Two-dimensional image detector

Details Two-dimensional image detector Two-dimensional image detector

1999-03-23

2001-06-05

Epps, Georgia (Department: 2873)

Radiant energy

Photocells; circuits and apparatus

Photocell controlled circuit

C250S370090

Reexamination Certificate

active

06242729

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a two-dimensional image detection device which can detect images of radioactive rays such as X rays, visible light rays, infrared rays, etc.
BACKGROUND OF THE INVENTION
Conventionally, with respect to two-dimensional image detectors for detecting images by using radioactive rays, there have been known a radioactive-ray two-dimensional image detector in which semiconductor sensors, each of which generates charges (electron-positive hole) upon sensing X-rays, are arranged in a two-dimensional format and electric switches are respectively attached to these sensors so that the electric switches are successively turned on for each row so as to read out charges of the sensors for each column. Specific structures and principles of such a two-dimensional image detector are described, for example, in documents “D. L. Lee, et al., ‘A New Digital Detector for Projection Radiography’, SPIE, 2432, pp. 237-249, 1995”, and “L. S. Jeromin, et al., ‘Application of a-Si Active-Matrix Technology in a X-Ray Detector Panel’, SID
97
DIGEST, pp. 91-94, 1997”, and Japanese Laid-Open Patent Application No. 342098/1994 (Tokukaihei 6-342098) (Published on Dec. 13, 1994).
The following description will discuss the structure and principle of the above-mentioned conventional radioactive-ray two-dimensional image detector.
FIG. 7
is an explanatory drawing that schematically shows the structure of the above-mentioned radioactive-ray two-dimensional image detector. Moreover,
FIG. 8
is an explanatory drawing that schematically shows the construction of each pixel of the above-mentioned radioactive-ray two dimensional image detector.
As illustrated in FIG.
7
and
FIG. 8
, the radioactive-ray two-dimensional image detector is constituted by an active-matrix substrate that is a glass substrate
51
on which XY-matrix electrode wiring (gate electrodes
52
and source electrodes
53
), thin-film transistors (TFT)
54
, and charge-storage capacitors (Cs)
55
, etc. are formed, and a photoconductive film
56
, a dielectric layer
57
and upper electrode
58
that are formed virtually on the entire surface of the active-matrix substrate.
The charge-storage capacitor
55
has a construction in which charge-storage capacity electrodes (Cs electrodes)
59
and pixel electrodes
60
connected to the drain electrodes of the TFTs
54
are aligned face to face, with an insulating film
61
located in between.
The photoconductive film
56
is made from a semiconductive member that generates charges (electron-positive hole) upon irradiation by radioactive rays such as X-rays. In accordance with the above-mentioned documents, amorphous selenium (a-Se), which has a high dark resistivity so that it exhibits a superior photoconductive property to X-ray irradiation, is adopted. The photoconductive film
56
is formed by the vacuum deposition method with a thickness of 300 to 600 &mgr;m.
Moreover, active-matrix substrates, which are formed during processes in manufacturing liquid crystal displays, can also be adopted as the above-mentioned active-matrix substrate. For example, an active-matrix substrate, used for an active-matrix-type liquid crystal display (AMLCD), has a construction having TFTs, XY-matrix electrodes and electric storage capacitors formed by amorphous silicon (a-Si) and polysilicon (p-Si). Therefore, those active-matrix substrates, which are formed during processes in manufacturing liquid crystal displays, are readily utilized as the active-matrix substrate used for a radioactive-ray two-dimensional image detector with slight designing modifications.
The following description will discuss the operation principle of a radioactive-ray two-dimensional image detector having the above-mentioned structure.
When the photoconductive film
56
such as an a-Se film is irradiated with radioactive rays, charges (electron-positive hole) are generated in the photoconductive film
56
. As illustrated in
FIGS. 7 and 8
, since the photoconductive film
56
and the charge-storage capacitors
55
are electrically connected in series with each other, when a voltage is applied between the upper electrode
58
and the Cs electrode
59
, charges (electron-positive hole) generated in the photoconductive film
56
respectively shift toward the positive (+) electrode side and the negative (−) electrode side, with the result that a charge is stored in each charge-storage capacitor
55
.
Here, a charge-blocking layer
62
consisting of a thin insulating layer is formed between the photoconductive film
56
and the charge-storage capacitors
55
, and this serves as a blocking type photodiode that blocks injection of a charge from one side.
With the above-mentioned function, the charges stored in the charge-storage capacitors
55
can be drawn outside from source electrodes S
1
, S
2
, S
3
, . . . , Sn, by making the TFTs
54
in an open state by using input signals of gate electrodes G
1
, G
2
, G
3
, . . . , Gn. Since the electrode wiring (the gate electrodes
52
and source electrodes
53
), the TFTs
54
, the charge-storage capacitors
55
, etc. are all installed in a XY-matrix format, image information of X-rays can be two-dimensionally obtained by scanning signals inputting to the gate electrodes G
1
, G
2
, G
3
, . . . , Gn in a line sequential manner.
Additionally, in the case when the photoconductive film
56
, used in the two-dimensional image detector, exhibits photoconductivity not only to radioactive rays such as X-rays, but also to visible light rays and infrared rays, the two-dimensional image detector also functions as a two-dimensional image detector for detecting images resulting from visible light rays and infrared rays.
The radioactive-ray two-dimensional image detector for detecting images by using radioactive rays is designed to use a-Se as the photoconductive film
56
. However, since a-Se has a dispersion-type conductive property to photocurrents which is inherent to the amorphous material, it has a poor response characteristic. Moreover, since a-Se does not have a sufficient sensitivity (S/N ratio) to X-rays, information can not be read until just after the charge-storage capacities
55
have been charged sufficiently through irradiation by X-rays for a long time.
Moreover, in an attempt to prevent a charge from being stored in the charge-storage capacitors
55
due to leakage current and to reduce a leak current (dark current) upon irradiation by X-rays, there is an dielectric layer
57
installed between the photoconductive film (a-Se)
56
and the upper electrode
58
. Since a charge remains in this dielectric layer
57
, a sequence needs to be added so as to eliminate the residual charge for each frame. The resulting problem is that the two-dimensional image detector is only used for picking up still images.
In order to obtain image data from motion images, it is necessary to use a photoconductive film
56
made from a photoconductive member which is a crystal material and also has a superior sensitivity to X-rays. The improvement of the sensitivity of the photoconductive film
56
allows the charge-storage capacitors
55
to be sufficiently charged even irradiation of X-rays for a short period, thereby eliminating the necessity for applying a high voltage to the photoconductive layer
56
and consequently eliminating the dielectric layer
57
. For this reason, it is not necessary to add the sequence for eliminating the residual charge for each frame, and it becomes possible to meet the demands for motion images.
With respect to photoconductive members which have a superior sensitivity to X-rays, CdTe, CdZnTe, etc. have been known. In general, the photoelectric absorption of X-rays is directly proportional to the effective atomic number of an absorbing material raised to 5th power; therefore, for example, supposing that the atomic number of Se is 34 and that the effective atomic number of CdTe is 50, the sensitivity is expected to be improved to approximately 6.9 times. However, when an attempt is made to use CdTe or CdZnTe as the photocond

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