Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-01-13
2001-07-17
Epps, Georgia (Department: 2873)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S370010, C250S370120
Reexamination Certificate
active
06262408
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a two-dimensional image detector for detecting an image by radiation such as X-rays, visible light, infrared light, etc., and a process for manufacturing such a two-dimensional image detector.
BACKGROUND OF THE INVENTION
A device conventionally known as a two-dimensional image detector for detecting an image by radiation has such a structure that semiconductor sensors for sensing X-rays to generate charges (electrons-holes) are two-dimensionally disposed, and electric switches are provided for the respective sensors so as to read out the charges of the sensors column by column by sequentially turning on the electric switches row by row. The specific structure and the principle of the two-dimensional image detector are described in, for example, references: 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 Publication of Unexamined Patent Application No. 342098/1994 (Tokukaihei 6-342098).
The following descriptions will explain the structure and principle of the conventional two-dimensional radiation image detector mentioned above.
FIG. 8
is a depiction of the structure of the two-dimensional radiation image detector.
FIG. 9
is a depiction of the cross section showing the structure of each pixel of the two-dimensional radiation image detector.
As shown in
FIGS. 8 and 9
, the two-dimensional radiation image detector includes an active matrix substrate having a glass substrate
51
on which XY matrix-form electrode wiring (gate electrodes
52
and source electrodes
53
), TFTs (thin film transistors)
54
, and charge storage capacitors (Cs)
55
, etc. are formed. On the almost entire surface of the active matrix substrate, a photoconductive film
56
, a dielectric layer
57
, and a top electrode
58
are formed.
The charge storage capacitor
55
includes a Cs electrode
59
and a pixel electrode
60
connected to a drain electrode of the TFT
54
, arranged so that the Cs electrode
59
faces the pixel electrode
60
through an insulating film
61
.
The photoconductive film
56
is formed by a semiconductor material for generating charges on exposure to radiation such as X-rays. According to the above references, amorphous selenium (a-Se) having a high dark resistance and showing satisfactory photoconduction characteristics on exposure to X-ray irradiation is used. The photoconductive film
56
is formed by a vacuum evaporation method to have a thickness ranging from 300 to 600 &mgr;m.
As the above-mentioned active matrix substrate, an active matrix substrate formed in the process of manufacturing a liquid crystal display device can be utilized. For example, an active matrix substrate used for an active-matrix type liquid crystal display device (AMLCD) includes TFTs formed by amorphous silicon (a-Si) or poly-silicon (p-Si), XY matrix electrodes, and charge storage capacitors. Therefore, by only slightly changing the design, the active matrix substrate formed in the process of manufacturing the liquid crystal display device can be easily utilized as the active matrix substrate for use in the two-dimensional radiation image detector.
Next, the following descriptions will explain the operational principle of the two-dimensional radiation image detector having the above-mentioned structure.
When the photoconductive film
56
is exposed to radiation, charges are generated in the photoconductive film
56
. As shown in
FIGS. 8 and 9
, since the photoconductive film
56
and the charge storage capacitor
55
are electrically connected in series, when a voltage is applied across the top electrode
58
and the Cs electrode
59
, the negative and positive charges generated in the photoconductive film
56
move toward the anode side and the cathode side, respectively. As a result, the charges are accumulated in the charge storage capacitor
55
. Note that a charge blocking layer
62
as a thin insulating layer is formed between the photoconductive film
56
and the charge storage capacitor
55
, and functions as a blocking-type photodiode for blocking the flow of the charges from one side.
With this function, the charges accumulated in the charge storage capacitors
55
can be taken out via source electrodes S
1
, S
2
, S
3
, . . . , Sn by setting the TFTs
54
to the open state in accordance with input signals of gate electrodes G
1
, G
2
, G
3
, . . . , Gn. Since the gate electrodes
52
, the source electrodes
53
, the TFTs
54
, the charge storage capacitors
55
, etc. are all provided in the XY-matrix form, X-ray image information can be obtained two-dimensionally by sequentially scanning the signals inputted to the gate electrodes G
1
, G
2
, G
3
, . . . , Gn line by line.
If the photoconductive film
56
used in the above two-dimensional image detector shows photoconductivity with respect to visible light and infrared light as well as radiation such as X-rays, the two-dimensional image detector also functions as a two-dimensional image detector for detecting an image by visible light and infrared light.
However, in the above conventional structure, a-Se is used as the photoconductive film
56
. The response of a-Se is not good because the photocurrent produced by a-Se has distributed conduction characteristics typical of amorphous materials. Further, since the sensitivity (S/N ratio) of a-Se to X-rays is not sufficient, information cannot be read out until the charge storage capacitors
55
are fully charged by long-time irradiation with X-rays.
In addition, the dielectric layer
57
is provided between the photoconductive film
56
and the top electrode
58
for reduction of the leakage current (dark current) and protection from the high voltage. Since the charges remain in the dielectric layer
57
, a sequence for removing the remaining charges every frame must be added, thereby causing such a problem that the two-dimensional image detector can be utilized for only shooting static images.
Meanwhile, in order to obtain image data corresponding to dynamic images, it is necessary to use the photoconductive film
56
made of a crystalline (or polycrystalline) photoconductive material having excellent sensitivity (S/N ratio) to X-rays in place of a-Se. When the sensitivity of the photoconductive film
56
is improved, the charge storage capacitors
55
can be sufficiently charged even by short-time irradiation with X-rays, and the application of a high voltage to the photoconductive film
56
becomes unnecessary, thereby eliminating the need for providing the dielectric layer
57
. As a result, adding the sequence for removing the remaining charges every frame is not required, thereby making it possible to shoot dynamic images.
Photoconductive materials known to have excellent sensitivity to X-rays are CdTe, CdZnTe, etc. In general, X-ray photoelectric absorption of a material is proportional to the fifth power of the effective atomic number of the absorbing material. For example, provided that the atomic number of Se is 34 and the effective atomic number of CdTe is 50, about 6.9-times improvement in sensitivity can be expected. However, when using CdTe or CdZnTe as the photoconductive film
56
of the two-dimensional radiation image detector instead of a-Se, the following problem arises.
In the conventional case of using a-Se, the vacuum evaporation method can be employed as the method for depositing the film, and the film can be deposited at room temperatures. Thus, the film deposition on the active matrix substrate was easy. Meanwhile, in the case of using CdTe and CdZnTe, the MBE method and the MOCVD method are known as methods for depositing the film, and particularly, the MOCVD method is suitable, considering deposition of the film over the large-area substrate.
However, the deposition of CdTe or CdZnTe by the MOCVD method requires a high temperature of about 400° C., because thermal decomposition of organic cadmium (DMCd) as
Izumi Yoshihiro
Sato Toshiyuki
Shinomiya Tokihiko
Teranuma Osamu
Tokuda Satoshi
Epps Georgia
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
Thompson Timothy J
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