Method for determining beam convergence of charged particle beam and charged particle beam system

Abstract

A method of determining a beam convergence of a charged particle beam (11) focused by a focusing lens (120) towards a sample (10) in a charged particle beam system (100), the method comprising: (a) taking one or more images (h1... N) of the sample when the sample is arranged at one or more defocusing distances (z1... N) from respective beam focuses of the charged particle beam, and retrieving one or more retrieved beam contours (g1... N) from the one or more images (h1... N); (b) simulating one or more beam profiles at the one or more defocus distances (z1... N) based at least on the estimated beam convergence value (initial C) of the charged particle beam to provide one or more simulated beam profiles (g '1... N); (c) determining a magnitude (R) of a difference between the one or more simulated beam profiles (g '1... N) and the one or more retrieved beam profiles (g1... N); and (d) changing the estimated beam convergence value in an iteration process to reduce or minimize the magnitude (R) to determine an actual beam convergence value (actual C). Further, a charged particle beam system for imaging and / or inspecting a sample is provided, the charged particle beam system configured for any of the methods described herein.

Description

Technical Field

[0001] Embodiment described herein relates to and determines in charged particle beam system, particularly in scanning electron microscope (SEM), by the method for the beam convergence of the charged particle beam focused towards sample by focusing lens.Particularly, can determine the numerical aperture (NA) that is provided by focusing lens in charged particle beam system according to method described herein.Embodiment further relates to the charged particle beam system for testing and / or imaging sample, and these charged particle beam systems are configured for carrying out any one in method described herein. Background Art

[0002] Modern semiconductor technology has created a high demand for structuring and probing samples at the nanometer or even sub-nanometer level. Micrometer and nanometer-scale process control, inspection or structuring are typically performed using charged particle beams (e.g., electron beams) that are generated, shaped, deflected, and focused in charged particle beam systems (such as electron microscopes or electron beam pattern generators). For inspection purposes, charged particle beams provide higher spatial resolution than, for example, photon beams.

[0003] The inspection apparatus of the use charged particle beam such as scanning electron microscope (SEM) has many functions in multiple industrial fields, including but not limited to the inspection of electronic circuits, exposure systems for photolithography, detection systems, defect detection tools, and test systems for integrated circuits. In particle beam systems, a fine beam probe with high current density can be used. For example, in the case of SEM, a primary electron beam generates signal particles that can be used for imaging and / or inspecting a sample, such as secondary electrons (SE) and / or backscattered electrons (BSE).

[0004] However, it is challenging to reliably inspect and / or image samples with good resolution using a charged particle beam system. Particularly, charged particle beams suffer from beam aberrations that limit resolution usually, and the actual cross-sectional shape of the charged particle beam focused can be different from the designed shape. The resolution limit of the charged particle beam system is determined by the numerical aperture (NA) of the charged particle beam focused on the sample surface by an objective lens.

[0005] The numerical aperture (NA) of charged particle beam is the quantity that is generally used to describe the beam convergence of the charged particle beam that is focused on the sample surface by object lens.Bigger numerical aperture provides improved resolution limit, if beam aberration is very low, then can reach described resolution limit in theory.Described system can be designed to provide the specific numerical aperture of outstanding operation, but actual numerical aperture may depart from desired value.Therefore, understanding the beam convergence of the charged particle beam focused by focusing lens and more specifically knowing the actual numerical aperture of charged particle beam will help determine beam aberration source, and help improve the resolution of system.

[0006] In view of the above, it will be beneficial to accurately and reliably determine the beam convergence of the charged particle beam focused by the focusing lens in the charged particle beam system, particularly accurately determine the numerical aperture of the charged particle beam focused by the objective lens. In addition, it will be beneficial to provide a charged particle beam system for testing and / or imaging a sample, the charged particle beam system being configured to operate according to any one of the methods described herein. Summary of the Invention

[0007] In view of the above, a method of determining beam convergence of a charged particle beam and a charged particle beam system configured therefor are provided according to the independent claims.

[0008] According to a first aspect, a method for determining a beam convergence of a charged particle beam focused by a focusing lens toward a sample in a charged particle beam system is provided. The method comprises: (a) taking one or more images of the sample when the sample is arranged at one or more defocus distances from the corresponding beam focus of the charged particle beam, and retrieving one or more retrieved beam profiles from the one or more images; (b) simulating one or more beam profiles at the one or more defocus distances based on at least an estimated beam convergence value of the charged particle beam to provide one or more simulated beam profiles; (c) determining a magnitude (R) of a difference between the one or more simulated beam profiles and the one or more retrieved beam profiles; and (d) varying the estimated beam convergence value in an iterative process to reduce or minimize the magnitude (R) to determine an actual beam convergence value.

[0009] In some embodiments, the actual beam convergence value comprises information about the beam width variation as a function of the defocus distance. In particular, the actual beam convergence value may comprise or may be the numerical aperture (NA) of the charged particle beam.

[0010] In some embodiments, a focused image of the sample is also taken (when the sample is positioned at a focal distance from the focusing lens), and the focused image of the sample can be used in (a) to retrieve one or more retrieved beam profiles from the one or more images taken at the one or more defocused distances. The focused image of the sample can also be known in other ways and can then be used to retrieve the one or more retrieved beam profiles in (a).

[0011] According to a second aspect, an alternative method for determining a beam convergence of a charged particle beam focused by a focusing lens toward a sample in a charged particle beam system is provided. The method comprises: (a) capturing one or more images of the sample to provide one or more captured images when the sample is arranged at one or more defocus distances from a corresponding beam focus of the charged particle beam; (b) simulating one or more images of the sample captured at the one or more defocus distances based on at least an estimated beam convergence value of the charged particle beam and a focus image of the sample to provide one or more simulated images; (c) determining a magnitude (R) of a difference between the one or more captured images and the one or more simulated images; and (d) varying the estimated beam convergence value in an iterative process to reduce or minimize the magnitude (R) to determine an actual beam convergence value.

[0012] According to another aspect, a charged particle beam system for imaging and / or inspecting a sample with a charged particle beam, in particular an electron beam, is provided. The charged particle beam system comprises: a charged particle source for emitting a charged particle beam propagating along an optical axis; a sample stage; a focusing lens for focusing the charged particle beam toward 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 that, when executed by the processor, cause the system to perform any of the methods described herein.

[0013] In particular, these instructions, when executed, cause the system to: (x1) simulate one or more beam profiles at one or more defocus distances based at least on an estimated beam convergence value of the charged particle beam to provide one or more simulated beam profiles; (x2) compare one or more retrieved beam profiles retrieved from one or more images of a sample taken at the one or more defocus distances with the one or more simulated beam profiles to determine a magnitude of a difference between the two; and (x3) vary the estimated beam convergence value in an iterative process to reduce or minimize the magnitude to determine an actual beam convergence value, in particular, to determine an actual numerical aperture (NA).

[0014] Alternatively, the instructions, when executed, cause the system to: (x1) simulate one or more images of the sample taken at one or more defocus distances based at least on an estimated beam convergence value of the charged particle beam and a focused image of the sample to provide one or more simulated images; (x2) determine a magnitude of a difference between one or more captured images of the sample taken at one or more defocus distances and the one or more simulated images; and (x3) vary the estimated beam convergence value in an iterative process to reduce or minimize the magnitude to determine an actual beam convergence value, in particular, to determine an actual numerical aperture (NA).

[0015] The embodiments also relate to apparatus for carrying out the disclosed methods, including apparatus components for performing the individual method actions. The described methods may be performed by hardware components, a computer programmed with appropriate software, any combination of the two, or any other means. Furthermore, the embodiments also relate to methods of operating the described apparatus.

[0016] Further advantages, features, aspects and details, which can be combined with the embodiments described herein, are apparent from the dependent claims, the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order that the above-described features of the present disclosure can be understood in detail, a more particular description, briefly summarized above, may be had by reference to embodiments. The accompanying drawings relate to one or more embodiments and are described below.

[0018] Figure 1 shows a schematic diagram of a charged particle beam system according to embodiments described herein, the charged particle beam system being adapted to operate according to any of the methods described herein;

[0019] Figure 2 shows a flow chart illustrating a method of determining beam convergence of a charged particle beam according to embodiments described herein;

[0020] Figure 3 Shown is a description of Figure 2 A more detailed diagram of the method shown; and

[0021] Figure 4 is a graph showing retrieved beam profile parameters as a function of defocus distance, determined using any of the methods described herein. DETAILED DESCRIPTION

[0022] Reference will now be made in detail to various embodiments, one or more examples of which are illustrated in the accompanying drawings. In the following description of the drawings, like reference numerals represent like parts. Generally, only the differences with respect to various embodiments will be described. Each example is provided by way of explanation and is not meant to be limiting. In addition, features illustrated or described as part of one embodiment may be used on or in conjunction with other embodiments to produce yet further embodiments. This description is intended to encompass such modifications and variations.

[0023] Figure 1is a schematic diagram of a charged particle beam system 100 for inspecting and / or imaging a sample 10 according to an embodiment described herein. The charged particle beam system 100 comprises a charged particle source 105, in particular an electron source, for emitting a charged particle beam 11, in particular an electron beam, propagating along an optical axis A. The charged particle beam system 100 further comprises a sample stage 108 and a focusing lens 120, in particular an objective lens, for focusing the charged particle beam 11 onto the sample 10 placed on the sample stage 108. The charged particle beam system 100 further comprises a charged particle detector 118, in particular an electron detector, for detecting signal particles (in particular secondary electrons and / or backscattered electrons) emitted from the sample 10.

[0024] An image generation unit 160 may be provided to generate 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 transmit the one or more images of the sample to a processing unit 170, which is configured to determine at least one beam convergence value of the charged particle beam according to the methods described herein.

[0025] At least one beam convergence value can be used, for example, to adjust the charged particle beam and to correct the charged particle beam. For example, a set of beam aberration coefficients can be determined in an iterative process using the beam convergence value as an input parameter. The beam aberration coefficients can be transmitted to a controller 180 of the beam-affecting element 109, such as an aberration corrector, so that the beam-affecting element 109 can be appropriately adjusted to correct the aberrations of the charged particle beam. Alternatively or additionally, the controller 180 can control other elements of the charged particle beam system 100, such as the charged particle source 105 or the focusing lens 120. The charged particle beam 11 can be adjusted in this manner, for example, so that the charged particle beam system 100 has a specific beam convergence value, such as a beam convergence value that is substantially the same as another charged particle beam system. For example, if the determined actual beam convergence value deviates from a desired value, one or more beam optical components of the system can be adapted to modify the actual beam convergence value to approximate the desired value.

[0026] The sample stage 108 may be a movable stage. In particular, the sample stage 108 can be moved in the Z direction, ie, in the direction of the optical axis A, so that the distance between the focusing lens 120 and the sample 10 can be changed (see Figure 1 By moving the sample stage 108 in the Z direction, the sample 10 can be moved to different defocus distances z1, z2, z3, that is, away from the focal plane p of the focusing lens 120. I , so that the out-of-focus image of the sample 10 can be taken by moving the corresponding stage, for example, in predetermined increments. In some embodiments, the out-of-focus image of the sample 10 is taken at a depth of less than 2 times the depth of focus, less than 1.5 times the depth of focus (d 1.5At least one of: capturing an out-of-focus image of the sample 10 at a defocus distance less than 1.2 times the depth of focus, or within the depth of focus. The depth of focus can be described as the distance from the beam focus at which nonlinear effects begin to significantly affect the correlation between the defocus distance and the beam profile diameter. In other words, the correlation between the defocus distance and the beam profile size is (significantly) nonlinear at defocus distances less than or equal to the "depth of focus." In some embodiments, the depth of focus can also be described as the Rayleigh length or the confocal parameter.

[0027] In some embodiments, the sample stage 108 can also be moved in a plane (also referred to herein as the XY plane) perpendicular to the optical axis A. By moving the sample stage 108 in the XY plane, a specific surface area of ​​the sample 10 can be moved to an area below the focusing lens 120, so that the specific surface area can be imaged by focusing the charged particle beam 11 thereon.

[0028] The beam optical components of the charged particle beam system 100 are typically placed in an evacuable vacuum chamber 101 so that the charged particle beam 11 can propagate along an optical axis A from a charged particle source 105 toward a sample stage 108 and at a subatmospheric pressure (e.g., less than 10 - 3 mbar pressure or less than 10 -5 mbar pressure) to irradiate the sample 10.

[0029] In some embodiments, the charged particle beam system 100 may be an electron microscope, in particular a scanning electron microscope. A scanning deflector 107 may be provided to scan the charged particle beam 11 over the surface of the sample 10 along a predetermined scanning pattern, for example in the X direction and / or the Y direction.

[0030] In some embodiments, a condenser lens system 106 can be arranged downstream of the charged particle source 105, in particular for collimating the charged particle beam 11 propagating toward the focusing lens 120. In some embodiments, the focusing lens 120 is an objective lens configured to focus the charged particle beam 11 onto the sample 10, in particular a magnetic objective lens, an electrostatic magnetic lens, or a combined magnetic-electrostatic lens. The objective lens can optionally include a deceleration field device, such as one or more deceleration electrodes, which are configured to decelerate the charged particle beam to a predetermined landing energy on the sample.

[0031] One or more surface areas of a sample 10 can be inspected and / or imaged using a charged particle beam system 100. As used herein, the term "sample" can refer to, for example, a substrate having one or more layers or features formed thereon, a semiconductor wafer, a glass substrate, a web substrate, or another sample to be inspected. The sample can be inspected for one or more of the following: (1) imaging the surface of the sample; (2) measuring the dimensions of one or more features of the sample, for example in a lateral direction, i.e., in the XY plane; (3) performing critical dimension measurements and / or metrology; (4) detecting defects; and / or (5) investigating the quality of the sample.

[0032] In order to examine the sample 10 with the charged particle beam 11, the charged particle beam 11 is typically focused onto the sample surface using a focusing lens 120. When the charged particle beam 11 is incident on the sample surface, secondary electrons and / or backscattered electrons are emitted from the sample. The signal electrons provide information about the spatial characteristics and characteristic structure sizes of the sample, and the signal electrons can be detected by a charged particle detector 118. By scanning the charged particle beam 11 over the sample surface, for example, using a scanning deflector 107, and detecting the signal electrons as a function of the generation position of the signal electrons, the sample surface or a portion thereof can be imaged, for example, using an image generation unit 160, which can be configured to provide an image of the sample 10 based on the received signal electrons.

[0033] Focusing the charged particle beam 11 to a small spot on the sample surface increases the achievable image resolution. Therefore, during inspection, the sample surface should be arranged in the focal plane p of the focusing lens 120. I In order to obtain a clear and focused image of the sample 10. The clear image of the sample 10 taken in focus (also referred to as a "high-resolution image") is also referred to as a "focus image h I ”, the subscript I indicates “focusing”. Similarly, the charged particle beam 11 is at the focal plane p I The beam profile in this paper is referred to as the "focal beam profile g I ", the subscript I represents "focus". In some embodiments, a clear image of the sample 10 taken in focus may also be provided by using measurements of other instruments or using simulations of the sample.

[0034] Images can be mathematically represented in real space (i.e., in the image domain, i.e., as a function of spatial coordinates) or in Fourier space (i.e., in the frequency domain, i.e., as a function of spatial frequencies). Images in Fourier space can be calculated from images in real space via a Fourier transform (FT). Both representations contain corresponding information about the image. In some of the embodiments described herein, images and beam profiles in real space can be Fourier transformed into Fourier space, and vice versa, via a fast Fourier transform (FFT) algorithm.

[0035] The actual value of the beam convergence of the charged particle beam focused onto the sample by the focusing lens 120 in a charged particle beam system is generally unknown. The beam convergence can be expressed in terms of the beam convergence angle (α) relative to the optical axis or in terms of the numerical aperture (NA) of the charged particle beam, values ​​which are difficult to determine and are "longitudinal characteristics" of the beam that are hidden and cannot be directly obtained from the image. The numerical aperture (NA) determines the resolution limit, so that knowledge of the actual value of the numerical aperture would be very valuable.

[0036] In some embodiments, the numerical aperture of the beam influencing element of the charged particle beam system is used to determine the numerical aperture of the beam influencing element. For example, the numerical aperture of the system is usually used to adjust and design the beam influencing element (such as an objective lens), which may cause inaccuracies and beam aberrations when the numerical aperture actually existing in the charged particle beam system deviates from the design numerical aperture assumed by the system design. The beam influencing element may include one or more elements in the group consisting of: a lens, a beam extractor, a beam deflector, a collimator, an aberration corrector, a scanning deflector, a beam splitter, and a charged particle detector. Therefore, determining the actual numerical aperture can help identify the source of inaccuracies in the system and help optimize the system design and resolution. Moreover, the numerical aperture as a measure of the beam convergence can be used as an input parameter in the aberration coefficient determination routine.

[0037] The numerical aperture (NA) of a charged particle beam system can be roughly estimated as follows (= first NA determination method): the ratio between the estimated beam width at the position of the objective lens and the distance between the sample and the objective lens (when the sample is approximately arranged at the focus position) can be calculated, and the NA can be calculated based on this ratio. However, the obtained NA value may be inaccurate.

[0038] A more accurate method for determining the numerical aperture (NA) of a charged particle beam system is as follows (= second NA determination method): one or more beam widths of the charged particle beam are retrieved from one or more defocused images of the sample, and the numerical aperture is calculated based on the average variation of the beam width with defocus distance. However, the accuracy of the second NA determination method may be impaired if one or more of the defocus distances lies within the depth of focus, because, as described above, the correlation between the beam width and the defocus distance is significantly nonlinear in the range close to the beam focus.

[0039] In view of the above, according to the method described herein, in particular independent of the choice of the defocus distance, at least one beam convergence value, in particular the numerical aperture (NA), of a charged particle beam may be reliably and accurately determined. Figure 1 The charged particle beam system 100 may include a processor and a memory (in Figure 1 170), the memory stores instructions that, when executed by the processor, cause the system to perform any one of the methods described herein.

[0040] Figure 2 and Figure 3 is a flow chart schematically illustrating such a method of determining at least one beam convergence value according to embodiments described herein. Figure 2 Compared to the more generalized graph of Figure 3 The figures show optional further details.

[0041] (a) If Figure 2 and Figure 3 As shown in blocks 210 and 220 in FIG. 1 , when the sample is arranged at one or more defocus distances (z 1…N ) at which one or more images of the sample are taken (h 1…N ), and from one or more images (h 1…N ) to retrieve one or more retrieved bundle profiles (g 1…N ), especially based on deconvolution. (b) Figure 2 and Figure 3 As shown in block 230 in FIG. 1 , the beam convergence at one or more defocus distances (z ) is simulated based at least on the estimated beam convergence value, in particular on the estimated numerical aperture, and optionally on other known or estimated beam parameter values. 1…N ) at one or more beam profiles (g') 1…N (c) Figure 2 and Figure 3 As shown in block 240 in FIG. 1 , one or more simulated beam profiles (g 1…N ) with one or more retrieved beam profiles (g' 1…N ) between the magnitude (R). (d) As Figure 2 and Figure 3 As shown in block 250 in FIG. 1 , the estimated beam convergence value is changed in an iterative process to reduce or minimize the magnitude (R). The beam convergence value that results in a reduction or minimization of the magnitude (R) is deemed to be the actual beam convergence value ( 实际 C), in particular the actual numerical aperture of the charged particle beam. Further details of the above method are described below.

[0042] exist Figure 2 and Figure 3 In block 210, when the sample is arranged at one or more defocus distances (z 1…N ), in particular 2 or more, 3 or more, 5 or more, 8 or more, or 10 or more different defocus distances, taking one or more images of the sample (h 1…N). The defocus distance is understood herein as the distance between the sample and the beam focus when the image is taken (≠0). If an image is taken when the sample is arranged at the defocus distance relative to the beam focus, the resulting image is an out-of-focus image of the sample. Naturally, an increase in the defocus distance leads to an increase in the blurriness of the corresponding image taken, since the beam width generally increases with the defocus distance and the resolution generally decreases with the defocus distance.

[0043] In some embodiments, a plurality of images, e.g., two, three, five, six, eight, ten or more images, are taken at a plurality of different defocus distances between the sample and the respective beam focus, e.g., at two, three, five, six, eight, ten or more different defocus distances. In particular, at least one image of the sample may be taken at an overfocus distance, i.e., at a defocus setting in which the sample is arranged further from the focusing lens 120 than the respective beam focus of the charged particle beam (see Figure 1 Furthermore, at least one image of the sample can be taken at an underfocus distance, i.e. at a defocus setting, in which the sample is arranged closer to the focusing lens 120 than the corresponding beam focus of the charged particle beam (see Figure 1 The image captured at the first defocus distance z1 is denoted as h1 in this document, and the image captured at the nth defocus distance z2 is denoted as h1 in this document. n The images taken at h are denoted in this paper. n A total of N images can be taken, which are denoted as (h 1…N ), in particular, N is six or greater, such as ten or greater, or fifteen or greater.

[0044] In some embodiments, at least one image of the sample may be captured at a defocus distance where the beam profile (e.g., beam profile size or width) is significantly nonlinearly dependent on the defocus distance. For example, at least one image of the sample may be captured at a defocus distance less than 2 times the depth of focus, 1.5 times the depth of focus (d 1.5 ), 1.2 times the depth of focus or less than the defocus distance of the sample. The depth of focus can be described as the distance from the minimum beam waist to the beam width along the propagation direction of the charged particle beam. 1 / 2 The depth of focus can also be described as the Rayleigh length.

[0045] In some embodiments, one or more images are captured. 1…N ) is located at one or more defocus distances (z 1…N) is known quantitatively, i.e., in absolute terms, e.g., in [μm] or in another unit of length. In order to determine the beam convergence from one or more defocused images, it is beneficial to know quantitatively at which defocus distance (e.g., in [μm]) each of the defocused images was taken. In some embodiments, a calibration may be performed before taking defocused images of the sample, such that each defocus distance at which an image was taken is quantitatively known from the corresponding settings of the charged particle beam system.

[0046] In some embodiments, which may be combined with other embodiments, the defocus distance is changed by moving the sample stage 108 in the Z direction, ie, along the optical axis A, relative to the focusing lens 120 . Figure 1 The stage movement for changing between a plurality of different defocus distances is schematically depicted. For example, the sample stage can be moved in predetermined increments, such as a plurality of equal or similar increments between 0.2 μm and 2 μm, and an image of the sample can be captured at each of the plurality of defocus distances. While the sample stage is moved and images are captured, a constant focus intensity of the focus lens 120 can be maintained.

[0047] In other embodiments, the defocus distance is varied by varying the focus strength of the focusing lens 120. Increasing the focus strength of the focusing lens moves the corresponding beam focus and focal plane toward the focusing lens relative to the sample, while decreasing the focus strength moves the corresponding beam focus and focal plane away from the focusing lens relative to the sample, thereby varying the defocus distance by varying the focal plane p. I The defocus distance can be changed by shifting relative to the sample 10. The sample can remain stationary. Specifically, the focusing lens 120 can apply multiple different focusing intensities to the charged particle beam to change between multiple different defocus distances, and an image can be captured at each of the multiple different focusing intensities. Alternatively or additionally, the energy of the charged particles of the charged particle beam 11 can be changed to change the defocus distance.

[0048] Note that changing the focus strength also changes the beam convergence. Thus, in the embodiments described herein, if a change is made, only the focus strength is changed such that the resulting change in the defocus distance is negligible relative to the overall focal length (f) of the focus lens 120. For example, by varying the focus strength used to capture multiple images, the defocus distance can be varied over a total range of a few microns (e.g., <10 μm), while the overall focal length (f) of the focus lens 120 can be in the range of a few millimeters or even a few centimeters. Thus, according to the embodiments described herein, by varying the focus strength of the focus lens, the beam convergence can be varied only to a negligible extent, and the determined numerical aperture (NA) or other beam convergence values ​​are substantially unaffected.

[0049] In some embodiments, the change in defocus distance as a function of the focus intensity of the focusing lens 120 is known in advance or determined in a previous calibration, such that for each change in focus intensity applied by the focusing lens, the corresponding change in defocus distance is quantitatively known, or for each focus intensity of the focusing lens, the corresponding defocus distance is quantitatively known (e.g., in [μm]).

[0050] Back to Figure 2 and Figure 3 In block 220, from one or more images (h 1…N ) to retrieve the charged particle beam at one or more defocus distances (z 1…N One or more beam profiles (g) at 1…N In particular, multiple beam profiles are retrieved from the multiple images, i.e., one beam profile is retrieved for each of the multiple images. Each retrieved beam profile corresponds to the beam profile of the charged particle beam at the defocus distance at which the corresponding image was captured. Several different beam profile extraction methods can be used to extract the defocused beam profile from the defocused image. An exemplary beam profile extraction method is exemplarily explained below.

[0051] In addition to one or more images taken out of focus (h 1…N ), you can also take a focused image of the sample (h I ), and the focus image (h I ) can be used to extract the image from one or more images (h 1…N ) to retrieve one or more bundle profiles (g 1…N ). Focus image h of the sample I It may also be known, for example because a sample with known geometry is used to determine the beam convergence according to the methods described herein. The focus image (h) of the sample may further be known from simulations or from measurements using another instrument. I ). The focused image of the sample is also referred to herein as a "high-resolution image" of the sample, and the resolution of the high-resolution image is improved relative to the resolution of one or more images of the sample taken at a defocused distance from the corresponding beam focus, for example, by at least 30% or at least 50%.

[0052] In some implementations, based on deconvolution, in particular with a high-resolution image of the sample (h I ) deconvolution, retrieve one or more beam profiles from one or more images. Specifically, the defocused image (h n ) and the high-resolution image deconvolution to obtain the corresponding defocused beam profile of the charged particle beam (g n )(Because h n ≈h I ×g n ). Alternatively or additionally, from one or more images (h1…N ) to retrieve one or more bundle profiles (g 1…N ) may comprise dividing one or more images in Fourier space by a high-resolution image of the sample in Fourier space. This division in Fourier space may be considered equivalent to deconvolution.

[0053] In some embodiments, from one or more images (h 1…N ) to retrieve one or more bundle profiles (g 1…N ) may include one or more images (h 1…N ) is Fourier transformed to provide one or more images in Fourier space (H 1…N ), and one or more images in Fourier space (H 1…N ) divided by the focal image of the sample in Fourier space (H I The above beam profile extraction method is based on the fact that in Fourier space, the defocused image of the sample (H n ) divided by the focal image of the sample (H I ) removes the structure of the sample so that the division can produce a pure beam profile, i.e. a beam profile without sample information. Each retrieved beam profile (G n ) is inverse Fourier transformed to obtain the corresponding retrieved beam profile in real space (g n ).

[0054] From one or more images (h 1…N ) to retrieve one or more bundle profiles (g 1…N ) may optionally include an adaptive filtering term The sum of the products and the focal beam profile in Fourier space (G I ) are multiplied. Adaptive filtering term It can be provided by an adaptive filter that can receive the captured image as input information. The adaptive filter term can be provided separately for each of the images captured by the adaptive filter. If there is no adaptive filter term Then the focus image H in the denominator of the above division is I A value close to zero may result in excessive weighting of noise in the focus image. 1…N ) in the calculation of the adaptive filtering term The focus image H can be reduced or avoided I For example, in order to ensure that an appropriate filter term is used for each image and the corresponding noise in the image, an adaptive filter may be used for each image (h 1…N ) to determine the corresponding filtering items.

[0055] Back to Figure 2 and Figure 3 In block 230, one or more beam profiles at one or more defocus distances are simulated to provide one or more simulated beam profiles (g' 1…N ). can be Figure 1 The processing unit 170 shown performs the simulation. The beam profile at one or more defocus distances at which the one or more images are captured may be simulated in block 210. In other words, the defocus distance used to capture the image in block 210 is considered for simulating the beam profile in block 230. For example, if the beam profile is simulated at one or more defocus distances (z 1…N ) to capture one or more images (h 1…N ), then one or more defocus distances (z 1…N ) is transmitted to or otherwise known to the processing unit, and one or more simulated beam profiles (g' 1…N ) is the defocused beam profile that looks like the beam profile at that defocus distance.

[0056] The beam profile at each defocus distance in block 230 may be simulated based on an estimated beam convergence value of the charged particle beam, in particular based on an estimated numerical aperture (NA), and optionally based on other parameter values ​​of the charged particle beam. The initial estimated beam convergence value ( 初始 C).

[0057] For example, an initial estimated numerical aperture (NA) of the charged particle beam may be calculated based on one or more retrieved beam profile size variations with defocus distance. Alternatively, an initial estimated beam convergence value ( 初始 C) may be based on known details of the beam optics of the charged particle beam system or on experience, eg on typical beam convergence values ​​for similar charged particle beam systems.

[0058] The beam profile in block 230 may be simulated by a wave optics simulation. The wave optics simulation may use an estimated beam convergence value of the charged particle beam as an input parameter for the simulation, optionally together with other input parameters characterizing the charged particle beam, such as one or more beam aberration coefficients, beam landing energy, beam brightness, beam spectral width, and / or beam current of the charged particle beam. The wave optics simulation may include a Fourier transform, in particular a fast Fourier transform, and / or may include electron optical phase information. If one or more other (known or estimated) parameters of the charged particle beam are used as input parameters in the simulation of the beam profile, the simulated beam profile may more accurately simulate the retrieved beam profile, allowing the iterative method to converge more quickly and / or provide more reliable results.

[0059] For example, one or more (known or estimated) beam aberration coefficients, such as astigmatism, spherical aberration, coma, and / or one or more chromatic aberrations of the charged particle beam, may also be utilized to simulate the one or more simulated beam profiles in block 230. Simulating one or more beam profiles based on one or more beam aberration coefficients of the actual beam may result in faster and / or better convergence of the iterative process toward a reduced or minimized magnitude (R).

[0060] In some embodiments, which may be combined with other embodiments described herein, one or more parameters of a charged particle beam may be provided (e.g., estimated), the one or more parameters may be used as one or more input parameters for a simulation, and at least some of the parameters may also be varied in an iterative process (along with and / or sequentially with changes in beam convergence values) to improve the accuracy of the estimated parameters. For example, one or more simulated beam profiles may also be simulated using one or more (known or estimated) beam aberration coefficients, and the one or more beam aberration coefficients may be varied in an iterative process (along with the estimated beam convergence values) to reduce or minimize the magnitude (R) to determine a beam aberration coefficient that is closer to the actual beam aberration coefficient. In this way, the methods described herein are suitable not only for determining actual beam convergence values, but also for determining actual values ​​of other beam parameters, such as one or more beam aberration coefficients.

[0061] In some embodiments, one or more simulated beam profiles (g' 1…N ) may include simulating at least one of measurement artifacts or retrieval artifacts that may occur during an image capture process, an image processing process, or a beam profile retrieval process.

[0062] like Figure 2 and Figure 3 As shown schematically, the input data simulated in block 230 may include the data previously used to capture the image (h 1…N ) of the defocus distance (z 1…N ), and the initial estimated beam convergence value 初始 C or updated beam convergence value i+1 C. Optionally, as described above, one or more other beam parameters may be used in the simulation.

[0063] In block 240, one or more retrieved beam profiles (g 1…N ) and one or more simulated beam profiles (g' 1…N ) to determine one or more retrieved beam profiles (g 1…N ) and one or more simulated beam profiles (g' 1…N ) is the magnitude of the difference between i ).like Figure 2 and Figure 3As shown schematically, in block 240, one or more retrieved beam profiles (g 1…N ) and one or more simulated beam profiles (g' 1…N ) are compared with each other.

[0064] In some embodiments, one or more retrieved beam profiles (g 1…N ) and one or more simulated beam profiles (g' 1…N ) for comparison. Comparison in Fourier space may be advantageous because linear shifts in real space appear only as phase terms in Fourier space, so inaccuracies due to incorrect superposition of the retrieved beam profile and the corresponding simulated beam profile used for comparison do not appear in Fourier space (when converting (G 1…N ) and (G' 1…N ) when comparing the absolute value (squared) of

[0065] The comparison in block 240 may include computing one or more retrieved beam profiles (g 1…N ) and the corresponding simulated beam profile (g' 1…N ) and summing the (optionally squared) differences to obtain the magnitude (R). When the initial beam convergence value is used in the simulation 初始 C, the value R1 is calculated. In subsequent iterations, the estimated beam convergence value is updated i+1 C can be used for the simulation respectively and the updated magnitude (R i+1 ), the goal is to determine the value that leads to the minimization of the quantity (R 最小值 ) of the beam convergence value.

[0066] In determining the value (R i ) and then change the beam convergence value i C to provide updated beam convergence value i+1 C, and uses the updated bundle convergence value during the iteration i+1 C repeats the simulation in block 230 and the comparison in block 240 with the goal of reducing or minimizing the magnitude (R) in the iterative simulation and comparison process.

[0067] In particular, the simulation in block 230, the comparison in block 240, and the steps for providing i+1 C's i C is varied until a value (R 最小值 ), and the corresponding updated bundle convergence value at the corresponding iteration is regarded as the actual bundle convergence value 实际 C. Specifically, Figure 2 and Figure 3 As schematically shown in block 250, it may be checked whether (R i <R 最小值 ). If (R i <R 最小值 ), the process can be stopped and C can be regarded as the actual beam convergence value 实际 C. Otherwise, the iterative process can continue.

[0068] In some embodiments, which may be combined with other embodiments described herein, an actual beam convergence value may be used. 实际 C to adjust the charged particle beam, such as modifying the actual beam convergence value 实际 C to match the technical specifications.

[0069] Figure 3 is better than Figure 2 The diagram shown is a more detailed diagram. Specifically, Figure 3 The method shown corresponds to Figure 2 The method shown, however, includes optional additional operations shown in blocks 420 and 430. Figure 3 Most of the operations of the method shown correspond to Figure 2 For the corresponding operations of the method shown, please refer to the above description and will not be repeated here.

[0070] In particular, in some embodiments, in block 420, the beam profiles (g 1…N ) determines one or more retrieved beam profile parameters, such as one or more beam profile sizes or beam profile widths (k 1…N ); and in block 430, from one or more simulated beam profiles (g ' 1…N ) Determine one or more simulated beam profile parameters, such as one or more simulated beam profile sizes or beam profile widths (k' 1…N ). Specifically, the beam profile parameters may include one or more parameters describing the beam profile size, the beam profile width (eg, profile radius or diameter), the beam profile ellipticity, and / or the beam profile homogeneity.

[0071] In some embodiments, the comparison in block 240 may include comparing one or more retrieved beam profile parameters (eg, one or more beam profile widths (k 1…N )) and one or more simulated beam profile parameters (e.g., one or more simulated beam profile widths (k' 1…N )) to determine one or more retrieved beam profiles (g 1…N ) and one or more simulated beam profiles (g' 1…N ) between the magnitude (R). Determine one or more retrieved beam profiles (g 1…N) and one or more simulated beam profiles (g' 1…N ) (in particular, between one or more retrieved beam profile parameters and one or more simulated beam profile parameters) may include calculating a difference value between each of the retrieved beam profile parameters of the one or more retrieved beam profiles and the corresponding simulated beam profile parameters, and summing the (optionally, squared) difference values ​​to obtain the magnitude (R).

[0072] In some embodiments, summing the difference values ​​between the beam profile parameters may include weighting the beam profile parameters, in particular using empirically determined weights.

[0073] In some embodiments, one or more retrieved beam profiles (g 1…N ) may be substantially rotationally symmetric, e.g. circular or Gaussian. In other words, the radial beam width may be substantially independent of the azimuth angle at which the radial beam width is determined. As used herein, "azimuth angle" refers to different directions within a cross-section of the beam profile, i.e. different directions perpendicular to the optical axis (A), at which the width of the beam profile may be measured. If the beam profile is substantially circular, it may be appropriate to retrieve (only) one beam profile parameter for each beam profile, e.g. the radial beam width (e.g. the beam diameter, FW50 value or FWHM value). Alternatively, an average radial beam width may be determined from each beam profile, e.g. as an FW50 value, which is the diameter of a circle around the optical axis through which half the beam current propagates, as a 10 / 90 or 20 / 80 blade width, or as a D4σ width. In some embodiments, the beam profile parameters may include two or more parameters describing the beam profile, e.g. the beam profile ellipticity.

[0074] In some embodiments, which may be combined with other embodiments, each of the one or more beam widths is determined in two or more directions (ie at two or more different azimuth angles). 1…N (θ)) to determine each of the one or more beam widths. The beam widths may be in two or more directions, for example, in azimuth angle θ x at (= in the x direction) and azimuth angle θ y The corresponding beam width k is determined at (= in the y direction) n Specifically, it can be obtained from the azimuth angle (k n The corresponding beam profile g n The search beam width k n . It can be seen from the azimuth ((k 1…N The ellipticity of the beam profile is determined by one or more beam widths of )(θ)).

[0075] In some implementations, the beam cross-sectional shape may be asymmetric, such as elliptical or oval. In this case, the beam convergence value may be described by two or three parameters, such as (1) ellipticity and (2) orientation of the major axis of the ellipse, or (1) major axis width, (2) minor axis width, and (3) angle. Multiple dimensions of the beam convergence value may be varied sequentially and / or in parallel until a minimized magnitude (R) of all dimensions that are a function of the beam convergence value is obtained. 最小值 ). In particular, a plurality of size changing routines may be used to change each of the sizes of the beam convergence values ​​to retrieve the beam convergence value by comparing the retrieved beam profile with the simulated beam profile.

[0076] In some embodiments, which may be combined with other embodiments described herein, at least one quantity from the following group is determined from each beam profile and is considered the corresponding beam width: (1) FWHM (=full width at half maximum), or average FWHM, at one or more azimuth angles; (2) FW50 (=full width 50), or average FW50, also known as D50 width, at one or more azimuth angles; (3) 1 / e 2 Width; and (4) Another commonly used quantity to describe beam width. The beam radius or beam diameter can be considered as the beam width, respectively.

[0077] In some embodiments, the actual numerical aperture (NA) of the system is determined at two or more different azimuth angles.

[0078] In some embodiments, which may be combined with other embodiments described herein, the charged particle beam is an electron beam and the charged particle beam system is an electron system, in particular an electron microscope such as a SEM. The charged particle source may be an electron source and the detector may be an electron detector.

[0079] The electron beam may have one or more of the following characteristics: a numerical aperture in the range below 30 mrad, in particular in the range of 1 mrad to 5 mrad; a probe current in the range of 10 pA to 10 nA; and / or a landing energy in the range of 100 eV to 100 keV. Any one or more of these parameters may be used in the simulation of the beam profile.

[0080] Figure 4 Two graphs are shown of normalized beam size, specifically normalized beam width, as a function of defocus distance normalized to the depth of focus DOF. Here, nine images are taken at different (quantitatively known) defocus distances and the corresponding beam profile widths are retrieved. Both sub-graphs show the same data. The beam width (k) determined in block 420 is 1…N ) at the corresponding defocus distance (z 1…N ) are shown as small squares.

[0081] In the upper subgraph, the variation of the beam width as a function of the defocus distance is determined as the slope of the line connecting the beam widths (or a linear fit thereof). Typically, the absolute values ​​of the slopes upstream and downstream of the beam focus (defocus distance = 0) substantially correspond to each other, since the charged particles of the beam propagate along a substantially linear path.

[0082] In the upper sub-graph, the nonlinearity of the beam width as a function of defocus distance can be seen at small defocus distances. In particular, the data points at a defocus distance of approximately -1 DOF deviate far from the linear fit of the beam width determined at larger defocus distances.

[0083] In the lower sub-figure, a beam convergence value, in particular a numerical aperture, is determined according to the method described in the present disclosure by simulating beam profiles at corresponding defocus distances and determining the magnitude (R) of the difference between one or more simulated beam profiles and one or more retrieved beam profiles. The simulated beam profile can take into account the aforementioned nonlinearity of the beam profile, in particular at small defocus distances, exemplarily at defocus distances less than twice the depth of focus.

[0084] Simulating the beam profile may further comprise simulating artifacts from the beam profile retrieval, in particular artifacts of the threshold value or artifacts of the selected filter size, and / or beam aberrations (if known), thereby improving the accuracy of the NA determination method.

[0085] In some embodiments that may be combined with other embodiments described herein, at two or more azimuth angles, in particular with azimuth angles ((k 1…N )(θ)) to determine one or more beam widths (k 1…N ), and the corresponding beam convergence value can be calculated at two or more azimuth angles or with the azimuth angle value.

[0086] In particular, at least one beam convergence value may include the charged particle beam at a first azimuth angle (θ x ) and the first beam convergence value at the second azimuth angle (θ y In some embodiments, at least one beam convergence value may be calculated as a function of azimuth, for example if the beam profile is highly asymmetric, elliptical, or otherwise distorted.

[0087] In some embodiments, the numerical aperture of a charged particle beam may be calculated at two or more azimuthal angles, and in particular as a function of the azimuthal angle (NA(θ)).

[0088] In one embodiment, the beam width of the charged particle beam can be determined and / or displayed along with the defocus distance. In another embodiment, the beam width of the charged particle beam can be determined and / or displayed along with the defocus distance. In another embodiment, the beam width of the charged particle beam can be determined and / or displayed along with the defocus distance. In another embodiment, the beam width of the charged particle beam can be determined and / or displayed along with the defocus distance. In another embodiment, the beam width of the charged particle beam can be determined and / or displayed along with the defocus distance. In another embodiment, the beam width of the charged particle beam can be determined and / or displayed along with the defocus distance. In another embodiment, the beam width of the charged particle beam can be determined and / or displayed along with the defocus distance. In another embodiment, the beam width of the charged particle beam can be determined and / or displayed along with the defocus distance.

[0089] In some embodiments described herein, particularly if a single defocused image is taken at a defocus distance z n is quantitatively known (and furthermore, a focused image h of the sample is taken or otherwise known). I , so that the retrieved beam profile g n From a single captured defocused image h n retrieval), then a single defocused image h n It may be sufficient to determine at least one beam convergence value therefrom. In particular, the at least one beam convergence value may be determined by: at least based on the defocus distance z n The estimated beam convergence value at is used to simulate the simulated beam profile g' of the charged particle beam n ; and by determining the retrieved beam profile g n with the retrieved bundle profile g' n and reducing or minimizing the magnitude (R) by varying the estimated beam convergence value in an iterative process to determine the actual beam convergence value. 1…N ) to capture multiple defocused images (h 1…N ), more accurate beam convergence values, in particular more accurate NA values, can be determined and from these the beam convergence values ​​are determined as explained herein, in particular in both underfocus and overfocus.

[0090] Known processes for determining beam convergence values ​​are based on an analysis of the beam convergence angle, deriving the beam convergence angle from a linear fit of retrieved beam profile widths at different known defocus distances. However, the known processes are based on a linear approximation of the beam convergence. At small defocus distances, in particular at defocus distances less than 2 times the depth of focus, the linear approximation may result in a large deviation from the actual beam convergence. In contrast, the method described herein is based on the fact that a defocused beam profile can be used for comparison with a simulated beam profile based at least on an estimated beam convergence value of a charged particle beam if the defocused beam profile is taken at a quantitatively known defocus distance or a difference between quantitatively known defocus distances, and on the fact that an iterative variation of the estimated beam convergence value minimizes the magnitude (R) of the difference between one or more simulated beam profiles and one or more retrieved beam profiles to allow determination of the actual beam convergence value. In particular, known beam aberrations or beam profile retrieval artifacts may be incorporated into the simulated beam profiles, which allows a more accurate comparison of the retrieved beam profiles with the simulated beam profiles. In very low numerical aperture embodiments, nonlinear effects may be relevant over a large range of defocus distances, leading to considerable deviations from the actual beam convergence values ​​in known processes. In contrast, the method described herein is suitable for accurately determining the actual beam convergence values ​​in very low numerical aperture embodiments.

[0091] The ability to accurately determine beam convergence values ​​(particularly numerical aperture) in charged particle beam systems allows, inter alia, (1) reproducibility and matching, since numerical aperture is the decisive quantity determining the achievable resolution; (2) determination of beam-affecting elements, e.g. to improve future designs; (3) determination of beam aberrations scaled to various powers of numerical aperture; and (4) generation of conditioned charged particle beams for certain critical dimension and defect review tasks, e.g. to inspect deep trenches in samples elongated along only one axis.

[0092] An alternative method for determining a beam convergence of a charged particle beam focused by a focusing lens toward a sample in a charged particle beam system does not compare a retrieved defocused beam profile and a simulated defocused beam profile to each other, but instead compares a captured defocused image and a simulated defocused image to each other. The alternative method comprises: (a) capturing one or more images of the sample when the sample is arranged at one or more defocused distances from a corresponding beam focus of the charged particle beam to provide one or more captured images; (b) simulating one or more images of the sample captured at the one or more defocused distances based on at least an estimated beam convergence value of the charged particle beam and a focus image of the sample to provide one or more simulated images; (c) determining a magnitude (R) of a difference between the one or more captured images and the one or more simulated images; and (d) varying the estimated beam convergence value in an iterative process to reduce or minimize the magnitude (R) to determine an actual beam convergence value.

[0093] In alternative method (b), the simulation may include: simulating a beam profile at each defocus distance based at least on the estimated beam convergence value (and optionally on other beam parameters); and calculating a corresponding simulated image in Fourier space based on a product of the beam profile in Fourier space and a focus image in Fourier space or based on a convolution in real space. The convolution in real space is considered to be equivalent to the product in Fourier space.

[0094] Alternative approaches may include Figure 2 and Figure 3 Some or all of the additional features described can be referred to the above description and will not be repeated here.

[0095] Specifically, the following embodiments are described herein:

[0096] Embodiment 1: A method for determining a beam convergence of a charged particle beam (11) focused by a focusing lens (120) toward a sample (10) in a charged particle beam system (100), comprising: (a) when the sample is arranged at one or more defocus distances (z) from the corresponding beam focus of the charged particle beam; 1…N ) at which one or more images of the sample are taken (h 1…N ), and from one or more images (h 1…N ) to retrieve one or more retrieved bundle profiles (g 1…N ); (b) based at least on an estimated beam convergence value of the charged particle beam ( 初始 C) to simulate the defocusing at one or more defocus distances (z 1…N ) to provide one or more simulated beam profiles (g' 1…N (c) determining one or more simulated beam profiles (g' 1…N ) with one or more retrieved bundle profiles (g 1…N ) between the magnitude (R); and (d) changing the estimated beam convergence value in an iterative process to reduce or minimize the magnitude (R) to determine the actual beam convergence value ( 实际 C).

[0097] Embodiment 2: The method as described in embodiment 1 further includes: determining one or more retrieved beam profile parameters from one or more retrieved beam profiles; and determining one or more simulated beam profile parameters from one or more simulated beam profiles, wherein determining the value (R) includes determining the value of the difference between the one or more retrieved beam profile parameters and the one or more simulated beam profile parameters.

[0098] Embodiment 3: The method of Embodiment 1 or 2, wherein the one or more retrieved beam profile parameters and the one or more simulated beam profile parameters are selected from the group consisting of: beam profile size, beam profile diameter, and beam profile ellipticity.

[0099] Embodiment 4: A method as described in any of embodiments 1 to 3, wherein one or more defocus distances at which one or more images are taken or the difference between one or more defocus distances is known in absolute terms, and the absolute values ​​are used to simulate one or more simulated beam profiles in (b).

[0100] Embodiment 5: The method of any one of Embodiments 1 to 4, wherein the estimated beam convergence value comprises at least one of a numerical aperture of the charged particle beam, a beam width variation as a function of defocus distance, and a beam convergence angle of the charged particle beam.

[0101] Embodiment 6: A method as described in any of embodiments 1 to 5, wherein in (b), one or more simulated beam profiles are simulated using one or more parameters of a charged particle beam, and the one or more parameters of the charged particle beam include one or more of a beam aberration coefficient, a beam energy, and a beam brightness.

[0102] Embodiment 7: A method as described in Embodiment 6, wherein the beam aberration coefficient is determined in a previous aberration determination process, or wherein the beam aberration coefficient utilized in (b) is the coefficient that is modified in (d) to determine an estimate of the actual beam aberration coefficient.

[0103] Embodiment 8: The method of any one of Embodiments 1 to 7, wherein in (b), the one or more beam profiles are simulated based on wave optics simulations.

[0104] Embodiment 9: The method of any one of Embodiments 1 to 8, wherein at least one of the one or more images is taken when the sample is arranged at a defocused position within 1.5 times the depth of focus of the charged particle beam.

[0105] Embodiment 10: A method as described in any one of embodiments 1 to 9, wherein in (a), when the sample is arranged at multiple different defocus distances, multiple images are taken and multiple beam profiles are retrieved from the multiple images; and in (b), multiple beam profiles at multiple different defocus distances are simulated.

[0106] Embodiment 11: The method of any one of Embodiments 1 to 10, wherein in (a), the focus position of the charged particle beam is changed to vary between a plurality of different defocus distances.

[0107] Embodiment 12: The method of embodiment 11, wherein the focus position of the charged particle beam is changed by changing at least one of a magnetic lens setting, an electron lens setting, or a beam energy.

[0108] Embodiment 13: The method of any one of Embodiments 1 to 12, wherein in (a), the sample stage is moved relative to the focal point of the charged particle beam to change between a plurality of different defocus distances.

[0109] Embodiment 14: The method of any one of Embodiments 1 to 13, further comprising modifying at least one beam-affecting element to adjust or correct the charged particle beam based on the actual beam convergence value.

[0110] Embodiment 15: The method of any one of embodiments 1 to 14, further comprising: capturing or providing a high-resolution image or a focus image of the sample at a second resolution, the second resolution being higher than the first resolution of the one or more images captured in (a).

[0111] Embodiment 16: The method of embodiment 15, wherein in (a), one or more retrieved beam profiles of the charged particle beam are retrieved by deconvolving the one or more images with a high-resolution image or a focus image of the sample.

[0112] Embodiment 17: A method as described in embodiment 15, wherein in (a), retrieving one or more retrieved beam profiles from one or more images includes dividing one or more images in Fourier space by a high-resolution image of the sample, or wherein the one or more retrieved beam profiles are retrieved from one or more images based on deconvolution.

[0113] Embodiment 18: The method of Embodiment 16 or 17, wherein retrieving one or more retrieved beam profiles from the one or more images further comprises application of an adaptive filter.

[0114] Embodiment 19: A method as described in any of embodiments 1 to 18, wherein (b), (c) and (d) are repeated until a minimized value (R) of the difference between one or more simulated beam profiles and one or more retrieved beam profiles is determined, and the estimated beam convergence value at the corresponding iteration is determined as the actual beam convergence value of the charged particle beam.

[0115] Embodiment 20: The method of any one of Embodiments 1 to 19, wherein the initial estimated beam convergence value used in (b) is calculated based on one or more retrieved beam profiles as a function of defocus distance.

[0116] Embodiment 21: A method as described in any of embodiments 1 to 20, wherein the charged particle beam is an electron beam focused onto the sample with one or more of the following parameters: numerical aperture in the range of less than 30 mrad, probe current in the range of 10 pA to 10 nA, landing energy in the range of 100 eV to 100 keV.

[0117] Embodiment 22: A method for determining the beam convergence of a charged particle beam focused by a focusing lens toward a sample in a charged particle beam system, comprising: (a) capturing one or more images of the sample to provide one or more captured images when the sample is arranged at one or more defocus distances from the corresponding beam focus of the charged particle beam; (b) simulating one or more images of the sample captured at the one or more defocus distances based at least on an estimated beam convergence value of the charged particle beam and a focus image of the sample to provide one or more simulated images; (c) determining a magnitude (R) of a difference between the one or more captured images and the one or more simulated images; and (d) changing the estimated beam convergence value in an iterative process to reduce or minimize the magnitude (R) to determine an actual beam convergence value.

[0118] Embodiment 23: The method of embodiment 22 in combination with the additional features of any one of embodiments 4 to 15 and / or 19 to 21.

[0119] Embodiment 24: A charged particle beam system comprises: a charged particle source for emitting a charged particle beam propagating along an optical axis; a sample stage; a focusing lens for focusing the charged particle beam toward 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 that, when executed by the processor, cause the charged particle beam system to perform a method as described in any one of embodiments 1 to 23.

[0120] While the foregoing is directed to embodiments, other and further embodiments may be devised without departing from the basic scope thereof, and the scope of the same is determined by the claims that follow.

Claims

1. A method for determining the beam convergence of a charged particle beam focused toward a sample by a focusing lens in a charged particle beam system, comprising: (a) taking one or more images of the sample when the sample is arranged at one or more defocus distances from a corresponding beam focus of the charged particle beam, and retrieving one or more retrieved beam profiles from the one or more images; (b) simulating one or more beam profiles at the one or more defocus distances based at least on the estimated beam convergence value of the charged particle beam to provide one or more simulated beam profiles; (c) determining a magnitude (R) of a difference between the one or more simulated beam profiles and the one or more retrieved beam profiles; and (d) varying the estimated beam convergence value to reduce or minimize the magnitude (R) in an iterative process to determine an actual beam convergence value.

2. The method of claim 1, further comprising: determining one or more retrieved beam profile parameters from the one or more retrieved beam profiles; as well as determining one or more simulated beam profile parameters from the one or more simulated beam profiles, Wherein determining said magnitude (R) comprises determining said magnitude of said difference between said one or more retrieved beam profile parameters and said one or more simulated beam profile parameters.

3. The method of claim 2, wherein the one or more retrieved beam profile parameters and the one or more simulated beam profile parameters are selected from the group consisting of: beam profile size, beam profile width, beam profile diameter, and beam profile ellipticity.

4. A method as described in any one of claims 1 to 3, wherein the one or more defocus distances at which the one or more images are taken or the differences between the one or more defocus distances are known in absolute terms, and the absolute values ​​are used to simulate the one or more simulated beam profiles in (b).

5. The method of any one of claims 1 to 3, wherein the estimated beam convergence value comprises at least one of a numerical aperture of the charged particle beam, a beam width variation as a function of defocus distance, and a beam convergence angle of the charged particle beam.

6. A method as described in any one of claims 1 to 3, wherein in (b), one or more parameters of the charged particle beam are used to simulate the one or more simulated beam profiles, and the one or more parameters of the charged particle beam include one or more of beam aberration coefficient, beam energy and beam brightness.

7. The method of claim 6, wherein the beam aberration coefficients are determined in a previous aberration determination process, or wherein the beam aberration coefficients utilized in (b) are coefficients that are modified in (d) to determine an estimate of an actual beam aberration coefficient.

8. The method of any one of claims 1 to 3, wherein in (b), the one or more beam profiles are simulated based on wave optics simulations.

9. The method of any one of claims 1 to 3, wherein at least one of the one or more images is taken when the sample is arranged at a defocused position within 1.5 times the focal depth of the charged particle beam.

10. The method according to any one of claims 1 to 3, wherein In (a), taking a plurality of images when the sample is arranged at a plurality of different defocus distances, and retrieving a plurality of beam profiles from the plurality of images; and In (b), a plurality of beam profiles at the plurality of different defocus distances are simulated.

11. The method of claim 10, wherein in (a), a focus position of the charged particle beam is changed to change between the plurality of different defocus distances.

12. The method of claim 10, wherein in (a), a sample stage is moved relative to a focal point of the charged particle beam to change between the plurality of different defocus distances.

13. The method of any one of claims 1 to 3, further comprising modifying at least one beam-affecting element to condition the charged-particle beam based on the actual beam convergence value.

14. The method of any one of claims 1 to 3, further comprising: capturing or providing a high resolution image or a focus image of the sample at a second resolution that is higher than the first resolution of the one or more images captured in (a), Wherein in (a), the one or more retrieved beam profiles of the charged particle beam are retrieved by deconvolving the one or more images with the high resolution image or focus image of the sample.

15. The method of claim 14, wherein retrieving the one or more retrieved beam profiles from the one or more images further comprises application of an adaptive filter.

16. A method as described in any one of claims 1 to 3, wherein (b), (c) and (d) are repeated until a minimized value (R) of the difference between the one or more simulated beam profiles and the one or more retrieved beam profiles is determined, and the estimated beam convergence value at the corresponding iteration is determined as the actual beam convergence value of the charged particle beam.

17. The method of any one of claims 1 to 3, wherein an initial estimated beam convergence value used in (b) is calculated based on a variation of the one or more retrieved beam profiles with defocus distance.

18. A method as described in any one of claims 1 to 3, wherein the charged particle beam is an electron beam focused onto the sample with one or more of the following parameters: numerical aperture in the range below 30 mrad, probe current in the range of 10 pA to 10 nA, landing energy in the range of 100 eV to 100 keV.

19. A method of determining the beam convergence of a charged particle beam focused by a focusing lens toward a sample in a charged particle beam system, comprising: (a) capturing one or more images of the sample to provide one or more captured images when the sample is arranged at one or more defocus distances from a corresponding beam focus of the charged particle beam; (b) simulating one or more images of the sample taken at the one or more defocus distances based at least on the estimated beam convergence value of the charged particle beam and the focus image of the sample to provide one or more simulated images; (c) determining a magnitude (R) of a difference between the one or more captured images and the one or more simulated images; and (d) varying the estimated beam convergence value to reduce or minimize the magnitude (R) in an iterative process to determine an actual beam convergence value.

20. A charged particle beam system comprising: a charged particle source for emitting a charged particle beam propagating along an optical axis; Sample stage; a focusing lens for focusing the charged particle beam toward 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 storing instructions that, when executed by the processor, cause the charged particle beam system to: (x1) simulating one or more beam profiles at one or more defocus distances based at least on the estimated beam convergence value of the charged particle beam to provide one or more simulated beam profiles, (x2) comparing one or more retrieved beam profiles retrieved from one or more images of the sample taken at the one or more defocus distances with the one or more simulated beam profiles to determine a magnitude of a difference therebetween, and (x3) changing the estimated beam convergence value in an iterative process to reduce or minimize the magnitude to determine an actual beam convergence value.

Images