On the fly local alignment

Abstract

A charged particle beam inspection method includes acquiring an image of an alignment mark during a constant velocity stage movement and performing alignment corrections prior to imaging a care area associated with the alignment mark. The alignment mark may be selected to be located on a straight path from a prior care area. The alignment mark may be selected such that it is separated from the care area by a predetermined minimum computation distance so that any alignment corrections may be performed during the constant velocity stage movement.

Description

ON THE FLY LOCAL ALIGNMENTCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 573,190 which was filed on April 02, 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The description herein relates to measurement schemes that may be useful in the field of charged particle beam systems, and more particularly, to systems and methods that may be applicable to charged particle inspection systems such as scanning electron microscope (SEM) tools.BACKGROUND

[0003] Inspection and metrology systems may be used for sensing physically observable phenomena. For example, charged particle beam tools, such as electron microscopes, may comprise detectors that receive charged particles projected from a sample and that output detection signals. Detection signals may be used to reconstruct images of sample structures under inspection and may be used for, e.g., metrology, overlay, or defect inspection.

[0004] For example, most semiconductor devices require a plurality of pattern layers to be formed and transferred onto the substrate. To ensure proper functioning of the devices and high yield in a manufacturing process, the semiconductor devices may be inspected. For example, a charged particle beam tool may be used to inspect a semiconductor wafer at a plurality of areas of interest (or care areas) at which a defect or other error is most likely to occur. With the continual desire in the lithographic art to increase yield and throughput while reducing the sizes of pattern features in semiconductor devices, the requirements on the number of care areas and speed of inspection are increasing.SUMMARY

[0005] Some embodiments of the present disclosure provide an inspection method. The inspection method may comprise: acquiring a first charged particle beam image of a first alignment mark on a substrate supported on the substrate stage while the substrate stage moves at a constant speed; determining a first alignment correction based on the first charged particle beam image; and acquiring a second charged particle beam image of a first care area on the substrate based on the first alignment correction.

[0006] Some embodiments of the present disclosure may provide a non-transitory computer-readable medium. The non-transitory computer-readable medium may store a set of instructions that is executable by at least one processor of an apparatus to cause the apparatus to perform operations comprising the above method.

[0007] Some embodiments of the present disclosure may provide a charged particle beam apparatus. The charged particle beam apparatus may comprise: a charged particle beam source configured to generate a charged particle beam; a charged particle optical system configured to direct the charged particle beam at a pattern region on a substrate; a substrate stage configured to support and move the substrate; and a controller comprising one or more processors and configured to cause the charged particle beam apparatus to perform operations comprising the above method.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] 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.

[0009] Fig. 1 is a diagrammatic representation of an exemplary electron beam inspection (EBI) system, consistent with embodiments of the present disclosure.

[0010] Figs. 2A-B are diagrams illustrating a charged particle beam apparatus that may be an example of an electron beam tool, consistent with embodiments of the present disclosure.

[0011] Figs. 3A-B are diagrammatic representations of example alignment and inspection processes, according to a comparative embodiment.

[0012] Figs. 4A-C are diagrammatic representations of example alignment and inspection processes, consistent with embodiments of the present disclosure.

[0013] Fig. 5 is a flowchart illustrating an example method that may be useful for an alignment and inspection process, consistent with embodiments of the disclosure.DETAILED DESCRIPTION

[0014] Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the 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 consistent with the invention. Instead, they are merely examples of apparatuses, systems, and methods consistent with aspects related to subject matter that may be recited in the appended claims. For example, although some embodiments are described in the context of utilizing charged-particle beams (e.g., electron beams), the disclosure is not so limited. Other types of charged particle beams (e.g., photon beams) may be similarly applied. Furthermore, other imaging systems may be used, such as optical imaging, photodetection, x-ray detection, or the like.

[0015] Electronic devices are constructed of circuits formed on a piece of silicon called a substrate. Many circuits may be formed together on the same piece of silicon and are called integrated circuits or ICs. With advancements in technology, the size of these circuits has decreased dramatically so that many more of them can fit on the substrate. For example, an IC chip in a smart phone can be as smallas a fingernail and yet may include over 2 billion transistors, the size of each transistor being less than 1 / 1, 000th the width of a human hair.

[0016] Making these ICs with extremely small structures or components is a complex, timeconsuming, 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, one goal of the manufacturing process is to avoid such defects to maximize the number of functional ICs made in the process, that is, to improve the overall yield of the process.

[0017] One component of improving yield is monitoring the chip making process to ensure that it is producing a sufficient number of functional integrated circuits. One way to monitor the process is to inspect the chip circuit structures at various stages of their formation. Inspection can be carried out using a charged particle beam apparatus such as a scanning electron microscope (SEM). A SEM can be used to image these extremely small structures, in effect, taking a “picture” of the structures. The image can be used to determine if the structure was formed properly and also if it was formed in the proper location. If the structure is defective, then the process can be adjusted so the defect is less likely to recur. To enhance throughput (e.g., the number of samples processed per hour), it is desirable to conduct inspection as quickly as possible.

[0018] The working principle of a SEM is similar to a camera. A camera takes a picture by receiving and recording intensity of light reflected or emitted from people or objects. A SEM takes a “picture” by receiving and recording energies or quantities of electrons reflected or emitted from the structures of the wafer. Before taking such a “picture,” an electron beam may be projected onto the structures, and when the electrons are reflected or emitted (“exiting”) from the structures (e.g., from the wafer surface, from the structures underneath the wafer surface, or both), a detector of the SEM may receive and record the energies or quantities of those electrons to generate an inspection image. To take such a “picture,” the electron beam may scan through the wafer (e.g., in a line-by-line or zig-zag manner), and the detector may receive exiting electrons coming from a region under electron-beam projection (referred to as a “beam spot”). The detector may receive and record exiting electrons from each beam spot one at a time and join the information recorded for all the beam spots to generate the inspection image. Some SEMs use a single electron beam (referred to as a “single-beam SEM”) to take a single “picture” to generate the inspection image, while some SEMs use multiple electron beams (referred to as a “multi-beam SEM”) to take in parallel multiple “pictures” of the wafer, which can be used separately or be stitched together to generate the inspection image. By using multiple electron beams, the SEM may provide more electron beams onto the structures for obtaining these multiple “pictures,” resulting in more electrons exiting from the structures. Accordingly, the detector may receive more exiting electrons simultaneously and generate inspection images of the structures of the wafer with higher efficiency and faster speed.

[0019] Typically, the detection process involves measuring the magnitude of an electrical signal generated when electrons land on the detector. In another approach, electron counting may be used, inwhich a detector may count individual electron arrival events as they occur. In either approach, intensity of the secondary beam may be determined based on electrical signals generated in the detector that vary in proportion to the change in intensity of the secondary beam.

[0020] In addition to defect inspection, the detection process may be used to, e.g., perform metrology measurements. For example, in some embodiments the process may be used to determine metrology parameters such as edge placement error (EPE), critical dimension (CD), overlay (OVL), linewidth roughness (LWR), line edge roughness (LER), line end shortening (LES), line end shortening (LES) or the like.

[0021] An inspection process may involve targeting a plurality of care areas on a substrate. A care area may comprise, e.g., a region on the substrate that is determined to be a likely location for a defect, a region that is determined to be representative of other locations on the substrate, or any other region that may be of interest in a manufacturing process. Care areas may be identified based on, e.g., a design file that was used to create the pattern area. Such files may take the form of, e.g., GDS, GDSII, OASIS or other circuit pattern file formats (referred to collectively as “GDS”).

[0022] During an inspection process, a moveable substrate stage may be used to position each care area to be inspected within a field of view of the SEM. The SEM may then scan an electron beam over the care area to take an image of it. Prior to imaging each care area, the substrate stage may first be moved to position the SEM field of view over a local alignment mark in the neighborhood of the care area. The SEM may take an image of the alignment mark to determine any deviations of the alignment mark from the intended design. For example, the SEM may compare the alignment mark image to an expected image based on, e.g., a GDS file of the alignment mark. Because the alignment mark and its associated care area are very close to each other, it may be assumed that any substrate or pattern distortions in the alignment mark are similar to those found in the care area. Therefore, by measuring positional shifts or distortions of the alignment mark, it may be possible to determine shifts or distortions of the care area. In this way, the precise location or orientation of the care area may be calibrated to achieve a higher quality image of it.

[0023] However, performing this alignment process requires an excessive amount of stage motion. For example, each time the stage moves the substrate to position a new alignment mark in the field of view, the stage must accelerate, decelerate, and stop at the new position. The stage must then sit idle for a period while vibrations caused by these accelerations dissipate (known as settling time) before acquiring an image. These operations slow down the inspection process, harming manufacturing throughput and placing unwanted constraints on the number of care areas that may be inspected in each substrate.

[0024] Embodiments of the present disclosure may provide an inspection apparatus and inspection method for accurately taking alignment measurements while the stage is in motion (“on the fly”). This may involve moving a substrate stage at a constant velocity between successive care areas to allow the stage vibrations to settle before the SEM field of view passes over a selected alignment mark. Bymeasuring the alignment mark while the substrate stage is on its way to the next care area position, it may be possible to eliminate time wasted in accelerating, decelerating and settling. Thus, throughput may be improved, and it may be possible to inspect more care areas than would otherwise be feasible. In some embodiments, both the alignment mark and the care area may be imaged on the fly. For example, in some embodiments, a series and alignment marks and care areas may be measured successively during a single constant-velocity stage movement to measure multiple care areas without stopping.

[0025] This on-the-fly local alignment process may require placing additional conditions on alignment mark selection compared to other inspection processes. For example, in a comparative inspection process, an alignment mark may be selected based on it being located in close proximity to its respective care area to ensure that the alignment mark and care area share similar distortion characteristics. However, an on-the-fly local alignment process may require an alignment mark that is placed farther away. This is because any alignment computations should ideally be completed during the time the stage travels from the alignment mark position to the care area position. Therefore, in some embodiments, an alignment mark may be selected based on a minimum separation distance between the alignment mark and the care area. Furthermore, comparative inspection processes may select from a plurality of candidate alignment marks that surround a care area on all sides. However, in some embodiments of the present disclosure, selection may be limited only to alignment marks that are located on a direct line path from a prior care area to a next one. By selecting appropriate alignment marks based on the above considerations, on-the-fly local alignment may be achieved to reduce the time required for an inspection process.

[0026] 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.

[0027] Without limiting the scope of the present disclosure, some embodiments may be described in the context of providing detection systems and detection methods in systems utilizing electron beams (“e-beams”). However, the disclosure is not so limited. Other types of charged particle beams (such as proton beams) may be similarly applied. Furthermore, systems and methods for detection may be used in other imaging systems, such as optical imaging, photon detection, proton detection, x-ray detection, ion detection, or the like. Photon detection may comprise light in the infrared, visible, UV, DUV, EUV, x-ray, or any other wavelength range. Therefore, while detectors in the present disclosure may be disclosed with respect to electron detection, some embodiments of the present disclosure may be directed to detecting other charged particles or photons. For example, in some embodiments the process may be applied to an image obtained by, e.g., scatterometry or other optical metrology processes.

[0028] Furthermore, while some embodiments may be described in the context of defect inspection, embodiments of the present disclosure are not limited to this. For example, in some embodiments the process may be used to determine metrology parameters such as critical dimension (CD), overlay (OVL), linewidth roughness (LWR), line edge roughness (LER), edge placement error (EPE), line end shortening (LES), or the like.

[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.

[0030] Reference is now made to Fig. 1, which illustrates an exemplary electron beam inspection (EBI) system 10 that may be used for wafer inspection, consistent with embodiments of the present disclosure. As shown in Fig. 1, EBI system 10 includes a main chamber Il a load / lock chamber 20, an electron beam tool 100 (e.g., a scanning electron microscope (SEM)), and an equipment front end module (EFEM) 30. Electron beam tool 100 is located within main chamber 11 and may be used for imaging. EFEM 30 includes a first loading port 30a and a second loading port 30b. EFEM 30 may include additional loading ports. First loading port 30a and second loading port 30b receive wafer front opening unified pods (FOUPs) that contain wafers (e.g., semiconductor wafers or wafers made of other materials) or samples to be inspected (wafers and samples may be collectively referred to as “wafers” herein).

[0031] One or more robotic arms (not shown) in EFEM 30 may transport the wafers to load / lock chamber 20. Load / lock chamber 20 is connected to a load / lock vacuum pump system (not shown) which removes gas molecules in load / lock chamber 20 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 20 to main chamber 11. Main chamber 11 is connected to a main chamber vacuum pump system (not shown) which removes gas molecules in main chamber 11 to reach a second pressure below the first pressure. After reaching the second pressure, the wafer is subject to inspection by electron beam tool 100. Electron beam tool 100 may be a single-beam system or a multi-beam system. A controller 109 is electronically connected to electron beam tool 100, and may be electronically connected to other components as well. Controller 109 may be a computer configured to execute various controls of EBI system 10. While controller 109 is shown in Fig. 1 as being outside of the structure that includes main chamber 11, load / lock chamber 20, and EFEM 30, it is appreciated that controller 109 can be part of the structure.

[0032] 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 centralprocessing 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), 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.

[0033] 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.

[0034] A charged particle beam microscope, such as that formed by or which may be included in EBI system 10, may be capable of resolution down to, e.g., the nanometer scale, and may serve as a practical tool for inspecting IC components on wafers. With an e-beam system, electrons of a primary electron beam may be focused at probe spots on a wafer under inspection. The interactions of the primary electrons with the wafer may result in secondary particle beams being formed. The secondary particle beams may comprise backscattered electrons, secondary electrons, or Auger electrons, etc. resulting from the interactions of the primary electrons with the wafer. Characteristics of the secondary particle beams (e.g., intensity) may vary based on the properties of the internal or external structures or materials of the wafer, and thus may indicate whether the wafer includes defects.

[0035] The intensity of the secondary particle beams may be determined using a detector. The secondary particle beams may form beam spots on a surface of the detector. The detector may generate electrical signals (e.g., a current, a charge, a voltage, etc.) that represent intensity of the detected secondary particle beams. The electrical signals may be measured with measurement circuitries which may include further components (e.g., analog-to-digital converters) to obtain a distribution of the detected electrons. The electron distribution data collected during a detection time window, in combination with corresponding scan path data of the primary electron beam incident on the wafer surface, may be used to reconstruct images of the wafer structures or materials under inspection. The reconstructed images may be used to reveal various features of the internal or external structures or materials of the wafer and may be used to reveal defects that may exist in the wafer.

[0036] Fig. 2A illustrates a charged particle beam apparatus that may be an example of electron beam tool 100, consistent with embodiments of the present disclosure. Fig. 2A shows an apparatus that uses a plurality of beamlets formed from a primary electron beam to simultaneously scan multiple locations on a wafer.

[0037] As shown in Fig. 2A, electron beam tool 100 A may comprise an electron source 202, a gun aperture 204, a condenser lens 206, a primary electron beam 210 emitted from electron source 202, a source conversion unit 212, a plurality of beamlets 214, 216, and 218 of primary electron beam 210, a primary projection optical system 220, a wafer stage (not shown in Fig. 2A), multiple secondary electron beams 236, 238, and 240, a secondary optical system 242, and electron detection device 244. Electron source 202 may generate primary particles, such as electrons of primary electron beam 210. A controller, image processing system, and the like may be coupled to electron detection device 244. Primary projection optical system 220 may comprise beam separator 222, deflection scanning unit 226, and objective lens 228. Electron detection device 244 may comprise detection sub-regions 246, 248, and 250.

[0038] Electron source 202, gun aperture 204, condenser lens 206, source conversion unit 212, beam separator 222, deflection scanning unit 226, and objective lens 228 may be aligned with a primary optical axis 260 of apparatus 100A. Secondary optical system 242 and electron detection device 244 may be aligned with a secondary optical axis 215 of apparatus 100 A.

[0039] Electron source 202 may comprise a cathode, an extractor or an anode, wherein primary electrons can be emitted from the cathode and extracted or accelerated to form a primary electron beam 210 with a crossover (virtual or real) 208. Primary electron beam 210 can be visualized as being emitted from crossover 208. Gun aperture 204 may block off peripheral electrons of primary electron beam 210 to reduce size of probe spots 270, 272, and 274.

[0040] Source conversion unit 212 may comprise an array of image-forming elements (not shown in Fig. 2A) and an array of beam-limit apertures (not shown in Fig. 2A). An example of source conversion unit 212 may be found in U.S. Patent No 9,691,586; U.S. Publication No. 2017 / 0021543; and International Application No. PCT / EP2017 / 084429, all of which are incorporated by reference in their entireties. The array of image-forming elements may comprise an array of micro-deflectors or micro-lenses. The array of image-forming elements may form a plurality of parallel images (virtual or real) of crossover 208 with a plurality of beamlets 214, 216, and 218 of primary electron beam 210. The array of beam-limit apertures may limit the plurality of beamlets 214, 216, and 218.

[0041] Condenser lens 206 may focus primary electron beam 210. The electric currents of beamlets 214, 216, and 218 downstream of source conversion unit 212 may 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. Condenser lens 206 may be an adjustable condenser lens that may be configured so that the position of its first principal plane is movable. The adjustable condenser lens may be configured to be magnetic, which may result in off-axis beamlets 216 and 218 landing onthe beamlet-limit apertures with rotation angles. The rotation angles change with the focusing power and the position of the first principal plane of the adjustable condenser lens. In some embodiments, the adjustable condenser lens may be an adjustable anti-rotation condenser lens, which involves an anti-rotation lens with a movable first principal plane. An example of an adjustable condenser lens is further described in U.S. Publication No. 2017 / 0021541, which is incorporated by reference in its entirety.

[0042] Objective lens 228 may focus beamlets 214, 216, and 218 onto a wafer 230 for inspection and may form a plurality of probe spots 270, 272, and 274 on the surface of wafer 230. Secondary electron beamlets 236, 238, and 240 may be formed that are emitted from wafer 230 and travel back toward beam separator 222.

[0043] Beam separator 222 may 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 electrostatic dipole field on an electron of beamlets 214, 216, and 218 may be equal in magnitude and opposite in direction to the force exerted on the electron 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 may also be non-zero. Beam separator 222 may separate secondary electron beams 236, 238, and 240 from beamlets 214, 216, and 218 and direct secondary electron beams 236, 238, and 240 towards secondary optical system 242.

[0044] Deflection scanning unit 226 may deflect beamlets 214, 216, and 218 to scan probe spots 270, 272, and 274 over an area on a surface of wafer 230. In response to incidence of beamlets 214, 216, and 218 at probe spots 270, 272, and 274, secondary electron beams 236, 238, and 240 may be emitted from wafer 230. Secondary electron beams 236, 238, and 240 may comprise electrons with a distribution of energies including secondary electrons and backscattered electrons. Secondary optical system 242 may focus secondary electron beams 236, 238, and 240 onto detection sub-regions 246, 248, and 250 of electron detection device 244. Detection sub-regions 246, 248, and 250 may be configured to detect corresponding secondary electron beams 236, 238, and 240 and generate corresponding signals used to reconstruct an image of the surface of wafer 230.

[0045] The generated signals may represent intensities of secondary electron beams 236, 238, and 240 and may be provided to an image processing system (e.g. such as image processing system 199 provided in Fig. 2B below) that is in communication with detection device 244, primary projection optical system 220, and motorized wafer stage. The movement speed of motorized wafer stage 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 wafer230 may have different resistance-capacitance characteristics that may cause different signal sensitivities to the movement of the scan probe spots.

[0046] The intensity of secondary electron 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 electron beams 236, 238, and 240 that may have different intensities. Therefore, by mapping the intensity of secondary electron beams 236, 238, and 240 with the areas of wafer 230, the image processing system may reconstruct an image that reflects the characteristics of internal or external structures of wafer 230.

[0047] Detection sub-regions 246, 248, and 250 may include separate detector packages, separate sensing elements, or separate regions of an array detector. In some embodiments, each detection subregion may include a single sensing element.

[0048] Another example of a charged particle beam apparatus will now be discussed with reference to Fig. 2B. An electron beam tool 100B (also referred to herein as apparatus 100B) may be an example of electron beam tool 100 and may be similar to electron beam tool 100 A shown in Fig. 2A. However, different from apparatus 100A, apparatus 100B may be a single-beam tool that uses only one primary electron beam to scan one location on the wafer at a time.

[0049] As shown in Fig. 2B, apparatus 100B includes a wafer holder 136 supported by motorized stage 134 to hold a wafer 150 to be inspected. Electron beam tool 100B includes an electron emitter, which may comprise a cathode 103, an anode 121, and a gun aperture 122. Electron beam tool 100B 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 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 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.

[0050] 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 be communicatively coupled with detector 144 of electron beam tool 100B througha 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 post-processed 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.

[0051] 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.

[0052] 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 100B may comprise a first quadrupole lens 148 and a second quadrupole lens 149. 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 149 may be controlled to adjust the beam spot size and beam shape.

[0053] 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 embodiment 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 embodiment 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 by an angle a toward an electron detection device 244, as shown in Fig. 2A.

[0054] A detector in a charged particle beam system may include one or more sensing elements. The detector may comprise a single -element detector or an array with multiple sensing elements. The sensing elements may be configured for charged particle counting. Sensing elements of a detector thatmay be useful for charged particle counting are discussed in U.S. Publication No. 2019 / 0379682, which is incorporated by reference in its entirety.

[0055] Sensing elements may include a diode or an element similar to a diode that may convert incident energy into a measurable signal. For example, sensing elements in a detector may include a PIN diode. Throughout this disclosure, sensing elements may be represented as a diode, for example in the figures, although sensing elements or other components may deviate from ideal circuit behavior of electrical elements such as diodes, resistors, capacitors, etc.

[0056] In some embodiments, machine learning may be employed in the generation of inspection images, reference images or other images associated with apparatus 100, 100A or 100B. For example, in some embodiments a machine learning system may operate in association with, e.g., controller 109, image acquisition unit 199, image acquirer 120, or storage unit 130 of Figs. 1-2B. In some embodiments, a machine learning system may comprise a discriminative model. In some embodiments, a machine learning system may include a generative model. For example, learning can feature two types of mechanisms: discriminative learning that may be used to create classification and detection algorithms, and generative learning that may be used to actually create models that, in the extreme, can render images. For example, as described further below, a generative model may be configured for generating an image from a design clip that resembles a corresponding location on a wafer in a SEM image. This may be performed by 1) training the generative model with design clips and the associated actual SEM images from those locations on the wafer; and 2) using the model in inference mode to feed the model design clips in locations for which simulated SEM images are desired. Such simulated images can be used as reference images in, e.g., die-to-database inspection.

[0057] If the model(s) include one or more discriminative models, the discriminative model(s) may have any suitable architecture and / or configuration known in the art. Discriminative models, also called conditional models, are a class of models used in machine learning for modeling the dependence of an unobserved variable “y” on an observed variable “x.” Within a probabilistic framework, this may be done by modeling a conditional probability distribution P(y|x), which can be used for predicting y based on x. Discriminative models, as opposed to generative models, may not allow one to generate samples from the joint distribution of x and y. However, for tasks such as classification and regression that do not require the joint distribution, discriminative models may yield superior performance. On the other hand, generative models are typically more flexible than discriminative models in expressing dependencies in complex learning tasks. In addition, most discriminative models are inherently supervised and cannot easily be extended to unsupervised learning. Application specific details ultimately dictate the suitability of selecting a discriminative versus generative model.

[0058] A generative model can be generally defined as a model that is probabilistic in nature. In other words, a “generative” model is not one that performs forward simulation or rule-based approaches and, as such, it may not be necessary to model the physics of the processes involved ingenerating an actual image or output (for which a simulated image or output is being generated). Instead, the generative model can be learned (in that its parameters can be learned) based on a suitable training set of data. Such generative models may have a number of advantages for the embodiments described herein. In addition, the generative model may be configured to have a deep learning architecture in that the generative model may include multiple layers, which may perform a number of algorithms or transformations. The number of layers included in the generative model may depend on the particular use case. For practical purposes, a suitable range of layers is from 2 layers to a few tens of layers.

[0059] Deep learning is a type of machine learning. Machine learning can be generally defined as a type of artificial intelligence (Al) that provides computers with the ability to learn without being explicitly programmed. Machine learning focuses on the development of computer programs that can teach themselves to grow and change when exposed to new data. In other words, machine learning can be defined as the subfield of computer science that “gives computers the ability to learn without being explicitly programmed.” Machine learning explores the study and construction of algorithms that can learn from and make predictions on data — such algorithms overcome following strictly static program instructions by making data driven predictions or decisions, through building a model from sample inputs.

[0060] The machine learning described herein may be further performed as described in “Introduction to Statistical Machine Learning,” by Sugiyama, Morgan Kaufmann, 2016, 534 pages; “Discriminative, Generative, and Imitative Learning,” Jebara, MIT Thesis, 2002, 212 pages; and “Principles of Data Mining (Adaptive Computation and Machine Learning)” Hand et al., MIT Press, 2001, 578 pages; which are incorporated by reference as if fully set forth herein. The embodiments described herein may be further configured as described in these references.

[0061] In some embodiments, a machine learning system may comprise a neural network. For example, a model may be a deep neural network with a set of weights that model the world according to the data that it has been fed to train it. Neural networks can be generally defined as a computational approach which is based on a relatively large collection of neural units loosely modeling the way a biological brain solves problems with relatively large clusters of biological neurons connected by axons. Each neural unit is connected with many others, and links can be enforcing or inhibitory in their effect on the activation state of connected neural units. These systems are self-learning and trained rather than explicitly programmed and excel in areas where the solution or feature detection is difficult to express in a traditional computer program.

[0062] Neural networks typically consist of multiple layers, and the signal path traverses from front to back. The goal of the neural network is to solve problems in the same way that the human brain would, although several neural networks are much more abstract. Modern neural network projects typically work with a few thousand to a few million neural units and millions of connections. The neural network may have any suitable architecture and / or configuration known in the art.

[0063] In a further embodiment, a model may comprise convolutional and deconvolution neural network. For example, the embodiments described herein can take advantage of learning concepts such as a convolution and deconvolution neural network to solve the normally intractable representation conversion problem (e.g., rendering). The model may have any convolution and deconvolution neural network configuration or architecture known in the art.

[0064] Figs. 3A-B schematically illustrate example alignment and inspection processes, according to a comparative embodiment. In Fig. 3 A, two graphs 301 and 302 illustrate stage velocity V as a function of time t for two such processes. Here, a moveable stage (similar to, e.g., wafer stage 134 in Fig. 2B) may be used to position various portions of a substrate under a field of view of a SEM or other charged particle beam apparatus (similar to, e.g., EBI system 10 of Fig. 1, or apparatus 100A or 100B of Figs. 2A-B). For example, graph 301 may illustrate an example “leap and scan” inspection process. In the leap and scan inspection process, a substrate may be moved to alternately position alignment marks 342 and care areas 352 under the field of view of the charged particle beam apparatus to perform stationary exposures of them. For example, the process of graph 301 may begin at a time ti with the completion of a prior exposure at a prior care area 3520. The substrate stage first moves to position a next alignment mark 342i in the field of view at time t2. As shown in the figure, this movement may include accelerating the stage up to a cruising velocity Vc, maintaining the cruising velocity for a duration, and decelerating the stage to zero when alignment mark 342i is in the field of view. Next, at time t2, the stage remains stationary at the alignment mark 342i position for a predetermined settling time (as indicated by the light rectangle). The settling time may be a minimum time required to allow stage vibrations dissipate below a threshold level at which an accurate SEM image may be obtained. After the settling time has passed, the SEM may perform an exposure period (as indicated by the light rectangle) to obtain an image of the alignment mark 342i at time b,. The SEM image may then be compared to a reference image (such as in a GDS file) to determine any deviations of the alignment mark 342i from its intended design. For example, a processor or other controller may determine an x-shift, y-shift, rotation, distortion, etc. with respect to the reference image and perform any necessary corrections, such as by adjusting a stage path or SEM exposure settings. Next, at time U, the stage again moves to position the next care area 352i in the field of view. This may involve, e.g., a short acceleration and deceleration as shown. After the stage arrives at the care area 352i and stops at time ts, the stage must again remain stationary for a predetermined settling time. The care area settling time at time ts may the same as, or different from, the alignment mark settling time at time t2 depending on, e.g., the exposure requirements, stage dynamics, or other considerations. Finally, at time te, next care area 352i is exposed. The cycle may then repeat from time ti at a further care area (not shown).

[0065] Alternatively, graph 302 may illustrate an example “continuous scan” inspection process. The continuous scan process may be used to, e.g., expose long strip-like care areas that are too large to fit within the SEM field of view in a stationary exposure. In the continuous scan process, the stagemay remain at the cruising velocity Vc as the SEM field of view passes over a care area to expose it. The cruising velocity Vc may be limited by the need to fully expose the care area while the stage is in motion. For example, cruising velocity Vc may be selected based on, e.g., dosage requirements for exposing the care area, electron beam deflection capabilities, or other exposure parameters. The process of graph 302 may begin at a time ti with the completion of a prior exposure at prior care area 3520while the stage is moving at the constant cruising velocity Vc. The substrate stage may then decelerate to stop at the next alignment mark 342i in the SEM field of view at time t2. Then, after a predetermined settling time, the SEM may obtain an image of the alignment mark 342i and perform corrections as discussed above at time fi. Next, at time fi, the stage may accelerate to the cruising velocity Vc at time ts. After a predetermined settling period during constant velocity motion, the next care area 352i may pass through the field of view and be imaged by the SEM. The cycle may then repeat from time ti at a further care area (not shown).

[0066] Fig. 3B schematically illustrates example travel paths 351 of a stage 334 in the alignment and inspection processes of Fig. 3A, according to a comparative embodiment. The dashed box 320 on the right side of Fig. 3B indicates a close-up view of the region around care area 352i of a wafer 350 under inspection. Note that the arrows of travel paths 351 indicate relative motion of a SEM field of view with respect to wafer 350. For example, the SEM may remain stationary while stage 334 moves the wafer 350 in the direction opposite to the direction shown by the arrows.

[0067] Wafer 350 may comprise a plurality of care areas 352 that are targeted for inspection. As illustrated at care area 352i , each care area may be surrounded by a plurality of alignment marks 342 (denoted by “x’s” in Fig. 3B). An alignment mark 342 may comprise a pattern feature that is determined to provide suitable alignment information. For example, an alignment mark 342 may comprise a dedicated alignment pattern that is designed specifically for performing alignment. Alternatively or additionally, an alignment mark 342 may comprise, e.g., a device feature or other pattern that is considered suitable for use in an alignment process. For each care area 352, a corresponding alignment mark 342 may be selected in advance from a plurality of candidate alignment marks. For example, an alignment mark 342 may be selected based on its structural suitability for providing alignment information, as well as the likelihood that any deformations will provide usable information about a corresponding deformation in its associated care area 352. For this reason, one condition of alignment mark selection may be a requirement that the alignment mark 342 be located in close proximity to its associated care area 352. In this way, it can be assumed that any wafer deformations or patterning aberrations will be common to both the alignment mark 342 and care area 352. In Fig. 3B, the “x” denoting alignment mark 342i is circled to indicate that it has been selected from the plurality of candidate alignment marks to provide alignment information for care area 352

[0068] The travel paths 351 may proceed according to, e.g., the velocity and time information shown graphs 301 or 302 of Fig. 3A discussed above. For example, as shown in the close-up region 320,travel path 351a may proceed from a prior care area 3520and stop at alignment mark 342i. Then, after a predetermined settling time, alignment mark 342i may be exposed, and alignment information such as an x-shift, y-shift, rotation, distortion, etc. may be obtained. Next, a SEM controller may perform any necessary alignment corrections to an exposure plan for the next care area 352i based on the alignment information, such as by adjusting a next travel path 351b or SEM exposure settings. The stage may then proceed along travel path 351b to the next care area 352i . After a further stage settling time (such as during a stationary period as shown at graph 301 or a constant velocity period as shown at graph 302), the SEM may acquire an image of care area 352i . This process may repeat through a plurality of alignment marks 342 and care areas 352. For example, stage may then proceed along travel path 351c to a further alignment mark in the neighborhood of a further care area 352N.

[0069] As discussed above, the alignment processes of Figs. 3A-B require an excessive amount of acceleration, deceleration, and stage settling time. This harms throughput and also puts unwanted constraints on the number of care areas that may be inspected per wafer, which may degrade the overall device yield.

[0070] Figs. 4A-C schematically illustrate example alignment and inspection processes, consistent with embodiments of the present disclosure. The alignment and inspection processes may be performed using, e.g., a SEM or other charged particle beam apparatus, such as EBI system 10 of Fig. 1, or apparatus 100A or 100B of Figs. 2A-B. In Fig. 4A, two graphs 401 and 402 illustrate stage velocity V as a function of time t for two such processes. Graphs 401 and 402 may be similar to, e.g., graphs 301 and 302 of Figs. 3A-B except as described below. In particular, graphs 401 and 402 illustrate examples of on-the-fly local alignment, consistent with embodiments of the present disclosure.

[0071] For example, graph 401 may illustrate an example on-the-fly local alignment for a leap and scan inspection process. For example, the process of graph 401 may begin at a time ti with the completion of a prior exposure at a prior care area 4520. The substrate stage may then accelerate to a cruising velocity Vc at time t2. The cruising velocity may be selected based on, e.g., dosage requirements for exposing an alignment mark 442i, electron beam deflection capabilities, or other exposure parameters. The substrate stage may maintain this cruising velocity Vc through a stage settling period (indicated by the light rectangle near time t2) and continue through on-the-fly exposure of the next alignment mark 442i at time . After alignment mark 442i is imaged by the SEM, the image may be compared to a reference image (such as in a GDS fde) at time t4 to determine any deviations of the alignment mark 442i from its intended design. For example, a SEM processor or other controller (such as, e.g., controller 109 of Fig. 1 or image processing system 199 of Fig. 2B), may determine an x-shift, y-shift, distortion, etc. with respect to the reference image and perform any necessary alignment corrections to an exposure plan for a next care area 452 such as by adjusting a stage path or SEM exposure settings. This correction process is performed during the period between times t4 and ts at least to the extent that the precise stopping location for exposing the next care area452i is corrected in advance of time ts. In other words, there may be a sufficient duration between times t4 and ts that at least the precise stopping location is calculated in advance. Further, any additional alignment corrections, such as exposure settings or complex deformation corrections, may be performed at least by the time exposure begins at time te, such as during the settling period after time ts. As further discussed below with respect to Fig. 4B, sufficient time may be ensured, e.g., by appropriate alignment mark selection, by reducing the constant velocity Vc, or by generating a more complex deceleration profile at time U that allows for a greater duration between times t4 and ts. Finally, at time ts, the next care area 452i may be exposed based on the calculated corrections. The cycle may then repeat from time ti at a further care area (not shown). The process illustrated in graph 401 may be preferable when, e.g., a high quality image is more desirable than high throughput.

[0072] Alternatively, graph 402 illustrates an example on-the-fly local alignment for a continuous scan inspection process. In addition to enabling exposure of larger care areas, using on-the-fly local alignment in the continuous scan process allows multiple alignment marks 442 and care areas 452 to be exposed in succession without any acceleration, deceleration, or settling time. For example, the process of graph 402 may begin at a time ti with the completion of a prior exposure at a prior care area 4520. However, unlike graph 401, the process shown in graph 402 may proceed to exposing a first alignment mark 442i at time t2 without any further acceleration or settling, and the process of performing any necessary corrections may begin at time ts. There may be a sufficiently long duration between times ts and U to perform any necessary corrections before care area 452i is exposed at time t4. At time ts, the cycle may repeat for a further pair of alignment marks and care areas. For example, the continuous scan process with on-the-fly local alignment as shown in graph 402 may be repeated indefinitely without any acceleration, deceleration or settling time. For example, the process may proceed across an entire substrate until it reaches a final care area 452N, where N may be, e.g., 10, 50, 100 or any number of care areas arranged in a row across a substrate under inspection.

[0073] Although graphs 401 and 402 illustrate a constant velocity Vcbetween care areas 452 (where Vc is a cruising velocity at which exposure may be performed), embodiments of the present disclosure are not limited to this. For example, in some embodiments a stage may be temporarily moved at a higher velocity to facilitate faster transitions between care areas. In such cases, it may be sufficient to reduce the stage velocity down to Vc in time to allow for stage settling before performing on-the-fly local alignment.

[0074] Further, although graphs 401 and 402 illustrate a single alignment mark 442 being imaged between each care area 452, embodiments of the present disclosure are not limited to this. For example, a plurality of alignment marks 442 may be imaged on a single travel path between two successive care areas. Alignment corrections may then be based on the plurality of alignment mark images to determine more accurate information about alignment conditions at the care area. For example, the alignment corrections may be based on, e.g., a weighed combination or extrapolation of the plurality of alignment mark images. For example, a weighted combination may be used to averageout any potential inaccuracies resulting from the use of an alignment mark that is situated far from the care area. Further, if an alignment parameter in each successive alignment mark is seen to change as a function of proximity to the care area, an alignment correction may applied that accounts for this trend based on extrapolation of the alignment parameter. Furthermore, in some embodiments a single alignment mark 442 may be used to perform alignment corrections for multiple successive care areas 452 arranged in a row, or for a single care area that forms a continuous strip across the entire substrate.

[0075] As discussed above, performing the on-the-fly local alignment process may require certain conditions on the layout or selection of alignment marks. For example, although it may be desirable to select an alignment mark that is close to its associated care area for the reasons discussed above, there may be a competing interest in locating the alignment mark sufficiently far away from the care area to allow computations and corrections to be performed during a constant velocity stage motion. Further, while comparative processes may allow for selection of any alignment mark surrounding a care area, in some embodiments of the present disclosure, candidate alignment marks may be selected only from those marks that lie on a direct path between a prior care area and the next one.

[0076] Figs. 4B-C schematically illustrate example alignment mark selection conditions in the alignment and inspection processes of Fig. 4A, consistent with embodiments of the present disclosure. In order to achieve a travel path that is not hindered by settling time between successive exposures, in some embodiments, alignment marks 442 (denoted by “x”) may be selected from a plurality of candidates based on the conditions illustrated in Figs. 4B-C. Here, an array of example candidate alignment marks 442 is illustrated around care area 452i . Alignment marks 442 that are circled indicate marks that meet the conditions discussed below. It should be understood that the conditions discussed below are not all required in any particular implementation of embodiments of the present disclosure, nor that they represent an exhaustive list of all conditions that may arise in mark selection. For example, other typical considerations, such as the suitability of a particular mark pattern itself, naturally may also apply.

[0077] For example, a first condition may be illustrated at the left side of Fig. 4B. The first condition may require that the alignment mark be situated on a straight travel path between a prior care area and a next care area to be imaged. Travel path 451 is illustrated as a straight strip bounded by dashed lines. In some embodiments, travel path 451 may comprise a straight path between two care areas, such as 4520and 452i. In some embodiments, travel path 451 may comprise a straight path across more than two care areas, such as care areas 4520, 452i ...452N. By selecting an alignment mark based on this condition, it may be possible to travel in one continuous direction between successive care areas without creating vibrations that require a stage settling time. As seen at the left in Fig. 4B, out of the array of schematically illustrated candidate alignment marks, only the circled marks satisfy the first condition (schematically illustrated here as a row of three circled marks; it should be understood that more or fewer marks may satisfy the first condition in practice).

[0078] A second condition is schematically illustrated on the right side of Fig. 4B. The second condition may require that the alignment mark be separated from the care area by a predetermined minimum computation distance Dmm. The minimum computation distance Dmmmay comprise a product of a predetermined stage speed and a computation time required to perform the necessary corrections based on the on-the-fly local alignment process. For example, in a leap and scan process as illustrated at graph 401 of Fig. 4A, the predetermined stage speed may comprise an average speed between times t4 and ts. For a continuous scan process as illustrated at graph 402 of Fig. 4A, the predetermined stage speed may comprise, e.g., the cruising velocity Vc. The computation time may depend on, e.g., the computational power of processing circuitry used to acquire images and perform the computations, as well as the complexity of the required computations. In some embodiments, the computation time may comprise, e.g., 0.3 seconds, 0.2. seconds, 0.15 seconds, 0.1 seconds, or 0.05 seconds. By selecting an alignment mark outside the minimum computation distance, the corrections may be performed in time to image are area 452i without decelerating or otherwise introducing unwanted delays or vibrations. For a given computational power, the minimum computation distance may be increased by, e.g., reducing the predetermined stage speed at which a scan is expected to be performed. As shown on the right in Fig. 4B, introducing this condition eliminates a further one of the three schematically illustrated circled alignment mark candidates 442. In some embodiments, even performing a scan at a stage speed below the maximum allowable speed may still achieve a net reduction in inspection time as compared to processes that involve starting and stopping as illustrated in Figs. 3A-B. Furthermore, reducing the predetermined stage speed may reduce the minimum computation distance Dmmto allow for the selection of alignment marks that are closer to the care area 452i.

[0079] A third condition is illustrated on the left of Fig. 4C. In some embodiments, the third condition may require that the alignment mark be separated from the care area by less than a predetermined maximum separation distance Dmax. This condition may be based on the desire to select an alignment mark that is in close proximity to its respective care area, as discussed previously. For example, in some embodiments the maximum separation distance Dmaxmay comprise a fixed threshold distance such as, e.g., 10 mm, 5 mm, 2 mm, or 1 mm. In some embodiments, the maximum separation distance Dmaxmay be chosen based on a function of the minimum computation distance Dmin. For example, it may be desired that the maximum separation distance is no more than 50% greater than the minimum computation distance such that D max < 1.5 Dmin- In some embodiments, another proportion may be chosen, such as, e.g., 10%, 25%, 75%, 100%, 500%, etc. In some embodiments, the maximum separation distance may be based on other considerations, such as on a map or model of wafer deformation, on feedback from prior inspections, or other similar considerations. As seen at the left of Fig. 4C, introducing the third condition eliminates a further one of the three schematically illustrated circled alignment mark candidates 442, such that only one remains. On the right side of Fig. 4C, the first second and third conditions are illustrated together. Byimposing one or more of the above discussed conditions on alignment mark selection, it may be possible to perform on-the-fly local alignment as discussed above.

[0080] Fig. 5 schematically illustrates a flowchart of an example method 500 of performing on-the- fly local alignment in an inspection process, consistent with embodiments of the present disclosure. The method may be performed according to embodiments disclosed in, e.g., Figs. 4A-C using charged particle beam apparatus as disclosed in, e.g., Figs. 1-2B. For example, method 500 may be performed using a controller, such as controller 109 in Fig. 1, or image acquisition unit 199 in Fig.2B

[0081] At step 501, a substrate stage may be moved at a constant cruising speed. For example, the cruising speed may be a speed at which the charged particle beam apparatus is capable of imaging a pattern feature of the substrate to a desired resolution. The stage may be moved at the constant cruising speed on a travel path from a prior care area position to a next care area position. The prior care area position may be a stage position at which the prior care area is located in a field of view of the charged particle beam apparatus. The next care area position may be a stage position at which the next care area is located in the field of view of the charged particle beam apparatus. The travel path between the prior care area and the next care area may include a predetermined alignment mark. In some embodiments, the movement may be at the constant cruising speed over the entire distance between the prior care area position and the next care area position. In some embodiments, the movement may be at the constant cruising speed for at least long enough to allow for stage settling time before the predetermined alignment mark enters the field of view of the charged particle beam apparatus. For example, the travel path may comprise a segment over which the stage speed is increased with respect to the constant cruising speed in order to improve throughput.

[0082] At step 502, the charged particle beam apparatus may acquire an image of the alignment mark while the stage is moving at the constant cruising speed. Because the stage is moving at a constant speed during the imaging process, imaging may be performed without significant disturbances from, e.g., stage vibrations due to acceleration or deceleration. For example, in some embodiments, the alignment mark may be a predetermined alignment mark that is selected based on a first condition that the alignment mark is located along a straight travel path from the prior care area to the next care area.

[0083] At step 503, alignment corrections may be performed based on the acquired image of the alignment mark. The alignment corrections may comprise corrections to a next image acquisition of the next care area as discussed below. For example, the alignment correction may comprise, e.g., an x- shift, a y-shift, a rotation, or a distortion or other deformation correction applied to the next care area.

[0084] The correction may be performed while the stage is still moving to position the next care area in the field of view of the charged particle beam apparatus. In some embodiments, there may be sufficient time to perform the alignment corrections due to appropriate selection of the predetermined alignment mark. For example, the predetermined alignment mark may be selected based on a second condition that the predetermined alignment mark be separated by at least a minimum computationdistance from the next care area. For example, the minimum computation distance may comprise a product of the predetermined stage speed and a computation time required to perform the alignment corrections. In some embodiments, the predetermined stage speed may comprise the constant cruising speed. In some embodiments, the predetermined stage speed may comprise an average stage speed between acquiring the predetermined alignment mark image and reaching the position of the next care area.

[0085] At step 504, the charged particle beam apparatus may acquire an image of the next care area. For example, in some embodiments the charged particle beam apparatus may acquire the image of the alignment mark while the stage continues to move at the constant cruising speed. In some embodiments, the stage may continue to move at the constant cruising speed through a plurality of successive alignment marks and care areas without stopping. In some embodiments, the stage may decelerate to a stop at the next care area position and allow the stage to settle before acquiring the image of the next care area.

[0086] At step 505, an inspection process may be performed based on the image of the care area. For example, the inspection may comprise a defect inspection, or a metrology process, such as a measurement of alignment, overlay, edge placement error, critical dimension, overlay, linewidth roughness, line edge roughness, line end shortening, line end shortening (LES) or the like.

[0087] A non-transitory computer-readable medium may be provided that stores instructions for a processor of a controller (e.g., controller 109 in Fig. 1, or image acquisition unit 199 in Fig. 2B) for detecting charged particles according to, e.g., of Figs. 4A-C, or the example method 500 of Fig. 5, consistent with embodiments of the present disclosure. For example, the instructions stored in the non-transitory computer-readable medium may be executed by the circuitry of the controller for performing measurements according to Figs. 4A-C or method 500 in part or in entirety. 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), a cache, a register, any other memory chip or cartridge, and networked versions of the same.

[0088] Embodiments of the present disclosure may further be described by the following clauses: 1. A non-transitory computer-readable medium that stores a set of instructions that is executable by at least one processor of an apparatus to cause the apparatus to perform operations comprising: acquiring a first charged particle beam image of a first alignment mark on a substrate supported on a substrate stage while the substrate stage moves at the constant speed; determining a first alignment correction based on the first charged particle beam image; andacquiring a second charged particle beam image of a first care area on the substrate based on the first alignment correction.2. The non-transitory computer-readable medium of clause 1, wherein the operations further comprise: acquiring a third charged particle beam image of a second alignment mark on the substrate while the substrate stage moves at the constant speed; determining a second alignment correction based on the third charged particle beam image; and acquiring a fourth charged particle beam image of a second care area on the substrate based on the second alignment correction.3. The non-transitory computer-readable medium of clause 2, wherein the operations further comprise: moving the substrate stage on a straight travel path from a position at which the second charged particle beam image is acquired to a position at which the fourth charged particle beam image is acquired.4. The non-transitory computer-readable medium of clause 3, wherein moving the substrate stage on the straight travel path comprises moving the substrate stage continuously.5. The non-transitory computer-readable medium of clause 4, wherein moving the substrate stage on the straight travel path comprises moving the substrate stage at a constant velocity.6. The non-transitory computer-readable medium of clause 2, wherein the operations further comprise: moving the substrate stage on a straight travel path at a constant velocity from a position at which the first charged particle beam image is acquired to a position at which the fourth charged particle beam image is acquired.7. The non-transitory computer-readable medium of clause 2, wherein the operations further comprise: selecting the second alignment mark for imaging from a plurality of alignment mark candidates on the substrate based on a condition that the second alignment mark is located on a straight travel path between the first care area and the second care area.8. The non-transitory computer-readable medium of clause 2, wherein the operations further comprise: selecting the second alignment mark for imaging from a plurality of alignment mark candidates on the substrate based on a condition that the second alignment mark is separated from the second care area by a predetermined computation distance, wherein the predetermined computation distance is based on a product of the constant speed and a computation time required to determine the second alignment correction.9. The non-transitory computer-readable medium of clause 8, wherein the operations further comprise: selecting the second alignment mark for imaging from the plurality of alignment mark candidates on the substrate based on a further condition that the second alignment mark is separated from the second care area by less than five times the predetermined computation distance.10. The non-transitory computer-readable medium of clause 2, wherein the operations further comprise: acquiring a fifth charged particle beam image of a third alignment mark on the substrate while the substrate stage moves at the constant speed, wherein determining the second alignment correction is further based on the fifth charged particle beam image.11. The non-transitory computer-readable medium of clause 10, wherein determining the second alignment correction is based on a weighted combination of the third charged particle beam image and the fifth charged particle beam image.12. The non-transitory computer-readable medium of clause 10, wherein determining the second alignment correction is based on an extrapolation of the third charged particle beam image and the fifth charged particle beam image.13. The non-transitory computer-readable medium of clause 1, wherein the operations further comprise: acquiring the second charged particle beam image of the first care area while the substrate stage is stationary.14. An inspection method, comprising: acquiring a first charged particle beam image of a first alignment mark on a substrate supported on the substrate stage while the substrate stage moves at a constant speed; determining a first alignment correction based on the first charged particle beam image; and acquiring a second charged particle beam image of a first care area on the substrate based on the first alignment correction.15. The method of clause 14, further comprising: acquiring a third charged particle beam image of a second alignment mark on the substrate while the substrate stage moves at the constant speed; determining a second alignment correction based on the third charged particle beam image; and acquiring a fourth charged particle beam image of a second care area on the substrate based on the second alignment correction.16. The method of clause 15, further comprising:moving the substrate stage on a straight travel path from a position at which the second charged particle beam image is acquired to a position at which the fourth charged particle beam image is acquired.17. The method of clause 16, wherein moving the substrate stage on the straight travel path comprises moving the substrate stage continuously.18. The method of clause 17, wherein moving the substrate stage on the straight travel path comprises moving the substrate stage at a constant velocity.19. The method of clause 15, further comprising: moving the substrate stage on a straight travel path at a constant velocity from a position at which the first charged particle beam image is acquired to a position at which the fourth charged particle beam image is acquired.20. The method of clause 15, further comprising: selecting the second alignment mark for imaging from a plurality of alignment mark candidates on the substrate based on a condition that the second alignment mark is located on a straight travel path between the first care area and the second care area.21. The method of clause 15, further comprising: selecting the second alignment mark for imaging from a plurality of alignment mark candidates on the substrate based on a condition that the second alignment mark is separated from the second care area by a predetermined computation distance, wherein the predetermined computation distance is based on a product of the constant speed and a computation time required to determine the second alignment correction.22. The method of clause 21, further comprising: selecting the second alignment mark for imaging from the plurality of alignment mark candidates on the substrate based on a further condition that the second alignment mark is separated from the second care area by less than five times the predetermined computation distance.23. The method of clause 15, further comprising: acquiring a fifth charged particle beam image of a third alignment mark on the substrate while the substrate stage moves at the constant speed, wherein determining the second alignment correction is further based on the fifth charged particle beam image.24. The method of clause 23, wherein determining the second alignment correction is based on a weighted combination of the third charged particle beam image and the fifth charged particle beam image.25. The method of clause 23, wherein determining the second alignment correction is based on an extrapolation of the third charged particle beam image and the fifth charged particle beam image.26. The method of clause 14, further comprising:acquiring the second charged particle beam image of the first care area while the substrate stage is stationary.27. A charged particle beam apparatus, comprising: a charged particle beam source configured to generate a charged particle beam; a charged particle optical system configured to direct the charged particle beam at a pattern region on a substrate; a substrate stage configured to support and move the substrate; and a controller comprising one or more processors and configured to cause the charged particle beam apparatus to perform operations comprising: acquiring a first charged particle beam image of a first alignment mark on the substrate while the substrate stage moves at a constant speed; determining a first alignment correction based on the first charged particle beam image; and acquiring a second charged particle beam image of a first care area on the substrate based on the first alignment correction.28. The apparatus of clause 27, wherein the operations further comprise: acquiring a third charged particle beam image of a second alignment mark on the substrate while the substrate stage moves at the constant speed; determining a second alignment correction based on the third charged particle beam image; and acquiring a fourth charged particle beam image of a second care area on the substrate based on the second alignment correction.29. The apparatus of clause 28, wherein the operations further comprise: moving the substrate stage on a straight travel path from a position at which the second charged particle beam image is acquired to a position at which the fourth charged particle beam image is acquired.30. The apparatus of clause 29, wherein moving the substrate stage on the straight travel path comprises moving the substrate stage continuously.31. The apparatus of clause 30, wherein moving the substrate stage on the straight travel path comprises moving the substrate stage at a constant velocity.32. The apparatus of clause 28, wherein the operations further comprise: moving the substrate stage on a straight travel path at a constant velocity from a position at which the first charged particle beam image is acquired to a position at which the fourth charged particle beam image is acquired.33. The apparatus of clause 28, wherein the operations further comprise:selecting the second alignment mark for imaging from a plurality of alignment mark candidates on the substrate based on a condition that the second alignment mark is located on a straight travel path between the first care area and the second care area.34. The apparatus of clause 28, wherein the operations further comprise: selecting the second alignment mark for imaging from a plurality of alignment mark candidates on the substrate based on a condition that the second alignment mark is separated from the second care area by a predetermined computation distance, wherein the predetermined computation distance is based on a product of the constant speed and a computation time required to determine the second alignment correction.35. The apparatus of clause 34, wherein the operations further comprise: selecting the second alignment mark for imaging from the plurality of alignment mark candidates on the substrate based on a further condition that the second alignment mark is separated from the second care area by less than five times the predetermined computation distance.36. The apparatus of clause 28, wherein the operations further comprise: acquiring a fifth charged particle beam image of a third alignment mark on the substrate while the substrate stage moves at the constant speed, wherein determining the second alignment correction is further based on the fifth charged particle beam image.37. The apparatus of clause 36, wherein determining the second alignment correction is based on a weighted combination of the third charged particle beam image and the fifth charged particle beam image.38. The apparatus of clause 36, wherein determining the second alignment correction is based on an extrapolation of the third charged particle beam image and the fifth charged particle beam image.39. The apparatus of clause 27, wherein the operations further comprise: acquiring the second charged particle beam image of the first care area while the substrate stage is stationary.

[0089] Block diagrams in 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. In this regard, each block in a schematic diagram may represent certain arithmetical or logical operation processing that may be implemented using hardware such as an electronic circuit. Blocks 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 block may occur out of the order noted in the figures. For example, two blocks shown in succession may be executed or implemented substantially concurrently, or two blocks may sometimes be executed in reverse order, depending upon the functionality involved. Some blocks may also be omitted. It should also be understood that each block of the block diagrams, andcombination of the blocks, 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.

[0090] 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 can be made without departing from the scope thereof. For example, a charged particle inspection system may be but one example of a charged particle beam system consistent with embodiments of the present disclosure.

Claims

CLAIMS1. A non-transitory computer-readable medium that stores a set of instructions that is executable by at least one processor of an apparatus to cause the apparatus to perform operations comprising: acquiring a first charged particle beam image of a first alignment mark on a substrate supported on a substrate stage while the substrate stage moves at the constant speed; determining a first alignment correction based on the first charged particle beam image; and acquiring a second charged particle beam image of a first care area on the substrate based on the first alignment correction.

2. The non-transitory computer-readable medium of claim 1, wherein the operations further comprise: acquiring a third charged particle beam image of a second alignment mark on the substrate while the substrate stage moves at the constant speed; determining a second alignment correction based on the third charged particle beam image; and acquiring a fourth charged particle beam image of a second care area on the substrate based on the second alignment correction.

3. The non-transitory computer-readable medium of claim 2, wherein the operations further comprise: moving the substrate stage on a straight travel path from a position at which the second charged particle beam image is acquired to a position at which the fourth charged particle beam image is acquired.

4. The non-transitory computer-readable medium of claim 3, wherein moving the substrate stage on the straight travel path comprises moving the substrate stage continuously.

5. The non-transitory computer-readable medium of claim 4, wherein moving the substrate stage on the straight travel path comprises moving the substrate stage at a constant velocity.

6. The non-transitory computer-readable medium of claim 2, wherein the operations further comprise: moving the substrate stage on a straight travel path at a constant velocity from a position at which the first charged particle beam image is acquired to a position at which the fourth charged particle beam image is acquired.

7. The non-transitory computer-readable medium of claim 2, wherein the operations further comprise: selecting the second alignment mark for imaging from a plurality of alignment mark candidates on the substrate based on a condition that the second alignment mark is located on a straight travel path between the first care area and the second care area.

8. The non-transitory computer-readable medium of claim 2, wherein the operations further comprise: selecting the second alignment mark for imaging from a plurality of alignment mark candidates on the substrate based on a condition that the second alignment mark is separated from the second care area by a predetermined computation distance, wherein the predetermined computation distance is based on a product of the constant speed and a computation time required to determine the second alignment correction.

9. The non-transitory computer-readable medium of claim 8, wherein the operations further comprise: selecting the second alignment mark for imaging from the plurality of alignment mark candidates on the substrate based on a further condition that the second alignment mark is separated from the second care area by less than five times the predetermined computation distance.

10. The non-transitory computer-readable medium of claim 2, wherein the operations further comprise: acquiring a fifth charged particle beam image of a third alignment mark on the substrate while the substrate stage moves at the constant speed, wherein determining the second alignment correction is further based on the fifth charged particle beam image.

11. The non-transitory computer-readable medium of claim 10, wherein determining the second alignment correction is based on a weighted combination of the third charged particle beam image and the fifth charged particle beam image.

12. The non-transitory computer-readable medium of claim 10, wherein determining the second alignment correction is based on an extrapolation of the third charged particle beam image and the fifth charged particle beam image.

13. The non-transitory computer-readable medium of claim 1, wherein the operations further comprise:acquiring the second charged particle beam image of the first care area while the substrate stage is stationary.

14. An inspection method, comprising: acquiring a first charged particle beam image of a first alignment mark on a substrate supported on the substrate stage while the substrate stage moves at a constant speed; determining a first alignment correction based on the first charged particle beam image; and acquiring a second charged particle beam image of a first care area on the substrate based on the first alignment correction.

15. A charged particle beam apparatus, comprising: a charged particle beam source configured to generate a charged particle beam; a charged particle optical system configured to direct the charged particle beam at a pattern region on a substrate; a substrate stage configured to support and move the substrate; a controller comprising one or more processors and configured to cause the charged particle beam apparatus to perform operations comprising: acquiring a first charged particle beam image of a first alignment mark on the substrate while the substrate stage moves at a constant speed; determining a first alignment correction based on the first charged particle beam image; and acquiring a second charged particle beam image of a first care area on the substrate based on the first alignment correction.

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