Single-photon avalanche diode-based image sensor and driving method thereof

Abstract

According to one embodiment of the present disclosure, a single-photon avalanche diode-based image sensor includes a single-photon avalanche diode (SPAD) that generates a plurality of pulses corresponding to each of a plurality of photons received during a predetermined exposure time, a front-end circuit that receives a set of pulses received during a portion of the exposure time among the plurality of pulses, a counter that counts the number of pulses in the set of pulses, and a global clock that provides a plurality of clock pulses to the front-end circuit after the portion of the exposure time, and the image quality of an image acquired using the single-photon avalanche diode can be determined based on the timing of the global clock.

Description

[Technical Field]

[0001] The present disclosure relates to a single-photon avalanche diode-based image sensor and a driving method thereof. [Background technology]

[0002] Single-Photon Avalanche Diode (SPAD) is a sensing technology that detects faint light signals at the photon level. In particular, SPAD uses avalanche multiplication, which amplifies a single incident photon, making it highly sensitive and making it very easy to take pictures in dark places.

[0003] Meanwhile, in such single-photon avalanche diodes, photons require the operation of counting trigger pulses generated by photons. However, if a counter with a large number of bits is used to count a large number of photons, the circuit size becomes large, which increases power consumption.

[0004] This invention was derived from research conducted as part of the development of a Global Shutter-based 20x20cm large-area Hybrid X-ray video detector (Project ID: 1711138024, Project ID: KMDF_PR_20200901_0048-01, Research Project Name: Ministry of Science and Technology, Ministry of Welfare and Trade, Ministry of Trade and Industry, Project Management Agency: Ministry of Science and Technology, Ministry of Health, Labour and Welfare, Ministry of Industry and Trade, Project Executing Agency: Rayence Corporation, Research Period: March 1, 2021 - December 31, 2022).

[0005] On the other hand, there is no financial interest in any aspect of this invention from the Korean government, which is the subject of the project. Summary of the Invention [Problem to be solved by the invention]

[0006] In order to solve the above problem, the present disclosure provides a single-photon avalanche diode-based image sensor that reduces power consumption by estimating the number of total pulses (i.e., total photons) using the overflow point of a counter. [Effects of the Invention]

[0007] According to some embodiments of the present disclosure, the power consumed by the counter can be significantly reduced by counting only a portion of the total photons received in a single-photon avalanche and estimating the total number of photons using the counter overflow time.

[0008] According to some embodiments of the present disclosure, images with excellent image quality can be obtained depending on the lighting environment. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a block diagram illustrating a configuration of a single-photon avalanche diode-based image sensor according to an embodiment of the present disclosure. [Figure 2] 1 is a graph illustrating a method for calculating the total number of photons received by a single-photon avalanche diode-based image sensor according to an embodiment of the present disclosure. [Figure 3] 4 is a graph illustrating clock pulses of a global clock provided to provide clock pulses to a front-end circuit according to one embodiment of the present disclosure. [Figure 4] 1 is an example illustrating the timing of a global clock according to one embodiment of the present disclosure. [Figure 5a] 10 is a graph illustrating extrapolated gains for each type of global clock according to an embodiment of the present disclosure. [Figure 5b] 10 is a graph illustrating extrapolated gains for each type of global clock according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0010] According to one embodiment of the present disclosure, a single-photon avalanche diode-based image sensor includes a single-photon avalanche diode (SPAD) that generates a plurality of pulses corresponding to each of a plurality of photons received during a predetermined exposure time, a front-end circuit that receives a set of pulses received during a portion of the exposure time among the plurality of pulses, a counter that counts the number of pulses in the set of pulses, and a global clock that provides a plurality of clock pulses to the front-end circuit after the portion of the exposure time, and the image quality of an image captured using the single-photon avalanche diode can be determined based on the timing of the global clock.

[0011] According to one embodiment, the timing of the global clock may be determined at least in part based on the number of bits in the counter.

[0012] According to one embodiment, at least a portion of the timing of the global clock can be configured to have a positive linear relationship to the timing of the exposure time.

[0013] According to one embodiment, at least a portion of the timing of the global clock can be configured to have a positive linear relationship to the square root of the timing of the exposure time.

[0014] According to one embodiment, at least a portion of the timing of the global clock can be configured to have a positive linear relationship to the log of the timing of the exposure time.

[0015] According to one embodiment, at least a portion of the timing of the global clock can be configured to have a negative linear relationship to the timing of the exposure time.

[0016] According to one embodiment, the system may further include a processor that determines the timing of the global clock based on the number of photons received during the exposure time.

[0017] According to one embodiment, the end of the portion of time may be based on the overflow time of a counter that counts the number of pulses in the pulse set.

[0018] According to another embodiment of the present disclosure, a method for driving a single-photon avalanche diode-based image sensor includes generating, by the single-photon avalanche diode, a plurality of pulses corresponding to each of a plurality of photons received during a predetermined exposure time; receiving, by a front-end circuit, a set of pulses received during a portion of the exposure time among the plurality of pulses; counting, by a counter, the number of pulses in the set of pulses; and providing, by a global clock, a plurality of clock pulses to the front-end circuit after the portion of the exposure time, wherein the image quality of an image acquired using the single-photon avalanche diode can be determined based on the timing of the global clock.

[0019] According to another embodiment of the present disclosure, a computer program recorded on a computer-readable recording medium may be provided to cause a method for driving a single-photon avalanche diode-based image sensor to be executed.

[0020] Hereinafter, specific details for implementing the present disclosure will be described in detail with reference to the accompanying drawings, provided that in the following description, detailed descriptions of well-known functions and configurations will be omitted if they may unnecessarily obscure the gist of the present disclosure.

[0021] In the accompanying drawings, identical or corresponding components are denoted by the same reference numerals. In addition, in the following description of the embodiments, redundant description of identical or corresponding components may be omitted. However, the omission of a description of a component does not mean that such a component is not included in a certain embodiment.

[0022] The advantages and features of the disclosed embodiments, and methods for achieving them, will become apparent from the following description of the embodiments in conjunction with the accompanying drawings. However, the present disclosure is not limited to the embodiments disclosed below, and may be embodied in various different forms. The present embodiments are merely provided so that the disclosure will be complete and will fully convey the scope of the invention to those skilled in the art.

[0023] The terms used in this specification will be briefly explained, and the disclosed embodiments will be specifically described. The terms used in this specification have been selected based on the functions of the present disclosure and are currently widely used and general terms that are possible. However, these terms may change depending on the intentions of engineers engaged in the relevant field, precedents, the emergence of new technologies, etc. In addition, in certain cases, the applicant may arbitrarily select terms, and in such cases, their meanings will be described in detail in the relevant description of the invention. Therefore, the terms used in this disclosure should be defined based on the meanings of the terms and the overall content of this disclosure, rather than simply by their names.

[0024] In this specification, the singular includes the plural unless the context clearly dictates otherwise. Furthermore, the plural includes the singular unless the context clearly dictates otherwise. Throughout the specification, when a certain part 'comprises' certain elements, this does not mean that other elements are excluded, but that other elements may also be included, unless specifically stated to the contrary.

[0025] FIG. 1 is a block diagram illustrating a configuration of a single-photon avalanche diode-based image sensor 100 according to an embodiment of the present disclosure. Briefly, the image sensor 100 receives a plurality of photons from an external source and generates an image by counting the number of received photons. In this case, the image sensor 100 may consume a considerable amount of power to count photons. To address this, the present disclosure provides an image sensor 100 that reduces power consumption by counting photons up to a predetermined number and estimating the total number of received photons using information regarding when the counting ends. As illustrated, the image sensor 100 includes at least one of a single-photon avalanche diode 110, a front-end circuit 120, and a counter 130.

[0026] The single-photon avalanche diode 110 can receive multiple photons from the outside and generate multiple pulses corresponding to each of the received photons. For example, if N photons (where N is a natural number) are received by the single-photon avalanche diode 110, the single-photon avalanche diode 110 can generate N pulses. The generated pulses can then be transmitted to a front-end circuit.

[0027] The single-photon avalanche diode 110 can receive multiple photons from the outside during a predetermined exposure time and generate multiple pulses corresponding to each of the received photons. For example, if the single-photon avalanche diode 110 receives N photons (where N is a natural number) from 0 ms to 16 ms, the single-photon avalanche diode 110 can generate N pulses. At least some of the generated pulses can then be transmitted to a front-end circuit.

[0028] The front-end circuit 120 can receive a plurality of pulses generated by the single-photon avalanche diode 110. In this case, the plurality of pulses can refer to the plurality of pulses received by the single-photon avalanche diode 110 during the exposure time. For example, when N (where N is a natural number) photons are received by the single-photon avalanche diode 110 from 0 [ms] to 16 [ms], the front-end circuit 120 can receive N pulses from the single-photon avalanche diode 110. Additionally or alternatively, the front-end circuit 120 can receive at least some of the plurality of pulses generated by the single-photon avalanche diode 110. For example, when N (where N is a natural number) photons are received by the single-photon avalanche diode 110 from 0 [ms] to 16 [ms], the front-end circuit 120 can receive N pulses received from 0 [ms] to T [ms] (where T OF <16) until the N OF received (where N OF <N). As another example, when N (where N is a natural number) photons are received by the single-photon avalanche diode 110 from 0 [ms] to 16 [ms], the front-end circuit 120 can receive the N pulses received from T OF1 [ms] to T OF2 [ms] (where 0 < T OF1 <T OF2 <= 16) until the N OF received (where N OF <N).

[0029] The front-end circuit 120 may receive multiple clock pulses generated by an external device (e.g., a global clock). In this case, the multiple clock pulses may refer to multiple clock pulses generated during the exposure time. For example, the front-end circuit 120 may receive M clock pulses from 0 ms to 16 ms. Additionally or alternatively, the front-end circuit 120 may receive multiple clock pulses generated during a portion of the entire exposure time. For example, the front-end circuit 120 may receive T clock pulses during the exposure time from 0 ms to 16 ms. OF In another example, the front-end circuit 120 may receive a clock pulse generated between 0 ms and T 16 ms within an exposure time range of 0 ms to 16 ms. OF It is possible to receive clock pulses generated during [ms].

[0030] The front-end circuit 120 may begin receiving a plurality of clock pulses generated by an external device in response to the signal received from the counter 130. In this case, the signal received from the counter 130 may refer to a signal generated in response to the counter 130 overflowing. More specifically, the signal received from the counter 130 may refer to a signal transmitted to the front-end circuit 120 in response to the counter 130 overflowing while counting a plurality of pulses received from the single-photon avalanche diode 110 through the front-end circuit 120. For example, if the exposure time is from 0 ms to 16 ms, the N-bit counter 130 may use bits from 0 ms to N-1 ms to count the two pulses received from the single-photon avalanche diode 110 through the front-end circuit 120. (N-1) After counting pulses, the counter 130 may overflow. In this case, a value indicating an overflow is input to the remaining bit of the N bits of the counter 130 that is not used for counting, and in response, the counter 130 counts T OF[ms], the front-end circuit 120 can transmit a signal to the front-end circuit 120. Thereafter, in response to the signal received from the counter 130, the front-end circuit 120 can stop receiving pulses from the single-photon avalanche diode 110 and start receiving clock pulses from the external device. That is, the front-end circuit 120 can receive a clock pulse from the external device from 0 [ms] to T OF [ms] or earlier, receive multiple pulses from the single-photon avalanche diode 110, and OF It is possible to receive multiple clock pulses from an external device from 1 ms to 16 ms later. Meanwhile, as described above, a MUX element can be used to switch the front-end circuit 120 between counting pulses received from the single-photon avalanche diode 110 and receiving clock pulses from an external device.

[0031] The counter 130 can count the number of pulses input to the front-end circuit 120. For example, the counter 130 can count the number of pulses from 0 [ms] to T OF The counter 130 can count the number of pulses input from the single-photon avalanche diode 110 to the front-end circuit 120 up to [ms]. OF The number of clock pulses input to the front-end circuit 120 can be counted from 1 [ms] to 16 [ms].

[0032] Meanwhile, although not shown in FIG. 1, the image sensor 100 may further include a global clock (not shown) that generates the above-mentioned clock pulses. In this case, the total number of clock pulses generated by the global clock during the exposure time is counted until just before the N-bit counter 130 (at this time, only N-1 bits of the counter 130 are used for counting) overflows (i.e., T in the above example). OFThe number of pulses counted during the exposure time (up to [ms]) may be equal to the number of pulses counted. That is, if an N-bit counter 130 is used, the total number of clock pulses generated by the global clock during the exposure time is 2 (N-1) This can be preset as an individual, as will be described in more detail later with reference to FIG.

[0033] 1 illustrates the image sensor 100 as including one single-photon avalanche diode 110 for ease of explanation, but is not limited to this. That is, the image sensor 100 may include multiple single-photon avalanche diodes. For example, the front-end circuit 120 may be connected to multiple single-photon avalanche diodes arranged in multiple pixels, respectively.

[0034] 2 is a graph 200 illustrating a method for calculating the total number of photons received by a single-photon avalanche diode-based image sensor (e.g., image sensor 100) according to an embodiment of the present disclosure. Here, it is assumed that a counter (e.g., counter 130) provided for counting pulses generated by a single-photon avalanche diode (e.g., single-photon avalanche diode 110) and / or a global clock is configured with N bits, and only N-1 bits are used for counting. In addition, T in FIG. OF indicates the point in time when the counter overflows as a result of counting multiple pulses received from the single-photon avalanche diode.

[0035] As mentioned above in Figure 1, the counter is switched from 0 to T OF The 2 generated by the single-photon avalanche diode during the time (N-1) In this case, the image sensor is exposed to the light source for a period of time T EXP Since has a predetermined value, the total number of photons received by the image sensor during the exposure time (N OH) can be calculated. In this case, N PH / N OF This allows us to save about N photon counting power. OF means the number of photons received by the image sensor from the time the image sensor starts being exposed to the light source until the counter overflows. However, the number of photons (N OH ), first calculate the T OF You must know. OF The method for estimating is described later in Figure 3.

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[0036] 3 is a graph 300 illustrating clock pulses of a global clock provided to provide clock pulses to a front-end circuit according to one embodiment of the present disclosure. Similarly, a counter (e.g., counter 130) provided to count pulses generated by a single-photon avalanche diode (e.g., single-photon avalanche diode 110) and / or a global clock is configured with N bits, and it is assumed that only N-1 bits are used for counting. Also, T OF and T EXP is T in Figure 2 OF and T EXP indicates the time when each counter overflows and the time the single-photon avalanche diode is exposed to the light source.

[0037] As mentioned above in Figure 1, the total number of clock pulses generated during the exposure time, M, ranges from 0 to T OF The number of multiple pulses generated by the single-photon avalanche diode during (N-1)The timing of each of the total clock pulses is set to include at least one of a log function, a linear function, or a square-root function with respect to the exposure time. For example, from 0 to T OF The clock pulses between are provided in the form of a linear function, and T OF From T EXP The clock pulses between T can be provided in a logarithmic form, i.e., each time instant of the total clock pulse has a predetermined value based on the shape of the selected function. OF The order of the earliest occurring clock pulse (here, n+1) and the time T n+1 Using T OF The latest previous clock pulse time T n At this time, the counter overflows by T n and T n+1 occurs at a time between T OF is expressed as N OF *(T EXP / T n ) and N OF *(T EXP / T n+1 ) can be estimated at any value between

number

[0038] FIG. 4 is an example diagram illustrating the timing of a global clock according to one embodiment of the present disclosure. GCLK can refer to a device that generates the above clock pulses according to a certain timing. In Figure 4, N indicates the number of bits used for pulse counting in the counter. EX is the time from the start of the exposure time to the end of the exposure time (e.g., T in Figure 2). EXP ) is the number of photons received by the single-photon avalanche diode up to N. In the following, we will consider how various forms of global clocks can be used to calculate N EXThe method for estimating is described below.

[0039] 4(a) shows an example in which at least a portion of the timing of the global clock is configured to have a positive linear relationship with the log (or logarithm) of the exposure time. Specifically, the timing of the global clock and the exposure time T EXP The log value of can have a proportional relationship, in which the counter overflow time T depends on the number of clock pulses of the global clock that the counter counts, M_LOG. SAT is obtained through a lookup table, and the overflow time T is obtained as shown in Equation 3 below. SAT Using the inverse of EX On the other hand, in this disclosure, T SAT is T OF is interpreted as having the same meaning.

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[0040] 4(b) shows an example in which at least a portion of the timing of the global clock is configured to have a positive linear relationship with the timing of the exposure time. Specifically, the interval between the first clock pulse (e.g., 65.1 us) and the next second clock pulse (e.g., 66.5 us) generated by the global clock is equal to or greater than the total exposure time T EXP 2 N In this case, the counter can have a value obtained by dividing N by the inverse of the number of clock pulses M_EQ of the global clock counted by the counter as shown in Equation 4 below. EX can be calculated.

number

[0041] 4(c) shows an example in which at least a portion of the timing of the global clock is configured to have a positive linear relationship with the square root of the timing of the exposure time. Specifically, the timing of the global clock and the exposure time T EXP The square root of can have a proportional relationship. In this case, the overflow time T of the counter depends on the number of clock pulses of the global clock that the counter counts, M_SQRT. SAT is obtained through a lookup table, and the overflow time T is obtained as shown in Equation 5 below. SAT Using the inverse of EX can be determined.

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[0042] Additionally or alternatively, although not shown in FIG. 4, at least a portion of the timing of the global clock may be configured to have a negative linear relationship with the timing of the exposure time, in which case N is calculated according to Equation 6 below: EX can be calculated.

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[0043] 5a and 5b are graphs showing extrapolated gains for each type of global clock according to an embodiment of the present disclosure. As shown, the EQ-type global clock maintains a constant gain, which may be advantageous for capturing images in low and / or medium illumination conditions. Therefore, although not shown in FIGS. 5a and 5b, a global clock according to Equation (6) may be advantageous for capturing images in medium and / or high illumination conditions. On the other hand, the SQRT-type global clock exhibits an increasing gain, but the rate of change of the gain decreases. Furthermore, the LOG-type global clock exhibits an increasing gain, and the rate of change of the gain also increases.

[0044] The single-photon avalanche diode-based image sensor of the present disclosure can determine the type of global clock according to the illumination environment based on the gain of each type of global clock described above. Specifically, the single-photon avalanche diode-based image sensor can include a processor that determines the timing of the global clock based on the number of photons received during an exposure time. In this case, the processor can select an EQ-type global clock as the global clock connected to a front-end circuit (e.g., front-end circuit 120) in a low-illumination environment. Also, the processor can select a global clock according to equation (6) as the global clock in a high-illumination environment.

[0045] Additionally or alternatively, the processor can use a combination of the four types of global clocks described above depending on the illumination environment. For example, the processor can determine to use an EQ type global clock from the start point to the intermediate point during the time the counter counts the global clock, and to use an SQRT type global clock from the intermediate point to the end point. That is, the processor can determine the timing of the global clock based on the number of photons received during the exposure time.

[0046] The previous description of the disclosure is provided to enable any person skilled in the art to make or use the disclosure. Various modifications of the present invention will be readily apparent to those skilled in the art, and the general principles defined in the present invention may be applied to various variations without departing from the spirit or scope of the present invention. Thus, the present disclosure is not intended to be limited to the examples described herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0047] Although the present disclosure has been described herein with reference to certain embodiments, it should be understood that various modifications and changes that would be apparent to one of ordinary skill in the art to which the present invention pertains can be made without departing from the scope of the present disclosure, and such modifications and changes are to be considered to fall within the scope of the claims appended hereto.

Claims

1. a Single Photon Avalanche Diode (SPAD) that generates a plurality of pulses corresponding to each of a plurality of photons received during a predetermined exposure time; a front-end circuit for receiving a set of pulses received during a portion of the exposure time among the plurality of pulses; a counter that counts the number of pulses in the set of pulses; a global clock that provides a plurality of clock pulses to the front-end circuitry from the time after the portion; Including, The image sensor is a single-photon avalanche diode-based image sensor, wherein the image quality of the image acquired using the single-photon avalanche diode is determined based on the timing of the global clock.

2. 10. The single-photon avalanche diode-based image sensor of claim 1, wherein the timing of the global clock is determined at least in part based on the number of bits of the counter.

3. 10. The single-photon avalanche diode-based image sensor of claim 1, wherein at least a portion of the timing of the global clock is configured to have a positive linear relationship with the timing of the exposure time.

4. 10. The single-photon avalanche diode-based image sensor of claim 1, wherein at least a portion of the timing of the global clock is configured to have a positive linear relationship to a square root of the timing of the exposure time.

5. 10. The single-photon avalanche diode-based image sensor of claim 1, wherein at least a portion of the timing of the global clock is configured to have a positive linear relationship to a log of the timing of the exposure time.

6. 10. The single-photon avalanche diode-based image sensor of claim 1, wherein at least a portion of the timing of the global clock is configured to have a negative linear relationship with the timing of the exposure time.

7. a processor that determines the timing of the global clock based on the number of photons received during the exposure time; The single-photon avalanche diode-based image sensor of claim 1 , further comprising:

8. 2. The single-photon avalanche diode-based image sensor of claim 1, wherein the end point of the partial time period is based on an overflow point of the counter that counts the number of pulses in the pulse set.

9. generating, by a single-photon avalanche diode, a plurality of pulses corresponding to each of a plurality of photons received during a predetermined exposure time; receiving, by a front-end circuit, a set of pulses from the plurality of pulses received during a portion of the exposure time; counting, with a counter, the number of pulses in the set of pulses; providing a plurality of clock pulses to the front-end circuitry from after the portion of time in accordance with a global clock; Including, The image quality of the image acquired using the single-photon avalanche diode is determined based on the timing of the global clock.

10. A non-transitory computer-readable recording medium having a program recorded thereon for executing the method of driving a single-photon avalanche diode-based image sensor according to claim 9.

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