Method of determining brightness of a charged particle beam, method of determining size of a source of a charged particle beam, and charged particle beam imaging apparatus

Abstract

A method of determining a brightness (B r ) of a charged particle beam (11) focused by a focusing lens (120) towards a sample (10) in a charged particle beam imaging apparatus (100) is described. The method comprises: (a) taking one or more images (h f,1...N ) of the sample with the charged particle beam imaging apparatus; (b) retrieving one or more beam profiles (g f,1...N ) of the charged particle beam from the one or more images; and (c) determining the brightness (B f,1...N ) of the charged particle beam (11) based on at least the one or more beam profiles (g P ), a probe current (I r ) of the charged particle beam, and a landing electric potential (LE) of the charged particle beam. Optionally, the brightness (B r ) determined as above can be used to determine a size (D virt ) of a source (105) of the charged particle beam (11). Further, a charged particle beam imaging apparatus (100) configured for any of the methods described herein is provided.

Description

TECHNICAL FIELD

[0001] The embodiments described herein relate to a method of determining the brightness of a charged particle beam, in particular an electron beam, generated by a charged particle source and focused towards a sample by a focusing lens, e.g. in an electron microscope, in particular a scanning electron microscope (SEM). In particular, the brightness of a charged particle beam in a charged particle beam imaging device is determined. The embodiments described herein further relate to a method of determining the size of a charged particle source, in particular a virtual size of an electron emission tip, using the determined brightness. Charged particle beam imaging devices configured for carrying out the described methods are further described. BACKGROUND

[0002] Modern semiconductor technology has created a high demand for structuring and detecting sample structures on a nanometer scale or even on a sub-nanometer scale. Micro- and nanometer scale process control, inspection or structuring is often done with charged particle beams, e.g. electron beams, which are generated, shaped, deflected and focused in a charged particle beam imaging device, such as an electron microscope or an electron beam pattern generator. For inspection purposes, charged particle beams provide superior spatial resolution compared to, e.g., photon beams.

[0003] Inspection apparatuses using charged particle beams, such as scanning electron microscopes (SEMs), have many functions in a number of industrial fields, including but not limited to electronic circuit inspection, exposure systems for lithography, detection systems, defect inspection tools, and test systems for integrated circuits. In such particle beam systems, fine beam probes with high current density can be used. For example, in the case of an SEM, a primary electron beam generates signal particles such as secondary electrons (SEs) and / or backscattered electrons (BSEs) that can be used to image and / or inspect a sample.

[0004] Charged particle beam imaging devices have a beam source, e.g. an electron source, for generating a charged particle beam for sample inspection and / or imaging. The brightness of the charged particle beam is a key parameter for obtaining high spatial resolution and at the same time a key parameter for obtaining high throughput of the microscope and sample analysis. At least in theory, the (reduced) brightness of the charged particle beam is conserved in the charged particle imaging device from the beam source to the sample. In particular, after generating the charged particle beam, the brightness defined by the characteristics of the beam source, i.e. by the emission tip and other charged particle emission parameters, cannot be increased.

[0005] However, there is often uncertainty about the parameters of the charged particle beam, in particular about the brightness of the charged particle beam. The brightness can be roughly estimated, e.g. based on an estimated physical size of the emission tip from which the charged particles are emitted or based on a spot size of a beam probe estimated by directing the focused charged particle beam (after magnification with a particular magnification optics) onto a beam profiler. However, it is challenging to accurately determine the beam brightness. Accurate knowledge of the brightness can facilitate identifying changes in beam source parameters, e.g. in a charged particle beam imaging device, and thus identifying beam probe parameters. Such changes can be identified by reliably determining and / or monitoring the brightness of the charged particle beam, which can be used for improving the matching.

[0006] In view of the above, it would be beneficial to provide a method of accurately and reliably determining the brightness of a charged particle beam focused by a focusing lens in a charged particle beam imaging device. Further, it would be beneficial to provide a charged particle beam imaging device configured to operate according to any of the methods described herein, such that the charged particle beam imaging system can be accurately characterized in terms of the brightness of the charged particle beam and / or the size of the charged particle beam source. SUMMARY

[0007] In view of the above, a method of determining the brightness of a charged particle beam, a method of determining the size of a source of a charged particle beam and a charged particle beam imaging device are provided according to independent claims.

[0008] According to a first aspect, a method of determining the brightness of a charged particle beam focused by a focusing lens towards a sample in a charged particle beam imaging device is provided. The method comprises: (a) taking one or more images of the sample with the charged particle beam imaging device; (b) retrieving one or more beam profiles of the charged particle beam from the one or more images; and (c) determining the brightness of the charged particle beam (B r ) based on at least the one or more beam profiles, a probe current of the charged particle beam and a landing potential of the charged particle beam.

[0009] In particular, a focused beam profile can be retrieved in (b) and the brightness can be determined in (c) based on at least the focused beam profile, a numerical aperture, the probe current and the landing potential. Alternatively or additionally, a numerical aperture of the charged particle beam can be determined based on the one or more beam profiles determined in (b) and the brightness can be determined in (c) based on at least the numerical aperture, the focused beam profile, the probe current and the landing potential.

[0010] According to a second aspect, the brightness of the charged particle beam as determined herein is used for determining the size of a source of the charged particle beam, in particular for determining a virtual size of a charged particle emission tip, in particular an electron emission tip.

[0011] According to a third aspect, a method of determining a size of a source of a charged particle beam, in particular a virtual size of an emission tip of an electron emitter, in a charged particle beam imaging apparatus is provided. The method comprises generating a charged particle beam with the source; focusing the charged particle beam towards a sample with a focusing lens; determining a brightness of the charged particle beam according to any of the methods described herein; and determining the size of the source based on at least the brightness, a beam potential of the charged particle beam, a charged particle beam current and an opening angle of the source.

[0012] According to a fourth aspect, a charged particle beam imaging apparatus is provided, comprising a source for emitting a charged particle beam propagating along an optical axis; a sample stage; a focusing lens for focusing the charged particle beam to a sample placed on the sample stage; a charged particle detector for detecting signal particles emitted from the sample; and a processor and a memory storing instructions which, when executed by the processor, cause the charged particle beam imaging apparatus to (a) take one or more images of the sample; (b) retrieve one or more beam profiles of the charged particle beam from the one or more images; and (c) determine a brightness of the charged particle beam based on at least the one or more beam profiles, a probe current of the charged particle beam and a landing potential of the charged particle beam. The imaging apparatus can be configured for performing any of the methods described herein.

[0013] Embodiments also relate to devices for performing the disclosed methods and comprise device parts for performing individual method actions. The methods can be performed by means of hardware parts, computers programmed with suitable software, any combination of these two or in any other way. Further, embodiments also relate to methods of operating the described devices.

[0014] Further advantages, features and details that can be combined with the embodiments described herein are apparent from the dependent claims, the description and the figures. BRIEF DESCRIPTION OF DRAWINGS

[0015] For a more detailed understanding of the above-recited features a more particular description will be rendered by reference to embodiments. The drawings relate to one or more embodiments and describe, by way of example only, the principles of the present disclosure:

[0016] Figure 1 A schematic diagram of a charged particle beam imaging apparatus according to embodiments described herein is shown, the charged particle beam imaging apparatus being configured for performing any of the methods described herein;

[0017] Figure 2 A schematic flow chart of a method of determining a brightness B of a charged particle beam according to embodiments described herein is shown; r

[0018] Figure 3 ​is a plot showing the determination of the numerical aperture (NA) of a charged particle beam for determining the brightness according to embodiments described herein;

[0019] Figure 4 is a schematic flow chart showing a method of determining the brightness B of a charged particle beam according to some embodiments described herein; r

[0020] Figure 5 is a schematic flow chart showing a method of determining the size of a charged particle source according to embodiments described herein; and

[0021] Figure 6 is a schematic illustration of a source area of a charged particle beam imaging apparatus according to embodiments described herein, to illustrate the method of determining the size of the source. DETAILED DESCRIPTION

[0022] Reference will now be made in detail to the various embodiments, one or more examples of which are illustrated in the drawings. Within the following description of each drawing, same reference numbers refer to same components. Generally, only the differences between respective embodiments will be described. Each example is provided by way of explanation of the embodiments and is not meant as a restriction thereon. Further, features illustrated or described as part of one embodiment can be used with other embodiments or in combination with other embodiments to produce yet other embodiments. It is intended that the description included such modifications and variations.

[0023] The brightness of a charged particle beam is a key parameter in a charged particle beam imaging apparatus, which can influence, for example, the achievable resolution, signal-to-noise ratio and throughput or inspection speed of the apparatus. Since the (reduced) brightness of a charged particle beam is a parameter that is generally conserved in a charged particle beam imaging system, the (reduced) brightness of the beam probe incident on the sample generally substantially corresponds to the “brightness of the beam source” (= brightness of the charged particle beam generated by the source) that emits the charged particle beam. A small size of the emission tip of the source that emits the charged particle beam generally increases the brightness of the beam, since a large current is emitted per solid angle from the small tip area, which can be focused towards a small spot size on the sample, thereby providing a high brightness beam probe. It is therefore generally an aim to provide a high brightness charged particle source with a small source size, i.e. when back-tracing the charged particle beam, the particles of the charged particle beam appear to originate from a small size area, while maintaining a large emission current.

[0024] While the “angular intensity” of a charged particle beam describes the number of charges (e.g. electrons) per unit time per solid angle, the brightness B is generally considered a more relevant parameter, since the brightness refers to the angular intensity per source size, which is a parameter that characterizes the source of the charged particle beam and influences the achievable resolution. If the “beam potential” varies along the optical axis (e.g. due to acceleration and / or deceleration sections), it can be reasonable to use the “reduced” brightness B​r which is referred to as the angular intensity per source dimension divided by the beam potential (in [A / (m 2 sr V)]), which is assumed to be conserved in the device, even if the beam potential (= energy of the particles of the charged particle beam) varies along the optical axis within the device. Instead of the term "reduced brightness", sometimes the term "normalized brightness" is used in the literature. "Beam potential" as used herein refers to the potential in [V] to which the charged particles of the electron beam are accelerated or decelerated, and in case of electrons as charged particles, this potential corresponds to the particle energy in electron volts [eV]. Where appropriate, for example in case of high beam potential values, a relative correction value of the beam potential can be used.

[0025] As used herein, the term "brightness" can refer to the reduced brightness B r of a charged particle beam, i.e. the normalized brightness, which is considered to be a conserved quantity in a charged particle beam imaging device (ignoring Coulomb interactions between charges). In particular, the reduced brightness B r is considered to be a conserved quantity in the entire beam column of a charged particle beam imaging device (ignoring Coulomb interactions), and is a parameter related to the source performance and the amount of current in the probe.

[0026] Determining the brightness of a charged particle beam is challenging. Conventionally, the brightness can be roughly estimated, for example based on assumed source parameters, which is not an accurate approach as the emission characteristics of the tip can differ from the expected ones. Alternatively, the brightness can be determined experimentally in a transmission electron microscope geometry by inserting specific magnifying optics in the charged particle beam such that the beam focus can be magnified onto a CCD camera and measured by the CCD camera. The brightness can be calculated based on the magnified beam profile. However, inserting specific magnifying optics is complex and can influence the determined brightness value as the optics and the beam path can differ from the optics used in the actual inspection device.

[0027] According to embodiments described herein, a method of accurately determining the (reduced) brightness B r of a charged particle beam in a charged particle beam imaging device is provided. The method is accurate as the brightness in a charged particle beam imaging system can be determined based on an actual image of a sample taken by the device without the need of using specific magnifying optics, and with the actual device settings without the need of using a specific microscope geometry.

[0028] More specifically, the brightness B geo may be determined based on at least the beam convergence angle of the charged particle beam towards the focal spot, in particular based on the numerical aperture NA of the charged particle beam, and based on the focal spot size of the charged particle beam, in particular based on the focal spot diameter d rAccording to the embodiments described herein, the beam convergence angle (especially NA) and focal spot size (especially d) are determined from actual images of the sample. geo At least one or both of these conditions ensure that the determined brightness value is accurate and reliable, and is retrieved based on an actual image captured by the imaging device.

[0029] Figure 1 This is a schematic diagram of a charged particle beam imaging apparatus 100 configured for the method of determining beam brightness described herein.

[0030] Figure 1 The charged particle beam imaging apparatus 100 is configured to examine and / or image a sample 10, and includes a charged particle source 105, particularly an electron source with an emission tip, for emitting a charged particle beam 11, particularly an electron beam, propagating along an optical axis A. The charged particle beam imaging apparatus 100 also includes a sample stage 108 and a focusing lens 120, particularly an objective lens, for focusing the charged particle beam 11 onto the sample 10 placed on the sample stage 108. The charged particle beam imaging apparatus 100 also includes a charged particle detector 118, particularly an electron detector, for detecting signal particles (e.g., secondary electrons and / or backscattered electrons) emitted from the sample 10. An image generation unit 160 may be provided, which generates one or more images of the sample 10 based on the charged particle signals received from the charged particle detector 118. The image generation unit 160 may forward one or more images of the sample to a processor 170, such as a computer, which may be configured to determine the brightness of the charged particle beam 11 according to the methods described herein.

[0031] Specifically, processor 170 may include a processor and a memory storing instructions that, when executed by the processor, cause charged particle beam imaging device 100 to determine the brightness of the charged particle beam according to any of the methods described herein.

[0032] The sample stage 108 can be a movable stage. Specifically, the sample stage 108 can move in the Z direction, i.e., in the direction of the optical axis A, such that the distance between the focusing lens 120 and the sample stage 108 can be varied (see [reference]). Figure 1 (Arrow 112 in the image). By moving the sample stage 108 in the Z direction, the sample 10 can be moved to a focal plane p away from the focusing lens 120. Idifferent defocus distances, such that“defocus images” of the sample 10 can be taken by corresponding movements of the stage away from the focus point of the beam. In some embodiments, the sample stage 108 can also be movable in a plane perpendicular to the optical axis A, also referred to herein as the X-Y plane. By moving the sample stage 108 in the X-Y plane, a predetermined surface area of the sample 10 can be moved into the area below the focusing lens 120, such that the predetermined surface area can be imaged or inspected by focusing the charged particle beam 11 thereon.

[0033] The beam optical components of the charged particle beam imaging apparatus 100 are typically placed in a vacuum chamber 101 which can be evacuated, such that the charged particle beam 11 can propagate along the optical axis A from the charged particle source 105 towards the sample stage 108 and impinge on the sample 10 at sub-atmospheric pressure, e.g. a pressure lower than 10 -3 millibar, a pressure lower than 10 -5 millibar or even a pressure lower than 10 -8 millibar.

[0034] In some embodiments, the charged particle beam imaging apparatus 100 can be an electron microscope, in particular a scanning electron microscope. A scanning deflector 107 can be provided for scanning the charged particle beam 11 along a predetermined scan pattern on the surface of the sample 10, e.g. in the X-direction and / or the Y-direction.

[0035] In some embodiments, a condenser lens system 106 comprising one or more condenser lenses can be arranged downstream of the charged particle source 105, in particular for collimating the charged particle beam 11 propagating towards the focusing lens 120. In some embodiments, the focusing lens 120 is an objective lens, in particular a magnetic objective lens, an electro-magnetic lens or a combined magnetic-electrostatic lens, configured to focus the charged particle beam 11 on the sample 10.

[0036] In some embodiments, an extraction electrode 104 can be arranged downstream of the charged particle source 105 and configured to extract the charged particle beam for propagation along the optical axis A by applying an extraction potential to the extraction electrode. In particular, a beam acceleration potential, in particular a positive potential difference with respect to the emission tip of the source 105, can be applied to the extraction electrode 104 to accelerate the charged particles, in particular electrons, towards and through an opening of the extraction electrode. In some embodiments, the charged particle source 105 can be a cold field emission (CFE), a hot field emission (TFE) or a Schottky emitter.

[0037] The charged-particle source 105 can comprise a point-like tip that emits charged particles of the charged-particle beam. A tip of small “physical size” (in particular < 1 pm, < 100 nm or even < 10 nm) is typically suitable for generating a high-brightness charged-particle beam. However, the “physical size” of the emitting tip can not necessarily define the brightness of the light source. Thus, the term “size” of a charged-particle source as used herein typically refers to a virtual size of the source (also referred to as “virtual source size” herein), which is the area within the beam source from which the charged particles appear to originate when back-tracing the charged-particle beam. In particular, the virtual size of the source can not necessarily specify the physical size of the emitting tip, but can refer to the size of the virtual or real intersection of the charged-particle beam formed by the field in the vicinity of the emitting tip.

[0038] One or more surface regions of the sample 10 can be inspected and / or imaged with the charged-particle beam imaging apparatus 100. The term “sample” as used herein can relate to a substrate, e.g. a substrate on which one or more layers or features are formed, a semiconductor wafer, a glass substrate, a web substrate or another sample to be inspected. One or more of the following can be inspected of the sample: imaging of a surface of the sample, measuring a size of one or more features of the sample (e.g. in lateral directions, i.e. in the X-Y plane), performing critical dimension measurements and / or metrology, detecting defects and / or investigating the quality of the sample.

[0039] For inspecting the sample 10 with the charged-particle beam 11, the charged-particle beam 11 is typically focused on the sample surface with the focusing lens 120. When the charged-particle beam 11 hits the sample surface, secondary electrons and / or backscattered electrons (referred to as “signal electrons”) are emitted from the sample. The signal electrons provide information about the spatial properties and feature sizes of the sample and can be detected with the charged-particle detector 118. By scanning the charged-particle beam 11 over the sample surface, e.g. with the scanning deflector 107, and detecting the signal electrons according to their generation position, the sample surface or a portion thereof can be imaged, e.g. with the image generation unit 160, which can be configured to provide an image of the sample 10 based on the received signal electrons.

[0040] The small spot of the focused charged-particle beam 11 on the sample surface typically increases the image resolution. Thus, during actual sample inspection, the sample surface is typically arranged substantially in the focal plane p I of the focusing lens 120 in order to obtain a focused image of the sample 10. The image of the sample 10 taken in focus is also referred to as “focused image h f ” herein. Similarly, the beam profile of the charged-particle beam 11 in the focal plane p I is referred to as “focused beam profile g f ” herein.

[0041] Images can be mathematically represented in real space (i.e., in the image domain, as a function of spatial coordinates) or in Fourier space (i.e., in the frequency domain, as a function of spatial frequency). An image in Fourier space can be computed from an image in real space using the Fourier transform (FT). Both representations contain corresponding information about the image. As used in this paper, images in real space are represented by the lowercase letter "h". n The figure in Fourier space is represented by the capital letter "H". n The term "" is used to indicate this. Similarly, the bundle profile in real space is represented by the lowercase letter "g" in this paper. n The term "G" indicates that the bundle profile in Fourier space will be represented by the capital letter "G" in this paper. n The symbol indicates that, through Fourier transform, such as the Fast Fourier Transform (FFT) algorithm, an image and bundle contour in real space can be Fourier transformed to Fourier space, and vice versa.

[0042] Figure 2 The schematic diagram illustrates the determination of charged particle beam imaging devices, for example, in... Figure 1 The brightness B of the charged particle beam focused by the focusing lens 120 toward the sample 10 in the charged particle beam imaging device 100 r The method.

[0043] In box 210, also referred to herein as method action (a), is to capture one or more images of the sample, particularly at least one focused image h. f (=When the sample is positioned at the focal point of the charged particle beam, for example in) Figure 1 Distance z in f Images captured at the time of capture) and / or one or more defocused images h 1...N (=When the sample is positioned at one or more defocusing distances from the corresponding focal point, for example at) Figure 1 One or more out-of-focus images taken at distances z1, z2 and / or z3).

[0044] In box 220, also referred to herein as method action (b), one or more beam profiles of a charged particle beam are retrieved from one or more images captured in (a). Specifically, this can be done from at least one focused image h captured in (a). f Retrieve Focused Beam Profile g f And / or may be obtained from one or more defocused images h taken in (a). 1...N Retrieve one or more defocused beam profiles g 1...N In some specific implementations, deconvolution is used to retrieve one or more bundle contours from one or more images. Specifically, for the focused bundle image h... f Deconvolution yields the focused beam profile g. fFurthermore, deconvolution of the defocused beam image h1 yields the corresponding defocused beam profile g1 of the charged particle beam.

[0045] One or more retrieved beam profiles (focused beam profile g) f and / or one or more defocused beam profiles g 1...N This can be used to determine the brightness B of a charged particle beam. r Specifically, this is determined through calculation, simulation, and / or fitting. Specifically, in box 230, also referred to herein as method action (c), the brightness B of the charged particle beam is determined based on one or more beam profiles retrieved in (b). r The additional beam parameters are typically used to determine, in particular, the current of the charged particle beam probe (I0). P ) and / or the landing potential (LE) (or landing energy) of the charged particle beam on the sample.

[0046] More specifically, in some embodiments, the numerical aperture (NA) of the charged particle beam can be retrieved from one or more defocused beam profiles g in (b). 1...N Found (by Figure 2 (as shown in box 240), and the brightness B of the charged particle beam. r The focused beam profile g of the charged particle beam retrieved from numerical aperture (NA) and (b) can be obtained. f Probe current (I) P The brightness is determined by the landing potential (LE) and the luminance. For example, the brightness can be calculated using the following formula:

[0047]

[0048] Where NA is the numerical aperture of the charged particle beam, and I P This is the probe current, LE corresponds to the beam potential in the landing plane, i.e., the landing potential (which usually has the same value if expressed as potential in [V] or landing energy in electron volts [eV], and is therefore usually abbreviated as LE in this document), and d geo It is based on the focused beam profile g f The focal spot diameter of the determined charged particle beam. Note that, where appropriate (e.g., in cases of high electron landing energies >10 keV), the relative correction value can be used for the landing potential, where the relative correction value U... cor The following can be calculated based on the corresponding uncorrected value U:

[0049] Probe current I P It refers to the current of the charged particle beam probe striking the sample, usually expressed in amperes [A], which can be determined, for example, by measuring the corresponding beam current.

[0050] The landing potential (LE) of a charged particle beam refers to the electrical potential to which the charged particles of the beam have been accelerated and / or decelerated when hitting the sample. If the charged particles are electrons, the landing potential in [V] usually corresponds to the energy of the electrons of the charged particle beam hitting the sample in [eV], such that the respective quantities of “landing potential” and “landing energy” are used interchangeably herein.

[0051] In some embodiments, which can be combined with other embodiments described herein, the method comprises, in block 240, determining a numerical aperture (NA) of the charged particle beam based on the one or more beam profiles g 1...N determined in (b), wherein in (c) the brightness is determined based on at least the numerical aperture (NA), the probe current (I P ), and the landing potential (LE).

[0052] The numerical aperture (NA) of a charged particle beam is a quantity commonly used by the skilled person to describe the size of the beam convergence of a charged particle beam focused on the sample surface by an objective lens. A charged particle beam imaging device can be designed for a specific numerical aperture, but the actual numerical aperture can deviate from the intended value. Knowing the actual value of the numerical aperture (NA) of a charged particle beam is beneficial for accurately determining other parameters, including the brightness B r of the charged particle beam (determined at the focal spot). The brightness B r is dependent on the solid angle of the charged particle beam focused by the focusing lens towards the sample (i.e. the beam convergence angle, denoted by the numerical aperture NA). Alternatively, the numerical aperture (NA) can be estimated (see dotted arrow in Figure 2 ) based on the distance of the focusing lens from the sample and based on the beam aperture provided by the focusing lens. Based on the one or more defocused beam profiles g 1...N , a more accurate value of the numerical aperture can be obtained as described herein, such that also a more accurate value of the brightness B r can be obtained.

[0053] The numerical aperture (NA) can be determined as follows: in (a) one or more defocused images h 1...N of the charged particle beam are taken when the sample is arranged at one or more defocused distances z 1...N from the respective beam focal point of the charged particle beam. In (b) one or more defocused beam profiles g 1...N are retrieved from the one or more defocused images h 1...N , in particular based on deconvolution, and one or more beam widths c 1...N are determined from the one or more defocused beam profiles g 1...N . The numerical aperture (NA) of the charged particle beam can be based on the one or more beam widths c 1...N and the one or more defocused distances z 1...Nto calculate, in particular based on dc / dz, which represents the change of the beam width as a function of the defocus distance.

[0054] In some embodiments, the absolute value of the difference between one or more defocus distances z 1...N at which one or more defocused images h 1...N are taken is known, and said absolute value is used to calculate the numerical aperture (NA). 1...N

[0055] In some embodiments, the numerical aperture (NA) of the charged particle beam can be calculated based on the average change of the beam width as a function of the defocus distance, optionally using a calibration factor (K).

[0056] Figure 3 is a graph showing the determination of the numerical aperture (NA) of the charged particle beam based on one or more defocused beam profiles g 1...N . In an exemplary defocus range from -20 pm to +20 pm relative to the beam focus point (here: z = 0), the beam width (c) is shown in the graph as a function of the defocus distance (z). In the depicted example, three images are taken at different (quantitatively known) underfocus distances, and three images are taken at different (quantitatively known) overfocus distances. The respective beam profiles g 1...N are retrieved from the images, e.g. based on deconvolution, and the respective beam widths c 1...N are determined from the beam profiles.

[0057] Here, the FW50 width is taken from the beam profile, but the methods described herein are not limited in this respect (e.g. the FWHM, the 1 / e 2 width, or another commonly used quantity describing the beam width can be used). The beam widths c 1...N are shown in the graph as small squares at the respective defocus distances.

[0058] In the graph, the change of the beam width as a function of the defocus distance corresponds to the slope of the line (or linear fit) connecting the beam widths. Typically, the absolute values of the slopes upstream and downstream of the beam focus point (z = 0) substantially correspond to each other, as the charged particles of the beam propagate along a substantially linear path.

[0059] In some embodiments, the average slope, in particular the slope of a linear fit of at least some of the beam width values as a function of the defocus distance, is determined, and from this the beam convergence angle can be calculated. In some embodiments, the beam convergence angle is calculated by dividing the slope value ​The numerical aperture (NA) of the charged particle beam is calculated from the (average or fitted) slope value multiplied with a calibration factor (K). The calibration factor (K) can take into account a measure for the beam width (e.g. FW50 or FWHM). In some embodiments, the numerical aperture (NA) is calculated as follows: NA = K • |slope(FW50)|, if “FW50” (= full width 50) is used as a measure for the beam width and “slope” is the absolute value of the (optionally, average or linearly fitted) slope of the beam profile, wherein the calibration factor (K) is a value between 0.5 and 1, in particular a value between 0.6 and 0.9.

[0060] The resulting numerical aperture (NA) of the charged particle beam (e.g. NA = 7 mrad) can be used to calculate or otherwise determine the brightness B r , e.g. using (formula 1).

[0061] In some embodiments, one or more defocussed images h 1...N are taken as follows. The focusing lens 120 can apply a plurality of different focusing strengths to vary between a plurality of different defocussed distances, and the charged particle beam imaging apparatus 100 can take an image at each of the plurality of different focusing strengths of the focusing lens 120. In particular, the defocussed distance variation as a function of the focusing strength of the focusing lens 120 can be known, or can be determined in a prior calibration. Alternatively, the sample stage 108 can be moved along the optical axis with respect to the focusing lens 120 to vary between a plurality of different defocussed distances, and an image can be taken at each of the plurality of different defocussed distances. In other words, the defocussed distance can be varied by changing the focusing strength of the focusing lens 120 and / or by changing the z-position of the sample stage 108, both of which are suitable to move the sample between different defocussed positions such that one or more defocussed images can be taken.

[0062] In some implementations, at least one image is taken at an overfocussed distance, and at least one image is taken at an underfocussed distance. In particular, several overfocussed images can be taken and several underfocussed images can be taken, from which the respective beam profiles can be retrieved, and the NA can be determined based on the beam profiles and the respective overfocussed / underfocussed distances, as Figure 3 schematically shown in

[0063] In some embodiments, one or more beam profiles g 1...N are retrieved from one or more images h 1...N comprises deconvolution, in particular deconvolution with a high resolution image of the sample. Alternatively or additionally, one or more beam profiles g 1...N are retrieved from one or more images h 1...N ​The dividing in Fourier space can be considered equivalent to deconvolution.

[0064] A“high-resolution image” can be understood as an image of the sample having a higher resolution than an image that is deconvolved using the high-resolution image, or an image that is divided in Fourier space with the high-resolution image to obtain a corresponding beam profile. For example, the resolution of the high-resolution image can be increased by at least 30% or at least 50% relative to the resolution of the image of the beam profile to be retrieved. In some embodiments, the high-resolution image of the sample is taken with a charged particle beam imaging device, in particular with a device setting configured to provide a high-resolution image (e.g., with a diffraction-limited focal spot on the sample), as a focused image of the sample. Alternatively, the high-resolution image of the sample can be obtained in other ways, e.g., because details of the surface of the sample are known, or because the sample is known to be used for brightness determination.

[0065] The above-described beam profile extraction method is based on the fact that dividing a taken image of the sample in Fourier space with a high-resolution image of the sample is equivalent to deconvolving the defocused image with the high-resolution image, removing the structure of the sample from the image, such that the dividing or deconvolution can generate a pure beam profile, i.e., a beam cross-section without sample information.

[0066] Returning to Figure 2 , in some embodiments, which can be combined with other embodiments described herein, at least one image h f of the sample is taken in method action (a) when the sample is arranged at the beam focus of the charged particle beam f , which is also referred to herein as a“focused image h f ” of the sample. In (b), at least one focused beam profile g f is retrieved from the at least one focused taken image h f .

[0067] In some embodiments, at least one focused beam profile g f is retrieved from the at least one image h f based on deconvolution, in particular based on deconvolution with a high-resolution image of the sample. Alternatively, at least one focused beam profile g f may comprise dividing the at least one image in Fourier space with a high-resolution image of the sample in Fourier space. As described above, a“high-resolution image” of the sample can be understood as an image of the sample having a higher resolution (e.g., increased by 30% or more, in particular by 50% or more) than an image that is deconvolved using the high-resolution image, or an image that is divided in Fourier space with the high-resolution image to obtain a corresponding beam profile.

[0068] In some embodiments, which can be combined with other embodiments described herein, the method further comprises taking a high-resolution image of the sample, e.g. at a different setting of the charged-particle beam imaging apparatus adapted to generate a high-resolution image, in particular at a second resolution better than a first resolution of the image(s) taken in (a). In (b), the one or more beam profiles can be retrieved by deconvolving the image(s) taken in (a) with the high-resolution image of the sample, or the retrieval can comprise dividing the image(s) by the high-resolution image in Fourier space. For example, to obtain the high-resolution image of the sample, the apparatus setting can provide a high-resolution image, e.g. by providing a diffraction-limited spot size with a low-current charged-particle beam having a large numerical aperture. Alternatively, the high-resolution image of the sample used for retrieving the one or more beam profiles from the image(s) taken can be obtained in other ways, e.g. because details of the sample are known beforehand, e.g. because the sample is known to be imaged in (a).

[0069] In (c), the brightness B of the charged-particle beam r may be determined based on the focused beam profile g f , the numerical aperture (NA) (which can be determined as described above or can be determined in other ways), the probe current (I P ) and the landing potential (LE).

[0070] In particular, the focal spot size, in particular the focal spot diameter d f , more particularly the full width 50 diameter (FW50 diameter) of the charged-particle beam can be retrieved from the focused beam profile g geo , and the brightness of the charged-particle beam can be determined based on the focal spot size, the numerical aperture (NA), the probe current (I P ) and the landing potential (LE), in particular using (Formula 1).

[0071] In some embodiments, which can be combined with other embodiments described herein, the charged-particle beam can be an electron beam having one or more of the following parameters: the probe current (I P ) can be in a range from 1 nA to 5 nA, the landing potential (LE) can correspond to a landing energy in a range from 200 eV to 5 keV, and / or the numerical aperture (NA) can be in a range from 3 mrad to 10 mrad. The focal spot diameter d f of the charged-particle beam retrieved from the focused beam profile g geo may be, e.g. in a range from 5 nm to 15 nm.

[0072] Example 1 (determination of luminance via calculation)

[0073] An exemplary method of determining the beam brightness according to embodiments described herein is performed as follows.

[0074] A high-resolution image of the sample can already be available, or a high-resolution image of the sample can be taken using a charged-particle beam, e.g. using a high-resolution device setting suitable for taking a high-resolution image (e.g. a small probe current (e.g. 50-100 pA) focused with a relatively large numerical aperture (e.g. 10 to 20 mrad)). For example, a high-resolution image of the sample can be taken with a diffraction-limited focal spot providing a resolution of e.g. 3 nm or less, e.g. about 1 nm. The high-resolution image of the sample can later be used to retrieve the beam profile from the taken image, e.g. based on deconvolution of the taken image.

[0075] Optionally, the actual brightness determination process can be performed at a different device setting, namely at a brightness-limited working point, which can not be optimized for taking a high-resolution image, but which can use a focused beam profile allowing to identify the actual geometric spot size, as the beam profile is not dominated by other contributions such as diffraction, chromatic blur and / or spherical blur contributions. At the brightness-limited working point, the charged-particle beam is focused by the focusing lens towards the sample such that the focal spot size is limited by the brightness, in particular such that the geometric focal spot size is larger than each of the diffraction, chromatic blur and spherical blur contributions to the focal spot size of the charged-particle beam.

[0076] The contributions of spherical and chromatic blur to the focal spot size typically increase with increasing numerical aperture, while the diffraction contribution decreases with the numerical aperture. The geometric spot size corresponding to the reduced virtual source size depends on the magnification of the optical system. Typically, the magnification decreases with increasing numerical aperture, and thus, the geometric spot size also decreases with the numerical aperture. The above-mentioned contributions to the focal spot size also typically depend on the probe current. The brightness determination according to embodiments described herein is advantageously performed at a brightness-limited working point, at which the focal spot size is mainly defined by the actual geometric spot size, and the contributions of diffraction and blur to the beam profile are relatively small. This can allow to retrieve the geometric spot size directly from the focused beam profile without considering distortions / variations of the focused beam profile caused by diffraction and / or aberrations. For example, the brightness determination according to embodiments described herein can be performed at a probe current in the range of 1 nA or more to 5 nA or less and / or at a numerical aperture in the range of 3 mrad or more to 10 mrad or less, which can provide a brightness-limited focal spot. The brightness-limited working point of the device is not necessarily suitable for taking a high-resolution image of the sample, but is rather more suitable for providing a focused beam profile which is mainly defined by the geometric spot, and thus, is beneficial for the brightness determination.

[0077] The landing potential (LE) can be known, e.g. because the electrostatic potential of the charged particle beam applied to the impinging sample is known. The probe current I P may also be known, e.g. because the setup of the beam limiting aperture defining the probe current is known or the probe current can be measured. For example, the landing potential can be about 500 V, corresponding to a landing energy of about 500 eV, and the probe current (I P ) can be about 2 nA.

[0078] The numerical aperture NA can be determined as described above, in particular by taking one or more images of the sample at one or more defocus distances, by retrieving one or more beam profiles from the one or more images, by determining one or more beam widths from the one or more beam profiles, and by calculating the numerical aperture NA of the charged particle beam based on the one or more beam widths and the one or more defocus distances. For example, the NA can be about 7 mrad (see above example).

[0079] The focused beam profile g f may be retrieved from at least one image h f of the sample taken in focus, in particular based on deconvolution with a high resolution image of the sample. The focal spot size, in particular the focal spot diameter d geo may be retrieved directly from the focused beam profile g f , e.g. as the FW50 diameter, in particular if the in-focus image h f is taken at a brightness-limited working point of the apparatus. For example, the focal spot diameter d geo may be determined to be about 9 nm (see Figure 2 ).

[0080] Based on the above exemplary parameter values, the brightness B r may be determined by a calculation based on (Formula 1):

[0081]

[0082] Thus, in “Example 1”, the (reduced) brightness B r of the charged particle beam is determined to be about

[0083] Example 2 (determination of luminance by fitting)

[0084] Figure 4 A schematic flowchart showing an alternative method of determining the brightness (B r ) of a charged particle beam according to the embodiments described herein is shown.

[0085] In box 410, also referred to herein as method action (a), one or more images of the sample are acquired using a charged particle imaging device, particularly but not necessarily in the brightness-limited device settings described above. The one or more images may include the sample's profile g that can be used to retrieve the focused beam profile. f At least one focused image h f And / or one or more defocused images h that can be used to determine the numerical aperture NA 1...N .

[0086] In box 420, also referred to herein as method action (b), one or more beam profiles of a charged particle beam are retrieved from one or more images, particularly but not necessarily based on deconvolution retrieval using high-resolution images as described above. Focused beam profile g f From at least one focused image h f Search.

[0087] Hereafter, it will also be referred to as method action (c) in this paper, which can be based on the focused beam profile g. f Numerical aperture (NA) (e.g.) Figure 3 (as shown or otherwise determined), probe current (I) P The brightness B of the charged particle beam is determined as follows, along with the landing potential (LE). r .

[0088] In box 430, referred to herein as method action (c1), at least numerical aperture (NA) and probe current (I0) can be used. P Landing potential (LE) and estimated brightness of charged particle beam (B) est () is used as an input parameter to simulate the focused beam profile, providing a simulated focused beam profile. sim g f The simulation of the beam profile can be performed, for example, based on wave optics simulation of the focused beam cross-section. The estimated brightness can be a rough estimate of the brightness of the charged particle beam, for example, based on commonly used estimates or values ​​published in the literature.

[0089] In box 440, referred to in this paper as method action (c2), the simulated focus beam profile will be... sim g f Compared with the focused beam profile g retrieved in (b) f Compare them to determine the magnitude (R) of the difference between them.

[0090] In box 450, referred to in this paper as the method action (c3), the estimated brightness (B) is changed. est To provide updated brightness (B) i+1 ), and during the iteration process, update the brightness (B) i+1 Instead of the initially used estimated brightness (B) estRepeat (c1) and (c2) to minimize the focused beam profile g. f The magnitude (R) of the difference between the corresponding simulated focused beam profile and the actual beam profile.

[0091] Specifically, the method actions (c1) and (c2) can be repeated until the simulated focused beam profile is obtained. sim g f With the retrieved focused beam profile g f The minimum value of the difference between them (R) min Furthermore, the updated brightness during iteration when minimizing the magnitude of the difference can be determined as the brightness corresponding to the charged particle beam (B). r ).

[0092] For example, the simulated focused beam profile can be minimized during the iterative or fitting process (by changing the brightness used for the corresponding simulation). sim g f With the retrieved focused beam profile g f The difference between the two is calculated until the minimum value of the difference between them is obtained, and the brightness value used to obtain the minimum difference is determined as the brightness of the charged particle beam.

[0093] like Figure 4 The lower part of the curve is schematically shown, if the same retrieved focused beam profile g is used. f and (NA), (I P The corresponding parameter values ​​of (LE) and "brightness fitting method" in "Example 2" result in a brightness value of the charged particle beam, for example, a brightness value of approximately This essentially corresponds to the value determined in the "Calculation Method" using (Formula 1) according to "Example 1" above. Therefore, the "Brightness Fitting Method" and the "Calculation Method" can be used as alternative methods to accurately determine the brightness of a charged particle beam, or both methods can be used to improve the reliability of the obtained brightness values.

[0094] Experiments show that the brightness values ​​of the charged particle beam obtained according to the method described herein are in excellent agreement with expectations. The method described herein is advantageous compared to available methods for the following reasons: The brightness of the charged particle beam can be measured in virtually any charged particle beam column, requiring no special tools or optical or device geometry. This allows for the analysis of beam emitter variations and also allows for comprehensive modeling of the charged particle beam: brightness can be used as a parameter for beam spot simulation.

[0095] Source size determination

[0096] According to another aspect described in this article, the brightness B of the charged particle beam r Used to determine the source of charged particle beams Size Especially virtual sources Sizea virtual source of the electron emission tip Size .

[0097] Figure 5 A method of determining a source 105 of a charged particle beam 11 in a charged particle beam imaging apparatus according to embodiments described herein is shown in a schematic flowchart and Size is a schematic diagram of a source region of a charged particle beam imaging apparatus for illustrating a method of Figure 5 .

[0098] A method of determining a source 105 of a charged particle beam 11 in a charged particle beam imaging apparatus according to embodiments described herein is shown in a schematic flowchart and Size (especially a virtual source of the electron emission tip Size Figure 5 D virt ) of the charged particle beam 11 comprises the following operations. The charged particle beam 11 is generated with the source 105, wherein the source can comprise an emission tip from which the charged particles, especially electrons, are emitted. The charged particle beam 11 is focused towards the sample 10 by a focusing lens 120. The brightness (B r ) of the charged particle beam is determined according to any of the methods described herein (e.g. using the “fitting method” described above or the “calculation method” described above), especially by taking one or more images of the sample with the charged particle beam imaging apparatus (block 210), retrieving one or more beam profiles of the charged particle beam from the one or more images (block 220) and determining the brightness B P ) of the charged particle beam based on at least one or more of the beam profile g r , the numerical aperture (NA), the probe current (I P ) and the landing potential (LE) (block 230). In particular, the brightness B r ) of the charged particle beam can be determined based on the focusing beam profile g geo , the numerical aperture (NA), the probe current (I r ) and the landing potential (LE). Alternatively, one of d virt and (NA) can also be determined in other ways (see dotted arrow in Figure 5 ).

[0099] As shown in block 540 in ​ , the size (D r ) of the source 105 can be determined based on at least the brightness (B ex ) determined in block 230. In particular, the size (D b ) can be determined based on at least the brightness (B virt ), the charged particle beam energy (U P ), the charged particle beam current (I b ) and the source opening angle (a).

[0100] The source size determination method described herein is based on the finding that the (reduced) brightness of the charged particle beam can be assumed to be a conserved parameter in the charged particle beam imaging apparatus. Thus, the (reduced) brightness can be determined at the beam probe focused on the sample (which is at a "downstream" position along the beam) and can be used to reliably determine the source size (which is at an "upstream" position along the beam). Thus, the probe current (I P ) impinging on the sample does not necessarily correspond to the beam current (I b ) of the charged particle beam initially generated by the source, and the beam potential of the charged particle beam (corresponding to the acceleration / deceleration potential to which the beam particles are exposed, respectively) can also vary along the optical axis. For example, the landing potential LE (corresponding to the landing energy on the sample) can be lower than the initial beam potential (corresponding to the initial particle energy in the beam column caused by the extraction potential), e.g. due to a retarding field lens component of the focusing lens 120.

[0101] The beam potential of the charged particle beam can correspond to the electron potential in [V] (or the energy in [eV]) caused by the potential difference between the beam acceleration potential, in particular the potential of the extraction electrode 104, and the potential of the source. In particular, the beam potential can correspond to the potential difference between the emission tip of the source 105 and the potential of the extraction electrode 104, which accelerates the electrons emitted by the emission tip to the charged particle beam energy within the beam column. An example value for the extraction potential is U ex = 7 kV

[0102] The source opening angle (a) can be determined based on the aperture size of the beam limiting aperture 103 arranged downstream of the source 105 and the aperture distance of the beam limiting aperture 104 to the source 105. The beam limiting aperture can be an aperture that limits the charged particle beam, i.e. it blocks the outer edge region of the charged particles emitted by the source 105 and thus forms the charged particle beam 11 emitted from the emission tip and propagating along the optical axis A within the beam column of the charged particle beam imaging apparatus.

[0103] In particular, the source opening angle (a) can be calculated by D a / 2 x a , where D a / 2 is half the diameter of the beam limiting aperture 103 and X a is the aperture distance of the beam limiting aperture 103 to the emission tip of the source, which is much larger than the diameter Da of the beam limiting aperture such that tan(a) « a. For example, the diameter D a of the beam limiting aperture 103 can be 0.08 mm and the distance X a may be 80 mm such that a = 0.5 mrad.

[0104] The charged particle beam current (I bThis can correspond to the current in the charged particle beam 11 within the column of a charged particle beam imaging device, particularly downstream of the beam confinement aperture 103, which defines the beam current within the column propagating along the optical axis A. For example, the beam current (I...) b It can be in the range of 20 to 200 pA, for example 150 pA.

[0105] According to embodiments that can be combined with other embodiments described herein, the virtual size (D) of source 105 is calculated using the following formula. virt )

[0106]

[0107] Among them (B) r ) is the brightness of the charged particle beam, (I b () is the charged particle beam current, (α) is the source opening angle, and (U) is the source opening angle. ex U is the charged particle beam potential. Similarly, here, the value of the relative correction can be used for the charged particle beam potential U. ex If the charged particle beam potential is high (e.g., >10 keV), i.e., if the beam accelerating voltage is high, then the relative correction value is reasonable. The relative correction value U... cor The following calculation can be performed based on the corresponding uncorrected value U: Using the above example parameter values, the following source dimensions are calculated.

[0108]

[0109] The example above calculates a virtual size of approximately 9nm for the source 105.

[0110] According to the implementation described herein, the size of the source is determined based on the actual brightness of the beam generated by the source. Therefore, when tracing the charged particle beam in reverse, the determined size corresponds precisely to the region from which the charged particles at the emission tip appear to originate.

[0111] Another implementation relates to a charged particle beam imaging system having a processor and a memory storing instructions, which, when executed by the processor, cause the charged particle beam imaging apparatus to perform any of the methods described herein, particularly methods for determining the brightness of a charged particle beam and / or methods for determining the size of a source of the charged particle beam. The charged particle beam imaging apparatus described herein can be configured to operate according to any of the methods described herein. Specifically, one or more methods described herein can be performed using a charged particle beam imaging apparatus.

[0112] Specifically, the following implementation methods are described in this document:

[0113] Implementation 1: A method for determining the brightness of a charged particle beam (11) focused by a focusing lens (120) toward a sample (10) in a charged particle beam imaging apparatus (100), the method comprising: (a) capturing one or more images (h) of the sample using the charged particle beam imaging apparatus. f,1...N (b) Retrieve one or more beam profiles (g) of the charged particle beam from the one or more images. f,1...N (c) based at least on the one or more bundle profiles (g f,1...N The probe current (I) of the charged particle beam P The brightness (B) of the charged particle beam (11) is determined by the landing potential (LE) of the charged particle beam (11). r ).

[0114] Implementation Method 2: According to the method of Implementation Method 1, wherein in (a), when the sample is arranged at one or more defocusing distances (z) from the corresponding focal point of the charged particle beam. 1...N When at the location, one or more images (h) of the sample (10) are captured. 1...N The method further includes: extracting data from the one or more bundle profiles (g...). 1...N Determine one or more bundle widths (c) 1...N ); and based on the one or more beam widths (c 1...N ) and the one or more defocus distances (z 1...N The numerical aperture (NA) of the charged particle beam is calculated using (c), wherein, at least based on the numerical aperture (NA) and the probe current (I0), the numerical aperture (NA) is calculated. P The brightness of the charged particle beam is determined by the landing potential (LE) and the landing potential (LE).

[0115] Implementation Method 3: According to the method of Implementation Method 2, wherein the one or more defocus distances (z) when capturing the one or more images are... 1...N Either the absolute value of the difference between the one or more defocusing distances is known, and the absolute value is used to calculate the numerical aperture (NA).

[0116] Implementation 4: The method according to Implementation 2 or 3, wherein the numerical aperture (NA) of the charged particle beam is calculated based on the average change in beam width as a function of defocus distance and optionally a calibration factor.

[0117] Implementation Method 5: The method according to any one of Implementation Methods 1 to 4, wherein in (a), when the sample is arranged at the focal point of the charged particle beam (11), at least one image (h) of the sample is captured. f In (b), from the at least one image (h)f ) a focused beam profile is retrieved, and in (c) the brightness of the charged particle beam is determined based on at least the focused beam profile (g f ), a numerical aperture (NA) of the charged particle beam, a probe current (I P ) and a landing electric potential (LE). The numerical aperture (NA) can be determined according to any of embodiments 2 to 4, or can be determined otherwise.

[0118] Embodiment 6: The method according to embodiment 5, wherein from the focused beam profile (g f ) a focal spot size, in particular a focal spot diameter (d geo ), more particularly a full width 50 diameter, of the charged particle beam is retrieved, and in (c) the brightness of the charged particle beam is determined based on the focal spot size, the numerical aperture (NA), the probe current (I P ) and the landing electric potential (LE).

[0119] Embodiment 7: The method according to any of embodiments 1 to 6, wherein in (c) the brightness (B r ) is determined by a calculation using the following formula:

[0120]

[0121] where NA is the numerical aperture of the charged particle beam, I P is the probe current, LE is the landing electric potential (corresponding to landing energy), and d geo is the focal spot diameter of the charged particle beam. A value of a relative correction can optionally be used for the landing electric potential, in particular if the landing electric potential is high, e.g. 10 keV or more.

[0122] Embodiment 8: The method according to embodiment 5, comprising: (cl) simulating a focused beam profile using at least the numerical aperture (NA), the probe current (I P ), the landing electric potential (LE) and an estimated brightness (B est ) as input parameters to provide a simulated focused beam profile; (c2) comparing the simulated focused beam profile with the retrieved focused beam profile to determine a magnitude (R) of a difference therebetween; and (c3) changing the estimated brightness (B est ) to provide an updated brightness (B i+1 ), and repeating (cl) and (c2) in an iterative process to minimize the magnitude (R).

[0123] Embodiment 9: The method according to embodiment 8, wherein (c1) and (c2) are repeated until a minimum magnitude of the difference between the simulated focused beam profile and the retrieved focused beam profile is obtained (R min ), and the updated brightness at which the magnitude of the difference is smallest or smallest is determined as the brightness (B r ) of the charged particle beam.

[0124] Embodiment 10: The method according to any one of embodiments 1 to 9, wherein the charged particle beam is an electron beam having one or more of the following parameter values: the probe current (I P ) is in a range from 1 nA to 5 nA, the landing potential (LE) corresponds to a landing energy in a range from 200 eV to 5 keV, and / or the numerical aperture (NA) is in a range from 3 mrad to 10 mrad.

[0125] Embodiment 11 : The method according to any one of embodiments 1 to 10, wherein in (b), retrieving the one or more beam profiles from the one or more images comprises dividing the one or more images in Fourier space with a high-resolution image (h HR ) of the sample in Fourier space, or wherein retrieving the one or more beam profiles in (b) from the one or more images is based on deconvolution.

[0126] Embodiment 12: The method according to any one of embodiments 1 to 11, wherein in (a), the charged particle beam (11) is focused by the focusing lens (120) towards the sample (10) such that a focal spot size is limited by brightness, in particular such that a geometric focal spot size is larger than each of a diffraction contribution, a chromatic blur contribution, and a spherical blur contribution to the focal spot size of the charged particle beam.

[0127] Embodiment 13: The method according to any one of embodiments 1 to 12, further comprising taking a high-resolution image (h HR ) of the sample, in particular at a second resolution which is higher than a first resolution of the one or more images taken in (a), wherein in (b), the one or more beam profiles of the charged particle beam are retrieved by deconvolution of the one or more images (h HR ) with the high-resolution image (h f,1...N ) of the sample. For example, the second resolution can be improved by at least 30% or more, in particular at least 50% or more, relative to the first resolution.

[0128] Embodiment 14: A brightness (B rThe purpose of this is to determine the size (D) of the source (105) of the charged particle beam. virt (In particular, the virtual source size at the electron emission tip.)

[0129] Implementation 15: A method for determining the size (D) of the source (105) of a charged particle beam (11) in a charged particle beam imaging apparatus (100). virt The method includes: generating the charged particle beam using the source (105); focusing the charged particle beam (11) toward the sample (10) using a focusing lens (120); and determining the brightness (B) of the charged particle beam according to any of the methods described herein. r ); and at least based on the brightness (B) r The beam potential (U) of the charged particle beam ex ), charged particle beam current (I) b The source (105) is determined by the source opening angle (α) and the source opening angle (α). virt ).

[0130] Implementation 16: According to the method of implementation 15, wherein the beam potential (U) ex This corresponds to the potential difference between the source (105) and the beam acceleration potential, particularly the potential of the extraction electrode.

[0131] Implementation 17: The method according to implementation 15 or 16, wherein the source opening angle (α) is determined based on the aperture size of the beam limiting aperture (103) downstream of the source (105) and the aperture distance of the beam limiting aperture (103) from the source.

[0132] Implementation 18: The method according to any one of Implementations 15 to 17, wherein the dimension (D) of the source (105) is calculated using the following formula. virt ):

[0133]

[0134] Among them B r The brightness of the charged particle beam, I b It is the current of the charged particle beam, α is the source opening angle, and U ex It is the beam potential. The relative correction value can be optionally used for the beam potential, especially if the beam potential is high, for example, 10 keV or higher.

[0135] Embodiment 19: A charged particle beam imaging apparatus (100) comprising: a source (105) for emitting a charged particle beam (11) propagating along an optical axis (A); a sample stage (108); a focusing lens (120) for focusing the charged particle beam to a sample (10) placed on the sample stage (108); a charged particle detector (118) for detecting signal particles emitted from the sample (10); and a processor (170) and a memory storing instructions which, when executed by the processor, cause the charged particle beam imaging apparatus to: (a) take one or more images (h f,1...N ) of the sample; (b) retrieve one or more beam profiles (g f,1...N ) of the charged particle beam from the one or more images; and (c) determine a brightness (B f,1...N ) of the charged particle beam based at least on the one or more beam profiles (g P ), a probe current (I r ) of the charged particle beam, and a landing electric potential (LE) of the charged particle beam.

[0136] Embodiment 20: The charged particle beam imaging apparatus according to embodiment 19, wherein the instructions, when executed by the processor (170), further cause the charged particle beam imaging apparatus (100) to determine a size (D r ) of the source (105) based on the brightness (B r ) of the charged particle beam, in particular based on the brightness (B ex ), a beam potential (U virt ) of the charged particle beam, a charged particle beam current, and / or a source opening angle (a). The processor (170) can calculate the size (D virt ) based on the above (formula 2).

[0137] While the foregoing is directed to implementations, other and further implementations may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

1. A method for determining the brightness of a charged particle beam focused toward a sample by a focusing lens in a charged particle beam imaging apparatus, the method comprising: (a) To capture one or more images of the sample using the charged particle beam imaging device; (b) Retrieving one or more beam profiles of the charged particle beam from the one or more images; and (c) Determine the brightness of the charged particle beam, wherein In (a), at least one image of the sample is captured when the sample is positioned at the focal point of the charged particle beam. In (b), the focus beam profile is retrieved from the at least one image, and In (c), the brightness of the charged particle beam is determined by at least one of a formula calculation method and a brightness fitting method, based at least on the focused beam profile, the numerical aperture of the charged particle beam, the probe current of the charged particle beam, and the landing potential of the charged particle beam.

2. The method of claim 1, wherein in (a), one or more images of the sample are captured when the sample is positioned at one or more defocus distances from the respective focal point of the charged particle beam, the method further comprising: Determine one or more bundle widths from the one or more bundle profiles; as well as The numerical aperture of the charged particle beam is calculated based on the one or more beam widths and the one or more defocusing distances. In (c), the brightness of the charged particle beam is determined at least based on the numerical aperture, the probe current, and the landing potential.

3. The method of claim 2, wherein the absolute value of the one or more defocus distances or the difference between the one or more defocus distances when capturing the one or more images is known, and the absolute value is used to calculate the numerical aperture.

4. The method of claim 2, wherein the numerical aperture of the charged particle beam is calculated based on the average variation of the beam width as a function of the defocusing distance and optionally a calibration factor.

5. The method of claim 1, wherein the focal spot size of the charged particle beam is retrieved from the focused beam profile, and In (c), the brightness of the charged particle beam is determined based on the focal spot size, the numerical aperture, the probe current, and the landing potential.

6. The method of claim 5, wherein in (c), the brightness is determined by calculation using the following formula: Where NA is the numerical aperture of the charged particle beam, I P It is the probe current, LE corresponds to the landing potential, and d geo It is the focal diameter of the charged particle beam.

7. The method according to claim 1, comprising: (c1) At least the numerical aperture, the probe current, the landing potential and the estimated brightness are used as input parameters to simulate the focused beam profile to provide a simulated focused beam profile; (c2) Compare the simulated focused beam profile with the focused beam profile retrieved in (b) to determine the magnitude of the difference between the two; as well as (c3) Change the estimated brightness to provide an updated brightness, and repeat (c1) and (c2) during the iteration process to minimize the magnitude.

8. The method of claim 7, wherein (c1) and (c2) are repeated until a minimum value of the difference between the simulated focused beam profile and the retrieved focused beam profile is obtained, and the updated brightness at this iteration is determined as the brightness of the charged particle beam.

9. The method according to any one of claims 1 to 8, wherein the charged particle beam is an electron beam having one or more of the following parameter values: the probe current is in the range of 1 nA to 5 nA, the landing potential corresponds to a landing energy in the range of 200 eV to 5 keV, and the numerical aperture of the charged particle beam is in the range of 3 mrad to 10 mrad.

10. The method according to any one of claims 1 to 8, wherein, In (b), retrieving the one or more bundle profiles from the one or more images includes dividing the one or more images in Fourier space using a high-resolution image of the sample in Fourier space, or wherein the one or more bundle profiles in (b) are retrieved from the one or more images based on deconvolution.

11. The method according to any one of claims 1 to 8, wherein in (a), the charged particle beam is focused toward the sample by the focusing lens such that the focal spot size is limited by brightness, and in particular, the geometric focal spot size is greater than each of the diffraction contribution, color blur contribution, and spherical blur contribution to the focal spot size of the charged particle beam.

12. The method according to any one of claims 1 to 8, further comprising: A high-resolution image of the sample is captured at a second resolution, which is higher than the first resolution of the one or more images captured in (a). In (b), the beam profile of the charged particle beam is retrieved by deconvolving the one or more images with the high-resolution image of the sample.

13. Use of the method of any one of claims 1 to 8 to determine the brightness of a charged particle beam for determining the size of the source of the charged particle beam.

14. A method for determining the size of a source of a charged particle beam in a charged particle beam imaging apparatus, comprising: The source is used to generate the charged particle beam; The charged particle beam is focused toward the sample using a focusing lens; The method of any one of claims 1 to 8 determines the brightness of the charged particle beam; and The size of the source is determined based at least on the brightness, the beam potential of the charged particle beam, the charged particle beam current, and the source opening angle.

15. The method of claim 14, wherein the beam potential is caused by the potential difference between the source and the beam acceleration potential.

16. The method of claim 14, wherein the source opening angle (α) is determined based on the aperture size of the beam confinement aperture downstream of the source and the aperture distance of the beam confinement aperture from the source.

17. The method of claim 14, wherein the dimension of the source is calculated using the following formula: Among them B r The brightness of the charged particle beam, I b Where α is the charged particle beam current, α is the source opening angle, and U ex It is the beam potential.

18. A charged particle beam imaging device, comprising: A source, the source being used to emit a beam of charged particles propagating along the optical axis; Sample stage; A focusing lens for focusing the charged particle beam onto a sample placed on the sample stage; A charged particle detector for detecting signal particles emitted from the sample; as well as A processor and a memory, the memory storing instructions that, when executed by the processor, cause the charged particle beam imaging apparatus to determine the brightness of the charged particle beam according to any one of claims 1 to 8.

19. The charged particle beam imaging apparatus of claim 18, wherein the instructions, when executed by the processor, further cause the charged particle beam imaging apparatus to determine the size of the source based at least on the brightness of the charged particle beam.

Images