JP2001094880A - Solid-state image pickup device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain fine picture quality by suppressing the fluctuation of saturation when a dynamic range is extended by a non-linear characteristic after saturating an image pickup element. SOLUTION: An image including saturation fluctuation is acquired by transferring charge to a vertical transfer register in a charge transfer period after saturating a photodiode by inversely injecting charge from a semiconductor substrate in a solid-state image pickup element into the photodiode by dropping voltage to be applied to the substrate in a charge inverse injection period and a difference between respective acquired images and the mean value of respective acquired images is added to a picture signal to suppress the saturation fluctuation.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid-state imaging device, and more particularly to a solid-state imaging device for suppressing a variation in saturation when a dynamic range is expanded by using a nonlinear characteristic of a solid-state imaging device.

[0002]

2. Description of the Related Art In a solid-state image pickup device such as an ordinary video camera or digital still camera, a luminance region in which the characteristic between the incident luminance and the output voltage of the solid-state image pickup device has a linear relationship is to be processed. In a solid-state imaging device having a vertical overflow drain structure, the dynamic range can be expanded by utilizing a luminance region having a nonlinear relationship after saturation.

A method for driving a solid-state image pickup device having a vertical overflow drain structure is disclosed in Japanese Patent Application No. 2002-110,087 as a method for expanding the dynamic range by controlling the amount of charge accumulated by stepwise or linearly increasing the voltage applied to a semiconductor substrate. There is one described in JP-A-9-65217. However, the conventional input / output sensitivity characteristic is not used in the prior art.

In addition to the above-mentioned method, a solid-state imaging device for expanding a dynamic range is described in the following document as a method for expanding a dynamic range by combining images of two or more frames having different shutter times. Are known. Kobuchi: "Wide Dynamic Range CCD
"Hyper-D CCD"", OPTRONICS,
No. 3, 1999.

[0005]

However, the solid-state imaging device according to the prior art described above has several problems to be solved as follows. First, it is difficult to expand the dynamic range of the linear region of the solid-state imaging device.

The reason is that it is difficult to suppress noise components such as dark current of the solid-state imaging device. In particular,
When the pixel size is reduced to reduce the manufacturing cost, the amount of charge stored is reduced, and the dynamic range may be narrowed.

In addition, when the amount of charge accumulated is controlled by changing the voltage applied to the semiconductor substrate of a solid-state imaging device having a vertical overflow drain structure, or when the non-linear characteristic after saturation is used, saturation variation may occur. There is a problem that it is large.

The reason is as follows. The photodiode capacitance of the solid-state imaging device has a variation for each pixel. When only the linear region is used, saturation is not reached, and all the accumulated charges are transferred, so that such a problem does not occur. However, in the case of saturation, the amount of charge accumulated and transferred varies, which causes fixed pattern noise to degrade image quality. Also,
Since the slope is smaller on average in the non-linear region than in the linear region, the noise component with respect to the signal component increases, and the S / N ratio relatively decreases.

Further, as a method of reducing the saturation variation, a method in which a median filter is provided as a noise removing circuit is described in Japanese Patent Application No. 7-89283. However, this noise removal circuit has a problem that the resolution is reduced.

The reason is that the median filter performs signal processing using adjacent pixels other than the saturated pixels.

Further, the method of expanding the dynamic range by synthesizing images of two or more frames having different shutter times has a problem that the subject is blurred when a fast-moving subject is imaged.

The reason is to combine a plurality of images taken at certain time intervals.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and when a dynamic range is widened by using a nonlinear characteristic after saturation of an image pickup device, good image quality can be obtained by suppressing variation in saturation. An object of the present invention is to provide a solid-state imaging device to be obtained and a driving method thereof.

[0014]

In order to achieve the above object, according to the first aspect of the present invention, there is provided a luminance region in which a characteristic between an incident luminance and an output voltage of a solid-state image sensor has a linear relationship and a non-linear relationship. In a solid-state imaging device targeted for processing, by lowering the voltage applied to the semiconductor substrate of the solid-state imaging device, charges are reversely injected from the semiconductor substrate to the photodiode to saturate the photodiode, and then the inside of the photodiode is An image including saturation variation is obtained by transferring electric charges to a vertical transfer register.

According to a second aspect of the present invention, in the first aspect of the invention, after the voltage applied to the semiconductor substrate is reduced,
It is characterized in that the unnecessary voltage in the photodiode is extracted to the semiconductor substrate by increasing the applied voltage again.

According to a third aspect of the present invention, in the first or the second aspect of the present invention, the electric charge is transferred from the semiconductor substrate to the photodiode and the vertical register by reducing the voltage applied to the semiconductor substrate of the solid-state imaging device to around 15 V. After forcibly saturating the photodiode and the vertical transfer register by back injection, first transfer the charge in the vertical register, and then transfer the charge in the photodiode to the vertical transfer register through the gate region. An image including saturation variation is obtained by the above method.

According to a fourth aspect of the present invention, in the first aspect of the present invention, the reverse injection of charges is performed at intervals of one frame or more.

According to a fifth aspect of the present invention, in the first aspect of the present invention, the reverse injection of charges is performed when the solid-state imaging device is started.

According to a sixth aspect of the present invention, in any one of the first to fifth aspects of the present invention, an optical shutter means is provided, and the shutter is optically closed during the reverse injection of the electric charge, so that the photodiode is used. It is characterized in that photoelectric conversion does not occur.

According to a seventh aspect of the present invention, in any one of the first to third aspects, an optical shutter means and a light emitting means are provided before the light receiving surface of the solid-state imaging device and after the optical shutter means. The method is characterized in that at the time of acquiring a saturation variation image, the light emitting unit emits light to saturate the photodiode of the solid-state imaging device.

According to an eighth aspect of the present invention, in the solid-state imaging device according to the seventh aspect, the light emission of the light emitting means is performed at intervals of one frame or more.

According to a ninth aspect of the present invention, in the seventh aspect of the present invention, the light emission of the light emitting means is performed when the solid state imaging device is started.

According to a tenth aspect of the present invention, in accordance with the seventh aspect of the present invention, there is provided an optical shutter means for blocking light to prevent outside light other than the light emitting body from entering during photoelectric conversion. .

According to an eleventh aspect of the present invention, in the invention of the seventh aspect, the sensitivity characteristics of the incident luminance and the output voltage of the solid-state imaging device are obtained by changing the light amount of the light emitting means.

The twelfth aspect of the present invention is the first aspect of the present invention.
In the invention according to any one of the first to third aspects, a first storage unit that stores an image signal including a saturation variation, an average brightness calculation unit that calculates an average brightness of an image including the saturation variation,
Means for calculating the difference between the image signal stored in the first storage means and the average luminance signal calculated by the average luminance calculation means, and addition for adding the calculated difference signal to the image signal including the saturation variation And a signal in which noise in the image signal is suppressed is output from the adding means.

According to a thirteenth aspect of the present invention, in the invention of the twelfth aspect, when the first storage means acquires a saturation variation image, the first storage means prevents the output of the moving image from being interrupted by one frame before the image. It has a second storage means for storing an image.

[0027]

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

FIG. 3 is a sectional view of a solid-state imaging device having a vertical overflow drain. On the surface side of the P-type impurity well layer 301 on the N-type semiconductor substrate 300, a photo-electric layer made of an N-type impurity that photoelectrically converts into a signal charge having a charge amount corresponding to the amount of incident light and stores the signal charge. A diode 305 and a P ⁺ -type impurity layer 304 for suppressing generation of dark current are formed. Also, P
A vertical register is formed by the type impurity layer 302 and the N type impurity layer 303 thereabove. Photodiode 3
From 05, electric charges are read out to the vertical register via the gate region. An insulating film 306 is formed on the upper surface of the semiconductor substrate, and a vertical transfer electrode 307 made of a polysilicon film or the like is formed thereon.

FIG. 4 is a diagram showing a potential of the semiconductor substrate in a vertical direction at a position including the photodiode 305. The depth direction (downward) of the semiconductor substrate in FIG. 3 is shown in the horizontal direction (rightward) in FIG. From the left, the potentials of the P ⁺ -type impurity layer 304, the N-type photodiode 305, the P-type impurity well layer 301, and the N-type semiconductor substrate are shown.

When light is incident on the solid-state image pickup device, it is photoelectrically converted by the photodiode 305 and the electric charge is accumulated. The potential of the photodiode 305 decreases in the direction of the arrow in FIG. 4 due to the accumulated charge. When strong light is incident, charges may flow into the vertical register and blooming may occur. A structure for avoiding this blooming is a vertical overflow drain. This is a structure in which by increasing the substrate voltage (NSub in the figure), the potential potential of the P-type impurity well layer 301 is increased to lower the barrier, and excess charges are swept out to the N-type semiconductor substrate 300. It is.

FIG. 6 is a diagram showing the relationship between the incident luminance and the output voltage of the linear characteristic and the non-linear characteristic of the solid-state imaging device having the vertical overflow drain structure. Until the solid-state imaging device is saturated, the characteristic between the incident luminance and the output voltage is linear. However, when the accumulated charges start to cross the barrier formed by the P-type impurity well layer 301, the characteristics between the incident luminance and the output voltage become non-linear. This is because the height of the barrier formed by the P-type impurity well layer 301 is smaller than that of the photodiode 3
This is because it changes in accordance with the amount of electric charge accumulated in 05.
To summarize the above, in a luminance region until the solid-state imaging device is saturated, the solid-state imaging device has a linear characteristic represented by the following equation 1.

[0032]

(Equation 1)

Here, x is the incident luminance and y is the output voltage. In a nonlinear region (logarithmic region) in which the incident luminance increases and saturates, it has a logarithmic characteristic as shown in Equation 2 below. The following literature is a reference for the logarithmic characteristic of the vertical overflow drain after saturation. Kawai, et. al: "Photo Response
se Anal_y_sis in CCD Imag
e Sensor with a VOD struc
cure ", IEEE Transactions o
n Electron Devices, Vol. 4
2, No. 4, Apr. 1995.

[0034]

(Equation 2)

In the linear region, the photodiode 3
Since all the electric charges accumulated in 05 are transferred to the vertical register, there is little variation between pixels. However, in the non-linear region, the height of the barrier formed by the P-type impurity well layer 301 is often different for each pixel in manufacturing. In this case, the saturation variation appears as a fixed pattern noise in the captured image, so that the image quality deteriorates. In the present invention, pixels are saturated using reverse charge injection or an optical light source, an image including saturation variations is acquired, and pixel variations are suppressed by a noise cancellation circuit.

FIG. 2 shows an example of a normal drive timing of a normal solid-state imaging device. The upper part shows the vertical transfer pulse, and the lower part shows the voltage applied to the semiconductor substrate. Unnecessary charges in the photodiode 305 are swept out to the N-type semiconductor substrate 300 during the electronic shutter period. Next, charge is accumulated in the photodiode 305 during the charge accumulation period, and a change in the potential as shown by an arrow in FIG. 4 occurs. The stored charges are transferred to the vertical register during the readout period. In the charge transfer period, charges in the vertical register are output via the horizontal register.

FIG. 1 shows an example of the drive timing of the solid-state imaging device at the time of reverse injection in the embodiment of the present invention. The upper part shows the vertical transfer pulse, and the lower part shows the voltage applied to the semiconductor substrate. Below,
A description will be given using a cross-sectional view of the solid-state imaging device with the vertical overflow drain in FIG. 3 and a potential at the time of reverse injection in FIG. By lowering the substrate voltage during the charge reverse injection period, charges are reverse injected into the photodiode 305 from the N-type semiconductor substrate 300. When the reverse injection is performed, the potential as shown in FIG. 4 changes to the potential as shown in FIG.

Further, in order to improve the in-plane uniformity of the injected charges, that is, the uniformity in the whole CCD, excessive charges are swept out by increasing the substrate voltage during the electronic shutter period. However, since not all charges are swept out, a voltage of about 15 V, which is lower than the substrate voltage of about 22 V during the electronic shutter period of normal driving, is applied. The voltage applied by the electronic shutter causes a change in the potential voltage as shown by an arrow in FIG.
Next, the charge accumulated in the charge accumulation period is transferred to the vertical register in the readout period. In the charge transfer period, charges in the vertical register are output via the horizontal register.

Next, a solid-state imaging device including a noise canceling circuit using the saturation variation image thus obtained will be described.

FIG. 7 is a block diagram showing a configuration example of the solid-state imaging device according to the present embodiment. Here, the solid-state imaging device includes a video camera for moving images and a digital still camera for still images. Light transmitted through an optical unit such as a lens is photoelectrically converted in the solid-state imaging device 700. The solid-state imaging device 700 converts an analog video output signal into a digital signal RAWin by an A / D converter 701. The digital video output of the A / D converter 701 is converted into a signal RAWout in which the saturation variation is suppressed by the noise suppressor 702.

FIG. 8 shows the noise suppression means 70 shown in FIG.
FIG. 2 is a block diagram showing one configuration example. The frame memory 801 is a frame memory for storing an image including saturation variations. The integration means 802 and the average value calculation means 803 are integration means and average value calculation means for calculating the average value of the image in the frame memory 801. The subtraction circuit 804 calculates a difference between signals from the frame memory 801 and the average value calculation unit 803 to calculate a signal for suppressing noise. This signal is added to the RAWin input signal in the adding circuit 805 to obtain a signal RAWout in which noise is suppressed.

The frame memory 800 stores an image one frame before the current image in order to prevent a moving image from being interrupted when the frame memory 801 is updated, that is, when the frame memory 801 obtains an image including saturation variation. Keep it. When the frame memory 801 is updated, the switching unit 807 is operated so that the input signal RAWin is connected to the frame memory 801 and the integrating unit 802.

However, in the normal state, it is assumed that the switching means 806 and 807 are connected as shown in FIG. Note that the reverse injection of the charge described with reference to FIG. 1 is performed at least at intervals of one frame or more or when the solid-state imaging device is started. At such a timing, the switching unit 806 is connected to the frame memory 801 side, and the switching unit 807 is connected to the addition circuit 805 side.

As for the acquisition of the saturation variation image, there is an embodiment using the following light emitting means other than the above-described method of performing the reverse injection of electric charge.

FIG. 9 is a block diagram showing an example of the configuration of a solid-state imaging device provided with an optical shutter means and a light emitting means. The basic configuration is the same as in the above embodiment.

The lens 901 is arranged so as to focus on the solid-state imaging device 900. In order to saturate the solid-state imaging device 900 and obtain a variation image, a surface light-emitting body 903 having a uniform in-plane luminance is placed so as to match the optical axis. When acquiring the saturation variation image, the surface light emitter 903 emits light, and the image at this time is acquired by the frame memory 801. The light emission of the surface light emitter is performed at intervals of at least one frame or at the time of starting the solid-state imaging device. At this time, an optical shutter means 902 for shielding light is used so as not to be affected by external light. Provide.

Even at the time of reverse injection of charges in FIG. 1, the influence of external light can be avoided by providing the same optical shutter means 902 as described above. This is because even at the time of reverse injection, photoelectric conversion occurs in the photodiode 305 by light incident on the solid-state imaging device. For this reason, light is optically shielded during one field period of the field for performing reverse injection in order to avoid the influence of external light on the saturation variation image. However, in this case, the surface light emitter 903 is not provided.

In the present embodiment, the amount of light incident on the solid-state image sensor from the surface light emitter is controlled, and the output voltage of the solid-state image sensor at that amount of light is detected.
The sensitivity characteristics of the solid-state imaging device can be obtained. By obtaining this sensitivity characteristic, for example, it is possible to detect a boundary point between a linear region and a nonlinear region, and it is possible to improve the linearity of the obtained image.

[0049]

As is apparent from the above description, as a first effect of the present invention, the variation in saturation can be suppressed.

The reason is that a circuit is provided which suppresses the saturation variation by adding an image containing the saturation variation acquired in advance and its average value to these difference signals for each pixel. With this circuit, fixed pattern noise when the solid-state imaging device is saturated can be suppressed by 3 dB or more (experimental value) in terms of output voltage.

Further, as a second effect of the present invention, it is possible to saturate the photodiode without using optical means and obtain an image including a variation in saturation.

The reason is that, in the driving method of the solid-state imaging device in which the charge is reversely injected, the photodiode is saturated by reversely injecting the charge from the N-type semiconductor substrate into the photodiode.
This is because the light emitting means is not required.

As a third effect of the present invention, when a light emitting means is used, the sensitivity characteristics of the incident luminance and the output voltage of the solid-state imaging device can be obtained.

The reason is that sensitivity characteristics can be obtained by controlling the amount of light incident on the solid-state imaging device and detecting an output voltage at the amount of light. By obtaining the sensitivity characteristics, for example, it is possible to detect a boundary point between a linear region and a non-linear region, and it is possible to improve the linearity of the obtained image.

As a fourth effect of the present invention, it is possible to obtain an image including a variation in saturation without being affected by external light.

The reason for this is that an optical shutter is provided so as not to be affected by external light when acquiring an image including saturation variations. When external light is incident on the solid-state imaging device, saturation at uniform incident luminance cannot be achieved, and it is difficult to determine whether or not a pixel is saturated from an output voltage pixel of the solid-state imaging device.

A fifth effect of the present invention is that a moving image is not interrupted during imaging.

The reason is that the provision of the storage means for storing the image of one or more frames earlier allows the output image to be obtained when an image including the saturation variation of the solid-state imaging device is obtained at an arbitrary frame interval. This is because the image can be supplemented by using an image from the storage unit so that the image is not interrupted. When acquiring an image including an image including a variation in saturation, it is not possible to capture an image because an unusual drive such as closing an optical shutter or performing reverse injection of electric charge is performed.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating driving timing of a solid-state imaging device according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating an example of a drive timing of a solid-state imaging device according to the related art.

FIG. 3 is a diagram illustrating an example of a cross-sectional view of the solid-state imaging device according to the embodiment of the present invention.

FIG. 4 is a diagram illustrating an example of a potential of a solid-state imaging device during photoelectric conversion according to an embodiment of the present invention.

FIG. 5 is a diagram illustrating an example of a potential of a solid-state imaging device at the time of back injection according to the embodiment of the present invention.

FIG. 6 is a diagram illustrating an example of a characteristic of an output voltage of a solid-state imaging device with respect to an incident luminance in the embodiment of the present invention.

FIG. 7 is a diagram illustrating a configuration example of a solid-state imaging device according to an embodiment of the present invention.

FIG. 8 is a diagram illustrating a configuration example of a noise suppression unit according to the embodiment of the present invention.

FIG. 9 is a diagram illustrating a configuration example of a solid-state imaging device including an optical shutter unit and a light emitting unit according to another embodiment of the present invention.

[Explanation of symbols]

Reference Signs List 300 N-type semiconductor substrate 301 P-type impurity well layer 302 P-type impurity layer 303 N-type impurity layer 304 P ⁺ -type impurity layer 305 N-type photodiode 306 Insulating film 307 Vertical transfer electrode 700 Solid-state image sensor 701 AD conversion means 702 Noise suppression Means 800, 801 Frame memory 802 Integrating means 803 Average value calculating means 804 Subtracting means 805 Addition circuit 806, 807 Switching means 900 Solid-state image sensor 901 Lens 902 Optical shutter means 903 Surface light emitter

Claims (13)

[Claims] 1. A solid-state imaging device for processing a luminance region in which a characteristic between an incident luminance and an output voltage of a solid-state imaging device has a linear relationship and a non-linear relationship, wherein a voltage applied to a semiconductor substrate of the solid-state imaging device is By lowering, after the charge is reversely injected from the semiconductor substrate to the photodiode to saturate the photodiode,
A solid-state imaging device, wherein an image including a variation in saturation is obtained by transferring charges in the photodiode to a vertical transfer register. 2. The solid-state imaging device according to claim 1, wherein after the voltage applied to the semiconductor substrate is reduced, the applied voltage is increased again to extract unnecessary charges in the photodiode to the semiconductor substrate. 3. A charge applied from the semiconductor substrate to the photodiode and the vertical register by lowering a voltage applied to the semiconductor substrate of the solid-state imaging device to around 15 V, thereby causing the photodiode and the vertical transfer register to be injected. After forcibly saturating, first transfer the charges in the vertical register, and then transfer the charges in the photoresistor to the vertical transfer register via the gate region to obtain an image including saturation variation. The solid-state imaging device according to claim 1, wherein the image is acquired. 4. The solid-state imaging device according to claim 1, wherein the reverse injection of the charge is performed at intervals of one frame or more. 5. The solid-state imaging device according to claim 1, wherein the reverse injection of the charge is performed when the solid-state imaging device is started. 6. The method according to claim 1, further comprising an optical shutter means, wherein the shutter is optically closed during the reverse injection of electric charge so that photoelectric conversion does not occur in the photodiode. A solid-state imaging device according to any one of claims 1 to 3. 7. An optical shutter means provided before the light receiving surface of the solid-state imaging device and a light emitting means provided after the optical shutter means, and the light emitting means emits light when a saturation variation image is obtained. The solid-state imaging device according to any one of claims 1 to 3, wherein a photodiode of the solid-state imaging device is saturated. 8. The solid-state imaging device according to claim 7, wherein light emission of said light emitting means is performed at intervals of one frame or more. 9. The solid-state imaging device according to claim 7, wherein said light-emitting means emits light when said solid-state imaging device is started. 10. The solid-state imaging device according to claim 7, further comprising an optical shutter unit that shields light to prevent external light other than the light-emitting body from entering during photoelectric conversion. apparatus. 11. The solid-state imaging device according to claim 7, wherein a sensitivity characteristic of an incident luminance and an output voltage of the solid-state imaging device is obtained by changing a light amount of the light emitting unit. apparatus. 12. An image signal stored in the first storage unit, wherein the first storage unit stores an image signal including the saturation variation, an average brightness calculation unit calculates an average brightness of the image including the saturation variation. Signal, a means for calculating a difference between the average luminance signal calculated by the average luminance calculation means, and an addition means for adding the calculated difference signal to the image signal including the saturation variation, The solid-state imaging device according to claim 1, wherein a signal in which noise in the image signal is suppressed is output from the adding unit. 13. When the first storage unit obtains a saturation variation image, a second storage unit that stores an image one frame before the image in order to prevent the output of a moving image from being interrupted is provided. 13. The solid-state imaging device according to claim 12, comprising: