JPH0775405B2 - Driving method for solid-state imaging device - Google Patents

Description

The present invention relates to a method for driving a vertical overflow drain type CCD solid-state image sensor.

(B) Conventional technology Conventionally, in the image pickup device using the CCD solid-state image pickup device,
It is considered to electronically control exposure by utilizing the operating principle of CD. In such an exposure control method,
In the middle of the photoelectric conversion period of each vertical scanning period of the electrically scanned image pickup device, the photocharges accumulated in the image pickup unit until then are discharged, and the photocharges obtained in the remaining photoelectric conversion period are accumulated. is doing. Here, the problem is the method of discharging the photocharges of the image pickup unit, and various methods have been considered for sufficiently discharging the unnecessary photocharges without affecting the video signal output from the image pickup device. .

For example, in the Nikkei Microdevice October 1987 issue P60-P67 "Solid-state image sensor reaching 380,000 pixels", the so-called vertical overflow drain CCD solid-state image sensor that discharges the photocharge of the image pickup section to the substrate side. Is listed.

Also, in Japanese Patent Application No. 63-95881 proposed by the present applicant, in a vertical overflow drain CCDB solid-state imaging device, the potential of the channel region under the transfer electrode is reduced by lowering the potential of the transfer electrode. Then, the photocharge is discharged to the substrate side.

FIG. 4 is a plan view showing an image pickup section of the vertical overflow drain type CCD solid-state image pickup device as described above.
The figure is a sectional view taken along line XX '.

A P-Well region (2) is formed on one surface of the N-type semiconductor substrate (1), and a plurality of P ⁺ -type channel stop regions (3) are formed in the P-Well region (2). Arrayed in parallel. An N-type diffusion region (4) is formed between each channel stop region (3) to form a buried storage transfer channel. Then, transfer electrodes (5a) and (5b) orthogonal to the channel stop region (3) are formed on the diffusion region (3) via the insulating film (6). This transfer electrode (5a)
(5b) has a two-layer structure, of which the upper transfer electrode (5b) has a narrower width on the channel stop region (3) and between the adjacent lower transfer electrodes (5a). It is provided across. These transfer electrodes (5a) (5b) are
It is pulse-driven by four-phase transfer clocks φ _{F1 to} φ _F4 , and four layers of transfer clocks φ _{F1 to} φ _F4 are sequentially applied to each transfer electrode (5a) (5b).

On the other hand, the semiconductor substrate (1) is fixed at a constant potential Vsub, and the P-Well region (2) is a channel stop region (3).
It is fixed to the ground potential via. FIG. 6 shows the potential state of the YY ′ line (in FIG. 5) when a specific potential is applied to the semiconductor substrate (1) and the P-Well region (2). At this time, the transfer electrode (5b) is held at a level higher than the ground level by a constant value, and P-We
A potential barrier is formed near the ll region (2). Therefore, the photocharges e are accumulated in the potential well formed between this potential barrier and the potential barrier on the surface of the semiconductor substrate (1). The transfer of the photocharge e is performed by changing the potentials of the transfer electrodes (5a) and (5b) within a range in which the potential barrier near the P-Well region (2) can maintain a sufficient height.

Here, when the potentials of the transfer electrodes (5a) and (5b) are set to a certain level or below or negative, the potential on the surface of the semiconductor substrate (1) becomes shallow as shown by the broken line in FIG. 4) Because the potential inside is shallow, P
The potential barrier near the -Well region (2) disappears and all the photocharges e flow to the semiconductor substrate (1) side. Therefore, the discharge of the photocharge e accumulated in the accumulation transfer channel can be performed by lowering the potentials of the transfer electrodes (5a) and (5b).

(C) Problems to be Solved by the Invention In the vertical overflow drain type CCD as described above, when the potentials of the transfer electrodes (5a) and (5b) are lowered, the photocharge e is on the semiconductor substrate (1) side. It is necessary to set the concentration and each potential of the P-Well region (2) so as to flow in the direction. That is,
With respect to the potentials of the transfer electrodes (5a) and (5b) when transferring and driving the photocharge e, a potential barrier is formed near the P-Well region (2) as shown by the solid line in FIG. For the potential of the transfer electrodes (5a) and (5b) when discharging,
The concentration and depth of the P-Well region (2) are set so that a potential barrier is not formed from the front surface to the back surface of the semiconductor substrate (1) as indicated by the broken line in FIG. So P-Well
Decrease the impurity concentration in region (2), or use P-Well
When the region (2) is provided at a shallow position of the semiconductor substrate (1),
Since the photocharge e easily flows into the semiconductor substrate (1) when the potentials of the transfer electrodes (5a) (5b) are lowered, the range of potential fluctuations of the transfer electrodes (5a) (5b) can be reduced. However, if the concentration of P-Well region (2) is lowered, P-Well
The potential barrier of the region (2) becomes low, the capacity for accumulating the photocharge e becomes small, and when the P-Well region (2) is made shallow, the effective region for photoelectric conversion becomes narrow, and thus the light receiving sensitivity decreases. To do.

On the contrary, in order to increase the storage capacity and the light receiving sensitivity, P
If the concentration of the -Well region (2) is increased or the P-Well region (2) is deeply formed, it becomes difficult for the photocharge e to be discharged. Therefore, the fluctuation range of the potential of the transfer electrodes (5a) (5b) is widened. In addition to this, there is a possibility that the discharge of the photocharge e may be incomplete.

Therefore, the present invention is a vertical overflow drain type CC.
The purpose of D is to increase the storage capacity and the light receiving sensitivity without impairing the operation of discharging the photocharge.

(D) Means for Solving the Problems The present invention has been made to solve the above problems.
A transfer electrode is formed on a diffusion region of a conductivity type opposite to that of the substrate provided on one main surface of the semiconductor substrate, and a channel region for accumulating photocharges generated by photoelectric conversion is formed in the diffusion region. In a vertical overflow drain type CCD solid-state imaging device driving method, the transfer electrode capable of forming a potential barrier between the channel region and the semiconductor substrate and the transfer electrode with respect to both potentials of the semiconductor substrate. Is set to a low potential and the semiconductor substrate is set to a high potential to eliminate a potential barrier between the channel region and the semiconductor substrate, and unnecessary photocharges in the channel region are discharged to the semiconductor substrate side. It is characterized by being busy.

(E) Action According to the present invention, by lowering the potential of the transfer electrode and raising the potential of the semiconductor substrate, the photocharge in the channel region is prevented even when the diffusion region is provided at a high concentration or deep in the substrate. Be sure to discharge to the semiconductor substrate side. On the contrary, by raising the potential of the transfer electrode and lowering the potential of the semiconductor substrate, the potential barrier formed in the vicinity of the diffusion region is increased and the storage capacitance of the channel region can be increased.

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

FIG. 1 is a diagram showing a potential state of a CCD solid-state image pickup device according to the driving method of the present invention. The CCD solid-state image pickup device itself is the same as that shown in FIGS. 4 and 5, and a description thereof will be omitted here.

The feature of the present invention resides in that the potential of the semiconductor substrate (1) is changed according to the change of the potentials of the transfer electrodes (5a) and (5b). That is, instead of the constant potential Vsub shown in FIG. 5, the semiconductor substrate (1) is applied with the discharge control clock φsub which becomes high level during the discharge operation. This discharge control clock φsub is held at the above-mentioned potential Vsub during the discharge operation, and is kept at the potential Vs during the period for accumulating photocharges.
It is kept at a lower potential than ub.

The transfer electrodes (5a) (5b) have a high potential and the semiconductor substrate (1)
When the potential becomes low, the potential of the CCD solid-state image sensor in the depth direction (corresponding to the line YY 'in FIG. 5) forms a potential barrier near the P-Well region (2) as shown by the solid line in FIG. To be done. Therefore, the photocharge e is accumulated between this potential barrier and the potential barrier on the surface of the semiconductor substrate (1).

On the contrary, when the transfer electrodes (5a) and (5b) have a low potential and the semiconductor substrate (1) has a high potential (Vsub), the potential of the semiconductor substrate (1) is changed from the front surface to the back surface as shown by the broken line in FIG. There is a gradient, and the photocharge e flows along the gradient toward the semiconductor substrate (1) side.

Here, comparing the potential state of FIG. 1 with the case of FIG. 6, the potential state during the discharging operation (shown by the broken line in the figure) is the same, but the potential state during the accumulation of the photocharge e. In (indicated by the solid line in the figure), it can be seen that the charge storage capacity is large.

FIG. 2 is a block diagram showing a configuration when performing exposure control of a CCD solid-state image pickup device by adopting the driving method of the present invention.

The CCD (10) obtains video information by photoelectrically converting a received image, and outputs a continuous video signal X _(t) on a screen-by-screen basis by being pulse-driven. The video signal X _(t) is subjected to processing such as sample hold and gamma correction in the signal processing circuit (11) and output as a video signal Y _(t) to an external device.

On the other hand, the transfer clock φ _F is supplied from the read clock generation circuit (12) to the transfer electrode of the CCD (10), and the discharge control clock φsub is supplied from the discharge clock generation circuit (13) to the substrate. The clock generation circuits (12) and (13) are supplied with the read timing signal FT and the discharge period setting signal DT for setting the operation timing and the operation period from the read timing generating circuit (14) and the discharge period setting circuit (15), respectively. To be done. The read timing generation circuit (14) and the discharge period setting circuit (15) are provided with a vertical scanning signal VD and a horizontal scanning signal.
The discharge period setting circuit (15) operates based on HD, and the discharge period is expanded / contracted according to the determination output of the exposure amount determination circuit (16).

The exposure amount determination circuit (16) determines the level of the video signal X _(t) obtained from the CCD (10), and outputs an exposure suppression signal CLOSE if the level is higher than the proper range, and conversely if the level is higher than the proper range. If it is low, the exposure promotion signal OPEN is output.

FIG. 3 is a timing diagram showing the operation of FIG.

The read timing signal FT generates a timing pulse I at a predetermined timing in each blanking period of the vertical scanning signal VD, and the read clock generation circuit (12) generates a clock pulse B at the input of this timing pulse I. On the other hand, the discharge period setting signal DT becomes low level at the rising edge of the vertical scanning signal VD and becomes high level again at the timing determined according to the exposure amount of the CCD (10), and the low level period is set as the discharge period. . Also, this discharge period setting signal
The timing of the rise of DT from low level to high level is
It is configured so that it is delayed by the exposure suppression signal CLOSE and accelerated by the exposure promotion signal OPEN. In such a configuration, for example, a step counter that counts up with the horizontal scanning signal HD and a rising timing are stored as the number of horizontal scanning lines, and the exposure suppression signal CLOSE counts up and the exposure promotion signal OPEN.
By using the up-down counter which is counted down by, the coincidence of the outputs of both counters is detected by the comparator,
The output of the comparator is used as the set input of the flip-flop, the rising edge of the vertical scanning signal VD is used as the reset input, and the output of the flip-flop is used as the discharge period setting signal DT.
It can be obtained by

When the discharge period setting signal DT becomes low level, the transfer clock φ _F becomes low level during the blanking period of the horizontal scanning signal HD and the discharge control clock φsub becomes high level. Therefore, during the blanking period of the horizontal scanning signal HD, CC
The potential in the depth direction of D (10) becomes as shown by the broken line in FIG. 1, and the photocharge is discharged.

After the discharge period setting signal DT rises from the low level to the high level, the transfer clock φ _F is held at the high level until the timing pulse y of the read timing signal FT is input, and the discharge control clock φsub is low. Hold on to the level. At this time, a potential as shown by the solid line in FIG. 1 is formed and accumulated in the photocharge. Therefore, the period from the timing when the discharge period setting signal DT rises from the low level to the high level to the timing when the read timing signal FT generates the timing pulse is set as the photoelectric conversion period E, and one screen is displayed in this photoelectric conversion period E. Minute video information is accumulated.

According to the above configuration, when the exposure amount of the CCD (10) becomes higher than the proper range, the rising timing of the discharge period setting signal DT is delayed by 1H (one cycle of the horizontal scanning signal HD), so that the photoelectric conversion period E Is shortened, and conversely, when it is lower than the proper range, the rising timing of the discharge period setting signal DT is advanced and the photoelectric conversion period E is extended.
The exposure amount of (10) is controlled so that it falls within an appropriate range.

By the way, in the normal CCD (10), since the video signal X _(t) is sequentially output even during the period when the photocharge discharging operation is performed, the noise during the discharging operation causes the video signal. X _(t)
May be superimposed on. Therefore, in the present embodiment, if the transfer clock φ _F is changed as shown in FIG. 3 and the photocharges are discharged in synchronization with the blanking period of the horizontal scanning signal HD, the influence on the video signal X _(t) is not affected. Extremely low. Similarly, by changing the discharge control clock φsub in synchronization with the blanking period of the horizontal scanning signal HD, the video signal X _(t)
It is possible to prevent the noise from being superimposed on.

In this embodiment, the discharge period of the photocharges is set to the period from the rising of the vertical scanning signal VD to the timing determined according to the exposure amount, but if the end time of the discharge period is the same. It is not necessary to match the start of the discharge period with the rising timing of the vertical scanning signal VD.

Particularly, in the driving method according to the present invention, it is possible to complete the discharge of the photocharges in an extremely short period, so that it is possible to set the discharge period to within 1H period.

(G) Effect of the Invention According to the present invention, it is expected that the light receiving sensitivity and the storage capacity of the CCD solid-state imaging device will be improved. In addition, since it is possible to reduce the variation range of the CCD drive clock without lowering the light receiving sensitivity and the storage capacity, power saving can be expected.

[Brief description of drawings]

FIG. 1 is a potential diagram for explaining the driving method of the present invention, FIG. 2 is a block diagram showing the structure of an exposure control method adopting the driving method of the present invention, FIG. 3 is its timing diagram, and FIG. 4 is a vertical overflow. FIG. 5 is a cross-sectional view taken along line XX ′ of FIG. 5, and FIG. 6 is Y-
It is a potential diagram of Y'line. (1) …… Semiconductor substrate, (2) …… P-Well region,
(3) …… Channel stop region, (4) …… Diffusion region, (5a) (5b) …… Transfer electrode, (10) …… CCD, (1
1) …… Signal processing circuit, (12) …… Read clock generation circuit, (13) …… Discharge control clock generation circuit, (14) ……
Readout timing generation circuit, (15) ... discharge period setting circuit, (16) ... exposure amount judgment circuit.

Claims (3)

[Claims] 1. A well region having a conductivity type opposite to that of the substrate is provided at a predetermined depth on one main surface of a semiconductor substrate, and a plurality of transfer electrodes are arranged on the semiconductor substrate so as to cover the well region. In a method of driving a vertical overflow drain type CCD solid-state imaging device in which a channel region for accumulating photocharges generated by photoelectric conversion is formed in the well region under the transfer electrode, the semiconductor substrate of the semiconductor substrate A first potential applied to the transfer electrode when the photo barrier is accumulated in the channel region by forming a potential barrier in the well region that blocks the movement of charges from the surface side to the deep region, and the semiconductor substrate The potential applied to the transfer electrode is lower than the first potential and the potential applied to the semiconductor substrate is higher than the second potential with respect to the applied second potential. It abolished the potential barrier in the region, the driving method of the solid-state imaging device unnecessary light charge in the channel region, characterized in that to discharge to the semiconductor substrate side. 2. The method for driving a solid-state image pickup device according to claim 1, wherein during the vertical scanning period of the solid-state image pickup device which is scanned in the horizontal and vertical directions, the first to the middle of the vertical scanning period. After the photocharges of the channel region are discharged to the semiconductor substrate side during the period of, the potential barrier that blocks the movement of the charge from the surface side of the semiconductor substrate to the deep side is applied to the well region during the remaining second period. A method of driving a solid-state image pickup device, comprising: forming a photoelectric charge in a channel region, accumulating photocharges in the channel region, and obtaining image information for one screen from the photocharges accumulated in the second period. 3. The method for driving a solid-state image pickup device according to claim 2, wherein the photocharges in the channel region are discharged to the semiconductor substrate side only within a blanking period of horizontal scanning. And a method for driving a solid-state image sensor.