Solid-state image sensor, imaging device, and imaging method

Abstract

The goal is to improve the dynamic range of image generation, in particular, while suppressing an increase in pixel size. [Solution] The overflow storage capacitance LOFIC is shared by a pair of photodiodes PD1 and PD2. Overflow gates OFG1 and OFG2 are provided between each photodiode PD1 and PD2 and the overflow storage capacitance LOFIC. Furthermore, the potential barriers of the overflow gates OFG1 and OFG2 are set lower than those of the respective transfer gates TX1 and TX2 during the charge storage period of the photodiodes PD1 and PD2.

Description

[Technical Field]

[0001] This specification discloses a solid-state image sensor, an imaging device, and an imaging method. [Background technology]

[0002] As described in Patent Document 1, an image sensor circuit structure called a shared pixel is known in which multiple photodiodes (photoelectric conversion units) share a floating diffusion. For example, in Non-Patent Document 1 and Patent Document 1, a group of four photodiodes (subpixels) share a floating diffusion. Furthermore, in this shared pixel structure, the potential barrier between adjacent subpixels is intentionally set lower than the potential barrier surrounding the subpixel group. As a result, when the charge of a certain subpixel becomes saturated, the charge overflows to the adjacent subpixel. Consequently, the reduction in dynamic range associated with the shared pixel structure can be suppressed.

[0003] Furthermore, in Patent Documents 2, 3, and 4, instead of allowing charge overflow between subpixels, a Lateral Overflow Integration Capacitor (LOFIC) is incorporated into the image sensor circuit to serve as a destination for saturated charge.

[0004] Furthermore, Patent Documents 5 and 6 disclose a dual-pixel structure. In a dual-pixel structure, a pair of photodiodes (subpixels) share a floating diffusion. For example, in a pixel array in which multiple dual pixels are arranged in a two-dimensional array, charges are read separately from each subpixel for a predetermined column or row. Phase detection autofocus (PDAF) is made possible using the read charges. Alternatively, image generation is performed by simultaneously reading or adding the charges of a pair of subpixels. In other words, during image generation, a pair of subpixels are treated as a single pixel. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] U.S. Patent Application Publication No. 2023 / 0156369 [Patent Document 2] U.S. Patent Application Publication No. 2024 / 0259705 Specification [Patent Document 3] U.S. Patent Application Publication No. 2020 / 0154066 [Patent Document 4] U.S. Patent No. 8184191 [Patent Document 5] U.S. Patent Application Publication No. 2016 / 0240570 [Patent Document 6] U.S. Patent Application Publication No. 2023 / 0143387 [Non-patent literature]

[0006] [Non-Patent Document 1] Sungsoo Choi et al., “World smallest 200Mp CMOS Image Sensor with 0.56μm pixel equipped with novel Deep Trench Isolation structure for better sensitivity and higher CG”, 2023 International Image Sensor Workshop, May 22, 2023 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] In a dual pixel structure, by providing a lateral overflow storage capacitance for each sub pixel, an improvement in dynamic range can be achieved in both image generation and phase difference autofocus. However, such a functional expansion may lead to an increase in pixel size. Therefore, in this specification, a solid-state imaging device, an imaging apparatus, and an imaging method are disclosed that can improve the dynamic range of image generation particularly while suppressing an increase in pixel size.

Means for Solving the Problems

[0008] The solid-state imaging device disclosed in this specification includes a pair of photodiodes, a floating diffusion, a pair of transfer gates, an overflow storage capacitance, and a pair of overflow gates. The floating diffusion is shared by the pair of photodiodes. The transfer gates are provided between the respective photodiodes and the floating diffusion. The overflow storage capacitance is shared by the pair of photodiodes. The overflow gates are provided between the respective photodiodes and the overflow storage capacitance. Further, the overflow gates are set to have a lower potential barrier than the respective transfer gates during the charge storage period of the photodiodes.

[0009] According to the above configuration, the overflow storage capacitance is shared by the pair of photodiodes. Therefore, an increase in pixel size is suppressed as compared with the case where an overflow storage capacitance is provided individually for each photodiode.

[0010] Further, in this specification, an imaging apparatus including the above solid-state imaging device is disclosed. The imaging apparatus includes a constant voltage source. The constant voltage source applies equal voltages to the pair of overflow gates respectively.

[0011] According to the above configuration, the saturation charge amounts of the pair of photodiodes can be made uniform.

[0012] This specification also discloses an imaging device comprising the above-described solid-state image sensor. The solid-state image sensor may include an overflow capacitance gate. The overflow capacitance gate is provided between the overflow storage capacitance and the floating diffusion. The imaging device may also include an AD conversion unit. The AD conversion unit reads the charge of the floating diffusion and converts it into a pixel value. For a pair of photodiodes designated as pixels for image generation, a pair of transfer gates are simultaneously turned on to transfer charge to the floating diffusion. Furthermore, the overflow capacitance gate is turned on for the floating diffusion to which charge has been transferred from the pair of photodiodes. During the period when the overflow capacitance gate is on, the AD conversion unit reads the charge of the floating diffusion.

[0013] With the above configuration, when the capacity is expanded by the amount of overflow storage capacity, the charge is read out, which improves the dynamic range during image generation.

[0014] This specification also discloses an imaging device comprising the above-described solid-state image sensor. The imaging device may include an exposure time control unit. The exposure time control unit controls the exposure time corresponding to the charge accumulation period. A pair of photodiodes designated as pixels for phase-difference autofocus have their charges transferred to floating diffusion at different timings. When at least one of the pair of photodiodes has reached a saturation charge amount, the exposure time control unit reduces the exposure time.

[0015] According to the above configuration, phase-difference autofocus becomes possible using a pair of photodiodes that are not yet saturated (unsaturated).

[0016] This specification also discloses an imaging device comprising the above-described solid-state image sensor. The solid-state image sensor may include an overflow capacitance gate. The overflow capacitance gate is provided between the overflow storage capacitance and the floating diffusion. The imaging device may also include an AD conversion unit. The AD conversion unit reads the charge from the floating diffusion and converts it into a pixel value. For a pair of photodiodes designated as pixels for phase-difference autofocus, a pair of transfer gates are turned on at different timings to transfer charge to the floating diffusion. When the photodiode to which charge transfer is being made has reached its saturation charge amount, the overflow capacitance gate is turned on for the floating diffusion from which charge has been transferred from the photodiode. The AD conversion unit calculates the pre-expansion pixel value before the overflow capacitance gate is turned on, and the post-expansion pixel value when the overflow capacitance gate is turned on.

[0017] With the above configuration, phase-detection autofocus becomes possible using the pre- and post-extension pixel values.

[0018] Furthermore, in the above configuration, the imaging device may include an autofocus processing unit. The autofocus processing unit controls the lens position based on phase-difference autofocus. When one of a pair of photodiodes has reached its saturation charge and the other is below its saturation charge, the autofocus processing unit sets the sum of the pre-expanded pixel value and the post-expanded pixel value of one of the photodiodes as the pixel value of the other photodiode.

[0019] The above configuration allows for an improvement in the dynamic range of phase-detection autofocus.

[0020] Furthermore, in the above configuration, the imaging device may include an exposure time control unit. The exposure time control unit controls the exposure time corresponding to the charge accumulation period. When both of the pair of photodiodes have reached their saturation charge, the exposure time control unit shortens the exposure time.

[0021] When both photodiodes in a pair reach their saturation charge, charge flows from each photodiode into the overflow storage capacitance. By shortening the exposure time, the mixing of charge into the overflow storage capacitance during phase-difference autofocus is suppressed.

[0022] This specification also discloses an imaging method, which uses a solid-state image sensor. The solid-state image sensor comprises a pair of photodiodes, a floating diffusion, a pair of transfer gates, an overflow storage capacitor, and a pair of overflow gates. The floating diffusion is shared by the pair of photodiodes. The transfer gates are provided between each photodiode and the floating diffusion. The overflow storage capacitor is shared by the pair of photodiodes. The overflow gates are provided between each photodiode and the overflow storage capacitor. The potential barrier of the pair of overflow gates is set lower than that of each transfer gate during the charge storage period of the photodiodes.

[0023] In the above configuration, an equal voltage may be applied to each of the pair of overflow gates.

[0024] Furthermore, in the above configuration, the solid-state image sensor may be equipped with an overflow capacitance gate. The overflow capacitance gate is provided between the overflow storage capacitance and the floating diffusion. The imaging device equipped with the above solid-state image sensor may also be equipped with an overflow capacitance gate and an AD conversion unit. The AD conversion unit reads the charge from the floating diffusion and converts it into a pixel value. For a pair of photodiodes designated as pixels for image generation, a pair of transfer gates are simultaneously turned on to transfer charge to the floating diffusion. Furthermore, the overflow capacitance gate is turned on for the floating diffusion to which charge has been transferred from the pair of photodiodes. During the period when the overflow capacitance gate is on, the AD conversion unit reads the charge from the floating diffusion.

[0025] Furthermore, in the above configuration, the imaging device equipped with a solid-state image sensor may also include an exposure time control unit. The exposure time control unit controls the exposure time corresponding to the charge accumulation period. Charge is transferred to the floating diffusion from a pair of photodiodes designated as pixels for phase-difference autofocus at different timings. When at least one of the pair of photodiodes reaches a saturation charge amount, the exposure time control unit shortens the exposure time.

[0026] In the above configuration, the solid-state image sensor may also be equipped with an overflow capacitance gate. The overflow capacitance gate is provided between the overflow storage capacitance and the floating diffusion. The imaging device equipped with the above solid-state image sensor may also be equipped with an overflow capacitance gate and an AD conversion unit. The AD conversion unit reads the charge from the floating diffusion and converts it into a pixel value. For a pair of photodiodes designated as pixels for phase-difference autofocus, a pair of transfer gates are turned on at different timings to transfer charge to the floating diffusion. When the photodiode to which charge transfer is to be made has reached its saturation charge amount, the overflow capacitance gate is turned on for the floating diffusion from which charge has been transferred from the photodiode. The AD conversion unit calculates the pre-expansion pixel value before the overflow capacitance gate is turned on, and the post-expansion pixel value when the overflow capacitance gate is turned on.

[0027] Furthermore, in the above configuration, the imaging device may include an autofocus processing unit. The autofocus processing unit controls the lens position based on phase-difference autofocus. When one of a pair of photodiodes has reached its saturation charge and the other is below its saturation charge, the autofocus processing unit sets the sum of the pre-expanded pixel value and the post-expanded pixel value of one of the photodiodes as the pixel value of the other photodiode.

[0028] Furthermore, in the above configuration, the imaging device may include an exposure time control unit. The exposure time control unit controls the exposure time corresponding to the charge accumulation period. When both of the pair of photodiodes have reached their saturation charge, the exposure time control unit shortens the exposure time. [Effects of the Invention]

[0029] The imaging apparatus and imaging method disclosed herein allow for the suppression of increasing pixel size while particularly improving the dynamic range of image generation. [Brief explanation of the drawing]

[0030] [Figure 1] This figure illustrates an imaging device according to this embodiment. [Figure 2] This diagram illustrates the hardware configuration of the control device. [Figure 3] This diagram illustrates the circuit surface structure on the side opposite to the light-receiving surface of a solid-state imaging device. [Figure 4] This diagram illustrates the circuit configuration of a solid-state imaging device. [Figure 5] This diagram illustrates the flow of overflowing electric charge. [Figure 6] This diagram illustrates the potential barriers between the transfer gate TX1 and the overflow gate OFG1. [Figure 7] This diagram illustrates the potential barriers of the transfer gate TX2 and the overflow gate OFG2. [Figure 8] This diagram illustrates a timing chart for image generation. [Figure 9] This diagram illustrates a timing chart for phase-difference autofocus (pre-saturation transfer type). [Figure 10] This figure illustrates the dynamic range during image generation and phase-difference autofocus (pre-saturation transfer) using the imaging device according to this embodiment. [Figure 11] This diagram illustrates the pixel value processing involved in phase-detection autofocus using overflow storage capacity. [Figure 12] This diagram illustrates a timing chart for phase-detection autofocus using overflow storage capacity (with saturation of only one pixel). [Figure 13] This figure illustrates a timing chart for phase-difference autofocus using overflow storage capacity (where a pair of pixels are saturated). [Figure 14] This diagram illustrates the dynamic range of phase-detection autofocus using overflow storage capacity. [Figure 15]This is a circuit diagram illustrating the configuration of shared pixels using a solid-state image sensor according to this embodiment. [Modes for carrying out the invention]

[0031] The imaging apparatus and imaging method according to this embodiment will be described below with reference to the drawings. The shapes, materials, quantities, and numerical values ​​described below are illustrative examples for illustrative purposes. These shapes, etc., can be appropriately changed according to the specifications of the imaging apparatus. In addition, the same reference numerals are used for equivalent elements in all drawings below.

[0032] 1. Configuration of the imaging device Referring to Figure 1, the imaging device 100 according to this embodiment comprises a solid-state imaging device 10, a control device 30, a display device 40, and a lens mechanism 45. In the solid-state imaging device 10, photoelectric conversion and A / D conversion are performed. That is, the charge photoelectrically converted in the dual pixel array 12 is converted into a digital pixel value in the CDS-ADC circuit 18. The detailed structure will be described later.

[0033] Based on the pixel values ​​obtained from the timing chart illustrated in Figure 8, the control device 30 generates an image. Furthermore, the control device 30 performs phase-difference autofocus based on the pixel values ​​obtained from the timing chart illustrated in Figure 9 or Figure 12. The detailed structure will be described later.

[0034] The display device 40 displays the image generated by the image generation unit 38. The lens mechanism 45 adjusts the position of the lens which is positioned in front of the light-receiving surface of the dual pixel array 12 (i.e., upstream along the incident direction).

[0035] 2. Configuration of the Solid State Imaging System The solid-state imaging device 10 includes a dual pixel array 12, a color filter array 14, a vertical scanning circuit 15, a horizontal scanning circuit 16, and a CDS-ADC circuit 18.

[0036] The color filter array 14 is placed on the light-receiving surface of the dual pixel array 12. For example, the color filter array 14 has a two-dimensional arrangement of red (R), green (Gr, Gb), and blue (B) color filters. This two-dimensional arrangement is, for example, a Bayer array.

[0037] The horizontal scanning circuit 16 is a circuit that selects the read row of the dual pixel array 12. The CDS-ADC circuit 18 holds the signal (voltage value) of each pixel of the dual pixel array 12 and performs analog-to-digital conversion (A / D conversion). The mechanism of signal holding and A / D conversion by the CDS-ADC circuit 18 is known, so a detailed explanation is omitted.

[0038] The ADC circuit portion of the CDS-ADC circuit 18 is also called the AD conversion unit. The AD conversion unit reads out the charge from the floating diffusion and converts it into a pixel value. The converted digital value is called the pixel value. For example, the pixel value can take values ​​from a minimum of 0 to a maximum of 255. The vertical scanning circuit 15 instructs the CDS-ADC circuit 18 (AD conversion unit) which column of the dual pixel array 12 to read.

[0039] Figures 3 and 4 illustrate the solid-state image sensor 20, which is an element circuit of the dual-pixel array 12. The solid-state image sensor 20 is also called a dual pixel. In the dual-pixel array 12, the solid-state image sensors 20 are arranged in two dimensions along the column and row directions.

[0040] The solid-state image sensor 20 is, for example, a back-illuminated CMOS image sensor. Figure 3 illustrates the circuit side opposite to the light-receiving surface. Figure 4 illustrates the circuit of the solid-state image sensor 20.

[0041] The solid-state image sensor 20 has a dual-pixel structure. That is, in the solid-state image sensor 20, a pair of photodiodes PD1 and PD2 are paired. In other words, the pair of photodiodes PD1 and PD2 share one floating diffusion FD. As will be described later, when the charge of the pair of photodiodes PD1 and PD2 is used for image generation, the pair of photodiodes PD1 and PD2 are treated as a single pixel. Also, when the charge of the pair of photodiodes PD1 and PD2 is used for phase-difference autofocus, the pair of photodiodes PD1 and PD2 are treated as independent subpixels.

[0042] A transfer gate TX1 is provided between photodiode PD1 and floating diffusion FD. A transfer gate TX2 is also provided between photodiode PD2 and floating diffusion FD.

[0043] Furthermore, a pair of photodiodes PD1 and PD2 share a single capacitance. This capacitance is called the overflow storage capacitance. This overflow storage capacitance is, for example, a lateral overflow integration capacitor (LOFIC). Because it is shared by a pair of photodiodes PD1 and PD2, the overflow storage capacitance LOFIC is also called a shared LOFIC.

[0044] A pair of photodiodes PD1 and PD2 are in contact with the overflow storage capacitance LOFIC. Furthermore, overflow gates OFG1 and OFG2 are provided in this conductive path (wiring). As illustrated in Figure 5, the potential barrier V of the overflow gates OFG1 and OFG2 in the photodiodes PD1 and PD2 OFG Any charge exceeding the limits shown in Figures 6 and 7 is stored in the overflow storage capacity LOFIC.

[0045] Figure 3 illustrates the circuit surface of a solid-state image sensor 20. The solid-state image sensor 20 comprises a pixel section 22 and a logic circuit section 24. The overflow storage capacitance LOFIC is located in the logic circuit section 24. For example, a contact C1 is formed on the wiring connecting overflow gates OFG1, OFG2 and an overflow capacitance gate LFG. The contact C1 extends in the depth direction (stacking direction) perpendicular to the circuit surface. The overflow storage capacitance LOFIC is connected to the contact C1.

[0046] In the solid-state image sensor 20 according to this embodiment, only one overflow storage capacitor (LOFIC) is required in the logic circuit section 24. Therefore, compared to, for example, the case where a pair of overflow storage capacitors (LOFIC) are provided, the increase in installation space is suppressed.

[0047] Furthermore, compared to the case where a pair of overflow storage capacities (LOFICs) are provided, the number of nodes can be reduced in the solid-state image sensor 20 according to this embodiment. In Figure 4, nodes are shown as black circles. Generally, nodes consist of N+ regions. It is known that dark current is easily generated in N+ regions. By suppressing the increase in the number of nodes, the increase in dark current is suppressed.

[0048] Referring to Figures 3 and 4, an overflow gate OFG1 is provided between the overflow storage capacitor LOFIC and the photodiode PD1 in the circuit. Additionally, an overflow gate OFG2 is provided between the overflow storage capacitor LOFIC and the photodiode PD2 in the circuit.

[0049] Furthermore, an overflow capacitance gate LFG is provided between the overflow storage capacitance LOFIC and the floating diffusion FD in the circuit. A reset gate RST, a source follower SF, and a row selection gate RS are provided in the path from the floating diffusion FD to the bit line.

[0050] In terms of the circuit configuration, the charges accumulated in the photodiodes PD1 and PD2 can be transferred to the floating diffusion FD. Also, the charges overflowing from the photodiodes PD1 and PD2 can be accumulated in the overflow storage capacitance LOFIC. Fig. 6 illustrates the potential distributions of the photodiode PD1, the transfer gate TX1, and the overflow gate OFG1 during the charge accumulation period (i.e., the exposure time). When light is incident on the photodiode PD1, charges are accumulated in the photodiode PD1 by photoelectric conversion. During this accumulation period, the potential barrier V OFG1 of the overflow gate OFG1 is set to a value lower than the potential barrier V TX1 of the transfer gate TX1 (V OFG1 < V TX1 ).

[0051] Also, Fig. 7 illustrates the potential distributions of the photodiode PD2, the transfer gate TX2, and the overflow gate OFG2 during the charge accumulation period (i.e., the exposure time). Similar to Fig. 6, when light is incident on the photodiode PD2, charges are accumulated in the photodiode PD2 by photoelectric conversion. During this accumulation period, the potential barrier V OFG2 of the overflow gate OFG2 is set to a value lower than the potential barrier V TX2 of the transfer gate TX2 (V OFG2 < V TX2 ). Also, for example, the potential barriers V TX1 , V TX2 are equal to each other (V TX1 = V TX2 ). Furthermore, the overflow potentials V OFG1 , V OFG2 are equal to each other (V OFG1 = V OFG2 ).

[0052] That is, during the charge accumulation period, the potentials of the photodiodes PD1 and PD2 are the overflow potentials V OFG1 , V OFG2When it reaches this point, the charge overflows through the overflow gates OFG1 and OFG2 and is stored in the overflow storage capacitance LOFIC. Therefore, the saturation charge amount of photodiodes PD1 and PD2 is equal to the overflow potential V. OFG1 ,V OFG2 This is the result. V OFG1 =V OFG2 By doing so, the photodiodes PD1 and PD2 can be set to the same saturation charge amount. For example, when the imaging device 100 is ON, the overflow gates OFG1 and OFG2 are supplied with an overflow potential V from the constant voltage source 17 (see Figure 1). OFG1 ,V OFG2 This is applied at all times.

[0053] 3. Configuration of the control device The control device 30 is comprised of a computer, for example, as illustrated in Figure 2. Specifically, the control device 30 includes a CPU 30A, RAM 30B, ROM 30C, storage 30D, and an input / output controller 30E.

[0054] The CPU 30A is the central processing unit, also called a processor. The RAM 30B is a volatile or non-volatile memory device that temporarily stores data being worked on. The ROM 30C is a memory device that can read data. The storage 30D is a memory device that can write and read data. The storage 30D consists of, for example, an HDD (Hard Disk Drive) or an SSD (Solid State Drive).

[0055] The CPU 30A executes a program stored in the storage 30D or ROM 30C, thereby configuring the control device 30 with the functional units illustrated in Figure 1. Specifically, the control device 30 includes an imaging signal acquisition unit 31, a pixel value determination unit 32, an autofocus processing unit 34, an exposure time control unit 36, and an image generation unit 38. Details of these functional units will be described later.

[0056] Figure 8 illustrates the operation of the solid-state imaging device 10 to obtain pixel values ​​for image generation. The pixel values ​​obtained through this operation are sent to the image generation unit 38 via the imaging signal acquisition unit 31. The image generation unit 38 generates an image based on the acquired pixel values. The generated image is displayed on the display device 40. Details of the timing chart in Figure 8 will be described later.

[0057] Figure 9 illustrates the operation of the solid-state imaging device 10 to obtain pixel values ​​for phase-difference autofocus. The pixel values ​​obtained through this operation are sent to the pixel value determination unit 32 and the autofocus processing unit 34 via the imaging signal acquisition unit 31. Phase-difference autofocus technology is known and will not be explained below. Details of the timing chart in Figure 9 will be described later.

[0058] For example, to perform phase-detection autofocus, the solid-state image sensors 20 of the dual-pixel array 12 are specified in column or row units. Based on the respective pixel values ​​of the photodiodes PD1 and PD2 obtained from each solid-state image sensor 20, it is calculated whether the focus position relative to the subject is in focus, front focus, or back focus. Furthermore, based on this calculation result, the amount of lens movement is determined. The determined amount of movement is sent to the lens mechanism 45.

[0059] 4. Operation during image generation Figure 8 shows an example of a timing chart for the solid-state imaging device 10 to obtain pixel values ​​for image generation. Note that in Figures 8, 9, 12, and 13, the overflow gates OFG1 and OFG2 are set to constant voltages and are therefore not shown.

[0060] Between times t1 and t3, charge accumulates in photodiodes PD1 and PD2. In many cases, the angles of incidence to photodiodes PD1 and PD2 are different. Therefore, the charge accumulation patterns differ between photodiodes PD1 and PD2. Figures 8, 9, and 12 show examples where more light is incident on photodiode PD1 than on photodiode PD2.

[0061] Referring to Figure 8, at time t2, when the charge of photodiode PD1 becomes saturated (reaches the saturation charge amount), the charge overflowing from photodiode PD1 is stored in the overflow storage capacitance LOFIC.

[0062] At time t3, the reset gate RST turns on (opens), and the voltage of the floating diffusion FD becomes the reference voltage. At time t4, the transfer gates TX1 and TX2 turn on (opens) simultaneously. The charge accumulated in photodiodes PD1 and PD2 is then transferred to the floating diffusion FD. In other words, the charges of photodiodes PD1 and PD2 are added together in the floating diffusion FD.

[0063] Furthermore, at time t5, the overflow capacitive gate LFG turns ON (open). For example, the ON state of the overflow capacitive gate LFG continues until time t8.

[0064] When the overflow capacitance gate LFG is turned on (opened), the overflow storage capacitance LOFIC and the floating diffusion become equipotential. Since the overflow storage capacitance LOFIC has a larger capacitance than the floating diffusion, the potential of the floating diffusion FD drops (is pulled) down to the potential of the overflow storage capacitance LOFIC. In other words, the capacitance that accepts charge is temporarily expanded by the overflow storage capacitance LOFIC.

[0065] Therefore, even if high-brightness light that would saturate the photodiodes PD1 and PD2 is incident, the pixel values ​​after the capacitance has been expanded will not saturate. In other words, the overflow storage capacitance (LOFIC) expands the dynamic range in image generation. The dynamic range in image generation refers to the range from the minimum signal intensity at which so-called black crushing occurs to the maximum signal intensity at which so-called white clipping occurs.

[0066] Furthermore, in conjunction with the overflow capacitance gate LFG, A / D conversion is performed in the CDS-ADC circuit 18 (see Figure 1). For example, A / D conversion is performed from time t6, which is a predetermined time delay from the time (time t5) when the overflow capacitance gate LFG switches from off (closed) to on (open). The A / D conversion period is from time t6 to time t7, when, for example, the reset gate turns on (open). During this A / D conversion period, the CDS-ADC circuit 18 reads out the charge of the floating diffusion FD. The process described above is then repeated.

[0067] 5. Pre-saturation transfer type phase-detection autofocus Figure 9 illustrates a timing chart for acquiring pixel values ​​for phase-difference autofocus. Specifically, Figure 9 shows a timing chart for pre-saturation transfer type phase-difference autofocus. In other words, in Figure 9, the accumulated charge of both photodiodes PD1 and PD2 is controlled to be less than the saturation charge amount.

[0068] Furthermore, in a pre-saturation transfer type phase-difference autofocus system, when at least one of the photodiodes PD1 and PD2 reaches a saturation charge amount, the exposure time control unit 36 ​​(see Figure 1) shortens the exposure time.

[0069] In phase-detection autofocus, a predetermined column or row of solid-state image sensors 20 (see Figure 3) in the dual-pixel array 12 (see Figure 1) is specified. The charge of the solid-state image sensors 20 that are not specified is read out based on Figure 8 and used for image generation.

[0070] In phase-difference autofocus, the charges of photodiodes PD1 and PD2 are acquired independently. This means that transfer gates TX1 and TX2 are turned on (opened) at different times. For example, in Figure 9, transfer gate TX1 is turned on (opened) at times t25, t29, t33, and t37. The periods during which the charge of photodiode PD1 is transferred to and read by the floating diffusion FD are t25-t26, t29-t30, t33-t34, and t37-t38.

[0071] On the other hand, at times t22, t27, t31, and t35, the transfer gate TX2 is turned on (open). Also, the period during which the charge of the photodiode PD2 is transferred to and read by the floating diffusion FD is t22-t24, t27-t28, t31-t32, and t35-t36.

[0072] At time t23, the stored charge in photodiode PD1 becomes saturated. The saturated charge flows into the overflow storage capacitance LOFIC. Since the stored charge in photodiode PD1 is saturated, at time t25, when charge is transferred from photodiode PD1 to the floating diffusion FD, the potential of the floating diffusion FD becomes the saturated charge amount Vth.

[0073] The saturation charge Vth is converted to AD by the CDS·ADC circuit 18, resulting in a pixel value of 255. When the pixel value determination unit 32 (see Figure 1) determines that the pixel value is at its maximum value, it sends a command to the exposure time control unit 36 ​​to shorten the exposure time. The exposure time control unit 36 ​​adjusts the timing chart for the dual pixel array 12. For example, the exposure time control unit 36 ​​shortens the charge accumulation period from the on (open) of the reset gate RST to the on (open) of the transfer gates TX1 and TX2, i.e., the exposure time.

[0074] After adjusting the exposure time, referring to times t27–t28 and t29–30, the accumulated charge in both photodiodes PD1 and PD2 becomes less than the saturation charge amount Vth. Once the accumulated charges in photodiodes PD1 and PD2 are converted using AD conversion, the pixel value determination unit 32 transmits these pixel values ​​to the autofocus processing unit 34.

[0075] The autofocus processing unit 34 adjusts the lens position based on phase-difference autofocus. The autofocus processing unit 34 performs phase-difference autofocus calculations based on a pair of pixel values ​​(more specifically, based on a pair of pixel values ​​in any column or row of the dual pixel array 12). As a result of this calculation, the amount of lens movement is determined. The determined amount of lens movement is sent to the lens mechanism 45.

[0076] Figure 10 shows the dynamic range (DR) in image generation. IMG And, the dynamic range (DR) in phase-detection autofocus. AF Examples are given. Note that the dynamic range (DR) is... IMG This is determined based on the pixel values ​​obtained from the timing chart illustrated in Figure 8. Furthermore, the dynamic range (DR) is also determined. AF This is determined based on the pixel values ​​obtained from the timing chart illustrated in Figure 9.

[0077] In the graph in Figure 10, the horizontal axis represents luminance, and the vertical axis represents the signal-to-noise ratio (SNR). As mentioned above, due to the difference in the incident angle on photodiodes PD1 and PD2, the charge accumulation patterns of each photodiode PD1 and PD2 are different. In Figure 10, more light is incident on photodiode PD1 than on photodiode PD2.

[0078] Referring to Figure 10, the brightness L1 at which either photodiode PD1 or PD2 reaches its saturation charge is the dynamic range DR of the phase-difference autofocus. AF This becomes the upper limit. On the other hand, the dynamic range DR of image generation IMGEven when the accumulated charge of both photodiodes PD1 and PD2 is saturated, the dynamic range is extended up to the maximum capacity of the overflow storage capacitance LOFIC. Thus, the imaging device according to this embodiment can particularly expand the dynamic range in image generation.

[0079] 6. Overflow storage capacity-based phase-detection autofocus In phase-difference autofocus, the charges of photodiodes PD1 and PD2 must be extracted independently (without mixing) as pixel values. On the other hand, as illustrated in the circuit diagram in Figure 4, the overflow storage capacitance LOFIC is shared by photodiodes PD1 and PD2. That is, the overflow storage capacitance LOFIC can receive charge from both photodiode PD1 and photodiode PD2.

[0080] Therefore, in the timing chart of Figure 9, once charge flows into the overflow storage capacitance LOFIC, the pixel values ​​obtained from photodiodes PD1 and PD2 are excluded and not used for phase-difference autofocus.

[0081] However, as mentioned above, due to differences in the angle of incidence, the charge accumulation patterns of photodiodes PD1 and PD2 differ. Therefore, it is possible that the charge flowing into the overflow storage capacitor LOFIC comes from either photodiode PD1 or PD2. In this case, there is no charge mixing between photodiodes PD1 and PD2 in the overflow storage capacitor LOFIC. When there is no charge mixing, phase-difference autofocus becomes possible using the charge stored in the overflow storage capacitor LOFIC.

[0082] Figure 11 illustrates the control flow of phase-difference autofocus using the charge of the overflow storage capacitance LOFIC. The pixel value determination unit 32 (see Figure 1) determines whether the pixel value P_PD1 obtained from the photodiode PD1 is less than the upper threshold Pth1 (S10).

[0083] Here, the pixel value P_PD1 obtained from photodiode PD1 is also called the pre-extension pixel value. The pre-extension pixel value is the pixel value read from the floating diffusion FD when the charge from photodiode PD1 is transferred to the floating diffusion FD and the overflow capacitive gate LFG is off (closed). The CDS-ADC circuit 18 calculates (AD converts) the pre-extension pixel value at the timing when the charge is transferred from either photodiode PD1 or PD2 to the floating diffusion FD and before the overflow capacitive gate is turned on.

[0084] If P_PD1 < Pth1, then no charge has flowed from photodiode PD1 to the overflow storage capacitance LOFIC. Next, the pixel value determination unit 32 determines whether the pixel value P_PD2 obtained from photodiode PD2 is less than the upper limit threshold Pth2 (S12). Here, both the upper limit threshold Pth1 and the upper limit threshold Pth2 may be the maximum value of the pixel. (Pth1=Pth2=Pixel value 255)

[0085] If P_PD2 < Pth2, then no charge flows into the overflow storage capacitance LOFIC for either photodiode PD1 or PD2. Therefore, the autofocus processing unit uses pixel values ​​P_PD1 and P_PD2 to perform phase-difference autofocus calculations (S22).

[0086] Returning to step S10, if P_PD1 ≥ Pth1, the photodiode PD1 is saturated, and the charge of the photodiode PD1 is accumulated in the overflow storage capacitance LOFIC. Next, the pixel value determination unit 32 determines whether the pixel value P_PD2 obtained from the photodiode PD2 is less than the upper limit threshold Pth2 (S16).

[0087] If P_PD2 ≥ Pth2, then charge is flowing into the overflow storage capacitance LOFIC from both photodiodes PD1 and PD2. In this case, the exposure time control unit 36 ​​controls the dual pixel array 12 to shorten the exposure time (S20).

[0088] In step S16, if P_PD2 < Pth2, the charge stored in the overflow storage capacitance LOFIC will be only the charge from the photodiode PD1. The autofocus processing unit 34 adds the pixel value P_LOFIC obtained from the overflow storage capacitance LOFIC to the pixel value P_PD1 obtained from the photodiode PD1 to obtain the new pixel value P_PD1 (S18).

[0089] The pixel value P_LOFIC obtained from the overflow storage capacitance LOFIC refers to the pixel value read from the floating diffusion FD when the overflow capacitance gate LFG is in the ON state (open state). The pixel value P_LOFIC is also called the expanded pixel value. In other words, the CDS·ADC circuit 18 (see Figure 1) calculates (AD converts) the expanded pixel value when the overflow capacitance gate is in the ON state. For example, the new pixel value P_PD1 can take a value of 255 or greater.

[0090] Furthermore, using the new pixel value P_PD1 and the pixel value P_PD2 obtained from the photodiode PD2, the autofocus processing unit 34 performs a phase-difference autofocus calculation (S22).

[0091] Returning to step S12, if P_PD2 ≥ Pth2, the charge stored in the overflow storage capacitance LOFIC will be only the charge from photodiode PD2. The autofocus processing unit 34 adds the pixel value P_LOFIC obtained from the overflow storage capacitance LOFIC to the pixel value P_PD2 obtained from photodiode PD2 to obtain a new pixel value P_PD2 (S14). Furthermore, using the new pixel value P_PD2 and the pixel value P_PD1 obtained from photodiode PD1, the autofocus processing unit 34 performs a phase-difference autofocus calculation (S22).

[0092] Figure 12 illustrates a timing chart corresponding to step S18 in Figure 11. The AD conversion period is determined in accordance with the flowchart in Figure 11. Specifically, when the charge of photodiode PD1 is saturated (the pixel value is at its maximum), the overflow capacitance gate LFG is turned ON (open). The charge during the ON period (open period) of the overflow capacitance gate LFG is then read from the floating diffusion FD.

[0093] Similarly, when the charge of photodiode PD2 is saturated, the overflow capacitance gate LFG turns ON (open). The charge during the ON period of the overflow capacitance gate LFG is then read from the floating diffusion FD.

[0094] For example, referring to the time t44-t45, the charge readout (AD conversion) for the floating diffusion FD is performed from the ON (open) time of the transfer gate TX1. During this period, the photodiode PD1 is the target of charge transfer. In other words, the calculation of the pre-extension pixel value is performed by the CDS·ADC circuit 18.

[0095] If the charge amount of the floating diffusion FD (i.e., the charge amount accumulated in the photodiode) reaches the saturation charge amount Vth, the calculation of the pixel value (pre-extension pixel value) based on photodiode PD1 is completed at time t45.

[0096] As the charge of the floating diffusion FD reaches the saturation charge Vth, the overflow capacitance gate LFG is turned on (open) at time t45. The potential of the floating diffusion FD decreases due to the overflow storage capacitance LOFIC. Then, during the period from time t46 to t47, the charge of the floating diffusion FD is read out. In other words, the expanded pixel value is calculated by the CDS·ADC circuit 18.

[0097] At times t48-t49 and t56-t57, the pixel value (pre-extension pixel value) based on photodiode PD2 is read. In this example, the amount of charge accumulated in photodiode PD2, which is the target of charge transfer, is less than the saturation charge amount. In this case, the overflow capacitance gate LFG is not opened. In other words, the extended pixel value is not read.

[0098] The timing chart above allows us to obtain the pixel value of the saturated photodiode PD1 (pixel value before expansion), the pixel value of the overflow storage capacitance LOFIC (pixel value after expansion), and the pixel value of the unsaturated photodiode PD2 (pixel value before expansion). In this case, the overflow storage capacitance LOFIC is exclusively used to store the charge of photodiode PD1.

[0099] The autofocus processing unit 34 takes the sum of the pixel value of the saturated photodiode PD1 and the pixel value of the overflow storage capacitance LOFIC as the new pixel value of photodiode PD1. Then, the autofocus processing unit 34 performs a phase-difference autofocus calculation based on the new pixel value of photodiode PD1 and the pixel value obtained from photodiode PD2.

[0100] Figure 13 shows an example of reaching step S20 in Figure 11. In this example, both photodiodes PD1 and PD2 become saturated, particularly at times t63, t73, and t64, t74. In such cases, charge flows into the overflow storage capacitance LOFIC from both photodiodes PD1 and PD2. In this case, neither the pixel values ​​obtained from photodiodes PD1 and PD2 (pre-expanded pixel values) nor the pixel values ​​obtained from the overflow storage capacitance LOFIC (post-expanded pixel values) are used for phase-detection autofocus (they are discarded). As described above, the exposure time is shortened.

[0101] Thus, in the phase-detection autofocus system illustrated in the flowchart of Figure 11, the dynamic range of the phase-detection autofocus is expanded in addition to the dynamic range of image generation. Note that expanding the dynamic range of the phase-detection autofocus means that the maximum pixel value for which phase-detection autofocus can be performed increases.

[0102] Figure 14 shows the dynamic range (DR) in image generation. IMG And, the dynamic range (DR) in phase-detection autofocus. AF This is an example. Furthermore, based on the pixel values ​​obtained based on the flowchart exemplified in Figure 11, the dynamic range DR AF This is required. Also, dynamic range (DR) IMG This is illustrated in Figure 10 by the dynamic range DR IMG It is identical to [the other one].

[0103] Similar to Figure 10, in the graph of Figure 14, the horizontal axis represents brightness, and the vertical axis represents the signal-to-noise ratio (SNR). As shown in this graph, the dynamic range (DR) of phase-detection autofocus... AF This extends further from the point (L1, A1) where one of the photodiodes PD1 or PD2 is saturated. In other words, the dynamic range DR continues until the other photodiode PD2 becomes saturated. AF It will be expanded.

[0104] 7. Another example of this embodiment Figure 15 shows a solid-state image sensor 20 according to another example of this embodiment. For example, the solid-state image sensor 20 has a so-called shared pixel structure.

[0105] The solid-state image sensor 20 is equipped with two sets of dual-pixel circuits 20A and 20B. In other words, the solid-state image sensor 20 is equipped with four photodiodes PD1-1, PD1-2, PD2-1, and PD2-2. The photodiodes PD1-1, PD1-2, PD2-1, and PD2-2 share a floating diffusion FD. Furthermore, the photodiodes PD1-1, PD1-2, PD2-1, and PD2-2 also share the logic circuit portion downstream from the floating diffusion FD. In addition, the dual-pixel circuit 20A is provided with an overflow storage capacitor LOFIC1. Furthermore, the dual-pixel circuit 20B is provided with an overflow storage capacitor LOFIC2.

[0106] For example, when performing a 2x2 binning process, the transfer gates TX1-1, TX1-2, TX2-1, and TX2-2 are simultaneously turned on (open). In addition, the overflow capacitance gates LFG1 and LFG2 are also turned on (open). At this time, the potential of the floating diffusion FD is determined by the charge accumulated in the photodiodes PD1-1, PD1-2, PD2-1, and PD2-2, and the charge accumulated in the overflow storage capacitances LOFIC1 and LOFIC2. [Explanation of Symbols]

[0107] 10 Solid-state imaging device, 12 Dual pixel array, 14 Color filter array, 15 Vertical scanning circuit, 16 Horizontal scanning circuit, 18 ADC circuit (AD conversion unit), 20 Solid-state image sensor, 20A, 20B Dual pixel circuit, 22 Pixel unit, 24 Logic circuit unit, 30 Control unit, 30A CPU, 30B RAM, 30C ROM, 30D Storage, 30E Input / Output controller, 31 Image signal acquisition unit, 32 Pixel value determination unit, 34 Autofocus processing unit, 36 Exposure time control unit, 38 Image generation unit, 40 Display device, 45 Lens mechanism, 100 Imaging device, FD Floating Diffusion, LFG Overflow Capacitance Gate, LOFIC Overflow Accumulation, OFG1, OFG2 Overflow Gate, PD1, PD2 Photodiode, RS Row Selection Gate, RST Reset Gate, SF Source Follower, TX1, TX2 Transfer Gate.

Claims

1. A pair of photodiodes, A floating diffusion shared by a pair of photodiodes, A pair of transfer gates are provided between each of the aforementioned photodiodes and the floating diffusion, The overflow storage capacitance shared by the pair of photodiodes, A pair of overflow gates are provided between each of the photodiodes and the overflow storage capacitance, and the potential barrier of each of the transfer gates during the charge storage period of the photodiodes is set lower than that of each of the transfer gates, Equipped with, Solid-state image sensor.

2. An imaging device comprising a solid-state image sensor as described in claim 1, An imaging device comprising a constant voltage source that applies equal voltage to each of the pair of overflow gates.

3. An imaging device comprising a solid-state image sensor as described in claim 1, The unit includes an AD conversion unit that reads out the charge of the floating diffusion and converts it into a pixel value. The solid-state image sensor includes an overflow capacitance gate provided between the overflow storage capacitance and the floating diffusion, For the pair of photodiodes designated as pixels for image generation, the pair of transfer gates simultaneously turn on and transfer charge to the floating diffusion. Furthermore, the overflow capacitance gate turns ON for the floating diffusion to which charge has been transferred from the pair of photodiodes. During the period when the overflow capacitance gate is ON, the AD conversion unit reads out the charge of the floating diffusion. Imaging device.

4. An imaging device comprising a solid-state image sensor as described in claim 1, The system includes an exposure time control unit that controls the exposure time corresponding to the charge accumulation period, The pair of photodiodes designated as pixels for phase-difference autofocus each have charge transferred to the floating diffusion at different timings. When at least one of the pair of photodiodes reaches a saturation charge amount, the exposure time control unit shortens the exposure time. Imaging device.

5. An imaging device comprising a solid-state image sensor as described in claim 1, The unit includes an AD conversion unit that reads out the charge of the floating diffusion and converts it into a pixel value. The solid-state image sensor includes an overflow capacitance gate provided between the overflow storage capacitance and the floating diffusion, For a pair of photodiodes designated as pixels for phase-difference autofocus, the pair of transfer gates are turned on at different timings to transfer charge to the floating diffusion. When the photodiode to which charge transfer is to be performed has reached its saturation charge, the overflow capacitance gate turns ON for the floating diffusion to which charge has been transferred from the photodiode. The AD conversion unit calculates the pre-expanded pixel value before the overflow capacitive gate is turned on, and the post-expanded pixel value when the overflow capacitive gate is turned on. Imaging device.

6. An imaging device according to claim 5, The system includes an autofocus processing unit that controls the lens position based on the aforementioned phase-difference autofocus, When one of the pair of photodiodes has reached a saturation charge amount and the other has less than a saturation charge amount, the autofocus processing unit sets the sum of the pre-expanded pixel value and the post-expanded pixel value of the one photodiode as the pixel value of the one photodiode. Imaging device.

7. An imaging device according to claim 5, The system includes an exposure time control unit that controls the exposure time corresponding to the charge accumulation period, When both of the pair of photodiodes have reached their saturation charge, the exposure time control unit shortens the exposure time. Imaging device.

8. A pair of photodiodes, A floating diffusion shared by a pair of photodiodes, A pair of transfer gates are provided between each of the aforementioned photodiodes and the floating diffusion, The overflow storage capacitance shared by the pair of photodiodes, A pair of overflow gates are provided between each of the aforementioned photodiodes and the overflow storage capacitor, An imaging method using a solid-state image sensor, comprising: The potential barrier of the pair of overflow gates is set lower than that of each of the transfer gates during the charge accumulation period of the photodiode. Imaging method.

9. The imaging method according to claim 8, An imaging method comprising applying equal voltages to each of the pair of overflow gates.

10. The imaging method according to claim 8, The solid-state image sensor includes an overflow capacitance gate provided between the overflow storage capacitance and the floating diffusion, The imaging device comprising the solid-state image sensor includes an AD conversion unit that reads out the charge of the floating diffusion and converts it into a pixel value, For the pair of photodiodes designated as pixels for image generation, the pair of transfer gates simultaneously turn on and transfer charge to the floating diffusion. Furthermore, the overflow capacitance gate turns ON for the floating diffusion to which charge has been transferred from the pair of photodiodes. During the period when the overflow capacitance gate is ON, the AD conversion unit reads out the charge of the floating diffusion. Imaging method.

11. The imaging method according to claim 8, The imaging apparatus comprising the solid-state image sensor includes an exposure time control unit that controls the exposure time corresponding to the charge accumulation period, Charge is transferred from the pair of photodiodes designated as pixels for phase-difference autofocus to the floating diffusion at different timings. When at least one of the pair of photodiodes reaches a saturation charge amount, the exposure time control unit shortens the exposure time. Imaging method.

12. The imaging method according to claim 8, The solid-state image sensor includes an overflow capacitance gate provided between the overflow storage capacitance and the floating diffusion, The imaging device comprising the solid-state image sensor includes an AD conversion unit that reads out the charge of the floating diffusion and converts it into a pixel value, For a pair of photodiodes designated as pixels for phase-difference autofocus, the pair of transfer gates are turned on at different timings to transfer charge to the floating diffusion. When the photodiode to which charge transfer is to be performed has reached its saturation charge, the overflow capacitance gate turns ON for the floating diffusion to which charge has been transferred from the photodiode. The AD conversion unit calculates the pre-expanded pixel value before the overflow capacitive gate is turned on, and the post-expanded pixel value when the overflow capacitive gate is turned on. Imaging method.

13. The imaging method according to claim 12, The imaging device includes an autofocus processing unit that controls the lens position based on the phase-difference autofocus, When one of the pair of photodiodes has reached a saturation charge amount and the other has less than a saturation charge amount, the autofocus processing unit sets the sum of the pre-expanded pixel value and the post-expanded pixel value of the one photodiode as the pixel value of the one photodiode. Imaging method.

14. The imaging method according to claim 12, The imaging device includes an exposure time control unit that controls the exposure time corresponding to the charge accumulation period, When both of the pair of photodiodes have reached their saturation charge, the exposure time control unit shortens the exposure time. Imaging method.

Images