical engagement and for example fixed by clamping or screw fitting. With the arrangement according to the examples illustrated in figure 11, a large axial extension Lb of the booster electrical field within the pole tube 169 can be maintained while the distance zl between exit 153 of beam tube 151 and wafer surface 25 can be kept short. Next, distance z2 between lower pole shoe segment 165 and wafer surface 25 can be reduced, and a focal length of the magnetic lens can be kept small. Thereby, aberrations are reduced while throughput can be increased with higher beam currents. Furthermore, landing energies LE can be adjusted in a wide range as described above. During use, a control unit 800 of a multi-beam charged-particle system 1 is configured to generate and provide voltages selected from the group of voltages comprising the voltages Ul, U2, U3, U4, and optionally at least one of the booster voltage Ub and shielding voltage Us to corresponding elements selected from the elements comprising upper pole shoe 167, booster electrode 175, lower pole shoe 165, beam tube 151, decelerating electrode 133, wafer support table 500 and shielding electrode 181. Accordingly, a method is provided comprising the steps of generating and providing the appropriate voltages selected from the group of voltages comprising Ul, U2, U3, U4, Ub, Us to electrodes selected from the group of electrodes comprising upper pole shoe 167, booster electrode 175, lower pole shoe 165, beam tube 151, decelerating electrode 133, wafer support table 500 and / or shielding electrode 181, and performing an inspection task at high resolution with large throughput with a plurality of J primary charged particles 3 with J > 90, for example J > 300 or even more. The appropriate voltages Ul, U2, U3, U4, and / or Ub, and / or Us are typically stored in a memory of the multi-beam charged-particle system 1 or accessible to a user selection via a user interface. For example, the landing energy LE may be subject to a specific inspection task or may be selected by an operator via a user interface configured for selecting the landing energy LE. The other voltages are then selected for example according to LE = U0 - dUl - dU2 - dU3 with additional constraints according to the extended maximum of field strength Emax along an optical axis 1105 for generating an almost homogeneous decelerating field for primary electrons.

[0079] With the design, the method, the voltages and the electrical potential differences generated during use, a resolution of a multi-beam charged-particle system 1 is increased, interaction at a beam cross-over is reduced and a throughput is increased. With the design, the voltages and the electrical potential differences generated during use, furthermore a variation in angles of incidence of the plurality of beamlets at a wafer surface is reduced and an isotropy and spatial invariance of an imaging result during an inspection task is increased. With the design, the voltages and the electrical potential differences generated during use, furthermore a variation in angles of the plurality of secondary beamlets emanating from the surface 25 of a wafer 7 is reduced, and an isotropy and spatial invariance of an imaging result during an inspection task is increased.

[0080] The disclosure can be described by following clauses:

[0081] Clause 1: A multi-beam charged particle beam system (1) for irradiation of a surface (25) of a sample (7) with a plurality of primary charged particle beamlets (3), comprising:

[0082] - a charged-particle multi-beamlet generator (300), comprising at least one charged particle source (301) and multi-beam forming unit (305) for generating the plurality of primary charged particle beamlets (3),

[0083] - an object irradiation unit (100) comprising an objective lens (102) for forming a plurality of focus spots (5) of the plurality of primary charged particle beamlets (3) in an image plane (101),

[0084] - a stage (500) for positioning the surface (25) of the sample (7) in the image plane (101),

[0085] - a beam tube (151), wherein the objective lens (102) is formed during use by:

[0086] - a beam exit opening (153) of the beam tube (151), connected to a first voltage supply

[0087] (173.1) for providing during use a first voltage U1 to beam exit opening (153) of the beam tube (151),

[0088] - a coil (161), an upper pole shoe (167) and a lower pole shoe (165), forming an axial gap (168) of width AG for forming an axially confined magnetic field,

[0089] - the lower pole shoe (165) being electrically isolated from the upper pole shoe (167) and connected to a second voltage supply (173.2) for providing during use a second voltage U2 to the lower pole shoe (165),

[0090] - a sample voltage supply (503), connected to the stage (500) and configured for providing during use a fourth voltage U4 to the sample (7),

[0091] - wherein the upper pole shoe (167) is connected to a fifth voltage supply (173.5) for providing during use a boost voltage Ub to the upper pole shoe (167), wherein Ub > Ul.

[0092] Clause 2: The multi-beam charged particle beam system (1) of clause 1, wherein Ub is selected within a range of 160% * UO / e | < Ub < 1160% * UO / e | , with e being the elementary charge of a primary charged particle. Clause 3: The multi-beam charged particle beam system (1) of clause 1 or 2, wherein the plurality of primary charged particle beamlets exit the beam exit opening (153) of the beam tube (151) at a kinetic energy U0 of U0 >= 30keV, for example U0 = 30keV, U0 = 60keV, or U0 = lOOkeV, and wherein a kinetic energy of the primary charged particle beamlets is increased during passage of the primary charged particle beamlets through the upper pole shoe (167).

[0093] Clause 4: The multi-beam charged particle beam system (1) of any of the clauses 1 to 3, wherein the axial gap AG is given between 10mm <= AG <= 20mm.

[0094] Clause 5: The multi-beam charged particle beam system (1) according to any of the clauses 1 to 4, wherein an electrode (175) is attached in electrical contact to the upper pole shoe (167), the electrode (175) having an opening for passing the plurality of charged particle beamlets (3).

[0095] Clause 6: The multi-beam charged particle beam system (1) according to clause 5, wherein the electrode (175) is made from material with low magnetic permeability such that the magnetic field lens generated during use is confined between the edge (179, 179a) of the upper pole shoe segment (167) and the lower pole shoe (165).

[0096] Clause 7: The multi-beam charged particle beam system (1) according to any of the clauses 1 to 6, wherein the electrode (175) is further connected to a micro-ampere meter (185) for measuring during use an electron beam current.

[0097] Clause 8: The multi-beam charged particle beam system (1) according to clause 7, wherein in the electrode (175), a faraday cup (187) is formed.

[0098] Clause 9: The multi-beam charged particle beam system (1) according to clauses 1 to 8, wherein the upper pole shoe (167) comprises and extended pole shoe tube (169) for passing the plurality of primary charged particle beamlets (3) with axial length Lb with Lb selected between 8mm <= Lb <= 18mm.

[0099] Clause 10: The multi-beam charged particle beam system (1) of clause 9, wherein the kinetic energy of the primary charged particle beamlets is increased during passage of the primary charged particle beamlets (3) through the extended pole shoe tube (169).

[0100] Clause 11: The multi-beam charged particle beam system (1) according to clause 9 or 10, wherein the extended pole shoe tube (169) is formed by the opening in the electrode (175).

[0101] Clause 12: The multi-beam charged particle beam system (1) according to any of the clauses 5 to 11, wherein the electrode (175) is arranged between the upper pole shoe segment (167) and the lower pole shoe segment (165).

[0102] Clause 13: The multi-beam charged particle beam system (1) of clause 12, wherein the axial gap AG is given between 15mm <= AG <= 20mm.

[0103] Clause 14: The multi-beam charged particle beam system (1) according to any of the clauses 5 to 13, wherein the electrode (175) is arranged between the beam tube (151) and the upper pole shoe segment (167).

[0104] Clause 15: The multi-beam charged particle beam system (1) of clause 14, wherein the axial gap AG is given between 10mm <= AG <= 15mm.

[0105] Clause 16: The multi-beam charged particle beam system (1) according to any of the clauses 1 to 15, wherein an inner radius r4 of an edge (179) of the upper pole shoe segment (167) is larger than the inner radius r2 of the lower pole shoe segment (165) with r4 > r2, thereby forming a radial gap RG. Clause 17: The multi-beam charged particle beam system (1) of any of the clauses 1 to 16, further comprising a shielding electrode (181) arranged between the lower pole shoe segment (165) and the image plane (101) in a circumference of a confined area (183) centered around the optical axis (1105), the shielding electrode (181) being connected to a sixth voltage supply (173.6) for providing during use the shielding electrode (181) with a shielding voltage Us with Us < U4.

[0106] Clause 18: The multi-beam charged particle beam system (1) according to clause 17, wherein Us = U4 - 100V.

[0107] Clause 19: The multi-beam charged particle beam system (1) of any of the clauses 1 to 18, further comprising a decelerating electrode (133), arranged in propagation direction of the primary charged particles downstream of the lower pole shoe (165), the decelerating electrode (133) being connected to a third voltage supply (173.3) for providing during use a third voltage U3 to the decelerating electrode (133).

[0108] Clause 20: The multi-beam charged particle beam system (1) according to clause 19, wherein the beam exit opening (153), the lower pole shoe (165), the decelerating electrode (133), the image plane (101) and first, second, third and fourth voltages Ul, U2, U3, U4 are configured to generate during use a maximum field strength Emax within an axial extension MzE with MzE > 3mm, for example MzE > 5mm, for example MzE > 10mm.

[0109] Clause 21: A multi-beam charged particle beam system (1) for irradiation of a surface (25) of a sample (7) with a plurality of primary charged particle beamlets (3), comprising:

[0110] - a charged-particle multi-beamlet generator (300), comprising at least one charged particle source (301) and multi-beam forming unit (305) for generating the plurality of primary charged particle beamlets (3), - an object irradiation unit (100) comprising an objective lens (102) for forming a plurality of focus spots (5) of the plurality of primary charged particle beamlets (3) in an image plane (101),

[0111] - a stage (500) for positioning the surface (25) of the sample (7) in the image plane (101),

[0112] - a beam tube (151), wherein the objective lens (102) is formed during use by:

[0113] - a beam exit opening (153) of the beam tube (151), connected to a first voltage supply (173.1) for providing during use a first voltage U1 to beam exit opening (153) of the beam tube (151),

[0114] - a coil (161), an upper pole shoe (167) and a lower pole shoe (165), forming an axial gap (168) of width AG for forming an axially confined magnetic field,

[0115] - the lower pole shoe (165) being electrically isolated from the upper pole shoe (167),

[0116] - a sample voltage supply (503), connected to the stage (500) and configured for providing during use a fourth voltage U4 to the sample (7),

[0117] - comprising a shielding electrode (181) arranged between the lower pole shoe segment (165) and the image plane (101) in a circumference of a confined area (183) centered around the optical axis (1105), the shielding electrode (181) being connected to a sixth voltage supply (173.6) for providing during use the shielding electrode (181) with a shielding voltage Us with Us < U4.

[0118] Clause 22: The multi-beam charged particle beam system (1) according to clause 21, wherein Us = U4 - 100V.

[0119] Clause 23: The multi-beam charged particle beam system (1) of any of the clauses 21 or 22, further comprising a decelerating electrode (133), arranged in propagation direction of the primary charged particles downstream of the lower pole shoe (165), the decelerating electrode (133) being connected to a third voltage supply (173.3) for providing during use a third voltage U3 to the decelerating electrode (133).

[0120] Clause 24: The multi-beam charged particle beam system (1) of any of the clauses 21 to 23, wherein the lower pole shoe (165) is connected to a second voltage supply (173.2) for providing during use a second voltage U2 to the lower pole shoe (165).

[0121] Clause 25: The multi-beam charged particle beam system (1) of any of the clauses 21 to 24, wherein the upper pole shoe (167) is connected to a fifth voltage supply (173.5) for providing during use a boost voltage Ub to the upper pole shoe (167), wherein Ub > Ul.

[0122] Clause 26: A multi-beam charged particle beam system (1) for irradiation of a surface (25) of a sample (7) with a plurality of primary charged particle beamlets (3), comprising:

[0123] - a charged-particle multi-beamlet generator (300), comprising at least one charged particle source (301) and multi-beam forming unit (305) for generating the plurality of primary charged particle beamlets (3),

[0124] - an object irradiation unit (100) comprising an objective lens (102) for forming a plurality of focus spots (5) of the plurality of primary charged particle beamlets (3) in an image plane (101),

[0125] - a stage (500) for positioning the surface (25) of the sample (7) in the image plane (101),

[0126] - a beam tube (151), wherein the objective lens (102) is formed during use by:

[0127] - a beam exit opening (153) of the beam tube (151), connected to a first voltage supply

[0128] (173.1) for providing during use a first voltage Ul to beam exit opening (153) of the beam tube (151), - a coil (161), an upper pole shoe (167) and a lower pole shoe (165), forming an axial gap (168) of width AG for forming an axially confined magnetic field,

[0129] - the lower pole shoe (165) being electrically isolated from the upper pole shoe (167) and connected to a second voltage supply (173.2) for providing during use a second voltage U2 to the lower pole shoe (165),

[0130] - a decelerating electrode (133), arranged in propagation direction of the primary charged particles downstream of the lower pole shoe (165), the decelerating electrode (133) being connected to a third voltage supply (173.3) for providing during use a third voltage U3 to the decelerating electrode (133),

[0131] - a sample voltage supply (503), connected to the stage (500) and configured for providing during use a fourth voltage U4 to the sample (7), wherein the beam exit opening (153), the lower pole shoe (165), the decelerating electrode (133), the image plane (101) and first, second, third and fourth voltages Ul, U2, U3 and U4 are configured to generate during use a maximum field strength Emax within an axial extension MzE with MzE > 3mm, for example MzE > 5mm, for example MzE > 10mm.

[0132] Clause 27: The multi-beam charged particle beam system (1) of clause 26, wherein the plurality of primary charged particle beamlets exit the beam exit opening (153) of the beam tube (151) at a kinetic energy U0 of U0 >= 30keV, for example U0 = 30keV, U0 = 60keV, or U0 = lOOkeV.

[0133] Clause 28: The multi-beam charged particle beam system (1) of clause 27, wherein an axial distance zl between the beam exit opening (153) of the beam tube (151) and the image plane (101) is given by zl [mm] <= U0 / 10 (keV / mm). Clause 29: The multi-beam charged particle beam system (1) of any of the clauses 26 to 28, wherein the axial gap g is given by AG <= 15mm, for example AG <= 12mm, for example AG about 10mm.

[0134] Clause 30: The multi-beam charged particle beam system (1) of any of the clauses 26 to 29, further comprising a control unit (800) configured for generating and providing during use first to fourth voltages Ul, U2, U3, U4, with U3 < U2 < Ul, and with U4 = U3 + / - lkV.

[0135] Clause 31: The multi-beam charged particle beam system (1) of clause 30, wherein U2 is selected in a range between 55% * U3 > U2 > 65% * U3.

[0136] Clause 32: The multi-beam charged particle beam system (1) of any of the clauses 26 to 31, wherein the upper pole shoe (167) connected to a fifth voltage supply (173.5) for providing during use a boost voltage Ub to the upper pole shoe (167), wherein Ub > Ul.

[0137] Clause 33: The multi-beam charged particle beam system (1) of clause 32, wherein Ub is selected within a range of 160% * UO / e | < Ub < 1160% * UO / e | , with e being the elementary charge of a primary charged particle.

[0138] Clause 34: The multi-beam charged particle beam system (1) according to any of the clauses 26 to 33, wherein an electrode (175) is attached in electrical contact to the upper pole shoe (167).

[0139] Clause 35: The multi-beam charged particle beam system (1) according to clause 34, wherein the electrode (175) is made from material with low magnetic permeability such that the magnetic field lens generated during use is confined between the edge (179, 179a) of the upper pole shoe segment (167) and the lower pole shoe (165). Clause 36: The multi-beam charged particle beam system (1) according to any of the clauses 33 to 35, wherein the electrode (175) is further connected to a micro-ampere meter (185) for measuring during use an electron beam current.

[0140] Clause 37: The multi-beam charged particle beam system (1) according to clause 36, wherein in the electrode (175), a faraday cup (187) is formed.

[0141] Clause 38: The multi-beam charged particle beam system (1) according to clauses 26 to 37, wherein the upper pole shoe (167) comprises and extended pole shoe tube (169) for passing the plurality of primary charged particle beamlets (3) with axial length Lb with Lb selected between 8mm <= Lb <= 18mm.

[0142] Clause 39: The multi-beam charged particle beam system (1) according to clause 38, wherein the extended pole shoe tube (169) is formed by an opening within the electrode (175).

[0143] Clause 40: The multi-beam charged particle beam system (1) according to clause 39, wherein the electrode (175) is arranged between the upper pole shoe segment (167) and the lower pole shoe segment (165).

[0144] Clause 41: The multi-beam charged particle beam system (1) according to clause 39, wherein the electrode (175) is arranged between the beam tube (151) and the upper pole shoe segment (167).

[0145] Clause 42: The multi-beam charged particle beam system (1) according to any of the clauses 26 to 41, wherein an inner radius r4 of an edge (179) of the upper pole shoe segment (167) is larger than the inner radius r2 of the lower pole shoe segment (165) with r4 > r2, thereby forming a radial gap RG. Clause 43: The multi-beam charged particle beam system (1) of any of the clauses 26 to 42, further comprising a shielding electrode (181) arranged between the decelerating electrode (133) and the image plane (101) in a circumference of a confined area (183) centered around the optical axis (1105), the shielding electrode (181) being connected to a sixth voltage supply (173.6) for providing during use the shielding electrode (181) with a shielding voltage Us with Us < U4.

[0146] Clause 44: The multi-beam charged particle beam system (1) according to clause 43, wherein Us = U4 - 100V.

[0147] Clause 45: The multi-beam charged particle beam system (1) of any of the clauses 1 to 44, further comprising a control unit (800) configured for generating and providing during use at least one of a voltage selected from the group of voltages Ul, U2, U3, U4, Us, Ub.

[0148] Clause 46: The multi-beam charged particle beam system (1) of any of the clauses 1 to 45, further comprising a beam divider unit (400) for separating a secondary electron beam path (13) from a primary beam path (11).

[0149] Clause 47: The multi-beam charged particle beam system (1) of any of the clauses 1 to 46, further comprising a detection unit (200) with an image sensor (600) for imaging and detecting a plurality of secondary electron beamlets (9).

[0150] Clause 48: The multi-beam charged particle beam system (1) of clause 47, wherein the detection unit (200) comprises an aperture stop (284) arranged in a pupil plane (258) of the detection unit (200), and wherein the objective lens (102) is configured such that the plurality of secondary electron beamlets (9) form a cross-over at the pupil plane (258). Clause 49: A method of operating a multi-beam charged particle beam system (1), comprising

[0151] - positioning a surface (25) of a sample (7) in an image plane (101) of the multi-beam charged particle beam system (1),

[0152] - generating and transmitting a plurality of primary charged particle beamlets (3) through an beam exit opening (153) of a beam tube (151) at a kinetic energy U0 of U0 >= 30keV,

[0153] - generating and providing, by a control unit (800), a current I to a coil (161) to generate a magnetic lens field,

[0154] - generating and providing, by a control unit (800), at least four voltages selected from a group of voltages including first voltage U1 provided to the beam tube, a second voltage U2 provided to a lower pole piece segment (165), a third voltage U2 provided to a deceleration electrode (133), a fourth voltage U4 provided to a wafer support table (500), a boost voltage Ub provided to an upper pole piece segments (167), and a shielding voltage Us provided to a shielding electrode with U2 < Ul, and with U4 = U3 + / - lkV or U4 = U2 + / - lkV , to provide a decelerating electrical field E to decelerate the plurality of primary charged particle beamlets (3) to a landing energy LE.

[0155] Clause 50: The method of clause 49, wherein the at least four voltages are selected to generate a maximum field strength Emax with 3kV / mm <= Emax <= 8kV / mm.

[0156] Clause 51: The method of clause 50, wherein the maximum field strength Emax is generated with an axial extension MzE with MzE > 3mm, for example MzE > 5mm, for example MzE > 10mm.

[0157] Clause 52: The method of any of the clauses 49 to 51, further comprising generating and providing the boost voltage Ub with Ub > Ul. Clause 53: The method of clause 52, wherein the boost voltage Ub is selected in a range 160% * UO / e | <= Ub <= 1160% * UO / e | , with e being an elementary charge of a primary charged particle.

[0158] The invention is however not limited to the embodiments, examples or clauses described above. The clauses, embodiments or examples can be fully or partly combined with one another, and various modifications within the scope of any person of ordinary skill in the art are covered by the embodiments and examples of the disclosure.

[0159] A list of reference numbers is provided:

[0160] I multi-beam charged-particle system

[0161] 3 primary charged particle beamlet or plurality of primary charged particle beamlets

[0162] 5 primary charged particle beam spot

[0163] 7 object or sample

[0164] 9 secondary electron beamlet, forming the plurality of secondary electron beamlets

[0165] II primary beam path

[0166] 13 secondary electron beam path

[0167] 15 focus spot of secondary electron beam

[0168] 25 surface of object or sample

[0169] 67 local isolated capacity

[0170] 100 object irradiation unit

[0171] 101 image plane

[0172] 102 objective lens

[0173] 103 field lens

[0174] 108 first beam cross over

[0175] 110 collective multi-beam raster scanner

[0176] 112 Multipole element

[0177] 133 decelerating electrode

[0178] 137 equipotential lines of electric field 151 beam tube

[0179] 153 Beam exit opening

[0180] 161 coil

[0181] 163 pole shoe

[0182] 165 lower pole shoe segment

[0183] 167 upper pole shoe segment

[0184] 168 axial gap

[0185] 169 Pole tube

[0186] 171 isolating gap

[0187] 173 voltage supply

[0188] 175 booster electrode

[0189] 179 magnetic field confining edge

[0190] 181 shielding electrode

[0191] 183 confined area

[0192] 185 micro ampere meter

[0193] 187 Faraday cup

[0194] 189 isolator

[0195] 200 detection unit

[0196] 205 magneto-dynamic lens

[0197] 211 correction lens

[0198] 220 multi-pole corrector

[0199] 222 second raster scanner

[0200] 225 secondary electron image plane

[0201] 230 monitoring system

[0202] 232 high resolution sensor

[0203] 235 imaging lens

[0204] 237 beam divider mirror

[0205] 258 cross over or pupil position

[0206] 264 multi-pole corrector 284 Aperture stop

[0207] 300 charged-particle multi-beamlet generator

[0208] 301 charged particle source

[0209] 303 collimating lenses

[0210] 304 filter plate

[0211] 305 multi-beam forming unit

[0212] 306 multi-aperture plates

[0213] 309 primary electron beam

[0214] 321 intermediate image surface

[0215] 331 first field lens

[0216] 333 second field lens

[0217] 400 beam splitter or divider unit

[0218] 500 sample stage

[0219] 503 Sample voltage supply

[0220] 600 image sensor

[0221] 602 electron to photon conversion unit

[0222] 605 collection lens

[0223] 607 folding mirror

[0224] 609 light beam bundle

[0225] 611 zoom lens

[0226] 613 fiber entrance aperture

[0227] 615 light guiding fiber

[0228] 617 rotation / adjustment actuator

[0229] 623 detection element

[0230] 630 retraction direction

[0231] 800 control unit

[0232] 810 imaging control module

[0233] 820 sensor module

[0234] 830 primary beam-path control module 840 secondary beam-path control module

[0235] 850 stage control module

[0236] 860 scanning control unit

[0237] 880 control operation processor 890 memory

[0238] 1105 optical axis

Claims

52ClaimsWhat is claimed is:

1. A multi-beam charged particle beam system (1) for irradiation of a surface (25) of a sample (7) with a plurality of primary charged particle beamlets (3), comprising:- a charged-particle multi-beamlet generator (300), comprising at least one charged particle source (301) and multi-beam forming unit (305) for generating the plurality of primary charged particle beamlets (3),- an object irradiation unit (100) comprising an objective lens (102) for forming a plurality of focus spots (5) of the plurality of primary charged particle beamlets (3) in an image plane (101),- a stage (500) for positioning the surface (25) of the sample (7) in the image plane (101),- a beam tube (151), wherein the objective lens (102) is formed during use by:- a beam exit opening (153) of the beam tube (151), connected to a first voltage supply(173.1) for providing during use a first voltage U1 to beam exit opening (153) of the beam tube (151),- a coil (161), an upper pole shoe (167) and a lower pole shoe (165), forming an axial gap (168) of width AG for forming an axially confined magnetic field,- the lower pole shoe (165) being electrically isolated from the upper pole shoe (167) and connected to a second voltage supply (173.2) for providing during use a second voltage U2 to the lower pole shoe (165),- a sample voltage supply (503), connected to the stage (500) and configured for providing during use a fourth voltage U4 to the sample (7),- wherein the upper pole shoe (167) is connected to a fifth voltage supply (173.5) for53 providing during use a boost voltage Ub to the upper pole shoe (167), wherein Ub > Ul.

2. The multi-beam charged particle beam system (1) of claim 1, wherein Ub is selected within a range of 160% * UO / e | < Ub < 1160% * UO / e | , with e being the elementary charge of a primary charged particle.

3. The multi-beam charged particle beam system (1) of claim 1 or 2, wherein the plurality of primary charged particle beamlets exit the beam exit opening (153) of the beam tube (151) at a kinetic energy U0 of U0 >= 30keV, for example U0 = 30keV, U0 = 60keV, or U0 = lOOkeV, and wherein a kinetic energy of the primary charged particle beamlets is increased during passage of the primary charged particle beamlets through the upper pole shoe (167).

4. The multi-beam charged particle beam system (1) of any of the claims 1 to 3, wherein the width AG of the axial gap is given between 10mm <= AG <= 20mm.

5. The multi-beam charged particle beam system (1) according to any of the claims 1 to 4, wherein an electrode (175) is attached in electrical contact to the upper pole shoe (167), the electrode (175) having an opening for passing the plurality of charged particle beamlets (3).

6. The multi-beam charged particle beam system (1) according to claim 5, wherein the electrode (175) is made from material with low magnetic permeability such that the magnetic field lens generated during use is confined between the edge (179, 179a) of the54 upper pole shoe segment (167) and the lower pole shoe (165).

7. The multi-beam charged particle beam system (1) according to any of the claims 1 to 6, wherein the electrode (175) is further connected to a micro-ampere meter (185) for measuring during use an electron beam current.

8. The multi-beam charged particle beam system (1) according to claim 7, wherein in the electrode (175), a faraday cup (187) is formed.

9. The multi-beam charged particle beam system (1) according to claims 1 to 8, wherein the upper pole shoe (167) comprises an extended pole shoe tube (169) for passing the plurality of primary charged particle beamlets (3) with axial length Lb with Lb selected between 8mm <= Lb <= 18mm.

10. The multi-beam charged particle beam system (1) of claim 9, wherein the kinetic energy of the primary charged particle beamlets is increased during passage of the primary charged particle beamlets (3) through the extended pole shoe tube (169).

11. The multi-beam charged particle beam system (1) according to claim 9 or 10, wherein the extended pole shoe tube (169) is formed by the opening in the electrode (175).5512. The multi-beam charged particle beam system (1) according to any of the claims 5 to 11, wherein the electrode (175) is arranged between the upper pole shoe segment (167) and the lower pole shoe segment (165).

13. The multi-beam charged particle beam system (1) of claim 12, wherein the width of axial gap AG is given between 15mm <= AG <= 20mm.

14. The multi-beam charged particle beam system (1) according to any of the claims 5 to 13, wherein the electrode (175) is arranged between the beam tube (151) and the upper pole shoe segment (167).

15. The multi-beam charged particle beam system (1) of claim 14, wherein the width of the axial gap AG is given between 10mm <= AG <= 15mm.

16. The multi-beam charged particle beam system (1) according to any of the claims 1 to 15, wherein an inner radius r4 of an edge (179) of the upper pole shoe segment (167) is larger than the inner radius r2 of the lower pole shoe segment (165) with r4 > r2, thereby forming a radial gap RG.

17. The multi-beam charged particle beam system (1) of any of the claims 1 to 16, further comprising a shielding electrode (181) arranged between the lower pole shoe segment (165) and the image plane (101) in a circumference of a confined area (183) centered around the optical axis (1105), the shielding electrode (181) being connected to a sixth voltage supply(173.6) for providing during use the shielding electrode (181) with a shielding voltage Us with Us < U4.

18. The multi-beam charged particle beam system (1) according to claim 17, wherein Us = U4 - 100V.

19. The multi-beam charged particle beam system (1) of any of the claims 1 to 18, further comprising a decelerating electrode (133), arranged in propagation direction of the primary charged particles downstream of the lower pole shoe (165), the decelerating electrode (133) being connected to a third voltage supply (173.3) for providing during use a third voltage U3 to the decelerating electrode (133).

20. The multi-beam charged particle beam system (1) according to claim 19, wherein the beam exit opening (153), the lower pole shoe (165), the decelerating electrode (133), the image plane (101) and first, second, third and fourth voltages Ul, U2, U3, U4 are configured to generate during use a maximum field strength Emax within an axial extension MzE with MzE > 3mm, for example MzE > 5mm, for example MzE > 10mm.

21. A multi-beam charged particle beam system (1) for irradiation of a surface (25) of a sample (7) with a plurality of primary charged particle beamlets (3), comprising:- a charged-particle multi-beamlet generator (300), comprising at least one charged particle source (301) and multi-beam forming unit (305) for generating the plurality of primary charged particle beamlets (3),- an object irradiation unit (100) comprising an objective lens (102) for forming a plurality of focus spots (5) of the plurality of primary charged particle beamlets (3) in an image plane (101),- a stage (500) for positioning the surface (25) of the sample (7) in the image plane (101),- a beam tube (151), wherein the objective lens (102) is formed during use by:- a beam exit opening (153) of the beam tube (151), connected to a first voltage supply (173.1) for providing during use a first voltage U1 to beam exit opening (153) of the beam tube (151),- a coil (161), an upper pole shoe (167) and a lower pole shoe (165), forming an axial gap (168) of width AG for forming an axially confined magnetic field,- the lower pole shoe (165) being electrically isolated from the upper pole shoe (167),- a sample voltage supply (503), connected to the stage (500) and configured for providing during use a fourth voltage U4 to the sample (7),- comprising a shielding electrode (181) arranged between the lower pole shoe segment (165) and the image plane (101) in a circumference of a confined area (183) centered around the optical axis (1105), the shielding electrode (181) being connected to a sixth voltage supply (173.6) for providing during use the shielding electrode (181) with a shielding voltage Us with Us < U4.

22. The multi-beam charged particle beam system (1) according to claim 21, wherein Us= U4 - 100V.5823. The multi-beam charged particle beam system (1) of any of the claims 21 or 22, further comprising a decelerating electrode (133), arranged in propagation direction of the primary charged particles downstream of the lower pole shoe (165), the decelerating electrode (133) being connected to a third voltage supply (173.3) for providing during use a third voltage U3 to the decelerating electrode (133).

24. The multi-beam charged particle beam system (1) of any of the claims 21 to 23, wherein the lower pole shoe (165) is connected to a second voltage supply (173.2) for providing during use a second voltage U2 to the lower pole shoe (165).

25. The multi-beam charged particle beam system (1) of any of the claims 21 to 24, wherein the upper pole shoe (167) is connected to a fifth voltage supply (173.5) for providing during use a boost voltage Ub to the upper pole shoe (167), wherein Ub > Ul.

26. A multi-beam charged particle beam system (1) for irradiation of a surface (25) of a sample (7) with a plurality of primary charged particle beamlets (3), comprising:- a charged-particle multi-beamlet generator (300), comprising at least one charged particle source (301) and multi-beam forming unit (305) for generating the plurality of primary charged particle beamlets (3),- an object irradiation unit (100) comprising an objective lens (102) for forming a plurality of focus spots (5) of the plurality of primary charged particle beamlets (3) in an image plane (101),- a stage (500) for positioning the surface (25) of the sample (7) in the image plane (101),59- a beam tube (151), wherein the objective lens (102) is formed during use by:- a beam exit opening (153) of the beam tube (151), connected to a first voltage supply (173.1) for providing during use a first voltage U1 to beam exit opening (153) of the beam tube (151),- a coil (161), an upper pole shoe (167) and a lower pole shoe (165), forming an axial gap (168) of width AG for forming an axially confined magnetic field,- the lower pole shoe (165) being electrically isolated from the upper pole shoe (167) and connected to a second voltage supply (173.2) for providing during use a second voltage U2 to the lower pole shoe (165),- a decelerating electrode (133), arranged in propagation direction of the primary charged particles downstream of the lower pole shoe (165), the decelerating electrode (133) being connected to a third voltage supply (173.3) for providing during use a third voltage U3 to the decelerating electrode (133),- a sample voltage supply (503), connected to the stage (500) and configured for providing during use a fourth voltage U4 to the sample (7), wherein the beam exit opening (153), the lower pole shoe (165), the decelerating electrode (133), the image plane (101) and first, second, third and fourth voltages Ul, U2, U3 and U4 are configured to generate during use a maximum field strength Emax within an axial extension MzE with MzE > 3mm, for example MzE > 5mm, for example MzE > 10mm.

27. The multi-beam charged particle beam system (1) of claim 26, wherein the plurality of primary charged particle beamlets exit the beam exit opening (153) of the beam tube60(151) at a kinetic energy U0 of UO >= 30keV, for example UO = 30keV, UO = 60keV, or UO = lOOkeV.

28. The multi-beam charged particle beam system (1) of claim 27, wherein an axial distance zl between the beam exit opening (153) of the beam tube (151) and the image plane (101) is given by zl [mm] <= UO / 10 (keV / mm).

29. The multi-beam charged particle beam system (1) of any of the claims 26 to 28, wherein the width of the axial gap is given by AG <= 15mm, for example AG <= 12mm, for example AG about 10mm.

30. The multi-beam charged particle beam system (1) of any of the claims 26 to 29, further comprising a control unit (800) configured for generating and providing during use first to fourth voltages Ul, U2, U3, U4, with U3 < U2 < Ul, and with U4 = U3 + / - lkV.

31. The multi-beam charged particle beam system (1) of claim 30, wherein U2 is selected in a range between 55% * U3 > U2 > 65% * U3.

32. The multi-beam charged particle beam system (1) of any of the claims 26 to 31, wherein the upper pole shoe (167) connected to a fifth voltage supply (173.5) for providing during use a boost voltage Ub to the upper pole shoe (167), wherein Ub > Ul.6133. The multi-beam charged particle beam system (1) of claim 32, wherein Ub is selected within a range of 160% * UO / e | < Ub < 1160% * UO / e | , with e being the elementary charge of a primary charged particle.

34. The multi-beam charged particle beam system (1) according to any of the claims 26 to 33, wherein an electrode (175) is attached in electrical contact to the upper pole shoe (167).

35. The multi-beam charged particle beam system (1) according to claim 34, wherein the electrode (175) is made from material with low magnetic permeability such that the magnetic field lens generated during use is confined between the edge (179, 179a) of the upper pole shoe segment (167) and the lower pole shoe (165).

36. The multi-beam charged particle beam system (1) according to any of the claims 33 to 35, wherein the electrode (175) is further connected to a micro-ampere meter (185) for measuring during use an electron beam current.

37. The multi-beam charged particle beam system (1) according to claim 36, wherein in the electrode (175), a faraday cup (187) is formed.

38. The multi-beam charged particle beam system (1) according to claims 26 to 37, wherein the upper pole shoe (167) comprises and extended pole shoe tube (169) for passing the plurality of primary charged particle beamlets (3) with axial length Lb with Lb selected62 between 8mm <= Lb <= 18mm.

39. The multi-beam charged particle beam system (1) according to claim 38, wherein the extended pole shoe tube (169) is formed by an opening within the electrode (175).

40. The multi-beam charged particle beam system (1) according to claim 39, wherein the electrode (175) is arranged between the upper pole shoe segment (167) and the lower pole shoe segment (165).

41. The multi-beam charged particle beam system (1) according to claim 39, wherein the electrode (175) is arranged between the beam tube (151) and the upper pole shoe segment (167).

42. The multi-beam charged particle beam system (1) according to any of the claims 26 to 41, wherein an inner radius r4 of an edge (179) of the upper pole shoe segment (167) is larger than the inner radius r2 of the lower pole shoe segment (165) with r4 > r2, thereby forming a radial gap RG.

43. The multi-beam charged particle beam system (1) of any of the claims 26 to 42, further comprising a shielding electrode (181) arranged between the decelerating electrode (133) and the image plane (101) in a circumference of a confined area (183) centered around the optical axis (1105), the shielding electrode (181) being connected to a sixth voltage supply (173.6) for providing during use the shielding electrode (181) with a shielding voltage63Us with Us < U4.

44. The multi-beam charged particle beam system (1) according to claim 43, wherein Us = U4 - 100V.

45. The multi-beam charged particle beam system (1) of any of the claims 1 to 44, further comprising a control unit (800) configured for generating and providing during use at least one of a voltage selected from the group of voltages Ul, U2, U3, U4, Us, Ub.

46. The multi-beam charged particle beam system (1) of any of the claims 1 to 45, further comprising a beam divider unit (400) for separating a secondary electron beam path (13) from a primary beam path (11).

47. The multi-beam charged particle beam system (1) of any of the claims 1 to 46, further comprising a detection unit (200) with an image sensor (600) for imaging and detecting a plurality of secondary electron beamlets (9).

48. The multi-beam charged particle beam system (1) of claim 47, wherein the detection unit (200) comprises an aperture stop (284) arranged in a pupil plane (258) of the detection unit (200), and wherein the objective lens (102) is configured such that the plurality of secondary electron beamlets (9) form a cross-over at the pupil plane (258).6449. A method of operating a multi-beam charged particle beam system (1), comprising- positioning a surface (25) of a sample (7) in an image plane (101) of the multi-beam charged particle beam system (1),- generating and transmitting a plurality of primary charged particle beamlets (3) through an beam exit opening (153) of a beam tube (151) at a kinetic energy U0 of U0 >= 30keV,- generating and providing, by a control unit (800), a current I to a coil (161) to generate a magnetic lens field,- generating and providing, by a control unit (800), at least four voltages selected from a group of voltages including first voltage U1 provided to the beam tube, a second voltage U2 provided to a lower pole piece segment (165), a third voltage U2 provided to a deceleration electrode (133), a fourth voltage U4 provided to a wafer support table (500), a boost voltage Ub provided to an upper pole piece segments (167), and a shielding voltage Us provided to a shielding electrode with U2 < Ul, and with U4 = U3 + / - lkV or U4 = U2 + / - lkV , to provide a decelerating electrical field E to decelerate the plurality of primary charged particle beamlets (3) to a landing energy LE.

50. The method of claim 49, wherein the at least four voltages are selected to generate a maximum field strength Emax with 3kV / mm <= Emax <= 8kV / mm.

51. The method of claim 50, wherein the maximum field strength Emax is generated with an axial extension MzE with MzE > 3mm, for example MzE > 5mm, for example MzE > 10mm.

52. The method of any of the claims 49 to 51, further comprising generating and providing the boost voltage Ub with Ub > Ul.

53. The method of claim 52, wherein the boost voltage Ub is selected in a range 160% * UO / e | <= Ub <= 1160% * UO / e | , with e being an elementary charge of a primary charged particle.

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