Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
1999-04-23
2002-01-29
Epps, Georgia (Department: 2873)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370120
Reexamination Certificate
active
06342700
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a 2-D image detector capable of detecting an image by using radial rays, such as X-rays, visible light, infrared light, etc.
BACKGROUND OF THE INVENTION
A conventionally known 2-D image detector for detecting an image by using radial rays is provided with two-dimensionally aligned semiconductor sensors which generate charges (electrons-holes) upon detection of X-rays and electric switches which are individually attached to the sensors, so that the charges are read out from the sensors per column by sequentially turning on the electric switches per row. Detailed arrangements and operation principle of such a radiographic 2-D image detector are described in, for example, “A New Digital Detector for Projection Radiography” SPIE, 2432, pp. 237-249, 1995, D. L. Lee, et al., “Application of a-Si Active-Matrix Technology in an X-Ray Detector Panel”, SID 97 DIGEST, pp 91-94, 1997, L. S. Jeromin, et al., Japanese Laid-open Patent Application No. 342098/1994 (Japanese Official Gazette, Tokukaihei No. 6-342098, published on Dec. 13, 1994), etc.
The following will explain an arrangement and an operation principle of the conventional radiographic 2-D image detector.
FIG. 12
is a view which schematically explains an arrangement of the conventional radiographic 2-D image detector, and
FIG. 13
is a cross section which schematically shows an arrangement of each pixel of FIG.
12
.
As shown in
FIGS. 12 and 13
, the conventional radiographic 2-D image detector comprises an active matrix substrate having a glass substrate
51
on which are formed an X by Y matrix (hereinafter, referred to X-Y matrix) of electrode wires (gate electrodes
52
and source electrodes
53
), thin film transistors (TFTs)
54
, charge accumulating capacitors (Cs′)
55
, etc. In addition, a photoconductive film
56
, a dielectric layer
57
, and a top electrode
58
are formed almost entirely on the active matrix substrate.
Each of the charge accumulating capacitors
55
comprises a charge accumulating capacitor electrode (Cs electrode)
59
, and a pixel electrode
60
which is connected to the drain electrode of the TFT
54
. The two electrodes
59
and
60
are placed to oppose each other through an insulating layer
61
.
The photoconductive film
56
is made of a semiconductor material which generates charges (electrons-holes) upon irradiation of radial rays, such as X-rays. The semiconductor material used in the aforementioned publications is amorphous selenium (a-Se) which has good resistance to darkness and shows excellent photoconductivity upon X-ray irradiation. The photoconductive film (a-Se)
56
is formed in a thickness ranging from 300 to 600 &mgr;m by means of vacuum deposition.
The active matrix substrate can be the one manufactured in the liquid crystal display manufacturing process. To be more specific, an active matrix substrate used for an active matrix liquid crystal display (AMLCD) is provided with thin film transistors (TFTs) made of amorphous silicon (a-Si) or poly silicon (p-Si), and an X-Y matrix of electrodes and charge accumulating capacitors (Cs′). Thus, the active matrix substrate manufactured in the liquid crystal display manufacturing process can be used as an active matrix substrate for the radiographic 2-D image detector by slightly changing the specification.
Next, the following will explain the operation principle of the above-arranged radiographic 2-D image detector.
When radial rays are irradiated on the photoconductive film
56
, such as the aforementioned a-Se film, the charges (electrons-holes) are generated therein. As shown in
FIGS. 12 and 13
, the photoconductive film
56
is electrically connected to the charge accumulating capacitors
55
in series. Thus, if a voltage is applied across the top electrode
58
and charge accumulating capacitor electrodes
59
, the charges (electrons-holes) generated in the photoconductive film
56
start to migrate to the positive electrode end and negative electrode end, whereby the charges are accumulated in the charge accumulating capacitors
55
.
Here, a charge blocking layer
62
made of a thin insulating layer is formed between the photoconductive film
56
and charge accumulating capacitors
55
. The charge blocking layer
62
functions as a blocking photodiode which inhibits charge injection from one of the surfaces thereof.
The charges accumulated in the charge accumulating capacitors
55
in the above-described manner are released to the outside from the source electrodes S
1
, S
2
, S
3
, . . . , Sn by opening the TFTs
54
with input signals to the gate electrodes G
1
, G
2
, G
3
, . . . , Gn. Since all of the electrode wires (gate electrodes
52
and source electrodes
53
), TFTs
54
, and charge accumulating capacitors
55
are provided in an X-Y matrix arrangement, by linesequentially scanning input signals to the gate electrodes G
1
, G
2
, G
3
, . . . , Gn, 2-D image information of the X-rays can be obtained.
In case that the 2-D image detector employs the photoconductive film
56
which shows the photoconductivity to visible light or infrared light in addition to the radial rays, such as the X-rays, the 2-D image detector can also detect an image by using visible light or infrared light.
As has been mentioned, the conventional radiographic 2-D image detector uses a-Se as the photoconductive film
56
, but a-Se has drawbacks that the responsivity is poor due to a unique property to an amorphous material, namely, dispersive photoconductivity for a photoelectric current, and that information can not be read out until the charge accumulating capacitors (Cs′)
55
are fully charged by irradiating X-rays for a long time due to its poor sensitivity (S/N ratio) to X-rays.
The dielectric layer
57
is formed between the photoconductive film (a-Se)
56
and top electrode
58
so as to (1) prevent the charge accumulation in the charge accumulating capacitors
55
caused by a leak current during X-ray irradiation, (2) reduce a leak current (dark current), and (3) protect the components from a high voltage. In this case, however, a sequence to remove residual charges in the dielectric layer
57
in every frame is additionally necessary. Thus, the radiographic 2-D image detector has a problem that it can be used only to shoot still-frame images.
In order to obtain image data for motion pictures, instead of a-Se, the photoconductive film
56
must be made of a photoconductive crystalline (or poly-crystalline) material having excellent sensitivity (S/N ratio) to X-rays. By using the photoconductive film
56
with better sensitivity, the charge accumulating capacitors
55
can be fully charged by irradiating X-rays in a short time without applying a high voltage to the photoconductive film
56
. Consequently, the dielectric layer
57
can be omitted.
Known examples of the photoconductive material with excellent sensitivity to X-rays are CdTe, CdZnTe, etc. In general, photoelectric absorption of X-rays is directly proportional to the fifth power of the effective atomic number of an absorbed substance. Thus, given
34
as the atomic number of Se and
50
as the effective atomic number of CdTe, then sensitivity improved by a factor of approximately 6.9 can be expected. However, when CdTe or CdZnTe is used as the photoconductive film
56
of the radiographic 2-D image detector instead of a-Se, the following problem occurs.
In conventional case of a-Se, a film can be readily formed on the active matrix substrate by means of vacuum deposition at room temperature. In contrast, in case of CdTe or CdZnTe, a film is formed by means of MBE (Molecular Beam Epitaxy) or MOCVD (Metal Organic Chemical Vapor Deposition), and particularly, the latter is considered suitable to form a film over a large-area substrate.
However, when a film of CdTe or CdZnTe is formed by means of MOCVD from raw materials including organic cadmium (DMCd), organic tellurium (DETe or DiPTe), etc., the film has to be formed at a temperature as high as 400° C. because the thermal decomposition temperature of DM
Izumi Yoshihiro
Sato Toshiyuki
Teranuma Osamu
Tokuda Satoshi
Epps Georgia
Hanig Richard
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
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