JPH06283695A - Two-dimensional solid state image sensor and image sensing system - Google Patents

Abstract

PURPOSE:To facilitate an increase in number of pixels and to read a still picture with high resolution and high sensitivity by forming each pixel of only a photoelectric converter and an access transistor to be manufactured in the same steps as those of a MOS transistor. CONSTITUTION:A pixel formed of a photoelectric converter 2 and an access transistor 3 arranged two-dimensionally is provided on a semiconductor substrate 1. It has vertical scanning lines 4a-4c connected commonly to a gate electrode of the transistor 3 at each row, horizontal scanning lines 5a-5c connected commonly to a drain electrode of the transistor 3 at each row, and horizontal scanning switches 6a-6c, etc. It also has a control electrode commonly for all the pixels deposited on an imaging surface. A reset voltage is applied to the electrode to project light to the entire imaging surface to reset floating gates to a reset voltage through a photoelectric conversion film 11.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a two-dimensional solid-state image pickup device for still images used in digital copying machines, high-definition facsimiles, 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 video cameras, digital copying machines,
It is widely used for facsimiles. 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 x 1036 pixels), and for this reason, the development of a solid-state imaging device is advanced by making full use of the latest fine process, and a single pixel size is about 7 x 7
It was reduced to μm ² . However, further reduction of the pixel size is considered to be the limit in terms of devices and processes. Further, since the state-of-the-art process is used, the process cost becomes high, and it is necessary to develop the technology for cost reduction before putting it into practical use. The MOS type solid-state imaging device uses the signal charge stored in the photodiode as M
This is to selectively take out sequentially by the switching operation of the OS transistor. Since the signal charge is weak and the noise due to capacitive coupling is large, the sensitivity and S / N are higher than those of the CCD type.
Inferior in terms of. On the other hand, the MOS type is a device and the process is C
Since it is simpler than the CD type, it has advantages that the aperture ratio of the pixel can be increased and the process is inexpensive. Recently, an amplification type MOS solid-state imaging device (Japanese Patent Publication No. 58-50030) has been developed in which the signal level is amplified in each pixel to improve the S / N and then an image signal is taken out through a switch composed of a MOS transistor. ing.
5A and 5B are cross-sectional views and equivalent circuit diagrams of a photoelectric conversion element of an amplification type MOS solid-state imaging device in a conventional example, respectively.
Shown in. The photoelectric conversion element 2 shown in FIG. 1A is a MOS transistor formed on a semiconductor substrate 1 and its gate electrode 2
When the light from the subject is incident on the photoconductive film 11 in a state where the collection electrode 25, the photoconductive film 11 and the transparent conductive film 12 provided on the transparent conductive film 12 are applied with an appropriate DC voltage, A signal voltage corresponding to the amount of incident light is collected by the collection electrode 25.
It is based on the principle that a signal current amplified and amplified by the gate electrode 20 appears through the source electrode 16 or the drain electrode 17 of the MOS transistor through the gate electrode 20. To the desired voltage. The equivalent circuit shown in FIG. 7B is the photoelectric conversion element 2 of FIG.
S transistor 30, reset MOS transistor 3
0 and the vertical selection lines 4a and 4a in which the gate electrodes of the access MOS transistors 3 are commonly connected in each row.
b, 4c, horizontal selection lines 5a, 5b, 5c formed by commonly connecting the source electrodes of the same transistor for each column, and reset line 31a formed by commonly connecting the gate electrodes of the reset MOS transistors 30 for each row, 31b, 31
c, horizontal selection MOS transistor switches 6a, 6
b, 6c, a vertical drive circuit 8, a horizontal drive circuit 9 and a reset drive circuit 32. 3 MOSs per pixel
Requires transistors, vertical select lines 4a, 4
b, 4c and the horizontal selection lines 5a, 5b, 5c and the reset lines 31a, 31b, 31c 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 ordinary printed matter is required, a larger number of pixels is required as compared with the solid-state image pickup device for high-definition television. That is, 4 million to 250
A pixel count of 0,000 is required. This is the conventional CCD
Type or MOS type solid-state imaging device is extremely difficult at this stage. 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 mechanically driven to read a page. Since the reading is performed for each line, the image pickup element can be simplified with only one line and high resolution can be realized, but the reading speed is slow.

[0004]

Since a document that needs to be read is generally two-dimensional, it is best to read it with a two-dimensional solid-state image pickup device. At present, the number of pixels of the high-definition television solid-state imaging device, which boasts the largest number of pixels, is about 2 million. The image input device in the information processing system requires high-definition reading equivalent to that of printed matter, and thus requires an imaging device having 4 to 25 million pixels. It is technically difficult to increase the number of pixels to this level in a conventional CCD type or MOS type solid-state imaging device, and at the same time, it is difficult in terms of cost and yield. In the one-dimensional image pickup device, it is necessary to perform one cycle of reading operation for each line, so that it takes several seconds or more to read one page, and mechanical driving is required, so that the device becomes large.

[0005]

The point of the solution is to simplify the device structure of the pixel. The two-dimensional solid-state imaging device of the present invention includes at least a photoelectric conversion element formed of a floating gate MOS transistor formed on a semiconductor substrate, a two-dimensionally arranged pixel including an access transistor, and one electrode of the access transistor. Horizontal selection line, commonly connected for each column, M for access
A vertical selection line formed by commonly connecting the gate electrodes of the OS transistors in each row, a photoelectric conversion film formed on the floating gate electrode, and a control electrode common to all pixels deposited on the imaging surface. The control electrode is composed of a transparent conductive film deposited on the entire image pickup surface or a light-shielding conductor thin film formed in a mesh shape around each photoelectric conversion element. A first process of resetting the floating gate to the reset voltage through the photoelectric conversion film by applying a reset voltage to the control electrode and irradiating the entire imaging surface with light, and the imaging voltage or the imaging charge. Is applied to the control electrode, and then a second process is performed to form an image on the imaging surface to transfer charges between the charging electrode and the floating gate according to the optical image pattern, and vertical selection line and horizontal selection line. By sequentially accessing one line, it is possible to pick up an image through the third process of reading an image signal by the XY matrix method.

[0006]

The pixel is basically composed of only one photoelectric conversion element and one transistor, and both the reset process and the imaging process are simultaneously performed via the control electrode and the photoelectric conversion film. By eliminating the need for a reset transistor and reset line,
The structure becomes extremely simple, and it becomes easy to increase the number of pixels and the pixel density. Since the charge signal is applied to the floating gate electrode of the MOS transistor, it can be read without destroying the charge signal, and an image signal amplified for each pixel can be obtained. It has the feature of sensitivity.

[0007]

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

FIG. 1 is an equivalent circuit (a) of a two-dimensional solid-state image pickup device formed on a semiconductor substrate 1 which is an embodiment of the present invention and a plan view (b) of the device structure. In FIG. 1A, a two-dimensional solid-state image pickup device is a vertical structure in which pixels each including a photoelectric conversion element 2 and an access transistor 3 arranged two-dimensionally and a gate electrode of the access transistor 3 are commonly connected for each row. Scan lines 4a, 4b, 4c
And horizontal scanning lines 5a, 5b, 5c formed by commonly connecting the drain electrodes of the access transistors 3 for each column,
Horizontal scan switches 6a, 6b, 6c, a vertical drive circuit 8 for connecting the drain electrodes of the horizontal scan switches in common and sequentially driving the image signal output line 7 and the vertical scan lines 4a, 4b, 4c, and the horizontal scan switch 6a. , 6
It is composed of a horizontal drive circuit 9 connected to and driving the gate electrodes of the transistors constituting the transistors b and 6c. The drain electrode of each photoelectric conversion element 2 is connected to the source electrode of each access transistor 3, and the source electrode 10 is connected to the semiconductor substrate 1. In the figure (b), elements and circuits having the same functions as those in the figure (a) are denoted by the same reference numerals. In addition, the photoelectric conversion element is Japanese Patent Publication Sho 58-50.
It is almost the same as No. 030, and the photoelectric conversion film 11 is deposited on the gate electrode of each photoelectric conversion element. A photoconductive material such as an a.Si film or a CdS film is suitable for the photoelectric conversion film. However, in the present invention, the transparent conductive film 12 as the control electrode controls the gate voltage of the photoelectric conversion element in the reset process and the imaging process,
All pixels are connected in common. Reference numeral 13 is a terminal for supplying a voltage or charge to the control electrode, and 14 is an output terminal for an image signal connected to the image signal output line 7.
Since the pixel is basically composed of only one photoelectric conversion element and one transistor, and the reset process and the imaging process are performed simultaneously through the photoelectric conversion film 11, the reset line is not necessary. As a result, the structure becomes extremely simple and the number of pixels and the pixel density can be easily increased.

FIG. 2 is a sectional view of a pixel of a solid-state image pickup device according to an embodiment of the present invention. The photoelectric conversion element 2 and the access transistor 3 are formed on the semiconductor substrate 1.
Reference numerals 15 and 16 respectively denote a drain diffusion layer and a source diffusion layer of a photoelectric conversion element formed by diffusing impurities of a conductivity type different from that of the substrate on the semiconductor substrate 1, and 17 denotes a drain of an access transistor similarly formed. It is a diffusion layer. The drain diffusion layer 15 of the photoelectric conversion element 2 and the source diffusion layer 15 of the access transistor are shared. Reference numeral 18 is a diffusion layer of the same conductivity type as the substrate for connecting the source electrode of the photoelectric conversion element to the substrate. Reference numeral 19 is a thick oxide film formed between pixels. Reference numeral 20 is a gate electrode of the photoelectric conversion element 2 formed on a thin oxide film, and 21 is a gate electrode of an access transistor similarly formed. A photoelectric conversion film 11 is deposited on the gate electrode 20, this is a photosensitive region of the solid-state imaging device, and an oxide film 23 is deposited on the surface except the photosensitive region. 24
Is a surface protective film. Photoelectric conversion film 11 and oxide film 23
A transparent conductive film as the control electrode 12 is deposited on the surface of the. In this photoelectric conversion element 2, when the photoelectric conversion film 11 is irradiated with light with a positive voltage applied to the control electrode 12, charges proportional to the amount of light are transmitted from the transparent conductive film 12 to the gate electrode 20 of the photoelectric conversion element. To flow to, the potential rises. As a result, the drain current of the photoelectric conversion element 2 changes, and this is output as an image signal at the timing when the access transistor 3 becomes conductive.

FIG. 3 is a sectional view of a pixel of a solid-state image pickup device according to the second embodiment of the present invention, in which a photoelectric conversion element 2 and an access transistor are formed on a semiconductor substrate.
Reference numeral 11 is a photoelectric conversion film, and 25 is a mesh-shaped control electrode. In the figure, elements and films having the same functions as those in FIG. 2 are given the same numbers. In this embodiment, the photoelectric conversion film 11 is deposited on the entire image pickup surface, not on the gate electrode, and a conductive thin film such as AL is provided around the photoelectric conversion element without using a transparent conductive film as a control electrode. It is attached to the mesh. The photoelectric conversion film is deposited on the entire image pickup surface, but since there is a mesh-shaped light-shielding film between the pixels as a control electrode, the separation performance of image signals between pixels does not deteriorate, and high-resolution image pickup is performed. It can also be applied to devices.

FIG. 4 schematically shows the image pickup process of the solid-state image pickup device in one embodiment of the present invention. For simplicity,
The solid-state imaging device does not show a fine structure but is represented by a three-layer film formed on a semiconductor substrate, that is, a gate electrode, a photoelectric conversion film, and a control electrode. FIG. 7A shows a reset process which is performed for all pixels at the same time.
After the reset voltage Vr is applied between the transparent conductive film and the substrate by turning on W, a current flows through the photoelectric conversion element by irradiating the entire surface of the solid-state imaging device with light, and the gate voltage of the photoelectric conversion element changes. This is the process of setting the reset voltage. In other words, by irradiating the exposure amount larger than the white reference of the image pickup, the gate voltage can be easily set to the reset voltage all the pixels all at once even without the reset wiring or the drive circuit. The figure (b) is an imaging process. After the image pickup voltage Vp is applied between the transparent conductive film and the substrate by turning on the image pickup SW, when the image pickup surface is irradiated with the light image pattern from the subject, the charge proportional to the light amount is generated by the photoelectric effect of the photoelectric conversion film. Flows from the transparent conductive film to the gate electrode of the photoelectric conversion element, and the gate potential rises in proportion to the amount of light received by each pixel. The saturated exposure amount is the voltage V
It increases in proportion to p-Vr. The figure (c) is a reading process. Vertical drive circuit 8 and horizontal drive circuit 9
By outputting a scanning signal from, the image signals from the selected pixels are sequentially output to the output line 7. That is, the scanning signal from the horizontal drive circuit 8 is activated by the scanning signal from the vertical drive circuit 8 to operate the horizontal scan switches 6a, 6b, and 6c to operate the horizontal scan switches 6a, 6b, and 6c. Is output to the output line. Next, when 4b is activated instead of the vertical scanning line 4a, the image signal of the second row is similarly output to the output line. In this reading process, the image signal stored in the gate electrode 19 of the photoelectric conversion element can be read nondestructively. When a shift register is used for the vertical drive circuit 8 and the horizontal drive circuit 9, only sequential reading is possible, but when a decoder circuit is used for them, random reading is possible like an IC memory. The solid-state imaging device of the present embodiment is extremely useful in the field of image recognition due to the functions of nondestructive read and random read.

[0012]

Since each pixel is formed of only the photoelectric conversion element and the access transistor which can be manufactured in the same process as the MOS transistor, it is extremely simple. As a result,
The number of pixels can be easily expanded, a still image can be read with high resolution and high sensitivity, and its industrial effect is extremely large.

[Brief description of drawings]

1A is an equivalent circuit of a two-dimensional solid-state imaging device of the present invention, and FIG. 1B is a plan view of a device configuration of a two-dimensional solid-state imaging device of the present invention.

FIG. 2 is a sectional view of a pixel of the two-dimensional solid-state imaging device according to the first embodiment of the present invention.

FIG. 3 is a sectional view of a pixel of a two-dimensional solid-state imaging device according to a second embodiment of the present invention.

4A is a schematic diagram of an imaging process of the solid-state imaging device of the present invention, FIG. 4B is a schematic diagram of an imaging process of the solid-state imaging device of the present invention, and FIG. Pattern diagram

5A is a sectional view of a photoelectric conversion element of an amplification type MOS solid-state imaging device in a conventional example, and FIG. 5B is an equivalent circuit diagram of an amplification type MOS solid-state imaging device in a conventional example.

[Explanation of symbols]

1 semiconductor substrate 2 photoelectric conversion element 3 access MOS transistor 7 image signal output line 8 vertical drive circuit 9 horizontal drive circuit 11 photoelectric conversion film 12 control electrode made of transparent conductive film 15 drain electrode of photoelectric conversion element and access transistor Source electrode 16 Source electrode of photoelectric conversion element 17 Drain electrode of access transistor 19 Thick oxide film formed between elements 20 Gate electrode of photoelectric conversion element 23 Oxide film 24 Surface protective film 25 Mesh-shaped control electrode

Claims (5)

[Claims] 1. A two-dimensional solid-state image pickup device comprising a vertical drive circuit and a horizontal drive circuit, and an image pickup plane composed of photoelectric conversion elements and switches formed by MOS transistors arranged two-dimensionally. Layer, drain diffusion layer, thin oxide film, two-dimensionally arranged MOS transistor consisting of floating gate electrode formed on thin oxide film, photoelectric conversion film deposited on floating gate electrode, deposited on or around it A two-dimensional solid-state imaging device comprising a control electrode common to all pixels. 2. The two-dimensional solid-state image pickup device according to claim 1, wherein the control electrode is a transparent conductive film deposited on the entire image pickup surface. 3. The two-dimensional solid-state image pickup device according to claim 1, wherein the control electrode is a conductive thin film formed around each photoelectric conversion element and electrically connected to the photoelectric conversion film. 4. The two-dimensional solid-state imaging device according to claim 1, wherein the photoelectric conversion film is an a.Si thin film. 5. A first process of resetting a floating gate of a photoelectric conversion element to a reset voltage through a photoelectric conversion film by applying a reset voltage to a control electrode and then irradiating the entire image pickup surface with light. After applying an image-capturing voltage or charge to the control electrode, an optical image pattern from a subject is formed on the image-capturing surface, and electric charge is transferred between the control electrode and the floating gate in accordance with the optical image pattern to capture an image. An image pickup method of a two-dimensional solid-state image pickup device comprising a second process and a third process for reading an image signal by sequentially accessing each one line of a vertical selection line and a horizontal selection line.