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

Abstract

A method is provided for determining beam convergence of a charged particle beam (11) focused by a focusing lens (120) towards a sample (HO) in a charged particle beam system (100), the method including (a) taking one or more images of the sample when the sample is located at one or more defocus distances from a respective 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. Also provided is a charged particle beam system for imaging and / or inspecting a sample, the charged particle beam system configured for any of the methods described herein.

Description

Technical Field

[0001] The embodiments described herein relate to a method for determining the beam convergence of a charged particle beam focused towards a sample by a focusing lens within a charged particle beam system, particularly within a scanning electron microscope (SEM). Specifically, according to the methods described herein, the numerical aperture (NA) provided by a focusing lens within a charged particle beam system can be determined from one or more images taken of a sample. The embodiments further relate to a charged particle beam system for inspecting and / or imaging a sample, the charged particle beam system being configured for any of the methods described herein.

Background Art

[0002] The latest semiconductor technology has created a great demand for constructing and probing test specimens on the nanometer or sub-nanometer scale. Micro- and nanometer scale process control, inspection, or construction are often carried out using a charged particle beam, such as an electron beam, which is generated, shaped, deflected, and focused within a charged particle beam system such as an electron microscope or an electron beam pattern generator. For inspection purposes, a charged particle beam provides excellent spatial resolution, for example, compared to a photon beam.

[0003] Inspection apparatuses using a charged particle beam, such as a scanning electron microscope (SEM), have many functions in a plurality of industrial fields, including, but not limited to, inspection of electronic circuits, exposure systems for lithography, detection systems, defect inspection tools, and test systems for integrated circuits. In such a particle beam system, a fine beam probe having a high current density can be used. For example, in the case of an SEM, a primary electron beam generates signal particles such as secondary electrons (SE) and / or backscattered electrons (BSE), and those signal particles can be used to image and / or inspect a sample.

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, it is difficult to inspect and / or image a sample with high resolution and high reliability using a charged particle beam system. Specifically, a charged particle beam usually has beam aberrations that limit the 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 surface of the sample by the objective lens.

[0005] The numerical aperture (NA) of a charged particle beam is a magnitude commonly used by those skilled in the art to describe the beam convergence of the charged particle beam focused on the sample surface by the objective lens. A large numerical aperture provides an improved resolution limit that can theoretically be achieved when the beam aberration is small. This system may be designed for a specific numerical aperture that provides excellent operation, but the actual numerical aperture may deviate from the expected value. Therefore, knowing the beam convergence of the charged particle beam focused by the focusing lens, more specifically, knowing the actual numerical aperture of the charged particle beam, would be beneficial for determining the cause of the beam aberration and for 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 the charged particle beam focused by the focusing lens within the charged particle beam system, particularly to accurately determine the numerical aperture of the charged particle beam focused by the objective lens. Furthermore, it would also be beneficial to provide a charged particle beam system for inspecting and / or imaging a sample, which is configured to operate according to any of the methods described herein.

[0007] In view of the above, there is provided a method for determining beam convergence of a charged particle beam according to an independent claim, and a charged particle beam system configured to determine beam convergence of a charged particle beam. **Means for Solving the Problem**

[0008] According to a first aspect, there is provided a method for determining beam convergence of a charged particle beam focused towards a sample by a focusing lens within a charged particle beam system. The method includes: (a) taking one or more images of the sample when the sample is disposed at one or more defocus distances from respective beam foci of the charged particle beam; (b) searching for 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 includes information regarding a change in beam width as a function of defocus distance. In particular, the at least one beam convergence value may include, or be, the numerical aperture (NA) of the charged particle beam.

[0010] In some embodiments, further, a focused image of the sample is taken (when the sample is disposed at the focal distance from the focusing lens), and the focused image of the sample can be used in (b) to search for one or more beam cross-sections from the one or more images taken out of focus in (a). The focused image of the sample can also be known in other ways and then used in the search for the one or more beam cross-sections in (b). Alternatively, in (b), the one or more beam cross-sections can be searched for without using the focused image of the sample.

[0011] According to another aspect, there is provided a charged particle beam system for imaging and / or inspecting a sample using a charged particle beam, in particular an electron beam. The charged particle beam system includes 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 towards 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. The memory stores instructions that, when executed by the processor, cause the system to perform any of the methods described herein.

[0012] In particular, the stored instructions, when executed, may cause the system to (x1) search for one or more beam cross-sections from one or more images of the sample taken 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 the one or more defocus distances.

[0013] Embodiments are also directed to an apparatus for performing the disclosed methods, including apparatus components for performing the individual method operations. The described methods can be performed by hardware components, a computer programmed by appropriate software, or any combination of the two, or in any other manner. Further, embodiments are directed to methods of operating the described apparatus.

[0014] Additional advantages, features, aspects, and details that can be combined with the embodiments described herein will be apparent from the dependent claims, this description, and the drawings.

[0015] To better understand the features of the present disclosure listed above in detail, a more detailed description can be obtained by referring to the embodiments, which provides a more detailed illustration of the overview briefly shown above. The accompanying drawings relate to one or more embodiments and are described below.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Figure 4

Modes for Carrying Out the Invention

[0017] Next, various embodiments are referred to in detail. One or more examples of those embodiments are shown in the figures. In the following description of the drawings, the same reference numerals refer to the same components. Generally, only the differences regarding individual embodiments are described. Each example is shown for illustrative purposes and is not intended to be limiting. Furthermore, features illustrated or described as part of one embodiment can be used with respect to or in combination with other embodiments to provide additional embodiments. This description is intended to include such changes and modifications.

[0018] FIG. 1 is 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 includes a charged particle source 105 for emitting a charged particle beam 11, particularly an electron beam, that propagates along an optical axis A, particularly an electron source. The charged particle beam system 100 further includes a sample stage 108 and a focusing lens 120, particularly an objective lens, for focusing the charged particle beam 11 onto a sample 10 placed on the sample stage 108. The charged particle beam system 100 further includes a charged particle detector 118, particularly an electron detector, for detecting signal particles (particularly secondary electrons and / or backscattered electrons) emitted from the sample 10.

[0019] An image generation unit 160 may be provided for generating one or more images of the sample 10 based on the charged particle signal received from the charged particle detector 118. The image generation unit 160 can transfer one or more images of the sample to a processing unit 170 configured to determine at least one beam convergence value of the charged particle beam according to the method described herein.

[0020] Optionally, this at least one beam convergence value may be used for the purpose of determining the beam aberration 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 can be transferred to a controller 180 of the aberration corrector 109 in such a manner that the aberration corrector 109 can be appropriately adjusted to compensate for one or more beam aberrations present in the system. A charged particle beam with corrected aberration can be provided.

[0021] The sample stage 108 may be a movable stage. In particular, the sample stage 108 may be movable in the Z direction, i.e., the direction of the optical axis A, in such a manner that the distance between the focusing lens 120 and the sample 10 can be changed (see arrow 112 in FIG. 1). By moving the sample stage 108 in the Z direction, the sample 10 can be moved to different defocus distances in such a manner that out-of-focus images of the sample 10 can be captured for each stage movement, for example, for each stage movement at a predetermined step width, i.e., away from the focal plane p of the focusing lens 120 I from the sample 10.

[0022] In some embodiments, furthermore, the sample stage 108 may be movable within a plane perpendicular to the optical axis A (also referred to herein as the X-Y plane). By moving the sample stage 108 within the X-Y plane, the designated surface area of the sample 10 can be moved to an area below the focusing lens 120 in such a manner that the area can be imaged by focusing the charged particle beam 11 on the designated surface area of the sample 10.

[0023] Under a pressure lower than atmospheric pressure, for example, under a pressure lower than 10 -3 millibar or under a pressure lower than 10 -5 millibar, the beam optical components of the charged particle beam system 100 are typically placed in a vacuum chamber 101 that can be evacuated, in such a manner that the charged particle beam 11 propagates along the optical axis A from the charged particle source 105 towards the sample stage 108 and hits the sample 10.

[0024] In some embodiments, the charged particle beam system 100 may be an electron microscope, particularly a scanning electron microscope. A scanning deflector 107 may be provided to scan the surface of the sample 10 with the charged particle beam 11 along a predetermined scanning pattern, for example, in the X direction and / or the Y direction.

[0025] In some embodiments, a condenser lens system 106, particularly a condenser lens system 106 for collimating the charged particle beam 11 propagating towards the focusing lens 120, may be disposed downstream of the charged particle source 105. In some embodiments, the focusing lens 120 is an objective lens configured to focus the charged particle beam 11 onto the sample 10, particularly a magnetic objective lens, an electromagnetic lens, or a combined magnetic and electrostatic lens. The objective lens may optionally include a retarding field device, such as one or more retarding electrodes, configured to decelerate the charged particle beam to a predetermined landing energy on the sample.

[0026] One or more surface regions of the sample 10 can be inspected and / or imaged using the charged particle beam system 100. As used herein, the term "sample" may relate to a substrate, such as a substrate on which one or more layers or features are formed, a semiconductor wafer, a glass substrate, a web substrate, or another sample to be inspected. The sample can be inspected for one or more purposes, including (1) imaging the surface of the sample, (2) measuring the dimensions of one or more features of the sample, such as lateral dimensions, i.e., dimensions in the X-Y plane, (3) performing critical dimension measurements and / or metrology, (4) detecting defects, and / or (5) investigating the quality of the sample.

[0027] In order to inspect the sample 10 using the charged particle beam 11, the charged particle beam 11 is usually focused on the sample surface using the focusing lens 120. When the charged particle beam 11 collides with the sample surface, secondary electrons and / or backscattered electrons are emitted from the sample. Those signal electrons provide information regarding the spatial characteristics and dimensions of the features of the sample, and those signal electrons can be detected using the charged particle detector 118. By scanning the sample surface with the charged particle beam 11 using, for example, the 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 of the sample surface can be imaged using, for example, the image generation unit 160, and the image generation unit 160 may be configured to provide an image of the sample 10 based on the received signal electrons.

[0028] The small spot of the focused charged particle beam 11 on the sample surface increases the achievable image resolution. Therefore, in order to obtain a sharp in-focus image of the sample 10, the sample surface should be disposed on the focal plane p I of the focusing lens 120 during the inspection. In the present specification, a sharp image of the sample 10 taken in focus is also referred to as the "focus image h I ". The subscript I indicates "in focus". Similarly, in the present specification, the beam cross-section of the charged particle beam 11 at the focal plane p I is referred to as the "focus beam cross-section g I ". The subscript I indicates "in focus".

[0029] It should be noted that the image can be presented mathematically in real space (= in the image domain. That is, as a function of spatial coordinates) or in Fourier space (= in the frequency domain. That is, as a function of spatial frequency). The image in Fourier space can be calculated from the image in real space via a Fourier transform (FT). Both of the above representations contain the corresponding information of the image. When used in the present specification, the image in real space is denoted using the lower case letter "h n ", and the image in Fourier space is denoted using the capital letter "H nis indicated by "", and the subscript "n" indicates the n-th image taken. For example, "h" I " indicates the focused image of the sample in real space, and "H" I " indicates the focused image of the sample in Fourier space, which is the Fourier transform of h I . Similarly, in this specification, the beam cross-section in real space is denoted by the lowercase letter "g" n ", and in this specification, the beam cross-section in Fourier space is denoted by the uppercase letter "G" n ", and the subscript "n" indicates the beam cross-section of the n-th image taken. For example, "g" I " indicates the focused beam cross-section of the charged particle beam in real space, and "G" I " indicates the focused beam cross-section of the charged particle beam in Fourier space, which is the Fourier transform of g I . In some embodiments of the embodiments described in this specification, the image and beam cross-section in real space can be Fourier-transformed into Fourier space via a fast Fourier transform (FFT) algorithm, and vice versa.

[0030] In a charged particle beam system, the actual value of the beam convergence of the charged particle beam focused onto a sample by the focusing lens 120 is usually unknown. The beam convergence may be expressed by the beam convergence angle (a) with respect to the optical axis, or by the numerical aperture (NA) of the charged particle beam, and these values are "longitudinal characteristics" of the beam that are hidden and cannot be directly obtained from the image, making it difficult to determine. The numerical aperture (NA) determines the resolution limit in such a way that it is very beneficial to know the actual value of the numerical aperture.

[0031] For example, the numerical aperture of a system is often used to adapt and design elements (such as an objective lens) that affect the beam of the system, which can lead to inaccuracies and beam aberrations when the actual numerical aperture present in the charged particle beam system deviates from the designed numerical aperture assumed for the system design. The elements that affect the beam may include one or more elements from the group consisting of lenses, beam extractors, beam deflectors, collimators, aberration correctors, scanning deflectors, beam splitters, and charged particle detectors. Thus, determining the actual numerical aperture can help identify the causes of inaccuracies within the system and optimize the system design and resolution. Additionally, the numerical aperture as a measure of beam convergence can be used as an input parameter for an aberration coefficient determination routine.

[0032] According to the method described herein, at least one beam convergence value of a charged particle beam, particularly the numerical aperture (NA), can be determined with high reliability and accuracy. The charged particle beam system 100 of FIG. 1 may include a processor and a memory (shown as processing unit 170 in FIG. 1), and this memory stores instructions that, when executed by the processor, cause the system to execute any of the methods described herein.

[0033] FIGS. 2 and 3 are flowcharts schematically showing such a method for determining at least one beam convergence value. The diagram of FIG. 3 shows optional additional details compared to the more general diagram of FIG. 2.

[0034] In box 210 of FIGS. 2 and 3, when a sample is placed at one or more defocus distances (z 1...N ) from each beam focus of the charged particle beam, one or more images (h 1...N) is taken. In this specification, the defocus distance is understood as the distance (≠0) between the sample and the beam focus when the image is taken. When an image is taken with the sample placed at the defocus distance of the sample with respect to the beam focus, the resulting image is a defocused image of the sample. Since the beam cross-section generally increases with an increase in the defocus distance and the resolution generally decreases with an increase in the defocus distance, it is natural that as the defocus distance increases, the blur of each image taken also increases.

[0035] In some embodiments, a plurality of images, for example, two, three, six or more images, are taken at a plurality of different defocus distances between the sample and each beam focus, for example, at two, three, six or more different defocus distances. Specifically, at the overfocus distance, that is, in the defocus setting where the sample is placed at a position further away from the focusing lens 120 than each beam focus of the charged particle beam (see the defocus distance z3 shown in FIG. 1), at least one image of the sample may be taken. Further, at the underfocus distance, that is, in the defocus setting where the sample is placed closer to the focusing lens 120 than each beam focus of the charged particle beam (see the defocus distances z1 and z2 shown in FIG. 1), at least one image of the sample may be taken. In this specification, the image taken at the first defocus distance z1 is called h1, and the image taken at the nth defocus distance z n is called h n . In this specification, a total of N images (called (h 1...N )) may be taken, and in particular, N is 6 or more, for example, 10 or more or 15 or more.

[0036] In some embodiments described in this specification, one or more defocus distances (z 1...N ) at which one or more images (h 1...N) is known quantitatively, i.e., its absolute value is known, or, when taking a plurality of images at a plurality of different defocus distances, at least the difference between each two of the plurality of defocus distances is known quantitatively, for example, known in [μm] or in another unit of length. In order to determine the beam convergence from one or more defocused images, it is beneficial that it is known quantitatively (e.g., in [μm]) at which defocus distance each of the defocused images was taken. Alternatively, in order to determine the beam convergence from two (or three or more) defocused images taken at two (or three or more) different defocus distances, at least the difference between each two different defocus distances is known quantitatively (e.g., in [μm]). In some embodiments, calibration may be performed prior to taking the defocused image of the sample in such a manner that the defocus distance at which the image was taken from each setting of the charged particle beam system is known quantitatively.

[0037] In some embodiments, which can be combined with other embodiments, the defocus distance is changed by moving the sample stage 108 in the Z direction, i.e., along the optical axis A, with respect to the focusing lens 120. The movement of the stage for changing the defocus distance between a plurality of different defocus distances is schematically shown in FIG. 1. For example, the sample stage may be moved at a predetermined step width, for example, a plurality of equal or similar step widths between 0.2 μm and 2 μm, and an image of the sample may be taken at each of the plurality of defocus distances. The focal strength of the focusing lens 120 may be maintained constant while the sample stage is being moved and while the image is being taken.

[0038] In other embodiments, the defocus distance is changed by changing the focusing strength of the focusing lens 120. With respect to the sample 10, the focal plane p IBy moving it in such a way as to change the defocus distance, by increasing the focusing strength of the focusing lens, each beam focus and focal plane are moved toward the focusing lens with respect to the sample, and by decreasing the focusing strength, each beam focus and focal plane are moved away from the focusing lens with respect to the sample. The sample does not have to be moved. In particular, in order to change the defocus distance between a plurality of different defocus distances, a plurality of different focusing strengths may be applied to the charged particle beam by the focusing lens 120, and an image may be taken at each of the plurality of different focusing strengths.

[0039] It should be noted that when the focusing strength changes, the beam convergence also changes. Therefore, in the embodiments described herein, when changing the focusing strength, the focusing strength is changed only in such a manner that the resulting change in the defocus distance can be ignored with respect to the overall focal length (f) of the focusing lens 120. For example, the defocus distance may be changed within a total range of several μm (e.g., <10 μm) by changing the focusing strength to take a plurality of images (see FIG. 4), while the overall focal length (f) of the focusing lens 120 may be in the range of several millimeters or several centimeters. Therefore, the change in beam convergence due to changing the focusing strength of the focusing lens according to the embodiments described herein can be kept to only an extent that can be ignored, and the determined numerical aperture (NA) or other beam convergence value is not substantially affected.

[0040] In some embodiments, the change in defocus distance as a function of the change in the focusing strength of the focusing lens 120 is known in advance or determined in a previous calibration in such a manner that each change in defocus distance is quantitatively known for each change in the focusing strength applied by the focusing lens, or in such a manner that each defocus distance is quantitatively known (e.g., in [μm]) for each focusing strength of the focusing lens.

[0041] Next, returning to FIGS. 2 and 3. In box 220, one or more images (h 1...N) from one or more defocus distances (z 1...N ) to search for one or more beam cross-sections (g 1...N ) of a charged particle beam. In particular, a plurality of beam cross-sections are searched from a plurality of images. That is, one beam cross-section is searched for each of the plurality of images. Each of the searched beam cross-sections corresponds to the beam cross-section of the charged particle beam at the defocus distance at which each image was taken. The defocused beam profile can be extracted from the defocused images through several different beam profile extraction methods. Hereinafter, one exemplary beam profile extraction method will be exemplarily described.

[0042] In addition to one or more images (h 1...N ) taken out of focus, a focused image (h I ) of the sample can be taken, and the focused image (h I ) can be used to search for one or more beam cross-sections (g 1...N ) from one or more images (h 1...N ). The focused image h I of the sample can also be known in other ways. This is because, for example, a sample having known shape dimensions is used to determine beam convergence according to the method described in this specification.

[0043] More specifically, as shown in more detail in box 220 of FIG. 3, searching for one or more beam cross-sections (g 1...N ) from one or more images (h 1...N ) may include performing a Fourier transform on one or more images (h 1...N ) taken in real space to provide one or more images (H 1...N ) taken in Fourier space, and dividing one or more images (H 1...N ) taken in Fourier space by the focused image (H I ) of the sample in Fourier space. The above beam profile extraction method, in Fourier space, divides the taken defocused image (Hn) of the sample by the focused image (H I) is removed in such a manner that the division may give a pure beam profile without sample information, i.e., a pure beam cross-section without sample information, based on removing the structure of the sample. Each beam cross-section (g n ) retrieved in real space may be obtained by inverse Fourier transforming each beam cross-section (G n ) retrieved in Fourier space.

[0044] As further shown in box 220 of FIG. 3, searching for one or more beam cross-sections (g 1...N ) from one or more captured images (h 1...N ) is optionally included in at least one of multiplying by an adaptive filter term [Number] and multiplying by the focused beam cross-section (G I ) in Fourier space. The adaptive filter term

[0045] [Number] can be provided by an adaptive filter 260, and the adaptive filter 260 may receive the captured image as input information. The adaptive filter term may be provided individually for each of the captured images by the adaptive filter 260. When the adaptive filter term

[0046] [Number] is not used, if the value of the focused image H I in the denominator of the above division is close to zero, the noise weight of the focused image may become very strong. The adaptive filter term [Number] is the focused image H 1...N in the calculation of the focused beam cross-section (G ISuch an influence for which the noise is not required can be reduced or avoided. Each filter term may be determined individually for each of the images (h 1...N ) by the adaptive filter 260. This is for example to ensure that appropriate filter terms are used for each image and the respective noise of that image.

[0047] The focus beam cross-section (g I ) in real space or the focus beam cross-section (G I ) in Fourier space can be simulated, for example, based on a wave-optical simulation of the focused beam cross-section. For example, for the simulation of the focus beam cross-section g I , a Gaussian beam cross-section at the focal plane of the focusing lens 120 can be assumed. Specifically, the focus beam cross-section g I of the charged particle beam can be determined by a resolution measurement, for example, a resolution measurement assuming a Gaussian beam profile.

[0048] In box 230, one or more beam widths (c 1...N ) of the charged particle beam at one or more defocus distances (z 1...N ) are determined from one or more beam cross-sections (g 1...N ). In particular, a plurality of beam widths are determined from a plurality of beam cross-sections. That is, for each of the plurality of beam cross-sections (for example, in one or more directions, i.e., for one or more azimuth angles), one beam width is searched for.

[0049] In some embodiments, in box 230, one beam width may be searched for from each beam cross-section. For example, in a manner such that since the beam width is essentially the same for each azimuth angle, it is sufficient to search for only one beam width from each beam cross-section, one or more beam cross-sections (g 1...N) may be essentially rotationally symmetric, such as circular or Gaussian symmetric. As used herein, "azimuth angle" refers to different directions within the cross-sectional plane of the beam cross-section where the width of the beam can be measured, i.e., different directions perpendicular to the optical axis (A). Alternatively, even if the beam cross-section is not rotationally symmetric, one beam width for the beam cross-section can be determined as the average beam width or as the FW50 value. The FW50 value is the diameter of the circle around the optical axis A through which half of the beam current propagates.

[0050] In some embodiments that can be combined with other embodiments, each of one or more beam widths (c 1...N ) is determined in two or more directions, i.e., at two or more different azimuth angles. In particular, each of one or more beam widths (c 1...N ) can be determined as a function of the azimuth angle ((c 1...N )(θ)). FIG. 3 illustratively shows a beam cross-section g n retrieved from the image h n which is not strictly rotationally symmetric. Each beam width c n can be determined in two or more directions, for example, at azimuth angles θ x (= X direction) and azimuth angle θ y (= y direction). Specifically, the beam width c n can be retrieved from the beam cross-section g n as a function of the azimuth angle (c n (θ)).

[0051] In some embodiments that can be combined with other embodiments described herein, at least one size from the following group is determined from each beam cross-section and that size is regarded as each 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 called D50 width, at one or more azimuth angles, (3) 1 / e 2The width, and another commonly used size that describes (4) the beam width. The beam radius or beam diameter may be regarded as the beam width respectively. Depending on the size searched for as the beam width in box 230, it should be noted that it may be necessary to multiply the resulting beam convergence value by the respective calibration coefficient (K) in order to determine the exact value of the numerical aperture from the beam convergence value.

[0052] In box 240, calculate at least one beam convergence value based on one or more beam widths (c 1...N ) and one or more defocus distances (z 1...N ). In some embodiments, at least one beam convergence value may be the change in beam width as a function of the defocus distance

Number

[0053] FIG. 4 shows a graph of beam width (c) as a function of defocus distance (f) in an exemplary defocus range from -4 μm to +4 μm relative to the beam focus. Here, 10 images are taken at different (quantitatively known) underfocus distances and 10 images are taken at different (quantitatively known) overfocus distances, and from those images, the respective beam widths are retrieved. The beam width c determined in box 230 1...N is shown as a small square at each defocus distance z 1...N . In the graph, the change in beam width as a function of defocus distance

[0054]

Number

[0055] In some embodiments, an average slope, particularly the slope of the fit of the line to at least some of the beam width values shown as a function of defocus distance, is determined, and from that slope, the beam convergence angle (α) can be calculated. The (averaged or fitted) slope value and / or the beam convergence angle (α) calculated from that slope value can be determined as at least one beam convergence value in some of the embodiments described herein.

[0056] In some embodiments, from the above (averaged or fitted) slope value, the numerical aperture (NA) of the charged particle beam is determined from the slope value

Number

[0057] [Number] the absolute value of (optionally averaged or linearly fitted) of, the numerical aperture is calculated as NA = K·|slope(FW50)|, and the calibration coefficient (K) is a value between 0.5 and 1, particularly a value between 0.70 and 0.85.

[0058] The resulting numerical aperture NA of the charged particle beam (especially after multiplying by the calibration coefficient) can then be directly used as an input parameter in various simulation and design programs for a charged particle beam system that expects the numerical aperture provided by the objective lens as an input parameter.

[0059] In some embodiments that can be combined with other embodiments described herein, this method further includes changing at least one element affecting the beam based on at least one beam convergence value, particularly based on the determined numerical aperture of the charged particle beam, to adjust the charged particle beam.

[0060] In some embodiments that can be combined with other embodiments described herein, the method may further include determining one or more beam aberration coefficients of a charged particle beam by an iterative fitting routine that uses at least one beam convergence value as an input parameter. Such an iterative fitting routine for determining one or more beam aberration coefficients using the actual numerical aperture of the charged particle beam as an input parameter is shown by box 250 in FIG. 2. If the actual numerical aperture NA of the charged particle beam is known, such a fitting routine may be able to converge more quickly towards the actual beam aberration coefficients.

[0061] In some embodiments that can be combined with other embodiments described herein, each of one or more beam widths (c 1...N ) is determined at two or more azimuthal angles, particularly as a function of the azimuthal angle ((c 1...N )(θ)), and at each of said azimuthal angles, respective beam convergence values may be calculated.

[0062] In particular, at least one beam convergence value may include a first beam convergence value of the charged particle beam at a first azimuthal angle (θx) and a second beam convergence value of the charged particle beam at a second azimuthal angle (θy). In some embodiments, at least one beam convergence value can be calculated as a function of the azimuthal angle, for example when the beam profile is highly asymmetric, oval, or otherwise distorted.

[0063] In some embodiments, the numerical aperture of the charged particle beam can be calculated at two or more azimuthal angles, particularly as a function of the azimuthal angle (NA(θ)). For example, first, the beam convergence angle (a) or tilt

Number

[0064] In particular, when the beam convergence value is determined as a function of the azimuth angle, a three-dimensional model of the focused charged particle beam can be generated and / or displayed. Alternatively or in addition, one or more two-dimensional representations of the charged particle beam in one or more selected azimuth planes can be generated and / or displayed. Alternatively or in addition, one or more one-dimensional representations of the beam width of the charged particle beam as a function of the defocus distance can be generated and / or displayed. Alternatively or in addition, the numerical aperture can be determined and / or displayed as a function of the azimuth angle, for example in a one-dimensional representation.

[0065] In some embodiments described herein, in particular, when a single defocused image is taken and the defocus distance z n is known quantitatively (and, further, when a single defocused image h n from which the beam cross-section g n and the beam width c n can be retrieved, and the focused image h I of the sample has been taken or is otherwise known), a single defocused image h I that has been taken is already sufficient to determine at least one beam convergence value. Specifically, at least one beam convergence value can be determined by calculating c n / z n and the NA can be calculated from the at least one beam convergence value. However, when multiple defocused images h n are taken at multiple different defocus distances z 1...N and the quantity dc / dz is determined from those images as described herein (e.g., as an average slope value or as a linear fit to the beam width as a function of the defocus distance), in particular for both underfocus and overfocus, more accurate beam convergence values and more accurate NA values can be determined. 1...N ​

[0066] It should be noted that the known process for determining the beam aberration coefficient is based on the analysis of the probe shape retrieved from the defocused image taken out of focus. However, only a relative estimate regarding the beam aberration coefficient can be made from the retrieved probe shape, and the defocus distance at which the defocused image was actually taken cannot be quantitatively known. Therefore, it is not possible to retrieve quantitative information regarding the convergence of the focused charged particle beam using the known process. In contrast, the method described herein uses the defocused image taken at a quantitatively known defocus distance, or the defocused image taken with a quantitatively known difference between defocus distances, to retrieve information regarding the beam convergence angle, and thus, based on the understanding that they can be used to determine the actual numerical aperture of the focused charged particle beam.

[0067] The numerical aperture of a charged particle beam focused by a focusing lens has hitherto been a useful quantity determined by making assumptions regarding the actual beam shape based on resolution measurements. However, as a result of such assumptions, the determination becomes inaccurate. According to the method described herein, the NA can be retrieved from the actual defocused beam cross-section retrieved from the defocused image in such a manner that no assumptions regarding the beam shape need to be made. Specifically, no assumptions regarding the beam cross-section or the sample need to be made, and as a result, the value of the numerical aperture becomes more accurate. Furthermore, the numerical aperture can be determined at any azimuth angle, which was not previously possible and enables the identification of (intentional or unintentional) discrepancies along different directions between the intended beam shape and the actual beam shape. Furthermore, the beam shape can be visualized in 3D and / or 2D, which may assist in the proper analysis and improvement / adjustment of the charged particle beam.

[0068] The ability to precisely determine the numerical aperture of a charged particle beam system is particularly enabled by: (1) reproducibility and matching when the numerical aperture is a decisive quantity that determines the achievable resolution, (2) analysis of factors affecting the beam, such as for improving future designs, (3) determination of beam aberrations that scale to various magnifications of the numerical aperture, and (4) generation of an adjusted charged particle beam for certain critically important dimensional determination and defect inspection tasks, such as inspecting deep trenches in an elongated sample along only one axis.

[0069] Specifically, the following embodiments are described herein.

[0070] Embodiment 1: A method for determining the beam convergence of a charged particle beam (11) focused towards a sample (10) by a focusing lens (120) within a charged particle beam system (100), the method comprising: (a) taking one or more images (h 1...N ) of the sample when the sample is disposed at one or more defocus distances (z 1...N ) from each beam focus of the charged particle beam; (b) searching for one or more beam cross-sections (g 1...N ) from the one or more images (h 1...N ); (c) determining one or more beam widths (c 1...N ) from the one or more beam cross-sections (g 1...N ); and (d) calculating at least one beam convergence value based on the one or more beam widths (c 1...N ) and the one or more defocus distances (z 1...N ).

[0071] Embodiment 2: The absolute value of the one or more defocus distances (z 1...N ) at which the one or more images (h 1...N ) are taken, or the one or more defocus distances (z 1...N) the absolute value of the difference between is known, and (d) the method according to embodiment 1, wherein the absolute value is used to calculate at least one beam convergence value.

[0072] Embodiment 3: at least one beam convergence value is the change in beam width as a function of the defocus distance

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[0073] Embodiment 4: the method according to any one of embodiments 1 to 3, wherein at least one beam convergence value includes the numerical aperture (NA) of the charged particle beam.

[0074] Embodiment 5: (a) a plurality of images are taken when the sample is placed at a plurality of different defocus distances, (b) a plurality of beam cross-sections are retrieved from the plurality of images, (c) a plurality of beam widths are determined from the plurality of beam cross-sections, and (d) at least one beam convergence value is calculated based on the plurality of beam widths and the plurality of defocus distances. The method according to any one of embodiments 1 to 4.

[0075] Embodiment 6: the numerical aperture (NA) of the charged particle beam is the change in beam width as a function of the defocus distance

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[0076] Embodiment 7: (a) a plurality of different focusing intensities are applied by the focusing lens (120) to change the defocus distance between a plurality of different defocus distances, and an image is taken at each of the plurality of different focusing intensities. The method according to embodiment 5 or 6.

[0077] Embodiment 8: The method according to embodiment 7, wherein the change in defocus distance as a function of the change in the focusing intensity of the focusing lens (120) is known or determined in a previous calibration.

[0078] Embodiment 9: The method according to embodiment 5 or 6, wherein in (a), the sample stage (108) is moved along the optical axis (A) with respect to the focusing lens (120) to change the defocus distance between a plurality of different defocus distances, and an image is taken at each of the plurality of different defocus distances.

[0079] Embodiment 10: The method according to any one of embodiments 5 to 9, wherein at least one of the plurality of images is taken at an overfocus distance and at least one of the plurality of images is taken at an underfocus distance.

[0080] Embodiment 11: In (b), searching for one or more beam cross-sections (g 1...N ) from one or more images (h 1...N ) includes dividing one or more images (H 1...N ) in the Fourier space by the focused image (H I ) of the sample in the Fourier space. The method according to any one of embodiments 1 to 10.

[0081] Embodiment 12: Searching for one or more beam cross-sections (g 1...N ) from one or more images (h 1...N ) further includes at least one of multiplying by an adaptive filter term

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[0082] Embodiment 13: Each of the one or more beam widths (c 1...N ) is in two or more directions, particularly the azimuth angle ((c 1...NThe method according to any one of Embodiments 1 to 12, which is determined as a function of )(θ)).

[0083] Embodiment 14: The method according to Embodiment 13, wherein at least one beam convergence value includes a first beam convergence value of a 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, at least one beam convergence value is calculated as a function of the azimuth angle.

[0084] Embodiment 15: The method according to Embodiment 14, wherein the numerical aperture (NA(θ)) of the charged particle beam as a function of the azimuth angle is calculated from at least one beam convergence value as a function of the azimuth angle.

[0085] Embodiment 16: The method according to any one of Embodiments 1 to 15, further including at least one or more of: displaying a three-dimensional model of the charged particle beam; displaying one or more two-dimensional representations of the charged particle beam in one or more selected azimuth 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 azimuth angle.

[0086] Embodiment 17: The method according to any one of Embodiments 1 to 16, further including changing at least one element affecting the beam based on at least one beam convergence value to adjust the charged particle beam.

[0087] Embodiment 18: The method according to any one of Embodiments 1 to 17, further including 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.

[0088] 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 towards 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, wherein the memory stores instructions that, when executed by the processor, cause the charged particle beam system to execute the method according to any one of the above embodiments.

[0089] 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 towards 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, wherein the memory stores instructions that, when executed by the processor, cause the charged particle beam system to perform: (x1) searching for one or more beam cross-sections (g 1...N ) from one or more images (h 1...N ) of the sample taken at one or more defocus distances (z 1...N ); (x2) determining one or more beam widths (c 1...N ) from the one or more beam cross-sections (g 1...N ); and (x3) calculating at least one beam convergence value based on the one or more beam widths (g 1...N ) and the one or more defocus distances (z 1...N ). The charged particle beam system (100) may be further configured for the method according to any one of the above embodiments.

[0090] The above description is directed to embodiments, but other embodiments and additional embodiments may be devised without departing from the basic scope of the embodiments, and the scope of the embodiments is determined by the following claims.

Claims

Claim 1 A method for determining the beam convergence of a charged particle beam focused towards 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 placed at one or more defocus distances from respective beam foci of the charged particle beam, such that the one or more images include one or more out-of-focus images of the sample; (b) searching for 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. A method as claimed in claim 1. Claim 2 The method according to claim 1, wherein the absolute value of the one or more defocus distances at which the one or more images are taken, or the absolute value of the difference between the one or more defocus distances, is known and is used in (d) to calculate the at least one beam convergence value. Claim 3 The method according to claim 1, wherein the at least one beam convergence value includes at least one of a change in beam width as a function of the defocus distance of the charged particle beam, a beam convergence angle, and a numerical aperture. Claim 4 The method according to claim 1, wherein the at least one beam convergence value includes the numerical aperture of the charged particle beam. Claim 5 The method according to claim 1, wherein in (a), a plurality of images are taken when the sample is placed at a plurality of different defocus distances; in (b), a plurality of beam cross-sections are searched from the plurality of images; in (c), a plurality of beam widths are determined from the plurality of beam cross-sections; and in (d), the at least one beam convergence value is calculated based on the plurality of beam widths and the plurality of defocus distances. A method as claimed in claim 1. Claim 6 The method according to claim 5, wherein the numerical aperture of the charged particle beam is calculated based on an average change in beam width as a function of the defocus distance and a calibration coefficient. Claim 7 The method according to claim 5, wherein in (a), a plurality of different focusing intensities are applied by the focusing lens to vary the defocus distance between the plurality of different defocus distances, and an image is taken at each of the plurality of different focusing intensities. Claim 8 The method according to claim 7, wherein a change in defocus distance as a function of a change in focusing intensity of the focusing lens is known or determined in a preceding calibration.

9. The method according to claim 5, wherein in (a), the sample stage is moved along the optical axis with respect to the focusing lens to change the defocus distance between the plurality of different defocus distances, and an image is taken at each of the plurality of different defocus distances.

10. The method according to claim 5, wherein at least one of the plurality of images is taken at an overfocus distance and at least one of the plurality of images is taken at an underfocus distance.

11. The method according to claim 1, wherein in (b), searching for the one or more beam cross-sections from the one or more images includes subtracting the one or more images in the Fourier space from the focused image of the sample in the Fourier space.

12. The method according to claim 11, wherein searching for the one or more beam cross-sections from the one or more images further includes at least one of multiplying by an adaptive filter term and multiplying by a focused beam cross-section in the Fourier space.

13. The method according to claim 1, wherein each of the one or more beam widths is determined at two or more azimuth angles, in particular as a function of the azimuth angle.

14. The method according to claim 13, wherein the at least one beam convergence value includes a first beam convergence value of the charged particle beam at a first azimuth angle and a second beam convergence value of the charged particle beam at a second azimuth angle, and in particular, the at least one beam convergence value is calculated as a function of the azimuth angle.

15. The method according to claim 13, wherein the at least one beam convergence value includes a numerical aperture of the charged particle beam as a function of the azimuth angle.

16. At least one of generating and displaying a three-dimensional model of the charged particle beam, At least one of generating and displaying one or more two-dimensional representations of the charged particle beam in one or more selected azimuth planes, and At least one of generating and displaying one or more one-dimensional representations of the beam width of the charged particle beam as a function of the defocus distance, The method according to claim 1, further comprising at least one or more of the above.

17. The method according to claim 1, further comprising changing at least one element affecting the beam based on the at least one beam convergence value to adjust the charged particle beam.

18. The method according to claim 1, further comprising determining one or more beam aberration coefficients of the charged particle beam by an iterative fitting routine that uses the at least one beam convergence value as an input parameter.

19. 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, a processor and a memory A charged particle beam system comprising: the memory stores instructions that, when executed by the processor, cause the charged particle beam system to execute the method according to claim 1. Charged particle beam system.

20. 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, a processor and a memory A charged particle beam system comprising: when the memory is executed by the processor, the charged particle beam system (x1) searching for one or more beam cross-sections from one or more images of the sample taken to include one or more out-of-focus images of the sample at one or more defocus distances, (x2) determining one or more beam widths from the one or more beam cross-sections, and (x3) calculating at least one beam convergence value based on the one or more beam widths and the one or more defocus distances stores instructions to cause execution of Charged particle beam system.

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