Alignment of electro-optical elements

Abstract

A stack of planar elements for a charged particle optical module configured to project charged particles along a beam path, the stack comprising an adjacent pair of planar elements arranged across the beam path, where one of the planar elements comprises an alignment reference, and the alignment reference is configured to project the charged particles along the beam path. And another one of the planar elements comprises a monitoring aperture; wherein the pair of planar elements are positioned relative to each other such that the alignment reference and the monitoring aperture are aligned with each other in a direction substantially perpendicular to the planes of the planar elements.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 22200582.9 filed on October 10, 2022, the entire contents of which are incorporated herein by reference. Technical Field

[0003] Embodiments provided herein generally relate to a method for aligning a charged particle optical element, a method of making a charged particle optical module, a charged particle optical element stack, a charged particle optical module, a charged particle optical device, a charged particle optical apparatus, and an alignment apparatus. Background Art

[0004] When manufacturing semiconductor integrated circuit (IC) chips, during the manufacturing process, undesirable pattern defects may occur on the substrate (e.g., wafer) or mask, thereby reducing the yield. Defects may occur due to, for example, optical effects and deposition of accompanying particles or other processing steps (such as etching, chemical mechanical polishing). Therefore, monitoring the degree of undesirable pattern defects is an important process when manufacturing IC chips. More generally, the evaluation (such as inspection and / or measurement) of the surface of a substrate or other object / material is an important process during and / or after its manufacture.

[0005] Pattern evaluation tools with charged particle beams have been used to evaluate objects, for example, to detect pattern defects. These tools typically use electron microscopy techniques, such as scanning electron microscopes (SEMs). In an SEM, a primary electron beam of relatively high energy electrons is targeted at a final deceleration step so as to land on the target with a relatively low landing energy. The electron beam is focused into a detection spot on the target. The interaction between the material structure at the detection spot and the landing electrons from the electron beam causes electrons, such as secondary electrons, backscattered electrons, or Auger electrons, to be emitted from the surface, which can be referred to as signal electrons or more generally as signal particles. The generated secondary electrons can be emitted from the material structure of the target.

[0006] By scanning the primary electron beam as a detection spot over the target surface, secondary electrons can be emitted across the surface of the target. By collecting these secondary electrons emitted from the target surface, the evaluation tool (device) can obtain image-like signals that represent the characteristics of the material structure of the surface of the target. In this evaluation, the collected secondary electrons are detected by a detector in the device. The detector generates a signal in response to the accompanying particles. When evaluating an area of ​​the sample, the signal includes being processed to generate the following data, which corresponds to an evaluation image of the evaluated area of ​​the sample. The image may include pixels. Each pixel may correspond to a portion of the evaluated area. Typically, an electron beam evaluation device has a single beam and may be referred to as a single-beam SEM. Attempts have been made to introduce multi-electron beam evaluation in a device (or 'multi-beam tool'), which may be referred to as a multi-beam SEM (MBSEM).

[0007] Another application of an electron optical device (or device or column) is photolithography. A beam of charged particles reacts with a resist layer on the surface of a substrate. By controlling the position on the resist layer towards which the charged particle beam is directed, a desired pattern in the resist can be produced.

[0008] Electron-optical devices may be apparatus for generating, irradiating, projecting and / or detecting one or more beams of charged particles. The paths of charged particle beams are controlled by electromagnetic fields (i.e., electrostatic and magnetic fields). Stray electromagnetic fields may undesirably steer the beams.

[0009] In some electron-optical devices, there may be multiple electron-optical elements stacked relative to each other. For example, in some electron-optical devices, typically, an electrostatic field is generated between two electrodes corresponding to two electron-optical elements. There is a need for accurate alignment between electron-optical elements within a stack. Summary of the invention

[0010] The present invention provides a suitable system architecture to enable verification of the alignment of charged particle optical elements. According to a first aspect of the present invention, there is provided a stack of planar elements for a charged particle optical module, the charged particle optical module being configured to project charged particles along a beam path, the stack comprising an adjacent pair of planar elements, the adjacent pair of planar elements being arranged to straddle the beam path, wherein one of the planar elements comprises an alignment reference and the other of the planar elements comprises a monitoring aperture; wherein the pair of planar elements are positioned relative to each other so that the alignment reference and the monitoring aperture are aligned with each other in a direction substantially perpendicular to the planes of the planar elements.

[0011] According to a second aspect of the present invention, a method for aligning a planar element for a charged particle optical module is provided, the charged particle optical module being configured to project charged particles along a beam path, the method comprising: providing a first planar element, the first planar element comprising a first alignment reference piece; providing a second planar element, the second planar element comprising a first monitoring aperture stacked relative to the first planar element; interrogating the first alignment reference piece using interrogation light passing through the first monitoring aperture; detecting the interrogation light reflected from the first planar element; and aligning the second planar element relative to the first planar element based on the detected interrogation light.

[0012] Advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings, in which certain embodiments of the invention are set forth by way of illustration and example. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The above and other aspects of the present disclosure will become more apparent from the description of exemplary embodiments with reference to the accompanying drawings.

[0014] Figure 1 is a schematic diagram illustrating an exemplary electron beam evaluation apparatus.

[0015] Figure 2 It is illustrated as Figure 1 Schematic diagram of an exemplary multi-beam electron optical apparatus of portions of an exemplary electron beam evaluation apparatus.

[0016] Figure 3 is a schematic diagram of an exemplary electro-optical device including a Figure 1 Part of an exemplary electron beam evaluation apparatus and includes an array of collimator elements and an array of scanning deflectors.

[0017] Figure 4 Yes Figure 3 Schematic diagram of an exemplary electron-optical device array of electron-optical devices.

[0018] Figure 5 As Figure 1 Schematic diagram of an alternative exemplary electron optical device that is a portion of an exemplary electron beam evaluation apparatus.

[0019] Figure 6 As Figure 3 , Figure 4 and Figure 5 Schematic diagram of an exemplary electron-optical assembly of a portion of an electron-optical device.

[0020] Figure 7 As Figure 3 , Figure 4 and Figure 5Schematic diagram of an exemplary electron-optical assembly of a portion of an electron-optical device.

[0021] Figure 8 As Figure 3 , Figure 4 and Figure 5 Schematic diagram of an exemplary electron-optical assembly of a portion of an electron-optical device.

[0022] Fig. 9 As Figure 3 , Figure 4 and Figure 5 Schematic diagram of an exemplary electron-optical assembly of a portion of an electron-optical device.

[0023] Fig.10 is a schematic view of an alignment reference.

[0024] Fig.11 is a schematic view of the alignment datums on the opposite side of the beam area.

[0025] Reference is now made in detail to the exemplary embodiments, examples of which are illustrated in the accompanying drawings. The following description refers to the accompanying drawings, wherein the same reference numerals in different drawings represent the same or similar elements unless otherwise indicated. The implementations set forth in the following description of the exemplary embodiments do not represent all implementations consistent with the present invention. Instead, these implementations are merely examples of apparatus and methods consistent with various aspects of the present invention as described in the appended claims. DETAILED DESCRIPTION

[0026] By significantly increasing the packaging density of circuit components (such as transistors, capacitors, diodes, etc.) on IC chips, the physical size of the device can be reduced and the computing power of electronic devices can be enhanced. This has been achieved by increasing the resolution, thereby enabling the production of even smaller structures. Semiconductor IC manufacturing is a complex and time-consuming process with hundreds of individual steps. An error in any step in the IC chip manufacturing process is likely to have an adverse effect on the function of the final product. Only one defect will cause equipment failure. It is expected to increase the overall yield of the process. For example, for a 50-step process (wherein the step can indicate the number of layers formed on the wafer), in order to obtain a 75% yield, the yield of each individual step must be greater than 99.4%. If the yield of the individual step is 95%, the total process yield will be as low as 7% to 8%.

[0027] Maintaining high substrate (i.e., wafer) yield (defined as the number of substrates processed per hour) is also desirable. The presence of defects may affect high process yield and high substrate throughput. This is especially true if operator intervention is required for detecting defects. In order to maintain high yield and low cost of IC chips, high throughput detection and identification of micron-scale defects and nano-scale defects by evaluation systems such as scanning electron microscopes ('SEM') is desired.

[0028] The scanning electron microscope comprises a scanning device and a detector device. The scanning device comprises an irradiation device, which comprises an electron source for generating primary electrons; and a projection device, which is used to scan a target such as a substrate using one or more focused beams of primary electrons. The primary electrons interact with the target and generate interaction products, such as signal particles, for example, secondary electrons and / or backscattered electrons. Secondary electrons can be considered to have an energy of up to 50eV. Although backscattered electrons have an energy spectrum from substantially zero to the maximum energy of a charged particle device, they are generally set to electrons (or signal electrons) with an energy exceeding 50eV. When scanning the target, the detection device captures signal particles (e.g., secondary electrons and / or backscattered electrons) from the target so that the scanning electron microscope can create an image of the scanned area of ​​the target. The design of the evaluation device that embodies these scanning electron microscope characteristics can have a single beam. For higher production volumes such as for evaluation, some designs of the device use multiple focused beams of primary electrons, i.e., multiple beams. The component beams of multiple beams can be referred to as sub-beams or beam waves. Multiple beams can scan different parts of the target simultaneously. Thus, for example, a multi-beam evaluation device can evaluate the object more quickly by moving the object at a higher speed than a single-beam evaluation device.

[0029] In a multi-beam evaluation apparatus, the path displacements of some of the primary electron beams are displaced away from the central axis of the scanning device, i.e., the midpoint of the primary electron optical axis (which is also referred to herein as the charged particle axis). In order to ensure that all electron beams arrive at the sample surface at substantially the same angle of incidence, beamlet paths having a larger radial distance from the central axis need to be manipulated to move through a larger angle than beamlet paths having paths closer to the central axis. This stronger manipulation may cause aberrations that cause the resulting image to be blurred and out of focus. An example is spherical aberration, which causes the focus of each beamlet path to enter a different focal plane. Specifically, for beamlet paths that are not located on the central axis, the change in the focal plane in the beamlet is greater with radial displacement relative to the central axis. When detecting signal particles (e.g., secondary electrons) from a target, these aberrations and defocusing effects may remain associated with these signal particles (e.g., secondary electrons), for example, the shape and size of the spots formed by the beamlets on the target will be affected. Therefore, these aberrations degrade the quality of the resulting image created during the evaluation.

[0030] The following describes the implementation of a known multi-beam evaluation device.

[0031] The drawings are schematic. Therefore, for the sake of clarity, the relative sizes of the components in the drawings are exaggerated. Within the following description of the drawings, the same or similar reference numerals refer to the same or similar components or entities, and only the differences with respect to individual embodiments are described. Although the description and drawings relate to an electron-optical device, it should be appreciated that these embodiments are not intended to limit the present disclosure to specific charged particles. Therefore, references to electrons and terms related to electrons throughout this document may be more generally considered as references to and terms referenced with respect to charged particles, wherein the charged particles are not necessarily electrons.

[0032] Now, refer to Figure 1 , Figure 1 is a schematic diagram illustrating an exemplary evaluation apparatus 100, which may be a type of electron beam evaluation apparatus or may be referred to as an electron optical apparatus. Figure 1 The evaluation apparatus 100 includes a vacuum chamber 10, a load lock chamber 20, an electron optical device 40 (also referred to as an electron beam device or electron beam device), an equipment front end module (EFEM) 30, and a controller 50. The electron optical device 40 may be located within the vacuum chamber 10. The evaluation apparatus 100 may include a motorized or actuated stage.

[0033] The EFEM 30 includes a first loading port 30a and a second loading port 30b. The EFEM 30 may include additional loading ports. For example, the first loading port 30a and the second loading port 30b may receive a substrate front opening wafer box (FOUP) to be evaluated, the FOUP containing a substrate (e.g., a semiconductor substrate or a substrate made of (multiple) other materials) or a target (substrate, wafer and sample are collectively referred to as "target" below). One or more robot arms (not shown) in the EFEM 30 transport the target to the load lock chamber 20.

[0034] The load lock chamber 20 is used to remove gas around the target. The load lock chamber 20 can be connected to a load lock vacuum pump system (not shown), which removes gas particles in the load lock chamber 20. The operation of the load lock vacuum pump system enables the load lock chamber to reach a first pressure lower than atmospheric pressure. The main chamber 10 is connected to a main chamber vacuum pump system (not shown). The main chamber vacuum pump system removes gas molecules in the main chamber 10 so that the pressure around the target reaches a second pressure lower than the first pressure. After reaching the second pressure, the target is transported to the electron optical device 40, through which the target can be evaluated. The electron optical device 40 can be configured to project a single beam or multiple beams.

[0035] The controller 50 is electronically connected to the electron optical device 40. The controller 50 may be a processor (such as a computer) configured to control the charged particle beam evaluation apparatus 100. The controller 50 may also include processing circuitry configured to perform various signal and image processing functions. Figure 1 Controller 50 is shown as being located outside of the structure including main chamber 10, load lock chamber 20 and EFEM 30, but it should be appreciated that controller 50 may be part of the structure. Controller 50 may be located in one of the constituent elements of the charged particle beam evaluation device, or controller 50 may be distributed on at least two of the constituent elements. Although the present disclosure provides an example of a main chamber 10 housing an electron beam evaluation device, it should be noted that aspects of the present disclosure in its broadest sense are not limited to chambers housing electron optical equipment. On the contrary, it should be appreciated that the above principles may also be applicable to other devices and other arrangements of devices operating at a second pressure.

[0036] Now, refer to Figure 2 , Figure 2 is an evaluation device (e.g. Figure 1Schematic diagram of an exemplary multi-beam electron optical device 40 of an evaluation device 100 of the present invention. In an alternative embodiment, the evaluation device 100 is a single-beam evaluation device. The electron optical device 40 may include an electron source 201, a beam former array 372 (which is also called a gun aperture plate, a Coulomb aperture array, or a pre-beamlet forming aperture array), a converging lens 310, a source converter (or micro-optical array) 320, an objective lens 331, and a target 308. In one embodiment, the converging lens 310 is magnetic. (The single-beam evaluation device can have the same characteristics as the multi-beam evaluation device, except that the electron optical components having array apertures 372, 320 can have a single aperture. The source converter 320 can be replaced with several electron optical components along the beam path.) The target 308 can be supported by a support on a stage. The stage can be motorized. The stage moves so that the target 308 is scanned by the incident electrons. The electron source 201, the beam former array 372, the converging lens 310 may be components of an irradiation device included in the electron optical device 40. The source converter 320 (also referred to as a source conversion unit) and the objective lens 331 described in more detail below may be components of a projection device included in the electron optical device 40.

[0037] The electron source 201, the beam former array 372, the converging lens 310, the source converter 320 and the objective lens 331 are aligned with the primary electron optical axis 304 of the electron optical device 40. The electron source 201 can generate a primary beam 302 substantially along the electron optical axis 304 and having a source crossover (virtual or real) 301S. During operation, the electron source 201 is configured to emit electrons. The electrons are extracted or accelerated by the extractor and / or the anode to form the primary beam 302.

[0038] The beam former array 372 cuts the peripheral electrons of the primary electron beam 302 to reduce the Coulomb effect that occurs therewith. The primary electron beam 302 can be trimmed into a specified number of sub-beams, such as three sub-beams 311, 312 and 313, by the beam former array 372. It should be understood that this description is intended to be applied to an electron optical device 40 having any number of sub-beams (such as one, two or more than three). The beam former array 372 is configured to block peripheral electrons in operation to reduce the Coulomb effect. The Coulomb effect can increase the size of each of the detection spots 391, 392, 393, thereby degrading the evaluation resolution. The beam former array 372 reduces the aberrations produced by the Coulomb interaction between the electrons projected in the beam. The beam former array 372 may include a plurality of openings for generating primary sub-beams even before the source converter 320.

[0039] Source converter 320 is configured to convert the beam (including beamlets, if present) transmitted by beamformer array 372 into beamlets that are projected toward target 308. In one embodiment, source converter is a unit. Alternatively, the term source converter may simply be used as a collective term for a group of components for forming a beam from beamlets.

[0040] like Figure 2 As shown, in one embodiment, the electron optical device 40 includes a beam limiting aperture array 321 having an aperture pattern (i.e., apertures arranged in a configuration) that is configured to define the outer dimensions of the beam (or beamlets) projected toward the target 308. In one embodiment, the beam limiting aperture array 321 is part of the source converter 320. In an alternative embodiment, the beam limiting aperture array 321 is part of a system upstream of the beam of the main device. In one embodiment, the beam limiting aperture array 321 divides one or more of the beamlets 311, 312, 313 into beamlets so that the number of beamlets projected toward the target 308 is greater than the number of beamlets transmitted through the beam former array 372. In an alternative embodiment, the beam limiting aperture array 321 maintains the number of beamlets incident on the beam limiting aperture array 321, in which case the number of beamlets may be equal to the number of beamlets projected toward the target 308.

[0041] like Figure 2 As shown, in one embodiment, the electron optical device 40 includes a pre-bend deflector array 323 having pre-bend deflectors 323_1, 323_2 and 323_3 to respectively bend the beamlets 311, 312 and 313. The pre-bend deflectors 323_1, 323_2 and 323_3 can bend the paths of the beamlets 311, 312 and 313 onto the beam limiting aperture array 321.

[0042] The electron optical device 40 may also include an image forming element array 322 having image forming deflectors 322_1, 322_2 and 322_3. There are corresponding deflectors 322_1, 322_2 and 322_3 associated with the path of each beam wave. The deflectors 322_1, 322_2 and 322_3 are configured to deflect the path of the beam wave toward the electron optical axis 304. The deflected beam wave forms a virtual image (not shown) of the source intersection 301S. In the current embodiment, these virtual images are projected onto the target 308 through the objective lens 331 and form detection spots 391, 392, 393 on the target. The electron optical device 40 may also include an aberration compensator array 324, which is configured to compensate for the aberrations that may exist in each sub-beam in the sub-beam. In one embodiment, the aberration compensator array 324 includes a lens configured to operate on the corresponding beam wave. The lens may take the form of a lens array. The lenses in the array can operate on different beam waves of the multiple beams. The aberration compensator array 324 can, for example, include a field curvature compensator array (not shown), for example, having microlenses. The field curvature compensator and microlens can, for example, be configured to compensate individual sub-beams for field curvature aberrations that are evident in the detection spots 391, 392, and 393. The aberration compensator array 324 can include an astigmatism compensator array (not shown) having a micro-stigmator. For example, the micro-stigmator can be controlled to operate on the sub-beams to compensate for astigmatism aberrations that are otherwise present in the detection spots 391, 392, and 393.

[0043] The source converter 320 may be an electron optical assembly including a stack 700 as described herein. The source converter 320 may include a pre-bend deflector array 323, a beam limiting aperture array 321, an aberration compensator array 324, and an image forming element array 322. The pre-bend deflector array 323 may include pre-bend deflectors 323_1, 323_2, and 323_3 to bend the beamlets 311, 312, and 313, respectively. The pre-bend deflectors 323_1, 323_2, and 323_3 may bend the paths of the beamlets onto the beam limiting aperture array 321. In one embodiment, the pre-bend micro deflector array 323 may be configured to bend the beamlet paths of the beamlets toward the orthogonal to the plane of the beam limiting aperture array 321. In an alternative embodiment, the converging lens 310 may adjust the paths of the beamlets onto the beam limiting aperture array 321. For example, the converging lens 310 may focus (collimate) the three beamlets 311, 312, and 313 to become substantially parallel beams along the primary electron optical axis 304, so that the three beamlets 311, 312, and 313 are substantially perpendicularly incident on the source converter 320, which may correspond to the beam limiting aperture array 321. In such an alternative embodiment, the pre-bend deflector array 323 may not be necessary.

[0044] The image forming element array 322, the aberration compensator array 324 and the pre-bend deflector array 323 may include multiple layers of beamlet steering devices, some of which may be in the form of arrays, such as micro-deflectors, micro-lenses or micro-stigmators. The beam path may be steered in a rotational manner. Rotational correction may be applied by a magnetic lens. Additionally or alternatively, rotational correction may be achieved by an existing magnetic lens such as a converging lens arrangement.

[0045] In the present example of the electron optical device 40, the deflectors 322_1, 322_2 and 322_3 of the image forming element array 322 respectively deflect the beams toward the electron optical axis 304. It should be understood that the beam paths may have corresponded to the electron optical axis 304 before reaching the deflectors 322_1, 322_2 and 322_3.

[0046] The objective lens 331 focuses the beam onto the surface of the target 308, i.e., the objective lens 331 projects three virtual images onto the target surface. The three images formed by the three sub-beams 311 to 313 on the target surface form three detection spots 391, 392 and 393 on the target surface. In one embodiment, the deflection angles of the sub-beams 311 to 313 are adjusted to pass through or approach the front focus of the objective lens 331 to reduce or limit the off-axis aberrations of the three detection spots 391 to 393. In one arrangement, the objective lens 331 is magnetic. Although three beams are mentioned, this is only by way of example. Any number of beams may exist.

[0047] The manipulator is configured to manipulate one or more charged particle beams. The term manipulator encompasses deflectors, lenses, and apertures. Because the pre-bent deflector array 323, the aberration compensator array 324, and the image forming element array 322 manipulate one or more sub-beams or beam waves of charged particles, they can be referred to as manipulator arrays individually or in combination with each other. Because the lenses and deflectors 322_1, 322_2, and 322_3 manipulate one or more sub-beams or beam waves of charged particles, they can be referred to as manipulators.

[0048] In one embodiment, a beam splitter (not shown) is provided. The beam splitter may be downstream of the beam of the source converter 320. The beam splitter may be, for example, a Wien filter including an electrostatic dipole field and a magnetic dipole field. The beam splitter may be upstream of the beam of the objective lens 331. The beam splitter may be positioned between adjacent sections of the shield in the direction of the beam path. The inner surface of the shield may be radially inward from the beam splitter. Alternatively, the beam splitter may be inside the shield. In operation, the beam splitter may be configured to apply an electrostatic force to individual electrons of a beamlet through an electrostatic dipole field. In one embodiment, the electrostatic force is equal in magnitude to the magnetic force applied to the individual primary electrons of the beamlet by the magnetic dipole field of the beam splitter but opposite in direction. Therefore, the beamlet may pass through the beam splitter at least substantially straight with at least substantially zero deflection angle. The direction of the magnetic force depends on the direction of motion of the electron, while the direction of the electrostatic force does not depend on the direction of motion of the electron. Therefore, because the secondary electrons and backscattered electrons (or signal particles) move in roughly opposite directions compared to the primary electrons, the magnetic forces exerted on the secondary electrons and backscattered electrons (or signal particles) will no longer offset the electrostatic forces, and as a result, the secondary electrons and backscattered electrons moving through the beam splitter will be deflected away from the electron optical axis 304.

[0049] In one embodiment, a secondary device (not shown) is provided, which includes a detection element for detecting a corresponding secondary charged particle beam. When the secondary beam is incident on the detection element, these elements can generate a corresponding intensity signal output. The output can be directed to an image processing system (e.g., controller 50). Each detection element can include an array that can be in the form of a grid. The array can have one or more pixels; each pixel can correspond to an element of the array. The intensity signal output of the detection element can be the sum of the signals generated by all pixels in the detection element.

[0050] In one embodiment, a secondary projection device and its associated electronic detection equipment (not shown) are provided. The secondary projection device and its associated electronic detection equipment can be aligned with the secondary electron optical axis of the secondary device. In one embodiment, the beam splitter is arranged to deflect the path of the secondary electron beam toward the secondary projection device. The secondary projection device then focuses the path of the secondary electron beam onto multiple detection areas of the electronic detection device. The secondary projection device and its associated electronic detection equipment can record and generate an image of the target 308 using secondary electrons or backscattered electrons (or signal particles).

[0051] Such a Wayne filter, secondary device and / or secondary projection device can be arranged in a single beam evaluation device. In addition and / or alternatively, the detection device may be present in the beam downstream of the objective lens, for example, facing the sample during operation. In an alternative configuration, the detector device is positioned along the path of the charged particle beam toward the sample. This arrangement does not have a Wayne filter, a secondary device and a secondary projection device. The detection device can be positioned at one or more positions along the path of the charged particle beam toward the sample, such as facing the sample during operation, for example, around the path of the charged particle beam. Such a detector device can have an aperture and can be annular. Different detector devices can be positioned along the path of the charged particles to detect signal particles with different characteristics. The electron optical element along the path of the charged particle beam can be arranged and controlled to focus the signal particles with different corresponding characteristics to the corresponding detector devices at different positions along the path of the charged particle beam, and the electron optical element can include one or more electrostatic plates, which have an aperture for the path of the charged particle beam. Such an electrostatic plate can be arranged in series with two or more adjacent plates along the path of the charged particle beam.

[0052] In one embodiment, the evaluation device 100 includes a single source.

[0053] In the electron-optical device, any element or a collection of elements may be replaceable or field replaceable. One or more electron-optical components in the electron-optical device (especially electron-optical components that operate or generate beamlets, such as aperture arrays and manipulator arrays) may include one or more micro-electromechanical systems (MEMS). The pre-bent deflector array 323 may be a MEMS. MEMS are miniaturized mechanical and electromechanical components made using micro-fabrication technology. In one embodiment, the electron-optical device 40 includes an aperture, a lens, and a deflector formed as a MEMS. In one embodiment, manipulators such as lenses and deflectors 322_1, 322_2, and 322_3 can be controlled as an entire array, individually, or in groups within an array, in a passive manner or in an active manner to control the beam wave of charged particles projected toward the target 308.

[0054] In one embodiment, the electron optical device 40 may include alternative and / or additional components on the charged particle path, such as lenses and other components, some of which have been previously described with reference to Figure 1 and Figure 2 Described in more detail later Figure 3 and Figure 4 An example of such an arrangement is shown. Specifically, an embodiment includes an electron optical device 40 that divides a charged particle beam from a source into a plurality of sub-beams. A plurality of corresponding objective lenses can project the sub-beams onto a sample. In some embodiments, a plurality of converging lenses are provided upstream of the beam of the objective lens. The converging lens focuses each of the sub-beams to an intermediate focus upstream of the beam of the objective lens. In some embodiments, a collimator is provided upstream of the beam of the objective lens. A corrector can be provided to reduce focusing errors and / or aberrations. In some embodiments, such a corrector is integrated into the objective lens or positioned directly adjacent to the objective lens. Where a converging lens is provided, such a corrector can be additionally or alternatively integrated into the converging lens or positioned directly adjacent to the converging lens, and / or located in the intermediate focus or positioned directly adjacent to the intermediate focus. A detector is provided to detect charged particles emitted by the sample. The detector can be integrated into the objective lens. The detector can be on the bottom surface of the objective lens so as to face the sample when in use. The detector can include an array, and the array of the detector elements can correspond to the array of beam waves of the multi-beam arrangement. The detectors in the detector array may generate detection signals, which may be associated with pixels of the generated image.The focusing lens, objective lens and / or detector may be formed as MEMS and / or CMOS devices, for example, CMOS devices made using MEMS processing.

[0055] Figure 3is a schematic diagram of another design of an exemplary electron-optical device 40. The electron-optical device 40 may include a source 201 and one or more electron-optical components. Alternatively, the evaluation device 100 may include an upper beam limiter 252, a collimator element array 271, a control lens array 250, a scanning deflector array 260, an objective lens array 241, a beam shaping limiter 242, and a detector array. The source 201 provides a beam of charged particles (e.g., electrons). Multiple beams focused on the sample 208 are derived from the beam provided by the source 201. The sub-beams can be derived from the beam, for example, using a defined beam limiter (which defines a beam limiting aperture array). Desirably, the source 201 is a high brightness thermal field emitter with a good compromise between brightness and total emission current.

[0056] The upper beam limiter 252 defines a beam limiting aperture array. The upper beam limiter 252 may be referred to as an upper beam limiting aperture array or a beam limiting aperture array upstream of the beam. The upper beam limiter 252 may include a plate (which may be a plate-like body) having a plurality of apertures. The upper beam limiter 252 forms a beamlet from a charged particle beam emitted by the source 201. Portions of the beam other than those that contribute to forming the beamlet may be blocked (e.g., absorbed) by the upper beam limiter 252 to avoid interfering with beamlets downstream of the beam. The upper beam limiter 252 may be referred to as a beamlet defining aperture array.

[0057] A collimator element array 271 is provided downstream of the upper beam limiter. Each collimator assembly collimates a corresponding sub-beam. The collimator element array 271 can be spatially compact and can be implemented using MEMS manufacturing technology. In some embodiments, as shown in FIG. Figure 3 As illustrated, the collimator element array 271 is the first deflection or focusing electron optical array element in the beam path downstream of the source 201. In another arrangement, the collimator can take the form of a giant collimator in whole or in part. Such a giant collimator can be upstream of the beam of the upper beam limiter 252, so that the giant collimator operates on the beam from the source before generating multiple beams. A magnetic lens can be used as a giant collimator.

[0058] There is a control lens array 250 downstream of the collimator element array. 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 corresponding potential source. The control lens array 250 may include two or more (e.g., three) plate-shaped electrode arrays connected to a corresponding potential source. 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 sub-beams (e.g., applies a focusing action to the sub-beams before the sub-beams reach the objective lens array 241). Pre-focusing can reduce the divergence of the sub-beams or increase the convergence rate of the sub-beams. Although the control lens array 241 can be indistinguishable from the objective lens array 250 and can be part of the objective lens array 250, in this specification, the control lens array 250 is regarded as different and separate from the objective lens array 241.

[0059] As mentioned, the control lens array 250 is associated with the objective lens array 241. As described above, the control lens array 250 can be considered to provide electrodes in addition to the electrodes 242, 243 of the objective lens array 241, for example, as part of the objective lens array assembly. The additional electrodes of the control lens array 250 allow other degrees of freedom to control the electron optical parameters of the sub-beams. In one embodiment, the control lens array 250 can be considered to be an additional electrode of the objective lens array 241, realizing additional functions of the corresponding objective lenses of the objective lens array 241. In one arrangement, such electrodes can be considered to be part of the objective lens array, thereby providing additional functions to the objective lenses of the objective lens array 241. In this arrangement, the control lens is considered to be part of the corresponding objective lens, even to the extent that the control lens is only referred to as a part of the objective lens, for example, in terms of providing one or more additional degrees of freedom to the objective lens.

[0060] For ease of illustration, the lens array is schematically depicted herein by an array of elliptical shapes. Each elliptical shape represents one of the lenses in the lens array. Conventionally, elliptical shapes are used to represent lenses, similar to the biconvex form often used in optical lenses. However, in the context of an electronic optical arrangement such as the one discussed herein, it should be understood that the lens array will typically operate electrostatically, and therefore any physical elements that may not require the use of biconvex shapes. As described above, the lens array may instead include multiple plates with apertures.

[0061] A scanning deflector array 260 comprising a plurality of scanning deflectors may be provided. The scanning deflector array 260 may be formed using MEMS manufacturing techniques. Each scanning deflector causes a corresponding beamlet to scan across the sample 208. Thus, the scanning deflector array 260 may include a scanning deflector for each beamlet. Each scanning deflector may deflect the beamlet 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). The deflection may be such that the beamlet scans the entire sample 208 in one or two directions (i.e., one-dimensionally or two-dimensionally). In one embodiment, the scanning deflector described in EP2425444 (the entire contents of which are incorporated by reference particularly with respect to scanning deflectors) may be used to implement the scanning deflector array 260. The scanning deflector array 260 (e.g., formed using MEMS manufacturing techniques as mentioned above) may be more spatially compact than a giant scanning deflector. In another arrangement, a giant scanning deflector may be used upstream of the upper beam limiter 252. The giant scanning deflector may function similarly or identically to a scanning deflector array, although it operates on the beam from the source before generating beam waves of multiple beams.

[0062] An objective lens array 241 including a plurality of objective lenses is provided to direct the sub-beams onto the sample 208. Each objective lens includes at least two electrodes (e.g., two or three electrodes) connected to a corresponding potential source. The objective lens array 241 may include two or more (e.g., three) plate-shaped electrode arrays connected to a corresponding potential source. Each objective lens formed by the plate-shaped electrode array may be a microlens that operates on a different sub-beam. Each plate defines a plurality of apertures (which may also be referred to as holes). The position of each aperture in the plate corresponds to the position of a corresponding aperture (or multiple apertures) in another plate (or multiple plates). The corresponding apertures define the objective lens, and each corresponding aperture set therefore operates on the same sub-beam in multiple beams when in use. Each objective lens projects a corresponding sub-beam of multiple beams onto the sample 208.

[0063] An objective lens array 241 having only two electrodes may have lower aberrations than an objective lens array 241 having more electrodes. A three-electrode objective lens may have a greater potential difference between the electrodes, thus achieving a stronger lens. Additional electrodes (i.e., more than two electrodes) provide additional degrees of freedom for controlling electron trajectories, for example, focusing secondary electrons as well as the incident beam. Such additional electrodes may be considered to form a control lens array 250. The benefit of a two-electrode lens over a single lens is that the energy of the incident beam is not necessarily the same as the outgoing beam. Advantageously, the potential difference on this two-electrode lens array enables it to be used as an accelerating lens array or a decelerating lens array.

[0064] The objective lens array may form part of an objective lens array assembly together with any or all of the scanning deflector array 260, the control lens array 250, and the collimator element array 271. The objective lens array assembly may also include a beam shaping limiter 242. The beam shaping limiter 242 defines a beam limiting aperture array. The beam shaping limiter 242 may be referred to as a lower beam limiter, a lower beam limiting aperture array, or a final beam limiting aperture array. The beam shaping limiter 242 may include a plate (which may be a plate-like body) having a plurality of apertures. The beam shaping limiter 242 is downstream of the beam from at least one electrode (optionally, all electrodes) of the control lens array 250. In some embodiments, the beam shaping limiter 242 is downstream of the beam from at least one electrode (optionally, all electrodes) of the objective lens array 241.

[0065] In one arrangement, the beam shaping limiter 242 is structurally integrated with the electrode 302 of the objective lens array 241. Desirably, the beam shaping limiter 242 is located in an area of ​​low electrostatic field strength. Each of the beam limiting apertures is aligned with a corresponding objective lens in the objective lens array 241. The alignment allows a portion of a beamlet from the corresponding objective lens to pass through the beam limiting aperture and impinge on the sample 208. The aperture of the beam shaping limiter 242 may have a smaller diameter than the aperture of at least one of the objective lens array 241, the control lens array 250, the detector array 240, and the upper beam limiter array 252. Each beam limiting aperture has a beam limiting effect, thereby allowing only a selected portion of the beamlets incident on the beam shaping limiter 242 to pass through the beam limiting aperture. The selected portion may allow only a portion of the corresponding beamlet that passes through the central portion of the corresponding aperture in the objective lens array to reach the sample. The central portion may have a circular cross-section and / or be centered on the beam axis of the beamlet.

[0066] In one embodiment, the electronic optical device 40 is configured to control the objective lens array assembly (e.g., by controlling the potential applied to the electrodes of the control lens array 250) so 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. Therefore, the control lens array 250 and the objective lens array 241 can 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. The control lens array and the objective lens array operate together to obtain a combined focal length to the same surface. Combined operation without an intermediate focus can reduce the risk of aberrations. In other embodiments, the objective lens array assembly can be configured to form an intermediate focus between the control lens array 250 and the objective lens array 241.

[0067] An electric power source may be provided to apply corresponding potentials to electrodes of the control lenses of the control lens array 250 and the objective lenses of the objective lens array 241 .

[0068] In addition to the objective lens array 241, a control lens array 250 is provided to provide additional degrees of freedom to control the properties of the sub-beams. For example, additional degrees of freedom are provided even when the control lens array 250 and the objective lens array 241 are provided relatively close together, so that no intermediate focus is formed between the control lens array 250 and the objective lens array 241. The control lens array 250 can be used to optimize the beam angle relative to the reduction rate of the beam and / or control the beam energy delivered to the objective lens array 241. The control lens may include two or three or more electrodes. If there are two electrodes, the reduction rate and the landing energy are controlled together. If there are three or more electrodes, the reduction rate and the landing energy can be controlled independently. Note that the beam-most downstream electrode of the control lens array 250 can be the beam-most upstream electrode of the objective lens array 241. That is, the control lens array 250 and the objective lens array 241 can share electrodes. The shared electrodes provide different lens effects for each lens, each lens effect relative to one of its two opposing surfaces (i.e., the beam upstream surface and the beam downstream surface). Therefore, the control lens can be configured to adjust the reduction rate and / or beam angle of the corresponding sub-beam and / or the landing energy on the substrate (for example, using an electric power source to apply a suitable corresponding potential to the electrodes of the control lens and the objective lens). This optimization can be achieved without having an excessively negative impact on the number of objective lenses and without excessively degrading the aberration of the objective lens (for example, without reducing the strength of the objective lens). The use of a control lens array enables the objective lens array to operate at its optimal electric field strength. Note that references to reduction rate and angle are intended to refer to changes in the same parameter. In an ideal arrangement, the product of a certain range of reduction rate and corresponding angle is constant. However, the angle may be affected by the aperture used.

[0069] In one embodiment, the landing energy can be controlled to a desired value within a predetermined range, for example, from 1000 eV to 5000 eV. Desirably, the landing energy changes primarily by controlling the energy of the electrons leaving the control lens. Preferably, the potential difference within the objective lens remains constant during the 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 angle and the reduction ratio. The control lens can be used to change the reduction ratio in view of the change in the landing energy. Desirably, each control lens includes three electrodes to provide two independent control variables. For example, one of the electrodes can be used to control the magnification, while a different electrode can be used to independently control the landing energy. Alternatively, each control lens may have only two electrodes. When there are only two electrodes, one of the electrodes may need to control both the magnification and the landing energy.

[0070] A detector array (not shown) is provided to detect charged particles emitted from the sample 208. The detected charged particles may include any of the charged particles (e.g., signal particles) detected by the scanning electron microscope, including secondary electrons and / or backscattered electrons from the sample 208. The detector may be an array providing a surface of an electron optical device facing the sample 208 (e.g., the bottom surface of the electron optical device). Alternatively, the detector array is upstream of the beam of the bottom surface, or, for example, in an objective lens array or a control lens array, or upstream of the beam of the objective lens array or the control lens array. The elements of the detector array may correspond to beam waves of a multi-beam arrangement. The signals generated by detecting electrons by the elements of the array are transmitted to a processor for generating an image. The signals may correspond to pixels of an image.

[0071] In other embodiments, both a giant scanning deflector and a scanning deflector array 260 are provided. In such an arrangement, scanning the beamlets across the sample surface can be achieved by controlling the giant scanning deflector and the scanning deflector array 260 together (preferably synchronously).

[0072] In one embodiment, Figure 4 As illustrated, an electron optical device array 500 is provided. The array 500 may include any of the multiple electron optical devices described herein. Each electron optical device in the electron optical device focuses corresponding multiple beams simultaneously onto different areas of the same sample. Each electron optical device may form a beamlet from a beam of charged particles from different corresponding sources 201. Each corresponding source 201 may be one of the multiple sources 201. At least one subset of the multiple sources 201 may be provided as a source array. The source array may include multiple sources 201, which are provided on a common substrate. Multiple multiple beams simultaneously focusing onto different areas of the same sample allows a larger area of ​​the sample 208 to be processed (e.g., evaluated) simultaneously. The electron optical devices in the array 500 may be arranged adjacent to each other so as to project corresponding multiple beams onto adjacent areas of the sample 208.

[0073] Any number of electron-optical devices may be used in array 500. Preferably, the number of electron-optical devices ranges from two (2) (desirably, nine (9)) to one hundred (100) or even two hundred (200). In one embodiment, the electron-optical devices are arranged in a rectangular array or a hexagonal array. In other embodiments, the electron-optical devices are provided in an irregular array or in a regular array having a geometric shape other than rectangular or hexagonal. When referring to a single electron-optical device, each electron-optical device in array 500 may be configured in any of the manners described herein, for example, as described above, particularly with respect to reference 500. Figure 6The details of this arrangement are described in EPA 20184161.6, filed on July 6, 2020, which is hereby incorporated by reference, regarding how the objective lens can be incorporated and adapted for use in a multi-device arrangement.

[0074] exist Figure 4 In the example of Figure 3 A plurality of electron-optical devices of the type described. Thus, in this example, each of the electron-optical devices includes both a scanning deflector array 260 and a collimator element array 271. As mentioned above, the scanning deflector array 260 and the collimator element array 271 are particularly well suited for incorporation into the electron-optical device array 500 due to their spatial compactness, which facilitates positioning the electron-optical devices close to each other. This arrangement of the electron-optical devices may be preferred over other arrangements that use magnetic lenses as collimators. For example, due to magnetic interference between columns, it may be challenging to incorporate a magnetic lens into an electron-optical device intended for use in a multi-device arrangement (e.g., a multi-column arrangement).

[0075] Except as described below and Figure 5 In addition to the features shown, alternative designs of multi-beam electron optics may have similar characteristics to those described above. Figure 3 The same characteristics as described. An alternative design of a multi-beam electron optical device may include a converging lens array 231 upstream of the beam of the objective lens array arrangement 241, as disclosed in EP application 20158804.3 filed on February 21, 2020, which is hereby incorporated by reference for the description of a multi-beam device with a collimator and its components. Because the beam limiting aperture array associated with the converging lens array 231 can shape the beam waves 211, 212, 213 of the multiple beams from the source 201, this design does not require a beam shaping limiter array 242 or an upper beam limiter array 252. The beam limiting aperture array of the converging lens can also act as an electrode in the lens array.

[0076] The paths of the beams 211, 212, 213 diverge away from the converging lens array 231. The converging lens array 231 focuses the generated beams to an intermediate focus (i.e., toward the control lens array and the objective lens array) between the converging lens array 231 and the objective lens array assembly 241. The collimator array 271 may be located at the intermediate focus rather than associated with the objective lens array assembly 241.

[0077] The collimator can reduce the divergence of the divergent beam wave path. The collimator can collimate the divergent beam wave path so that these divergent beam wave paths are generally parallel toward the objective lens array assembly. The corrector array may be present in the multi-beam path, for example, associated with the converging lens array, the intermediate focus and the objective lens array assembly. The detector 240 can be integrated into the objective lens 241. The detector 240 may be on the bottom surface of the objective lens 241 so as to face the sample when in use. For example, the detector 240 can be an array of detector elements, each element for a different beam wave.

[0078] In one Figure 5 As shown and referenced Figure 5 In the described arrangement, the detector may be located in the electron optical device 241 and in the embodiment of the reference Figure 3 The electron optical device described and as Figure 3 The detector 250 may be integrated into the objective lens array 241 and the control lens array 240 (when present, since the control lens array 250 is not in the Figure 5 ). The detector may have more than one detector at different locations along the path of the beamlets of the multi-beam, for example, each array being associated with a different electron-optical element (such as an electrode of an objective lens array and / or a control lens array). The objective lens array 241 and the associated electron-optical elements (such as the control lens array 240) may be included in an assembly, which may be a monolithic assembly, which may be referred to as an electron-optical assembly, which includes the stack 700. In one embodiment, the detector 240 is associated with a planar element of the electron-optical module 41, or even integrated into the electron-optical elements of the stack 700. For example, the detector 240 may be located on the bottom surface of the stack 700 including the objective lens 241. The detector 240 may be provided with an electrical connection 60, as described elsewhere in this document. In one variation, the detector has a detector array located upstream of the beam of the objective lens array (optionally and the control lens array 240) (e.g., upstream of the beam of the stack 700). Between the stack 700 and the detector array, there may be a Wayne filter array that directs the charged particle beam in a beam downstream direction toward the sample and directs signal particles from the sample to the detector array.

[0079] The electron optical device array may have a Figure 3 The multi-beam device described and as Figure 4Multiple multi-beam devices of this design are shown. Multiple multi-beam devices can be arranged in a multi-beam device array. Such an arrangement is shown and described in EP application 20158732.6 filed on February 21, 2020, which is hereby incorporated by reference with respect to a multi-device arrangement of a multi-beam device, which is characterized by the disclosed design of a multi-beam device with a collimator at an intermediate focus.

[0080] Another alternative design of a multi-beam device includes multiple single-beam devices. A single beam generated for the purposes of the invention described herein may be similar to or equivalent to multiple beams generated by a single device. Each device may have an associated detector. This multi-device device may be arranged in the form of an array of three, four, nine, nineteen, fifty, one hundred or even two hundred devices, each device generating (in the case of a single-beam device) a single beam or beam wave or (in the case of a multi-beam device) multiple beams. In this other alternative design, the device array may have a common vacuum system, each device having a separate vacuum system or a device group being assigned a different vacuum system. Each device may have an associated detector.

[0081] Electron optical device 40 may be a component of an evaluation (eg, inspection, metrology or metrology inspection) apparatus or part of an electron beam lithography apparatus.Multi-beam charged particle apparatus may be used in several different applications including electron microscopy in general (not just scanning electron microscopy and lithography).

[0082] The electron optical axis 304 describes the path of the charged particles through the source 201 and out of the source 201. Unless explicitly mentioned, all beamlets and beamlets of the multiple beams are at least referred to by, for example, reference Figure 2 The manipulator or electron-optical array of the arrangement shown and described is generally parallel to the electron-optical axis 304. The electron-optical axis 304 can be the same as or different from the mechanical axis of the electron-optical device 40. Figures 2 to 5 In the context of the arrangement shown and described, the electron optical axis can correspond to the path of the central beam of the multi-beam (e.g., beam 212). The beams of the multi-beam are collimated (e.g., at the position of the collimator array 271 corresponding to the intermediate focal plane (e.g., as shown in FIG. Figure 5 The surface of the sample 208 (eg, the electron optical axis 304 ) and the upper beam limiter 252 ) are substantially parallel to each other.

[0083] The electronic optical device 40 may include Figure 6A stack 700 for operating (e.g., steering) an electron beam wave is shown. For example, the stack 700 may include one or more of the following (in a non-limiting list): an objective lens array 241, and / or a converging lens array 231 and / or a collimator element array 271 and / or individual beam correctors and / or deflectors and / or a Wayne filter array. Specifically, the objective lens 331 and / or the converging lens 310 and / or the control lens 250 may include the stack 700.

[0084] Electro-optical components are configured to provide a potential difference between two or more plates (or substrates). An electrostatic field is generated between the plates, which act as electrodes. The electrostatic field creates an attractive force between the two plates. The attractive force may increase as the potential difference increases.

[0085] The stack 700 includes a plurality of planar elements. The planar elements may include plates or may be plates. In one embodiment, one or more planar elements are electronic optical elements (e.g., Figure 6 The electron optical element 61, 62 shown). For example, the electron optical element can be a plate or can include a plate having a surface with an applied voltage to provide a potential difference relative to the surface of another electron optical element. The potential difference generates an electric field that can manipulate the electron beam. However, the planar element is not necessarily an element with a specific voltage applied thereto. For example, in one embodiment, one or more planar elements are configured to shape or limit one or more electron beams. For example, the planar element can include one or more apertures for narrowing one or more corresponding electron beams. This function may not require the planar element to have an applied voltage.

[0086] In one embodiment, stack 700 is used in an electron optical module configured to project electrons along a beam path. Figure 6 In the orientation shown, the beam path extends generally vertically from top to bottom. In the description below, the planar elements are referred to as electron-optical elements. However, it should be understood that any of the planar elements may not need to be electron-optical elements and may be different types of planar elements, such as beam limiters. In one embodiment, the electron-optical element includes one or more of the planar elements.

[0087] In one embodiment, the thickness of at least one of the plates in the stack 700 is stepped so that the first electron-optical element 61 is thinner in the region corresponding to the aperture array than in another region of the first electron-optical element 61. It is advantageous to have a stepped thickness, for example, where two portions of the plate have different thicknesses, because at a high potential difference, the plate is subjected to higher electrostatic forces, which may cause bending if the plate has a consistent thickness and is, for example, too thin. Bending of the plate may adversely affect beam-to-beam uniformity. Therefore, thick plates are beneficial for alleviating bending. However, if the plate is too thick in the region of the aperture array, the plate may produce undesirable electron beam wave deformation. Therefore, a thin plate around the aperture array is beneficial for alleviating electron beam wave deformation. That is, the aperture array can be defined in the region of the plate that is thinner than the rest of the plate. Therefore, the stepped thickness of the plate reduces the possibility of bending without increasing the possibility of beam wave deformation. In one embodiment, the plate has a uniform thickness, including in the region corresponding to the aperture array.

[0088] Figure 6 The exemplary electron optical assembly shown includes a first electron optical element 61, a second electron optical element 62 and a spacer (or isolator) 76. Although the terms first and second are used to distinguish between the two electron optical elements 61, 62, any of these elements is referred to as the first electron optical element or the second electron optical element, so such terms are interchangeable. That is, in different descriptions of the same characteristic, the second electron optical element may be an electron optical element 61 located upstream of the beam of the first electron optical element, and the first electron optical element may be an electron optical element 62 located downstream of the beam of another electron optical element. These terms are only used to assist in the description in order to distinguish between the two electron optical elements and are not intended to be limiting. Unless stated to the contrary, the same annotations apply to all other numbered characteristics in this article. In one embodiment, the first electron optical element 61 is an array plate or includes an array plate. (Note that the term 'array plate' is a term used to distinguish the plate from other plates mentioned in the description). The second electron optical element 62 may be an adjacent plate or may include an adjacent plate, that is, a plate adjacent to the above-mentioned array plate. In the first electron optical element 61, the aperture array 711 is defined as a path for electron beam waves. The number of apertures in the aperture array can correspond to the number of beamlets in the multi-beam arrangement. In one arrangement, there are fewer apertures than beamlets in the multi-beam, so that the group of beamlet paths passes through the aperture. For example, the aperture can extend across the multi-beam path; the aperture can be a strip or a slit. In one arrangement, the apertures can be arranged in a grid (or a two-dimensional array) so that a plurality of groups with beams are arranged in a two-dimensional array of groups of beams. A first spacer 76 is disposed between the electron-optical elements to separate the electron-optical elements. The electron-optical assembly is configured to provide a potential difference between the first electron-optical element 61 and the second electron-optical element 62.

[0089] In the second electron optical element 62, another aperture array 721 is defined as a path for the electron beam wave. In one embodiment, one or more of the apertures (or openings) of the aperture array 711 have a midpoint. In one embodiment, one or more of the apertures (or openings) of the another aperture array 721 have a midpoint. In one embodiment, when the first electron optical element 61 and the second electron optical element 62 are properly aligned, the midpoint between the first electron optical element 61 and the second electron optical element is aligned.

[0090] In one embodiment, the thickness of the second electron-optical element 62 can also be stepped so that the second electron-optical element is thinner in the area corresponding to the aperture array than another area of ​​the second electron-optical element. (Alternatively, the second electron-optical element 62 is substantially planar and / or has a uniform thickness). Desirably, the aperture array 721 defined in the second electron-optical element 62 has the same pattern as the aperture array 711 defined in the first electron-optical element 61. In one arrangement, the patterns of the aperture arrays in the two plates may be different. For example, the number of apertures in the second electron-optical element 62 may be less than or greater than the number of apertures in the first electron-optical element 61. In one arrangement, there is a single aperture in the second electron-optical element 62, which is used for the entire path of the sub-beams of the multi-beam. Preferably, the apertures in the first electron-optical element 61 and the second electron-optical element 62 are substantially well aligned with each other. This alignment between the apertures is to limit lens aberrations.

[0091] The first electron-optical element 61 and the second electron-optical element 62 may each have a thickness of up to 1.5 mm, preferably 1 mm, more preferably 500 μm at the thickest point of the plate. In one arrangement, the beam downstream plate (i.e., the plate closer to the sample) may have a thickness of between 200 μm and 300 μm at its thickest point. The beam downstream plate preferably has a thickness of between 200 μm and 150 μm at its thickest point. The beam upstream plate (i.e., the plate further away from the sample) may have a thickness of up to 500 μm at its thickest point.

[0092] The coating may be provided on the surface of the first electron-optical element 61 and / or the second electron-optical element 62. Preferably, the coating is provided on both the first electron-optical element 61 and the second electron-optical element 62. The coating reduces surface charging which may otherwise cause undesired beam distortion.

[0093] The coating is configured to withstand possible electrical breakdown events between the first electron-optical element 61 and the second electron-optical element 62. Preferably, a low ohmic coating is provided, and more preferably, a coating of 0.5 ohm / square or less is provided. The coating is preferably provided on the surface of the plate downstream of the beam. The coating is more preferably provided between at least one of the electron-optical elements and the first spacer 76. The low ohmic coating reduces undesirable surface charging of the plate.

[0094] The first electron-optical element 61 and / or the second electron-optical element 62 may comprise a low bulk resistance material, preferably a material of 1 Ohm (ohm).m or lower, optionally 0.1 Ohm.m or lower, optionally 0.01 Ohm.m or lower, optionally 0.001 Ohm.m or lower and optionally 0.0001 Ohm.m or lower. More preferably, the first electron-optical element 61 and / or the second electron-optical element 62 comprises doped silicon. Plates with low bulk resistance have the advantage that these plates are less likely to fail because the discharge current is supplied / exhausted via the bulk rather than, for example, via a thin coating.

[0095] The first electron optical element 61 includes a first wafer. The first wafer can be etched into regions with different thicknesses. The first wafer can be etched in the region corresponding to the aperture array so that the first electron optical element 61 is thinner in the region corresponding to the aperture array. For example, the first side of the wafer can be etched or both sides of the wafer can be etched to produce a stepped thickness of the plate. Etching can be performed by deep reactive ion etching. Alternatively or additionally, the stepped thickness of the plate can be produced by laser drilling or machining.

[0096] Alternatively, the first electron optical element 61 may include a first wafer and a second wafer. The aperture array may be defined in the first wafer. The first wafer may be arranged to contact the first spacer 76. The second wafer is arranged on the surface of the first wafer in a region not corresponding to the aperture array, for example, the region is away from the aperture array. The first wafer and the second wafer may be bonded by wafer bonding. In the region corresponding to the aperture array, the thickness of the first electron optical element 61 may be the thickness of the first wafer. In another region other than the region of the aperture array (for example, radially outward from the aperture array), the thickness of the first electron optical element 61 may be the combined thickness of the first wafer and the second wafer. Therefore, the first electron optical element 61 has a stepped thickness between the first wafer and the second wafer.

[0097] One of the first electron optical element 61 and the second electron optical element 62 is upstream of the beam of the other electron optical element. One of the first electron optical element 61 and the second electron optical element 62 is desirably negatively charged relative to the other electron optical element during operation. Preferably, the beam upstream plate has a higher potential than the beam downstream plate relative to the ground potential of, for example, a source or a sample. The electron optical assembly can be configured to provide a potential difference of 5 kV or greater between the first electron optical element 61 and the second electron optical element 62. Preferably, the potential difference is 10 kV or greater. More preferably, the potential difference is 20 kV or greater, or less than 30 kV or even greater than 30 kV. In one embodiment, the evaluation device 100 includes a power supply. The power supply can be included in the electron optical device 40. In one embodiment, the power supply is electrically connected to one of the electron optical elements. The power supply can be configured to apply a known voltage to the electron optical element. In one embodiment, the power supply is configured to apply a known voltage to each of a plurality of electron optical elements. In one embodiment, a plurality of power supplies are configured to apply a known voltage to the corresponding electron optical element.

[0098] The first spacer 76 is preferably disposed between the first electron-optical element 61 and the second electron-optical element 62 so that the opposing surfaces of the plates are coplanar with each other. The first spacer 76 has an inner edge 731 facing the path of the beam wave. The first spacer 76 may be planar with its major surface coplanar with the first electron-optical element 61 and the second electron-optical element 62. The first spacer 76 defines a central aperture 732 for the path of the electron beam wave.

[0099] A conductive coating (e.g., coating 740) may be applied to the first spacer 76. Preferably, a low ohmic coating is provided, and more preferably a coating of 0.5 ohm / square or less is provided. In one embodiment, the major surfaces of the spacer (i.e., the surface facing upstream of the beam and the surface facing downstream of the beam) are provided with a conductive coating. The peripheral edge (i.e., the sidewall) of the spacer may be exposed, i.e., free of a conductive coating. Alternatively, the peripheral edge may be covered with a conductive material such as a metal. In one embodiment, the spacer is completely covered in metal.

[0100] The coating is preferably on the surface of the space facing the negatively charged plate, which is negatively charged relative to the other plate. The beam downstream plate is preferably negatively charged relative to the beam upstream plate. The coating should be placed at the same potential as the negatively charged plate. The coating is preferably on the surface of the first spacer 76 facing the negatively charged plate. The coating is more preferably electrically connected to the negatively charged plate. The coating ensures that an electrostatic field exists on any possible gap between the first spacer 76 and the negatively charged plate.

[0101] The stack 700 may include or may be a lens assembly for manipulating an electron beam wave. The lens assembly may be, for example, an objective lens assembly or a converging lens assembly, or may be a part of an objective lens assembly or a converging lens assembly. A lens assembly such as an objective lens assembly may also include an additional lens array, such as a control lens array 250, which includes at least two plates.

[0102] In one embodiment, the electron-optical device 40 includes an electron-optical module. The electron-optical module may be field replaceable. The electron-optical device 40 and / or the electron-optical module may include, for example, Figures 6 to 8 A stack 700 for operating an electron beam wave (e.g., steering an electron beam wave) as shown in any of the figures in FIG. 1 . In one embodiment, the stack 700 includes a plurality of electron optical elements 61 to 64. The electron optical elements may have the Figure 6 Features described.

[0103] For example, Figure 6 As shown, in one embodiment, the stack includes a first electron-optical element 61 and a second electron-optical element 62. Figure 7 As shown, in one embodiment, the stack includes other electron-optical elements, such as a third electron-optical element 63 and a fourth electron-optical element 64 .

[0104] like Figure 7 As shown, there are not necessarily four electron optical elements 61, 64. For example, Figure 6 As shown, the stack may include only two electron optical elements 61, 62. In alternative arrangements, the stack may include three electron optical elements, five electron optical elements, or more than five electron optical elements. In one embodiment, the electron optical element is a plate or includes a plate. The plate may be substantially planar. In one embodiment, the electron optical element is arranged to span the path of the electron beam. The plane of the plate of the electron optical element may be substantially perpendicular to the direction parallel to the path of the electron beam.

[0105] In one embodiment, each of the electron optical elements comprises an aperture array. However, each electron optical element does not necessarily comprise an aperture array. The aperture is for the electron beam to pass through. In one embodiment, one or more of the electron optical elements comprise a single aperture for the one or more electron beams to pass through. In one embodiment, one or more of the electron optical elements comprise, for example, a detector for detecting electrons.

[0106] like Figure 7As shown, in one embodiment, stacked electron optical elements 61 to 64 are positioned substantially parallel to each other. Alternatively, a predetermined angle may be set between two or more electron optical elements in the electron optical element. In one embodiment, the stack is formed by stacking electron optical elements 61 to 64 relative to each other. The stack can be gradually built by adding one electron optical element at a time. For example, in one embodiment, a first electron optical element 61 is set. Then, a second electron optical element 62 can be stacked relative to the first electron optical element 61. Subsequently, a third electron optical element 63 can be stacked relative to the first electron optical element 61 and the second electron optical element 62. Subsequently, a fourth electron optical element 64 can be stacked relative to the first electron optical element 61, the second electron optical element 62, and the third electron optical element 63. As described herein, it is desirable that the order of stacking plates in the stack is related to the effective alignment between the different electron optical elements of the stack. With reference to the assembly or manufacture of the stack, this applies to any numbered term in this article, for example, first, second, third, etc.

[0107] like Figure 7 As shown, in one embodiment, a spacer is provided between one or more pairs of adjacent electron optical elements. The spacer may have the same Figure 6 Features described.

[0108] For example, in one embodiment, the first spacer 76 is located between the first electron optical element 61 and the second electron optical element 62. In one embodiment, the second spacer 77 is located between the second electron optical element 62 and the third electron optical element 63. In one embodiment, the third spacer 78 is located between the third electron optical element 63 and the fourth electron optical element 64. In one embodiment, the spacer is configured to mechanically support each pair of adjacent electron optical elements relative to each other. In one embodiment, the first spacer 76 is provided to control (e.g., fix) the distance between the first electron optical element 61 and the second electron optical element 62 in a direction parallel to the electron beam path. The thickness of the first spacer 76 may correspond to the spacing between the first electron optical element 61 and the second electron optical element 62 in a direction parallel to the electron beam path. In one embodiment, the spacer is configured to electrically isolate each pair of adjacent electron optical elements from each other. However, the spacer does not necessarily provide electrical isolation. For example, when it is desired that two adjacent electron optical elements are at the same potential, it may not be necessary to electrically isolate the two adjacent electron optical elements from each other. In one embodiment, the spacer can be omitted from the stack.

[0109] Figure 7 A stack 700 of planar elements is schematically depicted. In an embodiment, one or more of the planar elements are electro-optical elements.

[0110] The electron optical elements 61 to 64 are arranged to straddle the beam path. The planes of the electron optical elements 61 to 64 are desirably substantially perpendicular to the beam path.

[0111] In one embodiment, the first electron optical element 61 and the second optical element 62 constitute a pair of electron optical elements. One of the electron optical elements in the pair of electron optical elements includes an alignment reference 66. The alignment reference may be referred to as an alignment mark or a reference marker. The reference is a reference point for alignment of the first electron optical element 61 relative to another component, such as the second electron optical element 62. The alignment reference 66 may include one or more visible lines and / or one or more apertures passing through the first electron optical element 61. The alignment reference 66 may be used to verify the alignment of the stack, specifically, the alignment between the first electron optical element 61 and another component, such as the second electron optical element 62.

[0112] Another of the electron optical elements 62 includes a monitoring aperture 71. The monitoring aperture may be referred to as an aligned aperture. The monitoring aperture may be referred to as a viewing area or port. The monitoring aperture 71 is associated with the alignment reference 66. The alignment reference 66 is visible through the monitoring aperture 71. Figure 7 As shown, in one embodiment, the alignment datum 66 is aligned with the monitoring aperture 71. An imaginary line connecting the alignment datum 66 and the monitoring aperture 71 is substantially perpendicular to the planes of the first electron optical element 61 and the second electron optical element 71. The imaginary line is substantially parallel to the electron beam path.

[0113] like Figure 7 As shown, in one embodiment, the stack includes multiple pairs of adjacent electron optical elements. In each pair of electron optical elements, one electron optical element has an alignment reference and the other electron optical element has a monitoring aperture. For example, in Figure 7 In the arrangement shown, the second electron-optical element 62 and the third electron-optical element 63 form a pair of electron-optical elements. The third electron-optical element 63 and the fourth electron-optical element 64 form a pair of electron-optical elements.

[0114] like Figure 7 As shown, in one embodiment, the pair of electron optical elements are positioned (or arranged) relative to each other so that the alignment datum 66 and the monitoring aperture 71 are aligned with each other in a direction substantially perpendicular to the plane of the electron optical elements 61, 62. Figure 7In the view shown, the plane of the electron-optical elements 61, 64 extends horizontally. The first alignment datum 66 and the first monitor aperture 71 are vertically aligned. Similarly, the second alignment datum 67 and the second monitor aperture 72 are vertically aligned. Similarly, the third alignment datum 68 and the third monitor aperture 73 are vertically aligned. An imaginary line is substantially perpendicular to the plane of the electron-optical elements 61, 62, which is straight and joins the first alignment datum 66 to the first monitor aperture 71.

[0115] In one embodiment, the monitoring aperture 71 and the alignment reference 66 are aligned in at least two degrees of freedom (e.g., in at least one of two orthogonal directions in a plane parallel to the pair of electron-optical elements 61, 62) and are rotationally aligned in that plane.

[0116] There may be a small offset between the center of the alignment datum and the center of the associated monitoring aperture. As a result, the line joining the alignment datum to the monitoring aperture may be slightly angled from the normal to the electron optics. However, the alignment datum and the monitoring aperture are sufficiently aligned that illumination light projected through the monitoring aperture and incident on the alignment datum may be directly reflected back through the monitoring aperture.

[0117] like Figure 6 As shown, one or more of the electron optical elements 61 to 64 include aperture arrays 711, 721. The apertures are for the corresponding electron beams to pass through. In one embodiment, in a direction parallel to the plane of the electron optical element, the aperture has a smaller size than the monitoring aperture. In one embodiment, the apertures of the aperture arrays 711, 721 have a diameter in the range of about 5 μm to about 100 μm and optionally about 10 μm to about 50 μm. In one embodiment, the monitoring aperture has a diameter in the range of about 100 μm to about 1000 μm and optionally about 300 μm to about 600 μm. In one embodiment, in a direction parallel to the plane of the electron optical element, the apertures of the aperture array have a smaller size than the alignment reference. The apertures of the aperture array may not be wide enough (or may not be of sufficient size) to image the alignment reference through these apertures.

[0118] In one embodiment, the stack includes a plurality of electron optical elements (including the pair of electron optical elements and another electron optical element). Adjacent electron optical elements in the plurality of electron optical elements may include corresponding pairs of planar elements. Adjacent electron optical elements may include aligned alignment datums and monitoring apertures.

[0119] In one embodiment, one or more apertures (or openings) in the aperture (or opening) of the electron optical element have a midpoint. In one embodiment, when aligning the alignment reference of the electron optical element (e.g., alignment between the reference of one electron optical element and the reference of another electron optical element, or alignment between the reference of one electron optical element and the monitoring aperture that serves as the reference of another electron optical element), align the midpoint between the first electron optical element 61 and the second electron optical element 62. That is, the purpose of aligning the alignment reference with the corresponding monitoring aperture is to align the electron optical elements relative to each other, the monitoring aperture is defined in these electron optical elements, and the alignment reference exists on these electron optical elements. When aligning the electron optical element, align other characteristics on and in the electron optical element. This characteristic is the aperture array in each plate. In one embodiment, the apertures are directly aligned with each other. In a different embodiment, the pattern that the aperture array may have means that the apertures are not aligned, but the midpoints of different aperture arrays are aligned.

[0120] like Figure 7 As shown, in one embodiment, stack 700 includes another electron-optical element, i.e., third electron-optical element 63, which is adjacent to the electron-optical element (i.e., second electron-optical element 62) including first monitoring aperture 71. Second electron-optical element 62 and third electron-optical element 63 form another pair of electron-optical elements. In one embodiment, the other electron-optical element (i.e., third electron-optical element 63) includes another monitoring aperture, i.e., second monitoring aperture 72.

[0121] like Figure 7 As shown, in one embodiment, the monitoring apertures 71, 72 are offset from each other when viewed in a direction perpendicular to the plane of the electron optical element. In one embodiment, for each element assembly step, each of the alignment datum-monitoring aperture pairs is defined at a different distance from the center of the beam path (which may be a beam grid).

[0122] like Figure 8As shown, in one embodiment, at least two of the monitoring apertures (e.g., the first monitoring aperture 71 and the second monitoring aperture 72) are aligned with each other in a direction substantially perpendicular to the plane of the electron optical element. Of course, there may be a slight misalignment of the monitoring apertures 71, 72 (e.g., in the range of about 0.1 μm to about 2 μm). However, the monitoring apertures 71, 72 are sufficiently aligned with the first alignment reference 66 so that the illumination light projected through the monitoring apertures 71, 72 may be directly reflected from the first alignment reference 66 and passed back through the monitoring apertures 71, 72. In one embodiment, there is a line of sight from the second monitoring aperture 72 to the first alignment reference 66. Any misalignment may adversely affect the imaging resolution.

[0123] exist Figure 7 and Figure 8 , the aspect ratio is exaggerated so that some features of stack 700 can be more clearly shown.

[0124] like Figure 7 and Figure 8 As shown, in one embodiment, one of the electron optical elements in each pair of electron optical elements includes a plurality of alignment references. The other electron optical element in the pair of electron optical elements includes a plurality of monitoring apertures. The monitoring apertures are aligned with the corresponding alignment references in a direction substantially perpendicular to the plane of the electron optical element. For example, Figure 7 and Figure 8 As shown, in one embodiment, the first electron optical element 61 includes two first alignment references 66. The second electron optical element includes two first monitoring apertures 71. The first monitoring apertures 71 are aligned with the corresponding first alignment references 66 in a direction parallel to the beam path (i.e., perpendicular to the plane of the electron optical elements 61, 62). The two pairs of first monitoring apertures 71 and first alignment references 66 can be equidistantly spaced from each other on opposite sides of the beam path, and / or equidistantly spaced from each other relative to the midpoints of the corresponding electron optical elements 61, 62. Although two pairs of first monitoring apertures 71 and first alignment references 66 are depicted, there may be any number as desired, for example, three or more pairs. These pairs of first monitoring apertures 71 and first alignment references 66 can be equidistantly spaced around the midpoints of the corresponding electron optical elements 61, 62 and / or the beam paths. Fig.11Two alignment datums 66 are schematically depicted on opposite sides of the beam path that passes through the beam region where the aperture array 711 is located. In addition to alignment in a plane parallel to the electron-optical elements 61, 62 (e.g., in two different directions within the plane (e.g., the x-axis and y-axis that may be orthogonal to each other)), multiple alignment datum-monitoring aperture pairs allow alignment in a rotational direction (e.g., around the beam path and / or a plane orthogonal to the electron-optical elements 61, 62, which may be referred to as Rz (rotation about the z-axis)) to be determined. By providing multiple monitoring apertures that are spaced apart from each other, alignment in Rz is expected to be accurate (e.g., within a range of about 50 μrad to about 500 μrad). In one embodiment, the use of monitoring apertures and corresponding datums can be used to achieve alignment between adjacent electron-optical elements in at least three degrees of freedom (e.g., two different directions in the plane of at least one of the adjacent electron-optical plates (e.g., as the electron-optical elements) and around the beam path). It should be noted that for effective rotational alignment between adjacent electron-optical elements, eg about the beam path, at least two pairs of references and monitoring apertures are associated with adjacent electron-optical elements.To improve alignment, the references are desirably spaced from the midpoint of the respective electron-optical element.

[0125] It is contemplated that embodiments of the present invention enable verification of alignment after each stack assembly step.In one embodiment, illumination light is projected coaxially with an axis extending between the alignment datum and its associated monitoring aperture.

[0126] In one embodiment, the step of evaluating alignment includes: focusing the optical system on the electron optical element (e.g., the second electron optical element 62). This allows the determination of a mark (e.g., an alignment mark or a monitoring aperture). Subsequently, the optical system can be focused in a direction parallel to the beam path so as to image the alignment reference of the paired electron optical elements (e.g., the first alignment reference 66 of the first electron optical element 61). The first alignment reference 66 can be imaged through the first monitoring aperture 71 of the second electron optical element 62 (because the first alignment reference 66 is visible). By verifying the alignment of the electron optical element 61, 62 pair in two different rotational positions, one or more errors can be calibrated, which are caused by the focusing in the direction parallel to the beam path and / or the tilt and / or illumination effects of the optical system. The two different rotational positions can be offset by 180° from each other. For example, the stack 700 can be rotated between aligned measurements.

[0127] exist Figure 7 In the orientation shown, the direction of the electron beam projected toward the sample position is downward. The sample position is below the stack 700. Figure 7As shown, in one embodiment, the distance between the center of the beam path and the monitoring aperture increases with increasing distance from the sample location. For example, the first monitoring aperture 71 is farther from the central axis of the stack 700 than the second monitoring aperture 72. Similarly, the second monitoring aperture 72 is farther from the center than the third monitoring aperture 73.

[0128] Although the locations of the monitoring aperture and the fiducial are shown as being further away from the corresponding midpoints in the beam path and / or stack upstream of the beam (towards Figure 7 The monitoring aperture may be located at the top of the sample position or further from the sample position, but these positions may be closer to the beam path further upstream of the beam. As the distance from the sample position increases, the monitoring aperture is not necessarily further from the center. In a different embodiment, the location of the monitoring aperture and the reference may be located at different lateral locations for different pairs of adjacent electron optical elements in the stack. For example, Fig. 9 As shown, the first monitoring aperture 71 may be closer to the center of the beam grid than the second monitoring aperture 72. The first monitoring aperture 71 is further away from the sample position. In one embodiment, the second monitoring aperture 72 is larger than Figure 7 In one embodiment, the second monitoring aperture is large enough so that there is a line of sight between the second monitoring aperture 72 and the corresponding first alignment datum 66. This allows the first alignment datum 66 to be viewed / measured by means of an optical system on the other side of the third electron-optical element 63. Alternatively, as Fig. 9 As shown, in one embodiment, an additional monitoring aperture is provided for providing a line of sight through the monitoring aperture of an adjacent electron-optical element to the alignment datum of the next electron-optical element (i.e., the electron-optical element on the opposite side of the adjacent electron-optical element). Fig. 9 As shown, in one embodiment, the fourth electron-optical element 64 includes an additional third monitoring aperture that is aligned with the first monitoring aperture 71 , the second monitoring aperture 72 , and the first alignment datum 66 .

[0129] like Figures 6 to 9 As shown, in one embodiment, stack 700 includes a spacer, for example, a first spacer 76. The spacer is located between a pair of adjacent electron optical elements. For example, the first spacer 76 can be located between the first electron optical element 61 and the second electron optical element 62. The spacer is configured to physically separate the first electron optical element 61 from the second electron optical element 62. In one embodiment, the spacer is configured to mechanically support the electron optical element. In one embodiment, the first spacer 76 is configured to fasten the first electron optical element 61 to the second electron optical element 62. For example, the first spacer 76 can be fixed to the first electron optical element 61 and the second electron optical element 62.

[0130] like Figures 7 to 9 As shown, in one embodiment, stack 700 includes a plurality of spacers. For example, in one embodiment, second spacer 77 is located between second electron-optical element 62 and third electron-optical element 63. In one embodiment, third spacer 78 is located between third electron-optical element 63 and fourth electron-optical element 64.

[0131] like Figure 7 As shown, in one embodiment, the spacers 76 to 78 can have inner edges at different distances from the center of the beam path (i.e., the central axis through the stack 700). For example, compared to the inner edges of the first spacer 76 and the second spacer 77, Figure 7 It is shown that the third spacer 78 may have an inner edge closer to the center of the beam path. Alternatively, as Figure 8 and Fig. 9 As shown, the inner edges of all spacers 76 to 78 can be similar to each other relative to the center of the beam path. The positions of the monitoring aperture and the reference member in the corresponding electron-optical element can be selected so that there is an imaginary straight line between the monitoring aperture and the reference member, so that the imaginary straight line is away from the spacer. In other words, the imaginary straight line is not blocked by the spacer in the middle of the corresponding electron-optical element.

[0132] like Figures 7 to 9 As shown, in one embodiment, one or more of the spacers 76 to 78 have an inner edge that is stepped. In one embodiment, one or more of the spacers 76 to 78 have an inner edge that is a consistent distance from the center of the beam path.

[0133] like Figures 6 to 9 As shown, one or more of the spacers 76-78 include a central aperture 732. The central aperture 732 is provided for electrons to pass through the central aperture 732 along the beam path.

[0134] like Figures 7 to 9 As shown, in one embodiment, the central aperture 732 has a larger size than the monitoring aperture 71 in a direction parallel to the plane of the electron-optical elements 61 to 64. In one embodiment, a plurality of monitoring apertures 71 may fit within the central aperture 732.

[0135] In one embodiment, when viewed in a direction perpendicular to the plane of the electron-optical elements 61 to 64, at least one monitoring aperture overlaps the central aperture 732. Figure 7 , it is shown that the first monitoring aperture 71 overlaps with the central aperture 732 of the first spacer 76. The first monitoring aperture 71 is within the size of the central aperture 732 of the first spacer 76. However, all monitoring apertures do not necessarily overlap with all central apertures of the spacer. For example, Figure 7 As shown, when viewed in a direction parallel to the beam path, the first monitoring aperture 71 and the second monitoring aperture 72 are radially away (e.g., radially outward) from the area defined by the central aperture of the third spacer 78. In one embodiment, the stack 700 includes a spacer located between the monitoring aperture and the corresponding reference of the monitoring aperture (i.e., the reference to which the monitoring aperture is aligned). The spacer can intersect an imaginary straight line between the monitoring aperture and the corresponding reference of the monitoring aperture. The spacer can be transparent to allow evaluation of the reference through the monitoring aperture.

[0136] The present invention may be embodied as a method for aligning electron-optical elements 61 to 64. In one embodiment, the method includes: forming a stack, the stack including a first electron-optical element 61 and a second electron-optical element 62. For example, the second electron-optical element 62 may be moved to be located in the stack including the first electron-optical element 61. In one embodiment, a tool such as a robot arm is used to move the electron-optical elements 61, 64. In one embodiment, a first spacer 76 is fastened to the first electron-optical element 61. Next, the second electron-optical element 62 is initially positioned so as to abut against the first spacer 76.

[0137] In one embodiment, a method for aligning an electron optical element includes interrogating a first alignment datum 66 with an interrogation light passing through a first monitoring aperture 71. In one embodiment, a light source for the interrogation light is positioned such that the first monitoring aperture 71 is between the light source and the first alignment datum 66. The light source is arranged to project the interrogation light toward the first alignment datum 66 through the first monitoring aperture 71. The interrogation light may be visible light.

[0138] In one embodiment, the method includes detecting interrogation light reflected from the first electron optical element 61. The interrogation light may be reflected from the first alignment datum 66 and / or from a surface of the first electron optical element 61 near the first alignment datum 66. By detecting the reflected interrogation light, the alignment between the first electron optical element 61 and the second electron optical element 62 may be evaluated (e.g., verified). In one embodiment, evaluating the alignment between the first electron optical element 61 and the second electron optical element 62 includes evaluating the position of the first alignment datum 66 relative to a characteristic of the second electron optical element 62. For example, the position of the first alignment datum 66 relative to the second alignment datum 67 may be measured. In one embodiment, the surface of the second electron optical element facing the first electron optical element 61 includes a datum for aligning the opposite sides of the electron optical elements 61, 62. In one embodiment, when an electron-optical element (e.g., second electron-optical element 62) is added to stack 700, the relative positions of the monitoring aperture (e.g., first monitoring aperture 71) of the electron-optical element and the alignment datum (e.g., first alignment datum 66) of the adjacent electron-optical element (e.g., first electron-optical element 61) are measured relative to the alignment datum (67) of the electron-optical element (e.g., second electron-optical element 62) that has just been placed. This enables: when the monitoring aperture (e.g., second monitoring aperture 72) of another electron-optical element (e.g., third electron-optical element 63) is aligned with the alignment datum (e.g., second alignment datum 67) of the electron-optical element (e.g., second electron-optical element 62) that has just been placed (i.e., with reference to and relative to the alignment datum of the adjacent electron-optical element enabled by the monitoring aperture of the electron-optical element that has just been placed), the relative position of the other electron-optical element (e.g., third electron-optical element 63) can be determined relative to the adjacent electron-optical element (e.g., first electron-optical element 61). In one embodiment, the surface of the second electron-optical element facing the first electron-optical element 61 comprises a datum for aligning the opposing sides of the electron-optical elements 61, 62. Additionally or alternatively, the position of the first alignment datum 66 relative to the first monitoring aperture 71 may be measured. The first monitoring aperture 71 may be considered to have a dual purpose, i.e., enabling viewing of the first alignment datum 66 and also acting as a datum (since the first monitoring aperture 71 is used as a reference feature).

[0139] As mentioned above, the first alignment datum 66 is associated with the first monitoring aperture 71. The first alignment datum 66 and the first monitoring aperture 71 may be considered to form an alignment datum-monitoring aperture pair. Figure 7As shown, in one embodiment, a plurality of alignment datum-monitoring aperture pairs are provided at different positions, ie, at different positions when viewing the stack in plan view (in a direction parallel to the electron beam path). Figure 7 A first electron-optical element 61 comprising two first alignment datums 66 is shown. Figure 7 It is shown that the second electron-optical element 62 comprises two first monitoring apertures 71 . Figure 7 Two alignment datum-monitoring aperture pairs are shown for aligning the second electron-optical element 62 relative to the first electron-optical element 61. By providing two alignment datum-monitoring aperture pairs, two-dimensional alignment in a plane parallel to the electron-optical elements, as well as rotational alignment about an axis parallel to the electron beam path, can be assessed.

[0140] It is not essential to provide two alignment datum-monitoring aperture pairs. In an alternative embodiment, only one alignment datum-monitoring aperture pair is provided. In another alternative embodiment, three alignment datum-monitoring aperture pairs (or more than three) are provided.

[0141] In one embodiment, the method includes aligning the second electron-optical element 62 relative to the first electron-optical element 61 based on the detected interrogation light. For example, if the detected interrogation light indicates that the second electron-optical element is desirably aligned with the first electron-optical element 61, the second electron-optical element 62 can remain in place. In one embodiment, the second electron-optical element 62 is secured relative to the first electron-optical element 61. For example, the second electron-optical element 62 can be fixed relative to the first spacer 76. If the detected interrogation light indicates a misalignment between the first electron-optical element 61 and the second electron-optical element 62, the method may include moving the second electron-optical element 62 so as to align with the first electron-optical element 61. In one embodiment, the controller is configured to control the movement of the second electron-optical element 62 relative to the first electron-optical element 61 based on the detected interrogation light. For example, the controller may control a tool such as a robotic arm to move the second electron-optical element. Alternatively, if the detected interrogation light indicates that the second electron-optical element 62 is not aligned with the first electron-optical element, the stack may be discarded.

[0142] It is contemplated that embodiments of the present invention enable verification of alignment between electron-optical elements within a stack.

[0143] As mentioned above and as Figure 7 As shown, in one embodiment, the stack includes more than two electron optical elements. In one embodiment, the method for aligning electron optical elements includes: adding a third electron optical element 63 to the stack. Figure 7As shown, in one embodiment, the third electron-optical element 63 includes a second monitoring aperture 72.

[0144] like Figure 7 As shown, in one embodiment, the second electron-optical element 62 includes a second alignment datum 67. A second monitoring aperture 72 can be associated with the second alignment datum 67. The second alignment datum 67 and the second monitoring aperture 72 can form an alignment datum-monitoring aperture pair. In one embodiment, the second alignment datum 67 is visible through the second monitoring aperture 72.

[0145] In one embodiment, the method includes interrogating the second alignment datum 67 of the second electron-optical element 62 using interrogation light passing through the second monitoring aperture 72. Figure 7 In the arrangement shown, one side of the second monitoring aperture 72 is blocked by the third spacer 78. However, during assembly of the stack, the second monitoring aperture 72 is arranged so that interrogation light can pass through the second monitoring aperture 72. At some time before the third spacer 78 is added to the stack, interrogation light can pass through the second monitoring aperture 72. The third spacer 78 can be added to the stack after the alignment of the third electron-optical element 63 relative to the first electron-optical element 61 and / or the second electron-optical element 62 has been verified.

[0146] In one embodiment, the method includes detecting interrogation light reflected from the second electron-optical element 62. For example, the interrogation light may be reflected from the second alignment datum 67 and / or from a portion of the second electron-optical element 62 proximate to the second alignment datum 67.

[0147] In one embodiment, the method includes: based on the detected interrogation light, aligning the third electron-optical element 63 relative to the second electron-optical element 62. For example, if the alignment is verified, the third electron-optical element 63 can be kept in its position. Alternatively, if the alignment is not verified, the third electron-optical element 63 can be moved or the stack can be discarded.

[0148] Figure 8 An alternative arrangement of stack 700 is schematically depicted. Figure 8 The stack shown is also Figure 7 The features shown in FIG. 1 are described in detail to avoid redundant description. Figure 8 The stack shown is different from Figure 7 The characteristics of the stack shown are described.

[0149] like Figure 8As shown, the second alignment reference 67 is not essential. In one embodiment, specifically, the method includes: using the interrogation light passing through the second monitoring aperture 72 to interrogate the first alignment reference 66 (rather than the second alignment reference 67), such as Figure 8 As shown, the first alignment reference 66 can be interrogated using interrogation light that passes through both the first monitoring aperture 71 and the second monitoring aperture 72. The light source can be positioned so that the interrogation light passes through the second monitoring aperture 72 and the first monitoring aperture 71 so as to reach the first alignment reference 66. That is, the first monitoring aperture 71 can act as an alignment reference relative to the second monitoring aperture 72. The second monitoring aperture 72 can be regarded as having two types of monitoring references: the first monitoring aperture and the first alignment reference 66. This may require that the first monitoring aperture is larger than a monitoring aperture that does not have the function of a reference, that is, such a monitoring aperture acts only as a monitoring aperture. When the first monitoring aperture is used as a reference, the reference consists of the first monitoring aperture because the first reference may be too far in the direction along the beam path (e.g., along the z-axis) so that the intensity difference (e.g., contrast) is insufficient.

[0150] In one embodiment, the method includes detecting interrogation light reflected from the first electron-optical element 61. For example, the interrogation light may be reflected from the first alignment datum 66 and / or from a portion of the first electron-optical element 61 near the first alignment datum 66.

[0151] In one embodiment, the method includes: aligning the third electron optical element 63 relative to the first electron optical element 61 based on the detected interrogation light. By aligning the third electron optical element 63 relative to the first electron optical element 61, the third electron optical element 63 is also aligned with the second electron optical element 62. This is because the second electron optical element 62 is already aligned with the first electron optical element. The first monitoring aperture 71 is aligned with the second monitoring aperture 72. An imaginary straight line connecting the first monitoring aperture 71 to the second monitoring aperture 72 is substantially parallel to the electron beam path.

[0152] like Figure 7 and Figure 8 As shown, in one embodiment, the third electron-optical element 63 includes a third alignment datum 68. In one embodiment, the fourth electron-optical element 64 includes a third monitoring aperture 73. In one embodiment, the third alignment datum 68 and the third monitoring aperture 73 are associated with each other. The third alignment datum 68 is visible through the third monitoring aperture 73. The third alignment datum 68 and the third monitoring aperture 73 form an alignment datum-monitoring aperture pair.

[0153] like Figure 7 and Figure 8As shown, in one embodiment, a plurality of alignment datum-monitoring aperture pairs are provided for each pair of electron optical elements to be aligned relative to each other. Figure 8 In the arrangement shown, the first alignment datum 66 and the second monitoring aperture 72 form an alignment datum-monitoring aperture pair when aligning the third electron-optical element 63 relative to the first electron-optical element 61 .

[0154] Fig.10 A type of alignment reference member according to an embodiment of the present invention is schematically shown. The alignment reference member can be used as an embodiment of the present invention (such as Figures 7 to 9 As shown and referenced Figures 7 to 9 The reference member in any of the embodiments described herein. Fig.10 As shown, in one embodiment, the alignment datum 66 includes a plurality of marks 82, 87. The marks 82, 87 may be spaced apart from each other in a direction parallel to the plane of the electron-optical element. Fig.10 The view is a plane parallel to the plane of the electron optical element. As depicted, the marking may be a Vernier. A Vernier is in the form of a two-dimensional pattern that can be used as a reference according to an embodiment of the present invention, for example, a reference Figures 7 to 9 as described and as Figures 7 to 9 In one embodiment, the markings may form any suitable two-dimensional pattern, such as a grid.

[0155] For example, Fig.10 An exemplary depiction of an alignment datum 66 is schematically shown, which alignment datum 66 includes, for example, a first plurality (81) of marks 82. The first plurality (81) may be a one-dimensional array of marks 82. The marks may be substantially linear. For example, the marks 82 may be lines. The first plurality (81) of marks 82 may be referred to as a first series of marks (or sub-patterns). The first plurality (81) of marks 82 may be verniers. As a line of marks, the first series of marks may be used to determine relative alignment between adjacent electron optical elements in the direction of the line of marks (e.g., at the location of the alignment datum 66); that is, the line of marks may be associated with a degree of freedom in the direction of the line of marks. In one embodiment, the first plurality (81) of marks 82 includes at least three, optionally at least four, optionally at least five, and optionally at least ten marks 82. In one embodiment, the marks 82 are aligned substantially parallel to each other. The marks 82 are visible to the first monitoring aperture 71. When illumination light is projected onto the first alignment datum 66, a transition in the intensity (or contrast) of the reflected illumination light may be detected. This transition may correspond to an edge of the mark 82. The edge of the mark 82 may be a distinct sharp edge. By detecting the intensity transition of the image of the first alignment datum 66, the alignment between the electron-optical elements 61, 62 may be evaluated.

[0156] By providing a larger number of edge transitions, the accuracy of measuring the alignment between the electron-optical elements may be increased.The position fit of the second electron-optical element 62 relative to the first electron-optical element 61 may be averaged over the monitored edge transitions.

[0157] In one embodiment, the distance between the marks 82 in the first plurality (81) of marks 82 is known. The marks 82 may be provided at a predetermined distance from each other. In one embodiment, a constant pitch is provided between the marks 82. However, the pitch need not be constant, in particular, assuming that the spacing between the marks 82 is known.

[0158] like Fig.10 As shown, in one embodiment, the first alignment reference 66 includes a plurality of other marks, such as a second plurality (86) of marks 87, which are spaced apart from each other in another direction parallel to the plane of the electron optical element. The second plurality (86) of marks 87 (or a second marking line) may be vernier marks.

[0159] like Fig.10 As shown, in one embodiment, the first plurality (81) of marks 82 and the second plurality (86) of marks 87 are arranged in different directions (e.g., vertical directions). Otherwise, the second plurality (86) of marks 87 can have substantially the same features as the first plurality (81) of marks 82. By providing two series of marks (desirably, orthogonal to each other), the alignment datum 66 can be used to evaluate the alignment between electron optical elements in two dimensions. For example, when used to align adjacent electron optical elements, a single alignment datum can enable relative alignment at the position of the alignment datum to be determined in two degrees of freedom (e.g., in two directions of the first and second marking lines of the alignment datum 66).

[0160] However, it is not essential that the first alignment datum 66 includes an orthogonal series of marks. In one embodiment, the first alignment datum 66 includes one series of marks, for example, a first plurality (81) of marks 82. A second plurality (86) of marks 87 may be provided in another first alignment datum 66 at a very different location of the first electron-optical element 61. Fig.11 As shown, in one embodiment, the alignment reference member 66 (e.g., Fig.10 ) are located on either side of the aperture array 711. For example, in either embodiment, two first alignment references 66 may be provided on opposite sides of the beam path. Figures 7 to 9 Shown and referenced in Figures 7 to 9is described. Spacing more than two datums from each other, for example in a direction opposite to the aperture array 711, enables effective rotational alignment, for example, between adjacent electron optical elements. In one embodiment, the alignment datums on opposite sides of the beam path include marks arranged in different directions from each other. Desirably, each alignment datum includes: at least two marking lines extending in two different directions and at an angle relative to each other. In one embodiment, each of the two alignment datums 66 has a plurality of marks arranged in a line in a direction orthogonal to the direction between the two alignment datums 66. In one embodiment, one of the two alignment datums 66 on opposite sides of the beam region may omit one of the plurality of marks extending in the same direction between the two alignment datums.

[0161] In one embodiment, one or more of the planar elements include a detector configured to detect signal electrons from the sample location. Figures 2 to 5 Such a detector is depicted and disclosed in any of the figures in the drawings. Such a detector may be a detector array, such as including an array of detector elements. The detector (or detector array) may be a plate. The detector (or detector array) may be an example of an electron optical element. As mentioned herein, the electron optical element may be any of the following (in a non-limiting list): a plate of a lens array (such as an objective lens array, a converging lens array, or a control lens array), a plate of a corrector array, a plate of a collimator array, a plate of a deflector array, or a beam limiter (such as a beam limiting aperture array, a beam shaper array, or an upper beam limiter array). Therefore, the electron optical element may include a planar electron optical element in the form of a plate.

[0162] In one embodiment, the electron optical module is or includes an objective lens assembly. The objective lens assembly may include an objective lens array for focusing the electron beam onto the sample position. For example, Figures 3 to 5 Such an objective lens assembly is depicted in . In one embodiment, surfaces of the electron-optical elements of at least one pair of electron-optical elements are configured to form an objective lens 241 when a potential difference is applied between the electron-optical elements thereof.

[0163] In one embodiment, the electron optical module is or includes a converging lens array for deflecting electrons toward the sample 208. In one embodiment, the converging lens array is used to deflect electrons in one or more electron beams toward the sample 208. In one embodiment, the converging lens array is used to collimate the electrons toward the sample 208. In one embodiment, the electron optical module is or includes a giant converging lens for deflecting electrons toward the sample 208. In one embodiment, the electron optical module is or includes a collimator that can be separated from the converging lens or the converging lens array.

[0164] The present invention can be embodied as an alignment device, which includes a stack 700, an interrogation light source and an alignment detector. The interrogation light source is configured to guide the interrogation light through one or more monitoring apertures in the monitoring aperture. The alignment detector is configured to detect the interrogation light reflected from at least one of the electron optical elements 61 to 64. In one embodiment, the interrogation light source is located at one side of the stack (in a direction parallel to the beam path). The alignment detector is located at the same side of the stack. The optical system for evaluating alignment via imaging of the alignment reference is reflective. Light reflected from the alignment reference or an electron optical element near the alignment reference is used to evaluate the alignment. This is different from a transmission system in which light is transmitted through the stack and detected on the opposite side of the stack.

[0165] In one embodiment, the stack 700 includes one or more electro-optical elements, which include elements that can be referred to as micro-electromechanical components (although such components may not include moving features or movable features), or can be made using a technology suitable for manufacturing micro-electromechanical components (e.g., 'MEMS technology'), some of which are designed to have electro-optical functionality. The stack 700 or at least a component of the stack 700 can be manufactured by such technology. The stack 700 may include one or more elements that can be considered as MEMS elements. One or more of these elements can be controlled to be set to a high potential difference relative to a reference potential (e.g., ground) during use. Such elements may need to be accurately positioned (e.g., aligned) within the stack 700, for example, relative to the path of the beam grid and relative to other electro-optical elements within the device (e.g., relative to the source, relative to the path of the sample and / or beam grid). It is expected that embodiments of the present invention allow such elements to be more accurately positioned (e.g., aligned), such as during operation, within a stack of such stacks 700, for example, without deforming the stack 700, for example, by externally applied forces or moments. Additionally or alternatively, embodiments of the present invention may enable more accurate positioning (e.g., alignment) of such elements relative to other elements in device 40, and thus more accurate positioning of a stack 700 including such elements within device 40.

[0166] As mentioned above, in one embodiment, stack 700 is an electron optical lens assembly. The electron optical lens assembly may include an objective lens assembly. The electron optical lens assembly may be an objective lens assembly. In an alternative embodiment, the electron optical lens assembly is an electron optical converging lens assembly.

[0167] In one embodiment, stack 700 includes a collimator. For example, in one embodiment, stack 700 includes a magnetic collimator combined with an electrostatic focusing lens array. Stack 700 can include a single aperture lens array with one or two giant electrodes, which is placed away from the virtual source conjugate plane.

[0168] In an alternative embodiment, stack 700 includes a magnetic giant lens combined with an electrostatic slit deflector. The magnetic giant lens can be used for collimation. As another alternative, in one embodiment, stack 700 includes: a combined magnetic and electrostatic giant lens, and a beam downstream slit deflector.

[0169] In general, stack 700 can include any plates, such as plates of detector arrays, plates of lens electrodes (multiple deflectors can be integrated into the plates), multiple deflector arrays, beam aperture arrays (e.g., upper beam aperture arrays and / or final beam limiting arrays), deflector arrays (e.g., strip deflector arrays), and other types of corrector elements. Such plates can be referred to as electron-optical elements. Such electron-optical elements operate on or interact with multiple beams of a beam grid. The electron-optical element can have multiple apertures, each aperture for a different beam of the beam grid.

[0170] The embodiments described within this document have focused on a multi-beam electron optical device 40. The present invention is equally applicable to a single beam electron optical device 40.

[0171] Although the present invention has been described in conjunction with various embodiments, other embodiments of the present invention will be apparent to those skilled in the art from consideration of this specification and practice of the invention disclosed herein. For example, as described above, in one embodiment, stack 700 includes a monitoring aperture and an alignment reference. However, the monitoring aperture and alignment reference of the present invention can be used in any location in the electron-optical device 40 where there may be a possible misalignment problem. The description and embodiments are to be considered exemplary only, with the true scope and spirit of the present invention being indicated by the following claims and clauses.

[0172] The above description is intended to be illustrative rather than limiting. Therefore, it will be apparent to one skilled in the art that modifications may be made from the description without departing from the scope of the claims and clauses set forth below.

[0173] The following terms are provided:

[0174] Item 1. A stack of planar elements for a charged particle optical module, the charged particle optical module being configured to project charged particles along a beam path, the stack comprising a pair of adjacent planar elements, the pair of adjacent planar elements being arranged to be arranged across the beam path, wherein one of the planar elements comprises an alignment reference and the other of the planar elements comprises a monitoring aperture; wherein the pair of planar elements are positioned relative to each other so that the alignment reference and the monitoring aperture are aligned with each other in a direction substantially perpendicular to the planes of the planar elements.

[0175] Clause 2. The stack of clause 1, comprising another planar element adjacent to the planar element comprising the monitoring aperture so as to form another pair of planar elements.

[0176] Clause 3. The stack of clause 2, wherein the further planar element comprises a further monitoring aperture.

[0177] Clause 4. The stack of clause 3, wherein the monitoring apertures are offset from one another when viewed in a direction perpendicular to the plane of the planar element.

[0178] Clause 5. A stack according to clause 3 or 4, wherein the further planar element comprises an additional monitoring aperture, desirably, the additional monitoring aperture being aligned with the monitoring apertures of the pair of planar elements.

[0179] Item 6. A stack according to any one of items 3 to 5, wherein the other pair of planar elements are arranged relative to each other so that the other monitoring aperture of the other planar element and the other alignment reference piece of the planar element paired therewith are aligned with each other in a direction substantially perpendicular to the plane of the planar elements.

[0180] Clause 7. The stack of Clause 3, wherein the monitoring apertures are aligned with each other in a direction substantially perpendicular to the plane of the planar element.

[0181] Item 8. A stack according to any preceding item, wherein the planar elements respectively include one or more openings for charged particles, preferably, the one or more openings have a midpoint, and preferably, when the alignment reference and the monitoring aperture are aligned, the midpoint between the corresponding pair of planar elements is aligned.

[0182] Item 9. A stack according to any of the preceding items, wherein one planar element in each pair of planar elements includes a plurality of alignment datum members and the other planar element in the pair of planar elements includes a plurality of monitoring apertures, the plurality of monitoring apertures being aligned with corresponding alignment datum members in a direction substantially perpendicular to the plane of the planar elements, desirably, the plurality of alignment datum members are two alignment datum members, desirably, the plurality of monitoring apertures are two monitoring apertures, desirably, the two alignment datum members are spaced apart from the midpoint in different directions (desirably, opposite directions), desirably, the alignment datum members are spaced apart from the midpoint by the same distance.

[0183] Clause 10. A stack according to any preceding clause, comprising a spacer located between the planar elements of at least one pair of planar elements.

[0184] Clause 11. The stack of clause 10, wherein the spacer comprises a central aperture through which charged particles pass along the beam path.

[0185] Clause 12. The stack of clause 11, wherein the central aperture has a larger dimension in a direction parallel to the plane of the planar element than each of the monitoring apertures.

[0186] Clause 13. The stack of clause 12, wherein at least one monitoring aperture overlaps the central aperture when viewed in a direction perpendicular to the plane of the planar element.

[0187] Clause 14. The stack of any preceding clause, wherein each alignment datum comprises a plurality of marks that are spaced apart from one another in the plane of the planar element.

[0188] Item 15. A stack according to Item 14, wherein at least some of the multiple marks are arranged in a direction parallel to the plane of the planar element, desirably, all of the multiple marks are arranged in the direction parallel to the plane of the planar element, desirably, the multiple marks are cursors.

[0189] Clause 16. The stack of clause 14 or 15, wherein at least some of the plurality of markings are arranged in different directions parallel to the plane of the planar element.

[0190] Clause 17. A stack according to any one of clauses 14 to 16, wherein the plurality of markings forms a pattern, such as a grid.

[0191] Clause 18. A stack according to clause 14 or 15, wherein each alignment datum comprises a plurality of further marks which are spaced apart from one another in another direction parallel to the plane of the planar element such that the plurality of marks are arranged in a vertical direction.

[0192] Clause 19. The stack of any one of clauses 14 to 18, wherein the markings are periodic.

[0193] Clause 20. The stack of any preceding clause, wherein at least one alignment datum comprises a through hole.

[0194] Clause 21. The stack of clause 20, wherein each monitoring aperture has a larger dimension than the through hole in a direction parallel to the plane of the planar element.

[0195] Clause 22. A stack according to any preceding clause, wherein each of the planar elements comprises or is a plate.

[0196] Clause 23. A stack according to clause 22, wherein each plate comprises an array of apertures through which a respective charged particle beam passes (desirably along a beam path), desirably during operation, the beam path corresponding to the midpoint.

[0197] Clause 24. The stack of clause 23, wherein the apertures of the aperture array have a smaller dimension in a direction parallel to the plane of the planar element than the monitoring aperture.

[0198] Clause 25. The stack of any preceding clause, wherein at least one of the planar elements comprises a micro-electromechanical component.

[0199] Clause 26. A stack according to any preceding clause, wherein at least one of the planar elements is a charged particle optical element.

[0200] Clause 27. A stack according to any preceding clause, wherein at least one of the planar elements comprises a detector configured to detect signal charge particles from a sample location.

[0201] Clause 28. A charged particle optics module comprising a stack according to any preceding clause.

[0202] Item 29. A charged particle optical module according to Item 28, wherein the charged particle optical module is or includes an objective lens assembly, the objective lens assembly including an objective lens array for focusing the charged particle beam onto a sample position or a converging lens array for deflecting the charged particles toward the sample.

[0203] Clause 30. The charged particle optics module of Clause 29, wherein surfaces of the planar elements of at least one pair of planar elements are configured to form the lens when a potential difference is applied between the planar elements.

[0204] Clause 31. A charged particle optical device for directing a charged particle beam onto a sample location, the charged particle optical device comprising a stack according to any one of clauses 1 to 27 or a charged particle optical module according to any one of clauses 28 to 30.

[0205] Clause 32. A charged particle optical device comprising a stack according to any one of clauses 1 to 27, a charged particle optical module according to any one of clauses 28 to 30, or a charged particle optical apparatus according to clause 31.

[0206] Clause 33. The charged particle optical device of clause 32, further comprising an actuatable stage for supporting a sample at the sample position.

[0207] Clause 34. An alignment device comprising a stack according to any one of clauses 1 to 27 or a charged particle optical module according to any one of clauses 28 to 30; an interrogation light source configured to direct the interrogation light through at least one monitoring aperture; and an alignment detector configured to detect the interrogation light reflected from at least one planar element.

[0208] Clause 35. The alignment device of Clause 34, wherein the interrogation light source is located at one side of the stack and the alignment detector is located at the same side of the stack.

[0209] Clause 36. The alignment device of clause 34 or 35, comprising a mover configured to align the planar elements relative to each other based on the detected interrogation light.

[0210] Item 37. A method for aligning planar elements for a charged particle optical module, the charged particle optical module being configured to project charged particles along a beam path, the method comprising: providing a first planar element, the first planar element comprising a first alignment reference piece; providing a second planar element, the second planar element comprising a first monitoring aperture stacked relative to the first planar element; interrogating the first alignment reference piece using interrogation light passing through the first monitoring aperture; detecting the interrogation light reflected from the first planar element; and aligning the second planar element relative to the first planar element based on the detected interrogation light.

[0211] Clause 38. The method according to clause 37 comprises: providing a third planar element, wherein the third planar element comprises a second monitoring aperture stacked relative to the second planar element; interrogating the first alignment reference piece or the second alignment reference piece of the second planar element using interrogation light passing through the second monitoring aperture; detecting interrogation light reflected from the first planar element or the second planar element; and aligning the third planar element relative to the first planar element or the second planar element based on the detected interrogation light.

[0212] Clause 39. A method according to clause 37 or 38, wherein the aligning step comprises monitoring one or more intensity changes of the reflected interrogation light corresponding to one or more edges of the first alignment datum, desirably in two dimensions, for example in the plane of the corresponding planar element.

[0213] Clause 40. The method of clause 39, wherein the aligning step comprises monitoring a plurality of intensity changes of the reflected interrogation light corresponding to edges of a plurality of marks of the first alignment datum, the marks moving away from one another in a direction parallel to a plane of the first planar element.

[0214] Clause 41. The method of any one of clauses 37 to 40, wherein the interrogation step comprises focusing the interrogation light on the first alignment datum.

[0215] Clause 42. The method of any one of clauses 37 to 41, wherein the interrogation light is directed perpendicular to the plane of the second planar element.

[0216] Clause 43. The method of any one of clauses 37 to 42, comprising, after the aligning step, securing the second planar element relative to the first planar element.

[0217] Clause 44. A method of making a charged particle optical module comprising the method according to any one of clauses 37 to 43.

Claims

1. A planar element stack for a charged particle optical module configured to project charged particles along a beam path, the stack comprising: an adjacent pair of planar elements arranged to be arranged across the beam path, wherein one of the adjacent pair of planar elements comprises an alignment datum and the other of the adjacent pair of planar elements comprises a monitoring aperture; The pair of planar elements are positioned relative to each other such that the alignment datum and the monitoring aperture are aligned with each other in a direction substantially perpendicular to the planes of the planar elements.

2. A stack according to claim 1, comprising a further planar element adjacent to the planar element comprising the monitoring aperture so as to form a further pair of planar elements.

3. A stack according to claim 2, wherein the further planar element comprises a further monitoring aperture.

4. A stack according to claim 3, wherein the monitoring apertures are offset from each other when viewed in a direction perpendicular to the plane of the planar element.

5. A stack according to claim 3 or 4, wherein the further planar element comprises an additional monitoring aperture, desirably aligned with the monitoring apertures of the pair of planar elements.

6. A stack according to any one of claims 3 to 5, wherein the other pair of planar elements are arranged relative to each other so that the other monitoring aperture of the other planar element and the other alignment reference piece of the planar element paired therewith are aligned with each other in a direction substantially perpendicular to the plane of the planar elements.

7. The stack of claim 3, wherein the monitoring apertures are aligned with each other in a direction substantially perpendicular to the plane of the planar element.

8. A stack according to any preceding claim, wherein the planar elements each comprise one or more openings for charged particles.

9. A stack according to any preceding claim, wherein one planar element in each pair of planar elements includes a plurality of alignment reference members and the other planar element in the pair of planar elements includes a plurality of monitoring apertures, wherein the plurality of monitoring apertures are aligned with corresponding alignment reference members in a direction substantially perpendicular to the plane of the planar elements.

10. A stack according to any preceding claim, comprising a spacer located between the planar elements of at least one pair of planar elements, wherein the spacer comprises a central aperture through which charged particles pass along the beam path.

11. A stack according to any preceding claim, wherein each alignment datum comprises a plurality of marks which are remote from one another in the plane of the planar element.

12. The stack of claim 11, wherein at least some of the plurality of markings are arranged in a direction parallel to the plane of the planar element.

13. A stack according to claim 11 or 12, wherein at least some of the plurality of markings are arranged in different directions parallel to the plane of the planar element.

14. A stack according to claim 11 or 12, wherein each alignment datum comprises a plurality of further marks which are spaced apart from each other in another direction parallel to the plane of the planar element such that the plurality of marks are arranged in a vertical direction.

15. A charged particle optical device for directing a charged particle beam onto a sample location, the charged particle optical device comprising a stack according to any one of claims 1 to 14.

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