Multibeam charged particle microscope design using anisotropic filtering for improved image contrast

Abstract

Multi-beam charged particle systems and methods of operating multi-beam charged particle systems can provide improved image contrast. The multi-beam charged particle systems include filter elements or active array elements in the detection system that can achieve improved anisotropic image contrast. The present disclosure can be applied to applications of multi-beam charged particle systems where higher requirements for beam uniformity and throughput can be important.

Description

[Technical Field]

[0001] The present disclosure relates to a multi-beam charged particle microscope that can provide improved imaging contrast and a method for inspection of semiconductor features that can provide improved image contrast. [Background technology]

[0002] WO 2005 / 024881 discloses an electron microscope system operating with multiple electron beamlets to parallel scan an object to be inspected with the electron beamlets. The primary charged particle beamlet bundle is generated by guiding a primary charged particle beam onto a multi-beam forming unit including at least one multi-aperture plate having a plurality of apertures. A portion of the electrons of the electron beam impinge on the multi-aperture plate and are absorbed there, while another portion of the beam transmits through the apertures of the multi-aperture plate, thereby forming electron beamlets having a cross section defined by the cross section of the aperture in the beam path downstream of each aperture. The primary charged particle beamlets are focused onto the surface of the sample by an objective lens, causing secondary electrons or backscattered electrons to emanate from the sample as secondary electron beamlets, which are collected and imaged onto a detector. Each secondary beamlet impinges on a separate detector element or group of detector elements, so that the detected secondary electron intensity provides information about the surface of the sample at the location where the corresponding primary beamlet impinges on the sample. The bundle of primary beamlets is systematically scanned over the surface of the sample and an electron microscope image of the sample is produced in the usual manner for scanning electron microscopes.

[0003] In general, the imaging contrast of a scanning electron microscope depends entirely on the signal generated by secondary electrons, which in turn depends on the secondary electron (SE) yield per primary electron and the geometric collection efficiency of the electron microscope. The SE yield generally depends on the material properties and the kinetic energy of the primary electrons. The SE yield typically has an angular component, i.e., the SE yield is typically a function of the polar angle relative to a plane normal to the sample. In other instances, the SE yield can be affected by topographical effects on the sample surface.

[0004] Various contrast mechanisms have been proposed to improve the imaging contrast of multi-beam electron microscopes. U.S. Patent No. 11,049,686 BB proposes the placement of a circular annular aperture filter in the pupil plane of a secondary electron imaging system. German Patent Application No. 102021124099, filed September 17, 2021, discloses a multi-beam electron microscope using a detector capable of detecting the angular components of each secondary beamlet. Image contrast can thereby be improved by selecting the appropriate angular components. The disclosed system offers a high degree of adaptability by expending significant effort on highly complex detection systems involving even faster signal channels. Furthermore, by separating the secondary electron signal in several angular components, the system becomes more sensitive to noise, which may require a longer dwell time or a larger primary electron flow rate. Summary of the Invention

[0005] This patent application claims priority to U.S. Patent Application No. 17 / 966,026, filed October 14, 2022, the disclosure of which is incorporated by reference in its entirety into this patent application.

[0006] The present disclosure seeks to provide a multi-beam charged particle system and a method of operating a multi-beam charged particle system for obtaining advanced images with higher contrast. This may involve anisotropic filtering of secondary electron beamlets using a selected aperture filter. In one example, the anisotropic filtering of at least one secondary electron beamlet is achieved by applying selected anisotropic shaping of the secondary electron beamlet in conjunction with the selected aperture filter. In one example, an anisotropically shaped aperture filter is applied, including, for example, an elongated rectangular or elliptical aperture, or a dipole or quadrupole filter with two or four off-axis aperture openings. Using anisotropic filtering according to the present disclosure can improve image contrast of semiconductor features of interest, resulting in, for example, increased measurement accuracy.

[0007] According to one aspect, the present disclosure provides a multi-beam charged particle beam system for wafer inspection. The system can provide a relatively high degree of adaptability for various image contrast methods. The multi-beam charged particle beam system comprises an objective radiation unit including a multi-beamlet generator for generating a plurality of primary charged particle beamlets. The multi-beam charged particle beam system comprises an objective lens for focusing the plurality of primary charged particle beamlets onto an image plane, hereinafter also referred to as an object plane, of the objective radiation unit during use. During use, a plurality of secondary electron beamlets are generated in an interaction volume between the plurality of primary charged particle beamlets and the wafer. The multi-beam charged particle beam system further comprises a detection unit configured to image the plurality of secondary electron beamlets onto an image sensor. The multi-beam charged particle beam system further comprises a beam splitter unit for directing the plurality of primary charged particle beamlets from the multi-beamlet generator to the objective lens and for directing the plurality of secondary electron beamlets from the objective lens to the detection unit. The detection unit includes an aperture filter module including at least one selected aperture filter for anisotropically filtering the at least one secondary electron beamlet. The multi-beam charged particle beam system further includes a control unit including a contrast control module configured to control, during use, the selected anisotropic filtering of at least one of the secondary electron beamlets by the selected aperture filter of the aperture filter module.

[0008] The multi-beam charged particle beam system may further comprise a voltage supply unit connected to the wafer during use for supplying to the wafer a voltage for generating a deceleration field for the primary charged particles corresponding to an acceleration or extraction field for the secondary electrons generated in the interaction volume. The detection unit may comprise a plurality of electron optical elements configured to form, for example, a crossover plane or a common pupil plane of the secondary electron beamlets, at which the plurality of secondary beamlets form a crossover.

[0009] The secondary electron yield in each interaction volume generally depends on the flux of the corresponding primary charged particle beamlet and the material composition within the interaction volume, and the angular distribution of secondary electrons generally depends on local charging effects of semiconductor features, including semiconductor features in the underlying layer of the wafer, the local influence of the extraction field for secondary electrons, or the local topography of the wafer surface proximate the interaction volume.

[0010] Secondary electrons from the underlying semiconductor features or underlying background can be at least partially blocked or filtered out during inspection by anisotropic filtering. Selected anisotropic filtering according to the present disclosure can improve image contrast of semiconductor features of interest, which can, for example, increase measurement accuracy.

[0011] In one example, the aperture filter module includes a movement mechanism configured to replace at least one aperture filter. The contrast control module may be configured to select and position a selected aperture filter during use by the movement mechanism at a common pupil plane of the detection unit. In one example, the selected aperture filter includes an aperture opening having an anisotropic shape, for example, an elliptical shape or an elongated rectangular shape. In one example, the selected aperture filter includes multiple aperture openings configured to be positioned outside the electronic optical axis of the detection unit. For example, the selected aperture filter includes at least two aperture openings symmetrically positioned about the electronic optical axis to form an aperture filter including a dipole or quadrupole opening. In one example, the contrast control module is configured to position an aperture filter having a dipole or quadrupole shape according to the horizontal or vertical structure of semiconductor features on the wafer. In one example, the contrast control module is configured to position an aperture filter having a dipole or quadrupole shape according to the topography of semiconductor features on the wafer.

[0012] In one example, the plurality of electron optical elements of the detection unit are further configured to form an intermediate image plane of the plurality of secondary electron beamlets. The detection unit may further include an active multi-aperture array arranged close to the intermediate image plane at a distance where the plurality of secondary electron beamlets do not overlap or intersect. The active multi-aperture array may include a plurality of apertures, each of which is a passageway for one of the plurality of secondary electron beamlets, and each of the apertures is configured with a plurality of electrodes connected to a contrast control module. The active multi-aperture array may be configured to individually shape or deflect each of the secondary electron beamlets during use. For example, the contrast control module may be configured to control the active multi-aperture array for first anisotropic shaping or first deflection of the first secondary electron beamlet and second anisotropic shaping or second deflection of the second secondary electron beamlet. The contrast control module may further be configured to position a selected aperture filter of a circular shape at the common pupil plane. Different anisotropic filtering of the first and second secondary electron beamlets can thereby be achieved, improving the imaging contrast of the multi-beam charged particle beam system for each of the multiple charged particle beamlets. Secondary electrons from local underlying semiconductor features or from the local background can be at least partially blocked or filtered out for each of the multiple primary charged particle beamlets. The different anisotropic filtering allows larger areas of the wafer having various semiconductor features to be inspected with greater imaging contrast, and inspection tasks to be performed with greater reliability and accuracy.

[0013] In one example, the contrast control module is configured to determine anisotropic filtering for each of the plurality of secondary electron beamlets to achieve enhanced image contrast. In one example, the contrast control module is configured to modify the anisotropic filtering to iteratively improve and optimize image contrast. In one example, the contrast control module is configured to select the anisotropic filtering according to previous information, for example, based on stored information of anisotropic filtering selected for the inspection site or based on CAD information of semiconductor features of the wafer.

[0014] In one example, the detection unit of the multi-beam charged particle beam system includes at least one first multipole corrector upstream of a common pupil plane of the detection unit. The at least one first multipole corrector is connected to a contrast control module, and the contrast control module is configured to drive the at least one first multipole corrector to achieve collective shaping of pupil distributions of the multiple secondary electron beamlets at the common pupil plane. In one example, the contrast control module is configured to place a circular-shaped aperture filter at the common pupil plane.

[0015] In one example, a voltage supply unit operating in a multi-beam charged particle beam system is configured to adjust the voltage on the wafer to achieve a wide range of landing energies of the primary charged particles, including low landing energies below 100 eV, e.g., 50 eV, and high landing energies above 2 keV, e.g., 3 keV. For some landing energies, astigmatism may be induced in imaging the secondary electron beamlets using the detection unit, resulting in an elliptical pupil distribution of the multiple secondary electron beamlets. Therefore, in one example, the voltage supply unit is configured to achieve shaping of the pupil distribution of the multiple secondary electron beamlets into a circular shape. In one example, the multi-beam charged particle beam system is configured to shape the elliptical pupil distribution of the multiple secondary electron beamlets into a circular-shaped pupil distribution and filter the circular-shaped pupil distribution. In one example, the filtering is performed by a circular aperture filter, thereby avoiding anisotropic filtering of the secondary electron beamlets at a common pupil plane. In an alternative example, filtering is performed by an elliptical, elongated, or multipole shaped aperture filter to achieve anisotropic filtering. In another example, a contrast control module configured to achieve shaping of a circular shaped pupil distribution into an elliptical or multipole shape in combination with a circular shaped aperture filter at a common pupil plane to achieve anisotropic filtering.

[0016] In one example, the detection unit of the multi-beam charged particle beam system further includes at least one second multipole corrector downstream of the common pupil plane of the detection unit. The at least one second multipole corrector is connected to the contrast control module and configured to correct the effect of shaping the pupil distribution of the multiple secondary electron beamlets by the first multipole corrector. The first multipole corrector upstream of the common pupil plane shapes the pupil distribution of the multiple secondary electron beamlets at the common pupil plane, for example, from a circular shape to an elliptical shape. The beam shaping further affects the focused spot in the image plane of the detection unit. For example, astigmatism is introduced by the beam shaping by the first multipole corrector to generate an elliptical or astigmatism-deformed focused spot. Using the second multipole corrector arranged downstream of the common pupil plane, the astigmatism shape is corrected so that an aberration-free focused spot of the multiple secondary electron beamlets is further formed in the image plane of the detection unit. This avoids crosstalk during filtering and improves detection efficiency.

[0017] According to one aspect, the present disclosure provides a method for improving contrast for wafer inspection tasks. The method includes irradiating a surface of a wafer with multiple primary charged particle beamlets of a multi-beam charged particle beam system, thereby exciting multiple secondary electron beamlets from multiple interaction volumes created by the multiple primary charged particle beamlets and the wafer. The method includes focusing the multiple secondary electron beamlets with an objective lens and anisotropically filtering at least one of the secondary electron beamlets with a selected aperture filter. The method includes collecting signals of each of the multiple secondary electron beamlets, including at least one anisotropically filtered secondary electron beamlet, with an image sensor to generate an image of the surface of the wafer with improved contrast of a semiconductor feature of interest. The selected aperture filter is positioned at a common pupil plane of a detection unit of the multi-beam charged particle beam system. The anisotropic filtering can at least partially block or filter out secondary electrons from underlying semiconductor features or underlying background during inspection.

[0018] In one example, the method includes selecting a selected aperture filter and positioning the selected aperture filter at a common pupil plane of the detection unit by a motion mechanism. The motion mechanism may further include a linear or rotary slider. In one example, the method further includes supplying a voltage to at least one electrode of the active array element to anisotropically shape or deflect at least one of the secondary electron beamlets. Using active array elements disposed in the detection unit or by providing an isotropic shape including a lateral offset, each secondary electron beamlet can be individually shaped into an anisotropic shape, for example, an elliptical shape.

[0019] In one example, the method includes providing a voltage or current to a first multipole corrector disposed in the detection unit upstream from a common pupil plane to collectively shape or deflect a common pupil distribution of the plurality of secondary electron beamlets. In one example, the method further includes providing at least a voltage or current to a second multipole corrector to correct an effect of shaping the pupil distribution on a shape of the plurality of focused spots at an image plane of the detection unit.

[0020] According to one example, the method further includes the steps of positioning an inspection position on the surface of the wafer in an object plane of a multi-beam charged particle beam system, determining a selected contrast mechanism at the inspection position, selecting a selected aperture filter and at least one voltage from a plurality of voltages and supplying them to the active array element or the first or second multipole corrector according to the selected contrast mechanism, and performing image acquisition of the surface of the wafer.

[0021] A digital image of the semiconductor features of the wafer can thereby be acquired at the inspection position. The inspection position on the surface of the wafer is typically positioned using a wafer stage with six-axis control and, for example, an interferometer for precise control of position and alignment. At least one of the secondary electron beamlets can be anisotropically filtered by a selected aperture filter and at least one of a plurality of voltages on the active array elements to enhance the first image contrast of the digital image.

[0022] In one example, the method further includes evaluating a first image contrast of the digital image and modifying the selected contrast mechanism by modifying at least one of a selected aperture filter or a voltage supplied to an electrode of an active array element or the first or second multipole corrector. Image acquisition of the surface of the wafer can be repeated using the modified contrast mechanism, and an improved contrast mechanism having an improved image contrast compared to the first image contrast can be determined. In one example, the modified or improved contrast mechanism is stored in memory for use with the inspection positions. The method can optimize the contrast mechanism to obtain improved imaging contrast at each inspection position. Further inspection tasks at comparable inspection positions, for example, on another wafer or another die, can then be performed using the predetermined contrast mechanism.

[0023] In one example, the method further includes performing an image evaluation of the digital images of the semiconductor features of the wafer to identify defects, which are generally described by at least one of excessive deviations in size, area, material composition of the semiconductor features, or excessive features, e.g., contaminant particles. In one example, the method further includes repeating image acquisition of the surface of the wafer at multiple inspection positions, e.g., including first and second different contrast features, and evaluating a distribution of the defects to identify at least one of random defects, regular defects, or clusters of defects.

[0024] In one example, the method further includes image processing of a first digital image from a first image acquisition with first filtering at the inspection site on the wafer and a second digital image from a second image acquisition with second filtering at the same inspection site on the wafer. According to one example, at least two images are acquired with different contrast mechanisms using at least one of different aperture filters, different voltages supplied to electrodes of the active array elements, or different voltages or currents supplied to the first or second multipole correctors. According to an example, the method further includes image processing of the at least two images acquired with different contrast mechanisms, including image combining, edge enhancement, stereoscopic or 3D image generation, or difference calculation. Different first and second digital images are obtained using different first and second filtering, including at least one anisotropic filtering, and the combined processing can, for example, enhance edge contrast or utilize various topographic contrasts, and a 3D or stereoscopic image can be generated and displayed.

[0025] In one aspect, the present disclosure provides a multi-beam charged particle beam system comprising an objective radiation unit including a multi-beamlet generator configured to generate a plurality of primary charged particle beamlets and an objective lens configured to focus the plurality of primary charged particle beamlets onto an object plane of the objective radiation unit. The multi-beam charged particle beam system further comprises: a detection unit configured to image a plurality of secondary electron beamlets generated by interaction of the plurality of primary charged particle beamlets with a surface of a wafer onto an image sensor, the detection unit including an aperture filter module including an aperture filter configured to anisotropically filter at least one secondary electron beamlet; a beam splitter unit configured to direct the plurality of primary charged particle beamlets from the multi-beamlet generator to the objective lens and direct the plurality of secondary electron beamlets from the objective lens to the detection unit; and a control unit including a contrast control module configured to control the anisotropic filtering of at least one secondary electron beamlet of the plurality of secondary electron beamlets via the aperture filter.

[0026] According to one example, the aperture filter module includes an aperture filter having a circular, elliptical, or elongated aperture opening and selected from aperture filters having two or four aperture openings. According to one example, the aperture filter module includes at least two aperture filters having a circular, elliptical, or elongated aperture opening and selected from aperture filters having two or four aperture openings.

[0027] According to one example of the present disclosure, a multi-beam charged particle beam system for wafer inspection comprises an objective radiation unit including a multi-beamlet generator for generating a plurality of primary charged particle beamlets and an objective lens for focusing the plurality of primary charged particle beamlets onto an objective plane of the objective radiation unit during use. The multi-beam charged particle beam system further comprises a detection unit configured to image a plurality of secondary electron beamlets generated in parallel during use at an interaction volume between the plurality of primary charged particle beamlets and the surface of the wafer onto an image sensor. The detection unit comprises an aperture filter module including an active aperture filter for filtering at least one component of the angle or pupil distribution of the plurality of secondary electron beamlets. The multi-beam charged particle beam system comprises a control unit including a contrast control module configured to control, during use, anisotropic filtering of at least one secondary electron beamlet of the plurality of secondary electron beamlets by the active aperture filter of the aperture filter module. The active aperture filter includes at least one aperture opening having at least one deflection electrode for deflecting components of the angle or pupil distribution of the multiple secondary electron beamlets passing through the at least one aperture opening. In one example, the at least one aperture opening is disposed outside the electronic optical axis of the detection unit. During use, the contrast control module is configured to supply at least one deflection voltage to the deflection electrode of the active aperture filter. This deflects selected portions of the common pupil distribution of the multiple secondary electron beamlets and directs them, for example, to separate detection areas or pixels of the image sensor. For example, the image sensor includes a plurality of M detection areas, where the number M of the detection areas is at least twice the number J of the multiple secondary electron beamlets, M>=2×J, preferably M=4×J, M=7×J, or M=9×J. This allows at least two different components of the pupil distribution to be detected separately for each secondary electron beamlet.

[0028] According to one embodiment, a contrast improvement method for a wafer inspection task includes irradiating a surface of a wafer with multiple primary charged particle beamlets, thereby exciting multiple secondary electron beamlets from an interaction volume created by the multiple primary charged particle beamlets and the wafer. The method further includes focusing the multiple secondary electron beamlets with an objective lens and deflecting at least one component of the angle or pupil distribution of the multiple secondary electron beamlets with an active aperture filter disposed in a common pupil plane of a detection unit of a multi-beam charged particle beam system. The active aperture filter includes at least one aperture opening with at least one deflection electrode to pass and deflect the component of the angle or pupil distribution of the multiple secondary electron beamlets. In one example, the method includes collecting signals of each of the multiple secondary electron beamlets with an image sensor, the image sensor including a plurality of M detection areas, where the number of detection areas M is at least twice the number J of the multiple secondary electron beamlets (9), M > 2 × J, and preferably M = 4 × J, M = 7 × J, or M = 9 × J.

[0029] In some embodiments, the aperture filter module comprises a movement mechanism configured to replace the aperture filter, and the contrast control module is configured to select and position the aperture filter in a common pupil plane of the detection unit via the movement mechanism.

[0030] In some embodiments, the aperture filter comprises an anisotropically shaped aperture opening.

[0031] In some embodiments, the aperture filter comprises a member selected from an elliptical aperture filter and an elongated rectangular aperture filter.

[0032] In some embodiments, the aperture filter comprises a plurality of aperture openings outside the electronic optical axis of the detection unit.

[0033] In some embodiments, at least two aperture openings of the plurality of aperture openings are arranged symmetrically about the electronic optical axis to achieve an aperture filter having a shape selected from a dipole shape and a quadrupole shape.

[0034] In some embodiments, the contrast control module is configured to position an aperture filter having a shape based on a structure of the semiconductor features of the wafer selected from a horizontal structure and a vertical structure.

[0035] In some embodiments, the contrast control module is configured to position an aperture filter having a shape based on the topography of semiconductor features on the wafer.

[0036] In some embodiments, the detection unit comprises a plurality of electron optical elements configured to realize an intermediate image plane for the plurality of secondary electron beamlets, and an active multi-aperture array proximate to the intermediate image plane, the active multi-aperture array including a plurality of apertures, each aperture of the multi-aperture array configured to pass one of the plurality of secondary electron beamlets, and each aperture of the multi-aperture array including a plurality of electrodes connected to a contrast control module for individually anisotropically shaping or deflecting one of the plurality of secondary beamlets passing therethrough.

[0037] In some embodiments, the contrast control module is configured to control the active multi-aperture array to i) anisotropically shape or deflect the first secondary electron beamlet and anisotropically shape or deflect the second secondary electron beamlet, and the contrast control module is configured to place a circular aperture filter at a common pupil plane of the detection unit.

[0038] In some embodiments, the multi-beam charged particle beam system further comprises a voltage supply unit configured to be connected to the wafer to supply a voltage to the wafer to generate a deceleration field for the primary charged particles that corresponds to the acceleration field for the secondary electrons.

[0039] In one aspect, the present disclosure provides a method, the method including: irradiating a surface of a wafer using a plurality of primary charged particle beamlets of a multi-beam charged particle beam system to generate a plurality of secondary electron beamlets generated by the plurality of primary charged particle beamlets and the wafer; focusing the plurality of secondary electron beamlets using an objective lens; anisotropically filtering the secondary electron beamlets using a selected aperture filter positioned at a common pupil plane of a detection unit of the multi-beam charged particle beam system; and collecting signals of each of the plurality of secondary electron beamlets, including the anisotropically filtered secondary electron beamlets, using an image sensor to generate an image of the surface of the wafer.

[0040] In some embodiments, the method further includes selecting an aperture filter and positioning the selected aperture filter at a common pupil plane of the detection unit.

[0041] In some embodiments, the method further includes supplying a voltage to electrodes of an active array element disposed within the detection unit to anisotropically shape or deflect at least one of the secondary electron beamlets.

[0042] In some embodiments, the method further includes positioning an inspection position on the surface of the wafer in an object plane of the multi-beam charged particle beam system, determining a selected contrast mechanism at the inspection position, selecting and supplying an aperture filter and at least a voltage to the active array element to anisotropically filter at least one of the secondary electron beamlets in accordance with the selected contrast mechanism, and performing image acquisition of the surface of the wafer to acquire a digital image of semiconductor features of the wafer at the inspection position.

[0043] In some embodiments, the selected contrast feature at an inspection position is determined by information including: i) a previously determined selected contrast feature at an equivalent inspection position; and ii) a selected member from the CAD information.

[0044] In some embodiments, the method further includes evaluating a first image contrast of the digital image, modifying the selected contrast mechanism by modifying at least one member selected from the group consisting of a preselected aperture filter and a voltage supplied to an electrode of the active array element, and determining a second contrast mechanism having an improved image contrast compared to the first image contrast.

[0045] In some embodiments, the method further includes storing a second contrast mechanism to be used along with the examination position.

[0046] In some embodiments, the method further includes performing an image evaluation of the digital image of the semiconductor features of the wafer to identify defects comprising at least one member selected from the group consisting of: a deviation in size of the semiconductor feature, a deviation in area of ​​the semiconductor feature, a deviation in material composition of the semiconductor feature, and a contaminant particle.

[0047] In some embodiments, the method further includes repeating image acquisition of the surface of the wafer at the multiple inspection positions and evaluating the distribution of the defects to identify at least one member selected from the group consisting of random defects, regular defects, and clusters of defects.

[0048] In some embodiments, the method includes beam shaping of a pupil distribution of at least one secondary electron beamlet. In one example, the method includes collective beam shaping of the pupil distributions of all secondary electron beamlets. The method further includes filtering components of the pupil distributions of the secondary electron beamlets. In one example, the filtering is achieved by selecting and adjusting a selected aperture filter having a selected aperture filter opening at a common pupil plane. In one example, an active aperture filter is used, and the filtering of the components of the pupil distribution is achieved by a deflection means of the active aperture filter.

[0049] In some embodiments, the contrast mechanism is optimized, and a second contrast mechanism utilizing second anisotropic filtering is achieved by adjusting the first contrast mechanism utilizing first anisotropic filtering. In some embodiments, at least a first digital image with a first contrast mechanism utilizing first pupil filtering and a second digital image with a second contrast mechanism utilizing second pupil filtering are achieved. The first and second digital images are further processed by image processing or utilized, for example, for 3D or stereoscopic image rendering.

[0050] According to embodiments or examples of the present disclosure, the present disclosure provides a multi-beam charged particle beam system that can provide improved image contrast, and a method of operating a multi-beam charged particle beam system that can provide improved image contrast, which can enable wafer inspection with relatively high precision and accuracy.

[0051] It will be understood that the present disclosure is not limited to the embodiments and examples, but further includes combinations and variations of the embodiments and examples.

[0052] Embodiments of the present disclosure will now be described in more detail with reference to the drawings. [Brief explanation of the drawings]

[0053] [Figure 1] 1 is a schematic cross-sectional view of a multi-beam charged particle system according to [Figure 2] 3A-3C are diagrams illustrating some examples of aperture filters of the aperture filter module. [Figure 3] 1A-1C illustrate examples of semiconductor features in a multi-layer structure of a wafer. [Figure 4] 1A-1C illustrate examples of semiconductor features having topography. [Figure 5] FIG. 1 shows an active array element. [Figure 6A] FIG. 10 illustrates anisotropic filtering of two of the three secondary electron beamlets by active array elements. [Figure 6B] FIG. 10 illustrates anisotropic filtering of two of the three secondary electron beamlets by active array elements. [Figure 6C] FIG. 10 illustrates anisotropic filtering of two of the three secondary electron beamlets by active array elements. [Figure 7] FIG. 10 illustrates an example of a method according to a second embodiment. [Figure 8A] FIG. 1 illustrates anisotropic filtering for image contrast. [Figure 8B] FIG. 1 illustrates anisotropic filtering for image contrast. [Figure 8C] FIG. 1 illustrates anisotropic filtering for image contrast. [Figure 9] FIG. 1 illustrates defect classification on a wafer surface. [Figure 10]1A and 1B show examples of active aperture filters and corresponding detector arrays. [Figure 11] 1A and 1B show examples of active aperture filters and corresponding detector arrays. [Figure 12] 1A-1C show examples of applications of active aperture filters and corresponding detector arrays. [Figure 13] FIG. 1 illustrates an example of a detection unit including a multi-pole corrector. [Figure 14] FIG. 1 illustrates a contrast improvement method according to an example. [Figure 15] FIG. 10 shows another example of a detection unit including a multipole corrector and an active array element. [Figure 16] FIG. 10 shows an example of pupil filtering with beam shaping by a multipole corrector. DETAILED DESCRIPTION OF THE INVENTION

[0054] In the exemplary embodiments of the present disclosure described below, components that are similar in function and structure are indicated by similar or identical reference numerals whenever possible.

[0055] Some array elements, e.g., multiple primary charged particle beamlets, are identified by reference numbers. Depending on the context, the same reference number can fully identify a single element or further identify an array element. Each primary charged particle beamlet (3.1, 3.2, 3.3) is one of multiple primary charged particle beamlets (3).

[0056] The schematic depiction in FIG. 1 illustrates basic features and functionality of a multi-beam charged particle system 1 according to a first embodiment of the present disclosure. Note that the reference numerals used in the figure are selected to symbolize their respective functionality. The type of system shown is a multi-beam scanning electron microscope using multiple primary charged particle beamlets 3 to generate multiple primary charged particle beam spots 5 on a surface 25 of an object 7, such as a wafer or mask substrate, positioned with its top surface 25 in the object plane 101 of an objective lens 102. For simplicity, only three primary charged particle beamlets 3.1-3.3 and three primary charged particle beam spots 5.1-5.3 are shown. The features and functionality of the multi-beamlet charged particle system 1 can be implemented using electrons or other types of primary charged particles, such as ions, e.g., helium ions. Further details of the microscope system 1 are provided in International Patent Application WO 2022 / 262970, filed June 16, 2021, which is incorporated herein by reference in its entirety.

[0057] The system 1 comprises an objective radiation unit (100), a detection unit 200 and a secondary electron beam divider or beam splitter unit 400 for separating a secondary charged particle beam path 11 from a primary charged particle beam path 13. The objective radiation unit 100 comprises a charged particle multi-beam generator 300 for generating a plurality of primary charged particle beamlets 3 and is adapted to focus the plurality of primary charged particle beamlets 3 onto an object plane 101, the surface 25 of an object or wafer 7 being positioned by a sample stage 500.

[0058] The primary beam generator 300 produces a plurality of primary charged particle beamlet spots at an intermediate image plane 321. The primary beamlet generator 300 comprises at least one source 301 of primary charged particles, e.g., electrons. The at least one primary charged particle source 301 emits a diverging primary charged particle beam, which is collimated by at least one collimating lens 303 to form a collimated or parallel primary charged particle beam 309. The collimating lens 303 typically includes one or more electrostatic or magnetic lenses, or a combined electrostatic and magnetic lens. The collimated primary charged particle beam 309 enters a primary multi-beam forming unit 305. The multi-beam generating unit 305 is described, for example, by U.S. Patent Application Publication No. 2019 / 0259575 and U.S. Patent No. 10,741,355 B1, both of which are incorporated herein by reference. The multi-beam forming unit 305 essentially comprises a first multi-aperture plate or filter plate 304 illuminated by a collimated primary charged particle beam 309. The first multi-aperture plate or filter plate 304 comprises a plurality of apertures in a raster configuration for generating a plurality of primary charged particle beamlets 3, which are generated by transmission of the collimated 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 arranged downstream from the first multi-aperture or filter plate 304 with respect to the direction of electron motion of the beam 309. For example, the second multi-aperture plate 306 comprises, for example, four or eight electrostatic elements for each of the plurality of apertures, for example, to individually deflect each of the plurality of beamlets. The multi-beamlet forming unit 305 according to some embodiments is configured with an end multi-aperture plate 307. The multi-beamlet forming unit 305 is further configured with an adjacent electrostatic field lens 308.1 that is in some examples coupled into the multi-beamlet forming unit 305.In combination with the second field lens 308.2, the multiple primary charged particle beamlets 3 are focused at or close to the intermediate image plane 321. The primary charged particle source 301 and each of the active multi-aperture plates 306 are controlled by a control unit 800.

[0059] The multiple focal points of the primary charged particle beamlets 3 passing through the intermediate image plane 321 are imaged by the field lens group 103 and the objective lens 102 onto the object plane 101, where the surface 25 of the object 7 is located. 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 specimen voltage source 503. The decelerating electrostatic field generated by the specimen voltage source 503 adjusts the landing energy of the primary electrons to, for example, less than 1 keV, less than 500 eV, less than 300 eV, or even lower.

[0060] The objective radiation system 100 further includes a collective multi-beam raster scanner 110 adjacent to the beam crossover 108, which deflects the multiple charged particle beamlets 3 in a direction perpendicular to the propagation direction of the charged particle beamlets. The propagation direction of the primary beamlets throughout the example is the positive z direction. The objective lens 102 and the collective multi-beam raster scanner 110 are adjusted to center an optical axis (not shown) of the multi-beam charged particle system 1, which is perpendicular to the wafer surface 25. The multiple primary charged particle beamlets 3, forming multiple beam spots 5 arranged in a raster configuration, are synchronously scanned over the wafer surface 25. In one example, the raster configuration of the focused spots 5 of the multiple J primary charged particle beamlets 3 is a hexagonal raster of approximately 100 or more primary charged particle beamlets 3, e.g., J=91, J=100, or J=approximately 300 or more beamlets. The primary beam spots 5 have a distance of approximately 6 μm to 45 μm and a diameter of less than 5 nm, e.g., 3 nm, 2 nm, or even smaller. In one example, the beam spot size is approximately 1.5 nm, and the distance between two adjacent beam spots is 8 μm. Multiple secondary electrons are generated at each scanning position of each of the multiple primary beam spots 5, forming multiple secondary electron beamlets 9 in the same raster configuration as the primary beam spot 5. The intensity of the secondary charged particle beamlets 9 generated at each beam spot 5 depends on the intensity of the impinging primary charged particle beamlets 3, the irradiating corresponding spot 5, the material composition and topography of the object 7 directly below the beam spot 5, and the charge state of the sample at the beam spot 5. The multiple secondary charged particle beamlets 9 are accelerated by the same electrostatic field between the objective lens 102 and the object surface 25 generated by the voltage source 530, focused by the objective lens 102, and pass through the first collective multi-beam raster scanner 110 in the opposite direction to the primary beamlets 3. The plurality of secondary beamlets 9 are deflected during scanning by a first collective multi-beam raster scanner 110 .The multiple secondary charged particle beamlets 9 are then guided by a secondary electron beam divider or beam splitter unit 400 to follow the secondary beam path 11 to the detection unit 200. The multiple secondary electron beamlets 9 travel in the opposite direction from the primary charged particle beamlets 3, and the beam splitter unit 400 is configured to separate the secondary beam path 11 from the primary beam path 13, typically by a magnetic field or a combination of magnetic and electrostatic fields.

[0061] The detection unit 200 images the secondary electron beamlets 9 onto the image sensor 600, forming multiple secondary charged particle image spots 15 thereon. The detector or image sensor 600 includes multiple detector pixels or individual detectors. The intensity for each of the multiple secondary charged particle beam spots 15 is detected individually, allowing the characteristics of the object surface 25 to be detected with high throughput and high resolution for a large image section of the object 7. For example, with a 10×10 beamlet raster having an 8 μm pitch, a single image scan using the collective multi-beam raster scanner 110 generates an image section of approximately 88 μm×88 μm, for example, with an image resolution of 2 nm or less. The image section is sampled at half the beam spot size, thus 8,000 pixels per image line for each beamlet, so that an image section generated by 100 beamlets has 6.4 gigapixels. Digital image data is collected by the control unit 800. For example, details of digital image data collection and processing using parallel processing are described in International Patent Application WO2020151904 and U.S. Patent No. 9,536,702, which are incorporated herein by reference.

[0062] The detection unit 200 further comprises at least a second collective raster scanner 222 connected to a scanning and imaging control unit 860. The scanning control unit 860 is configured to correct residual errors in the positions of the multiple focal points 15 of the multiple secondary electron beamlets 9 such that the positions of the multiple secondary electron focal spots 15 are kept constant at the image sensor 600.

[0063] The detection unit 200 further comprises electrostatic or magnetic lenses 205.1 to 205.5 and a second crossover 21 of the plurality of secondary electron beamlets 9, at which a contrast aperture filter module 214 is disposed. The second crossover corresponds to a pupil plane 21 of the detection unit 200. At the pupil plane, a lateral coordinate with respect to an optical axis 2105 corresponds to a propagation angle of a secondary electron trajectory at the object plane 101. The propagation angle of the secondary electron trajectory is measured relative to a normal to the wafer surface, which corresponds to the optical axis 2105 of the detection unit 200. The detection unit 200 further comprises at least a first multi-aperture corrector 216 having an aperture and an electrode for individually influencing each of the plurality of secondary electron beamlets 9. The multi-aperture corrector 216 is disposed close to an intermediate image plane 211, at which the secondary electron beamlets are separated from each other.

[0064] The image sensor 600 is configured with an array of sensing areas in a manner that matches the raster configuration of the secondary electron beamlets 9 focused onto the image sensor 600 by the projection lens. This allows detection of each individual secondary electron beamlet independently of other secondary electron beamlets incident on the image sensor 600. The image sensor 600 shown in FIG. 1 can be an electron-sensitive detector array, such as a CMOS or CCD sensor. Such an electron-sensitive detector array can include an electron-to-photon conversion unit, such as a scintillator element or an array of scintillator elements. In some embodiments, the image sensor 600 can be an electron-to-photon conversion unit or scintillator plate positioned at the focal plane of multiple secondary electron particle image spots 15. In such embodiments, the image sensor 600 can further include a relay optical system for imaging and directing photons generated by the electron-to-photon conversion unit at the secondary charged particle image spots 15 onto dedicated photon detection elements, such as multiple photomultiplier tubes or avalanche photodiodes (not shown). Such an image sensor is disclosed in US Pat. No. 9,536,702, cited above and incorporated by reference.

[0065] The stage 500 typically does not move during acquisition of an image section by scanning the multiple primary charged particle beamlets 3, and after acquisition of the image section, the stage 500 moves to the next image section to be acquired. In some embodiments, with the collective multi-beam raster scanner 110 in a first direction, the stage 500 is subsequently moved in a second direction while an image is acquired by scanning the multiple primary charged particle beamlets 3. Stage movement and stage position are typically monitored and controlled by sensors known in the art, such as laser interferometers, grating interferometers, confocal microlens arrays, or the like.

[0066] During image scanning, the control unit 800 triggers the image sensor 600 to detect multiple timely resolved intensity signals from the multiple secondary electron beamlets 9 at predetermined time intervals, and digital images of the image section are accumulated and stitched together from all scanning positions of the multiple primary charged particle beamlets 3.

[0067] The control unit 800 of the multi-beamlet charged particle system 1 further comprises an imaging control module 810 configured to receive a data stream from the image sensor 600 and to generate a digital image of the surface of the sample 7 during operation, a secondary beam path control module 820 configured to control the lens 205 and other components of the detection unit 200, a primary beam path control module 830 configured to control elements of the objective radiation unit 100 including the charged particle multi-beamlet generator 300, a stage control module 850 configured to control stage positioning and alignment, including control of the sample voltage supply unit 503, a scanning operation control module 860 configured to control scanning operations by the first collective multi-beam raster scanner 110 and the second deflection system 222, and a control operation processor unit 840 configured to perform inspection tasks of the sample and configured to control the modules 810, 820, 830, 850, 860, 870, as well as a memory 880 for storing software, instructions, and image data. The control operation processor unit 840 is further connected to an interface (not shown) for exchanging data, instructions, software, or user interaction.

[0068] The control unit 800 of the multi-beamlet charged particle system 1 according to the present disclosure further comprises a contrast control module 870 connected to the control operation processor unit 840. The contrast control module 870 is configured to receive instructions from the control operation processor unit 840 to control the contrast mechanism of the imaging of secondary electrons onto the image sensor 600. Accordingly, the contrast control module 870 is connected to the aperture filter module 214 and is configured to select an aperture filter 284 according to the selected contrast mechanism. For simplicity, only two different aperture filters 284a and 284b are shown, but three or more different aperture filters 284 may be provided. For the placement of the selected aperture filter 284a at the common pupil position 21 of the multiple secondary electron beamlets 9, the aperture filter 284 may be mounted on an exchange mechanism, such as a rotational or linear movement mechanism 215.

[0069] FIG. 2 shows examples of various apertures 284a-284d.

[0070] According to one example, the aperture filter module 214 comprises a first aperture filter 284a in which four aperture openings 286.1 to 286.4 arranged at different azimuthal locations form a quadrupole shaping aperture filter 284a to pass four segments of the angular spectrum of each secondary electron beamlet 9.

[0071] The aperture filter module 214 comprises a second aperture filter 284b in which four aperture openings 286.5-286.8 arranged at various azimuthal locations form a quadrupole shaping aperture filter 284b to pass four segments of the angular spectrum of each secondary electron beamlet 9, but rotated by 45° relative to the first aperture filter 284a.

[0072] The aperture filter module 214 further comprises a third aperture filter 284c of elongated rectangular shape configured to transmit the angular spectrum of each secondary electron beamlet 9 to a greater extent in the y direction compared to the x direction. The third aperture filter 284c is currently centered at the common pupil position 2105 of the detection unit 200 by the motion mechanism 215.

[0073] The aperture filter module 214 further comprises a fourth aperture filter 284d of elongated rectangular shape configured to transmit the angular spectrum of each secondary electron beamlet 9 to a greater extent in the x direction compared to the y direction. Other anisotropic shapes of aperture filters are possible as well, for example aperture filters with two off-center aperture openings (dipole filter) or ellipsoidally shaped aperture openings.

[0074] In general, the example multi-beam charged particle system 1 includes at least a filter element in the detection unit 200 with anisotropic properties for generating anisotropic image contrast. As used herein, anisotropic properties refer to properties that depend on the azimuthal angle relative to the propagation direction of the secondary electron beamlets 9. Therefore, the anisotropic aperture filters 284a-284d do not have perfect rotational symmetry. Instead, their properties change with the rotational or azimuthal angle. The anisotropic aperture filters can achieve anisotropic filtering of the angular spectral distribution of the secondary electron beamlets 9. The angular spectral distribution of the secondary electron beamlets is generally defined by a probability distribution of all possible propagation angles of individual electron trajectories in the secondary electron beamlets 9.

[0075] The anisotropic aperture filters 284a-284d allow filtering of the azimuthal components of the angular spectral distribution of the secondary electron beamlets 9, thereby realizing various contrast mechanisms during imaging by the multi-beam charged particle system 1. The control operation processor 840 is configured to determine instructions regarding the contrast mechanisms to be used, for example, by receiving instructions from an input or by determining instructions from a digital image output from the imaging control module 810.

[0076] The aperture filter module 214 further comprises fifth and sixth circular-shaped aperture filters 284e and 284f having different radii. In one example, anisotropic filtering is achieved using anisotropically shaped secondary electron beamlets 9 and circular-shaped aperture filters 284e or 284f. The use of the fifth and sixth aperture filters 284e and 284f is described in more detail below.

[0077] FIG. 3 shows an example of enhanced contrast imaging by secondary electron beamlets 9.i, including an anisotropic angular spectrum of secondary electrons. Each primary charged particle beamlet 3, e.g., beamlet 3.i, when focused on the surface 25 of wafer sample 7, forms an interaction volume 707 therein with a diameter DX and a depth DZ at wafer sample 7. The diameter DX and depth DZ typically depend on the kinetic energy of the primary charged particle beamlet 3.i after deceleration. For example, at a kinetic energy of 300 eV, the diameter DX and depth DZ are approximately 5 nm. At 200 eV, the diameter DX and depth DZ are approximately 3 nm. For example, at a kinetic energy of 1 keV, the diameter DX increases to approximately 20 nm and the depth increases to approximately 25 nm. Typically, high resolution is desired, and the deceleration field is generated by the sample voltage source 503 so that the kinetic energy of the primary charged particles is less than 500 eV, e.g., 300 eV, 200 eV, or even lower.

[0078] Typically, the wafer sample 7 includes several layers 711, of which only four layers 711.1-711.4 are shown. The layers are formed on a silicon substrate using semiconductor fabrication techniques and include insulators such as silicon nitride, silicon oxide, or silicon carbide, as well as metal structures such as metal lines 728, 729, and 731. The metal lines 728, 729, and 731, or interconnects, are arranged orthogonal to one another and extend in either the x or y direction. During an inspection task, the metal lines 728, 729, and 731 can become sufficiently charged to produce the effect of a deceleration or extraction field 505. Thus, the isoelectric lines of the deceleration field 505 exhibit local inhomogeneities. These local inhomogeneities have a subtle impact on the primary charged particle beamlets 3.i but can have a significant impact on the secondary electrons generated within the interaction volume 707. Because the secondary electrons travel in the opposite direction, the deceleration field 505 for the primary charged particles forms an acceleration field 505 for the secondary electrons. The secondary electrons follow trajectories, e.g., trajectories 99.1 or 99.2, and are accelerated by the field having equipotential lines 505 generated by supplying voltage to the wafer 7 via voltage supply unit 503. Due to non-uniformity in the extraction field 505 and non-uniform charge distribution in the semiconductor wafer sample 7, the secondary electrons are emitted with a non-uniform angular spectrum, e.g., an elliptical angular spectrum. In another example, the interaction volume intersects several layers 711, e.g., layers 711.1-711.3, within various layers, and the secondary electrons are emitted with different azimuthal spectral distributions. Because the metal interconnects 728, 729, or 731 are elongated in either the x or y direction, the angular spectral distribution of the secondary electron beamlet 9.i has distinct spectral distributions in the x and y directions. By filtering out these prominent spectral distributions with aperture filters 284a-284c, the imaging contrast of, for example, metal line 728 is improved.

[0079] FIG. 4 shows a further example of enhanced contrast imaging by secondary electron beamlet 9.i, including an anisotropic angular spectrum of secondary electrons. Primary charged particle beamlet 3.i is focused on the surface 25 of a wafer sample 7, which includes several fins, including a gate fin 703 and a channel fin 705. The fins 703 and 705 form a topography on the wafer surface 25. The primary charged particle beam 3.i forms an interaction volume 707 that includes the topography. The deceleration field 505, indicated by the isotropic lines, has local nonuniformities due to the topography. Due to the local nonuniformity and topography of the deceleration or extraction field 505, the angular spectral distribution of secondary electron beamlet 9.i emitted and extracted from the interaction volume exhibits significant contributions in certain azimuthal directions. Because the fins are arranged in the x and y directions, the dominant direction of significant contributions to the secondary electron angular spectrum is the x or y direction. Aperture filters 284c or 284d can filter these prominent spectral distributions, for example, enhancing the imaging contrast of gate fin 703 and suppressing the imaging contrast of channel fin 705. Aperture filter 284a can filter the prominent spectral distributions, making the imaging contrast of gate fin 703 and channel fin 705 higher than that of background structures. Anisotropic aperture filter 284b enables filtering of prominent angular spectral distributions, enhancing the imaging contrast of the background and suppressing the imaging contrast of gate fin 703 and channel fin 705.

[0080] Any of the anisotropic aperture filters 284a-284d enables anisotropic filtering of a pronounced angular spectral distribution, enhancing the imaging contrast of semiconductor structures or metal lines. By placing a common anisotropic aperture filter 284a-284d at the common pupil position 2105 of multiple secondary electron beamlets 9, an anisotropic filtering operation is performed for each of the multiple secondary electron beamlets 9. In a second example of the first embodiment, individual anisotropic filtering of individual secondary beamlets 9 is enabled. According to the second example, each secondary electron beamlet 9 is individually manipulated by an active array element 216. An example of the active array element 216 is shown in FIG. 5. The active array element 216 is formed as a multi-aperture plate 689 having multiple apertures 685 (only two of which are identified by 685.1 and 685.2). The aperture 685 transmits the secondary electron beamlets 9 while they are still separated from one another and close to the intermediate image plane 211 of the detection unit 200 (see FIG. 1 ). Each aperture 685 is provided with two or more electrodes 681, e.g., eight electrodes 681.1 to 681.8, which are connected to the contrast control module 870 via voltage supply lines 687 (not all shown). During operation of a selected contrast mechanism, each individual secondary charged particle beamlet 9 can be anisotropically shaped by the corresponding electrode 681 of the corresponding aperture 685, such that the secondary electron beamlet 9 is, for example, elliptically shaped or deflected in a specific off-axis direction. The individually anisotropically shaped secondary electron beamlets 9 are transmitted through a downstream aperture filter 284, e.g., a circular aperture filter 284f, at a common pupil plane 2105.

[0081] 6A-6C show the effect in three examples. FIG. 6A shows the focused spots of three secondary electron beamlets in the image plane of the detection unit 200. The first secondary electron beamlet 9.1a is not anisotropically shaped, but transmits through its corresponding aperture 685 of the active array element 216 without any shaping. The second secondary electron beamlet 9.2a is anisotropically shaped into an elliptical shape in the image plane by applying corresponding voltages to at least four of the eight electrodes 681 arranged in the corresponding aperture of the second secondary electron beamlet 9.2. The third secondary electron beamlet 9.3a is anisotropically shaped into an elliptical shape and displaced in the image plane by applying corresponding voltages to at least four of the eight electrodes 681 arranged in the corresponding aperture of the third secondary electron beamlet 9.3. Corresponding voltages on at least four of the eight electrodes 681 are configured and selected to generate anisotropic shapes for secondary electron beamlets 9.2 or 9.3. Figure 6B shows the corresponding angular spectral distributions 9.1b-9.3b of each beamlet, each filtered by a common circular aperture opening 286.12. The second secondary electron beamlet 9.2 is filtered by aperture opening 286.12 such that the angular spectrum of secondary electron beamlet 9.2 is blocked in regions with large angles in the positive and negative y directions. Only the third secondary electron beamlet 9.3 passes through aperture opening 286.12 in regions with large angles in the positive x direction. Figure 6C shows the effect of the filtering action on the angular spectra of secondary electron beamlets 9.1-9.3 in an angular spectrum depiction using angular spectrum coordinates p and q. The transmitted angular spectrum is thereby affected by the combined action of the individual active array elements 216 and the common aperture filter 284f such that, in this example, the full angular spectrum of the first secondary electron beamlet 9.1 is detected, but only partial angular spectra of the second secondary electron beamlet 9.2 and the third secondary electron beamlet 9.3 are detected, thereby improving image contrast for each beamlet individually in a manner similar to that described with respect to the first example of the first embodiment.The common aperture filter 284 may have a single circular aperture opening 286.12 or in general may have any other shape or any number of openings depending on the inspection task for a number of different image segments produced by scanning a number of primary charged particle beamlets 3. In general, apertures 284 of other shapes as shown in Figure 2 are likewise feasible, for example, a bipolar aperture, an annular aperture, or an aperture with five openings similar to aperture filters 284a or 284b with additional axial openings.

[0082] Such a system allows for multi-beam inspection with improved image contrast. Some embodiments of the present disclosure provide image enhancement methods for multi-beam image acquisition and wafer inspection. An example is shown in FIG.

[0083] In step S, an inspection site is selected and the surface 25 of the wafer 7 is placed in the object plane 101 of the multi-beam charged particle beam system 1 by the stage 500 .

[0084] In step A, a first contrast feature is selected and a corresponding aperture filter 284 is placed in the common pupil plane 2105 of the detection unit 200 of the multi-beam charged particle beam system 1. The aperture filter 284 is placed, for example, by the movement mechanism 215. Optionally, voltages are supplied to a plurality of electrodes 681 arranged in the aperture 685 of the active array element 216 such that the individually shaped secondary electron beamlets 9 are adapted to the at least first and second contrast features for the at least first and second secondary electron beamlets 9.

[0085] In one example, the selected contrast feature at the inspection position is determined by previous information, including one of a previously determined selected contrast feature at a comparable inspection position or from CAD information regarding the semiconductor feature at the inspection position on the wafer.

[0086] In step I, a scanning microscope image is obtained by scanning a plurality of primary charged particle beamlets 3 over the surface 25 of the wafer 7 and collecting corresponding secondary electron signals with the sensor unit 600. The image data generated by the sensor 600 is collected by the imaging control unit 810 and further processed, for example, by image processing, binding operations, and other operations, and optionally stored in an image memory portion of the memory 880.

[0087] In step C, the image contrast is evaluated. Optionally, if the image contrast from the selected first or second contrast mechanism in step A does not comply with a predetermined expected value or desired characteristic, improvements are made to the aperture filter selection and voltage generation for the plurality of electrodes 681, and the method continues with step A. The process may be repeated iteratively until the predetermined expected value or desired characteristic for the image contrast is achieved. The finally obtained optimized contrast mechanisms may be stored as third and fourth or further contrast mechanisms, optionally individually for each or multiple charged particle beamlets 3, for a particular inspection task.

[0088] The first and second contrast schemes may be initial contrast schemes, for example, using a standard circular aperture 284e, and the determination of the optimal aperture filter 284 in cooperation with the individual anisotropic beam shaping by the active array elements 216 may thus be determined by iteratively optimizing the image contrast for each image segment obtained by each primary charged particle beamlet 3 and the corresponding secondary electron beamlet 9. The optimized contrast scheme may be stored for repeated application of the same inspection task at similar inspection locations.

[0089] In one example, a selected aperture filter having an anisotropic shape is positioned at the common pupil plane 21 of the detection unit 200. During optimization of the image contrast for each of the secondary electron beamlets, voltages are supplied to the active array elements 216. Multiple voltages are varied and the change in image contrast is determined. By repeated application of this method, the image contrast is optimized for each of the secondary electron beamlets.

[0090] In step E, the final inspection task is performed and the inspection results are stored, for example, in memory 880 or visualized by a user interface. For example, the inspection results of several inspection locations on the wafer can be evaluated and used for process optimization of the semiconductor manufacturing process.

[0091] Figures 8A-8C show the results of the method according to the second embodiment. Figure 8A shows a simplified semiconductor structure with horizontal and vertical elements such as gate fins 703 and channel fins 705. Two cross images along lines A and B obtained by the multi-beam charged particle system 1 without and with the image enhancement method are shown in Figures 8B and 8C.

[0092] FIG. 8B shows the image contrast along line A obtained with a standard contrast mechanism using a circular aperture 284e. The intensity signal I1 has a maximum intensity Imax,1 at the gate fin 703 in the presence of a strong background intensity Imin,1, resulting in a low contrast C1. The contrast C is defined, for example, by C=(Imax-Imin) / (Imax+Imin). After application of the elongated aperture filter 284c, the overall intensity is reduced, but the background signal is reduced even further, so that the contrast C2 calculated from Imax,2 and Imin,2 is large compared to C1. Typically, the contrast mechanism according to the present disclosure improves the image contrast C2 by at least 10% over the image contrast C1 of the standard contrast mechanism. In an example, the contrast C2 is improved even further, for example, by more than 20%, for example, about 30%.

[0093] In addition to the overall contrast improvement, topographical effects at the edges of the example gate fin 703 are reduced, reducing the rounding effect in the image of the edges. The improved contrast mechanism according to the present disclosure allows for a larger normalized image log slope (NILS) of the remaining digital image, which allows, for example, measurement tasks to be performed with greater accuracy.

[0094] 8C shows the image contrast along line B obtained with the standard contrast setup using circular aperture 284e. Intensity signal I3 has a maximum intensity Imax,3 at channel fin 705 in the presence of a strong background intensity Imin,3, resulting in low contrast C3. After application of elongated aperture filter 284d, with opening 286.10 perpendicular to opening 286.9 of aperture filter 284c, the overall intensity is still reduced, but the background signal is reduced even further, so that contrast C4 calculated from Imax,4 and Imin,4 is large compared to C3.

[0095] Thus, for example, by individual shaping of the secondary electron beamlets 9 by the active array elements 216 of the multi-beam charged particle beam system 1, high-contrast images of the gate fins 703 and channel fins 705 can be obtained in parallel during the performance of one inspection task, without the additional expense of the image sensor unit 600 and with only a slight increase in image noise, compared to using a prior art multi-sensor array having many individual image sensors for each secondary electron beamlet 9. Thus, the systems and methods according to the present disclosure can enable the application of improved contrast mechanisms with minimal impact on the multi-beam charged particle beam system 1, including, for example, reduced energy consumption, reduced computational effort, and reduced amount of memory for processing and storing image data generated by only one sensor for each secondary electron beamlet. Of course, the improved contrast mechanisms according to the present disclosure can nevertheless be combined with an image sensor array comprising multiple individual sensor elements for each secondary electron beamlet, and the benefits of the improved image contrast can be utilized there as well.

[0096] In general, for example, by individually shaping the secondary electron beamlets 9 in an anisotropic manner by the active array elements 216 of the multi-beam charged particle beam system 1, imaging with high contrast images can be obtained for each of the secondary electron beamlets 9 for different areas on the wafer surface. For example, in a logic device, different subfields corresponding to the primary or secondary electron beamlets can comprise different semiconductor features with different charging characteristics or different topographies. Special process control monitors (PCMs) can be arranged according to the raster of multiple beamlets, and special process performance indicators of the different semiconductor features can be measured in parallel with the individually improved image contrast for each beamlet of the multi-beam charged particle beam system 1. This enables higher throughput of wafer inspection tasks.

[0097] Figure 9 shows an example of the results of step E. The processed wafer 7 typically includes several dies 71 arranged in rows and columns. Each die 71 corresponds to one lithography exposure by a semiconductor mask. Each die 71 contains at least one semiconductor device, such as a processor or memory device. Using the multi-beam charged particle beam system 1, a large area of ​​the wafer surface 25 can be inspected in a shorter time, allowing for measurements of the characteristics of semiconductor features such as fins, interconnects, HAR channels, transistor gates, doped areas, and process control monitors. Furthermore, defects, such as defects resulting from contamination during wafer processing or from mask defects, can be detected. Figure 9 shows some example measurement results. The first numbered random defects 75.1-75.4 are scattered across the wafer surface 25 and may be the result of random particle defects. A cluster 77 of defects in the wafer contour may be an indicator of a deviation in a particular process step. A repeating deviation 79 may be an indicator of a mask defect or contamination. During step E, the results of multiple inspection tasks on a single wafer or group of wafers are analyzed for random defects 75, characteristic clusters of defects 77, or repeating defects 79. The results can trigger improvements to the manufacturing process, such as, for example, mask repair operations, cleaning operations, or adjustments to manufacturing process parameters for individual manufacturing process steps during wafer fabrication.

[0098] FIG. 10 shows a further example of a contrast or aperture filter of the aperture filter module 214. FIG. 10 is shown as a comparison to the contrast filter 284a of FIG. 4. The pupil or angular distribution 288 of secondary electrons is filtered by the aperture openings 286.1-286.4, with only four portions of the entrance pupil distribution 288 being transmitted. However, all four portions of the entrance pupil distribution 288 that pass through the aperture openings 286.1-286.4 are collected for each beamlet of multiple secondary beamlets on the same detector element assigned to the corresponding beamlet. FIG. 10b shows an example of an active aperture filter 214b. The active aperture filter 214b includes five aperture openings 286.1-286.5. Deflector electrode pairs are arranged in the four aperture openings 286.1-286.4 of the aperture openings 286.1-286.5, and are configured to deflect the transmitted portions of the pupil distribution 288 of secondary electrons. This allows the individual angular distributions of the pupil distribution 288 to be deflected so that they do not affect the same detector element. An example is shown in FIG. 10c. The detector 600 is provided with more individual detection areas 602, each corresponding to one focused spot 15 of one secondary electron beamlet 9, so that multiple spare detection areas 602 are included between each focused spot 15.1-15.4. While FIG. 10c shows only four focused spots 15.1-15.4, the number of beamlets may of course be much greater, for example, J=100, J=300, or even more. 10, the number M of detection areas 602 is selected to be larger, e.g., 2, 3, or 4 times the number J of secondary electron beamlets, corresponding to M=2J, M=3J, or M=4J. A plurality of focused spots 15.1-15.4 are generated at a plurality of first corresponding detection regions by the active pupil filter 290. Each of the deflector elements 291.1-292.4 deflects a portion of the pupil or angular distribution 288 of the plurality of secondary electron beamlets so that, during use, it is directed towards an adjacent detection region 602.An example is shown by deflected beam spots 15.13, 15.23, 15.33, 15.43, which correspond to deflected portions of each secondary electron beamlet 15.1-15.4 that pass through aperture opening 286.3 and are synchronously deflected by quadrupole deflector 292.3 to isolate a second detection region. In a similar manner, deflected beam spots 15.14, 15.24, 15.34, 15.44 correspond to deflected portions of each secondary electron beamlet 15.1-15.4 that pass through aperture opening 286.4 and are synchronously deflected by quadrupole deflector 292.4 to isolate a third detection region. For example, beam spot 15.13 corresponding to the deflected portion of the first secondary electron beamlet 15.1 that passed through aperture opening 286.3 is deflected from detection region 602.10 to detection region 602.11, while beam spot 15.14 corresponding to the deflected portion of the first secondary electron beamlet 15.1 that passed through aperture opening 286.4 is deflected from detection region 602.10 to detection region 602.12.

[0099] 10 , not all apertures 286 are provided with deflecting elements 292. In general, an active pupil filter comprises at least one aperture opening with at least one dipole deflecting element 292.

[0100] In use, the plurality of deflectors 292 are controlled by a contrast control module 870. In use, the contrast control module 870 is configured to supply individual voltages to the plurality of deflection electrodes of the deflector 292. The deflection angles and required voltages for focusing the angular spectral contribution of each secondary electron beamlet onto adjacent detector pixels 602 may be determined during calibration of the multi-beam charged particle beam system 1 and stored in a memory 880 of the control unit 800. The contrast control module 870 communicates with a control operation processor 840 for processing image information received from an imaging control module 810 connected to the detector 600. The control operation processor 840 is configured to execute a software module to determine image characteristics from detection signals, including signals corresponding to the various angular spectral contributions of the angle or pupil distribution 288 of each secondary electron beamlet 9.

[0101] FIG. 11 shows a further example of an active pupil filter 290 combined with a high-resolution detector 600. The active pupil filter 290 of FIG. 11a comprises an array of apertures 286.ij with a plurality of deflection elements 292, each including a quadrupole electrode arranged at each aperture opening 286.ij. FIG. 11b shows a corresponding high-resolution detector with even more detection elements. In conventional detectors, the number M of detection areas (assigned to one secondary electron beamlet) is M=J, corresponding to the number J of secondary electron beamlets. In the example of FIG. 11, the number M of detection areas is selected to be larger, for example, 7 times or even larger than the number J of secondary electron beamlets, corresponding to M=7J. Thus, the angular spectral distribution of the secondary electron beamlets can be determined with high angular resolution.

[0102] 12 shows an example application of a high-resolution active pupil filter 290. Separation of the angular spectral distribution or pupil distribution into many individual components reduces the signal strength of each component and increases the noise level. However, different components corresponding to the at least two apertures 286 of the active pupil filter 290 can be combined to direct the at least two components corresponding to the at least two apertures 286 to the same detection region 296 of each secondary electron beamlet 9. The allocation of at least two components corresponding to the at least two apertures 286 of the active pupil filter 290 can increase the signal strength and reduce the noise level. Figure 12 shows an active pupil filter 290 having first and second groups 294.1 and 294.2 of apertures 286, whose deflection electrodes 292 are supplied with voltages by a contrast control unit 870 in use to form, for each group of apertures 294.1 and 294.2, a first focus 296.1 of secondary electrons corresponding to the first group of apertures 294.1 and a second focus 296.2 of secondary electrons corresponding to the second group of apertures 294.2.

[0103] The active pupil filter 290 enables the image enhancement method for multi-beam image acquisition and wafer inspection as described above and shown in Figure 7. The active aperture filter 290 is positioned, for example, at the common pupil plane 21 of the detection unit 200 of the multi-beam charged particle beam system 1 by the movement mechanism 215 of the aperture filter module 214. In step A, a first contrast mechanism is selected and a corresponding deflection voltage is selected for at least one deflection element 292 and supplied to the active pupil filter 290.

[0104] During the scanning imaging in step I, at least two secondary electron signals are obtained for each secondary electron beamlet 9, each corresponding to a component of the angle or pupil spectrum of one secondary electron beamlet 9. The components of the angle or pupil spectrum of the secondary electron beamlets 9 are collected by the sensor unit 600. The image data generated by the sensor 600 is collected by the imaging control unit 810 and further processed, for example, by image processing, stitching operations, and other operations, and optionally stored in an image memory portion of the memory 880.

[0105] In step C, the image contrast is evaluated, for example, for each component of the angle or pupil spectrum of each secondary electron beamlet 9. In one example, the processed image is generated by merging at least two digital images corresponding to different components of the angle or pupil spectrum of each secondary electron beamlet 9. In one example, each digital image corresponding to a component of the angle or pupil spectrum is analyzed separately and features are extracted in each digital image corresponding to a component of the angle or pupil spectrum.

[0106] Optionally, if the image contrast according to the selected contrast mechanism in step A does not comply with a predetermined expected value or desired characteristic, a corresponding improvement in deflection voltage for at least one deflection element 292 is adjusted and supplied to the active pupil filter 290.

[0107] A third embodiment is shown in Figure 13. Figure 13 shows further details of a detection unit 200 configured to image with improved image contrast using anisotropic filtering. The same reference numerals as in Figure 1 are used and reference is also made to the description of Figure 1.

[0108] The primary charged particle beamlet is represented diagrammatically by primary beam path 13 and enters beam splitter unit 400. FIG. 13 shows the secondary electron beam path 11 in the example of two selected electron trajectories 281 and 283 for two secondary electron beamlets 9.i and 9.o. Selected electron trajectory 281 shows a trajectory starting from axial beam spot 5.i at an oblique angle relative to the normal to object plane 101. Selected electron trajectory 283 shows a trajectory starting from off-axis beam spot 5.o that is perpendicular to object plane 101. There are many further secondary electron beamlets corresponding to the multiple primary charged particle beamlets focused on surface 25 of sample 7.

[0109] The detection unit 200 comprises a second branch 151.2 of the common beam tube 151, connected to a voltage supply line and set to a tube voltage VT. VT may be, for example, earth potential. Through the tube 151, primary charged particles propagate with a constant, high kinetic energy, for example, E1 = 30 keV. Through the tube 151, secondary electrons propagate with a constant, high kinetic energy, for example, E2 = E1 - EL (EL = landing energy of the primary electrons, adjusted by the sample potential voltage VL supplied by the sample voltage source 503), with E2 between 27 keV and 30 keV. The detection unit 200 further comprises a second beam tube segment 159 and a third beam tube segment 155. Between the second branch 151.2 and the second beam tube segment 159, a first fast electrostatic lens element 211.1 is arranged. A second fast electrostatic lens element 211.2 is arranged between the second beam tube segment 159 and the third beam tube segment 155. Regarding the first and second fast electrostatic lenses 211.1 and 211.2, reference is made to German Patent Application No. 102022213751.5, filed December 16, 2022, which is incorporated herein by reference. A first magnetic projection lens 205.1 is arranged upstream of the first fast electrostatic lens element 211.1 in the propagation direction of the secondary electron path 13. A second scanning deflector 222 is arranged between the first fast electrostatic lens element 211.1 and the second fast electrostatic lens 211.2. In this example, the second scanning deflector 222 is a two-stage electrostatic octupole scanner. A pair of two further magnetic projection lenses 205.2 and 205.3 are configured to form the focused spots 15.i, 15.o of the secondary electron beamlets 9.i, 9.o on the secondary electron image plane 225 and to adjust for image rotation of the secondary electron beamlets caused, for example, by a change in the objective plane 101 by the objective lens 102. The three magnetic projection lenses 205.1, 205.2 and 205.3 are connected to and controlled by a secondary beam path control module 820 (not shown in FIG. 13, again see FIG. 1).The elements are positioned and aligned so that they are centered on the optical axis 2105 of the detection unit 200, which for simplicity is shown as a straight line; however, the optical axis 2105 may also include curved segments, for example, within the beam divider or beam splitter unit 400.

[0110] In the detection unit 200, at least a first crossover 256 and a second crossover at the pupil lane 21 of the secondary electron beamlet 9 are formed. A crossover is defined as a position along the secondary electron beam path 11 where multiple secondary electron beamlets 9 intersect with each other. Generally, the pupil plane or crossover plane 256 or 21 is defined by a crossover formed by the intersection of secondary electron trajectories starting perpendicular to the object plane 101. An example is shown by the trajectory 283 of the secondary electron beamlet 9.o starting at the focal point 5.o perpendicular to the object plane 101. The detection unit 200 may of course include more than two crossovers, for example a third crossover. In the example of FIG. 13, the selected aperture stop 284g is positioned in a plane perpendicular to the optical axis 2105 of the secondary electron beam path 11 at the second crossover position or pupil plane 21.

[0111] Using the first to third magnetic projection lenses 205.1, 205.2, and 205.3, and the high-speed electrostatic lenses 211.1, 211.2, the magnification of the imaging of the multiple secondary electron beamlets 9 onto the image plane 225 of the detection unit 200 starting from the focused spot 5 on the object plane 101 and the diameter of the pupil distribution of the multiple secondary electron beamlets 9 at the pupil plane 21 can be adjusted independently.

[0112] In a first example according to the third embodiment, the detection unit 200 comprises at least a first multipole corrector 220a, 220b, or 220c upstream of the second pupil plane 21. The at least first multipole corrector 220a, 220b, or 220c is connected to the contrast control module 870. By manipulating the multiple secondary electron beamlets 9 with the at least one first multipole corrector 220a, 220b, or 220c and by placing selected aperture filters 284e, 284f, 284g, or 284h in the pupil plane 21, anisotropic filtering of the secondary electron beamlets 9 is enabled. As in the second example of the first embodiment described above, here each secondary electron beamlet 9 is individually manipulated by the active array element 216, and each secondary electron beamlet 9 is manipulated by at least the first multipole corrector 220a, 220b, or 220c. For example, each secondary electron beamlet 9 is shaped into an elliptical form or deflected in a specific off-axis direction. The equally anisotropically shaped secondary electron beamlets 9 are filtered downstream at the common pupil plane 2105 by a selected aperture filter 284, for example, by a circular aperture filter 284e or 284f. This achieves anisotropic filtering of the secondary electron beamlets 9 (see further FIG. 6 ). The shaping of the pupil distribution of the secondary electron beamlets 9 is not limited to an elliptical shape; however, other shapes, such as a trefoil shape, or a higher-order astigmatic shape or higher-order waviness, are also possible. Instead of using circular filters 284e or 284f, other aperture filters 284 are possible as well, such as, for example, bi-pole or multi-pole filters (see FIG. 2 and its description).

[0113] However, the first example of the third embodiment is not limited to beam shaping the secondary electron beamlets 9 to form an elliptical pupil distribution at the pupil plane 21 due to the anisotropic filtering action of the circular aperture filter 284e or 284f. The first multipole corrector 220a, 220b, or 220c upstream of the second pupil plane 21 can be used to further correct the astigmatism of the multiple secondary electron beamlets 9. For example, it may be desirable to perform inspection tasks within a wide range of landing energies EL. The multi-beam charged particle beam system 1 including the detection unit 200 can operate within a large range of landing energies EL. The various landing energies EL are adjusted by adjusting the sample potential VL and the electrode potential VE at the counter electrode 133 over a wide range between EL=50 eV, e.g., 100 eV, and several kV, e.g., 2 keV or more, and even up to EV=20 keV. It is impossible to completely avoid astigmatism and transformation of the secondary electron beamlets 9 within such a wide range of landing energies EL. A first multipole corrector 220a, 220b or 220c upstream of the second pupil plane 21 can be used to convert the elliptical shape of the pupil distribution of the multiple secondary electron beamlets 9 due to astigmatism and transformation into a circular shape.

[0114] In general, the system according to the third embodiment is configured to shape a common pupil distribution of multiple secondary electron beamlets 9 and to filter the common pupil distribution of multiple secondary electron beamlets 9 with a selected aperture filter. The predefined shaping and aperture filter selection is controlled by a contrast control module 870, which may be configured to adjust the shaping and filter selection according to the landing energy and previous information about the specimen 7 being inspected, such as, for example, CAD information or previous measurement settings at similar inspection sites.

[0115] After shaping the pupil distribution of the multiple secondary electron beamlets 9, in some examples, the focused spots 15 of the secondary electron beamlets 9 at the image plane 225 are deformed, for example, into an elliptical shape with a large diameter. In such examples, crosstalk may occur between the individual beamlets 9. In a second example according to the third embodiment, the detection unit 200 further includes at least a second multipole corrector 218a or 218b downstream from the second pupil plane 21. The at least one second multipole corrector 218a and 218b is connected to the contrast control module 870. At least a first multipole corrector 220a, 220b, or 220c is configured to shape the pupil distribution of the multiple secondary electron beamlets 9 at the pupil plane 21, and at least one second multipole corrector 218a and 218b is configured to shape the focused spots 15 of the multiple secondary electron beamlets 9 at the image plane 225 to form approximately circular focused spots 15, thereby avoiding or reducing crosstalk.

[0116] In one example, the detection unit 200 includes at least two first multipole correctors 220 a, 220 b, and 220 c, which enable beam shaping and pupil distribution adjustment, including, for example, asymmetric pupil distribution, of multiple secondary electron beamlets 9 in the pupil plane 21. Each of the first multipole correctors 220 a, 220 b, and 220 c can include, for example, four, eight, twelve, or more individual addressable electrodes to generate electrostatic fields for beam shaping and deflection. However, the first multipole correctors 220 a, 220 b, and 220 c do not have to be configured as electrostatic multipole elements with electrodes, but may further include multiple coils forming magnetic drive poles.

[0117] In one example, the detection unit 200 includes at least two second multipole correctors 218a and 218b, which enable deflection and beam shaping of the focused spots 15 of the multiple secondary electron beamlets 9 at the image plane 225. Each second multipole corrector 218a, 218b can include, for example, four, eight, twelve, or more individual addressable electrodes to generate electrostatic fields for beam shaping and deflection. However, the second multipole correctors 218a, 218b do not have to be configured as electrostatic multipole elements with electrodes, but may further include multiple coils forming magnetic drive poles.

[0118] By means of at least one second multipole corrector 218a, 218b, the stigmatically shaped multiple focused spots 15 and the focused spot positions can be controlled by the contrast module 870. In the example shown in Figure 13, five multipole correctors 220a, 220b, 220c, and 218a and 218b are shown as adjustment devices for the secondary electron beam path 11.

[0119] Therefore, in general, the desired contrast filtering operation is: - beam shaping or positioning of the pupil distribution of at least one secondary electron beamlet 9 by at least one beam shaping element selected from the active array elements 216 and by at least one first multipole corrector 220a, 220b, 220c; This is achieved by combining with the selection of an aperture filter 284 (see FIG. 2) of a desired shape, for example an aperture filter 284c or 284d with an asymmetric shape, a multipole aperture filter 284a, 284b, or a circular aperture filter 284e, 284f.

[0120] The contrast filter operation controlled by the contrast control module 870 is - an anisotropic pupil filtering action of at least one secondary electron beamlet 9; - isotropic pupil filtering of at least one anisotropically shaped or astigmatically shaped secondary electron beamlet 9; - beam deflection for adjusting the position of the pupil distribution of at least one secondary electron beamlet 9; - beam shaping of the focused spot 15 of at least one secondary electron beamlet 9 in the image plane 225 of the detection unit 200; - beam deflection for position adjustment of the focused spot 15 of at least one secondary electron beamlet 9 in the image plane 225 of the detection unit 200; and a filtering operation selected from the group consisting of:

[0121] The electrostatic multipole compensators 216, 218, and 220 can be operated at high speeds so that contrast filter operation can be adjusted or changed during use, for example, during an examination task at an examination site.

[0122] An image enhancement method for multi-beam image acquisition and wafer inspection according to a third embodiment is disclosed, the method being illustrated in Figure 7 and reference being made to the description of Figure 7.

[0123] In step S, an inspection site is selected and the surface 25 of the wafer 7 is positioned by the stage 500 in the object plane 101 of the multi-beam charged particle beam system 1. Next, the landing energy EL of the primary charged particles is adjusted by adjusting at least one of the voltages VE or VL supplied to the electrode 133 or the wafer 7 via the specimen voltage source 503.

[0124] In step A, a first contrast feature or a first array of contrast features is selected. The first array of contrast features can include a first contrast feature and a second contrast feature. The selection of the contrast feature can be performed by: - placement of a corresponding aperture filter 284 in the common pupil plane 2105 of the detection unit 200 of the multi-beam charged particle beam system 1; - controlling at least one first multipole corrector 220a, 220b, 220c; - controlling at least one second multipole corrector 218a, 218b; - control of at least one of lenses 205.1, 205.2, 205.3, or 211.1, 211.2; - controlling a plurality of electrodes 681 disposed in apertures 685 of the active array elements 216.

[0125] In one example, the first selected contrast feature or first arrangement of contrast features at the inspection position is determined by previous information including one of a previously determined selected contrast feature at an equivalent inspection position or from CAD information regarding the semiconductor feature at the inspection position on the wafer.

[0126] Additionally, a plurality of control signals are provided by the contrast control module 870. The plurality of control signals include signals or voltages including a plurality of voltages for electrodes of at least one first multipole corrector 218 a, 218 b, a plurality of voltages for electrodes of at least one second multipole corrector 218 a, 218 b, a plurality of voltages for the plurality of electrodes 681 of the active array element 216, a control signal for positioning or adjusting the motion mechanism 215 to adjust or replace a selected aperture filter 284, and a plurality of control signals including voltages or currents to at least one of the lenses 205, 211 to adjust magnification.

[0127] In step I, a first scanning microscope image is obtained by scanning the multiple primary charged particle beamlets 3 over the surface 25 of the wafer 7 and collecting corresponding secondary electron signals using the sensor unit 600. The image data generated by the sensor 600 is collected by the imaging control unit 810, further processed, for example, by image processing, binding operations, and other operations, and optionally stored in the image memory portion of the memory 880. In a first example, the first contrast mechanism is maintained during the image scan. In a second example, during the image scan at the inspection position, the arrangement of the contrast mechanisms is continuously adjusted during the image scan. For example, the acquisition of the scanning microscope image starts from the first contrast mechanism and switches to the second contrast mechanism when a specific location on the surface 25 of the wafer 7 at the inspection site is reached during the image scan by the multiple primary charged particle beamlets 3.

[0128] In optional step C, the image contrast is evaluated. Optionally, if the image contrast from the first contrast mechanism or from the first array of contrast mechanisms selected in step A does not conform to predetermined expectations or desired characteristics, improvements are made to the aperture filter selection and voltage generation for the multiple electrodes of the multipole correctors 216, 218, 220, and the method continues iteratively with step A. The process can be repeated iteratively until the predetermined expectations or desired characteristics for the image contrast are achieved. The finally obtained optimized contrast mechanism can be stored as a second, third, fourth, or further contrast mechanism for the specific inspection task, optionally with an individual contrast mechanism for each or multiple charged particle beamlets 3.

[0129] In optional step E, an inspection task is performed and the inspection results are stored, for example, in memory 880 or visualized by a user interface. For example, the inspection results of several inspection locations on a wafer can be evaluated and used for process optimization of a semiconductor production process.

[0130] 14 describes yet another modification of the method, step S being in accordance with the method step S described above. After step S of adjusting the examination site and examination settings, a sequence of images with different contrast mechanisms is obtained and a final examination image is processed from the sequence of images.

[0131] The sequence of images with different contrast mechanisms begins with a first step An for n=1, where the first contrast mechanism is adjusted as described in step A above. A first image corresponding to n=1 is then acquired in a step In for n=1. The sequence continues with a step An for n=2, where a second contrast mechanism is adjusted, followed by acquisition of a second image in a step In for n=2. The sequence continues until a number N of images have been acquired, where N is, for example, N=2, N=3, N=4, or more. Each image in the sequence of images is acquired with a different contrast setting selected from contrast settings including filtering of the secondary electron beamlets at a common pupil plane and beam shaping of the secondary electron beamlets, including, for example, anisotropic filtering or beam shaping of the pupil distribution of the secondary electron beamlets into an elliptical or multipole shape.

[0132] In step P, the images of the image array are processed until at least one inspection result is reached. The inspection result can include any result including combined or processed images, including, for example, enhanced contrast, e.g., enhanced edge contrast, 3D image representation, color images, difference images.

[0133] The detection unit 200 is not limited to the examples shown in FIG. 1 or FIG. 13, and variations are also possible. An example is shown in FIG. 15. The detection unit 200 of FIG. 15 comprises a first multipole corrector 220 including a pair of adjacent multipole correctors 220b and 220c, and an active array element 216. The detection unit 200 of FIG. 15 further comprises four imaging lenses 205.1 to 205.4 and two second multipole correctors 281a and 218b. The same reference numerals as in FIG. 1 or 13 are used, and further reference is made to the descriptions of FIGS. 1 and 13. The arrangement of elements such as lenses and multipole correctors is not limited to the arrangement shown in FIG. 15.

[0134] FIG. 16 shows some examples of contrast mechanisms, including beam shaping and filtering of a pupil distribution 288 of multiple secondary electron beamlets 9 by an aperture filter opening 286. FIG. 16a shows an example of an elliptical pupil distribution 288.1 of multiple secondary electron beamlets 9 at a specific landing energy LE due to imaging aberrations of the detection unit 200 on the left. The elliptical pupil distribution 288.1 is the result of, for example, astigmatism. The elliptical pupil distribution 288.1 is converted into a circular pupil distribution 288.3 by at least one of the first multipole correctors 220a, 220b, or 220c and passed through the circular aperture opening 286a. FIGS. 16b-16d show an example of a circular pupil distribution 288.2 of multiple secondary electron beamlets 9 at a specific landing energy LE without aberrations on the left. In Figure 16b, circular pupil distribution 288.2 is converted by at least one of first multipole correctors 220a, 220b, or 220c into an elliptical-shaped pupil distribution 288.4 and filtered by circular-shaped aperture filter opening 286a, thereby achieving anisotropic filtering. In Figure 16c, circular pupil distribution 288.2 is converted by at least one of first multipole correctors 220a, 220b, or 220c into an off-center pupil distribution 288.5 and filtered by circular-shaped aperture filter opening 286a, thereby achieving anisotropic filtering. In Figure 16d, circular pupil distribution 288.2 is converted by at least one of first multipole correctors 220a, 220b, or 220c into a quadrupole-shaped multipole pupil distribution 288.6 and filtered by circular-shaped aperture filter opening 286a. Here, a bipole filter with two openings 286b is used as the aperture filter, thereby achieving anisotropic filtering.

[0135] The features of the embodiments can improve the performance, and in particular the image contrast, of the multi-beam charged particle system 1 to achieve large image contrast at an image resolution of less than 5 nm, such as less than 3 nm, e.g., less than 2 nm or even less than 1 nm. The improvements can be useful for further development of multi-beam charged particle systems using a larger number of multiple primary beamlets, such as more than 100 beamlets, more than 300 beamlets, more than 1000 beamlets, or even more than 10000 beamlets. The improvements can be useful for routine applications of multi-beam charged particle systems, for example, in semiconductor inspection and inspection, where high image contrast, high reliability, high reproducibility, and low machine-to-machine variation are typically desired. The features or method steps described in the embodiments and combinations thereof provide each beamlet of the multiple beamlets with enhanced imaging performance.

[0136] The present disclosure is further explained by the following clauses.

[0137] Clause 1: A multi-beam charged particle beam system (1) for wafer inspection, comprising: - an objective radiation unit (100) including a multi-beamlet generator (300) for generating a plurality of primary charged particle beamlets (3) and an objective lens (102) for focusing the plurality of primary charged particle beamlets (3) onto an objective plane (101) of the objective radiation unit (100) during use; a detection unit (200) configured to image, on an image sensor (600), a plurality of secondary electron beamlets (9) generated in parallel during use in an interaction volume (707) between the plurality of primary charged particle beamlets (3) and the surface (25) of the wafer (7), the detection unit (200) including an aperture filter module (214) including at least one aperture filter (284) for anisotropically filtering at least one secondary electron beamlet (9); a beam splitter unit (400) for directing a plurality of primary charged particle beamlets (3) from the multi-beamlet generator (300) to the objective lens (102) and for directing a plurality of secondary electron beamlets (9) from the objective lens (102) to the detection unit (200); - a control unit (800) including a contrast control module (870) configured to control, during use, anisotropic filtering of at least one secondary electron beamlet among a plurality of secondary electron beamlets (9) by at least one aperture filter (284) of an aperture filter module (214).

[0138] Clause 2: The system of clause 1, wherein the aperture filter module (214) comprises a movement mechanism (215) configured to replace at least one aperture filter (284), and the contrast control module (870) is configured to select and position a selected aperture filter (284) during use in the common pupil plane (21) of the detection unit (200) by the movement mechanism (215).

[0139] Clause 3: The system of clause 1 or 2, wherein the selected aperture filter (284) comprises an anisotropically shaped aperture opening (286c, 286d) comprising a member selected from the group consisting of an elliptical aperture filter and an elongated rectangular aperture filter.

[0140] Clause 4: A multi-beam charged particle beam system according to clause 1, wherein at least one aperture filter (284) is formed as an active aperture filter (290) having at least one aperture opening (286) having at least a deflection electrode (292) for deflecting the angle or pupil distribution of a plurality of secondary electron beamlets (9) passing through the at least one aperture opening (286).

[0141] Clause 5: The system of clause 1, 2, or 4, wherein the selected aperture filter (284) comprises a plurality of aperture openings (286.1-286.8) outside the electronic optical axis (2105) of the detection unit (200).

[0142] Clause 6: The system of clause 5, wherein at least two aperture openings of the plurality of aperture openings (286.1 to 286.4) are arranged symmetrically with respect to the electronic optical axis (2105) to realize an aperture filter (284) having a shape selected from the group consisting of a dipole shape or a quadrupole shape.

[0143] Clause 7: The system of clause 6, wherein the contrast control module (870) is configured to position an aperture filter (284) having a shape based on a structure (728, 729, 731) of a semiconductor feature of the wafer (7) selected from the group consisting of a horizontal structure and a vertical structure.

[0144] Clause 8: The system of clause 6, wherein the contrast control module (870) is configured to position an aperture filter (284) having a shape based on the topography of the semiconductor features (703, 705) of the wafer (7).

[0145] Clause 9: A system according to any one of clauses 1 to 8, wherein the detection unit (200) comprises a plurality of electron optical elements (205) configured to form an intermediate image plane (211) of a plurality of secondary electron beamlets (9), and an active multi-aperture array (216) adjacent to the intermediate image plane (211), the active multi-aperture array (216) including a plurality of apertures (685), each aperture (685) of the active multi-aperture array (216) configured to pass one of the plurality of secondary electron beamlets (9), and each aperture (685) of the active multi-aperture array (216) including a plurality of electrodes (681) connected to a contrast control module (870) for individually anisotropically shaping or deflecting, during use, at least one of the plurality of secondary electron beamlets (9) passing therethrough.

[0146] Clause 10: A system of clause 9, wherein the contrast control module (870) is configured to control the active multi-aperture array (216) to anisotropically shape or deflect the first secondary electron beamlet (9.2) and anisotropically shape or deflect the second secondary electron beamlet (9.3), and the contrast control module (870) is configured to position circular-shaped aperture filters (284e, 284f) at the common pupil plane (21).

[0147] Clause 11: The system of any of clauses 1 to 10, further comprising a voltage supply unit (503) connected to the wafer for supplying a voltage to the wafer, during use, to generate a deceleration field for the primary charged particles corresponding to the acceleration field for the secondary electrons generated in the interaction volume (707).

[0148] Clause 12: The system of any of clauses 1 to 11, further comprising at least one first multipole corrector (220a, 220b, 220c) upstream of the common pupil plane (21) of the detection unit (200), wherein the at least one first multipole corrector (220a, 220b, 220c) is connected to a contrast control module (870) and configured to achieve shaping of the pupil distribution (288) of the multiple secondary electron beamlets (9) at the common pupil plane (21).

[0149] Clause 13: The system of claim 12, wherein the contrast control module (870) is configured to place circular-shaped aperture filters (284e, 284f) at the common pupil plane (21).

[0150] Clause 14: A multi-beam charged particle beam system (1) according to clause 12 or 13, wherein the contrast control module (870) is configured to effect shaping of the pupil distribution (288) into a circular shape.

[0151] Clause 15: A multi-beam charged particle beam system (1) according to clause 12 or 13, wherein the contrast control module (870) is configured to effect shaping of the pupil distribution (288) into an elliptical or multipole shape.

[0152] Clause 16: A multi-beam charged particle beam system (1) according to any of clauses 1 to 15, comprising at least one second multipole corrector (218a, 218b) downstream of the common pupil plane (21) of the detection unit (200), the at least one second multipole corrector (218a, 218b) being connected to a contrast control module (870) and configured to correct the effect of shaping the pupil distribution (288) of at least one secondary electron beamlet (9) in the image plane (225) of the detection unit (200).

[0153] Clause 17: A method for improving contrast for a wafer inspection task, comprising: - irradiating a surface (25) of a wafer (7) with a plurality of primary charged particle beamlets (3) of a multi-beam charged particle beam system (1), thereby exciting a plurality of secondary electron beamlets (9) from an interaction volume (707) created by the plurality of primary charged particle beamlets (3) and the wafer (7); - focusing the plurality of secondary electron beamlets (9) with an objective lens (102); - anisotropically filtering at least one of the secondary electron beamlets (9) by a selected aperture filter (284) arranged in a common pupil plane (21) of a detection unit (200) of the multi-beam charged particle beam system (1); - collecting signals of each of a plurality of secondary electron beamlets (9) with an image sensor (600), including at least one anisotropically filtered secondary electron beamlet (9.2, 9.3) to generate an image of the surface (25) of the wafer (7) with enhanced contrast.

[0154] Article 18: - selecting a selected aperture filter (285); - positioning the selected aperture filter (285) in the common pupil plane (21) of the detection unit (200) by the movement mechanism (215).

[0155] Clause 19: The method of clause 17 or 18, further comprising the step of supplying a voltage to at least one electrode (681) of an active array element (216) disposed within the detection unit (200) for anisotropically shaping or deflecting at least one of the secondary electron beamlets (9).

[0156] Clause 20: A method according to any one of claims 17 to 19, further comprising the step of supplying a voltage for shaping or deflecting the pupil distribution (288) of the plurality of secondary electron beamlets (9) to a first multipole corrector (220a, 220b, 220c) arranged in the detection unit (200) upstream of the common pupil plane (21).

[0157] Article 21: - positioning an inspection position of the surface (25) of the wafer (7) in the object plane (101) of the multi-beam charged particle beam system (1) using a wafer stage (500); - determining a selected contrast mechanism at the examination location; - selecting and supplying a preselected aperture filter (284) and at least a voltage to the active array element (216) or the first multipole corrector (220a, 220b, 220c) for anisotropic filtering of at least one of the secondary electron beamlets (9) according to a selected contrast mechanism; - performing image acquisition of the surface (25) of the wafer (7) to acquire digital images of the semiconductor features (728, 703, 705) of the wafer (7) in the inspection position.

[0158] Clause 22: The method of claim 21, further comprising a step of supplying at least a voltage to a second multipole corrector (218a, 218b) for correcting the effect of shaping the pupil distribution (288) on the shape of the multiple focused spots (15) in the image plane (225) of the detection unit (200).

[0159] Clause 23: The method of clause 21 or 22, wherein the selected contrast mechanism at the examination location is determined by previous information, including one of a previously determined selected contrast mechanism at an equivalent examination location or CAD information.

[0160] Article 24: - evaluating a first image contrast of the digital image; - modifying the selected contrast mechanism by modifying at least one of the preselected aperture filter (284) or at least one voltage supplied to the electrodes of the active array element (216), the first multipole corrector (220a, 220b, 220c), or the second multipole corrector (218a, 218b); - determining an improved contrast mechanism having an improved image contrast compared to the first image contrast.

[0161] Clause 25: The method of any of clauses 17 to 24, further comprising storing the improved contrast mechanism for use with the examination location.

[0162] Article 26: 26. The method of any of clauses 17 to 25, further comprising the step of performing an image evaluation of the digital images of the semiconductor features (728, 703, 705) of the wafer (7) to identify defects comprising at least one of size deviations, area deviations, or material composition deviations of the semiconductor features (728, 703, 705), or contaminant particles.

[0163] Article 27: - repeating the image acquisition of the surface (25) of the wafer (7) at a plurality of inspection positions; - assessing the distribution of the defects to identify at least one of random defects, regular defects, or clusters of defects.

[0164] Clause 28: A multi-beam charged particle beam system (1) for wafer inspection, comprising: - an objective radiation unit (100) including a multi-beamlet generator (300) for generating a plurality of primary charged particle beamlets (3) and an objective lens (102) for focusing the plurality of primary charged particle beamlets (3) onto an objective plane (101) of the objective radiation unit (100) during use; a detection unit (200) configured to image, on an image sensor (600), a plurality of secondary electron beamlets (9) generated in parallel during use in an interaction volume (707) between the plurality of primary charged particle beamlets (3) and the surface (25) of the wafer (7), the detection unit (200) including an aperture filter module (214) including an active aperture filter (290) for filtering at least one component of an angular or pupil distribution (288) of the plurality of secondary electron beamlets (9); a beam splitter unit (400) for directing a plurality of primary charged particle beamlets (3) from the multi-beamlet generator (300) to the objective lens (102) and for directing a plurality of secondary electron beamlets (9) from the objective lens (102) to the detection unit (200); - a control unit (800) including a contrast control module (870) configured to control, during use, the anisotropic filtering of at least one of a plurality of secondary electron beamlets (9) by an active aperture filter (290) of an aperture filter module (214).

[0165] Clause 29: A multi-beam charged particle beam system according to clause 28, wherein the active aperture filter (290) comprises at least one aperture opening (286) having at least one deflection electrode (292) for deflecting a component of the angle or pupil distribution of a plurality of secondary electron beamlets (9) passing through the at least one aperture opening (286).

[0166] Clause 30: The system of clause 28 or 29, wherein the active aperture filter (290) comprises at least one aperture opening (286) outside the electronic optical axis (2105) of the detection unit (200).

[0167] Clause 31: The system of clause 29, wherein the contrast control module (870) is configured to supply at least one deflection voltage to the deflection electrodes (292) during use.

[0168] Clause 32: A system of any of clauses 28 to 31, wherein the image sensor (600) has a plurality of M detection areas (602), and the number M of the detection areas (602) is at least twice the number J of the plurality of secondary electron beamlets (9), M >= 2 × J, and preferably M = 4 × J, M = 7 × J or M = 9 × J.

[0169] Clause 33: A method for improving contrast for a wafer inspection task, comprising: - irradiating a surface (25) of a wafer (7) with a plurality of primary charged particle beamlets (3) of a multi-beam charged particle beam system (1), thereby exciting a plurality of secondary electron beamlets (9) from an interaction volume (707) created by the plurality of primary charged particle beamlets (3) and the wafer (7); - focusing the plurality of secondary electron beamlets (9) with an objective lens (102); - deflecting at least one component of the angular or pupil distribution of the plurality of secondary electron beamlets (9) by an active aperture filter (290) arranged in a common pupil plane (21) of a detection unit (200) of the multi-beam charged particle beam system (1), the active aperture filter (290) comprising at least one aperture opening (286) with at least one deflection electrode (292) for passing and deflecting the component of the angular or pupil distribution of the plurality of secondary electron beamlets (9); - collecting signals of each of the plurality of secondary electron beamlets (9) with an image sensor (600), wherein the image sensor (600) comprises a plurality of M detection areas (602), and the number M of the detection areas (602) is at least twice the number J of the plurality of secondary electron beamlets (9), M >= 2 × J, and preferably M = 4 × J, M = 7 × J or M = 9 × J.

[0170] Article 34: - positioning an inspection position of the surface (25) of the wafer (7) in the object plane (101) of the multi-beam charged particle beam system (1) using the wafer stage (500); - determining a contrast mechanism at the examination location; - selecting and applying at least one deflection voltage to at least one deflection electrode (292) to deflect an angle or a component of the pupil distribution of the plurality of secondary electron beamlets (9) according to a selected contrast mechanism; - performing an image acquisition of the surface (25) of the wafer (7) to acquire a digital image of the semiconductor features (728, 703, 705) of the wafer (7) in the inspection position.

[0171] Clause 35: The method of clause 34, wherein the contrast mechanism at the test location is determined by prior information, including one of a previously determined selected contrast mechanism at an equivalent test location or CAD information.

[0172] Article 36: - evaluating a first image contrast of the digital image; - modifying the contrast mechanism by modifying at least one deflection voltage to at least one deflection electrode (292); - determining an improved contrast mechanism having an improved image contrast compared to the first image contrast.

[0173] Clause 37: A method for image acquisition using a multi-beam charged particle beam system (1), comprising: - a first beam shaping or positioning of a pupil distribution (288) of at least one secondary electron beamlet (9) by at least one beam shaping element arranged upstream of a common pupil plane (21) of the detection unit (200), wherein the at least one beam shaping element is selected from a group of elements comprising an active array element (216) and a first multipole corrector (220a, 220b, 220c); - filtering a pupil distribution (288) of a plurality of secondary electron beamlets (9) by an aperture filter (284) arranged in a common pupil plane (21), wherein the aperture filter (284) is selected from the group of aperture filters including aperture filters (284c, 284d) having asymmetric or anisotropic shapes, multipole aperture filters (284a, 284b), circular aperture filters (284e, 284f), and an active pupil filter (290).

[0174] Article 38: - The method according to clause 37, further comprising a second beam shaping or position adjustment of the multiple focused spots (15) of the multiple secondary electron beamlets by at least one second multipole corrector (218a, 218b) arranged downstream of the common pupil plane (21) of the detection unit (200).

[0175] Clause 39: The method according to clause 37 or 38, further comprising a step of adjusting the magnification of the pupil distribution (288) of the plurality of secondary electron beamlets (9) by at least one first lens (205.1, 205.2) arranged upstream of the common pupil plane (21).

[0176] However, the present disclosure is not limited to the above-described embodiments or clauses. The embodiments or examples may be combined with each other completely or partially, and many variations and modifications are possible. [Explanation of symbols]

[0177] 1. Multi-beamlet charged particle system 3 Primary charged particle beamlet or multiple primary charged particle beamlets 5 Primary charged particle beam spot 7 Object 9 Secondary electron beamlet forming multiple secondary electron beamlets 11 Secondary electron beam path 13 Primary beam path 15 Secondary charged particle image spot 21 Common pupil plane 25 Surface of an object 71 Die 75 scattered random defects 77 Defect Clusters 79 Regular Defects 100 Objective Radiation Units 101 Objective surface 102 Objective Lens 103 Field Lens 108 First beam crossover 110 Collective Multi-Beam Raster Scanner 200 detection units 205 Lens Elements 211 First Electrostatic Lens 214 Aperture Filter Module 215 Movement mechanism 216 Active Array Elements 218 Second Multipole Corrector 220 First Multipole Corrector 222 Second Deflection System 225 Image plane of detection unit 200 281Selected electron trajectories starting from axial beam spot 5.i 283Selected electron trajectories starting from off-axis beam spot 5.o 284 Aperture Filter 286 Aperture Opening 288 Pupil distribution of secondary electrons 290 Active pupil filter 292 Deflection electrode 294 Aperture Group Secondary electrons corresponding to a group of 296 apertures 300 Charged Particle Multi-Beamlet Generator 301 Charged Particle Source 303 Collimating Lens 304 Filter Plate 305 Primary Multibeamlet Formation Unit 306 Multi-aperture plate 307 Terminal Multi-Aperture Plate 308 Field Lens 309 Primary Electron Beam 321 Intermediate image plane 400 Beam Splitter Unit 500 sample stage 503 Specimen voltage source 505 Deceleration or extraction field indicated by equipotential lines 600 image sensor 602 multiple detector pixels 681 Electrode 685 Aperture 687 Voltage Supply Line 689 Multi-Aperture Plate 703 Gate Fin 705 Channel Fin 707 Interaction Volume 711 Wafer Layer 728 Conduction element for the first layer 729 Conduction element for the second layer 731 Conduction element for the third layer 800 Control Unit 810 Imaging control module 820 Secondary Beam Path Control Module 830 Primary Beam Path Control Module 840 Control Action Processor 850 Stage Control Module 860 Scanning Motion Control Module 870 Contrast Control Module 880 memory 2105 Optical axis of detection unit

Claims

1. An objective radiation unit (100), comprising: a multi-beamlet generator (300) configured to generate a plurality of primary charged particle beamlets (3); and an objective lens (102) configured to focus the plurality of primary charged particle beamlets (3) onto an object plane (101) of the objective radiation unit (100); an objective radiation unit (100) including: a detection unit (200) configured to image, onto an image sensor (600), a plurality of secondary electron beamlets (9) generated by the interaction of the plurality of primary charged particle beamlets (3) with the surface (25) of the wafer (7), the detection unit (200) including an aperture filter module (214) including an aperture filter (284) configured to anisotropically filter at least one secondary electron beamlet (9); a beam splitter unit (400) configured to direct the plurality of primary charged particle beamlets (3) from the multi-beamlet generator (300) to the objective lens (102) and to direct the plurality of secondary electron beamlets (9) from the objective lens (102) to the detection unit (200); a control unit (800) including a contrast control module (870) configured to control anisotropic filtering of the at least one secondary electron beamlet of the plurality of secondary electron beamlets (9) by the aperture filter (284); A multi-beam charged particle beam system (1) comprising:

2. 2. The multi-beam charged particle beam system (1) of claim 1, wherein the aperture filter module (214) comprises a movement mechanism (215) configured to replace the aperture filter (284), and the contrast control module (870) is configured to select and position the aperture filter (284) in a common pupil plane (21) of the detection unit (200) via the movement mechanism (215).

3. The multi-beam charged particle beam system (1) of claim 1 or 2, wherein the aperture filter (284) comprises an anisotropically shaped aperture opening (286).

4. The multi-beam charged particle beam system (1) of any one of claims 1 to 3, wherein the aperture filter (284) comprises a member selected from the group consisting of an elliptical aperture filter and an elongated rectangular aperture filter.

5. The multi-beam charged particle beam system (1) according to any one of claims 1 to 3, wherein the aperture filter (284) comprises a plurality of aperture openings outside the electronic optical axis (2105) of the detection unit (200).

6. 6. The multi-beam charged particle beam system (1) of claim 5, wherein at least two of the plurality of aperture openings (286) are arranged symmetrically with respect to the electron optical axis (2105) to realize an aperture filter (284) having a shape selected from the group consisting of a dipole shape and a quadrupole shape.

7. 7. The multi-beam charged particle beam system of claim 6, wherein the contrast control module is configured to position the aperture filter having a shape based on a structure of a semiconductor feature of the wafer, the shape being selected from the group consisting of a horizontal structure and a vertical structure.

8. 7. The multi-beam charged particle beam system of claim 6, wherein the contrast control module is configured to position the aperture filter having a shape based on a topography of semiconductor features of the wafer.

9. The detection unit (200) a plurality of electron optical elements (205, 211) configured to provide an intermediate image plane (211) for the plurality of secondary electron beamlets (9); an active multi-aperture array (216) proximate the intermediate image plane (211), the active multi-aperture array (216) including a plurality of apertures (685), each aperture (685) of the active multi-aperture array (216) configured to pass one of the plurality of secondary electron beamlets (9), each aperture (685) of the active multi-aperture array (216) including a plurality of electrodes (681) connected to the contrast control module (870) for individually anisotropically shaping or deflecting the one of the plurality of secondary beamlets (9) passing therethrough; A multi-beam charged particle beam system (1) according to any one of claims 1 to 8, comprising:

10. the contrast control module (870) is configured to: i) control the active multi-aperture array (216) to anisotropically shape or deflect a first secondary electron beamlet (9.2) and anisotropically shape or deflect a second secondary electron beamlet (9.3); 10. The multi-beam charged particle beam system (1) of claim 9, wherein the contrast control module (870) is configured to place a circular aperture filter (284) in a common pupil plane (21) of the detection unit (200).

11. 11. The multi-beam charged particle beam system (1) according to any one of claims 1 to 10, further comprising a voltage supply unit (503) configured to be connected to the wafer (7) for supplying a voltage to the wafer (7) so as to generate a deceleration field for primary charged particles corresponding to an acceleration field for secondary electrons.

12. 12. The multi-beam charged particle beam system (1) according to claim 1, further comprising at least one first multipole corrector (220a, 220b, 220c) upstream of a common pupil plane (21) of the detection unit (200), the at least one first multipole corrector (220a, 220b, 220c) being connected to the contrast control module (870) and configured to achieve shaping of a pupil distribution (288) of the plurality of secondary electron beamlets (9) in the common pupil plane (21).

13. 13. The multi-beam charged particle beam system (1) of claim 12, wherein the contrast control module (870) is configured to effect shaping of the pupil distribution (288) into a circular shape.

14. 13. The multi-beam charged particle beam system (1) of claim 12, wherein the contrast control module (870) is configured to effect shaping of the pupil distribution (288) into an elliptical or multipole shape.

15. 15. The multi-beam charged particle beam system (1) according to any one of claims 12 to 14, further comprising at least one second multipole corrector (218a, 218b) downstream of the common pupil plane (21) of the detection unit (200), the at least one second multipole corrector (218a, 218b) being connected to the contrast control module (870) and configured to correct the effect of the shaping of the pupil distribution (288) of the plurality of secondary electron beamlets (9) in an image plane (225) of the detection unit (200).

16. irradiating a surface (25) of a wafer (7) using a plurality of primary charged particle beamlets (3) of a multi-beam charged particle beam system (1) to generate a plurality of primary charged particle beamlets (3) and a plurality of secondary electron beamlets (9) generated by the wafer (7); focusing the plurality of secondary electron beamlets (9) using an objective lens (102); anisotropically filtering the secondary electron beamlets (9) using a selected aperture filter (284) placed in a common pupil plane (21) of a detection unit (200) of the multi-beam charged particle beam system (1); collecting signals of each of the plurality of secondary electron beamlets (9), including the anisotropically filtered secondary electron beamlet (9), using an image sensor (600) to generate an image of the surface (25) of the wafer (7); A method comprising:

17. selecting the aperture filter (284); positioning the selected aperture filter (284) at the common pupil plane (21) of the detection unit; 17. The method of claim 16, further comprising:

18. 18. The method of claim 16 or 17, further comprising the step of supplying a voltage to an electrode of an active array element (216) arranged in the detection unit (200) for anisotropically shaping or deflecting at least one of the secondary electron beamlets (9).

19. The method according to any one of claims 16 to 18, further comprising the step of supplying a voltage to a first multipole corrector (220a, 220b, 220c) arranged in the detection unit (200) upstream of the common pupil plane (21) for anisotropically shaping or deflecting a pupil distribution (288) of the plurality of secondary electron beamlets (9).

20. positioning an inspection position of the surface (25) of the wafer (7) in an object plane (101) of the multi-beam charged particle beam system (1); determining a selected contrast mechanism at the inspection location; selecting and supplying the aperture filter (284) and at least a voltage to an active array element (214) or a first multipole corrector (220a, 220b, 220c) to anisotropically filter at least one of the secondary electron beamlets (9) according to the selected contrast mechanism; performing an image acquisition of the surface (25) of the wafer (7) to acquire a digital image of semiconductor features of the wafer (7) in the inspection position; The method of any one of claims 16 to 19, further comprising:

21. 21. The method of claim 20, wherein the selected contrast mechanism at the inspection position is determined by information comprising a selected member from the group consisting of: i) a previously determined selected contrast mechanism at an equivalent inspection position; and ii) CAD information.

22. assessing a first image contrast of the digital image; modifying the selected contrast mechanism by modifying at least one member selected from the group consisting of the preselected aperture filter (284) and a voltage supplied to an electrode of the active array element (216) or a first multipole corrector (220a, 220b, 220c); determining a second contrast mechanism having an improved image contrast compared to the first image contrast; 22. The method of claim 20 or 21, further comprising:

23. 23. The method of claim 22, further comprising storing the second contrast mechanism used along with the inspection position.

24. 23. The method of any one of claims 20 to 22, further comprising performing an image evaluation of the digital images of semiconductor features of the wafer to identify defects comprising at least one member selected from the group consisting of semiconductor feature size deviations, semiconductor feature area deviations, semiconductor feature material composition deviations, and contaminant particles.

25. repeating the image acquisition of the surface of the wafer at a plurality of inspection positions; evaluating the distribution of defects to identify at least one member selected from the group consisting of random defects, regular defects, and clusters of defects; 25. The method of claim 24, further comprising:

Images