JP2000165749A - Solid-state image pickup element driving method and image pickup system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solid-state image pickup element driving method capable of reducing the dark signal of the sensor part at the time of low illuminance in a solid-state image pickup element in which length of the void layer of a sensor part is set to be long enough for absorbing near infrared rays. SOLUTION: At the time of driving a CCD image pickup element 11 in which the length of the void layer of a sensor part is set to be long enough for absorbing near infrared rays, for example, 3 μm or above, the illuminance of the image pickup face of the CCD image pickup element 11 is detected by an illuminance detector 13, and a second substrate bias voltage Vsub2 higher than a first substrate bias voltage Vsub1 at the time of a normal operation is selected by a bias changeover switch 15 at the time of low illuminance, and impressed to the substrate of the CCD image pickup element 11 under the control of a bias control circuit 14 for monitoring the detected output.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for driving a solid-state imaging device and an imaging system, and more particularly to a method for driving a solid-state imaging device having high sensitivity in a red to near-infrared region and using the solid-state imaging device as an imaging device. The imaging system used.

[0002]

2. Description of the Related Art A solid-state imaging device such as a CCD (Charge Co.)
FIG. 3 shows an example of a cross-sectional structure around a pixel portion of an upled device (hereinafter, referred to as a CCD image sensor).

[0003] In FIG. 3, in the pixel structure according to the present example,
The structure of the n-type substrate 51 and the p-well 52 is adopted.
Then, an n layer 53 serving as a signal charge storage region is provided on the substrate surface side.
Are formed, a sensor unit (photoelectric conversion unit) 54 composed of a pn junction photodiode is formed. The sensor unit 54 has a HAD (Hole Accumulated Diode) sensor structure having a p ⁺ layer 55 serving as a hole accumulation layer on the substrate surface side in order to improve sensitivity and reduce dark current.

An n-layer 56 is formed adjacent to the sensor section 54 as a charge transfer region (transfer channel).
The n-layer 56 constitutes a vertical transfer section 59 together with a polysilicon electrode (transfer electrode) 58 formed on the substrate surface thereover via a silicon oxide film 57. The vertical transfer unit 59 has a structure having a second p-well 60 below the n-layer 56 in order to suppress smear.

[0005] Between the sensor unit 54 and the vertical transfer unit 59, a read gate unit 61 for reading out the signal charges photoelectrically converted and accumulated in the sensor unit 54 to the vertical transfer unit 59 is interposed. Here, the polysilicon electrode 58 of the vertical transfer part 59 extends to the vicinity of the sensor part 54, and the extended part is also used as the gate electrode of the read gate part 61. The area excluding the sensor unit 54 is covered with the light shielding film 62.

By the way, in the pixel structure having the above configuration,
Red to near infrared (wavelength of near infrared is 0.7 μm to 2.5
In order to improve the sensitivity in the region,
4 The length of the depletion layer is long enough to absorb near infrared rays,
For example, a CCD imaging device having a structure set to 3 μm or more is known. A predetermined substrate bias voltage Vsub set for each device is applied to the n-type substrate 51.

[0007]

However, as described above, a CCD image pickup device having a configuration in which the sensitivity in the red to near infrared region is improved by extending the length of the depletion layer of the sensor portion 54 to 3 μm or more. Since the dark charge generated in the deep portion of the substrate of 3 μm or more does not flow to the substrate side and is easily taken into the n-layer 53 serving as the signal charge storage region, the variation of the dark signal for each pixel becomes fixed pattern noise and the screen becomes a fixed pattern noise. There is a problem that it appears as a white dot on the upper side and deteriorates image quality.

The present invention has been made in view of the above-mentioned problems, and has as its object to provide a solid-state sensor in which the length of a depletion layer in a sensor section is set to a length sufficient to absorb near-infrared rays. It is an object of the present invention to provide a driving method capable of reducing a dark signal of a sensor unit particularly at low illuminance in an image sensor.

The present invention further provides a case where a solid-state imaging device in which the length of a depletion layer of a sensor section is set to a length sufficient to absorb near-infrared rays is used as an imaging device.
In particular, an object of the present invention is to provide an imaging system capable of improving image quality deterioration caused by a dark signal at low illuminance.

[0010]

According to the present invention, there is provided a method for driving a solid-state image sensor, wherein the length of a depletion layer in a sensor section is set to a length sufficient to absorb near-infrared rays. At the time of illuminance, the substrate bias voltage of the solid-state imaging device is set to be higher than at the time of normal operation.

In this driving method, the length of the depletion layer of the sensor section is set to a length sufficient to absorb near-infrared rays. By setting the substrate bias voltage higher than in normal operation at low illuminance where dark signals are conspicuous, the potential of each layer in the sensor unit becomes deeper as a whole. Shorter than Then, the dark charges generated in the deep part of the substrate easily flow to the substrate side, and conversely, the dark charges hardly accumulate in the signal charge accumulation region of the sensor part.

An imaging system according to the present invention includes a solid-state imaging device in which the length of a depletion layer in a sensor section is set to a length sufficient to absorb near-infrared light, and an illuminance on an imaging surface of the solid-state imaging device. Illumination detection means for detecting, and bias switching means for setting the substrate bias voltage of the solid-state imaging device to be higher at low illuminance than at the time of normal operation based on the detection output of the illuminance detection means.

In the imaging system having the above structure, the sensitivity of the solid-state imaging device in the red to near-infrared region during normal operation is set by setting the length of the depletion layer of the sensor section to a length sufficient to absorb near-infrared rays. Is improved. On the other hand, the illuminance detection means detects the illuminance in the vicinity of the imaging surface of the solid-state imaging device, and supplies the detection output to the bias switching means.

The bias switching means monitors the detection output of the illuminance detecting means, and sets the substrate bias voltage higher at low illuminance where the dark signal is conspicuous than during normal operation.
Thereby, the potential of each layer in the sensor section is deepened as a whole, and the length of the depletion layer in the sensor section is shorter than that in the normal operation. Then, the dark charges generated in the deep part of the substrate are more likely to flow to the substrate side, and conversely, are less likely to be stored in the signal charge storage region of the sensor unit.

[0015]

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

FIG. 1 is a block diagram showing the configuration of an imaging system according to one embodiment of the present invention. In FIG.
The imaging system includes a CCD imaging device 11 as an imaging device, a lens 12 constituting an optical system, an illuminance detector 1
3, a configuration including a bias control circuit 14 and a bias switch 15.

In the imaging system having the above configuration, the CCD
By setting the length of the depletion layer of the sensor unit to a length sufficient to absorb near-infrared rays, for example, 3 μm or more, the sensitivity of the image sensor 11 in the red to near-infrared region during normal operation is improved. A known device is used. Image light from a subject (not shown) is formed on an imaging surface of the CCD imaging device 11 through an optical system including a lens 12.

The illuminance detector 13 is disposed, for example, in the vicinity of the imaging surface of the CCD imaging device 11 and directly detects the illuminance on the imaging surface. The detection output of the illuminance detector 13 is provided to a bias control circuit 14. Bias control circuit 14
Monitors the detection output of the illuminance detector 13 and supplies a switching control signal to the bias switching switch 15 for switching the substrate bias voltage when the output level is equal to or lower than a predetermined threshold level Vth. Here, the threshold level Vth is set corresponding to low illuminance in which a dark signal is conspicuous.

As the substrate bias voltage, for example, the first
Of the substrate bias voltage Vsub1 and the second
And the substrate bias voltage Vsub2. The bias switch 15 is in a state where the first substrate bias voltage Vsub1 is selected during the normal operation, and the first substrate bias voltage Vs1 is provided at a low illuminance to which a switching control signal is supplied from the bias control circuit 14.
ub1 is switched to the second substrate bias voltage Vsub2, and the second substrate bias voltage Vsub2 is transferred from the substrate bias terminal 16 to the substrate (n in FIG. 3) of the CCD image sensor 11.
(Corresponding to the mold substrate 51).

FIG. 2 shows the potential distribution of the sensor section of the CCD image sensor 11 in the direction of the substrate depth. Note that the pixel structure in the substrate depth direction in FIG. 2 corresponds to the pixel structure in FIG. 3, and in the drawing, the same parts are denoted by the same reference numerals.

This pixel structure is a so-called vertical overflow drain structure. As a result of imaging a high-luminance subject, if the amount of light incident on the sensor exceeds a predetermined amount, the n-layer 53 serving as a signal charge storage region is formed. The excess charge generated in the step (c) crosses the barrier in the depth direction, that is, the overflow barrier, and is discharged to the n-type substrate 51 side. Therefore, the n-layer 53
The amount of charge stored inside is saturated.

The potential (height) of the overflow barrier during the normal operation is equal to the first substrate bias voltage Vs
It is determined by the voltage value of ub1. That is, when the voltage value of the first substrate bias voltage Vsub1 is high, the potential of the overflow barrier decreases, and the saturation charge decreases. Conversely, when the voltage value of the first substrate bias voltage Vsub1 is low, the potential of the overflow barrier increases, and the saturation charge increases.

Here, the first substrate bias voltage Vsub
In 1, the voltage value is set so that a saturated charge amount set for each device is obtained under a predetermined light amount. That is, the first substrate bias voltage Vsub1 is device-specific. On the other hand, the second substrate bias voltage Vsub2 is set so that its voltage value is higher than the voltage value of the first substrate bias voltage Vsub1.

Next, the operation of the imaging system according to the present embodiment having the above configuration will be described.

First, during normal operation, that is, when the detection output level of the illuminance detector 13 exceeds the threshold level Vth, the bias switch 15 selects the first substrate bias voltage Vsub1 by the switching control by the bias control circuit 14. It is in a state of having done. In this normal operation state, the potential of the overflow barrier is determined by the first substrate bias voltage Vsub1, and a depletion layer is formed from the surface of the n-layer 53 to the overflow barrier. In this example, the length is set to 3 μm or more. Is set.

As described above, in the CCD imaging device 11, by setting the length of the depletion layer of the sensor section to 3 μm or more, as described above, red to near infrared (the wavelength of near infrared is 0.7 μm to 2 μm). (About 0.5 μm). Here, in this type of CCD image sensor 11,
Since the depletion layer is longer than a general device, dark charges generated in a deep portion of the substrate of 3 μm or more do not flow to the substrate side and are easily accumulated as signal charges in the n-layer 53.

In normal imaging, the amount of incident light is large, and therefore the amount of signal charges (electrons) stored in the n-layer 13 is very large. Therefore, the amount of dark charge is insignificant compared to the amount of charge. It is. Therefore, at the time of normal imaging, the dark signal does not stand out, that is, does not degrade the image quality of the reproduction screen.

On the other hand, when the illuminance on the imaging surface of the CCD image sensor 11 becomes low, that is, when the detection output level of the illuminance detector 13 becomes lower than the threshold level Vth, the bias control circuit 14 A switching control signal is provided. Then, in response to this switching control signal, the bias switch 15 selects the second substrate bias voltage Vsub2 instead of the first substrate bias voltage Vsub1.

Here, the second substrate bias voltage Vsub
2 is set to be higher than the voltage value of the first substrate bias voltage Vsub1, the second substrate bias voltage Vsub2 is applied to the substrate 51 of the CCD image pickup device 11 in FIG. As can be seen from the potential diagram, since the potential of each layer in the sensor section is deeper as a whole, the length of the depletion layer is shorter than that in the normal operation.

This makes it easier for the dark charges generated in the deep part of the substrate to flow to the substrate 51 side.
Is hardly accumulated in Therefore, when the illuminance is shifted to the low illuminance in the state of the normal operation, the ratio of the dark charge amount to the small signal charge amount increases, so that the dark signal becomes more conspicuous. This makes it difficult for the dark charges generated in the deep part of the substrate to be accumulated in the n-layer 53, so that the dark signal of the sensor unit can be reduced.

As described above, the length of the depletion layer in the sensor portion is set to a length sufficient to absorb near infrared rays (3 μm in this example).
m or more), the substrate bias voltage Vsub is set to be higher than that in the normal operation (Vsub1) at low illuminance in which the dark signal is conspicuous in the CCD image sensor 11 in which the sensitivity in the red to near-infrared region during normal operation is improved. Yes (Vsub
2) Since the length of the depletion layer in the sensor section is shorter than that in the normal operation, dark charges generated in the deep portion of the substrate are less likely to be accumulated in the n layer 53 in the sensor section.

As a result, the dark signal of the sensor section can be reduced, so that the reduction of the yield due to the dark signal can be eliminated, and a countermeasure for a later white point can be obtained. Further, in an imaging system using the CCD imaging device 11 as an imaging device, it is possible to improve image quality deterioration particularly caused by a dark signal at low illuminance.

In the above embodiment, the illuminance detector 13 for directly detecting the illuminance on the imaging surface of the CCD image sensor 11 is used as the illuminance detection means. However, the present invention is not limited to this. A configuration in which the illuminance on the imaging surface of the CCD imaging device 11 is detected from the output level of the device 11 is also possible.

Then, the maximum value Smax of the output level of the CCD image sensor 11 is monitored, and when the maximum value Smax falls below the threshold level Sth, the second substrate bias voltage Vsub1 is replaced with the second substrate bias voltage Vsub1. The voltage Vsub2 may be selected and applied to the substrate of the CCD image sensor 11. At this time, the second substrate bias voltage Vsub2 is set so that the saturation charge amount of the sensor unit is the maximum value S.
It is set to be larger than max.

Further, in the above-described embodiment, an example has been described in which the first and second substrate bias voltages Vsub1 and Vsub2 are prepared as the substrate bias voltage Vsub, and the configuration is switched to two stages. However, it is also possible to switch the substrate bias voltage Vsub to three or more levels according to the illuminance.

[0036]

As described above, according to the present invention,
In a solid-state imaging device in which the length of the depletion layer of the sensor unit is set to be long enough to absorb near-infrared light, the substrate bias voltage of the solid-state imaging device at low illuminance where dark signals are conspicuous is lower than during normal operation. By setting it higher, the length of the depletion layer becomes shorter than in normal operation, and it becomes difficult for dark charges generated in the deep part of the substrate to accumulate in the signal charge accumulation area of the sensor section. It is possible to reduce the dark signal of the section and improve the image quality.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of an imaging system according to an embodiment of the present invention.

FIG. 2 is a potential diagram of a sensor section in a substrate depth direction.

FIG. 3 is a cross-sectional view illustrating an example of a structure around a pixel portion of the CCD image sensor.

[Explanation of symbols]

11: CCD imaging device, 13: Illuminance detector, 14: Bias control circuit, 15: Bias switch

 ────────────────────────────────────────────────── ─── Continued on the front page F term (reference) 2G065 AA03 AB02 AB19 BA09 BA15 CA12 4M118 AA05 AB01 BA10 CA04 DA03 DB01 FA06 FA13 FA35 FA50 GA10 5C024 AA01 AA06 CA06 FA01 GA01 GA06 GA11

Claims (2)

[Claims] 1. A method for driving a solid-state imaging device in which a length of a depletion layer of a sensor section is set to a length sufficient to absorb near-infrared rays, wherein a substrate bias voltage of the solid-state imaging device is set at low illuminance. Is set higher than during normal operation. 2. A solid-state imaging device in which a length of a depletion layer of a sensor unit is set to a length sufficient to absorb near-infrared light; and illuminance detection means for detecting illuminance on an imaging surface of the solid-state imaging device. An imaging system comprising: a bias switching unit configured to set a substrate bias voltage of the solid-state imaging device to be higher at low illuminance than at the time of normal operation based on a detection output of the illuminance detection unit.