Improvements relating to electron-based imaging

Abstract

An electron-based imaging system (100) comprises an electron emission source (105) that emits electrons. At least some of the electrons traverse an object (101) to be imaged. A Cherenkov radiator (106) receives at least some of the electrons that have traversed the object (101). The Cherenkov radiator (106) emits photons in response to an electron having a kinematic energy above a threshold level while propagating through the Cherenkov radiator (106). The electron emission source (105) emits the electrons with a kinematic energy so that the kinematic energy of at least some of the electrons received by the Cherenkov after having traversed the object (101) is above the threshold level. A photon-detection arrangement (107) receives photons emitted by the Cherenkov radiator (106). The photon-detection arrangement (107) provides a photon-detection output result indicating a spatial distribution of photons received by the photon-detection arrangement (107) during a detection time-interval. A processor (103) recognizes a ring-shaped spatial distribution of photons in the photon-detection output result. The processor (103) records a center of the ring-shaped spatial distribution as a location where an electron has arrived at the Cherenkov radiator (106). The processor (103) generates an image from respective locations that have successively been recorded accordingly on the basis of successive photon-detection output results provided by the photon-detection arrangement (107).

Description

[0001] Improvements relating to electron-based imaging.

[0002] FIELD OF THE INVENTION

[0003] An aspect of the invention relates to an electron-based imaging system. The electron-based imaging system may be in the form of, for example, a transmission electron microscope commonly designated by the acronym TEM. Other aspects of the invention relate to an electron-based examining device for use in an electron-based imaging system, to use of an electron-based imaging system, and to a method of preparing for use an electron-based imaging system.

[0004] BACKGROUND ART

[0005] Electron-based imaging basically works as follows. An electron emission source emits electrons with a relatively high kinematic energy, which may be, for example, several tens of kilo electron volt (keV) to several hundreds of kilo electron volts. The electrons traverse an object to be imaged, which may be, for example, a biological sample or a non-biological sample. The electrons experience scattering, as well as other effects, while traversing the object. As a result, the electrons that have traversed the object carry information on the object that can be used to form an image thereof.

[0006] An image of the object may thus be obtained by detecting the electrons that have traversed the object. Specifically, the image may be obtained by detecting a location on a detection surface where an electron hits the detection surface. This location detection is then carried out for numerous electrons that have traversed the object to be imaged. Respective detection locations correspond to respective pixels in the image. A count of electrons that have hit a detection location determine an intensity of the pixel corresponding with the detection location.

[0007] In prior electron-based imaging techniques, scintillator plates were generally used for location detection and thus for obtaining images. A scintillator plate generates a light flash in response to an electron hitting the scintillator plate, assuming that the electron has sufficient kinematic energy. The light flash occurs at a location corresponding with that where the electron has hit the scintillator plate. An optical detector may then detect this location. The optical detector is thus used to count light flashes occurring at respective locations. An image may thus be obtained on the basis of respective counts for respective locations.

[0008] More recent electron-based imaging techniques use so-called direct detectors, which are generally made of silicon. Basically, a direct detector has a detection surface and incorporates an array of sensor electrodes, which are spatially distributed beneath the detection surface. An electron that has hit the detection surface propagates through a medium, such as, for example, silicon, toward the array of sensor electrodes. The electron creates charge carriers while propagating through the medium at locations relatively close to that where the electron has hit the detection surface. One or more sensor electrodes that are relatively close to these locations may collect at least some of these charge carriers. These sensor electrodes then indicate the location where the electron has hit the detection surface.

[0009] The article by Barnaby D A Levin entitled “Direct detectors and their applications in electron microscopy for materials science “ published in the Journal of Physics: Materials, 4 (2021) 042005, mentions that direct detectors are an important innovation in electron microscopy. Direct detectors offer improved signal to noise at a given electron beam dose compared to conventional detectors, such as scintillator-coupled detectors.

[0010] However, direct detectors entail a compromise between, on the one hand, image resolution and, on the other hand, signal-to-noise ratio as well as reliability, longevity. An electron that propagates through the medium of the direct detector experiences lateral scattering. That is, the electron laterally moves away from the location where the electron has hit the detection surface in an erratic manner. The electron may still create charge carriers while being relatively distant from this location in a rather random direction. This adversely affects a precision with which the array of sensors can indicate the location where the electron has hit the detection surface. This, in turn, adversely affects image resolution.

[0011] Image resolution may be enhanced by positioning an array of sensor electrodes relatively close to a detection surface. This shortens a path that an electron propagating through a medium follows to reach the array of sensor electrodes. Shortening the path reduces an extent to which the electron may erratically scatter while creating charged particles that will be detected. However, shortening the path also reduces the number of charge carriers that can be collected by the sensor electrodes. An electron may therefore go undetected, which may adversely affect signal-to-noise ratio. What is more, shortening the path makes that active electronic circuits belonging to the sensor electrodes may get bombarded by electrons having a relatively high kinematic energy. The active electronic circuits may suffer from this bombardment, which may cause reliability and longevity issues.

[0012] SUMMARY OF THE INVENTION

[0013] There is a need for an electron-based imaging technique that offers an improvement in at least one of the following aspects: image resolution, image signal -to- noise ratio, reliability and longevity.

[0014] An aspect of the invention, which is defined in claim 1, relates to an electron-based imaging system comprising: a support adapted to hold an object to be imaged; an electron emission source adapted to emit electrons that traverse the object to be imaged; a Cherenkov radiator having a surface arranged to receive electrons that have traversed the object to be imaged, whereby the Cherenkov radiator emits photons in response to an electron having a kinematic energy above a Cherenkov threshold while propagating through the Cherenkov radiator, whereby the electron emission source emits the electrons with a kinematic energy so that the kinematic energy of at least some of the electrons received by the Cherenkov radiator after having traversed the object is above the Cherenkov threshold; a photon-detection arrangement configured to receive photons emitted by the Cherenkov radiator, the photon-detection arrangement being adapted to provide a photon-detection output result indicating a spatial distribution of photons received by the photon-detection arrangement during a detection time-interval; and a processor adapted to recognize a ring-shaped spatial distribution of photons in the photon-detection output result and to record a center of the ring-shaped spatial distribution as a location where an electron has arrived at the surface of the Cherenkov radiator, the processor being further adapted to generate an image from respective locations that have successively been recorded accordingly on the basis of successive photon-detection output results provided by the photon-detection arrangement.

[0015] A further aspect of the invention, which is defined in claim 16, relates to an electron-based examining device for use in an electron-based imaging system as defined hereinbefore, the electron-based examining device comprising the support, the Cherenkov radiator, the electron emission source, and the photon-detection arrangement, and a transmission module adapted to transmit photon-detection output results to the processor, which is external to the electron-based examining device.

[0016] Yet a further aspect of the invention, which is defined in claim 17, relates to use of an electron-based imaging system for imaging an object.

[0017] Yet a further aspect of the invention, which is defined in claim 18, relates to a method of preparing for use an electron-based imaging system as defined hereinbefore, the method comprising: making the electron emission source emit electrons without an object being present in the support; and using the spatial distribution of photons indicated in the photondetection output result provided by the photon-detection arrangement to configure the processor for recognizing the ring-shaped spatial distribution of photons in photondetection output results later obtained when an object is present in the support.

[0018] In each of these aspects, the so-called Cherenkov effect is used to detect a location on a detection surface where an electron hits the detection surface. The Cherenkov radiator emits photons in response to an electron having arrived at its surface. In this respect, the Cherenkov radiator has at least two particular characteristics, which are exploited to advantage.

[0019] One particular characteristic is that the Cherenkov radiator emits photons only as long as the kinematic energy of the electron traversing the Cherenkov radiator is above the so-called Cherenkov threshold level. In general, the electron will have propagated over a relatively short distance only when its kinematic energy drops below the Cherenkov threshold level. This prevents significant lateral scattering while photons are emitted. Since lateral scattering adversely affects image resolution as discussed hereinbefore, preventing this contributes to achieving a relatively high image resolution.

[0020] Another particular characteristic is that the photons that are emitted have a defined cone-like distribution that can relatively easily be detected. Moreover, this conelike distribution has a center point that corresponds with relatively great precision to the location where the electron has hit the surface of the Cherenkov radiator. This further contributes to achieving a relatively high image resolution. In addition, since the cone-like distribution can relatively easily be detected, relatively few electrons, if any, will go undetected. This, in turn, contributes to achieving a relatively high signal -to-noise ratio. What is more, the Cherenkov radiator may exclusive be composed of a material, or a composition of materials, that is damage resistant to electrons with a relatively high kinematic energy. Contrary to the direct detectors discussed hereinbefore, the Cherenkov radiator need not comprise any electronics that may get bombarded by such electrons. The Cherenkov radiator may absorb all electrons that have entered the Cherenkov radiator, or at least a significant part of these. The Cherenkov radiator thus effectively protects the photon detection arrangement against damage by electrons. This contributes to reliability and longevity.

[0021] These and other factors may thus offer an improvement with respect to prior-art electron-based imaging techniques, whereby the improvement is in at least one of the following aspects: image resolution, image signal-to-noise ratio, reliability and longevity.

[0022] For the purpose of illustration, some embodiments of the invention are described in detail with reference to accompanying drawings. In this description, additional features will be presented, some of which are defined in the dependent claims, and advantages will be apparent.

[0023] BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG. l is a schematic block diagram of a first exemplary electron microscopy system.

[0025] FIG. 2 is a conceptual diagram of an electron propagating in a Cherenkov radiator causing the Cherenkov radiator to emit photons.

[0026] FIG. 3 is a graph in which a probability of a photon being emitted by the Cherenkov radiator is plotted as a function of a radial position from a location where an electron hits a surface of the Cherenkov radiator.

[0027] FIG. 4 is a pictorial diagram of a distribution of photons emitted by the Cherenkov radiator in response to the electron propagating in the Cherenkov radiator after having hit its surface.

[0028] FIG. 5 is a schematic block diagram of a second exemplary electron microscopy system.

[0029] FIG. 6 is a schematic block diagram of a third exemplary electron microscopy system.

[0030] FIG. 7 is a schematic block diagram of a fourth exemplary electron microscopy system. DESCRIPTION OF SOME EMBODIMENTS

[0031] FIG. 1 schematically illustrates a first exemplary electron microscopy system 100. FIG. 1 provides a schematic block diagram of the first exemplary electron microscopy system 100. The first exemplary electron microscopy system 100 may be used, for example, for obtaining one or more high-resolution images of an object 101 in which the object 101 is represented with a relatively high magnification factor. The object 101 may be a nonbiological sample or a biological sample and will be referred to as sample 101 hereinafter. The first exemplary electron microscopy system 100 may be, for example, of the TEM type, TEM being an acronym of transmission electron microscope. More specifically, the first exemplary electron microscopy system 100 may be used for cryoelectron microscopy, which is a form of transmission electron microscopy where the sample 101 to be imaged is at cryogenic temperatures.

[0032] The first exemplary electron microscopy system 100 basically comprises an electron-based examining device 102 and a processor 103. The electron-based examining device 102 includes a support 104 that holds the sample 101 to be imaged, an electron emission source 105, a Cherenkov radiator 106, and a photon-detection arrangement 107. In the first exemplary electron microscopy system 100, the aforementioned functional entities of the electron-based examining device 102 are comprised in a vacuum chamber 108. The processor 103 may comprise a server, or another type of data-processing entity, which may be remote from the electron-based examining device 102. In such an embodiment, the electron-based examining device 102 may comprise a transmission module that can be coupled to a network, or another communication medium, through which the server can be reached. Such a transmission module is not represented in FIG. 1 for the sake of simplicity.

[0033] In the first exemplary electron microscopy system 100, the photon-detection arrangement 107 is directly coupled to the Cherenkov radiator 106. That is, photondetection arrangement 107 may contact the Cherenkov radiator 106 or, if there is any spacing between these two, this spacing is relatively small. The photon-detection arrangement 107 may be in the form of, for example, a single-photon-sensitive camera. The single photon-sensitive camera may be, for example, a fast data-driven optical camera, Tpx3Cam, which is presented in the article by Andrei Nomerotski entitled “Imaging and time stamping of photons with nanosecond resolution in Timepix based optical cameras”, published in Nuclear Instruments and Methods, volume 937, pages 26-30 (2019). The first exemplary electron microscopy system 100 basically operates as follows. The electron emission source 105 emits electrons, which may have a relatively high kinematic energy for a reason discussed hereinafter. The electrons that are emitted may conceptually be regarded as a rain of electrons falling down on the sample 101. At least some of these electrons traverse the sample 101 to be imaged. The electrons experience scattering, as well as other effects, while traversing the sample 101. As a result, the electrons that have traversed the sample 101 carry an image of the sample 101. The Cherenkov radiator 106 receives at least some of the electrons that have traversed the sample 101. Specifically, the electrons arrive at a surface 109 of the Cherenkov radiator 106, which will be referred to hereinafter as the Cherenkov detection surface 109.

[0034] The Cherenkov radiator 106 emits photons in response to an electron having a kinematic energy above a threshold level while propagating through the Cherenkov radiator 106. This threshold level will be referred hereinafter as the Cherenkov threshold. The Cherenkov threshold depends on which material, or composition of materials, the Cherenkov radiator 106 is made of. Some examples will be provided hereinafter. The Cherenkov threshold may be in the order of several tens of kilo electron volt to several hundreds of kilo electron volts (keV).

[0035] The kinematic energy of at least some of the electrons emitted by the electron emission source 105 is sufficiently high so that the kinematic energy of at least some of the electrons arriving at the Cherenkov detection surface 109 is above the Cherenkov threshold. The kinematic energy of the electrons emitted by the electron emission source 105 may be, for example, at least several tens of kilo electron volt to several hundreds of kilo electron volts (keV). In general, the kinematic energy of the electrons emitted by the electron emission source 105 may be somewhat above the Cherenkov threshold level. In vacuum, electrons do not lose kinematic energy, at least not significantly, when traveling from the electron emission source 105 to the Cherenkov radiator 106.

[0036] The photon-detection arrangement 107 receives photons emitted by the Cherenkov radiator 106. The photon-detection arrangement 107 provides a photondetection output result indicating a spatial distribution of photons received by the photondetection arrangement 107 during a detection time-interval. A processor 103 recognizes a ring-shaped spatial distribution of photons in the photon-detection output result. The processor 103 records a center of the ring-shaped spatial distribution as a location where an electron has arrived at the Cherenkov radiator 106. The processor 103 generates an image from respective locations that have successively been recorded accordingly on the basis of successive photon-detection output results provided by the photon-detection arrangement 107.

[0037] FIG. 2 schematically illustrates an electron propagating in the Cherenkov radiator 106 causing the Cherenkov radiator 106 to emit photons. FIG. 2 provides a conceptual diagram of this effect, which will be referred to hereinafter as the Cherenkov effect. In this example, the electron propagates in the Cherenkov radiator 106 along a line 201 with a velocity v. The Cherenkov effect occurs when the velocity v of the electron is greater than the speed of light within the Cherenkov radiator 106. The speed of light within the Cherenkov radiator 106 depends on a refractive index of the material, or the composition of materials, that make up the Cherenkov radiator 106. Some examples will be provided hereinafter. In FIG. 2, c denotes the speed of light in vacuum, and n denotes the refractive index of the Cherenkov radiator 106.

[0038] In case the kinematic energy of the electron is above the Cherenkov threshold, the velocity with which the electron propagates through the Cherenkov radiator 106 will initially be greater than the speed of light within the Cherenkov radiator 106. The electron will lose kinematic energy while propagating through the Cherenkov radiator 106. At a certain point, the kinematic energy of the electron drops below the Cherenkov threshold.

[0039] Starting at the Cherenkov detection surface 109, the Cherenkov effect may thus occur over a relatively short distance covered by the electron while propagating in the Cherenkov radiator 106. Any scattering that the electron experiences may therefore be relatively small and even insignificant. The Cherenkov effects may stop before the electron experiences significant scattering. Photons may thus be emitted along a relatively straight path, corresponding with the line 201 represented in FIG. 2, rather than over an erratic path. This property is exploited to advantage to achieve relatively higher image resolution as will be discussed hereinafter.

[0040] The photons that are emitted as a result of the Cherenkov effect form an emission wavefront 202 whose normal has an angle Q relative to the line 201 along which the electron propagates as illustrated in FIG. 2. This angle Q is generally referred to as the emission angle. The emission angle Q is determined by the following equation: in which = v / c. In FIG. 2, four black dots are indicated on the line 201 along which the electron propagates in the Cherenkov radiator 106. A photon is emitted at each black dot. This photon emission is represented by a circle around the black dot concerned. The electron traverses a distance v \z from the leftmost black dot to the rightmost back dot during a time interval At. The photon emitted at the leftmost black dot then propagates over c a distance -AZ during this time interval AZ. In this simplified representation, it is assumed that the velocity v of the electron is constant.

[0041] The photons emitted by Cherenkov radiator 106 may be regarded as forming a cone having an opening angle corresponding to the aforementioned emission angle 0. More precisely, the cone represents a mean of a probability distribution for photon emission angles. An electron will be slowed down while propagating in the Cherenkov radiator 106. Consequently, the emission angle 0 is largest for photons emitted when the electron has just entered the Cherenkov radiator 106, directly below the Cherenkov detection surface 109. The emission angle 0 becomes smaller as the electron further propagates in the Cherenkov radiator 106 losing kinematic energy and, therefore, losing velocity. The emission angle collapses to zero when the kinematic energy has dropped to the Cherenkov threshold.

[0042] FIG. 3 schematically illustrates a photon emission probability distribution 300 of a Cherenkov radiator 106. FIG. 3 is a graph having a horizontal axis, which represents a radial position r on an observation plane normal to an axis aligned with a direction in which electrons propagate in the Cherenkov radiator 106. The radial position r is relative to a point where this axis intersects with the observation plane. A vertical axis represents a probability P of a photon being emitted by the Cherenkov radiator 106. A curve represents the photon emission probability distribution 300 according to which the probability P of a photon being emitted by the Cherenkov radiator 106 varies as a function of the radial position r on the observation plane.

[0043] The photon emission probability distribution 300 is essentially radially symmetric with relatively sharp rim where two peaks 301, 302 occur. This sharp rim lies at a radial position directly related to the emission angle for photons emitted when the electron has just entered the Cherenkov radiator 106, directly below the Cherenkov detection surface 109. Specifically, the radial position where the sharp rim lies is equal to the tangent of the emission angle multiplied by a thickness of the Cherenkov radiator 106. Starting from the sharp rim, the probability of a photon being emitted decreases with the radial position, as illustrated in FIG. 3. This is because fewer photons are emitted when the kinematic energy of an electron decreases and, therefore, its velocity decreases and the emission angle decrease accordingly, as discussed hereinbefore. The radial position zero corresponds with the kinematic energy being equal to the Cherenkov threshold.

[0044] The photons produced by the Cherenkov effect are predominantly spectrally located in an ultraviolet (UV) spectral range. This is because the photons have a spectral intensity distribution that may be expressed as follows: wherein cW / dX represents the spectral intensity distribution a represents a fine structure constant, which is equal to 1 / 137, X represents a photon wavelength, and d represents a thickness of the Cherenkov radiator 106. The smaller the wavelength is, the greater the number of photons emitted at this wavelength. It is therefore desired that the Cherenkov radiator 106 has a relatively high ultraviolet transparency. That is, it is desired that photons may propagate through the Cherenkov radiator 106 with relatively low loss. The same applies to other elements that may be present in an optical path between the Cherenkov radiator 106 and the photon-detection arrangement 107.

[0045] The following table lists some suitable materials for the Cherenkov radiator

[0046] 106, as well as certain relevant characteristics of these materials.

[0047] The UV cutoff wavelength is defined as the wavelength at which transparency of the material concerned is halfway between maximum transparency and minimum transparency in the UV spectral range. The kinematic energy of the electron entering the Cherenkov radiator 106 at the Cherenkov detection surface 109 was assumed to be equal to 300 keV. This corresponds to f> = 0.77. The emission angle and the Cherenkov threshold have been calculated assuming this energy.

[0048] The photon-detection arrangement 107 receives photons emitted by the Cherenkov radiator 106. The photon-detection arrangement 107 provides a photondetection output result indicating a spatial distribution of photons 400 received by the photon-detection arrangement 107 during a detection time-interval. A processor 103 recognizes a ring-shaped spatial distribution of photons in the photon-detection output result. The processor 103 records a center of the ring-shaped spatial distribution as a location where an electron has arrived at the Cherenkov radiator 106. The processor 103 generates an image from respective locations that have successively been recorded accordingly on the basis of successive photon-detection output results provided by the photon-detection arrangement 107.

[0049] FIG. 4 schematically illustrates a distribution of photons 400 emitted by the Cherenkov radiator 106 in response to an electron propagating in the Cherenkov radiator 106 after having entered through the Cherenkov detection surface 109. FIG. 4 provides a pictorial diagram of this distribution on an observation plane normal to an axis aligned with a direction in which the electron propagates in the Cherenkov radiator 106. An emitted photon observed on the observation plane is pictorially represented by a star symbol.

[0050] FIG. 4 indicates a circle 401 that constitutes a best circular fit relative to the photons emitted by the Cherenkov radiator 106 as observed on the observation plane. FIG. 4 further indicates a center point 402 of this best fit circle 401; the center point 402 being pictorially represented by a cross symbol. The center point 402 essentially corresponds with a location on the Cherenkov detection surface 109 where the electron has entered the Cherenkov radiator 106.

[0051] In the first exemplary electron microscopy system 100 illustrated in FIG. 1, the photon-detection output result provided by the photon-detection arrangement 107 may correspond to, for example, the pictorial diagram provided by FIG. 4. That is, the observation plane mentioned hereinbefore with respect to FIG. 4 may correspond to a photon capturing surface of the photon-detection arrangement 107. More specifically, the photo-detection output result may indicate a set of x,y-coordinates, each x,y coordinate indicating a photon and a location on the photon capturing surface. The processor 103 may then determine a center point by means of, for example, a best fit circle as discussed hereinbefore with reference to FIG. 4. The processor 103 may determine which pixel in the image to be formed is associated with the center point. The processor 103 may then increment a value of this pixel by one unit.

[0052] In the first exemplary electron microscopy system 100, the electron emission source 105 may emit electrons at a relatively high rate. For example, the electron emission source 105 may emit 1 million electrons per second with an average delay of one microsecond between two consecutively emitted electrons. In this respect, it is noted that photons are produced by the Cherenkov radiator 106 in response to an electron propagating in the Cherenkov radiator 106 within a few nanoseconds only Accordingly, a set of photons in response to an electron as the one illustrated in FIG. 4 may be produced within a few nanoseconds at an average rate of one set of photons each microsecond.

[0053] As mentioned hereinbefore, the photon-detection arrangement 107 may be in the form of, for example, a single-photon-sensitive camera. The single photon-sensitive camera may be relatively fast in the sense that this camera can provide a new photo detection output result every one microsecond. The fast data-driven optical camera named Tpx3Cam, which was also mentioned hereinbefore, is an example of a single-photonsensitive camera that may constitute the photon-detection arrangement 107, or at least a part of thereof.

[0054] The Tpx3Cam is event driven. This means that the Tpx3Cam provides a detection result in response to a photon that is detected. Specifically, the Tpx3Cam provides a detection result in response to a photon having an energy level that exceeds a detection threshold. For each detected photon, the detection result comprises two metrics. One of these two metrics indicates a time of arrival (ToA), which is an instant at which the detection threshold level is exceeded. This metric will be referred to hereinafter as the ToA metric. The other metric indicates a time over threshold (ToT), which is a length of time during which detection threshold level remains exceeded. This metrics will be referred to hereinafter as the ToT metric. The ToT metric indicates an intensity of the photon that has been detected.

[0055] The processor 103 may use the ToA metric to establish that detected photons belong to a same set of photons produced by the Cherenkov radiator 106 in response to an electron having entered though the Cherenkov detection surface 109. For example, referring to FIG. 4, the processor 103 may receive a ToA metric for each photon represented by a star symbol. The processor 103 may then determine that differences between these ToA metrics, if any, are relatively small. This indicates that the detected photons occurred almost simultaneously, within a time interval in the order of, for example, a few tens of nanoseconds, or even smaller. In this embodiment, this time interval may correspond with the aforementioned detection time-interval. Having established that the detected photons belong to the same set, the processor 103 may then determine a best fit circle and a center point of this best fit circle, as discussed hereinbefore with reference to FIG. 4. The processor 103 determines which pixel in the image to be formed is associated with the center point. The processor 103 then increments a value of this pixel by one unit.

[0056] The processor 103 may use the ToT metric for a detected photon to determine if the intensity of the detected photon is sufficient to be accounted for. The processor 103 may thus exclude detection results provided by the Tpx3Cam of which the ToT metric is below a predefined threshold value. For example, the processor 103 may exclude one or more detection results in determining a best fit circle. As another example, the processor 103 may give respective weighing factors to respective detected photons based on respective ToT metrics for these detected photons. The processor 103 may also refrain from determining a best fit circle if there are too few detection results having a ToT metric that is above the predefined threshold value, although the ToA metrics indicates almost simultaneous occurrences.

[0057] FIG. 5 schematically illustrates a second exemplary electron microscopy system 500. FIG. 5 provides a schematic block diagram of the second exemplary electron microscopy system 500. Basically, the second exemplary electron microscopy system 500 may be regarded as a modified version of the first exemplary electron microscopy system 100 in which the photon-detection arrangement 107 has been implemented differently. Like elements are therefore denoted by like reference signs for the sake of simplicity and clarity.

[0058] In the second exemplary electron microscopy system 500, the photondetection arrangement 107 comprises a photon-detection device 501 outside the vacuum chamber 108. The photon-detection device 501 may comprise the Tpx3Cam discussed hereinbefore, or another type of single-photon-sensitive camera. The vacuum chamber 108 comprises a window 502 in an optical path extending between the Cherenkov radiator 106 and the photon-detection device 501. The photon-detection arrangement 107 further comprises a collimating lens 503 in the optical path extending between the Cherenkov radiator 106 and the photon-detection device 501. The collimating lens 503 may essentially be UV transparent given that the photons produced by the Cherenkov effect are predominantly spectrally located in the UV spectral range as discussed hereinbefore. In this embodiment, the collimating lens 503 is inside the vacuum chamber 108, positioned between the Cherenkov radiator 106 and the window 502 of the vacuum chamber 108.

[0059] Since the photon-detection device 501 is outside the vacuum chamber 108, this device can be relatively easily maintained and reconfigured, if so desired. In contrast, in the first exemplary electron microscopy system 100, maintenance or reconfiguration of the photon-detection arrangement 107 involves cooling and outgassing the vacuum chamber 108 once such an operation has been completed. However, the first exemplary electron microscopy system 100 may have a better photon detection efficiency because the single-photon-sensitive camera, such as the Tpx3Cam, is directly coupled to the radiator.

[0060] FIG. 6 schematically illustrates a third exemplary electron microscopy system 600. FIG. 6 provides a schematic block diagram of the third exemplary electron microscopy system 600. Basically, the third exemplary electron microscopy system 600 may be regarded as a modified version of the first exemplary electron microscopy system 100 in which the photon-detection arrangement 107 has been implemented differently. Like elements are therefore denoted by like reference signs for the sake of simplicity and clarity.

[0061] In the third exemplary electron microscopy system 600, the photondetection arrangement 107 equally comprises a photon-detection device 601 outside the vacuum chamber 108. The photon-detection device 601 may comprise an image sensor, which will be further discussed hereinafter. The photon-detection device 601 will therefore be referred to as the image sensor 601 hereinafter. Like in the second electron microscopy system, the vacuum chamber 108 comprises a window 602 in an optical path extending between the Cherenkov radiator 106 and the photon-detection device 601. The photondetection arrangement 107 further comprises an optical intensifier 603 in the optical path extending between the Cherenkov radiator 106 and the photon-detection device 601. In this embodiment, the optical intensifier 603 is inside the vacuum chamber 108 and may be positioned against the Cherenkov radiator 106, being directly coupled to the Cherenkov radiator 106. That is, optical intensifier 603 may contact the Cherenkov radiator 106 or, if there is any spacing between these two, this spacing is relatively small.

[0062] The optical intensifier 603 may emit a fast flash of light, which is essentially a multitude of photons, in response to a photon produced by the Cherenkov radiator 106. An embodiment of the optical intensifier 603 may comprise a photocathode, a microchannel plate, and a scintillator. The photocathode converts a single photon to a photoelectron. The photoelectron is multiplied in the microchannel plate and directed to the scintillator. The scintillator emits a fast flash of light in response to the photoelectron that has internally been produced in response to the single photon.

[0063] A fast flash of light emitted by the optical intensifier 603 in response to a photon may essentially have the same position as the photon. A fast flash of light emitted by the optical intensifier 603 may thus be regarded as a stronger substitute for a photon produced by the Cherenkov radiator 106. Consequently, an electron having hit the Cherenkov detection surface 109 and having propagated in the Cherenkov detector will result in a set of fast flashes of light occurring almost simultaneously. The set of fast flashes of light resulting from the electron will have a ring-shaped spatial distribution as discussed hereinbefore.

[0064] FIG. 4 provides an example of such a ring-shaped spatial distribution, which may thus also apply to the set of fast flashes of light. That is, in FIG. 4, each star symbol may represent a fast flash of light emitted by the optical intensifier 603. The fast flashes of light traverse the window 602 of the vacuum chamber 108 to reach the image sensor 601. The fast flashes of light are predominantly spectrally located in a blue spectral range. Consequently, there is no need for the window 602 of the vacuum chamber 108 to be UV transparent as in the second exemplary electron microscopy system 500.

[0065] The image sensor 601 may capture images at a rate that is lower than the average rate at which the electron emission source 105 emits electrons. The image sensor 601 may nonetheless be relatively fast, capturing images at a rate of, for example, at least 1000 frames per second. In this embodiment, this rate may determine the aforementioned detection time-interval.

[0066] A captured image will be a superposition of multiple sets of flashes of light consecutively emitted by the optical intensifier 603 in response to photons produced by Cherenkov radiator 106. That is, the captured image may be regarded as a superposition of multiple distributions of fast flashes of light like the one illustrated in FIG. 4. The processor 103 may recognize ring-shaped distributions in the captured image in case these ring-shaped distributions are sufficiently spaced apart. This may be the case, for example, if the sample 101 has a regular internal structure, such as, for example, a lattice structure. In that case, the electrons will hit the Cherenkov detection surface 109 at relatively few different locations that are relatively widely spaced apart. That is, there are specific spots on the Cherenkov detection surface 109 that are hit by electrons with relatively wide gaps between these spots where relatively few electrons hit. The image sensor 601 may operate as an integrator. For example, let it be assumed that capturing an image covers a time interval of one millisecond (1 ms). Let it further be assumed that the electron emission source 105 emits 1 million electrons per second with an average delay of one microsecond between two consecutively emitted electrons. Accordingly, 1000 electrons may hit the Cherenkov detection surface 109 during the capturing of the image. Let it further be assumed that multiple electrons hit the Cherenkov detection surface 109 at a particular location, a particular spot. In that case, the image sensor 601 may receive multiple fast flashes of light that overlap during the capturing of the image. Each of these fast flashes of light will have a ring-shaped distribution.

[0067] As a result of the multiple fast flashes of light overlapping, the ring-shaped distribution will be relatively pronounced in the image making that the ring-shaped distribution is relatively easily recognizable. Moreover, the ring-shaped distribution can be determined with relatively great precision. A best fit circle will be more precise. As a result, its center point will better correspond to the particular location where the multiple electrons have hit the Cherenkov detection surface 109. In this example, where 1000 electrons hit the Cherenkov detection surface 109 during the capturing of the image, a statistical precision of reconstructed coordinates may improve by factor of the square root of 1000, which is 31.6.

[0068] FIG. 7 schematically illustrates a fourth exemplary electron microscopy system 700. FIG. 7 provides a schematic block diagram of the fourth exemplary electron microscopy system 700. Basically, the fourth exemplary electron microscopy system 700 may be regarded as a modified version of the third exemplary electron microscopy system 600 to which a mirror 701 has been added, and in which the window 602 of the vacuum chamber 108 is positioned differently. Like elements are therefore denoted by like reference signs for the sake of simplicity and clarity. The mirror 701 is in the optical path extending between the Cherenkov radiator 106 and the image sensor 601. The mirror 701 directs the flashes of light emitted by the optical intensifier 603 through the window 602 of the vacuum chamber 108 toward the image sensor 601. The fourth exemplary electron microscopy system 700 may operate in a manner similar to the third exemplary electron microscopy system 600. NOTES

[0069] The embodiments described hereinbefore with reference to the drawings are presented by way of illustration. The invention may be implemented in numerous different ways. In order to illustrate this, some alternatives are briefly indicated.

[0070] The invention may be applied in numerous types of products and methods that involve imaging an object on the basis of electrons that have traversed the object. The embodiments presented hereinbefore concern an application in transmission electron microscopy (TEM). However, the invention may be applied to advantage in other types of electron-based imaging. The term electron-based imaging should be interpreted broadly. This term encompasses any technique that involves detecting a location where an electron hits a detection surface, such as, for example electron beam monitoring.

[0071] There are numerous different ways of implementing a photon-detection arrangement in an electron-based imaging system in accordance with the invention. For example, a photon-detection arrangement may comprise one or more optical elements different from, or additional to, those in the presented embodiments. Such optical elements may include, for example, windows, filters, mirrors and lenses.

[0072] The term processor should be interpreted broadly. This term encompasses any entity, or set of entities, which may be present at different locations, capable of processing electrical signals or data, or both.

[0073] The remarks made hereinbefore demonstrate that the embodiments described with reference to the drawings illustrate the invention, rather than limit the invention. The invention can be implemented in numerous alternative ways that are within the scope of the appended claims. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope. Any reference sign in a claim should not be construed as limiting the claim. The verb “comprise” in a claim does not exclude the presence of other elements or other steps than those listed in the claim. The same applies to similar verbs such as “include” and “contain”. The mention of an element in singular in a claim pertaining to a product, does not exclude that the product may comprise a plurality of such elements. Likewise, the mention of a step in singular in a claim pertaining to a method does not exclude that the method may comprise a plurality of such steps. The mere fact that respective dependent claims define respective additional features, does not exclude combinations of additional features other than those reflected in the claims.

Claims

CLAIMS:

1. An electron-based imaging system (100, 500, 600, 700) comprising: a support (104) adapted to hold an object (101) to be imaged; an electron emission source (105) adapted to emit electrons that traverse the object to be imaged; a Cherenkov radiator (106) having a surface (109) arranged to receive electrons that have traversed the object to be imaged, whereby the Cherenkov radiator emits photons in response to an electron having a kinematic energy above a Cherenkov threshold while propagating through the Cherenkov radiator, whereby the electron emission source emits the electrons with a kinematic energy so that the kinematic energy of at least some of the electrons received by the Cherenkov radiator after having traversed the object is above the Cherenkov threshold; a photon-detection arrangement (107) configured to receive photons emitted by the Cherenkov radiator, the photon-detection arrangement being adapted to provide a photon-detection output result indicating a spatial distribution of photons received by the photon-detection arrangement during a detection time-interval; and a processor (103) adapted to recognize a ring-shaped spatial distribution of photons in the photon-detection output result and to record a center of the ring-shaped spatial distribution as a location where an electron has arrived at the surface of the Cherenkov radiator, the processor being further adapted to generate an image from respective locations that have successively been recorded accordingly on the basis of successive photon-detection output results provided by the photon-detection arrangement.

2. An electron-based imaging system according to claim 1, wherein the kinematic energy of at least some of the electrons emitted by the electron emission source (105) is at least 100 keV.

3. An electron-based imaging system according to any of claims 1 and 2, wherein the Cherenkov radiator (106) is substantially transparent to ultra-violet light.

4. An electron-based imaging system according to claim 3, wherein theCherenkov radiator (106) comprises at least one of the following materials: quartz, fused silica, diamond, sapphire, and magnesium fluoride.

5. An electron-based imaging system according to any of claims 1 to 4, wherein the support (104), the Cherenkov radiator (106), the electron emission source (105), and the photon-detection arrangement (107) are comprised in a vacuum chamber (108).

6. An electron-based imaging system according to any of claims 1 to 4, wherein the support (104), the Cherenkov radiator (106), and the electron emission source (105) are comprised in a vacuum chamber (108), and wherein the photon-detection arrangement (107) comprises a photon-detection device (601) outside the vacuum chamber, the vacuum chamber comprising a window (502, 602) in an optical path extending between the Cherenkov radiator and the photon-detection device.

7. An electron-based imaging system according to claim 6, wherein the photon-detection arrangement (107) comprises an optical intensifier (603) in the optical path extending between the Cherenkov radiator (106) and the photon-detection device (601).

8. An electron-based imaging system according to any of claims 6 and 7, wherein the photon-detection arrangement (107) comprises a collimating lens (503) in the optical path extending between the Cherenkov radiator (106) and the photon-detection device (601).

9. An electron-based imaging system according to any of claims 6 to 8, wherein the photon-detection arrangement (107) comprises a mirror (701) in the optical path extending between the Cherenkov radiator (106) and the photon-detection device (601).

10. An electron-based imaging system according to any of claims 6 to 9, wherein at least one element of the photon-detection arrangement (107), other than the photon-detection device (601), is comprised in the vacuum chamber (108).

11. An electron-based imaging system according to any of claims 1 to 10, wherein the photon-detection arrangement (107) comprises a single-photon-sensitive camera.

12. An electron-based imaging system according to claim 11, wherein the single-photon-sensitive camera is configured to produce a time-of-arrival metric indicating an instant when the photon was received by the single-photon-sensitive camera.

13. An electron-based imaging system according to claim 12, wherein the single-photon-sensitive camera is configured to produce a time-over-threshold metric indicating an energy level of the photon received by single-photon-sensitive camera.

14. An electron-based imaging system according any of claims 1 to 10, wherein the photon-detection arrangement (107) comprises an image sensor (601), the photondetection output result comprising a photon-detection image captured by the image sensor, the photon-detection image being formed by photons emitted by the Cherenkov radiator (106) in response to a series of electrons successively arriving at the surface of the Cherenkov radiator.

15. An electron-based imaging system according to claim 14, wherein the image sensor (601) is configured to operate at a rate of at least 1000 frames per second.

16. An electron-based examining device (102) for use in an electron-based imaging system (100, 500, 600, 700) according to any of claims 1 to 15, the electron-based examining device comprising the support (104), the Cherenkov radiator (106), the electron emission source (105), and the photon-detection arrangement (107), and a transmission module adapted to transmit photon-detection output results to the processor (103), which is external to the electron-based examining device.

17. Use of an electron-based imaging system according to any of claims 1 to 15 for imaging an object.

18. A method of preparing for use an electron-based imaging system according to any of claims 1 to 15, the method comprising: making the electron emission source (105) emit electrons without an object being present in the support (104); and - using the spatial distribution of photons indicated in the photon-detection output result provided by the photon-detection arrangement (107) to configure the processor (103) for recognizing the ring-shaped spatial distribution of photons in photondetection output results later obtained when an object is present in the support.

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