JPH07115182A - Photoelectric transducer having memory function and solid-state image pickup device - Google Patents

Abstract

PURPOSE:To increase the number of pixels and a pixel density by a method wherein the pixel is basically composed of one photoelectric transducer and one access MOS- FET only and a reset operation state and an image pickup operation state are performed through a transparent electrode and a photoelectric conversion films. CONSTITUTION:A well diffused layer 2 is formed on an Si substrate 1. A source diffused layer 3 and a drain diffused layer 4 are formed on the well diffused layer 2 and, further, a tunnel oxide film 5 is formed on the well diffused layer 2 so as to be provided between the source and drain diffused layers 3 and 4. A floating gate 6 composed of a polycrystalline Si film is deposited on the tunnel oxide film 5 and, further, a control gate 8 composed of a polycrystalline Si film is deposited on the floating gate 6 with an oxide film 7 therebetween. Then a photoconducting film 9 and a transparent electrode 10 are successrvely deposrted on the control gate 8 to obtain an photoelectric transducer. A source electrode 11 and a drain electrode 12 which are composed of Al films respectively are provided. The conductivity types of the substrate 1, the well diffused layer 2, the source and drain diffused layers 3 and 4 and a channel are n-type, p-type, n-type and n-type respectively.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoelectric conversion element with a memory function and a solid-state image pickup device used in high-definition facsimiles, digital copying machines, image input devices, still image cameras and the like.

[0002]

2. Description of the Related Art Conventionally, CCD type and MOS type solid state image pickup devices have been developed and put into practical use, and are widely used in video cameras, facsimiles, digital copying machines and the like. Semiconductor device,
Due to the rapid progress of the process technology, various performances such as the number of pixels, the sensitivity and the imaging speed of the CCD type solid-state imaging device have been dramatically improved. Now, development of a solid-state imaging device for a high-definition television is energetically advanced. A high-definition television requires 2 million pixels (1920 × 1036 pixels), and for that reason, the development has been advanced by making full use of the latest fine process, and the single pixel size has been reduced to about 7 × 7 μm ² . However, further reduction of the pixel size is approaching a difficult level in terms of device and process. The MOS-type solid-state imaging device uses the MOS-type field effect transistor (M
(OS-FET) is sequentially and selectively taken out by the switch operation. In the MOS type, the device and process are C
Since it is simpler than the CD type, it is suitable for increasing the number of pixels and lowering the cost. However, since the signal sensitivity is weak and the noise due to capacitive coupling is large, the sensitivity and S / Inferior in terms of N.

Recently, an amplifying type MOS solid-state image pickup device (Japanese Patent Publication No. 58-50030) for extracting an image signal through a switch composed of a MOS-FET after amplifying a signal level in each pixel for improving S / N. ) Is being developed.
A sectional view and an equivalent circuit of a photoelectric conversion element of an amplification type MOS solid-state imaging device in a conventional example are shown in FIGS. 4 (a) and 4 (b), respectively. The photoelectric conversion element 30 shown in FIG. 3A is a MOS-FET formed on the semiconductor substrate 1, a collection electrode 32 provided on the gate electrode 31, a photoconductive film 9 and a transparent electrode 10.
When the light from the subject is incident on the photoconductive film 9 in the state where an appropriate DC voltage is applied to the transparent electrode 10, a signal voltage corresponding to the incident light amount is induced in the gate electrode 31 through the collection electrode 32. Signal current amplified by MOS-
This is based on the principle of appearing in the source electrode 3 or the drain electrode 4 of the FET, and after reading by each horizontal scanning, the gate electrode 31 is reset to a desired voltage sequentially for each line. The equivalent circuit shown in FIG. 9B is the photoelectric conversion element 30, the access MOS-FET 17, the reset MOS-FET 33, and the access MOS-of FIG.
Vertical selection lines 18a, 18b, 18c, which are formed by commonly connecting the gate electrodes of the FETs 17 for each row, and the same MOS-FE.
Horizontal selection lines 19a, 19b, 19c formed by commonly connecting the source electrodes of T for each column, reset MOS-FE
Reset lines 34a, 34b, 34c formed by commonly connecting the gate electrodes of T33 for each row, horizontal selection MOS-
It comprises FETs 20a, 20b, 20c, a vertical drive circuit 22, a horizontal drive circuit 23 and a reset drive circuit 35.
Two MOS-FETs per pixel other than photoelectric conversion elements
Is required, and vertical selection lines 18a, 18b, 1
8c, horizontal selection lines 19a, 19b and 19c, and reset lines 34a, 34b and 34c are required, which is an obstacle to high integration.

Image input device in information processing system,
The solid-state image pickup device for a still image camera does not require high-speed image pickup as in the case of a moving image because it targets a still image. However, since a high image quality equivalent to that of an ordinary printed matter is required, a larger number of pixels is required even compared with a solid-state imaging device for high-definition television. That is, 4 million to 2
A pixel count of 5 million is required. This is the conventional C
It is extremely difficult to realize with a CD type or MOS type solid-state imaging device. Therefore, currently, a one-dimensional solid-state image pickup device is used for image reading in digital copying machines, facsimiles, and image input devices. In this case, main scanning is performed electrically, and sub-scanning is page reading by mechanical drive. Since the reading is performed for each line, the required image pickup element is simple with only one line, but the reading speed is slow.

[0005]

At present, the number of pixels of a solid-state image pickup device for a high-definition television, which boasts the largest number of pixels, is about two.
It is, 000,000 pixels. Although an image input device in an information processing system targets still images, it requires high-definition reading that is higher than that of a video camera, and requires an imaging device with 4 to 25 million pixels. It is technically difficult and costly to increase the number of pixels to this level in the conventional CCD type or MOS type solid-state imaging device.

The image information is once stored in the image memory and subjected to signal processing. Therefore, in the image input device in a broad sense, the image pickup device and the memory are used as a pair, and the circuit scale increases as the number of pixels increases.

[0007]

A photoelectric conversion element includes a source diffusion layer, a drain diffusion layer and a tunnel oxide film formed on a Si substrate, a floating gate provided on the tunnel oxide film, and an oxide film on the floating gate. The control gate is provided, a photoconductive film sequentially formed on the control gate, and a transparent electrode.

Pixels composed of the above photoelectric conversion elements and access switches are two-dimensionally arranged to form an image pickup surface, and the photoconductive film and transparent electrode of each pixel are formed into a thin film layer common to the pixels, thereby forming a solid-state image pickup. Configure the device. The access switch is composed of a MOS-FET.

[0009]

When a high voltage is applied to the Si substrate and the well diffusion layer and a reset voltage is applied to the transparent electrode and the entire image pickup surface is irradiated with light, the reset voltage is guided to the control gate by the photoconductive effect of the photoconductive film, and all the pixels are exposed. The charge on the floating gate is extracted to the well diffusion layer. After that, the Si substrate and the well diffusion layer are set to zero potential, and an imaging voltage is applied to the transparent electrode to connect an image from a subject to the imaging surface, and charges are injected into the floating gate according to the image pattern. After this imaging state, a reading voltage is applied to the transparent electrode to sequentially access each one of the vertical selection line and the horizontal selection line to read the image signal.

In the solid-state image pickup device of the present invention, the pixel is basically one photoelectric conversion element and one access MOS-
The MOS-FET for resetting and the reset line are not necessary because the FET is configured only, and the reset operation state and the imaging operation state are both performed through the transparent electrode and the photoelectric conversion film. Therefore, the planar structure of the chip is extremely simple, and it is easy to increase the number of pixels and the pixel density. Further, in the image input device, the system is simplified because the image pickup device and the memory device are integrated on the same Si substrate.

[0011]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a plan view of a photoelectric conversion element with a memory function formed on a semiconductor substrate according to the first embodiment of the present invention. In the photoelectric conversion element, a well diffusion layer 2 is provided on a Si substrate 1, and a source diffusion layer 3 and a drain diffusion layer 4 are formed on the well diffusion layer 2.
Further, a tunnel oxide film 5 is provided so as to be interposed between both diffusion layers, a floating gate 6 made of a poly-Si film is deposited on the tunnel oxide film 5, and a poly-Si film is further formed on the floating gate 6 via an oxide film 7. Then, the control gate 8 is deposited, and the photoconductive film 9 and the transparent electrode 10 are sequentially deposited on the control gate 8. Reference numerals 11 and 12 denote source and drain electrodes each made of an Al film. In this figure, the substrate 1 is an n-type conductor, the well diffusion layer is a p-type conductor, and the source diffusion layer 3
And the drain diffusion layer 4 is n-type and the channel is n-type.
It is a type. When the drain diffusion layer 4 is connected to a positive power supply, the source electrode 11 connected to the source diffusion layer 3 outputs a signal current controlled by the charges on the floating gate 6. The tunnel oxide film 5 makes it possible to transfer electrons between the well diffusion layer 2 and the floating gate 6 by applying a desired electric field, and the film thickness thereof is about 10 n.
m. The control gate 8 controls the tunnel current,
An electric field for setting the read condition is generated. The structure from the substrate to the control gate 8 is similar to the cell structure of flash memory. With the structure in which the photoconductive film 9 and the transparent electrode 10 are deposited on the memory cell, photoelectric conversion and a memory function are achieved. As the photoconductive film 9, a photoconductive film such as a.Si, CdS, or an organic photoconductor is suitable. An ITO film is suitable as the transparent electrode 10. With the well diffusion layer at the ground potential, the transparent electrode 10
When exposure is performed with a positive imaging voltage applied to, the imaging voltage is transmitted to the control gate by the operation of the photoconductive film 9, and the signal charge is injected from the well diffusion layer to the floating gate 6 through the tunnel oxide film 6. . As a result, the threshold voltage Vth of the MOS-FET composed of the source diffusion layer 3, the drain diffusion layer 4 and the two kinds of gate electrodes 6 and 8 becomes large, and the signal current decreases. When not exposed, the charge of the floating gate 6 does not change and Vth does not change because the photoconductive film does not conduct. Therefore, the signal current does not decrease.

FIG. 2 shows an equivalent circuit of the solid-state image pickup device according to the second embodiment of the present invention. Each pixel 15 arranged two-dimensionally
Is the photoelectric conversion element 16 and the access MOS-F of the first embodiment.
It consists of ET17. Reference numerals 18a, 18b and 18c are vertical scanning lines formed by commonly connecting the gate electrodes of the access MOS-FETs 17 for each row, and 19a, 19b, 19c are commonly connected for each column of the source electrodes of the access MOS-FET. Horizontal scanning line, 20a, 20b and 20c are horizontal scanning MOS-FETs, and 21 is a horizontal scanning MOS-FET.
An image signal output line formed by commonly connecting the source electrodes of the FETs 20a, 20b, and 20c, a vertical drive circuit 22 for sequentially driving the vertical scan lines 18a, 18b, and 18c, and a horizontal scan MOS-FET 20a and 20.
It is a horizontal drive circuit for sequentially driving b and 20c.
Reference numeral 24 is a common well line for commonly connecting wells forming photoelectric conversion elements. The source electrode of each photoelectric conversion element 16 is connected to the drain of each access MOS-FET, and the drain electrode of the photoelectric conversion element 16 is connected to the positive power supply. The pixel is basically composed of only one photoelectric conversion element 16 and one MOS-FET 17, and since both the reset process and the imaging process are simultaneously performed via the photoconductive film 9, the reset line is By eliminating the above, the structure becomes extremely simple and the number of pixels and the pixel density can be easily increased. In addition, a pixel including a photoelectric conversion element 16 and a MOS-FET for access, which are arranged one-dimensionally,
It is obvious that a one-dimensional solid-state image pickup device can be constructed from the horizontal scanning MOS-FET and the horizontal drive circuit.

Next, a typical image pickup system of the photoelectric conversion element 16 having three operation states of reset, image pickup and read will be described with reference to FIG. In the first reset operation state, the transparent electrode is set to zero potential and the well diffusion layer is kept at a high voltage (about 10 V), and intense light is emitted from the light source to the entire surface of the photoconductive film 9 separately from the subject. By
The photoconductive effect brings the control gate to zero potential, and the charge on the floating gate 6 is discharged to the well diffusion layer 2. As a result, the charge disappears from the floating gate 6, and the threshold voltage Vth of the MOS-FET forming the photoelectric conversion element becomes negative. That is, after completion of the reset operation state, Vth of all MOS-FETs constituting the photoelectric conversion element becomes negative. In the second imaging operation state, the imaging voltage (about 1
0v) is applied to expose the well diffusion layer at zero potential, and an imaging voltage is applied to the control gate according to the image pattern on the imaging surface due to the photoconductive effect of the photoconductive film 9. Signal charges are injected from the well diffusion layer 2 into the floating gate 6 through the tunnel oxide film according to the pattern. As a result, the threshold voltage Vth of the transistor including the source diffusion layer 3, the drain diffusion layer 4, and the two kinds of gate electrodes becomes positive according to the image pattern. Since the unexposed photoconductive film 9 does not conduct,
There is no change in the voltage of the control gate 8 and therefore the floating gate 6
The electric charge of Vth remains unchanged and Vth remains negative. The imaging time is determined by the photoconductive film 9 and the response time of the tunnel current, and is 100 μs or less when a.Si is used as the photoconductive film and the tunnel oxide film thickness is 10 nm. In the third read operation state, the transparent electrode is kept at zero potential, and the vertical drive circuit 22 and the horizontal drive circuit 23 respectively send scanning signals while the entire surface of the photoconductive film is irradiated with intense light from the same light source as at the time of reset. By outputting, the image signals from the selected pixels are sequentially output to the image signal output line 21. That is,
A MOS-FE that constitutes the photoelectric conversion element 16 by two-dimensionally scanning the access MOS-FET 17.
A binarized image signal is output from the image signal output line 21 according to the threshold voltage of T. It is also possible to change the binary slice level by controlling the voltage applied to the transparent electrode 10 during reading. Since the image signal is stored in the floating gate 8 of the non-volatile memory, it can be read nondestructively and at the same time the image information is not lost even when the power is turned off. Therefore, this memory function can also serve as a memory for image processing.

[0015]

In the photoelectric conversion element of the present invention, since the signal charge is held in the floating gate, not only non-destructive reading can be performed, but also the image signal does not disappear due to the memory function even when the power is turned off. Further, in the solid-state image pickup device of the present invention, the pixel is basically composed of one photoelectric conversion element and one access MOS-FET, and since the reset line is not necessary, the planar structure of the chip is extremely simple. Becomes
It is easy to increase the number of pixels and the pixel density. Furthermore, since the image pickup device and the memory device are integrated as the same Si chip, the system can be simplified. Due to the above various functions, the industrial effect of the present invention is extremely large.

[Brief description of drawings]

FIG. 1 is a sectional view of a photoelectric conversion element according to a first embodiment of the present invention.

FIG. 2 is an equivalent circuit diagram of a solid-state imaging device according to a second embodiment of the present invention.

FIG. 3 is a diagram showing an imaging method of the solid-state imaging device of the present invention.

4A is a cross-sectional view of a photoelectric conversion element of an amplification type MOS solid-state imaging device in a conventional example, and FIG. 4B is an equivalent circuit diagram of an amplification type MOS solid-state imaging device in a conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Si substrate 2 Well diffusion layer 3 Source diffusion layer 4 Drain diffusion layer 5 Tunnel oxide film 6 Floating gate 7 Oxide film 8 Control gate 9 Photoconductive film 10 Transparent electrode 16 Photoelectric conversion element 17 Access MOS-FET 18 Vertical scanning line 19 Horizontal scan line 20 Horizontal scan MOS-FET 21 Image signal output line 22 Vertical drive circuit 23 Horizontal drive circuit 24 Common well line

Claims (3)

[Claims] 1. A well diffusion layer formed on a Si substrate, a source diffusion layer and a drain diffusion layer and a tunnel oxide film formed in a well diffusion layer region, a floating gate provided on the tunnel oxide film, and an oxide film on the floating gate. A photoelectric conversion element comprising a control gate provided via a photoconductive film, a photoconductive film sequentially formed on the control gate, and a transparent electrode. 2. A pixel comprising a photoelectric conversion element according to claim 1 and an access switch is arranged one-dimensionally or two-dimensionally, and a photoconductive film and a transparent electrode are formed as a thin film layer common to each pixel. Solid-state imaging device. 3. A high voltage is applied to the Si substrate and the well diffusion layer, a reset voltage is applied to the transparent electrode, and the entire surface of the image pickup surface is irradiated with light, whereby the reset voltage is guided to the control gate, and the reset voltage is applied to the floating gate. The first operation state in which electric charges are removed, the Si substrate and the well diffusion layer are set to zero potential, the imaging voltage is applied to the transparent electrode, and then the image pattern from the subject is connected to the imaging surface, so that the well diffusion is performed. A second operation state in which charges are injected from the layer to the floating electrode and a third operation state in which an image signal is read by sequentially accessing each one of the vertical selection line and the horizontal selection line are characterized. The solid-state imaging device according to claim 2.