Patent Publication Number: US-2012025190-A1

Title: Radiation detector

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 USC 119 from Japanese Patent Application No. 2010-117822 filed on May 21, 2010, the disclosure of which is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a radiation detector. The present invention particularly relates to a radiation detector with plural pixels arrayed in a matrix, in which charges generated by irradiation with radiation are accumulated, and the amount of accumulated charges are detected as image information. 
     2. Description of the Related Art 
     Radiographic imaging apparatuses are recently being implemented that employ radiation detectors having a X-ray sensitive layer disposed on a TFT (thin film transistor) active matrix substrate, and directly converting X-ray information into digital information, such as, for example, a FPD (flat panel detector) radiation detector. Such radiation detectors have the advantage that, in comparison to related imaging plates, images can be more immediately checked and video images can also be checked. The introduction of such radiation detectors is proceeding rapidly. 
     Various types are proposed for such radiation detectors. There are, for example, direct-conversion-type radiation detectors that convert radiation directly to charge in a semiconductor layer, and accumulate the charge. There are also indirect-conversion-type radiation detectors that first convert radiation into light with a scintillator, such as CsI:Tl, GOS (Gd 2 O 2 S:Tb) or the like, then convert the converted light into charge and accumulate the charge. Radiation detectors output an electrical signal according to the charge accumulated in each photo diode. In a radiographic imaging apparatus, the electrical signal output from the radiation detector is converted into digital information in an analogue/digital (A/D) converter after the signal has been amplified by an amplifier. 
     However, amplifier is capable for amplifying a defined range of electrical signals. 
     Consequently there are sometimes cases when, out of the signal levels of electrical signals output from the radiation detector, the range of signal levels employed for a radiographic image (called the dynamic range) does not fall within the detection range of the amplifier. A mismatch can arise between the dynamic range of the electrical signal output from the photo diode and the detection range of the amplifier, particularly when a general purpose amplifier is employed and there is commonality between multiple products. 
     When such a mismatch arises, this can be addressed by employing an amplifier with a wider detection range. However, the dynamic range of the electrical signal may become smaller than the amplifier detection range, and may results in lowering the S/N ratio. 
     As related technology, a technique is proposed in Japanese Patent Application Laid-Open (JP-A) No. 2008-270765 for obtaining an output voltage over a wide range of illumination intensities. In this technique, the output terminal of a photo diode and the drain terminal of a MOS transistor are connected together, and the drain terminal and the gate terminal of the MOS transistor are also connected together. In this technique, the voltage generated is detected at the gate terminal of the MOS transistor according to the current generated in the photo diode. 
     There is also a technique proposed in JP-A No. 2002-350551 for enabling the amount of charges accumulate in an accumulation capacitor to be varied. In this technique, a MIS accumulation capacitor is provided on the ground side of a photo diode, and the MIS accumulation capacitor is connected to a switch such that connection can be made to the positive potential side of a power source or to a GND potential. In this technique, the potential of the accumulation capacitor is switched over by the switch, such that the accumulation capacitor operates in either an accumulation state or a depletion state. 
     However, the techniques of JP-A No. 2008-270765 and JP-A No. 2002-350551, cannot eliminate the mismatch between the dynamic range of the electrical signal output from the radiation detector and the detection range of the amplifier. 
     SUMMARY OF THE INVENTION 
     The present invention provides a radiation detector that may set output characteristics of an output electrical signal to match a detection range of an amplifier. 
     A first aspect of the present invention is a radiation detector including: a plurality of scan lines provided parallel to each other; a plurality of signal lines provided parallel to each other and intersecting with the scan lines; a plurality of sensor sections, provided at intersecting portions of the scan lines and the signal lines, that generate charges due to irradiation of radiation, and that accumulate charges according to an amount of irradiated radiation; a plurality of bias lines that apply a bias voltage to the plurality of sensor sections; arid a plurality of charge storage capacitors, provided for each of the plurality of sensor sections, that accumulate charges generated in the sensor sections and are electrically connected to the bias lines in parallel to the sensor sections. 
     In the first aspect of the present invention, plural charge storage capacitors that accumulate charges generated in the sensor sections are provided, and the charge storage capacitor is provided for each of the plural sensor sections. Accordingly, in the first aspect of the present invention, the gain characteristics of electrical signal output from the radiation detector can be set to desired slope, according to the capacity of the charge storage capacitors. Accordingly, in the first aspect of the present invention, the output characteristics of the electrical signal output can be set to match the detection range of the amplifier. 
     A second aspect of the present invention, in the above aspect, may further include a light emitting section, provided above a detection region where the plurality of sensor sections are provided, that generates light according to irradiated radiation, wherein the plurality of bias lines may be provided at a downstream side from the sensor portions in a direction of the light generated in the light emitting sections, such that the bias lines overlap with the sensor sections with an insulation film disposed between the sensor sections and the bias lines, the bias lines may be connected to the sensor sections through contacts formed through the insulation film. 
     A third aspect of the present invention, in the above aspects, may further include a thin film transistor that reads charge generated in the plurality of sensor sections, wherein the plurality of charge storage capacitors may be configured with two electrodes and an insulation film disposed between the two electrodes, and one of the electrodes may be formed in a wiring layer in which the thin film transistor is formed. 
     A fourth aspect of the present invention, in the above aspects, the insulation film of the plurality of charge storage capacitors may be formed by an insulation layer that configures a gate insulation film of the thin film transistor. 
     According to the above aspects, the present invention can set the output characteristics of an electrical signal output to match the detection range of an amplifier. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a configuration diagram of a radiographic imaging apparatus according to an exemplary embodiment; 
         FIG. 2  is plan view illustrating a configuration of a radiation detector according to an exemplary embodiment; 
         FIG. 3  is a cross-section of a radiation detector according to an exemplary embodiment, taken on line A-A of  FIG. 2 ; 
         FIG. 4  is a cross-section of a radiation detector according to an exemplary embodiment, taken on line B-B of  FIG. 2 ; 
         FIG. 5  is a cross-section of a radiation detector according to an exemplary embodiment, taken on line C-C of  FIG. 2 ; 
         FIG. 6  is a configuration diagram of a radiation detector of a radiographic imaging apparatus according to an exemplary embodiment; 
         FIG. 7  is an equivalent circuit diagram focusing on a single pixel of a radiation detector according to an exemplary embodiment; and 
         FIG. 8  is a graph illustrating sensitivity characteristics of a radiation detector according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Explanation follows regarding an exemplary embodiment for implementing the present invention, with reference to the drawings. 
       FIG. 1  is a diagram illustrating the overall configuration of a radiographic imaging apparatus  100  according to the present exemplary embodiment. 
     As shown in  FIG. 1 , the radiographic imaging apparatus  100  according to the present exemplary embodiment is equipped with an indirect conversion radiation detector  10 . 
     The radiation detector  10  is provided with plural pixels  7  disposed along one direction (the across direction in  FIG. 1 , referred to below as the “row direction”) and a direction that intersects with the row direction (the vertical direction in  FIG. 1 , referred to below as the “column direction) so as to form a 2-dimensional shape. Each of the pixels  7  is configured to include a sensor section  103 , a charge storage capacitor  47  and a TFT switch  4 . The sensor section  103  receives light emitted by a scintillator, described below, and accumulates charges. The charge storage capacitor  47  is electrically connected in parallel to the sensor section  103 . TFT switch  4  reads out the charge that has accumulated in the sensor section  103  and the charge storage capacitor  47 . 
     The radiation detection device  10  is provided with plural scan lines  101  that run parallel to each other along the row direction, and that switch the TFT switches  4  ON/OFF. The radiation detection device  10  is also provided with plural signal lines  3  that run parallel to each other along the row direction, and that read out the charges accumulated in the sensor sections  103 . There are also common electrode lines  109  provided in the radiation detector  10  running parallel to the signal lines  3 . 
     A line  107  is additionally provided in the radiation detector  10  so as to surround the 2-dimensional shaped detection region where the pixels  7  are provided. The line  107  is connected to a power supply  110  supplying a specific bias voltage. Both ends of each of the common electrode lines  109  are connected at to the line  107 . One end of the sensor section  103  and one end of the charge storage capacitor  47  of each of the pixels  7  are connected to the respective common electrode line  109 , and bias voltage is applied to each of the sensor sections  103  and each of the charge storage capacitors  47  through the line  107  and the common electrode line  109 . 
     An electrical signal, corresponding to the amount of accumulated charges in the sensor section  103  and the charge storage capacitor  47 , flows in each of the signal lines  3  by switching ON the TFT switch  4  in one or other of the pixels  7  connected to this signal line  3 . A signal detection circuit  105  is connected to each of the signal lines  3  for detecting the electrical signal flowing out from each of the signal lines  3 . A scan signal control circuit  104  is also connected to the scan lines  101  for outputting a control signal to each of the scan lines  101  for ON/OFF switching of the TFT switches  4 . 
     The signal detection circuits  105  are each inbuilt with an amplifier circuit for each of the respective signal lines  3 , and the amplifier circuits amplify input electrical signals. Electrical signals input by each of the signal lines  3  are amplified by the amplifier circuits in the signal detection circuit  105 . The signal detection circuits  105  thereby detect the charge amount that has been accumulated in each of the sensor sections  103  as information for each pixel configuring an image. 
     A signal processing device  106  is connected to the signal detection circuits  105  and the scan signal control circuit  104 . The signal processing device  106  executes specific processing on the electrical signals detected by the signal detection circuits  105 . The signal processing device  106  also outputs a control signal expressing the timing of signal detection to the signal detection circuits  105 , and outputs a control signal expressing the timing of scan signal output to the scan signal control circuit  104 . 
       FIG. 2  to  FIG. 5  show an example of a configuration of the radiation detector  10  according to the present exemplary embodiment.  FIG. 2  illustrates in plan view the structure of a single pixel  7  of the radiation detector  10  according to the present exemplary embodiment.  FIG. 3  shows a cross-section taken on line A-A of  FIG. 2 .  FIG. 4  shows a cross-section taken on line B-B of  FIG. 2 .  FIG. 5  shows a cross-section taken on line C-C of  FIG. 2 . 
     As shown in  FIG. 3  to  FIG. 5 , the radiation detector  10  of the present exemplary embodiment is formed with an insulating substrate  1  configured from alkali-free glass or the like, on which the scan lines  101 , and gate electrodes  2  are formed. The scan lines  101  and the gate electrodes  2  are connected together (see  FIG. 2 ). The wiring layer in which the scan lines  101  and the gate electrodes  2  are formed (this wiring layer is referred to below as the first signal wiring layer) is formed from Al and/or Cu, or a layered film mainly composed of Al and/or Cu. However, the material of the first signal wiring layer is not limited thereto. 
     A first insulation film  15 A is formed above the scan lines  101  and the gate electrodes  2  on one face of the first signal wiring layer, so as to cover the scan lines  101  and the gate electrodes  2 . The locations of the first insulation film  15 A positioned over the gate electrodes  2  are employed as a gate insulation film in the TFT switches  4 . The first insulation film  15 A is, for example, formed from SiN x  or the like by, for example, Chemical Vapor Deposition (CVD) film forming. 
     An island shape of a semiconductor active layer  8  is formed above the first insulation film  15 A on each of the gate electrodes  2 . The semiconductor active layer  8  is a channel portion of the TFT switch  4  and is, for example, formed from an amorphous silicon film. 
     A source electrode  9  and a drain electrode  13  are formed above the aforementioned layer. The wiring layer in which the source electrode  9  and the drain electrode  13  are formed also has the common electrode lines  109  formed therein. The wiring layer in which the signal lines  3 , the source electrodes  9  and the common electrode lines  109  are formed (this wiring layer is referred to below as the second signal wiring layer) is formed from Al and/or Cu, or a layered film mainly composed of Al and/or Cu. However, the material of the second signal wiring layer is not limited thereto. 
     A contact layer (not shown in the drawings) is formed between the semiconductor active layer  8 , and both the source electrode  9  and the drain electrode  13 . The contact layer is an impurity doped semiconductor layer of, for example, impurity doped amorphous silicon or the like. Each of the TFT switches  4  is configured by the gate electrode  2 , the semiconductor active layer  8 , the source electrode  9 , and the drain electrode  13 . 
     A second insulation film  15 B is formed over substantially the whole surface (substantially the entire region) of regions where the pixels  7  are situated above the substrate  1 , so as to cover the semiconductor active layers  8 , the source electrodes  9 , the drain electrodes  13  and the common electrode lines  109 . The second insulation film  15 B is formed, for example, from SiN x  or the like, by, for example, CVD film forming. 
     The signal lines  3 , contacts  24 , and contacts  36  are formed above the second insulation film  15 B. The wiring layer in which the signal lines  3 , the contact  24  and the contacts  36  are formed (referred to below as a third signal wiring layer) is formed from Al and/or Cu, or a layered film mainly composed of Al and/or Cu. However, the material of the third signal wiring layer is not limited thereto. 
     Contact holes  37  (see  FIG. 2 ) are formed in the second insulation film  15 B at locations where the signal lines  3  and the source electrodes  9  face each other. Contact holes  38  are formed in the second insulation film  15 B at locations where the contacts  36  and the drain electrodes  13  face each other. Contact holes  39 A are also formed in the second insulation film  15 B at locations where the contacts  24  and the common electrode lines  109  face each other. 
     The signal lines  3  are connected to the source electrodes  9  through the contact holes  37  (see  FIG. 1 ). The contacts  36  are connected to the drain electrodes  13  through the contact holes  38 . The contacts  24  are also connected to the common electrode lines  109  through the contact holes  39 A (see  FIG. 4 ). 
     In the radiation detector  10  according to the present exemplary embodiment, an electrode portion  47 A is formed in the area of each of the sensor sections  103  by spreading the contact  36 . Further, in the present exemplary embodiment, electrode portions  47 B is formed in the area of each of the sensor sections  103  by spreading the common electrode lines  109 . The electrode portions  47 B are formed to face the electrode portions  47 A. Accordingly, in the present exemplary embodiment, the charge storage capacitors  47  are formed by the electrode portions  47 A and the electrode portions  47 B. 
     A third insulation film  15 C is also formed on one face above the third signal wiring layer, with a coated intermediate insulation film  12  further formed thereon. The third insulation film  15 C is formed, for example, from SiN x  or the like by, for example, CVD film forming. Contact holes  40  are formed through both the intermediate insulation film  12  and the third insulation film  15 C at locations facing the contacts  36 . Contact holes  39 B are also formed through both the intermediate insulation film  12  and the third insulation film  15 C at locations facing the contacts  24 . 
     Lower electrodes  18  are formed on the intermediate insulation film  12  so as to fill the respective contact holes  40 . The lower electrodes  18  are connected to the contacts  36  at the contact holes  40 . The lower electrodes  18  are also connected to the drain electrodes  13  of the TFT switches  4  through the contacts  36 . When a semiconductor layer  6 , described later, is about 1 μm thick, there is substantially no limitation to the material of the lower electrodes  18  as long as it is a conductive material. The lower electrodes  18  are therefore formed with a conductive metal, such as an aluminum based material, ITO or the like. 
     However, when the film thickness of the semiconductor layer  6  is thin (about 0.2 to 0.5 μm) there is insufficient light absorption by the semiconductor layer  6 . Accordingly, in order to prevent an increase in leak current flow due to light illumination onto each of the TFT switches  4 , the lower electrode  18  is preferably an alloy or layered film with a metal having light-blocking ability as a main component. 
     The semiconductor layer  6  is formed on the lower electrode  18  and functions as a photodiode. In the present exemplary embodiment, a photodiode of PIN structure is employed as the semiconductor layer  6 . The semiconductor layer  6  is formed from the bottom with an n +  layer, an i layer and a p +  layer layered on each other. Note that in the present exemplary embodiment, the lower electrodes  18  are larger than the respective semiconductor layer  6  portions. When the thickness of the semiconductor layer  6  is thin (for example 0.5 μm or less), the TFT switches  4  are preferably covered with a metal having light-blocking ability, in order to prevent light from being incident onto the TFT switches  4 . 
     A separation of 5 μm or greater is preferably secured from the channel of each of the TFT switches  4  to the edge of the light-blocking metal lower electrodes  18 , in order to suppress light entry to the TFT switches  4  due to light scattering and reflection within the device. 
     Individual upper electrodes  22  are formed on each of the semiconductor layers  6 . The upper electrodes  22  are, for example, formed using a material having high transmissivity to light, such as ITO, Indium Zinc Oxide (IZO) or the like. 
     In the radiation detection device  10  according to the present exemplary embodiment, each of the sensor sections  103  is configured by the upper electrodes  22 , the semiconductor layers  6 , and the lower electrodes  18 . 
     A coated intermediate insulation film  23 , with opening  41  corresponding to a portion of the upper electrode  22 , is formed on the intermediate insulation film  12  and the upper electrode  22 , so as to cover the semiconductor layer  6 . Contact holes  39 B are formed through the intermediate insulation film  23  at locations corresponding to the contacts  24 . 
     Electrodes  45  are formed on the intermediate insulation film  23  so as to cover the pixel regions. The electrodes  45  are, for example, formed using a material that have high transmissive to light, such as ITO, IZO or the like. The electrodes  45  are connected to the upper electrodes  22  through the openings  41  and are also connected to the contacts  24  through the contact holes  39 B. Accordingly, the upper electrodes  22  are electrically connected to the common electrode lines  109  through the contacts  24  and the electrodes  45 . 
     In a radiation detector  10  configured as described, a protection layer  28 , as shown in  FIG. 6 , may be formed from an insulating material with low light absorption characteristics as required. A scintillator  70 , configured for example from GOS or the like, may then be attached using an adhesive resin with low light absorption characteristics to the surface of the protection layer  28 . The scintillator  70  converts irradiated radiation into light, and emits the light. As shown in  FIG. 6 , a reflective body made from a material that reflects light is provided at a lower portion of the scintillator  70  in the present exemplary embodiment. 
     Explanation now follows regarding the principles of operation of the radiographic imaging apparatus  100  constructed as described above. 
     When X-rays are irradiated from above in  FIG. 6 , the irradiated X-rays are absorbed by the scintillator  70  and are converted into visible light. Note that the X-rays may also be irradiated from below in  FIG. 6 . In such cases the irradiated X-rays are absorbed by the scintillator  70  and converted into visible light. The generated light passes through the protection layer  28  of adhesive resin, and is illuminated onto the respective sensor sections  103 . 
       FIG. 7  illustrates an equivalent circuit diagram focusing on a single of the pixels  7  of the radiation detector  10  according to the present exemplary embodiment. 
     Charges are generated in the sensor section  103  on irradiation with radiation. 
     The charge storage capacitors  47  are electrically connected in parallel to the sensor sections  103  in the radiation detector  10  according to the present exemplary embodiment. The charge generated in each of the sensor sections  103  accordingly accumulates in both the sensor section  103  and the charge storage capacitor  47  until the potential of the upper electrode  22 A reaches the same potential as the bias voltage Vd. 
     In the radiation detector  10  of the present exemplary embodiment, manner in which the potential of the upper electrode  22  rises due to the accumulation of charge, differs according to the capacity of each of the charge storage capacitors  47 . As shown by the solid line in  FIG. 8 , high gain characteristics are exhibited when the capacity of the charge storage capacitor  47  is small, with the potential of the upper electrode  22  rising fast. A dynamic range of the sensor section  103  is range DLA until the potential of the upper electrode  22 A reaches the same potential as the bias voltage Vd. However, the greater the capacity of the charge storage capacitor  47 , the lower the gain characteristics, as shown by the intermittent line in  FIG. 8 , and the dynamic range of the sensor section  103  exhibits as a range DLB. 
     For example, in a case in which the capacity of the photo diode of 1 pF with no charge storage capacitor provided, the saturated charge amount (namely, when the photo diode lower electrode reaches the bias voltage) will be achieved with a radiation amount of 3 mR. In contrast, by employing the configuration of the present invention with an accumulation capacitor of 0.5 pF disposed electrically connected in parallel to the photo diode, the saturated charge amount becomes 1.5 times the previous case, and as a result the dynamic range can be extended to 4.5 mR. 
     Accordingly, the radiation detector  10  of the present exemplary embodiment can change the gain characteristics by changing the saturated charge amount, through changing the capacity of the charge storage capacitors  47 . The capacity of the charge storage capacitor  47  can be determined and the charge storage capacitors  47  in the present exemplary embodiment configured to give gain characteristics such that the dynamic range of the output electrical signal (the saturated charge amount) matches the detection range of the amplifier. Accordingly, the radiation detector  10  of the present exemplary embodiment may match the detection range of the internal amplification circuit in each of the signal detection circuits  105  and the output characteristics of the output electrical signal set. Thus, the present exemplary embodiment may eliminate the mismatch between the dynamic range of the electrical signal output from the radiation detector  10  and the detection range of the amplifier. 
     In the radiation detector  10  according to the present exemplary embodiment, the common electrode lines  109  are provided in the sensor section  103  below where light is generated in the scintillator  70 . The common electrode lines  109  are provided overlapping with the sensor sections  103 , with the third insulation film  15 C and the intermediate insulation film  12  interposed between the common electrode lines  109  and the sensor sections  103 . The common electrode lines  109  are connected to the sensor sections  103  through the contacts  24 . The radiation detector  10  of the present exemplary embodiment can accordingly prevent the common electrode lines  109  from blocking light onto the reception surface area of the sensor section  103 . Consequently, the present exemplary embodiment may increase the light reception surface area of the radiation detector  10 . This is particularly effective for securing light reception surface area when sensor sections are miniaturized with a high degree of miniaturization. 
     In the radiation detector  10  of the present exemplary embodiment one electrode  47 B of each of the charge storage capacitors  47  is formed in the second signal wiring layer in which the source electrodes  9  and the drain electrodes  13  of the TFT switches  4  are formed. Accordingly, the radiation detector  10  of the present exemplary embodiment may suppress an increase in the number of wiring layers for forming the charge storage capacitors  47 . The present exemplary embodiment can thereby simplify fabrication processes for the radiation detector  10 . 
     Note that the configuration of the radiographic imaging apparatus  100  and the configuration of the radiation detector  10  explained in the above exemplary embodiment are merely examples thereof, and obviously various modifications are possible within a scope not departing from the spirit of the present invention. 
     In the above exemplary embodiment, a case in which each of the common electrode lines  109  is provided in parallel to each of the respective signal lines  3 , has been described. However, the present invention is not limited thereto. For example, each of the common electrode lines  109  may be provided in parallel to each of the respective scan lines  101 . 
     In the above exemplary embodiment, a case in which the common electrode lines  109  and the electrodes  47 B are formed in the second signal wiring layer, and the contacts  36  are formed in the third signal wiring layer, has been described. However, the present invention is not limited thereto. For example, the common electrode lines  109  and the electrodes  47 B may be formed in the first signal wiring layer. In such cases, the contact holes  40  may also be formed in the second insulation film  15 B, and the contacts  36  formed in the second signal wiring layer. 
     The common electrode lines  109  and the charge storage capacitors  47  may also be formed in the same layer as the sensor sections  103 , or may be formed in the sensor sections  103  above where light is generated in the scintillator  70 . 
     In the present exemplary embodiment, a case in which the present exemplary embodiment is applied to the radiographic imaging apparatus  100  that detects an image by detecting X-rays, has been described. However, the present invention is not limited thereto. For example, radiation employed may be X-rays and also visible light, ultraviolet light, infrared light, gamma rays, or a particle beam.