Patent Publication Number: US-6660555-B2

Title: X-ray image sensor and method for fabricating the same

Description:
This application is a Division of application Ser. No. 09/722,654 Filed on Nov. 28, 2000 U.S. Pat. No. 6,399,962. 
    
    
     This application claims the benefit of Korean Patent Application No. 1999-53712, filed on Nov. 30, 2000, which is hereby incorporated by reference for all purposes as if fully set forth herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to X-ray image sensors. More particularly, it relates to X-ray image sensors having a TFT (Thin Film Transistor) array, and to a method for fabricating the same. 
     2. Discussion of the Related Art 
     X-ray detection has been widely used for medical diagnosis. X-ray detection typically uses an X-ray film to produce a photograph. Therefore, some predetermined developing and printing procedures are required to produce the photograph. 
     However, digital X-ray image sensors that employ TFTs (Thin Film Transistors) have been developed. Such X-ray image sensors have the advantage that real time diagnosis can be obtained. 
     FIG. 1 is a schematic, cross-sectional view illustrating the structure and operation of an X-ray image sensing device  100 . Included are a lower substrate  1 , a thin film transistor  3 , a storage capacitor  10 , a pixel electrode  12 , a photoconductive film  2 , a protection film  20 , a conductive electrode  24  and a high voltage D.C. (direct current) power supply  26 . 
     The photoconductive film  2  produces electron-hole pairs in proportion to the strength of external signals (such as incident electromagnetic waves or magnetic waves). That is, the photoconductive film  2  acts as a converter that converts external signals, particularly X-rays, into electric signals. Either the electrons or the holes are then gathered by the pixel electrode  12  as charges. The pixel electrode is located beneath the photoconductive film  2 . Which charge species that is gathered depends on the voltage (Ev) polarity that is applied to the conductive electrode  24  by the high voltage D.C. power supply  26 . The gathered species charges are accumulated in the storage capacitor  10 , which is formed in connection with a grounding line. Charges in the storage capacitor  10  are then selectively transferred through the TFT  3 , which is controlled externally, to an external image display device that forms an X-ray image. 
     In such an X-ray image sensing device, to detect and convert weak X-ray signals into electric charges it is beneficial to decrease the trap state density (for the electric charge) in the photoconductive film  2 , and to decrease charge flow in non-vertical directions. Decreasing non-vertical charge flow is usually accomplished by applying a relatively high voltage between the conductive electrode  24  and the pixel electrode  12 . 
     Electric charges in the photoconductive film  2  are trapped and gathered not only on the pixel electrode  12 , but also over the channel region of the TFT  3 . Even during the OFF state, the electric charges trapped and gathered on the pixel electrode  12  and on the channel region of the TFT  3  induce a potential difference between the TFT  3  and the pixel electrode. This has a similar effect as the TFT  3  being in the ON state. This adversely affects the switching of the TFT  3  and increases the OFF state leakage current. Such can result in an undesired image. 
     FIG. 2 is a plan view illustrating one pixel  102  of the X-ray image sensor panel  100 . Shown are the TFT  3 , a storage capacitor “S” and a pixel electrode  62  that collects charges. 
     The TFT  3  includes a gate electrode  31 , which is formed by an elongation of a gate line  50 , and a drain electrode  32 , which is formed by an elongation of a drain line  52 . 
     The storage capacitor “S” is comprised of transparent first and second capacitor electrodes  58  and  60 . A ground line  42  acts as a common electrode that is shared by adjacent pixels. Also shown are first contact holes  54  that connects the pixel electrode  62  with a source electrode  33  of the TFT  3 , and a second contact hole  56  that connects the pixel electrode  62  with the second capacitor electrode  60 . 
     According to the conventional art, an X-ray image sensor includes a photoelectric conversion part that produces electric charges in accordance with received electromagnetic energy; a charge storage capacitor “S” having a first capacitor electrode  58 , a dielectric layer that is deposited on the first capacitor electrode  58 , a second capacitor electrode  60  on the dielectric layer, a protection film having multiple contact hole(s)  54  and  56  on the second capacitor electrode  60 , and a pixel electrode  62  that is formed on the protection film. The pixel electrode is in contact with the second capacitor electrode  60  through the contact hole(s)  56  and collects the electric charges produced in the photoelectric conversion part. A switching TFT  3  controls the release of the electric charges stored in the storage capacitor “S”. The switching TFT includes a gate electrode  31 , a drain electrode  32 , and a source electrode  33  that contacts the pixel electrode  62 . 
     FIGS. 3 a  to  3   f  are sectional views, taken along the line III—III of FIG. 2, that illustrate a manufacturing process. 
     Referring to FIG. 3 a , a metal layer is deposited and patterned on a substrate  1  to form a taper-shaped gate electrode  31 . The substrate  1  can be a quartz substrate or a glass substrate. However, the substrate  1  is beneficially a glass panel since quartz panels are relatively expensive. The gate electrode  31  can made of a metallic material selected from a group comprised of Molybdenum (Mo), Tantalum (Ta), Tungsten (W), Niobium (Nb), and Antimony (Sb). 
     FIG. 3 b  illustrates the steps of depositing a first insulation film  102  and a semiconductor layer  104 . The gate insulation film  102  is formed by a deposition of an inorganic insulation film (such as a silicon nitride (SiN x ) film or a silicon oxide(SiO x ) film) having 4000 Å thickness. Alternatively, an organic insulation material such as BCB (benzocyclobutene) or acrylic resin can be used. After the deposition of the first insulation film  102 , a dual layer semiconductor film  104  comprised of an amorphous silicon layer  104   a  and a doped amorphous silicon film  104   b  are deposited. Although vapor deposition or ion injection can be used for the formation of the doped amorphous silicon film  104   b , vapor deposition is usually employed. 
     Next, as shown FIG. 3 c , a second metal layer is deposited for both the source electrode  33  and the drain electrode  32 , and for the sound line  42 . That metal, beneficially Chromium (Cr) or a Cr-alloy, is then patterned to form the source electrode  33 , the drain electrode  32  and the ground line  42 . Moreover, the portion of the doped amorphous silicon film  104   b  between the source and drain electrodes  33  and  32  is eliminated by using the source and drain electrodes as masks. Then, a first capacitor electrode  58  is formed over the ground line  42 . The first capacitor electrode  58  is beneficially comprised of a transparent electrode material such as ITO (indium tin oxide). The region C in FIG. 3 c  designates a switching transistor. 
     Referring to FIG. 3 d,  a silicon nitride film having a thickness of 3000 Å forms a second insulation film  106  is deposited on the source and drain electrodes  33  and  32 , and on the first capacitor electrode  58 . The second insulating film  106  acts as protective layer for the TFT  3  and as a dielectric for a capacitor that is being formed with the first capacitor electrode  58 . 
     After the second insulation film  106  is deposited a second capacitor electrode  60  is formed on the second insulation film  105  and over the first capacitor electrode  58 . Beneficially, the second capacitor electrode is the same size as or a little larger than the first capacitor electrode  58 . 
     As shown in FIG. 3 e , an insulating protection film  108  is then formed. An organic substance such as BCB (benzocyclobutene) is beneficially used. BCB is a material that has a lower dielectric constant than silicon nitride, silicon oxide or acrylic resin. After formation of the protection film  108 , first and second contact holes  54  and  56  are formed through the protection film  108 . The first contact hole  54  exposes a portion of the source electrode  33 . The second contact hole  56  exposes a portion of the second capacitor electrode  60 . Although the first contact hole  54  penetrates down to the source electrode  33 , the second contact hole  56  can not go as deep since the second capacitor electrode  60  acts as an etch stop that prevents the second insulation film  106  from being etched. 
     FIG. 3 f  illustrates the step of forming a pixel electrode  62  (a third transparent electrode layer). The pixel electrode is formed over the second insulation film  106  such that the pixel electrode extends into the first and second contact holes  54  and  56  and electrically connects with the source electrode and the second capacitor electrode  60 . In addition, the pixel electrode  62  is formed such that it extends over the TFT  3 . 
     The next step is the application of a light-sensitive material  123 . That material converts received external signals (X-rays) into electric charges. The light-sensitive material  123  is beneficially comprised of an amorphous selenium compound that is deposited in a thickness of 100 to 500 μm by an evaporator. However, other X-ray-sensitive materials that having low dark conductivity and high sensitivity to external signals, for example HgI 2 , PbO 2 , CdTe, CdSe Thallium bromide or cadmium sulfide can also be used. When the light-sensitive material is exposed to X-rays, electron-hole pairs are produced in the light-sensitive material in accordance with the strength of the x-rays. 
     After the application of the X-ray-sensitive material, a transparent conductive electrode  133  that passes X-ray is formed. When a voltage is applied to the transparent conductive electrode  133  while X-rays arc being irradiated electron-hole pairs formed in the light-sensitive material are separated into charges that are gathered to the pixel electrode  62  and stored in the storage capacitor “S”. 
     According to the mentioned conventional X-ray image sensing device, however, the depositing and patterning of the electrodes are performed three times to fabricating the storage capacitor “S”. Moreover, the gate line, and the source and drain electrode are overetched while etching the ITO layers. 
     Moreover, as shown FIG. 4, due to the shortness of the length “ΔL” between the first capacitor electrode  58  and the source electrode  33 , a short-circuit can result. 
     Furthermore, a parasitic capacitor between the drain line  52  and the first capacitor electrode  58  can cause problems. 
     The present invention has been developed as a result of continuous effort by the inventors to solve the above-described problems. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to an x-ray image sensor and to a method for fabricating the same and that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     An object of the present invention is to provide an X-ray image sensor having simpler processing steps while forming ITO (indium tin oxide) electrodes. 
     Another object of the present invention is to provide an X-ray image sensor having improved yields. 
     A further object of the invention is to provide a method of forming an X-ray image sensor which can reduce processing error during production by preventing short-circuits and which can decrease noise due to a parasitic capacitor. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve the above objects, the present invention provides an X-ray image sensor, including: a substrate; a gate electrode on the substrate; a first insulation fill on the substrate that covers the gate electrode; a semiconductor film of amorphous silicon film on the first insulation film over the gate electrode; a doped amorphous silicon film on the semiconductor film; source and drain electrodes on the doped amorphous silicon film that are spaced apart from each other; a ground line on the first insulation film that is spaced apart from the source and gate electrodes; a second insulation film covering the whole substrate and having first and second contact holes that expose portions of the ground line and the source electrode, respectively; a first capacitor electrode on the second insulation film, the first capacitor electrode having an electrical connection with the ground line through the first contact hole; an electron transport electrode on the second insulation film, the electron transport electrode having an electrical connection with the source electrode through the second contact hole; a dielectric layer covering the second insulation film, the first capacitor electrode and the electron transport electrode, the dielectric layer having a third contact hole; and a pixel electrode on the dielectric layer having an electrical connection with the electron transport electrode. 
     Beneficially, the pixel electrode extends over the semiconductor film, and the insulation layers are made of a material selected from the group consisting of BCB (benzocyclobutene), acryl and polyamide. Moreover, the X-ray image sensor includes a light-sensitive material on the pixel electrode. 
     In order to achieve the above objects, the invention also provides a method for fabricating an X-ray image sensor including: providing a substrate; forming a gate electrode on the substrate; forming a first insulation film on the substrate and that covers the gate electrode; forming a semiconductor film of amorphous silicon film on the first insulation film over the gate electrode; forming a doped amorphous silicon film on the semiconductor film; forming source and drain electrodes on the doped amorphous silicon film that are spaced apart from each other, forming a channel region by eliminating the doped amorphous silicon film between the source and drain electrodes by using the source and drain electrodes as a mask; forming a ground line on the first insulation film that is spaced apart from the source and gate electrodes; forming a second insulation film covering the whole substrate; forming first and second contact holes that expose a portion of the ground line and a portion of the source electrode, respectively; forming a first capacitor electrode on the second insulation film, the first capacitor electrode having an electrical connection with the ground line through the first contact hole; forming an electron transport electrode on the second insulation film, the electron transport electrode having an electrical connection with the source electrode through the second contact hole; forming a dielectric layer that covers the second insulation film, the first capacitor electrode and the electron transport electrode; forming a third contact hole in the dielectric layer; forming a pixel electron on the dielectric layer, the pixel electrode having an electrical connection with the electron transport electrode; forming a light-sensitive material on the pixel electrode; and forming a transparent conductive electrode that passes X-rays on the light-sensitive material. 
     The insulation layers are beneficially made of a material selected from the group consisting of BCB (benzocyclobutene), acryl and polyamide. The capacitor electrode and the pixel electrode are beneficially made of transparent ITO (indium tin oxide). The X-ray-sensitive material is beneficially one of a group consisting of HgI 2 , PbO 2 , CdTe, CdSe, Thallium Bromide, and Cadmium Sulfide etc. 
     It is to be understood that both toe foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. 
     In the drawings: 
     FIG. 1 is a cross-sectional view illustrating the principle of operation of an X-ray image sensor; 
     FIG. 2 is a plan view illustrating one pixel of an X-ray image sensor according to a conventional art; 
     FIGS. 3 a  to  3   f  are processing diagrams corresponding to a cross-section of FIG. 2 (III—III) and sequentially illustrate the manufacture of an X-ray image sensor according to the conventional art; 
     FIG. 4 is an enlarged cross-sectional view illustrating a portion “G” of FIG. 3 f ; and 
     FIGS. 5 a  to  5   f  are processing diagrams illustrating the manufacture of an X-ray image sensor according to the present invention. 
    
    
     DETAILED DESCRIPTION OF AN ILLUSTRATED EMBODIMENT 
     Reference will now be made in detail to an illustrated embodiment of the present invention, the example of which is shown in the accompanying drawings. 
     According to the present invention, an X-ray image sensor comprises a photoelectric conversion part that produces electric charges in accordance with the received amount of light; a charge storage capacitor having a second insulation film with a plurality of contact holes for a first capacitor electrode, the first capacitor electrode and an electron transport electrode on the second insulation film, a dielectric layer deposited on the first capacitor and electron transport electrodes, and a pixel electrode formed on the dielectric layer in contact with the electron transport electrode through a contact hole, the electron transport electrode for collecting the electric charges produced in the photoelectric conversion part; and a switching TFT that controls the release of electric charges stored in the storage capacitor. 
     The switching TFT is comprised of a gate electrode, a first insulation film, a drain electrode, and a source electrode that contacts the electron transport electrode. 
     FIGS. 5 a  to  5   e  are diagrams that help illustrate the process of manufacturing an X-ray image sensor according to the principles of the present invention. Such an X-ray image sensor uses only two electrodes for a storage capacitor. 
     Referring to FIG. 5 a,  an opaque first metal layer is deposited on a substrate  1 . The first metal layer is then patterned to form a gate electrode  200 . 
     FIG. 5 b  is a cross-sectional view illustrating the depositing of a first insulation film  202  and of a semiconductor layer  208 . The (gate insulating) first insulation film  202  is formed over the substrate by deposition. Then, a dual layer semiconductor film  208  comprised of a pure amorphous silicon film  204  and of a doped amorphous silicon film  206  is deposited and patterned. The semiconductor film  203  can also be made of polycrystalline silicon. 
     As shown in FIG. 5 c,  the next step is the formation of a second metal layer for source and drain electrodes  212  and  210 , and for a ground line  214 . The second metal layer is deposited on the semiconductor layer  208  and on the first insulation layer  202 . Then, the source and drain electrodes  212  and  210  are formed such that they are spaced apart by patterning the second metal layer on the semiconductor layer  208 . The ground line  214  is also formed on the first insulation film  202 , again by patterning the second metal layer. 
     After that, a portion of the doped amorphous silicon film  206  between the source an drain electrodes  212  and  210  is eliminated by using the source and drain electrodes  212  and  210  as masks. This defines a channel region “Ch”. The thin film transistor (TFT) “T” is this completed. In operation the ground line  214  removes residual charge in a storage capacitor that is being formed. 
     FIG. 5 d  is a cross-sectional view illustrating the fabrication step of a first capacitor electrode  222  and of an electron transport electrode  224 . 
     A second insulation film  216  that acts as a protection film is formed over the TFT an the ground line  214 . After the formation of the second insulation film  216 , first and second contact holes  220  and  218  are formed through the second insulation film. The first contact hole  220  exposes a portion of the ground line  214 , while the second contact hole  218  exposes a portion of the source electrode  212 . 
     Then, a first capacitor electrode  222  and an electron transport electrode  224  are formed on the second insulation film. The electron transport electrode  224  is formed such that it electrically contacts the source electrode  212 , while the first capacitor electrode  222  is formed such that it electrically contacts the ground line  214 . 
     Organic BCB (benzocyclobutene), which is transparent material, is beneficially used for the second insulation film  216 . BCB is a material having a dielectric constant less than 3, and a superior planarizing ratio that results in a flat surface. An acryl or a polyamide can also be employed as the second insulation film  216 . 
     Referring to FIG. 5 e , a dielectric layer  226  is then formed over the second insulation film, over the first capacitor electrode  222 , and over the electron transport electrode  224 . Then, a third contact hole  228  is formed through the dielectric  226  such that a portion of the electron transport electrode  224  is exposed. Then, a pixel electrode  230  (alternatively referred to as a second capacitor electrode  230 ) is formed on the dielectric layer  226 . Moreover, the pixel electrode  230  is formed such that it has an electrical connection with the electron transport electrode  224 . As the pixel electrode acts as second capacitor electrode, the storage capacitor “Cst” is completed. The storage capacitor “Cst” is comprised of the first capacitor electrode  222 , the pixel electrode  230 , and the dielectric layer  226 . 
     FIG. 5 f  illustrates a step of applying a light-sensitive material  232  that is used as a converter that converts an external signal (X-rays) into electron-hole pairs. Beneficially, the light-sensitive material  232  is an amorphous selenium compound that is deposited to a thickness of 100 to 500 μm by an evaporator. Alternatively, an X-ray-sensitive material having low dark conductivity and a high sensitivity to external signals, for example HgI 2 , PbO 2 , CdTe, CdSe, Thallium bromide, or Cadmium sulfide can also be used. When such a light-sensitive material is exposed to X-rays, electron-hole pairs are produced in the light-sensitive material in accordance with the strength of the irradiated light. 
     After the application of the light-sensitive material  232 , a transparent conductive electrode  234  that transmits X-ray is formed over the light-sensitive material  232 . 
     When a voltage is applied to the conductive electrode  234  while X-rays are being irradiated, electron-hole pairs are formed in the light-sensitive material. These pairs are separated into electrons and holes, one species of which is gathered to the pixel electrode  230  and stored as charge in the storage capacitor “Cst”. 
     When the gate electrode turns the TFT ON, the stored electric charges are transferred to an external image display device (not shown) and are used to form an X-ray image. 
     After stopping the mentioned switching operation, the residual charges are transferred to the ground line  214 . Therefore, the ground line  214  acts as the reset switch. 
     The present invention provides an X-ray image sensor having a simplicity of processing steps while forming ITO (indium tin oxide) electrodes. Moreover, the present invention provides a method of fabricating an X-ray image sensor that can have improved yields owing to a reduction in the processing steps. 
     Furthermore, the invention provides a more reliable X-ray image sensor having fewer short-circuits between the TFT electrodes and capacitor electrodes. Furthermore, the invention can reduce parasitic capacitances that affect the drain line. Therefore, noise can be decreased. 
     It will be apparent to those skilled in the art that various modifications and variation can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.