Automatic correction for hardware-based SEM tool time offset

Abstract

A system and method for correction of image offsets in a multi-detector charged particle beam apparatus comprises a plurality of charged particle detectors and a hardware-based time offset correction system. The hardware-based time offset correction system is configured to correct a time difference among detection signals from the plurality of detectors based on a predetermined time offset. The predetermined time offset may be calibrated according to a calibration process.

Description

AUTOMATIC CORRECTION FOR HARDWARE-BASED SEM TOOL TIME OFFSETCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 621,581 which was filed on 17 January 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The description herein relates to charged particle beam systems, and more particularly, to systems and methods for determining and correcting offsets between multiple charged particle beam detector images in such charged particle beam systems.BACKGROUND

[0003] Detectors 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 example, in metrology processes or to reveal defects in the sample. Metrology relates to precision measurements of sample structures and other miniaturized features. For example, in a semiconductor wafer, metrology may include measurements of circuit pattern features such as critical dimension (width of the smallest device feature), critical dimension uniformity, linewidth, overlay, line edge roughness, line end shortening, floor tilt, sidewall angle, and other dimensional parameters. Detection of defects in a sample is also increasingly important in the manufacturing of semiconductor devices, which may include large numbers of densely packed, miniaturized integrated circuit (IC) components. Inspection systems may be provided for these and other purposes.

[0004] With continuing miniaturization of semiconductor devices, performance demands for inspection systems may continue to increase. For example, to obtain an accurate high-resolution image of a sample surface, a combined inspection image may be created from multiple images using multiple detectors. Therefore it may be necessary to ensure that the multiple images are properly registered with each other.SUMMARY

[0005] Some embodiments of the present disclosure provide a charged particle beam apparatus. The charged particle apparatus may comprise: a charged particle beam source configured to irradiate a surface with a charged particle beam to generate emitted charged particles from the surface; a first charged particle detector configured to detect a first portion of the emitted charged particles; a second charged particle detector configured to detect a second portion of the emitted charged particles; an image processor configured to receive a first signal based on detection of the first portion of emittedcharged particles at the first charged particle detector and a second signal based on detection of the second portion of emitted charged particles at the second charged particle detector; and a hardwarebased time offset correction system configured to generate a predetermined time offset of the first signal with respect to the second signal.

[0006] Some embodiments of the present disclosure provide a charged particle beam method. The charged particle apparatus may comprise: irradiating a surface with a charged particle beam to generate emitted charged particles from the surface; detecting a first portion of the emitted charged particles with a first charged particle detector; detecting a second portion of the emitted charged particles with a second charged particle detector; receiving, at an image processor, a first signal based on detection of the first portion of emitted charged particles at the first charged particle detector and a second signal based on detection of the second portion of emitted charged particles at the second charged particle detector; and generating a predetermined time offset of the first signal with respect to the second signal by a hardware -based time offset correction system.

[0007] Some embodiments of the present disclosure 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 method above.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. l 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 charged particle beam apparatus that may be examples of an electron beam tool, consistent with embodiments of the present disclosure.

[0011] Fig. 3 is a diagrammatic representation of an example charged particle beam apparatus, consistent with embodiments of the present disclosure.

[0012] Fig. 4 is a diagrammatic representation of example charged particle beam images, consistent with embodiments of the present disclosure.

[0013] Figs. 5A-B illustrate example calibration processes 500A-B for calibrating signal offsets between a plurality of charged particle detector images, consistent with embodiments of the present disclosure.

[0014] Figs. 6A-C diagrammatic representations of example time offset correction systems in charged particle beam apparatus, consistent with embodiments of the present disclosure.

[0015] Fig. 7 illustrates a flowchart of an example method of calibrating a time offset in a charged particle beam detection process, consistent with embodiments of the present disclosure.

[0016] Fig. 8 illustrates a flowchart of an example method of calibrating a time offset in a charged particle beam detection process, consistent with embodiments of the present disclosure.DETAILED DESCRIPTION

[0017] 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., proton beams) may be similarly applied. Furthermore, other imaging systems may be used, such as optical imaging, photodetection, x-ray detection, or the like.

[0018] 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 small as 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.

[0019] Making these 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, 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.

[0020] 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 scanning charged-particle microscope, 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.

[0021] 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, pixel by pixel. A SEM takes a “picture” by receiving and recording energies or quantities of electrons reflected or emitted from thestructures 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 multiple “sub-pictures” of the wafer in parallel and, in some instances, stitch them 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 “subpictures,” 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.

[0022] In some inspection processes, multiple charged particle images may be taken simultaneously at different detectors. In some cases it may be desirable to combine one or more of these images into a single composite image. However, this requires computationally intensive image alignment processes that increase costs and harm throughput. One reason the image alignment processes may be necessary is that detection signals from the different detectors do not reach an image processor at the same time due to, e.g., varying distances that the electrons must travel, different signal propagation times, and other factors. The result is that detection pixels in one image may be spatially offset with respect to corresponding detection pixels in another image.

[0023] Embodiments of the present disclosure provide systems and methods for hardware -based corrections to these time offsets so that the image signals are processed simultaneously and image misalignments may be reduced or eliminated. A hardware-based image offset correction may comprise, e.g., generating signal delays, or adjusting sampling windows or time stamp information based on a predetermined time offset. The predetermined time offset may be determined using a calibration process.

[0024] 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.

[0025] 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 statedotherwise 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.

[0026] 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 I l 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).

[0027] 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.

[0028] 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.

[0029] 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 accessibleby 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.

[0030] 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.

[0031] 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. There are two ways for SEM inspection and metrology systems to perform image reconstruction based on the signal: based on amplitude integral of the signal in each scanning pixel; or based on signal pulse edge detection and discrimination. At each scanned pixel location, multiple electrical signal pulses collected by different detector segments may have different pulse shapes. These various pulse shapes contain information about the electron energy distribution collected by the detector. Electrons with higher energy can cause faster rising times in the electrical pulse shape. With multiple segments of the detector (4 channels), up to 4 pulse edge detection image channels can be used to perform electron energy analysis, yielding higher SEM resolution.

[0032] 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 usesa plurality of beamlets formed from a primary electron beam to simultaneously scan multiple locations on a wafer.

[0033] As shown in Fig. 2A, electron beam tool 200A 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.

[0034] 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 electron beam tool 100A. Secondary optical system 242 and electron detection device 244 may be aligned with a secondary optical axis 252 of electron beam tool 100A.

[0035] 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.

[0036] 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 / 0025243; 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 microlenses. 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.

[0037] 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 on the 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, theadjustable condenser lens may be an adjustable anti-rotation condenser lens, which involves an antirotation lens with a movable first principal plane. An example of an adjustable condenser lens is further described in U.S. Publication No. 2017 / 0025241, which is incorporated by reference in its entirety.

[0038] 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.

[0039] 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 nonzero. 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.

[0040] 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.

[0041] 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 wafer 230 may have different resistance-capacitance characteristics that may cause different signal sensitivities to the movement of the scan probe spots.

[0042] 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.

[0043] 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.

[0044] While Fig. 2A shows detector 244 having several detection sub-regions aligned with secondary optical axis 252, it is appreciated that other multi-beam detector schemes may exist. For example, it is appreciated that a detector may correspond with each beamlet, such as a different detector for each of beamlets 214, 216, 218. It is appreciated that these different detectors may be positioned under the primary column corresponding to primary optical axis 260. For example, these different detectors could be positioned between primary projection optical system 220 and the wafer stage.

[0045] Another example of a charged particle beam apparatus will now be discussed with reference to Fig. 2B. An electron beam tool 200B (also referred to herein as charged particle apparatus 200B) may be an example of electron beam tool 100 and may be similar to electron beam tool 100A shown in Fig. 2A. However, different from electron beam tool 100A, electron beam tool 200B may be a single -beam tool that uses only one primary electron beam to scan one location on the wafer at a time.

[0046] As shown in Fig. 2B, electron beam tool 200B includes a wafer holder 136 supported by motorized stage 134 to hold a wafer 150 to be inspected. Electron beam tool 200B includes an electron emitter, which may comprise a cathode 103, an anode 121, and a gun aperture 122. Electron beam tool 200B may further include 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 104 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.

[0047] 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 acquirer120 may be communicatively coupled with detector 144 of electron beam tool 200B 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. In some embodiments, image acquirer 120 may be communicatively coupled with detector 144 at least in part by a signal wire 164, such as electrical conductor, optical fiber cable, or other material medium. 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.

[0048] 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.

[0049] The condenser and illumination optics of the electon beam tool may comprise or be supplemented by electromagnetic quadrupole electron lenses. For example, as shown in Fig. 2B, electron beam tool 200B 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.

[0050] Fig. 2B illustrates an electron beam tool 200B 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.

[0051] In some embodiments of the disclosure, a PIN detector may be used as an in-lens detector in a retarding objective lens SEM column of EBI system 10. The PIN detector may be placed between a cathode for generating an electron beam and the objective lens. The electron beam emitted from the cathode may be potentialized at -BE keV (typically around -10 kV). Electrons of the electron beam may be immediately accelerated and travel through the column. The column may be at ground potential. Thus, electrons may travel with kinetic energy of BE keV while passing through the opening of detector 144. Electrons passing through the pole piece of the objective lens, such as pole piece 132a of objective lens assembly 132 of Fig. 2B, may be steeply decelerated down to landing energy LE keV as the wafer surface potential may be set at -(BE - LE) keV. Emitted electrons 171, comprising e.g., secondary or backscattered electrons, may be emitted from the wafer surface by the impingement of electrons of the primary electron beam 104. A retarding electric field, which may slow the primary electrons as they approach probe spot 170, may act as an acceleration electric field to accelerate the emitted electrons backwards toward a detector 144 surface.

[0052] Detectors 244 of Fig. 2A or 144 of Fig. 2B may include sensing elements such as diodes, or elements similar to diodes, that may convert incident energy into a measurable signal. For example, sensing elements in a detector may include a SPAD, APD, scintillator, or PIN diode. Throughout this disclosure, sensing elements may be represented as a diode, although sensing elements or other components may deviate from ideal circuit behavior of electrical elements such as diodes, resistors, capacitors, etc. In embodiments of the present disclosure, a detector in a charged particle beam system may comprise a pixelated array of multiple sensing elements. In some embodiments, the sensing elements may be configured for charged particle counting. Pixelated sensing elements of a detector that may be useful for charged particle counting are discussed in U.S. Publication No. 2019 / 0378682, which is incorporated by reference in its entirety. In some embodiments, as further discussed below, a charged particle apparatus may comprise multiple detectors. The multiple detectors may be arranged at different locations or may be configured to detect charged particles having different energy levels or other characteristics.

[0053] 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. For instance, a source in a charged-particle beam tool can emit one or more charged particles, such as electrons, protons, ions, muons, or any other particle carrying electric charges. Furthermore, some embodiments of the present disclosure may use photons instead of charged particles, such as light in the 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.

[0054] Fig. 3 illustrates a charged particle beam apparatus 300 that may be an example of electron beam tool 100, consistent with embodiments of the present disclosure. Charged particle beam apparatus300 may comprise a plurality of charged particle detectors, such as four detectors 344a-d, for generating a plurality of charged particle images. For example, detector 344a may comprise a secondary electron detector, detector 344b may comprise an energy filter detector, and detectors 344c -d may comprise two segments of a bottom backscattered electron detector. Secondary electron detector 344a and energy filter detector 344b may be arranged at an upper side of an objective lens assembly 332 at a first height Hl from the wafer surface, and may be configured to detect first emitted electons 371a. First emitted electrons 371a may comprise, e.g., a mix of secondary and backscattered electrons with a relatively higher concentration of secondary electrons. Secondary electron detector 344a and energy filter detector 344b may be communicatively coupled to image processing system 399 by, e.g., signal wires 364a and 364b, respectively. Bottom backscattered electron detectors 344c-d may be arranged at lower side of objective lens assembly 332 near wafer 350. Bottom backscattered electron detectors 344c-d may be configured to detect second emitted electrons 371b. Second emitted electrons 371b may comprise, e.g., e.g., primarily backscattered electrons. Bottom backscattered electron detectors 344c-d may be communicatively coupled to image processing system 399 by, e.g., signal wires 364c-d, respectively. By providing multiple detectors 344a-d, a plurality of charged particle images may be simultaneously obtained.

[0055] As shown at the bottom right of Fig. 3, in some embodiments, bottom backscattered electron detectors 344c and 344d may be arranged along orthogonal directions in an x-y plane perpendicular the z-direction of electron beam 304. Because the topography of features on wafer 350 may affect the trajectory of second emitted electron 371b, this segmented configuration may allow electrons travelling in different directions to be detected separately, thus yielding greater information about the topography of the features on wafer 350. In some embodiments, a single backscattered electron detector, or more backscattered electron detectors, may be provided.

[0056] Fig. 4 illustrates an example composite charged particle image 467 formed from a plurality of charged particle images 465 and 466. For example, a first charged particle image may comprise a secondary electron image 465 obtained from, e.g., secondary electron detector 344a of Fig. 3. A second charged particle image 466 may comprise a backscattered electron image obtained from, e.g., backscattered electron detector 344c or 344d. The images 465-467 may show a plurality of pattern features 351 -352 in different layers of a wafer, such as wafer 350 of Fig. 3. For example, buried features 351 may comprise an array of pattern features located in a layer beneath a surface of the wafer. Surface features 352 may comprise an array of pattern features located at a surface of the wafer, or at a layer closer to the surface than the layer containing buried features 351. As seen in the charged particle image 466, backscattered electrons may be primarily emitted from deeper regions of the wafer corresponding to buried features 351. Therefore, charged particle image 466 may adequately represent the buried features 351. On the other hand, secondary electrons may be emitted from surface regions of the wafer, such as from surface features 352. However, because a secondary electron detector may receive a mix of both secondary and backscattered electrons, the charged particle image 465 may contain both theburied features 351 and the surface features 352. To produce a clearer image of surface features 352, a composite charged particle image 467 may be generated based on the charge particle images 465 and 466. For example, the composite may be generated by subtracting charged particle image 466 from charged particle image 465, or performing another operation based on the two images, to eliminate the buried features from composite charged particle image 467. This clearer image of an upper surface may then be used to, e.g., determine an overlay offset between the two layers. The above discussion of composite images is provided for illustrative purposes only. It should be understood that any number of combinations of charged particle images may be performed to yield different types of desired information.

[0057] To combine multiple charged particle beam images into a single composite image, the multiple charged particle beam images must be precisely aligned with each other. However, each of the charged particle images may be spatially offset from each other due to time offsets in the reception and processing of intensity signals at each detector. For example, as seen in Fig. 3, detectors 344a-b may be separated from the wafer 350 by a first height Hi, while detectors 344c-d may be separated from the wafer 350 by a smaller second height H2. For example, in some embodiments, the first height may be, e.g., between 100 and 300 mm, while the second height may be less than 15 mm. This height difference may result in longer travel times for first emitted electrons 371a than for second emitted electrons 371b before landing on a detector surface. This travel time may also be affected by the different travel speeds of the electrons, which relates to their energies when emitted and the electrical field energy level in the system. For example, backscattered electrons may be emitted from the wafer 350 with higher energies than secondary electrons, such that they are traveling this shorter distance H2 at higher speeds. Applying a different electrical field energy level at the wafer stage may also change the electron traveling speed. Thus the landing energy of the incident electrons (i.e., the energy of the incident electrons when they reach the wafer), which may be a function of the stage bias level and the energy level of the electron source, may affect the imaging offsets between detectors.

[0058] In addition to time differences in reception of electrons, the signal propagation time from each detector to its signal processing circuitry (such as at image processing system 399) may vary. For example, each detector 344a-d may be communicatively coupled to image processing system 399 by signal wires 364a-d, respectively. Each signal wire 364 may have a different length, resulting in a different signal propagation time, for example due to RC (resistance and capacitance) delay along the length of the wire. For example, a signal propagation speed Vgin the signal wire 364 may be represented by:(eqn. 1)where c represents the speed of light in a vacuum (approximately 3.0 x 108m / s) and 8rrepresents the relative permittivity of the signal wire insulating material. As an example for illustrative purposes, 8rmay be about 2.2 for a 35 MHz electrical signal, resulting in a signal propagation speed Vgof about 1.9987 x 108m / s. Under these conditions, a signal wire length difference of 30 cm would result in a time signal delay of 1.5 ns. At a scan rate of 100 MHz, this 1.5 ns signal delay would result in a 0.15 nm spatial offset in the images.

[0059] In addition to RC delay along the length of the signal wire, there may be delays introduced by signal processing circuitry such as, e.g., amplifiers, analog-to-digital converters (ADCs), and other circuit modules. As the signal processing circuitry for each detector may not be identical, timing differences may arise within the circuitry as well.

[0060] The degree to which a time offset between detector signals may translate to a spatial offset between corresponding image pixel images may also depending on factors such as, e.g., a scanning speed of the electron beam. For example, doubling the scanning speed may double the number of pixels scanned by the electron beam during a given time offset, such that the corresponding spatial offset in the images may also be doubled. Furthermore, because a delayed signal will appear to have originated from a wafer region that is “ahead” of the actual region, the direction of spatial offsets in an image may depend on the scanning direction of the electron beam when the image was taken.

[0061] One solution for aligning charged particle images (such as, e.g., charged particle images 465 and 466 of Fig. 4) may comprise performing complex post-acquisition processing on the images. For example, pattern recognition and other image processing algorithms may be used to determine and correct the shifts, distortions and other offsets between corresponding features. Because these image offsets are typically not an exact integer number of pixels, it may first be necessary to perform interpolation of the images at subpixel resolution before determining and applying the offset corrections. Furthermore, because the offsets depend on multiple factors, such as e-beam source energy level, detector height, stage bias level, scan direction and channel wire length, the post-acquisition image processing algorithms must keep track of all configuration settings in the charged particle beam apparatus. In addition to being computationally intensive and time-consuming, this image processing may alter the acquired images in ways that may degrade inspection accuracy and robustness.

[0062] Embodiments of the present disclosure provide systems and methods for building image offset corrections into the signal processing hardware of the charged particle detectors to produce charged particle images that are properly aligned with each other at the time of image generation. In some embodiments, the systems and methods may comprise calibrating the image offsets and providing one or more signal processing hardware-based corrections to the images. For example, “hardware -based” may refer to a correction that is performed to signals or image data during the process of generating the charged particle image, such as by introducing a detection signal delay or generating a digital time offset, rather than by aligning one image with another during a post-processing operation.

[0063] Figs. 5A-B illustrate example calibration processes 500A-B for calibrating signal offsets between a plurality of charged particle detector images, consistent with embodiments of the present disclosure. Process 500A may comprise a 1 -dimensional offset calibration process that may be performed for, e.g., the x and y directions.

[0064] Process 500A may be performed for, e.g., each field of view (FOV) of the charged particle apparatus on a wafer, or on a selection of FOVs. For example, the wafer may comprise calibration patterns designed for performing the calibration process. In some embodiments, to reduce calibration errors caused by accumulation of electrical charge, a calibration wafer may be configured with a sufficiently high electrical conductivity that charging effects in the wafer are reduced. For example, the calibration wafer can be N-doped for higher electrical conductivity. Each region on the wafer corresponding to a FOV may comprise a plurality of calibration patterns, such as a series of lines 575. Although a series of vertically oriented lines for an x-offset calibration are shown in the figure, a similar y-offset calibration may be performed using, e.g., horizontally oriented lines. For example, in some embodiments, a wafer may comprise sets of both horizontal and vertical lines, and each step below may be carried out for both x and y offsets.

[0065] First, a plurality of charged particle images 565a-d of the lines 575 may be simultaneously acquired by each of a plurality of detectors (such as, e.g., detectors 364a-d of Fig. 3). Then, within each image, line centers LC of each line 575 may be obtained for a plurality of positions along the line, each position corresponding to a unit cell UC on the wafer pattern as shown in the enlarged portion in Fig. 5A. For example, an H-bar center algorithm may be employed in which line edges 576 of each line 575 are identified at a plurality of unit cells along each line. Line centers LC may then be identified as a mid-point between the two line edges 576. While the line centers LC are shown only at one pair of corresponding lines 575 within charged particle images 565a and 565b, the measurements may be taken at each unit cell UC of each line in each image. While the line centers LC are identified with a + sign in the drawings, this is meant only to identify the location of the line centers LC and do not necessarily represent an actual pattern structure. For each unit cell UQ on the wafer, the x-values of line centers LC from each charged particle image 565a-d may be paired together and represented as:U = [LC(at), LCtbt'), LC(c , LC^)] (eqn. 2) where LC(ai) represents an x-value of the line center of a unit cell UQ as measured in charged particle image 565a, LC(bi) represents an x-value of the line center of the unit cell UQ as measured in charged particle image 565b, etc.

[0066] After each line center LC is identified, one charged particle image may be selected as a reference charged particle image to which the other charged particle images may be aligned. For example, a reference charged particle image may be selected based on a determination of which detector’s x-y image position is likely to be the most stable. In some embodiments, for example, abottom backscattered electron detector image may be selected as the reference charged particle image. For illustration purposes, the first charged particle image 565a will be used as the reference charged particle image.

[0067] Next, for each unit cell UC of the wafer pattern, an x-offset value may be determined for each charged particle image 565c-d with respect to the reference charged particle image 565a. The x-offset OSi (a;, bi, Ci, d) at each unit cell may be represented as a difference of each unit cell measurement in charged particle images 565b-d from its corresponding value in the reference charged particle image 565a:OSt = [0, LC bi) - LC(ai'), LC(Ci) - LC(ai'), LC(di) - LC(af)] (eqn. 3)

[0068] Next, a standard deviation of offset values o(OS) may be calculated for each charged particle image, represented as:(7(05) = [0, (7( Sb), (7( Sc), <7(0Sd)] (eqn. 4)

[0069] A calibrated offset for each charged particle image 565b-d may then be taken as an average of all offset values that lie within k*o(OS) where k = [0, 1, 1, 1]. Each image 565b-d may be shifted toward the reference charged particle image 565a by their respective calibrated offset value. In some embodiments, this process may be iterative. For example, the process may end if the images are determined to have converged to within a desired threshold. Otherwise, the line centers LC of each line 575 may again be obtained for each unit cell UC in the newly shifted images, and the process may repeat until a desired convergence is reached. Using the above-described process, a calibrated image shift in the x and y directions may be determined. This calibrated image shift may be correlated with time offsets between detector signals in each detector (such as detectors 344a-d of Fig. 3) as discussed above, and may be corrected by any of the systems and methods discussed below with respect to Figs.6A-8.

[0070] Fig. 5B illustrates an alternative calibration process 500B for calibrating signal offsets between a plurality of charged particle detector images, consistent with embodiments of the present disclosure. Unlike the dual 1 -dimensional calibration process 500A, process 500B may comprise a simultaneous 2-dimensional offset calibration process. Process 500B may proceed in a manner that is similar to process 500A except as discussed below.

[0071] In process 500B, the plurality of calibration patterns may comprise arrays of 2-dimensional shapes such as, e.g., circles 575. After each charged particle image 565a-d is acquired, rather than separately detecting line centers in the x and y directions, a 2-dimensional center of gravity coordinate COG may be measured for each circle 575. Thus each circle 575 may comprise a unit cell. For each circle 575 on the wafer, the four detector images may be paired together, and a vector offset value maybe taken with respect to a chosen reference charged particle image (such as charged particle image 565a). A standard deviation of the of offset values o(OS) may be calculated for each charged particle image, and a calibrated offset for each charged particle image 565b-d may then be taken as an average of all offset values that lie within a prescribed threshold, such as within k*o(OS) where k = [0, 1, 1, 1]. Like process 500A above, in some embodiments process 500B may be performed iteratively until a convergence is reached.

[0072] Process 500B may beneficially improve throughput by enabling the calculation of offsets in two dimensions simultaneously. However, because the unit cells are formed from discrete pattern features, process 500B may not be able to provide as many data points per unit area as could be achieved with process 500A.

[0073] In some embodiments, the calibration processes 500A-B may be performed using multiple scanned charged particle images for each detector. In some embodiments, charged particle images may be captured for a plurality of scanning speeds and directions. For example, although a direction of an image offset may change depending on the scan direction, the offset magnitude may not. Therefore, in some embodiments, a calibrated image offset may be acquired by averaging the offsets measured in opposite scanning directions. Additionally, because the magnitude of the offset may depend on scanning speed, a plurality of images may be acquired for different scan speed values, and an offset value may be averaged by scaling each image offset based on the scan speed.

[0074] The above-described calibration processes may be performed under a plurality of conditions in order to construct a model or table of condition-dependent image offsets. For example, the calibration processes may be performed for a plurality of predefined landing energy levels, such as in increments of 1 keV for every landing energy value between, e.g., 1 keV to 30 keV. Uncalibrated offsets may be derived based on the incremental values. For example, a calibrated offset at landing energy value of 15.5 keV may be determined by interpolation or other methods based on the calibrated offsets measured at, e.g., 15 keV and 16 keV. The offset measurements may then be correlated with time offsets in the reception and processing of intensity signals at each detector, based on the various causes of such timing offsets as discussed above, such as, e.g., the relative distances of detectors from a wafer surface, the differing energies of emitted charged particles, the scanning speed, and the resistance and capacitance delays in signal wires.

[0075] Having calibrated the image offsets for a plurality of conditions it may be possible to correct the image offsets during the image acquisition stage. This may be done to either replace or supplement the computationally intensive post-acquisition image processing discussed above.

[0076] Figs. 6A-C illustrates example charged particle apparatus 600A-C, each having an example hardware -based time offset correction system, consistent with embodiments of the present disclosure. The hardware-based time offset correction systems may be configured to introduce a desired correction of the time offsets discussed above. The correction systems illustrated in Figs. 6A-C may be referredto as a clock signal delay system, detection oversampling system, and time stamp delay system, respectively.

[0077] In Fig. 6A, each detector 644 may be communicatively coupled to a digital image computer (DIC) 681 via a dedicated signal detection branch 680. For instance, in Fig. 6A only signal detection branches 680b and 680c of detectors 644b and 644c, respectively, are illustrated. However, it should be understood that detectors 644a and 644d may also be coupled to DIC 681 via their own dedicated signal detection branches. DIC 681 may form part of, e.g., controller 109 as shown in Fig. 1 or image processing system 199 as shown in Fig. 2B. Each signal detection branch 680 may comprise: a preamplifier 682, main amplifier 683, analog to digital converter (ADC) 684, and adjustable timing delay 685.

[0078] When a detector 644 receives incoming electrons at its surface, charges may be released from the detectors and be conducted as current through signal wire 664. Pre-amplifier 682 may convert this current into a voltage signal that may be input to main amplifier 683. Main amplifier 683 may then output an amplified voltage signal to ADC 684, which may convert the analog voltage signal into a digital signal indicating the intensity of electron detection. ADC 684 may output this digital signal to DIC 681, which may process the digital signals as, e.g., pixel intensity measurements to generate charged particle images of a wafer 650 surface.

[0079] Each ADC 684 may operate based on a common clock signal from a clock 686 of DIC 681 (such as, e.g., multiple synchronized clocks 686b / c as shown, or a common clock). The clock signal determines the sampling window of time that is assigned to each pixel in the charged particle images. Therefore any digital signal received during a sampling window will be assigned to its corresponding image pixel. As discussed above, a time offset may exist between two different detectors (such as detectors 644b and 644c) due to, e.g., a difference in electron travel time to each of the detectors, a signal propagation speed through their respective signal detection branch 680b an 680c, etc. This may cause two electron detections from a single wafer location to be captured at DIC 681 from different detectors 644 during different sampling windows, and thus to be registered at different image pixel locations in their respective charged particle beam images. These different pixel locations result in a spatial offset of one charged particle image with respect to another.

[0080] The clock signal delay system provides a hardware-based time offset correction to this time offset in the form of adjustable timing delays 685, which may be configured to produce a prescribed time delay in the clock signal. For instance, under a given set of charged particle beam conditions (such as beam source energy, stage bias, scanning speed or direction, detector height, signal wire or circuitry parameters, lens optics settings, etc.) it may be determined that a signal from bottom backscattered electron detector 644c will typically reach DIC 681 sooner than a corresponding signal from energy filter detector 644b by a predetermined time offset. For example, the time offset determination may be made by using one of the calibration processes discussed above with respect to Figs. 5A-5B. Alternatively, the determination may be made by a different calibration, modelling, experimental, orother process. To correct the determined time offset, adjustable timing delay 685c may delay a clock signal from clock 686c to ADC 684c with respect to a clock signal from clock 686b to ADC 684b. In this way, although the two detection signals from detectors 644b and 644c may reach their respective ADC’s 684 at different times, the corresponding digital signals may be synchronized when output to DIC 681. The result is that each image pixel in charged particle images from detectors 644b and 644c will already be properly aligned with each other at the time the detection signals are acquired by DIC 681. Therefore, the images may be aligned with each other at the time they are generated. Thus charged particle images may be ready for combination or other multi-detector image processing without the need for complex and time-consuming post-processing operations, or with a greatly reduced computational burden of such post-processing operations.

[0081] In some embodiments, adjustable timing delays 685 may be adjusted in real-time during a scan based on changing conditions. For example, if the charged particle beam conditions are changed, the previously determined time offset may change as well. Therefore, adjustable timing delays may be changed according to, e.g., a calibration table based on changing beam conditions. In some embodiments, charged particle beam conditions may remain substantially fixed during an e-beam exposure process, and adjustable timing delays may maintain a fixed delay value during the e-beam exposure process.

[0082] When more than two detectors are in use, there may be more than one relative time offset. For example, it may be determined that a signal from energy filter detector 644b will reach DIC 681 later than a signal from bottom backscattered electron detector 644c by a first time offset. It may further be determined that a signal from secondary electron detector 644a will reach DIC 681 later than the signal from bottom backscattered electron detector 644c by a second time offset, where the second time offset is larger than the first time offset. In such a case, adjustable time delay 685c may delay its clock signal by the largest (second) time offset, and adjustable time delay 685b may delay its clock signal by a difference between the second time offset and the first time offset. In this way, corresponding detection signals from each detector 644a-c may be captured during a same sampling window.

[0083] Alternatively or additionally, predetermined time offsets may be mitigated by a detection oversampling system as illustrated with respect to charged particle apparatus 600B in Fig. 6B. Here, much of the functional elements may be similar to the description above with respect to Fig. 6A. For example, detectors 644a-d may transmit detections signals along their respective signal detection branches 680 as discussed above, and a common clock signal may be distributed each ADC 684. However, rather than introducing individual clock delays by an adjustable timing delay in each signal detection branch, each ADC 684 may perform signal sampling at a rate that is higher than the sampling window given by the clock rate. A sampling rate may be an integer multiple N times the pixel rate set by a clock in the DIC 681, such as e,g., 10 times or more. For example, for a 100 MHz clock pixel rate, ADC 684 may be configured for a sampling rate of, e.g., 1 GHz. In this way, although the digital signals from each ADC 684 may reach DIC 681 at different times, the higher timing resolution may allow DIC681 to select the appropriate signals from each ADC 684 according to predetermined time offset. In this way a time offset may still be applied to a signal detection branch image during image generation based on a predetermined time offset. For example, a sampling delay corresponding to the predetermined time offset may be introduced by, e.g., a field-programmable gate array (FPGA) within DIC 681.

[0084] The resolution of the detection oversampling system discussed above may be determined by the oversampling factor N. The higher the oversampling factor N is, the higher the resolution that is achievable by the detection oversampling system. However, this method is limited by the signal strength of the detection signals in each signal detection branch 680. For example, it is not possible to cut one electron in half when performing signal detection. Thus, an oversampling factor N should not be so high relative to the beam current or other charged particle conditions that the number of electrons within each oversampling period is less than one.

[0085] Fig. 6C illustrates a further charged particle apparatus 600C comprising a time stamp delay system, consistent with embodiments of the present disclosure. In Fig. 6C, detectors 644a-644d may comprise charged particle counting detectors. A charged particle counting detector may be configured to detect individual charged particle arrival events at the detector surface. For example, in some embodiments charged particle counting detector may comprise a pixelated surface having an array of detection pixels. Each detection pixel may comprise dedicated detection circuitry that is independently coupled to a counting readout 688. When a charged particle is incident on a detection pixel, its dedicated detection circuitry may output a digital signal indicating the event to a counting readout 688 of the detector. The counting readout may then generate a time stamp to record the charged particle arrival event. Pixel intensity values of a charged particle image may thus be generated based on the number of charged particles arriving at the entire detector surface during the sampling window for each pixel, according to the time stamps. In some embodiments, charged particle detectors may further be configured to perform energy level discrimination of each charged particle arrival event.

[0086] Because the charged particle detection signals are recorded as individual time stamps, the time offsets may be corrected based on the time stamps. For example, DIC 681 may receive time stamp information from counting readouts 688b and 688c. The time stamp information may comprise a first time stamp from an earlier arriving charged particle at backscattered electron detector 644c, and a second time stamp from a later arriving charged particle at energy filter detector 644b. The first time stamp and the second time stamp may have a time offset corresponding to a time offset that was predetermined based on, e.g., a calibration operation as discussed above. Therefore, DIC 681 may correct the time offset simply by applying a time correction to the time stamp information. Alternatively or additionally, time stamp generators within each counting readout 688 may be adjustable so that time stamps may be generated with the time offset built in. In this way, as schematically illustrated by the dashed lines in graph 601 of Fig. 6C, detection signals from, e.g., a backscattered electron detector BSE and secondary electron detector SE may be aligned with each other simply applying a calibrated time offset.

[0087] Fig. 7 illustrates a flowchart of an example method 700 for calibrating image offsets in charged particle beam images, consistent with embodiments of the present disclosure. The calibration method may be performed by a charged particle beam apparatus on a test wafer comprising a plurality of patterns. In some embodiments, method 700 may be performed at a plurality of different charged particle beam setting combinations, such as beam source energy, stage bias, scanning speed or direction, detector height, signal wire or circuitry parameters, lens optics settings, etc., and a table may be constructed based on the calibration results at each of the different combinations of settings.

[0088] At step 701, a plurality of charged particle images of a wafer may be simultaneously acquired by each of a plurality of detectors (such as, e.g., detectors 344a-d of Fig. 3). The wafer may comprise a plurality of patterns, such as lines as seen in Fig. 5 A or circles as seen in Fig. 5B. In some embodiments, the plurality of patterns may be repeated in each FOV region on a wafer. Due to the factors discussed above, the charged particle images from each detector may include a spatial offset caused by a time offset in the reception of detection signals.

[0089] At step 702, center values (such as line centers or centers of gravity) may be measured at a plurality of unit cells on the plurality of patterns in each charged particle image, and corresponding center values from each charged particle image may be paired with each other (e.g., as illustrated at eqn. 2 above).

[0090] At step 703, a reference charged particle image may be selected. The reference charged particle image may serve as the basis by which the other charged particle image offsets are determined. In some embodiments, the reference charged particle image may be selected after image generation based on, e.g., an evaluation of image quality. In some embodiments, the reference charged particle image may be selected based on, e.g., an expectation that its corresponding charged particle detector will provide the most stable x-y image position. In some embodiments, for example, a bottom backscattered electron detector image may be selected as the reference charged particle image. Therefore, in some embodiments, step 703 may take place before step 702 and be outside the iterative loop of steps 702- 705 discussed below.

[0091] At step 704, spatial image offsets of each charged particle image may be measured with respect to the reference image. For example, offset values may be measured between each unit cell of the reference image and corresponding unit cells in each of the remaining images. In some embodiments, an average of the offset values in each charged particle image may be determined as the spatial image offset for the charged particle image. In some embodiments, the average may include a weighted average, or may include only those offset values that lie within a predetermined range, such as within 1 standard deviation of the mean offset value.

[0092] At step 705, it may be determined whether the charged particle images have converged to within a desired tolerance when aligned using the spatial image offsets that were determined in step 705. If the answer is no, the process may return to step 702 and be repeated with the re-aligned charged particle images. If sufficient convergence has been achieved, the process may proceed to step 706.

[0093] At step 706, the determined spatial offsets may be correlated with a time offset of the detection signals based on known conditions of the charged particle beam apparatus, such as beam source energy, stage bias, scanning speed or direction, detector height, signal wire or circuitry parameters, lens optics settings, etc. The time offset may be employed in an image offset correction.

[0094] Fig. 8 illustrates a flowchart of an example method 800 for hardware -based time offset correction in a charged particle beam apparatus, consistent with embodiments of the present disclosure. In some embodiments, the charged particle beam apparatus may comprise, e.g., EBI system 10 of Fig. 1, electron beam tools 200A or 200B of Figs. 2A-B, or charged particle apparatus 600A-600C of Figs. 6A-C.

[0095] At step 801, a predetermined time offset value may be obtained for a first charged particle detector of the charged particle beam apparatus. For example, the time offset value may correspond to a time difference between the reception at an image processor (such as, e.g., controller 109 of Fig. 1, image processing system 199 of Fig. 2B, or DIC 681 of Figs. 6A-C) of a first signal from the first charged particle detector and a second signal from a second charged particle detector. The time offset value may be, e.g., a calibrated time offset determined according to a calibration process as discussed above with respect to, e.g., Figs. 5A-B or method 700 of Fig. 7. The time offset may be based on exposure conditions or settings of the charged particle apparatus, such, e.g., beam source energy, stage bias, scanning speed or direction, detector height, signal wire or circuitry parameters, lens optics settings, etc.

[0096] At step 802, the charged particle apparatus may irradiate a wafer with a charged particle beam to generate a plurality of emitted charged particles from the wafer. The emitted charged particles may comprise, e.g., secondary or backscattered electrons. A first portion of the plurality of emitted electrons may be incident on the first detector and a second portion of the plurality of emitted electrons may be incident on the second detector. The first and second detectors may be configured to output the first and second signals to the image processor via first and second signal detection branches, respectively. The first and second signal detection branches may comprise, e.g., signal wires, pre-amplifiers, main amplifiers, ADC modules, adjustable timing delays, electron counting readouts, and other signal processing circuitry.

[0097] At step 803, a hardware -based time offset correction may be applied to one of the first and second signals. For example, if it is determined that, in the absence of a time offset correction, the first signal will arrive at the image processor earlier than the second signal by the amount of time obtained in step 801, then a time offset correction may be applied to the first detection signal. In some embodiments, applying the hardware -based time offset correction may comprise, e.g., generating a timing delay on a clock signal to an ADC module based on the predetermined time offset value. In some embodiments, applying the hardware-based time offset correction may comprise, e.g., oversampling a signal in the ADC module at a rate of N times a pixel rate set by a clock of the image processor, and selecting an appropriate sampling window based on the predetermined time offset value. In someembodiments, applying the hardware-based time offset correction may comprise, e.g., generating an electron counting time stamp for each emitted electron of the first and second portions of emitted electrons, and applying a numerical correction to the time stamps of the first plurality of emitted electrons.

[0098] At step 806, a first charged particle image and a second charged particle mage may be generated based on the first and second signals from the first and second detectors. The first and second charged particle images may be well aligned without a need for, e.g., extensive post-processing interpolation, in view of the hardware-based time offset correction.

[0099] Embodiments of the present disclosure may further be described using the following clauses:1. A method of correcting image offsets in a charged particle beam process, comprising: irradiating a surface with a charged particle beam using a first charged particle beam apparatus to generate emitted charged particles from the surface; receiving a first portion of the emitted charged particles at a first charged particle detector and a second portion of the emitted charged particles at a second charged particle detector; outputting a first signal to an image processor based on the first portion of the emitted charged particles received at the first charged particle detector; outputting a second signal to the image processor based on the second portion of the emitted charged particles received at the second charged particle detector; performing a hardware-based time offset correction to the first signal with respect to the second signal based on a predetermined time offset; generating a first charged particle image by the image processor based on the first signal; and generating a second charged particle image by the image processor based on the second signal.2. The method of clause 1 , wherein performing the hardware -based time offset correction comprises generating a delayed clock signal of signal processing circuitry coupled to the first charged particle detector.3. The method of clause 2, wherein: the signal processing circuitry comprises an analog to digital converter configured to output the first signal to the image processor; and the delayed clock signal is input to the analog to digital converter.4. The method of clause 1, wherein performing the hardware-based time offset correction comprises: oversampling the first signal with respect to a clock rate of signal processing circuitry coupled to the first charged particle detector; and selecting a sampling window for the oversampled first signal based on the predetermined time offset.5. The method of clause 4, wherein oversampling the first signal comprises sampling the first signal with an analog to digital converter at a rate of at least 10 times a clock rate of the analog to digital converter.6. The method of clause 1, wherein:the first charged particle detector comprises a charged particle counting detector; and the first signal comprises time stamp information; and performing the hardware-based time offset correction comprises generating a correction to the time stamp information.7. The method of clause 6, wherein generating the correction to the time stamp information comprises adjusting a time stamp generator in signal processing circuitry coupled to the first charged particle detector.8. The method of clause 6, wherein generating the correction to the time stamp information comprises applying the correction to the time stamp information in the image processor.9. The method of clause 1 , wherein the predetermined time offset is based on one of a landing energy, beam source energy, stage bias, scanning speed, scanning direction, charged particle detector height, signal wire parameter, signal processing circuitry parameter, or lens optics setting of the first charged particle beam apparatus.10. The method of clause 1, wherein the predetermined time offset is determined based on a calibration process.11. The method of clause 10, wherein the calibration process comprises: generating a first calibration charged particle image of a pattern by a third charged particle detector of a second charged particle beam apparatus; generating a second calibration charged particle image of the pattern by a fourth charged particle detector of the second charged particle beam apparatus; comparing the first calibration charged particle image to the second calibration charged particle image; and determining the predetermined time offset based on the comparison.12. The method of clause 11, wherein comparing the first calibration charged particle image to the second calibration charged particle image comprises measuring a plurality of offsets between a plurality of corresponding unit cells of the pattern in the first calibration charged particle image to the second calibration charged particle image.13. The method of clause 11, wherein: the second charged particle beam apparatus comprises the first charged particle beam apparatus; the third charged particle detector comprises the first charged particle detector; and the fourth charged particle detector comprises the second charged particle detector.14. The method of clause 1, wherein performing the hardware-based time offset correction to the first signal with respect to the second signal results in a spatial offset correction between the first charged particle image and the second charged particle image.15. The method of clause 1, wherein the first charged particle detector is closer to the surface than the second charged particle detector.16. The method of clause 15, wherein:the first charged particle detector is located on a first side of an objective lens assembly of the charged particle beam apparatus, and the second charged particle detector is located on a second side of the objective lens assembly of the charged particle beam apparatus.17. The method of clause 15, wherein: the first charged particle detector comprises a bottom backscattered electron detector; and the second charged particle detector comprises a secondary electron detector.18. A charged particle beam apparatus, comprising: a charged particle beam source configured to irradiate a surface with a charged particle beam to generate emitted charged particles from the surface; a first charged particle detector configured to detect a first portion of the emitted charged particles; a second charged particle detector configured to detect a second portion of the emitted charged particles; an image processor configured to receive a first signal based on detection of the first portion of emitted charged particles at the first charged particle detector and a second signal based on detection of the second portion of emitted charged particles at the second charged particle detector; and a hardware -based time offset correction system configured to generate a predetermined time offset of the first signal with respect to the second signal.19. The charged particle beam apparatus of clause 18, wherein the hardware-based time offset correction system comprises: signal processing circuitry coupled to the first charged particle detector; and an adjustable timing delay configured to generate a delayed clock signal of the signal processing circuitry.20. The charged particle beam apparatus of clause 19, wherein: the signal processing circuitry comprises an analog to digital converter configured to output the first signal to the image processor; and the adjustable timing delay is configured to input the delayed clock signal to the analog to digital converter.21. The charged particle beam apparatus of clause 18, wherein the hardware-based time offset correction system comprises: signal processing circuitry coupled to the first charged particle detector, the signal processing circuitry being configured to oversample the first signal with respect to a clock rate of signal processing circuitry, wherein the image processor is configured to select a sampling window for the oversampled first signal based on the predetermined time offset.22. The charged particle beam apparatus of clause 21 , wherein the signal processing circuitry comprises an analog to digital converter configured to sample the first signal at a rate of at least 10 times a clock rate of the analog to digital converter.23. The charged particle beam apparatus of clause 18, wherein:the first charged particle detector comprises a charged particle counting detector; and the first signal comprises time stamp information; and the hardware -based time offset correction system is configured to generate a correction to the time stamp information.24. The charged particle beam apparatus of clause 23, wherein the hardware-based time offset correction system comprises: an adjustable time stamp generator configured to adjust a time stamp of the first signal.25. The charged particle beam apparatus of clause 23, wherein the image processor is configured to apply the correction to the time stamp information.26. The charged particle beam apparatus of clause 18, wherein the predetermined time offset is based on one of a landing energy, beam source energy, stage bias, scanning speed, scanning direction, charged particle detector height, signal wire parameter, signal processing circuitry parameter, or lens optics setting of the charged particle beam apparatus.27. The charged particle beam apparatus of clause 18, wherein the predetermined time offset is determined based on a calibration process.28. The charged particle beam apparatus of clause 18, wherein the first charged particle detector is closer to the surface than the second charged particle detector.29. The charged particle beam apparatus of clause 18, wherein: the first charged particle detector is located on a first side of an objective lens assembly of the charged particle beam apparatus, and the second charged particle detector is located on a second side of the objective lens assembly of the charged particle beam apparatus.30. The charged particle beam apparatus of clause 18, wherein: the first charged particle detector comprises a bottom backscattered electron detector; and the second charged particle detector comprises a secondary electron detector.31. A charged particle beam method, comprising: irradiating a surface with a charged particle beam to generate emitted charged particles from the surface; detecting a first portion of the emitted charged particles with a first charged particle detector; detecting a second portion of the emitted charged particles with a second charged particle detector; receiving, at an image processor, a first signal based on detection of the first portion of emitted charged particles at the first charged particle detector and a second signal based on detection of the second portion of emitted charged particles at the second charged particle detector; and generating a predetermined time offset of the first signal with respect to the second signal by a hardwarebased time offset correction system.32. 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: irradiating a surface with a charged particle beam to generate emitted charged particles from the surface;detecting a first portion of the emitted charged particles with a first charged particle detector; detecting a second portion of the emitted charged particles with a second charged particle detector; receiving, at an image processor, a first signal based on detection of the first portion of emitted charged particles at the first charged particle detector and a second signal based on detection of the second portion of emitted charged particles at the second charged particle detector; and generating a predetermined time offset of the first signal with respect to the second signal by a hardwarebased time offset correction system.

[0100] Some embodiments of the present disclosure have been described with respect to electron beam systems, such as SEM, having an electron detector for detecting electron arrivals. However, the present disclosure is not limited to this. It should be understood that the above disclosed embodiments may be applicable to other systems, such as other non-SEM electron beam systems or non-electron based charged particle beam systems. Further, it should be understood that other charged particles, or other classes of electrons are contemplated within the scope of the present disclosure.

[0101] 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, and combination 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.

[0102] 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 charged particle beam apparatus, comprising: a charged particle beam source configured to irradiate a surface with a charged particle beam to generate emitted charged particles from the surface; a first charged particle detector configured to detect a first portion of the emitted charged particles; a second charged particle detector configured to detect a second portion of the emitted charged particles; an image processor configured to receive a first signal based on detection of the first portion of emitted charged particles at the first charged particle detector and a second signal based on detection of the second portion of emitted charged particles at the second charged particle detector; and a hardware -based time offset correction system configured to generate a predetermined time offset of the first signal with respect to the second signal.

2. The charged particle beam apparatus of claim 1 , wherein the hardware-based time offset correction system comprises: signal processing circuitry coupled to the first charged particle detector; and an adjustable timing delay configured to generate a delayed clock signal of the signal processing circuitry.

3. The charged particle beam apparatus of claim 2, wherein: the signal processing circuitry comprises an analog to digital converter configured to output the first signal to the image processor; and the adjustable timing delay is configured to input the delayed clock signal to the analog to digital converter.

4. The charged particle beam apparatus of claim 1 , wherein the hardware-based time offset correction system comprises: signal processing circuitry coupled to the first charged particle detector, the signal processing circuitry being configured to oversample the first signal with respect to a clock rate of signal processing circuitry, wherein the image processor is configured to select a sampling window for the oversampled first signal based on the predetermined time offset.

5. The charged particle beam apparatus of claim 4, wherein the signal processing circuitry comprises an analog to digital converter configured to sample the first signal at a rate of at least 10 times a clock rate of the analog to digital converter.

6. The charged particle beam apparatus of claim 1, wherein: the first charged particle detector comprises a charged particle counting detector; and the first signal comprises time stamp information; and the hardware-based time offset correction system is configured to generate a correction to the time stamp information.

7. The charged particle beam apparatus of claim 6, wherein the hardware-based time offset correction system comprises: an adjustable time stamp generator configured to adjust a time stamp of the first signal.

8. The charged particle beam apparatus of claim 6, wherein the image processor is configured to apply the correction to the time stamp information.

9. The charged particle beam apparatus of claim 1, wherein the predetermined time offset is based on one of a landing energy, beam source energy, stage bias, scanning speed, scanning direction, charged particle detector height, signal wire parameter, signal processing circuitry parameter, or lens optics setting of the charged particle beam apparatus.

10. The charged particle beam apparatus of claim 1, wherein the predetermined time offset is determined based on a calibration process.

11. The charged particle beam apparatus of claim 1 , wherein the first charged particle detector is closer to the surface than the second charged particle detector.

12. The charged particle beam apparatus of claim 1, wherein: the first charged particle detector is located on a first side of an objective lens assembly of the charged particle beam apparatus, and the second charged particle detector is located on a second side of the objective lens assembly of the charged particle beam apparatus.

13. The charged particle beam apparatus of claim 1, wherein: the first charged particle detector comprises a bottom backscattered electron detector; and the second charged particle detector comprises a secondary electron detector.

14. A charged particle beam method, comprising: irradiating a surface with a charged particle beam to generate emitted charged particles from the surface; detecting a first portion of the emitted charged particles with a first charged particle detector; detecting a second portion of the emitted charged particles with a second charged particle detector; receiving, at an image processor, a first signal based on detection of the first portion of emitted charged particles at the first charged particle detector and a second signal based on detection of the second portion of emitted charged particles at the second charged particle detector; and generating a predetermined time offset of the first signal with respect to the second signal by a hardware -based time offset correction system.

15. 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: irradiating a surface with a charged particle beam to generate emitted charged particles from the surface; detecting a first portion of the emitted charged particles with a first charged particle detector; detecting a second portion of the emitted charged particles with a second charged particle detector; receiving, at an image processor, a first signal based on detection of the first portion of emitted charged particles at the first charged particle detector and a second signal based on detection of the second portion of emitted charged particles at the second charged particle detector; and generating a predetermined time offset of the first signal with respect to the second signal by a hardware -based time offset correction system.

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