Range image pickup element and range image pickup device

Abstract

To provide a distance image imaging element capable of suppressing defective discharge and defective transfer of electric charge.SOLUTION: The distance image imaging element consists of a pixel circuit formed on a semiconductor substrate. The pixel circuit includes at least a photoelectric conversion element for generating a charge, a charge storage unit for storing a charge, a transfer MOS transistor provided on a transfer path for transferring a charge from the photoelectric conversion element to the charge storage unit, and a charge discharge MOS transistor provided on a discharge path for discharging a charge from the photoelectric conversion element. In the distance image imaging element, the photoelectric conversion element is formed on the semiconductor substrate in the shape of an N-square in plan view (N is an integer of 4 or more). The transfer MOS transistor and the charge discharge MOS transistor are arranged on either side of the photoelectric conversion element. At least one of the transfer MOS transistor and the charge discharge MOS transistor has a tapered shape in which a width dimension, which is a dimension in a direction parallel to the side on which it is arranged, gradually decreases as it is separated from the side in the photoelectric conversion element.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a distance image pickup element, and also to a distance image pickup apparatus using this distance image pickup element. [Background technology]

[0002] Time-of-Flight (hereinafter referred to as "TOF") distance image sensors have been developed that utilize the known speed of light to measure the distance between a measuring device and an object based on the time of flight of light in space (measurement space). TOF distance image sensors irradiate the measurement object with a pulse of light (e.g., near-infrared light) and measure the distance between the measuring device and the object based on the difference between the time the light pulse is irradiated and the time the light pulse (reflected light) reflected by the object in the measurement space returns, that is, the time of flight of light between the measuring device and the object (see, for example, Patent Document 1).

[0003] When using such a distance imaging device to accurately measure the distance to an object at a specified distance, the amount of charge generated by the pixel due to reflected light from the subject must be read out accurately by transferring it using multiple gates. In a TOF distance image sensor, a photoelectric conversion element converts the amount of incident light into an electric charge, accumulates the converted electric charge in a charge accumulation unit, and then converts an analog voltage corresponding to the amount of accumulated electric charge into a digital value using an AD converter. Furthermore, TOF distance image sensors determine the distance between the measuring device and the object using an analog voltage corresponding to the amount of charge and information on the time of flight of light between the measuring device and the object, which is included in the digital value.

[0004] In the range imaging device, the charges generated by the photoelectric conversion elements are accumulated in each charge accumulation section at a predetermined cycle, and the delay time from the time a light pulse is emitted until the light pulse reflected by the subject returns is calculated based on the amount of charge accumulated in each charge accumulation section. The delay time and the speed of light are then used to calculate the distance from the range imaging device to the subject. To transfer charges from the photoelectric conversion element to the charge accumulation section, a transfer gate (transistor) for transferring charges is provided for each of the photoelectric conversion element and the charge accumulation section, and a drain gate (transistor) for discharging the charges converted by the photoelectric conversion element during a period (drain period) in which the charges are discharged without being stored. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 4235729 Summary of the Invention [Problem to be solved by the invention]

[0006] However, there are cases where discharge failure occurs, in which the charges cannot be completely discharged, due to the layout of the discharge gate. For example, if the drain gate is located away from the center of the photoelectric conversion element, the potential gradient formed when the drain gate is turned on will be gentler than if the drain gate is located closer to the center of the photoelectric conversion element. In this case, for example, if a large amount of external light is received during the drain period, the drain gate may not be able to fully drain the charge. If the charge that cannot be drained by the drain gate enters the transfer gate, and the charge that has entered the transfer gate is accumulated in the charge storage section, this will cause a decrease in the accuracy of distance measurement.

[0007] In view of the above circumstances, an object of the present invention is to provide a range image pickup device in which defects in discharge and transfer of electric charges are suppressed. Another object of the present invention is to provide a range image pickup device with improved accuracy in distance measurement. [Means for solving the problem]

[0008] A first aspect of the present invention is a distance imaging element in which a pixel circuit is formed on a semiconductor substrate, the pixel circuit including at least a photoelectric conversion element that generates electric charge, a charge accumulation unit that accumulates the electric charge, a transfer MOS transistor provided on a transfer path that transfers the electric charge from the photoelectric conversion element to the charge accumulation unit, and a charge discharge MOS transistor provided on a discharge path that discharges the electric charge from the photoelectric conversion element. The photoelectric conversion element is formed on a semiconductor substrate in an N-sided polygonal shape (N is an integer of 4 or more) in plan view. The transfer MOS transistor and the charge discharging MOS transistor are arranged on either side of the photoelectric conversion element. At least one of the transfer MOS transistor and the charge drain MOS transistor has a tapered shape in which the width dimension, which is the dimension in a direction parallel to the side on which it is arranged, gradually decreases as it moves away from the side within the photoelectric conversion element.

[0009] A second aspect of the present invention is a distance imaging device comprising a light receiving unit having a distance image imaging element according to the first aspect, and a distance image processing unit configured to be able to calculate the distance from the distance image imaging element to a subject based on a distance image captured by the distance image imaging element. [Effects of the Invention]

[0010] According to the present invention, a range image pickup device is provided in which discharge failures of charge and transfer are suppressed. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a block diagram showing a schematic configuration of a range imaging device according to an embodiment of the present invention; [Figure 2] 2 is a block diagram showing a schematic configuration of a distance image sensor used in the distance image capturing device. FIG. [Figure 3] 2 is a circuit diagram showing a pixel circuit arranged in the distance image pickup device used in the distance image pickup device. FIG. [Figure 4] FIG. 2 is a diagram showing the arrangement of transistors in the pixel circuit. [Figure 5] 10(a) and 10(b) are diagrams showing the difference in potential depending on the gate shape of a photoelectric conversion element and a transfer transistor. [Figure 6] 10(a) to 10(c) are diagrams showing other examples of the shape of the gate. [Figure 7] 10A and 10B are diagrams illustrating a photoelectric conversion element and a transistor according to a modified example of the present invention. [Figure 8] FIG. 10 is a diagram showing a distance image pickup element according to a modified example of the present invention. [Figure 9] 10(a) and 10(b) are diagrams showing a photoelectric conversion element and a transistor according to a modified example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Figure 1 is a block diagram showing the schematic configuration of a distance image pickup device 1. The distance image pickup device 1 includes a light source unit 2, a light receiving unit 3, and a distance image processing unit 4. Figure 1 also shows a subject S, which is an object to which the distance is to be measured by the distance image pickup device 1.

[0013] The light source section 2 irradiates a light pulse PO into a space to be photographed, in which a subject S, the distance of which is to be measured by the distance image pickup device 1, is present, under the control of the distance image processor 4. The light source section 2 is, for example, a surface-emitting semiconductor laser module such as a vertical cavity surface-emitting laser (VCSEL). The light source section 2 includes a light source device 21 and a diffuser plate 22.

[0014] The light source device 21 is a light source that emits laser light in a near-infrared wavelength band (for example, a wavelength band of 850 nm to 940 nm) that becomes the light pulse PO to be irradiated onto the subject S. The light source device 21 is, for example, a semiconductor laser light-emitting element. The light source device 21 emits pulsed laser light in response to control from the timing control unit 41. The diffusion plate 22 is an optical component that diffuses the laser light in the near-infrared wavelength band emitted by the light source device 21 to the extent of the surface to be irradiated onto the subject S. The pulsed laser light diffused by the diffusion plate 22 is emitted as a light pulse PO and is irradiated onto the subject S.

[0015] The light receiving unit 3 receives reflected light RL of the light pulse PO reflected by a subject S, the distance of which is to be measured in the range image pickup device 1, and outputs a pixel signal corresponding to the received reflected light RL. The light receiving unit 3 includes a lens 31 and a range image sensor 32. The lens 31 is an optical lens that guides the incident reflected light RL to the range image sensor 32. The lens 31 outputs the incident reflected light RL to the range image sensor 32 side, and causes the light to be received (incident) by pixel circuits provided in the light receiving region of the range image sensor 32.

[0016] The range image sensor 32 is a range image pickup element according to this embodiment. The range image sensor 32 has a plurality of pixels in a two-dimensional light receiving area. Each pixel circuit (pixel circuit 321) of the range image sensor 32 includes one photoelectric conversion element, a plurality of charge accumulation units corresponding to this one photoelectric conversion element, and components that distribute charge to each charge accumulation unit.

[0017] Range image sensor 32 distributes the charges generated by the photoelectric conversion elements to the respective charge accumulation sections under the control of timing control section 41 provided in range image processing section 4. Range image sensor 32 also outputs pixel signals according to the amount of charge distributed to the charge accumulation sections. Range image sensor 32 has multiple pixel circuits arranged in a two-dimensional matrix, and each pixel circuit outputs a corresponding pixel signal for one frame.

[0018] The distance image processing unit 4 controls the distance image pickup device 1 and calculates the distance to the subject S. The distance image processing unit 4 includes a timing control unit 41 and a distance calculation unit . The timing control unit 41 controls the timing of outputting various control signals required for distance measurement. The various control signals here include, for example, a signal that controls the irradiation of the light pulse PO, a signal that distributes the reflected light RL to multiple charge accumulation units, and a signal that controls the number of distributions per frame. The number of distributions refers to the number of times the process of distributing electric charges to the charge accumulation units CS (see FIG. 3) is repeated.

[0019] The distance calculation unit 42 outputs distance information calculated based on the pixel signals output from the range image sensor 32, which indicates the distance to the subject S. The distance calculation unit 42 calculates the delay time Td from when the light pulse PO is emitted until when the reflected light RL is received, based on the amount of charge accumulated in the multiple charge accumulation units CS. The distance calculation unit 42 calculates the distance from the range image pickup device 1 to the subject S in accordance with the calculated delay time Td.

[0020] With this configuration, in the distance image capturing device 1, the light source unit 2 irradiates a light pulse PO in the near-infrared wavelength band onto the subject S, and the light receiving unit 3 receives the reflected light RL reflected by the subject S, and the distance image processing unit 4 outputs distance information measuring the distance between the subject S and the distance image capturing device 1. Although FIG. 1 shows the distance image pickup device 1 having the distance image processing unit 4 built therein, the distance image processing unit 4 may be an element provided outside the distance image pickup device 1.

[0021] Next, there will be described the configuration of the distance image sensor 32 used as the imaging element in the distance image capturing device 1. Fig. 2 is a block diagram showing a schematic configuration of the imaging element (distance image sensor 32). As shown in FIG. 2, the distance image sensor 32 includes, for example, a light receiving area 320 in which a plurality of pixel circuits 321 are arranged, a control circuit 322, a vertical scanning circuit 323 having a distribution operation, a horizontal scanning circuit 324, and a pixel signal processing circuit 325.

[0022] Light receiving area 320 is an area in which a plurality of pixel circuits 321 are arranged, and in FIG. 2 an example is shown in which the pixel circuits are arranged in a two-dimensional matrix of 8 rows and 8 columns. Pixel circuits 321 accumulate electric charges corresponding to the amount of light received. Control circuit 322 controls the operation of the components of distance image sensor 32 in response to instructions from timing control unit 41 of distance image processing unit 4, for example.

[0023] The vertical scanning circuit 323 is a circuit that controls the pixel circuits 321 arranged in the light receiving region 320 for each row in accordance with control from the control circuit 322. The vertical scanning circuit 323 outputs a voltage signal corresponding to the amount of charge accumulated in each charge accumulation unit CS of the pixel circuit 321 to the pixel signal processing circuit 325.

[0024] The pixel signal processing circuit 325, under the control of the control circuit 322, performs predetermined signal processing (for example, noise suppression processing, A / D conversion processing, etc.) on the voltage signals output from the pixel circuits 321 of each column. Horizontal scanning circuit 324 is a circuit that outputs signals output from pixel signal processing circuit 325 sequentially in time series under control of control circuit 322. As a result, pixel signals corresponding to the amount of charge accumulated for one frame are sequentially output to distance image processing unit 4. In the following explanation, it is assumed that pixel signal processing circuit 325 performs A / D conversion processing and that the pixel signals are digital signals.

[0025] Here, we will explain the configuration of the pixel circuit 321 arranged in the light receiving region 320 provided in the distance image sensor 32. Fig. 3 is a circuit diagram showing an example of the configuration of the pixel circuit 321. The pixel circuit 321 in Fig. 3 is an example configuration including four pixel signal readout units.

[0026] The pixel circuit 321 includes one photoelectric conversion element PD, a charge discharging transistor GD (GD1 and GD2 described below), and four pixel signal readout units RU (RU1 to RU4) that output voltage signals from corresponding output terminals O. Each pixel signal readout unit RU includes a transfer transistor G, a floating diffusion FD, a charge storage capacitance C, a reset transistor RT, a source follower transistor SF, and a selection transistor SL. The floating diffusion FD and the charge storage capacitance C form a charge storage unit CS.

[0027] 3, the pixel signal readout unit RU1, which outputs a voltage signal from the output terminal O1, includes a transfer transistor (transfer MOS transistor) G1, a floating diffusion FD1, a charge storage capacitor C1, a reset transistor RT1, a source follower transistor SF1, and a selection transistor SL1. In the pixel signal readout unit RU1, the floating diffusion FD1 and the charge storage capacitor C1 form a charge storage unit CS1. The pixel signal readout units RU2, RU3, and RU4 have a similar configuration.

[0028] The photoelectric conversion element PD is a buried photodiode that photoelectrically converts incident light, generates charges corresponding to the incident light, and accumulates the generated charges. In this embodiment, the incident light is incident from the space to be measured. In the pixel circuit 321, the photoelectric conversion element PD photoelectrically converts incident light to generate electric charges, which are then distributed to each of the four charge accumulation units CS (CS1 to CS4), and voltage signals corresponding to the amount of the distributed electric charges are output to the pixel signal processing circuit 325. 3, the pixel circuit configuration arranged in the range image sensor 32 is not limited to the configuration including four pixel signal readout units RU (RU1 to RU4), but may be a pixel circuit configured to include 2M (M is an integer, M≧2) or more pixel signal readout units RU. In other words, the pixel circuit may be configured to include 2M (M is an integer, M≧2) or more transfer transistors G.

[0029] When the pixel circuit 321 of the range image pickup device 1 is driven, a light pulse PO is emitted for an irradiation time To, and reflected light RL is received by the range image sensor 32 after a delay time Td. The vertical scanning circuit 323 transfers the charges generated in the photoelectric conversion element PD to charge accumulation units CS1, CS2, CS3, and CS4 in this order in synchronization with the emission of the light pulse PO, and accumulates the charges in each of them. At this time, the vertical scanning circuit 323 turns on the transfer transistor G1, which is provided on a transfer path that transfers charges from the photoelectric conversion element PD to the charge accumulation unit CS1. As a result, the charges photoelectrically converted by the photoelectric conversion element PD are accumulated in the charge accumulation unit CS1 via the transfer transistor G1. Thereafter, the vertical scanning circuit 323 turns off the transfer transistor G1. This stops the transfer of charges to the charge accumulation unit CS1. In this way, the vertical scanning circuit 323 accumulates charges in the charge accumulation unit CS1. The same applies to the other charge accumulation units CS2, CS3, and CS4.

[0030] At this time, during the charge accumulation period in which charge is distributed to the charge accumulation sections CS, an accumulation cycle in which the accumulation drive signals TX1, TX2, TX3, and TX4 are supplied to the transfer transistors G1, G2, G3, and G4, respectively, is repeated. Then, charges corresponding to incident light are transferred from the photoelectric conversion element PD to the charge accumulation units CS1, CS2, CS3, and CS4, respectively, via the transfer transistors G1, G2, G3, and G4. A plurality of accumulation cycles are repeated during the charge accumulation period. As a result, charges are accumulated in the charge accumulation units CS1, CS2, CS3, and CS4, respectively, for each accumulation cycle of the charge accumulation units CS1, CS2, CS3, and CS4 during the charge accumulation period.

[0031] In addition, when the vertical scanning circuit 323 repeats the accumulation cycle of each of the charge accumulation units CS1, CS2, CS3, and CS4, after the transfer (allocation) of charge to the charge accumulation unit CS4 is completed, it turns on the charge discharge transistor GD provided on the discharge path that discharges charge from the photoelectric conversion element PD. As a result, before the accumulation cycle for the charge accumulation unit CS1 starts, the charge discharging transistor GD discards the charge generated in the photoelectric conversion element PD after the accumulation cycle for the immediately preceding charge accumulation unit CS4 (i.e., resets the photoelectric conversion element PD).

[0032] The vertical scanning circuit 323 then causes all of the pixel circuits 321 arranged in the light receiving region 320 to sequentially output voltage signals to the pixel signal processing circuit 325 in units of rows (horizontal arrangement) of the pixel circuits 321. The pixel signal processing circuit 325 then performs signal processing such as A / D conversion on each of the input voltage signals and outputs the signals to the horizontal scanning circuit 324 . The horizontal scanning circuit 324 outputs the processed voltage signals to the distance calculation unit 42 in the order of the columns of the light receiving regions 320 .

[0033] As described above, the vertical scanning circuit 323 repeatedly stores charge in the charge storage units CS and discards the charge photoelectrically converted by the photoelectric conversion elements PD over one frame. As a result, charge corresponding to the amount of light received by the distance image pickup device 1 over a predetermined time period is stored in each charge storage unit CS. The horizontal scanning circuit 324 outputs an electrical signal corresponding to the amount of charge stored in each charge storage unit CS for one frame to the distance calculation unit 42.

[0034] Due to the relationship between the timing of irradiating the light pulse PO and the timing of accumulating charges in each of the charge accumulation units CS (CS1 to CS4), the charge accumulation unit CS1 holds an amount of charge corresponding to external light components such as background light before irradiating the light pulse PO. Furthermore, the charge accumulation units CS2, CS3, and CS4 hold charges corresponding to the reflected light RL and external light components, respectively. The distribution (allocation ratio) of the charge amounts allocated to the charge accumulation units CS2 and CS3, or the charge accumulation units CS3 and CS4, is a ratio that corresponds to the delay time Td between when the light pulse PO is reflected by the subject S and when it enters the range image pickup device 1.

[0035] The distance calculation unit 42 uses this principle to calculate the delay time Td using the following equation (1) or (2). Td=To×(Q3-Q1) / (Q2+Q3-2×Q1) …(1) Td=To+To×(Q4-Q1) / (Q3+Q4-2×Q1) …(2) In the above equation, To is the period during which the light pulse PO is irradiated, Q1 is the amount of charge accumulated in the charge accumulation unit CS1, Q2 is the amount of charge accumulated in the charge accumulation unit CS2, Q3 is the amount of charge accumulated in the charge accumulation unit CS3, and Q4 is the amount of charge accumulated in the charge accumulation unit CS4. For example, when Q4=Q1, the distance calculation unit 42 calculates the delay time Td using equation (1), and when Q2=Q1, the distance calculation unit 42 calculates the delay time Td using equation (2).

[0036] In equation (1), charges generated by reflected light are accumulated in the charge accumulation units CS2 and CS3, but not in the charge accumulation unit CS4. On the other hand, in equation (2), charges generated by reflected light are accumulated in the charge accumulation units CS3 and CS4, but not in the charge accumulation unit CS2. In addition, equation (1) or (2) is based on the assumption that the amount of charge stored in the charge storage units CS2, CS3, and CS4 that corresponds to the external light component is the same as the amount of charge stored in the charge storage unit CS1.

[0037] The distance calculation unit 42 calculates the round trip distance to the subject S by multiplying the delay time obtained by equation (1) or (2) by the speed of light (velocity). Then, the distance calculation unit 42 obtains the distance to the subject S by dividing the calculated round trip distance by two.

[0038] FIG. 4 is a diagram showing an example of the arrangement (layout pattern) of each transistor of the pixel circuit 321 in this embodiment. FIG. 4 shows a layout pattern of the pixel circuit 321. 4 also shows the pattern layout of transfer transistors G1, G2, G3, and G4, source follower transistors SF1, SF2, SF3, and SF4, select transistors SL1, SL2, SL3, and SL4, reset transistors RT1, RT2, RT3, and RT4, charge discharge transistors (charge discharge MOS transistors) GD1 and GD2, and photoelectric conversion element PD. Each of the above-mentioned transistors is an n-channel MOS transistor formed on a p-type semiconductor substrate.

[0039] For example, the reset transistor RT1 is composed of a drain RT1_D (n diffusion layer (n-type impurity diffusion layer)), a source RT1_S (n diffusion layer), and a gate RT1_G on a p-type semiconductor substrate. The contact RT1_C is a pattern indicating a contact provided in each diffusion layer of the drain RT1_D (n diffusion layer) and the source RT1_S (n diffusion layer) of the reset transistor RT1, and connected to wiring (not shown). The other transfer transistors G1 to G4, source follower transistors SF1 to SF4, select transistors SL1 to SL4, reset transistors RT2 to RT4, and charge discharge transistors GD1 and GD2 also have the same basic configuration.

[0040] The photoelectric conversion element PD has a rectangular shape in a plan view, and has a long side PDL1, a long side PDL2 parallel to the long side PDL1, a short side PDS1, and a short side PDS2 parallel to the short side PDS1. 4 is an axis that is orthogonal to the short sides PDS1 (and PDS2) of the rectangle in the rectangular pattern of the photoelectric conversion element PD (i.e., parallel to the long sides PDL1 and PDL2 of the rectangle) and passes through the center O of the rectangle. Also, the y axis is orthogonal to the x axis, i.e., perpendicular to the long sides PDL1 (and PDL2) of the rectangle (parallel to the short sides PDS1 and PDS2 of the rectangle) and passes through the center O of the rectangle.

[0041] The charge draining transistor GD1 is arranged on the x-axis of the short side PDS1 and perpendicular to the short side PDS1. The charge drain transistor GD2 is arranged on the x-axis of the short side PDS2 and perpendicular to the short side PDS2. That is, the charge discharging transistor GD2 is arranged so as to be symmetrical to the charge discharging transistor GD1 with respect to the y-axis.

[0042] As described above, the charge discharging transistors GD1 and GD2 are arranged on the x-axis and at the same distance from the y-axis, and therefore are arranged at the same distance from the center O of the photoelectric conversion element PD.

[0043] The transfer transistor G1 and the transfer transistor G2 are arranged on the long side PDL1 so as to be symmetrical with respect to the y-axis. The transfer transistor G3 and the transfer transistor G4 are arranged on the long side PDL2 at positions that are symmetrical with respect to the y-axis. The pair of transfer transistors G3 and G4 is arranged in a position that is symmetrical with respect to the x-axis to the pair of transfer transistors G1 and G2.

[0044] With the above-described arrangement, the transfer transistors G1, G2, G3, and G4 are all disposed at the same distance from the x-axis and also at the same distance from the center O of the photoelectric conversion element PD. Moreover, the transfer transistors G1 to G4 have the same shape and size in the plan view shown in FIG. 4, and have similar transistor characteristics. This allows the transfer efficiency (transfer characteristics) of the charges generated by the photoelectric conversion elements PD to be uniform, and allows the charges to be stored in each of the charge storage units CS1 to CS4 with the same transfer characteristics, thereby making it possible to calculate the distance between the subject and the distance image pickup device with high accuracy.

[0045] The reset transistors RT1 and RT2 are arranged symmetrically with the reset transistors RT3 and RT4, respectively, with respect to the x-axis. The source follower transistors SF1 and SF2 are also arranged symmetrically with the source follower transistors SF3 and SF4, respectively, with respect to the x-axis. Furthermore, the select transistors SL1 and SL2 are also arranged symmetrically with the select transistors SL3 and SL4, respectively, with respect to the x-axis.

[0046] 4 shows only the arrangement of each transistor on the semiconductor substrate of pixel circuit 321, and omits the wiring patterns and charge storage capacitors (C1 to C4). Charge storage units CS1, CS2, CS3, and CS4 are arranged at the positions of floating diffusions FD1, FD2, FD3, and FD4, respectively.

[0047] Each of the charge discharging transistors GD1 and GD2 has a drain, a gate, and a source (n diffusion layer of the photoelectric conversion element PD). The drain is connected to a power supply VDD via a contact and a wiring. When a high-level gate voltage is applied to the gate of the charge discharging transistor, the charge (electrons) generated in the photoelectric conversion element PD is transferred to the drain, and the drain then discharges the charge transferred from the photoelectric conversion element PD to the power supply VDD.

[0048] Each of the transfer transistors G1, G2, G3, and G4 has a floating diffusion (such as FD1 or FD2 shown in FIG. 3) as a drain, a gate, and a source (the n-type diffusion layer of the photoelectric conversion element PD). A charge storage portion is formed in the floating diffusion. The drain is connected to the gate of the source follower transistor (such as SF1 or SF2 shown in FIG. 3) and the source of the reset transistor (such as RT1 or RT2 shown in FIG. 3) via contacts and wiring. When a high-level gate voltage is applied to the gate of the transfer transistor, the transfer transistor transfers the charges (electrons) generated in the photoelectric conversion element PD to the floating diffusion, which serves as the drain. The floating diffusion then accumulates the charges transferred from the photoelectric conversion element PD.

[0049] In this embodiment, the gates of both the transfer transistor and the charge drain transistor are pentagonal in plan view. More specifically, the portion from the floating diffusion to the photoelectric conversion element PD is rectangular, and after overlapping with the photoelectric conversion element PD, the portion extends a certain width and then gradually decreases in width to form a roughly triangular shape. The tip of the gate of each transistor is located at the center of the transistor's width. The tips of the charge drain transistors GD1 and GD2 are located on the X-axis.

[0050] In the process of studying optimal transistor layouts in pixel circuits, the inventors encountered a phenomenon in which charge transfer and discharge were not completed within the specified time period due to smooth charge transfer and discharge, resulting in charge moving to areas where it should not have gone. Further investigation by the inventors revealed that the planar shape of the gate was partially responsible for this phenomenon. Specifically, it was found that if the width dimension of the gate portion overlapping the photoelectric conversion element PD remains constant, the gradient of the gate potential becomes blunt, as shown in Figure 5(a), making it difficult for charge to move. This phenomenon was more pronounced in the charge discharge transistors GD1 and GD2, which have a longer distance between the transistors.

[0051] Based on this finding, the inventors solved this problem by tapering the planar shape of the gate, with the width decreasing toward the tip. Figure 5(b) shows the simulation results of the potential gradient at the gates GD1_G and GD2_G of the charge-discharging transistors GD1 and GD2 according to this embodiment. GD1_D and GD2_D represent the drains of the charge-discharging transistors GD1 and GD2, respectively. As indicated by the arrows, the gradient in the area where the gradient is gentle in Figure 5(a) is stronger in Figure 5(b), which is expected to result in smoother charge transfer. Although FIG. 5 shows an example of a charge discharging transistor, the same effect is expected to occur in the transfer transistors G1 to G4, resulting in smoother charge transfer.

[0052] As described above, the range image sensor 32 according to this embodiment has a structure that suppresses the occurrence of defects in the discharge and transfer of electric charges. As a result, it is possible to arrange transistors even in distances that make it difficult for conventional configurations to operate satisfactorily, and the degree of freedom in transistor layout is improved.

[0053] The gate of the transistor according to this embodiment can achieve the above-described effect as long as its width decreases with increasing distance from the periphery of the photoelectric conversion element. Therefore, the gate may have a rounded tip, as in the gate Ga shown in FIG. 6(a), or a flat tip, as in the gate Gb shown in FIG. 6(b). Alternatively, the gate may have a substantially triangular shape, as in the gate Gc shown in FIG. 6(c), which does not have a portion of constant width within the photoelectric conversion element. The gate shape according to this embodiment can be realized relatively easily using known techniques by changing the mask used during fabrication.

[0054] Another advantage of a tapered gate is that it is less likely to interfere with other gates placed around it when placed. For example, when transistors are placed on adjacent long and short sides near the corners of a rectangular photoelectric conversion element, if the gates have a rectangular shape in plan view, their tips are likely to interfere with each other, limiting their placement. However, if the gates are tapered, the width of the tips becomes smaller and slopes are formed at the widthwise ends of the gates, making it less likely for adjacent gates to interfere with each other. As a result, two transistors can be placed closer together than when the gates are rectangular, further increasing the flexibility of the transistor layout.

[0055] In view of the above, the tapered shape of the gate does not necessarily have to be symmetrical in the width direction. For example, as in the transfer transistors G1 to G4 in the modified example shown in FIG. 7, the tapered shape may have a slope only on the side facing the charge discharging transistor across the corner.

[0056] Although one embodiment of the present invention has been described in detail above, the present invention is not limited to a specific embodiment and includes configuration changes and combinations within the scope of the gist of the present invention. Some examples of changes are shown below, but these are not all inclusive and other changes are also possible. Two or more of these changes may be combined as appropriate.

[0057] The depth image pickup element according to the present invention may have a microlens on the side of the pixel circuit where light is incident. As shown in Figure 8, by aligning the optical axis of the microlens ML with the center O of the photoelectric conversion element and making the optical axis perpendicular to the light incident surface, the microlens ML can effectively guide the incident light to the photoelectric conversion element PD, thereby improving sensitivity.

[0058] The planar shape of the photoelectric conversion element according to the present invention is not limited to the rectangular shape described above, but may be any N-sided polygon (N is an integer equal to or greater than 4). As another example, Fig. 9(a) shows a configuration example in which the planar shape of the photoelectric conversion element is a regular hexagon, and Fig. 9(b) shows a configuration example in which the planar shape of the photoelectric conversion element is a regular pentagon. The regular hexagonal configuration example has a total of six transistors: four transfer MOS transistors G1 to G4 and two charge drain MOS transistors GD1 and GD2. The four transfer MOS transistors are arranged one on each side where the charge drain MOS transistors GD1 and GD2 are not provided, and are positioned symmetrically with respect to an axis (shown by a dashed line in the figure) that is perpendicular to the side where the charge drain MOS transistors are provided and passes through the center O of the photoelectric conversion element. The regular pentagonal configuration example has five transistors in total: four transfer MOS transistors G1 to G4 and one charge drain MOS transistor GD. The four transfer MOS transistors are arranged one on each side where no charge drain MOS transistor GD is provided, and are positioned symmetrically with respect to an axis (shown by a dashed line in the figure) that is perpendicular to the side where the charge drain MOS transistor is provided and passes through the center O of the photoelectric conversion element. Even with this configuration, the same effects as those of the distance image pickup device according to the above-described embodiment can be achieved.

[0059] Even if the photoelectric conversion element has a quadrangular shape in plan view, its specific shape is not limited to a rectangle and may be a square, etc. In the case of a square, all sides have the same length and there are no long or short sides, so the charge-discharging MOS transistor may be provided on either of the pair of opposing sides. [Explanation of symbols]

[0060] 1. Range imaging device 3 Light receiving section 4. Depth image processing section 32 Range image sensor (range image sensor) 321 Pixel Circuit CS charge storage section G1, G2, G3, G4 Transfer transistors (transfer MOS transistors) GD, GD1, GD2: Charge drain transistors (charge drain MOS transistors) ML Micro Lens PD photoelectric conversion element

Claims

1. A distance imaging device having a pixel circuit formed on a semiconductor substrate, the pixel circuit including at least a photoelectric conversion element that generates a charge according to light incident from a space to be measured, a charge accumulation unit that accumulates the charge, a transfer MOS transistor provided on a transfer path that transfers the charge from the photoelectric conversion element to the charge accumulation unit, and a charge discharge MOS transistor provided on a discharge path that discharges the charge from the photoelectric conversion element, the photoelectric conversion element is formed on the semiconductor substrate in an N-sided polygon (N is an integer of 4 or more) shape in a plan view, the transfer MOS transistor and the charge drain MOS transistor are arranged on either side of the photoelectric conversion element, At least one of the transfer MOS transistor and the charge drain MOS transistor has a tapered shape in which a width dimension, which is a dimension parallel to a side on which the transistor is arranged, gradually decreases as the transistor moves away from the side of the photoelectric conversion element within the photoelectric conversion element. Range imaging sensor.

2. the photoelectric conversion element has a rectangular shape in plan view having long sides and short sides, the number of the transfer MOS transistors is 2M (M is an integer of 2 or more) and the number of the charge discharge MOS transistors is 2n (n is an integer of 1 or more), M transfer MOS transistors are arranged on each of the long sides so as to be symmetrical with respect to an x-axis that is parallel to the long sides and passes through the center of the photoelectric conversion element, n number of the charge-discharge MOS transistors are arranged on each of the short sides; The distance image pickup device according to claim 1 .

3. the transfer MOS transistors are arranged so as to be symmetrical with respect to a y-axis that is parallel to the short side and passes through the center of the photoelectric conversion element; The distance image pickup device according to claim 2 .

4. N is an integer of 5 or more, the sum of the transfer MOS transistors and the charge discharge MOS transistors is N or more; At least one transfer MOS transistor is provided on each of the sides of the photoelectric conversion element except for the side on which the charge discharging MOS transistor is provided. The distance image pickup device according to claim 1 .

5. the transfer MOS transistors are arranged at positions that are line-symmetric with respect to an axis that is perpendicular to any side of the N-gon and passes through the center of the N-gon; 5. The distance image pickup device according to claim 4.

6. a microlens disposed on the side of the pixel circuit where the light is incident, the microlens having an optical axis perpendicular to the incident surface of the photoelectric conversion element and coinciding with the center of the incident surface; The distance image pickup device according to claim 1 .

7. a light receiving section having the distance image pickup element according to any one of claims 1 to 6; a distance image processing unit configured to be able to calculate a distance from the distance image pickup element to a subject based on a distance image captured by the distance image pickup element; Equipped with Range imaging device.

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