JP2005039053A - Solid-state image sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve a problem in a conventional solid-state imaging element that the black level reference gets out of order because of production of signal electric charges except a dark current component, in the case that part of the light collected by an on-chip lens may transmit through a light shading film and the transmitted light may be subjected to photoelectric conversion at a light receiving section when strong light is particularly made incident onto an optical black region. <P>SOLUTION: An on-chip lens 38 of the optical black region in a CMOS image sensor does not collect the incident light to a photoelectric conversion region of a photo diode 21 but collects the incident light to a region other than the photoelectric conversion region such as the vicinity of a floating diffusion section FD. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a solid-state imaging device typified by a CCD (Charge Coupled Device) image sensor or a CMOS image sensor, and in particular, a condensing lens called an on-chip lens is placed on the light receiving unit for each light receiving unit (pixel). The present invention relates to a solid-state image pickup device arranged.

  In recent years, in solid-state imaging devices such as CCD image sensors and CMOS image sensors, miniaturization of the device itself and higher density of pixels have further advanced, and accordingly, an imaging area in which pixels are two-dimensionally arranged in a matrix is reduced. This causes deterioration of characteristics such as sensitivity reduction. Conventionally, as a countermeasure against this sensitivity reduction, an on-chip lens is provided on the light receiving part for each light receiving part (pixel), and the light collection efficiency of each light receiving part is increased by the light collecting action of the on-chip lens. (For example, refer to Patent Document 1).

  By the way, on the imaging surface of the solid-state imaging device, an effective pixel region that uses pixel signals as actual imaging information, and an optical black that is located outside the effective pixel region and is used to obtain an optical reference black level An area (optical black area) exists. In this way, no optical light is incident by providing an optical black region and taking the difference between the reference signal and each pixel signal in the effective pixel region based on the black level that is each pixel signal in the optical black region. Even in the state, a noise component that is thermally generated, that is, a DC component of so-called dark current can be removed. This optical black region is shielded from light by a light shielding film made of metal such as aluminum.

Japanese Patent Laid-Open No. 11-103036 (in particular, paragraphs 0015 and 0020 and FIG. 1)

  As described above, in a solid-state imaging device having an on-chip lens, it is difficult to completely block incident light with a light-shielding film in the optical black region. When particularly strong light is incident, it is collected by the on-chip lens. In some cases, part of the transmitted light passes through the light shielding film and is photoelectrically converted by the light receiving unit. As a result, signal charges other than the dark current component are generated, and the black level reference is lost.

  When the black level reference is out of order, the pixel signal in the optical black region is slightly lower in level than the pixel signal in the effective pixel region. The captured image (picture) in such a state is like a black stripe running on a part of it, resulting in an unnatural picture.

  In order to solve the problem caused by the light transmitted through the light shielding film, a method of increasing the light shielding ability of the optical black region by increasing the thickness of the light shielding film in the optical black region can be considered. However, when this method is adopted, it is necessary to change the upper layer structure significantly between the effective pixel region and the optical black region, which may cause unevenness due to a step or the like.

  For the optical black region, a method of eliminating the on-chip lens and reducing the light collection efficiency is also conceivable. However, even if the condensing efficiency is lowered, it is not as much as in the case of condensing by an on-chip lens in a harsh imaging state where intense light is locally irradiated on the imaging surface. The portion passes through the light shielding film and enters the light receiving portion, and is photoelectrically converted by the light receiving portion.

  The present invention has been made in view of the above problems, and the object of the present invention is to transmit the light-shielding film even in a severe imaging state in which particularly strong light is locally irradiated on the imaging surface. An object of the present invention is to provide a solid-state imaging device that reliably prevents light from entering the light-receiving portion and eliminates the unnaturalness of a captured image caused by the transmitted light.

  In order to achieve such an object, the solid-state imaging device according to the present invention includes a pixel including a photoelectric conversion element, an effective pixel region in which a signal of the pixel is used as an effective signal, and a pixel including the photoelectric conversion element is shielded from light. An optical black area in which the signal of the pixel is used as a black level reference and a pixel unit on the effective pixel area, and the incident light is condensed in the photoelectric conversion area of each pixel. The first on-chip lens group and a second on-chip lens group that is arranged in units of pixels on the optical black area and collects incident light outside the photoelectric conversion area of each pixel. ing.

  In the solid-state imaging device having the above-described configuration, the light incident on the effective pixel region is condensed in the photoelectric conversion region of each pixel by the condensing action of the first on-chip lens group, and the signal charges are photoelectrically converted by each photoelectric conversion device. Converted. The signal of each pixel in this effective pixel area is used as an effective (pixel) signal. On the other hand, the light incident on the optical black region is condensed outside the photoelectric conversion region of each pixel by the condensing action of the first on-chip lens group.

  Here, since the light condensed by the first on-chip lens group is generally shielded by the light shielding film, it does not reach the light receiving surface of the pixel. However, for example, in a harsh imaging state where intense light is locally irradiated on the imaging surface, the light condensed by the first on-chip lens group may pass through the light shielding film. Even if the light passes through the light-shielding film in this way, the light is condensed outside the photoelectric conversion region of each pixel by the first on-chip lens group, so that it is photoelectrically converted by the photoelectric conversion element and a signal other than the dark current component. No charge is generated.

  According to the present invention, the on-chip lens in the optical black region is formed such that incident light is not condensed on the photoelectric conversion region of the photodiode, but is condensed on a place other than the photoelectric conversion region, Even when particularly intense light is incident, even if part of the light passes through the light-shielding film, the transmitted light does not enter the photoelectric conversion region, so that signal charges other than dark current components are not generated. Therefore, it is possible to eliminate the unnaturalness of the captured image due to the transmitted light, which has been a problem in the past.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing an outline of a configuration of a solid-state imaging device, for example, a CMOS image sensor according to an embodiment of the present invention.

  The CMOS image sensor according to this embodiment includes an imaging unit 11 in which pixels are two-dimensionally arranged in a matrix, a vertical scanning circuit 12 that selects each pixel of the imaging unit 11 in units of rows, and a row unit from the imaging unit 11. The signal processing circuit 13 for performing signal processing such as CDS (Correlated Double Sampling) on the signal of each pixel read out in step S1 and the pixel signals for one row output from the signal processing circuit 13 are sequentially provided. A horizontal scanning circuit 14 for selecting in units of pixels and a horizontal output circuit 15 for sequentially outputting pixel signals selected by the horizontal scanning circuit 15 are provided.

  In the imaging unit 11, pixels 16 including photoelectric conversion elements are two-dimensionally arranged in a matrix, and an effective pixel region 17 in which a signal of the pixel 16 is used as an effective (pixel) signal, and pixels 16 including the photoelectric conversion elements are shielded from light. In this state, the optical black region 18 is arranged for a plurality of columns and the signal of the pixel 16 is used as a black level reference. A vertical signal line 19 is wired for each vertical pixel column with respect to the arrangement of the pixels 16 of the imaging unit 11.

  The optical black area 18 further includes an invalid optical black area 18A provided adjacent to the effective pixel area 17, and an effective optical area provided adjacent to the invalid optical black area 18A on the opposite side of the effective pixel area 17. And an optical black region 18B. Here, the signal of each pixel in the effective optical black region 18B is used as a reference for the black level, whereas the signal of each pixel in the invalid optical black region 18A is adjacent to the effective pixel region 17, Since it is easily affected by light leaking from the effective pixel region 17 side, it is not used as an optical reference black level.

  In FIG. 1, for simplification of the drawing, an example is shown in which the vertical pixel column of the invalid optical black region 18A is one column and the vertical pixel column of the effective optical black region 18B is two columns. However, the present invention is not limited to this, and each can be set as an arbitrary number of vertical pixel columns.

  FIG. 2 is a circuit diagram illustrating an example of the circuit configuration of the pixel 16. As can be seen from the figure, the pixel 16 has, for example, a photodiode 21 as a photoelectric conversion element. A transfer transistor 22, an amplification transistor 23, an address transistor 24, and a reset transistor 25 are added to the one photodiode 21. The circuit configuration has four transistors as active elements.

  The photodiode 21 is grounded at its anode, and photoelectrically converts incident light into charges (here, electrons) in an amount corresponding to the amount of light. The transfer transistor 22 is connected between the cathode of the photodiode 21 and the floating diffusion portion FD, and a transfer signal is given to the gate through the transfer wiring 26, whereby the electrons photoelectrically converted by the photodiode 21 are transferred to the floating diffusion portion. Transfer to FD.

  The gate of the amplification transistor 23 is connected to the floating diffusion portion FD. The amplification transistor 23 is connected to a vertical signal line 27 via an address transistor 24. When an address signal is applied to the gate of the address transistor 25 through the address wiring 28 and the address transistor 25 is turned on, the amplification transistor 23 amplifies the potential of the floating diffusion portion FD and applies a voltage corresponding to the potential to the vertical signal line. 27. The vertical signal line 27 transmits the voltage output from each pixel to the signal processing circuit 13 (see FIG. 1).

  The reset transistor 25 is connected between the power supply Vdd and the floating diffusion portion FD, and resets the potential of the floating diffusion portion FD to the potential of the power supply Vdd when a reset signal is given to the gate through the reset wiring 29. In these operations, since the wirings 26, 28, and 29 to which the gates of the transfer transistor 22, the address transistor 24, and the reset transistor 25 are connected are wired in units of rows, the pixels for one row are simultaneously processed. Is called.

  Here, as the wiring for the unit pixel, three of the transfer wiring 26, the address wiring 28 and the reset wiring 29 in the horizontal direction, one of the vertical signal lines 27 in the vertical direction, the Vdd supply wiring, and the floating diffusion portion FD. And an internal wiring that connects the gate of the amplifying transistor 23 and a two-dimensional wiring that is not shown here and that is used as a light shielding film for shielding each pixel in the optical black region 18 from the pixel boundary portion.

  FIG. 3 is a cross-sectional view showing an example of a pixel structure in the effective pixel region 17. In FIG. 3, a photodiode 21 that is a photoelectric conversion element is formed on the surface layer portion of the semiconductor substrate 31, and a floating diffusion portion FD is formed in the vicinity thereof. On the surface of the semiconductor substrate 31 and above the channel region between the photodiode 21 and the floating diffusion portion FD, the gate electrode 33 of the transfer transistor 22 is formed via the gate insulating film 32. The pixels 16 are individually separated by the pixel separation layer 34.

  On the surface of the semiconductor substrate 31, metal wirings 36 are wired in, for example, two layers via an interlayer insulating film 35. The metal wiring 36 is formed avoiding the opening on the surface region (photoelectric conversion region) of the photodiode 21, and also serves as a light shielding film for the region outside the surface region of the photodiode 21. On the interlayer insulating film 35, an on-chip lens (OCL) 37 that collects incident light in the photoelectric conversion region is disposed. A group of the on-chip lenses 37 is a first on-chip lens group.

  In the pixel structure in the effective pixel region 17 having the above-described configuration, light incident on each pixel 16 is condensed on the photodiode 21 through the opening above the photodiode 21 by the condensing action of the on-chip lens 37. Then, the photodiode 21 photoelectrically converts it into a signal charge having a charge amount corresponding to the amount of incident light and accumulates it here. The accumulated signal charge is transferred to the floating diffusion portion FD by the transfer transistor 22.

  FIG. 4 is a cross-sectional view showing an example of the pixel structure in the optical black region 18. The pixel structure in the optical black region 18 is basically the same as the pixel structure in the effective pixel region 17. The only difference is the configuration of the on-chip lens 38 and the metal wiring 39.

  That is, the on-chip lens 38 in the optical black region 18 is formed so as to collect incident light outside the surface region (photoelectric conversion region) of the photodiode 21, preferably in the vicinity of the floating diffusion portion FD. A group of the on-chip lenses 38 becomes a second on-chip lens group. Of the two-layer wiring, the upper metal wiring 39 is also formed above the photodiode 21, and the optical black region 18 is realized by shielding the photodiode 21 from light.

  In the pixel structure in the optical black region 18 having the above configuration, light incident on each pixel 16 is collected by the on-chip lens 38. However, the condensed light is shielded by the metal wiring 39 that also serves as a light shielding film. However, for example, in a severe imaging state in which intense light is locally irradiated on the imaging surface, the light condensed by the on-chip lens 38 may pass through the metal wiring 39.

  However, since the on-chip lens 38 is formed so as to collect incident light outside the surface region (photoelectric conversion region) of the photodiode 21, for example, in the vicinity of the floating diffusion portion FD, the on-chip lens 38 collects the incident light. The light transmitted through the metal wiring 39 does not enter the photoelectric conversion region of the photodiode 21. Therefore, even if a part of the light is transmitted through the metal wiring 39 due to the strong light incident, a signal other than the dark current component is caused in the photodiode 21 of each pixel in the optical black region 18 due to the transmitted light. No charge is generated.

  When light transmitted through the metal wiring 39 enters the vicinity of the floating diffusion portion FD, photoelectric conversion is performed in the vicinity of the floating diffusion portion FD, and charges generated by the photoelectric conversion are accumulated. In general, in the CMOS image sensor, an operation of resetting the potential of the floating diffusion portion FD is performed by the reset transistor 25 before the signal charge is read from the photodiode 21 by the transfer transistor 22. Thus, the charges accumulated in the floating diffusion portion FD are also discarded. Therefore, even if the transmitted light is condensed on the floating diffusion portion FD, there is no problem in operation.

  The pixel structure in the effective pixel region 17 and the pixel structure in the optical black region 18 can be formed by basically the same process as in the conventional case. However, in the case of the present invention, in the optical black region 18, the on-chip lens 38 is formed because the incident light needs to be condensed outside the photoelectric conversion region of the photodiode 21. It is necessary to slightly change the mask pattern used at the time.

  In this case, when the incident light is condensed outside the photoelectric conversion region of the photodiode 21, the condensed light is set on the floating diffusion portion FD so that the collected light enters the photodiode 21. This can be surely prevented, and the floating diffusion portion FD is located near the photodiode 21, so that there is an advantage that a change amount when changing the mask pattern is small.

  As described above, for example, in the CMOS image sensor, with respect to the on-chip lens 38 in the optical black region 18, the incident light is not condensed on the photoelectric conversion region of the photodiode 21 but collected on a place other than the photoelectric conversion region. By forming it so as to emit light, even when particularly intense light is incident, even if part of the light is transmitted through the light shielding film, in this example, the metal wiring 39, the transmitted light is transmitted to the photoelectric conversion region of the photodiode 21. Since there is no incidence, no signal charge other than the dark current component is generated. As a result, it is possible to eliminate the unnaturalness of the captured image caused by the transmitted light, which has been a problem in the past.

  In the optical black region 18, the on-chip lens 38 that collects incident light at a place other than the photoelectric conversion region of the photodiode 21 does not necessarily have to be formed in the entire area of the optical black region 18. It suffices to form the region 18B. This is because the signal of each pixel in the invalid optical black region 18A is not used as an optical reference black level, and no problem occurs even if transmitted light is incident and signal charges other than dark current components are generated. Because. By forming the on-chip lens 38 only in the effective optical black region 18B, there is an advantage that a region for changing the mask pattern can be reduced in forming the on-chip lens 38.

  By the way, in the CMOS image sensor, in order to make a minor change to the existing CMOS process, as described above, the metal wiring 39 is used as the light shielding film. In this case, there is a concern that the light shielding ability is slightly inferior to using a dedicated light shielding film. However, even if the light shielding ability is somewhat lowered by using the metal wiring 39 as a light shielding film, the on-chip lens 38 is used to collect incident light at a place other than the photoelectric conversion region of the photodiode 21. Therefore, the decrease in the light shielding capability can be sufficiently covered, so that there is an advantage that the CMOS image sensor process can be close to the existing CMOS process.

  In the above embodiment, the case where the present invention is applied to a CMOS image sensor has been described as an example. However, the present invention is not limited to the application to a CMOS image sensor, and is applicable to all solid-state imaging devices such as a CCD image sensor. In any case, the on-chip lens in the optical black area is formed so that the incident light is not condensed on the photoelectric conversion area of the photodiode, but on a place other than the photoelectric conversion area. By doing so, the intended purpose can be achieved.

It is a block diagram which shows the outline of a structure of the CMOS image sensor which concerns on one Embodiment of this invention. It is a circuit diagram which shows an example of the circuit structure of a pixel. It is sectional drawing which shows an example of the pixel structure in an effective pixel area | region. It is sectional drawing which shows an example of the pixel structure in an optical black area | region.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 ... Imaging part, 12 ... Vertical scanning circuit, 14 ... Horizontal scanning circuit, 15 ... Horizontal output circuit, 16 ... Pixel, 17 ... Effective pixel area, 18 ... Optical black area, 18A ... Invalid optical black area, 18B ... Effective optical Black region, 21 ... photodiode, 22 ... transfer transistor, 23 ... amplification transistor, 24 ... address transistor, 25 ... reset transistor, 36,39 ... metal wiring, 37,38 ... on-chip lens, FD ... floating diffusion part

Claims (5)

An effective pixel region in which a pixel including a photoelectric conversion element is arranged, and a signal of the pixel is used as an effective signal;
An optical black region in which a pixel including a photoelectric conversion element is disposed in a light-shielded state, and a signal of the pixel is used as a black level reference;
A first on-chip lens group disposed on the effective pixel region in units of pixels and collecting incident light in a photoelectric conversion region of each pixel;
A solid-state imaging device, comprising: a second on-chip lens group that is arranged in units of pixels on the optical black region and collects incident light outside the photoelectric conversion region of each pixel. The optical black region is provided adjacent to the effective pixel region, and is provided adjacent to the invalid optical black region in which the signal of each pixel is not used as a black level reference, and the invalid optical black region. And an effective optical black area used as a black level reference,
The solid-state imaging device according to claim 1, wherein the second on-chip lens group is provided at least on the effective optical black region. 2. The solid according to claim 1, wherein each pixel in the effective pixel region and the optical black region has a floating diffusion portion that converts a signal charge photoelectrically converted by the photoelectric conversion element into a signal output. Image sensor. The solid-state imaging device according to claim 3, wherein the second on-chip lens group condenses incident light in the vicinity of the floating diffusion portion. The solid-state imaging device according to claim 3, wherein the wiring in the optical black region is used as a light shielding film that shields each pixel in the optical black region.

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