Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-09-16
2002-01-22
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S370090
Reexamination Certificate
active
06340812
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a two-dimensional image detector capable of detecting images in radioactive rays (such as X-rays), visible light, infrared light, etc.
BACKGROUND OF THE INVENTION
Conventionally, a known type of two-dimensional image detector for radioactive rays is a device comprising a two-dimensional arrangement of semiconductor sensors which detect X-rays and produce charge (electron-hole) pairs, each sensor being provided with an electrical switch, in which the electrical switches are sequentially turned ON by row, and the charge of each sensor in that row is read out.
The principle of such a two-dimensional image detector and specific structures therefor are discussed in, for example, D. L. Lee, et al, “A New Digital Detector for Projection Radiography” (
Physics of Medical Imaging
, Proc. 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 (
Society for Information Display
)
International Symposium, Digest of Technical Papers
, pp.91-94, 1997); and Japanese Unexamined Patent Publication No. 6-342098/1994 (
Tokukaihei
6-342098, published on Dec. 13, 1994).
The following will explain the principle and structure of the foregoing conventional two-dimensional image detector for radioactive rays.
FIG. 10
is a perspective view schematically showing the structure of the foregoing conventional two-dimensional image detector for radioactive rays. Further,
FIG. 11
is a cross-sectional view schematically showing the structure of one pixel thereof.
As shown in
FIGS. 10 and 11
, the foregoing conventional two-dimensional image detector for radioactive rays includes an active matrix substrate made up of electrode lines (gate electrodes
52
and source electrodes
53
) arranged in an XY matrix, TFTs (thin-film transistors)
54
, charge storage capacitors (Cs)
55
, etc., provided on a glass substrate
51
. Further, over substantially the entire surface of the active matrix substrate
51
are provided a photoconductive film
56
, a dielectric layer
57
, and an upper electrode
58
.
Each charge storage capacitor
55
is made up of a Cs electrode
59
and a pixel electrode
60
(which is connected to the drain electrode of the TFT
54
), provided opposite each other but separated by an insulating layer
61
.
The photoconductive film
56
is made of a semiconductor material which produces a charge (electron-hole) when radioactive rays such as X-rays are projected thereon; the examples discussed in the foregoing documents use amorphous selenium (a-Se), which has high dark resistance and good photoconductive characteristics for X-rays. The photoconductive film
56
is formed by vacuum vapor deposition with a thickness of 300 &mgr;m to 600 &mgr;m.
For the foregoing active matrix substrate, it is possible to use active matrix substrates formed in the process of manufacturing liquid crystal display devices. For example, an active matrix substrate used in an active matrix liquid crystal display device (AMLCD) has a structure which includes TFTs made of amorphous silicon (a-Si) or polycrystalline silicon (p-Si), electrodes arranged in an XY matrix, and charge storage capacitors (Cs). Accordingly, it is easy to use such an AMLCD as an active matrix substrate for a two-dimensional image detector for radioactive rays, necessitating only minor design changes.
The following will explain the operating principle of the foregoing conventional two-dimensional image detector for radioactive rays.
When radioactive rays are projected onto the photoconductive film
56
, charge (electron-hole) pairs are produced therein. As shown in
FIGS. 10 and 11
, the photoconductive film
56
and the charge storage capacitor
55
are electrically connected in series, and thus if a voltage is applied across the upper electrode
58
and the Cs electrode
59
in advance, the electron and hole members of the charge pairs produced in the photoconductive film
56
move to the + and − electrode sides, respectively. As a result, a charge (electron-hole) accumulates in the charge storage capacitor
55
. Between the photoconductive film
56
and the charge storage capacitor
55
is provided an electron blocking layer
62
made of a thin insulating layer, which serves as a blocking photodiode for blocking a charge injection from one side to the other.
By putting the TFTs
54
in an open state by means of input signals from gate electrodes G
1
, G
2
, G
3
, . . . , G
n
, the charges accumulated in the respective charge storage capacitors
55
due to the foregoing effect can be drawn out through source electrodes S
1
, S
2
, S
3
, . . . , S
n
. Since the gate electrodes
52
, the source electrodes
53
, the TFTs
54
, the charge storage capacitors
55
, etc. are all provided in the form of an XY matrix, X-ray image information can be obtained two-dimensionally by sequential scanning of the signals inputted to the gate electrodes G
1
, G
2
, G
3
, . . . , G
n
.
Incidentally, if the photoconductive film
56
used has photoconductivity not only for radioactive rays such as X-rays but also for visible light, infrared light, etc., the foregoing conventional two-dimensional image detector can also function as a two-dimensional image detector for visible light, infrared light, etc.
The foregoing conventional two-dimensional image detector for radioactive rays uses a-Se for the photoconductive film
56
, but a-Se has the following drawbacks: due to the insufficient sensitivity to X-rays (S/N ratio) of a-Se, information cannot be read out unless the charge storage capacitors
55
are sufficiently charged by long X-ray exposure.
Further, in the foregoing conventional two-dimensional image detector for radioactive rays, a dielectric layer
57
is provided between the photoconductive film
56
and the upper electrode
58
in order to reduce current leakage (dark current) and to protect from high voltage. However, since it is necessary to add a step (sequence) for eliminating a residual charge from the dielectric layer
57
after each frame, another drawback of the foregoing conventional two-dimensional image detector for radioactive rays is that it can only be used for pickup of still images.
In order to obtain image data corresponding to moving images, on the other hand, it is necessary to use, instead of a-Se, a photoconductive film
56
which has superior sensitivity to X-rays (S/N ratio) . By improving the sensitivity of the photoconductive film
56
, it is possible to sufficiently charge the charge storing capacitor
55
with X-ray exposure of short duration, and since a high voltage need not be applied to the photoconductive film
56
, the dielectric layer
57
itself is no longer necessary.
Known examples of this kind of photoconductive material with superior sensitivity to X-rays include CdTe and CdZnTe. Since photoelectric absorption of X-rays by a substance is generally proportional to the fifth power of its effective atomic number, if, for example, the effective atomic number of Se is 34 and that of CdTe is 50, then CdTe can be expected to have a sensitivity of approximately 6.9 times that of Se. However, if replacement of the a-Se in the photoconductive film
56
of the foregoing conventional two-dimensional image detector with CdTe or CdZnTe is attempted, the following problems arise.
With conventional a-Se, a film can be formed by vacuum vapor deposition, and in this case, since the film formation temperature can be set at room temperature, it is easy to form a film on the foregoing active matrix substrate. With CdTe or CdZnTe, on the other hand, film formation by MBE (molecular beam epitaxy) or MOCVD (metal organic chemical vapor deposition) are known; in view of film formation on, in particular, substrates of large surface area, MOCVD is considered most suitable.
However, when forming a film of CdTe or CdZnTe by MOCVD, since the starting materials organic cadmium and organic tellurium have heat decomposition temperatures of approximately 300° C. (for dimethyl cadmium—DMCd) and approximately 400°
Izumi Yoshihiro
Teranuma Osamu
Le Que T.
Luu Thanh X.
Nixon & Vanderhye P.C.
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