Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
1999-01-29
2001-06-05
Kim, Robert (Department: 2877)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370090, C250S370080
Reexamination Certificate
active
06242746
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a two-dimensional image detecting device which is capable of detecting an image for radiation such as an X-ray, a visible ray, or an infrared ray, and relates to 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 a generating electrical charge(electron-positive hole) are two-dimensionally disposed, each sensor is provided with an electrical switch, and the electrical switches are successively turned on for each line and electrical charge 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’, Proc. SPIE, Vol. 2432, Physics of Medical Imaging, pp. 237-249, 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”, and Japanese Laid-Open Patent Publication No.342098/1994 (Tokukaihei 6-342098).
The following explanation describes the specific structure and principle of the conventional two-dimensional image detecting device for radiation.
FIG. 12
is a perspective view showing a model of the construction of the two-dimensional image detecting device for radiation. Further,
FIG. 13
is a sectional view showing a model of the structure for one pixel.
As shown in
FIGS. 12 and 13
, 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
), TFT(thin film transistor)
54
and electrical charge storage capacity(Cs)
55
arranged 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.
The electrical charge storage capacity
55
has a construction in which a Cs electrode
59
opposes to a pixel electrode
60
connected with a drain electrode of the TFT
54
via an insulating film
61
.
For 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 reference books, amorphous selenium(a-Se), which has high dark resistance and favorable photoconductivity, has been used. 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 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 poly-silicon(P-Si), an XY matrix electrode, and electrical charge storage capacity. 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 is generated when the photoconductive film
56
is exposed to radiation. As shown
FIGS. 12 and 13
, the photoconductive film
56
and the electrical charge storage capacity
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 electrical charge storage capacity
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 electrical charge storage capacity
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
comes into an open state in response to input signals of gate electrode G
1
, G
2
, G
3
, . . . , and Gn so that the electrical charge stored in the electrical charge storage capacity
55
can be applied to the outside from source electrodes S
1
, S
2
, S
3
, . . . , and Sn. The gate electrodes
52
, the source electrodes
53
, the TFT
54
, and the electrical charge storage capacity
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, in the case 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 acts as a two-dimensional image detecting device for detecting the visible ray and the infrared ray.
However, the conventional arrangement 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 and the sensitivity(S/N ratio) to an X-ray is not sufficient. Therefore, the electrical charge storage capacity
55
needs to be fully charged by being exposed to the X-ray for a long time in order to read information.
Further, upon irradiation of X-ray, in order to prevent electrical charge from being stored in the electrical charge storage capacity due to leakage current and in order to reduce leakage current(dark current), the dielectric layer
57
is provided between the photoconductive film
56
and the upper electrode
58
. Since it is necessary to add a step(sequence) for removing electrical charge remained in the dielectric layer
57
for each frame, the above-mentioned two-dimensional image detecting device is available 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
which is made of crystal(or polycrystal) material and is superior in X-ray sensitivity(S/N ratio). If the sensitivity of the photoconductive film
56
improves, it becomes possible to sufficiently charge the electrical charge storage capacity
55
even when X-ray is applied for a short time. Further, the need for applying high voltage to the photoconductive film
56
is eliminated; thus, it is not necessary to arrange dielectric layer
57
.
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 effective atomic number of absorbed substance that is multiplied to the fifth power. 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 temperature; thus, it has been easy to form a film on the active-matrix substrate. 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 when forming a film on a large substrate is taken into consideration, it is understood that the MOCVD method is suitable.
However, in the case when a material selected from CdTe an
Fujii Akiyoshi
Izumi Yoshihiro
Shinomiya Tokihiko
Teranuma Osamu
Kim Robert
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
Smith Zandra
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