detector for detecting radiation

Abstract

A detector for detecting radiation is disclosed. In one configuration, the detector has a sensing element including a semiconductor sensor substrate (402) configured such that impact of target radiation generates charge carriers in the sensor substrate. A readout circuit provides an output in response to the charge carriers generated in the sensor substrate. The sensor substrate includes a low bandgap layer (408) and a high bandgap portion (409).

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to European Patent Application Publication No. 22216582.1, filed December 23, 2022, which is incorporated herein by reference in its entirety.

[0002]

[0002] The present disclosure relates to detectors and related devices and methods for detecting radiation, and is particularly applicable in connection with charged particle-based microscopy. [Background technology]

[0003]

[0003] Detectors can be used to detect physically observable phenomena. For example, a charged particle beam tool such as an electron microscope may 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 under inspection, for example, to reveal defects in the sample. Detection of defects in samples becomes increasingly important in the manufacture of semiconductor devices, which may include many densely packed miniature integrated circuit (IC) components. For this purpose, dedicated inspection tools can be provided.

[0004]

[0004] In some applications in the field of inspection, for example, microscopy using scanning electron microscopes (SEMs), an electron beam may be scanned across a sample to derive information from backscattered or secondary electrons generated from the sample. In a related technical field, an electron detection system in an SEM tool may include a detector configured to detect electrons incident from the sample. Existing detectors in SEM tools may only detect the intensity of the beam. Some detection methods may use a large-area semiconductor detector or a group of small-area semiconductor detectors with an area equal to, smaller than, or larger than the area of ​​the beam spot. In the detector, a current induced by the incident electron beam is generated and may then be amplified by an amplifier located after the detector. The detection performance may be limited due to the amplifier, resulting in, for example, a poor signal-to-noise ratio (SNR), especially when the beam current is reduced, for example, to the picoampere range.

[0005] As semiconductor devices continue to shrink, inspection systems may use smaller and smaller electron beam currents. As the beam current decreases, maintaining an acceptable signal-to-noise ratio (SNR) becomes even more challenging. For example, a decrease in probe current below 200 pA can dramatically reduce the SNR. Poor SNR can require measures such as image averaging or extending the integration time of the signal corresponding to each pixel in the image of the sample, which can result in increased electron dose on the sample surface and potentially cause surface charging artifacts or other deleterious effects. Such measures can also reduce the overall throughput of the inspection system.

[0006] Detectors that offer active internal gain through avalanche gain can be attractive because they can provide a stronger output signal. A stronger output signal is desirable because it allows for the use of simpler and / or lower-power readout electronics (e.g., amplifiers). Known detectors of this type include single-photon avalanche detectors (SPADs), silicon photomultipliers (SiPMs), and avalanche photodetectors (APDs). Some implementations of such detectors have drawbacks. For example, the detector may require recovery time after each avalanche. During this recovery time, the detector cannot detect incident radiation. Detectors may also have a relatively low fill factor due to the need to avoid crosstalk between adjacent detector pixels (e.g., to avoid an avalanche in one pixel causing an avalanche in a neighboring pixel). The fill factor in such detectors can be as low as 40-50%. Summary of the Invention

[0007]

[0007] It is an object of the present disclosure to provide an improved detector for detecting radiation.

[0008]

[0008] According to one aspect of the present invention, there is provided a detector for detecting radiation, the detector comprising a sensing element including a semiconductor sensor substrate, wherein the sensing element is configured so that impact of target radiation on the sensor substrate generates charge carriers in the sensor substrate, and a readout circuit configured to provide an output in response to the charge carriers generated in the sensor substrate, wherein the sensor substrate includes a low bandgap portion made of a low bandgap material and a high bandgap portion made of a high bandgap material, the low bandgap material having a lower bandgap than the bandgap of the high bandgap material.

[0009]

[0009] Providing a low bandgap section increases the passive internal gain. By configuring the sensor substrate to include both a low bandgap section and a high bandgap section, it is possible to achieve an increase in passive internal gain without excessively increasing the dark current.

[0010] In one embodiment, the readout circuit is configured to operate the sensing element as a reverse-biased diode with a depletion region in the sensor substrate, where charge carriers are generated in the depletion region in the sensor substrate. In one embodiment, the high bandgap portion forms a majority of the depletion region. Configuring the high bandgap portion to form a majority of the depletion region allows the depletion region to be relatively large without excessively increasing dark current. Having a relatively large depletion region can beneficially reduce junction capacitance and improve noise performance.

[0011] In one embodiment, the low band gap portion comprises a low band gap layer. Providing the low band gap portion in the form of a layer may facilitate positioning the low band gap layer to effectively interact with impinging target radiation.

[0012] In one embodiment, the low bandgap layer is substantially planar and perpendicular to the width of the depletion region in the direction of reverse bias and / or to a surface of the sensing element configured to receive target radiation. Orienting the low bandgap layer in this manner can promote effective interaction with impinging target radiation.

[0013] In one embodiment, at least a portion of the low bandgap layer is disposed at a surface of the sensing element configured to receive the target radiation, or closer to the surface of the sensing element configured to receive the target radiation than the depletion region. Locating the low bandgap layer at or near the surface may provide good performance for a range of target radiation energy distributions because at least a portion of the target radiation is absorbed relatively close to the surface.

[0014] In one embodiment, the low band gap layer is provided at a depth within the sensor substrate such that a portion of the depletion region is between the low band gap layer and the surface of the sensing element configured to receive the target radiation. Locating the low band gap layer at a depth may encourage electrons to be generated further away from the surface, thereby reducing charge carrier loss due to recombination events from the surface or associated with defects at or near the surface. Additionally or alternatively, locating the low band gap layer at a depth may enable the low band gap layer to accommodate a wider region of the expected spherical shape of the interaction volume between the impinging radiation and the sensor substrate. The interaction volume may, for example, have a maximum cross-sectional area (in a direction parallel to the surface) at a finite depth within the sensor substrate.

[0015] In one embodiment, the thickness and / or position of the low bandgap layer is selected according to the energy distribution of the target radiation, optionally the average energy of the target radiation. For example, the thickness and / or position may be selected to take into account that the cross-sectional area of ​​the interaction volume is at a maximum at a finite depth.

[0016] In one embodiment, the low band gap layer has a composition that varies as a function of position through the thickness of the low band gap layer. In one embodiment, the change in composition is such that the composition converges toward the composition of the material outside the low band gap layer and in contact with the low band gap layer on one or both sides of the low band gap layer. Arranging for such convergence in composition can reduce the mismatch in lattice at one or both interfaces between the low band gap layer and the surrounding material, thereby reducing the amount of lattice defects such as dislocations, thereby reducing recombination events and improving internal gain.

[0017] In one embodiment, the low bandgap portion includes multiple low bandgap layers. In one embodiment, each of the low bandgap layers has a thickness small enough to lower the bandgap of the material of the low bandgap layer. Lowering the bandgap can increase the passive internal gain. By providing multiple low bandgap layers, the total thickness of the low bandgap material is greater. By providing the total thickness as multiple thin layers rather than a single thicker layer, a lower average bandgap is achieved in the low bandgap material due to the suppression effect on the bandgap from the thinness of the individual layers, thereby improving the overall passive internal gain.

[0018]

[0018] In one embodiment, at least a subset of the low bandgap layers are configured to generate charge carriers primarily from target radiation having different respective energy distributions or different respective mean energies. For example, different layers of the subset may be located at significantly different depths within the sensor substrate. Spacing the layers in this manner increases operational flexibility and efficiency by allowing the detector to have high passive internal gain for a range of different target radiation.

[0019] In one embodiment, the low bandgap material comprises a material having the same crystal structure as the high bandgap material and / or a lattice constant within 10% of the lattice constant of the high bandgap material. Reducing the mismatch between the lattice structures of the low and high bandgap materials can reduce defects such as dislocations that can act as recombination centers, reducing passive internal gain.

[0020]

[0020] According to one aspect of the present invention, there is provided a method of manufacturing a sensing element for a detector for detecting radiation, the sensing element including a semiconductor sensor substrate configured to generate charge carriers upon impact of target radiation on the sensor substrate, the sensor substrate including a low band gap layer forming a part of the sensor substrate and having a band gap lower than the band gap of a high band gap material forming another part of the sensor substrate, the method comprising forming a plurality of candidate sensor substrates having different configurations of the low band gap layer, the different configurations including one or both of different thicknesses and different positions of the low band gap layer in the sensor substrate, measuring the internal gain of the candidate sensor substrates when exposed to the target radiation, selecting as the final configuration of the low band gap layer the configuration corresponding to the largest measured internal gain, and manufacturing the sensor element by forming a sensor substrate having the low band gap layer in accordance with the selected final configuration of the low band gap layer.

[0021]

[0021] It is to be understood that both the foregoing summary and the following detailed description are exemplary and explanatory only and are not intended to limit the disclosed embodiments, which may be set forth in the claims.

[0022]

[0022] The above and other aspects of the present disclosure will become more apparent from a reading of the description of exemplary embodiments in conjunction with the accompanying drawings. [Brief explanation of the drawings]

[0023] [Figure 1]FIG. 1 is a schematic diagram illustrating an exemplary electron beam inspection (EBI) system consistent with an embodiment of the present disclosure. [Figure 2A]

[0024] FIG. 2 is a schematic diagram illustrating an example electron beam tool that may be part of the example electron beam inspection system of FIG. 1, consistent with an embodiment of the present disclosure. [Figure 2B] FIG. 2 is a schematic diagram illustrating an example electron beam tool that may be part of the example electron beam inspection system of FIG. 1, consistent with an embodiment of the present disclosure. [Figure 3A]

[0025] 1 is a diagram of an exemplary structure of a detector, consistent with an embodiment of the present disclosure. [Figure 3B]

[0026] FIG. 1 illustrates a cross-sectional view of a detector consistent with an embodiment of the present disclosure. [Figure 3C]

[0026] FIG. 1 illustrates a cross-sectional view of a detector consistent with an embodiment of the present disclosure. [Figure 3D]

[0027] FIG. 1 illustrates a cross-sectional view of an individual detector element consistent with an embodiment of the present disclosure. [Figure 3E]

[0027] FIG. 1 illustrates a cross-sectional view of an individual detector element consistent with an embodiment of the present disclosure. [Figure 4A]

[0028] FIG. 2C is a view of a portion of FIG. 2B showing secondary electrons being projected from the sample towards a detector. [Figure 4B]

[0029] 1 illustrates an example of a secondary electron landing site distribution on a detector surface, consistent with embodiments of the present disclosure. [Figure 5]

[0030] 1 shows a schematic diagram of some of the elements in a MEMS-based miniature SEM array, according to one embodiment of the present disclosure. [Figure 6]

[0031] 1 shows a schematic diagram of a detector array according to one embodiment of the present disclosure. [Figure 7]

[0032] FIG. 1 is a schematic diagram of a detector having a sensor substrate with a low bandgap portion in the form of a single layer. [Figure 8]

[0033] 10 shows a schematic representation of the expected characteristic shape of the interaction volume for radiation impinging on the sensor substrate. [Figure 9]

[0034] 8 shows a modification to the situation in FIG. 8 for a sensor substrate with a higher mass density. [Figure 10]

[0035] 8 illustrates a variation to the sensor substrate of FIG. 7 in which the layer is provided at a depth within the sensor substrate. [Figure 11]

[0036] 11 illustrates a variation to the sensor substrate of FIG. 10 in which multiple layers are provided in closely spaced groups. [Figure 12]

[0037] 11 illustrates a variation on the sensor substrate of FIG. 10 in which multiple widely spaced layers are provided. [Figure 13]

[0038] 12 illustrates a variation on the sensor substrate of FIG. 11 in which multiple closely spaced layers are provided. DETAILED DESCRIPTION OF THE INVENTION

[0024]

[0039] Reference will now be made in detail to the exemplary embodiments, examples of which are illustrated in the drawings. The following description refers to the accompanying drawings in which, unless otherwise stated, like numbers in different drawings represent the same or similar elements. The implementations set forth in the following description of the exemplary embodiments do not represent all implementations that are consistent with the present invention. Instead, these implementations are merely examples of devices, systems, and methods that are consistent with aspects related to the subject matter that may be recited in the appended claims.

[0025]

[0040] Aspects of the present application relate to systems and methods for charged particle beam detection. The systems and methods can employ counting of charged particles, such as electrons, and can be useful in inspection tools such as scanning electron microscopes (SEMs). The inspection tools can be used in the manufacturing process of integrated circuit (IC) components. To achieve increased computing power in today's electronic devices, the physical size of the devices can shrink while significantly increasing the packing density of circuit components, such as transistors, capacitors, and diodes, on IC chips. For example, in a smartphone, an IC chip (which may be the size of a thumbnail) can contain over two billion transistors, each less than 1 / 1000 the size of a human hair. Naturally, manufacturing semiconductor ICs is a complex process involving hundreds of individual steps. An error in even one step can dramatically affect the functionality of the final product. Even a single "killer defect" can cause device failure. The goal of the manufacturing process is to improve the overall yield of the process. For example, in a 50-step process to achieve a 75% yield, the yield of each individual step must be greater than 99.4%, and if the yield of any individual step is 95%, the overall process yield drops to 7%.

[0026]

[0041] Ensuring the ability to detect defects with high accuracy and resolution while maintaining high throughput (e.g., defined as the number of wafers processed per hour) is becoming increasingly important. High process yields and high wafer throughput can be affected by the presence of defects, especially when operator intervention is involved. Therefore, the detection and identification of micro- and nano-sized defects by inspection tools (e.g., SEM) is important to maintain high yields and low costs.

[0027]

[0042] In some inspection tools, a sample may be inspected by scanning a beam of high-energy electrons across the sample surface. Interactions at the sample surface may produce secondary or backscattered electrons from the sample, which may then be detected by a detector.

[0028]

[0043] Without limiting the scope of the present disclosure, some embodiments may be described in connection with providing detectors and detection methods in systems utilizing electron beams. However, the present disclosure is not so limited. Other types of charged particle beams may be applied as well. Furthermore, the systems and methods for detection may also be used in other imaging systems, such as optical imaging, photon detection, x-ray detection, ion detection, etc.

[0029]

[0044] As used herein, unless specifically stated otherwise, the term "or" includes all possible combinations except where feasible. For example, if a component is stated to include A or B, the component may include A or B, or A and B, unless specifically stated otherwise or feasible. As a second example, if a component is stated to include A, B, or C, the component may include A, or B, or C, or A and B, or A and C, or B and C, or A, B, and C, unless specifically stated otherwise or feasible.

[0030]

[0045] Referring now to FIG. 1 , FIG. 1 illustrates an exemplary electron beam inspection (EBI) system 10 that may include a detector consistent with embodiments of the present disclosure. The EBI system 10 may be used for imaging. As shown in FIG. 1 , the EBI system 10 includes a main chamber 11, a load / lock chamber 20, an electron beam tool 100, and an equipment front-end module (EFEM) 30. The electron beam tool 100 is disposed within the main chamber 11. The EFEM 30 includes a first load port 30 a and a second load port 30 b. The EFEM 30 may include additional load ports. The first load port 30 a and the second load port 30 b receive wafers (e.g., semiconductor wafers or wafers made of other materials) or wafer front-opening integrated pods (FOUPs) containing samples to be inspected (wafers and samples may be collectively referred to herein as “wafers”).

[0031]

[0046] One or more robot arms (not shown) of the EFEM 30 can transfer wafers to the load / lock chamber 20. The load / lock chamber 20 is connected to a load / lock vacuum pumping system (not shown) that removes gas molecules from the load / lock chamber 20 to reach a first pressure below atmospheric pressure. After the first pressure is reached, the one or more robot arms (not shown) can transfer the wafers from the load / lock chamber 20 to the main chamber 11. The main chamber 11 is connected to a main chamber vacuum pumping system (not shown) that removes gas molecules from the main chamber 11 to reach a second pressure below the first pressure. After the second pressure is reached, the wafers are inspected by the electron beam tool 100. The electron beam tool 100 can be a single-beam system or a multi-beam system. A controller 109 is electronically connected to the electron beam tool 100 and can be electronically connected to other components as well. The controller 109 can be a computer configured to perform various controls of the EBI system 10. Although the controller 109 is shown in FIG. 1 as being external to the structure including the main chamber 11, the load / lock chamber 20, and the EFEM 30, it should be understood that the controller 109 may be part of the structure.

[0032]

[0047] 2A shows a charged particle beam device in which the inspection system may include a multi-beam inspection tool that uses multiple primary electron beamlets to simultaneously scan multiple locations on a sample.

[0033]

[0048] As shown in FIG. 2A , the electron beam tool 100A (also referred to herein as apparatus 100A) may include an electron source 202, a gun aperture 204, a condenser lens 206, a primary electron beam 210 emitted from the electron source 202, a source conversion unit 212, multiple beamlets 214, 216, and 218 of the primary electron beam 210, a primary projection optics 220, a wafer stage (not shown in FIG. 2A ), multiple secondary electron beams 236, 238, and 240, a secondary optics 242, and an electron detection device 244. The electron source 202 may generate primary particles, such as electrons in the primary electron beam 210. A controller, an image processing system, and the like may be coupled to the electron detection device 244. The primary projection optics 220 may include a beam separator 222, a deflection scanning unit 226, and an objective lens 228. The electron detection device 244 may include detection subregions 246, 248, and 250.

[0034]

[0049] Electron source 202, gun aperture 204, condenser lens 206, source conversion unit 212, beam separator 222, deflection scanning unit 226, and objective lens 228 may be aligned with a primary optical axis 260 of apparatus 100A. Secondary optics 242 and electron detection device 244 may be aligned with a secondary optical axis 252 of apparatus 100A.

[0035]

[0050] The electron source 202 may include a cathode, an extraction electrode, or an anode, and primary electrons may be emitted from the cathode and extracted or accelerated to form a primary electron beam 210 with a crossover (virtual or real) 208. The primary electron beam 210 may be visualized as being emitted from the crossover 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.

[0036]

[0051] The source conversion unit 212 may include an image-forming element array (not shown in FIG. 2A ) and a beam-limiting aperture array (not shown in FIG. 2A ). Examples of the source conversion unit 212 can be found in U.S. Pat. No. 9,691,586, U.S. Patent Application Publication No. 2017 / 0025243, and International Application No. PCT / EP2017 / 084429, all of which are incorporated by reference in their entireties. The image-forming element array may include an array of micro-deflectors or micro-lenses. The image-forming element array can form multiple parallel images (virtual or real images) of the crossover 208 with the multiple beamlets 214, 216, and 218 of the primary electron beam 210. The beam-limiting aperture array can limit the multiple beamlets 214, 216, and 218.

[0037]

[0052] The condenser lens 206 can focus the primary electron beam 210. The currents of the beamlets 214, 216, and 218 downstream of the source conversion unit 212 can be varied by adjusting the focusing power of the condenser lens 206 or by changing the radial size of the corresponding beam-limiting apertures in the beam-limiting aperture array. The condenser lens 206 can be a movable condenser lens that can be configured to have a movable first principal plane. The movable condenser lens can be configured to be magnetic, so that the off-axis beamlets 216 and 218 land on the beamlet-limiting aperture at a rotation angle. The rotation angle varies with the focusing power and the position of the first principal plane of the movable condenser lens. In some embodiments, the movable condenser lens can be a movable anti-rotation condenser lens that includes an anti-rotation lens having a movable first principal plane. Movable condenser lenses are further described in U.S. Patent Application Publication No. 2017 / 0025241, which is incorporated herein by reference in its entirety.

[0038]

[0053] The objective lens 228 can focus the beamlets 214 , 216 , and 218 onto the wafer 230 under inspection, forming multiple probe spots 270 , 272 , and 274 on the surface of the wafer 230 .

[0039]

[0054] Beam separator 222 may be a Wien filter-type beam separator that generates electrostatic and magnetic dipole fields. In some embodiments, when these fields are applied, the force exerted on electrons in beamlets 214, 216, and 218 by the electrostatic dipole field may be equal in magnitude and opposite in direction to the force exerted on electrons by the magnetic dipole field. Therefore, beamlets 214, 216, and 218 may pass through beam separator 222 in a straight line with zero deflection angle. However, the total dispersion of beamlets 214, 216, and 218 caused by beam separator 222 may be non-zero. Beam separator 222 may separate secondary electron beams 236, 238, and 240 from beamlets 214, 216, and 218 and direct secondary electron beams 236, 238, and 240 toward secondary optics 242.

[0040]

[0055] The deflection scanning unit 226 can deflect the beamlets 214, 216, and 218 to scan the probe spots 270, 272, and 274 across the surface area of ​​the wafer 230. In response to the beamlets 214, 216, and 218 impinging on the probe spots 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 can include electrons having an energy distribution, including secondary electrons and backscattered electrons. The secondary optics 242 can focus the secondary electron beams 236, 238, and 240 onto detection subregions 246, 248, and 250 of the electron detection device 244. The detection subregions 246, 248, and 250 can be configured to detect the corresponding secondary electron beams 236, 238, and 240 and generate corresponding signals used to reconstruct an image of the surface area of ​​the wafer 230.

[0041]

[0056] 2A shows an example of electron beam tool 100 as a multi-beam tool using multiple beamlets, embodiments of the present disclosure are not so limited. For example, electron beam tool 100 can be a single-beam tool that uses only one primary electron beam to scan locations on the wafer one at a time.

[0042]

[0057] As shown in FIG. 2B , electron beam tool 100B (also referred to herein as apparatus 100B) can be a single-beam inspection tool used in EBI system 10. Apparatus 100B includes a wafer holder 136 supported by a motorized stage 134 for holding a wafer 150 to be inspected. Electron beam tool 100B includes an electron emitter, which can include a cathode 103, an anode 121, and a gun aperture 122. Electron beam tool 100B further includes a beam-limiting aperture 125, a condenser lens 126, a column aperture 135, an objective lens assembly 132, and a detector 144. Objective lens assembly 132 can be a modified SORIL lens in some embodiments and includes a pole piece 132a, a control electrode 132b, a deflector 132c, and an excitation coil 132d. In the imaging process, an electron beam 161 emanating from the tip of the cathode 103 is accelerated by the anode 121 voltage, passes through the gun aperture 122, the beam limiting aperture 125, the condenser lens 126, and is focused by a modified SORIL lens to a probe spot 170, which can impinge on the surface of the wafer 150. A deflector, such as deflector 132c or other deflectors of a SORIL lens, can scan the probe spot 170 across the surface of the wafer 150. Secondary electrons or scattered primary particles, such as secondary electrons or scattered primary electrons emanating from the wafer surface, are collected by a detector 144 to determine the beam intensity, so that an image of the area of ​​interest on the wafer 150 can be reconstructed.

[0043]

[0058] An image processing system 199 may also be provided, including the image acquirer 120, the storage 130, and the controller 109. The image acquirer 120 may include one or more processors. For example, the image acquirer 120 may include a computer, a server, a mainframe host, a terminal, a personal computer, any type of mobile computing device, and the like, or a combination thereof. The image acquirer 120 may be connected to the detector 144 of the electron beam tool 100B through a medium such as an electrical conductor, a fiber optic cable, a portable storage medium, IR, Bluetooth, the Internet, a wireless network, wireless radio, or a combination thereof. The image acquirer 120 may receive signals from the detector 144 and construct an image. Thus, the image acquirer 120 may acquire an image of the wafer 150. The image acquirer 120 may also perform various post-processing functions, such as generating contours, overlaying indicators on the acquired image, and the like. The image acquirer 120 may be configured to perform adjustments, such as brightness and contrast, of the acquired image. The storage 130 may be a storage medium such as a hard disk, random access memory (RAM), cloud storage, other types of computer-readable memory, and the like. The storage 130 may be coupled to the image acquirer 120 and may be used to store scanned raw image data as original images and to store post-processed images. The image acquirer 120 and the storage 130 may be connected to the controller 109. In some embodiments, the image acquirer 120, the storage 130, and the controller 109 may be integrated together as one electronic control unit.

[0044]

[0059] In some embodiments, the image acquirer 120 can acquire one or more images of the sample based on an imaging signal received from the detector 144. The imaging signal can correspond to a scanning motion for performing charged particle imaging. The acquired image can be a single image including multiple imaging areas that can include various features of the wafer 150. The single image can be stored in the storage 130. The imaging can be performed based on imaging frames.

[0045]

[0060] The collector and illumination optics of an electron beam tool can include or be supplemented by electromagnetic quadrupole electron lenses. For example, as shown in FIG. 2B , electron beam tool 100B can 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, first quadrupole lens 148 can be controlled to adjust the beam current, and second quadrupole lens 158 can be controlled to adjust the beam spot size and beam shape.

[0046]

[0061] FIG. 2B illustrates a charged particle beam device. In a charged particle beam device, the inspection system can use a single primary beam that can be configured to generate secondary electrons by interacting with a wafer 150. As in the embodiment shown in FIG. 2B, the detector 144 can be positioned along the optical axis 105. The primary electron beam can be configured to travel along the optical axis 105. Therefore, the detector 144 can include a hole in its center to allow the primary electron beam to pass through and reach the wafer 150. However, some embodiments can use a detector that is positioned off-axis with respect to the optical axis along which the primary electron beam travels. For example, as in the embodiment shown in FIG. 2A, a beam separator 222 can be provided to direct the secondary electron beam toward the off-axis detector. The beam separator 222 can be configured to redirect the secondary electron beam by an angle α.

[0047]

[0062] A detector for detecting radiation may be used to detect electrons in the device 100 of the EBI system 10. The EBI system 10 may be a high-speed wafer imaging SEM including an image processor. The electron beam generated by the EBI system 10 may illuminate the surface of a sample or may pass through the sample. The EBI system 10 may be used to image the surface or subsurface structures of a sample, such as to analyze layer alignment. In some embodiments, the EBI system 10 may detect and report process defects associated with semiconductor wafer manufacturing, for example, by comparing an SEM image to a device layout pattern or to an SEM image of the same pattern elsewhere on the wafer being inspected. The detector may include a reverse-biased junction diode. The reverse-biased junction diode may be a PIN diode (e.g., a silicon PIN diode) operated with a negative (reverse) bias. In this case, the detector may be referred to as a PIN detector.

[0048]

[0063] Reference is now made to Figure 3A, which shows a schematic diagram of an exemplary structure of a detector 300. Detector 300 may be provided as detector 144 or electron detection device 244 in relation to Figures 2A and 2B. Although one array is shown in Figure 3A, it should be understood that detector 300 may include multiple arrays, such as one array for each secondary electron beam.

[0049]

[0064] Detector 300 may include an array of sensing elements, including sensing elements 311, 312, and 313. The sensing elements may be arranged in a planar, two-dimensional array, with the plane of the array being substantially perpendicular to the direction of incidence of incident radiation (e.g., charged particles or photons). In some embodiments, detector 300 may be arranged at an angle to the direction of incidence.

[0050]

[0065] The detector 300 may include a substrate 310. The substrate 310 may be a semiconductor substrate that may include sensing elements. The sensing elements may include diodes. The sensing elements may also be diode-like elements capable of converting incident energy into a measurable signal. The sensing elements may include, for example, PIN diodes, avalanche diodes, or the like, or a combination thereof. Areas 325 may be provided between adjacent sensing elements. Areas 325 may be separation areas that separate the sides or corners of adjacent sensing elements from each other. Areas 325 may include an insulating material that is different from the material of other areas of the detecting surface of the detector 300. Areas 325 may be provided as cross-shaped areas when viewed in the plan view of FIG. 3A . Areas 325 may be provided as squares. In some embodiments, areas 325 may not be provided between adjacent sides of sensing elements. For example, in some embodiments, there may be no separation areas provided on the detecting surface of the detector.

[0051]

[0066] Reference is now made to FIG. 3B , which shows a schematic diagram of a cross-sectional structure of a substrate 310, which may be an example of a structure included in a PIN detector. The substrate 310 may include one or more layers. For example, the substrate 310 may be configured to have multiple layers stacked in a thickness direction, which is substantially parallel to the incident direction of the electron beam. In some embodiments, the substrate 310 may have multiple layers stacked in a direction perpendicular to the incident direction of the electron beam. The substrate 310 may be provided with a sensor surface 301 for receiving incident radiation. Sensing elements (e.g., sensing elements 311, 312, and 313) may be provided in a sensing layer of the substrate 310. An area 325 may be provided between adjacent sensing elements. For example, the substrate 310 may include a trench or other structure made of or filled with an insulating material. In some embodiments, the area 325 may extend completely or partially through the substrate 310. In other words, the insulating material may extend from one surface to a certain depth.

[0052]

[0067] As shown in FIG. 3C , in some embodiments, area 325 is not provided between sensing elements. For example, there may be no insulating material provided between the sides of adjacent sensing elements in a cross-sectional view. Multiple sensing elements may be adjacent in a cross-sectional view. Separation between adjacent sensing elements may still be achieved by other means, such as by controlling an electric field. For example, an electric field may be controlled between each sensing element.

[0053]

[0068] Although the figures may show sensing elements 311, 312, and 313 as separate units, such division may not exist in practice. For example, the sensing elements of a detector may be formed by semiconductor devices that constitute a PIN diode device. A PIN diode device may be fabricated as a substrate having multiple layers, including p-type regions, intrinsic regions, and n-type regions. One or more of such layers may be adjacent in cross-section. However, in some embodiments, the sensing elements may be provided with a physical separation between them. For example, additional layers, such as a circuit layer and a readout layer, may be provided in addition to the sensor layer.

[0054]

[0069] As an example of additional layers, the detector 300 may include one or more circuit layers adjacent to the sensor layer. The one or more circuit layers may include wiring, interconnects, and various electronic circuit components. The one or more circuit layers may include a processing system. The one or more circuit layers may include signal processing circuitry. The one or more circuit layers may be configured to receive detected output charges or voltages from sensing elements in the sensor layer. The one or more circuit layers and the sensor layer may be provided on the same or separate dies, for example.

[0055]

[0070] 3D and 3E show schematic diagrams of exemplary individual sensing elements, each of which may be an example of sensing elements 311, 312, and 313. For example, in FIG. 3D, sensing element 311A ​​is shown. Sensing element 311A ​​may include a semiconductor structure of p-type layer 321, intrinsic layer 322, and n-type layer 323. Sensing element 311A ​​may include two terminals, such as an anode and a cathode. Sensing element 311A ​​may be reverse biased, and a depletion region may form and span a portion of the length of p-type layer 321, substantially the entire length of intrinsic layer 322, and a portion of the length of n-type layer 323. In the depletion region, charge carriers may be removed, and new charge carriers generated in the depletion region may be swept away according to their charge. For example, when an incident charged particle reaches the sensor surface 301, an electron-hole pair may be formed and the hole 351 may be attracted towards the p-type layer 321 while the electron 352 may be attracted towards the n-type layer 323. In some embodiments, a protective layer may be provided on the sensor surface 301.

[0056]

[0071] 3E, sensing element 311B may operate similarly to sensing element 311A, except for a changed orientation. For example, p-type layer 321 may include sensor surface 301. P-type layer 321 may be exposed to incident charged particles. Thus, the incident charged particles may interact with p-type layer 321 and the depletion region, generating electron-hole pairs. In some embodiments, a metal layer may be provided on p-type layer 321.

[0057]

[0072] The background noise of a detector can be caused, inter alia, by the dark current of the diode. For example, defects in the crystalline structure of a semiconductor device functioning as a diode can cause current fluctuations. The dark current of a detector can be due to defects in the material forming the detector and can occur even in the absence of incident illumination. "Dark" current can refer to the fact that the current fluctuations are not related to any incident charged particles.

[0058]

[0073] In some embodiments of the present disclosure, a detector such as a PIN detector can be used as an in-lens detector in a deceleration objective lens SEM column of the EBI system 10. The detector can be positioned between a cathode for generating an electron beam and the objective lens. The electron beam emitted from the cathode can be given a potential of −BE keV (typically about −10 kV). Electrons in the electron beam can be immediately accelerated and travel through the column. The column can be at ground potential. Thus, electrons can travel with a kinetic energy of BE keV while passing through the detector aperture. Electrons passing through an objective lens pole piece, such as pole piece 132a of the objective lens assembly 132 in FIG. 2B, can be rapidly decelerated to a landing energy of LE keV because the wafer surface potential can be set to −(BE-LE) keV.

[0059]

[0074] Secondary electrons emitted from the wafer surface by the impact of electrons from the primary electron beam can be accelerated by an acceleration field (e.g., a deceleration field near the wafer can act as an acceleration field for the secondary electrons) and travel backward toward the detector surface. For example, as shown in FIG. 4A, secondary electrons returning toward the detector 144 can be generated due to interaction with the wafer 150 at the probe spot 170. Secondary electrons emitted from the wafer surface and traveling along the optical axis 105 can arrive at the surface of the detector 144 at dispersed locations. The landing locations of the secondary electrons can be, for example, within a roughly circular area with a radius of several millimeters. The geometric spread of the landing locations of the secondary electrons can be due to the electrons having different trajectories, which can depend, for example, on the initial kinetic energy and emission angle of the electrons.

[0060]

[0075] FIG. 4B shows an example of a secondary electron landing site distribution on the detector surface. Electrons 300a may land at different points on the surface of the detector 144, but generally, most may be clustered around the central portion of the detector 144. The landing site distribution may shift depending on the secondary emission location and the SEM deflection field (e.g., scanning field). Therefore, in some applications, if a specific field of view (FOV) of the SEM image is required, the required size of the in-lens PIN detector may be substantially large. Typically, the detector may be, for example, 10 mm or larger in diameter. In some embodiments, the detector may be approximately 4-10 mm in diameter.

[0061]

[0076] The sensing element may be configured to generate a large number of electron-hole pairs in response to an electron arrival event. In some embodiments, the charge or voltage generated in response to an electron arrival event at the sensing element may be usable as a detection signal. The output of the sensing element in response to an electron arrival event may be used as is or may be amplified. The amplifier may apply amplification indiscriminately to all signals generated by the diode. Thus, even so-called "dark counts" may be amplified and contribute to an erroneous detection signal.

[0062]

[0077] In an exemplary PIN detector, holes can be excited in the depletion region of the intrinsic region of the PIN detector and drift toward the anode due to the electric field generated by the reverse bias in the PIN detector. The holes can then be collected at the anode. Electrons generated in the depletion region can drift in the opposite direction to the holes. Thus, the electrons can be collected at the cathode. The cathode can be grounded. Holes and electrons generated in the depletion region can recombine with opposite charges within the PIN detector. The recombination rate can be high outside the depletion region. The depletion region can include a portion of the P+ region, which can function as the anode due to the reverse bias. Recombination of holes or electrons on the side of the P+ region where the incident electrons enter the detector can contribute to energy loss and not contribute to the detector signal at the anode terminal. Therefore, for example, to reduce energy loss, it may be desirable to configure the electrode on the side where the incident electrons enter the detector thin. For example, in a PIN detector, it may be desirable to configure the thickness of the P+ layer as thin as possible.

[0063]

[0078] The reverse bias applied to the PIN detector may involve application of a voltage. The diode may be configured to operate at a reverse voltage below a certain amount. In some embodiments, the certain amount may be 100 volts. The diode may be operated within a linear region.

[0064]

[0079] In some embodiments, both secondary electrons and backscattered electrons can reach the detector. For example, in a comparative example, approximately 20-30% of the incident electrons on the detector may be backscattered electrons, which have energies approximately equal to the energy of the electrons in the primary beam (e.g., BE). Backscattered electrons may be the same electrons contained in the primary beam generated by the electron source, but which reflect back from the sample without losing a significant amount of energy.

[0065]

[0080] Furthermore, some electrons that are not backscattered may lose their kinetic energy by causing lattice atoms in the detector (e.g., Si atoms in a silicon substrate) to emit their characteristic X-ray photons. Other excitations, such as phonons, may also be generated. Thus, the number of charges generated by a single incident electron with a fixed kinetic energy may vary; that is, the electron gain (e.g., the number of charges collected at the diode terminals per incident electron) may vary from incident electron to incident electron.

[0066]

[0081] The charge collected at the terminals of the detector may form a voltage signal that may follow the modulation of the incident electron rate as the electron beam is scanned across the wafer surface.

[0067]

[0082] A MEMS-based miniature SEM array is essentially an array of miniature SEMs, where miniature refers to the size of each element in the array compared to a conventional SEM. In a conventional SEM, the detectors may be several millimeters in diameter. However, in a MEMS-based miniature SEM, the detectors may typically have diameters of less than 1 mm, or even less than 100 micrometers. Figure 6 shows a schematic diagram of an array 1200 of detectors 144 that may be used in a MEMS-based miniature SEM.

[0068]

[0083] 5 shows a schematic representation of a MEMS-based miniature array element 1100, which includes two annular electrodes 1110 and 1120. A voltage can be applied between the two annular electrodes 1110 and 1120, and between the lower annular electrode 1120 and a substrate W to be inspected using the miniature SEM. The electrodes 1110 and 1120 and the voltage applied between them are configured to direct an incident electron beam through holes 1111 in the electrode 1110 towards the substrate W, and to direct backscattered or secondary electrons from the substrate W to a detector 144 located at the bottom of the electrode 1110.

[0069]

[0084] As mentioned in the introduction, it may be desirable to increase the intensity of the detector output when the flux of the target radiation is relatively small, such as when the electron beam current is low, particularly in association with an inspection system. One approach to achieving this is to increase the active internal gain of the detector based on avalanche, but this can result in drawbacks such as the need for recovery time after each avalanche and / or a low fill factor. The embodiments of the present disclosure, particularly those described below, aim to increase the intensity of the detector output by increasing the passive internal gain of the detector. This is achieved by using a low bandgap material for the detector's sensor substrate. Examples and further details about low bandgap materials are provided below.

[0070]

[0085] Although this disclosure primarily refers to the detection of charged particles such as electrons, it should be understood that embodiments may be configured and / or used to detect other types of radiation, including uncharged particles such as photons.

[0071]

[0086] The internal gain G0 of a solid state detector can be defined by the following equation:

number

[0072]

[0087] FIG. 7 illustrates an example of a detector based at least in part on the above insights. The detector is for detecting radiation. The detector includes a sensing element. The detector may include a single sensing element or an array of sensing elements. The sensing element may be positioned and / or configured to operate in any of the manners described above with reference to FIGS. 3A-3E. The detector may be configured to operate as detector 144 or electronic sensing device 244 described above with reference to FIGS. 2A and 2B.

[0073]

[0088] 7 shows a single sensing element. The sensing element includes a semiconductor sensor substrate 402. The sensing element is configured such that impact of target radiation 405 on the sensor substrate 402 generates charge carriers (e.g., electrons and holes) in the sensor substrate 402. The sensor substrate 402 of the sensing element may be part of a larger substrate, such as the substrate 310 described above with reference to FIGS. 3A-3E. If an array of sensing elements is provided, some or all of the sensing elements in the array may be formed on a common substrate. In this case, the sensor substrates 402 may correspond to different respective portions of the common substrate.

[0074]

[0089] The target radiation 405 may include charged particles, such as electrons. Alternatively or additionally, the target radiation 405 may include photons. The detector includes a readout circuit 404 configured to provide an output in response to charge carriers generated in the sensor substrate 402. The readout circuit 404 may be connected to the sensor substrate 402 via electrical connections 406. The electrical connections may contact the sensor substrate 402 on opposite sides of the sensor substrate 402. The readout circuit 404 may be configured to apply a potential difference across the sensor substrate 402 via the electrical connections 406. The readout circuit 404 may be configured to measure a current associated with the charge carriers. The charge carriers may be driven to form a current by the applied potential difference. The readout circuit 406 may integrate the current over time to measure a total charge associated with the charge carriers generated by the impact of the target radiation. The sensor substrate 402 includes a low bandgap portion made of a low bandgap material. The sensor substrate 402 includes a high bandgap portion made of a high bandgap material. A low bandgap material has a bandgap lower than that of a high bandgap material. A low bandgap material may have a bandgap lower than that of silicon, for example. A high bandgap material may have a bandgap equal to or greater than that of silicon.

[0075]

[0090] In one embodiment, the readout circuit 404 is configured to operate the sensing element as a reverse-biased diode having a depletion region 410 in the sensor substrate 402. The reverse-biased diode may include a junction diode, such as a PN junction diode. The junction diode may be doped to define a p-type region adjacent to and / or in electrical contact with the electrical connection 406 on one side of the sensor substrate 402 and an n-type region adjacent to and / or in electrical contact with the electrical connection 406 on the other side of the sensor substrate 402. The junction diode may be a PIN diode having a large undoped intrinsic semiconductor region between the n-type and p-type regions. A reverse bias is applied by the electrical connection 406, forming the depletion region 410. Charge carriers generated by impinging radiation are generated in the depletion region 410 and form a current due to the reverse bias.

[0076]

[0091] The depletion region 410 defines the junction capacitance of the sensing element. The junction capacitance can contribute noise to the input of the readout circuit 404. The junction capacitance can adversely affect bandwidth and power consumption. Reducing the junction capacitance is desirable. The junction capacitance can be reduced by increasing the width of the depletion region 410 (i.e., increasing the vertical height of the depletion region 410 in the orientation shown in FIG. 7). This is similar to increasing the separation between the plates of a parallel-plate capacitor. The width of the depletion region 410 can be set, for example, to 100 microns or more. However, absorption of the target radiation 405 typically occurs over a much smaller depth, typically several microns for electrons as the target radiation.

[0077]

[0092] In one embodiment, the sensor substrate 402 is made significantly thicker than necessary to absorb the target radiation 405. By thickening the sensor substrate 402, junction capacitance can be reduced. In this type of embodiment, the target radiation may interact with only a small portion of the sensor substrate 402. The width of the depletion region 410 in the depth direction can be significantly greater than the average penetration depth of the target radiation 405. Any increase in dark current caused by the presence of low bandgap material can be proportional to the amount of low bandgap material present. In some embodiments, dark current is reduced by configuring the sensor substrate 402 to include both a low bandgap portion (illustrated as low bandgap layer 408 in FIG. 7 , as described below) and a high bandgap portion 409. The high bandgap portion 409 is made of a high bandgap material having a higher bandgap than the low bandgap material. In embodiments where the low bandgap material has a bandgap lower than that of silicon, the high bandgap material can therefore have a bandgap equal to or greater than that of silicon. The high bandgap material can include, consist essentially of, or consist of silicon. A high bandgap material can include, consist essentially of, or consist of a material that has a higher bandgap than silicon, such as a Group 3-5 material (eg, GaN) or SiC.

[0078]

[0093] In one embodiment, the high bandgap portion 409 forms a majority (e.g., by volume) of the depletion region 410. This may be appropriate, for example, when the target radiation is absorbed in a volume representing less than half of the depletion region 410. Most or all of the portion of the sensor substrate 402 that exceeds the penetration depth of the target radiation may, for example, form all or part of the high bandgap portion 409. By providing a higher bandgap material (e.g., silicon) in areas of the sensor substrate 402 that do not interact with the target radiation, dark current can be reduced without adversely affecting charge carrier generation. In one embodiment, as illustrated in FIG. 7 , the low bandgap portion may include a low bandgap layer 408. The low bandgap layer 408 may be substantially planar and perpendicular to the width of the depletion region 410 in the direction of reverse bias (i.e., parallel to the vertical dimension of the depletion region 410 in the orientation shown in FIG. 7 ). The low bandgap layer 408 may be substantially parallel to a surface 407 of the sensing element configured to receive the target radiation 405. In one embodiment, the thickness of the low bandgap layer 408 is less than 25%, optionally less than 10%, optionally less than 5% of the thickness of the sensor substrate 402 and / or less than the width of the depletion region 410 in the reverse bias direction.

[0079]

[0094] In one embodiment, as illustrated in FIG. 7 , at least a portion of the low band gap layer 408 is disposed at the surface 407 of the sensing element configured to receive the target radiation, or is closer to the surface 407 of the sensing element configured to receive the target radiation than the depletion region 410. Thus, the low band gap layer 408 can be at or near the surface 407. Disposing the low band gap layer 408 at or near the surface 407 may provide better performance for a range of target radiation energy distributions because at least a portion of the target radiation is absorbed relatively close to the surface 407. In one embodiment, the low band gap layer 408 provides the surface 407 (i.e., the low band gap layer is exposed to the environment outside the sensing element and / or is the first solid element encountered by the target radiation). In one embodiment, the low band gap layer 408 is disposed below the surface 407, optionally a passivation layer (not shown) that provides the surface 407 in direct contact with the passivation layer. The depletion region 410 overlaps the low band gap layer 408. The portion of depletion region 410 closest to surface 407 is within low bandgap layer 408 , and the portion of depletion region 410 away from surface 407 is outside low bandgap layer 408 .

[0080]

[0095] In one embodiment, the thickness of the low bandgap layer 408 is selected depending on the target radiation. The low bandgap layer 408 can be made thicker for higher energy target radiation and thinner for lower energy target radiation. This approach can avoid or minimize the presence of low bandgap material in areas not encountered by the target radiation, thereby keeping dark current low.

[0081]

[0096] FIG. 8 schematically illustrates the expected characteristic shapes of interaction volumes 414, 416 between target radiation 405 and sensor substrate 402. The two different volumes 414 and 416 correspond to different acceleration voltages for the incident radiation consisting of charged particles. The different acceleration voltages provide incident charged particles of different respective energies. Interaction volume 416 corresponds to a higher acceleration voltage than volume 414. The interaction volumes in both cases have a spherical shape (e.g., with a cross-sectional area greater than the surface 407 at a range of depths away from the surface 407). The interaction volume has a maximum cross-sectional area (in a direction parallel to the surface 407) at a finite depth within the sensor substrate 402. In some embodiments, the thickness and / or position (e.g., depth) of the low band gap layer 408 are selected to take into account the shape of the interaction volume. For example, as illustrated in FIG. 10 , the low band gap layer 408 may be disposed at a depth within the sensor substrate 402 such that a portion 411 of the depletion region 410 is between the low band gap layer 408 and a surface 407 of the sensing element configured to receive the target radiation. In the example shown in FIG. 10 , the depletion region 410 is the region between the horizontal dashed lines. The portion 411 may be disposed within the high band gap region 409. The depth of the low band gap layer 408 may be selected to take into account that the cross-sectional area of ​​the interaction volume is greatest at a finite depth. In one embodiment, the low band gap layer 408 is disposed such that at least a portion of the low band gap layer 408 is present at a depth where the cross-sectional area of ​​the interaction volume is greatest. Thus, the thickness and / or location of the low band gap layer 408 may be selected according to the energy distribution or average energy of the target radiation.

[0082]

[0097] The penetration depth of the target radiation into the sensor substrate 402 may depend on the mass density of the portion of the sensor substrate 402 encountered by the target radiation. For example, the penetration depth R of electrons into a solid of uniform composition may depend on the energy E of the electrons and the mass density ρ of the solid according to the following equation:

number

[0083]

[0098] The above problem may be mitigated by providing a low bandgap layer at some depth within the sensor substrate 402, which encourages electrons to be generated further away from the surface 407. Alternatively or additionally, the above problem may be mitigated by selecting a low bandgap material with a relatively low mass density, such that the target radiation penetrates deeper (i.e., the interaction volume looks more like FIG. 8 than like FIG. 9). In some embodiments, the low bandgap material comprises a material with a mass density lower than that of silicon. Silicon has a mass density of 2.329 g / cm 3 Therefore, in such an embodiment, the low bandgap material has a 3 In one embodiment, the low bandgap material may comprise, consist essentially of, or consist of a material having a mass density lower than 1.99 g / cm. 3The material may comprise, consist essentially of, or consist of magnesium silicide, Mg2Si, having a mass density of

[0084]

[0099] The low bandgap material may additionally or alternatively comprise, consist essentially of, or consist of various other semiconductor materials known to have bandgaps smaller than that of silicon. Such semiconductor materials include mercury cadmium telluride (Hg), in any combination. 1-x CD x Te), mercury zinc telluride (Hg 1-x Zn x The compounds may contain one or more of: lead(Te), lead selenide (PbSe), lead telluride (PbTe), indium arsenide (InAs), indium antimonide (InSb), gallium antimonide (GaSb), cadmium arsenide (Cd3As2), bismuth telluride (Be2Te3), tin telluride (SnTe), tin selenide (SnSe), silver(I) selenide (Ag2Se), and germanium (Ge).

[0085]

[0100] In one embodiment, the low band gap layer 408 has a uniform composition. Forming the low band gap layer 408 with a uniform composition may facilitate manufacturing of the low band gap layer 408. However, any differences between the lattice structure (e.g., symmetry and / or one or more lattice constants) of the material of the low band gap layer 408 and the lattice structure of the material of the sensor substrate 402 (e.g., silicon) outside and in contact with the low band gap layer 408 may contribute defects to the crystalline structure of the sensor substrate 402. For example, the low band gap layer 408 may be epitaxially grown on silicon. During this process, the material of the low band gap layer 408 may attempt to adopt the crystalline structure of silicon as much as possible, but differences in crystalline structure and / or lattice constants result in periodic dislocations of atoms at the interface between the low band gap layer 408 and the silicon. The dislocations may propagate throughout much of the low band gap layer 408. Dislocations are defects that can act as recombination centers and reduce internal gain.

[0086]

[0101] In some embodiments, this effect is reduced by selecting the low bandgap material to include, consist essentially of, or consist of the material of the sensor substrate 402 (e.g., silicon) outside the low bandgap portion (e.g., having a lattice constant within 10%, optionally within 7.5%, optionally within 5% of the lattice constant of the material of the sensor substrate 402 that is in contact with the low bandgap portion and / or outside the low bandgap material and in contact with the low bandgap material), e.g., a material having the same crystal structure as the high bandgap material. Reducing the mismatch between the lattice structures of the low bandgap material and the surrounding material can reduce defects such as dislocations. In one embodiment, the low bandgap material includes, consists essentially of, or consists of a material having the same crystal structure as silicon and / or having a lattice constant within 10%, optionally within 7.5%, optionally within 5% of the lattice constant of silicon. In one embodiment, the low bandgap material includes germanium, and the material of the sensor substrate 402 outside the low bandgap portion includes, consists essentially of, or consists of silicon. Silicon and germanium have the same crystal structure and very similar lattice constants (a = 0.5657 nm for Ge and a = 0.543 nm for Si). Growing germanium adjacent to or within silicon produces relatively low defects due to the similarity of their respective lattices.

[0087]

[0102] In one embodiment, the low bandgap layer 408 comprises a mixture of chemical elements having different bandgaps, at least one of which has a bandgap smaller than the bandgap of the high bandgap material (e.g., silicon). x B 1-x where A and B represent elements with different bandgaps. x Ge 1-x It can consist of Si x Ge 1-xMixtures such as these can be fabricated efficiently and in a highly controlled manner using well-known deposition techniques, such as epitaxial growth. Epitaxial growth can be performed in a variety of ways, including using, for example, chemical vapor deposition (CVD) or physical vapor deposition (PVD) in high- or low-vacuum environments. The process parameters of the epitaxial growth can be controlled to precisely and flexibly define the composition of the mixture (e.g., the relative proportions of the mixture's components, such as silicon and germanium). The composition of the mixture can be uniform throughout the thickness of the low-bandgap layer 408 or can vary as a function of position in the thickness direction. Thus, the average bandgap of the materials in the low-bandgap layer 408 can be tailored to a desired value to achieve, for example, a desirable balance between increased passive internal gain (signal strength) and an acceptable level of dark current. Forming a mixture with components having the same or similar crystal structure and / or the same or similar lattice constants can reduce defects and associated recombination centers, thereby enhancing internal gain.

[0088]

[0103] In one embodiment, the low band gap layer 408 has a non-uniform composition. This can be achieved by the low band gap layer 408 having a composition that varies as a function of position through the thickness of the low band gap layer 408. In one embodiment, the change in composition is such that the composition converges toward the composition of the material outside the low band gap layer 408 that is in contact with the low band gap layer 408 on one or both sides of the low band gap layer 408. For example, the proportion of high band gap material (e.g., silicon) in the composition can gradually decrease as a function of position through the thickness from one or both sides of the low band gap layer 408 toward the center of the low band gap layer 408. Thus, the proportion of high band gap material (e.g., silicon) can gradually increase toward one or both sides of the low band gap layer 408. Such a gradual convergence in composition can reduce the lattice mismatch at one or both interfaces between the low band gap layer 408 and the surrounding material, thereby reducing the amount of lattice defects, such as dislocations, which can reduce recombination events and improve internal gain.

[0089]

[0104] For example, germanium has a direct bandgap about 4.5 times smaller than that of silicon, which in principle could lead to an internal gain of 4.5 times. SiGe compounds are already widely used in the semiconductor industry to fabricate high frequency devices due to the relatively high mobility of electrons in germanium. It has been established that SiGe compounds can be routinely produced using any composition of Si and Ge. That is, for any value of x, Si x Ge 1-x In such SiGe compounds, the lattice constant varies approximately linearly as a function of the fraction of silicon x between the lattice constant of Si at x=1 and the lattice constant of Ge at x=0. Based on the above, in one embodiment, the low bandgap layer 408 is a SiGe layer embedded in silicon. x Ge 1-xThe composition of the low band gap layer varies, optionally monotonically, optionally linearly, through the thickness of the low band gap layer, e.g., in a central region of the low band gap layer in the thickness direction, from x≧0.8 near one or both of the two interfaces, respectively, to a lower value of x elsewhere in the low band gap layer. The lower value of x can be x≦0.2, optionally x≦0.1, or even x=0.

[0090]

[0105] It has been shown that the bandgap of a thin layer of material can vary depending on the thickness of the layer. For example, it has been shown that a thickness on the order of about 50 nm or less can reduce the bandgap of Ge to about 0.4 eV, which is twice the direct bandgap of a thick layer of Ge. Based on this insight, in some embodiments, the thickness of the low bandgap layer 408 is selected to be small enough to lower the bandgap of the material of the low bandgap layer 408 in this manner. As discussed above, lowering the bandgap can increase passive internal gain. In some configurations, it may be desirable to provide a greater total thickness of low bandgap material to achieve a desired increase in passive internal gain. For example, the desired total thickness of the low bandgap material may be such that, if the total thickness is embodied in a single layer of low bandgap material, the single layer has substantially the same bandgap as observed in a bulk sample of the material (i.e., not a thin layer). In such a scenario, multiple low bandgap layers 408A-C may be provided, as illustrated in FIG. 11. The low bandgap layers 408A-C are spaced apart from one another. Each of the plurality of low band gap layers 408A-C may have a thickness small enough to lower the band gap of the material of the low band gap layers 408A-C. The low band gap material of each layer may have a band gap lower than the band gap of a bulk sample (or thicker layer) of the material, optionally at least 10% lower, optionally at least 25% lower, or optionally at least 50% lower. In the illustrated embodiment, the plurality of low band gap layers 408A-C are configured to detect the same target radiation (e.g., particles having the same or similar energy). Thus, the plurality of low band gap layers 408A-C may be positioned relatively closely together. For example, the average spacing between the low and high gain layers is less than the average thickness of the low band gap layers 408A-C.

[0091]

[0106] In one embodiment, as illustrated in FIG. 12 , at least a subset of the low bandgap layers 408, 418, 428 are configured to generate charge carriers from target radiation having primarily different respective energy distributions (e.g., different respective average energies). Accordingly, each of the layers 408, 418, 428 of the subset may be disposed at significantly different depths within the sensor substrate 402. The depths of the layers 408, 418, 428 correspond to different expected penetration depths of the different energy distributions or average energies. For example, in the example of FIG. 12 , layer 408 may be disposed at a relatively shallow depth corresponding to the maximum width of the expected interaction volume for target radiation having a relatively low energy. Layer 418 may be disposed at an intermediate depth to target target radiation of intermediate energy. Layer 428 may be disposed at a deepest depth to target target radiation of higher energy. The low bandgap layers 408, 418, 428 of at least a subset may be spaced apart by a distance greater than the average thickness of the low bandgap layers 408, 418, 428. Spacing the layers in this manner increases operational flexibility and efficiency by allowing the detector to have high passive internal gain for a range of different target radiation (e.g., energy distributions and / or average energies) that may have significantly different characteristic interaction volumes within the sensor substrate 402.

[0092]

[0107] In one embodiment, the low band gap layers are provided in multiple groups of closely spaced low band gap layers, as illustrated in FIG. 13 . The example of FIG. 13 shows three groups. The shallowest group includes three layers labeled 408A-C. The intermediate group includes three layers labeled 418A-C. The deepest group includes three layers labeled 428A-C. These groups are configured to generate charge carriers primarily from target radiation having different respective energy distributions or different respective average energies. The groups of closely spaced low band gap layers may be spaced apart by a distance greater than the average sum of the layer thicknesses in each group. Thus, the spacing between each group may be greater than the average sum of the layer thicknesses within each group. Providing the low band gap layers in groups in this manner combines the advantages of the embodiments of FIGS. 11 and 12 . Each group is configured to work particularly well for different energy distributions or average energies of the target radiation, while providing multiple layers in each group allows the layers to be thinner, thereby reducing their band gap.

[0093]

[0108] Aspects of the present disclosure described above, particularly with reference to FIGS. 7-13 , may be embodied in a method for fabricating a sensing element. The method may be configured to fabricate a sensing element in any of the forms described above. As described above, the sensing element includes a semiconductor sensor substrate 402 configured to generate charge carriers upon impact of target radiation on the sensor substrate 402. The sensor substrate 402 includes a low bandgap layer 408 that forms a portion of the sensor substrate 402 and has a bandgap lower than the bandgap of a high bandgap material (e.g., silicon) that forms another portion of the sensor substrate. In one embodiment, the method for fabricating a sensing element includes acquiring information regarding the energy distribution of the target radiation. The information regarding the energy distribution may include an expected average energy of the target radiation. The method includes using the acquired information to select a thickness and / or location of the low bandgap layer 408 in the sensor substrate 402. The method includes forming the sensor substrate 402 having the low bandgap layer 408 according to the selected thickness and / or location. For example, the sensor substrate 402 may be formed by an epitaxial growth process. Processing parameters of the epitaxial growth process can be controlled during the growth process to define a composition profile of the sensor substrate 402 along its thickness. The composition profile can be controlled to form the low band gap layer 408 at a selected location and / or thickness. Alternatively or additionally, the method can include forming a plurality of candidate sensor substrates having different configurations of the low band gap layer. The different configurations can include either or both different thicknesses and different locations of the low band gap layer in the sensor substrate. In such an embodiment, the method can include measuring the internal gain of the candidate sensor substrates when exposed to the target radiation and selecting the configuration corresponding to the largest measured internal gain as the final configuration of the low band gap layer. The method can then include fabricating the sensor element by forming a sensor substrate having the low band gap layer according to the selected final configuration of the low band gap layer.Any of the methods described above may be extended to form any of the configurations described above, particularly with reference to Figures 7-13, including forming multiple low band gap layers 408, closely spaced and / or widely spaced low band gap layers, one or more low band gap layers of uniform composition and / or one or more low band gap layers of non-uniform composition.

[0094]

[0109] The following numbered clauses further define embodiments of the present disclosure: Clause 1: A detector for detecting radiation, comprising: a sensing element including a semiconductor sensor substrate, the sensing element being configured such that impact of target radiation on the sensor substrate generates charge carriers in the sensor substrate; a readout circuit configured to provide an output in response to charge carriers generated in the sensor substrate; and wherein the sensor substrate includes a low bandgap portion made of a low bandgap material and a high bandgap portion made of a high bandgap material, the low bandgap material having a bandgap lower than that of the high bandgap material. Clause 2: The detector of clause 1, wherein the low bandgap material has a bandgap lower than the bandgap of silicon. Clause 3: The detector of clause 1 or 2, wherein the high bandgap material has a bandgap equal to or greater than the bandgap of silicon. Clause 4: A detector described in any one of the preceding clauses, wherein the readout circuit is configured to operate the sensing element as a reverse-biased diode having a depletion region in the sensor substrate, and charge carriers are generated in the depletion region in the sensor substrate. Clause 5: The detector of clause 4, wherein the high bandgap portion forms a majority of the depletion region. Clause 6: The detector of clause 4 or 5, wherein the low bandgap portion comprises a low bandgap layer. Clause 7: A detector as described in clause 6, wherein the low band gap layer is substantially planar and perpendicular to the width of the depletion region in the direction of reverse bias and / or to the surface of the sensing element configured to receive target radiation. Clause 8: The detector of clause 7, wherein the thickness of the low bandgap layer is less than 25% of the thickness of the sensor substrate. Clause 9: A detector described in clause 7 or 8, wherein at least a portion of the low band gap layer is positioned on a surface of the sensing element configured to receive the target radiation or is closer to the surface of the sensing element configured to receive the target radiation than the depletion region. Clause 10: A detector as described in clause 7 or 8, wherein the low band gap layer is provided at a depth within the sensor substrate such that a portion of the depletion region is between the low band gap layer and a surface of the sensing element configured to receive target radiation. Clause 11: A detector according to clause 10, wherein the thickness and / or position of the low band gap layer is selected according to the energy distribution of the target radiation, optionally the average energy of the target radiation. Clause 12: The detector of any one of clauses 6 to 11, wherein the low band gap layer has a uniform composition. Clause 13: The detector of any one of clauses 6 to 11, wherein the low bandgap layer has a non-uniform composition. Clause 14: The detector of clause 13, wherein the low band gap layer has a composition that varies as a function of position across a thickness of the low band gap layer. Clause 15: A detector as described in clause 14, wherein the change in composition is such that the composition converges towards the composition of a material outside the low band gap layer and in contact with the low band gap layer on one or both sides of the low band gap layer. Clause 16: The detector of clause 15, wherein the proportion of high band gap material in the composition gradually decreases as a function of position in the thickness direction from one or both sides of the low band gap layer toward the center of the low band gap layer. Clause 17: The low band gap layer comprises a mixture of chemical elements having different band gaps, at least one of the chemical elements having a band gap smaller than the band gap of the high band gap material, and the mixture optionally comprises A x B 1-x 17. The detector of any one of clauses 6 to 16, comprising: A and B, each of which represents an element having a different band gap. Clause 18: The high bandgap material includes silicon, and the mixture includes Si x Ge 1-x 18. A detector according to clause 17, comprising: Clause 19: The detector of any one of clauses 6 to 18, wherein the low bandgap portion includes a plurality of low bandgap layers. Clause 20: The detector of clause 19, wherein the plurality of low bandgap layers are spaced apart from one another. Clause 21: A detector according to clause 19 or 20, wherein each of the low band gap layers has a thickness sufficiently small to lower the band gap of the material of the low band gap layer. Clause 22: The detector of clause 21, wherein an average spacing between the plurality of low band gap layers is less than an average thickness of the low band gap layers. Clause 23: At least a subset of the low bandgap layers are configured to generate charge carriers primarily from target radiation having different respective energy distributions or different respective mean energies; or 23. A detector as described in any one of clauses 19 to 22, wherein the low band gap layers are provided in a plurality of groups of closely spaced low band gap layers, the groups being configured to generate charge carriers primarily from target radiation having different respective energy distributions or different respective mean energies. Clause 24: the low band gap layers in at least a subset are spaced apart by more than the average thickness of those low band gap layers; or 24. The detector of clause 23, wherein the group of closely spaced low bandgap layers are spaced apart by more than the average sum of the thicknesses of the layers in each group. Clause 25: A detector according to any one of the preceding clauses, wherein the high bandgap material comprises silicon. Clause 26: A detector according to any one of the preceding clauses, wherein the low bandgap material comprises a material having a lower mass density than the high bandgap material. Clause 27: The detector of clause 26, wherein the low bandgap material comprises Mg2Si. Clause 28: A detector described in any one of the preceding clauses, wherein the low band gap material comprises a material having the same crystal structure as the high band gap material and / or having a lattice constant within 10% of the lattice constant of the high band gap material. Clause 29: The detector of clause 28, wherein the high bandgap material comprises silicon and the low bandgap material comprises germanium. Clause 30: A method for manufacturing a sensing element for a detector for detecting radiation, comprising: The sensing element includes a semiconductor sensor substrate configured to generate charge carriers upon impact of target radiation on the sensor substrate, the sensor substrate including a low bandgap layer forming a portion of the sensor substrate and having a bandgap lower than the bandgap of a high bandgap material forming another portion of the sensor substrate, and the method includes: forming a plurality of candidate sensor substrates having different configurations of the low band gap layer, the different configurations including one or both of different thicknesses and different locations of the low band gap layer in the sensor substrate; measuring the internal gain of the candidate sensor substrate when exposed to the target radiation; selecting as the final configuration of the low bandgap layer the configuration that corresponds to the maximum measured internal gain; fabricating a sensor element by forming a sensor substrate having a low bandgap layer according to a selected final configuration of the low bandgap layer; A method comprising:

[0095]

[0110] It is to be understood that the present invention is not limited to the exact construction described above and illustrated in the accompanying drawings, and that various modifications and changes may be made without departing from the scope of the present disclosure.

[0096]

[0111] It should be understood that elements shown in different figures may be combined.

[0097]

[0112] Furthermore, although scanning electron microscopy has been discussed in connection with some embodiments, other types of systems are equally applicable, for example, the detector may be used in a transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), or structured illumination microscopy (SIM) system.

Claims

1. A detector for detecting radiation, comprising: a sensing element including a semiconductor sensor substrate, the sensing element configured such that impact of target radiation on the sensor substrate generates charge carriers in the sensor substrate; a readout circuit that provides an output in response to charge carriers generated on the sensor substrate; the sensor substrate includes a low bandgap portion made of a low bandgap material and a high bandgap portion made of a high bandgap material, the low bandgap material having a lower bandgap than the high bandgap material, and a thickness and / or position of the low bandgap portion selected according to an energy distribution of the target radiation.

2. 2. The detector of claim 1, wherein the low bandgap material has a bandgap lower than that of silicon, and the high bandgap material has a bandgap equal to or greater than that of silicon.

3. 3. A detector according to claim 1 or 2, wherein the readout circuitry operates the sensing element as a reverse biased diode having a depletion region in the sensor substrate, the charge carriers being generated in the depletion region in the sensor substrate.

4. The detector of claim 3 , wherein the high bandgap portion forms a majority of the depletion region.

5. 5. The detector of claim 3 or 4, wherein the low bandgap portion comprises a low bandgap layer.

6. 6. The detector of claim 5, wherein the low bandgap layer is substantially planar and perpendicular to a width of the depletion region in the direction of reverse bias and / or to a surface of the sensing element that receives the target radiation.

7. 7. The detector of claim 6, wherein the low bandgap layer is disposed at a depth within the sensor substrate such that a portion of the depletion region is between the low bandgap layer and a surface of the sensing element that receives the target radiation.

8. 8. The detector of claim 7, wherein the thickness and / or location of the low bandgap layer is selected according to the average energy distribution of the target radiation.

9. 9. A detector according to any one of claims 5 to 8, wherein the low bandgap layer has a non-uniform composition that varies as a function of position across the thickness of the low bandgap layer.

10. 10. The detector of claim 9, wherein the change in composition is such that the composition converges toward a composition of a material outside the low band gap layer and in contact with the low band gap layer on one or both sides of the low band gap layer.

11. 11. The detector of claim 10, wherein the proportion of the high band gap material in the composition gradually decreases as a function of position in the thickness direction from one or both sides of the low band gap layer toward a center of the low band gap layer.

12. The low bandgap layer comprises a mixture of chemical elements having different bandgaps, at least one of the chemical elements having a bandgap smaller than the bandgap of the high bandgap material, the mixture optionally comprising A ｘ B １－ｘ 12. A detector according to any one of claims 5 to 11, consisting of: A and B, where A and B represent elements with different bandgaps.

13. The high bandgap material includes silicon, and the mixture includes Si ｘ Ge １－ｘ 13. The detector of claim 12, comprising:

14. The detector of any one of claims 5 to 13, wherein the low band gap portion includes a plurality of the low band gap layers, the plurality of low band gap layers being spaced apart from one another.

15. at least a subset of the low bandgap layers generate charge carriers primarily from target radiation having different respective energy distributions or different respective mean energies; or 15. The detector of claim 14, wherein the low band gap layers are provided in multiple groups of closely spaced low band gap layers, the groups generating charge carriers from target radiation having primarily different respective energy distributions or different respective mean energies.

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