JP2005033003A - Radiation detector and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a direct type planar radiation detector that outputs signals having high intensity characteristics (radiation-charge conversion efficiency), and to provide a method of manufacturing the detector. <P>SOLUTION: A radiation photoconductive film 2 which converts incident radiation R into signal charges contains a binder 13 which can hold the film 2 in a prescribed thickness in a state where radiation-charge converting medium particles 12 are mutually contacted with each other or approached to each other at a prescribed density. The dielectric constant of the binder 13 is higher than that of the radiation-charge converting medium particles 12. Consequently, the magnitude of a bias voltage Vd acting on the binder 13 is reduced and the occurrence of dielectric breakdown can be inhibited. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a radiation detector for detecting a radiographic image taken with radiation and a method for manufacturing the same.
[0002]
[Prior art]
As a radiation detector, for example, a planar detector in which a plurality of X-ray detection elements are two-dimensionally arranged is drawing attention. The planar detector can output an X-ray image taken with X-rays or a real-time X-ray image as a digital signal. Since the flat detector is a solid state detector, it is expected to improve image quality performance and stability.
[0003]
Planar detectors for general radiography and chest radiography that collect still images with relatively large doses have already been developed and commercialized.
[0004]
In the near future, X-ray moving images of 30 screens or more per second can be detected using, for example, fluoroscopic doses, and commercialization of products applied to fields such as circulatory organs and digestive organs is also predicted. For detection of such an X-ray moving image, improvement of S / N, development of a real-time processing technique of minute signals, and the like are required.
[0005]
There are two types of planar detectors: a direct method and an indirect method.
[0006]
The direct method is a method in which an X-ray is directly converted into a signal charge using an X-ray photoconductive material such as a-Se, and the converted signal charge is stored in a charge storage capacitor.
[0007]
On the other hand, the indirect method converts X-rays into visible light using a scintillator material, converts the converted visible light into signal charges with an a-Si photodiode or CCD, and converts the signal charges into a charge storage capacitor. This is an accumulation method.
[0008]
In the direct method, a pixel portion, a charge storage portion, a TFT, and a Zener diode are provided for each pixel that is two-dimensionally arranged on the detection surface, and the voltage input to the TFT input side by the Zener diode is a voltage that destroys the TFT. There has already been proposed a detector that prevents destruction of a TFT (readout switch) by outputting a charge to the charge storage section when a predetermined voltage lower than the predetermined voltage is reached (see, for example, Patent Document 1).
[0009]
[Patent Document 1]
Japanese Patent Laid-Open No. 10-10237 (abstract, FIG. 1, paragraphs [0038] to [0048]).
[0010]
[Problems to be solved by the invention]
In the direct radiation flat detector, the characteristics of the radiation photoconductive layer, that is, the intensity of the output signal with respect to the intensity of the incident radiation (conversion efficiency capable of outputting the incident radiation as a signal charge) is important. Therefore, the radiation - for the purpose of improving the signal charge conversion efficiency, the radiation photoconductive layer, for example, PbI 2 _(iodide lead) and HgI 2 _(iodide mercury) or the like of a metal iodide or metal halide-based A photoconductive material is used.
[0011]
When a metal iodide or metal halide compound such as PbI ₂ or HgI ₂ is used for the radiation photoconductive layer, the radiation photoconductive layer is used to increase the intensity of the output signal with respect to the incident radiation and to secure a sufficient amount of radiation absorption. The layer is required to have a thickness of several hundred μm or more.
[0012]
However, when a radiation photoconductive layer having a film thickness of several hundred μm or more is used, the output signal intensity is improved with respect to the intensity of the incident radiation, and the photoconductive charge in the direction perpendicular to the direction in which the bias electrode is applied. In order to prevent diffusion, a high voltage of several kV or more must be applied as the bias electrode.
[0013]
When the radiation photoconductive layer is formed, a vacuum deposition method or a sputtering method is usually used. However, a metal iodide such as PbI ₂ or HgI ₂ or a metal halide compound-based photoconductive material has poor crystal growth. Since it is stable, it is difficult to form a layer having a film thickness of several hundred μm by vacuum deposition or sputtering.
[0014]
Therefore, when configuring the radiation photoconductive layer by PbI ₂ and HgI metal iodide such as ₂ or metal halide-based, and the like PbI ₂ and HgI _2, for example, a binder which is a resin were mixed, a method of coating Is widely used.
[0015]
However, since the density of PbI ₂ , HgI _2, etc. in the radiation photoconductive layer is reduced by the amount of the binder mixed by using the binder, there is a problem in that the conversion efficiency for converting radiation into electric charge is reduced.
[0016]
In addition, since the dielectric constant of an epoxy resin or the like used as a binder is lower than that of a photoconductive material such as PbI ₂ or HgI ₂ , most of the bias voltage applied to the bias electrode is shared (applied) to the binder ( The electric field acting on the binder is higher than the electric field acting on the photoconductive material). This causes problems such as deterioration of the intensity of the output signal with respect to incident radiation and dielectric breakdown of the binder.
[0017]
The object of the present invention is to improve the intensity characteristic (radiation-signal charge conversion efficiency) of the output signal of the radiation flat detector in the direct method, and to manufacture a radiation flat detector capable of improving the intensity characteristic of the output signal. It is to provide a method.
[0018]
[Means for Solving the Problems]
The present invention has been made on the basis of the above-described problems. A circuit board in which a plurality of pixel electrodes and switching elements are provided one-dimensionally or two-dimensionally on a planar substrate, and the circuit board is brought into contact with the incident radiation. A radiation detector comprising a radiation photoconductive layer containing particles of a radiation-to-charge conversion medium that converts the charge into a magnitude corresponding to the intensity, and a bias electrode capable of applying a predetermined bias voltage to the radiation photoconductive layer The radiation photoconductive layer includes a binder capable of maintaining a predetermined film thickness in a state where the radiation-charge conversion medium particles are in contact with each other or close to each other at a predetermined density, and the radiation-charge conversion layer The radiation detector characterized in that the relationship between the dielectric constant of the medium particles and the dielectric constant of the binder is: the dielectric constant of the radiation-charge conversion medium particles <the dielectric constant of the binder A.
[0019]
In the present invention, a plurality of switching elements and charge storage capacitors are formed one-dimensionally or two-dimensionally on a planar substrate, and an insulating layer is interposed for each pixel region defined by the switching elements and charge storage capacitors. The pixel electrode is arranged, and the pixel electrode and the pixel region are brought into contact with each other by using a binder having a dielectric constant larger than that of the radiation-charge converting medium particle and the radiation-charge converting medium particle and the radiation-charge converting medium element. Or a radiation photoconductive layer having a predetermined thickness in a state of being close to each other at a predetermined density, and covering the radiation photoconductive layer with a bias electrode layer. is there.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, the present invention can be applied to X-rays, γ-rays, and other various radiations. However, in the following embodiment, a case of representative X-rays in radiation will be described as an example. Therefore, by replacing “X-ray” in the embodiment with “radiation”, the present invention can be applied to other radiation targeted by the present invention.
[0021]
Hereinafter, an example of an embodiment of the present invention will be described in detail with reference to the drawings.
[0022]
As shown in FIG. 1, a direct X-ray planar detector 101 can generate a signal charge having a magnitude corresponding to the intensity of input radiation (X-rays in this example) on the active matrix substrate 1. And an X-ray photoconductive layer 2 which is an X-ray-signal charge conversion layer. Note that FIG. 1 shows an active matrix substrate 1 in which a plurality of signal charge detection units and signal extraction elements are provided in a matrix of m rows × n columns and an X-ray, as will be described later with reference to FIG. The cross section of the photoconductive layer 2 cut out for two pixels is shown enlarged.
[0023]
As shown in FIG. 1, a bias electrode 3 capable of applying a bias voltage to the X-ray photoconductive layer 2 is provided on the X-ray photoconductive layer 2.
[0024]
The matrix substrate 1 is on a support substrate 4 made of, for example, glass, and divides signal charges generated by the X-ray photoconductive layer 2 into m rows × n columns with the X-ray photoconductive layer 2. The pixel electrode 5 that is taken in independently for each region, the charge storage capacitor 6 that stores the signal charge taken in by the pixel electrode 5, and the signal charge that is stored in the charge storage capacitor 6 is transferred at a predetermined timing. TFT (thin film transistor) 7 as a switching element for this purpose. The m × n unit pixels defined by the charge storage capacitor 6 and the TFT 7 and the X-ray photoconductive layer 2 are insulated by an insulating layer 8 formed of, for example, SiO ₂ (silicon oxide). ing.
[0025]
Each pixel electrode 5 is connected to the drain electrode 10 of the TFT 7 that is insulated by the insulating layer 8 by, for example, a through hole 9. The drain electrode 10 is connected to a transparent electrode 11 serving as one electrode of the charge storage capacitor 6.
[0026]
X-ray photoconductive layer 2 is, for example PbI 2 _(iodide lead) and HgI 2 X-ray as a main component _(iodide mercury) metal iodide such or metal halide - charge conversion medium is a predetermined size of It consists of X-ray-charge conversion particles 12 formed in the form of particles and a binder 13 that holds the X-ray-charge conversion medium particles 12. In addition, as the binder 13, resin materials, such as an epoxy resin, can be utilized widely, for example. Further, the binder 13 has a predetermined film thickness in a state where the X-ray-charge conversion medium (PbI ₂ or HgI ₂ ) particles 12 having unstable crystal growth properties are brought into contact with each other or close to each other at a predetermined density. Useful for depositing.
[0027]
The X-ray detector 101 shown in FIG. 1 has a plurality of charge storage capacitors 6 and TFTs 7 formed one-dimensionally or two-dimensionally on a glass substrate 4,
A pixel electrode 5 is disposed through an insulating layer 8 for each pixel region defined by the charge storage capacitor 6 and the TFT 7,
In the pixel electrode 5 and the pixel region, a binder 13 having a dielectric constant larger than that of the X-ray-charge conversion medium particles 12 (for example, the ratio of the dielectric constant of the X-ray-charge conversion medium particles 12 to the dielectric constant of the binder 13 is 1: X-ray photoconductive film having a predetermined film thickness in a state where the X-ray-charge conversion medium particles and the X-ray-charge conversion medium element are in contact with each other or close to each other at a predetermined density. Covered by layer 2,
Covering the X-ray photoconductive layer 2 with a bias electrode 3;
Thus, it is easily formed.
[0028]
When the bias electrode 3 is irradiated with incident X-rays R in a state where a predetermined bias voltage Vb is applied to the bias electrode 3 of the direct X-ray planar detector shown in FIG. 1, the X-ray photoconductive layer 2 The electric charge 14 generated in the step is moved to an arbitrary pixel electrode 5 on the active matrix substrate 1 by an electric field directed by the bias voltage Vb applied to the bias electrode 3, and passes through the drain electrode 10 of the TFT 7. The charge is stored in the charge storage capacitor 6. The charge 14 is illustrated in FIG. 1 as a state in which electrons e and holes (holes) h are directed to the bias electrode 3 and the pixel electrode 5, respectively.
[0029]
The charges 14 stored in the charge storage capacitor 6 are output to a signal (image) processing unit (not shown) at a predetermined timing by a signal reading unit described below with reference to FIG.
[0030]
FIG. 2 is an equivalent circuit schematically illustrating a signal extraction unit for reading out signal charges from the individual pixels illustrated in FIG.
[0031]
The gate electrodes 15 of the respective TFTs 7 arranged in the form of m × n lattices are individually connected to m control electrodes 16 connected to a scanning line control circuit (not shown). Further, the drain electrode 10 of the TFT 7 is individually connected to n readout electrodes 17 connected to a readout control circuit (not shown).
[0032]
Therefore, accumulation is performed in the capacitor 6 via the pixel electrode 5 provided in each pixel until an ON signal is input to the gate electrode 15 of an arbitrary row by the control electrode 16 and each TFT is turned on. The charged charges 14 are output to the readout electrode 17 when the TFT 7 is turned on.
[0033]
That is, the charges corresponding to the X-ray image incident on m × n pixels arranged in a matrix and converted into charges by the photoconductive layer 2 and stored in the capacitor 6 are stored in a read control circuit (not shown). By the control, it is fetched in order for every arbitrary row. The extracted signal charges are output to a signal (image) processing unit (not shown).
[0034]
The above-mentioned X-ray photoconductive layer 2 is shown in FIG. 3 as a state composed of two layers of a layer 2a of the X-ray-signal charge conversion particles 12 and a layer 2b of the binder 13 in a macro (macroscopic) manner. It can be approximated as follows. In FIG. 3, the X-ray photoconductive layer 2 is approximated by two layers of the X-ray-signal charge converting particle 12 layer 2a and the binder 13 layer 2b. The positional relationship between the layer 2a of the particles 12 and the layer 2b of the binder 13 may be reversed. That is, it goes without saying that the same holds true even when the layer 2b of the binder 13 is arranged on the bias side and the layer 2a of the X-ray-signal charge conversion particles 12 is arranged on the unit pixel side.
[0035]
Accordingly, the magnitude of the bias voltage Vb applied to the bias electrode 3 is V [V], the thickness of the X-ray photoconductive layer 2 is d, and the volume mixing ratio of the X-ray-signal charge conversion particles 12 and the binder 13 is d. _{When 1} : d ₂ , the dielectric constant ratio is e ₁ : e ₂ , and the applied electric field ratio is E ₁ : E ₂ , the electric field magnitude E ₁ acting on the X-ray-signal charge conversion particles 12 and the binder 13 are affected. The magnitude E _{2 of the} electric field is
E ₁ = e ₂ V / (e ₂ d ₁ + e ₁ d ₂ ) (A)
E ₂ = e ₁ V / (e ₂ d ₁ + e ₁ d ₂ ) (B)
It is written.
[0036]
Here, when the volume mixing ratio of the X-ray-signal charge conversion particles 12 and the binder 13 is 1: 1, the formulas (A) and (B) are
E ₁ = 2e ₂ V / (e ₁ + e ₂ ) d (C)
E ₂ = 2e ₁ V / (e ₁ + e ₂ ) d (D)
It is written.
[0037]
From the formulas (C) and (D), by selecting the material of the binder 13 so that the relationship between the dielectric constant ratios of the X-ray-signal charge conversion particles 12 and the binder 13 is e ₁ <e ₂ , - as compared with the case where the relationship of the dielectric constant ratio of the signal charge conversion particles 12 and the binder 13 has set the material of the binder 13 so that e ₁ ≧ e _2, X-ray - field acting on the signal charge conversion particles 12 E ₁ higher than the electric field _{E 2} acting on the binder 13 (the electric field low _{E 2} acting on the binder 13) can be set.
[0038]
Accordingly, the magnitude of the electric field acting on the binder 13 is reduced, so that dielectric breakdown can be prevented and the intensity characteristics of the output signal with respect to incident X-rays can be improved.
[0039]
For example, a ferroelectric polymer in which the volume mixing ratio of the X-ray-signal charge converting particles 12 and the binder 13 is 9: 1 and the dielectric constant ratio of the X-ray-signal charge converting particles 12 and the binder 13 is 1:10. (For example, it contains an organic substance or a heavy metal typified by a fluoride polymer such as a copolymer of vinylidene fluoride and trifluoroethylene, a polymer having a cyano group or a cyanoethyl group such as a copolymer of vinylidene cyanide and vinyl acetate, etc. When the binder 13 is selected as the binder 13, the electric field E ₁ acting on the X-ray-signal charge conversion particle 12 and the electric field E ₂ acting on the binder 13 are expressed by the equations (A) and (B):
E ₁ = 100V / 91d (E)
E ₂ = 10V / 91d (F)
It becomes.
[0040]
Therefore, when the dielectric constant ratio between the X-ray-signal charge conversion particles 12 and the binder 13 is 1:10, the dielectric constant ratio between the X-ray-signal charge conversion particles 12 and the binder 13 is 1: 1. Compared to the case, the electric field acting on the X-ray-signal charge conversion particles 12 can be increased by approximately 10%, and the electric field acting on the binder 13 can be lowered by approximately 90%.
[0041]
Therefore, the magnitude of the electric field acting on the binder 13 is reduced, so that dielectric breakdown can be prevented and the intensity characteristic of the output signal with respect to the incident X-ray can be improved. Even if the dielectric constant ratio between the X-ray-signal charge conversion particles 12 and the binder 13 is about 1: 2, for example, the electric field acting on the binder 13 is larger than when the dielectric constant ratio is 1: 1. Needless to say, the size of can be reduced. The material of the binder 13 is preferably a polymer containing an organic substance or heavy metal.
[0042]
As described above, according to the present invention, the conversion efficiency of the X-ray-charge conversion output obtained from the pixels arranged in a matrix can be increased. In addition, by suppressing the magnitude of the electric field acting on the binder that allows the particles of the X-ray-charge conversion medium to contact each other or to be deposited in a predetermined film thickness in a state of being close to each other at a predetermined density, Insulation breakdown can be prevented.
[0043]
Therefore, an X-ray planar detector with high X-ray-charge conversion efficiency and stable operation can be obtained.
[0044]
The present invention is not limited to the above-described embodiments, and various modifications and changes can be made without departing from the scope of the invention when it is implemented. Moreover, each embodiment may be implemented in combination as appropriate as possible, and in that case, the effect of the combination can be obtained.
[0045]
In the above-described embodiment, the X-ray detector in which a plurality of vertical and horizontal pixels are arranged has been described. However, the ratio of the vertical and horizontal pixels is different (for example, when one pixel is one). The present invention is also applicable to an X-ray detector configured in a shape.
[0046]
【The invention's effect】
As described above, according to the present invention, a radiation flat detector having high radiation-to-charge conversion efficiency and capable of operating stably can be obtained. In addition, since the efficiency with which radiation is converted into electric charges by each pixel is increased, it corresponds to a substantial improvement in sensitivity. Therefore, it is possible to reduce the amount of radiation that has to be irradiated on the inspection object (specimen). When is a human body, the amount of exposure can be reduced.
[Brief description of the drawings]
FIG. 1 is a schematic diagram illustrating an example of an X-ray detector to which an embodiment of the present invention can be applied.
FIG. 2 is a schematic diagram for explaining an example of an active matrix substrate in the X-ray detector shown in FIG. 1;
3 is a schematic view that approximates the X-ray photoconductive layer of the X-ray detector shown in FIG. 1 macroscopically.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Active matrix substrate, 2 ... X-ray photoconductive layer, 3 ... Bias electrode, 4 ... Support substrate, 5 ... Pixel electrode, 6 ... Charge storage capacitor, 7 ... Thin-film transistor (TFT), 8 ... Insulating layer, 9 ... Through hole, 10 ... drain electrode, 11 ... transparent electrode, 12 ... X-ray (radiation) -charge conversion medium particles (particles mainly composed of metal iodide or metal halide), 13 ... binder, 14 ... charge (light Conductive charge), 15 ... gate electrode, 16 ... control electrode, 17 ... readout electrode, R ... incident X-ray, e ... electron, h ... hole (hole), 101 ... X-ray planar detector.

Claims (7)

A circuit board in which a plurality of pixel electrodes and switching elements are provided one-dimensionally or two-dimensionally on a planar substrate, and radiation that is brought into contact with the circuit board and converts incident radiation into charges having a magnitude corresponding to the intensity thereof. In a radiation detector comprising a radiation photoconductive layer containing particles of a charge conversion medium, and a bias electrode capable of applying a predetermined bias voltage to the radiation photoconductive layer,
The radiation photoconductive layer includes a binder capable of maintaining a predetermined film thickness in a state where the radiation-charge conversion medium particles are in contact with each other or close to each other at a predetermined density, and the radiation-charge conversion medium particles The radiation detector characterized in that the relationship between the dielectric constant of the binder and the dielectric constant of the binder is such that the dielectric constant of the radiation-charge conversion medium particles <the dielectric constant of the binder. 2. The radiation detector according to claim 1, wherein the binder is a ferroelectric polymer containing an organic substance or a heavy metal. The binder is a polymer having a cyano group or a cyanoethyl group, such as a fluoride polymer such as a copolymer of vinylidene fluoride and trifluoroethylene, or a copolymer of vinylidene cyanide and vinyl acetate. 2. The radiation detector according to 2. The radiation detector according to any one of claims 1 to 3, wherein the radiation-to-charge conversion medium contains PbI ₂ (lead iodide) or HgI ₂ (mercury iodide). A plurality of switching elements and charge storage capacitors are formed one-dimensionally or two-dimensionally on a planar substrate,
A pixel electrode is disposed with an insulating layer interposed for each pixel region defined by the switching element and the charge storage capacitor,
The pixel electrode and the pixel region are brought into contact with each other by using a binder having a dielectric constant larger than that of the radiation-charge conversion medium particles, or at a predetermined density. Covered with a radiation photoconductive layer having a predetermined film thickness in a close proximity,
Covering the radiation photoconductive layer with a bias electrode layer;
The manufacturing method of the radiation detector characterized by the above-mentioned. The binder is a polymer having a cyano group or a cyanoethyl group, such as a fluoride polymer such as a copolymer of vinylidene fluoride and trifluoroethylene, or a copolymer of vinylidene cyanide and vinyl acetate. 6. A method for producing a radiation detector according to 5. The radiation - charge conversion medium particles, PbI 2 _(iodide lead) or HgI 2 method of manufacturing a radiation detector according to claim 5 or 6, wherein it contains a _(iodide mercury).

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