CN118435310A

CN118435310A - Electronic optical device and method for compensating for variation in properties of sub-beam

Info

Publication number: CN118435310A
Application number: CN202280085376.XA
Authority: CN
Inventors: M·J-J·维兰德
Original assignee: ASML Holding NV
Current assignee: ASML Holding NV
Priority date: 2021-12-23
Filing date: 2022-11-23
Publication date: 2024-08-02

Abstract

Electro-optical devices and related methods are disclosed. In one arrangement, the electron optical device projects a plurality of beamlets of charged particles onto the sample. A plurality of plates is provided in which respective aperture arrays are defined. The plates include an objective array configured to project beamlets towards the sample. The aperture arrays defined in at least two of the plates each have a geometric characteristic configured to impart a perturbation to a corresponding target property of the beamlets. The controller controls the electrical potentials applied to the plates having the geometric characteristics such that the applied disturbances substantially collectively compensate for the change in the target property over a range of parameters of the apparatus.

Description

Electronic optical device and method for compensating for variation in properties of sub-beam

Cross Reference to Related Applications

The present application claims priority from EP application 21217583.0 filed on 12/23 of 2021 and EP application 22163356.3 filed on 3/21 of 2022, which are incorporated herein by reference in their entireties.

Technical Field

Embodiments provided herein relate to compensating for variations in properties of beamlets of an electron optical apparatus over an operational configuration of the electron optical apparatus.

Background

When manufacturing semiconductor Integrated Circuit (IC) chips, undesired pattern defects due to, for example, optical effects and incidental particles inevitably occur on a substrate (i.e., wafer) or mask during a manufacturing process, thereby reducing yield. Therefore, monitoring the extent of undesired pattern defects is an important process in manufacturing IC chips. More generally, inspection and/or measurement of the surface of a substrate or other object/material is an important process during and/or after its manufacture.

Pattern detection tools with charged particle beams have been used to detect objects (which may be referred to as samples), for example, to detect pattern defects. These tools typically use electron microscopy techniques such as Scanning Electron Microscopy (SEM). In SEM, a final deceleration step is used to target the primary electron beam of electrons at a relatively high energy in order to land on the sample with a relatively low landing energy. The electron beam is focused to a probe spot on the sample. Interactions between the material structure at the probe spot and landing electrons from the electron beam cause signal electrons, such as secondary electrons, backscattered electrons or auger electrons, to be emitted from the surface. Signal electrons may be emitted from the material structure of the sample. By scanning the primary electron beam as a detection spot over the surface of the sample, signal electrons can be emitted across the surface of the sample. By collecting these signal electrons emitted from the surface of the sample, the pattern detection tool can obtain an image representing the characteristics of the material structure of the surface of the sample. The electro-optical device may be provided with correction features that reduce aberrations. The correction feature may be, for example, a change in shape, size and/or position of apertures of an array of apertures defined in a plate through which the electron beam passes. The apertures of the array of apertures may be nominally uniform so as to be at least of similar shape and size and located at grid points of a nominally regular array. The correction features may, for example, change, adjust or perturb the shape and/or size of the apertures, and/or change, adjust or perturb the position of the apertures relative to grid points of a regular array, depending on the position of the apertures in the array of apertures. Thus, the variation between correction features applied to different apertures of an aperture array depends on the positions of the apertures in the aperture array.

These correction features can be regarded as geometrical characteristics of the apertures of the aperture array. Since the correction features adjust the structural form of the aperture, they may be referred to as hard-coded corrections. The hard-coded correction can be contrasted with correction features achieved by controlling the potential applied to the plates defining the aperture array. These hard-coded corrections cannot be easily changed and may not be optimal in all situations. The expectations are: so that the hard-coded correction can be effective over a wide range of scenes.

Disclosure of Invention

It is an object of the invention to improve the control of a charged particle beam.

According to one aspect of the present invention there is provided an electron optical apparatus configured to project a plurality of beams of beamlets of charged particles onto a sample, the apparatus comprising a plurality of plates in which respective aperture arrays are defined, wherein the plurality of plates comprise an objective lens array configured to project the beamlets towards the sample, and the aperture arrays defined in at least two of the plates each have a geometrical characteristic configured to impart a perturbation to a corresponding target property of the beamlets; and a controller configured to apply and control the electrical potential applied to the plate having the geometric characteristic such that the applied disturbances substantially collectively compensate for the change in the target property over a range of parameters of the apparatus.

According to one aspect of the present invention, there is provided a method of compensating for variations in properties of beamlets of charged particles projected into a plurality of beams of a sample, the method comprising: projecting the beamlets towards the sample using a plurality of plates defining respective aperture arrays and including an objective lens array to project the beamlets towards the sample, wherein the aperture arrays defined in at least two of the plates each have a geometric characteristic configured to impart a perturbation to a corresponding target property of the beamlets; and controlling the potentials applied to the plates having the geometric characteristics such that the applied perturbations substantially collectively compensate for variations in the target properties over a range of parameters of the apparatus.

According to one aspect of the present invention there is provided a method of compensating for variations in properties of a sub-beam of charged particles in a plurality of beams of a sample projected into an electro-optical device, the electro-optical device comprising a plurality of plates in which respective aperture arrays are defined, the plurality of plates comprising an array of objective lenses, wherein the aperture arrays defined in at least two of the plates have geometric characteristics, the method comprising: projecting the beamlets towards the sample by operating the beamlets using a plate having an array of apertures with geometrical properties, the operation comprising: applying a perturbation to a target property of the sub-beam using the respective plate; and applying potentials to the aperture plate and controlling these potentials such that the respective perturbations substantially collectively compensate for variations in the target properties over a range of parameters of the apparatus.

Drawings

The above and other aspects of the present disclosure will become more apparent from the description of exemplary embodiments thereof, taken in conjunction with the accompanying drawings.

Fig. 1 is a schematic diagram illustrating an exemplary charged particle beam inspection apparatus.

Fig. 2 is a schematic diagram illustrating an exemplary multi-beam apparatus as part of the exemplary charged particle beam inspection apparatus of fig. 1.

Fig. 3 is a schematic illustration of an exemplary electron optical column including a converging lens array.

Fig. 4 is a plot of landing energy versus resolution for an exemplary arrangement.

Fig. 5 is an enlarged view of the objective lens and the control lens.

Fig. 6 is a schematic cross-sectional view of a portion of a schematically arranged objective lens array.

Fig. 7 is a bottom view of a portion of the objective lens array of fig. 6.

Fig. 8 is a bottom view of a modified version of a portion of the objective lens array of fig. 6.

Fig. 9 is a schematic enlarged cross-sectional view of a detector incorporated in the objective lens of fig. 6.

Fig. 10 is a schematic illustration of an exemplary electron optical apparatus including a macro collimator and a macro scanning deflector.

FIG. 11 is a top view of a portion of a plate defining an aperture array having apertures with a range of different aperture areas for compensating for off-axis aberrations such as field curvature.

Fig. 12 is a top view of a portion of a plate defining an aperture array having apertures with different ellipticity ranges for compensating for off-axis aberrations such as astigmatism.

Fig. 13 is a top view of a portion of a plate defining an aperture array having apertures displaced relative to a nominal position to compensate for off-axis aberrations such as distortion caused by telecentricity errors.

Fig. 14 is a schematic cross-sectional view of portions of a control lens array and an objective lens array of an electro-optical device.

Fig. 15 is a schematic illustration of an exemplary electron optical apparatus including a beam splitter.

Fig. 16 is a schematic cross-sectional view of a portion of an electrode in an objective lens array to illustrate electrode distortion (bow).

Fig. 17 is a graph showing beam current versus resolution, which is the smallest curve for two different landing energy resolutions.

Fig. 18 is the graph of fig. 17 additionally showing a plot in which the landing energy steps from 2.5keV to 1keV with the image plane fixed and the resolution minimum for each of eight different physical configurations of the system.

Fig. 19 is the graph of fig. 18, wherein a plot of stepped landing energy at a fixed image plane position (i.e., a hard-coded correction set) is shown for one of the physical configurations of the electron optical apparatus, and wherein the additional plot shows the change in beam current achieved by controlling the demagnification (by controlling the potential applied to the plates of the objective lens array assembly).

FIG. 20 is a schematic side view of an electron optical device including a plate having geometric features configured to impart turbulence to a beamlet property upstream of a beam of the plate and to vary the electron optical device.

Fig. 21 is a graph depicting how the sensitivity (ash/Elli) of astigmatism (ash) to changes in aperture ellipticity (Elli) varies for different plates over the Landing Energy (LE).

Fig. 22 is a graph showing how Defocus (ash) from astigmatism varies according to Landing Energy (LE) for three different combinations of plates used for compensation.

Fig. 23 is a graph depicting how the sensitivity (ash/Elli) of the change in astigmatism (ash) to aperture ellipticity (Elli) varies for different plates over the range of ratios of linear reduction to angular reduction (M/Ma).

Fig. 24 is a graph depicting how the sensitivity of Defocus (refocus) to changes in aperture diameter (diam) due to field curvature (refocus/diam) varies for different plates over a range of ratios of linear reduction rate to angular reduction rate (M/Ma).

Fig. 25 is a plot of how Defocus (ash) from astigmatism varies depending on the ratio of linear reduction rate to angular reduction rate for three different combinations of plates used for compensation.

Fig. 26 is a graph of fig. 25, in which the single plate case is omitted, to compare the performance of compensation using two plates with compensation using three plates.

Fig. 27 is a graph showing how the optimal resolution varies depending on the beam current when the beam current is changed by changing the reduction rate for the case of compensating for aberrations with and without using a plurality of plates having geometric characteristics.

Detailed Description

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

The computational power of electronic devices can be enhanced by significantly increasing the packing density of circuit components (such as transistors, capacitors, diodes, etc.) on an IC chip, which reduces the physical size of the device. This has been achieved by increasing the resolution, enabling even smaller structures to be manufactured. For example, an IC chip of a smartphone (which is the size of a thumb nail and available in 2019 or earlier) may include more than 20 hundred million transistors, each transistor having a size of less than 1/1000 of human hair. Thus, it is not surprising that semiconductor IC fabrication is a complex and time-consuming process with hundreds of individual steps. Even errors in one step can significantly affect the functionality of the final product. Only one "fatal defect" will lead to equipment failure. The goal of the manufacturing process is to increase the overall yield of the process. For example, for a 50 step process (one of which may dictate the number of layers formed on a wafer), the yield per individual step must be greater than 99.4% in order to achieve a yield of 75%. If the yield per individual step is 95%, the overall process yield will be as low as 7%.

Although high process yields are required in IC chip manufacturing facilities, it is also necessary to maintain high substrate (i.e., wafer) throughput (defined as the number of substrates processed per hour). The presence of defects can affect high process yields and high substrate throughput. This is especially true if operator intervention is required to review the defect. Thus, high throughput detection and identification of micro-scale and nano-scale defects by inspection tools such as scanning electron microscopy ("SEM") is necessary to maintain high yields and low cost.

The SEM includes a scanning device and a detector arrangement. The scanning device comprises an illumination means comprising an electron source for generating primary electrons; and a projection device for scanning a sample, such as a substrate, through one or more primary electron beams. At least the illumination device or illumination system and the projection device or projection system may together be referred to as an electron optical apparatus or electron optical column. The primary electrons interact with the sample and generate secondary electrons. As the sample is scanned, the detection device captures secondary electrons from the sample so that the SEM can produce an image of the scanned area of the sample. For high throughput inspection, some of the inspection devices use multiple focused beams, i.e., multiple beams, of primary electrons. The component beams of the plurality of beams may be referred to as sub-beams or beam waves. The multiple beams may simultaneously scan different portions of the sample. Thus, the multibeam inspection apparatus is capable of inspecting a sample at a much higher speed than the single-beam inspection apparatus.

An implementation of a known multibeam inspection apparatus is described below.

The figures are schematic. Accordingly, 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 numerals refer to the same or similar components or entities, and only differences are described with respect to individual embodiments. While both the description and the drawings refer to an electron optical apparatus, it should be appreciated that these embodiments are not intended to limit the disclosure to particular charged particles. Thus, references to electrons in this context may be more generally considered references to charged particles, which are not necessarily electrons.

Referring now to fig. 1, fig. 1 is a schematic diagram illustrating an exemplary charged particle beam inspection device 100, which exemplary charged particle beam inspection device 100 may also be referred to as a charged particle beam evaluation system or simply as an evaluation system. The charged particle beam inspection apparatus 100 (or charged particle apparatus) of fig. 1 includes a main chamber 10, a load lock chamber 20, an electron beam tool 40, an Equipment Front End Module (EFEM) 30, and a controller 50. An electron beam tool 40 is located within the main chamber 10.

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. For example, the first load port 30a and the second load port 30b may receive a front opening substrate cassette (FOUP) containing a substrate to be inspected (e.g., a semiconductor substrate or a substrate made of one or more other materials) or a sample to be inspected (the substrate, wafer, and sample are hereinafter collectively referred to as a "sample"). One or more robotic arms (not shown) in the EFEM 30 transport samples to the load lock chamber 20.

The load lock chamber 20 is used to remove gas from around the sample. This creates a vacuum with a partial gas pressure lower than the pressure in the surrounding environment. The load lock chamber 20 may be connected to a load lock vacuum pump system (not shown) that removes gas particles from the load lock chamber 20. The operation of the load lock vacuum pump system enables the load lock chamber to reach a first pressure below atmospheric pressure. After reaching the first pressure, one or more robotic arms (not shown) transfer the sample from the load lock chamber 20 to the main chamber 10. The main chamber 10 is connected to a main chamber vacuum pump system (not shown). The main chamber vacuum pump system removes gas particles in the main chamber 10 so that the pressure around the sample reaches a second pressure lower than the first pressure. After reaching the second pressure, the sample is transported to an electron beam tool where the sample can be inspected. The electron beam tool 40 may include multiple beam electron optics.

The controller 50 is electrically 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 device 100. The controller 50 may also include processing circuitry configured to perform various signal and image processing functions. Although the controller 50 is shown in FIG. 1 as being external to the structure including the main chamber 10, the load lock chamber 20, and the EFEM 30, it should be appreciated that the controller 50 may be part of the structure. The controller 50 may be located in one of the constituent elements of the charged particle beam inspection device, or it may be distributed over at least two of the constituent elements. Although the present disclosure provides an example of a main chamber 10 housing an electron beam inspection tool, it should be noted that aspects of the present disclosure are not limited in their broadest sense to chambers housing electron beam inspection tools. Conversely, it should be appreciated that the principles described above may also be applied to other arrangements of other tools and devices operating at the second pressure.

Referring now to fig. 2, fig. 2 is a schematic diagram illustrating an exemplary electron beam tool 40 including a multibeam inspection tool as part of the exemplary charged particle beam inspection apparatus 100 of fig. 1. The multi-beam electron beam tool 40 (also referred to herein as the apparatus 40) comprises an electron source 201, a projection apparatus 230, a motorized stage (or stage) 209 and a sample holder 207. The electron source 201 and the projection device 230 may together be referred to as an illumination device. The sample holder 207 is supported by a motorized stage or actuation stage 209 to hold a sample 208 (e.g., a substrate or mask) for inspection. The multi-beam electron beam tool 40 further comprises an electron detection device 240.

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

The projection device 230 is configured to convert the primary electron beam 202 into a plurality of beamlets 211, 212, 213 and to direct each beamlet onto the sample 208. Although three beamlets are illustrated for simplicity, there may be tens, hundreds, thousands, tens of thousands or even hundreds of thousands (or more) of beamlets. The beamlets may be referred to as beam waves.

The controller 50 may be connected to various parts of the charged particle beam inspection apparatus 100 of fig. 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 to govern the operation of the charged particle beam inspection device, including the charged particle beam device.

Projection device 230 may be configured to focus beamlets 211, 212, and 213 onto sample 208 for inspection, and may form three probe spots 221, 222, and 223 on a surface of sample 208. Projection device 230 may be configured to deflect primary beamlets 211, 212, and 213 to scan probe spots 221, 222, and 223 across individual scan areas in a section of the surface of sample 208. Electrons, including secondary electrons and backscattered electrons, which may be referred to as signal particles, are generated from the sample 208 in response to incidence of the primary beamlets 211, 212 and 213 on probe spots 221, 222 and 223 on the sample 208. Typically, the secondary electrons have an electron energy of 50eV or less, typically the electron energy of the backscattered electrons is between 50eV and the landing energy of the primary sub-beams 211, 212 and 213.

The electron detection device 240 is configured to detect secondary electrons and/or backscattered electrons and generate corresponding signals that are sent to the controller 50 or a signal processing system (not shown), for example, to construct an image of a corresponding scanned region of the sample 208. The electron detection device may be incorporated into the projection apparatus or may be separate from the projection apparatus, wherein a secondary optical column is provided to direct secondary electrons and/or backscattered electrons to the electron detection device.

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, computer, server, mainframe, terminal, personal computer, any kind of mobile computing device, etc., or a combination thereof. The image acquirer may include at least a portion of the processing functionality of the controller. Thus, the image acquirer may include at least one or more processors. The image acquirer may be communicatively coupled to the electronic detection device 240 of the apparatus 40, permitting signal communication, such as electrical conductors, fiber optic cables, portable storage media, IR, bluetooth, the internet, wireless networks, radios, or the like, or combinations thereof. The image acquirer may receive the signal from the electronic detection device 240, may process the data included in the signal, and may construct an image therefrom. Thus, the image acquirer can acquire an image of the sample 208. The image acquirer may also perform various post-processing functions, such as generating contours on the acquired image, superimposing indicators, and the like. The image acquirer may be configured to perform adjustment of brightness, contrast, and the like of the acquired image. The storage device may be a storage medium such as a hard disk, flash drive, cloud storage, random Access Memory (RAM), other types of computer readable memory, and the like. A storage device may be coupled to the image acquirer and may be used to save the scanned raw image data as raw images as well as post-processed images.

The image acquirer may acquire one or more images of the sample based on the imaging signals received from the electronic detection device 240. The imaging signal may correspond to a scanning operation for performing charged particle imaging. The acquired image may be a single image including a plurality of imaging regions. The single image may be stored in a storage device. A single image may be an original image that may be divided into a plurality of regions. Each of the plurality of regions may include an imaging region containing a feature of the sample 208. The acquired images may include multiple images of a single imaging region of the sample 208 that is sampled multiple times over a period of time. The plurality of images may be stored in a storage device. The controller 50 may be configured to perform the image processing steps using multiple images of the same location of the sample 208.

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

The controller 50 may control the motorized stage 209 (or stage) to move the sample 208 during inspection of the sample 208. The controller 109 may enable the motorized stage 209 to move the sample 208, preferably continuously, in one direction, at least during sample inspection, for example, at a constant speed. The controller 50 may control the movement of the motorized stage 209 such that it varies the speed of movement of the sample 208 in accordance with various parameters. For example, the controller may control the speed of the table (including its direction) in accordance with the inspection steps and/or characteristics of the scan of the scanning procedure disclosed by EPA 21171877.0, which was filed on 5, 3, 2021, which EPA 21171877.0 incorporates in terms of at least a combined step and scan strategy of the table.

FIG. 3 is a schematic illustration of an exemplary electron optical column for use in an evaluation system. An electron optical column is an example of an electron optical apparatus. For ease of illustration, the lens array is schematically depicted herein by an oval shaped array. Each oval represents one of the lenses in the lens array. Conventionally, lenses are represented using an oval shape, similar to the biconvex form commonly employed in optical lenses. However, in the context of a charged particle arrangement such as the charged particle arrangement discussed herein, it should be understood that the lens array will typically operate electrostatically and thus may not require any physical elements in the shape of biconvex surfaces to be employed. As described below, the lens array may alternatively include a plurality of plates having apertures. Each plate with an aperture may be referred to as an electrode. The electrodes may be arranged in series along the beamlet paths of the beamlets of the plurality of beamlets.

The electron source 201 directs electrons toward a converging lens array 231 that forms part of the projection device 230. Ideally, the electron source is a high brightness thermal field emitter with a good tradeoff between brightness and total emission current. There may be tens, hundreds or thousands or even tens of thousands of converging lenses 231. The converging lens array 231 may comprise a multi-electrode lens and have a configuration based on EP1602121A1, which is specifically incorporated by reference in its disclosure of a lens array for dividing an electron beam into a plurality of sub-beams, wherein the array provides a lens for each sub-beam. The converging lens array may take the form of at least two plates (preferably three plates) to act as electrodes, with the apertures in each plate aligned with apertures in the other plates to define paths of beamlets passing through the plates. During operation, at least two of the plates are maintained at different potentials to achieve the desired lensing effect. Between the plates of the converging lens array are electrically insulating plates, for example made of an insulating material such as ceramic or glass, with one or more apertures for the beamlets. In an alternative arrangement, one or more of the plates may have apertures, each having their own electrode, for example, an array of electrodes around their perimeter or an aperture group arranged with a common electrode.

In one arrangement, the converging lens array is formed of three plate arrays in which charged particles have the same energy as they enter and leave each lens, which arrangement may be referred to as an Einzel lens. Thus, chromatic dispersion occurs only within the Einzel lens itself (between the entrance and exit electrodes of the lens), thereby limiting off-axis chromatic aberration. When the thickness of the converging lens is small, for example a few millimeters, the effect of such aberrations is small or negligible.

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

Below the deflector 235 (i.e. downstream or further from the beam of the source 201) there is a control lens array 250, which control lens array 250 comprises a control lens 251 for each sub-beam 211, 212, 213. The control lens array 250 may comprise two or more (preferably at least three) flat electrode arrays connected to respective potential sources, preferably wherein the insulating plates are in contact with the electrodes, e.g. between 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 relative to the demagnification of the beam and/or to control the beam energy delivered to the objective lenses 234, each of which directs a respective sub-beam 211, 212, 213 onto the sample 208.

Optionally, a scan deflector array 260 is provided between the control lens array 250 and the objective lens array 234 (objective lens array). The scan deflector array 260 includes a scan deflector 261 for each beamlet 211, 212, 213. Each scan deflector is configured to deflect a respective beamlet 211, 212, 213 in one or both directions in order to cause the beamlet to scan across the sample 208 in one or both directions.

A detector module 402 of the detector is provided in or between the objective 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. Note that the detector may additionally or alternatively have detector elements along the beam upstream of the primary beam path of the objective lens array or even the control lens array.

The electron optical device of fig. 3 is configured to control landing energy of electrons on a sample by varying the potential applied to the electrodes controlling the lens and objective lens. The control lens and the objective lens work together and may be referred to as an objective lens assembly. Depending on the nature of the sample being evaluated, the landing energy may be selected to increase the emission and detection of secondary electrons. The controller may be configured to control the landing energy to any desired value or desired predetermined value of a plurality of predetermined values within a predetermined range. In one embodiment, the landing energy may be controlled to a desired value within a predetermined range (e.g., from 1000eV to 5000 eV). Fig. 4 is a graph depicting resolution as a function of landing energy, assuming that the beam opening angle/demagnification is re-optimized for changing landing energy. It can be seen that as the change in landing energy falls to a minimum value LE min, the resolution of the assessment tool can remain substantially constant. Resolution decreases below LE min because the lens strength of the objective lens and the electric field within the objective lens must be reduced in order to maintain a minimum separation between the objective lens and/or detector and the sample. As discussed further below, the exchangeable module may also be used to change or control landing energy.

Ideally, the landing energy is changed primarily by controlling the energy of electrons exiting the control lens. The potential difference within the objective lens is preferably kept constant during this change so that the electric field within the objective lens remains as high as possible. In addition, the potential applied to the control lens can be used to optimize the beam opening angle and demagnification. The control lens may also be referred to as a refocusing lens because it can correct the focus position in view of the change in landing energy. Ideally, each control lens includes three electrodes to provide two independent control variables, as discussed further below. For example, one of the electrodes may be used to control the reduction rate, while a different electrode may be used to independently control landing energy. Alternatively, each control lens may have only two electrodes. In contrast, when there are only two electrodes, one of the electrodes may need to control both the shrinkage rate and landing energy.

Fig. 5 is a schematic enlarged view of one objective lens 300 of the objective lens array and one control lens 600 of the control lens array 250. Objective lens 300 may be configured to demagnify the electron beam by a factor greater than 10, desirably in the range of 50 to 100 or more. The objective lens comprises a middle or first electrode 301, a lower or second electrode 302 and an upper or third electrode 303. The voltage sources V1, V2, V3 are configured to apply electric potentials to the first electrode, the second electrode, and the third electrode, respectively. Another voltage source V4 is connected to the sample to apply a fourth potential, which may be grounded. The potential may be defined relative to the sample 208. Each of the first, second and third electrodes is provided with an aperture through which a respective beamlet propagates. The second potential may be similar to the potential of the sample, for example, in the range of 50V to 200V positive to the sample. Alternatively, the second potential may be in the range of about +500V to about +1,500V relative to the sample. Higher potentials are useful if the detector module 402 is higher than the lowest electrode in the optical column. Each aperture or group of apertures may vary the first potential and/or the second potential to achieve focus correction.

Ideally, in one embodiment, the third electrode is omitted. An objective lens having only two electrodes has lower aberrations than an objective lens having more electrodes. A three-electrode objective lens can have a larger potential difference between the electrodes, thus realizing a stronger lens. The additional electrodes (i.e. more than two electrodes) provide additional degrees of freedom for controlling the electron trajectories, e.g. to focus the secondary electrons as well as the incident beam.

As mentioned above, it may be desirable to use a control lens to determine landing energy. However, the objective lens 300 may be additionally used to control landing energy. In this case, the potential difference across the objective lens is changed when a different landing energy is selected. One example of a situation where it may be desirable to partially change the landing energy by changing the potential difference across the objective lens is: preventing the focal point of the sub-beam from being too close to the objective lens. This situation may occur, for example, if the landing energy is reduced. This is because the focal length of the objective lens is approximately scaled with the chosen landing energy. By reducing the potential difference across the objective lens and thereby the electric field inside the objective lens, the focal length of the objective lens is again increased, resulting in a focal position further below the objective lens.

In the depicted arrangement, the control lens 600 includes three electrodes 601 to 603 connected to potential sources V5 to V7. The electrodes 601 to 603 may be separated 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, for example, 2mm to 200mm or more. The small spacing makes alignment easier, while the larger spacing allows weaker lenses to be used, reducing aberrations. Ideally, the potential V5 of the uppermost electrode 603 of the control lens 600 is maintained the same as the potential of the next electron-optical element (e.g., deflector 235) upstream of the beam of the control lens. The potential V7 applied to the lower electrode 602 may be varied to determine the beam energy. The potential V6 applied to the middle electrode 601 may be varied to determine the lens strength of the control lens 600, and thus the beam opening angle and demagnification. Ideally, the lower electrode 602 of the control lens and the uppermost electrode of the objective lens have substantially the same potential as the sample. In one design, the upper electrode V3 of the objective lens is omitted. In this case, the lower electrode 602 of the control lens and the electrode 301 of the objective lens ideally have substantially the same potential. It should be noted that the control lens can be used to control the beam opening angle even though the landing energy does not need to be changed or has been changed by other means. The position of the focal point of the sub-beam is determined by a combination of actions of the respective control lens and the respective objective lens.

In one example, to obtain landing energy in the range of 1.5kV to 2.5kV, the potentials V1, V2, V4, V5, V6, and V7 may be set, as indicated in table 1 below. The potentials in this table are given as beam energy values in keV, which correspond to the electrode potential relative to the cathode of the beam source 201. It will be appreciated that there is a considerable degree of design freedom in designing an electro-optical device as to which point in the electro-optical device is set to ground potential, and that operation of the electro-optical device is determined by the potential difference rather than the absolute potential.

TABLE 1

It can be seen that the beam energies at V1, V3 and V7 are the same. In various embodiments, the beam energy at these points may be between 10keV and 50 keV. If a lower potential is chosen, the electrode spacing can be reduced, especially in the objective lens, to limit the reduction of the electric field.

When a control lens, not a converging lens, is used to correct the opening angle/reduction ratio of the electron beam, the collimator is maintained at the intermediate focus, so that the astigmatic correction of the collimator is not required. In addition, the landing energy can be varied over a wide energy range while maintaining an optimal field strength in the objective lens. This minimizes the aberration of the objective lens. The intensity of the converging lens (if used) is also maintained constant, avoiding any additional aberrations introduced by the collimator not being in the intermediate focal plane or by the change of the path of the electrons through the converging lens.

In some embodiments, the charged particle evaluation tool further comprises one or more aberration correctors that reduce one or more aberrations in the beamlets. In one embodiment, each of the at least one subset of aberration corrector is located in or positioned directly adjacent (e.g., in or adjacent) a respective one of the intermediate foci. The beamlets have a smallest cross-sectional area in or near a focal plane such as the midplane. This provides more space for the aberration corrector than is available elsewhere (i.e. either upstream of the beam of the intermediate plane or downstream of the beam), or than may be available in alternative arrangements without an intermediate image plane.

In one embodiment, an aberration corrector located in or directly adjacent to the intermediate focus (or intermediate image plane) comprises a deflector to correct sources 201 that appear at different positions for different beams. The corrector may be used to correct macroscopic aberrations generated by the source that prevent good alignment between each sub-beam and the corresponding objective lens.

The aberration corrector can correct the aberration that prevents proper column alignment. Such aberrations may also lead to misalignment between the beamlets and the corrector. For this reason, it may additionally or alternatively be desirable to locate an aberration corrector at or near the converging lenses of the converging lens array 231 (e.g., where each such aberration corrector is integral with or directly adjacent to one or more of the converging lenses 231). This is desirable because at or near the converging lenses of the converging lens array 231, the aberrations also do not cause a shift in the corresponding beamlets, as the converging lenses are vertically close to or coincident with the beam aperture. However, the challenge of locating the corrector at or near the converging lens is: the beamlets each have a relatively large cross-sectional area and a relatively small pitch at that location relative to a further downstream location. The aberration corrector may be a CMOS based individually programmable deflector as disclosed in EP2702595A1 or a multipole deflector array as disclosed in EP2715768A2, the description of the beam manipulator in both documents being incorporated herein by reference.

In some embodiments, each of the at least one subset of aberration corrector is integrated with or directly adjacent to one or more of the objective lenses 234. In one embodiment, the aberration correctors reduce one or more of the following: field curvature, focus error and astigmatism. Additionally or alternatively, one or more scanning deflectors (not shown) may be integrated with or directly adjacent to the one or more objective lenses 234 for scanning the beamlets 211, 212, 214 throughout the sample 208. In one embodiment, the scanning deflector described in US2010/0276606, the entire contents of which are incorporated herein by reference, may be used.

In some embodiments, the objective lens array assembly includes a detector having a detector module 402 downstream of the beam 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. Integration of the detector module 402 into the objective lens array will replace the secondary post. The CMOS chip is preferably oriented to face the sample (because the distance between the wafer and the bottom of the electron optical system is small (e.g., 100 μm)). In one embodiment, electrodes for capturing secondary electronic signals are formed in the top metal layer of the CMOS device. The electrodes may be formed in other layers of the substrate, such as a CMOS chip. The power and control signals of the CMOS may be connected to the CMOS through silicon vias. For robustness, the bottom electrode is preferably composed of two elements: CMOS chips and passive Si plates with holes. The plate shields the CMOS from high electric fields.

In order to maximize detection efficiency, it may be desirable to make the electrode surface as large as possible so that substantially all of the area of the objective lens array (except for the aperture) is occupied by the electrodes, and each electrode has a diameter substantially equal to the array pitch. In one embodiment the outer shape of the electrode is circular, but this may be made square to maximize the detection area. Moreover, the diameter of the through-substrate holes can be minimized. Typical sizes of electron beams are about 5 microns to 15 microns.

In one embodiment, a single electrode surrounds each aperture. In another embodiment, a plurality of electrode elements are provided around each aperture. Electrons captured by the electrode elements surrounding an aperture may be combined into a single signal or used to generate separate signals. The electrode elements may be divided in a radial fashion (i.e., to form a plurality of concentric rings), in an angular fashion (i.e., to form a plurality of sectors), both in a radial fashion and in an angular fashion, or in any other convenient fashion.

However, a larger electrode surface results in a larger parasitic capacitance and thus a lower bandwidth. For this element, it may be desirable to limit the outer diameter of the electrode. Especially if the larger electrode gives only a slightly larger detection efficiency, but a significantly larger capacitance. Circular (ring) electrodes can provide a good compromise between collection efficiency and parasitic capacitance.

The larger outer diameter of the electrodes may also lead to larger cross-talk (sensitivity to signals of adjacent apertures). This may also be the reason for making the electrode outer diameter smaller. Especially if the larger electrode gives only a slightly larger detection efficiency, but gives significantly larger cross-talk.

The backscattered electron current and/or the secondary electron current collected by the electrodes is amplified by a transimpedance amplifier.

Fig. 6 shows an exemplary embodiment of a detector integrated into an objective lens array, which fig. 6 illustrates a schematic cross section of a part of a multibeam objective lens 401. In this embodiment, the detector comprises a detector module 402, which detector module 402 comprises a plurality of detector elements 405 (e.g. an array of detector elements) (e.g. sensor elements such as capture electrodes), which plurality of detector elements 405 preferably act as an array of detector elements (i.e. preferably a plurality of detector elements in a pattern or arrangement across a two-dimensional surface). In this embodiment, the detector module 402 is provided on 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 a detector module 402, the detector module 402 including a substrate 404 with a plurality of capture electrodes 405 disposed on the substrate 404, each capture electrode 405 surrounding a beam aperture 406. The beam aperture 406 may be formed by etching through the substrate 404. In the arrangement shown in fig. 7, the beam apertures 406 are shown in a rectangular array. The beam apertures 406 may also be arranged in different ways (e.g., in a hexagonal close-packed array as depicted in fig. 8).

Fig. 9 depicts a cross-section of a portion of the detector module 402 on a larger scale. The capture electrode 405 forms the bottom-most (i.e., closest to the sample) surface of the detector module 402. A logic layer 407 is provided between the trapping electrode 405 and the body of the silicon substrate 404. The logic layer 407 may include amplifiers (e.g., transimpedance amplifiers), analog-to-digital converters, and readout logic. In one embodiment, each capture electrode 405 has an amplifier and an analog-to-digital converter. The circuitry featuring these elements may be included in a unit area known as a cell associated with an aperture. The detector module 402 may have several cells each associated with an aperture; preferably, the units have similar shapes. The logic layer 407 and the capture electrode 405 may be fabricated using a CMOS process, wherein the capture electrode 405 forms the final metallization layer.

Wiring layer 408 is disposed on the backside of substrate 404 or within substrate 404 and is connected to logic layer 407 through silicon via 409. The number of silicon vias 409 need not be the same as the number of beam apertures 406. In particular, if the electrode signal is digitized in the logic layer 407, only a small number of silicon vias may be required to provide a data bus. The routing layer 408 may include control lines, data lines, and power lines. It should be noted that despite the beam aperture 406, there is still sufficient space for all necessary connections. The detector module 402 may also be fabricated using bipolar or other fabrication techniques. A printed circuit board and/or other semiconductor chips may be provided on the backside of the detector module 402.

The integrated detector module 402 described above is particularly advantageous when used with tools having tunable landing energies, since secondary electron capture can be optimized for the landing energy range. The detector modules in the form of an array may also be integrated into other electrode arrays, not just the lowermost electrode array. Further details and alternative arrangements of the detector modules integrated into the objective lens can be found in EP application number 20184160.8, which is incorporated herein by reference.

Fig. 10 is a schematic illustration of another exemplary electron-optical column for an evaluation system. The electron optical column is another example of an electron optical apparatus. The 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 comprises 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 array of the objective lens array 241 may be referred to as an objective lens electrode. Each objective lens formed by the array of plate electrodes may be a microlens that operates on a different sub-beam or group of sub-beams of the plurality. Each plate defines a plurality of holes (which may also be referred to as apertures). The position of each aperture in a plate corresponds to the position of a corresponding aperture (or corresponding hole) in another plate (or plates). The corresponding apertures define an objective lens so that each corresponding set of apertures operates on the same sub-beam or group of sub-beams of the plurality of beams when in use. Each objective lens projects a respective sub-beam of the plurality of beams onto the sample 208. See also the description of objective 234.

In some arrangements, the aperture in the objective array 241 is adapted to compensate for off-axis aberrations in the multiple beams. For example, the shape, size, and/or position of the aperture of one or more of the objective electrodes may be designed to compensate for off-axis aberrations. For example, the aperture holes may have different regional ranges (or diameter ranges) for compensating for field curvature, different ellipticity ranges for compensating for astigmatism, and/or different displacement ranges from the nominal grid position for compensating for distortion caused by telecentricity errors. See EPA 21166214.3, for example, filed on 3.31 of 2021, which is incorporated herein by reference for correction of off-axis aberrations thereof.

Fig. 11 depicts an example arrangement of apertures in an aperture array having 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 variation in aperture area depicted in fig. 11 is exaggerated for clarity and is actually smaller than the variation depicted. The solid circles represent apertures having different aperture area ranges. The dashed circle represents an unmodified aperture size to aid in visual recognition of the depicted change in aperture area. The different aperture areas may be described with reference to the diameter of circles having the same aperture area. Thus, even if the corresponding aperture is not exactly circular, the aperture area can be described with reference to the diameter. These variations generally involve an increase in aperture area as a function of increasing distance from the principal axis of the bundles (as schematically depicted in fig. 11, the principal axis is perpendicular to the page and passes through the centermost aperture). Proper correction may also involve the aperture area decreasing in accordance with an increase in distance from the principal axis of the bundles. In the example shown in fig. 11, the apertures are arranged on a regular grid defined by grid points 801 and grid lines 802.

Fig. 12 depicts an example arrangement of apertures in an aperture array having an ellipticity range to compensate for off-axis aberrations such as astigmatism. At least a subset of the apertures have different ellipticity ranges. The change in ellipticity depicted in fig. 12 is exaggerated for clarity and is actually less than the change in ellipticity depicted. The different ellipticity ranges are selected to compensate for off-axis aberrations such as astigmatism. These variations may involve the size of the radially oriented axis of the aperture (which may be the principal axis) increasing as a function of increasing distance from the principal axis of the bundles (as schematically depicted in fig. 12, where the principal axis is perpendicular to the page and passes through the centermost aperture). The proper correction may also involve that the size of the axis of azimuthal orientation of the aperture (which may be the principal axis) increases as a function of increasing distance from the principal axis of the multiple beams. In the example shown in fig. 12, the apertures are arranged on a regular grid defined by grid points 801 and grid lines 802.

Fig. 13 depicts an exemplary arrangement of apertures in an aperture array having a range of displacement different from the nominal grid position to compensate for off-axis aberrations such as distortion caused by poor telecentricity. At least a subset of the apertures are displaced relative to the nominal position. Such nominal positions may correspond to intersections between grid wires 802 of the grid. The apertures are each displaced from a corresponding nominal position 801 on the corresponding grid. The nominal positions may be provided on a regular grid. The regular grid may comprise, for example, a rectangular, square or hexagonal grid. The nominal position may represent a position corresponding to an ideal configuration in which no off-axis aberrations are present. The displacement shown in fig. 13 is exaggerated for clarity and is actually less than the depicted displacement. The displacement causes the aperture to lie on a grid (depicted by the thick dashed line) that is distorted relative to the nominal grid (depicted by grid line 802). The off-axis aberrations compensated by the displacement may include distortion caused by telecentricity errors. As illustrated in fig. 13, the displacement may be radially inward (toward the principal axis of the bundles) or radially outward. In both cases, the magnitude of the displacement may increase with radial distance. In the simplified example of fig. 13, this results in the corner aperture being shifted more than the lateral aperture.

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) flat panel electrode arrays connected to respective potential sources. The array of plate electrodes that control the 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 positioned close to each other and/or mechanically connected to each other and/or controlled together as a unit). The control lens array 250 is located upstream of the beam of the objective lens array 241. The control lens pre-focuses the beamlets (e.g., applies a focusing action to the beamlets before they reach the objective lens array 241). Prefocusing may reduce the divergence of the beamlets or increase the convergence rate of the beamlets. The lens array and the objective lens array are controlled to operate together to provide a combined focal length. A combined operation without intermediate focus may reduce the risk of aberrations.

In one embodiment, the electron optical device including the objective lens array assembly is configured to control the objective lens assembly (e.g., by controlling the potential applied to the electrodes of the control lens array 250) such that the focal length of the control lens is greater than the spacing 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 together, wherein the focusing action 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.

In one embodiment, the control lens array is an exchangeable module, which may be used alone or in combination with other elements such as an objective lens array and/or a detector module. The exchangeable module may be field replaceable, i.e. the module may be exchanged for a new module by a field engineer. In situ replaceable is intended to mean that the module can be removed and replaced with the same or a different module while maintaining the vacuum in which the electron optical tool 40 is located. Only the section of the column corresponding to the module is emptied in order to remove and return or replace the module.

The control lens array may be in the same module as the objective lens array 241, i.e. forming the objective lens array assembly or the objective lens arrangement, or it may be in a separate module.

Power may be provided to apply respective potentials to the electrodes of the control lenses of the control lens array 250 and the electrodes of the objective lenses of the objective lens array 241.

Providing a control lens array 250 in addition to the objective lens array 241 provides an additional degree of freedom for controlling the properties of the beamlets. Even when the control lens array 250 and the objective lens array 241 are arranged relatively close together, for example such that no intermediate focus is formed between the control lens array 250 and the objective lens array 241, an additional degree of freedom is provided. The control lens array 250 may be used to optimize the beam opening angle with respect to the demagnification of the beam and/or to control the beam energy delivered to the objective lens array 241. The control lens may include two or three or more electrodes. If two electrodes are present, the reduction rate and landing energy are controlled together. If there are three or more electrodes, the reduction rate and landing energy can be independently controlled. Accordingly, the control lens may be configured to adjust the demagnification and/or beam opening angle of the respective beamlets (e.g., using a power supply to apply respective suitable potentials to the electrodes of the control lens and the objective lens). Such optimization may be achieved by having an excessive negative impact on the number of objectives and without excessively degrading the aberrations of the objectives (e.g., without increasing the intensity of the objectives).

For example, the control lens array 250 may be considered to provide electrodes other than those of the objective lens array 241. The objective array 241 may have any number of additional electrodes, e.g., five, seven, ten, or fifteen additional electrodes, associated with the objective array 241 and proximate to the objective array 241. Additional electrodes, such as control lens array 250, allow more degrees of freedom for controlling the electron-optical parameters of the beamlets. Such additional associated electrodes may be considered as additional electrodes of the objective lens array 241, which electrodes perform the additional function of the respective objective lenses of the objective lens array 241. In one configuration, these electrodes may be considered part of the objective lens array 241, providing additional functionality to the objective lenses of the objective lens array 241. Thus, the control lens is considered to correspond to a portion of the objective lens, even in the range where the control lens is referred to as only a portion of the objective lens.

In the embodiment of fig. 10, the electron optical device comprises a source 201. The source 201 provides a beam of charged particles (e.g., electrons). A plurality of beams focused on the sample 208 are derived from the beams provided by the source 201. The beamlets may be derived from the beam, for example, using beam limiters that are used to define an array of beam limiting apertures. Ideally, source 201 is a high brightness thermal field emitter with a good tradeoff between brightness and total emission current. In the example shown, the collimator is provided upstream of the beam of the objective lens array assembly. The collimator may include a macro collimator 270. The macro collimator 270 acts on the beam from the source 201 before it is split into multiple beams. The macro-collimator 270 bends the respective portions of the beam by an amount effective to ensure that the beam axis of each of the sub-beams derived from the beam is incident on the sample 208 substantially perpendicular (i.e., substantially 90 deg. from the nominal surface of the sample 208). The macro collimator 270 applies a macro-collimation to the beam. Thus, instead of including an array of collimator elements each configured to act on a different individual portion of the beam, the macro-collimator 270 may act on all of the beams. The macro-collimator 270 may include a magnetic lens or a magnetic lens arrangement including a plurality of magnetic lens sub-units (e.g., a plurality of electromagnets forming a multipole arrangement). Alternatively or additionally, the macro-collimator may be at least partially electrostatically implemented. The macro collimator may comprise an electrostatic lens or an electrostatic lens arrangement comprising a plurality of electrostatic lens sub-units. The macro collimator 270 may use a combination of magnetic and electrostatic lenses.

In the embodiment of fig. 10, a macro-scan deflector 265 is provided to scan beamlets throughout the sample 208. The macro-scan deflector 265 deflects a corresponding portion of the beam so that the beamlets scan across the sample 208. In one embodiment, macro-scanning deflector 256 comprises a macro-multipole deflector having, for example, 8 poles or more. Deflection causes sub-beams derived from the beam to scan across the sample 208 in one direction (e.g., parallel to a single axis, such as the X-axis) or in two directions (e.g., relative to two non-parallel axes, such as the X-axis and the Y-axis). In some arrangements, scanning of the beamlets is coordinated with movement of the sample 208. For example, a combination of scanning beamlets parallel to the X-axis while moving the sample 208 parallel to the Y-axis may be repeated with the sample in an unsynchronized position to process a plurality of parallel slivers on the sample 208. The larger movement of the sample 208 may then be used to jump to a new processing location on the sample 208. An example of such movement is described in EPA 21171877.0 filed on 5.3.2021, which application is incorporated herein in terms of control of beam scanning as the stage moves. The macro-scan deflector 265 acts macroscopically on all beams, rather than comprising an array of deflector elements each configured to act on a different individual portion of the beams. In the illustrated embodiment, a macro-scan deflector 265 is disposed between the macro-collimator 270 and the control lens array 250.

Any of the objective lens array assemblies described herein may also include a detector (e.g., including 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 SEM, including secondary electrons and/or backscattered electrons emitted from the sample 208. Exemplary configurations of the detector module 402 are described above with reference to fig. 6-9.

In a variation on the embodiment of fig. 10, the objective lens array assembly may include a scanning deflector array. The scan deflector array includes a plurality of scan deflectors. Each scan deflector causes a respective sub-beam to scan across the sample 208. Thus, the scan deflector array may comprise a scan deflector for each sub-beam. Deflection causes the beamlets to scan across the sample 208 in one or two directions (i.e., one or two dimensions). In one embodiment, the scanning deflector described in EP2425444, which is specifically incorporated herein by reference in its entirety, may be used to implement a scanning deflector array. The scan deflector array is located between the objective lens array 241 and the control lens array 250. Instead of the macro scan deflector 265, a scan deflector array may be provided. In other embodiments, both macro scan deflector 265 and scan deflector arrays are provided and may be operated synchronously. In some embodiments, as shown in fig. 10, the control lens array 250 is a first deflecting or lensing electron optical array element in the beam path downstream of the beam of source 201.

Instead of the macro-collimator 270, an array of collimator elements may be provided. Although not shown, this variation can also be applied to the embodiment of fig. 3 to provide an embodiment with an array of macro-scanning deflectors and collimator elements. Each collimator element collimates a respective sub-beam. The array of collimator elements may be more spatially compact than the macro-collimator 270. Thus, providing the collimator element array and the scanning deflector array 260 together may save space. Such space savings are desirable where a plurality of electron optical devices including an objective lens array assembly are provided in an electron optical device array. In such an embodiment, there may be no macro-converging lenses or converging lens arrays. In this scenario, the control lens thus offers the possibility to optimize the beam opening angle and the demagnification for changes in landing energy.

In one embodiment, an electro-optical device in the form of an array is provided. The array may include any of the plurality of electron optical devices (e.g., electron optical columns) described herein. Each of the electro-optical devices in the array simultaneously focuses a respective plurality of beams onto different areas of the same sample. Each electron optical device in the array may form a sub-beam from a charged particle beam from a different respective source 201. Each respective source 201 may be one of a plurality of sources 201. At least a subset of the plurality of sources 201 may be provided as an array of sources. The source array may include a plurality of emitters on a common substrate. Focusing multiple beams simultaneously onto different areas of the same sample allows for the simultaneous processing (e.g., evaluation) of increased areas of the sample 208. The electron optical devices in the array may be arranged adjacent to each other so as to project respective bundles onto adjacent areas of the sample 208. Any number of electro-optical devices may be used in the array. Preferably, the number of electron-optical devices is in the range of 9 to 200. In one embodiment, the electro-optical devices are arranged in a rectangular array or a hexagonal array. In other embodiments, the electro-optical devices are provided in an irregular array or a regular array having a geometry other than rectangular or hexagonal. When referring to a single electron optical device or system or column, each electron 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 into an electron optical device in the form of an array due to their spatial compactness, which facilitates positioning of the electron optical devices in the array close to each other.

Fig. 14 depicts a portion of another example of an objective lens array assembly. The lens array assembly includes a series of electrodes 501-504 positioned orthogonally and/or serially along the beamlet paths of the multiple bundles. Although four electrodes are depicted and described, the variation is characterized by as many desired electrodes as possible; thus, the lens array assembly may comprise at least four electrodes. The objective lens array assembly may be used in the arrangement of fig. 10 and is an embodiment of the arrangement of fig. 5. The objective lens array assembly comprises a control lens array 250, 600 and an objective lens array 241, 300. In the arrangement depicted in fig. 5 and described with reference to fig. 5, the relative potentials of the beam upstream electrode 303 of the objective lens and the beam downstream electrode 602 of the control lens are substantially the same. It should also be noted that the lensing effect of the electrode is provided by the surface of the electrode. As depicted in fig. 14, the control lens array 250 may be defined by a plurality of electrodes 501 to 503, or at least by the beam upstream surfaces of the electrodes 501 and 503. Each electrode 501, 502, 503 contributing to the control lens array (and may be referred to as a control electrode, even though at least one electrode only partially contributes to the control lens array) may comprise a plate defining an aperture for each beamlet path 510 (these apertures together are an aperture array). The objective array 241 may be defined by a plurality of objective electrodes 503, 504 or at least the beam downstream surfaces of the electrodes 504 and 503. Each electrode 503, 504 contributing to the objective array (and may be referred to as an objective electrode, even though at least one electrode only partially contributes to the objective array) may comprise a plate with apertures for each beamlet path 510 (these apertures together being an array of apertures). The control electrodes 501 to 503 and the objective lens electrodes 503, 504 may be referred to as lens electrodes. (in one variation, the electrode 503, which may be referred to as either the objective electrode or the control electrode, may be two separate electrodes that are spaced apart along the path of the beamlets and have substantially the same applied potential).

Fig. 14 shows five exemplary beamlet paths 510. The control electrodes 501 to 503 are arranged in series along the beamlet path 510 and define respective apertures aligned with the beamlet path 510 to define a control lens. Thus, each control lens is aligned with the beamlet path 510 of the corresponding beamlet and operates (e.g., electrostatically manipulates) the beamlet. Each control electrode 501 to 503 may operate on part of the beamlets or on all of the beamlets. Each objective in the objective array 241 may be aligned with a beamlet path 510, which beamlet path 510 is aligned with a respective control lens. The objective array 241 directs the beamlets onto the sample 208.

The arrangement may be described as four or more lens electrodes as plates. Apertures are defined in the plate, for example, as an array of apertures that are aligned with the several beams in the corresponding array of beams. The electrodes may be grouped into two or more electrodes, for example, to provide a control electrode group and an objective electrode group. In one configuration, the objective electrode group has at least three electrodes, while the control electrode group has at least two electrodes.

In the example of fig. 14, the objective electrode 503 furthest from the sample 208 (which may be referred to as the most beam upstream electrode of the objective array 241) and the control electrode 503 closest to the sample 208 (which may be referred to as the most beam downstream electrode of the control lens array 250) are provided by a common electrode. Thus, the most beam upstream electrode of the objective electrode group is the common electrode, which is also a member of the control electrode group. The surface of the common electrode 503 remote from the sample 208 (which may be described as the beam upstream surface) contributes functionality to the control lens array and may therefore be considered to comprise part of the control lens array. The surface of the common electrode 503 facing the sample 208, which may be referred to as the beam downstream surface, contributes a function to the objective lens array 241 and may therefore be considered to comprise part of the objective lens array 241.

It is advantageous to provide a common electrode, wherein it may be desirable to control the positioning of the lens array 250 close to the objective lens array 241. This is more likely to be the case in an arrangement in which the scan deflector array 260 is not used (e.g., in which the macro scan deflector 265 is used instead). This is because in the case of using the scan deflector array 260, it may be desirable to locate the scan deflector array 260 between the control lens array 250 and the objective lens array 241, for example, so that the distance between the scan deflector 260 and the objective lens array 241 is as short as possible. An arrangement with a macro scan deflector 265 is illustrated in fig. 10. However, it should be noted that variations with respect to the arrangement of fig. 10, which still have a converging lens array but have a scanning deflector array, are possible. In these arrangements, it may also be desirable to locate the scanning deflector array between the control lens array and the objective lens array. Alternatively, the scanning deflector array may be located elsewhere, such as within the control lens array or upstream of the beam of the control lens array, such as between the control lens array and the beam limiting aperture array.

In this example of fig. 14, the objective lens array assembly further comprises a beam shaping limiter 242. The beam shaping limiter 242 defines an array of beam limiting apertures. The beam shaping limiter 242 may be referred to as a beam shaping limiting aperture array or a final beam limiting aperture array. The beam shaping limiter 242 may comprise a plate (which may be a plate-like body) having a plurality of apertures. The beam shaping limiter 242 is downstream of the beam controlling at least one electrode (optionally from all electrodes) of the lens array 250. In some embodiments, the beam shaping limiter 242 is downstream of the beam of at least one electrode (optionally all electrodes) of the objective lens array 241. In another embodiment, it may be an array, for example, the bottommost array of the objective lens array 241.

In one arrangement, the beam shaping limiter 242 is structurally integrated with the electrodes of the objective lens array 241. Each beam limiting aperture has a beam limiting effect allowing only a selected portion of the beamlets incident on the beam shaping limiter 242 to pass through the beam limiting aperture 124. The selected portions may be such that only a portion of the respective beamlets that pass through a central portion of the respective aperture in the objective lens array reach the sample 208.

In some embodiments, the electron optical apparatus further comprises an upper beam limiter 252. The upper beam limiter 252 defines a beam limiting aperture array or generates a beam array, for example, from a source beam from the source 201. The upper beam limiter 252 may comprise a plate (which may be a plate-like body) having a plurality of apertures. The upper beam limiter 252 forms beamlets from the charged particle beam emitted by the source 201. Portions of the beam other than those that contribute to the formation of the beamlets may be blocked (e.g., absorbed) by the upper beam limiter 252 so as not to interfere with the beamlets downstream of the beam.

The upper beam limiter 252 may form part of an objective lens array assembly. For example, the upper beam limiter 252 may be adjacent to and/or integrated with the control lens array 250 (e.g., adjacent to and/or integrated with an electrode of the control lens array 250 that is closest to the light source 201, or even the same electrode). 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. Accordingly, the beam limiting aperture of the beam shaping limiter 242 may have a smaller size than a corresponding aperture defined in the objective lens array 241 and/or the control lens array 250.

Ideally, the beam shaping limiter 242 is configured to have a beam limiting effect, i.e. to remove a portion of each sub-beam incident on the beam shaping limiter 242. For example, the beam shaping limiter 242 may be configured to ensure that each sub-beam exiting the objective lens of the objective lens array 241 has passed through the center of the respective objective lens. Further, the beam shaping limiter 242 reduces the length of the scanning operation performed on the beamlets. The distance is reduced to the length of the beam path from the beam shaping limiter 242 to the sample surface.

The beam shaping limiter 242 may be integrally formed with the bottom electrode of the objective lens array 241. In general, it may be desirable to locate the beam shaping limiter 242 adjacent to the electrode of each objective lens that has the strongest lensing effect. In one arrangement, it may be desirable to provide a beam shaping limiter 242 upstream of the beam of the detector module 402 of the detector. Providing the beam shaping limiter 242 upstream of the detector module 402 ensures that the beam shaping limiter 242 does not block charged particles emitted from the sample 208 and does not prevent them from reaching the detector module 402. Thus, the beam shaping limiter 242 may be provided directly adjacent to the detector module 402 in the beam upstream direction.

Fig. 15 schematically depicts another example of an electron optical apparatus (electron optical column). Features identical to those described above are given the same reference numerals. For brevity, these features are not described in detail with reference to fig. 15. For example, the source 201, the converging lens 231, the macro collimator 270, the objective array 241, and the sample 208 may be as described above. In one variation, the macro-collimator 270 may include a deflector array configured to collimate the beamlets. Alternatively, the collimator may be an array of deflectors configured to collimate the beamlets. In one arrangement, the converging lens may be a single plate for a beam limiting aperture array (defining a plurality of apertures in the array), wherein one or more macro-electrodes are associated with the single plate. Such a beam limiting aperture array and associated macro-electrode may also form a converging lens array to focus the generated beam on an intermediate focus, which ideally corresponds to the position of collimator 270.

As described above, in one embodiment, the detector 240 is located 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, an objective array 241 comprising a plurality of objectives is located between the detector 240 and the sample 208.

In one embodiment, deflector array 95 is located between detector 240 and objective lens array 241. In one embodiment, the deflector array 95 comprises a Wien (Wien) filter array, such that the deflector array may be referred to as a beam splitter. The deflector array 95 is configured to provide a magnetic field to separate charged particles projected onto the sample 208 from secondary electrons emitted from the sample 208 toward the detector 240.

In one embodiment, the detector 240 is configured to detect signal particles by referencing the energy of charged particles (depending on the bandgap, such as a semiconductor-type based detector). Such a detector 240 may be referred to as 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, such as a scintillator array of phosphor stripes between the beams, located upstream of the beams with respect to the wien filter, e.g. along the primary beam path. The primary beam passing through the wien filter array (magnetic and electrostatic stripes orthogonal to the primary beam path) has substantially parallel paths upstream and downstream of the wien filter array, while the signal electrons from the sample are directed toward the scintillator array by the wien filter array. The electron-to-photon converter may be photon coupled to the photon-to-electron converter to convert any photons generated and emitted in the electron-to-photon converter. The photon-electron converter may be electrically connected to the electronic circuitry to process the detection signal. In different embodiments, the photon-electron converter may be internal or external to the electron-optical device. In one embodiment, the photon coupling may be coupled via a photon transmission unit (e.g., an array of optical fibers) to a remote optical detector that generates a detection signal when a photon is detected.

In operation, the electron optical device generates a strong electric field between electrodes (e.g., plates) of the objective lens array 241. A significant electric field can also be generated between the electrodes elsewhere in the system. The strong electric field is associated with a corresponding strong electrostatic pressure. The electrostatic pressure is proportional to the field energy density η _E, which is η _E according toAnd is also proportional to E ² (where epsilon is the permittivity and E is the electric field strength). Thus, the electrostatic pressure increases rapidly with an increase in E.

In some arrangements, the electrostatic pressure causes a change in 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 deformation of the plate. Fig. 16 schematically depicts such electrode distortions of the electrodes 503 and 504 of the objective lens array 241. For ease of description, the electrodes 503, 504 are shown without apertures and are not drawn to scale. The dashed lines, which are rectangular in shape, depict example cross-sectional shapes of the electrodes 503, 504 prior to turning on the electron optical system (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 depicts an example cross-sectional shape of the electrodes 503, 504 when the electron optical system is on and an example electric field is present between the electrodes. Fig. 16 depicts a typical case where the electrode bows enter the high electric field strength region. This mode of electrode distortion may be referred to as bow. Bow may result in distortion having a parabolic or near parabolic form; that is, the distortion varies as an approximation of the square of the radial position.

Electrode distortion in the objective lens array 241 may affect multiple sub-beams. For example, electrode distortion may have an effect on field curvature. The field curvature is the case where the focal plane is different for different sub-beams of the plurality, which may result in a focus error at the planar surface of the sample 208. The objective array may be configured to compensate for predicted effects on the beamlets from predicted electrode distortions in the objective 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 correction includes a change in the size of an aperture defined in one or more of the electrodes (e.g., the diameter of the aperture being circular) that varies depending on the location in each electrode. Changing the size of the aperture in the electrode can compensate for the change in field curvature.

Having a limited build tolerance limits the accuracy of electrode distortion prediction. Limited manufacturing tolerances can result in small but significant variations between different manufacturing instances of the objective lens array 241, such as differences in electrode thickness and/or aperture size. These variations may affect the hardness of the electrode, which may result in a given electrostatic pressure being associated with different electrode distortions for different 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, electrode distortion is expected to result in surface displacements of up to about 10 microns. If a budget defocus amount of 100nm is assigned to this effect, this may imply that the reproducibility of the electrode distortion should be within 1% if hardware correction is to be effectively made (e.g. by changing the aperture diameter). It is not desirable to be limited by such tight manufacturing tolerances. For arrangements of the type shown in fig. 10, electrode distortion is expected to be much smaller, but it is particularly desirable to support tunable landing energies in such systems. Tuning the landing energy can result in significant changes in the electrostatic field in the objective lens array, which again can lead to hardware under-correction. The arrangement described below aims at improving the compensation of electrode distortion effects. The improved compensation may allow manufacturing tolerances to be relaxed (e.g., 1% to 10%) and/or support tunable landing energy functionality. An arrangement allowing the landing energy to be varied while suppressing both field curvature and astigmatism is described below.

The arrangement including 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 a control board (e.g., controlling the lens array 250 and/or the objective lens array 241), for example, by controlling the electrical potential applied to the board. For this purpose, a controller 500 (as schematically shown in fig. 3 and 10) may be provided. As described below, the controller 500 may be computer implemented, wherein any suitable combination of elements (e.g., CPU, RAM, etc.) are used to provide the desired functionality. The control electrode and the objective lens electrode as described above with reference to fig. 5 may be controlled by connecting them to a source of electrical potential. Thus, the controller 500 may include and/or control a source of electrical potential that applies electrical potential to the different lens electrodes.

Any reference herein to a device or system configured to perform a function is intended to encompass the following: the controller 500 is configured to perform this function (e.g., by being programmed in a suitable manner to provide the necessary control signals to a device such as a source of electrical potential).

In some arrangements, the electron-optical device (e.g., via the controller 500) is configured to implement a plurality of selectable landing energies for the beamlets of the plurality of beamlets (and optionally, for all of the beamlets). A plurality of selectable landing energies may be achieved by applying corresponding potentials (e.g., via corresponding potential sources) to the control electrodes 501-503 and the objective lens electrodes 503 and 504. A different potential may be applied to each of the selectable landing energies. Thus, the device allows to select different landing energies for the beamlets at different corresponding times. The selectable landing energy may include one or more continuous landing energy ranges. In this case, the device will be able to select any landing energy over one or more continuous ranges. Alternatively or additionally, the optional landing energy may comprise a plurality of predetermined discrete landing energies. The selection may be performed 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 select the selectable landing energy based at least in part (i.e., in whole or in part) on the received user input. Alternatively or additionally, the device may be at least partially (i.e., fully or partially) automatically operated. The device may select landing energy, for example, based at least in part on a predetermined program or in response to one or more input parameters, for example, determined by an application or model. The input parameters may represent, for example, measurements made by the system.

The choice of landing energy may depend on the particular inspection scenario. For example, the landing energy may be selected to optimize parameters of a particular type of signal particles, such as secondary electronic yield and contrast (which may be defined as the yield difference between the feature and the background). The landing energy to achieve this will depend on the material being inspected. The nature of the defect of interest may also play a role. When physical defects are of interest, the material properties will determine the secondary electronic yields. In the event of a voltage contrast defect of interest, the charging behavior and secondary electronic yield will depend on whether the circuit is capable of drawing charge. Alternatively or additionally, landing energy may be selected to control charging (which has an impact on distortion and secondary electronic yield). Alternatively or additionally, landing energy may be selected to achieve a desired electron optical performance. For example, degradation of resolution by selecting a lower landing energy may be compromised with an increase in secondary electronic yields.

The electro-optical device may be configured to select (e.g., via the controller 500) the corresponding potentials for the different 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 the objective electrodes 503 and 504 is the same for each of the selectable landing energies. Thus, the spacing between all electrodes and between each of the electrodes and the image plane of the device remains the same regardless of which alternative landing energy is achieved. Thus, the user may choose different landing energies without adjusting the position of any of the electrodes 501-504, sample 208, or detector module 402.

In some arrangements, the device is configured to: the same potential is applied (e.g., via controller 500) to the control electrode 501 furthest from the sample 208 (and the portion of the control lens aligned with the beamlet path of the beamlet for which landing energy was selected) for at least a portion of the selectable landing energy. 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 beamlets are derived. The control electrode 501 may be fixed at a potential corresponding to a beam energy between 10keV and 50keV, for example.

In some arrangements, the device is configured to: for each of at least a portion of the selectable landing energies (e.g., via controller 500), a different potential is applied to the objective electrode 503 furthest from the sample 208 (and a portion of the objective aligned with the beamlet path of the beamlet for which landing energy was selected). For example, each potential may be selected 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 in the objective array 241 and thus the focal length of each objective. Therefore, the position of the image plane can be controlled by controlling the electric potential applied to the objective lens electrode 503.

In some arrangements, the device is configured to: landing energy is controlled (i.e., a desired landing energy is selected from a range of available selectable landing energies) by controlling (e.g., via controller 500) at least the potential applied to the objective lens electrode 504 nearest the sample (and a portion of the objective lens aligned with the beamlet path of the beamlet for which landing energy is selected). For example, the objective lens electrode 504 may be set to a potential corresponding to a beam energy equal to the desired landing energy + the predetermined offset. The predetermined offset may be, for example, in the range of-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 for voltage contrast use cases. In the case where secondary electrons are to be detected, the offset voltage may be generally about 50V or higher to ensure sufficient detection efficiency, but a lower offset voltage may be sufficient if the distance between the detector and the sample is sufficiently small. If it is desired to repel secondary electrons, for example, if the backscatter signal is of interest, a negative voltage is used.

In some arrangements, the device is configured to: the lens array 250 is controlled (e.g., via the controller 500) to minimize the resolution of each of the plurality of selectable landing energies. This may be accomplished, at least in part, by adjusting the control lens array 250 at each selectable landing energy to maintain the demagnification of the device (from the electron source to the sample) divided by the angular demagnification of the device the same for each selectable landing energy. This may be achieved, for example, by controlling the potential applied to a middle (preferably central) control electrode 502 of the control lens array 250 (e.g., where the control lens array 250 is defined by three control electrodes 501 to 503; note that the middle electrode may be the only central control electrode of a lens array having an odd number of electrodes). Controlling the potential applied to the intermediate (preferably, middle) control electrode 502 will control the rate of reduction. Maintaining the demagnification divided by the angular demagnification is the same for different landing energies ensures that the off-axis aberrations remain constant. Thus, the hard-coded correction for off-axis aberrations is still effective, keeping the net aberrations (after correction) for different landing energies at a low level. This is achieved without the need to exchange the respective objective electrode or objective array 241, which may undesirably introduce downtime and/or inconvenience. It further eliminates or reduces the need for electronic components as field replaceable exchangeable modules. The complexity required for the vacuum chamber can be reduced.

Fig. 17 is a graph showing predicted changes in beam current versus beamlet resolution at sample 208 for an electron optical system using an electron optical device without a converging lens array (e.g., as depicted in fig. 10 featuring a macro converging lens 270 or additional macro electron optical components such as macro collimator 270 shown in fig. 15). Such macroelectron optical components may be magnetic. The curve is obtained by simulating an electrostatic field and electronically tracking the light passing through the electrostatic 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 a respective combination of beam current and resolution (including, for example, hard-coded off-axis compensation). In general, the larger the total current, the better to achieve good throughput, while the minimum resolution is required to provide measurements with good spatial resolution. The graph shows that a balance needs to be achieved between the two quantities: increasing the beam current increases the resolution and vice versa. Still further, the beam current versus resolution curve is different for different landing energies.

Fig. 18 is a graph showing eight example curves (solid curve 523 with open squares) 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 plurality of different landing energies are selected in steps of 250eV between a landing energy of 2.5keV corresponding to the curve 521 and a landing energy of 1keV corresponding to the curve 522. At each selected landing energy, the electron optical system controls the lens array 250 to minimize resolution by varying the demagnification. This may be achieved, for example, by: dividing the demagnification of the electron optical system by the angular demagnification of the electron optical system remains constant to ensure that the hard-coded off-axis aberration compensation remains effective, and/or the demagnification of the control lens array is changed to compensate for other effects such as changes in field curvature caused by distortion of the element (e.g., electrode) caused by the electric field. In each case, the potentials applied to the control electrodes and the objective electrodes are also selected to maintain the same spatial relationship between the image plane and all of the control electrodes and the objective electrodes. Thus, each curve 523 shows a different landing energy range available, as well as the corresponding beam current and minimum resolution.

In some arrangements, the resolution is deliberately not minimized, in contrast to the situation illustrated in fig. 18. Removing this constraint may allow a different beam current range to be selected for each selectable landing beam energy. Accordingly, the beam current may take other values, rather than being limited to having the beam current correspond to one of the curves 523 in fig. 18. Higher beam currents can be selected but at the cost of higher (less desirable) resolution. By allowing the beam current to be varied in this way, the electron optical system thus provides a plurality of selectable beam currents of the sub-beams for each of one or more of the selectable landing beam energies. Thus, in exchange for operating at a greater resolution, the beam current may be selected from a range of selectable beam currents.

Fig. 19 is a graph showing four example curves (solid curves 524 to 527) showing how the beam current changes in the manner described above. Each curve 524 to 527 corresponds to a different landing energy (524=2.5 keV, 525=2.0 keV, 526=1.5 keV, 527=1 keV), but with the same physical configuration (same beam limiting aperture diameter and hard-coded off-axis correction) and the same image plane. Each curve 524 to 527 has a parabolic form rotated 90 degrees. In curve 527, the upper and lower branches of the parabola are shown. In curves 524 through 526, only the upper branch (i.e., the branch in which beam current increases with increasing resolution) is shown for clarity, although there are two branches. The device may be configured to achieve each selectable beam current by selectively controlling a corresponding demagnification of the lens array 250. Each reduction rate corresponds to a different beam current. In the example of fig. 19, different points on each of the curves 524 through 527 correspond to different reduction rates. The reduction rate may be adjusted for each curve 524-527 to optimize resolution. Alternatively, the reduction rate may be adjusted to allow resolution degradation while achieving a larger beam current. As described above, in the case where the control lens array 250 includes three control electrodes 501 to 503, each reduction rate can be selected by applying a corresponding potential to the middle electrode 502 of the three control electrodes 501 to 503. Thus, the electron optical system may be configured to achieve each selectable beam current by applying a corresponding potential to the middle electrode 502 of the three control electrodes 501 to 503.

As mentioned in the introductory part of the description, hard-coded correction in an electro-optical device may not be optimal in all situations. Such hard-coded correction may be applied to the objective lens, control lens or other associated electrode or preferred electrode of the plate. For example, corrections for field curvature and astigmatism may be hard coded into the diameter and ellipticity of apertures in an aperture array in a plate of an objective array (such as a particular plate of an objective array). During use of the electro-optical device, field curvature and astigmatism may change, which results in the hard-coded correction becoming suboptimal.

In some configurations of electro-optical devices, for example, changing landing energy may cause a change in field curvature. This is because if the image plane remains in the same position, a change in landing energy may need to be accompanied by a change in the electric field between the plates of the objective array 241. As discussed above with reference to fig. 16, the electric field causes distortion of the plate, e.g., bow, due to the electrostatic pressure. These distortions have an effect on the aberrations of the sub-beam, such as field curvature.

In some embodiments, the varying electron optical device disposed upstream of the beam of the plate also affects the field curvature. For example, in a configuration such as depicted in fig. 10 and 15, this is the case: wherein the macro collimator 270 (which may also be referred to as a macro converging lens) also has a significant effect on the field curvature. The magnitude of the field curvature contributed by macro-collimator 270 may be as large as 1 to 3 microns in the image plane of the objective array, depending on the demagnification (which may be controlled using control lens array 250 as described above). Note that the demagnification referred to herein is the overall system demagnification determined by the combination of all lenses along the beam path of the electron optical apparatus; the overall system reduction ratio is a reduction ratio of the electron optical system. Each beamlet passes through its own set of lenses, but the demagnification is approximately the same for all beamlets. Thus, the overall system shrinkage may be quantified by averaging the sub-beams or by referencing the shrinkage of a reference sub-beam, such as an axial center sub-beam. The field curvature contributed by the macro collimator 270 may be of opposite sign and may be as large or larger than the change in field curvature caused by field induced distortion of the plates in the objective array 241. Thus, the magnitude of the field curvature applied by the macro collimator 270 may be controlled to compensate for the change in field curvature caused by field induced distortion of the plate. For example, the amplitude may be controlled by controlling the reduction rate using the control lens array 250. The amplitude is expected to vary in proportion to the linear reduction rate divided by the angular reduction rate. However, such varying electron-optical devices (e.g., macro-collimator 270) may contribute to variations in properties of the beamlets other than field curvature. For example, in the arrangement of fig. 10 and 15, the macro collimator 270 will contribute to astigmatism as well as field curvature. Calculation of the expected field induced distortion shows that: in a typical landing energy range, the plate may bow up to about 200nm to 400nm. The defocus amplitude caused by the astigmatism applied by the macro collimator 270 will be about half the amplitude of the field curvature applied by the macro collimator 270. It should be noted that astigmatism is not in terms of blur or resolution (e.g., in a direction substantially parallel to the plane of the sample 208, e.g., in the planes of the x-axis and the y-axis) with respect to focusing (e.g., in a direction from the source toward the sample, e.g., the z-axis relative to the plane of the sample 208). This implies that: within a typical landing energy range, astigmatism can vary between about 100nm to 200nm, which may represent a significant negative contribution to defocus performance.

As the landing energy changes, the demagnification applied by the control lens array 250 may be tuned such that the field curvature applied by the macro collimator 270 continues to cancel the field curvature applied by the plates downstream of the beam of the macro collimator 270 (e.g., the objective array). However, a change in demagnification will result in a change in astigmatism, which is typically not compensated for by a plate downstream of the beam. There may be hard-coded corrections that aim to compensate for astigmatism, but these cannot change with a change in the demagnification, and therefore typically become ineffective with a change in landing energy. A desired combination of low field curvature and low astigmatism can be achieved within a narrow landing energy range, but without the embodiments described herein, either or both of the field curvature and astigmatism can degrade relatively rapidly outside the narrow landing energy range.

The embodiments described below aim to increase the range of situations in which hard-coded corrections are effective (e.g., the range of operating parameters of an electronic optical device).

Fig. 20 schematically depicts another example of an electron optical apparatus (electron optical column). Features identical to those described above are given the same reference numerals; for example, the arrangements depicted in fig. 3, 10, 14 and 15 and described with reference to fig. 3, 10, 14 and 15. For brevity, these features are not described in detail with reference to fig. 20. Features having the same reference numerals adopt the same description as those previously stated unless stated to the contrary. For example, the source 201, the macro-collimator 270, the upper beam limiter 252, the electrodes 502 to 504, and the sample 208 may be as described above. Fig. 20 also schematically depicts a rigid mount 730 for supporting the electrodes 502-504. Further details of example arrangements for mounting electrodes can be found in PCT/EP2021/084737, at priority date 2021, 12, 23, the disclosure of which, in particular, is incorporated herein by reference in its entirety for all parts of the disclosure of a mount for electrodes.

As illustrated with reference to fig. 1-10, 14, 15, and 20, an electron optical device may be provided that projects multiple beams of beamlets of charged particles onto a sample 208.

The apparatus includes a plurality of plates. The plates may be electrically conductive or have an electrically conductive coating. Thus, the plate may define an equipotential surface. These plates may be referred to as electrodes. The plates define respective arrays of apertures. Thus, each plate defines an array of apertures. Each aperture array includes a plurality of apertures. Each plate may be arranged such that the perimeters of apertures in the array are electrically connected together to be at the same potential as each other (e.g., to form part of the same potential surface). The plates may be arranged in series along the beamlet path 510. Each beamlet path 510 may intersect a corresponding aperture in each of the plates. Thus, the apertures in the different plates may be aligned with each other along the respective beamlet paths 510. In fig. 5, 14 and 20, electrodes 301 to 303, 501 to 504 and 601 to 603 all represent examples of such plates. The electron optical device applies an electrical potential to the plate to control the multiple beamlets (e.g., to shrink the beamlets and/or to properly focus the beamlets on the sample 208 to be evaluated). The electric field generated near the aperture in such a plate is known to produce a lensing effect on charged particles passing through the aperture.

The plurality of plates includes an objective lens array 241. A subset or all of the plates may form an objective array 241. The objective lens array 241 projects the beamlets towards the sample 208. The objective lens array 241 may take any of the forms described with reference to fig. 3,5 to 10 and 14 to 16. In some arrangements, the plurality of plates further includes an electrode associated with and adjacent to the objective lens array (such as control lens array 250). Where present, the control lens array 250 may take any of the forms described with reference to fig. 3,5, 10, and 14, and may feature the electro-optical device 41 depicted and described with reference to fig. 15. The electron optical device may be configured to detect signal electrons emitted from the sample 208 (e.g., using a detector, which may include the detector module 402 as described above) to obtain information about the sample 208. Such a plate features the converging lens array 231 of fig. 3 and 15.

The electro-optical device may include a controller 500 (as schematically depicted in fig. 3, 10, and 15) to control the operation of the board (e.g., the application and control of the electrical potential applied to the board). As described below, the controller 500 may be computer implemented, wherein any suitable combination of elements (e.g., CPU, RAM, etc.) are used to provide the desired functionality. As described above with reference to fig. 5, the board may be controlled by connecting the board to a source of electrical potential. Accordingly, the controller 500 may include and/or control a source of electrical potential. The potential source may apply a potential to different plates, samples 208, and/or other elements. The controller 500 may also control a stage for supporting the sample 208.

Any reference herein to an electro-optical device (or associated evaluation system) configured to perform a function is intended to encompass the following: the controller 500 is configured to perform this function (e.g., by being programmed in a suitable manner to provide the necessary control signals to devices such as a potential source and/or station).

In one embodiment, the aperture arrays defined in at least two of the plates each have a geometric characteristic. The geometric properties may be applied to respective apertures or aperture arrays of respective plates, e.g. at least two of the plates. The geometric properties are configured to apply a perturbation to a corresponding target property (or properties) of the beamlets that pass through the apertures in the plate. Thus, the geometric properties correspond to specific target properties of the beamlets. The target properties may include astigmatism, field curvature, distortion, coma, or other properties of interest. A plate in which an aperture having geometric properties is defined may be referred to as a plate with hard-coded corrections.

In one embodiment, the target property comprises astigmatism and the geometric characteristic corresponding to the target property (and thus configured to apply a corresponding perturbation) comprises different aperture ellipticity ranges in the aperture array, as discussed above with reference to fig. 12.

In one embodiment, the target properties include field curvature, and the geometric characteristics corresponding to the target properties (and thus configured to apply corresponding perturbations) include a range of different aperture sizes (such as areas) in the aperture array, as discussed above with reference to fig. 11.

In one embodiment, the target property comprises distortion (e.g., due to telecentricity errors), and the geometric characteristic corresponding to the target property (thus configured to apply a corresponding perturbation) comprises a range of different aperture positions in the aperture array relative to a respective nominal position (preferably on a regular grid (e.g., rectangular or hexagonal grid), as discussed above with reference to fig. 13.

In one embodiment, the target property comprises coma and the geometric characteristic corresponding to the target property (thus configured to apply a corresponding perturbation) comprises a range of different aperture positions in the aperture array relative to the respective nominal positions (preferably on a regular grid (e.g., rectangular or hexagonal grid), as discussed above with reference to fig. 13.

Each aperture array may have a single geometric characteristic configured to impart such perturbations, or may have a plurality of different such geometric characteristics. Each additional geometric feature other than the first geometric feature may be referred to as another geometric feature and may take any of the forms described above. Each such further geometrical property is configured to impart a perturbation to a corresponding further target property of the sub-beam, and the further target property may comprise any of the target properties described above. Thus, the aperture array may be configured to impart perturbations to a plurality of target properties by having a corresponding plurality of geometric properties. In this way, a single aperture array may perturb any combination of the target properties mentioned above.

In some embodiments, the controller 500 is configured to apply and control the electrical potentials applied to the plates having the geometric characteristics such that the applied perturbations substantially collectively compensate for the variations in the target properties corresponding to the geometric characteristics. The change in the target property may be an aberration. Compensating for the change in the target property may suppress the aberration. The device is configured such that the variation is substantially compensated for within the parameters of the device. Thus, compensation is not limited to one particular configuration, as may be the case for typical hard-coded corrections to changes in target properties. By providing geometric features in a plurality of different plates, it is possible to apply the geometric features independently in the different plates and to control the plates themselves independently, enabling compensation over a wider range of configurations. The method effectively provides at least one additional degree of freedom relative to the case where the geometric properties are provided in only a single plate or are fixed to be the same in each of a plurality of plates. It has been found that the additional degrees of freedom allow tuning of the hard-coded correction so as to be applicable to a wider range of operating configurations of the device (e.g., a wider range of parameters of the device).

Providing geometric features (which may be referred to as hard-coded corrections) in more than two different plates (e.g., in three plates, four plates, or five plates) provides other degrees of freedom, allowing for higher degrees of compensation, and/or allowing for compensation over a longer (or larger) range of parameters and/or over different parameter ranges. In one arrangement, the same geometric characteristics may be applied to two or more different plates as hard coded corrections. The geometric characteristics of the aperture array applied to two or more plates as hard-coded corrections may include one or more of the following: ellipticity, diameter, and displacement from the regular grid, such as a change in one or more of ellipticity, diameter, and displacement from the regular grid. The variation between correction features applied to different apertures in an aperture array depends on the position of the apertures in the aperture array. The geometric characteristics of the apertures applied to the plate may be within a range of geometric characteristics such that the magnitude of the geometric characteristics of the apertures applied to the plate may be different from another aperture in the plate. It has been found that a particularly good balance of performance and device complexity is achieved when the aperture array defined in the three plates has geometric characteristics and the controller applies and controls potentials to all three plates. It has been found that the corresponding perturbations applied in such an embodiment substantially compensate for variations in the target properties to high levels within the parameters of the device.

The following discussion relates to theoretical models and simulations. In some of these simulations, different data points may represent different hard-coded corrections in one or more plates, but it should be understood that in practice, hard-coded corrections cannot generally be changed without mechanically changing and/or replacing one or more elements (e.g., plates) in which the hard-coded corrections are defined.

In the use case discussed above, the potentials of the plates in the objective lens array 241 and control lens 250 in the arrangement of fig. 10 and 15 are varied to compensate for field curvature over the landing energy range. The challenge in this scenario is that astigmatism also varies depending on the demagnification used to tune the contribution from the field curvature of the macro collimator 270. One aspect of an embodiment of the invention is based on the following observations: the ellipticity of the apertures of all the plates through which the beamlets pass (e.g., the apertures in the plates controlling the lens array 250 and the objective lens array 241) affects astigmatism and the sensitivity of each of these plates (i.e., the range of contributions of a given amount of ellipticity to astigmatism) varies differently depending on landing energy. This is illustrated in the simulation results shown in fig. 21 for example configuration based on the arrangements of fig. 10 and 14. These simulations were performed at a landing energy of 2.5eV and a resolution tuned to 5 nm. Curves 701 to 703 show the simulated variation of the sensitivity (referred to as "ash/Elli") of astigmatism (referred to as "ash") to ellipticity (referred to as "Elli") in the aperture array of the respective plate over the Landing Energy (LE) range. Curve 701 depicts the change in sensitivity of astigmatism to ellipticity for a plate corresponding to electrode 502 in fig. 14 (which may be referred to as the middle electrode in the control lens array). Curve 702 depicts the change in sensitivity of astigmatism to ellipticity for a plate corresponding to electrode 503 in fig. 14 (which may be referred to as the top electrode in the objective lens array). Curve 703 depicts the change in sensitivity of the astigmatism to ellipticity of the plate corresponding to electrode 504 in fig. 14 (which may be referred to as the bottom electrode in the objective lens array). Fig. 21 shows that the sensitivity is different for different plates at each landing energy and varies in a different manner (e.g., has a different slope) depending on the landing energy.

For a single plate, the variation in sensitivity in terms of landing energy means that even if the contribution of the macro collimator 270 to astigmatism remains constant (e.g., by keeping the ratio of linear reduction rate to angular reduction rate constant), the contribution of the plate to astigmatism will vary over the range of landing energies. In practice, however, it may be desirable to adjust the ratio of the linear reduction rate to the angular reduction rate to compensate for the field curvature change, so the astigmatism applied by the macrocollimator 270 will vary depending on the landing energy. By appropriate configuration of the plurality of plates (e.g., by providing each of the plurality of plates with a geometric characteristic that perturbs the astigmatism, such as the aperture ellipticity range described above with reference to fig. 12), the plurality of plates may be caused to impart a perturbation to the astigmatism that varies in a desired manner depending on the landing energy and thus may be configured to compensate for the astigmatic change imparted by the macrocollimator 270. Corrections from different plates may accumulate or cancel each other. That is, for some landing energy ranges, overcompensation may be applied by one plate and overcompensation by another plate. In a different configuration, the under-compensation may be applied by one plate and the re-compensation may be applied by another plate. A series of plates may be overcompensated and/or undercompensated, which may cancel and accumulate; the net effect may be an effect in which one of the plates applies a re-compensation. The increased freedom provided by the ability to control astigmatism correction using multiple plates allows near perfect correction of both astigmatism and field curvature over a large landing energy range.

Thus, embodiments of the present disclosure have been found to be effective where the parameter of the device being changed is landing energy and the target property to be compensated comprises astigmatism. The corresponding geometry of the aperture array allows for making the astigmatism substantially independent of landing energy in the landing energy range. At the same time, the controller 500 may control the electrical potential applied to the plate such that the field curvature disturbance applied by the plate substantially compensates for the field curvature disturbance applied by the macro-collimator 270 (an example of a changing electron optical device) upstream of the beam of the plate over the landing energy range. For example, the controller 500 may control the demagnification applied by the slab such that field curvature disturbances applied by the slab substantially compensate for field curvature disturbances applied by the macrocollimator 270. Thus, this approach allows simultaneous control of field curvature and astigmatism over the landing energy range.

The field curvature may be considered as a target property to be compensated by geometric properties in the plurality of plates. The respective geometric characteristics of the aperture arrays (e.g., such that each aperture array includes apertures having different aperture area ranges as described above with reference to fig. 11) also allow the field curvature to be substantially independent of landing energy over the landing energy range. At the same time, the controller 500 controls the demagnification applied by the plate such that the astigmatic disturbance applied by the plate substantially compensates for the astigmatic disturbance applied by the macro collimator 270 over the landing energy range.

The improved performance is illustrated in fig. 22. Fig. 22 is a graph showing changes in Defocus (referred to as "Defocus (ash)") in the landing energy range due to astigmatism for three different cases. Each case includes: an arrangement of fig. 14 is used in which different respective combinations of electrodes 502 to 504 are used to compensate for astigmatism changes. In this example, the astigmatism change is caused at least in part by astigmatism applied by a macro collimator 270 upstream of the beam of the plate (electrode), for example as shown in fig. 10 and 15. In each case, one geometry (e.g., the geometry that perturbs the astigmatism, such as the aperture ellipticity range described above with reference to fig. 12) or multiple geometries are selected so as to minimize the sum of squares of the astigmatism over the landing energy range. Curve 711 represents the case where the geometry in the aperture array of the plate corresponding to electrode 504 alone is used to compensate for the astigmatic change caused by macro collimator 270. Curve 712 represents such a case where the geometry of the aperture array in the two plates corresponding to electrodes 502 and 504, respectively, in fig. 14 is used to compensate for the astigmatism. Curve 713 represents the case where the geometry of the aperture array in the three plates corresponding to electrodes 502, 503 and 504, respectively, in fig. 14 is used to compensate for the astigmatism change. It can be seen that the case represented by curve 712 compensates for astigmatism much better than the case represented by curve 711 in the landing energy range. Thus, the additional degree of freedom provided by using geometric features in an aperture array of only two plates is significantly improved. Curve 713 demonstrates that providing another degree of freedom may further improve performance.

Fig. 23-27 are graphs relating to another example use case in which the geometric characteristics in the plurality of plates are tuned such that the astigmatism and field curvature are substantially independent of the ratio of linear reduction rate to angular reduction rate, M/Ma.

Fig. 23 and 24 show simulation results for determining a variation in sensitivity of a plate in an arrangement of the type depicted in fig. 10 and 14 as a function of M/Ma. FIG. 23 is a graph showing simulated variation of astigmatism (referred to as "Ast") versus ellipticity (referred to as "Elli") sensitivity (referred to as "Ast/Elli") in an aperture array of three different plates over the M/Ma range. FIG. 24 is a graph showing simulated variation of the sensitivity of Defocus (referred to as "refocus") versus diameter (referred to as "diam") due to field curvature (referred to as "refocus/diam") in an aperture array of three different plates in the M/Ma range. Curves 721 and 731 depict the change in the corresponding sensitivity of the plate corresponding to electrode 502 in fig. 14 (the middle electrode of the control lens array). Curves 722 and 732 depict the variation of the corresponding sensitivity of the plate corresponding to electrode 503 in fig. 14 (top electrode of the objective lens array). Curves 723 and 733 depict the variation of the corresponding sensitivity of the plate corresponding to electrode 504 in fig. 14 (bottom electrode of the objective lens array).

Such use cases include: the M/Ma is changed and therefore this degree of freedom cannot be used to correct the field curvature. This is why the astigmatism and field curvature have to be arranged independently of the M/Ma, allowing the astigmatism and field curvature correction to remain effective in the range of M/Ma.

Fig. 25-27 illustrate improved performance achieved by using geometric features to impart turbulence in multiple plates. Fig. 25 and 26 are graphs showing changes in Defocus (ash) due to astigmatism in the M/Ma range according to M/Ma for three different cases. Defocus in this context is the total defocus in the worst case direction (due to astigmatism and field curvature). There is a worst case direction because in some directions the defocus due to astigmatism will partially compensate for the defocus due to field curvature, while in other directions the defocus due to astigmatism and the defocus due to field curvature will add. The worst case direction is the direction in which the total defocus is greatest. Fig. 25 shows curves 741 to 743 of all three cases, while fig. 26 shows only curves 742 to 743 of the second case and the third case (so that the difference between these two curves can be seen more clearly). Each case includes: an arrangement of fig. 14 is used in which different respective combinations of electrodes 502 to 504 are used to compensate for the changes in astigmatism and field curvature. In each case, the geometry is chosen such that the sum of squares of the total defocus (due to astigmatism and field curvature) in the M/Ma range is minimized. Curve 741 (shown only in fig. 25) represents the case where the geometry of the apertures applied only to the aperture array in the plate corresponding to electrode 504 is used to compensate for astigmatism and field curvature. Curve 742 represents the case where the geometry of the aperture array in the two plates corresponding to electrodes 502 and 504, respectively, in fig. 14 is used to compensate for astigmatism and field curvature. Curve 743 represents the case where the geometry of the aperture array in the three plates corresponding to electrodes 502, 503 and 504, respectively, in fig. 14 is used to compensate for astigmatism and field curvature. It can be seen that the compensation for astigmatism in the M/Ma range is better in the case represented by curve 742 than in the case represented by curve 741, in a similar manner to that described above with reference to fig. 22. Thus, the additional degree of freedom provided by using geometric features in the aperture array of only two plates is improved. Curve 743 demonstrates that providing another degree of freedom can significantly improve performance. Since the total defocus variation in the case represented by curve 741 was modeled as about 4000nm, the overall improvement was large in absolute terms. In contrast, the defocus variation in the case represented by curve 711 in fig. 22 was simulated to be about 150nm. This is still significant, but much smaller.

Fig. 27 demonstrates how control of astigmatism and field curvature in the M/Ma range provides improved performance when changing beam current. Fig. 27 depicts a plot of the predicted change in beam current at sample 208 versus beamlet resolution for a landing energy of 2.5keV and different beam limiting aperture diameters in the upper beam limiter 252. Curves 741 to 744 correspond to beam limiting aperture diameters of 6 microns, 8 microns, 10 microns and 12 microns, respectively, and have fixed hard-coded corrections in the plates corresponding to at least the control electrodes 501, 502, 503 and the objective electrodes 503, 504, so all plates in the arrangement are shown and described with reference to at least fig. 14 and 20. The curve 521 shown in fig. 27 corresponds to the curve 521 described above with reference to fig. 17-19 (where different points on the curve correspond to different optimization hard-coded corrections). Thus, the curve 521 shown in fig. 27 shows a completely optimized case in which the hard-coded correction is allowed to be re-optimized. The different curves 741 to 744 all represent the following: wherein the geometry in the aperture array of the plurality of plates is used to compensate for astigmatism and field curvature when M/Ma is changed to adjust beam current without allowing any re-optimization of the hard-coded correction. Curve 752 represents an example case of a different curve 524 to 527 in form than discussed above with reference to fig. 19 without the approach of the present invention applied. Curve 752 represents the case where the beam limiting aperture diameter is 8 microns; curve 752 is directly comparable to curve 742. In all cases, M/Ma is varied within a range to achieve the observed beam current variation. At each point, the resolution is optimized using the available degrees of freedom. The increased number of degrees of freedom provided by embodiments of the present invention is believed to allow curves 741-744 to be much steeper than they would otherwise be (as illustrated by curve 752). In fact, curves 741 to 744 are very similar to best case 521, in which best case 521 the hard-coded correction is allowed to be re-optimized for different values of beam current in the simulation, but in the case of curves 741 to 744 performance is achieved without any re-optimization (i.e. change) of the hard-coded correction. Thus, curves 741 to 744 represent performance that may be achieved for fixed hard-coded corrections (i.e., without any exchange of physical components such as boards).

Thus, embodiments of the invention have been found to be effective in cases where the parameters of the device (within the scope of applying the compensation) include the beam current of the charged particles and the target properties include one or both of astigmatism and field curvature. In such a case, the corresponding geometrical characteristics of the aperture array make it possible to make the astigmatism and field curvature substantially independent of the ratio of the total angular reduction to the total linear reduction in the range of beam currents for which compensation of the target characteristics is applied. The beam current/resolution operating point is changed by changing the demagnification. Thus, arranging the astigmatism and field curvature to be substantially independent of the ratio of the total linear reduction rate to the total angular reduction rate means: the beam current/resolution operating point may be changed (e.g., to follow curve 743 in fig. 27) with minimal negative impact on performance. It should also be noted that in the graphs of fig. 23-26, the horizontal axis may alternatively be M instead of M/Ma because there is a 1:1 correspondence between M and M/Ma. It should also be noted that there is also a 1:1 correspondence between the linear reduction rate and the beam current for a fixed aperture size.

In some embodiments, macro-collimator 270 (e.g., as shown in fig. 10 and 15) is an example of a variable electro-optic device that may be disposed upstream of the beams of the plurality of plates. The varying electron optical device is configured to apply electron optical disturbance to charged particles oriented towards the sample. The changing electro-optic device may comprise any macro-electro-optic device. Thus, the variable electron optical device can operate on charged particles corresponding to a plurality of beamlets as a group. In some embodiments, the variable electron optical device includes a collimator configured to collimate charged particles corresponding to the beamlets. The variable electron optical device may or may not act directly on the charged particles in the beamlets. The variable electron optical device may act on the charged particles before or after the formation of the beamlets. In some embodiments, the variable electro-optical device includes a converging lens. In some embodiments, the variable electron optical device comprises a macro-collimator configured to apply macro-collimation to charged particles (either before or after they are formed into beamlets). The disturbance from the varying electron optical device affects at least the target properties of the beamlets. In some embodiments, the disturbance applied by the varying electron optical device is a change in a target property of the sub-beam that is compensated for by the disturbance from a geometric property (e.g., astigmatism and/or field curvature, as discussed above) in the plurality of plates.

In embodiments where such a varying electro-optical device is present, the controller 500 may be configured to control the varying electro-optical device such that the applied electro-optical disturbance and the corresponding disturbance applied by the aperture array substantially collectively compensate for the variation of the target property over a range of parameters of the device. The controller 500 may control the changing electron optical device by applying and controlling one or more electrical potentials applied to the changing electron optical device (i.e., the changing electron optical device may include an electrostatic element). The controller 500 may control the changing electro-optic device by applying and controlling one or more currents applied to the changing electro-optic device (i.e., the changing electro-optic device may include an electromagnetic element). Thus, the variable electro-optical device may operate electrostatically and/or magnetically. As demonstrated in fig. 22, contributions to aberrations (e.g., astigmatism) applied, for example, by changing the electron optical device (e.g., macro collimator 270) and by an aperture array having geometric characteristics, may substantially cancel each other out over the range of parameters (e.g., landing energy) of the electron optical device.

In some embodiments, the electro-optical device 41 further includes a beam limiting aperture array. The beam limiting aperture array may be disposed upstream of the beam of the variable electron optical apparatus or between the variable electron optical apparatus and the plate. The beam limiting aperture array may be configured to generate beamlets from the source beam. The beam limiting aperture array may be the most upstream plate of the beam that may be used as an electrode. The upper beam limiter 252 described above with reference to fig. 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 for the upper beam limiter 252. The source beam includes charged particles and the variable electron optical device applies electron optical disturbance to the charged particles. 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 fig. 2 and 3.

Compensation for the target property is achieved within the parameters of the device. The parameters include one or more of the following: landing energy of the charged particles, beam current of the charged particles, spacing between the sample and detector of the electron optical device, reduction rate (e.g., ratio of linear reduction rate to angular reduction rate), resolution. Examples of the case where the parameters are the landing energy (see discussion with reference to fig. 21 and 22) and the reduction rate (see discussion with reference to fig. 23 to 27) are discussed above.

In some embodiments, different aperture arrays are configured by geometric characteristics to apply perturbations that vary differently depending on parameters within a range, at least for perturbations corresponding to a target property. Arranging the aperture array to contribute differently over a range of parameters helps to effectively compensate for variations in target characteristics over that range. Fig. 21, 23 and 24 demonstrate how this function is achieved by showing how the sensitivity of the perturbations to the corresponding geometrical characteristics may be different for different plates (due to plate position) and vary in different ways depending on the parameters of the device.

The disturbances imposed by the different plates work together to compensate for the change in the target property. The contributions from the different plates may contribute or cancel each other out in a cumulative manner, as long as the overall effect is to compensate for the change in the target property. Thus, the controller 500 may control the applied potentials such that the perturbations applied by at least two of the aperture arrays cancel each other out over at least a portion of the range of parameters, at least for perturbations corresponding to the target property. Alternatively or additionally, the controller 500 may control the applied potentials such that the perturbations applied by at least two of the aperture arrays contribute in an accumulated manner at least for a perturbation corresponding to the target property over at least a portion of the range of parameters.

In some embodiments, the controller is configured to apply and control the electrical potential applied to the plate to maintain a substantially constant spatial relationship between the image plane of the device and the rigid mount 730 over a range of parameters of the device. For example, the constant spatial relationship may include a constant spacing between the image plane and the rigid mount 730. The mount 730 holds at least a portion of one or more plates in a fixed position in the frame of reference of the electro-optical device. Other portions of one or more plates may deform during use due to an electric field generated in an area adjacent to the plates, such as a bow as described herein with reference to fig. 16. Such deformation does not affect the spatial relationship between the image plane and the mount 730. In some embodiments, maintaining the spatial relationship between the image plane and mount 730 substantially constant is performed within the parameters of the device while additionally controlling the changing electro-optical device (e.g., macro-collimator 270).

Embodiments of the present disclosure may be provided as methods, including any of the methods using the apparatus described above. The method includes a method of compensating for a change in a property of a sub-beam of the plurality of charged particles projected onto the sample 208. The method may include: a plurality of plates are used to project beamlets towards the sample 208. The plates define respective arrays of apertures. These plates include an array of objective lenses to project beamlets towards the sample 208. The aperture arrays defined in at least two of the plates each have a geometric characteristic configured to impart a perturbation to a corresponding target property of the beamlets. The electrical potential applied to the plate having the geometric characteristic may be controlled such that the applied disturbances substantially collectively compensate for the change in the target property over the parameters of the device. A method of compensating for a change in a property of a sub-beam of a plurality of beams of charged particles projected onto a sample 208 in an electron optical apparatus may be provided. The electro-optical device may comprise a plurality of plates in which respective aperture arrays are defined. The plurality of plates includes an objective lens array. The array of apertures defined in at least two of the plates has geometric characteristics. The method comprises the following steps: the beamlets are projected towards the sample 208 by operating the beamlets using a plate having an array of apertures with geometrical properties. The operations include: a perturbation is applied to the target property of the sub-beam using the corresponding plate. An electrical potential is applied to the aperture plate and the electrical potential is controlled such that the respective perturbations substantially collectively compensate for variations in the target properties over a range of parameters of the apparatus.

References to upper and lower, above and below, etc. should be understood to refer to directions parallel to (typically but not always perpendicular to) the beam upstream direction and the beam downstream direction of the electron beam or beams impinging on the sample 208. Thus, references to upstream and downstream of the beam are intended to refer to directions relative to the beam path independent of any gravitational field present.

The embodiments described herein may take the form of a series of aperture arrays or electro-optical elements arranged in an array along a beam or paths. Such electron optical elements may be electrostatic. In one embodiment, all electron optical elements (e.g., the last electron optical element in the beamlet path from the beam limiting aperture array to the sample front) may be electrostatic, and/or may be in the form of an aperture array or a plate array. In some arrangements, one or more electro-optical elements are fabricated as microelectromechanical systems (MEMS) (i.e., using MEMS fabrication techniques). The electro-optical element may have a magnetic element and an electrostatic element. For example, the composite array lens may feature a macro-magnetic lens that encompasses multiple beam paths, with upper and lower plates disposed within the magnetic lens and along the multiple beam paths. In the plate may be an array of apertures for the beam paths of the plurality of beams. Electrodes may be present above, below or between the plates to control and optimize the electromagnetic field of the compound lens array.

Where electrodes or other elements are provided that may be set to different potentials relative to each other, it will be appreciated that such electrodes/elements are electrically isolated from each other. An electrically insulating connector may be provided if the electrodes/elements are mechanically connected to each other. For example, in case the electrodes/elements are provided as a series of conductive plates each defining an array of apertures, for example forming an array of objective lenses or an array of control lenses, an electrically insulating plate may be provided between the conductive plates. The insulating plate may be connected to the conductive plate so as to serve as an insulating connector. The conductive plates may be separated from each other along the beamlet path by an insulating plate. In the insulating plate, an aperture may be defined around the path of the multiple bundles of beamlets (e.g., around all of the beamlets).

An assessment tool or an assessment system according to the present disclosure may include means for qualitatively assessing a sample (e.g., pass/fail), means for quantitatively measuring a sample (e.g., size of a feature), or means for generating a mapped image of a sample. For example, when used for evaluation, the evaluation tool may be any one of the following: charged particle optical apparatus, e.g. as part of the charged particle beam device 100; or more specifically, a charged particle optical apparatus 40 (which may be a charged particle optical column); and/or 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 part of an evaluation tool, such as an inspection tool or a metrology inspection tool, or an electron beam lithography tool. Any reference herein to a tool is intended to encompass an apparatus, device or system comprising a plurality of components, which may or may not be co-located, and may even be located in a separate location, particularly for example for a data processing element.

References to a system of components or elements that can be controlled to manipulate the component or charged particle beam in some way include: the controller or control system or control unit is configured to control the components to manipulate the charged particle beam in the manner described, and optionally other controllers or devices (e.g., voltage supplies) are used to control the components to manipulate the charged particle beam in this manner. For example, under the control of a controller or control system or control unit, a voltage supply may be electrically connected to one or more components to apply an electrical potential to the components, such as to electrodes controlling lens array 250 and objective lens array 241. One or more controllers, control systems, or control units for controlling actuation of a component may be used to control an actuatable component (such as a stage) to actuate and thus move relative to another component (such as a beam path).

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

The terms "beamlet" and "beam wave" are used interchangeably herein and are each understood to encompass any radiation beam derived from a parent radiation beam by dividing or splitting the parent radiation beam. The term "manipulator" is used to encompass any element affecting the path of the beamlets or beams, such as lenses or deflectors. References to elements aligned along a beam path or a beamlet path should be understood to mean that the corresponding element is positioned along the beam path or beamlet path. References to optical devices should be understood to mean electron optical devices.

References to elements aligned along a beam path or a beamlet path should be understood to mean that the corresponding element is positioned along the beam path or beamlet path.

References to a component or system of components or elements are controllable to manipulate or operate the charged particle beam in a manner including: the controller or control system or control unit is configured to control the components so as to manipulate the charged particle beam in the manner described, and optionally other controllers or devices (e.g. voltage supply and/or current supply) are used to control the components so as to manipulate the charged particle beam in this manner. For example, under the control of a controller or control system or control unit, a voltage supply may be electrically connected to one or more components to apply an electrical potential to components such as those in a non-limiting list comprising control lens array 250, objective lens array 234, converging lens 231, corrector, and scanning deflector array 260. One or more controllers, control systems, or control units for controlling actuation of a component may be used to control an actuatable component (such as a stage) to actuate and thus move relative to another component (such as a beam path).

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 a charged particle beam towards the sample 208. In one embodiment, the controller 50 controls at least one charged particle optical element (e.g., a plurality of deflectors or an array of scanning deflectors 260) to operate on a charged particle beam in a charged particle beam path. Additionally or alternatively, in one embodiment, the controller 50 controls at least one charged particle optical element (e.g., detector 240) to operate on a 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.

Embodiments of the present invention provide the following clauses.

Clause 1. An electron optical apparatus for projecting a plurality of beams of charged particles towards a sample or configured to project a plurality of beams of beamlets of charged particles onto a sample, the apparatus comprising a plurality of plates in which respective aperture arrays are defined, wherein the plurality of plates comprise an objective lens array configured to project the plurality of beamlets towards the sample, and the aperture arrays defined in at least two of the plates each have a geometric characteristic configured to impart a perturbation to a corresponding target property of the beamlets; and a controller configured to apply and control an electrical potential applied to the plate having the geometric characteristic such that the applied disturbances substantially collectively compensate for variations in the target property over a range of parameters of the apparatus.

Clause 2 the device of clause 1, wherein the aperture arrays in the at least two of the plates each have one or more other geometric characteristics configured to impart a perturbation to a corresponding other target property of the beamlets.

Clause 3 the device of clause 1 or 2, wherein the different aperture arrays are configured by the geometric characteristics to: at least for disturbances corresponding to the target property, disturbances are applied that vary differently depending on the parameters within the range.

Clause 4 the apparatus of any preceding clause, wherein the controller is configured to control the applied electrical potentials such that perturbations applied by at least two of the aperture arrays cancel each other out over at least a portion of the range of the parameter, at least for perturbations corresponding to the target property.

Clause 5, the apparatus of any preceding clause, wherein the controller is configured to control the applied electrical potentials such that perturbations applied by at least two of the aperture arrays contribute in an accumulated manner at least for perturbations corresponding to the target property over at least a portion of the range of the parameter.

Clause 6 the device of any of the preceding clauses, wherein the array of apertures defined in three of the plates have the geometric characteristics, and the controller is configured to apply and control the electrical potentials applied to all three of the plates such that the applied respective perturbations substantially collectively compensate for the variation of the target property over the range of the parameter of the device.

Clause 7, the device of any of the preceding clauses, further comprising a varying electron optical device upstream of the beam of the plurality of plates, the varying electron optical device configured to apply electron optical perturbation to charged particles directed toward the sample, the perturbation being such that at least the target property of the beamlets is affected.

The apparatus of clause 8, wherein the controller is configured to control the varying electro-optical apparatus such that the applied electro-optical disturbance and the corresponding disturbance applied by the aperture array substantially collectively compensate for the variation of the target property over the range of the parameter of the apparatus.

Clause 9 the device of clause 7 or 8, wherein the applied electro-optical disturbance forms at least a portion of the change in the target property to be compensated.

Clause 10 the apparatus of any of clauses 7 to 9, comprising a beam limiting aperture array: upstream of the beam of the varying electron optical device, wherein the beam limiting aperture array is configured to generate the beamlets; or between the variable electron optical device and the plate, the beam limiting aperture array being configured to generate the beamlets from a source beam.

Clause 11 the device of any of clauses 7 to 10, wherein a source beam comprises the charged particles, the varying electron optical device applying the electron optical disturbance to the charged particles, the device preferably comprising a source for emitting the source beam.

The apparatus of any one of clauses 7 to 11, wherein the changing electron optical apparatus is a macroelectron optical apparatus and/or is configured to operate on charged particles corresponding to a plurality of beamlets as a group.

Clause 13 the device of any of clauses 7 to 12, wherein the varying electron optical device is a collimator configured to collimate charged particles corresponding to the beamlets, preferably acting on charged particles in the beamlets; and/or the variable electro-optical device is a converging lens.

The apparatus of clause 14, wherein the collimator comprises a macro-collimator configured to apply macrocollimation.

The apparatus of any preceding clause, wherein the parameters include one or more of: landing energy of charged particles, beam current of charged particles, separation between the sample and detector of the electron optical device, magnification, resolution.

The apparatus of any preceding clause, wherein the target attribute comprises one or more of: astigmatism, field curvature, distortion, coma.

Clause 17, the apparatus of any of the preceding clauses, wherein the target property comprises astigmatism and/or the geometric characteristic configured to apply the corresponding perturbation comprises different aperture ellipticity ranges in the aperture array.

Clause 18 the apparatus of any of the preceding clauses, wherein the target property comprises field curvature and/or the geometric characteristic configured to apply the corresponding perturbation comprises a range of different aperture sizes (such as areas) in the aperture array.

Clause 19 the apparatus of any of the preceding clauses, wherein the target property comprises distortion and/or the geometric characteristic configured to apply the corresponding perturbation comprises a range of different aperture positions in the aperture array relative to a respective nominal position (preferably on a regular grid).

Clause 20 the apparatus of any of the preceding clauses, wherein the target property comprises coma and/or the geometric characteristic configured to apply the corresponding perturbation comprises a range of different aperture positions in the aperture array relative to a respective nominal position (preferably on a regular grid).

Clause 21, the device of any of the preceding clauses, wherein the controller is configured to apply and control the electrical 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 the range of the parameter of the device, preferably the controller is configured to maintain the substantially constant spatial relationship while additionally controlling the changing electro-optical device over the range of the parameter of the device.

Clause 22 the device of any of the preceding clauses, wherein the parameter comprises landing energy of the charged particles and the target property comprises astigmatism.

Clause 23 the device of clause 22, wherein the controller is configured to control the electrical potential applied to the plate such that the field curvature disturbance applied by the plate substantially compensates for the field curvature disturbance applied by the varying electron optical device upstream of the beam of the plate within the range of the parameter.

Clause 24 the device of clause 23, wherein the controller is configured to control the demagnification applied by the plate such that the field curvature disturbance applied by the plate substantially compensates for the field curvature disturbance applied by the varying electron optical device upstream of the beam of the plate within the range of the parameter.

Clause 25 the device of any of the preceding clauses, wherein the parameter comprises landing energy of the charged particles and the target property comprises field curvature.

Clause 26, the device of clause 25, wherein the controller is configured to control the demagnification applied by the plate such that the astigmatic disturbance applied by the plate substantially compensates for the astigmatic disturbance applied by the varying electron optical device upstream of the beam of the plate within the range of the parameter.

Clause 27, the device of clauses 23, 24, or 26, wherein the varying electron optical device comprises a macro-collimator configured to apply macro-collimation.

The apparatus of any one of clauses 22 to 27, wherein the respective geometric characteristics of the aperture array are such that astigmatism and/or field curvature are substantially independent of landing energy over the landing energy range.

Clause 29, the apparatus of any preceding clause, wherein the parameter comprises a beam current of the charged particles, and the target property comprises one or both of astigmatism and field curvature.

Clause 30 the apparatus of clause 29, wherein the respective geometric characteristics of the aperture array are such that astigmatism and field curvature are substantially independent of a ratio of a total linear reduction rate to a total angular reduction rate over the range of beam currents.

Clause 31, the device of clause 29 or 30, wherein the controller is configured to control the electrical potential applied to the plate such that the field curvature disturbance and/or the astigmatism disturbance applied by the plate substantially compensates for the field curvature disturbance and/or the astigmatism disturbance applied by the varying electro-optical device upstream of the beam of the plate within the range of the parameter.

The apparatus of any preceding clause, wherein the change in the target property is an aberration.

Clause 33, an electron optical apparatus for projecting a plurality of beams of charged particles toward a sample, the apparatus comprising: a plurality of plates in which an aperture array is defined, the plurality of plates comprising an objective lens array configured to project a sub-beam of the plurality towards the sample, at least two of the plurality of plates having geometric properties applied to the respective aperture array, the geometric properties being configured or so as to impart a perturbation to a property (or target property) of the beam; and a controller configured to apply and control the electrical potentials applied to the at least two of the plates having the geometric characteristic such that the disturbances applied to the beam by the plates substantially jointly compensate for the variation of the property over a range of parameters of the apparatus.

Item 34. An electron optical apparatus for projecting a plurality of beams of charged particles toward a sample, the apparatus comprising: a plurality of plates, an aperture array being defined in at least two plates of the plurality of plates, the plurality of plates comprising an objective lens array configured to project the beam toward the sample, the aperture array in the at least two plates having a geometry configured to impart a perturbation to a target property of the beam; and a controller configured to apply and control the electrical potentials applied to the plates having the geometric characteristics such that the disturbances applied to the beams substantially collectively compensate for variations in the target property over a range of parameters of the apparatus.

Clause 35, the electron-optical device of clause 33 or 34, further comprising a changing macroelectron-optical device above the beam of the plurality of plates, the changing macroelectron-optical device configured to apply electron-optical perturbation to charged particles directed toward the sample, the electron-optical perturbation affecting at least the target property of the beam.

Clause 36 the electro-optic device of clause 33 or 34, further comprising a changing macro-electro-optic device over the beam of the plurality of plates, the changing macro-electro-optic device being configured to apply electro-optical disturbances to charged particles directed toward the sample, the disturbances being desirably such that at least the target property of the beam is affected.

Clause 37 the electro-optical device of clause 35 or 36, wherein the controller is configured to: the varying electron optical device and the aperture array are controlled such that the perturbations substantially collectively compensate for the variations in the target properties over the range of the parameters of the device.

Clause 38 the electro-optical device of clause 35 or 36, wherein the controller is configured to: the varying electro-optical device is controlled such that the applied electro-optical disturbance and the corresponding disturbance applied by the aperture array substantially jointly compensate for the variation of the target property over the range of the parameter of the device.

Clause 39 the electro-optical device of clause 33 or 38, wherein the controller is configured to: the applied potentials are controlled such that, within at least a portion of the range of the parameter, perturbations applied by at least two of the aperture arrays ideally cancel each other out and/or contribute in a cumulative manner at least with respect to perturbations corresponding to the target property.

Item 40. The electro-optical device of any one of items 33 to 39, wherein the parameters include one or more of: landing energy of charged particles, beam current of charged particles, separation between the sample and detector of the electron optical device, magnification, resolution.

Item 41. The electro-optical device of any one of items 33 to 40, wherein the target attribute comprises one or more of: astigmatism, field curvature, distortion, coma.

Clause 42 the device of any of clauses 1 to 32 or the electro-optical device of any of clauses 33 to 41, wherein the geometric property is a correction of hard coding of the aperture array of the two or more plates, and comprising one or more of the following: ellipticity, diameter, and displacement from a regular grid.

Clause 43 the device according to clause 42 or the electro-optical device according to clause 42, wherein the relationship between correction features applied to different apertures in the aperture array depends on the position of the aperture in the respective aperture array.

Clause 44 a charged particle device comprising the apparatus of any of clauses 1 to 32, 42 or 43 or the electron optical apparatus of any of clauses 33 to 43.

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

Clause 46. An evaluation system comprising the apparatus of any of clauses 1 to 32, 42 or 43 or the electro-optical apparatus of any of clauses 33 or 43 or the charged particle apparatus of any of clauses 44 or 45.

Clause 47. A method of compensating for a change in a property of a sub-beam of charged particles projected into a plurality of beams of a sample, the method comprising: projecting the beamlets towards the sample using a plurality of plates defining respective aperture arrays and including an objective lens array projecting the beamlets towards the sample, wherein the aperture arrays defined in at least two of the plates each have a geometric characteristic configured to impart a perturbation to a corresponding target property of the beamlets; and controlling the electrical potential applied to the plate having the geometric characteristic such that the applied perturbations substantially collectively compensate for variations in the target property over a range of parameters of the apparatus.

Clause 48. A method of compensating for a change in a property of a sub-beam of charged particles in a plurality of beams of a sample projected into an electron optical apparatus, the electron optical apparatus comprising a plurality of plates in which respective aperture arrays are defined, the plurality of plates comprising an objective lens array, wherein the aperture arrays defined in at least two of the plates have geometric characteristics, the method comprising: projecting a beamlet towards a sample by operating the beamlet with a plate having an array of apertures with the geometrical properties, the operations comprising: applying a perturbation to a target property of the beamlets using a respective plate; and applying an electrical potential to the aperture plate and controlling the electrical potential such that the respective perturbations substantially collectively compensate for variations in the target property over a range of parameters of the apparatus.

Claims

1. An electron optical apparatus for projecting a plurality of beams of charged particles onto a sample, the apparatus comprising:

A plurality of plates in which respective aperture arrays are defined, wherein the plurality of plates comprises an objective array configured to project a sub-beam of the plurality of beams toward the sample, and the aperture arrays defined in at least two of the plates each have a geometric characteristic configured to impart a perturbation to a corresponding target property of the sub-beam; and

A controller configured to apply and control an electrical potential applied to the plate having the geometric characteristic such that the applied disturbances substantially collectively compensate for variations in the target property over a range of parameters of the device.

2. The apparatus of claim 1, wherein the aperture arrays defined in the at least two of the plates each have one or more other geometric characteristics, each other geometric characteristic configured to impart a perturbation to a corresponding other target property of the beamlets.

3. The apparatus of claim 1 or 2, wherein the different aperture arrays are configured by the geometric characteristics to: at least for disturbances corresponding to the target property, disturbances are applied that vary differently depending on the parameters within the range.

4. The apparatus of any preceding claim, wherein the controller is configured to control the applied electrical potentials such that perturbations applied by at least two of the aperture arrays cancel each other out for at least a portion of the range of the parameter for at least a perturbation corresponding to the target property.

5. The apparatus of any preceding claim, wherein the controller is configured to control the applied electrical potentials such that perturbations applied by at least two of the aperture arrays contribute in an accumulated manner over at least a portion of the range of the parameter for at least perturbations corresponding to the target property.

6. The apparatus of any preceding claim, wherein the array of apertures defined in three of the plates has the geometric characteristic, and the controller is configured to apply and control the electrical potentials applied to all three of the plates such that the respective perturbations applied substantially jointly compensate for the variation of the target property over the range of the parameter of the apparatus.

7. The apparatus of any preceding claim, further comprising a varying electron optical device located upstream of the beams of the plurality of plates, the varying electron optical device configured to apply electron optical perturbation to charged particles directed towards the sample, the perturbation being such as to affect at least the target property of the beamlets.

8. The device of claim 7, wherein the controller is configured to control the varying electro-optical device such that the applied electro-optical disturbance and a corresponding disturbance applied by the aperture array substantially collectively compensate for the variation of the target property over the range of the parameter of the device.

9. The apparatus of claim 7 or 8, wherein the electro-optical disturbance applied forms at least a portion of the change in the target property to be compensated.

10. The device of any of claims 7 to 9, comprising a beam limiting aperture array upstream of a beam of the varying electron optical device, wherein the beam limiting aperture array is configured to generate the beamlets.

11. The device of any one of claims 7 to 10, wherein the varying electron optical device is a macroelectron optical device and/or is configured to operate on charged particles corresponding to a plurality of beamlets as a group.

12. The device of any one of claims 7 to 11, wherein the varying electron optical device is a collimator configured to collimate charged particles corresponding to the beamlets, preferably acting on charged particles in the beamlets; and/or the variable electro-optical device is a converging lens.

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

14. The apparatus of any preceding claim, wherein the geometric characteristic is a hard-coded correction of the aperture array of two or more of the plates, and comprises one or more of: ellipticity, diameter, and displacement from a regular grid.

15. The apparatus of claim 14, wherein the variation between correction features applied to different apertures in the array of apertures depends on the position of the apertures in the respective array of apertures.