Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-11-10
2002-05-21
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S370130
Reexamination Certificate
active
06392217
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a two-dimensional image detecting device which can detect an image of radiation such as an X-ray, a visible radiation, or an infrared radiation, and further concerns a manufacturing method thereof.
BACKGROUND OF THE INVENTION
Conventionally, a two-dimensional image detecting device for radiation has been known in which semiconductor sensors for detecting an X-ray by generating electrical charge (electron-hole pair) are two-dimensionally disposed in a matrix form, each sensor is provided with an electrical switch, and the electrical switches are successively turned on for each line and electrical charge of the sensors is read for each raw. A specific structure and principle of such a two-dimensional image detecting device are described in “D. L. Lee, et al., ‘A New Digital Detector for Projection Radiography’, SPIE, 2432, pp. 237-249, 1995”, “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 Publication No.342098/1994 (Tokukaihei 6-342098).
Referring to
FIGS. 11 and 12
, the following explanation describes the specific structure and principle of the conventional two-dimensional image detecting device for radiation.
FIG. 11
is a schematic diagram showing the conventional construction of the two-dimensional image detecting device for radiation. Further,
FIG. 12
is a schematic diagram showing a sectional structure for one pixel of the two-dimensional image detecting device for radiation.
As shown in
FIGS. 11 and 12
, the two-dimensional image detecting device for radiation is provided with an active-matrix substrate having electrode wires (gate electrode
52
and source electrode
53
) in an XY matrix form, a thin film transistor (TFT)
54
and a storage capacitor (Cs)
55
, on a glass substrate
51
. Moreover, a photoconductive film
56
, a dielectric layer
57
, and an upper electrode
58
are formed on virtually the entire surface of the active-matrix substrate.
The storage capacitor
55
has a construction in which a Cs electrode
59
is opposed via an insulating film
61
to a pixel electrode
60
connected with a drain electrode of the thin-film transistor
54
.
For photoconductive film
56
, a semiconductive material is used to generate electrical charge (electron-hole pair) by exposure to radiation such as an X-ray. According to the aforementioned literatures, amorphous selenium (a-Se), which has high dark resistance and favorable photoconductivity, has been used for the photoconductive film
56
. The photoconductive film (a-Se)
56
is formed with a thickness of 300~600 &mgr;m by using a vacuum evaporation method.
Further, an active-matrix substrate, which is formed in a manufacturing process of a liquid crystal display device, can be applied to the aforementioned active-matrix substrate. For example, the active-matrix substrate used for an active matrix liquid crystal display device (AMLCD) is provided with the TFT made of amorphous silicon (a-Si) or polysilicon (p-Si), an XY matrix electrode, and a storage capacitor. Therefore, only a few changes in arrangement make it easy to use the active-matrix substrate as that of the two-dimensional image detecting device for radiation.
The following explanation describes a principle of operations of the two-dimensional image detecting device for radiation having the above-mentioned structure.
Electrical charge (electron-hole pair) is generated in the photoconductive film
56
when the photoconductive film
56
such as an a-Se film is exposed to radiation. As shown
FIGS. 11 and 12
, in the two-dimensional image detecting device, the photoconductive film
56
and the storage capacitors (Cs)
55
are electrically connected in series with each other; thus, when voltage is applied between the upper electrode
58
and the Cs electrode
59
in the two-dimensional image detecting device for radiation, electrical charge (electron-hole pair) generated in the photoconductive film
56
moves to a positive electrode side and a negative electrode side. As a result, the storage capacitors (Cs)
55
stores electrical charge. Further, an electron blocking layer
62
made of a thin insulating layer is formed between the photoconductive film
56
and the storage capacitor (Cs)
55
. The electron blocking layer
62
acts as a blocking photodiode for preventing electrical charge from being injected from one side.
With the above-mentioned effect, the thin-film transistor (TFT)
54
comes into an open state in response to input signals of gate electrodes G
1
, G
2
, G
3
, . . . , and Gn so that the electrical charge stored in the storage capacitors (Cs)
55
can be applied to the outside from source electrodes S
1
, S
2
, S
3
, . . . , and Sn. The electrode wires (gate electrodes
52
and source electrodes
53
), the thin-film transistor (TFT)
54
, and the storage capacitors (Cs)
55
, etc. are made in a matrix form; therefore, it is possible to obtain two-dimensional image information of an X-ray by line sequentially scanning signals inputted to gate electrodes G
1
, G
2
, G
3
, . . . , and Gn.
Additionally, in the case when the photoconductive film
56
has photoconductivity for a visible radiation and an infrared radiation as well as for the radiation such as an X-ray, the above-mentioned two-dimensional image detecting device for radiation acts as a two-dimensional image detecting device for detecting the visible radiation and the infrared radiation.
However, the conventional two-dimensional detecting device for radiation has used a-Se as the photoconductive film
56
. Since the a-Se has dispersive conductivity of photoelectric current, that is peculiar to amorphous materials, the a-Se is inferior in response. The a-Se does not have sufficient sensitivity (S/N ratio) to an X-ray. Therefore, the conventional two-dimensional detecting device for radiation has a drawback as follow: the storage capacitor (Cs)
55
needs to be exposed to the X-ray for a long time to be fully charged, before reading information.
Further, in order to prevent electrical charge from being stored in the storage capacitor due to leakage current upon irradiation of X-ray, and in order to reduce leakage current (dark current) and to provide a protection against high voltage, the dielectric layer
57
is provided between the photoconductive film (a-Se)
56
and the upper electrode
58
. However, it is necessary to add a sequence for removing electrical charge remained in the dielectric layer
57
for each frame; thus, the above-mentioned two-dimensional image detecting device can be used only when photographing a static picture.
In response to this problem, in order to obtain image data corresponding to a moving image, it is necessary to use the photoconductive film
56
instead of the a-Se. The photoconductive film
56
is made of a crystal (or polycrystal) material and a material being superior in X-ray sensitivity (S/N ratio). If the sensitivity of the photoconductive film
56
improves, it becomes possible to sufficiently charge the storage capacitor (Cs)
55
even when X-ray is applied for a short time, and high voltage does not need to be applied to the photoconductive film
56
; thus, the dielectric layer
57
is not necessary.
As photoconductive materials which are superior in X-ray sensitivity, CdTe and CdZnTe have been known. Generally, photoelectricity absorption for X-ray proportionally increases to the fifth power of the effective atomic number of absorbed substance. For example, if it is assumed that the atomic number of Se is 34 and the effective atomic number of CdTe is 50, the sensitivity is expected to improve by approximately 6.9 times. However, in the case when CdTe or CdZnTe is adopted instead of a-Se as a material of the photoconductive film
56
of the two-dimensional image detecting device for radiation, the following problem arises:
In the case of the conventional a-Se, a vacuum evaporation method can be adopted as a film-forming method and a film can be formed at a normal temperatur
Izumi Yoshihiro
Teranuma Osamu
Le Que T.
Luu Thanh X.
Sharp Kabushiki Kaisha
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