JP4761491B2 - Solid-state imaging device and imaging system using the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a solid-state imaging device used in a digital imaging system or the like.
[0002]
[Prior art]
Conventionally, a CCD is often used as a solid-state imaging device because of its good SN ratio. On the other hand, so-called amplification type solid-state imaging devices have been developed which have the advantages of low power consumption and ease of use. An amplification type solid-state imaging device is a device that guides signal charges accumulated in a photodiode to a control electrode of a transistor provided in a pixel, amplifies an output corresponding to the amount of signal charge from the main electrode of the transistor, and outputs the amplified signal. . In particular, a so-called CMOS sensor using a MOS transistor as a transistor has a good matching with the CMOS process, and the drive circuit and the signal processing circuit can be made on-chip, and therefore, development is focused on.
[0003]
FIG. 4 is a circuit diagram showing a typical example of a CMOS sensor pixel. 1 is a unit pixel, 2 is a photodiode for accumulating signal charges generated by incident light, 3 is an amplifying MOS transistor serving as an amplifying means for outputting an amplified signal corresponding to the amount of signal charges, and 4 receives signal charges. The floating diffusion portion 5 connected to the gate electrode of the MOS transistor 3 is a MOS transistor as transfer means for transferring the signal charge accumulated in the photodiode 2 to the floating diffusion portion 4, and 6 is for resetting the floating diffusion portion 4. The MOS transistor 7 is a MOS transistor for selecting an output pixel.
[0004]
Reference numeral 8 is a control line for applying a pulse to the gate of the MOS transistor 5 to control the charge transfer operation, 9 is a control line for applying a pulse to the gate of the MOS transistor 6 to control the reset operation, A control line 11 for applying a pulse to the gate of the MOS transistor 7 to control the selection operation, 11 is a power supply wiring, which is connected to the drain of the amplifying MOS transistor 3 and the drain of the reset MOS transistor 6 and supplies power to them. Electric potential is supplied. Reference numeral 12 denotes an output line through which an amplified signal of the selected pixel is output, 13 operates as a constant current source, a MOS transistor that forms an amplification MOS transistor 3 and a source follower, and 14 operates so that the MOS transistor 13 operates at a constant current. This is a wiring for supplying a potential to the gate electrode of the MOS transistor 13.
[0005]
The pixels 1 arranged in a two-dimensional matrix form a pixel region of a two-dimensional solid-state imaging device. In the matrix configuration, the output line 12 is a common line of pixels in each column, control lines 8, 9, Reference numeral 10 denotes a common line for pixels in each row, and only the pixels in the row selected by the control line 10 are signal-outputted to the output line 12.
[0006]
Next, the operation of the pixel will be briefly described. For the pixels in the row where the selection MOS transistor is turned on by the control line 10, a pulse is first applied to the control line 9, and the floating diffusion portion 4 is reset. Since the amplifying MOS transistor 3 and the constant current MOS transistor 13 form a source follower, an output potential corresponding to the reset potential appears on the output line 12. Next, when the signal charge accumulated in the photodiode 2 is transferred to the floating diffusion portion 4 by applying a pulse to the control line 8, the potential of the floating diffusion portion 4 is changed by a voltage corresponding to the signal charge amount. The potential change appears on the output line 12. Since the reset potential appearing on the output line 12 includes noise such as variations in threshold voltage values of the amplifying MOS transistor 3 and reset noise when the floating diffusion portion 4 is reset, the potential change corresponding to the signal charge amount Minute is a signal that does not contain noise.
[0007]
In the two-dimensional CMOS sensor, a readout circuit for removing this noise and taking out only a signal is connected to the output line 12. In this readout circuit, the above-mentioned noise is removed by a clamp circuit, and the noise and noise + pure signal are held separately, and the noise is eliminated by guiding them to the differential amplifier at the final stage during horizontal scanning readout, etc. Several configurations have been proposed, but they are not directly related to the present invention and will not be described in detail.
[0008]
Next, FIG. 5 shows a cross-sectional structure of a pixel photodiode, a transfer MOS transistor, and a floating diffusion. In the figure, 15 is an N-type semiconductor substrate, 16 is a P-type well, 17 is an N-type semiconductor region formed in the well 16, and a photodiode is formed by 16 and 17. Signal charge is accumulated. Reference numeral 18 denotes an N-type semiconductor region formed in the well 16 and serves as a floating diffusion. Reference numeral 19 denotes a gate electrode of the transfer MOS transistor 5. Reference numerals 17 and 18 denote a source region and a drain region of the transfer MOS transistor. Become. A wiring 20 is connected to the region 18 and is connected to the gate of the amplification MOS transistor 3. The signal charges accumulated in the region 17 are all transferred to the floating diffusion 18 during the transfer operation, and the N-type impurity concentration in the region 17 is set so that the region 17 is completely depleted immediately after the transfer. Therefore, signal charge accumulation in the photodiode starts immediately after signal charge transfer, and when signal charge accumulation ends, the row to which the pixel belongs is selected again, and the signal charge accumulated in the photodiode is transferred.
[0009]
[Problems to be solved by the invention]
However, in the above conventional example, rows for reading are sequentially selected. Since the timing for starting and ending the accumulation of the photodiodes in the pixel is determined at the time of signal charge transfer, the timing for starting and ending the accumulation of the photodiodes is shifted for each row. In the two-dimensional solid-state imaging device, a signal of one screen, that is, one field is formed by pixel signals of all rows, so that the shape of a subject moving at a high speed is distorted when the accumulation timing for each row is shifted. was there.
[0010]
Accordingly, an object of the present invention is to provide a solid-state imaging device capable of obtaining a distortion-free image even with a moving subject by enabling the start and end timings of the signal accumulation operation simultaneously for all pixels.
[0011]
[Means for Solving the Problems]
A solid-state imaging device according to a first aspect of the present invention includes a photodiode for accumulating signal charges generated by incident light, a transfer gate for transferring the signal charges, and a floating diffusion to which the signal charges are transferred. In the solid-state imaging device configured by arranging a plurality of unit pixels, each of which includes a region and an amplifying means for outputting a signal amplified based on the signal charge transferred to the floating diffusion region, the transfer gate A signal charge holding region for holding the signal charge provided below, and a potential barrier unit for providing a potential barrier between the signal charge holding region and the floating diffusion region. The first and second wells of the second conductivity type opposite to the first conductivity type are arranged in the semiconductor region of the type The first conductivity type first semiconductor region constituting the photodiode in the first well and the first conductivity type second semiconductor constituting the signal charge holding region in the second well. An area is arranged.
A solid-state imaging device according to a second aspect of the present invention includes a photodiode having a first semiconductor region of a first conductivity type capable of storing a signal charge, a transfer gate for transferring the signal charge of the photodiode, A second semiconductor region of a first conductivity type that provides a signal charge holding region disposed under the transfer gate; a floating diffusion region to which charges in the signal charge holding region are transferred; the signal charge holding region; A plurality of unit pixels each including a potential barrier portion that provides a potential barrier between the floating diffusion region and amplification means for outputting a signal amplified based on the signal charge transferred to the floating diffusion region A first first of a second conductivity type that is arranged and constitutes the photodiode together with the first semiconductor region; A second well of a second conductivity type disposed under the second semiconductor region, and the impurity concentration of the second well is higher than that of the first well. And
A solid-state imaging device according to a third aspect of the present invention includes a photodiode having a first semiconductor region of a first conductivity type capable of storing a signal charge, a transfer gate for transferring the signal charge of the photodiode, A second semiconductor region of a first conductivity type that provides a signal charge holding region disposed under the transfer gate; a floating diffusion region to which charges in the signal charge holding region are transferred; the signal charge holding region; A plurality of unit pixels each including a potential barrier portion that provides a potential barrier between the floating diffusion region and amplification means for outputting a signal amplified based on the signal charge transferred to the floating diffusion region A first first of a second conductivity type that is arranged and constitutes the photodiode together with the first semiconductor region; A well and a second well of a second conductivity type disposed below the second semiconductor region, and the first well is formed deeper than the second well. It is characterized by.
[0012]
In a pixel configuration of an active pixel type CMOS sensor that performs signal amplification, a signal charge holding region having a low potential for holding signal charges accumulated by a photodiode is provided in a part of a channel portion under a signal charge transfer gate. A potential barrier portion made of a semiconductor region that provides a potential barrier and is fully depleted is provided between the floating diffusion region and the holding region. The signal charge holding region can perform operations such as signal charge transfer from the photodiode, signal charge holding, and signal charge transfer to the floating diffusion region by transfer gate potential control. In the above configuration, if the signal charge transfer from the photodiode to the charge holding region is performed simultaneously for all the pixels, and the charge transfer from the charge holding region to the floating diffusion portion is performed line-sequentially as before, the accumulation timing of the photodiode is all Since the same applies to the pixels, an image without distortion can be obtained even if a subject moving at high speed is imaged.
[0013]
It is also preferable to provide a light shielding means for blocking light incident on the holding region.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Next, embodiments of the present invention will be described with reference to the drawings.
[0015]
(First embodiment)
FIG. 1 is a cross-sectional view of a part of a pixel that best represents the features of the present invention. A floating diffusion portion connected to an input node of a photodiode, a charge transfer MOS transistor as transfer means, and an amplification means (not shown). The same members as those in FIG. 4 are denoted by the same reference numerals, and the description thereof is omitted. In FIG. 1, reference numeral 21 denotes an N-type semiconductor region serving as a signal charge holding portion formed in a channel portion immediately below the transfer gate 19, and 22 is formed between the charge holding region 21 and the floating diffusion region 18. The N-type semiconductor region 23 whose impurity concentration is controlled so as to be completely depleted when the potential of the region 18 is sufficiently reverse-biased with respect to the P-type well 16 blocks light incidence on the charge holding region 21. It is the light shielding layer formed in this way.
[0016]
In (a), (b), and (c), how the potential for electrons in the portion from the region 17 through the charge transfer channel portion, the region 21 and the region 22 to the floating diffusion region 18 changes depending on the potential of the gate electrode 19. It is shown. Since electrons are negatively charged, the lower the potential, the higher the potential for the electrons.
[0017]
Next, the operation of the present embodiment will be described. The operation of one pixel is as follows: (1) signal charge accumulation in the photodiode, (2) signal charge accumulated in the photodiode is transferred to the charge holding region 21, (3) signal charge holding in the charge holding region 21; (4) Signal charge transfer from the charge holding region 21 to the floating diffusion region 18 is fundamental.
[0018]
Since the operation of (1) is the same as the conventional one, the description is omitted.
[0019]
(2) The potential during the operation of transferring the signal charge accumulated in the photodiode to the charge holding region 21 is shown in FIG. 1A. At this time, a high potential VH is applied to the transfer gate electrode 19. Has been. The channel potential under the transfer gate that separates the region 17 and the region 21 is determined such that the value of VH is sufficiently higher than the potential of the region 17, and the signal charge accumulated in the photodiode is transferred to the charge holding region 21. Is done. At this time, the potential of the floating diffusion region 18 is set to a high potential so that the region 22 is completely depleted, and the charge transferred to the charge holding region 21 is caused by the potential barrier in the region 22 because of the potential barrier. It does not flow to 18.
[0020]
(3) The operation moves to the operation of holding the signal charge in the charge holding region 21. In this operation, the potential of the transfer gate electrode 19 is set to VM lower than VH, and the potential at that time is shown in FIG. At this time, the value of VM is determined so that the channel potential under the transfer gate separating the region 17 and the region 21 is lower than the potential of the region 17, but the potential of the charge holding region 21 is higher than the potential of the region 22. . The signal charge in the charge holding region 21 is isolated by the potential barriers existing at both ends thereof, and electrons generated by photoexcitation are also prevented from entering the region 21 because of the light shielding layer 23. Can operate.
[0021]
(4) The operation proceeds to signal charge transfer from the charge holding region 21 to the floating diffusion region 18. In this operation, the potential of the transfer gate electrode 19 is set to VL lower than VM, and the potential at that time is shown in FIG. At this time, the value of VL is determined so that the potential of the charge holding region 21 is lower than the potential of the region 22. In this state, there is no potential barrier between the charge holding region 21 and the floating diffusion region 18, and the signal charge in the charge holding region 21 is transferred to the floating diffusion region 18.
[0022]
If a two-dimensional sensor is formed by pixels having the above-described configuration and operation, the operation of “transferring the signal charge accumulated in the photodiode to the charge holding region 21” can be performed simultaneously for all the pixels. Since the start and end timings of the accumulation operation are the same for all pixels, an image without distortion can be obtained. As can be seen from FIG. 1, since the floating diffusion region 18 is not in contact with the transfer gate 19, the total parasitic capacitance of the floating diffusion portion 4 does not include the capacitance with the transfer gate as in the conventional configuration. As a result, the total parasitic capacitance of the floating diffusion portion can be reduced as compared with the conventional case, and there is an effect that the charge conversion coefficient is large and thus the sensitivity is increased.
[0023]
(Second Embodiment)
FIG. 2 is a cross-sectional view of a part of a pixel showing a second embodiment of the present invention, in which a photodiode, a charge transfer MOS transistor as transfer means, and a floating connected to an input node of an amplification means (not shown). It shows the structure of the diffusion part, and the same members as those in FIG. In FIG. 2, reference numeral 24 denotes a first P-type well, and 25 denotes a second P-type well. The photodiode 17 is formed in 24, and the charge holding region 21 is formed in 25.
[0024]
If charge generated by incident light near the photodiode enters the charge holding region 21 during the holding operation, it becomes a false signal. Therefore, if the P-type well 25 in which the charge holding region is formed is configured to have a high impurity concentration, a high potential barrier against electrons can be formed and a false signal can be suppressed.
[0025]
On the other hand, since the P-type well 24 wants to collect charges generated by incident light in the photodiode 17 as much as possible, it is preferable that the P-type well 24 has a deep and low impurity concentration. As described above, in the second embodiment, two P-type wells having different structures are formed according to the characteristics of the photodiodes and the charge holding regions having different purposes. As a result, it is possible to realize a sensor pixel in which the start and end timing of the signal accumulation operation can be performed simultaneously for all the pixels, the sensitivity is high, and the false signal generation in the charge holding region is small.
[0026]
In the first and second embodiments, the fully depleted regions such as the photodiode 17 and the potential barrier region 22 are buried type, that is, a diode having a surface layer of the same conductivity type as the well. May be. The charge holding region 21 may be a bulk channel type, that is, formed in a semiconductor bulk in which the charge holding region is slightly separated from the interface during the holding operation.
[0027]
(Third embodiment)
Next, an imaging system using the solid-state imaging device of the first and second embodiments will be described.
[0028]
FIG. 3 is a block diagram showing an example in which the solid-state imaging device according to the present invention is applied to a “still video camera”. Reference numeral 101 denotes a barrier that serves as a lens switch and a main switch, 102 denotes a lens that forms an optical image of a subject on the solid-state imaging device 104, 103 denotes a diaphragm for changing the amount of light passing through the lens 102, and 104 denotes a lens 102. A solid-state imaging device for capturing an imaged subject as an image signal, 106 is an A / D converter that performs analog-digital conversion of an image signal output from the solid-state imaging device 104 via the imaging signal processing circuit 105, 107 is The signal processing unit performs various corrections on the image data output from the A / D converter 106 and compresses the data. A timing generator 108 outputs various timing signals to the solid-state imaging device 104, the imaging signal processing circuit 105, the A / D converter 106, and the signal processor 107, and 109 controls various operations and the entire still video camera. An overall control / arithmetic unit, 110 a memory unit for temporarily storing image data, 111 an interface unit for recording or reading on a recording medium, and 112 a semiconductor memory for recording or reading image data A removable recording medium such as 113 is an interface unit for communicating with an external computer or the like.
[0029]
Next, the operation of the still video camera at the time of shooting in the above configuration will be described.
[0030]
When the barrier 101 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 106 is turned on.
[0031]
Then, in order to control the exposure amount, the overall control / arithmetic unit 109 opens the diaphragm 103, and the signal output from the solid-state imaging device 104 is converted by the A / D converter 106 and then sent to the signal processing unit 107. Entered. Based on this data, exposure calculation is performed by the overall control / calculation unit 109.
[0032]
The brightness is determined based on the result of the photometry, and the overall control / calculation unit 109 controls the aperture according to the result.
[0033]
Next, based on the signal output from the solid-state image sensor 104, the high-frequency component is extracted and the distance to the subject is calculated by the overall control / calculation unit 109. 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.
[0034]
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 image sensor 104 is A / D converted by the A / D converter 106 and is written to the memory unit by the overall control / calculation 109 through the signal processing unit 107. Thereafter, the data stored in the memory unit 110 is recorded on a removable recording medium 112 such as a semiconductor memory through the recording medium control I / F unit under the control of the overall control / arithmetic unit 109. Alternatively, the image may be processed by directly entering a computer or the like through the external I / F unit 113.
[0035]
【The invention's effect】
As described above, according to the present invention, since the start and end timing of the signal accumulation operation can be performed simultaneously for all pixels, an image without distortion can be obtained even with a moving subject. Moreover, since the parasitic capacitance of the floating diffusion portion is small, high sensitivity can be obtained.
[0036]
Further, if a photodiode is formed in the first well, a signal charge holding region is formed in the second well, and the first well has a lower impurity concentration and a deeper depth than the second well. In addition to the above effects, it is possible to realize a sensor with higher sensitivity and less generation of false signals.
[Brief description of the drawings]
FIG. 1 is a cross-sectional structure diagram of a pixel for explaining a first embodiment of the present invention.
FIG. 2 is a cross-sectional structure diagram of a pixel for explaining a second embodiment of the present invention.
FIG. 3 is a block diagram showing an example in which the solid-state imaging device of the present invention is applied to a still video camera.
FIG. 4 is a circuit diagram of a conventional pixel.
FIG. 5 is a cross-sectional structure diagram of a conventional pixel.
[Explanation of symbols]
1: Pixel 2: Photodiode 3: Amplifying MOS transistor 4: Floating diffusion part 5: Signal charge MOS transistor 6: Reset MOS transistor 7: Selection MOS transistor 8: Gate control line 9: Gate control line 10: Gate control line 11: Power supply line 12: Pixel output line 13: Constant current supply MOS transistor 14: Gate control line 15: Semiconductor substrate 16: Semiconductor well 17: Photodiode region 18: Floating diffusion region 19: Transfer gate 20: Wiring 21: Charge holding region 22: Complete depletion region 23: Light shielding layer 24: First well 25: Second well

Claims (11)

A photodiode for accumulating signal charges generated by incident light;
A transfer gate for transferring the signal charge;
A floating diffusion region to which the signal charge is transferred;
In a solid-state imaging device configured by arranging a plurality of unit pixels, including an amplifying means for outputting a signal amplified based on the signal charge transferred to the floating diffusion region,
A signal charge holding region for holding the signal charge provided under the transfer gate;
A potential barrier portion that provides a potential barrier between the signal charge holding region and the floating diffusion region;
First and second wells of the second conductivity type opposite to the first conductivity type are arranged in the semiconductor region of the first conductivity type, and the photodiode is configured in the first well. A solid-state imaging device, wherein the first conductive type first semiconductor region is provided with the first conductive type second semiconductor region constituting the signal charge holding region in the second well. .   The solid-state imaging device according to claim 1, wherein the potential barrier portion is completely depleted in a state where the signal charge is held in the signal charge holding region.   The solid-state imaging device according to claim 1, further comprising a light shielding layer for blocking incident light to the signal charge holding region.   The solid-state imaging device according to claim 1, wherein the first well has a lower impurity concentration and a deeper depth than the second well. An imaging optical system for imaging light from a subject;
The solid-state imaging device according to any one of claims 1 to 4, which photoelectrically converts the formed image;
An image pickup system comprising: a signal processing circuit for digitally converting and processing an output signal from the solid-state image pickup device. Provided is a photodiode having a first semiconductor region of a first conductivity type capable of storing signal charge, a transfer gate for transferring signal charge of the photodiode, and a signal charge holding region disposed under the transfer gate potential barrier for providing a second semiconductor region of a first conductivity type, a floating diffusion region charge of the signal charge holding region is transferred, a signal charge holding region the potential barrier between said floating diffusion region A plurality of unit pixels are arranged, and an amplifying means for outputting a signal amplified based on the signal charge transferred to the floating diffusion region,
A second conductivity type first well that constitutes the photodiode together with the first semiconductor region; and a second conductivity type second well disposed below the second semiconductor region;
The solid-state imaging device, wherein an impurity concentration of the second well is higher than that of the first well.   The solid-state imaging device according to claim 6, wherein the first well is formed to a position deeper than the second well. Provided is a photodiode having a first semiconductor region of a first conductivity type capable of storing signal charge, a transfer gate for transferring signal charge of the photodiode, and a signal charge holding region disposed under the transfer gate potential barrier for providing a second semiconductor region of a first conductivity type, a floating diffusion region charge of the signal charge holding region is transferred, a signal charge holding region the potential barrier between said floating diffusion region A plurality of unit pixels are arranged, and an amplifying means for outputting a signal amplified based on the signal charge transferred to the floating diffusion region,
A second conductivity type first well that constitutes the photodiode together with the first semiconductor region; and a second conductivity type second well disposed below the second semiconductor region;
The solid-state imaging device, wherein the first well is formed to a position deeper than the second well.   The solid-state imaging device according to claim 8, wherein the impurity concentration of the second well is higher than that of the first well.   The solid-state imaging device according to claim 1, wherein the photodiode is an embedded photodiode. The solid-state imaging device according to claim 1, wherein the signal charge holding region is a bulk channel type.

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