JP2009177601A - Imaging device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To inexpensively provide an imaging apparatus capable of preventing the deterioration of image quality by changing an OFD voltage in response to a photographing condition. <P>SOLUTION: A digital camera includes: a solid-state image sensor 5 having a photoelectric conversion element 21 formed in a p-well layer formed on a silicon substrate 20; an image sensor driving part 10 for driving the solid-state image sensor 5; and an overflow drain for discharging electric charges exceeding the saturation storage capacity of the photoelectric conversion element 21 to the silicon substrate 20. The image sensor driving part 10 changes an overflow drain voltage, which is applied to the overflow drain for controlling the saturation storage capacity of the photoelectric conversion element 21, according to set photography sensitivities. The setting value by each photography sensitivity of the overflow drain voltage is made to be a regular value. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a solid-state imaging device including a photoelectric conversion element formed in a semiconductor substrate, and an imaging apparatus having a driving unit that drives the solid-state imaging device.

  The digital camera can set the shooting sensitivity. For example, it is possible to switch between a low-sensitivity shooting mode in which shooting is performed at a shooting sensitivity equivalent to ISO sensitivity 100 of film sensitivity and a high-sensitivity shooting mode in which shooting is performed at a shooting sensitivity equivalent to ISO sensitivity 400 of film sensitivity. .

  Since the amount of light incident on the solid-state image sensor decreases as the imaging sensitivity increases, the amount of signal charge accumulated in the photoelectric conversion element also decreases as the imaging sensitivity increases, and the output voltage also decreases accordingly. Therefore, even if the same amount of dark current occurs, the amount of signal charge is large in the low-sensitivity shooting mode and the amount of signal charge is small in the high-sensitivity shooting mode. S / N deteriorates.

  Therefore, in the high sensitivity shooting mode, the charge transfer capacity in the vertical charge transfer path (the number of transfer electrodes forming a packet for accumulating charges in the vertical charge transfer path) is set in the vertical charge transfer path in the low sensitivity shooting mode. By making it smaller than the transfer capacity, the dark current generated in the vertical charge transfer path is reduced to prevent the deterioration of image quality (see Patent Document 1).

  As described above, when the transfer capacity of the vertical charge transfer path is changed in accordance with the photographing sensitivity, an overflow drain (hereinafter abbreviated as OFD) is used so that the charge amount accumulated in the photoelectric conversion element does not exceed the transfer capacity. It is necessary to control the OFD voltage to be applied and change the amount of charge that can be accumulated until the photoelectric conversion element is saturated (maximum accumulation in the photoelectric conversion element) (hereinafter referred to as saturation storage capacity). is there. In the manufacturer of the digital camera, information on the OFD voltage for each photographing mode is provided from the manufacturer of the solid-state imaging device, and the OFD voltage is set according to this information.

JP 2005-286470 A

  A manufacturer of a solid-state imaging device needs to measure the OFD voltage corresponding to the set number of imaging sensitivities. In recent years, since the photographing sensitivity has been increased, the number of times of measurement of the OFD voltage tends to increase accordingly. As a result, the time required for measuring the OFD voltage is increased, and there is a concern about an increase in manufacturing cost due to a decrease in yield.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an imaging apparatus capable of changing the OFD voltage according to imaging conditions and preventing deterioration in image quality at a low cost.

  An imaging apparatus according to the present invention is an imaging apparatus having a solid-state imaging device including a photoelectric conversion element formed on a semiconductor substrate and a driving unit for driving the solid-state imaging device, wherein the saturation storage capacity of the photoelectric conversion element is reduced. An overflow drain for discharging the excess charge to the semiconductor substrate is provided, and the driving means sets an overflow drain voltage to be applied to the overflow drain for controlling a saturated storage capacity of the photoelectric conversion element. The value is changed according to the conditions, and the set value of the overflow drain voltage for each photographing condition is a regular value.

  The imaging apparatus according to the present invention has a regular difference between the reference value and the set value other than the reference value when any of the set values of the overflow drain voltage for each imaging condition is set as a reference value. It has become a value.

  In the imaging apparatus according to the aspect of the invention, the difference may be obtained by calculating a difference between a saturation storage capacity of the photoelectric conversion element determined by the reference value and a saturation storage capacity of the photoelectric conversion element determined by the set value other than the reference value, as the overflow. The value is obtained by dividing the saturation storage capacity of the photoelectric conversion element per unit voltage of the drain voltage.

  In the imaging device according to the aspect of the invention, the solid-state imaging device includes a vertical charge transfer unit that transfers charges generated in the photoelectric conversion device in a vertical direction, and the driving unit is configured according to a saturated storage capacity of the photoelectric conversion device. The charge transfer capacity in the vertical charge transfer unit is changed.

  In the imaging apparatus of the present invention, the imaging condition is imaging sensitivity.

  ADVANTAGE OF THE INVENTION According to this invention, the imaging device which can change OFD voltage according to imaging | photography conditions and can prevent deterioration of an image quality can be provided at low cost.

  Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram illustrating a schematic configuration of a digital camera which is an example of an imaging apparatus for explaining an embodiment of the present invention.
The imaging system of the illustrated digital camera includes a photographing lens 1, a CCD (Charge Coupled Device) type solid-state imaging device 5, a diaphragm 2 provided between the two, an infrared cut filter 3, and an optical low-pass filter 4. With.

  A system control unit 11 that performs overall control of the electrical control system of the digital camera controls the flash light emitting unit 12 and the light receiving unit 13 and controls the lens driving unit 8 to adjust the position of the photographing lens 1 to the focus position and zoom. The exposure amount is adjusted by adjusting the aperture amount of the aperture 2 via the aperture drive unit 9.

  Further, the system control unit 11 drives the solid-state imaging device 5 via the imaging device driving unit 10 and outputs a subject image captured through the photographing lens 1 as a color signal. An instruction signal from the user is input to the system control unit 11 through the operation unit 14.

  The electric control system of the digital camera further includes an analog signal processing unit 6 that performs analog signal processing such as correlated double sampling processing connected to the output of the solid-state imaging device 5, and RGB output from the analog signal processing unit 6. And an A / D conversion circuit 7 for converting the color signals into digital signals, which are controlled by the system control unit 11.

  Furthermore, the electric control system of this digital camera generates image data by performing main memory 16, memory control unit 15 connected to main memory 16, interpolation calculation, gamma correction calculation, RGB / YC conversion processing, and the like. A digital signal processing unit 17, a compression / decompression processing unit 18 that compresses image data generated by the digital signal processing unit 17 into a JPEG format or decompresses compressed image data, and a digital signal processing unit 17 that integrates photometric data. The integration unit 19 for obtaining the gain of white balance correction performed by the camera, the external memory control unit 20 to which the removable recording medium 21 is connected, and the display control unit 22 to which the liquid crystal display unit 23 mounted on the back of the camera is connected. These are connected to each other by a control bus 24 and a data bus 25, and are controlled by commands from the system control unit 11. That.

  The digital camera shown in FIG. 1 can set a plurality of shooting sensitivities corresponding to the ISO sensitivity of the film, and the image sensor driving unit 10 performs driving according to the set shooting sensitivity. In the following description, it is assumed that the photographing sensitivity can be set to four types of ISO 100, 200, 400, and 800.

FIG. 2 is a schematic plan view showing a configuration example of the solid-state imaging device 5 shown in FIG. FIG. 3 is an enlarged view of a region 25 indicated by a broken line in FIG.
The solid-state imaging device 5 shown in FIG. 2 has a large number of photoelectric elements composed of n-type impurity layers arranged in a lattice pattern in a vertical direction Y in a p-well layer formed on a silicon substrate 20 and a horizontal direction X orthogonal thereto. Each of the conversion element 21, the plurality of vertical charge transfer units (VCCD) 22 that transfers charges generated in each of the large number of photoelectric conversion elements 21 in the vertical direction Y, and the plurality of vertical charge transfer units 22 is transferred. A horizontal charge transfer unit (HCCD) 23 that transfers the received charges in the horizontal direction Y and an output unit 24 that converts the charges transferred through the horizontal charge transfer unit 23 into signals and outputs the signals.

  As shown in FIG. 3, the vertical charge transfer unit 22 includes a charge transfer channel (not shown) formed of an n-type impurity region formed in a p-well layer on the silicon substrate 20 and a vertical direction Y above the charge transfer channel. It consists of a number of transfer electrodes 22a formed in an array. Each photoelectric conversion element 21 is provided with three transfer electrodes 22a. Transfer pulses φV1 to φV6 are applied from the image sensor driving unit 10 to a total of six transfer electrodes 22a corresponding to each of two photoelectric conversion elements 21 adjacent in the vertical direction Y, whereby the vertical charge transfer unit 22 6-phase driven.

  Each of the transfer pulses φV1 to φV6 can take a low level and a higher level than that. A packet capable of storing charges is formed in the charge transfer channel below the transfer electrode 22a to which the high-level transfer pulse is applied, and the charge transfer channel below the transfer electrode 22a to which the low-level transfer pulse is applied A barrier for the packet is formed.

  Similarly to the vertical charge transfer unit 22, the horizontal charge transfer unit 23 also includes a charge transfer channel (not shown) formed of an n-type impurity region formed in a p-well layer on the silicon substrate 20 and a horizontal direction above the charge transfer channel. It is composed of a large number of transfer electrodes (not shown) formed in an X array. Transfer pulses [phi] H1 and [phi] H2 are applied to the large number of transfer electrodes from the image sensor driving unit 10, respectively, whereby the horizontal charge transfer unit 23 is driven in two phases.

  The p-well layer below the photoelectric conversion element 21 functions as an overflow drain for discharging charges exceeding the saturation storage capacity of the photoelectric conversion element 21 to the silicon substrate 20. The OFD voltage φOFD is applied to the p-well layer from the image sensor driving unit 10. The barrier potential of the overflow drain can be controlled by the magnitude of the OFD voltage, and the saturation storage capacity of the photoelectric conversion element 21 can be changed.

  Since the incident light quantity decreases as the imaging sensitivity increases, the saturation storage capacity of the photoelectric conversion element 21 can be reduced as the sensitivity increases. For example, when the minimum required storage capacity (hereinafter referred to as required saturation) is 400 mV when converted to a signal voltage when the imaging sensitivity is 100, the required saturation is 400 mV / 2 = 200 mV when the imaging sensitivity is 200. When the photographing sensitivity is 400, the required saturation can be 400 mV / 4 = 100 mV, and when the photographing sensitivity is 800, the necessary saturation can be 400 mV / 8 = 50 mV.

  In the present embodiment, when a certain shooting sensitivity is set, the OFD voltage is controlled so that the saturation storage capacity of the photoelectric conversion element 21 becomes the required saturation at the shooting sensitivity.

  Further, in order to suppress dark current generated when charges are transferred by the vertical charge transfer unit 22, it is preferable to change the charge transfer capacity of the vertical charge transfer unit 22 according to the required saturation. This transfer capacity is the number of packets (the number of transfer electrodes 22a to which a high level transfer pulse is applied) to be formed corresponding to one photoelectric conversion element 21 at the time of charge transfer. When transferring the charges, the charges are transferred while increasing or decreasing the number of packets by one, such as n → (n + 1) → n → (n + 1) →. For this reason, in order to be able to transfer the charges accumulated in the photoelectric conversion element 21 without leakage, the transfer capacity is required for n packets, and this number n becomes the transfer capacity.

  The charge transfer capacity in the vertical charge transfer unit 22 can be determined according to the necessary saturation for each photographing sensitivity. For example, the transfer capacity at a photographing sensitivity of 100 may be a minimum capacity that can transfer charges of 400 mV without leakage. In the present embodiment, when a certain shooting sensitivity is set, the transfer capacity in the vertical charge transfer unit 22 is the minimum capacity that can transfer the necessary saturation charge at the shooting sensitivity without omission. In addition, the transfer pulses φV1 to φV6 are controlled.

  FIG. 4 is a diagram showing a design example of transfer capacity (number of packets), required saturation, and OFD voltage for each photographing sensitivity. Hereinafter, a design method of the design example shown in FIG. 4 will be described.

  The manufacturer of the solid-state imaging device 5 determines the photographing sensitivity (100, 200, 400, 800) and the necessary saturation (400, 200, 100, 50) corresponding thereto, and then the OFD voltage for realizing the necessary saturation. And the minimum transfer capacity (number of packets) that can transfer the necessary saturation charge without leakage. The number of packets can be determined in consideration of the capacity per packet, the required saturation, and the saturation unevenness.

  For the OFD voltage, first, one of the four imaging sensitivities is selected as the reference imaging sensitivity, and an OFD voltage value necessary to realize the necessary saturation at the reference imaging sensitivity is obtained by actual measurement. Here, the OFD voltage for realizing the required saturation of 400 mV is measured using the photographing sensitivity 100 as a reference, and this is used as the reference value Vref of the OFD voltage. Further, it is measured how much the saturation storage capacity of the photoelectric conversion element 21 changes when the OFD voltage is changed by a unit voltage (for example, 1 V here). This measured value is assumed to be ΔV.

  After actually measuring the reference values Vref and ΔV, an OFD voltage for realizing necessary saturation at a photographing sensitivity other than the reference is calculated according to the following equation (1).

OFD voltage for realizing necessary saturation at the photographing sensitivity to be calculated = Vref + {(required saturation at the reference photographing sensitivity−necessary saturation at the photographing sensitivity to be calculated) / ΔV} (1) )

Substituting the necessary saturation determined in advance, the measured reference value Vref (here 6V) and ΔV (here 50mV) into equation (1),
The OFD voltage for realizing the required saturation at the photographing sensitivity 200 is 6 + {(400−200) / 50} = 10V,
The OFD voltage for realizing the necessary saturation at the photographing sensitivity 400 is 6 + {(400−100) / 50} = 12V,
The OFD voltage for realizing the required saturation at the photographing sensitivity 800 is 6 + {(400−50) / 50} = 13V.

  In this way, the required saturation, OFD voltage, and number of packets are set when each imaging sensitivity is set.

  The digital camera of this embodiment is manufactured according to a design example as shown in FIG. For this reason, when the imaging sensitivity is set to 100, the OFD voltage is controlled to 6V, and the pattern of the transfer pulses φV1 to φV6 is controlled so that the minimum number of packets at the time of transfer is four. When the photographing sensitivity is set to 200, the OFD voltage is controlled to 10V, and the pattern of transfer pulses φV1 to φV6 is controlled so that the minimum number of packets at the time of transfer is three. When the photographing sensitivity is set to 400, the OFD voltage is controlled to 12V, and the pattern of transfer pulses φV1 to φV6 is controlled so that the minimum number of packets at the time of transfer is two. When the photographing sensitivity is set to 800, the OFD voltage is controlled to 13V and the pattern of transfer pulses φV1 to φV6 is controlled so that the minimum number of packets at the time of transfer is one.

  As described above, since all of the OFD voltages for each photographing sensitivity are not actually measured, only one is actually measured, and the others are obtained by calculation based on the actually measured values. Even in this case, an increase in time required for manufacturing the digital camera can be prevented, and the manufacturing cost can be reduced.

  Further, according to the digital camera of the present embodiment, the transfer capacity of the vertical charge transfer unit 22 is also changed according to the saturation storage capacity determined by the OFD voltage, so that it is possible to prevent image quality deterioration due to dark current.

  In the above description, the transfer capacity, the required saturation, and the OFD voltage are designed for each imaging sensitivity. However, the transfer capacity, the required saturation, and the OFD voltage are designed according to the amount of incident light, not the imaging sensitivity. You may make it do. For example, the transfer capacity, necessary saturation, and OFD voltage data are stored for each exposure value, and the transfer capacity, necessary saturation, and OFD voltage are changed according to the exposure value set at the time of shooting. Also good.

  In the above description, a vertical overflow drain mechanism having an overflow drain under the photoelectric conversion element 21 is taken as an example. However, a horizontal overflow drain mechanism having an overflow drain adjacent to the photoelectric conversion element 21 may be used.

  In the above description, the solid-state imaging device 5 is a CCD type, but it may be a MOS type. That is, instead of the vertical charge transfer unit 22, the horizontal charge transfer unit 23, and the output unit 24, a MOS circuit that converts the charge accumulated in the photoelectric conversion element 21 into a signal voltage and outputs the signal voltage may be provided. good.

  Also, the actual measurement value of the saturation storage capacity of the photoelectric conversion element 21 at the OFD voltage corresponding to each of the imaging sensitivities 200, 400, and 800 obtained by the above-described calculation method and the required saturation design value shown in FIG. , Vref and ΔV do not completely match due to measurement errors and the like, but the errors are negligibly small.

The figure which shows schematic structure of the digital camera which is an example of the imaging device for describing embodiment of this invention FIG. 1 is a schematic plan view showing a configuration example of the solid-state imaging device 5 shown in FIG. Enlarged view of the region 25 indicated by the broken line in FIG. Diagram showing design examples of transfer capacity, required saturation, and OFD voltage for each shooting sensitivity

Explanation of symbols

DESCRIPTION OF SYMBOLS 5 Solid-state image sensor 10 Image sensor drive part 20 Silicon substrate 21 Photoelectric conversion element 22 Vertical charge transfer part 23 Horizontal charge transfer part 24 Output part

Claims (5)

An imaging apparatus having a solid-state imaging device including a photoelectric conversion element formed on a semiconductor substrate, and a driving means for driving the solid-state imaging device,
An overflow drain for discharging charges exceeding the saturated storage capacity of the photoelectric conversion element to the semiconductor substrate;
The drive means changes an overflow drain voltage applied to the overflow drain for controlling a saturated storage capacity of the photoelectric conversion element according to a set photographing condition.
An imaging apparatus in which a set value of the overflow drain voltage for each imaging condition is a regular value. The imaging apparatus according to claim 1,
Imaging in which the difference between the reference value and the set value other than the reference value is a regular value when any of the set values of the overflow drain voltage for each imaging condition is set as a reference value apparatus. The imaging apparatus according to claim 2,
The difference between the saturated storage capacity of the photoelectric conversion element determined by the reference value and the saturation storage capacity of the photoelectric conversion element determined by the set value other than the reference value is calculated as a unit per unit voltage of the overflow drain voltage. An imaging apparatus having a value divided by the amount of change in saturation storage capacity of the photoelectric conversion element. The imaging apparatus according to any one of claims 1 to 3,
The solid-state imaging device includes a vertical charge transfer unit that transfers charges generated in the photoelectric conversion device in a vertical direction,
An imaging apparatus in which the driving unit changes a charge transfer capacity in the vertical charge transfer unit according to a saturated storage capacity of the photoelectric conversion element. The imaging apparatus according to any one of claims 1 to 4,
An imaging apparatus in which the imaging condition is imaging sensitivity.

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