JPH0322768A - Driving method for solid-state image pickup element - Google Patents

Abstract

PURPOSE:To lower the voltage of a driving pulse when light charges in a chan nel area are discharged by discharging light charges in an image pickup part from the channel area to an overflow drain in a process of transfer in the opposite direction from a read direction. CONSTITUTION:The CCD solid-state image pickup element 10 converts an image irradiating the image pickup part photoelectrically. Light charges accumulated in the CCD10 are read out and driven with a transfer clock phiF forward at constant intervals, transferred with an opposite-directional transfer clock phiB at timing set with a discharge timing signal RT in the opposite direction from the direction of the read driving, and also discharged to the overflow drain with a potential control clock phiOFD. Consequently, the light charges remaining in the channel area can be discharged to the overflow drain without increasing the driving voltage, so that the light charges can be discharged efficiently.

Description

DETAILED DESCRIPTION OF THE INVENTION <<A>> Industrial Application Field The present invention relates to a method for driving a solid-state image pickup device for discharging photocharges in a channel region to the outside of the channel region. <Background> Conventional technology Conventionally, in imaging devices such as television cameras that use CCD solid-state imaging devices, it has been considered to electronically control exposure by utilizing the operating principle of CCD. Such an exposure control method, as disclosed in, for example, Japanese Patent Application Laid-open No. 63-24764, transfers and discharges the photocharges accumulated in the imaging section in the middle of the photoelectric conversion period of each vertical scanning period, and removes the remaining photoelectric charges. It is configured to accumulate the photocharge obtained by photoelectric conversion during the photoelectric conversion period. That is, the photoelectric conversion period is expanded or contracted by changing the timing of photocharge discharge, and the timing of photocharge discharge is set according to the level of the output signal of the CCD. In this CCD driving method, unnecessary photocharges in the imaging section are discharged by transferring them in the opposite direction to the readout transfer direction. Smear occurs during ejection and transfer as well as during transfer. Therefore, various methods of discharging photocharges that do not cause smearing have been considered. For example, in Japanese Patent Application No. 1-96712 previously proposed by the present applicant, by increasing the potential of the overflow drain provided in parallel with the storage and transfer channel of the imaging section, By eliminating the potential barrier between the capacitor and the drain, the photocharge is discharged from the storage transfer channel to the barflow drain. FIG. 3 is a plan view of a main part of a CCD solid-state image pickup device employing the above-described driving method, and FIG. 4 is a cross-sectional view taken along line xx' in FIG. 3. Here, an imaging section of a frame transfer type CCD with a cross-gate structure is shown. A plurality of channel stops are provided on one side of the P-type substrate (1).
2> are arranged in parallel to each other by LOGOS, and an overflow drain <3) is formed below this channel stop <2>. An N-type channel region (4) is formed between each channel stop (2) by diffusion, and a plurality of lower electrodes (5) are formed on this channel region (4) in a direction orthogonal to the channel stop (2). ) are provided in parallel, and a plurality of upper layer electrodes (6) are further provided along the channel stop <2>.
6>, the protruding parts covering the gap between the lower layer electrodes <5> are formed so as to alternate with the adjacent upper layer electrodes (6>). ■〜φ
The upper layer electrode <6> is pulse-driven with the transfer clocks φ■, φ, . , transfer clocks φ, .φ■ are applied alternately to the lower layer electrode (5>), and potential control clocks φ, ., are applied to the overflow drain (3>). When accumulating photocharges, for example, by setting the transfer clock φ■ at a high level and the control clocks φ. (4) and the overflow drain (3>) to form a potential barrier. Therefore, the photocharge generated in the light receiving region (7>) flows along the potential gradient to the upper layer electrode (3).
6) and is accumulated in the channel region (4). On the other hand, when discharging the photocharges in the channel region (4), the transfer clock φ■ is activated, contrary to the case of accumulation. By setting the low level and the control clock φ, to high level, the potential of the channel region (4) is made shallow as shown by the broken line in FIG. 3, and the potential of the channel region (4) and the overflow drain (
3) Eliminates the potential barrier between When the potential barrier disappears, the photocharge in the channel region (4〉) flows along the potential gradient into the barflow drain (
Flows to 3). Therefore, the photocharges in the channel region <4) are discharged. According to this method of discharging photocharges, the photocharges of the entire imaging section can be discharged almost simultaneously and in an extremely short period of time, so smearing is significantly reduced compared to a method of discharging photocharges that uses reverse transfer. suppressed by (8) Problems to be Solved by the Invention However, when photocharges flow from the channel region (4) to the overflow drain (3), the channel region (0
Unnecessary photocharges may remain inside the device. This is due to manufacturing variations in the impurity concentration of the channel region (4), overflow drain (3), etc., and the thickness of the LOGOS in the channel stop (2) portion.
This is because there are areas where the potential barrier between the Photocharge remains. In order to prevent such photocharges from remaining, an extremely high potential difference is applied to the channel region (4) so that the Botenskel barrier between the channel region (4) and the overflow drain (3) is sufficiently eliminated in all regions. ) and the overflow drain <3>.Therefore, a problem arises in that the voltage of the driving pulse for driving the CCD becomes high.Therefore, the present invention aims to drain the photocharges in the channel region (4>). The purpose of the present invention is to reduce the voltage of the drive pulse when the drive pulse is applied and to prevent photocharges from remaining in the ticket channel region (4). It was done to solve
In a method for driving a CCD solid-state imaging device, a plurality of channel regions are arranged and separated from each other by a channel stop region, and an overflow drain provided in the channel stop region receives excess photocharge in the channel region. , with respect to the potential of the overflow drain that can form a potential barrier between the channel region and the overflow drain, the overflow drain is set at a high potential to eliminate the potential barrier, and the The present invention is characterized in that by pulse-driving the transfer electrode, photocharges in the channel region are discharged from the channel region to the overbroad drain. (E) Effect According to the present invention, since photocharges in the imaging section are discharged from the channel region to the overflow drain in the process of being transferred in the opposite direction to the readout direction, the channel region and the overflow drain are connected. The photocharges remaining in the region where the potential barrier is difficult to disappear between the transfer paths are discharged to the overflow drain in the region in the transfer path where the potential barrier is easy to disappear.

《《〉《〉Example〉An example of the present invention will be explained according to the drawings. FIG. 1 is a block diagram of an imaging device that performs automatic exposure control using the driving method of the present invention. The CCD solid-state imaging device (10) photoelectrically converts the image irradiated onto the imaging section, and the resulting photocharges are read out and driven at regular intervals to output a continuous video signal X (t) for each screen. This video signal X (t) is processed by the signal processing circuit (11)
Processing such as sample hold and gamma correction is performed on the signal, and the signal is output to an external device as a signal Y(t). C O D (10) is pulse driven by various clocks, and the forward transfer clock φ from the read clock generation circuit (12>) and the reverse transfer clock φ. from the discharge clock generation circuit (13) are sent to the imaging section. Furthermore, the potential control clocks φ., D are supplied from the control clock generation circuit <14> to the overflow drain of CCD (10).These clock generation circuits (12) ( 13) and <14> include a read timing setting circuit (15) and a discharge timing setting circuit (16).
A read timing signal FT and a discharge timing signal RT are respectively supplied from the terminals 1 and 2, and the operation is performed according to the read timing signal FT and the discharge timing signal RT. That is, C
The photocharges accumulated in the CD (10) are read out and driven by the forward transfer clock φ, at regular intervals, and are read out by the reverse transfer clock φ. The potential control clock φ is transferred in the direction opposite to the read drive direction. , , the water is discharged to the overflow drain. First, the exposure amount judgment circuit (17) is C C D (10
) detects the exposure amount of the video signal X (t) obtained from
If the exposure amount is above the appropriate range, the exposure suppression signal CLO
SE, if it is below the appropriate range, the exposure promotion signal OPEN is given to the discharge timing setting circuit (16).The discharge timing setting circuit (16) then outputs the exposure suppression signal CLOS.
Expansion and contraction of the exposure period is controlled according to E and the exposure promotion signal OPEN. Figure 2 is a timing diagram showing the operation of Figure 1. The read timing signal FT generates a timing pulse (a) at a predetermined timing during the blanking period of the vertical scanning signal VD.
), and upon input of this timing pulse (a), the readout clock generation circuit (12) generates a clock pulse (b) for vertically transferring photocharges for one screen.The discharge timing signal RT is , has a timing pulse (e) at a specific timing in the vertical scanning period, and at this timing the photocharges accumulated up to that point are discharged.The timing at which this timing pulse (C) is generated is determined by the exposure amount judgment circuit. The timing pulse (e) is input to the discharge clock generation circuit (13) and the control clock generation circuit (14). Then, during the blanking period of the horizontal scanning signal HD, the potential control clock φ. During the blanking period of the horizontal scanning signal HD from the rise to the fall of .,D, the reverse direction transfer clock φ, is a clock pulse (
d). During the photocharge discharge period, the potential control clock φ. . As a result, the potential of the barflow drain is increased and a deep potency hole is formed, and the photocharge in the channel region is transferred in the opposite direction with the barrier between the channel region and the overflow drain disappearing. It will be done. Therefore, if there is a region where the potential barrier does not completely disappear and a photocharge remains, if the residual charge enters the region where the potential barrier disappears while being transferred over a certain period, it will become a single bar broad. , is discharged into the rain. The period from the end of this discharge period until the next forward transfer clock φ generates a clock pulse (b) is set as an exposure period E, and photo charges for one screen are accumulated in this period E. ．． According to the driving method as described above, the reverse transfer clock φ.
Since it is sufficient that the photocharges are discharged from the channel region to the overflow drain in a part of a certain section where they are transferred by Even if there is a region, it is possible to reduce the drive voltage by controlling the potential according to this region. In this embodiment, a case is illustrated in which the photocharge discharge timing is made to correspond to the exposure amount of C O D (1G) (automatic exposure control), but it is also possible to use the driving method of the present invention. It is also possible to realize a variable speed electronic shutter. In this case, a still image at a predetermined shutter speed can be obtained by manually setting the ejection timing. (G) Effects of the Invention According to the present invention, the photocharges remaining in the channel region can be discharged to the overflow drain without increasing the drive voltage, so the photocharges can be efficiently discharged and the exposure period The expansion and contraction control can be easily realized.Also, since the drive voltage can be lowered, the circuit that generates the drive clock can be easily configured, and the cost can be reduced.

[Brief explanation of drawings]

Fig. 1 is a block diagram of an imaging device using the driving method of the present invention, Fig. 2 is an operation timing diagram of Fig. 1, and Fig. 3 is a CCD
A plan view of the main parts of the solid-state image sensor, FIG. 4 is a cross-sectional view of FIG. 3,
Figure 5 is a diagram of the potency inside the CCD solid-state image sensor.

Claims (2)

[Claims] (1) A plurality of channel regions that accumulate and transfer photocharges generated by photoelectric conversion are separated from each other by a channel stop region and arranged in an array, and an overflow drain provided in the channel stop region is connected to an overflow drain provided in the channel stop region. In a method for driving a CCD solid-state imaging device that receives a photoelectric charge, the potential of the overflow drain is set at a high potential with respect to the potential of the overflow drain that can form a potential barrier between the channel region and the overflow drain. Disappearing the barrier and driving the transfer of photocharges in the channel region by pulse-driving a transfer electrode provided on the channel region to discharge the photocharges from the channel region to the overflow drain. A method for driving a solid-state image sensor, characterized by: (2) In the method for driving a solid-state image sensor according to claim 1, during a vertical scanning period of the solid-state image sensor that is scanned in horizontal and vertical directions, photocharges in the channel region during a first period. is discharged to the overflow drain, photocharges are accumulated in the channel region during the remaining second period, and the photocharges obtained during the second period are output as one screen of video information. A method for driving a solid-state image sensor.