Television – Camera – system and detail – Solid-state image sensor
Reexamination Certificate
1998-11-16
2004-09-28
Christensen, Andrew (Department: 2615)
Television
Camera, system and detail
Solid-state image sensor
C348S308000, C378S098800
Reexamination Certificate
active
06798453
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a photoelectric conversion device for forming an image by visible light, radiation, or the like and, more particularly, to a photoelectric conversion device suitably applicable to a two-dimensional photoelectric conversion apparatus such as a still camera or an X-ray image pickup system.
2. Related Background Art
The conventional photography was mostly the silver salt photography using an optical camera and silver salt film. The development of the semiconductor technology has brought about development of an image pickup apparatus capable of taking images of moving picture, such as video camcorders, by a solid image pickup device using Si single-crystal sensors typified by CCD sensors or MOS sensors, but these images were inferior in the number of pixels and in S/N ratios to silver-salt photographs. Therefore, it was common practice to use the silver salt photography for taking still images with high quality.
On the other hand, demands are increasing in recent years for image processing by computer, storage of image in the form of an electronic file, and transmission of image by electronic mail. Under such circumstances, there are desires for electronic image pickup device capable of outputting a digital signal of an image comparable to those of the silver salt photography. This is not the case only in the field of ordinary photographs but also in the field of medical care.
The X-ray photography is popularly known as application of the silver salt photography technology in the field of medical care. This is the photography for exposing the affected part of a human body to X-rays emitted from an X-ray source and, for example, for determining whether a fracture or a tumor is present, based on information of X-ray transmission, which has been and is widely used long in medical diagnosis. Normally, X-rays transmitted by the affected part are made incident once to a fluorescent member to be converted into visible light; and the silver salt film is exposed to this visible light. The silver salt film has advantages of high sensitivity and high resolution, but also has disadvantages of taking some time for development, requiring much time and labor for storage and management, not allowing quick transmission of data to a remote place, and so on. There are thus desires for electronic X-ray image pickup device capable of outputting a digital signal of an image equivalent to those of the silver salt photography, as stated above. A suggestion to implement it was a method for forming an image by using a reducing optical system and a compact photoelectric conversion device using a single-crystal, such as the CCD sensors or the MOS sensors.
This method was, however, able to utilize only about one thousandth of light emitted from the fluorescent member and thus was still susceptible to improvement against the requirement that diagnosis should be made with as weak X-rays as possible where the human body was observed with X-rays. It is thus not easy to implement an X-ray diagnosis device for medical care by the compact photoelectric conversion device using the reducing optical system of poor light utilization efficiency.
In order to meet this requirement, development is under way of an image pickup apparatus using a large sensor with a two-dimensional array of image pickup devices using photoelectric conversion elements having hydrogenated amorphous silicon (hereinafter referred to as “a-Si”). The image pickup device of this type is constructed in such structure that a metal and a-Si are deposited in a desired order on an insulating substrate having each side of 30 to 50 cm by a sputtering apparatus, a chemical vapor deposition apparatus (CVD apparatus) or the like, for example, approximately 2000×2000 semiconductor diodes are formed therein, an electric field of a reverse bias is applied thereto, and charges flowing in the reverse direction in the respective diodes can be individually detected by a thin film transistor (hereinafter referred to as “TFT”) made at the same time as the diodes. It is popularly known that when the electric field of the reverse direction is applied to the semiconductor diode, a photocurrent flows corresponding to a quantity of light incident to the semiconductor layer. The above device utilizes this phenomenon. However, a current, so called a dark current, flows even in a state in which no light is present at all and this gives rise to shot noise, which is the cause of degradation of detection capability of the overall system, i.e., degradation of the sensitivity called the S/N ratio. The point of development is thus how much this dark current can be decreased.
EP-A-0660421 discloses structural examples of X-ray image pickup systems for satisfying these requirements.
FIG. 1
is a schematic circuit diagram of a photoelectric conversion device used in an X-ray image pickup system.
FIG. 2
is a schematic block diagram of another X-ray image pickup system.
The above image pickup systems were, however, still susceptible to improvement against the requirements of higher S/N, better operability, and lower cost. The reasons will be described below with the examples of the image pickup systems of FIG.
1
and FIG.
2
.
In
FIG. 1
, S
11
, S
12
, . . . , S
mn
(m and n are positive integers) represent photosensors, T
11
, T
12
, . . . , T
mn
(m and n are positive integers) switching elements such as thin film transistors, C
11
, C
12
, . . . , C
mn
(m and n are positive integers) capacitor elements, and SR
1
and SR
2
shift registers. One pixel is composed of a photosensor S
1
, . . . , or S
mn
, a capacitor element C
11
, . . . , or C
mn
, and a switching element T
11
, . . . , or T
mn
, and the pixels are arranged in a matrix pattern. The switching element T
11
, . . . , or T
mn
of each pixel is used for transmission of signal. Gates of the respective switching elements T
11
, . . . , T
mn
of pixels in each row are connected to a control line g
1
, g
2
, . . . , or g
m
(m is a positive integer), and the control lines g
1
, . . . , g
m
are connected to the shift register SR
1
. Each one main electrode of the respective switching elements T
11
, . . . , T
mn
of pixels in each column are connected to each signal line provided for every column. One electrode of each of the photosensors S
11
, . . . , S
mn
and one electrode of each of capacitor elements C
11
, . . . , C
mn
in each pixel are connected in common and then connected to a switch SWg and a switch SWs. The other electrode of each of the photosensors S
11
, . . . , S
mn
and the other electrode of each of capacitor elements C
11
, . . . , C
mn
in each pixel are connected to the other main electrode than the above one main electrode of each switching device T
11
, . . . , T
mn
. Each signal line is connected to a switch M
1
, M
2
, . . . , or Mn (n is a positive integer), and the switches M
1
, . . . , Mn are successively driven by the shift register SR
2
to output signals as outputs in order through an amplifier. Switches SWg and SWs are connected to desired power supplies Vg and Vs, respectively, and are driven so as to give a desired potential to the one electrodes of each of the photosensors S
11
, . . . , S
mn
and the one electrode of each of capacitor elements C
11
, . . . , C
mn
at desired timing.
In
FIG. 2
, numeral
6001
indicates a photoelectric conversion portion,
6002
an analog/digital signal converter for converting analog signals from the photoelectric conversion portion
6001
to digital signals,
6003
subtracters for correction of fixed pattern,
6004
a memory,
6005
a controller,
6006
a buffer, and
6007
an image processor.
FIG. 2
shows a example in which a plurality of shift registers SR
1
are arranged in series and a plurality of integrated circuits IC for detection are arranged. Outputs from the integrated circuits IC for detection are input to the analog-digital signal converters
6002
in the processing circuit
6008
to be digitized. Each digital output is supplied to the subtracte
Christensen Andrew
Tran Nhan T
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