JPH0552111B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Field of Industrial Application The present invention relates to a method for driving a solid-state image sensor, and particularly to a method for driving a solid-state image sensor using a frame transfer method.

(b) Conventional technology Solid-state imaging devices include interline transfer (IT) type CCDs (Charge Coupled
Device) and frame transfer (FT) method
There is a CCD. In an IT CCD, the imaging area and the storage area are arranged alternately, and in an FT CCD, the imaging section and the storage section are arranged separately.

The structure of the imaging section of the FT CCD is shown in FIGS. 4 and 5. This imaging section adopts a cross-gate structure, in which 1 is a P-type silicon substrate, 2 is a gate oxide film, 3 is a channel isolation region made of a field oxide film having a LOCOS structure, and 4 and 5 are horizontally extending regions. 6 and 7 are vertically extending upper gate electrodes made of polysilicon; 8 is an N ⁺ type overflow drain region; 9 is an N type meandering channel region; 10 is an opening window, 11 is an opening window 1
This is a P ⁺ type opening area provided below 0.

Such an FT type CCD is driven by a clock pulse shown in FIG. Clock pulses φ ₁ and φ ₃ are applied to every other upper layer gate electrode 6 and 7, and V _L ,
Clock pulses φ ₂ and φ ₄ having three levels, V _M and V _H , are applied to every other lower gate electrode 4 and 5, and have two levels, V _L and V _H.

In the odd field of the 2:1 interlaced drive, clock pulses φ ₂ , φ ₃ and φ ₄ are set to V _L and clock pulse φ ₁ is set to _VM to enter the light accumulation period. The charges photoelectrically converted during this photoaccumulation period are transferred to the clock φ ₁ around the aperture window 10.
is applied to the upper layer gate electrode 6, the clock φ ₂ ,
It is accumulated under the lower layer gate electrodes 4 and 5 to which the clock φ ₄ is applied, and is blocked under the upper layer gate electrode 7 to which the clock φ ₃ is applied. Note that excessive charges generated by the strong incident light overflow into the adjacent overflow drain region 8 through the channel separation region 3 under the upper layer gate electrode 6 to which the clock φ ₁ is applied. This accumulated charge is transferred from the imaging section to the storage section by the four-phase driving method in the next frame transfer period.

Then in the even field, clock pulses φ ₁ , φ ₂ and φ ₄ are set to V _L during the photoaccumulation period;
Clock pulse _φ3 is set to _VM . The charges photoelectrically converted during this period are transferred to the upper layer gate electrode 7 around the aperture window 10 to which clock φ ₃ is applied, the lower layer gate electrode 4 to which clocks φ ₂ and φ ₄ are applied,
5 and is blocked below the upper layer gate electrode 6 to which the clock _φ1 is applied. Similarly, this charge is transferred from the imaging section to the storage section by the four-phase driving method during the frame transfer period.

Note that it is necessary to give the OFD voltage of the overflow drain region 8 a deeper potential than that of the channel separation region 3 throughout the optical accumulation period and the frame transfer period. The reason for this is from the overflow drain region to the channel region 9.
This is to prevent charge from flowing backwards. Therefore, the potential of the channel separation region 3 becomes the deepest when V _H is applied to the upper layer gate electrodes 6 and 7 during the frame transfer period, and the OFD voltage becomes the deepest in the channel separation region when V _H is applied to the upper layer gate electrodes 6 and 7. It was fixed at a DC voltage higher than the potential of 3.

The above-mentioned conventional technology is described, for example, in the March 1986 issue of Electronic Materials published by Kogyo Kenkyukai, pages 123 to 127.

(c) Problems to be solved by the invention However, in the conventional FT CCD driving method described above, in the case of a 1/2 inch optical system,
When the resolution of a million pixel CCD is increased to 300,000 pixels,
The area of the aperture window 10 per pixel is reduced by about 70%. For this reason, there was a problem in that the sensitivity decreased due to the reduction in the area of the aperture window 10.

This decrease in sensitivity is considered to be due to an increase in the rate at which charges photoelectrically converted in the aperture window 10 flow out to the overflow drain region 8. FIG. 7 is an enlarged view of the CCD of the imaging section, showing the light accumulation period of an even field. At this time, the potential of the - line cross section is shown in FIG. That is,
Since the deepest potential is given to the overflow drain region 8, the charge generated in the opening window 10, shown by the dotted line, flows out to the overflow drain region 8 as shown by the dotted arrow due to the electric field of the overflow drain region 8. It's put away.

(d) Means for solving the problems The present invention has been made in view of the above-mentioned problems.
The present invention provides a method for driving a solid-state image sensor that improves the conventional problems by lowering the OFD voltage.

(E) Effect According to the present invention, by lowering the OFD voltage of the overflow drain region 8 during the photoaccumulation period, the electric field of the overflow drain region 8 is made weaker than that of the channel region 9, thereby channeling the charges generated in the aperture window 10. The rate of inflow into region 9 is greatly increased to prevent a decrease in sensitivity.

(F) Embodiment An embodiment of the present invention will be described in detail with reference to FIGS. 1 to 3.

FIG. 1 is an enlarged view of the CCD of the imaging section, showing the light accumulation period of an even field. FIG. 2 shows the potential of the cross section taken along the line -- in FIG. 1 and 2 correspond to FIGS. 7 and 8, and the same components are given the same reference numerals.

The feature of the present invention is that the overflow drain region 8
The reason for this is that the OFD voltage of the device was lowered from the conventional _VH to _VM . As a result, as is clear from the potential diagram in FIG. 2, the potential of the overflow drain region 8 becomes significantly shallower than in FIG.
The electric field due to the OFD voltage in the overflow drain region 8 is weakened. Therefore, the charge generated in the opening window 10, shown by the dotted line, has a significantly increased rate of flowing into the channel region 9, which is set deeper than the potential of the overflow drain region 8, and most of it flows into the channel region 9, as shown by the solid arrow. flows into.

On the other hand, the potential of the overflow drain region 8 is set deeper than the potential of the channel separation region 3. The reason for this is that the overflow drain region 8 has an original role of absorbing excessive charges caused by strong incident light.

FIG. 3 shows clock pulses used in the method for driving a solid-state image pickup device of the present invention. FIG. 3 corresponds to FIG. 6, and the clock pulses φ ₁ , φ ₂ , φ ₃ , and φ ₄ are the same as those of the prior art, and the driving method is also the same, so a description of the operation will be omitted. The feature of the present invention is that the OFD voltage applied to the overflow drain region 8 is pulsed with V _M during the optical accumulation period and V _H during the frame transfer period. Therefore, during the frame transfer period, the overflow drain region 8
OFD voltage is given the deepest potential,
During the frame transfer period, reverse flow of charges from the overflow drain region 8 to the channel region 9 is prevented as in the conventional case.

(g) Effects of the Invention According to the present invention, by lowering the OFD voltage of the overflow drain region 8 during the photoaccumulation period to V _M , the electric charge generated in the aperture window 10 is transferred to the overflow drain by the electric field of the overflow drain region 8. It has the advantage that the ratio of leakage to region 8 can be reduced compared to the conventional method, and the short wavelength light sensitivity can be improved by about 20% compared to the conventional method.

[Brief explanation of the drawing]

FIG. 1 shows the imaging section of the solid-state imaging device according to the present invention.
A top view explaining the CCD, Figure 2 is the − of Figure 1.
3 is a timing chart of clock pulses used in the driving method of the present invention, FIG. 4 is a top view illustrating an FT type CCD, and FIG. 5 is a sectional view taken along the line 4 in FIG. Fig. 6 is a timing chart of clock pulses used in the conventional driving method, Fig. 7 is a top view explaining the CCD of the imaging section of the conventional FT type CCD, and Fig. 8 is a timing diagram of the clock pulse used in the conventional driving method.
It is a potential diagram of the cross section taken along the line - in the figure. 1 is a semiconductor substrate, 2 is a gate oxide film, 3 is a channel isolation region, 4 and 5 are lower layer gate electrodes, 6,
7 is an upper layer gate electrode, 8 is an overflow drain region, 9 is a channel region, 10 is an opening window, 1
1 is an opening area.

Claims (1)

[Claims] 1. A frame transfer method in which an imaging section and a storage section are separated, the imaging section accumulates charges by photoelectric conversion during a photoaccumulation period, and the charges are transferred to the storage section during a frame transfer period. In the solid-state imaging device, during a light accumulation period, a voltage lower than a voltage applied during a frame transfer period is applied to an overflow drain region provided in a channel separation region of the imaging section independently of a transfer electrode of the imaging section. A method for driving a solid-state image sensor characterized by: