Method for determining aberrations in an image obtained by a charged particle beam tool, method for determining settings of a charged particle beam tool, and charged particle beam tool

Abstract

A method for determining aberrations in an image obtained by a charged particle beam tool includes the following steps: a) obtaining two or more images of a sample, wherein each image is obtained with a known relative difference in the measurement conditions of the charged particle beam tool; b) selecting estimated aberration parameters for aberrations representing a detection profile of the charged particle beam used by the charged particle beam tool; c) evaluating an error function that indicates the difference between the two or more images and the two or more estimated images, the two or more estimated images being a function of the estimated aberration parameters and the known relative difference in the measurement conditions; d) updating the estimated aberration parameters; e) iteratively performing steps c) and d); f) determining the final aberration parameters as the aberration parameters that provide a minimum of the error function.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 19204524.3 filed on October 22, 2019 and EP application 20165029.8 filed on March 23, 2020, which are incorporated herein by reference in their entirety. Technical Field

[0003] The present invention relates to a method of determining aberrations in an image acquired by a charged particle beam tool. The present invention also relates to a method of determining settings of a charged particle beam tool. The present invention also relates to a charged particle beam tool. Background Art

[0004] When manufacturing semiconductor integrated circuit (IC) chips, pattern defects and / or contaminant particles (residues) inevitably appear on the wafer and / or mask during the manufacturing process, thereby greatly reducing the yield. For example, contaminant particles may cause trouble for patterns with smaller critical feature sizes, which have been adopted to meet the increasingly advanced performance requirements of IC chips.

[0005] Pattern inspection tools using charged particle beams (such as electron beams) have been used to detect defects and / or contaminant particles. These tools typically use a scanning electron microscope (SEM) or other charged particle beam tools. In an SEM, a primary electron beam with a higher energy is decelerated to land on a sample with a lower landing energy and is focused to form a detection point thereon. Due to this focused detection point of the primary electrons, secondary electrons will be generated from the surface. By scanning the detection point on the sample surface and collecting the secondary electrons, the pattern inspection tool can acquire an image of the sample surface. Aberrations in the detection point profile may reduce the spatial resolution of the image. Summary of the invention

[0006] Thus, for example, it would be desirable to provide a method of determining aberrations in images acquired from a charged particle beam tool.

[0007] According to one aspect of the present invention, a method for determining aberrations in images acquired by a charged particle beam tool is provided, the method comprising the following steps: a) acquiring two or more images of a sample using the charged particle beam tool, wherein each image is acquired under a known relative difference in measurement conditions of the charged particle beam tool; b) selecting estimated aberration parameters for the aberrations of a detection profile, wherein the detection profile represents the charged particle beam used by the charged particle beam tool; c) evaluating an error function, the error function indicating the difference between two or more images and an estimated two or more images, wherein the estimated two or more images are a function of the estimated aberration parameters and the known relative difference in measurement conditions; d) updating the estimated aberration parameters; e) iteratively performing steps c) and d); and f) determining a final aberration parameter as the estimated aberration parameter that provides a minimum value of the error function.

[0008] According to one aspect of the present invention, a method for determining settings of a charged particle beam tool is provided, the method comprising: executing a method for determining aberrations in an image acquired by the charged particle beam tool two or more times, each time based on a different estimate for the settings of the charged particle beam tool; determining a residual error for each estimate of the settings by evaluating an error function based on final aberration parameters determined based on corresponding estimates for the settings; and determining the setting estimate that produces the minimum residual error as the settings of the charged particle beam tool.

[0009] According to another aspect of the present invention, a method for determining aberrations in an image acquired by a multi-beam charged particle tool is provided, the method comprising: executing a method for determining aberrations of a first beamlet of the multi-beam charged particle tool, wherein the estimated aberration parameters include a plurality of estimated aberration values, thereby determining final aberration parameters including a plurality of final aberration values ​​for the first beamlet; executing a method for determining aberrations for each beamlet of one or more additional beamlets of the multi-beam charged particle tool, wherein the step of selecting the estimated aberration parameters includes selecting one or more final aberration values ​​determined for the first beamlet as corresponding one or more aberration values ​​estimated for each beamlet of the one or more additional beamlets, and wherein the step of updating the estimated aberration parameters includes updating estimated aberration values ​​that do not correspond to one or more estimated aberration values ​​selected as corresponding to the one or more final aberration values ​​determined for the first beamlet.

[0010] According to another aspect of the present invention, a charged particle beam tool is provided, comprising: an irradiation system configured to generate a charged particle beam and scan the charged particle beam across a sample; a detection system configured to capture charged particles interacting with the sample, thereby creating an image of the sample; and a controller configured to execute a method.

[0011] According to another aspect of the invention, there is provided a computer program product comprising instructions which, when the program is executed by a computer, cause the computer to perform the method.

[0012] According to another aspect of the present invention, a computer-readable storage medium comprising instructions is provided, which, when executed by a computer, causes the computer to perform a method. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which corresponding reference numerals indicate corresponding parts, and in which:

[0014] Figure 1 A charged particle beam system is schematically depicted;

[0015] Figure 2 A charged particle beam tool is schematically depicted;

[0016] Figure 3 A method of determining aberrations in an image acquired by a charged particle beam tool is schematically depicted;

[0017] Figure 4 schematically depicts a method of determining settings for a charged particle beam tool; and

[0018] Figure 5 and Figure 6 The experimental results are shown. DETAILED DESCRIPTION

[0019] Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings. The following description refers to the accompanying drawings, wherein, unless otherwise specified, the same reference numerals in different drawings represent the same or similar elements. The implementations set forth in the following description of the exemplary embodiments do not represent all implementations consistent with the present invention. Instead, they are merely examples of devices and methods consistent with aspects related to the present invention as described in the appended claims.

[0020] The increased computing power of electronic devices, while reducing the physical size of the devices, can be achieved by significantly increasing the packing density of circuit components such as transistors, capacitors, diodes, etc. on IC chips. For example, a thumbnail-sized smartphone IC chip can include more than 2 billion transistors, each less than 1 / 1000 the size of a human hair. It is not surprising, therefore, that semiconductor IC manufacturing is a complex and time-consuming process with hundreds of individual steps. An error in even one step has the potential to greatly affect the functionality of the final product. Even one "fatal defect" can cause device failure. The goal of a manufacturing process is to improve the overall yield of the process. For example, to achieve a 75% yield for a 50-step process, the yield of each individual step must be greater than 99.4%, and if the yield of a single step is 95%, the yield of the entire process drops to 7%.

[0021] While high process yields are desired in IC chip fabrication facilities, it is also important to maintain high substrate (i.e., wafer) throughput (defined as the number of substrates processed per hour). Especially in cases where operator intervention is required to detect defects, the presence of defects can affect both high process yield and high substrate throughput. Therefore, high throughput detection and identification of micron and nanometer size defects by charged particle beam tools (such as electron beam tools, especially SEM) is essential to maintaining high yields and low costs.

[0022] An electron beam tool, such as an electron beam inspection tool, includes an illumination system that uses one or more focused primary electron beams to generate and scan a sample, such as a substrate. The primary electrons interact with the sample and generate secondary electrons. The electron beam tool also includes a detection system. The electron beam tool creates an image of the sample scanned area by capturing secondary electrons from the sample as it is scanned.

[0023] A known charged particle beam tool, such as a charged particle beam inspection tool, is described below.

[0024] For clarity, the relative sizes of components in the drawings may be exaggerated. In the following description of the drawings, the same or similar reference numerals refer to the same or similar components or entities, and only the differences with respect to the various embodiments are described. As used herein, unless otherwise explicitly stated, the term "or" covers all possible combinations that are feasible. For example, if it is stated that a device may include A or B, then unless otherwise explicitly stated or not feasible, the device may include A, or B, or A and B. As a second example, if it is stated that a device may include A, B, or C, then unless otherwise explicitly stated or not feasible, the device may include A, or B, or C, or A and B, or A and C, or B and C, or A and B and C.

[0025] Reference now Figure 1 , Figure 1is a schematic diagram illustrating an exemplary charged particle beam system 5. The charged particle beam system 5 may be a charged particle beam inspection system. The charged particle beam system 5 may be an electron beam system 5, such as an electron beam inspection system. Figure 1 As shown, the charged particle beam system 5 includes a main chamber 10, a load lock chamber 20, a charged particle beam tool 40 (such as a charged particle beam inspection tool 40) and an equipment front end module (EFEM) 30. The charged particle beam tool 40 is located in the main chamber 10. Although the description and drawings are directed to electron beams, it should be understood that the embodiments are not used to limit the present disclosure to specific charged particles. Charged particles other than electrons can be used in the present invention.

[0026] The EFEM 30 includes a first loading port 30a and a second loading port 30b. The EFEM 30 may include additional loading ports. The first loading port 30a and the second loading port 30b may, for example, receive a substrate front opening pod (FOUP) containing a substrate (e.g., a semiconductor substrate or a substrate made of other materials) or a sample to be inspected (substrate, wafer and sample are collectively referred to as "sample" below). One or more robotic arms (not shown) in the EFEM 30 transport the sample to the load lock chamber 20.

[0027] The load lock chamber 20 may be connected to a load lock vacuum pump system (not shown) that removes gas molecules in the load lock chamber 20 to reach a first pressure lower than atmospheric pressure. After reaching the first pressure, one or more robotic arms (not shown) transport the sample from the load lock chamber 20 to the main chamber 10. The main chamber 10 is connected to a main chamber vacuum pump system (not shown) that removes gas molecules in the main chamber 10 to reach a second pressure lower than the first pressure. After reaching the second pressure, the sample is subjected to inspection by the charged particle beam tool 40.

[0028] The controller 50 is electrically connected to the charged particle beam tool 40. The controller 50 may be a computer configured to perform various controls of the charged particle beam system 5. The controller 50 may also include processing circuits configured to perform various signal and image processing functions. Figure 1 Controller 50 is shown as being outside the structure including main chamber 10, load lock chamber 20 and EFEM 30, but it is understood that controller 50 may be part of the structure.

[0029] The charged particle beam tool 40 may be a charged particle beam inspection tool. The charged particle beam tool 40 may be an electron beam tool 40, such as an electron beam inspection tool 40. The charged particle beam tool 40 may include a single beam tool. Alternatively, the charged particle beam tool 40 may include a multi-beam tool. Figure 2A multi-beam tool is described, but it should be understood that the invention is not limited to such multi-beam tools, but may also be performed using a single beam tool.

[0030] Reference now Figure 2 , Figure 2 is a schematic diagram illustrating an exemplary charged particle beam tool 40, in particular an electron beam tool, which includes as Figure 1 A multi-beam tool as part of an exemplary charged particle beam system 5. The charged particle beam tool 40 may include a charged particle source 201 (e.g., an electron source 201), a gun hole plate 271, a beam focusing lens 210, a source conversion unit 220, a primary projection system 230, a motorized platform 209, and a sample holder 207, which is supported by the motorized platform 209 to hold a sample 208 to be inspected (e.g., a sample or a photomask). The charged particle beam tool 40 may also include a secondary projection system 250 and a detection device 240 (e.g., an electronic detection device). The primary projection system 230 may include an objective lens 231. The detection device 240 may include a plurality of detection elements 241, 242, and 243. A beam splitter 233 and a deflection scanning unit 232 may be positioned inside the primary projection system 230.

[0031] The charged particle source 201, the aperture plate 271, the condenser lens 210, the source conversion unit 220, the beam splitter 233, the deflection scanning unit 232 and the primary projection system 230 can be aligned with the primary optical axis 204 of the charged particle beam tool 40. The secondary projection system 250 and the detection device 240 can be aligned with the secondary optical axis 251 of the charged particle beam tool 40.

[0032] The primary optical axis 204 is the optical axis of the charged particle beam tool 40 as part of the illumination system. The secondary optical axis 251 is the optical axis of the charged particle beam tool 40 as part of the detection system. The primary optical axis 204 may also be referred to herein as the primary electro-optical axis or the charged particle optical axis. The secondary optical axis 251 may also be referred to herein as the secondary electro-optical axis or the secondary charged particle optical axis.

[0033] The charged particle source 201 may be an electron source and may include a cathode (not shown) and an extractor or anode (not shown), wherein during operation, the charged particle source 201 is configured to emit primary electrons or other charged particles from the cathode, and the primary electrons or other charged particles are extracted or accelerated by the extractor and / or the anode to form a primary charged particle beam 202, such as a primary electron beam 202, which forms a primary beam cross (virtual or real) 203. The primary charged particle beam 202 may be visualized as being emitted from the primary beam cross 203.

[0034] The source conversion unit 220 may include an image forming element array, an aberration compensator array, a beam limiting aperture array, and a pre-bent micro-deflector array. The pre-bent micro-deflector array may deflect a plurality of primary sub-beams 211, 212, 213 of the primary charged particle beam 202 to enter the beam limiting aperture array, the image forming element array, and the aberration compensator array in a normal direction. The sub-beams 211, 212, 213 are also referred to herein as sub-beams. The primary charged particle beam 202 may include a plurality of primary sub-beams 211, 212, 213. The condenser lens 210 may be designed to focus the primary charged particle beam 202 into a parallel beam and to be incident normally onto the source conversion unit 220. The image forming element array may include a plurality of micro-deflectors or micro-lenses to influence the plurality of primary beamlets 211, 212, 213 of the primary electron beam 202 and form a plurality of parallel images (virtual or real) of the primary beam cross 203, one parallel image for each of the primary beamlets 211, 212, 213. The aberration compensator array may include a field curvature compensator array (not shown) and an astigmatism compensator (i.e., stigmatism) array (not shown). The field curvature compensator array may include a plurality of micro-lenses to compensate for the field curvature aberration of the primary beamlets 211, 212, 213. The astigmatism compensator array may include a plurality of micro-stigmatisms to compensate for the astigmatism aberration of the primary beamlets 211, 212, 213. The beam limiting aperture array may be configured to limit the diameter of each primary beamlet 211, 212, 213. Figure 2 As an example, three primary beamlets 211, 212, 213 are shown, and it should be understood that the source conversion unit 220 can be configured to form any number of primary beamlets. Alternatively, a single charged particle beam 202 can be provided, and the source conversion unit 220 can include a single image forming element, a single aberration compensator (including a single field curvature compensator and a single astigmatism compensator), a single beam limiting aperture and a single pre-bent micro-deflector. The controller 50 can be connected to Figure 1 The controller 50 may be used to control various parts of the charged particle beam system 5, such as the source conversion unit 220, the electronic detection device 240, the primary projection system 230 or the motorized platform 209. As explained in further detail below, the controller 50 may perform various image and signal processing functions. The controller 50 may also generate various control signals to manage the operation of the charged particle beam system 5.

[0035] The condenser lens 210 is configured to focus the primary charged particle beam 202. The condenser lens 210 can also be configured to adjust the current of the primary beamlets 211, 212, 213 downstream of the source conversion unit 220 by changing the focusing power of the condenser lens 210. Alternatively, the current can be changed by changing the radial size of the beam limiting apertures corresponding to the respective primary beamlets within the beam limiting aperture array. The current can be changed by changing both the radial size of the beam limiting apertures and the focusing power of the condenser lens 210. The condenser lens 210 can be a movable condenser lens, which can be configured so that the position of its first principal plane is movable. The movable condenser lens can be configured to be magnetic, which can cause the off-axis beamlets 212 and 213 to irradiate the source conversion unit 220 at a rotation angle. The rotation angle changes with the focusing power or the position of the first principal plane of the movable condenser lens. The condenser lens 210 can be an anti-rotation condenser lens, which can be configured to keep the rotation angle unchanged while changing the focusing power of the condenser lens 210. The condenser lens 210 may be a movable anti-rotation condenser lens, and when its focusing power and the position of its first principal plane change, its rotation angle remains unchanged.

[0036] The objective lens 231 can be configured to focus the sub-beams 211, 212, 213 onto the sample 208 for inspection, and three detection points 221, 222, 223 can be formed on the surface of the sample 208. Alternatively, in a single beam tool, the objective lens 231 can be configured to focus the primary charged particle beam 202 onto the sample 208 and a single detection point 221 can be formed on the surface of the sample 208. The aperture plate 271 is configured to block the peripheral electrons of the primary electron beam 202 to reduce the Coulomb effect in operation. The Coulomb effect may enlarge the size of each detection point 221, 222, 223 of the primary sub-beams 211, 212, 213, and thus degrade the inspection resolution. The aperture plate 271 may also be referred to as a Coulomb aperture array.

[0037] The beam splitter 233 may be, for example, a device that generates an electrostatic dipole field and a magnetic dipole field ( Figure 2 213 or the primary charged particle beam 202. In operation, the beam splitter 233 can be configured to exert an electrostatic force on each charged particle of the primary sub-beams 211, 212, 213 or the primary charged particle beam 202 through an electrostatic dipole field. The electrostatic force is equal in magnitude to the magnetic force exerted on each electron by the magnetic dipole field of the beam splitter 233 but opposite in direction. The primary sub-beams 211, 212, 213 or the primary charged particle beam 202 can therefore pass through the beam splitter 233 at least substantially straight with a deflection angle that is at least substantially zero.

[0038] The deflection scanning unit 232 is configured in operation to deflect the primary beamlets 211, 212, 213 to scan the detection points 221, 222, 223 across respective scanning areas in a portion of the surface of the sample 208. In response to the primary beamlets 211, 212, 213 or the detection points 221, 222, 223 incident on the sample 208, electrons are ejected from the sample 208 and three secondary electron beams 261, 262, 263 are generated. Each of the secondary electron beams 261, 262, 263 typically includes secondary electrons (having an electron energy ≤ 50 eV) and backscattered electrons (having an electron energy between 50 eV and the landing energy of the primary beamlets 211, 212, 213). The beam splitter 233 is configured to deflect the secondary electron beams 261, 262, 263 to the secondary projection system 250. The secondary projection system 250 then focuses the secondary electron beams 261, 262, 263 onto the detection elements 241, 242, 243 of the electron detection device 240. The detection elements 241, 242, 243 are arranged to detect the corresponding secondary electron beams 261, 262 and 263 and generate corresponding signals, which are sent to the controller 50 or a signal processing system (not shown), for example to construct an image of the corresponding scan area of ​​the sample 208.

[0039] The detection elements 241, 242, 243 can detect the corresponding secondary electron beams 261, 262, 263, respectively, and generate corresponding intensity signal outputs (not shown) to an image processing system (e.g., controller 50). Each detection element 241, 242 and 243 can include one or more pixels. The intensity signal output of the detection element can be the sum of the signals generated by all pixels within the detection element. In a single beam tool, a single detection element 241 can be provided.

[0040] The controller 50 may include an image processing system, which includes an image collector (not shown) and a storage device (not shown). The image collector may include one or more processors. For example, the image collector may include a computer, a server, a mainframe, a terminal, a personal computer, any type of mobile computing device, etc., or a combination thereof. The image collector may be communicatively coupled to the detection device 240 of the charged particle beam tool 40 by means of a medium such as an electrical conductor, a fiber optic cable, a portable storage medium, IR, Bluetooth, the Internet, a wireless network, radio, etc., or a combination thereof. The image collector may receive a signal from the detection device 240 and may construct an image. The image collector may thus collect an image of the sample 208. The image collector may also perform various post-processing functions, such as generating contours, superimposing indicators on the collected image, etc. The image collector may be configured to perform adjustments to the brightness and contrast of the collected image, etc. The storage device may be a storage medium such as a hard disk, a flash drive, a cloud storage, a random access memory (RAM), other types of computer-readable memory, etc. The storage device may be coupled to the image collector and may be used to save the scanned raw image data as raw images and post-processed images.

[0041] The image collector can collect one or more images of the sample based on the imaging signal received from the detection device 240. The imaging signal can correspond to a scanning operation for performing charged particle imaging. The collected image can be a single image including multiple imaging areas. The single image can be stored in a storage device. The single image can be an original image that can be divided into multiple areas. Each area can include an imaging area containing features of the sample 208. The collected image can include multiple images of a single imaging area of ​​the sample 208 sampled multiple times in a time series. Multiple images can be stored in a storage device. The controller 50 can be configured to perform image processing steps on multiple images of the same position of the sample 208.

[0042] The controller 50 may control the motorized stage 209 to move the sample 208 during inspection of the sample 208. The controller 50 may enable the motorized stage 209 to continuously move the sample 208 in one direction at a constant speed. The controller 50 may enable the motorized stage 209 to change the movement speed of the sample 208 over time according to the steps of the scanning process.

[0043] Although Figure 2 The charged particle beam tool 40 is shown using three primary electron beams, but the charged particle beam tool 40 may use two or more primary electron beams. The charged particle beam tool 40 may also use a single primary charged particle beam 202 instead of beamlets 211, 212, 213. The present disclosure does not limit the number of primary electron beams used in the charged particle beam tool 40.

[0044] A charged particle beam tool 40, in particular an electron beam tool 40, can create an image of a sample 208. The image can be an image created by the primary charged particle beam 202 of a single beam tool or an image created by one of the beamlets 211, 212, 213 of a multi-beam tool. The created image includes a contribution due to secondary electrons emitted by the sample 208 (referred to herein as the object) and a contribution due to the shape or profile (referred to herein as the detection profile) of the primary charged particle beam 202 or one of the beamlets 211, 212, 213 incident on the sample.

[0045] The image I in the z plane (i.e., the plane at distance z relative to the focal plane) z (r) can be represented as an object O(r) and a detected contour H z (;ζ) is the convolution in the z plane, that is,

[0046] I z (r)=O(r)*H z (r;ζ).

[0047] Detection profile H z (r; ζ) is the shape or profile of the primary charged particle beam 202 in the z plane (i.e., a plane orthogonal to the primary optical axis 204 and at a position z along the primary optical axis 204 relative to the focal plane of the objective lens 231) where the sample 208 or, in particular, the surface of the sample 208 is located. In the focal plane of the objective lens 231, z = 0. The detection profile H z (r; ζ) is related to the aberration ζ for a specific charged particle wavelength λ (which is related to the kinetic energy or landing energy of the charged particle) and the numerical aperture NA of the objective lens 231. Similarly, for optical devices, the detection profile H z (r; ζ) corresponds to the intensity of the point spread function (PSF).

[0048] In the spatial frequency domain,

[0049]

[0050] in is image I z The Fourier transform of (r), is the Fourier transform of the object O(r), and is the detected contour H z(r; ζ), and k is the spatial frequency component. Therefore, the Fourier transform of the image is the product of the Fourier transform of the object and the Fourier transform of the detected profile. By analogy, for optical devices, the Fourier transform of the detected profile can also be called the optical transfer function. In the following, the terms "Fourier transform of the object" and "object", "Fourier transform of the image" and "image", and "Fourier transform of the detected profile" and "detected profile" will be used interchangeably.

[0051] The wavefront of an ideal focused charged particle beam is a perfect sphere, centered at the focus in the focal plane. In reality, aberrations distort the detected profile, deviating from the ideal wavefront. As a result, the image I z The spatial resolution of image I (r) may be negatively affected, even when the sample 208 is in the focal plane of the charged particle beam tool 40. Therefore, it is necessary to determine and remove or reduce the image I z (r) is the aberration from image I z (r) determining and removing or reducing the detection contour caused by aberration on image I z (r) contribution.

[0052] Figure 3A method 100 of determining aberrations in an image acquired by a charged particle beam tool 40 according to one embodiment of the present invention is shown. The image may be an image created by the primary charged particle beam 202 of a single beam tool or an image created by one of the beamlets 211, 212, 213 of a multi-beam tool. The aberrations in each beamlet 211, 212, 213 of a multi-beam tool may be different. If the aberrations of each beamlet 211, 212, 213 are relatively similar, the aberrations determined for one of the beamlets 211, 212, 213 may be considered to correspond to the aberrations of the other beamlets 211, 212, 213. Alternatively, if the aberrations of each beamlet 211, 212, 213 are relatively different (e.g., in a multi-column charged particle beam tool), the method 100 may be performed separately for each beamlet 211, 212, 213 to determine the corresponding aberrations of each of the beamlets 211, 212, 213. Further alternatively, if the aberration of each beamlet 211, 212, 213 is or may be relatively different (e.g., in a multi-beam charged particle beam tool where the beamlets 211, 212, 213 share a common macro-electro-optical element, for example when a single beam is split into multiple beams), the method 100 may be performed for one of the beamlets 211, 212, 213, i.e., based on one or more images created by one of the beamlets 211, 212, 213, and taking into account the aberration determined for one of the beamlets 211, 212, 213, the method 100 may be performed again for one or more other beamlets 211, 212, 213. When the beamlets 211, 212, 213 share a common macro-electro-optical element, the aberrations in each beamlet 211, 212, 213 due to the common macro-electro-optical element may be similar. However, considering the aberration determined for one of the beamlets 211, 212, 213 when determining the aberrations of the other beamlets 211, 212, 213 may also be used when the beamlets 211, 212, 213 do not share a common macro electro-optical element. This is true even if the aberrations in the beamlets 211, 212, 213 can be considered independent, such as when these independent aberrations are still relatively similar (e.g., because the respective electro-optical elements operate relatively similarly). Considering the aberration determined for one of the beamlets 211, 212, 213 when determining the aberrations of the other beamlets 211, 212, 213 will be explained in more detail below.

[0053] The method 100 includes a step 110 of acquiring two or more images of a sample 208 using a charged particle beam tool 40. The two or more images are images of the same sample 208. Each image is acquired with a known relative difference in the measurement conditions of the charged particle beam tool 40. In other words, a known phase diversity can be introduced between the two or more images. The method 100 may include acquiring an image of the sample 208 at a first value of the measurement condition, adjusting the first value to a second value of the measurement condition by a known difference in the value of the measurement condition (i.e., in a known manner), and acquiring an image of the sample 208 at the second value of the measurement condition. Optionally, the first and second values ​​of the measurement condition may also be known. The method 100 may also include adjusting the value of the second measurement condition to other values ​​of the measurement condition by one or more known relative differences, and acquiring an image of the sample 208 at each other value of the measurement condition. Optionally, the other values ​​of the measurement condition may also be known.

[0054] The measurement conditions of the charged particle beam tool 40 may include the position of the sample 208 along the primary optical axis 204 relative to the focal plane of the objective lens 231, in particular the position of the surface of the sample 208. Changing the position of the sample 208 relative to the focal plane of the objective lens 231 is a way to introduce a known phase diversity between two or more images. The position of the sample 208 along the primary optical axis 204 relative to the focal plane of the objective lens 231 can be adjusted by adjusting the focusing amount of the objective lens 231 and thus moving the focal plane 231 of the objective lens. To this end, the charged particle beam tool 40 may include the objective lens 231. Adjusting the focusing amount of the objective lens 231 may affect the numerical aperture NA of the charged particle tool 40. However, the influence of different numerical apertures on the image is negligible compared to the influence of different positions of the focal plane on the image.

[0055] Alternatively or additionally, the position of the sample 208 along the primary optical axis 204 relative to the focal plane of the objective lens 231 can be adjusted by moving the sample 208 in a direction along the primary optical axis 204. To this end, the charged particle beam tool 40 may include a motorized stage 209 for adjusting the position of the sample 208 in a direction along the primary optical axis 204. Moving the sample 208 does not affect the numerical aperture NA of the charged particle tool 40.

[0056] Alternatively or additionally, the measurement condition of the charged particle beam tool 40 may include an amount of beam shaping applied by the charged particle beam tool 40 to the primary charged particle beam 202. The amount of beam shaping may, for example, include an amount of aberration correction, such as an amount of field curvature compensation applied to the charged particle beam 202 and / or an amount of astigmatism compensation applied to the charged particle beam 202. To this end, the charged particle beam tool 40 may include an aberration compensator or any other beam shaping element. The aberration compensator may include a field curvature compensator and / or an astigmatism compensator. In one embodiment, the measurement condition of the charged particle beam tool 40 is the amount of astigmatism applied to the charged particle beam 202. Changing the amount of beam shaping or specifically changing the amount of astigmatism is an alternative way to introduce a known phase diversity in a controlled manner between two or more images. For example, this may be advantageous in cases where it is difficult to know the exact position of the sample 208 along the primary optical axis 204 relative to the focal plane of the objective lens 231, for example, if the exact position of the focal plane is difficult to determine. The amount of astigmatism can be adjusted using an astigmatism compensator or stigmator of the charged particle beam tool 40. In one embodiment, the amount of beam shaping or in particular the amount of astigmatism can be changed when the sample 208 is intentionally out of focus, i.e. when the sample 208 is not in the focal plane of the objective lens 231. This can amplify the effect of changing the amount of beam shaping or the amount of astigmatism.

[0057] Alternatively or additionally, the measurement conditions of the charged particle beam tool 40 may include an average landing energy of the charged particles used in the charged particle beam tool 40. The average landing energy of the charged particles may be adjusted, for example, by adjusting an operating condition of the charged particle source 201, such as a voltage applied to an extractor and / or an anode of the charged particle source 201. To this end, the charged particle beam tool 40 may include the charged particle source 201. Additionally or alternatively, the average landing energy of the charged particles may be adjusted by decelerating the charged particles between the objective lens 231 and the sample 208, for example by creating a suitable electric field between the objective lens 231 and the sample 208. To this end, the charged particle beam tool 40 may include the objective lens 231.

[0058] The measurement conditions may also include any combination of the measurement conditions provided above.

[0059] The method 100 further comprises a step 120 of selecting an estimated aberration parameter ζ for the aberration representing the detected profile of the primary charged particle beam 202. The estimated aberration parameter ζ may comprise one or more estimated aberration values ​​(ie, ζ=[ζ 1 ,ζ 2 ,…]), for example, one estimated aberration value for each Zernike polynomial is used to interpret the aberration in the image. The estimated aberration parameter ζ may be a vector comprising one or more estimated aberration values ​​(ζ = [ζ 1 ,ζ 2,…]), preferably 15 or more estimated aberration values, more preferably 37 or more estimated aberration values, or consisting of them. Selecting the estimated aberration parameter z may include selecting each estimated aberration value included by the estimated aberration parameter z. Selecting the estimated aberration parameter z may include selecting or setting the estimated aberration parameter z as an initial guess or estimate of the actual aberration parameter. The initial guess or estimate can be predetermined. For example, the initially estimated aberration values ​​can all be 0. Alternatively, for example when method 100 is last performed 100, the initial guess or estimate can be based on the estimated aberration parameter z previously determined for the charged particle beam tool 40. Further alternatively, the operator of the charged particle beam tool 40 can freely select the initial guess or estimate.

[0060] The method 100 also includes a step 130 of evaluating an error function. The error function indicates the difference between the two or more images and the two or more estimated images. The two or more estimated images are based on the known relative difference in the estimated aberration parameter ζ and the measurement conditions. The two or more estimated images can be based on the known values ​​of the estimated aberration parameter ζ and the measurement conditions. The two or more estimated images can also be based on the settings of the charged particle beam tool 40, such as the numerical aperture NA of the objective lens 231 and the wavelength λ of the charged particles. The settings of the charged particle beam tool 40 can be at least roughly known.

[0061] The step 130 of evaluating the error function may include calculating an estimated detection profile for each image based on the estimated aberration parameter ζ and the known relative difference in the measurement conditions. The estimated detection profile may be based on the known values ​​of the estimated aberration parameter ζ and the measurement conditions. The estimated detection profile may be further based on the settings of the charged particle beam tool 40. Step S130 may further include, for each of the two or more images, calculating the difference between the image and the estimated image and summing the calculated differences, the estimated image being based on the corresponding estimated detection profile.

[0062] For example, two or more images can be represented in the spatial frequency domain as a system of linear equations:

[0063]

[0064]

[0065]

[0066] in is the corresponding image, is an object, is the corresponding detected contour. The measurement conditions are different due to the known relative differences of each image. Represents sample 208 pairs of images The contribution of is therefore the same in each linear equation.

[0067] Can be targeted To solve the linear equations to provide Solution:

[0068]

[0069] in express The complex conjugate of .

[0070] The error function L(ζ) indicates that two or more images The error function L(ζ) can be expressed as the difference between two or more estimated images, which are functions of the estimated aberration parameters and the known relative differences in the measurement conditions (and optionally the known values ​​of the measurement conditions):

[0071]

[0072] in is a corresponding one of the two or more images in a plane at a distance z relative to the focal plane, and the corresponding estimated image is calculated as in is the corresponding estimated detection contour. Insert The solution and reordering of the terms give the error function L(ζ):

[0073]

[0074] The method 100 may further include a step 130' of calculating a gradient L'(ζ) of the error function with respect to the aberration parameter ζ based on the estimated aberration parameter ζ (wherein the aberration parameter ζ may be a vector ζ=[ζ 1 ,ζ 2 ,…]). Step S130' may be performed after step S130 of evaluating the error function L(ζ), before step S130 of evaluating the error function L(ζ), or in parallel with step S130 of evaluating the error function L(ζ). The gradient L'(ζ) of the error function with respect to the aberration parameter ζ may be calculated as:

[0075]

[0076] in represents the real part of the term in brackets, express The gradient with respect to the aberration parameter ζ.

[0077] The gradient L'(ζ) of the error function may indicate a difference between two or more images and two or more estimated images, the two or more estimated images being based on the estimated aberration parameters and a known relative difference in the measurement conditions (optionally, a known value of the measurement conditions). The gradient L'(ζ) may also provide information for updating the estimated aberration parameters. The estimated aberration parameters may be updated until the value of the error function is sufficiently small and / or the gradient L'(ζ) is so small that the value of the error function cannot be further reduced. Therefore, the step 130 of evaluating the error function may include evaluating the gradient L'(ζ) of the error function.

[0078] Detecting contours The aberration parameter ζ may be calculated based on the estimated aberration parameter ζ and the known relative difference in the measurement conditions (or a known value based on the measurement conditions). The settings of the charged particle tool 40, such as the numerical aperture NA of the charged particle tool 40 and the average wavelength of the charged particle tool 40, may also be used in calculating the detection profile. For example, detecting contours Contributions due to aberrations (as represented by the aberration parameter ζ) and contributions due to the focal plane z (z=0) of the sample 208 relative to the objective lens 231 may be included.

[0079] Because the contribution of the aberration can be expressed according to the pupil function P(k). The pupil function P(k) indicates the amplitude and phase of the charged particle beam in the plane of the objective lens 231 (i.e., in the pupil plane). In an ideal case (i.e., if there is no aberration), the pupil function P(k) has a uniform amplitude and phase. In practice, the aberration is expressed as a deviation from the uniform amplitude and phase.

[0080] Aberrations may result in deviations from uniform amplitude and uniform phase. Aberrations can be represented using Zernike polynomials. Each Zernike polynomial corresponds to a specific type of aberration. The pupil function P(k) can therefore be represented as:

[0081] P(k)=[∑ i ζ i Z i (k)]exp{i2π[∑ j ζ j Z j (k)]}

[0082] Where Z i,j (k) is the Zernike polynomial, ζ i,j is the corresponding weight (i.e., the corresponding aberration value of the aberration parameter ζ), i and j represent the amplitude and phase, respectively. j The set of is used to represent the aberration of the charged particle beam tool 40. Optionally, it is also possible to consider ζ jIn one embodiment, at least the first 15 Zernike polynomials are considered, and the aberration parameter is a set of at least 15 aberration values ​​ζ=ζ 1 ,ζ 2 ,…,ζ 15 In another embodiment, at least the first 37 Zernike polynomials are considered, and the aberration parameter is a vector including at least 37 aberration values ​​ζ=ζ 1 ,ζ 2 ,…,ζ 37 Vector.

[0083] The contribution due to the position z of the sample 208 relative to the focal plane (z=0) of the objective lens 231 can be determined by the defocus term, in particular, by the defocus term exp(iπz|k| 2 ). For example, the detected contour at position z in the image plane can be calculated as:

[0084] H z (r)=|FT{P(k)exp(iπz|k| 2 )}(r)| 2

[0085] where FT represents Fourier transform, and If the position z of the sample 208 relative to the focal plane (z=0) of the objective lens 231 is constant (e.g., if the measurement conditions include the amount of astigmatism applied to the charged particle beam, and the amount of defocus is kept constant), the defocus term exp(iπz|k| 2 ). As described above, when the measurement conditions include an amount of astigmatism applied to the charged particle beam, the sample 208 may be intentionally out of focus to amplify the effect of varying the amount of astigmatism. Similar to an optical device, the propagation of a charged particle (e.g., electron) from the pupil (i.e., the plane of the objective lens 231) to the image plane is described using a Fourier transform FT, where r and k are the coordinates of the pupil and image plane, respectively, P(k) is the pupil function of the charged particle beam, and exp(iπz|k| 2 ) represents the defocus term (equal to 1 at focal plane z=0). Here, r is normalized by the factor λ / NA, where λ is the wavelength of the charged particle and NA is the numerical aperture (semi-focusing angle) of the objective lens 231. The factor λ / NA determines the scaling of the detection profile size. The wavelength λ of the charged particle is related to the kinetic energy (or landing energy) of the charged particle. For example, for electrons, the wavelength can be calculated as in is Planck's constant, 9.1×10 -31 kg is the rest mass of the electron, 1.6×10 -19 C is the electron elementary charge and E is the kinetic energy. The numerical aperture NA and the wavelength λ may be settings of the charged particle beam tool 40 and may be at least approximately known to an operator of the charged particle beam tool 40 .

[0086] Optionally, it is possible to consider the detection profile H z (r) Additional and / or Other Contributions.

[0087] For example, in the charged particle beam tool 40, the charged particles emitted by the charged particle source 201 may have a kinetic energy distribution. This kinetic energy distribution induces a defocusing effect, because charged particles of different speeds will be focused at different positions along the primary optical axis 204. As a result, the entire detection profile will be a weighted sum of the detection profiles of the charged particles of each kinetic energy. The induced defocus term can be expressed, for example, as:

[0088]

[0089] Where E is the nominal or mean kinetic energy, ΔE is the deviation from E, and C c is the color difference coefficient. Therefore, the total detection profile can be calculated as:

[0090]

[0091] in represents the Gaussian distribution of ΔE, σ E is the variance of ΔE. The distribution of ΔE may be related to the exact nature of the charged particle source 201, and the Gaussian distribution used above is just one example of a possible distribution.

[0092] If the charged particle source 201 is not exactly an infinitesimal point source, the detection profile H z The formula for (r) can be further modified as follows:

[0093]

[0094] Where * is the convolution operator, represents the Gaussian distribution of source intensity, σ s is the variance of the source intensity. The distribution of the source intensity is related to the exact nature of the charged particle source 201, and the Gaussian distribution used above is just one example of a possible distribution. Alternatively, one can add the defocus term exp(iπz|k| 2 ) to account for the non-zero virtual source size. As described below, this can enable the determination of the virtual source size (e.g., in addition to the aberration coefficients and source intensity variance).

[0095] For example, when the stigmator value is deliberately adjusted to adjust the measurement conditions, the detection profile H z The formula for (r) can be further modified to take into account the change in the stigmator value. This can be done by adding the term exp(i2πΔζ j Z j (k)) where Z j(k) is the Zernike polynomial describing the astigmatism caused by the astigmatism filter value, Δζ j is the relative change of the corresponding aberration value. For the Zernike polynomials, j=5 and j=6 are two types of astigmatism: along the xy axis or at a 45 degree angle relative to the xy axis. However, any other type of astigmatism (and corresponding Zernike polynomial) that can be adjusted by the astigmatism compensator of the charged particle tool can be used in adjusting the detection profile H z (r) is considered when formulating.

[0096] The method 100 includes a step 160 of updating the estimated aberration parameters. In one embodiment, the estimated aberration parameters are updated based on the gradient of the error function. The gradient of the error function may provide an indication of whether to increase or decrease the estimated aberration parameters to decrease the value of the error function. The gradient of the error function may provide an indication of how much to increase or decrease the estimated aberration parameters. The gradient of the error function may be used to update the estimated aberration parameters so that the value of the error function is reduced compared to the value of the error function based on the previously estimated aberration parameters.

[0097] The method 100 comprises iteratively performing a step 130 of evaluating an error function, optionally a step 130' of calculating a gradient of the error function and a step 160 of updating the estimated aberration parameters. In other words, steps 130 and 160 and optionally step 130' are repeatedly performed. The number of iterations may be predetermined. The method 100 may comprise determining the final aberration parameters as the estimated aberration parameters that provide a minimum value of the error function.

[0098] In one embodiment, the method 100 comprises iteratively performing a step 130 of evaluating the error function, a step 130' of optionally calculating the gradient of the error function, and a step 160 of updating the estimated aberration parameters until the error value function is below a threshold, in particular a predetermined threshold. The method 100 may comprise iteratively performing the steps of comparing the value of the error function with a predetermined threshold, updating the estimated aberration parameters when the value of the error function is greater than the predetermined threshold, and evaluating the error function based on the updated estimated aberration parameters.

[0099] Additionally, the method 100 may include iteratively performing the step 130 of evaluating the error function, the step 130' of calculating the gradient of the error function, and the step 160 of updating the estimated aberration parameters until the gradient of the error function is below a second threshold, in particular a second predetermined threshold. The method 100 may include iteratively performing the steps of comparing the gradient of the error function with the second predetermined threshold, updating the estimated aberration parameters when the gradient of the error function is greater than the second predetermined threshold, and calculating the gradient of the error function based on the updated estimated aberration parameters.

[0100] The estimated aberration parameters are thus iteratively updated until the value of the error function and optionally the gradient of the error function are equal to or below a respective threshold value. The method 100 comprises a step 170 of determining final aberration parameters providing an error function value below a threshold value. The final aberration parameters may be set equal to the estimated aberration parameters providing an error function value or an error function gradient below a threshold value. Thus, evaluating the error function based on the final aberration parameters may provide a minimum or substantially minimum value of the error function. The final aberration parameters may thus be a fairly accurate representation of the actual aberrations in the two or more images.

[0101] The method 100 may include a step 180 of removing or reducing the contribution of aberrations from at least one image of the sample 208 acquired by the charged particle beam tool 40. The step 180 of removing or reducing the contribution of aberrations from at least one image of the sample 208 acquired by the charged particle beam tool 40 may occur inherently when performing the method 100, i.e., performing the method 100 may automatically generate an image of the sample 208 without or with reduced aberrations without the need for the final step 180 of removing or reducing the contribution of aberrations to be performed separately. The contribution of aberrations may be removed or reduced based on the final aberration parameters. The at least one image may be at least one of the two or more images acquired by the charged particle beam tool 40 in step 110. For example, considering I z (r)=O(r)*H z (r; ζ), from at least one image I z Removing or reducing the contribution of the aberration in (r) may include performing at least one image I z (r) and the detection profile H calculated based on the final aberration parameters z (r; ζ) deconvolution, so that from at least one image I z (r) to extract the object O(r).

[0102] Figure 4 A method 300 for determining the settings of a charged particle beam tool 40, in particular the actual settings, according to one embodiment of the present invention is shown. The settings of the charged particle beam tool 40 may be the numerical aperture NA of the objective lens 231. Alternatively, the settings of the charged particle beam tool 40 may be the nominal wavelength λ of the charged particles used in the charged particle beam tool 40. The nominal wavelength λ of the charged particles may be the average value of the mean wavelength λ of the charged particles. Alternatively, the settings of the charged particle beam tool 40 may be the virtual source size of the charged particle beam tool 40. The settings of the charged particle beam tool 40 may be any other settings or parameters that have an impact on the detection profile of the primary charged particle beam 202.

[0103] In some cases, the setting of charged particle beam tool 40 may be only roughly known. For example, in some cases, the numerical aperture NA of object lens 231 or the wavelength λ of charged particles may be only roughly known. The setting of charged particle beam tool 40 affects the detection profile of primary charged particle beam 202. Therefore, any inaccuracy in the setting of charged particle beam tool 40 may affect the error function. In the method 100 for determining aberrations, the approximate estimate of the setting of charged particle beam tool 40 may have an impact on the value of the error function. Specifically, when evaluating based on final aberration parameters, the residual of the error function will be affected for the approximate estimate of the setting. This residual can be considered to determine the actual setting of charged particle beam tool 40.

[0104] Method 300 may include a step 310 of selecting two or more estimates for the settings of charged particle beam tool 40. The two or more estimates may include two or more estimates at regular intervals within the estimated range of the settings. For example, an operator of charged particle beam tool 40 may know that the numerical aperture NA of charged particle beam tool 40 is in the range between 0mrad and 30mrad, but may not know the exact numerical aperture NA. The two or more estimates may include all numerical apertures in the range of 1mrad intervals.

[0105] The method 300 may further include performing step 320 of the method 100 of determining aberrations in the image for each of the selected two or more estimates of the settings. Each time the method 100 is performed, the error function may be evaluated based on a different estimate of the two or more estimates for the settings. For each of the two or more estimates of the settings, a corresponding final aberration parameter may be determined.

[0106] The method 300 may also include a step 330 of determining a residual for each estimate of the setting. The residual may be determined by evaluating an error function based on each estimate for the setting and a corresponding final aberration parameter determined for the corresponding setting. If the estimate for the setting is relatively accurate (i.e., close to the actual value of the setting), the residual is small; if the estimate for the setting is relatively inaccurate (i.e., far from the actual value of the setting), the residual is large.

[0107] The method 300 may also include step 340 of determining the settings that produce the minimum residual as the settings of the charged particle beam tool 40. This can be achieved by comparing the residuals of each corresponding estimate of the settings and identifying the minimum residual and the associated settings estimate. Optionally, determining the settings that produce the minimum residual can include plotting a curve of the determined residuals and identifying the settings corresponding to the minimum of the curve. The curve can be fitted to the determined residuals. The curve can be fitted to a polynomial curve, a spline curve, a Gaussian curve, or any other curve. The minimum residual can be determined from the fitted curve. This can reduce the number of different settings of the charged particle beam tool 40 that need to be considered to reach an accurate estimate of the actual settings.

[0108] The method 300 may include performing the method 100 using the settings of the charged particle beam tool 40 determined by step 340 of the method 300 to determine final aberration parameters, ie final aberration parameters that are considered to be closest to the actual aberration parameters.

[0109] Alternatively, the method 100 can be used to determine the settings of the charged particle beam tool 40 in a manner similar to determining the final aberration value. For this purpose, the estimated aberration parameters can (in addition to one or more estimated aberration values) include setting values ​​representing the settings of the charged particle beam tool 40. The setting value can be, for example, a numerical aperture NA or a nominal wavelength λ of the charged particles. Alternatively, the setting value can represent a virtual source size of the charged particle beam tool 40, such as a Gaussian distribution of source intensity The source intensity variance σ in s 2 Or the defocus term exp(iπz|k| 2 ) is the size of the imaginary component of the virtual source (related to the representation of the virtual source size). When executing method 100, the setting value can be iteratively updated in the same manner as other estimated aberration values. The final setting value can be determined as the estimated setting value that corresponds to the minimum value that provides the error function.

[0110] The inventors have appreciated that in some applications, such as in a multi-beam charged particle beam tool in which the beamlets 211, 212, 213 share a common macro-electro-optical element, the aberrations in an image acquired using a first beamlet 211, 212, 213 are not independent of the aberrations in an image acquired using one or more other beamlets 211, 212, 213. The common macro-electro-optical element may be a common charged particle beam source, a common focusing lens or collimating lens, a common objective lens, or any other common element used to manipulate two or more beamlets 211, 212, 213. This allows information related to the aberration determined for the first beamlet 211, 212, 213 to be used to help determine the aberrations for one or more other beamlets 211, 212, 213. When the beamlets 211, 212, 213 do not share a common macro-electro-optical component, the aberrations determined for the first beamlet 211, 212, 213 may also be used to help determine the aberrations for one or more other beamlets 211, 212, 213. Even when the aberrations of the sub-beams 211, 212, 213 are independent, the aberrations of each sub-beam 211, 212, 213 may be relatively similar (for example, when the individual electro-optical elements operate relatively similarly), thereby allowing the aberration determined for the first sub-beam 211, 212, 213 to help determine the aberrations of one or more other sub-beams 211, 212, 213.

[0111] In one embodiment, a method of determining aberrations in an image acquired by a multi-beam charged particle beam tool is provided. The multi-beam charged particle beam tool may be configured to optionally receive a common charged particle beam source, such as a reference beam source. Figure 2 A plurality of beamlets 211, 212, 213 are generated in the manner described. The method comprises performing method 100 for a first beamlet of a multi-beam charged particle tool. This comprises acquiring two or more images of a sample using the first beamlet of the charged particle beam tool. Each image is acquired with a known relative difference in the measurement conditions of the first beamlet of the charged particle beam tool. An estimated aberration parameter of the first beamlet is then selected for an aberration representing a detection profile of the first beamlet used by the charged particle beam tool. An error function indicating the difference between the two or more images and the two or more estimated images is then evaluated, the two or more estimated images being updated according to the estimated aberration parameter and the known relative difference in the measurement conditions, and the estimated aberration parameter. This is performed iteratively. The final aberration parameter for the first beamlet is selected as the estimated aberration parameter of the first beamlet that provides a minimum value of the error function.

[0112] The estimated aberration parameters of the first beamlet may include a plurality of estimated aberration values ​​of the first beamlet. Determining the final aberration parameters thus includes determining a plurality of final aberration values ​​of the first beamlet. The estimated aberration parameters and the final aberration parameters may represent a set of Zernike polynomials. Each estimated aberration value and each final aberration value represent a weight of a corresponding Zernike polynomial.

[0113] The final aberration value for the first beamlet 211, 212, 213 may then be used to help determine the final aberration value for each of the one or more other beamlets 211, 212, 213. This includes performing the method 100 for one or more other beamlets 211, 212, 213 of the multi-beam charged particle tool 40. The step of selecting the estimated aberration parameters may include selecting the one or more final aberration values ​​determined for the first beamlet 211, 212, 213 as the corresponding one or more estimated aberration values ​​for each of the one or more other beamlets 211, 212, 213. In other words, at least some of the estimated aberration values ​​for one or more other beamlets are set to the corresponding final aberration values ​​determined for the first beamlet. Optionally, all of the estimated aberration values ​​for one or more other beamlets are set to the final aberration value determined for the first beamlet (at least in the first iteration of the method 100). Alternatively, at least some of the estimated aberration values ​​may not be set to the corresponding final aberration values ​​determined for the first beamlet 211, 212, 213.

[0114] The inventors have found that, in particular, high-order aberration values ​​(i.e., aberration values ​​representing weights of relatively high-order Zernike polynomials) are the same or relatively similar between the multiple beamlets 211, 212, 213 of the multi-beam charged particle beam tool 40. Low-order aberration values ​​(i.e., aberration values ​​representing weights of relatively low-order Zernike polynomials) may be relatively different between the multiple beamlets 211, 212, 213 of the multi-beam charged particle beam tool. Therefore, the step of selecting estimated aberration parameters may include selecting one or more higher-order final aberration values ​​(e.g., at least the nth highest-order final aberration value, where n is 1, 3, 6, 10, 15 or 21, or where n is 5%, 10%, 25%, 50%, 75% or 90% of the total number of final aberration values) determined for the first beamlet 211, 212, 213 as the corresponding one or more high-order estimated aberration values ​​for each one or more other beamlets 211, 212, 213. Optionally, the step of selecting estimated aberration parameters may include not selecting one or more lower order final aberration values ​​(e.g., at least the m lowest order final aberration values, where m is 1, 3, 6, 10, 15 or 21, or where m is 5%, 10%, 25%, 50%, 75% or 90% of the total number of final aberration values) determined for the first sub-beam 211, 212, 213 as the corresponding one or more low-order estimated aberration values ​​for each one or more other sub-beams 211, 212, 213.

[0115] For multiple sub-beams 211, 212, 213 of the multi-beam charged particle beam tool 40, it can be assumed that the high-order estimated aberration values ​​are the same or relatively similar. Therefore, the high-order estimated aberration values ​​determined for the first sub-beam 211, 212, 213 can be assumed to be correct for one or more other sub-beams 211, 212, 213. The step of updating the estimated aberration parameters for each one or more other sub-beams 211, 212, 213 may include updating the estimated aberration values ​​that do not correspond to the one or more estimated aberration values ​​selected to correspond to the one or more final aberration values ​​determined for the first sub-beam. The step of updating the estimated aberration parameters may include not updating the estimated aberration values ​​corresponding to the one or more estimated aberration values ​​selected to correspond to the one or more final aberration values ​​determined for the first sub-beam. For example, the step of updating the estimated aberration parameters for each one or more other beamlets 211, 212, 213 may include updating one or more low-order estimated aberration values ​​(e.g., at least the lowest order final aberration value, or at least n lowest order final aberration values, where n is greater than 10% or 25% or 50% or 75% or 90% of the total number of final aberration values). The step of updating the estimated aberration parameters for each one or more other beamlets 211, 212, 213 may include not updating one or more higher-order estimated aberration values ​​(e.g., at least the highest order final aberration value, or at least n highest order final aberration values, where n is greater than 10% or 25% or 50% of the total number of final aberration values ​​or where n is 3, 6, 10, 15 or 21). Optionally, n+m may be equal to the total number of final aberration values.

[0116] In one embodiment, the charged particle beam tool 40 includes an irradiation system and a detection system. The irradiation system is configured to generate a charged particle beam and scan the charged particle beam across the sample 208. In particular (but not exclusively) when the measurement condition is the amount of astigmatism, the irradiation system may include an astigmatism compensator or an stigmatizer. The detection system is configured to capture charged particles interacting with the sample, thereby creating an image of the sample. The charged particle tool 40 includes a controller 50. The controller 50 is configured to perform the method 100 and / or the method 300. The charged particle beam tool 40 may be an electron beam tool.

[0117] In one embodiment, a computer program product is provided. The computer program product includes instructions, and when the program is executed by a computer, the instructions cause the computer to perform the method 100 and / or the method 300.

[0118] In one embodiment, a computer-readable storage medium is provided. The computer-readable storage medium includes instructions, which, when executed by a computer, cause the computer to perform method 100 and / or method 300 .

[0119] Figure 5 and Figure 6Experimental results obtained by performing method 100 to determine aberrations in an image captured by a SEM, and experimental results obtained by performing method 300 to determine the numerical aperture NA of the SEM are shown. The stigmator value (indicative of the astigmatism of the electron beam of the SEM) was deliberately varied to induce different amounts of aberrations in the images acquired by the SEM. For each stigmator value, three images were measured: one in the focal plane and two in the defocused plane (±400 nm). Method 100 was initially performed based on an estimate of the numerical aperture NA of 15 mrad, and the final aberration parameters were determined for each stigmator value. As Figure 5 As shown by the dashed line in C, for NA=15 mrad, a linear relationship between the determined final aberration parameters and the stigmator value can be observed.

[0120] Method 300 is then performed by selecting estimates of the numerical aperture NA in the range from 4 mrad to 24 mrad in 2 mrad steps, and performing method 100 based on each of these selected numerical aperture NA estimates. Residuals are determined for each combination of stigmator value (i.e., aberration level) and numerical aperture NA estimate. The results are plotted at Figure 5 In a, Figure 5 a shows the residuals versus the NA estimate for each stigmator value. In addition, for different stigmator values, different NA estimates are used to obtain the minimum residuals. This is also shown in Figure 5 b shows the numerical aperture NA estimate (i.e., corresponding to the actual numerical aperture NA) determined. Figure 5 The numerical aperture NA estimate of the minimum residual in a) is plotted against the stigmator value. Figure 5 b It can be clearly seen that the actual numerical aperture NA of the SEM changes as the stigmator value is adjusted. Figure 5 The dashed line in c shows the relationship between the final aberration parameters and the stigmator values ​​for the calibrated numerical aperture NA, i.e., when using the numerical aperture NA determined by method 300. It is apparent that the correlation between the determined final aberration parameters and the stigmator values ​​is improved compared to the case where the numerical aperture NA estimate is fixed to 15 mrad.

[0121] Figure 6 SEM images acquired in the focal plane of the SEM for different stigmator values ​​are shown, along with images restored based on the final aberration parameters determined by method 100 and based on the calibrated numerical aperture NA determined by method 300 for each stigmator value. Figure 6 The calculated detection profile and wavefront error at each stigmator value are also shown.

[0122] The embodiments may be further described using the following terms:

[0123] 1. A method for determining aberrations in an image acquired by a charged particle beam tool, the method comprising the following steps:

[0124] a) acquiring two or more images of the sample using a charged particle beam tool, wherein each image is acquired with a known relative difference in measurement conditions of the charged particle beam tool;

[0125] b) selecting estimated aberration parameters for the aberration of a detection profile representative of the charged particle beam used by the charged particle beam tool;

[0126] c) evaluating an error function indicative of a difference between the two or more images and two or more estimated images based on the estimated aberration parameters and known relative differences in the measurement conditions;

[0127] d) updating the estimated aberration parameters;

[0128] e) iteratively performing steps c) and d); and

[0129] f) determining final aberration parameters as the estimated aberration parameters that provide a minimum value of the error function.

[0130] 2. The method according to clause 1, wherein step e) comprises iteratively performing steps c) and d) until the value of the error function is below a threshold value.

[0131] 3. A method according to clause 1 or 2, wherein updating the estimated aberration parameters comprises calculating a gradient of an error function based on the estimated aberration parameters, and updating the estimated aberration parameters based on the gradient of the error function.

[0132] 4. The method according to clause 3, wherein step e) comprises iteratively performing steps c) and d) until the gradient of the error function is below a second threshold.

[0133] 5. A method according to any of the preceding clauses, wherein evaluating the error function comprises:

[0134] calculating an estimated detection profile for each image based on the estimated aberration parameters and the known relative differences in measurement conditions;

[0135] for each of the two or more images, calculating a difference between the image and an estimated image, the estimated image being estimated based on the corresponding estimated detection contour; and

[0136] The calculated differences are summed.

[0137] 6. A method according to any of the preceding clauses, wherein the error function L(ζ) is

[0138]

[0139] Each of these are two or more image corresponding images in a plane at a distance z relative to the focal plane, and each is the estimated detection profile of the corresponding image, where the estimated detection profile is calculated based on the estimated aberration parameter ζ and the known relative differences in the measurement conditions.

[0140] 7. A method according to clauses 3 and 6, wherein the gradient of the error function is

[0141]

[0142] in represents the real part of the term in brackets, and express The gradient with respect to the aberration parameter ζ.

[0143] 8. A method according to any of the preceding clauses, wherein the measurement conditions of the charged particle beam tool are one of the following:

[0144] a position of the sample along a primary optical axis of the charged particle beam tool relative to a focal plane of the charged particle beam tool;

[0145] the amount of beam shaping applied to the charged particle beam;

[0146] the average landing energy of the charged particles in the charged particle beam; or

[0147] Any combination thereof.

[0148] 9. The method according to any of the preceding clauses, further comprising removing or reducing aberrations in at least one image of the sample acquired by the charged particle beam tool based on the final aberration parameters.

[0149] 10. A method of determining settings for a charged particle beam tool, the method comprising:

[0150] performing a method according to any of the preceding clauses two or more times, each time based on a different estimate of the settings of the charged particle beam tool;

[0151] Determining for each estimate of the setting by determining a final aberration parameter based on a corresponding estimate for the setting; and determining the setting estimate that produces the smallest residual as the setting for the charged particle beam tool.

[0152] 11. The method according to clause 10, wherein the setting of the charged particle beam tool is the numerical aperture of an objective of the charged particle beam tool and / or the nominal wavelength of the charged particles used in the charged particle beam tool.

[0153] 12. A method according to clause 10 or 11, comprising determining final aberration parameters by performing a method according to any of clauses 1 to 8 using the determined settings of the charged particle beam tool.

[0154] 13. The method according to any of the preceding clauses, wherein the charged particle beam tool is an electron beam tool, and wherein the charged particle beam is an electron beam.

[0155] 14. A charged particle beam tool comprising:

[0156] an irradiation system configured to generate a charged particle beam and scan the charged particle beam across the sample;

[0157] a detection system configured to capture charged particles that interact with the sample, thereby creating an image of the sample; and

[0158] A controller configured to perform a method according to any of the preceding clauses.

[0159] 15. The charged particle beam tool according to clause 14, wherein the charged particle beam tool is an electron beam tool.

[0160] 16. A computer program product comprising instructions which, when the program is executed by a computer, cause the computer to perform the method according to any one of clauses 1 to 13.

[0161] 17. A computer-readable storage medium comprising instructions which, when executed by a computer, cause the computer to perform the method according to any one of clauses 1 to 13.

[0162] 18. A method according to any of clauses 1 to 13, wherein the measurement condition of the charged particle beam tool is the amount of astigmatism applied to the charged particle beam.

[0163] 19. A charged particle beam tool comprising

[0164] an illumination system configured to generate a charged particle beam and scan the charged particle beam across the sample, the illumination system including an astigmatism compensator configured to adjust an amount of astigmatism applied to the charged particle beam;

[0165] a detection system configured to capture charged particles that interact with the sample, thereby creating an image of the sample, and

[0166] A controller configured to perform the method according to clause 18.

[0167] 20. A method of determining aberrations in an image acquired by a multi-beam charged particle beam tool, the method comprising:

[0168] performing, for a first beamlet of a multi-beam charged particle beam tool, a method according to any of clauses 1 to 13 or 18, wherein the estimated aberration parameters comprise a plurality of estimated aberration values, thereby determining, for the first beamlet, final aberration parameters comprising a plurality of final aberration values;

[0169] performing, for each of one or more other beamlets of a multi-beam charged particle tool, a method according to any one of clauses 1 to 13 or 18,

[0170] wherein the step of selecting the estimated aberration parameters comprises selecting one or more final aberration values ​​determined for the first beamlet as the corresponding one or more estimated aberration values ​​for each of the one or more other beamlets, and

[0171] Wherein the step of updating the estimated aberration parameters comprises updating estimated aberration values ​​that do not correspond to one or more estimated aberration values ​​selected to correspond to the one or more final aberration values ​​determined for the first beamlet.

[0172] 21. A method according to clause 20, wherein when the method is performed on one or more other beamlets of a multi-beam charged particle tool,

[0173] The step of selecting estimated aberration parameters comprises selecting one or more high-order final aberration values ​​determined for the first beamlet as corresponding one or more high-order estimated aberration values ​​for each of the one or more other beamlets, and

[0174] The step of updating the estimated aberration parameters comprises updating one or more low-order estimated aberration values ​​that do not correspond to one or more high-order estimated aberration values ​​selected to correspond to one or more high-order final aberration values ​​determined for the first beamlet.

[0175] 22. A method according to clause 21, wherein one or more high-order estimated aberration values ​​are n highest-order estimated aberration values, and wherein one or more low-order estimated aberration values ​​are m lowest-order estimated aberration values, wherein n+m equals the total number of estimated aberration values.

[0176] 23. A method according to clause 22, wherein n is greater than or equal to 1 and less than the total number of estimated aberration values.

[0177] 24. A method according to clause 20 or 21, wherein the estimated aberration parameters and the final aberration parameters represent a set of Zernike polynomials, wherein each estimated aberration value and each final aberration value represents a weight of a corresponding Zernike polynomial.

[0178] Although specific embodiments of the present invention have been described above, it should be understood that the present invention may be practiced in a manner other than that described. The above description is intended to be illustrative rather than limiting. Therefore, it will be apparent to those skilled in the art that modifications may be made to the described invention without departing from the scope of the claims set forth below.

Claims

1. A method for determining aberrations in an image obtained by a charged particle beam tool, the method comprising the steps of: a) obtaining two or more images of a sample using the charged particle beam tool, wherein each image is obtained with a known relative difference in the measurement conditions of the charged particle beam tool; b) selecting estimated aberration parameters for the aberration of a detection profile, the detection profile representing the charged particle beam used by the charged particle beam tool; c) evaluating an error function that indicates the difference between the two or more images and two or more estimated images, the two or more estimated images being dependent on the estimated aberration parameters and the known relative difference in the measurement conditions; d) updating the estimated aberration parameters; e) iteratively performing steps c) and d); and f) determining the final aberration parameters as the estimated aberration parameters that provide a minimum value of the error function.

2. The method according to claim 1, wherein step e) comprises iteratively performing steps c) and d) until the value of the error function is below a threshold.

3. The method according to claim 1, wherein updating the estimated aberration parameters comprises calculating the gradient of the error function based on the estimated aberration parameters and updating the estimated aberration parameters based on the gradient of the error function.

4. The method according to claim 3, wherein step e) comprises iteratively performing steps c) and d) until the gradient of the error function is below a second threshold.

5. The method according to claim 1, wherein evaluating the error function comprises: calculating an estimated detection profile for each image based on the estimated aberration parameters and the known relative difference in the measurement conditions; calculating the difference between the image and an estimated image for each of the two or more images, the estimated image being estimated based on the corresponding estimated detection profile; and summing the calculated differences.

6. The method according to claim 1, wherein the error function L(ζ) is where each is the Fourier transform of the two or more images in a plane that is at a distance z relative to the focal plane, k is the spatial frequency component, and each is the estimated detection profile for a respective one of the two or more images, where the estimated detection profile is calculated based on the estimated aberration parameter ζ and the known relative differences in the measurement conditions.

7. The method according to claim 3, wherein the gradient of the error function is where each is the Fourier transform of the two or more images in a plane that is at a distance z relative to the focal plane, k is a spatial frequency component, and each is an estimated detection profile for a respective one of the two or more images, where the estimated detection profile is calculated based on the estimated aberration parameter ζ and the known relative differences in the measurement conditions; wherein denotes the real part of the term in parentheses, and denotes the gradient with respect to the aberration parameter ζ.

8. The method according to claim 1, wherein the measurement conditions of the charged particle beam tool are one of the following: the position of the sample along the primary optical axis of the charged particle beam tool relative to the focal plane of the charged particle beam tool; the amount of beam shaping applied to the charged particle beam; the average landing energy of the charged particles in the charged particle beam; or any combination thereof.

9. The method according to claim 1, further comprising removing or reducing the aberration in at least one image of the sample obtained by the charged particle beam tool based on the final aberration parameters.

10. A method for determining the settings of a charged particle beam tool, the method comprising: performing the method according to claim 1 two or more times, each time based on a different estimate of the settings of the charged particle beam tool; determining the residuals for each estimate of the settings based on the final aberration parameters determined for the corresponding estimates of the settings; and Determine the estimate of the setting that produces the minimum residual as the setting of the charged particle beam tool.

11. The method according to claim 10, wherein the measurement condition of the charged particle beam tool is the amount of astigmatism applied to the charged particle beam tool.

12. A method for determining aberrations in an image acquired by a multi-beam charged particle beam tool, the method comprising: For a first sub-beam of the multi-beam charged particle beam tool, perform the method according to claim 1, wherein the estimated aberration parameters include a plurality of estimated aberration values, thereby determining a final aberration parameter including a plurality of final aberration values for the first sub-beam; For each of one or more other sub-beams of the multi-beam charged particle beam tool, perform the method according to claim 1, wherein the step of selecting the estimated aberration parameters includes: selecting one or more final aberration values determined for the first sub-beam as the corresponding one or more estimated aberration values for each of the one or more other sub-beams, and wherein the step of updating the estimated aberration parameters includes: updating the estimated aberration values that do not correspond to the one or more estimated aberration values, the one or more estimated aberration values being selected to correspond to the one or more final aberration values determined for the first sub-beam.

13. The method according to claim 12, wherein when the method is performed for each of the one or more other sub-beams of the multi-beam charged particle beam tool, the step of selecting the estimated aberration parameters comprising: selecting one or more higher-order final aberration values determined for the first sub-beam as the corresponding one or more higher-order estimated aberration values for each of the one or more other sub-beams, and wherein the step of updating the estimated aberration parameters includes: updating one or more lower-order estimated aberration values that do not correspond to the one or more higher-order estimated aberration values, the one or more higher-order estimated aberration values being selected to correspond to the one or more higher-order final aberration values determined for the first sub-beam.

14. A charged particle beam tool, comprising: An irradiation system configured to generate a charged particle beam and scan the charged particle beam across a sample; A detection system configured to capture charged particles that interact with the sample, thereby creating an image of the sample, and A controller configured to perform the method according to claim 1.

15. The charged particle beam tool according to claim 14, wherein the irradiation system further includes an astigmatism compensator configured to adjust the amount of astigmatism applied to the charged particle beam; wherein the measurement condition of the charged particle beam tool is the amount of astigmatism applied to the charged particle beam.

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