Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
1999-12-13
2004-09-28
Wilczewski, M. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S443000, C257S444000, C257S448000, C438S057000
Reexamination Certificate
active
06798030
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 ray, or an infrared ray, 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 and for generating electrical charge (electron-hole pair) are two-dimensionally disposed, each sensor is provided with an electrical switch, and the electrical switches are successively turned on for each raw so as to read electrical charge for each column of the sensor.
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’, Proc. SPIE, Vol. 2432, Physics of Medical Imaging, pp. 237-249, 1995 (published on Feb. 26, 1995)”, “L. S. Jeromin, et al., ‘Application of a-Si Active-Matrix Technology in a X-Ray Detector Panel’, SID(Society for Information Display) International Symposium, Digest of Technical Papers, pp. 91-94, 1997 (published on May 13, 1997)”, and Japanese Laid-Open Patent Publication No.342098/1994 (Tokukaihei 6-342098, published on Dec. 13, 1994).
Referring to
FIGS. 15 and 16
, the following explanation describes the specific structure and principle of the conventional two-dimensional image detecting device for radiation.
FIG. 15
is a perspective view schematically showing the construction of the two-dimensional image detecting device for radiation. Further,
FIG. 16
is a sectional drawing schematically showing a structure for one pixel.
As shown in
FIGS. 15 and 16
, the two-dimensional image detecting device for radiation is provided with an active-matrix substrate
50
having electrode wires(gate electrode
52
and source electrode
53
), a TFT(thin film transistor)
54
and a storage capacitor(Cs)
55
, in an XY matrix form 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
50
.
The storage capacitor
55
has a construction in which a Cs electrode
59
opposes a pixel electrode
60
connected with a drain electrode of the TFT
54
via an insulating film
61
.
As for the photoconductive film
56
, semiconductive materials are used so as to generate electrical charge 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
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 active-matrix substrate
50
. For example, an 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 poly-silicon(P-Si), an XY matrix electrode, and a storage capacitor. Therefore, only a few changes in the arrangement make it easy to use the active-matrix substrate
50
for the two-dimensional image detecting device for radiation.
Referring to
FIGS. 15 and 16
, the following explanation describes a principle of operations of the two-dimensional image detecting device for radiation having the above-mentioned structure. Electrical charge is generated when the photoconductive film
56
is exposed to radiation. The photoconductive film
56
and the storage capacitor
55
are electrically connected in series with each other; thus, when voltage is applied between the upper electrode
58
and the Cs electrode
59
, electrical charge generated in the photoconductive film
56
moves to a positive electrode side and a negative electrode side. As a result, the storage capacitor
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
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 TFT
54
is turned on in response to input signals of gate electrode G
1
, G
2
, G
3
, . . . , and Gn so that the electrical charge stored in the storage capacitor
55
can be applied to the outside from source electrodes S
1
, S
2
, S
3
, . . . , and Sn. The gate electrodes
52
and source electrodes
53
, the TFT
54
, and the storage capacitor
55
, etc. are all formed in a matrix form; therefore, it is possible to two-dimensionally obtain image information of an X-ray by scanning signals for each line inputted to gate electrodes G
1
, G
2
, G
3
, . . . , and Gn.
Additionally, when the photoconductive film
56
has photoconductivity for a visible ray and an infrared ray as well as for the radiation such as an X-ray, the above-mentioned two-dimensional image detecting device for radiation also acts as a two-dimensional image detecting device for detecting the visible ray and the infrared ray.
Incidentally, the conventional two-dimensional detecting device for radiation has used a-Se for the photoconductive film
56
. Since the a-Se does not have sufficient responsivity to an X-ray, the storage capacitor
55
needs to be exposed to the X-ray for a long time to be fully charged, before reading information, and it takes a long time to return the photoconductive layer
56
to the initial state after X-ray irradiation is shielded.
Further, in order to reduce leakage current(dark current) and to provide a protection against high voltage, the dielectric layer
57
is provided between the upper electrode
58
and the photoconductive film
56
made of a-Se. However, it is necessary to add a step(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 for 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
which is superior in responsivity and sensitivity to X-ray. As the photoconductive materials, CdTe and CdZnTe, whose effective atomic number is larger than that of Se, have been known. However, 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 temperature; thus, it has been easy to form a film on the active-matrix substrate
50
. Meanwhile, in the case of CdTe and CdZnTe, film-forming methods such as an MBE(molecular beam epitaxy)method and an MOCVD(metal organic chemical vapor deposition)method have been known. Especially in view of forming a film on a large substrate, it is understood that the MOCVD method is appropriate. However, when a material such as CdTe and CdznTe is made into a film by using the MOCVD method, a high temperature of approximately 400° C. is required for forming a film.
Generally, in the TFT
54
which is formed on the active-matrix substrate
50
, an a-Si film or a p-Si film is used as a semiconductive layer. The a-Si film and the p-Si film are formed at a film-forming temperature of 300-350° C. while hydrogen(H
2
) being added, in order to improve the semiconductive property. The TFT element formed in such a process has a heat-resistance temperature of approximately 300° C. If the TFT element is exposed at a temperature exceeding the heat-resistance temperature, hydrogen is released from the a-Si film and the p-Si film; consequently, the conductive property is degraded.
Therefore, in view of the film-forming temperature, it has been pract
Izumi Yoshihiro
Teranuma Osamu
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
Wilczewski M.
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