JPH0434868B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field to Which the Invention Pertains] The present invention relates to a driving method for an electronic still image recording/reproducing device (electronic camera).

[Prior art and its problems]

Interline transfer method is a type of solid-state imaging device
There is a CCD image sensor. Below, this is IT-
It is abbreviated as CCD. Figure 1 shows a schematic diagram of this IT-CCD image sensor. In the figure, 1ij
(i = 1, 2, . . . , m, j = 1, 2, . . . n) are photosensitive parts arranged in a matrix, and are made of, for example, photodiodes, which accumulate signal charges obtained by photoelectrode conversion. A vertical CCD 2i for reading out the signal charges photoelectrically converted and then accumulated in the photosensitive parts 1ij along the arrangement of the photosensitive parts 1ij, and an overflow for removing excess signal charges photoelectrically converted in the photosensitive parts 1ij. drain (ever flw
drain: OFD) 4. An overflow control electrode 5i is provided between the OFD 4 and the photosensitive portion 1ij to reliably remove excess signal charges from the OFD 4. The signal charge accumulated in the photosensitive section 1ij is transferred to the vertical CCD 2i, and then transferred column by column to the horizontal CCD 3 shown in the figure.
After transferring the contents of CCD 3, the output section 6
Read more. The vertical CCD 2i and the horizontal CCD 3 can transfer signal charges by applying single-phase, two-phase, three-phase or four-phase clock pulses to the transfer electrodes.

In this IT-CCD image sensor, while signal charges are accumulated in the photosensitive section 1ij, the adjacent vertical CCD 2i transfers the signal charges of the previous field column by column to the horizontal CCD 3 and reads them out. For example _, the signal charges in the photosensitive parts 1 ₁₁ , 1 ₂₁ , .
You will stay there for a time close to the field period.
The signal charges accumulated in these photosensitive areas are read out by the vertical CCD 2i, and before the signal charges of the next field are transferred from the photosensitive area 1ij to the vertical CCD 2i, there are no signal charges in the vertical CCD 2i. Must not be left behind. However, in reality, signal charges are formed within the semiconductor substrate forming the solid-state imaging device due to light incident on the photosensitive section, so a small amount of signal charges diffused within the semiconductor substrate leak into the vertical CCD 2i. It's gone. This small amount of leakage of signal charges causes bright lines in the vertical direction on the reproduced image, that is, vertical smear. This becomes noticeable when capturing an image of a subject with high brightness, so it significantly changes the image quality. In order to reduce this vertical smear, there is a method that sweeps away the leakage charge that causes the vertical smear at high speed before transferring the signal charge from the photosensitive section 1ij to the vertical CCD 2i during the vertical blanking period. It has been adopted as a drive method for solid-state image sensors.

The above method is a method for preventing false signals such as smear from a solid-state image sensor intended for continuous imaging.

On the other hand, electronic still image recording and reproducing devices (hereinafter referred to as electronic cameras), which have been attracting attention recently, often use optical or electric shutters to capture images for a desired fixed period of time. A method to prevent this is desired.

[Purpose of the invention]

An object of the present invention is to provide a drive method suitable for solid-state imaging devices such as the above-mentioned electronic cameras, which prevents false signals such as vertical smear and cross talk.

[Summary of the invention]

The present invention provides an electronic camera or the like in which potential wells are formed at the same depth over the entire surface of the vertical CCD section under each transfer electrode over a desired period including the exposure period of light incidence during the shutter open period.
A drive that captures carriers generated within the substrate in its potential well, and after discharging these captured carriers by a sweeping operation, reads out the accumulated signal charge that becomes an effective signal of the photosensitive cell and converts it into a video signal. It is a method.

〔Effect of the invention〕

According to the present invention, good image quality that prevents smearing can be ensured in imaging with an electronic camera. Further, it is possible to obtain a good image with high resolution and less cross talk between adjacent pixels in the horizontal direction.

[Embodiments of the invention]

Embodiments of the present invention will be described using the drawings.

FIG. 2 is a horizontal cross-sectional view of the IT-CCD image sensor of FIG. A photodiode 12 consisting of an n-type region formed on a p-type Si substrate 11 by diffusion or ion implantation and a vertical transfer CCD 13 are arranged as constituent elements of each photosensitive cell. Additionally, an overflow drain 14, designated n, is provided. vertical transfer
On the top of the CCCD 13 is a transfer electrode 1 made of poly-Si.
There are 5 and 16. In this example, the transfer electrode 15 also serves as a transfer gate from the photodiode to the vertical CCD. Further, an Al electrode 18 serving as a light shield is formed on the top with an insulating film 17 such as SiO ₂ interposed therebetween. The figure shows an example in which this Al electrode also serves as an electrode that controls the barrier from the photodiode to the overflow drain.

19-1 of the light incident on the photodiode
The light shown by is absorbed in the depletion layer of the photodiode shown by the dotted line, generating carriers. Since this carrier is within the junction, it is accumulated as an effective signal charge. On the other hand, the light indicated by 19-2 generates a carrier inside the substrate. This carrier diffuses in any direction unless a specific electric field is formed inside the substrate. The figure shows an example of diffusion to the left, where it is captured in a potential well shown by a dotted line formed by a voltage applied to the transfer electrode of the CCD in advance.

Figure 3 shows a cross-sectional view of a vertical CCD. This is a developed view in the vertical direction of the vertical CCD in FIG.
Embedded CCD with phase-driven two-layer poly-Si electrode configuration
Here is an example.

Further, FIG. 4 is a diagram showing the phase relationship of electric pulses in the driving method of the present invention. The operation will be explained with reference to FIG. 3 or FIG. 2 mentioned above.

At time t=t ₀ , the transfer electrodes φ ₁ 16-1, φ ₂ 1
5-1, φ ₃ 16-2, and φ ₄ 15-2, for example, the high transfer electrode voltage during vertical transfer.
Apply level voltage _VH . This allows vertical transfer
A series of empty potential wells are formed at the bottom of the CCD, indicated by dotted lines. During this period, at t= _t1 , the object light image to be captured by an optical shutter, etc. is IT.
- Lead to CCD. Among the carriers generated by light incidence, the carriers generated within the depletion layer of the photodiode as described above are accumulated on the spot as signal charges, and some of the carriers generated outside the depletion layer are transferred to the transfer electrode during diffusion. Captured in the lower potential well. If the potential well under the transfer electrode can be formed deeply, it will capture most of the carrier that diffuses to adjacent pixels, making it possible to prevent cross talk. At time t= _t2 , when no light is incident and most of the generated carriers are trapped in one of the depletion layers, the transfer electrodes are driven in high-speed four-phase mode and a sweep operation is started. After sufficiently sweeping out the signal charge at t=t ₃ , the signal charge accumulated in the photodiode is applied to φ ₃ and φ ₄ at a high voltage.
Apply V _T and transfer to the vertical CCD side. After t= _t4 , the signal charges transferred to the vertical CCD are horizontally driven by 4-phase drive with pulses of _VH and _VL for _φ1 to _φ4 , respectively.
The operation is to transfer to the CCD, and although it is not shown in the figure,
The signal passes through the horizontal CCD and is output as a signal to the outside through an amplifier.

As shown in the above embodiment, the intermittent exposure in Fig. 4 is equivalent to the fact that the vertical CCD does not carry signal charges during exposure.
Since the carrier mixed in the potential well at the bottom of the CCD is vertically swept out in advance, vertical smearing during continuous imaging corresponding to conventional continuous exposure can be prevented. In addition, crosstalk caused by carriers leaking into adjacent pixels can also be improved.

[Other embodiments of the invention]

In the above embodiment, the transfer electrode voltage during exposure is set to _VH , but the voltage is not limited to this, and any voltage as high as the voltage of V _T may be used. Furthermore, it goes without saying that the sweeping drive is not limited to four-phase drive, but may also be operated by single-phase, two-phase, three-phase, etc. clock pulses.

[Brief explanation of drawings]

Figure 1 is a schematic diagram of an interline transfer type CCD, and Figure 2 is an interline transfer type CCD according to the present invention.
FIG. 3 is a horizontal cross-sectional view illustrating the driving method of the CCD, FIG. 3 is a vertical cross-sectional view, and FIG. 4 is a diagram showing the pulse voltage waveform of the driving method of the present invention. 1ij...Photosensitive part, 2i...Vertical CCD, 3...Horizontal
CCD, 4...overflow drain, 5i
...overflow drain control electrode, 6...output section, 11...p-type Si substrate, 12, 13...n ⁺ layer, 1
4...n layer, 15-1, 15-2...first layer poly-Si
Electrode, 16-1, 16-2... second layer poly-Si electrode,
17... _SiO2 , 18...Al.

Claims (1)

[Claims] 1. A plurality of mutually independent photosensitive cells provided for accumulating signal charges generated in a semiconductor substrate by light incidence over a shutter open period;
In an electronic camera having a plurality of vertical readout sections for reading out signal charges accumulated in these photosensitive cells and a horizontal readout section for reading out signal charges from these vertical readout sections, including the shutter open period. over a desired period of time,
Applying a constant voltage to each of the transfer electrodes so as to make the potential wells under each transfer electrode of the vertical readout part have the same depth over the entire surface, and then,
A driving method for an electronic camera, characterized in that after the carrier trapped in the potential well is discharged by a sweeping operation, the signal charge accumulated in the photosensitive cell is read out and converted into a video signal.