JP2007103408A - Radiation detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a radiation detector whose life can be prolonged and which can continuously obtain a stable output image. <P>SOLUTION: The radiation detector 1 is provided with a photoconduction layer 11 which converts a radial ray that is made incident from the outside into an electric signal and which is mainly composed of a halogenated compound and a circuit board 21 where a plurality of pixel electrodes and switching elements are two-dimensionally arranged. An upper electrode is formed on the surface of a photoconduction film of the photoconduction layer on a side opposite to the circuit board. An interface between the pixel electrode and the photoconduction film has a film which is mainly composed of one or more materials in tantalum, niobium, vanadium, titanium, zirconium and hafnium, and of nitride or carbide or an oxide. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a radiation detector that detects a radiographic image taken with radiation represented by X-rays.

  An X-ray image detector using an active matrix has attracted much attention as a new generation X-ray diagnostic detector. By applying X-rays to a planar detector, an X-ray image or a real-time X-ray image is output as a digital signal. Since it is a solid state detector, it is extremely promising in terms of image quality and stability. For this reason, research and development by many universities and manufacturers are being promoted.

  The first practical application has been developed for chest / general radiography that collects still images with a relatively large dose, and has recently been commercialized. Commercialization is expected in the near future for applications in the cardiovascular and gastrointestinal fields where it is necessary to clear higher technical hurdles and realize real-time video of 30 frames per second under fluoroscopic dose. . For this video application, improvement of S / N, real-time processing technology of minute signals, and the like are important development items.

  There are two types of flat detectors: a direct method and an indirect method. The direct method is a method in which X-rays are directly converted into a charge signal by a photoconductive film such as a-Se and led to a charge storage capacitor. The indirect method is a method in which X-rays are received by the scintillator layer and converted into visible light, and the visible light is converted into signal charges by an a-Si photodiode or CCD, and led to a charge storage capacitor. The direct method is a method in which photoconductive charges generated inside an X-ray photoconductor by incident X-rays are directly guided to a charge storage capacitor by a high electric field.

  As described above, there are two types of X-ray image detectors: a direct method and an indirect method. Although the indirect method occupies the majority of what has been announced at present, the direct method has a higher possibility of higher performance in the future.

  The direct method uses a semiconductor characterized by a detection wavelength as an “X-ray photoconductive material” for directly converting incident X-rays into a charge signal. The main application of the flat image detector is to transmit the human body and use the information for medical purposes in many cases, and it needs to be large enough to cover the human body. For this reason, a detector having a side of about 40 cm is often used as a normally used size. At this time, if an X-ray image detector of a direct method is to be realized, it is required to uniformly form an X-ray photoconductive film on a TFT circuit substrate having a larger size. In addition, in order to sufficiently detect incident X-rays, it is necessary to laminate a material having a large specific gravity made of heavy metal with a thickness of several hundreds of μm to form an X-ray photoconductive film. This requires that a semiconductor film with a size of about 40 cm square is formed on the TFT substrate.

  Since the X-ray photoconductive material is a kind of semiconductor, there is a very high possibility that the characteristics will change greatly depending on its crystal structure and composition. In addition, although the characteristics of ordinary semiconductor materials are the highest in single crystals, semiconductor single crystal materials that can cover the size of an X-ray image detector have not been realized. Therefore, in order to realize a direct X-ray image detector, it is necessary to form an X-ray photoconductive material directly on the TFT substrate.

In the direct method, a pixel portion, a charge storage portion, a TFT (readout switch), and a Zener diode are provided for each pixel that is two-dimensionally arranged on the detection surface, and a voltage input to the TFT input side by the Zener diode is There has already been proposed a detector that prevents the breakdown of the TFT by outputting a charge to the charge storage portion at a predetermined voltage lower than the voltage that destroys the TFT (see, for example, Patent Document 1).
Japanese Patent Laid-Open No. 10-10237

A TFT circuit board of a normal X-ray image detector is made by diverting a manufacturing process of a liquid crystal display device, for example. Therefore, aluminum or ITO is often used for the pixel electrode in direct contact with the X-ray photoconductive film, and a silicon oxide (SiO ₂ ) insulating film is also in contact with the X-ray photoconductive film so as to surround the pixel electrode. It has become.

In many cases, an X-ray photoconductive film having a length of several hundred μm is formed on a TFT substrate by vacuum deposition. As the X-ray photoconductive film, lead iodide (PbI ₂ ), mercury iodide (HgI ₂ ), bismuth iodide (BiI ₃ ), indium iodide (InI) having the characteristic of converting X-rays into charge signals efficiently. ), Thallium bromide (TlBr) or the like. Usually, an upper electrode responsible for supplying electric charge to the X-ray photoconductive film is further laminated on the X-ray photoconductive film.

  However, in the direct X-ray image detector, the X-ray photoconductive film is made of heavy metal in order to efficiently absorb X-rays, and those iodides and bromides are mainly used for providing sufficient performance. It is used. Many of these heavy metal halides are very corrosive, and when most metals are used as electrodes, the metals are often corroded and the performance as an X-ray image detector is often greatly impaired. In particular, in an operating X-ray image detector, it is necessary to apply an electric field of a high voltage (several tens to hundreds of volts) to the X-ray photoconductive film. May move to the anode side of the X-ray photoconductive film, and the interface with the pixel electrode or upper electrode mainly made of metal may be altered. This greatly affects the quality of the X-ray image at the interface between the X-ray photoconductive film and the electrode surface, which has strong properties as a semiconductor, so that the alteration of the interface due to the halogen component in the X-ray photoconductive film is suppressed. There is a problem that must be.

  An object of the present invention is to provide a radiation detector capable of extending the lifetime of the radiation detector and continuously obtaining a stable output image.

  The present invention provides a circuit board in which a plurality of pixel electrodes and switching elements are two-dimensionally arranged on a planar substrate, and a halogen compound that is provided in contact with the circuit board and converts radiation incident from the outside into an electrical signal. A radiation detector having a photoconductive layer as a main component and having an upper electrode formed on a surface of the photoconductive film of the photoconductive layer on the side facing the circuit board, the pixel electrode and the photoconductive film The radiation detector is characterized in that it has a film mainly composed of any one or more materials of tantalum, niobium, vanadium, titanium, zirconium, or hafnium and a nitride at the interface. is there.

  In the present application, it is possible to obtain a radiation detector capable of extending the lifetime of the radiation detector and obtaining a stable output image continuously.

  Hereinafter, embodiments and effects of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a diagram showing only the detector portion of the X-ray image detector. 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 the description of “X-ray” in the embodiment with “radiation”, the present invention can be applied to other radiation targeted by the present invention.

  An X-ray image detector 1 shown in FIG. 1 has an X-ray photoconductive layer (photoconductive layer) 11 that converts incident X-rays into an electric signal (electrons or holes (holes)), and electrons are generated by the X-ray photoconductive layer 11. Alternatively, a TFT circuit substrate 21 that takes out the output converted into holes (holes) in association with the position of incident X-rays incident on the X-ray photoconductive layer 11 is provided.

  The X-ray photoconductive layer 11, which will be described later with reference to FIGS. 3 to 5, includes an upper electrode 12 and an X-ray photoconductive film 13 that is prevented from contacting the atmosphere by the upper electrode 12.

  The TFT circuit substrate 21 usually includes a TFT circuit layer 23 and a pixel electrode 22 laminated on a holding substrate 24 that is a flat glass.

  An equivalent circuit inside the TFT circuit substrate 21 is shown in FIG.

  As shown in FIG. 2, the TFT circuit substrate 21 has a matrix shape defined by control electrodes 25 arranged at predetermined intervals and read electrodes 28 arranged at predetermined intervals in a direction orthogonal to the control electrodes 25. The output circuit portion includes a thin film transistor (TFT) 26 provided in a set at each intersection (intersection) of each electrode, a pixel electrode 22 connected to the TFT 26, and a capacitor 27.

  The pixel electrode 22 and the capacitor 27 are arranged in a lattice pattern as a set, and each set corresponds to a pixel of the X-ray image.

  The control electrode 25 is connected to the gate electrode of the TFT 26, and the readout electrode 29 is connected to the drain electrode of the TFT 26.

  With such a circuit configuration, the charge that has flowed into the pixel electrode 22 corresponding to each pixel is connected to each of the capacitors 27 connected until the gate electrode of the thin film transistor 26 connected thereto is turned on. In this state, only one of the control electrodes 25 is switched to the on state, so that the horizontal row of thin film transistors 26 connected to the control electrode 25 in the on state is turned on. The charge of the capacitor 27 connected to the TFT 26 flows to the readout electrode 28.

  As a result, image information (charge detected by the pixel electrode 22) corresponding to a specific row is output to the outside.

  The entire image information converted into an electrical signal by the X-ray photoconductive layer 11 (photoconductive film 13) is converted into a video signal outside by changing (shifting) the control electrode 25 to be turned on in order. It becomes possible to output.

  Next, the characteristic features of the present invention will be described with reference to FIGS.

  FIG. 3 is a simplified cross-sectional view of an X-ray image detector according to the present invention.

  The TFT circuit substrate 21 has a TFT circuit layer 23 on a holding substrate 24, and the pixel electrode 22 is disposed on the circuit layer 23 so as to cover most of the surface of the TFT circuit substrate.

  Further, a protective film 29 mainly composed of tantalum nitride (TaN), for example, is formed on each pixel electrode 22. The protective film 29 can be easily formed on the surface of the TFT circuit substrate 21 by reactive sputtering deposition in nitrogen gas. For example, a protective film 29 made of tantalum nitride is formed on the entire surface of the TFT circuit substrate 21, and the protective film 29 in a region other than the pixel electrode 22 is removed by, for example, dry etching, whereby the structure of the protective film 29 can be obtained.

  On the TFT circuit layer 23 and the protective film 29, as the X-ray photoconductive film 13 of the X-ray photoconductive layer 11, for example, lead iodide (PbI2) is formed to a thickness of several hundreds μm by vacuum deposition, By forming the upper electrode 12 thereafter, the main structure as the X-ray image detector 1 can be realized.

When lead iodide (PbI ₂ ) is used for the X-ray photoconductive film 13 as the X-ray photoconductive substance, X-rays incident from the outside into the X-ray photoconductive film 13 (X-ray photoconductive layer 11) are The X-ray photoconductive film 13 generates a large number of electron-hole pairs inside due to its energy. At this time, by applying a potential difference (voltage) of several hundred volts between the upper electrode 12 and the pixel electrode 15 (TFT circuit substrate 21), the generated electric charges, that is, electrons and holes move by the electric field, It reaches the pixel electrode 15 and is stored in the TFT circuit layer 23. By taking out the electric charge held (stored) by the TFT circuit layer 23 by the method described with reference to FIG. 2, an X-ray image corresponding to the input X-ray is obtained.

  Note that the pixel electrode 15 and the upper electrode 12 that are in contact with the highly corrosive X-ray photoconductive film 13 must be made of a highly corrosion-resistant substance. In particular, since the halogen ions that have moved from the inside of the X-ray photoconductive film 13 accumulate on the positive electrode side and may corrode the electrode, extremely strong corrosion resistance is required.

  In general, platinum, palladium and the like are known as substances having strong corrosion resistance. It is known that a metal having high corrosion resistance including these generally has a high work function value. As an example, when platinum is used as the pixel electrode, a schematic diagram of a band gap in the vicinity of the X-ray photoconductive film is shown in FIG.

In FIG. 7, a necessary electric field is applied to the X-ray photoconductive film 13 with the upper electrode 12 as a negative electrode and the pixel electrode 22 as a positive electrode. In this case, the work function of platinum constituting the pixel electrode 22 is 5.3 eV, which is larger than the work function of lead iodide (PbI ₂ ) often used as the X-ray photoconductive film 13.

  For this reason, as shown in FIG. 7, the energy band (level) 18 of the X-ray photoconductive film 13 in contact with the pixel electrode 22 is largely bent upward (becomes a barrier to the pixel electrode 22).

  Electrons 15 generated by X-ray R incident from the outside move to the right (pixel electrode side) by the electric field of the X-ray photoconductive film 13, but the conduction band energy level (band) generated at the interface of the pixel electrode 22. It cannot stay over the barrier generated by the 18 bends and stays in the X-ray photoconductive film 13.

  Due to the stagnation of electrons, many afterimage phenomena occur in the output X-ray image, and the quality of the X-ray image is remarkably deteriorated. In order to prevent deterioration in the quality of the (output) X-ray image due to this afterimage, it is effective to reduce the value (size) of the work function on the surface of the pixel electrode 22 on the positive electrode side, for example. However, with a general element, when the work function value is reduced, the corrosion resistance is remarkably lowered, and the lifetime of the X-ray image detector is reduced.

  In order to solve the above problem, a low work function value and high corrosion resistance can be obtained at the same time by using a compound containing a metal element having high corrosion resistance as an electrode material.

  As elements having high corrosion resistance, elements of group IV (4) in the periodic table, such as titanium, zirconium and hafnium, and elements of group V (5) (vanadium, niobium or tantalum) are known.

  It is known that these elements are further improved in corrosion resistance by forming nitrides, carbides, and oxides, and do not dissolve in ordinary acids and alkalis.

Moreover, the work functions of nitrides, carbides and oxides of group 4 and group 5 are low
TiN (titanium nitride): 2.9 eV,
ZrN (zirconium nitride): 2.9 eV,
HfN (hafnium nitride): 3.9 eV,
VN (vanadium nitride): 3.6 eV,
NbN (niobium nitride): 3.9 eV,
TaN (tantalum nitride): 4.0 eV,
Ta ₃ N ₅ (tantalum nitride): 4, 2 eV,
Ta ₂ O ₅ (tantalum oxide): 4.5 eV,
TaON (tantalum oxynitride): 4.4 eV,
TiC (titanium carbide): 3.8 eV,
ZrC (zirconium carbide): 4.0 eV,
HfC (hafnium carbide): 4.5 eV,
NbC (niobium carbide): 4.2 eV,
TaC (tantalum carbide): 4.3 eV
It is.

Among these, tantalum nitride (Ta ₃ N ₅ or TaN) that can be easily formed at a low temperature was used for the protective film 29 shown in FIG.

  FIG. 6 is a band gap (model) of the X-ray image detector shown in FIG.

  The difference from the general structure shown in FIG. 7 is that a tantalum nitride film (protective film) 29 is provided at the interface with the pixel electrode 22 on the positive electrode side of the X-ray photoconductive film 13.

  With this structure, the work function value of the surface of the pixel electrode 22 (interface with the protective film 29) becomes the work function value of the tantalum nitride film (protective film) 29, which is 5.3 eV which is the comparative example shown in FIG. To 4.0 eV.

  As a result, in the band model of the X-ray photoconductive film 12, the bending of the band near the pixel electrode 22 is reversed (compared to the example shown in FIG. 7).

  By providing this band structure, the electrons 15 generated by the incident X-ray R are output to the outside as an image signal without hindering movement in the direction of the pixel electrode 22 due to charges inside the X-ray photoconductive film 13.

Note that when tantalum nitride (Ta ₃ N ₅ ) having a high nitrogen content is used as the material of the protective film 29, the resistivity of tantalum nitride is extremely high (50 MΩcm or more), so FIG. 4 shows an X-ray image detector. A protective film 129 as shown as 101 can cover the entire surface of the TFT circuit layer 32 and the pixel electrode 22.

  With the structure shown in FIG. 4, a protective film 129 made of tantalum nitride is formed between the pixel electrodes 22, but since the resistivity of tantalum nitride is extremely high, there is almost no charge leakage between the pixel electrodes 22. No. Therefore, there is an advantage that the costly dry etching process (protective film (29) in the example shown in FIG. 3) can be omitted.

  FIG. 5 shows an embodiment of a protective film different from the protective film described with reference to FIGS.

  The X-ray image detector 201 shown in FIG. 5 is characterized in that a protective film 214 is provided on the X-ray photoconductive layer 211 side.

  In this embodiment, the upper electrode 12 is a positive electrode and the pixel electrode 22 is a negative electrode, and the electric field direction inside the X-ray photoconductive film 13 is opposite to the example already described with reference to FIG. 3 or FIG.

  In the example shown in FIG. 5, the corrosion resistance and the work function value at the interface between the upper electrode 12 on the positive electrode side and the X-ray photoconductive film 13 are important, and the protective film 214 similar to the example shown in FIG. Is formed at the interface between the upper electrode 12 and the X-ray photoconductive film 13.

  As described above, in the present application, it is possible to obtain a radiation detector that can extend the lifetime of the radiation detector and can continuously obtain a stable output image.

  The present invention is not limited to the above-described embodiments, and various modifications or changes can be made without departing from the scope of the invention when it is implemented. In addition, the embodiments may be implemented by appropriately combining them as much as possible, or by deleting a part thereof, and in that case, various effects resulting from the combination or deletion can be obtained.

Schematic which shows an example of the X-ray image detector which can apply embodiment of this invention. Schematic which shows the equivalent circuit inside the X-ray image detector shown in FIG. Schematic which shows an example of the cross-section of the X-ray image detector shown in FIG. Schematic which shows an example of the cross-section of the X-ray image detector shown in FIG. Schematic which shows an example of the cross-section of the X-ray image detector shown in FIG. The band model explaining the energy band between the X-ray photoconductive film, the upper electrode, and the pixel electrode in the X-ray image detector shown in FIG. 7 is a band model of an X-ray image detector having a known structure to be compared with the band model of the X-ray image detector of the present invention shown in FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1,101,201 ... X-ray (radiation) detector, 11, 211 ... X-ray photoconductive layer (photoconductive layer), 12 ... Upper electrode, 13 ... X-ray photoconductive film (photoconductive film), 14 ... Conductive Band energy level, 15 ... electron, 16 ... electron band energy level, 17 ... hole, 21, 121 ... TFT circuit substrate, data processing unit, 22 ... pixel electrode, 23 ... TFT circuit layer, 24 DESCRIPTION OF SYMBOLS ... Protective substrate, 25 ... Control electrode, 26 ... Thin film transistor, 27 ... Capacitor, 28 ... Read-out electrode, 29 (129) ... Protective film (entire protective film), 214 ... Upper protective film.

Claims (7)

A circuit board in which a plurality of pixel electrodes and switching elements are two-dimensionally arranged on a planar substrate, and a halogen compound that is provided in contact with the circuit board and converts radiation incident from the outside into an electrical signal are the main components. In a radiation detector having a photoconductive layer and an upper electrode formed on the photoconductive film surface of the photoconductive layer on the side facing the circuit board,
A film containing, as a main component, any one or more materials of tantalum, niobium, vanadium, titanium, zirconium, or hafnium and nitride is provided at the interface between the pixel electrode and the photoconductive film. Radiation detector. A circuit board in which a plurality of pixel electrodes and switching elements are two-dimensionally arranged on a planar substrate, and a halogen compound that is provided in contact with the circuit board and converts radiation incident from the outside into an electrical signal are the main components. In a radiation detector having a photoconductive layer and an upper electrode formed on the photoconductive film surface of the photoconductive layer on the side facing the circuit board,
A film containing, as a main component, any one or more materials of tantalum, niobium, vanadium, titanium, zirconium, or hafnium and nitride is formed at the interface between the upper electrode and the photoconductive film. Radiation detector. A circuit board in which a plurality of pixel electrodes and switching elements are two-dimensionally arranged on a planar substrate, and a halogen compound that is provided in contact with the circuit board and converts radiation incident from the outside into an electrical signal are the main components. In a radiation detector having a photoconductive layer and an upper electrode formed on the photoconductive film surface of the photoconductive layer on the side facing the circuit board,
Radiation having a film mainly composed of one or more materials of tantalum, niobium, vanadium, titanium, zirconium, or hafnium and carbide at an interface between the pixel electrode and the photoconductive film. Detector. A circuit board in which a plurality of pixel electrodes and switching elements are two-dimensionally arranged on a planar substrate, and a halogen compound that is provided in contact with the circuit board and converts radiation incident from the outside into an electrical signal are the main components. In a radiation detector having a photoconductive layer and an upper electrode formed on the photoconductive film surface of the photoconductive layer on the side facing the circuit board,
Radiation having a film mainly composed of one or more materials of tantalum, niobium, vanadium, titanium, zirconium, or hafnium and carbide at an interface between the upper electrode and the photoconductive film. Detector. A circuit board in which a plurality of pixel electrodes and switching elements are two-dimensionally arranged on a planar substrate, and a halogen compound that is provided in contact with the circuit board and converts radiation incident from the outside into an electrical signal are the main components. In a radiation detector having a photoconductive layer and an upper electrode formed on the photoconductive film surface of the photoconductive layer on the side facing the circuit board,
A film containing, as a main component, any one or more materials of tantalum, niobium, vanadium, titanium, zirconium, or hafnium and an oxide is provided at an interface between the pixel electrode and the photoconductive film. Radiation detector. A circuit board in which a plurality of pixel electrodes and switching elements are two-dimensionally arranged on a planar substrate, and a halogen compound that is provided in contact with the circuit board and converts radiation incident from the outside into an electrical signal are the main components. In a radiation detector having a photoconductive layer and an upper electrode formed on the photoconductive film surface of the photoconductive layer on the side facing the circuit board,
A film containing, as a main component, any one or more materials of tantalum, niobium, vanadium, titanium, zirconium, or hafnium and an oxide is provided at the interface between the upper electrode and the photoconductive film. Radiation detector.   7. The photoconductive film according to claim 1, wherein the photoconductive film contains at least one of lead iodide, mercury iodide, indium iodide, bismuth iodide, and thallium bromide as a main component. Radiation detector.

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