Time-gated detection, dual-layer SPAD-based electron detection

Abstract

Time-gated detection, dual-layer SPAD-based electron detection. SPAD elements are used to measure electron beam modulation in response to optical pump pulses applied to the sample. Individual detection events are used to form a histogram of the number of events in a time bin associated with the pump pulse timing. This histogram can be generated on the SPAD array, simplifying data transmission. In some examples, two SPAD arrays are stacked, and a coincidence circuit distinguishes signal events from noise events by determining that corresponding events are detected within a pre-set timing window.

Description

Technical Field

[0001] The present disclosure relates to time-resolved electron microscopy. Background Art

[0002] Traditional time-resolved electron microscopy revolves around synchronized pump pulses (such as laser pulses, microwave pulses, temperature pulses, mechanical or other excitations) to initiate the sample probe process. Typically, a pump pulse is directed to the sample to interact with the sample and induce a sample response. A probe pulse is then directed to the pumped sample and the response to the probe pulse is measured. The temporal response can be obtained by varying the time delay of the probe pulse relative to the pump pulse. The probe pulse can be an electron beam pulse generated in response to a pump laser, typically by doubling the pump pulse frequency and generating an electron beam pulse in response to the doubled frequency pump pulse. Repeating the pulse / probe pulse allows for an increased signal-to-noise ratio, and varying the pulse / probe time delay allows for reconstruction of the sample temporal response. These pump / probe measurements can require long acquisition times to provide an adequate signal-to-noise ratio. Furthermore, electron beam modulation and the associated time delay complicate the measurement setup.

[0003] Some electron detectors offer high temporal resolution but exhibit high noise, resulting in a low signal-to-noise ratio. For example, single-photon avalanche detectors (SPADs) generate an avalanche-based output signal in response to individual electrons. Unfortunately, SPADs also produce an output signal (dark counts) in the absence of an electron beam input. These dark counts can be reduced by cooling the SPAD, but they are indistinguishable from the real signal.

[0004] In view of the above, alternatives to the traditional pump / probe measurement approach as well as alternative detector configurations are needed. Summary of the Invention

[0005] A charged particle (CP) detector includes a first single photon detector (SPD) array and a second SPD array aligned relative to the first SPD array along a CP beam axis. A coincidence detector is affixed to one of the first and second SPD arrays and is operable to indicate a detection event corresponding to a charged particle detected within a predetermined timing window in an SPD array element of the first and second SPD arrays. In some examples, the first and second SPD arrays are defined on first and second substrates, respectively, and the first substrate is affixed relative to the second substrate. In other examples, the first and second substrates are semiconductor substrates, wherein the SPD array elements of the first and second SPD arrays are defined, respectively, and the first and second SPDs are single photon avalanche detectors (SPADs). In typical examples, the coincidence detector includes a corresponding coincidence detector element for each pair of corresponding SPD array elements of the first and second SPD arrays. In some cases, the first SPD array and the second SPD array include different numbers of SPD elements, wherein the coincidence detector is configured to indicate a detection event in response to detecting a charged particle in at least one SPD element of the first SPD array and at least one SPD element of the second SPD array. The coincidence detector can be defined on a third substrate fixed to at least one of the first substrate and the second substrate. In other instances, the frame is positioned to align the first SPD array and the second SPD array, and at least one spacer is positioned to define a separation of the first SPD array and the second SPD array. The first SPD array and the second SPD array can be defined on first and second substrates, respectively, and the first substrate can be fixed relative to the second substrate. The third substrate is coupled to the first SPD array and the second SPD array to receive detection events from the first SPD array and the second SPD array, and to generate a timestamp associated with each received detection event for a plurality of corresponding SPD array elements of the first SPD array and the second SPD array. In some examples, the third substrate is coupled to the first SPD array and the second SPD array to receive detection events from the first SPD array and the second SPD array and generate a histogram of detection events for a plurality of corresponding SPD array elements of the first SPD array and the second SPD array. The first SPD and the SPD array may be single photon avalanche detector (SPAD) arrays.

[0006] The method includes directing an electron beam toward a sample in an electron microscope and detecting events based on modulating the electron beam in response to corresponding pump beam pulses directed toward the sample from a first single-photon detector (SPD) array. Detected events are associated with time bins corresponding to the time elapsed since application of the corresponding pump beam pulses. Typically, a histogram of the number of detected events is formed. In some embodiments, the electron beam is repeatedly blanked for at least a portion of the time interval between the corresponding pump beam pulses. The electron beam may not be blanked based on the application of the pump pulses, and in some instances, the electron beam is not blanked with a fixed time delay relative to the pump beam pulses, and the pump beam pulses and electron beam blanking are applied at a fixed repetition rate. The fixed repetition rate may be between 100 Hz and 300 MHz, and the electron beam may be repeatedly blanked for at least 95% of the fixed time interval associated with the fixed repetition rate.

[0007] According to a representative embodiment, the first SPD array is a single photon avalanche detector (SPAD) array, and with a second SPAD array secured to the first SPAD array, a determination is made as to whether a coincidence event corresponding to an event detected by the first SPAD array is detected. A histogram is formed based on the coincidence events, wherein a temporal bin of each coincidence event is based on a time bin relative to an associated pump beam pulse. In some examples, the first SPAD array and the second SPAD array are defined on first and second substrates, respectively, and the first substrate is secured relative to the second substrate, and a third substrate is coupled to the first SPAD array and the second SPAD array and is operable to generate a histogram of detection events for a plurality of corresponding SPAD array elements of the first SPAD array and the second SPAD array.

[0008] The foregoing and other features and advantages of the disclosed technology will become more apparent from the following detailed description, which proceeds with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1A A representative electron microscope system configured to use a pump beam and a single-photon avalanche detector (SPAD) array is illustrated.

[0010] Figure 1B is with Figure 1A Representative timing diagrams associated with the system.

[0011] Figure 2A Representative electron microscope systems configured to use a pump beam, a SPAD array, and blanked and unblanked detection electron beams are illustrated.

[0012] Figure 2B is with Figure 2A Representative timing diagrams associated with the system.

[0013] Figure 3A A representative electron microscope system configured to use a pump beam, a probe electron beam generated in response to the pump beam, and a SPAD array is illustrated.

[0014] Figure 3B is with Figure 3A Representative timing diagrams associated with the system.

[0015] Figure 4A A stacked SPAD assembly is described that includes first and second SPAD arrays defined on respective substrates such as wafers secured together.

[0016] Figure 4B A stacked SPAD assembly is described that includes spaced-apart first and second SPAD arrays.

[0017] Figure 5 is a cross-sectional view of a stacked SPAD array comprising first and second SPAD arrays defined on opposing major surfaces of a common substrate.

[0018] Figure 6 A two-dimensional stacked SPAD array defined on first and second substrates including associated substrate circuitry areas is described.

[0019] Figure 7 A two-dimensional SPAD array defined on a first substrate and stacked in alignment with a one-dimensional SPAD array defined on a second substrate is described.

[0020] Figure 8 A two-dimensional SPAD array comprising a plurality of sub-arrays including respective circuit areas where event detection circuitry is located is described.

[0021] Figure 9 A representative method is described in which a SPAD array is used to detect modulation of a sample electron beam in response to a pump pulse to produce arrival time histograms for some or all of the SPAD elements of the SPAD array.

[0022] Figure 10 A representative method using a stacked SPAD array is described. DETAILED DESCRIPTION

[0023] introduction

[0024] Unlike conventional pump-probe methods used in electron microscopy, in the disclosed method, the electron beam does not need to be modulated, but the SPAD array and pump pulses are synchronized with the gating signal applied to the SPAD array. Even with scintillator-based detectors, a temporal resolution of 5-30 ps or better can be achieved. The electron beam used as the probe beam can be retained as in conventional transmission electron microscopy, or alternatively, modulated at a slower time scale using an electrostatic blanker or other modulator to reduce the dose to the sample.

[0025] The SPAD array can be operated in a histogram mode, where detection events are added to the acquired histogram, eliminating the need to store and transmit raw, individual arrival times. This type of "in-pixel" histogram is based on event times relative to a strobe pulse or other synchronization pulse, which can be provided by a dedicated clock circuit, or based on the repetition rate of the pump pulse. Because only summary statistics (such as the histogram) are transmitted, rather than individual event times, the data rate required to obtain an off-chip image is significantly reduced.

[0026] In measurements where only a small fraction of the probe beam electrons are captured by the detector (e.g., (weak) diffraction or (iron loss) EELS), the disclosed method can significantly increase throughput and greatly reduce complexity compared to conventional pump-probe techniques at similar time resolution. This increased throughput can lead to a reduction in the overall measurement time and better sample preservation over a large number of pump pulses.

[0027] The following examples generally involve single-photon avalanche detector (SPAD) arrays based on semiconductors such as Si, Ge, InGaAs, or other materials. The disclosed arrangements are generally suitable for detecting charged particles, such as electrons, which can be transferred from one SPAD to another appropriately aligned SPAD, but the disclosed methods are applicable to other charged particle beams. The disclosed methods can also be used with measurements based on, for example, short pump pulses and continuous detection. The pump pulses can be light or charged particle beam pulses, mechanical, thermal, or other excitations. In some of the following examples, one-dimensional SPAD arrays are shown for ease of illustration, but two-dimensional SPAD arrays can also be provided, in which SPAD elements are stacked in an N×M array, where N and M are positive integers. In typical examples, each stacked SPAD has the same number (N×M) of SPAD elements, but different numbers can be used. Furthermore, the SPAD elements can be semiconductor-based devices, but other types of elements, such as superconducting nanowire single-photon detectors, photomultiplier tubes, microchannel plate photomultipliers, superconducting transition edge sensors, and single-photon quantum dot detectors, can also be used. Detectors operating in Geiger mode (such as SPADs) can be used with time histograms as discussed below to record signal waveforms. Detectors that exhibit analog or other or non-Geiger responses can also be used with histograms if the histograms are appropriately processed. As used herein, a single-photon detector refers to a photon or charged particle detector that produces a Geiger mode response to a single photon or charged particle. The Geiger mode response is essentially independent of the number of photons or charged particles detected. This type of response can be referred to as a binary response that changes from on to off.

[0028] The SPAD element generates an avalanche response in response to received charged particles. This type of response is referred to herein as an event or detection event. This type of event can be used to reconstruct a temporal waveform by forming a histogram of the number of events as a function of time. The SPAD can be defined on a substrate, such as a semiconductor substrate, or as a discrete device that is then arranged and fastened to a substrate for support. In a typical example, a silicon substrate is used, but the SPAD element can be defined in other semiconductors, such as germanium and InGaAs.

[0029] In some examples disclosed herein, temporal resolution is determined by the SPAD element, and the probe (electron) pulse can be of almost arbitrarily long. However, due to the SPAD element dead time, the probe pulse duration is often limited to minimize dose and reduce pile-up distortion, but the probe electron pulse duration and timing are less critical than in traditional pump-probe measurements. In traditional pump-probe measurements, temporal resolution is determined by the probe pulse duration, and measurement throughput is low.

[0030] In an example, an optical pump pulse is applied to the sample and the modulation of the electron beam directed to the sample is detected by a SPAD array. Typically, the electron beam is blocked or attenuated during at least some time intervals in which the electron beam modulation of the sample in response to the optical pump pulse is not of interest, typically to reduce the electron dose on the sample. As used herein, "blanking" or "beam blanking" refers to attenuating, blocking, or deflecting the electron beam in this manner. Such blanking can be provided by modulation of the electron beam source or by the use of one or more electron beam deflectors or other electron optical elements.

[0031] While these examples are generally focused on pump / probe measurements, the disclosed method can be used to temporally characterize various electron beam modulation techniques, such as fast (electrostatic) beam blanking, RF chopping pulse trains, and so on, and can therefore be used to demonstrate and validate the performance of such techniques. In this case, the SPAD is synchronized to the appropriate output of the beam modulation technique, rather than to a pump source (e.g., a pump laser).

[0032] Example 1

[0033] refer to Figure 1A , a representative measurement system 100 includes a pump source 102, typically a light source that generates short pulses, such as pulses having a duration of less than 30 ns, 1 ns, 300 ps, 30 ps, 1 ps, or less. Light source 102 can be a mode-locked laser operating at a fixed repetition rate, or a light source that generates pulses at a fixed, variable, or arbitrary rate when desired and is positioned to deliver a beam 103 to a sample 111. Figure 1A As shown, the pump source 102 can generate pulses in response to a clock signal generator 104, but in some examples, the pump source operates at a fixed frequency and this fixed frequency is used as a clock signal. A gate signal generator 106 is responsive to the clock signal and generates a gate signal that can be coupled to one or both of the electron beam source 108 and the electron beam deflector 130 to provide beam blanking or other electron beam modulation. The electron beam source 108 generates an electron beam that propagates along a beam axis 109 that is directed toward the sample 111 and can be blanked to reduce beam dose.

[0034] The strobe signal generator 106 is also coupled to one or more SPAD arrays 112 that generate detection events based on the modulation of the electron beam in response to the pump pulses. Figure 1A In an example of , processing circuitry 114 generates a histogram of events detected in a time bin corresponding to the time elapsed since the application of a pump pulse associated with the detected event. Strobe signal generator 106 may operate with a fixed, variable, or random delay relative to the clock signal from clock signal generator 104.

[0035] like Figure 1BAs shown, the SPAD array 112 is gated to be in the on state using a gate pulse 140 to measure the detection pulse modulation in response to an optical pump pulse 142. The SPAD detector array 112 can generate output signals, such as output signal 146 including a detection event 147 at any element of the SPAD detector array 112. The SPAD detector array 112 is operated in Geiger mode so that the amplitude of the detection event 147 is essentially independent of the signal amplitude, but the time at which the detection event is generated for a series of detection pulses (called "arrival times") can be used to generate a histogram of arrival times. This histogram can be used to obtain the time response of the sample to the detection pulses. The relative signal timing can be set electronically by the gate signal generator 106, and the detection electron beam can be blanked with a blanking signal 144 to reduce the sample dose. As shown Figure 1B As shown, an event is assigned to one of the time bins 160A-160P corresponding to the detection time with reference to the timing of the associated pump pulse. The same time bin is used for all repeated pump pulses with reference to the timing of the associated pump pulse. More or fewer time bins may be used, and the time bins may be of the same or different sizes. The pump pulses are applied repeatedly such that Figure 1B The pulse sequence repetition diagram shown in , allows many events to be detected and assigned to time bins 160A-160P.

[0036] The beam deflector 130 can be controlled to deflect the electron beam until the pump pulse is applied. In this way, the SPAD array 112 and the sample 111 receive a reduced electron beam dose compared to using an unmodulated electron beam. In addition, blanking the electron beam can reduce pile-up distortion, which can cause a histogram generated using the SPAD array output signal to show a response to a probe pulse with a significantly reduced amplitude. For example, with an unblanked beam, the SPAD detector array elements may respond to portions of the electron beam that are unrelated to the response of the sample 110 to the pump pulse. As a result, the histogram of arrival times can be skewed to earlier histogram bins. Although in Figure 1B Not shown in , but after detecting electrons, SPAD array elements typically have a dead time typically between 30 ns and 1 μs, so responding to an un-blanked electron beam can increase the dead time and delay signal acquisition.

[0037] Pipeline distortion can be compensated or partially compensated by reducing the electron current or changing the timing of beam blanking, that is, blanking the beam for only a portion of the time window of interest after each pump pulse; then varying the blanking interval between subsequent pump pulses. Varying SPAD gate timing can also be used with histogram time library definitions that remain relative to the pump pulse rather than the SPAD gate.

[0038] Example 2

[0039] exist Figure 2A In another example, the pump beam and detector gating can be operated asynchronously, and the detection electron beam can be unmodulated. However, the time window of interest is often very short compared to the period between pump pulses, and this type of asynchronous operation is often inefficient. To reduce pile-up distortion, the SPAD gating can be varied over the duration of beam blanking or otherwise generated electron pulses.

[0040] System 200 includes an electron beam source 202 that directs an electron beam 206 to a sample 210 via an electron beam column 204. A beam blanker 208 can be used to reduce sample dose if desired. Sample 210 can be located on a sample stage 212, and portions of the electron beam from the sample are directed to a detector, such as a SPAD array 214A, which can be coupled to a signal processor substrate 214B for processing the SPAD array signals, such as for time correlation and histograms. Laser 216 directs a pulsed laser beam to an optical system 224, which directs a pump beam 222 to sample 210. In some alternatives, the electron beam 206 can be generated in response to a pulsed laser beam, if desired, typically via a second harmonic generator 218 to increase the laser photon energy. However, in typical embodiments, the electron beam 206 is modulated by the electron beam source 202 or with a beam deflector 208. As described above, the electron beam 206 is modulated to reduce the electron beam dose at the sample 210 outside of a time window of interest.

[0041] The SPAD array 214A is coupled to a gating generator 230 that receives a gating signal from a delay generator 232 that can activate the SPAD array 214A at random times or between predetermined timings associated with the pump pulses. As shown, the delay generator is included in a controller 250 that also provides a gating signal to one or more of the laser 216, the electron beam source 202, and / or the beam deflector 208. In some instances, the laser 216 provides pulses at a fixed frequency and the gating signal is a time-based fixed frequency. The SPAD array output signal is directed to a processor 240 for time correlation and time histograms, but these operations may also be performed on the signal processor substrate 214B. In some instances, the controller 250 is coupled to the laser 216 to establish laser pulse characteristics such as pulse duration, repetition rate, and pulse energy.

[0042] like Figure 2B As shown, the SPAD array 214A is gated to an on state using a gate pulse 240, which is also used to gate the laser 216 to produce a laser pulse 242 that can be used as a pump pulse. The sample 210 can also be stimulated in different ways. The electron beam can be gated to produce a laser beam with a variable delay T relative to the pump pulse. DDetection (electron beam) pulses 244 are generated. However, the electron beam does not need to be pulsed because the temporal resolution is determined by the SPAD array 214A. Each element of the SPAD array 214A can generate an output signal, such as output signal 246, which can include a representative detection event 247. As described above, the laser 216 can operate at a fixed frequency and can therefore be used to time the SPAD gating.

[0043] Example 3

[0044] Referring to 3A , a representative time-gated detection system 300 includes an electron beam source 302 and an electron beam optical column 304 with appropriate lenses, a deflector such as deflector 305, and other electron optical components that direct an electron beam 306 (probe beam) toward a sample 310, which may be located on a platform 312. In some cases, deflector 308 is positioned to sweep electron beam 306 toward different detector elements of a detector, such as a single-photon avalanche diode (SPAD) array 314A, after interacting with sample 330. Deflector 305 can be controlled to deflect electron beam 306 so that sample 310 is not exposed except at appropriate times. This electron beam blanking can be used to reduce the dose to sample 310 and the dose received by SPAD array 314A, particularly during the SPAD array recovery time. After interacting with electron beam 306, a portion of electron beam 306 is received by SPAD array 314A. In other examples, other radiation responsive to the interaction, such as secondary emissions or X-radiation, is detected using one or more suitable detectors instead of or in addition to the electron beam portion.

[0045] System 300 may be operated as follows. First, note that in a conventional optical pump / probe system using electron beam detection, a pulsed laser is used to generate pump pulses that are directed to the sample, and a portion of the pulsed laser output is used to generate electron beam probe pulses that are directed to the sample with various delays between the pump and probe pulses. By varying the pump / probe delays, the response of the sample to the pump can be measured with a slow detector. Using a sufficient number of pump / probe delays, the temporal response of the sample to the pump can be measured. In contrast, in a system such as system 300, the detector (e.g., SPAD array 314A) is strobed by SPAD strobe generator 311 to initiate data acquisition. Figure 3BAs shown, SPAD array 314A is gated to the on state using a gate pulse 340, which is also used to gate laser 316 to generate laser pulses 342, which can be used as pump pulses. While laser pulses facilitate time-resolved measurements on short timescales, sample 310 can also be stimulated in a different manner (i.e., without laser pulses). A portion of the pump pulses can be directed to electron beam source 302 to generate probe (electron beam) pulses 344. SPAD array 314A can generate multiple output signals, such as output signal 346, which includes detection events 347. Because SPAD array 314A operates in Geiger mode, the amplitude of output signal 346 is independent of signal amplitude. However, the time at which a series of probe pulses generate detection events 347 (referred to as "arrival times") can be used to generate a histogram of arrival times. This histogram can be used to obtain the temporal response of the sample to the probe pulses. The relative signal timing can be electronically set by controller 309 and / or variable optical pulse delay 320, which allows for varying the timing of the probe pulses.

[0046] Figure 3A The configuration is a representative example, and in other examples, the generation of the probe pulse is not based on an optical pump pulse. For example, the beam deflector 305 can be controlled to deflect the electron beam 306 until the pump pulse is applied. In this way, the SPAD detector array 314A receives a reduced electron beam dose compared to using an unmodulated electron beam. In addition, blanking the electron beam 306 reduces electron pile-up distortion, which causes the histogram generated using the SPAD array output signal to show a significantly reduced amplitude response to the probe pulse. Although Figure 3B Not shown in , but after detecting electrons, SPAD array elements typically have a dead time typically between 30 ns and 1 μs, so responding to an un-blanked electron beam can increase the dead time and delay signal acquisition.

[0047] Controller 309 may generate time correlation of the SPAD detector array signals (time correlated single photon detection - TCSPD) and generate a histogram for each element of SPAD detector array 314A. In some cases, some of this processing is provided by an adjacent or integrated signal processing substrate 314B.

[0048] In another approach, electron beam source 302 can be responsive to light pulses from laser 316, such that electron beam 306 is a pulsed electron beam or includes a pulsed component. In some examples, the electron beam pulses are generated by light emission, and the laser beam output can be frequency-multiplied at a second harmonic generator 318 to improve light emission efficiency based on the work function of the electron emitter. Furthermore, a variable delay generator 320 can be provided to control the relative timing of the electron pulses with respect to the light pulses (i.e., pump pulses) in laser beam 322, which is directed to sample 310 via optical system 324. Electron beam blanking is generally not necessary, as the electron beam can be pulsed. The electron and laser beams propagating toward sample 310 are located within vacuum enclosure 301.

[0049] Example 4. Stacked SPADs

[0050] SPADs can operate with a high temporal resolution of 30 ps or better. In some examples, two SPAD-based detection wafers (referred to as the top and middle tiers for convenience) containing arrays of SPAD elements (also called pixels) are attached to a third wafer (referred to as the bottom wafer for convenience) on which processing circuitry is defined. In some examples, the processing circuitry is located elsewhere to avoid exposure to the electron beam. Because high-energy electrons are not blocked by the upper detector, they can generate free charge carriers in the SPAD elements of both SPAD-based detection wafers. Consequently, a single electron can generate a detection event in a corresponding SPAD element in each of the two SPAD-based detectors. Timing circuitry can be used to time the detection event in each SPAD-based detector with an accuracy of approximately 30 ps or better. Simultaneous detection events in the top and middle tiers can be distinguished from noise events through correlation based on detection time. The wafers can be connected using copper-to-copper direct hybrid bonding and interconnected with through-silicon vias (TSVs) that enable 3D interconnection between layers.

[0051] Example 5. SPAD with integrated histogram

[0052] SPADs and other single-event detectors can generate a large number of detection events, which must be processed to determine the temporal waveform. This may require high-speed data transmission and / or data storage of the detection events. In the examples disclosed herein, the SPAD array is operated in so-called histogram mode, where individual detection events are processed on-chip at each SPAD array element or at each SPAD element to generate a per-pixel event histogram. In this histogram, bins correspond to electron arrival times relative to each gate or pump pulse. Pump-probe TEM measurements can be performed in stroboscopic mode, where the pump-probe sequence is repeated a large number of times to collect detection events for the histogram. Detection events can be collected within a predetermined time interval to generate a sufficient number of detection events, or at other time intervals. As detection events are collected, they are accumulated into a per-element (per-pixel) histogram to generate a histogram of the number of detection events over multiple time periods associated with the gate control signal. After the detection events are collected and processed to generate the histogram, the histogram can be transmitted from the SPAD array, significantly reducing overall data transmission requirements. No detection event data, such as event times, needs to be transmitted.

[0053] Example 6

[0054] refer to Figure 4A , the SPAD array system 400 includes a first SPAD array 402 defined on a first substrate 403 and a second SPAD array 404 defined on a second substrate 405. The SPAD arrays 402, 404 include corresponding SPAD elements, such as representative SPAD elements 412, 414. Figure 4A As shown, the SPAD detector system 400 is generally positioned to receive an electron beam or other particle beam that generally propagates along a charged particle beam axis 416. The SPAD elements are arranged so that electrons directed to a SPAD element in a first SPAD array 402 are received by a corresponding SPAD element in a second SPAD array 404. For example, electrons detected by SPAD element 412 may also be received and detected by SPAD element 414. Unlike SPAD detection of light signals, electrons are not absorbed in the SPAD elements but can propagate to be detected by multiple detectors.

[0055] exist Figure 4AIn the example, a third substrate 406 is provided, which may include electrical connections and circuit elements for the first and second SPAD arrays 402, and some or all of the SPAD elements in the second SPAD array 404. The third substrate 406 may be coupled to one or more external circuits, such as control, bias, and clock circuitry 420, which controls the biasing of the SPAD elements, and may provide output signals indicating detection events at the SPAD elements. The external circuitry may include a coincidence detector 418, which indicates simultaneous detection events in corresponding SPAD elements of the SPAD arrays 402 and 404. As used herein, "simultaneous" detection events are detection events that occur at predetermined timing intervals, typically corresponding to events occurring within a time interval of typically less than 1 ns, 300 ps, 30 ps, 10 ps, or less. Coincidence detection may alternatively be provided in circuitry defined on the third substrate 406. Using coincidence detection, individual noise events that may be mistaken for true signal events can be rejected. The coincidence detector 418 indicates an electronic detection only when corresponding SPAD elements of the SPAD arrays 402 and 404 both indicate a detection event within a selected time interval.

[0056] Coincidence detection, SPAD array biasing, and other processing and control circuitry can be defined in the third substrate 406. In some cases, some circuit elements are generally defined in the first substrate 402 and the second substrate 404, or at, near, or with the SPAD elements. In this example, the substrates 402, 404, 406 can be bonded to one another with the SPAD elements properly aligned relative to the axis 416.

[0057] refer to Figure 4B , stacked SPAD array system 450 includes a first SPAD array 452 and a second SPAD array 454, which are fixed relative to each other by a frame 458. Frame 458 positions the SPAD arrays so that corresponding SPAD elements, such as representative SPAD elements 462, 464, are aligned along axis 456. A circuit substrate 460 defining bias, control, coincidence detection, or other circuit elements may also be fixed by frame 458. Some or all of such circuitry may be provided with an external SPAD control system 470. The SPAD arrays 452, 454 and circuit substrate 460 are separated by respective gaps 474, 475.

[0058] Although not in Figures 4A-4B As shown in FIG. 1 , event histograms may be performed using circuitry / processing elements located on some or all of the substrate or provided externally.

[0059] Example 7

[0060] refer to Figure 5, stacked SPAD array 500 includes respective SPAD arrays 503, 505 including SPAD elements, such as representative SPAD elements 504, 506, defined on respective surfaces 508, 510 of substrate 502. Alternatively, separate substrates may be secured to one another, as illustrated at 520.

[0061] Example 8

[0062] refer to Figure 6 , a first two-dimensional stacked SPAD array includes a two-dimensional array of SPAD elements, such as representative element 604, defined on a first substrate 602. Additionally, SPAD control circuitry may be provided in substrate region 606. A corresponding second two-dimensional stacked SPAD array (shown in dashed lines) may be defined on a second substrate 612 and may include SPAD elements, such as representative SPAD element 614 corresponding to SPAD element 604. Second substrate 612 also includes substrate region 616 (shown in dashed lines), which may be provided with control, bias, electrical connections, and other circuitry. The first and second two-dimensional stacked SPAD arrays are coupled to external analog and / or digital circuitry, which may include amplifiers, signal processors, filters, counters, coincidence circuits, and processors for receiving coincidence events and generating histograms. In other examples, such circuitry is provided in one or more of substrate regions 606, 616.

[0063] Example 9

[0064] refer to Figure 7The dual-SPAD system includes a two-dimensional array of SPAD elements, such as representative elements 704A-704D, defined on a first substrate 702. Additionally, SPAD control circuitry may be provided in substrate region 706. A one-dimensional SPAD array (shown in dashed lines) may be defined on a second substrate 712 and may include SPAD elements, such as representative SPAD elements 714A-714C. SPAD element 714A is associated with SPAD elements 704A-704D. In this example, SPAD elements 714A-714C are associated with multiple SPAD elements of the two-dimensional SPAD array. While a one-dimensional array cannot confirm coincidence events with the same spatial accuracy as a stacked two-dimensional array, it can distinguish noise events. Second substrate 712 also includes substrate region 716, where control, biasing, electrical connections, and other circuitry may be provided. The SPAD array is stacked and can be connected to external analog and / or digital circuitry, which may include amplifiers, signal processors, filters, counters, coincidence circuits, and a processor that receives coincidence events and generates a histogram, as shown at 720. Typically, with such SPAD systems, events detected in the two-dimensional array are confirmed by coincidence detection in the one-dimensional array, and these detected events are used in a histogram. Stacked SPAD arrays can have different numbers and sizes of SPAD elements. For example, the first array can have multiple elements, while the second array can have fewer elements, including a single element used only to establish coincidence, but matching the arrays may be convenient.

[0065] Example 10

[0066] refer to Figure 8 , SPAD array 800 is defined on substrate 801 and includes respective SPAD sub-arrays 806, 816, 826, 836, which include SPAD elements, such as representative SPAD elements 804, 814, 824, 834, and control circuitry 802, 812, 822, 832. Histogram data is provided to serializer 840 for transmission from SPAD array 800, but the histogram data may be accumulated at substrate 801.

[0067] Example 11

[0068] refer to Figure 9, a representative method 900 includes applying a pump pulse to the sample at 902 and starting a timer at 904. Typically, the probe beam is also not blanked, but blanking may be provided before or after the pump pulse, and the time increment or time delay may be fixed or variable. At 906, an event is detected at one or more SPAD elements and the event is assigned to a time bin at 908. At 910, a SPAD dead time or recovery time may be allowed to pass, and at 912 it is determined whether a pulse interval has passed, typically a pulse time period corresponding to the pump pulse repetition rate. If not, then the method 900 returns to 906 to detect additional events associated with the pump pulse. If the pulse interval has passed, then at 914 it is determined whether an additional pump pulse is to be applied and measured, and if so, then the pump pulse is again applied at 902. Although the histogram may be generated by processing external to the SPAD array, if additional pulses are not used, then in some instances, the histogram is generated directly from the SPAD array, such as Figure 4A A SPAD detector assembly provides a histogram of time-divided bin event counts, the SPAD detector assembly including a suitable substrate on which processing circuitry is defined. The histogram can be read when a certain number of counts in one or more bins is reached, after a total number of counts is reached, or at a fixed rate independent of the number of counts.

[0069] Example 12

[0070] refer to Figure 10 , a representative method 1000 includes aligning first and second SPAD arrays at 1002. At 1004, detecting an event at one or more array elements of the first and second SPAD arrays. At 1006, evaluating the detected events to detect coincident events in corresponding elements of the first and second SPAD arrays. If there are coincident events, then these are identified as true detection events and added to an event histogram at 1008. Alternatively, due to a lack of coincidence, these are identified as noise and discarded at 1007. In a typical example, a plurality of such noise events are maintained and each additional noise event is associated with an incrementing noise event counter. Coincidence may be established when the event is received, or the event may be processed after data collection if appropriate event timestamps are recorded to allow for determination of coincidence. As Figure 10 As shown, after processing an event (or during event processing), the method continues by detecting an event. In this example, the event is associated with electron beam modulation in response to a pump beam applied to the sample. The electron beam can be continuous, but to reduce sample dose and electron buildup, the electron beam may not be blanked during the time period of interest, typically the time interval after the pump beam is applied to the sample.

[0071] General terms

[0072] As used in this application and the claims, the singular forms "a," "an," and "the" include the plural forms unless the context clearly dictates otherwise. Additionally, the term "comprising" means "including." Further, the term "coupled" does not exclude the presence of intervening elements between the coupled items.

[0073] The systems, devices, and methods described herein should not be construed as limiting in any way. Indeed, the present disclosure is directed to all novel and non-obvious features and aspects of the various disclosed embodiments, individually and in various combinations and subcombinations thereof. The disclosed systems, methods, and devices are not limited to any specific aspect or feature or combination thereof, nor are the disclosed systems, methods, and devices required to have any one or more specific advantages or solve any one or more specific problems. Any theories of operation are provided for ease of illustration, but the disclosed systems, methods, and devices are not limited to such theories of operation.

[0074] Although the operations of some of the disclosed methods are described in a particular sequential order for ease of presentation, it should be understood that unless the specific language set forth below requires a specific ordering, this description encompasses rearrangements. For example, in some cases, the operations described in sequence may be rearranged or performed simultaneously. In addition, for simplicity, the accompanying drawings may not illustrate the various ways in which the disclosed systems, methods, and devices can be used in conjunction with other systems, methods, and devices. In addition, this specification sometimes uses terms like "produce" and "provide" to describe the disclosed methods. These terms are high-level abstractions of the actual operations performed. The actual operations corresponding to these terms will vary depending on the specific implementation and are easily discernible by a person of ordinary skill in the art.

[0075] In some instances, values, procedures, or devices are referred to as "lowest," "optimal," "minimum," etc. It will be appreciated that such descriptions are intended to indicate that a choice may be made among many functional alternatives that may be used, and that such a choice is not necessarily better, lesser, or preferred over other choices.

[0076] Examples are described with reference to directions indicated as "above," "below," "upper," "lower," etc. These terms are used for convenience of description and do not imply any particular spatial orientation.

[0077] In view of the many possible embodiments to which the principles of the disclosed invention may be applied, it should be recognized that the illustrated embodiments are only preferred examples and should not be taken as limiting the scope of the disclosure.

Claims

1. A charged particle detector comprising: a first single photon detector array; a second single photon detector array aligned along the charged particle beam axis relative to the first single photon detector array, wherein the charged particle beam axis is directed toward the sample and is blanked; and a coincidence detector secured to one of the first single photon detector array and the second single photon detector array and operable to indicate a detection event corresponding to detection of a charged particle in corresponding single photon detector array elements of the first single photon detector array and the second single photon detector array within a predetermined time window; Among them, The first single photon detector array and the second single photon detector array operate in a histogram mode in which detection events are added to an acquired histogram. 2 . The charged particle detector according to claim 1 , wherein the first single photon detector array and the second single photon detector array are defined on a first substrate and a second substrate, respectively, and the first substrate is fixed relative to the second substrate.

3. A charged particle detector according to claim 2, wherein the first substrate and the second substrate are semiconductor substrates, in which the single photon detector array elements of the first single photon detector array and the second single photon detector array are respectively defined, and the first single photon detector and the second single photon detector are single photon avalanche detectors.

4. The charged particle detector of claim 2, wherein the coincidence detector comprises a corresponding coincidence detector element for each pair of corresponding single photon detector array elements of the first single photon detector array and the second single photon detector array.

5. The charged particle detector of claim 4, wherein the coincidence detector is defined on a third substrate secured to at least one of the first substrate and the second substrate.

6. A charged particle detector according to claim 2, wherein the first single-photon detector array and the second single-photon detector array include different numbers of single-photon detector elements, wherein the coincidence detector is configured to indicate a detection event in response to detecting a charged particle in at least one single-photon detector element of the first single-photon detector array and at least one single-photon detector element of the second single-photon detector array.

7. The charged particle detector according to claim 1 , further comprising: a frame positioned to align the first single photon detector array and the second single photon detector array; and At least one spacer is positioned to define a separation of the first and second single photon detector arrays.

8. The charged particle detector of claim 1 , wherein the first single photon detector array and the second single photon detector array are defined on first and second substrates, respectively, and the first substrate is fixed relative to the second substrate, and further comprising: and a third substrate coupled to the first single-photon detector array and the second single-photon detector array to receive detection events from the first single-photon detector array and the second single-photon detector array, and to generate a timestamp associated with each received detection event of a plurality of corresponding single-photon detector array elements of the first single-photon detector array and the second single-photon detector array.

9. The charged particle detector of claim 1 , wherein the first single photon detector array and the second single photon detector array are defined on first and second substrates, respectively, and the first substrate is fixed relative to the second substrate, and further comprising: a third substrate connected to the first single-photon detector array and the second single-photon detector array to receive detection events from the first single-photon detector array and the second single-photon detector array, and to generate a histogram of detection events of a plurality of corresponding single-photon detector array elements of the first single-photon detector array and the second single-photon detector array. 10 . The charged particle detector according to claim 9 , wherein the first single photon detector array and the second single photon detector array are single photon avalanche detector arrays. The charged particle detector of claim 10 , wherein the first, second, and third substrates are fixed relative to each other.

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