Electro-optical device, method for compensating for variations in the characteristics of sub-beams - Patents.com

Abstract

Electro-optical devices and associated methods are disclosed. In one configuration, the electron-optical device projects multiple sub-beams of charged particles onto a sample. A plurality of plates are provided having respective aperture arrays defined therein. The plates include an objective lens array configured to project the sub-beams towards the sample. The aperture arrays defined in at least two of the plates each have geometric features configured to apply disturbances to corresponding target properties of the sub-beams. A controller controls potentials applied to the plates having geometric features such that the applied disturbances together substantially compensate for variations in the target properties over a range of parameters of the device.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to European Patent Application No. 21217583.0, filed December 23, 2021, and European Patent Application No. 22163356.3, filed March 21, 2022, both of which are incorporated by reference in their entireties. [Background technology]

[0002] FIELD OF THE DISCLOSURE

[0002] Embodiments provided herein relate to compensating for variations in properties of sub-beams of an electro-optic device over a range of operating configurations of the device.

[0003]

[0003] When manufacturing semiconductor integrated circuit (IC) chips, undesirable pattern defects inevitably occur on a substrate (i.e., wafer) or mask during the fabrication process, e.g., as a result of optical effects and accidental particles, thereby reducing yield. Therefore, monitoring the extent of undesirable pattern defects is an important process in the manufacture of IC chips. More generally, inspection and / or measurement of the surface of a substrate or other object / material during and / or after its manufacture is an important process.

[0004]

[0004] Charged particle beam pattern inspection tools are used to inspect objects, which may be called samples, for example to detect pattern defects. These tools typically use electron microscopy techniques, such as scanning electron microscopes (SEMs). In an SEM, a primary electron beam of relatively high energy electrons is targeted with a final deceleration step to land on the sample with a relatively low landing energy. The electron beam is focused as a probing spot on the sample. Interaction of the landing electrons from the electron beam with material structures at the probing spot causes signal electrons, such as secondary electrons, backscattered electrons or Auger electrons, to be emitted from the surface. The signal electrons may be emitted from the material structures of the sample. Scanning the primary electron beam as a probing spot across the sample surface can cause signal electrons to be emitted across the surface of the sample. By collecting these emitted signal electrons from the sample surface, the pattern inspection tool can obtain an image that is representative of the material structure of the surface of the sample. The electron optical device can be equipped with correction features to reduce aberrations. The correction features may be, for example, variations in the shape, size and / or position of apertures of an aperture array defined in the plate through which the electron beam passes. Such apertures of the aperture array may be nominally uniform, having at least similar shapes and sizes, and nominally located at grid points of a regular array. The correction features may vary, adjust or perturb the shape and / or size of the apertures, for example, depending on the location of the apertures in the aperture array, and / or may vary, adjust or perturb the position of the apertures relative to the grid points of the regular array. Thus, the variation between correction features applied to different apertures in the aperture array depends on the location of the apertures in the aperture array.

[0005]

[0005] Such correction features can be considered as geometric characteristics of the apertures of the aperture array. Because the correction features adjust the structural shape of the apertures, they are sometimes called hard-coded corrections. Hard-coded corrections can be contrasted with correction features implemented by controlling potentials applied to the plates that define the aperture array. Such hard-coded corrections cannot be easily changed and may not be optimal in all situations. It would be desirable to make hard-coded corrections effective in a wider range of scenarios. Summary of the Invention

[0006]

[0006] It is an object of the present disclosure to improve control of charged particle beams.

[0007]

[0007] According to one aspect of the present invention, there is provided an electron-optical device configured to project a multi-beam of sub-beams of charged particles onto a sample, the device comprising a plurality of plates, each having an aperture array defined within the plurality of plates, the plurality of plates comprising an objective lens array configured to project the sub-beams towards the sample, the aperture arrays defined within at least two of the plates, each aperture array having geometric features configured to apply a disturbance to a corresponding target property of the sub-beam, and a controller configured to apply and control potentials applied to the plates having the geometric features such that together the applied disturbances substantially compensate for variations in the target property over a range of parameters of the device.

[0008]

[0008] According to one aspect of the invention, there is provided a method of compensating for variations in properties of sub-beams of charged particles in a multi-beam projected onto a sample, the method comprising using a plurality of plates for projecting the sub-beams towards the sample, the plates defining respective aperture arrays and including an objective lens array for projecting the sub-beams towards the sample, the aperture arrays defined in at least two of the plates each having geometric features configured to apply a disturbance to a corresponding target property of the sub-beam, and controlling a potential applied to the plates having geometric features such that the applied disturbances together substantially compensate for variations in the target property over a range of device parameters.

[0009]

[0009] According to one aspect of the invention, there is provided a method of compensating for variations in properties of sub-beams of charged particles in a multi-beam projected onto a sample in an electron-optical device comprising a plurality of plates on each of which an aperture array is defined, the plurality of plates comprising an objective lens array, the aperture arrays defined in at least two of the plates having geometric features, the method comprising projecting the sub-beams towards the sample by acting on the sub-beams with a plate having an aperture array with the geometric features, the operating comprising applying a disturbance to a target property of the sub-beam with each of the plates, the projecting comprising applying a potential to the aperture plates and controlling the potential such that the applied disturbances together substantially compensate for the variation in the target property over a range of parameters of the device.

[0010]

[0010] The above and other aspects of the present disclosure will become more apparent from the following description of exemplary embodiments taken in conjunction with the accompanying drawings. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram illustrating an exemplary charged particle beam inspection apparatus. [Diagram 2]

[0012] 2 is a schematic diagram illustrating an example multi-beam device that is part of the example charged particle beam inspection apparatus of FIG. 1. [Diagram 3]

[0013] 1 is a schematic diagram of an exemplary electron optical column including a focusing lens array. [Figure 4]

[0014] 1 is a graph of landing energy versus resolution for an exemplary arrangement. [Diagram 5]

[0015] FIG. 2 is an enlarged diagram of the objective lens and the control lens. [Figure 6]

[0016] 1 is a schematic cross-sectional view of a portion of an objective lens array in an exemplary arrangement. [Figure 7]

[0017] FIG. 7 is a bottom view of a portion of the objective lens array of FIG. 6. [Figure 8]

[0018] FIG. 7 is a bottom view of a modified version of a portion of the objective lens array of FIG. 6. [Figure 9]

[0019] FIG. 7 is an enlarged schematic cross-sectional view of a detector incorporated in the objective lens of FIG. [Figure 10]

[0020] 1 is a schematic diagram of an exemplary electro-optical device including a macro-collimator and a macro-scanning deflector. [Figure 11]

[0021] FIG. 13 is a top view of a portion of a plate defining an aperture array having apertures with various aperture area ranges for compensating for off-axis aberrations such as field curvature. [Figure 12]

[0022] FIG. 13 is a top view of a portion of a plate defining an aperture array having apertures with a range of different ellipticities for compensating for off-axis aberrations such as astigmatism. [Figure 13]

[0023] FIG. 1 is a top view of a portion of a plate defining an aperture array having apertures displaced relative to their nominal positions to compensate for off-axis aberrations, such as distortion caused by telecentricity errors. [Figure 14]

[0024] 2 is a schematic cross-sectional view of a portion of a control lens array and an objective lens array of an electro-optical device. [Figure 15]

[0025] 1 is a schematic diagram of an exemplary electro-optical device including a beam separator; [Figure 16]

[0026] 1 is a schematic cross-sectional view of a portion of an electrode in an objective lens array showing electrode distortion (deflection). [Figure 17]

[0027] 1 is a graph of beam current versus resolution showing curves of minimum resolution for two different landing energies. [Figure 18]

[0028] 18 is a graph of FIG. 17 for each of eight different physical configurations of the system, with additional curves shown in which the landing energy was stepped from 2.5 keV to 1 keV at minimum resolution with a fixed image plane. [Figure 19]

[0029] The graph of FIG. 18 is supplemented with curves of stepped landing energy at a fixed image plane position for one of the physical configurations of the electron-optical device (i.e., one set of hard-coded corrections), and an additional curve showing the variation of beam current achieved by controlling the demagnification ratio (by controlling the potential applied to the plates of the objective lens array assembly). [Figure 20]

[0030] 1 is a schematic side view of an electron-optical device comprising a plate having geometric features configured to apply perturbations to sub-beam properties and a variation electron-optical device on an up-beam side of the plate. [Figure 21]

[0031] 1 is a graph showing how the sensitivity (Ast / Elli) of astigmatism (Ast) to changes in aperture ellipticity (Elli) varies for various plates over a range of landing energies (LE). [Figure 22]

[0032] 1 is a graph showing how the astigmatism defocus (defocus (Ast)) varies as a function of landing energy (LE) for three different combinations of plates used for compensation. [Figure 23]

[0033] 1 is a graph showing how the sensitivity (Ast / Elli) of astigmatism (Ast) to changes in aperture ellipticity (Elli) varies for various plates over a range of linear to angular demagnification ratios (M / Mα). [Figure 24]

[0034] 1 is a graph showing how the sensitivity of blur due to field curvature (blur / diameter) to changes in aperture diameter (diameter) varies for various plates over a range of linear to angular demagnification ratios (M / Mα). [Diagram 25]

[0035] 1 is a graph showing how the astigmatism blur (Blur (Ast)) varies as a function of the ratio of linear to angular demagnification for three different combinations of plates used for compensation. [Figure 26]

[0036] 26 is a graph that omits the single plate case from the graph of FIG. 25, thereby comparing the performance of compensation using two plates with the performance of compensation using three plates. [Figure 27]

[0037] 13 is a graph showing how the optimum resolution varies as a function of beam current when the beam current is varied by varying the demagnification ratio in implementations with and without multiple plates having geometric features to compensate for aberrations. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012]

[0038] Reference will now be made in detail to the exemplary embodiments, examples of which are illustrated in the accompanying drawings. The following description refers to the accompanying drawings, in which the same numbers in different drawings represent the same or similar elements unless otherwise indicated. The implementations set forth in the following description of the exemplary embodiments do not represent all implementations consistent with the present invention. Instead, the implementations are merely examples of apparatus and methods consistent with related aspects of the present invention, as described in the appended claims.

[0013]

[0039] Increases in the computational power of electronic devices, which reduces the physical size of the device, can be achieved by greatly increasing the packing density of circuit components such as transistors, capacitors, and diodes on an IC chip. This has been made possible by improvements in resolution that allow for the creation of even smaller structures. For example, an IC chip in a smartphone the size of a thumbnail and available before 2019 can contain over 2 billion transistors, each less than 1 / 1000 the size of a human hair. It is therefore not surprising that semiconductor IC manufacturing is a complex and time-consuming process with hundreds of individual steps. An error in even one step can dramatically affect the functionality of the final product. Only one "fatal defect" can cause the device to fail. The goal of a manufacturing process is to improve the overall yield of the process. For example, to obtain a 75% yield for a process with 50 steps (where a step can refer to the number of layers formed on a wafer), the individual steps must have a yield of greater than 99.4%. If each individual step had a yield of 95%, the overall process yield would be as low as 7%.

[0014]

[0040] In IC chip manufacturing facilities, while high process yields are desirable, it is also essential to maintain high substrate (i.e., wafer) throughput, defined as the number of substrates processed per hour. High process yields and high substrate throughput can be affected by the presence of defects. This is especially true when operator intervention is required to investigate the defects. Thus, high throughput detection and identification of micro- and nano-scale defects by inspection tools, such as scanning electron microscopes ("SEMs"), is essential to maintain high yields and low costs.

[0015]

[0041] The SEM includes a scanning device and a detector device. The scanning device includes an illumination device, which includes an electron source for generating primary electrons, and a projection device for scanning a sample, such as a substrate, with one or more focused beams of primary electrons. Together, at least the illumination device or illumination system and the projection device or projection system may be referred to as an electron optical device or column. The primary electrons interact with the sample and generate secondary electrons. The detector device captures the secondary electrons from the sample as it is scanned so that the SEM can generate an image of the scanned area of ​​the sample. For high throughput inspection, some inspection devices use multiple focused beams of primary electrons, i.e., multibeams. The component beams of a multibeam may be referred to as sub-beams or beamlets. The multibeams can scan different parts of the sample simultaneously. Thus, multibeam inspection devices can inspect samples much faster than single-beam inspection devices.

[0016]

[0042] Known implementations of multi-beam inspection devices are described below.

[0017]

[0043] The drawings are schematic. Thus, the relative dimensions of the components in the drawings are exaggerated for clarity. In the following description of the drawings, the same or similar reference numbers refer to the same or similar components or entities, and only the differences to the individual embodiments are described. Although the description and drawings are directed to an electro-optical device, it is understood that the embodiments are not used to limit the present disclosure to a particular charged particle. Thus, throughout this specification, references to electrons can be considered to be references to charged particles more generally, and charged particles are not necessarily electrons.

[0018]

[0044] Reference is now made to Figure 1, which is a schematic diagram illustrating an exemplary charged particle beam inspection apparatus 100, which may also be referred to as a charged particle beam characterization system or simply characterization system. The charged particle beam inspection apparatus 100 (or charged particle apparatus) of Figure 1 includes a main chamber 10, a load lock chamber 20, an electron beam tool 40, a front end equipment module (EFEM) 30, and a controller 50. The electron beam tool 40 is located within the main chamber 10.

[0019]

[0045] The EFEM 30 includes a first load port 30a and a second load port 30b. The EFEM 30 may include one or more additional load ports. The first load port 30a and the second load port 30b may receive, for example, a substrate front-opening integrated pod (FOUP) that contains a substrate (e.g., a semiconductor substrate or a substrate made of other material) or a sample to be inspected (hereinafter, substrates, wafers and samples are collectively referred to as "samples"). One or more robotic arms (not shown) in the EFEM 30 transport the sample to the load lock chamber 20.

[0020]

[0046] The load lock chamber 20 is used to remove gas from around the sample. This creates a vacuum, which is a local gas pressure lower than the pressure of the surrounding environment. The load lock chamber 20 may be connected to a load lock vacuum pumping system (not shown), which removes gas particles in the load lock chamber 20. Operation of the load lock vacuum pumping system allows the load lock chamber to reach a first pressure below atmospheric pressure. After the first pressure is reached, 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 pumping system (not shown). The main chamber vacuum pumping system removes gas particles in the main chamber 10 such that the pressure around the sample reaches a second pressure below the first pressure. After the second pressure is reached, the sample is transported to an electron beam tool, where the sample can be inspected by the electron beam tool. The electron beam tool 40 may include a multi-beam electron optical device.

[0021]

[0047] The controller 50 is electronically connected to the electron beam tool 40. The controller 50 may be a processor (such as a computer) configured to control the charged particle beam inspection apparatus 100. The controller 50 may also include processing circuitry configured to perform various signal and image processing functions. Although in FIG. 1 the controller 50 is shown as being external to the structure including the main chamber 10, the load lock chamber 20 and the EFEM 30, it is understood that the controller 50 may be part of the structure. The controller 50 may be located within one of the component elements of the charged particle beam inspection apparatus, or the controller 50 may be distributed among at least two of the component elements. It is noted that while the present disclosure provides an example of a main chamber 10 housing an electron beam inspection tool, aspects of the present disclosure are not limited in the broad sense to a chamber housing an electron beam inspection tool. Rather, it is understood that the principles described above may be applied to other tools and other arrangements of the apparatus operating under a second pressure.

[0022]

[0048] Reference is now made to Figure 2, which is a schematic diagram illustrating an exemplary electron beam tool 40 including a multi-beam inspection tool that is part of the exemplary charged particle beam inspection apparatus 100 of Figure 1. The multi-beam electron beam tool 40 (also referred to herein as apparatus 40) includes an electron source 201, a projection apparatus 230, a motorized stage 209 (or stage), and a sample holder 207. The electron source 201 and projection apparatus 230 may collectively be referred to as an illumination apparatus. The sample holder 207 is supported by a motorized or actuated stage 209 to hold a sample 208 (e.g., a substrate or mask) for inspection. The multi-beam electron beam tool 40 further includes an electron detection device 240.

[0023]

[0049] The electron source 201 may include a cathode (not shown) and an extractor or anode (not shown). In operation, the electron source 201 is configured to emit electrons from the cathode as primary electrons. The primary electrons are extracted or accelerated by the extractor and / or anode to form a primary electron beam 202.

[0024]

[0050] The projection device 230 is configured to convert the primary electron beam 202 into multiple sub-beams 211, 212, 213 and direct each sub-beam onto the sample 208. Although three sub-beams are shown for simplicity, there may be tens, hundreds, thousands or hundreds of thousands (or more) of sub-beams. The sub-beams may be referred to as beamlets.

[0025]

[0051] 1, such as the electron source 201, the electron detection device 240, the projection device 230, and the motorized stage 209. The controller 50 may perform various image and signal processing functions. The controller 50 may also generate various control signals for controlling the operation of the charged particle beam inspection apparatus, including the charged particle multi-beam device.

[0026]

[0052] The projection device 230 may be configured to focus the sub-beams 211, 212 and 213 onto the sample 208 for inspection, forming three probe spots 221, 222 and 223 on the surface of the sample 208. The projection device 230 may be configured to deflect the primary sub-beams 211, 212 and 213 to scan the probe spots 221, 222 and 223 over respective scan areas within a section of the surface of the sample 208. In response to the incidence of the primary sub-beams 211, 212 and 213 on the probe spots 221, 222 and 223 on the sample 208, electrons including secondary electrons and backscattered electrons, which may be referred to as signal particles, are generated from the sample 208. The secondary electrons typically have an electron energy of 50 eV or less, and the backscattered electrons typically have an electron energy between 50 eV and the landing energy of the primary sub-beams 211, 212 and 213.

[0027]

[0053] The electron detection device 240 is configured to detect the secondary electrons and / or the backscattered electrons and generate corresponding signals, which are transmitted to the controller 50 or a signal processing system (not shown) (e.g., to construct an image of a corresponding scanned area of ​​the sample 208). The electron detection device may be integrated into the projection system or may be separate therefrom, and a secondary optical column is provided to direct the secondary electrons and / or backscattered electrons to the electron detection device.

[0028]

[0054] The controller 50 may include an image processing system including an image acquirer (not shown) and a storage device (not shown). For example, the controller may include a processor, a computer, a server, a mainframe host, a terminal, a personal computer, any type of mobile computing device, etc., or a combination thereof. The image acquirer may include at least a portion of the processing functions of the controller. Thus, the image acquirer may include at least one or more processors. The image acquirer may be communicatively coupled to an electronic detection device 240 of the apparatus 40 that allows for signal communication, such as electrical conductors, fiber optic cables, portable storage media, IR, Bluetooth, Internet, wireless networks, wireless radios, or combinations thereof, among others. The image acquirer may receive signals from the electronic detection device 240, process data contained in the signals, and construct an image therefrom. Thus, the image acquirer may acquire an image of the sample 208. The image acquirer may also perform various post-processing functions, such as generating contours and superimposing indicators on the acquired image. The image acquirer may be configured to make adjustments, such as brightness and contrast, of the acquired image. The storage may be a storage medium such as a hard disk, a flash drive, cloud storage, random access memory (RAM), other types of computer readable memory, etc. The storage may be coupled with the image acquirer and may be used to store raw scanned image data as original images or to store post-processed images.

[0029]

[0055] The image acquirer may acquire one or more images of the sample based on the imaging signal received from the electronic detection device 240. The imaging signal may correspond to a scanning operation to perform charged particle imaging. The acquired image may be a single image including multiple imaging areas. The single image may be stored in a storage device. The single image may be an original image that may be divided into multiple regions. Each of these regions may include one imaging area that includes a feature of the sample 208. The acquired image may include multiple images of a single imaging area of ​​the sample 208 sampled multiple times over a period of time. The multiple images may be stored in a storage device. The controller 50 may be configured to perform an image processing step using multiple images of the same location of the sample 208.

[0030]

[0056] The controller 50 may include measurement circuitry (e.g., an analog-to-digital converter) to obtain a distribution of detected secondary electrons. The electron distribution data collected during the detection time window, in combination with the corresponding scan path data of each of the primary sub-beams 211, 212 and 213 incident on the sample surface, may be used to reconstruct an image of the sample structure under inspection. The reconstructed image may be used to reveal various features of the internal or external structure of the sample 208. Thus, the reconstructed image may be used to reveal any defects that may be present in the sample.

[0031]

[0057] The controller 50 may control the motorized stage 209 (or stages) to move the sample 208 during inspection of the sample 208. The controller 50 may enable the motorized stage 209 to move the sample 208 in a direction, e.g., at a constant speed, preferably continuously, at least during inspection of the sample. The controller 50 may control the movement of the motorized stage 209 such that the motorized stage 209 varies the speed of the movement of the sample 208 depending on various parameters. For example, the controller may control the stage speed (including its direction) depending on the inspection steps and / or scanning characteristics of the scanning process, at least as far as the combined step / scan strategy of the stage is concerned, e.g., as disclosed in European Patent Application Publication No. A21171877.0, filed May 3, 2021, which is incorporated herein by reference.

[0032]

[0058] FIG. 3 is a schematic diagram of an exemplary electron-optical column used in the evaluation system. The electron-optical column is an example of an electron-optical device. For ease of illustration, the lens array is generally depicted herein by an array of ellipses. Each elliptical shape represents one of the lenses in the lens array. An elliptical shape is conventionally used to represent a lens by analogy with the biconvex shape often adopted by optical lenses. However, it should be understood that in the context of the charged particle arrangement as discussed herein, the lens array does not require any physical element to adopt a biconvex shape, since it usually operates electrostatically. As described below, the lens array may instead include multiple plates having apertures. Each plate having an aperture may be referred to as an electrode. The electrodes may be provided in series along the sub-beam paths of the sub-beams of the multi-beam.

[0033]

[0059] The electron source 201 directs electrons towards an array of condenser lenses 231 forming part of the projection device 230. The electron source is preferably a high brightness thermal field emitter with a good compromise between brightness and total emission current. There may be tens, hundreds, or thousands or even tens of thousands of condenser lenses 231. The array of condenser lenses 231 may include multi-electrode lenses and may have a structure based on EP 1602121 A1, which is incorporated herein by reference for its disclosure of a lens array for splitting an electron beam into a number of sub-beams (providing one lens per sub-beam). The condenser lens array may take the form of at least two (preferably three) plates acting as electrodes, with apertures in each plate aligned with apertures in the other plates to define the paths of the sub-beams through the plate. At least two of the plates are maintained at different potentials during operation to achieve the desired lens effect. Between the plates of the focusing lens array are electrically insulating plates, made of an insulating material such as ceramic or glass, with one or more apertures for each sub-beam. In an alternative arrangement, one or more of the plates may feature apertures, each with its own electrode, for example arranged with an array of electrodes around its periphery or as a group of apertures with a common electrode.

[0034]

[0060] In one configuration, the focusing lens array is formed from a three-plate array in which a charged particle has the same energy when it enters each lens as when it leaves each lens, and this configuration can be called an Einzel lens. Dispersion therefore occurs only within the Einzel lens itself (between the entrance and exit electrodes of the lens), thereby limiting off-axis chromatic aberration. If the focusing lens is thin, for example a few mm, the effect of such aberrations is small or negligible.

[0035]

[0061] Each collecting lens in the array directs electrons into a respective sub-beam 211, 212, 213 that is focused at a respective intermediate focus 233. A collimator or an array of collimators may be positioned to operate on each intermediate focus 233. The collimator may take the form of a deflector 235 provided at the intermediate focus 233. The deflector 235 is configured to bend each beamlet 211, 212, 213 by an effective amount to ensure that the chief ray (which may also be referred to as the beam axis) is incident on the sample 208 approximately normal (i.e. at approximately 90° to the nominal surface of the sample).

[0036]

[0062] Below the deflector 235 (i.e. on the down-beam side of or further from the electron source 201) there is a control lens array 250 which includes a control lens 251 for each sub-beam 211, 212, 213. The control lens array 250 may for example include two or more, preferably at least three, plate electrode arrays in contact with respective potential sources between the electrodes, preferably with insulating plates in contact with the electrodes. Each of the plate electrode arrays may be referred to as a control electrode. The function of the control lens array 250 is to optimize the beam opening angle with respect to the beam reduction ratio and / or to control the beam energy delivered to the objective lenses 234, each of which directs the respective sub-beam 211, 212, 213 onto the sample 208.

[0037]

[0063] Optionally, an array of scan deflectors 260 is provided between the control lens array 250 and the array of objective lenses 234 (objective lens array). The array of scan deflectors 260 includes one scan deflector 261 for each sub-beam 211, 212, 213. Each scan deflector is configured to deflect a respective sub-beam 211, 212, 213 in one or two directions to scan the sub-beam in one or two directions across the sample 208.

[0038]

[0064] A detector module 402 of the detector is provided between the objective lens 234 and the sample 208 to detect signal electrons / particles emitted from the sample 208. An exemplary configuration of such a detector module 402 is described below. It is noted that the detector may additionally or alternatively have detector elements on the up-beam side along the primary beam path of the objective lens array or even the control lens array.

[0039]

[0065] The electron optical device of FIG. 3 is configured to control the landing energy of electrons on the sample by varying the potentials applied to the electrodes of the control lens and the objective lens. The control lens and the objective lens work together and may be referred to as the objective lens assembly. The landing energy may be selected to increase the emission and detection of secondary electrons depending on the nature of the sample being evaluated. The controller may be configured to control the landing energy to any desired value within a predetermined range or to a desired one of a plurality of predetermined values. In one embodiment, the landing energy may be controlled to a desired value within a predetermined range (e.g., 1000 eV to 5000 eV). FIG. 4 is a graph depicting the resolution as a function of landing energy assuming that the beam opening angle / reduction ratio is reoptimized for varying landing energy. As can be seen, the resolution of the evaluation tool may remain approximately constant with the change in landing energy up to a minimum value LE_min. The resolution deteriorates below LE_min because it is necessary to reduce the lens strength of the objective lens and the electric field within the objective lens to maintain a minimum spacing between the objective lens and / or detector and the sample. Interchangeable modules may also be employed to vary or control the landing energy, as discussed further below.

[0040]

[0066] Preferably, the landing energy is varied primarily by controlling the energy of the electrons exiting the control lens. The potential difference in the objective lens is preferably kept constant during this variation so that the electric field in the objective lens remains as high as possible. The potential applied to the control lens can be used to optimize the beam opening angle and the beam demagnification. The control lens can also be called a refocusing lens because it can function to correct the focal position to account for changes in landing energy. Preferably, each control lens includes three electrodes to provide two independent control variables as discussed further below. For example, one of the electrodes can be used to control the demagnification while a different electrode can be used to independently control the landing energy. Alternatively, each control lens can have only two electrodes. In contrast, if there are only two electrodes, one of the electrodes may be required to control both the demagnification and the landing energy.

[0041]

[0067] FIG. 5 is an enlarged schematic diagram of one objective lens 300 of the objective lens array and one control lens 600 of the control lens array 250. The objective lens 300 may be configured to demagnify the electron beam by a factor of more than 10 (preferably in the range of 50-100 or more). The objective lens includes a central or first electrode 301, a lower or second electrode 302 and an upper or third electrode 303. Voltage sources V1, V2, V3 are configured to apply potentials to the first, second and third electrodes, respectively. Another voltage source V4 is connected to the sample for applying a fourth potential, which may be ground potential. The potentials may be defined relative to the sample 208. The first, second and third electrodes each include an aperture through which the respective sub-beam propagates. The second potential may be similar to the potential of the sample (e.g. in the range of 50V-200V more positive than the sample). Alternatively, the second potential can be in the range of about +500V to about +1,500V relative to the sample. A higher potential can be useful if the detector module 402 is higher in the optical column than the lowest electrode. The first and / or second potential can be varied for each aperture or group of apertures to achieve focus correction.

[0042]

[0068] Desirably, in one embodiment, the third electrode is omitted. Objective lenses with only two electrodes may have lower aberrations than objective lenses with more electrodes. Three-electrode objective lenses may allow for a more powerful lens since they may have a larger potential difference between the electrodes. Additional electrodes (i.e., more than two electrodes) provide additional degrees of freedom to control the electron trajectory, for example to focus not only the incident beam but also secondary electrons.

[0043]

[0069] As mentioned above, it is desirable to use a control lens to determine the landing energy. However, it is possible to additionally use the objective lens 300 to control the landing energy. In such a case, the potential difference across the objective lens is changed if a different landing energy is selected. An example of a situation where it is desirable to partially change the landing energy by changing the potential difference across the objective lens is to prevent the focus of the sub-beam from being too close to the objective lens. This situation can arise, for example, if the landing energy is lowered. This is because the focal length of the objective lens is roughly scaled depending on the landing energy selected. By lowering the potential difference across the objective lens, and thereby lowering the electric field inside the objective lens, the focal length of the objective lens is made larger again, resulting in a focus position further down the objective lens.

[0044]

[0070] In the depicted arrangement, the control lens 600 includes three electrodes 601-603 connected to potential sources V5-V7. The electrodes 601-603 may be spaced apart from each other by a few millimeters (e.g., 3 mm). The spacing between the control lens and the objective lens (i.e., the gap between the lower electrode 602 and the upper electrode of the objective lens) may be selected from a wide range (e.g., from 2 mm to 200 mm or more). A small separation distance makes alignment easier, while a larger separation distance allows weaker lenses to be used, reducing aberrations. Desirably, the potential V5 of the top electrode 603 of the control lens 600 is maintained the same as the potential of the next electron optical element on the up-beam side of the control lens (e.g., deflector 235). The potential V7 applied to the lower electrode 602 may be varied to determine the beam energy. The potential V6 applied to the central electrode 601 may be varied to determine the lens strength of the control lens 600 and thus to control the opening angle and demagnification of the beam. Preferably, the lower electrode 602 of the control lens and the top electrode of the objective lens and the sample have approximately the same potential. In one design, the upper electrode of the objective lens V3 is omitted. In this case, preferably, the lower electrode 602 of the control lens and the electrode 301 of the objective lens have approximately the same potential. It should be noted that the control lenses can be used to control the beam divergence angle even if the landing energy does not need to be changed or is changed by other means. The position of the focal points of the sub-beams is determined by a combination of the action of the respective control lens and the action of the respective objective lens.

[0045]

[0071] In one example, to obtain landing energies in the range of 1.5 kV to 2.5 kV, the potentials V1, V2, V4, V5, V6 and V7 may be set as shown in Table 1 below. The potentials in this table are given as values ​​of beam energy in keV that are equivalent to the electrode potential for the cathode of the beam source 201. It will be understood that there is considerable design freedom in designing the electro-optical device as to which points in the electro-optical device are set to ground potential, and that operation of the electro-optical device is determined by potential differences rather than absolute potentials.

[0046] [Table 1]

[0047]

[0072] It can be seen that the beam energies at V1, V3, and V7 are the same. In some embodiments, the beam energies at these points can be between 10 keV and 50 keV. If lower potentials are chosen, the electrode spacing (especially within the objective lens) can be reduced to limit the reduction in the electric field.

[0048]

[0073] If a control lens (rather than a condenser lens) is used for electron beam opening angle / demagnification correction, there is no need for a collimator stigmator since the collimator remains at the intermediate focal point. In addition, the landing energy can be varied over a wide range of energies while maintaining the optimum field strength inside the objective lens. This minimizes the aberrations of the objective lens. The strength of the condenser lens (if used) is also kept constant, avoiding the introduction of any additional aberrations due to the collimator not being at the intermediate focal plane or due to changes in the path of the electrons through the condenser lens.

[0049]

[0074] In some embodiments, the charged particle evaluation tool further includes one or more aberration correctors that reduce one or more aberrations in the sub-beams. In one embodiment, each of at least a subset of the aberration correctors is positioned at or directly adjacent to a respective one of the intermediate foci (e.g., in or adjacent to an intermediate image plane). The sub-beams have a minimum cross-sectional area in or near a focal plane such as the intermediate plane. This provides more space for the aberration correctors available elsewhere (i.e., on the up-beam or down-beam side of the intermediate plane) (or more space than would be available in an alternative configuration that does not have an intermediate image plane).

[0050]

[0075] In one embodiment, an aberration corrector positioned in or directly adjacent to the intermediate focus (or intermediate image plane) includes a deflector to correct the source 201 to appear at different positions for the different beams. The corrector can be used to correct macroscopic aberrations originating from the source that prevent good alignment between each sub-beam and the corresponding objective lens.

[0051]

[0076] The aberration correctors may correct aberrations that prevent proper column alignment. Such aberrations may also lead to misalignment between the sub-beams and the correctors. For this reason, it may be desirable to additionally or alternatively position the aberration correctors at or near the collection lens array 231 (e.g., each such aberration corrector is integrated with or directly adjacent to one or more of the collection lenses 231). This is desirable because aberrations at or near the collection lens array 231 do not yet lead to misalignment of the corresponding sub-beams because the collection lenses are close to or aligned perpendicular to the beam aperture. However, a challenge with positioning the correctors at or near the collection lenses is that each sub-beam has a relatively large cross-sectional area and a relatively small pitch at this location compared to locations further downstream. The aberration corrector may be a CMOS-based individual programmable deflector as disclosed in EP 2 702 595 A1, the description of which is incorporated herein by reference, or an array of multi-pole deflectors as disclosed in EP 2 715 768 A2.

[0052]

[0077] In some embodiments, at least a subset of the aberration correctors are each integrated with or directly adjacent to one or more of the objective lenses 234. In one embodiment, these aberration correctors reduce one or more of field curvature, focus error, and astigmatism. Additionally or alternatively, one or more scanning deflectors (not shown) may be integrated with or directly adjacent to one or more of the objective lenses 234 for scanning the sub-beams 211, 212, 214 across the sample 208. In one embodiment, a scanning deflector as described in U.S. Patent Application Publication No. 2010 / 0276606, the entirety of which is incorporated herein by reference, may be used.

[0053]

[0078] In some embodiments, the objective lens array assembly includes a detector having a detector module 402 on the down-beam side of at least one electrode of the objective lens array 241. The detector module 402 may take the form of a detector array. In one embodiment, at least a portion of the detector is adjacent to and / or integrated with the objective lens array 241. For example, the detector module 402 may be implemented by integrating a CMOS chip detector into the bottom electrode of the objective lens array 241. The integration of the detector module 402 into the objective lens array replaces the secondary column. The CMOS chip is preferably oriented to face the sample (due to the short distance between the wafer and the bottom of the electron optics, e.g., 100 μm). In one embodiment, electrodes for capturing secondary electron signals are formed in the top metal layer of the CMOS device. The electrodes may be formed in other layers of the substrate (e.g., of the CMOS chip). Power and control signals of the CMOS chip may be connected to the CMOS chip by through-silicon vias. For robustness, the bottom electrode preferably consists of two elements: the CMOS chip and a passive silicon plate with a hole. The passive silicon plate shields the CMOS chip from high electric fields.

[0054]

[0079] To maximize detection efficiency, it is desirable to make the electrode surface as large as possible so that nearly all the area of ​​the objective lens array (excluding the aperture) is occupied by electrodes and each electrode has a diameter approximately equal to the array pitch. In one embodiment, the electrode outline is circular, but can be made square to maximize the detection area. The diameter of the substrate through-holes can also be minimized. The typical size of the electron beam is about 5-15 micrometers.

[0055]

[0080] In one embodiment, a single electrode surrounds each aperture. In another embodiment, multiple electrode elements are provided around each aperture. Electrons captured by the electrode elements surrounding an aperture may be combined into a single signal or may be used to generate independent signals. The electrode elements may be divided radially (i.e., to form multiple concentric rings), angularly (i.e., to form multiple sectors), both radially and angularly, or in any other convenient manner.

[0056]

[0081] However, a larger electrode surface leads to a larger parasitic capacitance and therefore a narrower bandwidth. For this reason, it may be desirable to limit the outer diameter of the electrodes, especially if a larger electrode not only gives a slightly higher detection efficiency but also a slightly higher capacitance. Circular (annular) electrodes may offer a good compromise between collection efficiency and parasitic capacitance.

[0057]

[0082] A larger outer diameter of the electrode can also lead to a larger crosstalk (sensitivity to signals of neighboring holes). This can also be a reason to make the electrode outer diameter smaller, especially if a larger electrode gives slightly more detection efficiency but also significantly more crosstalk.

[0058]

[0083] The backscattered and / or secondary electron current collected by the electrode is amplified by a transimpedance amplifier.

[0059]

[0084] An exemplary embodiment of a detector integrated into an objective lens array is shown in FIG. 6, which shows a schematic cross-sectional view of a portion of a multi-beam objective lens 401. In this embodiment, the detector includes a detector module 402 including a plurality (e.g., an array) of detector elements 405 (e.g., sensor elements such as capture electrodes), preferably as an array of detector elements (i.e., a plurality of detector elements in a pattern or arrangement preferably over a two-dimensional surface). In this embodiment, the detector module 402 is provided at the output side of the objective lens array. The output side is the output side of the objective lens 401. FIG. 7 is a bottom view of the detector module 402 including a substrate 404 on which are provided a plurality of capture electrodes 405, each of which surrounds a beam aperture 406. The beam aperture 406 may be formed by etching the substrate 404. In the configuration shown in FIG. 7, the beam aperture 406 is shown in a rectangular array. The beam aperture 406 may also be arranged in a different manner (e.g., in a close-packed hexagonal array as depicted in FIG. 8).

[0060]

[0085] FIG. 9 depicts a portion of the detector module 402 in cross section, but on a larger scale. The capture electrodes 405 form the bottom (i.e., closest to the sample) surface of the detector module 402. Between the capture electrodes 405 and the body of the silicon substrate 404 is a logic layer 407. The logic layer 407 may include amplifiers (e.g., transimpedance amplifiers), analog-to-digital converters, and readout logic. In one embodiment, there is one amplifier and one analog-to-digital converter per capture electrode 405. Circuits featuring these elements may be contained within a unit area called a cell, which is associated with an aperture. The detector module 402 may have several cells, each associated with an aperture, and preferably the cells have a similar shape. The logic layer 407 and the capture electrodes 405 may be fabricated using a CMOS process, with the capture electrodes 405 forming the final metallization layer.

[0061]

[0086] The wiring layer 408 is provided on the backside or inside the substrate 404 and is connected to the logic layer 407 by through silicon vias 409. The number of through silicon vias 409 does not have to be the same as the number of beam apertures 406. In particular, if the electrode signals are digitized in the logic layer 407, only a few through silicon vias may be needed to provide a data bus. The wiring layer 408 may include control lines, data lines, and power lines. It is noted that there is enough space for all the necessary connections despite the beam apertures 406. The detector module 402 may also be manufactured by using bipolar manufacturing techniques or other manufacturing techniques. Printed circuit boards and / or other semiconductor chips may be provided on the backside of the detector module 402.

[0062]

[0087] The integrated detector module 402 described above is particularly advantageous when used with tools having adjustable landing energy since secondary electron capture can be optimized for a range of landing energies. Detector modules in the form of arrays can also be integrated into other electrode arrays as well as the bottom electrode array. Further details and alternative configurations of detector modules integrated into the objective lens can be found in EP 20184160.8, which is incorporated herein by reference.

[0063]

[0088] FIG. 10 is a schematic diagram of another exemplary electron-optical column used in the evaluation system. This electron-optical column is a further example of an electron-optical device. The electron-optical column includes an objective lens array assembly. The objective lens array assembly includes an objective lens array 241. The objective lens array 241 includes a plurality of objective lenses. Each objective lens includes at least two electrodes (e.g., two or three electrodes) connected to a respective potential source. The objective lens array 241 may include two or more (e.g., three) plate electrode arrays connected to respective potential sources. The plate electrode arrays of the objective lens array 241 may be referred to as objective electrodes. Each objective lens formed by the plate electrode arrays may be a microlens that serves a different sub-beam or group of sub-beams in the multi-beam. Each plate defines a plurality of apertures (which may also be referred to as holes). The position of each aperture in the plate corresponds to the position of a corresponding aperture (or corresponding hole) in the other plate (or multiple other plates). The corresponding apertures define an objective lens such that each set of corresponding holes serves, when in use, the same sub-beam or group of sub-beams in the multi-beam. Each objective lens projects a respective sub-beam of the multi-beam onto sample 208. See also the description of objective lens 234.

[0064]

[0089] In some arrangements, the apertures in the objective lens array 241 are adapted to compensate for off-axis aberrations in the multi-beam. For example, the apertures of one or more of the objective electrodes may be shaped, sized, and / or positioned to compensate for off-axis aberrations. The apertures may have, for example, a series of different areas (or a series of diameters) to compensate for field curvature, a series of different ellipticities to compensate for astigmatism, and / or a series of different displacements from a nominal grid position to compensate for distortions caused by telecentricity errors. See, for example, EP-A-21166214.3, which is incorporated herein by reference insofar as off-axis aberration correction is concerned.

[0065]

[0090] FIG. 11 shows an exemplary arrangement of apertures in an aperture array with different areas to compensate for off-axis aberrations such as field curvature. At least a subset of the apertures have different aperture area ranges. The aperture area variations shown in FIG. 11 are exaggerated for clarity and will be smaller than shown. The solid circles represent apertures with different aperture area ranges. The dashed circles represent unaltered aperture dimensions to aid in the visualization of the variations shown in the aperture areas. Different aperture areas can be described by reference to the diameter of circles with the same aperture area. Thus, aperture areas can be described by reference to the diameter even if the corresponding apertures are not necessarily circular. This variation will often involve an increase in aperture area as a function of increasing distance from the major axis of the multibeam (as shown diagrammatically in FIG. 11, the major axis is perpendicular to the page and passes through the most central aperture). It is also possible that a suitable correction will involve a decrease in aperture area as a function of increasing distance from the major axis of the multibeam. In the embodiment shown in FIG. 11, the apertures are arranged on a regular lattice defined by lattice points 801 and lattice lines 802 .

[0066]

[0091] FIG. 12 shows an example configuration of apertures in an aperture array with a range of ellipticities to compensate for off-axis aberrations such as astigmatism. At least a subset of the apertures have a range of different ellipticities. The variation in ellipticities shown in FIG. 12 is exaggerated for clarity and is smaller than shown. The range of different ellipticities is selected to compensate for off-axis aberrations such as astigmatism. This variation may involve an increase in the size of a radially oriented axis, which may be the aperture's primary axis, as a function of increasing distance from the multibeam's primary axis (as shown diagrammatically in FIG. 12, the primary axis is perpendicular to the page and passes through the central-most aperture). Appropriate compensation may also involve an increase in the size of an azimuthally oriented axis, which may be the aperture's primary axis, as a function of increasing distance from the multibeam's primary axis. In the embodiment shown in FIG. 12, the apertures are arranged on a regular grid defined by grid points 801 and grid lines 802.

[0067]

[0092] FIG. 13 shows an example configuration of apertures in an aperture array with a range of different displacements from a nominal grid position to compensate for off-axis aberrations such as distortion caused by telecentricity errors. At least a subset of the apertures are displaced with respect to the nominal positions. Such nominal positions may correspond to intersections between grid lines 802 of the grid. Each of the apertures is displaced with respect to a corresponding nominal position 801 on the grid. The nominal positions may be provided on a regular grid. The regular grid may include, for example, a rectangular, square, or hexagonal grid. The nominal positions may represent positions corresponding to an ideal configuration with no off-axis aberrations. The displacements shown in FIG. 13 are exaggerated for clarity and are smaller than shown. The displacements cause the apertures to be located on a grid that is distorted (shown by thick dashed lines) with respect to the nominal grid (shown by grid lines 802). The off-axis aberrations compensated for by the displacements may include distortion caused by telecentricity errors. The displacement can be radially inward (towards the major axis of the multibeam), as illustrated in Figure 13, or radially outward. In either case, the magnitude of the displacement can increase with increasing radial distance. In the simplified embodiment of Figure 13, this causes the corner apertures to have a larger displacement than the side apertures.

[0068]

[0093] The objective lens array assembly further includes a control lens array 250. The control lens array 250 includes a plurality of control lenses. Each control lens includes at least two electrodes (e.g., two or three electrodes) connected to a respective potential source. The control lens array 250 may include two or more (e.g., three) plate electrode arrays connected to a respective potential source. The plate electrode arrays of the control lens array 250 may be referred to as control electrodes. The control lens array 250 is associated with the objective lens array 241 (e.g., the two arrays are disposed near each other and / or mechanically connected to each other and / or controlled together as a unit). The control lens array 250 is disposed on the up-beam side of the objective lens array 241. The control lenses pre-focus the sub-beams (e.g., apply a focusing action to the sub-beams prior to the sub-beams reaching the objective lens array 241). Pre-focusing may reduce the divergence of the sub-beams or increase the convergence rate of the sub-beams. The control lens array and the objective lens array work together to provide a compound focal length. Compounding without intermediate foci may reduce the risk of aberrations.

[0069]

[0094] In one embodiment, the electro-optical device including the objective lens array assembly is configured to control the objective lens assembly (e.g., by controlling the potentials applied to the electrodes of the control lens array 250) such that the focal length of the control lens is greater than the separation distance between the control lens array 250 and the objective lens array 241. Thus, the control lens array 250 and the objective lens array 241 may be positioned relatively close to each other, and the focusing effect from the control lens array 250 is too weak to form an intermediate focus between the control lens array 250 and the objective lens array 241. In other embodiments, the objective lens array assembly may be configured to form an intermediate focus between the control lens array 250 and the objective lens array 241.

[0070]

[0095] In one embodiment, the control lens array is either a replaceable module by itself or in combination with other elements such as an objective lens array and / or a detector module. The replaceable module may be field replaceable, i.e., the module may be swapped with a new module by a field engineer. Field replaceable is intended to mean that the module may be removed and replaced with the same or a different module while maintaining the vacuum in which the electro-optical tool 40 is located. Only a section of the column corresponding to the replaceable module is ejected in order for the replaceable module to be removed and replaced back or replaced.

[0071]

[0096] The control lens array may be in the same module as the objective lens array 241 (ie may form an objective lens array assembly or objective lens arrangement) or may be in a separate module.

[0072]

[0097] A power supply may be provided for applying respective electrical potentials to electrodes of the control lenses of the control lens array 250 and to electrodes of the objective lenses of the objective lens array 241 .

[0073]

[0098] The provision of the control lens array 250 in addition to the objective lens array 241 provides an additional degree of freedom for controlling the properties of the sub-beams. This additional degree of freedom is provided even if the control lens array 250 and the objective lens array 241 are provided relatively close to each other, e.g. such that no intermediate focus is formed between the control lens array 250 and the objective lens array 241. The control lens array 250 can be used to optimize the beam divergence angle with respect to the beam demagnification and / or to control the beam energy delivered to the objective lens array 241. The control lens can include two or more electrodes. If there are two electrodes, the demagnification and landing energy are controlled together. If there are three or more electrodes, the demagnification and landing energy can be controlled independently. Thus, the control lens can be configured to adjust the demagnification and / or beam divergence angle of each sub-beam (e.g. by using a power supply to apply appropriate respective potentials to the electrodes of the control lens and the objective lens). This optimization can be achieved by unduly affecting the number of objective lenses and without unduly exacerbating the aberrations of the objective lenses (eg, without increasing the strength of the objective lenses).

[0074]

[0099] The control lens array 250 can be considered to provide electrodes in addition to, for example, the electrodes of the objective lens array 241. The objective lens array 241 can have any number of additional electrodes, for example 5, 7, 10, or 15, associated with and adjacent to the objective lens array 241. Such additional electrodes, for example of the control lens array 250, allow for additional degrees of freedom in controlling the electron-optical parameters of the sub-beams. Such additional associated electrodes can be considered to be additional electrodes of the objective lens array 241, enabling additional functionality of the respective objective lenses of the objective lens array 241. In one configuration, such electrodes can be considered to be part of the objective lens array 241, providing additional functionality to the objective lenses of the objective lens array 241. Thus, the control lenses are considered to be part of the corresponding objective lenses, even to the extent that the control lenses are only referred to as being part of the objective lenses.

[0075]

[0100] In the embodiment of FIG. 10, the electron optical device includes a source 201. The source 201 provides a beam of charged particles (e.g., electrons). The multi-beams that are focused on the sample 208 are derived from the beam provided by the source 201. The sub-beams may be derived from the beam, for example by using a beam limiter that defines an array of beam-limiting apertures. The source 201 is preferably a high-brightness thermal field emitter with a good compromise between brightness and total emission current. In the example shown, a collimator is provided on the up-beam side of the objective lens array assembly. The collimator may include a macro-collimator 270. The macro-collimator 270 acts on the beam from the electron source 201 before the beam is split into the multi-beams. The macro-collimator 270 bends the respective portions of the beam by an effective amount to ensure that the beam axis of each of the sub-beams derived from the beam is incident on the sample 208 approximately perpendicularly (i.e., approximately 90° to the nominal surface of the sample 208). The macro-collimator 270 applies macro-collimation to the beam. Thus, the macro-collimator 270 may act on all of the beam rather than including an array of collimator elements each configured to act on a different individual portion of the beam. The macro-collimator 270 may include a magnetic lens or may include a magnetic lens configuration including multiple magnetic lens subunits (e.g., multiple electromagnets forming a multi-pole configuration). Alternatively or in addition, the macro-collimator may be implemented at least in part electrostatically. The macro-collimator may include an electrostatic lens or may include an electrostatic lens configuration including multiple electrostatic lens subunits. The macro-collimator 270 may use a combination of magnetic and electrostatic lenses.

[0076]

[0101] In the embodiment of FIG. 10, a macro scan deflector 265 is provided to cause the sub-beams to scan across the sample 208. The macro scan deflector 265 deflects the respective portions of the beam to cause the sub-beams to scan across the sample 208. In one embodiment, the macro scan deflector 256 includes a macro multi-pole deflector, for example with 8 poles or more. The deflection is such that the sub-beams derived from the beam are scanned across the sample 208 in one direction (e.g. parallel to a single axis, such as the X axis) or two directions (e.g. with respect to two non-parallel axes, such as the X axis and the Y axis). In some arrangements, the scanning of the sub-beams is coordinated with the motion of the sample 208. For example, the combination of scanning the sub-beams parallel to the X axis while moving the sample 208 parallel to the Y axis can be repeated at various step positions of the sample to process multiple parallel elongated strips on the sample 208. The larger motion of the sample 208 can then be used to jump to a new processing location on the sample 208. An example of this motion is described in European Patent Application Publication No. A-21171877.0, filed 3 May 2021, which is incorporated herein by reference insofar as beam scanning with stage motion is concerned. The macro scan deflector 265 does not comprise a series of deflector elements, each configured to act on a different individual portion of the beam, but rather acts macroscopically on all of the beam. In the embodiment shown, the macro scan deflector 265 is provided between the macro collimator 270 and the control lens array 250.

[0077]

[0102] Any of the objective lens array assemblies described herein may further include a detector (e.g., including a detector module 402). The detector detects charged particles emitted from the sample 208. The detected charged particles may include any of the charged particles detected by an SEM (including secondary electrons and / or backscattered electrons emitted from the sample 208). An exemplary structure of the detector module 402 is described above with reference to Figures 6-9.

[0078]

[0103] In a variation of the embodiment of FIG. 10, the objective lens array assembly may include a scan deflector array. The scan deflector array includes a plurality of scan deflectors. Each scan deflector scans a respective sub-beam across the sample 208. Thus, the scan deflector array may include a scan deflector for each sub-beam. The deflection is such that the sub-beam is scanned across the sample 208 in one or two directions (i.e., one-dimensionally or two-dimensionally). In one embodiment, the scan deflectors described in EP 2425444, which is incorporated herein by reference in its entirety, particularly with regard to the scan deflectors, may be used to implement the scan deflector array. The scan deflector array is positioned between the objective lens array 241 and the control lens array 250. The scan deflector array may be provided in place of the macro scan deflector 265. In other embodiments, both the macro scan deflector 265 and the scan deflector array may be provided and operated synchronously. In some embodiments, as illustrated in FIG. 10, the control lens array 250 is the first deflection or lens electro-optic array element in the beam path on the down beam side of the source 201 .

[0079]

[0104] A collimator element array may be provided instead of the macro collimator 270. Although not shown, this variant can also be applied to the embodiment of FIG. 3 to provide an embodiment with a macro scanning deflector and a collimator element array. Each collimator element collimates a respective sub-beam. The collimator element array may be more spatially compact than the macro collimator 270. Thus, providing the collimator element array and the scanning deflector array 260 together may provide space savings. This space savings is desirable when multiple electro-optical devices including objective lens array assemblies are provided in the electro-optical device array. In such an embodiment, there may not be any macro condenser lens or condenser lens array. Thus, in this scenario, the control lens provides the possibility to optimize the beam opening angle and reduction ratio for varying landing energy.

[0080]

[0105] In one embodiment, an electro-optical device in the form of an array is provided. The array may comprise a plurality of any of the electro-optical devices (e.g., electro-optical columns) described herein. Each of the electro-optical devices in the array focuses a respective multi-beam simultaneously onto a different area of ​​the same sample. Each of the electro-optical devices in the array may form sub-beams from a beam of charged particles from a different respective source 201. Each source 201 may be one source of the plurality of sources 201. At least a subset of the plurality of sources 201 may be provided as a source array. The source array may comprise a plurality of emitters on a common substrate. By focusing the multiple multi-beams simultaneously onto different areas of the same sample, a larger area of ​​the sample 208 may be processed (e.g., evaluated) simultaneously. The electro-optical devices in the array may be positioned adjacent to each other to project the respective multi-beams onto adjacent areas of the sample 208. Any number of electro-optical devices may be used in the array. Preferably, the number of electro-optical devices is in the range of 9 to 200. In one embodiment, the electro-optical devices are arranged in a rectangular array or in a hexagonal array. In other embodiments, the electro-optical devices are provided in an irregular array or in a regular array having a geometric shape other than a rectangle or hexagon. When referring to a single electro-optical device or system or column, each electro-optical device in the array may be configured in any of the ways described herein. As mentioned above, the scanning deflector array 260 and the collimator element array 271 are particularly suitable for incorporation in the form of an array within the electro-optical device, since they are spatially compact, facilitating the placement of the electro-optical devices in the array close to each other.

[0081]

[0106] FIG. 14 shows a portion of a further example of an objective lens array assembly. The lens array assembly comprises a series of electrodes 501-504 arranged in series orthogonal and / or along the sub-beam paths of the multi-beam. Although four electrodes are shown and described, variants can feature any number of electrodes, and thus the lens array assembly may comprise at least four electrodes. This objective lens array assembly can be used in the configuration of FIG. 10 and is an embodiment of the configuration of FIG. 5. The objective lens array assembly comprises a control lens array 250, 600 and an objective lens array 241, 300. In the configuration shown in and described with reference to FIG. 5, the relative potential of the electrode 303 on the up-beam side of the objective lens and the electrode 602 on the most down-beam side of the control lens are substantially identical. It should also be noted that the lensing effect of the electrodes is provided by the surfaces of the electrodes. As shown in FIG. 14, the control lens array 250 can be defined by a number of electrodes 501-503, or at least by the up-beam surfaces of the electrodes 501 and 503. Each electrode 501, 502, 503 contributing to the control lens array (which may be referred to as a control electrode even if only at least one electrode partially contributes to the control lens array) may comprise a plate defining an aperture for each sub-beam path 510 (the apertures together are the aperture array). The objective lens array 241 may be defined by a number of objective electrodes 503, 504, or by at least the down-beam surface of the electrode 504 and the electrode 503. Each electrode 503, 504 contributing to the objective lens array (which may be referred to as an objective electrode even if only at least one electrode partially contributes to the objective lens array) may comprise a plate having an aperture for each sub-beam path 510 (the apertures together are the aperture array). The control electrodes 501-503 and the objective electrodes 503, 504 may be referred to as lens electrodes. (In a variant, electrode 503, which may be referred to as both the object electrode and the control electrode, may be two separate electrodes spaced along the path of the sub-beam and having substantially the same applied potential).

[0082]

[0107] Five exemplary sub-beam paths 510 are shown in FIG. 14. Control electrodes 501-503 are arranged in series along the sub-beam paths 510 and define respective apertures aligned with the sub-beam paths 510 to define control lenses. Thus, each control lens is aligned with the sub-beam path 510 of a respective sub-beam and acts on the sub-beam (e.g., electrostatically steers the sub-beam). Each control electrode 501-503 may act on some or all of the sub-beams. Each objective lens in the objective lens array 241 may be aligned with the sub-beam paths 510 aligned with the respective control lens. The objective lens array 241 directs the sub-beams onto the sample 208.

[0083]

[0108] This arrangement may be described as four or more lens electrodes that are a plate. Defined in the plate are apertures (e.g., an aperture array) that are aligned with the many beams in the corresponding beam array. The electrodes may be grouped into two or more electrodes, for example to provide a group of control electrodes and a group of objective electrodes. In one arrangement, the group of objective electrodes has at least three electrodes and the group of control electrodes has at least two electrodes.

[0084]

[0109] In the example of FIG. 14, the objective electrode 503 furthest from the sample 208 (which may be referred to as the up-beam-most electrode of the objective lens array 241) and the control electrode 503 closest to the sample 208 (which may be referred to as the down-beam-most electrode of the control lens array 250) are provided by a common electrode. Thus, the up-beam-most electrode of the objective electrode group is also a common electrode that is a member of the control electrode group. The surface of the common electrode 503 facing away from the sample 208 (which may be described as the up-beam surface) functionally contributes to the control lens array and thus may be considered to comprise a part of the control lens array. The surface of the common electrode 503 facing the sample 208 (which may be referred to as the down-beam surface) functionally contributes to the objective lens array 241 and thus may be considered to comprise a part of the objective lens array 241.

[0085]

[0110] The provision of a common electrode is beneficial when it is desired that the control lens array 250 be positioned close to the objective lens array 241. This is likely to be the case in arrangements where an array of scan deflectors 260 is not used (e.g. when a macro scan deflector 265 is used instead). This is because, when an array of scan deflectors 260 is used, it is desirable to position the array of scan deflectors 260 between the control lens array 250 and the objective lens array 241 (e.g. to keep the distance between the scan deflector 260 and the objective lens array 241 as short as possible). An arrangement with a macro scan deflector 265 is illustrated in FIG. 10. However, it should be noted that variations on the arrangement of FIG. 10 are possible that still do not have a focusing lens array but have an array of scan deflectors. In such an arrangement, it may be desirable to position the array of scan deflectors between the control lens array and the objective lens array. Alternatively, the array of scanning deflectors may be positioned within the control lens array or elsewhere, such as on the up-beam side of the control lens array (such as between the control lens array and the array of beam-limiting apertures).

[0086]

[0111] In this example of FIG. 14, the objective lens array assembly further includes a beam shaping limiter 242. The beam shaping limiter 242 defines an array of beam limiting apertures. The beam shaping limiter 242 may be called a beam shaping limiting aperture array or a final beam limiting aperture array. The beam shaping limiter 242 may include a plate having a plurality of apertures. The beam shaping limiter 242 is on the down-beam side of at least one electrode (optionally all electrodes) of the control lens array 250. In some embodiments, the beam shaping limiter 242 is on the down-beam side of at least one electrode (optionally all electrodes) of the objective lens array 241. In another embodiment, it may be an array (e.g. the bottom array of the objective lens array 241).

[0087]

[0112] In one arrangement, the beam-shaping restrictor 242 is structurally integrated with the electrodes of the objective lens array 241. Each beam-limiting aperture has a beam-limiting effect that allows only a selected portion of a sub-beam incident on the beam-shaping restrictor 242 to pass through the beam-limiting aperture 124. The selected portion may cause only a portion of each sub-beam that passes through a central portion of the respective aperture in the objective lens array to reach the sample 208.

[0088]

[0113] In some embodiments, the electron-optical device further includes an upper beam limiter 252. The upper beam limiter 252 defines an array of beam-limiting apertures or generates an array of beams from, for example, a source beam from the source 201. The upper beam limiter 252 may include a plate with a number of apertures. The upper beam limiter 252 forms sub-beams from the beam of charged particles emitted by the source 201. Portions of the beam other than those that contribute to the formation of the sub-beams may be blocked (e.g., absorbed) by the upper beam limiter 252 so as not to interfere with the sub-beams on the down beam side.

[0089]

[0114] The upper beam limiter 252 may form part of the objective lens array assembly. The upper beam limiter 252 may, for example, be adjacent to and / or integrated with the control lens array 250 (e.g., adjacent to and / or integrated with even the electrode of the control lens array 250 closest to the source 201). In one embodiment, the upper beam limiter 252 defines a beam-limiting aperture that is larger (e.g., has a larger cross-sectional area) than the beam-limiting aperture of the beam-shaping limiter 242. Thus, the beam-limiting aperture of the beam-shaping limiter 242 may be of smaller dimensions than the corresponding aperture defined in the objective lens array 241 and / or in the control lens array 250.

[0090]

[0115] The beam shaping limiter 242 is desirably configured to have a beam limiting effect (i.e. to remove a portion of each sub-beam that is incident on the beam shaping limiter 242). The beam shaping limiter 242 may, for example, be configured to ensure that each sub-beam exiting the objective lens array 241 has passed through the center of a respective objective lens. Furthermore, the beam shaping limiter 242 reduces the length over which the scan affects the sub-beams. This distance is reduced to the length of the beam path from the beam shaping limiter 242 to the sample surface.

[0091]

[0116] The beam shaping limiter 242 may be integrally formed with the bottom electrode of the objective lens array 241. It is usually desirable to position the beam shaping limiter 242 adjacent to the electrode of each objective lens having the strongest lens effect. In one arrangement, it is desirable to provide the beam shaping limiter 242 on the up-beam side of the detector module 402 of the detector. Providing the detector module 402 on the up-beam side of the beam shaping limiter 242 ensures that the beam shaping limiter 242 does not interfere with the charged particles emitted from the sample 208 and prevents these charged particles from reaching the detector module 402. Thus, the beam shaping limiter 242 may be provided directly adjacent to the detector module 402 in the up-beam direction.

[0092]

[0117] FIG. 15 shows a further embodiment of an electron-optical device (electron-optical column) in a schematic way. The same features as described above are given the same reference numbers. For the sake of brevity, such features will not be detailed with respect to FIG. 15. For example, the source 201, the condenser lens 231, the macro-collimator 270, the objective lens array 241 and the sample 208 may be as described above. In a variant, the macro-collimator 270 may comprise a deflector array configured to collimate the sub-beams. Alternatively, the collimator may be an array of deflectors configured to collimate the sub-beams. In one configuration, the condenser lens may be a single plate for a beam-limiting aperture array, in which multiple apertures are defined with one or more associated macro-electrodes having a single aperture. Such a beam-limiting aperture array and associated macro-electrodes may also form a condenser lens array to focus the generated beams to an intermediate focus that preferably corresponds to the position of the collimator 270.

[0093]

[0118] As mentioned above, in one embodiment, the detector 240 is between the objective lens array 241 and the sample 208. The detector 240 may face the sample 208. Alternatively, as shown in FIG. 15, in one embodiment, the objective lens array 241 comprising multiple objective lenses is between the detector 240 and the sample 208.

[0094]

[0119] In one embodiment, a deflector array 95 is between the detector 240 and the objective lens array 241. In one embodiment, the deflector array 95 comprises a Wien filter array, and thus the deflector array may be referred to as a beam separator. The deflector array 95 is configured to provide a magnetic field to separate the charged particles projected onto the sample 208 from secondary electrons emitted from the sample 208 towards the detector 240.

[0095]

[0120] In one embodiment, the detector 240 is configured to detect the signal particles with reference to the energy of the charged particles, i.e., depending on the band gap for such semiconductor-based type detectors. Such a detector 240 may be called an indirect current detector. The secondary electrons emitted from the sample 208 gain energy from the field between the electrodes. The secondary electrons have sufficient energy once they reach the detector 240. In a different arrangement, the detector 240 may be an electron-photon converter, e.g., a scintillator array, of fluorescent strips between the beams, located on the up-beam side along the main beam path relative to the Wien filter. The main beam passing through the Wien filter array (of magnetic and electrostatic strips orthogonal to the main beam path) has a substantially parallel path on the up-beam side and the down-beam side of the Wien filter array, while the signal electrons from the sample are guided by the Wien filter array towards the scintillator array. The electron-photon converter may be optically coupled to the photon-electron converter to convert any photons generated in and emitted by the electron-photon converter. The photon-electron converter may be electrically connected to an electronic circuit for processing the detection signal. In different embodiments, the photon-to-electron converter may be internal or external to the electro-optical device. In one embodiment, photons may be coupled via a photon transport unit (e.g., an array of optical fibers) to a remote photodetector, which generates a detection signal upon detecting a photon.

[0096]

[0121] During operation, the electro-optic device generates strong electric fields between the electrodes (e.g., plates) of the objective lens array 241. Significant electric fields may also be generated between electrodes elsewhere in the system. Strong electric fields are accompanied by correspondingly strong electrostatic pressures. The electrostatic pressures are characterized by a field energy density η E , and the field energy density is proportional to η E = 1 / 2εE 2 According to E 2 where ε is the dielectric constant and E is the electric field strength. Thus, the electrostatic pressure increases rapidly with increasing E.

[0097]

[0122] In some arrangements, the electrostatic pressure induces a change in the shape and / or position of one or more of the electrodes (plates). Such a change in shape and / or position may be referred to as electrode distortion or field-induced distortion of the plate. FIG. 16 shows such electrode distortions in schematic form for the electrodes 503 and 504 of the objective lens array 241. For ease of illustration, the electrodes 503, 504 are shown without apertures and are not to scale. The dashed lines in the form of a rectangle represent an exemplary cross-sectional shape of the electrodes 503, 504 before the electron-optical system is turned on (i.e., when there is no electric field between the electrodes 503, 504). In this example, the electrodes 503, 504 are substantially planar at this stage. The solid rectangle shows an exemplary cross-sectional shape of the electrodes 503, 504 when the electron-optical system is turned on and an exemplary electric field is present between the electrodes. FIG. 16 shows a typical case where the electrodes are deflected into a region of high electric field strength. This mode of electrode distortion may be referred to as bowing. The deflection may cause a strain that has a parabolic or nearly parabolic shape, that is, the strain varies as an approximate function of the square of the radial position.

[0098]

[0123] Electrode distortions in the objective lens array 241 may affect the sub-beams of the multibeam. The electrode distortions may contribute, for example, to field curvature. The field curvature is such that the focal plane is different for different sub-beams of the multibeam, which may cause focus errors at the flat surface of the sample 208. It is possible to configure the objective lens array to compensate for the predicted effects on the sub-beams from the predicted electrode distortions in the objective lens array. Thus, the objective lens array may be provided with hardware corrections (which may be referred to as hard-coded corrections). In some arrangements, the hardware corrections include variation in the size (e.g., diameter if the aperture is circular) of the apertures defined in one or more of the electrodes depending on the position within each electrode. By varying the size of the apertures in the electrodes, the change in field curvature can be compensated for.

[0099]

[0124] Finite manufacturing tolerances limit the accuracy of electrode distortion prediction. Finite manufacturing tolerances lead to small but significant variations (such as differences in electrode thickness and / or aperture size) between various manufacturing instances of the objective lens array 241. These variations can affect the stiffness of the electrodes, resulting in a given electrostatic pressure being accompanied by different electrode distortions for various manufacturing instances. This variability means that hardware corrections of the type described above may not achieve optimal compensation. For a typical implementation of the type shown in FIG. 3, it is predicted that electrode distortions can lead to surface displacements of up to about 10 micrometers. If a budgeted defocus amount of 100 nm is allocated to this effect, this would mean that electrode distortions should be reproducible within 1% if hardware corrections (e.g. by changing the aperture diameter) are effective. It is not desirable to be constrained by such tight manufacturing tolerances. For an arrangement of the type shown in FIG. 10, electrode distortions are expected to be much smaller, but supporting adjustable landing energy is particularly desirable in such systems. Adjusting the landing energy can lead to significant changes in the electrostatic field in the objective lens array, again leading to inadequate hardware correction. The arrangements described below aim to improve compensation for the effects of electrode distortion. Improved compensation may allow manufacturing tolerances to be relaxed (e.g., from 1% to 10%) and / or support adjustable landing energy functionality. An arrangement is described below that allows varying the landing energy while simultaneously suppressing field curvature and astigmatism.

[0100]

[0125] The arrangement comprising the plates defining the aperture array (e.g. including the control lens array 250 and / or the objective lens array 241) may be configured to perform various functions as described below. These functions may be performed by controlling the plates (e.g. the control lens array 250 and / or the objective lens array 241), for example by controlling potentials applied to the plates. A controller 500 (as shown diagrammatically in Figs. 3 and 10) may be provided for this purpose. As described below, the controller 500 may be computer implemented with any suitable combination of elements (e.g. CPU, RAM, etc.) used to provide the required functionality. As described above with reference to Fig. 5, the control and objective electrodes may be controlled by connecting the electrodes to potential sources. Thus, the controller 500 may comprise and / or control potential sources that apply potentials to the various lens electrodes.

[0101]

[0126] In this specification, references to a device or system being configured to perform a function are intended to include the case where the controller 500 is configured to perform the function (e.g., by being appropriately programmed to provide the necessary control signals to an apparatus such as a potential source).

[0102]

[0127] In some arrangements, the electro-optical device is configured (e.g., via the controller 500) to implement a plurality of selectable landing energies for the sub-beams of the multi-beam (optionally for all of the sub-beams). The plurality of selectable landing energies can be implemented by applying corresponding potentials to the control electrodes 501-503 and the object electrodes 503 and 504 (e.g., via respective potential sources). Different potentials may be applied to each of the selectable landing energies. Thus, the device allows different landing energies to be selected for the sub-beams at different corresponding times. The selectable landing energies may include one or more continuous ranges of landing energies. In this case, the device allows for any landing energy within one or more continuous ranges to be selected. Alternatively or additionally, the selectable landing energies may include a plurality of predetermined discrete landing energies. The selection may be made by a user. Thus, the device may receive user input (e.g., via a user interface of a computer system or as an input data stream) and may select the selectable landing energies based at least in part (i.e., fully or partially) on the received user input. Alternatively or in addition, the device may operate at least partly (i.e. fully or partly) automatically. The device may, for example, select the landing energy based at least partly on a predefined program or in response to one or more input parameters determined, for example, by an application or model. The input parameters may, for example, represent measurements made by the system.

[0103]

[0128] The choice of landing energy may depend on the particular inspection scenario. For example, the landing energy may be selected to be optimized for parameters of a particular type of signal particle, such as secondary electron gain and contrast (defined as the gain difference between the feature and background). The landing energy that achieves this is a function of the material being inspected. The nature of the defect of interest may also play a role. If physical defects are of interest, the material properties will determine the secondary electron gain. If voltage contrast defects are of interest, the charging behavior, and therefore the secondary electron gain, will depend on whether the circuit is able to drain the charge. Alternatively or in addition, the landing energy can be selected to control charging (which affects distortion as well as secondary electron gain). Alternatively or in addition, the landing energy can be selected to achieve the desired electron optical performance. For example, the degradation of resolution caused by choosing a lower landing energy may be traded off for improved secondary electron emission gain.

[0104]

[0129] The electro-optical device may be configured (e.g., via the controller 500) to select corresponding potentials for various landing energies such that the spatial relationship between the image plane of the electro-optical device (e.g., on the sample) and all of the control electrodes 501-503 and object electrodes 503 and 504 is the same for each of the selectable landing energies. Thus, the separation between all of the electrodes, and the separation between each of the electrodes and the image plane of the device, remains the same regardless of which selectable landing energy is implemented. Thus, a user may select various landing energies without having to adjust the position of any of the electrodes 501-504, the sample 208, or the detector module 402.

[0105]

[0130] In some arrangements, the device is configured (e.g., via the controller 500) to apply the same potential to the control electrode 501 furthest from the sample 208 (and to the portion of the control lens aligned with the sub-beam path of the sub-beam for which the landing energy is selected) for at least a portion of the selectable landing energies. The potential applied to the control electrode 501 may be determined, for example, by the beam energy delivered by the source module. The source module provides a charged particle beam from which the sub-beams are derived. The control electrode 501 may be fixed, for example, to a potential corresponding to a beam energy between 10 keV and 50 keV.

[0106]

[0131] In some arrangements, the device is configured (e.g., via the controller 500) to apply a different potential to the objective electrode 503 furthest from the sample 208 (and to the portion of the objective lens aligned with the sub-beam path of the sub-beam for which the landing energy is selected) for each of at least a portion of the selectable landing energies. Each potential may be selected, for example, to provide the same distance between the objective electrode 503 and the image plane of the system. The potential applied to the objective electrode 503 determines the field strength of the electric field at the objective lens array 241 and thus the focal length of each objective lens. Thus, by controlling the potential applied to the objective electrode 503, it is possible to control the position of the image plane.

[0107]

[0132] In some arrangements, the device is configured (e.g., via the controller 500) to control the landing energy (i.e., to select a desired landing energy from a range of available selectable landing energies) by controlling a potential applied to at least the objective electrode 504 closest to the sample (and to a portion of the objective lens aligned in the sub-beam path of the sub-beam for which the landing energy is selected). For example, the objective electrode 504 may be set to a potential corresponding to a beam energy equal to the desired landing energy plus a predetermined offset. The predetermined offset may be in the range of, for example, -50 eV to 300 eV. The offset is used to set the electric field strength at the sample surface. The electric field strength plays a role in determining the secondary electron contrast, especially when using voltage contrast. Where secondary electrons are to be detected, the offset voltage may typically be about 50 V or more to ensure adequate detection efficiency, although a lower offset voltage may be appropriate if the distance between the detector and the sample is small enough. For example, if backscattered signals are of interest, a negative voltage is used when it is desired to bounce the secondary electrons.

[0108]

[0133] In some arrangements, the device is configured to control (e.g., via the controller 500) the control lens array 250 to minimize the resolution for each of a plurality of selectable landing energies. This may be achieved, at least in part, by adjusting the control lens array 250 at each of the selectable landing energies to keep the demagnification ratio of the device (from the electron source to the sample) divided by the angular demagnification ratio of the device the same for each of the selectable landing energies. This is achieved, for example, by controlling a potential applied to a middle, preferably central, control electrode 502 of the control lens array 250 (note, for example, that if the control lens array 250 is defined by three control electrodes 501-503, the middle electrode may be only the central control electrode of a lens array having an odd number of electrodes). By controlling the potential applied to the middle, preferably central, control electrode 502, the demagnification ratio is controlled. Keeping the demagnification ratio divided by the angular demagnification ratio the same for various landing energies ensures that off-axis aberrations remain constant. Thus, the hard-coded corrections for off-axis aberrations remain effective, thereby keeping the net aberrations low for various landing energies (after corrections). This is achieved without the need to replace each objective electrode or objective lens array 241, which can undesirably introduce downtime and / or inconvenience. This further obviates or reduces the need to have field replaceable electronic components as interchangeable modules. Thus, the complexity required for the vacuum chamber can be reduced.

[0109]

[0134] FIG. 17 is a graph showing the predicted variation of the beam current of the sub-beams versus the sub-beam resolution at the sample 208 for an electron-optical system using an electron-optical device without a condenser lens array, e.g. as shown in FIG. 10 featuring a macro condenser lens 270, or without a macro electron-optical component, e.g. as shown in FIG. 15, such a macro electron-optical component may be magnetic. The curves were obtained by simulating an electrostatic field and ray tracing electrons through the field. Curve 521 corresponds to a landing energy of 2.5 keV. Curve 522 corresponds to a landing energy of 1 keV. For each curve, each distinct point on the curve represents a distinct physical configuration of the electron-optical column optimized for each combination of beam current and resolution (e.g. including hard-coded off-axis compensation). It is usually desirable that the total current is high to achieve good throughput, and that the resolution is minimized to provide good spatial resolution for the measurement. The graph shows that a balance must be achieved between the two quantities; that is, increasing the beam current increases the resolution and vice versa. The curves of beam current versus resolution are even more different for different landing energies.

[0110]

[0135] FIG. 18 is a graph showing eight example curves (solid curves 523 with open boxes) illustrating the selection of different landing energies in eight different physical configurations of the electro-optical device. For each configuration represented by a corresponding one of the curves 523, a number of different landing energies are selected in steps of 250 eV between a landing energy of 2.5 keV corresponding to curve 521 and a landing energy of 1 keV corresponding to curve 522. At each selected landing energy, the electro-optical system controls the control lens array 250 to minimize the resolution by varying the demagnification ratio. This can be achieved, for example, by keeping the demagnification ratio of the electro-optical system divided by the angular demagnification ratio of the electro-optical system constant to ensure that hard-coded off-axis aberration compensation remains effective and / or by varying the demagnification ratio of the control lens array to compensate for other effects such as changes in field curvature caused by distortion of elements (e.g., electrodes) caused by the electric field. In each case, the potentials applied to the control and object electrodes are also selected to maintain the same spatial relationship between the image plane and all of the control and object electrodes. Thus, each curve 523 shows a range of different landing energies and the corresponding beam currents and minimized resolution that are made available.

[0111]

[0136] In some arrangements, in contrast to the situation illustrated in FIG. 18, the resolution is not intentionally minimized. With the removal of this restriction, it is possible to select a range of different beam currents for each selectable landing beam energy. Thus, instead of restricting the beam current to correspond to one of the curves 523 of FIG. 18, the beam current can take other values. A higher beam current can be selected at the expense of a greater (less optimal) resolution. By allowing the beam current to vary in this way, the electron optical system thus provides a plurality of selectable beam currents of the sub-beams for each one or more of the selectable landing beam energies. Thus, the beam current can be selected from a range of selectable beam currents at the selected landing beam energy, in exchange for operating with a greater resolution.

[0112]

[0137] FIG. 19 is a graph showing four example curves (solid curves 524-527) illustrating how the beam current can be varied in the above manner. Each curve 524-527 has a different landing energy (524=2.5 keV, 525=2.0 keV, 526=1.5 keV, 527=1 keV), but corresponds to the same physical shape (same beam limiting aperture diameter and hard-coded off-axis correction), and the same image plane. Each curve 524-527 has a shape similar to a parabola rotated 90 degrees. In curve 527, both the upper and lower branches of the parabola are shown. In curves 524-526, only the upper branch (i.e., the branch where the beam current increases with increasing resolution) is shown for clarity, but both branches are present. The device can be configured to implement each of the selectable beam currents by selecting a corresponding demagnification ratio of the control lens array 250. Each demagnification ratio corresponds to a different beam current. In the embodiment of FIG. 19, different points on each curve 524-527 correspond to different demagnification ratios. The demagnification ratio may be adjusted for each curve 524-527 to optimize resolution. Alternatively, the demagnification ratio may be adjusted to allow for a larger beam current while accepting degradation in resolution. As mentioned above, if the control lens array 250 comprises three control electrodes 501-503, each demagnification ratio may be selected by applying a corresponding potential to the central electrode 502 of the three control electrodes 501-503. Thus, the electron-optical system may be configured to implement each of the selectable beam currents by applying a corresponding potential to the central electrode 502 of the three control electrodes 501-503.

[0113]

[0138] As mentioned in the introductory part of this specification, hard-coded corrections in the electro-optical device are not optimal in all situations. Such hard-coded corrections may be applied to the objective lens, the preferred electrodes of the control lens, or other relevant electrodes or plates. For example, corrections for field curvature and astigmatism may be hard-coded into the diameter and ellipticity of the apertures in the aperture array in a plate of the objective lens array (e.g., a specific plate of the objective lens array). Variations in field curvature and astigmatism may occur during the use of the electro-optical device, which makes the hard-coded corrections suboptimal.

[0114]

[0139] In some configurations of the electro-optic device, for example, changing the landing energy may result in a change in the field curvature, since if the image plane is kept in the same place, a change in the landing energy may need to be accompanied by a change in the electric field between the plates of the objective lens array 241. As described above with reference to Figure 16, the electric field induces distortions, e.g., bending, of the plates due to electrostatic pressure. These distortions contribute to aberrations, e.g., field curvature, of the sub-beams.

[0115]

[0140] In some embodiments, the variable electron-optical device on the up-beam side of the plate also contributes to the field curvature. This is the case, for example, in configurations such as those shown in Figures 10 and 15, where the macro-collimator 270 (which may also be called a macroscopic condenser lens) also contributes significantly to the field curvature. The magnitude of the field curvature contributed by the macro-collimator 270 can be on the order of 1-3 micrometers in the image plane of the objective lens array, depending on the demagnification (which can be controlled using the control lens array 250 as described above). Note that the demagnification referred to here is the overall system demagnification measured by the combination of all lenses along the beam path of the electron-optical device, and that the overall system demagnification is that of the electron-optical system. Although each sub-beam passes through its own set of lenses, the demagnification is approximately the same for all sub-beams. Thus, the overall system demagnification can be quantified by averaging over the sub-beams, or by referencing the demagnification of a reference sub-beam, such as the axially centered sub-beam. The field curvature contributed by the macro-collimator 270 may be of opposite sign and of equal or greater magnitude than the field curvature variation caused by the field-induced distortion of the plate in the objective lens array 241. Thus, to compensate for the field curvature variation caused by the field-induced distortion of the plate, the magnitude of the field curvature applied by the macro-collimator 270 can be controlled. This magnitude can be controlled, for example, by controlling the demagnification ratio using the control lens array 250. This magnitude is expected to vary in proportion to the linear demagnification ratio divided by the angular demagnification ratio. However, such a varying electro-optic device (e.g., the macro-collimator 270) may introduce variations in properties of the sub-beams other than the field curvature. For example, in the arrangements of Figs. 10 and 15, the macro-collimator 270 will contribute to the field curvature as well as astigmatism. Calculations of the expected field-induced distortion suggest that the plate may deflect by about 200-400 nm over a typical range of landing energies. The magnitude of the defocus caused by the astigmatism applied by the macro-collimator 270 is approximately half the magnitude of the field curvature applied by the macro-collimator 270 .Note that astigmatism is with respect to focus (e.g., in a direction from the source to the sample, e.g., along the z-axis relative to the plane of the sample 208), and not with respect to blur or resolution (e.g., in a direction substantially parallel to the plane of the sample 208, e.g., in the plane of the x and y axes). This means that over a typical landing energy range, astigmatism can vary by about 100-200 nm, which represents a very large negative contribution to the defocus performance.

[0116]

[0141] As the landing energy is changed, the demagnification applied by the control lens array 250 can be adjusted so that the field curvature applied by the macro-collimator 270 continues to cancel the field curvature applied by the plate (e.g., the objective lens array) on the down-beam side of the macro-collimator 270. However, a change in the demagnification will lead to a change in astigmatism, which is not typically compensated for by the down-beam side plate. Although there may be hard-coded corrections intended to compensate for astigmatism, these hard-coded corrections cannot be changed with changes in the demagnification and therefore typically become ineffective when the landing energy is changed. For a narrow range of landing energies, a desirable combination of low field curvature and low astigmatism can be achieved, but outside the narrow range of landing energies, either or both of the field curvature and astigmatism degrade relatively quickly in the absence of the embodiments described herein.

[0117]

[0142] The embodiments described below are intended to expand the range of situations (eg, the range of operating parameters of electro-optical devices) in which hard-coded corrections are effective.

[0118]

[0143] FIG. 20 shows a further example of an electron-optical device (electron-optical column) in a schematic manner. The same features as those described above, e.g., the arrangements shown in and described with reference to FIGS. 3, 10, 14 and 15, are given the same reference numbers. For the sake of brevity, such features will not be detailed with respect to FIG. 20. For features having the same reference numbers, the same description as previously described applies, unless otherwise stated. For example, the source 201, the macro collimator 270, the upper beam restrictor 252, the electrodes 502-504, and the sample 208 may be as described above. FIG. 20 further shows in a schematic manner a rigid mount 730 for supporting the electrodes 502-504. Further details regarding exemplary arrangements for mounting the electrodes can be found in PCT / EP2021 / 084737, having a priority date of December 23, 2021, which is incorporated herein by reference in its entirety, particularly with respect to the portions of the present disclosure relating to mounting of the electrodes.

[0119]

[0144] As illustrated with reference to Figures 1 to 10, 14, 15 and 20, an electro-optical device may be provided which projects multiple sub-beams of charged particles onto the sample 208.

[0120]

[0145] The device comprises a number of plates. The plates may be conductive or may have a conductive coating. The plates may therefore define equipotential surfaces. The plates may be referred to as electrodes. The plates define respective aperture arrays. Each plate therefore defines an aperture array. Each aperture array comprises a number of apertures. Each plate may be arranged such that the peripheries of the apertures in the array are electrically connected together and are at the same potential as each other (e.g. form part of the same equipotential surface). The plates may be arranged in series along sub-beam paths 510. Each sub-beam path 510 may intersect a respective aperture in each plate. The apertures in different plates may therefore be aligned with each other along the respective sub-beam paths 510. In Figs. 5, 14 and 20, electrodes 301-303, 501-504 and 601-603 all represent examples of such plates. The electro-optical device applies a potential to the plate to control the sub-beams of the multi-beam (e.g., to demagnify the sub-beams and / or properly focus the sub-beams on the sample 208 to be evaluated). It is known that an electric field generated near an aperture in such a plate creates a lens effect for charged particles passing through the aperture.

[0121]

[0146] The plurality of plates comprises an objective lens array 241. A subset or all of the plates may form the objective lens array 241. The objective lens array 241 projects the sub-beams towards the sample 208. The objective lens array 241 may take any of the forms described with reference to Figures 3, 5-10 and 14-16. In some arrangements, the plurality of plates further comprises an electrode associated with and proximate to the objective lens array, such as a control lens array 250. If present, the control lens array 250 may take any of the forms described with reference to Figures 3, 5, 10 and 14, or may be a feature of the electro-optical device 41 shown in and described with reference to Figure 15. The electro-optical device may be configured to detect signal electrons emitted from the sample 208 (e.g. using a detector which may comprise a detector module 402 as described above) to obtain information about the sample 208. Such a plate may be a feature of the condenser lens array 231 of Figures 3 and 15.

[0122]

[0147] The electro-optical device may comprise a controller 500 (as shown diagrammatically in Figs. 3, 10 and 15) for controlling the movement of the plate (e.g., to apply and control the potentials applied to the plate). As described below, the controller 500 may be computer-implemented with any suitable combination of elements (e.g., CPU, RAM, etc.) used to provide the required functionality. As described above with reference to Fig. 5, the plate may be controlled by connecting the plate to a potential source. Thus, the controller 500 may comprise and / or control the potential source. The potential source may apply potentials to the different plates, the sample 208, and / or other elements. The controller 500 may further control a stage for supporting the sample 208.

[0123]

[0148] In this specification, references to an electro-optical device (or associated evaluation system) being configured to perform a function are intended to include the case where the controller 500 is configured to perform the function (e.g., by being appropriately programmed to provide the necessary control signals to devices such as potential sources and / or stages).

[0124]

[0149] In one embodiment, the aperture arrays defined in at least two of the plates have geometric characteristics. The geometric characteristics may be applied to each aperture or to the array of apertures of each plate, for example of at least two of the plates. The geometric characteristics are configured to apply a disturbance to a corresponding target characteristic (or characteristics) of a sub-beam passing through the aperture in the plate. Thus, the geometric characteristics correspond to a particular target characteristic of the sub-beam. The target characteristic may include astigmatism, field curvature, distortion, coma, or other characteristics of interest. A plate in which apertures with geometric characteristics are defined may be referred to as a plate with hard-coded correction.

[0125]

[0150] In one embodiment, as described above with reference to FIG. 12, the target characteristic includes astigmatism, and the geometric features that correspond to the target characteristic (and are therefore configured to apply a corresponding disturbance) include a range of different aperture ellipticities in the aperture array.

[0126]

[0151] In one embodiment, as described above with reference to FIG. 11, the target characteristic includes field curvature, and the geometric features that correspond to the target characteristic (and are therefore configured to apply corresponding disturbances) include a range of different aperture dimensions, such as areas, in the aperture array.

[0127]

[0152] In one embodiment, as described above with reference to FIG. 13, the target characteristic includes a distortion (e.g. due to telecentricity error) and the geometric feature corresponding to the target characteristic (and therefore configured to apply a corresponding disturbance) includes a range of different aperture positions in an aperture array, preferably on a regular lattice (e.g. a rectangular or hexagonal lattice), relative to their respective nominal positions.

[0128]

[0153] In one embodiment, as described above with reference to FIG. 13, the target characteristic includes coma aberration, and the geometric feature corresponding to the target characteristic (and therefore configured to apply a corresponding disturbance) includes a range of different aperture positions in an aperture array, preferably on a regular lattice (e.g., a rectangular or hexagonal lattice), relative to their respective nominal positions.

[0129]

[0154] Each aperture array may have a single geometric feature configured to apply such a disturbance, or may have a plurality of different such geometric features. Each additional geometric feature beyond the first geometric feature may be referred to as a further geometric feature and may take any of the forms described above. Each such further geometric feature is configured to apply a disturbance to a corresponding further target feature of the sub-beam, which may include any of the target features described above. Thus, an aperture array may be configured to apply disturbances to multiple target features by having a corresponding plurality of geometric features. In this manner, a single aperture array may apply disturbances to any combination of the aforementioned target features.

[0130]

[0155] In some embodiments, the controller 500 is configured to apply and control potentials to plates having geometric features such that the applied disturbances together substantially compensate for variations in the target property corresponding to the geometric features. The variations in the target property may be aberrations. Compensation for the variations in the target property may suppress the aberrations. The device is configured such that this variation is substantially compensated for over a range of parameters of the device. Thus, the compensation is not limited to one particular configuration, but may be limited as is the case with typical hard-coded corrections for variations in the target property. By providing geometric features to multiple different plates, allowing the geometric features to be applied independently to the different plates, and allowing the plates themselves to be controlled independently, the ability to compensate over a wider range of configurations is realized. This approach effectively provides at least one additional degree of freedom compared to the case where the geometric features are provided to only a single plate, or where the geometric features are fixed the same in each of the multiple plates. It has been found that the additional degree of freedom allows the hard-coded corrections to be adjusted to be applicable over a wider range of operating configurations of the device (e.g., a wider range of parameters of the device).

[0131]

[0156] Providing geometric features (which may be referred to as hard-coded corrections) on two or more different plates, for example three, four, or five plates, provides additional degrees of freedom. This allows for a higher degree of compensation to be achieved and / or compensation to be achieved over a longer (or larger) range of parameters and / or over a range of different parameters. In one arrangement, the same geometric feature may be applied as a hard-coded correction to two or more different plates. The geometric features applied as hard-coded corrections to the aperture arrays of the two or more plates may include one or more variations in ellipticity, diameter, and deviation from a regular grid, for example one or more variations in ellipticity, diameter, and deviation from a regular grid. The variation between the correction features applied to different apertures in the aperture array depends on the location of the aperture in the aperture array. The geometric features applied to the apertures of a plate may span a range of geometric features, such that the magnitude of the geometric features applied to an aperture of a plate may be different from other apertures in the plate. It has been found that a particularly good balance between performance and device complexity is achieved when the aperture arrays defined in three of the plates have geometric features and a controller applies and controls potentials to all three of the plates. It has been found that each disturbance applied in such an embodiment substantially compensates for variations in the target characteristic to a high degree over the range of device parameters.

[0132]

[0157] The following discussion refers to theoretical models and simulations, in some of which different data points may represent different hard-coded corrections in one or more plates, although it will be understood that in practice, the hard-coded corrections typically cannot be changed without mechanically altering and / or replacing one or more elements (e.g., plates) on which the hard-coded corrections are defined.

[0133]

[0158] In the use case described above, the potentials on the plates in the objective lens array 241 and the control lens 250 in the arrangement of Figs. 10 and 15 are varied to compensate for the field curvature over a range of landing energies. The challenge in this scenario is that the astigmatism also varies as a function of the demagnification used to adjust the contribution from the macro collimator 270 to the field curvature. One aspect of an embodiment of the present disclosure is based on the observation that the ellipticity of the apertures of all plates through which the sub-beams pass (e.g., the apertures in the plates of the control lens array 250 and the objective lens array 241) affects the astigmatism, and that the sensitivity (i.e., the degree to which a given amount of ellipticity contributes to the astigmatism) for each plate varies differently as a function of landing energy. This is illustrated in the simulation results shown in Fig. 21 for an exemplary configuration based on the arrangement of Figs. 10 and 14. The simulation was performed at a landing energy of 2.5 eV with the resolution adjusted to 5 nm. Curves 701-703 show simulated variations in sensitivity of astigmatism (called "Ast") to ellipticity (called "Elli") (called "Ast / Elli") at the aperture array of each plate over a range of landing energies (LE). Curve 701 shows the variation of astigmatism sensitivity to ellipticity for the plate corresponding to electrode 502 in FIG. 14 (which may be called the center electrode in the control lens array). Curve 702 shows the variation of astigmatism sensitivity to ellipticity for the plate corresponding to electrode 503 in FIG. 14 (which may be called the top electrode in the objective lens array). Curve 703 shows the variation of astigmatism sensitivity to ellipticity for the plate corresponding to electrode 504 in FIG. 14 (which may be called the bottom electrode in the objective lens array). FIG. 21 shows that for different plates, the sensitivity varies for different landing energies and varies differently (e.g., with different slopes) as a function of landing energy.

[0134]

[0159] For a single plate, the variation in sensitivity as a function of landing energy means that even if the contribution to astigmatism from the macro-collimator 270 is kept constant (e.g., by keeping the ratio of linear to angular demagnification constant), the contribution to astigmatism from the plate will vary over a range of landing energies. In practice, however, it may be desirable to adjust the ratio of linear to angular demagnification to compensate for changes in field curvature, so that the astigmatism applied by the macro-collimator 270 will vary as a function of landing energy. By appropriate configuration of the multiple plates, the multiple plates can be made to apply perturbations to the astigmatism that vary in a desired manner as a function of landing energy (e.g., by providing each of the multiple plates with a geometric feature that perturbs the astigmatism, such as a range of aperture ellipticities as described above with reference to FIG. 12), and thus the multiple plates can be configured to compensate for the variation in the astigmatism applied by the macro-collimator 270. The corrections from the different plates may contribute cumulatively or may cancel each other out. That is, for a range of landing energies, overcompensation may be applied by one plate and recompensation by another. In a different arrangement, undercompensation may be applied by one plate and recompensation by the other. A series of plates may apply overcompensation and / or undercompensation, which may cancel and accumulate, and the net effect may be that one of the plates applies recompensation. The increased freedom provided by the ability to control the astigmatism correction using multiple plates makes it possible to achieve near perfect correction of astigmatism and field curvature over a wide range of landing energies.

[0135]

[0160] Thus, the embodiments of the present disclosure have been found to be effective when the parameter of the device being varied is the landing energy and the target characteristic to be compensated for includes astigmatism. The geometric features of each of the aperture arrays allow the astigmatism to be substantially independent of the landing energy over a range of landing energies. At the same time, the controller 500 may control the potential applied to the plate such that the disturbance of the field curvature applied by the plate compensates for the disturbance of the field curvature applied by the macro-collimator 270 (an example of a variable electro-optical device) on the up-beam side of the plate over a range of landing energies. For example, the controller 500 may control the demagnification applied by the plate such that the disturbance of the field curvature applied by the plate compensates for the disturbance of the field curvature applied by the macro-collimator 270. This approach therefore allows for simultaneous control of the field curvature and astigmatism over a range of landing energies.

[0136]

[0161] The field curvature can be considered as a target characteristic that is compensated for by the geometric features of the plates. The geometric features of each of the aperture arrays, e.g., each aperture array having apertures with a range of different aperture areas as described above with reference to Fig. 11, also allow the field curvature to be substantially independent of the landing energy over a range of landing energies. At the same time, the controller 500 controls the demagnification applied by the plates such that the astigmatic disturbance applied by the plates substantially compensates for the astigmatic disturbance applied by the macro-collimator 270 over a range of landing energies.

[0137]

[0162] The improved performance is illustrated in FIG. 22, which is a graph showing the variation of the defocus due to astigmatism (referred to as “defocus (Ast)”) as a function of landing energy (LE) over a range of landing energies for three different cases. Each case involves the use of the arrangement of FIG. 14, but with a different respective combination of electrodes 502-504 to compensate for the variation of astigmatism. In this example, the variation of astigmatism is caused at least in part by the astigmatism applied by the macro collimator 270 on the up-beam side of the plate (electrode), as shown, for example, in FIGS. 10 and 15. In each case, a geometric feature (e.g., a geometric feature that causes a disturbance in the astigmatism, e.g., a range of aperture ellipticities as described above with reference to FIG. 12), or multiple geometric features, is selected such that the sum of the squares of the astigmatism is minimized over the range of landing energies. Curve 711 represents the case where geometric features are used only in the aperture array of the plate corresponding to electrode 504 to compensate for the variation of astigmatism caused by the macro-collimator 270. Curve 712 represents the case where geometric features of the aperture array of two plates corresponding to electrodes 502 and 504 of FIG. 14, respectively, are used to compensate for the variation of astigmatism. Curve 713 represents the case where geometric features of the aperture array of three plates corresponding to electrodes 502, 503 and 504 of FIG. 14, respectively, are used to compensate for the variation of astigmatism. As can be seen, the compensation of astigmatism over the range of landing energies is better for the case represented by curve 712 than for the case represented by curve 711. Thus, the additional degree of freedom provided by using geometric features in the aperture array of just two plates provides a clear improvement. Curve 713 shows that providing an additional degree of freedom provides a further improvement in performance.

[0138]

[0163] 23-27 are graphs relating to further exemplary use cases in which the geometric features of the plates are adjusted to make the astigmatism and field curvature substantially independent of the ratio M / Ma of the linear demagnification to the angular demagnification.

[0139]

[0164] Figures 23 and 24 show the results of a simulation to determine the variation of sensitivity for plates in an arrangement of the type shown in Figures 10 and 14 as a function of M / Ma. Figure 23 is a graph showing the simulated variation of the sensitivity (called "Ast") of astigmatism (called "Ast") to ellipticity (called "Elli") at the aperture array (called "Ast / Elli") over a range of M / Ma for three different plates. Figure 24 is a graph showing the simulated variation of the sensitivity to defocus (called "defocus / diameter") due to field curvature (called "defocus") and diameter (called "diameter") at the aperture array over a range of M / Ma for three different plates. Curves 721 and 731 show the variation of the respective sensitivity for the plate corresponding to electrode 502 in Figure 14 (the central electrode of the control lens array). Curves 722 and 732 show the variation of the respective sensitivity for the plate corresponding to electrode 503 in Figure 14 (the top electrode of the objective lens array). Curves 723 and 733 show the respective sensitivity variations for the plate corresponding to electrode 504 in FIG. 14 (the bottom electrode of the objective lens array).

[0140]

[0165] Since this use case involves varying M / Ma, this degree of freedom cannot be used to correct for field curvature. For this reason, it is necessary to simultaneously adjust the astigmatism and field curvature to be independent of M / Ma, allowing the correction of astigmatism and field curvature to remain effective over the range of M / Ma.

[0141]

[0166] The improved performance achieved by applying disturbances using geometric features in multiple plates is illustrated in Figures 25-27. Figures 25 and 26 are graphs showing the variation of the defocus due to astigmatism (Defocus (Ast)) as a function of M / Ma over the range of M / Ma for three different cases. Defocus in this context is the total defocus (due to astigmatism and curvature of field) in the worst case direction. The reason for the existence of a worst case direction is that in some directions the defocus due to astigmatism will partially compensate for the defocus due to curvature of field, while in other directions the defocus due to astigmatism and the defocus due to curvature of field will be additive. The worst case direction is the direction in which the total defocus is the largest. FIG. 25 shows curves 741-743 for all three cases, whereas FIG. 26 shows only curves 742 and 743 for the second and third cases (to make the difference between these two curves more clearly visible). Each case involves the use of the arrangement of FIG. 14, but with a different respective combination of electrodes 502-504 to compensate for the variations in astigmatism and field curvature. The geometrical features are selected in each case such that the sum of the squares of the total defocus (due to astigmatism and field curvature) is minimized over the range M / Ma. Curve 741 (only shown in FIG. 25) represents the case where the geometrical features applied only to the apertures of the aperture array in the plate corresponding to electrode 504 are used to compensate for astigmatism and field curvature. Curve 742 represents the case where the geometrical features of the aperture array in two plates corresponding to electrodes 502 and 504 of FIG. 14, respectively, are used to compensate for astigmatism and field curvature. Curve 743 represents the case where the geometric characteristics of the aperture array in three plates corresponding respectively to electrodes 502, 503 and 504 in Figure 14 are used to compensate for astigmatism and field curvature. As can be seen, similar to the situation described above with reference to Figure 22, the compensation of astigmatism over the range M / Ma is better for the case represented by curve 742 than for the case represented by curve 741.Thus, the additional degree of freedom provided using the geometric features in just the two plate aperture arrays provides an improvement. Curve 743 shows that providing the additional degree of freedom provides a very large additional improvement in performance. The overall improvement is large in absolute terms, since the total defocus variation in the situation represented by curve 741 is simulated to be about 4000 nm. In comparison, the defocus variation in the situation represented by curve 711 in FIG. 22 is simulated to be about 150 nm. This is still significant, but much smaller.

[0142]

[0167] FIG. 27 shows how control of astigmatism and field curvature over the M / Ma range results in improved performance when beam current is varied. FIG. 27 shows curves of expected variation of beam current versus sub-beam resolution at sample 208 for a landing energy of 2.5 keV and different beam-limiting aperture diameters in upper beam limiter 252. Curves 741-744 correspond to beam-limiting aperture diameters of 6 micrometers, 8 micrometers, 10 micrometers, and 12 micrometers, respectively, and have fixed hard-coded corrections at least in the plates corresponding to control electrodes 501, 502, 503 and object electrodes 503, 504, and therefore at least in all of the plates in the arrangement shown and described with reference to FIG. 14 and FIG. 20. Curve 521 shown in FIG. 27 corresponds to curve 521 described above with reference to FIG. 17-FIG. 19 (with different points on the curve corresponding to different optimized hard-coded corrections). Thus, curve 521 shown in FIG. 27 represents a fully optimized case where reoptimization of the hard-coded corrections is allowed. The different curves 741-744 all represent cases where the geometric features in the aperture array of the multiple plates are used to compensate for astigmatism and field curvature while varying M / Ma to adjust the beam current without allowing any reoptimization of the hard-coded corrections. Curve 752 represents an exemplary case where the techniques of the present disclosure are not applied, thereby corresponding in shape to the different curves 524-527 described above with reference to FIG. 19. Curve 752 represents the case where the beam-limiting aperture diameter is 8 micrometers. Curve 752 corresponds directly to curve 742. For all cases, M / Ma was varied over a range to achieve the observed variation of the beam current. At each point, the available degrees of freedom were used to optimize the resolution. The increased number of degrees of freedom provided by embodiments of the present disclosure appears to allow curves 741-744 to be much steeper than would otherwise be the case (as exemplified by curve 752).In fact, curves 741-744 are very similar to optimal case 521, where re-optimization of the hard-coded corrections is permitted in the simulation for different values ​​of beam current, but in the case of curves 741-744, performance is achieved without requiring any re-optimization (i.e., change) of the hard-coded corrections. Thus, curves 741-744 represent the performance that can be achieved for fixed hard-coded corrections, i.e., without requiring any replacement of physical elements such as plates.

[0143]

[0168] Thus, embodiments of the present disclosure have been found to be effective when the device parameters (over the range over which compensation is applied) include the charged particle beam current and the target characteristics include one or both of astigmatism and field curvature. In such a case, the respective geometric features of the aperture array may be such that the astigmatism and field curvature are substantially independent of the ratio of the total linear demagnification to the total angular demagnification over the range of beam currents over which compensation of the target characteristics is applied. The beam current / resolution operating point is changed by changing the demagnification. Thus, configuring the astigmatism and field curvature to be substantially independent of the ratio of the total linear demagnification to the total angular demagnification means that the beam current / resolution operating point (e.g., following curve 743 in FIG. 27) can be changed with minimal adverse effect on performance. It should also be noted that in the plots of FIGS. 23-26, the horizontal axis may instead be M rather than M / Ma, since there is a one-to-one correspondence between M and M / Ma. It should also be noted that for a fixed aperture size, there is also a one-to-one correspondence between linear demagnification and beam current.

[0144]

[0169] The macro-collimator 270 (e.g., as shown in Figures 10 and 15) is an example of a variable electron optical device that may be provided on the up-beam side of the plates in some embodiments. The variable electron optical device is configured to apply an electron optical disturbance to the charged particles directed at the sample. The variable electron optical device may comprise any macro-electron optical device. Thus, the variable electron optical device can act on the charged particles corresponding to the sub-beams as a group. In some embodiments, the variable electron optical device comprises a collimator configured to collimate the charged particles corresponding to the sub-beams. The variable electron optical device may or may not directly act on the charged particles in the sub-beams. The variable electron optical device may act on the charged particles before or after the sub-beams are formed. In some embodiments, the variable electron optical device comprises a condenser lens. In some embodiments, the variable electron optical device comprises a macro-collimator configured to apply a macroscopic collimation to the charged particles (before or after the charged particles are formed into sub-beams). The disturbance from the variable electron optical device affects at least the target properties of the sub-beams. In some embodiments, the disturbance applied by the varying electro-optical device is a variation in a target characteristic of the sub-beam (e.g., astigmatism and / or field curvature as described above) that is compensated for by disturbances from geometric features in the multiple plates.

[0145]

[0170] In embodiments where such a variable electron-optical device is present, the controller 500 may be configured to control the variable electron-optical device such that the applied electron-optical disturbance and the respective disturbance applied by the aperture array together substantially compensate for changes in the target characteristic over a range of parameters of the device. The controller 500 may control the variable electron-optical device by applying and controlling one or more potentials applied to the variable electron-optical device (i.e., the variable electron-optical device may comprise electrostatic elements). The controller 500 may control the variable electron-optical device by applying and controlling one or more currents applied to the variable electron-optical device (i.e., the variable electron-optical device may comprise electromagnetic elements). Thus, the variable electron-optical device may operate electrostatically and / or magnetically. As shown in FIG. 22, for example, the contributions to aberrations, such as astigmatism, by the variable electron-optical device (e.g., macro-collimator 270) and by the aperture array having geometric features may substantially cancel each other over a range of parameters of the electron-optical device (e.g., landing energy).

[0146]

[0171] In some embodiments, the electron optical device 41 further comprises a beam-limiting aperture array. The beam-limiting aperture array may be provided on the up-beam side of the variable electron optical device or between the variable electron optical device and the plate. The beam-limiting aperture array may be configured to generate sub-beams from the source beam. The beam-limiting aperture array may be the most up-beam plate that may act as an electrode. The upper beam limiter 252 described above with reference to Figures 14 and 20 is an example of such a beam-limiting aperture array. The beam-limiting aperture array may take any of the forms described above with reference to the upper beam limiter 252. The source beam comprises charged particles to which the variable electron optical device applies an electron optical disturbance. The electron optical device may comprise a source 201 for emitting a source beam. The source 201 may take any of the configurations described above with reference to Figures 2 and 3.

[0147]

[0172] Compensation of the target characteristics is achieved over a range of device parameters, including one or more of: landing energy of the charged particles, beam current of the charged particles, separation between the sample and the detector of the electron-optical device, demagnification (e.g., ratio of linear demagnification to angular demagnification), and resolution. Examples are described above for the case where the parameters are landing energy (see discussion of Figures 21 and 22) and demagnification (see discussion of Figures 23-27).

[0148]

[0173] In some embodiments, the various aperture arrays are configured to apply disturbances that vary differently as a function of the parameter over the range, at least for the disturbances corresponding to the target characteristic, due to the geometric features. Tuning the aperture arrays to contribute differently over a range of parameters facilitates effectively compensating for the variation of the target characteristic over the range. Figures 21, 23 and 24 show how this functionality can be achieved by showing how the sensitivity of the disturbances to each geometric feature can be different for different plates (due to plate position) and how they can vary differently as a function of the device's parameters.

[0149]

[0174] The disturbances applied by the different plates cooperate to compensate for the variation of the target characteristic. The contributions from the different plates may contribute cumulatively or cancel each other out, as long as the overall effect is to compensate for the variation of the target characteristic. Thus, the controller 500 may control the applied potentials such that, for at least the disturbances corresponding to the target characteristic, the disturbances applied by at least two of the aperture arrays cancel each other out over at least a portion of the range of the parameter. Alternatively or in addition, the controller 500 may control the applied potentials such that, for at least the disturbances corresponding to the target characteristic, the disturbances applied by at least two of the aperture arrays contribute cumulatively over at least a portion of the range of the parameter.

[0150]

[0175] In some embodiments, the controller applies and controls the potentials applied to the plates to maintain a substantially constant spatial relationship between the image plane of the device and the rigid mount 730 over a range of device parameters. A constant spatial relationship may include, for example, a constant separation between the image plane and the rigid mount 730. The mount 730 holds at least a portion of the plate(s) in a fixed position in the reference frame of the electro-optic device. Other portions of the plate(s) may deform during use due to electric fields generated in areas adjacent to the plates, e.g., deflections as described herein with reference to 16. Such deformations do not affect the spatial relationship between the image plane and the mount 730. In some embodiments, a variable electro-optic device (e.g., macro collimator 270) is additionally controlled while maintaining a substantially constant spatial relationship between the image plane and the mount 730 over a range of device parameters.

[0151]

[0176] An embodiment of the present disclosure may be presented as a method including any of the methods of using the apparatus described above. The method includes a method of compensating for variations in a characteristic of a sub-beam of charged particles in a multi-beam projected onto a sample 208. The method may include using a plurality of plates for projecting the sub-beams towards the sample 208. The plates define respective aperture arrays. The plates include an objective lens array for projecting the sub-beams towards the sample 208. The aperture arrays defined in at least two of the plates each have a geometric characteristic configured to apply a disturbance to a corresponding target characteristic of the sub-beam. Potentials applied to the plates having the geometric characteristic may be controlled such that the applied disturbances together substantially compensate for the variation of the target characteristic over a range of parameters of the device. A method of compensating for variations in a characteristic of a sub-beam of charged particles in a multi-beam projected onto a sample 208 in an electron-optical device may be provided. The electron-optical device may include a plurality of plates having respective aperture arrays defined therein. The plurality of plates includes an objective lens array. The aperture arrays defined in at least two of the plates have a geometric characteristic. The method includes projecting the sub-beams towards a sample 208 by affecting the sub-beams with a plate having an aperture array with the geometric feature. The affecting includes applying a disturbance to a target property of the sub-beam with each plate. Potentials are applied to the aperture plates and the potentials are controlled such that, together, the respective disturbances substantially compensate for variations in the target property over a range of parameters of the device.

[0152]

[0177] References to upper and lower, up and down, above and below, etc. should be understood as referring to directions parallel to the (usually, but not always perpendicular) up-beam and down-beam directions of the electron beam or multi-beam impinging on the sample 208. Thus, references to up-beam and down-beam are intended to refer to directions relative to the beam path, independent of any prevailing gravitational fields.

[0153]

[0178] The embodiments described herein may take the form of a series of aperture arrays or electron optical elements arranged in an array along the beam path or multi-beam path. Such electron optical elements may be electrostatic. In one embodiment, for example, all the electron optical elements from the beam limiting aperture array to the final electron optical element in the sub-beam path before the sample may be electrostatic and / or in the form of aperture arrays or plate arrays. In some configurations, one or more of the electron optical elements are fabricated as microelectromechanical systems (MEMS) (i.e., by using MEMS fabrication techniques). The electron optical elements may have magnetic and electrostatic elements. For example, a compound array lens may feature a macro magnetic lens disposed along the multi-beam path, containing the multi-beam path by upper and lower pole plates within the magnetic lens. Within the pole plates there may be an array of apertures for the beam paths of the multi-beam. Electrodes may be above, below or between the pole plates to control and optimize the electromagnetic field of the compound lens array.

[0154]

[0179] Where electrodes or other elements are provided that can be set to different potentials relative to one another, it will be understood that such electrodes / elements are electrically isolated from one another. Where the electrodes / elements are mechanically connected to one another, electrically insulating connectors may be provided. For example, where the electrodes / elements are provided as a series of conductive plates each defining an aperture array (e.g. to form an objective lens array or a control lens array), electrically insulating plates may be provided between the conductive plates. The insulating plates may be connected to the conductive plates and thus act as insulating connectors. The conductive plates may be separated from one another along the sub-beam paths by insulating plates. Apertures may be defined in the insulating plates around the paths of the multi-beam of sub-beams (e.g. around all of the sub-beams).

[0155]

[0180] An evaluation tool or evaluation system according to the present disclosure may comprise a device that performs a qualitative measurement (e.g., pass / fail) of a sample, a device that performs a quantitative evaluation of a sample (e.g., size of a feature), or a device that generates an image of a map of a sample. For example, the evaluation tool, when used for evaluation, may be any of the charged particle optical devices, e.g., part of the charged particle beam device 100, or more specifically the charged particle optical device 40 (which may be a charged particle optical column) and / or a charged particle optical device as part of an optical lens array assembly. Examples of evaluation tools or systems are inspection tools (e.g., for identifying defects), review tools (e.g., for classifying defects), and metrology tools, or tools capable of performing any combination of evaluation functions associated with an inspection tool, a review tool, or a metrology tool (e.g., a metrology inspection tool). The charged particle beam tool 40 (which may be a charged particle optical column) may be a component of an evaluation tool, e.g., an inspection tool or a metrology inspection tool, or part of an electron beam lithography tool. In this specification, any reference to a tool is intended to encompass a device, apparatus or system, which may comprise various components, which may or may not coexist and may even be located in separate rooms, particularly with respect to, for example, data processing components.

[0156]

[0181] Reference to a component or a system of components or elements that is controllable to manipulate a charged particle beam in a certain manner includes configuring a controller, or control system, or control unit to control the components to manipulate the charged particle beam in the manner described above, and optionally using other controllers or devices (e.g., voltage sources) to control the components to manipulate the charged particle beam in this manner. For example, a voltage source may be electrically connected to electrodes of one or more components, such as the control lens array 250 and the objective lens array 241, to apply a potential to the components under the control of the controller, or control system, or control unit. An activatable component, such as a stage, may be controllable to be activated and thus moved relative to another component, such as the beam path, by using one or more controllers, control systems, or control units to control the activation of the component.

[0157]

[0182] The functions provided by the controller, or control system, or control unit may be computer-implemented. Any suitable combination of elements may be used to provide the required functionality, including, for example, CPU, RAM, SSD, motherboard, network connections, firmware, software and / or other elements known in the art that allow the required computing operations to be performed. The required computing operations may be defined by one or more computer programs. The one or more computer programs may be provided in the form of a medium, optionally a non-transitory medium, that stores computer-readable instructions. When the computer-readable instructions are read by the computer, the computer performs the required method steps. The computer may be comprised of a distributed computing system having multiple different computers connected to each other in a self-contained unit or via a network.

[0158]

[0183] The terms "sub-beam" and "beamlet" are used interchangeably herein and are both understood to encompass any radiation beam derived from a parent radiation beam by splitting or separating the parent radiation beam. The term "manipulator" is used to encompass any element that affects the path of a sub-beam or beamlet, such as a lens or deflector. References to elements aligned along a beam path or sub-beam path are understood to mean that the respective element is positioned along the beam path or sub-beam path. References to an optical system are understood to mean an electron optical system.

[0159]

[0184] Reference to a number of elements being aligned along a beam path or sub-beam path is understood to mean that each element is disposed along the beam path or sub-beam path.

[0160]

[0185] References to a component or system of components or elements being controllable to manipulate and act on the charged particle beam in a particular way include configuring a controller or control system or control unit to control the component to manipulate the charged particle beam in the described manner, as well as, optionally, using other controllers or devices (e.g., voltage supplies and / or current supplies) to control the component to manipulate the charged particle beam in this manner. For example, a voltage supply may be electrically connected to and apply a potential to one or more components under the control of a controller or control system or control unit, including, for example, the control lens array 250, the objective lens array 234, the condenser lens 231, the corrector, and the scan deflector array 260, as a non-limiting list. An actuatable component, such as a stage, may be controllable to be actuated and thus moved relative to another component, such as the beam path, using one or more controllers, control systems, or control units to control the actuation of the component.

[0161]

[0186] The computer program may include instructions to instruct the controller 50 to perform the following steps: The controller 50 controls the charged particle beam device to project the charged particle beam towards the sample 208. In one embodiment, the controller 50 controls at least one charged particle optical element (e.g., an array of multiple deflectors or scanning deflectors 260) to act on the charged particle beam in the charged particle beam path. Additionally or alternatively, in one embodiment, the controller 50 controls at least one charged particle optical element (e.g., the detector 240) to act on the charged particle beam emitted from the sample 208 in response to the charged particle beam. While the invention has been described in conjunction with various embodiments, other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims and clauses.

[0162]

[0187] An embodiment of the present invention provides the following provisions:

[0163]

[0188] Clause 1. An electro-optical device for projecting a plurality of beams of charged particles towards a sample, or configured to project a multi-beam of sub-beams of charged particles towards a sample, the device comprising a plurality of plates, each having a respective aperture array defined in the plurality of plates, the plurality of plates comprising an objective lens array configured to project sub-beams of the multi-beam towards the sample, the aperture array defined in at least two of the plates, each aperture array having geometric features configured to apply a disturbance to a corresponding target property of a sub-beam, and a controller configured to apply and control potentials applied to the plates having the geometric features such that together the applied disturbances substantially compensate for variations in the target property over a range of parameters of the device.

[0164]

[0189] Clause 2. A device as described in clause 1, wherein the aperture arrays defined in at least two of the plates each have one or more further geometric features, each of the further geometric features being configured to apply a disturbance to a corresponding further target characteristic of the sub-beam.

[0165]

[0190] Clause 3. A device as described in clause 1 or 2, wherein the various aperture arrays are configured by their geometric characteristics to apply disturbances that vary differently as a function of the parameter over the range, at least for the disturbance corresponding to the target characteristic.

[0166]

[0191] Clause 4. A device described in any one of clauses 1 to 3, wherein the controller is configured to control the applied potentials such that, for at least disturbances corresponding to the target characteristic, the disturbances applied by at least two of the aperture arrays cancel each other over at least a portion of the range of the parameter.

[0167]

[0192] Clause 5. A device described in any one of clauses 1 to 4, wherein the controller is configured to control the applied potentials such that disturbances applied by at least two of the aperture arrays contribute cumulatively to disturbances corresponding to at least the target characteristic over at least a portion of a range of the parameter.

[0168]

[0193] Clause 6. A device described in any one of clauses 1 to 5, wherein the aperture array defined in three of the plates has geometric features, and the controller is configured to apply and control potentials to all three of the plates such that each applied disturbance together substantially compensates for variations in the target characteristic over a range of parameters of the device.

[0169]

[0194] Clause 7. A device described in any one of clauses 1 to 6, further comprising a variable electron-optical device on the up-beam side of the plurality of plates, the variable electron-optical device configured to apply an electron-optical disturbance to the charged particles directed at the sample, the disturbance being such that it affects a target characteristic of at least the sub-beam.

[0170]

[0195] Clause 8. The device of clause 7, wherein the controller is configured to control the variable electro-optical device such that the applied electro-optical disturbances and the respective disturbances applied by the aperture array together substantially compensate for changes in the target characteristic over a range of parameters of the device.

[0171]

[0196] Clause 9. A device according to clause 7 or 8, wherein the applied electro-optical disturbances form at least part of the variation of the target characteristic to be compensated.

[0172]

[0197] Clause 10. A device described in any one of clauses 7 to 9, wherein the device is either provided with a beam-limiting aperture array on the up-beam side of the variable electron-optical device, the beam-limiting aperture array being configured to generate sub-beams, or the device is provided with a beam-limiting aperture array between the variable electron-optical device and the plate, the beam-limiting aperture array being configured to generate sub-beams from the source beam.

[0173]

[0198] Clause 11. A device described in any one of clauses 7 to 10, wherein the source beam comprises charged particles and the varying electron-optical device applies an electron-optical disturbance to the charged particles, the device preferably comprising a source for emitting the source beam.

[0174]

[0199] Clause 12. A device according to any one of clauses 7 to 11, wherein the variable electron-optical device is a macro-electron-optical device and / or is configured to act on charged particles corresponding to a plurality of sub-beams as a group.

[0175]

[0200] Clause 13. A device described in any one of clauses 7 to 12, wherein the variable electron-optical device is preferably a collimator configured to act on the charged particles in the sub-beam to collimate the charged particles corresponding to the sub-beam, and / or a condenser lens.

[0176]

[0201] Clause 14. The device of clause 13, wherein the collimator comprises a macrocollimator configured to apply macroscopic collimation.

[0177]

[0202] Clause 15. A device described in any one of clauses 1 to 14, wherein the parameters include one or more of the landing energy of the charged particles, the beam current of the charged particles, the separation distance between the sample and the detector of the electron optical device, the magnification, and the resolution.

[0178]

[0203] Clause 16. A device according to any one of clauses 1 to 15, wherein the target characteristics include one or more of astigmatism, field curvature, distortion and coma.

[0179]

[0204] Clause 17. A device described in any one of clauses 1 to 16, wherein the target characteristic includes astigmatism and / or the geometric feature configured to apply the corresponding disturbance includes a range of different aperture ellipticities in the aperture array.

[0180]

[0205] Clause 18. A device described in any one of clauses 1 to 17, wherein the target characteristic includes field curvature and / or the geometric feature configured to apply a corresponding disturbance includes a range of different aperture dimensions, e.g. area, in an aperture array.

[0181]

[0206] Clause 19. A device as described in any one of clauses 1 to 18, wherein the target characteristic includes a distortion and / or the geometric feature configured to apply a corresponding disturbance includes a range of different aperture positions in the aperture array relative to their respective nominal positions, preferably on a regular lattice.

[0182]

[0207] Clause 20. A device described in any one of clauses 1 to 19, wherein the target characteristic includes coma aberration and / or the geometric feature configured to apply a corresponding disturbance includes a range of different aperture positions in the aperture array relative to their respective nominal positions, preferably on a regular lattice.

[0183]

[0208] Clause 21. A device described in any one of clauses 1 to 20, wherein the controller is configured to apply and control the potential applied to the plates to maintain a substantially constant spatial relationship between the image plane of the device and a rigid mount supporting at least one of the plates over a range of device parameters, preferably the controller is configured to maintain a substantially constant spatial relationship whilst additionally being configured to control a varying electro-optical device over a range of device parameters.

[0184]

[0209] Clause 22. A device according to any one of clauses 1 to 21, wherein the parameters include landing energy of the charged particles and the target characteristics include astigmatism.

[0185]

[0210] Clause 23. The device described in clause 22, wherein the controller is configured to control the potential applied to the plate such that the field curvature disturbance applied by the plate compensates for the field curvature disturbance applied by the variation electro-optical device on the up-beam side of the plate, over a range of parameters.

[0186]

[0211] Clause 24. A device as described in clause 23, wherein the controller is configured to control the demagnification applied to the plate such that the field curvature disturbance applied by the plate compensates for the field curvature disturbance applied by the variation electro-optical device on the up-beam side of the plate, over a range of parameters.

[0187]

[0212] Clause 25. A device according to any one of clauses 1 to 24, wherein the parameters include landing energy of the charged particles and the target characteristics include field curvature.

[0188]

[0213] Clause 26. The device described in clause 25, wherein the controller is configured to control the demagnification applied to the plate such that the astigmatic disturbance applied by the plate compensates for the astigmatic disturbance applied by the variation electro-optical device on the up-beam side of the plate, over a range of parameters.

[0189]

[0214] Clause 27. A device according to clause 23, 24 or 26, wherein the variable electron-optical device comprises a macro-collimator configured to apply macroscopic collimation.

[0190]

[0215] Clause 28. A device described in any one of clauses 22 to 27, wherein the geometric characteristics of each of the aperture arrays are such that astigmatism and / or field curvature are substantially independent of landing energy over a range of landing energies.

[0191]

[0216] Clause 29. A device according to any one of clauses 1 to 28, wherein the parameters include a beam current of charged particles and the target characteristics include one or both of astigmatism and field curvature.

[0192]

[0217] Clause 30. A device as described in clause 29, wherein the geometric characteristics of each of the aperture arrays are such that astigmatism and field curvature are substantially independent of the ratio of total linear demagnification to total angular demagnification over the range of beam currents.

[0193]

[0218] Clause 31. A device as described in clause 29 or 30, wherein the controller is configured to control the potential applied to the plate such that the field curvature and / or astigmatism disturbance applied by the plate compensates for the field curvature and / or astigmatism disturbance applied by a variation electro-optical device on the up-beam side of the plate, over a range of parameters.

[0194]

[0219] Clause 32. A device according to any one of clauses 1 to 31, wherein the variation in the target property is an aberration.

[0195]

[0220] Clause 33. An electro-optical device for projecting multiple beams of charged particles onto a sample, the device comprising: a plurality of plates having an aperture array defined within the plurality of plates, the plurality of plates comprising an objective lens array configured to project the beams of the multiple beams towards the sample, at least two of the plurality of plates having geometric features configured to apply a disturbance to a property of the beam (e.g. a target property) or having a corresponding geometric feature applied to a respective array of apertures such that the disturbances applied by the plates to the beams together substantially compensate for variations in the property over a range of parameters of the device.

[0196]

[0221] Clause 34. An electro-optical device for projecting a plurality of beams of charged particles towards a sample, the device comprising a plurality of plates, at least two of which have an aperture array defined in them, the plurality of plates comprising an objective lens array configured to project the beams towards the sample, the aperture arrays in at least two of the plates having geometric features configured to apply disturbances to a target property of the beams, and a controller configured to apply and control potentials applied to the plates having the geometric features such that the disturbances applied to the beams together substantially compensate for variations in the target property over a range of parameters of the device.

[0197]

[0222] Clause 35. An electro-optical device as described in clause 33 or 34, further comprising a variable macro electro-optical device on the up-beam side of the plurality of plates, the variable macro electro-optical device being configured to apply an electro-optical disturbance to the charged particles directed towards the sample, the disturbance being such as to affect at least a target characteristic of the beam.

[0198]

[0223] Clause 36. An electro-optical device as described in clause 33 or 34, further comprising a variable macro electro-optical device on the up-beam side of the plurality of plates, preferably the variable macro electro-optical device being configured to apply an electro-optical disturbance to the charged particles directed towards the sample, preferably the disturbance being such as to affect at least a target characteristic of the beam.

[0199]

[0224] Clause 37. An electro-optical device as described in clause 35 or 36, wherein the controller is configured to control the varying electro-optical device and the aperture array such that the disturbances together substantially compensate for the variation in the target characteristic over a range of parameters of the device.

[0200]

[0225] Clause 38. An electro-optical device as described in clause 35 or 36, wherein the controller is configured to control the variation electro-optical device such that the applied electro-optical disturbances and the respective disturbances applied by the aperture array together substantially compensate for the variation of the target characteristic over a range of parameters of the device.

[0201]

[0226] Clause 39. An electro-optical device as described in clause 33 or 38, preferably configured to control the applied potentials such that, for at least a disturbance corresponding to the target characteristic, over at least a portion of the range of parameters, the disturbances applied by at least two of the aperture arrays cancel each other out and / or there is a cumulative contribution of at least two of the aperture arrays.

[0202]

[0227] Clause 40. An electron-optical device as described in any one of clauses 33 to 39, wherein the parameters include one or more of the landing energy of the charged particles, the beam current of the charged particles, the separation distance between the sample and a detector of the electron-optical device, the magnification, and the resolution.

[0203]

[0228] Clause 41. An electro-optical device according to any one of clauses 33 to 40, wherein the target characteristics include one or more of astigmatism, field curvature, distortion and coma.

[0204]

[0229] Clause 42. A device described in any one of clauses 1 to 32 or any one of clauses 33 to 41, wherein the geometric characteristics are hard-coded corrections to the aperture arrays of two or more plates and include one or more of ellipticity, diameter, and deviation from a regular lattice.

[0205]

[0230] Clause 43. A device according to clause 42 or a device according to clause 42, wherein the spacing between the correction features applied to different apertures in the aperture array depends on the location of the apertures in the respective aperture array.

[0206]

[0231] Clause 44. A charged particle apparatus comprising a device according to any one of clauses 1 to 32, 42 or 43, or an electron-optical device according to any one of clauses 33 to 43.

[0207]

[0232] Clause 45. The charged particle device of clause 44, further comprising a stage configured to support the sample.

[0208]

[0233] Clause 46. An evaluation system comprising a device according to any one of clauses 1 to 32, 42 or 43, or an electron-optical device according to any one of clauses 43 to 33, or a charged particle apparatus according to any one of clauses 44 or 45.

[0209]

[0234] Clause 47. A method of compensating for variations in properties of sub-beams of charged particles in a multi-beam projected onto a sample, the method comprising: using a plurality of plates for projecting the sub-beams towards the sample, the plates defining respective aperture arrays and including an objective lens array for projecting the sub-beams towards the sample, the aperture arrays defined in at least two of the plates each having geometric features configured to apply disturbances to corresponding target properties of the sub-beams; and controlling potentials applied to the plates having the geometric features such that together the applied disturbances substantially compensate for variations in the target properties over a range of parameters of the device.

[0210]

[0235] Clause 48. A method of compensating for variations in properties of sub-beams of charged particles in a multi-beam projected onto a sample in an electron-optical device comprising a plurality of plates in which respective aperture arrays are defined, the plurality of plates comprising an objective lens array, the aperture arrays defined in at least two of the plates having geometric features, the method comprising: projecting the sub-beams towards the sample by acting on the sub-beams with a plate having an aperture array with the geometric features, the acting comprising applying a disturbance to a target property of the sub-beams with the respective plate; applying a potential to the aperture plates; and controlling the potential such that the applied disturbances together substantially compensate for the variations in the target property over a range of parameters of the device.

Claims

1. 1. An electro-optical device for projecting multiple beams of charged particles onto a sample, said device comprising: a plurality of plates, each having a respective aperture array defined therein, the plurality of plates comprising an objective lens array configured to project sub-beams of the multi-beam towards the sample, the aperture arrays being defined in at least two of the plates, each aperture array having a geometric feature configured to apply a disturbance to a corresponding target feature of the sub-beam; a controller configured to apply and control a potential applied to the plate having geometric characteristics such that together the applied disturbances substantially compensate for variations in the target property over a range of parameters of the device; An electro-optical device comprising:

2. 2. The device of claim 1, wherein the aperture arrays defined in at least two of the plates each have one or more further geometric features, each of the further geometric features configured to apply a disturbance to a corresponding further target characteristic of the sub-beam.

3. 3. The device of claim 1 or 2, wherein different aperture arrays are configured to apply disturbances that vary differently as a function of the parameter across the range, at least for disturbances corresponding to the target characteristic, due to the geometric characteristics.

4. 4. The device of claim 1, wherein the controller is configured to control the applied potentials such that, for at least a disturbance corresponding to the target characteristic, disturbances applied by at least two of the aperture arrays cancel each other over at least a portion of the range of the parameter.

5. 5. A device according to claim 1 , wherein the controller is configured to control the applied potentials such that disturbances applied by at least two of the aperture arrays cumulatively contribute to a disturbance corresponding to at least the target characteristic over at least a portion of the range of the parameter.

6. 6. A device as claimed in any one of claims 1 to 5, wherein the aperture array defined in three of the plates has the geometric feature, and the controller is configured to apply and control potentials to all three of the plates such that the respective applied disturbances together substantially compensate for the variation in the target property over the range of parameters of the device.

7. The device of any one of claims 1 to 6, further comprising a varying electron-optical device on the up-beam side of the plurality of plates, the varying electron-optical device being configured to apply an electron-optical disturbance to the charged particles directed towards the sample, the disturbance being such that it affects the target characteristic of at least the sub-beam.

8. 8. The device of claim 7, wherein the controller is configured to control the varying electro-optical device such that the applied electro-optical disturbances and the respective disturbances applied by the aperture array together substantially compensate for the variation in the target characteristic over the range of the parameter of the device.

9. 9. A device according to claim 7 or 8, wherein the applied electro-optical disturbance forms at least part of the variation of the target characteristic that is to be compensated for.

10. A device according to any one of claims 7 to 9, comprising a beam-limiting aperture array on an up-beam side of the varying electro-optical device, the beam-limiting aperture array being configured to generate the sub-beams.

11. A device according to any one of claims 7 to 10, wherein the varying electron-optical device is a macro-electro-optical device and / or is configured to act on charged particles corresponding to a number of sub-beams as a group.

12. A device according to any one of claims 7 to 11, wherein the varying electron-optical device is a collimator, preferably configured to act on the charged particles in the sub-beams to collimate the charged particles corresponding to the sub-beam, and / or is a condenser lens.

13. The device of claim 12 , wherein the collimator comprises a macro-collimator configured to apply macroscopic collimation.

14. 14. The device of claim 1, wherein the geometric features are hard-coded corrections to the aperture arrays of the two or more plates and include one or more of ellipticity, diameter, and deviation from a regular lattice.

15. The device of claim 14 , wherein the variation between correction features applied to different apertures in the aperture arrays depends on the location of the apertures in their respective aperture arrays.

Images