Charged particle detector for microscope

Abstract

A charged particle evaluation system includes: a charged particle beam device configured to direct a charged particle beam onto a sample such that secondary particles and backscatter particles are generated in response to the charged particle beam; an array of sensing elements configured to generate an electrical signal in response to incident secondary particles or backscatter particles from the sample; and a controller configured to selectively activate a first subset (333) of the set of sensing elements, selectively deactivate a second subset (332) of the set of sensing elements, and combine electrical signals of selected subsets into a detector output signal, where the selective activation and selective deactivation are based on a predicted distribution of secondary particles or backscattering particles. The present application also relates to a non-transitory computer-readable medium comprising an instruction set executable by one or more processors of a controller such that the controller controls a charged particle evaluation system to execute a corresponding method of configuring a detector having an array of sensing elements.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to European patent application 22200760.1 filed on October 11, 2022, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The description herein relates to charged particle detection and, more particularly, to systems and methods applicable to charged particle beam detection. Background Art

[0004] Detectors can be used to sense physically observable phenomena. For example, a charged particle beam tool such as an electron microscope can include a detector that receives charged particles projected from a sample and outputs a detection signal. The detection signal can be used to reconstruct an image of the sample structure being inspected, and can be used, for example, to reveal defects in the sample. In the manufacture of semiconductor devices, it becomes increasingly important to detect defects in samples, which can include a large number of densely packed miniaturized integrated circuit (IC) components. For this reason, a dedicated inspection tool can be provided.

[0005] In some applications in the field of inspection, such as microscopy using a scanning electron microscope (SEM), an electron beam can be scanned across a sample to derive information from backscattered or secondary electrons generated by the sample. Backscattered electrons (or more generally backscattered particles) and secondary electrons (or more generally secondary particles) can be collectively referred to as return particles. In the related art, an electron detection system in a SEM tool may include a detector configured to detect electrons from a sample. Existing detectors in SEM tools can only detect the intensity of the beam. The sensitivity of conventional detection systems may be limited by a poor signal-to-noise ratio (SNR), especially when the beam current is reduced to, for example, the picoampere range. In some detection methods, a large area semiconductor detector or a group of small area semiconductor detectors with an area equal to, less than, or greater than the beam spot area can be used. The current induced by the incoming electron beam can be generated within the detector and then amplified by an amplifier behind the detector.

[0006] As semiconductor devices continue to miniaturize, inspection systems can use lower and lower electron beam currents. Maintaining SNR becomes more difficult as beam current decreases. For example, SNR can drop dramatically when probe current is reduced to 200pA or below. Poor SNR can require measures such as image averaging or extending the integration time corresponding to the signal for each pixel in the sample image, which can increase the electron dose on the sample surface, causing surface charging artifacts or other deleterious effects. Such measures can also reduce the overall throughput of the inspection system.

[0007] In the related art, particle counting can be used for low current applications. Particle counting can be used for detectors, such as Everhart-Thornley detectors (ETDs), which can use scintillators and photomultiplier tubes (PMTs). In the probe current range of some applications, such as 8pA to 100pA, ETDs can exhibit good SNR. However, the light yield of scintillators may decrease with the accumulated electron dose and therefore have a limited lifespan. Aging of scintillators may also cause system-level performance drift and may result in the generation of uneven images. Therefore, ETDs may not be suitable for use in inspection tools, especially when used in semiconductor manufacturing facilities, where they may need to operate 24 hours a day, 7 days a week.

[0008] A charged particle detector is needed that can achieve high SNR and good performance over a wide range of probe currents (e.g., from about 40 pA to 1 nA). At the same time, the detector should ensure stable quantum efficiency and long lifetime as well as low performance drift, e.g., even when using a probe current of 1 nA or greater in continuous operation.

[0009] Detection systems using related art methods may face limitations in detection sensitivity and SNR, especially at low electron doses. In order to improve the SNR, related art has proposed so-called pixelated electron counting detectors, in which the detector is subdivided into a large number of sensing elements, the outputs of which are combined to generate a detection signal. However, pixelated electron counting detectors are still affected by noise, and additional improvements are needed in this regard. Summary of the invention

[0010] An embodiment of the present disclosure provides a method of calibrating a charged particle detector in a charged particle evaluation device, the charged particle detector having an array of sensing elements configured to generate an electrical signal in response to incident secondary particles or backscattered particles from a sample; the method comprising:

[0011] scanning a charged particle beam across a calibration sample including features of interest such as edges and flat topography;

[0012] receiving an electrical signal from the sensing element in response to return particles generated in response to the charged particle beam to obtain a characteristic distribution of return particles as a function of position on the detector for characteristic topography and a flat distribution of return particles as a function of position on the detector for flat topography; and

[0013] Based on the characteristic distribution and the flat distribution, a subset of sensing elements is selected for evaluation of the sample.

[0014] An embodiment of the present disclosure provides a method for calibrating a charged particle detector in a charged particle evaluation device, the charged particle detector having an array of sensing elements configured to generate an electrical signal in response to incident secondary particles or backscattered particles from a sample; the method comprising:

[0015] scanning a charged particle beam across a calibration sample having a known topography;

[0016] receiving electrical signals from a sensing element in response to secondary particles and backscattered particles generated in response to the charged particle beam to obtain a combined distribution of the secondary particles and backscattered particles as a function of position on the detector;

[0017] estimating a distribution of backscattered particles among secondary particles and backscattered particles as a function of position on the detector;

[0018] subtracting the distribution of backscattered particles from the combined distribution of secondary particles and backscattered particles to obtain a distribution of secondary particles; and

[0019] Based on the distribution of backscattered particles and the distribution of secondary particles, a subset of sensing elements is selected for evaluation of the sample.

[0020] An embodiment of the present disclosure provides a charged particle evaluation system, comprising:

[0021] a charged particle beam device configured to direct a charged particle beam onto the sample such that secondary particles and backscattered particles are generated in response to the charged particle beam;

[0022] an array of sensing elements configured to generate electrical signals in response to incident secondary particles or backscattered particles from the sample; and

[0023] A controller configured to selectively activate a first subset of the set of sensing elements, selectively deactivate a second subset of the set of sensing elements, and combine electrical signals of the selected subsets into a detector output signal, wherein the selective activation and the selective deactivation are based on a predicted distribution of secondary particles or backscattered particles.

[0024] An embodiment of the present disclosure provides a method for detecting charged particles, comprising:

[0025] Configuring a detector of a charged particle evaluation system by the above method;

[0026] directing a charged particle beam onto the sample using a charged particle evaluation system such that secondary particles and backscattered particles are generated in response to the charged particle beam;

[0027] directing secondary particles and backscattered particles to a detector;

[0028] activating a first subset of the array of sensing elements;

[0029] combining the electrical signals of the first subset into a detector output signal; and

[0030] A second subset of the array of sensing elements is deactivated.

[0031] Embodiments of the present disclosure provide a method of configuring a detector of a charged particle evaluation system, the detector having an array of sensing elements configured to generate an electrical signal in response to incident secondary particles or backscattered particles from a sample, the method comprising:

[0032] selecting a first subset of the set of sensing elements for activation based on data derived from a predicted distribution of secondary particles or backscattered particles; and

[0033] selecting a second subset of the set of sensing elements for deactivation based on the predicted distribution;

[0034] Wherein the first subset has a different predicted ratio of incident secondary particles to incident backscattered particles than the second subset.

[0035] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the disclosed embodiments, as may be claimed. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The above and other aspects of the present disclosure will become more apparent through the description of exemplary embodiments in conjunction with the accompanying drawings, in which:

[0037] Figure 1 is a schematic diagram illustrating an exemplary electron beam inspection (EBI) system consistent with embodiments of the present disclosure.

[0038] Figure 2A , Figure 2B and Figure 2C is a schematic diagram illustrating an exemplary electron beam tool consistent with embodiments of the present disclosure, which electron beam tool can be used as Figure 1 A portion of an exemplary electron beam inspection system.

[0039] Figure 3 is a schematic diagram of a radiation detector of an embodiment.

[0040] Figure 4A , Figure 4B and Figure 4C Schematic diagram of the incident positions of electrons from different sources on the detector obtained from the simulation.

[0041] Figure 5 Is to indicate the first mode Figure 3 Schematic diagram of the active and inactive parts of a radiation detector.

[0042] Fig. 6A and Figure 6B Indicates the second mode Figure 3 Schematic diagram of the active and inactive components of a radiation detector.

[0043] Fig. 7A and Figure 7B When the scanning beam is incident on a flat feature and a feature including an edge, the electrons are incident on the surface at different radii. Figure 3 Histogram of the probabilities on the detector.

[0044] Figure 8 is a diagram explaining the contrast-to-noise ratio metric.

[0045] Fig. 9 is a sketch illustrating the secondary electron yield near the edge of the sample under evaluation.

[0046] Fig.10 is a schematic diagram illustrating selection of inner and outer radii of the activation area to maximize sensitivity to edges. DETAILED DESCRIPTION

[0047] Reference will now be made in detail to the exemplary embodiments, examples of which are illustrated in the accompanying drawings. The following description refers to the accompanying drawings, wherein the same numerals in the different drawings represent the same or similar elements, unless otherwise specified. The embodiments set forth in the following description of the exemplary embodiments do not represent all embodiments consistent with the present invention. However, they are merely examples of devices, systems, and methods consistent with aspects related to the subject matter that may be recited in the appended claims.

[0048] Aspects of the present application relate to systems and methods for charged particle beam detection. Systems and methods may employ counting of charged particles (such as electrons) and may be used in inspection tools, such as scanning electron microscopes (SEMs). Inspection tools may also be referred to as evaluation tools or evaluation devices. Detection tools may be used in the manufacturing process of integrated circuit (IC) components. In order to achieve the enhanced computing power of modern electronic devices, the physical size of the device may be reduced, while the packaging density of circuit components such as transistors, capacitors, diodes, etc. on the IC chip is significantly increased. For example, in a smartphone, an IC chip (which may be the size of a thumbnail) may include more than 2 billion transistors, each of which is less than 1 / 1000 the size of a human hair. Not surprisingly, semiconductor IC manufacturing is a complex process with hundreds of independent steps. Even an error in one step may greatly affect the functionality of the final product. The goal of the manufacturing process is to improve the overall yield of the process. For example, to achieve a 75% yield in a 50-step process, the yield of each individual step must be greater than 99.4%, and if the yield of the individual step is 95%, the yield of the entire process drops to 7%.

[0049] The ability to ensure that defects are detected with high accuracy and resolution while maintaining high throughput (e.g., defined as the number of wafers processed per hour) is increasingly important. The presence of defects can impact high processing yields and high wafer throughput, especially when operator intervention is involved. Therefore, the detection and identification of micron and nanometer-sized defects by inspection tools (such as SEM) is important to maintain high yields and low costs.

[0050] In some inspection tools, a sample can be inspected by scanning a high energy electron beam over the sample surface. Due to the interaction at the sample surface, secondary electrons or backscattered electrons may be generated from the sample and can then be detected by a detector.

[0051] Backscattered electrons are electrons from a beam scanning a sample that are scattered back toward the detector by atoms in the sample. Secondary electrons are electrons emitted by atoms in the sample as a result of the beam. In many cases, the rate of backscattered electrons is random, while the rate of secondary electrons varies depending on the properties of the surface of the sample. In such cases, the backscattered electrons can be considered noise, while the secondary electrons are the signal that is desired to be detected. Because all electrons are essentially the same, it is difficult to distinguish between backscattered electrons and secondary electrons. Backscattered electrons can have higher energy than secondary electrons, and proposals have been made to distinguish secondary electrons from backscattered electrons on this basis, but practical implementation of this idea is difficult.

[0052] The inventors have determined that backscattered electrons are typically emitted from a sample in a different direction than secondary electrons. The distributions of backscattered electrons and secondary electrons overlap, but the distribution of backscattered electrons is broader than the distribution of secondary electrons. Therefore, by using a pixelated detector (i.e., a detector having many independent pixels that emit independent signals when electrons are detected), the output from some pixels can be selected and the output from other pixels can be rejected in order to control the ratio of detected backscattered electrons to detected secondary electrons. Several desired effects can thereby be achieved: for example, the signal-to-noise ratio can be improved; the contrast-to-noise ratio can be improved; or the sensitivity to a particular feature type on a sample can be improved.

[0053] In some embodiments of the present disclosure, the sensing elements of the array may be sized so that no more than a certain number of charged particles are received in the area of ​​a single sensing element per sampling period. The specific number may be one. The size of the sensing element may be smaller than the geometric spread of the charged particles incident on the detector. Thus, a single sensing element may be configured to receive fewer charged particles than the total number of charged particles incident on the detector. Various aspects of the detector (such as the size of the sensing element, the sampling rate, and other characteristics) may be set according to various criteria to enable charged particle counting.

[0054] Without limiting the scope of the present disclosure, some embodiments may be described in the context of providing detectors and detection methods in systems utilizing electron beams. However, the present disclosure is not limited thereto. Other types of charged particle beams may be similarly applied. In addition, the systems and methods for detection may be used in other imaging systems, such as optical imaging, photon detection, x-ray detection, ion detection, and the like.

[0055] As used herein, unless otherwise specifically stated, the term "or" encompasses all possible combinations unless not feasible. For example, if a component is specified to include A or B, then, unless otherwise specifically stated or not feasible, the component may include A, or B, or A and B. As a second example, if a component is specified to include A, B, or C, then, unless otherwise specifically stated or not feasible, 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.

[0056] In addition, the term "detector element" may include or cover "sensing element", "sensor element", "detection cell" or "detector segment", etc. A sensing element may be a diode configured to have a depletion region, and in some embodiments discussed herein, the term "sensing element" may exclude an avalanche diode operating in Geiger mode. A detector element may include a diode, an interconnect, and a circuit, which may include, for example, front-end electronics. In addition, the term "frame" may include or cover "sampling period", "SEM image pixel period" or "pixel period", etc. A SEM image frame may refer to a pixel frame that may be refreshed frame by frame, and a data frame may refer to a group of data acquired by a detection system within a specific time period.

[0057] Embodiments of the present disclosure may utilize detection methods involving charged particle counting, such as electron counting. By counting the number of electrons received within a predefined period, the intensity of the incoming electron beam can be determined. The term "incoming electron" may include or cover incident electrons, such as electrons that strike the surface of a detector. According to some embodiments, noise from the charged particle detection process may be reduced. However, simply improving SNR may not meet the growing demands of various SEM applications.

[0058] Electron counting can involve determining individual electron arrival events occurring at a detector. For example, as electrons arrive at a detector, they can be detected one by one. In some embodiments, electrons incident on the detector can generate an electrical signal that is routed to an electro-optic modulator that selectively modulates the beam by changing a parameter of the beam, such as phase, amplitude, or polarization. Signal processing circuitry detects the modulation of the beam and then reads out to an interface, such as a digital controller. The detector can be configured to parse the signal generated by the incident electrons and distinguish individual electrons with discrete counts.

[0059] In some embodiments, electron counting can be applied to situations where the beam current is very small. For example, the electron beam can be set to irradiate the sample at a low dose. Low current can be used to prevent the electron counting detector from being oversaturated by large currents. For example, large currents can have the effect of introducing nonlinearities in the detection results. At the same time, for a detector to be used in an industrial environment, the detector should also be able to handle situations with large beam currents.

[0060] Some embodiments may utilize a detector having multiple relatively small sensing elements that may be used to detect an electron beam. Isolation may be provided between adjacent sensing elements, thereby reducing the probability that an incoming electron from one sensing element reaches its adjacent sensing element. In this way, crosstalk between adjacent sensing elements may be reduced.

[0061] In some embodiments, the detector can be constructed using digital circuitry and optoelectronic elements, rather than implementations that require extensive analog or digital electronic circuitry. As a result, various aspects of the detector implementation, such as operating speed, can be improved.

[0062] Reference Figure 1 , Figure 1 An exemplary electron beam inspection (EBI) system 10 consistent with embodiments of the present disclosure is illustrated and may include a detector. The EBI system 10 may be used for imaging. Figure 1 As shown, the EBI system 10 includes a main chamber 11, a load / lock chamber 20, an electron beam tool 100, and an instrument front end module (EFEM) 30. The electron beam tool 100 is located in the main chamber 11. The EFEM 30 includes a first load port 30a and a second load port 30b. The EFEM 30 may include (multiple) additional load ports. The first load port 30a and the second load port 30b receive a front opening wafer transport box (FOUP), which includes a wafer to be inspected (e.g., a semiconductor wafer or a wafer made of (multiple) other materials) or a sample (wafers and samples may be collectively referred to as "samples" in this article).

[0063] One or more robotic arms (not shown) in the EFEM 30 can transport the wafer to the load / lock chamber 20. The load / lock chamber 20 is connected to a load / lock vacuum pump system (not shown) that removes gas molecules in the load / lock chamber 20 to reach a first pressure lower than atmospheric pressure. After reaching the first pressure, one or more robotic arms (not shown) can transport the wafer from the load / lock chamber 20 to the main chamber 11. The main chamber 11 is connected to a main chamber vacuum pump system (not shown) that removes gas molecules in the main chamber 11 to reach a second pressure lower than the first pressure. After reaching the second pressure, the wafer is inspected by the electron beam tool 100. The electron beam tool 100 can be a single beam system or a multi-beam system. The controller 109 is electrically connected to the electron beam tool 100 and can also be electrically connected to other components. The controller 109 can be a computer configured to perform various controls of the EBI system 10. Although the controller 109 is Figure 1 1 is shown as being outside the structure including the main chamber 11, the load / lock chamber 20 and the EFEM 30, but it should be understood that the controller 109 can be part of the structure.

[0064] Figure 2A A charged particle beam apparatus is illustrated, wherein the inspection system may include a multi-beam inspection tool that uses multiple primary electron beam waves to simultaneously scan multiple locations on the sample.

[0065] like Figure 2A As shown, the electron beam tool 100A (also referred to herein as the apparatus 100A) may include an electron source 202, a gun aperture 204, a focusing lens 206, a primary electron beam 210 emitted from the electron source 202, a source conversion unit 212, a plurality of beams 214, 216, and 218 of the primary electron beam 210, a primary projection optical system 220, a wafer stage ( Figure 2A ), a plurality of secondary electron beams 236, 238, and 240, a secondary optical system 242, and an electron detection device 244. The electron source 202 may generate primary particles, such as electrons of the primary electron beam 210. A controller, an image processing system, etc. may be coupled to the electron detection device 244. The primary projection optical system 220 may include a beam splitter 222, a deflection scanning unit 226, and an objective lens 228. The electron detection device 244 may include detection sub-areas 246, 248, and 250.

[0066] Electron source 202, gun aperture 204, focusing lens 206, source conversion unit 212, beam splitter 222, deflection scanning unit 226, and objective lens 228 may be aligned with primary optical axis 260 of apparatus 100A. Secondary optical system 242 and electron detection device 244 may be aligned with secondary optical axis 252 of apparatus 100A.

[0067] The electron source 202 may include a cathode, an extractor, or an anode, where primary electrons may be emitted from the cathode and extracted or accelerated to form a primary electron beam 210 having a cross (virtual or real) 208. The primary electron beam 210 may be visualized as being emitted from the cross 208. The gun aperture 204 may block peripheral electrons of the primary electron beam 210 to reduce the size of the probe spots 270, 272, and 274.

[0068] The source conversion unit 212 may include an imaging element array (not shown). Figure 2A ) and a beam limiting aperture array (not shown in Figure 2A). Examples of source conversion unit 212 can 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 herein by reference in their entirety. The imaging element array may include a microdeflector or microlens array. The image forming element array may form multiple parallel images (virtual or real) of intersection 208 with multiple beam waves 214, 216, and 218 of the primary electron beam 210. The beam limiting aperture array may limit the multiple beam waves 214, 216, and 218.

[0069] The converging lens 206 can focus the primary electron beam 210. The current of the beams 214, 216 and 218 downstream of the source conversion unit 212 can be changed by adjusting the focusing ability of the converging lens 206 or by changing the radial size of the corresponding beam limiting apertures in the beam limiting aperture array. The converging lens 206 can be a movable converging lens, which can be configured so that the position of its first principal plane is movable. The movable converging lens can be configured to be magnetic, which can cause the off-axis beams 216 and 218 to fall on the beam limiting aperture at a rotation angle. The rotation angle changes with the focusing ability of the movable converging lens and the position of the first principal plane. In some embodiments, the movable converging lens can be a movable anti-rotation converging lens, which involves an anti-rotation lens having a movable first principal plane. The movable converging lens is also described in U.S. Publication No. 2017 / 0025241, the entire contents of which are incorporated herein by reference.

[0070] The objective lens 228 may focus the beams 214 , 216 , and 218 onto the wafer 230 for inspection, and may form a plurality of probe points 270 , 272 , and 274 on the surface of the wafer 230 .

[0071] The beam splitter 222 can be a Venn filter type beam splitter that generates an electrostatic dipole field and a magnetic dipole field. In some embodiments, if these two fields are applied, the force exerted by the electrostatic dipole field on the electrons of the beam waves 214, 216 and 218 can be equal in magnitude and opposite in direction to the force exerted by the magnetic dipole field on the electrons. The beam waves 214, 216 and 218 can therefore pass directly through the beam splitter 222 with a zero deflection angle. However, the total dispersion of the beam waves 214, 216 and 218 generated by the beam splitter 222 can also be non-zero. The beam splitter 222 can separate the secondary electron beams 236, 238 and 240 from the beam waves 214, 216 and 218, and guide the secondary electron beams 236, 238 and 240 to the secondary optical system 242.

[0072] The deflection scanning unit 226 can deflect the beams 214, 216, and 218 to scan the probe points 270, 272, and 274 over the surface area of ​​the wafer 230. In response to the incidence of the beams 214, 216, and 218 at the probe points 270, 272, and 274, secondary electron beams 236, 238, and 240 can be emitted from the wafer 230. The secondary electron beams 236, 238, and 240 may include electrons having an energy distribution including secondary electrons and backscattered electrons. The secondary optical system 242 may focus the secondary electron beams 236, 238, and 240 onto the detection sub-regions 246, 248, and 250 of the electron detection device 244. The detection sub-regions 246, 248, and 250 may be configured to detect the corresponding secondary electron beams 236, 238, and 240 and generate corresponding signals for reconstructing an image of the surface area of ​​the wafer 230.

[0073] Although Figure 2A An example of a multi-beam tool using multiple beams, namely an electron beam tool 100, is shown, but the embodiments of the present disclosure are not limited thereto. For example, the electron beam tool 100 may also be a single beam tool that uses only one primary electron beam to scan one location on the wafer at a time.

[0074] like Figure 2B As shown, the electron beam tool 100B (also referred to herein as the apparatus 100B) can be a single beam inspection tool for the EBI system 10. The apparatus 100B includes a wafer holder 136 supported by a motorized stage 134 to hold a wafer 150 to be inspected. The electron beam tool 100B includes an electron emitter, which may include a cathode 103, an anode 121, and a gun aperture 122. The electron beam tool 100B also includes a beam limiting aperture 125, a converging lens 126, a column aperture 135, an objective lens assembly 132, and a detector 144. In some embodiments, the objective lens assembly 132 can be a modified SORIL lens, which includes a pole piece 132a, a control electrode 132b, a deflector 132c, and an excitation coil 132d. During imaging, the electron beam 161 emitted from the tip of the cathode 103 may be accelerated by the voltage of the anode 121, pass through the gun aperture 122, the beam limiting aperture 125, the converging lens 126, and be focused by the improved SORIL lens into a probe point 170, and impinge on the surface of the wafer 150. The probe point 170 may be scanned across the surface of the wafer 150 by a deflector (such as the deflector 132c or other deflectors in the SORIL lens). The detector 144 may collect secondary particles or scattered primary particles emitted from the wafer surface, such as secondary electrons or scattered primary electrons, to determine the intensity of the beam so that an image of the region of interest on the wafer 150 may be reconstructed.

[0075] An image processing system 199 may also be provided, which includes an image acquisition device 120, a storage device 130, and a controller 109. The image acquisition device 120 may include one or more processors. For example, the image acquisition device 120 may include a computer, a server, a mainframe, a terminal, a personal computer, any kind of mobile computing device, or the like, or a combination thereof. The image acquisition device 120 may be connected to the detector 144 of the electron beam tool 100B through a medium such as an electrical conductor, an optical cable, a portable storage medium, IR, Bluetooth, the Internet, a wireless network, radio, or a combination thereof. The image acquisition device 120 may receive a signal from the detector 144 and may construct an image. The image acquisition device 120 may thus acquire an image of the wafer 150. The image acquisition device 120 may also perform various post-processing functions, such as generating a contour, superimposing an indicator on the acquired image, and the like. The image acquisition device 120 may be configured to perform adjustments to the brightness and contrast of the acquired image, and the like. The storage device 130 may be a storage medium, such as a hard disk, a random access memory (RAM), a cloud storage device, other types of computer-readable memory, and the like. The storage element 130 may be coupled to the image acquirer 120 and may be used to save the scanned raw image data as raw images and post-processed images. The image acquirer 120 and the storage element 130 may be connected to the controller 109. In some embodiments, the image acquirer 120, the storage element 130 and the controller 109 may be integrated together as an electronic control unit.

[0076] In some embodiments, the image acquirer 120 may acquire one or more images of the sample based on the imaging signal received from the detector 144. The imaging signal may correspond to a scanning operation for performing charged particle imaging. The acquired image may be a single image including a plurality of imaging regions, which may include various features of the wafer 150. The single image may be stored in the storage element 130. Imaging may be performed on an imaging frame basis.

[0077] The condenser and illumination optics of an electron beam tool may include or be supplemented by an electromagnetic quadrupole electron lens. Figure 2B As shown, the electron beam tool 100B may include a first quadrupole lens 148 and a second quadrupole lens 158. In some embodiments, the quadrupole lenses are used to control the electron beam. For example, the first quadrupole lens 148 may be controlled to adjust the beam current, and the second quadrupole lens 158 may be controlled to adjust the beam spot size and beam shape.

[0078] Figure 2B A charged particle beam arrangement is shown in which the inspection system may use a single primary beam that may be configured to generate secondary electrons by interacting with the wafer 150. The detector 144 may be positioned along the optical axis 105, such as Figure 2B The primary electron beam can be configured to travel along the optical axis 105. Thus, the detector 144 can include a hole at its center so that the primary electron beam can pass through to the wafer 150. However, some embodiments may use a detector that is placed off-axis relative to the optical axis along which the primary electron beam travels. For example, as in Figure 2A In the embodiment shown, a beam splitter 222 may be provided to direct the secondary electron beam to an off-axis placed detector. The beam splitter 222 may be configured to steer the secondary electron beam to an angle α.

[0079] Now refer to Figure 2C Another example of a charged particle beam apparatus is discussed. An electron beam tool 100C (also referred to herein as apparatus 100C) may be an example of an electron beam tool 100 and may be similar to Figure 2A An electron beam tool 100A is shown.

[0080] like Figure 2C As shown, beam splitter 222 can be a Venn filter type beam splitter that generates an electrostatic dipole field and a magnetic dipole field. In some embodiments, if both fields are applied, the force exerted on the electrons of beams 214, 216, and 218 by the electrostatic dipole field can be equal in magnitude and opposite in direction to the force exerted on the electrons by the magnetic dipole field. Beams 214, 216, and 218 can therefore pass directly through beam splitter 222 at a zero deflection angle. However, the total dispersion of beams 214, 216, and 218 generated by beam splitter 222 can also be non-zero. For the dispersion plane 224 of beam splitter 222, Figure 2C The beam 214 having a nominal energy V0 and an energy spread ΔV is shown dispersed into a beam portion 262 corresponding to energy V0, a ​​beam portion 264 corresponding to energy V0+ΔV / 2, and a beam portion 266 corresponding to energy V0-ΔV / 2. The resultant force exerted by the beam splitter 222 on the electrons of the secondary electron beams 236, 238, and 240 may be non-zero. The beam splitter 222 may separate the secondary electron beams 236, 238, and 240 from the beams 214, 216, and 218 and direct the secondary electron beams 236, 238, and 240 to the secondary optical system 242.

[0081] Semiconductor electron detectors (sometimes referred to as "PIN detectors") can be used in the apparatus 100 of the EBI system 10. The EBI system 10 can be a high-speed wafer imaging SEM including an image processor. The electron beam generated by the EBI system 10 can irradiate the surface of the sample or can penetrate the sample. The EBI system 10 can be used to image the structure on the surface or below the surface of the sample, such as for analyzing layer alignment. In some embodiments, the EBI system 10 can detect and report process defects related to the manufacture of semiconductor wafers by, for example, comparing the SEM image with the device layout pattern or the SEM image of the same pattern at other locations on the wafer being inspected. The PIN detector can include a silicon PIN diode that can operate under a negative bias. The PIN detector can be configured so that the incoming electrons generate relatively large and different detection signals. In some embodiments, the PIN detector can be configured so that the incoming electrons can generate multiple electron-hole pairs, while the photons can only generate one electron-hole pair. Compared with photodiodes for photon detection, PIN detectors for electron counting can have many differences, as described below.

[0082] Reference Figure 3 , Figure 3 FIG. 3 is a schematic diagram showing an exemplary structure of the detector 300. Figure 2A , Figure 2B and Figure 2C , the detector 300 may be provided as the detector 144 or the electronic detection device 244 .

[0083] Detector 300 may include an array of sensing elements including sensing elements 311, 312, and 313. There may be a large number of sensing elements, for example 1000 or more, desirably 5000 or more, and more desirably 10000 or more. The sensing elements may be arranged in a planar two-dimensional array, the plane of the array being substantially perpendicular to the incident direction of the incoming charged particles. In some embodiments, detector 300 may be arranged so as to be tilted relative to the incident direction. The array may include 10 or more rows, desirably 50 or more rows, and more desirably 100 or more rows, each row including 10 or more sensing elements, desirably 50 or more sensing elements, and more desirably 100 or more sensing elements. Although Figure 3 One array is shown in FIG. 3 , but it will be appreciated that the detector 300 may include multiple arrays, such as one array for each secondary electron beam.

[0084] Detector 300 may include substrate 310. Substrate 310 may be a semiconductor substrate that may include a sensing element. The sensing element may be a diode. The sensing element may also be an element similar to a diode that can convert incident energy into a measurable signal. The sensing element may include, for example, a PIN diode, an avalanche diode, an electron multiplier tube (EMT), etc., or a combination thereof. Region 325 may be provided between adjacent sensing elements. Region 325 may be an isolation region to isolate the sides or corners of adjacent sensing elements from each other. Region 325 may include an isolation material that is a material different from other regions of the detection surface of detector 300. As Figure 3 As shown in the plan view of , the region 325 may be arranged as a cross-shaped region. The region 325 may be arranged as a square. In some embodiments, the region 325 may not be arranged between adjacent sides of the sensing element. For example, in some embodiments, no isolation region may be provided on the detection surface of the detector.

[0085] The sensing element can generate an electrical signal commensurate with the charged particles received in the activation region of the sensing element. For example, the sensing element can generate a current signal commensurate with the energy of the received electrons. The preprocessing circuit can convert the generated current signal into a voltage, which can represent the intensity of the electron beam spot or a portion thereof. The preprocessing circuit device can include, for example, a preamplifier circuit device. The preamplifier circuit device can include, for example, a charge transfer amplifier (CTA), a transimpedance amplifier (TIA), an impedance conversion circuit coupled to the CTA or TIA, or a three-transistor amplifier. In some embodiments, a signal processing circuit device can be provided, which provides an output signal in arbitrary units on a time basis. One or more substrates (such as tube cores) can be provided, which can form a circuit layer for processing the output of the sensing element. The tube cores can be stacked together in the thickness direction of the detector. Other circuit devices can also be provided for other functions. For example, a switch drive circuit device can be provided, which can control the switch element used to connect the sensing elements to each other.

[0086] The sensing element is desirably configured to be individually switchable between an activated state and an inactivated state. In the inactivated state, the sensing element does not emit an output when a charged particle is incident on the sensing element. The sensing element can be set to an inactivated state by, for example, turning off a power supply to an amplifier associated with or incorporated into the sensing element. The sensing element can be set to an inactivated state by, for example, blocking a signal that would otherwise be output therefrom.

[0087] Although the accompanying drawings may show sensing elements 311, 312, and 313 as discrete units, such divisions may not actually exist. For example, the sensing elements of the detector may be formed by semiconductor devices that constitute PIN diode devices. The PIN diode device may be manufactured as a substrate having multiple layers including a p-type region, an intrinsic region, and an n-type region. One or more of such layers may be continuous in a cross-sectional view. However, in some embodiments, physical separation may be provided between the sensing elements. For example, in addition to the sensor layer, additional layers such as a circuit layer and a readout layer may also be provided.

[0088] As an example of an additional layer, the detector 300 may be provided with one or more circuit layers adjacent to the sensor layer. One or more circuit layers may include line conductors, interconnects, and various electronic circuit components. One or more circuit layers may include a processing system. One or more circuit layers may include a signal processing circuit device. One or more circuit layers may be configured to receive an output current detected from a sensing element in the sensor layer. For example, one or more circuit layers and the sensor layer may be arranged in the same or separate die.

[0089] Further details of the operation of a detection element (such as a diode) for detecting electrons can be found in US2019 / 0378682A1, the entire contents of which are incorporated herein by reference. As an alternative to a PIN diode, an embodiment may employ a low gain avalanche diode (LGAD).

[0090] Each of the sensing elements 311, 312, 313 outputs an electrical signal in response to incident charged particles (specifically electrons), which may be referred to as a detection signal. The detection signal may be, for example, a signal expressed in amperes, volts, or any unit commensurate with the energy of the electrons received at the corresponding sensing element. In some embodiments, the detection signal may represent the number of charged particles incident on the sensing element of the detector within a time period (e.g., a frame). The detection signal may indicate that a discrete number of charged particles have reached the sensing element. The number of charged particles may be viewed as an integer.

[0091] The radiation receiving portion of the sensing element (e.g., a PIN diode) may emit a brief current pulse upon incident charged particles. Thus, the sensing element may include a permanent element (such as an accumulator or a sample and hold circuit) that is set to an "on" state by the current pulse, or accumulates charge in response to the pulse of current until reset by a reset circuit at the end of a sampling period or frame. The sampling period or frame rate may be pre-set or automatically determined based on operating conditions.

[0092] Each sensing element 311, 312, 313 is connected to an electronic circuit device and / or an electro-optical device (not shown), which combines the outputs to generate a detector output signal for each detector 300. An electro-optical arrangement for summing the outputs of an array of sensing elements is described in European patent application No. EP22186172.7 filed on July 21, 2022, which is incorporated herein by reference at least for the disclosure of the electro-optical signal summing arrangement. The detector output signal may be a simple sum of the detection signals output by all sensing elements. In some embodiments, a weighted sum may be desired and implemented by introducing a weighting factor for each sensing element. The weighting factor may vary, for example, between 0 and 1. The weighting factor may be implemented in several different ways. For example, the weighting factor may be applied to the gain of an electronic amplifier associated with each sensing element. Each sensing element weighting factor may be fixed at the time of manufacture or calibration, or may be variable during the use of the device.

[0093] In some embodiments, the beam spot on the detector surface can be larger than the beam spot on the sample surface. Thus, the overall size of the detection surface of the detector can be configured to be large enough to accommodate the wide beam spot. The beam spot diameter on the detector surface can be on the order of a few millimeters. However, increasing the size of the detector may cause noise effects. For example, the capacitance of the detector may be proportional to the area of ​​the detector surface. Some noise sources, such as those due to components coupled to the detector (e.g., amplifiers), may be related to capacitance.

[0094] In some embodiments, the area ratio may vary, which may be the ratio of the area of ​​a single sensing element to the area of ​​the entire surface of the detector. The area ratio may have a corresponding relationship with the SNR. For example, in some embodiments, reducing the size of the sensing element to 1 / 1000 of the detector area may correspond to a 1000-fold increase in SNR.

[0095] The sensing element array may include a plurality of sensing elements, each sensing element having a size of, for example, Dx and Dy or less. For example, each sensing element may have a radiation receiving area having a size not exceeding 500 μm×500 μm, desirably not exceeding 200 μm×200 μm, more desirably not exceeding 150 μm×150 μm, or even as small as 50 μm×50 μm. In some embodiments, the radiation detector may include no less than 1000 sensing elements, desirably no less than 5000 sensing elements, and more desirably no less than 10000 sensing elements. The sensing elements may be arranged in a planar two-dimensional array, the plane of the array being substantially perpendicular to the incident direction of the incoming charged particles. In some embodiments, the detector may be arranged so as to be tilted relative to the incident direction.

[0096] In many use cases, the topographic signal obtained from the sample (i.e., the signal that depends on the sample's topography) is generally included in the secondary electrons. The measured backscattered electrons captured by a conventionally sized detector are only weakly dependent on the topography. Therefore, the backscattered electrons can be considered as noise. Conventional detectors cannot both distinguish between secondary electrons and backscattered electrons and provide information about the incident position, so the detector output signal has a lower signal-to-noise ratio and contrast-to-noise ratio than would be achieved by excluding backscattered electrons. Proposals for detectors that can distinguish between backscattered electrons and secondary electrons by energy have been proposed, but these are not currently feasible.

[0097] Reference Figure 8 and Fig. 9 The contrast-to-noise ratio metric (CNR) is explained. A histogram of an image of a region of a substrate obtained using a charged particle evaluation device ( Figure 8 ) can have two peaks derived from two feature types (e.g., line and groove or plane and edge). The contrast-to-noise ratio is defined as:

[0098]

[0099] where μ1 and μ2 are the signal levels (e.g., incident electron counts) of the corresponding peaks, and σ1 and σ2 are the half-widths at half-maximum of the corresponding peaks, representing the noise in the image. Fig. 9 is a sketch of the edge topography and the corresponding secondary electron yield. The values ​​of μ and σ are obtained from the image portions corresponding to the edge and flat areas. Other metrics measuring image contrast can be used and can also be improved by excluding backscattered electrons.

[0100] The inventors have found in simulations that the incident positions of backscattered electrons and secondary electrons on the detector are different. Secondary electrons hit the detector near the center of the detector, while backscattered electrons hit a larger detector area, such as FIG. 4A to FIG. 4C As shown, these figures show the simulated incident positions of secondary electrons and backscattered electrons on the detector plane. The landing energy (LE) of the sample is 800 eV. Figure 4A The secondary electrons are shown to be incident together with the backscattered electrons. Figure 4B Only secondary electron incidence (energy leaving the sample < 50 eV) is shown. Figure 4C Only backscattered electron incident (energy > 50 eV) are shown.

[0101] From the same simulations, it was also found that the secondary electron incident area depends on the landing energy at the sample, with smaller energy resulting in smaller incident area. Furthermore, it was found that the secondary electron distribution is spread over a slightly larger area when scanning edge topography on the sample compared to flat topography. Therefore, the spatial distribution of the secondary electrons depends on the topography.

[0102] Therefore, for a charged particle detector having an array of sensing elements, the array of sensing elements being configured to generate electrical signals in response to incident charged particles from a sample, it is proposed to provide a controller configured to selectively activate a first subset of a set of sensing elements and to combine the electrical signals of the first subset into a detector output signal. The first subset is selected based on a predicted distribution of backscattered particles and secondary charged particles (e.g., electrons) to achieve a desired effect. Based on the predicted distribution of backscattered particles and / or secondary particles, the predicted ratio of backscatter to secondary particles incident on a given sensing element or subset of sensing elements can be determined by simple calculations. Therefore, the sensing element to be activated or deactivated can be selected based on criteria related to the predicted ratio of backscatter to secondary particles. Several different effects can be achieved depending on the size, shape, and position of the (multiple) regions of the detector defined by the selected subset.

[0103] A second subset of sensing elements that are not part of the first subset may be deactivated, i.e. their outputs are not combined into the detector output signal. Desirably, the sensing elements of the second subset are switched off, which is advantageous because it reduces energy consumption and heat dissipation in the main vacuum chamber. The sensing elements may be switched off by deactivating power to the sensing elements, for example deactivating power to an amplifier associated with or incorporated in the sensing elements.

[0104] The controller may be integrated into the detector, or located elsewhere in the evaluation device. Integrating the controller into the detector may be desirable because it reduces the amount of data that needs to be transmitted from the detector. As described below, the controller may be configured to operate in a variety of different modes or combinations of modes.

[0105] For example, by selecting a subset of sensing elements corresponding to a variable inner portion of the sensing element array (e.g., selecting a first subset of sensing elements located within a selected radius of a detector reference point) as secondary electron detectors, and optionally selecting an outer portion as a backscattered electron detector, such as Figure 5 As shown, different effects can be achieved. For example, in contrast mode, a desired contrast-to-noise ratio can be achieved for the detector output signal, while in ratio mode, a desired ratio of detection of secondary particles to backscattered particles can be achieved. By way of example, three different criteria for selecting the radius of the secondary electron detector will be described below.

[0106] In a first example of an inward annular pattern, a secondary electron detector zone is selected to follow the secondary electron incident zone. The detector can be designed so that the entire detector zone captures more than 90% of all secondary electrons incident on the sample at the highest landing energy that the charged particle evaluation system is capable of capturing. At lower landing energies, the radius of the secondary electron incident zone will be reduced, and therefore a secondary electron detector zone can be selected as a first subset that is smaller than the entire sensing element array, but still large enough that a predetermined proportion (e.g. >95%) of the secondary electron incidents are captured. This can mean that the area of ​​the sensing element array that is activated is significantly reduced, for example, for low landing energies (e.g. LE=150eV), only a radius r i The sensing element within is activated, where r i is approximately the radius of the entire array (denoted by r a ). This enables detection of secondary electrons with reduced backscattered electron background. In an embodiment, the radius of the area occupied by the selected subset is less than 75% of the radius of the entire array, desirably less than 50%, and more desirably less than 20%.

[0107] like Figure 5 As depicted, in the inward annular mode, the sensing element array is controlled to select an inner detector portion 331 that primarily detects secondary electrons as an activated subset of the sensing element array, while an outer detector portion 332 detects backscattered electrons. The separation of the secondary electron detector and the backscattered electron detector stems purely from the different incident positions of the backscattered electrons and the secondary electrons; no additional separation / selection methods (such as energy selection) are included. It should be noted that in this example, the inner detector portion is a circle (or a pixelated approximation of a circle) centered at the center of the sensing element array. However, if for any reason the distribution of the secondary electrodes is not centered at the center of the sensing element array, the inner detector portion can be centered at another predetermined position, which is referred to as the detector reference point. For example, the detector reference point can be the center of the impact area of ​​the particle on the detector. As described below, the same applies to other selected areas.

[0108] The signal from the outer detector portion consists only of the backscattered electron incidence. In the outward annular mode, if this signal is considered valuable (e.g. for diagnostic purposes), it can be processed separately from the signals of the secondary electron detector pixels. Alternatively, this portion of the detector can be turned off. Turning off or deactivating the sensing elements that are not part of the selected subset can be advantageous because it reduces energy consumption and heat dissipation in the main vacuum chamber. The sensing element can be deactivated by deactivating power to the sensing element, such as deactivating power to an amplifier associated with or incorporated in the sensing element.

[0109] A variation of the inward annular mode is to select the secondary electron detector area so as to achieve at least the desired ratio of secondary particles to backscattered particles, thereby minimizing the contribution of backscattered electron incidents. This may require a slight reduction in the number of secondary electron incidents (no longer capturing >90-95% of all secondary electrons), but will reduce backscattered electron noise and therefore have better contrast (after longer integration times). This mode of operation can be beneficial for low contrast use cases for which there are currently no suitable SEM evaluation tools. If the contrast is increased, critical dimensions can be measured more accurately. Alternatively, the throughput of the evaluation tool can be increased while maintaining the contrast level.

[0110] The area of ​​the sensing element array used to achieve the desired effect, such as the desired contrast-to-noise ratio or the desired detection ratio of secondary particles to backscattered particles, such as the radius of the dividing line between the selected sensing element and the unselected sensing element, can be determined based on the predicted distribution of backscattered particles and / or secondary particles (e.g., the predicted distribution obtained from simulation or empirical data), for example. As discussed further below, in the empirically derived particle incidence distribution using the test sample, the particle incidence distribution at a large radius from the detector reference point can be used to estimate the distribution of backscattered particles across the entire detector. This estimated distribution of backscattered particles can then be subtracted from the total distribution to obtain an estimated distribution of secondary particles. Another method discussed in more detail below is to scan a test sample having a characteristic area and a flat area to obtain a flat particle incidence distribution and a characteristic particle incidence distribution. Because the incidence distribution of secondary particles is significantly more dependent on the morphology, the difference between the flat particle incidence distribution and the characteristic particle incidence distribution can be attributed to the secondary particles. Other methods for determining the predicted distribution of backscattered particles and / or secondary particles can also be used, and other methods use particle distribution data obtained from a test sample or a production sample.

[0111] A calibration method for determining a desired contrast-to-noise ratio or a desired detection ratio of secondary particles to backscattered particles comprises: scanning a charged particle beam across a calibration sample having a known morphology; receiving electrical signals from sensing elements in response to return particles, the return particles being generated in response to the charged particle beam to obtain a distribution of the return particles as a function of position on a detector; estimating the distribution of backscattered particles in the return particles as a function of position on the detector; subtracting the distribution of backscattered particles from the distribution of return particles to obtain a distribution of secondary particles; and selecting a subset of sensing elements for evaluation of the sample based on the distribution of backscattered particles and the distribution of secondary particles.

[0112] More specifically, the SEM is used to scan across the edge on a calibration wafer with a known topography, and the entire output of the sensing element array is obtained, including spatial information (e.g., the incident pixel position for each incident). Based on these measurements, the number of incidents with a radius of R divided by the number of incidents on the entire array can be determined. This can be expressed as a percentage of incidents as a function of R. The contribution of backscattered electrons to all incidents is estimated based on the rate of increase of the incidents at R at a larger radius. Outside a certain radius, it is assumed that all incidents are backscattered electrons, so the incident is expected to increase rapidly according to R first (SE and BSE contributions), while for R greater than the SE radius, the incident increases more slowly with R. Therefore, the maximum radius at which SE is still expected to contribute can be regarded as a change in the slope of the curve of the incident according to the detector radius. Based on Monte Carlo simulations of the detector incident positions, it is assumed that the distribution of backscattered incidents at small radii is consistent with the distribution of backscattered electrons at large radii, so that the secondary electron distribution can be obtained according to R by subtracting the estimated backscattered electron distribution from the total distribution. It is then straightforward to choose a radius that captures a desired proportion of secondary electrons or a desired ratio of backscatter to secondary electrons.

[0113] Another mode of operation is annular mode, in which sensing elements in an annular region of the sensing element array are selected. In this way, two effects can be achieved: obtaining a desired ratio of detection of secondary particles to backscattered particles (an example of ratio mode), or selecting a subset of sensing elements to enhance sensitivity to a specific feature type (e.g., edges) on the sample (an example of sensitivity mode).

[0114] To obtain a desired ratio of detection of secondary particles to backscattered particles, it may be helpful to deactivate the sensing elements in a small central region of the sensing element array, such as Fig. 6A This arrangement excludes the central peak in backscattered electrons and selects only the annular region 333 where the ratio of detected secondary electrons to detected backscattered electrons is high.

[0115] To enhance sensitivity to specific feature types on the sample (e.g., edges), the selected subset may be selected so that it captures the shift towards a larger incident radius that occurs when crossing an edge on the sample (rather than a flat surface). For example, a thin annular region 333 may be selected, e.g., 3 / 10 to 4 / 10 of the radius of the entire array at LE = 800 eV, such as Figure 6B Depicted. Reference Fig.10The selection of appropriate values ​​for the inner and outer radii of the activation area of ​​the array is explained. Based on the incident data for all sensing elements of the array obtained during the calibration process, the contrast can be calculated from the inner and outer radius values, and a pair of values ​​that gives the maximum contrast can be selected. An enhanced contrast is obtained because not only is the secondary electron yield higher at the sample edge, but the incidence ratio at the selected radius is also larger (e.g. 1.5 times). In other words, the signal change between the edge present and the edge not present under the beam is largest in this area. Although the total number of incidents of secondary electrons per unit time (and the resulting throughput) is reduced, the resulting image will have better contrast.

[0116] To enhance sensitivity to edges, the selected subset may define an area that is one or more portions of a ring rather than a complete ring. For example, the selected subset may define two portions of a ring centered on the center of the detector, with sensing elements on axes parallel to the edge to be detected excluded from the substrate. Another useful shape is a crescent (a concave-convex area defined by two arcs), such as a meniscus, or a similar shape, such as a bean or kidney. This type of area captures electrons incident on one side of the center of the sensing array (but not including the center). An area offset to one side may have a higher contrast relative to an edge from a lower to a higher area, while an area offset to the other side may have a higher contrast relative to an edge from a higher to a lower area of ​​the sample. Two such areas may be used to capture two types of edges.

[0117] It will be appreciated that, since individual sensing elements have a finite size, the actual shape of the area occupied by the selected subset will be an approximation of the given mathematical shape. Rules may be applied to determine whether a given sensing element will be included in the selected subset, such as if the center of the sensing element is within the mathematically defined optimal region, or if a certain percentage of the sensing elements are within the optimal region.

[0118] As described above, determining the area of ​​the sensing element array for achieving enhanced sensitivity to a particular feature type on a sample can be based on simulation or empirical results. A calibration method can include scanning a charged particle beam across a calibration sample that includes a topography of interest (e.g., an edge) and a flat topography; receiving electrical signals from sensing elements in response to return particles generated in response to the charged particle beam to obtain a characteristic distribution of the return particles as a function of position on the detector for the characteristic topography and a flat distribution of the return particles as a function of position on the detector for the flat topography; and selecting a subset of the sensing elements for evaluation of the sample based on the characteristic distribution and the flat distribution.

[0119] More specifically, the SEM is used to scan across an edge on a calibration wafer with known topography (as an example of a feature of interest), and the full output of the sensing element array is obtained, including spatial information such as the incident pixel location for each incident. From these measurements, the contrast is calculated based on the detection ring radius (or more generally, the detector area of ​​any shape). If the selected detector area is smaller, the contrast will be lower; within practical limits, this should be corrected because the goal is to increase contrast, and the reduction in throughput caused by the need for more frame averages is acceptable. It is then simple to choose the radius that optimizes the contrast. Fig.10 It is based on the pixel inner radius R 内 and pixel outer radius R 外 An exemplary graph of the contrast (the difference between the signal with the feature and without the feature) of the image, showing the image having the best contrast and located at R 内最大 With R 外最大 The pixel group at the position between .

[0120] Fig. 7A and Figure 7B The results for the flat sample area ( Fig. 7A ) and edge sample area ( Figure 7B ), distribution of electron incidence on the detector according to the radius of arbitrary units. It can be seen that at the edge, the electron distribution on the detector has a slightly larger radius; the distribution is a bit wider. The incidence percentage within the ring between 3 arbitrary units and 4 arbitrary units increases from 12.9% in the flat sample area to 19.4% at the edge. Therefore, by selecting sensing elements within such a ring or annulus as the selected subset, the contrast between the flat area and the edge area is enhanced.

[0121] Although the above description uses circular, annular, partially annular and concave-convex areas as sensing element arrays of selected subsets, it should be understood that in other cases, areas of other shapes may also be useful. For example, the selected subset may form an area having a shape selected from the group consisting of circular, elliptical, annular, square, rectangular, diamond, and rhombus. In the case of asymmetric distribution of backscattered or secondary electrons, asymmetric areas may be useful. Similarly, the selected subset of sensing elements does not need to occupy a single continuous area, but may form multiple separated areas. The incident data on each sensing element obtained in a calibration scan of a specific topology or by simulation (e.g., using a Monte Carlo method) can be used to identify a suitable subset.

[0122] While the above arrangement seeks to select secondary electrons in preference to backscattered electrons, it is certainly possible to select backscattered electrons in preference to secondary electrons. This can be achieved by reversing the above selection, for example, selecting only sensing elements outside a selected radius or deactivating sensing elements having an annular region. This can be useful for enhancing contrast (and thus increasing throughput) in high landing energy / high voltage SEM applications.

[0123] The detector can be configured to operate in several different modes, as selected by the user. These modes can include some or all of the following: selecting only sensing elements within a central region, such as a circle; selecting only sensing elements outside a central region, such as a circle; selecting sensing elements between two boundaries, which may be circular; and selecting sensing elements not between two boundaries, which may be annular.

[0124] Based on the same set of collected raw data, multiple results can be collected according to specific application requirements. The results can be used for post-processing instead of pre-processing. In some embodiments, pre-processing can include grouping of sensing elements. Data can also be customized to optimize specific purposes. For example, there can be a trade-off between optimizing detection parameters such as crosstalk and secondary charged particle collection efficiency. However, when post-processing is used to adjust the data, the trade-off can be adjusted without losing information. This can provide greater flexibility for the application layer and reduce the risk of having to repeat a specific operation just to obtain data with different detection parameter settings.

[0125] In some embodiments, the detector can communicate with the controller controlling the charged particle beam system. The controller can instruct the components of the charged particle beam system to perform various functions, such as controlling the charged particle source to generate the charged particle beam, and controlling the deflector to scan the charged particle beam. The controller can also perform various other functions, such as adjusting the sampling rate of the detector, resetting the sensing element, or performing image processing. The controller can include a storage component, which is a storage medium, such as a hard disk, a random access memory (RAM), other types of computer readable memory, etc. The storage component can be used to save the scanned raw image data as raw images and post-processing images. A non-transient computer readable medium can be provided, which stores instructions for the processor of the controller 109 to perform charged particle beam detection, sampling period determination, image processing, or other functions and methods that meet the present disclosure. Common forms of non-transitory media include, for example, floppy disks, flexible disks, hard disks, solid-state drives, magnetic tapes or any other magnetic data storage medium, CD-ROMs, any other optical data storage medium, any physical medium having a pattern of holes, ROMs, PROMs and EPROMs, FLASH-EPROMs or any other flash memory devices, NVRAMs, cache devices, registers, any other memory chip or cartridge memory, and networked versions thereof.

[0126] The block diagram in the figure can illustrate the architecture, functionality and operation of the possible implementation of the system, method and computer hardware / software product according to various exemplary embodiments of the present disclosure. In this regard, each block in the schematic diagram can represent a specific arithmetic or logic operation process that can be implemented using hardware such as an electronic circuit. The block can also represent a module, a code segment or a code portion, which includes one or more executable instructions for implementing a specified logical function. It should be understood that in some alternative embodiments, the functions indicated in the block may not occur in the order shown in the figure. For example, two blocks shown in succession can be performed or implemented substantially simultaneously, or two blocks can sometimes be implemented in reverse order, depending on the functionality involved. Some blocks can also be omitted. It should also be understood that each block of the block diagram and the combination of blocks can be implemented by a dedicated hardware-based system that performs a specified function or action, or by a combination of dedicated hardware and computer instructions.

[0127] Exemplary embodiments of the present disclosure are listed in the following numbered clauses:

[0128] 1. A method of configuring a detector of a charged particle assessment system, the detector having an array of sensing elements configured to generate an electrical signal in response to incident secondary particles or backscattered particles from a sample, the method comprising:

[0129] selecting a first subset of the set of sensing elements for activation based on data derived from a predicted distribution of secondary particles or backscattered particles; and

[0130] selecting a second subset of the set of sensing elements for deactivation based on the predicted distribution;

[0131] Wherein the first subset has a different predicted ratio of incident secondary particles to incident backscattered particles than the second subset.

[0132] 2. A method according to clause 1, wherein the first subset is selected to have a higher predicted ratio of incident secondary particles to incident backscattered particles than the second subset.

[0133] 3. A method according to clause 1, wherein the first subset is selected to have a predicted ratio of incident secondary particles to incident backscattered particles that is higher than a predetermined ratio.

[0134] 4. A method according to clause 1, wherein the first subset is selected to capture at least a predetermined proportion of secondary particles incident from the sample.

[0135] 5. A method according to clause 2, 3 or 4, wherein the first subset consists of sensing elements within a selected radius of a detector reference point, which detector reference point is expected to be the center of the impact area of ​​the particle on the detector, and the second subset consists of sensing elements outside the selected radius of the detector reference point.

[0136] 6. A method according to clause 1, wherein the first subset is selected to have a higher sensitivity to a predetermined feature type on the sample than the second subset.

[0137] 7. A method according to claim 6, wherein the first subset consists of sensing elements within a selected maximum radius of a detector reference point and outside a selected minimum radius of the detector reference point, wherein the detector reference point is expected to be the center of the impact area of ​​the particle on the detector, and the second subset consists of sensing elements outside the selected maximum radius of the detector reference point and sensing elements within the selected minimum radius of the detector reference point.

[0138] 8. The method according to any of clauses 1, 2, 3, 4 or 6, wherein the first subset consists of sensing elements forming a shape selected from the group consisting of: circle, oval, ring, square, rectangle, diamond, rhombus, concave-convex shape.

[0139] 9. A method according to any of clauses 1, 2, 3, 4 or 6, wherein the first subset consists of sensing elements forming a plurality of separate regions.

[0140] 10. The method according to any one of clauses 1 to 9, further comprising: obtaining a predicted distribution of secondary particles or backscattered particles by performing a simulation of directing a charged particle beam at the sample.

[0141] 11. The method according to any one of clauses 1 to 10, further comprising: obtaining a predicted distribution of secondary particles or backscattered particles by analyzing empirical results of directing a charged particle beam onto at least one sample.

[0142] 12. A method for detecting charged particles, the method comprising:

[0143] Configuring a detector of a charged particle assessment system by the method of any one of clauses 1 to 9;

[0144] directing a charged particle beam onto the sample using a charged particle evaluation system such that secondary particles and backscattered particles are generated in response to the charged particle beam;

[0145] directing secondary particles and backscattered particles to a detector;

[0146] activating a first subset of the array of sensing elements;

[0147] combining the electrical signals of the first subset into a detector output signal; and

[0148] A second subset of the array of sensing elements is deactivated.

[0149] 13. A non-transitory computer readable medium comprising a set of instructions executable by one or more processors of a controller to cause the controller to control a charged particle evaluation system to perform a method according to any of the preceding clauses.

[0150] 14. A charged particle evaluation system, the charged particle evaluation system comprising:

[0151] a charged particle beam device configured to direct a charged particle beam onto the sample such that secondary particles and backscattered particles are generated in response to the charged particle beam;

[0152] an array of sensing elements configured to generate electrical signals in response to incident secondary particles or backscattered particles from the sample; and

[0153] A controller configured to selectively activate a first subset of the set of sensing elements, selectively deactivate a second subset of the set of sensing elements, and combine electrical signals of the selected subsets into a detector output signal, wherein the selective activation and the selective deactivation are based on a predicted distribution of secondary particles or backscattered particles.

[0154] 15. A charged particle assessment system according to clause 14, wherein the controller is further configured to operate in a ratio mode, wherein the selected subset is selected to achieve a desired ratio of detection of secondary particles to backscattered particles on the sensing element array.

[0155] 16. A charged particle assessment system according to clause 14 or 15, wherein the controller is further configured to operate in a contrast mode, wherein the selected subset is selected to achieve a desired contrast-to-noise ratio of the detector output signal.

[0156] 17. A charged particle assessment system according to clause 14, 15 or 16, wherein the controller is further configured to operate in a sensitivity mode, wherein the selected subset is selected to achieve a desired sensitivity to features (eg edges) on the sample.

[0157] 18. A charged particle assessment system according to clause 14, 15, 16 or 17, wherein the controller is further configured to operate in an inward radial mode, wherein the selected subset consists of sensing elements within a selected radius of a detector reference point, which detector reference point is desirably the center of the impact area of ​​the particle on the detector.

[0158] 19. A charged particle assessment system according to any of clauses 14 to 18, wherein the controller is further configured to operate in an outward radial mode, wherein the selected subset consists of sensing elements outside a selected radius of a detector reference point, which detector reference point is desirably the center of the impact area of ​​the particle on the detector.

[0159] 20. A charged particle assessment system according to any of clauses 14 to 19, wherein the controller is further configured to operate in a circular radial mode, wherein the selected subset consists of sensing elements within a selected maximum radius of a detector reference point and outside a selected minimum radius of the detector reference point, the detector reference point being desirably the centre of the impact area of ​​the particle on the detector.

[0160] 21. A charged particle assessment system according to any one of clauses 14 to 20, wherein the controller is further configured to operate in a shape mode, wherein a selected subset consists of sensing elements forming a shape selected from the group consisting of: circular, elliptical, annular, square, rectangular, diamond, rhombus, and concave-convex shapes.

[0161] 22. A charged particle assessment system according to any of clauses 14 to 21, wherein the controller is further configured to operate in a separation mode, wherein the selected subset consists of sensing elements forming a plurality of separation regions.

[0162] 23. A charged particle assessment system according to any of clauses 14 to 22, wherein the controller is configured to deactivate sensing elements not included in the selected subset.

[0163] 24. A charged particle assessment system according to clause 23, wherein the controller is configured to deactivate power supply to sensing elements not included in the selected subset.

[0164] 25. A charged particle assessment system according to any of clauses 14 to 24, wherein the controller is configured to apply a respective weight to each of the electrical signals to combine the electrical signals into a detector output signal, preferably electrical signals from sensing elements not included in the selected subset having a zero weight.

[0165] 26. A charged particle assessment system according to any of clauses 14 to 25, wherein the charged particle beam device is configured to direct a plurality of charged particle beams onto the sample; and comprises an array of sensing elements for each of the plurality of charged particle beams.

[0166] 27. A method of calibrating a charged particle detector in a charged particle evaluation apparatus, the charged particle detector having an array of sensing elements configured to generate an electrical signal in response to incident secondary particles or backscattered particles from a sample; the method comprising:

[0167] Scanning a charged particle beam across a calibration sample of known topography;

[0168] receiving electrical signals from a sensing element in response to secondary particles and backscattered particles generated in response to the charged particle beam to obtain a combined distribution of the secondary particles and backscattered particles as a function of position on the detector;

[0169] estimating a distribution of backscattered particles among secondary particles and backscattered particles as a function of position on the detector;

[0170] subtracting the distribution of backscattered particles from the combined distribution of secondary particles and backscattered particles to obtain a distribution of secondary particles; and

[0171] Based on the distribution of backscattered particles and the distribution of secondary particles, a subset of sensing elements is selected for evaluation of the sample.

[0172] 28. The method according to clause 27, wherein estimating the distribution of backscattered particles is based on the distribution of returning particles at the periphery of the sensing element array.

[0173] 29. A method according to clause 27 or 28, wherein selecting the subset of sensing elements comprises selecting the subset of sensing elements to achieve detection of a desired proportion of secondary particles.

[0174] 30. A method of calibrating a charged particle detector in a charged particle evaluation apparatus, the charged particle detector having an array of sensing elements configured to generate an electrical signal in response to incident secondary particles or backscattered particles from a sample; the method comprising:

[0175] scanning a charged particle beam across a calibration sample including features of interest (e.g., edges) and flat topography;

[0176] receiving an electrical signal from the sensing element in response to return particles generated in response to the charged particle beam to obtain a characteristic distribution of return particles as a function of position on the detector for characteristic topography and a flat distribution of return particles as a function of position on the detector for flat topography; and

[0177] Based on the characteristic distribution and the flat distribution, a subset of sensing elements is selected for evaluation of the sample.

[0178] 31. The method of clause 30, wherein selecting the subset of sensing elements comprises selecting the subset of sensing elements to achieve a desired contrast of the feature of interest.

[0179] It should be understood that the present disclosure is not limited to the exact structures described above and shown in the accompanying drawings, and various modifications and changes may be made without departing from the scope thereof. For example, although PIN diodes have been discussed with reference to certain exemplary embodiments, other types of diodes, such as NIP diodes, may be similarly applied. In addition, other types of devices that can generate a measurable signal in response to receiving incident energy may be applied in the detector.

[0180] It is to be understood that elements shown in separate figures may be combined.

[0181] Furthermore, although a scanning electron microscope has been discussed with reference to some embodiments, other types of systems may also be applicable. For example, the detector may be used in a transmission electron microscope (TEM), a scanning transmission electron microscope (STEM), or a structured illumination microscope (SIM) system.

Claims

1. A charged particle evaluation system comprising: a charged particle beam device configured to direct a charged particle beam onto a sample such that secondary particles and backscattered particles are generated in response to the charged particle beam; an array of sensing elements configured to generate electrical signals in response to incident secondary particles or backscattered particles from the sample; as well as A controller configured to selectively activate a first subset of the set of sensing elements, to selectively deactivate a second subset of the set of sensing elements, and to combine the electrical signals of the selected subsets into a detector output signal, wherein the selective activation and the selective deactivation are based on a predicted distribution of secondary particles or backscattered particles.

2. A charged particle assessment system according to claim 1, wherein the controller is further configured to operate in a ratio mode, wherein the selected subset is selected to achieve a desired ratio of detection of secondary particles to backscattered particles across the sensing element array.

3. The charged particle assessment system of claim 1, wherein the controller is further configured to operate in a contrast mode, wherein the selected subset is selected to achieve a desired contrast-to-noise ratio of the detector output signal.

4. The charged particle assessment system of claim 1, wherein the controller is further configured to operate in a sensitivity mode, wherein the selected subset is selected to achieve a desired sensitivity to features, such as edges, on the sample.

5. A charged particle assessment system according to claim 1, wherein the controller is further configured to operate in an inward radial mode, wherein the selected subset consists of sensing elements within a selected radius of a detector reference point, wherein the detector reference point is expected to be the center of the impact area of ​​the particle on the detector.

6. A charged particle assessment system according to claim 1, wherein the controller is further configured to operate in an outward radial mode, wherein the selected subset consists of sensing elements outside a selected radius of a detector reference point, wherein the detector reference point is expected to be the center of the impact area of ​​the particle on the detector.

7. A charged particle assessment system according to claim 1, wherein the controller is further configured to operate in an annular radial mode, wherein the selected subset consists of sensing elements within a selected maximum radius of a detector reference point and outside a selected minimum radius of the detector reference point, wherein the detector reference point is expected to be the center of the impact area of ​​the particle on the detector.

8. The charged particle assessment system of claim 1 , wherein the controller is further configured to operate in a shape mode, wherein the selected subset consists of sensing elements forming a shape selected from the group consisting of: Circle, oval, ring, square, rectangle, diamond, rhombus, concave and convex shapes.

9. The charged particle assessment system of claim 1, wherein the controller is further configured to operate in a separation mode, wherein the selected subset consists of sensing elements forming a plurality of separation regions.

10. The charged particle assessment system of claim 1, wherein the controller is configured to deactivate sensing elements not included in the selected subset, optionally by deactivating power to the sensing elements not included in the selected subset.

11. The charged particle evaluation system of claim 1, wherein the charged particle beam device is configured to direct a plurality of charged particle beams onto the sample; and comprises an array of sensing elements for each of the plurality of charged particle beams.

12. A non-transitory computer readable medium comprising an instruction set executable by one or more processors of a controller to cause the controller to control a charged particle assessment system to perform a method of configuring a detector of the charged particle assessment system, the detector having an array of sensing elements configured to generate an electrical signal in response to incident secondary particles or backscattered particles from a sample, the method comprising: selecting a first subset of the set of sensing elements for activation based on data derived from a predicted distribution of secondary particles or backscattered particles; as well as selecting a second subset of the set of sensing elements for deactivation based on the predicted distribution; Wherein the first subset has a different predicted ratio of incident secondary particles to incident backscattered particles than the second subset.

13. A non-transitory computer-readable medium according to claim 12, wherein the first subset is selected to have a higher predicted ratio of incident secondary particles to incident backscattered particles than the second subset; or to have a predicted ratio of incident secondary particles to incident backscattered particles that is higher than a predetermined ratio; or to capture at least a predetermined proportion of secondary particles incident from the sample.

14. A non-transitory computer readable medium according to claim 13, wherein the first subset consists of sensing elements within a selected radius of a detector reference point, wherein the detector reference point is expected to be the center of the impact area of ​​the particle on the detector, and the second subset consists of sensing elements outside the selected radius of the detector reference point.

15. The non-transitory computer readable medium of claim 12, wherein the first subset is selected to have a higher sensitivity to a predetermined feature type on the sample than the second subset.

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