Charged particle detection system

Abstract

A charged-particle beam apparatus configured to direct a charged-particle beam onto a sample, the charged-particle beam apparatus comprising a detection system which comprises: a first detection surface configured to generate electrical signals in response to signal particles generated by the sample in response to the charged-particle beam; and a second detection surface configured to generate electrical signals in response to incident secondary particles or backscattered particles generated by the first detection surface in response to the signal particles, the second detection surface defining a second hole to enable the signal particles to pass to the first detection surface.

Description

CHARGED PARTICLE DETECTION SYSTEMCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP / US application 23190948.2 which was filed on August 10, 2023 and EP application 24186398.4 which was filed on July 03, 2024 which are incorporated herein in its entirety by reference.FIELD

[0002] The description herein relates to charged particle detection, and more particularly, to systems and methods that may be applicable to charged particle beam detection.BACKGROUND

[0003] Detectors may be used for sensing physically observable phenomena. For example, charged particle beam tools, such as electron microscopes, may comprise detectors that receive charged particles projected from a sample and that output a detection signal. Detection signals can be used to reconstruct images of sample structures under inspection and may be used, for example, to reveal defects in the sample. Detection of defects in a sample is increasingly important in the manufacturing of semiconductor devices, which may include large numbers of densely packed, miniaturized integrated circuit (IC) components. Dedicated inspection tools may be provided for this purpose.

[0004] In some applications in the field of inspection, for example, microscopy, using a scanning electron microscope (SEM), an electron beam may be scanned across a sample to derive information from backscattered or secondary electrons generated from the sample. Backscattered electrons (or more generally backscattered particles) and secondary electrons (or more generally secondary particles) generated by the sample may be referred to collectively as returning particles. In a related art, electron detection systems in SEM tools may include a detector configured to detect electrons coming from the sample. Existing charged particle beam tools typically include a semiconductorbased detector, which generate an electron-hole pair when a backscattered particle or secondary particle is incident on the detector. The electron-hole pairs that are generated in the detector as a backscattered particles or secondary particles form a current, which reflects the number of particles incident on the detector from the sample.

[0005] In the related art, the current generated by the detector may not accurately reflect the number of returning particles incident on the detector from the sample. One of the causes of this discrepancy is particle loss, due the interaction of returning particles with the surface of the detector. The returning particles can undergo an elastic interaction with the detection surface to form a further backscattered particle, or an inelastic interaction with the detection surface to form a further secondary particle or partially backscattered electron. In some cases, the returning particles undergopartial backscattering, wherein the particle deposits some of its energy in the detector and then undergoes a scattering event.

[0006] Most semiconductor-based detectors known in the art are silicon-based. Semiconductorbased detectors based on germanium offer a number of advantages over silicon detectors, such as improved signal and energy resolution. Furthermore, the germanium has a higher density and atomic number than silicon, meaning that the penetration depth of a charged-particle beam into the sample surface is lower for germanium than silicon. Thus, for a given energy of a charged-particle beam, most incoming charged particles undergo an interaction with the detector over a smaller distance for a germanium-based detector, than for a silicon-based detector. Therefore, germanium detectors are generally thinner than their silicon counterparts, making the detection system set more flexible. However, in germanium-based detectors, backscattering of retuning particles is exacerbated, due to the higher atomic number of germanium. In addition, there are not many semiconductor materials with a lower atomic number than silicon, meaning that there are few options of low-atomic number semiconductor-based detectors. Therefore, there is a need to improve the detection efficiency of a detector, comprising elements with a relatively high atomic number.

[0007] Patent Literature 1 discloses an optical column comprising two detectors, wherein the returning electrons pass via a secondary electron detector onto a backscattered electron detector. This configuration enables the secondary electrons and the backscattered electrons to be detected at two detectors respectively, however, returning electrons may nevertheless elastically or inelastically interact with either of the detectors, meaning that they go undetected.

[0008] [Patent Literature 1] H Jaksch, J-P Vermeulen, New Developments in Gemini® Fesem Technology, Microscopy Today, Volume 13, Issue 2, 1 March 2005, Pages 8-11, https: / / doi.org / 10.1017 / S1551929500051397SUMMARY

[0009] According to a first aspect of the present invention, there is provided a charged-particle beam apparatus configured to direct a charged-particle beam onto a sample, the charged-particle beam apparatus comprising a detection system which comprises: a first detection surface configured to generate electrical signals in response to signal particles generated by the sample in response to the charged-particle beam; and a second detection surface configured to generate electrical signals in response to incident secondary particles or backscattered particles generated by the first detection surface in response to the signal particles, the second detection surface defining a second hole to enable the signal particles to pass to the first detection surface.

[0010] According to a second aspect of the present invention, there is provided a non-transitory computer-readable medium having instructions that, when executed by a computer, cause the computer to execute a method of detecting particles in a charged-beam apparatus, the method comprising: detecting with a first detection surface one or more returning particles, which aregenerated by a sample in response to a charged-particle beam being incident on the sample, and detecting with a second detection surface one or more particles generated by the first detection surface, which are generated in response to the returning particles being incident on the first detection surface.

[0011] According to a third aspect of the present invention, there is provided a charged-particle beam apparatus configured to direct a charged-particle beam onto a sample, the charged-particle beam apparatus comprising a detection system which comprises: a first detector having a first detection surface configured to generate first electrical signals in response to first signal particles being incident on the first detection surface, the first detector defining a first hole to enable the charged-particle beam to pass therethrough to the sample, wherein the first signal particles are generated by the sample in response to the charged-particle beam being incident on the sample; and a second detector having a second detection surface configured to generate second electrical signals in response to second signal particles being incident on the second detection surface, the second detector defining a second hole to enable the charged particle beam to pass therethrough to the sample and to enable the first signal particles to pass therethrough to the first detector surface, wherein the second signal particles are generated by the first detector in response to the first signal particles being incident on the first detection surface.

[0012] Further embodiments, features and advantages of the present invention, as well as the structure and operation of the various embodiments, features and advantages of the present invention, are described in detail below with reference to the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0013] 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 in which:

[0014] FIG. 1 is a schematic diagram illustrating an exemplary electron beam inspection (EBI) system, consistent with embodiments of the present disclosure.

[0015] FIG. 2A, FIG. 2B, and FIG. 2C are schematic diagrams illustrating exemplary electron beam tools, consistent with embodiments of the present disclosure that may be a part of the exemplary electron beam inspection system of FIG. 1.

[0016] FIG. 3 is a schematic diagram illustrating a cross-sectional view of a charged-particle beam apparatus according to the prior art.

[0017] FIG. 4 is a schematic diagram illustrating a cross-sectional view of a charged-particle beam apparatus according to an embodiment of the present disclosure.

[0018] FIG. 5A is a schematic diagram illustrating a cross-sectional view of a charged-particle beam apparatus according to an embodiment of the present disclosure.

[0019] FIG. 5B is a schematic diagram illustrating a cross-sectional view of a charged-particle beam apparatus according to an embodiment of the present disclosure.

[0020] FIG. 6A is a schematic diagram illustrating a cross-sectional view of a charged-particle beam apparatus according to an embodiment of the present disclosure.

[0021] FIG. 6B is a schematic diagram illustrating a cross-sectional view of a charged-particle beam apparatus according to an embodiment of the present disclosure.

[0022] FIG. 6C is a schematic diagram illustrating a cross-sectional view of a charged-particle beam apparatus according to an embodiment of the present disclosure.

[0023] FIG. 6D is a schematic diagram illustrating a cross-sectional view of a charged-particle beam apparatus according to an embodiment of the present disclosure.

[0024] FIG. 7 is a graph illustrating the collection efficiency of the second detection surface as a function of distance between the first detection surface and the second detection surface.

[0025] FIG. 8A, 8B, 8C and 8D show possible embodiments of the second detection surface.DETAILED DESCRIPTION

[0026] Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the 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 consistent with the invention. Instead, they are merely examples of apparatuses, systems, and methods consistent with aspects related to subject matter that may be recited in the appended claims.

[0027] Aspects of the present application relate to systems and methods for charged particle beam detection. Systems and methods may employ counting of charged particles, such as electrons, and may be useful in an inspection tool, such as a scanning electron microscope (SEM). Inspection tools may also be referred to as assessment tools or assessment apparatuses. Inspection tools may be used in the manufacturing process of integrated circuit (IC) components. To realize the enhanced computing power of modern day electronic devices, the physical size of the devices may shrink while the packing density of circuit components, such as, transistors, capacitors, diodes, etc., is significantly increased on an IC chip. For example, in a smartphone, an IC chip (which may be the size of a thumbnail) may include over 2 billion transistors, the size of each transistor being less than 1 / 1, 000th of a human hair. Not surprisingly, semiconductor IC manufacturing is a complex process, with hundreds of individual steps. Errors in even one step have the potential to dramatically affect the functioning of the final product. The goal of the manufacturing process is to improve the overall yield of the process. For example, for a 50-step process to get 75% yield, each individual step must have a yield greater than 99.4%, and if the individual step yield is 95%, the overall process yield drops to 7%.

[0028] It is increasingly important to ensure the ability to detect defects with high accuracy and high resolution while maintaining high throughput (defined as the number of wafer processed per hour, for example). High process yields and high wafer throughput may be impacted by the presence of defects, especially when operator intervention is involved. Thus, detection and identification of micro and nano-sized defects by inspection tools (such as a SEM) is important for maintaining high yields and low cost.

[0029] In some inspection tools, a sample may be inspected by scanning a beam of high energy electrons over the sample surface. Due to interactions at the sample surface, secondary or backscattered electrons (returning electrons) may be generated from the sample that may then be detected by a detector. An image of the sample is formed pixel by pixel, wherein a value of a pixel is determined by the number of returning electrons detected by the detector.

[0030] The returning electrons, such as backscattered or secondary electrons generated by the sample, may undergo an elastic scattering event or an inelastic scattering event. In some cases, the electrons which are scattered by the detector may not be detected by the detector, and these undetected electrons may not contribute to the signal which is used to form the image of the sample. When an electron undergoes an elastic scattering event, such as an elastic scattering event with the detector, no energy is lost by the electron. When an electron undergoes an inelastic scattering event, such as an inelastic scattering event with the detector, the electron imparts some energy onto the detector. A plurality of factors determines the proportion of returning electrons which undergo a scattering event, leading to them being undetected. For example, detectors comprising elements of higher atomic number are more likely to backscatter the retuning electrons.

[0031] The detection of retuning particles is often carried out by solid state semiconductor-based detectors. When a returning particle of an energy higher than the bandgap of the semiconductor is incident on the detector, an electron-hole pair is generated in the detector and a current is generated. Often the semiconductor-based detectors are silicon-based detectors.

[0032] The present inventors have determined that efficiency of returning particle collection can be improved by providing a second detector, which is configured to detect the particles which have undergone an interaction with the first detector.

[0033] Without limiting the scope of the present disclosure, some embodiments may be described in the context of providing detectors and detection methods in systems utilizing electron beams. However, the disclosure is not so limited. Other types of charged particle beams may be similarly applied. Furthermore, systems and methods for detection may be used in other imaging systems, such as optical imaging, photon detection, x-ray detection, ion detection, etc.

[0034] As used herein, unless specifically stated otherwise, the term “or” encompasses all possible combinations, except where infeasible. For example, if it is stated that a component includes A or B, then, unless specifically stated otherwise or infeasible, the component may include A, or B, or A and B. As a second example, if it is stated that a component includes A, B, or C, then, unless specificallystated otherwise or infeasible, the component may include A, or B, or C, or A and B, or A and C, or B and C, or A and B and C.

[0035] Reference is now made to FIG. 1, which illustrates an exemplary electron beam inspection (EBI) system 10 that may include a detector, consistent with embodiments of the present disclosure. EBI system 10 may be used for imaging. As shown in FIG. 1, EBI system 10 includes a main chamber I l a load / lock chamber 20, an electron beam tool 100, and an equipment front end module (EFEM) 30. Electron beam tool 100 is located within main chamber 11. EFEM 30 includes a first loading port 30a and a second loading port 30b. EFEM 30 may include additional loading port(s). First loading port 30a and second loading port 30b receive wafer front opening unified pods (FOUPs) that contain wafers (e.g., semiconductor wafers or wafers made of other material(s)) or samples to be inspected (wafers and samples may be collectively referred to as “samples” herein).

[0036] One or more robotic arms (not shown) in EFEM 30 may transport the wafers to load / lock chamber 20. Load / lock chamber 20 is connected to a load / lock vacuum pump system (not shown) which removes gas molecules in load / lock chamber 20 to reach a first pressure below the atmospheric pressure. After reaching the first pressure, one or more robotic arms (not shown) may transport the wafer from load / lock chamber 20 to main chamber 11. Main chamber 11 is connected to a main chamber vacuum pump system (not shown) which removes gas molecules in main chamber 11 to reach a second pressure below the first pressure. After reaching the second pressure, the wafer is subject to inspection by electron beam tool 100. Electron beam tool 100 may be a single -beam system or a multi-beam system. A controller 109 is electronically connected to electron beam tool 100, and may be electronically connected to other components as well. Controller 109 may be a computer configured to execute various controls of EBI system 10. While controller 109 is shown in FIG. 1 as being outside of the structure that includes main chamber 11, load / lock chamber 20, and EFEM 30, it is appreciated that controller 109 can be part of the structure.

[0037] FIG. 2A illustrates a charged particle beam apparatus in which an inspection system may comprise a multi-beam inspection tool that uses multiple primary electron beamlets to simultaneously scan multiple locations on a sample.

[0038] As shown in FIG. 2A, an electron beam tool 100A (also referred to herein as apparatus 100 A) may comprise an electron source 202, a gun aperture 204, a condenser lens 206, a primary electron beam 210 emitted from electron source 202, a source conversion unit 212, a plurality of beamlets 214, 216, and 218 of primary electron beam 210, a primary projection optical system 220, a wafer stage (not shown in FIG. 2A), multiple secondary electron beams 236, 238, and 240, a secondary optical system 242, and an electron detection device 244. Electron source 202 may generate primary particles, such as electrons of primary electron beam 210. A controller, image processing system, and the like may be coupled to electron detection device 244. Primary projection optical system 220 may comprise a beam separator 222, deflection scanning unit 226, and objective lens 228. Electron detection device 244 may comprise detection sub-regions 246, 248, and 250.

[0039] Electron source 202, gun aperture 204, condenser lens 206, source conversion unit 212, beam separator 222, deflection scanning unit 226, and objective lens 228 may be aligned with a primary optical axis 260 of apparatus 100A. Secondary optical system 242 and electron detection device 244 may be aligned with a secondary optical axis 252 of apparatus 100A.

[0040] Electron source 202 may comprise a cathode, an extractor or an anode, wherein primary electrons can be emitted from the cathode and extracted or accelerated to form a primary electron beam 210 with a crossover (virtual or real) 208. Primary electron beam 210 can be visualized as being emitted from crossover 208. Gun aperture 204 may block off peripheral electrons of primary electron beam 210 to reduce size of probe spots 270, 272, and 274.

[0041] Source conversion unit 212 may comprise an array of image-forming elements (not shown in FIG. 2A) and an array of beam-limit apertures (not shown in FIG. 2A). An example of source conversion unit 212 may be found in U.S. Pat. No. 9,691,586; U.S. Publication No. 2017 / 0025 and International Application No. PCT / EP2017 / 084429, all of which are incorporated by reference m their entireties. The array of image-forming elements may comprise an array of micro-deflectors or micro-lenses. The array of image-forming elements may form a plurality of parallel images (virtual or real) of crossover 208 with a plurality of beamlets 214, 216, and 218 of primary electron beam 210. The array of beam-limit apertures may limit the plurality of beamlets 214, 216, and 218.

[0042] Condenser lens 206 may focus primary electron beam 210. The electric currents of beamlets 214, 216, and 218 downstream of source conversion unit 212 may be varied by adjusting the focusing power of condenser lens 206 or by changing the radial sizes of the corresponding beam-limit apertures within the array of beam-limit apertures. Condenser lens 206 may be a moveable condenser lens that may be configured so that the position of its first principle plane is movable. The movable condenser lens may be configured to be magnetic, which may result in off-axis beamlets 216 and 218 landing on the beamlet-limit apertures with rotation angles. The rotation angles change with the focusing power and the position of the first principal plane of the movable condenser lens. In some embodiments, the moveable condenser lens may be a moveable anti-rotation condenser lens, which involves an anti-rotation lens with a movable first principal plane. A moveable condenser lens is further described in U.S. Publication No. 2017 / 0025241, which is incorporated by reference in its entirety.

[0043] Objective lens 228 may focus beamlets 214, 216, and 218 onto a wafer 230 for inspection and may form a plurality of probe spots 270, 272, and 274 on the surface of wafer 230.

[0044] Beam separator 222 may be a beam separator of Wien filter type generating an electrostatic dipole field and a magnetic dipole field. In some embodiments, if they are applied, the force exerted by electrostatic dipole field on an electron of beamlets 214, 216, and 218 may be equal in magnitude and opposite in direction to the force exerted on the electron by magnetic dipole field. Beamlets 214, 216, and 218 can therefore pass straight through beam separator 222 with zero deflection angle. However, the total dispersion of beamlets 214, 216, and 218 generated by beam separator 222 mayalso be non-zero. Beam separator 222 may separate secondary electron beams 236, 238, and 240 from beamlets 214, 216, and 218 and direct secondary electron beams 236, 238, and 240 towards secondary optical system 242.

[0045] Deflection scanning unit 226 may deflect beamlets 214, 216, and 218 to scan probe spots 270, 272, and 274 over a surface area of wafer 230. In response to incidence of beamlets 214, 216, and 218 at probe spots 270, 272, and 274, secondary electron beams 236, 238, and 240 may be emitted from wafer 230. Secondary electron beams 236, 238, and 240 may comprise electrons with a distribution of energies including secondary electrons and backscattered electrons. Secondary optical system 242 may focus secondary electron beams 236, 238, and 240 onto detection sub-regions 246, 248, and 250 of electron detection device 244. Detection sub-regions 246, 248, and 250 may be configured to detect corresponding secondary electron beams 236, 238, and 240 and generate corresponding signals used to reconstruct an image of surface area of wafer 230.

[0046] Although FIG. 2A shows an example of electron beam tool 100 as a multi-beam tool that uses a plurality of beamlets, embodiments of the present disclosure are not so limited. For example, electron beam tool 100 may also be a single-beam tool that uses only one primary electron beam to scan one location on a wafer at a time.

[0047] As shown in FIG. 2B, an electron beam tool 100B (also referred to herein as apparatus 100B) may be a single-beam inspection tool that is used in EBI system 10. Apparatus 100B includes a wafer holder 136 supported by motorized stage 134 to hold a wafer 150 to be inspected. Electron beam tool 100B includes an electron emitter, which may comprise a cathode 103, an anode 121, and a gun aperture 122. Electron beam tool 100B further includes a beam limit aperture 125, a condenser lens 126, a column aperture 135, an objective lens assembly 132, and a detector system 144.Objective lens assembly 132, in some embodiments, may be a modified SORIL lens, which includes a pole piece 132a, a control electrode 132b, a deflector 132c, and an exciting coil 132d. In an imaging process, an electron beam 161 emanating from the tip of cathode 103 may be accelerated by anode 121 voltage, pass through gun aperture 122, beam limit aperture 125, condenser lens 126, and be focused into a probe spot 170 by the modified SORIL lens and impinge onto the surface of wafer 150. Probe spot 170 may be scanned across the surface of wafer 150 by a deflector, such as deflector 132c or other deflectors in the SORIL lens. Secondary or scattered primary particles, such as secondary electrons or scattered primary electrons emanated from the wafer surface may be collected by detector system 144 to determine intensity of the beam and so that an image of an area of interest on wafer 150 may be reconstructed.

[0048] There may also be provided an image processing system 199 that includes an image acquirer 120, a storage 130, and controller 109. Image acquirer 120 may comprise one or more processors. For example, image acquirer 120 may comprise a computer, server, mainframe host, terminals, personal computer, any kind of mobile computing devices, and the like, or a combination thereof. Image acquirer 120 may connect with detector system 144 of electron beam tool 100B through amedium such as an electrical conductor, optical fiber cable, portable storage media, IR, Bluetooth, Internet, wireless network, wireless radio, or a combination thereof. Image acquirer 120 may receive a signal from detector system 144 and may construct an image. Image acquirer 120 may thus acquire images of wafer 150. Image acquirer 120 may also perform various post-processing functions, such as generating contours, superimposing indicators on an acquired image, and the like. Image acquirer 120 may be configured to perform adjustments of brightness and contrast, etc. of acquired images. Storage 130 may be a storage medium such as a hard disk, random access memory (RAM), cloud storage, other types of computer readable memory, and the like. Storage 130 may be coupled with image acquirer 120 and may be used for saving scanned raw image data as original images, and postprocessed images. Image acquirer 120 and storage 130 may be connected to controller 109. In some embodiments, image acquirer 120, storage 130, and controller 109 may be integrated together as one electronic control unit.

[0049] In some embodiments, image acquirer 120 may acquire one or more images of a sample based on an imaging signal received from detector system 144. 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 that may contain various features of wafer 150. The single image may be stored in storage 130. Imaging may be performed on the basis of imaging frames.

[0050] The condenser and illumination optics of the electron beam tool may comprise or be supplemented by electromagnetic quadrupole electron lenses. For example, as shown in FIG. 2B, electron beam tool 100B may comprise a first quadrupole lens 148 and a second quadrupole lens 158. In some embodiments, the quadrupole lenses are used for controlling the electron beam. For example, first quadrupole lens 148 can be controlled to adjust the beam current and second quadrupole lens 158 can be controlled to adjust the beam spot size and beam shape.

[0051] FIG. 2B illustrates a charged particle beam apparatus in which an inspection system may use a single primary beam that may be configured to generate secondary electrons by interacting with wafer 150. Detector system 144 may be placed along optical axis 105, as in the embodiment shown in FIG. 2B. The primary electron beam may be configured to travel along optical axis 105. Accordingly, detector system 144 may include a hole at its center so that the primary electron beam may pass through to reach wafer 150. However, some embodiments may use a detector system placed off-axis relative to the optical axis along which the primary electron beam travels. For example, as in the embodiment shown in FIG. 2A, beam separator 222 may be provided to direct secondary electron beams toward a detector system placed off-axis. Beam separator 222 may be configured to divert secondary electron beams by an angle a.

[0052] Another example of a charged particle beam apparatus will now be discussed with reference to FIG. 2C. Electron beam tool 100C (also referred to herein as apparatus 100C) may be an example of electron beam tool 100 and may be similar to electron beam tool 100A shown in FIG. 2A.

[0053] As shown in FIG. 2C, beam separator 222 may be a beam separator of Wien filter type generating an electrostatic dipole field and a magnetic dipole field. In some embodiments, if they are applied, the force exerted by electrostatic dipole field on an electron of beamlets 214, 216, and 218 may be equal in magnitude and opposite in direction to the force exerted on the electron by magnetic dipole field. Beamlets 214, 216, and 218 can therefore pass straight through beam separator 222 with zero deflection angle. However, the total dispersion of beamlets 214, 216, and 218 generated by beam separator 222 may also be non-zero. For a dispersion plane 224 of beam separator 222, FIG. 2C shows dispersion of beamlet 214 with nominal energy VO and an energy spread AV into beamlet portions 262 corresponding to energy VO, beamlet portion 264 corresponding to energy VO+AV / 2, and beamlet portion 266 corresponding to energy VO-AV / 2. The total force exerted by beam separator 222 on an electron of secondary electron beams 236, 238, and 240 can be non-zero. Beam separator 222 may separate secondary electron beams 236, 238, and 240 from beamlets 214, 216, z 218 and direct secondary electron beams 236, 238, and 240 towards secondary optical system 242.

[0054] A semiconductor electron detector (sometimes called a “PIN detector”) may be used in apparatus 100 in EBI system 10. EBI system 10 may be a high-speed wafer imaging SEM including an image processor. An electron beam generated by EBI system 10 may irradiate the surface of a sample or may penetrate the sample. EBI system 10 may be used to image a sample surface or structures under the surface, such as for analyzing layer alignment. In some embodiments, EBI system 10 may detect and report process defects relating to manufacturing semiconductor wafers by, for example, comparing SEM images against device layout patterns, or SEM images of identical patterns at other locations on the wafer under inspection. A PIN detector may include a silicon PIN diode that may operate with negative bias. A PIN detector may include a PIN diode including an element other than silicon. A PIN detector may be configured so that incoming electrons generate a relatively large and distinct detection signal. In some embodiments, a PIN detector may be configured so that an incoming electron may generate a number of electron-hole pairs while a photon may generate just one electron-hole pair. A PIN detector used for electron counting may have numerous differences as compared to a photodiode used for photon detection, as shall be discussed as follows.

[0055] Reference is now made to FIG. 3, illustrating a cross-sectional view of a charged-particle beam apparatus according to the prior art comprising detector system 300, as a comparative example.

[0056] According to the comparative example illustrated on FIG. 3, detector system 300 comprises a single in-column detector 301, configured to detect returning electrons. When returning electrons are incident on the surface of the detector 301, some of the returning electrons are scattered, without any means to detect them. The detector 301 has radius to, and the detector 301 defines a hole having radius n to enable a charged-particle beam to pass therethrough to a sample surface.

[0057] Reference is now made to FIG. 4, which illustrates a schematic representation of an exemplary structure of a detector system 400. Detector system 400 may be provided as detector 144 or electron detection device 244 with reference to FIG. 2A, FIG. 2B, and FIG. 2C.

[0058] According to the configuration on FIG. 4, detection system 400 comprises a first detection surface 401 and a second detection surface 402. The first detection surface may be disposed on a first detector. The second detection surface may be disposed on a second detector. The charged-particle beam is indicated by a vertical line. The first detection surface 401 is configured to generate electrical signals in response to signal particles generated by the sample 150 (e.g. returning electrons) in response to the charged-particle beam. Returning particles (e.g. returning electrons) are shown schematically via curved arrows coming off the sample 150. In an embodiment, the first detection surface may define a first hole to enable the charged-particle beam to pass therethrough to the sample surface. This embodiment may be used when the detection system is disposed as detector 144, to enable the charged-particle beam to pass therethrough to the sample surface. In another embodiment the first detection surface may not define a hole. This embodiment may be used when the detection system is provided as electron detection device 244, i.e. when the detection system is provided off- axis. In an embodiment, the first hole is circular and has radius rn. The second detection surface 402 is configured to generate electrical signals in response to incident secondary particles (e.g. secondary electrons) or backscattered particles (e.g. backscattered electrons) generated by the first detection surface 401 in response to the signal particles (e.g. returning electrons). These particles (e.g. electrons) emitted by the first detection surface are indicated by arrows coming off the first detection surface. The second detection surface defines a second hole to enable the charged-particle beam to pass to the sample surface and to enable the signal particles to pass to the first detection surface. In an embodiment, the second hole is circular and has radius ra. First hole radius rn and second hole radius , may be the same, or second hole radius r ; may be greater than the first hole radius rn, or the first hole radius rn may be greater than the second hole radius ra. In an embodiment, wherein the detector system 400 is provided as the electron detection device 244 (which is not aligned with the primary optical axis) the first detection surface may not have a first hole. As described with reference to Figures 8A-8D, the first detection surface and / or the second detection surface may have a plurality of different configurations.

[0059] The first detection surface 401 may be any suitable detection surface for detecting returning particles. For example, the first detection surface 401 may be a semiconductor-based detector, a silicon-based PIN detector, a germanium-based PIN detector, etc. The second detection surface 402 may be any suitable detection surface for detecting incident backscattered or secondary particles generated by the first detection surface. For example, the second detection surface 402 may be a semiconductor-based detector, a silicon-based PIN detector, a germanium-based PIN detector, etc. The first detection surface 401 and the second detection surface 402 within a detection system 400 arenot necessarily the same type of detector. In an embodiment, a germanium-based detector may be used instead of a silicon-based detector.

[0060] Reference is now made to FIG. 5A and FIG 5B., which illustrate a cross-sectional view of a charged-particle beam apparatus according to an embodiment of the present disclosure

[0061] The first detection surface 401 may be disposed such that it is perpendicular to the charged- particle beam. Alternatively, a radial line of the first detection surface may a first angle Pi with a plane perpendicular to the charged-particle beam. Angle i may be equal to or less than 45°, or preferably equal to or less than 30°, or more preferably equal to or less than 15°.

[0062] The second detection surface 402 may be disposed such that it is perpendicular to the charged-particle beam. Alternatively, a radial line of the first detection surface may a first angle [T with a plane perpendicular to the charged-particle beam. Angle P2 may be equal to or less than 45°, or preferably equal to or less than 30°, or more preferably equal to or less than 15°.

[0063] Setting first angle Pi to be greater than 0°may increase the collection efficiency of backscattered electrons generated by the sample surface. Backscattered electrons are emitted by the sample at a variety of angles, with the most electrons emitted normal to the sample surface and close to the normal to the sample surface. When first angle Pi is set to be 0°, electrons emitted normal to the surface and at angles close to the normal of the surface may miss the second detection surface by passing through the second hole of the second detector. By inclining the first detection surface such that first angle Pi is formed, the proportion of electrons which are lost may be reduced.

[0064] By setting second angle P2 to be greater than 0°, the collection efficiency of electrons generated by the first detection surface increases, as described above.

[0065] First angle Pi and second angle 2 may be equal to each other, or may be different.

[0066] Reference is now made to FIG.6A, FIG.6B, FIG.6C and FIG.6D, which illustrate a cross- sectional view of a charged-particle beam apparatus according to embodiments of the present disclosure.

[0067] The charged-particle beam apparatus may comprise an annular detection surface disposed to surround the charge-particle beam. The annular detection surface may be included as a third detection surface, in addition to the first detection surface and the second detection surface, as illustrated on FIG. 6A, FIG. 6B and FIG. 6C. Alternatively, the annular detection surface may be provided as the second detection surface, as shown in FIG. 6D. The annular detection surface may be disposed on a detector.

[0068] The annular detection surface (third detection surface 403, as in FIG. 6A-6C, or second detection surface 402, as in FIG. 6D) is configured to generate electrical signals in response to incident secondary particles (e.g. secondary electrons) or backscattered particles (e.g. backscattered electrons) generated by the first detection surface 401 in response to the signal particles (e.g. returning electrons). The annular detection surface defines a hole to enable the charged-particle beam to pass therethrough to the sample surface. The hole may be circular and have radius r51.

[0069] Any of the first detection surface, the second detection surface and the third detection surface may comprise a plurality of segments.

[0070] Whilst the use of circular, part-circular, annular and part-annular detection surfaces has been described, the shapes of the detectors are not limited thereto. For example, the detection surfaces m comprise part annular or part circular segments. In addition, the detection surface segment may hav a shape selected from the group consisting of: a circle, an oval, an annulus, a square, a rectangle, a diamond, a rhombus.

[0071] Some possible configurations of the detection surfaces are described with reference to Figures 8A-8D, which show examples of the second detection surface 402a, 402b, 402c and 402d. The first detection surface 401 may be formed in any one of the shapes shown on Figures 8A-8D. The person skilled in the art will understand, based on the teaching herein that other shapes are also possible. Features of the second detection surface described with reference to Figures 8A-8D may also apply to the first detection surface.

[0072] In an embodiment, the hole of the second detection surface 402 (the second hole) may be disposed in a central portion of the second detection surface 402. The second detection surface 402 may surround the second hole. The second detection surface 402 may surround the second hole at least partially. Optionally, the second detection surface 402 may surround the second hole completely. For example, Figure 8A shows second detection surface 402a which completely surrounds the second hole.

[0073] The boundary between the second detection surface 402 and the second hole (i.e. the outer circumference of the hole) may be any suitable shape selected from a list comprising substantially circular, substantially elliptical, substantially rectangular or substantially hexagonal. Multiple holes may be provided and each may be of any suitable shape.

[0074] The second hole may be disposed in a central portion of the second detection surface 402. In an example, it may be disposed at the center of the second detection surface. In an alternative embodiment, the second hole may be disposed off-center. Optionally, the hole may be circular, substantially circular, or it may be another shape.

[0075] In an embodiment, the second hole may extend from one edge of the second detection surface 402 to another edge of the second detection surface 402. In other words, the second detection surface 402 may be provided as a plurality of segments. Figure 8B shows a second detection surface 402b defined by two semi-circular segments and Figure 8C shows a second detection surface 402c defined by two rectangular segments. In some embodiments, the number of segments may be greater than two. For example, the second detection surface comprise three or more segments which are separated by the second hole.

[0076] When the outer edge of the second detection surface 402 is circular, the second hole may extend from a point on the circular edge to a point which is diametrically opposite. In other words, the second hole may bisect the detection surface into semi-circular segments. In another embodiment,the hole may extend between two other points, resulting in two segments of a different shape. The hole does not necessarily extend in a straight line, and may be curved, angled, or have another shape.

[0077] When the outer edge of the second detection surface 402 is rectangular or square, the second hole may extend from a point on one side of the edge to the opposite edge. In other words, the second hole may bisect the detection surface. In another embodiment, the hole may extend between two other points, resulting in two segments of a different shape.

[0078] Other shapes of the second detection surface 402 are possible, and these may be provided with a second hole as described with the embodiments above. For example, the second detection surface may be hexagonal, or another shape.

[0079] In an embodiment, the second hole may extend from a central portion of the second detection surface to an edge of the second detection surface. Figures 8D shows an example of a second detection surface 402d, having a second hole which extends from a central portion to the edge of the second detection surface.

[0080] As illustrated with reference to the non-limiting examples in Figures 8A-8D, the second detection surface has a shape which permits passage of the secondary particles or backscattered particles generated by the first detection surface, which are generated in response to signal particles incident on the first detector surface. The second detector surface may have any suitable shape which fulfils this function.

[0081] The shape of the first detection surface 401 is not particularly limited. The first detection surface 401 may take on any of the shapes described with reference to Figures 8A-8D. In other words, the first detection surface 401 may define a first hole.

[0082] The first hole may be disposed in a central portion of the first detection surface 401. The first detection surface 401 may surround the first hole. The first detection surface 401 may completely surround the first hole. A boundary between the first detection surface 401 and the first hole may be any suitable shape selected from a list comprising substantially circular, substantially rectangular or substantially hexagonal. The first hole may extend from one edge of the first detection surface 401 to another edge of the first detection surface. The first hole may extend from a central portion of the first detection surface 401 to an edge of the first detection surface 401.

[0083] The shape of the outer edge of the first detection surface 401 and the second detection surface 402 is not particularly limited, and may be circular (as shown in Figures 8A, 8B and 8D) or rectangular / square (as shown in Figure 8C) or another shape. The shape of the first detection surface 401 and the second detection surface 402 may be different,

[0084] In some embodiments, an acceleration unit is provided which is configured to apply a potential difference between the first detection surface and second detection surface. The acceleration unit may be further configured to apply a potential different between the first detection surface and the third detection surface. According to this embodiment, electrons generated by the first detectionsurface are accelerated to the second detection surface (and optionally, the third detection surface), meaning that the detection efficiency is increased.

[0085] In some embodiments, a detector, such as one of the first detector, the second detector, and the third detector, may communicate with a controller that controls a charged particle beam system. The controller may instruct components of the charged particle beam system to perform various functions, such as controlling a charged particle source to generate a charged particle beam and controlling a deflector to scan the charged particle beam. The controller may also perform various other functions such as adjusting a sampling rate of a detector, resetting sensing element, or performing image processing. The controller may comprise a storage that is a storage medium such as a hard disk, random access memory (RAM), other types of computer readable memory, and the like. The storage may be used for saving scanned raw image data as original images, and postprocessed images. A non-transitory computer readable medium may be provided that stores instructions for a processor of controller 109 to carry out charged particle beam detection, sampling period determination, image processing, or other functions and methods consistent with the present disclosure. Common forms of non-transitory media include, for example, a floppy disk, a flexible disk, hard disk, solid state drive, magnetic tape, or any other magnetic data storage medium, a CD- ROM, any other optical data storage medium, any physical medium with patterns of holes, a ROM, a PROM, and EPROM, a FLASH-EPROM or any other flash memory, NVRAM, a cache, a register, any other memory chip or cartridge, and networked versions of the same.

[0086] A charged-particle beam apparatus according to the present invention may further comprise a charged-particle detection unit. A charged-particle detection unit is configured assign an electrical signal generated by the first detector and an electrical signal generated by the second detector to a single charged-particle count, in response to a determination that the sum of energies of a first electrical signal and a second electrical signal received within a given timeframe corresponds to a predicted value of the energy of a secondary particle or backscattered particle generated by the sample. In an embodiment, the predicted value of the energy of a secondary particle or backscattered particle generated by the sample (i.e. the full energy) is approximately equal to a landing energy of the charged-particles when they are incident on the sample. In an embodiment, the landing energy is in the range of 0.05keV to 50 keV. In an embodiment, the timeframe may be the sample pixel dwell time, which is equal to the interval during which charged particles incident on the first detection surface are assigned to the same pixel. For example, the pixel dwell time can be equal to or more th 2.5 ns and equal to or less than 40 ns. According to this embodiment, it is possible to detect electrons which are partially backscattered by the first detector.

[0087] For example, the first detector and the second detector may comprise multiple threshold values to allow correlating signals on both detectors. For example, there could be three threshold levels, with the highest threshold corresponding to full energy deposition and two others corresponding to 2 / 3 and 1 / 3 of the full energy, respectively. If an incidence with lower thresholdlevel is counted on the first detector (e.g. 1 / 3 of full energy), it should be checked whether within a given timeframe whether the second detector has detected a signal with the residual energy (1 / 3 or 2 / 3 of full energy). If there are matching incidences, the second signal is very likely to be caused by an electron generated by the first detection surface (i.e. a partially backscattered electron). In view of this, the two signals from the first detector and the second detector are combined to correspond to a single electron count. If there is no match, the lower threshold counts are likely noise and should be disregarded. In embodiments comprising a third detector, the same correlation can be performed between signals generated by the second detector and the third detector, and / or between signals generated by the first detector and the third detector.

[0088] Passages of the description which refer to an “electron beam”, “secondary electron beam” or an “electron” are understood to not be limited thereto, because according to the present invention the detection system may be configured to detect a charged-particle beam comprising charged-particles, which may interact with the sample to generate a secondary charged-particle beam.

[0089] Reference is now made to FIG. 7 which is a graph illustrating the collection efficiency of the second detection surface as a function of distance between the first detection surface and the second detection surface. In FIG. 7, the first detection surface and second detection surface are referred to as the first detector and second detector respectively, for brevity.

[0090] Exemplary embodiments of the invention are set out in the following numbered clauses:1. A charged-particle beam apparatus configured to direct a charged-particle beam onto a sample, the charged-particle beam apparatus comprising a detection system which comprises: a first detection surface configured to generate electrical signals in response to signal particles generated by the sample in response to the charged-particle beam; and a second detection surface configured to generate electrical signals in response to incident secondary particles or backscattered particles generated by the first detection surface in response to the signal particles, the second detection surface defining a second hole to enable the signal particles to pass to the first detection surface.2. The charged-particle beam apparatus according to clause 1, wherein the second hole is disposed in a central portion of the second detection surface.3. The charged-particle beam apparatus according to clause 1 or clause 2, wherein the second detection surface surrounds the second hole.4. The charged-particle beam apparatus according to any one of clauses 1-3, wherein the second detection surface completely surrounds the second hole.5. The charged-particle beam apparatus according to any one of clauses 1-4, wherein a boundary between the second detection surface and the second hole is substantially circular, substantially elliptical, substantially rectangular or substantially hexagonal.6. The charged-particle beam apparatus according to clause 1, wherein the second hole extends from one edge of the second detection surface to another edge of the second detection surface.7. The charged-particle beam apparatus according to clause 1, wherein the second hole extends from a central portion of the second detection surface to an edge of the second detection surface.8. The charged-particle beam apparatus according to any of the preceding clauses, wherein the field of view of the second detection surface includes the first detection surface.9. The charged-particle beam apparatus according to any of the preceding clauses, wherein the first detection surface defines a first hole to enable the charged-particle beam to pass therethrough t" the sample surface.10. The charged-particle beam apparatus according to clause 9, wherein the first hole is disposed in a central portion of the first detection surface.11. The charged-particle beam apparatus according to clause 9 or clause 10, wherein the first detection surface surrounds the first hole.12. The charged-particle beam apparatus according to any one of clauses 9-11, wherein the first detection surface completely surrounds the first hole.13. The charged-particle beam apparatus according to any one of clauses 9-12, wherein a boundary between the first detection surface and the first hole is substantially circular, substantially rectangular or substantially hexagonal.14. The charged-particle beam apparatus according to clause 9, wherein the first hole extends from one edge of the first detection surface to another edge of the first detection surface.15. The charged-particle beam apparatus according to clause 9, wherein the first hole extends from a central portion of the first detection surface to an edge of the first detection surface.16. The charged-particle beam apparatus according to any one of the preceding clauses, wherein a radial line of the first detection surface forms a first angle 01 with a plane perpendicular to the charged-particle beam that is equal to or less than 45°, preferably equal to or less than 30°, more preferably equal to or less than 15°.17. The charged-particle beam apparatus according to clause 16, wherein the first angle 0i is 0 °.18. The charged-particle beam apparatus according to any one of the preceding clauses, wherein a radial line of the second detection surface forms a second angle with a plane perpendicular to the charged-particle beam that is equal to or less than 45°, preferably equal to or less than 30°, more preferably equal to or less than 15°.19. The charged-particle beam apparatus according to clause 18, wherein the second angle 02 is 0 °.20. The charged-particle beam apparatus according to any one of clauses 1 to 19, wherein the second detection surface is an annular detection surface and the second detection surface is disposed to surround the charged-particle beam.21. The charged-particle beam apparatus according to any of the preceding clauses, further comprising a third detection surface, wherein the third detection surface is an annular detection surface, which is disposed to surround the charged-particle beam.22. The charged-particle beam apparatus according to any one of clauses 1-15, wherein the signal particles generated by the sample in response to the charged-particle beam are directed away from an axis defined by the charged-beam to form a secondary beam.23. The charged-particle beam apparatus according to clause 22, wherein a radial line of the first detection surface forms a first angle Pi with a plane perpendicular to the secondary beam that is equal to or less than 45°, preferably equal to or less than 30°, more preferably equal to or less than 15°.24. The charged-particle beam apparatus according to clause 23, wherein the first angle Pi is 0 °.25. The charged-particle beam apparatus according to any one of clauses 22-24, wherein a radial line of the second detection surface forms a second angle Pi with a plane perpendicular to the secondary beam that is equal to or less than 45°, preferably equal to or less than 30°, more preferably equal to or less than 15°.26. The charged-particle beam apparatus according to clause 25, wherein the first angle P2 is 0 °.27. The charged-particle beam apparatus according to any one of clauses 22-26, wherein the second detection surface is an annular detection surface and the second detection surface is disposed to surround the secondary beam.28. The charged-particle beam apparatus according to one of clauses 22-27, further comprising a third detection surface, wherein the third detection surface is an annular detection surface, which is disposed to surround the secondary beam.29. The charged-particle beam apparatus according to clause 21 or clause 28, wherein the third detection surface comprises a plurality of segments.30. The charged-particle beam apparatus according to any one of the preceding clauses, where the first detection surface comprises a plurality of segments.31. The charged-particle beam apparatus according to any one of the preceding clauses, wherein the second detection surface comprises a plurality of segments.32. The charged-particle beam apparatus according to any one of the preceding clauses further comprising an acceleration unit, configured to apply a potential difference between the first detection surface and the second detection surface.33. The charged-particle beam apparatus according to one any one of clauses 21 or 28 or, further comprising an acceleration unit which is configured to apply a potential difference between the first detection surface and the third detection surface.34. The charged-particle beam apparatus according to any one of the preceding clauses, further comprising: a charged-particle detection unit which is configured assign an electrical signal generated by the first detection surface and an electrical signal generated by the second detection surface to a single charged-particle count, in response to a determination that:the sum of energies of a first electrical signal and a second electrical signal received within a given timeframe corresponds to a predicted value of the energy of a secondary particle or backscattered particle generated by the sample.35. The charged-particle beam apparatus according to clause 34, wherein the given timeframe is equal to a pixel dwell time of the charged-particle beam on the sample.36. The charged-particle beam apparatus according to any one of the preceding clauses wherein the first detection surface is a semiconductor -based detector.37. The charged-particle beam apparatus according to any one of the preceding clauses wherein the second detection surface is a semiconductor-based detector.38. The charged-particle beam apparatus according to any one of the preceding clauses wherein the first detection surface and / or the second detection surface comprise germanium.39. The charged-particle beam apparatus according to any one of the preceding clauses wherein the first detection surface and / or the second detection surface comprise silicon.40. The charged particle beam apparatus of any one of the preceding clauses, wherein the charged-particle beam apparatus is configured to direct a plurality of charged particle beams onto the sample; and comprising an array of sensing elements for each of the plurality of charged particle beams.41. A method of detecting particles in a charged-beam apparatus, comprising the steps of: detecting with a first detection surface one or more returning particles, which are generated by a sample in response to a charged-particle beam being incident on the sample, and detecting with a second detection surface one or more particles generated by the first detection surface, which are generated in response to the returning particles being incident on the first detection surface.42. A charged-particle beam apparatus configured to direct a charged-particle beam onto a sample, the charged-particle beam apparatus comprising a detection system which comprises: a first detector having a first detection surface configured to generate first electrical signals in response to first signal particles being incident on the first detection surface, the first detector defining a first hole to enable the charged-particle beam to pass therethrough to the sample, wherein the first signal particles are generated by the sample in response to the charged-particle beam being incident on the sample; and a second detector having a second detection surface configured to generate second electrical signals in response to second signal particles being incident on the second detection surface, the second detector defining a second hole to enable the charged particle beam to pass therethrough to the sample and to enable the first signal particles to pass therethrough to the first detector surface, wherein the second signal particles are generated by the first detector in response to the first signal particles being incident on the first detection surface.43. The charged-particle beam apparatus of clause 42, wherein the field of view of the second detection surface includes the first detection surface.44. The charged-particle beam apparatus of clause 42 or clause 43, wherein a radial line of the first detection surface forms a first angle Pi with a plane perpendicular to the charged-particle beam that is equal to or less than 45°, preferably equal to or less than 30°, more preferably equal to or less than 15°.45. The charged-particle beam apparatus according to clause 44, wherein the first angle Pi is 0 °.46. The charged-particle beam apparatus according to any one of clauses 42 to 45, wherein a radial line of the second detection surface forms a second angle P2 with a plane perpendicular to the charged-particle beam that is equal to or less than 45°, preferably equal to or less than 30°, more preferably equal to or less than 15°.47. The charged-particle beam apparatus according to clause 46, wherein the second angle P2 is 0 °.48. The charged-particle beam apparatus according to any one of clauses 42 to 47, wherein the second detector is an annular detector and the second detection surface is disposed to surround the charged-particle beam.49. The charged-particle beam apparatus according to one of clauses 42 to 48 further comprising a third detector having a third detection surface, wherein a third detection surface is an annular detection surface disposed to surround the charged-particle beam.50. The charged-particle beam apparatus according to clause 49, wherein the third detection surface comprises a plurality of segments.51. The charged-particle beam apparatus according to any one of clauses 42-50, wherein the first detection surface comprises a plurality of segments.52. The charged-particle beam apparatus according to any one of clauses 42-51, wherein the second detection surface comprises a plurality of segments.53. The charged-particle beam apparatus according to any one of clauses 49-50, further comprising an acceleration unit, configured to apply a potential difference between the first detection surface and the second detection surface.54. The charged-particle beam apparatus according to any one of clauses, further comprising an acceleration unit which is configured to apply a potential difference between the first detection surface and the third detection surface.55. The charged-particle beam apparatus according to any one of clauses 42-54, further comprising: a charged-particle detection unit which is configured assign an electrical signal generated by the first detection surface and an electrical signal generated by the second detection surface to a single charged-particle count, in response to a determination that:the sum of energies of a first electrical signal and a second electrical signal received within a given timeframe corresponds to a predicted value of the energy of a secondary particle or backscattered particle generated by the sample.56. The charged-particle beam apparatus according to clause 55, wherein the given timeframe is equal to a pixel dwell time of the charged-particle beam on the sample.57. The charged-particle beam apparatus according to any one of clauses 42-56, wherein the first detector is a semiconductor-based detector.58. The charged-particle beam apparatus according to any one of clauses 42-57, wherein the second detector is a semiconductor-based detector.59. The charged-particle beam apparatus according to any one of clauses 42-58, wherein the first detector and / or the second detector comprise germanium.60. The charged-particle beam apparatus according to any one of clauses 42-59, wherein the first detection surface and / or the second detection surface comprise silicon.61. The charged particle beam apparatus according to any one of clauses 42-60, wherein the charged-particle beam apparatus is configured to direct a plurality of charged particle beams onto the sample; and comprising an array of sensing elements for each of the plurality of charged particle beams.62. A non- transitory computer-readable medium having instructions that, when executed by a computer, cause the computer to execute a method of detecting particles in a charged-beam apparatus, the method comprising: detecting with a first detection surface one or more returning particles, which are generated by a sample in response to a charged-particle beam being incident on the sample, and detecting with a second detection surface one or more particles generated by the first detection surface, which are generated in response to the returning particles being incident on the first detection surface.

[0091] 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 metrolo apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device).

[0092] Although specific reference may have been made above to the use of embodiments of the invention in the context of sample assessment, it will be appreciated that the invention, where the context allows, is not limited to sample assessment and may be used in other applications, for example electron beam lithography.

[0093] 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 abov are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art thatmodifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A charged-particle beam apparatus configured to direct a charged-particle beam onto a sample, the charged-particle beam apparatus comprising a detection system which comprises: a first detection surface configured to generate electrical signals in response to signal particles generated by the sample in response to the charged-particle beam; and a second detection surface configured to generate electrical signals in response to incident secondary particles or backscattered particles generated by the first detection surface in response to the signal particles, the second detection surface defining a second hole to enable the signal particles to pass to the first detection surface.

2. The charged-particle beam apparatus according to claim 1, wherein the field of view of the second detection surface includes the first detection surface.

3. The charged-particle beam apparatus according to claim 1, wherein the first detection surface defines a first hole to enable the charged-particle beam to pass therethrough to the sample surface.

4. The charged-particle beam apparatus according to claim 1, wherein a radial line of the first detection surface forms a first angle Pi with a plane perpendicular to the charged-particle beam that is equal to or less than 45°, preferably equal to or less than 30°, more preferably equal to or less than 15°.

5. The charged-particle beam apparatus according to claim 4, wherein the first angle Pi is 0 °.

6. The charged-particle beam apparatus according to claim 1, wherein a radial line of the second detection surface forms a second angle Pz with a plane perpendicular to the charged-particle beam that is equal to or less than 45°, preferably equal to or less than 30°, more preferably equal to or less than 15°.

7. The charged-particle beam apparatus according to claim 6, wherein the second angle Pz is 0 °.

8. The charged-particle beam apparatus according to claim 1, wherein the second detection surface is an annular detection surface and the second detection surface is disposed to surround the charged-particle beam.

9. The charged-particle beam apparatus according to claim 6 further comprising a third detection surface, wherein the third detection surface is an annular detection surface, which is disposed to surround the charged-particle beam.

10. The charged-particle beam apparatus according to claim 1, wherein the signal particles generated by the sample in response to the charged-particle beam are directed away from an axis defined by the charged-beam to form a secondary beam.

11. The charged-particle beam apparatus according to claim 10, wherein a radial line of the first detection surface forms a first angle Pi with a plane perpendicular to the secondary beam that is equal to or less than 45°, preferably equal to or less than 30°, more preferably equal to or less than 15°.

12. The charged-particle beam apparatus according to claim 11, wherein the first angle i is 0 °.

13. The charged-particle beam apparatus according to claim 10, wherein a radial line of the second detection surface forms a second angle P2 with a plane perpendicular to the secondary beam that is equal to or less than 45°, preferably equal to or less than 30°, more preferably equal to or less than 15°.

14. The charged-particle beam apparatus according to claim 13, wherein the second angle P2 is 0 °.

15. A non- transitory computer-readable medium having instructions that, when executed by a computer, cause the computer to execute a method of detecting particles in a charged-beam apparatus, the method comprising: detecting with a first detection surface one or more returning particles, which are generated by a sample in response to a charged-particle beam being incident on the sample, and detecting with a second detection surface one or more particles generated by the first detection surface, which are generated in response to the returning particles being incident on the first detection surface.

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