JPH0834568B2 - Driving method for solid-state imaging device - Google Patents

Description

TECHNICAL FIELD The present invention relates to a driving method of a vertical overflow drain type CCD solid-state imaging device.

(B) Prior Art FIG. 3 is a cross-sectional view of a vertical overflow drain type CCD solid-state imaging device. A P-Well region (1) is formed on one surface of the N type Si substrate (4), and an N ⁺ type diffusion region (7) is formed in the light receiving portion thereof. And P
-A storage gate electrode (6) and a transfer electrode (3) are formed on the well region (1) through an insulating film (2), and a control pulse φ _c for controlling the accumulation of photocharges and the photocharges are transferred. Drive pulses φ _t are applied respectively. Further, a back surface electrode (5) is formed on the other surface of the Si substrate (4). A CCD solid-state image pickup device having such a vertical overflow drain structure is disclosed in, for example, Japanese Patent Application Laid-Open No. 59-19480.

The charge accumulation period of the CCD solid-state image pickup device is usually set to 1/60 second, and when obtaining a still image in such a state, the shutter speed is 1/60 second. In order to further increase the shutter speed, it is necessary to shorten the charge accumulation period, and the method (electronic shutter) is described in, for example, Nikkei Microdevice October 1987 issue P60 to P67. This electronic shutter function is configured to sweep the photocharges accumulated in the channel of the image pickup unit to the substrate for a desired period during the accumulation period and accumulate the charges in the channel for the remaining period. For example, if the charge is swept out of the channel in the 2/3 period of the 1/60 second accumulation period and the charge is accumulated in the remaining 1/3 period, the shutter speed of 1/180 second is obtained. In the vertical overflow drain type CCD solid-state imaging device as shown in FIG. 3, the above-mentioned charge sweeping operation is performed by controlling the voltage V _S applied to the back surface electrode (5). Normally, a voltage V _{S of a} constant level is applied to the back surface electrode (5) at the time of storing and transferring charges, but if this voltage V _{S is set} to a certain level or higher, all the photo-charges generated in the imaging unit are Si. It flows to the substrate (4), and photocharges are no longer accumulated in the imaging section. Therefore, the back surface electrode (5) for a desired period
The pulse φ _s for sweeping out the electric charges is applied to the photo electric charges to sweep out the photo electric charges generated in the imaging unit.

FIG. 4 is a diagram showing the potential distribution along the line XX ′ in FIG. 3, where A is the charge accumulation and B is the electronic shutter operation. During storage, the drive pulses φ _t and φ _c applied to the transfer electrode (3) and the gate electrode (6) are driven at a level below the threshold value between the diffusion region (7) and the P-Well region (1), and the back electrode A constant voltage V _S is applied to (5) to form a potential well (10) as shown by the broken line in FIG. Here, when the voltage V _S becomes higher (the pulse φ _s is applied), the potential in the vicinity of the back surface electrode (5) becomes deeper, and the photocharges generated in the imaging section easily flow to the Si substrate (4). . Therefore, if this voltage V _S is set to a certain value or higher, the photocharge is not accumulated in the channel of the image pickup section, and the sweep operation is performed.

(C) Problems to be Solved by the Invention However, the voltage V applied to the back surface electrode (5) is
_{Since S} is applied substantially uniformly over the entire back surface of the device, if the voltage V _S is increased, the Si substrate (4) is also removed from the channel even in a region other than the imaging unit, for example, in the vertical transfer unit.
There is a possibility that photocharges easily flow into the device, resulting in deterioration of device characteristics and deterioration of reliability.

(D) Means for Solving the Problems The present invention has been made to solve the above-mentioned problems, and a transfer electrode is formed on one surface of a semiconductor substrate of one conductivity type via an insulating film, In a method of driving a solid-state image pickup device using a vertical overflow drain type CCD in which an overflow region of the opposite conductivity type is formed on the other surface, the transfer electrode of the image pickup unit has a lower level than the transfer electrodes of other regions. Is applied to drive the photocharge generated under the transfer electrode of the image pickup section to the semiconductor substrate.

(E) Action According to the present invention, by applying a drive pulse of a lower level to the transfer electrodes of the image pickup unit than the transfer electrodes of other regions, the potential near the transfer electrodes of only the image pickup unit is made shallow and the transfer of the image pickup unit is performed. Only the photocharges generated under the electrodes can be swept out to the overflow area efficiently, and the potential near the back surface electrode does not become deeper in the area other than the imaging section, so that the overflow from the area other than the imaging section does not occur. The photocharge does not flow to the area.

(F) Embodiment An embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a sectional view of a device for explaining the driving method of the present invention. On one surface of the N ⁺ type Si substrate (4), P-We
ll region (1) is formed, and further transfer electrodes (3a) to (3d) are formed via the insulating film (2). Further, a back surface electrode (5) is formed on the other surface of the Si substrate (4). A feature of the driving method of the present invention is that the driving pulses φ _{1 to} φ ₄ applied to the transfer electrodes (3a) to (3d) of the image pickup unit are fixed to low levels during the operation of the electronic shutter.

That is, even when the electronic shutter is operating, the back surface electrode (5)
Only voltage V _S is applied, the drive pulse phi ₁ to [phi] ₄ is applied to the transfer electrodes (3a) ~ (3d) is secured to a slightly higher level than V _S equal to or V _S to. Drive pulses φ _{1 to} φ ₄
When fixed at a level equivalent to V _S , the potential well (10) formed in the Si substrate (1) becomes extremely shallow as shown in FIG. ) Is not formed and photocharges are not accumulated.

FIG. 2 is a diagram showing a potential distribution in a cross section taken along the line XX 'in FIG. 1, where A'indicates charge accumulation and B'indicates an electronic shutter operation. During storage, the drive pulses conventional voltage V _S to the back electrode (5) in the same manner is applied phi ₁ to [phi] ₄ is fixed, the distribution of the same potential and Figure 4 A. Here, if the drive pulses φ _{1 to} φ ₄ are fixed at a low level, the potential in the vicinity of the insulating film (2) becomes shallow and it becomes difficult for photocharges to be accumulated under the transfer electrodes (3a) to (3d). Therefore, if the drive pulse is set to a certain level or less, all the photocharges generated in the P-Well region (1) under the transfer electrodes (3a) to (3d) of the image pickup unit are transferred to the Si substrate (4). Swept out.

In such a driving method, the photocharges can easily flow to the Si substrate (4) only by the imaging unit, so that the photocharges flow to the Si substrate (4) at the vertical transfer unit and the storage unit other than the imaging unit. Can be prevented.

(G) Effect of the Invention According to the present invention, it is possible to efficiently sweep out photocharges generated in the image pickup unit during the operation of the electronic shutter to the overflow region, and at the same time, to prevent photocharges from the region other than the image pickup unit in the overflow region. It can be prevented from flowing, the device characteristics can be improved, and the reliability can be improved.

[Brief description of drawings]

FIG. 1 is a sectional view of a CCD solid-state image pickup device for explaining a driving method of the present invention, FIG. 2 is a diagram showing potential distribution in a sectional view taken along line XX ′ of FIG. 1, and FIG. FIG. 4 is a sectional view of a CCD solid-state imaging device for explaining a conventional driving method, and FIG. 4 is a diagram showing a potential distribution in a section taken along line XX ′ of FIG. (1) ... Channel region, (3a) to (3d) ... Transfer electrode,
(4) ... Si substrate, (5) ... Back electrode, (10) ... Potential well.

Claims (1)

[Claims] 1. A vertical overflow drain system in which a diffusion region of opposite conductivity type is formed on one surface of a semiconductor substrate of one conductivity type, and a plurality of transfer electrodes are formed on the diffusion region via an insulating film. In a method of driving a solid-state image pickup device using a CCD, a drive pulse of a lower level than that of a transfer electrode in a region other than the image pickup unit is applied to the transfer electrode of the image pickup unit, and the diffusion under the transfer electrode of the image pickup unit is applied. A method for driving a solid-state imaging device, characterized in that photocharges generated in a region are swept out to the semiconductor substrate.