WO2022034558A1 - Method and sensor for measuring electrons - Google Patents

Abstract

The invention relates to a method, system, chip or set of chips and devices such as TEM microscope, involving improvement by way of a method of detecting fast high contrast transitions in time at image capturing, the method comprising the steps of providing a chip (26) of at least one silicon photomultiplier array, where the array can be a matrix or a line of pixels, directly coupling each pixel comprising at least a photomultiplier thereof directly to a distinct amplifier circuit (28), reading out an integrated analog charge of each of the pixels, providing the amplified signal to an analogue to digital convertor (30), and subsequently processing the amplified signal in a digital domain. Light intensity may be measured by using the parallelization of each photomultiplier or avalanche photodiodes signals in each pixel.

Description

METHOD AND SENSOR FOR MEASURING ELECTRONS

[0001] The present invention relates to a sensor, device and method of sensing photons. Such sensors are known per se and applied in a wide area of use, including e.g. automotive image retrieval or more in general applications of so-called LiDAR, acronym for light imaging, detection, and ranging, transmission electron microscopes and gamma spectroscopy.

[0002] While the known methods and devices, amongst which so-called LDR and silicon photomultiplier read out ASICs, are satisfactory for most of the fields they are used in, indications start arising that with contemporary developments in image retrieval, the known methods and devices are more and more falling short in desired performance.

[0003] One example in the latter respect is in the area of gamma spectroscopy as is e.g. provided by the 2017 poster on SIPHRA or Silicon Photomultiplier Readout ASIC titled "Using the SIPHRA ASIC with an SiPM array and scintillators for gamma spectroscopy". This disclosure indicates that "read out rates might be limited". Another examples is of application is in the field of automotive, in particular for enabling the practical use of autonomous vehicles. This is e.g. provided by the 2018 SensL technologies poster "SiPM for automotive 3D Imaging LiDAR systems". The publication discloses the use amongst others of a 1 by 16 array of silicon photomultiplier. Such a set-up is used in situations with low reflectivity and in complex environments with amongst others solar ambient light noise. Yet another example of application is in the field of electron microscopes, in particular transmission electron microscopes, as may be known by e.g. the 27 oct. 2016 article "Transmission electron imaging in the Delft multibeam scanning electron microscope" of by Yan Ren and Pieter Kruit.

[0004] While many more applications of electron sensing may be identified, all deal with the problem of an ever increasing desire and hence need for faster read-out rates or at least more sophisticated sensors in general, more in particular of how to make such available in an economic, i.e. cost-wise widely accessible and preferably compact manner.

[0005] In the latter respect an impression and overview of evolution of high speed imaging devices over the past three decades is provided by figure 1 of the 2017 article by Takeharu Goji Etoh et al, "the theoretical highest frame rate of silicon image sensors". The article identifies that highest sensing speeds may be attained by relatively costly and/or complex devices like Laser, Streak tube and holography, while the relatively simple and affordable Silicon Photomultiplier sensors as available attain between 10A7 and 10A8 frames per second for devices capable of capturing 100 or more consecutive images. Industrially available photomultipliers are e.g. known from chapter 3 of the Hamamatsu “e03 handbook si-apd-mppc.pdf”, which introduces their so-called MPPC device to be a multi pixel photon counter to be one of Silicium photomultiplier (Si-PM) devices, using multiple avalanche photodiode pixels (APD) operating in Geiger mode. While it “essentially is an opto-semiconductor device, it has excellent photon-counting capability and can be used for extremely weak light at photon counting level”. At experimenting with this feature, it was found however that this feature could not satisfactorily deliver under the extreme, at least most advanced circumstances with fast and high contrast transitions as is targeted by the present invention.

[0006] While the present invention is by no means limited to any particular one of various, partly above mentioned possible applications, one particular and exemplary area of application with contemporary, challenging requirements is that of the earlier mentioned transmission electron microscopes. The cited article discloses the conversion of electron into photons in a manner known per se, using a fluorescent material, the conversion is in this case desired for minutely analyzing biological tissue, in particular in a manner using the particular contrast that transmission electron beam microscope may provide at identifying small tissue particles. Departing from a single beam scanning electron microscope, hereinafter "SEM", the article indicates that acquiring a 3D image of a 400X400X1000 um mouse cortical volume, with a typical 4 nm resolution, may take about 400 days, depending on the detection method applied. The disclosure suggests that it would help if this imaging acquisition time would be reduced to one or two days while still keeping the imaging quality, and further suggests that "Multibeam technology would be "the only option" to achieve this".

[0007] Having this problem of acquiring small biological tissue elements at a highest possible rate in mind, while proposing an affordable and compact solution, practicable within contemporary industrial setting, the present invention hence seeks to improve on a sensing means, preferably even where the contemporary single e-beam transmission microscope beam would be replaced by a system as proposed and under development with multiple e-beams, more in particular with 14 x 14 or 196 e-beams as in this example.

TECHNICAL BACKGROUND

[0008] While the article on theoretical highest frame rate of silicon image sensors evaluates performance, it does not provide any design for the same, other than referring to academic efforts towards highest achievable frame rates like the therein cited articles by Etoh et al. A recent article on academic achievements also provided by Takehare Goji Etoh et al, Light in flight imaging by a silicon image sensor: toward the highest theoretical Frame Rate was published May 15, 2019. The present invention however seeks to depart from readily available sensors, hence to provide a readily industrially applicable sensing device.

[0009] One such earlier application seems to be that of SensL, which by its above cited 2018 poster on SiPM for Automotive 3D imaging LiDAR Systems indicates that the latter should amongst others be low cost and fully solid state, i.e. without moving parts, hence relatively simple. It departs from an eye safe laser diode array and a 16 array silicon photomultiplier as receiver, associated with readout and control electronics. The poster concludes that Silicon photomultipliers have emerged as a preferred sensor for automotive LiDAR applications. While in this application good range information results may be achieved by utilizing multiple, here 20 shots per measurement, the required frame rate is quite low with a mere 30 frames per second. [0010] Further, the above cited gamma spectroscopy application of silicon photomultipliers, indicates to be capable of handling signals from bright scintillators with energies of tens of MeV, in that respect relating to the above cited microscope application with electron beams and scintillator. The publication teaches to connect silicon photo multipliers in DC mode to a readout ASIC, using a 16 pixel SiPM array in three different readout configurations. The SiPM outputs in this embodiment are DC- connected to SIPHRA inputs using 30 cm long coaxial cables. Three readout configurations were examined: all 16 SiPM pixels connected to one SIPHRA input, 16 pixels connected to 4 SIPHRA inputs (4 pixels per input), and one single pixel connected to one SIPHRA input. Concluding, the experiments of this set-up the disclosure, without indicating how, remarks that "higher readout rates might be achievable with analogue readout". With readout usually meaning a conversion from analogue signals io another signal, like a spectrum, a counter or a count rate-meter, it is remarked that it seems difficult within contemporary technology to achieve state of art throughput with analogue systems without suffering stability and offset variation.

BRIEF SUMMARY OF THE INVENTION

[0011] For the purpose of measuring light intensity and the variations of light intensity incident on a sensor array, in particular with a rate of 10 Mfps to 40 Mfps, the present invention intelligently and advantageously combines an off-the-shelf Silicon Photon Multiplier array, henceforth denoted SiPM array, with a custom electronics readout design while unexpectedly using, at least reading out the SiPM in analogue fashion before converting and processing the signal in a digital domain. An insight underlying this manner of measuring, which in view of the capabilities of and SiPM combines an as it were old fashioned analogue manner of handling signals with contemporary digital processing, provides that absolute light intensity information may in this way be conserved at the moment of sampling, all the more if not especially for reason that, or else when, the signal handling and processing is performed in direct current form of the signal. The invention hence holds that only after such analogue processing, the signal may be converted into values for further processing thereof in contemporary digital domain.

[0012] The present invention thus relates to a method of measuring incident light flux on a pixels array using an Silicon Photon Multiplier (henceforthe SiPM) array availing of an analogue operational mode, the method comprising using the SiPM array in its analogue mode of operation; and functionally coupling the output of the array to a two- stage DC-coupled analogue amplifier with an output bandwidth within the range of 10 to 100 MHz, preferably within the range of 20MHz to 50 MHz, outputting the amplified signal to a multi-channel ADC with a per channel sampling frequency within a range of 10 to 100 MSPS, preferably within a range of 40MSPS to 100MSPS, and a sampling resolution of 10, preferably at least 12-bit. The incident light flux per single cell of the SiPM array may hence be acquired in analogue mode, where the analogue signal is the output of the SiPM single cell. In this respect, the analogue mode in fact may consist in measuring the average SiPM current output at the moment of sampling. It is to be noted that so called time of arrival is no longer possible using this analogue mode according to the invention. [0013] In further elaboration an integrated analogue signal may preferably have a time constant, e.g. at about 50ns, which is longer than the digital sampling period Ts, e.g. at about 25ns. It may be considered that in the new set up, time of arrival of a photon is in fact not of interest, nor is the number of photons. Rather the new concept of measuring considers as it were the average light power received per pixel in a period of time. The system Tau may here be 100ns, and Ts may be 25ns. It should further be remarked that while the digital domain is not at stake at the instant of sampling or right thereafter, in fact in a later stage several digital samples may per acquisition be further integrated to increased light intensity resolution.

[0014] According to yet further development of the invention, an analogue output signal output by the SiPM is coupled with an analogue front-end in a direct current or DC domain, preserving absolute light intensity information in the signal at the moment of sampling. Such analogue signal output by the SiPM is preferably amplified by a gain factor within the range of 50 to 400, typically within the range of 100 to 300, e.g. by a factor of 200, more specifically amplified in a two-stage amplifier circuitry, preferably with a bandwidth higher than 10 MHz. In this manner high timing dynamics may be allowed for, black and white patterns as it were at an ADC sampling period, and hence a high sampling resolution of an SiPM signal. It should be remarked that according to the invention a conversion of a thus collected signal into the digital domain allows digital processing towards extracting absolute light power intensity from dark current and gain calibration parameter.

[0015] The invention further provides to typically use only part of an ADC input range, with a bit-resolution of preferably at least 12, typically 14-bit, in order to sufficiently quantify the amount of light intensity at moment of sampling. The measured amount of light intensity sets forth as it were the difference between a light signal at moment of sampling and a reference offset signal, the reference offset signal typically combining an amplified sensor’s so called dark signal and an amplified electronics front-end offset signal. While the term offset signal is being used here, it may be understood to in practice take the form of an e.g. digitally stored value, representative for a signal received from a pixel in a condition where it does not receive any light.

[0016] Yet further, each pixel channel may individually be corrected for gain and offset errors. Because each individual pixel readout exhibits various physical response in terms of gain and offset, calibration values are used in the digital domain to correct for individual channel disparities. The results of each converted pixel current output from the SiPM array are then equalized. Hence it will be understood that the result of each pixel current output is eventually converted into a digital value for further processing in a digital domain, in which later calibration values are used to correct for individual channel disparities, in particular with the results of each converted pixel current output from the SiPM array then being equalized. These measures promote a so- called flat field response of the resulting image, i.e. support that for each pixel a same value will be output at a same light condition. Hence also, the SiPM analogue signal output may be amplified with a factor adapting or scaling the signal to an ADC input range with a bandwidth higher than 10MHz in a 2-stage circuitry, allowing a change of light intensity up to 5MHz, as well as relatively, i.e. compared to standard specification, conventionally departing from digital sampling high sampling resolution of the SiPM output signal to identify different levels of light intensity without being limited by the gain-bandwidth of operational amplifier. It is remarked that a large bandwidth ADC in in practice relatively expensive, so that normally it would be preferred to utilize the full range of an ADC converter.

[0017] In one typical embodiment, the SiPM array consists of 64 SiPM pixels where each individual SiPM output is connected to an electronics readout channel consisting of a two-stage amplifier of 12MHz bandwidth and a high performance ADC of 14-bit resolution and 40MSPS sampling frequency. In order to report a new image, also called frame, of 64 pixels at a rate of lOMfps to 40Mfps, each channel is acquired in parallel and converted within the acquisition period, which is, in fact obviously limited by the invert of the desired frame rate, i.e limited to 100 ns down to 25 ns.

[0018] It is hence a major goal of the invention to accurately measure an absolute SiPM output signal of each pixel within the acquisition period knowing that the absolute SiPM output signal can vary from dark current to saturated in the same acquisition period and that the signal history must not interfere with the measured signal at the moment of sampling The invention considers that like in most typical light conversion systems, each SiPM output signal is an electrical current proportional to the amount of incident light. More accurately, each SiPM output is, in one embodiment of the invention, made of several Silicon Avalanche Photo Diode, henceforth and commonly denoted SiAPD, connected in parallel, see FIG. 3B, such that the output current of each Si APD is added up. The Si APD output thus mainly consists of a photocurrent, generated by the amount of incident light, multiplied by the avalanche process. The invention recognizes that because of the avalanche process, Si APD can be very sensitive and are able to provide a signal triggered by individual photons. See in this respect e.g. the known SiPM characterization in FIG 14.

[0019] With an aim of the present invention to measure an image of the absolute incident light flux on each pixel, actually by means of the pixel current output, within a predefined acquisition period, according to the invention preferably as short as 25ns, the electrical coupling must be such that no signal charge is integrated for more than the pre-defined period, otherwise the signal history will affect the actual acquisition. Moreover, the variations of incident light flux on a pixel are completely dependent of the application and might even be close to null. Yet the absolute incident light flux must be measured. Whereas the parasitic dark current of any photo sensor needs to be discarded for a proper measurement of the absolute incident light flux, the invention does not apply any AC coupling between the sensor and the readout to solve this issue, but rather implements a DC coupling for the sake of the application.

[0020] In an embodiment of the invention, the SiPM pixel is made of 3600 Si APDs array where each Si APD signal adds up and a ‘piled-up’ signal is created in presence of light such as presented in FIG 15. When the flux of the incident light is in the order of 100 Mphoton/s (Milion photons per second), the ‘piled-up signal’ can not be used in photon-counting mode where every single photon is identify by a raising edge in the signal, typically depicted by the left most square in FIG.13. In the imaginary case that 1 to 3600 photons would instantaneously fall exactly onto 1 to 3600 different Si APD in a single pixel, the quantification of the output signal would allow to measure at best 3600 levels of light, corresponding to the exact amount of incident photons. In principle such pixel can thus not provide more output current that the sum of 3600 photocurrents. However, because of the random distribution in time of photons in a light beam and because of the electrical response of the Si APD, especially its decay, see as in FIG. 15 for instance, there is no such a defined output level but an integrated output current corresponding to the incident level of light for when the incident light flux is too high for photon-counting mode of operation. As to spatial distribution of photons it is remarked that photons may arrive on the same SiAPD while a neighbor Si APD is not hit. [0021] In further understanding of the new set up, the analogue readout mode is preferred, where the averaged pixel signal output level tends to be directly proportional to the incident light flux, from no-light to light saturation, where saturation of the pixel is given at about 1011 p/s in the example of FIG. 13. Because the analogue ‘piled-up’ signal as further elaborated upon in the description is at this stage yet a weak signal, it needs to be further amplified before an ADC can efficiently digitalize such signal and to eventually provide a digital value of the incident light flux. Moreover the amplified analogue signal must remain within the ADC input range to avoid over ranged sampling. This in practice gives a constraint on the ADC resolution, where 12-bit resolution ADC can detect 4096 electrical levels on its full scale range, allowing to fit the maximum expected 3600 levels of light. The invention further recognizes however, that the amplified current output signal when no light is received is not null, but rather is the result of several physical properties, involving the SiPM characteristics like dark current and analogue front-end characteristics. Such no-light signal, also called offset, is of importance for the proper quantification of the incident light flux when the pixel is illuminated and is recorded as a reference. Hence, further, each acquisition relies on the difference between the analogue signal of the illuminated pixel and its no-light signal reference, or offset, reasonably assuming that other physical conditions, like biasing voltage and temperature, remain stable or are corrected for.

[0022] Hence, the useable input range of an ADC is reduced by the level of the offset signal. The invention assumes variations in the entire detection chain up to half the output signal dynamic, so that the ADC resolution is in the new set up further increased to cope with the loss of input range. This gives an additional constraint on the ADC resolution, where 13-bit resolution ADC can convert 8192 electrical levels on its full scale range, allowing to map the 3600 maximum expected levels of incident light even when the offset is mapped around half the ADC input range. This is in the description illustrated along FIG. 8.

[0023] Another consideration of the present invention is the time dynamic of a piled- up signal that the present invention includes for measuring. Indeed, the signal resulting from photon impact can change in the order of 50 ns according to the characteristics of the SiPM. An example is provided along the decay plot from Hamamatsu type of SiPM, commercialized MMPC in FIG. 14. This rapid, virtually instant decay of a signal means that the analogue front-end must allow such fast time transition. This gives an additional constraint on the ADC sampling frequency and digital system, where the sampling frequency should, as assessed by developing the invention, be at least 40MHz. [0024] Evidently, the present invention equally relates to a signal acquiring system, in particular applying the method in accordance with any of the preceding claims, comprising a photomultiplier array (26) electrically connected to a front -end subsystem (28), e.g. by an interconnect (27), comprising a two-stage amplifier circuit for each pixel of the photomultipliers to be readout, supplying N, e.g. 64 amplified readout signals, e.g. via an interconnect (29), to a multi-channel ADC chip (30), adapted for digitizing an amplified signal digitally before being streamed out by a digital streamer or subsystem (31), connected to a further signal processing unit for rendering of an image, e.g. as depicted by FIG. 10.

[0025] It is remarked that a photosensitive array may be known per se from multiple publications, e.g. WO2018218298, that however still do not lead to the present invention and its superior measuring capability. In the mentioned case for instance, the front end and the photosensitive array are, unlike in the present invention, embedded on the same semiconductor die. These are in the new method and set-up rather separated so that an Off-the-Shelf photosensitive array can be connected to, i.e. functionally coupled with e.g. a discrete printed circuit board that ensures the front-end functionality. Also, in the present case the front-end does not use photon counting or a photo-counter and does not rely on photon timing but rather relies on the analog mode of operation of such SiPM. Hence, it does rely on a so called SPAD, since the contemporary SiPD chip as departed from and improved upon, in the present example the Hamamatsu MPPC device, is the integration of multiple Single-Photon Avalanche Diode (SPAD) per pixel, but it does not require to reset the avalanche multiplication, usually performed by means of a quencher, because of the application and use of a biasing mode in the invention. Also, no event detector used since the invented manner of application continuously acquires the average intensity of light and converts it periodically in a digital signal. Hence the invention does not collect so-called Time of Flight of singles photon, but rather collects the average intensity of light simultaneously on each pixel periodically with a pre-defined interval of in the present example of preferably 25ns. Further characterization of differences lies in the fact the in the application of the invention no simultaneous collection two images takes place, but a single image is collected, based on the analogue signal of each MPPC, thus based on the average intensity of light of each pixel. Also the present invention does not consist in arranging two types of photo sensors on the same die as in this published case, but rather uses a commercial SiPD array, in this example above mentioned MPPC, of 8x8 pixels. Also the present application is not intended for 3D hyperspectral image acquisition, rather optimizes 2D acquisition.

[0026] The present invention might, at least partly alternatively denoted, be considered to also be defined as a photons capturing device of the known type, allowing operation in at least a transmission electron microscope at a practically useable acquisition rate, preferably at reasonable costs and in a relatively modest volumetric envelope. In accordance to the present invention such may be essentially achieved by a method of detecting fast high contrast transitions at image capturing, the method comprising the steps of providing a chip of at least one silicon photomultiplier array, where the array can be a matrix or a line of pixels, directly coupling each pixel comprising at least a photomultiplier thereof directly to a distinct amplifier circuit, reading out an integrated analog charge of each of the pixels. The amplified signals are in a further elaboration of the invention be provided to an analogue to digital convertor or ADC, and subsequently processed in a digital domain.

[0027] The invention recognizes that sampling rate and dynamic range are key to certain applications like the application of TEM. It is recognized in this respect that most systems with a DC coupling are high frequency but low resolution, e.g. in comparison with AC coupling systems which are somewhat slower but with higher resolution. It is seen in practice to use AC coupling to increase signal resolution by especially removing leakage current from the detector. For obtaining an AC coupling, a DC coupling may be departed from, while inserting a capacitor in series between the sensor and the amplifier of the circuit. The present invention however, amongst others combines the advantage of a DC coupling, thereby measuring absolute light intensity, with a digital readout in which an AC coupling is performed by measuring the signal amplitude by subtracting the dark current from the average local light signal in the digital domain, thereby providing optimum dynamic range. Using a high resolution, typically above 12-bit, fast analogue-to-digital convertor or ADC, say above 10 million samples per seconds or MSPS, enables to get rid of the offset and obtain sufficient contrast in the digital domain even if only a smaller range of the full scale range or FSR of the ADC is utilized, e.g. as depicted in FIG. 8. The present invention hence combines precision and speed/high frequency. Where reading out is performed in analogue charges, the information is, amongst others for meeting contemporary industrial standards, transformed digitally for recording or archi ving and e.g. later processing, including correction of signal, hence utilizing and allowing the convenience in contemporary digital domain processing. In doing so, the photomultiplier of the system, preferably embodied with a number of diodes, or a diode array, is in a preferred embodiment read out as a single value.

[0028] It is remarked that some publications may at surface level seem to correspond with the present invention, as is the case with

[0029] Where it is known that the gain-bandwidth product of the amplifier often is a limiting factor to signal speed, and efforts to overcome the same are directed to dedicated and often scientific level only or otherwise highly expensive solutions, the present invention unexpectedly proposes the application in the present context of an elegant known per se, industrially applicable solution by performing amplification in two or more consecutive partial stages. In this respect it is in advance of other to be described factors remarked that where it may normally be considered that the gainbandwidth product of the amplifier would become a limitation to the signal speed, which deals with an optimal, typically high frequency level, e.g. a 50MHz in a presently targeted application, as a detected photon would generate an output signal of less than 20ns rise time. Splitting the amplifier in two stages in the set up and goal according to the present invention allows for a higher gain bandwidth product that a single amplifier could allow in order to cope with the application needs.

In yet further elaboration the new method may hold the feature that light intensity is measured by using the piling-up effect of the parallelization of each photomultiplier or avalanche photodiodes in each pixel, in particular actually creating a photomultipliers array or a diodes array as integration of charges, in particular in a biased photoconductive mode, and converting the amount of light received per pixel in an electrical signal with a short response time, more in particular with a quick decay, for time domain high dynamics. It will hence be evident that the invention uses an otherwise known, or known per se sensor in a quite different and unexpected manner, however leading to significantly improved measuring characteristics. Some counterintuitive measures may be involved in reaching at least part of such result, e.g. having generated in fact and using a pulse by an array of the sensor, and integrating the same in fact, as will effectively be explained in the description. Certainly, in fact the present invention may also, i.e. alternatively be defined as a method of measuring light intensity, in which light intensity is measured by using the piling-up effect of the parallelization of each photomultiplier or avalanche photodiodes in each pixel, in particular actually creating a photomultipliers array or a diodes array as integration of charges, in particular in a biased photoconductive mode, and converting the amount of light received per pixel in an electrical signal with short persistence, more in particular with a quick decay for time domain high dynamics.

[0030] In further elaboration each photomultiplier of the pixel array may be provided with a number of diodes, in particular in the form of a chip forming an array of arrays, alternatively denoted a pixel array of diodes arrays, more in particular in the order of at least hundreds per photomultiplier. At least part of the photomultiplier array, in particular the full array may be readout as a single value. In this context either one or both values of mean light and ultimately instantaneous absolute amount of light may be determined. Also, the absolute amount of light may be retrieved by correcting the digital signal for a resulting offset by means of a digital calibration procedure at the system level. Factors that may be taken into account in this respect may include e.g. the sensor temperature and bias.

[0031] Favorably, amplification may be performed with a high amplification gain, in particular gain bandwidth product of at least 20GHz, more in particular determined by considering a useful range of the ADC input, in particular while performing the amplification in two or more consecutive partial stages. The amplification of the amplifier circuit, in particular at each amplification stage, may be set with an at least relatively high amplification gain, i.e. is performed in the order of hundreds of times. In line with the preceding, the present invention and method may rely on the provision of a fast Analogue to Digital Convertor or ADC in the sampling rate of 10 to 100MSPS. For determining an actual light intensity difference, apreceding calibrated digital offset is subtracted from an acquired value. The ADC may herein again favorably be provided with a multiplexer, and may be embodied as one of a multi-channel with embedded analogue multiplexers, and as a multi ADC embedded in a single chip. In one favorably designed embodiment, the array may be comprised of at least 4 pixels or photomultipliers, arranged in line or in matrix. [0032] An industrially interesting embodiment of the invention may hold that the detection concept is incorporated into an ASIC. The invention may also hold an embodiment with the incorporation comprising a set of chips, comprising at least a silicon photomultiplier array chip and an ADC chip which is functionally coupled thereto. In yet a further embodiment of the method of the invention, the invention may hold a signal acquiring system, comprising a photomultiplier array electrically connected to a front-end subsystem, e.g. by an interconnect, comprising a two-stage amplifier circuit for each pixel of the photomultipliers to be readout, supplying N, e.g. 64 amplified readout signals, e.g. via an interconnect, to a multi-channel ADC chip, each amplified signal being digitized thereby, before being streamed out by a digital streamer or subsystem to a high level signal processing unit; an ASIC configured for either embodying the system or for executing the method of the invention; a set of chips for interconnected application, including an ASIC, configured for executing the method according to the invention in a manner distributed over each of the chips of the set; and a device, in particular TEM microscope, improved by the inclusion of any one of a chip, chip set, system and method in accordance with the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Various aspects of the invention and an example of a possible embodiment of the invention are illustrated in the drawings in which:

[0034] FIG. 1 by way of one possible application that is improved by the present invention, is a schematic representation of a transmission microscope system provided with a camera;

[0035] FIG. 2 is a typical pulse shape of a silicon characteristic of a photomultiplier;

[0036] FIG. 3A is a known per se schematic lay out of a basis configuration of an exemplary type of silicon photomultiplier array, with FIG 3B providing a simplified, electrical schematic thereof;

[0037] FIG. 4 is a schematic representation of a conventional measuring set up or so called acquisition chain using a silicon photomultipliers;

[0038] FIG. 5A illustrates a problematic situation of piling up of the pulse of two photons such as is recognized to occur in situations with e.g. high contrast and fast changes, i.e. high dynamic, in which a single photon would create a signal 23 and the result of piling-up of photon-effects is represented by the combined or resulting signal 23, 24 of photon effects, with part 24 representing the effect of a following photon; [0039] FIG. 5B is an illustration of possible outcome when using conventional measuring techniques such as photo counting for acquiring pixel intensity at high rate, with the upper graph representing periodic pulses, and the lower random pulses;

[0040] FIG. 6A is a schematic of a fairly standard, i.e. known per se and simple transimpedance amplifier circuit

[0041] FIG. 6B and6C represent modifications to the standard transimpedance amplifier circuit converting incident light to charges by way of biasing and for measuring the charges by way of direct and indirect coupling respectively;

[0042] FIG. 6D represents an otherwise basic amplification circuitry, adapted by splitting the amplifier in two stages, and unexpectedly applied as a DC type of measuring for imaging high contrast and high dynamic signal;

[0043] FIG. 7A represents the output signal of a photon multiplier under high level of incident light on top of its own dark current signal (DS).

[0044] FIG. 7B represents the output of a biased photon multiplier under pulses of high level of incident lights, providing a local average signal of light (LS), highlighting the instants of analogue, locally average dark current signal (DS) as wel, and analogue light signal difference to be readout as the pixel amplitude (PA), with distinction of the integral AC level (ACS) of this signal.

[0045] FIG. 8 illustrates the effect in accordance with the invention of using a fast analogue to digital convertor or ADC, in order to get rid of the signal offset SO while obtaining sufficient contrast of the signal dynamic (SD) in a digital domain and while applying merely a small range of the full scale range or FSR of the ADC;

[0046] FIG. 9A and 9B provide signal characteristics of the DC coupled and AC coupled amplifier circuits of FIG 7 A and 7B respectively, while a slow modulation of amplitude occurs in the input signal, and such modulation visible in FIG. 9A being filtered out in the AC coupled signal shown in FIG. 9B ;

[0047] FIG. 10 is an illustration of a possible signal acquiring architecture in accordance with the invention;

[0048] FIG. 11 is a schematic representation of an alternative architecture according to the invention, in which an ASIC cooperates with the photomultiplier array; [0049] FIG. 12 is yet a further alternative architecture in which the same is accommodated in a single ASIC or flip-chip assembly;

[0050] FIG 13-15 represent prior art representations from which the present invention departed from in part and as may e.g. be found in industrially released technical notes and handbooks, in this case Hamamatsu released document cl2332-01_kaccl233e.pdf for MPPC devices.

DETAILED DESCRIPTION

[0051] FIG. 1 represents a state of the art multi beam electron-optical system 1 for a transmission electron microscope, forming one possible application of the present invention. Such a system comes with an otherwise not represented source for electron beams 3, such as a known Schottky source, which in a manner known per se, using e.g. extraction electrodes and a current limiting array creates a multiplicity of electron beams, e.g. in an array of 14 by 14 beams. In the represented system the beams 3 are condensed by an accelerator lens 2 and passed through a variable aperture 5 via a second condenser lens 4, and scanned by scan coils 7 after having passed an intermediary lens 6. The scanned beams pass an ultra-high resolution lens 8 before being projected onto sample , which in this case sits right on top of a photon generator 9, e.g. embodied by a so called YAG or fluorescent material and forming part of a detector for capturing, at least detecting electrons transmitted through the sample.

[0052] The light beams 10 emitted by photon generator 9 are captured by a camera system 13 by projection of the light beams 10 through a window 12, here combined with a focusing lens. The projection may include redirecting the light beams 10 via a mirror 11.

[0053] FIG. 2 illustrates the typical signal as produced by contemporary silicon photomultipliers, henceforth SiPM, used for counting the number of pulses generated thereby as an indicator of incident light. The pulse signal generated typically comprises a very short rise phase 14 of several nanoseconds and a much longer decay or reset phase 15 of up to 100 nanoseconds. General teaching as per Wikipedia page in Silicon photomultiplier amongst others holds that signal decay time is inversely proportional to square root of photoelectrons number within an excitation event.

[0001] FIG. 3 A is a schematic representation of a known array 16 of silicon photomultipliers SiPM. Typically such SiPM come with a basic N-doped body layer 17, an Avalanche layer 18 and P-doped entrance windows 19 for receiving photons. The photon receiving windows may be provided with an anti-reflective layer 20 and are physically separated, here by a strip of the avalanche zone creating rectangular cells. The strip areas are used in this example for allocating a so-called quenching resistor 22 with which the window, or diode or PN-junction forms a cell in the one orientation, and a bias lead 21 on top of the strip oriented in transverse direction, each cell being a pixel or a sensor cell for the later image acquisition system. In order to increase the sensitive area, it is common to use photodiode array, i.e. several photodiodes in parallel, in the vicinity of each cell. Such Array is electrically represented in FIG. 3B where each diode symbol represents an avalanche photo diode connected to a serial resistor also called quenching resistor. This is the parallelization of these APD which make possible to use a summed signal of all current circulating through the diode array between its cathode K and anode A. FIG 3B provides a simplified, electrical schematic of FIG. 3A, where each diode symbol represents an avalanche photo diode connected to a serial resistor also called quenching resistor and where cathode K and anode A are the available electrical connections.

[0002] FIG. 4 is a schematic representation of a common use of a light acquisition chain with silicon photomultipliers S, in general applicable to any camera or spectrometer as e.g. may be applied in particle physics. Light intensity LI of a light signal is captured by a sensor S, coupled to an amplifier A, coupled to an analogue to digital convertor ADC, which provides a digital value DV for the measured light intensity. Main architectures in photo detection consist in either photo-counting or light integration, and either AC or DC coupling. The choice of architecture is dependent on the information to be retrieved and on the nature of the incoming light.

[0003] While the transition from previously known photomultiplier tubes towards above described solid state single photon sensitive devices has opened tremendous possibilities for measuring high frequency signals such as in LiDAR applications, measuring at high speed with these contemporary sensors may still pose a problem in situations where high acquisition rate, i.e. high frame rate is desired such as high speed camera, e.g. as used for research, typically for analyzing quick changing events, with more than 10.000 frames per second, yet not excluding high resolution imaging, and such as in high contrast situations. [0004] Hence, for a camera to provide ultimate time and intensity resolution of a highly dynamic amount of light, e.g. as in contrast between dark and bright spots of a sample, the present invention integrates a photo detector array in a complete readout system with direct coupling.

[0005] Where such may render the same global measuring architecture as provided by FIG. 4, in the present invention, the energy information or wavelength of the incoming light is now regarded not of interest and here assumed constant. Rather, the invention seeks to measure the amount of light received by each pixel of the photon-detector at a very high frequency, say a frame rate of over 1 Mega Hertz, (>1 MHz), i.e. an interval time below lus. It may be clear that such a requirement brings about a barrier to using a photo-counting architecture, in which the state-of-the-art maximum throughput is currently about 10 Mcps, i.e. 10 million counts per second.

[0006] It is an insight underlying the present invention, that this state of the art limitation is mainly due to a what is called a relatively slow signal, i.e. what is here, in accordance with underlying insight, concluded as to mean low system throughput capability, that is to say at least due to above mentioned pile up of signal due to signal decay time, obtained after electrical conversion of the photon energy. The present invention recognizes a circumstance that in order to measure light intensity by counting the numbers of incident photons, each photon must be resolved, and that such is challenging and in fact also not possible in case of so-called pile-ups. Such pile up phenomenon is illustrated by FIG. 5A, which depicts a situation where at least two photons with a signal 23 arrive in a short interval. As illustrated this situation caused a pulse pile-up 24 in the signal, at least compared to a non-piling up pulse signal 23, or 14 and 15 in FIG. 2. Since counting is performed by identifying a flank of a pulsed signal as presented by FIG. 2, the pile-up of two or more photons will eventually be seen as a single pulse signal, implying that at least one count is lost in case of each pile-up. The present invention recognizes the latter as a cause of attaining the high pulse count rates as desired in contemporary applications. In particular it further recognized and desires to solve measuring situations where the incoming photon rate is higher than the system throughput capability, and where hence the chance of pile-up occurrence is large.

[0007] FIG. 5B is an illustration of possible outcome when using conventional measuring techniques such as photo counting for acquiring pixel intensity at high rate. The top chart illustrates a periodic distribution of photons whereas the bottom chart represents a random distribution of photons. If photons can be counted per interval, the resulting countrate can be calculated, in the example of 100 ns interval, the resulting count rate is 10 million count per seconds as 1 count per interval is recorded (CPI). [0008] With a conventional photo-counting system with for instance 10MHz sampling rate and due to the random nature of photon arrivals, this means that about 1 photon may be counted in every 100ns sampling period. Contrary to a periodic signal, where interval between events is constant, the number of incident photons per time interval can in the desired situation to be measured, very well be 0 (zero) or 3 (three) due to the randomness of the light generation process. No signal, i.e. zero (0) counts per interval CPI, usually expressed as counts per second, abbreviated cps, would lead to 0 (zero) photon count per time interval or CPI measurement at any time. A maximum measureable signal can on the other hand also lead to a zero (0) count in a same time interval of for instance 100 ns. An example is depicted in FIG. 5C, where both measuring channels see and count an averaged flux of 10 Mcps. Nevertheless, whereas 1 count per interval is always reported with the periodic signal, zero (0) count can be reported in the same time interval for the random signal as illustrated between 0.2us and 0.3us. As amongst others underlying the present invention, this recognized situation is concluded to mean that a proper measurement of the light intensity requires a longer integration time, for instance one micro second, to average the number of photons per interval. Such a duration is however not convenient for an application where pixels intensity must be acquired at an even faster rate than 100ns.

[0009] An obvious measure to avoid pile-up or to correct for it, seems to be to keep the incoming photon rate low. However, on the other hand, in a sensing application such as is desired, and for even further improving high resolution TEM, however including other applications in this field as amongst others mentioned in the introductory part, the invention recognizes that the application requires short measurement times and a high dynamic in terms of signal amplitude, where 1 to about 10000 incident photons needs to be identified in this short measurement interval. This in fact leads to, or requires a high incoming photon rate.

[0010] The present invention recognizes that for such a latter mentioned pixel rate, photon counting may not be adapted, at least not adequately, and proposes a different approach of light integration. This new proposal still utilizes the same kind, state of the art silicon photomultiplier, however in a manner different from what is normally proposed. Usually, such ordinary silicon photomultiplier is used to detect a single photon per pixel and per time interval. The generated signal of the incident photon is thus generally quickly converted in a digital trigger instant. In this respect the invention recognizes that silicon photomultipliers as sensor cell may be used to convert incident light into charges by means of proper biasing. Hence it proposes to turn to analogue measuring or reading out of the photomultiplier, rather than digitally. In this respect it should be remarked that even in pulse counting mode the readout from the array can be analogue. Actually it might be represented as that the number of single photon avalanche diodes is of importance in the discretization of the light amount and in the requested signal dynamic to be readout. The photon multiplier, being based on an array of single photon avalanche diodes in parallel, is used and recognized to allow summing of the response of each diode so that the output signal is proportional to the amount of incident photons absorbed. The proposed analogue reading out method may, despite the common practice of counting pulses by way of detecting rise phase 14 and decay phase 15 of signal, at hindsight, in principle still be derived from the basic signal representation of FIG.2, where the signal is represented as an accurate measurement of Voltage against time in ns. However, a pulse duration would under contemporary conditions, in which incidentally the photomultiplier has developed, normally typically be measured digitally.

[0011] Once having recognized the possibility of such a new use and set-up, the invention recognizes the possibility to measure the charges with either direct (DC) or indirect (AC) coupling, in particular when using a known per se amplifier. One example of such is the simple transimpedance amplifier circuit proposed by the Analog Devices technical article MS-2624, here represented as FIG. 6A. Such standard set up converts light into charges, thus current, i.e. under the electrical field provided by the diode biasing. The photodiode, in this circuit connected to ground, is in the left hand side of the drawing referenced by the pair of Z-shaped light arrows, and the circuit provides an output terminal and signal VOUT at the right hand side of the diagram. Incidentally, it is further recognized by the present invention that the known per se acquisition chain of FIG. 4 can also be used to measure photon energy when the digital value thereof compares to that of the representation in FIG. 2 and using a high sampling rate.

[0012] FIG. 6B and 6C illustrate the above mentioned direct or DC coupling and the indirect or AC coupling respectively, showing a connection of the photodiode to bias voltage VBIAS- In case of the DC coupling of FIG. 6B, the photodiode output is directly coupled to an amplifier circuit. Hence without any capacitor as is included between the photodiode and the amplifier circuit in the AC coupled configuration of FIG. 6C.

[0013] While the AC coupling of FIG. 6B should from an electrical standpoint be preferred in order to get rid of the biasing voltage and to reduce total noise of the circuit output, the present invention recognized that counter-intuitively, a direct coupling should be applied despite the fact that noise will be maintained, i.e. be included in the signal, hence will also be measured and, what's more, will also be amplified. Yet, it was in view of the new manner of measuring to be set, recognized by the present invention that such an AC coupled solution would otherwise loose DC supplied information such as dark current or mean light and ultimately instantaneous absolute amount of light. As it comes to higher level of incident light, the response signal does not allow for individual pulse counting anymore as one can understand for the signal shape given in FIG. 7A and 7B. The local average value becomes the integration of charges equivalent to the incident light level. Hence, in accordance with the latter insight and again further underlying the present invention, measuring should for attaining high measuring rates, against a prejudice in the art, where signal noise is normally avoided by way of inclination, or not primary nature as it were, be performed using DC connection with, in fact despite the presence, at least initially, of "non cleaned" or raw signal in the measurement taking. Where in the ordinary case of one signal as in Fig. 7A or Fig. 7B, removing dark current can be profitable, such could in another case or in specific application, imply that a mean signal will also be lost, thereby reducing the contrast in total acquisition.

[0014] It is additionally recognized that the contemporary manner of measuring using AC coupling has a time constant which affects the measured signal amplitude. This will depend on the local mean amount of light received. In this respect, FIG. 9A illustrates the signal amplitude in case of DC coupled amplification. In contrast to such a type of signal, an AC coupled signal as in FIG. 9B does not provide information about the amount of light but only the local variation. In fact the AC coupled signal meanders above and below the zero current line, implying a loss of absolute light intensity information.

[0015] On the other hand, while DC coupling is known per se, it is often used to know the absolute amount of light or to enable time over threshold measurements. This might be performed, at least makes sense when single photons must be detected and characterized such as time of arrival or incident energy. It also makes sense to measure amount of light in slow changing application. In fast changing application, as is recognized to be the case e.g. a TEM application, DC coupling would not be trivial because of the chances of signal saturation. It is however, further recognized by the invention that by proper gain tuning, this effect may be limited, and that in case of an application with focus on imaging high contrast and high dynamic, a DC coupling should nevertheless be preferred, be it while adding a special measure of gain tuning. [0016] A DC coupling, of which basic circuitry is provided in FIG. 6B, with the diode used in photoconductive mode, i.e. with reverse bias applied, is hence proposed to improve speed and linearity of the diode as opposite to photovoltaic mode such as implemented in FIG. 6. The reverse bias, however increases the dark current and noise current. In such implementation, the amplifier stage also contributes to signal offset error on top of dark current. To improve the contrast and signal resolution, the invention sets a high amplification gain in the order of 100 to 1000 in order to fit the signal of the sensor , e.g. multi-pixel silicon photon multiplier, to the ADC input, the maximum gain being dictated by the maximum amount of incident light to be digitized without overranging the ADC input. Because any offset may be added to the amplified signal, the gain should be set by considering the remaining useful range of the ADC input, e.g. as indicated by FIG. 8. To retrieve the absolute amount of light, the digital signal is corrected in a digital domain for the resulting offset by means of a (digital) calibration procedure at the system level. The invention favorably applies a trick to reduce the effect of the amplifier offset, splitting the amplification in two stages so that parasitic offsets like input bias current and voltage offset are not amplified with the full gain factor. This is also valid for the noise level. Above that, the gain-bandwidth product of the amplifier would become a limitation to the signal speed which is dealing with an optimal, typically high frequency level, e.g. a 50MHz in a presently targeted application, as a detected photon would generate an output signal of less than 20ns rise time. Splitting the amplifier in two stages in the set up and goal according to the present invention allows for a higher gain bandwidth product that a single amplifier could allow in order to cope with the application needs. It is being recognized that amplifiers have a gain-bandwidth product (GBW) limit, nowadays in the range of 1 to 10 GHz, the invention according to yet further insight applies a measure of splitting the gain in two stages as schematically represented by FIG. 6D, recognizing that the GBW product of each of the split amplifiers allows for a larger bandwidth. Each gain bandwidth product being for instance 1000MHz (gain=20, Bandwidth = 50MHz) with a total chain gain bandwidth product of 20GHz (gain=400, bandwidth = 50MHz). Offset and gain correction however, may according to the present invention preferably be applied to the digital domain, removing complexity from the analogue path. Figure 8B illustrates the effect of a further measure in accordance with the invention, with a signal with relatively strong, i.e. useful signal dynamic SD, however with high offset SO. The invention, in a further elaboration proposes to get rid of the offset SO, so as to remain with sufficient signal contrast, at least in the digital domain, utilizing only a relatively small range SD of the full scale range or FSR of the analogue to digital convertor or ADC, it being provided that a fast ADC is utilized, i.e. working at least at about 20MHz to be able to acquire the desired pixel rate of 100ns.

[0017] FIG. 10 provides an example of a signal acquiring architecture in accordance with the invention, in which the photomultiplier array 26 is wired via an interconnect 27 to a front-end subsystem 28 comprising of a 2-stage amplifier circuit for each pixel to be readout, supplying N, e.g. 64 amplified channels via interconnect 29 to a multichannel ADC chip 30, each amplified signal being further digitized thereby, before being streamed out by a fully digital streamer or subsystem 31, for communication to a high level unit not represented in the illustration.

[0018] FIG. 11 represents the architecture of FIG. 10, at least the principles thereof, or at least the method of detecting fast contrast transitions at image capturing according to the invention, carried over into a so-called ASIC embodiment with DC signal coupling. Such figure highlight the fact that the required functions of amplifiers, multiplexer, ADC and transceiver can be integrated in a chip for higher integration level.

[0019] Figure 12 represents yet a further possible embodiment of the principles of the present invention, wherein the principle or method according to the invention is in application executed by means of a completely integrated ASIC comprising the photomultipliers array and the ASIC embodiment described above. One possible embodiment of such an ASIC instead of an above exemplified discrete implementation would require multi-channel amplifier/front-end and a multi-channel ADC in one chip, each of e.g. 64 channels. Such ASIC embodiment features a matching input impedance for reducing noise, trans impedance amplifiers and a high gain with a factor preferably being of at least about 100, thereby allowing a synchronous sampling of said multiple signals.

[0020] FIG.’s 13 to 15 are extracted from a Hamamatsu manual, hence related to an example of the shelf SiPM chip, here the so-called MMPC chip, for illustration purposes, with FIG. 14 a.o. illustrating the prescense a characteristics of a Analogue mode in addition to a these days normally used Digital mode thereof, with FIG. 14 illustrating the decay time as in an off the shelf chip, as used in the present invention at averaging pile up of signals, and where FIG. 15 may be representative of a so-called a ‘piled-up’ signal in presence of light.

CLAUSES: The present invention may further be described along the following clauses.

I. Method of detecting contrast transitions in the time domain, in particular so- called fast high contrast transitions at image capturing, the method comprising the steps of providing a chip (26) of at least one silicon photomultiplier array, where the array can be a matrix or a line of pixels, directly coupling each pixel comprising at least a photomultiplier thereof directly to a distinct amplifier circuit (28), reading out an integrated analog charge of each of the pixels, providing the amplified signal to an analogue to digital convertor or ADC (30), and subsequently processing the amplified signal in the digital domain.

II. Method according to clause I, in which light intensity is measured by using the piling-up effect of the parallelization of each of either a photomultiplier and an avalanche photodiode, in each pixel, in particular creating a photomultipliers array or a diodes array as integration of charges, in particular in a biased photoconductive mode, and converting the amount of light received per pixel in an electrical signal with a predefined response time, typically short response time, more in particular for coping with signals with a quick, virtually instant decay for time domain with so-called high dynamics.

III. Method of measuring light intensity, in particular in accordance with any of the preceding clauses, in which light intensity is measured by using the piling-up effect of the parallelization of each photomultiplier or avalanche photodiodes in each pixel, in particular actually creating a photomultipliers array or a diodes array as integration of charges, in particular in a biased photoconductive mode, and converting the amount of light received per pixel in an electrical signal with short persistence, more in particular with a quick decay for time domain high dynamics.

IV. Method in accordance with any of the preceding clauses, in which each photomultiplier of the pixel array is provided with a number of diodes, in particular in the form of a chip forming an array of arrays, alternatively denoted a pixel array of diodes arrays, more in particular at least hundred diodes per photomultiplier.

V. Method in accordance with any of the preceding clauses, in which at least part of the photomultiplier array, in particular the full array is readout as a single value.

VI. Method in accordance with any of the preceding clauses, in which the amplification is performed with a relatively high amplification gain, in particular a gain bandwidth product of at least 20GHz, more in particular determined by considering a useable range of the ADC input.

VII. Method in accordance with any of the preceding clauses, in which the amplification is performed in two or more consecutive partial stages.

VIII. Method according to any of the preceding clauses, in which the amplification of the amplifier circuit, in particular at each amplification stage, is set with an at least relatively high amplification gain, i.e. is typically performed at least hundred times.

IX. Method according to the preceding clause, in which the ADC is provided with a multiplexer.

X. Method according to the preceding clause, in which the ADC is embodied as one of a multi-channel with embedded analogue multiplexers, and a multi ADC embedded in a single chip.

XI. Method in accordance with any of the preceding clauses, in which the array is comprised of at least 4 pixels or photomultipliers, arranged in line or in matrix.

XII. Method in accordance with the preceding clauses, applied in a physical embodiment.

XIII. Method in accordance with any of the preceding claims, where the SiPM analogue signal output is amplified with a factor adapting or scaling the signal to an ADC input range with a bandwidth higher than 10 MHz in a 2-stage circuitry, allowing a change of light intensity higher than 5 MHz, typically in the range of 20 to 50 MHz.

XIV. A set of chips, in particular for executing a method in accordance with any of the preceding clauses, comprising at least a silicon photomultiplier array chip and an ADC chip which is functionally coupled thereto, which array chip and ADC chip may be embodied as a single unit, the set preferably at least configured for executing the method according to the preceding method clauses in a manner distributed over each of the chips of the set.

XV. ASIC configured, at least programmed for either embodying the system, or for executing the method in accordance with any of the preceding clauses.

XVI. Imaging device such as a microscope, in particular a TEM microscope, improved by, at least equipped with or arranged for either the inclusion or execution of any one of a chip, ASIC, chip set, system and method in accordance with any of the preceding clauses, in which in particular the image captured by such chip or ASIC is related to an image captured by the imaging device.

[0021] The present invention, apart from what has been described above, also relates to all details in the figures, at least for as far as these are directly and unambiguously retrievable by a skilled person, and to everything that is described in the following set of claims.

Claims

26 CLAIMS

1. Method of measuring incident light flux on a pixels array using an Silicon Photo Multiplier, henceforth SiPM, array availing of an analogue operational mode, the method comprising i) using the SiPM array in its analogue mode of operation; and ii) functionally coupling the output of the array to a two-stage DC-coupled analogue amplifier with an output bandwidth within the range of 10 to 100 MHz, preferably within the range of 20MHz to 50 MHz ; iii) outputting the amplified signal to a multi-channel Analogue to Digital Converter, henceforth ADC, with a per channel sampling frequency within a range of 10 to 100 Mega Sample Per Second, henceforth MSPS, preferably within a range of 40 MSPS to 100 MSPS, and a sampling resolution of at least 12, preferably 14-bit.

2. Method according to claim 1, in which the incident light flux per single cell of the SiPM array is acquired in analogue mode, where the analogue signal is the output of a SiPM single cell, alternatively denoted single pixel.

3. Method according to claim 1 or 2, in which the analogue mode consists in measuring an average SiPM current output at the moment of sampling, in particular the current output of a signal or else of piled up signals of a the Silicon Silicon Avalange Photon Diode or SPAD within the same pixel.

4. Method in accordance with any of the preceding claims, in which an integrated analogue signal has a time constant, e.g. at about 50ns, longer than the digital sampling period, e.g. at about 25ns.

5. Method according to any of the preceding claims, in which an analogue output signal output by the SiPM is coupled with an analogue front-end in a direct current or DC domain.

6. Method in accordance with any of the preceding claims, in which the analogue signal output by the SiPM is amplified by a gain factor within the range of 50 to 400, e.g 200.

7. Method in accordance with any of the preceding claims, in which the analogue signal output by the SiPM is amplified in a two-stage amplifier circuitry, preferably with a bandwidth higher than 10 MHz.

8. Method in accordance with any of the preceding claims, in which only part of the ADC input range is used with a bit-resolution of at least 12 bit, typically 14-bit, in order to sufficiently quantify the amount of light intensity at moment of sampling.

9. Method in accordance with any of the preceding claims, in which the measured amount of light intensity sets forth the difference between a light signal at moment of sampling and a reference offset signal, the reference offset signal typically combining an amplified sensor’s so called dark signal and an amplified electronics front-end offset signal.

10. Method in accordance with any of the preceding claims, in which each pixel channel is individually corrected for gain and offset errors.

11. Method in accordance with any of the preceding claims, in which the result of each pixel current output is eventually converted into a digital value for further processing in a digital domain, in which later calibration values are used to correct for individual channel disparities, in particular with the results of each converted pixel current output from the SiPM array being equalized.

12. Signal acquiring system, in particular applying the method in accordance with any of the preceding claims, comprising a photomultiplier array (26) electrically connected to a front-end subsystem (28), e.g. by an interconnect (27), comprising a two- stage amplifier circuit for each pixel of the photomultipliers to be readout, supplying N, e.g. 64 amplified readout signals, e.g. via an interconnect (29), to one or more multichannel ADC chip (30), e.g. in a number of eight, before being streamed out by a digital streamer or subsystem (31), connected to a further signal processing unit for rendering of an image.