Charged particle-optical module

Abstract

A charged particle-optical module (200) for a plurality of charged particle-optical devices configured to direct charged particle beams along respective beam paths towards a sample location, the charged particle-optical module comprising: at least one emitter (21) arranged at an emission surface (80) of an emitter layer, each emitter configured to emit charged particles for a source beam; at least one extractor electrode (27) for the at least one emitter, wherein the at least one extractor electrode is planar and a beam aperture is defined in each extractor electrode for the charged particles emitted by the respective emitter; and a spacer (88) configured to space each extractor electrode from the emission surface, wherein the spacer is an electrical insulator and comprises an insulator surface (94) between the extractor electrode and the emission surface, wherein at least part of the insulator surface is angled relative to a direction perpendicular to the emission surface.

Description

CHARGED PARTICLE-OPTICAL MODULECROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24167310.2 which was filed on 28 March 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a charged particle-optical module, a plurality of charged particle-optical devices, a charged particle-optical apparatus, an assessment apparatus and a method for manufacturing a charged particle-optical module.BACKGROUND

[0003] During manufacturing processes of, for example, semiconductor integrated circuit (IC) chips or displays, undesired defects may occur on a substrate (e.g. wafer) or a mask. Such defects may reduce yield. Defects may occur as a consequence of all kinds of processing necessary to produce an integrated circuit or display, for example, lithography, etching, deposition or chemical mechanical polishing.

[0004] Defects may include patterning defects, in which the created pattern lies outside the pattern tolerance for the process, and particles. Monitoring the extent of defects during the manufacturing processes is therefore important. Such monitoring (or more generally assessment) includes the determination of the existence of a defect, but also the classification of the types of defects found.

[0005] For the assessment of a sample, different types of inspection or metrology systems have been used, including charged particle systems such as electron microscopes. Such assessment for inspection may relates to defects, for example the existence and classification of such defects.Electron microscopes typically generate a probe beam (also often referred to as primary beam) which may, for example, be scanned across a part of the substrate (such as in a scanning electron microscopes (SEM)). Collecting interaction products that result from the interaction of the primary beam with the part of the substrate, allows the electron microscope to collect data representing the probed part of the substrate.

[0006] The data may be processed / rendered for example by the electron microscope to generate an image representation of the part of the substrate. The collected data for example as a generated image representation allows for measuring structures on the part of the substrate, or allows for identifying defective structures by comparing the image representation with a reference. Such measurement may be referred to as metrology; such defect inspection may be referred to as inspection. The interaction products may contain charged particles which may be referred to as signalparticles (e.g. signal electrons), such as secondary electrons and backscattered electrons, and may contain other interaction products, such as X-ray radiation.

[0007] The primary beam may be derived from a source beam emitted by a source. A plurality of sources may be provided. It can be difficult to align the various components of the sources. It is desirable to reduce the time required to perform the alignment.SUMMARY

[0008] According to an aspect of the present invention there is provided a charged particle- optical module for a plurality of charged particle-optical devices configured to direct charged particle beams along respective beam paths towards a sample location, the charged particle-optical module comprising: at least one emitter arranged at an emission surface of an emitter layer, each emitter configured to emit charged particles for a source beam; at least one extractor electrode for the at least one emitter, wherein the at least one extractor electrode is planar and a beam aperture is defined in each extractor electrode for the charged particles emitted by the respective emitter; and a spacer configured to space each extractor electrode from the emission surface, wherein the spacer is an electrical insulator and comprises an insulator surface between the extractor electrode and the emission surface, wherein at least part of the insulator surface is angled relative to a direction perpendicular to the emission surface.

[0009] According to another aspect of the present invention there is provided a method for manufacturing a charged particle-optical module for a charged particle-optical device configured to direct a charged particle beam along a beam path towards a sample location, the method comprising: arranging at least one emitter at an emission surface of an emitter layer, each emitter configured to emit charged particles for a source beam; defining in each of at least one extractor electrode for the at least one emitter a beam aperture for the charged particles emitted by the respective emitter, wherein the at least one extractor electrode is planar and; and spacing each extractor electrode from the emission surface with a spacer, wherein the spacer is an electrical insulator and comprises an insulator surface between the extractor electrode and the emission surface, wherein at least part of the insulator surface is angled relative to a direction perpendicular to the emission surface.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 is a schematic diagram of an exemplary assessment apparatus;Figure 2 schematically depicts a multi-beam charged particle-optical device, for example of the assessment apparatus of Figure 1 ;Figure 3 schematically depicts a charged particle-optical device array;Figure 4 schematically depicts a source of a charged particle-optical module;Figure 5 schematically depicts a charged particle-optical module comprising a plurality of the sources shown in Figure 4;Figure 6 is a plan view of the charged particle-optical module shown in Figure 5;Figure 7 is a plan view of an alternative arrangement of the charged particle-optical module shown in Figure 5; andFigure 8 schematically depicts a plurality of sources for a multi-beam charged particle- optical device.

[0011] The Figures are schematic. Within the following description of drawings, the same or like reference numbers refer to the same or like components or entities, and only the differences with respect to the individual embodiments are described. While the description and drawings are directed to an electron-optical apparatus, it is appreciated that the embodiments are not used to limit the present disclosure to specific charged particles. References to electrons, and items referred to with reference to electrons, throughout the present document may therefore be more generally be considered to be references to charged particles, and items referred to in reference to charged particles, with the charged particles not necessarily being electrons. For example, references to an electron-optical device may more generally be considered to be references to a charged particle- optical deviceDETAILED DESCRIPTION

[0012] There is a trend in the semiconductor industry (often known as “Moore’ s law”) to reduce the physical dimensions of structures representing circuit components on a substrate and / or to increase the packing density of such structures, in order to reduce the physical size of electronic devices and / or enhance the computing power of electronic devices. The physical dimensions of such structures may be reduced and / or the packing density of such structures may be increased by increasing lithographic resolution. Manufacturing processes of semiconductor IC chips can have 100s of individual steps. An error in any step of the manufacturing process has the potential to adversely affect the functioning of the electronic device. It is desirable to improve the overall yield of the manufacturing process. For example, to obtain a 75% yield for a 50-step manufacturing process (where a step may indicate the number of layers formed on a substrate), each individual step must have a yield greater than 99.4%. If an individual step has a yield of 95%, the overall yield of the manufacturing process would be as low as 7-8%. It is desirable to determine defects quickly so as to maintain a high substrate throughput, defined as the number of substrates processed per hour.

[0013] Figure 1 is a schematic diagram illustrating an exemplary assessment apparatus 100, e.g. a metrology apparatus or an inspection apparatus. The assessment apparatus 100 may be configured to scan a sample with one or more beams of electrons. The sample may be a semiconductor substrate, a substrate made of other material, or a mask, for example. The electrons interact with the sample and generate interaction products. The interaction products comprise signal electrons, e.g. secondaryelectrons and / or backscattered electrons, and possibly X-ray radiation. The assessment apparatus 100 may be configured to detect the interaction products from the sample so that a data set may be generated which may be processable into an image or any other data representation of the scanned area of the sample can be generated. For clarity, the description below focuses on embodiments in which the interaction products that are detected are signal electrons. The assessment apparatus 100 may comprise, for example during operation, a single beam or a plurality of beams, i.e. a multi-beam. The component beams of a multi-beam may be referred to as sub-beams or beamlets. A multi-beam may be used to scan different parts of a sample simultaneously. When the assessment apparatus 100 uses a multi-beam, the assessment apparatus 100 may assess a sample more quickly than when the assessment apparatus 100 uses a single-beam. For example, a high throughput of sample assessment may be achieved using a multibeam assessment apparatus than a single beam apparatus.

[0014] The assessment apparatus 100 of Figure 1 comprises a vacuum chamber 110, a load lock chamber 120, an electron-optical apparatus 140, an equipment front end module (EFEM) 130 and a controller 150. The electron-optical apparatus 140 (also known as an electron beam apparatus or an electron apparatus) may be within the vacuum chamber 110. The electron-optical apparatus 140 may comprise an electron-optical device (described in more detail below) and an actuatable stage. It should be appreciated that reference in the description to the electron-optical elements of the electron- optical apparatus 140 can be considered to be a reference to the electron-optical device.

[0015] The EFEM 130 includes a first loading port 130a and a second loading port 130b. The EFEM 130 may include additional loading port(s). The first loading port 130a and the second loading port 130b may, for example, receive substrate front opening unified pods that contain samples. One or more robot arms (not shown) in the EFEM 130 transport the samples to the load lock chamber 120.

[0016] The load lock chamber 120 is used to remove the gas around a sample. The load lock chamber 120 may be connected to a load lock vacuum pump system (not shown), which removes gas particles in the load lock chamber 120. The operation of the load lock vacuum pump system enables the load lock chamber to reach a first pressure below the atmospheric pressure. The vacuum chamber 110, which may be a main chamber of the assessment apparatus 100, is connected to a main chamber vacuum pump system (not shown). The main chamber vacuum pump system removes gas molecules from the vacuum chamber 110 so that the pressure around the sample reaches a second pressure equal to or lower than the first pressure. Different parts of the electron-optical apparatus 140 may have different levels of pressure below the atmospheric pressure. After reaching the required pressure, the sample leaves the load lock chamber 120 and is transported to the electron-optical apparatus 140 by which it may be assessed. The electron-optical apparatus 140 may use either a single beam or a multibeam for the assessment. Alternatively, an electron-optical device array comprising a plurality of electron-optical devices may be used, further also referred to as a multi-column electron-array, in which each electron-optical device (or each column in the multi-column array) comprises, for example during operation, either a single beam or a multi-beam.

[0017] The controller 150 is electronically connected to the electron-optical apparatus 140. The controller 150 may be a processor (such as a computer) configured to control the assessment apparatus 100. The controller 150 may also include processing circuitry configured to execute data, signal and image processing functions for example on the data set e.g. embodied as signals such as detection signals. The controller 150 may thus include processing circuitry configured to execute processing functions on signal, image and other data produced in the assessment apparatus 100. While the controller 150 is shown in Figure 1 as being outside of the structure that includes the vacuum chamber 110, the load lock chamber 120, and the EFEM 130, it is appreciated that the controller 150 may be part of the structure. The controller 150 may be located in one of the components of the assessment apparatus 100 or it may be distributed over at least two of the components.

[0018] Figure 2 is a schematic diagram illustrating an exemplary electron-optical apparatus 140. The electron-optical apparatus 140 may be provided as part of the assessment apparatus 100 of Figure 1. The electron-optical apparatus 140 includes a source 201 and an electron-optical device 230 (which may also be referred to as an electron-optical column). The source 201 may comprise an emitter (not shown), which may be a cathode, and an extractor and / or anode (not shown). During operation, the source 201 is configured to emit electrons from the emitter. The electrons may be extracted or accelerated by the extractor and / or the anode to form the source beam 202.

[0019] The electron-optical device 230 may be configured to convert the source beam 202 into a plurality of primary beams 211, 212, 213 (which may be referred to as sub-beams or beamlets). The electron-optical device 230 may be configured to direct the primary beams 211, 212, 213 along respective beam paths toward a sample location for the sample 208. Although three beams are illustrated, the number of beams may be of the order of 100s or 1,000s, for example up to 20,000 per electron-optical apparatus 140. The plurality of beams may be referred to collectively as a multibeam or a beam grid. The different beams may be arranged relative to each other across the beam grid in a pattern. The pattern of the beam grid may be referred to array. The electron-optical device 230 has a field of view which may be defined as the area of the surface of the sample 208 within which the primary beams 211, 212, 213 can scan while the aberrations of the electron-optical device 230 remain within a defined value. Alternatively, the field of view may be defined by the maximum scan range of the electron-optical device 230. The field of view may be of the order of millimeters, for example up to 20mm at the sample 208.

[0020] The electron-optical device 230 comprises a plurality of electron-optical elements positioned along the beam paths. The electron-optical elements are configured to manipulate the beams. For example, the electron-optical elements may be configured to lens, focus, deflect or correct the beams. The electron-optical elements may be arranged in at least one stack of electron-optical elements. Such an electron-optical element may be positioned upbeam or downbeam with respect to another of the electron-optical elements. The terms upbeam and downbeam relate to the direction ofthe beams from the source 201 to the sample 208 during use of the electron-optical device 230, which may be expressed as a direction along one or more of the beam paths. In an embodiment some of the different electron-optical elements may take a planar form, such as a plate 261. An electric field that manipulates the beams may be generated between two plates 261, e.g. by applying, in use, different potentials to neighboring / adjoining plates 261 such as along the beam path. An electric field that manipulates the beams may be generated between surfaces of plates 261 across the beam path for example between the neighboring plates 261. One or more beam apertures 266 may be defined in the plates 261 for the passage of one or more beams. The beam apertures 266 may be arranged in a pattern such as a regular grid e.g. hexagonal or square. Such a pattern of the beam apertures 266 may be referred to as an aperture array (i.e. a two-dimensional array over the surface of the plate). The pattern of the beam apertures 266 may correspond to the pattern of beams within the beam grid. Beam apertures 266 in different plates operating on the same beam(s) are typically aligned.

[0021] The electron-optical elements may comprise one or more corrector arrays. For example, a corrector array may be integrated into the shape, position and / or size of the beam apertures 266 of the plates 261. The disclosure of such a corrector array as described in WO 2022101072 Al is hereby incorporated by reference. One or more corrector arrays may comprise multipole deflectors with a specific superposition of potentials applied across the individually controllable electrodes. The disclosure in WO2012165955 of an array of multipole deflectors is hereby incorporated by reference.

[0022] One or more of the electron-optical elements may comprise an aperture for the path of a plurality of the beams. For example, the aperture may be a macro aperture for all of the beams. The disclosures of a slit aperture for a collimator or corrector comprising strip electrodes in WO 2021156121 Al and WO 2021204734 Al are hereby incorporated by reference. One or more electron-optical elements may comprise one or more plate electrodes that are curved across the path of the beam grid for use as a lens array, a corrector array and / or a collimator array such as disclosed in European patent application 23211553.5 filed 22 November 2023, which is hereby incorporated by reference at least so far as the use and application of curved plate electrodes.

[0023] In the current embodiment, the electron-optical device 230 may form three probe spots 281, 282, 283 on the surface of the sample 208. The electron-optical device 230 may be configured to deflect the primary beams 211, 212, 213 so as to scan the probe spots 281, 282, 283 across individual scanning areas of the sample 208. In response to incidence of the primary beams 211, 212, 213 on the sample 208, signal electrons are generated from the sample 208 which may include secondary electrons and backscattered electrons. Secondary electrons typically have electron energy of at most 50 eV. Backscattered electrons typically have electron energy of more than 50 eV and less than the landing energy of the primary beams 211, 212, 213.

[0024] The electron-optical apparatus 140 comprises a sample holder 207 that supports a sample 208. The sample holder 207 supports the sample 208 for assessment. The sample holder 207 is supported by an actuatable stage 209. The electron-optical apparatus 140 further comprises a detectorarray 240. The detector array 240 may be part of the electron-optical device 230. The detector array 240 e.g. detects signal electrons from the sample 208. The detector array 240 generates detection signals based on detection of the signal electrons.

[0025] In an embodiment, he detector array 240 may define the surface of the electron-optical apparatus 140 facing the sample 208, e.g. the bottom surface of the electron-optical device 230.There may be more than one detector array at different positions along the paths of the primary beams 211, 212, 213.

[0026] The detector array 240 may comprise a plurality of detector elements, with at least one detector element per beam. The detector elements may, for example, be charge capture electrodes, for example metal plates, which may be configured to detect at least some of the signal electrons. Alternatively or additionally, the detector elements may comprise detection diodes configured to detect at least some of the signal electrons. Alternatively or additionally, the detector elements may comprise a scintillator material (such as YAG) configured to convert signal electrons into photons that may be subsequently detected. The detector elements may be arranged around beam apertures 266 in the bottom surface of the electron-optical device 230 to allow the primary beams 211, 212, 213 to pass towards the sample 208. Each detector element may comprise a plurality of detection segments or may constitute a single sensitive surface for each beam. The detection signal generated by a detector element may be transmitted to a processor for generation of an image. For example, the detection signal may represent a grey value or an intensity value of a pixel of an image.

[0027] The detector array 240 may send the detection signals, for example as an imaging signal or a detection signal, to the controller 150 or to a signal processing system (not shown) which may be part of the controller 150. The controller 150 or the signal processing system may be configured to generate images of the corresponding scanned areas of the sample 208. The detector array 240 may be incorporated at least partly into the electron-optical device 230. Alternatively, the detector array 240 may be separate from the electron-optical device 230. For example, the electron-optical apparatus 140 may comprise a secondary electron-optical device configured to direct secondary electrons to the detector array 240. In such an embodiment, the secondary electron-optical device comprises a beam separator (such as a Wien filter, not shown). The beam separator may separate the paths of the primary electrons towards the sample 208 from the paths of the signal electrons away from the sample 208. Note, such a beam separator may be present in a different embodiment with a detector array within the electron-optical device 230 for directing the primary electrons towards the sample and the signal particles to detector elements of the detector array.

[0028] The controller 150 (for example a control system comprising distributed controllers) may be connected to various parts (e.g. components) of the electron-optical apparatus 140 of Figure 2, such as the source 201, the detector array 240, the electron-optical device 230, and the actuatable stage 209. The controller 150 may perform various image processing functions and signal processingfunctions. The controller 150 may also generate various control signals to govern operations of the assessment apparatus 100.

[0029] Figure 3 schematically depicts an electron-optical device array 299. In an embodiment the assessment apparatus 100 of Figure 1 comprises the electron-optical device array 299 instead of the electron-optical apparatus 140. Such an electron-optical device array 299 is also referred to as a multi-column array. The different columns (or electron-optical devices 230) may comprise in use a plurality of beams such as a beam grid. In an embodiment the electron-optical device array 299 comprises a plurality of electron-optical devices 230 of the type shown in Figure 2.

[0030] In an embodiment, one or more electron-optical elements may be shared between more than one of the electron-optical devices 230 of the electron-optical device array 299. The electron- optical elements may comprise one or more plates 261 in which a plurality of beam apertures 266 are defined for respective beam paths. In an embodiment, one or more sources 201 may be shared between more than one of the electron-optical devices 230 of the electron-optical device array 299. The sources may be comprised in a source array having different sources generating a source beam for different respective electron-optical device 230.

[0031] The electron-optical devices 230 may focus respective multi-beams simultaneously onto different regions of the same sample 208. In a different embodiment, the electron-optical devices 230 of the electron-optical device array 299 may project respective single beams towards the sample 208.

[0032] Each electron-optical device 230 of the electron-optical device array 299 may be configured in any of the ways described herein. The disclosure in WO 2022008286 Al of how the objective lens is incorporated and adapted for use in the multi-device arrangement is hereby incorporated by reference. The disclosure in WO 2021165135 Al of a multi-device arrangement of a multi-beam device comprising a collimator at, or proximate to, an intermediate focus is hereby incorporated by reference.

[0033] In an embodiment any electron-optical element or group of electron-optical elements may be replaceable or field replaceable within the electron-optical apparatus 140. Field replaceability means that the electron-optical element or group of electron-optical elements may be replaced without substantially disassembling the electron-optical apparatus 140. In an embodiment the electron-optical apparatus 140 comprises at least one module. Each module comprises a group of adjacent electron- optical elements. The electron-optical elements of a module may be secured to each other, for example as a stack.

[0034] Features in electron-optical plates may be manufactured using techniques from microelectromechanical systems (MEMS) (i.e. using MEMS manufacturing techniques). MEMS are miniaturized mechanical and electromechanical elements that are made using microfabrication techniques. Merely as an example, a collimator array may be formed using MEMS manufacturing techniques so as to be spatially compact. As another example, a scan deflector array may be formed using MEMS manufacturing techniques.

[0035] An electric power source may be provided to supply power to the assessment apparatus 100 and / or electron-optical apparatus 140. For example, such a power supply may apply respective potentials to electrodes of lenses of the electron-optical device 230.

[0036] In an embodiment, the controller 150 is configured to control the electron-optical device 230. The controller 150 may be configured to control potentials applied to electrodes of lenses of the electron-optical device 230.

[0037] In an embodiment the controller 150 is configured to control the actuatable stage 209 to move the sample 208 during inspection of the sample 208. The controller 150 may enable the actuatable stage 209 to move the sample 208 in a direction, for example continuously, such as at a constant speed, at least during sample inspection, which may be referred to as a type of scanning. The speed of the actuatable stage 209 may be referred to as the moving rate. The controller 150 may control movement of the actuatable stage 209 so as to change the speed of movement of the sample 208 relative to the beam paths dependent on one or more parameters. The controller 150 may control deflection of scan deflectors so that the beam paths move relative to the actuatable stage 209 and thus over the surface of the sample 208. The controller 150 may change a beam deflection of a scan deflector and thus the scanning of the primary beams 211, 212, 213 over the sample 208 dependent on one or more parameters. For example, the controller 150 may control a scan deflector and / or the speed of the actuatable stage 209 and / or the direction of movement of the actuatable stage 209 depending on characteristics of the assessment process. The disclosure in EP4086933 Al of a combined stepping and scanning strategy of the stage and scanning deflectors is hereby incorporated by reference. The moving rate may at different times comprise a stepping frequency and / or a stage scanning rate.

[0038] As shown in Figure 2, in an embodiment the electron-optical apparatus 140 comprises an electron-optical module 200. The electron-optical module 200 may comprise the source 201.

[0039] As shown in Figure 3, in an embodiment the electron-optical device array 299 comprises an electron-optical module 200. The electron-optical module 200 may comprise a plurality of sources 201. For example, the electron-optical module 200 may comprise a plurality of sources 201 for a respective plurality of electron-optical devices 230.

[0040] Figure 4 schematically depicts a source 201 of an electron-optical module 200. The electron-optical module 200 may comprise a plurality of sources 201. The electron-optical module 200 may have features as described above with reference to other drawings, except where differences are described below.

[0041] Figure 4 shows only one source 201 for ease of explanation. Figure 5 schematically depicts the electron-optical module 200 of Figure 4 comprising a plurality of sources 201. The sources 201 may be comprised in an array. Each source 201 corresponds to an emitter 21.

[0042] In an embodiment the source 201 comprises an avalanche diode structure. An avalanche diode structure comprises a stack of doped semiconductor junctions and is biased from twoconnections. For example, an avalanche diode structure may comprise a PN junction or a PIN junction, An avalanche diode structure may comprise a homo-junction or a hetero-j unction having stacks of semiconductors of different band gaps. In an embodiment the avalanche diode structure comprises a hetero-j unction of a silicon carbide P-type substrate with a gallium nitride N++ layer on top of it. Gallium nitride has a lower work function (~leV lower) and thus more electrons can escape from it. Meanwhile, the silicon carbide has a high thermal conductivity and the ability to make it P- type. The band gap structures influence the electron energy distribution in the avalanching region of the avalanche diode structure. The source 201 may be based on avalanche electron emitting diodes (AEEDs) as emitter technology. AEED emitters are semiconductor based emitters. The AEEDs may alternatively be referred to as avalanche cold cathodes or semiconductor junction cold cathodes. In an embodiment the source 201 is junction based. For example, the emitter 21 may comprise a diode junction such as a PN junction. In an embodiment the source 201 comprises a plurality of junctions. Each junction may be an interface between two layers or regions of similar semiconductors or dissimilar semiconductors. In an embodiment the junction is an interface between doped materials. The junction may be a junction between two or more than two materials. Such a junction may be a diode. In an embodiment the source 201 is configured such that an avalanching current is generated inside a diode of the emitter 201 that is perpendicular to the surface facing the sample 208. Some electrons are sufficiently energized in the avalanche region to overcome the work function of the surface and be emitted into the vacuum.

[0043] As shown in Figure 4, in an embodiment each source 201 comprises an emitter 21. The emitter 21 is configured to emit a source beam of electrons along a source path. In the orientation shown in Figure 4, the source path extends vertically upwards from the emitter 21. In the orientation shown in Figure 5, the source path extends vertically downwards from the emitters 21.

[0044] As shown in Figure 4, in an embodiment the electron-optical module 200 comprises a substrate element 84. The substrate element 84 may be planar. The substrate element 84 may be formed as a layer. For example, the electron-optical module 200 may comprise a base substrate 83. The base substrate 83 may comprise silicon. For example, the base substrate 83 may be a SiC substrate. In an embodiment the substrate element 84 is formed by epitaxy (e.g. molecular beam epitaxy) on the base substrate 83.

[0045] As shown in Figure 4, in an embodiment the substrate element 84 comprises the emitters 21. In an embodiment the substrate element 84 further comprises one or more electrical connectors. The electrical connectors may be for transmitting control signals and / or electrical power. The electrical connectors may be formed as traces within the substrate element 84. In an embodiment the electrical connectors comprise an electrical conductor such as a metal, for example copper.

[0046] In an embodiment the emitter 21 is formed by a PN junction. The area of the emitter 21 may correspond to the highly P doped region 87 of the PN junction. The highly P doped region 87 ismore highly doped than the substrate element 84. In an embodiment the substrate element 84 is a P doped layer.

[0047] As shown in Figure 4, in an embodiment the source 201 comprises an extractor electrode 27. The extractor electrode 27 may correspond to the emitter 21. In an embodiment the electron- optical module 200 comprises a plurality of extractors 27 for a corresponding plurality of emitters 21. As shown in Figure 4, in an embodiment the extractor electrode 27 is spaced from the emitter 21 along the source path.

[0048] As shown in Figure 4, in an embodiment the source 201 comprises an N doped region 85. In an embodiment the N doped region 85 substantially surrounds the highly P doped region 87 of the emitter 21. In an embodiment the N doped region 85 is distanced from the highly P doped region 87. For example, as shown in Figure 4 there may be a volume of the substrate element 84 between the N doped region 85 and the highly P doped region 87.

[0049] As shown in Figure 4, in an embodiment the source 201 comprises a highly N doped layer 86. The highly N doped layer 86 may be more highly N doped compared to the N doped layer 85. As shown in Figure 4, in an embodiment the highly N doped layer is located between the highly P doped layer 87 and the extractor electrode 27. The highly N doped layer 86 may be located at a surface of the substrate element 84. In an embodiment the highly N doped layer 86 is configured to divert electrons radially away from the highly P doped layer 87.

[0050] As shown in Figure 4, the emitters 21 may be located at an emission surface 80 of the electron-optical module 200.

[0051] As shown in Figure 4, in an embodiment the electron-optical module 200 comprises a spacer 88. The spacer 88 is configured to space each extractor electrode 27 from the emission surface 80. The emitters 21 are arranged at the emission surface 80 of an emitter layer. The emitter layer comprises the emitters 21. In an embodiment the spacer 88 is configured to electrically insulate the extractor electrode 27 from the emission surface 80. The spacer 88 may be an electrical insulator. During use of the electron-optical module 200, the extractor electrode 27 and the emission surface 80 may have different electric potentials. There may be a potential difference between the extractor electrode 27 and the emission surface 80. The spacer 88 is configured to facilitate the potential difference being maintained.

[0052] As shown in Figure 4, in an embodiment the spacer 88 comprises an insulator surface 94. The insulator surface 94 is between the extractor electrode 27 and the emission surface 80. As shown in Figure 4, in an embodiment at least part of the insulator surface 94 is angled relative to a direction perpendicular to the emission surface 80. For example, in the arrangement shown in Figure 4, the insulator surface 94 may comprise a first vertical section, a horizontal section and a second vertical section. The horizontal section is angled (e.g. perpendicular) to the direction perpendicular to the emission surface 80. The vertical sections of the insulator surface 94 may be parallel to the direction perpendicular to the emission surface 80. In an embodiment the angled part of the insulator surface94 may be referred to as an angled surface. The two vertical sections may form the interior surfaces of layers 89, 90 of the spacer 88. The angled surface, which is horizontal in the example shown in Figure 4, may be part of a down beam surface of a first layer 89 of the spacer 88.

[0053] The electron-optical module 200 shown in Figure 4 comprises a plurality of spacers 88. The spacers 88 are configured to space the substrate element 84 comprising the emitters 21 from the extractors 27. As shown in Figure 4, in an embodiment the spacer 88 substantially surrounds the source path in plan view. The spacers 88 may have a rim profile so as to increase the path length along the insulator surface 94 of the spacers 88 between the emitter 21 and the extractor electrode 27. For example, the insulator surface 94 may be stepped.

[0054] As shown in Figure 4, in an embodiment the spacer 88 comprises a plurality of layers 89, 90. The layers 89, 90 may have different dimensions across the source paths. For example, a first layer 89 closer to the emitter 21 may have a greater dimension across the source path. A second layer 90 at the extractor side may have a smaller dimension across the source path. The spacer 88 is arranged so as to reduce the possibility of electrical breakdown between the emitter 21 and the extractor electrode 27.

[0055] In an embodiment the first layer 89 and the second layer 90 of the spacer 88 are formed of different materials. The first layer 89 and the second layer 90 may be referred to as spacer layers 89, 90. For example, in an embodiment at least one of the spacer layers 89, 90 comprises silicon dioxide. In an embodiment at least one of the spacer layers 89, 90 comprises hafnium oxide. According to an embodiment of a method of manufacturing the electron-optical module 200, the method comprises etching a plurality of spacer layers 89, 90 of the spacer 88. For example, in an embodiment the etching is wet etching. In an embodiment, the spacer layers 89, 90 are formed of different materials having different etching rates to an etchant used for the wet etching.

[0056] As shown in Figure 5, in an embodiment the spacers 88 encircle each beam path from each emitter 21. The extractor electrodes 27 may be electrically separate from each other in an electron-optical plate element. The electron-optical plate element may be deposited and then etched to make doughnut shaped islands, as shown in Figure 6 of Figure 7, for example.

[0057] In an embodiment, the spacers 88 are made by growing a dieletric layer. Holes are etched through the dieletric layer, the spacer 88 is a layer with holes for the beam path from each source 201.

[0058] An embodiment of the invention comprising the spacer 88 is expected to reduce the electric field at a triple junction involving the spacer 88. The triple junction is a point at which the spacer 88 is in contact with the atmosphere and an electrical conductor (e.g. an extractor electrode 27). An embodiment of the invention is expected to reduce the possibility of undesirable electrical breakdown.

[0059] In an embodiment the extractor electrode 27 comprises a non-magnetic material. In an embodiment the extractor electrode 27 comprises a material that is not easily oxidised. For example, the extractor electrode 27 may comprise a material other than aluminium. The extractor electrode 27comprises an electrically conductive material. In an embodiment the extractor electrode 27 comprises tungsten.

[0060] Figure 4 schematically depicts an electrical circuit for applying potentials to the emitter 27 and the extractor electrode 27. As shown in Figure 4, different electric potentials are applied to each of the extractor electrode 27, the N doped region 85 and the highly P doped region 87. In an embodiment, electrical connections through the substrate element 84 connect one or more of the highly P doped region 87, the N doped region 85 and the extractor electrode 27 to one or more electric power supplies. In an embodiment a power supply 91 applies a potential difference between the highly P doped region 87 and the N doped region 85. A second power supply 92 may apply a potential difference between the N doped region 85 and the extractor electrode 27. In an embodiment the N doped region 85 functions as an anode. In an embodiment a common electric potential is applied to the plurality of n doped regions 85. Different electric potentials may be applied to the extractors 27 corresponding to different emitters 21.

[0061] The extractors 27 may be comprised in an extractor arrangement as an electron-optical plate element. In an embodiment the electron-optical module 200 comprises one or more further electron-optical plate elements. For example, although not shown in Figure 5, in an embodiment the electron-optical module 200 comprises a beam limiting aperture array and / or an individual beam corrector array. In an embodiment the beam limiting aperture array and / or an individual beam corrector array may be common to a plurality of (optionally all of) the emitters 21.

[0062] Figure 6 schematically depicts a plan view of an arrangement of the electron-optical module 200 shown in Figure 5. As shown in Figure 6, in an embodiment the extractors 27 are physically separate from each other. Each extractor electrode 27 substantially surrounds an emitter 21 when viewed in plan view. Between the extractors 27, the spacers 88 may be exposed.

[0063] As shown in Figure 6, in an embodiment the extractors 27 have an outer perimeter that is substantially circular. However, it is not essential for the outer perimeter of the extractor electrode 27 to be circular. In an alternative embodiment the outer perimeter is elliptical or polygonal, for example triangular, square (as shown in Figure 7), pentagonal or hexagonal. As shown in Figure 6, in an embodiment the extractor electrode 27 has an inner perimeter that is substantially circular. However, it is not essential for the inner perimeter to be circular. In an alternative embodiment, the inner perimeter of the extractor electrode 27 is elliptical, or polygonal, for example triangular, square, pentagonal or hexagonal.

[0064] As shown in Figure 6, in an embodiment the extractors 27 are arranged in an array. In an embodiment the array forms a grid. As shown in Figure 6, in an embodiment the grid may be a hexagonal grid. Alternatively, the grid may be a square or rectangular grid.

[0065] Figure 7 shows an alternative arrangement of the extractors 27. In the embodiment shown in Figure 7, the extractors 27 have an outer perimeter that is substantially rectangular.

[0066] Figure 8 schematically depicts a plurality of sources 201 for a multi-beam electron-optical device 230. Figure 8 schematically depicts the vacuum chamber 60 of the electron-optical module.In the arrangement shown in Figure 8, the sources 201 may comprise an avalanche diode structure, for example as described above.

[0067] As shown in Figure 8, in an embodiment a vacuum apparatus 61 is provided for the vacuum chamber 60. The vacuum apparatus 61 is configured, in use, to maintain the source underpressure within the vacuum chamber 60. The vacuum apparatus 61 may comprise one or more pumps, for example. In an embodiment the vacuum apparatus 61 comprises one or more getters.

[0068] As shown in Figure 8, in an embodiment the sources 201 are located between the vacuum apparatus 61 and the beam paths. The vacuum apparatus 61 is in an upbeam direction relative to the sources 201.

[0069] As shown in Figure 8, in an embodiment the electron-optical module comprises a plurality of sources 201. Each source 201 may comprise a respective emitter 21. The emitters 21 may be arranged in an array.

[0070] As shown in Figure 8, in an embodiment the electron-optical module comprises an anode element 81. The anode element 81 may be configured to function as an anode for the sources 201. In an embodiment the anode element 81 further comprises individual beam correctors for correcting individual source beams emitted by the emitters 21 of the sources 201. In an embodiment the anode element 81 is a planar element, such as a plate.

[0071] As shown in Figure 8, in an embodiment the electron-optical module comprises a beam limiting aperture array 82. The beam limiting aperture array 82 may have a plurality of beam limiting apertures defined in it. The beam limiting apertures may be configured to limit the source beams emitted by the sources 201. The beam limiting aperture array 82 may be configured to block a portion of the current of electrons and to transmit another portion of the current of electrons. In an embodiment the beam limiting aperture array 82 is substantially planar, for example a plate.

[0072] In an embodiment the anode element 81 is configured to steer source beams through the beam limiting apertures of the beam limiting aperture array 82.

[0073] As shown in Figure 3, in an embodiment a plurality of electron-optical devices 23 may comprise the electron-optical module 200. The electron-optical module may be configured to generate the source beams for the electron-optical devices 230.

[0074] In an embodiment each electron-optical device 230 comprises electron-optical elements downbeam of the electron-optical module 230. The electron-optical elements of the electron-optical devices 230 may be configured to operate on the source beams. The electron-optical elements may be configured to operate on primary beams that are derived from the source beams generated by the electron-optical module 200.

[0075] In an embodiment the electron-optical elements of the electron-optical devices 230 are configured to operate on a respective source beam or primary beam derived from the respectivesource beam. Alternatively, the electron-optical elements may be configured to operate on a plurality of the source beams or primary beams derived from a plurality of the source beams. There may be a source 201 for each electron-optical device 230. Alternatively, there may be a plurality of sources 201 for each electron-optical device 230. Alternatively, the primary beams for a plurality of electron- optical devices 230 may be derived from the source beam of a single source 201. The sources 201 may be comprised in the same electron-optical module 200.

[0076] In an embodiment an assessment apparatus 100 comprises the electron-optical apparatus 140. The electron-optical apparatus 140 may comprise a plurality of electron-optical devices 230. The electron-optical devices 230 may comprise at least one detector 240. The detector 240 may be configured to detect signal particles from a sample 208 when the sample is supported at the sample location. In an embodiment the at least one detector 240 comprises one or more of the electron- optical plate elements. The detector interacts with the electrons that it may be considered an electron- optical element.

[0077] Further embodiments according to the present invention are described in below numbered clauses:1. A charged particle-optical module for a plurality of charged particle-optical devices configured to direct charged particle beams along respective beam paths towards a sample location, the charged particle-optical module comprising: at least one emitter arranged at an emission surface of an emitter layer, each emitter configured to emit charged particles for a source beam; at least one extractor electrode for the at least one emitter, wherein the at least one extractor electrode is planar and a beam aperture is defined in each extractor electrode for the charged particles emitted by the respective emitter; and a spacer configured to space each extractor electrode from the emission surface, wherein the spacer is an electrical insulator and comprises an insulator surface between the extractor electrode and the emission surface, wherein at least part of the insulator surface is angled relative to a direction perpendicular to the emission surface.2. The charged particle-optical module of clause 1, wherein the insulator surface is stepped.3. The charged particle-optical module of clause 1 or 2, wherein the at least one extractor electrode is aligned relative to the at least one emitter.4. The charged particle-optical module of any preceding clause, comprising a plurality of emitters.5. The charged particle-optical module of clause 4, comprising a plurality of extractor electrodes for the plurality of emitters.6. The charged particle-optical module of clause 4 or 5, comprising a plurality of spacers for the plurality of emitters.7. The charged particle-optical module of any preceding clause, wherein the spacer comprises a plurality of spacer layers formed of different materials.8. The charged particle-optical module of clause 7, wherein at least one of the spacer layers comprises silicon dioxide.9. The charged particle-optical module of clause 7 or 8, wherein at least one of the spacer layers comprises hafnium oxide.10. The charged particle-optical module of any preceding clause, wherein the at least one extractor electrode is non-magnetic.11. The charged particle-optical module of any preceding clause, comprising: a planar charged particle-optical element configured to operate on the one ore more source beams individually.12. The charged particle-optical module of any preceding clause, comprising: a planar charged particle-optical element configured to shape the at least one source beam.13. The charged particle-optical module of any preceding clause, wherein a distance between the insulator surface and an axis of a beam path of the charged particles emitted by the emitter is greater at an emission surface end of the insulator surface than at an extractor electrode end of the insulator surface.14. The charged particle-optical module of any preceding clause, wherein the spacer is planar.15. The charged particle-optical module of any preceding clause, wherein the at least one emitter comprises an avalanche diode structure.16. A plurality of charged particle-optical devices configured to direct charged particle beams along respective beam paths towards a sample location, the plurality of charged particle-optical devices comprising the charged particle-optical module of any preceding clause.17. The plurality of charged particle-optical devices of clause 16, wherein each charged particle- optical device comprises charged particle-optical plate elements downbeam of the charged particle- optical module configured to operate on the source beams or to operate on primary beams derived from the source beams.18. The plurality of charged particle-optical devices of clause 17, wherein the charged particle- optical plate elements are configured to operate on a respective source beam or primary beams derived from the respective source beam, or to operate on a plurality of the source beams or primary beams derived from a plurality of the source beams.19. A charged particle-optical apparatus comprising: the plurality of charged particle-optical devices of any of clauses 16-18; and an actuatable stage configured to support a sample at the sample location.20. The charged particle-optical apparatus of clause 19, further comprising: a vacuum chamber comprising the plurality of the charged particle-optical devices and the actuatable stage.21. An assessment apparatus for assessing a sample comprising the charged particle-optical apparatus of clause 19 or 20, wherein the plurality of charged particle-optical devices comprise at least one detector configured to detect signal particles from a sample when supported at the sample location.22. The assessment apparatus of clause 21, wherein the at least one detector comprises at least one of the charged particle-optical plate elements.23. A method for manufacturing a charged particle-optical module for a charged particle-optical device configured to direct a charged particle beam along a beam path towards a sample location, the method comprising: arranging at least one emitter at an emission surface of an emitter layer, each emitter configured to emit charged particles for a source beam; defining in each of at least one extractor electrode for the at least one emitter a beam aperture for the charged particles emitted by the respective emitter, wherein the at least one extractor electrode is planar and; and spacing each extractor electrode from the emission surface with a spacer, wherein the spacer is an electrical insulator and comprises an insulator surface between the extractor electrode and the emission surface, wherein at least part of the insulator surface is angled relative to a direction perpendicular to the emission surface.24. The method of clause 23, comprising: aligning the at least one extractor electrode and the spacer relative to the emitter.25. The method of clause 24, comprising: after aligning the at least one extractor electrode and the spacer relative to the emitter, incorporating the charged particle-optical module into the charged particle-optical device.26. The method of any of clauses 23-25, wherein the spacing comprises: etching a plurality of spacer layers of the spacer.27. The method of clause 26, wherein the etching is wet etching.28. The method of clause 27, wherein the spacer layers are formed of different materials having different etching rates to an etchant used for the wet etching.

[0078] Although specific reference may be made in this text to embodiments of the invention in the context of an electron microscope, embodiments of the invention may be used in other types of apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device).

[0079] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art thatmodifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A charged particle-optical module for a plurality of charged particle-optical devices configured to direct charged particle beams along respective beam paths towards a sample location, the charged particle-optical module comprising: at least one emitter arranged at an emission surface of an emitter layer, each emitter configured to emit charged particles for a source beam; at least one extractor electrode for the at least one emitter, wherein the at least one extractor electrode is planar and a beam aperture is defined in each extractor electrode for the charged particles emitted by the respective emitter; and a spacer configured to space each extractor electrode from the emission surface, wherein the spacer is an electrical insulator and comprises an insulator surface between the extractor electrode and the emission surface, wherein at least part of the insulator surface is angled relative to a direction perpendicular to the emission surface.

2. The charged particle-optical module of claim 1, wherein the insulator surface is stepped.

3. The charged particle-optical module of claim 1 or 2, wherein the at least one extractor electrode is aligned relative to the at least one emitter.

4. The charged particle-optical module of any preceding claim, comprising a plurality of emitters.

5. The charged particle-optical module of claim 4, comprising a plurality of extractor electrodes for the plurality of emitters.

6. The charged particle-optical module of claim 4 or 5, comprising a plurality of spacers for the plurality of emitters.

7. The charged particle-optical module of any preceding claim, wherein the spacer comprises a plurality of spacer layers formed of different materials.

8. The charged particle-optical module of claim 7, wherein at least one of the spacer layers comprises silicon dioxide.

9. The charged particle-optical module of claim 7 or 8, wherein at least one of the spacer layers comprises hafnium oxide.

10. The charged particle-optical module of any preceding claim, wherein the at least one extractor electrode is non-magnetic.

11. The charged particle-optical module of any preceding claim, comprising: a planar charged particle-optical element configured to operate on the one ore more source beams individually.

12. The charged particle-optical module of any preceding claim, comprising: a planar charged particle-optical element configured to shape the at least one source beam.

13. The charged particle-optical module of any preceding claim, wherein a distance between the insulator surface and an axis of a beam path of the charged particles emitted by the emitter is greater at an emission surface end of the insulator surface than at an extractor electrode end of the insulator surface.

14. The charged particle-optical module of any preceding claim, wherein the spacer is planar.

15. The charged particle-optical module of any preceding claim, wherein the at least one emitter comprises an avalanche diode structure.

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