Abstract:
Conventionally, a metal which is opaque to light has been employed for the common electrode of the pixels of a photosensor. Consequently, a common electrode line has concealed the light receiving portions of photodiodes and has lowered the opening degree of these portions, thereby decreasing light outputs and degrading S/N (signal to noise) ratios. In view of this situation, a common electrode line to which one of a pair of electrodes of each of light receiving elements is connected is formed over signal lines for transferring the light signals of the light receiving elements.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a photosensor for detecting optical information, or a photosensor which is well suited to detect radiations, such as X-rays and γ-rays, directly or after converting the wavelength regions thereof into the photosensitive wavelength region such as that of visible light. It also relates to a radiation detection apparatus and system each of which is constructed by adopting the photosensor. 
     2. Related Background Art 
     Heretofore, a film has generally been used for observing a radiation transmission image in a radiation diagnosis. Besides, an image intensifier (hereinbelow, expressed as “I/I”) has been used in order to observe a transmission image in real time during the irradiation of radiation. The I/I, however, has had the problems of a large size and a heavy weight. 
     Recently, a large-sized sensor employing amorphous silicon (hereinbelow, expressed as “a-Si”) has been proposed as a detector which is smaller in size and lighter in weight and with which a transmission image can be observed in real time. This sensor has a construction wherein a photodiode being a light receiving element formed of the a-Si, and a thin film transistor (hereinbelow, abbreviated to “TFT”) being a switching element are combined in one-to-one correspondence so as to form one pixel, and wherein the light receiving elements formed of the a-Si are arrayed in the shape of a two-dimensional matrix (see, for example, U.S. Pat. No. 5,262,649). 
     Although the above photodiode is of pin type, photodiodes include ones of pn type, Schottky type, etc. In addition, a photosensor of MIS type has been proposed by the inventors of the present invention (see the official gazette of Japanese Patent Application Laid-Open No. 8-116044). In this photosensor, a transparent electrode is disposed on an entrance side for light, and a wiring line Vce for the common electrode in the sensor is formed of metal. The common electrode line Vce is laid passing parts of the upper portions of the photodiodes, in parallel with signal lines Vsig which transfer light signals from the photodiodes to an amplifier via the switching TFTS. 
     Herein, the metal employed for the common electrode in the sensor is one which is opaque to light (for example, Al, Cr, W, Ta or Mo). In consequence, the common electrode line Vce conceals the light receiving portions of the photodiodes and lowers the opening degree of these portions, thereby to decrease light inputs to the light receiving areas of the photodiodes and to degrade the S/N (signal-to-noise) ratios of the light signals. 
     SUMMARY OF THE INVENTION 
     The present invention has its object to provide a photosensor in which, in spite of a large screen and a high definition, the opening degree of a light receiving portion can be held higher, thereby to increase a light input to this portion and to ensure a good S/N ratio. 
     Another object of the present invention is to provide a radiation detection apparatus and a radiation detection system each of which adopts the above photosensor, especially a two-dimensional photosensor in which light receiving elements are arrayed in two dimensions. 
     In one aspect of performance of the present invention, there is provided a photosensor having a plurality of pixels each of which includes a light receiving element and a switching element, a common electrode line to which one of a pair of electrodes of each of the light receiving elements is connected in common with each of the pixels, and signal lines to which light signals of the light receiving elements are transferred by simultaneously driving the switching elements of the pixels. A main wiring portion of said common electrode line is formed over said signal lines. 
     In another aspect of performance of the present invention, there is provided a radiation detection apparatus wherein a photosensor has a plurality of pixels each of which includes a light receiving element and a switching element, a common electrode line to which one of a pair of electrodes of each of the light receiving elements is connected in common with each of the pixels, and signal lines to which light signals of the light receiving elements are transferred by simultaneously driving the switching elements of the pixels. The photosensor is overlaid with wavelength conversion means for converting a wavelength region of radiation into a sensible wavelength region of the light receiving elements. A main wiring portion of the common electrode line is formed over the signal lines. 
     In a further aspect of performance of the present invention, there is provided a radiation detection system comprising a radiation detection apparatus including a photosensor having a plurality of pixels each of which includes a light receiving element and a switching element, a common electrode line to which one of a pair of electrodes of each of the light receiving elements is connected in common with each of the pixels, and signal lines to which light signals of the light receiving elements are transferred by simultaneously driving the switching elements of the pixels and over which a main wiring portion of the common electrode line is formed. The system also includes wavelength conversion means disposed over the photosensor, for converting a wavelength region of radiation into a sensible wavelength region of the light receiving elements, and image processing means for processing image information delivered for the radiation detection apparatus. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic plan view showing the first aspect of performance of the present invention; 
     FIG. 2A is a schematic sectional view of part  2 A— 2 A indicated in FIG. 1; 
     FIG. 2B is a schematic sectional view of part  2 B— 2 B indicated in FIG. 1; 
     FIGS. 3A,  3 B,  3 C,  3 D,  3 E and  3 F are schematic views for explaining an example of the fabricating process of the photosensor of the present invention, as to part  3 F— 3 F indicated in FIG. 1; 
     FIGS. 3G,  3 H and  3 I are schematic views for explaining the steps of the process subsequent to the step of FIG. 3F, as to the part  2 A— 2 A indicated in FIG. 1 (FIG. 3G shows the part  2 A— 2 A in FIG. 1 at the same step as in FIG.  3 F); 
     FIG. 4 is a schematic plan view showing the second aspect of performance of the present invention; 
     FIG. 5 is a schematic sectional view of part  5 — 5  indicated in FIG. 4; 
     FIGS. 6A and 6B are sectional views schematically showing an example of a constructed in which the sensor constructed in the first aspect of performance of the present invention is applied to a radiation detection apparatus, as to the parts  2 A— 2 A and  2 B— 2 B indicated in FIG. 1, respectively. 
     FIG.  7  and FIG. 8 are a schematic arrangement view and a schematic sectional view in the case of applying the sensor of the present invention to a radiation detection apparatus, for example, an X-ray detection apparatus, respectively; and 
     FIG. 9 illustrates an example of application of the two-dimensional photosensor of the present invention to a radiation inspection system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic plan view showing the first aspect of performance of the present invention. FIG. 2A is a schematic sectional view of part  2 A— 2 A indicated in FIG. 1, while FIG. 2B is a schematic sectional view of part  2 B— 2 B indicated in FIG.  1 . 
     Referring to FIG. 1, numeral  11  designates a photodiode, which is a photodiode of pin type, pn type or Schottky type (including MIS type) formed of a-Si. The photodiode  11  is formed on a drain electrode  12  of a TFT  13 . The TFT  13  transfers to a signal line (Vsig line)  14  a light output which has been generated by the incidence of light on the photodiode  11 . Shown with numeral  16  is a gate line for driving the TFT  13 . 
     Referring to FIGS. 2A and 2B, the photodiode  11  has a predetermined bias potential applied thereto by a common electrode line (Vce line)  15  via a transparent electrode  104  disposed on the light entrance side of this photodiode and through an opening provided in an insulating protective film  103 . As shown in FIG.  1  and FIGS. 2A and 2B, most of the part of the common electrode line (Vce line)  15  except the lugs thereof leading to the photodiodes  11  is formed over the signal lines (Vsig lines)  14 . Accordingly, the wiring portion of the common electrode line (Vce line)  15  hardly shields the entrance portions of the photodiodes  11  from the light. As seen particularly from FIG. 2B, the wiring portion of the common electrode line  15  is formed within an extent overlying the signal lines  14 , under the condition that it has a width equal to the width of the signal lines  14  or that at least one end face thereof has a width equal to or smaller than the width of the signal lines  14 . Numerals  101  and  102  in FIG. 2A indicate a substrate and an insulator film, respectively. 
     FIGS. 3A to  3 F are schematic views for explaining an example of the fabricating process of the photosensor of the present invention. The fabricating process will be described in conjunction with the section of part  3 F— 3 F indicated in FIG.  1 . 
     As shown in FIG. 3A, the gate wiring line  16  of each TFT is formed on a glass substrate  101  (step A). 
     As shown in FIG. 3B, an insulator film  102 , a semiconductor layer  105  and a channel protection film  106  are deposited on the resulting substrate (step B). The insulator film  102  is made of, for example, silicon nitride, and it becomes a gate insulator film. The channel protection film  106  is an insulator film made of, for example, silicon nitride. 
     Thereafter, as shown in FIG. 3C, the channel protection film  106  is patterned so as to leave only the channel portion of each TFT behind (step C). 
     Subsequently, as shown in FIG. 3D, an ohmic contact layer  107  for the TFTs as is made of, for example, a-Si or microcrystalline silicon (expressed as “μc-Si”) doped into the n+-type is deposited on the resulting structure, whereupon the ohmic contact layer  107  and the semiconductor layer  105  are patterned so as to be left behind only at each TFT part (step D). 
     Further, as shown in FIG. 3E, a metal such as Al is deposited in a thickness of 1 μm by sputtering, and the deposited metal is patterned into the signal line (source electrode)  14  and drain electrode  12  of each TFT  13  (step E). 
     As shown in FIG. 3F, a p-layer, an i-layer and an n-layer of a-Si are stacked by deposition on the drain electrodes  12  of the TFTs in the above-mentioned order, a transparent film of ITO (indium tin oxide) or the like is formed on the deposited layers, and the film and the layers are patterned into each light receiving portion, whereby each photodiode  11  of pin type and each transparent electrode  104  are formed (step F). The stacking order of the types of the a-Si layers for forming each photodiode  11  may well be in the order of the n-layer, i-layer and p-layer reverse to the aforementioned order. Since, however, the diffusion length of a positive hole is greater than that of an electron, the efficiency of the sensor (namely, the light receiving portion) is bettered by locating the p-layer on the light entrance side of each photodiode  11 . 
     The above steps A thru F concern the section of the part  3 F— 3 F indicated in FIG.  1 . The same step as the step F is illustrated in FIG. 3G as to the section of the part  2 A— 2 A indicated in FIG.  1 . The ensuing description taken with reference to FIGS. 3G to  3 I will concern the section of the part  2 A— 2 A. 
     As shown in FIG. 3G, the photodiode  11  and the transparent electrode  104  have been formed on each drain electrode  12  in the section  2 A— 2 A which does not include any TFT portion other than this drain electrode. 
     An insulating protective film  103  is deposited on the structure depicted in FIG. 3G, and each contact hole  108  for connecting the corresponding transparent electrode  104  to a common electrode line  15  is provided in the film  103  as shown in FIG.  3 H. 
     A metal such as Al is deposited on the structure depicted in FIG.  3 H. Then, as shown in FIG. 3I, the deposited metal is connected with each transparent electrode  104  through the corresponding contact hole  108  provided in the insulating protective film  103 . Further, the deposited metal is patterned to lay the common electrode line  15  so that the wiring portion thereof necessary for connection with a power source may be arranged over the signal lines  14 . In particular, except the lugs of the common electrode line  15  leading to the sensors (namely, the light receiving portions), this line is formed over the signal lines  14  so as to have a width smaller than or equal to the width of the lines  14 . Thus, the main wiring portion of the common electrode line  15  comes to overlie the signal lines  14 . 
     In this way, most of the part of the common electrode line  15  can be formed on the signal lines  14 , thereby to readily enhance the space factor of the photosensor and the opening degree of the light receiving portions. 
     Next, the second aspect of performance of the present invention will be described with reference to FIGS. 4 and 5. 
     In the embodiment shown in FIG. 4, a common electrode line  15  protrudes only over TFTs  13 . Over each TFT  13  over which the common electrode line  15  extends, a light shield  17  is formed by the extension of the common electrode line  15  so as to conceal the channel portion of the TFT  13 . Thus, the channel portion of the TFT  13  is shielded from light which otherwise enters this channel portion from above. 
     FIG. 5 is a schematic sectional view taken along plane  5 — 5  indicated in FIG.  4 . 
     With the embodiment shown in FIGS. 4 and 5, a predetermined potential is usually applied to the common electrode line  15  extended over the TFTs  13 . If necessary, therefore, the common electrode line  15  can be operated as a bias electrode for further lowering a leakage current at the turn-OFF of each TFT or for further increasing the response rate of each TFT. Such an operation depends also upon the thickness of an insulating protective layer  103  and the sign and magnitude of the applied voltage. 
     Incidentally, the TFT  13  shown in FIG. 5 is of so-called “channel etch type”, in which an ohmic contact layer  107  in the channel portion of the TFT is removed after the metal electrodes (source and drain electrodes) of the TFT have been formed without providing a channel protection layer. In this case, also a semiconductor layer  105  is somewhat over-etched, but a mask for providing the channel protection layer is dispensed with to bring forth the advantage of a simpler fabricating process. 
     An example of construction in which the photosensor constructed in the foregoing first aspect of performance is applied to a radiation detection apparatus, will now be described with reference to schematic sectional views depicted in FIGS. 6A and 6B. Sections shown in FIGS. 6A and 6B correspond to the planes  2 A— 2 A and  2 B— 2 B indicated in FIG. 1, respectively. 
     In the photosensor adopted here, a second insulator film  109  made of silicon nitride for protecting the sensor portions (light receiving portions) has been further disposed after the formation of the TFTs, photodiodes and wiring portion. When the second insulator film  109  is formed as an inorganic film in this manner, water and moisture can be perfectly prevented from intruding into the semiconductor elements such as TFTs and photodiodes, and the wiring lines. A scintillator  111  which serves as a wavelength converter is stuck on the second insulator film  109  by the use of a binder layer  110  of epoxy resin, silicone resin or the like. Usable for the scintillator  111  is a rare-earth-based phosphor, CsI, CsI(Tl) or the like. The scintillator  111  converts X-rays  112  into visible light having a wavelength to which the sensor portions of a-Si are highly sensitive. Of course, the scintillator  111  should preferably be furnished with a protective member, such as thin aluminum film, capable of transmitting the X-rays  112  and serving to protect the phosphor or the like from moisture and mechanical damages. 
     FIG.  7  and FIG. 8 show a schematic arrangement view and a schematic sectional view in the case where the photosensor of the present invention is applied to a radiation detection apparatus, for example, an X-ray detection apparatus, respectively. Referring to FIG. 7, a plurality of light receiving pixels each including an a-Si photodiode and an a-Si TFT are formed in a matrix shape within an a-Si sensor substrate  200 . First flexible circuit boards  201  on each of which a shift register IC (SRI) is mounted are connected on the surface of an edge of the a-Si sensor substrate  200 , while second flexible circuit boards  202  on each of which a detecting IC for amplifying and detecting the light signals of the sensor pixels is mounted are connected on the surface of another edge orthogonal to the first-mentioned edge. Besides, printed circuit boards PCB 1  and PCB 2  are respectively connected on the sides of the first and second flexible circuit boards ( 201  and  202 ) remote from the sensor substrate  200 . 
     Referring to FIG. 8, a plurality of (for example, four) a-Si sensor substrates  200  explained above are stuck on a base  203 , thereby to construct a large-sized two-dimensional photosensor. A lead plate  204  for protecting electric circuits or ICs, such as the memories  205  of a processing circuit  206 , from X-rays  212  is mounted on the side of the base  203  remote from the incidence side of the photosensor for the X-rays  212 . Further, each flexible circuit board is bent into the shape of letter U, whereby the correspondent detecting IC  202  is arranged so as to be shaded by the protecting lead  204  against the incident X-rays  212 . A scintillator of, for example, CsI(Tl)  210  for converting the X-rays  212  into visible light is stuck on the a-Si sensor substrates  200 , or it is formed directly on the surfaces of the a-Si sensor substrates  200 . 
     The X-ray detection apparatus thus constructed can detect the dose or quantity of the X-rays  212  by detecting the quantity of the light into which the X-rays  212  have been converted by the same principle as in the foregoing. 
     In the embodiment shown in FIGS. 7 and 8, the whole structure including the sensor substrates and the electric circuits is held in a case  211  made of carbon fiber. 
     FIG. 9 illustrates an example of the application of the two-dimensional photosensor of the present invention to a radiation inspection system. 
     X-rays  302  generated by an X-ray tube  301  are transmitted through the breast  304  of a patient or subject  303 , and are entered into a photoelectric conversion apparatus (image sensor)  305  including a screen of phosphor arranged thereon. Information on the interior of the body of the patient  303  is contained in the image of the entered X-rays. The phosphor phosphoresces in correspondence with the entrance of the X-rays  302 , and the resulting phosphorescence is photoelectrically converted to obtain electrical information. The electrical information is digitized and is subsequently processed by an image processor  306  into an image, which can be observed on a display device  307  installed in a control room. 
     Moreover, the image information can be transmitted to a remote site by transmission (communication) means such as a telephone line  308 . In a doctor&#39;s room or the like in a place separate from the X-ray room or the control room, the transmitted information can be displayed on a display device  309 , in reverse fashion if necessary, or it can be saved and stored in saving means such as an optical disk. It is thus possible to utilize the radiation inspection system for diagnosis by a doctor in the remote site. Furthermore, the transmitted information can be recorded on a film  311  (or paper) by using a laser printer included in a film processor  310  which serves also as save means. Of course, it is also possible that the patient  303  maybe replaced with any structure or article, with the breast  304  being replaced with the part of the structure or article desired to be examined. In this case, the radiation inspection system can be utilized for the inspection of an internal construction or internal contents. 
     As described above, according to the present invention, the open areas of light receiving elements can be enlarged to produce greater sensor outputs. It is therefore possible to provide a two-dimensional photosensor with high S/N ratio and a radiation detection apparatus as well as a radiation inspection system adopting the photosensor. Further, a pixel size required for a sensor output can be reduced by enlarging the open area. It is therefore possible to provide a two-dimensional photosensor of high definition and a radiation detection apparatus as well as a radiation inspection system adopting the photosensor. 
     Besides, in the present invention, a leakage current at the turn-OFF of each TFT can be diminished by employing a common electrode line in order to shield TFTs from light. It is accordingly possible to provide a two-dimensional photosensor of still higher S/N ratio and stabler characteristics, and a radiation detection apparatus as well as a radiation inspection system adopting the photosensor.