JP4250857B2 - Solid-state image sensor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a solid-state image sensor, and more particularly, to a solid-state image sensor provided with an overflow drain that sweeps excess charge generated in a photoelectric conversion unit.
[0002]
[Prior art]
FIG. 4 is a diagram showing a CCD solid-state imaging device using a CCD (charge coupled device) in the charge transfer portion. FIG. 4 shows a schematic configuration of an interline transfer type solid-state imaging device in which a photodiode and a vertical CCD register are separately provided.
[0003]
In FIG. 4, first, photodiodes 401 are arranged in rows on a semiconductor substrate 400, and a vertical CCD register 402 is provided corresponding to each photodiode row. A charge readout gate region 403 is formed between the photodiode 401 and the vertical CCD register 402. A horizontal CCD register 404 is provided at one end of the vertical CCD register 402, and a charge detection unit 405 and an output unit 406 are formed at one end of the horizontal CCD register 404. A portion surrounded by a broken line is a unit pixel 407.
[0004]
The charge photoelectrically converted by the photodiode 401 is transferred to the vertical CCD register 402 via the read gate region 403. The read charges are transferred to the horizontal CCD register 404 by the vertical CCD register 402, further transferred to the charge detection unit 405 by the horizontal CCD register 404, and output to the outside via the output unit 406.
[0005]
By the way, in such a solid-state imaging device using a CCD, an overflow drain mechanism for sweeping excess charges is provided for each pixel in order to suppress blooming. There are two overflow drain mechanisms, a vertical overflow drain and a horizontal overflow drain.
[0006]
FIG. 5 is a diagram showing a horizontal sectional structure of a unit pixel of an interline transfer type solid-state imaging device having a conventional vertical overflow drain. In FIG. 5, a P ⁻ type well 501 is provided on the surface of an N type semiconductor substrate 500 made of silicon. An N type photoelectric conversion unit 502 constituting the photodiode 401 is provided on the surface portion of the P ⁻ type well 501, and a P ⁺ type diffusion layer 503 for reducing dark current is provided on the surface thereof. ing. Further, an N-type diffusion layer 504 constituting the vertical CCD register 402 and a P-type diffusion layer 505 are provided in contact therewith. A charge readout gate region 506 is provided between the photoelectric conversion unit 502 and the corresponding N-type diffusion layer 504, and a P ⁺ -type element isolation layer 507 is provided between the photoelectric conversion unit 502 and the corresponding N-type diffusion layer 504. ing.
[0007]
Further, an insulating film 508 made of a silicon dioxide film, a silicon nitride film or the like is provided on the surface, and a transfer gate electrode of the vertical CCD register 402 made of a polycrystalline silicon film or the like is formed in an upper region of the N-type diffusion layer 504. 509 is provided. Further, a light shielding film 510 made of a tungsten film, an aluminum film, or the like is provided on the upper portion with an insulating film 508 interposed therebetween. Excess charge generated in the photoelectric conversion unit 502 beyond the storage capacity is swept out to the semiconductor substrate 500 through the P ⁻ type well 501. That is, the P ⁻ type well 501 serves as a barrier region for accumulated charges, and the semiconductor substrate 500 serves as a drain region for sweeping out excessive charges. C51 represents the coupling capacitance between the P ⁻ type well 501 and the photoelectric conversion unit 502, and C52 represents the coupling capacitance between the P ⁻ type well 501 and the semiconductor substrate 500.
[0008]
FIG. 6 is a diagram showing a horizontal sectional structure of a unit pixel of an interline transfer type solid-state imaging device having a horizontal overflow drain of a conventional example. In FIG. 6, first, an N-type photoelectric conversion unit 601 that constitutes a photodiode 401 is provided on the surface of a P-type semiconductor substrate 600 made of silicon, and a P for reducing dark current is provided on the surface. ^{A +} -type diffusion layer 602 is provided. Further, an N-type diffusion layer 603 constituting the vertical CCD register 402 is provided. A charge readout gate region 604 is provided between the photoelectric conversion unit 601 and the corresponding N-type diffusion layer 603. A drain region 605 is provided on the opposite side, and a barrier region 606 is provided between the photoelectric conversion unit 601 and the drain region 605. A P ⁺ type element isolation layer 607 is provided between the drain region 605 and the N type diffusion layer 603 on the opposite side.
[0009]
Further, an insulating film 608 made of a silicon dioxide film, a silicon nitride film or the like is provided on the surface, and a transfer gate electrode 609 of the vertical CCD register 402 made of a polycrystalline silicon film or the like is formed in a region above the N-type diffusion layer 603. Is provided. Further, a light shielding film 610 made of a tungsten film, an aluminum film, or the like is provided on the upper portion with an insulating film 608 interposed therebetween. Excess charge generated in the photoelectric conversion unit 601 beyond the storage capacity is swept out to the drain region 605 through the barrier region 606. C61 represents the coupling capacitance between the barrier region 606 and the photoelectric conversion unit 601, and C62 represents the coupling capacitance between the barrier region 606 and the drain region 605.
[0010]
[Problems to be solved by the invention]
The conventional solid-state imaging device has a problem that cannot be dealt with in either of FIGS. 5 and 6 if the amount of incident light is large. The reason will be described below. In the solid-state imaging device having the vertical overflow drain shown in FIG. 5 and the solid-state imaging device having the horizontal overflow drain shown in FIG. 6, charges are accumulated in the photoelectric conversion unit according to the incident light amount, and the incident light amount is small. Sometimes the accumulated charge increases in proportion to the amount of incident light. This region is called a linear region. When the amount of incident light increases and charges exceeding the storage capacity of the photoelectric conversion unit are generated, the excess charges are swept out to the drain region, and further increase in stored charges is suppressed. This region is called a saturation region. Even in the saturation region, the output is not constant, and gradually increases as the amount of incident light increases. This phenomenon is also described in, for example, IEEE Transaction on Electron Devices, Vol. 42, No. 4 (April 1995) pp. 652-655. In the saturation region, the accumulated charge amount increases linearly with the logarithm of the incident light amount.
[0011]
FIG. 7 is a graph showing the dependence of the amount of charge accumulated in the photoelectric conversion unit on the amount of incident light. The incident light amount a is a linear region, and when the incident light amount exceeds a, a saturated region is obtained. The maximum output in the linear region is A. However, the maximum output charge amount of the solid-state imaging device is limited by the minimum value among the maximum transfer charge amount of the vertical CCD register, the maximum transfer charge amount of the horizontal CCD register, and the maximum charge amount of the charge detection unit. Therefore, when the accumulated charge amount of the photoelectric conversion unit exceeds the maximum output charge amount B, a false signal such as a vertical stripe or a horizontal stripe appears on the reproduction screen.
[0012]
In order to solve this problem, the applicant of the present application has previously filed a method for expanding the dynamic range using an output signal in the saturation region as Japanese Patent Application No. 11-256,002. In this method, the dynamic range can be greatly expanded by using the output signal in the saturation region. That is, when only the linear region is used, the maximum amount of light that can be detected is a. However, if the output signal in the saturation region is also used, the maximum amount of light that can be detected is b. Therefore, the dynamic range can be greatly increased. .
[0013]
However, in FIG. 7, in the linear region, the amount of change in the accumulated charge is A with respect to the change a in the incident light amount, whereas in the saturation region, the amount of change in the accumulated charge is (B−A) with respect to the change in the incident light amount (b−a). Therefore, the amount of change in accumulated charge with respect to the change in the amount of incident light is smaller in the saturation region than in the linear region. On the other hand, since the output signal of the solid-state imaging device includes output fluctuation due to noise in addition to the output proportional to the amount of accumulated charge, the above method changes the accumulated charge in the saturation region with respect to fluctuations in the output signal due to noise. When the amount (B-A) is too small, there is a problem that a reproduced image has a poor SN ratio.
[0014]
The present invention has been made in view of the above-described conventional problems, and an object of the present invention is to increase the amount of change in accumulated charge with respect to the change in the amount of incident light in the saturation region, thereby improving the SN ratio of the reproduced image. The object is to provide a solid-state imaging device.
[0015]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides a solid-state imaging device including a photoelectric conversion unit that detects accumulated charges in a linear region and a saturation region as an output signal, and an overflow drain that sweeps out excess charges generated in the photoelectric conversion unit . A high concentration region having a higher impurity concentration than other regions is formed in a part of the photoelectric conversion portion on the overflow drain side .
[0016]
In the present invention, by forming a high concentration region having a higher impurity concentration than the other regions in a part of the photoelectric conversion portion on the overflow drain side, the change in accumulated charge can be increased with respect to the change in the amount of incident light in the saturation region. Even if the output signal in the saturation region is used, the SN ratio of the reproduced image can be improved.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. First, the principle of the present invention will be described. The amount of change in accumulated charge in the saturation region relative to the amount of incident light depends on the coupling capacity between the barrier region and the photoelectric conversion unit and the ratio between the coupling capacity between the barrier region and the drain region. The amount of change in charge increases.
[0018]
The reason for this can be explained as follows. First, when the potential barrier height (potential difference between the barrier region and the photoelectric conversion unit) rapidly decreases when the potential of the photoelectric conversion unit fluctuates due to an increase in the accumulated charge amount, a large amount of charge generated in the photoelectric conversion unit is discharged, The increase in the amount of accumulated charge is small. On the contrary, if the height of the potential barrier does not decrease so much, the discharge amount of the charge generated in the photoelectric conversion unit decreases, and the increase amount of the accumulated charge amount increases. At this time, if the coupling capacity between the barrier region and the photoelectric conversion unit is large, the potential of the barrier region also fluctuates according to the fluctuation of the potential of the photoelectric conversion unit, and the height of the potential barrier does not decrease so much. Becomes larger.
[0019]
Therefore, the larger the coupling capacitance C51 with respect to the coupling capacitance C52 in FIG. 5, the larger the amount of change in accumulated charge. The larger the coupling capacitance C61 with respect to the coupling capacitance C62 in FIG. Therefore, if the coupling capacitances C51 and C61 are increased, the amount of change in the accumulated charge can be increased, so that the S / N ratio of the reproduced image can be improved. In the present invention, in order to increase the coupling capacitance C51 or C61 based on such a principle, a high concentration region having a higher impurity concentration than other regions is formed in a part of the photoelectric conversion portion. Therefore, a specific embodiment of the present invention will be described.
[0020]
FIG. 1 is a view for explaining a horizontal sectional structure of a unit pixel of an interline transfer type solid-state imaging device having a vertical overflow drain according to the first embodiment of the present invention. In FIG. 1, first, a P ⁻ type well 501 is provided on the surface of an N type semiconductor substrate 500 made of silicon. On the surface portion of the P ⁻ type well 501, an N type photoelectric conversion unit 502 constituting the photodiode 401 is provided. The photoelectric conversion portion 502 is formed of a high concentration region 101 having a high impurity concentration and a low concentration region 102 having a low impurity concentration.
[0021]
A P ⁺ -type diffusion layer 503 for reducing dark current is provided on the surface of the photoelectric conversion unit 502. Further, an N-type diffusion layer 504 constituting the vertical CCD register 402 and a P-type diffusion layer 505 are provided in contact therewith. A charge readout gate region 506 is provided between the photoelectric conversion unit 502 and the corresponding N-type diffusion layer 504, and a P ⁺ -type element isolation layer 507 is provided between the photoelectric conversion unit 502 and the corresponding N-type diffusion layer 504. ing. An insulating film 508 made of a silicon dioxide film, a silicon nitride film or the like is provided on the surface, and a transfer gate electrode 509 of the vertical CCD register 402 made of a polycrystalline silicon film or the like is formed in an upper region of the N-type diffusion layer 504. Is provided. Further, a light shielding film 510 made of a tungsten film, an aluminum film, or the like is provided on the upper portion with an insulating film 508 interposed therebetween.
[0022]
Excess charge generated in the photoelectric conversion unit 502 beyond the storage capacity is swept out to the semiconductor substrate 500 through the P ⁻ type well 501. That is, the P ⁻ type well 501 serves as a barrier region for accumulated charges, and the semiconductor substrate 500 serves as a drain region for sweeping out excessive charges. C51 represents the coupling capacitance between the P ⁻ type well 501 and the photoelectric conversion unit 502, and C52 represents the coupling capacitance between the P ⁻ type well 501 and the semiconductor substrate 500. In the present embodiment, since the high concentration region 101 is provided on the semiconductor substrate 500 side of the photoelectric conversion unit 502, the coupling capacitance C51 between the P ⁻ type well 501 and the photoelectric conversion unit 502 is increased, and the amount of change in accumulated charge is increased. it can.
[0023]
Here, for example, the size of the photodiode is 2 μm × 2 μm, the concentration of the N-type semiconductor substrate is 2.0E14, the concentration of the P ⁻ -type well is 1.5E15, and the depth is 3.5 μm. Further, the concentration of the P ⁺ -type diffusion layer is 1.0E18, the depth is 0.3 μm from the surface, the concentration of the photoelectric conversion portion is 8.0E15, and the depth is 0.3 μm to 2.0 μm. As a result of simulation for this structure, the amount of change in the accumulated charge with respect to the change in the amount of incident light in the saturation region was 14000 electrons per 10-fold increase in the amount of incident light.
[0024]
On the other hand, when the photoelectric conversion part is formed in a high concentration region having a concentration of 1.5E16 and a depth of 1.5 μm to 2.0 μm and in a concentration region of 2.0E15 and a low concentration region having a depth of 0.3 μm to 1.5 μm, The amount of change in the accumulated charge with respect to the change in the amount of incident light was 28000 electrons per 10-fold increase in the amount of incident light. Therefore, by configuring the photoelectric conversion unit 502 from two regions, the high concentration region 101 on the semiconductor substrate 500 side and the low concentration region 102 on the surface side, the amount of change in the accumulated charge in the saturation region is doubled, and the reproduced image The SN ratio can be improved by a factor of two.
[0025]
FIG. 2 is a diagram for explaining a horizontal sectional structure of a unit pixel of an interline transfer solid-state imaging device having a horizontal overflow drain according to a second embodiment of the present invention. In FIG. 2, first, an N-type photoelectric conversion unit 601 that constitutes a photodiode 401 is provided on a surface portion of a P-type semiconductor substrate 600 made of silicon. The photoelectric conversion unit 601 is formed of a high concentration region 201 having a high impurity concentration on the barrier region 606 side and a low concentration region 202 having a low impurity concentration on the opposite side. A P ⁺ -type diffusion layer 602 for reducing dark current is provided on the surface of the photoelectric conversion unit 601.
[0026]
Further, an N-type diffusion layer 603 constituting the vertical CCD register 402 is provided. A charge readout gate region 604 is provided between the photoelectric conversion unit 601 and the corresponding N-type diffusion layer 603. A drain region 605 is provided on the opposite side, and a barrier region 606 is provided between the photoelectric conversion unit 601 and the drain region 605. A P ⁺ type element isolation layer 607 is provided between the drain region 605 and the N type diffusion layer 603 on the opposite side.
[0027]
Further, an insulating film 608 made of a silicon dioxide film, a silicon nitride film or the like is provided on the surface, and a transfer gate electrode of the vertical CCD register 402 made of a polycrystalline silicon film or the like is formed in an upper region of the N-type diffusion layer 603. 609 is provided. Further, a light shielding film 610 made of a tungsten film, an aluminum film, or the like is provided on the upper portion with an insulating film 608 interposed therebetween. Excess charge generated in the photoelectric conversion unit 601 beyond the storage capacity is swept out to the drain region 605 through the barrier region 606. C61 represents the coupling capacitance between the barrier region 606 and the photoelectric conversion unit 601, and C62 represents the coupling capacitance between the barrier region 606 and the drain region 605. In this embodiment, since the impurity concentration of a part of the photoelectric conversion unit 601 on the side in contact with the barrier region 606 is increased, the coupling capacitance C61 between the barrier region 606 and the photoelectric conversion unit 601 can be increased, and the amount of change in accumulated charge Can be increased.
[0028]
Here, for example, the size of the photodiode is 2 μm × 2 μm, the concentration of the P-type semiconductor substrate is 1.0E17, the concentration of the P ⁺ -type diffusion layer is 1.0E18, the depth is 0.3 μm from the surface, and the barrier region 606 The density is 1.0E15 and the width is 1.5 μm. Further, the density of the photoelectric conversion portion 601 is 2.0E16, and the depth is 0.3 μm to 2.0 μm. As a result of performing a simulation on this structure, the amount of change in accumulated charge with respect to the change in incident light amount in the saturation region was 1200 electrons per 10-fold increase in incident light amount.
[0029]
On the other hand, when the photoelectric conversion unit 601 is formed in a high-concentration region having a density of 6.0E16 and a width of 0.5 μm and a low-concentration region having a density of 1.0E16 and a width of 1.5 μm, The amount of change was 1600 electrons per 10-fold increase in the amount of incident light. Therefore, by constituting the photoelectric conversion unit from two regions of the high concentration region on the drain region side and the low concentration region on the opposite side, the amount of change in the accumulated charge in the saturation region is 1.3 times, and the SN of the reproduced image is increased. The ratio can be improved 1.3 times.
[0030]
FIG. 3 is a graph showing the dependence of the amount of charge accumulated in the photoelectric conversion unit of the solid-state imaging device having an overflow drain on the amount of incident light. The broken line indicates the dependence of the accumulated charge amount of the conventional solid-state imaging device, and the solid line indicates the dependence of the accumulated charge amount of the solid-state imaging device according to the present invention. The accumulated charge amount according to the present invention is a linear region up to the incident light amount a ′, and becomes a saturated region when the incident light amount exceeds a ′. The maximum output in the linear region is A ′.
[0031]
In the saturation region, the amount of change in the accumulated charge is (B-A ') with respect to the change in the amount of incident light (b−a ′), and the ratio of the amount of change in the accumulated charge to the amount of incident light is larger than in the conventional solid-state imaging device. You can see that Therefore, it is possible to improve the SN ratio of the reproduced image of the image device using the output signal in the saturation region.
[0032]
In the above embodiment, the interline transfer type solid-state imaging device has been described as an example. However, the present invention is not limited to this, and a linear type, frame transfer type, full frame transfer type, or frame interline transfer type CCD is used. It can also be used for a solid-state image sensor or a CMOS solid-state image sensor. In the embodiment, the case where the photoelectric conversion unit is separate from the charge transfer unit is taken as an example, but the present invention can also be used for a solid-state imaging device having a configuration in which the photoelectric conversion unit also serves as the charge transfer unit.
[0033]
【The invention's effect】
As described above, according to the present invention, a high concentration region having a higher impurity concentration than other regions is formed in a part of the photoelectric conversion unit on the overflow drain side. The change in accumulated charge can be increased, and the SN ratio of the reproduced image can be greatly improved even when the output signal in the saturation region is used.
[Brief description of the drawings]
FIG. 1 is a diagram showing a cross-sectional structure of a unit pixel of a solid-state imaging device having a vertical overflow drain according to a first embodiment of the present invention.
FIG. 2 is a diagram showing a cross-sectional structure of a unit pixel of a solid-state imaging device having a horizontal overflow drain according to a second embodiment of the present invention.
FIG. 3 is a diagram comparing the relationship between the amount of incident light and the amount of accumulated charge of a solid-state imaging device according to the present invention and the related art.
FIG. 4 is a diagram showing a conventional interline transfer type solid-state imaging device.
FIG. 5 is a diagram illustrating a cross-sectional structure of a unit pixel of a solid-state imaging device including a conventional vertical overflow drain.
FIG. 6 is a diagram showing a cross-sectional structure of a unit pixel of a solid-state imaging device having a horizontal overflow drain of a conventional example.
FIG. 7 is a diagram showing the relationship between the amount of incident light and the amount of accumulated charge in a solid-state imaging device having a conventional overflow drain.
[Explanation of symbols]
101, 201 High concentration region 102, 202 Low concentration region 400 Semiconductor substrate 401 Photodiode 402 Vertical CCD register 403 Read gate region 404 Horizontal CCD register 405 Charge detection unit 406 Output unit 407 Unit pixel 500 Semiconductor substrate 501 P ^- type well 502 Photoelectric Conversion unit 503 P ⁺ type diffusion layer 504 N type diffusion layer 505 P type diffusion layer 506 Read gate region 507 P ⁺ type element isolation layer 508 Insulating layer 509 Transfer gate electrode 510 Light shielding film 600 Semiconductor substrate 601 Photoelectric conversion unit 602 P ⁺ type Diffusion layer 603 N-type diffusion layer 604 Read gate region 605 Drain region 606 Barrier region 607 P ⁺ type element isolation layer 608 Insulating layer 609 Transfer gate electrode 610 Light shielding film

Claims (7)

In a solid-state imaging device including a photoelectric conversion unit that detects accumulated charge in a linear region and a saturation region as an output signal, and an overflow drain that sweeps out excess charge generated in the photoelectric conversion unit ,
A solid-state imaging device, wherein a high concentration region having a higher impurity concentration than other regions is formed in a part of the photoelectric conversion portion on the overflow drain side .   The said overflow drain consists of a vertical overflow drain, The high concentration area | region where an impurity concentration is higher than another area | region was formed in a part by the side of a semiconductor substrate among the said photoelectric conversion parts. Solid-state image sensor.   2. The overflow drain according to claim 1, wherein the overflow drain is a lateral overflow drain, and a high concentration region having a higher impurity concentration than other regions is formed in a part of the photoelectric conversion portion on the drain region side. Solid-state image sensor.   4. The solid-state imaging device according to claim 1, wherein the photoelectric conversion unit and the charge transfer unit are provided separately. 5.   The solid-state imaging device according to claim 1, wherein the photoelectric conversion unit also serves as a charge transfer unit.   6. The solid-state imaging device according to claim 1, wherein the solid-state imaging device is any one of a linear type, an interline transfer type, a frame transfer type, a full frame transfer type, and a frame interline type. Solid-state image sensor.   The solid-state imaging device according to claim 1, wherein the solid-state imaging device is a CMOS solid-state imaging device.

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