JP2000077646A - Solid-state image pickup device and its driving method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent deterioration of the transfer efficiency of information charges from an image sensing part to an accumulating part, in a solid state image sensing element of a frame transfer system. SOLUTION: In this solid-state image pickup element, a P-type diffusion layer 12 turning to an element region is formed on an N-type silicon substrate 11, the respective transfer electrodes 13i, 13s, 13h are formed on the diffusion layer 12, and an image sensing part, an accumulating part and a horizontal transfer part are constituted. An image pickup part region 12i and an accumulating part region 12s of the diffusion layer 12 are so formed that impurity concentrations are uniform. A horizontal transfer region 12h is so formed that impurity concentration is higher than that of the image pickup region 12i and that of the accumulating region 12s.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frame transfer type solid-state imaging device and a driving method thereof.

[0002]

2. Description of the Related Art FIG. 5 is a schematic diagram showing a configuration of a frame transfer type solid-state imaging device.

The frame transfer type CCD solid-state image pickup device has an image pickup section i, a storage section s, a horizontal transfer section h, and an output section d. The imaging unit i includes a plurality of shift registers extending in the vertical direction and arranged in parallel with each other, and each bit of each shift register forms a light receiving pixel. The accumulation unit s
It consists of a plurality of light-shielded shift registers that are continuous with the shift register of the imaging unit i, and each bit of each shift register constitutes a storage pixel. The horizontal transfer unit h is composed of a single shift register extending in the horizontal direction, and the output of the shift register of the storage unit s is connected to each bit. Output section d
Includes a capacitor that temporarily stores the charge transferred and output from the horizontal transfer unit h and a reset transistor that discharges the charge stored in the capacitor. As a result, the information charges accumulated in each light receiving pixel of the imaging unit i are independently transferred to the accumulation pixels of the accumulation unit s for each pixel, and thereafter, stored in the accumulation unit s row by row.
From the horizontal transfer unit h to the output unit d in pixel units. The output unit d converts the amount of charge for each pixel into a voltage value, and the change in the voltage value is supplied to an external circuit as a CCD output.

FIG. 6 is a sectional view taken along line XX of FIG.

On one main surface of an N-type silicon substrate 1, a P-type diffusion layer 2 serving as an element region is formed. An isolation region (not shown) is formed in the surface region of the P-type diffusion layer 2,
A channel region is formed between the separation regions. The channel region is vertically continuous from the imaging unit i to the storage unit s, and extends horizontally in the horizontal transfer unit h. On the silicon substrate 1 on which the channel region is formed, a plurality of transfer electrodes 3 i, 3
s and 3h are arranged. The transfer electrodes 3i and 3s formed in the imaging unit i and the storage unit s are each formed in two layers extending in the horizontal direction, and transfer the information charges in the channel region in one direction by receiving a multilayer transfer clock. Output. The transfer electrode 3h formed in the horizontal transfer section h is formed so as to extend in the vertical direction so as to intersect with the transfer electrodes 3i and 3s of the imaging section i and the storage section s, and information charges vertically transferred from the storage section s. Are sequentially transferred and output in the horizontal direction.

[0006] The diffusion layer 2 is formed in the region 2i of the image pickup section i so that information charges in the channel region can be easily discharged to the substrate side.
The impurity concentration is formed lower than that of the region 2s of the accumulation unit s and the region 2h of the horizontal transfer unit h. That is, in the silicon substrate 1, as the impurity concentration of the diffusion layer 2 becomes lower, the depletion layer is more likely to spread in that region, and the information charge is held at a deeper position. Therefore, in the imaging unit i in which overflow charge may be generated due to excessive exposure, the overflow charge is formed by forming the impurity concentration of the imaging unit region 2i lower than the impurity concentration of the accumulation unit region 2s and the horizontal transfer unit region 2h. It can be easily discharged to the substrate side. Conversely, in the accumulation section s and the horizontal transfer section h, in order to prevent information charges from being inadvertently discharged to the substrate side, the impurity concentration of each of the regions 2s, 2h is formed higher than the impurity concentration of the imaging section region 2i. are doing.

[0007]

In the case of a frame transfer type solid-state imaging device, information charges are continuously transferred through a continuous channel region from an imaging section i to a storage section s.
At this time, since the impurity concentration of each of the regions 2i and 2s of the diffusion layer 2 is different at the connection portion between the imaging unit i and the storage unit s, even if the same transfer clock is applied to each of the transfer electrodes 3i and 3s, Potentials in the channel region do not match. Such a difference in potential between the imaging unit i and the storage unit s degrades the transfer efficiency when transferring information charges from the imaging unit i to the storage unit s.

Accordingly, an object of the present invention is to efficiently transfer information charges from an imaging unit to a storage unit in a frame transfer type solid-state imaging device.

[0009]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and is characterized in that a semiconductor substrate of one conductivity type is provided on one principal surface of a semiconductor substrate of one conductivity type. A plurality of light receiving pixels are arranged in a matrix in the semiconductor layer to store information charges in each of the light receiving pixels; and an image pickup unit is disposed adjacent to the image pickup unit and stored in the plurality of light receiving pixels. And a horizontal transfer unit that is arranged adjacent to the storage unit and that takes out the information charges stored in the storage unit in units of rows. In the transfer type solid-state imaging device,
The region of the semiconductor layer where the imaging unit and the accumulation unit are formed has the same impurity concentration as each other, and the region of the semiconductor layer where the horizontal transfer unit is formed is the same as that of the imaging unit and the accumulation unit. It has a higher impurity concentration than a region to be formed.

According to the present invention, since the imaging section and the accumulation section are formed in the diffusion layer having the same impurity concentration, the potential in each channel region with respect to each transfer electrode is determined by the imaging section and the accumulation section. Always matches. Therefore, when information charges are transferred from the imaging unit to the storage unit, it is possible to prevent the transfer efficiency from being deteriorated at the connection unit.

Further, the present invention is characterized in that an image pickup unit in which a plurality of light receiving pixels are arranged in a matrix and information charges are stored in each light receiving pixel, and the plurality of light receiving pixels arranged adjacent to the image pickup unit. A storage unit that stores information charges stored in the pixels in units of one screen, and a horizontal transfer unit that is arranged adjacent to the storage unit and takes out the information charges stored in the storage unit in units of rows. A method for driving a solid-state imaging device of a frame transfer system formed on a substrate, comprising: a first step of accumulating information charges generated by photoelectric conversion in each light receiving pixel of the imaging unit for a predetermined period; A second step of transferring information charges accumulated in each light receiving pixel of the imaging unit from the imaging unit to the storage unit by a number of rows smaller than the number of rows of the light receiving pixels, In the second step A third step of transferring the information charges stored in the respective light receiving pixels of the imaging unit by a number of rows smaller than the number of rows of the light receiving pixels from the imaging unit to the storage unit while driving the storage unit. And a fourth step of, following the third step, discharging the information charge remaining in each light receiving pixel of the imaging unit while the imaging unit and the accumulation unit are stopped.

In the second step, a part of the information charges on the storage section side of the image pickup section is discharged, and in the third step, the information charges in the middle row of the image pickup section are read out to the storage section. Then, in the fourth step, the information charges located on the side opposite to the storage section of the imaging section left by the reading in the third step are discharged. Therefore, information charges can be read from an arbitrary row of the imaging unit by setting the discharging operation in the second step and the reading operation in the third step.

[0013]

FIG. 1 is a cross-sectional view showing the structure of a solid-state imaging device according to the present invention, and shows the same portion as FIG.

On one main surface of an N-type silicon substrate 11, a P-type diffusion layer 12 serving as an element region is formed. In the diffusion layer 12, the impurity concentration in the region 12i serving as the imaging unit and the region 12s serving as the storage unit are formed uniformly. Then, the impurity concentration of the region 12h serving as the horizontal transfer unit is changed to the imaging unit region 1
2i and the storage region 12s are formed to have higher impurity concentrations. An isolation region (not shown) is formed in the surface region of such a diffusion layer 12, and a channel region is formed between the isolation regions. This separation region is the same as in FIG. 6, and the channel region is formed so as to be continuous in the vertical direction from the imaging unit to the storage unit, and to extend in the horizontal direction in the horizontal transfer unit.

Silicon substrate 1 with channel region formed
A plurality of transfer electrodes 13i, 13s, 13h are arranged on 1 so as to cross each channel region. The transfer electrodes 13i and 13s formed in the imaging unit and the storage unit are formed in two layers extending in the horizontal direction, for example, four-phase vertical transfer clocks φv1 to φv4 and four-phase storage transfer clocks φs1 to φs4 is applied respectively. Thus, the information charges generated in the channel region of the imaging unit are independently transferred for each pixel from the imaging unit to the storage unit, and then sequentially transferred from the storage unit to the horizontal transfer unit. The transfer electrodes 3h formed in the horizontal transfer unit include transfer electrodes 3i of the imaging unit and the storage unit,
3s, and is formed to extend in the vertical direction, for example,
Two-phase horizontal transfer clocks φh1 and φh2 are applied. These horizontal transfer clocks φh1 and φh2 are stored transfer clocks φs1
The information charges transferred and output from the storage section are sequentially transferred and output in the horizontal direction in synchronization with .about..phi.s4. In addition, the silicon substrate 11
Is the substrate clock φ that controls the potential on the substrate side.
b is applied.

In the imaging region 12i and the accumulation region 12s of the diffusion layer 12 formed with a low impurity concentration, information charges in the channel region are more easily discharged to the silicon substrate 11 side than in the horizontal transfer region 12h. I have. Therefore, during the information charge transfer period, the substrate clock φb
Is lowered to increase the potential on the silicon substrate 11 side so that information charges are not inadvertently discharged to the silicon substrate 11 during the transfer process.

In the above solid-state imaging device, the diffusion layer 1
Since the impurity concentration is uniformly formed between the second imaging unit region 12i and the accumulation unit region 12s, the potential in the channel region is controlled by the same condition in the imaging unit and the accumulation unit. Therefore, if the levels of the vertical transfer clocks φv1 to φv4 and the accumulation transfer clocks φs1 to φs4 are matched, no potential step is formed in the channel region at the connection between the imaging unit and the accumulation unit, and the transfer is performed. Efficiency degradation can be prevented.

In the diffusion layer 12 formed with a low impurity concentration, a potential well for storing information charges is formed at a deep position. That is, the diffusion layer 1
2, the depletion layer easily spreads in the silicon substrate 11, and the potential profile in the silicon substrate 11 is, as shown by a solid line a in FIG. Then, the potential well for storing information charges moves to the deep part of the silicon substrate 11. Therefore, in the storage unit, as in the imaging unit, S
Since information charges can be transferred at a position distant from the i-SiO ₂ interface, transfer efficiency can be improved.

FIG. 3 is a timing chart for explaining a method of driving the solid-state image sensor according to the present invention. This driving method is shown in FIG.
The present invention is applied to the solid-state imaging device of the present invention shown in FIG.

The vertical transfer clocks φv1 to φv4 all fall to the low level, and thereafter are fixed for the period L with only φv1 and φv2 rising. In response to the vertical transfer clocks φv1 to φv4, the substrate clock φb is once raised to a high level, and thereafter fixed at a low level during a period L. Thereby, in the channel region of the imaging unit,
After the information charges are once discharged, the information charges generated during the period L are accumulated.

[0021] In the subsequent time t _1, when the vertical transfer clock φv1~φv4 is clocked while keeping a phase difference of [pi / 2 with each other, the information charges in the channel region of the imaging unit is transferred to the storage unit side. At this time, the accumulation transfer clocks φs1 to φs4 are all fixed to a low level, and information charges transferred and output from the imaging unit are discharged to the silicon substrate 11 side without being taken into the accumulation unit.

[0022] In the timing t ₂ has elapsed period D1, storage transfer clock φs1~φs4 vertical transfer clocks φ
When clocking is performed in synchronization with v1 to φv4, information charges which have been discharged to the silicon substrate 11 side until then are taken into the storage section. Incorporation of this information charges, the vertical transfer clock at time t ₃ φv1~φv
4 and the clocking of the accumulation transfer clocks φs1 to φs4 is stopped. Then, the vertical transfer clock at time t _3? V1 to? V4 and the storage transfer clock φ
After the clocking of s1 to φs4 is stopped, a period D2
Until time t ₄ when the expiration of the vertical transfer clock φv1~φv4 is secured all at the low level. At the same time, of the stored transfer clocks φs1 to φs4, φs1 and φs2 are fixed at low level, and φs3 and φs4 are fixed at high level. Thereby, the information charges taken into the storage unit from the imaging unit are held in the channel region of the storage unit, and the information charges left in the imaging unit are discharged to the silicon substrate 11 side. Timing t ₄ and later, store-and-forward clock φs1~
φs4 is clocked in the horizontal scanning cycle, and the information charges held in the storage unit are transferred and output to the horizontal transfer unit row by row.

According to the above-described driving method, information charges can be read from an arbitrary row of the imaging section by setting the driving timings t ₂ and t ₃ . That is, as shown in FIG. 4, in the period D1 which is set between the timing t ₁ of the timing t _2, information charges in the region of the imaging unit i adjacent to the storage section s is discharged, subsequent to, the information charges Reading is started. Then, in the period D2 to be set between the timing t ₃ of a timing t _4, the information charges in the region of the imaging section i of opposite side is discharged from the storage section s. The information charges in the area may be left in the imaging unit i, and all the information charges may be once discharged in the next imaging operation. Therefore, the timing t
₂ sets the reading start position in the imaging unit i,
Reading end position is set by the timing t _3.

[0024]

According to the present invention, the imaging section and the storage section can be operated under the same conditions, the setting of the drive circuit can be simplified, and the transfer efficiency of information charges from the imaging section to the storage section can be improved. Deterioration can be prevented.

Further, according to the driving method of the present invention, it is possible to read out information charges from an arbitrary position of the imaging section, and to simplify a signal processing for an image signal when extracting a part of the imaging area. Can be.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a structure of a solid-state imaging device of the present invention.

FIG. 2 is a profile diagram showing a potential state in a substrate.

FIG. 3 is a timing chart illustrating a method for driving a solid-state imaging device according to the present invention.

FIG. 4 is a schematic diagram showing a reading region of information charges.

FIG. 5 is a plan view schematically showing a conventional frame transfer type solid-state imaging device.

FIG. 6 is a sectional view showing a sectional structure taken along line XX of FIG. 5;

[Explanation of symbols]

 i Imaging unit s Storage unit h Horizontal transfer unit d Output unit 1,11 Silicon substrate 2,12 P-type diffusion layer 3i, 3s, 3h, 13i, 13s, 13h Transfer electrode

Claims (3)

[Claims] 1. A semiconductor layer of opposite conductivity type is formed on one main surface of a semiconductor substrate of one conductivity type, and a plurality of light receiving pixels are arranged in a matrix in this semiconductor layer to store information charges in each light receiving pixel. An imaging unit, a storage unit disposed adjacent to the imaging unit and storing information charges stored in the plurality of light receiving pixels in units of one screen, and a storage unit disposed adjacent to the storage unit and stored in the storage unit. In a solid-state imaging device of a frame transfer system in which a horizontal transfer unit for taking out stored information charges in units of one row is formed, the region of the semiconductor layer where the imaging unit and the storage unit are formed is the same as each other. A solid-state imaging device having an impurity concentration, wherein a region of the semiconductor layer where the horizontal transfer unit is formed has a higher impurity concentration than a region where the imaging unit and the accumulation unit are formed; . 2. An imaging section in which a plurality of light receiving pixels are arranged in a matrix and accumulates information charges in each of the light receiving pixels, and an information charge arranged adjacent to the imaging section and accumulated in the plurality of light receiving pixels is one. A frame transfer method in which a storage unit that stores data in units of a screen and a horizontal transfer unit that is disposed adjacent to the storage unit and takes out information charges stored in the storage unit in units of one row are formed on a semiconductor substrate. A first step of accumulating information charges generated by photoelectric conversion in each light-receiving pixel of the imaging unit for a predetermined period, and a method of driving the imaging unit in a state where the accumulation unit is stopped. A second step of transferring the information charges accumulated in each light receiving pixel from the imaging unit to the accumulation unit side by a smaller number of rows than the number of rows of the light receiving pixels; With the unit driven A third step of transferring information charges accumulated in each light receiving pixel of the imaging unit from the imaging unit to the storage unit by a number of rows smaller than the number of rows of the light receiving pixels; A fourth step of discharging information charges remaining in each light-receiving pixel of the imaging unit while the imaging unit and the accumulation unit are stopped, a method of driving a solid-state imaging device. 3. The solid according to claim 2, wherein the potential of the semiconductor substrate is set higher during the second step to the fourth step than during the first step. A method for driving an image sensor.