Abstract:
Disclosed is a flat panel X-ray detector, comprising an X-ray-electric charge conversion film converting an incident X-ray into an electric charge, a pixel electrode contiguous to the X-ray-electric charge conversion film and arranged for every pixel, a switching element connected to the pixel electrode, a signal line connected to the switching element, and a scanning line supplying a driving signal to the switching element, wherein the X-ray-electric charge conversion film contains phosphor particles, a photosensitive material, and a carrier transfer material.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a flat panel X-ray detector.  
           [0002]    In recent years, preparation of data base on the medical data on a patient is being promoted in a medical field in order to perform the medical treatment promptly and appropriately. It should be noted in this connection that the patient utilizes in general a plurality of medical organizations. Therefore, if there is no data prepared in another medical organization, there is possibility that an appropriate medical treatment is not performed.  
           [0003]    The data base preparation is also required in respect of the image data of the X-ray photography, and it is hoped from this point that a digital system be developed in respect of the X-ray photograph. It was customary in the past to use a silver halide film in the medical X-ray diagnostic apparatus. For employing a digital system in the medical X-ray diagnostic apparatus, it was necessary to develop the photograph film image and scan again the developed film image with a scanner, which was laborious and time-consuming.  
           [0004]    In recent years, a system of directly converting the image into digital data has been realized by using a CCD cameral sized about one inch. However, in photographing, for example, a chest, a region of about 40 cm×40 cm is photographed, making it necessary to use an optical apparatus for collecting light, leading to the problem that the apparatus is rendered bulky.  
           [0005]    As a system for overcoming the above-noted problems inherent in the two systems described above, proposed is a flat panel X-ray detector of an indirect conversion system using an amorphous silicon thin film transistor (a-Si TFT). FIG. 1 shows the circuit construction of the flat panel X-ray detector. The operation of the flat panel X-ray detector will now be described with reference to FIG. 1.  
           [0006]    The flat panel X-ray detector shown in FIG. 1 is a flat panel X-ray detector of an indirect conversion type, in which an incident X-ray is converted into a visible light by, for example, a phosphor, and the converted visible light is further converted into an electric charge for each pixel by a photo-conduction film.  
           [0007]    As shown in FIG. 1, the flat panel X-ray detector comprises pixels e (i, j) (i=1 to 2000, j=1 to 2000). Each pixel e comprises a switching TFT  401  formed of a-Si, a photo-conduction film  402  and a Cst  403 . These pixels e are arranged to form an array, the row of the array consisting of hundreds of to thousands of pixels e and the column of the array also consisting of hundreds of to thousands of pixels. A negative bias voltage is applied from a power source  404  to the photo-conduction film  402 . The switching TFT  401  is connected to a signal line  405  and to a scanning line  406  and is subjected to an on-off control by a scanning line driving circuit  407 . The terminal of the signal line  405  is connected to an amplifier  410  for the signal detection via a change-over switch  409  that is controlled by a signal line control circuit  408 .  
           [0008]    If an X-ray is incident, the phosphor (not shown) irradiated with the X-ray emits a fluorescent light. The fluorescent light is then converted into an electric charge by the photo-conduction film  402 , and the electric charge is accumulated in the Cst  403 . If a scanning line  406  is driven by the scanning line driving circuit  407  so as to turn on a column of switching TFTs  401  connected to one of the scanning lines  406 , the accumulated charge is transferred through the signal line  405  toward the amplifier  410 . By the change-over switch  409 , the charge is supplied to the amplifier  410  for each pixel so as to be converted into a dot sequential signal.  
           [0009]    The amount of the electric charge differs depending on the amount of light incident on the pixels (i, j) so as to change the output amplitude of the amplifier  410 . By subjecting the output signal of the amplifier  410  to an A/D conversion, the electric charge can be converted directly into a digital image. Further, the pixel region can be made thin and large by utilizing the array of the switching TFTs  401 .  
           [0010]    [0010]FIG. 2 is a plan view showing the construction of the pixel  501  included in the flat panel X-ray detector. As shown in the drawing, the pixel  501  comprises a switching TFT  401  for the reading, a Cst  403 , a Cst line  502  connected to the Cst  403 , an auxiliary electrode  503  facing the Cst  403 , a pixel electrode  504 , a signal line  405 , and a scanning line  406 . A contact portion  505  is formed in each of the switching TFT  401  and the auxiliary electrode  503 .  
           [0011]    It should be noted that the layers above the pixel electrode  504  and the region outside the pixel  501  are omitted from the drawing of FIG. 2. Incidentally, it is possible to utilize the floating capacitance of the other elements and the wiring in place of arranging the Cst  403 .  
           [0012]    [0012]FIG. 3 is a cross sectional view along the line II-II shown in FIG. 2, which shows the constructions of the layers formed above the pixel electrode  504 .  
           [0013]    As shown in FIG. 3, a pixel electrode  504 , a p-type contact film  601 , a photo-conduction film  402 , an n-type contact film  602 , a common electrode  603 , a phosphor layer  604  and a reflective layer  605  are laminated in the order mentioned on the structure including the switching TFT  401 , the Cst  403 , the auxiliary electrode  503 , the signal line  405  and the scanning line (not shown).  
           [0014]    If an X-ray is incident on the phosphor layer  604  through the reflective layer  605 , a fluorescent light is emitted from the phosphor layer  604  irradiated with the X-ray, and the fluorescent light thus emitted is scattered. The fluorescent light then enters the photo-conduction film  402  directly or is reflected from the reflective layer  605  and, then, the reflected fluorescent light enters the photo-conduction film  402 . In the photo-conduction film  402 , the fluorescent light is converted into an electric charge. It should be noted that, since voltage is applied across the photo-conduction film  402 , the generated electric charge is attracted by the pixel electrode  504  for each pixel  501  so as to be accumulated in the Cst  403  through the pixel electrode  504 .  
           [0015]    In the flat panel X-ray detector of the construction described above, a fluorescent light is emitted in every direction from the phosphor layer  604  upon irradiation with the X-ray. The fluorescent light thus emitted is scattered and reflected from the reflective layer  605 . It follows that it is highly possible for the fluorescent light emitted from the phosphor layer  604  in a certain pixel to arrive at the photo-conduction film  402  of the adjacent pixel. It should be noted that voltage is applied to the photo-conduction film  402  and, thus, the electric charge converted from the fluorescent light is scarcely scattered so as to arrive at the pixel electrode  504  corresponding to the particular region. Also, there is a problem that the light emitted from the phosphor is attenuated by the absorption within the phosphor film and by the reflection from the upper surface and the bottom surface of the film so as to lower the efficiency.  
           [0016]    As a result, the fluorescent light emitted from the phosphor layer  604  is scattered so as to arrive at the adjacent pixel. The fluorescent light is converted into an electric charge in the photo-conduction film  402  of the adjacent pixel, and the electric charge thus generated is accumulated in the pixel electrode  504  of the adjacent pixel. It follows that a problem is generated that the resolution is deteriorated.  
         BRIEF SUMMARY OF THE INVENTION  
         [0017]    An object of the present invention is to provide a flat panel X-ray detector, which has a high resolution and permits manufacturing a large apparatus with a low cost.  
           [0018]    According to the present invention, there is provided a flat panel X-ray detector, comprising an X-ray-electric charge conversion film converting an incident X-ray into an electric charge, a pixel electrode contiguous to the X-ray-electric charge conversion film and arranged for every pixel, a switching element connected to the pixel electrode, a signal line connected to the switching element, and a scanning line supplying a driving signal to the switching element, wherein the X-ray-electric charge conversion film contains phosphor particles, a photosensitive material, and a carrier transfer material.  
           [0019]    According to the present invention, there is provided an X-ray-electric charge conversion film, containing a photosensitive material, phosphor particles covered with the photosensitive material, and a carrier transfer material. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0020]    [0020]FIG. 1 is a circuit diagram of a flat panel X-ray detector;  
         [0021]    [0021]FIG. 2 is a plan view showing a pixel included in the flat panel X-ray detector shown in FIG. 1;  
         [0022]    [0022]FIG. 3 is cross sectional view showing the pixel of the conventional flat panel X-ray detector along the line II-II shown in FIG. 2;  
         [0023]    [0023]FIG. 4 is a cross sectional view showing a flat panel X-ray detector according to a first embodiment of the present invention;  
         [0024]    [0024]FIG. 5 is a band view of a flat panel X-ray detector according to the first embodiment of the present invention;  
         [0025]    [0025]FIG. 6 is a cross sectional view showing a flat panel X-ray detector according to a second embodiment of the present invention; and  
         [0026]    [0026]FIG. 7 is a cross sectional view showing a flat panel X-ray detector according to a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]    A flat panel X-ray detector of the present invention comprises an X-ray-electric charge conversion film and is featured in that the X-ray-electric charge conversion film contains phosphor particles, a photosensitive material, and a carrier transfer material.  
         [0028]    In the present invention, it is possible for the phosphor particles to be covered with the photosensitive material. Alternatively, it is possible for the phosphor particles to be dispersed in the X-ray-electric charge conversion film containing a photosensitive material and a carrier transfer material.  
         [0029]    It is desirable for the phosphor particle used in the present invention to contain at least one material selected from the group consisting of a metal oxide, a metal iodide, and a metal sulfide. Also, it is desirable for the photosensitive material used in the present invention to contain at least one material of a first group consisting of trinitrofluorenone (TNF), diphenyl hydrazone, tris(8-quinolinolato)aluminum (Alq 3 ), and oxytitanium, and at least one material of a second group consisting of an azo pigment, a squarylium pigment, a phthalocyanine pigment, a titanyl phthalocyanine pigment, and a perylene pigment.  
         [0030]    Further, it is desirable for the carrier transfer material used in the present invention to be at least one material selected from the group consisting of triphenylene, polyvinyl carbazole, N,N-diphenyl-N,N-di(m-tolyl)benzidine, poly(para-phenylene vinylene), polyalkyl thiophene, 4-(dicyano methylene)-2-methyl-6-(p-dimethylamino styryl)-4H-pyrane, and pentacene.  
         [0031]    It is possible for the phosphor particles to contain a plurality of kinds of particles differing from each other in the particle diameter. In this case, it is desirable for the particle of the smaller diameter to have a particle diameter about 0.1 to 0.3 times as much as the particle diameter of the particle having a larger diameter.  
         [0032]    It is possible for each pixel to include a capacitor accumulating the electric charge generated in the X-ray-electric charge conversion film and connected to the switching element in each pixel.  
         [0033]    The phosphor used in the present invention represents a material that emits an ultraviolet light, a visible light and an infrared light upon irradiation with an X-ray. The photosensitive material used in the present invention represents a material that generates a carrier such as electrons and holes upon irradiation with an ultraviolet light, a visible light or an infrared light. Further, the carrier transfer material used in the present invention represents a material that transfers the carrier such as electrons and holes.  
         [0034]    Some embodiments of the present invention will now be described.  
         [0035]    The flat panel X-ray detector according to the first embodiment of the present invention has a circuit construction equal to that shown in FIG. 1 and has a pixel construction equal to that shown in FIG. 2. FIG. 4 is a cross sectional view showing the pixel used in the flat panel X-ray detector according to the first embodiment of the present invention. The flat panel X-ray detector of the first embodiment will now be described with reference to FIG. 4.  
         [0036]    As shown in FIG. 4, a switching TFT  401 , a Cst  403 , an auxiliary electrode  503  facing the Cst  403 , and a pixel electrode  503  electrically connected to the auxiliary electrode  503  are formed for each pixel on a glass substrate  101 . Also, a hole transfer layer  108 , an X-ray-electric charge conversion film  109 , an electron transfer layer  113  and a common electrode  603  are uniformly formed in the order mentioned on the entire surface of the pixel electrode  504 .  
         [0037]    The switching TFT  401  comprises a gate electrode  102 , an insulating film  103  for the gate insulation, and an undoped a-Si layer  104  used as an active layer. A stopper  105  and an n + -type a-Si layer  106  are formed on the undoped a-Si layer  104 . A source-drain electrode connected to the signal line  405  is formed on one part of the n + -type a-Si layer  106 , and a drain electrode connected to the auxiliary electrode  503  is formed on the other part of the n + -type a-Si layer  106 . Further, the X-ray-electric charge conversion film  109  used in this embodiment comprises phosphor particles  110  each covered with a photosensitive material  111  and a carrier transfer material  112 .  
         [0038]    How to manufacture the flat panel x-ray detector of the first embodiment will now be described.  
         [0039]    In the first step, a predetermined material such as MoTa, Ta, TaN, Al alloy, Cu, MoW, or Ta/TaNx is deposited on the glass substrate  101  in a thickness of about 300 nm, followed by selectively etching the deposited layer so as to form a pattern including the gate electrode  102  of the switching TFT  401 , a scanning line (not shown), the Cst  403 , and a Cst line (not shown).  
         [0040]    In the next step, the insulating film  103  is formed by depositing by a plasma CVD method a SiOx in a thickness of about 300 nm, followed by depositing by a plasma CVD method a SiNx on the SiOx layer in a thickness of about 50 nm. Then, the undoped a-Si layer  104  and a SiNx layer acting as the stopper  105  are deposited in a thickness of about 10 nm and about 200 nm, respectively.  
         [0041]    Then, the stopper  105  is patterned by a back surface light exposure method to conform with the gate electrode  102 , followed by depositing the n + -type a-Si layer  106  in a thickness of about 50 nm and subsequently etching selectively the undoped a-Si layer  104  and the n + -type a-Si layer  106  to conform with the shape of a transistor, thereby forming an a-Si island.  
         [0042]    In the next step, those regions of the insulating film  103  which correspond to the contact portions both inside and outside the pixel area are selectively etched so as to form contact holes. Then, a laminate structure consisting of a Mo film about 50 nm thick, an Al film about 350 nm thick and a Mo film about 20 to about 50 nm thick is formed on the insulating film  103  by a sputtering method so as to form the auxiliary electrode  503 , the signal line  405 , and the other wirings.  
         [0043]    Then, a SiNx layer is formed in a thickness of about 200 nm, followed by laminating a benzocyclobutene (BCB) layer on the SiNx layer in a thickness of about 1 to 5 μm, preferably about 3 μm, so as to form a protective film  107 . After a contact hole is formed in the protective film  107 , an ITO film is formed in a thickness of about 100 nm, followed by patterning the ITO film so as to form a pixel electrode  504  connected to the switching TFT  401  and to the auxiliary electrode  503 . Further, the pixel electrode  504  is coated with diphenyl-3-methylphenyl-biphenyl-diamine (TPD) in a thickness of about 10 nm to about 100 μm so as to form the hole transfer layer  108 .  
         [0044]    In the next step, the hole transfer layer  108  is coated with the X-ray-electric charge conversion film  109  in a thickness of about 200 μm. For forming the X-ray-electric charge conversion film  109 , phosphor particles each having a particle diameter of about 10 μm are prepared by using GdO 2 S:Pr (GOS). Then, the surface of the phosphor particle is coated with the photosensitive material  111  prepared by mixing about 40% by weight of diphenyl hydrazone in polycarbonate. The coated phosphor particles are mixed with the carrier transfer material  112  formed of polyvinyl carbazole (PVK), and the surface of the hole transfer layer  108  is coated with the resultant mixture so as to form the X-ray-electric charge conversion layer  109 .  
         [0045]    It is possible to prepare the phosphor particles  110  by pulverizing and classifying a polycrystalline phosphor or by using a plasma melt vapor deposition. As a method of coating the phosphor particle  110  with the photosensitive material  111 , it is possible to dip the phosphor particles  110  in a solution of the photosensitive material  111 , followed by removing the solvent by evaporation.  
         [0046]    It is desirable for the particle diameter of the phosphor particles to fall within a range of between 0.5 and 100 μm, preferably between 1 and 30 μm. It should be noted that the light emitting efficiency is lowered by the surface defect in the surface region of the phosphor particle. Also, the X-ray absorption efficiency is lowered by the decrease in the packing density of the phosphor particles in the X-ray-electric charge conversion film. It is necessary to determine the optimum particle diameter of the phosphor particles in view of these two factors.  
         [0047]    It is necessary to determine the thickness of the photosensitive layer  111  covering the surface of the phosphor particle  110  such that the photosensitive layer  111  is capable of sufficiently absorbing the fluorescent light emitted from the phosphor particle. To be more specific, it is desirable for the thickness of the photosensitive layer  111  to fall within a range of between about 1 μm and about 10 μm.  
         [0048]    The X-ray-electric charge conversion film  109  is coated with an oxazole derivative in a thickness of about 10 nm to about 100 nm so as to form the electron transfer layer  113 . Further, the common electrode  603  is formed by film-forming, for example, an Al or MgAg film in a thickness of about 100 μm on the electron transfer layer  113 , followed by connecting the common electrode  603  to the driving circuit, thereby finishing manufacture of the flat panel X-ray detector according to the first embodiment of the present invention.  
         [0049]    [0049]FIG. 5 is a band diagram of the flat panel X-ray detector according to the first embodiment of the present invention. The drawing shows only the portions of the hole transfer layer  108 , the X-ray-electric charge conversion film  109  including the phosphor particles  110 , the photosensitive material  111 , and the carrier transfer material  112 , and the electron transfer layer  113 . Upon irradiation with the X-ray, the phosphor particle  110  contained in the X-ray-electric charge conversion film  109  shown in FIG. 5 emits a fluorescent light, and upon irradiation with the fluorescent light, the photosensitive material  111  covering the phosphor particle  110  generates carriers. The carriers thus generated are migrated within the carrier transfer material  112  such that holes  201  and electrons  202  are transferred into the hole transfer layer  108  and the electron transfer layer  113 , respectively, with the result that the electric charges are accumulated in the pixel electrode.  
         [0050]    As shown in FIG. 5, the band energy of the photosensitive material  111  is made higher than that in the portion of the carrier transfer material  112  on the side of the conduction band and is made lower than that in the portion of the carrier transfer material  112  on the side of the valence band. It should be noted that, since the carrier migration is carried out mainly within the carrier transfer material  112 , the carriers generated within the photosensitive material  111  are migrated promptly into the carrier transfer material  112 , leading to the situation described above. It should also be noted that the phosphor particle  110  has in general a band gap not smaller than about 3 eV, and each of the photosensitive material  111  and the carrier transfer material  112  has a band gap smaller than that of the phosphor particle  110 . Incidentally, FIG. 5 shows that the X-ray-electric charge conversion film  109  contains only one phosphor particle  110  covered with the photosensitive material  111 . However, the X-ray-electric charge conversion film  109  actually contains a large number of phosphor particles  110  covered with the photosensitive material  111 .  
         [0051]    In the first embodiment of the present invention, the phosphor particle  110  is covered with the photosensitive material  111 . Therefore, when a fluorescent light is emitted from the phosphor particle  110  upon irradiation with the X-ray, the fluorescent light is incident on the photosensitive material  111  before the fluorescent light is scattered so as to generate electric charges. The electric charges thus generated are migrated within the carrier transfer material  112  to which an electric field is applied, with the result that the electric charges are not scattered but are accumulated in the pixel electrode  504  of each pixel  501 .  
         [0052]    It should be noted that, in the conventional flat panel X-ray detector, the fluorescent light generated within the phosphor layer upon irradiation with an X-ray is scattered, and the fluorescent light is incident on the adjacent pixel, and then the fluorescent light is converted into electric charges. As a result, the electric charges are accumulated in a certain pixel and adjacent pixels, giving rise to the problem that the resolution is rendered poor. However, according to the flat panel x-ray detector according to the first embodiment of the present invention, after the X-ray is converted into a fluorescent light in the phosphor particle  110 , the fluorescent light is not scattered so as to be incident as it is on the photosensitive material  111  so as to generate the electric charges. Then, since an electric field is applied to the carrier transfer material  112 , the electric charges are not scattered but are accumulated in the pixel  501  corresponding to the particular region so as to obtain a high resolution.  
         [0053]    Also, in the prior art, it is necessary to form each of the photo-conduction film and the phosphor layer by vapor deposition or CVD method. However, in the flat panel X-ray detector according to the first embodiment of the present invention, the X-ray-electric charge conversion film  109  performs the functions which are conventionally performed by the two layers, the photo-conduction film and phosphor layer, in single layer. In addition, the X-ray-electric charge conversion film  109  can be formed easily, because the film  109  can be formed by the coating of the carrier transfer material  112  containing the phosphor particles  110  covered with the photosensitive material  111 . Since the X-ray-electric charge conversion film  109  can be formed easily, a large flat panel X-ray detector can be obtained with a low manufacturing cost.  
         [0054]    A second embodiment of the present invention will now be described. The circuit construction of the flat panel X-ray detector according to the second embodiment of the present invention is equal to that shown in FIG. 1, and the pixel construction of the flat panel X-ray detector of the second embodiment is equal to that shown in FIG. 2. FIG. 6 is a cross sectional view of the pixel of the flat panel X-ray detector according to the second embodiment of the present invention. The flat panel X-ray detector according to the second embodiment of the present invention will now be described with reference to FIG. 6. The flat panel X-ray detector according to the second embodiment comprises an X-ray-electric charge conversion film  301 , which differs from the X-ray-electric charge conversion film included in the X-ray detector according to the first embodiment. The X-ray-electric charge conversion film  301  in the second embodiment comprises phosphor particles  303  and a photosensitive-transfer material  302 , which is photosensitive and has carrier transfer property.  
         [0055]    How to manufacture the flat panel X-ray detector according to the second embodiment of the present invention will now be described. The second embodiment will be described mainly in respect of the portion differing from the first embodiment, and the same reference numerals are put in the drawing to the portions equal to the first embodiment for avoiding the overlapping description.  
         [0056]    In the second embodiment, the process ranging between the step of forming the gate electrode  102  of the switching TFT  401 , the scanning line, the Cst  403  and the Cst line on the glass substrate  101  and the step of forming the hole transfer layer  108  is carried out by employing the methods and the materials equal to those employed in the first embodiment.  
         [0057]    In the next step, the X-ray-electric charge conversion film  301  is formed on the hole transfer layer  108  in a thickness of about 100 μm to about 300 μm. The X-ray-electric charge conversion film  301  is formed by coating the hole transfer layer  108  with a mixture of phosphor particles  303  in the photosensitive-transfer material  302  prepared by mixing about 50% by weight of TNF used as a photosensitive material and about 50% by weight of PVK used as a transfer material. The phosphor particles  303  are formed to have about 1 μm to about 30 μm of the particle diameter by using the material and the method used in the first embodiment. In the second embodiment, however, the surface of the phosphor particle  303  is not covered with the photosensitive material.  
         [0058]    After formation of the x-ray-electric charge conversion film  301 , the electron transfer layer  113  and the common electrode  603  are formed by the materials and methods equal to those employed in the first embodiment. Finally, the common electrode  603  is connected to the driving circuit so as to finish manufacture of the flat panel X-ray detector according to the second embodiment of the present invention.  
         [0059]    In the second embodiment, the phosphor particle  303  is covered with the photosensitive material. However, since the phosphor particles  303  are surrounded by the photosensitive-transfer material  302 , it is possible to obtain the effect similar to that obtained in the first embodiment. To be more specific, the fluorescent light emitted from the phosphor particle  303  after irradiation with an X-ray is converted into the electric charge by the photosensitive-transfer material  302  before the fluorescent light is scattered to the adjacent pixels. As a result, the electric charge finally obtained from the X-ray incident on a certain pixel does not leak into the adjacent pixel and is accumulated in the particular pixel, with the result that it is possible to obtain a high resolution.  
         [0060]    It should be noted that an electric field is applied to the X-ray-electric charge conversion film  301 . Therefore, after the fluorescent light is converted into an electric charge by the photosensitive-transfer material  302 , the electric charge is attracted to the pixel electrode of the pixel corresponding to the particular region. It follows that the resolution is scarcely lowered.  
         [0061]    What should also be noted is that, in the second embodiment, the X-ray-electric charge conversion film can be formed by the coating as in the first embodiment so as to simplify the forming method of the X-ray-electric charge conversion film. As a result, a large flat panel X-ray detector can be manufactured with a low manufacturing cost. Incidentally, in the second embodiment, the X-ray-electric charge conversion film  301  can be formed by only the coating of a mixture of the phosphor particles  301  and the photosensitive-transfer material  302  containing both a photosensitive material and a transfer material. Since the phosphor particle is not covered with a photosensitive material in the second embodiment, the X-ray-electric charge conversion film  301  can be formed more easily than in the first embodiment.  
         [0062]    A third embodiment of the present invention will now be described. The circuit construction of the flat panel X-ray detector according to the third embodiment of the present invention is equal to that shown in FIG. 1, and the pixel construction of the flat panel X-ray detector of the third embodiment is equal to that shown in FIG. 2. FIG. 7 is a cross sectional view of the pixel of the flat panel X-ray detector according to the third embodiment of the present invention. The flat panel X-ray detector according to the third embodiment of the present invention will now be described with reference to FIG. 7. The flat panel X-ray detector according to the third embodiment comprises an X-ray-electric charge conversion film  109 , which differs from the X-ray-electric charge conversion film included in the X-ray detector according to the first embodiment.  
         [0063]    In the third embodiment of the present invention, phosphor particles  110   a  having a relatively small particle diameter are mixed in the X-ray-electric charge conversion film  109  in addition to the phosphor particles  110  having a relatively large particle diameter, which are used in the first embodiment. The surface of the phosphor particle  110   a  having a relative small particle diameter is also covered with a photosensitive material  111   a.    
         [0064]    In the case of using the small particles  110   a  together with the large particles  110 , it is possible to further increase the packing density of the phosphor particles in the X-ray-electric charge conversion film  109  so as to improve the absorption efficiency of the X-ray and to decrease the thickness of the X-ray-electric charge conversion film  109 . As a result, it is possible to increase the intensity of the electric field applied to the X-ray-electric charge conversion film  109  so as to increase the mobility of the carriers generated in the X-ray-electric charge conversion film and, thus, to take out efficiently the carriers. Thus, it is possible to diminish the voltage applied to the X-ray-electric charge conversion film  109 .  
         [0065]    It is desirable for the particle diameter of the smaller phosphor particle  110   a  to be about 0.1 to 0.3 times as large as the particle diameter of the larger phosphor particle  110  because the packing density of the phosphor particles can be improved in this case. In the third embodiment of the present invention, arranged is a protective diode  701  serving to release the electric charge when the potential of the pixel electrode is elevated by the high voltage of the electrode in the upper portion of the photosensitive film so as to prevent the pixel potential from exceeding the breakdown voltage of the gate insulating film of the switching TFT.  
         [0066]    It is possible to change the kinds of the large particle of the X-ray sensitive material and the small particle of the X-ray sensitive material depending on the purpose. For example, it is possible to obtain an X-ray-electric charge conversion film having a high mechanical strength and a high conversion efficiency by mixing large diameter phosphor particles of Gd 2 O 2 S:Tb having a high mechanical strength and a high light emission efficiency with small diameter phosphor particles of CsI having a high absorption coefficient and a high conversion efficiency.  
         [0067]    In the conventional laminate structure consisting of a phosphor film and a photosensitive film, the fluorescent light is absorbed within the phosphor or is reflected from the upper surface and the bottom surface of the phosphor film, with the result that the fluorescent light is attenuated. In the third embodiment of the present invention, however, the phosphor particles are covered with the photosensitive film so as to prevent the fluorescent from being attenuated and, thus, to improve the utilization rate of the fluorescent light.  
         [0068]    The present invention is not limited to the various embodiments described above.  
         [0069]    For example, it is possible to use any kind of phosphor as far as the phosphor emits a fluorescent light of an ultraviolet light, a visible light and an infrared light upon irradiation with an X-ray. For example, a metal oxide, a metal iodide and a metal sulfide exhibits a large absorption coefficient of an X-ray and, thus, the X-ray can be absorbed sufficiently by a thin film of any of these materials, though the phosphor used in the present invention is not limited to these materials. Particularly, it is desirable to use, for example, GOS, GdO 2 S:Tb, CsI, ZnS, YxGd 1 - 2x O 3 , or CdWO 4  as the phosphor because these materials exhibit a large X-ray absorption coefficient and, thus, the X-ray can be sufficiently absorbed by a thin film of these materials. In addition, the fluorescent light emission efficiency upon irradiation with an X-ray is particularly high.  
         [0070]    It is desirable for the particle diameter of the phosphor particle to fall within a range of between about 1 μm and about 100 μm, particularly, to be about 10 μm. If the particle diameter is smaller than about 1 μm, the surface defects such as the surface state may be increased so as to lower the light emitting efficiency. On the other hand, if the particle diameter exceeds about 100 μm, it may be difficult to mix sufficiently the phosphor particles with the other materials forming the X-ray-electric charge conversion film for formation of the X-ray-electric charge conversion film by coating.  
         [0071]    It is also desirable for the phosphor particle used in the present invention to be shaped substantially spherical because the thickness of the X-ray-electric charge conversion film is likely to be made uniform.  
         [0072]    Further, the equivalent thickness of the phosphor in the X-ray-electric charge conversion film is selected depending on the X-ray absorption coefficient of the phosphor material. To be more specific, it is desirable for the equivalent thickness of the phosphor to fall within a range of between about 1 and about 10 times the thickness at which the phosphor absorbs at least about 50% of the X-ray. The phosphor having an equivalent thickness of this level is capable of sufficiently absorbing the X-ray. It is also desirable for the thickness of the X-ray-electric charge conversion film to fall within a range of between about 1.3 and about 3 times the equivalent thickness noted above. Where the thickness of the X-ray-electric charge conversion film falls within the range noted above, it is possible to ensure a sufficient fluidity required for forming the X-ray-electric charge conversion film by means of coating.  
         [0073]    The photosensitive material used in the flat panel X-ray detector of the present invention, which generates carriers such as holes and electrons upon irradiation with light such as an ultraviolet light, a visible light or an infrared light, comprises at least one material of a first group consisting of trinitrofluorenone (TNF), diphenyl hydrazone, tris (8-quinolilato) aluminum (Alq), and oxytitanium. It is also possible for the photosensitive material to comprise at least one material of a second group consisting of an azo pigment, a squalylium pigment, a phthalocyanine pigment, a titanyl phthalocyanine and a perylene pigment together with at least one material of the first group. However, the photosensitive material used in the present invention is not limited to the materials exemplified above.  
         [0074]    It is desirable for the X-ray-electric charge conversion film to contain the photosensitive material in an amount falling within a range of between about 0.1 mol % and about 40 mol %. If the amount of the photosensitive material contained in the X-ray-electric charge conversion film is smaller than about 0.1 mol %, the sensitivity as the photosensitive material may be lowered. Also, the absorption coefficient of the fluorescent light tends to be lowered. On the other hand, if the amount of the photosensitive material is larger than about 40 mol %, it is possible for the amount of the carrier transfer material to be rendered insufficient so as to deteriorate the carrier transfer characteristics.  
         [0075]    Where the phosphor particle is covered with the photosensitive material, it is desirable for the thickness of the photosensitive layer to fall within a range of between about 1 μm and about 100 μm. If the thickness of the photosensitive material layer is less than about 1 μm, it may be difficult for the phosphor particle to be coated uniformly with the photosensitive material. Also, the absorption of the fluorescent light may be rendered insufficient. On the other hand, if the thickness of the photosensitive layer exceeds about 100 μm, it is possible for the photosensitive material to be mixed with the carrier transfer material and for the coated photosensitive layer to be rendered nonuniform in thickness.  
         [0076]    It is desirable for the photosensitive material used in the present invention to be selected from the group consisting of the azo-series pigment such as 4,4′(9-oxo-9H-fluorene-2,7-diyl) bis-(azo)-bis [N-(2-chlorophenyl)-3-hydroxy-2-naphthalene carboxyamide] and the trisazo pigment such as tris {4,[3-(2-ethylphenyl carbamoyl)-2-hydroxybenzo]-1-ylazo] phenyl} amine.  
         [0077]    A material capable of transferring the carrier such as electrons and holes can be used in the present invention as the carrier transfer material. To be more specific, the carrier transfer material used in the present invention includes compounds in which at least one element selected from the group consisting of hydrogen, oxygen and nitrogen is bonded to carbon or silicon. Particularly, it is desirable to use metal complexes of these compounds because these metal complexes exhibit a high carrier mobility, though the carrier transfer material used in the present invention is not limited to the materials exemplified above.  
         [0078]    Among the materials exemplified above, it is desirable to use polyvinyl carbazole (PVK), N,N-diphenyl-N,N-di (m-tolyl) benzidine (TPD), poly-para-phenylene vinylene (PPV), polyalkyl thiophene, 4-(dicyanomethylene)-2-methyl-6-(p-dimethylamino styryl)-4H-pyran (DCM), pentacene, NPD, polyphenylene vinylene, polyalkyl thiophene and liquid crystal molecules. Particularly, it is more desirable to use a naphthalene series organic liquid crystal material and 4-N,N-bis(4-methylphenyl)amino-α-phenyl-sitylben because these materials exhibit a high carrier mobility. It is also possible to mix these organic materials with a polymer such as polycarbonate.  
         [0079]    Also, it is desirable for the carrier transfer material to exhibit a carrier mobility not lower than about 1×10 −7  cm 2 /(V·sec) in order to obtain a sufficient conductivity and, to have a resistivity not lower than about 1×10 10  Ωcm in order to prevent an increase of the dark current.  
         [0080]    It is desirable for each of the electron transfer layer and the hole transfer layer to exhibit a carrier mobility not lower than about 1×10 −7  cm 2 /(V·sec) in order to obtain a sufficient conductivity, and to have a resistivity not lower than about 1×10 8  Ωcm in order to prevent an increase of the dark current.  
         [0081]    In the flat panel X-ray detector of the present invention, it is possible to use at least one material selected from the group consisting of an oxazoazole derivative, Alq 3 , n-type Si, and n-type GaAs for forming the electron transfer layer. These materials produce the effect of a high electron mobility.  
         [0082]    It is possible to use at least one material selected from the group consisting of TPD, diamine, p-type Si and p-type GaAs for forming the hole transfer layer. These materials produce the effect of a high hole mobility.  
         [0083]    In the present invention, it is possible to use any kind of the substrate as far as a TFT can be formed on the substrate. In each of the embodiments described previously, the X-ray-electric charge conversion film is formed at low temperatures by means of coating. Therefore, it is possible to use a plastic material having a low heat resistance for forming the substrate. In this case, the entire flat panel X-ray detector can be made flexible. In the case of using a plastic substrate, the flat panel x-ray detector can be made light in weight so as to facilitate the transportation of the flat panel X-ray detector.  
         [0084]    In each of the embodiments described previously, a-Si was used as the silicon for forming a TFT. Alternatively, it is also possible to use a polycrystalline silicon (polysilicon) for forming the TFT. In the case of using a polysilicon for forming the TFT, the TFT can be made small so as to enlarge the effective area of the pixel. Also, the peripheral circuit can also be formed on the same glass substrate, leading to a low manufacturing cost including the peripheral circuit.  
         [0085]    The TFT can be of a top gate type or bottom gate type.  
         [0086]    Further, the protective film  107  can be formed of, for example, an inorganic SiNx or SiO 2 , organic polyimides (∈=about 3.3; breakdown voltage of about 300 V/mm), BCB (∈=about 2.7; breakdown voltage of about 400 V/mm), acrylic photosensitive resin HRC (∈=about 3.2) manufactured by JSR Inc., and a black resist. It is also possible to laminate films of these materials one upon the other. Also, a fluorine-based resin has a small dielectric constant (∈=about 2.1) and, thus, can be used effectively for forming the protective film  107 . It is not absolutely necessary for the protective film  107  to be photosensitive. However, a photosensitive material is effective because the patterning can be facilitated.  
         [0087]    As described above, the present invention provides a flat panel X-ray detector having a high resolution, which can be formed as a large flat panel X-ray detector with a low manufacturing cost.  
         [0088]    Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.