WO2025201799A1

WO2025201799A1 - Charged particle-optical module

Info

Publication number: WO2025201799A1
Application number: PCT/EP2025/055693
Authority: WO
Inventors: Roy Ramon VEENSTRA; Erwin Paul SMAKMAN
Original assignee: ASML Netherlands BV
Current assignee: ASML Netherlands BV
Priority date: 2024-03-28
Filing date: 2025-03-03
Publication date: 2025-10-02
Anticipated expiration: 2026-09-28
Also published as: TW202546886A

Abstract

A charged particle-optical module (200) for a charged particle-optical device configured to direct charged particle beams along respective beam paths towards a sample location, the charged particle-optical module comprising: an emitter (21) configured to emit a source beam of charged particles; a plurality of charged particle-optical plate elements (24-26) configured to operate on the source beams and defining a plurality of beam apertures (266) for passage of the source beam; and a spacer (33) configured to space two of the charged particle-optical plate elements from each other, wherein the spacer comprises at least one interface surface that interfaces with one of the charged particle-optical plate elements, the interface surface being planar.

Description

CHARGED PARTICLE-OPTICAL MODULE

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24167258.3 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 operating 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. 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.

[0004] 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. 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 signal particles (e.g. signal electrons), such as secondary electrons and backscattered electrons, and may contain other interaction products, such as X-ray radiation.

[0005] The primary beam may be derived from a source beam emitted by a source. It is desirable to make the source more compact without unduly increasing a risk of electrical breakdown. SUMMARY

[0006] According to an aspect of the present invention there is provided a charged particle- optical module for a charged particle-optical device configured to direct charged particle beams along respective beam paths towards a sample location, the charged particle-optical module comprising: a plurality of emitters configured to emit respective source beams of charged particles; a plurality of charged particle-optical plate elements configured to operate on the source beams and defining a plurality of beam apertures for passage of the source beams, wherein at least one of the charged particle-optical plate elements is common to a plurality of the emitters; and a spacer configured to space two of the charged particle-optical plate elements from each other, wherein the spacer comprises at least one interface surface that interfaces with one of the charged particle-optical plate elements, the interface surface being planar.

[0007] According to another aspect of the present invention there is provided a charged particle- optical module for a charged particle-optical device configured to direct charged particle beams along respective beam paths towards a sample location, the charged particle-optical module comprising: an emitter configured to emit a source beam of charged particles; a plurality of charged particle-optical plate elements configured to operate on the source beams and defining a plurality of beam apertures for passage of the source beam; and a spacer configured to space two of the charged particle-optical plate elements from each other, wherein the spacer comprises at least one interface surface that interfaces with one of the charged particle-optical plate elements, the interface surface being planar. [0008] According to another aspect of the present invention there is provided a method for operating a charged particle-optical module for a charged particle-optical device configured to direct charged particle beams along respective beam paths towards a sample location, the method comprising: emitting source beams of charged particles from respective emitters; and operating on the source beams with a plurality of charged particle-optical plate elements that define a plurality of beam apertures for passage of the source beams, wherein at least one of the charged particle-optical plate elements is common to a plurality of the emitters; wherein two of the charged particle-optical plate elements are spaced from each other by a spacer, wherein the spacer comprises at least one interface surface that interfaces with one of the charged particle-optical plate elements, the interface surface being planar.

[0009] According to another aspect of the present invention there is provided a method for operating a charged particle-optical module for a charged particle-optical device configured to direct charged particle beams along respective beam paths towards a sample location, the method comprising: emitting a source beam of charged particles from an emitter; and operating on the source beams with a plurality of charged particle-optical plate elements that define a plurality of beam apertures for passage of the source beam; wherein two of the charged particle-optical plate elements are spaced from each other by a spacer, wherein the spacer comprises at least one interface surface that interfaces with one of the charged particle-optical plate elements, the interface surface being planar.

BRIEF DESCRIPTION OF THE DRAWINGS

[00010] 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 charged particle-optical module comprising a plurality of sources;

Figure 5 schematically depicts a spacer, for example of the charged particle-optical module of Figure 4;

Figure 6 schematically depicts a plan view of a stack of the charged particle-optical module shown in Figure 4;

Figure 7 schematically depicts a side on view of the stack shown in Figure 6;

Figure 8 schematically depicts a charged particle-optical module comprising a single source; and

Figure 9 schematically depicts a vacuum chamber.

[00011] 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 device

DETAILED DESCRIPTION

[00012] 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.

[00013] 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. secondary electrons 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.

[00014] 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.

[00015] 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. [00016] 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.

[00017] 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.

[00018] 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.

[00019] 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 multi- beam 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.

[00020] 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 of the 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.

[00021] 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. [00022] 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.

[00023] 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.

[00024] 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 detector array 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.

[00025] 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.

[00026] 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.

[00027] 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.

[00028] 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 processing functions. The controller 150 may also generate various control signals to govern operations of the assessment apparatus 100.

[00029] 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.

[00030] 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.

[00031] 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. [00032] 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. [00033] 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.

[00034] 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.

[00035] 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.

[00036] 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.

[00037] 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.

[00038] 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. [00039] 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.

[00040] Figure 4 schematically depicts an electron-optical module 200 according to an embodiment of the invention. The electron-optical module 200 may be referred to as a multi-source module. The electron-optical module 200 may comprise a plurality of emitters 21. As shown in Figure 3, in an embodiment the electron-optical module 200 is for a plurality of electron-optical devices 230. The electron-optical devices 230 may be configured to direct electron beams along respective beam paths towards the sample location at which the sample 208 is located.

[00041] As shown in Figure 4, in an embodiment the electron-optical module 200 comprises a plurality of emitters 21. The emitters 21 may be configured to emit respective source beams 202 of electrons. The source beams 202 may be emitted along respective source paths. A source path may be the path that the electrons of a source beam 202 travel along. Figure 4 schematically shows two emitters 21 of respective sources 201. Two emitters 21 are shown for ease of explaining the features of the electron-optical module 200. The electron-optical module 200 may comprise at least three, optionally at least five, optionally at least 10, optionally at least 20, optionally at least 50, optionally at least 100, optionally at least 200, optionally at least 500 and optionally at least 1000 emitters 21 of respective sources 201.

[00042] In an embodiment each source 201 is configured to generate a source beam 202 for a respective electron-optical device 230. Each electron-optical device 230 may comprise one or more electron-optical elements downbeam of the source 201. In an embodiment the sources 201 comprise Schottky sources. Additionally or alternatively, the sources 201 may comprise cold field emitters. [00043] As shown in Figure 4, in an embodiment the electron-optical module 200 comprises a plurality of electron-optical plate elements 24-26. The electron-optical plate elements 24-26 are configured to operate on the source beams. For example, the electro-optical p late elements 24-26 may contribute to defining one or more electromagnetic fields that affect electrons of the source beams.

[00044] As shown in Figure 4, in an embodiment a plurality of beam apertures 266 are defined in the electron-optical plate elements 24-26. The beam apertures 266 are configured for passage of the source beams. As shown in Figure 4, in an embodiment the beam apertures 266 are aligned with respect to the emitters 21.

[00045] As shown in Figure 4, in an embodiment the sources 201 comprise respective emitters 21. In an embodiment the electron-optical plate elements comprise an extractor arrangement 25. The extractor arrangement 25 is configured to operate on the source beams. In an embodiment, during operation the emitters 21 are configured to emit electrons. The electrons may be extracted or accelerated by the extractor arrangement 25 to form the source beams 202. [00046] As shown in Figure 4, in an embodiment at least one of the electron-optical plate elements 24-26 is common to a plurality of the emitters 21. As shown in Figure 4, in an embodiment at least two of the electron-optical plate elements 24-26 are common to a plurality of the emitters 21. At least one of the electron-optical plate elements 24-26 are common to all of the emitters 21 of the electron-optical module 200. Alternatively, at least one of the electron-optical plate elements 24-26 are common to a subset of the emitters 21 of the electron-optical module 200.

[00047] As shown in Figure 4, in an embodiment the electron-optical plate elements 24-26 extend across the source paths of the source beams. The electron-optical plate elements 24-26 may extend between neighboring emitters 21. As shown in Figure 4, in an embodiment the electron-optical plate elements 24-26 may extend substantially perpendicular to the source paths of the source beams emitted by the emitters 21.

[00048] As shown in Figure 4, in an embodiment the electron-optical plate elements 24-26 are substantially planar. The electron-optical plate elements 24-26 may comprise major planar surfaces. Electromagnetic fields may be set up between facing surfaces. The electromagnetic fields may affect the electrons of the source beams emitted by the emitters 21.

[00049] It is not essential for the electron-optical plate elements 24-26 to be perfectly planar. For example, one or more of the electron-optical plate elements 24-26 may be curved, for example bowlshaped. Such a bowl-shaped electron-optical plate element has a plate-like shape.

[00050] In an embodiment the extractor arrangement 25 is configured to operate on the source beams individually. The extractor arrangement 25 may contribute to defining one or more electromagnetic fields that affect electrons in the source beams. The extractor arrangement 25 may be configured such that at least one of the electromagnetic fields may be different for source beams from different emitters 21. For example, the extractor arrangement 25 may be arranged such that the magnitude and/or shape of the electromagnetic fields may be different for different source beams.

[00051] An embodiment of the invention is expected to make it easier to align the emitters and extraction electrodes of a plurality of sources 201. An embodiment of the invention is expected to reduce the time required to align the emitters and extraction electrodes of the sources 201. For example, the emitters 21 may be aligned relative to the extractor arrangement 25 during assembly of the electron-optical module 200. The sources 201 may subsequently be integrated into an electron- optical device array 299, for example as shown in Figure 3. By aligning the emitters 21 relative to the extractor arrangement 25 during assembly of the electron-optical module 200, it may not be necessary to align the emitters relative to the extraction electrodes after the sources 201 have been integrated into the electron-optical device array 299.

[00052] An embodiment of the invention is expected to improve the accuracy of controlling beam currents of the source beams generated by a plurality of sources 201. By operating on the source beams individually with the extractor arrangement 25, the beam currents of the source beams can be controlled individually. For example, it may be possible to provide a plurality of sources 201 that have more consistent beam currents relative to each other.

[00053] Figure 6 schematically depicts a plan view of the stack 23 of electron-optical plate elements 24-26 shown in Figure 4. Figure 7 schematically depicts a side on view of the stack 23 of electron-optical plate elements 24-26. The side on view shown in Figure 7 is a cross sectional view taken along the dot-chain line shown in Figure 6.

[00054] As shown in Figures 4, 6 and 7, in an embodiment the electron-optical module 200 comprises a plurality of spacers 33, 36. The spacers 33, 36 are configured to space a plurality of the electron-optical plate elements 24-26 from each other.

[00055] For example, in an embodiment the electron-optical plate elements comprise a suppressor 24. The suppressor 24 may be common to a plurality of the emitters 21. The suppressor 24 may be configured to suppress any undesirable currents of electrons. As shown in Figure 4, in an embodiment the electron-optical plate elements comprise an anode 26. For example, the emitter 21 may form a corresponding cathode. During operation, the emitter 21 may be configured to emit electrons. The electrons may be accelerated by the anode 26 to form the source beam. As shown in Figure 4, in an embodiment the anode 26 is common to a plurality of the emitters 21. For example, the anode 26 shown in Figure 4 is shared by the two sources 201 shown.

[00056] For example, Figure 4 schematically depicts a plurality of spacers 33 configured to space the suppressor 24 from the extractor arrangement 25. As another example, Figure 6 and Figure 7 schematically depict a plurality of spacers 36 configured to space the suppressor 24 from the anode 26. In an embodiment, other pairs of electron-optical plate elements are spaced from each other by one or more spacers. For example, in an embodiment the electron-optical module 200 comprises one or more spacers configured to space the extractor arrangement 25 from the anode 26.

[00057] In an embodiment the electron-optical plate elements comprise one or more electron- optical plate elements other than the suppressor 24, the extractor arrangement 25 and the anode 26. In an embodiment, the suppressor 24 may be omitted. In an embodiment the anode 26 may be omitted. In an embodiment the electron-optical module 200 comprises one or more spacers configured to space such other electron-optical plate elements from each other and/or from one or more of the suppressor 24, the extractor arrangement 25 and the anode 26.

[00058] As shown in Figure 4, in an embodiment each emitter 21 comprises a tip 22. The emitter

21 is configured to emit electrons from the tip 22. As shown in Figure 4, in an embodiment the tip 22 is located upbeam of the extractor arrangement 25. As shown in Figure 4, in an embodiment the tip

22 of the emitter 21 extends through a beam aperture 266 defined in at least one of the electron- optical plate elements (e.g. the suppressor 24). As shown in Figure 4, in an embodiment a distance between the tip 22 of the emitter 21 and the extractor arrangement 25 is less than a distance between the extractor arrangement 25 and an electron-optical plate element upbeam of the extractor arrangement 25 (e.g. the suppressor 24). [00059] In an embodiment the spacers 33, 36 are configured to mechanically support the electron- optical plate elements relative to each other. For example, the spacers 33, 36 may be secured to the electron-optical plate elements 24-26. The electron-optical plate elements 24-26 may be secured relative to each other via the spacers 33, 36. An embodiment of the invention is expected to improve mechanical stability of the plurality of sources 201, for example the stack of electrodes comprised within the plurality of sources. Additionally or alternatively, the electron-optical module 200 may comprise one or more frames configured to mechanically support the electron-optical plate elements relative to each other. Such a frame is optional. The spacers 33, 36 may provide sufficient mechanical support for the electron-optical plate elements 24-26.

[00060] In an embodiment the spacers 33, 36 are configured to electrically isolate the electron- optical plate elements 24-26 from each other. For example, the spacers 33, 36 may be arranged such that a gap is provided between neighboring electron-optical plate elements 24-26. The spacers 33, 36 may comprise an electrically insulating material, e.g. quartz. The spacers 33, 36 may be configured to reduce the possibility of undesirable electrical breakdown between the electron-optical plate elements 24-26.

[00061] During operation of the electron-optical module 200, the electron-optical plate elements 24-26 may be maintained at different electrical potentials. For example, one or more power supplies may be electrically connected to the electron-optical plate elements 24-26. The one or more power supplies may be configured to apply an electric potential to each of the electron-optical plate elements 24-26. For example, the controller 150 of an electron-optical apparatus 140 may be configured to control one or more power supplies so as to control the electric potential of the electron-optical plate elements 24-26. An embodiment of the invention is expected to reduce the possibility of undesirable electrical breakdown.

[00062] In an embodiment the spacers 33, 36 are configured to mechanically support the extractor arrangement 25 positionally relative to beam paths of the source beams. For example, the spacers 33 may be configured to mechanically support the extractor arrangement 25 a predetermined distance from the suppressor 24 for a respective source beam emitted from a respective emitter 21. In an embodiment, the spacers 33, 36 are configured to mechanically support the extractor arrangement 25 positionally relative to the beam paths of the source beams individually such that the extractor arrangement 25 is configured to operate on the source beams individually. By controlling the position of the extractor arrangement 25 relative to the beam path of the source beams, the extractor arrangement may affect the source beams differently for different source beams. An embodiment of the invention is expected to improve the accuracy of controlling current beams of respective source beams.

[00063] In an embodiment the extractor arrangement 25 comprises a plurality of extractor electrodes 27, as shown in Figure 4, for example. In the arrangement shown in Figure 4, the extractor arrangement 25 comprises an extractor electrode 27 for each emitter 21. The extractor electrodes 27 may be arranged in a common plane with each other. The extractor arrangement 25 may form a layer comprising a plurality of extractor electrodes 27.

[00064] As shown in Figure 6, in an embodiment for each source beam a plurality of the spacers 33 are configured to mechanically support the extractor arrangement 25. By providing a plurality of spacers 33 to mechanically support the extractor arrangement 25, the orientation of the extractor arrangement 25 relative to the emitters 21 may be controlled more accurately. For example, undesirable tilting of the extractor arrangement 25 relative to the emitter 21 may be reduced.

[00065] As shown in Figure 6, in an embodiment for each source beam at least three spacers 33 are configured to mechanically support the extractor arrangement 25. By providing at least three spacers 33, the orientation of the plate-like shape of extractor arrangement 25 may be controlled relative to the emitter 21. By providing three spacers 33, a plane of the extractor arrangement 25 may be controlled for each source beam. As shown in Figure 6, in an embodiment only three spacers 33 are configured to mechanically support the extractor arrangement 25 for each source beam. In an alternative embodiment, more than three spacers 33 are provided to mechanically support the extractor arrangement 25 for each source beam.

[00066] As shown in Figure 6, in an embodiment the spacers 33 are evenly distributed in a circumferential direction around the source path of each source beam. Where more than three spacers 33 are provided for each source beam, in an embodiment the spacers 33 are arranged to be evenly distributed around the circumferential direction around the source path.

[00067] As shown in Figure 4 and Figure 6, for example, in an embodiment the extractor arrangement 25 comprises a plurality of extractor electrodes 27. As shown in Figure 6, the extractor electrodes 27 are separate from each other. The extractor electrodes 27 may be arranged such that there is a gap between neighboring extractor electrodes 27. In the arrangement shown in Figure 6, there are three extractor electrodes 27. In an alternative embodiment, the number of extractor electrodes 27 is at least five, optionally at least ten, optionally at least 20, optionally at least 50, optionally at least one 100, optionally at least 200, optionally at least 500 and optionally at least 1000. [00068] As shown in Figure 6, in an embodiment each extractor electrode 27 is configured to operate on a different source beam. For example, each extractor electrode 27 may be configured to operate on a different source beam separately. By providing different extractor electrodes 27, the extractor electrodes may be individually controllable. For example, one or more power supplies may be configured to apply different electric potentials to different extractor electrodes 27. By applying different electric potentials to the different extractor electrodes 27, the source beams may be controlled individually. For example, the beam current of the different source beams output by the different emitters 21 may be individually controlled.

[00069] As shown in Figure 6, in an embodiment there are a plurality of spacers 33 per extractor electrode 27. For example, there may be three spacers per extractor electrode 27 as shown in Figure 6. By providing three spacers 33 per extractor electrode 27, the plane of the extractor electrode 27 may be determined by the spacers 33. By providing three spacers 33 per extractor electrode 27, the effect of the extractor electrode 27 on the source beam may be controlled more accurately and more reliably.

[00070] As shown in Figure 4 or Figure 7, for example, in an embodiment the spacers 33 are substantially planar. For example, the spacers 33 may be disc-shaped. However, it is not essential for the spacers 33 to be substantially planar. For example the spacers 33 may have a dimension along the source path that is greater than their dimension across the beam paths. In the orientation of Figure 7, the spacers 33 may have a height that is at least as great, or greater than, their width.

[00071] As shown in Figure 7, in an embodiment the spacers 33 are substantially parallel to one or more of the electron-optical plate elements 24-26. For example, as shown in Figure 7, the spacers 33 may be substantially parallel to the suppressor 24. In an embodiment the spacers 33 are substantially parallel to the extractor arrangement 25 (e.g. to the extractor electrodes 27 of the extractor arrangement 25). In an embodiment the spacers 33 are substantially parallel to the anode 26.

[00072] As shown in Figure 6, in an embodiment the spacers 33 have substantially a similar shape in at least two dimensions. For example, in plan view the spacers 33 may be circular. Alternatively, the spacers 33 may be square or hexagonal, for example, in plan view. Plan view refers to a view along the source paths of the source beams. In an embodiment the spacers 33 have substantially a similar shape in cross-section. In an embodiment the spacers 33 are substantially dimensioned the same in directions across the source paths.

[00073] As shown in Figure 4 or Figure 7, in an embodiment the spacers 33, 35 have a rim profile. For example, the spacers 33, 36 may comprise a radially inner portion 34 and a radially outer portion 35. The radially outer portion 35 is radially outward of the radially inner portion 34. The radially inner portion 34 and the radially outer portion 35 may be formed integrally with each other.

[00074] As shown in Figure 4 and Figure 7, in an embodiment the radially inner portion 34 and the radially outer portion 35 may have different dimensions. For example, the radially inner portion 34 may have a greater dimension than the radially outer portion 35 in a direction along the source paths of the source beams. The radially inner portion 34 and the radially outer portion 35 may be arranged such that the spacers 33, 36 have a rim profile.

[00075] As shown in Figure 7, for example, in an embodiment the radially inner portion 34 is secured to the surfaces of two neighboring electron-optical plate elements 24-26 that are spaced by the spacer 33, 36. The radially outer portion 35 may be secured to the surface of only one of the two electron-optical plate elements 24-26 that the spacer 33, 36 is configured to space. As shown in Figure 7, in an embodiment the radially outer portion 35 is secured to only the upbeam electron- optical plate element of the two electron-optical plate elements that the spacer 33, 36 is configured to space. For example, the spacer 33 may have its radially outer portion 35 secured to only the suppressor 24. The radially outer portion 35 of the spacer 33 may be distanced from the extractor electrode 27 of the extractor arrangement 25. The spacer 36 may have a radially outer portion 35 that is secured to only the suppressor 24. The radially outer portion 35 of the spacer 36 may be distanced from the anode 26.

[00076] As shown in Figure 7, for example, in an embodiment the rim profile has a path length over a surface over the rim that is larger than a thickness of the spacer 33, 36. By providing the rim profile, the possibility of undesirable electrical breakdown between the spaced electron-optical plate elements may be reduced.

[00077] As shown in Figure 7, in an embodiment the rim profile comprises a step. The step provides an abrupt change in the dimension of the spacer 33, 36 in a direction across the source paths. However, it is not essential for the rim profile to have a step. In an alternative embodiment, the rim profile may comprise a gradually changing dimension of the spacer 33, 36 in a direction across the source paths. By providing that the rim profile comprises a step, the spacers 33, 36 may have a stepped rim.

[00078] In an embodiment the extractor arrangement 25 comprises an extractor electrode 27 common to a plurality of the source beams. For example, the extractor electrodes 27 shown in Figure 4 or Figure 6, for example, may be joined together to form a single integral plate. The resulting extractor electrode may be common to the plurality of source beams. By providing the extractor arrangement 25 with an extractor electrode 27 common to a plurality of the source beams, the total number of components of the electron-optical module 200 may be reduced. The construction of the electron-optical module 27 may be simplified.

[00079] In an embodiment the extractor arrangement 25 comprises a plurality of extractor electrodes 27, each extractor electrode 27 configured to operate on a selection of the plurality of source beams. For example, each extractor electrode may be configured to operate on a different plurality of the source beams. In an embodiment the extractor electrodes 27 are configured to operate on respective source beams. For example, the extractor electrodes may be configured to operate on different individual source beams. The extractor electrodes 27 may be controllable individually. For example, the controller 150 may be configured to control one or more power supplies to control the electric potential applied to each extractor electrode 27 individually. The electric potential supplied to the different extractor electrodes 27 may be controlled independently of each other.

[00080] Figure 5 schematically depicts a close-up view of a spacer 33. The spacer 33 shown in Figure 5 is a spacer 33 that is configured to space the suppressor 24 from an extractor electrode 27. However, the features shown in Figure 5 apply also to other types of spacer, for example a spacer 36 configured to space the suppressor 24 from the anode 26.

[00081] As shown in Figure 5, in an embodiment the spacer 33 comprises at least one interface surface 95. The interface surface 95 interfaces with one of the electron-optical plate elements 24-26. As shown in Figure 5, in an embodiment the interface surface 95 is planar.

[00082] In the arrangement shown in Figure 5, the spacer 33 comprises two interface surfaces 95. One of the interface surfaces 95 interfaces with the suppressor 24. The other interface surface 95 interfaces with the extractor electrode 27. In an alternative embodiment, the spacer 33 may comprise only one planar interface surface 95 that interfaces with one of the electron-optical plate elements 24- 26. The spacer 33 may be secured to the other electron-optical plate element via an alternative fixing mechanism, or by a non-planar surface, for example.

[00083] As shown in Figure 5, in an embodiment the spacer 33 is located wholly between the two electron-optical plate elements that it spaces. The spacer 33 is wholly downbeam of the suppressor 24. The spacer 33 is wholly upbeam of the extractor electrode 27.

[00084] By providing the planar interface surface 95, the spacer 33 may space the electron-optical plate elements from each other without unduly adding to the overall size of the electron-optical module 200. An embodiment of the invention is expected to provide a more compact electron-optical module 200. The interface surfaces 95 may form the most upbeam and/or most downbeam surfaces of the spacer 33.

[00085] As shown in Figure 5, in an embodiment a first electron-optical plate element of the two electron-optical plate elements that the spacer 33 is configured to space from each other is common to a plurality of the emitters 21. For example, in the arrangement shown in Figure 5, the suppressor 24 is common to the emitters 21.

[00086] As shown in Figure 5, in an embodiment a second electron-optical plate element of the two electron-optical plate elements is individual to one of the emitters 21. For example, in the arrangement shown in Figure 5, the extractor electrode 27 may be individual to one of the emitters 21. Alternatively, the extractor electrode 27 may correspond to a subset of the emitters 21, for example a group of the emitters 21 but not all of the emitters 21.

[00087] In an alternative embodiment, the second electron-optical plate element may be common to the same set of emitters 21 to which the first electron-optical plate element is common. For example, when the spacer 36 spaces the suppressor 24 from the anode 26, both the suppressor 24 and the anode 26 may be common to a plurality of (e.g. the same plurality of) the emitters 21. The plurality of the emitters 21 may be all of the emitters 21.

[00088] In the arrangement shown in Figure 5, the first electron-optical plate element may be upbeam of the spacer 33. In Figure 5, the upbeam direction is upwards. The second electron-optical plate element may be downbeam of the spacer 33. In Figure 5, the downbeam direction is downwards. As shown in Figure 7, in an embodiment the second electron-optical plate element (e.g. the extractor arrangement 25) is located between the first electron-optical plate element (e.g. the suppressor 24) and a third electron-optical plate element (e.g. the anode 26). The spacer 33 is configured to secure the location of an electron-optical plate element that is at an intermediate position within the stack 23.

[00089] As shown in Figure 7, in an embodiment the second electron-optical plate element (e.g. the extractor arrangement 25) and the third electron-optical plate element (e.g. the anode 26) are secured to each other only indirectly via at least one other of the plurality of electron-optical plate elements (e.g. the suppressor 24). In other words, as shown in Figure 7, there is not required to be a spacer between each pair of adjacent electron-optical plate elements in the stack 23. For example, it is not necessary to provide a spacer that spaces the extractor arrangement 25 from the anode 26. In an alternative embodiment, a further spacer is provided to space the extractor arrangement 25 from the anode 26.

[00090] By omitting a spacer between the extractor arrangement 25 and the anode 26, undesirable mechanical stress in the electron-optical module 200 may be reduced. For example, if spacers are provided between each pair of adjacent electron-optical plate elements of the stack 23, then a given electron-optical plate element may have its location determined by a plurality of different mechanical connections. For example, when a spacer spaces the extractor arrangement 25 from the anode 26, then the location of the anode 26 may be defined by spacers 33, 36 relative to two different components. The location of the anode 26 may be determined by the spacer 36 relative to the suppressor 24. The position of the anode 26 may also be determined by the further spacer relative to the extractor arrangement 25. This may cause the position of the anode 26 to be over-determined. This can cause undesirable stress in the electron-optical module 200. Any undesirable stress may cause undesirable deformation of the electron-optical module 200. An embodiment of the invention is expected to improve the reliability of the electron-optical module 200.

[00091] As shown in Figure 7, in an embodiment the first electron-optical plate element (e.g. the suppressor 24) and the third electron-optical plate element (e.g. the anode 26) are spaced from each other by a spacer 35.

[00092] In an embodiment, one of the electron-optical plate elements that the spacer 33 spaces from each other is configured to control a current density of the respective source beams individually. For example, one of the electron-optical plate elements may be the extractor arrangement 25. The extractor arrangement 25 may be configured to control the current density of the respective source beams individually. For example, the extractor arrangement 25 may comprise a plurality of extractor electrodes 27. Each extractor electrode 27 may be configured to control the current density of an individual source beam.

[00093] Figure 8 schematically depicts an electron-optical module according to an alternative embodiment. As shown in Figure 8, in an embodiment the electron-optical module comprises a single source 201. It is not essential for the electron-optical module to comprise a plurality of sources 201. [00094] As shown in Figure 8, in an embodiment the electron-optical module comprises a single emitter 21. The emitter 21 is configured to emit a source beam of electrons. As shown in Figure 8, in an embodiment the electron-optical module comprises a plurality of electron-optical plate elements 24-26.

[00095] Some of the features shown in Figure 8 are described elsewhere with reference to other figures. These features may be as described elsewhere. For example, the emitter 21, the suppressor 24, the extractor electrode 27, the anode 26, and the spacers 33, 36 may be as described with reference to Figures 4-7, for example.

[00096] As shown in Figure 8, the electron-optical module comprising a single source 201 may comprise a spacer 33, 36 comprising at least one interface surface (e.g. as described elsewhere with reference to Figure 5 particularly).

[00097] As shown in Figure 5, in an embodiment the interface surface 95 is substantially parallel to the electron-optical plate element with which the interface surface 95 interfaces. For example, the upbeam interface surface 95 may be substantially parallel to the suppressor 24. The downbeam interface surface 95 may be substantially parallel to the extractor electrode 27 or the anode 26. [00098] As shown in Figure 5, in an embodiment the spacer 33 comprises an insulator surface 94. The insulator surface 94 is between the two electron-optical plate elements that the spacers 33 spaces from each other. For example, in the arrangement shown in Figure 5, the insulator surface 94 is between the suppressor 24 and the extractor electrode 27 of the extractor arrangement 25. In an alternative embodiment, the insulator surface 94 of the spacer 36 may be between the suppressor 24 and the anode 26.

[00099] In an embodiment, at least part of the insulator surface 94 is angled relative to a direction perpendicular to the two electron-optical plate elements. In Figure 5, the direction perpendicular to the two electron-optical plate elements is the vertical direction. The insulator surface 94 shown in Figure 5 comprises a part that extends substantially parallel to the electron-optical plate elements. This is shown by the insulator surface 94 (i.e. the surface of the spacer 33 other than the interface surfaces 95) being represented by a horizontal line in Figure 5. In particular, Figure 5 shows a downbeam end of the radially outer portion 35 being substantially parallel to the electron-optical plate elements.

[000100] As shown in Figure 5, in an embodiment the insulator surface 94 comprises at least part that is in a direction perpendicular to the two electron-optical plate elements. For example, the radially outer edge of the radially outer portion 35 is substantially perpendicular to the two electron- optical plate elements. However, not all of the insulator surface 94 needs to be perpendicular to the electron-optical plate elements.

[0100] As shown in Figure 4, for example, in an embodiment the electron-optical module 200 comprises a plurality of spacers 33 arranged in a common plane. In an embodiment, each spacer 33 comprises at least one interface surface 95 that is planar.

[0101] As shown in Figures 4, 6, 7 and 8, for example, in an embodiment the spacers 33, 36 are distanced from the beam apertures 266. The beam apertures 266 are defined in the two electron- optical plate elements. In an embodiment the spacers 33, 36 are distanced from the beam apertures 266 by at least a threshold distance. By providing a minimum threshold distance, the possibility of electrons from the source beams undesirably charging a surface of the spacer 33, 36 may be reduced. [0102] As shown in Figure 6, for example, in an embodiment the spacers 33 are distanced from the beam apertures 266 by substantially the same distance. For example, Figure 6 shows that each beam aperture 266 has three associated spacers 33. Those three spacers 33 are located the same distance from the beam aperture 266. In an embodiment, the spacers 33 are arranged such that adjacent spacers 33 are distanced from each other by substantially the same distance.

[0103] As shown in Figure 4, for example, in an embodiment at least one of the electron-optical plate elements comprises a feature located between the beam aperture 266 and the spacer 33. The feature is configured to reduce electrons reaching the spacer 33 from the beam aperture 266. For example, the electron absorbers 41, 42 are examples of such a feature.

[0104] As shown in Figure 8, in an embodiment the electron-optical module 200 comprises a shield 37. The shield 37 may be a field shield. In an embodiment the shield 37 is configured to shield the emitter 21. For example, the shield 37 may be configured to shield the emitter 21 from external electromagnetic fields.

[0105] As shown in Figures 4, 6 and 7, in an embodiment a vent 46 is defined in one or more of the electron-optical plate elements 24-26. The vent 46 is separate from the beam apertures 266 defines in the electron-optical plate elements 24-26. In an embodiment, a plurality of vents 46 are defined in one or more of the electron-optical plate elements 24-26.

[0106] As shown in Figure 6, in an embodiment the vents 46 defined in different electron-optical plate elements 24-26 may be aligned with each other, for example in a direction along the source paths. For example, the three vents 46 shown closest to the centre of the stack 23 shown in Figure 6 are aligned in a direction along the source path in both the suppressor 24 and the anode 26. In an embodiment, one or more vents 46 are provided that extend only partway through the stack 23. For example, as shown in Figure 7 the vent 46 provided to the right hand side of the drawing is defined in the suppressor 24 but is not aligned with a corresponding vent in the anode 26. The vent may be referred to as a vent hole.

[0107] In an embodiment the vent 46 is configured to provide gas conductance through the electron-optical plate elements 24-26. It is not essential for each electron-optical plate element to have at least one vent 46 defined in it. For example, as shown in Figure 7, in an embodiment the extractor arrangement 25 has no vent defined in it. The extractor arrangement 25 may have beam apertures 266 defined in it. In an embodiment the beam apertures 266 are the only apertures or holes defined in the extractor electrodes 27 of the extractor arrangement 25. By providing a vent, the gas conductance through the stack 23 may be increased. The beam apertures 266 may provide some gas conductance through the stack 23 by providing the vents 46, the gas conductance may be increased compared to if only the beam apertures 266 were provided for gas conductance.

[0108] In an embodiment the gas conductance is for maintaining a vacuum within the electron- optical module 200. For example, in an embodiment the pressure within the electron-optical module 200 is maintained to be lower compared to the pressure in one or more volumes within the electron- optical apparatus 140 outside of the electron-optical module 200. It may be desirable to increase gas conductance through the stack 23 so as to enable the low pressure to be maintained.

[0109] Figure 9 schematically depicts part of an electron-optical apparatus 140. As shown in Figure 9, an embodiment the emitters 21 and the stack 23 of the electron-optical module 200 are located within a vacuum chamber 60. In an embodiment a vacuum apparatus 61 is provided for maintaining a vacuum for the emitters 21. In an embodiment the vacuum apparatus 61 comprises an ion pump. In an embodiment the vacuum apparatus comprises one or more getters. In an embodiment the vacuum apparatus 61 is configured to receive gas from a volume containing the emitters 21. The vacuum apparatus 61 may be configured to prevent gas from flowing back towards the emitters 21. In an embodiment magnetic shielding 62 is provided between the vacuum apparatus 61 and the emitters 21.

[0110] As shown in Figure 9, in an embodiment the electron-optical device 230 may comprise electron-optical elements downbeam of the sources 201. For example, in an embodiment one or more deflector arrays 235 and/or one or more collimator arrays 260 may be provided downbeam of the sources 201. In an embodiment the electron-optical elements are provided in different volumes from the volume in which the sources 201 are located. In an embodiment, different volumes are maintained at different pressures. In an embodiment an electron-optical element defines a border between two different volumes that are maintained at different pressures.

[0111] For example, in an embodiment the sources 201 comprising the emitters 21 and the stack 23 are located in a volume that is maintained at a vacuum pressure. In an embodiment the condenser array 231 defines a downbeam boundary of the volume containing the sources 201. In an embodiment the volume immediately downbeam of the condenser array 231 may be maintained at a different pressure. The condenser array 231 may define an upbeam boundary of the volume. As shown in Figure 9, in an embodiment a vacuum pump 63 and/or a valve (e.g. a revolver valve) defines a downbeam end of the volume.

[0112] In an embodiment the valve 64 defines an upbeam boundary of a volume containing the deflector array 235. The downbeam end of the volume may be defined by the collimator array 260. The volume containing the deflector array 235 may be maintained at a different pressure independently of the other volumes. In an embodiment the collimator array 260 defines an upbeam end of another volume. The downbeam end of the volume may be defined by the objective end array 234 and/or detector 240. The volume may be maintained at a different pressure, independently of the other volumes.

[0113] As shown in Figure 4, in an embodiment the electron-optical module 200 comprises a condenser array 231. In an embodiment a condenser array 231 is provided for each emitter 21. The condenser array 231 may be considered to be part of the electron-optical module 200. For example, the condenser array 231 may be secured to the stack 23 of the electron-optical module 200 during assembly of the electron-optical module 200. The condenser array 231 may be secured relative to the emitters 21 before the electron-optical module 200 is incorporated into an electron-optical device 230 or an electron-optical apparatus 140. Alternatively, the condenser array 231 may be omitted from the electron-optical module 200. A condenser array 231 of an electron-optical device 230 may be secured relative to the emitters 21 after the electron-optical module 200 has been incorporated into the electron-optical device 230 or electron-optical apparatus 140.

[0114] As shown in Figure 4, in an embodiment the electron-optical module 200 comprises a condenser frame 32. The condenser frame 32 is configured to secure the condenser array 231 to the stack 23. In an embodiment a plurality of condenser frames 32 are provided for securing respective condenser arrays 231 to the respective emitters 21.

[0115] As shown in Figure 4, in an embodiment the condenser array 231 is secured to a most downbeam element of the stack 23. For example, as shown in Figure 4 in an embodiment the most downbeam element of the stack 23 is the anode 26. In an embodiment the condenser frame 32 is configured to secure the anode 26 to the condenser array 231. For example, the condenser frame 32 may be secured directly to the anode 26 and to the condenser array 231. The condenser frame 32 is configured to fix the position of the condenser array 231 relative to the stack 23.

[0116] As shown in Figure 4, in an embodiment the electron-optical module 200 comprises one or more electron absorbers 41, 42. For example, electron absorbers 41 may be provided at a surface of the anode 26. For example, the electron absorbers 41 may be provided at an upbeam surface of the anode 26. In an embodiment, electron absorbers 42 are provided at a surface of the extractor arrangement 25. For example, the electron absorbers 42 may be located at a downbeam surface of the extractor arrangement 25.

[0117] As shown in Figure 4, in an embodiment an electron absorber 41 is provided at the anode 26 corresponding to each emitter 21. An electron absorber 42 may be provided at the extractor arrangement 25 for each emitter 21. In an embodiment the electron absorbers 41, 42 are formed to substantially surround the source path when viewed in plan view (i.e. when viewed along the source path). For example, the electron absorbers 41, 42 may form annuluses.

[0118] In an embodiment one or more electron absorbers are provided at an upbeam surface of the extractor arrangement. In an embodiment one or more electron absorbers are located at a downbeam surface of the suppressor 24.

[0119] In an embodiment the electron absorbers 41, 42 are configured to reduce the current of electrons in a radially outward direction from the source paths. It is not essential for the electron absorbers 41, 42 to be formed as annuluses. In an alternative arrangement, the electron absorbers 41, 42 may comprise a plurality of arcs, or a plurality of elongate sections when viewed in plan view (i.e. when viewed along the beam path).

[0120] As shown in Figure 4, in an embodiment recesses 49 may be provided at surfaces facing (or opposing) the electron absorbers 41, 42. For example, recesses 49 are provided in the downbeam surface of the extractor arrangement 25 opposite the electron absorbers 41. Similarly, recesses 49 are provided at the upbeam surface of the anode 26 to oppose the electron absorbers 42 provided at the downbeam surface of the extractor arrangement 25. By providing the recesses, the size of the gap between neighbouring electron-optical plate elements may be made more consistent in a direction across the beam paths. By making the size of the gap more consistent, the magnitude of the electric field may be more consistent. Undesirable peaks in the amplitude of the electromagnetic field may be reduced. By avoiding high peaks in the magnitude of the electromagnetic field, the possibility of undesirable electrical breakdown may be reduced.

[0121] By reducing the current of electrons radially outwards from the source paths, the possibility of parts of the source module 200 becoming undesirably charged may be reduced. For example, the possibility of the spacers 33, 36 becoming undesirably charged may be reduced. By reducing charging of components which has the spacers 33, 36, the possibility of undesirable electrical breakdown may be reduced.

[0122] As shown in Figure 4, for example, in an embodiment the elements of the electron-optical module 200 are aligned. For example, the elements of the electron-optical module 200 may be aligned before assembly. Alignment may be performed before the electron-optical module 200 is incorporated as part of an electron-optical apparatus 140 or an electron-optical device 230. A further alignment that may be performed to align the electron-optical module 200 relative to other electron- optical elements of an electron-optical device 230 or an electron-optical apparatus 140.

[0123] 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.

[0124] 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.

[0125] 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 respective source 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.

[0126] 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 such that it may be considered an electron-optical element.

[0127] Further embodiments according to the present invention are presented in below numbered clauses:

1. A charged particle-optical module for a charged particle-optical device configured to direct charged particle beams along respective beam paths towards a sample location, the charged particle- optical module comprising: a plurality of emitters configured to emit respective source beams of charged particles; a plurality of charged particle-optical plate elements configured to operate on the source beams and defining a plurality of beam apertures for passage of the source beams, wherein at least one of the charged particle-optical plate elements is common to a plurality of the emitters; and a spacer configured to space two of the charged particle-optical plate elements from each other, wherein the spacer comprises at least one interface surface that interfaces with one of the charged particle-optical plate elements, the interface surface being planar.

2. The charged particle-optical module of clause 1, wherein a first charged particle-optical plate element of the two charged particle-optical plate elements that the spacer is configured to space from each other is common to a plurality of the emitters.

3. The charged particle-optical module of clause 2, wherein a second charged particle-optical plate element of the two charged particle-optical plate elements is individual to one of the emitters.

4. The charged particle-optical module of clause 2, wherein a second charged particle-optical plate element of the two charged particle-optical plate elements is common to a subset of the emitters to which the first charged particle-optical plate element is common.

5. The charged particle-optical module of clause 3 or 4, wherein the second charged particle- optical plate element is located between the first charged particle-optical plate element and a third charged particle-optical plate element of the plurality of charged particle-optical plate elements.

6. The charged particle-optical module of clause 5, wherein the second charged particle-optical plate element and the third charged particle-optical plate element are secured to each other only indirectly via at least one other of the plurality of charged particle-optical plate elements.

7. The charged particle-optical module of clause 5 or 6, wherein the first charged particle-optical plate element and the third charged particle-optical plate element are spaced from each other by a spacer.

8. The charged particle-optical module of any preceding clause, wherein one of the charged particle-optical plate elements is configured to control a current density of the respective source beams individually. 9. A charged particle-optical module for a charged particle-optical device configured to direct charged particle beams along respective beam paths towards a sample location, the charged particle- optical module comprising: an emitter configured to emit a source beam of charged particles; a plurality of charged particle-optical plate elements configured to operate on the source beams and defining a plurality of beam apertures for passage of the source beam; and a spacer configured to space two of the charged particle-optical plate elements from each other, wherein the spacer comprises at least one interface surface that interfaces with one of the charged particle-optical plate elements, the interface surface being planar.

10. The charged particle-optical module of any preceding clause, wherein the interface surface is substantially parallel to the charged particle-optical plate element with which the interface surface interfaces.

11. The charged particle-optical module of any preceding clause, wherein the interface surface is at an upbeam end or a downbeam end of the spacer.

12. The charged particle-optical module of any preceding clause, wherein the spacer comprises two interface surfaces that interface with the two charged particle-optical plate elements that the spacer spaces from each other.

13. The charged particle-optical module of any preceding clause, wherein the spacer comprises an insulator surface between the two charged particle-optical plate elements that the spacer spaces from each other, wherein at least part of the insulator surface is angled relative to a direction perpendicular to the two charged particle-optical plate elements.

14. The charged particle-optical module of clause 13, wherein the insulator surface is stepped.

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

16. The charged particle-optical module of clause 15, wherein the spacer is substantially disc shaped.

17. The charged particle-optical module of any preceding clause, comprising a plurality of spacers, each spacer configured to space the same two charged particle-optical plate elements from each other and comprising at least one interface surface that interfaces with one of the charged particle-optical plate elements, the interface surface being planar.

18. The charged particle-optical module of clause 17, wherein the spacers are arranged in a common plane.

19. The charged particle-optical module of clause 17 or 18, wherein the spacers are distanced from the beam apertures defined in the two charged particle-optical plate elements by at least a threshold distance.

20. The charged particle-optical module of any of clauses 17-19, wherein the spacers are distanced from the beam apertures defined by the two charged particle-optical plate elements by substantially the same distance. 21. The charged particle-optical module of any of clauses 17-20, wherein the spacers are arranged such that adjacent spacers are distanced from each other by substantially the same distance.

22. The charged particle-optical module of any preceding clause, wherein a vent is defined in one or more of the charged particle-optical plate elements, the vent configured to provide gas conductance through the charged particle-optical plate elements, the gas conductance for maintaining a vacuum within the charged particle-optical module.

23. The charged particle-optical module of any preceding clause, wherein at least one of the charged particle-optical plate elements comprises a feature located between a beam aperture and the spacer configured to reduce charged particles reaching the spacer from the beam aperture.

24. The charged particle-optical module of any preceding clause, wherein the charged particle- optical plate elements comprise an anode.

25. The charged particle-optical module of any preceding clause, wherein the charged particle- optical plate elements comprise a suppressor.

26. The charged particle-optical module of any preceding clause, wherein the charged particle- optical plate elements comprise an extractor arrangement.

27. The charged particle-optical module of any preceding clause, wherein beam apertures in different charged particle-optical plate elements are aligned relative to a corresponding emitter.

28. At least one charged particle-optical device configured to direct charged particle beams along respective beam paths towards a sample location, the at least one charged particle-optical device comprising the charged particle-optical module of any preceding clause.

29. A charged particle-optical apparatus comprising: the charged particle-optical device of clause 28; and an actuatable stage configured to support a sample at the sample location.

30. An assessment apparatus for assessing a sample, comprising: a vacuum chamber; and the charged particle-optical apparatus of clause 29 in the vacuum chamber.

31. A method for operating a charged particle-optical module for a charged particle-optical device configured to direct charged particle beams along respective beam paths towards a sample location, the method comprising: emitting source beams of charged particles from respective emitters; and operating on the source beams with a plurality of charged particle-optical plate elements that define a plurality of beam apertures for passage of the source beams, wherein at least one of the charged particle-optical plate elements is common to a plurality of the emitters; wherein two of the charged particle-optical plate elements are spaced from each other by a spacer, wherein the spacer comprises at least one interface surface that interfaces with one of the charged particle-optical plate elements, the interface surface being planar. 32. A method for operating a charged particle-optical module for a charged particle-optical device configured to direct charged particle beams along respective beam paths towards a sample location, the method comprising: emitting a source beam of charged particles from an emitter; and operating on the source beams with a plurality of charged particle-optical plate elements that define a plurality of beam apertures for passage of the source beam; wherein two of the charged particle-optical plate elements are spaced from each other by a spacer, wherein the spacer comprises at least one interface surface that interfaces with one of the charged particle-optical plate elements, the interface surface being planar. [0128] 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). [0129] 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 that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

1. A charged particle-optical module for a charged particle-optical device configured to direct charged particle beams along respective beam paths towards a sample location, the charged particle- optical module comprising: an emitter configured to emit a source beam of charged particles; a plurality of charged particle-optical plate elements configured to operate on the source beams and defining a plurality of beam apertures for passage of the source beam; and a spacer configured to space two of the charged particle-optical plate elements from each other, wherein the spacer comprises at least one interface surface that interfaces with one of the charged particle-optical plate elements, the interface surface being planar.

2. The charged particle-optical module of any preceding claim, wherein the interface surface is substantially parallel to the charged particle-optical plate element with which the interface surface interfaces.

3. The charged particle-optical module of any preceding claim, wherein the interface surface is at an upbeam end or a downbeam end of the spacer.

4. The charged particle-optical module of any preceding claim, wherein the spacer comprises two interface surfaces that interface with the two charged particle-optical plate elements that the spacer spaces from each other.

5. The charged particle-optical module of any preceding claim, wherein the spacer comprises an insulator surface between the two charged particle-optical plate elements that the spacer spaces from each other, wherein at least part of the insulator surface is angled relative to a direction perpendicular to the two charged particle-optical plate elements.

6. The charged particle-optical module of claim 5, wherein the insulator surface is stepped.

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

8. The charged particle-optical module of claim 7, wherein the spacer is substantially disc shaped.

9. The charged particle-optical module of any preceding claim, comprising a plurality of spacers, each spacer configured to space the same two charged particle-optical plate elements from each other and comprising at least one interface surface that interfaces with one of the charged particle-optical plate elements, the interface surface being planar.

10. The charged particle-optical module of claim 9, wherein the spacers are arranged in a common plane.

11. The charged particle-optical module of claim 9 or 10, wherein the spacers are distanced from the beam apertures defined in the two charged particle-optical plate elements by at least a threshold distance.

12. The charged particle-optical module of any of claims 9 to 11, wherein the spacers are distanced from the beam apertures defined by the two charged particle-optical plate elements by substantially the same distance.

13. The charged particle-optical module of any of claims 9 to 12, wherein the spacers are arranged such that adjacent spacers are distanced from each other by substantially the same distance.

14. The charged particle-optical module of any preceding claim, wherein a vent is defined in one or more of the charged particle-optical plate elements, the vent configured to provide gas conductance through the charged particle-optical plate elements, the gas conductance for maintaining a vacuum within the charged particle-optical module.

15. The charged particle-optical module of any preceding claim, wherein at least one of the charged particle-optical plate elements comprises a feature located between a beam aperture and the spacer configured to reduce charged particles reaching the spacer from the beam aperture.