Light detection device

Abstract

The present invention relates to a light detection device. The light detection device can include an avalanche photodiode having a first terminal and a second terminal, the first terminal of the avalanche photodiode connected to a first potential; a current source configured to supply a current from a second potential to the avalanche photodiode; an output circuit connected to the second terminal of the avalanche photodiode and configured to output a light reception signal; a pulse generation circuit configured to output a pulse signal based on the light reception signal; a transistor between the second terminal and a third potential; and a switch configured to selectively limit the current based on the pulse signal, wherein an output of the pulse generation circuit is connected to a gate of the transistor and to the switch.

Description

[0001] This application is a divisional application of patent application No. 201880007598.3 with a filing date of October 17, 2018, entitled “Imaging device and imaging system”. TECHNICAL FIELD

[0002] The present technology relates to an imaging device and an imaging system. More particularly, the present technology relates to an imaging device and an imaging system using a single photon avalanche photodiode (SPAD). BACKGROUND

[0003] Hitherto, an imaging device that measures a distance to an object using pixels each equipped with a SPAD has been developed.

[0004] With a SPAD, a so-called after-pulse phenomenon in which some of the carriers (electrons and holes) generated by avalanche multiplication are captured by an impurity level before being discharged later, thereby triggering another avalanche multiplication in the absence of a photon incident, sometimes occurs. The after-pulse phenomenon can cause the SPAD to extend its dead time in which it cannot detect a photon, or let the SPAD erroneously detect the incidence of a photon despite the absence of a photon. Thus, the accuracy of distance measurement deteriorates.

[0005] To cope with the above phenomenon, a technology has been proposed by which a SPAD and a capacitor for biasing the SPAD are connected to a power supply during a refresh period and disconnected from the power supply during a detection period, so as to suppress the generation of after-pulses (see, for example, Patent Literature 1).

[0006] Specifically, the invention described in Patent Literature 1 involves charging the capacitor during the refresh period so that the capacitor applies a reverse voltage equal to or higher than a breakdown voltage to the SPAD. When the voltage of the capacitor exceeds the breakdown voltage, a switch is turned off to disconnect the SPAD and the capacitor from the power supply, thereby starting the detection period. When a photon enters the SPAD during the detection period to generate avalanche multiplication, the carriers accumulated in the capacitor are discharged via the SPAD. This causes the voltage applied to the SPAD by the capacitor to drop below the breakdown voltage, thereby causing the avalanche multiplication to stop. When a delay time elapses after the voltage of the capacitor drops below the breakdown voltage by a delay circuit, the switch is turned on to connect the SPAD and the capacitor to the power supply, thereby starting the refresh period. In this way, the current flowing through the SPAD during the generation of avalanche multiplication is suppressed, thereby suppressing the generation of after-pulses.

[0007] Likewise using the SPAD, the following phenomenon can occur: where the current flowing into the SPAD after the avalanche multiplication is generated cannot fall below the latching current, typically by a quenching resistance, thereby preventing the SPAD from recovering its voltage and prolonging the dead time (see, for example, Patent Literature 2). As a result, the accuracy of distance measurement deteriorates.

[0008] Bibliographic List

[0009] Patent Literature

[0010] Patent Literature 1: Japanese Patent Laid-Open No. 2008-542706

[0011] Patent Literature 2: Japanese Patent Laid-Open No. 2014-160042 SUMMARY

[0012] Technical Problem

[0013] However, according to the invention described in Patent Literature 1, the excess pulse generated in the refresh period can cause a feed-through between the power supply and the ground. Because the cathode potential of the SPAD is in a floating state in the detection period, the noise immunity performance deteriorates in this period. Furthermore, the refresh period starts after a delay in the delay circuit after the voltage of the capacitor falls below the breakdown voltage. This causes the dead time to vary significantly.

[0014] Furthermore, Patent Literature 2 does not mention measures to deal with the latching current.

[0015] The present technology was designed in view of the above circumstances, and aims to improve the accuracy of distance measurement performed using a SPAD.

[0016] Technical Solution to the Technical Problem

[0017] According to a first aspect of the present technology, there is provided an imaging device including a pixel array section in which pixel sections are arranged. Each of the pixel sections includes: a SPAD (single photon avalanche photodiode); a resistance component configured to be connected in series with the SPAD; an output configured to output a light reception signal representing that a photon is incident on the SPAD; and a pulse generation configured to output a pulse signal in synchronization with the output of the light reception signal. Each of the pixel sections further includes at least one of: a switch configured to be connected in an interposed manner between the SPAD and the resistance component, and configured to be turned off in synchronization with the pulse signal; and an introduction configured to introduce, in synchronization with the pulse signal, an input current flowing through the SPAD via the resistance component, thereby suppressing the input current from flowing through the SPAD.

[0018] According to a second aspect of the present technology, there is provided an imaging system including: an illuminating device configured to emit irradiation light; and an imaging device configured to receive reflection light originating from the irradiation light. The imaging device includes a pixel array section in which pixel sections are arranged, each of the pixel sections including: a SPAD (single photon avalanche photodiode); a resistance component configured to be connected in series with the SPAD; an output configured to output a light reception signal representing that a photon is incident on the SPAD; and a pulse generation configured to output a pulse signal in synchronization with the output of the light reception signal. Each of the pixel sections further includes at least one of: a switch configured to be connected in an interposed manner between the SPAD and the resistance component, and configured to be turned off in synchronization with the pulse signal; and an introduction configured to introduce, in synchronization with the pulse signal, an input current flowing through the SPAD via the resistance component, thereby suppressing the input current from flowing through the SPAD.

[0019] According to the first aspect of the present technology, the pulse signal is output in synchronization with the output of the light reception signal representing that a photon is incident on the SPAD. The switch connected in an interposed manner between the SPAD and the resistance component is turned on in synchronization with the pulse signal. The input current flowing through the SPAD is introduced via the resistance component in synchronization with the pulse signal. This suppresses the input current from flowing through the SPAD.

[0020] According to a second aspect of the present technology, illumination light is emitted, and reflected light originating from the illumination light is received. The pulse signal is output in synchronization with the output of the light reception signal, which represents the incidence of a photon on the SPAD. The switch connected in an interposed manner between the SPAD and the resistive component is turned on in synchronization with the pulse signal. The input current flowing through the SPAD is introduced via the resistive component in synchronization with the pulse signal. This suppresses the input current from flowing through the SPAD.

[0021] Advantages of the Invention

[0022] According to the first aspect or the second aspect of the present technology, it is possible to prevent the dead time of the SPAD from being prolonged or significantly changed. As a result, the accuracy of distance measurement by using the SPAD is improved.

[0023] Incidentally, the advantages stated in this specification are merely examples and do not limit the present technology. There can be other advantages derived from this specification and not covered by this specification. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 is a circuit diagram showing a configuration example of a pixel using a SPAD.

[0025] Figure 2 is a graph showing an example of a cathode potential characteristic.

[0026] Figure 3 is a graph schematically showing the relationship between the cathode potential and the light reception signal.

[0027] Figure 4 is a circuit diagram showing an equivalent circuit of the pixel in Figure 1

[0028] Figure 5 is a graph showing an example of a cathode potential characteristic in a case where the input current is smaller than the latch-up current.

[0029] Figure 6 is a graph showing an example of a SPAD current characteristic in a case where the input current is smaller than the latch-up current.

[0030] Figure 7 is a schematic diagram schematically showing the change of carriers in the depletion layer of the SPAD.

[0031] Figure 8 is a graph showing an example of a cathode potential characteristic in a case where the input current is larger than the latch-up current.

[0032] Figure 9 ​is a graph showing an example SPAD current characteristic in a case where the input current is greater than the latch-up current.

[0033] Figure 10 is a graph showing an example cathode potential characteristic in a case where a residual pulse is generated.

[0034] Figure 11 is a graph showing an example distribution of the pulse count of the light reception signal in the time direction.

[0035] Figure 12 is a graph showing an example distribution of the time interval between adjacent pulses of the light reception signal.

[0036] Figure 13 is a block diagram showing an embodiment of an imaging system to which the present technology is applied.

[0037] Figure 14 is a block diagram showing a configuration example of an imaging device.

[0038] Figure 15 is a circuit diagram showing a configuration example of a pixel.

[0039] Figure 16 is a cross-sectional view showing a configuration example of a pixel.

[0040] Figure 17 is a plan view showing a configuration example of a wiring of a pixel.

[0041] Figure 18 is a timing chart that explains the operation of a pixel.

[0042] Figure 19 is a schematic view showing a use example of an imaging system.

[0043] Figure 20 is a block diagram showing a schematic configuration example of a vehicle control system.

[0044] Figure 21 is a view that assists in explaining an example of the mounting positions of the outside-vehicle information detection unit and the imaging unit. DETAILED DESCRIPTION

[0045] Preferred embodiments (referred to as embodiments) for practicing the present technology are described below. The description is given under the following headings:

[0046] 1. Effects of Latch-Up Current and Residual Pulse

[0047] 2. Embodiments

[0048] 3. Modifications

[0049] 4. Use Example of Imaging System

[0050] 1. Influence of latch-up current and after-pulse

[0051] Reference will be made below Figures 1 to 12 The influence of latch-up current and after-pulse in the case where distance is measured using a SPAD is briefly described.

[0052] Figure 1 A configuration example of a pixel 1 in an imaging element that measures distance by using a Time of Flight (ToF) method of a SPAD is shown.

[0053] The pixel 1 includes a SPAD 11, a transistor 12 made of a P-type MOSFET, and an output section 13. The output section 13 includes inverters 21 and 22.

[0054] The cathode of the SPAD 11 is connected to the source of the transistor 12 and the input terminal of the inverter 21. The anode of the SPAD 11 is connected to a power supply Vspad (not shown). The drain of the transistor 12 is connected to a power supply Ve (not shown). The output terminal of the inverter 21 is connected to the input terminal of the inverter 22. The inverter 21 is supplied with an operating voltage of the power supply Ve.

[0055] For example, the drain of the transistor 12 is set to a positive potential Ve by the power supply Ve. For example, the anode of the SPAD 11 is set to a negative potential Vspad by the power supply Vspad. The power supply Ve and the power supply Vspad apply a reverse voltage equal to or higher than a breakdown voltage Vbd to the SPAD 11, thereby causing the SPAD 11 to enter a Geiger mode. When a photon enters the SPAD 11 in the Geiger mode, avalanche multiplication occurs, and a current flows through the SPAD 11.

[0056] The transistor 12 is a current source that operates in a saturation region, and functions as a quenching resistance that performs passive quenching. That is, when avalanche multiplication occurs in the SPAD 11, causing a current to flow through the SPAD 11, a current also flows through the transistor 12, thereby causing a resistive component of the transistor 12 to generate a voltage drop. This lowers the cathode potential Vs of the SPAD 11. When the voltage applied to the SPAD 11 becomes equal to or lower than the breakdown voltage Vbd, the avalanche multiplication stops. Thereafter, the carriers accumulated in the SPAD 11 by the avalanche multiplication are discharged through the transistor 12. This enables the cathode potential Vs to recover to a level close to the initial potential Ve, thereby again setting the SPAD 11 to the Geiger mode.

[0057] The output section 13 outputs a light reception signal PFout that indicates that a photon has been incident on the SPAD 11.

[0058] Specifically, when the input voltage is equal to or higher than a predetermined threshold voltage Vth, the inverter 21 outputs a predetermined low-level voltage. When the input voltage is lower than the threshold voltage Vth, the inverter 21 outputs a predetermined high-level voltage. Thus, when a photon enters the SPAD 11 and generates avalanche multiplication, which in turn causes the cathode potential Vs to drop below the threshold voltage Vth, the output voltage of the inverter 21 inverts from low to high. On the other hand, when the avalanche multiplication in the SPAD 11 stops and the cathode potential Vs rises to be equal to or higher than the threshold voltage Vth, the output voltage of the inverter 21 inverts from high to low.

[0059] The inverter 22 outputs the light reception signal PFout by inverting and outputting the output voltage of the inverter 21. Thus, the light reception signal PFout is a low-active pulse signal. That is, when a photon entering the SPAD 11 generates avalanche multiplication, that is, when the SPAD 11 detects photon incidence, a low-level pulse-type light reception signal PFout indicating that a photon has hit the SPAD 11 is output.

[0060] Figure 2 An exemplary characteristic of the cathode potential Vs of the pixel 1 is shown.

[0061] Before time t1, the cathode potential Vs is approximately equal to the potential Ve. The SPAD 11 is set to the Geiger mode with a reverse voltage equal to or higher than the breakdown voltage Vbd applied to the SPAD 11.

[0062] At time t1, a photon entering the SPAD 11 generates avalanche multiplication, causing a current to flow through the SPAD 11. This in turn causes a current to flow through the transistor 12, resulting in a voltage drop and a decrease in the cathode potential Vs. When the cathode potential Vs drops to the breakdown voltage Vbd + the potential Vspad, the avalanche multiplication stops because the applied voltage of the SPAD 11 reaches the breakdown voltage Vbd. Thereafter, the carriers accumulated in the SPAD 11 by the avalanche multiplication are discharged through the transistor 12. This causes the cathode voltage Vs to gradually rise and eventually recover to the potential Ve.

[0063] The time period from the time t1 at which the photon enters the SPAD 11 to the time t2 at which the cathode potential Vs recovers to a level close to the potential Ve is a dead time during which the SPAD 11 cannot detect photon incidence. The shorter the dead time, the greater the number of photons detected, and as a result, the higher the accuracy of distance measurement performed with the pixel 1.

[0064] Figure 3 The relationship between the cathode potential Vs of the pixel 1 and the light reception signal PFout is schematically shown.

[0065] At time t11, a photon enters the SPAD 11, an avalanche multiplication occurs in the SPAD 11, and the cathode potential Vs decreases. When the cathode potential Vs falls below the threshold voltage Vth, the output voltage of the inverter 21 inverts from low to high, and the output voltage of the inverter 22 inverts from high to low. That is, the inverter 22 outputs the low active light-receiving signal PFout.

[0066] Then, ideally, as shown by the solid line, the cathode potential Vs rapidly starts to recover. At time t12 when the cathode potential Vs reaches the threshold voltage Vth, the output voltage of the inverter 21 inverts from high to low, and the output voltage of the inverter 22 inverts from low to high. That is, the inverter 22 stops outputting the low active light-receiving signal PFout.

[0067] Thereafter, the cathode potential Vs recovers to a level close to the potential Ve, sets the SPAD 11 to the Geiger mode again, and the dead-time ends. Then, as shown at times t13 and t14, whenever a photon enters the SPAD 11, similar operations are repeated.

[0068] However, as described later, due to the effect of the hold-off current and after-pulse of the SPAD 11, as shown by the dashed line in the figure, the cathode potential Vs can recover slowly, the pulse width of the light-receiving signal PFout can widen, and the dead-time of the SPAD 11 can extend. This causes the count rate, i.e., the frequency of continuous readout, to drop. As a result, the accuracy of distance measurement by the pixel 1 deteriorates.

[0069] The following describes Figures 4 to 9 the effect of the hold-off current of the SPAD 11.

[0070] Figure 4 An equivalent circuit of the pixel 1 in Figure 1 is shown. In this equivalent circuit, the SPAD 11 is represented by a diode element Ds, an internal resistance Rs, and a parasitic capacitance Cs. The transistor 12 is represented by a resistance Rin.

[0071] The cathode of the diode element Ds is connected to the resistance Rin and the input terminal of the inverter 21 via the internal resistance Rs. The anode of the diode element Ds is connected to the negative terminal of the power supply Vspad. The parasitic capacitance Cs is connected in a manner of insertion between the input terminal of the inverter 21 and the anode of the diode element Ds. The positive terminal of the power supply Ve is connected to the input terminal of the inverter 21 via the resistance Rin. The negative terminal of the power supply Ve is grounded. The positive terminal of the power supply Vspad is grounded.

[0072] For simplicity, the following describes the case where the voltage Vspad of the power supply Vspad is equal to the breakdown voltage Vbd of the diode element Ds. Therefore, the anode potential of the diode element Ds is -Vbd.

[0073] Figure 5 An exemplary characteristic of the cathode potential Vs in the case where the input current Iin (= Ve / (Rin+Rs)) input to the SPAD 11 via the resistance Rin is smaller than the quenching current Iq of the SPAD 11 is shown. Figure 5 Also shown is the output voltage waveform of the inverter 21, indicated by a dotted line. Figure 6 An exemplary characteristic of the current Id flowing through the diode element Ds in the case where the input current Iin is smaller than the quenching current Iq is shown.

[0074] At time t21, a photon entering the diode element Ds generates avalanche multiplication in the diode element Ds and causes a current to flow through the diode element Ds. This in turn causes a current to flow through the resistance Rin, resulting in a voltage drop and a decrease in the cathode potential Vs. At this time, the current Id flowing through the diode element Ds is maximized to reach Ve / Rs. Thereafter, as the cathode potential Vs decreases, the current Id decreases.

[0075] Figure 7 Subgraph A in FIG. 10 schematically shows the situation in the depletion layer 31 of the diode element Ds when the current Id reaches point A in FIG. 10. Figure 6 Subgraph A in FIG. 10 schematically shows the situation in the depletion layer 31 of the diode element Ds when the current Id reaches point A in FIG. 10.

[0076] Thereafter, in the case where the input current Iin becomes smaller than the quenching current Iq, as shown in FIG. 11, the current Id reaches the quenching current Iq. Figure 6

[0077] Subgraph B in FIG. 10 schematically shows the situation in the depletion layer 31 of the diode element Ds when the current Id reaches point B in FIG. 10, i.e., when the current Id reaches the quenching current Iq. As shown in the figure, when the current Id reaches the quenching current Iq, there are few carriers in the depletion layer 31. In this case, there are few carriers that generate impact ionization, so that avalanche multiplication stops. As shown in subgraph C in FIG. 10, there are virtually no carriers in the depletion layer 31, thereby increasing the impedance of the diode element Ds to a very high level. Then, as shown by point C at time t22 in FIG. 10, the current Id suddenly drops to approximately equal to the input current Iin. At this time, the cathode potential Vs becomes approximately equal to the resistance Rs x the quenching current Iq. Figure 7 Figure 6 Subgraph B in FIG. 10 schematically shows the situation in the depletion layer 31 of the diode element Ds when the current Id reaches point B in FIG. 10, i.e., when the current Id reaches the quenching current Iq. As shown in the figure, when the current Id reaches the quenching current Iq, there are few carriers in the depletion layer 31. In this case, there are few carriers that generate impact ionization, so that avalanche multiplication stops. As shown in subgraph C in FIG. 10, there are virtually no carriers in the depletion layer 31, thereby increasing the impedance of the diode element Ds to a very high level. Then, as shown by point C at time t22 in FIG. 10, the current Id suddenly drops to approximately equal to the input current Iin. At this time, the cathode potential Vs becomes approximately equal to the resistance Rs x the quenching current Iq. Figure 7 Figure 6 Subgraph B in FIG. 10 schematically shows the situation in the depletion layer 31 of the diode element Ds when the current Id reaches point B in FIG. 10, i.e., when the current Id reaches the quenching current Iq. As shown in the figure, when the current Id reaches the quenching current Iq, there are few carriers in the depletion layer 31. In this case, there are few carriers that generate impact ionization, so that avalanche multiplication stops. As shown in subgraph C in FIG. 10, there are virtually no carriers in the depletion layer 31, thereby increasing the impedance of the diode element Ds to a very high level. Then, as shown by point C at time t22 in FIG. 10, the current Id suddenly drops to approximately equal to the input current Iin. At this time, the cathode potential Vs becomes approximately equal to the resistance Rs x the quenching current Iq.

[0078] ​​Thereafter, the carriers accumulated in the parasitic capacitance Cs by avalanche multiplication are discharged through the transistor 12. This causes the cathode potential Vs to recover to a level close to the initial potential Ve. At time t23, the dead time ends.

[0079] Here, the time constant τ indicating the recovery speed of the cathode potential Vs is represented by the resistance Rin x the parasitic capacitance Cs. Therefore, the higher the resistance Rin, the smaller the input current Iin, which slows the recovery speed of the cathode potential Vs. As a result, the dead time of the SPAD 11 is lengthened.

[0080] In contrast, the lower the resistance Rin, the larger the input current Iin, which increases the recovery speed of the cathode potential Vs. As a result, the dead time of the SPAD 11 is shortened. However, when the input current Iin becomes larger than the latch-up current Iq, the dead time is lengthened.

[0081] Figure 8 An exemplary characteristic of the cathode potential Vs in a case where the input current Iin is larger than the latch-up current Iq is shown. Figure 9 An exemplary characteristic of the current Id in a case where the input current Iin is larger than the latch-up current Iq is shown.

[0082] At time t31, the photons entering the diode element Ds generate avalanche multiplication, so that the cathode potential Vs decreases as in the case of time t21 of Figure 5 This time, the current Id flowing through the diode element Ds is maximized to reach Ve / Rs.

[0083] Thereafter, the decrease in the cathode potential Vs requires the current Id to decrease. With the input current Iin larger than the latch-up current Iq, the current Id becomes approximately equal to the input current Iin, but does not start to decrease from the input current Iin. Therefore, the current Id does not reach the latch-up current Iq. For this reason, the state where a large number of carriers exist in the depletion layer 31 of the diode element Ds continues. As the avalanche multiplication slowly stops, the state where the cathode potential Vs is approximately equal to Ve x (Rs / (Rs+Rin) continues. Therefore, the cathode potential Vs takes time to recover to the potential Ve. Therefore, the dead time of the SPAD 11 is lengthened and changes.

[0084] As described above, for the pixel 1, it is necessary to make the input current Iin smaller than the latch-up current Iq of the SPAD 11. However, making the input current Iin too small delays the recovery of the cathode potential Vs and lengthens the dead time.

[0085] Next, reference will be made to Figures 10 to 12 the influence of the afterpulse in the SPAD 11 will be described.

[0086] As in Figure 2 , Figure 10An exemplary characteristic of the cathode potential in the pixel 1 is shown.

[0087] At time t41, the photon entering the SPAD 11 generates avalanche multiplication and the cathode potential Vs decreases.

[0088] When the cathode potential Vs falls to the breakdown voltage Vbd + the potential Vspad, the avalanche multiplication stops because the applied voltage of the SPAD 11 reaches the breakdown voltage Vbd. Thereafter, the cathode potential Vs starts to recover, but when the afterpulse is generated, the avalanche multiplication occurs again and the cathode potential Vs decreases again. When the cathode potential Vs falls to the breakdown voltage Vbd + the potential Vspad, the avalanche multiplication stops because the anode-to-cathode voltage of the SPAD 11 reaches the breakdown voltage Vbd, and the cathode potential Vs starts to recover again.

[0089] Therefore, if the afterpulse is not generated, the cathode potential Vs recovers to the threshold voltage Vth at time t42. If the afterpulse is generated, the time at which the cathode potential Vs recovers to the threshold voltage Vth is delayed to time t43. Obviously, the generation of the afterpulse also prolongs the dead time.

[0090] Although Figure 10 An example in which the afterpulse is generated during the dead time is shown, but the afterpulse can also be generated after the cathode potential Vs recovers to a level close to the potential Ve and the dead time ends. In this case, although no photon is incident on the SPAD 11, the avalanche multiplication occurs. Then, the cathode potential Vs falls below the threshold voltage Vth, and the output light-receiving signal PFout is generated. That is, the photon is erroneously detected.

[0091] Figure 11 An exemplary output characteristic of the light-receiving signal PFout from the pixel 1 is shown. Specifically, the horizontal axis of the graph indicates the time elapsed from the emission of the irradiation light for distance measurement. The vertical axis of the graph indicates the pulse count indicating the number of pulses of the light-receiving signal PFout. That is, Figure 11 The exemplary distribution of the pulse count of the light-receiving signal PFout in the time direction is illustrated.

[0092] In this example, the number of pulses of the light-receiving signal PFout reaches the maximum at around 1.5 x 10 -9 seconds from the emission of the irradiation light. Therefore, it is estimated that there is some object at the time point of (1.5 x 10 -9 ) / 2 seconds from the emission of the irradiation light.

[0093] Meanwhile, after the number of pulses of the light-receiving signal PFout reaches the maximum at around 1.5 x 10 -9 seconds, as Figure 11As shown by the dotted rectangle Fl, the counts of the pulse-to-pulse time intervals do not decrease smoothly, but rather, they increase and decrease repeatedly. This is mainly attributed to the false outputs of the light-receiving signal PFout due to the generation of afterpulses after the dead time, although no photons are incident on the SPAD 11.

[0094] Incidentally, the probability of the generation of afterpulses depends on the carrier concentration in the depletion layer of the SPAD 11. Therefore, immediately after the photons incident on the SPAD 11 generate avalanche multiplication, the probability of the generation of afterpulses becomes maximum. Thereafter, the probability of the generation of afterpulses decreases exponentially with time. Therefore, the higher the speed of recovery of the cathode potential Vs becomes after the large input current lin causes the avalanche multiplication to stop, the more likely the generation of afterpulses is.

[0095] Figure 12 Fig. 6 illustrates other exemplary output characteristics of the light-receiving signal PFout of the pixel 1. More specifically, Figure 12 Fig. 7 shows an exemplary distribution of the time intervals between adjacent pulses of the light-receiving signal PFout. Figure 12 The horizontal axes of the upper and lower graphs in Fig. 8 represent the time intervals between adjacent pulses of the light-receiving signal PFout. The vertical axes of the two graphs represent the counts of the pulse-to-pulse time intervals. The upper graph shows the distribution of the time intervals between adjacent pulses of the light-receiving signal PFout in the case where a large number of afterpulses are generated. The lower graph illustrates the distribution of the time intervals between adjacent pulses of the light-receiving signal PFout in the case where a small number of afterpulses are generated. Further, the dotted waveform in the upper and lower graphs represents an ideal gradient of the graphical distribution of the time intervals between adjacent pulses of the light-receiving signal PFout.

[0096] In the case where a small number of afterpulses are generated, the counts of the pulse-to-pulse time intervals of the light-receiving signal PFout reach a maximum at approximately 100 ns, for example. This time point is estimated to be close to the dead time of the SPAD 11. The counts are approximately 0 at less than 100 ns. After 100 ns, the counts decrease in an approximately linear manner.

[0097] On the other hand, in the case where a large number of afterpulses are generated, the counts at less than 100 ns are higher than the counts at 100 ns. As shown by the dotted rectangle F2, as the time intervals are shortened step by step, the counts increase in an exponential manner. This growth is estimated to be attributed to the generation of a large number of afterpulses immediately after the end of the dead time to trigger the output of the light-receiving signal PFout, although no photons are incident on the SPAD 11.

[0098] 2. Embodiments

[0099] The following describes embodiments of the present technology with reference to Figures 13 to 18 Embodiments of the present technology are described below.

[0100] <Configuration example of imaging system>

[0101] Figure 13 is a block diagram illustrating an imaging system 101 as an embodiment of the present technology. The imaging system 101 captures a distance image using a ToF method, for example. Here, the distance image is constituted by distance pixel signals based on a distance of an object detected for each pixel in a depth direction from the imaging system 101.

[0102] The imaging system 101 includes an illumination device 111 and an imaging device 112.

[0103] The illumination device 111 includes an illumination control section 121 and a light source 122.

[0104] The illumination control section 121 controls the light source 122 to emit a pattern of irradiation light under the control of a control section 132 in the imaging device 112. Specifically, the illumination control section 121 controls the light source 122 to emit a pattern of irradiation light in accordance with an irradiation code included in an irradiation signal supplied from the control section 132. The irradiation code is composed of two values of 1 (high) and 0 (low), for example. When the value of the irradiation code is 1, the illumination control section 121 turns on the light source 122. When the value of the irradiation code is 0, the illumination control section 121 turns off the light source 122.

[0105] The light source 122 emits light (irradiation light) of a predetermined wavelength band under the control of the illumination control section 121. The light source 122 is constituted by an infrared laser diode, for example. Incidentally, the type of the light source 122 and the wavelength band of the irradiation light can be set as needed in accordance with the purpose of the imaging system 101.

[0106] The imaging device 112 is an imaging device 112 that receives reflected light that is irradiation light reflected by the objects 102 and 103, for example. The imaging device 112 includes an imaging section 131, a control section 132, a display section 133, and a storage section 134.

[0107] The imaging section 131 includes a lens 141, an imaging element 142, and a signal processing circuit 143.

[0108] The lens 141 forms an image of incident light on an imaging plane of the imaging element 142. The lens 141 can be constituted as needed. The lens 141 can be constituted with a plurality of groups of lenses, for example.

[0109] The imaging element 142 is configured with a complementary metal oxide semiconductor (CMOS: Complementary Metal Oxide Semiconductor) image sensor using SPAD, for example. Under the control of the control section 132, the imaging element 142 captures images of the objects 102 and 103, and supplies a pixel signal obtained from the capturing to the signal processing circuit 143. The pixel signal is formed of a light-reception signal PFout output from each pixel, for example.

[0110] The signal processing circuit 143 processes the pixel signal supplied from the imaging element 142 under the control of the control section 132. Based on the pixel signal from the imaging element 142, for example, the signal processing circuit 143 detects a distance to an object pixel by pixel to generate a distance image representing a distance from each pixel to the object. Specifically, a timer included in the signal processing circuit 143 measures a time period required for each pixel of the imaging element 142 to receive light after the light source 122 emits light a plurality of times (e.g., thousands of times). The signal processing circuit 143 prepares a histogram corresponding to the measured time. By detecting a peak of the histogram, the signal processing circuit 143 determines a time required for light irradiated by the light source 122 to return after being reflected by the object 102 or 103. Further, the signal processing circuit 143 performs a calculation based on the determined time and the speed of light to obtain a distance to the object. The signal processing circuit 143 supplies the distance image thus generated to the control section 132.

[0111] The control section 132 is configured of a control circuit and a processor (e.g., a field programmable gate array (FPGA: Field Programmable Gate Array) and a digital signal processor (DSP: Digital Signal Processor)), for example. The control section 132 controls the illumination control section 121, the imaging element 142, and the signal processing circuit 143. Specifically, the control section 132 supplies a signal to the timer of the signal processing circuit 143 at the same time as feeding an irradiation signal to the illumination control section 121 to start measuring time. In response to the irradiation signal, the light source 122 emits irradiation light. The timer starts measuring time according to the signal for starting time measurement, and stops measuring time when the irradiation light is received, thus receiving the pixel signal supplied from the imaging element 142. Further, the control section 132 supplies the distance image obtained from the imaging section 131 to the display section 133, thus causing the display section 133 to display the distance image. Further, the control section 132 stores the distance image obtained from the imaging section 131 in the storage section 134. In addition, the control section 132 outputs the distance image obtained from the imaging section 131 to the outside.

[0112] The display section 133 is configured, for example, of a panel-type display device such as a liquid crystal display device or an organic electroluminescence (EL) display device.

[0113] The storage section 134 is configured, for example, of an appropriate storage device and storage medium, and stores distance images.

[0114] <Configuration Example of Imaging Element 142>

[0115] Figure 14 A configuration example of the imaging element 142 is shown.

[0116] The imaging element 142 includes a pixel array section 201, a vertical drive section 202, a column processing section 203, a horizontal drive section 204, a system control section 205, a pixel drive line 206, a vertical signal line 207, a signal processing section 208, and a data storage section 209.

[0117] The pixel array section 201 is configured of a plurality of pixels for detecting incident photons and outputting a pixel signal representing a detection result. In the drawing, for example, the pixels configuring the pixel array section 201 are arranged horizontally (in the row direction) and vertically (in the column direction).

[0118] In the pixel array section 201, for example, each row of pixels arranged in the row direction is wired to the pixel drive line 206, and each column of pixels arranged in the column direction is wired to the vertical signal line 207.

[0119] The vertical drive section 202 is configured, for example, of a shift register and an address decoder. The vertical drive section 202 supplies a signal to the pixels through a plurality of pixel drive lines 206. In this way, the vertical drive section 202 drives, for example, all the pixels of the pixel array section 201 simultaneously or in units of rows.

[0120] The column processing section 203 reads a signal from each column of pixels in the pixel array section 201 via the vertical signal line 207. Then, the column processing section 203 generates a pixel signal by performing processing such as noise suppression, correlated double sampling, and analog to digital (A / D) conversion.

[0121] The horizontal drive section 204 is configured, for example, of a shift register and an address decoder. The horizontal drive section 204 sequentially selects a unit circuit in the column processing section 203 corresponding to a column of pixels. The selective scanning of the horizontal drive section 204 causes the pixel signal processed by each unit circuit in the column processing section 203 to be sequentially output to the signal processing section 208.

[0122] The system control section 205 is constituted by a timing generator that generates various timing signals, for example. Based on the timing signals generated by the timing generator, the system control section 205 performs drive control on the vertical drive section 202, the column processing section 203, and the horizontal drive section 204.

[0123] The signal processing section 208 temporarily stores data in the data storage section 209 as necessary. In this way, the signal processing section 208 performs signal processing such as arithmetic processing on the pixel signals supplied from the column processing section 203 in order to output an image signal constituted by the pixel signals.

[0124] <Configuration Example of Pixel>

[0125] Figures 15 to 17 A configuration example of a unit pixel 301 (hereinafter simply referred to as pixel 301) that constitutes the pixel array section 201 in the imaging element 142 is shown.

[0126] Figure 15 is a circuit diagram showing a configuration example of the pixel 301. In Figure 15 correspond to the components of the pixel 1 in Figure 1 correspond to the components of the pixel 1 in

[0127] The pixel 301 includes a SPAD 311, a transistor 312 formed of a P-type MOSFET, an output section 313, a pulse generation section 314, a switch 315, and a transistor 316 constituted by an N-type MOSFET. The output section 313 includes inverters 321 and 322.

[0128] The cathode of the SPAD 311 is connected to the source of the transistor 312 via the switch 315, the cathode of the SPAD 311 is connected to the input terminal of the inverter 321, and the cathode of the SPAD 311 is connected to the drain of the transistor 316. Thus, the switch 315 is interposed between the SPAD 311 and the transistor 312. More specifically, the switch 315 is provided in an interposed manner between a branching point at which a signal line from the cathode of the SPAD 311 branches in the direction of the transistor 312 and in the direction of the inverter 321 and the transistor 312. The anode of the SPAD 311 is connected to a power supply Vspad (not shown). The drain of the transistor 312 is connected to a power supply Ve (not shown). The output terminal of the inverter 321 is connected to the input terminal of the inverter 322. The inverter 321 is supplied with an operating voltage of the power supply Ve. The output terminal of the inverter 322 is connected to the input terminal of the pulse generation section 314. The output terminal of the pulse generation section 314 is connected to the control terminal of the switch 315 and to the gate of the transistor 316. The source of the transistor 316 is grounded.

[0129] For example, the drain of the transistor 312 is set to a positive potential Ve by a power supply Ve. For example, the anode of the SPAD 311 is set to a negative potential Vspad by a power supply Vspad. The power supply Ve and the power supply Vspad apply a reverse voltage equal to or higher than the breakdown voltage Vbd to the SPAD 311, thereby causing the SPAD 311 to enter the Geiger mode. When a photon enters the SPAD 311 in the Geiger mode, avalanche multiplication occurs and a current flows through the SPAD 311.

[0130] The transistor 312 is a current source operating in the saturation region and functions as an arc resistance that performs passive arc extinction. That is, when avalanche multiplication occurs in the SPAD 311, causing a current to flow through the SPAD 311, a current also flows through the transistor 312, thereby causing the resistive component of the transistor 312 to generate a voltage drop. This lowers the cathode potential Vs of the SPAD 311. When the voltage applied to the SPAD 311 becomes equal to or lower than the breakdown voltage Vbd, the avalanche multiplication stops. Thereafter, the carriers accumulated in the SPAD 311 by the avalanche multiplication are discharged through the transistor 312. This enables the cathode potential Vs to recover to a level close to the initial potential Ve, thereby again setting the SPAD 311 to the Geiger mode.

[0131] The output section 313 outputs a light reception signal PFout representing that a photon is incident on the SPAD 311.

[0132] Specifically, the inverter 321 outputs a predetermined low-level voltage when the input voltage is equal to or higher than a predetermined threshold voltage Vth. The inverter 321 outputs a predetermined high-level voltage when the input voltage is lower than the threshold voltage Vth. Thus, when a photon enters the SPAD 311, causing avalanche multiplication, in turn causing the cathode potential Vs to drop below the threshold voltage Vth, the output voltage of the inverter 321 inverts from low to high. On the other hand, when the avalanche multiplication in the SPAD 311 stops and the cathode potential Vs rises to equal to or higher than the threshold potential Vth, the output voltage of the inverter 321 inverts from high to low.

[0133] The inverter 322 outputs the light reception signal PFout by inverting and outputting the output voltage of the inverter 321. Thus, the light reception signal PFout is a low active pulse signal. That is, when a photon entering the SPAD 311 causes avalanche multiplication, that is, when the SPAD 311 detects photon incidence, a low-level pulse-type light reception signal PFout representing that a photon is incident on the SPAD 311 is output. The light reception signal PFout is a pixel signal output from the pixel 301.

[0134] Incidentally, the output section 313 is not limited to the configuration having the inverter 321 and the inverter 322. The output section 313 can be configured as needed as long as it can output the light-receiving signal PFout. For example, the inverter 321 can be replaced with a transistor composed of a P-type MOSFET.

[0135] The pulse generation section 314 outputs a pulse signal Pc of a predetermined width in a manner synchronized with the output of the light-receiving signal PFout. More specifically, the pulse generation section 314 outputs the pulse signal Pc of the predetermined width when the light-receiving signal PFout is detected (i.e., when the edge of the pulse-type light-receiving signal PFout falls).

[0136] The switch 315 is turned off in a manner synchronized with the pulse signal Pc from the pulse generation section 314. More specifically, the switch 315 is turned off for the period in which the pulse signal Pc is input, and is turned on for other periods. An example of the switch 315 is a complementary switch that combines a NMOS transistor and a PMOS transistor. The switch 315 is not limited to the complementary switch, and alternatively, the switch 315 can be configured with a plurality of transistors. The switch 315 can have many other alternative configurations as long as they allow connection and disconnection of current.

[0137] The transistor 316 constitutes a pull-in section that pulls in the input current Iin flowing through the SPAD 311 via the transistor 312 so as to suppress the input current Iin from flowing to the SPAD 311. The transistor 316 is turned on in a manner synchronized with the pulse signal Pc from the pulse generation section 314. More specifically, the transistor 316 is turned on for the period in which the pulse signal Pc is input to the gate, and is turned off for other periods.

[0138] Initially, the transistor 316 is needed to pull the cathode potential Vs to the ground. When the transistor 316 is operated in a manner synchronized with the pulse signal, the transistor 316 can be used for the excess pulse countermeasure discussed later without adding a new circuit component. In addition, a single switch (i.e., the transistor 316) can be used as a switch for blocking current from passing to the unused pixel 301, or as a switch for suppressing generation of the excess pulse. That is, both purposes can be achieved by one switch for the excess pulse countermeasure without adding a new circuit component.

[0139] The pixel array section 201 in the imaging element 142 can be a stacked pixel array section. For example, the SPAD 311 can be formed on a first semiconductor substrate, and the transistor 312, the output section 313, the pulse generation section 314, the switch 315, and the transistor 316 are formed on a second semiconductor substrate that is stacked on the first semiconductor substrate. This configuration expands the light-receiving region of the SPAD 311, allowing more photons to be received. The configuration also provides a fine design for the size of each pixel, making it possible to produce a distance image with high resolution. Incidentally, the components formed on the first semiconductor substrate and the second semiconductor substrate can be changed as needed. For example, the transistor 312 can be formed on the first semiconductor substrate.

[0140] Figure 16 is a cross-sectional view showing a configuration example of the pixel 301. Figure 17 is a plan view showing a configuration example of the wiring layer of the pixel 301.

[0141] As shown in Figure 16 , the imaging element 142 has a multilayer structure in which the sensor board 331, the sensor-side wiring layer 332, and the logic-side wiring layer 333 are stacked in this order. A logic circuit board (not shown) is stacked on the logic-side wiring layer 333. For example, formed on the logic circuit board are Figure 14 the vertical drive section 202, the column processing section 203, the horizontal drive section 204, the system control section 205, the signal processing section 208, and the data storage section 209 in the Figure 15 the transistor 312, the output section 313, the pulse generation section 314, the switch 315, and the transistor 316 in the Figure 16 For example, the imaging element 142 is manufactured by a method in which the sensor-side wiring layer 332 is formed on the sensor board 331 and the logic-side wiring layer 333 is formed on the logic circuit board before the sensor-side wiring layer 332 and the logic-side wiring layer 333 are joined together on a joining surface (indicated by a broken line in

[0142] For example, the sensor board 331 is a semiconductor substrate formed of a single-crystal silicon wafer. On this substrate, the concentration of p-type or n-type impurities is controlled. The SPAD 311 is formed in each pixel 301. In Figure 16 , the bottom surface of the sensor board 331 is a light-receiving surface. The sensor-side wiring layer 332 is stacked on the top surface opposite the light-receiving surface.

[0143] Formed in the sensor-side wiring layer 332 and the logic-side wiring layer 333 are a wiring for supplying a voltage to the SPAD 311, a wiring for pulling out an electron generated by the SPAD 311 from the sensor board 331, and the like.

[0144] The SPAD 311 is configured with an N-well 341, a P-type diffusion layer 342, an N-type diffusion layer 343, a hole storage layer 344, a pinning layer 345, and a high-concentration P-type diffusion layer 346 formed in the sensor plate 331. In the SPAD 311, an avalanche multiplication region 347 is formed by a depletion layer generated in a region where the P-type diffusion layer 342 and the N-type diffusion layer 343 are connected to each other.

[0145] The N-well 341 is formed by controlling the impurity concentration in the sensor plate 331 to be n-type. The N-well 341 generates an electric field that transports electrons generated by photoelectric conversion in the SPAD 311 to the avalanche multiplication region 347. Alternatively, the N-well 341 can be replaced with a P-well formed by controlling the impurity concentration in the sensor plate 331 to be p-type.

[0146] The P-type diffusion layer 342 located near the surface of the sensor plate 331 is a high-concentration P-type diffusion layer (P+) formed on the back side (bottom side in the drawing) of the N-type diffusion layer 343. The P-type diffusion layer 342 is configured to substantially cover the entire surface of the SPAD 311. Figure 16

[0147] The N-type diffusion layer 343 located near the surface of the sensor plate 331 is a high-concentration N-type diffusion layer (N+) formed on the surface side (top side in the drawing) of the P-type diffusion layer 342. The N-type diffusion layer 343 is configured to substantially cover the entire surface of the SPAD 311. In addition, the N-type diffusion layer 343 is formed in a partially concave shape to reach the surface side of the sensor plate 331 so as to be connected to a contact electrode 361 that supplies a negative voltage to generate the avalanche multiplication region 347. Figure 16

[0148] The hole storage layer 344 is a P-type diffusion layer (P) formed to surround the side surface and the bottom surface of the N-well 341 and store holes. In addition, the hole storage layer 344 is electrically connected to the anode of the SPAD 311 to allow bias adjustment. This increases the hole concentration in the hole storage layer 344 to enhance pinning including the pinning layer 345, thereby suppressing, for example, generation of dark current.

[0149] The pinning layer 345 is a high-concentration P-type diffusion layer (P+) formed on the surface outside the hole storage layer 344, that is, the side surface in contact with the back side of the sensor plate 331 and the insulating film 352. Like the hole storage layer 344, the pinning layer 345 suppresses, for example, generation of dark current.

[0150] ​​The high-concentration P-type diffusion layer 346 located near the surface of the sensor plate 331 is a high-concentration P-type diffusion layer (P++) formed in a manner that surrounds the outer periphery of the N-well 341. The high-concentration P-type diffusion layer 346 is used to connect to the contact electrode 362, which electrically connects the hole storage layer 344 to the anode of the SPAD 311.

[0151] The avalanche multiplication region 347 is a high electric field region formed above the boundary surface between the P-type diffusion layer 342 and the N-type diffusion layer 343 by applying a large negative voltage to the N-type diffusion layer 343. The avalanche multiplication region 347 increases the number of electrons (e-) generated by photons entering the SPAD 311.

[0152] Furthermore, in the imaging element 142, adjacent SPADs 311 are isolated and separated from each other by inserting a dual-structure pixel-to-pixel separation section 353, which includes a metal film 351 and an insulating film 352. For example, the pixel-to-pixel separation section 353 is formed in a manner that penetrates from the rear side of the sensor plate 331 to the surface side.

[0153] The metal film 351 is formed of a light-reflecting metal (e.g., tungsten). The insulating film 352 is a film with insulating properties, such as a SiO2 film. For example, the pixel-to-pixel spacing 353 is formed with the metal film 351 embedded in the sensor plate 331 such that the surface of the metal film 351 is covered by the insulating film 352. The pixel-to-pixel spacing 353 thus formed electrically and optically separates adjacent SPADs 311 from each other.

[0154] Formed in the sensor-side wiring layer 332 are contact electrodes 361-363, metal wiring 364-366, contact electrodes 367-369, and metal pads 370-372.

[0155] Contact electrode 361 connects the N-type diffusion layer 343 to the metal wiring 364. Contact electrode 362 connects the high-concentration P-type diffusion layer 346 to the metal wiring 365. Contact electrode 363 connects the metal film 351 to the metal wiring 366.

[0156] For example, such as Figure 17 As shown, the metal wiring 364 is formed to at least cover the avalanche multiplication region 347, that is, to be wider than the avalanche multiplication region 347. Figure 16 As shown by the hollow arrow, the metal wiring 364 reflects the light passing through SPAD 311 back into SPAD 311.

[0157] For example, such as Figure 17As shown, the metal wiring 365 is formed so as to surround the outer periphery of the metal wiring 364 and overlap the high-concentration P-type diffusion layer 346. For example, as shown in FIG. 3B, the metal wiring 365 is formed so as to surround the outer periphery of the metal wiring 364 and overlap the high-concentration P-type diffusion layer 346. Figure 17 As shown, the metal wiring 366 is formed so as to be connected to the metal film 351 at the four corner portions of the pixel 301.

[0158] The contact electrode 367 connects the metal wiring 364 with the metal pad 370. The contact electrode 368 connects the metal wiring 365 with the metal pad 371. The contact electrode 369 connects the metal wiring 366 with the metal pad 372.

[0159] The metal pads 370 to 372 are respectively used for electrical and mechanical joining with the metal pads 391 to 393 formed in the logic-side wiring layer 333, which joining is achieved by using the metal (Cu) constituting the pads.

[0160] Formed in the logic-side wiring layer 333 are the electrode pads 381 to 383, the insulating layer 384, the contact electrodes 385 to 390, and the metal pads 391 to 393.

[0161] The electrode pads 381 to 383 are respectively used for connection with a logic circuit board (not shown). The insulating layer 384 insulates the electrode pads 381 to 383 from each other.

[0162] The contact electrodes 385 and 386 connect the electrode pad 381 with the metal pad 391. The contact electrodes 387 and 388 connect the electrode pad 382 with the metal pad 392. The contact electrodes 389 and 390 connect the electrode pad 383 with the metal pad 393.

[0163] The metal pad 391 is joined with the metal pad 370. The metal pad 392 is joined with the metal pad 371. The metal pad 393 is joined with the metal pad 372.

[0164] In the above-described wiring structure, the electrode pad 381 is connected with the N-type diffusion layer 343, for example, via the contact electrodes 385 and 386, the metal pads 391 and 370, the contact electrode 367, the metal wiring 364, and the contact electrode 361. Thus, in the pixel 301, a high negative voltage applied to the N-type diffusion layer 343 is supplied from the logic circuit board to the electrode pad 381.

[0165] In addition, the electrode pad 382 is configured to be connected with the high-concentration P-type diffusion layer 346 via the contact electrodes 387 and 388, the metal pads 392 and 371, the contact electrode 368, the metal wiring 365, and the contact electrode 362. Thus, in the pixel 301, the anode of the SPAD 311 electrically connected with the hole storage layer 344 is connected with the electrode pad 382. This allows bias adjustment of the hole storage layer 344 via the electrode pad 382.

[0166] In addition, the electrode pad 383 is configured to be connected with the metal film 351 via the contact electrodes 389 and 390, the metal pads 393 and 372, the contact electrode 369, the metal wiring 366, and the contact electrode 363. Thus, in the pixel 301, a bias voltage supplied from a logic circuit board to the electrode pad 383 is applied to the metal film 351.

[0167] As described above, in the pixel 301, the metal wiring 364 is formed to cover at least the avalanche multiplication region 347, that is, to be wider than the avalanche multiplication region 347. In addition, the metal film 351 is formed to penetrate the sensor board 331. That is, the pixel 301 is configured to completely surround the SPAD 311 with the metal wiring 364 and the metal film 351 except for a light incidence plane in the light reflecting structure of the SPAD 311. Due to the effect of the metal wiring 364 and the metal film 351 reflecting light, this structure enables the pixel 301 to prevent generation of optical cross-talk. This simultaneously improves the sensitivity of the SPAD 311.

[0168] In addition, the pixel 301 is configured to surround the side surface and the bottom surface of the N-well 341 with the hole storage layer 344, and the pixel 301 is configured to electrically connect the hole storage layer 344 with the anode of the SPAD 311, thereby realizing bias adjustment. Further, the pixel 301 has a bias voltage applied to the metal film 351 of the pixel-to-pixel spacing portion 353 so as to generate an electric field for assisting carriers in the avalanche multiplication region 347.

[0169] The pixel 301 configured as described above can prevent generation of cross-talk and can improve the sensitivity of the SPAD 311. As a result, pixel characteristics are improved.

[0170] <Operation of the pixel>

[0171] Next, the operation of the pixel 301 is described with reference to a timing chart of FIG. 10. Figure 18

[0172] Before time t101, the cathode potential Vs is approximately equal to the potential Ve. With a reverse voltage equal to or higher than the breakdown voltage Vbd applied to the SPAD 311, the SPAD 311 is set to the Geiger mode.

[0173] ​At time t101, a photon entering the SPAD 311 generates avalanche multiplication, thereby causing a current to flow through the SPAD 311. This in turn causes a current to flow through the transistor 312, thereby generating a voltage drop and lowering the cathode potential Vs. When the cathode potential Vs falls below the threshold voltage Vth, the output voltage of the inverter 321 is inverted from low to high, and the output voltage of the inverter 322 is inverted from high to low. That is, the inverter 322 outputs a low active light-receiving signal PFout.

[0174] The pulse generation section 314 outputs a pulse signal Pc of a predetermined width upon detecting the light-receiving signal PFout.

[0175] The pulse signal Pc is input to a control terminal of the switch 315. The switch 315 is turned off at the same time as the pulse signal Pc is input. This blocks a path through which the input current Iin is input to the SPAD 311. That is, the pulse signal Pc is output in synchronization with the output of the light-receiving signal PFout. The switch 315 is turned off in synchronization with the pulse signal Pc. This blocks the current flowing through the SPAD 311.

[0176] In addition, the pulse signal Pc is input to a gate of the transistor 316. The transistor 316 is turned on at the same time as the pulse signal Pc is input to the gate. This causes the path through which the input current Iin flows to be diverted from the SPAD 311 to the transistor 316. That is, the input current Iin is introduced into the transistor 316, making it possible to suppress the input current Iin from flowing to the SPAD 311. In other words, the pulse signal Pc is output in synchronization with the output of the light-receiving signal PFout, and the transistor 316 is turned on in synchronization with the pulse signal Pc, thereby introducing the input current Iin. This suppresses the current from flowing through the SPAD 311.

[0177] As described above, the switch 315 is able to block the current from flowing through the SPAD 311, and the transistor 316 is able to suppress the current from flowing through the SPAD 311. This causes the current flowing through the SPAD 311 to become smaller than the latch-up current Iq regardless of the magnitude of the input current Iin, thereby virtually eliminating carriers from the inside of the depletion layer of the SPAD 311. This in turn stops the avalanche multiplication in the SPAD 311 and increases the impedance of the SPAD 311.

[0178] The generation of an afterpulse can be suppressed while blocking or suppressing the current flow through the SPAD 311. As described above, the probability of the generation of an afterpulse reaches a maximum immediately after the generation of avalanche multiplication in the SPAD 311, and thereafter, the probability decreases exponentially with time. Therefore, in a case where the generation of an afterpulse is suppressed immediately after the generation of avalanche multiplication in the SPAD 311, the probability of the generation of an afterpulse is significantly reduced. With the generation of an afterpulse thus suppressed, the dead time can be prevented from being prolonged or varied. Further, false detection of a photon incident on the SPAD 311 can be suppressed.

[0179] Thereafter, the pulse signal Pc is cut off, the switch 315 is turned on, and the transistor 316 is cut off. This enables a current to start flowing through the SPAD 311. When the carriers accumulated in the SPAD 311 by avalanche multiplication are discharged through the transistor 312, the cathode potential Vs rises.

[0180] At time t102, the cathode potential Vs reaches the threshold voltage Vth. This causes the inverter 321 to invert its output voltage from high to low, and causes the inverter 322 to invert its output voltage from low to high. That is, the output of the light reception signal PFout from the inverter 322 is stopped.

[0181] Thereafter, the cathode potential Vs returns to a level close to the initial potential Ve, thereby setting the SPAD 311 in the Geiger mode again and terminating the dead time. That is, when no pulse signal Pc is input, the switch 315 is turned on and the transistor 316 is cut off. This causes a current to be continuously supplied to the SPAD 311, thereby enabling the SPAD 311 to reliably output a light reception signal in response to a photon incident.

[0182] Then, as shown at time t103 and time t104, similar operations are repeated each time a photon enters the SPAD 311.

[0183] As described above, the pixel 301 can reliably stop avalanche multiplication regardless of the magnitude of the input current Iin. This enables the input current Iin to be larger and the dead time to be shorter than before.

[0184] Because the probability of the generation of an afterpulse is significantly reduced, the dead time can be prevented from being prolonged or varied. Further, false detection of a photon incident on the SPAD 311 can be suppressed. Because the dead time can be prevented from being prolonged, the number of times of distance measurement performed per unit time increases. As a result, the proportion of a noise component in components required for distance measurement decreases.

[0185] Further, the cathode potential of the SPAD 311 is only momentarily brought into a floating state. Therefore, the anti-noise performance decreases very little.

[0186] Accordingly, the accuracy of detecting photons entering the SPAD 311 is improved. As a result, the accuracy of distance measurement is improved.

[0187] The above-described advantageous effects are merely examples, and do not limit the present technology. There can be other advantageous effects derived from the present specification and not encompassed herein.

[0188] 3. Modification

[0189] Hereinafter, some modifications of the above-described embodiments of the present technology will be described.

[0190] For example, a resistor can be used instead of the transistor 312 in the pixel 301.

[0191] As another example, the switch 315 or the transistor 316 in the pixel 301 can be omitted.

[0192] As yet another example, the introduction portion that introduces the input current Iin can be configured differently from the transistor 316.

[0193] As still another example, the light-receiving signal PFout from the pixel 301 can be a high active signal.

[0194] For example, it has been described that the signal processing circuit 143 generates a distance image. Alternatively, the imaging device 142 can generate a distance image internally, and can output the distance image.

[0195] For example, it has also been described that the present technology is applied to the imaging device to obtain a distance image. Alternatively, the technology can be applied to an imaging device that includes a color filter and is configured to obtain a picture for imaging purposes.

[0196] 4. Use example of the imaging system

[0197] Figure 19 is a schematic diagram showing a use example of the above-described imaging system 101.

[0198] The above-described imaging system 101 can be used in various ways in cases of sensing various types of light including visible light, infrared rays, ultraviolet radiation, or X-rays, for example, as described below.

[0199] - cases of devices that take images for visual appreciation, such as digital cameras and mobile phones equipped with a camera function, and the like.

[0200] - the case of a device for the field of vehicle traffic, including: a vehicle-mounted sensor that captures images of the front, rear, surrounding environment, and interior of the vehicle to ensure safe operation such as automatic parking and to identify the state of the driver; a monitoring camera that monitors passing vehicles and the road on which they travel; and a distance measurement sensor for measuring the distance between vehicles.

[0201] - the case of a device that captures an image of a user's posture in order to operate a home appliance such as a television, refrigerator, and air conditioner in a manner that reflects the posture.

[0202] - the case of a device for the field of medical care, such as an endoscope and an instrument that uses received infrared radiation to capture images of blood vessels.

[0203] - the case of a device for the field of security, such as a monitoring camera for preventing crime and a camera for personal authentication.

[0204] - the case of a device for the field of beauty care, such as a skin measurement instrument and a microscope for capturing images of the scalp.

[0205] - the case of a device for the field of sports, such as a sports camera and a wearable camera.

[0206] - the case of a device for the field of agriculture, such as a monitoring camera for monitoring fields and crops.

[0207] <Examples of Applications to Mobile Bodies>

[0208] The technology of the present application (the present technology) can be applied to various products. For example, the present technology can be implemented as a device mounted on a mobile body such as a car, an electric car, a hybrid electric car, a motorcycle, a bicycle, a personal mobility device, an airplane, a drone, a ship, a robot, and the like.

[0209] Figure 20 is a block diagram showing an example of the schematic configuration of a vehicle control system that is an example of a mobile body control system to which the technology according to an embodiment of the present application can be applied.

[0210] The vehicle control system 12000 includes a plurality of electronic control units connected to each other through a communication network 12001. In the present embodiment, the electronic control units include an engine control unit 12002, a transmission control unit 12003, a brake control unit 12004, a steering control unit 12005, a body control unit 12006, a navigation control unit 12007, a vehicle exterior monitoring camera 12008, a vehicle interior monitoring camera 12009, a driver monitoring camera 12010, a distance measurement sensor 12011, and a vehicle-to-vehicle communication unit 12012. Figure 20In the example shown, the vehicle control system 12000 includes a drive system control unit 12010, a body system control unit 12020, an outside information detecting unit 12030, an inside information detecting unit 12040, and an integrated control unit 12050. Further, a microcomputer 12051, a sound / image output section 12052, and a vehicle-mounted network interface (I / F) 12053 are illustrated as the functional configuration of the integrated control unit 12050.

[0211] The drive system control unit 12010 controls the operation of devices related to the drive system of the vehicle in accordance with various programs. For example, the drive system control unit 12010 functions as a control device for a drive force generating device, such as an internal combustion engine or a drive motor, for example, for generating the drive force of the vehicle; a drive force transmission mechanism for transmitting the drive force to the wheels; a steering mechanism for adjusting the steering angle of the vehicle; and a brake device for generating the braking force of the vehicle, and the like.

[0212] The body system control unit 12020 controls the operation of various devices provided on the body of the vehicle in accordance with various programs. For example, the body system control unit 12020 functions as a control device for a keyless entry system; a smart key system; a power window device; or various lamps such as a headlamp, a backup lamp, a brake lamp, a turn signal lamp, or a fog lamp, and the like. In this case, radio waves transmitted from a mobile device that replaces a key or signals of various switches can be input to the body system control unit 12020. The body system control unit 12020 receives these input radio waves or signals, and controls the door lock device, the power window device, or the lamps of the vehicle, and the like.

[0213] The outside information detecting unit 12030 detects information related to the outside of the vehicle including the vehicle control system 12000. For example, the outside information detecting unit 12030 is connected with an imaging section 12031. The outside information detecting unit 12030 causes the imaging section 12031 to capture an image of the outside of the vehicle, and the outside information detecting unit 12030 receives the captured image. Based on the received image, the outside information detecting unit 12030 can perform a process of detecting an object such as a human, a vehicle, an obstacle, a sign, or a character on a road surface, or perform a detection process of the distance to the above object.

[0214] The imaging section 12031 is an optical sensor for receiving light and outputting an electric signal corresponding to the light amount of the received light. The imaging section 12031 can output the electric signal as an image, or can output the electric signal as information related to the measured distance. Further, the light received by the imaging section 12031 can be visible light, or can be non-visible light such as infrared rays.

[0215] The in-vehicle information detecting unit 12040 detects information about the inside of the vehicle. For example, the in-vehicle information detecting unit 12040 is connected to a driver state detecting portion 12041 for detecting the state of the driver. For example, the driver state detecting portion 12041 includes a camera for taking an image of the driver. Based on the detection information input from the driver state detecting portion 12041, the in-vehicle information detecting unit 12040 can calculate the degree of fatigue of the driver or the degree of concentration of the driver, or can determine whether the driver is dozing off.

[0216] Based on the information about the outside or inside of the vehicle obtained by the outside-vehicle information detecting unit 12030 or the in-vehicle information detecting unit 12040, the microcomputer 12051 can calculate a control target value of the driving force generating device, the steering mechanism, or the braking device, and the microcomputer 12051 can output a control command to the drive system control unit 12010. For example, the microcomputer 12051 can perform cooperative control aimed at realizing functions of an advanced driver assistance system (ADAS) including vehicle collision avoidance or vehicle impact mitigation, following travel based on an inter-vehicle distance, vehicle speed maintenance travel, collision warning of the vehicle, or lane departure warning of the vehicle, or the like.

[0217] Further, based on the information about the outside or inside of the vehicle obtained by the outside-vehicle information detecting unit 12030 or the in-vehicle information detecting unit 12040, the microcomputer 12051 can perform cooperative control aimed at realizing automatic driving or the like, in which the vehicle autonomously travels without having to rely on the operation of the driver, by controlling the driving force generating device, the steering mechanism, the braking device, or the like.

[0218] Further, based on the information about the outside of the vehicle obtained by the outside-vehicle information detecting unit 12030, the microcomputer 12051 can output a control command to the body system control unit 12020. For example, the microcomputer 12051 can perform cooperative control aimed at preventing glare by controlling the headlamp so as to change from a high beam to a low beam in accordance with the position of the preceding vehicle or the oncoming vehicle detected by the outside-vehicle information detecting unit 12030.

[0219] The sound / image output portion 12052 transmits an output signal of at least one of a sound and an image to an output device that can visually or aurally notify information to the passenger of the vehicle or the outside of the vehicle. In Figure 20 In the example, an audio speaker 12061, a display portion 12062, and an instrument panel 12063 are illustrated as the output device. For example, the display portion 12062 can include at least one of a vehicle-mounted display and a head-up display.

[0220] Figure 21 FIG. 17 is a diagram showing an example of a mounting position of the imaging section 12031.

[0221] In Figure 21 , the vehicle 12100 includes imaging sections 12101, 12102, 12103, 12104, and 12105 as the imaging section 12031.

[0222] For example, the imaging sections 12101, 12102, 12103, 12104, and 12105 are provided at positions on the front nose, side mirrors, rear bumper, and rear door of the vehicle 12100 and at a position on the upper portion of the windshield inside the vehicle. The imaging section 12101 provided to the front nose and the imaging section 12105 provided to the upper portion of the windshield inside the vehicle mainly obtain images of the front of the vehicle 12100. The imaging sections 12102 and 12103 provided to the side mirrors mainly obtain images of both sides of the vehicle 12100. The imaging section 12104 provided to the rear bumper or the rear door mainly obtains an image of the rear of the vehicle 12100. The front images obtained by the imaging sections 12101 and 12105 are mainly used for detecting a preceding vehicle, a pedestrian, an obstacle, a signal, a traffic sign, or a lane, and the like.

[0223] Incidentally, Figure 21 FIG. 17 is a diagram showing an example of a photographing range of the imaging sections 12101 to 12104. The imaging range 12111 indicates the imaging range of the imaging section 12101 provided to the front nose. The imaging ranges 12112 and 12113 respectively indicate the imaging ranges of the imaging sections 12102 and 12103 provided to the side mirrors. The imaging range 12114 indicates the imaging range of the imaging section 12104 provided to the rear bumper or the rear door. For example, by superimposing the image data photographed by the imaging sections 12101 to 12104, a bird's-eye image of the vehicle 12100 viewed from above can be obtained.

[0224] At least one of the imaging sections 12101 to 12104 can have a function of obtaining distance information. For example, at least one of the imaging sections 12101 to 12104 can be a stereo camera constituted by a plurality of imaging elements, or can be an imaging element having pixels for phase difference detection.

[0225] For example, based on distance information obtained from the imaging sections 12101 to 12104, the microcomputer 12051 can determine the distance to each three-dimensional object within the imaging ranges 12111 to 12114 and the change in the distance over time (relative speed with respect to the vehicle 12100), and thereby particularly extract, as a preceding vehicle, the closest three-dimensional object that exists on the travel path of the vehicle 12100 and travels in substantially the same direction as the vehicle 12100 at a predetermined speed (for example, equal to or greater than 0 km / h). Further, the microcomputer 12051 can set in advance a following distance to be maintained ahead of the preceding vehicle, and can perform automatic brake control (including follow-up stop control), or automatic acceleration control (including follow-up start control), and the like. Thus, cooperative control aimed at realizing automatic driving and the like, in which the vehicle autonomously travels without depending on the operation of the driver, can be performed.

[0226] For example, based on distance information obtained from the imaging sections 12101 to 12104, the microcomputer 12051 can classify three-dimensional object data on three-dimensional objects into three-dimensional object data of two-wheeled vehicles, standard-size vehicles, large vehicles, pedestrians, utility poles, and other three-dimensional objects, can extract the classified three-dimensional object data, and can automatically avoid obstacles using the extracted three-dimensional object data. For example, the microcomputer 12051 identifies obstacles around the vehicle 12100 as obstacles that the driver of the vehicle 12100 can visually recognize and obstacles that the driver of the vehicle 12100 cannot visually recognize. Then, the microcomputer 12051 judges a collision risk indicating the risk of collision with each obstacle. In a case where the collision risk is equal to or higher than a set value and thus there is a possibility of collision, the microcomputer 12051 outputs a warning to the driver through the audio speaker 12061 or the display section 12062, and performs forced deceleration or avoidance steering through the drive system control unit 12010. Thereby, the microcomputer 12051 can assist driving to avoid collision.

[0227] At least one of the imaging sections 12101 to 12104 can be an infrared camera for detecting infrared rays. For example, the microcomputer 12051 can recognize a pedestrian by determining whether a pedestrian is present in the captured image of the imaging section 12101 to 12104. For example, such pedestrian recognition is performed by extracting feature points in the captured image of the imaging section 12101 to 12104 as an infrared camera, and determining whether the object is a pedestrian by performing pattern matching processing on a series of feature points representing the outline of the object. When the microcomputer 12051 determines that a pedestrian is present in the captured image of the imaging section 12101 to 12104 and thus recognizes the pedestrian, the sound / image output section 12052 controls the display section 12062 so that a rectangular outline for emphasis is displayed superimposed on the recognized pedestrian. The sound / image output section 12052 can also control the display section 12062 so that an icon or the like representing the pedestrian is displayed at a desired position.

[0228] The above describes an example of a vehicle control system to which the technology of the present application can be applied. The technology can be applied to the imaging section 12031 among the components described above. Specifically, the imaging system 101 in Figure 13

[0229] In the present specification, the term "system" refers to a collection of a plurality of components (for example, devices or modules (parts)). It is not important whether all the components are packaged in the same housing. Thus, a system can be configured with a plurality of devices that are packaged in different housings and are connected to each other through a network, or the system can be configured with a single device that packages a plurality of modules in a single housing.

[0230] Embodiments of the present technology are not limited to the above-described embodiments, and various modifications or changes can be made to the embodiments of the present technology within the scope of the present technology.

[0231] Further, the present technology can be preferably configured as follows:

[0232] (1) An imaging device including:

[0233] a pixel array section (201) in which pixel sections each including:

[0234] a SPAD (Single Photon Avalanche Photodiode) (311);​

[0235] a resistance component (312) configured to be connected in series with the SPAD (311);

[0236] an output section (313) configured to output a light reception signal indicating that a photon is incident on the SPAD (311); and

[0237] a pulse generation section (314) configured to output a pulse signal in synchronization with the output of the light reception signal;

[0238] wherein each of the pixel sections further includes at least one of: a switch (315) configured to be connected in an intervening manner between the SPAD (311) and the resistance component (312) and configured to be turned off in synchronization with the pulse signal; and an introduction section (316) configured to introduce, in synchronization with the pulse signal, an input current flowing through the SPAD (311) via the resistance component (312), thereby suppressing the input current from flowing through the SPAD (311).

[0239] (2) The imaging device according to the above paragraph (1), wherein the switch (315) is turned off when the pulse signal is input, and the switch (315) is turned on when the pulse signal is not input.

[0240] (3) The imaging device according to the above paragraph (1) or (2), wherein the switch (315) is connected in an intervening manner between a cathode of the SPAD (311) and the resistance component.

[0241] (4) The imaging device according to the above paragraph (3), wherein the switch (315) is connected in an intervening manner between a branching point from which a signal line from the cathode of the SPAD (311) extends in a direction of the resistance component (312) and in a direction of the output section, and the resistance component (312).

[0242] (5) The imaging device according to any one of the above paragraphs (1) to (4), wherein the introduction section (316) introduces the input current when the pulse signal is input, and the introduction section (316) does not introduce the input current when the pulse signal is not input.

[0243] (6) The image pickup device according to any one of the above (5), wherein the introduction section (316) includes a transistor configured to be connected in an interposed manner between the cathode of the SPAD (311) and a ground, a gate of the transistor receiving an input of the pulse signal.

[0244] (7) The image pickup device according to any one of the above (1) to (6), wherein the pulse generation section (314) outputs the pulse signal of a predetermined width upon detection of the light reception signal.

[0245] (8) The image pickup device according to any one of the above (1) to (7), wherein the resistive component (312) includes a MOSFET.

[0246] (9) The image pickup device according to any one of the above (1) to (8), wherein the output section (313) outputs the light reception signal based on a cathode potential of the SPAD (311).

[0247] (10) The image pickup device according to any one of the above (1) to (9), wherein the switch includes a plurality of transistors.

[0248] (11) The image pickup device according to any one of the above (1) to (10), wherein the SPAD (311) is formed on a first semiconductor substrate, and at least one of the resistive component (312), the output section (313), the pulse generation section (314), the switch (315), and the introduction section (316) is formed on a second semiconductor substrate, the second semiconductor substrate being laminated on the first semiconductor substrate.

[0249] (12) An image pickup system, comprising:

[0250] an illumination device configured to emit irradiation light; and

[0251] an image pickup device configured to receive reflected light originating from the irradiation light;

[0252] wherein the image pickup device includes:

[0253] a pixel array section (201) in which pixel sections are arranged, each of the pixel sections including:

[0254] a SPAD (single photon avalanche photodiode) (311);

[0255] a resistive component (312) configured to be connected in series with the SPAD;

[0256] an output section (313) configured to output a light reception signal representing that a photon is incident on the SPAD; and

[0257] a pulse generation section (314) configured to output a pulse signal in synchronization with the output of the light reception signal;

[0258] wherein each of the pixel sections further includes at least one of: a switch (315) configured to be connected in an intervening manner between the SPAD (311) and the resistance section (312), and configured to be turned off in synchronization with the pulse signal; and an introduction section (316) configured to introduce, in synchronization with the pulse signal, an input current flowing through the SPAD (311) via the resistance section (312), thereby suppressing the input current from flowing through the SPAD (311).

[0259] The advantageous effects stated in this specification are merely examples, and do not limit the present technology. There can be other advantageous effects derived from this specification.

[0260] List of Reference Signs

[0261] 101 imaging system, 111 illumination device, 112 imaging device, 121 illumination control section, 122 light source, 131 imaging section, 132 control section, 142 imaging element, 143 signal processing circuit, 201 pixel array section, 301 pixel, 311 SPAD, 312 transistor, 313 output section, 314 pulse generation section, 315 switch, 316 transistor, 321, 322 inverter.

Claims

1. An optical detection device comprising: an avalanche photodiode having an anode and a cathode, the anode of the avalanche photodiode being connected to a first potential; a current source configured to supply a current from a second potential to the cathode of the avalanche photodiode; an output circuit connected to the cathode of the avalanche photodiode and configured to output a light reception signal; a pulse generation circuit configured to output a pulse signal based on the light reception signal; a transistor having a drain connected to the cathode of the avalanche photodiode and a source grounded; and a switch disposed between the current source and the cathode of the avalanche photodiode and configured to limit the current in a manner synchronized with the pulse signal, wherein an output of the pulse generation circuit is connected to a gate of the transistor and to a control terminal of the switch.

2. The light detecting device of claim 1, wherein, The switch is open when the pulse signal is input and is closed when the pulse signal is not input.

3. The light detecting device of claim 1, wherein, An input current is introduced to the transistor when the pulse signal is input and flows through the avalanche photodiode when the pulse signal is not input.

4. The light detecting device of claim 1, wherein, The pulse generation circuit outputs the pulse signal upon detection of the light reception signal, wherein the pulse signal is output with a predetermined width.

5. The light detecting device of claim 1, wherein, The current source comprises a transistor.

6. The light detecting device of claim 1, wherein, The output circuit outputs the light reception signal based on a potential of the cathode of the avalanche photodiode.

7. The light detecting device of claim 1, wherein, The switch comprises a transistor.

8. The light detecting device of claim 1, wherein, The output circuit comprises at least one inverter.

9. The light detecting device of claim 1, wherein, A first switch terminal of the switch is connected to the current source and a second switch terminal of the switch is connected to the cathode of the avalanche photodiode.

Images