Counteracting interfering magnetic fields in a charged-particle beam system

Abstract

An apparatus for generating a counteracting magnetic field for a charged-particle beam apparatus comprises a coil, individual coils or pairs of coils spaced apart and arranged about each one of an X axis, a Y axis, or a Z axis. A controller may be configured to control the pairs of coils to generate a substantially uniform magnetic field at a central location between the pairs of coils.

Description

COUNTERACTING INTERFERING MAGNETIC FIELDS IN A CHARGED-PARTICLE BEAM SYSTEMCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 562,680 which was filed on March 07, 2024 and US application 63 / 753,444 which was filed on February 04, 2025 which are incorporated herein in its entirety by reference.TECHNICAL FIELD

[0002] The description herein generally relates to systems and methods to counteract, or reduce the effect of, magnetic field interference in a charged-particle beam apparatus.BACKGROUND

[0003] In manufacturing processes of integrated circuits (ICs), unfinished or finished circuit components are inspected or measured to ensure that features are fabricated according to design and are free of defects. Inspection or metrology systems utilizing charged particle (e.g., electron) beam microscopes, such as a scanning electron microscope (SEM) can be employed. As feature sizes of IC components continue to shrink, determination of the resolution of the particle beam (e.g., electron beam) becomes important, for example, for the accurate determination of feature sizes. Increased resolution makes even the smallest disturbances to the beam path detrimental to performance of charged particle beam instruments, such as SEM.SUMMARY

[0004] Some embodiments provide a coil arrangement for generating a counteracting magnetic field. The coil arrangement may include a first pair of coils extending about an X axis and spaced apart from each other along the X axis, a second pair of coils extending about a Y axis and spaced apart from each other along the Y axis, and a third pair of coils extending about a Z axis and spaced apart from each other along the Z axis. The coil arrangement may also include one or more additional coils located between at least one of the first pair of coils, the second pair of coils, or the third pair of coils. At least one of the first, second, or third pair of coils may be controlled to collectively generate a substantially uniform magnetic field at a location between the first, second, and third pair of coils. And the one or more additional coils may be controlled to cause (a) a movement of the substantially uniform magnetic field from the location, or (b) a change in shape of the substantially uniform magnetic field.

[0005] Some embodiments provide an apparatus for generating a counteracting magnetic field for a charged-particle beam apparatus. The apparatus may include a pair of coils spaced apart and arranged about each one of an X axis, a Y axis, or a Z axis. At least one pair of coils of the pairs of coils maybe controlled to generate a substantially uniform magnetic field at a central location between the pairs of coils.

[0006] Some embodiments provide a non-transitory computer readable medium that stores a set of instructions that is executable by at least one processor of a computing device to cause the computing device to perform operations for generating a counteracting magnetic field for a charged-particle beam apparatus. The operations may include directing current through at least one pair of coils of the pairs of coils spaced apart and arranged about each one of an X axis, a Y axis, or a Z axis to generate a substantially uniform magnetic field at a central location between the pairs of coils.

[0007] Some embodiments provide a method of generating a counteracting magnetic field for a charged-particle beam apparatus. The method may include directing current through at least one pair of coils of the pairs of coils spaced apart and arranged about each one of an X axis, a Y axis, or a Z axis to generate a substantially uniform magnetic field at a central location between the pairs of coils.

[0008] Other advantages of the embodiments of the present disclosure will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of the present invention.BRIEF DESCRIPTION OF FIGURES

[0009] 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.

[0010] Fig. 1 is a schematic illustration of an exemplary charged-particle beam system, consistent with some embodiments of the present disclosure.

[0011] Fig. 2A is a schematic illustration of an exemplary multi-beam tool that may be part of the charged-particle beam system of Fig. 1.

[0012] Fig. 2B is a schematic illustration of an exemplary single-beam tool that may be part of the charged-particle beam system of Fig. 1.

[0013] Fig. 3 is a schematic illustration of electromagnetic coils in an exemplary active magnetic field cancellation (MFC) system of the charged-particle beam system of Fig. 1.

[0014] Figs. 4A-4B are schematic illustrations of an exemplary arrangement of electromagnetic coils in an active magnetic field cancellation system, consistent with some embodiments of the present disclosure.

[0015] Figs. 5A-5C are schematic illustrations of an exemplary arrangement of auxiliary electromagnetic coils in a magnetic field cancellation system, consistent with some embodiments of the present disclosure.

[0016] Fig. 6 is a schematic illustration of an exemplary configuration of magnetic field sensors in a magnetic field cancellation system, consistent with some embodiments of the present disclosure.

[0017] Fig. 7 is a schematic illustration of an exemplary arrangement of electromagnetic coils in a magnetic field cancellation system, consistent with some embodiments of the present disclosure.

[0018] Fig. 8 is a flow chart of an exemplary method of counteracting interfering magnetic field variations, consistent with some embodiments of the present disclosure.

[0019] Fig. 9 is a schematic illustration of an exemplary configuration of a magnetic field suppression system configured to suppress the magnetic field at a magnetic field source, consistent with some embodiments of the present disclosure.

[0020] Fig. 10 is a flow chart of an exemplary method of counteracting interfering magnetic field variations in the system of Fig. 9, consistent with some embodiments of the present disclosure.DETAILED DESCRIPTION

[0021] Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings. The following description refers to the accompanying drawings in which the same numbers in different drawings represent the same or similar elements unless otherwise represented. The implementations set forth in the following description of exemplary embodiments do not represent all implementations. Instead, they are merely examples of apparatuses and methods consistent with aspects related to the disclosed embodiments as recited in the appended claims. For example, although some embodiments are described in the context of utilizing electron beams, the disclosure is not so limited. Other types of charged-particle beams (e.g., including protons, ions, muons, or any other particle carrying electric charges) may be similarly applied. Furthermore, other imaging systems may be used, such as optical imaging, photon detection, x-ray detection, ion detection, etc.

[0022] Electronic devices are constructed of circuits formed on a piece of semiconductor material called a substrate. The semiconductor material may include, for example, silicon, silicon nitride, gallium arsenide, indium phosphide, silicon germanium, diamond, or the like. Many circuits may be formed together on the same piece of semiconductor material and are called integrated circuits or ICs. The size of these circuits has decreased dramatically so that many more of them can be fit on the substrate. For example, an IC chip in a smartphone can be as small as a thumbnail and yet may include over 2 billion transistors, the size of each transistor being less than 1 / 1000th the size of a human hair.

[0023] Making ICs with extremely small structures or components is a complex, time-consuming, and expensive process, often involving hundreds of individual steps. Errors in even one step have the potential to result in defects in the finished IC, rendering it useless. Thus, the manufacturing process highly relies on the defect inspection and metrology (critical dimensions (CD), edge placement error (EPE), pattern pitch, etc.) so that the fabrication process can be optimized to maximize the number of functional ICs made in the process; that is, to improve the overall yield of the process.

[0024] Defect inspection and metrology of semiconductor circuits can be conducted using a scanning charged particle microscope (“SCPM”). For example, an SCPM may be a scanning electron microscope (SEM). A SCPM can be used to image these extremely small structures, in effect, taking a“picture” of the structures of the wafer. The image can be used to determine if the structure was formed properly in the proper location. If the structure is defective, then the process can be adjusted, so the defect is less likely to recur.

[0025] As the physical sizes of IC components continue to shrink, accuracy and yield in defect detection becomes increasingly important. In the context of an SEM used for inspection in semiconductor fabrication, resolution refers to the microscope’s ability to distinguish fine details in the sample being observed. Resolution is typically described in terms of spatial resolution, which is the smallest distance between two points on a sample’s surface that can be distinguished as separate entities in the SEM image. In the context of SEM imaging, fidelity refers to the microscope’s ability to reproduce and display fine details of the sample being observed, with displayed image being an accurate representation of the sample being observed. The resolution and fidelity of an SEM are critical performance parameters in the semiconductor industry where extremely small features need to be accurately characterized.

[0026] SEMs (and other charged-particle beam systems) are sensitive to changes in environmental magnetic field (or interfering magnetic field) since they interact with the electron beam and affect image quality. Typically, a sensor (e.g., a magnetometer) is used to detect the interfering magnetic field, and an arrangement of electromagnetic coils is used to create a uniform cancellation magnetic field to try to nullify or cancel the effect of the interfering magnetic field at that location. However, these conventional SEMs may have several limitations. For example, in some conventional SEMs, these electromagnetic coils generate the uniform cancellation field in a zone located at the center of the coils. However, in some SEMs, the electron beam column (or SEM column) may not be located at the center of the coils. In other words, the SEM column may not be located in the uniform cancellation field zone. As another example of a drawback, in some conventional SEMs, the magnetometer used to detect the interfering magnetic field only detects the magnetic field at one location (e.g., where the magnetometer is located). Therefore, the electromagnetic coils used to generate the uniform cancellation field only attempts to cancel the magnetic field at that location (e.g., the location of the magnetometer). As a further example of a drawback, in some conventional SEMs, control circuitry used to generate the uniform cancellation field may not be capable of cancelling a strong environmental DC magnetic field simultaneously with the interfering AC magnetic field. As a further example of a drawback, in some conventional SEMs, the electromagnetic coils used generate the uniform cancellation field may be positioned proximate to conductive members (e.g., a frame) of the SEM system such that magnetic interference (e.g., mutual inductance) between the coil and the conductive members reduces the effectiveness of the coils. In some embodiments of the present disclosure, the electromagnetic coils used to generate the uniform cancellation field may be separated from the conductive members of the frame to reduce magnetic interference.

[0027] Embodiments of the present disclosure provide systems and techniques to decrease the influence of magnetic field disturbances on the resolution and fidelity of SEMs and other charged-particle beam tools and systems. In some embodiments of the present disclosure, one or more additional electromagnetic coils may be provided in one or more of the X, Y, and Z dimensions to enable the generated uniform cancellation field to be moved to the location of the SEM column. In some embodiments of the present disclosure, two or more sensors may be located around the SEM column to measure the magnetic field in the vicinity of the SEM column such that uniform cancellation field is generated at the column location. As yet another example, in some embodiments, the sensors used to detect the interfering magnetic field in conventional SEMs may be located near devices (e.g., an ion pump) that generate a DC magnetic field, and get saturated by the DC magnetic field such that their ability to detect the interfering magnetic field is affected. In some embodiments of the present disclosure, one or more permanent magnets may be positioned near each sensor to cancel the DC magnetic field at that location so that the sensors can effectively measure the interfering magnetic field. In some embodiments of the present disclosure, pairs of electromagnetic coils may be arranged in one or more of the X, Y, and Z dimensions and driven by DC current to generate a DC magnetic field to cancel the DC magnetic field in the vicinity of the SEM column. In some embodiments of the present disclosure, the electromagnetic coils used to generate the uniform cancellation field may be separated from the conductive members of the frame to reduce magnetic interference.

[0028] Relative dimensions of components in drawings may be exaggerated for clarity. Within the following description of drawings, the same or like reference numbers refer to the same or like components or entities, and only the differences with respect to the individual embodiments are described. Other objects and advantages of the disclosure may be realized by the elements and combinations as set forth in the embodiments discussed herein. However, embodiments of the present disclosure are not necessarily required to achieve such exemplary objects or advantages, and some embodiments may not achieve any of the stated objects or advantages.

[0029] As used herein, unless specifically stated otherwise, the term “or” encompasses all possible combinations, except where infeasible. For example, if it is stated that a component includes A or B, then, unless specifically stated otherwise or infeasible, the component may include A, or B, or A and B. As a second example, if it is stated that a component includes A, B, or C, then, unless specifically stated otherwise or infeasible, the component may include A, or B, or C, or A and B, or A and C, or B and C, or A and B and C. Expressions such as “at least one of’ do not necessarily modify an entirety of a following list and do not necessarily modify each member of the list, such that “at least one of A, B, and C” should be understood as including only one of A, only one of B, only one of C, or any combination of A, B, and C. The phrase “one of A and B” or “any one of A and B” shall be interpreted in the broadest sense to include one of A, or one of B.

[0030] All relative terms such as “about,” “substantially,” “approximately,” etc., indicate a possible variation of ±10% (unless noted otherwise or another variation is specified). For example, a feature disclosed as being about “t” units long (wide, thick, etc.) may vary in length from (t-O.lt) to (t+0. It)units. Similarly, a width within a range of about 100-150 mm can be any width between (100 - 10%) and (150 + 10%). Further, a range described as varying from, or between, 5 to 10 (5-10), includes the endpoints (i.e., 5 and 10). In some cases, the specification also provides context to some of the relative terms used. Similarly, term “generally” implies a degree of approximation or similarity rather than an exact replication. For example, a structure described as being substantially circular or generally circular, it means that the structure shares similarities with the geometric characteristics of a circle but may not precisely match its form. For example, its shape may deviate slightly (e.g., 10% variation in diameter or curvature at different locations, etc.) from being perfectly circular.

[0031] Fig. 1 illustrates an exemplary electron beam (EB) system 100 consistent with embodiments of the present disclosure. EB system 100 may be used for imaging in any application (metrology, defect detection, etc.). As shown in Fig. 1, EB system 100 includes a main chamber 101, a load / lock chamber 102, a beam tool 104, and an equipment front end module (EFEM) 106. Beam tool 104 is located within main chamber 101. EFEM 106 includes a first loading port 106a and a second loading port 106b. EFEM 106 may include additional loading port(s). First loading port 106a and second loading port 106b receive wafer front opening unified pods (FOUPs) that contain wafers (e.g., semiconductor wafers or wafers made of other material(s)) or samples to be inspected (wafers and samples may be used interchangeably). A “lot” is a plurality of wafers that may be loaded for processing as a batch.

[0032] One or more robotic arms (not shown) in EFEM 106 may transport the wafers to load / lock chamber 102. Load / lock chamber 102 is connected to a load / lock vacuum pump system (not shown) which removes gas molecules in load / lock chamber 102 to reach a first pressure below the atmospheric pressure. After reaching the first pressure, one or more robotic arms (not shown) may transport the wafer from load / lock chamber 102 to main chamber 101. Main chamber 101 is connected to a main chamber vacuum pump system (not shown) which removes gas molecules in main chamber 101 to reach a second pressure below the first pressure. After reaching the second pressure, the wafer is subject to inspection by beam tool 104. Beam tool 104 may be a single-beam system or a multi-beam system.

[0033] A controller 109 is electronically connected to beam tool 104. Controller 109 may be a computer configured to execute various controls of EB system 100. While controller 109 is shown in Fig. 1 as being outside of the structure that includes main chamber 101, load / lock chamber 102, and EFEM 106, it is appreciated that controller 109 may be a part of the structure. In some embodiments, controller 109 may include one or more processors (not shown). A processor may be a generic or specific electronic device capable of manipulating or processing information. For example, the processor may include any combination of any number of a central processing unit (or “CPU”), a graphics processing unit (or “GPU”), an optical processor, a programmable logic controllers, a microcontroller, a microprocessor, a digital signal processor, an intellectual property (IP) core, a Programmable Logic Array (PLA), a Programmable Array Logic (PAL), a Generic Array Logic(GAL), a Complex Programmable Logic Device (CPLD), a Field-Programmable Gate Array (FPGA), a System On Chip (SoC), an Application-Specific Integrated Circuit (ASIC), a neural processing unit (NPU), and any other type of circuit capable of data processing. The processor may also be a virtual processor that includes one or more processors distributed across multiple machines or devices coupled via a network.

[0034] In some embodiments, controller 109 may further include one or more memories (not shown). A memory may be a generic or specific electronic device capable of storing codes and data accessible by the processor (e.g., via a bus). For example, the memory may include any combination of any number of a random-access memory (RAM), a read-only memory (ROM), an optical disc, a magnetic disk, a hard drive, a solid-state drive, a flash drive, a security digital (SD) card, a memory stick, a compact flash (CF) card, or any type of storage device. The codes and data may include an operating system (OS) and one or more application programs (or “apps”) for specific tasks. The memory may also be a virtual memory that includes one or more memories distributed across multiple machines or devices coupled via a network.

[0035] Fig. 2A illustrates a schematic diagram of an example multi-beam beam tool 104A (also referred to herein as apparatus 104A) and an image processing system 290 that may be configured for use in EB system 100 (Fig. 1), consistent with embodiments of the present disclosure. Beam tool 104A comprises a charged particle source 202, a gun aperture 204, a condenser lens 206, a primary charged-particle beam 210 emitted from charged particle source 202, a source conversion unit 212, a plurality of beamlets 214, 216, and 218 of primary charged-particle beam 210, a primary projection optical system 220, a motorized wafer stage 280, a wafer holder 282, multiple secondary charged- particle beams 236, 238, and 240, a secondary optical system 242, and a charged particle detection device 244. Primary projection optical system 220 can comprise a beam separator 222, a deflection scanning unit 226, and an objective lens 228. Charged particle detection device 244 can comprise detection sub-regions 246, 248, and 250.

[0036] Charged particle source 202, gun aperture 204, condenser lens 206, source conversion unit 212, beam separator 222, deflection scanning unit 226, and objective lens 228 can be aligned with a primary optical axis 260 of apparatus 104A. Secondary optical system 242 and charged particle detection device 244 can be aligned with a secondary optical axis 252 of apparatus 104A.

[0037] Charged particle source 202 can emit one or more charged particles, such as electrons, protons, ions, muons, or any other particle carrying electric charges. In some embodiments, charged particle source 202 may be an electron source. For example, charged particle source 202 may include a cathode, an extractor, or an anode, wherein primary electrons can be emitted from the cathode and extracted or accelerated to form primary charged-particle beam 210 (in this case, a primary electron beam) with a crossover (virtual or real) 208. For ease of explanation without causing ambiguity, electrons are used as examples in some of the descriptions herein. However, it should be noted that any charged particle may be used in any embodiment of this disclosure, not limited to electrons.Primary charged-particle beam 210 can be visualized as being emitted from crossover 208. Gun aperture 204 can block off peripheral charged particles of primary charged-particle beam 210 to reduce Coulomb effect. The Coulomb effect may cause an increase in size of probe spots.

[0038] Source conversion unit 212 can comprise an array of image-forming elements and an array of beam-limit apertures. The array of image-forming elements can comprise an array of micro-deflectors or micro-lenses. The array of image-forming elements can form a plurality of parallel images (virtual or real) of crossover 208 with a plurality of beamlets 214, 216, and 218 of primary charged-particle beam 210. The array of beam-limit apertures can limit the plurality of beamlets 214, 216, and 218. While three beamlets 214, 216, and 218 are shown in Fig. 2 A, embodiments of the present disclosure are not so limited. For example, in some embodiments, the apparatus 104 A may be configured to generate a first number of beamlets. In some embodiments, the first number of beamlets may be in a range from 1 to 1000. In some embodiments, the first number of beamlets may be in a range from 200-500.

[0039] Condenser lens 206 can focus primary charged-particle beam 210. The electric currents of beamlets 214, 216, and 218 downstream of source conversion unit 212 can be varied by adjusting the focusing power of condenser lens 206 or by changing the radial sizes of the corresponding beam-limit apertures within the array of beam-limit apertures. Objective lens 228 can focus beamlets 214, 216, and 218 onto a wafer 230 for imaging, and can form a plurality of probe spots 270, 272, and 274 on a surface of wafer 230.

[0040] Beam separator 222 can be a beam separator of Wien filter type generating an electrostatic dipole field and a magnetic dipole field. In some embodiments, if they are applied, the force exerted by the electrostatic dipole field on a charged particle (e.g., an electron) of beamlets 214, 216, and 218 can be substantially equal in magnitude and opposite in a direction to the force exerted on the charged particle by magnetic dipole field. Beamlets 214, 216, and 218 can, therefore, pass straight through beam separator 222 with zero deflection angle. However, the total dispersion of beamlets 214, 216, and 218 generated by beam separator 222 can also be non-zero. Beam separator 222 can separate secondary charged-particle beams 236, 238, and 240 from beamlets 214, 216, and 218 and direct secondary charged-particle beams 236, 238, and 240 towards secondary optical system 242.

[0041] Deflection scanning unit 226 can deflect beamlets 214, 216, and 218 to scan probe spots 270, 272, and 274 over a surface area of wafer 230. In response to the incidence of beamlets 214, 216, and 218 at probe spots 270, 272, and 274, secondary charged-particle beams 236, 238, and 240 may be emitted from wafer 230. Secondary charged-particle beams 236, 238, and 240 may comprise charged particles (e.g., electrons) with a distribution of energies. For example, secondary charged-particle beams 236, 238, and 240 may be secondary electron beams including secondary electrons (energies < 50 eV) and backscattered electrons (energies between 50 eV and landing energies of beamlets 214, 216, and 218). Secondary optical system 242 can focus secondary charged-particle beams 236, 238, and 240 onto detection sub-regions 246, 248, and 250 of charged particle detection device 244.Detection sub-regions 246, 248, and 250 may be configmed to detect corresponding secondary charged-particle beams 236, 238, and 240 and generate corresponding signals (e.g., voltage, current, or the like) used to reconstruct an inspection image of structures on or underneath the surface area of wafer 230.

[0042] The generated signals may represent intensities of secondary charged-particle beams 236, 238, and 240 and may be provided to image processing system 290 that is in communication with charged particle detection device 244, primary projection optical system 220, and motorized wafer stage 280. The movement speed of motorized wafer stage 280 may be synchronized and coordinated with the beam deflections controlled by deflection scanning unit 226, such that the movement of the scan probe spots (e.g., scan probe spots 270, 272, and 274) may orderly cover regions of interests on the wafer 230. The parameters of such synchronization and coordination may be adjusted to adapt to different materials of wafer 230. For example, different materials of wafer 230 may have different resistance-capacitance characteristics that may cause different signal sensitivities to the movement of the scan probe spots.

[0043] The intensity of secondary charged-particle beams 236, 238, and 240 may vary according to the external or internal structure of wafer 230, and thus may indicate whether wafer 230 includes defects. Moreover, as discussed above, beamlets 214, 216, and 218 may be projected onto different locations of the top surface of wafer 230, or different sides of local structures of wafer 230, to generate secondary charged-particle beams 236, 238, and 240 that may have different intensities. Therefore, by mapping the intensity of secondary charged-particle beams 236, 238, and 240 with the areas of wafer 230, image processing system 290 may reconstruct an image that reflects the characteristics of internal or external structures of wafer 230.

[0044] In some embodiments, image processing system 290 may include an image acquirer 292, a storage 294, and a controller 296. Image acquirer 292 may comprise one or more processors. For example, image acquirer 292 may comprise a computer, server, mainframe host, terminals, personal computer, any kind of mobile computing devices, or the like, or a combination thereof. Image acquirer 292 may be communicatively coupled to charged particle detection device 244 of beam tool 104A through a medium such as an electric conductor, optical fiber cable, portable storage media, IR, Bluetooth, internet, wireless network, wireless radio, or a combination thereof. In some embodiments, image acquirer 292 may receive a signal from charged particle detection device 244 and may construct an image. Image acquirer 292 may thus acquire inspection images of wafer 230. Image acquirer 292 may also perform various post-processing functions, such as generating contours, superimposing indicators on an acquired image, or the like. Image acquirer 292 may be configured to perform adjustments of brightness and contrast of acquired images. In some embodiments, storage 294 may be a storage medium such as a hard disk, flash drive, cloud storage, random access memory (RAM), other types of computer-readable memory, or the like. Storage 294 may be coupled with image acquirer 292 and may be used for saving scanned raw image data as original images, and post-processed images. Image acquirer 292 and storage 294 may be connected to controller 296. In some embodiments, image acquirer 292, storage 294, and controller 296 may be integrated together as one control unit.

[0045] In some embodiments, image acquirer 292 may acquire one or more inspection images of a wafer based on an imaging signal received from charged particle detection device 244. An imaging signal may correspond to a scanning operation for conducting charged particle imaging. An acquired image may be a single image comprising a plurality of imaging areas. The single image may be stored in storage 294. The single image may be an original image that may be divided into a plurality of regions. Each of the regions may comprise one imaging area containing a feature of wafer 230. The acquired images may comprise multiple images of a single imaging area of wafer 230 sampled multiple times over a time sequence. The multiple images may be stored in storage 294. In some embodiments, image processing system 290 may be configured to perform image processing steps with the multiple images of the same location of wafer 230.

[0046] When electrons of primary charged-particle beam 210 are projected onto a surface of wafer 230 (e.g., probe spots 270, 272, and 274), the electrons of primary charged-particle beam 210 may penetrate the surface of wafer 230 for a certain depth, interacting with particles of wafer 230. Some electrons of primary charged-particle beam 210 may elastically interact with (e.g., in the form of elastic scattering or collision) the materials of wafer 230 and may be reflected or recoiled out of the surface of wafer 230. An elastic interaction conserves the total kinetic energies of the bodies (e.g., electrons of primary charged-particle beam 210) of the interaction, in which the kinetic energy of the interacting bodies does not convert to other forms of energy (e.g., heat, electromagnetic energy, or the like). Such reflected electrons generated from elastic interaction may be referred to as backscattered electrons (BSEs). Some electrons of primary charged-particle beam 210 may inelastically interact with (e.g., in the form of inelastic scattering or collision) the materials of wafer 230. An inelastic interaction does not conserve the total kinetic energies of the bodies of the interaction, in which some or all of the kinetic energy of the interacting bodies convert to other forms of energy. For example, through the inelastic interaction, the kinetic energy of some electrons of primary charged-particle beam 210 may cause electron excitation and transition of atoms of the materials. Such inelastic interaction may also generate electrons exiting the surface of wafer 230, which may be referred to as secondary electrons (SEs). Yield or emission rates of BSEs and SEs depend on, e.g., the material under inspection and the landing energy of the electrons of primary charged-particle beam 210 landing on the surface of the material, among others. The energy of the electrons of primary charged- particle beam 210 may be imparted in part by its acceleration voltage (e.g., the acceleration voltage between the anode and cathode of charged particle source 202 in Fig. 2A). The quantity of BSEs and SEs may be more or fewer (or even the same) than the injected electrons of primary charged-particle beam 210.

[0047] In some embodiments, image processing system 290 may include measurement circuits (e.g., analog-to-digital converters) to obtain a distribution of the detected secondary charged particles (e.g., secondary electrons). The charged particle distribution data collected during a detection time window, in combination with corresponding scan path data of beamlets 214, 216, and 218 incident on the wafer surface, can be used to reconstruct images of the wafer structures under inspection. The reconstructed images can be used to reveal various features of the internal or external structures of wafer 230, and thereby can be used to reveal any defects that may exist in the wafer.

[0048] Another example of a charged-particle beam apparatus will now be discussed with reference to Fig. 2B. Beam tool 104B (also referred to herein as apparatus 104B) may be an example of beam tool 104 and may be similar to beam tool 104 A shown in Fig. 2A. However, different from apparatus 104A, apparatus 104B may be a single-beam tool that uses only one primary electron beam to scan one location on the wafer at a time. As shown in Fig. 2B, apparatus 104B includes a wafer holder 136 supported by motorized stage 134 to hold a wafer 150 to be inspected. Beam tool 104B includes an electron emitter, which may comprise a cathode 103, an anode 121, and a gun aperture 122. Beam tool 104B further includes a beam limit aperture 125, a condenser lens 126, a column aperture 135, an objective lens assembly 132, and a detector 144. Objective lens assembly 132, in some embodiments, may be a modified SORIL lens, which includes a pole piece 132a, a control electrode 132b, a deflector unit 132c, and an exciting coil 132d. In a detection or imaging process, an electron beam 161 emanating from the tip of cathode 103 may be accelerated by anode 121 voltage, pass through gun aperture 122, beam limit aperture 125, condenser lens 126, and be focused into a probe spot 170 by the modified SORIL lens and impinge onto the surface of wafer 150. Probe spot 170 may be scanned across the surface of wafer 150 by a deflector, such as deflector unit 132c or other deflectors in the SORIL lens. Secondary or scattered particles, such as secondary electrons or scattered primary electrons emanated from the wafer surface may be collected by detector 144 to determine intensity of the beam and so that an image of an area of interest on wafer 150 may be reconstructed.

[0049] There may also be provided an image processing system 199 that includes an image acquirer 120, a storage 130, and controller 109. Image acquirer 120 may comprise one or more processors. For example, image acquirer 120 may comprise a computer, server, mainframe host, terminals, personal computer, any kind of mobile computing devices, and the like, or a combination thereof. Image acquirer 120 may connect with detector 144 of beam tool 104B through a medium such as an electrical conductor, optical fiber cable, portable storage media, IR, Bluetooth, internet, wireless network, wireless radio, or a combination thereof. Image acquirer 120 may receive a signal from detector 144 and may construct an image. Image acquirer 120 may thus acquire images of wafer 150. Image acquirer 120 may also perform various post-processing functions, such as image averaging, generating contours, superimposing indicators on an acquired image, and the like. Image acquirer 120 may be configured to perform adjustments of brightness and contrast, etc. of acquired images. Storage 130 may be a storage medium such as a hard disk, random access memory (RAM), cloud storage,other types of computer readable memory, and the like. Storage 130 may be coupled with image acquirer 120 and may be used for saving scanned raw image data as original images, and postprocessed images. Image acquirer 120 and storage 130 may be connected to controller 109. In some embodiments, image acquirer 120, storage 130, and controller 109 may be integrated together as one electronic control unit.

[0050] In some embodiments, image acquirer 120 may acquire one or more images of a sample based on an imaging signal received from detector 144. An imaging signal may correspond to a scanning operation for conducting charged particle imaging. An acquired image may be a single image comprising a plurality of imaging areas that may contain various features of wafer 150. The single image may be stored in storage 130. Imaging may be performed on the basis of imaging frames.

[0051] The condenser and illumination optics of the electron beam tool may comprise or be supplemented by electromagnetic quadrupole electron lenses. For example, as shown in Fig. 2B, electron beam tool 104B may comprise a first quadrupole lens 148 and a second quadrupole lens 158. In some embodiments, the quadrupole lenses may be used for controlling the electron beam. For example, first quadrupole lens 148 may be controlled to adjust the beam current and second quadrupole lens 158 may be controlled to adjust the beam spot size and beam shape.

[0052] Fig. 2B illustrates a charged-particle beam apparatus that may use a single primary beam configured to generate secondary electrons by interacting with wafer 150. Detector 144 may be placed along optical axis 105, as in the example shown in Fig. 2B. The primary electron beam may be configured to travel along optical axis 105. Accordingly, detector 144 may include a hole at its center so that the primary electron beam may pass through to reach wafer 150. Fig. 2B shows an example of detector 144 having an opening at its center. However, some embodiments may use a detector placed off axis relative to the optical axis along which the primary electron beam travels. For example, as in the example shown in Fig. 2A, discussed above, a beam separator 222 may be provided to direct secondary electron beams toward a detector placed off axis. Beam separator 222 may be configured to divert secondary electron beams toward an electron detection device 244, as shown in Fig. 2A. While Figs. 2A and 2B generally show an electron beam inspection system, the disclosed embodiments can be used by any electron beam system.

[0053] Charged-particle beam systems, such as electron beam (EB) system 100, are sensitive to changes of magnetic field (from sources internal or external to EB system 100), as it directly interacts with charged-particle beams and adversely affect image quality by shifting the charged-particle beam in both time and space. For example, variations in the environmental magnetic fields (or interfering magnetic fields) affect charged-particle beams by exerting forces that cause deflection or distortion of their trajectories and thereby impact the performance of the system. Stray magnetic fields generated by sources located within the particle beam instrument may adversely affect image quality by causing unintended shift of the particle beam in space and time. Magnetic field disturbances, or interferingmagnetic fields, may be generally categorized as quasi-DC (slowly varying) and alternating current (AC) fields. Quasi-DC (or simply DC) magnetic fields may result from movements of relatively large ferromagnetic objects / structures, such as, for example, elevators, swing doors, rotating fan blades, etc. AC magnetic fields may result from power transformers, electrical cabinets, motors and motor drivers, power lines, etc. Magnetic field disturbances (or interfering magnetic fields) vary from site to site and generally result from a combination of unknown sources. To mitigate interference to charged- particle beam systems from such magnetic field disturbances, passive, active, or a combination of passive and active magnetic field cancellation systems and techniques may be employed.

[0054] As schematically illustrated in Fig. 3, active magnetic field cancellation (MFC) systems may be implemented with a pair of electromagnetic coils per axis and a 3D magnetometer to protect a volume of interest between the pairs of coils. Typically, the magnetometer detects the magnetic field in the volume of interest and current is directed through the pairs of coils by a controller 390 (or controller 109 of Fig. 1, etc.) to generate a counteracting or compensating magnetic field. Each electromagnetic coil in the coil structure may include one or more coils of wire wound around a central axis, sometimes in a circular, square, or rectangular shape, among others. When an electric current flows through the coil, it generates a magnetic field around the coil according to Ampere’s circuital law.

[0055] In some embodiments, as illustrated in Fig. 3, each electromagnetic coil extends around a coordinate axis. For example, coils 310’x and 3102xextend around the X axis. In other words, the X axis (or a line parallel to the X axis) forms a central axis around which coils 3101. and 3102xextend. Similarly, coils 3101,.. and 3102yextend around the Y axis, and coils 3 l ()'zand 3102zextend around the Z axis. Further, electromagnetic coil pairs are spaced apart and arranged along each coordinate axis (e.g., 310’x and 3102xalong the X axis, 310’y and 3102yalong the Y axis, and 310’z and 3102zalong the Z axis). In general, any pair of coils of wire wound around a common axis can generate a magnetic field at the volume of interest. When a current is passed through these coil pairs, a uniform magnetic field is generated in a relatively small zone or region (e.g., volume 300) between the pairs of coils. As used herein, a uniform magnetic field refers to a magnetic field that is generally consistent. In other words, although the magnetic field strength and direction across the entire defined region may not be perfectly constant, they remain relatively constant with small possible deviations (e.g., ±10%) that are considered acceptable or negligible in the application. A magnetic field sensor (e.g., a 3 -axis magnetometer 320) may detect the magnetic field interferences along each axis (X, Y, and Z) between the pairs of coils. Based on measured magnetic field by this sensor, the controller 390 drives current to the coils in a closed-loop structure such that the substantially uniform magnetic field generated by the rectangular coil structure counteracts (negates, nullifies, cancels, compensates for, etc.) the effect of the detected variations in magnetic field.

[0056] Counteracting variations of the magnetic field involves generating a magnetic field that opposes and significantly reduces, or cancels out, the changes of the magnetic field. By counteractingthe magnetic field variations, the effects of the disturbance in the magnetic field (or the interfering magnetic field) can be greatly reduced or completely nullified, allowing sensitive equipment (e.g., a charged-particle beam apparatus) to operate accurately and reliably without interference. Although the electromagnetic coils are illustrated as being arranged in a rectangular configuration in Fig. 3, this is only exemplary. In general, these coils may be arranged in any pattern around the different coordinate axes (X, Y, and Z). Charged-particle beam pathways (e.g., the SEM column) of a charged-particle beam apparatus may generally be positioned between the coil pairs (e.g., 3 IO1, and 3102x, 3 IO1,.. and 3102y, and 31 ()'zand 3102z) to try to minimize the effect of interfering magnetic fields on the charged- particle beams.

[0057] A rectangular coil arrangement with two electromagnetic coils per axis (e.g., as illustrated in Fig. 3) may create magnetic field disturbance protection only within a limited volume (e.g., volume 300) at the center of the coil structure. Therefore, while such an arrangement may offer adequate protection (from variations of magnetic fields) to charged-particle beams at the center of an SEM column (when the SEM column is centrally positioned in volume 300), protection may be diminished for off-center charged-particle beams within the SEM column. Furthermore, in some charged-particle beam systems, due to design constraints, the SEM column may be located off-center relative to all or some of the electromagnetic coil pairs (e.g., in volume 300). In such cases, any coil arrangement (such as, for example, Helmholtz coil arrangement) may offer limited protection from variations of magnetic fields to charged-particle beams in the off-center SEM column due to the beam passing through non-uniform areas of the Helmholtz coil field. A Helmholtz coil arrangement is a specific geometric arrangement of electromagnetic coils with two electromagnetic coils per axis.

[0058] Moreover, in some cases, as illustrated in Fig. 3, the electromagnetic coil pairs of the active magnetic field cancellation system may be routed on a frame 350 positioned around the charged- particle beam apparatus (not shown in Fig. 3). Frame 350 may be made of an electrically conductive material, such as, for example, steel, aluminum, or another metal. Due to the inductive coupling between the electromagnetic coils and the metal frame, the frame may act as a short-circuited turn of each coil. The inductive coupling of electromagnetic coils with the short-circuited turn may decrease the compensatory magnetic field generated by the coil and reduce the effectiveness of the magnetic field cancellation system.

[0059] Magnetic fields affect the trajectories of charged-particle beams in the SEM column of a charged-particle beam apparatus and lead to distorted images of a sample being imaged (e.g., wafer 230 of Fig. 2A, wafer 150 of Fig. 2B., etc.). Static magnetic field effects are addressed by the design of beam optics, however variations of the field result in distortion of the beam path. This distortion can obscure defects or features of interest in the imaged sample (e.g., a semiconductor device) and thereby detrimentally affect the defect detection and metrology. As semiconductor devices shrink in size, the demand for higher-resolution charged-particle beam systems (e.g., electron beam tools) with higher fidelity of imaging increases, which in turn increases the sensitivity of such tools to magneticfield disturbances. The systems and methods of the present disclosure provide protection for a charged-particle beam apparatus from magnetic field disturbances (or interfering magnetic fields).

[0060] To reduce the inductive coupling between the electromagnetic coils (e.g., 3 IO1,. 3102x, 310’y, 3102y, 3 l()'z. 3102z) and the metal frame 350 resulting from their close proximity, in some embodiments of the present disclosure, the electromagnetic coils of Helmholtz or other geometrical configuration may be physically spaced apart from the metal frame 350. For example, as illustrated in Fig. 4A, coil 31 ()' (of the X axis rectangular coil pair) may be connected to members 352A, 354B, 352C, and 352D of metal frame 350 via spacers or standoffs 360 such that this coil is separated from these metal members. Instead, the standoffs 360 that mechanically connect coil 3101, to members 352A, 352B, 352C, and 352D maintain a physical spacing between the coil and these members. Similarly, the second coil of the X axis coil pair, coil 3102x, may be mechanically connected to members 354A, 354B, 354C, and 354D of metal frame 350 using standoffs 360 such that this coil is spaced apart from members 354A, 354B, 354C, and 354D. It should be noted that, although only the coil pair arranged along the X axis (coils 310!xand 3102x) are illustrated in Fig. 4A, this is only exemplary. In some embodiments, the coils of all coil pairs (e.g., 310’x and 3102x, 310’y and 3102y, and 3 l ()'zand 3102z) may be similarly connected to metal frame 350 using standoffs 360 such that each coil is physically spaced apart from metal frame 350. Electromagnetic coils of any geometrical configuration could be spatially separated from the conductive frame to reduce inductive coupling between the coil and the frame.

[0061] Using standoffs 360 to maintain physical spacing between the electromagnetic coil and the metal frame may isolate the coil from the frame electromagnetically. The spatial separation between the electromagnetic coils and the metal frame 350 (of the charged-particle beam apparatus) reduces the mutual inductance between the coils and the frame, which in turn improves its electromagnetic response and increases the cancellation bandwidth of the magnetic field cancellation system. Cancellation bandwidth refers to the range of frequencies, or the spectrum of magnetic fields, that the cancellation system can be used to mitigate or neutralize. A wider cancellation bandwidth means that the magnetic field cancellation system is effective at canceling or counteracting a broader range of magnetic field frequencies, and therefore provides more comprehensive protection against interference from interfering magnetic fields.

[0062] To isolate the electromagnetic coils (Helmholtz or other geometrical configuration of electromagnetic coils) and the metal frame electromagnetically, the standoffs 360 may be made of a non-ferromagnetic material. In some embodiments, the material of the standoffs 360 may also be electrically insulating. The specific material used may depend on the application (e.g., environmental conditions, mechanical requirements, etc.). In some embodiments, the standoffs 360 may be made of one or more of nylon, polyethylene, polypropylene, rubber, ceramic, glass (e.g., borosilicate glass), air, pyrolytic graphite, bismuth, ferrite, or a composite material (e.g., fiberglass, carbon fiber, or aramid fibers in an epoxy resin).

[0063] In some embodiments, to reduce inductive coupling between the coil and the metal frame 350 to be within acceptable levels, the standoffs 360 may maintain a spacing greater than or equal to (>) about 20 mm between the coil and the members (e.g., 352A-352D, 354A-354D, etc.) of metal frame 350. In some embodiments, the standoffs 360 may maintain a spacing between about 20 - 50 mm between the coils and the metal frame 350. The term “about” indicates that the spacing may not be precisely the specified value and accounts for a small margin of error or possible practical variations (e.g., ±10%) in the spacing. It should be noted that the shape and configuration of the standoffs 360 illustrated in Fig. 4 A is only exemplary. In general, the standoffs 360 may have any suitable shape and configuration. For example, in some embodiments, the standoffs 360 may be configured as washers, screws, sleeves, bushings, spacers, cable ties, etc. that may maintain the desired spacing between the coils and the metal frame 350.

[0064] Thus, in embodiments of the present disclosure, the electromagnetic coils of the magnetic field cancellation system are separated from the conductive components of the SEM (e.g., parts of the metal frame 350 surrounding the SEM) by the use of a standoff or a spacer of a suitable material to reduce the magnetic interaction between the coil and the frame component. In some exemplary embodiments, reducing the mutual inductance enables the coils to generate a counteracting or cancelling magnetic field that may counteract or cancel an interfering magnetic field with a frequency, for example, greater than 50 Hz.

[0065] In addition to providing a physical spacing between the electromagnetic coils (e.g., 313102x, 310’y, 3102y, 3 l()'z. 3102z) and the metal frame 350, or as an alternative to providing the physical spacing, in some embodiments, some or all members (e.g., 352A-352D, 354A-354D, etc.) of metal frame 350 may be connected to each other using shims, washers, or spacers made of an electrically non-conductive material to reduce the possibility of the metal frame forming a short- circuited turn of a coil. For example, as illustrated in Fig. 4B, where coil 31 ()', is routed through members 352A, 352B, 352C, and 352D of frame 350', to prevent (or reduce the possibility of) these members from forming a short-circuited turn of coil 31electrically non-conductive spacers 380 may be connected between these members. It should be noted that the specific arrangement of spacers illustrated in Fig. 4B is only exemplary. In general, electrically non-conductive spacers 380 may be positioned at any suitable location to break or disrupt the electrical continuity between members 352A, 352B, 352C, and 352D. For example, spacers 380 may only be positioned between some members (e.g., between members 352A and 352B and between members 552C and 352D) and not between other members (e.g., between members 352B and 352C and between members 352D and 352A).

[0066] As explained previously with reference to Fig. 3, an active magnetic field cancellation system with only two electromagnetic coils per axis (e.g., coils 310’x and 3102xalong the X axis, etc.) yields a volume with relatively uniform magnetic field protection only within a limited volume between the two coils, while uniformity of the field and therefore effectiveness of protection is diminished for off-center charged-particle beams within the SEM column or for an SEM column positioned off-center relatively to electromagnetic coils of the compensation system. As explained previously, a specific geometric configuration or arrangement of coils with two electromagnetic coils per axis is generally referred to as a Helmholtz coil arrangement. In some embodiments, to enclose substantially the entire SEM column and the entire charged-particle beam path within the protective volume of a uniform magnetic cancellation field, or to shape and extend the volume of the generated magnetic cancellation field, one or more auxiliary driving coils may be incorporated between the pair of coils (e.g., of Figs. 3, 4A, 4B) along one or more axes (X, Y, and Z). For example, Fig. 5A is a schematic illustration of an exemplary arrangement of auxiliary electromagnetic coils in a magnetic field cancellation system of an exemplary charged-particle beam apparatus. In the example arrangement illustrated in Fig. 5A, a pair of auxiliary coils 3201xand 3202xis positioned between the pair of electromagnetic coils 313102xarranged along the X axis. Powering the pair of coils 313102xgenerates a uniform magnetic cancellation field in a volume between these two coils. By powering the auxiliary coils 32()',:and 320:x, the volume of the uniform magnetic cancellation field generated by the pair of coils 310^, 3102Xmay be stretched (e.g., along the X axis) or shaped (e.g., along the Y or Z axes) to enclose the off-center charged-particle beams in the SEM column, or a column physically positioned off-center relatively to the electromagnetic coil structure, for effective suppression of magnetic disturbances (or interfering magnetic field) in the entire SEM column. It should be noted that although two auxiliary coils 3201xand 3202xare positioned between 310’x, 3102xin Fig. 5A, this is only exemplary. In general, any number (e.g., one or more) of auxiliary coils may be positioned between 310’x, 3102x.

[0067] In some embodiments, one or more auxiliary coils may be positioned between the pair of coils along all axes (X, Y, and Z). However, in some embodiments, the one or more auxiliary coils may only be positioned between the pair of coils along selected axes (e.g., the X axis). In some embodiments, one or more auxiliary coils may also be positioned between the pair of coils along the Y axis or the Z axis. For example, as illustrated in Fig. 5B, a single auxiliary coil 320ymay be positioned between the pair of coils 31 ()'yand 3102yarranged along the Y axis. Alternatively, or additionally, in some embodiments, as illustrated in Fig. 5C, a pair of auxiliary coils 3201z, 3202zmay be positioned between the pair of coils 3 lO’z, 3102zarranged along the Z axis. In general, the auxiliary coils may have any configuration (e.g., circular, rectangular, square, etc.) and size. The auxiliary coils may be aligned with the pair of electromagnetic coils that they are positioned between. For example, if one or more auxiliary coils are positioned between the pair of electromagnetic coils 310’x, 3102xarranged along the X axis, these auxiliary coils may be aligned with coils 310’x, 3102x. In some embodiments, as illustrated in Figs. 5A and 5C, the size and configuration of the auxiliary coils may be generally similar to that of the coils they are positioned between. However, this is not a requirement. For example, as illustrated in Fig. 5B, in some embodiments, the auxiliary coil(s) may be smaller, or larger, or have geometrical shape that is different than the coils that they are positioned between.

[0068] In some embodiments, as illustrated by double headed arrows in Figs. 5A-5C, the auxiliary coils may be independently movable (e.g., slidable) along the respective axis relative to the outer coils. For example, with reference to Fig. 5A, the pair of auxiliary coils 3201xand 3202xmay be independently movable along the X axis to vary (e.g., increase or decrease) the spacing between the auxiliary coils. Similarly, the pair of auxiliary coils 320!zand 3202zmay be independently movable along the Z axis (see Fig. 5C), and auxiliary coil 320Y may be movable along the Y axis. The auxiliary coils may be coupled to frames or other structural members that may be selectively moved (e.g., using actuators, links, gears, or other suitable mechanisms) by controller (e.g., controller 109 of Fig. 1, controller 390 of Fig. 3, etc.) to move, extend, or shape the protective volume of the uniform magnetic cancellation field. For example, the protective volume may be moved or stretched along the X axis by selectively powering auxiliary coil 320',:or 3202x. By selectively powering the auxiliary coils along the X, Y, and Z axes, the protective volume of the uniform magnetic cancellation field may be moved or stretched in any direction to enclose the entire SEM column and the whole beam path within the protective volume.

[0069] Thus, in some embodiments of the present disclosure, one or more additional electromagnetic coils may be provided along some or all of the X, Y, and Z axes to enable the uniform compensating or counteracting magnetic field (generated, for example, by the coil arrangement of Figs. 3, 4 A, 4B, etc. to counteract the interfering magnetic field variations) to be moved or shaped. The controller may control (e.g., provide power to) these additional coils to move the uniform counteracting magnetic field (e.g., horizontally, vertically, etc.), or change its shape (e.g., stretch, etc.), such that the charged- particle beam path in the SEM column is encapsulated by the counteracting magnetic field. The auxiliary coils (e.g., discussed with reference to Figs. 5A-5C) may be used as an alternative to, or in addition to, the standoffs and spacers (e.g., discussed with reference to Figs. 4A and 4B). Moreover, the auxiliary coils may also incorporate standoffs, spacers, or other means for physical separation of the auxiliary coil from the metal elements. In other words, some embodiments of the disclosed magnetic field cancellation systems may include the auxiliary coils in addition to the standoffs (of Fig. 4A) or spacers (of Fig. 4B). While some embodiments of the disclosed magnetic field cancellation systems may include the auxiliary coils or the standoffs (or spacers).

[0070] Notwithstanding the discussed exemplary embodiments, peripheral electromagnetic coils, possibly forming a Helmholtz configuration, and auxiliary electromagnetic coils shaping or extending magnetic field may have same or different geometric shapes, contain same or different number of windings, and be excited by the same or different electrical current source or voltage source.

[0071] Typically, as illustrated in Fig. 3, a sensor block with a three-axis magnetometer 320 is positioned at a location near the SEM column of a charged-particle beam apparatus (not shown in Fig. 3) to monitor magnetic field near the column and detect variations of the magnetic field (e.g., the interfering magnetic field). The pair of electromagnetic coils along one or more of X, Y, and Z axis are powered by the controller (e.g., controller 390) to minimize or cancel the effect of variations in thedetected magnetic field. In some cases, a pair of sensor blocks, each with a three-axis magnetometer, may be positioned near the SEM column to monitor the magnetic environment, detect the magnetic field variations at these locations, and control the electromagnetic coils based on these measurements. However, there is a considerable difference between the magnetic field variations (e.g., disturbance) at the magnetometer location(s) and the charged-particle beam path that is spatially separated from the location(s) of magnetometer(s). This spatial offset between the charged-particle beam path and the magnetometers hampers an accurate sampling of the magnetic field at locations along the beam path. Since the variations of magnetic field at the charged-particle beam path may not be accurately measured by these magnetometers, the controller may not be able to suitably drive current into the coil pairs to nullify the effect of the magnetic field variations.

[0072] In some embodiments of the present disclosure, to accurately measure the magnetic field and its variations around the SEM column and the spatial gradient of the magnetic field in this region, as illustrated in Fig. 6, multiple pairs of magnetic field sensors, or magnetometers, with each pair configured to measure the environmental or interfering magnetic field along a single axis (e.g., X axis, Y axis, or Z axis) may be arranged around the SEM column 550 of a charged-particle beam apparatus 500. Each pair of magnetometers may be arranged such that the charged-particle beam path is located between the two magnetometers of the pair. In some embodiments, each pair of magnetometers configured to measure the magnetic field environment along a single axis (X, Y, or Z axis) are arranged such that a central axis of each magnetometer of the pair is parallel to (or coaxial with) the central axis of the electromagnetic coils (that generate the uniform counteracting magnetic field) extending around that single axis. After signals collected by the magnetometers arranged in the abovedescribed manner are processed by the controller, they form a combined signal emulating a virtual magnetic field sensor spatially coincident with the charged-particle beam, and therefore, accurately representing the interfering magnetic field at the charged-particle beam path.

[0073] For example, with reference to Fig. 6, a first pair of magnetometers 6001, and 6002xaligned with the X axis are arranged on opposite sides of the SEM column 550 to measure the interfering magnetic field along the X axis. These pair of magnetometers 6001, and 6002xare aligned with the X axis such that a central axis of each of these two magnetometers is parallel to the central axis around which the pair of coils 3101, and 3102xextend. Since the pair of magnetometers 6001, and 6002xare arranged on opposite sides of the charged-particle beam path (within the SEM column 550), they create a distributed sensor capable of detecting spatial gradient (e.g., variation in space) of the field along the X axis. Processing signals from the two (or more) magnetometers spatially distributed around SEM column emulates a virtual sensor spatially coincident with the charged-particle beam path, and accurately measures the interfering magnetic field along the X axis at the charged-particle beam path. Similarly, a second pair of magnetometers 60()'yand 6002yaligned with the Y axis are arranged on opposite sides of the SEM column 550 to measure the interfering magnetic field at the beam path along the Y axis, and a third pair of magnetometers 60()'zand 6002zaligned with the Z axisare arranged on opposite sides of the SEM column 550 to measure the interfering magnetic field at the beam path along the Z axis. Based on inputs from the first, second, and third pair of magnetometers, the controller (e.g., controller 109 of Fig. 1, controller 390 of Fig. 3, etc.) may determine the interfering magnetic field distribution in the X, Y, and Z directions at the beam path, and may direct current through the electromagnetic coils to generate a counteracting magnetic field to cancel or minimize the disturbances of magnetic field (or the interfering magnetic field) at the beam path. In some embodiments, as an alternative to the above-described arrangement of sensors, two or more sensors in X, Y, and Z axes may be arranged within the space surrounding the electron beam path to sample the magnetic field in this region. The magnetic field and variations (e.g., disturbance) of the magnetic field in X, Y, and Z directions at the charged-particle beam path may then be calculated (e.g., by the controller) based on signals from one of, or a combination of multiple sensors.

[0074] Thus, in some embodiments of the present disclosure, two or more sensors are located around the SEM column (or the charged-particle beam path) to measure the magnetic field along each axis in the vicinity of the column. These sensor measurements may then be processed to determine a counteracting magnetic field at the beam location, and drive current to the coil structure to generate a uniform counteracting magnetic field at the beam location. Although the magnetic field sensors are described as magnetometers, any type of magnetic field sensor (Hall Effect sensor, magneto-resistive sensor, fluxgate sensor, Superconducting Quantum Interference Device or SQUID sensor, search coil sensor, magnetometer, etc.) may be used.

[0075] The exemplary sensor arrangement illustrated in Fig. 6 shows two single-direction sensors arranged along each axis (X, Y, and Z) around the column coaxial with coils (e.g., along a coil axis). However, this arrangement is only exemplary. In general, any number (e.g., three, four, etc.) and any type (e.g., single-direction, bi-directional, tri-directional, etc.) of sensors may be positioned around the column. In some embodiments, these multiple sensors may be arranged along a coil axis (e.g., positioned on-axis) as described with reference to Fig. 6. While in other embodiments, they may not be arranged or aligned along a coil axis. Instead, they may be misaligned with the coil axis (e.g., positioned off-axis). Regardless of how many and what type of sensors are used, and whether they are positioned on-axis or off-axis, the controller (e.g., algorithms in the controller) may process signals from these sensors to determine the interfering magnetic field along the beam path. For example, in some embodiments of the present disclosure, multiple (e.g., three or more) bi-direction sensors (e.g., X / Y sensors) may be positioned around the column (on-axis or off-axis), and the controller may process signals from these three or more bi-directional sensors to determine the interfering magnetic field in the XY plane of the beam path. As another example, two groups of bi-directional X / Y sensors could be spatially arranged along the beam path to accurately determine the interfering magnetic field in three-dimensional space. As yet another example, in some embodiments, multiple tri-directional sensors (e.g., X / Y / Z sensors) may be positioned at two planes, for example, one plane close to one end of the beam path along Z axis and another plane near another end of the beam path along Z axis.Processing signals from these tri-directional sensors may allow the controller to determine the interfering magnetic field in the Z direction (or in other directions) in the beam path. In general, any arrangement (e.g., a random arrangement) of any number and type of sensors, aligned with a coil axis (on-axis) or misaligned with a coil axis (off-axis), can be incorporated, and the control algorithm may be designed to determine the interfering magnetic field in the beam path from these sensor signals. The controller may then drive current(s) through the electromagnetic coils of the coil structure(s) to generate a uniform counteracting magnetic field at the beam location to counteract the interfering magnetic field. In some embodiments, the controller may drive current through the coils such that the sum of the square of all magnetic fields (3D) measured by the sensors is minimized.

[0076] With further reference to Fig. 6, charged-particle beam apparatus 500 typically includes many components and devices (e.g., magnetic lens coils, magnets of ion pumps 510, 520, 530, etc.) that generate a local DC magnetic field. In some applications, the magnetic field sensors (e.g., magnetometers) used in magnetic field cancellation system of apparatus 500 may get saturated by the strong DC magnetic fields originating from these local sources. In other words, this strong local DC magnetic field may overwhelm the magnetometer’s ability to provide accurate measurements of magnetic field environment and its variations, and thereby affect the performance of the magnetometer. Although passive shielding of local DC field sources may be employed to mitigate the saturation to some extent, it may be expensive and result in a heavy and bulky structure.

[0077] To prevent (or reduce the possibility of) the magnetic field sensors (e.g., magnetometers SOO’x, 6002X, 600’y, 6002y, 600^, 6002z, magnetometer 320 of Fig. 3, or any other magnetic field sensor) in the magnetic field cancellation system (of the charged-particle beam apparatus) from getting saturated by strong local DC magnetic fields (e.g., from the ion pumps, magnetic lenses, and other components of the apparatus), in some embodiments of the present disclosure, one or more (e.g., an array of) permanent magnets may be positioned near the magnetic field sensors to neutralize or minimize the effect of the DC magnetic field from these local sources. In some embodiments, as an alternative to permanent magnets, auxiliary coils driven by DC current (referred herein as DC auxiliary coils) or an electromagnetic device may be positioned near the magnetic field sensors to generate a DC magnetic field in the location of the magnetometer that is opposing, or counteracting, the DC field from other components and therefore preventing saturation (e.g., overwhelming) of the sensor by a stray magnetic field. Since the generated DC magnetic field by these permanent magnets or auxiliary coils is time invariant, it may have no impact on the performance of the charged-particle beam apparatus, while preventing the sensors from saturation by the DC field and therefore enabling detection of magnetic field variations.

[0078] For example, as illustrated in Fig. 6, an array of permanent magnets 620 may be positioned near selected magnetic field sensors (e.g., magnetometers 600'yand 6001z) to counteract the DC magnetic field generated by ion pump 510. Permanent magnets 620 may be oriented such that their magnetic field opposes the direction of the external DC magnetic field generated by ion pump 510 inthe location of magnetometer. Although a single array of permanent magnets 620 is shown to be located near magnetometers 600'yand 6001zin Fig. 6, this is only exemplary. In general, permanent magnets (or DC auxiliary coils) may be positioned near any magnetometer that is located within a region of a strong DC magnetic field. In some embodiments, permanent magnets may be positioned near each magnetic-field sensor, and these magnets may be sized to cancel the external DC magnetic field at the associated sensor location. In general, experiments or simulations may be used to determine the magnitude and direction of external DC magnetic fields in the vicinity of the magnetometers, and permanent magnets (or DC auxiliary coils) may be suitably oriented and positioned near the magnetometers located in strong (or saturating) external DC magnetic fields to counteract its effect. Thus, in some embodiments of the present disclosure, to position magnetometers (or other magnetic field sensors) at optimal locations in the vicinity of the SEM column 550 and create a spatially distributed arrangement emulating a virtual sensor detecting magnetic field variations in the location coincident with the beam path (as described previously), interfering DC magnetic fields at the sensor locations are locally attenuated using permanent magnets or auxiliary coils driven by DC current.

[0079] As explained previously, variations of magnetic field environment (e.g., disturbances) may be categorized as quasi-DC (slowly varying) and alternating current (AC) magnetic fields. In some applications, a major source of magnetic field disturbance is intermittent changes of quasi-DC magnetic field (e.g., from nearby tools or components) and associated step-like changes of the quasi- DC magnetic field. Active magnetic field cancellation systems (e.g., the coil arrangement of Figs. 3- 4B) may not be able to fully compensate for such interferences as the change of DC magnetic field may exceed the compensation range (e.g., the minimum and maximum intensities of the magnetic field that the system can counteract) provided by magnetic field cancellation system. Moreover, in some applications, compensation for the step-like changes in the quasi-DC magnetic field may be hampered by the cancellation system bandwidth or the speed at which the magnetic field cancellation system can effectively respond to changes in the magnetic field.

[0080] In some embodiments of the present disclosure, simultaneous compensation of strong variations in DC and AC magnetic field (e.g., disturbances) may be accomplished by an additional set of compensation coils driven by DC power supplies to nullify the external DC magnetic fields that are, for example, induced by nearby machines. For example, in addition to the previously described coil arrangement (described previously, for example, with reference to Figs. 3-4B) to counteract or compensate for interfering AC magnetic field disturbance, in some embodiments, additional pairs of coils in each of the X, Y, and Z axes may be provided and driven with a DC current to generate a DC magnetic field to counteract (or compensate for) the interfering DC magnetic field in the vicinity of the SEM column.

[0081] For example, as illustrated in Fig. 7, coil pairs (e.g., 310’x and 3102xin the X axis, 310’y and 3102yin the Y axis, and 31 ()'zand 3102zin the Z axis), powered by an AC power supply 740, may bearranged along each spatial axis (X, Y, Z) to generate a counteracting AC magnetic field to counteract or compensate for the AC variations of magnetic field (i.e., disturbance). Additionally, pairs of coils (e.g., 330!xand 3302xin the X axis, 330!yand 3302yin the Y axis, and 330!zand 3302zin the Z axis) may also be provided along each spatial axis. Note that only coils 3 I O1,. 330^, 310^, 330!y, 3102z, and 3302Zare shown in Fig. 7. Coils 3102xand 3302xmay be spaced apart from coils 310’x, 33O’xin the positive X direction, coils 3102y, 3302ymay be spaced apart from coils 310’y, 330!yin the positive Y direction, and coils 3 l ()'z. and 330'zmay be spaced apart from coils 3102z, and 3302zin the negative Z direction. These additional coils may be powered using DC current by a DC power supply 760 to generate a counteracting DC magnetic field that compensates for the external DC magnetic field and quasi-DC field variations (e.g., disturbance). In some embodiments, as explained previously with reference to Fig. 4A, the electromagnetic coils may be physically spaced apart from metal frame 350 (e.g., using standoffs) to reduce inductive coupling between the coils and the frame 350. As illustrated in Fig. 7, to avoid performance deterioration of the AC magnetic field compensation due to presence of additional DC compensation coils, the DC coils may be routed on the frame 350 or driven by DC power supplies working in current source mode.

[0082] Although each of the AC and DC power supplies 740, 760 are shown to be connected to a single coil in Fig. 7 (to avoid obscuring details of the figure), each coil may be connected to a power supply (the same or a different power supply) that provides current or voltage excitation to the coil. Further, although Fig. 7 illustrates a pair of DC compensation coils along each of X, Y, and Z axes, this is only exemplary. In some embodiments, a pair of DC compensation coils may only be provided along one or more selected axes. For example, simulations may indicate that the external DC magnetic field may be predominantly along some axes (e.g., X and Y axes). In some such embodiments, a pair of DC compensation coils may only be provided along these axes (e.g., X and Y axes). It is also contemplated that, in some embodiments, only a single coil may be provided along some axes. For example, instead of providing a pair of DC compensation coils 3301, and 3302xalong the X axis, only a single DC compensation coil (e.g., 3301,) may be provided along the X axis.

[0083] Based on signals from the magnetic field sensors associated with the magnetic field cancellation system (of charged-particle beam apparatus) that are indicative of the magnetic field environment in the vicinity of the charged-particle beam path of the apparatus, a controller (e.g., controller 109 of Fig. 1) of the system may control (e.g., provide controlled power to) the AC or DC compensation coils of the system to generate AC or DC compensating magnetic fields to nullify (or reduce) the effect of the interfering magnetic fields and their variations. In some embodiments, a non- transitory computer readable medium may be provided that stores instructions for a processor of the controller to carry out, among other things, control the AC or DC compensation coils to generate AC or DC compensating magnetic fields (e.g., process 800 described below). Common forms of non- transitory media include, for example, a floppy disk, a flexible disk, hard disk, solid state drive, magnetic tape, or any other magnetic data storage medium, a Compact Disc Read Only Memory (CD-ROM), any other optical data storage medium, any physical medium with patterns of holes, a Random Access Memory (RAM), a Programmable Read Only Memory (PROM), and Erasable Programmable Read Only Memory (EPROM), a FLASH-EPROM or any other flash memory, Non-Volatile Random Access Memory (NVRAM), Field Programmable Gate Array (FPGA), Application Specific Integrated Circuit (ASIC), a cache, a register, any other memory chip or cartridge, and networked versions of the same.

[0084] Fig. 8 illustrates an exemplary process 800 that may be used by a controller to nullify (or reduce) the effect of interfering magnetic fields in the vicinity of the changed-particle beam path of a changed-particle beam apparatus.

[0085] In step 810, the controller may receive signals from magnetic field sensors (e.g., magnetometers bOO’x, 6002x, bOO’y, 6002y, bOO’z, and 6002zof Fig. 6) associated with the magnetic field cancellation system of charged-particle beam apparatus 500. These signals may be indicative of the environmental magnetic field in the vicinity of the charged-particle beam path (e.g., SEM column 550) of apparatus 500. For example, as explained with reference to Fig. 6, signals from separate pairs of magnetometers arranged on opposite sides of the charged-particle beam path along the X, Y, and Z axes may indicate the magnetic field in the charged-particle beam path.

[0086] In step 820, based on the signals received in step 810, the controller may process signals received from the magnetic field sensors to determine information about the magnetic field (e.g., the interfering magnetic field) at the location of the beam path.

[0087] In step 830, the controller may then control (e.g., provide power to) the AC compensation coils, or the DC compensation coils to generate AC or DC magnetic fields to compensate for, or counteract, the detected interfering magnetic field or variations of environmental magnetic field. For example, with reference to Fig. 7, in step 830, the controller may provide AC current to some or all of the electromagnetic coils (e.g., 310’x and 3102xin the X axis, 310’y and 3102yin the Y axis, and 3 l ()'zand 3102zin the Z axis) to generate uniform AC magnetic field in the vicinity of the charged-particle beam path to compensate for the effect of detected variations of AC magnetic field. In some embodiments, in step 830, the controller may also provide DC current or control DC current or voltage power sources to one or more of the DC compensating coil pairs (e.g., 330',:and 3302xin the X axis, 330'y and 3302yin the Y axis, and 3301zand 3302zin the Z axis) to generate a counteracting DC magnetic field in the vicinity of the charged-particle beam path to compensate for the effect of detected interfering DC magnetic field. In some embodiments, the controller may control the compensation coils such that the sum of the square of all magnetic fields measured by the sensors is minimized.

[0088] In some embodiments, in step 840, the controller may move (e.g., change the location of) one or more of the auxiliary coils to thereby move, or change the shape of, the generated uniform AC magnetic field such that it encapsulates the entire charged-particle beam pathway. In some embodiments, the compensatory coils may be connected to electric motors that may be activated tomove the coils. For example, with reference to Figs. 5A, the controller may provide power to motors connected to additional coils 32O’xand 3202x(positioned between the pair of coils 31 ()' . and 3102x) to move these coils and thereby move (or stretch) the generated uniform AC magnetic field in the X direction. The controller may also similarly provide power to motors connected to additional coils positioned between the compensatory coil pairs along other axes (e.g., see Figs. 5B and 5C) to also move or stretch the uniform AC magnetic field in other directions.

[0089] In some embodiments, to negate or reduce the effect of strong local DC magnetic fields on magnetic field sensors positioned in their region of influence, in step 830, the controller may provide a DC current to an auxiliary coil positioned proximate to such a magnetic field sensor to counteract (or reduce) the effect of the DC magnetic field on the sensor reading. For example, with reference to Fig. 6, the controller may activate and power an auxiliary coil (not shown) positioned, for example, near magnetometer 60()'yto nullify the effect of a strong DC magnetic field (generated, for example, by ion pump 510) on magnetometer 600'yor the signals measured by magnetometer 6001,.,

[0090] Both DC and AC external magnetic field disturbances may be generated by sources remote from the charged-particle beam tool and by sources located proximate to, but separate from, the charged-particle beam tool. For example, sources that may be located proximate to the charged- particle beam tool include power cabinets with transformers or other sources of interfering AC or DC magnetic fields. The principles described in the present disclosure in connection with MFC systems to protect the charged-particle beam tool may also be deployed as active MFC systems to suppress the magnetic field around a source (e.g., an external power cabinet or other source of interfering AC or DC field).

[0091] Fig. 9 is a schematic illustration of an exemplary configuration of a magnetic field suppression system 900 configured to suppress the magnetic field at a magnetic field source, consistent with some embodiments of the present disclosure. A charged-particle beam tool 902 (e.g., an SEM tool) has a first sensor 904 located proximate to the charged-particle beam tool 902. The first sensor 904 is configured to measure the magnetic field in the vicinity of the charged-particle beam tool 902. For example, the first sensor 904 may include the magnetometer 320 described in connection with Fig. 3 or may include the pairs of magnetometers (e.g., magnetometers 6001,. 6002x, 600’y, 6002y, 600’z, and 6002z) described in connection with Fig. 6. In some embodiments, a frame 906 including suppression coils (not shown in Fig. 9) may be positioned around the charged-particle beam tool 902. For example, the frame 906 may be similar to the frame 350 described in connection with Fig. 4 A and the suppression coils may be similar to the suppression coils 310’x, 3102x, 310^, 310^, 310’z, and 3102zdescribed in connection with Fig. 3.

[0092] A controller and coil driver 908 may be implemented as controller 390 described in connection with Fig. 3 or controller 109 described in connection with Fig. 1. The controller and coil driver 908 is configured to receive signals from the first sensor 904.

[0093] The system 900 also includes a noise source 910. It is noted that one noise source 910 is shown in Fig. 9 for purposes of illustration and that any number of noise sources may be present in the system. The operation of the system 900 is the same regardless of the number of noise sources present. A second sensor 912 is located proximate to the noise source 910. The second sensor 912 is configured to measure the magnetic field in the vicinity of the noise source 910 and to provide measurements to the controller and coil driver 908. For example, the second sensor 912 may include the magnetometer 320 described in connection with Fig. 3 or may include the pairs of magnetometers (e.g., magnetometers SOO’x, 6002x, SOO’y, 6002y, SOO’z, and 6002z) described in connection with Fig. 6. In some embodiments, a frame 914 (shown in dashed outline in Fig. 9) including suppression coils (not shown in Fig. 9) may be positioned around the noise source 910. In some embodiments, the frame 914 may be similar to the frame 350 described in connection with Fig. 4 A and the suppression coils may be similar to the suppression coils 310’x, 3102x, 310’y, 3102y, 310’z, and 3102zdescribed in connection with Fig. 3. In some embodiments, one suppression coil in each of the X direction, the Y direction, and the Z direction may be used. In some embodiments, one suppression coil spatially oriented to generate a field counteracting the interference from the noise source 910 may be used. It is noted that while the term “coils” may be used to describe generating the compensating magnetic field, it is to be understood that the term “coil” may also apply for embodiments in which one suppression coil is used. The principles of operation of the system 900 are the same regardless of the number of suppression coils used. In some embodiments, the coil or coils may be connected to one or more components proximate to the noise source 910 and not connected to a frame (e.g., frame 914 may not be present).

[0094] The controller and coil driver 908 processes the measurements received from the first sensor 904 and the second sensor 912 to determine the magnetic interference generated by each source 910 (e.g., differentiating between the magnetic interference measured by the first sensor 904 at the charged-particle beam tool 902 and the magnetic interference measured by the second sensor 912 at the noise source 910). The controller and coil driver 908 is configured to generate control signals to control the suppression coils to generate a counteracting or compensating magnetic field to cancel out the magnetic interference generated by the noise source 910. In some embodiments, the compensating magnetic field may not be uniform but may be configured such that the compensating magnetic field suppresses the magnetic interference generated by the noise source 910. In some embodiments, the compensating magnetic field may uniform to suppress the magnetic interference generated by the noise source 910.

[0095] In some embodiments, there may be a plurality of noise sources and each noise source may have a corresponding sensor to determine the magnetic interference generated by that source. The controller may be configured to receive the signals from sensors corresponding to each of the plurality of noise sources and may generate multiple control signals, each control signal to control suppression coils at each of the plurality of noise sources. For example, if there are five noise sources, each noisesource may have a corresponding sensor. The controller may receive signals from each of the five sensors and may generate five control signals, each control signal specific to one of the five noise sources.

[0096] In some embodiments, the control signals may also be based on an attenuation of the magnetic field generated by the noise source 910 as detected by the first sensor 904, such as an attenuation caused by the distance between the charged-particle beam tool 902 and the noise source 910. For example, the first sensor 904 may detect a lower level of magnetic interference at the charged-particle beam tool 902 than the level of magnetic interference detected by the second sensor 912 at the noise source 910. If the control signals were only based on the magnetic interference detected by the second sensor 912, the control signals may result in a stronger magnetic field generated to suppress the magnetic interference generated by the noise source 910. Because the magnetic interference as detected by the first sensor 904 may be lower than the magnetic interference detected by the second sensor 912, the control signals may result in a weaker field needing to be generated to suppress the magnetic interference generated by the noise source 910 and detected at the charged-particle beam tool 902.

[0097] Fig. 10 is a flow chart of an exemplary method 1000 of counteracting interfering magnetic field variations in the system of Fig. 9, consistent with some embodiments of the present disclosure. In some embodiments, the method 1000 may be implemented by the controller and coil driver 908 described in connection with Fig. 9.

[0098] At step 1002, magnetic field measurements are obtained from a first sensor located proximate to a charged-particle beam tool. For example, the first sensor may include first sensor 904 located proximate to charged-particle beam tool 902 described in connection with Fig. 9.

[0099] At step 1004, magnetic field measurements are obtained from a second sensor located proximate to a noise source. For example, the second sensor may include second sensor 912 located proximate to noise source 910 described in connection with Fig. 9. In some embodiments, multiple second sensors may be present (e.g., one second sensor corresponding to each of a plurality of noise sources) and magnetic field measurements may be obtained from all of the second sensors.

[0100] At step 1006, information about the magnetic field generated by the noise source is determined, using the measurements received from the first sensor and the second sensor. In embodiments in which there are a plurality of second sensors, the magnetic field measurements corresponding to each noise source may be separately identified. For example, an identifier may be associated with each noise source and the corresponding magnetic field measurements from that noise source.

[0101] At step 1008, control signals are applied to one or more suppression coils located around the noise source to generate a counteracting or compensating magnetic field to cancel out the magnetic interference generated by the noise source. In embodiments in which there are a plurality of noise sources, separate control signals may be applied to respective suppression coils located around each ofthe noise sources to generate a counteracting or compensating magnetic field specific to that noise source to cancel out the corresponding magnetic interference.

[0102] The figures may illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer hardware or software products according to various exemplary embodiments of the present disclosure. For example, each block or step in process 800 or process 1000 may represent certain arithmetical or logical operation that may be implemented using hardware such as an electronic circuit. The steps may also represent a module, segment, or portion of code that comprises one or more executable instructions for implementing the specified logical functions. It should be understood that in some alternative implementations, functions indicated in a step may occur out of the order noted in the figures. For example, two steps shown in succession may be executed or implemented substantially concurrently, or two steps may sometimes be executed in reverse order, depending upon the functionality involved. Further, some steps may also be omitted. It should also be understood that each step of the flow chart of Figs. 8 or 10, and combination of the steps, may be implemented by special purpose hardware-based systems that perform the specified functions or acts, or by combinations of special purpose hardware and computer instructions.

[0103] The embodiments may be further described using the following clauses:1. A coil arrangement for generating a counteracting magnetic field, comprising: a first pair of coils extending about an X axis and spaced apart from each other along the X axis; a second pair of coils extending about a Y axis and spaced apart from each other along the Y axis; a third pair of coils extending about a Z axis and spaced apart from each other along the Z axis; and one or more additional coils located between at least one of the first pair of coils, the second pair of coils, or the third pair of coils, wherein at least one of the first, second, or third pair of coils are controlled to collectively generate a substantially uniform magnetic field at a location between the first, second, and third pair of coils, and the one or more additional coils are controlled to cause (a) a movement of the substantially uniform magnetic field from the location, or (b) a change in shape of the substantially uniform magnetic field.2. The coil arrangement of clause 1, wherein the first, second, or third pair of coils, or the one or more additional coils are connected to a frame of a charged particle beam apparatus via standoffs made of an electrically non-conductive material that provide a physical spacing between coils of the first, second, and third pair of coils or the one or more additional coils and metal components of the frame to reduce magnetic interaction between the coils and the frame.3. The coil arrangement of clause 2, the physical spacing is between about 20-50 mm.4. The coil arrangement of any of clauses 2 or 3, wherein the standoffs are made of a nonferromagnetic material.5. The coil arrangement of clause 4, wherein the standoffs include at least one of nylon, polyethylene, polypropylene, rubber, ceramic, glass, air, pyrolytic graphite, bismuth, ferrite, or a composite material.6. The coil arrangement of any of clauses 1-3, wherein the one or more additional coils comprise at least one first additional coil located between the first pair of coils.7. The coil arrangement of clause 6, wherein the one or more additional coils further comprises at least one second additional coil located between the second pair of coils.8. The coil arrangement of clause 7, wherein the one or more additional coils further comprises at least one third additional coil located between the third pair of coils.9. The coil arrangement of clause 8, wherein one or more of the at least one first additional coil, the least one second additional coil, and the least one third additional coil comprises a pair of coils.10. The coil arrangement of any of clauses 1-3, further including a controller including circuitry, wherein the controller is configured to control a current or a voltage of the first, second, or third pair of coils to collectively generate the substantially uniform magnetic field and control a current or a voltage of one or more additional coils to cause the movement of the substantially uniform magnetic field or the change in shape of the substantially uniform magnetic field.11. The coil arrangement of clause 10, further including two or more sensors positioned between the first, second, and third pair of coils, wherein the controller is configured to determine a magnetic field environment along the X, Y, and Z axes at a region within a charged-particle column of a charged-particle beam apparatus that is configured to be disposed between the first, second, and third pair of coils, based on signals from the two or more sensors.12. The coil arrangement of clause 11, wherein the two or more sensors comprise at least three pairs of sensors, wherein a first pair of the at least three pairs include a pair of sensors positioned on opposite sides of the charged-particle column, a second pair of the at least three pairs include a pair of sensors positioned on opposite sides of the charged-particle column, and a third pair of the at least three pairs include a pair of sensors positioned on opposite sides of the charged-particle column.13. The coil arrangement of clause 12, wherein the pair of sensors of the first pair are aligned with the X axis, the pair of sensors of the second pair are aligned with the Y axis, and the pair of sensors of the third pair are aligned with the Z axis.14. The coil arrangement of any of clauses 11-13, wherein the controller is configured to control the current or the voltage of the first, second, and third pair of coils and generate the substantially uniform magnetic field to counteract the determined magnetic field environment along the X, Y, and Z axes.15. The coil arrangement of any of clauses 11-13, further comprising one or more permanent magnets positioned proximate to at least one sensor of the two or more sensors, wherein the one or more permanent magnets is configured to generate a DC magnetic field that counteracts an interfering DC magnetic field at a position of the at least one sensor.16. The coil arrangement of any of clauses 11-13, further comprising at least one electromagnetic device positioned proximate to at least one sensor of the two or more sensors, wherein the at least one electromagnetic device is configured to generate a DC magnetic field that counteracts an interfering DC magnetic field at a position of the at least one sensor when powered by DC current.17. The coil arrangement of any of clauses 1-3, further comprising: a fourth pair of coils extending about the X axis and spaced apart from each other along the X axis; a fifth pair of coils extending about the Y axis and spaced apart from each other along the Y axis; and a sixth pair of coils extending about the Z axis and spaced apart from each other along the Z axis, wherein the fourth, fifth, and sixth pair of coils are controlled to collectively generate a DC magnetic field at the location to counteract an interfering DC magnetic field in the location.18. An apparatus for generating a counteracting magnetic field for a charged-particle beam apparatus, comprising: pairs of coils spaced apart and arranged about each one of an X axis, a Y axis, or a Z axis, wherein at least one pair of coils of the pairs of coils are controlled to generate a substantially uniform magnetic field at a central location between the pairs of coils.19. The apparatus of clause 18, further including one or more additional coils positioned between pairs of coils arranged about at least one of the X, Y, or Z axes, wherein the one or more additional coils are controlled to cause (a) a movement of the substantially uniform magnetic field from the central location, or (b) a change in shape of the substantially uniform magnetic field.20. The apparatus of any one of clauses 18-19, wherein the pairs of coils are connected to a frame of the charged-particle beam apparatus using standoffs made of an electrically non-conductive and non-ferromagnetic material to provide a spacing of between about 20 - 50 mm between coils of the pairs of coils and metal components of the frame to reduce magnetic interaction between the coils and the frame.21. The apparatus of clause 20, wherein the standoffs include at least one of nylon, polyethylene, polypropylene, rubber, ceramic, glass, air, pyrolytic graphite, bismuth, ferrite, or a composite material.22. The apparatus of clause 19, wherein the one or more additional coils are positioned between pairs of coils arranged about each one of the X, Y, and Z axes.23. The apparatus of clause 22, wherein the one or more additional coils include a pair of additional coils.24. The apparatus of clause 19, further including a controller including circuitry, wherein the controller is configured to control a current or a voltage of the at least one pair of coils to generate thesubstantially uniform magnetic field and control a current or a voltage of the one or more additional coils to cause the movement of the substantially uniform magnetic field or the change in shape of the substantially uniform magnetic field.25. The apparatus of clause 18, further including two or more sensors positioned between the pairs of coils, wherein the controller is further configured to, determine a magnetic field environment along the X, Y, and Z axes at a location within a charged- particle column of the charged-particle beam apparatus configured to be disposed between the pairs of coils based on signals from the two or more sensors, and control a current or a voltage of the at least one pair of coils to generate the substantially uniform magnetic field to counteract the determined magnetic field environment.26. The apparatus of clause 25, wherein the two or more sensors comprise at least three pairs of sensors, wherein a first pair of the at least three pairs include a pair of sensors aligned with the X axis and positioned on opposite sides of the charged-particle column, a second pair of the at least three pairs include a pair of sensors aligned with the Y axis and positioned on opposite sides of the charged-particle column, and a third pair of the at least three pairs include a pair of sensors aligned with the Z axis and positioned on opposite sides of the charged-particle column.27. The apparatus of clause 25, further including one or more permanent magnets positioned proximate to at least one sensor of the two more sensors, wherein the one or more permanent magnets is configured to generate a DC magnetic field that counteracts an interfering DC magnetic field at a position of the at least one sensor.28. The apparatus of clause 25, further including at least one electromagnetic device positioned proximate to at least one sensor of the two more sensors, wherein the at least one electromagnetic device is configured to generate a DC magnetic field that counteracts an interfering DC magnetic field at a position of the at least one sensor when powered by DC current.29. The apparatus of any of clauses 18-19, further comprising pairs of auxiliary coils spaced apart and arranged about each one of the X, Y, and Z axes, wherein the pairs of auxiliary coils are controlled to collectively generate a DC magnetic field at the central location to counteract an interfering DC magnetic field in the central location.30. A non-transitory computer readable medium that stores a set of instructions that is executable by at least on processor of a computing device to cause the computing device to perform operations for generating a counteracting magnetic field for a charged-particle beam apparatus, the operations comprising: directing current through at least one pair of coils of pairs of coils spaced apart and arranged about each one of an X axis, a Y axis, or a Z axis to generate a substantially uniform magnetic field at a central location between the pairs of coils.31. The non-transitory computer readable medium of clause 30, the operations further comprising directing current through one or more additional coils positioned between pairs of coils arranged about at least one of the X, Y, or Z axes to cause (a) a movement of the substantially uniform magnetic field from the central location, or (b) a change in shape of the substantially uniform magnetic field.32. The non-transitory computer readable medium of any of clause 30-31, wherein the pairs of coils are connected to a frame of the charged-particle beam apparatus using standoffs made of an electrically non-conductive and non-ferromagnetic material to provide a spacing of between about 20 - 50 mm between coils of the pairs of coils and metal components of the frame to reduce magnetic interaction between the coils and the frame.33. The non-transitory computer readable medium of clause 32, wherein the standoffs include at least one of nylon, polyethylene, polypropylene, rubber, ceramic, glass, air, pyrolytic graphite, bismuth, ferrite, or a composite material.34. The non-transitory computer readable medium of clause 31, wherein the one or more additional coils are positioned between pairs of coils arranged about each one of the X, Y, and Z axes.35. The non-transitory computer readable medium of clause 34, wherein the one or more additional coils includes a pair of additional coils.36. The non-transitory computer readable medium of clause 30, further including two or more sensors positioned between the pairs of coils, wherein the operations further comprise, determining a magnetic field environment along the X, Y, and Z axes at a location within a charged- particle column of the charged-particle beam apparatus configured to be disposed between the pairs of coils based on signals from the two or more sensors, and controlling a current or a voltage of the at least one pair of coils to generate the substantially uniform magnetic field to counteract the determined magnetic field environment.37. The non-transitory computer readable medium of clause 36, wherein the two or more sensors comprise at least three pairs of sensors, wherein a first pair of the at least three pairs include a pair of sensors aligned with the X axis and positioned on opposite sides of the charged-particle column, a second pair of the at least three pairs include a pair of sensors aligned with the Y axis and positioned on opposite sides of the charged-particle column, and a third pair of the at least three pairs include a pair of sensors aligned with the Z axis and positioned on opposite sides of the charged-particle column.38. The non-transitory computer readable medium of clause 36, further including one or more permanent magnets positioned proximate to at least one sensor of the two more sensors, wherein the one or more permanent magnets is configured to generate a DC magnetic field that counteracts an interfering DC magnetic field at a position of the at least one sensor.39. The non-transitory computer readable medium of clause 36, further including at least one electromagnetic device positioned proximate to at least one sensor of the two more sensors, wherein the at least one electromagnetic device is configured to generate a DC magnetic field that counteracts an interfering DC magnetic field at a position of the at least one sensor when powered by DC current.40. The non-transitory computer readable medium of any of clauses 30-31, further comprising pairs of auxiliary coils spaced apart and arranged about each one of the X, Y, and Z axes, wherein the operations further comprise controlling the pairs of auxiliary coils to collectively generate a DC magnetic field at the central location to counteract an interfering DC magnetic field in the central location.41. A method of generating a counteracting magnetic field for a charged-particle beam apparatus, the method comprising: directing current through at least one pair of coils of pairs of coils spaced apart and arranged about each one of an X axis, a Y axis, or a Z axis to generate a substantially uniform magnetic field at a central location between the pairs of coils.42. The method of clause 41, further comprising directing current through one or more additional coils positioned between pairs of coils arranged about at least one of the X, Y, or Z axes to cause (a) a movement of the substantially uniform magnetic field from the central location, or (b) a change in shape of the substantially uniform magnetic field.43. The method of any of clauses 41-42, wherein the pairs of coils are connected to a frame of the charged-particle beam apparatus using standoffs made of an electrically non-conducting and nonferromagnetic material to provide a spacing of between about 20 - 50 mm between coils of the pairs of coils and metal components of the frame to reduce magnetic interaction between the coils and the frame.44. The method of clause 43, wherein the standoffs include at least one of nylon, polyethylene, polypropylene, rubber, ceramic, glass, air, pyrolytic graphite, bismuth, ferrite, or a composite material.45. The method of clause 42, wherein the one or more additional coils are positioned between pairs of coils arranged about each one of the X, Y, and Z axes.46. The method of clause 45, wherein the one or more additional coils includes a pair of additional coils.47. The method of clauses 41, further including two or more sensors positioned between the pairs of coils, wherein the method further comprises, determining a magnetic field environment along the X, Y, and Z axes at a location within a charged- particle column of the charged-particle beam apparatus configured to be disposed between the pairs of coils based on signals from the two or more sensors, and controlling a current or a voltage of the at least one pair of coils to generate the substantially uniform magnetic field to counteract the determined magnetic field environment.48. The method of clause 47, wherein the two or more sensors comprise at least three pairs of sensors, wherein a first pair of the at least three pairs include a pair of sensors aligned with the X axis and positioned on opposite sides of the charged-particle column, a second pair of the at least three pairs include a pair of sensors aligned with the Y axis and positioned on opposite sides of the charged-particle column, and a third pair of the at least three pairs include a pair of sensors aligned with the Z axis and positioned on opposite sides of the charged-particle column.49. The method of clause 47, further including one or more permanent magnets positioned proximate to at least one sensor of the two more sensors, wherein the one or more permanent magnets is configured to generate a DC magnetic field that counteracts an interfering DC magnetic field at a position of the at least one sensor.50. The method of clause 47, further including at least one electromagnetic device positioned proximate to at least one sensor of the two more sensors, wherein the at least one electromagnetic device is configured to generate a DC magnetic field that counteracts an interfering DC magnetic field at a position of the at least one sensor when powered by DC current.51. The method of any of clauses 41-42, further comprising pairs of auxiliary coils spaced apart and arranged about each one of the X, Y, and Z axes, wherein the method further comprises controlling the pairs of auxiliary coils to collectively generate a DC magnetic field at the central location to counteract an interfering DC magnetic field in the central location.52. An apparatus for generating a counteracting magnetic field for a noise source, comprising: a coil controller configured to generate in a coil the counteracting magnetic field in an orientation to suppress a magnetic field generated by the noise source.53. The apparatus of clause 52, wherein the coil is arranged about an X axis and the apparatus further comprises: a second coil arranged about a Y axis; and a third coil arranged about a Z axis.54. The apparatus of clause 53, wherein: the coil includes a pair of coils spaced apart and arranged about the X axis; the second coil includes a pair of coils spaced apart and arranged about the Y axis; and the third coil includes a pair of coils spaced apart and arranged about the Z axis.55. The apparatus of clause 52, wherein the coil is connected to a component located proximate to the noise source using a standoff made of an electrically non-conductive and non-ferromagnetic material.56. The apparatus of clause 55, wherein the standoff includes multiple standoffs and each standoff is comprised of at least one of nylon, polyethylene, polypropylene, rubber, ceramic, glass, air, pyrolytic graphite, bismuth, ferrite, or a composite material.57. The apparatus of clause 55, wherein the component located proximate to the noise source includes a frame positioned around the noise source.58. The apparatus of clause 55, wherein the component is a structure associated with the noise source.59. The apparatus of clause 58, wherein the structure is the noise source.60. The apparatus of any one of clauses 52-59, wherein the coil controller includes circuitry and is configured to control a current or a voltage of the coil to generate the counteracting magnetic field.61. The apparatus of any one of clauses 52-60, further comprising: a sensor positioned near the coil, wherein the coil controller is further configured to: determine a magnetic field environment at a location proximate to the noise source; and control a current or a voltage of the coil to generate the counteracting magnetic field to counteract the determined magnetic field environment.62. The apparatus of any one of clauses 52-61, further comprising: a first sensor positioned at the noise source and configured to determine a magnetic field environment at the noise source; and a second sensor positioned at a charged-particle beam apparatus and configured to determine a magnetic field environment at the charged-particle beam apparatus.63. The apparatus of clause 62, wherein the coil controller is configured to: receive signals from the first sensor; receive signals from the second sensor; and control a current or a voltage of the coil to generate the magnetic field to suppress the magnetic field at the noise source, wherein the current or the voltage is based on the received signals.64. The apparatus of clause 63, wherein the coil controller is further configured to: adjust the current or the voltage based on the signals received from the second sensor whereby the current or the voltage may be reduced if the signals received from the second sensor indicates a lower magnetic field environment than the signals received from the first sensor.65. The apparatus of any one of clauses 52-64, further comprising: a plurality of noise sources; and a plurality of sensors, each of the plurality of sensors corresponding to one of the plurality of noise sources.66. The apparatus of clause 65, wherein the coil controller is configured to: receive signals from each of the plurality of sensors; and control a current or a voltage of the coil at each of the plurality of noise sources to generate a counteracting magnetic field at each of the plurality of noise sources to suppress the magnetic field at each of the plurality of noise sources, wherein the current or the voltage is based on the received signal from the corresponding one of the plurality of sensors.67. The apparatus of clause 66, further comprising: at least one second sensor positioned at a charged-particle beam apparatus and configured to determine a magnetic field environment at the charged-particle beam apparatus, wherein the coil controller is further configured to adjust the current or the voltage of the coil at each of the plurality of noise sources based on the signals received from at least one corresponding second sensor whereby the current or the voltage of the coil at each of the plurality of noise sources may be reduced if the signals received from the at least one corresponding second sensor indicates a lower magnetic field environment than the signals received from a corresponding sensor of the plurality of sensors.

[0104] It will be appreciated that the embodiments of the present disclosure are not limited to the exact construction that has been described above and illustrated in the accompanying drawings, and that various modifications and changes may be made without departing from the scope thereof. The present disclosure has been described in connection with various embodiments, other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

Claims

CLAIMS1. An apparatus for generating a counteracting magnetic field for a charged-particle beam apparatus, comprising: pairs of coils spaced apart and arranged about each one of an X axis, a Y axis, or a Z axis, wherein at least one pair of coils of the pairs of coils are controlled to generate a substantially uniform magnetic field at a central location between the pairs of coils.

2. The apparatus of claim 1, further including one or more additional coils positioned between pairs of coils arranged about at least one of the X, Y, or Z axes, wherein the one or more additional coils are controlled to cause (a) a movement of the substantially uniform magnetic field from the central location, or (b) a change in shape of the substantially uniform magnetic field.

3. The apparatus of claim 1, wherein the pairs of coils are connected to a frame of the charged-particle beam apparatus using standoffs made of an electrically non-conductive and nonferromagnetic material to provide a spacing of between about 20 - 50 mm between coils of the pairs of coils and metal components of the frame to reduce magnetic interaction between the coils and the frame.

4. The apparatus of claim 3, wherein the standoffs include at least one of nylon, polyethylene, polypropylene, rubber, ceramic, glass, air, pyrolytic graphite, bismuth, ferrite, or a composite material.

5. The apparatus of claim 2, wherein the one or more additional coils are positioned between pairs of coils arranged about each one of the X, Y, and Z axes.

6. The apparatus of claim 5, wherein the one or more additional coils include a pair of additional coils.

7. The apparatus of claim 2, further comprising a controller including circuitry, wherein the controller is configured to control a current or a voltage of the at least one pair of coils to generate the substantially uniform magnetic field and control a current or a voltage of the one or more additional coils to cause the movement of the substantially uniform magnetic field or the change in shape of the substantially uniform magnetic field.

8. The apparatus of claim 1, further comprising two or more sensors positioned between the pairs of coils, wherein the controller is further configured to:determine a magnetic field environment along the X, Y, and Z axes at a location within a charged-particle column of the charged-particle beam apparatus configured to be disposed between the pairs of coils based on signals from the two or more sensors; and control a current or a voltage of the at least one pair of coils to generate the substantially uniform magnetic field to counteract the determined magnetic field environment.

9. The apparatus of claim 8, wherein the two or more sensors comprise at least three pairs of sensors, wherein: a first pair of the at least three pairs include a pair of sensors aligned with the X axis and positioned on opposite sides of the charged-particle column; a second pair of the at least three pairs include a pair of sensors aligned with the Y axis and positioned on opposite sides of the charged-particle column; and a third pair of the at least three pairs include a pair of sensors aligned with the Z axis and positioned on opposite sides of the charged-particle column.

10. The apparatus of claim 8, further comprising one or more permanent magnets positioned proximate to at least one sensor of the two more sensors, wherein the one or more permanent magnets is configured to generate a DC magnetic field that counteracts an interfering DC magnetic field at a position of the at least one sensor.

11. The apparatus of claim 8, further comprising at least one electromagnetic device positioned proximate to at least one sensor of the two more sensors, wherein the at least one electromagnetic device is configured to generate a DC magnetic field that counteracts an interfering DC magnetic field at a position of the at least one sensor when powered by DC current.

12. The apparatus of claim 8, further comprising pairs of auxiliary coils spaced apart and arranged about each one of the X, Y, and Z axes, wherein the pairs of auxiliary coils are controlled to collectively generate a DC magnetic field at the central location to counteract an interfering DC magnetic field in the central location.

13. A non-transitory computer readable medium that stores a set of instructions that is executable by at least on processor of a computing device to cause the computing device to perform operations for generating a counteracting magnetic field for a charged-particle beam apparatus, the operations comprising: directing current through at least one pair of coils of pairs of coils spaced apart and arranged about each one of an X axis, a Y axis, or a Z axis to generate a substantially uniform magnetic field at a central location between the pairs of coils.

14. The non-transitory computer readable medium of claim 13, wherein the operations further comprise directing current through one or more additional coils positioned between pairs of coils arranged about at least one of the X, Y, or Z axes to cause (a) a movement of the substantially uniform magnetic field from the central location, or (b) a change in shape of the substantially uniform magnetic field.

15. The non-transitory computer readable medium of claim 13, wherein the pairs of coils are connected to a frame of the charged-particle beam apparatus using standoffs made of an electrically non-conductive and non-ferromagnetic material to provide a spacing of between about 20 - 50 mm between coils of the pairs of coils and metal components of the frame to reduce magnetic interaction between the coils and the frame.

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