Image sensor and distance measuring device

Abstract

This ranging device comprises a plurality of pixels (30). Each pixel (30) comprises: a light-receiving element (31); a capacitor (36); and a constant current source device that outputs a constant current to the capacitor (36) from when exposure of the pixel (30) begins until the light-receiving element (31) detects light.

Description

[Technical field]

[0001] The present disclosure relates to an imaging element and a distance measuring device. [Background technology]

[0002] 2. Description of the Related Art There are distance measuring devices and distance measuring systems that measure the distance to a subject by using a pixel array including a plurality of Single Photon Avalanche Diodes (SPADs).

[0003] For example, the distance measuring device of Patent Document 1 includes a pulsed light source that emits an optical signal, a detector array including single-photon detectors that output respective detection signals indicative of arrival times of multiple incident photons, and a processing circuit that receives the respective detection signals. The processing circuit includes a correlator circuit configured to output respective correlation signals representative of detection of one or more of the photons having arrival times that are within a predetermined correlation time relative to one another, and a time processing circuit including a counter circuit configured to increment a count value based on the respective correlation signals or the detection signals, and a time integrator circuit configured to generate an integrated time value. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2021-513087 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in Patent Document 1, it is necessary to provide a counter circuit and a time integrator circuit in each pixel (see FIG. 19 in Patent Document 1), which increases the circuit scale per pixel.

[0006] An object of the present disclosure is to provide an image sensor and a distance measuring device in which the size of each pixel is reduced. [Means for solving the problem]

[0007] In order to solve the above problem, an imaging element according to one embodiment of the present disclosure includes a plurality of pixels, each of which includes a light receiving element, a storage element, and a constant current source device that outputs a constant current to the storage element from the start of exposure of the pixel to the end of exposure. Effect of the Invention

[0008] According to the present disclosure, the size of each pixel can be reduced. [Brief description of the drawings]

[0009] [Figure 1] 1 is a block diagram showing an example of the overall configuration of a distance measuring device according to a first embodiment. [Diagram 2] FIG. 2 is a block diagram showing the configuration of a light receiving sensor according to the first embodiment. [Diagram 3] FIG. 2 is a diagram showing a circuit configured in a pixel according to the first embodiment. [Figure 4] 13 is a timing chart relating to a distance measurement operation during one frame period of a pixel according to the second embodiment. [Diagram 5] FIG. 2 is a block diagram showing the configuration of a readout circuit according to the first embodiment. [Figure 6] 11A and 11B are diagrams for explaining the principle of distance measurement by a distance measuring device according to a second embodiment. [Figure 7] 13A to 13C are diagrams for explaining a method of generating a subrange image according to the second embodiment. [Figure 8] FIG. 11 is a diagram showing a circuit configured in a pixel according to a second embodiment. [Figure 9] 13 is a timing chart relating to a distance measurement operation during one frame period of a pixel according to the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, the embodiments of the present invention will be described in detail with reference to the drawings. The following description of the preferred embodiments is merely illustrative in nature and is not intended to limit the present invention, its application, or its uses.

[0011] (First embodiment) -Overall configuration of distance measuring device- Fig. 1 is a block diagram showing an example of the overall configuration of a distance measuring device according to the first embodiment. As shown in Fig. 1, the distance measuring device according to this embodiment includes a light source 1, a light receiving sensor 2, a signal processing device 3, and a timing signal generator 4.

[0012] The light receiving sensor 2 receives light that is irradiated by the light source 1 and reflected by the subject. The light receiving sensor 2 outputs an output signal indicative of the result of the light reception to the signal processing device 3.

[0013] The signal processing device 3 calculates the distance to the subject based on the signal received from the light receiving sensor 2. The signal processing device 3 outputs a signal indicating the calculation result.

[0014] The timing signal generator 4 outputs signals indicating the drive timing of each of the light source 1, the light receiving sensor 2, and the signal processing device 3. Specifically, the timing signal generator 4 outputs a signal whose phase is synchronized with the frame rate of the light receiving sensor 2 so that the light source 1, the light receiving sensor 2, and the signal processing device 3 perform simultaneous imaging of all pixels (global shutter) operation. Note that the frequencies of the signals output by the timing signal generator 4 may be different from each other.

[0015] -Light receiving sensor configuration- Fig. 2 is a block diagram showing the configuration of the light receiving sensor according to the first embodiment. As shown in Fig. 2, the light receiving sensor 2 includes a bias generating circuit 20, a pixel array 21, a readout circuit 22, a horizontal output circuit 23, a vertical drive circuit 24, and a sensor timing generator 25.

[0016] The bias generating circuit 20 supplies a bias signal (details omitted) necessary for driving the light receiving sensor 2. Note that the bias signal may be configured to be supplied from an external source.

[0017] The pixel array 21 includes a plurality of pixels 30 arranged in an array. The pixels 30 are arranged in rows in response to a row selection signal V SEL , reset signal V RST , PD bias control signal V D and the constant current source bias signal V I Each pixel 30 is supplied with a row selection signal V SEL , reset signal V RST , PD bias control signal V D and the constant current source bias signal V I In response, a pixel signal indicating the detection result is output to an output line 26.

[0018] The readout circuit 22 includes a plurality of column circuits 221. The column circuits 221 include an amplifier and an AD converter, which will be described later, and are provided for each column of a plurality of pixels 30. The readout circuit 22 reads out signals output from each pixel 30 via the output line 26 using the column circuits 221.

[0019] The horizontal output circuit 23 sequentially outputs the signals output from the readout circuit 22 as output signals.

[0020] The vertical drive circuit 24 receives a row selection signal V SEL , reset signal V RST , PD bias control signal V D and the constant current source bias signal V I and outputs it to each pixel 30 at a predetermined timing.

[0021] The sensor timing generator 25 outputs a drive timing signal indicating the drive timing of the horizontal output circuit 23 and the vertical drive circuit 24 .

[0022] -About pixel configuration- Fig. 3 is a diagram showing a circuit configured in a pixel according to embodiment 1. As shown in Fig. 3, a pixel 30 includes a light receiving element 31, a reset transistor 32, a constant current source transistor 33, a source follower transistor 34, a selection transistor 35, and a capacitance 36.

[0023] The light receiving element 31 is, for example, a photodiode (PD) such as a SPAD or an avalanche photodiode (APD), and a high voltage of −20 V is supplied to the anode terminal from the outside.

[0024] The reset transistor 32 has a source (or drain) connected to a PD bias control signal V D , the drain (or source) of the transistor 33 is connected to the cathode terminal of the light receiving element 31 and the gate of the constant current source transistor 33, and a reset signal V RST Receive.

[0025] The constant current source transistor 33 has a source (or drain) connected to a constant current source bias control signal V I The input is connected to a floating diffusion (FD) which is connected to the drain (or source).

[0026] The source follower transistor 34 has a source (or drain) connected to a pixel power supply bias signal Vc, a drain (or source) connected to the source (or drain) of the selection transistor 35, and a gate connected to FD.

[0027] The selection transistor 35 has a drain (or source) connected to the output line 26 and a gate connected to the selection signal V SEL Receive.

[0028] The capacitor 36 has one end connected to the FD and the other end connected to a ground voltage (earth).

[0029] The constant current source transistor 33 is set to a floating state during the exposure period. At this time, a charge according to the distance of the subject is accumulated in the capacitor 36. When the selection transistor 35 is turned on, the source follower transistor 34 outputs a pixel signal according to the charge accumulated in the capacitor 36 to the output line 26.

[0030] -About pixel operation- FIG. 4 shows a timing chart of distance measurement operation during one frame period of a pixel according to the first embodiment. In this embodiment, a laser pulse is used as the light source 1, and distance measurement results obtained by one laser pulse correspond to one frame. In FIG. 4, from the top, a drive signal (exposure start signal) for the light source 1 (laser pulse), a reset signal V RST , the cathode voltage APDC of the light receiving element 31, the constant current source bias control signal V I , a reflected light pulse signal (exposure end signal) output when the light receiving sensor 2 receives reflected light, and an FD voltage V indicating the voltage level of the capacitor 36. FD and are respectively indicated. The drive signal for the light source 1 is generated by a vertical drive circuit 24 that receives a signal from a timing signal generator 4. The reflected light pulse signal goes high when the light receiving element 31 detects light. Typically, each signal and voltage is 3V at high level (H) and 0V at low level (L).

[0031] It is assumed that one frame period starts at an initial time t0.

[0032] At time t1, the reset signal V RST and the PD bias control signal V D As a result, the reset transistor 32 is turned on and the cathode voltage APDC of the light receiving element 31 becomes high level, so that the light detection signal and the dark current component in the previous frame are reset.

[0033] At time t1, the constant current source bias control signal V IAt this time, the cathode voltage APDC of the light receiving element 31 is at a high level, so the gate of the constant current source transistor 33 is also at a high level, and therefore the FD voltage V FD will be at a high level.

[0034] At time t2, the light source 1 starts to be driven, and the reset signal V RST At this time, the constant current source bias control signal V I is set to a middle level (M) between the high level and the low level so that the subthreshold voltage is output from the drain of the constant current source transistor 33. Here, the subthreshold voltage of the constant current source transistor 33 is V th and the voltage at high level is V H , the voltage at the middle level is V M Then, V H -V M <V th As a result, from time t2 to time t3, the constant current source transistor 33 is biased in the subthreshold region, so that the constant current source bias control signal V I As a result, the FD voltage V FD The potential of decreases in proportion to time due to the constant current injected from the constant current source transistor 33.

[0035] At time t3, when the light receiving sensor 2 receives reflected light (the reflected light pulse signal is at a high level), the light receiving element 31 (for example, a SPAD) detects the reflected light and generates a Geiger mode pulse. At this time, since the reset transistor 32 is in an off state, the light receiving element 31 self-quenches, and the cathode voltage APDC of the light receiving element 31 drops to a low level due to the charge generated by avalanche multiplication. This causes the constant current source transistor 33 to turn off, and charge injection into the FD stops.

[0036] At time t4, the reset signal V RSTbecomes high level, and the reset transistor 32 is turned on. As a result, the injection of charges into the capacitor 36 in all the pixels 30 is stopped.

[0037] After time t4, the readout period begins, and the output signal from each pixel 30 is read out by the readout circuit 22, after which the system enters a standby state until the start of the next frame.

[0038] -About the configuration of the readout circuit- Fig. 5 is a block diagram showing the configuration of the readout circuit according to the first embodiment. As shown in Fig. 5, the column circuit 221 of the readout circuit 22 includes a column amplifier circuit 41, a CDS (correlated double sampling) circuit 42, and a single-slope AD converter (SSADC) 43.

[0039] The column amplifier circuit 41 is connected to the output line 26 and amplifies the output signal output from each pixel 30 .

[0040] The CDS circuit 42 outputs the difference between the output signal amplified by the column amplifier circuit 41 and a zero level signal that has been read out in advance.

[0041] The single-slope AD converter 43 converts the signal output from the CDS circuit 42 into an 8-bit digital signal (Q0 to Q7) and outputs it to the horizontal output circuit .

[0042] Here, the current I outputted to the FD by the constant current source transistor 33 from time t2 to time t3 is

[0043]

number

[0044] Here, ψ is expressed as V H -V M <V th The surface potential barrier from the source to the gate of the constant current source transistor 33 is set by I 0is a constant determined by the surface impurity concentration and the device size, and a is a constant that depends on temperature.

[0045] The distance resolution in this embodiment is shown below. As described above, the switching noise of the capacitor 36 is removed by the CDS circuit 42, and the noise limit is determined by the shot noise of the constant current source transistor 33 as a current source. The distance to the nearest subject is Z min , and the speed of light constant is c, the flight (exposure) time from when the light source 1 emits a laser pulse until the light receiving sensor 2 detects the light reflected by the subject is Δt min , 2·Z min / C. Therefore, the charge stored in the capacitor 36 during the exposure period is

[0046]

number

[0047] The shot noise for the charge amount is its square root, and therefore the signal-to-noise ratio (S / N ratio) is also given by this square root. Therefore, the minimum amount required as a signal in this embodiment is S / N>1, that is,

[0048]

number

[0049] For example,

[0050]

number

[0051] In the case of Z min = 1.6 cm, which is a small enough value for practical use.

[0052] As described above, according to the distance measuring device of the first embodiment, intra-pixel TDC (Time to Digital Converter) operation is performed simultaneously for all pixels in the same frame, making it possible to perform distance measuring and imaging with high accuracy over the entire range.

[0053] Moreover, the distance measuring device according to the first embodiment includes a plurality of pixels 30. Each pixel 30 includes a light receiving element 31, a capacitance 36 (electrical storage element), and a constant current source transistor (constant current source device) that outputs a constant current to the capacitance 36 from the start of exposure of the pixel 30 until the light receiving element 31 detects light. This makes it possible to measure the distance to the subject by measuring the charge stored in the capacitance 36. Also, since it is no longer necessary to provide a counter circuit or a time integrator circuit for each pixel, it is possible to reduce the size of each pixel. Also, since the size of each pixel is reduced, it is possible to increase the number of pixels that allow simultaneous distance measurement of all pixels.

[0054] In addition, the light receiving element 31 is an avalanche photodiode. This improves the sensitivity of the light receiving sensor 3, thereby extending the distance measurement range. In addition, the S / N ratio in the TDC operation can be improved, thereby improving the distance resolution.

[0055] Second embodiment 6 is a diagram for explaining the principle of distance measurement by the distance measuring device according to the second embodiment. The distance measuring device according to the second embodiment can generate sub-range (SR) images SR1 to SR5 and a full-range (FR) image FR1 consisting of the sub-range images SR1 to SR5. In the following description, the same reference numerals are used for configurations similar to those of the above embodiment, and detailed description may be omitted.

[0056] For example, the flight time (the time it takes for light to be emitted from the light source 1, reflected by the subject, and returned to the light receiving sensor 2) differs depending on the distance from the light source 1 to the subject. By setting the exposure time of the light receiving sensor 2 based on the flight time, it is possible to detect a subject at a specified distance.

[0057] In the second embodiment, the exposure time in each subrange is set to a timing delayed by a round-trip flight time of a distance corresponding to the center position between the previous and next subranges (for example, in the case of subrange image SR3, subrange images SR2 and SR4) after the light source emits light. By repeating exposure for the exposure time (counting the returning light (photons)), a photon count value at a position corresponding to each subrange can be obtained. When the count value exceeds a certain threshold, the light receiving sensor 2 determines that a subject is present, outputs a signal of a predetermined output level, and generates an image of the subrange. The light receiving sensor 2 also generates a full-range image FR1 by superimposing the obtained multiple subrange images (subrange images SR1 to SR5 in FIG. 6).

[0058] Fig. 7 is a diagram for explaining a method for generating a sub-range image according to the second embodiment. Fig. 7 shows the generation timing of the sub-range image SR3.

[0059] As shown in Fig. 7, in the second embodiment, an exposure+exposure end pulse (a pulse whose rising edge corresponds to the start of exposure and whose falling edge corresponds to the end of exposure) is generated at a timing delayed by a time EX3 (distance measurement period) corresponding to the flight time corresponding to the subrange image SR3 after light (pulse) is emitted from the light source 1. That is, when generating the subrange image SR3, the light receiving sensor 2 performs exposure during a period when the exposure+exposure end pulse is high. In order to create the subrange image SR3, the light receiving sensor 2 performs this exposure operation multiple times (frames, n times in this embodiment) and counts the number of photons reflected back from the subject.

[0060] -About pixel configuration- Fig. 8 shows a circuit configuration of a pixel according to the second embodiment. As shown in Fig. 8, in the second embodiment, the pixel 30 further includes a charge transfer transistor 37, a constant current source control transistor 38, and a signal charge storage capacitor 39. Note that the configuration of the light receiving sensor 2 is almost the same as that in Fig. 2, and therefore a description thereof will be omitted.

[0061] The charge transfer transistor 37 has a source (or drain) connected to the drain (or source) of the reset transistor 32 and the cathode of the light receiving element 31, a drain (or source) connected to the gate of the constant current source transistor 33, a drain (or source) of the constant current source control transistor 38, and one end of the signal charge storage capacitance 39, and a gate connected to a charge transfer gate signal V TRN The other end of the signal charge storage capacitor 39 is connected to the ground voltage.

[0062] The constant current source control transistor 38 has a source (or drain) connected to a constant current source control signal V A The gate receives the signal charge capacitance reset signal V B Receive.

[0063] In addition, the charge transfer gate signal V TRN and the constant current source control signal V A is generated by the vertical drive circuit 24.

[0064] -About pixel operation- 9 shows a timing chart of distance measurement operation during one frame period of a pixel according to the second embodiment. The exposure start pulse (exposure start signal) is generated by the vertical drive circuit 24 upon receiving a signal from the timing signal generator 4. As described above, the exposure start pulse is generated (high level) with a delay of a time (distance measurement period) equivalent to the flight time corresponding to the sub-range image after light (pulse) is emitted from the light source 1. The exposure end pulse signal goes high when the light receiving element 31 detects light.

[0065] In the second embodiment, as in the first embodiment, a laser pulse is used as the light source 1, and distance measurement results from one laser pulse are taken as one frame. Then, distance measurement is performed for a predetermined number of frames in one sub-range section. Then, a signal charge proportional to both the number of photon detections and the distance to the subject at that time is accumulated in the capacitance 36, and the pixel 30 outputs the result as a pixel signal to the signal line 26.

[0066] Specifically, one frame period starts at an initial time t10.

[0067] At time t11, the reset signal V RST , PD bias control signal V D and the charge transfer gate signal V TRN As a result, the reset transistor 32 and the charge transfer transistor 37 are turned on, and the cathode of the light receiving element 31 goes to high level, so that the light detection signal and the dark current component in the previous frame are reset.

[0068] Also, at time t11, the constant current source bias control signal V I At this time, the gate of the constant current source transistor 33 is also at a high level, so the FD voltage V FD will be at a high level.

[0069] At time t12, the exposure start pulse (a pulse indicating the exposure start timing for generating a subrange image) becomes high level, and the reset signal V RST At this time, the constant current source bias control signal V I is set to a middle level between the high level and the low level so that the subthreshold voltage is output from the drain of the constant current source transistor 33. Here, the subthreshold voltage of the constant current source transistor 33 is V th and the voltage at high level is V H , the voltage at the middle level is V M Then, V H -V M <V th As a result, from time t12 to time t13, the constant current source transistor 33 is driven in response to the constant current source bias control signal V I As a result, the FD voltage V FD The potential of decreases in proportion to time due to the constant current injected from the constant current source transistor 33.

[0070] At time t13, when the light receiving sensor 2 receives reflected light (the reflected light pulse signal is on), the light receiving element 31 (for example, a SPAD) detects the reflected light and generates a Geiger mode pulse. At this time, since the reset transistor 32 is in the off state, the light receiving element 31 self-quenches, and the cathode voltage APDC of the light receiving element 31 drops to a low level due to the charge generated by avalanche multiplication. This causes the constant current source transistor 33 to turn off, and charge injection into the FD stops.

[0071] At time t14, the reset signal V RST becomes high level, and the reset transistor 32 is turned on. As a result, the injection of charges into the capacitor 36 in all the pixels 30 is stopped.

[0072] After time t4, the readout period begins, and the output signal from each pixel 30 is read out by the readout circuit 22, after which the system enters a standby state until the start of the next frame.

[0073] As described above, according to the distance measuring device of the second embodiment, the timing at which the light receiving sensor 2 receives photons can be distinguished, thereby improving the resolution of the sub-range image. Also, in the second embodiment, as in the first embodiment, the pixels can perform TDC operation, making it possible to switch between generating a sub-range image and performing TDC operation.

[0074] (Other embodiments) As described above, the embodiments have been described as examples of the technology disclosed in the present application. However, the technology in the present disclosure is not limited to these, and may be applied to embodiments in which modifications, substitutions, additions, omissions, etc. are made as appropriate.

[0075] In the above embodiments, the constant current source device is the constant current source transistor 33, but the constant current source device is not limited to this and may have any configuration as long as it can inject a constant current into the capacitance 36. For example, the constant current source device may be configured with a low voltage and a resistor. [Explanation of symbols]

[0076] 1 light source 2 Light receiving sensor 3. Signal Processing Device 4 Timing Generator 30 pixels 31 Photodetector 33 Constant current source transistor (constant current source device) 36 Capacity (storage element)

Claims

1. Equipped with multiple pixels Each pixel is A light receiving element; A storage element; a constant current source device provided in each of the pixels, the constant current source device outputting a constant current to the storage element from the start of exposure of each of the pixels until the light receiving element detects light; Equipped with The image pickup device, wherein a constant voltage is supplied to the constant current source device from the start of exposure of each of the pixels to the time when the light receiving element detects light.

2. The image sensor according to claim 1 , wherein the constant current source device stops outputting the constant current in response to a voltage change at an output terminal of the light receiving element.

3. the constant current source device includes a constant current source transistor having a gate connected to an output terminal of the light receiving element; The image sensor according to claim 1 , wherein the constant current source transistor is biased in a subthreshold region when outputting the constant current.

4. The imaging element according to claim 1 , wherein each of the pixels performs exposure a plurality of times for one frame, and outputs a signal indicating electric charge accumulated in the storage element by the plurality of exposures.

5. The imaging device according to claim 1 , wherein the light receiving element is an avalanche photodiode.

6. The imaging device according to claim 5 , wherein the avalanche photodiode operates in a Geiger mode when it detects light.

7. The imaging element according to any one of claims 1 to 6, A light source; a timing signal generator that outputs an exposure start signal indicating a timing for starting the exposure to the plurality of pixels; and a signal processing device that calculates a distance to a subject from pixel signals output from the plurality of pixels.

8. 8. The distance measuring device according to claim 7, wherein said timing signal generator outputs said exposure start signal when a flight time corresponding to a predetermined distance has elapsed since said light source emitted light.

Images