JPH01268269A - Driving method for solid image pick-up device - Google Patents

Abstract

PURPOSE:To improve the characteristic of an element by impressing a driving pulse, whose level is lower than a transferring electrode in another area, to the transferring electrode of an image pick-up part and sweeping-out an optical charge, which is generated in the image pick-up part, to a semiconductor substrate. CONSTITUTION:Driving pulses phi1 to phi4, which are impressed to transferring electrodes 3a to 3d when an electronic shutter is operated, are respectively fixed to the low level. Namely, even when the electronic shutter is operated, only a voltage VS is impressed to a back electrode 5 and the driving pulses phi1 to phi4, which are impressed to the transferring electrodes 3a to 3d, are fixed to the same level as the VS or the level to be slightly higher than the VS. When the driving pulses are fixed to the same level as the VS, the depth a potential well 10 to be formed in an Si substrate 1 goes to be extremely shallow or the potential well 10 is not formed and the optional charge is not accumulated. Thus, the characteristic of the element can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Field of Industrial Application The present invention relates to a method for driving a vertical overflow drain type CCD solid-state imaging device.

(b) Conventional technology Figure 3 shows a vertical overflow drain type CCDC.
C is a top view of the 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 of the P-Well region (1). (1) A storage gate electrode (6) and a transfer electrode (3) are formed on the insulating film (2) via an insulating film (2), and a control pulse φ for controlling the accumulation of photocharges.

and a driving pulse 4 for transferring photocharges. are applied respectively. Further, a back electrode (5) is provided on the other surface of the Si substrate (4).
) is formed. A CCD solid-state imaging device with such a vertical overflow drain structure is disclosed in, for example, Japanese Unexamined Patent Publication No. 59-1
It is disclosed in Japanese Patent No. 9480.

The charge accumulation period of a CCD solid-state imaging device is usually 1760
If the shutter speed is set to 1,760 seconds and a still image is to be obtained under such conditions, the shutter speed will be 1760 seconds. In order to further increase this shutter speed, the charge accumulation period must be shortened. The method (electronic shutter) is described, for example, in Nikkei Microdevice October 1987 issue P60~
It is described on page 67. This electronic shutter function is configured to sweep the photocharge accumulated in the channel of the imaging section to the substrate during a desired period during the accumulation period, and to accumulate the charge in the channel during the remaining period. For example, if the charge is swept out from the channel during 2/3 of the 1760 second accumulation period and the charge is accumulated during the remaining 173 seconds, then the
A scintillation speed of 7180 seconds 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 (2) that can be applied to the back electrode (5). Normally, during charge accumulation and transfer, the back electrode (
A certain level of voltage ■ is applied to 5), but when this voltage 5 is increased to a certain level or higher, all of the photocharges generated in the imaging section flow to the Si substrate (4), and the photocharges are transferred to the imaging section. will no longer be accumulated. Therefore, the back electrode (
5), a charge sweeping pulse φ5 is applied to sweep out the photocharges generated in the imaging section.

FIG. 4 is a diagram showing the potential distribution along the line xx' in FIG. 3, where A shows the time of charge accumulation and B shows the time of electronic shutter operation. During storage, drive pulses φ and φ are applied to the transfer electrode (3) and the gate electrode (6). is driven at a level below the threshold between the diffusion region (7) and the P-well region (1), and a constant voltage (2) is applied to the back electrode (5), causing the potential well to rise as shown by the dotted line in Fig. 3. (10) can be formed. Here, the voltage ■5 becomes high (
When a pulse ≠ 5 is applied), the potential near the back electrode (5) becomes deeper, and photocharges generated in the imaging section tend to flow to the Si substrate (4). Therefore, this voltage V
If s is set to a certain value or more, no photocharge is accumulated in the channel of the imaging unit, and a sweeping operation is performed.

(c) Problems to be Solved by the Invention However, since the voltage (5) applied to the back electrode (5) is applied approximately evenly over the entire back surface of the device, if this voltage (5) is increased, Even in regions other than the imaging section, such as the vertical transfer section, photocharges tend to flow from the channel to the Si substrate (4), which may lead to deterioration of device characteristics and reduced reliability.

(d) Means for Solving the Problems The present invention has been made to solve the above-mentioned problems, and includes a transfer electrode formed on one surface of a semiconductor substrate of one conductivity type via an insulating film, In a method for driving a solid-state imaging 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 imaging section has a lower level than the transfer electrodes of other regions. The present invention is characterized in that a driving pulse of 1 is applied to sweep out photocharges generated in the imaging section onto the semiconductor substrate.

(e) Effect According to the present invention, by applying a drive pulse of a lower level to the transfer electrode of the imaging section than the transfer electrodes of other areas, the potential near the transfer electrode of only the imaging section is made shallow, and the potential of the transfer electrode of the imaging section is made shallow. Only the generated photocharges can be efficiently swept out to the overflow region, and the potential near the back electrode will not become deep in areas other than the imaging area. No photocharge will flow.

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

FIG. 1 is a sectional view of the device for explaining the driving method of the present invention. One side of the N+ type Si substrate (4) has P
- A well region (1) is formed, and an insulating film (2) is formed.
Transfer electrodes (3a) to (3d) can be formed through the transfer electrodes (3a) to (3d).

Further, a back electrode (5) is formed on the other surface of the Si substrate (4). The driving method of the present invention is characterized in that the driving pulses φ and φ4 applied to the transfer electrodes (3a) to (3d) during electronic shutter operation are fixed at a low level.

That is, even during the electronic shutter operation, only the voltage ■ is applied to the back electrode (5), and the transfer electrodes (3a) to (3d
) is fixed at a level equal to or slightly higher than Vs. When drive pulses -8 to i4 are fixed at a level equivalent to Vs, Si
The potential well (10) formed in the substrate (1) has a very shallow depth, as shown in FIG. 1, or the potential well (10) is not formed and no photocharge is accumulated.

FIG. 2 is a diagram showing the potential distribution in a cross section along the line xx' in FIG. 1, where A' shows the time of charge accumulation and B' shows the time of electronic shutter operation. During storage, a voltage V is applied to the back electrode (5) as in the conventional case, and the drive pulse ≠1 to ≠4.
is fixed and shows a potential distribution similar to that shown in FIG. 4A. Here, if the driving pulse ≠8 to φ is fixed at a low level, the potential near the insulating film (2) becomes shallow, making it difficult for photocharges to be accumulated. Therefore, if the driving pulse is set below a certain level, all the photocharges generated in the imaging section can be swept out to the Si substrate (4).

In such a driving method, only the imaging section transfers photocharges to the Si substrate (4
), it is possible to prevent photocharges from flowing to the Si substrate (4) in a vertical transfer section, storage section, etc. outside the imaging device.

(G) Effects of the Invention According to the present invention, the photocharges generated in the imaging section during electronic shutter operation can be efficiently swept to the overflow region, and the photocharges can be transferred from the region outside the imaging device to the overflow region. This eliminates the flow of water, improves device characteristics, and increases reliability.

[Brief explanation of the drawing]

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

Claims (1)

[Claims] (1) A solid-state using a vertical overflow drain type CCD in which a transfer electrode is formed on one side of a semiconductor substrate of one conductivity type via an insulating film, and an overflow region of the opposite conductivity type is formed on the other side. In the driving method of the imaging device,
A method for driving a solid-state imaging device, comprising: applying a drive pulse at a lower level to a transfer electrode of an imaging section than to transfer electrodes in other areas, and causing photocharges generated in the imaging section to be swept to the semiconductor substrate.