Alignment of electron 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 pairs of adjacent planar elements arranged across the beam path, one of the planar elements including an alignment fiducial and the other of the planar elements including a monitoring aperture, the pair of planar elements positioned relative to each other such that the alignment fiducial and the monitoring aperture are aligned with each other in a direction substantially perpendicular to the plane of the planar elements.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to European Patent Application No. 22200582.9, filed October 10, 2022, the entire contents of which are incorporated herein by reference.

[0002]

[0002] The embodiments provided in this specification generally relate to methods for aligning charged particle optical elements, methods for fabricating charged particle optical modules, stacks of charged particle optical elements, charged particle optical modules, charged particle optical devices, charged particle optical apparatuses, and alignment apparatuses. [Background technology]

[0003]

[0003] During the manufacture of semiconductor integrated circuit (IC) chips, unwanted pattern defects can occur on substrates (e.g., wafers) or masks during the assembly process, thereby reducing yield. Defects can arise, for example, as a result of optical effects and accidental particles or other processing steps, such as etching, chemical deposition, and mechanical polishing. Therefore, monitoring the extent of unwanted pattern defects is an important process in the manufacture of IC chips. More generally, evaluation (e.g., inspection and / or measurement) of substrate surfaces or other objects / materials is an important process during and / or after their manufacture.

[0004]

[0004] Pattern evaluation tools using 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 for a final deceleration step to land on the target with a relatively low landing energy. The electron beam is focused on the target as a probe spot. Interaction of the landing electrons from the electron beam with the material structure at the probe spot causes electrons to be emitted from the surface, such as secondary electrons, backscattered electrons, or Auger electrons, which may collectively be referred to as signal electrons or, more generally, signal particles. The generated secondary electrons may be emitted from the material structure of the target.

[0005]

[0005] By scanning primary electrons as a probe spot across the target surface, secondary electrons can be emitted across the target's surface. By collecting these emitted secondary electrons from the target's viewing surface, an evaluation tool (or device) can obtain a signal, such as an image, that represents the characteristics of the material structure of the target's surface. In such evaluation, the collected secondary electrons are detected by a detector within the device. The detector generates a signal in response to an incident particle. As an area of ​​the sample is evaluated, the signal contains data, and the data is processed to generate an evaluation image corresponding to the evaluated area of ​​the sample. The image may contain pixels. Each pixel may correspond to a portion of the evaluated area. Typically, electron beam evaluation devices have a single beam and may be called single-beam SEMs. Attempts have been made to introduce multi-electron beam evaluation in devices (or "multi-beam tools") that may be called multi-beam SEMs (MBSEMs).

[0006] Another application of electro-optical devices (or devices or columns) is lithography. A charged particle beam reacts with a resist layer on the surface of a substrate. A desired pattern in the resist can be created by controlling the locations on the resist layer where the charged particle beam is directed.

[0007]

[0007] An electro-optical device can be an apparatus that generates, illuminates, projects, and / or detects one or more beams of charged particles. The beam path of the charged particles is controlled by electromagnetic fields (i.e., electrostatic and magnetic fields). Stray electromagnetic fields can undesirably deflect the beam. Summary of the Invention [Problem to be solved by the invention]

[0008]

[0008] In some electro-optical devices, multiple electro-optical elements may be stacked relative to one another. For example, in some electro-optical devices, an electrostatic field is typically generated between two electrodes corresponding to two electro-optical elements. There is a need to precisely align the electro-optical elements within the stack. [Means for solving the problem]

[0009]

[0009] The present invention provides an architecture suitable for enabling alignment of charged particle optical elements to be verified. According to a first aspect of the present invention, there is provided a stack of planar elements of a charged particle optics module configured to project charged particles along a beam path, the stack including pairs of adjacent planar elements disposed across the beam path, one of the planar elements including an alignment fiducial and the other of the planar elements including a monitoring aperture, the pairs of planar elements positioned relative to each other such that the alignment fiducial and the monitoring aperture are aligned with each other in a direction substantially perpendicular to the planes of the planar elements.

[0010]

[0010] According to a second aspect of the present invention, there is provided a method for aligning a planar element of a charged particle optical module configured to project charged particles along a beam path, the method comprising: providing a first planar element including a first alignment fiducial; providing a second planar element including a first monitoring aperture stacked on the first planar element; inspecting the first alignment fiducial using inspection light passing through the first monitoring aperture; detecting the inspection light reflected from the first planar element; and aligning the second planar element to the first planar element based on the detected inspection light.

[0011]

[0011] Advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings which set forth, by way of illustration and example, specific embodiments of the invention.

[0012]

[0012] The above and other aspects of the present disclosure will become more apparent from the description of exemplary embodiments taken in conjunction with the accompanying drawings. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic diagram illustrating an exemplary electron beam evaluation apparatus. [Figure 2]

[0014] 2 is a schematic diagram illustrating an exemplary multi-beam electron optical device that is part of the exemplary electron beam evaluation apparatus of FIG. 1. [Figure 3]

[0015] 2 is a schematic diagram of an exemplary electron-optical device including a collimator element array and a scanning deflector array that are part of the exemplary electron beam evaluation apparatus of FIG. 1. [Figure 4]

[0016] 4 is a schematic diagram of an exemplary electro-optical device array including the electro-optical device of FIG. 3. [Figure 5]

[0017] 2 is a schematic diagram of an alternative exemplary electron-optical device that is part of the exemplary electron beam evaluation apparatus of FIG. 1. [Figure 6]

[0018] FIG. 6 is a schematic diagram of an exemplary electro-optical assembly that is part of the electro-optical device of FIGS. 3, 4, and 5. [Figure 7]

[0019] FIG. 6 is a schematic diagram of an exemplary electro-optical assembly that is part of the electro-optical device of FIGS. 3, 4, and 5. [Figure 8]

[0020] FIG. 6 is a schematic diagram of an exemplary electro-optical assembly that is part of the electro-optical device of FIGS. 3, 4, and 5. [Figure 9]

[0021] FIG. 6 is a schematic diagram of an exemplary electro-optical assembly that is part of the electro-optical device of FIGS. 3, 4, and 5. [Figure 10]

[0022] FIG. 1 is a schematic diagram of an alignment fiducial. [Figure 11]

[0023] FIG. 1 is a schematic diagram of alignment fiducials on either side of the beam area. DETAILED DESCRIPTION OF THE INVENTION

[0014]

[0024] Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings. The following description refers to the accompanying drawings, in which like numbers in different drawings represent the same or similar elements unless otherwise indicated. The implementations described in the following description of exemplary embodiments do not represent all implementations consistent with the present invention. Instead, they are merely examples of apparatus and methods consistent with aspects related to the present invention as set forth in the appended claims.

[0015]

[0025] Reducing the physical size of devices and increasing the computing power of electronic devices can be achieved by significantly increasing the packing density of circuit components such as transistors, capacitors, and diodes on IC chips. This has been made possible by increasing resolution, allowing for the creation of smaller structures. The fabrication of semiconductor ICs is a complex and time-consuming process, involving hundreds of individual steps. An error at any step in the IC chip manufacturing process has the potential to adversely affect the functionality of the final product. A single defect can cause device failure. It is desirable to improve the overall yield of a process. For example, to achieve a 75% yield in a 50-step process (steps can refer to the number of layers formed on a wafer), each individual step must have a 99.4% yield. If the yield of an individual step is 95%, the overall process yield will be as low as 7–8%.

[0016]

[0026] It is also desirable to maintain high substrate (i.e., wafer) throughput, defined as the number of substrates processed per hour. High process yields and high substrate throughput can be affected by the presence of defects. This is especially true when operator intervention is required to examine the defects. To maintain high yields and low costs of IC chips, high-throughput detection and identification of macroscale and nanoscale defects by characterization systems (e.g., scanning electron microscopes ("SEMs")) is desirable.

[0017]

[0027] A scanning electron microscope includes a scanning device and a detector. The scanning device includes an illumination device, including an electron source, that generates primary electrons, and a projection device that scans a target, such as a substrate, with one or more focused beams of primary electrons. The primary electrons interact with the target and generate interaction products, such as signal particles, e.g., secondary electrons and / or backscattered electrons. Secondary electrons can be considered to have energies up to 50 eV. Backscattered electrons have an energy spectrum ranging from near zero to the maximum energy of charged particle devices, but are conventionally set to electrons (or signal electrons) with energies above 50 eV. A detector captures signal particles (e.g., secondary electrons and / or backscattered electrons) from the target as it is scanned, allowing the scanning electron microscope to create an image of the scanned area of ​​the target. Evaluation device designs that embody these scanning electron microscope functions can have a single beam. To achieve higher throughput for evaluation, some device designs use multiple focused beams of primary electrons, i.e., multibeams. Component beams of a multibeam can be called subbeams or beamlets. The multiple beams may scan different portions of the target simultaneously, and thus a multi-beam evaluation device may evaluate a target much faster, for example, by moving the target faster than a single-beam evaluation device.

[0018]

[0028] In a multi-beam evaluation system, some paths of the primary electron beam are displaced away from the central axis of the scanning device, i.e., the midpoint of the primary electron optical axis (also referred to herein as the charged particle axis). To ensure that all electron beams reach the sample surface at approximately the same angle of incidence, sub-beam paths at greater radial distances from the central axis must be manipulated to travel through larger angles than sub-beam paths closer to the central axis. This stronger manipulation can result in aberrations that smear and blur the resulting image. One example is spherical aberration, which causes the focus of each sub-beam path to be at a different focal plane. In particular, for sub-beam paths that are not on the central axis, the change in focal plane of the sub-beam increases with radial displacement from the central axis. Such aberrations and defocusing effects can remain associated with signal particles (e.g., secondary electrons) from the target when the signal particles are detected; for example, the shape and size of the spot formed by the sub-beam on the target are affected. Therefore, such aberrations degrade the quality of the resulting image produced during evaluation.

[0019]

[0029] An embodiment of a known multi-beam evaluation device is described below.

[0020]

[0030] The figures are schematic. Accordingly, relative dimensions of components in the figures are exaggerated for clarity. Within the following description of the figures, the same or similar reference numbers refer to the same or similar components or entities, and only differences relative to individual embodiments are described. While the description and figures are directed to electron-optical systems, it is understood that the embodiments are not used to limit the present disclosure to specific charged particles. Thus, throughout this document, references to electrons and items referenced with reference to electrons can be considered as more general references to charged particles and items referenced with reference to charged particles, which are not necessarily electrons.

[0021]

[0031] Reference is now made to FIG. 1 , which 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. The evaluation apparatus 100 of FIG. 1 includes a vacuum chamber 10, a load lock chamber 20, an electron-optical device 40 (also known as an electron beam device or e-beam device), a front-end equipment module (EFEM) 30, and a controller 50. The electron-optical device 40 may be within the vacuum chamber 10. The evaluation apparatus 100 may include a motorized or actuable stage.

[0022]

[0032] The EFEM 30 includes a first load port 30a and a second load port 30b. The EFEM 30 may include additional load ports. The first load port 30a and the second load port 30b may receive, for example, a substrate front-opening integrated pod (FOUP) containing a substrate (e.g., a semiconductor substrate or a substrate made of other material) or a target to be evaluated (hereinafter, substrates, wafers, and samples are collectively referred to as "targets"). One or more robotic arms (not shown) in the EFEM 30 transfer the target to the load lock chamber 20.

[0023]

[0033] The load lock chamber 20 is used to remove gas from around the target. The load lock chamber 20 may be connected to a load lock vacuum pumping system (not shown) that removes gas particles from within the load lock chamber 20. Operation of the load lock vacuum pumping system allows the load lock chamber to reach a first pressure below atmospheric pressure. The main chamber 10 is connected to a main chamber vacuum pumping system (not shown). The main chamber vacuum pumping system removes gas molecules from within the main chamber 10 so that the pressure around the target reaches a second pressure that is lower than the first pressure. After the second pressure is reached, the target can be transferred to and evaluated by the electron-optical device 40. The electron-optical device 40 may be configured to project either a single beam or multiple beams.

[0024]

[0034] The controller 50 is electronically connected to the electron-optical device 40. The controller 50 may be a processor (e.g., 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. While the controller 50 is shown in FIG. 1 as being external to the structure including the main chamber 10, the load lock chamber 20, and the EFEM 30, it is understood that the controller 50 may be part of the structure. The controller 50 may be located in one of the component elements of the charged particle beam evaluation apparatus or distributed across at least two of the component elements. While the present disclosure provides an example of a main chamber 10 housing an electron beam evaluation apparatus, it should be noted that aspects of the present disclosure are not limited, in the broadest sense, to chambers housing electron-optical devices. Rather, it is understood that the principles described above may also be applied to other apparatus and apparatuses of other configurations operating at a second pressure.

[0025]

[0035] Reference is now made to FIG. 2, which is a schematic diagram of an exemplary multi-beam electron-optical device 40 of an evaluation apparatus, e.g., evaluation apparatus 100 of FIG. 1. In an alternative embodiment, evaluation apparatus 100 is a single-beam evaluation apparatus. Electron-optical device 40 may include an electron source 201, a beam former array 372 (also known as a gun aperture plate, a Coulomb aperture array, or a pre-sub-beam forming aperture array), a condenser lens 310, a source converter (or micro-optic array) 320, an objective lens 331, and a target 308. In one embodiment, condenser lens 310 is magnetic. (A single-beam evaluation apparatus may have the same features as a multi-beam evaluation apparatus, except that the electron-optical components with array apertures 372, 320 may have a single aperture. The source converter 320 may be replaced with several electron-optical components along the beam path.) The target 308 may be supported by a support on a stage. The stage may 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, and the condenser lens 310 may be components of an illumination system included in the electron-optical device 40. The source converter 320 (also known as a source conversion unit), which is described in more detail below, and the objective lens 331 may be components of a projection system included in the electron-optical device 40.

[0026]

[0036] The electron source 201, beam former array 372, condenser lens 310, source converter 320, and objective lens 331 are aligned with a primary electron optical axis 304 of the electron-optical device 40. The electron source 201 generates a primary beam 302 generally along the electron optical axis 304 with a source crossover (virtual or real) 301S. In operation, the electron source 201 is configured to emit electrons. The electrons are extracted or accelerated by an extractor and / or an anode to form the primary beam 302.

[0027]

[0037] The beam former array 372 cuts peripheral electrons from the primary electron beam 302 to reduce the resulting Coulomb effect. The primary electron beam 302 can be trimmed by the beam former array 372 into a specified number of sub-beams, such as three sub-beams 311, 312, and 313. It should be understood that the description is intended to apply to an electron-optical device 40 having any number of sub-beams, such as one, two, or more than two. In operation, the beam former array 372 is configured to block peripheral electrons to reduce the Coulomb effect. The Coulomb effect can increase the size of each of the probe spots 391, 392, and 393, thus reducing the evaluation resolution. The beam former array 372 reduces aberrations resulting from Coulomb interactions between electrons projected in the beam. The beam former array 372 can include multiple apertures to generate primary sub-beams, even before the source converter 320.

[0028]

[0038] The source converter 320 is configured to convert the beams (including sub-beams, if any) transmitted by the beamformer array 372 into sub-beams that are projected toward the target 308. In one embodiment, the source converter is monolithic. Alternatively, the term source converter may simply be used as a collective term for the components that form beamlets from the sub-beams.

[0029]

[0039] As shown in FIG. 2 , 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 formation) configured to define the contours of beamlets (or sub-beams) 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 on the up-beam side of the main device. In one embodiment, the beam-limiting aperture array 321 splits one or more of the sub-beams 311, 312, 313 into beamlets such that the number of beamlets projected toward the target 308 is greater than the number of sub-beams sent through the beam former array 372. In an alternative embodiment, the beam-limiting aperture array 321 maintains the number of sub-beams incident on the beam-limiting aperture array 321, in which case the number of sub-beams may be equal to the number of beamlets projected toward the target 308.

[0030]

[0040] 2 , in one embodiment, the electro-optical device 40 includes a pre-bend deflector array 323 having pre-bend deflectors 323_1, 323_2, and 323_3 for bending the sub-beams 311, 312, and 313, respectively. The pre-bend deflectors 323_1, 323_2, and 323_3 may bend the paths of the sub-beams 311, 312, and 313 to the beam-limiting aperture array 321.

[0031]

[0041] 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 is a respective deflector 322_1, 322_2, and 322_3 associated with each beamlet path. The deflectors 322_1, 322_2, and 322_3 are configured to deflect the beamlet path toward the electron-optical axis 304. The deflected beamlets form virtual images (not shown) of the source crossover 301S. In this embodiment, these virtual images are projected onto the target 308 by the objective lens 331, forming probe spots 391, 392, and 393 on the target. The electron-optical device 40 may also include an aberration compensator array 324 configured to compensate for aberrations that may be present in each sub-beam. In one embodiment, the aberration compensator array 324 includes a lens configured to operate on each beamlet. The lens may take the form of a lens or an array. The lenses in the array can operate on different beamlets of the multi-beam. The aberration compensator array 324 can include, for example, a field curvature compensator array (not shown) having, for example, microlenses. The field curvature compensators and microlenses can be configured, for example, to compensate individual sub-beams for field curvature aberrations apparent at the probe spots 391, 392, and 393. The aberration compensator array 324 can include an astigmatism compensator array (not shown) having microastigmatism correctors. The microastigmatism correctors can be controlled to operate on the sub-beams to compensate for astigmatism that would otherwise be present at the probe spots 391, 392, and 393.

[0032]

[0042] The source converter 320 may be an electron-optical assembly including the stack 700 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 that bend sub-beams 311, 312, and 313, respectively. The pre-bend deflectors 323_1, 323_2, and 323_3 may bend the paths of the sub-beams to the beam-limiting aperture array 321. In one embodiment, the pre-bend micro-deflector array 323 may be configured to bend the sub-beam paths of the sub-beams orthogonal to the plane of the beam-limiting aperture array 321. In an alternative embodiment, the condenser lens 310 may adjust the path directions of the sub-beams to the beam-limiting aperture array 321. The condenser lens 310 may, for example, focus (collimate) the three sub-beams 311, 312, and 313 so that they are substantially perpendicularly incident on the source converter 320 into substantially parallel beams along the primary electron optical axis 304, and may correspond to the beam-limiting aperture array 321. In such an embodiment, the pre-bending deflector array 323 may not be necessary.

[0033]

[0043] The image forming element array 322, the aberration compensator array 324, and the pre-bending deflector array 323 may include multiple layers of sub-beam steering devices: micro-deflectors, micro-lenses, or micro-astigmatism correctors, some of which may be in the form of arrays. The beam paths may be rotated and steered. The rotational correction may be applied by magnetic lenses. Additionally or alternatively, the rotational correction may be achieved by existing magnetic lenses, such as in a focusing lens arrangement.

[0034]

[0044] In the present example electron-optical device 40, the beamlets are deflected toward the electronic optical axis 304 by deflectors 322_1, 322_2, and 322_3 of the array of imaging elements 322, respectively. It should be understood that the beamlet paths may already correspond to the electronic optical axis 304 before reaching the deflectors 322_1, 322_2, and 322_3.

[0035]

[0045] The objective lens 331 focuses the beamlets onto the surface of the target 308, i.e., projects three virtual images onto the target surface. The three images formed on the target surface by the sub-beams 311-313 form three probe spots 391, 392, and 393 on the target surface. In one embodiment, the deflection angles of the sub-beams 311-313 are adjusted to pass through or approach the front focus of the objective lens 331, reducing or limiting off-axis aberrations of the three probe spots 391-393. In one configuration, the objective lens 331 is a magnetic lens. Although three beamlets are mentioned, this is by way of example only. Any number of beamlets may be present.

[0036]

[0046] The manipulators are configured to manipulate one or more beams of charged particles. The term manipulator encompasses deflectors, lenses, and apertures. The pre-bending deflector array 323, the aberration compensator array 324, and the imaging element array 322 may be referred to individually or in combination with each other as manipulator arrays because they manipulate one or more sub-beams or beamlets of charged particles. The lenses and deflectors 322_1, 322_2, and 322_3 may be referred to as manipulators because they manipulate one or more sub-beams or beamlets of charged particles.

[0037]

[0047] In one embodiment, a beam separator (not shown) is provided. The beam separator may be on the down-beam side of the source converter 320. The beam separator may be, for example, a Wien filter including an electrostatic dipole field and a magnetic dipole field. The beam separator may be on the up-beam side of the objective lens 331. The beam separator may be located between adjacent sections of a shield in the direction of the beam path. The inner surface of the shield may be radially inside the beam separator. Alternatively, the beam separator may be within the shield. In operation, the beam separator may be configured to exert an electrostatic force on individual electrons of the sub-beam due to the electrostatic dipole field. In one embodiment, the electrostatic force is of the same magnitude but opposite direction as the magnetic force exerted by the magnetic dipole field of the beam separator on individual primary electrons of the sub-beam. Thus, the sub-beams may pass through the beam separator in at least a substantially straight line with at least substantially zero deflection angle. The direction of the magnetic force depends on the direction of electron motion, but the direction of the electrostatic force does not depend on the direction of electron motion. Thus, because secondary and backscattered electrons (or signal electrons) generally move in the opposite direction compared to primary electrons, the magnetic force exerted on the secondary and backscattered electrons (or signal particles) no longer cancels the electrostatic force, and as a result, the secondary and backscattered electrons moving through the beam separator are not deflected away from the electron optical axis 304.

[0038]

[0048] In one embodiment, a secondary device (not shown) is provided that includes a detector element for detecting a corresponding secondary charged particle beam. When the secondary beam is incident on the detector element, the element can generate a corresponding intensity signal output. The output can be directed to an image processing system (e.g., controller 50). Each detector element can include an array that can be in the form of a grid. The array can have one or more pixels, and each pixel can correspond to an element of the array. The intensity signal output of a detector element can be the sum of signals generated by all pixels in that detector element.

[0039]

[0049] In one embodiment, a secondary projection apparatus and its associated electron detection device (not shown) are provided. The secondary projection apparatus and its associated electron detection device may be aligned with a secondary electron optical axis of the secondary device. In one embodiment, a beam separator is positioned to deflect the path of the secondary electron beam toward the secondary projection apparatus. The secondary projection apparatus subsequently focuses the path of the secondary electron beam onto multiple detection regions of the electron detection device. The secondary projection apparatus and its associated electron detection device may register the secondary electrons or backscattered electrons (or signal particles) and use the secondary electrons or backscattered electrons (or signal particles) to generate an image of the target 308.

[0040]

[0050] Such a Wien filter, secondary device, and / or secondary projection device may be provided in a single-beam evaluation system. Additionally and / or alternatively, for example, a detection device facing the sample during operation may be present on the down-beam side of the objective lens. In an alternative arrangement, the detector device is located along the path of the charged particle beam toward the sample. Such an arrangement does not have a Wien filter, secondary device, or secondary projection device. The detection device may be located at one or more positions along the path of the charged particle beam toward the sample, for example, around the path of the charged particle beam and facing the sample during operation. Such a detector device may have an aperture and may be annular. Different detector devices may be located along the path of the charged particle beam to detect signal particles with different characteristics. The electron optical elements along the path of the charged particle beam may include one or more electrostatic plates with apertures for the charged particle beam, which may be positioned and controlled to focus each signal particle with a different characteristic onto a respective detector device at a different position along the path of the charged particle beam. Such electrostatic plates may be arranged in series with two or more adjacent plates along the path of the charged particle beam.

[0041]

[0051] In one embodiment, the assessment device 100 includes a single source.

[0042]

[0052] Any element or group of elements may be replaceable or field replaceable within the electro-optical device. One or more electro-optical components within the electro-optical device, particularly components that operate on or generate sub-beams, such as the aperture array and the manipulator array, may include one or more microelectromechanical systems (MEMS). The pre-bending deflector array 323 may be a MEMS. MEMS are miniaturized mechanical and electromechanical elements made using microfabrication techniques. In one embodiment, the electro-optical device 40 includes apertures, lenses, and deflectors formed as MEMS. In one embodiment, the lenses and manipulators, such as the deflectors 322_1, 322_2, and 322_3, may be passively, actively, as an entire array, individually, or in groups within the array to control the beamlets of charged particles projected toward the target 308.

[0043]

[0053] In one embodiment, the electron-optical device 40 may include alternative and / or additional components in the charged particle path, such as lenses and other components, some of which are described above with reference to FIGS. 1 and 2. Examples of such arrangements are shown in FIGS. 3 and 4, which are described in more detail below. In particular, an embodiment includes the electron-optical device 40 splitting a charged particle beam from a source into multiple sub-beams. Multiple respective objective lenses may project the sub-beams onto a sample. In some embodiments, multiple focusing lenses are provided up-beam from the objective lens. The focusing lenses focus each of the sub-beams to an intermediate focus upstream of the objective lens. In some embodiments, a collimator is provided upstream of the objective lens. Correctors may be provided to reduce focusing errors and / or aberrations. In some embodiments, such correctors are integrated into the objective lens or located directly adjacent to the objective lens. If a collecting lens is provided, additionally or alternatively, such a corrector may be integrated into or located immediately adjacent to the collecting lens and / or located at or immediately adjacent to the intermediate focus. A detector is provided for detecting charged particles emitted by the sample. The detector may be integrated into the objective lens. The detector may be on the bottom surface of the objective lens so as to face the sample in use. The detectors may form an array that may correspond to the array of beamlets in the multi-beam arrangement. Detectors in the detector array may generate detection signals that may be associated with pixels of the generated image. The collecting lens, objective lens, and / or detector may be formed as MEMS and / or CMOS devices, for example as CMOS devices fabricated using MEMS processing.

[0044]

[0054] FIG. 3 is a schematic diagram of an exemplary electro-optical device 40 of another design. The electro-optical device 40 may include a source 201 and one or more electro-optical assemblies. Alternatively, the evaluation apparatus 100 including the electro-optical device 40 may include the source 201. The electro-optical device 40 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 at the sample 208 are derived from the beam provided by the source 201. Sub-beams may be derived from the beam using, for example, a beam limiter defining an array of beam-limiting apertures. The source 201 is preferably a high-brightness thermal field emitter with a good compromise between brightness and total emission current.

[0045]

[0055] The upper beam limiter 252 defines an array of beam-limiting apertures. The upper beam limiter 252 may be referred to as an upper beam-limiting aperture array or an up-beam beam-limiting aperture array. The upper beam limiter 252 may include a plate (which may be a plate-like body) having multiple apertures. The upper beam limiter 252 forms sub-beams from the beam of charged particles emitted by the source 201. Portions of the beam other than those contributing to the formation of the sub-beams may be blocked (e.g., absorbed) by the upper beam limiter 252 so as not to interfere with the sub-beams on the down-beam side. The upper beam limiter 252 may be referred to as a sub-beam-defining aperture array.

[0046]

[0056] A collimator element array 271 is provided on the down-beam side of the upper beam limiter. Each collimator element collimates a respective sub-beam. The collimator element array 271 can be spatially compact and can be achieved using MEMS fabrication techniques. In some embodiments illustrated in FIG. 3, the collimator element array 271 is the first deflection or focusing electron-optic array element in the beam path on the down-beam side of the source 201. In another arrangement, the collimator can take the form of a macro-collimator, in whole or in part. Such a macro-collimator can be on the up-beam side of the upper beam limiter 252, thereby operating on the beam from the source before generating the multiple beams. A magnetic lens can be used as the macro-collimator.

[0047]

[0057] On the down-beam side of the collimator element array is a control lens array 250. The control lens array 250 includes a plurality of control lenses. Each control lens includes at least two electrodes (e.g., two or three electrodes) connected to respective potential sources. The control lens array 250 may include two or more (e.g., three) plate electrode arrays connected to respective potential sources. The control lens array 250 is associated with the objective lens array 241 (e.g., two arrays located near each other and / or mechanically connected to each other and / or controlled together as a unit). The control lens array 250 is located on the up-beam side of the objective lens array 241. The control lenses pre-focus the sub-beams (e.g., apply a focusing action to the sub-beams before they reach the objective lens array 241). Pre-focusing may reduce the divergence of the sub-beams or increase the convergence rate of the sub-beams. Although control lens array 241 is indistinguishable from and can be part of objective lens array 250, for purposes of this description control lens array 250 is considered to be separate and distinct from objective lens array 241.

[0048]

[0058] As noted, the control lens array 250 is associated with the objective lens array 241. As discussed above, the control lens array 250 may 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 for additional degrees of freedom for controlling the electro-optical parameters of the sub-beams. In one embodiment, the control lens array 250 may be considered to be additional electrodes of the objective lens array 241 that enable additional functionality for each objective lens of the objective lens array 241. In one arrangement, such electrodes may be considered part of the objective lens array that provides additional functionality to the objective lenses of the objective lens array 241. In such an arrangement, the control lenses are considered part of the corresponding objective lenses, even to the extent that the control lenses are only referred to as being part of the objective lenses, for example, with respect to providing one or more additional degrees of freedom to the objective lenses.

[0049]

[0059] For ease of explanation, lens arrays are generally represented herein by an array of ellipses. Each ellipse represents one of the lenses in the lens array. The ellipse is conventionally used to represent a lens, analogous to the biconvex shape often adopted by optical lenses. However, it will be understood that in the context of charged particle arrangements such as those discussed herein, lens arrays typically operate electrostatically and therefore may not require any physical elements adopting a biconvex shape. As mentioned above, lens arrays may instead include multiple plates having apertures.

[0050]

[0060] A scan deflector array 260 including a plurality of scan deflectors may be provided. The scan deflector array 260 may be formed using MEMS fabrication techniques. Each scan deflector scans a respective sub-beam across the sample 208. Thus, the scan deflector array 260 may include a scan deflector for each sub-beam. Each scan deflector may deflect the sub-beam in one direction (e.g., parallel to one axis, such as the X-axis) or two directions (e.g., about two non-parallel axes, such as the X-axis and the Y-axis). The deflection may be such that the sub-beam scans across the sample 208 in one or two directions (i.e., one or two dimensions). In one embodiment, the scan deflector array 260 may be implemented using the scan deflectors described in EP 2 425 444, which is incorporated herein by reference generally, and in particular with respect to the scan deflectors. The scanning deflector array 260 (e.g., formed using MEMS fabrication techniques as mentioned above) may be more spatially compact than a macro scanning deflector. In another arrangement, a macro scanning deflector may be used on the upbeam side of the upper beam limiter 252. Its function may be similar or equivalent to the scanning deflector array, but it operates on the beam from the source before the beamlets of the multi-beam are generated.

[0051]

[0061] An objective lens array 241 including multiple objective lenses is provided to direct the sub-beams toward the sample 208. Each objective lens includes at least two electrodes (e.g., two or three electrodes) connected to respective potential sources. The objective lens array 241 may include two or more (e.g., three) plate electrode arrays connected to respective potential sources. Each objective lens formed by the plate electrodes may be a microlens operating on a different sub-beam. Each plate defines multiple apertures (sometimes called holes). The position of each aperture in a plate corresponds to the position of a corresponding aperture (or apertures) in another plate (or plates). The corresponding apertures define an objective lens, and therefore, each set of corresponding apertures operates on the same sub-beam in the multi-beam during use. Each objective lens projects a respective sub-beam of the multi-beam onto the sample 208.

[0052]

[0062] An objective lens array 241 with only two electrodes can have lower aberrations than an objective lens array 241 with more electrodes. A three-electrode objective lens can have a larger potential difference between the electrodes, thereby enabling a more powerful lens. Additional electrodes (i.e., three or more electrodes) provide additional degrees of freedom to control electron trajectories, for example, to focus secondary electrons and the incident beam. Such additional electrodes can be considered to form a control lens array 250. A benefit of a two-electrode lens over an Einzel lens is that the energy of the input beam does not necessarily need to be the same as the output beam. Advantageously, the potential difference across such a two-electrode lens array allows it to function as either an acceleration or deceleration lens array.

[0053]

[0063] The objective lens array, together with any or all of the scanning deflector array 260, the control lens array 250, and the collimator element array 271, may form part of an objective lens array assembly. The objective lens array assembly may further include a beam-shaping limiter 242. The beam-shaping limiter 242 defines an array of beam-limiting apertures. The beam-shaping limiter 242 may be referred to as a 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 multiple apertures. The beam-shaping limiter 242 is located on the down-beam side from at least one electrode (optionally from all electrodes) of the control lens array 250. In some embodiments, the beam-shaping limiter 242 is located on the down-beam side from at least one electrode (optionally from all electrodes) of the objective lens array 241.

[0054]

[0064] In one configuration, 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 is such that a portion of the sub-beam from the corresponding objective lens can 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 apertures of at least one of the objective lens array 242, 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, transmitting only a selected portion of the sub-beam incident on the beam-shaping limiter 242 through the beam-limiting aperture. The selected portion may be such that only the portion of each sub-beam that passes through the central portion of the respective aperture in the objective lens array reaches the sample. The central portion may have a circular cross section and / or may be centered on the beam axis of the sub-beam.

[0055]

[0065] In one embodiment, the electro-optical device 40 is configured to control the objective lens array assembly (e.g., by controlling the potentials applied to the electrodes of the control lens array 250) so that the focal length of the control lens is greater than the separation between the control lens array 250 and the objective lens array 241. Thus, the control lens array 250 and the objective lens array 241 may be positioned relatively close together, with the focusing effect from the control lens array 250 being 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 form a combined focal length relative to the same surface. The combined operation without an intermediate focus may reduce the risk of aberrations. In other embodiments, the objective lens array assembly may be configured to form an intermediate focus between the control lens array 250 and the objective lens array 241.

[0056]

[0066] A power source may be provided to apply respective electrical potentials to the electrodes of the control lenses of the control lens array 250 and the objective lenses of the objective lens array 241 .

[0057]

[0067] Providing the control lens array 250 in addition to the objective lens array 241 provides additional degrees of freedom for controlling the properties of the sub-beams. This additional degree of freedom is provided even when the control lens array 250 and the objective lens array 241 are provided relatively close to each other, for example, 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 divergence angle with respect to beam demagnification and / or to control the beam energy delivered to the objective lens array 241. The control lens array 250 can include two or more electrodes. When there are two electrodes, the demagnification and landing energy are controlled together. When there are three or more electrodes, the demagnification and landing energy can be controlled independently. Note that the electrode closest to the down beam of the control lens array 250 can be the electrode closest to the up beam 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 each lens with a different lens effect, with each lens effect being relative to one of its two opposing surfaces (i.e., the up-beam surface and the down-beam surface). Thus, the control lens can be configured to adjust the demagnification and / or beam divergence angle and / or the landing energy of each sub-beam on the substrate (e.g., by applying appropriate respective potentials to the electrodes of the control lens and the objective lens using a power source). This optimization can be achieved without excessively adversely affecting the number of objective lenses and without excessively worsening the aberrations of the objective lenses (e.g., without reducing the strength of the objective lenses). The use of a control lens array allows the objective lens array to operate at its optimal electric field strength. Note that references to demagnification and divergence angle are intended to refer to variations of the same parameter. In an ideal configuration, the product of various demagnifications and the corresponding divergence angle is constant. However, the divergence angle can be affected by the use of apertures.

[0058]

[0068] In one embodiment, the landing energy can be controlled to a desired value within a predetermined range, for example, between 1000 eV and 5000 eV. Desirably, the landing energy is changed primarily by controlling the energy of the electrons exiting the control lens. The potential difference within the objective lens is preferably kept constant during this change so that the electric field within the objective lens remains as high as possible. Additionally, the potential applied to the control lens can be used to optimize the beam divergence angle and demagnification. The control lens can function to change the demagnification in light of changes in landing energy. Desirably, each control lens includes three electrodes, thereby providing two independent control variables. For example, one electrode can be used to control magnification, while a different electrode can be used to independently control landing energy. Alternatively, each control lens may have only two electrodes. If there are only two electrodes, one of the electrodes may be required to control both magnification and landing energy.

[0059]

[0069] 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 a scanning electron microscope, including secondary electrons and / or backscattered electrons from the sample 208. The detector may be an array serving a surface of the electron-optical device facing the sample 208, for example, the bottom surface of the electron-optical device. Alternatively, the detector array may be on the up-beam side of the bottom surface, or, for example, on the in-beam or up-beam side of an objective lens array or a control lens array. Elements of the detector array may correspond to beamlets in a multi-beam configuration. Signals generated by detection of electrons by elements of the array are sent to a processor for image generation. The signals may correspond to pixels of the image.

[0060]

[0070] In other embodiments, both a macro scan deflector and a scan deflector array 260 are provided. In such configurations, scanning of the sub-beams across the sample surface can be achieved by controlling the macro scan deflector and the scan deflector array 260 together, preferably synchronously.

[0061]

[0071] In one embodiment, an electron-optical device array 500 is provided, as illustrated in FIG. 4 . The array 500 may include a plurality of any of the electron-optical devices described herein. Each of the electron-optical devices simultaneously focuses a respective multibeam onto a different region of the same sample. Each electron-optical device may form sub-beams from a respective charged particle beam from a different source 201. Each respective source 201 may be one source among the plurality of sources 201. At least a subset of the plurality of sources 201 may be provided as a source array. The source array may include multiple sources 201 provided on a common substrate. Simultaneous focusing of multiple multibeams onto different regions 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 positioned closely to each other to project the respective multibeams onto adjacent regions of the sample 208.

[0062]

[0072] Any number of electro-optical devices may be used in array 500. Preferably, the number of electro-optical devices ranges from two, preferably from nine to one hundred, or even two hundred. In one embodiment, the electro-optical devices are arranged in a rectangular array or a hexagonal array. In other embodiments, the electro-optical devices are provided in an irregular array or a regular array having a geometry other than rectangular or hexagonal. Each electro-optical device in array 500 may be configured in any of the ways described herein with reference to a single electro-optical device, for example, as described above with particular reference to the embodiment shown in and described with reference to FIG. 6. Details of such configurations are described in European Patent Application No. 20184161.6, filed July 6, 2020, which is incorporated herein by reference, regarding how objective lenses are incorporated and adapted for multi-device configurations.

[0063]

[0073] In the example of Figure 4, array 500 includes a plurality of electro-optical devices of the type described above with reference to Figure 3. Thus, each of the electro-optical devices in this example includes both a scan deflector array 260 and a collimator element array 271. As previously mentioned, scan deflector array 260 and collimator element array 271 are particularly well suited for incorporation into electro-optical device array 500 due to their spatial compactness, which facilitates positioning of the electro-optical devices near each other. This configuration of electro-optical devices may be preferable over other configurations that use magnetic lenses as collimators. Magnetic lenses may be difficult to incorporate into electro-optical devices intended for use in multi-device configurations (e.g., multi-column configurations) due to, for example, magnetic interference between the columns.

[0064]

[0074] An alternative design of the multi-beam electron-optical device may have the same features as those described with respect to FIG. 3 , except as described below and shown in FIG. 5 . An alternative design of the multi-beam electron-optical device may include a condenser lens array 231 on the up-beam side of the objective lens array arrangement 241, as disclosed in European Patent Application No. 20158804.3, filed February 21, 2020, which is incorporated herein by reference solely for purposes of describing a multi-beam device with a collimator and its components. Because the condenser lens array 231 and associated beam-limiting aperture arrays can shape the beamlets 211, 212, 213 of the multi-beams from the beam of the source 201, such a design does not require the beam-shaping limiter array 242 or the upper beam limiter array 252. The condenser lens's beam-limiting aperture array may also function as an electrode in the lens array.

[0065]

[0075] The paths of beamlets 211, 212, 213 diverge away from the collector lens array 231. The collector lens array 231 focuses the generated beamlets to an intermediate focus between the collector lens array 231 and the objective lens array assembly 241 (i.e., towards the control lens array and the objective lens array). The collimator array 271 may reside at the intermediate focus instead of being associated with the objective lens array assembly 241.

[0066]

[0076] The collimator may reduce the divergence of diverging beamlet paths. The collimator may collimate diverging beamlet paths so that they are substantially parallel toward the objective lens array assembly. The corrector array may be present in a multi-beam path, for example, in conjunction with a focusing lens array, an intermediate focus, and the objective lens array assembly. The detector 240 may be integrated with the objective lens 241. The detector 240 may be on the bottom surface of the objective lens 241 so that it faces the sample during use. For example, the detector 240 may be an array of detector elements, each element for a different beamlet.

[0067]

[0077] In one embodiment of the configuration shown and described with reference to FIG. 5 , the detector may be located in a similar location within the electro-optical device 241 as described and shown with reference to the electro-optical device of FIG. 3 . The detector 250 may be integrated into the objective lens array 241 and the control lens array 240 (if present, not shown in FIG. 5 ). The detector may have two or more detectors at different positions along the paths of the sub-beams of the multi-beam, for example, each array associated with a different electro-optical element, such as electrodes of the objective lens array and / or the control lens array. The associated electro-optical elements, such as the objective lens array 241 and the control lens array 240, may be included in an assembly that may be a monolithic assembly that may be referred to as an electro-optical assembly that includes the stack 700. In one embodiment, the detector 240 is associated with or even integrated with the electro-optical elements of the stack 700. For example, the detector 240 may be at the bottom of the stack 700 that includes the objective lens 241. The detector 240 may be provided with electrical connections 60, as described elsewhere in this document. In one variant, the detector comprises a detector array located on the up-beam side of the objective lens array (optionally also the control lens array 240), for example on the up-beam side of the stack 700. Between the stack 700 and the detector array there may be a Wien filter array, which directs the charged particle beam in a down-beam direction towards the sample and directs signal particles from the sample towards the detector array.

[0068]

[0078] An electro-optical device array may have a plurality of multi-beam devices of this design described with reference to the multi-beam device of Figure 3, as shown in Figure 4. Multiple multi-beam devices may be arranged in an array of multi-beam devices. Such an arrangement is shown and described in European Patent Application No. 20158732.6, filed February 21, 2020, which is incorporated herein by reference with respect to a multi-device configuration of a multi-beam apparatus having the disclosed multi-beam device design with a collimator at an intermediate focus.

[0069]

[0079] A further alternative design of the multi-beam apparatus includes multiple single-beam devices. A single beam generated for purposes of the invention described herein may be similar or equivalent to multiple beams generated by a single device. Each device may have an associated detector. Such a multi-device apparatus may be arranged in an array of 3, 4, 9, 19, 50, 100, or even 200 devices, each generating a single beam or single beamlet (for a single-beam device) or multiple beams (for a multi-beam device). In this further alternative design, the array of devices may have a common vacuum system, each device may have a separate vacuum system, or different vacuum systems may be assigned to groups of devices. Each device may have an associated detector.

[0070]

[0080] The electron-optical device 40 may be a component of an evaluation (e.g., inspection, metrology, or metrology inspection) apparatus or may be part of an electron beam lithography apparatus. Multi-beam charged particle apparatus can be used in several different applications, including not only scanning electron microscopy but also electron microscopy in general and lithography.

[0071]

[0081] The electron optical axis 304 describes the path taken by the charged particles output from the source 201. All sub-beams and beamlets of the multi-beam may be substantially parallel to the electron optical axis 304, at least, for example, through the manipulator or electron optical array of the configuration shown in and described with reference to FIG. 2, except where expressly stated. The electron optical axis 304 may be the same as or different from the mechanical axis of the electron optical device 40. For the configurations shown and described with respect to FIGS. 2-5, the electron optical axis may correspond to the path of the central beam of the multi-beam, e.g., beam 212. The beams of the multi-beam are substantially parallel to each other (e.g., to the electron optical axis 304) between collimation (e.g., the plane of the intermediate focus (e.g., shown in FIG. 5) or the location of the collimator array 271 corresponding to the upper beam limiter 252) and the surface of the sample 208.

[0072]

[0082] 6 to operate on (e.g., manipulate) the electron beamlets. For example, the stack 700 may include one more of the objective lens array 241 and / or the condenser lens array 231 and / or the collimator element array 271 and / or individual beam correctors and / or deflectors and / or Wien filter arrays (a non-limiting list). In particular, the objective lens 331 and / or the condenser lens 310 and / or the control lens 250 may comprise the stack 700.

[0073]

[0083] The electro-optical assembly is 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 can increase as the potential difference increases.

[0074]

[0084] The stack 700 includes multiple planar elements. The planar elements may include or be plates. In one embodiment, one or more of the planar elements are electron-optical elements (e.g., electron-optical elements 61, 62 shown in FIG. 6). For example, an electron-optical element may be or include a plate having a surface to which a voltage is applied, thereby providing a potential difference relative to the surface of another electron-optical element. The potential difference generates an electric field that can manipulate the electron beams. However, it is not necessary for the planar elements to be specifically energized. For example, in one embodiment, one or more of the planar elements are configured to shape or restrict one or more electron beams. For example, the planar elements may include one or more apertures to narrow one or more respective electron beams. This function may not require the planar elements to have a voltage applied to them.

[0075]

[0085] In one embodiment, stack 700 is for an electron-optics module configured to project electrons along a beam path. In the orientation shown in FIG. 6 , the beam path extends substantially vertically from top to bottom. In the following description, planar elements refer to electron-optics elements. However, it should be understood that none of the planar elements need be electron-optics elements, but may be different types of planar elements, such as beam limiters. In one embodiment, the electron-optics elements include one or more of the planar elements.

[0076]

[0086] In one embodiment, at least one of the plates in the stack 700 has a stepped thickness such that the first electron-optical element 61 is thinner in a region corresponding to the array of apertures than in another region of the first electron-optical element 61. At high potential differences, it is advantageous for the plate to have a stepped thickness, e.g., two portions of the plate having different thicknesses, because if the plate were of consistent thickness, e.g., too thin, it would experience higher electrostatic forces that could cause bending. Bending of the plate could adversely affect beam-to-beam uniformity. Therefore, a thick plate advantageously reduces bending. However, if the plate is too thick in the region of the array of apertures, it could cause undesirable electron beamlet deformation. Therefore, a thin plate around the array of apertures is advantageous for reducing electron beamlet deformation. That is, the array of apertures can be defined in a region of the plate that is thinner than the rest of the plate. Thus, a stepped thickness plate reduces the possibility of bending without increasing the possibility of beamlet deformation. In one embodiment, the plate has a uniform thickness, including the area corresponding to the array of apertures.

[0077]

[0087] The exemplary electron-optical assembly shown in FIG. 6 includes a first electron-optical element 61, a second electron-optical element 62, and a spacer (or isolator) 76. While the terms first and second are used to distinguish between the two electron-optical elements 61, 62, either of the elements may be referred to as the first or second electron-optical element, and thus such terms are interchangeable. That is, in different descriptions of the same feature, the second electron-optical element may be the electron-optical element 61 located on the up-beam side of the first electron-optical element, which may be the electron-optical element 62 located downstream of the other. These terms are used only to aid in the description to distinguish between the two electron-optical elements and are not intended to be limiting. The same comments apply to all other ordinal-numbered features herein, unless stated to the contrary. In one embodiment, the first electron-optical element 61 is or includes an array plate. (Note that the term "array plate" is a term used to distinguish that plate from other plates referred to in the description.) The second electron-optical element 62 may be or may include an adjacent plate, i.e., a plate adjacent to the array plate. In the first electron-optical element 61, an array of apertures 711 is defined in the path of the electron beamlets. The number of apertures in the array of apertures may correspond to the number of sub-beams in the multi-beam configuration. In one configuration, there are fewer apertures than sub-beams in the multi-beam, such that a group of sub-beam paths passes through one aperture. For example, the aperture may extend across the multi-beam paths, and the aperture may be a strip or slit. In one configuration, the apertures may be arranged in a grid (or two-dimensional array) such that the groups of beams are arranged in a two-dimensional array of beam groups. A first spacer 76 is disposed between the electron-optical elements to separate them. 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.

[0078]

[0088] In the second electron optical element 62, another array of apertures 721 is defined in the path of the electron beamlets. 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 other aperture array 721 also 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 of the first electron optical element 61 aligns with the midpoint of the second electron optical element.

[0079]

[0089] In one embodiment, the second electron optical element 62 may have a graduated thickness such that the second electron optical element is thinner in areas corresponding to the array of apertures than in other areas of the second electron optical element. (Alternatively, the second electron optical element 62 is substantially planar and / or has a uniform thickness.) Desirably, the array of apertures 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 configuration, 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 fewer or more than the number of apertures in the first electron optical element 61. In one configuration, there is one aperture in the second electron optical element 62 for every path of a sub-beam 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.

[0080]

[0090] 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, and more preferably 500 μm at the plate's thickest point. In one configuration, the down-beam plate (i.e., the plate closest to the sample) may have a thickness of 200 μm to 300 μm at its thickest point. The down-beam plate is preferably 200 μm to 150 μm thick at its thickest point. The up-beam plate (i.e., the plate away from the sample) may have a thickness of up to 500 μm at its thickest point.

[0081]

[0091] A coating may be provided on a surface of the first electron-optical element 61 and / or the second electron-optical element 62. Preferably, a coating is provided on both the first electron-optical element 61 and the second electron-optical element 62. The coating reduces surface charging that may otherwise cause unwanted beam distortion.

[0082]

[0092] The coating is configured to survive electrical breakdown events that may occur between the first electron-optical element 61 and the second electron-optical element 62. Preferably, a low-resistivity coating is provided, more preferably a coating of 0.5 Ω / sq or less. The coating is preferably provided on the surface of the down beam plate. More preferably, the coating is provided between at least one of the electron-optical elements and the first spacer 76. The low-resistivity coating reduces undesirable surface charging of the plate.

[0083]

[0093] The first electro-optical element 61 and / or the second electro-optical element 62 may comprise a material with a low bulk resistivity, preferably 1 Ω·m or less, optionally 0.1 Ω·m or less, optionally 0.01 Ω·m or less, optionally 0.001 Ω·m or less, and optionally 0.0001 Ω·m or less. More preferably, the first electro-optical element 61 and / or the second electro-optical element 62 comprise doped silicon. Plates with low bulk resistivity have the advantage that they are less likely to fail, since discharge currents are supplied / drained through the bulk rather than, for example, through a thin coating layer.

[0084]

[0094] The first electronic optical element 61 includes a first wafer. The first wafer can be etched to create regions with different thicknesses. The first wafer can be etched in areas corresponding to the aperture array so that the first electronic optical element 61 is thinner in those areas. For example, to create the stepped thickness of the plate, the first side of the wafer can be etched or both sides of the wafer can be etched. The etching can be by deep reactive ion etching. Alternatively or additionally, the stepped thickness of the plate can be created by laser drilling or machining.

[0085]

[0095] Alternatively, the first electronic optical element 61 may include a first wafer and a second wafer. The aperture array may be defined on the first wafer. The first wafer may be disposed in contact with the first spacer 76. A second wafer may be disposed on the surface of the first wafer in a region not corresponding to the aperture array, e.g., the region may be spaced apart from the aperture array. The first and second wafers may be bonded by wafer bonding. The thickness of the first electronic optical element 61 in the region corresponding to the aperture array may be the thickness of the first wafer. For example, the thickness of the first electronic optical element 61 in another region radially outward from the aperture array, other than the region of the aperture array, may be the combined thickness of the first wafer and the second wafer. Thus, the first electronic optical element 61 has a stepped thickness between the first wafer and the second wafer.

[0086]

[0096] One of the first electron optical element 61 and the second electron optical element 62 is on the up-beam side of the other. Preferably, one of the first electron optical element 61 and the second electron optical element 62 is more negatively charged than the other electron optical element during operation. Preferably, the up-beam plate has a higher potential than the down-beam plate, e.g., with respect to ground, the source, or the sample. The electron optical assembly may be configured to provide a potential difference of 5 kV or more between the first electron optical element 61 and the second electron optical element 62. Preferably, the potential difference is 10 kV or more. More preferably, the potential difference is 20 kV or more, or less than 30 kV, or even more than 30 kV. In one embodiment, the evaluation apparatus 100 includes a power supply. The power supply may 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 may 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 the multiple electron optical elements. In one embodiment, multiple power supplies are configured to apply known voltages to respective electro-optical elements.

[0087]

[0097] The first spacer 76 is preferably disposed between the first electron optical element 61 and the second electron optical element 62 so that opposing surfaces of the plates are in the same plane as each other. The first spacer 76 has an inner edge 731 facing the path of the beamlets. The first spacer 76 may be planar with a major surface that is in the same plane as the first electron optical element 61 and the second electron optical element 62. The first spacer 76 defines a central aperture 732 in the path of the electron beamlets.

[0088]

[0098] A conductive coating, such as coating 740, may be applied to the first spacer 76. Preferably, a low resistivity coating is provided, more preferably a coating of 0.5 Ω / sq or less. In one embodiment, the major surfaces of the spacer (i.e., the surfaces facing the up beam and the down beam) are provided with a conductive coating. The peripheral edges (i.e., sidewalls) of the spacer may be exposed, i.e., free of a conductive coating. Alternatively, the peripheral edges may be covered with a conductive material, such as metal. In one embodiment, the spacer is completely covered with metal.

[0089]

[0099] The coating is preferably on the surface of the spacer facing the negatively charged plate, which is preferably negatively charged relative to the other plate. The down beam plate is preferably negatively charged relative to the up beam plate. The coating is to be 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 there is an electrostatic field across any possible gap between the first spacer 76 and the negatively charged plate.

[0090]

[0100] The stack 700 may include or be a lens assembly for manipulating electron beamlets. The lens assembly may be, for example, or part of, an objective lens assembly or a condenser lens assembly. A lens assembly, such as an objective lens assembly, may further include an additional lens array including at least two plates, such as the control lens array 250.

[0091]

[0101] 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 a stack 700 as shown in any of Figures 6-8 for operating on (e.g., manipulating) electron beamlets. In one embodiment, the stack 700 includes a plurality of electron-optical elements 61-64. The electron-optical elements may have the features described above in connection with Figure 6.

[0092]

[0102] For example, as shown in Figure 6, in one embodiment the stack includes a first electro-optical element 61 and a second electro-optical element 62. As shown in Figure 7, in one embodiment the stack includes further electro-optical elements, such as a third electro-optical element 63 and a fourth electro-optical element 64.

[0093]

[0103] It is not necessary that there are four electron optical elements 61, 64 as shown in FIG. 7. For example, as shown in FIG. 6, the stack may include only two electron optical elements 61, 62. In alternative configurations, the stack may include three electron optical elements, five electron optical elements, or six or more electron optical elements. In one embodiment, the electron optical elements are or include plates. The plates may be substantially planar. In one embodiment, the electron optical elements are positioned across the electron beam path. The plane of the plates of the electron optical elements may be substantially perpendicular to a direction parallel to the electron beam path.

[0094]

[0104] In one embodiment, the electron optical elements each include an array of apertures. However, it is not necessary for each electron optical element to include an array of apertures. The apertures are for the passage of electron beams. In one embodiment, one or more of the electron optical elements includes a single aperture for the passage of one or more electron beams. In one embodiment, one or more of the electron optical elements includes a detector, for example, for detecting electrons.

[0095]

[0105] As shown in FIG. 7 , in one embodiment, the electro-optic elements 61-64 of the stack are positioned substantially parallel to one another. Alternatively, a predetermined angle may be provided between two or more of the electro-optic elements. In one embodiment, the stack is formed by stacking the electro-optic elements 61-64 relative to one another. The stack may be built incrementally by adding one electro-optic element at a time. For example, in one embodiment, a first electro-optic element 61 is provided. A second electro-optic element 62 may then be stacked relative to the first electro-optic element 61. Subsequently, a third electro-optic element 63 may be stacked relative to the first electro-optic element 61 and the second electro-optic element 62. Subsequently, a fourth electro-optic element 64 may be stacked relative to the first electro-optic element 61, the second electro-optic element 62, and the third electro-optic element 63. As described herein, the order of stacking the plates in the stack desirably relates to efficient alignment between the different electro-optic elements of the stack. This applies to any ordinal terminology herein relating to the assembly or manufacture of a stack, eg, first, second, third, etc.

[0096]

[0106] In one embodiment, spacers are provided between one or more pairs of adjacent electron-optical elements, as shown in Figure 7. The spacers may have features such as those described above in connection with Figure 6.

[0097]

[0107] For example, in one embodiment, a first spacer 76 is disposed between the first electron optical element 61 and the second electron optical element 62. In one embodiment, a second spacer 77 is disposed between the second electron optical element 62 and the third electron optical element 63. In one embodiment, a third spacer 78 is disposed between the third electron optical element 63 and the fourth electron optical element 64. In one embodiment, the spacers are configured to mechanically support pairs 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. A 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 spacers are configured to electrically insulate pairs of adjacent electron optical elements from each other. However, it is not necessary for the spacers to provide electrical insulation. For example, if it is desired that two adjacent electro-optical elements are at the same potential, it may not be necessary to electrically isolate them from each other. In one embodiment, the spacers may be omitted from the stack.

[0098]

[0108] 7 shows a schematic of a stack of planar elements 700. In one embodiment, one or more of the planar elements are electro-optical elements.

[0099]

[0109] The electron optical elements 61 to 64 are arranged across the beam path, and the planes of the electron optical elements 61 to 64 are preferably substantially perpendicular to the beam path.

[0100]

[0110] In one embodiment, the first electron optical element 61 and the second electron optical element 62 constitute a pair of electron optical elements. One of the pair of electron optical elements includes an alignment fiducial 66. An alignment fiducial is sometimes referred to as an alignment mark or a fiducial marker. The fiducial is a reference point for aligning the first electron optical element 61 with another component, such as the second electron optical element 62. The alignment fiducial 66 may include one or more visible lines and / or one or more apertures through the first electron optical element 61. The alignment fiducial 66 may be for verifying stack alignment, particularly for aligning the first electron optical element 61 with another component, such as the second electron optical element 62.

[0101]

[0111] The other electron optical element 62 includes a monitoring aperture 71. The monitoring aperture is sometimes called an alignment aperture. The monitoring aperture is sometimes called a viewport or a port. An alignment fiducial 66 is associated with the monitoring aperture 71. The alignment fiducial 66 is visible through the monitoring aperture 71. As shown in FIG. 7 , in one embodiment, the alignment fiducial 66 is aligned with the monitoring aperture 71. A straight imaginary line connecting the alignment fiducial 66 and the monitoring aperture 71 is substantially perpendicular to the plane of the first electron optical element 61 and the second electron optical element 62. The straight imaginary line is substantially parallel to the electron beam path.

[0102]

[0112] As shown in Figure 7, in one embodiment, the stack includes multiple pairs of adjacent electron-optical elements. Within each pair of electron-optical elements, one electron-optical element has an alignment fiducial and the other electron-optical element has a monitoring aperture. For example, in the configuration shown in Figure 7, the second electron-optical element 62 and the third electron-optical element 63 form an electron-optical element pair. The third electron-optical element 63 and the fourth electron-optical element 64 form an electron-optical element pair.

[0103]

[0113] As shown in FIG. 7 , in one embodiment, the pair of electron-optical elements are positioned (or arranged) relative to each other such that the alignment fiducial 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. In the view shown in FIG. 7 , the planes of the electron-optical elements 61, 62 extend horizontally. The first alignment fiducial 66 and the first monitoring aperture 71 are aligned vertically. Similarly, the second alignment fiducial 67 and the second monitoring aperture 72 are aligned vertically. Similarly, the third alignment fiducial 68 and the third monitoring aperture 73 are aligned vertically. An imaginary line that is straight and connects the first alignment fiducial 66 to the first monitoring aperture 71 is substantially perpendicular to the plane of the electron-optical elements 61, 62.

[0104]

[0114] In one embodiment, the monitoring aperture 71 and alignment fiducial 66 are aligned in at least two degrees of freedom, for example, in at least one of two orthogonal directions in a plane parallel to the pair of electron-optical elements 61, 62, and rotation in that plane.

[0105]

[0115] It is possible for there to be a small offset between the center of an alignment fiducial and the center of the associated monitoring aperture, so that the line connecting the alignment fiducial to the monitoring aperture may be slightly tilted from the normal to the electron-optical element. However, the alignment fiducial and monitoring aperture are sufficiently aligned so that illumination light projected through the monitoring aperture and incident on the alignment fiducial can be directly reflected back through the monitoring aperture.

[0106]

[0116] As shown in FIG. 6 , one or more of the electron optical elements 61-64 include aperture arrays 711, 721. The apertures are for passing the respective electron beams. In one embodiment, the apertures have a smaller dimension than the monitoring apertures in a direction parallel to the plane of the electron optical elements. In one embodiment, the apertures of aperture arrays 711, 721 have diameters ranging from about 5 μm to about 100 μm, optionally from about 10 μm to about 50 μm. In one embodiment, the monitoring apertures have diameters ranging from about 100 μm to about 1,000 μm, optionally from about 300 μm to about 600 μm. In one embodiment, the apertures of the aperture array have a smaller dimension than the alignment fiducials in a direction parallel to the plane of the electron optical elements. The apertures of the aperture array may not be wide enough (or may be of insufficient size) to image the alignment fiducials therethrough.

[0107]

[0117] In one embodiment, the stack includes a plurality of electron optical elements (including a pair of electron optical elements and a further electron optical element), wherein adjacent electron optical elements of the plurality of electron optical elements may include respective pairs of planar elements, and wherein the adjacent electron optical elements may include aligned alignment fiducials and monitoring apertures.

[0108]

[0118] In one embodiment, one or more of the apertures (or openings) of the electron-optical elements have a midpoint. In one embodiment, when the alignment fiducials of the electron-optical elements align (e.g., when a fiducial of one electron-optical element aligns with a fiducial of another electron-optical element, or when a fiducial of one electron-optical element aligns with a monitoring aperture that serves as a fiducial for the other electron-optical element), the midpoint of the first electron-optical element 61 aligns with the midpoint of the second electron-optical element 62. That is, the purpose of aligning the alignment fiducials with their respective monitoring apertures is to align the electron-optical elements for which the monitoring apertures are defined and for which the alignment fiducials reside with each other. When the electron-optical elements align, other features on and within the electron-optical elements also align. Such features are aperture arrays in each plate. In one embodiment, the apertures align directly with each other. In different embodiments, the aperture arrays may have a pattern that means that the apertures do not align, but that the midpoints of different aperture arrays do align.

[0109]

[0119] 7, in one embodiment, stack 700 includes a further electron optical element, i.e., third electron optical element 63, adjacent to the electron optical element, i.e., second electron optical element 62, which includes first monitoring aperture 71. The second electron optical element 62 and the third electron optical element 63 form a further pair of electron optical elements. In one embodiment, the further electron optical element, i.e., third electron optical element 63, includes a further monitoring aperture, i.e., second monitoring aperture 72.

[0110]

[0120] 7, in one embodiment, the monitoring apertures 71, 72 are offset from one another when viewed perpendicular to the plane of the electron optical elements. In one embodiment, each alignment fiducial-monitoring aperture pair is defined at a different distance from the center of the beam path (which may be a beam grating) for each element assembly step.

[0111]

[0121] As shown in FIG. 8 , in one embodiment, at least two of the monitoring apertures, e.g., first monitoring aperture 71 and second monitoring aperture 72, are aligned with each other in a direction substantially perpendicular to the plane of the electron-optical elements. Of course, there may be slight misalignment of monitoring apertures 71, 72 (e.g., in the range of about 0.1 μm to about 2 μm). However, monitoring apertures 71, 72 are sufficiently aligned with first alignment fiducial 66 so that illumination light projected through monitoring apertures 71, 72 can be reflected directly off first alignment fiducial 66 and returned through monitoring apertures 71, 72. In one embodiment, there is a line of sight from second monitoring aperture 72 to first alignment fiducial 66. Any misalignment can adversely affect imaging resolution.

[0112]

[0122] 7 and 8, the aspect ratio has been exaggerated so that some features of stack 700 can be more clearly seen.

[0113]

[0123] As shown in FIGS. 7 and 8 , in one embodiment, one electron optical element of each pair of electron optical elements includes multiple alignment fiducials. The other electron optical element of the pair of electron optical elements includes multiple monitoring apertures. The monitoring apertures align with the respective alignment fiducials in a direction substantially perpendicular to the plane of the electron optical elements. For example, as shown in FIGS. 7 and 8 , in one embodiment, the first electron optical element 61 includes two first alignment fiducials 66. The second electron optical element includes two first monitoring apertures 71. The first monitoring apertures 71 align with the respective first alignment fiducials 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 fiducials 66 may be on opposite sides of the beam path and / or may be spaced an equal distance apart from each other relative to the midpoint of the respective electron-optical elements 61, 62. Although two pairs of first monitoring apertures 71 and first alignment fiducials 66 are shown, there may be any number, e.g., three or more, as desired. The pairs of first monitoring apertures 71 and first alignment fiducials 66 may be spaced an equal distance apart around the midpoint of the respective electron-optical elements 61, 62 and / or the periphery of the beam path. Figure 11 schematically shows two alignment fiducials 66 on opposite sides of the beam path through the beam area in which the aperture array 711 is located. Multiple alignment fiducial-monitoring aperture pairs allow, for example, determination of alignment in a plane parallel to the electron-optical elements 61, 62 in two different directions, e.g., the x-axis and y-axis, which may be orthogonal to each other in the plane, as well as alignment in a rotational direction (e.g., around the beam path and / or orthogonal to the plane of the electron-optical elements 61, 62, which may be referred to as Rz (rotation about the z-axis)). By providing multiple monitoring apertures spaced apart from each other, the 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 respective fiducials 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 and about the beam path). Note that to effectively achieve rotational alignment between adjacent electron-optical elements, e.g., about the beam path, at least two pairs of fiducials and monitoring apertures are associated with adjacent electron-optical elements. To improve alignment, the fiducials are desirably spaced away from the midpoint of the respective electron-optical element.

[0114]

[0124] It is anticipated that one embodiment of the present invention will allow alignment to be verified after each stack assembly step. In one embodiment, illumination light is projected simultaneously onto an axis extending between the alignment fiducials and their associated monitoring apertures.

[0115]

[0125] In one embodiment, assessing the alignment includes focusing the optical system on an electron-optical element, such as the second electron-optical element 62. This allows a mark (e.g., an alignment mark or a monitoring aperture) to be identified. The optical system then focuses in a direction parallel to the beam path to image the alignment fiducial of the paired electron-optical element, such as the first alignment fiducial 66 of the first electron-optical element 61. The first alignment fiducial 66 can be imaged through the first monitoring aperture 71 of the second electron-optical element 62 (e.g., because the first alignment fiducial 66 is visible). One or more errors caused by focusing in a direction parallel to the beam path and / or tilt of the optical system and / or lighting effects can be calibrated by checking the alignment of the pair of electron-optical elements 61, 62 at two different rotational positions. The two different rotational positions can be offset by 180° from each other. For example, the stack 700 can be rotated between alignment measurements.

[0116]

[0126] In the orientation shown in Figure 7, the direction of the electron beam projected toward the sample location is downward. The sample location is below the stack 700. As shown in Figure 7, in one embodiment, the distance between the center of the beam path and the monitoring apertures 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 is farther from the central axis. Similarly, the second monitoring aperture 72 is farther from the center than the third monitoring aperture 73.

[0117]

[0127] Although the positions of the monitoring apertures and fiducials are shown away from the respective midpoints of the beam path and / or up-beam side of the stack (toward the top of FIG. 7 ) or away from the sample location, the positions can be closer to the beam path the further up the beam. It is not necessary for the monitoring apertures to be farther from the center as the distance from the sample location increases. In different embodiments, the positions of the monitoring apertures and fiducials can be at different lateral positions for different adjacent pairs of electron-optical elements in the stack. For example, as shown in FIG. 9 , the first monitoring aperture 71 can be closer to the center of the beam grid than the second monitoring aperture 72. The first monitoring aperture 71 is farther from the sample location. In one embodiment, the second monitoring aperture 72 is larger than shown in FIG. 7 (i.e., larger than the first monitoring aperture). 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 each of the first alignment fiducials 66. This allows the first alignment fiducial 66 to be viewed / measured by the optical system opposite the third electron-optical element 63. Alternatively, as shown in Figure 9, in one embodiment, an additional monitoring aperture is provided to provide a line of sight through the monitoring aperture of an adjacent electron-optical element to the alignment fiducial of the next electron-optical element (i.e., the electron-optical element opposite the adjacent electron-optical element). For example, as shown in Figure 9, in one embodiment, the fourth electron-optical element 64 includes a first monitoring aperture 71, a second monitoring aperture 72, and an additional third monitoring aperture aligned with the first alignment fiducial 66.

[0118]

[0128] As shown in FIGS. 6-9 , in one embodiment, the stack 700 includes a spacer, such as a first spacer 76. The spacer is disposed between adjacent pairs of electron optical elements. For example, the first spacer 76 may be disposed 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 elements. In one embodiment, the first spacer 76 is configured to secure the first electron optical element 61 to the second electron optical element 62. For example, the first spacer 76 may be secured to the first electron optical element 61 and the second electron optical element 62.

[0119]

[0129] 7-9, in one embodiment, stack 700 includes a plurality of spacers. For example, in one embodiment, second spacer 77 is disposed between second electro-optical element 62 and third electro-optical element 63. In one embodiment, third spacer 78 is disposed between third electro-optical element 63 and fourth electro-optical element 64.

[0120]

[0130] As shown in FIG. 7 , in one embodiment, spacers 76-78 can have inner edges at different distances from the center of the beam path, i.e., the central axis through stack 700. For example, FIG. 7 shows that third spacer 78 can have an inner edge closer to the center of the beam path than the inner edges of first spacer 76 and second spacer 77. Alternatively, as shown in FIGS. 8 and 9 , the inner edges of all of spacers 76-78 can be similar to each other with respect to the center of the beam path. The locations of the monitoring apertures and fiducials at each electron optical element can be selected so that an imaginary straight line lies between them away from the spacers. That is, the imaginary straight line is not blocked by the spacers intermediate each electron optical element.

[0121]

[0131] 7-9, in one embodiment, one or more of the spacers 76-78 have a stepped inner edge. In one embodiment, one or more of the spacers 76-78 have an inner edge that is a fixed distance from the center of the beam path.

[0122]

[0132] 6-9, one or more of the spacers 76-78 includes a central aperture 732. The central aperture 732 allows electrons to pass through the central aperture 732 along the beam path.

[0123]

[0133] 7-9, in one embodiment, the central aperture 732 has a dimension parallel to the plane of the electron optical elements 61-64 that is larger than the monitoring aperture 71. In one embodiment, multiple monitoring apertures 71 can fit within the central aperture 732.

[0124]

[0134] In one embodiment, at least one monitoring aperture overlaps with the central aperture 732 when viewed in a direction perpendicular to the plane of the electron-optical elements 61-64. For example, FIG. 7 shows 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 dimensions of the central aperture 732 of the first spacer 75. However, it is not necessary for all monitoring apertures to overlap with the central apertures of all spacers. For example, as shown in FIG. 7, the first monitoring aperture 71 and the second monitoring aperture 72 are radially (e.g., radially outwardly) spaced from the area defined by the central aperture of the third spacer 78 when viewed in a direction parallel to the beam path. In one embodiment, the stack 700 includes a spacer disposed between a monitoring aperture and its corresponding fiducial (i.e., the fiducial with which the monitoring aperture aligns). The spacers may intersect a straight imaginary line between the monitoring aperture and its corresponding fiducial. The spacers may be transparent to allow evaluation of the fiducial through the monitoring aperture.

[0125]

[0135] The present invention may be implemented as a method for aligning electron-optical elements 61-64. In one embodiment, the method includes forming a 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 so that it is positioned within the stack including the first electron-optical element 61. In one embodiment, a tool such as a robotic arm is used to move the electron-optical elements 61, 64. In one embodiment, a first spacer 76 is fixed to the first electron-optical element 61. The second electron-optical element 62 is then first positioned so that it abuts the first spacer 76.

[0126]

[0136] In one embodiment, a method for aligning an electron-optical element includes inspecting a first alignment fiducial 66 with inspection light through a first monitoring aperture 71. In one embodiment, a source of the inspection light is positioned such that the first monitoring aperture 71 is located between the light source and the first alignment fiducial 66. The light source is positioned to project the inspection light through the first monitoring aperture 71 toward the first alignment fiducial 66. The inspection light can be visible light.

[0127]

[0137] In one embodiment, the method includes detecting test light reflected from the first electron-optical element 61. The test light may be reflected from the first alignment fiducial 66 and / or from a surface of the first electron-optical element 61 near the first alignment fiducial 66. Detecting the reflected test light can evaluate, e.g., confirm, the alignment between the first electron-optical element 61 and the second electron-optical element 62. 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 fiducial 66 relative to a feature of the second electron-optical element 62. For example, the position of the first alignment fiducial 66 relative to the second alignment fiducial 67 can be measured. In one embodiment, a surface of the second electron-optical element 61 facing the first electron-optical element 61 includes a fiducial for aligning the facing sides of the electron-optical elements 61, 62. In one embodiment, when an electronic optical element (e.g., the second electronic optical element 62) is added to the stack 700, the relative position of its monitoring aperture (e.g., the first monitoring aperture 71) and the alignment fiducial (e.g., the first alignment fiducial 66) of the adjacent electronic optical element (e.g., the first electronic optical element 61) is measured with respect to the alignment fiducial (e.g., the second alignment fiducial 67) of the electronic optical element (e.g., the second electronic optical element 62) just placed. This allows the relative position of the further electron optical element (e.g., the third electron optical element 63) to be determined with respect to the adjacent electron optical element (e.g., the first electron optical element 61) when the monitoring aperture (e.g., the second monitoring aperture 72) of the further electron optical element (e.g., the third electron optical element 63) is aligned with the alignment fiducial (e.g., the second alignment fiducial 67) of the just-placed electron optical element (e.g., the second electron optical element 62) (i.e., with reference to and relative to the alignment fiducial of the adjacent electron optical element, made possible by the monitoring aperture of the just-placed electron optical element).In one embodiment, the surface of the second electron-optical element facing the first electron-optical element 61 includes a fiducial for aligning the facing sides of the electron-optical elements 61, 62. Additionally or alternatively, the position of the first alignment fiducial 66 relative to the first monitoring aperture 71 may be measured. The first monitoring aperture 71 may be considered to have a dual purpose: to make the first alignment fiducial 66 visible and to function as a fiducial (because the first monitoring aperture 71 is used as a reference feature).

[0128]

[0138] As mentioned above, the first alignment fiducial 66 has an associated first monitoring aperture 71. The first alignment fiducial 66 and the first monitoring aperture 71 can be considered to form an alignment fiducial-monitoring aperture pair. As shown in FIG. 7 , in one embodiment, multiple alignment fiducial-monitoring aperture pairs are provided at different locations, i.e., different positions, when the stack is viewed in a plan view (a direction parallel to the electron beam path). FIG. 7 shows a first electron optical element 61 including two first alignment fiducials 66. FIG. 7 shows a second electron optical element 62 including two first monitoring apertures 71. FIG. 7 shows two alignment fiducial-monitoring aperture pairs for aligning the second electron optical element 62 with respect to the first electron optical element 61. By providing two alignment fiducial-monitoring aperture pairs, two-dimensional alignment in a plane parallel to the electron optical elements can be assessed, as well as rotational alignment about an axis parallel to the electron beam path.

[0129]

[0139] It is not necessary that two alignment fiducial-monitoring aperture pairs be provided. In an alternative embodiment, only two alignment fiducial-monitoring aperture pairs are provided. In another alternative embodiment, three alignment fiducial-monitoring aperture pairs (or four or more) are provided.

[0130]

[0140] In one embodiment, the method includes aligning the second electron-optical element 62 with respect to the first electron-optical element 61 based on the detected inspection light. For example, if the detected inspection light indicates that the second electron-optical element is desirably aligned with the first electron-optical element 61, the second electron-optical element 62 may remain in a fixed position. In one embodiment, the second electron-optical element 62 is fixed with respect to the first electron-optical element 61. For example, the second electron-optical element 62 may be fixed with respect to the first spacer 76. If the detected inspection 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 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 with respect to the first electron-optical element 61 based on the detected inspection 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 test light indicates that the second electron-optical element 62 is out of alignment with the first electron-optical element, the stack may be discarded.

[0131]

[0141] It is anticipated that one embodiment of the present invention will allow for verification of alignment between electro-optical elements within a stack.

[0132]

[0142] As discussed above and shown in Figure 7, in one embodiment, the stack includes three or more electron-optical elements. In one embodiment, the method for aligning the electron-optical elements includes adding a third electron-optical element 63 to the stack. As shown in Figure 7, in one embodiment, the third electron-optical element 63 includes a second monitoring aperture 72.

[0133]

[0143] 7 , in one embodiment, the second electron-optical element 62 includes a second alignment fiducial 67. A second monitoring aperture 72 may be associated with the second alignment fiducial 67. The second alignment fiducial 67 and the monitoring aperture 72 may form an alignment fiducial-monitoring aperture pair. In one embodiment, the second alignment fiducial 67 is visible through the second monitoring aperture 72.

[0134]

[0144] In one embodiment, the method includes inspecting the second alignment fiducial 67 of the second electron-optical element 62 with inspection light through the second monitoring aperture 72. In the configuration shown in FIG. 7 , the second monitoring aperture 72 is blocked on one side by the third spacer 78. However, during assembly of the stack, the second monitoring aperture 72 is positioned to allow the inspection light to pass through the second monitoring aperture 72. The inspection light can pass through the second monitoring aperture 72 before the third spacer 78 is added to the stack. The third spacer 78 can be added to the stack after the alignment of the third electron-optical element 63 with respect to the first electron-optical element 61 and / or the second electron-optical element 62 is confirmed.

[0135]

[0145] In one embodiment, the method includes detecting the inspection light reflected from the second electron-optical element 62. For example, the inspection light may be reflected from the second alignment fiducial 67 and / or from a portion of the second electron-optical element 62 proximate to the second alignment fiducial 67.

[0136]

[0146] In one embodiment, the method includes aligning the third electronic optical element 63 with respect to the second electronic optical element 62 based on the detected interrogation light. For example, if alignment is confirmed, the third electronic optical element 63 may be kept in place. Alternatively, if alignment is not confirmed, the third electronic optical element 63 may be moved or the stack may be discarded.

[0137]

[0147] Figure 8 shows a schematic representation of an alternative configuration for stack 700. To avoid redundancy, features of the stack shown in Figure 8 that are also shown in Figure 7 will not be described in detail below. Features of the stack shown in Figure 8 that differ from the stack shown in Figure 7 will be described below.

[0138]

[0148] As shown in FIG. 8 , the second alignment fiducial 67 is not required. In one embodiment, the method includes inspecting the first alignment fiducial 66 (instead of the second alignment fiducial 67) using inspection light, particularly through the second monitoring aperture 72; as shown in FIG. 8 , the first alignment fiducial 66 may be inspected using inspection light through both the first monitoring aperture 71 and the second monitoring aperture 72. The light source may be positioned such that the inspection light passes through the second monitoring aperture 72 and the first monitoring aperture 71 to reach the first alignment fiducial 66. That is, the first monitoring aperture 71 may function as an alignment fiducial for the second monitoring aperture 72. The second monitoring aperture 72 may be considered to have two types of monitoring fiducials: the first monitoring aperture and the first alignment fiducial 66. This may require the first monitoring aperture to be larger than a monitoring aperture that does not function as a fiducial, i.e., functions only as a monitoring aperture. When the first monitoring aperture is used as a fiducial, the fiducial is configured with the first monitoring aperture because the first fiducial may be too far away such that there is insufficient intensity difference (e.g., contrast) in a direction along the beam path (e.g., along the z-axis).

[0139]

[0149] In one embodiment, the method includes detecting test light reflected from the first electron-optical element 61. For example, the test light may be reflected from the first alignment fiducial 66 and / or from a portion of the first electron-optical element 61 near the first alignment fiducial 66.

[0140]

[0150] In one embodiment, the method includes aligning the third electron-optical element 63 with respect to the first electron-optical element 61 based on the detected interrogation light. By aligning the third electron-optical element 63 with the first electron-optical element 61, the third electron-optical element 63 also aligns with the second electron-optical element 62 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. A straight imaginary line connecting the first monitoring aperture 71 to the second monitoring aperture 72 is substantially parallel to the electron beam path.

[0141]

[0151] 7 and 8, in one embodiment, the third electron-optical element 63 includes a third alignment fiducial 68. In one embodiment, the fourth electron-optical element 64 includes a third monitoring aperture 73. In one embodiment, the third alignment fiducial 68 and the third monitoring aperture 73 are associated with one another. The third alignment fiducial 68 is visible through the third monitoring aperture 73. The third alignment fiducial 68 and the third monitoring aperture 73 form an alignment fiducial-monitoring aperture pair.

[0142]

[0152] 7 and 8, in one embodiment, multiple alignment fiducial-monitoring aperture pairs are provided for each pair of electron-optical elements that are to be aligned with respect to one another. In the configuration shown in FIG. 8, when aligning the third electron-optical element 63 with respect to the first electron-optical element 61, the first alignment fiducial 66 and the second monitoring aperture 72 form an alignment fiducial-monitoring aperture pair.

[0143]

[0153] FIG. 10 schematically illustrates one type of alignment fiducial according to an embodiment of the present invention. The alignment fiducial may be used as a fiducial in an embodiment of the present invention, such as any of the embodiments shown and described with reference to FIGS. 7-9. As shown in FIG. 10, in one embodiment, the alignment fiducial 66 includes a plurality of marks 82, 87. The marks 82, 87 may be spaced apart from one another in a direction parallel to the plane of the electron-optical elements. The view in FIG. 10 is of a plane parallel to the plane of the electron-optical elements. As shown, the marks may be verniers. A vernier is one form of two-dimensional pattern that may be used as a fiducial according to an embodiment of the present invention, for example, as described and illustrated with reference to FIGS. 7-9. In one embodiment, the marks may form any suitable two-dimensional pattern, such as a grating.

[0144]

[0154] For example, FIG. 10 schematically illustrates an exemplary diagram of an alignment fiducial 66 including, 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 marks 82 of the first plurality 81 may be referred to as a first mark series (or subpattern). The marks 82 of the first plurality 81 may be verniers. As a line of marks, the first mark series may be used, for example, to determine relative alignment between adjacent electro-optical elements in the direction of the line of marks at the position of the alignment fiducial 66; i.e., the line of marks may be associated with a degree of freedom in the direction of the line of marks. In one embodiment, the marks 82 of the first plurality 81 include at least 3, at least 4, optionally at least 5, and optionally at least 10 marks 82. In one embodiment, the marks 82 are aligned substantially parallel to one another. The mark 82 is visible to the first monitoring aperture 71. When illumination light is projected onto the first alignment fiducial 66, intensity transitions (or contrasts) in the reflected illumination light can be detected. The transitions can correspond to edges of the mark 82. The edges of the mark 82 can be discrete, sharp edges. By detecting the intensity transitions in the image of the first alignment fiducial 66, the alignment between the electron-optical elements 61, 62 can be assessed.

[0145]

[0155] The more edge transitions that are provided, the more accurate the alignment between the electron-optical elements can be measured. The location fit of the second electron-optical element 62 relative to the first electron-optical element 61 can be averaged over the edge transitions that are monitored.

[0146]

[0156] 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 one another. In one embodiment, a constant pitch is provided between the marks 82. However, it is not necessary for the pitch to be constant, especially if the spacing between the marks 82 is known.

[0147]

[0157] 10, in one embodiment, the first alignment fiducial 66 includes a further plurality of marks spaced apart from one another in a further direction parallel to the plane of the electron-optical element, e.g., a second plurality of marks 87 86. The marks 87 of the second plurality 86 (or a second line of marks) may be verniers.

[0148]

[0158] 10 , in one embodiment, the marks 82 of the first plurality 81 and the marks 87 of the second plurality 86 are arranged in different directions, e.g., orthogonal directions. Otherwise, the marks 87 of the second plurality 86 may have substantially the same characteristics as the marks 82 of the first plurality 81. By providing two mark series that are desirably orthogonal to one another, the alignment fiducial 66 may be used to assess alignment between electro-optical elements in two dimensions. For example, when used to align adjacent electro-optical elements, a single alignment fiducial may enable relative alignment at the position of the alignment fiducial in two degrees of freedom, e.g., in two directions of the first and second lines of marks of the alignment fiducial 66.

[0149]

[0159] However, it is not necessary for the first alignment fiducial 66 to include orthogonal mark series. In one embodiment, the first alignment fiducial 66 includes one mark series, a first plurality 81 of marks 82. A second plurality 86 of marks 87 may be provided within the further first alignment fiducial 66 at significantly different locations on the first electron-optical element 61. As shown in FIG. 11 , in one embodiment, the alignment fiducial 66 (e.g., shown in FIG. 10 ) is positioned on one side of the aperture array 711. For example, in either embodiment, two first alignment fiducials 66 may be provided on either side of the beam path. This is shown, for example, in and described with reference to FIGS. 7-9 . For example, having two or more fiducials spaced apart from each other in both directions from the aperture array 711 allows for effective rotational alignment, for example, between adjacent electron-optical elements. In one embodiment, the alignment fiducials on either side of the beam path include marks arranged in different directions from each other. Desirably, each alignment fiducial includes at least two lines of marks extending in two different directions at an angle from each other. In one embodiment, each of the two alignment fiducials 66 has a plurality of marks arranged in a line in a direction perpendicular to the direction between the two alignment fiducials 66. In one embodiment, one of the two alignment fiducials 66 on either side of the beam area may omit one of the plurality of marks extending in the same direction between the two alignment fiducials.

[0150]

[0160] In one embodiment, one or more of the planar elements includes a detector configured to detect signal electrons from the sample location. For example, such a detector is shown and disclosed with reference to any of FIGS. 2-5. 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 (non-limiting list): a lens array (such as an objective lens array, a condenser lens array, or a control lens array), a corrector array, a collimator array, a deflector array, or a beam limiter such as a beam-limiting aperture array, a beam shaper array, or a plate of an upper beam limiter array. Thus, the electron-optical element may include a planar electron-optical element in the form of a plate.

[0151]

[0161] In one embodiment, the electron-optical module is or includes an objective lens assembly. The objective lens assembly may include an array of objective lenses for focusing the electron beam at a sample location. Such an objective lens assembly is shown, for example, in FIGS. 3-5. 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 them.

[0152]

[0162] In one embodiment, the electron optics module is or includes a condenser lens array for deflecting electrons towards the sample 208. In one embodiment, the condenser lens array is for deflecting electrons in one or more electron beams towards the sample 208. In one embodiment, the condenser lens array is for collimating the electrons towards the sample 208. In one embodiment, the electron optics module is or includes a macro condenser lens for deflecting electrons towards the sample 208. In one embodiment, the electron optics module is or includes a collimator, which may be separate from the condenser lens or condenser lens array.

[0153]

[0163] The present invention may be embodied as an alignment apparatus including a stack 700, an inspection light source, and an alignment detector. The inspection light source is configured to direct inspection light through one or more of the monitoring apertures. The alignment detector is configured to detect inspection light reflected from at least one of the electron-optical elements 61-64. In one embodiment, the inspection light source is located on one side of the stack (in a direction parallel to the beam path). The alignment detector is located on the same side of the stack. The optical system used to assess alignment via imaging of the alignment fiducial is reflective. Light reflected from the alignment fiducial or an electron-optical element close to the alignment fiducial is used to assess alignment. This differs from a transmission system, in which light is transmitted through the stack and detected on the opposite side of the stack.

[0154]

[0164] In one embodiment, stack 700 includes one or more electro-optical elements, some of which may be referred to as microelectromechanical components (even though such components may not include moving or movable features) designed to have electro-optical functionality, or may be fabricated using techniques suitable for fabricating microelectromechanical components (e.g., “MEMS techniques”). Stack 700, or at least components of stack 700, may be fabricated by such techniques. Stack 700 may include one or more elements that may be considered MEMS elements. One or more of such elements may be controlled, during use, to be set to a high potential difference relative to a reference potential (e.g., ground). Such elements may require precise positioning (e.g., alignment) within stack 700, e.g., with respect to the path of the beam grating and with respect to the paths of other electro-optical elements in the device, e.g., the source, sample, and / or beam grating. One embodiment of the present invention is expected to enable more precise positioning (e.g., alignment) of such elements within such stack 700, e.g., during operation, without distortion of stack 700, e.g., due to externally applied forces or moments. Additionally or alternatively, one embodiment of the present invention allows for more accurate positioning, e.g., alignment, of such elements relative to other elements within device 40, and thus of stacks of stacks 700 including such elements within device 40.

[0155]

[0165] As previously mentioned, 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 condenser lens assembly.

[0156]

[0166] In one embodiment, stack 700 includes a collimator. For example, in one embodiment, stack 700 includes a magnetic collimator in combination with an electrostatic focusing lens array. Stack 700 may include a single aperture lens array with one or two macroelectrodes positioned away from the virtual source conjugate plane.

[0157]

[0167] In an alternative embodiment, stack 700 includes a magnetic macrolens in combination with an electrostatic slit deflector. The magnetic macrolens may be for collimation. As a further alternative, in an embodiment, stack 700 includes a magnetic-electrostatic coupled microlens and a down-beam slit deflector.

[0158]

[0168] In general, stack 700 may include any of the following plates: a plate of a detector array, a plate of lens electrodes (on which multiple detectors may be integrated), 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 plates of other types of corrector elements. Such plates may be referred to as electron-optical elements. Such electron-optical elements operate or interact with multiple beams of the beam grid. The electron-optical elements may feature multiple apertures, each one for a different beam of the beam grid.

[0159]

[0169] The embodiments described within this document have focused primarily on multi-beam electron-optical devices 40. The present invention is equally applicable to single-beam electron-optical devices 40.

[0160]

[0170] While the present invention has been described in connection with various embodiments, other embodiments of the present invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. For example, as described above, in one embodiment, stack 700 includes a monitoring aperture and alignment fiducial. However, the monitoring aperture and alignment fiducial of the present invention may be used anywhere in electro-optical device 40 where potential misalignment problems may exist. It is intended that the specification and examples be considered exemplary only, with the true scope and spirit of the invention being indicated by the following claims and clauses.

[0161]

[0171] The above description is intended to be illustrative and not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made as described without departing from the scope and provisions of the claims set out below.

[0162]

[0172] The following provisions are provided:

[0163]

[0173] Clause 1. A stack of planar elements of a charged particle optical module configured to project charged particles along a beam path, the stack comprising a pair of adjacent planar elements arranged across the beam path, one of the planar elements including an alignment fiducial and the other of the planar elements including a monitoring aperture, the pair of planar elements positioned relative to each other such that the alignment fiducial and the monitoring aperture are aligned with each other in a direction substantially perpendicular to the plane of the planar elements.

[0164]

[0174] Clause 2. The stack of clause 1, comprising a further planar element adjacent to the planar element comprising the monitoring aperture, thereby forming a further pair of planar elements.

[0165]

[0175] Clause 3. The stack of clause 2, wherein the further planar element comprises a further monitoring aperture.

[0166]

[0176] 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 elements.

[0167]

[0177] Clause 5. A stack according to clause 3 or 4, wherein the further planar element comprises an additional monitoring aperture, preferably the additional monitoring aperture aligns with the monitoring aperture of the pair of planar elements.

[0168]

[0178] Clause 6. A stack described in any one of clauses 3 to 5, wherein the pair of further planar elements are arranged relative to each other such that the further monitoring aperture of the further planar element and the further alignment fiducial of the pair of planar elements are aligned with each other in a direction substantially perpendicular to the plane of the planar elements.

[0169]

[0179] Clause 7. The stack of clause 3, wherein the monitoring apertures are aligned with one another in a direction substantially perpendicular to the plane of the planar elements.

[0170]

[0180] Clause 8. A stack described in any one of the preceding clauses, wherein each of the planar elements includes one or more apertures for charged particles, preferably the one or more apertures have midpoints, and preferably the midpoints between each pair of planar elements align when the alignment fiducial and the monitoring fiducial are aligned.

[0171]

[0181] Clause 9. A stack described in any one of the preceding clauses, wherein one planar element of each pair of planar elements includes a plurality of alignment fiducials, and the other planar element of the pair of planar elements includes a plurality of monitoring apertures that align with the respective alignment fiducials in a direction substantially perpendicular to the plane of the planar elements, preferably the plurality of alignment fiducials is one alignment fiducial, preferably the plurality of monitoring apertures is two, preferably the two alignment fiducials are spaced from the midpoint in different directions, preferably opposite directions, and preferably the alignment fiducials are spaced from the midpoint by the same distance.

[0172]

[0182] Clause 10. A stack according to any one of the preceding clauses, including a spacer disposed between the planar elements of at least one pair of planar elements.

[0173]

[0183] Clause 11. The stack of clause 10, wherein the spacer includes a central aperture for passing charged particles along the beam path.

[0174]

[0184] Clause 12. The stack of clause 11, wherein the central aperture has a dimension that is larger than each monitoring aperture in a direction parallel to the plane of the planar element.

[0175]

[0185] Clause 13. The stack of clause 12, wherein at least one monitoring aperture overlaps with the central aperture when viewed in a direction perpendicular to the plane of the planar element.

[0176]

[0186] Clause 14. A stack according to any one of the preceding clauses, wherein each alignment fiducial comprises a plurality of marks spaced apart from one another in the plane of the planar element.

[0177]

[0187] Clause 15. A stack as described in clause 14, wherein at least some of the plurality of marks are arranged in a direction parallel to the plane of the planar element, preferably all of the plurality of marks are arranged in a direction parallel to the plane of the planar element, preferably the plurality of marks are verniers.

[0178]

[0188] Clause 16. A stack according to clause 14 or 15, wherein at least some of the marks are arranged in different directions parallel to the plane of the planar element.

[0179]

[0189] Clause 17. A stack according to any one of clauses 14 to 16, wherein the plurality of marks form a pattern such as a grid.

[0180]

[0190] Clause 18. A stack according to clause 14 or 15, wherein the alignment fiducials are spaced apart from each other in a further direction parallel to the plane of the planar element such that the marks are aligned vertically.

[0181]

[0191] Clause 19. A stack according to any one of clauses 14 to 18, wherein the marks have periodicity.

[0182]

[0192] Clause 20. The stack of any one of the preceding clauses, wherein at least one alignment fiducial comprises a through-hole.

[0183]

[0193] Clause 21. The stack of clause 20, wherein each monitoring aperture has a dimension parallel to the plane of the planar element that is larger than the through-hole.

[0184]

[0194] Clause 22. A stack according to any one of the preceding clauses, wherein each of the planar elements comprises or is a plate.

[0185]

[0195] Clause 23. A stack according to clause 22, wherein each plate preferably includes an array of apertures for passing a respective charged particle beam along the beam path, preferably in operation the beam path corresponding to a midpoint.

[0186]

[0196] Clause 24. A stack according to 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.

[0187]

[0197] Clause 25. A stack according to any one of the preceding clauses, wherein at least one of the planar elements comprises a microelectromechanical component.

[0188]

[0198] Clause 26. A stack according to any one of the preceding clauses, wherein at least one of the planar elements is a charged particle optical element.

[0189]

[0199] Clause 27. A stack according to any one of the preceding clauses, wherein at least one of the planar elements includes a detector configured to detect signal charged particles from the sample location.

[0190]

[0200] Clause 28. A charged particle optics module comprising a stack according to any one of the preceding clauses.

[0191]

[0201] Clause 29. A charged particle optics module according to clause 28, wherein the charged particle optics module comprises an array of objective lenses to focus the charged particle beam at a sample location or an array of condenser lenses to deflect the charged particles towards the sample.

[0192]

[0202] Clause 30. A charged particle optics module according to clause 29, wherein surfaces of the planar elements of at least one pair of planar elements are configured to form a lens when a potential difference is applied between them.

[0193]

[0203] Clause 31. A charged particle optical device for directing a charged particle beam to a sample location, 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.

[0194]

[0204] Clause 32. A charged particle optical apparatus 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 device according to clause 31.

[0195]

[0205] Clause 33. The charged particle optical apparatus of clause 32, further comprising an actuatable stage for supporting the sample at the sample location.

[0196]

[0206] Clause 34. An alignment device comprising a stack described in any one of clauses 1 to 27 or a charged particle optical module described in any one of clauses 28 to 30, an inspection light source configured to direct inspection light through at least one monitoring aperture, and an alignment detector configured to detect inspection light reflected from at least one planar element.

[0197]

[0207] Clause 35. An alignment apparatus according to clause 34, wherein the inspection light source is located on one side of the stack and the alignment detector is located on the same side of the stack.

[0198]

[0208] Clause 36. An alignment apparatus according to clause 34 or 35, comprising a mover configured to align the planar elements relative to one another based on the detected interrogation light.

[0199]

[0209] Clause 37. A method for aligning a planar element of a charged particle optical module configured to project charged particles along a beam path, the method comprising: providing a first planar element including a first alignment fiducial; providing a second planar element including a first monitoring aperture stacked on the first planar element; inspecting the first alignment fiducial using inspection light passing through the first monitoring aperture; detecting the inspection light reflected from the first planar element; and aligning the second planar element to the first planar element based on the detected inspection light.

[0200]

[0210] Clause 38. A method according to clause 37, comprising: providing a third planar element including a second monitoring aperture stacked relative to the second planar element; inspecting a first alignment fiducial or a second alignment fiducial of the second planar element using inspection light passing through the second monitoring aperture; detecting inspection 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 inspection light.

[0201]

[0211] Clause 39. A method according to clause 37 or 38, wherein the alignment step comprises monitoring, preferably in two dimensions, for example in the plane of each planar element, one or more intensity variations of the reflected test light corresponding to one or more edges of the first alignment fiducial.

[0202]

[0212] Clause 40. The method of clause 39, wherein the alignment step includes monitoring a plurality of intensity fluctuations of the reflected inspection light corresponding to edges of a plurality of marks of the first alignment fiducial, the marks moving away from each other in a direction parallel to the plane of the first planar element.

[0203]

[0213] Clause 41. The method of any one of clauses 37 to 40, wherein the inspecting step includes focusing the inspection light onto the u-cial in a first alignment fI.

[0204]

[0214] Clause 42. The method according to any one of clauses 37 to 41, wherein the inspection light is directed perpendicular to the plane of the second planar element.

[0205]

[0215] Clause 43. The method of any one of clauses 37 to 42, comprising, after the aligning step, fixing a second planar element relative to the first planar element.

[0206]

[0216] Clause 44. A method of making a charged particle optics module, comprising the method according to any one of clauses 37 to 43.

Claims

1. a stack of planar elements of a charged particle optics module configured to project charged particles along a beam path, the stack comprising: a pair of adjacent planar elements disposed across the beam path, one of the planar elements including an alignment fiducial and the other of the planar elements including a monitoring aperture; The stack, wherein the pair of planar elements are positioned relative to each other such that the alignment fiducial and the monitoring aperture are aligned with each other in a direction substantially perpendicular to the plane of the planar elements.

2. The stack of claim 1 , comprising a further planar element adjacent to the planar element containing the monitoring aperture, thereby forming a further pair of planar elements.

3. The stack of claim 2 , wherein the further planar element comprises a further monitoring aperture.

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

5. 5. A stack according to claim 3 or 4, wherein the further surface-area element comprises an additional monitoring aperture, preferably the additional monitoring aperture being aligned with the monitoring aperture of the pair of surface-area elements.

6. 6. The stack of claim 3, wherein the pair of further planar elements are arranged relative to each other such that the further monitoring aperture of the further planar element and the further alignment fiducial of the pair of planar elements 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 one another in a direction substantially perpendicular to the plane of the planar element.

8. 8. The stack according to any one of claims 1 to 7, wherein each of the surface-area elements comprises one or more apertures for charged particles.

9. 9. The stack of claim 1, wherein one planar element of each pair of planar elements includes a plurality of alignment fiducials, and the other planar element of the pair includes a plurality of monitoring apertures aligned with the respective alignment fiducials in a direction substantially perpendicular to the plane of the planar elements.

10. 10. The stack of claim 1, further comprising a spacer disposed between the planar elements of at least one pair of planar elements, the spacer comprising a central aperture for passing charged particles along the beam path.

11. 11. The stack of claim 1, wherein each alignment fiducial comprises a plurality of marks spaced apart from one another in the plane of the planar element.

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

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

14. 13. The stack of claim 11 or 12, wherein each alignment fiducial includes a further plurality of marks spaced apart from each other in a further direction parallel to the plane of the planar element such that the marks are arranged in a vertical direction.

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

Images