Method for determining the beam convergence of a focused charged particle beam and charged particle beam system

Abstract

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) is provided. The method comprises: (a) acquiring 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; (b) retrieving one or more beam cross sections from the one or more images; (c) determining one or more beam widths from the one or more beam cross sections; and (d) calculating at least one beam convergence value based on the one or more beam widths and the one or more defocus distances. In addition, a charged particle beam system for imaging and / or inspecting a sample is provided, the charged particle beam system being configured for use in any of the methods described herein.

Description

Technical Field

[0001] Embodiments described herein relate to methods for determining the beam convergence of a charged particle beam focused by a focusing lens toward a sample in a charged particle beam system, particularly in a scanning electron microscope (SEM). Specifically, the numerical aperture (NA) provided by the focusing lens in the charged particle beam system can be determined from one or more acquired images of the sample according to the methods described herein. Embodiments further relate to a charged particle beam system configured for use in any of the methods described herein for inspecting and / or imaging a sample. Background Art

[0002] Modern semiconductor technology has created a high demand for constructing and probing samples at nanometer or even sub-nanometer scales. Micrometer and nanometer scale process control, inspection or construction is usually performed by charged particle beams (e.g., electron beams), which 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 excellent spatial resolution compared to, for example, photon beams.

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

[0004] However, it is challenging to reliably inspect and / or image samples with good resolution using charged particle beam systems. Specifically, charged particle beams often suffer from beam aberrations that limit resolution, and the actual cross-sectional shape of the focused charged particle beam may differ from the designed shape. The resolution limit of a 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 a charged particle beam is a value that is usually used by technicians to describe the beam convergence of a charged particle beam focused on a sample surface by an objective lens. A large numerical aperture provides an improved resolution limit, which can be achieved in theory if the beam aberration is low. Systems can be designed for a specific numerical aperture that provides superior operation, but the actual numerical aperture may deviate from an expected value. Therefore, knowing the beam convergence of a charged particle beam focused by a focusing lens, and more specifically, knowing the actual numerical aperture of a charged particle beam will be useful for determining the source of beam aberration and improving the resolution of the system.

[0006] In view of the above, it would be beneficial to accurately and reliably determine the beam convergence of a charged particle beam focused by a focusing lens in a charged particle beam system, and in particular, it would be beneficial to accurately determine the numerical aperture of the charged particle beam focused by an objective lens. In addition, it would be beneficial to provide a charged particle beam system for inspecting 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 a beam convergence of a charged particle beam and a charged particle beam system configured for said method 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 toward a sample in a charged particle beam system by a focusing lens is provided. The method comprises: (a) acquiring 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; (b) retrieving one or more beam cross sections from the one or more images; (c) determining one or more beam widths from the one or more beam cross sections; and (d) calculating at least one beam convergence value based on the one or more beam widths and the one or more defocus distances.

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

[0010] In some embodiments, a focused image of the sample is also acquired (when the sample is arranged at a focusing distance from the focusing lens), and the focused image of the sample can be used to (b) retrieve one or more beam cross sections from one or more images acquired out of focus in (a). The focused image of the sample may also be known in other ways and may then be used in (b) to retrieve one or more beam cross sections. Alternatively, one or more beam cross sections may be retrieved in (b) without using a focused image of the sample.

[0011] According to another aspect, a charged particle beam system is provided, the charged particle beam system being used for imaging and / or inspecting a sample by a charged particle beam, in particular by an electron beam. 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, which, when executed by the processor, cause the system to perform any one of the methods described herein.

[0012] In particular, the stored instructions, when executed, may cause the system to: (x1) retrieve one or more beam cross sections from one or more images of a sample acquired at one or more defocus distances; (x2) determine one or more beam widths from the one or more beam cross sections; and (x3) calculate at least one beam convergence value based on the one or more beam widths and one or more defocus distances.

[0013] Embodiments are also directed to an apparatus for performing the disclosed method, and the apparatus includes apparatus components for performing the various method actions. The method may be performed by means of hardware components, by means of a computer programmed by appropriate software, by any combination of the two, or in any other manner. In addition, embodiments are also directed to methods of operating the apparatus.

[0014] Further advantages, features, aspects and details which can be combined with the embodiments described herein will be apparent from the dependent claims, the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order that the manner in which the above-described features of the present disclosure may 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.

[0016] Figure 1 A schematic diagram showing 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;

[0017] Figure 2 A flow chart showing a method of determining a beam convergence of a charged particle beam according to an embodiment described herein;

[0018] Figure 3 The diagram shows Figure 2 A more detailed diagram of the method shown in ;

[0019] Figure 4 is a graph showing beam width as a function of defocus distance determined using any of the methods described herein. DETAILED DESCRIPTION

[0020] 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 in the drawings, the same reference numerals indicate the same parts. Generally, only the differences with respect to the various embodiments are described. Each example is provided by way of explanation and is not meant as a limitation. In addition, features illustrated or described as part of one embodiment may be used on other embodiments or in combination with other embodiments to produce yet further embodiments. It is intended that the description includes such modifications and variations.

[0021] Figure 1 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 includes 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 includes a sample stage 108, and a focusing lens 120, in particular an objective lens, the focusing lens 120 being used to focus the charged particle beam 11 on the sample 10 placed on the sample stage 108. The charged particle beam system 100 further includes a charged particle detector 118, in particular an electron detector, the charged particle detector 118 being used to detect signal particles (in particular, secondary electrons and / or backscattered electrons) emitted from the sample 10.

[0022] An image generation unit 160 may be provided for generating 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 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.

[0023] Optionally, at least one beam convergence value may be used to determine beam aberrations of the charged particle beam. For example, a set of beam aberration coefficients may be determined in an iterative process using the beam convergence value as an input parameter. The beam aberration coefficients may be forwarded to a controller 180 of the aberration corrector 109 so that the aberration corrector 109 may be appropriately adjusted to compensate for one or more beam aberrations present in the system. An aberration-corrected charged particle beam may be provided.

[0024] The sample stage 108 may be a movable stage. In particular, the sample stage 108 may be movable 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 may be varied (see Figure 1 By moving the sample stage 108 in the Z direction, the sample 10 can be moved to different defocus distances, i.e., away from the focal plane p of the focusing lens 120. I, so that an out-of-focus image of the sample 10 can be acquired by corresponding stage movement (eg, in predetermined increments).

[0025] 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 designated surface area of ​​the sample 10 can be moved to an area below the focusing lens 120, so that the designated surface area can be imaged by focusing the charged particle beam 11 on the designated surface area.

[0026] 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 is heated at a subatmospheric pressure (e.g., less than 10 -3 mbar pressure or less than 10 -5 mbar pressure) impacts the sample 10.

[0027] 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 for scanning the charged particle beam 11 over the surface of the sample 10 along a predetermined scanning pattern (eg, in the X direction and / or in the Y direction).

[0028] In some embodiments, a bunching lens system 106 may 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 on the sample 10, in particular a magnetic objective lens, an electrostatic magnetic lens or a combined magnetic-electrostatic lens. The objective lens may optionally include a deceleration field device, for example, one or more deceleration electrodes, configured to decelerate the charged particle beam to a predetermined landing energy on the sample.

[0029] One or more surface areas of the sample 10 may be inspected and / or imaged by the charged particle beam system 100. As used herein, the term "sample" may relate 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 may 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 an XY plane; (3) performing critical dimension measurements and / or metrology; (4) detecting defects; and / or (5) investigating the quality of the sample.

[0030] In order to examine the sample 10 by means of the charged particle beam 11, the charged particle beam 11 is usually focused on the sample surface by means of a focusing lens 120. When the charged particle beam 11 hits the sample surface, secondary electrons and / or backscattered electrons are emitted from the sample. The signal electrons provide information about the spatial properties and characteristic dimensions of the sample, and the signal electrons can be detected by means of a charged particle detector 118. By scanning the charged particle beam 11 over the sample surface, for example with a scanning deflector 107, and detecting the signal electrons as a function of the position at which the signal electrons are generated, the sample surface or a portion thereof can be imaged, for example, by means of an image generating unit 160, which can be configured to provide an image of the sample 10 based on the received signal electrons.

[0031] A small beam spot of the focused charged particle beam 11 on the sample surface increases the achievable image resolution. Therefore, the sample surface should be arranged in the focal plane p of the focusing lens 120 during inspection. I In order to obtain a clear focused image of the sample 10. The clear image of the sample 10 obtained by focusing is also referred to as a "focused image h I ”, the subscript I indicates “focused”. Similarly, the charged particle beam 11 is in the focal plane p I The beam cross section in the present invention is referred to as the "focused beam cross section g I ”, with the subscript I indicating “in-focus”.

[0032] It should be noted that an image can be represented mathematically in real space (= in the image domain, i.e. as a function of spatial coordinates) or in Fourier space (= in the frequency domain, i.e. as a function of spatial frequencies). An image in Fourier space can be calculated from an image in real space via a Fourier transform (FT). Both of the above representations contain the corresponding information of the image. As used herein, images in real space are represented by lowercase letters "h n ", and the image in Fourier space is represented by the capital letter "H n " indicates that the subscript "n" indicates the image acquired for the nth time. For example, when "h I ” represents the focused image of the sample in real space, “H I ” represents the focused image of the sample in Fourier space, H I h I Similarly, beam cross sections in real space are referred to herein by the lowercase letter “g n ” and the beam cross section in Fourier space is represented herein by the capital letter “G n " indicates that the subscript "n" indicates the beam cross section of the nth acquired image. For example, when "g I ” represents the focused beam cross section of a charged particle beam in real space, “G I” represents the focused beam cross section of the charged particle beam in Fourier space, G I g I Via the Fast Fourier Transform (FET) algorithm in some of the embodiments described herein, images and beam cross sections in real space can be Fourier transformed into Fourier space, and vice versa.

[0033] In charged particle beam systems, the actual value of the beam convergence of the charged particle beam focused onto the sample by the focusing lens 120 is usually unknown. The beam convergence can be expressed in terms of the beam convergence angle (a) relative to the optical axis or in terms of the numerical aperture (NA) of the charged particle beam, the values ​​of which are difficult to determine because they are "longitudinal properties" of the beam that are hidden and cannot be directly obtained from the image. The numerical aperture (NA) determines the resolution limit, so it is very valuable to know the actual value of the numerical aperture.

[0034] For example, the numerical aperture of the system is usually used for the beam-affecting element (such as an objective lens) of the adaptation and design system, and the numerical aperture actually existing in the charged particle beam system deviates from the design numerical aperture assumed for the system design, which may cause inaccuracy and beam aberration. The beam-affecting element may include one or more elements in the group consisting of the following: lens, beam extractor, beam deflector, collimator, aberration corrector, scanning deflector, beam splitter and charged particle detector. Therefore, determining the actual numerical aperture can help identify the source of inaccuracy in the system and optimize system design and resolution. Moreover, the numerical aperture as a value representing the degree of beam convergence can be used as an input parameter in the aberration coefficient determination routine.

[0035] According to the method described herein, 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 storing instructions (in Figure 1 170), which, when executed by a processor, causes the system to perform any of the methods described herein.

[0036] Figure 2 and Figure 3 1 is a flow chart schematically illustrating the method of determining at least one beam convergence value. Figure 2 Compared to the more general graph of Figure 3 The diagrams show optional further details.

[0037] exist Figure 2 and Figure 3 In block 210 of the present invention, when the sample is arranged at one or more defocus distances (z 1…N), obtain one or more images of the sample (h 1…N ). The defocus distance is understood in this context as the distance between the sample and the beam focus when the image is acquired (≠0). If an image is acquired when the sample is arranged at the defocus distance of the sample relative to the beam focus, the resulting image is an out-of-focus image of the sample. Naturally, an increased defocus distance leads to an increased blurriness of the corresponding acquired image, since with increasing defocus distance the beam cross section generally increases and the resolution generally decreases.

[0038] In some embodiments, multiple images (e.g., two, three, six or more images) are acquired at multiple different defocus distances between the sample and the corresponding beam focus (e.g., at two, three, six or more different defocus distances). In particular, the images may be acquired at an overfocus distance, i.e., a defocus setting where the sample is arranged further away from the focusing lens 120 than the corresponding beam focus of the charged particle beam (see Figure 1 In addition, at least one image of the sample can be acquired at an underfocus distance z3) , 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 At least one image of the sample is acquired at the defocus distances z1 and z2 as shown in FIG. The image acquired at the first defocus distance z1 is denoted as h1 herein, and the image acquired at the nth defocus distance z2 is denoted as h1 herein. n The image obtained at is represented as h n A total of N images can be obtained, which is represented in this paper as (h 1…N ), in particular, N is six or greater, for example, ten or greater, or fifteen or greater.

[0039] In some embodiments described herein, acquiring one or more images (h 1…N ) of one or more defocus distances (z 1…N ) is quantitatively known (i.e., in absolute value), or when multiple images are acquired at multiple different defocus distances, at least the difference between corresponding two defocus distances of the multiple defocus distances is quantitatively known (e.g., in "μm" or another length unit). In order to determine the beam convergence from one or more defocused images, it is beneficial to know quantitatively (e.g., in "μm") at which defocus distance each of the defocused images was acquired. Alternatively, in order to determine the beam convergence from two (or more) defocused images acquired at two (or more) different defocus distances, it is beneficial to know at least the difference between two corresponding different defocus distances quantitatively (e.g., in "μm"). In some embodiments, calibration is performed before acquiring defocused images of the sample so that each defocus distance at which the image is acquired is quantitatively known from the corresponding setting of the charged particle beam system.

[0040] In some embodiments, which may be combined with other embodiments, the defocus distance is changed by moving the sample stage 108 relative to the focusing lens 120 in the Z direction (ie, along the optical axis A). Figure 1 Schematically depicting stage movement for changing between a plurality of different defocus distances. For example, the sample stage may be moved in predetermined increments, e.g., in a plurality of equal or similar increments between 0.2 μm and 2 μm, and an image of the sample may be acquired at each of the plurality of defocus distances. While moving the sample stage and acquiring images, a constant focus strength of the focus lens 120 may be maintained.

[0041] In other embodiments, the defocus distance is changed by changing the focus strength of the focusing lens 120. Increasing the focus strength of the focusing lens shifts the corresponding beam focus and focal plane toward the focusing lens relative to the sample, and decreasing the focus strength shifts the corresponding beam focus and focal plane away from the focusing lens relative to the sample, so that by making the focal plane p I The defocus distance is changed relative to the offset of the sample 10. The sample can remain stationary. In particular, a plurality of different focusing intensities can be applied to the charged particle beam by the focusing lens 120 for changing between a plurality of different defocus distances, and an image can be acquired at each of the plurality of different focusing intensities.

[0042] It should be noted that changes in focus strength also change the degree of beam convergence. Therefore, in the embodiments described herein, the focus strength, if changed, is only changed so that the resulting change in defocus distance is negligible relative to the overall focal length (f) of the focusing lens 120. For example, the focus strength used to acquire multiple images (see Figure 4 ) to vary the defocus distance in a total range of a few microns (e.g., <10 μm), while the overall focal length (f) of the focusing lens 120 may be in the range of a few millimeters or even a few centimeters. Thus, according to the embodiments described herein, by varying the focusing strength of the focusing lens, the beam convergence may only be varied in a negligible manner, and the already determined numerical aperture (NA) or other beam convergence values ​​are substantially unaffected.

[0043] In some embodiments, the change in defocus distance as a function of the change in focus intensity of the focusing lens 120 is known in advance or determined in a previous calibration, so 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 units of "μm").

[0044] Reference Figure 2 and Figure 3 In block 220, from one or more images (h 1…N) to detect the charged particle beam at one or more defocus distances (z 1…N ) at one or more beam cross sections (g 1…N ). In particular, multiple beam cross sections are retrieved from multiple images, i.e., one beam cross section is retrieved for each of the multiple images. Each retrieved beam cross section corresponds to the beam cross section of the charged particle beam at the defocus distance at which the corresponding image was acquired. The out-of-focus beam profile can be extracted from the out-of-focus image via several different beam profile extraction methods. An exemplary beam profile extraction method is exemplarily explained below.

[0045] In addition to one or more images acquired in an out-of-focus manner (h 1…N ), a focused image of the sample (h I ), and the focused image (h I ) can be used to extract the image from one or more images (h 1…N ) retrieve one or more beam cross sections (g 1…N ). The focused image h of the sample can also be known in other ways. I , for example, because a sample with a known geometry is used to determine the beam convergence according to the methods described herein.

[0046] More specifically, as in Figure 3 As further described in block 220 of FIG. 1 , from one or more images (h 1…N ) retrieve one or more beam cross sections (g 1…N ) may include one or more acquired images (h 1…N ) to provide one or more acquired images (H) in Fourier space 1…N ), and one or more acquired images (H 1…N ) divided by the focused 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 acquired defocused image (H n ) divided by the focused 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 cross section without sample information. Each retrieved beam cross section (G) in Fourier space n ) can be inverse Fourier transformed to obtain the corresponding retrieved beam cross section (g n ).

[0047] As in Figure 3 As further illustrated in block 220 of FIG. 1 , from one or more acquired images (h 1…N ) retrieve one or more beam cross sections (g 1…N) may optionally include an adaptive filter term Multiplication and with the focused beam cross section in Fourier space (G I ) are multiplied. The adaptive filter term The adaptive filter 260 may be provided, which may receive the acquired image as input information. The adaptive filter term may be provided independently for each of the acquired images by the adaptive filter 260. In the absence of the adaptive filter term In the case of I A value close to zero of may result in excessive noise weighting in the focused image. The adaptive filter term The focused beam cross section (G 1…N ) to reduce or avoid focusing the image H I Such unwanted noise effects in the image can be removed by adaptive filtering 260 independently for the image (h 1…N ) determines a corresponding filter term, for example, to ensure that an appropriate filter term is used for each image and the corresponding noise in the image.

[0048] The real space (g I ) or Fourier space (G I ). For example, a Gaussian beam cross section on the focal plane of the focusing lens 120 may be assumed for the focusing beam cross section g I Specifically, assuming, for example, a Gaussian beam profile, the focused beam cross section g of the charged particle beam can be determined from resolution measurements I .

[0049] In block 230, from one or more beam cross sections (g 1…N ) determines the charged particle beam at one or more defocus distances (z 1…N ) at one or more beam widths (c 1…N ). In particular, a plurality of beam widths are determined from the plurality of beam cross sections, there being one retrieved beam width for each of the plurality of beam cross sections (eg in one or more directions, ie for one or more azimuth angles).

[0050] In some embodiments, a beam width may be retrieved from each beam cross section in block 230. For example, one or more beam cross sections (g 1…N) may be rotationally symmetric in nature, e.g. circular or Gaussian distribution, so that in any case it is sufficient to retrieve only one beam width from each beam cross section, since the beam width is essentially the same at each azimuth angle. As used herein, "azimuth angle" refers to different directions within the cross-sectional plane of the beam cross section, i.e. different directions perpendicular to the optical axis (A) where the width of the beam cross section can be measured. Alternatively, even if the beam cross section is not rotationally symmetric, one beam width of the beam cross section may still be determined as the average beam width or as the FW50 value, which is the diameter of the circle around the optical axis A through which half of the beam propagates.

[0051] In some embodiments, which may be combined with other embodiments, one or more beam widths (c 1…N In particular, one or more beam widths (c 1…N ) is determined as a function of the azimuth angle ((c 1…N )(θ)). Figure 3 The example shows the image h which is not completely rotationally symmetrical. n The beam cross section g n The corresponding beam width c can be determined in two or more directions n , for example, at an azimuth angle θ x up (= in the X direction) and at an azimuth angle θ y In particular, the beam cross section g n Detection beam width c n As a function of azimuth (c n (θ)).

[0052] In some embodiments that may be combined with other embodiments described herein, at least one quantity from the following group is determined from each beam cross section and is considered as the corresponding beam width: (1) FWHM (=full width at half maximum) at one or more azimuth angles, or average FWHM; (2) FW50 (=full width 50) at one or more azimuth angles, or average FW50, also called D50 width; (3) 1 / e 2 width; and (4) another commonly used quantity describing the width of the beam. The outer beam diameter or beam diameter may be considered the beam width, respectively. It should be noted that, depending on the quantity retrieved as the beam width in block 230, the resulting beam convergence value may need to be multiplied by a corresponding calibration factor (K) for determining the correct value of the numerical aperture therefrom.

[0053] In block 240, based on one or more beam widths (c 1…N ) and one or more defocus distances (z 1…N) to calculate at least one beam convergence value. In some embodiments, at least one beam convergence value may be or may include a function of the defocus distance. The beam width variation as a function of the defocus distance and / or the beam convergence angle (a) calculated from the beam width variation, for example, in units of "degrees" or "radians". For example, an average beam width variation as a function of the defocus distance may be determined, and the beam convergence angle (a) may be calculated from the average beam width variation. If at least one image is acquired at an overfocus distance and at least one image is acquired at an underfocus distance, the average beam width variation as a function of the defocus distance may be determined separately in front of the focal plane and behind the focal plane. The average beam convergence angle (a) may be calculated from the average beam width variation and the average beam convergence angle (a) may be considered as at least one beam convergence value.

[0054] Figure 4 A graph of beam width (c) as a function of defocus distance (f) is shown in an exemplary defocus range from -4 μm to +4 μm relative to the beam focal point. Here, ten images are acquired at different (quantitatively known) underfocus distances, and ten images are acquired at different (quantitatively known) overfocus distances, and the corresponding beam widths are retrieved from these images. The beam width c determined in block 230 is 1…N Shown as the corresponding defocus distance z 1…N In the graph, the beam width changes as a function of the defocus distance corresponds to the slope of the line (or its linear fit) connecting the beam widths. Typically, the absolute values ​​of the slopes upstream and downstream of the beam focus (z=0) substantially correspond to each other, since the charged particles of the beam propagate along substantially rectilinear paths. However, it is possible to calculate the slopes for over-focus distances and under-focus distances separately, as in Figure 4 Schematically depicted in .

[0055] In some embodiments, an average slope, in particular a slope of a linear fit for at least some of the beam width values ​​plotted as a function of the defocus distance, is determined, and the beam convergence angle (a) can be calculated from these slopes. In some of the embodiments described herein, a (average or fitted) slope value and / or a beam convergence angle (a) calculated from the slope value can be determined as at least one beam convergence value.

[0056] In some embodiments, by making the slope value The numerical aperture (NA) of the charged particle beam is calculated from the above (average or fitted) slope value by multiplying with a calibration factor (K). The calibration factor (K) may be taken into account in the quantity used as the beam width in block 230. In some embodiments, the numerical aperture is calculated as follows: NA = K·|slope(FW50)|, if "FW50" is used as the quantity representing the beam width and "slope" is The (optionally, averaged or linearly fitted) absolute value of , wherein the calibration factor (K) is a value between 0.5 and 1, in particular a value between 0.70 and 0.85.

[0057] The resulting numerical aperture NA of the charged particle beam (particularly after multiplication with the calibration factor) can then be used directly as an input parameter for various simulation and design programs for charged particle beam systems which expect the numerical aperture provided by the objective as input parameter.

[0058] In some embodiments, which may be combined with other embodiments described herein, the method may further comprise modifying at least one beam influencing element for shaping the charged particle beam based on at least one beam convergence value, in particular based on a determined numerical aperture of the charged particle beam.

[0059] In some embodiments, which may be combined with other embodiments described herein, the method may further comprise determining one or more beam aberration coefficients of the charged particle beam by an iterative fitting routine using at least one beam convergence value as an input parameter. Figure 2 Such an iterative fitting routine for determining one or more beam aberration coefficients, which uses the actual numerical aperture of the charged particle beam as an input parameter, is depicted in FIG by block 250. If the actual numerical aperture NA of the charged particle beam is known, the fitting routine may be able to converge more quickly towards the actual beam aberration coefficients.

[0060] In some embodiments, which may be combined with other embodiments described herein, one or more beam widths (c 1…N ), in particular as a function of the azimuth angle ((c 1…N )(θ)), and a corresponding beam convergence value can be calculated at each of the azimuth angles.

[0061] In particular, at least one beam convergence value may include a first beam convergence value of the charged particle beam at a first azimuth angle (θx), and a second beam convergence value of the charged particle beam at a second azimuth angle (θy). In some embodiments, for example, if the beam profile is highly asymmetric, elliptical, or otherwise deformed, at least one beam convergence value may be calculated as a function of the azimuth angle.

[0062] In some embodiments, the numerical aperture of a charged particle beam may be calculated at two or more azimuth angles, particularly as a function of azimuth angle (NA(θ)). For example, the beam convergence angle (a) or the slope (θ) may be first calculated. Calculated as a function of azimuth and can be obtained from the beam convergence angle (a) or the slope The calculation of the numerical aperture as a function of the azimuth angle is based in particular on multiplication with a calibration factor (K).

[0063] A three-dimensional model of a focused charged particle beam can be generated and / or displayed, particularly when the beam convergence value is determined as a function of the azimuth angle. Alternatively or additionally, one or more two-dimensional representations of a charged particle beam in one or more selected azimuth planes can be generated and / or displayed. Alternatively or additionally, one or more one-dimensional representations of the beam width as a function of the defocus distance of the charged particle beam can be generated and / or displayed. Alternatively or additionally, the numerical aperture can be determined and / or displayed, for example, with a one-dimensional representation, as a function of the azimuth angle.

[0064] In some embodiments described herein, a single acquired defocused image h n It may be sufficient to determine at least one beam convergence value therefrom, in particular when the defocus distance z at which the single acquired defocused image was acquired is quantitatively known. n When (and, in addition, the focused image h of the sample I has been acquired or is otherwise known, so that it is possible to obtain from the single acquired defocused image h n Pick up beam cross section g n and beam width c n Specifically, by calculating c n / z n At least one beam convergence value is determined, and the NA can be calculated from the at least one beam convergence value. 1…N Get multiple defocused images h 1…N and as explained herein from the plurality of defocused images h 1…N Determining the quantity dc / dz (eg as an average slope value or a linear fit of the beam width as a function of defocus distance), in particular both when underfocus and when overfocus, allows more accurate values ​​of beam convergence and more accurate values ​​of NA to be determined.

[0065] It should be noted that the known process for determining the beam aberration coefficient is based on the analysis of the probe shape extracted from the defocused image acquired out of focus. However, only a relative estimate of the beam aberration coefficient is made from the extracted probe shape, without quantitatively knowing the defocus distance at which the defocused image was actually acquired. Therefore, the known process cannot be used to extract quantitative information about the convergence of the focused charged particle beam. In contrast, the method described herein is based on the recognition that if the defocused image is acquired at a quantitatively known defocus distance or at a difference between quantitatively known defocus distances, then the defocused image can be used to extract information about the beam convergence angle, and therefore can be used to determine the actual numerical aperture value of the focused charged particle beam.

[0066] The numerical aperture of the charged particle beam focused by the focusing lens is a valuable quantity that has been determined so far based on resolution measurement, making assumptions about the actual beam shape, but this can lead to inaccuracies in the determination. According to the method described herein, the NA can be retrieved from the actual out-of-focus beam cross section retrieved from the defocused image, so that no assumptions about the beam shape are needed. Specifically, no assumptions about the beam cross section or sample need to be made, which leads to a more accurate value of the numerical aperture. In addition, the numerical aperture can be determined at any azimuth, which was not possible before, and this allows the (intentional or unintentional) difference between the expected beam shape and the actual beam shape to be identified along different directions. In addition, the beam shape can be visualized in 3D and / or 2D, which can help analyze and appropriately improve / trim the charged particle beam.

[0067] The ability to accurately determine the numerical aperture in a charged particle beam system would allow, among other things, (1) reproducibility and matching, since the numerical aperture is the determining quantity in determining the achievable resolution; (2) analysis of beam-affecting elements, e.g., for improving future designs; (3) determination of beam aberrations, which scale with various powers of the numerical aperture; and (4) generation of tailored charged particle beams for certain critical dimension and defect review tasks, e.g., for inspecting deep trenches in a sample that extend along only one axis.

[0068] Specifically, the following embodiments are described herein:

[0069] Embodiment 1: A method for determining the beam convergence of a charged particle beam (11) focused toward a sample (10) in a charged particle beam system (100) by a focusing lens (120), the method comprising: (a) when the sample is arranged at one or more defocusing distances (z) from the corresponding beam focus of the charged particle beam; 1…N ) at which one or more images of the sample are acquired (h 1…N ); (b) from one or more images (h 1…N ) retrieve one or more beam cross sections (g 1…N); (c) from one or more beam cross sections (g 1…N ) determines one or more beam widths (c 1…N ); and (d) based on one or more beam widths (c 1…N ) and one or more defocus distances (z 1…N ) calculates at least one beam convergence value.

[0070] Embodiment 2: The method of embodiment 1, wherein one or more images (h 1…N ) of one or more defocus distances (z 1…N ), or one or more defocus distances (z 1…N ) is known in absolute value, and said absolute value is used to calculate at least one beam convergence value in (d).

[0071] Embodiment 3: The method of embodiment 1 or 2, wherein at least one beam convergence value comprises a beam width variation as a function of defocus distance

[0072] Embodiment 4: The method of any one of Embodiments 1 to 3, wherein at least one beam convergence value comprises a numerical aperture (NA) of the charged particle beam.

[0073] Embodiment 5: A method as in any one of embodiments 1 to 4, wherein in (a), multiple images are acquired when the sample is arranged at multiple different defocus distances; in (b), multiple beam cross sections are retrieved from the multiple images; in (c), multiple beam widths are determined from the multiple beam cross sections; and in (d), at least one beam convergence value is calculated based on the multiple beam widths and the multiple defocus distances.

[0074] Embodiment 6: A method as in embodiment 5, wherein based on the variation of beam width as a function of defocus distance The numerical aperture (NA) of the charged particle beam is calculated by multiplying the average absolute value of by a calibration factor (K).

[0075] Embodiment 7: A method as in Embodiment 5 or 6, wherein in (a), a plurality of different focusing intensities are applied by the focusing lens (120) for varying between a plurality of different defocus distances, and an image is acquired at each of the plurality of different focusing intensities.

[0076] Embodiment 8: A method as in embodiment 7, wherein the variation in defocus distance as a function of the variation in the focus strength of the focusing lens (120) is known or determined in a previous calibration.

[0077] Embodiment 9: A method as in Embodiment 5 or 6, wherein in (a), the sample stage (108) is moved along the optical axis (A) relative to the focusing lens (120) for changing between a plurality of different defocus distances, and an image is acquired at each of the plurality of different defocus distances.

[0078] Embodiment 10: The method of any one of Embodiments 5 to 9, wherein at least one of the plurality of images is acquired at an over-focus distance, and at least one of the plurality of images is acquired at an under-focus distance.

[0079] Embodiment 11: A method as in any one of embodiments 1 to 10, wherein in (b), from one or more images (h 1…N ) retrieve one or more beam cross sections (g 1…N ) includes: converting one or more images (H 1…N ) divided by the focused image of the sample in Fourier space (H I ).

[0080] Embodiment 12: A method as in embodiment 11, wherein from one or more images (h 1…N ) retrieve one or more beam cross sections (g 1…N ) further comprises: with the adaptive filter term Multiplication and with the focused beam cross section in Fourier space (G I ) are multiplied together.

[0081] Embodiment 13: The method of any one of embodiments 1 to 12, wherein one or more beam widths (c 1…N ), in particular as a function of the azimuth angle ((c 1…N )(θ)).

[0082] Embodiment 14: A method as in Embodiment 13, wherein at least one beam convergence value comprises a first beam convergence value of the charged particle beam at a first azimuth angle (θx), and a second beam convergence value of the charged particle beam at a second azimuth angle (θy), and in particular, wherein at least one beam convergence value is calculated as a function of the azimuth angle.

[0083] Embodiment 15: The method of Embodiment 14, wherein the numerical aperture (NA(θ)) of the charged particle beam as a function of azimuth angle is calculated from at least one beam convergence value as a function of azimuth angle.

[0084] Embodiment 16: The method of any one of embodiments 1 to 15, further comprising at least one or more of the following: displaying a three-dimensional model of a charged particle beam; displaying one or more two-dimensional representations of the charged particle beam in one or more selected azimuthal planes; displaying one or more one-dimensional representations of the beam width of the charged particle beam as a function of the defocus distance; and displaying the numerical aperture of the charged particle beam as a function of the azimuthal angle.

[0085] Embodiment 17: The method of any one of Embodiments 1 to 16, further comprising modifying at least one beam-affecting element for shaping the charged-particle beam based on at least one beam convergence value.

[0086] Embodiment 18: The method of any one of Embodiments 1 to 17, further comprising determining one or more beam aberration coefficients of the charged particle beam by an iterative fitting routine using at least one beam convergence value as an input parameter.

[0087] Embodiment 19: A charged particle beam system, comprising: a charged particle 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 toward a sample (10) placed on the sample stage (108); a charged particle detector (118) 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 system to perform the method of any one of the above embodiments.

[0088] Embodiment 20: A charged particle beam system (100), comprising: a charged particle 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 toward a sample (10) placed on the sample stage; a charged particle detector (118) 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 system to: (x1) focus the charged particle beam at one or more defocus distances (z 1…N One or more images (h ) of a sample acquired at 1…N ) retrieve one or more beam cross sections (g 1…N ); (x2) from one or more beam cross sections (g 1…N ) determines one or more beam widths (c 1…N ); and (x3) based on one or more beam widths (c 1…N ) and one or more defocus distances (z 1…N ) calculates at least one beam convergence value. The charged particle beam system (100) may further be configured for the method of any one of the above embodiments.

[0089] While the foregoing is directed to embodiments, other and further embodiments may be devised without departing from the basic scope of the embodiments, and the scope of the embodiments is to be determined by the claims that follow.

Claims

1. A method for determining the beam convergence of a charged particle beam focused toward a sample in a charged particle beam system by a focusing lens, the method comprising the following steps: (a) acquiring 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; (b) retrieving one or more beam cross sections from the one or more images; (c) determining one or more beam widths from the one or more beam cross sections; as well as (d) calculating a change in beam width as a function of defocus distance based on the one or more beam widths and the one or more defocus distances for determining a numerical aperture of the charged particle beam.

2. The method of claim 1 , wherein the one or more defocus distances of the one or more images are acquired, or the difference between the one or more defocus distances is known in absolute value, and the absolute value is used for the calculation in (d).

3. The method of claim 1, wherein In (a), a plurality of images are acquired when the sample is arranged at a plurality of different defocus distances; In (b), a plurality of beam cross sections are retrieved from the plurality of images; In (c), determining a plurality of beam widths from the plurality of beam cross sections; as well as In (d), the variation of the beam width as a function of the defocus distance is calculated based on the plurality of beam widths and the plurality of defocus distances.

4. The method of claim 3, wherein the numerical aperture of the charged particle beam is calculated based on an average beam width variation as a function of defocus distance and a calibration factor.

5. The method of claim 3, wherein in (a), a plurality of different focus intensities are applied by the focusing lens for varying between the plurality of different defocus distances, and an image is acquired at each of the plurality of different focus intensities.

6. A method as claimed in claim 5, wherein the change in defocus distance as a function of the change in focus strength of the focusing lens is known or determined in a previous calibration.

7. The method of claim 3, wherein in (a), a sample stage is moved along an optical axis relative to the focusing lens for varying between the plurality of different defocus distances, and an image is acquired at each of the plurality of different defocus distances.

8. The method of claim 3, wherein at least one of the plurality of images is acquired at an over-focus distance, and at least one of the plurality of images is acquired at an under-focus distance.

9. The method of claim 1, wherein in (b), the step of retrieving the one or more beam cross sections from the one or more images comprises the steps of: The one or more images in Fourier space are divided by a focused image of the sample in Fourier space.

10. The method of claim 9, wherein the step of retrieving the one or more beam cross sections from the one or more images further comprises at least one of multiplication with an adaptive filter term and multiplication with a focused beam cross section in Fourier space.

11. The method of claim 1 , wherein each of the one or more beamwidths is determined at two or more azimuth angles.

12. The method of claim 11, wherein the variation of beam width as a function of defocus distance is calculated as a function of azimuth angle.

13. The method of claim 11, wherein the numerical aperture of the charged particle beam is determined as a function of azimuth angle.

14. The method of claim 1, further comprising at least one or more of the following steps: at least one of generating and displaying a three-dimensional model of the charged particle beam; generating and displaying at least one of one or more two-dimensional representations of the charged particle beam in one or more selected azimuthal planes; and At least one of one or more one-dimensional representations of a beam width of the charged particle beam as a function of a defocus distance is generated and displayed.

15. The method of claim 1, further comprising the steps of: At least one beam-affecting element is modified based on the numerical aperture for shaping the charged-particle beam.

16. The method of claim 1, further comprising the steps of: One or more beam aberration coefficients of the charged particle beam are determined by an iterative fitting routine using the numerical aperture as an input parameter.

17. A charged particle beam system comprising: a charged particle source, the charged particle source being used to emit a charged particle beam propagating along an optical axis; Sample table; a focusing lens, the focusing lens being used to focus 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 which, when executed by the processor, cause the charged particle beam system to perform the method of claim 1.

18. A charged particle beam system comprising: a charged particle source, the charged particle source being used to emit a charged particle beam propagating along an optical axis; Sample table; a focusing lens, the focusing lens being used to focus 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) extracting one or more beam cross sections from one or more images of the sample acquired at one or more defocus distances; (x2) determining one or more beam widths from the one or more beam cross sections; as well as (x3) calculating a change in beam width as a function of defocus distance based on the one or more beam widths and the one or more defocus distances for determining a numerical aperture of the charged particle beam.

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