JP2006216985A - Solid state imaging device and imaging system using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solid state imaging device capable of suppressing also the generation of OB steps caused by the increase of a dark current in an OB pixel part without increasing imaging time. <P>SOLUTION: The solid state imaging device comprises a photoelectric conversion element equipped with a PN junction area 20 in each of an image formation area pixel 14 and an optical black area pixel 13. The ratio of the size of a PN junction area of a photoelectric conversion element formed in the image formation area pixel 14 to the size of a semiconductor junction area of a photoelectric conversion element formed in the optical black area pixel 13 is set so that the dark current value of the photoelectric conversion element formed in the image formation area pixel 14 and the dark current value of the photoelectric element formed in the optical black area pixel 13 are the same or a difference between both the dark current values is small. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a solid-state imaging device and an imaging system using the same, and more particularly to a solid-state imaging device having a photoelectric conversion element having a semiconductor junction region in an image forming region and an optical black region, respectively, and an imaging system using the same.

  In general, a solid-state imaging device has an image forming area for image formation and a light-shielded optical black (optical black) area (hereinafter referred to as an OB area) for obtaining a signal level reference signal (black reference signal). The signal from the image forming area is processed based on the level obtained in the OB area.

  In such a solid-state imaging device, the amount of dark current generated may differ between the image forming area and the OB area. This is called an OB step, and as a technique for correcting such an OB step, for example, Patent Document 1 discloses a luminance signal level of at least two frames exposed at an appropriate exposure time and an exposure time different therefrom. A video signal processing method is disclosed in which an OB step is calculated from the signal and corrected at the input stage of the signal processing circuit.

  As a technique related to the present invention, Patent Document 2 discloses that the size of the PN junction region in the OB region is reduced in order to prevent an abnormal increase in read signal due to white scratches or the like. However, there is no disclosure of setting the size of the PN junction region in relation to the dark current value.

Further, in Patent Document 3, in a CCD solid-state imaging device, when a large amount of light is incident on an OB region, the light cannot be sufficiently shielded, and a part of the light enters the vertical transfer register below the vertical transfer electrode. The width of the vertical transfer electrode in the OB region is formed wider than the width of the vertical transfer electrode in the effective pixel portion, with respect to the problem that the image quality is disturbed due to a black level shift caused by transmission and photoelectric conversion there. A solid-state imaging device is disclosed. However, there is no disclosure regarding a solid-state imaging device having an amplification element in an effective pixel and a pixel in the OB region, and only a case where dark current increases in the OB region is disclosed as a configuration.
JP 2002-354348 A JP 2003-134400 A JP-A-10-209424

  However, the configuration of Patent Document 1 requires a procedure of fetching a signal from an image in which incident light is shielded once in order to remove an adverse effect due to an OB step, and there is a problem that an imaging time becomes long. Further, when the OB step is generated, as disclosed in Patent Document 3, generally, the dark current in the OB region increases, so that it does not coincide with the accurate implicit output in the effective pixel region.

  However, according to studies by the present inventors, it has been found that depending on the configuration of the solid-state imaging device, there is a configuration in which the dark current is smaller in the OB pixel portion.

  The present invention has been made in order to solve the above-described problems. A solid-state imaging device that suppresses occurrence of an OB step due to an increase in dark current in an OB pixel unit without increasing an imaging time. The purpose is to provide.

  In order to solve the above problems, the present invention provides a first conductivity type first semiconductor region for accumulating charges, and a second conductivity type second semiconductor that forms a PN junction with the first semiconductor region. A photoelectric conversion element having a semiconductor region; an image forming region having a plurality of transistors in a pixel for reading a signal based on the charge accumulated in the first semiconductor region; and a first one for accumulating the charge. A third semiconductor region of one conductivity type, and a fourth semiconductor region of a second conductivity type that forms a PN junction with the third semiconductor region, and the charge accumulated in the third semiconductor region A solid-state imaging device having a plurality of transistors for reading a reference signal, and the image forming region and the optical black region each include a part of the wiring of the transistor on the solid-state imaging device. When the first semiconductor region and the third semiconductor region are provided in an element region divided by an element isolation region, and the dark current of the optical black region is larger than the dark current of the image forming region, The second distance between the third semiconductor region and the element isolation region is made larger than the first distance between the first semiconductor region and the element isolation region.

  In the present application, the semiconductor junction region is a region where a semiconductor junction is formed. For example, a photoelectric conversion element is formed by a P well, an N-type semiconductor region formed in the P well, and a P layer serving as a surface layer. In this case, it means a region where a PN junction between the P well and the N-type semiconductor region is formed.

  The present invention will be further described below.

  As a result of the study by the present inventors, a case where the dark current is newly reduced in the OB region pixel has been found. The following models can be considered for this.

  In many cases, the light shielding layer on the OB area pixel is fixed with a constant bias. In this case, the interface is depleted and the dark current of the OB area pixel increases or decreases. For example, in a configuration in which the well is p-type and the carrier is electron (n-type), the dark current of the OB region pixel decreases.

Or, for some reason, the number of Si / SiO ₂ interface defects on the surface of the pixel portion in the OB area is larger in the image forming area pixel than in the OB area pixel. However, the dark current becomes smaller.

  First, the first solid-state imaging device of the present invention will be described.

Since the dark current increases when the PN junction region serving as the semiconductor junction region is brought close to the element isolation region provided with the conductive layer thereon, the present inventors increase the OB from the element isolation region as shown in FIG. The area of the PN junction region 21 of the OB region pixel 13 is gradually decreased in a direction away from the element isolation region by separating the PN junction region 21 of the region pixel 13 by a certain distance (t0) or more (the PN junction region and the element isolation). When the dark current value was measured), the dark current value was found to decrease substantially corresponding to the size (area) of the semiconductor junction region. This is shown in FIG. Here, the horizontal axis of FIG. 8A is S = Sa-Sb (S is the area of the PN junction region 21, Sa is the area of the PN junction region 21 at t0, Sb is the area of the decrease), and the vertical axis is the image. It is a dark current ratio between a pixel in the formation region and a pixel in the OB region. S ₁ to S of the horizontal shaft ₅ represents the area when you release a certain distance Δt by one PN junction region from t0, the area of the OB region pixels and the image forming region pixels when an area S ₂ are equal.

  It can be seen from the characteristic diagram of FIG. 8A that the dark current ratio linearly decreases as the size of the OB region pixel decreases. Therefore, by appropriately designing (increasing or decreasing) the ratio of the area of the semiconductor junction region of the pixel in the OB region and the area of the pixel in the image forming region, the dark current value of the photoelectric conversion element formed in the image forming region and the OB region The dark current value of the photoelectric conversion element formed in the same or the difference between both dark current values can be reduced.

  Next, the second solid-state imaging device of the present invention will be described.

  When the photoelectric conversion elements are arranged one-dimensionally or two-dimensionally, an element isolation region such as a selective oxide film is provided between adjacent elements, and a photoelectric conversion element is formed in an element region divided by the element isolation region. . A gate wiring of a field effect transistor for driving a photoelectric conversion element or selecting a pixel, a gate electrode of the field effect transistor, or the like is provided on the element isolation region. Here, when the semiconductor junction region of the pixel is brought close to the element isolation region provided with the conductive layer constituting the wiring or the like, the dark current increases due to the potential of the conductive layer. Therefore, for example, when the dark current of the OB region pixel is smaller than the dark current of the image forming region in the same structure design, the semiconductor junction region of the OB region pixel is increased in order to increase the dark current of the pixel of the OB region pixel. And the element isolation region may be shortened. In this case, if the distance between the semiconductor junction region and the element isolation region of the OB region pixel is shortened and at the same time the area of the semiconductor junction region is increased, the dark current increases due to the approach to the element isolation region and the area increases. The dark current is increased by this, and the dark current can be further increased. On the contrary, in the design of the same structure, when the dark current of the OB region pixel is larger than the dark current of the image forming region, the distance between the semiconductor junction region and the element isolation region of the image forming region pixel may be shortened.

  Next, the third and fourth solid-state imaging devices of the present invention will be described.

  In order to suppress a current (charge) flowing into the semiconductor junction region of the photoelectric conversion element, a semiconductor region called a guard ring layer is formed around the semiconductor junction region. If a part or all of the semiconductor region is removed, the dark current can be increased.

  Therefore, for example, when the dark current of the OB region pixel is smaller than the dark current of the image forming region in the design of the same structure, the semiconductor region that becomes a guard ring layer around all or part of the semiconductor junction region of the image forming region pixel The semiconductor region around the semiconductor junction region of the OB region pixel is not provided, or the periphery of the semiconductor junction region of the OB region pixel is larger than the length of the semiconductor region surrounding the semiconductor junction region of the image formation region pixel. The length of the surrounding semiconductor region may be shortened. On the contrary, in the same structure design, when the dark current of the OB region pixel is larger than the dark current of the image forming region, the OB region is longer than the length of the semiconductor region surrounding the semiconductor junction region of the image forming region pixel. The length of the semiconductor region surrounding the semiconductor junction region of the pixel may be increased.

  According to the present invention, the dark offset (OB step) between the dark reference output from the OB area and the dark output from the image forming area can be suppressed, and the long exposure in which the offset appears more noticeably. It can be suitably used at times, at high temperatures, and at high ISO.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(Embodiment 1)
FIG. 1A is a diagram showing one pixel of the OB region of the solid-state imaging device according to the first embodiment of the present invention, and FIG. 1B is an image forming region of the solid-state imaging device according to the first embodiment of the present invention. It is a figure which shows one pixel.

  In FIG. 1, reference numeral 13 denotes an optical black (OB) area pixel, and reference numeral 14 denotes an image forming area pixel. The OB area pixel 13 and the image forming area pixel 14 have a PN that is a semiconductor junction area in the photoelectric conversion element forming area 20. A junction region 21 is provided. The charge accumulated in the PN junction region 21 is transferred to the charge / voltage conversion unit 23 (floating diffusion region) via the charge transfer switch 22, and the charge / voltage conversion unit 23 is connected to the gate (input unit) of the amplifier MOS transistor 24. Connected, amplified and output. In FIG. 1, the reset switch 103 and the row selection switch 105 for selecting pixels shown in FIG. 3 are omitted. Further, a configuration may be adopted in which charges are directly transferred from the PN junction region to the charge-voltage converter 23.

  In the present embodiment, when the OB area pixel and the image forming area pixel have the same structure, and the dark current of the OB area pixel is smaller than the dark current of the image forming area pixel, the characteristics shown in FIG. Therefore, in order to increase the dark current of the OB region pixel, the area of the PN junction region of the OB region pixel is made larger than that of the PN junction region of the image forming region pixel. In the case where the OB area pixel and the image forming area pixel have the same structure, if the dark current of the OB area pixel is larger than the dark current of the image forming area pixel, the OB area pixel has a dark current difference so that the dark current difference is eliminated. The PN junction region of the pixel may be reduced.

  The extent to which the area of the PN junction region of the OB region pixel can be increased or decreased from the PN junction region of the image forming region pixel can be determined by, for example, obtaining a characteristic diagram as shown in FIG. In the solid-state imaging device in which the areas of the PN junction region of the OB region pixel and the PN junction region of the image formation region pixel are designed to be the same, the variation in the area of the PN junction region of the OB region pixel and the image formation region pixel It may be obtained by measuring the variation in the dark current ratio of the OB region pixel with respect to.

  FIG. 2 is a diagram illustrating a configuration of the light receiving unit of the solid-state imaging device. In FIG. 2, reference numeral 10 denotes a light receiving unit, and an image forming region 11 and an OB region around the image forming region 11 are provided in the light receiving unit 10. Image forming area pixels 14 are arranged in the image forming area 11, and OB area pixels 13 are arranged in the OB area 12 in a two-dimensional manner.

  FIG. 3 is a block diagram showing the overall configuration of the solid-state imaging device according to the first embodiment of the present invention. Although two pixels are shown in FIG. 3, a larger number of pixel rows may be used. The solid-state imaging device is shown as an area sensor in which pixels are arranged in a two-dimensional manner, but may be a line sensor in which pixels are arranged in a one-dimensional manner.

  3, the pixel includes a photodiode 101 and a pixel transfer switch 102 (charge transfer switch of FIG. 1) that transfers charges accumulated in the photodiode 101 to a floating diffusion region (corresponding to the charge-voltage converter 23 of FIG. 1). 22), an input MOS transistor 104 (corresponding to the amplifier MOS transistor 24 in FIG. 1) serving as an amplifying means, an input unit is connected to the floating diffusion region, a reset switch 103 connected to the floating diffusion region, a pixel It is comprised from the row selection switch 105 which selects.

  The pixel transfer switch 102, the reset switch 103, and the selection switch 105 are controlled by control signals PTX, PRES, and PSEL from the vertical scanning circuit 123, and the signals from the pixels in the row selected by the selection switch 105 are in the vertical direction in the figure. Are output alternately to the vertical output line 106 which is a signal output line. The vertical output line 106 connects the pixel and one terminal (+) of the operational amplifier 120 serving as an amplifier circuit via a clamp capacitor 108. Reference numeral 107 denotes a load MOS transistor, and 109 denotes a clamp switch.

  The signal amplified by the operational amplifier 120 is accumulated in the storage capacitor 112 via the transfer gate 110, and is sequentially output to the horizontal output line 116 via the horizontal transfer switch 114 controlled by the horizontal scanning circuit 119. Is output via. Reference numeral 132 denotes a common GND wiring of the operational amplifier 120.

FIG. 4 is a diagram illustrating a configuration example of the clamp circuit. As shown in FIG. 4, the sensor output output from the output amplifier 118 is input to one terminal of the clamp capacitor. With the clamp switch controlled by the clamp control signal φCLP, the other terminal of the clamp capacitor is set to a reference voltage based on a signal (black reference signal) that is obtained from the OB region and serves as a reference of the pixel signal level. When a sensor signal is input to one terminal of the clamp capacitor, a clamped signal is output.
(Embodiment 2)
FIG. 5A is a diagram showing one pixel in the OB region of the solid-state imaging device according to the second embodiment of the present invention, and FIG. 5B is a cross-sectional view along the line BB ′. The same components as those in FIG. 1 are denoted by the same reference numerals. The overall configuration of the solid-state imaging device of the present embodiment is the same as the configuration shown in FIG. 3, and the configuration of the light receiving unit of the solid-state imaging device is the same as the configuration shown in FIG.

  In FIG. 5A, reference numeral 25 denotes a wiring or gate electrode (hereinafter referred to as wiring) made of polysilicon or the like, which is provided on a selective oxide film (LOCOS). Electric charges are generated under the selective oxide film due to the potential of the wiring 25, and the electric charges flow into the PN junction region, increasing the dark current in the OB region 13.

  For example, the wiring 25 is a gate wiring or a gate electrode of the reset switch 103, the amplification MOS transistor 104, and the selection switch 105 shown in FIG.

  In the present embodiment, when the pixels in the OB region and the pixels in the image forming region have the same structure, the dark current of the pixels in the OB region is smaller than the dark current of the pixels in the image forming region. In order to increase the dark current, the PN junction region of the OB region pixel is brought close to the selective oxide film (distance is shortened), the current flowing from the selective oxide film side is increased, and the dark current of the OB region pixel is reduced. It is the same as the dark current of the image forming area pixels.

  If the pixels in the OB region and the image forming region have the same structure, and the dark current of the pixels in the OB region is larger than the dark current of the pixels in the image forming region, the dark current difference is eliminated. The dark current may be increased by bringing the PN junction region of the image forming region pixel closer to the selective oxide film side. When it is not desired to increase the dark current of the pixels in the image forming area, the dark current of the pixels in the OB area can be reduced by reducing the PN junction area of the pixels in the OB area.

(Embodiment 3)
6A and 6C are views showing one pixel of the OB region of the solid-state imaging device according to the third embodiment of the present invention, and FIG. 6B is a cross-sectional view taken along the line BB ′. The same components as those in FIG. 1 are denoted by the same reference numerals. The overall configuration of the solid-state imaging device of the present embodiment is the same as the configuration shown in FIG. 3, and the configuration of the light receiving unit of the solid-state imaging device is the same as the configuration shown in FIG.

  In order to prevent the charge from the periphery of the PN junction region from flowing into the PN junction region and increasing the dark current of the OB region pixel, the PN junction region is interposed between the selective oxide film (LOCOS) and the PN junction region. A semiconductor region (guard ring layer) having a conductivity type different from that of the PN junction region is provided.

  In the present embodiment, when the pixels in the OB area have the same structure as the pixels in the OB area, and the dark current in the pixels in the OB area is smaller than the dark current in the pixels in the image formation area, In order to increase the dark current, as shown in FIG. 6C, a part of the semiconductor region (guard ring layer) 26 provided around the PN junction region of the OB region pixel as shown in FIG. By removing (making the length of the semiconductor region surrounding the PN junction region of the OB region pixel shorter than the length of the semiconductor region surrounding the PN junction region of the image forming region pixel) The current is increased to make the dark current of the OB area pixel the same as the dark current of the image forming area pixel. For example, the configuration shown in FIG. 6A can be an image forming region pixel, and the configuration shown in FIG. 6B can be an OB region pixel. Here, the length of the semiconductor region 26 is the length of the semiconductor region 26 formed so as to cover the periphery of the PN junction region 26. As shown in FIG. In the case of being formed, the lengths of the two semiconductor regions are totaled to obtain the length of the semiconductor region.

  If the pixels in the OB region and the image forming region have the same structure, and the dark current of the pixels in the OB region is larger than the dark current of the pixels in the image forming region, the dark current difference is eliminated. The dark current may be increased so as not to form a part of the semiconductor region provided around the PN junction region of the image forming region pixel. When it is not desired to increase the dark current of the image forming area pixel, the dark current of the OB area pixel can be reduced by reducing the PN junction area of the OB area pixel.

  Next, an imaging system using the solid-state imaging device of the first to third embodiments will be described. Examples of the imaging system using such a solid-state imaging device include a still camera, a video camera, a copying machine, and a facsimile. Here, based on FIG. 7, an embodiment when the solid-state imaging device according to the present invention is applied to a still camera will be described in detail.

  FIG. 7 is a block diagram showing a case where the solid-state imaging device according to the present invention is applied to a “still video camera”.

  In FIG. 7, 201 is a barrier that doubles as a lens protect and main switch, 202 is a lens that forms an optical image of a subject on the solid-state imaging device 4, 203 is a diaphragm for changing the amount of light passing through the lens 202, and 204 is A solid-state imaging device for capturing an object imaged by the lens 202 as an image signal, 206 an A / D converter that performs analog-digital conversion of an image signal output from the solid-state imaging device 204, and 207 an A / D conversion A signal processing unit 208 performs various corrections on the image data output from the imager 206 and compresses the data. 208 denotes a solid-state image sensor 204, an image signal processing circuit 205, an A / D converter 206, and a signal processing unit 207. A timing generator for outputting a timing signal, 209 is an overall control / arithmetic unit for controlling various operations and the entire still video camera, 10 is a memory unit for temporarily storing image data, 211 is an interface unit for performing recording or reading on a recording medium, and 212 is a detachable recording such as a semiconductor memory for recording or reading image data. A medium 213 is an interface unit for communicating with an external computer or the like.

  Next, the operation of the still video camera at the time of shooting in the above configuration will be described.

  When the barrier 201 is opened, the main power supply is turned on, the control system power supply is turned on, and the power supply of the imaging system circuit such as the A / D converter 206 is turned on.

  Then, in order to control the exposure amount, the overall control / arithmetic unit 209 opens the diaphragm 203, and the signal output from the solid-state imaging device 204 is converted by the A / D converter 206 and then sent to the signal processing unit 207. Entered. Based on the data, the exposure calculation is performed by the overall control / calculation unit 209.

  The brightness is determined based on the result of the photometry, and the overall control / calculation unit 209 controls the aperture according to the result.

  Next, based on the signal output from the solid-state imaging device 204, the high-frequency component is extracted and the distance to the subject is calculated by the overall control / calculation unit 209. Thereafter, the lens is driven to determine whether or not it is in focus. If it is determined that the lens is not in focus, the lens is driven again to perform distance measurement.

  Then, after the in-focus state is confirmed, the main exposure starts. When the exposure is completed, the image signal output from the solid-state imaging device 204 is A / D converted by the A / D converter 206, passes through the signal processing unit 207, and is written in the memory unit 210 by the overall control / calculation 209. Thereafter, the data stored in the memory unit 210 is recorded on a removable recording medium 212 such as a semiconductor memory through the recording medium control I / F unit 211 under the control of the overall control / arithmetic unit 209. Further, the image may be processed by directly inputting to a computer or the like through the external I / F unit 213.

  The present invention is a solid-state imaging device that is required to suppress dark offset (OB level difference) between a dark reference output from an optical black area and a dark output from an image forming area, particularly long-time exposure, use at a high temperature, The present invention can be applied to a solid-state imaging device suitable for use in high ISO, and can be used for a solid-state imaging device for a still camera, a video camera, a copying machine, and a facsimile.

It is a figure which shows one pixel of the OB area | region and image formation area of the solid-state imaging device concerning 1st Embodiment of this invention. It is a figure which shows the structure of the light-receiving part of a solid-state imaging device. 1 is a block diagram illustrating an overall configuration of a solid-state imaging apparatus according to a first embodiment of the present invention. It is a figure which shows one structural example of a clamp circuit. It is a figure which shows one pixel of the OB area | region of the solid-state imaging device concerning 2nd Embodiment of this invention. It is a figure which shows one pixel of the OB area | region of the solid-state imaging device concerning 3rd Embodiment of this invention. It is a block diagram which shows the case where the solid-state imaging device by this invention is applied to a still video camera. It is a figure for demonstrating the principle of this invention.

Explanation of symbols

13 Optical Black (OB) Area Pixel 14 Image Forming Area Pixel 20 Photoelectric Conversion Element Forming Area 21 PN Junction Area 22 Charge Transfer Switch 23 Charge Voltage Converter (Floating Diffusion Area)
24 MOS transistor for amplifier

Claims (3)

A photoelectric conversion element comprising: a first conductivity type first semiconductor region for accumulating electric charge; and a second conductivity type second semiconductor region constituting a PN junction with the first semiconductor region; An image forming region having a plurality of transistors in a pixel for reading a signal based on charges accumulated in the first semiconductor region;
A third semiconductor region of a first conductivity type for accumulating electric charge; and a fourth semiconductor region of a second conductivity type constituting a PN junction with the third semiconductor region, the third semiconductor region In a solid-state imaging device having an optical black region including a plurality of transistors for reading a reference signal based on the electric charge accumulated in
Each of the image forming region and the optical black region has the first semiconductor region and the third semiconductor region in an element region divided by an element isolation region having a part of the wiring of the transistor thereon, When the dark current in the optical black region is larger than the dark current in the image forming region, the third semiconductor region and the element are more than the first distance between the first semiconductor region and the element isolation region. A solid-state imaging device, wherein a second distance between the separation region and the separation region is increased. In the image forming area and the optical black area, pixels each including a photoelectric conversion element and a reading unit that reads an electric signal converted by the photoelectric conversion element are formed in a one-dimensional or two-dimensional manner. The solid-state imaging device according to claim 1. The solid-state imaging device according to claim 1, an optical system that forms an image of light on the solid-state imaging device, and a signal processing circuit that processes an output signal from the solid-state imaging device. An imaging system characterized by that.

Images