Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
2001-11-16
2004-04-06
Porta, David (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370130
Reexamination Certificate
active
06717152
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an electromagnetic wave detecting device which is capable of detecting electromagnetic waves including radiation (such as X-rays), visible light, and infrared light.
BACKGROUND OF THE INVENTION
Conventionally, in the field of medical diagnosis, an image pickup device adopting an S/F (Screen/Film) mode, a CR (Computed Radiography) mode or an I.I-TV (Image Intensifier TV) mode has been used as a means to photograph an X-ray image. Note that, the S/F mode utilizes intensifying screens and films. The CR mode reads a latent image recorded on an imaging plate with laser scanning. The I.I-TV mode utilizes an electron multiplier tube and a CCD in combination. In addition, as a new type of the image pickup device replacing all of them, a flat panel two-dimensional image detecting device has been developed more actively in recent years.
The new two-dimensional image detecting device is made up of a combination of an active-matrix array which is used as a key device having a switching element disposed in a two-dimensional state and a converting element (detecting element) which converts electromagnetic wave information to an electric charge.
This two-dimensional image detecting device falls roughly into an “indirect converting system” and a “direct converting system” depending on a difference of the principles of detecting the electromagnetic wave. The “indirect converting system” first converts electromagnetic wave information, such as X-rays, to light through Scintillator, thereafter converting the light to an electric signal through a photodiode. On the other hand, the “direct converting system” converts the electromagnetic wave information, such as X-rays, directly to the electric signal through a semiconductor film. Note that, as the latter, the “direct converting system”, specific structures and principles of the device are described in, for example, {circle around (1)} US patent gazette U.S. Pat. No. 5,132,541 (Date of Patent: Jul. 21, 1992), {circle around (2)} D. L. Lee, et al., “A New Digital Detector for Projection Radiography”, SPIE, 2432, pp.237-249, 1995, and the like.
Here, the principles of an electromagnetic wave detecting device (two-dimensional image detecting device)
100
which is described in the above-mentioned document {circle around (2)} is shown in FIG.
8
.
The electromagnetic wave detecting device
100
has a single common bias electrode
102
and a plurality of charge collector electrodes
103
which are respectively formed on upper and lower layers of a semiconductor film
101
made of Se showing electromagnetic wave conductivity. Further, the charge collector electrodes
103
are respectively connected to charge storage capacitance (hereinafter referred to as “Cs”)
104
and a switching element (TFT)
105
. Note that, a dielectric substance layer
106
which is a charge rejection layer is provided between the semiconductor film
101
and the bias electrode
102
. Further, an electron rejection layer
107
which is a charge rejection layer is provided between the semiconductor film
101
and the charge collector electrodes
103
. In addition, an external high-voltage power source
109
for applying bias voltage to the bias electrode
102
is provided.
When an electromagnetic wave, such as an X-ray, is incident onto the electromagnetic wave detecting device
100
thus arranged, a charge (an electron-positive hole pair) is generated inside of the semiconductor film
101
. At this stage, the semiconductor film
101
and the Cs
104
are serially connected electrically. Therefore, by previously applying a bias voltage to the bias electrode
102
, an electron of the charge (electron-positive hole pair) generated in the semiconductor film
101
moves to a positive (+) electrode side, and a positive hole of the charge (electron-positive hole pair) moves to a negative (−) electrode side, thereby storing the charge in the Cs
104
. Furthermore, by turning on the switching element
105
, the charge stored in the Cs
104
can be taken outside. By thus disposing the charge collector electrode
103
, the Cs
104
and the switching element
105
in a two-dimensional state, and reading out charges in a line-sequential manner, it becomes possible to obtain two-dimensional information of an electromagnetic wave which is a detection target.
Further, generally, Se, CdTe, CdZnTe, PbI
2
, HgI
2
, SiGe, Si, etc. are used as a semiconductor film which has electromagnetic wave conductivity. Among these, an Se film shows desirable electromagnetic wave conductivity with respect to X-ray application. Also, the Se film is capable of large-area deposition at a low temperature by vacuum evaporation. For those reasons, the Se film is widely used for the electromagnetic wave detecting device having a structure (the structure disclosed in the foregoing documents {circle around (1)} and {circle around (2)}) in which a semiconductor film is formed directly on an active matrix substrate.
Further, CdTe and CdZnTe are the materials that show desirable electromagnetic wave conductivity with respect to X-ray application. However, since CdTe and CdZnTe need to be deposited at a high deposition temperature, it is not possible to form them direct on the active matrix substrate. Therefore, a semiconductor film of CdTe or the like is formed on a different supporting substrate first, thereafter joining the active matrix substrate to the substrate having the semiconductor film, thereby making up an electromagnetic wave detecting device of a hybrid structure. The electromagnetic wave detecting device of the hybrid structure thus using the CdTe or CdZnTe film is described in document {circle around (3)} Y. Izumi, et al., “A Direct Conversion X-ray Sensor with A Novel Hybrid Panel Structure”, AM-LCD99 DIGEST OF TECHNICAL PAPERS, pp. 49-52, 1999.
Incidentally, the switching element array
105
(active matrix substrate) used for the electromagnetic wave detecting device as discussed is formed under normal circumstances by having a glass substrate as a base, on which a metal film (Al, Ta, etc.), a semiconductor film (a-Si, p-Si, etc.) and an insulating film (SiNx, SiOx, etc.) are deposited. Further, it is possible to form components, such as electrical wiring, a TFT element and the like, by patterning switching element arrays
105
in a predetermined form.
However, as the electromagnetic wave detecting device discussed above, in the case that an inorganic material such as Se or the like is deposited on the active matrix substrate having the glass substrate
108
as its base, a problem described below occurs.
The thermal expansion coefficient of a glass substrate is 3-8(×10
−6
/° C.), and the thermal expansion coefficient of an Se film is 30-50(×10
−6
/° C.). Namely, as the glass substrate and the Se film have about 10 times difference in their thermal expansion coefficients, the semiconductor film peels off the glass substrate when an environmental temperature varies to the extent of ±20° C. to ±30° C. Especially, as shown in
FIG. 8
of a prior art example, in the case of an arrangement such that a semiconductor film (Se) covers substantially the whole surface of the active matrix substrate as a continuous film, the influence of the difference between the two thermal expansion coefficients is likely to be pronounced as the size of the substrate becomes larger, that is, the removal of the semiconductor film is likely to occur in the vicinity of the substrate. Therefore, an electromagnetic wave detecting device using the Se film as a semiconductor film can be used in an environment under the limited, small range of temperature. Accordingly, an environment under a constant temperature should be maintained in the case of using or carrying the electromagnetic wave detecting device using the Se film, thereby arising such a problem as to cause extra works and costs.
Besides, the glass substrate has high solidity and poor flexibility. This flattens a detecting surface of the electromagnetic wave detecting device,
Porta David
Sharp Kabushiki Kaisha
Sung Christine
LandOfFree
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