Distance image pickup element and distance image pickup device

Abstract

To provide a distance image pickup element capable of suppressing crosstalk more securely.SOLUTION: In a distance image pickup element which comprises a plurality of pixels including: a photoelectric conversion element PD for generating charge; a charge accumulation part for accumulating the charge; a transfer MOS transistor provided on a transfer path which transfers the charge from the photoelectric conversion element to the charge accumulation part; a charge discharging MOS transistor provided on a discharge path which discharges the charge from the photoelectric conversion element; and a pixel signal read part for outputting a voltage signal from a corresponding output terminal, the pixel signal read part has a reset transistor, a source follower transistor, a selection transistor, and a charge accumulation capacitor. Furthermore, a separation wall 50 provided at a boundary part of two neighboring pixels and extending to a thickness direction, and a second separation wall 51 provided between the photoelectric conversion element and the separation wall in a plan view of the pixel.SELECTED DRAWING: Figure 7

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] A time-of-flight (TOF) type range image sensor has been developed that utilizes the known speed of light to measure the distance between a measuring device and an object based on the flight time of light in space (measurement space). In a TOF type range image sensor, a pulse of light (e.g., near-infrared light, etc.) is irradiated onto the object, and the distance between the measuring device and the object is measured based on the difference between the time when the light pulse is irradiated and the time when the light pulse (reflected light) reflected by the object in the measurement space returns, that is, the flight time of light between the measuring device and the object (see, for example, Patent Document 1).

[0003] When using such a range image pickup device to accurately measure the distance to an object at a specified distance, the amount of charge generated from the pixels by light reflected from the subject must be transferred using multiple gates and read out with high accuracy. In a TOF type 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 converts an analog voltage corresponding to the amount of accumulated electric charge into a digital value by an AD converter. In addition, TOF type distance image sensors determine the distance between the measuring device and the target object using an analog voltage corresponding to the amount of charge and information on the flight time of light between the measuring device and the target object, which is contained in the digital value.

[0004] In general, imaging devices including distance imaging devices have a large number of pixels. Each pixel is separated by an element isolation structure such as a partition (see, for example, Patent Document 1). This prevents light incident on a pixel from entering another adjacent pixel (hereinafter, this may be referred to as "crosstalk"). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2021-190466 A Summary of the Invention [Problem to be solved by the invention]

[0006] Although the details will be described later, the inventors have found that the element isolation structure as described in Patent Document 1 is still insufficient in suppressing crosstalk, and have completed the present invention based on this finding.

[0007] An object of the present invention is to provide a distance image pickup element that can more reliably suppress crosstalk. 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 image pickup element comprising a plurality of pixels, each including a photoelectric conversion element that generates an electric charge in response to light incident from the space to be measured, a charge accumulation section 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 section, a charge discharge MOS transistor provided on a discharge path that discharges the electric charge from the photoelectric conversion element, and a pixel signal readout section that outputs a voltage signal from a corresponding output terminal. The pixel signal readout unit includes a reset transistor, a source follower transistor, a selection transistor, and a charge storage capacitor. The distance image pickup element further includes a separation wall provided at the boundary between two adjacent pixels and extending in the thickness direction, and a second separation wall provided between the photoelectric conversion element and the separation wall in a planar view of the pixel.

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

[0010] According to the present invention, it is possible to provide a distance image pickup element capable of suppressing crosstalk more reliably. [Brief description of the drawings]

[0011] [Figure 1] 1 is a block diagram showing a schematic configuration of a range image capturing device according to a first embodiment of the present invention. [Diagram 2] 2 is a block diagram showing a schematic configuration of a distance image sensor used in the distance image pickup device. FIG. [Diagram 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 an arrangement of transistors in the pixel circuit. [Diagram 5] FIG. 1 is a diagram showing an aspect of a separation wall in a general pixel structure. [Figure 6] 1A to 1C are diagrams showing some patterns in which incident light leaks into adjacent pixels in a typical pixel. [Figure 7] 2 is a plan view of a pixel region of the distance image capturing device. FIG. [Figure 8] FIG. [Figure 9] 13 is a plan view of a pixel region according to a modified example of the distance image pickup device. FIG. [Figure 10] 13 is a plan view of a pixel region according to a modified example of the distance image pickup device. FIG. [Figure 11] 13 is a plan view of a pixel region according to a modified example of the distance image pickup device. FIG. [Figure 12] 13 is a plan view of a pixel region according to a modified example of the distance image pickup device. FIG. [Figure 13]FIG. 11 is a cross-sectional view showing a pixel region of a range image pickup device according to a second embodiment of the present invention. [Figure 14] FIG. 11 is a cross-sectional view showing a pixel region according to a modified example. [Figure 15] FIG. 13 is a plan view showing a pixel region of a range image pickup device according to a modified example of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, a first embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a block diagram showing a 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. Fig. 1 also shows a subject S, which is an object to which the distance is to be measured in 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, exists, 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 diffusion 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 a light pulse PO to be irradiated to 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 a surface area 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 to be measured for distance 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 a pixel circuit provided in the light receiving region of the range image sensor 32.

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

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

[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 are, for example, a signal for controlling the irradiation of the light pulse PO, a signal for distributing the reflected light RL to a plurality of charge accumulation units, a signal for controlling the number of distributions per frame, etc. 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 obtained by calculating the distance to the subject S based on the pixel signals output from the distance image sensor 32. 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 distance 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 that is 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 section 4 therein, the distance image processing section 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 an imaging element in the distance image pickup 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 Fig. 2 shows an example 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 section 41 of distance image processing section 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 response to 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 (eg, 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 sequentially outputs signals output from pixel signal processing circuit 325 in time series according to control from 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 circuit 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, a description will be given of the configuration of the pixel circuit 321 arranged in the light receiving region 320 of 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 configure 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 capacitance 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 capacitance 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 according 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 the incident light to generate electric charges, which are then distributed to each of the four charge accumulation units CS (CS1 to CS4), and each voltage signal corresponding to the amount of electric charge distributed is output to the pixel signal processing circuit 325. 3, the pixel circuit in the distance image sensor 32 is not limited to the configuration having four pixel signal readout units RU (RU1 to RU4), but may be a pixel circuit having a plurality of pixel signal readout units RU, 2M (M is an integer, M≧2) or more. In other words, the pixel circuit may be a pixel circuit having 2M (M is an integer, M≧2) or more transfer transistors G.

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

[0030] At this time, during a charge accumulation period in which charges are 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 in 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 (relocation) 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 previous charge accumulation unit CS4 (i.e., resets the photoelectric conversion element PD).

[0032] Then, the vertical scanning circuit 323 causes all of the pixel circuits 321 arranged in the light receiving region 320 to output voltage signals to the pixel signal processing circuit 325 in sequence for each row (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 voltage signals after signal processing 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 charges in the charge storage units CS and discards the charges photoelectrically converted by the photoelectric conversion elements PD over one frame. This causes charges corresponding to the amount of light received by the distance image pickup device 1 during a predetermined time period to be stored in each of the charge storage units CS. The horizontal scanning circuit 324 outputs an electrical signal corresponding to the amount of charge for one frame stored in each of the charge storage units CS 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. The charge accumulation units CS2, CS3, and CS4 hold charges corresponding to the reflected light RL and external light components. The distribution (allocation ratio) of the amount of charge allocated to the charge accumulation units CS2 and CS3, or the charge accumulation units CS3 and CS4, is a ratio according to the delay time Td from when the light pulse PO is reflected by the subject S to when it is incident on the range image pickup device 1.

[0035] Using this principle, the distance calculation unit 42 calculates 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 formula, 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 formula (1), and when Q2=Q1, the distance calculation unit 42 calculates the delay time Td using formula (2).

[0036] In formula (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 formula (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, in equation (1) or (2), it is assumed that the component corresponding to the external light component among the amount of charge accumulated in the charge accumulation units CS2, CS3, and CS4 is the same amount as the amount of charge accumulated in the charge accumulation unit CS1.

[0037] Distance calculation unit 42 calculates the round trip distance to 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 round trip distance calculated above by two.

[0038] FIG. 4 is a diagram showing the arrangement (layout pattern) of each transistor of a pixel circuit 321 in this embodiment. FIG. 4 shows a layout pattern of the pixel circuit 321. 4 also shows the pattern arrangement of the transfer transistors G1, G2, G3, and G4, the source follower transistors SF1, SF2, SF3, and SF4, the selection transistors SL1, SL2, SL3, and SL4, the reset transistors RT1, RT2, RT3, and RT4, the charge discharge transistors (charge discharge MOS transistors) GD1 and GD2, and the photoelectric conversion element PD. Each of the above-mentioned transistors is an n-channel type 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 of the diffusion layers 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, the source follower transistors SF1 to SF4, the selection transistors SL1 to SL4, the reset transistors RT2 to RT4, and the charge discharging 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 perpendicular to the short side PDS1 (and PDS2) of the rectangle in the rectangular pattern of the photoelectric conversion element PD (i.e., parallel to the long sides PDL1, PDL2 of the rectangle) and passes through the center O of the rectangle. Also, the y axis is perpendicular to the x axis, i.e., perpendicular to the long sides PDL1 (and PDL2) of the rectangle (parallel to the short sides PDS1, PDS2 of the rectangle) and passes through the center O of the rectangle.

[0041] The charge drain transistor GD1 is disposed on the x-axis of the short side PDS1 and perpendicular to the short side PDS1. The charge drain transistor GD2 is disposed 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, the charge discharging transistors GD1 and GD2 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 linearly symmetric with respect to the y-axis. The pair of transfer transistors G3 and G4 is disposed in a position that is symmetrical with respect to the x-axis to the pair of transfer transistors G1 and G2.

[0044] The reset transistors RT1 and RT2 are arranged so as to be symmetrical with respect to the x-axis with respect to the reset transistors RT3 and RT4, respectively. The source follower transistors SF1 and SF2 are also arranged symmetrically with respect to the x-axis with respect to the source follower transistors SF3 and SF4, respectively. Furthermore, the select transistors SL1 and SL2 are also arranged symmetrically with respect to the x-axis with respect to the select transistors SL3 and SL4, respectively.

[0045] 4 shows only the arrangement of each transistor on the semiconductor substrate of the pixel circuit 321, and omits wiring patterns and charge storage capacitors (C1 to C4). The charge storage units CS1, CS2, CS3, and CS4 are disposed at the positions of the floating diffusions FD1, FD2, FD3, and FD4, respectively.

[0046] Each of the charge discharging transistors GD1 and GD2 has a drain, a gate, and a source (the n-type diffusion layer of the photoelectric conversion element PD). The drain is connected to a power supply VDD via a contact and a wiring. When an "H" level gate voltage is applied to the gate of the charge drain transistor, the charge drain transistor transfers the charge (electrons) generated in the photoelectric conversion element PD to the drain. The drain then discharges the charge transferred from the photoelectric conversion element PD to the power supply VDD.

[0047] Each of the transfer transistors G1, G2, G3, and G4 has a floating diffusion (FD1, FD2, etc. shown in FIG. 3) as a drain, a gate, and a source (n-diffusion layer of the photoelectric conversion element PD). A charge storage portion is formed in the floating diffusion. The drain is connected to each of the gates of the source follower transistors (SF1, SF2, etc. shown in FIG. 3) and the sources of the reset transistors (RT1, RT2, etc. 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 charge (electrons) generated in the photoelectric conversion element PD to the floating diffusion acting as the drain. The floating diffusion then accumulates the charge transferred from the photoelectric conversion element PD.

[0048] The periphery Pp indicated by the dashed line in Fig. 4 is the boundary between adjacent pixels. In a typical pixel structure, a separation wall 50 that divides each pixel is formed along the periphery Pp as shown in Fig. 5. The separation wall 50 extends downward from the upper surface of the pixel in the thickness direction of the pixel, reaching the vicinity of each of the transistors described above and the wiring connecting them. The separation wall 50 is formed of a material having a refractive index different from that of the photoelectric conversion element PD etc. Examples of the material of the separation wall include silica, metal, other metal oxides and the like.

[0049] The isolation wall 50 is well known, and a representative structure is deep trench isolation (DTI). The isolation wall 50 has a function of reflecting light that is obliquely incident on the upper surface of a pixel and suppressing the light from leaking to an adjacent pixel. However, when the inventors conducted an investigation using light ray simulation, it became clear that typical separation walls provided at pixel boundaries alone do not necessarily completely prevent light from leaking into adjacent pixels.

[0050] FIG. 6 shows some patterns found by the inventors in which incident light leaks into adjacent pixels in a pixel having a separation wall 50 only on the periphery. First, separation wall 50 only completely reflects light when total reflection occurs (arrow N in Figure 6), so that light incident at an angle different from the angle at which total reflection occurs is partially reflected by separation wall 50 while partially refracted and enters separation wall 50, and either passes through separation wall 50 as is and enters an adjacent pixel (arrow A in Figure 6) or is repeatedly reflected within separation wall 50 before entering an adjacent pixel (arrow B in Figure 6). Further, below where the separation wall 50 does not exist, the same silicon material as the photoelectric conversion element PD is connected across adjacent pixels, so that light propagates through this portion and leaks into the adjacent pixel (arrow C in FIG. 6). Furthermore, there is light that propagates through the oxide film 70 covering the upper surface of the pixel and leaks to an adjacent pixel (arrow D in FIG. 6). Such light becomes prominent in a so-called back-illuminated (BSI) configuration in which there is no wiring layer in the oxide film. Therefore, in a range image pickup device, it is expected that crosstalk can be further reduced and accuracy improved by suppressing the leakage of such light to adjacent pixels.

[0051] A plan view of a pixel region according to this embodiment is shown in Fig. 7. Based on the above findings, the inventors have succeeded in further reducing crosstalk by providing a second separation wall 51 around the photoelectric conversion element PD in the pixel in addition to the separation wall 50 provided at the boundary portion of the pixel. In this embodiment, the second separation wall 51 has a rectangular shape in plan view that is smaller than the separation wall 50, is disposed without overlapping with the rectangular region of the photoelectric conversion element PD in plan view, and seamlessly surrounds the periphery of the photoelectric conversion element PD. The second separation wall 51 can be formed of a material that can be used for the separation wall 50. The material and the length of the downward extension of the second separation wall 51 can be the same as or different from those of the separation wall 50.

[0052] FIG. 8 is a cross-sectional view of the pixel region of the range image pickup device according to this embodiment configured as described above. By providing the second separation wall 51 in each pixel, a total of three separation walls, consisting of the conventional separation wall 50 and two second separation walls 51, are present between the two photoelectric conversion elements PD of adjacent pixels, as shown in FIG. 8. For this reason, in order for light incident on a pixel to enter another adjacent pixel, it must pass through separation walls with different refractive indices three times. Since a part of the light is reflected each time it enters a separation wall, the light gradually attenuates (arrow A1 shown in FIG. 8). Furthermore, the light that has passed through a separation wall may be incident on the next separation wall at an angle that causes total reflection due to refraction, in which case leakage to adjacent pixels is completely prevented. Furthermore, the frequency with which a portion of the light propagating below each separation wall is reflected by the lower surface of one of the separation walls increases (arrow C1 shown in FIG. 8), which also contributes to reducing light leakage to adjacent pixels.

[0053] In this way, in the distance image pickup device 1 according to this embodiment, by providing the separation wall 50 and the second separation wall 51, various changes in the behavior of light, including the above-mentioned phenomena, occur compared to the conventional pixel structure. As a result, crosstalk can be more reliably suppressed and distance measurement accuracy can be improved. The inventors' study using simulation also confirmed that crosstalk can be reduced by about 30% compared to the conventional configuration in which separation walls are provided only at pixel boundaries.

[0054] The second partition wall 51 in this embodiment is rectangular like the partition wall 50, but since the technical idea of ​​the present invention is to provide an additional partition wall between the photoelectric conversion element PD and the partition wall 50, there are no strict limitations on the shape of the second molecular wall in a planar view. For example, the shape of the second separating wall in plan view may be a polygon different from that of separating wall 50. Or, even if it is a similar polygon, it may be provided in an orientation that does not have a side parallel to separating wall 50, like second separating wall 51A of a modification shown in Fig. 9. Furthermore, the second separating wall may be a circle or an ellipse, like second separating wall 51B of a modification shown in Fig. 10, that does not have a linear portion in plan view.

[0055] As in the above-mentioned examples, the second separation wall is preferably in a form surrounding the photoelectric conversion element PD without any breaks, but this is not essential. Therefore, the second separation wall may be formed in an incomplete ring shape having one or more interrupted portions. For example, the second separation wall 51C of the modified example shown in FIG. 11 is formed of four linear portions that are not connected to each other, and does not surround the photoelectric conversion element PD without any breaks. However, since the second separation wall 51C is further present between the photoelectric conversion element PD and the separation wall 50 except for the portions corresponding to the corners of the rectangle formed by the four linear portions, it has a sufficient effect that is comparable to the form surrounding the photoelectric conversion element PD without any breaks.

[0056] As long as the second separation wall has a portion surrounding the periphery of the photoelectric conversion element PD, the second separation wall may have a portion connected to the separation wall 50 as in the modified example shown in FIG. As described above, the second separation wall is formed at a depth that does not contact the various transistors and wirings provided in the circuit section. Therefore, the position at which the second separation wall is provided is not restricted in any way by the arrangement of the various mechanisms of the circuit section in a planar view of the pixel, and there is no problem even if the two overlap in a planar view of the pixel.

[0057] A second embodiment of the present invention will be described with reference to Figures 13 and 14. In the following description, configurations common to those already described will be given the same reference numerals and duplicated description will be omitted.

[0058] Fig. 13 is a cross-sectional view of a pixel portion according to this embodiment, and is shown in a manner corresponding to Fig. 8. In the pixel region of this embodiment, a shield 60 made of metal is disposed above the separation wall 50 and the second separation wall 51. The shield 60 according to this embodiment overlaps with the separation wall 50 and the second separation wall 51 in a plan view of the pixel, and completely covers the separation wall 50 and the second separation wall 51. Examples of the material for the shield 60 include aluminum, copper, silver, and tungsten, and the shield 60 can be formed by vapor deposition, sputtering, or the like.

[0059] By covering the upper sides of the separation walls 50 and 51 with the shield 60, in addition to the effects of the first embodiment, the amount of light indicated by the arrow B in Fig. 6 that enters the separation walls from above is reduced by the shield 60 (arrow B1 in Fig. 13). As a result, the light leaking to adjacent pixels can be further reduced. In this embodiment, there is no essential condition for the positional relationship between the shield 60 and the oxide film 70, and even when the shield 60 is below the oxide film 70 or above the oxide film 70, the effect of reducing light incident on the separation wall from above is achieved. However, as shown in Fig. 13, when the shield 60 is located within the oxide film 70, part of the light traveling within the oxide film 70 is also reflected by the shield 60 (arrow D1 shown in Fig. 13), which has the advantage of further improving the effect of reducing light leaking to adjacent pixels.

[0060] The planar shape of the shield according to this embodiment does not necessarily have to be the same as the separation walls. For example, a continuous shield may cover a plurality of separation walls, as in a modified shield 60A shown in FIG. In addition, although the effect is slightly reduced compared to the above-described embodiment, it is also possible to provide a shield that covers a portion of the separation wall in a planar view (for example, covering only one of the separation wall 50 and the second separation wall 51, or only a portion of each separation wall).

[0061] Although each embodiment of the present invention has been described above in detail, 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 modifications are exemplified below, but these are not all inclusive, and other modifications are also possible. Two or more of these modifications may be appropriately combined.

[0062] The distance image pickup element according to the present invention may have a microlens on the side of the pixel circuit on which light is incident. As shown in Fig. 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 incidence surface, the microlens ML can suitably guide the incident light to the photoelectric conversion element PD, improving sensitivity. The light concentration by the microlens ML increases the ratio of light directed toward the photoelectric conversion element PD and reduces the light directed toward an adjacent pixel, which has the advantage of further reducing light leakage into adjacent pixels. [Explanation of symbols]

[0063] 1. Range imaging device 3 Light receiving section 4. Distance image processing section 32 Range image sensor (range image sensor) 321 Pixel Circuit 50 Separation wall 51, 51A, 51B, 51C Second separation wall 60, 60A Shielded C, C1, C2, C3, C4 Charge storage capacity CS charge storage section G1, G2, G3, G4 Transfer transistors (transfer MOS transistors) GD, GD1, GD2 Charge drain transistor (charge drain MOS transistor) ML Micro Lens PD photoelectric conversion element RT, RT1, RT2, RT3, RT4 Reset transistors RU, RU1, RU2, RU3, RU4 Pixel signal readout unit SF, SF1, SF2, SF3, SF4 Source follower transistors SL, SL1, SL2, SL3, SL4 select transistors

Claims

1. A distance image pickup element including a plurality of pixels, each pixel including 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, a charge discharge MOS transistor provided on a discharge path that discharges the charge from the photoelectric conversion element, and a pixel signal readout unit that outputs a voltage signal from a corresponding output terminal, the pixel signal readout unit includes a reset transistor, a source follower transistor, a selection transistor, and a charge storage capacitor; a separation wall provided at a boundary between two adjacent pixels and extending in a thickness direction; a second separation wall provided between the photoelectric conversion element and the separation wall in a plan view of the pixel; Equipped with Distance imaging element.

2. the second separation wall seamlessly surrounds the periphery of the photoelectric conversion element in a plan view of the pixel; 2. The distance image pickup device according to claim 1.

3. a metal shield provided above the separation wall and the second separation wall and overlapping at least a portion of the separation wall and the second separation wall in a plan view of the pixel; 2. The distance image pickup device according to claim 1.

4. a microlens arranged on a surface side of the pixel where the light is incident, the microlens having an optical axis perpendicular to the incident surface of the photoelectric conversion element and coinciding with a center of the incident surface; 2. The distance image pickup device according to claim 1.

5. A light receiving section having the distance image pickup element according to claim 1 ; 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.

Images