Patent Publication Number: US-2023154725-A1

Title: Emitter for emitting charged particles

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of International application PCT/EP2021/069785, which was filed on 15 Jul. 2021, which claims priority of EP application 20186333.9, which was filed on 16 Jul. 2020. These applications are incorporated herein by reference in their entireties. 
    
    
     FIELD 
     The embodiments provided herein generally relate the provision of an emitter configured to emit charged particles. Embodiments provide a charged particle source, an illumination apparatus, a charged particle beam tool and a charged particle beam inspection apparatus. Embodiments also provide a method for making an emitter and a method for emitting a beam of charged particles. 
     BACKGROUND 
     When manufacturing semiconductor integrated circuit (IC) chips, undesired pattern defects, as a consequence of, for example, optical effects and incidental particles, inevitably occur on a substrate (i.e. wafer) or a mask during the fabrication processes, thereby reducing the yield. Monitoring the extent of the undesired pattern defects is therefore an important process in the manufacture of IC chips. More generally, the inspection and/or measurement of a surface of a substrate, or other object/material, is an import process during and/or after its manufacture. 
     Pattern inspection tools with a charged particle beam have been used to inspect objects, for example to detect pattern defects. These tools typically use electron microscopy techniques, such as a scanning electron microscope (SEM). In a SEM, a primary electron beam of electrons at a relatively high energy is targeted with a final deceleration step in order to land on a sample at a relatively low landing energy. The beam of electrons is focused as a probing spot on the sample. The interactions between the material structure at the probing spot and the landing electrons from the beam of electrons cause electrons to be emitted from the surface, such as secondary electrons, backscattered electrons or Auger electrons. The generated secondary electrons may be emitted from the material structure of the sample. By scanning the primary electron beam as the probing spot over the sample surface, secondary electrons can be emitted across the surface of the sample. By collecting these emitted secondary electrons from the sample surface, a pattern inspection tool may obtain an image representing characteristics of the material structure of the surface of the sample. 
     Another application for a charged particle beam is lithography. The charged particle beam reacts with a resist layer on the surface of a substrate. A desired pattern in the resist can be created by controlling the locations on the resist layer that the charged particle beam is directed towards. A charged particle apparatus may be an apparatus for generating and/or projecting one or more beams of charged particles. 
     “A high-brightness large-diameter graphene coated point cathode field emission electron source” by Shao et al (Nature Communications, 9. 10.1038; 29 Mar. 2018; see also Supplementary Communication) discloses a few layer graphene-coated nickel wire cathode as the emitter for an electron source. 
     There is a general need for improving an emitter, for example to increase stability and/or total emitted current for a given brightness of a current of emitted charged particles and/or increase reproducibility of making the emitter. 
     SUMMARY 
     According to some embodiments of the present disclosure, there is provided an emitter configured to emit charged particles, the emitter comprising: a body having a point; a metal layer of a first metal on at least the point; and a charged particle source layer on the metal layer, wherein the point comprises a second metal other than the first metal. 
     According to some embodiments of the present disclosure, there is provided a method for making an emitter configured to emit charged particles, the method comprising: providing a body having a point; disposing a metal layer of a first metal on at least the point; and forming a charged particle source layer on the metal layer, wherein the body comprises a second metal other than the first metal. 
     According to some embodiments of the present disclosure, there is provided a method of emitting a beam of charged particles, the method comprising: providing an emitter configured to emit charged particles, the emitter comprising a charged particle source layer on a metal at a point of a body of the emitter; and heating the point to a temperature of greater than 500° C. so as to promote thermionic emission. 
     According to some embodiments of the present disclosure, there is provided an emitter configured to emit charged particles, the emitter comprising: a body having a point; a metal layer on at least the point; and a charged particle source layer on the metal layer, wherein the point comprises a different metal from the metal layer 
     Advantages will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and certain examples of the embodiments of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
       The above and other aspects of the present disclosure will become more apparent from the description of exemplary embodiments, taken in conjunction with the accompanying drawings. 
         FIG.  1    is a schematic diagram illustrating an exemplary charged particle beam inspection apparatus. 
         FIG.  2    is a schematic diagram illustrating an exemplary multi-beam apparatus that is part of the exemplary charged particle beam inspection apparatus of  FIG.  1   . 
         FIG.  3    is a schematic diagram of exemplary multi-beam apparatus illustrating an exemplary configuration of source conversion unit of the exemplary charged particle beam inspection apparatus of  FIG.  1   . 
         FIG.  4    is a schematic diagram of an emitter for charged particle emission according to some embodiments of the present disclosure. 
         FIG.  5    is a schematic diagram of a charged particle source according to some embodiments of the present disclosure. 
         FIGS.  6  to  8    are schematic diagrams showing different stages of a method of manufacturing an emitted structure according to some embodiments of the present disclosure. 
         FIG.  9    is a schematic diagram of restoring the graphene-based layer of the emitter according to some embodiments of the present disclosure. 
         FIG.  10    is a schematic diagram of an emitter according to some embodiments of the present disclosure. 
         FIG.  11    is a schematic diagram of an emitter with a suppressor according to some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings. The following description refers to the accompanying drawings in which the same numbers in different drawings represent the same or similar elements unless otherwise represented. The implementations set forth in the following description of exemplary embodiments do not represent all implementations. Instead, they are merely examples of apparatuses and methods consistent with aspects related to the invention as recited in the appended claims. 
     The reduction of the physical size of devices, and enhancement of the computing power of electronic devices may be accomplished by significantly increasing the packing density of circuit components such as transistors, capacitors, diodes, etc. on an IC chip. This has been enabled by increased resolution enabling yet smaller structures to be made. For example, an IC chip of a smart phone, which is the size of a thumbnail and available in, or earlier than, 2019, may include over 2 billion transistors, the size of each transistor being less than 1/1000th of a human hair. The semiconductor IC manufacturing is a complex and time-consuming process. Errors may cause device failure. The goal of the manufacturing process is to improve the overall yield of the process. For example, to obtain a 75% yield for a 50-step process (where a step may indicate the number of layers formed on a wafer), each individual step must have a yield greater than 99.4%. If an individual step has a yield of 95%, the overall process yield would be as low as 7-8%. 
     While high process yield is desirable in an IC chip manufacturing facility, maintaining a high substrate (i.e. wafer) throughput, defined as the number of substrates processed per hour, is also essential. High process yield and high substrate throughput may be impacted by the presence of a defect. This is especially if operator intervention is required for reviewing the defects. Thus, high throughput detection and identification of micro and nano-scale defects by inspection tools (such as a Scanning Electron Microscope (SEW)) is essential for maintaining high yield and low cost. 
     A SEM comprises a scanning device and a detector apparatus. The scanning device comprises an illumination apparatus that comprises an electron source, for generating primary electrons, and a projection apparatus for scanning a sample, such as a substrate, with one or more focused beams of primary electrons. The primary electrons interact with the sample and generate interaction products, such as secondary electrons and/or backscattered electrons. The detection apparatus captures the secondary electrons and/or backscattered electrons from the sample as the sample is scanned so that the SEM may create an image of the scanned area of the sample. For high throughput inspection, some of the inspection apparatuses use multiple focused beams, i.e. a multi-beam, of primary electrons. The component beams of the multi-beam may be referred to as sub-beams or beamlets. A multi-beam may scan different parts of a sample simultaneously. A multi-beam inspection apparatus may therefore inspect a sample at a much higher speed than a single-beam inspection apparatus. 
     In a multi-beam inspection apparatus, the paths of some of the primary electron beams are displaced away from the central axis, i.e. a mid-point of the primary electron-optical axis (also referred to herein as the charged particle axis), of the scanning device. To ensure all the electron beams arrive at the sample surface with substantially the same angle of incidence, sub-beam paths with a greater radial distance from the central axis need to be manipulated to move through a greater angle than the sub-beam paths with paths closer to the central axis. This stronger manipulation may cause aberrations that cause the resulting image to be blurry and out-of-focus. 
     An implementation of a known multi-beam inspection apparatus is described below. 
     The figures are schematic. Relative dimensions of components in drawings are therefore exaggerated for clarity. 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 throughout the present document may therefore be more generally be considered to be references to charged particles, with the charged particles not necessarily being electrons. 
     Reference is now made to  FIG.  1   , which is a schematic diagram illustrating an exemplary charged particle beam inspection apparatus  100 . The charged particle beam inspection apparatus  100  of  FIG.  1    includes a main chamber  10 , a load lock chamber  20 , an electron beam tool  40 , an equipment front end module (EFEM)  30  and a controller  50 . 
     EFEM  30  includes a first loading port  30   a  and a second loading port  30   b . EFEM  30  may include additional loading port(s). First loading port  30   a  and second loading port  30   b  may, for example, receive substrate front opening unified pods (FOUPs) that contain substrates (e.g., semiconductor substrates or substrates made of other material(s)) or samples to be inspected (substrates, wafers and samples are collectively referred to as “samples” hereafter). One or more robot arms (not shown) in EFEM  30  transport the samples to load lock chamber  20 . 
     Load lock chamber  20  is used to remove the gas around a sample. This creates a vacuum that is a local gas pressure lower than the pressure in the surrounding environment. The load lock chamber  20  may be connected to a load lock vacuum pump system (not shown), which removes gas particles in the load lock chamber  20 . The operation of the load lock vacuum pump system enables the load lock chamber to reach a first pressure below the atmospheric pressure. After reaching the first pressure, one or more robot arms (not shown) transport the sample from load lock chamber  20  to main chamber  10 . Main chamber  10  is connected to a main chamber vacuum pump system (not shown). The main chamber vacuum pump system removes gas molecules in main chamber  10  so that the pressure around the sample reaches a second pressure lower than the first pressure. After reaching the second pressure, the sample is transported to the electron beam tool by which it may be inspected. An electron beam tool  40  may comprise either a single beam or a multi-beam electron-optical apparatus. 
     Controller  50  is electronically connected to electron beam tool  40 . Controller  50  may be a processor (such as a computer) configured to control the charged particle beam inspection apparatus  100 . Controller  50  may also include a processing circuitry configured to execute various signal and image processing functions. While controller  50  is shown in  FIG.  1    as being outside of the structure that includes main chamber  10 , load lock chamber  20 , and EFEM  30 , it is appreciated that controller  50  may be part of the structure. The controller  50  may be located in one of the component elements of the charged particle beam inspection apparatus or it may be distributed over at least two of the component elements. While the present disclosure provides examples of main chamber  10  housing an electron beam inspection tool, it should be noted that aspects of the disclosure in their broadest sense are not limited to a chamber housing an electron beam inspection tool. Rather, it is appreciated that the foregoing principles may also be applied to other tools and other arrangements of apparatus, that operate under the second pressure. 
     Reference is now made to  FIG.  2   , which is a schematic diagram illustrating an exemplary electron beam tool  40  including a multi-beam inspection tool that is part of the exemplary charged particle beam inspection apparatus  100  of  FIG.  1   . Multi-beam electron beam tool  40  (also referred to herein as apparatus  40 ) comprises an electron source  201 , a gun aperture plate  271 , a condenser lens  210 , a source conversion unit  220 , a primary projection apparatus  230 , a motorized stage  209 , and a sample holder  207 . The electron source  201 , gun aperture plate  271 , condenser lens  210  and source conversion unit  220  are the components of an illumination apparatus comprised by the multi-beam electron beam tool  40 . The sample holder  207  is supported by motorized stage  209  so as to hold a sample  208  (e.g., a substrate or a mask) for inspection. Multi-beam electron beam tool  40  may further comprise a secondary projection apparatus  250  and an associated electron detection device  240 . Primary projection apparatus  230  may comprise an objective lens  231 . Electron detection device  240  may comprise a plurality of detection elements  241 ,  242 , and  243 . A beam separator  233  and a deflection scanning unit  232  may be positioned inside primary projection apparatus  230 . 
     The components that are used to generate a primary beam may be aligned with a primary electron-optical axis of the apparatus  40 . These components may include: the electron source  201 , gun aperture plate  271 , condenser lens  210 , source conversion unit  220 , beam separator  233 , deflection scanning unit  232 , and primary projection apparatus  230 . Secondary projection apparatus  250  and its associated electron detection device  240  may be aligned with a secondary electron-optical axis  251  of apparatus  40 . 
     The primary electron-optical axis  204  is comprised by the electron-optical axis of the of the part of electron beam tool  40  that is the illumination apparatus. The secondary electron-optical axis  251  is the electron-optical axis of the of the part of electron beam tool  40  that is a detection apparatus. The primary electron-optical axis  204  may also be referred to herein as the primary optical axis (to aid ease of reference) or charged particle optical axis. The secondary electron-optical axis  251  may also be referred to herein as the secondary optical axis or the secondary charged particle optical axis. 
     Electron source  201  may comprise a cathode (not shown) and an extractor or anode (not shown). During operation, electron source  201  is configured to emit electrons as primary electrons from the cathode. The primary electrons are extracted or accelerated by the extractor and/or the anode to form a primary electron beam  202  that forms a primary beam crossover (virtual or real)  203 . Primary electron beam  202  may be visualized as being emitted from primary beam crossover  203 . 
     The formed primary electron beam  202  may be a single beam and a multi-beam may be generated from the single beam. At different locations along the beam path, the primary electron beam  202  may therefore be either a single beam or a multi-beam. By the time it reaches the sample, and preferably before it reaches the projection apparatus, the primary electron beam  202  is a multi-beam. Such a multi-beam may be generated from the primary electron beam in a number of different ways. For example, the multi-beam may be generated by a multi-beam array located before the cross-over  203 , a multi-beam array located in the source conversion unit  220 , or a multi-beam array located at any point in between these locations. 
     Gun aperture plate  271 , in operation, is configured to block off peripheral electrons of primary electron beam  202  to reduce Coulomb effect. The Coulomb effect may enlarge the size of each of probe spots  221 ,  222 , and  223  of primary sub-beams  211 ,  212 ,  213 , and therefore deteriorate inspection resolution. A gun aperture plate  271  may also include multiple openings for generating primary sub-beams (not shown) even before the source conversion unit  220  and may be referred to as a coulomb aperture array. 
     Condenser lens  210  is configured to focus (or substantially collimate) primary electron beam  202 . In some embodiments, the condenser lens  210  may be designed to focus (or collimate) primary electron beam  202  to become a parallel beam and be substantially normally incident onto source conversion unit  220 . Condenser lens  210  may be a movable condenser lens that may be configured so that the position of its principle plane is movable. In some embodiments, the movable condenser lens may be configured to physically move, e.g. along the optical axis  204 . Alternatively, the movable condenser lens may be constituted of two or more electro-optical elements (lenses) in which the principle plane of the condenser lens moves with a variation of the strength of the individual electro-optical elements. The (movable) condenser lens may be configured to be magnetic, electrostatic or a combination of magnetic and electrostatic lenses. In a further example, the condenser lens  210  may be an anti-rotation condenser lens. The anti-rotation condenser lens may be configured to keep the rotation angles unchanged when the focusing power (collimating power) of condenser lens  210  is changed and/or when the principle plane of the condenser lens moves. 
     In some embodiments of the source conversion unit  220 , the source conversion unit  220  may comprise an image-forming element array, an aberration compensator array, a beam-limit aperture array, and a pre-bending micro-deflector array. The pre-bending micro-deflector array may, for example, be optional and may be present in some embodiments in which the condenser lens does not ensure substantially normal incidence of sub-beams originating from the coulomb aperture array onto e.g. the beam-limit aperture array, the image-forming element array, and/or the aberration compensator array. The image-forming element array may be configured to generate the plurality of sub-beams in the multi-beam path, i.e. primary sub-beams  211 ,  212 ,  213 . The image forming element array may, for example, comprise a plurality electron beam manipulators such as micro-deflectors micro-lenses (or a combination of both) to influence the plurality of primary sub-beams  211 ,  212 ,  213  of primary electron beam  202  and to form a plurality of parallel images (virtual or real) of primary beam crossover  203 , one for each of the primary sub-beams  211 ,  212 , and  213 . The aberration compensator array may, for example, comprise a field curvature compensator array (not shown) and an astigmatism compensator array (not shown). The field curvature compensator array may, for example, comprise a plurality of micro-lenses to compensate field curvature aberrations of the primary sub-beams  211 ,  212 , and  213 . The astigmatism compensator array may comprise a plurality of micro-stigmators to compensate astigmatism aberrations of the primary sub-beams  211 ,  212 , and  213 . The beam-limit aperture array may be configured to define the diameters of individual primary sub-beams  211 ,  212 , and  213 .  FIG.  2    shows three primary sub-beams  211 ,  212 , and  213  as an example, and it should be understood that source conversion unit  220  may be configured to form any number of primary sub-beams. Controller  50  may be connected to various parts of charged particle beam inspection apparatus  100  of  FIG.  1   , such as source conversion unit  220 , electron detection device  240 , primary projection apparatus  230 , or motorized stage  209 . As explained in further detail below, controller  50  may perform various image and signal processing functions. Controller  50  may also generate various control signals to govern operations of the charged particle beam inspection apparatus, including the charged particle multi-beam apparatus. 
     Condenser lens  210  may further be configured to adjust electric currents of primary sub-beams  211 ,  212 ,  213  down-beam of source conversion unit  220  by varying the focusing power (collimating power) of condenser lens  210 . Alternatively, or additionally, the electric currents of the primary sub-beams  211 ,  212 ,  213  may be changed by altering the radial sizes of beam-limit apertures within the beam-limit aperture array corresponding to the individual primary sub-beams. 
     Objective lens  231  may be configured to focus sub-beams  211 ,  212 , and  213  onto the sample  208  for inspection and, in the current example, may form three probe spots  221 ,  222 , and  223  on the surface of sample  208 . 
     Beam separator  233  may be, for example, a Wien filter comprising an electrostatic dipole field and a magnetic dipole field (not shown in  FIG.  2   ). 
     Deflection scanning unit  232 , in operation, is configured to deflect primary sub-beams  211 ,  212 , and  213  to scan probe spots  221 ,  222 , and  223  across individual scanning areas in a section of the surface of sample  208 . In response to incidence of primary sub-beams  211 ,  212 , and  213  or probe spots  221 ,  222 , and  223  on sample  208 , electrons are generated from the sample  208  which include secondary electrons and backscattered electrons. In the current example, the secondary electrons propagate in three secondary electron beams  261 ,  262 , and  263 . The secondary electron beams  261 ,  262 , and  263  typically have secondary electrons (having electron energy ≤50 eV) and may also have at least some of the backscattered electrons (having electron energy between 50 eV and the landing energy of primary sub-beams  211 ,  212 , and  213 ). The beam separator  233  is arranged to deflect the path of the secondary electron beams  261 ,  262 , and  263  towards the secondary projection apparatus  250 . The secondary projection apparatus  250  subsequently focuses the path of secondary electron beams  261 ,  262 , and  263  onto a plurality of detection regions  241 ,  242 , and  243  of electron detection device  240 . The detection regions may, for example, be the separate detection elements  241 ,  242 , and  243  that are arranged to detect corresponding secondary electron beams  261 ,  262 , and  263 . The detection regions may generate corresponding signals which are, for example, sent to controller  50  or a signal processing system (not shown), e.g. to construct images of the corresponding scanned areas of sample  208 . 
     The detection elements  241 ,  242 , and  243  may detect the corresponding secondary electron beams  261 ,  262 , and  263 . On incidence of secondary electron beams with the detection elements  241 ,  242  and  243 , the elements may generate corresponding intensity signal outputs (not shown). The outputs may be directed to an image processing system (e.g., controller  50 ). Each detection element  241 ,  242 , and  243  may comprise one or more pixels. The intensity signal output of a detection element may be a sum of signals generated by all the pixels within the detection element. 
     The controller  50  may comprise an image processing system that includes an image acquirer (not shown) and a storage device (not shown). For example, the controller may comprise a processor, computer, server, mainframe host, terminals, personal computer, any kind of mobile computing devices, and the like, or a combination thereof. The image acquirer may comprise at least part of the processing function of the controller. Thus the image acquirer may comprise at least one or more processors. The image acquirer may be communicatively coupled to an electron detection device  240  of the apparatus  40  permitting signal communication, such as an electrical conductor, optical fiber cable, portable storage media, IR, Bluetooth, internet, wireless network, wireless radio, among others, or a combination thereof. The image acquirer may receive a signal from electron detection device  240 , may process the data comprised in the signal and may construct an image therefrom. The image acquirer may thus acquire images of sample  208 . 
     The image acquirer may acquire one or more images of a sample based on an imaging signal received from the electron detection device  240 . An imaging signal may correspond to a scanning operation for conducting charged particle imaging. An acquired image may be a single image comprising a plurality of imaging areas. The single image may be stored in the storage. The single image may be an original image that may be divided into a plurality of regions. Each of the regions may comprise one imaging area containing a feature of sample  208 . The acquired images may comprise multiple images of a single imaging area of sample  208  sampled multiple times over a time period. The multiple images may be stored in the storage. The controller  50  may be configured to perform image processing steps with the multiple images of the same location of sample  208 . 
     The controller  50  may include measurement circuitry (e.g., analog-to-digital converters) to obtain a distribution of the detected secondary electrons. The electron distribution data, collected during a detection time window, may be used in combination with corresponding scan path data of each of primary sub-beams  211 ,  212 , and  213  incident on the sample surface, to reconstruct images of the sample structures under inspection. The reconstructed images may be used to reveal various features of the internal or external structures of sample  208 . The reconstructed images may thereby be used to reveal any defects that may exist in the sample. 
     The controller  50  may, e.g. further control the motorized stage  209  to move the sample  208  during, before or after inspection of the sample  208 . In some embodiments, the controller  50  may enable the motorized stage  209  to move sample  208  in a direction, e.g. continuously, for example at a constant speed, at least during sample inspection. The controller  50  may control movement of the motorized stage  209  so that the speed of the movement of the sample  208  changes, e.g. dependent on various parameters. For example, the controller may control the stage speed (including its direction) depending on the characteristics of the inspection steps of scanning process. 
     Although  FIG.  2    shows that apparatus  40  uses three primary electron sub-beams, it is appreciated that apparatus  40  may use two or more number of primary electron sub-beams. The present disclosure does not limit the number of primary electron beams used in apparatus  40 . 
     Reference is now made to  FIG.  3   , which is a schematic diagram of exemplary multi-beam apparatus illustrating an exemplary configuration of source conversion unit of the exemplary charged particle beam inspection apparatus of  FIG.  1   . The apparatus  300  may comprise an election source  301 , a pre-sub-beam-forming aperture array  372  (further also referred to as coulomb aperture array  372 ), a condenser lens  310  (similar to condenser lens  210  of  FIG.  2   ), a source conversion unit  320 , an objective lens  331  (similar to objective lens  231  of  FIG.  2   ), and a sample  308  (similar to sample  208  of  FIG.  2   ). The election source  301 , the coulomb aperture array  372 , the condenser lens  310  may be the components of an illumination apparatus comprised by the apparatus  300 . The source conversion unit  320  and objective lens  331  may be the components of a projection apparatus comprised by the apparatus  300 . The source conversion unit  320  may be similar to source conversion unit  220  of  FIG.  2    in which the image-forming element array of  FIG.  2    is image-forming element array  322 , the aberration compensator array of  FIG.  2    is aberration compensator array  324 , the beam-limit aperture array of  FIG.  2    is beam-limit aperture array  321 , and the pre-bending micro-deflector array of  FIG.  2    is pre-bending micro-deflector array  323 . The election source  301 , the coulomb aperture array  372 , the condenser lens  310 , the source conversion unit  320 , and the objective lens  331  are aligned with a primary electron-optical axis  304  of the apparatus. The electron source  301  generates a primary-electron beam  302  generally along the primary electron-optical axis  304  and with a source crossover (virtual or real)  301 S. The coulomb aperture array  372  cuts the peripheral electrons of primary electron beam  302  to reduce a consequential Coulomb effect. The primary-electron beam  302  may be trimmed into a specified number of sub-beams, such as three sub-beams  311 ,  312  and  313 , by the coulomb aperture array  372  of a pre-sub-beam-forming mechanism. Although three sub-beams and their paths are referred to in the previous and following description, it should be understood that the description is intended to apply an apparatus, tool, or system with any number of sub-beams. 
     The source conversion unit  320  may include a beamlet-limit aperture array  321  with beam-limit apertures configured to define the outer dimensions of the sub-beams  311 ,  312 , and  313  of the primary electron beam  302 . The source conversion unit  320  may also include an image-forming element array  322  with image-forming micro-deflectors,  322 _ 1 ,  322 _ 2 , and  322 _ 3 . There is a respective micro-deflector associated with the path of each sub-beam. The micro-deflectors  322 _ 1 ,  322 _ 2 , and  322 _ 3  are configured to deflect the paths of the sub-beams  311 ,  312 , and  313  towards the electron-optical axis  304 . The deflected sub-beams  311 ,  312  and  313  form virtual images (not shown) of source crossover  301 S. In the current example, these virtual images are projected onto the sample  308  by the objective lens  331  and form probe spots thereon, which are the three probe spots,  391 ,  392 , and  393 . Each probe spot corresponds to the location of incidence of a sub-beam path on the sample surface. The source conversion unit  320  may further comprise an aberration compensator array  324  configured to compensate aberrations that may be present in each of the sub-beams. The aberration compensator array  324  may, for example, include a field curvature compensator array (not shown) with micro-lenses. The field curvature compensator and micro-lenses may, for example, be configured to compensate the individual sub-beams for field curvature aberrations evident in the probe spots,  391 ,  392 , and  393 . The aberration compensator array  324  may include an astigmatism compensator array (not shown) with micro-stigmators. The micro-stigmators may, for example, be controlled to operate on the sub-beams to compensate astigmatism aberrations that are otherwise present in the probe spots,  391 ,  392 , and  393 . 
     The source conversion unit  320  may further comprise a pre-bending micro-deflector array  323  with pre-bending micro-deflectors  323 _ 1 ,  323 _ 2 , and  323 _ 3  to bend the sub-beams  311 ,  312 , and  313  respectively. The pre-bending micro-deflectors  323 _ 1 ,  3232 , and  323 _ 3  may bend the path of the sub-beams onto the beamlet-limit aperture array  321 . 
     The image-forming element array  322 , the aberration compensator array  324 , and the pre-bending micro-deflector array  323  may comprise multiple layers of sub-beam manipulating devices, some of which may be in the form or arrays, for example: micro-deflectors, micro-lenses, or micro-stigmators. 
     In the current example of the source conversion unit  320 , the sub-beams  311 ,  312  and  313  of the primary electron beam  302  are respectively deflected by the micro-deflectors  322 _ 1 ,  322 _ 2  and  322 _ 3  of image-forming element array  322  towards the primary electron-optical axis  304 . It should be understood that the sub-beam  311  path may already correspond to the electron-optical axis  304  prior to reaching micro-deflector  322 _ 1 , accordingly the sub-beam  311  path may not be deflected by micro-deflector  322 _ 1 . 
     The objective lens  331  focuses the sub-beams onto the surface of the sample  308 , i.e., it projects the three virtual images onto the sample surface. The three images formed by three sub-beams  311  to  313  on the sample surface form three probe spots  391 ,  392  and  393  thereon. In some embodiments, the deflection angles of sub-beams  311  to  313  are adjusted to pass through or approach the front focal point of objective lens  331  to reduce or limit the off-axis aberrations of three probe spots  391  to  393 . 
     In some embodiments of a multi-beam inspection tool  300  as shown in  FIG.  3    the beam path of the secondary electrons, beam separator (similar as Wien filter  233 ), secondary projection optics (similar as secondary projection optics  250  of  FIG.  2   ) and electron detection device (similar as electron detection device  240 ) have been omitted for clarity reasons. Is should be clear however that similar beam separator, secondary projection optics and electron detection device may be present in the current example of  FIG.  3    to register and generate an image of the sample surface using the secondary electrons or backscattered electrons. 
     At least some of the above-described components in  FIG.  2    and  FIG.  3    may individually, or in combination with each other, be referred to as a manipulator array, or manipulator, because they manipulate one or more beams, or sub-beams, of charged particles. 
     The above described embodiments of multi-beam inspection tools comprise a multi-beam charged particle apparatus, that may be referred to as a multi-beam charged particle optical apparatus, with a single source of charged particles. The multi-beam charged particle apparatus comprises an illumination apparatus and a projection apparatus. The illumination apparatus may generate a multi-beam of charged particles from the beam of electrons from the source. The projection apparatus projects a multi-beam of charged particles towards a sample. At least part of the surface of a sample may be scanned with the multi-beam of charged particles. 
     A multi-beam charged particle apparatus comprises one or more electron-optical devices for manipulating the sub-beams of the multi-beam of charged particles. The applied manipulation may be, for example, a deflection of the paths of sub-beams and/or a focusing operation applied to the sub-beams. The one or more electron-optical devices may comprise MEMS (Micro-Electro-Mechanical Systems). 
     The charged particle apparatus may comprise beam path manipulators located up-beam of the electron-optical device and, optionally, in the electron-optical device. Beam paths may be manipulated linearly in directions orthogonal to the charged particle axis, i.e. optical axis, by, for example, two electrostatic deflector sets operating across the whole beam. The two electrostatic deflector sets may be configured to deflect the beam path in orthogonal directions. Each electrostatic deflector set may comprise two electrostatic deflectors located sequentially along the beam path. The first electrostatic deflector of each set applies a correcting deflection, and the second electrostatic deflector restores the beam to the correct angle of incidence on the electron-optical device. The correcting deflection applied by the first electrostatic deflector may be an over correction so that the second electrostatic deflector can apply a deflection for ensuring the desired angle of incidence to the MEMS. The location of the electrostatic deflector sets could be at a number of locations up-beam of the electron-optical device. Beam paths may be manipulated rotationally. Rotational corrections may be applied by a magnetic lens. Rotational corrections may additionally, or alternatively, be achieved by an existing magnetic lens such as the condenser lens arrangement. 
     In some embodiments, a charged particle apparatus may comprise alternative and/or additional components on the charged particle path, such as lenses and other components some of which have been described earlier with reference to  FIGS.  1  to  3   . In particular, embodiments include a charged particle projection apparatus that divides a charged particle beam from a source into a plurality of sub-beams. A plurality of respective objective lenses may project the sub-beams onto a sample. In some embodiments, a plurality of condenser lenses is provided up-beam from the objective lenses. The condenser lenses focus each of the sub-beams to an intermediate focus up-beam of the objective lenses. In some embodiments, collimators are provided up-beam from the objective lenses. Correctors may be provided to reduce focus error and/or aberrations. In some embodiments, such correctors are integrated into or positioned directly adjacent to the objective lenses. Where condenser lenses are provided, such correctors may additionally, or alternatively, be integrated into, or positioned directly adjacent to, the condenser lenses and/or positioned in, or directly adjacent to, the intermediate foci. A detector is provided to detect charged particles emitted by the sample. The detector may be integrated into the objective lens. The detector may be on the bottom surface of the objective lens so as to face a sample in use. The condenser lenses, objective lenses and/or detector may be formed as MEMS or CMOS devices. 
       FIG.  4    is a schematic diagram of an emitter  60  according to some embodiments of the present disclosure. The emitter  60  is configured to emit charged particles. In some embodiments, the emitter  60  is part of a charged particle source  201 ,  301 . For example, in some embodiments, the emitter  60  is part of a charged particle source  201 ,  301  of an illumination apparatus of a charged particle beam tool such as the electron beam tool  40  shown in  FIG.  2    or  FIG.  3   . In an alternative example, the emitter  60  is part of an apparatus for lithography, such as a source. 
     As shown in  FIG.  4   , in some embodiments the emitter  60  comprises a body  61 . The body  61  has a point  62 . The point  62  may be a tip. In some embodiments, the body  61  has an elongate shape with one end of the elongate shape being pointed. The elongate body may be cylindrical, for example with a conical end that is the point  62 . The point  62  forms the pointed end of the body  61 . The elongate shape may have an axis for example substantially aligned with the point. The elongate shape may have a surface which is curved around the axis. The point may have a surface that is curved for example with respect to the axis of the elongate shape. In some embodiments, the body  61  defines the shape of the surface from which charged particles are emitted. The shape of the body  61  contributes to defining the direction in which charged particles are emitted from the emitter  60 . The cross-sectional shape of the body  61  is not limited to the shapes described here. The body  61  may have other cross-sectional shapes known to the skilled person. The cross-sectional shape may be of any shape. In some embodiments, the cross-sectional shape is approximately circular, for example with one side. However, other shapes for example with at least one corner, or two or more sides or both are also possible, for example hexagonal. The body may comprise metal. 
     As shown in  FIG.  4   , in some embodiments, the emitter  60  comprises a metal layer  63 . The metal layer  63  comprises a metal, referred to as a first metal. The metal layer  63  is on at least the point  62  of the body  61 . The metal layer  63  covers the point of the body  61 . As shown in  FIG.  4   , in some embodiments, the metal layer  63  is disposed along the sides of the elongate shape of the body  61 . However, in an alternative example the metal layer  63  is provided on a smaller portion of the surface of the body, for example the portion being curved around the axis or comprising at least part of two sides of the surface around the axis. In some embodiments, the metal layer  63  is provided on the point  62 . Such a metal layer covering curved surface or at least part of two surfaces beneficially has increased rigidity and stability in view of stresses applied within the metal layer  63  during use. The rigidity of the metal layer on the body is enabled by having between the metal layer  63  to the body being a metal to metal interface. The metal layer  63  may have a thickens of for example 200 nm as described later herein. The sides of the body  61  may define the surface of the emitter  60 ; that is the side of the body may not be covered by the metal layer  63 . As shown in  FIG.  4   , in some embodiments, the end of the body  61  opposite the point  62  defines the surface; that is the surface is not provided with the metal layer  63 . In an alternative example, the metal layer  63  is additionally provided at the end of the body  61  opposite to the point  62 . The body  61  may be completely covered by the metal layer  63 . 
     As shown in  FIG.  4   , in some embodiments, the emitter  60  comprises a charged particle source layer  64 . The charged particle source layer  64  is the source of the charged particles. The charged particle source layer  64  is on the metal layer  63 . As shown in  FIG.  4   , in some embodiments, the charged particle source layer  64  is on the metal layer  63  at least at the point  62  of the body  61 . In some embodiments, the charged particle source layer  64  forms the surface from which charged particles are emitted from the emitter  60 . As the charged particle source layer  64  is a layer on the metal layer  63 , the charged particle source layer  64  takes the shape and form and geometry of the underlying metal layer  63 . Such a charged particle source layer  64  covering a curved surface of, or at least part of two surfaces of, the metal layer  63 , and thus of the elongate body  61 . The metal layer  63  and the charged particle source layer  64  may be concentric layers on the curved surface of the elongate body  61 . 
     In some embodiments, the charged particle source layer  64  comprises a material such that the charged particle source layer  64  exhibits a relatively low work function. That is the material, when providing the charged particle source layer  64  may have a relatively low work function. Such a charged particle source layer  64  comprises a graphene-based layer. Graphene as used, i.e. selected, in embodiments has a relatively low work function. Graphene typically has a work function of for example 4.5 eV, but when graphene is used as the charged particle source layer the work function of the graphene, for example at the surface of the charged particle source layer  64 , is lower. The work function is the minimum thermodynamic work needed to remove a charged particle from a solid to a point in the vacuum immediately outside the solid surface. In some embodiments, the charged particle source layer  64  is configured to provide chemical and mechanical protection for the emitter  60 . Graphene has a relatively high mechanical strength combined with electrical conductivity and thermal stability. 
     Such a charged particle source layer  64  covering a curved surface of, or at least part of two surfaces of, the metal layer  63  has been noted to, increased rigidity and stability beneficially in view of stresses applied within the metal layer  63  during use. Such improved rigidity would enable such a coating to maintain its shape and form, so it can withstand more elevated temperatures. In a structure with a geometry having curved or angled surfaces, the geometry of the structure enables the structure to withstand greater stresses than otherwise, within the structure of the emitter, for example in the charged particle source layer  64 , in the metal layer  63  and/or between these layers and the body  61 . 
     However, it is not essential for the charged particle source layer  64  to comprise a graphene-based layer. In some embodiments, the charged particle source layer  64  comprises a structured material. The structured material may be based on graphene. In some embodiments, the charged particle source layer  64  comprises a two-dimensional compound having metallic atoms in its lattice. 
     In some embodiments, the body comprises metal, referred to a as ‘a second metal’. Preferably at least the point  62  comprises the second metal. The second metal is other than the first metal of the metal layer  63 . The two metals may have different physical properties. In some embodiments, the second metal has a higher melting point than the first metal of the metal layer  63 . The metal of the point  62  is configured to conduct heat to the emitting surface formed by the charged particle source layer  64  on the metal layer  63 . The point  62  can be heated to relatively high temperatures, for example above 500° C. 
     By providing that the emitter  60  is suitable to be heated to high temperatures, contaminants at the surface of the emitter  60  can be removed by the heating. By removing contaminants, the current of emitted charged particles is more stable. 
     In some embodiments, there is provided a method of emitting a beam of charged particles. The emitter  60  comprises the charged particle source layer  64  on a metal at the point  62  of the body  61  of the emitter  60 . In some embodiments, the method of generating the beam of charged particles comprises heating the point  62  to a temperature of greater than 500° C. so as to promote thermionic emission. The heat conducted from the point  62  to the charged particle source layer  64  provides the energy for the charged particles to overcome the work function of the charged particle source layer  64 . In some embodiments, the point  62  is heated to a temperature of at least 700° C., preferably of at least 900° C., optionally at least 1000° C., at least 1100° C., optionally at least 1500° C. and optionally at least 2000° C. The inventor recognizes that graphene is stable at such elevated temperatures. At such elevated temperatures, graphene with a stable performance may have a low rate of generation of lattice irregularities (i.e. defects) by temperature dependent lattice vibrations. (See for example: “ Graphene, a material for high temperature devices—Intrinsic carrier density, carrier drift velocity, and lattice energy ”. Yin, Yan &amp; Cheng, Zengguang &amp; Wang, Li &amp; Jin, Kui-Juan &amp; Wang, Wenzhong. (2014). Scientific reports. 4. 5758. 10.1038; or “ High - Field Electrical and Thermal Transport in Suspended Graphene ” Dorgan, Vincent &amp; Behnam, Ashkan &amp; Conley, Hiram &amp; Bolotin, Kirill &amp; Pop, Eric. (2013). Nano letters. 13. 10.1021). 
     By providing that the second metal of the point  62  is different from the first metal of the metal layer  63 , the metals can be selected to have different properties. For example, the second metal may be selected to be mechanically stable at high temperatures. The first metal may be selected for its properties in helping formation of the charged particle source layer  64 . Alloying may occur between the metals of the metal layer  63  and the second metal because diffusion may occur at the interface between the metals, for example at an elevated temperature. Such alloying may increase the strength between of the bonding the metal layer and the second metal 
     By providing that the second metal of the point  62  has a higher melting point than the first metal of the metal layer  63 , the emitter  60  can be used for thermionic emission without unduly deteriorating the emitter  60 . The point  62  can be heated to relatively high temperatures without the point  62  of the body  61  deforming (e.g. flattening). The point  62  can be heated to relatively high temperatures while maintaining the directionality of the emitted beam of charged particles. As noted, the geometry of the body  61  of the emitter  60 , for example, may help to ensure the structural integrity of the emitter at such temperatures, for example, in having an elongate shape with a curved surface and/or multiple edges around the axis of the elongate shape and, for example, the point. 
     By providing that the emitter  60  is suitable for thermionic emission, the stability of the current of emitted charged particles is greater. The current stability for charged particles is greater for thermionic emission than for cold field emission. Some embodiments of the present disclosure are expected to improve current stability of the beam of charged particles For example, improved current stability through thermo-ionic emission may be achieved by operating the emitter  60  at temperatures of at least 700° C., preferably of at least 900° C., optionally at least 1000° C., at least 1100° C., optionally at least 1500° C. Some embodiments of the present disclosure are expected to achieve improved current stability while maintaining directionality of the beam of emitted charged particles. 
     In some embodiments, the metal of the point  62  is suitable for being heated to high temperatures without unduly deteriorating. For example, the metal of the point  62  can be heated without cracking or shattering. As noted herein, the geometry of the point  62  (for example as being part of the body  61  with an elongate shape) may help to maintain the structural integrity of the point when exposed to internal stresses, such as thermally induced stress such as a high temperatures. The metal of the point  62  has high thermal conductivity. Materials having an appropriately high mechanical stability at high temperatures can be used for the body  61  and point  62 . In some embodiments, the body  61  comprises a refractory metal. A refractory metal is a heat-resistant metal. In some embodiments, the body  61  comprises a refractory metal, namely a metal selected from the group consisting of tungsten, molybdenum, tantalum, niobium, rhenium, titanium, vanadium, chromium, zirconium, hafnium, ruthenium, rhodium, osmium and iridium. In some embodiments, the body  61  comprises a metal selected from the group consisting of tungsten, molybdenum, tantalum, niobium and rhenium. 
     In some embodiments, the point  62  comprises tungsten. In some embodiments, the body  61  is made of tungsten. Bodies made of tungsten are relatively cheap and readily available. Some embodiments of the present disclosure are expected to improve the emission current stability without unduly increasing the cost of manufacturing the emitter  60 . In an alternative example, the point  62  comprises molybdenum. In some embodiments, the body  61  is made of molybdenum. A metal such as tungsten or molybdenum is thermally more stable than other material such as silicon. A point comprising a metal such as tungsten or molybdenum is structurally stable at a suitable elevated temperature. Such metals have a high melting point temperature; tungsten has a melting point of 3422 degrees Celsius and molybdenum has a melting point of 2623 degrees Celsius. For example, silicon may shatter when heated to the temperatures required for significant thermionic emission. Silicon has a melting point of 1414 degrees Celsius. Silicon has a melting point which is relatively lower than a material such as tungsten. 
     In some embodiments, the material of the point  62  has a lower remanence than the metal of the metal layer  63 . When charged particles are emitted from the emitter  60 , remnant magnetism may be caused in the emitter  60 . For example, the point  62  may have a remnant magnetism as a result of the charged particle emission. Other components of the apparatus in which the emitter  60  is used, e.g. lenses, may cause a remnant magnetism in the emitter  60 . 
     By providing that the material of the point  62  has a lower remanence than the metal of the metal layer  63 , the potential remnant magnetism in the point  62  is reduced and/or limited. By reducing the potential remnant magnetism of the point  62 , the effect of magnetism on the direction of the emitted beam of charged particles is reduced. Some embodiments of the present disclosure are expected to improve accuracy of the direction of the emitted beam of charged particles. 
     For example, in some embodiments, the target direction for the beam of charged particles is along the axis of the elongate body  62 . Any remnant magnetism in the point  62  can undesirably change the direction away from the axial direction. Some embodiments of the present disclosure are expected to decrease deviation of the emitted beam of charged particles away from the axial direction. 
     In some embodiments, the metal layer  63  comprises nickel. By providing that the charged particle source layer  64  is on metal, the work function of the charged particle source layer  64  is reduced. By providing that the charged particle source layer  64  is on a layer of nickel, the work function of the charged particle source layer  64  is reduced. The stability of the emitted current of charged particles is increased. For example, the work function value for graphene-coated nickel has been calculated and measured to be 1.1 eV for example under certain conditions of a high electric field. It is noted that the typical work function of Nickel is 5.5 eV. The work function of graphene is typically 4.5 eV. The point  62  of the emitter operating under other conditions may have a work function of around a 3.4 eV for example without application of an electric field. (See for example “ Doping Graphene with Metal Contacts ” Giovannetti, G &amp; Khomyakov, P A &amp; Brocks, G &amp; Karpan, Volodimir &amp; Brink, Jeroen &amp; Kelly, Paul. (2008). Physical review letters. 101. 026803. 10.1103/PhysRevLett.101.026803.) 
     Since Nickel has a melting point of 1450° C., the operating temperature of the emitter may be less than the melting point temperature of nickel and preferably more than 900° C. The emitter may thus operate as a thermionic emitter with structural integrity up to at least 1400° C., at least without the point melting. (It is noted that the second metal comprised in the point  62  is beneficial because the second metal has a much higher melting point than the metal of the metal layer  63 ; unlike Silicon which has a melting point even lower than nickel). 
     In some embodiments, the charged particle source layer  64  is doped by the first metal of the metal layer  63 . For example, when the metal layer  63  is nickel and the charged particle source layer  64  is a graphene-based layer, the graphene-based layer may be doped by the nickel. In another arrangement, the carbon may be used to dope the nickel of the first metal (i.e. in the metal layer  63 ) so as to generate the charged particle source layer, such as the graphene based layer. In some embodiments, the work function of the charged particle source layer  64  is reduced by n-type doping of graphene due to chemisorption on nickel. 
     In some embodiments, there is provided a method for making an emitter  60  configured to emit charged particles. The method comprises providing the body  61  having the point  62 , disposing the metal layer  63  on at least the point  62 , and forming the charged particle source layer  64  on the metal layer  63 . In some embodiments, the charged particle source layer  64  is formed using a chemical vapor deposition (CVD) method. In some embodiments, a solid carbon source poly(methyl methacrylate) (PMMA) is used as feedstock for the CVD method. Such a feedstock for the CVD method may be a source of carbon for example for generation of graphene on the metal layer. By using a CVD method, the charged particle source layer  64  can be formed while avoiding extremely high temperatures which could otherwise deform the point  62 . In some embodiments, the body  61  with the metal layer  63  disposed thereon is placed in a ceramic holder in a furnace such as a tube furnace. An Al 2 O 3  boat loaded with the feedstock is placed at the inlet slide of the tube (e.g. quartz tube), just outside of the heating zone. In some embodiments, the body  61  is heated to, for example 800 to 900° C. In some embodiments, the feed source is heated to about 150° C. The furnace is opened for the formation of the charged particle source layer  64  on the metal layer  63 . In some embodiments, the metal layer  63  comprises nickel that catalyzes formation of the charged particle source layer  64  on the metal layer  63 . The charged particle source layer  64  may be 20 nm or less, for example 10 nm or less, 5 nm or less or 2 nm or less. The charged particle source layer  64  should cover the metal layer for example with carbon. The quantity of material to form the charged particle source layer  64  may be sufficient to have a thickness to provide a sufficient lifetime. In the case of the charged particle source layer  64  comprising carbon, the applied carbon to the metal layer  63  may be sufficient, for example a number of layers thick, such as several layers or even a few layers thick, so that in use the carbon diffuses towards the surface of the charged particle source layer  64  so as to prolong the lifetime of the charged particle source layer and thus the emitter  60 . 
     However, it is not essential for the metal layer  63  to comprise nickel. Other metals may be used, for example palladium, copper, silver, cobalt, iridium or platinum. Such metals can catalyze formation of a charged particle source layer  64  such as a graphene-based layer. Such metals can reduce the work function of the charged particle source layer  64 . Chemisorption on nickel, cobalt and palladium has been found to significantly reduce the work function of graphene. For example, with a graphene coating, cobalt has a work function reduced from 5.4 to 3.8 eV, palladium has reduced work function from 5.7 to 4.0 eV and platinum has a reduced work function of 4.2 to 4.0 eV; (see for example “ Doping Graphene with Metal Contacts ” Giovannetti, G &amp; Khomyakov, P A &amp; Brocks, G &amp; Karpan, Volodimir &amp; Brink, Jeroen &amp; Kelly, Paul. (2008). Physical review letters. 101. 026803. 10.1103/PhysRevLett.101.026803.) Weaker adsorption on copper and silver has been found to slightly decrease the work function of graphene. Some of these metals, such as cobalt, palladium and platinum may be suited for use as the metal layer  63  of an emitter that is operated as a thermionic emitter. The melting point of cobalt, palladium and platinum is, respectively, 1495° C. 1550° C. and 1700° C. Such metals may be suited for use as the metal layer  63  in the emitter operated at such elevated temperatures for example without risk to the structural integrity of the emitter, for example by melting. 
     In some embodiments, the metal layer  63  has a thickness of at most 500 nm, optionally at most 200 nm, optionally at most 100 nm, optionally at most 50 nm, optionally at most 20 nm, optionally at most 10 nm, and optionally at most 5 nm. The metal layer may have a thickness in the range of 20 to 100 nm, preferably 30 to 80 nm and more preferably 40 to 60 nm. By providing that the metal layer  63  is relatively thin, the volume of the metal layer  63  is reduced and/or limited. A thinner layer may help ensure the sharpness of the point  62 ; a point  62  that is too blunt is likely to have reduced performance. The potential remnant magnetism of the metal layer  63  is reduced. For example the potential reduction in the remnant magnetism may be achieved by way of the second metal within the point  62 . Any adverse effect of remnant magnetism in the metal layer  63  on the direction of the emitted beam of charged particles is reduced. In some embodiments, the metal layer  63  is disposed on at least the point  62  by a thin film deposition method. Other methods known to the skilled person may be used for depositing the metal layer  63 . 
     As shown schematically in  FIG.  5   , in some embodiments, a charged particle source  201  comprises the emitter  60  and an electric field generator  70 . Details of the electric field generator  70  are not shown in  FIG.  5   . The electric field generator  70  is configured to generate an electric field relative to the emitter, for example at the point  62 . In some embodiments, the electric field generator  70  comprises at least one electric component and/or a power supply. Operation of the emitter  60  in a high electric field has been found to lower the work function. For example, the work function value for graphene-coated nickel of 1.1 eV was determined using such a high field. The electric field lowers the work function of the charged particle source layer  64  at the point  62  of the emitter  60 . That is, operation of the emitter  60  in the high electric field lowers the work function of the point  62  relative to the typical work functions of the metal layer  62  and the charged particle source layer  64 . A suitable electric field may in the range of 100 to 200 kV/mm; preferably 200 kV/mm which may desirably be an upper limit. Such a high field may be achieved through local field enhancement of the point  62 . Operation of such emitter without such a high electric field has been found to have a lower work function, but not to the extent under application of a high electric field; for example 3.7 eV for a graphene as a charged particle source layer  63  on a metal layer consisting of nickel. 
     By providing an electric field at the point  62 , cold field emission is promoted. By providing an electric field at the point  62 , thermionic emission (or ‘high thermal field emission’) is promoted. In some embodiments, when the electric field is generated at the point  62  and the emitter  60  is heated, thermionic emission and cold field emission contribute to the current of charged particles emitted. In some embodiments, thermionic emission contributes at least 95%, optionally at least 98%, optionally at least 99%, and optionally at least 99.5% of the current of the beam of charged particles. The thermionic emission contribution to the current is more stable than the cold field emission contribution. By providing a higher percentage for the thermionic emission contribution, the overall stability of the current of charged particles is improved. The inventor noted that in view of the stability of the charged particle source layer, such a graphene layer, at elevated temperatures for example 900 degrees Celsius and above, that an emitter with improved emission current is provided. 
     An emitter according to some embodiments of the present disclosure features a charged particle source layer on a metal layer. The metal layer is on at least the point of the emitter  61  having an elongate shape. The metal layer is on a second metal comprised in the point, such as tungsten having a high melting point and relatively low remanence for example compared to the metal of the metal layer. Having materials such as graphene as the charged particle source and nickel as the metal layer enables a lower work function to be achieved. Operation of the emitter in a high electric field enables the work function of the surface of the point achieve a lower work function. To ensure a sufficiently stable emission current the emitter  60  is operated at an elevated temperature. To realize an emitter capable of being operated under such conditions, the inventor applied the thin metal layer (with the charged particle source layer) to the point of low remanence material. The emitter may be operated as a thermionic emitter. For example the temperature may be 900° C. or more. An emitter with one or more of these improvements can achieve an emitter of improved operational specifications, for example at least a more stable emissions current. 
     The type of electric field generator  70  is not limited to the electric field generator described here. Other types of electric field generator known to the skilled person may be used. As shown in  FIG.  10   , in some embodiments, an electrode  71  is provided facing the point  62  of the body  61 . The electrode  71  may be a plate in which is defined an opening. The opening may allow passage of the beam of charged particles through the electrode  71 . In some embodiments, the body  61  is a cathode and the electrode  71  is an anode such than an electric field is applied at the point  62  of the body  61 . In an alternative example, a plurality of anodes are provided. 
     By providing that the charged particle source layer  64  is adjacent to the first metal and applying the electric field at the point  62 , the work function of the charged particle source layer  64  is reduced. By heating the point  62  so as to promote thermionic emission, the likelihood of an electron having enough energy to overcome the work function is increased. In some embodiments, at an electric field of about 100 MV/m, the work function is reduced by about 0.3 eV. 
       FIGS.  6  to  8    schematically depict different stages of a method of making an emitter  60  according to some embodiments of the present disclosure. As shown in  FIG.  6   , in some embodiments, the method comprises providing a length of wire  81 . In some embodiments, the wire  81  has a circular cross-section. 
     In some embodiments, the method comprises sharpening one end of the wire  81  so as to form the body  61 , as shown in  FIG.  7   . By sharpening one end of the wire  81 , the point  62  is produced, as shown in  FIG.  7   . As shown in  FIG.  7   , in some embodiments, the point  62  gradually tapers towards the point. In the arrangement shown in  FIG.  7   , the tapering angle is approximately equal from the wide-diameter portion of the body  61  up to the point  62 . However, this is not necessarily the case. In some embodiments, the tapering angle decreases towards the end of the point  62  so as to increase sharpness at the point  62 . 
     The method of sharpening the wire  81  so as to form the point  62  is not limited to method described here. Other methods known to the skilled person may be used. In some embodiments, an etching process is used for preparing the point  62 . In some embodiments, an electrochemical etching process is used. In some embodiments, the point  62  comprises a second metal other than the first metal used for the metal layer  63 . For example, in some embodiments, the metal layer  63  is formed of nickel and the wire  81  from which the body  61  is formed is made of another metal such as tungsten or molybdenum. Metals such as tungsten and molybdenum have higher etching reproducibility compared to nickel. Some embodiments of the present disclosure are expected to make manufacturing of the emitter  60  more reproducible. 
     As shown in  FIG.  8   , in some embodiments, the method comprises applying the metal layer  63  onto the body  61 . In some embodiments, the method comprises forming the charged particle source layer  64  on the metal layer  63  so as to produce the emitter  60  as shown in  FIG.  4   . As shown in  FIG.  4   , in some embodiments, the charged particle source layer  64  covers substantially all of the metal layer  63 . In some embodiments, substantially the whole of the projection system facing surface of the emitter  60  is provided by the charged particle source layer  64  on the metal layer  63 . However, this is not necessarily the case. In an alternative example, the charged particle source layer  64  covers the metal layer  63  at the point  62  but not at one or more other portions of the metal layer  63  (e.g. at the wide-diameter portion of the emitter  60 ). 
     It is not essential for the metal layer  63  to be provided. In an alternative example, the charged particle source layer  64  is formed directly onto the point  62  of the body  61  without an intervening metal layer  63 . In some embodiments, the point  62  comprises a metal that reduces the work function of the charged particle source layer  64 . For example, in some embodiments, the point  62  comprises nickel, cobalt, palladium, copper, silver, iridium or platinum. The body  61  may be made of nickel, cobalt, palladium, copper, silver, iridium or platinum. In some embodiments, the body  61  is made of a metal having a lower remanence than nickel. In some embodiments, the body  61  is made of a metal having a higher melting point than nickel. 
       FIG.  9    schematically depicts a method of restoring the charged particle source layer  64  of the emitter  60 . As shown in  FIG.  9   , in some embodiments, an environment  83  comprising hydrocarbons is provided. In some embodiments, a chamber  82  is filled with hydrocarbons  83 . The chamber provides an environment  83  having a hydrocarbon laden atmosphere. The emitter  60  is placed into the chamber  82 . In the chamber  82 , the charged particle source layer  64  is in a hydrocarbon laden atmosphere, The charged particle source layer  64  may come into contact with the hydrocarbons in the environment  83 . In some embodiments, the point  62  of the body  61  is heated, or is placed into the chamber  82  when it is in a heated state. The charged particle source layer  64  is at least partially restored through contact with the hydrocarbons in the environment  83 . In some embodiments, the charged particle source layer  64  is restored in situ. 
     Due to the elevated temperature, operating temperature defects build up in the structure of the charged particle source layer, e.g. the graphene layer. The structure of the material of the charged particle source layer may have the structural form of a lattice, i.e. repeating structure in at least two with respect to the internal structure of the material; that is for graphene a two dimensional lattice in three dimensional space, for example on the curved surface of the metal layer  63 . Such defects may be generated by lattice vibration, for example Brownian motion at an elevated temperature, within the structure which may be more likely at such elevated temperatures as during operation. The defects may be present as irregularities in the regular repeating lattice. 
     In the environment in which the charged particle source layer is restored, the hydrocarbon laden atmosphere is at an elevated temperature, i.e. heated environment, which may be under vacuum relative to the ambient environment, the hydrocarbons in the atmosphere decompose on the charged particle source layer, e.g. graphene layer. In restoring the charge particle source layer, the defects are removed from the charged particle source layer. The lattice structure of the charged particle source layer, e.g. the graphene, is restored; so the structure of the charged particle source layer has a regular repeating structure in the three dimensions of the lattice. As shown in  FIG.  10   , in some embodiments, the emitter  60  comprises a heating element  65 . The heating element  65  is attached to the body  61 . In some embodiments, the heating element  65  is welded to the body  61 . In some embodiments, the body  61  and the heating element  65  are made of materials that can be welded together. For example, in some embodiments, the body  61  and the heating element  65  are made of the same material. 
     In some embodiments, the heating element  65  forms a resistive heater. An electric current may be passed through the heating element  65  so as to heat the heating element  65 . The heating element  65  conducts heat to the body  61  for thermionic emission. For example, in some embodiments, the heating element  65  is made of tungsten. In an alternative example, the heating element  65  comprises molybdenum, nickel, cobalt, palladium, copper, silver, iridium or platinum. 
     As shown in  FIG.  11   , in some embodiments, a charged particle source comprises an electrically conductive suppressor  90 . in the suppressor  90  is defined an opening  91  through which the body  61  extends. The opening  91  has a larger dimension, e.g. wider, than the body  61 . The body  61  does not contact the suppressor  90 . The point  62  of the tip body  61  is external to the suppressor. The suppressor is set a potential relative to the emitter. The point  62  is exposed to electro-magnetic fields external to the suppressor  90 , such as the electro-optical elements of the illumination and projection systems. The point  62  may emit the beam of charged particles from the charged particle source layer  64  at the point of the body  61 . The suppressor  90  is configured to reduce emission of unwanted charged particles from parts of the emitter  60  other than the point  62 . Thus from the perspective of the emitter, the suppressor  90  interferes with electrons emitted from the emitter within the suppressor  90 . 
     In some embodiments, the suppressor  90  is electrically negatively biased relative to the body  61 . Unwanted charged particles that may be emitted from the wide-diameter portion of the body  61  may be prevented from being emitted because of the presence of the suppressor  90 . The suppressor  90  helps to improve the quality of the beam of charged particles. 
     The emitting surface at the end of the point  62  of the emitter  60  can come in different sizes. In some embodiments, the emitter surface has a diameter of at least 100 nm, optionally at least 200 nm, optionally at least 500 nm and optionally at least 1000 nm. In some embodiments, the emitting surface has a diameter of at most 1000 nm, optionally at most 500 nm, optionally at most 200 nm, and optionally at most 100 nm. 
     Calculations have been performed for an emitter  60  in which the body  61  is made of tungsten and the metal layer  63  is made of nickel. The emitting surface has a diameter of 800 nm. The current density Jsc can be calculated using the following formula 
     
       
         
           
             
               Jsc 
               ⁡ 
               ( 
               T 
               ) 
             
             := 
             
               A 
               · 
               
                 T 
                 2 
               
               · 
               
                 e 
                 
                   
                     - 
                     
                       ( 
                       
                         
                           1.1 
                               
                           eV 
                         
                         - 
                         
                           0.3 
                               
                           eV 
                         
                       
                       ) 
                     
                   
                   
                     
                       k 
                       B 
                     
                     · 
                     T 
                   
                 
               
             
           
         
       
     
     where A is the surface area from which the charged particles are emitted, T is the temperature at the point  62  and k B  is the Boltzmann constant. 
     The brightness Br can be calculated using the formula 
     
       
         
           
             
               Br 
               ⁡ 
               ( 
               T 
               ) 
             
             := 
             
               ec 
               · 
               
                 
                   Jsc 
                   ⁡ 
                   ( 
                   T 
                   ) 
                 
                 
                   π 
                   · 
                   
                     k 
                     B 
                   
                   · 
                   T 
                 
               
             
           
         
       
     
     where ec is the charge of an electron 1.602×10 −19  C, Jsc is the current density, k B  is the Boltzmann constant and T is the temperature at the point  62 . 
     Based on the calculations, some embodiments of the present disclosure are expected to increase brightness and/or current density of the beam of charged particles. The disclosed embodiments may achieve similar brightness enhancements in arrangements of the emitter using the different materials disclosed herein, for example platinum, cobalt or palladium or a similar metal as a metal layer with a graphene coating 
     As mentioned above, in some embodiments, the electron source  201  comprises a gun aperture plate  271 . The gun aperture plate  271  defines an aperture. In some embodiments, the aperture is distanced from the point  62  by at most 100 mm, optionally at most 50 mm, optionally at most 20 mm, and optionally at most 10 mm. By reducing the distance to the aperture, the maximum effective brightness that can be achieved is increased. 
     While the embodiments of the present disclosure have been described in connection with various examples, other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the technology disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims. 
     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 as described without departing from the scope of the claims and clauses set out below. 
     Clause 1: An emitter configured to emit charged particles, the emitter comprising: a body having a point; a metal layer of a first metal on at least the point; and a charged particle source layer on the metal layer, wherein the point comprises a second metal other than the first metal. 
     Clause 2: An emitter configured to emit charged particles, the emitter comprising: a body having a point; a metal layer on at least the point; and a charged particle source layer on the metal layer, wherein the point comprises a different metal from the metal layer 
     Clause 3: The emitter of clause 1 or 2, wherein the second metal has a higher melting point than the first metal. 
     Clause 4: The emitter of clause 1, 2 or 3, wherein the second metal has a lower remanence than the first metal. 
     Clause 5: The emitter of any preceding clause, wherein the second metal is a refractory metal. 
     Clause 6: The emitter of any preceding clause, wherein the second metal or different metal is a metal selected from the group consisting of tungsten, molybdenum, tantalum, niobium, rhenium, titanium, vanadium, chromium, zirconium, hafnium, ruthenium, rhodium, osmium and iridium; wherein the second metal is preferably Tungsten. 
     Clause 7: The emitter of any preceding clause, wherein the first metal is selected from a group consisting of nickel, cobalt, palladium, copper, silver, platinum and iridium, preferably the group consisting of nickel, cobalt, palladium and platinum, more preferably the first metal is nickel. 
     Clause 8: The emitter of any preceding clause, wherein the charged particle source layer is doped by the first metal. 
     Clause 9, The emitter of any preceding clause, wherein the body has an elongate shape, preferably a point which is for example conical, preferably the elongate shape has an axis, preferably the elongate shape is cylindrical and preferably a surface of the body at least around the axis the body is curved and/or has at least two sides. 
     Clause 10: The emitter of any preceding clause, wherein the metal layer and the charged particle source layer are concentric layers for example on at last part of the surface of the body. 
     Clause 11: The emitter of any preceding clause, comprising a heating element configured to apply a heat load to the body so as to promote thermionic charged particle emission, preferably the heat load applies a temperature to the body of at least 900° C., more preferably 1000° C., more preferably 1100° C. 
     Clause 12: The emitter of any proceeding clause, wherein the charged particle source layer comprises graphene and preferably consists of graphene. 
     Clause 13 A charged particle source comprising the emitter of any preceding clause. 
     Clause 14: The charged particle source of clause 13, comprising an electric field generator configured to generate an electric field at the point so as to lower a work function of the charged particle source layer. 
     Clause 15: The charged particle source of clause 14, comprising an electrically conductive suppressor comprising a hole through which the body extends such that the point is exposed, wherein the suppressor is configured to reduce emission of charged particles from parts of the emitter other than the point. 
     Clause 16: An illumination apparatus comprising the charged particle source of any of clauses 13 to 15. 
     Clause 17: The illumination apparatus of clause 16, comprising an aperture plate defining an aperture. 
     Clause 18: The illumination apparatus of clause 17, wherein the aperture is distanced from the point by at most 50 mm. 
     Clause 19: A charged particle beam tool comprising the illumination apparatus of any of clauses 16 to 18. 
     Clause 20: A charged particle beam tool of clause 19, wherein the charged particle beam tool is a single beam tool or a multi-beam tool 
     Clause 21: A charged particle beam inspection apparatus comprising the charged particle beam tool of clause 19 or 20. 
     Clause 22: A method for making an emitter configured to emit charged particles, the method comprising: providing a body having a point; disposing a metal layer of a first metal on at least the point; and forming a charged particle source layer on the metal layer, wherein the body comprises a second metal other than the first metal. 
     Clause 23: The method of clause 22, wherein the first metal catalyzes formation of the charged particle source layer on the metal layer. 
     Clause 24: The method of clause 22 or 23, comprising: providing an environment comprising hydrocarbons with which the charged particle source layer can come into contact while the point is in a heated state so as to restore the charged particle source layer. 
     Clause 25: A method of emitting a beam of charged particles, the method comprising: providing an emitter configured to emit charged particles, the emitter comprising a charged particle source layer on a metal at a point of a body of the emitter; and heating the point to a temperature of greater than 500° C. so as to promote thermionic emission 
     Clause 26: The method of clause 25, wherein the emitter comprises: a metal layer of a first metal on at least the point; and the charged particle source layer on the metal layer, wherein the point comprises a second metal other than the first metal. 
     Clause 27: The method of clause 26, wherein the body comprises an elongate body of the first metal defining the point at one end. 
     Clause 28: An emitter configured to emit charged particles, the emitter comprising: a body having a point; a metal layer on at least the point; and a charged particle source layer on the metal layer, wherein the point comprises a different metal from the metal layer.