JP2015005880A - Imaging element and imaging device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small-sized imaging element which can reduce a package size constituting an imaging element and increases the degree of freedom in a layout even when constituting an imaging device, and also to provide an imaging device.SOLUTION: By adopting a read-out configuration in which signals of photoelectric conversion elements in first pixel regions a02-b23 are outputted from a first or second read-out means and the signals of the photoelectric conversion elements in second pixel regions a00-b21 are outputted from the first and second read out means, the signals of the photoelectric conversion elements in the second pixel region required for OB clamp or the like are used also for the signals of the photoelectric conversion elements in the first pixel region read out by the first and second read-out means.

Description

  The present invention particularly relates to an image sensor used in a video camera, a digital still camera, and the like, and more particularly to a signal readout component from a pixel array of the image sensor.

  In general, a signal from an image sensor is output including an offset component in addition to a light reception signal of subject light due to an increase in dark current or the like. In order to extract only the signal from the subject light from the output, a reference pixel (OB pixel) that is not photosensitive is arranged by covering a part of the pixels of the image sensor with a metal light shielding film. When a signal from the image sensor is captured as an image signal, an operation called an OB clamp is performed so that the reference pixel (OB pixel) is at a predetermined level as described in Patent Document 1.

FIG. 8 shows the pixel arrangement of the image sensor, and the OB clamping operation will be described with reference to FIG. The reference pixel VOB pixel group is arranged in the upper part of the drawing and the reference pixel HOB pixel group is arranged on the left side of the pixel group (effective pixel) of the opening as the photosensitive part.
Signals from the pixels are output line by line from the upper left to the right in the figure, and when the signal output for the first line is completed, the signals for the second line are sequentially output from the left to the right. (Arrow in the figure)

  At this time, a clamping operation is performed by the subsequent processing unit of the image sensor so that the output of the VOB area becomes a predetermined level based on the pixel signals of the plurality of VOB areas. Subsequently, in the row including the effective pixel region, the clamping operation is performed for each row so that the output of the HOB region becomes a predetermined level based on the pixel signals of the plurality of HOB regions. The signal clamped in this way can be converted into a light reception signal by subject light by subtracting the clamp level.

  On the other hand, in recent years, various methods as described in Patent Document 2 have been proposed as a method for acquiring and displaying a stereoscopic image.

  FIG. 9 is a diagram illustrating a method for acquiring two images with parallax. In the figure, 91 is an exit pupil of the photographing lens, 92 is a microlens disposed immediately above the light receiving element of the image sensor, 93 is an A image light receiving element, and 94 is a B image light receiving element. The light beam indicated by 51 A image pupil 95 and the light beam indicated by B image pupil 96 can be received. A unit pixel is constituted by two light receiving elements of the A image light receiving element 93 and the B image light receiving element 94 and a microlens 92 immediately above the light receiving element, and the unit pixels are arranged in a lattice shape so that the light receiving portion of the image pickup element is formed. Forming.

  In such a configuration, an image generated only from the output from the A image light receiving element 93 and an image generated only from the output from the B image light receiving element 94 have a parallax of the distance M between the center of gravity of the A image pupil and the B image pupil. Image.

  Here, 96 indicates the incident angle of light to the A image light receiving element 93 and its intensity distribution, 97 indicates the incident angle of light to the B image light receiving element 94 and its intensity distribution, and the left side of the horizontal axis is the same figure. The incident angle from the left direction, the right side indicates the incident angle from the right direction in the figure, and the vertical axis indicates the incident light intensity.

  In the example of FIG. 9, the unit pixel is composed of two light receiving elements, the A image light receiving element 93 and the B image light receiving element 94, but there is also an example in which the unit pixel is composed of a larger number of light receiving elements.

JP 2005-175930 A JP-A-58-24105

  In the OB clamping operation as described above, strict accuracy is required in order to correctly obtain a light reception signal from a low-luminance subject. Generally, the clamp target value is calculated from the average value of the output of a plurality of reference pixels (OB pixels) and the clamp operation is performed to eliminate the influence of noise in the pixel portion and the influence of pixel defects in the reference pixel. I have to. In particular, in a high temperature environment, dark current increases and noise and defective pixel output abnormalities also increase. Therefore, it is important to prevent a clamping error by using a large number of reference pixels for averaging.

  However, when a large number of reference pixels are arranged, there are problems such as an increase in the cost of the apparatus due to an increase in the size of the semiconductor image sensor and a restriction on the degree of freedom of layout. Further, in the image sensor in which the unit pixel is configured by a plurality of light receiving elements as described in FIG. 9, the A image signal and the B image signal are processed separately. Therefore, it is necessary to provide the reference pixel (OB pixel) separately for the A image light receiving element and the B image light receiving element, which leads to further cost increase and restriction on the degree of layout freedom.

  In order to solve the above problem, an image sensor having a pixel array in which unit pixels including at least one photoelectric conversion element are two-dimensionally arranged in a first pixel region and a second pixel region is a photoelectric conversion of the pixel array. First reading means and second reading means for reading the signal of the element for each row of unit pixels, and the signal of the photoelectric conversion element in the first pixel region is transmitted by the first reading means or the second reading means. The signal is read and the signal of the photoelectric conversion element in the second pixel region is read by the first reading unit and the second reading unit. Further, the signal of the light receiving element in the second pixel area necessary for the OB clamp or the like is also used as the signal of the photoelectric conversion element in the first pixel area read by the first and second reading means.

  According to the present invention, since the output from the photoelectric conversion element that is the reference pixel can be shared by a plurality of effective pixels, it is possible to reduce the total number of pixels, that is, the size of the image sensor while ensuring a sufficient number of necessary pixels. is there. Therefore, the size of the package constituting the image sensor can be reduced, and when the image pickup apparatus is configured, the degree of freedom in layout increases, and the size of the image sensor is small, so that the cost can be significantly reduced.

The figure which shows the structure of the pixel of the image pick-up element based on the Example of this invention. The figure which shows the structure of the image pick-up element based on the Example of this invention. The figure for demonstrating the pixel arrangement | sequence of the image pick-up element based on the Example of this invention. The figure for demonstrating the output image of the image pick-up element based on the Example of this invention. 1 is a conceptual diagram of a readout configuration of an image sensor according to an embodiment of the present invention. The figure which shows the structural example of the imaging device to which the image pick-up element based on the Example of this invention is applied. The figure for demonstrating the effect of the image pick-up element based on the Example of this invention. Diagram for explaining clamp operation Illustration for explaining the optical system

[Example]
FIG. 1 is a diagram for explaining a pixel portion of a CMOS image sensor according to an embodiment of the present invention. For example, an A image (first image) and a B image (second image) having parallax are shown. The pixel structure of the CMOS type image pick-up element in which a unit pixel is comprised by the several light receiving element (pixel) to produce | generate is shown. Specifically, the pixel configuration of an A image light receiving element (first light receiving element) or a B image light receiving element (second light receiving element) is shown.

  In this figure, the photodiode 102p for generating the optical signal charge is grounded on the anode side in this example. The cathode side of the photodiode 102p is connected to a floating diffusion (Cfd) (not shown) via the transfer MOS transistor 102. The floating diffusion is connected to the gate of the amplification MOS transistor 104. The gate of the amplification MOS transistor 104 is connected to the source of the reset MOS transistor 103 for resetting the floating diffusion. The drain of the reset MOS transistor 103 is connected to the power supply voltage VDD. Further, the amplification MOS transistor 104 has a drain connected to the power supply voltage VDD and a source connected to the drain of the selection MOS transistor 105.

  The gate terminal of the transfer MOS transistor 102 is driven by the Ptx signal, and transfers the signal of the photodiode 102 p to the floating diffusion and the gate of the amplification MOS transistor 104. The gate terminal of the reset MOS transistor 103 is driven by the Pres signal, and resets the floating diffusion and the photodiode 102p. The gate terminal of the selection MOS transistor 105 is driven by the Psel signal, and the signal is output from the Vout terminal. The Vout terminal is connected to a vertical output line (not shown), and the amplification MOS transistor 104 is connected to the vertical output line load via the selection MOS transistor 105, thereby functioning as a source follower amplifier.

  In FIG. 2, the light receiving element (pixel) described in FIG. 1 is an A image light receiving element and a B image light receiving element, and these two light receiving elements constitute a unit pixel as shown in FIG. The configuration of the image sensor when four pixels are arranged in the direction and three pixels are arranged in the vertical direction is shown. In the figure, a00 to a23 are A image light receiving elements, and b00 to b23 are B image light receiving elements, respectively. For example, a unit pixel is constituted by pixels a00 and b00. The pixels a00 to b21 are non-photosensitive reference pixels (HOB pixels) covered with a metal light shielding film (light shielding means), and a02 to b23 are photosensitive effective pixels. Each light receiving element (pixel) is supplied with a pres signal, a ptx signal, and a psel signal from the vertical shift register 240 for each row, and sequentially scanned in units of rows.

  Each light receiving element output is input to the capacitors (c0) 201a and 201b in the column circuit via a vertical output line to which the current source load 200 is connected. Reference numerals 203a and 203b denote operational amplifiers, and feedback capacitors (cf) 202a and 202b and capacitors (c0) 201a and 201b constitute an inverting amplifier. By short-circuiting both ends of the feedback capacitors (cf) 202a and 202b with an analog switch (not shown), the capacitors c0 and cf are reset and the capacitors cts and ctn in the subsequent stage are reset. The outputs of the operational amplifiers 203a and 203b are held in holding capacitors (cts) 210a and 210b and (ctn) 211a and 211b via analog switches 208a, 208b, 209a and 209b driven by pulses pts and ptn, respectively. Here, the signal immediately after the floating diffusion (Cfd) of the light receiving element portion is reset is held in the holding capacitors (ctn) 211a and 211b. Further, the signal immediately after the signal from the pixel is transferred to the floating diffusion (Cfd) is held in the holding capacitors (cts) 210a and 210b.

  When the pixel signals for one row are held in the holding capacitors (cts) 210a, 210b, (ctn) 211a, 211b for each column, the pulses ph (n) are sequentially driven by the horizontal scanning units 230a, 230b. Thereby, the analog switches 214a, 214b, 215a, and 215b are sequentially opened and closed. As a result, the pixel signals for one row are input to the subsequent differential read amplifiers 216a and 216b and output to the outside.

  Here, in the image sensor of this embodiment, the outputs of the A image light receiving elements a02 to a23 out of the effective pixels a02 to b23 (first pixel region) are downward in the figure, and the B image light receiving elements b02 to b23 (21st pixel). The output of the pixel region) is read out in the upward direction of the figure. Accordingly, the effective pixel signal from the A image light receiving element is output from the differential read amplifier 216a, and the effective pixel signal from the B image light receiving element is output from the differential read amplifier 216b.

  On the other hand, the outputs of the reference pixels (HOB pixels) a00 to b21 (second pixel regions) are simultaneously read from the vertical direction in the figure. In this case, since the load capacitance (c0) of the amplification MOS transistor 104 becomes two, 201a and 201b, the time constant slightly increases, but there is no problem of the signal output level.

  That is, from the differential read amplifier 216a, the reference pixel signal is read without distinction of the A image light receiving element / B image light receiving element, and the effective pixel signal from the A image light receiving element is read. Further, from the differential read amplifier 216b, the reference pixel signal is read out without distinction between the A image light receiving element and the B image light receiving element, and the effective pixel signal from the B image light receiving element is read out.

  3 to 4 show a two-dimensional pixel array of the image pickup device according to the present embodiment of FIG. 2 and an output from the two-dimensional pixel array. FIG. 3 shows a light receiving device of four horizontal pixels and three vertical pixels on the image pickup device. The arrangement is shown, and these correspond to the pixels a00 to b23 in FIG. FIGS. 4A and 4B show the output signals of the image sensor having such an array of light receiving elements. 4A shows an output image from the differential read amplifier 216a, and FIG. 4B shows an output image from the differential read amplifier 216b (see the description of FIG. 2).

  FIG. 5 is a diagram illustrating an outline of the readout configuration of the image sensor according to the present embodiment illustrated in FIGS. 2 to 3 with respect to the pixel arrangement for one row of the pixels a00, b00 to a03, and b03. In the figure, reference numeral 506 denotes a unit pixel, and reference numeral 505 denotes a microlens disposed on the light receiving surface of the unit pixel, and a plurality of light receiving elements of the unit pixel correspond to one microlens. In the figure, four unit pixels are arranged to form one row. Further, since the pixels a00, b00, a01, and b01 are HOB portions, they are read out to both the A image pixel reading portion 508 and the B image pixel reading portion 507, and the pixels a02 and a03 are read out from the A image pixel reading portion 508 ( Read to first reading means). The pixels b02 and b03 are read out to the B image pixel reading unit 507 (second reading means).

  As shown in FIGS. 4A and 4B, there are two effective pixel signals in the horizontal direction, but there are four reference pixel (HOB pixel) signals. This is because the output of the reference pixel (HOB pixel) is output in two directions, as shown in FIG. 2, and the effective pixel signal is output only in one direction, either up or down.

  With such an output configuration, the reference pixel (HOB pixel) signal can be increased (in this case, doubled), and the number of reference pixels used for averaging can be increased, so that the clamping operation can be performed with high accuracy. In other words, the reference pixel for the A image light receiving element and the reference pixel for the B image light receiving element are arranged independently, and the total number of pixels can be increased while ensuring a sufficient number of reference pixels as compared with the case of reading in one of the upper and lower directions. It is possible to reduce.

  As described above, the clamp operation is correction for removing offset fluctuation due to dark current applied to the photoelectric conversion signal caused by the subject light. Generally, the dark current difference between the A image light receiving element and the B image light receiving element is almost the same. Therefore, the configuration shared by this embodiment is effective.

[Application example of the embodiment]
FIG. 6 is a block diagram of an imaging apparatus using the imaging device according to the present embodiment described with reference to FIGS.

  In the figure, reference numeral 601 denotes a CMOS image sensor, and its configuration is assumed to be the same as that in FIG. The imaging device captures an optical image of a subject formed by an optical system including a photographing lens (not shown) with an effective pixel and outputs a pixel signal.

  Reference numeral 602a denotes an AFE (Analog Front End) which is a signal processing unit that performs A / D conversion and the like after amplifying the signal after clamping and correcting the A image light receiving element signal from the image sensor 601 with a reference pixel (HOB pixel). Reference numeral 602b denotes an AFE that is a signal processing unit that clamps a B-image light-receiving element signal from the image sensor 601 with a reference pixel (HOB pixel) and then amplifies the signal to perform AD conversion and the like. Each of the signal processing units 602a and 602b receives a timing signal from a timing generation unit 604 described later, and operates according to the timing signal. Reference numerals 603a and 603b denote DSPs (Digital Signal Processors), which perform various correction processes on the data from the signal processing units 602a and 602b. Further, under the control of 605 CPU described later, various memories such as ROMs 606a and 606b and RAMs 607a and 607b are controlled, and video data is written to the recording medium 608.

  Reference numeral 604 denotes a TG (Timing Generator) which is a timing generation unit that supplies a clock signal and a control signal to the image sensor 601, signal processing units 602a and 602b, and DSPs 603a and 603b, and is controlled by a CPU 605 described later. A CPU 605 controls the DSPs 603a, 603b, and TG 604, and controls camera functions using each unit such as the distance measuring unit 613 and the photometric unit 614. The switches 609 to 611 are connected to the CPU 605, and processing corresponding to each state is executed.

  Reference numerals 606a and 606b denote ROMs for storing camera control programs and correction tables. Reference numerals 607a and 607b denote RAMs that temporarily store video data and correction data processed by the DSPs 603a and 603b. The RAMs 607a and b can be accessed at a higher speed than the ROMs 606a and b. Reference numeral 608 denotes a recording medium such as a compact flash card that stores the captured video, and is connected to the camera via a connector (not shown).

  Reference numeral 609 denotes a power switch for activating the camera, and reference numeral 610 denotes a shutter switch SW1 for instructing the start of a photographing preparation operation such as a so-called EVF operation in which photometry processing, distance measurement processing, and subject video are displayed in real time. A shutter 611 drives a mirror and a shutter (not shown), and instructs the start of a series of imaging operations for writing a signal read from the imaging device 601 to the recording medium 608 via the signal processing units 602a, b and DSPs 603a, b. The switch SW2. Reference numeral 612 denotes a mode dial switch for instructing a camera shooting mode (for example, a normal mode, a 3D mode, and a moving image mode). Reference numeral 613 denotes a shooting lens (not shown) that detects the distance to the subject and forms an optical image of the subject. This is a distance measuring unit for adjusting the focus. Reference numeral 614 denotes a photometric unit for measuring subject luminance and determining an exposure amount to the image sensor, and 615 is a display device for displaying a photographed image on the outside.

  Here, 651 a indicated by a broken line is a processing block (first signal processing means) for processing the A image light receiving element signal output from the image sensor 601, and 651 b is a B image light receiving element signal output from the image sensor 601. Is a processing block (second signal processing means). By these processing blocks, an A image image and a B image image can be generated to form a stereoscopic image.

  As described above, according to the configuration of the present invention described with reference to FIGS. 1 to 6, it is possible to reduce the total number of pixels while sufficiently securing the number of reference pixels used for clamping. FIGS. 7A and 7B are diagrams for explaining this effect. In FIG. 7A, a reference pixel for the A image light receiving element and a reference pixel for the B image light receiving element are arranged independently. The pixel arrangement in an image sensor configured to read in one direction, either up or down, is shown. On the other hand, FIG. 7B is an array of pixels of an image sensor having a pixel readout configuration according to the present invention that outputs the output of a reference pixel (HOB pixel) in two directions, and is a reference compared to FIG. 7A. The horizontal size of the pixel array can be reduced. Therefore, the size of the package constituting the image sensor can be reduced, and when an image pickup apparatus having a pixel arrangement as shown in FIG. 7B is configured, the degree of freedom in layout increases and the size of the image sensor is small, resulting in significant cost reduction. Is possible.

  Although FIGS. 1 to 7 illustrate the case where the light receiving element (pixel) is an A image light receiving element and a B image light receiving element, and the unit pixel is configured by these two light receiving elements, the present invention is limited to this example. Is not to be done. For example, the present invention can also be applied to an image sensor that generates a normal image in which one light receiving element is configured as a unit pixel, and the same effect can be obtained. That is, it is possible to obtain the same effect by outputting a plurality of outputs from the reference pixel for reading one signal of the effective pixel.

  The above-described embodiments are merely examples of embodiments for carrying out the present invention, and the technical scope of the present invention should not be construed as being limited thereto. That is, the present invention can be implemented in various forms without departing from the technical idea or the main features thereof.

Claims (6)

In an imaging device having a pixel array in which unit pixels including at least one light receiving element are two-dimensionally arranged in a first pixel region and a second pixel region,
First reading means and second reading means for reading the signals of the light receiving elements of the pixel array for each row of unit pixels,
The signal of the light receiving element in the first pixel region is read by the first reading means or the second reading means, and the signal of the light receiving element in the second pixel region is read by the first reading means and the second reading means. An image pickup device configured to be read by the reading means. The unit pixel includes a first light receiving element and a second light receiving element,
The first reading means is configured to read a signal of the first light receiving element of the first pixel region and a signal of the first and second light receiving elements of the second pixel region,
The second reading means is configured to read a signal of the second light receiving element in the first pixel region and a signal of the first and second light receiving elements in the second pixel region. The imaging device according to claim 1.   3. The image sensor according to claim 1, wherein the unit pixel of the second pixel region includes a light-shielding unit of a light-receiving element.   4. The imaging device according to claim 1, further comprising a microlens disposed on a light receiving surface of the unit pixel corresponding to each unit pixel. 5. A photographic lens that forms an optical image of the subject;
The image sensor according to any one of claims 1 to 4, which captures the optical image;
First signal processing means for generating a first image based on the signal read by the first reading means;
An image pickup apparatus comprising: a second signal processing unit that generates a second image based on the signal read by the second reading unit.   The first signal processing means and the second signal processing means have correction means for correcting a signal of the light receiving element in the first pixel region using a signal of the light receiving element in the second pixel region. The imaging apparatus according to claim 5.

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