Method of determining an energy width of a charged particle beam

Abstract

The present disclosure relates to a method of determining an energy width of a charged particle beam, comprising the steps of: providing a charged particle beam; directing the beam towards a sample; and forming an energy-dispersed beam from a stream of charged particles transmitted through the sample. As defined herein, the method comprises the steps of: providing a slit element in a slit plane, and using the slit element to block a portion of the energy-dispersed beam, and modifying the energy-dispersed beam at the location of the slit plane in such a way that the energy-dispersed beam is partially blocked at the slit element. The unblocked portion of the energy-dispersed beam is imaged, and an intensity gradient of the imaged energy-dispersed beam is determined, with which the energy width of the charged particle beam can be determined.

Description

[0001] This invention relates to a method for determining the energy width of a charged particle beam.

[0002] Charged particle microscopy, particularly in the form of electron microscopy, is a well-known and increasingly important imaging technique for microscopic objects. Historically, the basic class of electron microscopes has evolved into many well-known instrument types, such as transmission electron microscopes (TEM), scanning electron microscopes (SEM), and scanning transmission electron microscopes (STEM), and has also evolved into various sub-species, such as so-called "dual-beam" instruments (e.g., FIB-SEM), which additionally employ "processing" focused ion beams (FIB) to allow supporting activities such as ion beam grinding or ion beam induced deposition (IBID). Technicians will be familiar with different types of charged particle microscopy.

[0003] In SEM, the irradiation of a sample by a scanning electron beam promotes the emission of "auxiliary" radiation from the sample in the form of secondary electrons, backscattered electrons, X-rays, and cathodoluminescence (infrared, visible, and / or ultraviolet photons). One or more components of this emitted radiation can be detected and used for sample analysis.

[0004] In TEM, an electron beam is transmitted through a sample to form an image through the interaction of electrons with the sample as the beam passes through. The image is then magnified and focused onto an imaging device, such as a fluorescent screen, a photographic film layer, or a sensor, such as a scintillator connected to a charge-coupled device (CCD). The scintillator converts the electrons in the microscope into photons, which the CCD can then detect.

[0005] In (S)TEM, the interactions of electrons in the beam can be categorized into elastic and inelastic interactions. In elastic interactions, the proto-electrons are scattered across the sample and thus change direction, or equivalently, their wavefunctions change phase, but they do not experience significant energy loss. These phase transitions are used to create images in TEM mode (which is why TEM imaging is sometimes called phase-contrast imaging). Changes in direction (or angle) are used to create images in STEM mode.

[0006] In inelastic interactions, the atom electron transfers energy to electrons in the sample, for example, by exciting electrons in the sample to higher atomic orbitals, by exciting plasmon oscillations, or by exciting bandgap transitions. If the energy loss of the atom electron can be measured, it can be advantageously used, for example, to extract information about the chemical composition or electrical properties of the sample. This technique is called electron energy loss spectroscopy (EELS). EELS is often combined with STEM to collect EELS spectra pixel by pixel.

[0007] However, inelastic interactions can also be detrimental, especially in TEM imaging, because the primary electrons that have experienced energy loss may not be properly focused by the TEM imaging system due to their energy deviation. Generally, the minimum energy diffusion in the primary electrons yields the best resolution in TEM imaging.

[0008] Therefore, modern TEMs (especially TEMs used for cryo-electron microscopy of biological samples) are typically equipped with so-called energy filters. Such filters, located in or after the imaging system of a TEM, typically include: energy dispersing elements (e.g., curved magnets) for dispersing the original electrons in a spectrum of electrons with different energies; energy-selective slits for selecting and transmitting only a certain range of electron energies; optical elements for constructing (or imaging) a TEM image of those selected electrons; and a camera for recording the TEM image of the energy filter.

[0009] Such cameras can be scintillators attached to a charge-coupled device (CCD), where the scintillator converts atomic electrons in the microscope into photons, allowing the CCD to detect them. Modern energy filters typically employ some type of direct detection camera. In this type of camera, atomic electrons directly strike and enter the pixels of the sensing array (therefore there is no scintillator and the associated intermediate step of conversion into photons), and their energy distribution within the pixels is measured directly and electronically. The advantage of such direct detection cameras is that they are essentially noise-free; that is, they can detect each incoming atomic electron non-destructively and with perfect accuracy. However, due to their direct detection, the performance of the electronics within the pixels of such cameras can be degraded due to radiation damage caused by the incoming atomic electrons. When the total dose in a pixel exceeds approximately 10... 10 This damage occurs when there are only 30 electrons per second per pixel. When such direct-detection cameras continuously record images at 30 electrons per second per pixel, this limits the lifespan of such cameras to approximately 10 years.

[0010] Energy-filtered TEM (EFTEM) is known to significantly improve contrast in imaging biological samples (see, for example, Nature, Vol. 587, No. 7832, November 5, 2020, https: / / www.nature.com / nature / volumes / 587 / issues / 7832). It can be noted that such energy filters can also typically be configured to operate in a so-called "EELS mode." In EELS mode, the spectrum of electrons generated by the energy dispersive element is not filtered and "fanned out" to construct a TEM image (as in EFTEM mode); instead, the spectrum is amplified and imaged on a spectral detector. This spectral detector can be the same camera used to record EFTEM images in EFTEM mode, or it can be a dedicated spectral detector optimized, for example, for a large dynamic range and / or for fast readout and / or for low noise. Such energy filters operating in EELS mode can be referred to as EELS spectrometers.

[0011] Elastic interactions are typically the dominant type of interaction in TEM imaging. Therefore, the energy distribution in the electron beam behind the sample is dominated by the so-called zero-loss electrons, or "zero-loss peak" (ZLP), and optimal spatial resolution in the TEM can be obtained when the energy diffusion within this ZLP is as small as possible. Thus, the achievable spatial resolution is limited by the energy diffusion within the ZLP.

[0012] The width of the zero-loss peak is primarily attributed to the inherent energy diffusion effect of the electron gun in the original electron source. This energy diffusion, ΔE, depends on the type of electron gun but is practically on the order of 0.3–1 eV. Energy diffusion in TEM imaging or energy resolution in EELS experiments is typically defined as the full width at half maximum of the zero-loss peak. Various factors can influence this energy diffusion. For example, the energy diffusion of electrons emitted by a field emitter gun depends on the operating temperature of the emitter, the electrical extraction field applied to the emitter, and the radius of the emitter's apex (which can gradually vary over its lifetime). Another example is the energy diffusion of electrons from a monochromatic electron gun. Here, the diffusion depends (in particular) on the monochromator's excitation, the width of the monochromator slit, and adjustments to any possible aberrations of the monochromator. To obtain and maintain optimal spatial resolution in TEM imaging, it is desirable to be able to accurately measure and monitor the energy diffusion of the zero-loss peak.

[0013] Typically, the energy width ΔE is measured in EELS mode. In this mode, the optics of the energy filter are configured such that the spectrum at the energy-selective slit is magnified and imaged on a camera or a dedicated spectral detector. This allows the entire beam entering the energy filter to be focused into a small ZLP image on the camera. Such an image on the camera is typically approximately (height × width) = (20 × 10) pixels. If the beam entering the energy filter has a current density of 0.1 nA, the current density in the ZLP image is 0.5 pA / pixel or 3,000,000 electrons / pixel / second. This current density is very high for direct-detection cameras, thus requiring dedicated detector equipment to measure the ZLP.

[0014] An improved method is needed for measuring the energy width ΔE of the zero-loss peak (ZLP). According to one aspect, the energy width is useful not only in EELS applications but also in non-EELS applications. The energy width can be used to optimally adjust specific parts of a charged particle microscope. For example, a charged particle microscope may include a monochromator, which can be adjusted using knowledge of the energy width to reduce color blur, thereby improving the spatial resolution of the charged particle microscope. The energy width can also be used, additionally or optionally, to minimize the effects of external influences—such as stray AC fields on the microscope.

[0015] In view of the above-mentioned problems, the object of this disclosure is to provide an improved method for determining the energy width of a charged particle beam. In particular, the object of this disclosure is to provide a method for determining the energy width of a charged particle beam for non-EELS applications. It is desirable to be able to measure the energy width of a charged particle beam without having to revert to EELS-specific equipment, such as dedicated detectors capable of handling the relatively high current densities in ZLP images.

[0016] Therefore, this disclosure provides a method as defined in claim 1. The method defined herein relates to determining the energy width of a charged particle beam, particularly a charged particle beam in a charged particle microscope. The method defined herein includes the steps of providing a charged particle beam and directing said beam toward a sample. The method further includes the steps of forming an energy-dispersed beam from the stream of charged particles transmitted through the sample and imaging said energy-dispersed beam. Imaging may include the step of directing the energy-dispersed beam onto a detector or image sensor, etc.

[0017] As defined herein, the method includes the step of providing a slit element in a slit plane. The slit element can be used to block a portion of the energy dispersion beam, as will be explained later. The slit plane is located downstream of the dispersion element, at or near the location where the energy dispersion beam is formed, and upstream of the location where the energy dispersion beam is imaged.

[0018] As defined herein, the method includes the step of modifying the position of the energy dispersion beam at the slit plane. The modification step includes the following steps: forming a shadow portion of the energy dispersion beam and an unblocked portion of the energy dispersion beam by partially blocking the energy dispersion beam with the slit element. The shadow portion and the unblocked portion of the energy dispersion beam are then imaged. Thus, the energy dispersion beam is modified in such a way that, at the position of the slit plane, only a portion of the energy dispersion beam can be blocked, while another portion of the energy dispersion beam can move freely further downstream. By blocking a portion of the energy dispersion beam, a shadow portion is created. At least a portion of the shadow portion and at least a portion of the unblocked portion are then imaged using, for example, an image sensor or detector.

[0019] As defined herein, modification may include having defocus and / or other aberrations on the energy-dispersive beam at the location of the slit plane. As defined herein, modification may include the slit blocking the energy-dispersive beam at some portion of the image sensor or detector. In other words, the spectral plane of the energy dispersion can be intentionally modified, for example, by slightly defocusing it from the slit plane. This means manipulating the energy-dispersive beam in such a way that the focal point of the energy-dispersive beam is located upstream or downstream of the slit plane. The defocus can be relatively small, meaning the slit plane can be relatively close to the slit plane. This defocus can be the same for every location in the incident beam (circular defocus), or it can be astigmatic (non-circular) for different directions / locations in the incident beam, or any combination of circular, non-circular, or higher-order aberrations. However, defocus can be done in such a way that the ZLP in the spectral plane acts as an electron source, and the slit element produces a shadow on the image plane.

[0020] The method defined herein also includes the step of imaging at least a portion of the shadowed portion and the unblocked portion of the energy dispersion beam. For this purpose, a detector or image sensor may be used.

[0021] As defined herein, the method further includes the step of determining the intensity gradient of the imaged energy-dispersive beam to determine the energy width. Therefore, the method may include the step of determining the intensity gradient of at least an unobstructed portion of the energy-dispersive beam. This step may include determining the intensity gradient between the unobstructed portion and the obstructed portion (i.e., the shadow portion) of the energy-dispersive beam. The unobstructed portion of the energy-dispersive beam is imaged in an image plane. A slit element obstructs at least a portion of the energy-dispersive beam from being imaged in the image plane. This obstructed portion manifests as the shadow of the slit in the image. Because the ZLP in the spectral plane has a finite width, different portions of the ZLP in the spectral plane will be obstructed by the slit element in slightly different ways, and the shadow of the slit will not have infinitely sharp edges. Therefore, across the edge of the shadow in the image, and thus from the obstructed portion (i.e., the shadow portion) of the energy-dispersive beam toward the unobstructed portion, the intensity gradient will be visible. This shadow and its variations can be used to determine the intensity distribution (or shape) of the ZLP, thereby determining the energy width of the charged particle beam. Because the characteristics of the beam shadow are used to determine the energy width, the method defined herein is also referred to as the "shadow method".

[0022] The shading method, in principle, offers better resolution than the traditional EELS method. This is because, unlike EELS, the shading method suffers negligibly from spectral broadening due to the camera's point spread function. Furthermore, the shading method is insensitive to optical aberrations.

[0023] Furthermore, the shadowing method does not require the use of relatively expensive dedicated detector equipment. Due to defocusing, the current density at the image plane is drastically reduced compared to the current density observed during the conventional EELS method.

[0024] Therefore, the method defined herein provides an improved way to determine the energy width of a charged particle beam. This achieves the purpose defined herein.

[0025] Advantageous embodiments are defined by the dependent claims. Some advantageous embodiments will be described below.

[0026] In one embodiment, the step of modifying the energy dispersion beam includes the step of providing defocus on the energy dispersion beam. Additionally or alternatively, the step of modifying the energy dispersion beam may include the step of providing aberrations on the energy dispersion beam.

[0027] The step of modifying the energy dispersion beam can introduce non-isochromatism. non =No, iso = Same, chroma =color). In this case, the image of the energy dispersion beam contains different energies across the image because the chosen energy is not the same for all locations in the image.

[0028] The method may include a step of measuring anisochromaticity. In one embodiment, the measurement can be accomplished by providing multiple settings for high tension and recording which area of ​​the camera is illuminated at which corresponding setting (i.e., offset) under high tension. Alternatively, the measurement can be performed by providing multiple settings for exciting the dispersion element (e.g., multiple currents for bending a magnet), or by providing multiple excitations for the deflector to scan the beam through the energy-selective slit, or by adjusting the potential of the electron beam in the dispersion element (e.g., by multiple voltage offsets on the potential inside the element), or by providing multiple positions for the slit element, or by any combination of these methods.

[0029] The steps of introducing and / or measuring anisochromaticity can be performed with the slit element at a distance from the energy dispersion beam, i.e., in a non-obstructing position relative to the energy dispersion beam. Therefore, the method can include the steps of introducing anisochromaticity and subsequently providing a slit element with anisochromaticity in the energy dispersion beam. The step of measuring anisochromaticity can be performed prior to the step of providing the slit element with anisochromaticity in the energy dispersion beam.

[0030] In one embodiment, the unobstructed energy-scattering beam images onto an image sensor. The slit element can be provided in such a way that it blocks the energy-scattering beam on a portion of the image sensor. In other words, the slit element blocks a portion of the energy-scattering beam, and at least a portion of the shadow formed by the slit element is imaged onto the image sensor.

[0031] In one embodiment, the slit element includes two adjustable (movable) slit edges, one edge being adjusted to block a portion of the image on the sensor, while the other edge is adjusted not to block any portion of the image on the image sensor. In another embodiment, both edges are adjusted to block a portion of the image on the image sensor.

[0032] In one embodiment, the method includes the step of determining the intensity gradient of the imaged energy dispersion beam, particularly the intensity gradient between the unblocked and blocked portions (i.e., shadow portions) of the energy dispersion beam on the image sensor.

[0033] In one embodiment, the method includes the steps of: providing relative motion between the slit element and the energy dispersion beam; and imaging a plurality of intermediate locations of the relative motion to determine the intensity gradient.

[0034] In one embodiment, the method includes the step of adjusting parameters of a charged particle microscope using at least one result obtained through a shading method as defined herein. The adjustment may involve AC field compensation or monochromator focusing or astigmatism correction devices. The method may also include the steps of determining multiple energy widths for different parameters of the charged particle microscope and determining optimal settings.

[0035] According to one aspect, a transmission charged particle microscope (TCPM) as claimed in claim 8 is provided. The TCPM, as defined herein, includes a sample holder for holding a sample, a source for generating a beam of charged particles, and an illuminator for directing the beam onto the sample. Furthermore, the TCPM includes an imaging system for receiving and directing a stream of charged particles transmitted through the sample onto a sensing device, wherein the imaging system includes a post-column filter (PCF) module having an incident plane, an image plane, and a slit plane between the incident plane and the image plane, wherein the PCF module further includes a dispersion device disposed between the incident plane and the slit plane for forming an energy-dispersed beam. The PCF module includes a slit element at the location of the slit plane. The TCPM also includes a controller for controlling at least some operational aspects of the microscope.

[0036] The TCPM as defined herein is configured to define the energy width of the charged particle beam by performing a method according to any one of the preceding claims. Specifically, the controller of the TCPM may be configured and / or programmed to perform at least a portion of the method as defined herein. The TCPM may be configured and / or programmed to define the energy width based on signals emitted from a sensing device in response to at least a portion of the method performed by the TCPM.

[0037] The advantages of this type of TCPM have been explained above regarding the method defined here.

[0038] The apparatus and methods disclosed herein will now be illustrated in more detail based on exemplary embodiments and the accompanying schematic diagrams, in which: Figure 1 A longitudinal cross-sectional view of a charged particle microscope is shown. Figure 2 An enlarged cross-sectional view of the spectral device, including the projection system, is shown. Figure 3 An example of an EELS spectrum is shown; Figure 4 An example of the shadow method as defined here is shown; Figures 5a to 5f Embodiments of the shading method defined herein are shown in more detail; and Figures 6a to 6d An example of adjusting the settings of a charged particle microscope using a shading method as defined herein is shown.

[0039] In the figures, corresponding parts are indicated using corresponding reference symbols when relevant. It should be noted that, generally, the figures are not drawn to scale.

[0040] Figure 1 This is a highly schematic depiction of an embodiment of a transmission charged particle microscope M, in which case the transmission charged particle microscope M is a TEM / STEM (however, in the context of this disclosure, it may properly be, for example, an ion-based microscope or a proton microscope). Figure 1 Within a vacuum enclosure E, an electron source 4 (e.g., a Schottky emitter) generates an electron beam (B) that traverses an electron optical illuminator 6, which is then directed / focused onto a selected portion of the sample S (which may, for example, be (locally) thinned / planarized). This illuminator 6 has an electron optical axis B' and typically includes various electrostatic / magnetic lenses, (scanning) deflectors D, correctors (e.g., astigmatism correctors), etc.; it may also typically include a condenser system (the entire object 6 is sometimes referred to as the "condenser system").

[0041] The sample S is held in a sample holder H. As illustrated herein, a portion of this holder H (inside the housing E) is mounted in a bracket A', which can be positioned / moved in multiple degrees of freedom by a positioning device (stage) A; for example, the bracket A' can be (in particular) movable in the X, Y, and Z directions (see the depicted Cartesian coordinate system) and can rotate about a longitudinal axis parallel to X. This movement allows different portions of the sample S to be irradiated / imaged / detected by an electron beam traveling along axis B' (and / or, for example, allows scanning motion as an alternative to beam scanning [using deflector D], and / or allows selected portions of the sample S to be processed by a (not depicted) focused ion beam).

[0042] The (focused) electron beam B, traveling along axis B', interacts with the sample S, causing various types of "stimulated" radiation to be emitted from the sample S, including (e.g.) secondary electrons, backscattered electrons, X-rays, and photoradiance (catholuminescence). If desired, one or more of these radiation types can be detected by means of detector 22, which can be, for example, a combination of scintillator / photomultiplier tubes or EDX (energy-dispersive X-ray spectroscopy) modules; in this case, the image can be constructed using essentially the same principles as in SEM. However, alternatively or additionally, electrons that traverse (through) the sample S, emerge from it, and continue to propagate along axis B' (generally, although typically with some deflection / scattering) can be studied. This transmitted electron stream enters the imaging system (combined mirror / projection lens) 24, which will typically include various electrostatic / magnetic lenses, deflectors, correctors (e.g., astigmatism correction devices), etc.

[0043] In normal (non-scanning) TEM mode, this imaging system 24 focuses the transmitted electron flux onto a fluorescent screen 26, which, if desired, can retract / withdraw (as schematically indicated by arrow 26') away from the path of axis B'. An image (or diffraction pattern) of a portion of sample S is formed by the imaging system 24 on screen 26, and this can be observed through an observation port 28 located in a suitable portion of the wall of the housing E. The retraction mechanism of screen 26 may be, for example, mechanical and / or electrical in nature, and is not depicted herein.

[0044] As an alternative to viewing the image on screen 26, the fact that the focusing depth of the electron flux emanating from imaging system 24 is typically quite large (e.g., approximately 1 meter) can be utilized. Therefore, various types of sensing / analysis devices can be used downstream of screen 26, such as: -TEM camera 30. At camera 30, the electron flux can form a still image (or diffraction pattern), which can be processed by controller C and displayed on a display device (not depicted), such as a flat panel display. When not needed, camera 30 can be retracted / withdrawn (as schematically indicated by arrow 30') to move it away from the path of axis B'.

[0045] - STEM recorder 32. The output from recorder 32 can be recorded as a function of the (X,Y) scan position of beam B on sample S, and an image can be constructed, which is a "map" of the output from recorder 32 as a function of X and Y. Recorder 32 may include single pixels with a diameter of, for example, 20 mm, rather than a pixel matrix characteristically present in camera 30. Furthermore, recorder 32 will typically have a larger sensor size than camera 30 (e.g., 10 Hz). 2 A much higher acquisition rate (e.g., 10 images per second) 6 (Points). Again, when not needed, the recorder 32 can be retracted / withdrawn (as schematically indicated by arrow 32') so as to move away from the path of axis B' (but such retraction would not be necessary in the case of, for example, annular dark field recorders 32; in such recorders, the central aperture would allow the beam to pass through when the recorder is not in use).

[0046] - As an alternative to using camera 30 or recorder 32 for imaging, a spectroscopic device 34 may also be invoked, which may be, for example, an EELS spectrometer.

[0047] It should be noted that the order / position of items 30, 32, and 34 is not strict, and many possible variations can be considered. For example, the spectroscopic device 34 can also be integrated into the imaging system 24.

[0048] Note that the controller (which may be a combination of controller and processor) C is connected to the various described components via control lines (buses) C'. The controller may be connected to a computer screen 51, which may be equipped with a user interface (UI). This controller C can provide various functions, such as synchronizing actions, providing setpoints, processing signals, performing calculations, and displaying messages / information on a display device (not depicted). It should be understood that the controller C (schematically depicted) may be (partially) inside or outside the housing E, and may have an integrated or modular structure as needed. Those skilled in the art will understand that the interior of the housing E need not be maintained under a strict vacuum; for example, in so-called “ambient TEM / STEM,” a background atmosphere of a given gas is intentionally introduced / maintained within the housing E. Those skilled in the art will also understand that, in practice, it may be advantageous to limit the volume of the housing E such that, where possible, it is substantially close to the axis B' in the form of a small tube (e.g., about 1 cm in diameter) through which the electron beam is passed, but widened to accommodate structures such as the source 4, the sample holder H, the screen 26, the camera 30, the recorder 32, the spectroscopic device 34, etc.

[0049] Now go to Figure 2 It shows the method used to determine the energy width of a charged particle beam. Figure 1 An enlarged and more detailed view of an embodiment of the spectroscopic device 34. Here, the spectroscopic device 34 is an EELS module. Figure 2 The diagram shows an electron stream 1 (which has passed through sample S and imaging system 24) propagating along the electron optical axis B'. This stream 1 enters a dispersion device 3 (“electron prism”), where it is dispersed (fanned out) into an energy decomposition (energy differentiation) array 5 of spectral sub-beams distributed along the dispersion direction; for illustrative purposes, three of these sub-beams are shown in... Figure 3 The numbers are marked as 5a, 5b, and 5c.

[0050] Downstream of the dispersion device 3, the sub-beam array 5 encounters a post-dispersion electron optics device 9, where it is amplified / focused, for example, and ultimately guided / projected onto detector 11. Detector 11 may comprise an assembly of sub-detectors arranged along the dispersion direction, wherein the different sub-detectors are adjustable to have different detection sensitivities. Note that other detector configurations for measuring EELS spectra are known to those skilled in the art.

[0051] Figure 3 An example of an EELS spectrum is shown. The graph presents intensity I (arbitrary units, au) as a function of the energy loss E (in eV) of electrons that have traversed the sample containing carbon and titanium. From left to right, the main features of the spectrum are: -Zero-loss peak ZLP; - Plasmon resonance peak components / segmented PRP; and -Core loss peak component / segmented CLP.

[0052] According to existing technology, the energy width ΔE of a charged particle beam is measured in EELS mode. In this mode, the optics of the energy filter are configured such that the spectrum at the energy selective slit is magnified and imaged on a camera or dedicated spectral detector. The energy width of the ZLP is then measured, and this is used to determine the energy width of the charged particle beam.

[0053] Turn now Figure 4 This schematically illustrates the basic principles of the methods and apparatus as defined herein. Figure 4 An embodiment of a spectroscopic device 34 in the form of an EFTEM module is shown. Corresponding features are indicated by the same reference numerals. Here, a beam of charged particles is provided and directed toward a sample S (see figure). Figure 2 The energy dispersion beam 5 is formed by the flow 1 of charged particles transmitted through the sample S. A slit element 63a is disposed in the slit plane Psl. The energy dispersion beam 5 is also modified at the location of the slit plane Psl in such a way that the energy dispersion beam is partially blocked at the slit element 63a or the edge of the slit. Figure 4 As shown, there is a blocking portion 66 and an unblocked portion 67 of the energy dispersion beam. The modification of the energy dispersion beam causes it to be defocused and / or distorted at the location of the slit plane Psl. In the illustrated embodiment, the spectral plane Psp is intentionally slightly defocused from the slit plane Psl. The ZLP in the spectral plane Psp then acts as an electron source, and the slit edge 63a from the slit element produces a shadow portion 11c on the camera 11. Due to the finite width of the ZLP, the shadow has a soft edge with a transition 11b from bright 11a to dark 11c. By imaging the unblocked portion 67 of the energy dispersion beam 5, the intensity gradient 11a-11c of the imaged energy dispersion beam can be measured and determined. In particular, the intensity passing through this soft shadow 11b can be measured, and the shape of the ZLP can be calculated from this.

[0054] It should be noted that Figure 4 A simplified sketch is shown, but the imaging optics between the slit and the image sensor are not shown. However, these details are known to those skilled in the art.

[0055] Now refer to Figures 5a-5f To discuss in more detail Figure 4 The basic principle is shown. Figure 5a A well-tuned filter is shown, wherein prism 3 in the filter forms a spectrum at the plane of energy-selective slits 63a, 63b, and the final image 11 is formed only by electrons of a few specific selected energies (see...). Figure 2(Left). According to the method defined here, and as... Figure 5b As shown, filter 3 is adjusted in such a way that different energies are selected for different locations in the final image 11. Therefore, the energy dispersion beam 5 is modified in such a way that the spectral plane is not precisely focused on the slit plane. In this case, the slit does not select a specific energy. Instead, different energies are selected for different locations in the image. The effect of selecting different energies for all locations in the image is called anisochromaticity. Anisochromaticity can be measured by scanning high tension and recording which area of ​​the camera is illuminated at which offset of high tension. Alternatively, this measurement can be performed by providing multiple settings for exciting the dispersion element (e.g., multiple currents for bending magnets), or by providing multiple excitations for the deflector to scan the beam through the energy-selective slit, or by adjusting the potential of the electron beam in the dispersion element (e.g., by multiple voltage offsets on the potential inside the element), or by providing multiple locations for the slit element, or by any combination of these methods.

[0056] In currently available shadowing methods, the spectral focus at the slit plane is intentionally misaligned, resulting in anisochromaticity of several eV on the camera, such as... Figure 5c As shown. The method disclosed herein may include the step of determining the anisochromaticity of the energy-dispersed beam. The method may include the step of measuring the anisochromaticity. The method may include the step of fitting an equation, such as a polynomial fit, to the measured anisochromaticity. Figure 5c The left image shows an example of an isochromatic map measured over a 7x7 area in the camera's field of view. A polynomial fit f(x,y) can be performed on this isochromatic map to produce an inset, such as... Figure 5c As shown in the right figure.

[0057] The method may include the step of inserting a slit element into an energy-dispersing beam having a defined anisochromaticity. The slit element is positioned such that a portion of the energy-dispersing beam is blocked (forming a shadow region), while another portion of the energy-dispersing beam is not blocked. The unblocked portion of the energy-dispersing beam is then imaged in such a way that the image includes the unblocked portion, and the image also includes a portion of the shadow region formed by the defined anisochromaticity introduced into the energy-dispersing beam by the slit element. Figure 5d Example images are shown. Here, the bright portion 11a corresponds to the unblocked portion that is recorded, while the dark portion 11c (shadow portion 11c) corresponds to the blocked portion that is blocked by the slit element and produces a shadow on the image sensor or detector.

[0058] Figure 5e Will Figure 5d The intensity of the pixels in the graph is drawn as Figure 5c These are functions of energy in isochromatic graphs. Figure 5e The gray dots in the image. Noise reduction is achieved by collecting data from points in multiple energy boxes, for example, a total of 60 boxes. These are... Figure 5e The image shows 60 black dots. These black dots represent the overall energy distribution of the source. . Figure 5f The derivative is shown f(E) = dF (E) / dE This is energy distribution. For example... Figure 5f As shown, f(E) fits perfectly with Young's analytical expression for the energy distribution of the field emission source: f(E) = J FN • exp(E / d) / d / [1+exp(- E / kT)] J FN This is the Fowler-Nordheim current, d is a function of the introduced field, and k is the Boltzmann constant. The three numbers J... FN d and T are used as fitting parameters.

[0059] Filter 34 can compensate for the effects of stray AC fields (typically occurring at 50Hz / 60Hz and higher harmonics, such as 150Hz / 180Hz) by applying sine / cosine corrections at 50Hz / 60Hz (and 150Hz / 180Hz, etc.) to the TCPM or energy filter. These corrections can be sinusoidal / cosine shifts on the high tension, on the excitation of the dispersion element, on the deflector, or on the potential of the electron beam within the dispersion element (e.g., by a sinusoidal / cosine voltage shift on the potential within the element), or a combination thereof. Figures 6a to 6d An example in which the shading method is used to compensate for stray AC fields is shown.

[0060] Figures 6a to 6c The shading method for measuring the ZLP of a monochromatic FEG in the presence of a stray AC field is shown. Without compensation, the stray AC field interferes with the electron beam and causes a clearly visible ZLP split at 0.2 eV. Note that in Figure 6a In this case, the applied anisochromaticity is a saddle-shaped function, the opposite of the previous plot, where the applied anisochromaticity is essentially linear. To find the optimal excitation for the AC compensation function, the following method can be applied. The AC compensation is set to several values ​​over an interval, for example, -3 to +3 units, and for each setting, the integral of the energy distribution is determined (…). Figure 6b ) and ZLP fitting ( Figure 6c Therefore, the shading method disclosed in this paper is used to measure the ZLP value for each value. The obtained data ( Figure 6d Fits the following function: △Etot = (△E ZLP 2 + △E AC 2 ) 1 / 2 ,△E AC =c•(ACComp – ACComp0) And the optimal compensation is ACComp = ACComp0.

[0061] In a similar manner, the shading method can be used to adjust the monochromator. The monochromator's astigmatism correction device can be set within a certain range, the ZLP is determined at each setting, and the obtained data is fitted in a manner similar to adjusting AC compensation.

[0062] The shading method, in principle, offers better resolution than the traditional EELS method. This is because, unlike EELS, the shading method suffers negligibly from spectral broadening due to the camera's point spread function. Furthermore, the shading method is insensitive to optical aberrations. For example, the shading method gives an FWHM of 0.23 eV for CFEG, while an EELS measurement might give an FWHM of 0.27 eV; the EELS measurement deviates by 0.04 eV due to a 0.02 eV resolution loss caused by signal spread to adjacent pixels (“point spread”) in the EELS detector and another 0.02 eV resolution loss due to aberrations (incomplete focus) in the energy spectrum of the EELS detector.

[0063] The shading method can handle very low doses and very long exposure times. This is because, unlike the EELS mode, the entire camera is used. For example, for 25pA on direct electronic detectors such as the Falcon™4 and ThermoScientific™, the method works well in the exposure time range of 0.04s to 10s.

[0064] It is not necessary to use positionally linear aberrations to offset color. In principle, shading algorithms can handle any offset. Quadratic offsets (such as...) Figure 6a The advantage of (as shown) is that it increases the sampling f(E) around the energy of interest (E ≈ 0 eV).

[0065] The shading method is preferably performed using a direct electronic detection camera in counting mode, such as the Falcon™ 4. Such cameras avoid artifacts introduced by nonlinearity or nonuniformity of camera gain. Furthermore, the optimal statistics of the counting mode result in better energy resolution (however, AC adjustment and monochromator adjustment can also be performed on scintillator-based cameras, such as the Thermo Scientific™ CETA-D, using the shading method).

[0066] The shading method is applicable to many different sources, such as cold FEG, Schottky FEG, and monochrome FEG.

[0067] Having a method for measuring energy width ΔE is not only necessary for adjusting AC compensation or monochromator settings. It also facilitates monitoring source operation (e.g., verifying that the emitter is operating at optimal settings for the extraction field or temperature) and for troubleshooting.

[0068] The method described herein relates to the determination of an energy distribution obtained from the derivative of the intensity distribution on the knife-edge shadow in front of the energy spectrum. This method can be used to adjust AC compensation in an energy filter and / or to adjust a monochromator. The desired protection is granted by the appended claims.

Claims

1. A method for determining the energy width of a charged particle beam without using electron energy loss spectroscopy (EELS) techniques, comprising the following steps: - Provide a beam of charged particles and direct the beam toward the sample; - An energy-dispersing beam is formed from the stream of charged particles transmitted through the sample; and - Image the energy dispersion beam; Its characteristics include the following steps: - Provide slit elements in the slit plane; - Modify the energy dispersion beam at the location of the slit plane to provide defocus and / or aberration on the energy dispersion beam, such that the defocus and / or aberration exists on the energy dispersion beam at the location of the slit plane, wherein the modification step comprises the steps of: forming a shadow portion of the energy dispersion beam by partially blocking the energy dispersion beam with the slit element, and forming an unblocked portion of the energy dispersion beam; - Image at least a portion of the shadowed portion and the unblocked portion of the energy dispersion beam; as well as - Determine the intensity gradient of the energy dispersion beam of the imaging to determine the energy width.

2. The method of claim 1, wherein the unobstructed energy dispersion beam is imaged on an image sensor.

3. The method of claim 2, wherein the slit element is provided in such a manner that the slit element blocks the energy dispersion beam on a portion of the image sensor.

4. The method according to any one of claims 1 to 3, comprising the step of determining an intensity gradient between the unblocked portion and the shadowed portion of the energy dispersion beam on the image sensor.

5. The method of claim 4, further comprising the step of determining at least a first derivative of the intensity gradient for determining the energy diffusion function.

6. The method according to any one of claims 1 to 3, comprising the following steps: -Providing relative motion between the slit element and the energy dispersion beam; and - Image the multiple intermediate locations of the relative motion to determine the intensity gradient.

7. The method according to any one of claims 1 to 3, comprising the step of determining a plurality of energy widths for different settings of at least one parameter of a charged particle microscope.

8. The method of claim 7, further comprising the step of determining an optimal setting for the at least one parameter.

9. A transmission charged particle microscope, comprising: - Sample holder, used to hold samples in place; - A source used to generate beams of charged particles; - An illuminator for directing the beam onto the sample; An imaging system for receiving and guiding a stream of charged particles transmitted through the sample to a sensing device, wherein the imaging system includes a post-column filter (PCF) module having an incident plane, an image plane, and a slit plane between the incident plane and the image plane, wherein the PCF module further includes a dispersion device disposed between the incident plane and the slit plane for forming an energy dispersion beam, and wherein the PCF module includes a slit element at the location of the slit plane; and - A controller for controlling at least some operational aspects of the microscope; Its features are, The transmission charged particle microscope is configured to define the energy width of the charged particle beam by performing a method according to any one of the preceding claims.

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