Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2002-04-08
2003-10-21
Pyo, Kevin (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S370090, C257S072000, C257S444000
Reexamination Certificate
active
06635860
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a direct conversion type radiation detector for use in a medical field, an industrial field, and a nuclear energy field. More particularly, the present invention relates to techniques for improving an environment resistance of a radiation sensitive type semiconductor film in the radiation detector and for suppressing a creeping discharge due to a bias voltage applied to the radiation sensitive type semiconductor film.
2. Description of the Related Art
Detectors for detecting radiation, such as X-rays, include indirect conversion type detectors, and direct conversion type detectors. The indirect conversion type detectors are adapted to first convert radiation into light and then perform photoelectric conversion of the converted light into electric signals. The direct conversion type detectors are adapted to convert incident radiation directly into electrical signals, such as a radiation sensitive type semiconductor film.
In the latter direct conversion type detector, a predetermined bias voltage is applied onto a voltage application electrode formed on a front surface of a radiation sensitive type semiconductor film. A carrier collection electrode is formed on a back surface of the semiconductor film and collects carriers, which are generated by radiation irradiation and extracts the collected carriers as a radiation detection signal thereby to perform detection of radiation.
Further, when a thick film of an amorphous semiconductor, such as amorphous selenium, is used as a radiation sensitive type semiconductor film, a thick and large film of an amorphous semiconductor can easily be formed by a vacuum evaporation method. Thus, the amorphous semiconductor is suitable for constructing a two-dimensional array type radiation detector, which requires a large area thick film, among the related art conversion type radiation detectors
As illustrated in
FIG. 8
, a related art two-dimensional array type radiation detector comprises an insulating substrate
86
, an amorphous semiconductor thick film
81
and a voltage application electrode
82
. The insulating substrate
86
has plural charge storage capacitors Ca and plural charge-reading switching elements
88
, which are formed in a crosswise or two-dimensional matrix-like arrangement thereon. The charge-reading switching element
88
is constituted by thin film transistors and is normally put in an OFF-state. The amorphous semiconductor thick film
81
is electrically connected to the plural charge storage capacitors Ca and formed on the insulating substrate
86
through plural carrier collection electrodes
87
. In amorphous semiconductor thick film
81
, Charge transfer media (that is, carriers) are generated by incidence of radiation. The voltage application electrode
82
is formed on a surface of the amorphous semiconductor thick film
81
. Incidentally, one charge storage capacitor Ca and one charge reading switching element
88
are provided correspondingly to each of the carrier collection electrodes
87
. Each set of the charge storage capacitor Ca, the charge reading switching element
88
, and the carrier collection electrodes
87
constitutes a detecting element DU serving as a radiation detection unit.
Incidentally, when radiation is irradiated during a state in which a bias voltage is applied to the voltage application electrode
82
, the charge transfer media (that is, carriers) generated by incidence of radiation are respectively moved by the bias voltage to the voltage application electrode
82
and the carrier collection electrode
87
. Electric charges are stored in the charge storage capacitor Ca according to the number of the generated carriers. The stored charges are read as radiation detection signals by putting the switching element
88
into an ON-state.
When the radiation detector of the two-dimensional array configuration shown in
FIG. 8
is used for detecting, for example, an X-ray fluoroscopic image obtained by an X-ray fluoroscopic imaging system, the X-ray fluoroscopic image is obtained according to the radiation detection signal outputted from the radiation detector.
However, in the case of the related art radiation detector, the thermal expansion coefficient of the amorphous semiconductor thick film
81
is large. Thus, there is a danger that thermal warpage occurs owing to the difference in the thermal expansion coefficient between the amorphous semiconductor thick film
81
and the substrate
86
. When such warpage occurs, the amorphous semiconductor thick film
81
may crack. In such a case, such cracks may result in image defects. Further, discharge breakdown may occur at a crack portion, and the detector may be brought into an inoperable state.
Furthermore, in the case of the related art radiation detector, as illustrated in
FIG. 8
, read lines
810
, gate lines
811
, and ground lines
812
have parts
810
a,
811
a,
and
812
a
exposed on the insulating substrate
86
. Therefore, there is a danger that a creeping discharge is caused by an occurrence of dielectric breakdown in a portion from an end edge
82
a
of the voltage application electrode
82
along the surface of an end edge
81
a
of an amorphous semiconductor thick film
81
to the parts
810
a,
811
a,
and
812
a.
Further, for example, in the case of an X-ray fluoroscopic image, an occurrence of a creeping discharge causes noises of the radiation detection signal. This results in degradation in the picture quality of the image. Creeping discharge can be suppressed by setting the bias voltage at a low value as a countermeasure there against. However, in such a case, the amorphous semiconductor is inferior in carrier transit characteristics to a monocrystalline semiconductor. Thus, the related art radiation detector has encountered a drawback in that the detector cannot obtain sufficient detection sensitivity.
Incidentally, to deal with the problem of the environment resistance, such as that of warpage due to change in temperature, there has been proposed a radiation detector A having an insulating plate member
95
as shown in FIG.
9
. The insulating plate member
95
is formed on the surfaces of the top layer of the amorphous semiconductor thick film
91
and the voltage application electrode
92
formed on the insulating substrate
96
. The insulating plate member
95
has thermal expansion coefficient which is comparable to that of the insulating substrate
96
. The insulating plate member
95
is fixed by a high-withstand-voltage hardening synthetic resin
94
in such a manner as to cover the entire surfaces of the top layer.
However, in such a radiation detector A, a result of an experiment is obtained, which reveals that the surface of the amorphous semiconductor thick film
91
is deteriorated by a solvent ingredient of the high-withstand-voltage hardening synthetic resin
94
, so that a creeping discharge occurs and the withstand voltage lowers (see a result of an experiment corresponding to a first comparative detector, which is shown in FIG.
7
).
Moreover, to prevent an occurrence of a creeping discharge, there has been proposed a radiation detector B having a carrier-selective high-resistance film
103
made of a material, such as Sb
2
S
3
, which is formed between an amorphous semiconductor thick film
101
and a voltage application electrode
102
, as shown in
FIG. 10
(in Japanese Patent Application No.
11-240026
). The carrier-selective high-resistance film
103
is formed in such a way as to entirely cover the surface of the amorphous semiconductor thick film
101
, which is liable to deteriorate.
However, such a radiation detector B has a drawback in that the carrier-selective high-resistance film
103
made of a material, such as Sb
2
S
3
, is inferior in tensile strength and thus cannot withstand warpage of the amorphous semiconductor thick film
101
, which is caused owing to change in temperature, and that cracks are apt to occur. Further, it is difficult for the carrier-selective high-resistance film
103
to have a t
Sato Kenji
Tokuda Satoshi
Yoshimuta Toshinori
Pyo Kevin
Rankin, Hill Porter & Clark LLP
Shimadzu Corporation
Sohn Seung C.
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