Multi electron-beam system for inspection with backscattered electrons

Abstract

A multi-beam electron system with a backscattered electron detector is provided. In an example, the backscattered electron detector is comprising a plurality of apertures with locally confined regions of scintillating material in vicinity of the apertures. Thereby, backscattered electrons are converted to photons, which are then guided by optical means such as light guides of optical imaging lens system to a spatially resolving photon detectors.

Description

[0001] Multi electron-beam with backscattered electrons

[0002] Field of the invention

[0003] The disclosure relates to a multi electron-beam system with a detector for detection of backscattered electrons. of the invention

[0004] WO 2005 / 024881 A2 discloses an electron microscope system which operates with a multiplicity of electron beamlets for the parallel scanning of an object to be inspected with a bundle of electron beamlets. The bundle of primary charged particle beamlets is generated by directing a primary charged particle beam onto a multi-beam forming unit, comprising at least one multi-aperture plate, which has a multiplicity of openings. One portion of the electrons of the electron beam is incident onto the multi-aperture plate and is absorbed there, and another portion of the beam transmits the openings of the multi-aperture plate and thereby in the beam path downstream of each opening an electron beamlets is formed whose cross section is defined by the cross section of the respective opening. The plurality of primary charged particle beamlets are focused by an objective lens on a surface of a sample and trigger secondary electrons or backscattered electrons to emanate as secondary electron beamlets from the sample, which are collected and imaged onto a detector. Each of the secondary beamlets is incident onto a separate detector element or group of detector elements, so that the secondary electron intensities detected therewith provide information relating to the surface of the sample at the location where the corresponding primary beamlet is incident onto the sample. The signal generally depends on the signal generated by secondary electrons, which depends on the secondary electron (SE) yield per primary electron and a geometrical collection efficiency of the electron microscope. The SE yield depends on material characteristics and the kinetic energy of the primary electrons.

[0005] Recently, there is a growing demand in inspection of samples such as wafers by using backscattered electrons (BSE) instead of secondary electrons. Typically, single beam scanning electron microscopes use an in-lens detector for detection of BSEs. For example, from single beam systems it is well known to arrange an electron-to-light converting element in the circumference of a backscattered electron beam. In US 8,598,525 B2, a szintillator is connected to a photon detector via a light guiding element. In WO 2023 / 011824 Al, this idea is illustrated at a single beam in the context of a multi-beam system, with however lacking any practical examples how to implement such a solution. Generally, with such an inlens-detector, the individual information for each beamlet of a multi electron-beam system is lost. In a further example schematically illustrated in WO 2023 / 011824 Al, it is proposed to apply free space imaging instead of a light guiding element between scintillators and a photon detector. However, the systems described in WO 2023 / 011824 Alare of very low efficiency, and improvement is required.

[0006] WO 2023 / 280642 Al discloses a detector for use in a charged particle device for an assessment apparatus to detect charged particles from a sample, wherein the detector comprises: a backscatter detector component set to a backscatter bias electric potential and configured to detect higher energy charged particles; and a secondary detector component set to a secondary bias potential and configured to detect lower energy charged particles. There is a potential difference between the backscatter bias electric potential and the secondary bias electric potential.

[0007] It is therefore a need for an improved multi-beam electron beam imaging system with the capability to detect backscattered electrons for each beamlet separately. It is therefore a need for an improved multi-beam electron beam imaging system with a high efficiency of signal detection arising from backscattered electrons.

[0008] Description of the invention

[0009] The objective technical problem is solved by the subject matter of the independent claims. Dependent claims are directed to advantageous embodiments.

[0010] The present patent application claims the priority of German patent application No. 10 2024 202 846.0 filed on 25 March 2024, the disclosure of which in the full scope thereof is incorporated in the present patent application by reference.

[0011] A solution to the problem is provided with a multi-beam system for wafer inspection with a backscattered electron detector. In an embodiment, a multi-beam system for inspection with backscattered electrons is comprising a multi-beam generation apparatus configured for generating a plurality of primary electron beamlets from at least one electron source and at least one electron optical imaging lens for forming a plurality of spatially separated focus spots in an image plane of the multi-beam system. The backscattered electron detector is arranged parallel to the image plane at a distance D of for example 10pm, wherein the backscattered electron detector comprises a plurality of apertures for transmitting the plurality of primary electron beamlets and a plurality of electron detection elements. At each aperture, at least one electron detection element is arranged adjacent to the corresponding aperture, such that backscattered electrons detected by the electron detection element are individually assigned to a specific primary electron beam passing a corresponding aperture. The electron detection element is comprising scintillating material configured for converting backscattered electrons into photons. The multi-beam system is further comprising at least one photon detector for spatially resolved detecting of photons generated within each of the electron detection elements. The multi-beam system is further comprising a wafer stage configured to arrange a surface of a wafer in the image plane. In an example, the at least one photon detector is formed by a spatially resolving image sensor, and wherein the system is further comprising a photon imaging lens system for imaging photons excited within the plurality of electron detection elements onto the at least one photon detector. In an example, the backscattered electron detector further comprises an array of photon optical lenses arranged at an upper side of the backscattered electron detector, wherein the upper side faces the incident primary electron beamlets, and wherein each aperture extends through at least one of the photon optical array lenses. In an example, the backscattered electron detector further comprises a photon optical field lens attached to the backscattered electron detector, and wherein each aperture extends through the photon optical field lens. With either one or a combination of both or array lenses and field lenses (each for photon optical imaging, respectively), a collection efficiency of light or photons generated within the backscattered electron detector is increased. With the photon optical field lens, photons excited from the plurality of electron detection elements are guided to a common overlapping or pupil area, where an imaging lens may be arranged to collect the photons. With the array lenses, for each of the electron detection elements, the collection efficiency of the isotropically excited photons is increased. The at least one photon optical lens attached to or in vicinity of the backscattered electron detector therefore increases a collection efficiency of photons generated by backscattered electrons, and thus increases throughput or reduces noise.

[0012] In an example, the multi-beam system is comprising a beam tube segment having a conic shape with decreasing diameter in direction of propagation of the plurality of primary electron beamlets. The beam tube segment of conic shape can be arranged within an upper pole shoe of the at least one electron optical imaging lens. Thereby, a collection efficiency of light generated within the backscattered electron detector is even further increased.

[0013] In an example, the backscattered electron detector is connected to a lower pole shoe of the at least one electron optical imaging lens, and wherein the lower pole shoe is electrically insulated from the upper pole shoe. In an example, a voltage VE is provided to the lower pole shoe in electrical connection to the backscattered electron detector to generate a decelerating or accelerating electrical field between backscattered electron detector and the beam tube segment. Thereby, a voltage for decelerating or accelerating of electrons can be provided to the backscattered electron detector. In an example, the upper pole shoe and the lower pole shoe form an axial gap. In an example, the backscattered electron detector comprises a Nickel-Steel alloy. Thereby, a magnetic field can be confined within an area between the lower pole shoe and the upper pole shoe.

[0014] In an example, at least one of the photon detectors is connected to a corresponding electron detection element by a light guide. A light guide can be integrated within the backscattered electron detector or formed on the upper side of it. In an example, the at least one electron optical imaging lens is a magnetic lens with an axial gap, and wherein the backscattered electron detector is arranged between a lower pole shoe of the magnetic lens and the image plane.

[0015] Generally, the backscattered electron detector can comprise a thin metallic reflection coating at the backside, which is facing the image plane. Backscattered electrons may penetrate the thin metallic reflection coating, while light generated within the backscattered electron detector is reflected and directed upwards in direction of the light guide or optical imaging lens system.

[0016] Generally, the backscattered electron detector can comprise a conducting coating on the upper side which faces the incident primary electron beamlets. The conductive coating is connected to ground. Thereby, a generation of surface charges is avoided. The conducting coating can be formed as transparent coating such as an ITO-coating for covering at least one of the arrays of photon optical array lenses, the photon optical field lens or a light guide.

[0017] In an example, at least one of the apertures of the backscattered electron detector is inclined by an inclination angle with respect to a normal N to the image plane. A primary electron beamlet is then inclined accordingly. Thereby, a maximum of the backscattered electron distribution is impinging on the backscattered electron detector adjacent to an aperture and a backscattered electron collection is increased. In an example, the inclination angle of the apertures increases with increasing distance to an optical axis of the at least one electron optical imaging lens. Thereby, inclination angles are for example adjusted to align with a Larmor rotation of primary electron beamlets within a magnetic field lens.

[0018] The invention is useful for wafer inspection. It is however not limited to wafer inspection but can be used for inspection of other objects as well, for example semiconductor masks. The examples and embodiments can also be combined with each other. Embodiments of the present disclosure will be explained in more detail with reference to drawings, in which:

[0019] Fig. 1 shows a sectional view of a multi-beam electron beam system with a backscattered electron detector according to an example

[0020] Fig. 2a,b,c show examples of arrangements of backscattered electron detectors

[0021] Fig. 3a, b shows an example of a backscattered electron detector

[0022] Fig. 4a, b shows an example of a backscattered electron detector

[0023] Fig. 5a,b,c show examples of a backscattered electron detector with an optical imaging system

[0024] Fig. 6 shows an example of a backscattered electron detector with an optical imaging system

[0025] Fig. 7a, b show an example of a backscattered electron detector with increased backscattered electron efficiency

[0026] Fig. 8 shows a further example of a backscattered electron detector with an optical imaging system

[0027] Fig. 9a, b shows a further example of a backscattered electron detector for optimized image acquisition Fig. 10 shows a sectional view of a multi-beam electron beam system with a backscattered electron detector and a secondary electron detection system

[0028] In the exemplary embodiments of the invention described below, components similar in function and structure are indicated as far as possible by similar or identical reference numerals. Some array elements, for example the plurality of primary electron beamlets, are identified by a reference number. Depending on the context, the same reference number may also identify a single element out of the array elements. Each primary electron beamlet (3.1, 3.2, 3.3) is one of the plurality of primary electron beamlets (3). It is understood that details or features of the examples can be combined or modified without hindrance.

[0029] Figure 1 is a schematic illustration of a multi-beam electron imaging system 1 (in short also multi-beam system 1) according to an embodiment. The multi-beam system 1 uses a plurality of electron beams for forming an image of an object 7. The multi-beam system 1 generates a plurality of J primary particle beams 3 which strike the object 7 to be examined in order to generate interaction products, e.g. backscattered electrons, which emanate from the object 7 and are subsequently detected. The multi-beam system 1 is of an electron microscope, which uses a plurality of primary electron beams 3 which are incident on a surface of the object 7 at a plurality of locations and generate there a plurality of primary electron beam focus spots 5, that are spatially separated from one another. The object 7 to be examined can be of any desired type, e.g., a semiconductor wafer or a semiconductor mask, and can comprise an arrangement of miniaturized elements. The surface 25 of the object 7 is arranged in an object plane 101 of an objective lens 102 of an object irradiation unit 100. The object 7 can be a wafer or a semiconductor mask. A diameter of the minimal beam spots or focus spots 5 can be shaped to be small in the object plane 101. Exemplary values of this diameter are below four nanometers, for example three nm or less. The focusing of the primary electron beamlets 3 for shaping the focus spots 5 is carried out by the objective lens system 102. In this case, the objective lens system 102 can comprise a magnetic immersion lens. Further examples of focusing means are described in the German patent DE 102020125534 B3, the entire content of which is herewith incorporated in the disclosure.

[0030] The plurality of focus spots 5 of the primary electron beamlets 3 form a regular raster arrangement of incidence locations, which are formed in the object plane 101. The number J of primary beamlets 3 may be five, twenty-five, or more. In practice, the number J of beamlets 3, and hence the number of incidence locations or focus spots 5, can be chosen to be significantly greater, such as, for example, J = 10 x 10, J = 2791, J = 20 x 30 or J = 100 x 100. Exemplary values of the pitch P between the incidence locations are 1 micrometer, 10 micrometers, or more, for example 40 micrometers. For sake of simplicity, only three primary beamlets 3.1, 3.2 and 3.3 with corresponding focus points 5.1, 5.2 and 5.3 are shown in figure 1. The primary electron beamlets 3 striking the object 7 generate interaction products, for example back-scattered electrons, which emanate from the surface of the object 7. A backscattered electron detector 601 is arranged in the vicinity of the object plane 101. The gap distance D between surface 25 and the backscattered electron detector is for example D = 10pm or more, for example D = 15mm or D = 20pm (see for illustration also figure 2a below). It is however also possible to select a smaller distance of for example D = 8pm or even less, for example D = 5pm. The backscattered electron detector 601 is comprising a plurality of apertures, allowing the for passage of the plurality of primary electron beamlets 3. The detector 601 is comprising a plurality of detection elements, arranged in the vicinity of the plurality of apertures. Detection elements can for example be diodes such as PMDs, or CMOS detection elements, and can be provided with electron-to-light conversion elements, or can be formed as direct electron detection elements. In an example, the detector 601 comprises an electron-to-light conversion element, such as formed by scintillating material, by which backscattered electrons are converted into light, and a plurality of light detection elements. The combination of the electron-to-light conversion element and the plurality of light detection elements hereby form together a plurality of electron detection elements. The detector or image sensor 601 can further comprise a relay optical system for imaging and guiding the photons generated by the electron to photon conversion unit on dedicated photon detection elements, such as a plurality of photomultipliers or avalanche photodiodes (not shown).

[0031] The primary particle beams 3 are generated in a beam generation apparatus 300 comprising at least one electron emitter 301, at least one collimation lens 303, a multi-aperture arrangement 305 and a first electron optical field lens 331 and a second electron optical field lens 333. The electron emitter 301 is connected to a voltage supply for providing an emitter voltage VK to the emitter 301 and generates at least one diverging electron beam 309, which is at least substantially collimated by the at least one collimation lens 303, and which illuminates the multi-aperture arrangement 305. The multi-aperture arrangement 305 comprises at least one first multi-aperture or filter plate, which has a plurality of J openings formed therein in a first raster arrangement. Particles of the illuminating particle beam 309 pass through the J apertures or openings of the first multi-aperture plate and form the plurality J of primary beamlets 3. Particles of the illuminating beam 309 which strike the first aperture plate are absorbed by the latter and do not contribute to the formation of the primary beamlets 3. A multi-aperture arrangement 305 usually comprises further array elements, for example an electron optical lens array, a stigmator array or an array of electron optical deflection elements. In this example, the particle beam 309 is perfectly collimated by collimation lens 303. However, it is also possible to design the multi-aperture arrangement 305 for a diverging or converging incident particle beam 309.

[0032] Together with the electron-optical field lens 331 and a electron-optical second field lens 333, the multi-aperture arrangement 305 focuses each of the primary beamlets 3 in such a way that focal points are formed in an intermediate image surface. The intermediate image surface can be real or virtual. The intermediate image surface can be curved to pre-compensate a field curvature and image plane tilt of the electron imaging system arranged downstream of the intermediate image surface.

[0033] The at least one electron optical field lens 103 and the electron optical objective lens 102 provide a first imaging particle optical unit for imaging the intermediate image surface onto the object plane 101 such that a second raster configuration of focus spots 5 of the primary electron beamlets 3 is formed there. Typically, the surface 25 of the object 7 is arranged in the object plane 101, and the focal points 5 are correspondingly formed on the object surface 25. The plurality of primary beamlets 3 form a crossover point 108, in the vicinity of which a multipole corrector or deflector 110 is arranged. The multi-pole corrector deflector 110 is used to deflect the plurality of primary beamlets 3 collectively and synchronously such that the plurality of focus spots 5 can be moved simultaneously over the surface 25 of the object 7. The multi-pole corrector deflector 110 is driven by a multi-electron beamlet control unit 860. Additionally, the multi-beam system 1 can comprise further static deflectors and multipole elements 112 configured to adjust the position and beam shapes of the plurality of the primary beamlets 3. Further information relating to such multi-beam particle beam systems and components used therein, such as, for instance, particle sources, multi-aperture plate and lenses, can be obtained from the international patent applications WO 2005 / 024881, WO 2007 / 028595, WO 2007 / 028596, WO 2011 / 124352 and WO 2007 / 060017 and the German patent applications having the publication numbers DE 10 2013 016 113 Al and DE 10 2013 014 976 Al, the disclosure of which in the full scope thereof is incorporated by reference in the present application.

[0034] The multi-beam electron imaging system 1 furthermore comprises a control unit 800 configured both for controlling the individual particle optical components of the multiple electron beam system and for evaluating and analyzing the signals obtained by the detector 601. In this case, the control or controller unit 800 can be constructed from a plurality of individual electronic computers or electronic components. Byway of example, the control unit 800 comprises a control operation processor 880 and a control module 830 for the control of the electron-optical elements of the primary beamlet generation unit 300. The control unit 800 further comprises a stage control module 850 for positioning the sample surface 25 or sample 7 by stage 500 within the object plane 101. The control unit 800 further comprises a control module to adjust a sample voltage VS, which is connected to a module 503 for supplying the sample voltage VS to the sample 7, said sample voltage VS also being referred to as extraction voltage. Thereby, during use, an extraction field is generated between the backscattered electron detector 601 and the surface 25 of the object 7. During use, the extraction field decelerates the primary electrons of the primary electron beamlets 3 before the sample surface 25 is reached and generates an additional focusing effect on the plurality of primary electron beamlets 3. At the same time, the extraction field serves during use to accelerate the backscattered electrons from the surface 25 of the object 7 to the backscattered electron detector 601.

[0035] The detector 601 comprises a plurality of sets of detection elements with one set of detection elements for each primary electron beamlet 3. During use, each set of detection elements is configured to record the intensity signal of the assigned backscattered electrons. The plurality of intensity signals for the plurality of backscattered electrons is transferred to the image data acquisition unit or imaging control module 810, where the image data is processed and stored in memory 890.

[0036] During an acquisition of an image patch by the plurality of primary electron beamlets 3, the stage 500 is continuously moved at least in a first or x-direction, and after the acquisition of an image patch, the stage 500 is moved to the next image patch to be acquired. In an implementation, the stage 500 is continuously moved in a first or x-direction while an image is acquired by scanning of the plurality of primary electron beamlets 3 with the raster scanner 110 in a first direction. Stage movement and stage position is monitored and controlled by sensors known in the art, such as Laser interferometers, grating interferometers, confocal micro lens arrays, or similar.

[0037] During an image scanning operation step, the control unit 800 is configured to trigger the image sensor 601 to detect in predetermined time intervals a plurality of timely resolved intensity signals from the plurality of backscattered electrons, and the digital image of an image patch is accumulated and stitched together from all positions of the plurality of primary electron beamlets 3.

[0038] Figure 2 illustrates examples of the backscattered electron detector 601. Same reference numbers are used as in figure 1 and reference is made also to the description of figure 1. In figure 2a, a backscattered electron detector 601 is arranged between the pole shoe 165 of magnetic objective lens 102 and the image plane 101, in which a surface 25 of a wafer 7 is placed by wafer stage 500. The primary electron beamlets 3 pass an electron beam tube 151 at a given kinetic energy. The beam tube 151 is connected to a tube potential VT, which is for example given by ground potential. The backscattered electron detector 601 is connected to a potential VE, which generates a decelerating force to the primary electrons of the primary electron beamlets 3.1 to 3.3. The backscattered electron detector 601 comprises a plurality of apertures 85.1 to 85.3, thereby passing the plurality of primary electron beamlets 3.1 to 3.3. The backscattered electron detector 601 is arranged at distance D above the image pane 101, in which by stage 500 (not shown) a surface (25) of a wafer 7 is arranged. Electron beamlets 3.1 to 3.3 are focused by objective lens 102 by a magnetic field generated by coil 161, to which a current I is provided. In this example, the magnetic lens 102 with pole shoe 165 forms an axial gap lens, wherein the gap 167 between the lower pole shoe and higher pole shoe for forming a magnetic field is oriented in axial direction almost perpendicular to the image plane 101. The pole shoe may not be formed by a single piece but can be formed by two electrically isolated parts forming an upper and a lower pole shoe.

[0039] The focusing magnetic field is thus generated above the backscattered electron detector 601 and the wafer surface 25. The wafer is further connected to a voltage supply 503, configured for providing during use a voltage VL to the wafer 7. Thereby, primary electrons are further decelerated before reaching the wafer surface 25. For example, primary electrons are accelerated upstream the tube 151 to a kinetic energy of about 30keV and propagate through the tube 151 at constant kinetic energy of 30keV. Primary electrons are focused by the magnetic field of magnetic lens 102 and decelerated in two steps by voltage VE provided to the backscattered electron detector 601 and by voltage VL provided to the wafer, such that the primary electrons impinge on the surface 25 of the wafer 7 at landing energies of for example below 2keV, below 1 keV or even less, for example 300 eV or 200eV. For example, the absolute value of voltage | VE | provided to the backscattered electron detector 601 is smaller than the absolute value voltage | VL| provided to the wafer, i.e. | VE | < | VL| . However, for backscattered electron detection, higher landing energies are as well possible, for example voltage VL provided to the wafer 7 can be adjusted for landing energies of more than 2keV, more than 5 keV, for example even up to 50 keV. At the interaction volumes generated at the focus spots 5.1 to 5.3 of the three beamlets 3.1 to 3.3 shown, backscattered electrons are generated (not shown), which are backscattered from the wafer surface 25 at kinetic energies like the landing energy. Backscattered electrons are then accelerated to the backscattered electron detector 601 and partially focused into detecting areas of the backscattered electron detector 601 (see below). By selecting VE, the kinetic energy of backscattered electrons when impinging onto the backscattered electron detector 601 can be adjusted.

[0040] Figure 2b illustrates another example. A backscattered electron detector 601 is arranged between the lower pole shoe 169 of magnetic objective lens 102. The primary electron beamlets 3 pass the beam tube 151 at a given kinetic energy. The beam tube 151 is connected to a tube potential VT, which is for example given by ground potential. In this example, the magnetic lens 102 with pole shoe 165 forms a radial gap lens, wherein the gap 167 between the lower pole shoe 169 and higher pole shoe for forming a magnetic field has an extension almost parallel to the image plane 101. The focusing magnetic field is thus generated directly above the surface 25 of a wafer 7. Such a magnetic lens 102 is sometimes also called an immersion lens. The backscattered electron detector 601 is thus arranged within the magnetic field of the immersion lens. The backscattered electron detector 601 can be connected to a potential VE, which optionally generates a decelerating force to the primary electrons of the primary electron beamlets 3.1 to 3.3 before entering the backscattered electron detector 601. The backscattered electron detector 601 comprises a plurality of apertures 85.1 to 85.3, thereby passing the plurality of primary electron beamlets 3.1 to 3.3. The wafer is further connected to a voltage supply 503, configured for providing during use a voltage VL to the wafer 7. Thereby, primary electrons are decelerated before reaching the wafer surface 25. Primary electrons are therefore focused by the magnetic immersion lens of magnetic lens 102 and decelerated in in at least one step by voltage VL provided to the wafer, such that the primary electrons impinge on the surface 25 of the wafer 7 at landing energies below 2keV, below 1 keV or even less, for example 300 eV or 200eV. At the interaction volumes generated at the focus spots 5.1 to 5.3 of the three beamlets 3.1 to 3.3 shown, backscattered electrons are generated (not shown), which are backscattered from the wafer surface 25 at kinetic energies similar to the landing energy. Backscattered electrons are then accelerated to the backscattered electron detector 601 and partially focus into detecting areas of the backscattered electron detector 601 (see below).

[0041] In an example, the backscattered electron detector 601 comprises paramagnetic materials with a permeability pR close to one, such as Aluminum, Magnesium or Tungsten. Other paramagnetic materials are given by paramagnetic semiconductors. In an example, backscattered electron detector 601 is comprises diamagnetic material such as Bismuth or Copper. In an example, backscattered electron detector 601 is comprises quartz (SiO2). Thereby, a magnetic field of objective lens 102 is at least partially transferred or increased through the backscattered electron detector 601. In an example, the backscattered electron detector 601 comprises Nickel-Iron alloys such as Permanorm with higher permeability pR of for example pR about 10000. An example is illustrated in Figure 2c, where the backscattered electron detector 601 forms an extension of the lower pole shoe of the magnetic objective lens 102. Thereby, the magnetic field generated by objective lens 102 is confined within a region between objective lens 102 and backscattered electron detector 601. Thereby, a magnetic field between backscattered electron detector 601 and surface 25 of the wafer 7 is reduced and Larmor rotation of the electron beamlets below the backscattered electron detector 601 is reduced.

[0042] In an example, the backscattered electron detector 601 comprises hard magnetic materials, such as Samarium Cobalt, Ferrite or Neodymium. Thereby, a magnetic field from the objective lens 102 is modified. Thereby, for example a Larmor rotation of electrons within or between the backscattered electron detector 601 and the surface 25 of the wafer 7 is reduced.

[0043] Figure 3 illustrates a first example of a backscattered electron detector 601 with a plurality of apertures 85.1 to 85.3 for transmitting of primary electron beamlets 3.1 to 3.3. Figure 3a shows a cross section through the backscattered electron detector 601, while figure 3b shows a view on the backside 611 of the backscattered electron detector 601, which is directed to and opposing the surface 25 of the wafer 7. Figure 3a shows only three primary electron beamlet 3.1 to 3.3, but there can be more, for example 91 beamlets, more than 100 beamlets, more than 300 beamlets, for example 2791 beamlets. Figure 3b shows seven apertures 85.1 to 85.7, but there can be more apertures, for example corresponding to the number of primary electron beamlets 3. Each primary electron beamlet 3 is focused by objective lens 102 (see Figure 1 and 2) and by decelerating fields generated by voltages VL and VE and transmits the corresponding aperture 85.1 to 85.7. Each primary electron beamlet 3 impinges on the surface 25 of wafer 7 at focus points 5, including focus points 5.1 to 5.3 in figure 3a. At each focus point 5, some electrons are scattered back and form backscattered electrons 9 including backscattered electrons 9.1 to 9.3. Backscattered electrons 9.1 to 9.3 as propagating in direction of backscattered electron detector 601. With the electrostatic field generated by VL and VE, backscattered electrons 9.1 to 9.3 are accelerated and slightly focused and impinge on detection elements 607.1 to 607.7 arranged on the backside 611 of backscattered electron detector 601. At least one detection element 607 is assigned to each aperture 85 and is for example arranged as circular detection element 607 of ring shape around each corresponding aperture 85. Thereby, backscattered electrons 9 can be detected for each primary beamlet 3 individually and separately.

[0044] Here, only one individual detection element 607.1 to 607.7 for each of the individual apertures 85.1 to 85.7 is shown. However, it is also possible to provide more than one detection element 607 for a single aperture 85. Thereby, an angular distribution of backscattered electrons can be detected.

[0045] In an example, electron detection elements 607 comprise a scintillating material. Within a scintillating material, photons are generated by backscattered electrons 9 within the scintillating material. An example is illustrated in figure 4a. The electron detection elements

[0046] 607.1 to 607.3 are comprising scintillating material and are connected to optical light-guides

[0047] 609.1 to 609.3 (only two shown in the cross section of Figure 4a). Photons excited within the electron detection elements 607.1 to 607.3 are guided within lightguides 609.1 to 609.3 to photon detectors 613.1 to 613.3. Photon detectors 613.1 to 613.3 are connected to imaging control module 810, configured to capture image data corresponding to the backscattered electron count received by backscattered electron detector 601. The lightguides 609.1 to 609.3 can be small in cross section of about few urn and can be formed by well-known planar fabrication techniques of integrated optics, for example by structured ion exchange within glass, or as SiO2-structures within of a conducting semiconductor sample. The lightguides 609.1 to 609.3 have the advantage that they are formed by linear dielectric material and photons do not have any interaction with electrons and electrical or magnetic fields present in vicinity of the apertures 85. In an example, the bulk material of the backscattered electron detector 601 is comprising conducting material such as doped silicon or Permanorm. The bulk material is connected to voltage supply for providing voltage VE to assist the generation of the accelerating or decelerating electrical field between backscattered electron detector 601 and wafer surface 25. Using a conductive material also prevents the backscattered electron detector 601 from charging up during exposure with for example backscattered electrons. The lightguides 609.1 to 609.3 can be embedded (as shown in Figure 4a) within the conductive material. In another example, lightguides 609.1 to 609.3 can be covered by a conductive coating formed by for example ITO or Aluminum to avoid a buildup of surface charges on exposed surfaces of the lightguides 609.1 to 609.3.

[0048] A further example is illustrated in figure 4b. The electron detection elements 607.1 to 607.3 are comprising scintillating material and are directly connected to photon detectors 613.1 to 613.3. Photon detectors 613.1 to 613.3 are connected by embedded and isolated electrical connections 619.1 to 619.3 to imaging control module 810, configured to capture image data corresponding to the backscattered electron count received by backscattered electron detector 601. Embedded electrical connections 619.1 to 619.3 can be formed by local regions of doped semiconductors of metal lines embedded within for example an isolating SiO2-structure. Embedded electrical connections 609.1 to 609.3 can be smaller in diameter compared to optical light guides 609, for example 1pm or even less.

[0049] In both examples illustrated in figure 4a and 4b, the backside 411 of backscattered electron detector 601 is covered by reflective coating 621, for example comprising a thin Aluminum coating. Aluminum has a low stopping power to backscattered electrons, but it reflects photons generated in electron detection elements 607.1 to 607.3 and thus reduces light loss from electron detection elements 607.1 to 607.3. In an example, electron detection elements 607.1 to 607.3 are completely encapsulated by a reflection coating except for an aperture opening to a lightguide 609 or a photon detector 613.

[0050] Figure 5 illustrates an example of a backscattered electron detector 601 with detection elements 607 comprising scintillating material. Same reference numbers are used as with the previous figures and reference is also made to the description of the previous figures. In figure 5a and 5b, the illustration is reduced and the beam tube 151 is shown with the backscattered electron detector 601 below. The beam tube 151 can have a diameter of about 10mm or more.

[0051] Here, photons generated within the detection elements 607.1 to 607.3 are imaged by photon optical imaging lens system 627 onto a detector camera (in light propagation direction 631, not shown). In the first example shown in Figure 5a, the backscattered electron detector 601 comprises a photon optical lens array with photon optical array lenses

[0052] 629.1 to 629.3. Each photon optical array lens 629.1 to 629.3 is arranged at the upper side 603 of backscattered electron detector 601, opposing at least one of the detection elements

[0053] 607.1 to 607.3. Photons generated within a detection element 607 are thus collimated by an photon optical array lens 629 and collection efficiency by photon optical lens system 627 is increased even when limited within the electron beam tube 151. At the electron beam cross over 108, a photon optical folding mirror 651 is arranged with aperture 653 for passing the plurality of electron beamlets comprising beamlets 3.1 to 3.3. Primary electron beamlets 3.1 to 3.3 are propagating through aperture 653 arranged at the electron beam cross over 108 and are further through beam tube 151. By electron objective lens 102 (not shown here, see figure 1 or 2), primary electron beamlets 3.1 to 3.3 are focused and are passing apertures

[0054] 85.1 to 85.3 (see also figures 3 and 4) of backscattered electron detector 601 before impinging onto the surface 25 or a wafer 7. Backscattered electrons 9.1 to 9.3 are scattered back from focus points 5.1 to 5.3 or primary electron beamlets 3.1 to 3.3 and directed in direction of individual electron detection elements 607.1 to 607.3. Backscattered electrons

[0055] 9.1 to 9.3 transmit the reflection coating 621 on the backside of backscattered electron detector 601 and are converted within electron detection elements 607.1 to 607.3 into photons. Photons are collimated by photon optical array lenses 629.1 to 629.3 assigned to the electron detection elements 607.1 to 607.3 and imaged by photon optical imaging lens system 627 along folded light path 631 by folding mirror 651 onto a spatially resolving detector (not shown), which assigns light detected within different detection areas to the different individual primary electron beamlets 3.1 to 3.3. Photon optical array lenses 629.1 to 629.3 can be formed by optical transparent lens material such as Silicon dioxide or fused silica and can be formed by usual planar fabrication techniques of photon optical micro-lens arrays for example for CMOS sensors. An aperture 85 is formed through each of the photon optical array lenses 629.1 to 629.3. Lens surfaces including the open surfaces within the apertures 85 of the photon optical array lenses 629.1 to 629.3 can be covered by conducting materials such as ITO to prevent charges to stick on the surface of the isolating lens material. Figure 5b shows another example of an photon optical imaging lens system 627 with a photon optical field lens 623 arranged on the upper side 603 of backscattered electron detector 601. Photon optical field lens 623 and photon optical array lenses 629 can also be combined. Thereby, collection efficiency is further increased. A photon optical field lens 623 can for example be formed from fused silica and connected to backscattered electron detector 601 by wringing or using Van-der-Waals-forces. Apertures 85 can be formed in a photon optical field lens for example by directional RIE etching. Open surfaces of photon optical field lens 623, including the open surfaces within the apertures 85, can be covered by a conductive coating 633 comprising conducting materials such as ITO to prevent charges to stick on the surface of the isolating lens material.

[0056] All lens surfaces of photon optical imaging optical system 627, which are arranged within vacuum, can be covered by conducting material such as ITO to prevent electrons and other charged particle to stick to the surfaces of the typically isolating lens materials. Photon optical folding mirror 651 can be a mirror with a high reflective metal coating such as an aluminum or silver coating. Folding mirror 651 and photon optical lenses of imaging optical system 627 are connected to ground, such that no charges are build up within the light optical system.

[0057] Figure 5c illustrates an example with increased collection efficiency for light generated within the backscattered electron detector 601. Here, photon optical array lenses 629 for collimating light and the function of photon optical field lens 623 is combined within the spatially variable photon optical array lenses 629, such that photon optical lenses with a larger distance to axis 105.1 have a larger prismatic power to refract light in direction of axis

[0058] 105.1. Folding angle 117 is here selected to be larger than 45°, such that the optical axis 105.2 after reflection is pointing upwards (or, within the coordinate system of Figure 5c, in negative z-direction). Electron optical objective lens 102 has an upper pole shoe 163, which has an inner section of conic shape with increasing diameter in direction of light along the negative z-direction. Inside the upper pole shoe 163, a conic section of electron beam tube

[0059] 151.2 is arranged with decreasing diameter in direction of the primary electrons 3, here in positive z-direction. An upper diameter LI is for example given by 20mm or more, for example 30mm or even 40mm. A lower diameter L2 close to the end section of the upper pole shoe 163 is for example less than 15mm, for example 10mm or even less, for example 8mm. Between the end section of the upper pole shoe 163 and the lower pole shoe 169, an axial gap 167 is formed. The lower pole shoe 169 is electrically isolated from the upper pole shoe 163 by isolator 171. The lower pole shoe 169 is connected to backscattered electron detector 601, which can be formed by the same material as the pole shoe 163, for example a Nickel-Steel Alloy. In the Nickel-steel alloy, a plurality of apertures 85 is provided and in vicinity of each aperture 85, an optically transparent electron detection element 607 comprising scintillating material is provided. The lower pole shoe is set to a potential by voltage VE, such that primary electrons are for example decelerated before passing the apertures 85 of backscattered electron detector 601. With the conic design of the beam tube 151.2 and the combined action of photon optical array lenses 629 with additional collecting photon optical field lens properties 623, a collection efficiency for light generated within the electron detection elements 607 (not shown in Figure 5c) is increased. The multi- beamlet charged-particle system 1 further comprises an upper section of beam tube 151.1 including a branching section for passage of the light trajectories (for example light trajectories 617.11 and 617.31) after being reflected at metallic mirror 651. Beam tube 151.1 and 151.2 and metallic mirror 651 is connected to ground, such that no charged are build up and primary electrons propagate within the beam tube 151.2 and 151.2 through vacuum free of disturbing electrical fields.

[0060] In the example of figure 5c, the floating space between lower surface of beam tube 151.2 set to ground level and the backscattered electron detector 601 set to voltage VE determines the major contribution for decelerating or accelerating electrons. Nevertheless, wafer 7 may be set to voltage VL for generating an additional decelerating or accelerating electrical field between the surface 25 or the wafer and the backside of the backscattered electron detector 601.

[0061] In an example, primary electron beamlets 3.1 to 3.3 can be scanned across the surface 25 by multipole deflector element 110 within a limited range according to diameters of apertures 85.1 to 85.3. For example, with a beam pitch of 10pm, a scanning range of 2pm, 3pm or even more is possible. For example, a typical aperture angle of a primary beamlet 3 is less than lOmrad, such that a diameter of a beamlet at 100pm from the wafer surface 25 is about 1pm. With an aperture diameter of for example 4pm of apertures 85 of the backscattered electron detector 601, a scanning range of for example 2pm is possible. After repeatedly image scanning to acquire a plurality of image patches according to the allowed scanning range of for example 2pm, and laterally moving the wafer 7 by wafer stage 500 by approximately the allowed scanning range, for example 2pm, a spatially resolved backscattered electron image of a closed segment of the surface 25 of a wafer 7 is acquired by repetitive image scanning and stitching.

[0062] Figure 6 illustrates an example of a photon optical imaging system. The photon optical imaging lens system 627 is hereby formed by a first photon optical field lens 623.1 arranged at the backscattered electron detector 601, an intermediate photon optical imaging lens 625 and a second photon optical field lens 623.1 arranged in front of the spatially resolving detector 635 with several detection areas including detection area 633 assigned to the different individual primary electron beamlet 3.1 and the backscattered electrons generated by this beamlet 3.1. The photon optical imaging lens system 627 is schematically illustrated in an unfolded system with photon optical mirror plane 651, at which the system is folded by a half folding angle of for example 20°, 30° or 45°, or more. The photon optical folding mirror 651 is again placed at the beam cross over 108, such that primary electron beamlets can transmit the photon optical mirror 651 at an aperture 653 of small size.

[0063] In an example, the backscattered electron detector 601 can be made in its entirety from dielectric material such as fused silica with doped areas forming the electron detection elements 607 for converting electrons to photons. The reflective coating can be formed by a stack of dielectric layers. Thereby, the impact on a magnetic field of the magnetic lens 102 is minimized. The surface of backscattered electron detector 601 is further completely covered by an ITO coating to prevent the build-up of local surface charges. In an example, as described above, backscattered electron detector 601 may still comprise at the back side 611 facing the wafer surface 25 a thin aluminum coating 621.

[0064] In an example, the primary electron beamlets 3 forms close to the image plane 101 a telecentric bundle of beamlets 3, meaning that all electron beamlets 3 are parallel to each other. However, the telecentricity property does not require the beamlet to be perpendicular to the image plane 101, in which the surface 25 of a wafer 7 is arranged. In an example, the telecentric bundle of primary electron beamlets 3 is inclined with respect to the optical axis 105 by inclination angle 17, wherein the optical axis 105 is perpendicular to the image plane 101. In such an example, apertures 85 can be arranged within the backscattered electron detector 601 at the same angle 17 to allow transmission of oblique primary electron beamlets 3.1 to 3.3. Figure 7a illustrates an example of such a backscattered electron detector 601. Since backscattered electrons 9.1 to 9.3 are generated including ballistic scattering processes, the backscattered electron distribution comprises a strong directional or angular distribution which is mirrored to the direction or angular distribution of the primary electrons. Therefore, backscattered electron collection efficiency is increase by the backscattering of electrons at mirrored angle 17 of incident primary beamlets 3.1 to 3.3, such that the maximum of a backscattered electron distribution is not directed back into an aperture 85 but on an electron detection element 607, which is arranged adjacent to an aperture 85 instead. The angle of obliquity 17 can be about 5°, 10° or more, for example 15°. In such an example, electron detection elements 607.1 to 607.3 can be arranged for example only on one side of the apertures 85 in direction of the angle of incidence 17. The principle of using oblique primary electron beamlets is not limited to telecentric bundles but can as well be applied to homocentric bundles of primary electron beamlets 3. It is understood that for example within an immersion lens field, electron beamlets are further inclined to the optical axis 105 perpendicular to the wafer surface 25 by the Larmor rotation with increasing inclination angle with respect to the optical axis 105. In the example illustrated in Figure 7b, inclination angles 17.2 and 17.3 of apertures 85.2 and 85.3 are individually adjusted to compensate such an increasing inclination of each corresponding electron beamlet due to the Larmor rotation. The inclination of apertures is here shown in the x-z-plane, it is however understood that a Larmor-Rotation typically leads to an inclination angle of primary and backscattered electron beamlets in azimuthal direction, i.e. in the illustration in figure 7b, the oblique apertures can be inclined in y- direction perpendicular to the x-z-plane with radially increasing inclination angle 17. It is also T1 understood that the lateral displacement K of the intersection of the ballistic backscattered electrons with respect to the center of an aperture 85 increases with gap distance D by K = 2 D tan (g) with inclination angle g (label number 17 in Figure 7b). Therefore, with larger inclination angles within immersion lenses 102 with for example up to several mrad, for example up to 5mrad, a smaller gap distance D of D < 15 or even D < 10pm, is preferable.

[0065] Figure 8 shows another example of a multi-beamlet charged-particle system 1. In this example, backscattered electron detector 601 comprises an array of apertures 85, in each of which an electron source 301 is arranged. Only two electron sources 301.1 and 301.2 are shown, but there can be many more, for example 469 electron sources 301 forming 469 beamlets. The backscattered electron detector 601 further comprises a plurality of isolating layers 693 and extracting electrode layer 697. By providing appropriate voltages to the extracting electrode layer 697, electrons are accelerated from the electron sources 301.1, 303.3 and focused by electrostatic lens fields within the apertures 85 formed by lens electrode layer 698. Electron beamlets 3.1 and 3.2 are impinging on the surface 25 of wafer 7 and cause backscattered electrons to be directed backwards in direction of backscattered electron detector 601. Backscattered electron detector 601 comprises electron detection elements 607.1 and 607.2, each arranged in vicinity of an aperture 85. Electron detection elements 607.1 and 607.2 comprise scintillating material to convert electrons into photons. Photons emitted from the electron detection elements 607.1, 607.2 are then imaged by photon optical imaging lens system 627 onto spatially resolving photon detector 635. The example of figure 8 has the advantage that the detection of backscattered electrons by conversion of backscattered electrons into photons within the localized and confined electron detection elements 607.1, 607.2 does not need any electrical connection. Photons are inert to interaction within linear material and can be imaged without interference with electron-optical elements or charged particles.

[0066] A typical photon detector is formed by a spatially resolving image sensor 635 such as a CMOS sensor. An image field of a multi-beam system 1 for example has a diameter of 100pm or more. Small spatially resolving image sensor 635 with high resolution are familiar from camera sensors from smartphones. With small sensors, a moderate magnification M of for example M = 1, M = 2, or M = 5, of an imaging lens system 627 is sufficient. Thereby, system extension of the imaging lens system 627 can be keep small.

[0067] In one example of a backscattered electron detector 601 as shown in figure 8, electron sources 301.1, 303.2 are formed by a photocathode which is exposed by focus light. In such an example, photon optical system 627 further comprises a photon optical beam-splitter for guiding focused excitation light to specific positions at the photocathode to excite electrons at the source position within the apertures 85.

[0068] Figure 9 illustrates another example of a backscattered electron detector 601. Same reference numbers as within the previous figures are used and reference is also made to the description of the previous figures. Here, the apertures 85.1 to 85.5 are formed as elongated slits to allow for a larger scanning range of the primary electron beamlets (see figure 8b). For example, primary electrons are scanned in y-direction which the wafer 7 is continuously moved by stage 500 in x-direction. Thereby, a complete surface area of a surface 25 of a wafer 7 can be imaged by parallel scanning of the plurality of beamlets 3 in a first direction while moving the wafer 7 in a second direction.

[0069] Figure 10 illustrates another example of a multi-beamlet charged-particle system 1 with a backscattered electron detector 601. The backscattered electron detector 601 of figure 10 can be any of the backscattered electron detector 601 shown in figure 1 to 7 or 9. In addition the backscattered electron detector 601, multi-beamlet charged-particle system 1 comprises an electron-optical beam divider 400. Beam divider 400 is separating backscattered electrons and secondary electrons 19 which pass the apertures 85 of backscattered electron detector 601 along secondary electron beam path 11 into an electron-optical detection system 200. Both types of electrons are further described without limit as secondary electrons 19. Electron-optical detection system 200 comprises electron optical lenses 206, 208 and 210 to image secondary electrons 19 onto a spatially resolving detector 209, onto which focus points 15.1 to 15.3 of secondary electrons 19 are formed. With the multi- beamlet charged-particle system 1 according to figure 10, backscattered electrons 9 as well as secondary electrons 19 of typically lower kinetic energy can be detected separately and more information of a surface 25 of a wafer 7 can be acquired.

[0070] Throughout the embodiments and examples, for the purpose of illustration, only few primary beamlets, light paths or backscattered electrons are shown. It is understood, however, that the embodiments and examples are generally applicable to large numbers J of primary electron beamlets, for example J = 91 beamlets, J = 300 beamlets, J = 469 beamlets, or even more, for example J = 2791 beamlets.

[0071] The disclosure is summarized by following clauses, but the invention shall not be considered as limited to the following clauses, but various combinations and modifications thereto are covered as well.

[0072] Clause 1: A multi-beam system (1) for wafer inspection with backscattered electrons (9), comprising

[0073] - a multi-beam generation apparatus (300) configured for generating a plurality of primary electron beamlets (3) from at least one electron source (301),

[0074] - at least one electron optical imaging lens (102, 698) for forming a plurality of spatially separated electron focus spots (5) in an image plane (101) of the multi-beam system (1),

[0075] - a wafer stage (500) configured to arrange a surface (25) of a wafer (7) in the image plane (101),

[0076] - a backscattered electron detector (601) arranged parallel to the image plane (101) at a distance D, wherein the backscattered electron detector (601) comprises a plurality of apertures (85) for transmitting the plurality of primary electron beamlets (3) and a plurality of electron detection elements (607), wherein for each aperture (85), at least one electron detection element (607) is arranged adjacent to the corresponding aperture (85), wherein each of the electron detection element (607) is comprising a scintillating material configured for converting backscattered electrons (9) into photons,

[0077] - at least one photon detector (613, 635) for spatially resolved detecting of photons generated within each of the electron detection elements (607),

[0078] - a photon imaging lens system (627) for imaging photons excited by the electron detection elements (607) onto the at least one photon detector (635), wherein the photon imaging lens system (627) comprises at least one photon optical lens (623, 629) attached to the backscattered electron detector (601), and wherein each aperture (85) extends through the at least one photon optical lens (623, 628).

[0079] Clause 2: The system of clause 1, wherein the least one photon optical lens (623, 629) is formed as a single photon optical field lens (623) arranged at an upper side (603) of the backscattered electron detector (601), wherein the upper side (603) faces the incident primary electron beamlets (3), and wherein each aperture (85) extends through the photon optical field lens (623). Clause 3: The system of clause 1, wherein the least one photon optical lens (623, 629) is comprising an array of photon optical lenses (629) arranged at an upper side (603) of the backscattered electron detector (601), wherein the upper side (603) faces the incident primary electron beamlets (3), and wherein each aperture (85) extends through one photon optical lens of the array of photon optical lenses (629).

[0080] Clause 4: The system of any of the clauses 1 to 3, wherein the least one photon optical lens (623, 629) is formed as a hybrid lens formed by a single photon optical field lens integrated into an array of photon optical lenses (629), arranged at an upper side (603) of the backscattered electron detector (601), wherein the upper side (603) faces the incident primary electron beamlets (3), and wherein each aperture (85) extends through the photon optical lenses (629).

[0081] Clause 5: The system of any of the clauses 1 to 4, wherein the at least one photon detector (613, 635) is formed by a spatially resolving image sensor (635).

[0082] Clause 6: A multi-beam system (1) for wafer inspection with backscattered electrons (9), comprising

[0083] - a multi-beam generation apparatus (300) configured for generating a plurality of primary electron beamlets (3) from at least one electron source (301),

[0084] - at least one electron optical imaging lens (102, 698) for forming a plurality of spatially separated electron focus spots (5) in an image plane (101) of the multi-beam system (1),

[0085] - a wafer stage (500) configured to arrange a surface (25) of a wafer (7) in the image plane (101),

[0086] - a backscattered electron detector (601) arranged parallel to the image plane (101) at a distance D, wherein the backscattered electron detector (601) comprises a plurality of apertures (85) for transmitting the plurality of primary electron beamlets (3) and a plurality of electron detection elements (607), wherein for each aperture (85), at least one electron detection element (607) is arranged adjacent to the corresponding aperture (85), wherein each of the electron detection element (607) is comprising a scintillating material configured for converting backscattered electrons (9) into photons,

[0087] - at least one photon detector (613, 635) for spatially resolved detecting of photons generated within each of the electron detection elements (607),

[0088] - wherein the backscattered electron detector (601) comprises a plurality of light guides (609) for guiding photons from a lower side of the backscattered electron detector (601) to an upper side (603), wherein the upper side (603) faces the incident primary electron beamlets (3).

[0089] Clause 7: The system of clause 6, wherein the backscattered electron detector (601) comprises a conducting material, for example a Nickel-Steel alloy, and wherein the plurality of light guides (609) is embedded within the conducting material.

[0090] Clause 8: A multi-beam system (1) for wafer inspection with backscattered electrons (9), comprising

[0091] - a multi-beam generation apparatus (300) configured for generating a plurality of primary electron beamlets (3) from at least one electron source (301),

[0092] - at least one electron optical imaging lens (102, 698) for forming a plurality of spatially separated electron focus spots (5) in an image plane (101) of the multi-beam system (1),

[0093] - a wafer stage (500) configured to arrange a surface (25) of a wafer (7) in the image plane (101),

[0094] - a backscattered electron detector (601) arranged parallel to the image plane (101) at a distance D, wherein the backscattered electron detector (601) comprises a plurality of apertures (85) for transmitting the plurality of primary electron beamlets (3) and a plurality of electron detection elements (607), wherein the backscattered electron detector (601) comprises a conducting coating (633) on an upper side (603) which faces the incident primary electron beamlets (3).

[0095] Clause 9: A multi-beam system (1) for wafer inspection with backscattered electrons (9), comprising

[0096] - a multi-beam generation apparatus (300) configured for generating a plurality of primary electron beamlets (3) from at least one electron source (301),

[0097] - at least one electron optical imaging lens (102, 698) for forming a plurality of spatially separated electron focus spots (5) in an image plane (101) of the multi-beam system (1),

[0098] - a wafer stage (500) configured to arrange a surface (25) of a wafer (7) in the image plane (101),

[0099] - a backscattered electron detector (601) arranged parallel to the image plane (101) at a distance D, wherein the backscattered electron detector (601) comprises a plurality of apertures (85) for transmitting the plurality of primary electron beamlets (3) and a plurality of electron detection elements (607), wherein for each aperture (85), at least one electron detection element (607) is arranged adjacent to the corresponding aperture (85), wherein each of the electron detection element (607) is comprising a scintillating material configured for converting backscattered electrons (9) into photons,

[0100] - at least one photon detector (613, 635) for spatially resolved detecting of photons generated within each of the electron detection elements (607),

[0101] - a beam tube segment (151.2) having a conic shape with decreasing diameter in direction of propagation of the plurality of primary electron beamlets (3).

[0102] Clause 10: The system of clause 9, wherein the beam tube segment (151.2) of conic shape is arranged within an upper pole shoe (163) of the at least one electron optical imaging lens (102).

[0103] Clause 11: The system of clause 9 or 10, wherein the backscattered electron detector (601) is connected to a lower pole shoe (169) of the at least one electron optical imaging lens (102), and wherein the lower pole shoe (169) is electrically insulated from the upper pole shoe (163).

[0104] Clause 12: The system of clause 11, wherein the upper pole shoe (163) and the lower pole shoe (169) form an axial gap (167).

[0105] Clause 13: A multi-beam system (1) for wafer inspection with backscattered electrons (9), comprising

[0106] - a multi-beam generation apparatus (300) configured for generating a plurality of primary electron beamlets (3) from at least one electron source (301),

[0107] - at least one electron optical imaging lens (102, 698) for forming a plurality of spatially separated electron focus spots (5) in an image plane (101) of the multi-beam system (1),

[0108] - a wafer stage (500) configured to arrange a surface (25) of a wafer (7) in the image plane (101),

[0109] - a backscattered electron detector (601) arranged parallel to the image plane (101) at a distance D, wherein the backscattered electron detector (601) comprises a plurality of apertures (85) for transmitting the plurality of primary electron beamlets (3), wherein during use, a voltage VE is provided to the backscattered electron detector (601) to generate a decelerating or accelerating electrical field.

[0110] Clause 14: The system of clause 13, wherein, during use, a voltage VL is provided to the wafer (7), and wherein |VE | < | VL| .

[0111] Clause 15: A multi-beam system (1) for wafer inspection with backscattered electrons (9), comprising - a multi-beam generation apparatus (300) configured for generating a plurality of primary electron beamlets (3) from at least one electron source (301),

[0112] - at least one electron optical imaging lens (102, 698) for forming a plurality of spatially separated electron focus spots (5) in an image plane (101) of the multi-beam system (1),

[0113] - a wafer stage (500) configured to arrange a surface (25) of a wafer (7) in the image plane (101),

[0114] - a backscattered electron detector (601) arranged parallel to the image plane (101) at a distance D, wherein the backscattered electron detector (601) comprises a plurality of apertures (85) for transmitting the plurality of primary electron beamlets (3), wherein the backscattered electron detector (601) comprises a conducting material, for example a Nickel-Steel alloy.

[0115] Clause 16: A multi-beam system (1) for wafer inspection with backscattered electrons (9), comprising

[0116] - a multi-beam generation apparatus (300) configured for generating a plurality of primary electron beamlets (3) from at least one electron source (301),

[0117] - at least one electron optical imaging lens (102, 698) for forming a plurality of spatially separated electron focus spots (5) in an image plane (101) of the multi-beam system (1),

[0118] - a wafer stage (500) configured to arrange a surface (25) of a wafer (7) in the image plane (101),

[0119] - a backscattered electron detector (601) arranged parallel to the image plane (101) at a distance D, wherein the backscattered electron detector (601) comprises a plurality of apertures (85) for transmitting the plurality of primary electron beamlets (3), wherein the backscattered electron detector (601) comprises a dielectric material, for example fused silica or glass, and wherein the backscattered electron detector (601) is coated with a conducting material such as ITO.

[0120] Clause 17: A multi-beam system (1) for wafer inspection with backscattered electrons (9), comprising

[0121] - a multi-beam generation apparatus (300) configured for generating a plurality of primary electron beamlets (3) from at least one electron source (301),

[0122] - at least one electron optical imaging lens (102, 698) for forming a plurality of spatially separated electron focus spots (5) in an image plane (101) of the multi-beam system (1),

[0123] - a wafer stage (500) configured to arrange a surface (25) of a wafer (7) in the image plane (101),

[0124] - a backscattered electron detector (601) arranged parallel to the image plane (101) at a distance D, wherein the backscattered electron detector (601) comprises a plurality of apertures (85) for transmitting the plurality of primary electron beamlets (3), wherein the backscattered electron detector (601) comprises a thin metallic reflection coating at the backside (611), which is facing the image plane (101).

[0125] Clause 18: A multi-beam system (1) for wafer inspection with backscattered electrons (9), comprising

[0126] - a multi-beam generation apparatus (300) configured for generating a plurality of primary electron beamlets (3) from at least one electron source (301),

[0127] - at least one electron optical imaging lens (102, 698) for forming a plurality of spatially separated electron focus spots (5) in an image plane (101) of the multi-beam system (1),

[0128] - a wafer stage (500) configured to arrange a surface (25) of a wafer (7) in the image plane (101),

[0129] - a backscattered electron detector (601) arranged parallel to the image plane (101) at a distance D, wherein the backscattered electron detector (601) comprises a plurality of apertures (85) for transmitting the plurality of primary electron beamlets (3), wherein at least one of the apertures (85) of the backscattered electron detector (601) is inclined by an inclination angle (17) with respect to a normal N to the image plane (101).

[0130] Clause 19: The system of clause 18, wherein an inclination angle (17, 17.2, 17.3) increases with increasing distance to an optical axis (105) of the at least one electron optical imaging lens (102).

[0131] Clause 20: A multi-beam system (1) for wafer inspection with backscattered electrons (9), comprising

[0132] - a multi-beam generation apparatus (300) configured for generating a plurality of primary electron beamlets (3) from at least one electron source (301),

[0133] - at least one electron optical imaging lens (102, 698) for forming a plurality of spatially separated electron focus spots (5) in an image plane (101) of the multi-beam system (1),

[0134] - a wafer stage (500) configured to arrange a surface (25) of a wafer (7) in the image plane (101),

[0135] - a backscattered electron detector (601) arranged parallel to the image plane (101) at a distance D, wherein the backscattered electron detector (601) comprises a plurality of apertures (85) for transmitting the plurality of primary electron beamlets (3), wherein at least one of the apertures (85) of the backscattered electron detector (601) has an elongated shape in one direction for allowing a larger scanning range in the one direction.

[0136] A multi-beam electron system with a backscattered electron detector is provided. In an example, the backscattered electron detector is comprising a plurality of apertures with locally confined regions of scintillating material in vicinity of the apertures. Thereby, backscattered electrons are converted to photons, which are then guided by optical means such as light guides of optical imaging lens system to a spatially resolving photon detectors.

[0137] A list of reference numbers is provided:

[0138] 1 multi-beam system

[0139] 3 primary charged particle beamlets, or plurality of primary charged particle beamlets

[0140] 5 primary charged particle beam spot

[0141] 7 object

[0142] 9 backscattered electrons

[0143] 13 secondary electron beam path

[0144] 15 secondary charged particle image spot

[0145] 17 inclination angle

[0146] 19 secondary electrons

[0147] 25 surface of object

[0148] 85 aperture or apertures

[0149] 100 object irradiation unit

[0150] 101 object plane

[0151] 102 objective lens

[0152] 103 field lens

[0153] 105 optical axis

[0154] 108 beam cross over 110 multipole deflector element

[0155] 112 multipole corrector element

[0156] 117 folding angle

[0157] 151 electron beam tube 161 lens coil

[0158] 163 upper pole shoe

[0159] 165 pole shoe

[0160] 167 pole gap

[0161] 169 lower pole shoe 171 isolating bridge

[0162] 200 detection system

[0163] 206 electron-optical lens

[0164] 208 electron-optical lens

[0165] 209 electron detector 210 electron-optical lens

[0166] 300 beam generation apparatus

[0167] 301 charged particle source

[0168] 303 collimating lenses

[0169] 305 multi-aperture arrangement 309 primary electron beam

[0170] 331 first field lens

[0171] 333 second field lens

[0172] 400 beam divider 500 sample stage

[0173] 503 sample voltage supply

[0174] 601 backscattered electron detector

[0175] 603 upper side

[0176] 607 electron detection elements 609 light-guide

[0177] 611 backside of backscattered electron detector

[0178] 613 photon detector or photon detectors

[0179] 617 light ray

[0180] 619 electrical connection 621 reflection coating

[0181] 623 field lens

[0182] 625 imaging lens

[0183] 627 imaging lens system

[0184] 629 array lenses 631 light path

[0185] 633 conductive coating

[0186] 635 spatially resolving detector

[0187] 651 folding mirror 653 aperture

[0188] 693 isolating layer

[0189] 697 extracting electrode

[0190] 698 lens electrode layer

[0191] 800 control unit 810 imaging control module

[0192] 830 primary beam-path control module

[0193] 850 stage control module

[0194] 860 scanning operation control unit

[0195] 880 control operation processor 890 memory

Claims

ClaimsWhat is claimed is:

1. A multi-beam system (1) for wafer inspection with backscattered electrons (9), comprising- a multi-beam generation apparatus (300) configured for generating a plurality of primary electron beamlets (3) from at least one electron source (301),- at least one electron optical imaging lens (102, 698) for forming a plurality of spatially separated electron focus spots (5) in an image plane (101) of the multi-beam system (1),- a wafer stage (500) configured to arrange a surface (25) of a wafer (7) in the image plane (101),- a backscattered electron detector (601) arranged parallel to the image plane (101) at a distance D, wherein the backscattered electron detector (601) comprises a plurality of apertures (85) for transmitting the plurality of primary electron beamlets (3) and a plurality of electron detection elements (607), wherein for each aperture (85), at least one electron detection element (607) is arranged adjacent to the corresponding aperture (85), wherein each of the electron detection element (607) is comprising a scintillating material configured for converting backscattered electrons (9) into photons,- at least one photon detector (613, 635) for spatially resolved detecting of photons generated within each of the electron detection elements (607),- a photon imaging lens system (627) for imaging photons excited by the electron detection elements (607) onto the at least one photon detector (635), wherein the photon imaging lens system (627) comprises at least one photon optical lens (623, 629) attached to the backscattered electron detector (601), and wherein each aperture (85)extends through the at least one photon optical lens (623, 628).

2. The system of claim 1, wherein the least one photon optical lens (623, 629) is formed as a single photon optical field lens (623) arranged at an upper side (603) of the backscattered electron detector (601), wherein the upper side (603) faces the incident primary electron beamlets (3), and wherein each aperture (85) extends through the photon optical field lens (623).

3. The system of claim 1, wherein the least one photon optical lens (623, 629) is formed as an array of photon optical lenses (629) arranged at an upper side (603) of the backscattered electron detector (601), wherein the upper side (603) faces the incident primary electron beamlets (3), and wherein each aperture (85) extends through one photon optical lens of the array of photon optical lenses (629).

4. The system of any of the claims 1 to 3, wherein the least one photon optical lens (623, 629) is formed as a hybrid lens formed by a single photon optical field lens integrated into an array of photon optical lenses (629), arranged at an upper side (603) of the backscattered electron detector (601), wherein the upper side (603) faces the incident primary electron beamlets (3), and wherein each aperture (85) extends through the photon optical lenses (629).

5. The system of any of the claims 1 to 4, wherein the at least one photon detector (613, 635) is formed by a spatially resolving image sensor (635).

6. The system of any of the claims 1 to 5, wherein the backscattered electron detector (601) further comprises a plurality of light guides (609), for guiding photons from a lower side of the backscattered electron detector (601) to an upper side (603), wherein the upper side (603) faces the incident primary electron beamlets (3).

7. The system of any of the claims 1 to 6, wherein the multi-beamlet charged-particle system (1) is comprising a beam tube segment (151.2) having a conic shape with decreasing diameter in direction of propagation of the plurality of primary electron beamlets (3).

8. The system of claim 7, wherein the beam tube segment (151.2) of conic shape is arranged within an upper pole shoe (163) of the at least one electron optical imaging lens (102).

9. The system of claim 7 or 8, wherein the backscattered electron detector (601) is connected to a lower pole shoe (169) of the at least one electron optical imaging lens (102), and wherein the lower pole shoe (169) is electrically insulated from the upper pole shoe (163).

10. The system of claim 9, wherein the upper pole shoe (163) and the lower pole shoe (169) form an axial gap (167).

11. The system of any of the claims 1 to 10, wherein a voltage VE is provided to the backscattered electron detector (601) to generate a decelerating or acceleratingelectrical field between backscattered electron detector (601) and the beam tube segment (151.2).

12. The system of claim 11, wherein, during use, a voltage VL is provided to the wafer (7), and wherein |VE | < | VL| .

13. The system of any of the claims 1 to 12, wherein the backscattered electron detector (601) comprises a conducting material, for example a Nickel-Steel alloy.

14. The system of claim 13, wherein at least one optical lightguide (609) is embedded within the conducting material.

15. The system of any of the claims 1 to 12, wherein the backscattered electron detector (601) comprises a dielectric, for example fused silica or glass, and wherein the backscattered electron detector (601) is coated with a conducting material such as ITO.

16. The system of any of the claims 1 to 15, wherein the at least one electron optical imaging lens (102) is a magnetic lens with an axial gap (167), and wherein the backscattered electron detector (601) is arranged between a lower pole shoe (169) of the magnetic lens (102) and the image plane (101).

17. The system of any of the claims 1 to 16, wherein the backscattered electron detector (601) comprises a thin metallic reflection coating at the backside (611), which is facing the image plane (101).

18. The system of any of the claims 1 to 17, wherein the backscattered electron detector(601) comprises a conducting coating (633) on an upper side (603) which faces the incident primary electron beamlets (3), the conducting coating (633) covering at least one of the at least one photon optical lens (623, 629) or a light guide (609).

19. The system of any of the claims 1 to 18, wherein at least one of the apertures (85) of the backscattered electron detector (601) is inclined by an inclination angle (17) with respect to a normal N to the image plane (101).

20. The system of claim 19, wherein an inclination angle (17, 17.2, 17.3) increases with increasing distance to an optical axis (105) of the at least one electron optical imaging lens (102).

21. A multi-beam system (1) for wafer inspection with backscattered electrons (9), comprising- a multi-beam generation apparatus (300) configured for generating a plurality of primary electron beamlets (3) from at least one electron source (301),- at least one electron optical imaging lens (102, 698) for forming a plurality of spatially separated electron focus spots (5) in an image plane (101) of the multi-beam system (1),- a wafer stage (500) configured to arrange a surface (25) of a wafer (7) in the image plane (101),- a backscattered electron detector (601) arranged parallel to the image plane (101) at a distance D, wherein the backscattered electron detector (601) comprises a plurality of apertures (85) for transmitting the plurality of primary electron beamlets (3) and aplurality of electron detection elements (607), wherein for each aperture (85), at least one electron detection element (607) is arranged adjacent to the corresponding aperture (85), wherein each of the electron detection element (607) is comprising a scintillating material configured for converting backscattered electrons (9) into photons,- a plurality of photon detectors (613, 635) comprising at least one photon detector (613, 635) for detecting of photons generated within each of the plurality of electron detection elements (607),- a plurality of light guides (609).

22. The system of claim 21, wherein the backscattered electron detector (601) comprises a Nickel-Steel alloy.

23. The system of any of the claims 21 to 22, wherein the at least one electron optical imaging lens (102) is a magnetic lens with an axial gap (167), and wherein the backscattered electron detector (601) is arranged between a lower pole shoe (169) of the magnetic lens (102) and the image plane (101).

24. The system of any of the claims 21 to 23, wherein the backscattered electron detector (601) comprises a thin metallic reflection coating at the backside (611), which is facing the image plane (101).

25. The system of any of the claims 21 to 24, wherein the backscattered electron detector(601) comprises a conducting coating (633) on an upper side (603) which faces theincident primary electron beamlets (3).

26. The system of any of the claims 21 to 25, wherein at least one of the apertures (85) of the backscattered electron detector (601) is inclined by an inclination angle (17) with respect to a normal N to the image plane (101).

27. The system of claim 26, wherein an inclination angle (17, 17.2, 17.3) increases with increasing distance to an optical axis (105) of the at least one electron optical imaging lens (102).

28. A multi-beam system (1) for wafer inspection with backscattered electrons (9), comprising- a multi-beam generation apparatus (300) configured for generating a plurality of primary electron beamlets (3) from at least one electron source (301),- at least one electron optical imaging lens (102, 698) for forming a plurality of spatially separated electron focus spots (5) in an image plane (101) of the multi-beam system (1),- a wafer stage (500) configured to arrange a surface (25) of a wafer (7) in the image plane (101),- a backscattered electron detector (601) arranged parallel to the image plane (101) at a distance D, wherein the backscattered electron detector (601) comprises a plurality of apertures (85) for transmitting the plurality of primary electron beamlets (3) and a plurality of electron detection elements (607), wherein for each aperture (85), at least one electron detection element (607) is arranged adjacent to the corresponding aperturewherein the multi-beam system (1) is configured that at least one of primary electron beamlets (3) is traversing the image plane (101) at an inclination angle (17) with respect to a normal N to the image plane (101), and wherein at least one of the apertures (85) of the backscattered electron detector (601) is inclined by the inclination angle (17) with respect to a normal N to the image plane (101).

29. The system of claim 28, wherein an inclination angle (17, 17.2, 17.3) of an aperture (85) increases with increasing distance to an optical axis (105) of the at least one electron optical imaging lens (102).

30. The system of claim 28 or 29, wherein each of the electron detection element (607) is comprising a scintillating material configured for converting backscattered electrons (9) into photons, and wherein the multi-beam system (1) is further comprising at least one photon detector (613, 635) for spatially resolved detecting of photons generated within each of the electron detection elements (607).

31. The system of any of the claims 28 to 30, further comprising a photon imaging lens system (627) for imaging the electron detection elements (607) onto the at least one photon detector (635), wherein the photon imaging lens system (627) comprises at least one photon optical lens (623, 629) attached to the backscattered electron detector (601), and wherein each aperture (85) extends through at least one photon optical lens(623, 629) at the inclination angle (17, 17.2, 17.3).

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