JPH0714199B2 - Driving method for charge transfer imaging device - Google Patents

Description

The present invention relates to a driving method of a charge transfer image pickup device.

(Structure of Conventional Example and Problems Thereof) FIG. 1 shows the structure of a conventional charge transfer imaging device.
A photoelectric conversion storage diode (hereinafter referred to as a diode) 1 formed by an n ⁺ layer that is two-dimensionally separated and arranged on a p-well layer on an n-type silicon substrate, and a charge of the diode 1 is applied in a vertical direction for each horizontal period. And a horizontal transfer charge reading register 3 for sequentially reading the parallel transfer charges of the vertical charge transfer register 2.

In addition, since it is necessary to perform a 2-to-1 interlace operation to prevent image flicker, one diode 1 is provided for each of the first vertical electrode φ _{V1 and} the second vertical transfer electrode φ _V2 .
Is provided, and the electric charge of the diode 1 is read out by the structure which forms the potential barrier (FIG. 2) provided adjacent to the diode 1 under the vertical transfer electrode. A method of forming a diode with an npn structure is used to suppress blooming in which an image is crushed when an excessive charge is generated.

FIG. 2 is a sectional view showing the structure in the vicinity of the pixel. The voltage V _SUB is biased between the p-well 5 and the n-type substrate 6, and the p-layer 7 of the pn junction diode 1 is depleted.

In FIG. 2, 8 is a potential barrier portion, 9 is a channel (buried layer) of the vertical charge transfer register, and 10 is a channel.
Is a vertical transfer electrode, 11 is a channel stopper, 12 is a light shielding film, and 13 is an insulating film.

FIG. 3 (a) is an explanatory diagram showing the blooming suppressing function.

The charge of the diode 1 becomes an empty state shown by 14 in the figure by opening the potential barrier section 8 and transferring the signal charge to the channel 9 of the vertical charge transfer register. Electric charges are accumulated by the light incident on the diode 1, and the potential well becomes shallow as shown by 14. The potential of the p-layer 7 of the diode is the potential of the transfer gate 16
The bias voltage V _SUB between the p layer 7 of the diode and the n-type substrate 6 is adjusted so as to be always deeper. For this reason,
The charge accumulated in the diode 1 flows out toward the n-type substrate 6 as indicated by 15 before flowing into the channel 9 of the vertical charge transfer register in a state of being an excess charge,
This allows the blooming charge to be absorbed.

FIG. 3B shows an example of a clock pulse applied to the vertical charge transfer register. When the signal charge is transferred from the diode 1 to the vertical charge transfer register 2, the vertical charge transfer register 2 is transferred to the horizontal transfer charge read register 3. A voltage V _H higher than the clock pulse V _M for transferring the signal charge is applied.
The potential barrier section 8 is common with the transfer electrode 10 of the vertical charge transfer register 2 and does not open when transferring the signal charge of the vertical charge transfer register 2 to the horizontal transfer charge read register 3; It has a barrier structure that opens when the signal charge is transferred to.

FIG. 4 shows the applied clock pulse for the field accumulation operation, and shows the clock pulse applied to the vertical charge transfer register when the image pickup apparatus as described above performs image pickup by the standard television system.

A clock pulse for driving φ _V1 and a clock pulse for driving φ _V2 are applied to the vertical charge transfer register 2. First
In the vertical blanking period t ₁ in the field, due to the φ _V1 clock pulse to which a high clock pulse is applied,
The charge of the diode 1 corresponding to this is read to the vertical charge transfer register 2 and then vertically and horizontally transferred and output sequentially. In the second field, φ in the vertical blanking period t ₂
Diode 1 corresponding to φ _V2 by _V2 clock pulse
Read out to the vertical charge transfer register 2,
It is driven to transfer and output as in the field. According to such a driving method, in the first and second fields, charges of different diodes in the vertical direction are read out, so that a perfect 2: 1 interlace operation is realized and a high vertical resolution is obtained. Also, each diode is 1 in 2 fields.
Since the reading is performed once, a 2-field accumulation, that is, a frame accumulation operation is performed. Although it is possible to capture images with almost no obstacles with such frame accumulation operation, when capturing an object that moves at high speed, blurring of the image occurs due to the influence of the accumulation time, and a satisfactory image is obtained. You won't get it.

FIG. 5 shows a drive pulse in the conventional example, and shows a clock pulse applied to reduce the blur of the accumulation time as described above.

Basically, it suffices to read each diode for each field, and in the image sensor shown in FIG. 1, the accumulated charge in the diode is such that φ _V1 and φ _V2 applied to the vertical transfer electrodes are simultaneously set to a high voltage. For example, the charges of the respective diodes corresponding to φ _V1 and φ _V2 can be read simultaneously in each field. Also after the charge is essential interlacing operation in a standard television system read simultaneously, for example, in the first field mixed with charge of phi transferred to _V2 phi _V2 by one vertical transfer charge of phi _V1, sequentially vertical, horizontal transfer Then, in the second field, the electric charges of φ _V2 are transferred to φ _V1 by vertical transfer, mixed, and then vertically and horizontally transferred and then output.

However, despite the field storage driving method that is feasible from the element configuration, it can be seen that when the elements are actually driven, a large signal level difference occurs in each field, causing flicker on the screen and failing to capture a satisfactory image. ing. There are the following driving methods for suppressing this flicker. That is, in the first field, after the accumulated charge of the diode corresponding to the first or second vertical transfer electrode is read out to the vertical charge transfer register, this charge is vertically transferred one by one. The accumulated charges of different diodes are read to the vertical charge transfer register, mixed with the charges read first, and then sequentially read and output. In the second field, the accumulated charges of the diode different from the first field are vertically transferred. This is a method of reading out to a register, carrying out 1 vertical transfer, reading out accumulated charges from the other diode, mixing, and sequentially outputting. However, it has been found that this method is not perfect in reducing flicker. Although a reduction effect is recognized, this method is emphasized when a bright subject is projected or when flicker that is not negligible occurs near saturated illuminance and color imaging requires a large gain. I've found that it can't be removed. When investigating the cause, it was found that there was a problem in the transfer from the diode to the vertical charge transfer register.

That is, in the transfer from the diode to the vertical charge transfer register, first, the accumulated charge of the diode is transferred to the vertical charge transfer register in which there is no charge, but then this charge is transferred vertically and then the read charge of the diode is read. It has been moved to the position. In this state, the charges accumulated in the diode are transferred to the vertical charge transfer register and mixed. That is, in the transfer from the first diode to the vertical charge transfer register and the transfer from the next diode to the vertical charge transfer register, the former is transfer to a place where no charge exists in the vertical charge transfer register, whereas the latter transfer is It was found that is due to the fact that the transfer is to the place where the electric charge exists and the transfer efficiency is different. In particular, the transfer from the diode to the vertical charge transfer register is an incomplete transfer of the BBD transfer type and is easily affected by this, and it has been found that the effect is large when a large amount of charges are present in the vertical charge transfer register. This causes flicker when the subject illuminance increases.

As described above, it is difficult to suppress flicker in the field accumulation operation.

From these things, we repeated the detailed examination experiment on the flicker associated with the reading from the diode to the vertical register and found the following.

A vertical transfer register having a two-phase structure easily causes flicker.

The transfer from the diode to the vertical charge transfer register by providing the transfer gate electrode is more likely to cause flicker than the transfer electrode of the vertical charge transfer register and the barrier structure (FIG. 2).

In the vertical charge transfer register having a two-phase structure, flicker changes due to complementarity of clock pulses.

The four-phase structure is highly resistant to flicker, and flicker does not occur even if the accumulated charges are simultaneously transferred from the diode to the vertical charge transfer register.

Even in the four-phase structure, the method of transferring the accumulated charge from one diode to the vertical charge transfer register, vertically transferring the same one charge, and transferring the accumulated charge of the other diode to the vertical charge transfer register and mixing them is flicker.

From the above experimental results, the vertical charge transfer register has a four-phase structure, and the charge transfer from the diode to the vertical charge transfer register uses the transfer electrode common to the vertical charge transfer register.
It was found that flicker does not occur in a driving method in which a potential barrier is provided and the accumulated charges of the diode are simultaneously transferred to the vertical charge transfer register, and then vertically transferred to mix the two charges. By such a method, field accumulation operation without flicker becomes possible. However, one problem arose here. It is associated with a p-well structure that has nothing to do with flicker.

As described above, since the excess charge due to blooming is absorbed in the deep portion of the substrate, the bias voltage V _SUB is applied between the p well layer and the n substrate, but it has been found that the stable range of this voltage can be set only to an extremely narrow range. It was That is, V _SUB
The larger V _SUBMX is determined by the voltage that absorbs the excess charge and the signal charge is not _extracted , and the smaller V _SUBMN is the voltage at which the pseudo charge is not injected from the n substrate to the p well. This V
_{It turns out} that the voltage range of _SUBMX and V _SUBMN can be only about 0.5V. This poses a practical problem when considering the temperature and the stability of the power supply. Despite the fact that the device design is designed to allow for about 5V, we investigated the reason why it could only take about 0.5V and found that the p-well layer fluctuated significantly when applied to the clock pulse applied to the transfer electrode. I found out that The p-well layer has a structure in which a contact is provided to stabilize the potential as much as possible, and thus some improvement is recognized, which is still a problem in practical use.

(Object of the Invention) The present invention is intended to solve the above-described problems, and an object of the present invention is to provide a stable method of driving a charge transfer imaging device without flicker.

(Structure of the Invention) In the present invention, photoelectric conversion regions that are two-dimensionally separated and arranged, and every other one of the photoelectric conversion regions corresponding to the photoelectric conversion regions, the first and second photoelectric conversion regions.
A vertical transfer electrode group is arranged, and a four-phase drive pulse is applied to the first and second vertical transfer electrode groups to transfer the charge accumulated in the photoelectric conversion region to the first and second vertical transfer electrode groups, and A method of driving a charge transfer imaging device, comprising: a vertical charge transfer register that transfers the accumulated charge in the photoelectric conversion region in a predetermined direction; and a horizontal transfer charge read register that sequentially reads parallel transfer charges of the vertical charge transfer register. Transfer of the accumulated charge in one pixel of the accumulated charges of two adjacent pixels in the photoelectric conversion region to the first vertical transfer electrode group is started, and then the accumulated charge in the other pixel at a timing different from that of the one pixel. Transfer to the second vertical transfer electrode group, and after the charge transfer to the first and second vertical transfer electrode groups is completed, the first and second vertical transfer in the vertical charge transfer register. Mixing of a charge of pole group, by transferring the mixed charges to the horizontal transfer charge read register, flicker without, in which a structure to have a broad stability on the bias voltage V _SUB.

(Explanation of Embodiment) FIG. 6 is a diagram showing an example of a waveform of a clock pulse according to an embodiment of the present invention, which is an enlarged view of an A field at a four-phase drive pulse and a vertical blanking time t ₁ . is there.

Since the accumulated charge of the diode is transferred to the transfer electrodes φ _V2 and φ _V4 of the vertical charge transfer register, the highest voltage is applied to this transfer electrode. In this embodiment, this highest voltage is applied to φ _V2 and then applied to φ _V4 after 0.5 μsec. That is, from the start transferring charges accumulated in the phi _V2 corresponding photodiodes phi _V2, after 0.5 .mu.sec, phi _V4 accumulated charge begins transfer phi _V4 of the corresponding photodiode, phi _V2
After the transfer of the accumulated charge to Φ _V4 is completed, the transfer of the accumulated charge to φ _V4 is completed. And after the transfer of accumulated charge is completed,
The charge of φ _{V2 and} the charge of φ _V4 are mixed and sequentially output. For φ _V1 and φ _V3 , the accumulated charge of the photodiode is φ _V2 ,
It is closed when it is transferred to φ _V4 , and serves to prevent flicker due to leakage of charges due to capacitive coupling, which has been a problem in the past. Therefore, flicker does not occur although the clock pulses are not complementary.

In the present embodiment, the voltage range between V _SUBMX and V _SUBMN ΔV _SUB
Showed stable operation at about 5V. When the time difference t ₃ between the pulse application times of φ _V2 and φ _V4 and ΔV _SUB were examined, it was found that when they were applied at the same time, a value of about 0.5 V yielded a substantially constant ΔV _SUB in about 0.2 μsec, and about 5 V could be secured. Became clear.

In the above example, the case where there is an overlap between the pulses to which a high voltage of φ _V2 and φ _V4 is applied has been described, but even if there is a time difference in the state where there is no overlap, the same effect can be obtained. High pulse
When V _H is added, it is expected that it will be most affected by this application, and it is considered that this is alleviated by increasing the time.

(Effects of the Invention) When the image pickup device picks up an image by the driving method of the present invention as described above, no flicker is observed, and even in color image pickup in which the gain of the amplifier is increased, the flicker disappears over the entire illuminance. In such a state, the stable range added to the p well layer and the n substrate for suppressing blooming is also
The effect of eliminating practical problems at 5V was obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a conventional charge transfer imaging device, and FIG.
FIG. 3 is a cross-sectional view showing a structure in the vicinity of a pixel, FIG. 3 (a) is an explanatory view showing a blooming suppressing function, and FIG. 3 (b).
Is a diagram showing an example of a clock pulse applied to the vertical charge transfer register, FIG. 4 is a diagram showing an applied clock pulse of the field accumulation operation, FIG. 5 is a diagram showing a drive pulse in a conventional example, and FIG. 6 is a book. It is a figure which shows an example of the waveform of the clock pulse by one Example of invention. 1 ... Photoelectric conversion storage diode, 2 ... Vertical charge transfer register, 3 ... Horizontal transfer charge read register, 9 ... Channel (buried layer) of vertical charge transfer register, 10 ...
… Vertical transfer electrode, 11 …… Channel stopper, 12 ……
Light shielding film, 13 ... Insulating film.

Claims (2)

[Claims] 1. A photoelectric conversion region in which a plurality of two-dimensionally separated arrays are arranged, and first and second vertical transfer electrode groups are arranged corresponding to the photoelectric conversion regions, and the first and second vertical transfer electrode groups are arranged. A four-phase drive pulse is applied to the vertical transfer electrode group to transfer the accumulated charge in the photoelectric conversion region to the first and second vertical transfer electrode groups and to transfer the accumulated charge in the photoelectric conversion region in a predetermined direction. In a driving method of a charge transfer imaging device having a vertical charge transfer register and a horizontal transfer charge read register for sequentially reading parallel transfer charges of the vertical charge transfer register, one of accumulated charges of two adjacent pixels in the photoelectric conversion region. Transfer of the accumulated charge in the first vertical transfer electrode group to the second vertical transfer electrode group is started at a timing different from that of the one pixel. Then, after the charge transfer to the first and second vertical transfer electrode groups is completed, the charges of the first and second vertical transfer electrode groups are mixed in a vertical charge transfer register, and the mixed charges are transferred to the horizontal direction. A method for driving a charge transfer image pickup device, comprising: transferring to a transfer charge read register. 2. A point of time when the accumulated charge of one pixel is completely transferred to the vertical transfer electrode group and a point of time when the accumulated charge of the other pixel is completely transferred to the vertical transfer electrode group is different. A method of driving a charge transfer imaging device according to claim 1, which is characterized in that: