Method for correcting first order astigmatism and first order distortion under a multi-beam scanning electron microscope

Abstract

Method of correcting first order astigmatism and first order distortion under a multi-beam scanning electron microscope. An example multi-beam scanning electron microscope (MB-SEM) for correcting both astigmatism and linear distortion includes at least: an electron source coupled to provide an electron beam; an aperture plate including an array of apertures arranged to form an array of electron beamlets from the electron beam; and an electron column including a plurality of lenses and a first and second stigmators coupled to direct the array of electron beamlets toward a sample, wherein the first and second stigmators are arranged and energized to correct both astigmatism and linear distortion.

Description

Technical Field

[0001] The present invention relates generally to multi-beam charged particle microscopy, and more particularly to multi-beam charged particle microscopy including multiple astigmatism correctors for correcting astigmatism and first-order distortion. Background Technology

[0002] Charged particle microscopy can encounter various image quality problems, many of which are caused by column misalignment. These problems can lead to astigmatism and distortion, for example. Astigmatism causes a change in focus along orthogonal directions, while distortion causes tilting or changes in magnification along orthogonal directions in the field of view. While first-order distortion present in single-beam charged particle microscopy is generally of little concern, it can distort individual images in an image array in multi-beam microscopy, thus affecting overall image quality and the throughput of stitching individual images together. Although astigmatism correctors arranged in charged particle microscopes can adjust for either astigmatism or first-order distortion, adequate correction of both is difficult, especially in multi-beam charged particle microscopy. Solutions for correcting both astigmatism and first-order distortion are desired due to the expectation of increasing microscope throughput. Summary of the Invention

[0003] An example of a multi-beam scanning electron microscope (MB-SEM) for correcting both astigmatism and linear distortion includes at least an electron source coupled to provide an electron beam; an aperture plate containing an array of apertures arranged to form an array of electron small beams from the electron beam; and an electron column including a plurality of lenses and a first and a second astigmatism reducer coupled to guide the array of electron small beams to a sample, wherein the first and second astigmatism reducers are arranged and excited to correct both astigmatism and linear distortion.

[0004] Another example for correcting both astigmatism and linear distortion is a multi-beam scanning electron microscope (MB-SEM), which includes at least an electron source coupled to provide an electron beam; an aperture plate containing an array of apertures arranged to form an array of small electron beams from the electron beam; a plurality of lenses; a first astigmatism reducer and a second astigmatism reducer; and an electron column comprising the plurality of lenses and the first and second astigmatism reducers. The electron column is coupled to guide the array of small electron beams to a sample, wherein the column forms at least one individual beam crossing plane due to the plurality of lenses, wherein each of the electron beams forms an intermediate image of the electron source; and at least one single common beam crossing plane, wherein the electron beams in the array cross each other, wherein an odd number of crossing planes, including both the individual beam crossing planes and the common beam crossing plane, are located between the first and second astigmatism reducers.

[0005] Another example of correction for both astigmatism and linear distortion is a multi-beam scanning electron microscope (MB-SEM), which includes at least a source coupled to provide an electron beam, an aperture array plate arranged to provide multiple probe beams from the electron beam, a first astigmatism reducer and a second astigmatism reducer, and a controller coupled to at least control the excitation of the first and second astigmatism reducers. The controller includes code that, when executed by the controller, causes the controller to determine the deformation intensity of the two astigmatism reducers, determine the astigmatism intensity of the two astigmatism reducers based on the deformation intensity and the astigmatism intensity, determine at least two linear combinations of excitation of the two astigmatism reducers to affect astigmatism and linear distortion, and excite the first and second astigmatism reducers based on the two linear combinations to adjust for astigmatism and linear distortion.

[0006] An example method for exciting multiple astigmatism reducers to correct astigmatism and first-order distortion under a multi-beam scanning electron microscope includes at least determining the deformation intensity of two astigmatism reducers, determining the astigmatism intensity of the two astigmatism reducers based on the deformation intensity and astigmatism intensity of the two astigmatism reducers, determining at least two linear combinations of excitation of the two astigmatism reducers to affect astigmatism and linear distortion, and exciting the two astigmatism reducers based on the two linear combinations to adjust astigmatism and linear distortion.

[0007] Another example of a multi-beam scanning electron microscope (MB-SEM) includes at least a source coupled to provide an electron beam, an aperture array plate arranged to provide multiple probe beams from the electron beam, a first astigmatism reducer and a second astigmatism reducer, and a controller coupled to at least control the excitation of the first and second astigmatism reducers. The controller includes code that, when executed by the controller, causes the controller to determine the deformation intensity of the two astigmatism reducers, determine the astigmatic intensity of the two astigmatism reducers based on the deformation intensity and astigmatic intensity, determine at least two linear combinations of excitation of the two astigmatism reducers to affect astigmatism and linear distortion, and excite the first and second astigmatism reducers based on the two linear combinations to adjust for astigmatism and linear distortion. Attached Figure Description

[0008] Figure 1A This is a schematic diagram of a multi-beam charged particle microscope according to an embodiment of the present disclosure;

[0009] Figure 1B These are examples of astigmatism reducers placed relative to a common intersection plane and individual intersection planes according to embodiments of the present disclosure, and are used to illustrate the effect of astigmatism reducers on astigmatism and linear distortion.

[0010] Figure 2 This is an illustration of source generation using a aperture array plate to generate multiple beams according to embodiments of the present disclosure;

[0011] Figure 3 This is an illustration of a portion of a multibeam electron microscope according to an embodiment of the present disclosure;

[0012] Figure 4 This is an example method according to embodiments of the present disclosure for exciting two astigmatists to correct both astigmatism and linear distortion; and

[0013] Figure 5 This is an example functional block diagram based on an embodiment of the present disclosure.

[0014] Throughout the various views in the accompanying drawings, similar reference numerals refer to the corresponding parts. Detailed Implementation

[0015] The embodiments disclosed herein relate to a charged particle beam column designed for multi-beam operation, wherein the column includes at least two astigmatism cancellers positioned and energized to correct both astigmatism and linear distortion. In some instances, the two astigmatism cancellers are arranged within the column such that they can operate substantially independently of each other, meaning that one astigmatism canceller can be operated to primarily control astigmatism, while the other can be operated to primarily control linear distortion. However, it should be noted that complete independence is not possible, as astigmatism and linear distortion are intrinsically related physical phenomena. In some instances, the astigmatism cancellers are positioned such that there is an odd number of intersections between their positions. Although they can operate substantially independently even if there is an even number of intersections between the positions of two astigmatism cancellers, an odd number of intersections ensures stronger independent operation. Additionally, the embodiments disclosed herein relate to a method for determining an excitation to be provided to each astigmatism canceller so that both astigmatism and linear distortion are corrected. It should be understood that the methods described herein are generally applicable to a variety of different methods and apparatuses and are not limited to any particular apparatus.

[0016] As used in this application and claims, unless the context clearly indicates otherwise, the singular forms “a,” “an,” and “the” include the plural forms. Furthermore, the term “comprising” means “including.” Further, the term “joint” does not exclude the presence of intermediate elements between joined items.

[0017] The systems, apparatuses, and methods described herein should not be construed as limiting in any way. In fact, this disclosure is directed to all novel and non-obvious features and aspects of the various disclosed embodiments, whether individually or in various combinations and sub-combinations formed with each other. The disclosed systems, methods, and apparatuses are not limited to any particular aspect or feature or combination thereof, nor are they required to provide any one or more specific advantages or solve any one or more specific problems. Any operational theory is provided for ease of interpretation, but the disclosed systems, methods, and apparatuses are not limited to such operational theories.

[0018] Although some operations of the disclosed methods are described in a specific order for ease of presentation, it should be understood that this descriptive approach encompasses rearrangement unless the specific language used below requires a particular order. For example, in some cases, the operations described in sequence may be rearranged or performed simultaneously. Furthermore, for simplicity, the accompanying drawings may not show the various ways in which the disclosed systems, methods, and apparatus can be used in conjunction with other systems, methods, and apparatuses. Additionally, this specification sometimes uses terms such as “produce” and “provide” to describe the disclosed methods. These terms are high-level abstractions of the actual operations performed. The actual operations corresponding to these terms will vary depending on the specific embodiment and are readily discernible to those skilled in the art.

[0019] In some instances, values, programs, or devices are referred to as “lowest,” “best,” “smallest,” etc. It will be recognized that such descriptions are intended to indicate that there are many functional alternatives available for use, and that such choices are not necessarily better, smaller, or preferred over others.

[0020] Multi-beam charged particle microscopy, such as multi-beam scanning electron microscopy (MB-SEM) which uses multiple small beams of charged particles, provides the ability to simultaneously image large areas of a sample by forming multiple beams into an array. Each beam generates a corresponding image of a small region of the sample, and the individual images can be stitched together to provide a composite image of the sample. While this technique can improve the throughput of image acquisition, problems can arise when stitching together the individual images. Such problems can be caused by distortions such as tilt, stretching, and / or anisotropic magnification. Distortions caused by, for example, stretching can lead to higher crosstalk on the detector and may also require large overscans of the corresponding sampling area for each beam. Crosstalk problems limit resolution and contrast, while overscan can limit throughput.

[0021] One technique for reducing or eliminating distortion and reducing overscan requirements is to add one or more additional astigmatism correctors to a multi-beam charged particle column. Such additional astigmatism correctors provide the flexibility to correct not only astigmatism but also first-order distortion. This first-order distortion may be referred to herein as warping, linear distortion, or elliptic distortion. First-order distortion results in different magnifications in orthogonal directions, such as the x and y directions in the sample plane, and can also manifest as tilting in individual beam-based images. Correction of first-order distortion can lead to a reduction in the amount of overscan required for each beam and improve signal quality on the detector, thereby improving image quality. Although astigmatism and linear distortion are correlated, they can be corrected substantially independently if the individual astigmatism correctors are arranged in a manner that reduces their interrelationship within the charged particle column. For example, a first astigmatism corrector may be arranged adjacent to or coincident with the common beam crossing plane, while a second astigmatism corrector may be arranged adjacent to or downstream of the beam crossing plane. For clarity, the common crossing plane is the plane in the column of charged particles where all the small bundles cross each other, while the small bundle crossing plane is the plane in the column where each small bundle crosses only with itself. Generally, it is desirable to arrange the two astigmatism correctors such that an odd number of crossings occur between their positions, where the odd number of crossings includes both the common crossing and the individual or small bundle crossings. This arrangement results in the fundamental operation of the two astigmatism correctors being independent, allowing astigmatism and linear distortion to be corrected substantially independently of each other. While it is impossible to affect linear distortion if astigmatism is not changed simultaneously, this arrangement may cause one astigmatism corrector to be more dedicated to linear distortion correction than astigmatism correction. However, the astigmatism corrector will inevitably affect astigmatism as well. It should be noted that any arrangement of the two astigmatism correctors within a column is within the scope of this disclosure, with both even and odd number of crossings occurring between the two astigmatism correctors.

[0022] In summary, the two astigmatism correctors in the disclosed technology may have different effects on astigmatism and linear distortion. If we let A... tot and D tot If the desired astigmatism and distortion are generated by astigmatism correctors a and b, then e a and e b Let A1 and A2 be the excitations of astigmatism cancellers a and b, respectively, and let D1 and D2 be the distortion sensitivity constants of astigmatism cancellers a and b. Then the following relationship can be formed:

[0023]

[0024] The last two terms provide two equations with two unknowns in terms of excitation, which can be solved under certain conditions, such as: Where A1 and A2 should not both be zero and D1 and D2 should not both be zero. It should be noted that since each equation includes the excitation of both astigmatism reducer a and astigmatism reducer b, A... tot and D tot They are interconnected. Although this interconnection may make it more difficult to control the astigmatism reducer to correct both astigmatism and linear distortion, there may be two locations where the astigmatism reducer can be placed in an MB-SEM column, which may reduce the interconnection and simplify their operation.

[0025] In general, two astigmatism correctors can be placed at most locations along the optical path of the pillar between the emitter and the sample, provided they are sufficiently separated. Sufficient separation is based on the relative positions of the two astigmatism correctors with respect to the two types of intersections occurring in the pillar. However, in some instances, it may be desirable to avoid placing the astigmatism correctors in the same plane as the individual intersections, or to place them on opposite sides of the individual intersection planes due to a lack of influence the astigmatism correctors might have in and around that plane. On the other hand, placing the astigmatism correctors in a plane with a common intersection can provide astigmatism correction without affecting linear distortion. Furthermore, placing the astigmatism corrector between the common intersection plane and the individual intersection plane allows the astigmatism corrector to have a greater effect on linear distortion than on astigmatism. In some instances, placing one astigmatism corrector at the common intersection plane and the other between the individual intersection plane and the subsequent common intersection plane allows the two astigmatism correctors to operate substantially independently, such that one astigmatism corrector primarily affects astigmatism, while the other has a greater effect on linear distortion than on astigmatism. See below for further details. Figure 3 .

[0026] The disclosed technique further determines the excitation to be applied to each astigmatism reducer so that both astigmatism and linear distortion are corrected, for example, reduced or eliminated. This technique involves determining the astigmatic intensity and deformation intensity per unit excitation for each astigmatism reducer, determining a combination of excitations for two astigmatism reducers to handle both linear distortion and astigmatism, and then applying one or more excitations to the astigmatism reducers. As used herein, deformation intensity refers to the degree of linear distortion correction per unit excitation that the astigmatism reducer can provide for the electron beam and / or small beam. The concept of deformation intensity may also be referred to herein as the linear distortion correction capability per unit excitation. It should be noted that the relative position of the astigmatism reducers within the microscope affects the excitation of each astigmatism reducer.

[0027] Figure 1A This is a schematic diagram of a multi-beam charged particle microscope 100 according to an embodiment of the present disclosure. The multi-beam charged particle microscope (MBCPM) 100 illustrates an embodiment of SEM, but the microscope 100 could also be, for example, a STEM or photolithography imaging system. Microscope 100 especially The microscope 100 includes an electron source 3, an electron beam column 5, a vacuum chamber 7, an orifice plate 9, a stage 17, a detector 23, and a detector 27. The microscope 100 provides a plurality of small beams of charged particles to the sample S and acquires an image of each of these beams. To reduce or eliminate astigmatism and first-order distortion (e.g., linear distortion), the microscope 100 includes at least two astigmatism reducers arranged such that their performance is substantially independent. Reducing or eliminating linear distortion will at least increase throughput and improve image quality.

[0028] The emitter 3 can be any electron emitter that generates the electron beam B, such as a Schottky emitter, a cold FEG, or a LaB6 filament. The electron beam B can propagate along the particle optical axis B'.

[0029] The electron beam column 5 may include multiple lenses, an aperture array plate 9, and multiple astigmatism correctors. The multiple lenses may include at least lenses 11, 13, and 14, which can guide / focus the electron beam B onto the sample plane. A deflection unit 15 arranged within the column 5 can deflect and / or scan the beam B. In some instances, the column 5 may also include an aberration corrector, a field stop, etc.

[0030] The aperture array plate 9 can be placed between the emitter 3 and the lens 11, and can include an array of apertures within a grid of n×n apertures. Of course, other aperture structures, such as linear or circular apertures, are also possible. The number of apertures can be any desired number and is a non-limiting aspect of this disclosure. In some instances, the aperture plate 9 includes an array of 8×8 apertures, which can generate 64 small beams from the electron beam B.

[0031] Multiple astigmatism correctors, such as astigmatism corrector 12 and astigmatism corrector 16, can be arranged within column 5 to correct both astigmatism and linear distortion according to their respective excitations. If the astigmatism correctors are arranged as disclosed herein, a combination of astigmatism correctors 12 and 16 can be excited so that astigmatism and linear distortion are corrected almost independently of each other. In some instances, depending on the relative arrangement of astigmatism correctors 12 and 16, each astigmatism corrector can adjust linear distortion or astigmatism substantially independently. Furthermore, whether an astigmatism corrects astigmatism or linear distortion can depend on the type of cross that coincides with or is adjacent to the astigmatism corrector. For example, if an astigmatism corrector is adjacent to or coincides with a common beam cross, then that astigmatism corrector is arranged to primarily correct astigmatism. On the other hand, if another astigmatism corrector is placed at the location of a common cross, and if the astigmatism corrector is arranged between the individual beam cross and the common beam cross, then that astigmatism corrector is arranged to primarily correct linear distortion. Although linear distortion and astigmatism are correlated, the correction of either linear distortion or astigmatism can be largely decoupled if the astigmatism correctors within the column are positioned such that they can operate substantially independently of each other. This independence can be achieved by physically arranging the astigmatism correctors such that there is at least one bundle crossover between their respective positions. In some instances, the astigmatism correctors are arranged such that there is an odd number of crossovers between their positions, including both common and individual crossovers. While in some instances there may be an even number of intermediate crossovers between two astigmatism correctors, an odd number improves the independent operation of the two astigmatism correctors. It should be noted that their placement order does not affect the correction of either astigmatism or linear distortion. It is their relative positions to each other and their relative positions with respect to crossovers that determine their independence and relative influence.

[0032] Figure 1B This is an example illustration of an astigmatism reducer placed relative to a common intersection plane and individual intersection planes, and will be used to illustrate the effect of the astigmatism reducer on astigmatism and linear distortion. Figure 1B It shows the intersections placed between the common intersection plane and the individual intersection planes, each with its own distance L. C and L i The astigmatism eraser 12. Distance L C and L i Determine the effect of the astigmatism reducer 12 on astigmatism and linear distortion, such as on deformation. The effect of the astigmatism reducer 12 on the intensity of astigmatism, such as the effect of the astigmatism reducer 12 on astigmatism, is approximately related to L. i 2 Proportional, while for the intensity of linear distortion, for example, its effect on L, it is proportional. i *L C Proportional. It should be noted that these distances are directional and refer to the plane of the astigmatism reducer, thus producing both positive and negative values.

[0033] To correct both linear distortion and astigmatism, it is desirable that both astigmatism canceller 12 and astigmatism canceller 16 have an effect on astigmatism, and that at least one of astigmatism canceller 12 or astigmatism canceller 16 has a sufficient effect on linear distortion. For example, to have an effect on astigmatism, it is desirable to place the astigmatism cancellers sufficiently far away from individual intersecting planes, and to have an effect on linear distortion, it is desirable to place the astigmatism cancellers sufficiently far away from common intersecting planes. It is also desirable to place each astigmatism canceller so that they are sufficiently independent of each other. Placing the astigmatism cancellers so that they are sufficiently independent of each other simplifies their operation, but is not a requirement of the disclosed art. Thus, it is desirable that the values ​​of astigmatism canceller 12 are such that (L... i 2 , L i *L C The value pair between (L) and the astigmatism canceller 16 i 2 , L i *L C As orthogonal as possible. In general, the desired outcome is to place two astigmatism correctors such that they operate substantially independently of each other, and both astigmatism and linear distortion can be corrected.

[0034] It should also be noted that placing the astigmatism reducer in the same plane as the individual intersections may not affect either linear distortion or astigmatism, while placing it in the same plane as the common intersection may not cause image distortion but will affect astigmatism. Therefore, placing the astigmatism reducer at the common intersection to affect astigmatism without affecting linear distortion is operationally advantageous, but this arrangement is not necessary.

[0035] Given the above constraints, there may be various possible locations for the astigmatism corrector 12 and astigmatism corrector 16 within column 5 to provide the disclosed advantages regarding both astigmatism correction and linear distortion correction. However, some locations may be easier to implement in terms of column design and engineering than others. Additionally, some arrangements may allow for better separation of the operation of the two astigmatism correctors than others. However, it should be noted that such arrangements are not necessary for the implementation of the technology disclosed herein, and all layouts are considered herein. Reference will be made below. Figure 3 The various positions of the astigmatism reducers will be discussed in more detail, and the positions of astigmatism reducers 12 and 16 in Figure 1 are for illustrative purposes only. It should also be noted that the positions of astigmatism reducers 12 and 16 can also affect their operation, as changes in the position of one or both astigmatism reducers will alter the degree to which each reducer affects both astigmatism and linear distortion.

[0036] Vacuum chamber 7, containing sample holder 17 and associated stage / actuator 19, is used to hold / place sample S. Vacuum chamber 7 (and column 5 / source 3) is evacuated using a vacuum pump (not shown). If desired, sample holder 17 or at least sample S can be biased (floated) to a potential relative to ground by means of voltage source 21.

[0037] By scanning an electron beam B over a sample S, output radiation (including, for example, X-rays, infrared / visible / ultraviolet light, secondary electrons, and / or backscattered electrons) is emitted from the sample S. Detectors 23 and 27 can be selected from a variety of possible detector types that can be used to examine different types / morphologies of this output radiation.

[0038] Detector 23 is, for example, a segmented electron detector comprising multiple independent detection segments (e.g., quadrants) arranged about the central opening 25 (around the optical axis B'). For example, such a detector can be used to study the angular dependence of (second / backscattered) electrons emitted from the sample S.

[0039] Detector 27 is, for example, an X-ray detector, which can be used to record X-rays emitted from sample S, thereby performing energy-dispersive X-ray spectroscopy (EDX). Alternatively, it can be, for example, a cathodoluminescence detector.

[0040] Alternatively or additionally, a backscattered electron detector may be included. Since the detected output radiation is position-dependent (due to the scanning motion and beam position), the information obtained from detectors 23 and 27 will also be position-dependent and can therefore be used to assemble an image, which is essentially a graph of the detector outputs as a function of the scan path position on the sample S. Signals from detectors 23 and 27 are transmitted along control lines (buses) 29', processed by controller 29, and displayed on display unit 31. Such processing may include operations such as combination, integration, subtraction, pseudo-coloring, edge enhancement, and other processing known to those skilled in the art. Furthermore, automatic identification processes (e.g., for particle analysis) may be included in such processing.

[0041] During operation, electron beam B0 (see...) Figure 2An n×n array of small beams B'' is generated, guided by pillar 5 to the sample S. As the array of small beams B'' passes through pillar 5, each beam propagates, for example, through multiple lenses 11, 13, and 14, as well as astigmatism correctors 12 and 16. If astigmatism corrector 12 is arranged in or near the common beam crossing plane, it will primarily correct astigmatism when excited. Conversely, if astigmatism corrector 16 is arranged downstream of the individual beam crossing planes, it will primarily correct first-order distortion when excited. The position of astigmatism corrector 16 relative to the individual and common beam crossing planes determines the amount of correction it can provide per unit of excitation. Generally, placing astigmatism corrector 16 in the middle of the two crossing planes provides the most robust level of correction, as the midpoint results in Li equal to Lc, leading to the maximum distortion correction / effect. To ensure that the two astigmatism correctors 12 and 16 correct astigmatism and linear distortion substantially independently, they are arranged within pillar 5 such that an odd number of crossings occur between their positions. Odd numbers can be 1, 3, 5, etc. While instances with an odd number of intersections between the two astigmatism correctors may be preferred, instances with an even number of intersections are also possible and are considered herein. The difference between the two types of instances can be seen in the excitation and control of the astigmatism correctors used to address both astigmatism and linear distortion. Due to the rasterization of the small beams B'', each small beam forms a corresponding image of the region of the sample S. Due to the arrangement of the two astigmatism correctors according to the technique disclosed herein, the overscan of each small beam can be reduced, and the microscope throughput can be increased.

[0042] Furthermore, controller 29 includes code, or receiving code, that instructs microscope 100 to determine how to excite astigmatism reducers 12 and 16 so that both astigmatism and linear distortion are corrected. This code can proceed through multiple steps to obtain a suitable combination of excitations for the two astigmatism reducers, which may be influenced by their relative positions within column 5. The following is about... Figure 4 The method, described in more detail, includes at least using at least three linearly independent excitations to determine the astigmatism and distortion intensity (e.g., the degree of linear distortion correction) of each of the two astigmatism cancellers, such as individually shutting down each canceller and simultaneously shutting down both cancellers. To determine the astigmatism and / or distortion intensity, a small bundle is imaged to measure the position and / or astigmatism of each bundle for each excitation setting. For example, images of each bundle can be acquired with both cancellers off, only the first canceller on, and only the second canceller on. By measuring the position of each bundle for each setting, the distortion intensity per unit excitation of each of the two cancellers can be determined. For example, the distortion per unit excitation can be determined by measuring the relative movement of the bundles for each setting.

[0043] This determination results in obtaining the astigmatism and distortion intensity per unit excitation for each of the astigmatism reducers 12 and 16. Next, based on the determined intensity of astigmatism and linear distortion, astigmatism can be corrected using only reducer 12. Then, linear distortion is measured. Subsequently, based on the measured linear distortion and reducer intensity, the excitation of reducer 16 (and possibly reducer 12) can be determined to correct linear distortion without altering the astigmatism. Excitation of both reducers 12 and 16 thus produces an astigmatism-corrected image and a conformal image.

[0044] However, in other instances, the process can be performed to first correct linear distortion, for example, by exciting astigmatism canceller 16, and then determine a linear combination of excitations for astigmatism cancellers 12 and 16 that corrects astigmatism while preserving linear distortion. In yet another instance, the two astigmatism cancellers 12 and 16 can be excited with any linear combination of energy, and then one of the astigmatism cancellers 12 or 16 can undergo additional positive or negative excitation to correct any additional astigmatism and / or linear distortion introduced by any linear combination of excitation energy.

[0045] In another instance, if the astigmatism canceller 12 is arranged to coincide with a common cross plane, then it is only necessary to determine the deformation strength of the astigmatism canceller 16, and then the astigmatism canceller 12 can be excited to correct astigmatism while the excitation of the astigmatism canceller 16 is maintained to correct linear distortion.

[0046] Figure 2 This is an illustration of source 200 generating multiple beams using a aperture array plate according to an embodiment of this disclosure; precursor electron beam B o The electron beam exits the electron emitter 3, passes through a series of electrodes 3', and impacts a perforated plate 9 containing multiple holes 9'. This perforated plate 9 can be fabricated, for example, by etching an array of small holes in a silicon wafer using thin-film (MEMS) technology. The perforated plate / ALA 9 is placed, for example, within the electron beam column 5, and is typically positioned relatively close to the electron emitter 3, for example, at a spacing of approximately 5 mm to 10 mm downstream of it. As a result of impacting the ALA 9, the precursor beam B... o The bundle array B is converted into multiple sub-bundles / small bundles / component bundles B'', which have the same geometric configuration as the multiple apertures 9'' used to generate them. The bundle array B then follows its path along axis B' through column 5, guiding the bundle array B onto the sample S.

[0047] In some instances, the orifice plate / ALA 9 comprises an array of 14 × 14 wells 9', each well having a diameter of approximately 10 μm and a spacing / interval of approximately 20 μm. In other instances, the orifice plate / ALA 9 comprises an array of 8 × 8 wells 9'. The examples of orifice plates 9 provided are non-limiting aspects of this disclosure, and any orifice plate design is considered herein.

[0048] Electrode 3' generates a potential change at each aperture 9', causing each aperture 9' to act as an electrostatic microlens. In this way, the individual sub-beams B'' are focused onto the (intermediate) image array of source 3 downstream of the aperture plate / ALA 9. In general, it may only be necessary for one electrode 3' to work in conjunction with the aperture plate / ALA 9 to produce this lensing effect; however, the use of more than one electrode 3' gives more possibilities regarding the manipulation of small beams.

[0049] The distance between source 3 and each aperture 9' (electrostatic microlens) of aperture plate / ALA 9 increases slightly with increasing distance from the optical axis / cylindrical axis B'. If all apertures / microlenses 9' have the same focusing intensity, this will cause a variation in the image distance of the (intermediate) image of source 3, such that these images will not lie in a common plane perpendicular to the optical axis / cylindrical axis B', but will lie on a curved surface (essentially part of a sphere). To prevent this curvature of the plane and the (intermediate) source image, a correction electrode can be used, sized such that its field primarily affects the focusing intensity of the outer microlens. By appropriately fine-tuning the potential on this correction electrode, the aforementioned curvature can be mitigated / eliminated. Such a correction electrode can be included in the group of electrodes 3' shown, for example, as described by Y. Zhang and P. Kruit in the journal article, "Design of a high brightness multi-electron-beam source," Proc. Seventh Int. Conf. on Charged Particle Optics, Physics Procedia 1 (2008), pp. 553-563, Elsevier pub. Alternatively, this type of correction electrode can be placed downstream of ALA 9, for example, as described in U.S. Patent US 8,598,545 (incorporated herein by reference).

[0050] Figure 3This is an illustration of a portion of a multi-beam electron microscope 300 according to an embodiment of the present disclosure. The microscope 300 is configured to provide multiple small electron beams B to a sample S, whose astigmatism and linear distortion are corrected due to at least two astigmatism correctors included in the system 300. By correcting, reducing, or eliminating both astigmatism and linear distortion, overscan requirements can be reduced, image quality improved, and microscope throughput increased.

[0051] Microscope 300 includes a source and a column, which together guide an electron beam B to a sample S. The source includes an emitter 3, an electrode 3', and a aperture array plate 9. These components have been discussed above, and for the sake of brevity, such discussion will not be repeated. The source generates an electron beam B0, which is the origin of a smaller beam B''. The smaller beam B'' is guided to the sample S via the column.

[0052] The column includes multiple lenses, such as lens 11, lens 13, lens 14, and lens 17. Although four lenses are shown, fewer or more lenses may be included without affecting the technology disclosed herein and within the scope of this disclosure. Lenses 11, 13, 14, and 17 are arranged along the column to guide and focus the small beam B'' onto the sample S. Such lenses may be electromagnetic or electrostatic, and each lens may be formed by one or more electrodes. Additionally, the column includes astigmatism correctors 12 and 16, both of which are operated such that their combined effect on the small beam corrects both astigmatism and linear distortion. In general, the relative placement of each astigmatism corrector determines not only what it primarily corrects but also its level of independent operation. However, some combinations of positions may reduce their independence and may simplify operation.

[0053] Due to the various lenses, beam crossings are formed at different locations within the lens. While this isn't always entirely accurate, the pillar can be designed in this way to reduce aberrations. Beam crossings include common beam crossings and individual beam crossings. A common beam crossing is the location where each small beam intersects at the same position in a plane. In other words, a common crossing is the location where each small beam intersects with all other small beams. On the other hand, an individual beam crossing is the intersection of each small beam with itself but not with any other small beams. For example, the common beam crossing plane appears at the positions of lenses 13 and 17, while the individual beam crossing plane appears at the positions of lenses 11 and 14.

[0054] The placement of astigmatism correctors 12 and 16 can be chosen to ensure that both astigmatism and linear distortion are corrected. Generally, a single astigmatism corrector can be placed to correct essentially only astigmatism, while a combination of two astigmatism corrects linear distortion in terms of excitation. For example, one astigmatism corrector can be arranged to coincide with or be adjacent to lens 13, i.e., at Pos. 2. In some embodiments, lens 13 can be a condenser lens. Since this lens is also the location of the common beam crossing, this astigmatism corrector can primarily correct astigmatism. Another astigmatism corrector can then be placed at Pos. 4, which is between lenses 14 and 17, for example, downstream of the individual crossing planes and midway to the next common crossing plane. In some embodiments, lens 17 can be an objective lens. The astigmatism corrector at Pos. 4 can primarily correct linear distortion but will also provide some astigmatism correction intensity. While such positional combinations, such as positions 2 and 4, provide correction for both astigmatism and linear distortion, other combinations of positions 1 through 5 are also possible and within the scope of this disclosure. Other example combinations include positions 1 and 2, positions 2 and 3, positions 3 and 4, positions 1 and 4, positions 4 and 5, etc. Alternatively, two astigmatism cancellers can be placed at a certain distance around position 1 or position 4 to provide some examples.

[0055] In some instances, placing astigmatism cancellers to create an odd number of intersections at their positions, such as positions 1, 3, 5, etc., may be advantageous, considering both common and individual intersections. However, the disclosed technique is equally applicable to an odd number of intermediate intersections, including 0, 2, 4, etc.

[0056] Figure 4 This is an example method 401 for exciting two astigmatism reducers to correct both astigmatism and linear distortion, according to embodiments of the present disclosure. Method 401 can be implemented on an electron microscope including two astigmatism reducers, such as those discussed with respect to microscope 100. In such an example, the two astigmatism reducers can be arranged as discussed herein such that they operate substantially independently of each other. However, while one astigmatism reducer may only affect astigmatism, the other may affect both astigmatism and linear distortion. Thus, the method of establishing the excitation of the two astigmatism reducers involves determining the excitation level of each astigmatism reducer such that the overall effect is a reduction or elimination of astigmatism and linear distortion in each sub-beam of the MB-SEM. It should be noted that the excitation level of each astigmatism reducer that corrects both astigmatism and linear distortion may be affected by their relative positions to the common plane and the sub-beam intersection plane.

[0057] Method 401 may begin at process block 403, which includes determining the deformation intensity of two astigmatism correctors. The two astigmatism correctors may, for example, be astigmatism corrector 12 and astigmatism corrector 16 as described above. The deformation intensity of each of the two astigmatism correctors, such as the degree of linear distortion correction per unit excitation, can be determined using at least three linearly independent astigmatism corrector settings and by measuring the position of each sub-beam for each setting. For example, images of each sub-beam can be acquired with both astigmatism correctors off, only the first astigmatism corrector on, and only the second astigmatism corrector on. By measuring the position of each sub-beam for each setting, the deformation intensity per unit excitation of each of the two astigmatism correctors can be determined. For example, the distortion per unit excitation can be determined by measuring the relative movement of the sub-beams for each setting.

[0058] Process block 403 may be followed by process block 405, which includes determining the astigmatic intensity of the two astigmatism cancellers. Similar to process block 403, the two astigmatism cancellers may be, for example, astigmatism canceller 12 and astigmatism canceller 16. The determination of the astigmatic intensity may be based on the deformation intensity and the entire multi-beam trajectory, including the common crossover location and the minor crossover location, or the multi-beam trajectory between the two astigmatism cancellers. Knowing the trajectory of the system, or at least the trajectory between the astigmatism cancellers, allows the determination of the ratio between the deformation intensity and the astigmatic intensity, leading to the determination of the astigmatic intensity. In some instances, one astigmatism canceller may be placed at the common crossover plane, which may eliminate the need to determine the astigmatic intensity of that canceller. In such instances, it may only be necessary to determine the deformation intensity of the other astigmatism cancellers.

[0059] In some instances, astigmatism and / or deformation intensity can be determined using a camera to image a small beam. Imaging can provide an indication of astigmatism and / or deformation intensity based on the position of individual probes and the relative tilt and / or focus of individual or composite images. Using a camera to determine the intensity allows for rapid and periodic determination of these values. However, it should be noted that for each astigmatism reducer and a given beam trajectory, the deformation and astigmatism intensity of the reducer are fixed values ​​and may only need to be measured once.

[0060] It should also be noted that procedure blocks 403 and 405 can be executed in reverse order, and procedure block 403 does not need to be executed before procedure block 405.

[0061] Process block 405 or 403 may be followed by process block 407, which includes determining at least two linear combinations of excitations for the two astigmatism reducers to affect both astigmatism and linear distortion. The two linear combinations may result in one combination changing only astigmatism while keeping linear distortion unchanged. The other combination may change only linear distortion while keeping astigmatism unchanged.

[0062] Process block 407 can be followed by process block 409, which includes energizing two astigmatism reducers to correct astigmatism and linear distortion. In one instance, the process may include first eliminating astigmatism in the image using conventional methods. Once the astigmatism is corrected, the position of the probe is measured to determine the linear distortion. Based on the determined linear distortion, a combination of one of the combinations of excitations from the two astigmatism reducers is applied to remove the linear distortion while preserving the astigmatism.

[0063] Figure 5 This is an example functional block diagram 500 according to an embodiment of the present disclosure. Figure 5 This is a block diagram illustrating a computer system 500 that can be used to implement embodiments of the present disclosure. For example, the computing system 500 may be an example of computing hardware, such as controllers 30 / 230, included in system 100 and / or EM 200, and may be incorporated into memory 32 and coupled to display 31 and user input device 33. The computer system 500 includes at least a hardware processor, such as core 530 for processing information, which may be coupled to a communication bus. The computing system 500 can be used to implement the methods and techniques disclosed herein, such as method 301, and can also be used to acquire images based on the synchronization of sample scanning with the pulse period of a pulsed electron beam.

[0064] Computer system 500 also includes main memory 532, such as random access memory (RAM) or other dynamic storage devices, coupled to a bus for storing information and instructions to be executed by core 530. Main memory 532 can also be used to store temporary variables or other intermediate information during the execution of instructions to be executed by core 530. When such instructions are stored in a non-transitory storage medium accessible to core 530, computer system 500 becomes a dedicated machine customized to perform the operations specified in the instructions. Main memory 532 may be memory 32 or separate from memory 32.

[0065] The computer system 500 further includes a read-only memory (ROM) 534 or other static storage device coupled to the bus for storing static information and instructions of the core 530. A storage device 536, such as a magnetic disk or optical disk, is provided and coupled to the bus for storing information and instructions.

[0066] Computer system 500 can be coupled to a display, such as display 31, via a bus for displaying information to the computer user. Input device 33 (including alphanumeric keys and other keys) is coupled to the bus for transmitting information and command selections to core 530. Another type of user input device is a cursor control, such as a mouse, trackball, or cursor arrow keys, used to transmit directional information and command selections to core 530 and to control cursor movement on the display. This input device typically has two degrees of freedom on two axes (a first axis (e.g., x) and a second axis (e.g., y)), allowing the device to specify a position in a plane.

[0067] Computer system 500 may implement the techniques described herein using custom hard-wired logic, one or more ASICs or FPGAs, firmware, and / or program logic combined with or programming computer system 500 to make it a dedicated machine. According to one embodiment, the techniques herein are executed by computer system 500 in response to core 530 executing one or more sequences of one or more instructions contained in main memory 532. Such instructions may be read into main memory 532 from another storage medium (such as storage device 536). Execution of the sequence of instructions contained in main memory 532 causes core 530 to perform the process steps described herein. In alternative embodiments, hard-wired circuitry may be used instead of or in combination with software instructions.

[0068] As used herein, the term "storage medium" means any non-transitory medium that stores data and / or instructions that enable a machine to operate in a specific manner. Such storage media can include non-volatile media and / or volatile media. Non-volatile media include, for example, optical discs or magnetic disks, such as storage device 536. Volatile media include dynamic memory, such as main memory 532. Common forms of storage media include, for example, floppy disks, floppy hard disks, hard disks, solid-state drives, magnetic tape or any other magnetic data storage media, CD-ROMs, any other optical data storage media, any physical media with a perforated pattern, RAM, PROMs and EPROMs, flash memory EPROMs, NVRAMs, any other memory chips or cartridges, content-addressable memory (CAM), and ternary content-addressable memory (TCAM).

[0069] Storage media differ from transmission media, but can be used in conjunction with them. Transmission media participate in the information transmission between storage media. For example, transmission media include coaxial cables, copper wires, and optical fibers, including conductors containing bus 640. Transmission media can also take the form of sound waves or light waves, such as those generated during wireless and infrared data communication.

[0070] Various forms of media can involve carrying one or more sequences of one or more instructions onto the core 530 for execution. For example, the instructions may initially be carried on a disk or solid-state drive of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a network.

[0071] Computer system 500 also includes a communication interface 538 coupled to a bus. Communication interface 538 provides bidirectional data communication coupled to a network link (not shown) connected to a local network. For example, communication interface 538 may be an Integrated Services Digital Network (ISDN) card, a cable modem, a satellite modem, or a modem for providing data communication connectivity with a corresponding type of telephone line. As another example, communication interface 538 may be a local area network (LAN) card for providing data communication connectivity to a compatible LAN. A wireless link may also be implemented. In any such implementation, communication interface 538 transmits and receives electrical, electromagnetic, or optical signals carrying digital data streams representing various types of information.

[0072] Computer system 500 can send messages and receive data, including program code, via a network, network link, and communication interface 538. In an Internet instance, the server can transmit application request code via the Internet through an ISP, local network, and / or communication interface 538. Received code can be executed by core 530 upon receipt and / or stored in storage device 536 or other non-volatile storage device for later execution.

[0073] The embodiments discussed herein to illustrate the disclosed techniques should not be considered limiting, but rather merely examples of implementations. For instance, multiple astigmatism correctors can be used to correct both astigmatism and linear distortion, rather than just the two astigmatism correctors discussed. Different numbers of astigmatism corrector components can be placed along the column at the discussed location and similar operations can be performed, for example, with only minor variations in their excitation energy. Those skilled in the art will understand that numerous other ways in which the disclosed techniques can be implemented are considered herein and within the scope of this disclosure.

[0074] An example of a multi-beam scanning electron microscope (MB-SEM) for correcting both astigmatism and linear distortion includes at least an electron source coupled to provide an electron beam; an aperture plate containing an array of apertures arranged to form an array of electron small beams from the electron beam; and an electron column including a plurality of lenses and a first and a second astigmatism reducer coupled to guide the array of electron small beams to a sample, wherein the first and second astigmatism reducers are arranged and excited to correct both astigmatism and linear distortion.

[0075] In the above example, the first astigmatism canceller is positioned at the common beam crossing plane.

[0076] In the above example, the first astigmatism reducer is arranged in a plane with a condenser lens.

[0077] In the above example, the first astigmatism corrector corrects astigmatism.

[0078] In the above example, the second astigmatism canceller is arranged between the individual beam crossing plane and the common crossing plane.

[0079] In the above example, the second astigmatism reducer is arranged between the objective lens and the lens located between the objective lens and the condenser lens.

[0080] In the above example, the first astigmatism reducer is arranged between the second astigmatism reducer and the objective lens.

[0081] In the above example, the second astigmatism corrector essentially corrects linear distortion.

[0082] In the above example, both the first astigmatism canceller and the second astigmatism canceller are octet devices.

[0083] In the above example, the first astigmatism corrector corrects astigmatism, and the first and second astigmatism correctors are combined to correct linear distortion.

[0084] In the above example, the first astigmatism reducer is arranged in the plane with the condenser lens, and the second astigmatism reducer is arranged between the objective lens and the lens located between the condenser lens and the objective lens.

[0085] Another example for correcting both astigmatism and linear distortion is a multi-beam scanning electron microscope (MB-SEM), which includes at least an electron source coupled to provide an electron beam; an aperture plate containing an array of apertures arranged to form an array of small electron beams from the electron beam; a plurality of lenses; a first astigmatism reducer and a second astigmatism reducer; and an electron column comprising the plurality of lenses and the first and second astigmatism reducers. The electron column is coupled to guide the array of small electron beams to a sample, wherein the column forms at least one individual beam crossing plane due to the plurality of lenses, wherein each of the electron beams forms an intermediate image of the electron source; and at least one single common beam crossing plane, wherein the electron beams in the array cross each other, wherein an odd number of crossing planes, including both the individual beam crossing planes and the common beam crossing plane, are located between the first and second astigmatism reducers.

[0086] In the above examples, the first astigmatism reducer is arranged at the common beam crossing plane or in the plane with the condenser lens, or arranged adjacent to the common beam crossing plane or in the plane with the condenser lens.

[0087] In the above example, the first astigmatism corrector corrects astigmatism.

[0088] In the above examples, the second astigmatism reducer is arranged between the individual beam crossing plane and the common beam crossing plane, or between the objective lens and the lens located between the objective lens and the condenser lens.

[0089] In the above example, the first and second astigmatism correctors are combined to correct linear distortion.

[0090] In the above example, the first astigmatism corrector corrects astigmatism, and the first and second astigmatism correctors are combined to correct linear distortion.

[0091] In the above example, the first astigmatism reducer is arranged in the plane with the condenser lens, and the second astigmatism reducer is arranged between the objective lens and the lens located between the condenser lens and the objective lens.

[0092] In the above examples, the multiple lenses include at least a gun lens, a condenser lens, an intermediate lens, and an objective lens.

[0093] In the above example, multiple lenses are arranged in the order listed from the electron source to the sample.

[0094] In the above example, the perforated plate includes an n×n array of holes.

[0095] Another example of correction for both astigmatism and linear distortion is a multi-beam scanning electron microscope (MB-SEM), which includes at least a source coupled to provide an electron beam, an aperture array plate arranged to provide multiple probe beams from the electron beam, a first astigmatism reducer and a second astigmatism reducer, and a controller coupled to at least control the excitation of the first and second astigmatism reducers. The controller includes code that, when executed by the controller, causes the controller to determine the deformation intensity of the two astigmatism reducers, determine the astigmatism intensity of the two astigmatism reducers based on the deformation intensity and the astigmatism intensity, determine at least two linear combinations of excitation of the two astigmatism reducers to affect astigmatism and linear distortion, and excite the first and second astigmatism reducers based on the two linear combinations to adjust for astigmatism and linear distortion.

[0096] An example method for exciting multiple astigmatism reducers to correct astigmatism and first-order distortion under a multi-beam scanning electron microscope includes at least determining the deformation intensity of two astigmatism reducers, determining the astigmatism intensity of the two astigmatism reducers based on the deformation intensity and astigmatism intensity of the two astigmatism reducers, determining at least two linear combinations of excitation of the two astigmatism reducers to affect astigmatism and linear distortion, and exciting the two astigmatism reducers based on the two linear combinations to adjust astigmatism and linear distortion.

[0097] In the above example method, determining the deformation strength of the two astigmatism cancellers includes determining the probe position of each of the multiple probe beams for various astigmatism canceller settings.

[0098] In the above example method, the various astigmatism cancellation settings for the two astigmatism cancellers include both astigmatism cancellers being off, the first astigmatism canceller being on and the second astigmatism canceller being off, and the second astigmatism canceller being on and the first astigmatism canceller being off.

[0099] In the above example method, determining the position of each of the multiple detector beams for various astigmatism correction settings includes imaging each detector beam position.

[0100] In the example method described above, the deformation strength is the linear distortion per unit excitation caused by each of the two astigmatism cancellers.

[0101] In the above example method, determining the astigmatism intensity of the two astigmatism cancellers includes based on the multi-beam trajectory that appears between the first and second astigmatism cancellers, and determining the ratio between the deformation intensity and the astigmatism intensity of the two astigmatism cancellers.

[0102] In the above example method, the astigmatic strength is determined based on the ratio and the determined deformation strength.

[0103] In the above example method, one of the two linear combinations changes only the linear distortion but keeps the astigmatism unchanged, while the other of the two linear combinations changes only the astigmatism but keeps the linear distortion unchanged.

[0104] In the above example method, adjusting astigmatism and linear distortion by exciting two astigmatism reducers based on two linear combinations includes eliminating astigmatism by using a combination of excitations from the two astigmatism reducers based on two linear combinations; measuring the position of each of the multiple probe beams based on the probe beam position; determining the amount of linear distortion; and eliminating the determined linear distortion based on the two linear combinations while continuing to eliminate astigmatism.

[0105] In the above example method, eliminating the determined linear distortion based on two linear combinations while continuing to eliminate astigmatism includes adjusting the excitation of at least one of the two astigmatism cancellers to remove the determined linear distortion while maintaining astigmatism.

[0106] Another example of a multi-beam scanning electron microscope (MB-SEM) includes at least a source coupled to provide an electron beam, an aperture array plate arranged to provide multiple probe beams from the electron beam, a first astigmatism reducer and a second astigmatism reducer, and a controller coupled to at least control the excitation of the first and second astigmatism reducers. The controller includes code that, when executed by the controller, causes the controller to determine the deformation intensity of the two astigmatism reducers, determine the astigmatic intensity of the two astigmatism reducers based on the deformation intensity and astigmatic intensity, determine at least two linear combinations of excitation of the two astigmatism reducers to affect astigmatism and linear distortion, and excite the first and second astigmatism reducers based on the two linear combinations to adjust for astigmatism and linear distortion.

[0107] In the MB-SEM example above, the code used to determine the deformation intensity of the two astigmatism cancellers includes code that, when executed by the controller, causes the controller to determine the probe position of each of the multiple probe beams for various different astigmatism canceller settings.

[0108] In the MB-SEM example above, the various astigmatism correction settings for the two astigmatism corrections include both astigmatism cancellers being off, the first astigmatism canceller being on and the second astigmatism canceller being off, and the second astigmatism canceller being on and the first astigmatism canceller being off.

[0109] In the MB-SEM example above, a camera was used to determine the position of each probe beam.

[0110] In the MB-SEM example above, the deformation intensity is a linear distortion per unit excitation caused by the two astigmatism reducers.

[0111] In the MB-SEM example above, the code used to determine the astigmatism intensity of the two astigmatism reducers includes code that, when executed by the controller, causes the controller to determine the ratio between the deformation intensity of the electron column and the astigmatism intensity based on the complete multi-beam trajectory, including the common cross plane and the individual cross planes.

[0112] In the MB-SEM example above, the astigmatic intensity is determined based on the ratio and the determined deformation intensity.

[0113] In the MB-SEM example above, one of the two linear combinations changes only the linear distortion but keeps the astigmatism unchanged, while the other of the two linear combinations changes only the astigmatism but keeps the linear distortion unchanged.

[0114] In the MB-SEM example above, the code used to adjust astigmatism and linear distortion by exciting the first and second astigmatism reducers based on a combination of two linear combinations includes code that, when executed by the controller, causes the controller to eliminate astigmatism by combining the excitations of the two astigmatism reducers based on the code combination of the two linear combinations, measure the position of each of the multiple probe beams, determine the linear distortion based on the probe beam positions, and eliminate the determined linear distortion while continuing to eliminate astigmatism based on the two linear combinations.

[0115] In the MB-SEM example above, the code for eliminating the determined linear distortion while continuing to eliminate astigmatism includes code that, when executed by the controller, causes the controller to adjust the excitation of at least one of the two astigmatism cancellers to remove the determined linear distortion while maintaining astigmatism.

Claims

1. A device comprising: An electron source, coupled to provide an electron beam; An orifice plate comprising an array of orifices, the orifice plate being arranged to form an array of small electron beams from the electron beam; An electron column, comprising multiple lenses and a first and a second astigmatism corrector, is coupled to guide the electron beam array toward a sample. The first and second astigmatism correctors are arranged and activated to correct both astigmatism and linear distortion. The first astigmatism corrector is arranged in the plane of a condenser lens, and the second astigmatism corrector is arranged between an objective lens and a lens positioned between the objective lens and the condenser lens. The electron column is formed by the multiple lenses. At least one individual beam crossing plane, wherein each electron beam in the electron beam forms an intermediate image of the electron source, the at least one individual beam crossing plane appearing in the plane of the lens between the objective lens and the condenser lens, and At least one single common beam crossing plane, wherein the electron beams in the array cross each other, the at least one single beam crossing plane appearing in the plane of the condenser lens.

2. The device according to claim 1, wherein the first astigmatism corrector corrects astigmatism.

3. The device of claim 1, wherein the second astigmatism reducer is arranged between the individual beam crossing plane and the common crossing plane.

4. The device according to claim 1, wherein the first astigmatism reducer is disposed between the second astigmatism reducer and the objective lens.

5. The device of claim 3, wherein the second astigmatism corrects substantially linear distortion.

6. The device of claim 1, wherein the first astigmatism corrects astigmatism, and the first astigmatism corrector and the second astigmatism corrector together correct linear distortion.

7. An apparatus comprising: An electron source, coupled to provide an electron beam; An orifice plate comprising an array of orifices, the orifice plate being arranged to form an array of small electron beams from the electron beam; Multiple lenses; First astigmatism reducer and second astigmatism reducer; and An electron column, comprising the plurality of lenses and a first astigmatism reducer and a second astigmatism reducer, is coupled to guide the electron beam array toward the sample, wherein the electron column is formed by the plurality of lenses. At least one individual beam crossing plane, wherein each electron beam in the electron beam forms an intermediate image of the electron source, the at least one individual beam crossing plane appearing in the plane of the lens between the objective lens and the condenser lens, and At least one single-beam crossing plane, wherein the electron beams in the array intersect each other, and the at least one single-beam crossing plane appears in the plane of the condenser lens. Some of the beam crossing planes appear between the first and second astigmatism correctors. The first astigmatism reducer is arranged in the plane of the condenser lens, and the second astigmatism reducer is arranged between the objective lens and the lens located between the objective lens and the condenser lens.

8. The device according to claim 7, wherein the first astigmatism corrector corrects astigmatism.

9. The device of claim 7, wherein the first astigmatism corrector and the second astigmatism corrector are combined to correct linear distortion.

10. The apparatus of claim 7, wherein the first astigmatism corrects astigmatism, and the first astigmatism and the second astigmatism are combined to correct linear distortion.

11. The device of claim 7, wherein the plurality of lenses are arranged between the electron source and the sample.

Images