JP2008098330A - Radiological image detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To specify a specific configuration that enables compatibility between suppression of point defects and suppression of afterimages in a TFT radiological image detector. <P>SOLUTION: The radiological image detector has a structure in which an electric charge generation layer 20 for generating electric charges when irradiated with radiation, and a detection layer 10 are laminated. In the detection layer, a number of collection electrodes 8 for collecting electric charges generated at the electric charge generation layer 20 and a number of pixel portions 11 having switch elements 3 for reading the electric charges collected by the collection electrode 8 are two-dimensionally arranged. The entire periphery of the collection electrode 8 is covered with a protection film 18. The ratio (fill factor) of the area of the collection electrode 8 which is not covered with the protection film 18 to the entire area of the collection electrode 8 is set to be 40% to 99.8%. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a radiation image detector in which a charge generation layer that generates a charge upon irradiation of radiation and a detection layer in which a large number of pixel portions having switch elements are two-dimensionally arranged are stacked. .

  2. Description of the Related Art Conventionally, in the medical field and the like, various radiological image detectors that generate a charge upon irradiation of radiation transmitted through a subject and record a radiation image related to the subject by accumulating the charge have been proposed and put into practical use.

As a radiation image detector system, a direct conversion system in which radiation is directly converted into charges and accumulated, and radiation is converted into light once by a scintillator such as CsI: Tl or GOS (Gd ₂ O ₂ S: Tb). There is an indirect conversion method in which the light is converted into charges by the photoconductive layer and accumulated. In addition, as a reading method, an optical reading method using a semiconductor material that generates a charge by irradiation of light, a charge generated by irradiation of radiation is accumulated in a collecting electrode, and the accumulated charge is thin film transistor (thin film transistor: TFT) is roughly classified into a TFT system that reads by turning on and off an electrical switch such as a TFT one pixel at a time.

In the radiation image detector as described above, when the electric field applied to the photoelectric conversion layer exceeds a certain threshold value, charge injection (dark current) from the electrode suddenly increases, crystallization occurs, and point defects may occur. Are known. Therefore, Patent Document 1 proposes that in an optical reading type radiographic image detector, an electrode end is covered with a protective film made of an insulating film in order to prevent dark current. Patent Document 2 proposes that in a TFT-type radiation image detector, the entire surface of the collection electrode is covered with a protective film made of an organic film in order to prevent dark current from the collection electrode.
JP 2005-183670 A JP 2006-156555 A

  However, Patent Document 1 describes that “the insulating film is preferably about 30% of the width of the upper surface of the electrode”, but the rationale is not shown, and the width and shape of the insulating film are specific. Not shown.

  Further, in the radiological image detector, not only point defects but also an afterimage in which a previous image pattern remains is a serious problem. In particular, in a TFT radiation image detector, since charges are directly accumulated in the collecting electrode, it is important to solve the problem of afterimage. In contrast, the optical reading type radiological image detector includes a power storage unit separately from the electrode, and charges are stored in the power storage unit, so that the afterimage problem of the electrode itself is different from that of the TFT method. I can say that. In general, TFT radiation image detectors have wiring such as data lines and gate lines between collection electrodes, whereas optical reading radiation image detectors have no wiring between electrodes. The presence / absence of the wiring and the distance between the wiring and the collecting electrode affect the configuration of the protective film to be formed. From the above, it is considered that it is difficult to effectively prevent both point defects and afterimages even if the configuration of the protective film of the optical reading method is applied to the TFT method as it is.

  Patent Document 2 proposes imparting conductivity to the protective film in order to improve the afterimage characteristics. However, since the protective film covers the entire collection electrode, it is difficult to avoid deterioration of the afterimage characteristics. It is considered difficult to achieve both defect prevention and afterimage prevention.

  In view of the above circumstances, an object of the present invention is to clarify a specific configuration that enables both point defect suppression and afterimage suppression in a TFT radiation image detector.

  The radiological image detector of the present invention reads a charge generation layer that generates charges when irradiated with radiation, a collection electrode that collects charges generated in the charge generation layer, and the charges collected by the collection electrode In the radiographic image detector in which a plurality of pixel layers having switch elements for the detection are arranged in a two-dimensional manner, a protective film is provided on the entire periphery of the collection electrode, The ratio of the area of the collecting electrode not covered with the protective film to the area is 40% or more and 99.8% or less.

  In the radiation image detector, the ratio is preferably 60% or more and 99.5% or less, and more preferably the ratio is 80% or more and 96% or less.

  In the radiological image detector, the protective film may be an insulating film.

  In the present specification, the ratio of the area (Sn) of the portion of the collection electrode not covered with the protective film to the total area (S) of the collection electrode (= Sn / S × 100 [%]) "Is called" fill factor ".

  A specific example of “radiation” is, for example, X-rays.

  According to the radiation image detector of the present invention, a protective film is provided on the entire circumference of the collecting electrode, and the ratio of the area of the portion not covered with the protective film of the collecting electrode to the total area of the collecting electrode (fill factor) is calculated. By setting it to 40% or more and 99.8% or less, both the point defect characteristic and the afterimage characteristic can be maintained at a level acceptable for the product or more. When the fill factor is 60% or more and 99.5% or less, both the point defect characteristic and the afterimage characteristic can be maintained at a level sufficient for a product. When the fill factor is 80% or more and 96% or less, both the point defect characteristic and the afterimage characteristic can be maintained at an excellent level as a product.

  A radiation image detector according to an embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram of the radiation image detector 100, and FIG. 2 is a layout of a pixel portion of the radiation image detector 100 shown in FIG.

  The radiation image detector 100 of this embodiment is of the TFT type, and as shown in FIG. 1, an upper electrode layer 21, a charge generation layer 20 that generates a charge upon irradiation with radiation, and a charge generation layer The collection electrode 8 that collects the charges generated in 20 and the detection layer 10 in which a large number of pixel portions 11 having switch elements 3 for reading out the charges collected by the collection electrode 8 are two-dimensionally arranged are sequentially arranged. It has a laminated structure.

  The upper electrode layer 21 is made of a low-resistance conductive material such as Au. The upper electrode layer 21 is connected to a high voltage power supply 22 for applying a bias voltage.

  The charge generation layer 20 has electromagnetic wave conductivity, and generates charges therein by irradiation with radiation. As the charge generation layer 20, for example, an amorphous a-Se film having a film thickness of 100 to 1000 μm mainly composed of selenium can be used.

  The detection layer 10 is made of an active matrix substrate, and includes a collection electrode 8, a storage capacitor 4 that stores the charges collected by the collection electrode 8, and a switch element 3 that reads the charges stored in the storage capacitor 4. The pixel portion 11 is arranged in an array.

  The collection electrode 8 can be formed using a material such as Al, Au, Cr, ITO (Indium Tin Oxide), IZO (Indium Zinc Oxide), and the thickness is preferably in the range of 0.05 μm to 1 μm.

  A protective film 18 is provided on the entire circumference of the peripheral edge of the collection electrode 8. The cover width W in which the protective film 18 covers the collecting electrode 8 is uniform. In FIG. 2, the outer shape of the collecting electrode 8 is indicated by a thick dotted line, and the protective film 18 is indicated by a hatched portion surrounded by a thick solid line. 1 and 2 are conceptual diagrams, and the dimensions of each part in the drawings are not necessarily the same as actual ones. In this example, adjacent protective films 18 are separated, but may be configured to be continuous between adjacent collecting electrodes 8.

  By providing the protective film 18, the ratio of the electric field strength at the end of the collecting electrode 8 to the electric field strength at the center of the collecting electrode 8 (electric field ratio) can be reduced, dark current is reduced, and point defects are reduced. it can. In order to obtain these effects stably, the cover width W is preferably uniform, and in this embodiment, it is configured as such. Note that “the cover width W is uniform” means that the cover width of each part is within ± 10% of the average value of the cover width W over the entire circumference.

  The protective film 18 can be composed of, for example, an insulating film, and as a material, a novolac resin, an acrylic resin, a PVA (polyvinylalcohol) film, a PVP (polyvinyl pyrrolidone) film, a PAA (Polyacrylic Acid) film, or the like can be used. . The thickness of the protective film 18 can be, for example, in the range of 0.05 μm to 5 μm.

  In addition, the detection layer 10 includes a large number of scanning lines 1 for turning on / off the switch element 3 and a large number of data lines 5 for reading out charges accumulated in the storage capacitor 4.

  As the switch element 3, an a-Si TFT using amorphous silicon as an active layer is generally used. A scanning line 1 for turning on / off the switching element 3 is connected to the gate electrode 2 of the switching element 3, and data for reading out the electric charge accumulated in the storage capacitor 4 is connected to the source electrode 6. The line 5 is connected, and the drain electrode 7 is connected with a storage capacitor electrode 9 which is one electrode constituting the storage capacitor 4. An amplifier 23 is connected to the end of the data line 5. The other electrode of the storage capacitor 4 is connected to the storage capacitor wiring 12. In addition, in order to avoid complication of a figure, in FIG. 1, a part of code | symbol of the same site | part is abbreviate | omitted.

  Next, an example of the operation of the radiation image detector 100 will be described. A recording radiation carrying a radiographic image such as an X-ray is irradiated from above in FIG. 1, and the radiation passes through the upper electrode layer 21 and is irradiated onto the charge generation layer 20. By this irradiation, the charge generation layer 20 generates charges inside. Of the generated charges, the holes are collected on the collection electrode 8 by the bias between the upper electrode layer 21 and the collection electrode 8 and accumulated in the storage capacitor 4 electrically connected to the collection electrode 8. Since the charge generation layer 20 generates an amount of charge corresponding to the amount of irradiated radiation, the charge corresponding to the image information carried by the radiation is stored in the storage capacitor 4 of each pixel unit 11. Thereafter, a signal for turning on the switching element 3 is sequentially applied via the scanning line 1, and the electric charge accumulated in each storage capacitor 4 is taken out via the data line 5. Furthermore, image information can be read by detecting the charge amount of each pixel by the amplifier 23.

  Hereinafter, the relationship between the structure of the protective film, the point defect characteristic, and the afterimage characteristic will be described using the fill factor defined in the above-mentioned means. The fill factor is the ratio of the area of the collecting electrode not covered with the protective film to the total area of the collecting electrode. FIG. 3 is a result of evaluating the afterimage characteristics and the point defect characteristics by simulation experiments in the radiation image detector 100 by changing the fill factor by changing the cover width W. Each symbol in FIG. 3 represents an evaluation result, ◎: excellent performance as a product, ○: sufficient performance as a product, Δ: insufficient performance as a product, but acceptable performance, × : Means not allowed as a product.

  In the above experiment, the length of one side of the collecting electrode is 100 μm, the thickness of the protective film is 1 μm, and the contact angle θ of the protective film is 40 degrees. As shown in FIG. 4, the contact angle is an angle of the protective film contacting the upper surface of the collection electrode with respect to the upper surface of the collection electrode.

  As shown in FIG. 3, when the fill factor is 40% or more and 99.8% or less, both the point defect characteristic and the afterimage characteristic are at a level acceptable for the product. When the fill factor is 60% or more and 99.5% or less, both the point defect characteristic and the afterimage characteristic are at a level sufficient for a product. When the fill factor is 80% or more and 96% or less, both the point defect characteristic and the afterimage characteristic are at an excellent level as a product.

  When the same experiment was conducted for a contact angle θ of 60 degrees, the same result as that obtained when the contact angle θ was 40 degrees was obtained.

  That is, in the radiation image detector 100, the point defect characteristics can be improved by covering the peripheral edge of the collection electrode with a protective film, but it is found that if the area covered is too large, the afterimage characteristics important as image characteristics deteriorate. It was. It was also found that both the defect characteristics and the afterimage characteristics can be favorably maintained by defining the fill factor value as described above.

  In addition, the radiographic image detector of this invention is not limited to the said embodiment. For example, the layer configuration in the radiation image detector of the present invention is not limited to the layer configuration as in the above embodiment, and other layers may be added.

Schematic configuration diagram of an embodiment of a radiation image detector of the present invention The layout of the pixel portion of the radiation image detector shown in FIG. Diagram showing afterimage characteristics and point defect characteristics due to changes in fill factor Diagram for explaining the contact angle of the protective film

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Scan line 2 Gate electrode 3 Switch element 4 Storage capacity 5 Data line 6 Source electrode 7 Drain electrode 8 Collection electrode 9 Storage capacity electrode 10 Detection layer 11 Pixel part 12 Storage capacity wiring 18 Protective film 20 Charge generation layer 21 Upper electrode layer 22 High voltage power supply

Claims (4)

A charge generation layer that generates a charge when irradiated with radiation; and
A collecting electrode for collecting charges generated in the charge generation layer, and a detection layer in which a plurality of pixel portions having switch elements for reading the charges collected by the collecting electrode are two-dimensionally arranged are stacked. In the radiation image detector,
A protective film is provided on the entire circumference of the collecting electrode,
The ratio of the area of the portion of the collection electrode not covered with the protective film to the total area of the collection electrode is 40% or more and 99.8% or less.   The radiation image detector according to claim 1, wherein the ratio is 60% or more and 99.5% or less.   The radiation image detector according to claim 1, wherein the ratio is 80% or more and 96% or less.   The radiation image detector according to claim 1, wherein the protective film is an insulating film.

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